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Photomicrographs of garnet crystals in eclogite TM-15. a Crossed polarized image of garnet TM-15G#3. b BSE image of garnet TM-15G#3, showing the traverse of EPMA-measured garnet compositional profile (A–Aʹ). c XMapTools processed image of garnet TM-15G#3 based on an X-ray map, showing the garnet zonation and inclusion phases. d BSE images of the mineral inclusions in garnet (TM-15G#2) core. Mineral abbreviations as in Table 2
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Thermodynamic modeling is an important technique to simulate the evolution of metamorphic rocks, particularly the poorly preserved prograde metamorphic reactions. The development of new thermodynamic modeling techniques and availability of updated thermodynamic databases and activity–composition (a–X) relations, call for an evaluation of best pract...
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... 3.1 | Phase equilibrium modelling P-T pseudosections were calculated with PERPLE_X (see Connolly, 2005) using two different data sets to better realize possible errors of this method as suggested by Pan et al. (2020), Li et al. (2021), Massonne and Li (2022) and . Thermodynamic Data Set 1 is based on the data set for end-members of rock-forming minerals (broadly silicates + carbonates + oxides) published by Holland and Powell (1998, and subsequent updates). ...
An eclogite from the Early Palaeozoic Fleur‐de‐Lys Supergroup in Newfoundland was studied because of its biotite porphyroblasts, which very rarely occur in this rock type. Thermodynamic modelling suggests that eclogitic biotite in common metabasite (former basalt–gabbro) is limited to (1) bulk‐rock compositions, which are relatively rich in Fe ²⁺ and K and poor in Fe ³⁺ , and (2) the low‐pressure range of the eclogite facies. The latter reason is supported by the determination of the pressure–temperature (P–T) path of the Newfoundland eclogite. Chemical zonation of garnet, presence of phengite with Si contents of ~3.4 per formula unit, Zr contents in rutile and petrographic observations resulted in a P–T trajectory starting at medium‐pressure conditions. Nearly isothermal burial led to a peak pressure of 18–19 kbar at ~575°C, followed by exhumation and slight heating. Deformation occurred at or close to the peak pressure. Subsequent introduction of hydrous fluids caused the formation of porphyroblasts of biotite and Ca–amphibole in the pressure range of 12–17 kbar at peak temperatures of 625–640°C. Retrogression led to very fine‐grained symplectites around omphacite and phengite and marginal replacement of biotite porphyroblasts by plagioclase and titanite. Geodynamic scenarios invoking either a flat subduction of oceanic crust followed by continent–continent collision or intracontinental subduction along a transpressional fault system might best explain the formation of eclogite with biotite porphyroblasts in general. For the Newfoundland eclogite, the latter scenario is preferred.
... Metamorphic modelling carried out by various workers on the eclogitic enclaves of the TMCC (e.g. St-Onge et al. 2014;Palin et al. 2017;review by O'Brien, 2019;Pan et al. 2020) suggests peak metamorphism at a P-T condition of ≥ 2.8 GPa and 550°C-650°C followed by near isothermal decompression up to 1 GPa and a subsequent high-temperature overprint (0.7-0.8 GPa and 680°C-720°C). This was followed by a clockwise path towards garnetamphibolite, and finally, greenschist conditions ( Figure 3). ...
The Tso Morari Crystalline complex (TMCC) of eastern Ladakh, India, is part of the north Indian continental margin and is characterized by eclogitic enclaves embedded within ortho- and paragneisses known as the Puga Gneiss. Two fault zones bound the TMCC: the Karzok fault to the southwest and the Zildat fault to the northeast. In the present study, we carried out Electron Backscatter Diffraction study of quartz of 10 samples collected from the Puga Gneiss. The relict and recrystallized quartz grains were treated separately to understand the deformation conditions of the Puga Gneiss during early and late deformation stages related to UHP metamorphism and final stage of exhumation during retrogression, respectively. Microstructural observations suggest dynamic recrystallization in quartz and plagioclase at different temperature ranges. Misorientation analysis of both relict and recrystallized quartz grains reveals presence of Dauphiné Twins. Lattice preferred Orientation (LPO) of axis of relict quartz grains generally shows more than one point maxima indicating that the relict grains preserve LPO developed during different stages of metamorphism/deformation. On the other hand, LPO of axis of recrystallized grains from Karzok and Zildat fault zones shows asymmetric single girdle either normal or at an angle to the foliation plane, which suggests simple shear. We conclude that grain size reduction and recrystallization of the Puga Gneiss was greatly influenced by Dauphiné Twin and the final exhumation of the TMCC took place in a simple shear environment aided by activity along its two binding fault zones.
