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Reconstructions of the tectonometamorphic architectures of orogenic systems rely on petrogenetic indicator minerals, such as garnet, to trace the transport of rocks through space and time. We show the results of new garnet Lu–Hf geochronology and inclusion U-(Th-)Pb geochronology from exhumed midcrustal metamorphic rocks exposed in the Kanchenjunga region of the eastern Nepalese Himalaya. Garnet in three of the five specimens analyzed yielded pre-Himalayan Lu–Hf dates of ca. 292–230 Ma, which contrasts with Himalayan-aged inclusions therein. Garnet in these specimens either grew entirely during the Early Permian opening of the Neo-Tethys Ocean or grew partly during Cambrian–Ordovician orogenesis and partly during the Cenozoic. The remaining two specimens yield Lu–Hf ages of ca. 50 Ma, which are some of the oldest recorded dates for Himalayan metamorphism. The apparent discordance between the geochronological techniques highlights a potential issue with interpretations that rely on single-geochron-method inclusion records. These results further show that some pressure-temperature determinations used in Himalayan models may not be Himalayan in age.

The rheology and composition of arc crust and the overall evolution of continental magmatic arcs can be affected by sediment incorporation events. The exhumed Cretaceous-Eocene North Cascades arc exposes abundant meta-sedimentary rocks that were incorporated into the arc during multiple events. This study uses field relationships, detrital zircon geochronology, bulk rock geo-chemistry, geothermometry, and quartz-in-garnet geobarometry to distinguish approximate contacts and emplacement depths for different metasedimentary units to better understand their protolith incorporation history and impact on the arc. The Skagit Gneiss Complex is one of the main deep crustal units of the North Cascades arc. It includes metasedimentary rocks with distinct detrital zircon signatures: Proterozoic-Cretaceous (Group 1) or Triassic-Cretaceous (Group 2) zircon populations. Both metasedimentary groups achieved near-peak metamorphic conditions of 640-800 degrees C and 5.5-7.9 kbar; several Group 2 samples reveal the higher pressures. A third group of meta sedimentary rocks, which was previously interpreted as metamorphosed equivalents of backarc sediments (Group 3), exhibited unimodal Triassic or bimodal Late Jurassic- Early Cretaceous detrital zircon signatures and achieved near-peak conditions of 570-700 degrees C and 8.7-10.5 kbar. The combined field and analytical data indi-cate that protoliths of Group 1 and Group 2 metasedimentary rocks were successively deposited in a forearc basin and underthrusted into the arc as a relatively coherent body. Group 3 backarc sediments were incorporated into the arc along a transpressional step-over zone. The incorporation of both forearc and backarc sediments was likely facilitated by arc magmatism that weakened arc crust in combination with regional transpression. National Science Foundation [EAR-1419810, EAR-1419787]; U.S. Geological Survey EDMAP [G19AC00219]; Geological Society of America Graduate Student Research grant Published version We thank Joel DesOrmeau for assistance in sample preparation, obtaining mineral energy-dispersive detector and zircon-cathodoluminescence images, and U-Pb measurements at the University of Nevada, Reno. We also thank Andrew Kylander-Clark for help with U-Pb measurements at the University of California, Santa Barbara; Anette von der Handt of the University of Minnesota and Nick Botto of the University of California, Davis, for assistance with electron probe microanalyzer data collection; Jade Star Lackey and Jonathan Harris of Pomona College for help with wholerock geochemical measurements; and Stephen Hanson for assisting in the field. We also thank William Matthews and an anonymous reviewer, Associate Editor Craig Jones, and Science Editor David Fastovsky for their helpful comments on this manuscript. This research was funded by National Science Foundation Grant EAR-1419810 to S.M. Gordon and EAR-1419787 to R.B. Miller, U.S. Geological Survey EDMAP grant G19AC00219 to S.M. Gordon, and a Geological Society of America Graduate Student Research grant to A.E.H. Hanson.

