CONTACT-METAMORPHISM AND DEPTH OF EMPLACEMENT OF THE MANASLU GRANITE (CENTRAL NEPAL) - IMPLICATIONS FOR HIMALAYAN OROGENESIS

The Manaslu massif (central Nepal) provides a well-exposed example of a deeply eroded pluton: its contact aureole can be followed from the b ase to the top along medium- to low-grade (mesozonal to anchizonal) Te thyan metasediments. Contact metamorphic mineral assemblages and therm obarometric estimations suggest that the granite was emplaced at 18-21 km for the base and 9-13 km for the roof. Calculated temperatures wit hin the aureole, at 550 +/- 40 degrees C, are compatible with the intr usion temperature of a leucogranitic magma. Microstructural evidence s hows that the temperature remained high (> 500 degrees C) at the base of the massif during and after granite emplacement, whereas towards th e top of the granite deformation proceeded rapidly at lower temperatur e. Heating of the abundant calcareous rocks in the contact aureole rel eased a local CO2-rich fluid, whereas a H2O- and boron-rich fluid seem s to have pervaded the whole zone; this fluid is probably exsolved fro m the granitic melt during its crystallization. The depth of emplaceme nt of the massif has an important implication for the reconstruction o f the Himalayan geodynamic evolution, implying a burial of the Tethyan metasediments by a major refolding of the sedimentary cover or, more probably, by extensive development of the North Himalayan nappes towar ds the south for more than 70 km, before granite emplacement, i.e., be fore the Miocene. The young age of the granite and its depth of emplac ement suggest a rapid tectonic denudation, in the order of 1 mm a(-1), probably by normal faulting north of the massif during its cooling.