... Previously published work Guillot et al., 1997;Mukherjee et al., 2003;St. Onge et al., 2013;Chatterjee and Jagoutz, 2015;Wilke et al., 2015;Palin et al., 2014Palin et al., , 2017Pan et al., 2020Pan et al., , 2022 show at least three different stages of metamorphism. The peak HP/UHP metamorphic stage is followed by a decompression stage and a high-temperature overprint. ...
... Geothermobarometric analysis of Mukherjee et al. (2003) and metamorphic modelling of Wilke et al. (2015) suggested a peak pressure ≥4.0 GPa, which was further corroborated by the presence of high-pressure polymorphs of carbonate and coesite ( Fig. 15a and b). Pan et al. (2020) discussed this disparity in P-T estimation and argued that the cool prograde paths at higher pressures reported by St Onge et al. (2013), Chatterjee and Jagoutz (2015), and Palin et al. (2014Palin et al. ( , 2017 were incompatible with the petrographic features because these studies White et al. (2007). Pan et al. (2020) suggested a warmer prograde path for the TMCC eclogites, supporting Warren et al. (2008) and Beaumont et al. (2009). ...
... Pan et al. (2020) discussed this disparity in P-T estimation and argued that the cool prograde paths at higher pressures reported by St Onge et al. (2013), Chatterjee and Jagoutz (2015), and Palin et al. (2014Palin et al. ( , 2017 were incompatible with the petrographic features because these studies White et al. (2007). Pan et al. (2020) suggested a warmer prograde path for the TMCC eclogites, supporting Warren et al. (2008) and Beaumont et al. (2009). ...
... Evaluation of metamorphic P-T evolution using THERMOCALC by Powell et al. [14] has proven to be a powerful approach to elucidate phase relations in metamorphic rocks [15][16][17][18][19]. This method has also proven useful in the study of UHP eclogites, because it could quantify the evolution of mineral assemblages and constrain a robust P-T path [20]. ...
Eclogites from the Guanshan and Yangkou areas of the Sulu orogen consist of garnet, omphacite, phengite, amphibole, quartz/coesite, rutile, and ilmenite. Garnet exhibits weak compositional zoning where Xgr decreases from the core to the mantle and then increases towards the rim, coupled with an increase in Xpy from the core to the mantle and then decrease towards the rim. Phase equilibria modelling with pseudosections calculated using THERMOCALC in the NCKFMASHTO system for the Guanshan and Yangkou eclogites records two stages of metamorphism: (I) prograde associated with quick subduction (Stage-I) and (II) retrograde associated with quick exhumation (Stage-II). Stage-I is recorded in the core-mantle zoning of garnet and Si content in phengite in the Guanshan and Yangkou eclogites with a mineral assemblage of Grt-Omp-Amp-Phg-Qtz-Rt ± Lws, and the P-T conditions are constrained at 22–26 kbar and 600–615 °C in Guanshan, while 24–26 kbar and 595–600 °C in Yangkou. The peak P-T conditions (Pmax = 33 kbar; T = 685 °C) of Guanshan eclogites are revealed by the maximum Si content in phengite and the minimum Xgr in the garnet mantle with the mineral assemblage of Grt-Omp-Phg-Coe-Rt ± Lws. The value of Pmax suggests that the subduction depth of the Guanshan eclogites exceeds 110 km. Stage-II is recorded in the mantle-rim zoning of garnet, and its P-T conditions are estimated to be 12–15 kbar and 780–820 °C for the Guanshan eclogites reflected by the assemblage of Grt-Omp-Amp-Pl-LL-Qtz-Rt ± ilm, and 13–14 kbar and 770–790 °C for the Yangkou eclogites by the assemblage of Grt-Omp-Amp-Pl-LL-Qtz-Rt. The two stages of metamorphism in the study areas are overall consistent with the regional metamorphic events, from ultra-high-pressure eclogite facies, through high pressure eclogite facies, to amphibole eclogite facies, with the ages of 245, 227 and 195 Ma, respectively.
