Composition and evolution of the archaean crust in central Henan, China

Abstract

The upper Archaean in central Henan Province is composed chiefly of rocks of the Dengfeng Complex and the Taihua Complex. Rocks of the Dengfeng Complex show essential features of a granite—greenstone terrane. They consist of greenstone belts and surrounding granitic and migmatite-gneiss terranes. When the late superposed structures are removed the Archaean conspicuously exhibits alternating gneiss domes and synformal basins which are occupied by granitic rocks and greenstone belts, respectively. The succession of the greenstone belts is divided into three groups: 1.(1) The lower group consists of ultramafic complexes which are only locally exposed.2.(2) The middle group, which comprises most of the greenstone belts, contains tholeiitic metabasalts and felsic volcanics which are bimodal in character.3.(3) The upper sedimentary group consists of molasse and turbidite formations.The metamorphic grade in the Dengfeng Complex is up to the medium-pressure almandine—amphibolite facies.The mafic volcanics are characterized by flat to slightly enriched REE patterns that resemble the TH2 type of Condie. In most other elements the rocks are also similar to the tholeiitic metabasalts in typical greenstone belts throughout the world. However, they possess certain unique characteristics. As for the granitic rocks, the early plutons tend to be Na-rich and are geochemically similar to tonalitic rocks, while the later intrusives tend to be K-rich.The Taihua Complex is composed primarily of felsic gneisses and metamorphosed supracrustal rocks interbedded with mafic-ultramafic rocks. The metamorphic grade in the complex ranges from granulite to upper amphibolite facies. Geochemical characteristics of the rocks are also similar to those of other high-grade terranes throughout the world. In general, an overall structural grain characterizes the Taihua Complex which constitutes an elongate fold belt with characteristic features of linear mobile belts that are frequently found in high-grade terranes.The extensive Dengfeng granite—greenstone terrane and the linear Taihua high-grade gneissic terrane are bounded by thrusts or contacts of unknown nature and together make up the late Archaean crust in central Henan. They may have developed more or less contemporaneously on a pre-existing sialic crust in this region. Controlled by shallow convection cells in the upper mantle, their development may have occurred in two different tectonic settings; the rift-type and the subsiding-type setting, respectively.