Permian bimodal magmatism in the southern margin of the Central Asian Orogenic Belt, Beishan, Xinjiang, NW China: Petrogenesis and implication for post-subduction crustal growth

Abstract

Previous studies show that Permian bimodal magmatism is widespread in the Tianshan region, the southern part of the Central Asian Orogenic Belt in northern Xinjiang, NW China. We have found that such bimodal magmatism extended farther to the south in the Beishan region. Our new zircon U-Pb age data show that the dolerite and granite dykes in the Poyi area of the region were emplaced at ~270 Ma, >20 myr after subduction ended in the region. The dated dolerite dyke is characterized by pronounced negative Nb-Ta anomalies, high εNd(t) from +4.3 to +5.1 and low initial 87Sr/86Sr ratios from 0.7051 to 0.7060 for whole rocks, plus high εHf(t) from +8.96 to +13.0 and elevated δ18O from 5.8 to 6.5‰ for zircon crystals, which indicate a subduction-modified mantle source and moderate degrees of crustal contamination (<10 wt%). Whole-rock 1000Zn/Fe, Mn/Zn, Dy/Yb and La/Yb ratios of the Poyi dolerite dyke are consistent with a peridotite-dominant mantle source. Between two possible causes of mantle partial melting, such as mantle plume impingement and lithospheric delamination coupled with asthenosphere upwelling, we favor the latter because existing data indicate that the Permian mafic-ultramafic intrusive rocks in the region were emplaced over a long period (>25 myr) and show no age progression in a single direction, which are inconsistent with the results of mantle plume magmatism. Whole rocks from the Permian granite dyke in the Poyi area are characterized by strong negative Nb-Ta anomalies, negative εNd(t) from −2.4 to −1.6 and high initial 87Sr/86Sr from 0.7080 to 0.7088. Magmatic zircon crystals from the granite dyke have εHf(t) from +0.86 to +5.64 and δ18O from 6.5 to 7.2‰. The isotope compositions of the granite dyke are within the ranges of the pre-Permian crustal rocks in the region, indicating a predominantly crustal source. Based on these data, we propose that partial melting in the crust was induced by underplating of mantle-derived mafic magmas, some of which were emplaced in the upper parts of the crust to form the broadly coeval dolerite dykes and mafic-ultramafic intrusions. The results from this study support the view that post-subduction bimodal magmatism is important for continental crustal growth and differentiation.