南岭东段枫树洞稀土矿中辉长质包体锆石U-Pb年代学、地球化学特征及其成岩作用

Zircons in the gabbro enclaves of Fengshudong REE deposit in Eastern Nanling Mountains:Chronology,geochemistry and diagenesis

南岭东段是我国稀土矿产资源集中分布区,枫树洞稀土矿是南岭东段风化壳淋积型矿床,通过对该矿床含矿花岗岩层中的包体进行锆石U-Pb年代学和地球化学研究,发现稀土矿层中存在2种辉长质包体:早侏罗世包体((188±2.8)Ma)和晚三叠世包体((227±3.7)Ma)。包体中锆石阴极发光图像(CL)和锆石Th/U值普遍>0.4,指示为岩浆锆石,大多数锆石稀土元素分布在热液锆石与岩浆锆石过渡区,说明包体中的岩浆锆石遭受后期热液的强烈改造,晚三叠世包体改造最强烈。早侏罗世包体主要锆石Ti温度为694~1279℃,平均值为816℃,大多数>800℃;晚三叠世包体主要锆石Ti温度为687~925℃,平均值为671℃,大多数<750℃,包体可能来源于含水条件下发生的部分熔融。包体锆石年龄中存在多组年龄数据,暗示南岭东段晚三叠世—早侏罗世,该地区曾发生了多次岩浆活动,反映了古太平洋板块对华南板块的影响由弱到强的变化过程,在这种俯冲背景下的岩浆由陆壳部分熔融形成,幔源物质活动也呈现出由弱至强的趋势。

The eastern Nanling Mountains is the concentrated area of rare earth element(REE)mineral resources in China.By studying the zircon U-Pb age and trace elements of enclaves in ore-bearing granite beds of weathered leaching deposits in Eastern Nanling area,the results show that there are at least two types of gabbro enclaves in REE deposits,namely the Early Jurassic enclave(188±2.8)Ma and Late Triassic enclave(227±3.7)Ma.The zircon CL image and Th/U>0.4 indicate the type is magmatic zircon.The variation characteristics of zircon REEs in the enclave reveal that most zircons are distributed in the transition zone between hydrothermal zircons and magmatic zircons,indicating that the zircons undergo intense hydrothermal reformation in later period and the Late Triassic enclave is most strongly reformed.The Ti temperature of major zircons in the Early Jurassic enclave is 694~1279℃,averagely 816℃,nearly half above 800℃,while that for the Late Triassic inclusions is 687~925℃,averagely 671℃,most below 750℃,which may originate from the partial melting of various components under the condition of water near saturation.There are multiple groups of zircon age data for the enclaves,showing several magmatic activities occurred in the eastern Nanling during Triassic-Early Jurassic,and the Late Triassic rock mass was melted by Early Jurassic magma,then by Early-Middle Jurassic magma,which reflects the influence of paleo-Pacific Plate on South China Plate is from weak to strong,magma formed by the partial melting of continental crust under the subduction background and the mantle-derived material activity also from weak to strong.