Scientific Sessions ABSTRACTS
ORAL SESSIONS
-247-
The Rheic Ocean: Its Origin, Evolution and Correlatives -248-
Scientific Sessions Vertical-axes rotations during and after development of the Ibero-Armorican Arc recorded by
inclusion-trail patterns
Domingo Aerden1 & Mohammad Sayab2
1
Depto. de Geodinámica y Instituto Andaluz de Ciencias de la Tierras (CSIC), Universidad de Granada, 18071
GRANADA; Spain. E-mail: [email protected]
2
National Centre of Excellence in Geology, University of Peshawar, Peshawar 25120, Pakistan
Based on a compilation of field data and orientation data for porphyroblast inclusion trails Aerden
(2004) recognized four sets of overprinting folds and associated fabrics in the Iberian Massif. The
older sets are mainly preserved in low-strain domains and within metamorphic porphyroblasts
(inclusion trails). Regionally consistent orientations of these four groups of (micro-)structures
across the Ibero-Armorican Arc indicate a late-orogenic timing of this orocline, when NE-SW
shortening and associated sinistral shearing produced major vertical-axes rotations in an originally
NE-SW running fold belt. We further tested this model by collecting new inclusion-trail data from
Sierra de Guadarrama (Iberian Central System) and Brittany (Armorican Massif). As shown in the
accompanying Figure, preliminary data from these terranes are apparently consistent with the
earlier data of Aerden (2004), but conflict with the presumed 35° anticlockwise rotation of Iberia
during the opening of the Gulf of Biscay in Cretaceous times. No solution has yet been found for
this enigma. At the conference date (15 June 2009) we expect to also present inclusion-trail data
from the Sierra Albarrana (northern Ossa-Morena Zone) in order to discuss tectonometamorphic
correlations between this terrane and the Central Iberian Zone.
Aerden, D.G.A.M., 2004. Correlating deformation in Variscan NW-Iberia using porphyroblasts; implications for the Ibero-Armorican
Arc. Journal of Structural Geology 26, 177–196.
-249-
The Rheic Ocean: Its Origin, Evolution and Correlatives -250-
Scientific Sessions Pre-Variscan evolution of the Pre-Silurian rocks of the Eastern Pyrenees
Casas, J.M. 1, Castiñeiras, P. 2, Navidad, M. 2, Liesa, M. 3 and Carreras, J. 4
1
Dpt. Geodinàmica i Geofísica, Universitat de Barcelona, Martí i Franquès s/n. 08028 Barcelona, Spain
Dpto. Petrología y Geoquímica-Instituto de Geología Económica, Universidad Complutense de Madrid, José Antonio
Novais 2, 28040 Madrid, Spain
3
Dpt. Geoquímica, Petrologia i Prospecció Geològica, Universitat de Barcelona, Martí i Franquès s/n. 08028 Barcelona,
Spain
4
Dpt. Geologia, Universitat Autònoma de Barcelona, 08193 Bellaterra (Cerdanyola del Vallès) Barcelona, Spain
2
Recent advances in geochronological (U-Pb SHRIMP in zircon) and regional geological
studies in the Canigó, Roc de Frausa, Albera and Cap de Creus massifs, furnish new insights in the
evolution of the pre-Silurian rocks of the Eastern Pyrenees from Late Neoproterozoic to Late
Ordovician:
1) A Late Neoproterozoic-Early Cambrian (560-540 Ma) plutonic and volcanic activity is well
recorded and characterized by metavolcanic plagioclasic gneisses (metatuffs) coeval with
sedimentation and by sheets of granitic orthogneiss emplaced in the lower part of the
metasedimentary series. The metatuffs are spatially associated with metabasites and both lithologies
occur interbedded in the lower and middle part of the pre-Upper Ordovician metasedimentary
succession. This magmatism is bimodal and has a tholeiitic and calc-alkaline affinity. No tectonic or
metamorphic Late Neoproterozoic-Early Cambrian activity related to this igneous event has been
described in the study area to date.
2) The age of the lowermost series of the Canigó, Roc de and Cap de Creus massifs should be
Neoproterozoic (older than 600Ma?), an age slightly older than the magmatic interlayered rocks.
Nevertheless, the age of the lowermost series outcropping in the studied massifs remains
unresolved.
3) An Early Ordovician (475-460 Ma) magmatism is responsible for the emplacement of thick
laccoliths (more than 2000m) of stratoid porphyritic granitic gneisses in the middle part of the preUpper Ordovician succession. The intrusion of these Early Ordovician granites is apparently
unrelated to any deformational or metamorphic episode and predates the Upper Ordovician
unconformity.
4) A Middle (?) Ordovician folding event gives rise to NW-SE to N-S oriented, metric to
hectometric sized folds, without cleavage formation or related metamorphism. These folds can
account for the deformation and uplift of the pre-Upper Ordovician sequence and for the formation
of the Upper Ordovician unconformity. Ordovician folds control the orientation of the structures of
the Variscan main-folding phase in the pre-Upper Ordovician sediments.
5) A Late Ordovician (453-445 Ma) plutonic activity is responsible for the emplacement of granitic
-251-
The Rheic Ocean: Its Origin, Evolution and Correlatives and dioritic bodies in the metasedimentary series. This plutonism is coeval with a synsedimentary
volcanism and a fracture episode that gives rise to normal faults affecting the lower part of the
Upper Ordovician series, the basal unconformity and the underlying Cambro-Ordovician sediments.
As a result:
1) The schemes proposing the presence of a Cadomian basement in the Pyrenees have to be
abandoned.
2) The existence of a Middle Ordovician contractional event prevents us from assuming the
existence of a continuous extensional regime through the Ordovician and Silurian times, and
suggests a more complex evolution of this segment of the northern Gondwana margin during the
Ordovician.
3) The Cambrian igneous activity related to a rifting episode that is widespread in the Iberian massif
has not been recognized in the Pyrenees to date.
4) The largely unfossiliferous character of the pre-Upper Ordovician sequence prevents accurate
stratigraphic correlations with the pre-Ordovician sequences of other parts of the Variscan orogen.
Further geochronological studies are needed to gain insight into the age of the deepest series
cropping out in the Eastern Pyrenees.
-252-
Scientific Sessions Evidence for lithospheric delamination under Iberia
Javier Fernández-Suárez and Gabriel Gutiérrez-Alonso
Departamento de Petrología y Geoquímica
Facultad de Ciencias Geológicas, Universidad Complutense, C/ José Antonio Novais, 2
28040 Madrid, Spain
Departamento de Geología, Facultad de Ciencias
Plaza de los Caídos s/n, 37008 Salamanca, Spain
Lithospheric delamination, although this term is often used for the effects of the slab breakoff after the complete subduction of an ocean, is a mechanically and thermally viable phenomenon
that may occur during or at the end of orogenic processes whereby the lithosphere is thickened
beyond its gravitational stability limits.
The occurrence of lithospheric delamination is widely accepted and has been extensively
modelled and “imaged” by geophysical techniques but there is considerable debate regarding the
identification of this phenomenon in the geological record as only indirect evidences can be used to
prove such a lithospheric tectonic event.
From this point of view, the thermomechanical thinning of the lithosphere must have vast,
relatively fast and unequivocal consequences at orogenic belt scale. If lithospheric delamination
occurs, evidence for it must be found in those geological events –and resulting materials- that
record a profound change in the thermal regime of an orogenic belt or a segment of it.
Implicitly, if lithospheric delamination takes place, the sinking lithospheric “root” makes space for
and must be eventually replaced by ascending asthenospheric mantle, which in turn must have a
first order influence in the geochemical profile of the resulting rejuvenated lithosphere.
In this presentation we will endeavour to show that lithospheric delamination occurred in
Iberia at the end of the variscan collisional orogenesis (ca. 308-290 My ago, Stephanian to Early
most Permian). A companion presentation will further explore the regional to global phenomena
that eventually led to delamination under the Iberian segment of the variscan belt.
Our pivotal arguments to postulate lithospheric delamination in the aforementioned region
are based on the coeval occurrence of:
1) Voluminous granitoid magmatism and volcanism at orogenic belt scale, including the
foreland basin. Notably, these granitoids and volcanics are not spatially related with tectonic
structures generated either by crustal thickening or the subsequent extensional collapse. The
presence of this voluminous magmatism is the ultimate consequence of extensive mantle and crustal
melting across the entire orogenic belt. Nd isotopic composition of mantle derived rocks points to
an isotopic rejuvenation (replacement) of the Iberian sub-continental lithospheric mantle coinciding
-253-
The Rheic Ocean: Its Origin, Evolution and Correlatives with the emplacement of these granitoids (ca 308-290 Ma).
2) U-Pb ages of zircons from lower crustal granulite xenoliths indicate pervasive
granulitization of the lower crust in the exact same time interval.
3) Hydrothermal dolomitization and the genesis of gold mineralisations indicate active fluid
circulation in the crust coinciding with the thermal episode unleashed by delamination.
4) An important orogen scale uplift leading to a missive sediment discharge.
5) High ranks of Stephanian coal seams indicate a high geothermal gradient that cannot be
explained by depth of burial alone.
6) Apatite fission track data indicate annealing of traces in the same time interval.
In our view, the only mechanism -amongst those so far postulated- that can explain the
simultaneous occurrence of the above phenomena in a short time interval at orogenic belt scale is
delamination of the lithosphere.
-254-
Scientific Sessions A peri-Gondwanan arc in NW Iberia: Isotopic and geochemical constraints to the origin of the
arc - The sedimentary approach
Fuenlabrada, J.M.1, Arenas, R.2, Sánchez Martínez, S.3, Díaz García, F.4 and Castiñeiras, P.2
1
CAI de Geocronología y Geoquímica Isotópica. Facultad de Geología. Universidad Complutense de Madrid. 28040
Madrid, Spain. [email protected]
2
Departamento de Petrología y Geoquímica e Instituto de Geología Económica (CSIC). Universidad Complutense de
Madrid. 28040 Madrid, Spain.
3
Institut für Geowissenschaften, Facheinheit Petrologie und Geochemie, J.W. Goethe Universität Frankfurt am Main,
D-60438 Frankfurt am Main, Germany.
4- Departamento de Geología. Universidad de Oviedo. 33005 Oviedo, Spain.
The arc-derived upper terrane in NW Iberia contains a 3000 m thick turbiditic formation
located at the top structural position. The greenschist facies metagreywackes from the Órdenes
Complex show intense shearing but scarce weathering, and most of their primary sedimentological
and geochemical features are well preserved. From a lithological point of view the uppermost
terrigenous series can be divided in two members: a) a lower part with a maximum thickness of c. 1
km which consists of black metapelites with intercalations of very thick to thin beds of grey and
black quartzites and lydites; b) an upper part, c. 2 km thick, that has a flyschoid appearance and
consists of alternations of metagreywackes and grey to black metapelites with conglomeratic
intervals and minor green phyllites and calcsilicate layers. The whole seems to represent an upward
shoaling megasequence. Two detailed partial stratigraphic columns were previously measured and
studied by Gutierrez Alonso et al. (2000), where the identified facies indicate different associations
within a deep submarine fan model (lower-middle fan and upper fan mainly with some scarce
associations representing the slope). The identified facies association seems to indicate a type II
turbiditic system (Mutti, 1985), mostly formed by channel and sand lobes complexes.
Conglomeratic levels consist of pebbles of granitic rocks, quartz and greywacke intraclasts. Most of
the sandstones could be classified as feldespathic greywackes with a framework of quartz and
mainly fresh plagioclase. Rock fragments of vitric and microgranular texture are common in
polymictic conglomerates and coarse grained greywackes, together with slates, chert fragments and
bipyramidal volcanic quartz porphyroclasts. Althought recristallization under greenschists facies
conditions of the chlorite and biotite zones and the presence of two cleavages hinder detailed
textural analysis, the sandstones seems to be immature, typical first-cycle sandstones; grains are
angular to subangular, poorly sorted, and have a muddy matrix. Heavy minerals are mostly
dominated by unabraded zircon grains followed by epidote and rutile.
The metagreywackes have an average major and trace elements chemical composition
-255-
The Rheic Ocean: Its Origin, Evolution and Correlatives similar to the PAAS (Post Archean Australian Shale), which is considered to reflect the composition
of the upper continental crust. Their trace elements composition is very consistent and clearly
shows a convergent tectonic setting for the deposition of the greywackes, probably within an intraarc basin located in a volcanic arc developed over a thinned continental margin. The detrital zircon
populations suggest a Middle Cambrian maximum depositional age for the turbiditic formation
(530-500 Ma), and a Gondwanic provenance located in the periphery of the West African Craton
(Fernández Suárez et al., 2003). The Nd isotope data suggest mixing sources (Ediacaran Paleoproterozoic) for the provenance of the greywackes, with TDM ranging between 720 and 1215
Ma and the average at 995 Ma (n = 20), within an age range not represented at all in the detrital
zircon population. These Nd model ages are similar to those exhibited by West Avalonia, Florida or
the Caroline terrane, but younger than others of Cambrian and Ordovician sandstones and shales
from the Bohemian Massif. These data may suggest a westernmost provenace for the upper terrane
of NW Iberia in relation to the other terranes located in the footwall to the Variscan suture. This
suggested provenance is consistent with some previously proposed paleogeographic models for the
NW Iberia terranes (Gómez Barreiro et al., 2007).
References
Fernández-Suárez, J., Díaz García, F., Jeffries, T.E., Arenas, R., Abati, J. (2003) Constraints on the provenance of the
uppermost allochthonous terrane of the NW Iberian Massif: Inferences from detrital zircon U-Pb ages. Terra Nova,
15: 138-144.
Gómez Barreiro, J., Martínez Catalán, J.R., Arenas, R., Castiñeiras, P., Abati, J., Díaz García, F., Wijbrans, J.R. (2007)
Tectonic evolution of the upper allochthon of the Órdenes complex (northwestern Iberian Massif): Structural
constraints to a polyorogenic peri-Gondwanan terrane. In: Linneman, U., Nance, R.D., Kraft, P., Zulauf, G. (Eds.),
The evolution of the Rheic Ocean: From Avalonian-Cadomian active margin to Alleghenian-Variscan collision.
Geological Society of America Special Paper, 423: 315-332.
Gutiérrez Alonso, G., Barba, P., Díaz García, F. (2000) Sedimentology and structure of the low grade uppermost unit of
the Órdenes Complex at the coastal section. Superposition of structural trends and processes. In: Diaz García, F.,
González Cuadra, P., Martínez Catalán, J.R., Arenas, R. (Eds.), Variscan-Appalachian dynamics: The building of the
Upper Paleozoic basement. 15th International Conference on Basement Tectonics. A Coruña, Spain, Program and
Abstracts Volume: 234-235.
Mutti, E. (1985) Turbidite systems and their relations to depositional sequences. Provenance of arenites. NATO A.S.I.
Series, 148: 65-93.
-256-
Scientific Sessions How to bend an orogen
Gabriel Gutiérrez- Alonso1, Stephen T. Johnston2, Arlo B. Weil3, Javier Fernández-Suárez4, Daniel
Pastor-Galán1 & J. Brendan Murphy5.
1
Departamento de Geología, Universidad de Salamanca, Salamanca 37008, Spain. [email protected].
School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055 STN CSC, Victoria, British Columbia,
V8W 3P6 Canada. [email protected].
3
Department of Geology, Bryn Mawr College, Bryn Mawr, PA 19010, USA. [email protected].
4
Departamento de Petrología y Geoquímica, Universidad Complutense, Madrid 28040, Spain. [email protected].
5
Department of Earth Sciences, St.Francis Xavier University, Antigonish, Nova Scotia, B2G 2W5 Canada.
[email protected].
2
The origin and development of arcuate orogens is a current subject of intense debate. A key issue of
contention in this regard is the lack of kinematic data/criteria to constrain the relative timing of bending
relative to orogenic belt formation in a “classic” plate convergence scenario. The relative chronology of the
kinematic interplay of those two major overlapping processes (and how they are recorded in the structural
architecture of orogenic belts) is crucial in unravelling the processes that cause lithospheric bending around
vertical axes whose ultimate effect is the formation of arcuate mountain belts.
Several case studies in modern orogenic belts worldwide are arguably a good starting point to investigate the
above issue and further our understanding of the links and cause-effect relationships between large scale
lithospheric processes and orogen-scale bends.
For example, it is relatively straightforward to link the bending of the Central American ribbon continent
(especially in Panamá) with the change in relative motion between the North and South American plates
which has caused a dramatic change in the stress regime responsible of the sinusoidal shape of the Central
American volcanic arc.
Another well known and extensively studied example of a modern bent orogen can be found in the two sharp
syntaxes located at the edges of the Himalayan mountain range which resulted from the indentation of the
Indian continent into the Eurasian plate.
In contrast, the kinematic development and relative chronology of major tectonic events in ancient curved
orogenic belts are much more difficult to constrain. The horse-shoe shaped variscan orogenic belt of western
Europe is a prime example of an arcuate belt whose kinematic development has been explained by several
hypothesis that involve significantly different processes. Nevertheless, most of these models do not fully take
into account the wealth of paleomagnetic data that constrain the kinematic history of the bending of the
variscan belt, especially in its inner arc (i.e. the Cantabrian Zone).
Herein we propose a mechanism to explain the processes that caused the bending of the Iberian-Armorican
arc. This mechanism is consistent with and integrates geological, paleomagnetic, geochemical evidence that
point to a post orogenic (post-collisional and post-collisional collapse) origin of the Iberian-Armorican arc;
that is to say a true orocline.
-257-
The Rheic Ocean: Its Origin, Evolution and Correlatives The ultimate goal in explaining the origin of large curved orogenic belts is linking the kinematics of their
development with global scale plate motion driven processes. Here we will hypothesize on the origin of the
Iberian-Armorican arc in relation to Pangea amalgamation and subsequent evolution.
One of the most dramatic and rare occurrences in Earth history is the amalgamation of most of the
continental lithosphere into one super-continent. The most recent super-continent, Pangaea, lasted from ca.
320 to 200 million years ago. Here we hypothesize that after the continental collisions that led to the
amalgamation of Pangaea, plate convergence continued in a large, wedge shaped oceanic tract. We suggest
that plate strain at the periphery of the super-continent would eventually result in self-subduction of the
Pangean global plate, with the ocean margin of the continent subducting beneath the continental edge at the
other end of the same plate. This scenario would result in a stress regime within Pangaea that explains the
development of a large oroclinal fold structure, the Iberian-Armorican Arc still visible in the crustal rocks of
Western Europe, near the apex of the Palaeotethys Ocean, the extensive lower crustal heating and
voluminous magmatism at the core of the continent as well as the development of radially arranged
continental rifts in more peripheral regions of the plate.
In this large scale scenario the role of the mantle lithosphere still remains almost unexplored. Although
delamination of continental lithosphere in the core of collisional orogens is a well established process, the
mechanisms responsible for it are still poorly understood. Contemporaneous with the waning stages of
Pangea amalgamation and the generation of the Iberian-Armorican Arc (the orocline in the core of Pangea),
in Iberia a contrasting Sm-Nd isotopic signature between pre- and post- ca. 285 Ma mantle-derived mafic
rocks can be recognized. This change in the Sm-Nd isotopic signature suggests that the sub-continental
lithospheric mantle (SCLM) under Iberia was replaced in Early Permian times. We propose that the
delamination of thickened continental lithosphere in the core of the orocline triggered asthenospheric
upwelling and replacement of the ancient SCLM, providing a mechanism and explanation for the contrasting
Sm-Nd isotopic characteristics of pre- and post- ca. 285 Ma mafic rocks. This replacement was triggered by
the orocline formation that in turn resulted from the effects of the self-subduction of Pangea.
-258-
Scientific Sessions The sands of Pangea –
U-Pb-LA-ICP-MS geochronology of detrital zircon grains:
a case study of the Mesozoic of Central Europe
Mandy Hofmann1, Thomas Voigt2 & Ulf Linnemann3
1
c/o: Senckenberg Naturhistorische Sammlungen Dresden, Museum für Mineralogie und Geologie, Koenigsbruecker
Landstraße 159, D-01109 Dresden, E-mail: [email protected]
2
Institut für Geowissenschaften, Friedrich-Schiller-Universitaet Jena, Burgweg 11, D-07749 Jena
3
Senckenberg Naturhistorische Sammlungen Dresden, Museum für Mineralogie und Geologie, Koenigsbruecker
Landstraße 159, D-01109 Dresden
The formation of the supercontinent Pangea was finished during Permian times. The closure of the
Rheic Ocean is only one geotectonic process connected to the combination of all continents at that time.
Whereas in some areas this process was still active, in other areas e.g. pull-apart basins opened as first signs
for the beginning break-up of Pangea.
The Triassic period represents the time during and after the initial break-up of the supercontinent
Pangea. In this time period extreme amounts of sand were accumulated in the interior of the continent,
whereas the source areas are not yet identified completely. Examples are the sandstones in the continental
facies of the Triassic in Central Europe (“German Triassic”). Tension and weathering during the Permian and
intense subsidence during the Triassic are responsible for the formation of several hundred of meters thick
and widespread sandstone packages in this time. In the Early Jurrassic the central Atlantic Ocean opened,
separating Africa from North America. During Cretaceous time the southern Atlantic Ocean opened while
North America was still connected to Europe. Central Europe was a part of the European continental plate.
Due to high spreading rates of mid-ocean ridges and intra-oceanic plateau volcanism large areas of the
epicontinental shelf were flooded.
We have analysed detrital zircon grains from the Buntsandstein (Lower Triassic) and the Keuper
(Upper Triassic) using U and Pb isotopes by LA-ICP-MS (Laser Ablation combined with Inductive Coupled
Mass Spectrometry). These analyses yielded different zircon ages with a main age spectrum between ca. 250
Ma and ca. 700 Ma. Distinct zircon grains of Meso- and Paleoproterozoic ages were found. In addition to the
Triassic samples we analysed detrital zircon grains from the Middle Jurassic (Dogger) and the Cretaceous
(Cenomanian) concerning their LA-ICP-MS U-Pb zircon ages. The Cretaceous samples show a similar age
distribution to the Triassic ones: the main spectrum of zircon ages lies between ca. 240 Ma and 700 Ma.
Also, there are a few isolated zircon grains with Meso- to Paleoproterozoic ages. An important change shows
the Jurassic Sandstone, as the zircons of this sample have a main age distribution between ca. 950 Ma and
1900 Ma.
The zircon age distribution shows, that the source areas for the Mesozoic sedimentary record changed
-259-
The Rheic Ocean: Its Origin, Evolution and Correlatives clearly. We interpret the Paleozoic to Neoproterozoic ages of all samples as the influx of reworked local
material, such as the Avalonian/Armorican basement units and the Variscan Basement. In our interpretation,
the enormous amounts of Mesoproterozoic to upper Paleoproterozoic zircon ages in the Jurassic sample
originated in a southward sedimentary transport due to an oceanic current between Middle Europe and
Baltica, as these specific zircon ages are typical for Baltica. This arm of the sea was due to the ongoing
break-up of Pangea.
