Source, evolution and emplacement of Permian Tarim Basalts: Evidence from U–Pb dating, Sr–Nd–Pb–Hf isotope systematics and whole rock geochemistry of basalts from the Keping area, Xinjiang Uygur Autonomous region, northwest China

Abstract

Permian basalts distribute at least 250,000km2, and underlie the southwest Tarim Basin in Xinjiang Uygur Autonomous region, northwest China. This vast accumulation of basalt is the main part of the Tarim Large Igneous Province (LIP). The basaltic units in the Lower Permian Kupukuziman and Kaipaizileike Formations in the Keping area, Tarim Basin; were the best exposure of the Permian basalt sequence in the basin. LA–ICP–MS U–Pb dating of zircon from the basal basaltic unit in the section gives an age of 291.9±2.2Ma (MSWD=0.30, n=17); this age, combined with previously published geochronological data, indicates that the basalts in the Tarim Basin were emplaced between 292Ma and 272Ma, with about 90% of the basalts being emplaced between 292 and 287Ma. Basalts from the Keping area have high FeOT (10.8–18.6wt.%), low Mg#s (0.26–0.60), and exhibit primitive mantle normalized patterns with positive Pb, P and Ti but negative Zr, Y and Ta anomalies.The basalts from both formations have similar 206Pb/204Pb (18.192–18.934), 207Pb/204Pb (15.555–15.598) and 208Pb/204Pb (38.643–38.793) ratios. The basalts also have high εSr(t) (45.7–62.1), low εNd(t) (−3.6 to −2.2) and low zircon εHf(t) (−4.84 to −0.65) values. These characteristics are typical of alkali basalts and suggest that the basalts within the Tarim Basin were derived from an OIB-type mantle source and interacted with enriched mantle (EMI-type) before emplacement. Rare earth element systematics indicate that the parental melts for the basalts were high-degree partial melts derived from garnet lherzolite mantle at the base of the lithosphere. Prior to emplacement, the Tarim Permian Basalts (TPB) underwent fractional crystallization and assimilated crustal material; the basalts were finally emplaced during crustal extension in an intra-plate setting. The wide distribution, deep source and high degree partial melting of the TPB was consistent with a mantle plume origin. The TPB and other coeval igneous rocks in the Tarim Basin constitute a Permian LIP formed by a mantle plume in a similar fashion to the plume-related Emeishan LIP in southwest China.