The Neoproterozoic arc-type and OIB-type mafic-ultramafic rocks in the western Jiangnan Orogen: Implications for tectonic settings

Abstract

We report zircon U–Pb ages, zircon Hf isotopic data, mineral chemistry and the whole rock geochemistry of three representative Neoproterozoic mafic–ultramafic intrusions (Jinche gabbros, Huaqiao clinopyroxenite and Maodu gabbroic rocks) from the western Jiangnan Orogen in this manuscript, to decipher their petrogenesis and tectonic setting, and further provide constraints on the reconstruction of the Rodinia. The Jinche gabbros with a zircon SHRIMP U–Pb age of 806.4 ± 4.6 Ma, and it is suggested that the Huaqiao clinopyroxenites emplaced nearly synchronously as indicated by field geological evidences. They are typical tholeiitic arc rocks, characterized by the enrichment of light rare earth elements ([La/Yb]N = 2.27–3.64), significantly negative Nb, Ta and Ti anomalies, and positive εNd(t) (0.7–3.4) and εHf(t) (7.4–12.0) values. They are inferred to be derived from high-degree 5–10% partial melting of mantle wedge in the subduction setting from the garnet to spinel stability field. The Maodu gabbroic rocks yielded a zircon LA–ICP–MS U–Pb age of 773.6 ± 4.1 Ma. They are of alkaline series, and display ocean island basalt (OIB)-like geochemical signatures, with the enrichment of LREE and large-ion lithophile element relative to heavy rare earth elements ([La/Yb]N = 11.28–13.42), and have negative εNd(t) (−1.4 to −3.6) and positive εHf(t) (3.5–8.6) values. They are suggested to be derived from small degree (2%) partial melting of the upwelling asthenospheric mantle in an extensional setting in the garnet stability field. Based on comprehensive analyses of the Neoproterozoic mafic-ultramafic rocks reported in both this study and previous publications and regional geology in the western Jiangnan Orogen, we infer that the opening of the Nanhua rift was resulted from the back-arc extension related to the roll-back of the subducted slab, and the South China Block may be located at the margin of the Rodinia, as part of the Neoproterozoic Circum-Rodinia Subduction System.