Quartz and calcite microfabric transitions in a pressure and temperature gradient, Sivrihisar, Turkey

Abstract

Interlayered quartzite and marble in the southern Sivrihisar Massif, Turkey, record metamorphic conditions ranging from high-pressure/low-temperature through a Barrovian overprint from chlorite- to sillimanite-zone conditions. This sequence was exhumed under transtension, producing macroscopic constrictional fabrics (L-tectonites) during crustal thinning. Quartz microstructures consist of dynamically recrystallized aggregates in the dislocation creep regime dominated by grain boundary migration. Quartz microstructures are relatively constant across the high metamorphic gradient, and crystallographic fabric patterns transition from plane strain to constriction strain. Calcite fabrics are characterized by progressive overprinting of a columnar texture inherited from the high-pressure polymorph aragonite. In the low-temperature Barrovian domain (<400 °C), shearing of calcite rods produced a very strong c-axis point maximum. At moderate temperature, calcite rods were partially to totally recrystallized and the strong preferred orientation maintained. At temperature >500 °C and high constriction strain, marble has no crystallographic fabric, likely reflecting a transition from dislocation creep to diffusion creep. Phengite in high-pressure/low-temperature marble and quartzite yields relatively simple age spectra with Late Cretaceous (88–82 Ma) ⁴⁰Ar/³⁹Ar ages. Barrovian muscovite records significantly younger ages (63–55 Ma). The transtension system and associated metamorphism may have occurred above a subduction zone in Paleocene–Eocene time as a precursor to intrusion of Eocene (~53 Ma) arc plutons.

Keywords:

• quartz
• calcite
• microstructure
• crystallographic fabrics
• constriction
• argon-argon dates
• Sivrihisar
• Turkey

1. Introduction

The rheology of the continental crust is strongly influenced by the deformation behavior of quartz (Hacker, Yin, Christie, & Davis, 1992; Hirth, Teyssier, & Dunlap, 2001; Hirth & Tullis, 1992; Jessell, 1987; Tullis, 1977; Tullis, Christie, & Griggs, 1973; Tullis & Yund, 1987). In orogens with abundant carbonate rocks (e.g. the Tethyan Alpine-Himalayan belt), the deformation behavior of carbonate minerals is also important for understanding rheology (Barber, Wenk, Gomez-Barreiro, Rybacki, & Dresen, 2007; Barnhoorn, Bystricky, Burlini, & Kunze, 2004; Barnhoorn, Bystricky, Burlini, & Kunze, 2005; Casey, Kunze, & Olgaard, 1998; Pieri, Burlini, Kunze, Stretton, & Olgaard, 2001; Pieri, Kunze, et al., 2001; Wenk, Takeshita, Van Houtte, & Wagner, 1986). Field-based studies of naturally deformed quartz and calcite provide a critical link between deformation mechanisms determined from experiments and those experienced in nature (Bestmann, Kunze, & Matthews, 2000; Dresen, Duyster, Stöckhert, & Wirth, 1997; Oesterling, Heilbronner, Stünitz, Barnhoorn, & Molli, 2007; Stöckhert, Wachmann, Kuster, & Bimmermann, 1999; Weber, Ferrill, & Roden-Tice, 2001). Of particular interest are field sites in which an array of rock types and temperature (T), pressure (P) and deformation (d) conditions are represented across a region, allowing a broad view of microstructures. In the case of interlayered quartzite and marble, microstructures in quartz and calcite can be compared to understand the response of these rheologically important minerals to different P–T–d conditions.

An investigation of quartz and calcite microstructures in the southern Sivrihisar Massif, Turkey (Figure 1(A) and (B)), is useful for understanding deformation microstructures, mechanisms, and conditions of naturally deformed quartz and carbonate because the massif contains abundant interlayered quartzite and marble and is comprised of a subduction complex that has been partially to completely overprinted by later metamorphism at lower pressures and a range of temperatures in a transtensional setting (Seaton, Whitney, Teyssier, Toraman, & Heizler, 2009; Whitney, 2002; Whitney, Teyssier, Toraman, Seaton, & Fayon, 2011). Recorded metamorphic conditions range from HP to LT blueschist and eclogite facies conditions (~18–22 kbar, 400 °C) to low/medium-P (~3–6 kbar) regional metamorphism at a range of T (from <400 °C to >650 °C) (Whitney et al., 2011) (Figure 1(C)). The entire sequence contains marble and quartzite, allowing for comparison of deformation behavior across a broad spectrum of P–T conditions.

Figure 1. (A) Location of the Tavşanlı Zone (TZ), the Afyon Zone (AF) and Sivrihisar (S) Massif in Turkey. (B) Simplified map of the Sivrihisar Massif showing the high-pressure (HP) domain to the northwest and at the northern tip of the southern submassif, and the Barrovian domain (+ granites) comprising most of the southern submassif. (C) P–T summary figure after Whitney et al. (2011).

