DETRITAL ZIRCON ANALYSIS OF CENOZOIC BASINS IN CENTRAL TIBET: PALEOGEOGRAPHIC AND TECTONIC IMPLICATIONS

The Eastern Kunlun Range (EKR) in central Tibet consists of Proterozoic gneisses, Paleozoic marine sediments, and a composite batholith of E. Ordovician-Devonian and L. Permian-E. Jurassic plutons. No emplacement ages of these ranges are known in Tibet south of the EKR, though E. Paleozoic plutons are common to the north. U-Pb dating of detrital zircons from four sandstones from the Cenozoic Qaidam basin north of the EKR and three sandstones from the Tertiary Hoh Xil basin in the Qiangtang terrane south of the EKR indicates that Kunlun batholith-age zircons are ubiquitous in central Tibetan Tertiary sediments. Analysis of a M. Jurassic sandstone from southern Qiangtang also reveals Kunlun batholith-age zircons, as well as a major peak at ~1.9 Ga. Samples collected from Eocene Hoh Xil sediments and Eocene-Oligocene Qaidam sediments exhibit age clusters at ~1.9 Ga. Miocene Hoh Xil samples show age distributions similar to the Eocene Hoh Xil sample, while younger Qaidam sediments exhibit only Kunlun batholith ages.

Kunlun batholith zircons in Jurassic sandstones suggest that the batholith underwent an unroofing event in the E.-M. Jurassic, consistent with Jurassic cooling in the EKR identified by ⁴⁰Ar/³⁹Ar studies. Despite the presence of E. Proterozoic basement rocks in northern Tibet, significant ~1.9 Ga zircons are not found in late Tertiary Qaidam samples. Their presence in both Paleogene Hoh Xil and Qaidam sediments suggests that the source of ~1.9 Ga zircons was the Jurassic sequence of southern Qiangtang, supported by paleocurrent data and sandstone petrography of Eocene Hoh Xil sediments. Thus, uplift of southern Qiangtang, possibly associated with collision of the Lhasa terrane to the south, provided a source for recycled detritus. Paleogene Qaidam sediments with ~1.9 Ga zircons and a ~42 Ma grain, with potential sources only to the south, imply that the two basins were part of a single basin that was later partitioned by EKR uplift. This is consistent with rapid Neogene cooling indicated by low-temperature thermochronology studies. The flat-lying lacustrine Miocene Hoh Xil sediments were deposited after initiation of EKR uplift and derived from the EKR and older Hoh Xil sediments. Neogene sediments in Qaidam basin were cut off from the ~1.9 Ga zircon source and are dominated by ages reflecting the uplifting EKR.