冀东朱杖子群定年新证据

NEW EVIDENCE FOR DATING THE ZHUZHANGZI GROUP OF EASTERN HEBEI

青龙河绿岩带的层状岩系由双山子群和朱杖子群变质地层组成,分布在北自青龙县黄杖子、经双山子到南部庙岭一带。多数地质工作者认为双山子群的时代为新太古代,但对朱杖子群的时代归属则意见不一,至今尚未获得统一的认识。朱杖子群为浊积岩建造,由下部以变质砾岩为主的张家沟组和上部以黑云变粒岩等岩石的木孛罗台组所组成。区内广泛分布花岗斑岩脉,断续延伸十余千米,岩脉最宽处达300—400m,岩脉明显穿切朱杖子群木孛罗台组黑云变粒岩。花岗斑岩主要由钾长石、石英、更长石及黑云母组成。对花岗斑岩样品进行TI MS法锆石U-Pb同位素年龄测定,获得(2492±8)Ma年龄数据。说明被侵入的朱杖子群形成年龄应大于此数据,结合下伏的双山子群的年龄数值,朱杖子群形成在2641Ma(或2793Ma)—2492Ma之间,属新太古代而不是古元古代。同时获得青龙河绿岩带最晚期的浅成侵入岩的确切年龄和青龙河绿岩带一个较完整的太古宙花岗质岩石系列演化和成矿作用的年代格架。

The bedded rock series of the Qinglonghe Greenstone Belt consists of metamorphic strata of the Shuangshanzi and Zhuzhangzi Groups and is distributed from Huangzhangzi of Qinglong County in the north, through Shuangshanzi to the Miaoling in the south. Most geologists regard the Shuangshanzi Group to be Neoarchean in age but hold different views on its age assignment up to now. The Zhuzhangzi Group is a turbidite formation, with its lower part being the Zhangjiagou Formation consisting mainly of metamorphic conglomerate and its upper part being the Boluotai Formation consisting of biotite granulite. Granite-porphyry veiws are widespread in this area and extend intermittently for over ten km with the widest veiws reaching 300-400 m. Veiws evidently intercalate into the biotite granulite of the Boluotai Formation of the Zhuzhangzi Group. Granite is made up chiefly of K-feldspar, quarts, oligoclastite and biotite. The zircon is U/Pb dated to the 2 492±8 Ma by using the TIMS method, indicating that the age for the formation of the Zhuzhangzi Group should be greater than this datum. Combined with the age of the underlying bhuang-shanzi Group, the Zhuzhangzi Group might be formed between 2 461 （or 2 793） and 2 492 Ma, belinging to the Neoarchean, not the Paleoproterozoic. The actual age of the latest hypabyssal intrusive rocks of the Qinglonghe Greenstone Belt and the age framework of a fairly complete serial evolution and the ore-forming process of the Qinglonghe Greenstone Belt are also obtained at the same time.