Proveniências das rochas siliciclásticas do Ediacariano e Câmbrico
Inferior da Zona Ossa-Morena: dados geocronológicos de U-Pb com
LA-ICP-MS em zircão
Provenances of Ediacaran and Early Cambrian siliciclastic
sediments from the Ossa-Morena Zone: Laser ablation U-Pb zircon
data
Pereira, M.F. (1), Linnemann, U. (2), Dorst, K. (2), Chichorro, M. (1), Jeffries, T.E.(3),
(1) Departamento de Geociências, Centro de Geofísica de Évora, Universidade de Évora,
Apt.94, 7001-554 Évora, Portugal
(2) Staatliche Naturhistorische Sammlungen Dresden, Museum Mineralogie und
Geologie, Konigsbrucker Landstraβe 159, D-01109 Dresden, Germany
(3) Department of Mineralogy, The Natural History Museum, London SW7 5BD, UK
E-mail (s): [email protected]
SUMÁRIO
As amostras estudadas de sedimentos siliciclásticos do Ediacariano e Câmbrico Inferior do Nordeste Alentejano
(Zona de Ossa-Morena) mostram padrões similares de distribuição de idades U-Pb de LA-ICP-MS em zircão.
Apenas estão presentes zircões Precâmbricos com idades do Neoproterozóico Superior (eventos CadomianosAvalonianos ca. 540-850 Ma), Paleoproterozóico (eventos Eburniano e Icartiano) e Arcaico. A falta de idades de
zircão do Mesoproterozóico sugerem que estas rochas derivaram do Cratão Oeste Africano (Gondwana).
Palavras-chave: limite Precâmbrico/Câmbrico, Sudoeste do Maciço Ibérico, Cratão Oeste Africano, Zona de
Ossa-Morena
SUMMARY
The analysed samples of Ediacaran and Early Cambrian siliciclastic sediments from the Northeast Alentejo
(Ossa-Morena Zone) show a similar distribution of detrital LA-ICP-MS U-Pb zircon ages. Only Precambrian
zircons are present displaying Late Neoproterozoic (Cadomian-Avalonian events), Paleoproterozoic (Eburnian
or Icartian events) and Archean ages. The lack of Mesoproterozoic zircon ages suggests a derivation from the
West African Craton (Gondwana).
Key-words: Precambrian/Cambrian boundary, SW Iberian Massif, West African Craton, Ossa-Morena Zone
the Ossa-Morena Zone (SW Iberian Massif,
Portugal).
Similar age rocks were studied in Spain (Serie
Negra), using the same analytical procedure, and the
results show a characteristic zircon age signature
(absence of Mesoproterozoic zircons) interpreted to
be related with a West African Craton provenance
[1]. This peculiar feature was used to distinguish the
Ediacaran Ossa-Morena sediments (Serie Negra:
Introduction
In order to precise age constraints on the
Precambrian-Cambrian
transition
evolutionary
history of the SW Iberian Massif and correlate this
area with other peri-Gondwana realm segments from
Europe, we used LA-ICP-MS to study zircons.
The obtained detrital zircons were extracted from
Ediacaran and Early Cambrian detrital rocks from
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dominated magmatism (Nave de Grou-Azeiteiros
volcanic-sedimentary complex [8-9]) composed by
acid-to-intermediate
tuffs,
reworked
tuffs,
conglomerates (with pebbles of mainly felsicintermediae volcanics and granitoids and also, black
cherts and slates from the Serie Negra), rhyolites,
rhyo-dacites, dacites and andesites. Towards the top
this magmatism is followed by detrital and carbonate
sequences (Ouguela detritic-carbonate complex [89]) with conglomerates, arkosic sandstones, shales
and limestones with associated andesitic basalts and
basalts.
The selected samples used in this study
(Neoproterozoic Serie Negra greywacke- SNM;
Early Cambrian arkosic sandstone- OLG) were
taken from a cross-section located a few kilometers
towards the Northwest of Ouguela (Northeast
Alentejo), nearby the Spanish border.
[2]), and also, the Cambrian and Lower Ordovician
rocks from sediments of the same age with CentralIberian affinity which yielded abundant 0.9-1.2 Ga
zircon ages (NW Iberian Massif; [3]). For the same
authors the presence of 0.9-1.2 Ga ages in the
Neoproterozoic, Cambrian and Ordovician detrital
rocks from the NW Iberian Massif indicate that
these sediments were probably deposited close to
exposures
of
basement
rocks
containing
Mesoproterozoic zircons (Amazonian Craton).
These differences on provenances (Amazonian
versus West African) are also known in other periGondwana realm segments of Europe [1, 4].
In this work, we decided to test this hypothesis by
studying the Ediacaran-Cambrian boundary in the
northern domains of the Ossa-Morena (Northeast
Alentejo, Portugal).
Geological setting
In the Ossa-Morena Zone, the Cadomian basement
(Serie Negra; [2]), composed by variable deformed
and metamorphosed Neoproterozoic (Ediacaran)
rocks, is unconformably overlain by Early Cambrian
volcanic-sedimentary complexes and sedimentary
sequences (Fig.1).
