Evolution and chronology of the Pangong Metamorphic Complex adjacent to the Karakoram Fault, Ladakh: constraints from thermobarometry, metamorphic modelling and U-Pb geochronology

摘要

Sillimanite- and staurolite-grade metamorphic rocks exhumed along the Pangong fault, the NE branch of the right-lateral Karakoram strike-slip fault in northern Ladakh, NW India, show multiple episodes of metamorphism and fabric development. Debate has centred on whether these metamorphic rocks were formed as a result of shear heating during strike-slip faulting, or whether they are exhumed earlier metamorphic rocks unrelated to movement on the Karakoram fault. Here we constrain the burial and exhumation history of the Pangong Metamorphic Complex combining the pressure-temperature evolution with accessory phase geochronology. Sillimanite-grade metamorphism in graphitic pelites was superseded by the preserved P-T conditions of a Bt + Ms + St + Grt + Qtz + Fsp assemblage at 585-605°C and 6.05-7.25 kbar, equivalent to c. 20-25 km of burial. Laser ablation monazite U-Pb geochronology reveals that sillimanite-grade metamorphism occurred at 108.0 ± 0.6 Ma in rocks immediately adjacent to the Pangong strand of the Karakoram fault, implying that most metamorphic rocks along the Karakoram fault were not formed by shear heating during Miocene strike-slip faulting. This age correlates closely with the ages of the Hunza granite-granodiorite batholith, and the K2 orthogneiss in northern Pakistan, and confirms that some high-grade metamorphism occurred before collision and accretion of the Kohistan arc and the Indian plate to Asia; protracted high-grade metamorphism, and accompanying crustal thickening lasted at least 100 Ma along the South Asian plate margin. Our P-T and geochronology results also demonstrate the continuity of Cretaceous metamorphism across the Karakoram fault. © 2009 Geological Society of London.