Initial breakup of supercontinent Rodinia as recorded by ca 860-840Â Ma bimodal volcanism along the southeastern margin of the Yangtze Block, South China

It is considered that mantle plumes play an important role in the breakup of supercontinents, but continental rifting and associated bimodal volcanism often predate mantle-plume magmatism and the major stage of supercontinent breakup. Therefore, the petrogenesis of those bimodal volcanic rocks is crucial for understanding the causal relationship between magmatism and supercontinent breakup. In this paper, new SIMS U-Pb zircon ages, Nd isotopic data, and whole-rock major and trace element data are reported for Huangshan and Meiling bimodal volcanic rocks that crop out along the southeastern margin of the Yangtze Block. The Huangshan and Meiling bimodal volcanic rocks were formed respectively at 860 ± 9 Ma and 840 ± 5 Ma, about 35 to 15 Mys earlier than the recorded Neoproterozoic plume event in South China. The Huangshan basalts have arc-like geochemical signatures with enrichment of water (pre-eruption magma water content of ∼3.6%) and fluid-mobile elements, and depletion of high-field-strength elements (HFSE), as well as homogeneous Nd isotopic compositions (εNd(t) = +4.4 to +5.3). Although such geochemical characteristics suggest a hydrous asthenospheric mantle source for the Huangshan basalts, their incompatible trace elements plot within an intraplate tectonic setting, offset from true arc basalts. In contrast, the younger Meiling basalts have intraplate type (OIB-like) geochemical and homogeneous Nd isotopic compositions (εNd(t) = +4.3 to +4.7), implying a normal asthenospheric mantle source. Geochemical analyses indicate that the Huangshan basaltic rocks underwent olivine → olivine + amphibole + clinopyroxene + magnetite and then plagioclase fractional crystallization during magma ascent. Although mafic and felsic end-members have distinctive chemical compositions, the two end-members share similar Nd isotopic compositions. This implies that the felsic volcanic rocks were derived from basaltic rocks through fractional crystallization. The assemblage of fractional crystallization minerals may be amphibole-dominated and then plagioclase, coincident with fractioned mineral assemblage of the basaltic rocks. In contrast, Meiling felsic rocks have varied εNd(t) values that are correlated negatively with silica contents. Geochemical analyses indicate that the precursor magma underwent fractional crystallization of amphibole and plagioclase with minor titanite. This implies that Meiling felsic rocks were produced by partial melting of hydrous juvenile basaltic rocks, possibly ca 860 Ma underplated basaltic rocks, and then followed by variable crustal contamination. The petrogenesis of the 860-840 Ma bimodal volcanic rocks suggests that multi-stage rift-related anorogenic magmatism occurred before ca 825 Ma mantle-plume magmatism in South China. Wet upwelling, probably within or on top of the mantle transition zone, may have been the main cause of the hydration of the shallow asthenospheric mantle, which drove the mantle partial melting that generated these bimodal volcanic rocks. We conclude that mantle hydration was the trigger for early intracontinental rift-related magmatism, probably corresponding to the initial break-up of the Rodinia supercontinent.