ORIGIN AND EVOLUTION OF MIDDLE-PLEISTOCENE BASANITIC MAGMAS FROM NUSHAN, CHINA
女山中更新世碧玄岩岩浆的起源和演化

The results of study on the middle-pleistocene basanites and the mantle-derived lherzolite xenoliths entrained by the formers from Nushan indicate that the volatils dissolved in high-pressure solid mineral phases of mantle peridotite at mantle depth might be exsolved under the decompressive conditions at welling of asthenospheric mantle plume during preliminary continental rifting. These exsolvedly initial free fluid phases, which mainly consist of CO2, CO, H2O and small amounts of N2, SO2, H2S, CH4, H2, Cl2 and F, accumulate in some regions of mantle, and they resulted in lowring of mantle solidus (and liquidus ) and might initiate partial melting in the upper mantle. The mantle fluids and the melts produced by partial melting of mantle peridotites under inducement of the formers would be highly enriched in incompatible elements. Interaction between them and depleted mantle could bring the latter about substantial local enrichments of LREE and incompatible elements. namely resulted in mantle metasomatic effect. The authors' studies reveal that the mantle metasomatism and partial melting had been synchronized under the Inducement of initial mantle fluids before 30 Ma and below the 37km depth within the mantle peridotites beneath Nushan. and the primary basanitic magmas, which would have the eutectic composition for mantle lherzolite system, would be simultaneously generated. The latters, segregated from their source region, ascended more rapidly and they might not undergo storage in high-level magma chamber within the crust and neither undergo contamination processes of the continental crust. The primary mantle-derived basanitic magmas might only undergo a modification to some extent during transport to the surface. being accompanied by a slight crystal fractionation