DETRITAL ZIRCON GEOCHRONOLOGY FROM THE MESOZOIC QAMDO (CHANGDU) BASIN, SOUTHEASTERN TIBET: IMPLICATIONS FOR THE PALEO- AND MESO-TETHYS EVOLUTION

To better constrain the Mesozoic accretionary tectonics of southeastern Tibet, as well as contribute essential data required for a working model of Tibetan Plateau evolution, we report firsthand U-Pb detrital zircon geochronology data to investigate sediment provenance of the Mesozoic Qamdo basin. The Qamdo basin contains six major zircon age groups which include 170-190, 220-250, 300-330, 400-500 Ma, and 1.8-2.0, 2.4-2.5 Ga. The Upper Triassic strata of the Qamdo basin was sourced primarily from inversion and erosion of the Yidun Group associated with development of a widespread Late Triassic (ca.200-220 Ma) fold-thrust belt in northeastern Tibet related to closure of the Paleo-Tethyan Ganzi-Liang ocean. In contrast, the Jurassic and Cretaceous strata show more localized provenance, receiving sediment derived primarily from the basin bounding ranges, as well as internally recycled Upper Triassic strata. A lack of Jurassic-age zircons in the Lower Jurassic strata was interpreted to reflect a geographical segregation between the Bagong arc and the Qamdo basin during Early Jurassic, probably due to the opening of a short-lived oceanic backarc basin between the Qiangtang block and Bangong arc caused by slab rollback as a result of Meso-Tethys subduction, whereas the abundant Bangong arc-derived zircons in the Middle and Upper Jurassic strata may reflect localized exhumation of the Bangong arc as a result of closure of the Meso-Tethys ocean and subsequent collision between the Qiangtang and Lhasa blocks during Late Jurassic-Early Cretaceous.

Provenance data also revealed that the Jurassic and Cretaceous Qamdo basin was dominated by a long-lived internally drained basin system, which was most likely attributed to active contractional tectonics, as well as arid climate conditions in southeastern Tibet during mid-Mesozoic time.