Multi-stage origin of Roberts Victor eclogites: Progressive metasomatism and its isotopic effects

Abstract

Xenolithic eclogites are high-pressure, high-temperature garnet–clinopyroxene rocks brought from the mantle to the surface by kimberlites and other primitive magmas. Their origins have been controversial for decades: do they represent metamorphosed subducted oceanic crust or magmatic rocks originally crystallized in the deep earth? The answer has important implications for the definition of major Earth processes.The extensive eclogite suite from the Roberts Victor kimberlite (South Africa) has previously been divided into two types (Type I and II); this study proposes five subgroups (IA, IB, IK; IIA, IIB) based on mineral assemblages and compositions. All of these eclogites were derived from depths of 170–200km, near the base of the contemporary subcontinental lithospheric mantle.The fresh, microstructurally equilibrated Type IIA eclogites are the protoliths of Type I, which were heavily metasomatized by carbonatitic–kimberlitic melts/fluids to form Types IAIBIK. All the Type I rocks show extensive textural disequilibrium, and have abundant fluid inclusions, secondary phases, and higher δ18O than Type II (IA: 5.0–8.0‰; IB: 4.1–6.8‰; IK:2.2–6.8‰; IIA=3.5‰; IIB:2.2–3.9‰). Type I are richer in LREE and LILE than Type IIA; they also are isotopically more homogeneous, with higher 87Sr/86Sr and less radiogenic Nd and Hf isotopes. Type I eclogites give gnt–cpx isochron ages (SmNd (103±10Ma); LuHf (132±16Ma)) similar to the kimberlite eruption age (128±15Ma). Type IIA and IIB eclogites have unradiogenic Sr and radiogenic Nd and Hf isotopic compositions, and give a range of Proterozoic two-mineral “ages” (SmNd: 738–1143Ma; LuHf: 1148–1544Ma), reflecting some preservation of their original isotopic compositions. Type IIB are broadly similar to Type IIA, but have lower MgO; their mutual relationships are not clear.Neither Type I nor Type II eclogites are similar to modern or Archean oceanic crust when all the chemical evidence is compared. In any case, the data from Type I eclogites cannot be used to support a “subduction model” of origin because their major- and trace-element chemistry, and radiogenic- and stable-isotope compositions, reflect the extensive metasomatism, rather than any primary characteristics.Type II eclogites are the key samples for the study of eclogite origins. Their bulk compositions and microstructures (exsolution of gnt and rutile) suggest that most or all the garnet in Type II samples might have formed by exsolution from high-Al clinopyroxenes, crystallized from mafic–ultramafic magmas in the deep mantle. Therefore, the xenolithic eclogites from Roberts Victor Mine probably were originally magmatic rocks crystallized at depth, rather than subducted oceanic crust; as a suite, they record a major metasomatic event in the deep lithospheric mantle, close to the time of kimberlite eruption.