Abstract

Interpreting isotopic ages as deformation ages when they are acquired from moderate-temperature metamorphic environments can be a challenging task. Syros Island (Cyclades, Greece) is famous for Eocene high-pressure metamorphic rocks reworked by localized Miocene greenschist-facies deformation. In this work, we investigate phengites from coarse-grained marbles, which experienced the high-pressure event, and phengites from fine-grained localized marble shear zones attributed to the low-grade Miocene deformation. Based on structural criteria, both events can be easily discriminated because of their opposing kinematics. Laser-heating 40Ar/39Ar analysis on phengite yielded a 40 ± 1.6 Ma age for the host rock and a 37 ± 1.3 Ma age for the shear zone. Both ages are statistically indistinguishable, consistent with the regional Eocene event, and not the Miocene deformation event responsible for the formation of the shear zone. Thermodynamic modeling indicates that the observed high-variance mineral assemblage is stable without compositional change along the pressure-temperature path followed by the rocks of Syros. Although the marble within the shear zone was deformed at extremely fast strain rates (10−10 s–1), we observed no intracrystalline deformation of phengite grains and no resetting in the isotopic system, because strain was mostly accommodated by calcite. Consequently, a high strain rate does not necessarily create deformation ages in rocks with high-variance assemblages, such as marble mylonites.

INTRODUCTION

The 40Ar/39Ar geochronology method on white mica is a popular method used to date deformation under greenschist-facies temperature conditions (e.g., Dunlap, 1997; de Sigoyer et al., 2000; Bröcker et al., 2004; Rolland et al., 2009; Gébelin et al., 2011; Sanchez et al., 2011; Lanari et al., 2012; Schneider et al., 2013; Cossette et al., 2015). Most of the investigations applying such an approach have studied shear zones in a variety of quartzofeldspathic rock (e.g., quartzite, pelites, granites, gneisses) and concluded high strain events result in a distribution of apparent ages that scatter over 10–20 m.y.

In metasedimentary packages under upper-greenschist to lower-amphibolite metamorphic conditions, deformation tends to localize in calcite marbles, resulting in the formation of mylonites and ultramylonites (Bestmann et al., 2000). In such rocks, however, little is known about the behavior of the K/Ar system in mica and the influence of high strain, particularly with reference to potential inefficient removal of 40Ar from the grain boundary (Warren et al., 2012).

Several models invoke deformation-related Ar loss and therefore may justify the interpretation of mica dates as deformation ages. For example, Goodwin and Renne (1991) proposed that due to deformation-related ductile or brittle grain-size reduction, diffusion length scales are reduced, and consequently volume diffusion becomes more efficient. Moreover, Ar loss during mica recrystallization or neocrystallization resets the K/Ar system, potentially resulting in mixed ages (e.g., Wijbrans and McDougall, 1986; Forster and Lister, 2004; Cossette et al., 2015). From a crystallographic perspective, strong intracrystalline deformation is always accompanied by an increase in dislocation density in the crystals, which may act as high-diffusivity pathways, resulting in Ar loss by increased pipe diffusion along dislocation lines (Lee, 1995; Dunlap and Kronenberg, 2001). Consequently, pipe diffusion has a lower activation energy and larger diffusion coefficient than volume diffusion and is therefore the dominant mechanism (Kramar et al., 2001; Dunlap and Kronenberg, 2001).

Because of possible physiochemical mechanisms leading to Ar loss during deformation, the interpretation of geochronological data requires detailed microstructural and geochemical analyses. Building off of our recent study in which we characterized microfabrics and corresponding deformation mechanisms at different strain rates (Rogowitz et al., 2014), we use the same outcrop on Syros (Cyclades, Greece) to assess the extent of K/Ar resetting due to extremely localized deformation in marbles. Our study presents 40Ar/39Ar ages from two marble samples that followed the same pressure-temperature-time (P-T-t) path but witnessed contrasting strain intensities at different strain rates. We suggest that high strain rate in high-variance assemblages does not necessarily reset the K/Ar system in phengite.

GEOLOGICAL SETTING AND OUTCROP DESCRIPTION

Syros Island is part of the Cycladic blueschist belt, which is situated in the backarc of the Hellenic subduction zone (Papanikolaou, 1987; Wortel et al., 1990). The Cycladic area is composed of three main units, separated by tectonic contacts, which are, from bottom to top: the parautochthonous Basement unit, the Cycladic blueschist unit, and the Upper unit (e.g., Bonneau, 1984; Jolivet and Brun, 2010).

Syros is dominated by the Cycladic blueschist unit, with an Upper unit klippe exposed in the southeast (Tomaschek et al., 2000; Keiter et al., 2011; Soukis and Stockli, 2013). Two major Cenozoic metamorphic events have affected the Cyclades (Jolivet and Brun, 2010; Ring et al., 2010). An Eocene M1 eclogite-blueschist–facies event, which is associated with top-to-the-west sense of shear along the western margin of Syros and an Oligocene–Miocene M2 greenschist-facies event that is associated with dominant top-to-the-east sense of shear. Both events are well documented on Syros (Gautier and Brun, 1994; Trotet et al., 2001a; Bond et al., 2007; Keiter et al., 2011; Philippon et al., 2011). Geochronology performed on eclogite- and blueschist-facies rocks, including 40Ar/39Ar and Rb-Sr ages on white mica, U-Pb on zircons, and Lu-Hf on garnet, has yield ages between 52 and 37 Ma for M1 tectonism on Syros (Tomaschek et al., 2003; Lagos et al., 2007; Bröcker et al., 2013), whereas 40Ar/39Ar and Rb-Sr geochronology performed on greenschist-facies rocks resulted in ages around 23–19 Ma for the greenschist-facies overprint (Bröcker et al., 2013).

