Water Contents And Element Geochemistry Of Peridotite Xenoliths Hosted By Cenozoic Basalt From Muchang,Maguan,Southeastern Yunnan

This paper has study detailedly the water contents and element geochemistry of peridotite xenoliths hosted by Cenozoic basalt from Muchang,Maguan,Southeastern Yunnan.The major element of xenoliths and basalt were obtained by EMP and melting method respectively.The trace element of Cpx in xenoliths and basalt were obtained by LA-ICPMS.The Sr-Nd isotopic compositions were also analyzed by LA-ICPMS.Water content in major minerals of the xenoliths were measured by FTIR.Based on the above research contents,We have obtained the following preliminary understandings:Muchang basalts have relatively high Mg#(0.67-0.74)and total alkaline contents(5.34-6.82%).Sr-Nd isotopic compositions are high in 143 Nd/144 Nd and low in 87Sr/86Sr,and ?Nd is all positive(5.01-6.87),representing depleted mantle source region.Basalts obvious enrich HFSE and LILE,suggests that source region has been metasomatized by small volume silicate melt or fluid derived from the asthenosphere.The basalt magma has not been obvious crystallization differentiation during the rising stage,so its chemical composition probably is close to the primary magma.Peridotite xenoliths in the basalt is captured by accident in the process of magma rising,and there is no origin relationships between host basalts and xenoliths.The Mg#of olivines in Muchang peridotite are in the range of 88.87-90.77,show the character of fertile mantle.Elemental fractionation suggests that the peridotite xenoliths are relicts of primary mantle with partial melting.Low(La/Yb)N(0.22-0.57)and high Ti/Eu(407-6772)of clinopyroxenes illustrates Muchang lithospheric mantle has undergone low mantle metasomatism by silicate melt.The trace elemental of peridotite xenoliths are similar to MORB.Muchang peridotites may represent the newly accreted lithospheric mantle,which is formed from asthenosphere mantle's decompression and thermal diffusion.Most of 01 and all the pyroxene grains contain structural water in the form of OH.The water content of Cpx,Opx and 01 from Muchang peridotite are 160-557×10-6,85-207×10-6 and 5-12×10-6 respectively.Based on the water content and mineral model,it is estimated that the whole-rock water content of peridotites should vary in the range of 46-137×10-6.It is obvious that the water content of the Southeastern Yunnan Province lithospheric mantle is much higher than that of the North China Craton lithospheric mantle.It may be related to the Neo-tethys oceanic crust large-scale subduction and fluid metasomatism.The difference water content between the South China and the North China Craton probably reflects the different evolutionary phase of lithospheric mantle.