We have examined the suitability of a quartz-inclusions-in-epidote (qtz-in-ep) mineral barometer to better constrain P-T histories of epidote-bearing lithologies. Theoretical calculations applying an isotropic elastic model suggest that the qtz-in-ep barometer exhibits minimal temperature dependence, and thus, offers the potential to constrain growth conditions of epidote in various geologic environments, including skarn deposits, epidote-bearing granitoids, and metamorphic rocks.To test if the applied equations of state and isotropic elastic model reasonably simulate the elastic evolution of two anisotropic minerals, we measured Raman shifts of the 464 cm–1 band of quartz inclusions relative to that of an unencapsulated quartz standard. We calculated a quartz inclusion pressure (Pincl464) at various temperatures and compared these values with temperature-dependent Pincl predicted by elastic modeling (Pinclmod) at elevated temperatures. Three epidote-bearing samples with reasonably well-constrained P-T histories were also examined: (1) sample HF14C from the Upper Schieferhuelle in the Western Tauern Window, Italy (Pincl464=0.01 GPa); (2) sample LdC-31C from Lago di Cignana, Italy (Pincl464≈0.16 GPa); and (3) sample FT1E from the Frosnitz Tal in the Western Tauern region, Austria (Pincl464=0.57 GPa).Entrapment pressures (Pent464) calculated from Pincl464 determined at various temperatures show nominal differences from Pent calculated from Pinclmod, suggesting that for qtz-in-ep pairs, the calculated Pent does not significantly vary with the temperature of measurement. Furthermore, our calculated Pent464 for a sample from the Upper Schieferhuelle is in agreement with petrographic context and previously established P conditions, and the Pent464 determined for the Frosnitz Tal sample closely approximate previously reported pressures. The Lago di Cignana sample is derived from an epidote vein that is encased in a high-P foliation, and the calculated Pent464 is consistent with early, low-P epidote vein formation that pre-dates high-P metamorphism, or alternatively, late vein formation during exhumation, and confirms that the epidote did not form at or near peak conditions (~2.0 GPa). The results of this study indicate that the qtz-in-ep barometer potentially provides another tool that geoscientists can employ to better constrain P-T conditions in some epidote-bearing environments, where conventional thermobarometric techniques cannot be applied.

Palaeoproterozoic orogenesis within the Archean southeastern Rae craton is related to the initial amalgamation of Laurentia. Characterizing the accompanying tectonic processes during this time has been complicated due to polymetamorphism, which results in the obscuring of the age record of the terranes involved. To improve the knowledge of the tectonic evolution of the South Rae Craton, petrologic and structural analyses are applied in conjunction with in situ trace element chemistry, inclusion barometry, U–Pb monazite and titanite, and Lu–Hf garnet chronology. The data robustly constrain Palaeoproterozoic pressure–temperature–time paths of major deformational events along the southeastern Rae craton margin. D1 occurred between 1.94 and 1.93 Ga in the Dodge-Snowbird domain, which included prograde burial of metasedimentary rocks, deposited at 2.2–2.0 Ga, and the development of migmatitic layering and east-southeast trending folds (S1, F1). Peak metamorphism is recorded in metasedimentary units at c. 1.93 Ga when rocks reached conditions of 9.0–10.5 kbar and 810–830°C. Within the Dodge-Snowbird domain, D2 imparted north-northeast trending open folds and associated axial planar cleavage (S2, F2) between 1.93 and 1.90 Ga during east-west compression that appears to have been synchronous with cooling and exhumation. Later D2 deformation, localized within the Wholdaia Lake shear zone (WLsz; ST1), developed in the footwall of this thrust-sense structure at 1,873 ± 5 Ma at conditions of 9.5–11.0 kbar and 820–850°C. The hangingwall Dodge-Snowbird domain had already cooled to below 300°C by then, indicating a significant structural and metamorphic break across the domain's western boundary. A new phase of unroofing (D3) involved pervasive amphibolite- to greenschist facies extensional shearing (ST2) within the WLsz, which overprinted ST1 foliations between 1.87 and 1.86 Ga. Continued greenschist facies shearing younger than 1.86 Ga likely ended by c. 1.83 Ga when lamprophyre dykes cut the structure, which was followed by cooling until c. 1.80 Ga. This work highlights the utility and application of multiple chronometers (zircon, monazite, titanite, garnet) along with structural and petrologic analysis that together can resolve precise orogenic cycles in polymetamorphic terranes that may otherwise be undetected. The time-resolved P–T–D histories derived here enable more robust interpretations regarding the nature and evolution of 1.9 Ga tectonism along the southeast Rae craton margin, which may be used to refine models for Laurentian terrane amalgamation.

