再论红旗营子群的时代问题

THE AGE OF THE HONGQIYINGZI GROUP─ A FURTHER DISCUSSION

红旗营子群相当于单塔子群（广义）上部，为一套低角闪岩相火山沉积岩。该群上部刘营组的斜长变粒岩，测得单颗粒锆石Ｕ－ｐｂ年龄为２４２３±２３Ｍａ；侵入该群的花岗岩锗石Ｕ－ｐｂ年龄与以上数值相近（或稍大）。凤凰咀组上部的斜长角闪岩测得Ｓｍ－Ｎｄ全岩等时线年龄２５８７±１７８Ｍａ、２６３９±１７４Ｍａ（相对应的ＴＤＭ为２７８８Ｍａ、２７１４Ｍａ）。根据测定样品产于不同层位或岩石之中，经受后期变质与变形作用的改造亦有差异，对测定的年龄值所反映的地质意义（即代表生成或者变质年龄）进行了分析与对比。作者认为上述碎屑锆石Ｕ－ｐｂ年龄值，是红旗营子群的变质年龄，该群形成时限为２６５０－２４５０Ｍａ区间，其时代应归属于新太古代晚期。

The Hongqiyingzi Group,equivalent to tbe upper part of the Dantazi Group(sensu lato),isrepresented by a suite of metapyroclastic rocks of low amphibolite facies. The plagioclase gran-ulitite of the Liuying Formation in the upper part of this group was determined to have a single-grain zircon U-pb age of 2423±23 Ma; the zircon U-pb age of granites intruded into this groupis close(or slightly older) than the above age value. Dating of amphibolite in the upper part ofFenghuangzui gave whole-rock Sm-Nd isochron ages of 258±178 Ma and 2639± 174 Ma (thecorresponding TDM ageas are 2788 Ma and 2714 Ma respectively).The samples were taken from different horizons or rock types, the rocks underwent late-stage metamorphism and deformation and hydrothermal modification,and methods and objectsof determination and different all these will give rise to notable differences and contradicto-ry phenomena;so the authors analyze and discuss the geological significance of these age values(referring to whether they represent formation or metan1orphism).Finally the authors holdthat from an analysis of substantial isotopic data the above clastic zircon ages are not terrige-nous inherited ages but should be metamorphic ones. The time range for the formation of theHongqiyingzi is between 2650 and 2450 Ma;thus its age should be late Neo-archean.