Late Paleozoic and Mesozoic evolution of the Lhasa Terrane in the Xainza area of southern Tibet

Abstract

Models for the Mesozoic growth of the Tibetan plateau describe closure of the Bangong Ocean resulting in accretion of the Lhasa terrane to the Qiangtang terrane along the Bangong-Nuijiang suture zone (BNSZ). However, a more complex history is suggested by studies of ophiolitic melanges south of the BNSZ “within” the Lhasa terrane. One such mélange belt is the Shiquanhe-Namu Co mélange zone (SNMZ) that is coincident with the Geren Co-Namu Co thrust (GNT). To better understand the structure, age, and provenance of rocks exposed along the SNMZ we conducted geologic mapping, sandstone petrography, and U-Pb zircon geochronology of rocks straddling the SNMZ. The GNT is north-directed and places Paleozoic strata against the Yongzhu ophiolite and Cretaceous strata along strike. A gabbro in the Yongzhu ophiolite yielded a U-Pb zircon age of 153Ma. Detrital zircon age data from Permian rocks in the hanging wall suggests that the Lhasa terrane has affinity with the Himalaya and Qiangtang, rather than northwest Australia. The Late Jurassic- Early Cretaceous Rila Formation overlies the ophiolite and shows signatures of syntectonic deposition. Detrital zircon ages from the Cretaceous Duoni Formation in the footwall of the GNT suggest that the provenance is mainly Early Cretaceous igneous rocks in the northern Lhasa terrane and Paleozoic strata in the hanging wall of the GNT. Our results support the interpretation that the SNMZ is a Late Jurassic-Early Cretaceous suture zone that resulted from closure of a small Jurassic ocean basin, thereby suggesting more Jurassic shortening than conventional models. The GNT developed along the SNMZ in the Early Cretaceous and facilitated exhumation of Paleozoic strata, which became a source for Cretaceous basins along the GNT. Early Cretaceous igneous rocks in northern Lhasa are interpreted to be a consequence of subduction of ocean lithosphere along the SNMZ and its later delamination.