The Studies Of Mantle Xenoliths From North Daba Mts. In Northern Epicontinental Region Of Yangzi Plate

The volcanic activity of host lamprophyric complex occurred in rift zone of continentalmargin in northern epicontinental region of Yangzi Massif in Early Paleozoic, the lamprophyre,which is ultrahypabyssal subexplosive volcanic rocks, includes three subgroups containing 9kinds of rock types and belongs to alkali-subalkali series. The geochemical characteristicsindicate that the lamprophyric complex are derived from the magma of same source and thereexist two crystallization differentiation evolution series for the lamprophyric magma.â… -typeclinopyroxene xenocrysts, amphibole xenocrysts and phlogopite megacrysts contained in thehost lamprophyre were crystallized from main magma in deep source. Green-pyroxenes werederived from a kind of volatile-rich, alkali-rich magma, namely green pyroxenite magma, theirsource was also metasomatized enriched mantle.Phlogopite-amphibole-pyroxenite xenoliths are metasomatized mantle xenoliths, which arecharacterized by subhedral mosaic textures and metasomatic textures with well-developedsubsolidus metamorphism and deformation textures, such as equigranular mosaic textures andcataclastic boundaries textures. Compared to clinopyroxene phenocrysts andâ… -typeclinopyroxene xenocrysts being formed in magam stage, clinopyroxenes in phlogopite-amphibole-pyroxenite are richer in MgO, Si₂O and poorer in TiO₂, Al₂O₃, which were formed atdepth of 70km-150km, amphiboles and phlogopites marked with the composition characteristicsof amphibole and phlogopite formed by mantle metasomatism. Mantle metasomatism occurredin 650Ma ago and the metasomatic agent were derived from the mantle plume source similar toOIB source. The mantle metasomatism in early stage were induce by mantle fluid composed ofCO₂,H₂O,CH₄ and a small amount of CO,H₂S,SO₂,C₂H₄,C₃H₆, following that, theprimary mantle suffered a strong mantle metasomatism by a kind of melt which was rich in Ti,Fe, alkali and volatile and formed anomalous mantle represented by phlogopite amphibolepyroxenite, at last, the amomalous mantle was metasomatized by a kind of Ti-, Ca-, P- and Fe-rich fluid derived later from silicate melt. The metasomatized anomalous mantle below thisregion were in veined distribution and the anomalous mantle represented by phlogopite-amphibole-pyroxenite was distributed below the primary residual mantle marked with Cr-diopside pyroxenite xenoliths. The anomalous mantle represented by phlogopite-amphibole-pyroxenite was the source of host lamprophyre complex, which underwent partial melting atgreat degree and formed the primary magma of lamprophyre, Mantle metasomatism occurring inthe source was temporally closer to the emplacement of the host lamprophyric complex. Theprimary magma of the lamprophyre generated two parental magma after initial differentiation,one corresponding to olivine kimberlite magma and the other to olivine pyroxene porphyritemagma. The crystallization differentiation evolution trend of olivine kimberlite parental magmawas from olivine kimberlite magma to phlogopite olivine kimberlite magma, to leucitephlogopite+leucite diopside lamprophyre magma, to phlogopite siopside lamprophyre magma,finally, to biotite diopside lamprophyre magma; the crystallization differentiation evolution trendof olivine pyroxene porphyrite parental magma was from olivine pyroxene porphyrite magma topyroxene porphyrite magma, at last to alkali-feldspar pyroxene porphyrite magma.