In this study, we collected systematic field and microstructural data for quartz in quartzite and calcite in marble along a traverse from the blueschist/eclogite domain through the adjacent, highly attenuated Barrovian sequence. Primary goals were to characterize the grain-scale deformation mechanisms that prevailed under a wide range of conditions in quartz and calcite, to determine sense-of-shear across the blueschist-to-Barrovian zones, and to understand the processes responsible for the collapse (thinning) of the Barrovian crust, the development of a strong constrictional fabric, and the exhumation of the sequence.

2. Geologic overview of the Sivrihisar Massif

The Sivrihisar Massif, west-central Turkey, is part of the Tavşanlı zone (Okay, 1980) (Figure 1(A)), an exhumed subduction complex that formed during Late Cretaceous closure of a Neo-Tethyan seaway (Okay, Harris, & Kelley, 1998) and suturing of the Tauride-Anatolide microcontinent (related to Gondwana) with Eurasia (Robertson, 2002; Şengör, 1990; Stampfli & Borel, 2002). The zone was intruded by Eocene granitoids related to arc magmatism during Paleogene subduction of a slab to the south; this subduction also produced HP–LT metamorphism in the Afyon zone (Pourteau, Candan, & Oberhänsli, 2010) (Figure 1).

The Sivrihisar Massif is dominated by blueschist facies rocks (Gautier, 1984; Okay, 1986) but also contains eclogite (Kulaksız, 1978), some of it lawsonite-bearing (Cetinkaplan, Candan, Oberhänsli, & Bousquet, 2008; Davis & Whitney, 2006, 2008; Whitney & Davis, 2006). The southern part of the massif was overprinted by regional (Barrovian) metamorphism up to sillimanite zone conditions (Whitney, 2002; Whitney et al., 2011). Here, rock layering and foliation dip generally northward, exposing a section through a sequence of increasing metamorphic temperature toward the south. Mineral lineation typically plunges toward the N–E at a moderate angle (Figure 2). The entire sequence (HP to Barrovian) is dominated by marble that contains quartz- and mica-rich layers. Compositional layering is oriented approximately E–W and dips steeply (>40°) to the N, forming a tabular, ~500 m thick zone of highly deformed rocks characterized by planar and linear fabrics (S–L tectonite). Lineation is defined by the preferred orientation of calcite rods and aggregates and by elongate quartz and mica grains, and is fairly constant in orientation (NE plunge).

Figure 2. (A) Geologic map of the southern Sivrihisar submassif, with sample locations of quartzite and marble, as well as representative foliation and lineation data.

The Barrovian domain is characterized by massive marble interlayered with quartzite, mica schist and mafic rocks. Layering and foliation are variable owing to folding. In the central region, which contains abundant quartzite, foliation strikes northward and dips steeply (Figure 2). Linear fabric tectonites (L tectonites) are common here and in the southern, highest-grade part of the sequence; layering and foliation are variably oriented and folded around the prominent lineation. The strong lineation plunges to the NE and is defined by quartz, white mica and Al-rich porphyroblasts such as chloritoid, staurolite, kyanite and sillimanite.

The Sivrihisar granitoid exposed south of the Barrovian sequence is part of a belt of shallowly emplaced, post-collisional, mostly calc-alkaline plutons that intruded the Tavşanlı Zone (Harris, Kelly, & Okay, 1994; Kibici, Ilbeyli, Yıldız, & Bağci, 2008). Zircon U–Pb ages range from 78.4 ± 8.5 Ma to 42.4 ± 2.3 Ma (Shin, Catlos, Jacob, & Black, in press). The hornblende ⁴⁰Ar/³⁹Ar age for the granitoid is 53 ± 3 Ma (Sherlock, Kelley, Inger, Harris, & Okay, 1999) (ca. 54 Ma when recalculated using the Renne et al. (2010) age of GA1550 biotite). We also obtained a new apatite fission-track age of 41.6 ± 4 Ma from a sample collected near the margin of the pluton (Figure 2).

In the traverse discussed in this paper, the granite is essentially undeformed and has a well-exposed intrusive contact with the high-grade end of the Barrovian sequence (sillimanite zone) (Figure 2). The intrusion crosscuts lithologic layers, although some m-scale, layer-parallel leucogranite sills occur within metasedimentary rocks near the contact. Where in contact with schist, texturally late sprays of fibrolite crosscut foliation. The Barrovian isograds pre-date the intrusion; index minerals define the strong lineation of the L-tectonites, and the sequence represents ~10 km of structural thickness that is now collapsed in a section <1 km thick (Whitney et al., 2011).

The most significant contact metamorphic effects of the pluton, as determined by petrographic and microstructural observations of rocks at increasing distance from the pluton, are confined to within a few meters of the contact (Seaton et al., 2009). However, a fluid-related event that produced texturally late muscovite and chlorite may have been related to the intrusion and extends ~100 m (structural thickness) from the contact into the Barrovian sequence. Nevertheless, the deformation textures, isograd distribution, age relationships, and petrologic and textural effects of the intrusion on its host rocks all suggest that the major metamorphic and deformation features observed in the Barrovian sequence were related to a pre-intrusion event (Whitney et al., 2011).

