Thermochronology of the South Cyclades Shear Zone, los, Greece' Effects of ductile shear in the argon

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. B4, PAGES , APRIL 10, 1998 Thermochronology of the South Cyclades Shear Zone, los, Greece' Effects of ductile shear in the argon partial retention zone Suzanne L. Baldwin Geosciences Department, University of Arizona, Tucson Gordon S. Lister Australian Crustal Research Centre, Monash University, Melbourne, Australia Abstract. Micas and potassium feldspars from the South Cyclades Shear Zone, los, Cyclades, Greece, yield varied and complex 4øAr/39Ar apparent age spectra. A correlation exists between 4øAr/39Ar apparent ages and the relative timing of different episodes ofrecrystallization and grain growth, as indicated by fabric and microstructural analysis. The 4øAr/39Ar apparent age spectra record the effects of variation in the degree of recrystallization and grain growth, and partial to complete resetting of argon systematics in potassium-bearing minerals during Hercynian (Mo), Alpine (M1) and late Oligocene - early Miocene (M2) metamorphism. Deformation was strongly partitioned within the shear zone and this led to localized recrystallization and heterogeneous resetting of argon systematics within preexisting minerals. Modeling suggests the Oligo-Miocene thermal events were of insufficient magnitude and/or duration to completely reset the isotopic systematics in these samples. Our data lead to the concept of the argon partial retention and resetting zone (PRZ), defined as that portion of the crust where temperatures are insufficiento completely reset argon systematics within preexisting potassium-bearing minerals. Within the PRZ, some radiogenic argon in preexisting potassiumbearing minerals will be outgassed and only partially retained. Tectonic exhumation of the PRZ involves movement on crustal-scale ductile shear zones, accompanied by strongly partitioned deformation and localized recrystallization. Recrystallization leads to resetting of argon systematics, and thus will result in heterogeneous 4øAr/39Ar age distributions within these ductile shear zones (e.g., in the South Cyclades Shear Zone). 1. Introduction In the Aegean region, Eocene collision was followed by extension which began by at least the early Miocene and resulted in the formation of crustal-scale shear zones and metamorphi core complexes [Lister et al., 1984; Faure and Bonneau, 1988; Gautier et al., 1993; Gautier and Brun, 1994; Jolivet et al., 1994]. At least four metamorphic events have been recognized in the Central Cyclades, Greece, where the Aegean core complexes are well exposed (Figure 1). These events have been documented in pre-alpine amphibolite facies gneissic basement (Mo), Eocene blueschist and eclogite facies metamorphic rocks (M ), Oligo-Miocene greenschist and amphibolite facies metamorphic rocks (M2), and contact metamorphic rocks associated with the intrusion of granitoids (M3) in mid-late Miocene time [Altherr et al., 1982; Henjes- Kunst and Kreuzer, 1982; van der Maar and Jansen, 1983; Andriessen et al., 1987]. P-T-t paths for rocks from the Aegean metamorphicore complexes indicate that the region was characterized by low Copyright 1998 by the American Geophysical Union. Paper number 97JB /98/97JB geothermal gradients during Eocene convergence and by significantly higher geothermal gradients during Oligo- Miocene backarc extension [Jansen and Schuiling, 1976; van der Maar and Jansen, 1983; Wijbrans and McDougall, 1986, 1988; Baldwin, 1996]. This has been related to rollback of the subducting slab [Lister et al., 1984]. The increase in geothermal gradients in the Tertiary resulted from localized advective heat transfer during magmatism and metamorphism [e.g.,altherr et al., 1982; Griitter, 1993] and led to variable recrystallization, and incomplete resetting of argon [e.g., Wijbrans and McDougall, 1986, 1988; Baldwin, 1996], zircon U-Th-Pb [e.g., Henjes-Kunst et al., 1988], and Rb-Sr systematics [Altherr et al., 1982] within this polymetamorphosed region. We present here results of a detailed microstructural and 4øAr/39Ar thermochronological study from the lowermost structural levels of the South Cyclades Shear Zone on Ios. This major top-to-the-south crustal shear zone was the first recognized of a number of shear zones associated with Miocene extension of the Aegean crust [Lister et al., 1984; Vandenberg and Lister, 1996; Lister and Forster, 1996]. Heterogeneous and complex 4øAr/39Ar apparent age spectra were obtained for adjacent samples from within the shear zone. However, these data can be interpreted in the context of regional fabric and microstructural analysis. Results are comparable with

2 7316 BALDWIN & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE Syros Mykonos Kythnos... Serifos AEGEAN SEA Paros axos M0 Pre-Alpine basement M1 Blueschists &Eclogites M2 Greenschists [ Amphibolites M3 Contact Granodiorite aureole Pliocene- Recent Volcanism ['--[ Unmetamorphosed allochthonous units Antiparos 0 3o I I I I <: c km o Thera 25 ø I Figure 1. Simplified geology of the Attic-Cycladic metamorphic belt in the Aegean Sea, Greece [after Altherr et al., 1982; van der Maar and Jansen, 1983]. The Aegean metamorphicore complexes formed as the result of north-south stretching of the Aegean continental crust and are exposed on Naxos, Ios, Sikinos, Mykonos, and Paros. Diagonal patterns indicate blueschist, eclogite, and greenschist facies metamorphic rocks. Crosshatched pattern indicates M blueschists that have been partially overprinted by M 2 greenschist facies assemblages. geochronologic data for rocks found throughout the Attic- Cycladic metamorphic belt and provide additional constraints on the timing of metamorphic and thermal events which have Detailed mapping of the lower plate of the Ios metamorphic core complex has led to the recognition of five generations of penetrative ductile structures (S -Ss)and three main phases of affected the lower plate of the Ios metamorphicore complex. deformation (D2-D4) [Vandenberg and Lister, 1996]. Deformation occurred during and after at least three separate metamorphic mineral growth events, including Hercynian 2. General Geology of Ios amphibolite facies metamorphism (M0) [Henjes-Kunst and The island of Ios comprises one of the metamorphicore complexes in the central Aegean that formed as the result of continental crustal extension during Miocene time [Lister et al., 1984;Vandenberg and Lister, 1996; Lister and Forster, Kreuzer, 1982; Andriesson et al., 1987], Ma Alpine (high P/T; M )metamorphism [van der Maar and Jansen, 1983; Andriesson et al., 1987], and greenschist facies (M2) metamorphism at-25 Ma [Kreuzer et al., 1978; van der Maar 1996]. The lower plate of the Ios metamorphicore complex and Jansen, 1983]. A whole rock-phengite Rb/Sr age of 13 Ma consists of strongly deformed granitic gneisses structurally for meta-aplite dikes [Henjes-Kunst and Kreuzer, 1982] that overlain by amphibolite facies garnet-mica schists. Granites apparently intruded the garnet-mica schists during the Hercynian orogeny in late Paleozoic time [van der Maar and Jansen, 1983], but the contact relations have been obscured intrude basement rocks and are deformed in D 4 shear zones [Vandenberg and Lister, 1996] was the only evidence, prior to this study, that a mid-miocene contact metamorphism (M3) may have affected the basement rocks on Ios. This may date one by intense deformation during and subsequento the Alpine orogeny. Upper plate rocks on Ios consist of marbles and schists that were metamorphosed under peak high P-T conditions of 1.26 GPa and 475øC (M [van der Maar and Jansen, 1983; Griitter, 1993]). Blueschist nappes were thrust over the gneisses and schists during the Alpine orogeny. Subsequently, the lower plate rocks were tectonically exhumed from beneath the nappes during Oligo-Miocene extension [Vandenberg and Lister, 1996]. of the D4 shear zones at -13 Ma if Rb-Sr systematics were reset by fluid activity and metamorphic recrystallization at this time. A domed-up shear zone (i.e., the South Cyclades Shear Zone [Lister et al., 1984; Vandenberg and Lister, 1996]) forms the carapace to the lower plate of the Ios metamorphic core complex. Garnet-mica schists occur in the highest structural levels of the lower plate exposed on Ios. They are intruded by Hercynian granitoids which now define a complex of augen gneisses and relict granitoid "augen" enclosed within

