三江口东岩体岩石成因及产铀潜力分析

摘要

诸广山南体是中国重要的铀资源基地之一,复式岩体内与铀矿床关系密切的花岗岩均为S型,I型花岗岩一般无铀成矿潜力.三江口岩体位于诸广山南体西部,与产有铀矿床的燕山期长江岩体相邻,岩性也与长江岩体相似.目前对三江口岩体研究程度相对薄弱,缺乏高精度年代学研究,岩石成因类型也未明确.文章对三江口岩体东部(简称三江口东岩体)进行U-Pb年代学和岩石地球化学研究,并与产铀的长江岩体进行对比.LA-ICP-MS锆石U-Pb年龄为161.9±2.1 Ma,属于燕山早期花岗岩.岩体富SiO2(73.5％～76.1％),贫FeOT(1.12％～3.25％)、MgO(0.07％～0.83％)、CaO(0.64％～1.27％),具高分异指数DI(86.4～93.6),A/CNK值为1.00～1.35,属过铝-强过铝质花岗岩;微量元素Ba、Sr、Nb、Eu、Ti亏损,Rb、Th、U富集,属于典型的低Ba、Sr花岗岩;稀土总量中等(∑REE=119×10-6～268×10-6),稀土配分模式为右倾的轻稀土富集型;(87Sr/86Sr)i值为0.70789～0.71488,εNd(t)值较低(-10.8～-9.6),两阶段Nd模式年龄为1.73～1.83 Ga.三江口东岩体与长江岩体年龄相近,具有相似的岩石地球化学特征和同位素组成,均为S型花岗岩.结合两岩体形成年龄和区域构造背景,认为其形成与华南燕山早期陆内伸展作用有关,是由华南基底麻源群泥质岩、砂质岩部分熔融形成,在成岩过程中有少量幔源物质参与.通过与产铀的长江岩体对比研究认为,三江口东岩体具有较强的产铀潜力.%The southern Zhuguangshan composite pluton(SZCP)is one of the important uranium resource bases of China.The S-type granites in SZCP have a close relationship with uranium ore deposits, while the I-type granites in SZCP possess no uranium metallogenic potential. The Sanjiangkou granitic pluton in the western part of SZCP has similar lithology to the adjacent Yanshanian Changjiang granite which is a S-type granite and hosts uranium ore deposits. Till present, the Sanjiangkou granite has rarely been studied, its precise chronology is lacked and its genetic type needs to be determined. In this paper, an integrated study including geochronology, petrogeochemistry and Sr-Nd isotopic geochemistry of eastern Sanjiangkou granite and comparison with uranium-bearing Changjiang granite has been conducted.LA-ICP-MS zircon U-Pb dating for the eastern Sanjiangkou granite yields an age of 161.9±2.1 Ma,suggesting its emplacement in the early Yanshanian period.Chemically,the granite has high SiO2contents(73.5％～76.1%)and low FeOT(1.12%～3.25%),MgO(0.07%～0.83%)and CaO(0.64%～1.27%)contents,thus is highly fractionated with high differentiation index(DI)values(86.4～93.6).A/CNK values of the granite vary from 1.00 to 1.35,indicating that it is a peraluminous to strongly peraluminous granite.The granite is depleted in Ba,Sr,Nb,Eu and Ti,but enriched in Rb,Th and U,showing characteristics of a typical low Ba and Sr granite.The rock samples display medium total REE contents(∑REE=119×10-6～268×10-6),and their REE distribution patterns are right-inclined.Isotopic geochemical analysis results reveal variable initial 87Sr/86Sr ratios(0.70789～0.71488), yet low and constant εNd(t) values (-10.8～-9.6) with two stage model ages of 1.73～1.83 Ga. The geochemical and geochronological characteristics of the eastern Sanjiangkou granite are similar to those of the Changjiang granite.Both of them are believed to be S-type granitoids.Combining zircon U-Pb dating and regional tectonic setting,we suggest that the genesis of these granites were likely related to the crustal extension during the early Yanshanian in South China,and they were generated through partial melting of the Mayuan Group para-metamorphic rocks, with possibly minor involvement of mantle materials. Compared with the Changjiang granite, the eastern Sanjiangkou granite is believed to possess prosperous uranium mineralization potential.