Implications for Thrust-Related Shortening Punctuated by Extension From P-T Paths and Geochronology of Garnet-Bearing Schists, Southern (Çine) Menderes Massif, SW Turkey

Abstract

The Menderes Massif, Turkey, is a type locality for deciphering the plate tectonic response from collision- to extension-driven exhumation. Conventional thermobarometry and garnet pressure-temperature (P-T) paths from isochemical phase diagrams were calculated across a major fault (Selimiye Shear Zone, SSZ) bounding the southern edge of the Menderes Massif. Both approaches yield similar garnet rim temperatures (from 555 to 671 °C), but estimated P differs by between 8 and 15 kbar. Three garnets north of the SSZ reveal N-shaped P-T paths, whereas paths from three samples south of the SSZ show a simple increase in P-T. Monazite and zircon were dated in thin section from the same rocks using Secondary Ion Mass Spectrometry and Laser Ablation Inductively Coupled Plasma-Mass Spectrometry, respectively. Textural relationships of monazite within garnet appears indicative of post-garnet growth. The amount of monazite common ²⁰⁴Pb and ¹³⁷Ba⁺/Th⁺ significantly exceeds what is observed for the monazite age standard, suggesting their ages mark fluid-driven events, loosely constrained to Late Eocene-Early Miocene. Some zircon ages are consistent with Cambro-Ordovician ages reported elsewhere in the region, and other ages are Neoproterozoic and Permian-Triassic, a period not previously recognized in this area. Despite the lack of age constraints for the duration of garnet growth, we present a thermal model to understand the meaning of the N-shaped path. These paths are best reproduced by thermal models incorporating SSZ thrusting before and after denudation. This paper presents an example of the insight from high-resolution P-T paths, and an example of denudation within a prograde metamorphic event.

Key Points

• High-resolution N-shaped P-T paths from the S. Menderes Massif (Turkey) are most consistent with a model of burial with intermediate exhumation
• Monazite in garnet appears to be secondary based on textural relationships and their high common Pb and 137Ba/Th⁺ contents
• Neoproterozoic and Permian-Triassic age zircons are reported, a timeframe not yet recognized in the S. Menderes Massif

Plain Language Summary

The Menderes Massif, SW Turkey, is abundant in garnet-bearing metamorphic rocks formed during an ancient mountain building event. Reconstructing the history of this event has been difficult due to a variety of factors that have altered much of the rocks. Luckily, the mineral garnet largely survived alteration here and retains its original growth chemistry. This chemical information is useful for estimating the pressure and temperature conditions at which the mineral grew. Here we study six garnets and find two growth histories: one suggesting garnet grew as the rock was continuously buried to a depth >20 km and another suggesting garnet experienced fluctuations in burial depth during growth, that is, certain rocks were buried, partially exhumed (<1 km) and then reburied to a maximum depth >20 km. This variation in burial depth is consistent with a model calling on a period of surface erosion during overall mountain building as responsible for partial exhumation.

1 Introduction

The ability to efficiently model high-resolution changes in garnet chemical zoning offers new possibilities for acquiring precise and reliable metamorphic records from collisional tectonic locales (e.g., Catlos et al., 2018; Kelly et al., 2015; Moynihan & Pattison, 2013a, 2013b). The Menderes Massif of western Turkey (Figure 1) contains abundant garnet-bearing metapelitic schists for which other thermobarometric techniques have produced mostly broad and uncertain constraints. High-resolution pressure-temperature (P-T) paths from the region offer insight that has not been possible with previous techniques. Here high-resolution refers to the spatial resolution in estimating garnet-growth pressures and temperature from crystal core to rim (Moynihan & Pattison, 2013b).

Figure 1

Geologic map of the Southern (Çine) Menderes Massif (modified after Diniz, 2005 and Candan et al., 2011). Sample locations (ED-#) are identified (see Table 1). The inset shows the location of the Southern (Çine) Menderes Massif with respect to Turkey. IAESZ = Izmir-Ankara-Erzincan-Suture-Zone; PZ = Pontides Zone; ATZ = Anatolides-Taurides Zone.

The region is part of the Alpine-Himalyan Belt and formed during repeated collisions between continental Gondwana and derived microcontinents with Laurasia (e.g., Dewey & Şengör, 1979; Şengör & Yılmaz, 1981; Stampfli et al., 2002). The result is a multiphase collisional deformation history recording tectonic events since the Late Proterozoic (e.g., Bozkurt, 1996; Candan et al., 2001; Catlos & Çemen, 2005; Dűrr, 1975; Hetzel et al., 1998; Hetzel & Reischmann, 1996; Loos & Reischmann, 1999; Şengör & Yılmaz, 1981). Differences in the nature, style, and formation of the Menderes Massif during a transition from Eocene-Oligocene collision to Late Oligocene to present extension have been explored (e.g., Çemen et al., 2006; Erkul et al., 2013; Gessner et al., 2001; Pourteau et al., 2016; Rimmelé et al., 2003; Ring et al., 1999; Seyitoglu & Scott, 1996a, 1996b; van Hinsbergen, 2010). The extension qualifies the Menderes Massif as a metamorphic core complex. Therefore, the Massif has been a central location for studying syn to postcollisional tectonics (e.g., Bozkurt et al., 1995; Çemen et al., 2006; Gessner et al., 2013; Hetzel et al., 1995, 1995; Işık & Tekeli, 2001; Jolivet et al., 2004; Şengör et al., 1984).

Reconstructing the tectonic history of the Menderes Massif has been facilitated by thermobarometric constraints (e.g., Ashworth & Evirgen, 1984; Iredale et al., 2013; Régnier et al., 2003; Ring & Layer, 2003; Whitney & Bozkurt, 2002). However, conflicting P-T results have been reported from the same rock units, and only a few P-T paths have been reported (e.g., Ashworth & Evirgen, 1984; Ring & Layer, 2003; Whitney & Bozkurt, 2002). Challenges (e.g., retrograde metamorphism, overprinting, and modification by reactive fluids) exist regarding the application of conventional thermobarometers to Menderes Massif mineral assemblages (Iredale et al., 2013; Ring et al., 2001; Whitney & Bozkurt, 2002), leading to an incomplete understanding of the overall tectonic evolution of the massif. With the introduction of an efficient, high-resolution technique for P-T path construction (Moynihan & Pattison, 2013a), reevaluation of this region is possible.

In this paper, we use garnet chemical zoning and the P-T path approach of Moynihan and Pattison (2013a) to construct high-resolution paths for rocks collected in the Çine (southern) Menderes Massif. Sampling of rocks and extensive field mapping along the Selimiye Shear Zone (SSZ; Figure 1) was conducted in the summer of 2005 (Diniz, 2005). We dated both zircon and monazite grains occurring as inclusions in garnet and in the matrix. The data lend insight into the history of the massif before, and during, its transition from collision- to extension-driven exhumation. The paths provide insight into the thermal state of the crust large-scale extension (i.e., tectonically driven) in Late Oligocene.

2 Geological Background and Previous Work

2.1 General Overview

Western Anatolia is a part of the Alpine-Himalayan orogenic belt located in the eastern Mediterranean region. It is subdivided into several zones based on distinct paleogeographic origin and tectonometamorphic histories. In western Turkey, three important zones include the Izmir-Ankara-Erzincan (IAESZ), the Pontides zone to the north and the Anatolides-Tauride zone to the south (Figure 1; e.g., Collins & Robertson, 1997, 1998; Okay & Tüysüz, 1999; Pourteau et al., 2016; Şengör & Yılmaz, 1981). The IAESZ separates the Pontides from the Anatolide-Tauride zone and is a prominent structural feature marking the closure site of the northern branch of the Neo-Tethys Ocean (e.g., Çemen et al., 1999; Okay & Tüysüz, 1999; Şengör & Yılmaz, 1981). Subduction occurred along the IAESZ from Mesozoic to early Cenozoic (e.g., Okay & Tüysüz, 1999; Pourteau et al., 2016; Şengör & Yılmaz, 1981). The Pontides unit is an amalgamation of Pan-African basement blocks and Phanerozoic sedimentary cover units initially connected to the southern Eurasia margin before Late Cretaceous back-arc extension responsible for the opening of the Black Sea (e.g., Okay et al., 2013). The Anatolide-Tauride zone is believed to be a microcontinent rifted away from the northern margin of Gondwana beginning in the early Triassic (e.g., Okay & Tüysüz, 1999; Şengör & Yılmaz, 1981).