3 BALDWIN & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE 7317 anastomosing ductile shear zones. Four of the five generations of ductile structures (S -S4) have been documented at the contact between the augen gneiss core and the structurally overlying garnet mica schists within the South Cyclades Shear Zone where this study was undertaken (Figure 2). 3. Sample Descriptions The lower structural levels of the South Cyclades Shear Zone, including the lower part of the garnet-mica schist unit and the uppermost part of the mylonitized augen gneiss unit structurally below it, were analyzed in this study (Figure 2). Thin sections were examined, and detailed fabric and microstructural observations were made for each sample in order to aid in the interpretation of 4øAr/39Ar results. 4øAr/39Ar analytical results are presented in Table I and procedures are listed in the appendix. Electron probe analyses were performed on polished grain mounts in order to determine mineral compositions (Table 2) Garnet-Mica Schist Unit The texturally oldest mineral assemblage in sample consists of blue-green amphibole+gamet+biotite, interpreted to have formed during the Hercynian amphibolite facies metamorphism (i.e., M 0 [Vandenberg and Lister, 1996]). Younger fabrics anastomose around the M0 garnets, including D 4 shear bands (Plate l a) in which garnet and biotite are retrogressed to chlorite. S inclusion trails are preserved within the garnet porphyroclast cores. Minor amounts of reddish brown M0 biotite are also present in microlithons, but texturally younger biotite occurs in D 4 pressure shadows around the M0 garnets. The texturally older biotites are in general oriented such that the (001) plane is roughly parallel to the S inclusion trails within M0gamets, and these trails are at a high angle to the (younger) dominant S2 foliation. They are interpreted to be part of a relict (Hercynian) S fabric. As is the case for the biotites in the coarse granitoid (sample 89640, see below), the texturally old biotites are commonly overgrown and/or replaced by phengitic white mica which is oriented parallel to S2 (Plate l a). The white mica contains minor amounts of sphene and epidote, and in some grains ubiquitous rutile needles are present. Presumably, these phases derived some of their Fe-Mg-Ti content from the breakdown of preexisting biotite during Eocene high P-T metamorphism (M ). However, white mica exhibits sharp extinction and has generally well-preserved (001) impingement boundaries as the result of decussate intergrowth. Quartz aggregates in the S2 cleavage have Shear Zone. During D4 the garnet-mica schist was subjected to intense plastic strain, and mylonitic microstructures developed locally. Texturally young biotite grew during this event in microdilation sites caused by the fracturing and pulling apart of garnet porphyroclasts. There is also some microstructural evidence for new growth of garnet, but this may have predated D 4. In places, texturally young biotite has replaced older biotite that has been distorted, pulled apart, kinked, and microfolded during D 4. Later D 4 shear bands [Vandenberg and Lister, 1996] cause local zones of dynamic recrystallization and grain size reduction in quartz aggregates and shred, split, or tear the decussate white micas. White micas in these zones recrystallized as the result of in situ grain boundary migration. This later deformation also leads to the development of mica fish in zones of dynamically recrystallized quartz and to the formation of type II S-C mylonites [Lister and Snoke, 1984] Pegmatite Dikes in the Garnet-Mica Schists Pegmatite dikes crosscut garnet-mica schists and were folded and sheared during Alpine deformation [Vandenberg and Lister, 1996]. Quartz grains are deformed with highly irregular shapes and exhibit undulose extinction. The smallest recrystallized quartz grains (< gm) occur within localized higher strain zones. In contrast, muscovite and potassium feldspar are largely undeformed. This suggests that ductile deformation was partitioned into narrow zones and resulted in partial preservation of relatively unaltered igneous textures (90351, Plate lb). Zones of recrystallized decussate white mica occur parallel to S 2, suggesting recrystallization in these localized high strain zones occurred after D Mylonitic Augen Gneiss Four samples of mylonitic augen gneiss (90354, 88607, 90368, 88609) were studied (Figure 2 and Plates lc-lf). A well-developed gneissic foliation defines S2. The augen gneiss is also crosscut by anastomosing (D4) shear zones in which S2 stretched, disrupted, and locally folded (e.g., Plate l d). Spectacular S-C mylonites are particularly well developed where S2 is transected by D 4 shear bands. The mylonitic augen gneiss samples were variably affected by D 4 deformation. In some samples (90368 and 88607) the effects of D 4 deformation were minimal, although local microscale D 4 ductile shear zones are present. The phengitic white mica in zones outside these areas exhibit decussate intergrowth, and (001) impingement textures are prevalent. Phengitic white mica within these mylonitic augen gneiss samples may have recrystallized subsequent to D2, but because low-angle (001) impingement microstructures dominate, the developed foam textures [Vernon, 1975; Drury and Urai, amount of neocrystallized white mica is minimal. In contrast, 1990] with most grain boundaries intersecting at-120 ø. potassium feldspars were brittlely deformed and/or Decussate fabrics in mica aggregates and foam textures in quartz aggregate suggests that elevated temperatures during recrystallized. Quartz microstructures are dominated by the effects of crystal plasticity and dynamic recrystallization [Urai M outlasted D 2 deformation or that a separate recrystallization et al., 1990], and locally well developed oblique foliations are event occurred after D 2 ceased. Locally, these textures have been statically overgrown by albite, subsequent to the period preserved [Lister and Snoke, 1984]. Phengitic white mica in sample shows textures dominated by the effects of highof decussate recrystallization of the phengitic white mica. This angle decussate recrystallization subsequent to D 2 overgrowth may have occurred during a subsequent period of (greenschist facies) recrystallization (i.e., M 2 of van der Maar and Jansen [1976]). The most recent deformation (D4) in the garnet-mica schist unit was associated with formation of the South Cyclades deformation (Plate l c). Although decussate recrystallization has been recognized in all the samples examined, its effects are particularly marked in sample Recrystallization of white mica also occurred during, or after, D 4 deformation. Phengitic white mica in the S 2 cleavage

4 BALDWIN & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE 100: c"' 4o- 30- Q- 2oi <: J' OO o lso. <: 2o phengite 4- paragonite %' Cumulative Release of 39Ar(%) muscovite c.- 9o i Cumulative Release of 39Ar(%) ] ) Q... o phengite K-feldspar Cumulative Release of 39Ar(%) p e _.,.a- 50-3o 20 0 K-feldspar o Cumulative Release of 39Ar(%) km ' '400- : 350.., v (1) 3oo )250 biotite t, (1) so N [- : Garnet-mica schists Mylonitic augen gneiss S2 gneissic foliation S 3 axial planar foliation Q.loo < 5o S 4 S-C mylonites intrusive contact- deformed Cumulative Release of 39Ar(%) 88609, v 2o,,._, ß,.-, o-lo o K-feldspar Cumulative Release of 39Ar(%) : phengite o-'. o 0'"ii3" b"3b"4 "g0"&f"76"e /"' 0'i Cumulative Release of 39Ar(%) Figure 2. Map of the lowermost structural levels of the South Cyclades Shear Zone [Location map modified from Vandenberg and Lister, 1996] showing sample locations and 4øAr/39Ar age spectra. Inset shows location of study area. Mo muscovite (90351) and Mo biotite (89640) gave pre-alpine 4øAr/39Ar apparent ages. M 1 phengitic white mica (90350, 88607, 90368) yielded Paleocene-Eocene (i.e., Alpine) 4øAr/39Ar apparent ages. M2(?) phengitic white mica (90354 and 88609) yielded Oligocene and early Miocene ages. Potassium feldspars (90354, 88607, 90368) yielded Eocene (Mo) to middle Miocene (M3)4øAr/39Ar apparent ages

5 BALDWIN & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE 7319

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8 7322 BALDWIN & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE

