Protolith affiliation and tectonometamorphic evolution of the Gurla Mandhata core complex, NW Nepal Himalaya

Abstract Assigning correct protolith to high metamorphic-grade core zone rocks of large hot orogens is a particularly important challenge overcome when attempting constrain the early stages orogenic evolution and paleogeography lithotectonic units from these orogens. The Gurla Mandhata complex in NW Nepal exposes Himalayan metamorphic (HMC), sequence gneiss, migmatite, granite, hinterland orogen. Sm-Nd isotopic analyses indicate that HMC comprises Greater (GHS) Lesser (LHS) rocks. Conventional interpretation such provenance data would require Main Central thrust (MCT) be also outcropping within complex. However, new situ U-Th/Pb monazite petrochronology coupled with petrographic, structural, microstructural observations reveal composed solely hanging wall MCT. Rocks record Eocene late Oligocene Miocene (re-)crystallization periods (monazite age peaks 40 Ma, 25–19 19–16 Ma) overprinting pre-Himalayan Ordovician Bhimphedian metamorphism magmatism (ca. 470 Ma). combination analysis demonstrates both GHS LHS were captured MCT experienced Cenozoic during south-directed extrusion. Monazite ages do not coeval extensional exhumation, suggesting peak generation anatectic melt had ceased prior onset orogen-parallel extension at ca. 15–13 Ma. geometry requires significant crustal thickening 16 which attributed ductile footwall accretion associated ramp below We demonstrate signatures as should used characterize rock structures across Himalaya only conjunction supporting petrochronological structural data.