Variation of crustal thickness in central west Junggar orogenic belt: insight into its Late Palaeozoic tectonic evolution

Abstract

ABSTRACT The variation of crustal thickness of an orogenic belt through time will provide critical insights into the switch of tectonic setting. The geochemistry of subduction and collision-related magmatism is indicative of the crustal thickness. Late Palaeozoic magmatic products are prevalent in central west Junggar orogenic belt (C-WJOB), and can be employed to determine the temporal changes in crustal thickness as well as the tectonic evolution. Here, we constrain the crustal thickness and discuss the tectonic evolution of C-WJOB through by integrating Sr/Y and La/Yb ratios, zircon U-Pb ages and Hf isotopic compositions of the Late Palaeozoic igneous rocks. Our results show that the crust of C-WJOB was much thinner (20–25 km) than normal continental crust (average at 33 km) in the Early Carboniferous, and began to thicken from 350 Ma to 330 Ma at a slow rate. Crustal thickening was caused by the addition of subduction-related magma sourced from depleted-mantle but with contributions of subducted sediments. The crust started to thicken significantly at ~330 Ma and reached to a maximum thickness of ~40 km between 322 Ma and 310 Ma. The timing of maximum thickness corresponded with the emplacement of calc-alkaline to high-K calc-alkaline I-type dioritic magmas as well as adakitic magma. These magmas were generated by melting of both depleted mantle and juvenile lower crust, which was probably triggered by underplated mantle-derived basaltic magma. Lastly, crustal thinning happened between 310 Ma and 295 Ma due to regional post-collisional extension, although voluminous granitic magmatism occurred. The granitic magmas were formed by remelting of the juvenile lower crust, with a few contribution of mantle material. This study on the variation of crustal thickness in the Late Palaeozoic C-WJOB provides insight into how integrated geochemical and isotopic data can elucidate the complex histories of orogenic belts.