Contrib Mineral Petrol (2005) 150: 523–545
DOI 10.1007/s00410-005-0032-9
O R I GI N A L P A P E R
M. A. Forster Æ G. S. Lister
Several distinct tectono-metamorphic slices in the Cycladic
eclogite–blueschist belt, Greece
Received: 9 March 2005 / Accepted: 18 August 2005 / Published online: 13 October 2005
Ó Springer-Verlag 2005
Abstract Several relatively thin tectono-metamorphic
slices have been recognized in the Cycladic eclogite–
blueschist belt, through detailed studies on Ios, Sifnos,
Syros, and Tinos. A sequence of distinct metamorphic
mineral growth events has been documented. These recur in each tectonic slice, although individual slices are
dominated by different events. To constrain the timing
of these processes, the method of asymptotes and limits
has been used to reanalyze published 40Ar/39Ar apparent
age spectra. This reanalysis supports the concept that
there were separate and quite distinct high-pressure
metamorphic mineral growth events, and allows potential constraints as to the timing of some of these events
to be developed. M1B eclogite-facies metamorphism is
estimated to have occurred at some time in the period
53–49 Ma, the M1C blueschist-facies metamorphic event
at some time in the period 44–38 Ma, and the M1D
transitional blueschist-facies metamorphic event is estimated to have occurred at some time in the period 35–
30 Ma. A kinematic model is proposed to explain the
geometry of a thinly sliced tectono-metamorphic stratigraphy, as observed, and the reason as to why individual tectonic slices in this ‘tectono-metamorphic
stratigraphy’ should display distinctive patterns of fabrics and micro-structures, as well as characteristic temperature-time curves as inferred by 40Ar/39Ar
geochronology.
Introduction
The Cycladic archipelago (Fig. 1) is located within an
Alpine-type blueschist belt in which Eocene deformation
Communicated by A. Hofmann
M. A. Forster (&) Æ G. S. Lister
Research School of Earth Sciences,
The Australian National University, 0200 Canberra,
ACT, Australia
E-mail: [email protected]
and metamorphism was followed by intense Miocene
extension (Altherr et al. 1982; Avigad and Garfunkel
1991; Avigad et al. 1997; Gautier and Brun 1994). This
intense Miocene extension of the Aegean continental
crust was related to the southward retreat (or roll-back)
of the hinge of the subducting African slab, to the south,
which caused collapse and extension of previously
overthickened crust in the over-riding plate of this subduction zone (Gautier et al. 1993; Jolivet and Patriat
1999; Lister et al. 1984; Martinod 2000). Medium pressure (Barrovian facies series) metamorphic rocks were
exhumed beneath crustal-scale ductile shear zones as the
result of this extension, in the over-riding plate. Thereafter the terrane was sliced by several generations of
detachment faults (Fig. 2), as the stretching of the Aegean continental crust continued (Avigad and Garfunkel
1989; Forster 2002; Forster and Lister 1999a; Gautier
and Brun 1994; Lister and Forster 1996). The Miocene
metamorphic rocks are now well-exposed, in the cores of
partially eroded domal culminations, particularly in islands such as Naxos, Paros and Ios (Lister and Forster
1996; Vandenberg and Lister 1996).
The Miocene domes are mantled by tectonic slices
that allow us to examine the effects of the earlier tectonic
history. The uppermost tectonic slices occur above the
major detachment faults (Forster and Lister 1999a). At
different locations across the archipelago remnants of
these slices are preserved, including Cretaceous (70–
84 Ma, e.g. Bröcker and Enders 1999) granitoids, lowpressure/high-temperature metamorphic rocks, nonmetamorphic and sub-greenschist clastic sediments, as
well as fragments and detritus from the Cycladic ophiolite nappe (e.g. Altherr et al. 1982; Lister et al. 1984).
The underlying groups of tectonic slices were derived
from terranes that were subject to high-pressure eclogite–blueschist facies metamorphism, and then coherently
exhumed. These rocks derive from a deformed and
metamorphosed platform sequence defined by metaophiolite, marbles, calc-schists and volcano-sedimentary
sequences (e.g. Keay 1998). It is apparent that they were
exhumed well before the onset of Miocene extensional
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Fig. 1 Location of the islands
of Syros, Sifnos and Tinos, in
the context of the Cyclades,
Greece. Low-angle normal
faults marked on the islands of
Ios (Forster and Lister 1999a),
Naxos and Paros (Gautier et al.
1993), Sifnos and Syros (e.g.,
Trotet et al 2001) Tinos (e.g.,
Gautier and Brun 1994) have
been mapped, extrapolation
undertaken in this study.
LANFs marked on Amorgos,
Ikaria and Mykonos were
extrapolated from work in this
study and existing geological
maps and our knowledge of the
metamorphic slices
tectonism (e.g. Avigad 1998; Avigad et al. 1998; Wijbrans and McDougall 1986, 1988; cf. Trotet et al. 2001).
The early stage of tectonism involved large-scale
thrusting. For example the sheet of high-pressure
eclogite–blueschist facies metamorphic rock was thrust
over a Hercynian basement complex. This was also
subject to high-pressure metamorphism, but only in its
outer part where fluids were able to percolate (Grutter
1993). However in the central Cyclades this earlier history is less transparent. To understand the beginnings of
this orogeny one must travel to the more external domains of the Cyclades, to islands such as Sifnos, Syros
and Tinos, where spectacular outcrops of thinly sliced
remnants of this coherently exhumed high-pressure terrane are well-outcropped and exceptionally well-preserved. This study is restricted solely to those tectonic
slices that comprise the Cycladic eclogite–blueschist belt
in which aspects of the earlier history of eclogite and
blueschist facies metamorphism can be clearly discerned,
in particular on the islands of Ios, Sifnos, Syros and
Tinos (Fig. 1).
Metamorphic events in the Aegean crust
Three metamorphic events (M1, M2, M3) have been
distinguished in the Cycladic rocks by previous
researchers. These have been recognized because each
set of pressure conditions is widely different, and
therefore as a result indicator minerals have grown
that represent distinct metamorphic facies. M1 involved classic HP/LT eclogite/blueschist facies. M2
involved Barrovian amphibolite to greenschist facies
series. M3 involved low-pressure contact metamorphism. M1, M2 and M3 are distinguished on the basis
that the metamorphic mineral parageneses that grew
during these time periods reflect existing facies classifications.
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Fig. 2 Schematic
representation of geometrical
relations between multitudinous
low-angle faults that slice the
Cycladic blueschist belt. The
major detachment faults are
marked with the thickest
trajectories. Samples of interest
to this study lie between the two
major detachment faults
In detail however, this classification has led to
difficulty because of the lack of clear paragenetic distinctions between minerals that have grown at different
times during different events in the same or similar
pressure–temperature conditions. For example, previous
researchers have assumed that eclogite–blueschist
transitions were the result of geochemical variation (e.g.
Matthews and Schliestedt 1984; Schliestedt 1986;
Schliestedt and Matthews 1987), whereas in many cases,
blueschist facies assemblages overprint eclogitic assemblages, and geochemical variation has been induced as
the result of metasomatism.
Most authors adhere to the view that there is a
single high-pressure event in the Cycladic eclogite–
blueschist unit, and they debate as to exactly when this
event took place (e.g. Altherr et al. 1979, 1982;
Andriessen et al. 1979; Bröcker et al. 1993; Bröcker
and Enders 1999; Maluski et al. 1987; Wijbrans and
McDougall 1986; Wijbrans et al. 1993; Tomaschek
et al. 2003). According to Tomaschek et al. (2003), the
currently accepted Alpine tectono-metamorphic evolution
(cf Okrusch and Bröcker 1990) involves regional highpressure blueschist–eclogite facies metamorphism (M1)
during the Eocene, followed by a regional Oligo–Miocene medium pressure overprint (M2) that locally
reaches high-amphibolite facies conditions (Buick
1991), and local contact metamorphism (M3) accompanying Miocene granitoid emplacement (Altherr et al.
1979, 1982). Yet there is abundant field and microstructural evidence that several separate and distinctive
high-pressure metamorphic mineral growth events have
taken place, both in the Cycladic eclogite–blueschist
belt (Forster 2002; Forster and Lister 1999b; Lister and
Forster 1996; Lister and Raouzaios 1996), and more
regionally (Lips et al. 1998; Ring and Layer 2003;
Schermer et al. 1990).