References
Stampfli, G.M. & G.D. Borel G.D. (2002): A plate tectonic model for the Paleozoic and Mesozoic constrained by
dynamic plate boundaries and restored synthetic oceanic isochrons. Earth and Planetary Science Letters 196. 17-33.
Walter, R. (2003): Mesozoikum. In: Walter, R. (ed.): Erdgeschichte-Die Entstehung der Kontinente und Ozeane. Walter
de Gruyter Verlag. 175-223. Berlin, New York
-260-
Scientific Sessions Contrasting Orogens?:
The Accretionary North American Cordillera versus the Collisional Variscan
Stephen T. Johnston1 & Gabriel Gutierrez-Alonso 2
1
School of Earth & Ocean Sciences, University of Victoria, PO Box 3065 STN CSC, Victoria, British Columbia,
Canada V8W 3P6 [email protected]
2
Departamento de Geología, Universidad de Salamanca, Salamanca 37008, Spain [email protected]
Constraining the paleogeographic evolution of Earth back through time into the ancient past
is, a difficult task. The oceanic record of plate motions spans only the last 180 Ma of Earth history,
and even this record is incomplete due to the consumption of oceanic lithosphere within subduction
zones. Hence, for most of Earth history we have to rely upon the continental record of plate motions
and interactions in order to constrain paleogeography. Amongst the most important of continental
records of plate motion are orogenic belts or mountain systems, as these develop along plate
margins, and are attributable to the interaction of, and hence provide constraints on the locations of,
two or more lithospheric plates. Understanding and developing coherent interpretations of modern
and ancient orogens has, therefore, been one of the major focuses of the Earth Sciences.
Amongst the most critical of paleogeographic events in Earth history was the formation (and
subsequent demise) of the supercontinent Pangea. The initiation of break-up of the supercontinent,
at the end of the Permian, corresponds with the greatest biological crisis in Earth history – the
Permo-Triassic extinction event, in which >90% of all species on Earth were lost. One of the largest
magmatic events ever recorded, the eruption of the Siberian Traps, was coeval with the PermoTriassic extinction event, and may be a product of the same processes responsible for break-up of
the supercontinent (Gutiérrez-Alonso et al., 2008). Formation of the supercontinent, which is
thought to have occurred during or slightly before the end of the Carboniferous era, was attended by
the proliferation of bark-bearing trees across the continental landmass. It was those forests that
gave rise to the extensive coal deposits that in turn gave their name to the Carboniferous era.
Two major orogenic belts, the Variscan of western Europe and the Cordilleran of western
North America, are intimately related to, and provide us with constraints on the formation and
subsequent break-up, respectively, of Pangea. The Paleozoic Variscan orogen is interpreted as the
product of the continental collisions that amalgamated the disparate continents of Earth into a single
entity – Pangea. In contrast, the Mesozoic Cordilleran orogen of western North America (hereafter
referred to simply as the Cordillera), which is interpreted to have formed along the active western
margin of Pangea during its demise
is commonly held to have developed in response to steady-
state processes, including oblique subduction and transcurrent faulting, along a convergent margin
-261-
The Rheic Ocean: Its Origin, Evolution and Correlatives that faced west toward the open ocean of eastern Panthalassa, the one ocean that surrounded Pangea
and whose progeny is the Pacific.
One might expect, given the strongly contrasting geodynamic settings of the two orogens,
that they would be readily distinguishable in terms of their geometry, crustal architecture, thermal
structure, and magmatic and sedimentary records. It is this very point that we address, because
contrary to expectations, the two orogens are, in many ways, remarkably similar. We focus on six
main shared traits: (1) orogenic architecture; (2) late-tectonic dextral strike-slip faults; (3)
voluminous, syn- to post-tectonic, mixed I- and S-type foreland magmatism; (4) oroclinal bending;
(5) relative temporal evolutions; and (6) discordant paleomagnetic estimates of paleolatitude. How
and why such different orogens should share so many traits remains to be discerned. Howevwer,
the similarity of the two orogens implies that the use of continental orogenic belts as a constraint on
the paleogeographic evolution of Earth must be treated with caution.
-262-
Scientific Sessions New Techniques in 40Ar/39Ar Thermochronology to Derive Temperature-Time Histories of
Orogenic Terranes
Lee, James K.W.
Department of Geological Sciences and Geological Engineering,
Queen’s University,
Kingston, Ontario, Canada K7L 3N6
The
40
Ar/39Ar dating method has proven to be one of the most versatile of all of the
geochronological techniques used to elucidate the thermal history of earth. Because Ar is a noble
gas, it is more sensitive to temperature effects than most other geochronometers. Furthermore, the
large range of Ar closure temperatures (typically 250-550 ˚C) spanned by a variety of common, Kbearing minerals (feldspar, micas, amphiboles) has meant that
40
Ar/39Ar thermochronological
studies have the ability to elucidate a significant portion to the temperature-time (T-t) history of a
geologic terrane.
40
Ar/39Ar thermochronology has been successfully applied in a wide variety of geological
studies in fields such as volcanism, economic geology, sedimentary basin evolution, tectonics,
metamorphism,
and
the
evolution
of
the
solar
system.
Fundamentally,
40
Ar/39Ar
thermochronological methods are based on the physical principles of solid-state (volume) diffusion
(Crank 1975). Using these principles, it is possible to calculate an Ar closure temperature (which is
a characteristic temperature at which approximately 42% of the argon is retained in the crystal) for a
given mineral (Dodson 1973). Combining the Ar closure temperature with a knowledge of the
40
Ar/39Ar age of that mineral yields a very powerful tool to deduce the T-t history of a geologic
region.
Recent technological advances in the 40Ar/39Ar method now give us the ability to measure Ar
diffusion profiles within mineral grains. The
40
Ar/39Ar step-heating method can utilize either a
resistance furnace or a laser (commonly a CO2 or Ar-ion laser) with a defocused beam to evenly
heat single mineral grains or small aliquots of grains to obtain 40Ar/39Ar age spectra which can be
modelled to derive possible T-t histories. An alternative method is to use a finely focused laser
beam to obtain high spatial-resolution laser spot dates within single mineral grains; in effect, using
the laser as an age microprobe. The resultant age profile can also be modelled to obtain possible T-t
histories consistent with the observed data.
The earth’s crust is often affected by thermal pulses, where the rocks may experience elevated
temperatures for brief periods of time, e.g. dyke injection, magma recharge, heat advection by hot
fluids, etc. Although geochemical techniques such as geothermometry can be used to estimate peak
temperatures, it is much more difficult to estimate the duration of such thermal pulses, and hence,
-263-
The Rheic Ocean: Its Origin, Evolution and Correlatives the resultant T-t history experienced by the rocks. By coupling solid-state diffusion theory with the
40
Ar/39Ar microanalytical techniques described above, there are new and powerful methods of
determining the integrated temperature and duration of such thermal pulses. For any given Kbearing mineral in a rock, a measured Ar diffusion profile directly corresponds to a mean Ar
diffusion distance (commonly called ”x-bar”) which is defined as the average distance that an Ar
atom migrates over a certain time t at a certain temperature T. This mean diffusion distance is not
unique in T-t space, however, as it can be shown that there are actually an infinite family of T-t
pairs, each of which could yield that particular mean diffusion distance; this family of pairs defines
a single curve in T-t space. If there are at least two different minerals from a rock sample in which
diffusion profiles can be measured, it is therefore possible to use the two mean Ar diffusion
distances to derive two different T-t curves, and the intersection of these two curves will this yield
the unique T-t conditions experienced by the rock.
This method has been used to tightly constrain the flow of hot fluids in shear zones in the
lower crust of Norway (Bergen Arcs) – a process which was demonstrated to be of very short
duration (kyrs). More significantly, this result was incorporated into further models utilizing Ar
diffusion modelling coupled with numerical tectonic models of subduction and exhumation during
the Caledonian. In this way, the overall duration of an entire orogenic cycle (subduction and
exhumation) in the Bergen Arcs was shown to be extremely short – only ~13 Myr – derived solely
by using these recently developed methodologies in 40Ar/39Ar thermochronology. Consequently, the
combined use of
40
Ar/39Ar microanalytical techniques, Ar diffusion modelling, and numerical
tectonic modelling can be an extremely powerful tool that can be used in tectonic studies to
elucidate the thermal histories of geologic terranes.
References
Camacho A, Lee JKW, Hensen BJ, and Braun J (2005) Short-lived orogenic cycles and the eclogitization of cold crust
by spasmodic hot fluids. Nature 435: 1191-1996.
Crank J (1975) The Mathematics of Diffusion (2nd ed.) Clarendon Press, Oxford
Dodson MH (1973) Closure temperature in cooling geochronological and petrological systems. Contributions to
Mineralogy and Petrology 40: 259-274.
-264-
Scientific Sessions A piece of Avalonia in Turkey - The İstanbul Block: Its provenance, geotectonic
setting and palaeogeography constrained by U-Pb-LA-ICP-MS geochronology of
detrital zircon
Ulf Linnemann1, Kerstin Drost2, Erdin Bozkurt3, Erdinç Yiğitbaş4, Aral I. Okay5
1
2
Senckenberg Naturhistorische Sammlungen Dresden, Museum für Mineralogie und Geologie, Königsbrücker
Landstraße 159, Dresden, D-01109, Germany.
(e-mail: [email protected]; [email protected])
AEON EarthLAB, Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, South Africa.
3
Middle East Technical University, Department of Geological Engineering, TR-06531 Ankara, Türkiye.
4
Department of Geological Engineering, Çanakkale Onsekiz Mart University, TR-17000 Çannakale, Türkiye
5
Eurasia Institute of Earth Sciences and Department of Geology, Istanbul Technical University, Türkiye
The İstanbul Block is interpreted to be an exotic terrane of Avalonian affinity that was
derived from the periphery of Gondwana (Winchester et al., 2006; Okay et al., 2006; Bozkurt et al.,
2008). Unlike to the other Turkish terranes, a Variscan metamorphic event is not known. Ediacaran
plutonic rocks are about 565-576 Ma old, which intrude Ediacaran calc-alkaline metavolcanics
(Bozkurt et al., 2008). The latter ones overlay amphibolite-facies mafic and subordinate ultramafic
rocks. The geological record of Palaeozoic rocks (“İstanbul Palaeozoic”, e.g. Paeckelmann, 1925)
represents a sedimentary sequence of conglomerates, sandstones, shales and carbonates in the range
from the Ordovician to the Lower Carboniferous. We have analysed detrital zircon grains from
siliciclastic sediments of Ordovician and Silurian strata using U and Pb isotopes by LA-ICP-MS
(Laser Ablation combined with Inductive Coupled Mass Spectrometry). U-Pb ages of detrital zircon
grains indicate Avalonia as a potential source area. Passive margin setting on the Gondwanan
margin is indicated by Ordovician high mature quartzites. An active geotectonic setting is
documented by immature Lower Silurian greywackes that most probably reflect the closure of the
Tornquist Sea and the docking event on to Baltica.
References
Bozkurt, E., Winchester, J.A., Yiğitbaş, E., Ottley, C.J., 2008. Proterozoic ophiolithes and mafic-ultramafic complexes
marginal to the İstanbul Block: An exotic terrane of Avalonian affinity in NW Turkey. Tectonophysics, 461, 240251.
Okay, A.I., Satir, M., Siebel, W., 2006. Pre-Alpide Palaeozoic and Mesozoic orogenic events in the Eastern
Mediterranean region. In: Gee, D. G., Stepehnson R.A. (Eds.): European Lithospere Dynamics. Geological Society,
London, Memoirs, 32, 389-405.
Paeckelmann, W, 1925, Beitrage zur Kenntnis der Devons am Bosporus, insbesondere in Bithynien. Abh. Preussische
Geol. Landesanstalt N.F., 98, 150 p., Berlin.
Winchester, J.A., Pharaoh, T.C., Verniers, J., Ioana, D., Seghedi, A., 2006, Palaeozoic accretion of Gondwana-dereived
terranes to the East European Craton: recognition of detaches terrane fragments dispersed after collision with
promontories. Geological Society, London, Memoirs, 32, 323-332.
-265-
The Rheic Ocean: Its Origin, Evolution and Correlatives Phase diagram modeling of high-pressure metapelites from the Malpica-Tui Unit: constrains
on the subduction of the N Gondwana margin
López Carmona, A.1, Abati, J.1 and Pitra, P.2
1
Departamento de Petrología y Geoquímica and Instituto de Geología Económica, Universidad Complutense-Consejo
Superior de Investigaciones Científicas, 28040 Madrid, Spain
2
Géosciences Rennes, UMR CNRS 6118, Université Rennes 1, 35042 Rennes, France
The Malpica-Tui unit in the NW Iberian Massif is reputed for the relics of high-pressure,
eclogite and blueschist facies metamorphism. Basal Units form a huge and complex sheet emplaced
upon the sequences deposited on the passive margin of northern Gondwana, and show the imprints
of a late Devonian high-P metamorphism that has been interpreted as related with its subduction
below the southern border of Laurussia at the onset of the Variscan convergence (e.g. Martínez
Catalán et al., 2009). The higher structural level of the unit comprises a volcanosedimentary
sequence that shows a marked variation in mineralogy and texture. In the basal part pelitic schists
(referred to hereafter as “Lower schists”) are interbedded with amphibolites containing lawsonite
pseudomorphs, and the most complete and best preserved blueschist facies parageneses are found
here. We report in particular the assemblage chloritoid + glaucophane, preserved as inclusions in
garnet porphyroblasts. This mineral association, described in many subduction-related terranes
around the world as one of the high-pressure indicators for metapelites has not been reported
previously in the NW section of the Iberian Massif.
The Lower schists contain an initial blueschist facies assemblage formed by chloritoid +
garnet (Alm0.58 Prp0.03 Grs0.38 Sps0.09) ± glaucophane + phengite (3.5-3.4 Si p.f.u.) + paragonite +
chlorite + epidote + rutile + ilmenite + quartz, preserved as inclusions in garnet, chloritoid and
albite porphyroblasts, in which they define an internal fabric (S1). The matrix foliation (S2) contains
an association formed by garnet (Alm0.68 Prp0.04 Grs0.25 Sps0.03) + phengite (3.4-3.3 Si p.f.u.) +
paragonite + katophorite + taramite + hornblende + chloritoid + chlorite + epidote + rutile +
ilmenite + albite + quartz ± biotite .
Upwards in the sequence chloritoid+glaucophane bearing schists disappear leading to
micaschists (“Upper schists”) containing garnet (Alm0.67 Prp0.04 Grs0.24 Sps0.05) + phengite (3.4-3.3
Si p.f.u.) + chlorite + epidote + rutile + ilmenite + sphene + albite + quartz, defining a first foliation
(S1) preserved as micro-inclusions in garnet and albite porphyroblasts. The second fabric (S2)
contains garnet (Alm0.61 Prp0.05 Grs0.18 Sps0.16) + phengite (3.3-3.2 Si p.f.u.) + paragonite + chlorite +
epidote + rutile + ilmenite + magnetite + sphene + albite + biotite + quartz. Post-S2 deformations
include the development of C´planes that contain “retrograde” minerals such as chlorite, sericite,
-266-
Scientific Sessions stilpnomelane, carbonates and sulfurs indicating very low-grade greenschist facies conditions.
Finally, in the upper levels of the series, the content of mica decreases, garnet disappears and a
gradual increase of opaque phases and quartzite beds can be observed. The paragenesis becomes
restricted to fine grained white mica, chlorite and quartz, giving to this rock a slaty texture. This
suggests that the metamorphic gradient decreases rapidly upwards. This transition occurs in
approximately 500 m of maximum thickness from the base to the upper part of the sequence
indicating an important condensation of metamorphic isograds, compatible with extensional
detachment faulting (López-Carmona et al, 2008).
Two P-T pseudosections in the MnNCKFMASHTO system calculated with
THERMOCALC
3.31 (Powell and Holland, 1988) using the internally consistent thermodynamic dataset (Holland
and Powell, 1998; updated Nov. 2003) show that the chloritoid+glaucophane-bearing paragenesis of
the Lower schists is stable at ca. 16-20 kbar. This suggests that the rocks have been subducted to
depths of ca. 60-75 km. The subsequent growth of biotite and albite porphyroblasts, according to
their stability fields in the P-T pseudosection, indicates a strong decompression accompanied by
slight heating to reach the metamorphic peak at ∼ 500 ºC. In the Upper schists, the observed M1
assemblage is best approached by the field g+chl+ep+ilm+ru+ab+sph (Fig. 2B) indicating
conditions of ca. 5 kbar, 400°C. Crystallisation of magnetite and biotite during M2 suggests and
evolution characterised by decompression and possibly
cooling.
In order to further refine the P-T conditions of the M1
and M2 metamorphic events in both types of schists we have
used multi-equilibrium thermobarometry. Average P-T
calculations (Powell and Holland, 1988) using inclusion
compositions in the chloritoid+glaucophane-bearing Lower
schists indicate M1 conditions at about P ∼ 19 kbar and T ∼
450 ºC. The M2 values obtained using the matrix mineral
compositions are P ∼ 14 kbar and T ∼ 500 ºC. In the Upper
schists the calculations indicate conditions around P ∼ 14
Kbar and T ∼ 550 ºC for both, the inclusion and the matrix
assemblages (Fig. 1).
-267-
The Rheic Ocean: Its Origin, Evolution and Correlatives For the chloritoid+glaucophane-bearing Lower schists, there is a good agreement between
both methods. The MnNCKFMASTO P-T pseudosection predicts for these average P-T results a
paragenesis formed by gl + g + ctd + ru + ilm +law for M1 and, a gl + g + chl + ep + ru + ilm + pa
mineral assemblage for M2 (Fig.2A). This is essentially
in agreement with the petrographic observations, but for the absence of lawsonite, that is however
easily destabilised during the subsequent metamorphic evolution (e.g. Ballèvre et al., 2003). In
addition, pseudomorphs of this mineral are not rare in the interbedded mafic rocks; thus, probably
some of the abundant crystals of zoisite/clinozoisite found in the matrix of the schists could have
been formed at the expense of lawsonite.
On the other hand, for the Upper schists, there is a serious disagreement between both
methods. This can be related to serveral problems, including an incorrect interpretation of the
equilibrium assemblages and compositions, inappropriate identification of the bulk composition
(equilibration volume) or the insufficient complexity of the mixing models for some solid solutions.
Our preference goes to the pseudosection results that are further supported, and supplemented, by
different types of isopleths calculated for the schists composition (silica content in phengites and
calcium and magnesium content in garnet).
An interesting fact is the presence of “glaucophane” almost in the whole pseudosection, up to
-268-
Scientific Sessions 600 ºC. However, the solid solution model used (Diener et al., 2007) considers ferric iron and
“glaucophane” may then have a largely variable composition between the different assemblages.
Nevertheless, chloritoid + glaucophane is only stable at pressures higher than ca. 15 kbar, bearing
out that this assemblage is one of the high pressure indicators for metapelites, as it has been
reported in a number of high-P terranes around the world and so in the Malpica-Tui Unit. Therefore,
in this context, qualitative P-T paths and quantitative P-T values obtained with different modeling
techniques for both types of schists, support the thinning of the series with a rapid decrease of
pressure upwards and provide an approximate depth of subduction for the Malpica-Tui unit.
Ballèvre, M., Pitra, P. and Bohn, M. (2003) Lawsonite growth in the epidote blueschists from the Ile de Groix
(Armorican Massif, France): a potential geobarometer. Journal of Metamorphic Geology : 21(7), 723-735.
Diener, J.F.A., Powell, R., White, R.W. and Holland, T.J.B. (2007) A new thermodynamic model for clino-and
orthoamphiboles in Na2O - CaO - K2O - FeO - Fe2O3 - MgO - Al2O3 - SiO2 - H2O (NCKFMASHO). Journal of
Metamorphic Geology: 25, 631-656.
Holland, T. J. B. and Powell, R. (1998) An internally consistent thermodynamic data set for phases of petrological
interest. Journal of Metamorphic Geology: 16, 309-343.
López-Carmona, A., Abati, J. and Reche, J. (2008) Evolución Metamórfica de los Esquistos de AP/BT de Ceán (Unidad
de Malpica-Tui, NW del Macizo Ibérico). Geogaceta: 43, 3-6.
Martínez Catalán, J. R., Arenas, R., Abati, J., Sánchez Martínez, S., Díaz García, F., Fernández Suárez, J., González
Cuadra, P., Castiñeiras, P., Gómez Barreiro, J., Díez Montes, A., González Clavijo, E., Rubio Pascual, F. J.,
Andonaegui, P., Jeffries, T. E., Alcock, J. E., Díez Fernández, R. and López Carmona, A. (2009) A rootless suture
and the loss of the roots of a mountain chain: The Variscan belt of NW Iberia. Comptes Rendus Geosciences: 341(23), 114-126.
Powell, R. and Holland, T. J. B. (1988) An internally consistent dataset with uncertainties and correlations: 3.
Applications to geobarometry, worked examples and a computer program. Journal of Metamorphic Geology: 6, 173204.
-269-
The Rheic Ocean: Its Origin, Evolution and Correlatives Lower-Paleozoic rifting-related magmatism in two Northeastern Iberian Variscan massifs.
Geochronology and geochemistry.
F. J. Martínez, R. Capdevila, A. Iriondo, J.Reche
Orthogneiss bodies occurring in the pre-Mesozoic Variscan basement massifs of the Pyrenean axial
zone and related Variscan massifs North (Montagne Noire) and South (Catalan Coastal Ranges)
have since long been the subject of interest and controversy.
As more precise dating of these gneisses has accumulated it seems that the presence of a preVariscan crystalline basement can be ruled out since all the radiometric ages of orthogneiss
protoliths from the Pyrenees and vicinity, or from equivalent rocks in Iberia are mainly Ordovician.
Two massifs are the subject of this study; the Núria massif belongs to the Variscan basement of the
Pyrenees, and around 50 kms to the south, within the Catalonian Coastal Ranges, is located the
Guilleries massif. Both of them are constituted by lower Paleozoic Cambro-Ordovician
metasediments affected by a metamorphism of Variscan age that reaches the sillimanite grade in
metapelites of the southermost part of the Guilleries massif, whereas in Núria only reaches biotite
grade to the north of the massif.
