Protolith Affiliation and Tectonometamorphic Evolution of the Gurla Mandhata Core Complex, NW Nepal Himalaya

Abstract
Migmatite-cored gneiss domes in the Himalaya are valuable structures that expose the mid-crust, allowing for the investigation of tectonic and structural processes governing the evolution of the Himalayan orogen. Located in NW Nepal, the Gurla Mandhata core complex exposes the Himalayan Metamorphic Core (HMC), a sequence of high metamorphic-grade gneiss, migmatite, and granite, in the hinterland of the Himalayan orogen. Sm-Nd isotopic analyses indicate that these rocks have both Greater Himalayan sequence (GHS) and Lesser Himalayan sequence (LHS) protolith affinity. In-situ U-Th/Pb monazite petrochronology coupled with petrographic, structural, and microstructural observations reveal that the core complex is composed solely of rocks in the hanging wall of the Main Central Thrust (MCT). Rocks from the core complex record Eocene Eohimalayan and late-Oligocene to early-Miocene Neohimalayan metamorphic pulses (U-Th/Pb monazite age peaks of 40 Ma, 25-19 Ma, and 19-16 Ma) along with a pre-Himalayan Ordovician pulse (ca. 470 Ma). The combination of Sm-Nd isotopic analysis and U-Th/Pb monazite petrochronology demonstrates that both GHS and LHS protolith material is captured in the hanging wall of the MCT and experienced Cenozoic Himalayan metamorphism during south-directed extrusion in west Nepal. Monazite ages do not record metamorphism coeval with late Miocene east-directed exhumation, demonstrating that peak metamorphism and generation of anatectic melt in the core complex had ceased prior to the onset of hinterland orogen-parallel extension at ~15-13 Ma. The geometry of the Gurla Mandhata core complex requires significant hinterland crustal thickening prior to 16 Ma, which is attributed to ductile HMC thickening and footwall accretion of LHS protolith material associated with a ramp-flat MHT geometry below the core complex. The Gurla Mandhata core complex shares many structural and tectonometamorphic characteristics with the North Himalayan domes. This thesis demonstrates that isotopic signatures such as Sm-Nd should only be used to characterize lithotectonic units and structural features across the Himalaya in conjunction with supporting petrochronological and structural data. Further, this thesis highlights the necessity for clear definitions of Himalayan metamorphic rocks that can differentiate between their protolith and tectonometamorphic characteristics.
External DOI