PRECAMBRIAN GEOCHRONOLOGY, CHRONOTECTONIC FRAMEWORK AND MODEL OF CHRONOCRUSTAL STRUCTURE OF THE ZHONGTIAO MOUNTAINS
Sun Dazhong, Li Huimin, Lin Yunxian, Zhou Huifang, Zhao Fengqing and Tang Min (Tianjin Institute of Geology and Mineral Resources, CAGS, Tianjin)
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.