OIB-like, heterogeneous mantle sources of Permian basaltic magmatism in the western Tarim Basin, NW China: Implications for a possible Permian large igneous province

Abstract

Permian basalts are widely distributed in the Tarim Basin and surrounding areas of NW China. The magmatism is represented by basaltic flows in Keping and mafic and ultramafic dykes in Silurian–Devonian strata in Bachu, southwestern Tarim Basin. The basalts in Keping have SiO2 (44.1%–55.5 wt.%) and total alkalis (Na2O+K2O=3.20–7.79 wt.%) similar to the mafic dykes in Bachu, but with much higher TiO2 (3.53–4.33 wt.%). An ultramafic dyke has relatively low SiO2 (44.6–43.2 wt.%) and high MgO (19.0–20.2 wt.%), reflecting the abundance of cumulate olivine. All the rocks, including both lavas and dykes, have parallel, mantle-normalized trace element patterns enriched in Rb, Ba, Th, Nb, Ta, Zr, Hf, and light rare earth elements (LREE). The basalts have higher initial 87Sr/86Sr ratios (0.7064 to 0.7080) and lower εNd(t) values (−2.66 to −9.27) than the dykes (initial 87Sr/86Sr ratios range from 0.7048 to 0.7052 and εNd(t) values from +1.64 to +5.16). Both the basalts and dykes show a narrow range of 206Pb/204Pb (17.87–18.77), 207Pb/204Pb (15.52–15.58) and 208Pb/204Pb (38.38–39.04) ratios. The dykes do not show significant crustal contamination and were derived from an OIB-like, asthenospheric mantle source. In contrast, the basaltic flows show variable degrees (up to 10%) of crustal contamination and were derived from an OIB-like, but isotopically more enriched, asthenospheric mantle source. Olivine from the ultramafic dyke has Fo values up to 85, corresponding to a melt temperature of 1300 °C and a melt Mg# of 63. The Tarim Basin magmatism reflects partial melting of heterogeneous mantle sources related to a major mantle plume. Spatially and temporally associated mafic–ultramafic and syenitic intrusions and volcanic rocks form the ~275 Ma Tarim large igneous province.