New petrology and geochemistry data of lower cryogenian mafic rocks of Precambrian Igherm inlier (Western Anti-Atlas, Morocco)

The lower Cryogenian mafic magmatism from the Igherm inlier (Moroccan western Antiatlas) have tholeiitic, calc-alkaline and alkaline affinities. Doleritic dyke swarms with various directions and gabbroic intrusive bodies were emplaced during Neoproterozoic times (PH), cutting across either Eburnean (PI) schists and granites or Neoproterozoic (PH) limestones and quartzites. All these rocks were deformed by the main panafrican schistosity and covered by molassic and volcanic Upper Neoproteroroic series. Three main groups of mafic rocks were recognized by the means of petrography and relative chronology. Using incompatible trace elements and rare earth elements, this magmatism appears as truly heterogeneous: the group 1 has tholeitic affinities whereas the groups 2 and 3 have calc-akaline and alkaline affinities respectively. The group 1 is earlier, as shown by the intrusive character of the dykes of the other two groups, into the gabbroic bodies of the first group. The geodynamical environment of the mafic magmatism in the inlier of Igherm can be described as linked to a major distensive regime contemporaneous of, or occuring immediately after, the oceanic opening already described in Central Anti-Atlas (Bou Azzer and Siroua ophiolites). The chemical heterogeneities observed in the three groups can be related to heterogeneities of mantellic sources and/or various partial melting ratios of the sub-continental mantle.