Thermal evidence for early Cretaceous metamorphism in the Shyok suture zone and age of the Khardung volcanic rocks, Ladakh, India

Abstract

The Dras island arc (NW India) is intruded by the Ladakh Batholith and rimmed along its southern margin by the Indus suture zone, which developed ca. 50Ma at the start of the India–Asia collision. Along its northern margin the Ladakh Batholith intrudes the Shyok Formation, a series of folded and faulted metasedimentary and metavolcanic rocks that are thought to mark an older suture of Cretaceous age. Restoration of Miocene and younger strike-slip movement of ∼150km on the Karakoram fault suggests that the Shiquanhe suture in China was once continuous with the Shyok suture in Kohistan, but no geochronologic evidence for this connection has been demonstrated in the intervening region in Ladakh.The Khardung calc-alkaline volcanic rocks were deposited unconformably on the Shyok Formation and are thought to be of Late Cretaceous age on the basis of fossils and regional correlations, yet no reliable radiometric ages have been published. New Sensitive High Resolution Ion Microprobe (SHRIMP) U/Pb ages on single zircon grains from Khardung volcanic rocks have confirmed that a ∼7km thick section was deposited between 67.4 and 60.5Ma. The underlying Shyok Formation has been difficult to date due to strong thermal overprinting related to both intrusion by the ca. 102–50Ma Ladakh granites and movement on the younger Karakoram fault. Near Digar a series of metasedimentary and metavolcanic rocks in structural and metamorphic continuity with the Shyok Formation has experienced less thermal overprinting and a muscovite from a marble unit yields a 40Ar/39Ar maximum age ca. 124Ma, which indicates that greenschist facies metamorphism took place prior to this time. The geochronological evidence is consistent with an Early Cretaceous age for the Shyok Formation, but it further suggests an Early Cretaceous metamorphic and deformational event related to convergence in an oceanic arc setting between the Dras island arc and the Shiquanhe island arc. This metamorphism was followed in the Late Cretaceous by suturing of the Dras island arc to the continental rocks of the Qiangtang block in westernmost Tibet along the Bangong suture.