Geochemical evolution and metamorphic development of the early Precambrian in eastern Hebei, China

Abstract

Within the Archaean Qianxi Group in eastern Hebei, China, two roughly coeval tectonic units can be recognized: the Qian-an granulite complex and the Zunhua high grade greenstone belt. Both belong to the high grade terranes, and have recorded almost the same history of metamorphism. The granulite facies metamorphism was generally followed by a retrograde metamorphism of amphibolite facies and multistage anatexis. Despite the similar metamorphic histories, the geochemical features of the two units are different. 1.(1) the protoliths of the Qian-an granulite complex consist of a calc-alkaline volcanic series dominated by andesitic composition and a shallow-water sedimentary series, whereas that of the Zunhua greenstone belt clearly show the character of a bimodal magmatic series.2.(2) Most metabasalts of the Qian-an complex belong to the continental basalt type and grade into andesite, whereas in the Zunhua belt they are characterized by a suite of high-magnesian series, low-K2O tholeiites and a large amount of ultramafic—mafic layered complexes.3.(3) In terms of regional geochemistry the Qian-an complex is rich in potassium, and potassium charnockites of autochthonous anatectic and intrusive origin are widespread. In contrast, the Zunhua belt is rich in sodium and tonalitic gneisses of deep-seated mixed origin are widespread.Finally, a model for Archaean crustal evolution of high grade terranes is suggested.