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10 ß 7324 ß BALDWIN & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE Table 2. Representative Microprobe Analyses of Micas Used for 4øAr/39Ar Analyses Muscovite Paragonite Phengite Phengite Phengite Phengite Phengite Biotite SiO ! TiO A Cr < < na na na MgO ! FeO* /2.82 MnO 0.01 <0.01 <0.01 <0.01 < !5 CaO < <0.01 < BaO 0.02 < Na K F < CI <0.01 < Total Si AI (IV) A1 (VI) Ti Cr na na na Mg Fe c K Na Ba Ca F Cl (OH) !.822 All white mica data normalized to six cations and iron assumed to be Fe 2+. Biotite normalized to 22 cations and Fe2+: Fe 3+ assumed to have a ratio of 9:1. OH = 2-F-C1. Samples with low totals may be due to small grain size and morphology causing analysis to include underlying epoxy. na, not available. microlithons is decussate in zones adjacento large potassium zones of gently varying undulose extinction. Biotite, where feldspar augen. The S2 cleavage was subsequently ductilely found adjacent to quartz grain boundaries, appears relatively deformed in adjacent microscale shear zones, where this unaltered, although it may exhibit either fine decussate texture is substantially modified. In contrast to microcrenulations or tight kinks as the result of later surface dominated microstructures in the decussate (001) deformation (Plate lg). This texturally old biotite was impingement boundaries, recrystallization of white mica within these zones involves sutured grain boundaries, the mica exhibits undulos extinction, and deformation has locally led to shredding and splitting of the mica grains along the cleavage Granite Within Mylonitic Augen Gneiss progressively modified during later tectonic events. The first alteration began with the intergrowth and replacement of biotite by phengitic white mica, with (001) of the new grown mineral subparallel to (001) of the biotite host. This intergrowth and/or replacement was accompanied by the growth of sphene and epidote, as the Ti-rich biotite was reduced in its Fe-Mg component. Based on correlation with similar microstructures within augen gneiss outside the boudin, this period of metamorphic mineral growth took place A relatively undeformed granite, within the shear zone augen gneisses and structurally below the contact with the garnet mica schists, was also studied. It was previously mapped as a Miocene stock (i.e., ¾4 [van der Maar and dansen, 1983]), but subsequent work [Vandenberg and Lister, 1996; Lister and Forster, 1996] has shown that it is actually a granitic boudin within the augen gneiss. In thin section this granite (89640) is slightly deformed, but substantially altered. Most of the plagioclase feldspar has been partially replaced by felted white mica intergrowths (10-50 tm in size). Potassium feldspar exhibits microcline twinning where it has been deformed against adjacent mineral grains. Quartz is only slightly deformed and exhibits broad during M. The final stage of microstructural development was marked by the physical tearing apart of the kinked biotite along fractures and/or alteration zones. In many cases these kinks served as the locus of subsequent alteration and reaction fronts can be discerned where fine grained masses of garnet (-1-5!.tm) rim the older biotite (Plate l h). Such textures are typically found where biotite has been altered adjacent to a plagioclase grain. The older biotite grains have been progressively overgrown by interlocking, decussate medium-grained white mica (up to !.tm)and very fined grained, felted, and/or decussate intergrowth of

11 BALDWIN & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE 7325 white mica (<5 gm). There has been limited grain boundary migration in the biotite during this event, but recrystallization fronts can be discerned, involving the migration of kink band boundaries over distances of 5-50 gm. 4. The 4øAr/S9Ar Results and Interpretations 4.1. Garnet-Mica Schist Unit White mica (90350; paragonite and phengitic muscovite; Table 2 and Figure 2) yielded 4øAr/39Ar apparent ages ranging from Ma to Ma. Together with the microstructural and compositional data, the 4øAr/3øAr apparent ages suggest that the growth and recrystallization of white mica occurred during M. Decussate fabrics and micas with sharp extinction and (001) dominated grain boundaries imply that recrystallization either continued after D 2 terminated or that D 2 was followed by a static recrystallization event. The range of apparent ages obtained is likely the result of the degassing of M micas and those that were subsequently partially recrystallized to produce decussate micas during or after D Pegmatite in Garnet-Mica Schist Results of an 4øAr/3øAr step heating experiment on muscovite (90351) from a pegmatite that crosscuts the garnet mica schist unit yielded a complex age spectrum with apparent ages ranging from Ma to Ma (Table 2 and Figure 2). In thin section the muscovite is essentially undeformed (Plate lb), so apparent age spectra may reflect the age of post-crystallization cooling, with the effects of partial loss during later thermal events superimposed. If this interpretation is correct, the pegmatites may have intruded subsequent to the Hercynian orogeny, possibly during the Early Triassic. Alpine and later metamorphism (M1-M3) and deformation apparently did not result in recrystallization of the muscovite to phengitic compositions and/or resetting of the argon systematics in the muscovite [cf. Monid, 1990]. Isochron analysis does not aid in the interpretation of results as the first three steps define a linear trend with an atmospheric 4øAr/36Ar composition and the remaining data ( % radiogenic) plot along the x axis, suggesting that progressively older and more retentive domains were outgassed with increasing temperature during the course of the step heating experiment Mylonitic Augen Gneiss next -40% of the gas released corresponds to apparent ages of -14 Ma, while the remaining 50% of 3øAr released yielded progressively increasing apparent ages ranging from 14 to 54 Ma. The oldest potassium feldspar apparent ages are similar to results from coexisting phengitic white mica. These results are interpreted to reflect argon loss from pre-alpine (i.e., deformed Hercynian orthogneiss) potassium feldspar which was reset and recrystallized during M and subsequently partially reset during Miocene thermal event(s) (see discussion below). The form of the age spectrum for sample 90368, with a steep age gradient associated with 60-80% of the gas released, is similar to those obtained for potassium feldspars which contain excess argon within large diffusion domains [Foster et al., 1990]. Foster et al. [1990] describe spectra which are characterized by little or no excess argon in low-temperature steps, sensible apparent ages over the first % 3øAr released, followed by an abrupt increase to an age exceeding the permissable age of the sample. The highest-temperature steps are characterized by either a gradual decrease in age or an anomalously old plateau segment. Although the possibility of excess argon trapped in large diffusion domains cannot be ruled out, the oldest apparent ages obtained for feldspars analyzed in this study are not geologically implausible. The form of these feldspar spectra can also be compared to theorectical age spectra obtained for samples that have experienced >95% loss of argon due to a thermal event assuming aggregates containing a lognormal distribution of spheres [cf. Turner, 1968]. Results for another augen gneiss sample (88607) are similar to those described for Apparent ages for phengitic white mica in increase from -41 to 63 Ma over the first -40% of the gas released. Thereafter, apparent ages decrease to -59 Ma and increase again to 70 Ma. The apparent age spectrum obtained for coexisting potassium feldspar yielded a saddle-shaped spectra with apparent ages progressively decreasing to-14 Ma at-23% of the gas released and subsequently rising to-48 Ma. Potassium feldspar from mylonitic gneiss (Figure 2) also yielded pronounced age gradients similar to results for potassium feldspars from and Minimum apparent ages of- 15 Ma are interpreted to result from partial outgassing due to the effects of a mid-miocene thermal event. Phengitic white mica from this sample is dominated by the effects of decussate recrystallization subsequento the D 2 deformation. Although these textures have been recognized in all of the other samples examined, the decussate white micas are particularly obvious in this sample (Plate l c), and they likely dominated the mineral separate used for 4øAr/3øAr analysis. In Similar age spectra were obtained for phengitic white mica and potassium feldspar separates from two mylonitic augen gneiss samples (90368 and 88607; Figure 2). Phengitic white mica from sample yielded a complex spectrum, with the first 40% of the gas released yielding progressively increasing apparent ages ranging from Ma to Ma. The higher-temperature portion of the age spectrum is characterized by apparent ages that decrease to Ma then increase again to Ma. A detailed step heating experiment was performed on potassium feldspar from sample to determine diffusion domain parameters (Figures 2 and 3). In contrast to results this case it is possible that the 4øAr/3øAr apparent ages of Ma for sample record the age of post-d 2 recrystallization, subsequent to the M 1 event. Another phengitic white mica (88609, Figure 2)came from mylonitic augen gneiss sampled a few meters from 90368, and this yielded apparent ages ranging from 19.0 to 21.5 Ma, with a corresponding total fusion age of Ma. Microstructures are dominated by the effects of D 4 mylonitization, with recrystallization of phengitic white mica apparently occurring during or after deformation associated with the D 4 event (Plate l d). We thus infer that some D 4 shear obtained for phengitic white mica from the same sample, zones were active at-20 Ma. Note that Lister and Forster potassium feldspar gave progressively decreasing apparent ages corresponding to the first 7% of the gas released. The [1996] record evidence for two generations of ductile shear zones in the Ios "basement", and these samples come from the

