Study Of Petrology And Geochemistry Of The Permian Alkaline Basalts In Guizhou Province

In recent years the researchs of ELIP gain much achievement. Most researchersfocus on picrites and basalts from the inner and intermediary zone of ELIPs. Thepetrological and geochemical characteristics of Permian alkaline basalts in ofShuicheng，Xiongjiachang and Fuquan in Guizhou were studied which located in theouter zone of ELIPs. The petrology genesis，evolution of magma and its implicationwere analysed and discussed，the results shows that:1) The studied baslts are alkaline basalts，its total alkaline content are4.79%～8.86%，average to7.25%；they have SiO₂contents between44.5%⁵0.04%，theTiO₂content of2.38%².74%，MgO of4.15%⁶.49%，Mg#of0.37-0.51；therocks contain a small quantity of olivine, titanaugite and alkali feldspar, so therock belong to alkalic basalts；2) The rocks have the right deviation in rare-earth element distribution patterns withenriched total rare earth elements （ΣREE），and are characterized by slightlyradiogenic （87Sr/86Sr）iratios （0.70482–0.70503）， the range of εNd（t） and（206Pb/204Pb）tvariations are narrow:1.3¹.8，and17.21–17.62，and they have thegeochemistry characteristics of ocean island basalts （OIB）.3) The isotope values of143Nd144Nd-87Sr86Sr of Lijiang Picrites were simulatedwhich represented the primary magma，it shows that the alkaline baslts in Guizhouwere contaminated by1-2%of the lower Yangtze crust. The Fractionalcrystallization were model using Melts software，the result indicates that theformation temperature of Guizhou Alkaline basalts are1131℃～1231℃，and thepressure are10-18Kbar，they are smaller than the Picrites located at the innerzone.4) Compare with the basalts in Heishitou and Zhijin in Guizhou，the basalts inShuicheng have lower TiO₂contents，but higher Na₂O，MgO and Al₂O₃. Thedifferent mantle sources，fractional crystallization and crustal contamination maybe the causes of the different chemical compositions of these basalts in Guizhou. 5) The Guizhou alkaline basalts possibly originated from partial melting of mantleplume which arised to the garnet stability field，because of lower temperaturewhich located in the outer zone-the edge away from the mantle plume axis，theyhave lower partial melting degree. And with the long distance away from themantle plume axis, the depth and pressure increase. The melt interacted with thevolatile-rich metasomatized subcontinental lithospheric mantle，and arised to theearth surface with minor contamination of Yangze lower crust formed alkali basalt.It accord with the theory of Emeishan mantle plume model.