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Two distinct granite plutons occur above the Main Central Thrust north of Uttarkashi in the upper valley of the Bhagirathi River and its source, the Gangotri Glacier, in Garhwal, India. The structurally lower pluton is a biotite granite with mineralogic and major and trace element characteristics similar to late Precambrian to early Paleozoic plutons of the Northern and Lesser Himalayan belts of Indian shield granites. The structurally higher pluton, intruded into the Martoli Formation and Vaikrita Group of the Tethyan sedimentary rocks, is an aluminous S-type muscovite-tourmaline leucogranite similar to other Cenozoic High Himalayan leucogranites with respect to its mineralogy and major element chemistry, as well as its high concentrations of Rb, Cs, and U, combined with low concentrations of Sr, Zr, Th, and rare-earth elements. Whole-rock Rb-Sr data for five samples from the main leucogranite define two clusters on an isochron of 64 ± 11 Ma. This isochron may reflect (1) a significant age, (2) variations of initial 87Sr/86Sr in the magma at the time of crystallization, or (3) multiple pulses of magma with different isotopic compositions. An Rb-Sr mineral isochron for one of the leucogranite samples yields an age of 21.1 ± 0.9 Ma, whereas a K-Ar age on a muscovite separate from the same rock yields an age of 18.9 ± 1.3 Ma.

Whatever the actual age of the leucogranite magma within the range of these different determinations, it clearly had a high initial 87Sr/86Sr, greater than 0.746. The initial 143Nd/144Nd is correspondingly low, less than 0.51190. These data suggest that the granitic magma developed by anatectic melting of older continental crust. Although the timing of anatectic melting is equivocal, constraints on the conditions required to produce the melts imply that postcollisional, intracontinental subduction along the Main Central Thrust (MCT) could not produce the leucogranites without some preheating of the Indian continental crust. This heating may have occurred when delaminated Indian subcontinental lithosphere was replaced by hot asthenosphere in the initial stages of the continental collision. Crustal anatexis to produce the leucogranites might have already occurred at this early stage of collision. Alternatively, once the crust was so heated, crustal anatexis may have occurred within the hot crystalline Higher Himalayan slab due to influx of volatiles from the Lesser Himalayan slab as the former was emplaced over the latter by intracontinental subduction. Continued continental convergence led to uplift of the Higher Himalaya due to displacement along a major crustal footwall ramp of the Main Central Thrust. The mineral ages are interpreted as dating cooling related to this uplift, which may have coincided with termination of active motion of the MCT and the initiation of motion along the Main Boundary Thrust.

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