Migmatization, Granite Generation and Melt Accumulation in the Himalayan Orogenic Channel, Central and Eastern Bhutan

Abstract

In Central and Eastern Bhutan Himalaya, the Great Himalayan Sequence (GHS) reveals mesoscopic structures within the migmatite–leucogranite association due to crustal anataxis above the Main Central Thrust (MCT). The first phase of dominant melting generates stromatitic migmatite along the main foliation during high grade of metamorphism, possibly by dehydration melting. Subsequent ductile strike–slip shearing caused in situ melting in dilatational sites to produce structureless, non-foliated patchy leucogranite leucosome as well as in boudin necks and post-tectonic patches. In addition, melt-enhanced deformation caused doming of accumulated melt and subsidiary ductile shear zones on either margins of these domes. Surrounded by biotite-rich melanosome, leucosomes destroy the pre-existing foliation during new anatectic phase, which post-dates earlier stromatitic migmatite. These migmatites are the snapshot of mutual relations between newly-developed migmatite and leucogranite melt, and signify the transportation of Himalayan Orogenic Channel to the extreme south in Central and Eastern Bhutan over the Lesser Himalayan sedimentary belt along the MCT.