摘要
牛塘界花岗岩体的时代，以往曾定为华力西期。通过研究岩体的地质特征、岩性特征、稀土配分、钻石晶体标型、同位素年龄资料，结合区域对比，认为其时代应为加里东（中）期。以往所用的Ｒｂ－Ｓｒ全岩等时线年龄２８９Ｍａ，因岩石强烈蚀变，不能作为衡量岩体结晶年龄的尺度。研究表明，锆石Ｕ－Ｔｈ－Ｐｂ四个不一致年龄中，只有￣（２０７）Ｐｂ／￣（２０６）Ｐｂ比值计算的年龄，最接近岩体的结晶年龄。牛塘界岩体锆石￣（２０７）Ｐｂ／￣（２０６）Ｐｂ年龄为４６０Ｍａ，形成于中奥陶世，时代为加里东（中）期。
The time of granite body at Niutanngjie，Ziyuan County，Guangxi，was ever ddfined tobe formed in the Variscan Stage. But the research on the geological characteristics， thepetrological property， the rare-earth distribution，the typomorphic crystal of zircon andisotopic age of the granite body combined with the regional correlation shows that the bodyshould be formed in the（middle）Caledonian Stage. The 289 Ma of Rb-Sr isochron age ofthe total rock can not be used as the criterion for judging the crystallizing age of thegranite body due to the intense alteration on the body. The research reveals that only theage calculated with (207)Pb／^(206)Pb ratio among the four different ages of U-Th-Pb isotopes ofzircon is the most close to the crystallizing age of the body. The^(207)Pb／^(206)Pb age of thezircon of the body is 460 Ma，showing that the Niutangjie granite body was formed in theMiddle Ordovician（middle Caledonian Stage）.