Fabrics and microstructures suggest quite specific
temporal sequences of metamorphic mineral growth
events (Forster and Lister 1999b; Lister and Raouzaios
1996; Ridley 1984), and in many cases fabrics and microstructures reveal the history of metamorphic mineral
growth unequivocally, and in considerably more detail
than the M1, M2, M3 scheme (as described above) would
portray (Figs. 3, 4). As part of research done within a
larger project, we have systematically analyzed the
metamorphic evolution of several islands in the Cycladic
eclogite–blueschist belt (specifically Ios, Naxos, Syros,
and Sifnos, Fig. 1). On each island, key locations have
been examined in detail (e.g. Forster and Lister 1999b
report on the evolution of eclogitic boudins on Ios). A
detailed structural framework has also been established
at these key locations and a fabric and micro-structural
analysis has allowed the history of deformation and
metamorphism to be linked and analyzed. At each site, a
relative chronology has been established, describing the
evolution of a sequence of distinctive fabrics and microstructures.
Several distinct episodes of metamorphic mineral
growth appear to have taken place, some under quite
similar conditions of pressure and temperature (Forster
2002; Forster and Lister 1999b; Lister and Raouzaios
1996; Lister and Forster 1996). These distinct episodes
of metamorphic mineral growth have been referred to as
M1A, M1B, M1C, M1D, M2A and M2B, retaining the
framework offered by the previous M1, M2 classification,
to enable consistency of description in relation to previously published papers. It has been demonstrated that
this relative chronology of metamorphic mineral growth
events and a related relative chronology of fabric
forming and/or fabric modifying events remain consistent across the entire extent of the Cycladic eclogite–
blueschist belt. Note however that there is no particular
reason to label the four high-pressure events (M1) as
peculiarly distinct from later greenschist facies events
(M2) except that broadly different facies are involved in
the metamorphic mineral growth sequences.
M1A
M1A was an early episode of blueschist metamorphism,
characteristically involving growth of glaucophane and
lawsonite. Early glaucophane is typically aligned in well
developed lineations. M1A assemblages were first recognized in the interior of M1B and M1C porphyroblasts.
Detailed structural geology then allowed recognition of
outcrops dominated by D1 with strong L1 mineral lineations (Fig. 3a, b).
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Fig. 3 Multiple episodes of HP-LT metamorphism have taken
place: a and b show M1A relicts, finer grained intensely foliated
and lineated glaucophanite overprinted statically by M1C porphyroblasts of garnet, zoisite and white mica; c large crystals of
omphacite grown during M1B eclogite facies metamorphism as
it overprinted a 78–80 Ma gabbro; d, e, f strongly foliated
omphacitites in a shear zone that was later recumbently folded
and then statically overgrown by M1C porphyroblastic garnet; e
shows the effects of even later recumbent folds; g and h show
metasomatic fronts with glaucophane overprinting earlier omphacitite assemblages. Scale bars are each 1 cm across
527
M1B
M1B involved eclogite facies metamorphism, with widespread growth of omphacite and jadeite (Fig. 3c).
Intensely developed lineated foliations imply the operation of eclogite (transitional to blueschist) facies shear
zones subsequent to this growth event. These highpressure shear zones (and the associated alteration-related shear zones) were later recumbently folded
(Fig. 3d–f), prior to the reinitiation of (largely static)
porphyroblastic mineral growth in the M1C event.
Metasomatic fronts are commonly developed (usually
associated with early shear zones) showing conversion of
omphacitites to glaucophanites (Fig. 3g, h). These
metasomatic recrystallization fronts were developed
prior to the subsequent period of porphyroblastic
growth.
M1C
M1C involved widespread porphyroblastic growth across
the Cycladic eclogite–blueschist belt, this time under P–
T conditions appropriate to the glaucophane + garnet
blueschist facies (Evans 1990; Lister and Raouzaios
1996). Prolific large porphyroblasts (up to 1–5 cm) grew
under static conditions during this event (Fig. 3a–f),
including glaucophane, garnet, zoisite, and white mica
(including both phengite and paragonite). Porphyroblasts developed during the M1C event are readily recognized in slices of the eclogite–blueschist nappe from
locations across the entire Cyclades.
There are several occurrences where it appears that
M1C porphyroblast assemblages have overgrown M1A
fabrics, defined by lineated foliations with strongly
aligned glaucophane (Fig. 3a, b). These fabrics may
have survived later eclogite facies metamorphism without being obliterated, in which case it appears that new
metamorphic mineral parageneses were not pervasively
developed throughout the rock mass during M1B times.
The blueschist facies M1C growth event has thus been
able to overprint the earlier formed M1A blueschist
fabric without evidence of the intervening episode of
eclogite facies metamorphism.
It is relatively common to find M1A blueschist
assemblages directly overprinted by the M1C blueschist
facies porphyroblastic event. These surviving M1A
blueschist facies assemblages are intensely foliated and
lineated, and are in general statically overprinted by
later blueschist facies metamorphic mineral growth
events. For example, Fig. 3a, b shows strongly lineated
glaucophane statically overgrown by M1C garnet and
white mica porphyroblasts.
M1D
M1D involved transitional blueschist–greenschist facies
metamorphism (Fig. 4a, d), with simultaneous growth
of glaucophane, epidote, white mica and albite (the
epidote–albite blueschist facies of Evans 1990). This
paragenesis is readily recognized where static growth of
albite and epidote (± glaucophane) overprint intensely
developed foliations and lineations developed subsequent to the M1C metamorphism, or in veins (Fig. 4c,
d) where glaucophane selvages transect earlier formed
fabrics. Late recumbent folds refold shear zone fabrics
and veins formed during the M1D period (Fig. 4b).
M2
All previously formed fabrics are statically overgrown
by albite–chlorite assemblages formed during the first
greenschist facies overprint. On some islands (e.g. Ios,
Naxos) this later greenschist metamorphism can itself
also be seen to have been divided into two distinct
events, M2A and M2B (Forster 2002; Forster and Lister
1999b).
A thinly-sliced tectono-metamorphic stratigraphy
Detailed structural mapping has shown that the Aegean eclogite–blueschist belt is exposed as a thinly
sliced tectono-metamorphic stratigraphy. The basic
relations between individual tectonic slices are as
shown in Fig. 2. The geometry is made complex by the
many different generations of (extensional?) ductile
shear zones, some developed on a regional extent, and
the many different generations of low-angle normal
faults and/or detachment faults (Forster and Lister
1999a). In general, however, it is possible to recognize
tectonic slices that appear to have distinctive tectonothermal and metamorphic histories. Individual tectonic
slices display variable degrees of preservation of different metamorphic mineral growth events, as well as
distinctive occurrences of lithologies differently affected
by these individual metamorphic events. For example,
the M2 events are most evident in tectonic slices on Ios
and Naxos in the central Cyclades, but they are nevertheless still evident on Sifnos and Syros. Similarly
the M1C and the M1D events dominate tectonic slices
exposed on Sifnos, but yet they are still evident in
higher level tectonic slices on Ios, and Naxos.
The tectonic slices that best exhibit the effects of
blueschist and eclogite facies metamorphism occur
immediately beneath the highest level detachments
(Fig. 2). Within this scope the earlier history of deformation and metamorphism is best preserved in the tectonic slices that are highest in the structural pile,
although fabrics and microstructures in these slices are
nevertheless variably overprinted by later metamorphic
and deformational events. Lower tectonic slices display
fabrics that are more strongly overprinted by later
metamorphic events. For example, M1D parageneses are
best developed in the lowermost tectonic slice on Sifnos,
whereas in this same slice there are small lenses that were
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Fig. 4 Deformation associated with multiple episodes of HP-LT
metamorphism: a M1D shear zone with ankerite veins and quartz
± glaucophane veins; b these have been recumbently folded in
kilometer-scale axial zones; c and d quartz + glaucophane veins in
M1D epidote+albite rocks; e and f M1C garnet porphyroblasts in a
major extensional shear zone showing asymmetrically developed
pressure shadows adjacent to garnet M1C porphyroblasts; g rotated
M1D albite porphyroblasts in S–C crenulations in a post-M1D shear
zone; h mylonitized quartz veins in a post-M1D shear zone that has
later been locally overprinted by recumbent folds
529
almost certainly once eclogites, although the early mineralogy is almost completely retrogressed.
Ductile shear zones are associated with each tectonic
slice, and with each metamorphic event. Some of these
structures have been suggested to be extensional in their
origin (e.g. Jolivet and Patriat 1999; Trotet et al. 2001).
For example, a major ductile shear zone formed immediately subsequent to the M1C event, and this is spectacularly exposed in the upper levels of the M1C slice on
Sifnos. Figure 4e, f shows statically grown M1C garnet
porphyroblasts that have rotated while pressure shadows developed, in a syn- to post-M1C ductile shear zone.
Similarly, an intense ductile shear zone developed subsequent to the M2A event, the first period of blasthesis,
under greenschist facies conditions. This shear zone is
spectacularly exposed in the lowermost structural levels
on Sifnos, near Faros. This extensional (?) shear zone
had started forming while some albite porphyroblasts
were still growing (Fig. 4g).