3. Microstructure and microfabrics

Quartz and calcite microstructures are described from a traverse from north to south, from structurally high to structurally lower levels of the sequence. In the northern part of the traverse, HP–LT assemblages are preserved, but it is less clear under what conditions microstructures developed. The Barrovian metamorphic overprint increases down section from greenschist to amphibolite facies. We investigated quartz and calcite microstructures through the transition from the HP–LT metamorphic rocks to the Barrovian sequence.

3.1. Quartzite

There are various types of quartzite in the field area, including black (graphitic) quartzite, white (nearly pure) quartzite and white mica-bearing quartzite (±Al₂SiO₅ polymorphs, chloritoid, staurolite, garnet, tourmaline and magnetite/rutile). In the HP domain, quartzite occurs as m-scale pods and discontinuous layers. In the Barrovian domain, quartzite occurs as layers that can be traced for meters to hundreds of meters. In both domains, quartzite has a strong foliation defined by white mica and/or compositional layering, and a lineation defined by elongate minerals.

All quartzites examined show evidence for dynamic recrystallization by grain boundary migration (Regime 3, Hirth & Tullis, [1992]). Regime III microstructures occur throughout the field area, but are best developed in the Barrovian sequence, particularly at the deeper structural levels (Figure 3). Throughout the field area, there is no clear asymmetry displayed in quartz microstructures or other textural elements of the rocks; both top-to-north and top-to-south sense of shear is observed, in some cases in the same rock or thin section, with no clear crosscutting relationship. These relations suggest that the rocks were deformed by dominantly coaxial strain.

Figure 3. Photomicrographs showing representative microstructures in quartzite from the high-pressure domain preserved in the chlorite-zone of Barrovian sequence (A, B, C) and in the southern, amphibolite-facies portion of the Barrovian sequence (D, E, F). Photomicrographs in A, D, E and F correspond to samples in Figure 5A (EBSD pole figures for quartz c-axis and a-axis).

In the HP–LT domain and at the low-grade (greenschist facies) end of the Barrovian sequence, quartz microstructures are not uniform and depend on position in meso-scale folds. In hinge regions, mica folia anastomose around quartz lenses preserve a microstructural history. The central regions of quartz lenses display large grains (Figure 3(A)), 0.3 mm in diameter, with straight boundaries and triple junctions. More typically these large grains are partially recrystallized (Figure 3(B)); they contain deformation bands and subgrains, and their boundaries are serrated or decorated with newly recrystallized grains, particularly toward the tails of quartz lenses. Subgrain and recrystallized grain size is typically 0.05 mm. In fold limbs, mica layers define a planar fabric that separates entirely recrystallized quartz ribbons (Figure 3(C)). Quartz recrystallized grain size is ~0.05 mm, and grains are equant. In this part of the traverse, mica grains are dominantly planar and are not boudinaged as they are in the quartzite further south. Locally, quartzite contains crosscutting cataclastic zones of very fine-grained quartz. These cm-scale cataclastic zones overprint earlier fabrics and are commonly associated with hematite.

At deeper structural levels in the Barrovian domain, highly lineated quartzite also displays Regime 3 microstructures. Quartz grains are polygonal, equigranular, with equilibrated grain boundaries showing 120° triple junctions. Quartz grains appear to have overgrown a finer grained microstructure that is preserved in the spacing of now-discontinuous mica layers (Figure 3(D)). The mica grains are highly elongated and boudinaged, and small mica flakes are isolated within quartz grains (Figure 3(E)). Isolated mica grains have phengite cores and muscovite rims (Whitney et al., 2011), suggesting that constrictional fabrics and quartz dynamic recrystallization have prevented them from equilibrating at Barrovian conditions. In more mica-rich quartzite, lenses and layers of white mica consist of muscovite.

Quartzite located within a few meters of the contact with the intrusion shows evidence for annealing, but the pre-existing fabric has not been completely overprinted. In some quartzite located near the intrusive contact, there is evidence for grain boundary migration: small mica grains defining a relict foliation are embedded within quartz grains and some mica flakes appear to have impeded the migration of grain boundaries (Figure 3(F)).

3.2. Marble

Calcite marble dominates the field area, but dolomite marble also occurs, and some calcite marble contains minor dolomite in layers or isolated crystals. The texture and petrology of marble in this field area have been described in detail (Seaton et al., 2009); only the main features are summarized here.

In the HP domain, marble is typically comprised of mm- to cm-scale compositional layers that alternate between pure calcite layers and calcite + quartz + phengite layers. In the pure calcite layers, single grains of elongate calcite (rods) are aligned in a columnar texture that is typically at a high angle to the foliation defined by mica flakes (Figure 4(A)). This columnar texture is interpreted to indicate pseudomorphs of calcite after aragonite (Brady, Markley, Schumacher, Cheney, & Biancardi, 2004; Seaton et al., 2009).