The Menderes Massif is located at the western-most end of the Anatolide-Tauride zone and was assembled during the closure of the Neo-Tethyan Ocean (Figure 1). The massif is today a large-scale metamorphic core complex consisting of approximately 40,000 km² of metamorphic and igneous rocks (e.g., Bozkurt & Park, 1994; Hetzel, Passchier, et al., 1995; Hetzel, Ring, et al., 1995; Işık & Tekeli, 2001; Şengör et al., 1984). Low-angle detachment faults and high-angle normal faults bound sedimentary basins and separate the massif into northern (Gördes), central, and Çine submassifs (Figure 1; e.g., Çemen et al., 2006; Emre & Sözbilir, 1997; Hetzel, Passchier, et al., 1995, Hetzel, Ring, et al., 1995). Extensional structures, including the Alaşehir Detachment (located in the Central Menderes Massif) and SSZ (Figure 1), facilitated regional exhumation (e.g., Bozkurt & Park, 1994; Catlos & Çemen, 2005; Hetzel, Passchier, et al., 1995; Hetzel, Ring, et al., 1995; Yılmaz et al., 2000).

The Menderes Massif is mapped as a series of four nappes stacked during south-directed thrusting and is considered the structurally lowest unit in western Anatolia (Figure 2a; e.g., Collins & Robertson, 1999; Gessner et al., 2001; Ring et al., 1999). Structurally lowest (i.e., in the “core” of Şengör et al., 1984) to highest are termed: the Bayındır, Bozdağ, Çine, and Selimiye nappes. Mineral assemblages present in nappe metapelites, eclogite, and amphibolite rocks preserve evidence of a Barrovian style metamorphic and tectonic history (e.g., Candan et al., 2001; Catlos & Çemen, 2005; Şengör et al., 1984; Whitney & Bozkurt, 2002). These are ideal targets for understanding how rocks in this area evolved during collision.

Figure 2

(a) Interpretative thrust sequence during formation of Anatolide belt, including the Menderes Massif, after Gessner et al. (2013). The focus of this study are the Çine and Selimiye nappes, which lie beneath the Cyclades Blueschist Unit (CBU). SSZ = Selimiye Shear Zone; BT = Bozdag Thrust; CMT = Cyclades-Menderes Thrust. Note that this cross-section scenario applies to the region prior to large-scale extension. (b) Previously reported pressure-temperature paths from Menderes Massif nappes (Régnier et al., 2007; Rimmelé et al., 2005; Ring et al., 2001; Whitney & Bozkurt, 2002).

2.2 Menderes Massif Metamorphism

Rock fabrics in the Çine Massif may have formed under amphibolite to greenschist grade during a time frame designated as the Main Menderes Metamorphism (MMM; e.g., Bozkurt et al., 1995; Şengör et al., 1984; Whitney & Bozkurt, 2002). Although ⁴⁰Ar/³⁹Ar and Rb-Sr mica ages tenuously constrain MMM timing between the Paleocene to Miocene, the MMM is typically attributed to the Eocene-Oligocene during the closure of the northern branch of the Neo-Theys Ocean, also referred to as the IAESZ Ocean (e.g., Bozkurt & Satır, 2000; Çemen et al., 1999; Lips et al., 2001; Satır & Friedrichsen, 1986). For example, one Çine massif garnet has a reported Lu-Hf core age of 42.6±1.6 Ma and rim age of 34.8±3.1 Ma (Schmidt et al., 2015).

Although the MMM event is likely responsible for observed rock fabric in the Menderes Massif, some evidence suggests the influence of multiple deformation episodes. For example, Th-Pb ages of monazite inclusions in garnet and rock matrix that show crystallization during the Cambro-Ordovician suggest a garnet-growth event during that time (Catlos & Çemen, 2005). Altered garnets characterized by diffusionally modified chemical zoning provide evidence of previous metamorphism not associated with the closure of the Neo-Tethys Ocean (e.g., Baker et al., 2008; Candan et al., 2011; Catlos & Çemen, 2005; Régnier et al., 2003; Satır & Taubald, 2001). Backscattered electron and cathodoluminescence images document garnet zoning in a Çine Massif garnet consistent with polymetamorphism (Baker et al., 2008). Other observational evidence supporting the polymetamorphism includes relict lenses of high-temperature granulite within the Çine unit, high-pressure eclogite blocks, and upper amphibolite facies assemblages suggesting a multistage history (e.g., Candan et al., 2011; Oberhänsli et al., 1997). In addition, a major challenge in working with metamorphic rocks from the Menderes Massif is retrograde overprinting during Cenozoic extension of the core complex.

Menderes Massif thermal and kinematic models rely on P-T-time (P-T-t) estimates to describe metamorphic events (e.g., Ashworth & Evirgen, 1984; Candan et al., 2001; Régnier et al., 2003; Rimmelé et al., 2003; Ring et al., 2001; Satır & Taubald, 2001; Whitney & Bozkurt, 2002). However, the number and timing of garnet-growth events recorded in the Menderes Massif rocks remain unclear. Compilations of P-T data for the Menderes Massif metamorphic assemblages have shown that individual nappes have a range of reported peak P-T conditions, and temperatures recorded by garnet-bearing rocks ascribed to the Cambro-Ordovician overlap those achieved during Late-Cretaceous to Eocene (see reviews in Bozkurt, 2001; Bozkurt & Oberhaensli, 2001; Çemen et al., 2006). Conditions consistent with inverted metamorphic gradients within and between individual nappes have also been reported (e.g., Okay, 2001; Ring et al., 2001) and have implications for understanding heat transport within an evolving orogen.

Menderes Massif rocks have yielded problematic P-T estimates due to challenges such as disequilibrium among phases and the application of barometers to inappropriate (uncalibrated) mineral compositions (Ashworth & Evirgen, 1984, 1985a, 1985b). Many reported conditions also appear at odds with observed mineral assemblages and structural data (Ring et al., 2001; Whitney & Bozkurt, 2002). Iredale et al. (2013) forgo baric estimates altogether due to a “lack of P-sensitive assemblages” and report thermometry results calculated at 5 and 10 kbar.

The limited number of P-T paths for the Menderes Massif nappes are shown (Figure 2b) and present a difficult framework for interpretation. The paths were redrawn or generated from the literature by connecting peak metamorphic conditions of individual rocks, or inferring from mineral assemblages, pseudosections, or Gibbs method thermodynamic modeling. The cover series (Lycian nappe; Figure 2a) assemblages record high pressure/low temperature (HP/LT) conditions (e.g., Pourteau et al., 2013; Rimmelé et al., 2005). An overall P-T path from south to north has been proposed for the Çine Massif from 425 °C and ~4 kbar to 550 °C and ~6 kbar (Whitney & Bozkurt, 2002). Some Çine nappe rocks appear to have experienced two stages of garnet growth in the sillimanite stability field during prograde burial followed by “discordant” overgrowth (Ring et al., 2001). Other Çine nappe studies suggest one stage of prograde growth during decompression in the kyanite stability field (Régnier et al., 2007). Studies of Bozdağ nappe rocks show prograde burial, but conditions decrease downward by ~40 °C/kbar per km of structural section (Ring et al., 2001). Selimiye nappe rocks record exhumation and retrogression (Régnier et al., 2007). Although not shown in Figure 2b, Cenki-Tok et al. (2016) recently reported that metasedimentary rocks in the northern Menderes Massif reached amphibolite-facies conditions (625-670 °C and 7-9 kbar) during Alpine metamorphism.

P-T paths in Figure 2 that decrease in P or T suggest the potential for tectonic switching as unloading and refrigeration occurs when fault systems with compressional stress reverse and facilitate extension (Beltrando et al., 2007; Lister & Forster, 2009; Wells et al., 2012). The approach we apply allows us to obtain higher-resolution P-T paths to further test the hypothesis that the Menderes Massif continued to experience punctuated shortening during the switch of the tectonic regime to overall extension (Yılmaz, 2008).

2.3 Çine Massif samples

This study focuses on constraining the P-T-t conditions from eight garnet-bearing assemblages from the Çine Massif (Table 1; Figure 1; Ataktürk, 2014; Diniz, 2005). These metamorphic schists were collected across the SSZ (Figure 1; Diniz, 2005) and include three samples from the Selimiye nappe (ED01, ED02, and ED34) and five from the Çine nappe (ED09, ED17, ED27, ED28, and ED35).

Table 1. Samples and Mineral Assemblages From the Çine Massif Analyzed in This Study

Samplea	Latitude (N)	Longitude (E)	Mineral assemblageb	Geochronometerc
Selimiye nappe
ED01	37°25′22.64″	28°08′34.56″	Ilm+Xt+Zrn+Mnz	Mnz
ED02	37°25′09.72″	28°08′41.73″	Ilm+Chl+Mnz	Mnz
ED34	37°22′28.83″	27°57′48.10″	Ilm+FeO+Chl+Aln+Zrn+Mnz	Zrn
Çine nappe
ED09	37°22′27.03″	27°57′19.33″	Chl+Ilm+Ap+Zrn+Mnz	Mnz
ED17	37°28′18.30″	28°12′58.92″	Ilm+Ap+Rt+Pyr+Zrn+Mnz	Zrn
ED27	37°21′01.80″	28°11′31.78″	Chl+Ilm+Zrn+Mnz	Mnz and Zrn
ED28	37°26′51.52″	28°11′49.34″	Chl+Ilm+Zrn+Mnz	Zrn
ED35	37°22′30.50″	27°57′36.47″	Chl+Ilm+Zrn+Mnz	Mnz and Zrn

• ^a High-resolution P-T paths were generated for all samples, except samples ED09 and ED27. Peak P-T conditions were generated for all samples using garnet chemical zoning and conventional approaches.
• ^b All samples include assemblage Grt + Bt + Pl + Ms + Qz. Abbreviations after Whitney and Evans (2010).
• ^c Minerals dated within the sample.

