The nature of subduction system in the Neoarchean: Magmatic records from the northern Yangtze Craton, South China

Abstract

The Neoarchean is an important era during which plate tectonics began to operate widely on Earth. The style of plate tectonics in the Neoarchean should be different but poorly constrained. Here we report geochemical studies on felsic to mafic igneous rocks from the northern Yangtze Craton, which record a subduction at ca. 2.65–2.70 Ga with distinctive magmatic and geodynamic features from modern subduction system. The metaigneous rocks of felsic composition, trondhjemite gneisses (TTG), from the northern margin show high sodic contents, positive Eu anomalies, high (La/Yb)cn and Sr/Y ratios, low Yb and Y contents. The metaigneous rocks of mafic-intermediate composition from the northern margin are mainly calc-alkaline and show less fractionated REE patterns. They have similar zircon U-Pb ages of 2651 ± 30 Ma to 2703 ± 9 Ma, similar chondritic zircon εHf(t) values and whole-rock εNd(t) values of −1.6 to 3.4. Their zircon δ18O values are mostly 4.6–6.6‰, similar to or slightly higher than normal mantle values. Additionally, they all record a metamorphic event at ca. 2.47–2.50 Ga. The potassic granites from the interior are A-type in geochemistry and show similar zircon U-Pb ages and εHf(t) values to the metaigneous rocks in the northern margin but obviously lower zircon δ18O values from 3.6‰ to 5.4‰. The two types of rocks exhibit quartz δ18O values of 12.0–15.2‰ and 3.5–8.2‰, respectively. The trondhjemite gneisses and calc-alkaline metaigneous rocks were derived from partial melting of the subducted crust and the metasomatized mantle wedge, respectively. The A-type granites are the products of high temperature magmatism in an extensional setting in response to the contemporaneous subduction. This demonstrates the development of a subduction system in the northern Yangtze Craton during 2.65–2.70 Ga. The contemporaneous development of both slab-derived and mantle wedge-derived magmatism is suggested as a feature of the Neoarchean subduction system that is distinguished from the modern ones. The lack of TTG magmatism at 2.50–2.65 Ga but the reburst of TTG and metamorphism at 2.47–2.50 Ga imply that the subduction in the Neoarchean is intermittent.