ID-TIMS U-PB GEOCHRONOLOGY OF LOWER CRUSTAL CUMULATE COMPLEXES IN THE KOHISTAN TERRANE, NORTH-EAST PAKISTAN: IMPLICATIONS FOR ISLAND ARC FORMATION

The Kohistan Arc Complex (KAC) of northeast Pakistan is recognized as a preserved island arc formed by northward subduction of the Neo-Tethys lithosphere beneath the Karakoram plate during the Cretaceous. During the India-Asia collision the KAC was obducted onto India exposing the world’s most complete oblique cross section through ~50 km of arc generated crust, providing a unique window into deep crustal processes and formation of continental crust.

Miller et al. (1991) and Dhuime et al. (2009), outline the formation of the KAC by accretion and delamination. However, uncertainty exists in each model as the four cumulate complexes constituting the lower arc crust, known as the Jijal, Patan, Kayal and Dasu Complexes, lack precise geochronology. Reported data consist primarily of low resolution, low precision radio-isotopic ages (Yamamoto et al., 2005), hampering efforts to establish a robust model of formation. Therefore, U-Pb ID-TIMS analyses of zircons separated from an agmatite breccia and tonalite, from the Kayal and Patan Complexes respectively, were carried out to evaluate these models.

Five zircons from the Kayal Complex yielded an average weighted ²³⁸U/²⁰⁶Pb age of 107.01 ± 0.16 Ma, and three low uranium zircons from the Patan Complex yield an age of 100.41 ± 0.61 Ma. New geochronological data, with reference to published ages (Yamamoto & Nakamura, 2000), indicate a preliminary age gradient of 107 Ma to 95 Ma from the Kayal to Jijal Complex. The existence of an age gradient supports the Miller et al. (1991) model which infers that the upper, most metamorphosed Dasu Complex is the oldest complex in the KAC. Underlying complexes are inferred to decrease in both age and metamorphic deformation intensity, with the Jijal Complex being the deepest, least metamorphosed, and youngest complex. However, these new U-Pb ID-TIMS ages are still insufficient to dismiss the three stage formation model of Dhuime et al. (2009).

Identifying accurate and precise ages for each cumulate complex of the KAC will allow refinement of current models describing continental crust formation in island arc settings. Furthermore, the addition of high precision geochronology will provide an absolute time constraint on the formation of the KAC, and by extension provide a timeframe by which arc crust has been produced in island arcs since the Archean.