THE SONGPAN-GANZI TERRANE OF TIBET: WAS IT A PART OF SOUTH CHINA SINCE THE NEOPROTEROZOIC?

The Songpan-Ganzi terrane, which extends from the western margin of South China to the Pamirs over a distance of ~2000 km, is the least understood tectonic unit in Tibet. As it is covered almost exclusively by Triassic flysch deposits, its origin has been related to the Triassic development of an accretionary wedge, a remanent ocean, or a backarc basin. The accretionary-wedge model predicts the flysch to be in a tectonic contact with its underlying basement, the remanent-ocean model requires the flysch to be in depositional contact with a Neoproterozoic oceanic or continental basement, and the backarc-basin model predicts the flysch to be deposited on a highly stretched southern Kunlun-Qaidam terrane dominated by an Cambro-Ordovician arc. In eastern Tibet, the Triassic flysch rests on top of Permian or Devonian strata, which are parts of an 8-9 km thick Neoproterozoic-Early Paleozoic passive margin sequence overlying the South China basement consisting of 750-824 Ma felsic orthogneiss. In the eastern Songpan-Ganzi terrane, U-Pb zircon dating and Nd isotope analysis suggest the Triassic plutons to have formed from partial melting of a 1.1-1.6 Ga basement, whereas in the central Songpan-Ganzi terrane xenoliths in Cenozoic volcanic rocks suggest the presence of a Proterozoic mafic lower crust. In the westernmost Songpan-Ganzi terrane, meta-volcanic rocks yielding a U-Pb zircon age of 2481±14 Ma is interpreted to as representing the basement age. The presence of a Proterozoic basement and its depositional contact with South China suggest the Songpan-Ganzi terrane may have been connected with South China since the Neoproterozoic. This interpretation provides an explanation for two-phase arc magmatism (i.e., Cambro-Ordovician and Permo-Triassic) in the Kunlun Range of central Tibet. It also requires a reevaluation of the existing tectonic reconstructions of South China in the Neoproterozoic.