In many cases, late in the history of individual ductile
shear zone events, shear zone fabrics were thrown into
recumbent folds, apparently while the ductile shear zone
was still operating (e.g. Fig. 4h shows non-cylindrical
recumbent folds that formed late in the history of the
Faros shear zone on Sifnos). This led to a highly noncylindrical episodes of recumbent folding, with structures
forming that are text book examples of the early stage of
development of sheath folds. These abrupt changes in
behavior may be examples of ‘‘pull–push’’ tectonic mode
switches, in this case from overall extension (the ‘‘pull’’
phase) to overall crustal shortening (the ‘‘push’’ phase).
Since examples of this behaviour are associated with the
operation of early as well as late shear zones, inversion
cycles (or tectonic mode switches) may be relatively
commonplace during the tectonic evolution of the
Aegean continental crust.
It is noteworthy that many of the metamorphic
mineral growth events took place under static conditions, i.e. the porphyroblasts overgrew pre-existing fabrics without discernable distortion. While these episodes
might appear to be ‘static’ it is possible that they took
place remarkably rapidly, in time frames so short that
the effects of accumulating strain could not become
evident. It is also noteworthy that episodes of metamorphic mineral growth appear to take place after
periods of recumbent folding. If the preceding period of
folding (F) was the result of overall crustal shortening,
and the succeeding period of shear zone (SZ) operation
was caused by overall crustal extension, the episode of
metamorphic mineral growth (D) may mark the tectonic
mode switch. The FDSZ sequences are then examples of
‘‘push–pull’’ (shortening following by extension) tectonic mode switches marked by metamorphic events.
It is evident that ductile shear zones began to form
after many of the metamorphic events that we have listed
took place. This is certainly the case for the M1B, M1C,
M1D, and M2A growth events. In some cases it can be
shown that individual ductile shear zones formed
immediately subsequent to (or during the last stages of)
the preceding period of metamorphic mineral growth.
The individual shear zones continued to develop long
after the preceding period of porphyroblastic mineral
growth had ceased, developing intense foliations and
lineations defined by the newly grown minerals. The M1C
garnet porphyroblasts may grow to several centimeter in
diameter, and these porphyroblasts roll during the subsequent operation of these ductile shear zones, developing asymmetric pressure shadows, S–C fabrics, and other
indicators of non-coaxial laminar flow (Fig. 4e, f). Such
fabrics do not develop without large-scale tectonism, and
one would expect that several million years would elapse
to allow the formation of such intense fabrics.
In some cases it can be demonstrated that the ductile
shear zones commenced operation immediately subsequent to, or even during the preceding period of
metamorphic mineral growth. For example, M1C porphyroblasts were still growing when kilometer-scale
extensional (?) ductile shear zones began to form in
northern Sifnos. Inclusion trails are straight, without
distortion in the core of such porphyroblasts. Similarly
M2A albite porphyroblasts were still growing when
kilometer-scale extensional (?) ductile shear zones began
to form in a subsequent episode (Fig. 4g). Only at the
rim do the inclusion trails begin to curl. Lister and Raouzaios (1996) suggested that the variation in 40Ar/39Ar
apparent age spectra suggested rapid cooling during the
operation of these shear zones, and this is consistent
with the evolution of microstructure.
In view of the evidence for inversion cycles and tectonic mode switches, it is not surprising that many of the
contacts between these tectonic slices exhibit characteristics as described for the Ios Detachment Fault (Forster
and Lister 1999a), overlying the South Cyclades Shear
Zone (Vandenberg and Lister 1996; Lister et al. 1984).
These are the features that are to be expected of lithosphere-scale dislocations that have reversed their shear
sense. The overall initial juxtaposition of the tectonic
units is what is to be expected of thrust tectonics, with
rocks metamorphosed at great depth thrust over rocks
that apparently have not been affected by such great
pressure. However, when data from thermochronology
is considered (Baldwin and Lister 1998), it is apparent
that the upper plate of individual detachment faults or
extensional shear zones comprises rocks that had already been at relatively shallow crustal levels for a
considerable period of time. The extensional shear zones
and detachment faults exhumed the lower plate from
relatively deeper levels, where ambient temperatures at
that time were higher. If this process repeats time and
time again, a thinly sliced tectono-metamorphic stratigraphy will result, and individual tectonic slices will
display a characteristic tectono-thermal evolution.
Application of the method of asymptotes and limits
To test the hypothesis that individual tectono-metamorphic slices have distinctive tectono-thermal
530
Fig. 5 Different types of mixing sequences (modified from Forster
and Lister 2004) showing mixing and patterns of gas release: a
abruptly from a less retentive reservoir then changing to gas release
from a more retentive reservoir; b a more progressive evolution of
release from one reservoir to the other; c mixing without
appearance of the reservoir ages; d in comparison to nature, a
retentivity reversal as the result of dehydroxylation; e degassing of
the less retentive reservoir dominates gas release for a time, while
release from the more retentive reservoir continues throughout; f
more chaotic mixing, but still the limits constrain the reservoir ages
evolution, we have applied the method of asymptotes
and limits (Forster 2002; Forster and Lister 2004) to
ascertain whether or not there are clusters (or frequently
measured ages, FMAs) in published 40Ar/39Ar apparent
age spectra from these islands. The findings from this
analysis can be related back to the basic structural and
metamorphic relative chronological framework derived
as part of a larger study across the blueschist belt of the
Aegean. Data considered in this paper comes from the
islands of Sifnos, Syros and Tinos (Fig. 1) in the western
zone of the Cycladic archipelago, where mineral and
stretching lineations generally have a NNE to NE trend
(Blake et al. 1981), and where the Cycladic eclogite–
blueschist belt is also best preserved.
Extensive 40Ar/39Ar geochronology has been carried
out on these islands: on Syros, by Maluski et al. (1987)
and Baldwin (1996); on Sifnos (an unpublished study);
and on Tinos (Bröcker et al. 1993). All sample sites
have been revisited, and the apparent age spectra
linked back to the outcrop, and into the sequence of
fabrics and microstructures that has been recognized as
the result of independent research, potentially providing constraint as to the timing of individual metamorphic mineral growth events, and/or the timing and
duration of deformation/recrystallization subsequent to
these metamorphic mineral growth events. It is possible that 40Ar/39Ar geochronology records the timing of
mineral growth, and of the effect of deformation and
recrystallization in subsequently formed ductile shear
zones. Reanalysis of apparent age spectra measured in
these individual tectonic slices using the method of
asymptotes and limits thus has the potential to reveal
531
a correlation of FMAs with fabric and microstructure,
and this may provide time constraints for individual
metamorphic mineral growth events, and the duration
of the operation of the shear zones that formed
immediately after each individual metamorphic mineral
growth episode.
Summary of the method
In this section, we provide a summary of the method,
because it is relatively new. The method assumes that
in a complex microstructure there will be distinct argon reservoirs from which characteristic ages might be
obtained if a carefully designed step heating experiment is able to release gas from one reservoir then the
other. Figure 5 (modified from Forster and Lister
2004) shows how gas can be released from one reservoir then mixed with gas released from another. The
limits of the mixing age sequences may in some cases
(Fig. 5a, b) define the original age of the gas in the
reservoir.
No assumption is made as to the mechanism of gas
release in the mass spectrometer, i.e. the method does
not require solid-state diffusion and it does not fail when
dehydroxylation and delamination of the white mica
begin to take place. Circumstances will exist that
prevent a meaningful constraint from being obtained
(e.g. Fig. 5c) but it is assumed that limits obtained in
such sequences still provide bounds for the age of the
reservoir. This will be true even when vagaries of the
dehydroxylation process lead to breakdown of the
retentive (i.e. in the natural environment) reservoir
ahead of gas release from less-retentive reservoirs (e.g.
Fig. 5d) or more complex patterns of gas release as
shown in Fig. 5e, f.
The key to the method is a numerical technique to
allow recognition of FMAs. Asymptotes and limits are
first defined for individual apparent age spectra. Figures 6 and 7 show a selection of apparent age spectra
from Syros reanalyzed in this way. The ages of individual asymptotes and limits are then tabulated (here
in an Microsoft Excel spreadsheet), and correlated
with the percentage gas released in the step used to
define the particular asymptote or limit. In this way,
the inclusion of mixing ages which vary due to the
amount of mixing from more than one gas population
will be diminished or eliminated due to their low
count rate. A computer program me is then used to
produce cumulative age plots for each apparent age
spectrum.
This final step is achieved by defining a normal (or
Gaussian) distribution centered on the age determined
for each asymptote or limit, in this paper with an
(arbitrary) standard deviation, here chosen as ±0.5 Ma.
The Gaussian distribution for each asymptote or limit is
then weighted using the percentage gas released in the
step used to define the particular asymptote or limit. The
individual distributions are then summed for each
Fig. 6 Re-analysis of apparent age spectra from Syros: a, c, e are
representative data measured by Baldwin (1996) from phengitic
white mica with asymptotes and limits derived in this study
superimposed: b, d, f are the corresponding Gaussian plots
calculated from the distribution of asymptotes and limits in the
individual 40Ar/39Ar apparent age spectra (see text for further
details)
apparent age spectrum. This data is then cumulated for
groups of apparent age spectra (e.g. Fig. 8).
