冀东迁安地区太古代片麻杂岩的地球化学和演化

曹庄片麻杂岩包括35亿年的表壳岩及三个不同时期的复期正片麻岩。水厂片麻杂岩包括水厂表壳岩及侵入其中的淡花岗岩和紫苏花岗岩,后者年龄为26.5亿年。

GEOCHEMISTRY AND EVOLUTION OF ARCHEAN GNEISS COMPLEXES IN QIAN'AN AREA, EAST HEBEI

The Caozhuang and Shuichang gneiss complexes on the western margin of the Qian’an granite-gneiss dome, eastern Hebei, North China, were studied from their isotopic and trace element geochemistry. The oldest rocks so far recognised in China occur near Caozhuang, south of the area.The Caozhuang gneiss complex consists of supracrustal rocks and multiphase orthogneisses. The supracrustal rocks represent a sequence of amphibolite, quartzite, felsic gneisse, marble and BIF, which are thought to be laid down in a shallow water environment, A ca. 3.5 Ga sm-Nd isochron has been obtained for the amphibolite, and also for four felsic gneisses of the Caozhuang supracrustal rocks. The orthogncisses of the Caozhuang complex appear to have been generated in three main cycles: at ca. 3.5 Ga, ca. 2.7 Ga and ca. 2.5 Ga respectively. The earliest Huangbaiyu gneiss was broken up and contaminated by the intrusions of grano-diorites and late granitic gneisses. The granodiorites have yielded a ca. 2,7 Ga Rb-Sr isochron and relatively high I_Sr (= 0.7057±2(2σ)). It suggests that the granodiorites were formed by anatexis of older continental crust. Late granitic gneisses have ist U-Pb zircon age of 2.5 Ga,. and are remarkably similar in appearance and chemical composition to the Huangbaiyu gneisses. It suggests that the gneiss complex as a whole is a mixture of different types of gneisses in a wide age range.The Shuichang gneiss complex contains supracrustal rocks, granites and charnockites. The supracrustal rocks include greywackes, semi-pelites and BIF, which have been metamorphosed in to granulite facies and occur within a polyphase complex of deformed leucogranite, micro-cline granite and charnockite. The age of the supracrustal rocks is undetermined, but may be larger than charnockites of 2.65 Ga. There is a close relationship between the production of the granites and the charnockites. It is suggested that differences in mineralogical and chemical compositions of granites and charnockites could result from partial melting processes occurred in the Shuichang metasedimentary gneisses.