Figure 4. Photomicrographs showing representative microstructures in marble. (A) Columnar texture in marble showing calcite rods oblique to mica foliation. (B) Partially recrystallized columnar calcite. (C) Completely recrystallized calcite aggregate in graphitic marble (D) showing pencil shape graphite rods. A, B and C correspond to sampled 101, 124 and 150, respectively, in Figure 5.

At the southern end of the domain where HP–LT minerals are preserved in mafic rocks, the columnar texture is still observed in some layers, but most calcite rods are partially replaced by recrystallized calcite (Figure 4(B)), and some layers are entirely comprised of recrystallized calcite. The columnar texture is progressively obliterated from north to south in a transition zone (Seaton et al., 2009). Microstructural analysis reveals that the transition zone grades into a region in which calcite is recrystallized (equant grains, no rods), but the marble retains the same strong crystallographic preferred orientation as measured in the HP and transition zones (Seaton et al., 2009).

In the Barrovian domain, marble is characterized by straight grain boundaries and well-developed triple junctions among equant, coarse (typically 0.5–1 mm) grains (Figure 4(C)). There is no crystallographic preferred orientation of calcite (Seaton et al., 2009). However, highly elongated rods of graphite define a strong linear fabric (Figure 4(D)). Locally, marble is highly brecciated in m-scale, graphite-rich zones. Marble within ~1 m of the pluton contact contains calcite with lobate boundaries and exsolved blebs of dolomite surrounded by smaller calcite grains.

Calcite throughout the field area exhibits similar twinning. There are no systematic differences in twinning characteristics (width, spacing) in marble in the HP vs. Barrovian domains.

4. EBSD methods and results

Quartz and calcite microstructures were characterized using electron backscatter diffraction (EBSD) analysis. Samples were polished in colloidal silica solution (Syton) and given a thin carbon coat. We present EBSD data from 20 quartzite samples and 28 marble samples (five of which were previously published (Seaton et al., 2009)) (Figure 2). Additional samples were examined petrographically (no EBSD data). EBSD data were acquired using a JEOL 6500 FEG-SEM and the Oxford Instruments/HKL Channel 5 software. SEM conditions were 70° tilt, 20 kV accelerating voltage and a ~15 nA beam current. The EBSD data were collected as a series of maps with a 10 μm step size.

4.1. Quartz

Quartz fabrics define four groups that are organized spatially into four domains (Figure 5, Appendix 1 in Supplemental Material). Domain I is restricted to the northern margin of the study area, where HP–LT metamorphic assemblages are preserved; this fabric is defined by a single girdle of c-axes (samples 133C, 108; Appendix 1) or an ill-defined girdle with a point maximum at the edge of the stereonet, corresponding to dominant basal <a> slip.

Figure 5. (A) Quartz EBSD data representative of fabric domains I (preserved high-pressure domain and chlorite zone in Barrovian sequence), II (constrictional fabric patterns in chlorite to sillimanite zone), III (sillimanite zone with dominant prism-a slip) and IV (random fabric at contact with pluton). The complete set of studied samples is in Appendix 1, Supplemental Material; (B) EBSD inverse pole figures corresponding to pole figures shown in Figure 5A.

Domain II occupies most of the study area, from the transition zone between HP and Barrovian sequences in the north to within ~1 km of the contact with the granite in the south (Figure 5(A)). These quartz fabrics are observed over the region in which constriction strain is suspected from the strongly lineated tectonites. In the 13 samples from this domain, c-axes form two girdles that follow a small circle with wide opening angle around the lineation (Figure 5(B); Appendix 1); c-axes are distributed on a cone about the lineation, with an opening angle of 60–70°. Accordingly, a-axes are arranged along a small circle (a cone around the lineation) with an opening angle of 20–30° (Figure 5(B); Appendix 1). These crystallographic fabrics have been attributed to constriction strain on the basis of numerical modeling and rare field documentation (Burg & Teyssier, 1983; Lister & Hobbs, 1980; Price, 1985; Schmid & Casey, 1986; Sullivan & Beane, 2010).

Sample 08-159 (Figure 5) shows the main features of this fabric domain most clearly; patterns in other samples are commonly more diffuse (Appendix 1), but the main characteristics of c- and a-axes distribution are visible; these indicate a combination of basal <a>, prism <a> and rhomb <a> slip systems operated under these moderate T conditions.

Domain III occurs within 1 km of the contact with the pluton and is defined by a tighter distribution of c-axes near the center of the stereonet. Sample 08-148 shows the weak outline of a constrictional fabric, with branches extending away from the center, and sample 08-136 displays a short girdle in the center part of the stereonet that contains maxima and can be interpreted as dominant prism <a> and rhomb <a> slip. The grouping of c-axes toward the center of stereonets in Domain III relative to Domain II is consistent with increased deformation temperatures and the inhibition of basal <a> slip to the profit of prism and rhomb <a> slip (Passchier & Trouw, 2005; Tullis, 1977).

