THE EMEISHAN LARGE IGNEOUS PROVINCE ORIGINATED FROM MAGMATISM OF A PRIMITIVE MANTLE PLUS SUBDUCTED SLAB

The Emeishan flood basalts (EFB), which occur in southwestern China, are tightly associated with the Paleo-Tethyan volcanism. In order to find out relationships between the EFB and Paleo-Tethyan volcanism, a comprehensive geochemical study of EFB, MORB and IAB was carried out. Based on geochemical features, EFB can be divided into five groups, in which HM (high-Mg basalts and picrites), HT1 (high-Ti and high-HFSE basalts) and LT1 (low-Ti and high-HFSE basalts) occur in the eastern district, and LT2 (low-Ti and low-HFSE basalts) and HT2 (high-Ti and low-HFSE) occur in the western and central districts. Geochemically, the western EFB show transition features between the eastern EFB and the Paleo-Tethyan MORB-IAB in the Jinshajian-Alaoshan zones. HFSE and REE tracing indicates that HT1 flooded from an aggregate magma layer derived from fraction melting of a primitive mantle, and LT1 magmas had a major component derived from partial melting of subducted slabs. HT1, LT1 and HM with mg# varying from 82 to 32 covered an area of about 150,000 km² yield consistent Nd isotopic compositions from primitive to slight depleted mantles (ε_Nd(T) =0…+3) and a mg#–ε_Nd(T) inverse correlation. Their (²⁰⁶Pb/²⁰⁴Pb)–(²⁰⁷Pb/²⁰⁴Pb) data can fit NHLR. These isotopic features cannot be explained by mixing between a strong depleted upper mantle and subducted material, which implies that there existed plume magma components from the primitive lower mantle. Whereas Nd and Pb isotopic compositions for LT2, HT2 and MORB-IAB related to the Tethyan opening and subduction indicate that the magmas were derived from random mixing between strong depleted upper mantle, primitive mantle and recycled continental crust components. Sr isotopic compositions for all rocks demonstrate mainly sea-water influence from subducted oceanic crusts. Therefore, the geochemical data from the Permian EFB and MORB-IAB recorded the whole process from the Tethyan plate subduction through asthenosphere upwelling to development of the Emeishan mantle plume.