Inner Mongolia Chifeng District Hanno Dam Basalt Rock Type And Its Geochemical Characteristics

Hannuoba basalts will be analyzed in Chifeng, Inner Mongolia, which is located in the northern margin of the North China Plate. After collectting the geological data from previous researches and outdoor works, I mainly analyze the rock types and geochemical characteristics of Hannuoba basalts and discuss its tectonic setting, magma source and the petrogenesis in Chifeng, Inner Mongolia, by the theories of Igneous Petrology, Mineralogy and Petro-Geochemistry and the methods of microscopic identification and geochemical laboratory tests.Firstly the classification of the Hannuoba basalts, morphological characteristics show that Hannuoba basalt usually performs as a large scale of lava platform with multiple cyclic features, which belongs to the products of multi-stage magma eruption. Columnar structure basalt, dense massive basalt and vesicular-almond basalt are common for each period macroscopically. Hannuoba basalts are mainly olivine basalts and pyroxene basalts by the microscopic identification. Hannuoba basalts mainly distribute in the north of Chifeng, which belongs to Miocene.Secondly study on the characteristics of petrological geo-chemistry. Major elements contents are in a relatively narrow range, SiO2=47.35-49.9wt%, MgO=6.35-8.55wt%, Fe2O3=11.12～13.78wt%. According to SiO2-ALK and AFM diagrams, the Hannuoba basalts can be devided into alkaline and tholeiite basalts in Chifeng area. Primitive mantle-normalized spider diagram show that basalts are enriched in large ion lithophile elements (LILE) and high field strength elements (HFSE), especially Nb and Ta. relatively depleted in Yb and U. In Harker variation diagram, compatible elements Cr and Ni are positively correlated with MgO, suggesting that the fractional crystallization of olivine and clinopyroxene have happened. Incompatible elements Zr and Y are poorly correlated with MgO, suggesting that fractional crystallization of amphibole is unclear. The amounts of rare earth elements is in a larger scale,∑REE=92.5～207.71ppm. Chondrite-normalized curve is a negative slope. Basalts are enriched in LREE relatively, while depleted in HREE. The enropium anomaly is not obvious, suggesting that the fractional crystallization of plagioclase is unclear. The geochemical characteristics of Hannuoba basalt are similar to OIB, which indicate that they may originate from a similar mantle source.Thirdly discussion on the petrogenesis of Hannuoba basalts, the trace elements ratios and Harker variation diagrams show that the fractional crystallization of olivine and clinopyroxene has happened, but no continental crust contamination during magma evolution. Combining the discrimination diagrams of basalt tectonic setting with the regional tectonic revolution suggest that the Hannuoba basalt belongs to continental intraplate basalt, formed in a continental rift stretching environment. The characteristics of primitive mantle-normalized spider diagram and chondrite-normalized curve suggest that the alkaline and tholeiite basalts are originated from the same mantle source, the top of the asthenosphere enriched mantle. Hannuoba basalt is the low degrees of partial melting of garnet lherzolite and spinel lherzolite in studied area. The alkali basalt melt degree is slightly lower than the tholeiite. Mantle plume may be the dynamic mechanism of Hannuoba basalt in Chifeng area.The classification and geochemical characteristics of Hannuoba basalt in Chifeng Inner Mongolia will provide a new geological confidence for further research on Hannuoba basalt.