Geochemical constraints on the mantle source of the upper permian Emeishancontinental flood basalts, southwestern China

The widespread Emeishan igneous province in southwestern China comprises th e Emeishan continental flood basalts (ECFB) and associated mafic-ultramafic intrusions. The ECFB have variable SiO2, ranging from 43.6 to 52.1 wt%, Al 2O3 from 5.0 to 12.6 wt%, and total alkali (K2O + Na2O) from 0.7 to 6.5 wt% . These oxides exhibit negative correlations with MgO (5.4 - 23.1 wt%), imp lying fractional crystallization of olivine and clinopyroxene, which occur as phenocrysts in the rocks. Linear correlations between Zr, Nb, and La sug gest that crustal contamination is not important. The primitive-mantle-norm alized trace-element patterns show that the ECFB are enriched in high-field -strength trace elements, large-ion-lithophile elements, and light-rare-ear th elements, similar to ocean-island basalt. Incompatible element ratios of the ECFB, such as Zr/Nb (7-10), Th/La (0.1-0.15), and Rb/Nb (0.9-1.7), dif fer from those of primitive mantle, N-MORB, and continental crust, but are similar to ocean-island basalts from an enriched mantle source (EM-1). Howe ver, the ECFB have isotopic ratios (Nd-143/Nd-144 = 0.51229 - 0.51276 and S r-87/Sr-86 = 0.70480-0.70647) that imply that the ECFB were derived from a homogeneous, primitive lower mantle carried upward by a mantle plume. We propose that the original melts derived from the mantle plume were conta minated through interaction at shallower depth with an enriched lithospheri c mantle. This model suggests that the lithospheric mantle beneath the ECFB was modified by subduction of an oceanic slab.