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RESEARCH PRODUCT

Eclogite xenoliths from the Kuruman kimberlites, South Africa: geochemical fingerprinting of deep subduction and cumulate processes☆

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description

The mineralogically diverse eclogite xenolith suite sampled by the >1.6 Ga Zero kimberlite in the Kuruman group, close to the western edge of the Kaapvaal craton consists of bimineralic eclogites, orthopyroxene-bearing eclogites with and without sanidine, and kyanite eclogites. Garnet exsolutions in clinopyroxene occur in all Zero eclogite types, and orthopyroxene and, where present, sanidine are also exsolved from clinopyroxene in orthopyroxene-bearing eclogites. Mineral thermobarometry indicates that the last P–T condition prior to the kimberlite eruption was 900–950 °C at 3.5–4.0 GPa. However, inclusions of quartz, rutile and rarely K-feldspar, plus former fluid inclusions in garnets, indicate a complex earlier history. δ18O values of 5.18–5.47 are all mantle-like. Trace element concentrations in minerals of the Zero eclogite suite have been analysed by Laser-ICP-MS, and whole-rock compositions for both major and trace elements were reconstructed from mineral compositions and modal proportions. Results show that the eclogite protoliths consisted of varying proportions of clinopyroxene, plagioclase and olivine with minor spinel. Abundances of mafic minerals are constrained by transition elements and plagioclase by ratios of Sr and Eu to other rare earth elements. Calculations show that <25% plagioclase may not result in a noticeable Eu anomaly in the rock pattern. Forward modelling of mineral accumulation from melts with the program pMELTS shows that almost the entire eclogite suite can be explained by fractionation of a picritic parental magma at lower crustal depths in thick Archean oceanic crust. Sanidine–orthopyroxene eclogites appear to be cogenetic with the other eclogite types, and carry more evidence for the P–T–t history of the suite. They are interpreted to originate from phlogopite or phengite-bearing protoliths which were displaced to depths of 200–250 km during subduction or collision, where the hydrous minerals dehydrated. The released potassium was taken up in clinopyroxene, whereas evidence for fluid and its solutes is entrapped in garnet, later forming quartz. Tectonic relaxation caused uplift of the eclogites from the 7–8 GPa indicated by K solubility in clinopyroxene to 3.5–4.5 GPa, during which sanidine, orthopyroxene and garnet exsolved from clinopyroxene. The fluid inclusions in garnet decrepitated at a late stage.