P-T-D histories from quartz: A case study of the application of the TitaniQ thermobarometer to progressive fabric development in metapelites

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1 Article Volume 5, Number 00 0 MONTH 2013 doi: ISSN: P-T-D histories from quartz: A case study of the application of the TitaniQ thermobarometer to progressive fabric development in metapelites Kyle T. Ashley Department of Geology, University of Vermont, Burlington, Vermont, USA Now at Department of Geosciences, Virginia Tech, 4044 Derring Hall (0420), Blacksburg, Virginia, USA (ktashley@vt.edu) Laura E. Webb Department of Geology, University of Vermont, Burlington, Vermont, USA Frank S. Spear and Jay B. Thomas Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, New York, USA [1] This study investigates the ability of quartz to record segments of the pressure-temperaturedeformation (P-T-D) path in poly-deformed metamorphic terranes and the associated application of the Ti-in-quartz thermobarometer (TitaniQ). Metapelites from the Strafford Dome (eastern Vermont) were selected for this study because they record progressive fabric development during prograde metamorphism and have well constrained P-T-D histories from previous work. Results of this investigation are in agreement with previous studies and demonstrate that quartz in these samples records additional distinct intervals of the P-T-D path. Six preserved quartz equilibration events include: (1) early prograde growth during burial; (2) kyanite-in quartz-producing reactions; (3) prenappe emplacement equilibration during isobaric heating; (4) precipitation of quartz in quartzfeldspar domains during crenulation cleavage development due to solution transfer; (5) quartzproducing (chlorite-out) reactions during heating postnappe emplacement; and (6) retrograde quartz overgrowths from an influx of siliceous fluids. Modeling quartz volume allows for the identification of quartz-producing reactions in P-T space; the same may be done for Ti-phases to better constrain activities during reequilibration events. An integration of cathodoluminescence imaging, microstructural and petrographic investigation, isochemical forward stability modeling, and thermobarometry allows identification and interpretation of zoning patterns in quartz to unravel histories that would otherwise not be obtainable. Components: 13,437 words, 11 figures, 4 tables. Keywords: Ti-in-quartz; TitaniQ thermobarometry; Strafford Dome; cathodoluminescence; metamorphic tectonites; pseudosection modeling. Index Terms: 3651 Thermobarometry: Mineralogy and Petrology; 3660 Metamorphic petrology: Mineralogy and Petrology; 3625 Petrography, microstructures, and textures: Mineralogy and Petrology; 3612 Reactions and phase equilibria: Mineralogy and Petrology; 3611 Thermodynamics: Mineralogy and Petrology; 1011 Thermodynamics: Geochemistry; 1012 Reactions and phase equilibria: Geochemistry; 8411 Thermodynamics: Volcanology; 8412 Reactions and phase equilibria: Volcanology; 0766 Thermodynamics: Cryosphere. Received 20 May 2013; Revised 1 August 2013; Accepted 4 August 2013; Published 00 Month American Geophysical Union. All Rights Reserved. 1

2 Ashley, K. T., L. E. Webb, F. S. Spear, and J. B. Thomas (2013), P-T-D histories from quartz: A case study of the application of the TitaniQ thermobarometer to progressive fabric development in metapelites, Geochem. Geophys. Geosyst., 14, doi:. 1. Introduction [2] Quartz has been widely used to estimate temperatures of deformation in metamorphic rocks based on microstructures associated with dynamic recrystallization [Stipp et al., 2002] and the quartz c-axis fabric opening angle thermometer of Kruhl [1996]. Geochemical analysis of this common crustal phase however has only recently been exploited for thermobarometry purposes. In recent years, the solubility of Ti in quartz has been calibrated experimentally, resulting in the Ti-in- Quartz (TitaniQ) thermobarometer, and can be used to infer conditions of quartz equilibration with a precision of temperature estimates on the order of 65 C(2; if pressure and the activity of TiO 2 are well constrained) [Thomas et al., 2010; Wark and Watson, 2006]. [3] The applications of TitaniQ are potentially extensive and significant across a spectrum of disciplines due to the abundance and stability of quartz in continental crust [see, for example: Lang and Gilotti, 2007; Sato and Santosh, 2007; Wark et al., 2007; Wiebe et al., 2007; Holness and Sawyer, 2008; Rusk et al., 2008; Kohn and Northrup, 2009; Vazquez et al., 2009; Behr and Platt, 2011; Grujic et al., 2011; Behr and Platt, 2012; Korchinski et al., 2012; Kidder et al., 2013]. A study by Spear and Wark [2009] demonstrated that quartz, depending on textural context and metamorphic grade, (1) records temperatures of metamorphism, (2) may show metamorphic zoning and/or modification by volume diffusion, and (3) has the potential to record temperatures of fabric formation. Studies of Ti volume diffusion in quartz suggest that either high temperatures, small grain sizes and/or long times are required for grains to fully reequilibrate via volume diffusion during metamorphism [Cherniak, 2010; Cherniak et al., 2004, 2007] and Spear et al. [2012] have modeled Ti volume diffusion in quartz to infer metamorphic time scales. [4] This study assesses the application of the TitaniQ thermobarometer to constraining pressuretemperature-deformation (P-T-D) histories in metamorphic tectonites that record progressive deformation during crenulation cleavage development. The Strafford Dome in eastern Vermont was selected for this study because the petrological and structural histories of the rocks have been well constrained (Figure 1) [Menard and Spear, 1994; T. Menard, Rensselaer Polytechnic Institute, unpublished thesis, 1991], providing exceptional context for the samples being analyzed. The metapelitic rocks of the Strafford Dome record evidence of multiple stages of fabric development associated with prograde metamorphism (up to peak kyanite/staurolite-grade) during the Acadian Orogeny ( Ma) [Spear and Harrison, 1989]. The presence of quartz in distinct microstructural domains such as inclusion suites that define internal foliations in garnet porphyroblasts, garnet pressure shadows, and in matrix foliation microlithons facilitate the comparison of relationships between garnet growth, deformation and quartz recrystallization/(re)equilibration. The metapelitic samples also tightly confine TiO 2 activities to c. 1.0 [Ghent and Stout, 1984]. 2. Background 2.1. Regional Geology of the Strafford Dome [5] The Strafford Dome is the northern-most of a series of N S-trending domes in eastern Vermont (Figure 1) [Doll et al., 1961; Menard and Spear, 1994]. The Monroe Fault to the east and the Richardson Memorial Contact to the west separate the generalized anticlinorium structure from New Hampshire and pre-silurian rocks, respectively [Hatch, 1988; Menard and Spear, 1994; Spear et al., 2002]. Lithologies that define the Strafford Dome principally comprise the Silurian to Early Devonian Waits River and Gile Mountain Formations, which collectively make up the Connecticut Valley Trough. The older Waits River Formation consists of micaceous limestones, pelitic schists, and the Standing Pond amphibolites [Doll, 1944; Fisher and Karabinos, 1980]. This formation is a calcareous flysch deposit with dm- to 10 m scaled interleaving of pelitic and carbonate layers. The Standing Pond amphibolite is a thin ( m) unit of basic metavolcanic material [Evans et al., 2