... The Puga gneiss consists of an assemblage of plagioclase + quartz + phengite/muscovite + K-feldspar ± garnet ± biotite (O'Brien 2019). UHP conditions are recorded by coesite in eclogite and previous studies on peak P-T conditions (e.g., Mukherjee and Sachan 2001;Sachan et al. 2004;Singh et al. 2013;St-Onge et al. 2013;Chatterjee and Jagoutz 2015;Palin et al. 2017;Pan et al. 2020). ...
... The peak metamorphic pressure of the Tso Morari terrane is still debated, with estimates ranging from ~ 20 to ~ 48 kbar. Peak pressure calculations using mineral (e.g., garnet) isopleth thermobarometry from thermodynamic modeling range from ~ 20 to ~ 29 kbar (e.g., Konrad-Schmolke et al. 2008;Lanari et al. 2013;Singh et al. 2013;St-Onge et al. 2013;Chatterjee and Jagoutz 2015;Palin et al. 2017;Pan et al. 2020), while calculations using conventional geothermobarometry from carbonate assemblages range from ~ 33 to ~ 48 kbar (e.g., Mukherjee et al. 2003;Wilke et al. 2015). Magnesite has been observed in eclogite and gneiss, but aragonite has never been reported, indicating that the peak pressure is lower than 5 GPa (Massonne 2011). ...
... Magnesite has been observed in eclogite and gneiss, but aragonite has never been reported, indicating that the peak pressure is lower than 5 GPa (Massonne 2011). Past studies suggest that exhumation following peak pressure metamorphism (PPM) occurred along a close-to-isothermal decompression path crossing through amphibolite-facies conditions, but there is little agreement on the peak metamorphic temperature or the temperatures associated with exhumation (Guillot et al. , 2008de Sigoyer et al. 2004;Leech et al. 2007;St-Onge et al. 2013;Palin et al. 2014Palin et al. , 2017Long et al. 2020;Pan et al. 2020). The duration of exhumation of the terrane is proposed by multiple studies to be < ~ 21 Mya (Leech et al. 2007;Guillot et al. 2008;Palin et al. 2014), from ~ 51 Ma for peak pressure (St-Onge et al. 2013) to ~ 30 Ma for greenschist facies during exhumation (de Sigoyer et al. 2004;Long et al. 2020). ...
