Rb [sbnd]Sr dating of the Kohistan arc-batholith in the Trans-Himalaya of north Pakistan, and tectonic implications

Abstract

The Kohistan arc-batholith in northern Pakistan is situated between the Indus Suture and the Northern Suture. It belongs to the Trans-Himalayan belt which continues eastwards as the Ladakh arc-batholith in northwest India and the Kangdese batholith in southern Tibet. In Kohistan the island arc consists (upward sequence) of the Chilas stratiform complex of norites, noritic gabbros and chromite-layered dunites which formed in the sub-arc magma chamber, plutons of tonalite and diorite, one of which has a Rb Sr isochron age of 102 ± 12 Ma, the Chalt volcanics of basaltic tholeiites succeeded by andesites to rhyolites, and the Yasin Group sediments which formed in overlying intra-arc basins and which contain Albian-Aptian faunas. All these rocks were deformed by major fold structures which are correlated with the formation of the Northern Suture. NE-SW trending basic Jutal-Nomal dykes (north of Gilgit) cross-cut all the above structures and have a 39Ar/ 40Ar hornblende age of 75 Ma. Thus the Northern Suture must have formed in the period 102 ± 12 to 75 Ma. With the island arc attached to the Eurasian plate, further northward subduction of the Tethyan plate gave rise to an Andean-type batholith, two plutons of which have Rb Sr isochron ages of 54 ± 4 Ma and 40 ± 6 Ma. The Dir-Utror Group of calc-alkaline lavas (and sediments with Eocene fossils) are remnants of the volcanic cover of the Andean-type batholith. We suggest that late Cretaceous blueschists formed during subduction under the active continental margin, and that continental collision and formation of the Indus Suture was in the Eocene. The batholith was intruded by layered aplite-pegmatite sheets at 34 ± 14 Ma and 29 ± 8 Ma (Rb Sr ages) in post-collisional times. The 87Sr/ 86Sr initial ratios of all the dated rocks range between 0.7039 and 0.7052.