3 Methods

3.1 P-T Determinations

Characterization of mineral chemistry was performed using electron probe microanalysis (EPMA). X-ray maps of garnet porphyroblasts were obtained for Mg, Ca, Mn, Fe, and Y using the JEOL 8200 Superprobe at The University of Texas at Austin. Mapping conditions include a focused beam, 15-kV accelerating voltage, 50-nA beam current on brass, 1- to 2-μm step size (pixel size), and 50-ms dwell time. The maps were processed to enhance relative differences in element concentrations using image processing software.

Quantitative analyses of garnet, biotite, muscovite, and plagioclase were obtained using a Cameca SX50 EMP at the University of Oklahoma in Norman with an accelerating voltage of 20kV and beam current of 150 nA. Natural and synthetic standards were used for calibration. Three to six high-quality spots on each matrix phase produced averages that were used in subsequent calculations. Data were collected from grains both adjacent to and away from (up to 2 mm) the chosen garnet to gauge the range of compositional variations. Chlorite, ilmenite, and rutile were analyzed on the JEOL 8200 Superprobe at The University of Texas at Austin using an accelerating voltage of 15 KV, beam current of 15 nA on brass, and a 5-μm spot diameter. Natural and synthetic standards were used for calibration. One to five spots were measured depending on the size of the grain, and average compositions were used in the subsequent calculations. For all EPMA measurements, poor analyses due to overlap with inclusions and cracks were culled by referencing X-ray maps and considering divergence from ideal phase stoichiometry.

The locations of each of the quantitative EPMA traverses spanning core to rim of each garnet were chosen by referencing X-ray maps with an effort to avoid inclusions and cracks. For the garnet in sample ED28, evidence for retrograde re-equilibration along a crack below the surface of the traverse was discovered after the traverse had been collected. To avoid the re-equilibrated portion of the grain, the X-ray maps of the garnet were calibrated using XMapTools (Lanari et al., 2014) to produce quantitative X-ray maps. From the new maps, a second traverse was selected to avoid the re-equilibrated portion of the grain. The new core and near-rim chemistry is a close match to the preserved portions of the measured traverse, and the new mantle zoning is consistent with observed changes in chemistry viewed in X-ray maps elsewhere in the garnet.

For thermobarometry, rim compositions with the lowest garnet Mn concentrations and Fe/(Fe+Mg) combined with matrix minerals were used with the garnet-biotite thermometer (Berman, 1990; Ferry & Spear, 1978) and the garnet-muscovite-biotite-plagioclase barometer (Hoisch, 1990) to determine peak P-T conditions. Other, more recent calibrations can be applied but would likely result in ±25 °C to ±1 kbar differences. Supplementary data files show the detailed geochemical analyses.

For P-T path modeling, bulk-rock compositions were calculated using mineral chemistry measured in matrix phases and zoned garnets combined with estimates of phase volumes and converted to moles of elements as in Kelly et al. (2015). Mineral modes were estimated visually from thin sections and hand samples. The bulk compositions were used with Theriak-Domino (de Capitani & Brown, 1987; de Capitani & Petrakakis, 2010) and the Holland and Powell (1998) data set with updates through 2010 in the system MnO-Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂ (MnNCKFMASHT) to generate isochemical phase diagrams representing the thermodynamic conditions at the beginning of garnet growth. Solution models are the same as in Kelly et al. (2015) and Catlos et al. (2018). The intersection of almandine, pyrope, spessartine, and grossular concentrations (isopleths) of the apparent garnet core were used to estimate initial garnet growth conditions.

The isochemical phase diagrams and profiles of grossular, spessartine, almandine, and pyrope along garnet traverses were used with the automated approach of Moynihan and Pattison (2013a), which steps through the compositions, from core to rim, fractionating garnet from the bulk rock and generating a new diagram for the next zone of garnet growth, depending mostly on the distance between chemical analysis points along a radial profile. For points that are far apart relative to changes in the chemical zoning, the necessary discretization of a garnet zoning profile can be problematic. An error that can arise is the overestimate of garnet fractionation, which unnaturally forces the garnet-in line on a diagram to higher P-T conditions and introduces a garnet-free assemblage midway through garnet growth. By resampling the profile at a higher frequency (interpolation), we tend to see fewer failures in the automation and more geologically sound results. Smoothing of the profile to reduce spikes and drops, more likely due to EPMA errors from pits in the polished sample and overlap with inclusions than with recorded changes in P-T during garnet growth, can also reduce errors in the automation while producing geologically reasonable results. Therefore, some of our modeling results benefit from smoothing or interpolation along the garnet zoning profiles. Note that the profile approximated from a quantitative X-ray map in ED28 used a different technique (described above). Occasionally, a naturally low observed XSps (0.01-0.001) can prevent the automated routine from running successfully as the model will consume all Mn before finishing. To prevent this, we used a penalty function that prohibits XSps from failing below a specified minimum (e.g., XSps = 0.001).

For each sample, numerous variants of rock bulk composition were calculated by adjusting the visually estimated mineral modes (within uncertainty). In contrast to whole-rock chemical analysis, for these relatively low-grade rocks, we find that this technique helps to adjust the smoothed fit composition toward the effective bulk chemistry from which the garnet grew. Adjustments were made until the final assemblages and compositions approximated the preserved characteristics of the samples. Given the lack of preserved prograde matrix remaining in each sample, only broad constraints are possible from mineral assemblages and compositions. Instead, the goal was to focus on the preserved garnet chemical zoning as closely as possible while generating final rock characteristics in agreement with broad constraints from the area.

3.2 Geochronology

After obtaining the P-T conditions, zircon and monazite grains were identified in the rock thin sections and extracted using a slow-speed saw. Chips were ultrasonically cleaned in deionized water and soap mixture and mounted with monazite age standards into one-inch diameter epoxy plugs. All samples were examined using a scanning electron microscope to identify grains suitable for geochronology.

Zircon grains from samples ED17, ED27, ED28, ED34, and ED35 were dated using Laser-Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) at The University of Texas at Austin. The methods applied are similar to Jackson et al. (2004). A laser spot size of 20 m and a beam energy frequency of 10 Hz was used. Two age standards, GJ-1 (Jackson et al., 2004) and Pak1 (internal standard), were analyzed after every fourth sample to correct for any potential instrument fluctuations and to determine fractionation factors. Grains were located in a variety of textural relationships and range in length from ~10 to ~75 μm. Most grains were dated using a single laser spot, but those > 60μm were analyzed with multiple spots targeting the core and rim.

Monazite grains both in the rock matrix and as inclusions in garnet were dated in samples ED01, ED02, ED09, ED27, and ED35 using a CAMECA ims1270 ion microprobe at the University of California Los Angeles. Th-Pb monazite age standard 554 (45±1 Ma, Harrison et al., 1999) and U-Pb Amelia monazite (274.6±0.6 Ma, Peterman et al., 2006) were used as age standards. The ratios of ²⁰⁸Pb⁺/Th+ versus ThO₂⁺/Th⁺ for Th-Pb geochronology and ²⁰⁶Pb⁺/U⁺ versus UO⁺/U⁺ for U-Pb dating form the calibration, with the slope and intercept of the curve controlled by the age of the standard grain. Although the Amelia has a reported range of ages from 279±14 Ma (Rb-Sr age by Deuser & Herzog, 1962) to 250-255 Ma (Kohn & Vervoort, 2008), we used the TIMS reference age of 274.6±0.6 Ma (Peterman et al., 2006) and avoided analyzing recrystallized areas. We separated the analyses based on the calibration in two sessions. The first set of analyses includes 17 spots on monazite 554 and 17 spots of Amelia that produced calibration lines of ThO₂⁺/Th⁺= 0.116(²⁰⁸Pb⁺/Th⁺) + 1.423±0.076 and UO⁺/U⁺= 0.166(²⁰⁶Pb⁺/U⁺) + 6.412±0.446. These reproduce the ages of the standards to 45.4±1.3 Ma (±1σ) (monazite 554) and 270.5±10.0 Ma (Amelia). The second session includes nine spots on monazite 554 and five spots of Amelia that produced calibration lines of ThO₂⁺/Th⁺= 0.157(²⁰⁸Pb⁺/Th⁺) + 0.721±0.186 and UO⁺/U⁺= 0.137(²⁰⁶Pb⁺/U⁺) + 8.586±0.600. These reproduce the ages of the standards to 45.5±2.5 Ma (±1σ; monazite 554) and 276.7±13.7 Ma (±1σ; Amelia). Reported ages were corrected for common Pb using ²⁰⁴Pb and an assumed ²⁰⁸Pb/²⁰⁴Pb=38.34 (Stacey & Kramers, 1975). In addition to the isotopes for geochronology, we measured ¹³⁷Ba⁺ based on a suggested spatial covariation of ¹³⁷Ba⁺ with ²⁰⁴Pb⁺ within regions of the Amelia monazite grain (see Catlos & Miller, 2016). These measurements were only made during day 2 of the calibration. The peak was first identified using NIST610 glass, which has 435±23 ppm Ba (Jochum & Nohl, 2008). To avoid overlap of potential Ba-related polyatomic interferences on Pb isotopes the ion microprobe operated at a mass resolving power 5000.

