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).

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).

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).

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).

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.

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.