Early Proterozoic oceanic crust and the evolution of subcontinental mantle: Eclogites and related rocks from southern Africa
Seven eclogitic nodules from kimberlites in southern Africa have been studied in detail for whole-rock, major- and trace-element geochemistry, petrography, and mineral chemistry by electron microprobe; high-purity mineral separates from six of these samples have been analyzed for trace elements and for the isotopic composition of Sr, Nd, and oxygen. Three eclogite groups are recognized: group A eclogites have very high Mg/Fe, low Na in pyroxene, moderate δ18O (+4.7 to +5.3), and low 87Sr/86Sr and 143/144Nd. Olivine and enstatite may also be present as accessory phases in this group. Group B eclogites have moderate to high Mg/Fe ratios, high Na in pyroxene, low δ18O (+3.0 to +3.4), high 87Sr/86Sr and 143Nd/144Nd ratios, and extremely LREE-depleted/ HREE-enriched garnets with no Eu anomalies. Group C eclogites have low Mg/Fe ratios, high Na in pyroxene, and variable Sr-, Nd-, and O-isotope compositions. Accessory feldspar is present in one sample that may be of crustal origin. Mineral separates of garnet and pyroxene have positive Eu anomalies.

The group A eclogites are too refractory to represent magma compositions and must have formed as cumulate dike rocks in the upper mantle that contain a minor trapped liquid component. This is supported by the presence of accessory olivine and enstatite, and by their KSm and KNd, which are similar to empirical pyroxene/garnet partition coefficients. The Group B eclogites are extremely depleted in incompatible elements and have ϵNdvalues 10x to 20x MORB. The high 87Sr/86Sr of these eclogites is not consistent with their strongly depleted LREE and high ϵNd, and cannot be primary—it must have been imposed on the protolith. The low δ18O of these eclogites cannot form by mantle fractionation processes and must also be inherited from the protolith. Both the Sr- and oxygen-isotope data are consistent with high- temperature hydrothermal alteration of a basaltic protolith, followed by partial melting to form the refractory compositions observed now. The hydrothermal fluid may have been sea water, but secondary enrichment of the protolith in Rb is required to generate the observed high Sr ratios. The high Na content of the pyroxenes supports a spilitic alteration event. The major- and trace-element characteristics of the group C eclogites are consistent with recrystallization of a cumulate gabbro protolith. One group C rock is probably a garnet granulite derived from the lower crust. The other may represent the plutonic portion of oceanic crust.

A reconstructed whole-rock isochron for the three Bellsbank eclogites yields an age of 2.1 ± 0.1 b.y. This implies that plate-tectonic processes involving the generation and subduction of oceanic crust have been active since the early Proterozoic. The early Proterozoic ocean crust consisted of two components: a plutonic section of gabbro cumulates (group C eclogites) and a volcanic section of basalt (group B eclogites). The volcanic component has undergone substantial hydro-thermal alteration and subsequent partial melting to form a refractory residue; the plutonic component is less modified.