赞比亚伊索卡东部片麻岩锆石U-Pb年代学、地球化学特征及岩石成因

Zircon U-Pb geochronology,geochemical characteristics and petrogenesis of the gneisses in the eastern Isoka,Zambia

赞比亚伊索卡东部位于伊鲁米德带上,伊鲁米德带是一个北东走向的褶皱俯冲带,位于赞比亚中东部,南部以新元古代赞比西带为界,北东至赞比亚-坦桑尼亚-马拉维边境,并以北西向古元古代乌本迪带为界。锆石U-Pb测年结果表明,黑云母二长片麻岩、二云母石榴斜长片麻岩、含石榴黑云母钾长片麻岩和含石榴黑云母斜长片麻岩的岩浆侵位年龄为1949±18 Ma~2025±15 Ma,形成于古元古代。岩石地球化学分析结果表明,伊索卡东部片麻岩岩浆为过铝质高钾钙碱性—钾玄系列;稀土配分模式明显呈右倾型,富含轻稀土,贫重稀土,铕和铈表现为极微弱的负异常;微量元素原始地幔标准化蛛网图显示,富集Rb、K、La、Pb、Nd、Sm等元素,亏损Ba、Nb、Ta、Ce、Sr、P、Ti等元素。根据伊鲁米德带的构造演化阶段,可以判断伊索卡东部片麻岩形成于乌萨加兰期。

The eastern Isoka is located in the Irumide Belt.The Irumide Belt is a northeast-trending fold and thrust belt that stretches from central Zambia,where it is truncated by the Neoproterozoic Zambezi belt,to the Zambia-Tanzania-Malawi border in the northeastwhere it terminates against northwest trending Paleoproterozoic Ubendian Belt.Zircon U-Pb dating results show that the magmatic emplacement ages of biotite monzogneiss,two-mica garnet plagioclase gneiss,garnet bearing biotite K-feldspar gneiss and garnet biotite plagiogneiss range from 1949±18 Ma to 2025±15 Ma,indicating that theywere formed in the Paleoproterozoic.The geochemical analysis results show that the gneiss magma in the eastern Isoka belongs to peraluminous high-k calc alkaline-shoshonitic series;the REE distribution pattern is obviously right dipping,rich in LREE and poor in HREE;Eu and Ce show extremely weak negative anomalies;the primitive mantle-normalized trace elements distribution patterns of the gneisses shows that the magma is enriched in Rb,K,La,Pb,Nd,Sm,and depleted in Ba,Nb,Ta,Ce,Sr,P,Ti and other elements.Based on the tectonic evolution of the Irumide Belt,it can be concluded that the gneisses in the eastern Isoka were formed in the Usagaran Epoch.