Tectonic significance of 24 Ma crustal melting in the eastern Hindu Kush, Pakistan

Abstract

Northward-directed subduction has continued beneath the Hindu Kush from the Mesozoic, when the region formed part of the southern margin of Asia, until today. The Kohistan arc was accreted at ca. 100 Ma, followed by the Indian plate at 54–50 Ma. After the India-Asia collision, a regional crustal-melting event occurred at ca. 25–21 Ma to the east of the Hindu Kush in the Karakoram Range, producing the Baltoro pluton. In the eastern Hindu Kush Range, most of the granitoids appear to be subduction-related plutons that predate the India-Asia collision. However, the Gharam Chasma pluton is a two-mica (± garnet ± tourmaline) leucogranite that has intruded into staurolite-grade schists and (sillimanite ± K-feldspar)–grade migmatites. U-Pb ages on monazite, xenotime, and uraninite from undeformed samples of the Gharam Chasma pluton and a leucogranite dike that crosscuts the migmatites indicate that crustal melting occurred at ca. 24 Ma, synchronous with the Baltoro melting event. This age also provides an upper limit on the age of the regional staurolite-grade metamorphism in the eastern Hindu Kush.