ZIRCON U-PB GEOCHRONOLOGY AND ELEMENTAL AND SR-ND ISOTOPE GEOCHEMISTRY OF PERMIAN MAFIC ROCKS IN THE FUNING AREA, SW CHINA

Abstract

Mafic-layered intrusions and sills and spatially associated andesitic basalts are well preserved in the Funing area, SW China. The 258±3 Ma-layered intrusions are composed of fine-grained gabbro, gabbro and diorite. The 260±3 Ma sills consist of undifferentiated diabases. Both the layered intrusions and volcanic rocks belong to a low-Ti group, whereas the diabases belong to a high-Ti group. Rocks of the high-Ti group have FeO, TiO2 and P2O5 higher but MgO and Th/Nb ratios lower than those of the low-Ti group. They have initial 87Sr/86Sr ratios (0.706-0.707) lower and εNd (-1.5 to -0.6) higher than the low-Ti equivalents (0.710-0.715 and -9.6 to -4.0, respectively). The high-Ti group was formed from relatively primitive, high-Ti magmas generated by low degrees (7.3 -9.5%) of partial melting of an enriched, OIB-type asthenospheric mantle source. The low-Ti group may have formed from melts derived from an EM2-like, lithospheric mantle source. The mafic rocks at Funing are part of the Emeishan large igneous province formed by a mantle plume at ~260 Ma. © Springer-Verlag 2005.