Algerian iron ores. Mines of ain-sedma, near Collo.

Abstract

This study presents the first geochemical, mineralogical and petrological data on the mafic and ultramafic rocks of Texenna (Lesser Kabylia, north-eastern Algeria) with the aim of constraining their tectonic setting in the context of the Maghrebide belt. The magmatic-sedimentary complex of Texenna comprises serpentinite, metabasites (metagabbro, metadolerite and metabasalt with pillow-lava structures), and oceanic metasediments (radiolarite and calcschists). Serpentinites consist mainly of mesh textures and large bastites of lizardite, Cr-spinels altered to “ferritchromit” and chlorite, with chrysotile and chlorite veins. Their features are very similar to those of serpentinites collected from the seafloor. Pillow-lava metabasalts contain mineral assemblages of albite, epidote, chlorite, actinolite and titanite, and, despite metamorphism, have preserved many of the pillow-lava microstructures (former glassy, variolitic and spherulitic zones; former devitrification spherulites and gas vesicles; pseudomorphs after phenocrysts and microliths). Metadolerites and metagabbros have sometimes preserved their magmatic plagioclase, but are generally transformed into an assemblage of albite, calcic amphiboles, chlorite, epidote and titanite, a mineral assemblage typical of the static greenschist-facies hydrothermal metamorphism of an oceanic crust. Some deformation in the conditions of phengite-bearing greenschist-facies metamorphism is linked to the tectonic emplacement of the ophiolites in their current setting during the Maghrebide orogeny. Geochemically, the whole-rock compositions of the serpentinites indicate a harzburgite protolith, while the metabasites are tholeiitic and show the N-MORB signature for the pillow lavas and T-MORB to E-MORB affinities for the intrusive rocks (metadolerites and metagabbros). These results demonstrate for the first time that the magmatic-sedimentary complex of Texenna belongs to a true ophiolitic slice and indicate that the sedimentary cover of the Maghrebian Flysch domain was deposited over a true oceanic rather than a thinned continental crust. The Texenna ophiolites show analogies with many other ophiolites in the Western Mediterranean region, indicating that they represent remnants of the ancient oceanic lithosphere of the western Tethys (i.e., Maghrebian Tethys Ocean) that formed between Africa and Iberia during the Middle-Late Jurassic.