Garnet pyroxenite, eclogite and alkremite xenoliths from the off-craton Gibeon Kimberlite Field, Namibia: A window into the upper mantle of the Rehoboth Terrane

Abstract

Different types of mantle xenoliths, including garnet peridotite, garnet pyroxenite, eclogite, and alkremite, occur in kimberlites of the Gibeon Kimberlite Field (GKF), Namibia. The off-craton GKF is situated within the Palaeoproterozoic Rehoboth Terrane, about 400–500 km west of the inferred margin of the Archaean Kaapvaal Craton. Petrological data are available only for garnet peridotite; the other three rock types are described here for the first time. Both types of eclogite, Type I and II, are present in the GKF. The criteria to distinguish between the two types, i.e. Na 2O in garnet and K 2O in clinopyroxene, based on cratonic samples are less suited for off-craton eclogite xenoliths. As demonstrated in this study, Na 2O contents and Cr 2O 3/K 2O ratios in clinopyroxene are useful proxies for classifying off-craton eclogites. All GKF eclogites are bi-mineralic garnet-clinopyroxene rocks with rare accessory spinel and phlogopite. Minor rutile, amphibole and K-feldspar occur only in Type I eclogites. Alkremite is dominated by primary garnet and corundum and contains minor spinel and clinopyroxene of probably secondary origin. Garnet pyroxenite is websteritic and contains clinopyroxene, orthopyroxene and garnet. The garnet pyroxenite protolith was a high-pressure clinopyroxene-rich cumulate, in which garnet exsolution from Al-rich clinopyroxene took place by cooling below 1350 °C. Genetically linked with the garnet pyroxenite is Type II eclogite, which also formed by crystallization from a melt in the mantle. In contrast, the nature of Type I eclogite remains ambiguous. It may be of similar origin as Type II eclogite, or it may represent fragments of subducted oceanic crust. The equilibrium temperature of garnet pyroxenite of ca. 770 °C is consistently lower than that of eclogite (965–1190 °C). The pressure estimate for garnet pyroxenite (21–23 kbar) implies that the rocks formed and re-equilibrated at some 65–75 km depth. The inferred eclogite depth range is 90–130 km. A similar difference in temperature, and by implication also in depth, between pyroxenite and eclogite is known from other xenolith suites in southern Africa. These localities are more than 1000 km apart and extend from the Namibian GKF in the west to the Rietfontein and Kaalvallei kimberlites in South Africa and beyond to the area around the Premier diamond mine close to Pretoria. This implies a common thermal and lithological depth pattern for large parts of southern Africa, with a layer rich in garnet pyroxenite being underlain by an eclogite-rich layer. This supports a recent proposition ( Bell et al., 2003; Janney et al., 2010) that the thermal state and thickness of Proterozoic off-craton and Archaean cratonic lithosphere were largely similar prior to Mesozoic Gondwana break-up, which caused thermal erosion of the off-craton lithosphere with respect to the Kaapvaal Craton. In many cases, Proterozoic lithospheric mantle may represent reworked Archaean rather than juvenile Proterozoic material (e.g. Begg et al., 2009). If this also applies to the GKF mantle lithosphere has yet to be shown. The correlation inferred from this study may also be explained in terms of similar processes that formed and modified both Archaean and Proterozoic mantle lithosphere.