Figure 6 shows the Gaussian plots obtained for three
individual apparent age spectra, illustrating the power of
the technique in terms of summarizing the information
contained in the sequence of age steps, and in defining the
potential bounds for the age populations mixed together
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Fig. 7 Re-analysis of 40Ar/39Ar
apparent age spectra from
Syros: a–c measured by
Baldwin (1996), d–f measured
by Maluski et al. (1987) with
asymptotes and limits derived
in this study superimposed
in the one sample. Figure 6a shows a step heating experiment in which insufficient steps were performed to allow
the method to be applied with any precision. Nevertheless
useful age estimates can still be obtained (Fig. 6b). Figure 6c shows an apparent age spectrum in which the age
sequence ‘asymptotically’ converges towards a ‘median
asymptote’. The Gaussian plot again provides a useful
summary of the age information inherent in this sequence.
The final example (Fig. 6e) shows an apparent age spectrum with a ‘plateau’ initially marred by partial loss.
Again the Gaussian plot (Fig. 6f) usefully summarizes the
age information contained in the data sequence. The
plateau naturally emerges because small variations about
the median age define multiple limits.
We emphasize that the key to the method is the recognition of FMAs. The assumption is that FMAs will
not exist if the microstructure does not contain distinct
reservoirs as described in the previous paragraph. It is
also presumed that a carefully designed step heating
experiment will provide sequences of ages, some of
which converge asymptotically towards a limit. In other
cases, limits must be defined more pragmatically, by the
bounds of sequences of apparent ages obtained during
an experiment, as shown in Fig. 5 (modified from Forster and Lister 2004). We also note that routine application of the method has revealed that FMAs are
commonplace in the complex apparent age spectra typ-
ically obtained when conducting step heating experiments on white mica separated from metamorphic
tectonites.
Apparent age spectra from Syros
In this section, the method of asymptotes and limits
(Forster 2002; Forster and Lister 2004) is systematically
applied to the analysis of 40Ar/39Ar apparent age spectra
published by Maluski et al. (1987) and Baldwin (1996).
Figures 6a, c, e and 7a–c show a sample of Baldwin’s
(1996) 40Ar/39Ar apparent age spectra with limits as
marked. Baldwin (1996) interprets her ages using either
a weighted mean age, a fusion age or where gradients
occur in an age spectra, the upper and lower ages of that
gradient are constrained with a ‘‘fusion age’’. Asymptotes and limits are difficult to define in flat spectra such
as these, and generally cluster close to the integrated age.
Even so, the age spectra can be reinterpreted using the
method of asymptotes and limits, although in many
cases a multitude of different asymptotes and limits
(with very similar ages) must be defined in the same age
spectrum.
Individual Gaussian plots were then produced for
each apparent age plot, and graphs produced for each
individual apparent age spectrum. Figure 6 illustrates
533
Sample location sites used in Baldwin (1996) have
been revisited in this study, and the evolution of these
samples redefined within the context of the M1A, M1B,
M1C, M1D metamorphic history recognized as the result of our previous research. Note that the degree of
overprinting and preservation within this sequence will
most likely be different in comparison to the original
samples, even if samples are collected from closely
adjacent locations within the same particular metamorphic slice. Except for SY 89647, samples were
from the M1B dominated uppermost tectonic slice in
the Hermoupolis unit, defined by retrograded M1B
eclogite, M1C blueschist and retrograded blueschist
assemblages.
Maluski et al. (1987) previously undertook 40Ar/39Ar
geochronology on Syros using white mica (both phengite
and paragonite) and glaucophane. Some of the apparent
age spectra produced are illustrated in Fig. 7d–f, with
limits marked as shown. The glaucophane samples were
low in K-content (0.01%) and analyzed samples gave a
scattered spectra with no plateau ages, the age spectra
produced have relatively high errors and have been
analyzed with only a few steps (7–8 steps). White mica
samples overall represent a range of different metamorphic conditions, varying between eclogite, blueschist
and greenschist facies. To complicate the situation,
individual samples do not always display the effects of a
single metamorphic event. Based on the parageneses
given by Maluski et al. (1987), and by comparison with
observations made when we revisited the actual sample
sites, the samples analyzed generally represent a mixture
of metamorphic assemblages, for example either eclogite/blueschist mineral assemblages or blueschist/greenschist mineral assemblages. This information can be
taken into account when re-interpreting the significance
of the 40Ar/39Ar apparent age spectra using the method
of asymptotes and limits.
Apparent age spectra from Sifnos
Fig. 8 Cumulative age diagrams produced from the statistical
analysis of the asymptotes or limits in all age spectra measured by:
a Baldwin (1996) from Syros; b Maluski et al. (1987) from Syros; c
from Sifnos (G.S. Lister et al., unpublished data); d from Tinos
(Bröcker et al. 1993). The ages obtained allow an estimate of the
timing of three different metamorphic events, M1B, M1C, M1D and
M2A
individual Gaussian plots for some of the apparent age
spectra measured by Baldwin (1996). Gaussian plots for
all apparent age spectra measured by Baldwin (1996)
were then summed (Fig. 8).
Sifnos is located south west of Syros (Fig. 1). In its
higher structural levels, it contains tectono-metamorphic
slices dominated by the M1C blueschist event, with relict
M1B eclogite assemblages. In its lowest structural levels,
it contains a tectono-metamorphic slice dominated by
the M1D and M2 events. A set of samples was collected
after a period of detailed 1:5,000 scale mapping, and
carefully constrained in terms of the evolution of their
fabrics and microstructures. Samples were taken from
the four tectonic slices, each of which is dominated by
different metamorphic events. Figure 9a, b is from the
upper schist slice which is dominated by the M1C
blueschist event. Figure 9c, d is from the basal levels of
the overlying upper marble slice. Figure 9e, i is from the
underlying middle marble slice. Figure 9f–h is from the
lower schist slice which is the lowermost structural entity
that is exposed. The samples at or near to Vroulidia Bay
(Fig. 9a–c) are dominated by the effects of ductile shear
534
Fig. 9 Representative apparent
age spectra from phengitic
white mica in samples from
Sifnos, with asymptotes and
limits superimposed on the
appparent age spectra: a from
an (pre-M1C) F3 fold zone with
crenulated S2 fabrics on the
boundary of a post-M1C ductile
shear zone; b from the core of
the same 50 cm wide post-M1C
ductile shear zone, with the
main foliation, S2, crenulated
by F3 and then recrystallized,
stretched and disrupted in the
shear zone; c from the core of a
50 m thick boudin of schist in
the upper marble slice, near its
lower boundary; d from the
upper marble slice, immediately
adjacent to a low-angle fault at
its lowermost boundary; e a
post-M1D mylonitized schist at
the lowermost boundary of the
middle marble slice,
immediately adjacent to a
window exposing the lower
schist unit; f uppermost levels of
the lower schist unit,
immediately beneath an
extensional low-angle fault; g
post-M1D mylonitic schist
15 m beneath the detachment
fault separating the middle
marble slice from the lower
schist slice; h middle of lower
schist slice, in an pre-M1D axial
zone; i 230 m above basal
detachment of the middle
marble slice. Sample locations
shown in GPS coordinates
using the greek geodetic
reference system (GGRS
EGSA87)
zones post-M1C. Samples from the lower schist zone and
from the two marble slices (Fig. 9e–i) show the effects of
ductile shear zones post-M1D. The effects of deformation
overprinting metamorphic growth events are evident in
some samples (e.g. Fig. 9d, e), while other samples show
the effects of metamorphic growth events without subsequent deformation. The method of asymptotes and
limits was applied to a set of 18 40Ar/39Ar apparent age
spectra measured from these samples. Figure 9 shows a
representative sample set of the apparent age spectra
that have been obtained from phengitic white mica, and
the reanalysis of asymptotes and limits for this data. The
results are consistent within the context of the M1A,
M1B, M1C, M1D metamorphic history recognized as the
result of our research.
Apparent age spectra from Tinos
Tinos is located in the north west of the Cycladic
archipelago (Fig. 1). It contains several tectono-metamorphic slices (Fig. 2) each with its own distinctive
535
pattern of tectono-thermal evolution. Previous work
(Bröcker et al. 1993) has resulted in the recognition of a
single high-pressure metamorphic event, estimated to
have occurred under conditions of 15 kbar and temperatures of 450–500°C. This eclogite–blueschist facies
event was followed by exhumation during which retrogression took place under greenschist facies conditions
(e.g. Baldwin 1996; Maluski et al. 1987; Okrusch and
Bröcker 1990; van der Maar and Jansen 1983). The effects of an overprinting prograde Barrovian greenschist
to amphibolite facies metamorphism are also recognized
(Altherr et al. 1982).
