BACK-ARC BASIN ASSEMBLAGES IN KOHISTAN, NORTHERN PAKISTAN

The east central part of the Kohistan magmatic are is made up principa lly of the Jaglot Group. From bottom to top it consists of I) paragnei sses and schists intercalated with amphibolites and calc-silicates (Gi lgit Formation), II) Gashu-Confluence Volcanics (GCV) and III) the The lichi Formation comprising a volcanic base (Majne volcanics) and turbi dites, marble, volcanoclastic sediments and lava flows. Metamorphic gr ade varies up to the sillimanite zone. The GCV are correlated with the Chalt volcanics and the Thelichi Formation with the Yasin Group. Othe r lithologies include the Chilas Complex, the Kohistan Batholith and p art of the Kamila Amphibolite. Metavolcanics show a broad range in che mical composition. Geochemical parameters used to specify the tectonom agmatic regime suggest affinities of both island are and MORE-like bac k-are basin basalts. Kohistan can be divided into three tectonic zones , I) the southern (Kamila) zone comprises amphibolitized basalts, and mafic and ultramafic rocks, II) the central Chilas Complex, and III) t he northern (Gilgit) zone i.e., the Jaglot Group. Previous tectonic mo dels considered the southern two zones as the crust of a Cretaceous is land are. This investigation concludes that only the southern zone rep resents a true island arc. The Jaglot Group derives from back-arc basi n assemblages and the Chilas Complex is a basin assemblages and the Ch ilas Complex is a magmatic diapir emplaced in the back-are basin.