3 ASHLEY ET AL.: TRACKING P-T-D AND SI-FLUX WITH TITANIQ Figure 1. (a) Geologic map of the Strafford Dome including: metamorphic isograds, sample locations where P-T paths were constructed by T. Menard (unpublished thesis, 1991) and Menard and Spear [1994], strike and dip of bedding and foliations (S1 and S2) outlining dome structure (data from Howard [1969]), and samples analyzed for this study. Inset figure is an overview map showing location of geologic map (shaded area) with Eastern Vermont and Merrimack tectonic belts emphasized (modified from Spear et al. [2002]). (b) Cross section (A A0, shown in a) along the traverse of samples collected in this study modified after Howard [1969]. Generalized isobars are overlain on the cross section (from T. Menard (unpublished thesis, 1991)). Samples from this study are grouped with nearby samples with P-T histories from T. Menard (unpublished thesis, 1991) and Menard and Spear [1994], and labeled A (eastern flank nearing Monroe Fault), B (near core of dome), and C (northeast flank of dome, parallel to dome trends and isograds). Inset P-T plot shows generalized P-T paths for the metamorphic belts, displaying the typical Barrovian and Buchan/Abukuma facies series for the Eastern Vermont and Merrimack belts, respectively. (c) Compiled P-T histories for various samples across the Strafford Dome [Menard and Spear, 1994]. Note how peak conditions decrease east of the dome toward the Monroe Fault. 2002]. The Gile Mountain Formation has similar lithologies as the Waits River Formation, although it has a paucity of limestones and has a profusion of quartz-mica schists and amphibolites [Doll, 1944], with pelites and psammites being the main constituents. [6] The Gile Mountain Formation preserves sedimentary bedding locally [Fisher and Karabinos, 3

4 1980; Menard and Spear, 1994; Woodland, 1977], with bedding-parallel micaceous schistosity (S 1 ) [Menard and Spear, 1994;White and Jahns, 1950; T. Menard, unpublished thesis, 1991]. Previous workers have attributed development of crenulation cleavage (S 2 schistosity) and large-scale recumbent folds across the dome [White and Jahns, 1950] to progressive deformation associated with emplacement of a nappe [Woodland, 1977]. Map patterns and structural data from the core of the dome reveal the doming of S 2 [Doll, 1944; Menard and Spear, 1994; White and Eric, 1944; Woodland, 1977]. Widely spaced or kink banding (S 3 ) locally deforms S 2 [Fisher and Karabinos, 1980; Woodland, 1977], but the relationship between S 3 and metamorphism is not well constrained Metamorphism and Paragenesis [7] Metamorphism of the rocks of the Strafford Dome was studied in detail by Menard and Spear [1994] and is associated with the Devonian Acadian Orogeny ( Ma) [Spear and Harrison, 1989]. The paragenesis of muscovite þ biotite þ quartz 6 ilmenite constitutes the majority of the biotite grade metamorphic zone. Higher metamorphic-grade parageneses consist of garnet þ biotite þ muscovite þ quartz 6 plagioclase 6 tourmaline (garnet-grade metamorphism) and kyanite þ staurolite þ garnet þ plagioclase þ biotite þ quartz (staurolite/kyanite-grade metamorphism); the highest grade rocks are found in the core of the dome. Chlorite þ K-feldspar 6 biotite 6 calcite 6 sericite are found locally replacing prograde and peak assemblages. [8] Figure 1c illustrates P-T conditions for the Strafford Dome compiled from T. Menard (unpublished thesis, 1991) and Menard and Spear [1994]. Peak P-T conditions range from ca. 450 C, 5 kbar near the Monroe Fault to the east to ca. 600 C, 10 kbar near the core of the dome. P-T paths for lowgrade rocks are ill-constrained but higher grade rocks experienced an episode of near isothermal loading, attributed to nappe emplacement and associated with progressive crenulation cleavage development, followed by isobaric heating to peak conditions. Retrograde paths are poorly preserved suggesting both rapid exhumation and limited access of retrograde fluids. 3. Methods [9] Samples used in this study are from the garnet and staurolite/kyanite grades (Figure 1) and include both oriented samples ( SD in sample name) as well as unoriented samples from the Menard thesis collection ( TM in sample name) (T. Menard, unpublished thesis, 1991). The biotite grade rocks in this region are the focus of a subsequent study due to there being a less developed P- T framework from previous studies and additional complications when analyzing these lowtemperature rocks. Microstructural analysis was conducted to identify quartz in different fabric domains such as matrix foliation, inclusions defining foliations within garnet, and pressure shadows. These areas were examined by cathodoluminescence (CL) imaging (see below) and target areas for application of the TitaniQ thermobarometer were microdrilled from the thin sections and mounted in 1 00 epoxy rounds. [10] Cathodoluminescence (CL) imaging and spectroscopy were used in the 415 nm -specific mode to characterize Ti distribution within a single crystal and between neighboring grains [Rusk et al., 2006; Spear and Wark, 2009; Wark and Spear, 2005]. The Gatan MonoCL attached to the Cameca SX-100 electron microprobe at Rensselaer Polytechnic Institute (RPI) was used, with a 10 na beam current and 15 kv accelerating potential maintained during analysis. Dwell times of c mspixel 1 were held constant without any pronounced streaking (see, Spear and Wark [2009] for details). Back-scattered electron (BSE) imaging for textural analysis and phase identification was done with a 20 na, 15 kv incident electron beam. X-ray elemental mapping of garnet and surrounding matrix phases was done with beam currents of 200 na and 15 kv. Garnet maps for the elements Al, Ca, Fe, Mg, Mn, and Ti were collected, with tiling required due to large porphyroblast size (several mm in diameter) resulting in scan times between 4 and 6 h. Low-resolution thin section maps were conducted for the elements Ti, Fe, Ca, and Mg to locate and distinguish Tistoichiometric essential accessory phases (e.g., ilmenite, sphene, rutile), which are required for constraining Ti activity. [11] The electron microprobe was also utilized to conduct quantitative spot analyses on garnet porphyroblasts to construct chemical profiles and calibrate X-ray maps, and on garnet-biotite and garnet-ilmenite pairs for geothermometry [Docka, 1984; Feenstra and Engi, 1998; Ferry and Spear, 1978; Martin et al., 2010; Pownceby et al., 1987, 1991; Tracy, 1982; Tracy et al., 1976; Woodsworth, 1977]. Garnet was analyzed for Si, Ti, Al, Cr, V, Mg, Ca, Mn, Zn, and Fe using a 15 kv 4

