The International Institute for Science, Technology and Education (IISTE)
Abstract
Gneissic terranes under studied are one of the various formations constituting the Ourika Old massif. They underwent a metamorphic evolution characterized by a first amphibole facies event and a second greenschist facies metamorphism. The high-grade metamorphism is related to a Pan-African orogenesis that produced subduction-related granitoids preserved as GAA and GBA gneisses. These two gneissic groups have different geochemical compositions which were likely linked to the protolith nature. Petrology and geochemical investigations reveal that the protolith of GBA gneisses is calc-alkali peraluminous S-type granodiorite and thus of GAA gneisses is calc-alkali metaluminous diorite. The GBA protolith showed a continental active margin characteristic that may belong to the earlier Pan-African event, at ~780 to 750 Ma, whereas the GAA protolith could be formed in the island arc/fore-arc event, at ~753 Ma. Both groups were ordered in two lines suggesting two different sources where the crustal intervention is more or less marked, by juvenile upper continental crust for GBA protolith, and by young lower continental crust for GAA protolith. Correlated to the anti-atlasic formations of the same age, the geochemical similarities suggest a comparable geodynamic evolution that is closely linked to a Neoproterozoic continental convergent margin in the north of West-African Craton (WAC), collided at late Pan-African orogenesis. This collision induced the strongly N-S deformation that was materialized by the overthrusting of the GAA protolith onto the GBA protolith, and by the forming of the Ourika gneissic massif as a submeridian dome. Keywords: Ourika old massif; gneissic protoliths; Pan-African orogenesis; mineralogy and geochemistry; geodynamic evolution
