GEOCHRONOLOGICAL STUDY OF ARCHAEAN GRANITE-GNEISSES IN ANSHAN AREA, NORTHEAST CHINA
Three suites of rocks (granite-gneisses, potash granites and iron formation) occur in the Anshan area, Northeast China. Th granite-gneisses exposed in the Tiejiashan Hill are considered as the oldest rocks based on the stratigraphic and tectonic relations. Rb-Sr and Pb-Pb whole-rock isochron dating and U-Pb zircon dating of Tiejiashan gneisses have been done. The Rb-Sr isochron age is 2829±62m.y. (2σ) with an initial 67Sr/86Sr ratio of 0.70261±106. The Pb-Pb systematics yields t1=2862±51 m.y. (2σ) and t2=130 m.y. The consistent results indicate that the Tiejiashan gneisses were formed at 2830-2860 m.y. ago. Ion probe analyses of two samples by W. Compston showed that U-Pb age given by the, upper intersection of the discordia was 2,970 m.y. Ho wever, the two samples deviate far from the Pb-Pb whole-rock isochron. The initial 87Sr/ 86Sr ratio,μ0 and petrochemical data indicate that these rocks were derived from remelting of crustal materials. The formation age of the primordial crust in this area was estimated at 3.0-3.3 b.y. At least two events of magmatism (2.37 and 1.70 b. y.) have been confirmed by Rb-Sr dating, which caused the increase of K, and Rb/Sr, U/Pb and Th/U ratios in the gneisses. These rocks have a Th/U range from 3.3 to 6.5, which is in agreement with that of ore lead from China's continent, indicating that the ore lead enriched in 208Pb in China was associated with Archaean craton.