Chronology and geochemistry of Taxinan basalts from the Tarim basin: evidence for Permian plume magmatism

Abstract

A new K-Ar age of 289.6Ma has been obtained for Taxinan basalt in the southwest Tarim basin of the northwest China. This age is consistent with the Ar-Ar age of 290. 1Ma from the same sample. This dating confirms the formation age of the basalt is the Taxinan basaltic lavas are differentiated alkaline basalts, with ~45% \(SiO_2\) and 4% MgO contents. The Taxinan basalts and Keping basalts in the inner Tarim basin have similar major element contents, rare earth element and trace elements spider patterns with enriched light rare earth elements, and elevated contents of other incompatible elements including the high field strength elements. However, the Taxinan basalts have higher \(Al_2O_3\) and \(CaO\) contents and total rare earth elements \((288 \times 10^{-6} \sim 258 \times 10^{-6})\), and lower \(Na_2O\), \(P_2O_5\) and \(FeO\) contents compared to the Keping basalts. Abundances of K, Rb, and Cs are variable and non-systematic, suggesting an important influence of alteration on the abundances of these elements. For more immobile elements, the Taxinan basalts have high Ti \((Ti/Y=522\) \(\sim624)\) and low Zr/Nb ratios with high Nb contents \((30 \times 10^{-6} \sim 40\times10^{-6})\), indicating derivation from enriched mantle sources. White there is not a pronounced Nb anomaly relative to La, the lavas have slightly low Nb/U \((\sim30)\) and Ce/Pb \((\sim 15)\) suggesting either derivation from continental lithosphere or modest amounts of crustal contamination. The large volumes of eruptives combined with the enriched geochemical signature support the hypothesis that the Taxinan basalt was derived from plume volcanism, either from the plume itself or from melting of enriched lithospheric mantle that was heated and uplifted by ascending mantle plume. Based on the affinity between Taxinan and Keping basalts in time and composition, the areal distribution of the Permian basalts in Tarim basin can be extended to the southwest part from the Tazhong-Keping of the inner area of Tarim basin.