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

The Kohistan are terrane comprises an intra oceanic island are of Cretaceou s age separating the Indian plate to the south from the Karakoram (Asian) p late to the north within the Indus suture zone of north Pakistan. The: intr a-oceanic are volcanics (Chalt, Dras Group) were built on a foundation of d ominantly mid-ocean ridge basalt (MORB)related amphibolites of the Kamila G roup. The subarc magma chamber is represented by multiple intrusions of a h uge gabbro-norite complex (Chilas complex), which includes some ultramafic assemblages of residual mantle harzburgite and dunite, layered cumulates, a nd hornblendites cut by late stage dikes of hornblende + plagioclase pegmat ites. The Chilas complex norites intrude the Gilgit metasediments of lower amphibolite and greenschist facies in northern Kohistan, which also form xe nolithic roof pendants within the top of the Chilas complex. Along the sout hern margin of Kohistan, Jijal and Sapat complex ultramafics (dunites, harz burgites and websterites) form remnant suprasubduction zone ophiolitic mant le rocks along the hanging wall of the Main Mantle Thrust, the Cretaceous o bduction plane along which Kohistan was emplaced onto Indian plate rocks. G arnet granulites of the Jijal complex, formed at 12-14 kbars, represent ori ginal magmatic lower crustal rocks subducted to depths of at least 45 km an d metamorphosed during high-pressure and high-temperature subduction of ear lier are; related rocks. Obduction of the Sapat ophiolite and Kohistan are occurred between similar to 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 bac kthrusting 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, similar to 60-50 Myr ago. Postcollisi onal crustal thickening along the Karakoram led to multiple episodes of met amorphism from latest Cretaceous and throughout the Tertiary. Sillimanite g rade metamorphism in Hunza was actually pre-India-Asia collision and may ha ve resulted from the earlier Kohistan collision. Localized and sporadic cru stal melting episodes across northern Kohistan (Indus confluence and Parri granite sheets) and the southern Karakoram (Hunza dikes and Sumayar and Man go 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 M a and form the final phase of intrusion in the Haramosh area and parts of t he southern Karakoram area.