Miocene andalusite leucogranite in central-east Himalaya (Everest–Masang Kang area): Low-pressure melting during heating

The studied Miocene andalusite-bearing leucogranites intrude the upper part of the High Himalayan Crystallines (HHC) and the north Himalayan domes and outcrop in an area stretching from Mt. Everest to the Kula Khangri massif (Bhutan) towards the east. The leucogranites constitute both dykes as well as sills and parts of larger andalusite-free leucogranite plutons (e.g., Makalu). They represent mainly of two-mica (muscovite + biotite ± tourmaline ± cordierite ± andalusite ± sillimanite ± dumortierite) leucogranite, and tourmaline (muscovite + tourmaline ± biotite ± andalusite ± sillimanite ± garnet ± kyanite ± spinel ± corundum) leucogranites. Microstructures reveal several generations of andalusite (from residual/peritectic early magmatic to cotectic late magmatic), even in the same sample. The occurrence of residual and/or peritectic andalusite, together with inclusions of sillimanite + biotite in cordierite, indicates that melts formed by dehydration melting of biotite at T = 660–700 °C during prograde heating at low-pressure conditions (P < about 400 MPa). According to current models, leucogranites are produced by dehydration melting of muscovite and/or biotite during exhumation of the HHC. In this case, micas are consumed in the sillimanite stability field. As a consequence, these models cannot explain the occurrence of residual and/or peritectic magmatic andalusite. Conditions for anatexis in the andalusite field may have been achieved by heat transfer within the exhuming (extruding) HHC, from structurally lower and hotter rocks towards upper and colder fertile lithologies.