12 7326 BALDWIN & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE 'a ph c.%, kspar'. 'bio gt 'bi

13 BALDWIN & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE 7327 older (and more pervasively developed) South Cyclades Shear Zone. In summary, age spectra for phengitic white mica from mylonitic augen gneisses are complex but in general give Alpine M apparent ages (i.e., Ma; samples and 88607) and possibly apparent ages (e.g., sample 88609) related to M 2 at-- 20 Ma. While the oldest apparent ages for potassium feldspar are interpreted as Alpine ages, the low temperature portion of the feldspar spectra indicates significant argon loss occurred due to a later Miocene thermal event Granite Within Mylonitic Augen Gneiss The 4øAr/39Ar apparent ages ranging from 355 to 490 Ma were obtained from strongly kinked unrecrystallized biotites from within the relatively undeformed core of a granitoid boudin (sample 89640; Table I and Figure 2 and Plate l g). The biotite may contain excess argon which has been incorporated from the pore fluid of the rock mass into high diffusivity pathways in the biotite [e.g., Lee, 1995]. Nevertheless, subsequent metamorphic events (M, M2, and M3) were insufficient to cause complete resetting of the argon systematics in biotite from the granite boudin, in spite of the fact that peak temperatures inferred for M and M 2 from mineral assemblages in blueschists from the upper plate on Ios (>400øC [Griitter, 1993]) exceed the "blocking temperature" for argon diffusion in biotite [Lister and Baldwin, 1996] Additional Considerations In summary, samples from the lowermost structural levels of the South Cyclades Shear Zone yielded 4øAr/39Ar apparent ages which vary considerably over relatively small distances (< m). One might be tempted to dismiss this heterogeneous age distribution as being geologically meaningless and perhaps the result of the incorporation of excess argon. However, despite their complexity, the results obtained are consistent with regional trends of isotopic results and with the detailed structural history of the shear zone. Inverse isochron analysis [Heizler and Harrison, 1988] does not aid in the interpretation of results for the majority of samples as the data scatters and/or plots very close to the x axis. In cases where data define a linear trend (e.g., phengitic white mica. from samples and 88609), the composition of the nonradiogenic trapped argon component is atmospheric. The minerals crystallized in a shear zone which may have acted as a conduit for the flushing of argon toward the surface, so perhaps it is possible for the minerals to incorporate excess argon from fluids as they crystallized [e.g., Scaillet, 1996]. However, we deem it unlikely, for example, that the phengitic white mica has systematically incorporated excess argon to produce Alpine (M ) apparent ages. Because sized multigrain splits were used in the step heating experiments, we can not rule out the possibility that different generations of potassium-bearing phases were preferentially analyzed. For example, if a sample contains multiple generations of different sized white micas, results could vary depending on the size split used in the experiment. In other words, the complex argon systematics may reflect outgassing of a mixed population of white micas with each population having undergone a distinct but different thermal history. Further work (e.g., laser analysis) is needed to document the possible presence of age variations among different generations of white micas. Given the correlations between textural, compositional, and age relationships for the samples analyzed, we propose that the variation in 4øAr/39Ar apparent ages from within the South Cyclades Shear Zone is the result of volume diffusion, as well as mineral growth, in response to a complex thermal history. The degree to which argon systematics in the shear zone are reset depends on the mechanism of argon loss (e.g., volume diffusion versus recrystallization). This in turn is dependent on a number of variables including protolith composition, grain size, permeability, fluid flow, P-T conditions, and strain rate [cf. Goodwin and Renne, 1991]. In section 4.6 to 4.9, we further discuss the possibility that the heterogeneous 4øAr/39Ar data set obtained from the South Cyclades Shear Zone is the result of a polymetamorphic history Preservation of M 0 Ages Biotite is from a- 5 m ovoid granitic body that is transitional into mylonitic augen gneisses [see Vandenberg and Lister, 1996, Figure 10] within the shear zone. Given this structural evidence and the textural relationships discussed above, we interpret the biotite apparent ages from this granitic augen as recording an earlier M0, pre-alpine history. Previously reported geochronologic results on orthogneisses and paragneisses from Ios range from 300 to 500 Ma [Andriesson et al., 1987; Henjes-Kunst and Kreuzer, 1982; van der Maar and Jansen, 1983]. U-Pb zircon ion microprobe Plate 1. (Opposite) Photomicrographs of samples used for 4øAr/39Ar analyses showing minerals and microstructures relevanto interpretation of results. (a) Sample garnet-mica schist showing M 0 garnet (gt) porphyroblast. M phengitic white mica (ph) and paragonite is oriented parallel to S2; 3.6 mmwide. (b) Sample muscovite (musc) in pegmatite; 5 mm wide. (c) Sample augen gneiss showing recrystallized -32 Ma phengitic white mica (ph) with decussate structure; I mm wide. (d) Sample augen gneiss with prominent D 4 shear bands and albite (ab) porphyroblasts. Matrix foliation defined by phengitic white mica (ph) in cleavage septae separating quartz and phengitic white mica (-20 Ma) in the microlithons; 2.5 mm wide. (e) Sample augen gneiss showing potasssium-feldspar augen, dynamically recrystallized quartz, and M phengitic white mica; 12.0 mmwide. (f)close up of sample potassium feldspar augen showing microcline twinning; 1.0 mm wide. (g) Sample granite boudin within augen gneiss showing kinked and partially altered biotite (bio), adjacento quartz and altered plagioclase; 2.5 mm wide. (h) Texturally young garnet (gt) porphyroblasts which has overgrown rim of an M0 biotite. The garnet porphyroblasts are in contact with M (?) white mica and recrystallized biotite in a kink band; mm wide.