Domain IV is located within a few meters of the contact with the pluton. The fabric is diffuse and does not define clear patterns. Sample 08-134A displays a diffuse set of c-axis maxima near the lineation and an associated weak grouping of a-axes. This fabric may be interpreted as a weak prism <c> fabric that is typically developed at high T (>650 °C) (Blumenfeld, Mainprice, & Bouchez, 1986; Garbutt & Teyssier, 1991; Tullis, 1977); this is consistent with deformation close to the intrusive body.

Inverse pole figures also offer insight into the fabrics that developed in the different domains (Figure 5(B)). Inverse pole figures plot the position of X, Y and Z axes, in which X is lineation and Z is the pole to foliation (XY). The distribution of X-axes relative to crystal axes displays no particular pattern in Domain IV (nearly random fabric). In contrast, Domain III reveals a concentration of X near the quartz a-axes, whereas Y concentrates on the c-axis, as expected from prism <a> slip. The Y-axes are also smeared toward moderate angles away from the c-axis, a likely reflection of rhomb <a> slip. Domain II fabrics show a concentration of X-axes at 70–80° to the c-axes and a complementary distribution at 10–20° to a-axes, consistent with constrictional fabrics (axially symmetric extension [Kibici et al., 2008]). Domain I reveals a band of X-axes at a high angle to the c-axis, corresponding to the single girdle in the pole figure (Figure 5(A)), and a distribution of X-axes along quartz a-axes. A moderate concentration of Z-axes (pole to foliation) near the c-axis indicates an important role of basal <a> slip.

4.2. Calcite

The acquisition of 28 calcite fabric patterns along the HP-to-Barrovian transect confirms the previous results (Seaton et al., 2009) and reveals the location of the microstructural and fabric transitions more exactly (Figure 6). Three domains are identified. In a northern domain, marble contains calcite rods and a strong crystallographic preferred orientation (nine samples). The c-axes are slightly oblique to the perpendicular to foliation (typically < 40°) and are generally located in the upper northern quadrant (Figure 6), consistent with top-to-north sense of shear (Seaton et al., 2009). The calcite rods are systematically oriented in the upper northern quadrant as well, but lie at a low to intermediate angle to the foliation (20–40°). The southern limit of this domain corresponds to the fabric transition identified for quartz.

Figure 6. Calcite c-axis fabrics determined by EBSD; sample locations shown in inset map.

In the next domain to the south (10 samples, Figure 6), the calcite c-axis fabric displays a strong point maximum in the upper northern quadrant, but calcite is partially to completely recrystallized. Relict calcite rods are still present in samples 120, 124, 127 and 130, where calcite rods are generally inclined at a small angle to foliation. This fabric domain is approximately 2 km wide and extends southward to sample locality 209, within quartz fabric Domain II.

From the location of sample 208 and south, a series of nine samples shows a completely recrystallized calcite microstructure where no relict calcite rods are observed. Although macroscopic foliation defined by compositional layering and lineation defined by graphite rods are prominent, calcite c-axes form a weak to random fabric (Figure 6).

5. ⁴⁰Ar/³⁹Ar white mica thermochronology

White mica mineral separates from 10 samples were analyzed by the ⁴⁰Ar/³⁹Ar method: three samples from the northern (HP) domain (1 marble; 2 quartzite) and seven samples from the southern domain (1 marble, 6 quartzite or quartz-rich schist) (Table 1; Figure 7; Appendix 2 in Supplemental Material). White mica in the HP samples is phengite, and in the Barrovian samples is dominantly muscovite, although two quartzite samples from the low-grade end of the Barrovian sequence contain a mixed population of phengite and muscovite, including single grains that have muscovite rims on phengite cores; no phengite cores are found beyond the staurolite-in isograd, just muscovite (Whitney et al., 2011). Argon data for two samples (one HP, one Barrovian) were published in Seaton et al. (2009) and are included here for comparison with the complete data-set.

Table 1. Summary of ⁴⁰Ar/³⁹Ar data.

			Weighted mean age				Total gas age
Sample	Domain	min	n	%^39Ar	MSWD	Age (Ma) ± 1σ	Age (Ma) ±1σ
Summary of ^40Ar/^39Ar results
SV03A29A, Q	Trans	M/P	4	52.3	30	90.04 ± 0.85	84.57 ± 0.21
SV03-95, Q	Thans	M/P	8	71.3	6.0	55.33 ± 0.27	57.50 ± 0.13
SV03A150, Q	BV	M	4	58.7	19	63.02 ± 0.41	64.43 ± 0.13
SV03A31, Q	BV	M	8	80.0	57	60.36 ± 1.05	63.29 ± 0.16
SV01-01, M	BV	M	7	71.3	10	58.88 ± 0.24	59.35 ± 0.15
SV02–32C, Q	BV	M	3	48.8	49	69.41 ± 0.65	72.88 ± 0.12
SV01-21A, Q	BV	M	5	40.2	0.8	59.46 ± 0.12	61.00 ± 0.12
SV02-15E, M	HP	P	11	94.7	6.1	88.76 ± 0.32	87.67 ± 0.20
SV02-08, Q	HP	P	5	85.1	3.0	82.10 ± 0.28	81.93 ± 0.19
SV02-18, Q	HP	P	7	86.9	17	82.73 ± 0.30	80.93 ± 0.18

