SHRIMP Zircon U-Pb Age, Geochemistry, and Nd-Sr Isotopes of the Gaojiacun Mafic-Ultramafic Intrusive Complex, Southwest China

摘要

The Gaojiacun mafic-ultramafic intrusive complex in the Yanbian area, Sichuan Province, Southwest China, is a layered intrusive body that underwent intensive magmatic differentiation during two cumulative cycles. SHRIMP (sensitive high-resolution ion microprobe) zircon U-Pb data show that the Gaojiacun complex was formed at 825 + - 12 Ma. These mafic-ultramafic rocks are relatively enriched in light rare-earth elements (LREE) and large-ion lithophile elements (LILE), but are relatively depleted in high-field-strength elements (HFSE). They are characterized by low initial ~(87)Sr/~(86)Sr (0.7045 to 0.7050) and positive epsilon_(Nd)(t) values (+1.3 to +4.5). The geochemical data indicate that the parental magma of the mafic/ultramafic rocks was derived from a depleted mantle source, and underwent fractional crystallization and crustal contamination. We suggest that the Gaojiacun complex was formed in a continental rift related to mantle superplume activity beneath the Neoproterozoic supercontinent Rodinia.