The Tso Morari terrane within the Himalayan orogenic belt underwent ultrahigh-pressure (UHP) coesite-eclogite metamorphism due to northward subduction of the Indian continent under the Eurasian continent during the early Eocene. The Tso Morari UHP terrane has been intensely studied petrologically, mineralogically, and geochemically over the past several decades. However, the fluid history (e.g., phases and pressure–temperature conditions, fluid compositions and sources, and processes of fluid–rock interactions) and thermal structure during exhumation remain unresolved. To address these issues, we sampled a traverse from the center of an eclogite boudin out into the host orthogneiss. Three major fluid evolution stages (FESs) were identified and characterized using petrography, mineral and bulk-rock chemistry, and thermodynamic modeling. FES 1 constrained mineral dehydration and hydration reactions during prograde metamorphism before reaching peak pressure at 29.0 ± 0.8 kbar and 591 ± 9 °C by modeling garnet growth in the eclogites. FES 2 constrained mineral reactions in the eclogite matrix due to destabilization of internal hydrous minerals. This FES caused the formation of epidote at 22.8 ± 0.6 kbar, amphibole core domains (glaucophane) at 19.0 ± 0.4 kbar, amphibole rim domains (barroisite) at 14.5 ± 1.0 kbar, and symplectite at 9.0 ± 1.0 kbar, during isothermal decompression (600–650 °C). FES 3 caused amphibolization of eclogite at the boudin rim at 625 ± 50 °C and 9.0–14.0 kbar. Metasomatism resulted in increased K2O, CO2, and bulk-rock Fe³⁺/ΣFe in the amphibolized eclogites. Large ion lithophile elements (LILE) (e.g., K, Rb, Cs, Sr, Ba) and trace element ratios of Ba/Rb and Cs/Rb are also elevated relative to the eclogite core. The fluid most likely originated from dehydrating host orthogneiss and/or metasediments. Thermodynamic modeling also predicts that the Tso Morari complex was exhumed through a low-temperature (< 650 ± 50 °C) regime in the subduction channel.
... The following minerals and solution models were used: biotite, white mica, chlorite (White et al., 2014), amphibole and clinopyroxene (Green et al., 2016), epidote (Holland & Powell, 2011) and plagioclase (Holland & Powell, 2003). Garnet a-x relations of White et al. (2007) were used instead of those of White et al. (2014) following the suggestions of Pan et al. (2020). Albite, lawsonite, quartz/coesite, rutile and titanite were considered as pure end-members. ...
In the Western Gneiss Region in Norway, mafic eclogites form lenses within granitoid orthogneiss and contain the best record of the pressure and temperature evolution of this ultrahigh‐pressure (UHP) terrane. Their exhumation from the UHP conditions has been extensively studied, but their prograde evolution has been rarely quantified although it represents a key constraint for the tectonic history of this area. This study focused on a well‐preserved phengite‐bearing eclogite sample from the Nordfjord region. The sample was investigated using phase‐equilibrium modelling, trace‐element analyses of garnet, trace‐ and major‐element thermo‐barometry and quartz‐in‐garnet barometry by Raman spectroscopy. Inclusions in garnet core point to crystallisation conditions in the amphibolite facies at 510–600 °C and 11–16 kbar, while chemical zoning in garnet suggests growth during isothermal compression up to the peak pressure of 28 kbar at 600 °C, followed by near‐isobaric heating to 660–680 °C. Near‐isothermal decompression to 10–14 kbar is recorded in fine‐grained clinopyroxene‐amphibole‐plagioclase symplectites. The absence of a temperature increase during compression seems incompatible with the classic view of crystallization along a geothermal gradient in a subduction zone and may question the tectonic significance of eclogite‐facies metamorphism. Two end‐member tectonic scenarios are proposed to explain such an isothermal compression: (1) either the mafic rocks were originally at depth within the lower crust and were consecutively buried along the isothermal portion of the subducting slab, or (2) the mafic rocks recorded up to 14 kbar of tectonic overpressure at constant depth and temperature during the collisional stage of the orogeny.
... These enclaves and intrusions are garnet amphibolite to eclogite in nature. Metamorphic studies carried out by previous workers suggest the peak (HP) metamorphism of eclogite facies in the TMCC at ≥2.8 GPa [51][52][53][54][55][56]. Detailed petrochronological study [57] suggests that peak (HP) metamorphism of TMCC took place at~47-43 Ma. ...
... Detailed petrochronological study [57] suggests that peak (HP) metamorphism of TMCC took place at~47-43 Ma. Based on the P-T pseudosection modelling carried out by previous workers on the metabasic enclaves of TMCC [51][52][53][54][55][56] (Figure 3), it can be inferred that the high-pressure mineral omphacite was stable from ≥2.8 GPa to~1.5 GPa within a temperature range of~530°C to~650°C defining the P-T path from peak (HP) to postpeak (HP) metamorphic stages. Quartz represents mostly the late garnet amphibolite and granulite facies of metamorphism at ≤1.5 GPa to~0.5 GPa with temperature ranging from~500°C up to 750°C during granulite metamorphism. ...
