DEFORMATION AND MELT CRYSTALLIZATION IN LESSER HIMALAYAN SEQUENCE ROCKS AT 7 MA REVEALED BY U-TH/PB ZIRCON GEOCHRONOLOGY AND IN SITU MONAZITE PETROCHRONOLOGY

Prograde metamorphism induced by the India–Asian collision is recorded in the metamorphic core of the Himalaya in western Nepal as early as 48 Ma. The Main Central Thrust zone (MCTZ), one of the major shear zones that accommodated this convergence, records deformation beginning around 25 Ma and decreasing in age with structural depth as a result of accretion of Lesser Himalayan sequence (LHS) rocks from the footwall. The MCTZ therefore contains metamorphosed rocks from the base of the metamorphic core and the top of the LHS imbricate stack beneath it. To test for a structurally downward younging of deformation, samples were collected across a continuous exposure of the MCTZ in the Karnali valley of western Nepal. In situ U-Th/Pb petrochronology on monazite shows a progressive decreasing age of deformation from 18 Ma in the middle of the metamorphic core to 10 Ma within the MCTZ. ⁴⁰Ar/³⁹Ar thermochronology on muscovite indicates cooling below 450°C by 6 Ma. At the base of the MCTZ, an asymmetrically-boudinaged kyanite-bearing leucocratic pod is hosted in a Grt + Bt + Chl schist with a top-to-the-south C-S fabric. U-Pb geochronology on zircon from the leucocratic pod yields ~1.8 Ga Proterozoic cores, interpreted as inherited detrital grains consistent with a LHS source, and zircon rims as young as ~7 Ma, interpreted as neo-crystallized igneous zircon constraining top-to-the-south shearing to <7 Ma. The MCTZ of the Karnali section underwent partial melting and south-directed ductile shearing as recently as 7 Ma, which is later than in the MCTZ exposed in the external crystalline klippen to the south. Such young shearing implies out-of-sequence thrusting along a structure that has been inactive for 8-10 m.y. in the foreland. Partial melting at such low structural levels is anomalous across the Himalaya; our results indicate that the LHS imbricate stack must have been buried to considerable depth to instigate partial melting in late Miocene time, prior to rapid exhumation and cooling by 6 Ma.