5 accelerating potential and 20 na current. Data was collected for 5 7 spots to assure consistency and the results averaged. Analyses were taken on regions of elemental highs and lows as revealed by X-ray maps. Ilmenite inclusions within garnet were analyzed for Na, Al, K, Ca, Ti, Mn, Fe, Cr, Mg, Si, and F using 20 s dwell times, 15 kv accelerating potential and 20 na current. [12] Quartz grains and domains in different structural contexts were first imaged by CL using a blue filter to reveal Ti zoning. Based on the results of these analyses, representative grains of different zoning patterns were analyzed by SIMS to quantify [Ti]. [13] Titanium concentrations were measured via secondary ionization mass spectrometry (SIMS) spot analyses utilizing the Cameca IMS 1280 at the Northeast National Ion Microprobe Facility at the Woods Hole Oceanographic Institute. All analysis spot locations were guided by CL imagery of target domains, where representative grains with different zoning patterns from various domain and structural contexts were analyzed. An O - primary beam under ultrahigh vacuum (< Pa) was used to sputter atoms from the top several atomic layers of the sample surface. An incident ion beam with an impact energy of c. 23 kv acceleration potential and 4.05 na current was used. A 240 s presputter was used during sample analyses to remove surface contamination, followed by 10 analytical cycles analyzing 30 Si, 40 Ca, and 48 Ti with dwell times of 5 s, 5 s, and 10 s, respectively; 48 Ti was corrected for mass interference of 48 Ca based on measured 40 Ca intensities. Ion beam spot size varied slightly with focusing, and was typically mindiameter. Titanium concentrations were calculated based on a calibration curve of [Ti] versus 48 Ti/ 30 Si constructed from analyses of TitaniQ standards QTiP- 7, QTiP-14, QTiP-38, and QTiP-39 [Thomas et al., 2010]. If zoning is present in quartz crystals, analyses from the same zone were averaged since analyses often were statistically identical and weighted averaging of these domains would reduce statistical uncertainty for the zone being interpreted. [14] X-ray fluorescence bulk rock analysis of sample 09SD08A was collected to allow for forward stability modeling and modeling of volume percent quartz and Ti-oxides in P-T space (system MnO-Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 - H 2 O-TiO 2, or MnNCKFMASHT), computed with the program Perple_X [Connolly, 2009]. These models are used in interpreting quartz and Tiphase production/consumption to make interpretations regarding Ti growth or consumption of quartz in association with metamorphic reactions to explain zoning patterns, and to qualitatively assess activities of TiO 2 during the rock s evolution. A representative 5 g sample of whole rock from sample 09SD08A was prepared for bulk rock chemical analysis, with all weathered edges removed. The sample was powdered in an aluminum ball mill and passed through 80 m mesh sieve. The powders were sent to Franklin and Marshal s XRF lab and measured for major element chemistry with titration (for ferric iron estimation) and loss on ignition. Solution models utilized in the computed pseudosections include: ideal solution model for the ilmenite-geikielite-pyrophanite; omphacitic-pyroxene [Diener and Powell, 2010] (modification of the Green et al. [2007], omphacite); feldspar [Fuhrman and Lindsley, 1988]; chloritoid, hydrous cordierite, staurolite, garnet, and chlorite [Holland and Powell, 2011]; white mica [Auzanneau et al., 2010; Coggon and Holland, 2002]; Ti-Fe 3þ -biotite [Tajmanova et al., 2009] (extended to Mn solution after Tinkham et al. [2001]). In the models, quartz, rutile and sphene were considered as pure phases and water was considered to be in excess. [15] Results of this study are presented for sample groupings based on proximity and metamorphic grade (Figures 1a and 1b), and are integrated with the previously established P-T-D context of T. Menard (unpublished thesis, 1991) and Menard and Spear [1994]. Reported errors are calculated to include all sources of analytical uncertainty to 1 (supporting information Appendix A 1 ), and TiO 2 activities were assumed to be 0.97 (as per Ghent and Stout [1984]). The thermobarometric calibration published by Thomas et al. [2010] was used in this study for P-T estimation. Reasons for selecting this choice over the recently published calibration by Huang and Audetat [2012] will be discussed later in this manuscript (section 5.3). A data repository of all analyses collected can be found in supporting information Appendix B. 4. Results 4.1. Eastern Flank of the Dome: Garnet Grade Petrography and Microstructures [16] Sample TM623 is a crenulated garnet schist with the assemblage garnet þ quartz þ feldspar þ 1 Additional supporting information may be found in the online version of this article. 5

