The Petrology of Basanite–Tephrite Intrusions in the Erongo Complex and Implications for a Plume Origin of Cretaceous Alkaline Complexes in Namibia

Basanite intrusions from the Early Cretaceous Erongo complex, Namibia, have compositions consistent with near-primary mantle melts derived from a depth of at least 100 km. These rocks provide a key reference for the mantle component(s) involved in breakup-related magmatism in this region. Initial Sr–Nd–Pb isotope ratios of the Erongo basanites and associated tephrites and phonotephrites (87Sr/86Sr = 0·70425–0·70465; εNd = +1·8 to +2·7; 206Pb/204Pb = 18·63–18·91) are independent of the degree of differentiation and correspond closely to an estimated range for the Tristan plume at 130 Ma. Incompatible trace element ratios also overlap with ratios of ocean island basalt (OIB) from the South Atlantic islands of Tristan da Cunha, Gough and Inaccessible associated with the modern Tristan hotspot. The Tristan plume signature of Erongo basanite–tephrite intrusions is shared by at least six other Early Cretaceous mafic alkaline complexes in Namibia, whereas the associated flood basalts in general lack a plume signature. We attribute the contrast in mantle sources for the flood basalts and alkaline complexes to their relative timing with respect to lithospheric thinning. Thick lithosphere during the main flood basalt event prevented direct melting of the Tristan plume and magmas were generated mostly from the lithosphere. The alkaline complexes intruded later, when the lithosphere was sufficiently thinned to allow decompression melting of the underlying plume mantle.