Geochemistry, petrogenesis and tectonic setting of Neoproterozoic mafic–ultramafic rocks from the western Jiangnan orogen, South China

The Jiangnan orogenic belt (JOB) has been interpreted as a suture zone between the Yangtze craton and Cathaysian terranes in South China. The Neoproterozoic mafic–ultramafic rocks are extensively exposed in the western JOB, providing an ideal opportunity to study the Neoproterozoic assembly and tectonic evolution of South China. We present integrated field and geochemical studies including LA-ICP-MS zircon U–Pb dating, and whole-rock major and trace element and Sm–Nd isotope analyses of the Neoproterozoic mafic–ultramafic rocks exposed in the northern Guangxi Province, South China. Geochronological results show that the magmatic events took place in two distinct periods: the early Neoproterozoic (861–834Ma) and the late Neoproterozoic (770–750Ma). Early Neoproterozoic ultramafic rocks of the Sibao Group have positive ε Nd(t) values (+2.7 to +6.6) whereas mafic rocks exhibit negative ε Nd(t) values (−5.8 to −0.9). The basaltic rocks show TiO2 contents of 0.62–0.69wt.% and Mg-number of 59–65, and also display an enrichment of light rare earth elements (LREEs) and pronounced negative Nb, Ta and Ti anomalies on chondrite- and primitive mantle-normalized diagrams, consistent with subduction-related geochemical signatures. Late Neoproterozoic rocks of the Danzhou Group show ε Nd(t) values (−1.23 to +3.19) for both ultramafic and mafic rocks. The basaltic rocks have TiO2 contents of 1.01–1.33wt.% and Mg-number of 57–60, and have a mixture of MORB- and arc-like geochemical affinities, inferred to have formed in an extensional arc environment. Geochemical signatures suggest that all rock types in this study were derived from subarc mantle wedge sources and underwent various degrees of crustal contamination. Thus, we suggest that subduction may have continued to ca. 750Ma in the western JOB, implying that the amalgamation event between the Yangtze craton and Cathaysian terranes was later than 750Ma.