Our understanding of the Venusian surface composition is limited to the in-situ bulk rock chemical analyses collected by the Vega and Venera missions. However, these analyses have exceedingly large analytical uncertainties – up to 50% by weight (1σ) for certain oxide components. In this study, we use the Venera 14 lander data and apply a Monte Carlo approach to assess how significant uncertainties affect modeling of lava solidification. Thermodynamic modeling of mineral-melt equilibria is conducted over 1,000 iterations of simulated bulk composition within the Gaussian probabilistic bounds of the reported analytical uncertainty along a cooling path from 1350 °C to 950 °C, with a fixed pressure of 90 bars. The results are used to calculate melt and apparent viscosity ( η melt and η app , respectively). Despite the significant analytical uncertainty of the lander data, lava viscosity is tightly constrained. Mean log η melt increases from 1.65 ± 0.46 Pa s at 1350 °C to 8.57 ± 1.42 Pa s at 950 °C (1σ). Log η app is less well-defined due to increased scatter in crystal fraction with solidification, increasing to 17.7 ± 1.3 Pa s at 950 °C ( γ = 10 − 6 s − 1 ). A significant increase in η app occurs between 1240 °C and 1080 °C due to a rapid increase in crystal mass fraction (Φ increases from ∼0.05 at 1250 °C to 0.8 at ∼1100 °C). Slow cooling through this 150 °C window must occur so as to not drastically increase lava viscosity and impede flow. These results show that despite limited geochemical data and large analytical uncertainties, reasonable constraints for the physical and chemical evolution of lava solidification can be obtained. Most of the Venusian surface is composed of volcanic plains and rises, containing abundant landforms characteristic of fluid basaltic lava. Our results provide new insights into the crystallization processes in Venusian lava flows, which are fundamental for understanding Venusian igneous processes, geodynamics, and resurfacing. This work provides a necessary framework for future thermorheological flow models to determine flow volume and effusion rates.

The Northern Highlands Terrane of Scotland hosts several thrust nappes that were deformed and metamorphosed during the Silurian Scandian orogeny. Quantitative petrological analysis of metamorphic assemblages indicates that the hinterland-positioned Naver nappe experienced decompression heating from 8–9 kbar and 600°C to 6–7 kbar and 700°C. Monazite–xenotime thermometry and geochronology delineate a detailed temperature–time history for the Naver nappe. Monazite often exhibits compositional zoning, which is used to establish multiple temperature–time points in several samples. These data indicate that the Naver nappe experienced relatively fast heating (c. 50°C myr−1) and relatively slow cooling (15–20°C myr−1), with peak temperatures occurring at c. 425 Ma. This temperature–time evolution is compatible with the early Emsian (407–403 Ma) deposition of unmetamorphosed conglomerates that rest on high-grade metamorphic rocks in the Naver nappe, but requires an acceleration in the cooling rate to 40–50°C myr−1 at 420–410 Ma. Geochronological constraints from this study and previous work suggest that deformation and metamorphism in the hinterland of the Scandian orogen in northern mainland Scotland are younger than the c. 430 Ma deformation in the foreland-positioned Moine thrust zone. We postulate that heat from pervasive granitic intrusions in the Naver nappe weakened the crust, allowing deformation to retreat to the hinterland of the orogen. Supplementary material: A description of our analytical methods, all U–Pb-trace element data, additional figures explaining our petrological analysis and other relevant data are available at: https://doi.org/10.6084/m9.figshare.c.4458041

New 40Ar/39Ar thermochronological and deformation temperature analyses in the Scandian (c. 435–420 Ma) orogenic retrowedge of northern Scotland demonstrate accelerated cooling during late syn- to post-orogenic exhumation of the high-grade orogenic core. Initial cooling rates of 10–30°C myr−1 immediately following peak orogenesis transitioned to rapid rates of 45–90°C myr−1 during final exhumation of the Naver thrust sheet in the orogenic core. The flanking ductile thrust sheets exhibit a similar, albeit less pronounced, acceleration of cooling, with rates increasing by c. 150–300% following peak orogenesis. Closer to the foreland, the Moine thrust sheet did not experience increased cooling rates. Calculated unroofing rates of 3.75 mm a−1 in the high-grade Naver thrust sheet suggest increasing, rapid exhumation in the orogenic core during a presumed collapse phase of orogenesis. This is contrary to the expectation of decreasing erosional efficiency as topography is diminished and is interpreted to suggest that unroofing of the Scottish Caledonides may have been partially enhanced by upper crustal extensional deformation during ductile flow of the infrastructure of the orogenic core. Similar processes have been interpreted in the East Greenland Caledonides, which form the northern extension of the Scandian retrowedge. Supplementary material:40Ar/39Ar analytical data for muscovite (Supplementary Data Table 1), 40Ar/39Ar analytical data for amphibole (Supplementary Data Table 2), and electron microprobe analytical data for amphibole samples (Supplementary Data Table 3) is available at: https://doi.org/10.6084/m9.figshare.c.5087057

