Pretectonic tholeiitic volcanism and related transitional plutonism in the Kidal assemblage (Iforas Pan-African Belt, Mali)

Abstract

The Kidal assemblage corresponds to a high-grade tectonometamorphic unit situated beneath the IGU granulitic nappe. Pretectonic magmatic rocks of intermediate to ultrabasic compositions form more than 60% of the volume and were intensely foliated during the D1 event. Four groups have been defined: (i) alkaline leptynites associated with aluminous quartzites and dated at 1837 + 17/−9 Ma (Caby and Andreopoulous-Renaud 1983, J. Afr. Earth Sci. 1, 193–197); (ii) metadiorites-metatonalite plutons, far the most abundant; (iii) amphibolite bands, locally associated with metaconglomerates (O. Bourhessa), lying directly upon the quartzites and marbles attributed to the Upper Proterozoic; (iv) narrow lenes and large bodies of metagabbros associated with metaperidotites and some meta-anorthosites. The amphibolites show T. MORB affinities (low alkali and incompatible element contents and almost flat REE patterns), and this suggests a spreading environment. The gabbroic suite magmas, with various cumulates, are distributed between three end-members (olivine peridotites, pyroxenites, anorthosites). The fine grained gabbros show a close chemical similarity with the amphibolites and this suggests a direct genetic relationship between the two suites. The chemical characters of the metadiorite-metatonalite suite conform to typical diorite-tonalite-trondhjemite continental suites in a spreading environment. The magmatic units of the Kidal assemblage show significant lithological and geochemical similarities with the supracrustal basic volcanics and intrusive complexes of Archean high-grade belts. The postulated extensional behaviour of the Kidal assemblage may be considered to have indirectly induced the high-strain and dominantly rotational tectonic regime evidenced in this domain.