Notes: Q is quartzite, M is marble. L# = Lab ID. Irrad = Irradiation. %39Ar = Total 39Ar comprising the steps used for weighted mean calculations. min = mineral dated, M = muscovite, P = Phengite, M/P = muscovite + phengite. n = number of steps for weighted mean calculations. Domain = Metamorphic domain: Trans = Transitional, HP = Hiigh Pressure, BV = Barrovian.

Figure 7. ⁴⁰Ar/³⁹Ar spectra of phengite and muscovite.

⁴⁰Ar/³⁹Ar analyses were conducted at the New Mexico Geochronology Research Laboratory, at the New Mexico Bureau of Geology and Mineral Resources. Analytical methods and isotopic measurements are summarized in the data repository and closely follow methods reported by Whitney, Teyssier, and Heizler (2007).

The furnace incremental step-heating method was used to create an age spectrum for each sample (Figure 7). Weighed mean plateau ages are shown for the flattest part of the spectra; however, in several cases, complex spectra do not yield statistically rigorous normal distributions. We assign plateau ages for the relatively flat parts of the age spectra to facilitate description and distinction between samples and recognize that the choice for steps comprising the plateau segment is somewhat arbitrary.

Phengite results display mostly flat spectra with Late Cretaceous plateau ages ranging from 88.7 ± 0.3 to 82.1 ± 0.3 Ma (Figure 7). In contrast, white micas from the Barrovian domain yield significantly disturbed spectra with younger total gas ages between ~57 and 64 Ma. Muscovite from quartzite sample SV03A-150 is ~1 km from the ~53 Ma pluton contact (at current exposure levels) and yields an age of ~63 Ma that contrasts with a much younger age (~55 Ma) for sample SV03-95 that is located ~200 m from the pluton contact (Figure 7).

At the structural level of the transition from greenschist to lower amphibolite facies, white mica in two samples yields complex spectra with intermediate ages. These samples are >2 km from the pluton contact (Figure 2) and contain some muscovite with phengitic cores (Whitney et al., 2011). Sample SV03A-29A has a total gas age of ~85 Ma and SV0232C yields a total gas age of ~73 Ma (Figure 7).

6. Strain modeling

Results of structural and metamorphic analyses suggest that the thermal gradient in this region increased following subduction and that the crust was thinned while constrictional strain developed. Geochronologic data based on ⁴⁰Ar/³⁹Ar white mica dating show that Late Cretaceous HP–LT metamorphism and deformation at ~90–80 Ma was severely overprinted by a Paleocene/Eocene event at ~60–55 Ma. To explore the boundary conditions that led to this evolution, we conducted analytical strain modeling.

HP fabrics of the northern Sivrihisar Massif developed during dextral/normal shearing (Teyssier, Whitney, Toraman, & Seaton, 2011), with relatively flat foliations and lineations oriented consistently E–NE. In contrast, the southern Sivrihisar massif has N–NE lineation trends and is dominated by L-tectonites; this zone has been interpreted as a domain of transtension in a pull-apart structure that developed by a combination of dextral wrenching and N–S extension (Whitney et al., 2011).

Here, we simulate this transtension boundary condition in a forward model in order to explore the finite strain consequences, particularly in terms of crustal thinning and constrictional strain. The boundary conditions are simultaneous N–S extension and dextral shear along an E–W zone. Progressive strain under this transtension (E–W wrench simple shear added to N–S pure shear, Figure 8(A)), for reasonable values of bulk wrench shear (1 < γ_wrench < 2), shows that foliation (λ₁λ₂ plane) would begin vertical but would rapidly switch to horizontal (Figure 8(B)), as the strain ellipsoid goes through a purely constrictional shape (Fossen & Tikoff, 1993; Teyssier & Tikoff, 1999). The finite λ₁ orientation is in the N–E quadrant, consistent with the prominent and robust lineation direction in the southern Sivrihisar Massif (Figure 8(B)).

Figure 8. Transtension strain modeling results; (A) boundary conditions for the transtension model; (B) position of λ₁; (C) field of likely solutions in the constriction region of the Flinn diagram; see text for discussion.

Transtension modeling also investigates the relationship between the shape of the finite strain ellipsoid (Figure 8(C)) and motion of boundaries and predicts strong constriction for many reasonable values of γ_wrench and β, the thinning factor. The field of likely solutions (Figure 8(C)) is derived from several criteria:

• Although no finite strain marker can be used in this region, the qualitative aspect of tectonites in the field suggests that average λ₁–λ₃ ratio is in the order of up to 20:1.

