Provenance analysis of Late Triassic turbidites in the eastern Songpan-Ganzi Flysch Complex: Sedimentary record of tectonic evolution of the eastern Paleo-Tethys Ocean

摘要

The Xikang Group represents the main stratigraphic sequence of the Songpan-Ganzi complex (SGC), a flysch basin with a wide distribution and monotonous lithology. Strong tectonic deformation and accumulation of folds have led to destruction of the original sedimentary sequence and sedimentary structure of the strata, such that there exists no reasonable sedimentary model that can explain the massive flysch. Herein, we investigated in detail the sedimentary structures, paleocurrent direction, particle sizes, major and trace elements, and magmatic zircon and detrital zircon U-Pb ages of the typical sections of the Xikang Group in the eastern SGC. Our analysis led to the following findings: 1) The U-Pb zircon ages from the Galazi (215.6 +/- 1.1 Ma), Sudi (214.5 +/- 1.5 Ma and 221.1 +/- 1.5 Ma), and Gulu (221.3 +/- 1.2 Ma) plutons, which intrude the Xikang Group, and detrital zircon samples from the uppermost part of the Xinduqiao Fm. indicated the youngest mean age (YC1 sigma[2+]) of 222.4 +/- 2.5 Ma. This in turn indicates a pre-221 Ma depositional age for the Xikang Group. 2) The provenance was derived from the North China Block and Qiangtang Terrane, Qinling-Dabie orogenic belt, South China Block, and East Kunlun-Qaidam, and the construction of the flysch in the SGC belongs to the multi-source or line source provided by the peripheral orogenic belt. 3) Trace elements suggested that the Xikang Group may have been deposited in a sedimentary basin associated with a continental island arc. 4) The sedimentary environment in this area belonged to the distal fan of a submarine, and the new sedimentation age and sedimentation rate indicate that the Carnian pluvial event may have been the trigger mechanism. 5) The formation of the flysch deposition in SGC related to the Paleo-Tethys Ocean underwent a scissor-like closure from east to west.