U-Pb Shrimp-RG dating has been carried out in zircons in these two massifs.
Three pre-Variscan gneiss bodies occur in the Núria massif: the Núria main gneiss body, the
external ring gneiss, and the Ribes de Freser isotropic granophyre whose respective upper
Ordovician coeval ages are 457.1 ± 3.6 Ma, 456.5 ± 4.6 Ma, 458.1 ± 2.6 Ma.
Several tabular, leucocratic, fine-grained gneissic bodies, concordant with the regional foliation
appear in the Guilleries massif. They have yielded ages grading from 487.3 ± 2.5 Ma (upper
Cambrian - lower Ordovician) in one thin gneiss layer, to 459± 3 Ma, 464± 3 (middle to upper
Ordovician) for the thicker (tens to hundred of meters) gneiss slabs, which indicate the climatic
intrusive event. Llandeilian-Caradocian (Upper Ordovician) volcanics and volcanoclastics dated
from interbedded fosiliferous strata have given 451.7± 3.7 totally in agreement with their
paleontological age.
The Nuria main gneissic body is a fractionated leucogranite sourced from metapelites. The external
ring of this massif is a calc-alkaline suite going from non-fractionated to strongly fractionated, POG
granitoids. The Ribas de Freser granophyre is also a calc-alkaline, fractionated, POG granite.
The Guilleries gneisses are non- or slightly-fractionated calc-alkaline granites, related to extensional
-270-
Scientific Sessions events and originated from deep-seated igneous protoliths. The Caradocian metavolcanics, despite a
strong hydrothermal alteration, show the striking adakitic signature of rocks coming from a garnetamphibolite, low-crustal source.
Therefore, three magmatic types, usually spacially and temporally separated in orogens, are present
at the same spot and aproximately the same time.
A likely interpretation is that the Ordovician extension of what was going to be the Variscan realm
can lead to either a true oceanic basin from which the ophiolitic nappes come from, or, as shown in
the present study, a crustal thinning. The heat supplied in this latter case induces the simultaneous
melting of different non-oucropping Precambrian fertile crustal sources like garnet amphibolites,
calc-alkaline granitoids and metapelites.
-271-
The Rheic Ocean: Its Origin, Evolution and Correlatives Understanding the interplay between the Coimbra-Cordoba shear zone and
the Porto-Tomar fault zone, in the SW Iberian Massif (Portugal)
Pereira, M.F.1, Silva, J.B.2, Chichorro, M.3, Drost, K.4, Apraiz, A.5
1- Departamento de Geociências, Escola de Ciências e Tecnologia, CGE, Universidade de Évora, Portugal;
2- Departamento de Geologia, Faculdade de Ciências, IDL, Universidade de Lisboa, Portugal;
3- Departamento de Ciências da Terra, FCT, CICEGE, Universidade Nova de Lisboa, Portugal;
4- Department of Earth Sciences, University of Cape Town, AEON, CGE, South Africa
5- Geodinamika Saila, Zientzia Teknologia Fak., Euskal Herriko Unibertsitatea. Apt. 644, 48080 Bilbo, Spain
In this work, we present field relationships, new U-Pb LA-ICP-MS zircon and monazite dating
of Variscan migmatites, gneisses and granites from two key sections of the western Iberian Massif: (1)
the Caia section where the Coimbra-Cordoba shear zone (CCSZ; Burg et al., 1981; Pereira et al., 2008)
is well-exposed; and (2) the Martinchel section where is possible to study the interplay between the
CCSZ and the Porto-Tomar fault zone (PTFZ; Ribeiro et al., 1980).
Different kinematic models have been invoked to explain the formation and evolution of these
two major structures of the Variscan belt of Western Europe. These models are mainly supported on
assumptions and, with no allusion to detailed field structural mapping and reliable geochronology.
Our model based on interpretation of field relationships and new U-Pb zircon and monazite ages
proposes that: (1) ductile deformation and metamorphism were active in the CCSZ during the Visean
and created conditions for partial melting and coeval emplacement of granites; (2) later ductile-brittle
deformation linked to dextral displacements along the PTSZ overprint the earlier CCSZ ductile
deformation as well as, the Visean-Serpukhovian granites, after the Kasimovian .
The new obtained results of U-Pb dating on zircons and monazites yielded c.335-318 Ma
(Visean) ages for the ductile deformation and partial melting developed under amphibolite
metamorphic conditions in the Martinchel gneisses and, c. 335-334 Ma (Tournaisian-Visean) ages for
the Caia HP migmatites and basic granulites. The Martinchel gneisses were intruded by the Tramagal
muscovitic granite at c. 334 Ma (Visean) and, by dykes of leucogranites at c.319Ma (SerpukhovianBashkirian). The Martinchel gneisses and associated granites, probably related to a progressive
tectonothermal evolution, were later deformed under a brittle-ductile regime of deformation by the
PTFZ dextral displacement.
According to our results and recent published radiometric data, we assume that: (1) the CCSZ is
a major Variscan structure that was active before the Kasimovian give that is intruded by the Nisa
granite (c.307-305Ma; Solá et al., 2009); (2) the PTFZ is a late Variscan structure which was active
after the Kasimovian since it deforms the CCSZ and the Junqueiro granite (c.307Ma; Vale Aguado et
-272-
Scientific Sessions al. 2005).
Our contribution is critical to reinterpret the meaning of kilometre-scale dextral displacements
and their interplay with the sinistral shear zones of the Variscan belt of Western Europe and,
consequently improve the global models of lithosphere deformation during the assembly of Pangea in
the northern margin of Gondwana.
References
Burg, J.P., Iglesias, M., Laurent, P., Matte, P., Ribeiro, A. (1981). Variscan intracontinental deformation: the CoimbraCordoba Shear Zone (SW Iberian Peninsula). Tectonophysics 78, 161–177.
Pereira, M.F., Apraiz, A., Silva, J.B., Chichorro, M., (2008). Tectonothermal analysis of high-temperature
mylonitization in the Coimbra-Cordoba shear zone (SW Iberian Massif, Ouguela tectonic unit, Portugal): Evidence
of intra-continental transcurrent transport during the amalgamation of Pangea. Tectonophysics 461: 378-394.
Ribeiro, A., Pereira, E., and Severo, L., (1980). Análise da deformaçao da zona de cisalhamento Porto-Tomar na
transversal de Oliveira de Aeméis: Comunicações dos Serviços Geológicos de Portugal, v. 66, p. 3–9.
Solá, A.R., Williams, I.S., Neiva, A.M., Ribeiro, M-L. (in press). U-Th-Pb SHRIMP ages and oxygen isotope
composition of zircon from two contrasting late Variscan granitoids, Nisa-Albuquerque batholith, SW Iberian
Massif: petrologic and regional implications. Lithos,
Valle Aguado, B., Azevedo, M.R., Schaltegger, U., Martínez Catalán, J.R., and Nolan, J., 2005, U-Pb zircon and
monazite geochronology of Variscan magmatism related to syn-convergence extension in central northern Portugal,
Lithos, v. 82, p. 169–184.
-273-
The Rheic Ocean: Its Origin, Evolution and Correlatives U-Pb-LA-SF-ICP-MS zircon ages from the Saxonian Granulite Massif (Saxo-Thuringian
Zone, Bohemian Massif): Implications for the crustal evolution of the Central European
Variscides
Anja Sagawe1, Mandy Hofmann1, Klaus Thalheim1 & Ulf Linnemann1
1
Senckenberg Naturhistorische Sammlungen Dresden, Museum für Mineralogie und Geologie, Koenigsbruecker
Landstrasse 159, Dresden, D-01109, Germany.
Actually, there are two contrasting models concerning the Central European Variscides.
Main disparities are given with the direction of subduction and the involved crustal type. Franke &
Oncken (1996) and later Franke & Stein (2000) prefer a top-SE directed subduction of a part of the
Rheno-Hercynian Zone (Laurussia) below pre-Variscan Gondwana (Saxo-Thuringian Zone, and the
Teplá-Barrandian Unit). In contrast, Kroner et al. (2007) favour a subduction of Gondwana-crust
below Laurussia towards NE. Formation of HP-HT-rocks of the Saxonian Granulite Massif (SGM)
was related to the metamorphic processes in the subduction zone. After peak metamorphism at c.
340 Ma the granulites became exhumed in a subduction channel. Zircon populations of these rocks
indicate the type of crust that was subducted (Rheno-Hercynian vs. Saxo-Thuringian) and imply the
direction of subduction (top-SE vs. top-NE). From selected granulite samples of the SGM zircon
grains were separated by standard methods. Measurement of U-Pb isotope ratios on zircons was
performed by LA-SF-ICP-MS (Laser Ablation-Sector Field-Inductively Coupled Plasma-Mass
Spectrometry). First obtained zircon ages range from Devonian to Neoproterozoic. In our
presentation we show preliminary results of our provenance analysis from the Saxonian Granulite
Massif.
References
Franke, W. & Oncken, O. (1996) Auswege aus dem saxothuringischen Paradoxon. Terra Nostra, 96 (2): 55–57.
Franke, W. & Stein, E. (2000) Exhumation of high-grade rocks in the Saxo-Thuringian Belt: geological constraints and
geodynamic concepts. In: Franke, W., Haak, V., Oncken, O. & Tanner, D. (eds). Orogenic Processes: Quantification
and Modelling in the Variscan Belt. Geological Society of London, Special Publications, 179: 337–354.
Kroner, U., Hahn, T., Romer, R. L. & Linnemann, U. (2007) The Variscan orogeny in the Saxo-Thuringian zone –
Heterogenous overprint of Cadomian/Paleozoic Peri-Gondwana crust. In: Linnemann, U., Nance, R. D., Kraft, P. &
Zulauf, G. (eds). The evolution of the Rheic Ocean: From Avalonian-Cadomian active margin to AlleghenianVariscan collision. Geological Society of America, Boulder, Colorado, Special Paper, 423: 153–172.
-274-
Scientific Sessions Early Devonian Rhenohercynian (Schiefergebirge, Germany) - central European delta and
shelf sea depositional environments and basin formation.
Andreas Schäfer & Johannes Stets
Steinmann Institute – Geology, University of Bonn, Nussallee 8, 53115 Bonn, Germany ([email protected])
A rift-like genesis of the Rhenohercynian Basin in central Rheinisches Schiefergebirge
(Rhenish Massif) is suggested. A depositional thickness of up to 10 km was initiated by rapid
subsidence of the basin during a 20 myrs Early Devonian period (Stets & Schäfer 2002). The basin
today is devided by two thrust faults delimiting marked facies changes while it was formed in the
Siegenian (Pragian).
The northern part of the basin consists of a sediment body of a thickness of up to more than
8 000 m. It had been limited by a synsedimentary fault in its south, the later Siegen Main Thrust.
Thicknesses of up to 1 500 m for the Lower Devonian correspond to a subsidence rate of about 7.4
cm/ky. A sedimentary wedge formed in a land-sea transition zone by fluvio-deltaic environments,
affected by marine ingressions occasionally (Stets & Schäfer in print). As subsidence and sediment
input had been in a balance during the Siegenian, no severe changes in the sedimentary
environments are evident.
The central part of the basin suffered rapid subsidence by syngenetic normal faulting during
the Early Devonian. Subsidence rates were moderate and differ along its margins, yet, the central
basin – the Hunsrückschiefer facies belt - has a sediment thickness of up to 10 000 m and more, and
thus shows a subsidence rate of up to 70 cm/ky generating a persistent shelf environment (Stets &
Schäfer 2008). Synsedimentary normal faulting caused facies contrasts in the Siegenian and Lower
Emsian Hunsrückschiefer by local horsts and deeps. Synsedimentary tectonic shocks along the
faults may have favoured deep-water turbidites locally.
The southern part of the basin separated from the central part of the basin by the later
Taunuskamm Thrust. Located in the northern foreland of the Mid German High, Upper Gedinnian
continental alluvial red beds and fluvio-deltaic sediments were deposited. They were followed by
coastal to neritic environments during the Siegenian. During the Early Emsian, the Mid-German
High disappeared again due to land subsidence and sea-level rise, offshore evironments being
installed at this time. The subsidence rate of the southern facies belt was as moderate as the northern
one.
-275-
The Rheic Ocean: Its Origin, Evolution and Correlatives Conditions changed at about 397 Ma with the Late Emsian: Smaller-scaled local highs and
basins coincided with later structural units, and persisted throughout the rest of the Devonian,
during about 40 myrs. In the same time, another rift system, the oceanic Red Sea-type
Rhenohercynian (Rheic) Ocean, came into being for rather a short time. It was closed again in late
Devonian, and shows up today as the Northern Phyllite Zone of the Variscan structural domain in
Central Europe.
Early Devonian depositional environments and their relationship to the fill and the
geohistory of the Rhenohercynian Basin will be discussed.
References
Stets, J. & Schäfer, A. (2002): Depositional Environments in the Lower Devonian Siliciclastics of the Rhenohercynian
Basin (Rheinisches Schiefergebirge, W-Germany) - Case Studies and a Model.- Contrib. Sed. Geol., 22, 78 pp.,
Stuttgart.
Stets. J. & Schäfer, A. (2008): The Early Devonian Rhenohercynian Basin (Middle Rhine valley, Rheinisches
Schiefergebirge) – land-sea transitions in the northern part.- In: Königshof, P. & Linnemann, U. (eds): The RhenoHercynian, Mid-German Crystalline and Saxo-Thuringian Zones (Central European Variscides), Excursion Guide,
20th International Senckenberg Conference and 2nd Geinitz Conference: "From Gondwana and Laurussia to
Pangea: Dynamics of Oceans and Supercontinents", 159 pp., Final Meeting IGCP 497 and IGCP 499, Frankfurt am
Main, Dresden.
Stets, J. & Schäfer, A. (in print): The Siegenian delta - land-sea transitions at the northern margin of the Rhenohercynian Basin.- Geological Society
of London, Special Publication.
-276-
Scientific Sessions ‘Bretonian’ contraction-dominated deformation in Central Armorica caused by the docking of
the Léon microcontinental Block at the southern margin of the Rheic Ocean
Sintubin, M., Berwouts, I., Muchez, P. and van Noorden, M.
Department of Earth and Environmental Sciences,
Katholieke Universiteit Leuven, Celestijnenlaan 200E, B-3001 Leuven, Belgium
In the Monts d’Arrée (western Brittany, France) a high-strain slate belt is well-exposed. The
slate belt is located in the Central Armorican Terrane, a low-grade middle- to upper-crustal domain
in the Armorican Massif, composed of a Cadomian basement and its Neoproterozoic and Palaeozoic
metasedimentary cover sequence. The slate belt consists of highly deformed siliciclastic rocks of
the Pridolian to Lochkovian Plougastel Formation. A structural analysis has demonstrated that the
slate belt primarily reflects a coaxial, contraction-dominated deformation history, resulting from a
top-to-the-NW shearing on top of a weakly dipping décollement. It resulted in NW-verging folding
and a pervasive cleavage development, giving rise to a pronounced subvertical mechanical
anisotropy. Only during the later stages of the deformation history, incipient strain partitioning on
this anisotropy lead to the development of punctuated strain heterogeneities, consistently reflecting
dextral, belt-parallel, strike-slip strain (van Noorden et al. 2007). The deformation largely occurred
prior to the emplacement of the early Carboniferous granitoid intrusions in Central Armorica, and
can
therefore
be
correlated
with
the
late
Devonian-early
Carboniferous
‘Bretonian’
tectonometamorphic event (cf. Rolet 1982).
Our work in the Monts d’Arrée shows that the ‘Bretonian’ orogenic event affected the entire
north-western part of the Central Armorican Terrane. In a middle-crustal setting, as exposed in the
Monts d’Arrée and the Montagnes Noires, this event is primarily expressed by a top-to-the-NW
Figure 1: Conceptual model of the Variscan, contraction-dominated, top-to-the-NW thrust tectonics in Central
Armorica (Sintubin et al. 2008). (a) Top-to-the-NW thrusting during the ‘Bretonian’ event. (b) Development of the
Châteaulin Basin as a piggy-back basin. (c) Main Variscan deformation affecting the Montagnes Noires and the
Châteaulin Basin. MA: Monts d’Arrée; CB: Châteaulin Basin; MN: Montagnes Noires.
-277-
The Rheic Ocean: Its Origin, Evolution and Correlatives thrusting (Figure 1). This is in contrast to the upper-crustal setting, as exposed in the Crozon
peninsula, where SW-verging backthrusting is considered (Ballèvre et al. 2009). This regional
‘Bretonian’ deformation coincides with the onset of the development of the Châteaulin Bain (cf.
Rolet & Thonon 1979), as reflected by the dynamic sedimentation of Strunian age. In a geodynamic
context of ‘Bretonian’ thrust tectonics, this basin should be seen as a piggy-back basin, developing
as a result of large-scale buckling of the nappe stack (Figure 1). Another implication of our model is
the diachronous nature of the cleavage development within Central Armorica. While the cleavage in
the Monts d’Arrée slate belt is of late Devonian-early Carboniferous age, the cleavage in the
Carboniferous metasediments of the Châteaulin basin, as well as in the Montagnes Noires, is of
Namurian-Westphalian age, i.e. at least 30 Ma younger (Figure 1). This diachronous nature of
cleavage development should be taken into account in future geodynamic models.
The kinematics, inferred in the Monts d’Arrée, are consistent with the top-to-the-NW
thrusting and nappe stacking inferred in the Léon Terrane, situated to the northwest of the Central
Armorican Terrane (cf. Rolet et al. 1994) (Figure 2). These new insights allow the linkage of the
early Variscan, contraction-dominated deformation in Central Armorica to the continental collision
of the Léon microcontinental block with the northern margin of the Armorica microcontinent at the
southern margin of the Rheic Ocean (Faure et al. 2005) (Figure 3).
The crustal thickening, caused by the continental collision and the underthrusting of the Léon
microcontinental block subsequently resulted in partial melting of the thickened crust, generating
Figure 2: Tentative cross section across the Léon (Rolet et al. 1994) and Central Armorican Terrane (Sintubin et al.
2008). LCS: Le Conquet suture.
granitoid intrusions. These late Variscan granites occur at both sides of the suture (LCS on Figure 2)
and conclude the early Variscan tectonic history. Subsequently, middle to late Carboniferous
intracontinental deformation resulted in significant localised wrenching (e.g. Gapais & Le Corre
1980).
Our findings in wester Brittany have demonstrated that an early Variscan, contraction-278-
Scientific Sessions dominated deformation affected the rocks of the Plougastel Formation in the entire Central
Armorican Terrane. This deformation is linked to the late Devonian-early Carboniferous top-to-theNW thrusting and nappe stacking, caused by the continental collision of the Léon microcontinental
block with Armorica, and hence to the closure of the Rheic Ocean.
References
Ballèvre, M., Bosse, V., Ducassou, C. & Pitra, P. 2009.
Palaeozoic history of the Armorican Massif: Models for
the tectonic evolution of the suture zones. Comptes Rendus
Geoscience 341, 174-201.
Faure, M., Mézème, E. B., Duguet, M., Cartier, C. &
Talbot, J.-Y. 2005. Paleozoic tectonic evolution of medioeuropa from the example of the french massif central and
massif armoricain. Journal of the Virtual Explorer 19(5),
1-26.
Gapais, D. & Le Corre, C. 1980. Is the Hercynian belt of
Brittany a major shear zone? Nature 288, 574-576.
Rolet, J. 1982. La "phase bretonne en Bretagne": état des
connaissances. Bulletin de la Société géologique et
minéralogique de Bretagne 14(2), 63-71.
Rolet, J., Gresselin, F., Jegouzo, P., Ledru, P. & Wyns, R.
1994. Intracontinental hercynian events in the Armorican
Massif. In: Pre-Mesozoic geology in France and related
areas (edited by Keppie, J. D.). Springer-Verlag, Berlin,
195-219.
Rolet, J. & Thonon, P. 1979. Mise en évidence de trois
complexes volcano-détritiques d'âge Dévonien inférieur à
moyen, Strunien et Viséen inférieur sur la bordure nord du
bassin de Châteaulin (feuille Huelgoat 1/50000, Finistère).
Implications paléogéographiques et tectoniques. Bulletin
du B.R.G.M. 2(1/4), 303-315.
Sintubin, M., van Noorden, M. & Berwouts, I. 2008. Late
Devonian-early Carboniferous contraction-dominated
deformation in Central Armorica (Monts d'Arrée, Brittany,
France) and its relationship with the closure of the Rheic
Ocean. Tectonophysics 461, 343-355.
van Noorden, M., Sintubin, M. & Darboux, J.-R. 2007.
Incipient strain partitioning in a slate belt: Evidences from
the early Variscan Monts d'Arrée slate belt (Brittany,
France). Journal of Structural Geology 29(5), 837-849.
Figure 3: (a) Late Devonain plate tectonic reconstruction,
predating the continental collision of the Léon
microcontinental block with Armorica (after Faure et al.
2005). (b) lithospheric cross section. MGCH: MidGerman Crystalline High; PBMA: Paris Basin Magnetic
Anomaly; SBS: Sout-Brittany Suture.
-279-
The Rheic Ocean: Its Origin, Evolution and Correlatives Inclined transpression and multiple deformation events along an arcuate terrane boundary:
the Precambrian 'Mylonite Zone' of the Sveconorwegian orogen
Viola, G. and Henderson, I.C.
Geological Survey of Norway, 7491, Trondheim, Norway
The late Mesoproterozoic Sveconorwegian orogeny took place between 1.13 and 0.90 Ga and
accommodated the collision between Fennoscandia and an unknown large continent, possibly Amazonia. The
large-scale architecture of the resulting orogen is characterized by the tectonic juxtaposition of several
distinct Palaeo-to Mesoproterozoic terranes. In southeast Norway and Sweden, the tectonic boundary
between a parautochtonous terrane to the east (Eastern Segment) and an allochtonous block to the west
(Idefjorden terrane) is a top-to-the-SE thrusted contact, the so-called “Mylonite Zone” (MZ), with the
Idefjorden terrane emplaced above the Eastern Segment. The MZ has a remarkably arcuate shape, with a
NW/WNW strike in Norway and a N-S to NE/SW trend in southwestern Sweden. At its southernmost
exposed segment the MZ strike changes again and south of Göteborg there occurs a dramatic orientation
change as the shear zone swings to the west and attains an E-W trend. The Norwegian sector and the
Swedish E-W-trending segment south of Göteborg represent the sinistral and dextral lateral ramps to the topto-the-SE frontal thrust exposed in the ca. N-S to NNE trending intervening segment. Structural
characterization of the MZ mylonites in two different localities in Sweden (Värmlandsnäs along the frontal
thrust and Bua along the oblique dextral lateral ramp) reveals a triclinic strain pattern of Sveconorwegian
age. Inclined transpression is inferred on the basis of coexisting (and broadly coeval) foliation-parallel
oblique shearing (resolvable in a strike-slip and dip-slip component) and across-foliation shortening.