6 Table 1. Sample Locations and Mineralogy Sample Name UTM Coordinates a Northing Easting Mineralogy Quartz Abundance Zone Rock Type TM Garnet þ quartz þ plagioclase þ biotite þ domains Garnet Garnet schist muscovite þ ilmenite 6 magnetite 6 rutile 6 zircon 6 apatite 09SD08A Garnet þ quartz þ biotite þ muscovite þ abundant Staurolite Garnet schist plagioclase 6 titanite 6 ilmenite 6 rutile 6 zircon 6 opaques TM Garnet þ plagioclase þ muscovite þ opaques domains Staurolite Garnet schist; retrograde TM Kyanite þ garnet þ quartz þ plagioclase þ plagioclase þ biotite þ muscovite þ tourmaline þ ilmenite 6 opaques 6 zircon 10SD03B Garnet þ amphibole þ quartz þ plagioclase þ biotite þ muscovite þ ilmenite 6 opaques 6 zircon 10SD03D Kyanite þ garnet þ quartz þ plagioclase þ biotite þ muscovite þ ilmenite 6 opaques 6 zircon a UTM Zone: 18T. mylonitization abundant Staurolite Garnet-kyanite schist abundant Staurolite Garnet-amphibole schist abundant Staurolite Garnet-kyanite schist biotite þmuscovite þ ilmenite þ magnetite þ rutile 6 apatite 6 zircon 6 opaques (Table 1; Figures 1a and 1b). In outcrop, the dominant foliation, defined by compositional banding and inferred to be the regional S 1 schistosity, dips moderately steeply to the east-southeast; crenulation lineations plunge shallowly to the south. In thin section, the S 1 foliation is defined by compositional layering, preferred orientation of micas and ilmenite that is parallel to the compositional layering, and by grain size variation (Figure 2a). Crenulation cleavages (S 2 ) tend to be parallel or anastomosing and asymmetric and are defined by pressure solution seams in the micaceous domains. Rare kink-banding (S 3 ) is present and nearly perpendicular to the S 2 crenulation cleavage throughout the sample. Quartz grains in S 1 microlithons are flattened parallel to S 1 ; quartz in hinges of crenulated S 1 folations is more equant. Garnet porphyroblasts lack pressure shadows and include the S 2 crenulation cleavage (Table 2). Large, randomly oriented biotite porphyroblasts also include the crenulation cleavage. A quartz-rich domain in a crenulation cleavage hinge zone, where quartz grains predominantly display polygonal textures (i.e., grain boundaries meet at 120 triple junctions) and minor undulose extinction, was selected for further analysis. The quartz in this domain comprises a microlithon defining the S 1 foliation that has been crenulated during S 2 development. We denote crenulated S 1 foliations in S 2 microlithons as S 1 /S 2 in subsequent sections Laboratory Results [17] CL imaging of the matrix quartz from sample TM623 reveals quartz grains with homogeneous dark cores with brighter rims that range from being very narrow (<5 m) to wider and patchy in appearance (Figure 3a). Transitions from dark cores to brighter rims appear fairly discrete. Linear bands of higher CL intensity are locally present. Five TitaniQ analyses yield [Ti] from to ppm for the dark cores and ppm for a bright rim (Figures 3a and 4). [18] Samples that define the location of the garnetisograd in proximity to TM623 constrain peak pressures c. 6 kbar, with peak temperatures in the range of C[Menard and Spear, 1994; T. Menard, unpublished thesis, 1991]. We attempted to confirm T max for this sample by measuring garnet-biotite pairs (using larger biotite porphyroblasts in contact with garnet, both of which overgrew the S 2 crenulation cleavage) for Fe-Mg exchange geothermometry (Table 3). Four calibrations were selected for this study: Thompson [1976], Ferry and Spear [1978], and two calibrations from Bhattacharya et al. [1992] considering mixing parameters for the pyrope-almandine asymmetric regular solution presented by Ganguly and Saxena [1984] and Hackler and Wood [1989]. The range of calculated temperatures for the four calibrations is small ( C; assuming P ¼ 6 kbar) with an uncertainty of around 625 C Core of the Strafford Dome: Kyanite-Staurolite Grade Petrography and Microstructures [19] Sample 09SD08A (Figure 2b) is a garnet schist from the western flank of the dome s core with the assemblage garnet þ quartz þ feldspar þ 6

7 Figure 2. Cross-polarized, transmitted light photomicrographs for samples from the Strafford Dome. (a) Montage photomicrograph along the length of the thin section for sample TM623. The S 1 foliation is defined by quartz and mica domains, which has been crenulated in response to nappe emplacement resulting in an S 2 crenulation cleavage. Both garnet and large randomly oriented biotite porphyroblasts include the S 2 foliation. (b) Garnet porphyroblasts with linear inclusion trails (S i )ofs 1 that are rotated relative to the external foliation (S e ) at an oblique angle ( up to 35 ; sample 09SD08A). Blue, dashed line shows the approximate grain boundary location for the garnets. Pressure shadows developed in response to inhomogeneous strain due to the rigidity of the garnet porphyroblasts. Sample is cut perpendicular to foliation and parallel to the lineation. (c) Crenulated quartz vein with dynamic subgrain rotation recrystallization, resulting in ribbon grains with associated subgrains (forming an oblique foliation; sample TM455). Box shows location of CL image from Figure 3d. biotite þ muscovite þ rutile 6 ilmenite 6 apatite 6 sphene 6 zircon 6 opaques (Table 1). In outcrop, WNW-dipping S 2 schistosity dominates and open crenulation cleavage (S 1 /S 2 ) is locally apparent. The S 2 foliation is a spaced foliation defined by compositional layering and the preferred orientation of micas and opaques. Locally, crenulations of S 1 are observed in S 2 microlithons. There is a relatively small volume of S 2 cleavage domains present (<5 10%) that consists of biotite þ muscovite þ ilmenite. Quartz grains locally display a shape-preferred orientation (long axes parallel to S 2 ) where bound above and below by biotite grains in cleavage domains. Quartz-rich S 2 microlithon domains contain grains that often display more equant grain shapes and foam textures. The thin section contains several garnets 3 4 mm in diameter with linear inclusion trails (primarily quartz and ilmenite) that define an internal foliation (S 1 ) oblique to the matrix foliation ( up to 35 is typical; Table 2; Figure 2b). Some garnets have flattened quartz inclusions that define the included S1 foliation. In other garnets, there is no apparent shape-preferred orientation of quartz inclusions. Garnets typically have strain shadows consisting of quartz, feldspar and mica. Coarsegrained quartz veins cut through the sample and are concordant with the S 2 foliation. Representative spots from a garnet inclusion suite, a garnet strain shadow, the matrix (quartzo-feldspathic domain), and a quartz vein were selected for further analysis. 7

