鄂尔多斯盆地西南缘双峰式火成岩年代学、成因及构造意义OA

Mesoproterozoic magmatic rocks are extensively exposed along the southwestern margin of the Ordos Basin.Relevant studies are of great significance for revealing the breakup process of the Columbia Supercontinent.To clarify the magmatic genesis and the tectonic evolution characteristics of this era,this paper conducts systematic petrological,zircon U-Pb geochronological,and geochemical studies on the bimodal igneous rocks in the Qi'angou area of the southwestern Ordos Basin.Zircon U-Pb dating results show that the ages of the granite porphyry and ba-saltic andesite are 1 794±10 Ma and 1 803±21 Ma,respectively,indicating their formation in the Mesoproterozoic.Geochemical characteristics reveal that the granite porphyry is a peraluminous A2-type granite,enriched in largeion lithophile elements(LILE)and depleted in high field strength elements(HFSE),with relatively high Rb/Sr ratios(2.95～3.55),reflecting its origin from crustal melting.The basaltic andesite belongs to the calc-alkaline series,exhibiting island arc-type distribution characteristics of selective LILE enrichment and significant HFSE depletion,with right-inclined rare earth element(REE)distribution patterns.Compared with the continental crust,it has higher La/Nb(3.83～4.01)and La/Ta(342～355)ratios,indicating its derivation from the enriched lithospheric mantle and influence of crustal material contamination(Nb/Ta=17.8～18.6,obvious Rb-Th enrichment).The in-significant Eu anomaly(δEu=0.96～0.97)suggests that the magma did not undergo obvious plagioclase fractional crystallization during its evolution.Comprehensive analysis based on tectonic discrimination diagrams and regional tectonic settings indicates that this set of bimodal igneous rocks formed in a post-orogenic intracontinental extension-al environment during the Mesoproterozoic,reflecting the breakup event of the North China Craton around 1.80 Ga.