Lower Cryogenian calc-alkaline mafic rocks of the Western Anti-Atlas (Morocco): An example of orogenic-like magmatism in an extensional setting

Abstract

The lower Cryogenian mafic magmatism from the Igherm, Ifni and Kerdous inliers (Moroccan Western Anti-Atlas) have calc-alkaline, tholeiitic and alkaline affinities. The calc-alkaline dolerite dykes and gabbros bodies emplaced before the conglomeratic formations of the Upper Cryogenian and after the tholeiitic mafic rocks that characterize the pre-Pan-African rifting. They are similar to rocks from orogenic setting and characterized by high LILE, Th, Ce, P, Sm contents and La/Nb ratio and a low HFSE content with negative anomalies in Nb, Zr and Ti. The geodynamic environment of the sedimentary country rocks corresponds to that of a passive margin in a distensive tectonic context. The calc-alkaline affinity of these magmas can be attributed to the influence of a Palaeoproterozoic subduction zone that contributed to the enrichment of the sub-continental mantle. During the extensional event of the Pan-African orogenesis, the mantle would have produced tholeiitic, alkaline and/or transitional magmas before melting (caused by adiabatic decompression) reached the enriched sub-continental mantle. This mantle was previously enriched during the Eburnean subduction, where it would have generated calc-alkaline magmas.