Proterozoic Blueschist-Bearing Melange in the Anti-Atlas Mountains, Morocco: Implications for Pan-African Subduction

Blueschists from the Bou Azzer inlier provide compelling evidence for Late Proterozoic subduction in the Anti-Atlas Mountains of Morocco. High-pressure/low-temperature metabasites containing blue amphibole minerals crossite and magnesioriebeckite record pressures in excess of 5 kbars. Blueschist facies rocks crop out in a heterogeneous assemblage of variably deformed and metamorphosed tectonic slices of ophiolitic fragments enclosed in a schistose serpentinite matrix. The mélange belt containing the blueschist facies rocks is intruded by a number of diorite plutons, one of which has yielded a U/Pb radiometric age of 650 Ma. Together with regional relationships, the geologic setting of the blueschists constrains the polarity of Pan African subduction in this region, which occurred from ~750-600 Ma. The Neoproterozoic blueschists exposed within the Bou Azzer inlier provide compelling evidence for north-dipping subduction during the Pan African Orogeny. Alternative hypotheses suggesting that the southern Anti-Atlas represents an aulacogen or back arc basin environment are inconsistent with the occurrence of blueschist-bearing mélange in the Bou Azzer inlier. Together with Transaharan Belt to the southeast, the Anti-Atlas suture zone exposed within the Bou Azzer inlier contains among the oldest known blueschist-bearing, ophiolitic mélanges in the world.