塔里木西北缘阿克苏地区大陆拉斑玄武岩对新元古代裂解事件的制约

Continental tholeiitic basalt of the Akesu area(NW China)and its implication for the Neoproterozoic rifting in the northern Tarim

塔里木西北缘新元古界苏盖特布拉克组不整合覆盖在前寒武纪阿克苏群蓝片岩及侵入其中的基性岩墙之上,苏盖特布拉克组底部发育两层玄武岩夹层,其形成时代和成因背景对认识塔里木板块前寒武纪构造演化及有关的超大陆循环和地球动力学过程具有重要意义。本文对这两层玄武岩进行了全岩地球化学和锆石U-Pb年代学研究。结果显示,玄武岩均属于大陆拉斑玄武岩系列,微量元素地球化学特性与典型的大陆溢流玄武岩非常相似,其岩浆来源于富集地幔,并遭受了一定程度的地壳混染作用。玄武岩中锆石的LA-ICP-MS U-Pb年龄分布在1945～755Ma,这些锆石均属于玄武岩浆上升过程中从地壳岩石捕获的继承锆石,记录了塔里木北缘元古代期间多期变质和岩浆事件。这一年龄范围表明,玄武岩形成的时代应晚于755Ma。本文的研究结果表明,阿克苏大陆拉斑玄武岩形成于板内裂谷环境,可能与新元古代Rodinia超大陆之下的地幔柱活动有关,是塔里木板块从Rodinia超大陆裂解出来的直接证据。

In northwestern Tarim,Neoproterozoic Sugetbrak Formation unconformably overlies the Akesu Precambrian blueschist and intruding mafic dike swarms.Two Layers of basalt are intercalated in the lower part of the Sugetbrak Formation,their age and tectonic setting are important for the understanding of Precambrian tectonics of the Tarim,and related supercontinent evolution as well as the geodynamics.This study presents our new data of whole-rock geochemistry and zircon U-Pb dating from two basaltic interlayers.The results reveal that all analyzed samples belong to continental tholeiitic basalt showing trace element features similar to that of the typical continental flood basalt,they were derived from an enriched mantle source and suffered variable crustal contamination.The LA-ICPMS U-Pb dating on the zircons from the basalts yielded ages ranging from 1945Ma to 755Ma,indicating that these zircons were inherited from the crustal materials during the ascending of the basaltic magma,the age of the basalt is therefore considered to be younger than 755Ma.These results suggest that the tholeiitic basalts formed in an intraplate rifting setting probably resulted from a mantle plume beneath the Rodinia supercontinent,the tholeiitic basalt is a line of evidence for the Neoproterozoic rifting of the Tarim from the Rodinia supercontinent.