Age, petrogenesis, and metamorphic modelling of high-pressure garnet-amphibolite from the Tethyan Himalayan Sequence of Bhagirathi Valley, Western Garhwal Himalaya


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The Cenozoic Himalayan Orogeny is a manifestation of subduction and subsequent collision of the Indian lithosphere with the Asian Plate in the Eocene, resulting in the formation of intra-terrane shear zones and regional metamorphic belt along the strike of the entire orogen. This metamorphic belt contains high-pressure (HP) metamorphic rocks (garnet-amphibolite, late-stage granulite, and host gneisses) at various structural levels in the Himalayas. In the present study, we carried out geochemical, chronological, and metamorphic modelling of an exotic tschermakite-bearing garnet-amphibolite unit exposed within the Tethyan Himalayan Sequence of Western Garhwal Himalaya of India. Bulk-rock geochemistry suggests the protolith's tholeiitic fractionation trend and trachy-andesitic nature. U-Pb geochronology of zircon reveals an age spectrum with pronounced clusters at similar to 1,550 Ma, similar to 1,350 Ma, 1,100-1,150 Ma, 900-930 Ma, and similar to 550 Ma. There is a possibility that the protolith of the HP garnet amphibolite is magmatic. However, near-concordant ages at certain intervals might suggest crustal components from multiple sources. Integrated geothermobarometry and P-T pseudosection modelling indicate high-P/medium-T conditions (similar to 1.65 GPa and 650 degrees C) of peak metamorphism. Based on these results, we infer that this garnet-amphibolite represents pre-Himalayan Indian lower crust and during India-Asia collision. The leading edge of the Indian continental lithosphere subducted where mafic and some of the felsic lithologies underwent HP garnet-amphibolite facies metamorphism, and after a possible slab break-off, these rocks exhumed during the Himalayan orogeny.
Bhagirathi Valley, garnet amphibolite, Himalaya, P-T pseudosection, U-Pb geochronology, zircon
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