Tectonic dismemberment of Shona Hotspot volcano: Insights from Sites 698, 699, 701, and 703 bulk-rock Sr-Nd-Pb-Hf isotopic geochemistry

Abstract

A genetic link between the Georgia-Islas Orcadas and Meteor guyots on opposite sides of the South Atlantic was proposed based on seafloor morphology and tectonic reconstructions. Here we test this model with geochemistry. We present new major and trace element and Sr, Nd, Pb, Hf radiogenic isotope data from volcanic and sedimentary rocks from Meteor Rise (ODP Site 703) and Georgia-Islas Orcadas Rise (ODP Sites 698, 699, and 701). Site 701 samples have slightly depleted isotopic compositions similar to southern Mid-Atlantic Ridge (SMAR) basalts, representing the upper mantle. Site 698, 699 and 703 volcanic rocks have initial Sr-Nd-Pb-Hf isotope ratios ((87Sr/86Sr)i = 0.70278–0.70660; (143Nd/144Nd)i = 0.51217–0.51283; (206Pb/204Pb)i = 18.09–18.75; (176Hf/177Hf)i = 0.28246–0.28299), extending from the Shona Hotspot track field (consisting of Richardson guyot, Agulhas Ridge and Meteor guyot samples) towards Proterozoic continental crust. These data support derivation from a common mantle plume, having formed contemporaneously as a single continuous hotspot track, which was subsequently divided by a ridge jump. Modelling of the isotope data show that binary mixing between an average Shona anomaly composition and an upper continental crustal component with a composition similar to the Site 698 sandstones can explain the data by 10–30% upper crustal assimilation to generate the composition of the Site 698, 699 and 703 volcanic rocks. Therefore, our geochemical data confirm the dismemberment of the Shona Hotspot track by a ridge jump and subsequent seafloor spreading.