Garnet Lherzolites from the Kaapvaal Craton (South Africa): Trace Element Evidence for a Metasomatic History


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Abstract

Kimberlites from the Kaapvaal craton have sampled numerous mantle garnet lherzolites in addition to garnet harzburgites. Trace element characteristics of constituent clinopyroxenes allow two groups of garnet lherzolites to be distinguished. Trace element compositions of all clinopyroxenes are characterized by enrichment in light rare earth elements (LREE) and large ion lithophile elements and by a relative depletion in Ti, Nb, Ta, and to a lesser extent Zr and Hf. However, the LREE enrichment and the depletion in Nb and Zr (Hf) are less in the Type 1 clinopyroxenes than in the Type 2 clinopyroxenes. Our study suggests that the two melts responsible for the metasomatic imprints observed in the two garnet lherzolite groups are highly alkaline mafic silicate melts. Type 1 clinopyroxenes that have trace element similarities to those of PIC (Phlogopite–Ilmenite–Clinopyroxene) rocks appear to have crystallized from, or been completely equilibrated with, the same melt related to Group I kimberlite magma. The Type 2 clinopyroxenes have trace element similarities to those of MARID (Mica– Amphibole–Rutile–Ilmenite–Diopside) rocks and are therefore probably linked to melt related to Group II kimberlite magma.

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