New Petro-Geochemical Data on Carboniferous Mafic Rocks in the Achemmach Area (NW, Fourhal Basin-Moroccan Central Massif)

Abstract

The Achemmach region is located 40 km to the SW from Meknes city and 6 km to the NE frm the Hammam mine. It is part of the Paleozoic massif of Central Morocco. The studied area is formed by two Paleozoic rock-types: (i) a meta-sedimentary sequence composed of Middle Visean limestone and shale-sandstone withflyschoid of Upper Visean-Namurian age, and (ii) a magmatic rock series represented by volcanic rocks (pillow-lavas), hypovolcanic rocks (dolerites) and olivine-bearing gabbros.Based on the emplacement model, structural framework, relative chronology and petrogeochemistry of the magmatic rocks, for the first time in this area we distinguish: (i) dm to m-sized greenish pillow-lavas, with sharp borders and radius fractures underlined by fine greenish pelitic sedimentary intercalations, indicating recurrent volcanic activity in short episodes. Plagioclases and pyroxenes (augite) microlites, and more rarely phenocrystals, are recognizable in a glassy matrix devoid of recognizable olivine. (ii) Deformed, metamorphosed and altered dolerites dikes intrude the Middle to Upper Visean shale-sandstone formations. They have an overall NE-SW direction with a NW dip. They are composed of sericitized plagioclases, associated with partially to totally amphibolitized pyroxenes, tourmaline with differentdegrees of chlorite substitution, rutile and opaque minerals, in a microliticmesostasis and (iii) olivine-bearing gabbros, outcropping in variable dimensions (a few meters to 20 m). The olivine-bearing gabbros have a granular texture and are mainly made of plagioclases, pyroxenes, olivine, titanite, rutile, apatite and opaque minerals. All igneous minerals have undergone different degrees of replacement by secondary minerals; plagioclases are sericitized and albitized, pyroxenes are amphibolitized and epidotized and olivine is serpentinized and chloritized. Thepetro-geochemical study of these magmatic bodies demonstrates that pillow-lavas basalts and olivine-bearing gabbros have an alkaline affinity, while dolerites are thought to have a transitional alkaline affinity (alkaline-tholeiitic). Therefore, these formations would have been set up in anorogenic intra-continental geodynamic context, corresponding to a basin magmatism in the little evolved opening.