The Himalayan leucogranites: Constraints on the nature of their crustal source region and geodynamic setting

Late Oligocene–Miocene leucogranites within southern Tibet form part of an extensive intrusive igneous province within the Himalayan orogen. The main rock types are tourmaline leucogranites (Tg) and two-mica leucogranites (2 mg). They have high SiO2 (70.56–75.32 wt.%), Al2O3 (13.55–15.67 wt.%) and (87Sr/86Sr)i (0.724001–0.797297), and low MgO (0.02–0.46 wt.%) and (143Nd/144Nd)i (0.511693–0.511906). Chondrite-normalized rare earth element (REE) patterns display strong negative Eu anomalies. Whole-rock major and trace element and Sr–Nd isotope data for the leucogranites suggest that their source region was a two-component mixture between a fluid derived from the Lesser Himalayan (LH) crustal sequence and the bulk crust of the Higher Himalayan (HH) sequence. Trace element and Sr–Nd isotope modeling indicate that the proportion of fluid derived from the LH sequence varied from 2% to 19% and the resulting metasomatised source experienced 7–16% melting. The amount of fluid derived from the LH sequence increases from north to south. Northward underthrusting of the Indian continent resulted in infiltration of the LH-derived fluid into the overlying HH sequence. Subsequent decompression melting of this metasomatised crust, mostly during the Miocene (25–9 Ma), generated the leucogranites. This may be linked to steepening of the subducted slab of Indian lithosphere beneath the orogenic belt.