Neoproterozoic mafic magmatism (ca. 790–780 Ma) in the western Jiangnan Orogen of South China, and its implication for a slab rollback origin

In this paper, we present geochemical data for a Neoproterozoic basalt (from Ximalian) two clinopyroxenite intrusions (Tangtou, Mahai), and three gabbroic intrusions (Datan, Longsheng, and Youziping) in the Longsheng area of northern Guangxi, China. This area is located in the western part of the Jiangnan Orogen of the South China Block. Secondary ion mass spectrometry (SIMS) UPb zircon dating of the Xiamalian basalt, Youziping gabbro, and Mahai clinopyroxenite yielded concordia ages of 786.0 ± 5.6 Ma, 788.9 ± 4.8 Ma, and 780.1 ± 4.7 Ma. Geochemically, the Ximalian basalt and Tangtou clinopyroxenite, as well as three gabbroic rocks, have arc-like geochemical affinities characterized by moderate light rare earth element (LREE) enrichment (La_N/Yb_N = 2.79–9.45), pronounced negative NbTa anomalies, and relatively low Nb/La ratios (with an average of 0.52). They have variable εNd(t) (−4.5 to 3.9) and positive εHf(t) (6.5–8.5) values. We thus conclude that these rocks were derived from the mantle wedge in a subduction setting via partial dehydration melting of the lithospheric mantle metasomatized by dehydrated fluids. In contrast, the Mahai clinopyroxenite exhibits ocean island basalt (OIB)-like signatures, including LREE enrichment (La_N/Yb_N = 11.87–22.25), a lack of negative Nb, Ta, and Ti anomalies; and positive εNd(t) (0.5–1.5, with one exception of −2.9) and εHf (t) (3.0–3.5) values. We infer that the Mahai clinopyroxenite was generated by partial decompression melting of the upwelling asthenospheric mantle in an extensional setting. Based on the regional geology, as well as the results of this study and previous publications, we conclude that the arc-type rocks analyzed in this study were generated under geodynamic processes triggered by the rollback of the subducted slab in a subduction setting. The OIB-like rocks were related to the break-off of the rolled-back slab, which created an extensional setting and enabled asthenospheric upwelling and partial melting.