Neoproterozoic Bimodal Intrusive Complex in the Southwestern Tarim Block, Northwest China; Age, Geochemistry, and Implications for the Rifting of Rodinia

摘要

We report here the first SHRIMP U-Pb zircon ages and bulk-rock geochemistry for a Neoproterozoic bimodal intrusive complex in the Tarim block and discuss the tectonic implications. The Kudi igneous complex (K1C) crystallized at 783 + - 10 Ma. Granite and gabbro comingled, indicating that the intrusions were coeval. The granite displays: high total alkalis (Na_2O + K_9O = 8-11 wt percent ); high abundances of Zr, Nb, Y, and total REE; and strong LREE enrichment [(LaA'b)N = 9-59] with a significant negative Eu anomaly (SEu = 0.17-0.44), characteristic of A-type granites. Furthermore, samples of the granite have homogeneous positive sNd(T) values of 1.4 to 2.1 and Mesoproterozoic Nd modal ages (-1.3 Ga). We thus interpret the granite as probably generated by partial melting of the regional Mesoproterozoic arc and/or back-arc crust. The KIC gabbro is geochemically similar to intraplate alkaline basalts, with trace elements and isotope compositions indicating an OIB-type magma source, variably contaminated by continental crust. Field observations, petrology, and geochemistry indicate that the KIC was a bimodal alkaline complex formed by mixing of these two magma types of different origins in a continental rift, possibly related to a mantle superplume beneath the Rodinian supercontinent.