Quantifying the partial melting of Himalayan Metamorphic core in Eastern Himalaya: implications for crustal rheology

The Himalayan orogeny caused partial melting of rocks within the Greater Himalayan Sequence (GHS), forming migmatites. The extensive occurrence of such migmatites in the lower structural level of the GHS (GHS(L)) is a distinctive feature of the Western Arunachal Himalaya (WAH), situated in eastern part of the orogen; meanwhile leucogranite is predominantly found in the highest reaches of the GHS(L.) A comprehensive multi-method study incorporating field observations, petrography, phase equilibrium modelling, geochemical analysis, and zircon U-Pb and monazite U-Th-Pb geochronology was conducted on migmatitic paragneiss and leucogranites from the GHS(L) along the Bomdila-Tawang section of the WAH. P-T pseudosection modelling reveals a clockwise P-T path characterized by prograde burial and heating, significant melt production, and nearly isothermal decompression during melt solidification. Structural observations, including concordant and discordant relationships between leucosomes and gneissic bands, suggest that deformation established pathways for melt migration. Zircon U-Pb dates reveal bimodal protolith ages of similar to 1350 Ma (Ectasian) and similar to 900 Ma (Tonian). Insufficient zircon overgrowth (< 20 mu m), likely due to extensive melt extraction during suprasolidus metamorphism, precludes younger age determination. Monazite U-Th-Pb age indicates peak metamorphism of the GHS(L) at ca. 25-26 Ma, synchronous with MCT initiation in the WAH. Melt generation at peak metamorphic conditions in the GHS(L) reached similar to 16 vol% in stromatic metatexites and similar to 26 vol% in layered diatexites and of these generated melts, > 50% escaped at depths of similar to 30-34 km. This extensive migration formed complex leucosome networks, contributing to regional leucogranite distribution and rheological weakening, enabling ductile flow within the GHS.