We postulate that the inverted metamorphic sequence in the Lesser Himalayan Sequence of the Himalayan orogen is a finite product of its deformation and temperature history. To explain the formation of this inverted metamorphic sequence across the Lesser Himalayan Sequence with a focus on the Main Central Thrust (MCT) in eastern Bhutan, we determined the metamorphic peak temperatures by Raman spectroscopy of carbonaceous material and established the deformation temperatures by Ti-in-quartz thermobarometry and quartz c axis textures. These data were combined with thermochronology, including new and published Ar-40/Ar-39 ages of muscovite and published apatite fission track, and apatite and zircon (U-Th)/He ages. To obtain accurate metamorphic, deformation, and closure temperatures of thermochronological systems, pressures and cooling rates for the period of interest were derived by inverse modeling of multiple thermochronological data sets, and temperatures were determined by iterative calculations. The Raman spectroscopy of carbonaceous material results indicate two temperature sequences separated by a thrust. In the external sequence, peak temperatures are constant across the structural strike, consistent with the observed hinterland-dipping duplex system. In the internal temperature sequence associated with the MCT shear zone, each geothermometer yields an apparent inverted temperature gradient although with different temperature ranges, and all temperatures appear to be retrograde. These observations are consistent with the quartz microfabrics. Further, all thermochronometers indicate upward younging across the MCT. We interpret our data as a composite peak and deformation temperature sequence that formed successively and reflects the broadening and narrowing of the MCT shear zone in which the ductile deformation lasted until similar to 11 Ma.

Compiled volumetric data for fluor-, chlor-, and hydroxyl-apatite has been fitted to a pressure-volume-temperature ( P-V-T ) equation of state for volume calculation at elevated P and T . The regressions were used to assess the potential of apatite inclusions in garnet for thermobarometric applications, according to the pressurization of inclusions resulting from elastic differences between the inclusion and host minerals. Isomeke contours (lines in pressure temperature space, representing permissible entrapment conditions that yield a given inclusion pressure) were calculated and show that apatite inclusions in garnet are a particularly useful barometer, owing to the large differences in the bulk moduli and similar thermal expansivities between apatite and garnet. Heating experiments were conducted on fluorapatite inclusions in andraditic garnets from the Casting Copper skarn, Nevada, to assess the variation in measured inclusion pressure with heating relative to that predicted with isotropic elastic theory. Negligible departures between theoretical and measured pressurization suggests no significant correction is needed for applying room- T inclusion pressure measurements for barometry constraints.

Accessory phase geochronology records several phases of metamorphism that affected Moine metasedimentary rocks in the Northern Highlands Terrane of Scotland. Within the Sgurr Beag nappe, monazite and xenotime record initial metamorphism at c. 750 Ma, with later resetting at c. 600 Ma and c. 425 Ma. Monazite, xenotime, titanite and zircon also cryptically record c. 750 Ma and c. 600 Ma metamorphism in the Naver and Ben Hope nappes prior to the dominant 435-415 Ma Scandian orogeny. Textural, geochemical, and isotopic data indicate that c. 600 Ma and c. 425 Ma isotopic resetting in the Sgurr Beag nappe was caused by fluid-mediated dissolution and reprecipitation of monazite and xenotime. The compositions of fluid-altered monazite fall into two distinct trends, with high and low Y+HREE apparently reflecting the distinct timing and temperature conditions of dissolution and reprecipitation. Monazite-xenotime thermometry is applied to constrain the temperatures of fluid alteration. High Y+HREE, low Th/U monazite is related to high temperature (∼650°C) alteration at c. 600 Ma and low Y+HREE, high Th/U monazite is related to lower temperature (∼400°C) reactivity at c. 425 Ma. These data show that the composition of reprecipitated monazite is sensitive to metamorphic conditions during fluid alteration and is therefore useful in understanding the evolution of polyphase metamorphic terranes. These data suggest that a widespread metamorphic or metasomatic event affected northern Scotland at c. 600 Ma, likely related to higher heat flux and magmatic activity during the break-up of Rodinia. Additionally, the now adjacent Sgurr Beag and Moine nappes appear to have contrasting metamorphic records at least until Caledonian orogenesis, which suggests that final regional scale motion on the Sgurr Beag thrust must be of Caledonian age.