8 Table 2. Microstructure Summary of Samples Sample Name Qtz. Recryst. Mechanism a Included Foliation in Grt (S i ) Grt Growth Relative to Foliation Development b Quartz Occurrence TM623 Static S 1 ;S 2 S 2 <P Matrix, inclusion, vein (posttectonic) 09SD08A Static S 1 S 1 <P<S 2 Matrix, inclusion, vein (posttectonic) TM455 SGR S 2 S 2 <P Matrix, inclusion, vein (pretectonic) TM783 Static S 1 /S 2 ;S 2 S 2 P Matrix, inclusion 10SD03B2 Static S 1 /S 2 ;S 2 S 2 P Matrix, inclusion 10SD03D Static S 2 S 2 <P Matrix, inclusion, vein (posttectonic) a SGR: subgrain rotation recrystallization. b : syn-tectonic; P: porphyroblast growth. Figure 3. Summary blue CL images of quartz in various microstructural settings, displaying different Tidistribution patterns. Ti spot analyses where shown indicate [Ti] in ppm. Image widths are c m. Qtz ¼ quartz; bt ¼ biotite; feld ¼ feldspar. (a) Image of sample TM623 showing hinge zone of S 1 /S 2 crenulation cleavage with zoned quartz. (b) Quartz from a garnet pressure shadow in S 2 foliation displaying significantly larger bright rims than typically observed in inclusions (sample 09SD08A). Some grains are homogeneous and also display bright cores. (c) Quartz vein concordant with S 2 foliation displays homogeneous Ti distribution (sample 09SD08A). (d) Deformed quartz vein exhibiting subgrain rotation recrystallization (sample TM455). The dynamically recrystallized grains (darker) and ribbon subgrains (brighter) form an oblique foliation. 8

9 Figure 4. Summary diagram of Ti analyses collected with respect to sample and microstructural location. Concentrations are in ppm. See supporting information Appendix B for complete data tables corresponding to all data shown in the figure. [20] TM455 (Figure 2c) is mylonitized garnet schist that contains asymmetric C 0 -type shear banding, with the foliation defined by compositional layering and preferred orientation of micas. Two types of garnet are present within the sample: numerous small (sub-mm) inclusion-free grains (in micaceous domains) and larger (up to 2 mm) porphyroblasts that preserve a crenulation cleavage (crenulated S 1 foliations) defined by quartz and ilmenite inclusions present in more quartz-rich domains (Table 2). Mica fish are present and, locally, some of the larger garnets are porphyroclastic and display chlorite and biotite tails. Pressure solution seams are present in micaceous domains (composed of muscovite þ biotite þ ilmenite). Highly strained quartz veins exhibit extensive subgrain rotation recrystallization. Subgrains adjacent to ribbon quartz define an oblique foliation at a low angle relative to the ribbon grain (Figure 2c). A large garnet with an included foliation and a sample of the recrystallized quartz vein (including ribbon and subgrains) were selected for further analysis. 9

10 Table 3. Garnet-Biotite Chemical Analyses for TM623 a Garnet (n ¼ 12) Biotite (n ¼ 3) Oxide Wt % cpuf b Wt % cpuf SiO TiO Al 2 O Cr 2 O n.a. d n.d. e Fe 2 O c n.a. n.d. FeO MnO MgO CaO Na 2 O n.a. n.d K 2 O n.a. n.d Total a Cation per unit formula takes into consideration Al in the M1 site mixing solution models with total tetrahedral and octahedral sum of 6.9 with 11 oxygens for biotite. b cpuf ¼ cations per unit formula. c Determined by two-site mixing and cation sum of 8 for 12 oxygens. d n.a. ¼ not analyzed. e n.d. ¼ not determined Laboratory Results [21] X-ray mapping of garnet in 09SD08A reveals a slight decrease in Mn from core to rim, with a very thin (<10 m) bright rim in Mn that surrounds the grain (supporting information Appendix C). Fe and Mg increase slightly from core to rim, while Ca exhibits an unusual blocky zonation. CL images of quartz inclusions defining the internal, S 1 foliation from the core of the garnet reveal them to have dark cores ([Ti] of ppm) with thin (<5 m), bright rims (Figures 4 and 5c); the rims were too narrow to analyze with SIMS. Brighter patches occur within dark grains, sometimes adjacent to the rims, and yield [Ti] in the range of ppm. Bright, linear features are locally observed in CL images of included grains; an analysis from a high density area of these bright bands yielded [Ti] of ppm. [22] Quartz grains in three distinct matrix fabric domains of 09SD08A were also analyzed: strain shadows on garnet porphyroblasts, quartzofeldspathic microlithons that define the S 2 foliation, and an undeformed quartz vein. Grains for the quartz-rich S 2 domains contain larger, ribbon quartz grains and finer grained quartz. In CL, the finer-grained crystals typically show dark cores that grade to brighter mantles (from ppm Ti; Figure 6d) surrounded by dark rims of variable thickness. Some grains are homogeneous in CL and completely dark. Locally, a sharp contact between the bright mantles and dark rims is very apparent; where analyzed, a dark rim yields [Ti] of ppm and exhibits polygonal texture with the neighboring grains. The larger quartz grain in the matrix (S 2 foliaton) is more homogeneous in CL ( ppm Ti; Figure 6d). Quartz grains in the strain shadows (Figure 3b) have dark cores, bright mantles that are much more extensive than in the matrix S 2 quartzo-feldspathic domains, and thin dark rims. The bright mantles yield [Ti] up to ppm, with similar low [Ti] cores (c ppm). Quartz from the vein in the sample is homogeneous in CL ( ppm Ti; Figure 3c). [23] X-ray mapping of a large garnet porphyroblast in sample TM455 showed the garnet to be fairly chemically homogeneous, except for a distinct rim that contains a significant increase in Ca and a decrease in Mn. CL images of quartz inclusions within the garnet display grains with patchy zoning (dark regions as low as ppm Ti; brighter regions up to ppm Ti; Figure 4). Ti analysis of the recrystallized quartz vein revealed Ti concentrations of c ppm for the ribbon grains, with lower [Ti] for the dynamically recrystallized grains (c ppm; Figure 3d) Northeast of the Strafford Dome, Along Strike: Kyanite-Staurolite Grade Petrography and Microstructures [24] Samples TM783, 10SD03B2, and 10SD03D (Figure 1) are kyanite-garnet schists with the assemblage garnet þ kyanite þ quartz þ plagioclase þ biotite þ muscovite þ ilmenite 6 amphibole 6 tourmaline 6 rutile 6 opaques 6 apatite 6 zircon (Table 1). Crenulation cleavage is apparent in outcrop and both S 1 and S 2 foliations dip moderately to the NE; S 3 kink-banding is locally apparent. As in outcrop, asymmetric crenulation cleavage (S 1 /S 2 ) is apparent in thin section. There are gradational transitions between the S 2 cleavage domains (composed of biotite, muscovite, kyanite; c. 40% mica domains) and microlithons, with smooth, parallel-anastomosing cleavage domains present. Large biotite porphyroblasts statically overgrew the matrix to include the crenulation cleavage. Undulose extinction in quartz is common, and quartz domains in both the matrix and included foliations often display polygonal textures. Some quartz grains are locally elongated parallel to the foliation they define; in the matrix this apparent shape-preferred orientation is limited to grains neighboring micas in S 2 cleavage domains. The samples contain numerous large (up to c. 12 mm) garnet porphyroblasts. Large garnet 10

