a b s t r a c tThe Archean Guelb el Azib layered complex (GAC) in the West African craton of Mauritania is composed of an association of serpentinites, chromitites, amphibolites and anorthosites with few fine-grained amphibolite dykes. The complex forms tectonic slices in 2.9-3.5 Ga TTG gneiss terrains in close association with supracrustal rocks (BIFs, impure marbles, amphibolites). It was affected by a main granulite-facies grade metamorphism (up to 900• C at 5-6 kbar) with subsequent retrogression in amphibolite and greenschist facies conditions.The preserved igneous macrostructures, the mineral compositions and the nature of relic magmatic assemblages have been used to constrain the composition of the parental melts and the conditions of crystallization. According to petrological observations and to comparison with experimental data, the formation of the complex can be explained by fractionation of a slightly hydrous high-alumina basaltic melt at low pressure. The early fractionation of olivine and the absence of massive clinopyroxene fractionation before plagioclase saturation led to crystallization of highly calcic plagioclase with Fe-, Al-rich but Cr-poor chromite from a hydrous tholeiitic parental magma, similar to worldwide Archean tholeiites.The complex shares many similarities with Archean anorthosite layered complexes, possibly formed in a supra-subduction zone environment according to results obtained on similar 2.9-3.0 Ga complexes from Greenland and India (namely Fiskenaesset and Sittampundi). Three phases of PGE mineralization affected the GAC chromitites: (i) igneous crystallization of laurite; (ii) formation of late magmatic IPGE sulpho-arsenides (irarsite-hollingworthite) and (iii) hydrothermal Pt-Pd mineralization represented by sperrylite and rustenburgite.

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A xenolith made of exceptionally Cr-rich minerals (mostly omphacite, kyanite and minor rutile) has been found in the diamondiferous Mbuji-Mayi kimberlite province, Democratic Republic of Congo (DRC). Chromium contents (Cr 2 O 3) are heterogeneously distributed in the bulk sample, as well as in a single crystal: 3.34-5.70 wt% in omphacite, 4.45-11.81 wt% in kyanite and 2.68-4.65 wt% in rutile. Such Cr-rich kyanite is extremely rare; only grospydite xenoliths from the Zagadochnaya kimberlite (Yakutia) have been reported with kyanite containing up to 12.86 wt% Cr 2 O 3. However, unlike typical grospydites, no garnet is observed in the Mbuji-Mayi Cr-rich kyanite-bearing clinopyroxenite. The textural features of this rock and the tentative thermobarometric equilibration conditions (25-35 kbar, 700-800 C) argue for an origin related to that of eclogites, which are the predominant xenoliths in the Mbuji-Mayi kimberlites. The calculated whole-rock composition suggests an Al-, Ca-and Cr-rich protolith that resembles Cr-rich leucogabbros or anorthosites commonly found in Archean anorthosite massifs. The peak P-T conditions registered by this unusual rock can be reached either by Archean subduction or by mafic magma underplating at the crust-to-mantle boundary with subsequent delamination into the upper mantle.

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