湘东北仁里-传梓源铌钽矿床隐伏花岗岩锆石U-Pb年龄、Hf同位素特征及其地质意义

幕阜山复式花岗岩基位于扬子板块与华夏板块交接处的江南隆起带中段,是我国重要的稀有金属矿集区。仁里-传梓源超大型铌钽矿床位于幕阜山复式花岗岩基南缘的冷家溪群片岩中,是我国少有的高品位花岗伟晶岩型铌钽矿床,其下盘隐伏花岗岩为粗中粒似斑状黑云母二长花岗岩。全岩地球化学分析结果显示,隐伏岩体属过铝质高钾钙碱性岩系列,明显富集轻稀土元素(LREE),相对亏损高场强元素(如Nb、Ti)。锆石LA-ICP-MS U-Pb定年结果表明,该隐伏岩体形成于晚侏罗世(146.2±0.2 Ma)。岩石的εHf(t)值和二阶段模式年龄tDM2分别为-10.8～-5.3和1884～1537 Ma。结果表明,仁里-传梓源铌钽矿床隐伏花岗岩原岩源于中元古代地壳的熔融,并受到一定程度下地壳物质的影响。结合区域地质背景,该岩体形成于陆内挤压造山阶段结束后,挤压环境向伸展环境转变的大地构造背景,深部物质的加入也为仁里-传梓源铌钽矿床提供了成矿物质来源。

Zircon U-Pb Ages and Hf Isotopic Compositions of the Concealed Granite of Renli-Chuanziyuan Deposit,NE Hunan and Geological Significance

The Mufushan granite batholith in the middle part of the Jiangnan orogen,which is located at the junction of the Yangtze and Cathaysia plates,is an important rare metal ore concentration area of China.The Renli-Chuanziyuan super-large rare metal deposit is one of the few high grade granitic pegmatite Nb-Ta deposits in our country,which occurs in the schist of the Lengjiaxi Group at the south margin of the Mufushan granite batholith.The concealed granite is coarse-medium grain porphyroid biotite monzogranite,belonging to peraluminous high-K calc-alkaline series.The granite is enriched in LREE(large ion lithophile element)and relatively depleted in HFSE(high field strength elements,such as Nb,Ti).LA-ICP-MS U-Pb dating of zircon grains from the monzogranite indicate that the concealed granite was formed in the Late Jurassic(146.2±0.2 Ma).TheεHf(t)values of the concealed granite vary from–10.8 to–5.3,and two-stage model ages tDM2 vary from 1884 to 1537 Ma.These characteristics suggest that the primary magma was likely derived from melting of the Mesoproterozoic crust.Combined with the regional geological background,it can be inferred that the concealed granite was formed subsequent to intracontinental extrusion in a tectonic regime of transformation from extrusion to extension.