All monazite and zircon grains were imaged using the scanning electron microscope in backscattered electron mode to determine where the ion microprobe or laser spots were located. In some cases, the SIMS oxygen beam was larger than the grain itself. Thus, aperture windows were used to constrict the measured ions sputtered from monazite grain to a ~20-μm² region in the center of the beam. Aperture windows are an essential part of SIMS in situ analyses, as they significantly decrease the amount of nonradiogenic Pb from the surrounding minerals and matrix (e.g., Catlos et al., 2002). In some cases, the monazite grains contain high amounts of common Pb and did not yield reliable ages. This observation is similar to Menderes Massif monazites reported in Baker et al. (2008) and is discussed further in the results section.

4 Results

4.1 Mineralogy and Bulk Rock Compositions

Figures 3-5 show BSE images and X-ray maps of garnets that were used to generate P-T conditions. Garnets in samples from across the Çine Massif appear similar in their inclusion patterns, shape, and alteration textures. All have large quartz inclusions, and some are cut by quartz veins (e.g., samples ED01, ED09, and ED27). Many garnets are rounded, except sample ED17, which appears fragmented with parts replaced by chlorite. Monazite and zircon appear in all samples (Figures 3-5), but not all grains were dated due to their small size. In all samples, some monazite grains appear in a reaction texture replacing the previous allanite. Monazite inclusions in garnet appear to be secondary as a crack filling phase or show the same allanite reaction textures.

Figure 3

BSE images and X-ray element maps across Çine Massif garnets in samples (a) ED17, (b) ED28, and (c) ED35 from the Çine nappe. Yellow lines in the BSE images indicate the position of the EPMA transects. Maps use a rainbow palette (red = high concentrations, and blue = lower concentrations). We indicate regions where trends in zoning changes in some garnets. X-ray map locations are shown on the BSE images. We include BSE images of identified radiogenic phases. Note that monazite grains in ED17 and ED28 were unable to be dated using SIMS. We include BSE images of all dated monazite in sample ED35. X-ray maps of Fe, Mg, Ca, and Mn for ED28 are reprocessed using XMapTools (Lanari et al., 2014); transect points from core to rim were used as smoothed fit composition values.

Figure 4

BSE images and X-ray element maps across Çine Massif garnets in samples (a) ED01, (b) ED02, and (c) ED34 from the Selimiye nappe. Yellow lines in the BSE images indicate the position of the EPMA transects. X-ray maps use a rainbow palette (red = high concentrations, and blue = lower concentrations). Their locations are shown on the BSE images. We include BSE images of identified radiogenic phases. Note that monazite grains in ED34 were unable to be dated using SIMS. Two monazite grains in sample ED34 were unable to be dated using SIMS, but we include BSE images of the grains to demonstrate their textural relationships.

Figure 5

BSE images and X-ray element maps of Çine Massif garnets in samples (a) ED09 and (b) ED27 from the Çine nappe. X-ray maps use a rainbow palette (red = high concentrations, and blue = lower concentrations). Their locations are shown on the BSE images. We indicate regions where trends in zoning changes in some garnets and include BSE images of identified radiogenic phases. We include BSE images of all dated monazite and some zircon grains.

Table 2 shows the bulk compositional data used as input in the P-T path modeling. All rocks are aluminum poor, and comparable in magnesium and iron content to average pelite reported by Caddick and Thompson (2008).

Table 2. Çine Massif Bulk Compositional Data (mol %) Used for Generating Phase Diagrams

Sample	ED01	ED02	ED34	ED17	ED28	ED35	Average pelitea
	Selimiye nappe			Çine nappe
Si	80.56	76.25	75.81	75.05	74.34	69.21	60.83
Al	8.26	9.54	12.23	11.47	9.79	13.28	19.87
Fe	4.14	5.89	4.33	5.27	6.50	7.09	4.49
Mn	0.04	0.02	0.06	0.12	0.08	0.03	0.09
Mg	2.96	3.90	2.20	2.76	4.01	4.33	3.97
Ca	0.31	0.67	0.53	0.50	0.66	0.24	1.19
Na	0.52	1.05	1.19	1.07	0.59	1.15	3.42
K	2.36	2.14	2.85	2.89	2.50	4.04	4.58
Ti	0.85	0.54	0.82	0.88	1.53	0.62	0.57
Total	100	100	100	100	100	100	100
Ab	11.0	19.2	30.8	21.6	14.2	7.1	25.62
Fc	34.4	32.7	32.5	36.3	36.2	39.4	41.14
Md	54.6	48.1	36.7	42.2	49.6	53.5	33.24

• Note. Bulk compositional data were computed using electron microprobe data from major minerals from each sample. Values shown here have been renormalized excluding hydrogen to allow for a comparison against an average pelite composition.
• ^a Average pelite from Caddick and Thompson (2008). Element mole % values reported here were converted from oxide weight percent.
• ^b A = Al₂O₃ – 3K₂O.
• ^c F = FeO.
• ^d M = MgO.

X-ray element maps of garnets show different zoning patterns depending on the locality. All Çine nappe garnets have at least one element that demonstrates a detectable gradient from core to rim. These garnets show an overall increase in Fe at midrim, although in samples ED09 and ED27, this change is patchy (Figure 5). Ca zoning in Çine nappe garnets ED17, ED28, and ED35 decrease midrim, but this is not detectable in samples ED09 or ED27. The Ca decrease in sample ED28 appears to correlate with a decrease in Mn, but this is not observed in other rocks. In samples ED28 and ED35, the decrease in Ca appears to negatively correlate with Y. Mg overall increases from core to rim in all Çine nappe samples, except sample ED27. Some garnets show Y annuli (isolated sharp increases in Y mid-rim, ED35, ED28, and ED27), whereas others show high Y in the centers of the mapped crystals (ED09 and ED17).

Çine nappe garnets differ from Selimiye nappe samples in that Selimiye rocks show smooth compositional trends from core to rim. In these samples, Mn decreases and Mg increases from core to rim. Fe concentrations appear to increase from core to rim, but we observe slight decreases in Fe at near the rim boundary. In ED01 and ED02, Ca zoning is relatively flat, though occasionally patchy, but sample ED34 shows a definite decrease from core to rim. Sample ED01 and ED34 have high Y core at the centers of the mapped garnet surfaces.

Figure 6 shows compositional transects across garnets from both nappes. In general, these garnets are high in almandine and lowest in spessartine. Trends in the zoning profiles are consistent with prograde zoning in all cases. Garnets from the Selimiye nappe show relatively flat zoning in Fe compared to Çine massif garnets, which also have slightly more Fe-enriched rims. The grossular profile of the Selimiye garnets is also flat compared to Çine massif garnets, which decrease in Ca from core to rim. Sample ED17, from the Çine nappe, is notably enriched in Mn compared all other samples. The pyrope content across all transects in general increase from core to rim.

Figure 6

Compositional core-to-rim traverses across Çine Massif garnets in samples (a) ED01, (b) ED02, and (c) ED34 from the Selimiye nappe and (d) ED17, (e) ED28, and (f) ED35 from the Çine nappe. Each transect panel includes spessartine, grossular, almandine, and pyrope mole fractions obtained using electron probe microanalysis (black dots), smoothed fit composition (red dots), and modeled profile (black lines). Profiles for ED02 and ED35 were smoothed, interpolated for ED17 and approximated (XMapTools) for ED28. We did not smooth the data for samples ED01 and ED34; thus, electron probe microanalysis compositions and smoothed values are identical for these garnets. The dashed line in (d)–(f) denotes the point corresponding to the midrim P minimum observed in Figures 8a–8c. See the supporting information for transect data for samples ED09 and ED27. These samples did not fit criteria for generating the modeled P-T paths (see section 3).

