Petrogenesis of the alkali basalt and trachy-andesite suite in the northern Tarim Basin, NW China: Implications for crust-mantle interactions controlled by the Permian mantle plume

Abstract

The Permian Tarim large igneous province (LIP), which contains both large volumes of basalt and silicic volcanic sequences, has been intensively studied in the past decades, but some issues still remain controversial, including the petrogenesis of each component in the LIP, their interrelationships, and the geodynamic processes that produced them. In the Halahatang area of the northern Tarim Basin, a well-developed eruptive suite composed of basalt and trachy-andesite has been penetrated by numerous boreholes, which provide us an opportunity to better understand these issues. In this paper, we carried out an integrated study on the eruptive suite, including petrography, geochronology and geochemistry. The results indicate that the eruptive suite formed at around ∼290 Ma, generally synchronous with the main episode of magmatism within the Permian Tarim LIP. The basalts have similar compositions to typical oceanic island basalts (OIBs), which have alkaline, high-Ti and sodic compositions and show enrichments in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs). Whole-rock Sr–Nd isotopes are slightly enriched, with εNd(t) and (87Sr/86Sr)i values from −4.5 to −3.4 and 0.7065 to 0.7086, respectively. The trachy-andesites are alkaline, metaluminous and ferroan, and display highly enriched LREEs and LILEs. High Ga/Al ratios and Zr + Nb + Ce + Y values constrain their A-type affinity and A1-type nature. They have εNd(t) values from −8.2 to −4.5 and (87Sr/86Sr)i values from 0.7083 to 0.7189, more enriched than those of the basalts. Based on these results, we proposed that the different components in the igneous suite have close petrogenetic relationships. The alkali basalts may have been formed by mantle plume-derived melts coupled with somewhat assimilation of continental crust, while the trachy-andesites were products of variable fractionation of the basaltic magmas involving more continental crustal components in the magma evolution. In combination with the existing results in Tarim, we further suggested that the alkali basalt and trachy-andesite suite may have been generated via a crust–mantle interaction process related to the Permian mantle plume activity.