铀矿区花岗岩锆石定年的“高铀效应”:来自鲁溪和下庄花岗岩SHRIMP锆石U-Pb年龄的启示  

“High-uranium Effect”on Zircon U-Pb Dating of Ore-related Granites in the Uranium Ore District:Evidence from Zircon SHRIMP U-Pb Geochronology of the Luxi and Xiazhuang Granites

作　　者：吴佳[1,2] 巫建华[1,2] 刘晓东[1,2] 王凯兴[1,2] 刘帅[1,2] WU Jia;WU Jianhua;LIU Xiaodong;WANG Kaixing;LIU Shuai(State Key Laboratory of Nuclear Resources and Environment,East China University of Technology,Nanchang 330013,Jiangxi,China;School of Earth Science,East China University of Technology,Nanchang 330013,Jiangxi,China)

机构地区：[1]东华理工大学核资源与环境国家重点实验室,江西南昌330013 [2]东华理工大学地球科学学院,江西南昌330013

出　　处：《大地构造与成矿学》2023年第2期449-460,共12页Geotectonica et Metallogenia

基　　金：中国核工业地质局“中国中–新生代铀成矿作用的基本特征与规律研究”(201475)资助。

摘　　要：下庄铀矿田是我国最重要的花岗岩型铀矿田之一,其中的复式花岗岩体已获得了一批锆石U-Pb年龄,基本上构筑了下庄铀矿田加里东、印支和燕山多期多阶段的岩浆演化系列。分析前人数据发现,不同学者获得的同一岩体的锆石U-Pb年龄差别较大,最大相差达17 Ma,主要原因之一可能是高U锆石的存在导致了加权平均年龄可靠程度的降低。针对这一问题,笔者以下庄铀矿田鲁溪粗粒黑云母花岗岩和下庄中粒黑云母花岗岩为研究对象,采用SHRIMP锆石U-Pb法测试U-Th-Pb含量,重点分析了锆石U含量对^(206)Pb/^(238)U年龄的影响,并厘定了这两个岩体的侵位年龄。测试结果显示,鲁溪粗粒黑云母花岗岩38颗锆石的U含量变化于152×10^(–6)~5687×10^(–6)之间,^(206)Pb/^(238)U年龄变化于234~1345 Ma之间,可分为五组,其中年龄最小一组由27颗锆石组成,变化于234~265 Ma之间,加权平均年龄为247.3±3.2 Ma,MSWD=3.9;下庄中粒黑云母花岗岩18颗锆石的U含量变化于243×10^(–6)~2071×10^(–6)之间,^(206)Pb/^(238)U年龄为227~871 Ma,可分为三组,其中年龄最小一组由15颗锆石组成,加权平均值236.5±3.3 Ma,MSWD=1.6。鲁溪粗粒黑云母花岗岩年龄最小一组的锆石U含量变化大,^(206)Pb/^(238)U年龄变化也比较大,且U含量>2000×10^(–6)的锆石^(206)Pb/^(238)U年龄存在不确定性,大部分呈明显偏大的现象,若剔除U含量>2000×10^(–6)的锆石^(206)Pb/^(238)U年龄数据,加权平均年龄为240.9±2.4 Ma(n=11,MSWD=0.38);下庄中粒黑云母花岗岩也有一颗锆石的U含量>2000×10^(–6),^(206)Pb/^(238)U年龄为245.4±4.7 Ma,存在年龄偏大的问题,若剔除这个数据,加权平均年龄为235.9±3.3 Ma(n=12,MSWD=1.4)。再结合前人的测试数据表明,铀矿区普遍存在锆石的“高铀效应”,锆石的U含量>2000×10^(–6)时,虽然也存在部分颗粒年龄变化不大的情况,但多数颗粒的^(206)Pb/^(238)U年龄明显偏大或偏小。因此,The Xiazhuang uranium ore field,one of the most important granite-related uranium deposits in China,is believed to be associated with Caledonian,Indosinian to Yanshanian phases of granite intrusions.However,the zircon U-Pb ages of the granites are highly varied,with age differences for single intrusion as high as more than 17 Ma.It may be due to the“high-uranium effect”of zircon.In this paper,the influence of zircon U content on^(206)Pb/^(238)U age is discussed on the basis of zircon U-Pb chronology of the Luxi coarse-grained biotite granite and the Xiazhuang medium-grained biotite granite in the Xiazhuang uranium ore field.The results show that the U contents of zircon from the Luxi coarse-grained biotite granite vary from 152×10^(–6)to 5687×10^(–6),and^(206)Pb/^(238)U age vary from 234 Ma to 1345 Ma.Five groups of zircon^(206)Pb/^(238)U ages can be discerned for the Luxi biotite granite,within which the minimum age group has a weighted average age of 247.3±3.2 Ma(MSWD=3.9).While zircon from the Xiazhuang medium-grained biotite granite has U contents varying from 243×10^(–6)to 2071×10^(–6),with^(206)Pb/^(238)U ages of 227 Ma to 871 Ma,and the minimum age group has a weighted average age of 236.5±3.3 Ma(MSWD=1.6).It is worth noting that zircon grains in the minimum age group of the Luxi coarse-grained biotite granite have highly varied U contents.It is remarkable that the zircon grains with U contents higher than 2000×10^(–6)have highly varied and likely meaningless^(206)Pb/^(238)U ages.If the zircon grains with U contents more than 2000×10^(–6)were excluded,the weighted average^(206)Pb/^(238)U age of the Luxi granite will be 240.9±2.4 Ma(n=11,MSWD=0.38).Similarly,if the only one of zircon grain with U content more than 2000×10^(–6)were excluded,the weighted average^(206)Pb/^(238)U age of the Xiazhuang medium-grained biotite granite will be 235.9±3.3 Ma(n=12,MSWD=1.4).Therefore,this study indicate that zircon grains with U contents of more than 2000×10^(–6)exhibit obvious“high-ur

关 键 词：花岗岩 高U锆石 SHRIMP锆石U-PB年龄 下庄铀矿田 

分 类 号：P597[天文地球—地球化学]