14 7328 BALDWIN & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE 4.7. Preservation of M Ages Compared to Late Permian to Early Cretaceous muscovites, 15 white micas analyzed from within the lower structural levels of the South Cyclades Shear Zone gave Alpine ages (90350, 90368, 88607) and are phengitic with Si contents of 3.34, 3.37, and 3.45 (pfu, Table 2). Corresponding pressure 10- estimates using Massone and Shreyer's [1987] barometer are GPa assuming temperatures of 350øC. Preservation of M apparent phengitic white mica ages, and lack of complete 3.3 resetting during subsequent metamorphic and deformation events, has implications for the thermal history of the shear zone as discussed in section These results are similar to K-Ar and 4øAr/39Ar ages 3.1 obtained for phengitic white mica from Eocene high P-T I metamorphic rocks on Naxos [Wijbrans and McDougall, ], Syros [Maluski et al., 1987; Baldwin, 1996], and Ios [Griitter, 1993; Baldwin, 1996]. In addition, apparent ages T (øc) associated with the high-temperature portions of potassium feldspar spectra (88607, 90354, 90368) may be interpreted as Figure 3. Comparison of muscovites with late Permian to early M apparent ages. Although the 4øAr/39Ar results indicate that Cretaceous K-Ar and 4øAr/39Ar ages (light gray field) to some potassium-bearing minerals within the South Cyclades phengitic white mica with Paleocene to early Miocene K-Ar Shear Zone have been reset during Alpine metamorphism and and 4øAr/39Ar ages dark gray field) from Ios. Data compiled accompanying deformation, with the exception of the high Si TM from Henjes-Kunst [1980], van der Maar [1981], Henjes- in phengitic white mica, there is little petrologic evidence for Kunst and Kreuzer [1982], Andriesson et al. [1987], Grfitter M mineral assemblages present within the lower plate [1993], Baldwin [1996], and this work. Si isopleths for basement rocks of Ios [Henjes-Kunst, 1980; van der Maar, phengite Si contents (pfu)for the limiting assemblage with 1981; Griitter, 1993]. potassium feldspar, quartz, and phlogopite after Massone and Shreyer [1987] Apparent Ages Related to M2(? ) Phengitic white mica from sample yielded a flat age spectrum corresponding to 4øAr/39Ar apparent ages of Ma which may reflect complete resetting due to post-d2 decussate recrystallization. In contrast, phengitic white mica studies in progress also confirm the presence of Hercynian from sample yielded a flat age spectrum corresponding ages for basement orthogneisses from Ios (S. Keay, personal to 4øAr/39Ar apparent age of Ma; we interpret this communication, 1996). age to be the result of recrystallization during deformation Electron probe results of the mineral separate used in the which completely reset the argon systematics within this step heating experiment for the pegmatite within the garnet sample. Pressure estimates [Massone and Shreyer, 1987] are mica schist unit indicate a homogeneous muscovite -0.7 and 1.0 GPa assuming temperatures of 350øC for composition (90351, Table 2). No visible impurities within phengitic white mica from samples and 90354, the muscovite separate were found using backscatter electron respectively. imaging. In thin section, igneous textures are preserved (Plate lb). Thus muscovite 4øAr/39Ar apparent ages may record a pre Apparent Ages Related to Ms Alpine history including crystallization in Permo-Triassic time. If the muscovite partially retained radiogenic argon since it crystallized, the kinetics of argon diffusion were insufficient to reset argon systematics during subsequent M -M3 events. This was the only muscovite sample analyzed from the South Cydlades Shear Zone, and 4øAr/S9Aresults gave significantly older apparent ages than the Eocene-Miocene (Alpine) phengitic white mica analyzed in this study. K-Ar and 4øAr/39Ar analyses for white micas from Ios indicate Late Permian to Early Cretaceous ages for muscovites in contrast to Paleocene to Early Miocene phengitic white mica ages (Figure 3). In the western Alps (e.g., the Brianconnais Zone, [Monid, Thermochronologic results from potassium feldspars 90354, 88607, and can be interpreted to reflect argon loss during a-14 Ma thermal event. This event was of insufficient magnitude and/or duration to cause complete resetting of the most retentive domains in the feldspars and resetting of argon systematics from coexisting phengitic white mica. Despite the fact that no obvious correlation exists between the feldspar apparent age spectra, microstructures, and mineral assemblages, the youngest apparent ages are nearly concordant with a whole rock-phengite Rb/Sr age of Ma for a metaaplite dike which intrudes basement rocks on Ios [Henjes- 1990]) and in Crete [Jolivet et al., 1996], Hercynian 4øAr/39Ar Kunst and Kreuzer, 1982]. These dikes are volumetrically apparent ages have also been documented in rocks that have undergone Alpine high P, low T metamorphism. Monid [1990] attributes the preservation of Hercynian ages to the high P-T gradient associated with a relatively dry overprinting event of insignificant, and other igneous rocks associated with the M3 event have not yet been identified on Ios. However, on a regional scale, mid-late Miocene ages have been documented for granitoids emplaced during extensional tectonism. Related short duration. This combination of crustal conditions can contact (M3) metamorphism and resetting of argon systematics also be used to satisfactorily explain the preservation of pre- Alpine apparent ages in the basement rocks on Ios. in basement rocks have been documented on Naxos [Wo'brans and McDougall, 1986], Paros [Baldwin and Lister, 1994],

15 BALDWIN & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE 7329 and Mykonos [Lee and Lister, 1992] where deeper structural which the rocks remained at ambient temperatures of 280- levels of the lower plates of Aegean metamorphi core 300øC for- 35 Ma and then were rapidly cooled at- 14 Ma complexes are exposed. Ductiley deformed Miocene granites (Figures 4c). Fits to the experimental data using this T-t are also present on Tinos, Mykonos, and Ikaria [Faure et al., 1991]. history are shown in Figure 4d. Although it is possible to model portions of the age spectrum using this T-t history, it is difficult to obtain satisfactory fits to apparent ages corresponding to % of the gas released. 5. Thermal History of the South Cyclades Temperature-time histories characterized by thermal events Shear Zone of short duration were also used to model the potassium Modeling of 4øAr/3øAr results using the MacArgon program feldspar experimental results. The input thermal history (Figure [Lister and Baldwin, 1996] and programs [Lovera et al., 1989] for Arrhenius data and age spectra for potassium feldspars with 4e) is consistent with known geologic constraints and is characterized by low ambient temperatures during the multidiffusion domains was undertaken to further assess Paleozoic to mid-tertiary (so that 4øAr* is retained) followed possible interpretations of this heterogeneous data set. These models assume that argon loss is via volume diffusion. If our interpretation of pre-alpine apparent ages (samples and 90351) is correct, it implies that the duration and/or magnitude of subsequent metamorphic events was not sufficient to completely reset argon systematics within all potassium-bearing minerals within the shear zone. If the results reflect an "inherited" argon component which had not by two thermal pulses with maximum temperatures of 320øC and 380øC corresponding to the M 2 and M3 events, respectively. An uncertainty in this thermal history is related to the possible effects of the M event on the argon systematics within these rocks (see additional discussion in section 5.2). The modeled age spectrum obtained using the "agesme" routine is shown in Figure 4f and provides a better fit to the experimental data. been completely outgasse during subsequent events, this has two implications for the thermal history. First, if a thermal event has occurred subsequento mineral crystallization and cooling, then the duration of the thermal event must have been 5.2. Additional Constraints on the Thermal History From Modeling of Mica and Potassium Feldspar Apparent Ages relatively short. Second, prior to the thermal event the mineral We next considered whether it was possible to model the must have remained at low ambientemperatures, such that 4øAr/39Ar mica (biotite, muscovite, and phengitic white mica) radiogenic argon produced from the decay of 4øK was retained. apparent ages using temperature-time histories similar to those In section 5.1, we discuss additional constraints offered by used in the potassium feldspar models (Figures 4c and 4e). modeling thermochronologic results. Numerical models using MacArgon [Lister and Baldwin, 1996; also were 5.1. Constraints on the Thermal History From Feldspar used to examine the effects of ambient temperatures on the Multidiffusion Domain Models apparent fusion ages. Figure 5a shows the form of the The results of a detailed step heating experiment on potassium feldspar from augen gneiss sample were temperature-time histories used in the modeling. Temperature initially falls from 500øC at 70 Ma (assuming 10 kbar pressure) modeled using the method and programs developed by Lovera to a constant "ambient" temperature (in the range øC) et al. [ 1989; see also www page which is maintained from 50 to 14 Ma (assuming 5 kbar edu/argon.html]. The "autoarr.f" routine was used to model loss of 3øAr via volume diffusion from pressure). Temperature then rapidly decays to 120øC at 8 Ma. Figures 5b, 5c and 5d show plots of apparent bulk fusion ages potassium-feldspar during the step heating experiment. versus ambient temperature for biotite and muscovite, Diffusion (E, Do/r 2) and domain distribution parameters used to obtain fits to the experimental results as plotted on Arrhenius and log r/r o plots, were determined (Figures 4a and phlogopite, and muscovite (used as an upper and lower limit for argon retentivity in phengitic muscovite) and potassium feldspar, respectively. The range in relative sizes of diffusion 4b). The experimental results were modeled using an domains (shown as shaded region in Figure 5d) corresponds activation energy of 46.5 kcal/mol and log Do/ro to r/ro[lovera et al., 1989] values obtained for sample Domain distribution parameters obtained were then used to forward model age spectrafter a T-t history was specified. By fitting model age spectra to the experimentally derived age potassium feldspar (see Figure 4b). The oldest apparent ages modeled (see Figure 4b) correspond to r/ro values in the range of and are indicated by the shaded region in Figure 5d. spectrum possible thermal histories were evaluated. Note that These results illustrate that the maximum ambient the lowest-temperature portion of the age spectrum is likely to temperature cannot exceed øc without resetting the be affected by incorporation of adsorbed argon and/or excess argon systematics. Values for the diffusion parameters used in argon associated with fluid inclusions. The low-temperature the modelling are discussed by Lister and Baldwin [1996 and steps could not be corrected using the methods of Harrison et al. [1994] as isothermal duplicates were not made. Fits to the refer. ences therein]. Biotite is the least retentive (using available diffusion parameters)and apparent fusion ages will highest-temperature portion of the age spectrum (i.e., >80% be significantly reset if the 'ambient' temperaturexceeds 3øAreleased) were not made because at temperatures >1100øC -280øC. Phengite apparent ages (using the range indicated by the feldspar has likely broken down in the furnace and therefore loss of 3øAr is not via volume diffusion. Attempts were made to evaluate possible cooling histories using the automatic fitting routine "autoage-mon". We considered histories which involved initially cooling from high temperatures ( øC) followed by a period in the shading in Figure 5c)will be relatively unaffected unless ambient temperature significantly exceeds - 300øC. Potassium feldspar apparent fusion ages will be significantly reset if ambient temperatures exceed - 250øC. In summary, these models limit the maximumambient temperature subsequent to the Alpine orogeny to < øC. Note, however, that these

