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    赞比亚东北部卡萨马群形成环境:碎屑锆石U-Pb年龄与Hf同位素的限定

    Depositional environment of the Kasama Group, northeastern Zambia: Evidence from detrital zircon U-Pb-Hf isotopic compositions

    • 摘要: 赞比亚东北部班韦乌卢地块受到其东北部的古元古代乌本迪构造带、西北部的中元古代基巴拉构造带、东南部的中元古代伊鲁米德构造带及西南部泛非期卢弗里安构造带的影响。班韦乌卢地块主要由基底和沉积盖层两部分组成。其中, 基底主要由片岩带、安山质-流纹质变质火山岩、花岗岩类岩基及其他侵入体组成。沉积盖层从老至新依次为姆波罗科索群、卡萨马群、加丹加超群及新生代的河流相和湖泊相沉积。本文对卡萨马地区的卡萨马群地质特征、碎屑锆石U-Pb-Hf同位素及地球化学特征进行了系统研究, 获得卡萨马群中紫红色粉砂岩的32颗碎屑锆石U-Pb年龄多集中在(1849±23)~(1993±21)Ma, 卡萨马群的形成时代可能晚于(1434±14)Ma, 属于中元古代, 其中22颗锆石(176Hf/177Hf)i为0.281049~0.281618, εHft)为-12.8~-1.6, 二阶段模式年龄变化范围为2406~3487 Ma。对卡萨马群的砂岩进行了稀土元素分析和微量元素分析, 结果显示卡萨马群砂岩δEu负异常明显, 投点主要落在沉积岩区; 微量元素显示砂岩形成于被动大陆边缘环境。

       

      Abstract: The Bangweulu Block of northeastern Zambia which consists mainly of basement and sedimentary rocks is affected by northeastern Paleoproterozoic Ubendian Belt, northwestern Mesoproterozoic Kibaran Belt, southeastern Mesoproterozoic Irumide Belt and southwestern Pan African Lufulian Belt. The basement rocks are mainly composed of schist, andesitic-rhyolitic metamorphic volcanic rocks, granitoids and other intrusions. From lower to upper part the sedimentary rocks are mainly composed of the Mporokoso Group, the Kasama Group, the Katanga supergroup and Cenozoic fluvial and lacustrine sediments. The authors systematically studied the geological characteristics of the Kasama Group, U-Pb-Hf isotopic compositions and geochemical characteristics of detrital zircon from the Kasama Group in Kasama area and obtained 32 detrital zircons U-Pb ages of the purplish red siltstone from the Kasama Group. Specifically, their ages mainly vary from (1849±23) to (1993±21) Ma, suggesting that the Kasama Group was formed younger than (1434±14) Ma and should belong to Mesoproterozoic. (176Hf/177Hf)i values of 22 zircons are 0.281049-0.281618 and εHf(t) values are-12.8--1.6 with corresponding TDM2 ages of 2406-3487 Ma. The REE and trace elements of sandstones from Kasama Group were analyzed. The REE have obviously negative δEu anomalies and the points mainly fall in the sedimentary area; the trace elements characteristics of sandstones show that sandstones formed in the passive continental margin.

       

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