Metasomatized eclogite xenoliths from the central Kaapvaal craton as probes of a seismic mid-lithospheric discontinuity

Abstract

The central region of the Kaapvaal craton is relatively understudied in terms of its lithospheric mantle architecture, but is commonly believed to be significantly impacted by post-Archean magmatism such as the ca. 2056 Ma Bushveld large igneous event. We investigate a collection of 17 eclogite xenoliths from the Cretaceous Palmietfontein kimberlite at the Western Limb of the Bushveld Complex for their mineralogical compositions (major and trace elements, plus Fe3+ contents), as well as stable oxygen and radiogenic Pb isotopic compositions to gain further insights into the nature and evolution of the central Kaapvaal cratonic mantle lithosphere. New U/Pb age determinations on mantle-derived zircon yield a magma emplacement age of ca. 75 Ma for the Palmietfontein Group-1 kimberlite, which means that the entrained eclogite xenoliths may record a protracted metasomatic history from the Proterozoic through to most of the Phanerozoic eon.Garnet δ18O values of up to 6.9‰ and positive Eu anomalies for the bulk rocks suggest seawater-altered oceanic crustal protoliths for the Palmietfontein eclogite xenolith suite, which is typical for the eclogitic components of the Kaapvaal root and other cratonic mantle sections worldwide. However, several features of the Palmietfontein eclogites are commonly not observed in other mantle-derived eclogite xenolith suites. Firstly, the samples studied yield relatively low equilibration pressures and temperatures between 2.7 and 4.5 GPa and 740–1064 °C, indicating a relatively shallow residence between 90 and 150 km depths. Secondly, euhedral coarse amphibole is present in several eclogite nodules where it is in equilibrium with ‘touching’ garnet, supporting eclogite residence within the amphibole stability field at uppermost lithospheric mantle conditions. Thirdly, primary omphacitic clinopyroxene is often overgrown by diopside, and is significantly enriched in incompatible trace elements. The clinopyroxene is also characterized by elevated 206Pb/204Pb of 17.28–19.20 and 207Pb/204 Pb of 15.51–16.27, and these Pb isotopic compositions overlap with those of Mesozoic Group-2 kimberlites from the Kaapvaal craton.Our results show that eclogites reside at ~85 km depth beneath the central Kaapvaal craton as part of a layer that corresponds to an approximately 50 km thick seismically-detected mid-lithospheric discontinuity. Mid-lithospheric discontinuities have been interpreted as metasomatic fronts formed by focussed crystallization of hydrous mineral phases from enriched volatile-bearing melts, and as such the strongly overprinted amphibole-bearing eclogite xenoliths from Palmietfontein may represent a physical expression of such seismically anomalous metasomatic layer at mid-lithospheric depth. Our Pb isotope data suggest that the focussed metasomatism can be attributed to volatile-rich melts reminiscent of potassic Group-2 kimberlites, which have been invoked in MARID-style metasomatic overprinting of the lower lithospheric mantle beneath the western Kaapvaal craton. However, the relatively low fO2 recorded by the Palmietfontein eclogites (minimum FMQ-4.5) suggests that the metasomatism at mid-lithospheric depth was less protracted compared to the more intensive and oxidizing metasomatism typically observed near the base of cratonic mantle roots. While it is possible that Proterozoic magmatic events were responsible for the focussed mid-lithospheric metasomatism of the Kaapvaal mantle, on the basis of the Pb isotope constraints the Palmietfontein eclogites were most likely overprinted during ca. 120 Ma Group-2 kimberlite magmatism.