Petrogenesis of Early Permian basalts in the Turpan-Hami basin, NW China: Implications for the spatial limits of the Tarim mantle plume

Abstract

Controversies remain as to the area of the Tarim Large Igneous Province (TLIP), in particular, the relationship between extensive Permian magmatism in the Central Asian Orogenic Belt (CAOB) and the activity of Tarim plume. Here we report the geochronological, mineralogical and geochemical characteristics of the Permian basalts from the Turpan-Hami (Tu-Ha) basin, located in southwestern CAOB. LA-ICP-MS zircon U-Pb dating yielded an age of 298.4 ± 2.6 Ma, temporally close to that of the Tarim plume. The basalts could be classified into lower porphyritic basalt with phenocrysts of plagioclase, olivine, minor clinopyroxene, and upper aphyric basalts. Geochemically, the former is characterized by relatively higher Na2O + K2O (3.97–5.13 wt%), low (La/Yb)N (∼2) and depleted Sr-Nd values [εNd(t)= +8.2 to +8.7; (87Sr/86Sr)i = 0.7032–0.7034], whereas the latter have lower Na2O + K2O (4.19–4.43 wt%) content, higher (La/Yb)N (∼4–6) and more enriched Sr-Nd values [εNd(t)= +4.6 to +6.2; (87Sr/86Sr)i = 0.7033–0.7048]. The compositional variation of plagioclase phenocrysts and pressure estimation (∼7kbar) indicate that Tu-Ha magmas were stored and evolved in a lower crustal magma chamber which experienced at least one-time magma replenishment. The estimated mantle potential temperature (1270–1480 °C) in Tu-Ha basin is similar to MOR, with no obvious thermal anomaly. Furthermore, higher Na/Ti ratio of quartz-normative Tu-Ha basalts suggest a shallower melting depth for Tu-Ha basin than central area of TLIP, which against a typical plume model. These characteristics alongside regional geologic records collectively suggest that the formation of Permian Tu-Ha basalts is more closely linked post-orogenic extension, implying that the extensive Permian magmatism in the CAOB is unrelated to the Tarim plume.