• γ_wrench is sufficient to impart transtension but not sufficient to control the orientation of foliation, which is dominantly horizontal and therefore influenced by the pure shear (thinning) component; therefore, a range of γ_wrench = 1–2 is adopted for this system.

• The pure shear component, which represents the thinning factor β, is considered between 1/2 and 1/4, because increased thinning drags strain solutions toward plane strain and this would be inconsistent with the strong constrictional fabrics observed.

• We also see from modeling results that the angle of divergence relative to the field of likely solutions is most likely around 40–50°, or in the N–E direction, toward the fabric attractor in transtension (Passchier, 1997; Teyssier & Tikoff, 1999).

These results illustrate the relationship among the wrench shear, the thinning factor, the finite stretch and the point where the switch between horizontal and vertical foliation occurs (Figure 8(C)). The field of likely solutions is also indicated, with the most likely values for crustal thinning around 1/2 to 1/4.

Finally, exhumation of the Barrovian sequence likely occurred through top-to-N shearing, as indicated by calcite fabrics (Seaton et al., 2009) and crustal thinning (Whitney et al., 2011). Modeling this shear as an additional boundary condition to the strain modeled above (Figure 9) shows that increasing top-to-N shear drags the finite strain solutions toward plane strain and away from the field of constriction. It is unlikely that this distributed shear strain γ_{hor. shear} is >2 (Figure 9).

Figure 9. Transtension strain modeling showing (left) the stability fields of horizontal and vertical foliation as a function of the thinning factor and increasing finite strain (field of likely solutions in gray); (right) field of likely solutions shifted by the addition of a top-to-N shear that simulates unroofing of the Barrovian sequence.

7. Discussion

From north (HP) to south (Barrovian), calcite marble shows a significant change from well-developed columnar textures (calcite pseudomorphs after aragonite) to partially obliterated HP textures to completely recrystallized marble (Figure 4). In contrast, quartzite microstructures are similar across the transect, although Regime III microstructures are less well developed in the HP and low-grade Barrovian domains as compared to the deeper structural levels of the Barrovian sequence. Quartz crystallographic fabrics display variations in pattern and strength across the area. The switch in c-axes patterns from Domain I to Domain II (single girdle in HP domain to high-angle small circle girdle in the greenschist facies) reflects the increased participation of constrictional strain (Schmid & Casey, 1986; Sullivan & Beane, 2010). In both domains quartz deformed in the dislocation creep regime is dominated by grain boundary migration and by a combination of basal-a, prism-a and rhomb-a slip systems. Constriction strain prevails through the Barrovian zone, but the c- and a-axes patterns reflect dominant prism-a slip toward the south, in the sillimanite zone.

7.1. Microfabric switches

The observed fabric variations in quartz and calcite (Figure 10) reflect the T- and strain rate-dependent slip systems that were active and the strain and kinematic framework under which fabric developed. In the north, the change in quartz c-axis and a-axis fabric patterns is related to a transition from plane strain to constrictional strain (Lister & Hobbs, 1980; Schmid & Casey, 1986; Sullivan & Beane, 2010), coinciding with a change in calcite microstructure (Seaton et al., 2009). Given the southward increase in deformation T, this calcite microstructure transition is likely related to the activation of dynamic recrystallization processes; however, it is also conceivable that increased finite strain, as suggested by the rotation of calcite rods toward the shear plane (Figure 4(B)), was the main driver for dynamic recrystallization (Stipp et al., 2002). Although less likely, another possibility is that constrictional strain was more efficient than plane strain at obliterating the columnar texture by favoring dynamic recrystallization. Constrictional strain may have forced the elongation, necking and boudinage of existing calcite rods, thereby creating local flow stress gradients that may have enhanced calcite dynamic recrystallization.

Figure 10. (A) Graph representing the evolution of recrystallized grain sizes in quartz and calcite as well as the distribution of ⁴⁰Ar/³⁹Ar ages; (B) Summary of fabric variations in quartz and calcite across southern Sivrihisar with approximate metamorphic temperatures recorded in the Barrovian sequence; (C) Cross section of field area (location, Figure 2) and schematic block diagrams illustrating outcrop-scale strain relations (plane strain SL tectonite in north; constriction strain L-tectonite in south).

The next transition to the south is the obliteration of calcite crystallographic fabrics (Figure 10). The marble still deformed in constriction to high finite strain (Figure 4(D)), but the deformation mechanism switched from dislocation creep, with strong crystallographic fabrics, to a mechanism that does not involve intracrystalline glide. This transition is located within a continuous metamorphic gradient (Whitney et al., 2011) and was likely driven by increased temperature. Given that calcite grain growth is associated with this transition (Figure 10), a switch to a temperature-driven process such as diffusion creep, rather than grain-boundary sliding, is most likely. In this particular field area, the transition from dislocation creep to diffusion creep in calcite was likely ~450 °C (upper greenschist facies).