Foliation-parallel oblique shearing accommodated a transpressive component of the overall MZ strain
history, and its kinematics is either sinistral or dextral depending on the local strike of the MZ with respect to
the regional thrust shortening vector. Across-foliation shortening led to pure-shear shortening perpendicular
to the thrust sheet and subsequent lateral extrusion in a direction parallel to the mylonitic foliation via the
development of antithetic displacements. No significant strain partitioning is observed at the outcrop-scale
and strain is thus truly triclinic.
A different deformational history is preserved along the sinistral, transport-parallel lateral ramp in the
most northerly part of the exposed MZ. Here sinistral shearing is only the last of a long series of deformation
events. Mylonites in upper greenschist to amphibolite facies record a two-fold shearing and folding history.
Early, dip-slip, top-to-the-SW/W mylonites underwent three phases of folding prior to sinistral shearing.
Based on regional geometrical correlations, kinematic and metamorphic studies, we assign the sinistral
shearing phase to the Sveconorwegian orogeny, whereas we interpret the top-to-the-SW/W dip-slip tectonics
as relic of an older tectonic history, possibly the Gothian orogeny (~1.8Ga-1.5Ga) or alternatively very early
stages of the Sveconorwegian event. This earlier phase appears to be locally present also farther south in the
-280-
Scientific Sessions hanging wall of the more orthogonal thrust front of the MZ.
-281-
The Rheic Ocean: Its Origin, Evolution and Correlatives Constraining the kinematics and mechanics of complex curved orogens: case studies using
combined structural, paleomagnetic and geochronologic techniques
Arlo Brandon Weil & Gabriel Gutierrez-Alonso
Bryn Mawr College Department of Geology, Bryn Mawr College, Bryn Mawr, PA 19010, [email protected]
Univ Salamanca, Facultad de Ciencias, Salamanca, 37008
Reconstructing the kinematic evolution, and understanding the mechanical processes of
orogenic curvature are two long standing questions within the structural geology and tectonics
community. At the roots of these questions are when and how orogens acquire curvature relative to
their protracted deformation histories. A complete kinematic model for any curved orogenic system
should describe the three-dimensional displacement field, which is comprised of: bulk translation
(related to slip on major faults), horizontal- and vertical-axis rotations (produced by large-scale
folding and motion of coherent blocks), and internal strain (accommodated by cleavage, vein and
fracture networks, minor folds, minor faults, and grain-scale fabrics). Traditionally, displacements
across fold-thrust belts have been evaluated from cross sections that only incorporate map-scale
faults and folds. Contributions from internal strain and vertical-axis rotation, however, are generally
not included in cross section restoration due to the difficulty of acquiring these data at appropriate
spatial resolution. Yet, the incorporation of internal strain and vertical-axis rotation data greatly
affects the accuracy of fold-thrust belt restorations and consequently the interpretation of wedge
mechanics. Historically, the best way to accurately quantify orogenic curvature has been through
paleomagnetic analysis. Such analysis, if carefully done at appropriate scales, can track absolute
and relative magnitudes of block rotations by measuring changes in paleomagnetic declination
between individual sites. Paleomagnetic studies thus provide a key data set to test kinematic
models that predict different spatial and temporal distributions of rotations along and across
orogenic strike. For a model to be valid, however, it must also agree with available structural and
strain data. Furthermore, by integrating paleomagnetic rotations with structural and strain patterns,
more robust kinematic models can be constructed and the complex development of curved orogens
can be adequately deciphered. Ultimately, these composite models can be used to constrain
mechanical processes responsible for the 3-D development of mountain systems and critical-wedge
dynamics.
To highlight the importance of robust datasets in understanding complex orogenic systems, a
case study from the Cantabrian-Asturian Arc (CAA), northern Spain will be presented. The presentday arcuate geometry of the CAA describes ~180o of curvature, which is concave towards the east
-282-
Scientific Sessions (i.e., towards the foreland). Several hundred paleomagnetic sites have been collected from the outer
fold-thrust belt and inner core of the CAA. These data demonstrably show that the present-day
curvature is mainly secondary in nature and is a consequence of a protracted two-phase oroclinal
model. Closure of the Rheic Ocean resulted in E-W shortening (in present-day coordinates) in the
Carboniferous, which produced a near linear N-S trending, east verging, fold-thrust belt. A
subsequent change in the regional stress-field near the Carboniferous-Permian boundary resulted in
oroclinal bending. This late-stage orogenic event remains an enigmatic part of the final
amalgamation of Pangea. Age constraints for the deformation phases come from comparing
secondary and primary paleomagnetic directions with known Iberian reference directions for
magnetizations that were acquired before, during and after oroclinal bending. Confirmation of these
age estimates come from recent geochronologic analysis of clays associated with remagnetization
processes in the studied CAA carbonates. Detailed X-ray diffraction patterns of these samples
indicate that mixed-layer clays are mostly illitic, with an increasing proportion of authigenic illite in
finer grain size fractions.
40
Ar/39Ar age data from the different grain size fractions, coupled with
quantitative polytype modeling, indicates an authigenic age that is coeval with the established late
Paleozoic remagnetization age of these rocks. Importantly, the timing of remagnetization is
penecontemporaneous with the proposed stress-field change that produced oroclinal bending.
Originally thought of as strictly a thin-skinned fold-thrust belt, geodynamic models of late Variscan
tectonics suggest that oroclinal bending of the CAA was lithospheric in scale and resulted in
thickening and eventual detachment of the lithospheric root of the orogen. This hypothesis is
consistent with the chronology of tectonic, metamorphic, magmatic, and hydrothermal events in the
larger Ibero-Armorican Arc system.
-283-
Scientific Sessions ABSTRACTS
POSTER SESSION
-285-
The Rheic Ocean: Its Origin, Evolution and Correlatives -286-
Scientific Sessions A comparison of one- and two-dimensional models of the Variscan
thermal response to crustal thickening and thinning
in the Lugo Dome, NW Iberia
Alcock, J.1, Martínez Catalán, J.R.2, and Arenas, R.3
1. Department of Geosciences, Abington College, Penn State University, Abington, PA 19001, USA
2. Departamento de Geología, Universidad de Salamanca, 37008 Salamanca, Spain
3. Departamento de Petrología y Geoquímica e Instituto de Geología Económica [CSIC], Universidad Complutense,
28040 Madrid, Spain
Thermal modeling has been used for several decades to explore the thermal effects of crustal
thickening during orogeny (England and Thompson 1984, Thompson and England 1984, Peacock,
1989) and more recently to use the thermal models to evaluate potential Pressure-Temperature-time
(P-T-t) paths of specific terrains that had been proposed to explain data from structural,
metamorphic, and geochronological studies (e.g. Burg et a. 2004, Gerya et al. 2004, Alcock et al.
2009). Originally restricted to one-dimensional (1-D) models (England and Thompson 1984), twodimensional (2-D) models that include advection of heat through tectonic and magmatic activity
have become popular with the advent of faster and more accessible computing (Peacock 1989,
Gerya el al. 2004). One aspect of 2-D models, however, is that their added complexity requires a
significant increase in computing time, usually by at least two orders of magnitude. As a result, a 1D model can take less than 5 minutes to run while a comparable 2-D model can require several
hours.
In this paper a 2-D model of the thermo-tectonic history of the Lugo Dome, northwestern
Spain is compared to previously published results of 1-D modeling (Alcock et al. 2009). Although
the 2-D model includes incremental advection of heat by thrusting, the resultant thermal history is
very similar to1-D model results. It follows that, at least in this instance, the advantage of more
rapid computation using a 1-D model that allows for more extensive testing of the impact of
different variables on model outcomes is more significant than improvements in accuracy obtained
with 2-D modeling.
The Lugo gneiss dome is geologically well known and representative of the Variscan
thickening and subsequent extension by gravitational collapse of the orogen. The dome is a N-S,
140 km long and 35 km wide structure going from Viveiro, on the Cantabrian coast, to the Serra do
Courel. Its core is occupied by the Mondoñedo nappe, a set of kilometric-scale recumbent folds
floored by a thrust fault. Its autochthon, cropping out in the Xistral tectonic window has provided
information essential to understanding the extensional collapse of the Variscan crust in this region
(Martínez Catalán et al. 2003).
-287-
The Rheic Ocean: Its Origin, Evolution and Correlatives The dome is characterized by high-grade Variscan metamorphism, reaching the sillimaniteorthoclase zone (Arenas and Martínez Catalán 2003) and by a pervasive schistosity related to
extensional tectonics (Martínez Catalán et al. 2003). Partial melting is evident in migmatites
developed at its core and in the voluminous Variscan granitoids (Fernández-Suárez et al. 2000). The
structural evolution includes shortening and crustal thickening, related to plate convergence, as well
as extension and crustal thinning linked to gravitational collapse. Several generations of structures
permit the recognition of three periods of contraction, C1 – homogeneous thickening, C2 – nappe
emplacement, and C3 – minor thickening related to orogen parallel shear forces; and two of
extension, E1 – homogeneous thinning by erosion and gravitational collapse and E2 – normal
faulting associated with doming (fig. 1). (See Alcock et al. 2009 and references therein for a more
complete discussion.)
The tectono-thermal history of the Lugo Dome has been modeled using 1-D and 2-D finitedifference models that track position and temperature of 1-km spaced nodes during contraction and
extension. In 1-D models homogeneous thickening and thinning were treated as gradual processes.
Thickening by thrusting was modeled as an instantaneous event. In the 2-D models advection by
thrusting was also treated as a gradual event as the model allowed both vertical and horizontal
movement.
Figure 1: Timing of events included in 1D and 2-D models. In the 1-D model C-2 is an
instantaneous event at 325 Ma. Ages are from
Dallmeyer et al. (1997) and Fernandez-Suarez et
al. (2000).
Figure 2 presents results from representative 1-D and 2-D model runs using similar parameters for
thickening and thinning. The model results vary by less than 15° C and do not affect model
predictions of metamorphic or magmatic history, the most important constraints on evaluating the
models’ ability to predict the thermal response of the crust to orogeny.
-288-
Scientific Sessions Figure 2: Comparison of geotherms from 1-D
(solid lines) and 2-D models (dashed) of Lugo Dome
thermal history. Numbers beside lines are model ages in
millions of years. Saw-toothed pattern of 1-D model
results
from
instantaneous
emplacement
of
the
Mondoñedo thrust sheet. The 1-D model is the same as
that described in Alcock et al. (2009). 25 Ma geotherm
for 2-D model occurs within period of thrusting. Initial
geotherm is the same for both models. Models include
C1, C2 and E1.
The small differences in the thermal
histories derived from 1-D and 2-D models of the Lugo Dome are taken to confirm the validity of
the use of 1-D models to assist our understanding of the thermal processes related to orogeny. The
relative ease of writing 1-D code and the speed with which 1-D computations can be run suggest
that 1-D thermal modeling remains a valuable tool for improving our understanding of the thermal
histories and for evaluating the potential accuracy of pressure-temperature-time paths inferred from
structural, metamorphic and geochronological study.
References
Alcock, J.E., Martínez Catalán, J.R, Arenas, R., Díez Montes, A. (2009) Use of thermal modeling to assess the tectonometamorphic history of the Lugo and Sanabria Domes, Northwest Iberia, Bull. Soc. Geol..Fr., 180, 85-103.
Arenas, R., and Martínez Catalán, J.R. (2003) Low-P metamorphism following a Barrovian-type evolution. Complex
tectonic controls for a common transition, as deduced in the Mondoñedo thrust sheet (NW Iberian Massif).
Tectonophysics, 365, 143-164.
Burg, J.-P., Kaus, B.J.P. and Podladchikov, Y.Y (2004) Dome Structures in collision orogens: Mechanical investigation
of the gravity/compression interplay. In: Whitney, D.L., Teyssier, C., and Siddoway, C.S., Eds., Gneiss domes in
orogeny. – Geol. Soc. Am. Sp. Paper, 380, 47-66.
Chatterjee, N.D., and Johannes, W. (1974) Thermal stability and standard thermodynamic properties of synthetic 2M1muscovite (KAl2(AlSi3O10(OH)2). Contr. Mineral. Petrol., 48, 89-114.
Dallmeyer, R.D., Martínez Catalán, R.R., Arenas, R., Gil Ibarguchi, J.I. Gutíerrez Alonso, G., Farias, P., Aller, J., and
Bastida, F. (1997). Diachronous Variscan tectonothermal activity in the NW Iberian Massif: Evidence from
40
Ar/39Ar dating of regional fabrics. Tectonophysics, 277, 307-337.
England, P.C., and Thompson, A.B., (1984). Pressure-temperature-time paths of regional metamorphism I. Heat transfer
during the evolution of regions of thickened continental crust. J. Petrology, 25, 894-928.
Fernández-Suárez, J. Dunning, G.R., Jenner, G.A., and Gutíerrez-Alonso, G. (2000). Variscan collisional magmatism and
deformation in NW Iberia: constraints from U-Pb geochronology of granitoids. J. Geol. Soc., London, 157, 565-576.
Gerya, T.V., Perchuk, L.L., Maresch, W.V. and Willner, A.P. (2004). Inherent gravitational instability of hot continental
crust: Implications for doming and diapirism in granulite facies terrains. In: Whitney, D.L., Teyssier, C., and
Siddoway,C.S., Eds., Gneiss domes in orogeny. Geol. Soc. Am. Sp. Paper, 380, 97-115.
Martínez Catalán, J.R., Arenas, R. and Díez Balda, M.A. (2003). Large extensional structures developed during
emplacement of a crystalline thrust sheet: the Mondoñedo nappe (NW Spain). J. Struct. Geol., 25, 1815-1839.
Peacock, S.M. (1989). Thermal modeling of metamorphic-pressure-temperature-time paths. In:Spear, F.S., and Peacock,
S.M., Eds., Metamorphic pressure-temperature-time paths. Am. Geophys. Union, Short Course in Geology, 7, 57102.
Powell, R. and Holland, T. (1990). Calculated mineral equilibria in the pelite system, KFMASH (K2O-FeO-MgOAl2O3-SiO2-H2O). Am. Miner., 75, 367-380.
Thompson, A.B., and England, P.C. (1984). Pressure-temperature-time paths of regional metamorphism II. Their
-289-
The Rheic Ocean: Its Origin, Evolution and Correlatives inference and interpretation using mineral assemblages in metamorphic rocks. J. Petrology, 25, 929-955.
-290-
Scientific Sessions Assessment of inheritance, magmatic and metamorphic history
of the Corredoiras orthogneiss by zircon trace element geochemistry
Castiñeiras, P.1, Andonaegui, P.1, Gómez Barreiro, J.1,
González Cuadra, P.2, Díaz García, F.3 and Arenas, R.1
1
Dpto. Petrología y Geoquímica-Instituto de Geología Económica (UCM-CSIC), Facultad de Ciencias Geológicas.
Universidad Complutense, 28040 Madrid, Spain
2
Dpto. Geografía y Geología, Facultad de Filosofía y Letras. Universidad de León, 24071 León, Spain.
3
Dpto. Geología, Facultad de Geología. Universidad de Oviedo, 33005 Oviedo, Spain
In spite of the failure to infer the rock type based on the rare earth element (REE)
composition of detrital zircon in provenance studies (e.g., Hoskin and Ireland 2000), the chemical
characterization of previously dated zircons is a powerful tool in order to obtain petrogenetic
information from this mineral. Using cathodoluminescence imaging together with trace element
geochemistry and Ti-in-zircon thermometry, it is possible to distinguish continental from oceanic
zircon, metamorphic from magmatic zircon, and to know their formation temperature and the
evolution of the magma composition during zircon growth.
This procedure has been applied to the Corredoiras orthogneiss zircons using SHRIMP-RG
at Stanford University. The zircons were first dated in an analytical session on October 2005.
Cathodoluminescence images exposed complex grains, with abundant and variably luminescent
inherited cores mantled by several areas of oscillatory or homogeneous zoning. U-Th-Pb results in
the xenocrystic cores reveal a limited Archean, Paleoproterozoic (Group 1) and CambrianNeoproterozoic (Groups 2 and 3) inherited component, whereas oscillatory mantles and
homogeneous rims range from 525 to 480 Ma, with an acme at 492 Ma, considered the
crystallization age of the granodioritic protolith (Group 4).
In a later SHRIMP session (September 2008), some grains were selected for trace element
analyses. The preliminary assessment of the data provides information on the nature of the
inheritance component (Groups 1, 2 and 3), the evolution of the granodioritic magma during zircon
crystallization and the subsequent metamorphism (Group 4).
Attending to their trace element characteristics, group 1 zircons (2000-3000 Ma) derive from
continental evolved magmas with moderate Ce contents and Pl fractionation. One of the analyses is
from a metamorphic zircon, which Ti-in-zircon temperature (~840ºC) indicates high-grade
conditions. The absence of a flat HREE pattern suggests that if garnet was present during the
growth of this zircon, the HREE reservoir was open, probably owning to the presence of a partial
-291-
The Rheic Ocean: Its Origin, Evolution and Correlatives melt (Rubatto 2002).
The compositional variability observed in group 2 zircons (550-600 Ma) suggests that they
are derived from different sources. In fact, some of them come from continental magmas and others
are metamorphic in origin. Regarding group 3 zircons (500-550 Ma), their trace element
composition suggests that they were formed from wet crustal melts. Finally, zircons from group 4
(450-500 Ma) could have also derived from wet crustal melts, with a strong Pl fractionation,
probably during an anatectic episode. Most of the zircons from this group have Ti-in-zircon
formation temperatures equal or below the zircon saturation temperature, calculated using the
Watson and Harrison (1983) model, which provides an approximate maximum constraint for the
temperature of zircon crystallization in the orthogneiss protolith (~820ºC for the massive
orthogneisses).
References
Hoskin PWO and Ireland TR (2000). Rare earth element chemistry of zircon and its use as a provenance indicator.
Geology 28, 627-630.
Rubatto D (2002). Zircon trace element geochemistry: partitioning with garnet and the link between U–Pb ages and
metamorphism. Chemical Geology 184, 123-138.
Watson EB and Harrison TM (1983). Zircon saturation revisited: temperature and composition effects in a variety of crustal magma
types. Earth and Planetary Science Letters 64, 295-304.
-292-
Scientific Sessions Age and structure of A Silva granodiorite: constraints on the evolution of the Upper units of
the Iberian Allochthonous complexes
Castiñeiras, P.1, Díaz García, F.2, González Cuadra, P.3, Gómez Barreiro, J.1
and Martínez Catalán, J.R.4
1
Dpto. Petrología y Geoquímica-Instituto de Geología Económica (UCM-CSIC), Facultad de Ciencias Geológicas.
Universidad Complutense, 28040 Madrid, Spain
2
Dpto. Geología, Facultad de Geología. Universidad de Oviedo, 33005 Oviedo, Spain
3
Dpto. Geografía y Geología, Facultad de Filosofía y Letras. Universidad de León, 24071 León, Spain.
4
Dpto. Geología, Facultad de Ciencias. Universidad de Salamanca, 37008 Salamanca, Spain.
A Silva granodiorite is a metamorphosed plutonic body intruding the metasediments of the
Upper units of the Órdenes Complex, located to the northeast of the Monte Castelo gabbro, which
has been classically related to the late Variscan A Coruña granodiorite, a trapezoidally-shaped
pluton which transects upright regional Variscan folds.
However, new field mapping and structural analysis, showing both the migmatitic character of A
Silva granodiorite and its close relationship with the host metasediments, together with SHRIMP UPb zircon dating, demonstrate that this pluton must be regarded as another arc-related CambrianOrdovician plutonic body. Thus, A Silva granodiorite is equivalent to the Monte Castelo gabbro and
the Corredoiras orthogneiss, which are typical of the intermediate-pressure upper units of the
Allochthonous complexes of the Iberian Variscan belt.
Two samples from A Silva granodiorite were selected for geochronology. One sample was
collected near the contact with the A Coruña granodiorite, in the northern outer area of the pluton to
trace its extent as far as possible. The second sample was picked up from a type locality at the
internal part of the pluton.
U-Th-Pb analyses of zircon were conducted on the Bay SHRIMP-RG operated by the
SUMAC facility (USGS-Stanford University). La to Yb and Hf were measured concurrently with
the U-Th-Pb analyses as additional masses on each pass through the mass range.
The results from eighty-three analyses performed in 73 zircon grains from both A Silva samples
suggest that they are equivalent, and the data is accordingly considered as a whole henceforth. Most
of the spots were typically aimed to the most external oscillatory zones, disregarding their
luminescence; however, the analyses define an age smear between 540 and 460 Ma along the
concordia line in a Tera-Wasserburg plot. The smooth variation in age observed in A Silva samples
can be interpreted as a result of analytical scatter, some combination of Pb loss and inheritance, or it
may represent a true difference in age.
-293-
The Rheic Ocean: Its Origin, Evolution and Correlatives Taking into account the analytical strategy used in these samples, analytical scatter can be
ruled out. Even so, obtaining an age with such smooth variation in the data is not straightforward.
Nonetheless, it will benefit from a coupled assessment with the REE and Hf composition of the
zircons.
Chondrite-normalized REE plots are characteristic of magmatic zircon, with a pronounced
negative Eu anomaly (Eu/Eu*=0.02-0.05) and moderately fractionated heavy REE patterns.
In a Th/U versus Yb/Gd plot, zircons crystallized in an evolving magma usually define an
asymptotic curve. However, most of A Silva zircons have low Th/U contents, ranging from 0.04 to
0.3, and Yb/Gd values show a limited variation from 10 to 20. Hf can also be used as a marker of
fractional crystallization, but A Silva zircons show a narrow variation (11000-14000 ppm) in this
element as well.
This restricted compositional variation in extremely sensible fractionation indices can be
used to rule out the possibility that the age variation is real or owning to the presence of an inherited
component in the zircons. Thus, unless the zircon REE composition had been subsequently
modified, the process responsible of the age smear should be lead loss. We used the TuffZirc
algorithm, designed to obtain an age from a dataset affected by lead loss or slight inheritance, and
we obtained a
206
Pb/238U age of 518 (+1.77 -1.85) Ma as the best statistical estimate for the
crystallization of A Silva granodiorite.
Even if the crystallization age for A Silva granodiorite is not precisely constrained, our new
data entails us to consider this plutonic body as equivalent to other Cambrian-Ordovician plutons,
widely documented in the upper units of the Allochthonous complexes. Furthermore, the
preservation of the original field relationships between A Silva granodiorite and its country rocks
makes it a good place to obtain key information on the tectonometamorphic evolution of the upper
units, including the relationships between granite emplacement, regional extension and exhumation
of the HP-HT rocks situated below.