We revisited sample sites on Tinos, and then applied
the method of asymptotes and limits (Forster and Lister
2004) to the reinterpretation of 40Ar/39Ar apparent age
spectra published by Bröcker et al. (1993). Representative samples reanalyzed in this way are shown in Fig. 10
(based on the data tabulated by Bröcker et al. 1993).
This geochronological data represents data from a range
of metamorphic assemblages, from relatively pristine
blueschist assemblages, blueschist with a ‘greenschist’
overprint, to greenschists with prograde development of
new mineral assemblages.
Bröcker et al. (1993) carried out extensive 40Ar/39Ar
thermochronology, producing high resolution age spectra with low percentage errors. There are in general
>10–15 steps in each age spectrum, thus facilitating the
application of the method of asymptotes and limits
(Forster and Lister 2004) to the reanalysis of this data.
However Bröcker et al. (1993) present all their age
spectra on one single plot (their Fig. 7). This method of
presentation obscures the data, and therefore all
apparent age spectra (Fig. 10) were replotted (using the
MACARGON
program, Lister and Baldwin 1996).
Asymptotes and limits were superimposed, and tabulated, then analyzed as before. Data from blueschists
that were relatively unaffected by the later greenschist
overprint fall into a distinct cluster (Fig. 8d). Other
clusters are defined by data from blueschists that were
strongly affected by later greenschist facies events, or
completely retrogressed and/or overgrown by newly
developed greenschist facies assemblages. FMAs can be
extracted from this data and these allow correlation with
individual samples. It is evident that there are three
distinct groups of FMAs and these may correlate with
the three periods of metamorphic mineral growth observed to have occurred in these rocks.
FMAs
Statistical plots for all islands (e.g. Fig. 8) show clusters
of FMAs (Forster and Lister 2004). In each case, the
FMAs recognized in the 40Ar/39Ar apparent age spectra
(Fig. 8) can be correlated with the observed metamorphic assemblages. There is a reasonable degree of consistency between the different datasets, although more
work will be necessary to determine the best way to
proceed with statistical analysis.
Figure 8a is based on the results obtained by Baldwin
(1996) and suggests FMAs at 22, 40.4, 42.7, 49.7
and 52.3 Ma. Figure 8b is based on Maluski et al.
(1987) and suggests FMAs at 29.8, 38, 42.5, and
52.6 Ma. FMAs in the dataset from Sifnos (Fig. 8c)
are at 19.5, 29.6, 32, 34.5, and 41.4 Ma, FMAs
in the dataset from Tinos (Fig. 8d) are at 22, 32.2,
40.1, 42.2, and 43.8 Ma.
Based on the assemblages in the different samples,
these FMAs might be correlated with (minimum) estimates for the age of the M2A, M1D, M1C, M1B, and M1A
metamorphic mineral growth events, respectively, as
follows: M2A (to have occurred at some time in the
period between 22 and 19 Ma), M1D (to have occurred
at some time in the period between 30 and 35 Ma), M1C
(to have occurred at some time in the period between 38
and 44 Ma), and M1B (to have occurred at some time in
the period between 50 and 53 Ma). The age of the M1A
event is unconstrained, except that we know it must
occur after a magmatic event recorded in northern Syros
at 78–80 Ma (Keay 1998) prior to the M1B event (as
above). Note that the range specified above for each of
the clusters does not mean that a metamorphic growth
event lasted for that entire period at each location. It is
more likely that metamorphic growth periods occurred
early within this time range and argon distributions were
modified by diffusion as well as recrystallisation during
subsequent deformation.
Discussion
Limitations of the techniques used
The data re-analyzed in this paper comes from a variety
of sources. In a perfect world we would have taken a
new set of samples, and redone all analyses on the same
sample set, along with microprobe analyses, thermobarometry, and micro-structural analysis. This objective
has been partially accomplished, based on detailed field
work on Sifnos. But the added detail adds little to our
conclusions, and the work is still in progress. Hence we
have elected to publish the story as we now see it,
highlighting the differences in respect to conventional
interpretations to focus the issues involved.
The statistical techniques used to reinterpret the
40
Ar/39Ar data presented in this paper are imperfect, but
they are based on rules defined a priori and they allow a
move away from the sometimes ad hoc criteria that are
sometimes applied to extract a single ‘meaningful’ age
from a 40Ar/39Ar apparent age spectrum. There is no
hindrance, for example, in respect to the incorporation
of the method of asymptotes and limits in terms of
automatic data analysis, to provide a statistical assessment of the significance of the results obtained during a
particular study. Nevertheless, bias can be introduced
into a dataset because the actual relative weighting of
different ages can be changed in several ways, for
example as the result of samples focused on a single
536
Fig. 10 Representative
apparent age spectra measured
by Bröcker et al. (1993) from
phengitic white mica in samples
from Tinos. Asymptotes and
limits superimposed on the age
spectra
location, or during the mineral separation process if one
micro-structural type is preferentially selected. This
could explain why the 30 Ma age is not significant in the
Baldwin (1996) results from Syros, but is prominent in
the Maluski et al. (1987) data.
From these previously published analyses there are
also a wide range of uncertainties associated with details
of sample separation, and micro-structural context.
However sample sites have been revisited, and it was
possible to identify the sequence of mineral growth
events and the metamorphic events that dominated
particular areas. Individual samples can be linked to
specific tectono-metamorphic slices, and the data is
consistent with the hypothesis that each tectonometamorphic slice had a distinctive thermochronological
evolution.
Excess argon
Kelley (2002) provides a review of the issue of
argon in 40Ar/39Ar geochronology, noting the
cases in which this phenomenon has been well
mented. The possibility that there is excess
excess
many
docuargon
537
introduces uncertainty in respect to the significance of
data obtained anywhere that one uses 40Ar/39Ar geochronology to time events in high-pressure metamorphic
rocks. In particular Sherlock and Kelley (2002) identify
the Cyclades as a problem area in this aspect. Are the
uncertainties associated with excess argon of sufficient
import to negate the data reported in this paper?
We think not, in part because some of the results
described in this paper can be verified using metamorphic rim overgrowths on zircon (Keay 1998; Tomaschek
et al. 2003; Pulitz et al. 2005). Tomaschek et al. (2003)
recognize a population of zircons in a meta-ophiolite on
Syros that they ascribe to magmatism in developing
ocean crust, dated by SHRIMP at 80–76 Ma (cf Keay
1998). They suggest subsequent replacement-recrystallization of this zircon with a skeletal generation of zircons
grown at 52.4±0.8 Ma during high-pressure metamorphism. This agrees well with our estimate of the age
of M 1B metamorphic mineral growth. More importantly as noted by Tomaschek et al. (2003): ‘‘Intergrowth
relations between zircon and peak-metamorphic garnet,
and excellent agreement of the U-Pb ages with white mica
Ar-Ar ages of the same samples, support that Eocene is
the true age of high-pressure metamorphism’’ (see p.
1991). This coincidence of ages obtained using U–Pb
geochronology on zircon overgrowths independently
suggests that we need to revisit issues associated with use
of the label ‘‘excess argon’’ attached to problematic
40
Ar/39Ar apparent age spectra.
Sherlock and Kelley (2002) describe the Tavsanli
Zone of NW Turkey as a well-documented example of
the effects of excess argon. But from their own description these rocks are undoubtedly not so simple as they
would at first appear. For example, coarse porphyroblasts of lawsonite overprint the ‘‘main’’ penetrative
foliation, attesting to at least two distinct periods of
metamorphic mineral growth. Foliations require large
strains, and thus one would reasonably expect a minimum of 3–5 Ma to have elapsed between the first and
the second growth events. Perhaps we are too ready to
accept a single age from such terranes. Moreover, when
it comes to comparing Rb–Sr ages with 40Ar/39Ar geochronology, note that Rb–Sr ages rarely reflect cooling,
and bulk separates for Rb–Sr analysis provide fertile
ground for the same issues as bedevilled K–Ar geochronology. Of more concern is the fact that these ‘‘dry’’
rocks were apparently capable of dynamic recrystallization, and are thus not so dry at all. There is thus
another side to the question of excess argon in metamorphic rocks. We are able to analyze rocks from these
terranes in excruciating detail in a mass spectrometer.
We need to expend similarly detailed effort in terms of
fabric and micro-structural analysis! In many cases
where excess argon has been used as the explanation for
odd ages, there is also a long history of micro-structural
evolution that has not been properly taken into account.
It would be remarkable should the Tavsanli Zone of
NW Turkey not have the same complexity as other high
pressure zones in the region.
There are many places where the label ‘‘excess argon’’
has been applied because 40Ar/39Ar data is inconsistent
with single ages derived from other techniques. Each of
these terranes needs to be systematically re-investigated.
