FIELD RELATIONSHIPS AND GEOCHEMISTRY OF PRE-COLLISIONAL (INDIA-ASIA) GRANITOID MAGMATISM IN THE CENTRAL KARAKORAM, NORTHERN PAKISTAN

In the central Karakoram Range, northern Pakistan, seven major granitic units which were emplaced prior to the Eocene (ca. 50 Ma) India-Asia collision have been mapped and analysed geochemically. The mid-Cretaceous Hunza plutonic complex dominates the Karakoram batholith in the west and is composed of quartz diorite-granodiorite plutons which have been deformed in the south by later collision-related thrusting. The Sost and Khunjerab plutons intrude the Palaeozoic-early Mesozoic sediments north of the batholith. In the east pre-collision magmatic units occur both north of (K2 gneiss, Broad Peak quartz diorite, Muztagh Tower unit) and south of (Hushe gneiss) the batholith which is dominated by a Miocene monzogranite-leucogranite pluton (Baltoro unit). The plutonic units to the north of the batholith are mid- to late Cretaceous with an age range of ca. 115-80 Ma whereas the Hushe gneiss to the south is dominantly Jurassic spanning the age range 200-145 Ma. All the pre-collision magmatic units display similar chemical and isotopic characteristics. Major, trace and REE variations in the Hunza plutonic complex are controlled by high-level fractionation of clinopyroxene, hornblende, plagio-clase, biotite, ilmenite and allanite. High LILE/HFSE and LREE/HREE ratios, together with negative Nb, P and Ti anomalies suggest an ultimate source in the mantle wedge above the subducting slab. Sr-87/Sr-86 initial ratios (0.7055-0.7157) in the Hunza plutonic complex can best be explained by contamination of a mantle-derived magma by a crustal component with highly radiogenic Sr. All the units except the Hushe gneiss are probably related to subduction during closure of a back-arc basin between the Karakoram to the north and the Kohistan-Dras arc to the south during the mid- to late Cretaceous, and correlate with widespread plutonism from the same period in Kohistan, Ladakh and southern Tibet along the southern margin of Asia. The Hushe gneiss is probably related to an earlier northward-directed subduction phase during the Jurassic.