11 Figure 5. Forward stability model for 09SD08A in the system MnNCKFMASHT. Representative bulk rock chemistry is (determined from X-ray fluorescence analysis, in weight percent oxides): 77.22% SiO 2, 0.80% TiO 2, 13.23% Al 2 O 3, 4.56% FeO tot, 0.06% MnO, 2.45% MgO, 1.31% CaO, 2.02% Na 2 O, 2.52% K 2 O. Water is considered to be in excess. White-colored fields are invariant (eight phases stable), with increasing intensive degrees of freedom toward dark blue. Chl, chlorite; mica, white mica; zo, zoisite; law, lawsonite; sph, sphene; ab, albite; plag, plagioclase; gt, garnet; ru, rutile; feld, feldspar; bio, biotite; ky, kyanite; ilm, ilmenite; sill, sillimanite; hcrd, hydrouscordierite; and, andalusite; acti, actinolite; mic, microcline; ctd, chloritoid; st, staurolite. porpyroblasts have inclusion suites defining an S 1 /S 2 crenulation cleavage in the core that grade outward into spiral S 2 inclusion trails. Smaller porphyroblasts have linear inclusion trails that extend continuously into the external S 2 foliation (see TM783 in Figure 7 and 10SD3D in supporting information Appendix C; Table 2). Ilmenite grains in the sample are limited to inclusions in the cores of garnet porphyroblasts (S 1 /S 2 crenulations). Coarse-grained quartz veins are largely concordant with the S 2 foliation. Two large garnet porphyroblasts with spiral inclusion trails and one small garnet porphyroblast with linear inclusion trails were selected for further analysis. 11

12 Figure 6. Integrated modeling, zoning patterns, and Ti analyses for sample 09SD08A. (a) Previously constrained P-T history [Menard and Spear, 1994] is overlain on the modeled volume percent of quartz. Warmer colors (e.g., red) represent more volume of quartz in the rock than cooler colors. (b) Volume of rutile allows for better constraint on activity-constraining phases in P-T space. (c) CL image showing quartz inclusions defining S 1 foliation in garnet with narrow, bright rims (resulting from Ti volume diffusion from garnet into quartz; cf Spear and Wark [2009]). (d) Matrix quartz (S 2 foliation) that display typical dark core and bright rims, but with an additional dark rim that forms a sharp contact with the other zoning present in the grain. Secondary growth resulting from a quartz-producing reaction is the probable cause of the rim. Electron beam damage caused by BSE imaging is the cause of the false-bright region outlined by the dashed line. Dark striations in this bright region are artifacts of polishing after the BSE imaging. Low Ti cores in inclusion quartz preserves early prograde growth, which must have occurred c. 350 C based on isopleth and the general intersection with a steady state geotherm (1). Menard and Spear [1994] identified a c. 2 kbar pressure increase due to nappe emplacement at c. 500 C, with peak-p at 8.7 kbar (2). An isobaric heating path following peak-p must have occurred and can be constrained through the Ti isopleths from gradationally zoned matrix quartz (3). The polygon for this region represents the Ti increase observed through this gradation to determine the heating path experienced by the rock. A retrogressed rim resulting from an influx of Si-fluids (since no-quartz producing reactions would occur during retrogression) would occur c C, depending on the depth of the rocks during fluid infiltration (4) Laboratory Results [25] X-ray mapping of the garnet porphyroblasts show a notable decrease in Mn and an increase in Mg from core to rim for TM783 and 10SD03B2 (e.g., TM783 in Figure 8; both garnets have spiral inclusion trails); 10SD03D (linear inclusion trail) is chemically homogeneous except for a slight variation in Mn that appears to form an annulus, the zoning of which crosscuts the included foliation (supporting information Appendix C). CL images were collected from inclusions throughout the garnet porphyroblasts and surrounding matrix grains. While concentric zoning in quartz grains (e.g., darker cores and brighter rims) is locally apparent in these samples, quartz zoning is more variable in terms of the prevalence of features such as patchy zoning and bright banding. [26] In TM783 (snowball garnet), the lowest [Ti] obtained (3.7 ppm) is associated with quartz 12

