Petrogenesis of Archean PGM-bearing chromitites and associated ultramafic–mafic–anorthositic rocks from the Guelb el Azib layered complex (West African craton, Mauritania)

• Published in: Precambrian Research
• Main Authors: Berger, Julien, Diot, Hervé, Lo, Khalidou, Ohnenstetter, Daniel, Femenias, Olivier, Pivin, Marjorie, Demaiffe, Daniel, Bernard, Alain, Charlier, Bernard
• Other Authors: Fonds National de la Recherche Scientifique Bruxelles (FNRS), Université libre de Bruxelles (ULB), Laboratoire de Planétologie et Géodynamique UMR 6112 (LPG), Université d'Angers (UA)-Université de Nantes - UFR des Sciences et des Techniques (UN UFR ST), Université de Nantes (UN)-Université de Nantes (UN)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Université de Nouakchott, université de Nouakchott, Centre de Recherches Pétrographiques et Géochimiques (CRPG), Institut national des sciences de l'Univers (INSU - CNRS)-Université de Lorraine (UL)-Centre National de la Recherche Scientifique (CNRS), IAMGOLD Corporation-West Africa, Department of Earth, Atmospheric and Planetary Sciences MIT, Cambridge (EAPS), Massachusetts Institute of Technology (MIT)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2013
• Subjects: Anorthosite; Layered complex; Archean tholeiite; Granulites; Mafic–ultramafic rocks; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; Greenland
• Online Access: https://hal.univ-lorraine.fr/hal-01764070; https://doi.org/10.1016/j.precamres.2012.10.005

International audience The 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.