Picrites from the Emeishan Large Igneous Province: Evidence for the Mantle Plume Activity
ZHANG Zhao-chong, WANG Fu-sheng, HAO Yan-liInstitute of Geology , Chinese Academy of Geological Sciences , Beijing 100037, China; China University of Geosciences , Beijing 100083,China
Electronic microprobe analyses for olivine and spinel in the picrites, which were discovered recently in the Emeishan continental flood basalt province (ECFBP), show that the olivine is rich in Mg and the spinel is rich in Cr. Based on the principle of olivine-melt equilibrium, the calculated primary magma contains about 20% MgO. This indicates that there is the picrite formed directly by the primary magma and the picrite formed by the magma differentiation and accumulation in the area. The high Mg-olivine-bearing picrite can be regarded as the parental melt. Thus, the estimated melting temperature and pressure are 1600℃ and 4. 5Gpa respectively. This high temperature suggests that the formation of picrite is probably associated with the mantle plume activity. All rocks are characterized with the LREE enrichment, HFSE depletion, absence of Nb and Ta negative anomalies, and existence of P and K negative anomalies. The limited variation of some ratios of incompatible elements, such as La/Ta, La/Sm, (La/Nb)PM , (Th/Ta)PM ,show that these rocks were genetically linked with mantle plume activity, without or with little lithospheric or crustal contamination during the process of magma ascent en route to surface. It is believed that these picrites were formed by the magma derived by 7% partial melting of garnet Iherzolite and the axis of the mantle plume could be located beneath the Lijiang town, Yunnan Province.