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

Abstract

Prior to accretion of the Kohistan island arc during the Late Cretaceous and 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 Hindu Kush range in Pakistan demonstrate that localized crustal melting and leucogranite intrusion took place in the Gharam Chasma area at ca. 24 Ma. More regionally developed and widespread deformation, metamorphism, and magmatism took place before collision between both India and the Kohistan island arc with Asia. Ca. 195 Ma U-Pb monazite ages on a deformed leucogranite dike from the upper Lutkho valley indicate an Early Jurassic phase of crustal melting. U-Pb monazite ages of 135–126 Ma on a staurolite schist from near Gharam Chasma are interpreted as a minimum age for staurolite-grade metamorphism. Within the Tirich Mir fault zone, pegmatite dikes crosscut the staurolite schists. U-Pb dating of uraninites from one of these pegmatite dikes 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 Jurassic–Early Cretaceous constraints on metamorphism and magmatism relate to subduction and accretion processes, perhaps including the suturing of the Karakoram and Hindu Kush terranes along the Tirich Mir fault. In general, high-temperature, low-pressure metamorphism and subduction-related granitoid magmatism in the eastern Hindu Kush suggest a high thermal gradient in an active-margin setting from Early Jurassic to Cretaceous time.