Geochemical and SmNd isotopic study of Neoproterozoic ophiolites from southeastern China: petrogenesis and tectonic implications

Abstract

Geochemical and SmNd isotopic data are presented for two Neoproterozoic ophiolite suites (∼970 Ma), which were tectonically emplaced into the Banxi Group and equivalent sequences of the Yangtze Block in the southern Anhui and northeastern Jiangxi Provinces, southeast China during the Neoproterozoic (0.9-0.8 Ga). The two ophiolite suites are similar to each other in terms of geochemical feeatures, with volcanics showing affinity to basalts forming in subduction-related environments, typical of the ‘supra-subduction zone (SSZ)’ ophiolites. However, the two suites differ significantly in their initial Nd isotopic compositions. The Jiangxi suite displays uniform initial ϵ Nd values of +5.5 ± 1.2, implying derivation from a depleted upper mantle source without significant contamination by evolved continental materials. In contrast, the Anhui suite shows a large range of initial ϵ Nd values (+4.5 to −1.0) which are strongly correlated with Sm Nd , Nd, MgO and SiO 2, consistent with binary mixing between a depleted mantle component (initial ϵ Nd ∼ +5.5) and an evolved crustal component (initial ϵ Nd ∼ −1), accompanied by crystal fractionation at different stages. The geochemical and Nd isotopic signatures of the two distinct ophiolite suites, as well as the spatial and temporal distribution of different types of syntectonic granitoids and volcanic series in the region, can be explained in terms of continent-arc-continent collision between the Yangtze and Huanan Blocks during the Neoproterozoic. In this model, it is proposed that the Yangtze and Huanan Blocks were separated by a multi-arc oceanic basin during the early Neoproterozoic (∼1.0 Ga), with both oceanic and continental margin island arcs being developed to the south of the Yangtze Block. The Jiangxi and Anhui ophiolite suites are considered as tectonically emplaced fragments of oceanic crust forming in the inter-arc and continental-margin basins, respectively.