13 Figure 7. (a) Syn-tectonic snowball garnet from sample TM783, with (b f) inset CL images of quartz inclusions. Boxes illustrate the location the CL images were captured from. Figure 7b shows linear bright features are commonly present in quartz inclusions and correspond to planar defects. Figures 7c 7f show falsecolor CL images of inclusions in which quantitative spot analyses were conducted, including matrix S 2 foliaion (Figure 7c), rim (S 2 foliation) (Figures 7d and 7e), and core (S 1 /S 2 crenulation cleavage) (Figure 7f) grains. Ti spot analyses are shown with measured [Ti] indicated in ppm. Note the CL mirror was offset from center in these images resulting in erroneous grading in CL intensity. inclusions defining S 1 /S 2 crenulations in the core of the garnet. There, some zoning from darker cores to brighter rims is apparent, but core [Ti] varies from ppm. The highest [Ti] ( ppm) are obtained from the garnet-matrix interface (i.e., where grains are only partially included in the garnet and therefore communicate with the matrix grain boundary network) and matrix domain where CL images reveal zoned grains with dark cores and bright rims neighboring relatively homogenous bright grains (e.g., Figure 7e). Quartz from the S 2 linear inclusion trail (near top of porphyroblast, where box d is located) yielded intermediate [Ti] ( ppm) relative to garnet core and matrix domains. In comparison, 10SD03B2, which has similar garnet chemical zoning and microstructures to TM783, has Ti of ppm associated with dark quartz cores (in CL) defining S 1 /S 2 crenulations in the core of the garnet. More brightly zoned regions reach ppm Ti near the garnet s core and up to ppm near the rim. Garnet-ilmenite thermometry was conducted on TM783 (Table 4) resulting in T max estimates of C (2) [Martin et al., 2010, calibration]. 13

14 ASHLEY ET AL.: TRACKING P-T-D AND SI-FLUX WITH TITANIQ Figure 8. X-ray element maps of syn-tectonic garnet from sample TM783 showing a decrease in Mn and an increase in Mg from core to rim (attributed to increasing P during nappe emplacement). Profile plot (below) is from a to a0. Secondary growth results in increased Ca (decreased Mn) around the rim. [27] CL imaging of quartz grains associated with S2 in sample 10SD03D locally reveals slightly more systematic zoning. Quartz inclusions in garnet in 10SD03D have low [Ti] ( ppm) dark cores and bright rims with [Ti] up to ppm (Figure 9). Quartz in the matrix or near the garnet-matrix interface shows similar zoning, but the bright rims present only reach ppm where analyzed. 5. Discussion 5.1. TitaniQ Results Relative to P-T-D Path Constraints [28] In order to determine where [Ti] isopleths lie in P-T space, the activity of TiO2 must be known or assumed. In this study, a TiO2 activity of the growth medium of 0.97 was assumed for all 14

15 Table 4. Ilmenite-Garnet Exchange Thermometer Analyses for TM783 a Garnet Ilmenite Trav. No. X Mn b X FeII c X Mg d n X Mn b X FeII c n Final (lnk D ) T( C) 2 ( C) a Note: Other cations present in garnet s X-site (e.g., Zn) were excluded from mole fraction calculations for these samples due to the negligible partitioning of these elements. b X Mn ¼ Mn/(MgþCaþMnþFe 2þ ). c X FeII ¼ Fe 2þ /(MgþCaþMnþFe 2þ ). d X Mg ¼ Mg/(MgþCaþMnþFe 2þ ). samples based on the reaction studies for typical metapelites by Ghent and Stout [1984]. An activity of c. 1.0 is supported by using the AX program (v. 2 for windows platform) [Holland, 2008] on the electron microprobe chemical analyses of the ilmenite traverses in TM783. The program calculates activities of mineral endmembers using microprobe data for ilmenite assuming ideal twosite mixing for three oxygens for the Ti-Fe-Mn- Figure 9. (a) Photomicrograph of a garnet porphyroblast from sample 10SD03D with linear S 2 foliation inclusion trails that extends continuously with external foliation, S 2. (b and c) Matrix quartz near the rim of the garnet that show quartz that grade from low-ti cores to high-ti rims (similar to that observed in 09SD08A). The thin section was cut perpendicular of the dominant S 2 foliation and parallel to the dip direction (049 /29 ¼ dip azimuth, dip. Garnet growth was late syn to postkinematic with respect to S 2 foliation development, with inclusions being primarily quartz and ilmenite. Mg solid solution between ilmenite-hematitepyrophanite-geikielite, consistently returning ilmenite activities >0.90 [Ghent and Stout, 1984] TM623 (Garnet Grade) [29] Figure 9a presents results for TM623 along with P-T paths established for nearby samples by T. Menard (unpublished thesis, 1991) and Menard and Spear [1994]. Two isopleths are shown for quartz grains in the hinge of the S 1 /S 2 crenulation cleavage: 2.2 ppm represents the lowest [Ti] from a dark core (in CL images) of a zoned quartz grain and 5.6 ppm represents the highest [Ti] obtained from a bright rim. Neither isopleth intersects with the T max estimate from garnet-biotite thermometry (Figure 10a), or with the polygons representing P- T path constraints for nearby samples. Microstructural evidence suggests that the garnet and biotite both postdate crenulation cleavage development because the crenulation cleavage is included in both minerals, with no evidence for tightening or modification of crenulations in the matrix relative to inclusion suites. Biotite porphyroblasts also have random orientations, supporting static recrystallization. Therefore, no analyses of quartz appear to represent peak P-T conditions, but are instead likely recording intervals of the early prograde path. Sluggish Ti diffusivities and the lack of quartz-producing reactions (none are observed in the pseudosection model for this sample) account for the lack of quartz equilibration during peak metamorphism in this sample. This may also be attributed the lack of deformation near peak T, resulting in no dissolution-precipitation creep to drive new quartz growth. We also note that CL images show relatively sharp boundaries between dark and bright zones, suggesting no substantial modification of these zones via volume or pipe diffusion. 15

