Melt Depletion and Enrichment beneath the Western Kaapvaal Craton: Evidence from Finsch Peridotite Xenoliths

Abstract

We present major- and trace-element analyses of mineral phases present in a suite of 16 garnet-peridotite xenoliths from the western terrane of the Kaapvaal craton. The xenoliths were entrained by a Group II Finsch kimberlite at 118 Ma, shortly prior to a major metasomatic event that caused widespread enrichment of the Kaapvaal lithospheric mantle. Compositionally homogeneous grains of olivine, orthopyroxene, garnet and clinopyroxene and coarse–equant textures indicate equilibrium relationships between mineral phases in the majority of xenoliths. Pressure and temperature estimates suggest that clinopyroxene-bearing garnet peridotites last equilibrated at 1130–1270°C and 45–59 kbar whereas clinopyroxene-free xenoliths record temperatures of 1000–1070°C and pressures of 34–42 kbar. The Finsch xenoliths plot on a conductive palaeogeotherm with a surface heat flux of ~46 mW m2. Combined Ca and Cr abundances of Finsch pyrope garnets suggest that both lherzolitic and harzburgitic parageneses are present. Samples bearing sub-calcic (harzburgitic) garnets are from the shallowest depths. The lherzolitic garnets are depleted in light rare earth elements (LREE) relative to the middle and heavy REE (MREE and HREE) and have ‘smooth’ chondrite-normalized patterns. In contrast, the sub-calcic garnets are characterized by sinusoidal chondrite-normalized REE patterns that peak at Nd and Lu and exhibit lows at La and Er. The sub-calcic garnets also have lower Zr, Hf, Ti and HREE, and higher LREE and Sr, than lherzolitic garnets. The variations in REE ratios correlate with temperature and pressure and also Cr/Ca ratio. The high Cr content of harzburgitic and some lherzolitic Finsch garnets may have a significant effect on the crystal framework. Substitution of the larger Cr3+ ion for the smaller Al3+ ion increases with decreasing temperature and pressure and distorts the crystal lattice; this permits a greater substitution of Ca by large cations, such as Sr and the LREE, but also limits the replacement of Al by Ti, Zr and Hf. Positive HREE slopes displayed by harzburgitic garnets on chondrite-normalized plots are believed to result from metasomatic enrichment by a melt that had already undergone significant garnet fractionation during ascent through the lithospheric mantle. The low-temperature Finsch peridotites are characterized by much lower orthopyroxene (< 17%) and higher olivine (up to 96%) modal abundances than have been reported from xenolith suites elsewhere in the Kaapvaal craton. Significantly, they resemble residues generated in partial melting experiments. The Finsch harzburgites have very low Al2O3 (0·18 wt %) and CaO (0·38 wt %) and high MgO contents (49·75 wt %) and appear to be highly refractory. They also have high bulk-rock Mg/(Mg + Fe) and high modal olivine contents, and in this respect resemble some of those recently described from NW Canada and Greenland. We suggest that some of the Finsch low-temperature peridotites represent Kaapvaal lithospheric mantle that formed as a residue of adiabatic decompression melting between 4·5 and 1·5 GPa. The inferred mantle potential temperature of 1550°C would have been similar to that of ambient Archaean mantle. Importantly, it appears that the sub-Finsch lithospheric mantle has remained unmodified by the silica enrichment that has been so prevalent elsewhere in the craton. This may reflect the remoteness from the subduction zone that is believed to have been in existence at 2·9 Ga on the eastern margin of the craton.