4.2 P-T Determinations

Table 3 lists the estimated P-T conditions for all samples in this study. Using conventional thermobarometry (i.e., Ferry & Spear, 1978, with Berman, 1990; Hoisch, 1990) applied to rims of garnets, Çine nappe samples record peak T estimates between 572±20 °C and 671±27 °C and peak P between 15.3±0.2 kbar and 19.0±1.4 kbar. Samples ED17, ED27, and ED28 were collected within 5 km of each other, whereas samples ED17 and ED27 are located within meters of each other, but calculated pressures differ between them by ~4 kbar. Selimiye nappe sample estimates range between 556±10°C and 608±22°C and between 15.4±2.3 kbar and 22.4±0.5 kbar. Two samples with nearly identical estimated P (ED01 and ED02) differ in T by ~50°C, whereas the two samples with similar T (ED02 and ED34) differ in P by ~7 kbar. Note, the variation in these conventional thermobarometric estimates is consistent with previously reported variability for Menderes Massif rocks (Ashworth & Evirgen, 1984; Candan et al., 2001; Régnier et al., 2003; Rimmelé et al., 2003; Ring et al., 2001; Satır & Taubald, 2001; Whitney & Bozkurt, 2002) and demonstrates the problems outlined regarding obtaining reliable P-T estimates from these assemblages.

Table 3. Calculated P-T Conditions for Garnet-Bearing Metamorphic Rocks From the Çine Massif

Samplea	G-B Peak T (°C)b	G-B-M-P Peak P (kbar)b	GZM Core T (°C)c	GZM Core P (kbar)c	GZM Rim T (^oC)c	GZM Rim P (kbar)c
Selimiye nappe
ED01	556±10	15.8±3.6	570±11	5.9±0.1	583±5	7.7±0.3
ED02	604±18	15.4±2.3	555±6	6.0±0.3	586±10	7.3±1.1
ED34	608±22	22.4±0.5	565±10	6.0±0.3	582±5	7.4±0.1
Çine nappe
ED09	572±20	17.1±0.3	—	—	—	—
ED17	641±23	19.0±1.4	510±5	5.1±0.1	570±31	6.3±0.5
ED27	601±9	15.6±0.4	—	—	—	—
ED28	671±27	15.3±0.2	535±7	5.4±0.2	568±10	7.0±1.2
ED35	613±26	17.1±2.7	551±6	5.9±0.2	583±22	6.1±2.1

• ^a See Figure 1 for sample locations.
• ^b Conditions calculated using garnet-biotite thermometry (G-B; Ferry & Spear, 1978, Berman, 1990) and garnet-biotite-plagioclase-muscovite barometry (G-B-P-M; Hoisch, 1990) and the software Thermobarometry v2.1 (Spear & Kohn, 1999). Uncertainty is graphically estimated using intersections between equilibria lines calculated for the lowest and highest conditions estimated for mineral compositions in the sample.
• ^c Garnet zoning model (GZM) conditions calculated from isopleth intersections on isochemical phase diagrams generated with Theriak-Domino (de Capitani & Petrakakis, 2010). Uncertainty is graphically estimated using overlaps between garnet isopleths.

Isochemical phase diagrams were produced for six samples in which the modeled end-member garnet compositions are within ±0.01 mole fraction of the observed compositions (Figure 7). The isochemical phase diagrams predict garnet likely grew at the expense of chlorite during dehydration, and all garnet core assemblages are Grt + Bt + Ms + Pl + Ilm ± Chl + Qz + H₂O, similar to petrographic observations. Çine nappe samples have average garnet core growth conditions of 532±6 °C and 5.5±0.2 kbar, whereas structurally higher Selimiye nappe samples record higher average conditions of core garnet growth of 563±9 °C and 6.0±0.3 kbar (Table 3). A complete list of all predicted phase reactions occurring over P-T space for each sample are provided in Supporting Information S5.

Figure 7

Isochemical phase diagrams constructed for samples from the Çine massif using bulk compositions reported in Table 2. Sample numbers are indicated on each panel. Garnet isopleths are also plotted using the observed mole fractions (±0.01) at each core; the region of intersection is designated as the predicted approximate P-T conditions for which garnet cores began growth. Pertinent predicted stable mineral assemblages are given, corresponding to the numbers within the open circles. Quartz and H₂O are present in every predicted stable mineral assemblage. See Table 3 for a summary of these results.

In total, six garnet P-T paths were generated using the automated approach of Moynihan and Pattison (2013a); Figure 8). The quality of each modeled path is evaluated by comparing the modeled garnet chemical zoning to the observed chemical zoning (Figure 6) and constructing an isochemical phase diagram using the final (garnet fractionated) effective bulk composition. In the Çine Massif, two P-T path shapes are recorded as garnet grew over increasing T. The first, observed in Çine nappe samples, is an “N-shaped path” defined by a P increase, decrease, and a second increase (Figures 8a–8c). Selimiye nappe samples, however, show only continuous P and T increase.

Figure 8

Isochemical phase diagrams and predicted P-T paths from the Çine and Selimiye nappes. The phase diagrams were constructed using the final, effective bulk compositions from the corresponding model. Sample numbers are indicated on each panel. Garnet compositional isopleths are also plotted using the observed mole fractions at the crystal rim (±0.01 mole fraction); the region of intersection (yellow field) is designated as the predicted P-T space for which garnet growth ceased. Pertinent predicted stable mineral assemblages are also given, correlating to the numbers within the open circles. Quartz and H₂O are present in every predicted stable mineral assemblage.

For every sample in the Çine nappe, garnet growth appears to have occurred along a clockwise prograde P-T path during early and middle stages of crystallization, which reached 6.1-6.5 kbar and T of 547-553 °C prior to a decrease in P toward 5.3–6.0 kbar. However, in the next (last) stage of growth, Çine nappe samples experienced a second episode of increasing P with increasing T toward final rim conditions of 6.1-7.0 kbar and 568-583 °C, thus producing an N-shaped path. Models of the Selimiye nappe garnets reveal P-T paths that are relatively simple, showing increases in P and T of ~0.2 to 1.6 kbar and ~32 to 60 °C.

The garnet zoning models suggest pressures for the duration of garnet growth in both the Çine and Selimiye nappes are approximately 5–8 kbar. Pressure estimates are commonly more uncertain than temperature estimates on isochemical phase diagrams due to steeply sloping isopleths. Despite steep slopes of some isopleths in our diagrams (Figure 8), the isopleth intersections for garnet cores in all models are relatively tight, and for the rims of some models yield tight intersections (Figure 8; ED17, ED01, and ED34). Absolute uncertainty in these pressure estimates is unknown, but overlap of the isopleth intersections from a single nappe is in the range ±1 kbar.

Temperatures estimated by the garnet zoning models suggest garnet crystallized over a continuous increase in T of 13-60 °C, with Çine nappe samples experiencing a larger magnitude of change from core to rim. As described above, estimates of temperature are typically less uncertain compared to pressure; this holds true here, as demonstrated by the tight overlap of isopleths for core and rim (Figures 7 and 8).

4.3 Geochronology

Twenty-eight zircons from five Çine Massif samples were dated in situ using LA-ICP-MS; in total, 35 spots were analyzed (Table 4; Figure 9). Only one zircon was dated in the Selimiye nappe rock ED34 (inclusion in garnet 463±10 Ma; Figure 4c). Three zircons yield ~2 Ga ²⁰⁷Pb/²⁰⁶Pb ages (in samples ED17, ED27, and ED35) and a single grain in sample ED35 is Neoproterozoic with a 933±40 Ma core and ~807±16 Ma rim. A matrix grain in sample ED27 also yields a Neoproterozoic result (776±8 Ma). However, the majority of zircons in these rocks are Late Neoproterozoic to Ordovician (from 646±15 Ma to 484±37 Ma). Five grains are younger than the Ordovician and are Devonian (435±13 Ma to 371±10 Ma) to Late Permo-Triassic (254±5 Ma and 246±20 Ma). A variety of morphologies are observed, but no trend was found between age and grain shape. In sample ED35, the oldest zircon is an inclusion in garnet. In sample ED28, the oldest age zircon inclusion in garnet overlaps within uncertainty with the sample's oldest matrix grain. However, in sample ED27, the oldest zircon is a matrix grain, and the youngest is an inclusion in garnet. Zircon Th/U ratios are all >0.1 except for the youngest grain in sample ED35 (Th/U of 0.05). The oldest zircon dated in this study (ED17, grain 07) has the highest Th/U of 2.10. Most grains are concordant with only 11 ages having discordance of >10%.

