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

Abstract

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 ~1,550 Ma, ~1,350 Ma, 1,100–1,150 Ma, 900–930 Ma, and ~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 (~1.65 GPa and 650°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.