Paleoproterozoic metavolcanic rocks in the Ji’an Group and constraints on the formation and evolution of the northern segment of the Jiao-Liao-Ji Belt, China

Abstract

In this paper we present new petrological and whole-rock geochemical data for the Paleoproterozoic metavolcanic rocks in the lower part of the Ji’an Group within the northern segment of the Jiao-Liao-Ji Belt, China, as well as U–Pb dates and Lu–Hf isotope data for the zircons in these rocks. The new data help in understanding the original nature of these volcanic rocks, as well as their tectonic setting and the formation and evolution of the belt. The zircons can be divided into two groups, of which one is of a typical magmatic origin, whereas the other shows a metamorphic origin. Zircon U–Pb dating and Lu–Hf isotopic results show that the magmatic zircons have peak ages at 2184, 2365, 2503, and 2710Ma with εHf(t) values ranging from +1.40 to +8.95, −10.86 to +1.48, −14.68 to +7.53, and −1.05, and with corresponding model ages of 2.18–2.69, 2.80–3.57, 2.54–3.83, and 3.22Ga, respectively. The metamorphic zircons show two peak ages at 1857 and 1914Ma. Geochemical data show that the protolith of the metavolcanic rocks consists mainly of basaltic andesite, andesite, and dacite, along with minor basalt and rhyolite, and that these rocks formed a continuous calc-alkaline magmatic sequence. In this sequence, the intermediate–basic rocks have relatively low SiO2 contents, are enriched in MgO, FeOT, the light rare-earth elements (LREEs), Cr, Co, and Ni, and depleted in heavy rare-earth elements (HREEs) and high-field strength elements (HFSEs, such as Nb, Ta, and Ti), indicating derivation by partial melting of a depleted lithospheric mantle that had been metasomatized by fluids or melts derived from a subducted slab. On the other hand, the acidic rocks have high contents of SiO2 and low contents of MgO and FeOT, are enriched in alkalis, Ba, Th, U, and K, and depleted in Nb, Ta, P, and Ti. These geochemical characteristics, together with their Hf isotope data, indicate the acidic rocks could have been derived from the partial melting of juvenile crust and a small amount of ancient crustal material, possibly as old as late Archean. We consider, therefore, that the metavolcanic rocks in the lower part of the Ji’an Group were formed at ca. 2.18Ga as an assemblage of volcanic rocks typical of an active continental margin, and that they were affected by two-stage metamorphic events at 1.90 and 1.85Ga. Taking into account the regional geological data, we consider that the evolution of the northern segment of the Jiao-Liao-Ji Belt was related to an arc–continent collision before the middle Paleoproterozoic.