Features of the Early Palaeozoic Mantle beneath Langao County and Its Formation Mechanism

AbstractPhlogopite‐amphibole pyroxenite xenoliths contained in an Early Palaeozoic alkali subvolcanic lamprophyre complex in Langao County, Shaanxi Province, are metasomatized mantle xenoliths, composed mainly of clinopyroxene, amphibole, phlogopite, apatite, pervoskite, ilmenite and sphene with well‐developed subsolidus metamorphism‐deformation textures, such as “triple points” and “cataclastic boundaries”. Minerological studies indicate that clinopyroxene is rich in SiO2 and MgO and poor in TiO2 and Al2O3, which is notably different from magmatogenic deep‐seated megacrysts and phenocrysts formed in the range of mantle pressure. Amphibole and phlogopite have the compositional feature of mantle‐derived amphibole and phlogopite. Sm‐Nd isotope studies suggest that the metasomatized mantle beneath Langao County is the product of metasomatism of primitive mantle by melt (fluid) derived from the mantle plume, and the mantle metasomatism occurred 650 Ma ago. The process of mantle metasomatism changed from mantle metasomatism induced by CO2‐, H2O‐ and CH4‐rich fluid in the early stage to that induced by melt and P‐, Ti‐, Ca‐ and Fe‐rich fluid in the late stage.