The Archaean gneiss complex in E. Hebei Province, North China: Geochemistry and evolution

Abstract

Abstract The Caozhuang and Shuichang gneiss complexes from the western margin of the Qianan granite-gneiss dome, eastern Hebei, North China were investigated for their isotopic and trace element geochemistry. The oldest rocks so far recognized in China occur near Caozhuang in the south of the area. The Caozhuang gneiss complex consists of supracrustal rocks and multiphase orthogneisses. The supracrustals comprise a sequence of amphibolites, quartzites, felsic gneisses, 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 from the amphibolites, and also from four felsic gneisses of the Caozhuang supracrustals. The orthogneisses from the Caozhuang complex appear to have been generated in three main cycles: at ca. 3.5, ca. 2.7 and ca. 2.5 Ga. The earliest Huangbaiyu gneiss was broken up and contaminated by the intrusions of granodiorites and late granitic gneisses. The granodiorites have yielded a ca. 2.7 Ga Rb-Sr isochron, the relatively high ISr[=0.7057±2 (2σ)] for the rocks suggesting that the granodiorites were formed by anatexis of older continental crust. Late granitic gneisses have a U-Pb zircon age of 2.5 Ga, and they are remarkably similar in appearance and chemical composition to the Huangbaiyu gneisses, suggesting that the gneiss complex as a whole is a mixture of gneiss types of widely different ages. The Shuichang gneiss complex contains supracrustal rocks, granitic rocks and charnockites. The supracrustal rocks include greywackes, semi-pelites and BIF, which have been metamorphosed in the granulite facies and occur within a polyphase complex of deformed leucogranites, microcline granites and charnockites. The age of the supracrustals is not known, except that they predate 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 mineral and chemical composition between granites and charnockites could result from partial melting processes occurring in the Shuichang metasedimentary gneisses.