东川汤丹铜矿床石英真空击碎及其粉末阶段加热^(40)Ar-^(39)Ar年龄谱的含义

^(40)Ar-^(39) Ar DATING FOR A QUARTZ SAMPLE FROM THE TANGDAN COPPER DEPOSIT,DONGCHUAN,YUNNAN,BY CRUSHING IN VACUUM AND BY INCREMENTAL HEATING ON ITS POWDER

采用真空击碎技术提取东川汤丹铜矿床石英流体包裹体进行40Ar-39Ar法年龄测定，获得了逐渐下降的阶梯形年龄谱，表明流体包裹体含有过剩氩;数据点在40Ar／36Ar-39Ar／36Ar图解上构成等时线，等时线年龄为（712±33）Ma，代表了矿床的形成年龄。随后对其粉末进行40Ar-39Ar阶段加热（100—800℃）分析，形成相对比较平坦的年龄谱，坪年龄为（317±6）Ma（39Ar占45％，含真空击碎分析在内），粉末加热分析全部数据点构成的等时线年龄为（321±13）Ma。电子探针分析和显微镜观察证实了该石英样品中含有粒径大于100μm，K2O含量为8％-10％，形成时间明显晚于石英的白云母类富钾矿物，粉末阶段加热分析结果表明～320Ma应为后期白云母类矿物的年龄。

A quata sample DC-51-1Q from an ore hand specimen of the Tangdan copper deposit Dongchuan, middle Yunnan, was analysed with 40Ar-39Ar method by crushing in vacuurn and then by incremental heahng with temperature ranging from 100℃ to 800℃ on its powder through directly heahng outside the stainless steel tube which contained the sample during crushing. The crushing experiment forms a falling-down age spectrum and the apparent ages of last three stages c0nsist of a plateau with age of (710±5)Ma. The isochron plot of 40Ar/36Ar vs. 39Ar/36Ar based on all argon isotope data by crushing yields an isochron of (712±33)Ma, with inihal 40Ar/36Ar raho of 935±62. The age of^710 Ma is interpreted as the ore-forming age. The initial 40Ar/36Ar raho and the shape of the age spectrum by crushing indicate that excess 40Ar existS within the fluid inclusions and the larger ones contain more excess 40Ar. The fact of the isochron age in concordance with the plateau age of the last three crushing stages may imply that the smallest fluid inclusions do not contain excess 40Ar inside.A flat age spectrum is obtained by the incremental heahng experiment for the crushed powder with plateau age of (317±6)Ma. The regression line on the diagram of 40Ar/36Ar vs. 39Ar/36 Ar corresponds to an isochron age of (321±13)Ma.Although heating temperature was only up to 800℃, the frachon of 39Ar released by heahng takes up 69% to the total 39Ar, which indicates that K-rich minerals exist in the qurt sample. The electron-probe analyses show the size of K-rich minerals range from 10μm to 100μm. Their chemicaI components indicate that the K-rich minerals might be muscovite. Therefore, the age of ~ 320Ma is interpreted as the age of the muscovite formed by a later-period fluid.The remarkable difference between the age spectrum by crushing and that by heahng may indicate that the argon contained within the solid mineral is not released by crushing.