Garnet is a versatile and useful indicator mineral exploited in numerous geological studies. Despite its utility in providing thermobarometry and geochronology constraints, many difficulties remain in making meaningful interpretations of such data. In this paper, we characterize garnet grains from over 140 garnet-bearing metasedimentary rock samples collected from the northern part of the Moine Supergroup (Scottish Caledonides). Large, euhedral garnet grains are interpreted to be indicative of prograde metamorphic growth during the most recent (Scandian, c. 430 Ma) phase of orogenesis. Anhedral garnet is largely restricted to the relatively low-grade (greenschist – lower amphibolite facies) Moine thrust sheet, with an abrupt change in morphology and grain size when traced across the overlying Ben Hope and Sgurr Beag thrusts into the higher grade, more hinterland-positioned thrust sheets. Our results suggest that caution should be exercised in using anhedral garnet in the Moine thrust sheet to estimate peak P–T conditions associated with low temperature (< ~500 °C) Scandian metamorphism, because in at least some cases garnet growth may have occurred during an earlier metamorphic event. However, chemical and isotopic data from this structurally lower anhedral garnet may still be useful for deconvolving a possible polymetamorphic history for this thrust sheet. In the immediately overlying Ben Hope thrust sheet, garnet has prograde euhedral (Scandian?) rims, indicating that the Ben Hope thrust must represent a significant thermal and/or chemical break. Inclusion distribution and mineral assemblages in garnet have been used to gain further insight on garnet growth conditions and to distinguish garnet that likely contains multiple generations of growth. Although our results are specific to the Caledonides of northern Scotland, this work highlights the general necessity of a comprehensive petrographic assessment in advance of interpreting large suites of garnet-derived thermodynamic and geochronological data.

The 100 km 2 Mount Hay block (Arunta Region, central Australia) is a >15 km thick crustal cross-section comprised of granulite facies tectonites containing a tectonothermal record from ca. 1803 to ca. 1552 Ma, as demonstrated by the combination of prior work and new monazite geochronology and thermobarometry. Igneous and sedimentary protoliths formed ca. 1803–1798 Ma during 1810–1790 Ma Stafford–Tanami time. Between ca. 1790 and 1740 Ma, nearly continuous magmatism in the Aileron Province, and possibly ages of recrystallized zircon and monazite in the Mount Hay block, record subduction along the southern margin of the Proterozoic Australia-Mawson continent. Geologic mapping and zircon and monazite geochronology demonstrate that the Mount Hay block was penetratively deformed between ca. 1720 Ma and 1700 Ma during 1735–1690 Ma Strangways time. This includes the >8 km thick Mount Hay sheath fold, which records NE-SW subhorizontal shear, once Paleozoic tilting of the Mount Hay block is removed. While still in the deeper crust (>798 ± 33 °C; >7.6 ± 0.7 kbar), fabrics along the northern edge of the Mount Hay block were transposed by the cross-cutting >7 km thick Capricorn ridge shear zone at ca. 1551.7 ± 5.5 Ma, during Chewings time. Once restored, the Capricorn ridge shear zone records NE-SW extension. A jump in strain rate while at high temperatures (∼725 °C) initiated severe localization accommodated initially by pseudotachylite formation and cataclasis then mylonitization during Fe-rich fluid flux. Shearing may have continued to pressure–temperature conditions as low as

