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

Abstract

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.

Following the Amphibolite to Greenschist Metamorphic Path through the Structural Parameters of Spinels from Amsaga (Mauritania)

“…The observed oxygen parameters are rather different from samples from ophiolites [14,[16][17][18]. According to the petrological observations and in comparison with experimental data by [39], 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-and Al-rich, but Cr-poor, chromite, from a hydrous tholeiitic parental magma, similar to worldwide Archean tholeiites.…”

“…Amphibolite-hosted spinels are generally characterized by high Al and Mg contents, according to [53], and their paragenesis seems to be the result of pyroxene-consuming retrograde metamorphism that produces amphibole as a secondary phase. The composition of these spinels is typical of high temperature (granulite to amphibolites facies conditions) meta-troctolites [39]. Their composition suggests that high-grade metamorphism has led to enrichment in the spinel end-member with leaching of Cr [39].…”

“…The composition of these spinels is typical of high temperature (granulite to amphibolites facies conditions) meta-troctolites [39]. Their composition suggests that high-grade metamorphism has led to enrichment in the spinel end-member with leaching of Cr [39].…”

“…Chromitites are found in dissected complexes formed by the association of serpentinites (former olivine-rich cumulates), mafic olivine-amphibole-spinel-plagioclase rocks (meta-troctolites), amphibolites and recrystallized anorthosites [39]. The whole complex is in tectonic contact with TTG orthogneiss and supracrustal BIF, cipolin and amphibolites.…”

“…Rollinson et al (2010) [40] supposed an igneous origin for the amphibole present in the anorthositic chromitites of Fiskenaesset that seem to be analogous to the chromitites we examined herein. The presence of associated talc-serpentine and chlorite-bearing chromitite rocks in Amsaga make us confident in hypothesizing one or more metamorphic events affecting these rocks, so that the presence of amphiboles could be related to the metamorphism [39]. According to these authors, it is not possible to establish which were the primary igneous structures, textures, modal and mineralogical compositions because of the metamorphism that pervaded the Guelb el Azib complex, which means that the "primary" composition of the here-studied spinels is unclear.…”