DETRITAL ZIRCON U-PB AGES AND HF ISOTOPES OF TRIASSIC SONGPAN-GANZI AND YIDUN TURBIDITES AND IMPLICATIONS FOR THE CLOSURE OF EASTERN PALEO-TETHYS OCEAN

Sedimentary provenance and basin filling history of the Triassic Songpan-Ganzi and Yidun turbidites is an important and rich record of tectonism associated with evolution and closure of the Eastern Paleo-Tethys Ocean. This study presents the detrital zircon U-Pb age and Hf isotope results from Songpan-Ganzi and Yidun turbidite sandstone samples to unravel the provenance and the Triassic tectonic evolution of the Eastern Tibet area. The detrital zircon ages mainly consists of 5 populations: 240–310 Ma, 400–480 Ma, 750–1000 Ma, 1700–2000 Ma and 2300–2600 Ma. Most of >1.7Ga zircons have εHf (t) values ranging from -5 to 10, suggesting that the Archean and Paleoproterozoic zircons of those turbidites were dominantly sourced from North China rather than from South China or Qiangtang blocks. Integration of detrital zircon U-Pb ages and Hf isotopes and paleocurrent data indicates that the turbidites from north and central Songpan-Ganzi areas were mainly derived from East Kunlun-Qinling-Dabie belt and North China block, while the turbidites from the south area and Yidun Group were mainly fed by erosion of the Yidun terrane and inverted material along the Jinsha suture. The similarity among detrital zircon age populations of the southern Songpan-Ganzi and Yidun Group turbidites, which all exhibit a major age peak at ca. 1.85Ga and subordinate peaks at Paleozoic and Neoproterozic ages, indicate close association of Songpan-Ganzi and Yidun terranes by at least Middle Triassic time. Furthermore, regional comparison of detrital zircon ages from Mesozoic strata suggests that the subsequent deformation and uplift of both Songpan-Ganzi and Yidun terrane Triassic turbidite strata shed detritus into the Sichuan foreland basin to the east and the Qamdo basin (Eastern Qiangtang) to the west. We infer that deformation of the Songpan-Ganzi and Yidun terrane was coeval with or possibly predated Late Triassic shortening within Longmenshan thrust belt. Furthermore, both zircon U-Pb age and Hf isotope results indicate no affinity between Yidun terrane basement rocks and the rocks of the Longmenshan fold belt and East Kunlun-Qinling-Dabie belt.