长城系团山子组火山岩颗粒锆石U－Pb年龄及其地质意义

GRAIN ZIRCON U-Pb AGES FOR VOLCANIC ROCKS FROM TUANSHANZI FORMATION OF CHANGCHENG SYSTEM AND THEIR GEOLOGICAL IMPLICATIONS

蓟县中－上元古界典型剖面之邻区平谷县境内，长城系团山子组连续发育两层厚度各为１ｍ左右的富钾粗面安山岩，发育气孔构造和绳状构造。其中存在三种不同成因类型锆石，应用颗粒锆石Ｕ－Ｐｂ测定年龄技术，得到：（１）自生锆石，其Ｕ－Ｐｂ不一致上交点年龄为１６８３±６７Ｍａ，代表该火山岩形成年龄；（２）捕获锆石，多为原碎屑锆石，较年轻的一种捕获锆石的２０７Ｐｂ／２０６Ｐｂ表面年龄为１８２３±６８Ｍａ，这限定了长城系底界年龄应新于１８２３Ｍａ；（３）后生热液锆石，其Ｕ－Ｐｂ年龄为４６０Ｍａ左右，反映后期热液事件发生的时代。

Two about 1m thick layers of K-rich trachyandesite were found in the Tuanshanzi Formation of the Changcheng System in Pinggu County adjacent to the typical Mid-Upper Proterozoic profile of Jixian County. Ropy structure and porous structure are well-developed in the volcanic rock. Three genetically different zircons from the volcanic rock have been dated using the single zircon U-Pb method.The results show that: (1) the authogenic zircon has an upper intercept age of 1683±67Ma from the U-Pb concordia curve, which should be interpreted as the formation age of the volcanic rock;(2) the captured sedimentary detrital zircon has an appearance 207Pb/208Pb age of 1823±68Ma, suggesting that the basal Changcheng System be younger than 1823Ma;and (3) the catagenetic hydrothermal zircon has a U-Pb age of about 460Ma, an estimate for the age that late hydrothermal process occured.