Melting and Exhumation of the upper structural levels of the Greater Himalaya Sequence and Makalu granite: constraints from thermobarometry, metamorphic modeling and U-Pb geochronology

The Makalu massif of Eastern Nepal displays a complex suite of Leucogranites and host sillimanite grade gneisses. These leucogranites are linked to the intrusions at the base of the Everest-Lhotse-Nuptse massif immediately to the west (Searle et al 2003) The Makalu intrusion is multiphase and forms the structurally highest foliationparallel sheets of leucogranite along the top of the Greater Himalayan Sequence on the Nepal-Tibet border. It is comprised of massive Grt + Tur + Ms ± Bt leucogranites that also occasionally contain large cordierite crystals. The abundance of cordierite in the upper granite sheet is unlike other granites in the Nepalese Himalaya. The cordierite bearing leucogranite overlies lower sheets of ‘normal’ Himalayan granites intruded into black sillimanite gneisses and is thought to be the most recent phase of magmatism. A few crosscutting feeder dykes mapped adjacent to the upper Barun glacier have channeled magma to the upper sheet. Petrology shows evidence for muscovite dehydration melting (~<700°C) in the upper part of the Barun gneiss which is a likely mechanism by which to produce the Makalu granite melts. Host gneisses retain biotite and so melting temperatures did not exceed 800°C (White et al. 2001). Secondary cordierite rims around garnets in these gneisses and the presence of cordierite in leucogranites record the last low pressure phase of melting. We use these field and petrographic observations for the basis of detailed metamorphic modeling of decompression and geochronology work for the upper parts of the GHS. P-T determinations (THERMOCALCv.3.30) detail peak