Study On Garnet Minerals In Kimberlite Mantle Xenoliths In Hebi Area, Henan Province

There are a large number of olivine pyroxene porphyrites,olivine basalts and kimberlites in Hebi area.These mantle derived magmas capture a large number of mantle inclusions in the process of rising.These mantle inclusions have become an important object for us to study the composition,formation and evolution of the mantle.On the basis of predecessors,this paper focuses on the mineralogy and geochemistry of garnet in the mantle xenolith of jiangjiading kimberlite in Hebi.Compared with garnet in kimberlite enclaves in Shandong and Liaoning,this paper discusses the properties and evolution of lithospheric mantle in Hebi area and the cause of destruction of North China Craton.This paper mainly obtains the following understanding:(1)The mantle xenoliths in kimberlite are garnet peridotite xenoliths and lherzolite xenoliths.Due to the weak weathering resistance of kimberlite,there is only a large amount of garnet and a small amount of apatite and pyroxene in garnet peridotite xenoliths.Garnet single mineral is dark red and reddish brown,with particle size between 1.0 ～ 2.0mm,euhedral semi euhedral granular structure,glassy luster,transparent translucent and well preserved.(2)According to the study on the major elements of garnet in kimberlite xenoliths,the garnet in garnet peridotite xenoliths in this study is characterized by high Fe O and Al2O3,medium Mg O,low Cr2O3 and Mn O,which is similar to garnet formed in relatively high pressure environment.Garnet is divided into G3 and G4 low chromium magnesia aluminum garnet and magnesia iron aluminum garnet according to different ways.(3)There is obvious rare earth element imbalance in the garnet particles of the sample.The overall content of large ion lithophile element(lile)is less,Rb and th are lost,u is normal,and there is no Sr and Ba;In high field strength elements(his F),Nb is depleted,Zr and HF are enriched,and there is no TA;Among the transition elements,Cr,Sr and Ni are all depleted.(4)The formation temperature of garnet is 600 ～ 630 ℃,which is lower than that in the western region.It is speculated that the temperature and pressure of mantle material decrease due to the thinning of lithospheric mantle in North China Craton.The formation pressure range is 13.61 ～ 13.89 kbar,and the formation depth range is 45.26～ 46.11 km,which is the product of the upper mantle.The calculated mantle heat flow value is high,which is 60 ～ 65 w / m2,suggesting that the craton is in an unstable state.(5)The study of major elements,trace elements and rare earth elements of garnet shows that it is mainly G3 and G4 low chromium magnesium aluminum garnet and magnesium iron aluminum garnet.It is proved that peridotites in Hebi area have undergone mantle metasomatism and the North China Craton has been destroyed.There is an imbalance between rare earth elements and other trace elements,which is speculated to be largely due to the influence of mantle asthenosphere melting / fluid metasomatism.