甘肃省文县刘家坪蓟县系变基性火山岩地球化学特征及构造环境

详细的野外地质调查显示，甘肃省刘家坪蓟县系为一套变基性火山岩，岩石以变玄武岩为主。岩石地球化学特征表明:A/CNK平均值为059%，具有铝不饱和特征；Na2O/K2O值为4700～14400，表现为钠质；在TAS图解中，显示原岩为亚碱性系列；在AFM图解中，样品点均落入拉斑玄武岩系列；稀土元素含量∑REE为5822×10-6～9559×10-6，平均为6781×10-6；δEu=083～117，δCe=088～095，稀土配分模式为平坦型；微量元素中，高场强元素Zr、P、Y、Yb相对亏损，Th、Hf、Nb相对富集，低场强元素K亏损明显，Rb、Ba相对富集，且Zr/Y值为238～391，Ti/Zr值为9230～14723，Th/Ta值为177～221，大于160。Sm Nd同位素测试结果表明，变玄武岩等时线年龄为(1 387±32) Ma。根据地球化学构造环境判别图，结合区域地质背景，认为刘家坪蓟县系变基性火山岩主要形成于与岛弧相关的构造环境，这一认识对研究碧口地块的构造演化具有重要意义。

Geochemical Characteristics and Tectonic Setting of Meta mafic Volcanic Rocks from Jixian Group in Liujiaping,Wenxian County,Gansu Province

Detailed field geological survey suggested that the Jixian Group volcanic rocks in Liujiaping are mainly meta basalts. Geochemical analysis showed that the rocks are aluminum unsaturated (average A/CNK=059%) and sodic (Na2O/K2O=4700-14400). In the TAS diagram, the basalts are sub alkaline. In the AFM diagram, the samples fall into the tholeiitic basalt field. The total REE contents (∑REE) vary in the range of 5822×10-6-9559×10-6(average 6781×10-6 ). The rocks show anomalies of Eu (δEu=083-117) and Ce (δCe=088-095), with flat chondrite normalized REE distribution pattern. The trace element geochemistry is characterized by distinct depletions of Zr, P, Y and Yb, and relative enrichments of Th, Hf and Nb. There is also strong depletion in K and relative enrichments in Rb and Ba. The values of Zr/Y are 238-391; Ti/Zr are 9230-14723; and Th/Ta are 177-221, more than 160. The meta basalt yielded a whole rock Sm Nd isochron age of (1,387±32) Ma. Based on geochemical tectonic discrimination and regional geological background, it is considered that the Jixian Group meta basalts in Liujiaping were formed in an island arc setting, which is important to the tectonic evolution study of the Bikou terrane.