Table 4. In Situ U-Pb Zircon Ages for Samples From the Çine Massif

Samplea	Locationb	Age (±2σ) Mac	Discordance (%)	U (ppm)	Th/U
Selimiye nappe
ED34
01	m	463±10	0.7	397	0.48
Çine nappe
ED17
06	m	254±5	13.4	1,149	0.34
01	m	371±10	12.1	638	0.30
04	m	410±18	9.9	1,664	0.66
02	m	435±13	3.8	130	0.56
03	m	530±19	15.9	2,170	0.34
05	m	570±7	2.8	313	0.85
07	m	2,022±30	13.4	64	2.10
ED27
01	i	246±20	29.9	144	0.33
05_2	m	484±37	6.4	390	0.41
03	i	520±17	12.2	203	0.87
05_1	m	550±15	2.0	900	0.28
07	i	572±10	2.1	492	0.44
04	m	599±12	1.7	35	0.54
06	m	776±8	0.7	289	1.95
02	m	1,945±75	38.5	93	0.60
ED28
05	i	513±38	27.3	138	0.54
08_2r	m	514±11	5.7	1,131	1.06
10	i	514±22	7.2	81	0.21
09	m	539±22	3.4	191	2.05
03	m	548±14	13	382	0.93
08_3	m	557±33	12.8	431	1.12
02	m	563±10	3.3	799	1.65
04	m	600±12	31	281	0.07
01	m	615±34	44.7	101	0.64
07	i	617±23	6.4	122	0.94
08_1c	m	627±12	0.1	297	1.72
06	m	646±15	18	354	0.70
ED35
02	m	361±11	17	1,234	0.05
03_5r	m	435±9	14.3	781	0.42
03_4r	m	657±13	0.8	463	0.44
03_3r	m	807±16	1.4	251	0.22
03_1c	m	932±40	1.8	166	0.12
03_2c	m	933±38	9.1	102	0.12
01	i	1,995±16	20.9	940	1.04

• ^a Nomenclature for zircon grains: (grain #)_(spot #)(c or r) where c = core and r = rim were easily identified.
• ^b “i” indicates grain is a garnet inclusion, and “m” indicates the grain is in the matrix.
• ^{c 238}U/²⁰⁶Pb age is reported if <800 Ma and younger; ²⁰⁶Pb/²⁰⁷Pb age is reported for grains >800 Ma.

Figure 9

(a) Plot of zircon ages from the Çine Massif. See Table 4 for details. Pz = Paleozoic; Mz = Mesozoic; Cz = Cenozoic. Gray bands highlight the timing of previously recorded tectonic events suggested to be related to Pre-Alpine events (Reischmann et al., 1991) or Pan-African event (Catlos & Çemen, 2005; Gessner et al., 2004; Hetzel & Reischmann, 1996; Koralay et al., 2012; Loos & Reischmann, 1999). Alpine to Neogene ages after Şengör et al. (1984), Lips et al. (2001), and Hetzel, Passchier, et al. (1995). (b) Concordia diagram (²⁰⁷Pb/²³⁵U vs ²⁰⁶Pb/²³⁸U) of zircons dated by Laser-Ablation Inductively Coupled Plasma Mass Spectrometry. Inset shows the region between ~450 and 700 Ma.

Nineteen total spots on 18 different monazite grains were dated in situ from three Çine Massif samples (Table 5; Figures 3-5 and 10). In general, Çine Massif monazites show elevated amounts of common Pb (²⁰⁴Pb). Low amounts of radiogenic ²⁰⁸Pb (²⁰⁸Pb*) have been previously reported from Çine Massif monazites (Baker et al., 2008); this observation has recently been related to monazite Ba content and fluid interactions (Catlos & Miller, 2016). We include SIMS ¹³⁷Ba⁺/Th⁺ ratios of these monazites from the analyzed spot, which are significantly elevated relative to the standard monazite ¹³⁷Ba⁺/Th⁺ ratio (0.106±0.005 to 2.192±0.049 compared to the average standard value of 2.934×10-04±1.579×10-05). The small size and lower amount of Pb* results in higher analytical uncertainty for both the ²³⁸U-²⁰⁶Pb and Th-Pb age estimates (Table 5). The textural relationship of most grains suggests they formed due to a reaction with allanite, a mineral known for higher common Pb contents. However, some U-Pb ages are concordant (Figure 10). Both the oldest and youngest Th-Pb and ²³⁸U-²⁰⁶Pb ages are found as matrix grains.

Table 5. In Situ Monazite Ages for Grains From the Çine Massif

Sample_ Grain	Locationa	Th-Pb age (±1σ; Ma)	% ^208Pb* (±1σ)b	^238U^-206Pb age (±1σ; Ma)	% ^206Pb* (± 1σ)c	^137Ba^+/Th^+d
Selimiye nappe
ED01
01_01	m	n.r.	n.r	27.7±8.6	2.4±0.5	n.m
ED02
02_09	m	20.5±7.6	11.2±3.7	14.1±5.0	13.1±3.1	n.m
02_04_1	i	33.3±2.9	29.8±1.6	22.6±3.0	27.3±1.9	n.m
02_04_2	i	40.2±4.2	19.6±1.6	23.4±3.2	19.2±1.5	n.m
02_02	m	50.8±6.5	23.6±1.1	17.5±2.4	23.3±1.1	n.m
Çine nappe
ED09
09_06	m	21.6±4.9	9.0±1.9	40±10	10.0±1.9	0.419±0.002
09_09	i	28.6±5.7	7.0±1.1	23.8±3.9	9.1±1.1	0.425±0.034
09_08	i	29.6±6.9	7.8±1.6	20.6±4.3	8.6±1.6	0.475±0.016
09_07	m	32.9±6.1	9.0±1.3	60±15	11.8±1.2	0.311±0.010
09_10	m	47±16	8.3±2.7	42±11	11.9±2.6	0.505±0.006
09_01	m	120±15	24.1±1.3	61.8±8.5	27.2±1.2	n.m
09_04	i	130±16	26.2±1.1	100±16	31.3±1.0	n.m
09_02	m	162±43	19.3±2.0	60±11	23.7±1.8	n.m
09_03	m	n.r	35.3±2.4	59±11	38.6±2.3	n.m
ED27
27_06	i	26.6±5.2	23.9±4.2	40±20	27.1±4.0	1.046±0.014
27_07	m	32.3±5.0	16.1±2.1	28.9±6.0	20.7±2.0	2.192±0.049
ED35
35_09	m	18.8±5.0	6.2±1.6	13.8±5.9	6.0±1.7	0.347±0.011
35_04	m	47.1±5.0	7.3±0.6	102±27	8.8±0.6	0.519±0.008
35_08	m	65±12	22.4±2.6	28.7±8.4	14.6±3.0	0.106±0.005

• Note. n.m. = not measured; n.r. = not reported.
• ^a “i” indicates the grain is within a garnet crystal, and “m” indicates a matrix grain. See Figures 3-5 for images of the dated grains.
• ^b Percent radiogenically derived ²⁰⁸Pb.
• ^c Percent radiogenically derived ²⁰⁶Pb.
• ^{d 137}Ba+/Th measured in the dated grain. This analysis was only performed during day 2. The standard monazite 554 grain yielded a ¹³⁷Ba⁺/Th of 2.934×10^-04±1.579×10^-05.

Figure 10

Concordia plots for monazite analyses in samples (a) ED01 and ED02, (b) ED09 and ED35, and (c) ED27. See Table 5 for details. Figures 3-5 show images of each dated grain.

5 Discussion

5.1 Thermobarometry

Considering the range of P-T results from previous workers, as well as the range of P-T conditions determined from conventional thermobarometry in this study, the overprinting of prograde assemblages seems to have strongly affected attempts to extract reliable P-T conditions in this part of the Menderes Massif. Garnet chemical zoning in rocks that did not experience temperatures higher than roughly 600 °C for long durations tends to retain prograde chemical zoning of the major elements (Caddick et al., 2010; Carlson, 2006). Here we compare our estimates from conventional thermobarometry with garnet isopleth estimates taken from the end of garnet growth (rim) in our models of garnet chemical zoning. Our goal is to provide P-T estimates from garnet chemical zoning using a method that should retain prograde P-T conditions through subsequent overprinting and retrogression.

Ideally, one would anticipate estimating the same P-T conditions from both conventional and pseudosection isopleth approaches. Although the same mineral compositions are used, the results reflect several differences, including those of calibrations and available thermodynamic data (e.g., Berman, 1990, vs. Holland & Powell, 1998). We also recognize that the bulk compositions of the rocks have been altered since the peak of metamorphism, so this should introduce error as well. However, it appears that the bulk compositions are not far from those that were present during prograde metamorphism considering that the retrograde phases (primarily Chl) are the results of garnet consumption and the other phases likely experienced mostly exchange reactions; thus, the bulk compositions appear to be generally intact. The reasonable starting (garnet core) and ending (rim) P-T conditions and assemblages in each modeled rock, as well as the close fits between the measured zoning profiles and the calculated profiles, provide further confidence that the P-T conditions derived from the models give more realistic estimates of the prograde conditions than from conventional thermobarometry.

As described in the geological background section, reliable P estimates of Menderes Massif garnet-bearing rocks have been problematic in previous research. In this study, the peak P estimated by GBMP barometry is ~8-15 kbar higher than the final (rim) P estimated from the chemical zoning model. These high pressures are inconsistent with petrological observations (e.g., observed metamorphic grade, rock textures, and observed mineralogy), so the higher P conditions are unlikely.

