Mineralogical Deduction Of Characteristics Of The Mantle Beneath North Hetian Area

This paper is a part of National Scientific Foundation Project 'The Activity of Mantle-Derived Magma and The Characteristics of Its Earth Chemistry in North Hetian Area, Xinjiang', which is held by Professor Zhao Lei. Its aim lies in researching into the characteristics of the mantle chemistry in the research field.Research field lies in south margin of Tarim platform in which the natural and geographic conditions are very bad and fundamental research degree is very low.Because of the absence of mantle xenoliths and xenocrysts, it is the first time that this paper looks on the native heavy minerals deriving from mantle, which were collected at the terraces of the two rivers, Kalakshi river and Yulongkashi river, as objects and takes into account the geochemical characteristics of the rocks deriving from mantle such as Keliyang lamproite and so on to study the composition of lithospheric mantle, the characterics of palaeozoic era lithospheric mantle, mantle thermal state, and diamond metallogentic mantle geological conditions in research field.It is difficult to conform the derivations of the native heavy minerals, so the author assumes corresponding procedures as follows to accomplish the research task:Firstly, according to the composition typomorphic peculiarities of minerals deriving from mantle, the author selected a part of minerals as preliminary reseach objects from the native heavy minerals and then further investigate the mantle ones and their host rocks in two ways: one is the method of comparation with the corresponding minerals of rocks deriving mantle in the research field and other places; aNorther is the method of multivariate statistical analysis.Secondly, based on the microbeam analytical technique, on the one hand, through investigating the characteristic of major elements in the mantle minerals the author acquired the static information from mantle; on the other hand, through multi-point analysis of a part of the minerals from mantle, the auther got dynamic information from mantle.Finally, generalizating the fruits of this study and predecessors, the autor holds that the constituents of lithospheric mantle possibly includes spinel lherzolites, clinopyroxenites, websterite, dunite, harzburgites, garnet lherzite, phlogopite lherzite, eclogites, clinopyroxene megacrysts; compared with East China and North China platform, the research field mantle shows the characteristics of higher degree of partial melting processes and more depleted mantle; the. upper mantle beneath North Hetian area is heterogeneous, with a tendency of deficit in A12O3 and LREE from Kaliyang in west Hetian to the river basin of Kalakshi river and Yulongkashi river; the subduction of the crust beneath North Hetian has ever occurred in geological history and caused the mixing of mantle-crust; the depth of the origination of basaltic magmas beneath North Hetian exceeds 73km; the thickness of the lithosphere beneath the research area amounts to 204.9km; the mantle beneath North Hetian has geological condtions for forming diamond deposits.