16 Figure 10. Summary P-T data for groupings A C designated in Figure 1b using the TitaniQ thermobarometer calibration by Thomas et al. [2010], with comparison P-T paths from T. Menard (unpublished thesis, 1991) and Menard and Spear [1994]. Note the striking similarities between this study and previous studies in Figures 10b and 10c. (a) TM623 structurally lies between TM637 and TM549; P-T conditions are expected to reflect this structural position relative to the isograds. Due to the lack of definitive rims/mantles in quartz of different [Ti], interpreting the analyses is cautioned (see discussions for more detail). However, garnet-biotite exchange thermometry constrained peak-t to c. 525 C. This reflects expected T max for the sample. (b) With 09SD08A collected from the same outcrop as TM458a, the generated P-T history from TitaniQ thermobarometry should overlap the results from previous studies. Fabric development of S 2 occurred at peak P resulting from nappe emplacement and clearly overlays the previously constrained histories. Polygon graded from blue to red with increasing temperature represented the heating path constrained by graded matrix quartz (Figure 6). Analyses collected for TM455 are excluded because dynamically recrystallized grains and quartz veins were measured, which may contain [Ti] not representative of temperatures of deformation (see discussion for more details). (c) This sample has the advantage of constraining three points between peak P associated with burial and fabric development, better confining the thermobarometric histories of these rocks. Garnet-ilmenite exchange thermometry (this study) constrained peak temperatures at c. 520 C, which overlaps with expected T max from TM445. [30] While the dark cores (CL) in quartz observed in this sample (and in virtually all samples analyzed in this study) could be either relics of early prograde (low T) quartz formation during burial or inherited from the protoliths, our preferred interpretation is that these cores most likely preserve early prograde conditions since the protoliths are pelites that would have received detrital quartz from igneous, metamorphic, or recycled sedimentary sources and thus would have variable [Ti] (C. K. McWilliams, Indiana University, unpublished thesis, 2008). The cores and rims of zoned quartz grains likely formed via precipitation during the two main stages of fabric development: (1) S 1 spaced foliation defined by quartz-feldspar (Q-F) and mica (M) domains, and (2) and S 2 crenulation cleavage. Precipitation of quartz in Q-F domains and hinge zones of S 1 /S 2 crenulation cleavage is likely the result of pressure-solution attending foliation development and/or production of quartz via metamorphic reactions [c.f. Williams et al., 2001] SD08A & TM455 (Kyanite-Staurolite Grade) [31] For sample 09SD08A, garnet growth was intertectonic with respect to burial and nappe emplacement, with S 1 preserved as the internal 16

17 foliation. Pressure shadows formed on the grain as a result of inhomogeneous strain (contemporaneously with crenulation cleavage development) due to the rigidity of the porphyroblast relative to the groundmass during S 2 foliation development; garnet rotation occurred during this fabric development and causes the included S 1 foliation to become oblique with the external S 2 foliation. Lack of abrupt or steep compositional gradients of major element zoning in garnet suggests garnet growth was associated with only a small segment of the P-T path. A bright garnet rim (Mn X-ray maps) is attributed to garnet resorption by prograde reactions releasing Mn that diffuses back into the garnet [e.g., Kohn and Spear, 2000]. [32] Figure 9b presents a summary of the results obtained for 09SD08A integrated with existing P- T data. Four isopleths are shown: 2.6 ppm represents minimum [Ti] of dark cores of zoned quartz in the garnet-matrix interface (also the interface between quartz of the included S 1 and external S 2 where quartz grains are not fully enclosed in garnet and foliation associations are not clear cut); 7.8 ppm represents brighter mantle regions (around dark cores) grading to 9.8 outward; and 6.7 ppm is associated with the surrounding dark rims in the garnet pressure shadow and matrix S 2 foliation (Figure 3c). Relative ages inferred from the textural relationships are: 2.6 ppm is the oldest isopleth; 7.8 and 9.8 ppm are intermediate in age; and 6.7 ppm is the youngest. T. Menard (unpublished thesis, 1991) estimated peak burial (prenappe emplacement) conditions to be 7.3 kbar and 500 C for nearby sample TM458a (Figures 1a and 1c); minor heating was interpreted to follow before a c. 1.5 kbar isothermal pressure increase occurred (up to 8.7 kbar), resulting from nappe emplacement. Peak T of 600 C was determined for a sample to the southeast, in the core of the dome (TM732) (T. Menard, unpublished thesis, 1991). [33] A pseudosection model for sample 09SD08A can be found in Figure 5. Contouring quartz volume in P-T space (Figure 6a) indicates a dominant quartz-producing reaction due to a chlorite-out, biotite-producing reaction. This reaction requires balancing by removal of K from muscovite, which drives the chemistry to the muscovite-paragonite-margarite solvus, resulting in the stability of two white micas. Plagioclase is also required for balancing, and the result is the production of quartz. A small region of staurolite-production is observed, which produces quartz in the process. However, due to the Al-undersaturation in this sample, Al is largely incorporated by plagioclase, and the stability of staurolite is very reserved in P- T space. Ilmenite and rutile are stable around this reaction, suggesting the reaction would be occurring in a TiO 2 -saturated system (a TiO2 is 1.0). [34] Similar to TM623, the oldest isopleths associated with the lowest [Ti] do not intersect polygons representing P-T path constraints for nearby samples. The low Ti cores of zoned quartz found as S 1 inclusions in or near matrix S 2 domains likely represent recrystallization of quartz during the early prograde path, perhaps during burial. The thin bright rims present on quartz inclusions are probably attributed to volume diffusion of Ti from the garnet, postinclusion [Spear and Wark, 2009] and potentially provides insight into the time scales of prograde metamorphism postgarnet growth to cooling below 450 C[Spear et al., 2012]. [35] The 7.8 and 9.8 ppm isopleths more firmly overlap with existing constraints. The associated gradient of [Ti] is observed (Figure 5d) increasing toward the rim of quartz in pressure shadows. The isopleths plot in a region in P-T space that corresponds to quartz production (determined through isochemical forward stability modeling), suggesting that growth due to quartz-producing reactions is a probable explanation. The models (Figure 5) also suggest that the metamorphic reaction may be a chlorite-out, quartz-producing reaction occurring at peak pressure. This strongly suggests new quartz was being precipitated on matrix quartz during isobaric heating postnappe emplacement due to metamorphic reactions, starting at c. 500 C (assuming kbar from Menard and Spear [1994]) and increasing to peak T (540 C). We infer that the lower-ti (6.7 ppm) overgrowths must have occurred during the retrograde path because there is no evidence for another event to drive an increase in pressure needed to account for a decrease in [Ti] as part of the prograde path. Pseudosection analysis suggests that quartz should be consumed by retrograde metamorphic reactions (Figure 6a), therefore an influx in fluids saturated with Si must have occurred during exhumation, probably at c C or lower, depending on the depth of the fluid influx. Dissolutionprecipitation creep involving Ti mobilization during retrogression is also possible, although considering the short metamorphic time scales suggested by Spear et al. [2012], this mechanism may not be sufficient for the amount of material transport observed in such short time scales. 17