16 7330 BALDWIN & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE -1 T (C) 1: ' '" i... ß... i,' ' i ß i,' i' ' o o 50 o - 40 o (a) -8-9 _. O experimental % I ß I ß i ß, I, 8 10! /T (K) o < 20 lo o d) Cumulative %39Ar Released i ß - i, I Ill I I mondeled :expe ri,,,,,e,,,,,n:tal 6O0 400 o 0.6..J 0.2 ' (b) ' I ", I ' I I I I I I I II I Cumulative %39Ar Released 0 e) Age (Ua) 000 j- ß, modeled experimental Q) 400! < ) 0 20 Age 40 (Ua) 6o 80 (f) 10 J ß i i I I i I ß Cumulative %39Ar Released Figure 4. The 4øAr/39Ar results for K-feldspar from mylonitic augen gneiss of the South Cyclades Shear Zone and results of numerical modeling using Lovera et al.'s [1989] programs for multidiffusion domains. (a) Arrhenius plot, (b) log r/r o plot, (c) T-t history characterized by temperatures of-300øc for-35 Ma followed by relatively rapid cooling at 14 Ma, (d) modeled fit to experimental data using T-t history shown in Figure 4c, (e) T-t history characterized by a pre-alpine low ambient temperatures of-100øc and two thermal spikes at 20 and 14 Ma, and (f) modeled fit to experimental data using T-t history shown in Figure 4e.

17 BALDWIN & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE highest ambient temperature 400 :i:i: ],,,,,,,,, lowest ambient temperature (a) Time (Ma) (c) i!i i ii ii i!i '"'"':':'"" :.. % 0 Model phengite :::::::::::::::::::::::::::::::::::::::::: 200 2S S0 400 Ambient Temperature (øc). 80 (b) SO O OO Ambient Temperature (øc) ,E rolo (d) K-feldspar Ambient Temperature (øc) Figure 5. Numerical models [Lister and Baldwin, 1996] which examine the effects of ambient temperatures on the apparent fusion ages of (b) biotite and muscovite, (c)phlogopite and muscovite (used as an upper and lower limit for argon retentivity in phengitic white mica, shaded), and (d) potassium feldspar with a range in relative sizes of diffusion domains (i.e., sample r/ro [Lovera et al., 1989] values). The range of r/ro values corresponding to the oldest apparent ages modelled (see Figure 3b) are shaded. (a) The range of temperaturetime histories used in the modeling. These curves illustrate that the maximum ambient temperature cannot exceed øC without resetting the argon systematics in these potassium-bearing minerals. ambient temperatures are not sufficient to explain the metamorphic mineral assemblages preserved in these rocks. Estimates for the P-T conditions in the "basement"(i.e., lower plate) during M are 9-11 kbar, øC, and 5-7 kbar, øC during M2 [van der Maar and Jansen, 1983]. Grtitter [1993] estimates conditions during M in the overlying "series" (i.e., upper plate) as kbar at assumed thermal decay constant ( ). The apparent fusion age is calculated for each "experiment" and plotted against the peak temperature. Individual curves for = 0.001, 0.01, 0.1, and 1.0 Myr are shown (Figures 6b-6h). These models suggesthat the duration of Miocene heating event(s) were short-lived and that maximum temperatures did not exceed øC. Decay constants are <0.1 Myr for these temperatures. 475+_25øC, with M 2 conditions at-4 kbar, >400øC. In the rocks examined in this study, texturally young garnet-biotite is present(see Plate lh), suggesting temperatures of øc [Spear, 1993] were attained during M 2. While it is 6. Discussion As the modeling results indicate, if we consider only the possible that garnet-biotite can grow below 450øC adjacent to thermochronologic data, several interpretations can be used to plagioclase, the rate of growth may be very slow [e.g.,stiiwe, explain the heterogeneous 4øAr/39Ar data set obtained from the 1995]. This suggests that M 2 involved an increase in South Cyclades Shear Zone. However, if the data set is temperature which must be considered in addition to the constraints discussed in the preceding paragraph. Therefore we next consider models which examine the effect of variation in the peak temperature and the subsequent rate of considered in its entirety (i.e., fabrics, microstructures, metamorphic petrology, thermochronology), less ambiguous conclusions can be drawn. We need to further explore the reasons that might explain why heterogeneous 4øAr/39Ar age decay of a thermal pulse at 14 Ma. The form of the temperature- distributions are characteristic of rocks within this crustaltime curve used in these calculations is shown in Figure 6a. The model is allowed to remain at 0øC from 300 to 14 Ma (for scale ductile shear zone. Effects as we have described in this paper take place at a muscovite and biotite) or from 60 to 14 Ma (for potassium- level in the Earth's crust which we refer to as the argon partial feldspar and phengitic white mica). This ensures that there is no argon lost until the thermal spike takes place at 14 Ma. retention and resetting zone (PRZ) (Figure 7). Within stable continental crust, characterized by a constant geothermal Temperature is assumed to rise abruptly at 14 Ma to a peak gradient, the argon PRZ is that portion of the crust where value between 250 and 500øC. Thereafter temperature temperatures are insufficient to completely reset argon immediately begins to decrease, at a rate determined by the systematics within preexisting potassium-bearing minerals.