Quartz inclusions in garnet are suitable for barometry because quartz is highly compressible relative to garnet, and the garnet host can maintain large stress differences generated as pressure-temperature conditions change. However, experimental validation of the quartz-in-garnet approach has been limited, raising questions concerning the accuracy of calculated entrapment pressure. Here we test the results of quartz-in-garnet barometry by conducting in-situ Raman analysis of natural inclusions over a range of temperatures (–175 to 600 °C). We assess the temperature ( T ) dependence of inclusion pressure ( P incl ) at 1 bar and compare calculated entrapment pressures derived from measurements at different temperatures. Experiments used two quartz standards (oriented [c] and ) and fully encapsulated quartz inclusions from three different terranes, retaining different P incl at room T (–444, 296, and 755 MPa). The stretched quartz inclusion ( P incl < 0) had the greatest increase in P incl (+264 MPa, during heating from 25 to 500 °C), whereas the high- P inclusions underwent less change (+138 MPa) in P incl over the same T interval. The greater T sensitivity of inclusions with low P incl reflects the greater thermal expansivity of quartz near the α to β quartz transition. While measured P incl - T trends are consistent with predictions, numerical models tend to overestimate P incl at elevated T , and calculated entrapment pressures show an unrealistic dependence on reference T . Raman spectroscopic measurements conducted in situ at elevated T provide optimal results. In addition, we have recalibrated the thermal portion of the numerical method based on the present results, and provide new empirical expressions for improved quartz-in-garnet barometry.

Recent thermometric analyses of samples collected in thrust-parallel structural transects across the Scandian (435–415 Ma) orogenic wedge in northwest Scotland provide a comprehensive characterization of the synorogenic retro-wedge thermal architecture. However, the paucity of petrologically-important metamorphic mineral phases (e.g., staurolite, Al-silicates) has limited investigation of pressure–temperature ( P – T ) histories, which hinders our ability to examine the nature of orogen-scale kinematic and thermal coupling. New data collected along a foreland-to-hinterland transect from the Moine to the Naver thrust sheets provides additional constraints for characterizing the prograde metamorphic evolution. In addition, we characterized Ti diffusion profiles in quartz inclusions in garnet to constrain duration of metamorphic heating. These results are used to develop coupled kinematic-thermal models of Scandian orogenic evolution. Early garnet core growth conditions are constrained by isopleth intersections, with peak P – T estimates determined by conventional exchange and net transfer thermobarometry and thermodynamic calculations. Most samples follow normal prograde heating and burial profiles, with peak conditions of 450 °C and 5.0 kbar in the immediate hanging wall to the Moine thrust, increasing in temperature and pressure to 733 °C and 9.5 kbar in the immediate hanging wall to the Naver thrust. These normal prograde pressure trajectories are interpreted to reflect burial of incipient thrust sheets beneath the overriding wedge at the leading edge of the orogen. Prograde heating coeval with burial is interpreted to result from surface-directed isotherm perturbation due to thrust-related advection in the overriding wedge. One exception to this is a sample from the top of the Moine thrust sheet, where prograde heating occurs during decompression (540 °C and 8.1 kbar to 590 °C and 7.0 kbar). In this case, the short lag times between motion on the Moine and Ben Hope thrusts may have limited advectionary heating until after exhumation associated with motion on the underlying Moine thrust was underway. Ti diffusion profiles in quartz inclusions in garnet suggest the near-peak thermal evolution of these rocks occurred over very short time scales ( 9 kbar are probably of detrital origin.

The ubiquity of quartz in continental crust, and the involvement of SiO 2 in multiple metamorphic processes such as reactions, fluid flux, and solution-transfer processes, makes quartz an obvious choice for reconstructing prograde metamorphic conditions in various rock types. Recent studies have shown the usefulness of analyzing Ti distribution in quartz to constrain pressure-temperature-(relative) time-deformation ( P-T-t-D ) in metamorphosed tectonites. New high-precision single-crystal X-ray diffraction volume constraints on Ti-doped and chemically pure quartz provide further evidence for substitution of Ti 4+ for Si 4+ in the tetrahedral site in quartz, with resultant lattice strain on the structure. Recent applications of the Ti-in-quartz thermobarometer to dynamically recrystallized quartz have identified recrystallized subgrains that contain lower Ti concentrations ([Ti]) than their host porphyroclasts. In addition, [Ti] are lower than expected for the temperatures of recrystallization. Atomistic simulations that estimate energetic perturbations resulting from Ti incorporation into the quartz lattice indicate that significant increases in strain energy occur only at very high [Ti]; the strain-energy increase is negligible for [Ti] typical of quartz grown under mid-crustal conditions. This suggests that lattice strain rarely provides an appreciable driving force for Ti loss from quartz; instead, it appears that subgrain boundaries and dislocation arrays migrating through recrystallizing quartz crystals can promote localized re-equilibration, thermodynamically regulated by the composition of the intergranular medium (typically undersaturated in Ti). It therefore appears that analyses from dynamically recrystallized quartz cannot be meaningfully interpreted until methods are developed that can account quantitatively for the reduction of Ti resulting from crystal plastic flow.

