Isotope characteristics of the Okenyenya igneous complex, northwestern Namibia: constraints on the composition of the early Tristan plume and the origin of the EM 1 mantle component

Abstract

Sr, Nd and Pb isotope data are presented for a variety of intrusive rocks from the Mesozoic age Okenyenya igneous complex, which is temporally and spatially associated with the Etendeka Group volcanic rocks in northwestern Namibia. On the basis of bulk rock geochemistry the Okenyenya intrusions can be subdivided into tholeiitic and alkaline suites. The tholeiitic suite has a wide range in isotope composition; for example, initial ϵ Sr ( ϵ Sr(i)) from 1.2 to 150 with decrease in initial ϵ Nd ( ϵ Nd(i)) from 4.8 to −3.9. In contrast, the undersaturated rock types show a more restricted range and, in terms of ϵ Sr(i) (− 11.0–15.1) and ϵ Nd(i) (0.3–5.0), plot within the mantle array and close to Bulk Earth values. The range in isotope composition shown by the Okenyenya intrusions is similar to that shown by the Etendeka Group volcanic rocks. The tholeiitic suite is comparable in isotope composition to the Etendeka low TiZr (LTZ) basalts and defines a trend towards continental crust, whereas the alkaline suite is similar to the Etendeka Tafelkop basalts. The Etendeka high TiZr (HTZ) basalts do not have an isotopic equivalent amongst the Okenyenya intrusions, but are indistinguishable from basalts in DSDP Hole 525A on the Walvis Ridge; both are strongly displaced towards enriched mantle (EM 1) sources. The large variation in ϵ Sr(i) shown by the tholeiitic suite and Etendeka LTZ basalts appears to reflect extensive crustal contamination of the magmas, whereas the HTZ basalts, which trend towards EM 1, owe their isotope composition to melting of ancient continental lithospheric mantle. The alkaline gabbros and the Tafelkop basalts have compositions similar to the present-day composition of the Tristan plume and are interpreted as direct melts of the upwelling Tristan mantle plume at the time of continental break-up. An analogous relationship is observed between the Marion plume, Madagascan Upper Cretaceous basalts, and MORB erupted at the intersection between the Southwest Indian Ridge (SWIR) and the Madagascar Ridge. The strong EM 1 signature of basalts found in DSDP Hole 525A on the Walvis Ridge, and at 39–41°E on the SWIR is attributed to melting at shallow depth of ancient continental lithospheric mantle thermally eroded and rafted into the surrounding asthenosphere at the time of continental breakup.