Geochemical Characteristics And Geological Significance Of The Aoyougou Basalt In The Western Section Of The North Qilian

The Kawa shoshonite(SHO),in the Aoyougou formation of the Zhulongguan group of the Changcheng System in the Western North Qilian,that has important scientific significance for rebuilding tectonic evolution process of the Western North Qilian in the Proterozoic.The paper,on the basis of detailed field geological survey and the systematic study of petrology,rock geochemistry and tectonic,the main understandings as following:(1)The Kawa SHO,composed of dense massive andesitic basalt within the Aoyougou formation of Zhulongguan group of the Changcheng as interbed,belongs to the early Mesoproterozoic rocks(the Changcheng).(2)The volcanic rocks,in the Aoyougou formation of the Zhulongguan group,are characterized by SiO₂(44.56?48.07 wt.%),Al₂O₃(15.16?16.15 wt.%),A/CAK(0.64?0.75).^TFe₂O₃(10.49?13.01 wt.%),MgO(5.37?6.71 wt.%),averaged by 5.95 wt.%,Mg^#(48?51),averaged by 49,exclude a low levels of the magma crystallization differentiation,?Nb(0.31?0.51)<1,illustrates it suffered a certain amount of assimilation and contamination.The volcanic are into the domain of the tephrite and the basanite in the silicon alkali(TAS)diagram,which belong to the alkaline basalt and the shoshonite in the Zr/Ti-Nb/Y diagram and the SiO₂-K₂O diagram.These samples in the chondrite-normalized REE pattern,presenting the right type,are enriched in LREEs but depleted in HREEs(La/Yb)_N=6.92?9.44)and possess weak positive Eu anomalies,with high?REE(191?240ppm)contents.They are enriched in LILEs(but Sr)in the primitive mantle-normalized trace elements pattern,with Ba relatively strong enrichment of Rb,K,no obvious negative anomaly of Nb,Ta,but with a high ratio of Zr/Yb.these geochemical characteristics are similar to the OIB's.(3)The basaltic volcanic rocks,homologous magatism product,derived from enrichment mantle similar to the oceanic island basalt's(OIB,source rocks are composed by garnet+spinel lherzolite),was the product of the mantle plume magmatism.Indicating that the North Qilian area,in the early proterozoic,is in stretch tectonic setting,which may be respond to Columbia supercontinent global cracking incident.(4)The lithosphere of the Kawa and its adjacent regions,in the Early Changcheng System,in the stretch tectonic setting,partial melting because of decompression with deep mantle material upwelling,provided the advantageous geological background for the formation of the Kawa iron deposit.At the same time,the magma evolution provided a source of ore-forming materials for the formation of the Kawa iron deposit.