INSIGHTS INTO METAMORPHISM, CRUSTAL MELTING, AND EXHUMATION OF THE LEO PARGIL DOME, WESTERN HIMALAYA, FROM MONAZITE GEOCHRONOLOGY

Advances in monazite geochronology have created a tool that is increasingly valuable for constraining the timing of events during the evolution of orogens. We apply monazite geochronology to rocks from the Leo Pargil dome in NW India to constrain the relationship between metamorphism, crustal melting, and the onset of exhumation. The structural dome strikes northeast-southwest and is composed of amphibolite to granulite-facies metamorphic rocks and leucogranites. The dome is bound by oppositely dipping normal sense shear zones that accommodated exhumation of the dome during orogen-parallel extension in the convergent Himalaya. The distributed Leo Pargil shear zone defines the west flank of the dome and separates the footwall rocks from the overlying metasedimentary rocks in the hanging wall to the west. A leucogranite injection complex intrudes the amphibolites-facies metamorphic rocks. The leucogranite injection complex transitions into melt-present migmatites in the core of the dome. The pressure-temperature-time-deformation (P-T-t-D) path of the metamorphic rocks in the dome was constrained by combining P-T estimates with in-situ U-Th-Pb monazite geochronology from rocks within the dome and shear zone. Ages from monazite grains included within staurolite and kyanite porphyroblasts and grains in the matrix combined with P-T estimates demonstrate that rocks in the dome experienced prograde Barrovian metamorphism during crustal thickening until 30 Ma. These porphyroblasts are overprinted by a top-down-to-the-west to northwest fabric defined by sillimanite and cordierite, associated with exhumation of the dome during extension. Ages of syn-kinematic monazite grains combined with P-T estimates suggest that the top-down-to-the-west to northwest fabric occurred by 23 Ma, synchronous with near-isothermal decompression of the rocks within the dome. U-Th-Pb monazite ages from the leucogranite injection complex suggest that leucogranite injection occurred soon after initiation of exhumation and continued until 18 Ma. These data suggest that the onset of exhumation of the Leo Pargil dome induced decompression-melting in the core. Decompression-melting potentially enhanced exhumation of the dome.