Our study focuses on an outcrop in the western part of Syros, north of Delfini (Universal Transverse Mercator [UTM] 35, 414840N, 313839E). It exposes a decameter-scale thick calcite marble layer intercalated with quartz and dolomite lenses, preserving an E-W–trending lineation and two different shear kinematic directions. Quartz and dolomite layers are deformed with asymmetric shear-band boudinage indicating top-to-the-west shearing (Figs. 1A and 1B; Goscombe et al., 2004). The higher temperatures required for ductile deformation of dolomite and quartz together with the observed top-to-the-west sense of shear suggest that these structures are related to the Eocene M1 event. Conversely, flanking structures, calcite sigma clasts, and localized shear zones in calcite marble show M2 related to top-to-the-east shearing (Figs. 1C and 1D). We collected samples from a 5-m-long, a-type flanking structure located in an almost pure calcite marble as part of a larger study (Fig. 1D; Rogowitz et al., 2014). The flanking structure developed due to the rotation of a crack (i.e., crosscutting element) during top-to-the-east shearing, resulting in antithetic slip along the crosscutting element and the formation of a secondary shear zone with a maximum displacement of 120 cm (Grasemann and Stüwe, 2001; Passchier, 2001). As a consequence of overall top-to-the-east shearing, a local antithetic top-to-the-west sense of shear is observed within the secondary shear zone (Rogowitz et al., 2014).

Figure 1.

Outcrop photographs exhibiting two opposing shear directions. (A, B) Asymmetric boudinaged dolomite layers in marble showing evidence of top-to-the-west shear. (C) Sheared calcite sigma clast indicating top-to-the-east sense of shear. (D) An a-type flanking structure indicating overall top-to-the-east sense of shear resulting in local antithetic shearing. HR and SZ indicate the location of the sampled host rock and shear zone, respectively.

Figure 1.

Outcrop photographs exhibiting two opposing shear directions. (A, B) Asymmetric boudinaged dolomite layers in marble showing evidence of top-to-the-west shear. (C) Sheared calcite sigma clast indicating top-to-the-east sense of shear. (D) An a-type flanking structure indicating overall top-to-the-east sense of shear resulting in local antithetic shearing. HR and SZ indicate the location of the sampled host rock and shear zone, respectively.

METHODS

Two samples, one from the host rock and one from the shear zone, were collected for detailed microstructural, geochemical, and 40Ar/39Ar analyses (Fig. 1D). Carbon-coated, mechanically polished thin sections were prepared for microstructural analysis. Modal composition and microstructures were analyzed via optical microscopy (Leica DM4500 P) and scanning electron microscopy in backscattered electron mode (SEM-BSE; FEI Inspect S, FEI Quanta 3D FEG, University of Vienna, Austria) operated at an accelerating voltage in the range 10–15 kV and with a current of 4 nA. Quantitative electron microprobe (EMP) analyses were performed on a Cameca SX-100 (University of Vienna, Austria) at an accelerating voltage of 15 kV and a current of 20 nA with a defocused beam up to 7 µm in diameter. In order to evaluate the scattering of chemical analyses due to analytical uncertainties, we carried out a Monte Carlo simulation following the method of Lanari et al. (2014). For each sample, a population of 50 analyses was generated using the sample average analysis, and analytical uncertainties were calculated based on EMP counts. The structural formula was then calculated for each simulated analysis.

Incrementally step-heated 40Ar/39Ar geochronology on handpicked phengite separates was performed with a Photon Machines CO2 laser coupled to a Nu Instruments Noblesse multicollector mass spectrometer housed at the Geological Survey of Canada (Ottawa, Canada). Grain size ranged between 106 and 250 µm, and 3 to 4 grain aliquots were used for the analysis following the protocol of Kellett and Joyce (2014; the complete methodology and analytical details are described in the Data Repository1). Our preferred ages were calculated as the weighted mean of a selection of mostly contiguous increments that represent >50% of 39Ar gas released and result in concordant ages.

RESULTS

Flanking structures develop at relatively low strain (γ < 5; Kocher and Mancktelow, 2005). Calculations by Rogowitz et al. (2014) showed that the host rock on Syros experienced a shear strain of γ < 3 during the formation of the flanking structure. However, the rocks within the shear zone experienced a much higher shear deformation, having a maximum displacement of 120 cm at the center of the shear zone, where the width is only 1.5 cm. This corresponds to a shear strain of γ ∼ 80. A differential stress–grain-size deformation mechanism map for calcite at 300 °C was calculated, showing that the host rock has been deformed at strain rates of ∼10−12 s–1, whereas within the shear zone, strain rates reached 10−10 s–1 (Rogowitz et al., 2014). Based on strain and strain rate data, the interval of deformation is estimated to have lasted ∼25 k.y.

The marble is composed of nearly pure calcite with minor amounts of dolomite (<10%), quartz (<1%), and phengite (<1%). The microstructure of the host rock is characterized by coarse calcite grains with an average grain size of 280 µm. Minor undulatory extinction and slightly curved grain boundaries indicate that minor deformation took place within the calcite dislocation creep field (Fig. 2A). Within the shear zone, strong intracrystalline deformation, subgrain formation, and subsequent recrystallization led to the formation of alternating protomylonitic and ultramylonitic calcite layers (Fig. 2B).

