The Bikou basalts in the northwestern Yangtze block, South China: Remnants of 820-810 Ma continental fl ood basalts?

Abstract

Mantle plume or superplume activities have often been invoked as a cause for the breakup of the Neoproterozoic supercontinent Rodinia. However, associated Neoproterozoic continental flood basalts, a requisite product of mantle plume ac tivities, have rarely been identified. New sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages, geochemical, and Hf-Nd isotopic data are reported here for the Bikou Group basalts, the largest Neoproterozoic volcanic units in the northwestern Yangtze block, South China. The Bikou basalts are mainly tholeiitic in composition, and they can be stratigraphically subdivided into the lower and upper groups. SHRIMP U-Pb zircon dating of intercalated rhyolites indicates that the lower and upper group basalts were erupted at 821 ± 7 Ma and 811 ± 12 Ma, respectively. The two basalt groups define two populations on most elemental and isotopic diagrams. The lower group basalts display strong sub-continental lithospheric mantle affinities and large variable εNd(t) and εHf(t) values that correlate negatively with Th/Ta and positively with MgO. The upper group basalts, on the other hand, show ocean-island basalt (OIB) affinities and highly positive εHf(t) and εNd(t) values. Numerical modeling indicates that the lower and upper group basalts were formed at melting temperatures of >1350 °C and >1450 °C and mantle potential temperatures of 1400–1488 °C and 1550 ± 30 °C, respectively. Thus, the upper group basalts were derived from an anomalously hot asthenosphere mantle, ~160 °C hotter than the contemporary ambient mid-ocean-ridge basalt (MORB) source mantle. Our results are at clear variance with the alternative tectonic model that the Bikou basalts were part of a continental magmatic arc. Our work suggests that the Bikou basalts are likely the remnants of Neoproterozoic continental flood basalts that formed in response to a mantle plume starting ca. 825 Ma during the breakup of the supercontinent Rodinia.