Geochemical constraints on the mantle source of the upper Permian Emeishan continental flood basalts, Southwestern China

Abstract

The widespread Emeishan igneous province in southwestern China comprises the Emeishan continental flood basalts (ECFB) and associated mafic-ultramafic intrusions. The ECFB have variable SiO 2, ranging from 43.6 to 52.1 wt%, A1 2O 3 from 5.0 to 12.6 wt%, and total alkali (K 2O + Na 2O) from 0.7 to 6.5 wt%. These oxides exhibit negative correlations with MgO (5.4-23.1 wt%), implying fractional crystallization of olivine and clinopyroxene, which occur as phenocrysts in the rocks. Linear correlations between Zr, Nb, and La suggest that crustal contamination is not important. The primitive-mantle-normalized trace-element patterns show that the ECFB are enriched in high-field-strength trace elements, large-ion-lithophile elements, and light-rare-earth 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), differ from those of primitive mantle, N-MORB, and continental crust, but are similar to ocean-island basalts from an enriched mantle source (EM-1). However, the ECFB have isotopic ratios ( 143Nd/ 144Nd = 0.51229 -0.51276 and 87Sr/ 86Sr = 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 contaminated through interaction at shallower depth with an enriched lithospheric mantle. This model suggests that the lithospheric mantle beneath the ECFB was modified by subduction of an oceanic slab.