Geochemistry of Etendeka magmatism: Spatial heterogeneity in the Tristan-Gough plume head

Abstract

The origin and distribution of geochemically distinct source components in continental flood volcanism (generally associated with the initial phase of a mantle plume head) are poorly understood. Here we present new geochemical (major and trace element and Sr-Nd-Pb-Hf isotope) data from the Etendeka flood basalts and associated dikes from northern and central Namibia that are believed to have been produced during the initial stage of the Tristan-Gough hotspot. Following earlier studies, the Etendeka lava flows and dikes are divided into high-Ti and low-Ti groups. The trace element and isotopic composition of the high-Ti tholeiitic basalts, exclusively outcropping in northern Etendeka (northwestern Namibia), are similar to the Gough-type enriched mantle I (EMI) composition found on the Walvis Ridge (the Atlantic type locality for the EMI end member). The low-Ti tholeiitic basalts, primarily outcropping in Southern Etendeka (central western Namibia), have higher 143Nd/144Nd and 207Pb/204Pb but lower 208Pb/204Pb ratios than the Gough composition. Combining our data with newly published 3He/4He data and estimates of the magma source's potential temperature from 1520-1680°C, we conclude that the source of the low-Ti basalts was also intrinsic to the Tristan-Gough plume, consistent with a spatially-zoned plume head. The low-Ti basalts were derived from a distinct EMI-type source component that has thus far only been detected in the initial Tristan-Gough plume head (∼132 Ma), but not the later submarine hotspot track.