Modification of fabric in pre-Himalayan granitic rocks by post-emplacement ductile deformation: insights from microstructures, AMS, and U-Pb geochronology of the Paleozoic Kinnaur Kailash Granite and associated Cenozoic leucogranites of the South Tibetan Detachment zone, Himachal High Himalaya

The present day South Tibetan Detachment (STD) of Higher Himalaya is a system of low-angle normal faults. In the Himachal High Himalaya, the STD hanging wall is characterized by the presence of S-type per-aluminous Paleozoic (similar to 475 Ma) granite called the Kinnaur Kailash Granite (KKG). This granite is later intruded by Cenozoic leucogranites (similar to 18 Ma) in vicinity of the STD zone. In this work, microstructures, anisotropy of magnetic susceptibility (AMS), and U-Pb geochronology were carried out on the KKG and the leucogranites with an aim to (a) understand the conditions of fabric development and (b) decipher the tectonic relationship between deformation along the STD and the evolution of these granites. Microstructural features and magnetic anisotropy indicate that the granites are intensely deformed in vicinity of the STD and preserve their emplacement-related fabric in the interior parts. It is inferred that close to the STD zone, fabrics of both the KKG and the leucogranite are tectonic and are modified by the Cenozoic (similar to 20 Ma) right-lateral slip and extensional tectonics. Magnetic fabric in the interior parts of the KKG is related to its emplacement indicating that original fabric was preserved. U-Pb geochronology of zircons from two samples of the KKG yields crystallization age of 477.6 +/- 3.4 and 472 +/- 4 Ma. The leucogranite gives a crystallization age of 18.5 +/- 0.6 Ma. Zircons from the KKG also reveal signatures of a deformation event (20.6 +/- 2.3 Ma) at its rim. It is inferred that deformation of the external rim of the KKG and crystallization of the leucogranites are synchronous and triggered by ductile deformation along the STD.