STRUCTURAL AND THERMAL EVOLUTION OF THE SOUTH ASIAN CONTINENTAL MARGIN ALONG THE KARAKORAM AND HINDU KUSH RANGES,NORTH PAKISTAN

摘要

Prior to the collision and accretion of the Kohistan arc terrane during the late Cretaceous and the Indian plate after the early Eocene, the southern margin of Asia along the Hindu Kush, Karakoram and Lhasa block terranes was an active Andean|type continental margin. In south Tibet this margin was dominated by the calc|alkaline Ladakh—Gangdese granite batholith, associated andesitic volcanic rocks and continental red|beds. In contrast, the southern Karakoram exposes deep crustal metamorphic rocks and crustal melt leucogranites. New U|Pb age dating from the Hunza valley and Baltoro glacier region has revealed four spatially and temporally distinct metamorphic episodes. M1 sillimanite grade metamorphism in Hunza was a late Cretaceous event, probably caused by the accretion of the Kohistan arc to Asia. M2 was the major kyanite and sillimanite grade event during late Eocene—Oligocene crustal thickening and shortening, following India|Asia collision. Numerous melting events resulted in the formation of crustal melt granites throughout the last 50Ma with multiple generations of dykes and very large scale crustal melting along the Baltoro monzogranite|leucogranite ba tholith during the late Oligocene—early Miocene. M3 metamorphism was a high| T , low| p contact thermal metamorphism around the Baltoro granite. In Hunza, younger staurolite grade metamorphism has been dated by U|Pb monazites at 16Ma, with the Sumayar leucogranite intruded at 9 5Ma cross|cutting the metamorphic isograds. In the Baltoro region the youngest metamorphism, M4, is the sillimanite grade Dassu gneiss core complex dated by U|Pb on monazites as late Miocene—Pliocene (5 4±0 25)Ma with Precambrian protolith zircon cores (1855±11)Ma. Numerous gem|bearing pegmatite dykes cross|cut these rocks and are thought to have been intruded within the last 2～3Ma. Structural mapping, combined with U|Pb geochronology shows that major metamorphic events can be both long|lasting (up to 20Ma) and very restrictive, both in time and space.