Geochemistry and petrogenesis of the Xiong'er group in the southern regions of the North China Craton

Abstract

The volcanics of the Xiong'er Group are widely distributed in Henan, Shanxi provinces, distributed over an area of more than 60000 km 2. Most of the volcanic sequences vary from 3 km to 7 km in thickness, overlying unconformably the crystalline basement of Archean and palaeo-Proterozoic age, and covered with the middle-upper Proterozoic terrestrial clastic rocks and carbonate rocks. The volcanic rocks of the Xiong'er Group appear as a bimodal association, and rocks with SiO 2 = 62% ± are rare. The rocks are composed predomiantly of basaltic andesites and andesites, and minor dacitic-rhyolitic rocks. The mineral assemblage of mafic-intermediate rocks mainly contains pyroxene and plagioclase Amphiboles and bitotites have not been observed, suggesting an anhydraous environment of their formation. The volcanic rocks of the Xiong'er Group are rich in K 2O and FeO and poor in Al 2O 3, MgO and CaO. Geochemically the rocks are highly enriched in large-ion lithophile elements (LILE, e. g., K, Rb, Ba) and light REEs (LREE) and are depleted in high field strength elements (HFSE, e. g., Nb, Ta and Ti), probably reflecting the involvement of the continental materials during their eruption, or a derivation from an enriched lithospheric mantle source. The chemical and isotopic data presented in this paper show that the characteristics of the rocks are mainly controlled by a modified enriched mantle source with ancient subduction belt materials during the formation and consolidation of the Archean to paleo-Proterozoic continental crust. Based on the geological features of the Xiong'er Group and the underlying basement, it is suggested that the Xiong'er Group is originated from a failed trident continental rift environment.