THE NEOPROTEROZOIC BOU AZZER OPHIOLITE (MOROCCO): PETROLOGY, GEOCHEMISTRY AND GEODYNAMIC IMPLICATIONS

The Neoproterozoic Bou Azzer Ophiolite, located in the central part of the Panafrican Anti-Atlas belt (Morocco), represents oceanic lithosphere obducted onto the West African Craton. The ophiolitic sequence is strongly dismembered but it can be reconstructed to a 5 - 6 km thick sequence that includes volcanic rocks, sheeted dikes, gabbros and layered gabbros, ultramafic cumulates, serpentinized mantle peridotites, and chromitites, all with variable metamorphic/alteration imprint and deformation. The geochemical signatures of the rocks indicate that the oceanic lithosphere formed in a supra-subduction zone setting, but there is still a debate on whether it formed in a backarc or a forearc. The mantle sequence is strongly affected by metamorphic and hydrothermal processes and the original peridotites are completely serpentinized. Nonetheless, unaltered Cr-spinel cores are preserved and their compositions indicate that the ultramafic protolith was a highly depleted forearc peridotite. Chromitites within the mantle section also have chromite which pristine composition indicates equilibrium with forearc basalts (FAB) and boninite-like melts. The crustal sequence shows greenschist (locally amphibolite) facies metamorphism. However, immobile trace elements in the magmatic rocks that form the crustal sequence show island arc tholeiite (IAT) affinities and a clear evolution from mafic to intermediate compositions, which is reflected in a progressive enrichment in immobile incompatible elements. Therefore, our results indicate that the Bou Azzer ophiolite formed in a forearc setting, during a typical subduction-initiation geodynamic evolution, showing magmatic evolution from common forearc basalts towards boninitic and IAT melts. Similar Neoproterozoic ophiolites have been reported in other inliers within the Anti-Atlas domain, such as the Sirwa ophiolite, and also in the Arabian-Nubian shield related to the Panafrican Orogeny. However, in the South-American margin of Gondwana, Neoproterozoic ophiolites have only been reported in Peru (Tapo, 718 Ma) and Argentina (Pampean Ranges, 647 Ma), which cannot be correlated to the Moroccan Bou Azzer ophiolite. Other ophiolite relicts from the SE Brazilian coast represent the remnants from an Ediacaran Ocean that predated the Gondwana amalgamation.