THE TARIM PICRITE-BASALT-RHYOLITE SUITE OF NORTHWEST CHINA, A PERMIAN FLOOD BASALT WITH CONTRASTING, COEVAL RHYOLITES PRODUCED BY FRACTIONAL CRYSTALLIZATION AND ANATEXIS

In association with the the Tarim Oil Field Company, petrologic work on 13 boreholes has uncovered a picrite-basalt-rhyolite suite from the northern Tarim uplift, northwest China. The picritic samples have high MgO (14.5-16.8 wt. %, volatiles included), are enriched in incompatible element and have radiogenic Sr-Nd isotopic compositions (εNd (t) = -5.3; Sr_i = 0.707), resembling the Karoo high-Ti picrites. All the basaltic samples are enriched in TiO₂ (2.1-3.2 wt %), have high FeOt abundances (11.27-15.75 wt %), are enriched in incompatible elements and have radiogenic Sr and Nd isotopic compositions (ε Nd (t) = -4.1~-0.4; Sr_i = 0.7049 ~ 0.7065). High Nb/La ratios (0.91-1.34) of basalts attest that they are mantle-derived magma with negligible crustal contamination. The rhyolite samples can be subdivided into two coeval groups with overlapping U-Pb zircon ages between 291±4 and 272±2 Ma. Group 1 rhyolites are enriched in Nb, Ta, have similar Nb/La and Nb/U to the associated basalts, and relatively unradiogenic Sr and Nd isotopic compositions, implying that they formed by fractional crystallization of the associated basalts. Group 2 rhyolites are depleted in Nb, Ta, have low Nb/La ratios, and have very radiogenic Sr and Nd isotopic ratios, implying that crustal materials have been extensively, if not exclusively, involved in their source. The picrite-basalt-rhyolite suite from the northern Tarim uplift, together with Permian volcanic rocks from elsewhere Tarim basin, constitute a large igneous province (LIP) that is characterized by large areal extent (>300,000km²), rapid eruption, OIB-type chemical composition and eruption of high temperature picritic magma and thus this Early Permian magmatism is named the Tarim Flood Basalt.