Figure 2.

Photomicrographs of calcite marble (crossed-polarizers). (A) Host-rock marble showing minor undulatory extinction and grain-boundary migration. (B) Transition from host rock to protomylonitic and ultramylonitic marble within the shear zone. (C) Host-rock marble showing preferred alignment of mica grains. (D) Shear zone showing locally brittlely deformed mica surrounded by recrystallized calcite. (E) Backscattered-electron image of prismatic white mica in host-rock marble. Note that a lack of gray shade variation is an indication of no chemical zoning. (F) Backscattered-electron image of brittlely deformed white mica within the shear-zone marble.

Figure 2.

Photomicrographs of calcite marble (crossed-polarizers). (A) Host-rock marble showing minor undulatory extinction and grain-boundary migration. (B) Transition from host rock to protomylonitic and ultramylonitic marble within the shear zone. (C) Host-rock marble showing preferred alignment of mica grains. (D) Shear zone showing locally brittlely deformed mica surrounded by recrystallized calcite. (E) Backscattered-electron image of prismatic white mica in host-rock marble. Note that a lack of gray shade variation is an indication of no chemical zoning. (F) Backscattered-electron image of brittlely deformed white mica within the shear-zone marble.

In both samples, phengite (long axis <400 µm) is preferentially orientated parallel to the foliation (Figs. 2C and 2D), defining the lineation together with the shape-preferred orientation of calcite. Phengite grains behaved brittlely rather than ductilely and experienced minor grain-size reduction by splitting and breaking preferentially along the cleavage, resulting in prismatic or columnar shapes (Figs. 2C–2F). In ultrafine-grained (3 µm) layers within the shear zone, fracturing perpendicular to the phengite cleavage plane can be observed (Fig. 2F).

Mineral chemistry analyses reveal that in both samples, the phengite has a relatively high Si content (3.4–3.6; Fig. 3A). The Fe content is almost below the detection limit of around 500 ppm, resulting in an XMg greater than 0.97. The end-member composition of the phengite in both samples is around 50% muscovite, 45% celadonite, and smaller amounts of paragonite and pyrophyllite (Fig. 3B). Chlorine and fluorine concentrations are below the detection limit. Except for the slight Fe content variance, there is no distinct difference in chemical composition of phengite located in the host rock and the shear zone. Mineral chemical analyses and SEM-BSE images indicate homogeneous phengite composition (Figs. 2E and 2F).

Figure 3.

Mineral geochemistry results from the calcite marble on Syros. (A) XMg vs. Si plot of phengite from within the host rock (light gray) and the shear zone (dark gray) illustrating high-Si and extremely low-Fe content in mica. Shaded areas represent the range of compositions generated with Monte Carlo simulations. (B) Ternary diagrams of white mica composition. Note that there is no compositional difference between host-rock and shear-zone mica (see Table DR2 [see text footnote 1]). End-member abbreviations: Ms—muscovite, Cel—celadonite, Prl—pyrophyllite, and Pg—paragonite.

Figure 3.

Mineral geochemistry results from the calcite marble on Syros. (A) XMg vs. Si plot of phengite from within the host rock (light gray) and the shear zone (dark gray) illustrating high-Si and extremely low-Fe content in mica. Shaded areas represent the range of compositions generated with Monte Carlo simulations. (B) Ternary diagrams of white mica composition. Note that there is no compositional difference between host-rock and shear-zone mica (see Table DR2 [see text footnote 1]). End-member abbreviations: Ms—muscovite, Cel—celadonite, Prl—pyrophyllite, and Pg—paragonite.

Ages obtained by step-heated 40Ar/39Ar geochronology from both the host rock and shear zone are statistically undistinguishable (Fig. 4A; Table DR1 in the Data Repository [see footnote 1]), and total gas ages are concordant to preferred ages. The age spectrum for phengite located within the host rock exhibits a slightly disturbed age spectra, with single step ages varying between 32 and 41 Ma, yielding a preferred 40Ar/39Ar age of 40.2 ± 1.6 Ma. The age spectrum for phengite located within the shear zone is less disturbed, with little variation in single step ages between 36 and 40 Ma, yielding a preferred 40Ar/39Ar age of 37.4 ± 1.3 Ma. A 38Ar/39Ar versus 37Ar/39Ar diagram shows a clear data cluster for the shear-zone phengite, indicating an isochemical Ar population, whereas data for the host rock are variable, consistent with the more disturbed age spectra for host-rock mica (Fig. 4B).

Figure 4.

(A) 40Ar/39Ar age step-heated release spectra for phengite located within the host rock (HR, light gray) and shear zone (SZ, dark gray) on Syros. Note that both spectra yield concordant ages. Tg—total gas age; Tp—preferred age (see Table DR1 [see text footnote 1]). (B) 38Ar/39Ar vs. 37Ar/39Ar graph illustrating a potential isochemical population for the shear-zone mica analyses and a more heterogeneous chemical population for host-rock mica. The individual steps are labeled and correspond to the steps on the spectra (Table DR1 [see text footnote 1]).

Figure 4.