18 7332 BALDWIN & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE 700 5oo oo 200 (a) Elapsed Time (MYr) lo 5 o 35øt o r/to K_feldspar = 100 (e) Peak Temperature (øc) ' z 01 (b) 0 Model Biotite... ' ; oo Peak Temperature (øc) K-feldspar r/r0= ' do... ' ;0... do... io... 5 (f) Peak Temperature (øc) r/r0 = 25 (c) ' Peak Temperature (øc) (g) Peak Temperature (øc) 70 g ] 20 lo (d) o Figure 6. Numerical models that examine the effects of a thermal pulse at 14 Ma on apparent fusion ages for biotite, muscovite, phengitic white mica, and potassium-feldspar. (a) Example of a temperature-time history used in the modeling, involving a 500øC thermal pulse with decay constant, x = 1.0 Myr. (b)-(h) Peak temperatures used vary from øC. Prior to 14 Ma, ambient temperature is held at 0øC, thus ensuring that the only argon lost is via diffusion during decay of the thermal spike. The duration of the thermal pulse is thus overestimated. In Figures 6b-6h numbers on curves refer to values of x = 1.0, 0.1, 0.01, or Myr. Diffusion dat are taken from Lister and Baldwin [1996] or from potassium feldspar (E=46.5 kcal/mol, log (D0/r 2) = ) using r/ro values shown (see Figure 3b). For peak temperatures that exceed 400øC, the thermal decay constant must be less than Myr to avoid resetting the most retentive domains in potassium feldspar

19 BALDWIN & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE Ar* ] bio musc coexisting 40Ar* hbl retained --r.. -taie:-...-n-e.- i... l 4OAr* 40Ar* Kspar reset bio reset.-.'...--'.,',.--..-;':. -.'- - : -:'::::..-::: ::.*.'c?-'...:..'. ½}.?..:?.. ; Z-;'.': E ::-.':::; r--*"-" '..,...-.,, : : a- -..::. ::."....-:. muse reset retained PRZ. hbl reset aggregates) indicate that deformation continued as temperatures decreased. Cooling subsequent to thermal events may have involved timescales < Myr. Yet large strains Figure 7. Schematic diagram illustrating the concept of the accumulated in the South Cyclades shear zone during the argon partial resetting/retention zone (PRZ) for stable Miocene. Strain rates must have been rapid (in the order of 10 '9 continental crust. A steady state geothermal gradient of s -l. while strains involving as much as 70-80% 20øC/km is assumed. Potassium-bearing minerals residing in shallow portions of the crust above the PRZ will retain shortening took place). Rapid cooling followed the Miocene thermal events as the lower plate rocks of the Ios core complex radiogenic argon (4øAr*) produced from the radioactive decay were unroofed during ongoing extension. of nøk over geologic time. Hence they will record 4øAr/39Ar We propose that the South Cyclades Shear Zone formed apparent ages. Within the PRZ, at depths corresponding to during Oligo-Miocene extension of the Aegean continental relatively high temperatures, 4øAr* in potassium-bearing crust. Rather than slow, continuous deformation occurring in a minerals produced over geologic time will be partially retained and partially outgassed via volume diffusion. The ductile shear zone over the course of millions of years, we infer intermittent and very rapid shear events, perhaps coincident 4øAr/39Ar apparent ages for potassium-bearing minerals within with and related to (regionally observed) intrusion of magmas the PRZ will be partially reset. At depths in the crust below and accompanied by migration of fluids. Lower plate rocks on the PRZ, potassium-bearing minerals will not retain 4øAr* over Ios were subjected to at least one thermal pulse during the geologic time and hence their corresponding 4øAr/39Ar mid-miocene (at---14 Ma), which partially reset potassium apparent ages will be reset to zero. Note that recrystallization feldspars. Given regional geologic constraints, we infer this can also occur over a wide range of P-T conditions and will thermal event (M3) was related to intrusion of granitoids. result in partial to complete resetting of argon systematics. Greenschist facies retrogression occurred, accompanied by Movement on shear zones during crustal extension will bring deformation (D4), involving a major episode of continental rocks from above, within, and below the PRZ toward the extension, and the formation of crustal-scale ductile shear zones. surface. Mineral abbreviations are hornblende (hbl), muscovite (muse), biotite (bio), and potassium feldspar (Kspar). Although spars evidencexists for M s events at the surface on Ios, mid-miocene igneous rocks may be present at depth. This magma could have provided the heat to partially outgas potassium feldspars. We further speculate that intrusion of Within the PRZ radiogenic argon in preexisting potassiumbearing minerals will be partially outgassed (and partially retained). Subsequent tectonic exhumation of portions of the PRZ involves movement on crustal-scale ductile shear zones, such magma may have coincided with movement on shear zones during extensional tectonism which produced the metamorphic core complexes. Shallow multiple intrusion of magmas is a possible mechanism which could produce shortaccompanied by strongly partitioned deformation and lived thermal pulses, although rapid strain in a ductile shear localized recrystallization. This will lead to further resetting zone can also result in shear heating which could prolong the of argon systematics within preexisting potassium-bearing duration of a thermal event once deformation has initiated. The minerals and will result in heterogeneous 4øAr/39Ar age distributions within these ductile shear zones, such as those documented in the South Cyclades Shear Zone. The argon PRZ will be characterized by considerable heterogeneity in the degree of recrystallization and grain growth, as documented in the South Cyclades Shear Zone Correlations will exist between the fabrics and microstructures other source of heat that needs to be considered is the heat released during exothermic hydration reactions. Rapid metamorphic mineral growth events are capable of producing heat at a rate sufficiento generate a short-lived thermal pulse. In summary, the interpretation that is most consistent with the available data from the South Cyclades Shear Zone suggests the rocks within the shear zone were subjected to and the 4øAr/39Ar thermochronologic results. For example, several thermal pulses during which time localized phengitic white mica in the garnet mica schist unit and some augen gneisses recrystallized during M and were not significantly affected by subsequent thermal events, perhaps due to the relatively high retentivity for argon. Phengitic recrystallization occurred, with accompanying resetting of argon systematics, while loss of argon occurred via volume diffusion in minerals that were not recrystallized (Figure 8). Based on the results presented here, we propose that the Ios white mica from samples and did not record evidence for an Alpine history despite the fact that "Alpine" apparent ages were obtained for the most retentive domains in potassium-feldspar from We suggesthat the argon systematics in these phengitic white micas were completely reset during a (re)crystallization event which produced textures dominated by decussate recrystallization subsequent to D e deformation. Modeling of the argon results suggests that "basement" rocks on Ios remained at relatively low ambient temperatures (i.e., less than the temperatures required to form greenschist facies mylonites), prior to the Miocene. The onset of ductile deformation in the South Cyclades Shear Zone appears to have taken place subsequent to M e recrystallization, as indicated by the relative timing of the growth of albite porphyroblasts. Microstructures (e.g., dynamically recrystallized quartz basement consisted of a composite terrane of igneous and metamorphic rocks of perhaps different (pre-alpine) ages.

