TIMING OF MAGMATISM IN THE XAINZE RIFT, CENTRAL LHASA TERRANE - EVIDENCE FOR EXTENSIONAL UNROOFING AND EXHUMATION OF MIDDLE MIOCENE GRANITES WITH THE CENTRAL GANGDESE BATHOLITH

The evolution of the central Lhasa terrane prior to the Indo-Asian collision is characterized by northward subduction of oceanic lithosphere during closing of the Neo-Tethys, resulting in extensive magmatism and volcanism associated with the Gangdese Batholith (~120-30 Ma) and the Linzizong volcanic sequence (~60-40 Ma). This study presents new on zircon LA-MC-ICPMS U-Pb and (U-Th)/He data from the Xainza rift, Lhasa terrane, which is terminated by the Gyaring-Co fault in the north and the Indus Yalu Suture Zone (IYSZ) in the south. Granitic to granodioritic basement rocks are predominately exposed along rift-bounding normal faults and the majority of the hanging wall consists of Linzizong volcanic rocks. Based on new U-Pb data, three different age populations can be differentiated along a N-S transect through the Lhasa terrane. In the northern and central part of the Xainza rift, early Cretaceous (129-132 Ma) and early Tertiary (53 and 60 Ma) granitic intrusions dominate, whereas the southern part of the rift close to the IYSZ predominately consists of early Tertiary granodiorite (50-56 Ma) and a middle Miocene (~14 Ma) granite. Tuffs and ignimbrites of the Linzizong volcanic sequence yielded early Tertiary (~50-60 Ma) zircon ages. The middle Miocene granite is exposed as a ~30 km long normal fault-bounded, exhumed extensional fault block within the massive southern portion of the Gangdese Batholith. It appears to be similar in age to rift-related dikes and potassic volcanic rocks of the central Lhasa terrane and intrusives in the Nyainqentanglha Range. Two samples of the Miocene granite yield a well-defined U-Pb age of ~14 Ma. Zircon and apatite (U-Th)/He data suggest rapid exhumation of the Miocene granite in late Miocene times (~7-9 Ma). Additional structural, geochronological, and geochemical investigations are being carried out to elucidate the origin and affinity of this middle Miocene granite with the south-central portion of the Gangdese batholith. Exhumed Miocene intrusives from within the Tibetan middle crust, as documented in the Nyainqentanglha and Lunggar Ranges, should be considered as the most likely analogue for this post-Gangdese granite, but one might also speculate on its possible relationship with roughly coeval granites of the Pum Qu rift and North Himalayan gneiss domes immediately the south of the IYSZ.