陕西勉略宁三角区弧后裂解型火山岩特征及其构造意义

陕西勉略宁三角区新元古代火山作用强烈,发育碧口群大安岩组、陈家坝岩组、巨亭岩组、黑木林岩组等陆缘裂谷、岛弧、弧后裂解及蛇绿岩在内不同构造属性火山岩系。野外调查和岩石化学研究表明,以巨亭岩组为代表的火山岩系总体为受裂隙式火山机构控制的一套完整火山喷发序列酸-基性火山岩建造,发育溢流相-爆发相火山喷发旋回和韵律,具有分布范围广,岩石变形强烈特点。岩石类型以流纹岩、玄武岩及相应火山碎屑岩为主,玄武岩地球化学特征显示总体为钙碱性-拉斑玄武岩组合,其微量元素和稀土元素分配型式与裂解型玄武岩(CFB)特征相似,火山岩形成时代为新元古代晚期。它的出现标志着新元古代扬子地块北缘裂解背景下以火山-沉积作用为记录的洋陆俯冲-消减-弧后裂解事件,对研究勉略宁地区碧口地块构造演化机制具有重要意义。

Characteristics and Tectonic Significance of the Back-arc CFB Volcanic Rock in the Mianlue-ning triangle area, Shaanxi

There is strong Neoproterozoic volcanism in the Mianluening triangle area in Shaanxi Province, which develops volcanic rock series including Dan formation, Chenjiaba formation, Juting formation and Heimulin formation from differentiate tectonic structures of land edge rift, island arc, post-arc crack and ophioliteetc..Investigation and petrochemistry show the volcanic series mainly presented by Juting formation is a set of complete volcanic eruption sequence of acidic-basic volcanic rock fomation. Overflow phase - explosive phase volcanic eruption and rhythm are developed., characterized by wide distribution and strong alteration of the rocks..Rock types are mainly rhyolite, basalt and corespodent pyroclastic rocks.. Geochemical characteristics show that the basalt is mainly a combination of calc-alkali and tholeiite, its distribution patterns of microelement and rare earth element are similar to fractured basalt (CFB)..The rocks formed in the late of Neoproterozoic. Those volcanic rocks indicate the event of ocean-land subduction, subduction, back-arc cracking in disintegration of the northern margin of the Neoproterozoic Yangtze block which was recorded by the volcano-sedimentation. This research has great significance in the study of tectonic evolution in Mian-Lue-Ning area of Bikou plot.