Our preliminary investigations in the Western Alps
demonstrate multiple high-pressure metamorphic mineral growth events, very much as observed in the Aegean. The practice of ascribing a single age to describe
‘‘peak metamorphism’’ is not appropriate in such rocks.
There are many different growth events, all of which
need to be independently dated.
In summary; complex 40Ar/39Ar apparent age spectra from high-pressure terranes are too often too
readily ascribed to the effects of excess argon, for
example in cases such as those that we have illustrated
in this paper. Our results suggest that one should not
too readily dismiss complex data obtained from
40
Ar/39Ar geochronology in high-pressure rocks as due
to perceived difficulties in respect to the existence of
excess argon. Perhaps the difficulty is more related to a
lack of sophistication in respect to fabric and microstructural analysis. Researchers need to be able to
recognize complexity in respect to the history of
deformation and metamorphism, particularly in respect
to the preservation of relict mineral lattices derived
from earlier phases.
Correlating FMAs with microstructure
Frequently measured ages determined by application of
the method of asymptotes and limits can be correlated
with specific elements of fabric and micro-structure in
individual samples. This allows constraint as to the
timing (and the duration—how long it took to develop)
specific elements of the fabrics and microstructures in
the analyzed samples. But there will always be differences of opinion in this aspect, because correlation is
required, and opinions may differ. For example, Bröcker
et al. (1993) recognize three main age groups in the data
they produced. The age groups correlate with the
metamorphic state of the sample. Distinct differences
can be observed in spectra obtained from white mica
from blueschists, as opposed to white mica from retrograde blueschist assemblages or prograde greenschist
assemblages. The analysis in this paper confirms this
conclusion, using the method of asymptotes and limits.
Figure 8d should be compared with the synoptic plot of
the age spectra produced by Bröcker et al. (1993, their
Fig. 7). Nevertheless, because a wider sample set was
analyzed, and because more detail in the sequence of
metamorphic mineral growth events was recognized, we
came to a different conclusion as to the significance of
the three age groups.
Bröcker et al. (1993) suggest that many of the
40
Ar/39Ar apparent age spectra represent incomplete reequilibrium during later events, recognizing that their
spectra are disturbed and that most samples were affected to a greater or a lesser degree by growth of
538
‘‘retrograde greenschist facies assemblages during exhumation’’. They suggest that the effects of exhumation
and overprinting greenschist facies metamorphism can
be recognized in spectra with ages in the range 32–
28 Ma in some greenschist and late-stage blueschist
facies rocks, and in the low-temperature steps in
apparent age spectra obtained from some blueschist
facies white micas, in the range 30–20 Ma. Some
greenschist facies white mica samples yielded ages in the
range 23–21 Ma, which Bröcker et al. (1993) relate to
incomplete resetting caused by a renewed prograde phase
of greenschist metamorphism.
In our analysis the pervasive blueschist metamorphism of Bröcker et al. (1993) can be correlated with the
eclogite-overprinting blueschist facies metamorphism
(M1C), with local development of M1D transitional
blueschist–greenschist facies assemblages (albite–epidote–glaucophane) in lower tectonic slices defining the
‘‘retrograde assemblages’’. The effect of later greenschist
metamorphic event(s) can also be recognized, defining
‘‘prograde’’ M2A assemblages.
Thermal pulses during high-pressure metamorphism
Across the entire Cycladic eclogite–blueschist belt it is
evident that individual samples or outcrops can display
marked variation in 40Ar/39Ar apparent age spectra. For
40
Ar/39Ar apparent age spectra to behave in this way the
rock must have resided in an ancient argon partial
retention zone (Baldwin and Lister 1998; Forster and
Lister 2004) and/or the retentivity of argon must be
considerably higher than previously estimated. In an
argon partial retention zone 40Ar/39Ar, apparent age
spectra may yield widely varying ages because temperatures are not sufficiently high enough to allow anything
more than partial resetting of relict mineral grains as the
result of diffusional loss, whereas deformation and
recrystallization efficiently reset ages in material that is
regrown, or intensely sheared. This age heterogeneity is
thus observed even in samples that are in close spatial
proximity, and there is a strong correlation between
apparent age and microstructure.
Under these conditions 40Ar/39Ar apparent age
spectra allow conclusions to be drawn as to the nature
of the pressure–temperature–time (P–T–t) path followed by individual rocks, based on limited data and/
or estimates for the retentivity of argon in phengitic
white mica. Baldwin and Lister (1998) thus concluded
that M1 high-pressure metamorphism involved thermal
excursions of less than 1 Myears in their duration, on
the basis of modelling using the MACARGON software
(Lister and Baldwin 1996). High-pressure metamorphism has been estimated to have taken place at temperatures above 450°C (Avigad 1993, 1998;
Schliestedt and Matthews 1987; Wijbrans et al. 1990).
Older apparent ages were preserved in minerals relict
from previous metamorphic episodes. Therefore the
duration of the period of elevated temperatures must
have been short, and both the rise in temperature, and
the rate of cooling after each event must have been
rapid. Otherwise older ages could not have been preserved. The only escape from this conclusion is to argue that the retentivity for phengite has been
significantly underestimated (either via the use of too
low a value for the activation energy for diffusion, or
too low a value for the activation volume), in which
case the retentivity at pressure and temperature can be
significantly increased.
It is possible that significantly higher closure temperatures could apply to phengite, but any revision upwards of estimates as to the retentivity of phengite
requires similar migration of estimates for retentivity in
other minerals. This creates inconsistencies. For example
there are many cases where the ages of other minerals
(e.g. the most retentive domains in K-feldspar) have the
same ages as that observed in phengite.
Extensive 40Ar/39Ar geochronology has been
undertaken in the high-pressure metamorphic belt of
northeastern New Caledonia (Rawling 1998). These
data show ages that vary relatively smoothly across the
terrain, in spite of a multiplicity of metamorphic mineral growth events. This suggests that these data are
cooling ages, and/or that phengite has been reset because more elevated temperatures applied in later
growth events (e.g. during later greenschist facies events
that led to pervasive growth of small garnet). If closure
temperatures were as high as suggested by Ring and
Layer (2003), the homogeneity of age distributions
would not be observed. These are similar rocks developed under broadly similar metamorphic conditions to
the rocks studied in the Cycladic eclogite–blueschist
belt.
Should the retentivity of phengite be as high as
suggested by Ring and Layer (2003), the 40Ar/39Ar
system would allow direct measurement of the timing
of individual metamorphic growth events, and later
thermal events would have little impact on the measured ages. We do not observe this to be true. For
example, in the high-pressure metamorphic belt of
northeastern New Caledonia, there is a remarkably
homogeneous distribution of phengite ages in comparison with age distributions in the Aegean. Microstructures reflect the pervasive influence of a later
thermal event (Rawling 1998). Therefore we contend
that the survival of older ages in mineral grains such as
phengite requires us to reconsider the nature of P–T–t
paths inferred for high-pressure terranes such as the
eclogite–blueschist belt of the Cyclades, and to consider
the geodynamic implications of thermal pulses at depth
in the Earth’s crust.
Note that Bröcker et al. (1993) show that 40Ar/39Ar
spectra for the white micas they measured from
blueschist facies assemblages gave plateau ages of 44–
40 Ma and suggest these ages reflect dynamic recrystallisation under peak or slightly post-peak high-pressure
metamorphic conditions. Application of the method of
asymptotes and limits yields a similar estimate of age
539
variation, but based on the type of analysis as applied
by Baldwin and Lister (1998) or Lister and Raouzaios
(1996) it may be argued that there was rapid cooling
after the period of metamorphic mineral growth, and
that the thermal excursions associated with subsequent
metamorphic events must have been of short duration.
Otherwise these older ages would not have been retained. As on Sifnos, the period 44–40 Ma may have
commenced with M1C metamorphic mineral growth,
with subsequent deformation and recrystallisation in
ductile shear zones, rapidly cooling during the exhumation process. Lister and Raouzaios (1996) estimated
that the temperature subsequent to M1C had to have
dropped rapidly beneath 280°C during exhumation in
the tectonic slice affected.
The role of fluids
The role of fluids can also be debated. Rocks affected
by later metamorphic events occur in close proximity
to rocks containing pristine blueschist assemblages.
This may be the result of variable pH2 O because
dynamically-maintained dilatancy associated with the
operation of ductile shear zones channels the flow of
metamorphic fluids (and thus retrograde assemblages
tend to be localized in shear zones). Carbonate dominated lithologies protect adjacent rock masses to
some extent because they may in some cases actually
hinder the flow of aqueous fluids, by acting as a
barrier, and because they contribute to elevated pCO2 ,
thus reducing the efficacy of water in facilitating retrograde reactions.
Experiments show that ductile deformation of the
rock mass induces dynamically-maintained dilatancy
(e.g. Peach and Spiers 1996; Zhang et al. 1994).
