GEOLOGIC FRAMEWORK OF THE UPPER LIKHU KHOLA REGION, EAST-CENTRAL NEPAL

Recent models for the evolution of the Himalaya and adjacent regions have changed our understanding of how large, hot mountain belts form. The key data that govern these models has largely been extracted from the exhumed mid-crustal core of the orogen, the Greater Himalayan sequence, and its bounding structures. Targeted mapping in the upper Likhu Khola was carried out across the Greater Himalayan sequence as the initial phase of a project aimed to evaluate the viability of those models. The exhumed mid-crustal core in the study area exposes upper greenschist to upper amphibolite grade metamorphic rocks that have been pervasively deformed by ductile shearing. Mantled porphyroclasts and c, c’ and s fabrics record top-to-the-south directed shear. As with most transects across the Himalaya, metamorphic grade increases up structural section. Micaceous phyllite and schist with intercalacated limestone and calc-silicate rocks give way up structural section to migmatitic paragneiss and granitic orthogneiss. Metamorphic index minerals record increasing metamorphic grade up-section, from garnet and biotite in the lower portions to sillimanite throughout the structurally higher portions of the map-area. Evidence of crustal melting appears first as local mm-cm scale lenses within micaceous schist at lower structural levels. Anatectite content increases up structural section and at higher levels it is spatially associated with melanosome residuum, which is interpreted to indicate in-situ melting. At least two distinct phases of anatectite were identified, one of which is stromatic and generally parallel to the main foliation, which implies that it predates or was synchronous with the main ductile deformation; a younger phase cross-cuts the main foliation. Preliminary insitu U-Th-Pb monazite ages from migmatitic paragneiss in the region shows that structurally higher specimens record monazite growth phases during the early and middle Miocene, while a structurally lower specimen yields only late Miocene ages. The relationships between metamorphism, crustal melting and monazite growth and resorption are critical to evaluating if the models proposed for the evolution of the Himalaya are applicable in east-central Nepal.