Oligocene initiation of the South Tibetan Detachment System: Constraints from syn-tectonic leucogranites in the Kampa Dome, Northern Himalaya

Abstract

© 2019 Elsevier B.V. Himalayan leucogranites provide a robust record of the thermal and tectonic processes that occurred during convergence of the Eurasian and Indian tectonic plates. We present structural, petrological, mineralogical, whole-rock geochemical, Sr[sbnd]Nd isotope, and geochronological results for Cenozoic leucogranites from the Kampa Dome, Northern Himalaya. The Kampa leucogranites are typical syn-tectonic granites and occur as a series of sills, dikes, and puddings in the Kampa Shear Zone, which is considered to be the exhumed portion of the South Tibetan Detachment System (STDS) in the Northern Himalaya. The Kampa syn-tectonic leucogranites provide information about the timing and nature of regional kinematics, and record thermal conditions and the initiation of the STDS. Whole-rock geochemistry and Sr[sbnd]Nd isotope analyses show that the Kampa leucogranites are characterized by the following: (1) they are peraluminous and have high SiO2, Al2O3, and A/CNK, low TiO2, MgO, P2O5, and TFe2O3, and variable CaO, Na2O, K2O, and total alkalis; (2) they are enriched in large-ion lithophile elements (e.g., K and Rb) and depleted in high-field-strength elements (e.g., Zr and Ti), with negative Eu anomalies; and (3) they display variable initial 87Sr/86Sr compositions (0.750758-0.826065), and their εNd(i) values range between −7.6 and −12.2. Anatexis of graywackes and metapelites in the Great Himalayan Crystalline Series played an important role in the formation of the Kampa leucogranites. Anatexis was controlled by fluid-absent partial melting of muscovite. Mineralogical and petrological analyses show that zircons belong to an early crystallization phase, whereas monazites are remnants of the final crystallization phase of the granitic melt. Zircon U[sbnd]Pb dating reveals that anatexis in the deep Himalayan crust occurred at ca. 32–23 Ma, during the Oligocene. Monazites yield homogeneous 208Pb/232Th ages of ca. 23.5 Ma and record rapid emplacement in the Kampa Shear Zone. These results indicate that movement on the STDS was initiated during the early Oligocene and became well-developed during the late Oligocene in the Kampa region. The Oligocene was a unique period of counterbalance between continued plate convergence and intense extension in the Himalayan Orogen.