(A) 40Ar/39Ar age step-heated release spectra for phengite located within the host rock (HR, light gray) and shear zone (SZ, dark gray) on Syros. Note that both spectra yield concordant ages. Tg—total gas age; Tp—preferred age (see Table DR1 [see text footnote 1]). (B) 38Ar/39Ar vs. 37Ar/39Ar graph illustrating a potential isochemical population for the shear-zone mica analyses and a more heterogeneous chemical population for host-rock mica. The individual steps are labeled and correspond to the steps on the spectra (Table DR1 [see text footnote 1]).

DISCUSSION

Observed marble microstructures from the outcrop on Syros are consistent with calcite deformation at ∼300 °C (Bestmann et al., 2000; Rogowitz et al., 2014). Qualitative temperature estimates together with the top-to-the-east kinematics of the structure indicate that the flanking structure developed during the Miocene greenschist-facies deformation. However, the 40Ar/39Ar mica ages for the host rock and shear zone of ca. 37–40 Ma correlate to the regional high-pressure event (Bröcker et al., 2013), indicating that the phengite preserves the age of the older M1 event and not the formation of the Miocene flanking structure. Despite the expectation that the 40Ar/39Ar analyses from the calcitic mylonite should yield Miocene dates, apparently there was no resetting of the K/Ar system during deformation associated with shear-zone formation. Although we have documented strain rates up to 10−10 s–1 (Rogowitz et al., 2014), we did not observe any phengite recrystallization, and instead the calcite shows strong intracrystalline deformation by subgrain formation, undulatory extinction, and recrystallization (Fig. 2B). We therefore consider that the phengite was stronger than calcite during shearing, which fostered strain partitioning between the phases. This phenomenon has been documented elsewhere in the Cyclades on similar lithologies (Cossette et al., 2015) and is consistent with experiments on calcite-muscovite aggregates (Delle Piane et al., 2009) and evolution of two-phase systems (Etchecopar, 1977; Handy, 1990) showing that once mica is rotated in the shear direction, it behaves rather rigidly. The only deformation mechanism recorded in the phengite is therefore minor brittle deformation preferentially along the cleavage, which has been shown to be negligible for Ar loss at low temperatures (Dunlap and Kronenberg, 2001). Due to the short deformation interval (∼25 k.y.) at low temperatures (∼300 °C), which is below the muscovite 40Ar/39Ar closure temperature (Hames and Bowring, 1994; Harrison et al., 2009), evidence for enhanced effective volume diffusion, due to reduced diffusion path lengths by brittle deformation perpendicular to the cleavage, can be excluded for our samples.

In addition to potassium, Ca and Cl concentrations are important factors when interpreting 40Ar/39Ar spectra, since 37Ar and 38Ar can be derived from these elements, respectively (McDougall and Harrison, 1999), and they may shed light on the homogeneity of a sample. The shear-zone phengite is clearly isochemical, tightly clustering into a single population (Fig. 4B), indicating a single Ar reservoir. The host-rock phengite is comparatively scattered, particularly with respect to 37Ar. The somewhat heterogeneous 37Ar/39Ar versus 38Ar/39Ar distribution for host-rock phengite may be explained by the presence of different Ar reservoirs (e.g., Forster and Lister, 2004), or it could be a result of minor Ca (as 37Ar) contamination of calcite intergrown within or on the mica. Interestingly, although the 40Ar/39Ar analysis reveals two distinct chemical behaviors in Ar-isotope space, our mineral analyses illustrate that neither phengite sample exhibits chemical zoning, and both samples are chemically homogeneous with respect to the major-element chemistry. The observed scattering in Si content can be attributed to EMP uncertainties, as shown by a Monte Carlo analysis (Fig. 3A; Table DR2 [see footnote 1]).

The apparent homogeneous chemical composition of phengite for the host rock and shear zone can be explained by thermodynamic modeling (Fig. 5). An equilibrium phase diagram calculated for the rock composition suggests that the documented mineral assemblage (calcite/aragonite + quartz + dolomite + phengite) is stable over a wide range of fluid compositions and P-T conditions along the P-T path of Syros rocks (Trotet et al., 2001b; Schumacher et al., 2008). The parameters of the model include information on fluid composition, demonstrating that the mineral assemblage is also stable for CO2 fractions between 0.005 and 0.03. Similar fluid compositions (XCO2 <0.03) have been reported for impure marbles of Syros (Schumacher et al., 2008). With phengite being the only stable K- and Al-bearing phase over a wide range of P-T conditions, a change in mica chemical composition by Tschermak and pyrophyllite substitution is not probable. The phengite chemistry is therefore stable within the marble’s mineral assemblage and is not required to reequilibrate through prograde and retrograde conditions. If chlorite would have been present, the activation of these substitutions might have been possible, resulting in potential resetting of the K/Ar system.

Figure 5.

