Palaeoproterozoic granitoids from the Bas Draa and Tagragra d'Akka Inliers[western Anti-Atlas, Morocco]: part of the jigsaw puzzle concerning the West African Craton

Two Palaeoproterozoic plutonic units are recognised in the Bas Draa and Tag ragra d'Akka Inliers (western Anti-Atlas, Morocco) using mineralogical and chemical data. The first one consists of a calc-alkaline suite of diorites, monzogabbrodiorites, granodiorites and granites (50.9 < SiO2 < 70.9%). Its mineralogy (oligoclase-andesine, amphibole, biotite, titanite +/- magnetit e +/- epidote) and their chemical features are those of calc-alkaline I-typ e granites. Available isotopic data and rare earth element patterns suggest a tower crustal, or mantle, origin with variable contamination by crustal material. The second one corresponds to peraluminous granodiorites, granite s and leucogranites (61.7 < SiO2 < 74.8%). Its mineralogy (oligoclase, biot ite, muscovite +/- garnet +/- tourmaline) and peraluminous composition refl ects a crustal source. The nature and chemical signature of these two coeval plutonic units imply interaction between mantellic magmas and a continental crust. The silicicla stic nature of their sedimentary host rocks (Palaeoproterozoic in age) corr oborates the existence of an older continental domain of probable Archaean age. The lack of a volcaniclastic series in the two inliers precludes any v olcanic-are at that time. A back-arc geodynamic context, similar to that de scribed in the Palaeozoic of Argentina, is proposed. A comparison with the Reguibat and the Leo Rises shows the existence of two large Palaeoproterozoic domains in the West African Craton: (i) an older, recycled Archaean crust to the west and north (Man, Tiris and Anti-Atlas); and (ii) a Palaeoproterozoic juvenile domain located to the southeast and n ortheast (Baoule-Mossi Block and eastern Reguibat Rise). (C) 2001 Elsevier Science Limited. All rights reserved.