AGE AND COOLING HISTORY OF THE MANASLU GRANITE - IMPLICATIONS FOR HIMALAYAN TECTONICS

The Manaslu granite is one of the High Himalayan leucogranites which are believed to be derived by partial melting of quartzo-feldspathic gneissess of the High Himalayan Crystallines (HHC) and emplaced at the contact between the HHC and the Tethyan Sedimentary Series. We have analyzed a suite of muscovite (15), biotite (3), and alkali feldspar (10) by the 40Ar/39Ar method to help constrain the age and cooling history of the granite. 40Ar/39Ar ages ranges are 18.4 to 13.3 Ma, 17.0 to 14.7, and 16.4 to 3.4 for muscovite, biotite, and K-feldspar, respectively. Based on the muscovite analyses and a re-interpretation of a previously published U-Pb monazite age we conclude that the crystallization age of the Manaslu granite is everywhere approximately 20 Ma. The post-crystallization cooling history given by the micas and the feldspars suggests the presently exposed parts of the pluton were emplaced at depths of 8 to 15 km. These data place a minimum age of movement on the MCT in this area of 20 Ma. We estimate the time of magma segregation and transport to be no more than approximately 5 Ma.