-294-
Scientific Sessions Evaluating the lithostratigraphic evidence for Marinoan or Gaskiers glaciation in the
Battleground and Blacksburg Formations of the South Carolina and adjacent North Carolina
Piedmont, U.S.A.
Dennis, Allen J.
Biology and Geology, University of South Carolina Aiken; Aiken, SC 29801-6309, USA; [email protected]
Distinctive dolomitic marbles, stratiform barites, and manganiferous schists and gondites
overlie latest Precambrian metavolcanic rocks in northwestern South Carolina and adjacent North
Carolina. The association of sea floor barites, BIFs / manganiferous formations, and dolomitic cap
carbonates is typical of Marinoan (ca. 635 Ma) and Sturtian (ca. 710 Ma) global glacial events
(Hoffman and Schrag, 2002). Primarily metavolcanic Battleground Formation (Fm) is interpreted to
be overlain by metasedimentary rocks of the Blacksburg Fm (Horton, 2008). The contact between
the formations is obscured by later shearing. Both formations include unusual members. In the
Battleground these include metaconglomerates (e.g., Draytonville) and quartzites, Jumping Branch
manganiferous member, and massive and disseminated barites. Blacksburg Fm is dominated by
epiclastic phyllites and siltstones, and includes the Gaffney and the Dixons Branch marble
members. Deposition in the upper part of the Battleground Formation was coeval with either the
Marinoan or Gaskiers glaciation (ca. 580 Ma). The seafloor barite / BIF/MnF association is ancient,
and is generally restricted to Paleoarchean (>3.2 Ga) rocks (e.g., North Pole, Pilbara). The
association yields particular inferences regarding atmospheric [O2] chemistry and chemical
stratification of the ocean at that time. In the Cryogenian, the association is interpreted to represent
loss of atmospheric O2 and loss of communication between the ocean and the atmosphere through
pack ice. The marbles of the Blacksburg Fm may comprise the cap carbonate deposited as an
inorganic precipitate at the close of a global glacial event. Caveats to these interpretations include 1)
the lacks of variety in lithology and size of clasts in metaconglomerate members probably precludes
their being glaciogenic; 2) while barites, Mn schists and gondite are interpreted as signature mineral
deposits of the infracambrian glaciations, other well known mineral deposits of the Kings Mountain
belt (e.g., Au, kyanite/ sillimanite) are purely volcanogenic (or are much younger, i.e., spodumene
pegmatite, Sn) with no paleoclimatic significance; 3) volcanic activity, epiclastic deposition, soft
sediment deformation, and tectonic deformation occurred throughout deposition of barite-MnF and
cap carbonate.
Remarkably, there are no modern radiometric igneous crystallization ages from Blacksburg
and Battleground Fms (i.e., the fault-bounded Kings Mountain belt). The closest dated rocks
include metamorphosed and unmetamorphosed plutonic rocks of the Mean Crossroads Complex
-295-
The Rheic Ocean: Its Origin, Evolution and Correlatives (571±16 Ma – 535±4 Ma, Dennis and Wright, 1997). It is expected that Kings Mountain belt
crystallization ages would lie in the age range 620-630 Ma or 580-530 Ma comparable to other
rocks of Carolinia (Dennis and Wright, 1997; Dennis et al. 2004; Hibbard et al, 2007). Each of the
distinctive units discussed here- Kings Creek stratiform barite, Jumping Branch manganiferous
member (of Battleground Fm), Dixon’s Branch and Gaffney marble members (of Blacksburg Fm) –
is framed by laminated quartzites or metaconglomerates (e.g., Crowders Creek, Draytonville, Dixon
Gap). Work is ongoing to separate and date detrital zircons recovered from these units, and thus
establish the ages of the putative “Snowball” lithologies they frame, as well as provide the first
high-precision ages for rocks of the Kings Mountain belt.
It has been suggested that the
metaquartzites and metaconglomerates are not detrital sedimentary units but highly altered volcanic
units (LaPoint, pers. comm.). If this is the case, interpretation of ages may be much simpler. A
detailed C-isotope profile is being prepared concurrently for the marble members of the Blacksburg
Fm that will be temporally constrained and integrated into the existing Ediacaran isotope composite
stratigraphy (e.g., Halverson et al, 2005, Halverson, 2006). One consequence of the work described
above will be a test of the hypothesis that the Kings Creek shear zone reactivates a lowstand
unconformity corresponding to one of the Snowball events.
The rocks discussed here are presumably overlain by rocks exposed just to the east that
contain three different Ediacaran genera (Pteridinium, Swartpuntia, Aspidella; Gibson et al, 1988;
Weaver et al, 2006; Hibbard et al, 2006). Thus, Neoproterozoic rocks of the westernmost Carolina
terrane, U.S. Appalachians, include an unusual stratigraphy that may be related to one or more of
the Snowball Earth events; the potential exists to tie these rocks to one of three known events using
(detrital?) zircon geochronology, and create a temporally constrained
detailed C-isotope
stratigraphy for dolomitic marbles near the top of the section. Regional relationships established
during detailed mapping strongly suggest a link to rocks that have yielded Ediacaran fossils.
References
Dennis, A.J., Shervais, J.W., Mauldin, J., Maher, H.D., Jr., Wright, J.E. (2004) Petrology and geochemistry of
Neoproterozoic arc terranes beneath the Atlantic Coastal Plain, Savannah River Site, South Carolina. Geological
Society of America Bulletin, 116: 572-593.
Dennis, A.J., Wright, J.E. (1997) The Carolina terrane in northwestern South Carolina, USA: Age of deformation and
metamorphism in an exotic arc. Tectonics, 16: 460-473.
Gibson, G.G., Teeter, S.A., Fedonkin, M.A. (1984) Ediacaran fossils from the Carolina slate belt, Stannly County, Nortth
Carolina. Geology, 12: 387-390.
Halverson, G.P., (2006) A Neoproterozoic Chronology. In: Xiao, S. and Kaufman, A.J. (Eds.), Neoproterozoic
Geobiology and Paleobiology. 231-271.
Halverson, G.P., Hoffman, P.F., Schrag, D.P., Maloof, A.C., Rice, A.H. (2005) Towards a Neoproterozoic composite
carbon isotope record. Geological Society of America Bulletin, 117: 1181-1207.
Hibbard, J.P., McMenamin, M.A.S., Pollock, J., Weaver, P.G., Tacker, R.C., Miller, B.V., Samson, S.D., Secor, D.T.
(2006) Significance of a new Ediacaran fossil find in the Carolina terrane of North Carolina. Geological Society of
-296-
Scientific Sessions America Abstracts with Programs, 38 (2): 91.
Hibbard, J.P., van Staal, C.R., Miller, B.V. (2007) Links among Carolinia, Avalonia, and Ganderia in the Appalachian
peri-Gondwanan realm. In: Sears, J.W., Harms, T.A., and Evenchick, C.A. (Eds.), Whence the Mountains?
Inquiries into the evolution of orogenic systems: A volume in honor of Raymond A. Price: Geological Society of
America Special Paper, 433: 291-311.
Hoffman P.F., Schrag, D.P. (2002) The snowball Earth hypothesis: Testing the limits of global change. Terra Nova, 14:
129-155.
Horton, J.W., Jr. (2008) Geologic map of the Kings Mountain and Grover quadrangles, Cleveland and Gaston Counties,
North Carolina, and Cherokee and York Counties, South Carolina. U.S. Geological Survey Scientific Investigations
Map 2981, 1 sheet, scale 1:24,000, with 15-p. pamphlet.
Weaver, P.G., McMenamin, M.A.S., Tacker, R.C., (2006) Paleoenvironmental and paleogeographic implications of a new Ediacaran
body fossil from the Neoproterozoic Carolina terrane, Stanly County, North Carolina. Precambrian Research, 150: 123-135.
-297-
The Rheic Ocean: Its Origin, Evolution and Correlatives Late Variscan deformation in Iberia and Morocco
Rui Dias1,2,3, António Ribeiro4, Mohamed Hadani2, Caterina Basile2, Youssef Hendaq3
1
Dep. Geociências, Univ. Évora, Portugal, Rua Romão Ramalho, nº 59, 7000 Évora, Portugal
2
Centro Geofísica Évora, Rua Romão Ramalho, nº 59, 7000 Évora, Portugal.
3
Lab. Investigação Rochas Industriais Ornamentais (LIRIO), Univ. Évora, Convento das Maltesas, 7110-513
Portugal
4
Centro and Dep. Geologia, Fac. Ciências, Univ. Lisboa, Portugal.
* corresponding author: [email protected]; fax + 351 268 334 285
Abstract: The late Variscan deformation, both in Iberia and Morocco, is mainly characterized by NE-SW to NNE-SSW
shear zones with a sinistral kinematics. These shears could be emphasized at, either the orogenic scale (e.g. Vilariça
fault in Iberia or Smaâla-Oulmès one in Morocco), or the regional one, where they are expressed by decametric to
hectometric anisotropies with a metric to decametric offset. The absence of the dextral conjugated family, as well as the
importance of major E-W dextral shears, favor a genetical model where the local shears are the result of a bookshelf
mechanism controlled by major E-W dextral anisotropies; this model, which has already been proposed for the late
Variscan deformation in Iberia is here extrapolated for the Moroccan Variscan sector.
INTRODUCTION
The establishment of any geodynamical model for the Ibero-Moroccan late-Variscan
deformation must take into account some major constraints related with the evolution of the
rheological properties during the Fold Belt genesis. Indeed, the observed discrete late-Variscan
structures emphasize a predominant brittle-ductile behaviour which has been superimposed on
penetrative older Variscan ductile fabrics. This well established pattern is the result of the
metamorphic evolution related with the interaction between tectonic thickening, erosion and
isostatic compensation processes during the compressive stages of the Variscan Wilson cycle. But
the late orogenic stages can't also be understand ignoring the role of the anisotropies produced,
either during the main Variscan event or inherent from previous tectonic cycles. At this level, the
lithospheric anisotropies with a general E-W trend (fig. 1) play a major role at the Ibero-Moroccan
segment.
The classical genetical models for the late-Variscan deformation (Arthaud & Matte, 1975;
1977) assumed a fracture pattern which is consistent with a dextral simple shear model induced by
the oblique collision between Laurasia and Gondwana. Although the dextral movement between the
two major continents is in accordance with most of the recent paleogeographic models (e.g. Shelley
& Bossière, 2000; 2002; Simancas et al, 2005; Ribeiro et al, 2007), the predicted conjugated
fracture pattern has not been possible to emphasize in subsequent studies; the sinistral NNE-SSW
shear zones are clearly predominant. Another weakness of these models is their incapacity to fully
explain the observed structures. Indeed, although in Iberia most of the late-Variscan structures are
-298-
Scientific Sessions explained by a stress field with a N-S major shortening, in western Iberia a second stress field with
an E-W major compression must have been proposed (Ribeiro et al, 1979) in order to explain some
incompatible structures.
Fig. 1- Tectonic framework of the upper Paleozoic orogens around the Laurentia-Gondwana margin (continent
distribution adapted from Arthaud & Matte, 1977; the black stars indicates the studied sectors).
A completely different genetical approach was more recently proposed for Iberia by Ribeiro
(2002); using a bookshelf model it was possible, not only to explain the absence of conjugated shear
zones but also to use a unique stress field for all the observed late-Variscan structures.
In this work, the previous models have been tested using recent data from the SW Portuguese coast
and the western High Atlas in Morocco.
LATE-VARISCAN DEFORMATION IN SW PORTUGAL
The excellent outcrops in the SW Portuguese coast allow a detailed study of the Variscan
deformation in the pervasive turbiditic Carboniferous metasediments. If the first and main Variscan
tectonic event gives rise to a penetrative NW-SE folding with
a SW facing, the late Variscan deformation has only a local
behavior. This late deformation is mostly restricted to NE-SW
shear zones which produce the distortion of previous Variscan
fabrics; the induced steeply plunging asymmetrical chevrons
frequent in the wave-cut platforms (fig. 2A) clearly indicate a
sinistral kinematics.
Fig. 2- Major late-Variscan shear zones in the Almograve sector:
A- major chevron like fold developed at the shear zone boundary;
B- local structures induced by a dextral shear along the layers inside the
-299-
The Rheic Ocean: Its Origin, Evolution and Correlatives shear zones
Inside the shear zones a pervasive late Variscan deformation occurs mostly related with a
dextral shear sub-parallel to the layering (fig. 2B); this shear is the result of a local accommodation
in relation to the regional NNE-SSW sinistral rotation according to a flexural shear folding
mechanism.
It is important to emphasize that, although the sinistral shear zones are widespread along the
SW coast, their continuity is usually restricted to less than a couple of hundreds meters.
Concerning the age constrains of the late Variscan event, in SW Portugal it is possible to emphasize
that the sinistral shears are younger than Westephalian D age (the age of the younger deformed
metasediments) and older than lower Triassic; indeed, these shears have been reworked as normal
faults during the beginning of the Alpine orogeny controlling the evolution of the Triassic basins in
the SW Portugal (Dias & Ribeiro, 2002).
LATE-VARISCAN DEFORMATION IN WESTERN HIGH ATLAS (MOROCCO)
In Morocco we have characterize the late Variscan deformation in two sectors of the Western
High Atlas: Adassil and Azegour regions.
ADASSIL- The Twarirt intrusion (fig. 3) occurred during the late stages of the first and main
Variscan tectonic phase being controlled by a WNW-ESE sinistral shear zone (Hadani, 2003). If the
granite presents a ductile fabric related with its intrusion along the shear zone, latterly it has been
cut by discrete sinistral NE-SW shears that present a more brittle behavior in the granite (being
underline by metric quartz veins) and a brittle-ductile one in the middle Cambrian schists where
they induces the folding of the main cleavage.
Concerning the age constraints, the late Variscan deformation in Adassil is younger that the
Visean (the age of the main Variscan deformation) and older than the Alpine deformation that
induces the reworking of the brittle reverse fault at the northern boundary of the Tawrirt intrusion.
Fig. 3- Geological sketch of the Tawrirt intrusion in the
Adassil region (Hadani, 2003).
-300-
Scientific Sessions AZEGOUR- In this region the Permian granite (269 ± 9 My; Mrini, 1985), which is clearly
younger than the first and main Variscan deformation event, is cut by a NE-SW sinistral brittleductile shear zones with a sinistral kinematics. These shears, could be traced to the surrounding
Cambrian schists and, most of the mineralizations have been controlled by them (Hendaq, 2003)
Fig. 4- Geological sketch of the Azegour intrusion (Hendaq, 2003).
CONCLUSIONS
In all the studied sectors the late Variscan deformation is characterize by the predominance of
sinistral NE-SW to ENE-WSW shear zones with no evidences of any conjugated family. This
pervasive behavior, as well as the importance of major dextral shears at the orogen scale (fig. 1),
indicates that local studied sinistral shear zones are antithetic of the higher order faults (fig. 5B) and
not synthetic (fig. 5C). This shows that the genetical model for the late Variscan times proposed by
Ribeiro (2002) for Iberia (fig. 5A) is also suitable for the Moroccan sector.
Fig. 5- Late-Variscan genetical interpretations:
-301-
The Rheic Ocean: Its Origin, Evolution and Correlatives A- proposed model for Iberia (adapted from Ribeiro, 2002);
B- the antithetic approach;
C- the synthetic approach.
AKNOWLEDGEMENTS
Concerning Portugal sector, this work has been supported by the project FRIDS (POCTI/CTA/48595/ 2002). For the Moroccan
regions it has been supported by some bilateral grants -CNRS (Maroc)/GRICES (Portugal): IBMAVAR, MINVAR & TANTAN
dextral shears. M. Hadani benefits of a FCT PhD grant (SFRH/BD/19002/2004).
References
Arthaud, F., Matte, Ph. (1975). Les décrochements tardi-hercyniens du sud-ouest de l'Europe. Géométrie et essai de
reconstitution des conditions de la déformation. Tectonophysics, 25, pp. 139-171.
Arthaud, F., Matte, Ph. (1977). Late Paleozoic strike-slip faulting in southern Europe and northern Africa: result of a
right-lateral shear zone between the Appalachians and the Urals. Geol. Soc. Am. Bull., 88, pp. 1305-1320.
Dias, R. & Ribeiro, C. (2002). “O Triásico da Ponta Ruiva (Sagres); um fenómeno localizado na Bacia Mesozóica
Algarvia”, Comun. Inst. Geol. Min Portugal, 89, pp. 39-46.
Hadani, M. (2003). Contrôle structurale de l'intrusion granitique de Tawrirt (secteur d'Adassil); implications à
l'évolution géodynamique du Haut-Atlas occidental (Maroc). M. Sci. Thesis, Univ. Évora, 108 p.
Hendaq, Y. (2003). Relation du magmatisme, déformation et minéralisations qu secteur d'Azegour (Haut Atlas
occidental - Maroc). M. Sci. Thesis, Univ. Évora, 105 p.
Mrini, Z. (1985). Age et origine des granitoides hercyniens du Maroc: apport de la géochronologie et de la géochimie
isotopique (Sr,Nd,Pb). PhD Thesis, Univ. Clermont-Ferrand, 156 p
Ribeiro, A. (2002). Soft Plate Tectonics. Springer-Verlag, 324 p.
Ribeiro, A., Antunes, M., Ferreira, M., Rocha, R., Soares, A., Zbyszewski, G., Almeida, F., Carvalho, D., Monteiro, J.
(1979). Introduction à la géologie générale du Portugal. Serv. Geol. Portugal (26º Congr. Intern. Géol. – Paris –
1980), 142 p.
Ribeiro, A., Munhá, J., Dias, R., Mateus, A., Pereira, E., Ribeiro, L., Fonseca, P., Araújo, A., Oliveira, T., Romão, J.,
Chaminé, H., Coke, C., Pedro, J. (2007). "Geodynamic evolution of the SW Europe Variscides". Tectonics, 26, Art.
Nº TC6009, doi:10.1029/2006TC002058, 24 p.
Shelley, D., Bossière, G. (2000). A new model for the Hercynian Orogen of Gondwanan France and Iberia. J. Struct.
Geol., 22, pp. 757-776.
Shelley, D., Bossière, G. (2002). Megadisplacements and the Hercynian orogen of Gondwanan France and Iberia. in
Martínez Catalán, J. R., Hatcher, R.D., Jr., Arenas, R., and Díaz García, F. (Eds), "Variscan-Appalachian dynamics:
the building of the late Paleozoic basement", Boulder, Colorado, Geological Society of America, Special Paper 364,
pp. 209-222.
Simancas, J., Tahiri, A., Azor, A., Lodeiro, F., Poyatos, D., Hadi, H. (2005). The tectonic frame of the VariscanAlleghanian orogen in southern Europe and Northern Africa. Tectonophysics, 398, pp. 181-198.
-302-
Scientific Sessions Style and timing of the earliest tectonic events in the upper units of the Órdenes Complex.
F. Díaz García1, S. Sánchez Martínez2, P. Castiñeiras3,
J.M. Fuenlabrada4, R. Arenas3
1- Departamento de Geología. Universidad de Oviedo. 33005 Oviedo, Spain. [email protected]
2- Institut für Geowissenschaften, Facheinheit Petrologie und Geochemie, J.W. Goethe Universität Frankfurt am Main,
D-60438 Frankfurt am Main, Germany.
3- Departamento de Petrología y Geoquímica e Instituto de Geología Económica (CSIC). Universidad Complutense de
Madrid. 28040 Madrid, Spain.
4- CAI de Geocronología y Geoquímica Isotópica. Facultad de Geología. Universidad Complutense de Madrid. 28040
Madrid, Spain.
The upper units of the Órdenes Complex are emplaced above the Variscan suture and
contain a low-grade metasedimentary uppermost section intruded by a number of mafic dykes. This
unique section includes important geological information that allows to improve the general
knowledge about the evolution of the upper terrane of the allochthonous complexes of NW Iberia.
Three different deformative events affected the metasedimentary section. The youngest deformation
event D3 consists of metric to decametric folds represented by closed upright folds with axes gently
plunging to N-20E. These folds belong to the widespread refolding phase of the Órdenes Complex,
that affected the stacking of allochthonous units and their limiting shear zones and to which an
undifferentiated Variscan age is currently assigned. Structures belonging to the older D2
deformation event affected nearly the whole of the upper units, but in the studied coastal section
their higher deformation front can be traced running parallel to the “biotite in” boundary. The most
important structure is a regional S2 foliation axial planar of minor folds with dextral asymmetry,
sometimes non-cylindrical and with the short limbs intensely sheared. The presence of a stretching
lineation parallel to the D2 fold axes allows to deduce a top-to-the-North shearing. The oldest D1
structures can be observed in the low-grade turbidites devoid of D2 deformation from the coastal
section near Ares. Metric-size D1 folds developed in suitable greywacke-pelite alternancies consist
of tight folds with chevron and similar morphologies and axes gently plunge to N20E. Axial planar
S1 cleavage is a continuous foliation mainly defined by flattened and elongate quartz grains and
non-domainal homogeneous distribution of platy mineral grains with a preferred orientation. In the
western limb of the major Órdenes D3 synform, decametric and centimetric “facing up” and
senestral in asymmetry D1 folds can be observed, and allow us to deduce a major West vergent fold.
In the Eastern limb of the same large Órdenes D3 synform, suitable minor D1 folds have not been
found but microscopic analysis in oriented samples of the S1, S3 and S0 angular relationships also
suggest a West vergence for this first deformation event. Thus, the essential characteristic of the D1
-303-
The Rheic Ocean: Its Origin, Evolution and Correlatives deformation event is depicted by a set of West vergent folds with reverse limbs of less than 2 km,
affected in their lower part by the generalised presence of the regional foliation S2 that culminates in
the development of the Ponte Carreira detachment, the whole being affected by D3 upright Variscan
folding.
Mafic dykes intruding the low-grade turbidites are mainly composed of medium-grained
gabbros and diabases showing chilled margins and consisting of plagioclase, hornblende, epidote
and sphene with minor amounts of quartz and pyroxene relicts. Individual dike segments strike near
parallel and perpendicular to the regional composite foliation. They transect D1 folds and their field
relationships seem to suggest that they were emplaced at the end of the D2 shearing and prior to
upright D3 Variscan folds. U-Pb analyses on zircons obtained from one of the diabase dykes were
conducted using the SHRIMP-RG facility of the Standford University. Thirty-two analyses were
obtained in 31 zircon grains. The data can be roughly sorted in four age groups: <400 Ma, 480-530
Ma, 545-735 Ma and >2000 Ma. In the first group, three scattered ages around 320 Ma could
represent extreme Pb-loss of older zircons during the Variscan orogeny. From the second group, the
best age estimate is obtained by pooling together six analyses that yield ca. 510 Ma. This age is
interpreted as the crystallization age of the Ares dyke. The last two groups represent inherited
components, with a cluster of six analyses at ca. 565 Ma; three single analyses ranging between 625
and 650, and other two analyses at ca. 730 Ma. Finally, five analyses reveal a Paleoproterozoic
signature at ca. 2010 and 2080 Ma.
