Subduction-Accretion History of the Central Asian Orogenic Belt: Constraints from Mongolia

Abstract

Despite abundant research efforts, no consensus exists about the Carboniferous tectonic evolution of the Eastern Tianshan. The tectonic environment of the Yamansu arc belt and the geodynamic mechanisms of the Kangguer shear zone are not fully understood. To refine the proposed tectonic history, we studied three granitoids with a variety of shear deformation characteristics and emplacement relationships in the Yamansu–Kangguer area. Bulk geochemistry indicates that the mylonitic granite porphyry is I-type, while the undeformed two-mica granite and quartz porphyry are S-type granites. Zircon UPb dating results give concordant ages of the granite porphyry, two-mica granite, and quartz porphyry at 336.4 ± 2.3 Ma (MSWD = 1.9, n = 16), 331.5 ± 6.1 Ma (MSWD = 2.6, n = 6), and 332.8 ± 1.7 Ma (MSWD = 1.5, n = 25), respectively, suggesting an emplacement age in the middle Carboniferous. The whole-rock SrNd and zircon Hf isotopic analyses of the three granitoids indicate that the granitic magmas were derived from a Mesoproterozoic to Neoproterozoic crustal basement. The whole rock geochemical and isotopic data of the studied granitoids indicate partial melting with insignificant crustal contamination (except the two-mica granites) and fractional crystallization during magma origin and evolution. These granitoids show enrichment of LILEs (e.g., Rb, Ba, K) and Pb, depletion of Nb, Ta, P, Ti, and P, with low Sr/Y ratios (3–73) and Sr contents (33–160 ppm), displaying affinities to arc magma, and they were classified as continental arc granites. By combining the structural analysis and geological evidence, we propose that southward subduction of the Kangguer Ocean in the middle–late Carboniferous resulted in the ductile shear deformation of strata in the Kangguer shear zone and voluminous magmatism in the Yamansu–Kangguer continental arc–fore-arc basin system.