Equilibrium assemblage diagram demonstrating the stability field of the observed high-variance mineral assemblage (Ph + Cal + Qtz + Dol) of the calcite marble on Syros. Calculation was done in the CaKMASHC system with the PERPLEX 6.6.8 (Connolly, 1990) package using the Holland and Powell (1998) database. Solid solutions were considered for white mica (Coggon and Holland, 2002), carbonates (Holland and Powell, 1998), and H2O-CO2 fluid (Holland and Powell, 1998). Excluded phases are zoisite, sanidine, and vesuvianite. Bulk-rock composition was calculated using mineral fractions and electron microprobe analyses (SiO2 = 4.47 wt%, Al2O3 = 1.68 wt%, MgO = 0.40 wt%, CaO = 92.67 wt%, K2O = 0.78 wt%). Pressure-temperature and fluid composition are likely to have varied during the metamorphic evolution of the studied rocks. The phase diagram was therefore calculated along variable H2O-CO2 fluid composition (x-axis) and a linear gradient (from 550 °C, 18 kbar to 300 °C, 4 kbar; y-axis) that approximates the pressure-temperature path of Syros (Trotet et al., 2001b; Schumacher et al., 2008). Mineral abbreviations: Qtz—quartz, Dol—dolomite, Clc—clinochlore, Phl—phlogopite, Ph—phengite, Tr—tremolite, Kfs—potassium feldspar, Di—diopside, Tlc—talc, Arg—aragonite, and Cal—calcite.

Figure 5.

Equilibrium assemblage diagram demonstrating the stability field of the observed high-variance mineral assemblage (Ph + Cal + Qtz + Dol) of the calcite marble on Syros. Calculation was done in the CaKMASHC system with the PERPLEX 6.6.8 (Connolly, 1990) package using the Holland and Powell (1998) database. Solid solutions were considered for white mica (Coggon and Holland, 2002), carbonates (Holland and Powell, 1998), and H2O-CO2 fluid (Holland and Powell, 1998). Excluded phases are zoisite, sanidine, and vesuvianite. Bulk-rock composition was calculated using mineral fractions and electron microprobe analyses (SiO2 = 4.47 wt%, Al2O3 = 1.68 wt%, MgO = 0.40 wt%, CaO = 92.67 wt%, K2O = 0.78 wt%). Pressure-temperature and fluid composition are likely to have varied during the metamorphic evolution of the studied rocks. The phase diagram was therefore calculated along variable H2O-CO2 fluid composition (x-axis) and a linear gradient (from 550 °C, 18 kbar to 300 °C, 4 kbar; y-axis) that approximates the pressure-temperature path of Syros (Trotet et al., 2001b; Schumacher et al., 2008). Mineral abbreviations: Qtz—quartz, Dol—dolomite, Clc—clinochlore, Phl—phlogopite, Ph—phengite, Tr—tremolite, Kfs—potassium feldspar, Di—diopside, Tlc—talc, Arg—aragonite, and Cal—calcite.

Our study is certainly not an exhaustive one, and it is meant to challenge the common approach of a field geologist sampling a shear zone in an attempt to resolve the age of deformation. In our investigation, we have surprising 40Ar/39Ar results from a field-based perspective. If, instead, we were able to conduct the same experiment on a set of quartzofeldspathic rocks, our results would be markedly different, and similar to other studies that report 10–20 m.y. scatter in apparent ages (e.g., Mulch et al., 2002; Cossette et al., 2015). Thus, we urge caution when dating micas from deformed calcite-dominated assemblages.

CONCLUSION

  • (1) Step-heated 40Ar/39Ar geochronology performed on phengite from host-rock and shear-zone marbles collected on Syros results in indistinguishable ages of ca. 40 Ma, reflecting the overall Eocene high-pressure event and not the Miocene deformation, which is recorded in structures with opposing kinematics.

  • (2) Metamorphic conditions modeled from the preserved mineral assemblage do not require phengite reequilibration during the Miocene event.

  • (3) Although marbles were deformed during the Miocene at extremely fast strain rates (10−10 s–1) under greenschist-facies conditions, calcite accommodated most of the strain, thus inhibiting phengite recrystallization.

  • (4) Neither mechanical nor chemical processes caused any disturbance of the phengite crystal lattice, and therefore the K/Ar system remained closed during the Miocene deformation event.

  • (5) In accord with previous studies, it is emphasized that calcite marbles may not be the ideal host rock for resolving deformation ages. Moreover, it is the degree of mica recrystallization and not the amount of finite strain the rock has experienced that controls the resetting of K/Ar systems at low temperatures.

We thank the University of Vienna (grant number IK543002) for supporting the doctoral school DOGMA (“Deformation of Geological Materials”) and the Austrian Science Foundation (Fonds zur Förderung der wissenschaftlichen Forschung [FWF]) for funding the project “Mineral reactions and deformation in host-inclusion settings” (grant number I471-N19) as part of the international research group FOR741-DACH. Funding for Schneider was provided by a Natural Sciences and Engineering Research Council of Canada Discovery grant. Detailed reviews by reviewers Pierre Lanari, Yvette Kuiper, and Klaus Gessner and efficient editorial handling by Arlo Weil are gratefully appreciated.

1GSA Data Repository Item 2015236, a short description of sample preparation for 40Ar/39Ar geochronology as well as a description of the analytical method itself; Table DR1 displays the 40Ar/39Ar isotopic data of the analyses; Table DR2 displays a representative mineral composition of host rock and shear zone phengite together with results of a Monte Carlo simulation, is available at www.geosociety.org/pubs/ft2015.htm, or on request from editing@geosociety.org, Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.

