Petrology and geochemistry of eclogite xenoliths from the Rietfontein kimberlite,Northern Cape,South Africa |
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| Authors: | C M Appleyard D R Bell A P le Roex |
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| Institution: | (1) Department of Geological Sciences, University of Cape Town, Rondebosch, 7700, South Africa;(2) De Beers Geoscience Center, Ormonde, 2091, South Africa;(3) Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA |
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| Abstract: | Petrographic and geochemical features of a suite of eclogite xenoliths from the Rietfontein kimberlite that erupted through
probable Proterozoic crust west of the Kaapvaal Craton in the far Northern Cape region of South Africa, are described. Group
II eclogites dominate the suite both texturally and chemically, but can be subdivided into bimineralic, opx-bearing and kyanite-bearing
groups. Temperature estimates from different geothermometers range from 700 to 1,000°C, indicating derivation from relatively
shallow mantle depths. Orthopyroxene-bearing eclogites are inferred to originate from depths of 85 to 115 km and lie close
to the average cratonic thermal profile for southern Africa. These uppermost mantle temperatures during the late Cretaceous
provide evidence for equilibration of the off-craton lithosphere to craton-like thermal conditions following Namaqua-Natal
orogenesis. The kyanite eclogites are distinct from the remaining eclogites in terms of both major and trace element compositions
and their lesser degree of alteration. Garnets are richer in Ca, and are Cr-depleted relative to garnets from the bimineralic
and opx-bearing eclogites, which tend to be more magnesian. Clinopyroxenes from the kyanite eclogites are more sodic, with
higher Al2O3 and lower MgO contents than the bimineralic and opx-bearing eclogites. LREE-depletion, positive Sr and Eu anomalies, and
the Al-rich, Si-poor bulk composition suggest a plagioclase-rich, probably troctolitic protolith for the kyanite eclogites.
In contrast, the major and trace element bulk compositions of the high-MgO bimineralic and orthopyroxene-bearing eclogites
are consistent with gabbroic or pyroxenitic precursors, or high-pressure cumulates, rather than mafic to ultramafic lavas.
δ18O values for garnets do not deviate significantly from typical mantle values. The observations reported do not discriminate
unambiguously between continental and oceanic origins for the various eclogite components in the mantle lithosphere of this
region. |
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