A common challenge with applying conventional barometry to Menderes Massif mineral assemblages is low anorthite (An) plagioclase. The An content of plagioclase in our samples is relatively low (An_0–33, average An₁₄; see the supporting information) and has been reported in other Çine Massif rocks (e.g., Régnier et al., 2003; Whitney & Bozkurt, 2002). This can lead to erroneously high P estimates using the Hoisch (1990) calibration, or other barometers dependent on anorthitic plagioclase (Spear, 1993; Todd, 1998). Following Todd (1998), the products of mole fractions of grossular and anorthite corresponding with the rims of the modeled garnets are all lower than 0.05, which suggests errors with magnitudes greater than 3 kbar should be expected.

Pressure estimated by the conventional approach is variable throughout the Çine Massif and within each nappe (Table 3), most noticeably in the Selimiye nappe with estimates ranging from 15.4 ± 2.3 kbar to 22.4 ± 0.4 kbar. Estimates of rim P made using the garnet chemical zoning approach are lower, and more consistent, ranging from 6.1 ± 2.1 kbar to 7.7 ± 0.3 kbar.

Temperatures appear to be less affected by factors external to garnet zoning, perhaps due to the lack of dependence on anorthite content in plagioclase. Three rocks overlap in rim T using both chemical zoning models and conventional approaches: samples ED02 (586 ± 10 °C and 604 ± 18 °C), ED34 (582 ± 5 °C and 608 ± 22 °C), and ED35 (583 ± 22 °C and 613 ± 26 °C). In others, the rim temperatures from the models are higher (ED01) or lower (ED17 and ED28) compared to those estimated using G-B thermometry. Considering that the uncertainty of thermometry tends to fall in the range of ± 25-50 °C (Holdaway, 2000; Powell & Holland, 2008), all of the temperatures derived from conventional thermometry in this study are consistent with the temperatures derived from the chemical zoning models and give a range of temperatures of 556-671 °C with an average value of 596 °C. Using only the temperatures from the chemical zoning models yields a range of 568-586 °C with an average of 579 °C.

It is necessary to point out that the abrupt midrim pressure inflection of the “N-shaped” paths may not be immediately anticipated when evaluating compositional zoning profiles alone (Figures 6d–6f). Because the reversal in XGrs and XAlm zoning beyond the transect points associated with the modeled pressure inflections (i.e., kink) is gradual, a more subtle P inflection might be expected. One possible explanation is a major phase reaction has occurred at this point. To determine whether this is the case, a series of isochemical phase diagrams at the midrim P minimum and the next point immediately after were made for each sample (Supporting Information S6). The bulk compositions used correspond to the effective bulk compositions listed in Supporting Information S3 for each point. These isochemical phase diagrams do not reveal any major reactions occurring at or immediately beyond the kink. The incipient breakdown of feldspar could have occurred here, which might explain why XGrs in garnet slightly increases at the kink and for the remainder of each profile. Again though, this is not revealed by thermodynamic modeling. Another plausible explanation for the abrupt change might be that the XSps minimum penalty function used during modeling inadvertently exaggerated the magnitude of the pressure inflections. However, because the observed XSps at and beyond the kink is naturally low (0.01-0.001), this penalty function should not dramatically impact our final P-T estimates nor call into question the credibility of the overall Çine nappe path shape.

Ultimately, the P-T paths derived from the garnet zoning models provide much more information than starting and peak conditions. They describe the changes in P-T during metamorphism. The consistency of the shapes of the paths from rocks collected from different outcrops in the same nappe helps to show their reliability for understanding changes in P-T during metamorphism. Path shape should be a robust indicator of changes in P-T, despite uncertainty in absolute position in P-T space, because each path segment is the result of a difference in garnet composition within the same modeled environment (Kohn, 1993). The shapes of the paths provide detailed insight into the movement of the nappes on either side of the SSZ during garnet growth and are essential for thermal modeling (below).

5.2 Geochronology

Compared to monazite, zircon is more durable and less likely to reset in changing P-T conditions (Hoskin & Schaltegger, 2003). The mineral has a closure temperature of >900 °C for grains 10 to 100 μm (Lee et al., 1997) and commonly preserves relict crystallization ages (e.g., Hoskin & Schaltegger, 2003; Jackson et al., 2004). Unlike monazite, zircon ages from Çine Massif samples do not reveal evidence of Cenozoic crystallization. The majority of zircons likely time Pan-African magmatism and metamorphism. Cambro-Ordovician age zircons have long been known to be entrained within Menderes Massif metamorphic rocks (e.g., Gessner et al., 2004; Hetzel & Reischmann, 1996; Koralay et al., 2012; Loos & Reischmann, 1999), but the Neoproterozoic and Permian-Triassic ages from our samples have not yet been reported from the region. The results have implications for understanding the older history of the massif and its tectonic relationship to units in its central and northern portions. The Th/U ratio may indicate if a detrital zircon is igneous or metamorphic in origin (Cavosie et al., 2004; Vavra et al., 1996; Williams & Claesson, 1987). Typically, primary igneous grains have Th/U > 0.10. Previously reported Th/U ratios from Çine Massif zircons range from 0.04 to 1.68 (Gessner et al., 2004). In this study, all zircons have Th/U in the range 0.05 to 2.08, indicating an igneous origin.

Timing Cenozoic tectonic activity across the Menderes Massif is of primary interest for understanding the construction of western Anatolia. The origin of monazite in the samples is clearly due to the breakdown of allanite (Figures 3-5). The textural relationships suggest that the ages time events after garnet growth and therefore do not provide insight into the metamorphic events related to the development and growth of garnet. Their high ¹³⁷Ba⁺/Th⁺ (Table 5) suggest significant fluid-mediated alteration (Catlos & Miller, 2016), and because most of the crystals are found within the rock matrix, they would be susceptible to Pb-loss and recrystallization during the extensive retrogressive phase recorded in the Menderes Massif. Given these observations, the monazite ages reported here might provide the timing of the transition from shortening to extension after the MMM event. A range of ages would be expected for monazite as its precursor allanite reacted with fluids with heterogeneous compositions during retrogression and extension. The MMM event was likely an allanite producing event, a mineral that altered to monazite at peak metamorphism and continued as the region started to experience large-scale extension. Eocene to Oligocene Th-Pb monazite ages (Th-Pb 40.2±4.2 Ma and 33.3±2.9 Ma) fit with the growth of garnet elsewhere in the MMM (Schmidt et al., 2015); however, grains of this age in sample ED02 appear as filling cracks and are thus potentially postgarnet growth. The youngest ages we report (13.8±5.9 Ma, 14.1±5.0 Ma, 17.5±2.4 Ma ²³⁸U-²⁰⁶Pb, and 18.8±5.0 Ma Th-Pb) are observed elsewhere in the Menderes Massif and are linked to extension, suggesting that the young ages in our samples could be the result of monazite reprecipitation or resetting during retrograde metamorphism and is consistent with the timing of fluid-driven deformation (e.g., Catlos, 2013; Pyle & Spear, 2003; Zhu & O'Nions, 1999). Alternatively, monazite could have formed from the breakdown of allanite during decompression heating at peak metamorphism, particularly for our grains with ages that coincide with published garnet Lu-Hf ages (~40 Ma; Schmidt et al., 2015). However, it is unlikely this phase of metamorphism carried on over a >20 million period.

5.3 Thermal Modeling

Two models may be applicable to explain the N-shaped P-T paths observed in the Çine nappe garnets: erosional denudation followed by reactivation of the SSZ (model 1; Figure 11) or tectonic switching, in which the SSZ switched motion from shortening to extension (model 2) (Figure 12). To understand how each of these models would be recorded via garnet P-T paths, we developed a model framework for each, consisting of a grid with reflecting side boundaries and top and bottom maintained at 25 and 700 °C and an initial geothermal gradient at 25 °C/km indicated by colored zones. The thermal grid consists of depth versus horizontal distance with the initial position of the SSZ structure selected at 30°, and fault displacement that varies linearly across the shear zone. Parameters appropriate to crustal rocks in terms of thermal conductivity (2.5 W/[m-K]), diffusivity (8×10-7 m²/s), heat capacity (1 kJ/kgK), latent heat of fusion (400 kJ/kg), basal and initial surface flux (30 and 70 mW/m²), and radioactivity length scale (15 km) are incorporated. We ran each model over a 7-million-year timescale. The position of the Selimiye samples is inferred by a hatch area in the models. The gray bar represents the approximate initial location of Çine nappe sample ED28. In both scenarios, the SSZ is activated, and a finite difference solution to the diffusion-advection equation is used to examine the P-T variations in the hanging wall and footwall as a result of motion. The rock sample experiences the path from a to b on the P-T path inset. Motion stops and denudation occurs in model 1 (Figure 11), whereas extension occurs in model 2 (Figure 12). The amount of denudation and the constraints on the P-T conditions are based on the midrim lower P portion of the garnet P-T path and are represented by paths b to c on the P-T path insets. Paths c to d on the P-T path insets represent the time when the SSZ is reactivated. Although not depicted in either model, a second period of extension occurs after thrust reactivation terminates.

