Composition and Cretaceous thermal structure of the upper mantle beneath the Damara Mobile Belt: evidence from nephelinite-hosted peridotite xenoliths, Swakopmund, Namibia

Abstract

Abstract: Mantle xenoliths have been discovered in a Cretaceous nephelinite plug in the Damara Belt of NW Namibia, providing a new window into the thermal and compositional structure of the lithospheric mantle that separates the Congo and Kaapvaal cratons of southern Africa. The xenoliths range in size from 1.5 to 5 cm, and comprise lherzolite and harzburgite (± spinel), with predominantly granuloblastic textures. Two groups of spinel peridotites are identified on the basis of mineral chemistry. The first group has refractory compositions with Mg-number progressively increasing from olivine (91.3–92.0), through orthopyroxene (91.8–92.5) to clinopyroxene (93.7–94.3), low Al 2 O 3 (2.4–2.9 wt%), Na 2 O (0.25–0.63 wt%) and TiO 2 (<0.05 wt%) contents in clinopyroxene, and Cr-rich spinels (Cr/(Cr+Al)=0.34–0.47). The second group has less refractory mineral compositions with Mg-number of 90–91 for all three silicate phases, clinopyroxene with elevated Al 2 O 3 ( c . 6.5 wt%), Na 2 O ( c . 1.7 wt%) and TiO 2 ( c . 0.45 wt%) contents, and Al-rich spinels (Cr/(Cr+Al)=0.11). In terms of mineral chemistry and textures, both groups of xenoliths can be classified as Type I peridotites and, unlike spinel lherzolites from the Okenyenya complex 100 km to the NE, they show no signs of modal metasomatism. Calculated oxygen fugacities indicate equilibration of the xenoliths at −0.29 to 1.0 log units above fayalite–magnetite–quartz (FMQ), which for the fertile peridotites places them among the most oxidized samples recorded from continental settings. Temperatures of equilibration range from 871 to 1236 °C for T BKN and from 844 to 1128 °C for T Ca-in-opx . No significant difference in temperature was found between the two groups of lherzolites. Pressures calculated from the Ca content of olivine in equilibrium with clinopyroxene are in the range 1.4–2.0 GPa, with the fertile lherzolites originating at deeper levels than the refractory group. A local geotherm substantially hotter ( c . 90 mW m −2 ) than that beneath the contemporaneous Gibeon kimberlites ( c . 45 mW m −2 ) is indicated and probably relates to tectonothermal events associated with local and regional Cretaceous magmatism. The inferred mantle stratigraphy suggests the juxtapositioning of relatively fertile Phanerozoic-type mantle beneath refractory peridotite similar to that of Proterozoic lithosphere sampled by the Gibeon kimberlites 300 km to the SE. This may have occurred during formation of the Damara Belt and be related genetically to the linear belt of mafic volcanic rocks emplaced during late Proterozoic time and preserved today as the Matchless amphibolite.