EPISODIC UNROOFING OF THE KOHISTAN BATHOLITH, PAKISTAN - IMPLICATIONSFROM K-FELDSPAR THERMOCHRONOLOGY

New Ar-40/Ar-39 and U-Pb mineral ages from plutonic rocks help constra in the thermal and tectonic evolution of the Kohistan island are terra ne following its collision with India in early Tertiary time. Kohistan has experienced a prolonged tectonomagmatic history extending from th e Early Cretaceous through the Tertiary. Thermal histories derived fro m multi diffusion domain analyses of K-feldspar within the Kohistan ba tholith reveal rapid cooling events (70-140 degrees C/m.y.) distribute d through space and time. The cooling histories show a systematic vari ation along the length of the batholith suggesting that Kohistan exper ienced differential unroofing. An episode of rapid cooling in the midd le Eocene, is recognized in western Kohistan, whereas rapid cooling oc curs substantially later, during the middle and late Miocene, in easte rn Kohistan. Rapid cooling in western Kohistan might have been caused by postemplacement cooling of hot magma against cold country rocks at relatively shallow crustal levels. Within the NW region of eastern Koh istan, rapid cooling at 13-12 Ma is recorded in K-feldspars similar to 60 km from the contact with the Nanga Parbat-Haramosh Massif (NPHM), whereas cooling does not occur until 11 Ma only 2.5 km from the contac t. This temporal and spatial variation in cooling histories may record differential unroofing in response to the development and propagation of the NPHM structure beneath Kohistan. The NPHM has experienced rapi d cooling and unroofing over the last 10 m.y., and our results are con sistent with this mid-late Miocene event.