Late Paleozoic post-collisional magmatism in the Eastern Tianshan Belt, Northwest China: New insights from geochemistry, geochronology and petrology of bimodal volcanic rocks

Abstract

The Late Carboniferous to Permian marks an important phase of terrane amalgamation through accretion–collision in the Central Asian Orogenic Belt (CAOB). The Tianshan Belt forms part of this sequence, where the post-collisional event generated an E–W trending bimodal volcanic zone that extends for ca. 500km from the southern Bogda Mt. to the easternmost Tianshan Belt. Here we report the petrology, geochemistry and geochronology of the bimodal volcanic rocks to characterize the post-collisional tectonic evolution in Late Paleozoic. The major element chemistry of the basalt and rhyolite samples indicates a bimodal high-K calc-alkaline affinity. The basalts are characterized by high abundance in TiO2, Th, U and Pb and depletion in Nb, Ta and P, together with slight enrichment in LREE contents and low HFSE/LREE ratios. The rhyolites show a close affinity to aluminous A-type granites, with enrichment in K+Na, Zr, Ce, Y, and depletion in P, Nb, Ta and Ti. They also exhibit fractionated REE patterns with prominent negative Eu anomalies. The petrographic and geochemical data suggest that this bimodal rock series was generated in a post-collisional setting. The mafic rocks are likely derived from a metasomatized lithospheric mantle. Assimilation of different crustal components played a minor role in the genesis of these basaltic rocks. The LA-ICP-MS analysis on zircons from four rhyolites and two basalts yielded similar ages ranging from 295.8±2.8 to 293.3±1.7Ma, suggesting an Early Permian event. Zircons from the basalts show a wide εHf(t) range from −0.49 to +13, whereas those from the rhyolites show two groups of εHf(t) values: from −5 to 3 and from 6 to 12, suggesting a mixing of crust–mantle material. The magmatic zircons from the bimodal volcanic rocks yielded two peaks of TDM2 between 600–820Ma and 1160–1360Ma, probably representing two significant episodes of continental crust growth. The bimodal volcanic rocks are correlated with the early stage of post-collision. Our results suggest that vertical crustal growth through mantle-derived magmas in post-collisional setting also played a major role in the construction of the CAOB.