北秦岭光石沟花岗伟晶岩型铀矿床成矿时代:来自独居石原位微区U-Th-Pb定年约束

Metallogenic Age of the Guangshigou Pegmatite Type Uranium Deposit,North Qinling:Constraints From U-Th-Pb Dating of Monazite

丹凤秦岭地区光石沟铀矿床是我国典型伟晶岩型铀矿床之一。为探究该矿床的形成时代,文章利用微区、原位分析技术SEM、LA-ICP-MS对5件与晶质铀矿共生的独居石样品开展了形貌特征、稀土元素及U-Pb年龄测定。结果表明:独居石BSE图像没有明显的成分环带,化学成分具有高Th、Th/U的特征,稀土元素标准化图解表明其为高温岩浆作用相关成因;独居石的U-Th-Pb同位素年龄为(403.5±3.6)Ma、(400.2±4.3)Ma、(399.6±0.78)Ma、(400.8±2.4)Ma,表明它们具有相近的形成时代(早泥盆世)。结合前人研究工作,认为在早泥盆世,北秦岭地区商丹洋壳北向俯冲导致秦岭微陆块发生变质、深熔、岩浆作用,同时发生铀成矿作用,富集形成了光石沟铀矿床。

The Guangshigou uranium deposit in the north Qinling area is one of the typical pegmatite type uranium deposits in China.In order to know the age of the deposit,the morphology,REE and U-Th-Pb ages of 5 monazite samples related with uraninite deposits were determined by using in situ analysis techniques SEM and LA-ICP-MS.The results showed that the BSE image of monazite showed no obvious composition zone,and the chemical compositions were characterized by high Th and Th/U ratios,the normalized diagram of rare earth elements indicates that the origin is related to high temperature magmatism.The U-Th-Pb isotopic age of monazite is(403.5±3.6)Ma,(400.2±4.3)Ma(,399.6±0.78)Ma(,400.8±2.4)Ma,which suggest that they had the same age of formation(Early Devonian).Combined with previous studies,it was believed that the Shangdan oceanic crust subduction led to the metamorphism,deep melting,magmatism and uranium mineralization in the Qinling micro continental block in the Danfeng area during the Early Devonian,and finally formed the Guangshigou uranium deposit.