Provenance and thermal history of the Bayan Har Group in the western-central Songpan-Ganzi-Bayan Har terrane: Implications for tectonic evolution of the northern Tibetan Plateau

Triassic turbidites dominate the Songpan-Ganzi-Bayan Har (SGBH) terrane of the northern Tibetan Plateau. U-Pb dating on single detrital zircon grains from the Triassic Bayan Har Group turbidites yield peaks at 400-500 m.y., 900-1000 m.y., 1800-1900 m.y., and 2400-2500 m.y., These results are consistent with recently published U-Pb zircon ages of pre-Triassic bedrock in the East Kunlun, Altyn, Qaidam, Qilian and Alaxa areas to the north, suggesting that provenance of the Bayan Har Group may include these rocks. The similarities in the compositions of the lithic arkosic sandstones of the Bayan Har Group with the sandstones of the Lower-Middle Triassic formations in the East Kunlun terrane to the north also suggests a common northern provenance for both. A well exposed angular unconformity between the Carboniferous-Middle Permian melange sequences and the overlying Upper Permian or Triassic strata indicates that regional deformation occurred between the Middle and Late Permian. This deformation may have been the result of a soft collision between the Qiangtang terrane and the North China Plate and the closure of the Paleo-Tethyan oceanic basin. The Bayan Har Group turbidites were then deposited in a re-opened marine basin on a shelf environment. Fission-track dating of detrital zircons from the Bayan Har Group sandstones revealed pre- and post-depositional age components, suggesting that the temperatures did not reach the temperatures necessary to anneal retentive zircon fission tracks (250-300 degrees C). A 282-292 m.y. peak age defined by low U concentration, retentive zircons likely reflects a northern granitic source. Euhedral zircons from two lithic arkoses with abundant volcanic fragments in the southern area yielded a similar to 237 m.y. zircon fission track (ZFT) peak age, likely recording the maximum age of deposition. A dominant post-depositional 170-185 m.y. ZFT peak age suggests peak temperatures were reached in the Early Jurassic. Some samples appear to record a younger thermal event at similar to 140 m.y., a short lived event that apparently affected only the least retentive zircons.