CRUSTAL THICKENING OF THE LOWER CRUST OF THE KOHISTAN ARC (N. PAKISTAN) DEDUCED FROM AL ZONING IN CLINOPYROXENE AND PLAGIOCLASE

The lower-crustal rocks of the Kohistan complex (northern Pakistan) ar e mostly composed of metabasic rocks such as pyroxene granulites, garn et granulites and amphibolites. We have investigated P-T trajectories of the relic two-pyroxene granulites, which are the protolith of the a mphibolites within the Kamila amphibolite belt. Aluminous pyroxene ret ains igneous textures such as exsolution lamellae developed in the cor e. The significant amount of Al in clinopyroxene is buffered by breakd own reactions of plagioclase accompanied by film-like quartz as a prod uct at grain boundaries between plagioclase and clinopyroxene. Distinc t Al zoning profiles are preserved in pyroxene with exsolution lamella e in the core and in plagioclase adjacent to clinopyroxene in pyroxene granulites. In the northern part of the Kamila amphibolite belt, Al i n clinopyroxene increases towards the rim and abruptly decreases at th e outer rim, and anorthite in plagioclase decreases towards the rim an d abruptly increases near the grain boundary between plagioclase and c linopyroxene. In the southern part of the Kamila amphibolite belt, Al in clinopyroxene and anorthite in plagioclase simply increase towards the margins of the grains. The anorthite zoning in plagioclase is in a greement with the zoning profiles of Ca-Tschermaks and jadeite compone nts inferred from variations of Al, Na, Ti and Fe3+ in clinopyroxene. Assuming that the growth surface between them was in equilibrium, geot hermobarometry based on Al zoning in clinopyroxene coexisting with pla gioclase indicates that metamorphic pressures significantly increased with increasing temperature under granulite facies metamorphism. The p eak of granulite facies metamorphism occurred at conditions of about 8 00 degrees C and 80-1100 MPa. These prograde P-T paths represent a cru stal thickening process of the Kohistan are during the Early to Middle Cretaceous. The crustal thickening of the Kohistan are was caused by accretion of basaltic magma at mid-crustal depths.