THE UPPER PROTEROZOIC OF IMITER (EASTERN SAGHRO, MOROCCO) - A GEODYNAMIC BACK-ARC ENVIRONMENT

The Upper Proterozoic (PII) of Imiter shows a large amount of turbidit ic greywackes which are organized into deep sea fans. They were deposi ted at the foot of the continental slope in an extensional basin prior to the pan-African orogeny. The extension is associated with mafic vo lcanism (MORE) enriched into light REE by crustal contamination. Petro logical and geochemical data show that the greywackes are immature (23 to 42% feldspars, 11 to 24% lithic fragments) with high concentration s of Na2O, Ba, Co, Cr and Ni. All these features are consistent with a n active back-are environment, the are itself being situated to the so uth of the Saghro mountains. However, the source of the sedimentary in fill of the basin shows continental characteristics with calc-alkaline affinities. Furthermore, this material comes from the northeast, whic h is a direction opposite to the are. The REE pattern is characteristi c of an active domain. This domain would correspond to a marginal or b ack-are basin, supplied with sediment from two opposing directions.