The Tso Morari Crystalline Complex (TMCC) of trans-Himalaya (eastern Ladakh, India) contains enclaves of ultrahigh-pressure eclogites that underwent deep burial (≥80 km) and subsequent rapid exhumation during continental subduction, collision, and final accretion of the Indian plate with the Eurasian plate. We present an electron backscatter diffraction (EBSD) study of eight eclogite samples to investigate the deformation mechanism and strain regimes active during peak (HP) metamorphism and subsequent postpeak rapid exhumation of the TMCC. Our study shows that the least retrogressed eclogite exhibits strong linear fabric (L tectonite) characterized by omphacite, having [001] axes parallel to and (110) poles perpendicular to lineation. These features concur with constrictional strain during peak (HP) metamorphism. A transitional planolinear fabric (LS tectonite) is shown by other eclogites that show petrographic evidence of omphacite alteration to amphibole and the presence of lower metamorphic grade minerals like actinolite and chlorite. Characteristics of lattice preferred orientation (LPO) of omphacite and quartz, indicated, respectively, by the LS and B indices, also suggest variation in strain regime from pristine eclogites to their altered counterparts. Based on these results, it is suggested that a constrictional strain regime prevailed during peak (HP) metamorphism in the TMCC due to the buoyant rise of TMCC in response to slab break-off and reverse slab pull during and after the deepest continental subduction. This buoyant rise was also facilitated by compression related to the ongoing India-Eurasia collision. This regime evolved later to plane strain that was superimposed on the UHP rocks at a shallower depth. It is plausibly associated with foliation-parallel extension during exhumation at midcrustal depths. A high-temperature prism c-slip in quartz shown by few samples is interpreted to have formed due to a subsequent granulite facies metamorphic overprint on the eclogite during collisional thickening.
... The following minerals and solution models were used: biotite, white mica, chlorite (White et al., 2014), amphibole and clinopyroxene (Green et al., 2016), epidote (Holland & Powell, 2011) and plagioclase (Holland & Powell, 2003). Garnet a-x relations of White et al. (2007) were used instead of those of White et al. (2014) following the suggestions of Pan et al. (2020). Albite, lawsonite, quartz/coesite, rutile and titanite were considered as pure end-members. ...
... "inclusion" refers to the garnet core. Li et al. (2021a) and Pan et al. (2020). Then, pseudosections, obtained with two different data sets (outlined below), were constructed in detail for the reduced P-T range 6-26 kbar and 450-750 • C and various sub-ranges. ...
Eclogites form in different geotectonic environments such as subduction zones or roots of overthickened continental crust. The specific environment can be identified by the construction of pressure-temperature (P-T) trajectories. We determined the P-T path of eclogite from the Les Essarts high-pressure unit, Southern Armorican zone in western France. The eclogite contains unusual atoll garnet characterized by a younger generation (Grt2) that replaced the core of an older generation (Grt1). Large-area (cm²) X-ray mapping with the electron microprobe was applied to determine the quantities of Grt1 and Grt2 in the studied eclogites. Detailed characterization of the mineral chemistry was undertaken followed by thermodynamic modelling. The assemblage of amphibole, quartz, rutile, and ilmenite enclosed in Grt1 formed in the amphibolite facies during a first metamorphic event. Grt1 grew at P-T conditions around 20 kbar and 540 °C probably after nearly isothermal burial during a second metamorphic cycle. An isobaric heating to ca. 650 °C followed at which Grt2 and, thus, atoll garnet formed. A time period of ca. 15 million years for exhumation in the temperature range 620–655 °C was estimated by diffusional modelling of the contact between Grt1 and Grt2. We correlate the Southern Armorican zone with the Malpica-Tuy zone (NW Spain) that contains eclogite and related rocks with a similar metamorphic evolution. We outline a geodynamic scenario in which the eclogite-facies rocks of both zones were tectonically eroded from the upper continental plate by a subducting plate to reach maximum depths of 70–90 km in the Late Devonian. Exhumation to depths of ca. 45–50 km occurred in a subduction channel just before continent-continent collision started.