Migration of dilatant zones during the operation of a
ductile shear zone can drive fluid migration. The
presence of migrating fluids provides components and
enhances reaction pathways, and may be the factor
that allows (or even drives) each episode of metamorphic mineral growth to take place. Once deformation
ceases, dilatancy disappears, and with it any connectivity of fluid in the rock mass. Permeability becomes
negligible. The rate of metamorphic reactions then
decreases, and the episode of metamorphic mineral
growth is then over (e.g. Schliestedt and Matthews
1987). Eventually, this process may re-initiate and another growth episode takes place, but then it takes
place under different P–T conditions. In this way a
discrete sequence of metamorphic mineral growth
events can be explained, although pressure–temperature
conditions can change continuously (cf Schliestedt
1986).
This explanation in fact has no bearing on the
argument that the sequence of distinct episodes of
metamorphic mineral growth is the result of a sequence
of thermal pulses. Recognition of the role of water in
promoting metamorphic reactions allows us to explain
the existence of greenschist facies assemblages in close
proximity to preserved blueschists, for example, but
this observation has no implications in respect to the
thermal history. It is difficult to argue that the later
formed greenschist assemblages did not require temperatures in excess of 400–450°C for their formation
(e.g. Baldwin and Lister 1998). These elevated temperatures cannot have lasted long, because otherwise
the white mica in the relatively pristine blueschist
samples would also have been subject to temperatures
of 400–450°C for long periods of time. If the period of
elevated temperature lasted longer than 1 Myears,
based on estimates of diffusivity (Baldwin and Lister
1998), this would have been more than sufficient time
to have reset the argon clock. The relict mineral grains
are not reset. Therefore the elevated temperatures have
not lasted more than 1 Myears. Therefore a thermal
pulse has to have taken place. The basis of this argument does not depend on the kinetics of mineral
growth, and it is immaterial whether or not it was the
presence of water that facilitated growth of greenschist
facies assemblages, sometimes within a few centimeters
of pristine blueschists.
The nature of P–T–t trajectories
Concepts applied to the question of determining P–T–t
paths for individual rocks in orogenic belts have progressively evolved over the past decades (Fig. 11).
Smooth P–T loops derive mainly from concepts advocated by England and Thompson (1984) but pertain to a
model that is inapplicable except in the case of orogens
that first deform and then quietly erode. Such models
predict a smoothly varying P–T path, such as shown in
Fig. 11a, illustrating the effects of isothermal decompression.
While such isothermal decompression paths are
consistent with peak temperatures inferred from metamorphic assemblages, such curves do not take account
of thermochronological data. Wijbrans and McDougall
(1986, 1988) recognized that such smoothly varying P–
T–t loops (Fig. 11a) would not allow older 40Ar/39Ar
apparent ages to survive, and therefore were not a valid
approximation to the actual trajectories that must have
been followed. Wijbrans et al. (1990), concluded that the
survival of old apparent ages in spite of the greenschist
facies overprints observed in these rocks requires thermal excursions to have taken place after periods of
cooling. They proposed a P–T path as shown in
Fig. 11b.
Further constraints as to the duration of these
greenschist events were provided by Baldwin and Lister
(1998). Their analysis suggested remarkably short thermal pulses were involved during garnet–biotite metamorphism on Ios, implying that the P–T–t paths
involved temperature spikes and near isobaric cooling
(Fig. 11c). Forster and Lister (1999b) applied the same
argument to constrain the nature of the P–T–t path
540
during high-pressure metamorphism. Older ages would
not survive unless younger growth events took place
during relatively short periods during which temperatures could be elevated (Fig. 11d).
In spite of this detail, the concept that the metamorphic history can be separated into several individual
and quite distinct episodes of metamorphic mineral
growth has not found wide acceptance. The available
P–T data at first sight suggest that the evolution of
metamorphic parageneses in the Cyclades is progressive
during a single excursion to high pressure, and then back
towards the surface. This is In part due to the fact that it
is difficult to infer fine detail in a P–T path, although the
type of information that can be obtained might be
considerably enhanced by examination of the evolution
of metamorphic parageneses in combination with
microstructural studies and in combination with focused
thermochronology. For example, Parra et al. (2002)
have demonstrated temperature spikes in the P–T evolution of metamorphic rocks on Tinos, and these are
compatible to the type of P–T path inferred by Forster
and Lister (1999b). The modelling approach advocated
by Lister and Raouzaios (1996) allows a reasonable
degree of predictive capability in respect to the shape of
the P–T–t curve, although the results are considered
controversial (Dunlap and Lister 1998). Nevertheless it
is this combination of detailed fabric and microstructural analysis in conjunction with geodynamic modelling, geochronology and metamorphic petrology that we
believe has the capacity to allow further progress.
Geodynamic consequences of our observations
Why were there several distinctive deformational and
metamorphic events in the Cycladic blueschist belt? To
answer this question we need to reassess mechanisms for
the formation and exhumation of these high pressure
rocks. The P–T paths illustrated above imply a single
prograde path for metamorphism, while the rocks are
taken to depth, and then various episodes of metamorphic mineral growth while the high-pressure belt is
progressive exhumed. A final step in the evolution of our
thinking in respect to the nature of P–T–t trajectories in
such complex rocks is to accept that individual tectonometamorphic slices have distinctive thermochronological histories, and then to consider the implications of
their juxtaposition.
Lister et al. (2001) propose that each high-pressure
episode reflects the influence of individual tectonic mode
switches. Each episode of metamorphism is followed by
a period of extension during which high-pressure rocks
are (partially) exhumed. Individual tectonic slices may
thus exhibit variable P–T–t paths, for example of the
form as shown in Fig. 11e. This shows a gradual increase in pressure during the period of crustal shortening
that takes place during the first stages of an accretion
event. A rapid increase in temperature may mark the
onset of gravitational collapse as the orogenic welt is
Fig. 11 Schematics of P–T–t paths inferred for the eclogite–
blueschist belt of the Cyclades: a isothermal decompression on a
smooth P–T loop, relevant to a model that is inapplicable except in
the case of orogens that first deform and then quietly erode;
b thermal excursions after periods of cooling, required by the
survival of old apparent ages in spite of later greenschist facies
overprints; c a short thermal pulse during greenschist metamorphism, based on constraints as to the duration of this event (at
specific structural levels) on Ios and Naxos; d a sequence of short
thermal pulses during high-pressure metamorphism, based on the
same principles; e a thermal pulse during a rapid metamorphic
mineral growth event, with rapid exhumation thereafter, based on
the notion that collapse of the orogenic welt takes place subsequent
to an accretion event; f a schematic version of the type of P–T–t
path that may have been applicable to individual tectonometamorphic slices in the Cyclades, with a sequence of four highpressure metamorphic mineral growth events and two subsequent
greenschist facies events
541
torn apart as the result of renewed roll-back of the
adjacent lithospheric slab. This may culminate in a
short-lived episode of metamorphic mineral growth,
followed by decompression as the result of the operation
of extensional shear zones that lead to partial exhumation of the tectonic slice in question.
This process may take place over and over again.
Since individual tectonic slices show the influence of
multiple periods of high-pressure metamorphism, more
complex P–T–t paths as shown in Fig. 11f are likely.
Figure 11f represents schematically the nature of the
P–T–t path that we believe may have been applicable to
many of the rocks in the central Cyclades.
Orogenic surges
How could such thermal pulses occur at such great
depths in the Earth? The time scales are so short that we
can rule out all other sources of heat except for effects
related to latent heat released during metamorphic
hydration reactions, and/or the conversion of mechanical work into heat during plastic flow of the rock mass.
We can also estimate the amount of heat involved. The
amount of heat must be relatively small, otherwise it is
difficult to explain rapid cooling. We can also estimate
how far distant might be the source of heat. The source
of heat must be local, otherwise again, the time involved
for its dissipation cannot be so short. Similarly, the heat
produced must be produced rapidly. If all three of these
conditions can be fulfilled, there is no physical difficulty
in respect to providing an explanation of the thermal
pulses that we infer, and for example all of these conditions can be fulfilled if motion in an extensional ductile
shear zone is relatively rapid (e.g. strain rates as high as
10 7 /s; Baldwin and Lister 1998), operating under
conditions of relatively high deviatoric stress (e.g. 50–
100 MPa; cf Harrison et al. 1997).
Lister et al. (2001) and Ring and Layer (2003)
suggest individual episodes of high-pressure metamorphism as recognized in the Aegean are the consequence
of the accretion of continental ribbons as the result of
convergence during the Alpine orogeny. Lister et al.
(2001) suggest that exhumation of high-pressure metamorphic rocks immediately subsequent to an episode of
high-pressure metamorphic mineral growth is a natural
consequence of renewed roll-back of an adjacent subduction zone immediately subsequent to the accretion
event. If the above interpretations are correct, this
re-analysis of the geo-chronological data provides
constraint as to the timing of individual accretion
events, and as to the duration of subsequent extensional tectonism.