REFERENCES CITED

1.
Bestmann
M.
Kunze
K.
Matthews
A.
,
2000
,
Evolution of a calcite marble shear zone complex on Thassos Island, Greece: Microstructural and textural fabrics and their kinematic significance
:
Journal of Structural Geology
 , v.
22
, p.
1789
1807
, doi:10.1016/S0191-8141(00)00112-7.
2.
Bond
C.E.
Butler
R.W.H.
Dixon
J.E.
,
2007
,
Co-axial horizontal stretching within extending orogens: The exhumation of HP rocks on Syros (Cyclades) revisited
, in
Ries
A.C.
Butler
R.W.H.
Graham
R.H.
, eds.,
Deformation of the Continental Crust: The Legacy of Mike Coward: Geological Society of London Special Publication 272
 , p.
203
222
, doi:10.1144/GSL.SP.2007.272.01.12.
3.
Bonneau
M.
,
1984
,
Correlation of the Hellenide nappes in the south-east Aegean and their tectonic reconstruction
, in
Dixon
J.E.
Robertson
A.H.F.
, eds.,
The Geological Evolution of the Eastern Mediterranean: Geological Society of London Special Publication 17
 , p.
517
527
, doi:10.1144/GSL.SP.1984.017.01.38.
4.
Bröcker
M.
Bieling
D.
Hacker
B.
Gans
P.
,
2004
,
High Si phengite records the time of greenschist-facies overprinting: Implications for models suggesting mega-detachments in the Aegean Sea
:
Journal of Metamorphic Geology
 , v.
22
, p.
427
442
, doi:10.1111/j.1525-1314.2004.00524.x.
5.
Bröcker
M.
Baldwin
S.
Arkudas
R.
,
2013
,
The geological significance of 40Ar/39Ar and Rb-Sr white mica ages from Syros and Sifnos, Greece: A record of continuous (re)crystallization during exhumation
:
Journal of Metamorphic Geology
 , v.
31
, p.
629
646
, doi:10.1111/jmg.12037.
6.
Coggon
R.
Holland
T.J.B.
,
2002
,
Mixing properties of phengitic micas and revised garnet-phengite thermobarometers
:
Journal of Metamorphic Petrology
 , v.
20
, p.
683
696
, doi:10.1046/j.1525-1314.2002.00395.x.
7.
Connolly
J.A.D.
,
1990
,
Multivariable phase diagrams: An algorithm based on generalized thermodynamics
:
American Journal of Science
 , v.
290
, p.
666
718
, doi:10.2475/ajs.290.6.666.
8.
Cossette
É.
Schneider
D.A.
Warren
C.
Grasemann
B.
,
2015
,
Lithological, rheological and fluid infiltration control on 40Ar/39Ar ages in polydeformed rocks from the West Cycladic detachment system, Greece
:
Lithosphere
 , v.
7
, no.
2
, p.
189
205
, doi:10.1130/L416.1.
9.
Delle Piane
C.
Wilson
C.J.L.
Burlini
L.
,
2009
,
Dilatant plasticity in high strain experiments on calcite-muscovite aggregates
:
Journal of Structural Geology
 , v.
31
, p.
1084
1099
, doi:10.1016/j.jsg.2009.03.005.
10.
Dunlap
W.J.
,
1997
,
Neocrystallization or cooling? 40Ar/39Ar ages of white micas from low-grade mylonites
:
Chemical Geology
 , v.
143
, p.
181
203
, doi:10.1016/S0009-2541(97)00113-7.
11.
Dunlap
W.J.
Kronenberg
A.K.
,
2001
,
Argon loss during deformation of micas: Constraints from laboratory deformation experiments
:
Contributions to Mineralogy and Petrology
 , v.
141
, p.
174
185
, doi:10.1007/s004100000217.
12.
Etchecopar
A.
,
1977
,
A plane kinematic model of progressive deformation in a polycrystalline aggregate
:
Tectonophysics
 , v.
39
, p.
121
139
, doi:10.1016/0040-1951(77)90092-0.
13.
Forster
M.A.
Lister
G.S.
,
2004
,
The interpretation of 40Ar/39Ar apparent age spectra produced by mixing: Application of the method of asymptotes and limits
:
Journal of Structural Geology
 , v.
26
, no.
2
, p.
287
305
, doi:10.1016/j.jsg.2003.10.004.
14.
Gautier
P.
Brun
J.-P.
,
1994
,
Crustal-scale geometry and kinematics of late-orogenic extension in the central Aegean (Cyclades and Evia Island)
:
Tectonophysics
 , v.
238
, p.
399
424
, doi:10.1016/0040-1951(94)90066-3.
15.
Gébelin
A.
Mulch
A.
Teyssier
C.
Heizler
M.
Vennemann
T.
Seaton
N.
,
2011
,
Oligo-Miocene extensional tectonics and fluid flow across the Northern Snake Range detachment system, Nevada
:
Tectonics
 , v.
30
, p.
TC5010
, doi:10.1029/2010TC002797.
16.
Goodwin
L.B.
Renne
P.R.
,
1991
,
Effects of progressive mylonitization on Ar retention in biotite from the Santa Rosa mylonite zone, California, and thermochronological implications
:
Contributions to Mineralogy and Petrology
 , v.
108
, p.
283
297
, doi:10.1007/BF00285937.
17.