U-Th-Pb Laser ablation-ICP-MS results
In the present study was performed a set of 123 LAICP-MS analysis and obtained varying degrees of
concordance and precision from the U-Pb
measurements. We only considered in this study the
analyses with [206Pb/238U] /[207Pb/206Pb] ratio values
lower than 1.0 and higher than 0.5 (37 analysis were
rejected).
All the results of single grain analysis are shown in
the Concordia diagrams of Fig. 2, where ellipses
represent uncertainties.
The measured 207Pb/206Pb ages of the Serie Negra
greywacke
indicate
a
predominance
of
Neoproterozoic zircons (36 grains) in the range ca.
540-850 Ma (Ediacaran and Cryogenian follow the
ISC [10]). Only one grain gave a Mesoproterozoic
age (1242 Ma). Remaining analysis displayed
Paleoproterozoic and Archean ages (1924, 2082,
2550, 2662 and 3042 Ma).
The Early Cambrian arkosic sandstones (OGL)
followed a similar age distribution. The
Neoproterozoic grains are dominant (20 zircons) and
the older grains are Paleoproterozoic (ranging from
1706 to 2182 Ma) and Archean (2596 and 2810 Ma).
This sample lacks Cambrian and Mesoproterozoic
zircons.
If we use instead of 207Pb/206Pb the 206Pb/238U ages to
analyse the Neoproterozoic distributions, as
suggested by [3], the differences are not significant.
Figure 1 – Schematic stratigraphic column of the
Precambrian/Cambrian boundary at the northern
domains of the Ossa-Morena Zone (Ouguela crosssection; Northeast Alentejo, Portugal).
Discussion
The analysed samples of Ediacaran and Early
Cambrian detrital rocks from the Northeast Alentejo
show a lack of Mesoproterozoic zircon ages. They
have zircon age content (with West African Craton
affinity) different from the same age rocks from the
NW Iberian Massif, in Spain (with Amazonian
signature; see for comparison [1,3]).
In the Serie Negra sample, Late Neoproterozoic
zircons are abundant (in particular from Ediacaran
and Cryogen) indicating a Cadomian-Avalonian
provenance. The presence of such detrital zircons
At the Northeast Alentejo (Portugal), the Serie
Negra rocks (Mosteiros Formation) mainly consists
of quartzwackes, quartz phyllites, greywackes and
graphitic pelites with intercalations of black cherts,
basalts, and minor limestones [5-9]. These rocks
were deformed and metamorphosed before the
deposition of the Early Cambrian sequences. The
Early Cambrian includes at the bottom a felsic22
by the stratigraphy and whole-rock geochemistry [9]
and isotope chemistry data obtained for these two
segments of the Western Europe Variscan Belt.
indicates the derivation from Gondwana (West
African Craton). This sediment also includes as the
oldest age an Archean grain with 3042 Ma and
Paleoproterozoic grains probably related with
Eburnian or Icartian events. These data are in
agreement with other studies made by [1, 11], in
similar Ediacaran/Cambrian boundary rocks from
the Ossa-Morena Zone in Spain (Tentudia
Formation).
Aknowledgements
This work is a contribution for the IGCP Project
497.
References
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Figure 2 – Concordia plots of U-Pb analytical data.
A- Arkosic sandstones, OGL; B- greywackes, SNM.
In the studied arkosic sandstone, which is correlative
of the Early Cambrian sequences described in Spain
(Torrearboles Formation; [9]), were not reported
Cambrian and Mesoproterozoic grains. Only
Precambrian zircons are present displaying ages that
range from ca. 560 Ma to 2810 Ma, the same age
cluster as the Serie Negra greywackes. This means
that in the Early Cambrian time existed a strong
input of old cratonic zircons probably due to rifting
and fast exhumation of rift shoulders.
The similarity between the data obtained in the
Ossa-Morena Zone (Portugal) and in the SaxoThuringia Zone (Germany; [4]) is another strong
argument to believe on their close paleogeographical
relationship, as part of the Northern Gondwana
margin. The most significant Ediacaran ages (540630 Ma) match the span of the main episodes of
magmatic activity related with arc construction
along the northern Gondwana continental margin
[12]. This correlation is also consistently supported
23
[10] Gradstein, F.M., Ogg, J.G., Smith, A.G.,
Bleeker, W. & Lourens, L.J. (2004). A new
Geological Time Scale with special reference to
Precambrian and Neogene. Episodes 27 (82), 83-100
[11] Schafer, H.-J., Gebauer, D., Nagler, T.F. &
Eguiluz, L. (1993). Conventional and ionmicroprobe U-Pb dating of detrital zircons of the
Tentudia Group (Serie Negra, SW Spain):
implications for zircon systematics, stratigraphy,
tectonics and the Precambrian/Cambrian boundary.
Contrib. Mineral. Petrol. 113: 289-299.
[12] Nance, R.D. & Thompson, M.D. (eds.) (2006).
Avalonian and related peri-Godwanan terranes of
the circum-North atlantic, Geol. Soc. Am. Spec.
Publ. 304.
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