Thomas Hopkinson, Nigel Harris, Nick Roberts, Clare Warren, Sam J. Hammond, Christopher Spencer, Randall Parrish

The chemical compositions of growth zones of magmatic zircon provide powerful insight into evolving magma compositions due to their ability to record both time and the local chemical environment. In situ U-Pb and Hf isotope analyses of zircon rims from Tertiary leucogranites of the Bhutan Himalaya reveal, for the first time, an evolution in melt composition between 32-12 Ma. The data indicate a broadly stable melt source from 32 Ma to 17 Ma, and the progressive addition of an older source component to the melt from at least ~17 Ma, and possibly from as early as 21 Ma. Age-corrected εHf ratios decrease from between -10 to -15 down to values as low as 23 by 12 Ma. Complementary whole-rock Nd isotope data corroborate the Hf data, with a progressive decrease in ɛNd(T) from ~18 to 12 Ma. Published zircon and whole-rock Nd data from different lithotectonic units in the Himalaya suggest a chemical distinction between the younger Greater Himalayan Series (GHS) and the older Lesser Himalaya Series (LHS). The time-dependent isotopic evolution shown in the leucogranites demonstrates a progressive increase in melt contribution from older lithologies, indicative of increasing LHS involvement in Himalayan melting over time. The time-resolved data therefore are consistent with a model wherein LHS material was incorporated into the base of the hanging wall of the Main Central thrust from ~17 Ma. Exhumation of this hanging wall material along the thrust triggered decompression melting under fluid-absent conditions during the later stages of orogenesis.