| Cenozoic basalts widely occur in China, of which the Neogene basalt is especially the most common one. Hannuoba basalt has been achieved a great number of success in forming age, types, lithology classification and its genesis since it has been found. This paper chiefly investigates the mineralogy, petrology and geochemistry of Hongmiaozi basalt to discover whether Hongmiaozi basalt belongs to the Hannuoba basalt or not.In this thesis, comprehensive collection, collation and study on the previous data of Hannuoba basalt have been done to analyze the tectonic settings, lithological types and the distribution of volcanic facies, geochemical characteristics of Hongmiaozi basalt, which’s guided by theories of volcanic rock and petrology geochemistry, then it uses the results above to discuss the tectonic environment, petrogenesis, the source and differentiation evolution of magma and the characteristics of the source region. Meanwhile, In contrast with Hannuoba basalt, severval research conclusions are found as follows:Lithological investigations:Chifeng Hongmiaozi basalt can be divided into three eruption rhythms by observation descriptions and measurements of field outcrops, each rhythm includes three subfacies, where the lower is columnar jointed basalt, the central is dense olivine basalt, the upper is vesicular olivine basalt.Petrology geochemistry:the geochemical characteristics of Hongmiaozi basalt display mainly as follows:by the major elements and the TAS diagram, the Hongmiaozi basalt consists of alkaline basalt and sub-alkaline basalt series. The sub-alkaline basalt is subject to the tholeiite as the diagram of w(SiO2)-w(Fe2O3)/w(MgO) indicates. The relationship of the major elements change’s close, especially MgO and other major elements shows significant correlations, demonstrating positive correlation with K2O、SiO2、P2O5and Na2O, and weakly negative correlation with TiO2and Al2O3; the characteristics of rare earth element display LREE enrichment and HREE deficit, LREE and HREE fractionation significantly, the shape of all REE chondrite-normalized curves is consistent, Eu and Ce have positive anomaly and weak negative anomaly; incompatible elements in the chondrite-normalized diagram shows the enrichment of Ba、Zr, while Rb and K are strongly depleted; the content of compatible elements in Hongmiaozi basalt is generally lower than the primitive mantle values, Cr、Ni elements are strongly depleted, it may be interpreted to have something to do with the fractional crystallization of mafic minerals.Petro-genesis:according to what’ve been gained from the petrology types and systematical geochemical analysis of Hongmiaozi basalt, and also from using the graphs of the major elements, the incompatible elements and its ratio, the results reveal that Hongmiaozi basalt is continental flood basalt forming in intraplate environment, moreover, its cause may be related to the mantle plume. On the other side, as the highly incompatible elements not affected by the fractional crystallization and partial melting, the content and its ratio of highly incompatible elements can manifest the properties of the magma source, from which the results show that the original magma of Hongmiaozi basalt deprive from enriched mantle-a product of low degree of partial melting of garnet iherzolite mantle. Magma during the process of lifting and erupting is not significantly contaminated by continental crustal material, however, a certain weak degree of fractional crystallization can be seen instead.Rock type identification:compared with typical Hannuoba basalt in lithological combination, rock facies, geochemical characteristics, tectonic environment, genesis, magma formation and the evolution discipline, we found the two have significantly similarity in these aspects, which determined that Hongmiazi basalt is a part of the Hannuoba basalt.These research fruits not only provide more detailed information for further study of Hannuoba basalt, but also enrich the basical geology data for basalt in Chifeng region, Neimeng. |
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