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1. Thermodynamic modelling on the UHP metamorphism and fluid infiltration of the Tso Morari coesite-bearing eclogite in NW India

   Pan, R.; Macris, C.; Menold, C. (January 2021, Annual VM Goldschmidt Conference)

   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. Accessory phases as recorders of subduction redox: Sulfide–oxide– silica equilibria during high-pressure metamorphism

   Walters, JB; Cruz-Uribe, AM; Marschall, HR (December 2018, 2018 Fall Meeting, AGU)

   Examination of a global suite of eclogite-facies metabasites and metasediments suggests that eclogites tend to exhibit reduced mineral assemblages relative to their protoliths. High-pressure rocks tend to lack sulfides and Fe3+-bearing oxides in the eclogite facies. We suggest that eclogite-facies mineral assemblages are consistent with prograde reactions that balance the oxidation of S2- or S- to S6+ by reducing Fe3+in silicates or oxides: (1)8Fe3+Si O (OH) +S2-=8Fe2+Si O +SO 2-+(H O) abc de42f The oxidation of one mole of S2-or S-is balanced by the reduction of 7 to 8 moles of Fe3+, and typical S concentrations in the oceanic crust are capable of fully reducing the entire Fe3+ budget of metabasites. As most eclogite facies rocks do not preserve peak metamorphic sulfides, petrographic evidence for prograde S oxidation reactions are cryptic; however, textures associated with sulfate reduction in response to influx of external fluids are common (reaction 1 in reverse). These reactions produce Fe3+-rich phases and are observed in both metasedimentary and metabasic rocks across a range of retrograde P-T paths (blueschist to granulite facies). For example, high-P calc- schists exhibit reaction textures that suggest the breakdown of garnet and white mica to produce pyrite + chalcopyrite + epidote + biotite + magnetite. Our thermodynamic models of aS2 and aO2 at subduction zone P-T conditions suggest assemblages of this type are indicative of aO2 0.7 to 4.5 log units above the quartz-fayalite-magnetite buffer. In rehydrated eclogites, pyrite is commonly associated with the breakdown of garnet + omphacite to amphibole + pyrite. Additionally, direct precipitation of sulfide from sulfate is observed in two samples: 1) The retrograde assemblage pyrite + ilmenite + gypsum occurs in one retrogressed metagabbroic eclogite, and 2) Coronas of secondary pyrite + barite + gypsum enclose early retrograde pyrite in a retrogressed garnet blueschist. In many eclogites, S- is reduced to S2- as pyrite is replaced by pyrrhotite, chalcopyrite, and mixed valence Co-Ni sulfides. These reactions are balanced by oxidation of divalent to trivalent Fe-Co-Ni. Reactions of this type are consistent with increasing aS2 during retrograde metamorphism. Thus, ample evidence exists for oxidized S-bearing fluids released from subducting slabs. 

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3. Chemical and mechanical controls on metasomatic assimilation of eclogite blocks in high-pressure host gneiss

   Menold, C.A.; Grove, M.; Macris, C.A. (December 2019, AGU Fall Meeting 2019)

   Mica- and garnet-rich selvages are often developed around eclogitized mafic blocks within felsic gneiss in HP to UHP metamorphic terranes. The development of these metasomatic features ranges from readily identified reaction zones between the eclogite and host gneiss to shear zones where the spatial relationships between eclogite blocks and host gneiss are completely obscured. Block-selvage relationships within the Luliang Shan HP/UHP belt (North Qaidam, China) and the Tso Morari UHP terrane (NW Himalaya, India) approximate end members of the selvage preservation process. Here we apply whole-rock and incompatible trace element compositions coupled with B and O isotopic data in white mica to constrain the relationship of metasomatism vs. deformation during selvage formation. Within the Luliang Shan, extensive fluid flow formed thick, compositionally hybridized phengite- and garnet-bearing selvages between eclogite (SiO2 ~ 50%) and quartzofeldspathic gneiss (SiO2 ~ 80%). The Luliang Shan HP selvages have intermediate SiO2 and range from 5-10 m in thickness as "halos" around spheroidal eclogite blocks. Volatile enrichment at near-UHP conditions in the selvage is indicated by enrichment of Li, Cs, Ba, Ar, and δ18O and very light δ11B values in phengite. The retrograde muscovite from the host gneiss is low in Li, Cs, Rb, and Sr but possess remarkably high B concentrations (up to 3000 ppm) and positive δ11B values that are best explained by interaction with fluids devolatilized from accreted sediments within cooler regions of the subduction zone. Alternatively, the Tso Morari UHP terrane features boudinaged discoids of eclogite encased within highly strained quartzofeldspathic gneiss. Whole rock major element sampling performed normal to the foliation reveal consistently high SiO2 (78-80%). Highly variable degrees of metasomatic recrystallization occur within the phengite-rich rocks spatially associated with eclogite. The selvage rocks exhibit heterogeneous degrees of enrichment in Li, Be, B, and Ba and yield δ11B values of -4 to -6‰ typical of undevolatilized oceanic and continental crust. We conclude that fluid-mediated metasomatic reaction between eclogite and gneiss at Tso Morari is sheared out into lenses that are incorporated into, and heterogeneously distributed throughout, the host gneiss. 

