川西峨边地区金口河辉绿岩脉SHRIMP锆石U-Pb年龄及其对Rodinia裂解的启示

摘要

There are abundant mafic dykes chat intruded the Mesoproterozoic Ebian Group in Jinkouhe area of western Sichuan Province. Their precise emplacement time is significant for revising the time of the Ebian Group and studying the Precambrian tectonic setting of the western margin of Yangtze Craton. One of the diabase dykes from Jiadanqiao Formation of Ebian Group was dated by SHRIMP zircon U-Pb dating. The result shows that the zircons are of typical mafic rrugmatic origin with clear and wide internal growth zoning and high Th/U ratios, and the weighted mean 206Pb/236U age is (813.4±8.2) Ma (MSWD=0.56). This result represents the emplacement time of the diabase dykes, and also constrains the minimum age of Jiadanqiao Formation in Ebian Group. Based on synthesizing the latest isotopic ages of the Mid-Neoproterozoic mafic magmaric activities along the western margin of Yangtze Craton, the authors confirmed that there were two major active phases: 830-800Ma and 780-745 Ma, and the peak age of the mafic magmatic activities during 830-800Ma was (821±2)Ma. This is perfectly consistent with die previously-reported age distribution characteristics of mafic magmatic activities in South China and Australia, implying that the diabase dykes in Jinkouhe area were probably related to the episodic activities of the mantle plume which led to the breakup of Rodinia supercontinent.%川西金口河地区中元古代峨边群中发育大量基性侵入岩,其具体侵位时间对厘定峨边群的时限、研究扬子西缘前寒武纪构造属性具有重要意义.对峨边群枷担桥组层型剖面中一条辉绿岩脉进行了SHRIMP锆石U-Pb定年.结果表明,该辉绿岩脉的锆石发育清楚且宽缓的内部环带结构,具有较高的Th/U比值,属典型基性岩浆成因的锆石,其206Pb/238U年龄加权平均值为(813.4±8.2)Ma(MSWD=0.56),代表了该辉绿岩脉的侵位时间,也限定了峨边群枷担桥组的上限年龄.综合扬子西缘新元古代中期基性岩浆活动的最新同位素年龄,证实存在2个主要的活动期:830～800Ma和780～745 Ma,且830～800Ma期间基性岩浆活动的峰值年龄为(821±2)Ma.这与前人总结的华南和澳大利亚同期基性岩浆活动的年龄分布特征完全一致,暗示川西金口河辉绿岩脉的形成可能与新元古代导致Rodinia超大陆裂解的地幔柱幕式活动有关.