Provenances and tectonic implications of Paleozoic siliciclastic rocks from the Baishuijiang Group of the southern Qinling belt, central China

Abstract

The Paleozoic Baishuijiang Group is exposed in the southern Qinling belt and consists of turbidite sediments. The provenances of the Paleozoic sedimentary rocks is constrained by the integration of major and trace elements and detrital zircon U–Pb dating, which can help to understand the connection between the provenance and the Paleozoic tectonic evolution of the Qinling orogenic belt. The sandstones and mudstones have intermediate SiO2/Al2O3 and K2O/Na2O ratios, and high Fe2O3+MgO contents. In comparison with average upper continental crust, they show strong negative Nb–Ta and Sr anomalies, slight depletion of Zr–Hf and Th, but moderate enrichment of Sc, Ni and Cr. These rocks show LREE enrichment, and pronounced negative Eu anomalies in chondrite-normalized REE patterns, similar to post-Archean shales. The weathering trend of the sandstones and mudstones suggests andesitic and granodioritic provenances. These sediments are geochemically similar to continental island arc sediments, and therefore were probably deposited at an active continental margin. Detrital zircon U–Pb dating of five sandstones from the Baishuijiang Group yielded ages ranging from 407 to 3000Ma, with six peaks at ca. 425, 710, 780, 930, 1800, and 2500Ma. The youngest age peak at 425Ma and two zircon grains with a weighted average age of 414Ma indicate that the maximum depositional age of the Baishuijiang Group is late Paleozoic. Our new age data thus suggest that these turbidite sediments were mainly derived from the northern Qinling belt and the northern margin of the Yangtze Block. We propose that the Paleozoic sediments were deposited in an ocean basin between the northern Qinling belt and the Yangtze Block. Combined with regional geological evidences, our results indicate that there was a paleo-ocean in the Qinling orogenic belt since Neoproterozoic era. The oceanic crust was continually subducted northward, resulting in multiple accretion events from the early Paleozoic to early Triassic, which ultimately led to the amalgamation of the north China and Yangtze plates.