Late Paleozoic back-arc basin in West Junggar (northwestern China): New geochronological and petrogenetic constraints from basalts and cherts in the western Karamay area

Abstract

The Dagun basalts and cherts in the western Karamay area are critical for understanding the Late Paleozoic tectonic evolution of West Junggar, southern Altaids. In this study, we present new geochronological and geochemical data for the basalts and cherts from the Dagun area. Our U-Pb dating of zircons from a basalt sample yielded a weighted mean age of 418 ± 7 Ma. The Dagun basalts are tholeiitic in composition and have low K2O (0.19–1.05 wt.%) and TiO2 (1.01–1.14 wt.%) contents. They are characterized by depletion in high field strength elements (HFSE) with pronounced negative Nb, Ta, and Hf anomalies. The geochemical features indicate that the Dagun basalts were generated from a spinel-lherzolite facies depleted mantle source with the degree of partial melting between 7% and 12%; the source has been overprinted by subduction-derived components in a subduction-related setting. The cherts display relatively high Al2O3 (1.10–7.36 wt.%) contents and low MnO (0.05-0.48 wt.%) contents with slightly negative to positive Ce (Ce/Ce* = 0.81–1.18) anomalies, suggesting a transitional depositional environment between pelagic and continental margin settings. On the basis of previous studies and our new work, we conclude that the Dagun basalts and cherts were formed in a back-arc basin setting related to the northwestward subduction of the paleo-Junggar oceanic crust in the Devonian.