CONSTRAINING THE METAMORPHIC AND KINEMATIC EVOLUTION OF THE LEO PARGIL DOME, NORTHWEST INDIAN HIMALAYA, THROUGH INTEGRATED PRESSURE-TEMPERATURE-TIME-DEFORMATION DATA

The Leo Pargil dome, northwest India, is a 30 km-wide, northeast trending structure cored by migmatite and mantled by amphibolite-facies metamorphic rocks and leucogranite that is bound by oppositely dipping, normal-sense shear zones. The Leo Pargil shear zone on the southwest side of the dome is a broadly distributed shear zone that records top-down-to-the-west displacement throughout the dome. The rocks within the dome are intruded by a leucogranite injection complex. Estimates of deformation temperatures from quartz microstructures and electron backscatter diffraction (EBSD) analyses on rocks from the shear zone were integrated with pressure-temperature estimates and monazite U-Th-Pb ages to evaluate the metamorphic and kinematic evolution of the southwest flank of the dome.

Rocks within the dome contain garnet, kyanite, and staurolite porphyroblasts that record prograde Barrovian metamorphism during crustal thickening that reached ~530-630 °C and ~7-8 kbar, ending by ~30 Ma. Cordierite and sillimanite overgrowths on Barrovian assemblages within the dome record dominantly top-down-to-the-west shearing during near-isothermal decompression of the footwall rocks to ~4 kbar by 23 Ma. Quartz microstructures combined with EBSD analyses show that top-down-to-the-west ductile deformation is recorded at temperatures from >650 °C at the deepest levels within the dome and from 400-500 °C through 400-280 °C at the shallowest structural levels within the dome. These data suggest that the rocks within the shear zone were exhumed from mid-crustal depths by west-directed deformation following Barrovian metamorphism.

Contemporaneous onset of near-isothermal decompression and top-down-to-the-west shearing following Barrovian metamorphism, combined with ages from leucogranites within the dome from other studies, suggests that early Barrovian metamorphism and crustal melting may have created a weakened crust that was proceeded by localization of strain and shear zone development. Exhumation along the shear zone accommodated decompression by 23 Ma in a kinematic setting that favored orogen-parallel extension.