Abstract
Two Palæoproterozoic plutonic units are recognised in the Bas Drâa and Tagragra 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 < SiO 2 < 70.9%). Its mineralogy (oligoclase-andesine, amphibole, biotite, titanite ± magnetite ± epidote) and their chemical features are those of calc-alkaline I-type granites. Available isotopic data and rare earth element patterns suggest a lower crustal, or mantle, origin with variable contamination by crustal material. The second one corresponds to peraluminous granodiorites, granites and leucogranites (61.7 < SiO 2 < 74.8%). Its mineralogy (oligoclase, biotite, muscovite ± garnet ± tourmaline) and peralaminous composition reflects a crustal source. The nature and chemical signature of these two coeval plutonic units imply interaction between mantellic magmas and a continental crust. The siliciclastic nature of their sedimentary host rocks (Palæoproterozoic in age) corroborates the existence of an older continental domain of probable Archæan age. The lack of a volcaniclastic series in the two inliers precludes any volcanic-arc at that time. A back-arc geodynamic context, similar to that described in the Palæozoic of Argentina, is proposed. A comparison with the Reguibat and the Leo Rises shows the existence of two large Palæoproterozoic domains in the West African Craton: (i) an older, recycled Archæan crust to the west and north (Man, Tiris and Anti-Atlas); and (ii) a Palæoproterozoic juvenile domain located to the southeast and northeast (Baoulé-Mossi Block and eastern Reguibat Rise).
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