CENOZOIC MAGMATISM IN GANGDESE, SOUTHERN TIBET: RECORDS OF COLLISION AND SUBDUCTION BETWEEN INDIA AND ASIA

The geological record of Cenozoic magmatism in Gangdese (Lhasa terrane), southern Tibet, provides unique information on the collisional and postcollisional processes following the collision and partial subduction of India beneath Asia. The Linzizong volcanic succession (LVS, ~65-45 Ma) and the coeval batholiths (~60-40 Ma) of andesitic to rhyolitic composition represent a magmatic response to the India-Asia continental collision, which began ~70-65 Ma and culminated at ~45-40 Ma with convergence continuing to the Present. These syncollisional felsic magmatic rocks are widely distributed along much of the 1500 km long Gangdese Belt immediately north of the India-Asia suture (Yarlung-Zangbo) in southern Tibet. Our study of the Linzizong volcanic rocks from the Linzhou Basin (near Lhasa) suggests that syncollisional felsic magmatism likely accounts for much of the continental crustal growth in this collision zone. These volcanic rocks show a first-order temporal change from the andesitic lower Dianzhong Formation (64.4–60.6 Ma), to the dacitic middle Nianbo Formation (~54 Ma), and to the rhyolitic upper Pana Formation (48.7–43.9 Ma). Our detailed geochemical study suggests that the remarkable compositional similarity between the andesitic lower Dianzhong Formation and the modal bulk continental crust corroborates our proposal that continental collision zones may be the sites of net crustal growth (juvenile crust) through process of syncollisional felsic magmatism (Mo et al. 2007, 2008). Undeformed, subaerial LVS volcanic strata unconformably overlie the strongly deformed Cretaceous strata in the Gangdese Belt, providing an important temporal constraint for the timing of widespread volcanism in the region. Recent 1:250,000-scale geological mapping of the Gangdese Belt (Pan et al. 2004) revealed that the LVS occurs widely, extending E-W for more than 1500 km and taking up ~50% of the outcrop area in the entire Gangdese Belt. This unconformity goes with the LVS throughout the entire Gangdese Belt, suggesting a major tectonic event, which, together with the geochronological data (Dong, 2002; Mo et al., 2002, 2003; Zhou et al., 2004; Mo et al., 2006, 2007 allows us to advocate that the unconformity, which is ≥65 Ma, represents the onset of the India-Asia collision, supporting the inferred timing of initial collision at ~70–65 Ma by Yin and Harrison (2000) based on a multitude of observations. Postcollisional potassic and ultrapotassic rocks (28-8 Ma) reveal the mantle contribution to their source regions and the involvement of recycled subducted Indian continental material in response to the post-collisional lithosphere delamination (Mo et al., 2006; Zhao et al., 2009). The 14-18 my old volcanic rocks and adakitic ore-bearing granite porphyries, on the other hand, possibly represent the products of partial melting of the thickened lower crust of the plateau. The chronological order and geochemical characteristics of these magmatic events point to the processes of Tethyan oceanic crust subduction, followed by India-Asia collision and subsequent continental subducting/underthrustion of India beneath southern Tibet. This tectonic scenario is reminiscent of that of the eastern Mediterranean region, involving the collision of Africa with Eurasia in the late Cenozoic, offering new insights into the magmatism and crust accretion associated with continental collision, which may also suggest a common tectono-magmatic pathway for the evolution of continental collision zones.