Neoproterozoic sedimentary basin evolution in southwestern Tarim, NW China: New evidence from field observations, detrital zircon U Pb ages and Hf isotope compositions
Abstract Sedimentary basin evolution is intimately related to tectonic background and thus can serve as one of the most important indicators in deciphering the regional tectonic evolution process. Neoproterozoic volcanic sedimentary sequences are well preserved along the southwestern margin of the Tarim Block in NW China and provide new insights into the Neoproterozoic tectonic evolution of the Tarim. In this contribution, we report the metamorphic deformation features and systematic detrital zircon U Pb ages and Hf isotope compositions of the Neoproterozoic strata in SW Tarim. Geochronological data reveal that the greenschist facies metamorphic and tightly folded Sailajiazitage Group (SG) volcanic sedimentary sequence deposited during 860 830 Ma, the Ailiankate Group (AG) clastic rocks of low greenschist facies metamorphism and intensive deformation deposited during 820 800 Ma, and the unmetamorphosed and undeformed late Neoproterozoic carbonate clastic tillite sequences, including the Silu Group (SLG) and the Qiakemakelieke Group (QG) deposited after 760 Ma. Two phases of glaciations from the QG in SW Tarim could be equivalent to the Beiyixi glaciation and the Altungol Tereeken glaciation in north Tarim, respectively. Rock assemblages coupled with geochemistry of the SG bimodal volcanic rocks indicate its deposition in a back arc basin. Rock assemblages, detrital zircon age spectra and its metamorphism and deformation features suggest that the AG clastic sequence most likely deposited at a foreland basin on the back arc basin sedimentary package while the middle to late Neoproterozoic SLG and QG carbonate clastic tillite sequences deposited at a passive marginal and/or rift basins. The Neoproterozoic evolution process of the southwestern Tarim demonstrates that (1) the Neoproterozoic assemblage of the Tarim basement could have lasted till 760 Ma as evidenced by the Aksu blueschist facies metamorphism and (2) its Pre Nanhuaian basement could be composed of independent continental terranes. Furthermore, positive eHf(t) deviations in these detrital zircons suggest that the Rodinia plume could have effects on the Neoproterozoic igneous activities.