石榴子石U-Pb定年在矽卡岩矿床中的应用:以鄂东南高椅山硅灰石(-铜)矿床为例

Application of Garnet U-Pb Dating in the Skarn Deposit:A Case Study of Gaoyishan Wo(-Cu)Deposit in Southeast Hubei Province

石榴子石原位U-Pb定年是近年来新发展的低铀矿物同位素定年方法,目前在矿床中成功应用的实例较少,尤其是在非金属矿床中更为罕见.基于详细的岩相学观察,在鄂东南高椅山硅灰石(-铜)矿床中厘定出两期石榴子石,分别为第一期深棕色石榴子石Grt1和第二期浅棕色石榴子石Grt2.电子探针成分分析(EMPA)表明,两期石榴子石均属于钙铁榴石-钙铝榴石固溶体系列,其中Grt1相对富Fe(Adr62.4Gro36.5~Adr94.4Gro0),而Grt2相对富Al(Adr32.6Gro66.4~Adr40.2Gro58.6).对Grt1和Grt2石榴子石进行LA-ICP-MS U-Pb定年,获得T-W下交点206Pb/238U年龄分别为142.5±2.0 Ma(2σ,MSWD=1.30,n=38)和136.0±14.0 Ma(2σ,MSWD=0.42,n=17),与矿区内广泛出露的石英二长闪长岩锆石206Pb/238U加权平均年龄(139.8±1.5 Ma;2σ,MSWD=0.10,n=22)在误差范围内一致,证明二者之间存在密切的成因联系.高椅山Grt1石榴子石具有较高的U含量和较低的普通铅含量,此为U-Pb同位素测年成功的主要因素.

In recent years,in situ U-Pb dating of garnet is a newly developed isotopic dating method about the low-U minerals,yet it has been scarcely applied in the ore deposits and especially in the nonmetal deposits.Based on detailed petrographic observations,two generation of garnets have been distinguished from the Gaojiashan Wo(-Cu)deposit in Southeast Hubei Province,namely the first generational garnet(Grt1)with dark-brown color and the second generational garnet(Grt2)with lightbrown color.Electron probe composition analysis(EMPA)shows that both of them belong to andradite to grossularite solid solution series,in which Grt1 is relatively rich in Fe(Adr62.4Gro36.5 to Adr94.4Gro0),while Grt2 is relatively rich in Al(Adr32.6Gro66.4 to Adr40.2Gro58.6).LA-ICP-MS U-Pb dating on Grt1 and Grt2 yield Tera-Wasserburg lower intercept 206Pb/238U age of 142.5±2.0 Ma(2σ,MSWD=1.30,n=38)and 136.0±14.0 Ma(2σ,MSWD=0.42,n=17),which is consistent with the weighted mean 206Pb/238U ages(139.8±1.5 Ma;2σ,MSWD=0.10,n=22)of zircon in the quartz monzodiorite at Gaoyishan within the error range,indicating a close genetic relationship between them.The Grt1 with high U concentration and low common Pb content are the major factors for the success of U-Pb isotopic dating.