Isotopic constraints on the collision age from the Kohistan-Ladakh Arc crust

In the Himalayas, despite intensive research, the age of collision between India and Eurasia is still debated, with estimates ranging from 70 to 35 Ma. In the Western Himalaya, separated from the Eurasian plate to the north by the Shyok suture, and from the Indian plate to the south by the Indus suture, lies the Kohistan-Ladakh Arc (KLA). This geological entity is recognized as a fully preserved oceanic arc, formed in the Tethys Ocean and now incorporated in the Himalayan collision system. The age collision estimate between the Arc and Eurasia range from Mid-Creataceous to Oligocene, whereas the India-Arc collision ranges from Early Paleocene to Late-Eocene. The middle to upper crustal part of the KLA is made of plutonic rocks ranging from diorite to granites, together with Volcano-sedimentary units. The crust-forming granitoids records a magmatic activity between >120 and 30 Ma, covering the range of estimated India-Arc and Arc-Eurasia collisions. The limited available data in the literature for the KLA indicates that Jurassic/Cretaceous rocks are juvenile in composition, reflecting a subduction setting whereas lower Eocene rocks have a distinctively different isotopic composition involving evolved crust. Here, we present a detailed geochronological and isotopic study of the KLA granitoids in order to: (1) pinpoint the exact timing of the change in isotopic composition; and (2) to constrain the different possible sources accounting for those differences. We present U-Pb and Hf isotopic data on zircon coupled with Nd isotopic data on whole rocks from the KLA granitoids. Our results indicate a pronounced and abrupt shift in isotopic composition from typical juvenile oceanic arc isotopic signature (ɛNd(i) ≈ +1; ɛHf(i) ≈ +10) during the Jurassic/Early Paleocene to a sudden abrupt change to more evolved but highly variable crustal like composition in the Eocene/Oligocene (-10 < ɛNd(i) , -4; -15 < ɛHf(i) < 0). Inherited Paleozoic zircon crystals in the Eocene/Oligocene rocks indicate the participation of old evolved continental crust like the Indian continental crust in their formation. Therefore we speculate that the Indian crust was underplated below the juvenile KLA since the Eocene, contributing to the build up of the post-collisional KLA crust. The change in isotopic composition can thus be used to give a minimum constraint on the India-KLA collision.