RB-SR MICROCHRONS IN THE MANASLU GRANITE - IMPLICATIONS FOR HIMALAYANTHERMOCHRONOLOGY

Samples of minerals from the Manaslu leucogranite in the central Himal aya were microsampled and measured for Rb-Sr mineral ages. The ages ob tained are used in conjunction with models for Rb-Sr closure temperatu res, to reconstruct the thermochronological history of the Manaslu gra nite. 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, indica tes 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 Mana slu granite and its thermal aureole. The results confirm that parts of the Himalaya, in addition to the Gangdese belt, were eroded rapidly i n the 20-17 Ma time period, and this erosion could account for the obs erved 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 i sotopic 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 Ar-40-Ar -39 approach.