内蒙古东部建设屯火山岩锆石U-Pb测年、地球化学特征及成因

锆石LA-ICP-MSU-Pb同位素测年结果显示,建设屯火山岩形成时代为早三叠世早期(248.4±4.8Ma).该套火山岩为一套中性—中酸性的钙碱性—高钾钙碱性系列岩石;Na2O/K2O平均值为2.04,富钠低钾;A/CNK=0.81¹.40,A/NK=1.481.40,A/NK=1.48².50,为准铝质—过铝质中酸性岩石;Sr>300,Yb=1.372.50,为准铝质—过铝质中酸性岩石;Sr>300,Yb=1.37².91,δEu平均值为0.62,显示了埃达克质岩石的地球化学特征,原始岩浆应形成于加厚下地壳榴辉岩相物质部分熔融作用.结合同时期建设屯埃达克侵入岩的地球化学特征以及区域构造背景分析,认为建设屯火山岩应是古亚洲洋沿西拉木伦缝合带闭合后,碰撞造山初期加厚的新生下地壳部分熔融作用的产物.

ZIRCON U-Pb AGE,GEOCHEMISTRY AND PETROGENESIS OF THE JIANSHETUN VOLCANIC ROCKS IN EASTERN INNER MONGOLIA

The LA-ICP-MS zircon U-Pb dating for the Jianshetun volcanic rocks in Eastern Inner Mongolia yields an age of Early Triassic （248.4±4.8 Ma）.According to the major element compositions,the volcanic rocks,with a high Na2O/K2O ratio （2.04）,A/CNK of 0.81-1.40 and A/NK of 1.48-2.50,should be assigned to acidic-intermediate and intermediate calc-alkaline to high potassium calc-alkaline series.With respect to the features of Sr 〉 300,Yb =1.37-2.91,δEu =0.62 （average）,the rocks can be regarded as adakites,whose original magmas were most likely produced by the partial melting of granulite facies materials in the thickened lower crust.Considering the geochemical characteristics of the simultaneous Jianshetun granites and regional tectonic background,the volcanic rocks of this study should be the product of partial melting of the thickened newborn lower crust after the Late Permian closure of the Paleo-Asian Ocean along the Xar Moron suture zone.