Lithospheric Mantle Metasomatism In Hebi Area,Henan Province

The Hebi area in Henan is located in the Taihang Mountains tectonic belt,where Kimberlite,Olivine porphyrite and Olivine basalt are exposed.These deep-sourced magmatic rocks carry a large number of mantle xenoliths,which are a window to study the mantle evolution of the North China Craton lithosphere.In this paper,we analyzed the geochemical characteristics and U-Pb isotope chronology of apatite in the mantle xenoliths of Jiangjiading Village kimberlite in He Bi area to reveal the lithospheric mantle metasomatism event and explore the nature and evolution of the lithospheric mantle in He Bi area;we analyzed the mineral chemical composition of ferric small-balls in the mantle xenoliths of Jiangjiading Village kimberlite in He Bi area to explore the genesis of ferric small-balls.The following insights were obtained.(1)The kimberlite mantle xenoliths are mainly garnet peridotite and lherzolite xenoliths.The garnet peridotite xenoliths are rounded,and the xenoliths are about 30 cm in diameter.The xenoliths are strongly weathered,and the garnet peridotite xenoliths contain a large amount of garnet and a small amount of apatite.The apatite is well preserved,white,fresh,with semi-automorphic,granular and short columnar crystals,with grain size around 1-2mm.The lherzolite xenoliths are ellipsoidal in shape,with captives generally 2-10 cm in size and a few up to 40 cm.The lherzolite xenoliths are strongly weathered,and most of the olivine has been altered,but a few remain.The exterior of the xenoliths is a shell of altered minerals,grayish green and yellowish green in color.The ferric small-balls are well preserved,round and spherical,with fresh surface and uneven size,and the diameter of the spherules are between 0.5mm and 1mm in size.(2)The Backscattered electron images(BSE)of apatite in the mantle xenoliths of the Jiangjiading kimberlite show that the darkness and brightness characteristics of the Backscattered electron images of each apatite grain are uniform,which indicates that the distribution of major elements in each apatite grain is relatively uniform.The Cathodoluminescence images(CL)of apatite show that apatite does not show ring-band characteristics,and only individual apatites differ in Cathodoluminescence intensity,indicating some variability in some trace element characteristics of apatite.(3)The analysis of the main elements,trace elements and rare earth elements of apatite in the mantle xenoliths of kimberlite in Hebi area shows that apatite is characterized by enrichment of F,Ba,Th,U,Sm,P,LREE elements,loss of K,Rb,Sr and Nb,Zr,Ti high field strength elements,as well as Eu medium negative anomaly and Ce weak positive anomaly.This enrichment feature is the result of mantle metasomatism.The loss of high field strength elements(Nb,Zr,Ti)of apatite indicates that it is the result of fluid metasomatism,which indicates that the mantle xenoliths in Hebi area represented by apatite suffered from the effect of mantle fluid metasomatism.Based on this,it is believed that the lithospheric mantle of Hebi region in Henan experienced mantle fluid metasomatism,which may be closely related to the destruction of North China Craton(thinning of lithospheric mantle).(4)The U-Pb isotopic age of apatite in the mantle xenoliths in Hebi area is 1154±80Ma,which belongs to the Middle-Neoproterozoic.Therefore,the formation age of apatite cannot represent the intrusion age of kimberlite in the Hebi area.The U-Pb isotopic age of apatite indicates that the lithospheric mantle metasomatism in the Hebi area occurred during the Middle-Neoproterozoic period,and the tectonic context of apatite formation is considered to be related to the convergence and disintegration of Rodinia supercontinent,which is important for studying whether the North China Craton responded to the convergence and disintegration of Rodinia supercontinent.(5)A small amount of ferric small-balls was sorted out from the lherzolite xenoliths.The ferric small-balls were round in shape,gray-black and black in color,mostly less than1 mm in diameter,uneven in size,varying with 0.5 mm-1 mm,with a typical geometry of rapid condensation by the melt.The ferric small-balls have milky bulge and are mainly composed of iron.According to the iron content,it can be roughly classified into two groups: low iron content group and high iron content group.With the increase of iron content in different samples,the lithosphere elements Al,Si,Ca,and Ti content are accordingly decreased.The O content is independent on the Fe content.In the low iron content group,the Mg content is positive correlation with that of Fe;while in the high iron content group,their correlation becomes negative.According to the source,shape and chemical characters of the small-balls,this study infers that the small-balls were formed by partial of melt of pyroxene in mantle xenolith through thermal isolation and decompression,and these small-balls record the short-term geological process of decompression melting and rapid condensation of the mantle xenoliths with the emplacement of kimberlite to the near surface.which is important significance for studying basic geological issues such as the non-equilibrium melting of the mantle and the chemical composition of the initial magma.