Goscombe
B.D.
Passchier
C.W.
Hand
M.
,
2004
,
Boudinage classification: End-member boudin types and modified boudin structures
:
Journal of Structural Geology
 , v.
26
, p.
739
763
, doi:10.1016/j.jsg.2003.08.015.
18.
Grasemann
B.
Stüwe
K.
,
2001
,
The development of flanking folds during simple shear and their use as kinematic indicators
:
Journal of Structural Geology
 , v.
23
, p.
715
724
, doi:10.1016/S0191-8141(00)00108-5.
19.
Hames
W.E.
Bowring
S.A.
,
1994
,
An empirical evaluation of the argon diffusion geometry in muscovite
:
Earth and Planetary Science Letters
 , v.
124
, p.
161
169
, doi:10.1016/0012-821X(94)00079-4.
20.
Handy
M.R.
,
1990
,
The solid-state flow of polymineralic rocks
:
Journal of Geophysical Research
 , v.
95
, p.
8647
8661
, doi:10.1029/JB095iB06p08647.
21.
Harrison
T.M.
Célérier
J.
Aikman
A.B.
Hermann
J.
Heizler
M.T.
,
2009
,
Diffusion of 40Ar in muscovite
:
Geochimica et Cosmochimica Acta
 , v.
73
, p.
1039
1051
, doi:10.1016/j.gca.2008.09.038.
22.
Holland
T.J.B.
Powell
R.
,
1998
,
An internally-consistent thermodynamic dataset for phases of petrological interest
:
Journal of Metamorphic Geology
 , v.
16
, p.
309
344
, doi:10.1111/j.1525-1314.1998.00140.x.
23.
Jolivet
L.
Brun
J.-P.
,
2010
,
Cenozoic geodynamic evolution of the Aegean
:
International Journal of Earth Sciences
 , v.
99
, p.
109
138
, doi:10.1007/s00531-008-0366-4.
24.
Keiter
M.
Ballhaus
C.
Tomaschek
F.
,
2011
,
A New Geological Map of the Island of Syros (Aegean Sea, Greece): Implications for Lithostratigraphy and Structural History of the Cycladic Blueschist Unit
:
Geological Society of America Special Paper 481
 ,
43
p.
25.
Kellett
D.
Joyce
N.
,
2014
,
Analytical Details of Single- and Multicollection 40Ar/39Ar Measurements for Conventional Step-Heating and Total Fusion Age Calculation Using the Nu Noblesse at the Geological Survey of Canada
:
Geological Survey of Canada Technical Note 8
 ,
27
p., doi:10.4095/293465.
26.
Kocher
T.
Mancktelow
N.S.
,
2005
,
Dynamic reverse modelling of flanking structures: A source of quantitative kinematic information
:
Journal of Structural Geology
 , v.
27
, p.
1346
1354
, doi:10.1016/j.jsg.2005.05.007.
27.
Kramar
N.
Cosca
M.A.
Hunziker
J.C.
,
2001
,
Heterogeneous 40Ar distribution in naturally deformed muscovite: In situ UV-laser ablation evidence for microstructurally controlled intragrain diffusion
:
Earth and Planetary Science Letters
 , v.
192
, p.
377
388
, doi:10.1016/S0012-821X(01)00456-3.
28.
Lagos
M.
Scherer
E.E.
Tomaschek
F.
Münker
C.
Keiter
M.
Berndt
J.
Ballhaus
C.
,
2007
,
High precision Lu-Hf geochronology of Eocene eclogite-facies rocks from Syros, Cyclades, Greece
:
Chemical Geology
 , v.
243
, p.
16
35
, doi:10.1016/j.chemgeo.2007.04.008.
29.
Lanari
P.
Guillot
S.
Schwartz
S.
Vidal
O.
Tricart
P.
Riel
N.
Beyssac
O.
,
2012
,
Diachronous evolution of the alpine continental subduction wedge: Evidence from P-T estimates in the Briançonnais zone houillère (France, western Alps)
:
Journal of Geodynamics
 , v.
56–57
, p.
39
54
, doi:10.1016/j.jog.2011.09.006.
30.
Lanari
P.
Vidal
O.
De Andrade
V.
Dubacq
B.
Lewin
E.
Grosch
E.
Schwartz
S.
,
2014
,
XMapTools: A MATLAB©-based program for electron microprobe X-ray image processing and geothermobarometry
:
Computers & Geosciences
 , v.
62
, p.
227
240
, doi:10.1016/j.cageo.2013.08.010.
31.
Lee
J.K.W.
,
1995
,
Multipath diffusion in geochronology
:
Contributions to Mineralogy and Petrology
 , v.
120
, p.
60
82
, doi:10.1007/BF00311008.
32.
McDougall
I.
Harrison
T.M.
,
1999
,
Geochronology and Thermochronology by the 40Ar/39Ar Method
(2nd ed.):
New York
,
Oxford University Press
,
288
p.
33.
Mulch
A.
Cosca
M.
Handy
M.
,
2002
,
In-situ UV-laser 40Ar/39Ar geochronology of a micaceous mylonite: An example of defect-enhanced argon loss
:
Contributions to Mineralogy and Petrology
 , v.
142
, no.
6
, p.
738
752
, doi:10.1007/s00410-001-0325-6.
34.
Papanikolaou
D.J.
,
1987
,
Tectonic evolution of the Cycladic blueschist belt (Aegean Sea, Greece)
, in
Helgeson
H.C.
, ed.,
Chemical Transport in Metasomatic Processes
 :
Netherlands
,