The structural analysis carried out in the uppermost metasedimentary levels of the Órdenes
Comples reveals the development of D1 West vergent fold nappes, followed by a generalised top-tothe-north shearing event (D2). Field observations indicate that mafic dykes were emplaced at the
end of the D2 shearing. Considering the 510 Ma age obtained for one of the diabase dykes and the
530-500 Ma maximum depositional age of the turbiditic series (Fuenlabrada et al.), the
development of the two first deformative events D1 and D2 should be restricted to a short time span
during the Middle Cambrian. The new obtained data in the top levels of the Órdenes Complex,
suggest that the accretionary processes related to the development of D1 West vergent fold nappes
continued until ca. 510 Ma, when they changed to a regime of north-directed extension and mafic
diking. This tectonic and magmatic evolution can be tentatively related with the final subduction of
an oceanic ridge. The subduction caused the change to an extensional regime which culminates in
the opening of the Rheic Ocean.
References
Fuenlabrada, J.M., Arenas, R., Sánchez Martínez, S., Díaz García, F., Castiñeiras, P. (This volume) A peri-Gondwanan
-304-
Scientific Sessions arc in NW Iberia: Isotopic and geochemical constraints to the origin of the arc - The sedimentary approach.
-305-
The Rheic Ocean: Its Origin, Evolution and Correlatives Late Lower and Middle Devonian miospore stratigraphy of Oued Saoura, western Saharan,
Algeria
M. Hassan Kermandji, F. Khelifi Touhami
Department of Nature and Life, College of Nature and Life, University of Mentouri-Constantine, 25000 Constantine,
Algeria.
SUMMARY
A thick extensive sequence of Middle Devonian rocks in Oued Saoura is studied. Miospore
assemblages from Mongar Debad Km 30 and Chefar el Ahmar exposures south Béni Abbès have
been identified and keyed into previously described palynostratigraphic miospore assemblage
biozones of deep wells from the Tidikelt Plateau and Gourara, central Saharan, Algeria. The
biostratigraphical data show that the basal strata of Teferguenite Formation confirm faunal ages that
are established on limited rock interval, are of Eifelian age. Subsequently the presences of
characteristic miospore taxa in the higher studied sequence above the basal strata of the same
formation indicate Givetian age. Six miospore assemblage biozones, including three spore Interval
zones are proposed. Comparison with data from Algeria, Libya, Tunis promising constructive
correlation across the North Gondwana region. Differences are accentuating with cotemporary
Euro-American biozones.
Keywords: MIOSPORE; DEVONIAN; PALYNOSTRATIGRAPHY; WEST SAHARAN; ALGERIA.
INTRODUCTION
Oued Saoura is well known for its Palaeozoic exposures (Deleau 1962; Legrand 1965, 1983;
Legrand-Blain 2002; Alieve et al 1970; Fabre 1976, 1983; Ndjari et al. 2003). The Devonian rock
sequences of Oued Saoura are subdivided into numerous formations on lithologic criteria. Some
information on the lithology of these formations may be found in the above references but for
clarification, Teferguenite Formation will be described briefly in this paper. The precise ages of
these layers are difficult to establish due to the scarcity, low diversity and rarity of characteristic
faunas that they had been previously recorded. Devonian miospore data from five boreholes of the
Tidikelt Province and Gourara region, Western Algerian Sahara obtained by Hassan Kermandji et
al. (2008) are integrated here. Miospores from these rocks are of biogeographic and biostratigraphic
significance and can be of practical use to correlate the floras of northern Gondwana with those of
Laurussia. This paper documents in brief for the first time the description of miospore assemblages
-306-
Scientific Sessions from Lower and Upper Members of Teferguenite Formation fig.(2).
Oued Saoura extends through the provinces of Bechar, Béni Abbès and Adrar fig.(1). This
study concerns that part of depression south of Béni Abbès with its well-exposed Paleozoic strata.
Geologically it belongs to Bechar Basin and Béni Abbès depression occupies a zone between the
Ougerta chain to the west and the Grand Erg Occidental to the east. (Aliev et al. 1971, Fabre 1976).
The Saoura Depression underwent intensive subsidence during the Silurian and Devonian and
formed a graben linked to the Anti-Atlas miogeosynclinal furrow over the northern border of the
Dalle. Geostratigraphically characterized by a series deformation but within this depression an
almost complete succession of Silurian and Devonian deposits was preserved.
Siluro-Devonian sequence of Oued Saoura rests unconformably on Cambro-Ordovian strata (Faber
1976; Legrand 2002) and consists of five formations including Teferguenite Formation.
PALYNOLOGY
Most samples contain poorly preserved palynofloras, some of which are moderately well
preserved.
The Lower Member of the formation contain Emphanisporites annulatus, E.schultzii,
Chelinospora perforate, Stenozonotriletes furtivus, Geminospora cf. spinosa, G.svalbardiae,
Acinosporites conatus and Camarozonotriletes.sextantii. This is placed within the E.annulatusC.sextantii Assemblage Zone of Richardson and McGregor (1986) of early and early late Emsian.
Many of these species were recorded by: (Hassan Kermandji et al. 2008, Jardiné and Yapaudjian,
1968, Abdesselam-Rouighi, 1996, 2003, Moreau Benoît et al., 1993, Loboziak and Streel and
Loboziak et al., 1989 and 1992 respectively). On broad sense, E. foveolatus and Verrucosisporites
dubia Oppel Zone of Streel et al. (1987) may be similar but neither nominal species nor
characteristic taxa of FD Zone occur in both Tidikelt Plataeau and in Oued Saoura regions.
Miospores of the lowest strata of the Upper Member of the formation were obtained from a
sample sited approximately 5.2m above proposed boundary between Lower and Upper Members of
the formation. The first stratigraphically significant species occur in this sample are Grandispora
protea and Hystricosporites microancyreus. Thus, the entry of H.microancyreus and G.protea marks
the base of microancyreus-protea Biozone of Hassan Kermandji et al. (2008).
The successive first occurrence of diagnostic taxa: Calyptosporites velatus, Rhabdosporites
langii, Acinosporites lindlarensis, Samarisporites mediconus, and Ancyrospora ancyrea var.
ancyrea in higher samples correspond to the base of C.velatus-R.langii Assemblage Zone of early
-307-
The Rheic Ocean: Its Origin, Evolution and Correlatives Eifelian age of Richardson and McGregor (1986). Loboziak and Streel (1989), Loboziak et al.
(1992), Streel et al. (1988), Moreau-Benoit et al.1993 and Abdessalem Roughi, 1996,2003, reported
many of these species from Eifelian core samples of Ghadamès-Hammadah and Illizi Basins. In
spite of regional differences, there are a number of miospore species that were found both in early
Middle Devonian of the Old Red Sandstone continent and adjacent regions and Northern Gondwana
and current study.
Investigations of samples from the higher strata of the same section revealed the presence of
Late Eifelian and Early Givetian microfossils. They are represented by Cristatisporites orcadenis,
Samarisporites inaequus, Spinozonotriletes libyensis, Perotrilites conatus, Rhabdosporites
pervulus, Camptozonotriletes asdminthus and Convolutispora disparalis. Also many of these taxa
were recorded from late Eifelian-early Givetian of Northern Gondwana and many Euramerica
regions.
It is very interesting, that just few centimetres above the sample contains C.orcadensis,
S.libyensis appeared microfloral assemblage consists of: Geminospora lemurata, Aneurospora
goensis, Perotrilites ergatus, Verrucosisporites devonicus and Samarisporites libyensis, this is
accompanied by great number of miospores possessing anchor spines. It is quite reasonable to
presume that these strata are of Givetian age.
The highest studied samples contain the following species: Cristatisporites triangulatus,
Acantotriletes grandispinosus, Convolutispora tegula, Rhabdosporites scamnus, Samarisporites
libyensis and Convolutispora flexuosa. Numerous of these taxa are suggesting late Givetian-Early
Frasnian (Richardson and McGregor, 1986, Streel et al. 1987, Loboziak et al.1992).
Many of the record species were also recorded from Middle Devonian beds of Northern Gondwana
regions: Orsine Formation, Illizi Basin, Moreau-Benoit, 1993; Aouinet-Ouenine I and II, Ghadamis
Basin, Massa and Moreau-Beniot, 1976; northeast Libya, Streel et al., 1988; Ghadamis-Hammadah
Basin, Loboziak and Streel, 1989; Loboziak et al., 1992; northern part of Saudi Arabia, Loboziak
and Streel, 1995; Parnaiba Basin, Loboziak et al. 1992.
References
Abdesselam-Rouighi,
F.
1996.
Biostratigraphie
des
spores
du
Dévonien
de
la
synéclise
Illizi-Ghadamès, Algérie. Bull. Serv. Géol .Alg., 7, 171-209.
Abdesselam-Rouighi, F. 2003. Biostratigraphie des spores, acritarches et chitinozoaires du Dévonien Moyen et
Superieur du Bassin d’ Illizi ( Algérie). Bull. Serv. Géol .Alg., 14, 97- 117.
Aliev, M., Korj, M., Oulmi, M., Mazanev, V., Medvedev, E., Oriev, L., Korotkov, V., 1970. Lithologie, faciès et
paléogéographie du Paléozoïque du Sahara Algérien.2- ème Colloque scientifique, Boumerdes, Alger. 189 p.
Aliev, M., Ait Laoussine, N., Avrov, V., Aleksine, G., Barouline, G., Lakovlev, B., Korj, M- Kouvykine, J., Makarov, V.,
Mazanov, V., Medvedev, E., Mktchiane, O., Moustafinov, R., Oriev, L., Oroudjeva, D., Oulmi, M., Said, A., 1971.
Geological structures and estimation of oil and gas in the Sahara in Alegria. Altamira Rcto press, S.A Spain. 265 p.
-308-
Scientific Sessions Deleau, P., 1962. Le bassin Houiller d’Abdallah (Sud Oranais). Publication du Service de la carte Géographique de
l’Algérie. Bull., 14, 319 p.
Fabre, J., 1976 : Introduction à la géologie du Sahara Algérien. S. N . E . D . Alger , 421 p.
Fabre, J., Bertrand, J.M., Bertrand-Sarfati, J., Bessoles, B., Black, R., Moullier, A.M., Caby, R., Choubert, G., Conrad,
G., Conrad, J., Deynoux, M., Donzeau, M., Elouard, P., Faure-Muret, H., Hebrad, L., Kogbe, C.A., Lang, J.,
Latouche, I., Leblank, M., Lefrance, J., Legrand, Ph., Legrand-Blain, M., Mathieu, P., Moussine-Pouchkine, A.,
Reyment, R.A., Schobel, J., Tait, E. A., Trompette, R..1983. West Africa geological introduction and stratigraphic
terms. Lexique stratigraphique international. Nouvelle Série n° 1. Pergamon Press, 396p..
Hassan Kermandji, A.M., Kowalski, W.M., Khelifi Touhami, F., 2008. Miospore stratigraphy of Lower and middle
Devonian deposits from Tidikelt, Central Sahara, Algeria. Geobios, 41, 227-251.
Jardiné, S., Yapaudjian, L. 1968. Lithostratigrapgie et palynologie du Dévonien - Gothlandien Gréseux du basin de
Polignac (Sahara).Rev. Inst. Fr. Pétrole, 23, 439-468.
Legrand-Blain., M., 2002. Le Strunien et le Tournaisien au Sahara Algérien: Limites, échelles lithostratigraphiques et
bio stratigraphiques régionales. Mem. Serv. Géol. Alg., 11, 61-85.
Legrand, P., 1965. Nouelles connaissances acquises sur la limite du système Silurien et Dévonian au Sahara. Résumé
Mémoires du Bur.Rech.Géol.Min., 33, 50-58.
Legrand, P., 1983. Aperçu sur l’histoire géologique de l'Algérie Paléozoïque: Le Paléozoïque Inférieur et le Dévonien.
96-104. In: Fabre, J, West Africa, Geological introduction and stratigraphic terms. Pergamon Press.
Legrand, P., 2002. La limite Cambro-Ordovicien: Définition, Application au Sahara Algérien. Mem. Serv. Géol. Alg. 11,
45-59.
Loboziak, S., Steemans, P. Streel, M., Vachard, D. 1992. Biostratigraphy par miospores du Dévonien inférieur à
supérieur du sondage MG-1 (Bassin d’Hammadah, Tunisie)-comparaison avec les données des faunes. Rev.
Palaeobot. Palynol., 74, 193-205.
Loboziak, S., Streel, M., 1989. Middle-Upper Devonian miospores from the Rhadamès Basin (Tunisia-Libya):
Systematic and stratigraphy. Rev. Palaeobot. Palynol., 58, 173-196.
Loboziak,S.,Streel,M,1995.Late Lower and Middle Devonian miospores from Saudi Arabia. Rev. Palaeobot. Palynol.,
89, 105-113.
Loboziak,S.,Streel,M.Caputo, M.V.,Melo,J.H.G.,1992.Middle Devonian to lower Carboniferous miospore stratigraphy
in the central Parnaiba Basin (Brazil).Ann.Soc.Géol.Belg.,115,215-226.
Massa, D., Moreau-Benoît, A., 1976. Essai de synthèse stratigraphique et palynologique du système Dévonien en Libye
Occidentale. Rev., Inst., Fr., Pétrole. 31, 287-333.
Moreau-Benoit, A. Coquel, R., Latreche, S., 1993. Étude palynologique du Dévonien du Bassin d’Illizi (Sahara Oriental
Algérien). Approche Biotratigraphique. Géobios, 26, 3-31.
Nedjari, A., Ait ouali, R., Chikhi-Aouimeur, F., Bitam, L., 2003. Le bassin de l’Ougarta au Paléozoique: une mobilité
permanente (Liveret guide du field trip), 103p.. Publ. Service Géologique de l’Algérie. Off.Rech.Géol.Min.Boumerdès.
Richardson, J. B., McGregor, D. C., 1986. Silurian and Devonian spore zones of the Old Red Sandstone Continent and
adjacent regions. Geol. Surv. Can. Bull., 364, 79p.
Streel, M., Higgs, K., Loboziak, S, Reigel, W., Steemans, P., 1987. Spore stratigraphy and correlation with faunas and
floras in the type marine Devonian of the Ardenne - Rhenish regions. Rev. Palaeobot. Palynol., 50, 211 – 229
Streel, M., Paris, F., Riegel, W., Vanguestaine, M., 1988. Acritarchs, chitinozoan and spore stratigraphy from the Middle
and Late Devonian of northeast Libya. 11-126. In: A., El Arnauti, B. Owens and B.Thusu (eds).Subsurface
palynostratigraphy of northeast Libya. (1988) Benghazi.
-309-
The Rheic Ocean: Its Origin, Evolution and Correlatives Figure 1: Timing of events included in 1-D and 2-D models. In the 1-D model C-2 is an instantaneous event at 325 Ma.
Ages are from Dallmeyer et al. (1997) and Fernandez-Suarez et al. (2000).
-310-
Scientific Sessions -311-
The Rheic Ocean: Its Origin, Evolution and Correlatives Emplacement of the Cuera and Picos de Europa imbricate system at the Core of the IberianArmorican Arc (Cantabrian-Zone, NW Spain)
O. Merino-Tomé1, J.R. Bahamonde2, J.R. Colmenero 3, N. Heredia4 & E. Villa2
1
Instituto Geológico y Minero de España, León, Spain ([email protected])
2
Departamento de Geología, Universidad de Oviedo, Oviedo, Spain.
3
Departamento de Geología, Universidad de Salamanca, Salamanca, Spain
4
Instituto Geológico y Minero de España, Oviedo, Spain
The final stages of the Variscan Orogeny (Carboniferous) in the Western European Variscan Belt
(WEVB) triggered the development of the Ibero-Armorican Arc which is cored by the Cantabrian Zone
(NW Iberian Peninsula). The Cantabrian Zone represents the foreland of the WEVB and records the
waning stages of the closure of the Rheic Ocean. In the distal sectors of this foreland basin (Cuera Unit
and Picos de Europa Province) a giant Bashkirian-Moscovian carbonate platform developed. This
carbonate platform was progressively incorporated into the Variscan orogenic wedge and broken up into
numerous E-W oriented thrust sheets from the late Moscovian to the Gzhelian times. The thrust sheets
emplaced from north to south (according present day coordinates) following a forward sequence and
forming an imbricate system characterized by a low dip of the upper topographic surface ( <1º); thus
allowing the development of wedge-top basins.
The syntectonic successions accumulated in these piggy-back basins record a four-stage evolution
of the imbricate system: 1) Late Myachkovian to early Krevyakinian.- Advancement of the orogenic
front (at least 110 km) across the northern domain of the carbonate platform (Cuera and GamonedoCabrales units) and development of a piggy-back basin at the frontal part of the orogenic wedge. 2) Mid
Krevyakinian to late Khamovnikian.- Propagation of the deformation (~80 km) to the southern domain
of the carbonate platform. Thrust-top carbonate ramps nucleated in tectonic uplifts linked with blind
thrusts, and shelf and deep-water clastic deposits filled the adjacent marine depressions. 3)
Khamovnikian-Dorogomilovian transition.- Imbrications of new thrust slices. The southward transport of
the imbricate system (overriding the Pisuerga-Carrión Province) and the accommodation of internal
tectonic shortening caused the emergence of broad areas of the orogenic wedge. 4) Dorogomilovian and
Gzhelian.- Reactivation of previous thrusts and out-of-sequence thrusting that enhanced the tectonic
shortening and consequently led to the development of new depocentres in the Northern and Central
units while a piggy-back basin continued to subside in the Frontal Unit.
The thrust sheets were emplaced roughly perpendicular to previous tectonic units of the Cantabrian
foreland fold-and-thrust belt, most probably during the oroclinal bending of the SW European Variscan
belt that formed the Ibero-Armorican Arc. The enormous tectonic shortening (>150±15 kms) and the
-312-
Scientific Sessions complex models of propagation and distribution of the deformation over time are envisaged to have been
the result of mechanical constraints imposed by the increasing dip angle of the basal décollement ( ),
due to the northwards bending of the underthrust Gondwana lithosphere, and by the spatial and
geometric constraints imposed by the closure of the Ibero-Armorican Arc.
-313-
The Rheic Ocean: Its Origin, Evolution and Correlatives New data about stratigraphy and structure of the Vila Velha de Ródão complex syncline (SW
sector of the Centro-Iberian Zone)
Daniel Metodiev1, José Romão1, Rui Dias2
1
Geology Depart., LNEG (ex-INETI), Apartado 7586, 2721-866 Alfragide, Lisbon, Portugal, [email protected] ,
[email protected]
2
Geosciences Depart., Évora University, Largo dos Colegiais, 2-Apartado 94, 7002-554 Évora, Portugal,
[email protected]
Abstract: New data about the lithostratigraphy and the progressive Variscan structure of the Vila Velha de Ródão
complex syncline is presented. During the geological mapping revision was established an Ordovician-Silurian
succession, very similar to the Amêndoa-Carvoeiro synform D3 (Romão, 2000). The Variscan deformation includes:
early overthrusts subsequently transformed in forethrusts with duplex geometry facing to NE, folds, cleavage and linear
structures (D1a and D1b) and latter crenulation, backthrusts and backfolds with an opposite facing (D1c). Their
compatible progressive deformation with NE-SW maximum compression induces décollement in depth of thin-skinned
type and creates triangular structures. This macrostructure is affected by late-Variscan faults, probably associated with a
domino structures, of which the most important is the Ponsul reverse fault.
INTRODUCTION
The Vila Velha de Ródão complex syncline is located in the Central-Iberian Zone (CIZ), SW
sector, NE of the Amêndoa-Carvoeiro synform (Romão, 2000) and NW of the Serra de São
Mamede syncline. This study refers the Lower to Middle Paleozoic succession, situated above the
unconformity that separates the Beiras Group (BG), shale and greywacke complex, of the
Armorican Quartzitic Formation (AQF). Our main objective is to present the Ordovician-Silurian
lithostratigraphic succession and the Variscan structure of the Vila Velha do Ródão syncline, based
on new data that the review of geological mapping on the 1/25000 scale, allowed to establish. It
also correlates the lithostratigraphic succession established in the syncline of Vila Velha de Ródão
with the D3 Amêndoa-Carvoeiro synform succession and characterizes the Variscan deformation
events and their associated kinematics.
LITHOSTRATIGRAPHY
The revision of Vila Velha de Ródão geologic mapping emphasizes the presence of a
lithostratigraphic succession with Lower to Middle Paleozoic age, already recognized in the
Amêndoa-Carvoeiro synform (Romão, 2000; 2001; 2006). Above the older unit, BG, still not
differentiated in the studied area, overlaps with a high angle unconformity the Ordovician-Silurian
sequence.
This sequence initiates with the AQF (±80m) which is composed mostly by massive beds of
coarse-grained quartzites. Near the base sometimes quartzitic conglomerates occur (Vilas Ruivas
and Sobral Fernando) and to the top fine quartzites appear often laminated and with trace-fossils
marks. Inside these fine quartzites were recognized Skolithus and Cruziana (NE of Foz do Cobrão).
Above this unit occur fine pelites and siltstones, occasionally with Didymograptus and trilobites
(Ribeiro et al., 1965; 1967; Teixeira, 1981) which were included in the Brejo Fundeiro Formation
(BFF, ±120m). They follow arenites and impure quartzites with storm characteristics that were
integrated in the Monte de Sombadeira Formation (MSF, ±15m). On top of the MSF we identified
the Fonte da Horta (FHF) and Ribeira do Casalinho (RCF) Formations with thicknesses of few
meters. They are characterized respectively, by pelites and quartzo-arenites, intercalated with dark
pelites. The Upper Ordovician (Caradocian) is initiated by a regressive sequence of bioturbated
pelites and massive arenites which form the Cabeço do Peão Formation (CPF, ±25m). The previous
units are overlaid by regressive layers of micaceous impure quartzites of the Ribeira de Laje
Formation (RLF, ±5m) and, afterwards, by massive packets of pelite-siltitic sediments, inside which
elongated fragments of quartzo-arenitic and quartzitic composition occur. This last unit, with
-314-
Scientific Sessions glaciogenic sedimentary structure, was recognized as the Casal Carvalhal Formation (CCF, ±50m).
