Melting of the subcontinental lithospheric mantle by the Emeishan mantle plume;evidence from the basal alkaline basalts in Dongchuan,Yunnan,Southwestern China

摘要

The Emeishan continental flood basalt(ECFB)sequence in Dongchuan,SW China comprises a basal tephrite unit overlain by an upper tholeiitic basalt unit.The upper basalts have high TiO2 contents(3.2-5.2 wt.%),relatively high rare-earth element(REE)concentrations(40 to 60 ppm La,12.5 to 16.5 ppm Sm,and 3 to 4 ppm Yb),moderate Zr/Nb and Nb/La ratios(9.3-10.2 and 0.6-0.9,respectively)and relatively high psilon_((Nd)(t))values,ranging from-0.94 to 2.3,and are comparable to the high-Ti ECFB elsewhere.The tephrites have relatively high P2O5(1.3-2.0 wt.%),low REE concentrations(e.g.,17 to 23 ppm La,4 to 5.3 ppm Sm,and 2 to 3 ppm Yb),high Nb/La(2.0-3.9)ratios,low Zr/Nb ratios(2.3-4.2),and extremely low psilon_((Nd)(t))values(mostly ranging from-10.6 to-11.1).The distinct compositional differences between the tephrites and the overlying tholeiitic basalts cannot be explained by either fractional crystallization or crustal contamination of a common parental magma.The tholeiitic basalts formed by partial melting of the Emeishan plume head at a depth where garnet was stable,perhaps >80 km.We propose that the tephrites were derived from magmas formed when the base of the previously metasomatized,volatile-mineral bearing subcontinental lithospheric mantle was heated by the upwelling mantle plume.