The structure, petrology and geochemistry of the Kohistan batholith, Gilgit, Kashmir, N. Pakistan.

The Kohistan batholith is the most north-westerly part of the Trans-Himalayan batholith which extends in an arcuate linear belt from Lhasa in the east to Pakistan in the west. Detailed fieldwork of an area of 2500 square kms, centered on the town of Gilgit, has shown that the batholith can be sub-divided into two major units: (1) An early bi-modal sequence of gabbroic diorites and quartz-rich tonalites which have been deformed and folded, together with the meta-volcanic and meta-sedimentary country rocks around a major syncline (the Jaglot syncline). The early plutonics, which comprise 1/3 of the batholith, have a penetrative, often gneissose fabric which is orientated parallel to the major structural trends of the area. (2) A later sequence of basic dykes, gabbros, diorites, granodiorites, granites and leucogranitic, layered aplite-pegmatite sheets which cut the structures associated with the Jaglot syncline. There is a general basic-acid trend with time. Five rock units have yielded Rb-Sr whole-rock isochron ages. There are 102 + 12 Ma for an early, deformed, tonalite, 54 + 4 Ma and 40 + 6 Ma for two second stage granitoids and 34 + 14 Ma and 29 + 8 Ma for two late-aplite-pegmatite sheets. Even the least evolved gabbros are enriched in Rb, K, Ba, Sr, P and L.R.E.E. relative to Nb, Zr, Ti, Y and H.R.E.E. With fractionation L.F.S./H.F.S., K/Na and L.R.E.E./H.R.E.E. element ratios increase and the batholith displays a calc-alkaline trend with respect to Mg, Fe, Na and K. Geochemical and 87sr/86sr initial ratio data (87sr/6Sr initial ratios vary between 0.7039 and 0.7052) suggest that the ultimate source for the plutonics is the upper mantle. The main magmatic trend of the batholith can be explained by the fractionation of amphibole-plagioclase-magnetite - CPX in the basic-intermediate rocks and plagioclase-K-feldspar-biotite and magnetite in the acid rocks. Zircon, apatite and sphene were important accessory minerals. A limited amount of crustal melting has occurred. The early, deformed, plutonics were formed in an island arc environment, whereas the second stage of the batholith was formed in an Andean-type continental margin environment.