鞍山地区太古代岩石同位素地质年代学研究

GEOCHRONOLOGICAL STUDY OF ARCHAEAN ROCKS IN ANSHAN, LIAONING PROVINCE

鞍山本溪地区太古代变质岩可分为三套,即含铁的表壳岩建造、侵入于铁建造中的花岗质片麻岩和铁架山奥长花岗质-花岗质片麻岩,后者为表壳岩的基底。原划为上鞍山群樱桃园组(齐大山矿带)和山城子组(歪头山-北台矿带)的斜长角闪岩分别获得2729 Ma和2724Ma的Sm-Nd等时线年龄。这就为有争议的鞍本地区铁建造属于同一时代提供了依据,并讨论了表壳岩中的变质沉积岩以及铁架山基底片麻岩的同位素年代。

Three suites of Archaean rocks occurred in the Anshan-Benxi area, northeast China. They are supracrustal rocks (iron formation), Anshan granitic gneisses intruded in the supracrustal rocks, and Tiejiashan trodhjemite-granite gneisses as the basement of the Anshan group. The Sm-Nd and Rb-Sr whole rock isochron ages have been determined.The Sm-Nd age of amphibolites in the Yingtaoyuan formation of Anshan group in Qida-shan Mine is 2729 ± 245 (2o) Ma with an initial ratio INd = 0.509252 ± 253 and εNdl(T) = 3.03 ±4.97. The Sm-Nd age of amphibolites from Bianniu site of Benxi area is 2724 ±102 (2σ) Ma with an initial ratio of 0.509265±113 (2σ), and εNd (T)=3.2 ± 2.2. The Rb-Sr analysis data indicate that the Rb-Sr decay system was opened. So the Anshan group has been formed about 2,7 Ga ago.The Sm-Nd isochron age of Anshan granitic gneisses is 2520 ± 220 (2σ) Ma and εNd(T) = -8.72. It implies that these gneisses were formed from the melted crustal materials.The Sm-Nd age of the metasediments in Nanfen area is 3278 ± 278 (2σ) Ma. This age is not real age of the metasediments, but the age of protolith derived from the mantle. This age implies that the old crust in Anshan-Benxi area was formed about 3.2 Ga ago.