The Archaean high grade gneiss terrain in E Hebei Province, NE China: geological framework and conditions of metamorphism

Abstract

Summary The Archaean high-grade gneiss terrain of eastern Hebei Province, N.E. China, is unusual in that it contains abundant banded iron-formations at amphibolite or granulite facies. The Archaean terrain is divisible into three: an area between Taipingzhai and Malanyu comprising granulite-facies tonalitic gneisses formed about 2.5 Ga (the Qianxi Gneisses); farther south is an amphibolite-granulite-facies terrain consisting of enclaves of iron-formations and metasediments (Shuichang supracrustal association) within orthogneisses (Qianan Gneisses) which are in general more potassic; in the extreme south of the Archaean outcrop the oldest rocks, the Caozhuang group ( c. 3.5 Ga in age) consist of a series of amphibolites, iron-formations and metasediments enclosed within granodioritic and granitic gneisses. The Qianan Gneisses comprise ‘charnockites’ of c. 2.7 Ga age, tonalitic to granodioritic gneisses and a variety of highly deformed granodioritic and granitic gneisses of uncertain age. P-T conditions range from 7–8 kb and c. 700°C in the north near Taipingzhai to 5–6.5 kb and 650–750°C near Shuichang and 4.5–6.0 kb and 600–650°C in the southernmost area. Thus deeper crustal levels are exposed in the north, but it is not known whether the transition is gradual or whether different crustal levels have been brought together by thrusting or by vertical movements along shear zones.