摘要
中条山既是一个典型的前寒武纪岩石出露区,又是不同类型铜矿的集中区。使用6种测年方法,共取得55个年龄数据。据此:1)比较和讨论了不同方法测年结果的可靠性,指出在中条山单颗锆石U-Pb和离子探针质谱等年龄是较为可靠的;2)用较可靠的年龄建立了年代构造格架,表明该元古宙活动带年龄正落于全球岩浆活动稀少的2400—2000 Ma的时限内。格架还反映了活动带的重大事件;3)用捕获锆石年龄和亏损源钕模式年龄(TDM)以及地球化学和地质学为依据,建立的中条山前寒武纪年代地壳结构模式,是华北克拉通的一种地壳结构模式的代表。
Five bimodal volcanic successions and five granitic rocks of the Zhongtiao Mountains have been dated by single zircon U-Pb, ion microprobe, single zircon Pb-evaporation, conventional zircon U-Pb and Sm-Nd and Rb-Sr whole-rock isochron methods, and 55 age data have been obtained for these rocks. This combined isotopic study has enabled us to compare the reliability of the six methods as well as to constrain the age of chronotectonic framework. The results show good concordance between single zircon U-Pb and ion microprobe methods. The combination of these precise zircon U-Pb ages and a few of reliable Rb-Sr whole-rock isochron ages representing geological events permits constructing a perfect chronotectonic framework of the mobile belts and defining the ages of the Jiangxian movement (2100 Ma) and Zhongtiao movement (1900—1850 Ma). The age of 1850 Ma is a benchmark for division between the basement and cover for the North China Craton. It is particularly interesting that the ages of igneous rocks and their associated copper deposits fall in a time span (2400—2000 Ma) during which global magmatic records are lacking.
Xenocrystal and inherited zircon U-Pb ages for the abovementioned igneous rocks can be usually obtained by the three single zircon U-Pb methods that might contrain the age of felsic rocks at depths. The Nd model age (T_(DM)) of basic rocks can be used to infer the age of initial seperation of deep mafic rocks from depleted mantle. The characteristics of deep rocks and their positions can be studied by major and trace elements and Nd and Sr isotopic geochemistry. This comprehensive study is called a probe for the lithosphere using igneous rocks and it can give a model of chronocrustal structure of the mobile belt, which has been proved by geological observation from surrounding high-grade terrains.
出处
《地质学报》
EI
CAS
CSCD
北大核心
1991年第3期216-231,共16页
Acta Geologica Sinica