7.2 Age of fabrics and regional implications

The 88–82 Ma ⁴⁰Ar/³⁹Ar results for phengite in the northern domain are similar to other Late Cretaceous ages obtained in other studies (Harris et al., 1994; Okay & Kelley, 1994; Okay et al., 1998; Sherlock et al., 1999). These ages are interpreted as indicating cooling during exhumation of HP rocks from the subduction zone.

Rocks that equilibrated at the staurolite-in isograd and above (in temperature) attained T > 560 °C, higher than a typical closure temperature for Ar in white mica (Harrison, Celerier, Aikman, Hermann, & Heizler, 2009). Therefore, ⁴⁰Ar/³⁹Ar results of between 63 and 55 Ma from these samples likely represent cooling that followed the regional metamorphic thermal maximum and/or partial resetting related to heating from the nearby Eocene (53 ± 3 Ma) intrusion. The complexity and variability of the age spectra from samples near the pluton suggest at least partial argon loss.

The calc-alkaline Eocene plutons in the Tavşanlı zone have been interpreted as post-collisional because they are undeformed, cut across regional structures and intruded most metamorphic units (Figure 2) (Harris et al., 1994). In our field area, regional metamorphic isograds are locally highly oblique to the pluton contact (Whitney et al., 2011). These observations, as well as the strong lineated fabrics involving Barrovian index minerals and hornblende indicate that primary metamorphic crystallization was related to a regional event rather than thermal effects of the intrusion.

Although most observations of the petrology and structure of the Barrovian sequence suggest that the intrusion did not drive the main metamorphism that produced the index minerals and L-tectonite fabric, the effect of the intrusion must be considered in interpretation of the ⁴⁰Ar/³⁹Ar muscovite ages. Muscovite likely experienced at least partial argon loss caused by heating during pluton emplacement, and texturally late muscovite and chlorite in the region may have formed by interaction with magmatic fluids.

Because most of the ⁴⁰Ar/³⁹Ar muscovite ages at 57–64 Ma are older than the hornblende ⁴⁰Ar/³⁹Ar age of the Sivrihisar pluton (~53 Ma (Sherlock et al., 1999)), muscovite likely grew or recrystallized during regional metamorphism prior to pluton emplacement. However, samples most proximally located to the pluton likely experienced some to complete resetting of the argon clock. It is also possible that the pluton had several phases of intrusion, but unless the Sivrihisar granitoid had a more complex magmatic history than other Eocene intrusions in the Tavşanlı zone, intrusion was unlikely to have occurred much before 53 Ma because all dates from Eocene plutons in the Tavşanlı zone are between 53 and 48 Ma (biotite and hornblende ⁴⁰Ar/³⁹Ar ages; [Okay & Kelley, 1994; Sherlock et al., 1999]). We therefore interpret the argon ages for the Barrovian rocks to indicate cooling following regional metamorphism, and that in most cases the cooling occurred prior to intrusion by the granite. Given the preservation of compressed isograds across the studied transect, cooling in the transtension zone was likely rapid and associated with localized transtensional exhumation of the Barrovian metamorphic sequence.

On the regional scale, there is increasing recognition of the role of the Afyon subduction zone in the development of Anatolia (Pourteau et al., 2010, 2013). Early Cenozoic HP–LT metamorphism in the Afyon subduction complex was synchronous with the youngest ages we have obtained from the Sivrihisar Barrovian sequence. Therefore, the zone of transtension and Barrovian metamorphism documented here may reflect local oblique tectonism and extension in the upper plate of the Afyon subduction system.

8. Conclusion

A detailed investigation across a sequence of interlayered quartzite and marble has recognized quartz and calcite fabric transitions in a zone of localized transtension. In this zone, Barrovian metamorphism progressively overprinted a blueschist metamorphic complex while rocks developed constrictional fabrics. HP fabrics are recorded by calcite columnar texture that is preserved in marble up to the chlorite zone of the Barrovian sequence (~400 °C). During shearing that accommodated unroofing of the Barrovian sequence, very strong calcite crystallographic fabrics developed. Progressive strain and heating resulted in dynamically recrystallized marble that preserved a prominent asymmetric crystallographic fabric. Around the 500 °C isograd, the calcite crystallographic fabric was obliterated by grain growth and a likely switch from dislocation creep to diffusion creep. Quartz microstructures are relatively constant across the Barrovian sequence and developed in the grain boundary migration regime of dislocation creep. Fabric transitions are associated with the progressive role of constrictional strain from chlorite to sillimanite zones and with an increase in the participation of the prism-a slip system in the higher T domain.

Quartz and calcite fabrics developed in a zone of transtension and thinned Barrovian sequence in late Paleocene time. Argon ages indicate that this deformation and metamorphism is likely related to local oblique extension above the Afyon subducted slab in a region that was then intruded by Eocene arc-related plutons.

Supplemental data

Supplemental data for this article can be accessed here. [http://dx.doi.10.1080/09853111.2013.858952]

Acknowledgments

This project was supported by NSF grant EAR-0711263 to Whitney and Teyssier. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program.