Phlogopite-amphibole-pyroxenite xenoliths in Langao, Shaanxi Province, China: Evidence for mantle metasomatism

Abstract

Phlogopite-amphibole-pyroxenite xenoliths contained in the alkali basic-ultrabasic subvolcanic complex in Langao, Shaanxi Province, are composed of diopside, Ti-rich pargasite, phlogopite apatite, sphene and ilmenite, which have subsolidus metamorphism-deformation textures such as triple-points, cataclastic boundaries and kink-bands. Mineral chemical characteristics show that the diposide, Ti-rich paragasite and phlogopite are derived from the mantle and are the products of mantle metasomatism. Compared with normal mantle-derived spinel-lherzolites, the xenoliths are enriched in TiO2, Fe2O3, CaO, Na2O and K2O, with apparent depletion in MgO. Chondrite-normalized REE patterns and primordial-mantle normalized trace elements data show that they are enriched in REE (especially LREE) and incompatible trace elements. The petrographic, mineralogical and petrochemical characteristics indicate that the xenoliths are metasomatized mantle xenoliths, which offers the evidence for mantle metasomatism and represents the anomalous mantle beneath the Early Paleozoic rift in northern Daba Mountains. The agents of mantle metasomatism are probably derived from the rising of mantle hot plumes. The processes of metasomatism varied from limited-range fluid metasomatism in deep mantle (>90 km) to pervasive metasomatism of silicate melt.