20 7334 BALDW1N & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ZONE Time (My) I I I I I I I I I I I I I I Caledonian Hercynian exhumation Alpine Backarc collision extension c3? D 2 ' D3. 4 S 1 inclusion trails in gt S 2 penetrative foliation? --,. M1 M 2!-- M 0 relict gt and bio ~7-11 kbar 5-7 kbar 6 kbar, 600øC oc oc : musc pheng _o. wr, musc bio pheng parag pheng... -'-'-'-... '-... S 2 gneissic foliation!-- kspars 6 kbar -350oc 0 pheng pheng v!-- -,, rnusc pheng pheng 5oo, ii'i... wr bio Time (My) Figure 8. Schematic D-P-T-t history of the lower plate of the Ios core complex. Range in 4øArPøAr apparent ages for the South Cyclades Shear Zone indicated by solid lines. Additional P-T-t constraints for lower plate rocks from Henjes-Kunst [1980], Henjes-Kunst and Kreuzer [1982], van der Maar [1981] and Andriesson et al. [1987]. K-Ar ages indicated by dashed lines, Rb-Sr ages indicated by gray lines, and diamond indicates U- Pb age. Abbreviations are wr, whole rock; gt, garnet; musc, muscovite; bio, biotite; kspar, potassium feldspar. Constraints on the T-t history based on thermochronologic, microstructural, and petrologic data. See text for discussion. Prior to the onset of the Alpine orogeny these rocks occupied that some samples retain an earlier fabric (e.g., S2) while in a crustal setting above or within the PRZ when they were other samples the effects of D 4 dominate the fabric [Vandenberg overridden by Alpine nappes. Pressures increased to and Lister, 1996]. Heterogeneous recrystallization was likely GPa, and phengitic white mica grew during the Alpine affected by limited and focussed fluid circulation as well as deformation events, with temperatures remaining < 400øC. differences in the bulk composition of rocks within the shear Temperatures did not exceed those required for feldspar zone. Although it has been suggested that the rocks in the plasticity (450øC[Scholtz, 1988]). Subsequent cooling to Attic Cycladic metamorphic belt followed nearly isothermal relatively low ambientemperatures occurred while the rocks decompression paths based on mineral parageneses [van der began to be exhumed [cf. Lister and Raouzaios, 1996]. Maar, 1983], thermochronologic data suggest deformation and During Miocene time, temperatures within the shear zone metamorphismore likely occurred during transient thermal increased when the region was stretched in a crustal-scale D 4 pulses [Wijbrans and McDougall, 1986, 1988; Wijbrans et ductile shear zone. The markedly heterogeneous 4øArPøAr al., 1990; Lister and Baldwin, 1993; Lister and Raouzaios, apparent ages obtained from adjacent samples and from within 1996]. the same sample requires that the Miocene thermal pulse(s) Similar conclusions can be drawn with respect to the during which the D 4 mylonites formed were short-lived. We duration of the M event. However, more uncertainty is infer that magmatism at depth resulted in an increase in introduced because the retentivity of the "inherited" argon temperature that partially reset potassium feldspar 4øAr/39Ar component in pre-alpine phases may have increased during apparent ages, although mid-miocene granitoids have not yet the Alpine (M ) event due to the high P-T gradient. We infer been identified and may not be presently exposed at the surface that an increase in pressure increases argon retentivity [cf. on Ios. Dahl, 1996] since increasing the pressure decreases the The pre-alpine basement of los has been partially recycled diffusivity by an amount determined by the activation volume. during subsequent deformation (D -D4) and recrystallization For example, a 10 kbar increase in pressure can be expected to events (M -M3). Deformation was strongly partitioned such increase argon retentivities and to lead to an increase in

21 BALDWIN & LISTER: THERMOCHRONOLOGY OF THE SOUTH CYCLADES SHEAR ;ONE 7335 "closure" temperatures by 30-40øC, assuming an activation volume of- 10 cm 3 [Lister and Baldwin, 1996]. Although the heterogeneous resetting of argon systematics in preexisting minerals. potassium feldspars gave Alpine apparent ages corresponding to the highest temperature steps, modeling of the experimental results assuming loss of argon solely via volume diffusion Appendix: Analytical Procedures suggests incomplete outgassing of a pre-alpine "inherited" argon component. More work is needed, however, to fully High purity mineral separates (>99% pure) were prepared understand the effects of M high P/T metamorphism on the from crushed and sized rock chips using conventional heavy argon systematics in these samples. liquid and magnetic separation techniques. The separates were irradiated for 120 hours in position X34, or X33 of the HIFAR Rocks now exposed in the lower plate of the Ios core were resident in the argon partial retention/resetting zone (the argon PRZ) prior to the onset of the Alpine collision, enabling preservation of relatively old apparent ages recorded by muscovite and biotite. The effect of Alpine metamorphism on the argon systematics was most pronounced in cases where recrystallization occurred. The retentivity of the inherited argon component in pre-alpine phases may have increased during the Alpine (M )event due to an increase in pressure. Alpine high P-T metamorphism may have been of rather short duration [cf. Monid, 1990]. The lower plate of the Ios core complex was subjected to at least two thermal events during the Miocene. At-20 Ma, greenschist facies overprinting of older metamorphic fabrics may have taken place (with growth of garnet and biotite), and the D 4 deformation event initiated, involving a major episode of continental extension and formation of crustal-scale ductile shear zones. A second thermal pulse may have taken place at -14 Ma which reset the least retentive diffusion domains in feldspars but which had apparently little effect on the 4øAr/39Ar nuclear reactor of the Australian Nuclear Science and Technology Organization (ANSTO) at Lucas Heights, New 7. Conclusions South Wales. Irradiation procedures followed those described by McDougall and Harrison [1988]. Biotite standard As crustal rocks are exhumed within shear zones during GA1550 (97.9 Ma old [McDougall and Roksandic, 1974]) continental extension, the extent to which isotopic and hornblende standard (414.1 Ma old [Harrison, systematics are reset is strongly dependent on the T-t history and the degree of recrystallization that occurs. Structural and petrologic control is necessary to interpret thermochronologic results from ductile shear zones. Likewise, kinematic studies of shear zones require detailed thermochronologic control before tectonic implications can be drawn from results. Heterogeneous resetting of argon systematics in potassiumbearing minerals from the South Cyclades Shear Zone on Ios reflect the effects of several metamorphic and deformational 1981]) were used to monitor the neutron dose. The 4øAr/39Ar procedures followed those described by McDougall [1985]. Gas was extracted from the samples using a double-vacuum, resistance-heated tantalum furnace with temperature control monitored via a thermocouple in contact with the bottom of the crucible and mounted on the outer (low) vacuum side of the furnace. Three SAES getters were used for purification of the extracted gas. For age calculation, the data have been corrected for events. A correlation exists between 4øAr/39Ar apparent ages machine mass discrimination, decay of 37Ar and 39Ar, and and the relative timing of different episodes of recrystallization and grain growth, as indicated by fabric and microstructural analysis. The heterogeneous distribution of apparent ages from within the shear zone can be best explained with a history characterized by crystallization during early Eocene high P-T neutron-induced interfering isotopes [Tetley et al., 1980]. Correction factors used to account for interfering nuclear reactions are listed in Table 1 and were determined by analyzing argon extracted from irradiated CaF 2 and K2SO 4. Machine discrimination was determined from repeated analysis (Alpine) metamorphism followed by Miocene event(s) of atmospheric argon, and line blanks were measured at associated with continental extension. different temperatures prior to analyses. Blanks ranged from 1 x 10 '14 to 3 x 10-3 mol 4øAr and were generally atmospheric in composition. Results of 4øAr/39Ar step heating experiments and total fusion experiments are shown in Table 1, and age spectra are shown in Figure 2. All ages are calculated using the decay constants recommended by Steiger and dager [1977]. Stated precisions for 4øAr/39Ar ages include all uncertainties in the measurement of isotope ratios and are quoted at the lo level. The errors do not include an error associated with the J parameter which is < 0.5%. Data for each sample were plotted on inverse isochron diagrams to assess whether the nonradiogenic 4øAr/36Ar trapped component was atmospheric in composition or not. A SUN-based CAMECA SX-50 electron microprobe, automated with four wavelength dispersive spectrometers and one energy-dispersive spectrometer at the University of Arizona was used to characterize samples used for 4øAr/39Ar thermochronology. Operating voltage was 15 kv, beam current was 20 na, and the beam diameter varied from 1 to 10 systematics in other potassium-bearing minerals. Acknowledgements. Irradiations were made possible through We speculate that the mid-miocene thermal event is related support from the Australian Institute of Nuclear Science and to the intrusion of granitoids similar to those now exposed on Engineering and the Australian Nuclear Science and Technology Organization. Support from the Australian National University, the the islands of Mykonos, Paros, and Naxos. The preservation of Australian Research Council (a grant to study Continental Extension pre-alpine and Alpine ages indicates that the duration of Tectonics), the University of Arizona, and the National Science subsequent Miocene thermal events was short-lived and Foundation (EAR ) is gratefully acknowledged. The Institute of insufficient to completely reset the isotopic systematics within Geological and Mining Exploration (IGME), Athens, Greece, provided the South Cyclades shear zone. Deformation was strongly considerable assistance during field work, in particular, S. partitioned within the Shear Zone and coincided with these Kalogeropolous and V. Avdis. We are grateful to J. Mya, H. Kokkonen, thermal events, leading to recrystallization in some cases, and and R. Maier for technical support, F. Mazdab for electron microprobe

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