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4. Distinct P-T histories in a subduction mélange reveal underplating/mixing processes at the plate interface (North Motagua Mélange, Guatemala)

   Bonnet, G; Flores, K; Martin, C; Harlow, G (December 2020, American Geophysical Union, Fall Meeting 2019)

   Relicts of subducted and exhumed ocean floor preserved in suture zones record the events occurring at the plate interface. In particular, underplating and exhumation are the two main processes required to recover rocks from mantle depths. High-grade blocks exposed in serpentinite mélanges of the Motagua Valley record evidence of past subduction events between the North American Plate and the Caribbean Plate. Previous works suggest the existence of two subduction zones during Cretaceous, with cold metamorphism (lawsonite eclogite and blueschists) in the South (South Motagua Mélange), and warmer eclogites and amphibolites in the North (North Motagua Mélange, NMM). Although little work as been done so far to characterize the P-T paths and variability of the metabasite blocks embedded within serpentinite matrix in the NMM. Here we present new thermobarometric estimates using conventional thermobarometry, pseudosection modeling and thermometry of carbonaceous matter on a set of metabasites of different grades. There a minimum of four kinds of P-T paths: (1) (lawsonite-bearing) garnet-blueschists with peak P-T around 2.1 GPa and 480°C, (2) "cold eclogites" at ~2.2 GPa and 550°C experiencing isothermal decompression and epidote-amphibolite overprints, (3) "warm eclogites" at ~2.3 GPa and 600°C exhumed in cold environments and affected by blueschist-facies overprints, and (4) garnet-bearing epidote-amphibolites that may represent either retrogression of some eclogites, or prograde metamorphism under warm conditions. We find that garnet fractionation has a limited impact on isopleth-derived P-T estimates and that lawsonite breakdown may drive retrograde metamorphism and rheological switches at the plate interface. These new P-T estimates suggest that high-pressure rocks of the NMM may be recovered from different depths of a unique subduction zone, between 65 and 80 km, and exhumed in a relatively cold (and serpentinized) environment. This suggests a more complicated story than previously described, and calls for additional geochronological evaluation (in process). 

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5. Deep crustal source of gneiss dome revealed by eclogite in migmatite (Montagne Noire, French Massif Central)

   https://doi.org/10.1111/jmg.12523  

   Whitney, Donna L.; Hamelin, Clémentine; Teyssier, Christian; Raia, Natalie H.; Korchinski, Megan S.; Seaton, Nicholas C. A.; Bagley, Brian C.; von der Handt, Anette; Roger, Françoise; Rey, Patrice F. (February 2020, Journal of Metamorphic Geology)

   Abstract In orogens worldwide and throughout geologic time, large volumes of deep continental crust have been exhumed in domal structures. Extension‐driven ascent of bodies of deep, hot crust is a very efficient mechanism for rapid heat and mass transfer from deep to shallow crustal levels and is therefore an important mechanism in the evolution of continents. The dominant rock type in exhumed domes is quartzofeldspathic gneiss (typically migmatitic) that does not record its former high‐pressure (HP) conditions in its equilibrium mineral assemblage; rather, it records the conditions of emplacement and cooling in the mid/shallow crust. Mafic rocks included in gneiss may, however, contain a fragmentary record of a HP history, and are evidence that their host rocks were also deeply sourced. An excellent example of exhumed deep crust that retains a partial HP record is in the Montagne Noire dome, French Massif Central, which contains well‐preserved eclogite (garnet+omphacite+rutile+quartz) in migmatite in two locations: one in the dome core and the other at the dome margin. Both eclogites recordP ~ 1.5 ± 0.2 GPa atT ~ 700 ± 20°C, but differ from each other in whole‐rock and mineral composition, deformation features (shape and crystallographic preferred orientation, CPO), extent of record of prograde metamorphism in garnet and zircon, and degree of preservation of inherited zircon. Rim ages of zircon in both eclogites overlap with the oldest crystallization ages of host gneiss atc.310 Ma, interpreted based on zircon rare earth element abundance in eclogite zircon as the age of HP metamorphism. Dome‐margin eclogite zircon retains a widespread record of protolith age (c.470–450 Ma, the same as host gneiss protolith age), whereas dome‐core eclogite zircon has more scarce preservation of inherited zircon. Possible explanations for differences in the two eclogites relate to differences in the protolith mafic magma composition and history and/or the duration of metamorphic heating and extent of interaction with aqueous fluid, affecting zircon crystallization. Differences in HP deformation fabrics may relate to the position of the eclogite facies rocks relative to zones of transpression and transtension at an early stage of dome development. Regardless of differences, both eclogites experienced HP metamorphism and deformation in the deep crust atc.310 Ma and were exhumed by lithospheric extension—with their host migmatite—near the end of the Variscan orogeny. The deep crust in this region was rapidly exhumed from ~50 to <10 km, where it equilibrated under low‐P/high‐Tconditions, leaving a sparse but compelling record of the deep origin of most of the crust now exposed in the dome. 

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