Old origin for an active mountain range: Geology and geochronology of the eastern Hindu Kush, Pakistan

Prior to accretion of the Kohistan island are during the Late Cretaceous an d final suturing of India with Asia at ca, 50 Ma, the Hindu Kush mountains along the border of Afghanistan and northwest Pakistan were situated on the active southern margin of Asia. Geology and geochronology of the eastern H indu Kush range in Pakistan demonstrate that localized crustal melting and leucogranite intrusion took place in the Gharam Chasma area at ca, 24 Ma. M ore regionally developed and widespread deformation, metamorphism, and magm atism took place before collision between both India and the Kohistan islan d are with Asia. Ca. 195 Ma U-Pb monazite ages on a deformed leucogranite d ike from the upper Lutkho valley indicate an Early Jurassic phase of crusta l melting. U-Pb monazite ages of 135-126 Ma on a staurolite schist from nea r Gharam Chasma are interpreted as a minimum age for staurolite-grade metam orphism. Within the Tirich Mir fault zone, pegmatite dikes crosscut the sta urolite schists. U-Pb dating of uraninites from one of these pegmatite dike s reveals an age of 114 +/- 2 Ma. Monazites from the same rock give ages of 125-121 Ma, possibly due to inheritance of older cores. These Jurassie-Ear ly Cretaceous constraints on meta-morphism and magmatism relate to subducti on and accretion processes, perhaps including the suturing of the Karakoram and Hindu Kush terranes along the Tirich Mir fault. In general, high-tempe rature, low-pressure metamorphism and subduction-related granitoid magmatis m in the eastern Hindu Kush suggest a high thermal gradient in an active-ma rgin setting from Early Jurassic to Cretaceous time.