This Paleozoic succession finishes with one packet of gray quartzites layers, sometimes with
aggregates of pyrite and syn-sedimentary structures, typical characteristics of the Vale da Ursa
Formation (VUF, ±20m). On top of this last unit were observed some meters of dark laminated
graphitic pelites with fossils (brachiopods and Monograptus), which should correspond to the
Aboboreira Formation (AF). The two last units can already be considered Lower Silurian (Romão,
2000; 2001; 2006).
VARISCAN STRUCTURE
The geological mapping of Vila Velha de Ródão complex syncline permitted evidence that its
geometry and limits show strong tectonic control. It is the result of a progressive superposition of
deformation events related with the Variscan orogeny.
The main synclinoric structure ends in SSE with a monocline (Campos & Pereira, 1991) in the
Serra de São Miguel. In the SW limb of this monocline there is an overthrust with a NE facing. The
NNW final part of this macrostructure culminates in a triangular zone (Foz do Cobrão), limited to
SW by a forethrust and to NE by a backthrust (Fig.1), with opposite facings. Their SW limb is
imbricated by the Vinagra-Foz do Cobrão forethrust, which is characterized by a duplex geometry
(Ramsay & Huber, 1987) and it is caused the displacement of the BG metasediments above the
AQF quartzites. The forethrust and backthrust terms are related with the dominant regional facing
of the thrust-fold system; where their interference originates a triangular zone (Butler, 1982).
Fig.1 - Schematic framework and geological cross sections of the Vila Velha de Ródão complex syncline
The D1a Variscan deformation induced folds with primary penetrative foliation (S1) and early
overthrusts with a NE facing. The coeval folds, often asynchronous, present geometry and different
styles with metric to centimetric amplitudes and, more rarely, decametrics. The axial planes of the
-315-
The Rheic Ocean: Its Origin, Evolution and Correlatives folds shows a dominant facing to NE and a NW-SE to NNW-SSE general trend. Their axis are subhorizontals (values less than 20º-25º) or mostly plunges to SE (Serra de Perdigão and Serra de Foz
do Cobrão) and, more rarely, to NW (Serra de São Miguel). The axial plane S1 foliation has a NWSE to NNW-SSE trend. Their axis are sub-horizontals and present a general SW dip (≈70º). The L1
intersection lineation shows a N35º-40ºW orientation and a sub-horizontal to relatively low dip,
mainly to SE and, locally, to NW. Stretching lineation was not observed in the Ordovician-Silurian
formations, but only at a lower structural level in the BG metasediments where it presents a subhorizontal geometry; an extension sub-parallel to the a kinematic axis could than be emphasized
(Ribeiro et al., 1990; Romão, 2000).
The D1bVariscan deformation, in continuity with the D1a one, will retakes the early overthrusts,
generating larger forethrusts with duplex geometries and with orientation NNW-SSE. They are
including in this event the Vinagra-Foz do Cobrão, Portas do Ródão-Perdigão and Vale do Cobrão
forethrusts. The Vinagra-Foz do Cobrão forethrust (NNW-SSE, 45º-80ºSW) was due to overlap of
the BG shale and greywacke above the AQF quartzites. The kinematic markers on the plane of the
forethrust indicate thrusting movement with a slight dextral component.
The Portas do Ródão-Perdigão forethrust (N15º-20ºW, 70ºSW) must correspond to an
imbrication of the previously described forethrust. It presents a pure thrust movement, inferred from
the striations, and caused the displacement of the AQF beds above the metapelites of the FBF,
inducing an inversion of the polarity in the layers of the FBF-MSF-FHF-RCF-CPF succession. In
the core of the Vila Velha de Ródão syncline was identified the Vale do Cobrão forethrust (N25ºW,
70ºSW), with a similar facing and sub-parallel to the previous forethrusts. This overthrust was
responsible for the superposition of the inverted succession of Middle to Upper Ordovician from the
SW limb of the Vila Velha de Ródão syncline above the metapelitic FBF, from the opposite limb of
the same syncline.
In continuity the D1c event generates backfolds (N10ºW, 0º-20ºSE) and backthrusts with a SW
facing. Among the backthrusts stands out the Chão das Servas-Carregais backthrust, which is
induced by the superposition of the BG lithologies above the AQF quartzites on the NE limb of the
main structure. On the opposite limb of this structure develops the Vinagra-Foz do Cobrão
forethrust, already described. These two overthrusts with opposite facings define one triangular
structure, in the core of which was formed the Sobral Fernando anticline, with a NE facing. It is the
result of a progressive compression with a NE-SW orientation. Locally, the D1c event also produced
a crenulation cleavage with N10º-30ºW trend and a SW dip.
A late-Variscan brittle deformation affects the Vila Velha de Ródão complex syncline. Sinistral
strike-slip faults dominate with a NE-SW to NNE-SSW orientation. Among them stands out the
Ponsul fault (Ribeiro, 1943; Dias & Cabral, 1989), which was reactivated as a reverse fault during
the Alpine movements and is responsible for the overlapping of the Variscan substrate upon the
continental Tertiary deposits. The NE-SW to NNE-SSW strike-slips could be interpreted as domino
structures, related with the E-W dextral strike-slips, which were cutting the entire Variscan orogeny.
This brittle deformation of the Lower Permian, observed at orogenic scale, resulted from an E-W
maximum shortening that is locally accompanied by a N-S smaller shortening, which generated in
the same time constriction (Ribeiro et al., 2007).
CONCLUSIONS
In the Vila Velha de Ródão complex syncline was recognized and mapped an OrdovicianSilurian succession very similar to the sequence established by Romão (2000; 2001; 2006) in the
Amêndoa-Carvoerio synform. The comparative analyse between the two successions indicates that
the units are generally thinner and have less fossils than in the Amêndoa-Carvoeiro series. This
sedimentary succession was deposited during the development of a larger sedimentar cycle with
duration within of 50Ma. The transgressive phase occurred from Arenigian to Dobrotivian and is
-316-
Scientific Sessions characterized by the transition from coastal to external platform environments. In the Caradocian
the regressive phase is characterized by a coastal deposition environment, where the presence of
glacio-marine sedimentation stands out, which originated from sub-glacial waters, close to the
continent during Hirnantian (Romão & Oliveira, 1997; Romão, 2000; 2006).
The studied region, in scale of the Variscan orogeny, is located between two transpressive zones
with a WNW-ESE to NW-SE trend and opposite movements: the Tomar-Badajoz-Córdoba Shear
Zone and the outermost Arc of NW Iberia. The first is characterized by a left transpressive regime
and the second by a right movement. The fold’s geometry and attitude, as well as their dominating
NE facing and the primary S1 foliation, which corresponds to the flattening surface of the preexisting objects, are compatible with a maximum NE-SW compression during the early events of
the Variscan deformation phases. Still in these events the sub-vertical stretching in “a” indicates a
vertical escape of material, thus we can deduce one stress field which is characterized by horizontal
shortening and vertical stretching.
The presence of highly inclined NE facing forethrusts, backthrusts and backfolds with NNWSSE trend and SW facing, as well as secondary S2 foliation with NE facing, indicates that
deformation gradually continued during the rest of the D1 orogenic phase with a similar stress field,
eventually with one slight rotation of the major compression towards ENE-WSW due to the
progressive deformation of the Iberian-Armorican Arc (Ribeiro et al., 2007). The identification of
triangular structure in metric and decametric scale, suggests that there may exist a décollement in
depth of thin-skinned type (Butler, 1982).
REFERENCES
Butler, R. W. H. (1982) The terminology of structures in thrust belts. Jour. Structural Geology, 4, 3: 239-245.
Campos, A., Pereira, G. (1991) Aspectos da estrutura do Complexo Xisto-Grauváquico ante-Ordovícico e do Ordovícico da Serra de
São Miguel-Nisa (Alto Alentejo). Mem. Not. Publ. Mus. Lab. Min. Geol., 112 (a): 81-97.
Dias, R., Cabral, J. (1989) Neogene and Qaternary Reactivation of the Ponsul Fault in Portugal. Comun. Serv. Geol. Portugal, Lisboa,
75: 3-28.
Ramsay, J., Huber, M. (1987) The techniques of modern structural geology. Folds and fractures. London: 522.
Ribeiro, A., Quesada, C., Dallmeyer, R. D. (1990) Geodynamic evolution of the Iberian Massif. In: Dallmeyer, R. D. & Martinez
Garcia, E. (Eds.), Pre-Mesozoic Geology of Iberia. 399-410 (Springer-Verlag).
Ribeiro, A., Munhá, J., Dias, R., Mateus, A., Pereira, E., Ribeiro, L., Fonseca, P., Araújo, A., Oliveira, T., Romão, J., Chaminé, H.,
Coke, C., Pedro, J. (2007) Geodynamic evolution of SW Europe Variscides. Tectonics, 26: TC6009.
Ribeiro, O. (1943) Evolução da falha de Ponsul. Comun. Serv. Geol. Portugal, Lisboa, 24: 109-123.
Ribeiro, O., Teixeira, C., Carvalho, H., Peres, A., Fernandes, H. P. (1965) Carta Geológica de Portugal, escala 1:50 000. Notícia
explicativa da folha 28-B (Nisa). Serv. Geol. Portugal, Lisboa, 29.
Ribeiro, O., Teixeira, C., Ferreira, C., R. (1967) Notícia Explicativa da Folha 24-D, Castelo Branco. Serv. Geol. Portugal, Lisboa, 24.
Romão, J. (2000) Estudo tectono-estratigráfico de um segmento do bordo SW da Zona Centro-Ibérica, e as suas relações com a Zona
Ossa Morena. Dissertação de Doutoramento em Geologia, Fac. Cien., Univ. Lisboa: 322p.
Romão, J. (2001) O Paleozóico no bordo SW da Zona Centro Ibérica. Geonovas, 15: 33-43.
Romão, J. (2006) Notícia explicativa da folha 28-A Mação. Carta Geol. Portugal 1:50 000, Inst. Geol. Min., Lisboa.
Romão, J., Oliveira, J. T. (1997) Geoquímica dos diamictitos glaciomarinhos da Formação de Casal Carvalhal na estrutura sinclinal
Amêndoa-Carvoeiro. XIV Reunião Geol. do Oeste Penisular, 215-216.
Teixeira, C. (1981) Geologia de Portugal. Fundação Calouste Gulbenkian, Lisboa, vol. I: 333-337.
-317-
The Rheic Ocean: Its Origin, Evolution and Correlatives HYDROTHERMAL DOLOMITIZATION SPATIAL DISTRIBUTION AND RELATION TO
TECTONIC SETTING: THE PALEOZOIC OF THE CANTABRIAN ZONE, NORTHERN
SPAIN
Natalia Muñoz-Quijano1, Thilo Bechstädt1, Maria Boni2, Gabriel Gutiérrez-Alonso3, Paola Ronchi4
1
Inst. of Earth Sc. Univ. Heidelberg, D 69120 Heidelberg ([email protected])
2
Dip. Scienze della Terra, Univ. Napoli, I 80138 Napoli
3
Geodinamica Interna, Depart. Geologia, Univ. Salamanca, ES 37008 Salamanca
4
Eni Exploration & Production Division, SPES Dpt., I-20097 S. Donato Milanese
The Cantabrian Zone in NW Spain represents the Foreland Fold and Thrust Belt of the
Variscan orogen. The succession comprises Cambrian to Carboniferous sediments on top of a
Precambrian basement. Most of the succession is in the diagenetic realm, only locally a
metamorphic (mainly epizonal) overprint occurs. After the Variscan orogenesis, porosity and
permeability were low. Our research focuses on a very widespread, post-orogenetic hydrothermal
dolomitization, which strongly increased porosity and permeability. It affected mainly
Carboniferous but locally also older carbonate successions. Our earlier research (Gasparrini et al.,
2006; Lapponi et al., 2007) concentrated on the type of dolomite and its geochemical and isotopic
characteristics, the fluids generating this dolomite (very saline, originally evaporitic brines of higher
temperature), the timing of dolomitization (probably Permian), and the relation with the
geodynamic setting (associated, at least timewise, with the bending of the Cantabrian Arch -see
Gutiérrez-Alonso et al., 2004- together with slab break-off and a thermal event).
Study of various parameters such as crystal size, eventual rhythmicity of pores, porosity
orientation and permeability in relation to dolomite texture and fabric are used to identify common
patterns. With the spatial distribution analysis of an attribute (in the case of this study, the different
descriptions and characterizations of the dolomite bodies and structures) is possible to measure the
spatial autocorrelation in relation with the structural setting.
Macroscopically, three different types of dolomite bodies occur: Bedded, Breccia and
Crystalline. The four basic types of internal structures are: Zebra, Vuggy, Transitional and Tight.
The zebra structures strongly contribute to porosity and permeability are widely present in the
different dolomite bodies and can be characterized further.
By early variograms was identified that eastern region in the area studied, the innermost region
of the core arc of the Cantabrian Zone Oroclinal, have the most values with Poroperm properties
favourable conditions. This rocks show low average angle relationships (less than 30°) between the
bedding attitude (So) and the voids shape fabric (Sp) and corresponding to Bedded dolomite bodies,
mainly zebra structures, planar shape in the rhythmic band, size above 5mm in the rhythmic band
-318-
Scientific Sessions and planar relationship between the rhythmic band direction and the bedding.
The authors gratefully acknowledge Eni E&P Management for permission to present this study,
as part of a wider R&D study promoted by Eni E&P
References
Gasparrini, M., Bechstädt, T. & Boni, M. (2006): Massive hydrothermal dolomite in the southwestern
Cantabrian Zone (Spain) and its relation to the late Variscan evolution.- Marine and Petroleum Geology,
23, 543-568.
Lapponi, F., Bakker, R., Bechstädt, T. (2007): Low temperature behaviour of natural saline fluid inclusions in
saddle dolomite (Paleozoic, NW Spain).- Terra Nova 19/6, 440-444.
Gutiérrez-Alonso, G., Fernández-Suárez, J., Weil, A.B., (2004): Orocline triggered lithospheric
delamination. Geological Society of America Special Paper, 383, 121-130.
-319-
The Rheic Ocean: Its Origin, Evolution and Correlatives Chemostratigraphic features of Ediacaran carbonates in CIZ
(Iberian Massif, Salamanca, Spain)
M. D. Rodríguez Alonso1, M. Peinado1, M .P. Franco1 M. Navidad2
1-Dept. Geology. University of Salamanca. Spain
2-Dept. Petrology and Geochemistry. University Complutense of Madrid. Spain
This study reports a practical application of chemostratigraphic studies aimed at characterizing
the carbonate rocks from Ediacaran sequences in the Central Iberian Zone (CIZ) with a view to
providing some elements for age determination and regional-to-global correlations. We offer C, Sr,
and O isotopic data as well as data on selected major and trace elements, used to evaluate the effects
of post-depositional alteration of 87Sr/86Sr, δ18O, and δ13C. Comparison with the Sr and C
isotopic record was made combining their isotopic data with other biostratigraphic age-controls.
The studied area is placed in the Ciudad Rodrigo-Hurdes-Sierra de Gata Domain (CRHSG),
characterized by an abundance of synorogenic granitoid bodies and very thick sequences of
anteordovician metasediments upon which Ordovician-Silurian-Devonian? metasediments lie
unconformable. Upper Precambrian-Lowermost Cambrian sedimentation is featured by a very thick
sandstone-mudstone succession with some conglomeratic and carbonate intercalations, known as
Schist Greywacke Complex (SGC). Two different units have been described in the SGC (Rodríguez
Alonso, 1985), which can also be distinguished all over the CIZ. The Lower Unit is the most
monotonous lithologically and occupies the greatest part of the area, while the Upper Unit outcrops
toward the borders and constitutes a lithological succession with more variety. These sediments
were deposited in an active tectono-magmatic context associated with the last stages of the
Cadomian Orogeny. The geotectonic context of the sedimentation explains the facies characteristics
and their vertical evolution as a continuous succession, strongly controlled by tectonic activity, in
which some catastrophic episodes must have produced the development of folds and local
intraformational unconformities. Thus, the whole stratigraphic succession is interpreted as a
siliciclastic sedimentation with the development of turbiditic facies in the deepest parts of the basin
that evolve into a mixed siliciclastic-carbonate slope platform sedimentation toward the borders
(Rodríguez Alonso, 1985; Vidal et al., 1994b; Rodríguez Alonso & Palacios, 1994). Both units have
been interpreted as two depositional sequences separated by a disconformity of the type 1 boundary
sequence (Valladares et al., 2000, 2002). The presence of magmatism contemporaneous with
sedimentation in some areas includes massive volcanic rocks (metabasalts, metaandesites,
-320-
Scientific Sessions metarhyolites) and volcaniclastic rocks. Geochemical data reveal tholeiitic and calc-alkaline
affinities consistent with an active margin setting (Rodríguez Alonso, 1985; Rodríguez Alonso et
al., 2004 a and b). Currently, the only available geochronologic data come from U-Pb detrital
zircons in greywackes of the Lower Unit (620 Ma approx., Gutiérrez Alonso et al., 2003).
The stratigraphic succession of the Upper Unit in this area display a predominance of lowgrade metamorphism grey pelitic rocks, with several levels of black shales, some pelite-sandstone
alternances and conglomeratic beds. In certain places, they include discontinuous carbonatesiliciclastic levels, limestone breccias and olistostrome deposits, as well as thin phosphate
intercalations or nodules and some volcanic and volcaniclastic rocks (Rodríguez Alonso et al., 2004
a and b), all of them showing great lateral and vertical variations of facies (Valladares and
Rodríguez Alonso, 1988; Rodríguez Alonso and Alonso Gavilán, 1995). They represent a
progradant succession that has evolved from platform-turbiditic deposits to a mixed siliciclasticcarbonate slope-outer platform domain, recording episodes of strong tectonic activity associated
with volcanic activity, in which Valladares et al., (2000, 2002) recognized a major fall in the sea
level.
The carbonate-siliciclastic samples correspond to the Aldea del Obispo, Pastores and
Fuenteguinaldo outcrops. They form lenticular discontinuous bodies of no more than 200m in
thickness, deposited over black and/or grey shales. They consist of thin-bedded carbonate facies and
calcareous breccias, with thin pelitic laminations and some quartzose, lithic and quartz-amphibole
sandstones as well as volcaniclastic intercalations. Under the microscope they are crystalline
arenaceous limestones resembling previous wackestones or packstones that have been strongly
recrystallized. Quartz, plagioclase, zircon, tourmaline, biotite and scarce volcanic lithic fragments
are found among the non-carbonate components. There are also crystalline arenaceous dolomites
(Rodríguez Alonso, 1985; Valladares and Rodríguez Alonso, 1988; Martín Herrero et al., 1990;
Rodríguez Alonso et al., 1990; Rodríguez Alonso and Alonso Gavilán, 1995; Valladares et al., 2000,
2002 a and b, 2006; Rodríguez Alonso et al., 2004 a). In general, the Variscan metamorphism has
only produced recrystallization but neoformed metamorphic phases such as amphibole, diopside
and titanite may occasionally appear in one sample from Pastores outcrop. Besides, there are two
other samples placed within different contact aureoles: the sample from Aldea del Obispo displays
wollastonite, diopside and titanite, whereas the one from Fuenteguinaldo contains actinolite,
clinozoisite and titanite .
All samples considered are situated below the first Lowermost-Cambrian (Lower Corduban,
Liñán et al., 2002) trace fossils found in the region (Psammichnites ichnosp.). Cloudina (630-321-
The Rheic Ocean: Its Origin, Evolution and Correlatives 542Ma), the only Upper-Ediacaran fossil precisely recognized until now in the succession, is placed
in a limestone boulder inside the conglomeratic-calcareous megabreccia of La Encina outcrop,
representing the basal part of a siliciclastic-carbonate succession filling a large scale submarine
erosional surface (channel or gullie) placed at the base of the slope, over the Lower Unit sandstones
and mudstones. Samples from Pastores correspond to thin-bedded facies deposited together with
calcareous breccias filling a submarine channel close to the slope. Looking at the general succession
of the siliciclastic-carbonate sequences in the region, it seems that the catastrophic event of La
Encina deposits is situated above the Pastores levels (Rodríguez Alonso and Alonso Gavilán, 1995).
Taking into account that Valladares et al. (2006) collected their samples in the same
stratigraphic profile of Pastores, we added in our diagrams their 87Sr/86Sr, δ13C and δ18O values
from 12 samples, disregarding those the authors consider altered.
The population of samples shows a range of variation of Mn/Sr from 0.04 to 0.31. However,
most of them display relatively homogeneous 87Sr/86Sr values (from 0.70838 to 0.70853) that
could correspond to the primary seawater composition.
Plotting Mn/Sr versus Sri shows that even though Mn/Sr contents are very similar in the whole
population, when considering the Sri values, two populations can be observed, one with moderate
Sri values (accepted as primary) and the other with very high Sri (considered as altered). When the
Sri versus C and O isotope values are compared, the same features can be seen. That points to a
primary isotopic signature for the population with lower Sri values, whereas the others are
considered altered. Moreover, their high Sr contents (ca. or >1000 ppm) indicate that either the
rocks retain the isotopic values of the coeval seawater or that post-sedimentary modifications were
produced so early that the water composition was very little modified. In the same way, the high Sr
and the low δ18O values suggest that aragonite was the primitive carbonate (Peinado et al., 1999;
Valladares et al., 2006).
Examining the isotopic values of the whole population, no important variations in the
87Sr/86Sr ratio are observed, and they mainly depend on the diagenetic, hydrothermal,
metamorphic or meteoric alteration they might have undergone. On the other hand, no important
variations are seen in the δ13C values, ranging from –5.6 to -0.5 ‰. In contrast, δ18O values vary
from –8 to –15 ‰.
Plotting δ18O versus δ13C values, a Rayleigh-type evolution (Baumgartner and Valley, 2001)
due to normal calc-silicate decarbonation can be seen in the samples from Fuenteguinaldo, situated
in the distal part of a granite contact aureole, where accessory metamorphic minerals such as
-322-
Scientific Sessions amphibole and clinozoisite have been developed. In contrast, the sample from Aldea del Obispo (a
carbonate layer intercalated within other silica-rich beds), also situated within the distal contact
aureole of a Variscan granite, does not show the same behaviour. Although wollastonite has
developed in the rock as a consequence of thermal metamorphism, apparently it did not cause any
fluid contamination/variation because its Sr isotopic signature reflects primary values. One possible
explanation might be in the fact that the fluids source required for wollastonite to form were not
external, but coming from the siliciclastic levels intercalated within the limestones.
