THE EARLY PALEOZOIC TECTONIC EVOLUTION OF THE NORTHEASTERN MARGIN OF TARIM BLOCK, NW CHINA: CONSTRAINTS FROM INTEGRATED DEPOSITIONAL AND GEOCHRONOLOGICAL RECORDS

The northeastern Tarim continental margin is a key tectonic-unit for us to understand the evolution process and geodynamic mechanism of basin-range coupling between the Paleo-Asian tectonic realm to the north and the Tarim block to the south during Paleozoic. Three Upper Ordovician to Silurian sandstone samples collected from Tabei and Quruqtagh area were studied for detrital zircon U-Pb dating and Hf isotopic compositions in an effort to decipher their provenances and excavate their tectonic implications. The results show that the sandstone samples all yield evident detrital zircon U-Pb age groups of ca. 1100-720 Ma and ca. 2100-1700 Ma, demonstrating that Tarim block was even constituent of Rodinia and Columbia supercontinent during Neoproterozoic and Paleoproterozoic, respectively. Remarkably, Upper Ordovician sandstone sample from Tabei area yields higher proportion on age group of ca. 1100-900 Ma than that of ca. 860-720 Ma, meanwhile, Lower Silurian sample yields opposite result. The former and the latter are consistent with the age patterns of the tectonothermal events in the Central Tianshan microcontinent and small Tarim block, respectively, combined with valuable igneous rock information, indicating that Central Tianshan microcontinent drifted away from big Tarim block most likely at Early Silurian. Upper Silurian sandstones yield primarily different U-Pb dating and Hf isotopic compositions from the Lower Silurian and Upper Ordovician ones in Quruqtagh area, most notably, a small amount ages of ca. 430-420 Ma, very close to the depositional age, are detected from the Upper Silurian detrital zircons, furthermore, CL images and Th/U ratios of above young zircons indicate their provenances of proximal magmatic rocks. Coincidently, conspicuous arc-related magmatic rocks with the formation ages of ca. 420 Ma have been gradually found around Korla area during recent years, which most likely be the source rocks of abovementioned ca. 430-420 Ma detrital materials deposited in Late Silurian. Combining the matched age evidences from both the detrital and magmatic rocks, we propose that the northeastern Tarim margin changed into an active continental margin due to the southward subduction of the South Tianshan Ocean during Late Silurian.