Northward subduction of the leading edge of the Indian continental margin to depths greater than 100 km during the early Eocene resulted in high-pressure (HP) quartz-eclogite to ultrahigh-pressure (UHP) coesite-eclogite metamorphism at Tso Morari, Ladakh Himalaya, India. Integrated pressuretemperature-time determinations within petrographically well-constrained settings for zircon-and/or monazite-bearing assemblages in mafic eclogite boudins and host aluminous gneisses at Tso Morari uniquely document segments of both the prograde burial and retrograde exhumation path for HP/ UHP units in this portion of the western Himalaya. Poikiloblastic cores and inclusion-poor rims of compositionally zoned garnet in mafic eclogite were utilized with entrapped inclusions and matrix minerals for thermobarometric calculations and isochemical phase diagram construction, the latter thermodynamic modelling performed with and without the consideration of cation fractionation into garnet during prograde metamorphism. Analysis of the garnet cores document (M1) conditions of 21.5 AE 1.5 kbar and 535 AE 15°C during early garnet growth and re-equilibration. Sensitive high resolution ion microprobe (SHRIMP) U-Pb analysis of zircon inclusions in garnet cores yields a maximum age determination of 58.0 AE 2.2 Ma for M1. Peak HP/UHP (M2) conditions are constrained at 25.5-27.5 kbar and 630-645°C using the assemblage garnet rim-omphacite-rutile-phengite-lawsonite -talc-quartz (coesite), with mineral compositional data and regional considerations consistent with the upper P-T bracket. A SHRIMP U-Pb age determination of 50.8 AE 1.4 Ma for HP/UHP metamorphism is given by M2 zircons analysed in the eclogitic matrix and that are encased in the garnet rim. Two garnet-bearing assemblages from the Puga gneiss (host to the mafic eclogites) were utilized to constrain the subsequent decompression path. A non-fractionated isochemical phase diagram for the assemblage phengite-garnet-biotite-plagioclase-quartz-melt documents a restricted (M3) P-T stability field centred on 12.5 AE 0.5 kbar and 690 AE 25°C. A second non-fractionated isochemical phase diagram calculated for the lower pressure assemblage garnet-cordierite-sillimanite-biotite-plagioclase-quartz-melt (M4) documents a narrow P-T stability field ranging between 7-8.4 kbar and 705-755°C, which is consistent with independent multiequilibria P-T determinations. Th-Pb SHRIMP dating of monazite cores surrounded by allanite rims is interpreted to constrain the timing of the M4 equilibration to 45.3 AE 1.1 Ma. Coherently linking metamorphic conditions with petrographically constrained ages at Tso Morari provides an integrated context within which previously published petrological or geochronological results can be evaluated. The new composite path is similar to those published for the Kaghan UHP locality in northern Pakistan, although the calculated 12-mm a À1 rate of post-pressure peak decompression at Tso Morari would appear less extreme.
The Danba Structural Culmination is a tectonic window into the late Triassic to early Jurassic Songpan‐Garzê Fold Belt of eastern Tibet, which exposes an oblique section through a complete Barrovian‐type metamorphic sequence. Systematic analysis of a suite of metapelites from this locality has enabled a general study of Barrovian metamorphism, and provided new insights into the early thermotectonic history of the Tibetan plateau. The suite was used to create a detailed petrographic framework, from which four samples ranging from staurolite to sillimanite grade were selected for thermobarometry and geochronology. Pseudosection analysis was applied to calculate P–T path segments and determine peak conditions between staurolite grade at ∼5.2 kbar and 580 °C and sillimanite grade at ∼6.0 kbar and 670 °C. In situ U–Pb monazite geochronology reveals that staurolite‐grade conditions were reached at 191.5 ± 2.4 Ma, kyanite‐grade conditions were attained at 184.2 ± 1.5 Ma, and sillimanite‐grade conditions continued until 179.4 ± 1.6 Ma. Integration of the results has provided constraints on the evolution of metamorphism in the region, including a partial reconstruction of the regional metamorphic field gradient. Several key features of Barrovian metamorphism are documented, including nested P–T paths and a polychronic field gradient. In addition, several atypical features are noted, such as P–T path segments having similar slopes to the metamorphic field gradient, and Tmax and Pmax being reached simultaneously in some samples. These features are attributed to the effects of slow tectonic burial, which allows for thermal relaxation during compression. While nested, clockwise P–T–t loops provide a useful framework for Barrovian metamorphism, this study shows that the effects of slow burial can telescope this model in P–T space. Finally, the study demonstrates that eastern Tibet experienced a significant phase of crustal thickening during the Mesozoic, reinforcing the notion that the plateau may have a long history of uplift and growth.
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