RbSr microchrons in the Manaslu granite: implications for Himalayan thermochronology

Samples of minerals from the Manaslu leucogranite in the central Himalaya were microsampled and measured for RbSr mineral ages. The ages obtained are used in conjunction with models for RbSr closure temperatures, to reconstruct the thermochronological history of the Manaslu granite. The RbSr results indicate a rapid change in temperature between 21 Ma and 17 Ma of 60–70°C/million yr, which, in turn, indicates unroofing at 2 mm/yr. The temperature-time curve inferred from RbSr data is in excellent agreement with existing ArAr data on the Manaslu granite and its thermal aureole. The results confirm that parts of the Himalaya, in addition to the Gangdese belt, were eroded rapidly in the 20-17 Ma time period, and this erosion could account for the observed rapid increase in seawater 87Sr86Sr. A tourmaline-whole rock age of 29.5 Ma was obtained on a sample from the Chokkang Arm, but may be unreliable because of post-crystallization alteration of feldspar. Microsampling approaches are demonstrated that allow high precision isotopic data to be obtained on minerals sampled at scales of 100 μm or less. Microanalysis, combined with models for Sr redistribution, can be a useful thermochronological tool that can augment the 40Ar39Ar approach.