Metamorphic zonation, migmatization and leucogranites along the Everest transect of Eastern Nepal and Tibet: record of an exhumation history

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Abstract
A polyphase evolution and a thermal and mineral zonation characterize metamorphism of the High Himalayan Crystallines (HHC) in the Everest transect. After an early Barrovian-type metamorphism (M1: T = 550–680° C; P = 8–10 kbar) caused by subduction and crustal thickening during India-Eurasia collision, exhumation of the HHC occurred at rates which allowed the isotherms to rise and the thickened crust to suffer partial melting. In the upper structural levels, the thermal conductivity contrast between crystalline and upper sedimentary rocks together with the infiltration of metamorphic fluids released during exhumation, favoured migmatization and formation of leucogranite magmas through partial melting of muscovite-bearing metasediments chiefly at medium (M2: T = 650–750° C; P = 4–7 kbar) but also at low-pressure (M3: T = 600–700° C; P = 2–4 kbar). The leucogranite melts segregated as dykes and kilometre-sized bodies into extensional shear zones situated at the uppermost levels of the HHC and at the base of the overlying Tibetan Series. During exhumation, a metamorphic zonation was imposed on the HHC in response to the complex interplay between rise of the isotherms in the thickened crust, emplacement of leucogranites in the uppermost levels of the HHC and conductive cooling near the ‘cold’ Lesser Himalaya and Tibetan Series coupled with thrust imbrication.