[1] Burial histories of subduction zone rocks are often difficult to accurately constrain, owing to a lack of robust mineral geobarometers applicable to high pressure mineral assemblages. Knowledge of the depth-histories of subduction is, however, required for our understanding of global geochemical cycles, subduction-related seismicity, and the evolution of destructive tectonic boundaries. The high spatial resolution of quartz inclusion geobarometry can be used to determine pressure evolution during metamorphic growth of individual garnet crystals. Quartz inclusions in garnet from Sifnos, Greece, preserve such a record of the pressure of garnet growth, allowing detailed reconstruction of the metamorphic evolution of these rocks. Pressure-dependent Raman spectra of quartz inclusions were combined with elastic modeling to infer the conditions at which they were trapped during garnet growth. All measured inclusions suggest that garnet growth occurred between 19 and 20.5 kbars, with little evidence for significant pressure variation during the garnet growth interval, which is interpreted to record ∼100°C of heating. Coupled with thermometry and geochronology, these results show that early, cold burial was followed by a phase of rapid heating, which immediately preceded exhumation. Garnet growth occurred primarily during this heating phase.

[1] This study investigates the ability of quartz to record segments of the pressure-temperature-deformation (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 quartz-feldspar domains during crenulation cleavage development due to solution transfer; (5) quartz-producing (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.

We report new deformation temperature and flow vorticity data from the base of the Greater Himalayan Series (GHS) exposed in the Sutlej Valley and Shimla Klippe of NW India. We focus on three groups of transects across the hanging wall of the Main Central Thrust (MCT). In order of relative foreland – hinterland positions, they are the Shimla Klippe, Western and Eastern Sutlej transects. Deformation temperatures indicated by quartz c-axis fabric opening-angles increase both from foreland to hinterland at a given structural distance above the MCT and up structural section from the MCT within individual transects. Deformation temperatures in the immediate hanging wall to the MCT are estimated at ∼510–535, 535–550 and 610 °C on the Shimla, Western Sutlej and Eastern Sutlej transects, respectively. The steepest inferred field gradients in deformation temperatures are recorded adjacent to the MCT and progressively decrease up structural section following a power law relationship. Comparison with temperature estimates based on multi-mineral phase equilibria data suggests that penetrative shearing occurred at close to peak metamorphic conditions. Vorticity analyses indicate that shearing along the base of the GHS occurred under sub-simple shear conditions (Wm values of 0.9–1.0) with a minor component of pure shear.

polymorph kyanite also has the capacity to retain tensile stresses if exhumed to Earth's surface after being trapped as an inclusion in plagioclase at P-T conditions within the kyanite stability field, with the stresses developed during exhumation sufficient to produce a transformation to anda - lusite. These results highlight the elastic environment that may arise during exhumation and provide a potential expla- nation of observed inclusions whose stability fields are well removed from P-T paths followed during exhumation. Abstract Analysis of mineral inclusions in chemically and physically resistant hosts has proven to be valuable for reconstructing the P-T exhumation history of high-grade metamorphic rocks. The occurrence of cristobalite-bear- ing inclusions in garnets from Gore Mountain, New York, is unexpected because the peak metamorphic conditions reached are well removed (>600 °C too cold) from the sta- bility field of this low-density silica polymorph that typi - cally forms in high temperature volcanic environments. A previous study of samples from this area interpreted pol- ymineralic inclusions consisting of cristobalite, albite and ilmenite as representing crystallized droplets of melt gen- erated during a garnet-in reaction, followed by water loss from the inclusion to explain the reduction in inclusion pressure that drove the transformation of quartz to cristo- balite. However, the recent discovery of monomineralic inclusions of cristobalite from the nearby Hooper Mine can- not be explained by this process. For these inclusions, we propose that the volume response to pressure and tempera- ture changes during exhumation to Earth's surface resulted in large tensile stresses within the silica phase that would be sufficient to cause transformation to the low-density (low- pressure) form. Elastic modeling of other common inclu- sion-host systems suggests that this quartz-to-cristobalite example may not be a unique case. The aluminosilicate

