Petrography and Geochemistry of the Kohistan Batholith Granitoids from the Central Kohistan Magmatic Arc, Northern Swat, Pakistan

The E-W trending Kohistan magmatic arc developed as an intra-oceanic island arc during the Cretaceous and became an Andean-type margin during Early Tertiary after collision with Karakoram plate. The Kohistan batholith constitutes the major unit of the arc and comprises a range of granitoids, and minor gabbro-diorite and metamorphosed volcanic and sedimentary rocks. This research focuses on the petrologic evolution of the rocks of plutonic association from west central part of the batholith in northernmost Swat to elucidate their geodynamic setting and relationship to crustal growth processes over time. The area was geologically and structurally remapped to obtain new field, petrographic and whole rock major and trace element data.

The studied area contains stock-size to batholithic bodies of granitoids, subordinate migmatites, stocks and plugs of gabbronorite, doleritic dykes and a small intrusion of a diorite containing spectacular orbicules. The granitoids show textural and mineralogical variation, with some being strongly gneissose and or porphyritic. Amphibole and biotite are common to most rocks. Based on petrographic and geochemical data, the investigated granitoids cover a range of compositions, i.e., granite, granodiorite, tonalite, quartz monzonite, and (quartz) diorite which stretches for 70 km up to eastern Afghanistan. They are calc alkaline, metaluminous to peraluminous, calcic to alkalic, and dominantly magnesian in composition, except a few more fractionated granites which fall in the ferroan field. Our new data suggests, that the different granitoids were emplaced as independent plutons over a considerable length of time.