The tectonic evolution of the Kohistan‐Karakoram collision belt along the Karakoram Highway transect, north Pakistan

Abstract

The Kohistan arc terrane comprises an intra‐oceanic island arc of Cretaceous age separating the Indian plate to the south from the Karakoram (Asian) plate to the north within the Indus suture zone of north Pakistan. The intra‐oceanic arc volcanics (Chalt, Dras Group) were built on a foundation of dominantly mid‐ocean ridge basalt (MORB)‐related amphibolites of the Kamila Group. The subarc magma chamber is represented by multiple intrusions of a huge gabbro‐norite complex (Chilas complex), which includes some ultramafic assemblages of residual mantle harzburgite and dunite, layered cumulates, and hornblendites cut by late stage dikes of hornblende + plagioclase pegmatites. The Chilas complex norites intrude the Gilgit metasediments of lower amphibolite and greenschist facies in northern Kohistan, which also form xenolithic roof pendants within the top of the Chilas complex. Along the southern margin of Kohistan, Jijal and Sapat complex ultramafics (dunites, harzburgites and websterites) form remnant suprasubduction zone ophiolitic mantle rocks along the hanging wall of the Main Mantle Thrust, the Cretaceous obduction plane along which Kohistan was emplaced onto Indian plate rocks. Garnet granulites of the Jijal complex, formed at 12–14 kbars, represent original magmatic lower crustal rocks subducted to depths of at least 45 km and metamorphosed during high‐pressure and high‐temperature subduction of earlier arc‐related rocks. Obduction of the Sapat ophiolite and Kohistan arc occurred between ∼75 and 55 Ma.The closure of the Shyok suture zone separating Kohistan from the Karakoram plate must have occurred prior to 75 Ma, the age of the Jutal basic dikes which crosscut the closure‐related fabrics, mainly late north directed backthrusting in the lower Hunza valley. Andean‐type granitoid (gabbrodiorite‐granodiorite‐granite) emplacement along the Kohistan‐Ladakh batholith ended at the time of India‐Asia collision, ∼ 60–50 Myr ago. Postcollisional crustal thickening along the Karakoram led to multiple episodes of metamorphism from latest Cretaceous and throughout the Tertiary. Sillimanite grade metamorphism in Hunza was actually pre‐India‐Asia collision and may have resulted from the earlier Kohistan collision. Localized and sporadic crustal melting episodes across northern Kohistan (Indus confluence and Parri granite sheets) and the southern Karakoram (Hunza dikes and Sumayar and Mango Gusar leucogranites) occurred from 51 to 9 Ma and culminated in the huge Baltoro monzogranite‐leucogranite intrusion 25–21 Myr ago. A vast network of leucogranitic and pegmatitite dikes containing gem quality aquamarine + muscovite ± tourmaline ± garnet ± biotite quartz are younger than 5 Ma and form the final phase of intrusion in the Haramosh area and parts of the southern Karakoram area.