A REEVALUATION OF THE STRATIGRAPHY AND EVOLUTION OF THE KOHISTAN ARC SEQUENCE, PAKISTAN HIMALAYA - IMPLICATIONS FOR MAGMATIC AND TECTONIC ARC-BUILDING PROCESSES

New field mapping and structural data, combined with published geochem ical data, from the Kohistan are in the NW Himalaya, enable a re-evalu ation of the are stratigraphy. Key lithological units and their relati onships are more clearly defined, permitting the construction of a rev ised magmatic-tectonic history for the are. The oldest units are trans itional oceanic-type basalts, which form the basement to the subductio n related sequence. Arc-type gabbroic sheets and plutons intrude the o ceanic basalts; together these form the Kamila Amphibolite Belt. Metas ediments and basaltic lavas were deposited, within an extensional basi n, onto the Kamila Amphibolite Belt basement. This sequence, exposed a cross the are. forms a distinct stratigraphic unit which is formally d efined here as the Jaglot Group. Sediment-charged turbidiry currents t ransported material into the basin, whilst submarine eruptions contrib uted the basaltic component. This period of extension culminated in th e eruption of high-Mg boninites of the Chalt Volcanic Group which over lie the rocks of the Jaglot Group. The earliest granitoids of the Kohi stan Batholith predate suturing and intrude the Jaglot and Chalt seque nces. At c. 100 Ma Kohistan sutured to Asia, suturing being accompanie d by thickening of the are with the development of major intra-arc she ar zones and a penetrative, regionally developed steep cleavage. At c. 85 Ma intra-arc rifting permitted the emplacement into the are of the voluminous gabbronorites of the Chilas Complex which clearly intrudes the Kamila Amphibolite Belt to the south and the Jaglot Group to the north. The Chilas Complex has been regarded as part of the pre-suturin g, juvenile are sequence. Field evidence summarized here show this to be not so. Heat advection associated with emplacement of the Complex c aused amphibolite facies regional metamorphism. melting of the lower a re and plutonism. Some of the resultant granitoid plutons were unroofe d and eroded during a compressional phase at between 80 and 55 Ma, bef ore emplacement of further plutons and extrusion of basaltic through t o rhyolitic volcanic rocks at between 55 and 40 Ma. At least three pha ses of extension and rifting, each separated by short lived phases of compression, characterized are evolution. Much of the magmatism is con trolled by extensional tectonics within the overriding plate of the ki nd commonly associated with a retreating subduction zone.