D. Reidel Publishing
, p.
429
450
.
35.
Passchier
C.W.
,
2001
,
Flanking structures
:
Journal of Structural Geology
 , v.
23
, p.
951
962
, doi:10.1016/S0191-8141(00)00166-8.
36.
Philippon
M.
Brun
J.P.
Gueydan
F.
,
2011
,
Tectonics of the Syros blueschists (Cyclades, Greece): From subduction to Aegean extension
:
Tectonics
 , v.
30
, p.
TC4001
, doi:10.1029/2010TC002810.
37.
Ring
U.
Glodny
T.
Will
T.
Thomson
S.
,
2010
,
The Hellenic subduction system: High-pressure metamorphism, exhumation, normal faulting and large-scale extension
:
Annual Review of Earth and Planetary Sciences
 , v.
38
, p.
45
76
, doi:10.1146/annurev.earth.050708.170910.
38.
Rogowitz
A.
Grasemann
B.
Huet
B.
Habler
G.
,
2014
,
Strain rate dependent calcite microfabric evolution—An experiment carried out by nature
:
Journal of Structural Geology
 , v.
69
, p.
1
17
, doi:10.1016/j.jsg.2014.08.004.
39.
Rolland
Y.
Cox
S.F.
Corsini
M.
,
2009
,
Constraining deformation stages in brittle-ductile shear zones from combined field mapping and 40Ar/39Ar dating: The structural evolution of the Grimsel Pass area (Aar Massif, Swiss Alps)
:
Journal of Structural Geology
 , v.
31
, p.
1377
1394
, doi:10.1016/j.jsg.2009.08.003.
40.
Sanchez
G.
Rolland
Y.
Schneider
J.
Corsini
M.
Oliot
E.
Goncalves
P.
Verati
C.
Lardeaux
J.
Marquer
D.
,
2011
,
Dating low-temperature deformation by 40Ar/39Ar on white mica, insights from the Argentera-Mercantour Massif (SW Alps)
:
Lithos
 , v.
125
, p.
521
536
, doi:10.1016/j.lithos.2011.03.009.
41.
Schneider
S.
Hammerschmidt
K.
Rosenberg
C.
,
2013
,
Dating the longevity of ductile shear zones: Insight from 40Ar/39Ar in situ analyses
:
Earth and Planetary Science Letters
 , v.
369–370
, p.
43
58
, doi:10.1016/j.epsl.2013.03.002.
42.
Schumacher
J.C.
Brady
J.B.
Cheney
J.T.
Tonnsen
R.R.
,
2008
,
Glaucophane-bearing marbles on Syros, Greece
:
Journal of Petrology
 , v.
49
, p.
1667
1686
, doi:10.1093/petrology/egn042.
43.
de Sigoyer
J.
Chavagnac
V.
Blichert-Toft
J.
Villa
I.M.
Luais
B.
Guillot
S.
Cosca
M.
Mascle
G.
,
2000
,
Dating the Indian continental subduction and collisional thickening in the northwest Himalaya: Multichronology of the Tso Morari eclogites
:
Geology
 , v.
28
, p.
487
490
, doi:10.1130/0091-7613(2000)28<487:DTICSA>2.0.CO;2.
44.
Soukis
K.
Stockli
D.F.
,
2013
,
Structural and thermochronometric evidence for multi-stage exhumation of southern Syros, Cycladic Islands, Greece
:
Tectonophysics
 , v.
595–596
, p.
148
164
, doi:10.1016/j.tecto.2012.05.017.
45.
Tomaschek
F.
Baumann
A.
Villa
I.M.
Kennedy
A.
Ballhaus
C.
,
2000
,
Geochronological constraints on a Cretaceous metamorphic event from the Vari Unit (Syros, Cyclades, Greece)
:
Beihefte zum European Journal of Mineralogy
 , v.
12
, p.
214
.
46.
Tomaschek
F.
Kennedy
A.K.
Villa
I.M.
Lagos
M.
Ballhaus
C.
,
2003
,
Zircons from Syros, Cyclades, Greece—Recrystallization and mobilization of zircon during high-pressure metamorphism
:
Journal of Petrology
 , v.
44
, p.
1977
2002
, doi:10.1093/petrology/egg067.
47.
Trotet
F.
Jolivet
L.
Vidal
O.
,
2001a
,
Tectono-metamorphic evolution of Syros and Sifnos Islands (Cyclades, Greece)
:
Tectonophysics
 , v.
338
, p.
179
206
, doi:10.1016/S0040-1951(01)00138-X.
48.
Trotet
F.
Vidal
O.
Jolivet
L.
,
2001b
,
Exhumation of Syros and Sifnos metamorphic rocks (Cyclades, Greece). New constraints on the P-T paths
:
European Journal of Mineralogy
 , v.
13
, p.
901
920
, doi:10.1127/0935-1221/2001/0013/0901.
49.
Warren
C.
Hanke
F.
Kelley
S.
,
2012
,
When can muscovite 40Ar/39Ar dating constrain the timing of metamorphic exhumation?
:
Chemical Geology
 , v.
291
, p.
79
86
, doi:10.1016/j.chemgeo.2011.09.017.
50.
Wijbrans
J.R.
McDougall
I.
,
1986
,
40Ar/39Ar dating of white micas from an Alpine high-pressure metamorphic belt on Naxos (Greece): The resetting of the argon isotopic systems
:
Contributions to Mineralogy and Petrology
 , v.
93
, p.
187
194
, doi:10.1007/BF00371320.
51.
Wortel
M.J.R.
Goes
S.D.B.
Spakman
W.
,
1990
,
Structure and seismicity of the Aegean subduction zone
:
Terra Nova
 , v.
2
, p.
554
562
, doi:10.1111/j.1365-3121.1990.tb00120.x.