One pattern of movement that is capable of generating the inferred pattern of movement in a thinly-sliced
tectono-stratigraphy is that illustrated in Fig. 12, based
on the orogenic surge model described by Lister et al.
(2001). Figure 12a, b shows the effect of continued
thrusting after a metamorphic event (M1A) with exhu-
mation accomplished by erosion in the upper plate of the
thrust. Finally (as shown in Fig. 12c) a new thrust
breaks, incising into the foreland. Driven by gravitational potential energy, the orogen collapses, and an
orogenic surge takes place (between Fig. 12c–e). The
tectonic mode switch (Fig. 12d) is marked by a metamorphic event (M1B). Rocks in the lower plate record
the effects of M1B, but retain relicts of M1A. The original
thrust reactivates as a detachment fault, allowing exhumation of these rocks while the orogen (in a state of
overall extension) surges towards the south (Fig. 12e).
Once the orogenic surge has commenced, motion on
the basal thrust must involve a greatly increased relative
velocity to compensate for extensional displacements in
its over riding plate. In the simplest of terms, consider
that the indenting microcontinent is moving northward
at a rate of 60km/Myears, and that the north-dipping
basal thrust is accommodating convergence at 50 km/
Myears. The missing 10 km/Myears must be accommodated by movement on thrusts higher in the sequence. For the moment ignoring the effects of erosion,
this means that the mountain front will be pushed
northward at a rate of 10km/Myears and that the
orogen will be in an overall state of horizontal shortening. This can all change once the over-riding plate has
been switched into overall extension, and the orogenic
front surges southward. Suppose that exhumation of
metamorphic rocks takes place on detachment faults
that accommodate a cumulated motion of 30km/
Myears (projected onto the horizontal plane) as the
mountain front collapses southward (at 30km/
Myears). We assume that northward motion of the
indenting microcontinent continues at a rate of 60km/
Myears. Therefore the basal thrust must begin to
accomplish motion at a rate of 90km/Myears. At the
time collapse initiates, the entire orogen must undergo a
tectonic mode switch (from overall shortening to overall
extension). At that time, thrust faults invert their motion
and become (or superimposed on by) extensional
detachments (Fig. 12c, d).
It is necessary to be careful about the frame of
reference used to describe processes in any one tectonic
slice. Misleading conclusions can be arrived at if a
frame of reference is established that solely considers
the motion of the leading wedge of rock, bounded
below by the basal thrust, and above by the first active
extensional detachment. In this limited perspective, the
relative motion of the leading wedge is consistent with
the notion that extrusion (or channel flow) is taking
place. This is not necessarily the correct geodynamic
interpretation, however. The relative motion of the
frontal wedge can just as well be related to the pattern
of movement to be expected during orogenic collapse,
with the pattern of movement solely the consequence
of the switch to overall extension in the over-riding
plate.
Does a significant role for erosional denudation alter
these conclusions? It may be that the pattern of erosion
is influenced by the climatic conditions that prevail, and
542
Fig. 12 A schematic model to show how tectonic inversion cycles could produce the pattern of structural, fabric, metamorphic and
geochronological variation as observed in the Cycladic eclogite–blueschist facies belt
543
by the impact of the migrating mountain front. Removal
of material by erosion from the mountain front may
mean that the collapsing orogen significantly advects
through the erosion surface defined by the southward
migrating front. In this way erosion may significantly
accelerate the relative movement of the frontal wedge.
Nevertheless the basic geometry is that of collapse and
this will be evident in the formation of an arcuate
mountain belt that progressively increases its curvature.
After the orogenic surge, the entire orogen may
once again undergo a tectonic mode switch (from
overall extensional back to overall shortening). A new
failure plane will form, once again incising into the
lower plate (Fig. 12f). Overall crustal shortening will
then lead to the accumulation of gravitational potential energy that once again will eventually drive the
orogen to collapse, and to surge southward on a yet
again newly created basal thrust. Again, earlier formed
thrusts in its upper plate will be reactivated as extensional detachments.
The geometry of each inversion cycle leads to a stepmigration southward of a newly created basal thrust
(Fig. 12c, f) and each new break occurs in sympathy
with inversion of motion on earlier formed thrusts that
then act as extensional detachments. The pattern of
inversion cycles can continue indefinitely, as indicated
in Fig. 12e–h. The orogen is subjected alternately to
overall shortening, then overall extension, but this all
takes place above a basal thrust that accommodates
continued convergence between the two opposing
‘plates’.
During an orogenic surge thermo-mechanical coupling leads to increased temperatures, focused in the
ductile regime where pervasive deformation is taking
place. This ‘wave’ of heat leads to metamorphic mineral
growth, changing rock rheology, initiating the formation
of extensional shear zones. These extensional shear
zones focus strain, mechanically soften the rock mass,
reduce the rate of heat production, and produce a
geometry conducive to rapid cooling of individual tectonic slices. Thrusts that reactivate as detachment faults
exhume newly metamorphosed rock in their footwalls,
while at the same time the footwalls retain relicts of
earlier metamorphic events.
The overall pattern of motion results in a thinlysliced tectono-metamorphic stratigraphy, defined by
tectonic slices with distinctive tectono-thermal histories.
The more youthful slices have the capability of
recording the effects of all preceding events, although
these will be retained only as increasingly modified
relicts. The older metamorphic events will be best
preserved in tectonic slices at higher structural levels.
The model thus provides a description of a possible
movement history that is capable of producing the
observed geometries, and the observed distribution of
metamorphic rocks. More importantly the model
makes it possible to make specific predictions as to the
tectono-thermal evolution of individual tectonic slices,
and this work is in progress.
Conclusions
Fieldwork on Ios, Sifnos, Syros, and Tinos has shown
that the Cycladic eclogite–blueschist belt has been dissected into several relatively thin tectono-metamorphic
slices. These have been examined in detail, leading to an
understanding of the evolution of their fabrics and micro-structures. A sequence of distinct metamorphic
mineral growth events has been documented, with each
tectonic slice characterized by the influence of these
events. The method of asymptotes and limits has then
been applied to reinterpret 40Ar/39Ar apparent age
spectra that have been measured from these rocks. These
geochronological data represent the effects of highpressure low-temperature metamorphism in several of
these tectonic slices.
The following conclusions have been reached as the
result of the re-analysis of geo-chronological data: (1)
the M1B eclogite-facies metamorphic mineral growth
event and subsequent deformation and recrystallization
(during exhumation?) is estimated to have occurred from
53–49 Ma; (2) the M1C blueschist-facies metamorphic
mineral growth event and subsequent deformation and
recrystallization (again during exhumation?) is estimated
to have occurred from 44 to 38 Ma; (3) the M1D transitional blueschist-facies metamorphic mineral growth
event and subsequent deformation and recrystallization
is estimated to have occurred from 35 to 30 Ma; (4)
the first of the greenschist metamorphic events is estimated to have occurred at some time between 22 and
19 Ma.
Overall, the conclusion is that data obtained using
40
Ar/39Ar geochronology in high-pressure rocks should
not be too readily dismissed as due to the effects of
‘excess argon’. The analysis of fabrics and microstructures may reveal a more complex evolution than has
hitherto been recognized.
A tectonic model has been proposed that explains
the geometry of a thinly sliced tectono-metamorphic
stratigraphy, as observed. This model has the capacity
to explain why individual tectonic slices in this ‘tectono-metamorphic stratigraphy’ should display distinctive patterns of fabrics and micro-structures, as
well as characteristic temperature–time curves as inferred by 40Ar/39Ar geochronology. Individual metamorphic mineral growth events may assisted by
deformation enhanced dilatancy driving fluid flow, but
the temperatures reached during each event may be
the result of transient thermal pulses caused by the
conversion of mechanical work to heat during orogenic surges.
The P–T–t path of the tectono-metamorphic slices in
the Aegean Cycladic blueschist belt cannot be represented by a simple burial and exhumation loop with a
single metamorphic peak, but by P–T paths that include
pressure-variation (including pressure increases) related
to individual deformation events, reflecting the complex
structural evolution of the region.
544
Acknowledgements This research was supported by an Australian
Research Council Discovery Grant. We gratefully acknowledge the
support and help of Dr. Vaios Avdis, IGME, Athens. 40Ar/39Ar
data from Sifnos were measured by A. Raouzaios, D. Foster and S.
Szczepanski using samples collected by G. Lister. Dimitrios and
Maria Koukoletsos on Sifnos, and Vangelis Thaskalakis, Gina
Gana, Kirakios and Ourania Batsalis on Ios, are thanked for their
logistic and moral support. Al Hoffman is thanked for his guidance
and careful review of this manuscript.
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