other Si-mobilization events occur during the prograde and retrograde history of metapelites, and assuming TiO2 activities associated with the peak metamorphic paragen- esis may be misleading and result in significant errors in P-T calculations. Abstract Since its calibration, the Ti-in-quartz thermo- barometer has been applied to a wide variety of geologic scenarios. The abundance of quartz in the continental crust and the involvement of silica in metamorphic reactions, deformation, and fluid flux processes make it a particularly powerful tool for constraining the pressure and tempera- ture evolution of rocks, which is essential for developing tectonic models. Being able to quantitatively determine the solubility of Ti in quartz, however, is dependent upon being able to determine the activity of TiO2 in the rock dur- ing quartz growth or re-equilibration. Here we calculate TiO2 chemical potentials of the system relative to that of rutile (as a standard state), projected in P-T space for an average sub-aluminous pelite composition. Titania activi- ties are calculated from these dependent potentials, with resultant activities used to correct for Ti isopleth projec- tion in the Thomas et al. (2010) solubility equation. The modeling results are in good agreement with previous stud- ies that suggest ilmenite-bearing assemblages buffer high TiO2 activities and titanite-bearing assemblages have much lower activities (≥0.5). At elevated temperatures, however, significant deviation from an assumed average pelite activ- ity of 1.0 occurs, where the projected Ti concentration in quartz is up to 400 % different when assuming a dynamic system activity. This is due, in part, to the sequestering of Ti in biotite during heating and the destabilization of Ti- oxides at higher temperatures. With quartz-producing reac- tions, deformation-driven solution-transfer processes and

Quartz inclusions in garnet from samples collected from the staurolite zone in central New England are zoned in cathodoluminescence (CL). The CL intensity is interpreted to be a proxy for Ti concentration and the zoning attributed to Ti diffusion into the quartz grains driven by Ti exchange between quartz and enclosing garnet as a function of changing temperature. The CL zoning has been interpreted using a numerical diffusion model to constrain the time scales over which the diffusion has occurred. Temperature–time histories are sensitive to the presumed peak temperature but not to other model parameters. The total time of the metamorphic heating and cooling cycle from around 450 °C to the peak temperature (550–600 °C) back to 450 °C is surprisingly short and encompasses only 0.2–2 million years for peak temperatures of 600–550 °C. The metamorphism was accompanied by large-scale nappe and dome formation, and it is suggested that this occurred as a consequence of in-sequence thrusting resulting in a mid-crustal ductile duplex structure.

This is the first detailed study of the coastal exposure of the Springhill Mines Formation within the Joggins Fossil Cliffs World Heritage Site. A 16.9-m-thick interval of dark laminated mudrocks and sharpbased sandstones at the base of our section is reassigned to the top of the Joggins Formation. This interval records a rapid, presumably widespread flooding event and the temporary establishment of a marginalmarine to brackish bay. The overlying 697 m of strata represent deposition in poorly drained and well-drained environments, and are assigned to the Springhill Mines Formation. Strata reflecting poorly drained environments contain green and grey mudrocks, thin coals, sheet sandstones, and channel bodies interpreted to have been deposited in coastal swamps and low-lying parts of a floodplain. Intervals reflecting well-drained conditions contain reddish brown mudrocks, sheet sandstones, and channel bodies interpreted to have been deposited on a vegetated floodplain that was periodically exposed to oxidizing conditions. Strata reflecting poorly drained conditions are thick and abundant in the lower half of the formation and well-drained intervals become thick and more abundant in the upper half. The shift in facies abundance is accompanied by an interpreted evolution in fluvial style from predominantly anastomosed channels (below 376 m) to sheet-like channel bodies (376–449 m) and ultimately to predominantly meandering-channel bodies (449–697 m). The formation-scale changes in drainage conditions and fluvial style records decreased halokinetic subsidence and aggradation of the alluvial surface as sediments shed from the Caledonia Highlands prograded into this part of the basin.