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

Abstract

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