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  1. null (Ed.)
    The Tso Morari terrane within the Himalayan orogeny underwent ultrahigh-pressure (UHP) metamorphism due to northward subduction under the Eurasian continent during the early Eocene. The advancement of computational petrology and availability of relevant thermodynamic databases provide the mechanism to more precisely quantify metamorphic processes. In this study, we model the eclogite’s prograde pressure-temperature (P-T) path as well as multiple fluid infiltration events during exhumation using Theriak-Domino with dataset ds62 and garnet[1] and other metabasic mineral activity-composition relations. The effect of garnet fractionation on the rock’s effective bulk composition is considered in simulating prograde garnet growth. A “fishhook” shape clockwise P-T path is obtained with a peak pressure of ~28.5 kbar at ~563 °C, followed by a peak temperature of ~613 °C at ~24.5 kbar[2]. Thermodynamic modelling using P-M(H2O) pseudosections on Tso Morari eclogites indicates three distinct phases of fluid infiltration during exhumation. Fluid infiltration Ⅰ occurs at ~610 °C and ~23.5 kbar with ~3.1 mol % fluid expulsion due to the destabilization of lawsonite. The modelling results are consistent with petrographic observations in the eclogite: we found ~6.0 vol % epidote and ~21.0 vol % amphibole and the possible pre-existence of lawsonite evidenced by its pseudomorph (as epidote and paragonite aggregates) in a garnet core and rim[3], and CNASH modelling on the epidote and its inclusion paragonite. Fluid infiltration Ⅱ occurs at ~9.2 kbar and ~608 °C with >2.6 mol % fluid infiltration at amphibolite-facies. This phase of fluid infiltration is characterized by aggressive amphibolization from the boudin core to rim. Fluid infiltration Ⅲ occurs at ~610 °C and ~8.7 kbar, caused by breakdown of phengite as predicted through modelling the symplectitic association (plagioclase, biotite, and amphibole) surrounding omphacite. In summary, this study not only illustrates the application of thermodynamic modelling in quantifying metamorphic processes, but also the need of comparison between modeling predictions and petrographic observations. [1] White et al. (2007), J Metamorph Geol 25, 511–527. [2] Pan et al. (2020), Contrib Mineral Petrol 175, 1–28. [3] St-Onge et al. (2013), J Metamorph Geol 31, 469–504. 
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  2. null (Ed.)
  3. Ultra-high pressure (UHP) metamorphism of the Tso Morari coesite-eclogite during burial in NW Himalaya has been intensively studied over the past several decades. However, amphibolite-facies metamorphism and accompanying metasomatism occurring at lower-crustal depths in the Tso Morari terrane are less well-constrained. In this study, we characterize the eclogite amphibolization and related metasomatic fluids by systematically sampling and analyzing the eclogites at the core of an eclogite boudin and the amphiblolized eclogite (amphibolite) at the rim. Integrated techniques including modal mineralogy, mineral chemistry, whole-rock geochemistry, Mössbauer spectroscopy, and thermodynamic modelling are used to constrain the fluid-induced eclogite amphibolization and associated fluid behaviors. Petrographic observations show that infiltration of an external fluid caused complete amphibolite-facies overprinting of the eclogites at the boudin rim. This is recorded petrographically as increased modal proportions of amphibole, biotite, epidote, plagioclase, and calcite in the amphibolites. The infiltrating fluid caused increased K2O and CO2 concentrations and higher bulk-rock Fe3+/ΣFe ratio for the amphibolites, as well as increased LILE (e.g., K, Rb, Cs, Sr, Ba) and ratios of Ba/Rb and Cs/Rb. Phase equilibria modelling using P–T–M(H2O) pseudosections on the amphibolite and the surrounding gneiss indicate that the fluid infiltration occurred at 9.0–12.5 kbar and ~608 °C with >2.6–3.1 mol % H2O infiltration. The abrupt increase of bulk-rock Fe3+/ΣFe ratio from 0.192 to 0.395 near the boudin rim indicate that this phase of fluid most likely derived from the mixing of dehydrated host orthogneiss and/or metasediments during uplift at the amphibolite-facies zone in the subduction channel. This study also demonstrates the need for using careful petrographic observations and geochemical analysis in parallel with thermodynamic modelling to achieve realistic results. 
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  7. Thermodynamic modeling is an important technique to interpret metamorphic phase relations and calculate model pressure-temperature (P-T) paths for metamorphic rocks. This study uses representative, coesite-bearing eclogites from the Tso Morari UHP terrane of the NW Himalaya to simulate its prograde metamorphism using multiple modeling programs and thermobarometry. Our modeling yields a peak metamorphism P-T of ~32-33 kbar and ~560-570 °C by the THERMOCALC345 and Theriak-Domino programs (Green et al., 2016), which is ~5 kbar higher in pressure and ~15 °C lower in temperature than that determined by using THERMOCALC333 (White et al., 2007) (~27.8 kbar and ~580 °C). The significantly higher pressure obtained using the THERMOCALC345 and Theriak-Domino is likely a result of the upgrade of thermodynamic parameters of minerals (i.e. garnet Wpy-gr and agr) in the newer a-x relations. The modeled effective bulk compositions and mineral stabilities along the calculated P-T path show different patterns under the two modeling techniques. Modeling by the Theriak-Domino programs is preferred in this case because the results are more consistent with the measured mineral compositions of our rocks. Multiple thermobarometers by garnet-omphacite-phengite, garnet-omphacite, garnet-phengite on the garnet rim, high-Si phengite and matrix omphacite yield a peak metamorphism of ~ 28.5-29.0 kbar and ~ 650-728 °C, which is generally consistent with the modeled P-T path. Based on our model calculations, the initial bulk composition measured by XRF does not represent the reactant bulk composition at the time of garnet nucleation, and this compositional discrepancy possibly is caused by the crystallization of pre-garnet minerals (i.e. hematite), reaction overstepping, or partial reequilibration. In summary, by implementing and evaluating multiple modeling strategies and considering the petrography and metamorphic mineralogy of the rocks, this study finds that the eclogite modeling using Theriak-Domino programs in the Tso Morari terrane provide more consistent metamorphic phase relations and more reasonable thermodynamic simulations regarding fractionation of the bulk composition and prograde metamorphism. References: Green et al. J Metamorph Geol 34, 845-869 (2016) White et al. J Metamorph Geol 25, 511-527 (2007) 
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