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

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Journal of the Geological Society
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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 predominantt granuloblastic textures. Two groups of spinel peridotites are identified on the basis of mineral chemistry, The first group has refractors 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 Al2O3 (2.4-2.9 wt%), Na2O (0.25-0.63 wt%) and TiO2 (<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 Al2O3 (c. 6.5 wt%), Na2O (c. 1.7 wt%) and TiO2 (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 spinet lherzolites from the Okenyen a complex 100 km to the NE. them 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 degreesC for T-BKN and from 844 to 1128 degreesC for Tca-in-opx. No significant difference in temperature was found bet keen 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 refractor group. A local geotherm substantially hotter (c. 90 mW m(-2)) than that beneath the contemporaneous Gibeon kimberlites (c. 45mW m(-2)) is indicated and probable relates to tectonothermal events associated with local and regional Cretaceous magmatism. The inferred mantle stratigraphy suggests the juxtapositioning of retatively fertile Phanerozoic-type mantle beneath refractory peridotite similar to that of Proterozoic lithosphere sampled by the Gibeon kimberlites 300 km(-1) 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.


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