Structural and metamorphic evolution of the Haimanta Group: Insights from P-T-t-D path modeling of rocks near the Leo Pargil dome, NW India

The metasedimentary rocks of the Haimanta Group in the upper Sutlej Valley, NW India, record an extensive history of Himalayan deformation and metamorphism. Situated beneath the low-grade metasediments of the Tethyan Sedimentary sequence in the hanging wall of the South Tibetan detachment system (STDS), these poly-deformed rocks record Eocene–Oligocene burial metamorphism, crustal thickening and shortening, decompression and mid-crustal melting in the early Miocene, and exhumation along extensional shear zones related to the orogen-parallel exhumation of the Leo Pargil dome (LPD). Low-pressure assemblages (cordierite + sillimanite + quartz) overgrow Barrovian porphyroblasts in Haimanta Group rocks in the NE portion of Sutlej Valley. Detailed petrography on samples collected near the dome indicates a gradient in the intensity of the low-pressure overprint in the Haimanta Group that increases with proximity to the LPD, suggesting that exhumation of this portion of the Haimanta Group was coupled with exhumation of the dome. Quantitative thermobarometry and pressure-temperature-time-deformation (P-T-t-D)­­ path modeling suggests that Haimanta Group schists proximal to the LPD record near-isothermal decompression from peak conditions similar to published thermobarometric estimates for the LPD. Combined with previously reported age limits for peak burial, these results indicate that this portion of the Haimanta Group was initially exhumed from mid-crustal depths at a rate similar to the rocks to the SW that do not show a low-pressure overprint on Barrovian assemblages, although their respective retrograde P-T paths show markedly different trajectories. The results indicate that the transition from moderately sloped to steep retrograde P-T­ paths within the Haimanta Group is gradational and does not coincide with a tectonic boundary, and orogen-parallel extension along the Leo Pargil shear zone likely contributed to the differences in the P-T histories of these rocks. I present new U-Th-Pb geochronologic results of monazite that place maximum age limits on the development of the crenulation cleavage (S₂) at ~31 Ma and the development of low-pressure overgrowths on Barrovian porphyroblasts at ~27 Ma. The ­P-T-t-D analyses of the Haimanta Group schists may yield implications for orogen-parallel dome exhumation and the evolution of the hanging wall rocks of the STDS in this portion of the Himalaya.