Different sources involved in generation of continental arc volcanism: The Carboniferous–Permian volcanic rocks in the northern margin of the North China block

Abstract

New zircon U–Pb dating results on the Carboniferous–Permian volcanic rocks in the northern margin of the North China block (NCB) indicate their eruption during the Early Carboniferous to Late Permian from 347±3Ma to 258±1Ma and a slight decrease of the upper limits of the volcanic sequences from west to east. They have a main rock association of basalt, basaltic andesite, andesite, dacite, rhyolite, tuff, and tufaceous sandstone. Most of them have calc-alkaline compositions and exhibit variable SiO2 contents from 48.2wt.% to 77.1wt.%. There is no significant gap between the mafic and felsic volcanic rocks in major and trace element classification diagrams, indicating that they are not bimodal in composition. The Carboniferous–Permian volcanic rocks exhibit subduction-related geochemical features such as negative Nb and Ta anomalies of mafic to intermediate rocks on primitive mantle-normalized diagrams, indicating they were formed in an Andean-type continental arc during southward subduction of the Paleo-Asian oceanic plate beneath the northern NCB. However, their wide range of whole-rock Sr–Nd and zircon Hf isotopic compositions indicate that their source areas are very complex and different sources were involved in generation of these volcanic rocks. Geochemical and Sr–Nd–Hf isotopic results show that the basalt and some andesite were produced by fractional crystallization of mafic magma derived from partial melting of mantle wedge and subducted oceanic crust; however, most of the intermediate to felsic volcanic rocks were derived from partial melting of lower continental crust. There is an increasing input of crustal materials from the Carboniferous to Permian as indicated by increasing volumes of felsic volcanic rocks in the volcanic sequences. The results show that origin of the continental arc volcanism is very complex and both materials from the subducted oceanic crust and sediments, mantle wedge and arc continental crust could be involved in their generation. For the Permian andesite, dacite and rhyolitic volcanic rocks, there is a striking correlation between their Sr–Nd–Hf isotopic compositions and those of the underlying basement. Sr–Nd–Hf isotopic differences between the Permian intermediate to felsic volcanic rocks from either side of the northern boundary fault of the North China craton suggest that mixture and homogenization of the magmas in the melting–assimilation–storage–hybridization (MASH) zone within this continental arc was incomplete, and incomplete homogenization of magmas is likely a common feature of many MASH zones within continental arcs. Our results on the volcanic rocks also show that the final closure of the Paleo-Asian Ocean along the Solonker Suture Zone has not been finished prior to the Middle Permian and the decease of the Paleo-Asian oceanic subduction beneath the northern NCB in its middle-western parts is a little earlier than that in its eastern part.