Petrogenesis and tectonic implications of the Yadong leucogranites, southern Himalaya

Abstract

The leucogranites in the Higher Himalayan Sequence (HHS) provide a probe to elucidate the crustal melting of continental collisional orogen. An integrated geochemical and geochronological study of the Yadong leucogranites, southern Himalaya, shows that these rocks have relatively high SiO2 contents of 69.77 to 75.32wt.% and alumina saturation index (A/CNK) of 1.09–1.40, typical of peraluminous granites. They show moderately fractionated REE patterns with negative Eu anomalies, and are characterized by enriched LILE (Rb and Cs) and depleted HFSE (Zr, Hf, Nb and Ta). LA–ICP–MS U–Pb zircon dating of ten samples yields crystallization ages ranging from 21.0 to 11.7Ma. The zircons have variable εHf(t) values of −26.3 to −3.5 and corresponding Hf two-stage model ages of 2.77–1.33Ga. The present study reveals that the muscovite–biotite leucogranites (2ML) have higher TiO2, MgO, CaO, Sr, Ba and Zr contents, lower Rb/Sr ratios than the tourmaline–muscovite leucogranites (TML). Zircon and monazite saturation thermometry results show that the melt temperatures (681–784°C) of the 2ML are 20–80°C higher than those (663–705°C) of the TML. Combining with previous results, we propose that the TML were derived from the muscovite-dehydration melting, whereas the 2ML dominantly resulted from the biotite-dehydration melting during the prograde metamorphism of the pelitic and felsic granulites of the HHS. Therefore, the Himalayan leucogranites were probably formed during the subduction of the Indian crust following the India and Asia collision.