Timescales of Partial Melting and Melt Crystallization in the Eastern Himalayan Orogen: Insights From Zircon Petrochronology

Abstract

AbstractRevealing the timescales of metamorphic and anatectic processes is central to our understanding of tectonic evolution of collisional orogens. High‐temperature migmatites and leucogranites are well exposed in the Himalayan orogenic core, making it an ideal region to study the timing and duration of partial melting and melt crystallization of the orogen. Here, we report an integrated and comprehensive data set of petrography, U‐Pb age, and trace element data for zircon from a pelitic granulite and associated leucosomes of the Greater Himalayan Sequence (GHS) in the Yadong area, eastern Himalaya. Zircon grains with complex internal structure retain variable ages ranging from 32 Ma to 13 Ma that correlate systematically with changes in the concentrations of Y, Th, U, Hf, Nb, Ta, and HREE, and ratios of Th/U, Eu/Eu*, and Nb/Ta. Combined with petrologic analysis, we conclude that the granulite witnessed high‐temperature metamorphism, melting, and melt crystallization over ∼20 Myr. Prograde, simultaneous increases in pressure and temperature and associated dehydration melting began at least by ∼32 Ma and lasted until ∼24 Ma. Subsequent quasi‐isothermal decompression‐melting occurred between ∼22 and 19 Ma, and late melt crystallization spanned ∼19 to 13 Ma. Large volumes of melt generated during prograde metamorphism could have triggered exhumation of GHS rocks, increasing melt fraction through a positive feedback between exhumation and melting. More comprehensive analysis of different rock types led to more complete and different interpretations for the timing of exhumation and melt crystallization in the Yadong‐Sikkim region and might enable alternate interpretations elsewhere in the Himalayas.