热离子质谱测定的颗粒级锆石U-Pb不一致年龄数据处理的多边形准则——以大别山和东昆仑山深变质岩为例

POLYGONAL RULE FOR ASSESSING THE DISCORDANT U-Pb DATA OF SINGLE-GRAIN ZIRCON BY TIMS∶TAKING HIGH-GRADE METAMORPHIC ROCKS FROM DABIE AND EAST KUNLUN MOUNTAINS AS EXAMPLES

时代古老的前寒武纪中深变质岩中的锆石往往经历了不止一期构造热事件的改造。在理想情况下 ,不同期次的事件应有对应的不一致线 ,这一系列的不一致线可以构成一个封闭的多边形 ,相邻的高位不一致线的下交点与低位不一致线的上交点重合。然而 ,多期构造热事件的叠加使多数相邻不一致线的上、下交点并不重合。如果这两个上、下交点的年龄与其平均年龄的偏差在 5 %以内 ,则可视为有效年龄 ,否则 ,与数据点线性排列最接近于平行的不一致线可能是有效的 ,该不一致线的上、下交点年龄可能具有地质意义。若两条不一致线之下的数据点均不具线性排列 ,则各自的上、下交点年龄多半无地质意义。应用此准则处理大别山杂岩麻粒岩和东昆仑山片麻状混合花岗岩的锆石 U

Zircons from Precambrian high grade metamorphic rocks have undergone commonly more than one tectonothermal modification. Ideally,individual event should have one responding discordia;a set of such lines may form a closed polygonal.For two adjacent discordia,the lower intercept for the earlier event(LIUD) will have the same position as the upper intercept for the later(UILD).However,the LIUD and UILD are scarcely superposed because of multiphase tectonothermal superimposition.If the age difference LIUD and UILD is not larger than 5% from their average,then the mean age is considered to be effective, otherwise,the discordant line parallel to the better linearity defined by most data points might be effective and the ages of both upper and lower intercepts might have geological meaning.If there is no linearity of data arrangement for such line to parallel with,then both upper and lower intercept ages will have no geological meaning.Better results have been achieved for granulites in Dabie complex and gneissoid felsic migmatite from Eastern Kunlun Mountains using this assessing.