Initial-exploration Of Lithosphere Tectonic Environment In Bazhai,Southeast Yunnan And The Water Content Of Peridotite Xenoliths Hosted By Basalt

The study of upper mantle is the focal point of the earth science.Peridotite xenoliths hosted by Genozoic alkaline basalts from Bazhai,Southeast Yunnan are natural rocks of upper mantle,the studies of their petrography,geochemistry and mineral water can provide us some important information of the upper mantle,such as structure,composition,evolution history et al.The research object of this paper is peridotite xenoliths from Bazhai,using electron microprobe(EMP)to analyze the major elements concentrations of minerals and using laser ablation-inductively coupled plasma mass spectrometry(LA-ICPMS)to analyze the trace elements concentrations of minerals.We also using Fourier-transform infrared spectroscopy technique(FTIR)to measure water content in minerals of the xenoliths,First reported the water content of the data area.Characteristics of major and trace elements and water content distribution of the xenoliths,we initial-explore the nature and evolution history of the lithospheric mantle.The present research results are mainly from the following several aspects:(1)Most samples of the xenoliths are spinel double pyroxene peridotite,the Mg~# of olivines is 89.2?90.6,it behave fertile characteristics of mantle on the whole.The two kinds of pyroxenes showed good correlation,and the balance distribution coefficient is reasonable.these characters suggest the pyroxenes save the information of staying at the upper mantle.(2)The xenoliths have single element.The results suggest that the peridotite xenoliths are relicts of primary mantle after different degree of partial melting.The normalized curve about(Y-Yb)_N and(Ti-Yb)_N of clinopyroxenes show me the degree of partial melting is less than five percent,about at 1%?4%,and it have not undergone mantle metasomatism induced by silicate melt.(3)In spinel two pyroxene peridotite from Zhengjiadi,Bazhai area,the water content of olivine(Ol)is 2 to 4ppm,the orthopyroxene(Opx)range from 95 to 174ppm and clinopyroxene range from 141 to 600ppm.In peridotite from Haoziba,the water content of olivines(Ol)is 3 to 7ppm,the orthopyroxene(Opx)is 114 to 156ppm and clinopyroxene range from 317 to 383ppm.We get the water of whole rock accord to percent volume of mineral,and it range from 37 to 115ppm.In summary,the peridotite from Zhengjiadi have higher water content and the large variation in water content may be resulted from the heterogeneity of the primary water content in the mantle source and combined with the superimposition of partial melting and metasomatism events.(4)The water of upper mantle in southern China is more than the water in North China.Around the combination of the the water content of the data shows that the water content of the lithospheric structure there is a large-scale lateral heterogeneity.Implying that the mantle evolution brought on difference.(5)The peridotite xenoliths from Bazhai represents the Phanerozoic lithospheric mantle(LM).The LM beneath Bazhai area possessed a relatively complex structure.Alternatively,ancient melt extraction process of primitive mantle gave birth to the original LM,which underwent modification and metasomatism by Silicate melt derived from later geological processes,leading to parts of origninally depleted peridoties transformed into relatively fertile peridotites.