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 degrees 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 Sr-87/Sr-86. 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 0 mu m or less. Microanalysis, combined with models for Sr redistribution, can be a useful thermochronological tool that can augment the 40Ar-39Ar approach.