After plotting the 87Sr/86Sr and δ13C values on the composite curves (Halverson, 2006,
Halverson et al., 2007; Fike et al., 2006; Jacobson and Kaufman, 1999, Le Guerroué et al., 2006)
for the terminal Ediacaran-Cambriam times, and combining them with additional biostratigraphic
age-control criteria (skeletal or trace fossil biota), in absence of more precise geochronological data,
we concluded that the sedimentation of the siliciclastic-carbonate rocks took place at or very near
the Ediacaran-Cambrian boundary. Considering the 87Sr/86Sr ratios they are within the range of the
rocks of both time periods (Halverson et al., 2007). According to the negative δ13C values of the
samples considered, there are two time periods in which the samples from Pastores and Aldea del
Obispo fit both curves reasonably well: the older one would be around 546-547Ma, and the second
alternative could correspond to the Ediacaran-Cambrian limit, around 542 Ma, a time in which there
is a global negative δ13C record. As Cloudina (630-542Ma) is present in a limestone boulder placed
in an olistostrome deposit (La Encina), presumably younger than the thin-bedded carbonates of
Pastores, the first option may be more plausible. Nevertheless, only geochronological data can
determine more precisely the age of these siliciclastic-carbonate deposits. On the other hand, the
presence of black shale levels with phosphate nodules or laminae intercalated, together with
volcanic and volcaniclastic rock alternances in the succession, can explain the negative δ13C values
as the result of oceanic oxygen deficiency episodes that caused stagnation of the deep ocean waters,
as well as the influx of the volcanic derived CO2 (Kimura et al., 1997, Kimura and Watanabe,
2001).
ACKNOWLEDGEMENTS
Financial support for this research was provided by project SA-53/97 from the Junta of Castilla and León (Spain) and
project PRI/03.11649 from the Universidad Complutense de Madrid (Spain).
References
Baumgartner, L.P. and Valley, J.W., 2001, Stable Isotope Transport and Contact Metamorphism Fluid Flow, in Valley
J.W. and Cole, D.R. eds, Stable Isotope Geochemistry: Reviews in Mineralogy and Geochemistry, v. 43, p. 415-461.
Fike, D.A., Grotzinger, J.P., Pratt, L.M. and Summons, R.E. 2006, Oxidation of the Ediacaran Ocean, Nature, v. 444, p.
744-747
-323-
The Rheic Ocean: Its Origin, Evolution and Correlatives Gutiérrez-Alonso, G., Fernández-Suárez, J., Jeffries, T.E., Jenner, G.A., Tubrett, M.N., Cox, R. and Jackson, S. E., 2003,
Terrane accretion and dispersal in the northern Gondwana margin. An Early Palaeozoic analogue of a long-lived
active margin: Tectonophysics, v. 365, p. 221-232.
Halverson, G.P., Dudás, F.Ö., Maloof, A.C. and Bowring,S.A., 2007, Evolution of the 87Sr/86Sr composition of
Neoproterozoic seawater: Palaeogeography, Palaeoclimatology and Palaeoecology v. 256, 103-129.
Halverson, G.P., 2006, A Neoproterozoic Chronology, in: S. Xiao and Kaufman (eds.), Neoproterozoic Geobiology and
Palaeobiology, 231-271.
Jacobsen, S.B. and Kaufman, A.J., 1999, The Sr, C and O isotopic evolution of Neoproterozoic seawater: Chemical
Geology, v. 161, p. 37-57.
Kimura, H., Matsumoto, R., Kakuwa, Y., Bahaeddin, H. and Zibaseresht, H., 1997, The Vendian-Cambrian δ13C record,
North Iran: evidence for overturning of the ocean before the Cambrian explosion: Earth and Planetary Science
Letters, v. 147, p. E1-E7.
Kimura, H. and Watanabe, Y., 2001, Oceanic anoxia at the Precambrian-Cambrian boundary: Geology, v.29, p. 995-998.
Le Guerroué, E., Allen, P.A., Cozzi, A., Etienne, J.L. and Fannig, M., 2006, 50Myr recovery from the largest negative.
δ13C excursión in the Ediacaran ocean: Terra Nova, v. 18, 2, p. 147-153.
Liñán, E., (coord.), Gozalo, R., Palacios, T., Gómez-Vintaned, J.A., Ugidos, J.M. and Mayoral, E., 2002, Cambrian, in:
Gibbons, W. and Moreno, T., eds., The Geology of Spain: London, The Geological Society, p. 17-29.
Martín Herrero, D., Albert Colomer, V., López Plaza, M. and Rodríguez Alonso, M.D., 1990, in: ITGE (Instituto
Geológico y Minero de España) ed., Madrid, Spain, Mapa geológico de España 1: 50.000. Ciudad Rodrigo, no. 525,
96 p.
Peinado, M., López-Plaza, M., Rodríguez Alonso, M.D., Carnicero Gómez-Rodulfo, A., Franco, P., Gonzalo Corral,
J.C., Navidad, M. and Alonso Gavilán, G., 1999, Caracterización geoquímica preliminar de los materiales
carbonatados y sus asociaciones litológicas en los dominios centrales de la Zona Centroibérica: XV Reunión de
Geología del Oeste Peninsular (International Meeting on Cadomian Orogens), Badajoz, Spain, Extended Abstracts,
p.190-200.
Rodríguez Alonso, M.D., 1985, El Complejo Esquisto-Grauváckico y el Paleozoico en el Centro-Oeste español: Acta
Salmanticensia, Ciencias , Spain, Ediciones Universidad de Salamanca, 174 p.
Rodríguez Alonso, M.D., García Figuerola L.C., Bascones Alvira, L., Robles Casas, R., Álvarez Nava, H. and Picart
Boira, 1990, in: ITGE (Instituto Geológico y Minero de España), Madrid, Spain, Mapa geológico de España
1:50000, Fuenteguinaldo, no. 550, 71 p.
Rodríguez Alonso, M.D. and Alonso Gavilán, G., eds., 1995, Neoproterozoic-Lower Cambrian in the central-western
part of the Iberian Peninsula: Post-Conference Field Guide, XIII Geological Meeting of West Iberian Peninsula.
Salamanca, Spain, 120 p.
Rodríguez Alonso, M.D., Peinado, M., López-Plaza, M., Franco, P., Carnicero A. and Gonzalo, J.C., 2004 a,
Neoproterozoic-Cambrian synsedimentary magmatism in the Central Iberian Zone (Spain): geology, petrology and
geodynamic significance: International Journal of Earth Sciences (Geologische Rundschau), v. 93, p. 897-920.
Rodríguez Alonso, M.D , Díez Balda, M.A.,Perejón, A., Pieren, A., Liñán, E., López Díaz, F., Moreno, F.,Gámez
Vitaned, J.A., González Lodeiro, F., Martínez Poyatos, D. and Vegas, R., 2004 b, La secuencia litoestratigráfica del
Neoproterozoico-Cámbrico Inferior. Dominio del Complejo Esquisto-grauváquico, in: Vera J.A. ed., Geología de
España, SGE-IGME (Sociedad Geológica de España–Instituto Geológico y Minero de España), Madrid, Spain, p.
78-81.
Valladares, M.I. and Rodríguez Alonso, M.D., 1988, Depositional processes of channel fill carbonate: Revista de la
Sociedad Geológica de España, v. 1 no. (1-2), p. 165-175.
Valladares, M.I., Barba, P., Ugidos, J.M., Colmenero, J.R. and Armenteros, I., 2000, Upper Neoproterozoic-Lower
Cambrian sedimentary successions in the Central Iberian Zone (Spain): sequence, stratigraphy, petrology and
chemostratigraphy. Implications for other European zones: International Journal of Earth Sciences (Geologische
Rundschau), v. 89, p. 2-20
Valladares, M.I., Barba, P. and Ugidos, J.M., 2002 a, Precambrian in: Gibbons W. and Moreno, T., eds., The Geology of
Spain: Geological Society, London, p. 7-16
Valladares, M.I.; Ugidos, J.M.; Barba, P.; Colmenero, J.R,. 2002 b, Contrasting geochemical features of the Central
Iberian Zone shales, Iberian Massif, Spain; implications for the evolution of Neoproterozoic-Lower Cambrian
sediments and their sources in other peri-Gondwanan areas, Tectonophysics,v. 352, 1-2, 121-132
Valladares., M.I, Ugidos, J. M., Barba, P., Fallick, A. E., Ellam, R. M., 2006. Oxygen, carbon and strontium isotope
records of Ediacaran carbonates in Central Iberia (Spain). Precambrian Research, v.147. 354-365
Vidal, G., Jensen, S. and Palacios, T., 1994 a, Neoproterozoic (Vendian) ichnofossils from Lower Alcudian strata in
central Spain: Geological Magazine, v. 131, p. 169-179.
Vidal, G., Palacios, T., Gómez-Vintaned, J.A., Díez Balda, M.A. and Grant, S.W.F., 1994 b, Neoproterozoic-early
-324-
Scientific Sessions Cambrian geology and palaeontology of Iberia: Geological Magazine, v. 131, p. 729-765.
-325-
The Rheic Ocean: Its Origin, Evolution and Correlatives Linking different levels of Lower Carboniferous magmatic activity in the SW sector of the
Ossa-Morena Zone: preliminary study at the Almansor outcrop (Évora Massif)
Santos, J.F. 1, Moita, P. 2, Pereira, M.F.2
1
Geobiotec, Departamento de Geociências, Universidade de Aveiro, 3810-193 Aveiro, Portugal
2
Centro de Geofísica de Évora, Departamento de Geociências, Universidade de Évora,
Apartado 94, 7002-554 Évora, Portugal
One of the characteristic features of the Ossa-Morena Zone (OMZ), when compared with
other major geotectonic units of the Iberian Variscan Chain, is the significant compositional
diversity of the intrusive igneous rocks. The Évora Massif (Carvalhosa, 1983; Pereira et al., 2003,
2007), in the SW sector of the OMZ, testifies for that diversity, since it includes a wide range of
plutonic rocks, from gabbros to leucogranites, with a very important representation of tonalites and
granodiorites. The available geochronological data (e.g.: Moita et al., 2005a; Chichorro, 2006;
Moita, 2008; Pereira et al., in press) reveal that the syn-tectonic plutonism is mainly represented by
Lower Carboniferous ages (mostly, in the range 340-320 Ma). The calc-alkaline signature of the
intrusive rocks that outcrop in Évora Massif (Moita et al., 2005b; Antunes, 2006; Pereira et al.,
2007; Moita et al., 2009) fits into a geodynamic setting, in that period, where subduction under a
continental margin played a major role.
Coeval calc-alkaline volcanic and sub-volcanic rocks (basalts, andesites, dacites and
rhyolites) were also described in Visean basins (Pereira et al., 2006) located in the Évora Massif
(Cabrela basin; Chichorro, 2006) and along the contact between the Ossa-Morena and the SouthPortuguese zones (Toca de Moura basin; Santos et al., 1987). These basins were probably formed
by intra-orogenic extension.
Recently, Moita et al. (2009) studied, in the Évora Massif (EM), a complex outcrop in the
Almansor area (near Montemor-o-Novo), interpreted as the locus of injection of magmas formed by
different processes – fractional crystallization of mantle derived melts, crustal anatexis, and
mingling/mixing between different magmas– upwards an active shear zone. The dominant granitoid
lithologies (diatexites of monzogranitic composition and calc-alkaline tonalites to granodiorites) at
Almansor enclose both metamorphic and igneous enclaves. In the last category, the enclaves may be
divided, according to their petrographic features, into tonalites and andesites-dacites. Tonalitic
enclaves have lengths from 0.4 to 1.5 m and are most commonly isolated equidimensional bodies.
Andesitic-dacitic enclaves have similar dimensions but sometimes the association of several of
-326-
Scientific Sessions them in alignments suggests that they may represent fragments of larger tabular bodies (dykes).
Geochemically, both tonalitic (TE) and andesitic-dacitic (ADE) enclaves reveal strong
resemblances with the lithologies of the calc-alkaline plutonic suite of the EM, including those
occurring at Almansor. However, for some elements, the andesitic-dacitic enclaves show some
differences to the main calc-alkaline trend: compared to lithologies with similar silica contents,
these enclaves show higher K2O (3.09 - 4.12 %) and Rb (159 - 211 ppm) concentrations, whilst
CaO (4.36 - 4.82 %) and Na2O (1.34 - 2.49 %) are lower. The A/CNK ratios of the andesitic-dacitic
enclaves range between 1.1 and 1.2, clearly higher than in typical calc-alkaline lithologies. These
discrepancies are related to the modal abundance of biotite, which constitutes the most important
mafic mineral in the ADE. Since these enclaves are hosted by diatexites rich in restitic biotite
(Moita et al., 2009), it would be plausible to consider that the interaction of (partially molten?)
diatexite and andesite-dacite could cause the incorporation, in the ADE, of biotite proceeding from
the enclosing material. However, the Sr and Nd isotopic initial ratios (0.706907 and 0.512050,
respectively) as well as the Th (3.7 – 3.9 ppm) and U (1.3 – 2.1 ppm) contents in the ADE are
almost indistinguishable from those found in comparable rocks of the EM calc-alkaline suite,
showing that the ADE have no significantly higher crustal component. Additionally, the Mg# values
in ADE biotites (0.44 - 0.45) lie in the range of those obtained in biotites of the calc-alkaline rocks
but are lower than values found in diatexite biotites. Therefore, the major contribution for the
geochemical peculiarities of the studied ADE probably results from concentration of igneous biotite
crystallized from a calc-alkaline melt, rather than in situ incorporation of diatexitic material.
Considering that the andesitic-dacitic enclaves seem to represent stretched, boudinaged and
fractured dykes and that they show an anisotropic fabric defined by the preferred orientation of
biotite crystals, flow segregation may be viewed as a likely mechanism to explain the high biotite
abundance. Those dykes are probably almost contemporaneous of the host lithologies: they may
correspond to upward channels of the more mafic magma, through a crystallizing mush, that have
been disrupted before the complete solidification of the melts.
If the ADE represent biotite-enriched portions, via flow segregation in dykes, of andesiticdacitic melts, where is the whole composition of those melts represented? The small number of
samples of these type of enclaves studied until now may explain the absence of ADE analyses
showing “normal” calc-alkaline compositions. Another possible explanation is that TE represent
different (inner) parts of the same bodies, less affected by special concentration of one particular
mineral phase.
In conclusion, some of the igneous enclaves occurring at the Almansor outcrop may
-327-
The Rheic Ocean: Its Origin, Evolution and Correlatives represent syn-plutonic dykes that operated as channel magma flow between different crustal levels
feeding magma chambers, and probably also linking the widespread Lower Carboniferous calcalkaline intrusions found in the Évora Massif to the volcanic structures active in the same period. In
the western border of the EM, calc-alkaline volcanic rocks have been studied in basins such as Toca
da Moura (Santos et al., 1987) and Cabrela (Chichorro, 2006). The lack of many other testimonies
of volcanic activity probably result from the long period of erosion of the Variscan orogen, which,
as expected, must have affected particularly rocks formed in sub-aerial environments or small
intracontinental basins. Therefore, Lower Carboniferous plutons in SW part of the OMZ probably
represent magma chambers of a magmatic arc whose volcanic counterparts have been mostly
eroded, with the exception of a few remnants preserved in some tectonic depressions.
References
Antunes, I. (2006) - Rochas Granitóides da Zona de Ossa-Morena: Magmatismo, Geodinâmica e Reconstituição GeoHistórica. Tese de Mestrado, Univ. Aveiro, 181 pp.
Carvalhosa, A. (1983) - Esquema geológico do Maciço de Évora. Comun. Serv. Geol. Portugal 69: 201-208
Chichorro, M. (2006) - A Evolução Tectónica da Zona de Cisalhamento de Montemor-o-Novo (Sudoeste da Zona de
Ossa Morena – Área de Santiago do Escoural – Cabrela). Tese de Doutoramento, Univ. Évora, 521 pp.
Moita, P., Santos, J.F., Pereira, M.F. (2005a) - Dados geocronológicos de rochas intrusivas e sin-tectónicas do Maciço
dos Hospitais (Montemor-o-Novo, Zona de Ossa-Morena). Actas do XIV Semana de Geoquímica/VIII Congresso de
Geoquímica dos Países de Língua Portuguesa, Univ. Aveiro, vol. 2: 471-474.
Moita, P., Santos, J.F. & Pereira, M.F. (2005b) - Tonalites from the Hospitais Massif (Ossa-Morena Zone, SW Iberian
Massif, Portugal). II: Geochemistry and petrogenesis. Geogaceta 37: 55-58.
Moita, P., Santos, J.F. & Pereira, M.F., (2009) Layered granitoids: Interaction between continental crust recycling
processes and mantle-derived magmatism. Examples from the Évora Massif (Ossa–Morena Zone, southwest Iberia,
Portugal). Lithos, doi:10.1016/j.lithos.2009.02.009.
Pereira, M.F., Chichorro, M., Williams, I.S., Silva, J.B., Fernandez, C., Diaz-Azpiroz, M., Apraiz, A., Castro, A., (in
press) - Variscan intra-orogenic extensional tectonics in the Ossa-Morena Zone (Évora-Aracena-Lora del Río
metamorphic belt, SW Iberian Massif): SHRIMP zircon U-Th-Pb geochronology. Geol. Soc. London Special
Publication.
Pereira, M.F., Silva, J.B., Chichorro, M., (2003) - Internal Structure of the Évora High-grade Terrains and the
Montemor-o-Novo Shear Zone (Ossa-Morena Zone, Portugal), Geogaceta, 33, p. 79-82
Pereira, M., Silva, J., Chichorro, M., Moita, P., Santos, J., Apraíz, A., Ribeiro, C., 2007. Crustal growth and
deformational processes in the northern Gondwana margin: constraints from the Évora Massif (Ossa-Morena Zone,
SW Iberia, Portugal). Geol. Soc. America Special Paper 423: 333-358.
Pereira, Z., Oliveira, V., Oliveira, J.T. (2006) - Palynostratigraphy of the Toca da Moura and Cabrela Complexes, Ossa
Morena Zone, Portugal. Geodynamic implications. Rev. Palaeobotany and Palynology 139: 227–240.
Santos, J., Mata, J., Gonçalves, F., Munhá, J., 1987. Contribuição para o conhecimento geológico-petrológico da região
de Santa Susana: o complexo vulcano-sedimentar da Toca da Moura. Comun. Serv. Geol. Portugal 73, 29-48.
-328-
First discovery of a Neoproterozoic pebbly mudstone from the Lausitz Group (Bohemian
Massif, eastern Saxothuringian Zone, Lausitz Block, Germany): Provenance from detrital
modes and U-Pb dating of detrital zircon grains
Jens Ulrich1,*, Olaf Tietz2, Kerstin Drost3, and Ulf Linnemann1
1 Senckenberg Naturhistorische Sammlungen Dresden, Museum für Mineralogie und Geologie, Königsbrücker
Landstraße 159, D-01109 Dresden, Germany
* Corresponding author: [email protected]
2 Senckenberg Museum für Naturkunde Görlitz, Germany, Am Museum 1, D-02826 Görlitz, Germany
3 University of Cape Town, Department of Geological Sciences, AEON EarthLAB, Rondebosch 7701, Cape Town,
South Africa
Provenance-indicative diamictites and conglomeratic intercalations rarely occur within the
different monotonous Neoproterozoic greywacke successions of the eastern Saxothuringian Zone at
the northern margin of the Bohemian Massif. However, the intense Variscan tectonometamorphic
overprinting which has affected the Neoproterozoic rock complexes of the Erzgebirge and the Elbe
Zone hinders a comprehensive provenance study of these marker beds. Otherwise, such coarsegrained sedimentary rock types are unknown in some low-grade Neoproterozoic sequences of the
Saxothuringian Zone (Schwarzburg antiform and North Saxon antiform). Therefore the best outcrop
conditions for provenance studies are given in the Lausitz Block because of its low-grade
tectonometamorphic overprint.
Obviously, diamictites compose characteristic layers within the initial back-arc basin of the
Cadomian evolution in Saxo-Thuringia, whereas (micro)conglomeratic intercalations are more
typical for the subsequent retro-arc foreland basin formed by back-arc inversion during the
Cadomian orogeny at the northern margin of West Gondwana (Linnemann et al., 2007). But within
the Neoproterozoic greywacke succession of the Lausitz Block (Lausitz Group) both lithologies
stratigraphically occur close to each other.
A recently mapped road cut near the town of Görlitz (eastern Lausitz Block) revealed a cross
section of an unusually discontinuous alternating stratification between greywackes and mudstones
with intercalated layers of granule- to pebble-sized conglomerates in the footwall and a massive
pebbly mudstone bed of 7 m thickness in the hanging wall. The conglomerate layers are rich in
subangular chert fragments, and single layers attain a maximum thickness of 1 m. In contrast, the
framework of the pebbly mudstone mostly consists of rounded oblate pebbles with a maximum size
of 10 cm, and contains only a few pebbles of chert. Therefore, most of the chert fragments within
the conglomerate layers are interpreted to be of an intrabasinal origin. Because of the fact that both
a striated bedrock and scratched pebbles are unknown, the interpretation of the depositional
-329-
environment for the pebbly mudstone as glaciomarine diamicite or submarine debris flow remains
ambiguously. Detrital zircon grains separated from samples of a conglomerate layer and the pebbly
mudstone matrix were dated by U-Pb Laser ablation ICP-MS and yielded a Late Neoproterozoic
maximum age of sedimentation for both rock types.
Petrological and geochronological data from these siliciclastic rocks were interpreted to
indicate inversion of a Cadomian back-arc basin that was caused by magmatic arc-continent
collision at the West African margin of Gondwana (Linnemann et al., 2007). Subsequently, the
progressive development of a foreland fold-thrust belt gave rise to recycling of chert-hosting backarc sequences, basin cannibalism, and the rapid redeposition of this reworked material as chert
fragment-bearing greywackes and conglomerates into a retro-arc foreland basin.
References
Linnemann, U., Gerdes, A., Drost, K., and Buschmann, B. (2007). The continuum between Cadomian orogenesis and
opening of the Rheic Ocean: Constraints from LA-ICP-MS U-Pb zircon dating and analysis of plate-tectonic setting
(Saxo-Thuringian zone, northeastern Bohemian Massif, Germany). In: Linnemann, U., Nance, R. D., Kraft, P., and
Zulauf, G. (Eds.), The evolution of the Rheic Ocean: From Avalonian-Cadomian active margin to AlleghenianVariscan collision. Geol. Soc. Am., Spec. Pap., vol. 423, p. 61-96.
-330-