赣南蔡江A型花岗岩年代学、地球化学特征及地质意义

Chronology,Geochemistry and the Geological Significances of Caijiang A-type Granite in South Jiangxi Province

为系统研究赣南桃山复式岩体不同侵入体与铀成矿关系、评价总体铀资源潜力、摸清资源家底、指导铀矿勘查工作部署,对桃山复式岩体南西部的蔡江岩体开展岩石学、LA-ICP-MS锆石U-Pb年代学、岩石地球化学及产铀性等综合研究。结果表明,蔡江岩体主要为中粗粒似斑状黑云母二长花岗岩,锆石LA-ICP-MS U-Pb年龄为(227.4±1.6)Ma(MSWD=1.5),成岩时代为晚三叠世。岩石富硅、富碱、具较高K_(2)O/Na_(2)O及FeO^(T)/MgO值、低镁、低钙,属弱过铝质-过铝质系列。稀土含量较高,轻稀土明显富集、轻重稀土分馏明显,铕负异常明显,稀土配分模式图呈右倾“V”字形。高场强元素Th、U等明显富集,Zr、Y、Ga、Nb、Ta等含量普遍较高,亲石元素Rb、K及LREE明显富集,而Ba、Sr、P、Ti等强烈亏损,具有较高Zr+Nb+Ce+Y及1000×Ga/Al值。岩石成因类型属A型花岗岩,物源区以砂屑岩为主、含少量泥质组分,为伸展构造环境下形成的A型花岗质岩浆被动侵位于复式背斜核部虚脱部位而成。综合分析认为,印支期蔡江岩体铀含量较高,地球化学特征与华南其他的典型产铀花岗岩既有相似性又存在差异性,产铀能力一般;花岗岩的源区性质、成因类型是判别花岗岩产铀和不产铀的重要依据之一。

In order to systematically study the relationship between different intrusive rocks of the Taoshan composite granite and uranium mineralization,to evaluate the overall uranium resource potential,to find out the resources and to guide the uranium exploration,the Caijiang granite has been comprehensively studied in terms of petrology,LA-ICP-MS zircon U-Pb chronology,rock geochemistry and uranium production.The results show that the Caijiang granite mainly consists of Medium to coarse-grained porphyritic biotite adamellite granite,the age of zircon is(227.4±1.6)Ma(MSWD=1.5).The Caijiang granite is characterized by high silica content,high alkaline and high K_(2)O/Na_(2)O,FeO^(T)/MgO ratios,low content of MgO and CaO,and belongs to metaluminous to peraluminous.It has relatively high amounts of REE,and is characterized by significant enrichment of LREE,REE fractionation significantly,and depleted in Eu.REE pattern diagram showed asymmetric“V”shape.It has high quantity of HFSE Th,U,relatively high amounts of Zr,Y,Ga,Nb,Ta,and is characterized by enrichment of lithophile elements Rb,K,LREE,and depletion of Ba,Sr,P,Ti etc.It is high Zr+Nb+Ce+Y and 1000×Ga/Al contents.The geochemical characteristics indicate that the Caijiang granite belongs to A-type,it should be derived from anatexis of clay-poor clastic sandstone rocks at the extensional tectonic setting during the late Indosinian.It is concluded that the Caijiang granite contains higher U contents,and its geochemistry characteristics are similar to and different from those of other typical uranium-producing granites in south China.It has the normal uranium-production capacity.The source nature and genetic type of granites are one of the important criteria for distinguishing uranium-producing and non-uranium-producing granites.