Figure 11

Snapshots of thermal models of the Çine nappe. (a) The upper equilibrium thermal grid (depth vs. horizontal distance) prior to faulting with the position of fault (gray line) arbitrarily selected at 30°. Fault displacement varies linearly across shear zones. The grid includes reflecting side boundaries and top and bottom maintained at 25 and 700 °C and an initial geothermal gradient at 25 °C/km indicated by colored zones. The position of the Selimiye samples is inferred by a hatch area, and the gray bar represents the approximate initial location of the Çine nappe garnet ED28. This is also represented by point a in P-T path inset. (b) Fault is activated, and a finite difference solution to the diffusion-advection equation is used to examine the pressure-temperature (P-T) variations in the hanging wall and footwall as a result of motion. The rock sample experiences the path from a to b in the inset. (c) Motion stops, and denudation occurs. This is represented by paths b to c on the P-T path insets. (d) Fault is reactivated, represented by paths c to d on the P-T path insets.

Figure 12

Snapshots of thermal models of the Çine nappe. (a) The upper equilibrium thermal grid (depth vs. horizontal distance) prior to faulting with the position of fault (gray line) arbitrarily selected at 30°. Fault displacement varies linearly across shear zones. The grid includes reflecting side boundaries and top and bottom maintained at 25 and 700 °C and an initial geothermal gradient at 25 °C/km indicated by colored zones. The position of the Selimiye samples is inferred by a hatch area, and the gray bar represents the approximate initial location of the Çine nappe garnet ED28. This is also represented by point a in pressure-temperature (P-T) path inset. (b) Fault is activated and a finite difference solution to the diffusion-advection equation is used to examine the P-T variations in the hanging wall and footwall as a result of motion. The rock sample experiences the path from a to b on the P-T path inset. (c) Motion stops, and extension occurs. This is represented by paths b to c on the P-T path insets. (d) Fault is reactivated, represented by paths c to d on the P-T path insets.

The models represent only what is possible with these data assuming the duration of garnet growth, a specified geometry, and slip rate. Although the end surface geometry in the denudation phase (Figure 11c) and extensional phase (Figure 12c) are similar, the shape of the isotherms differs. The N-shaped P-T path is more consistent with the development of a drop in pressure due to erosional exhumation, as opposed to the P-T loop in the tectonic switching model. In this case, an N-shape P-T path does not form, as heat is unable to advect to the sample and the rock experiences a drop in T due to refrigeration during uplift. Note that the T-D approach we apply here is unable to record any decrease in T as; in that scenario, garnet is consumed (Figure 13a). However, we recognize that a P-T loop is possible. Consider that the garnet zoning models in this study are unable to record any decrease in T as, in that scenario, garnet is consumed (Figures 13a, 7, and 8, garnet-in line). Thus, it is possible that the garnets with N-shaped P-T paths actually experienced a hiatus in growth during a decrease in T (and/or P; e.g., Kelly et al., 2015). In ED28 (Figure 3b), the Mg map shows an irregular (nonconcentric) pattern between the low concentration near the core (blue) and higher concentration in the mantle (green) that resembles the irregular rim of a partially consumed garnet. Other samples from the Çine nappe (e.g., ED35; Figure 3c) show similar zoning patterns that are associated with inclusions. However, the inclusions make interpretation difficult; given that diffusion of matrix components is relatively fast along grain boundaries, while a garnet overgrows another crystal (inclusion), the composition of garnet along the boundary with the inclusion is likely to maintain near equilibrium with the matrix up to the point at which the inclusion is completely encapsulated by garnet. The midrim Y high in these samples may also be due to garnet consumption, or it could be the result of an Y-bearing accessory phase reaction during continuous garnet growth (Pyle & Spear, 1999; Spear & Pyle, 2010). These chemical zoning patterns suggest the possibility of garnet consumption during a P-T loop, but without more definitive evidence, the denudation P-T path is our preferred scenario.

Figure 13

Garnet pressure-temperature (P-T) paths developed using model 2. In panel (a), the P-T path is used to further clarify the meaning of the loop. Growth of the garnet core is described by blue squares that monotonically increase in T. The dash-open square portion corresponds to decreasing temperatures (retrograde path), and garnet will not grow during that time due to negative thermodynamic garnet-mode gradients. The red square shows a period of the garnet overgrowth or external rim. Panel (b) shows results if extensional and reactivation occur three times faster than in panel (a). If an idle period exists between extension and reactivation, the gap between b-c and c-d paths in both panels will increase. Note that the a-b-c-d portions in both paths correspond to model motions in Figure 12.

Increasing the rate of the change from shortening to extension along the SSZ has the potential to preserve or conserve the amount of heat lost due to advection and available for garnet growth (i.e., a “fast switch”). To understand if we can create an N-shaped path by a fast switch process, we increased the rate of extension by three times in model 2 (Figure 13b). This process affects the width of the b-c portion of the paths but does not result in the development of an N-shaped path as the shape of the loop decreases.

The decrease in temperature suggested by the tectonic switching (model 2) is consistent with some previously reported P-T paths that decrease in temperature (Figure 2b). However, our data for P-T paths from Çine nappe rocks (Figure 8) are consistent with the alternative hypothesis of denudation (model 1). Çine nappe garnets record ~0.5-2.0 kbar fluctuations in P. The reproducibility of the paths (three from each unit) lends confidence that they accurately record the P-T condition changes experienced by the nappes at this location and structural position. The Selimiye P-T path overlaps the latter portion of the path from the Çine nappe, and so the two units may have experienced the same metamorphic event. The second rise in P recorded by ED28 and inferred by small rises in ED17 and ED35 yield reasonable geothermal gradients (26.9-21.3 °C/km) and depths (from 18.9 to 25.9 km), which lends confidence in the conditions estimated using the garnet zoning approach.

The thermal modeling provides additional constraints for interpreting the P-T paths and allows differentiation between tectonic events that influence changes in pressure and temperature. The overall increase in T and the fluctuation in P in the Çine nappe are best interpreted as thrusting and burial interrupted by a period of erosional exhumation midway through the contractional event that grew garnet.

6 Conclusions

Estimates on P-T using conventional thermobarometry and high-resolution P-T paths were extracted from garnet-bearing assemblages across the SSZ in the Çine Massif, part of the larger Menderes Massif metamorphic core complex in southwest Turkey. Both approaches yield similar thermal conditions 556-671 °C, but P differs significantly (by up to 15 kbar). High P conditions are often reported for Menderes Massif garnet assemblages, and obtaining reliable barometric conditions is a challenge. However, our P-T path modeling results for Çine and Selimiye nappe garnets are an improvement on the conventional approaches and suggest much lower pressures 6.1-7.7 kbar. More importantly, they provide detailed changes in P-T that allow for more insightful interpretations of the driving mechanisms that produced the paths. Three Çine nappe garnets reveal N-shaped P-T paths, whereas three Selimiye nappe garnets show a simple prograde burial path of increasing P-T consistent with the second pulse of increase in P for N-shaped Çine nappe garnets.

Monazite grew as a replacement of allanite, and the timing of growth appears to be after garnet growth (Th-Pb 18.8±5.0 Ma to 40.2±4.2 Ma). Some ages are consistent with monazite growth immediately following garnet, and others are significantly younger. The amount of monazite common ²⁰⁴Pb and ¹³⁷Ba+/Th+ is high and suggests the monazite ages time events associated with fluids in the Oligo-Miocene. In situ zircon ages (246 ± 20 Ma to 2022 ± 30 Ma) are consistent with commonly reported Cambro-Ordovician ages in the Menderes Massif, and some Çine Massif zircon grains are Neoproterozoic and Permian-Triassic, a timeframe not previously recognized in the Menderes Massif.

We used thermal models to test driving mechanisms for the P-T paths. The first simulates thrusting, erosion, and continued thrusting along the SSZ. The second model simulates a “tectonic switch” where the SSZ experienced thrusting, extension with normal shearing, and continued thrusting. The N-shaped P-T path is best reproduced by the first model that simulates erosion and denudation midway through thrusting. Another model explored increasing the rate of the switch in shear motion, but this was unsuccessful in producing an N-shaped path.

These results represent one of the first applications of a high-resolution P-T path method in the Menderes Massif, making use of preserved garnet chemical zoning to extract information from prograde events despite retrograde alteration of matrix phases. The resolution of the paths shows even subtle effects on garnet growth, beyond fault-driven tectonics, in this case a denudation event.

Acknowledgments

No real or perceived financial conflicts of interests for any author exist. No affiliations for any author that may be perceived as having a conflict of interest with respect to the results of this paper exist. Data supporting the conclusions can be obtained as supporting information. This work was largely supported by the National Science Foundation Grant 0937254 and funds provided by the Jackson School of Geosciences. We thank Danny Stockli with assistance with zircon geochronology. We appreciated discussions regarding the approach with Kyle Ashley and thank Daniel Lizzardo-McPherson for assisting with garnet P-T modeling. We thank Nathan A. Niemi for editorial handling and Nilanjan Chatterjee for a thoughtful and constructive review.