The Emeishan Large Igneous Province: Evidence for mantle plume activity and melting conditions
XU Yi-Gang1, CHUNG Sun-Lin2 (1. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China; 2. Department of Geology, National Taiwan University, Taibei, China)
Electron microprobe analyses on olivine phenocrysts in picrites and their trapped melt inclusions confirmed the existence of high magnesian (MgO 16％ ) primary magmas for the Emeishan basaltic province. The application of McKenzie and O Nions (1991) rare earth element inversion scheme to the geochemistry of the Emeishan basalts reveals an enhanced mantle potential temperature of 1 450～ 1 550° C. These, together with the vast volume of basalts and the OIB- like signatures preserved in some samples, strongly suggest the involvement of mantle plumes in the generation of the Emeishan basalts. It is further suggested that two distinct mantle components may have been involved in generation of the low- Ti and high- Ti lavas, respectively. Whereas the high- Ti basalts originated most likely from an OIB- like mantle source (ISr≈ 0.704,ε Nd(t)≈＋ 5), a slightly enriched mantle component (ISr≈ 0.705,ε Nd(t)≈＋ 2) is required for the low- Ti lavas. The low- Ti lavas were generated in the plume axis where the mantle temperature is high and the lithosphere is relatively thin. The melting was initiated at a relatively great depth (140 km, garnet stability) and continued to the shallow level (60 km, spinel stability). The total degree of partial melting is 16％ . The low- Ti lavas may thus represent the main phase of this large igneous province. In contrast, the high-Ti lavas resulted from melting of the mantle at plume periphery or during a waning stage of plume activity. The low mantle potential temperature and thick lithosphere led the depth to the top of melting column confined within the garnet stability field and a relatively low degree of melting (1.5％ ).