... Investigations of the metamorphic conditions and timing of eclogite facies UHP metamorphism in the TMN, as well as the Barrovian-style amphibolite facies metamorphism that occurred at midcrustal depths along its exhumation path (e.g., de Sigoyer et al., 2000;Girard, 2001;Guillot et al., 1997;Leech et al., 2005Leech et al., , 2007Pan et al., 2020;Schlup et al., 2003;St.-Onge et al., 2013), have provided foundational P-T-t datapoints that have informed regional tectonic models for the geodynamic evolution of the nappe (most notably de Sigoyer et al., 2004;Epard & Steck, 2008). ...
... Not all data and interpretations converge on a consistent UHP P-T-t history for the TMN; many argue for a depth of~100 km at~600°C between~54 and~51 Ma (e.g., de Sigoyer et al., 2000;Leech et al., 2005Leech et al., , 2007St.-Onge et al., 2013), while others argue for depths up to~160 km at~650-750°C with ages as young as 45 Ma (Donaldson et al., 2013;Mukherjee et al., 2003;Wilke et al., 2015), and still others point to large P-T differences depending on what type of thermodynamic modeling is applied (Pan et al., 2020). Post-UHP detachment and extrusion of the TMN at rates of 1. 2 (St.-Onge et al., 2013) to 12 cm/yr (Wilke et al., 2015) are interpreted to have been facilitated by its low density relative to surrounding mantle peridotites (Chatterjee & Jagoutz, 2015;de Sigoyer et al., 2004;Epard & Steck, 2008) and accommodated by top-down-toeast shearing within the TMN (Epard & Steck, 2008). ...
... The orthogneiss commonly contains boudins of eclogitic metabasite that are up to 3 m thick, which are interpreted to represent syn-granitic mafic dikes (e.g., Epard & Steck, 2008). Thermobarometric studies of these eclogitized boudins have provided evidence for burial of the TMN to at least~27 kbar (e.g., Bidgood et al., 2020;Pan et al., 2020;Sachan et al., 2004;St.-Onge et al., 2013). ...
Documenting the processes that facilitate exhumation of ultrahigh‐pressure (UHP) rocks at convergent margins is critical for understanding orogen dynamics. Here, we present structural and temperature data from the Himalayan UHP Tso Morari nappe (TMN) and overlying nappes, which we integrate with published pressure‐temperature‐time constraints to refine interpretations for their structural evolution and exhumation history. Our data indicate that the 5.5‐km‐thick TMN is the upper portion of a penetratively deformed ductile slab, which was extruded via distributed, pure shear‐dominated, top‐down‐to‐east shearing. Strain in the TMN is recorded by high‐strength quartz fabrics (density norms between 1.74 and 2.86) and finite strain data that define 63% transport‐parallel lengthening and 46% transport‐normal shortening. The TMN attained peak temperatures of ~500–600°C, which decrease in the overlying Tetraogal and Mata nappes to ~150–300°C, defining a field gradient as steep as 67°C/km. Within the overlying nappes, quartz fabric strength decreases (density norms between 1.14 and 1.21) and transport‐parallel lengthening and transport‐normal shortening decrease to 14% and 18%, respectively. When combined with published ⁴⁰Ar/³⁹Ar thermochronometry, quartz fabric deformation temperatures as low as ~330°C indicate that the top‐to‐east shearing that exhumed the TMN continued until ~30 Ma. Peak temperatures constrain the maximum depth of the overlying Mata nappe to 12.5–17.5 km; when combined with published fission‐track thermochronometry, this provides further support that the TMN was not underplated at upper crustal levels until ~30 Ma. The long‐duration, convergence‐subnormal shearing that exhumed the TMN outlasted rapid India‐Asia convergence by ~15 Myr and may be the consequence of strain partitioning during oblique convergence.
