Geochemistry of high-pressure granulites from the Pan-African Dahomeyide orogen, West Africa: constraints on the origin and composition of the lower crust

Abstract

Major and trace element concentrations in high-pressure (HP) mafic granulites which mark the suture zone of the Pan-African Dahomeyide orogen have been determined. The data suggest that the HP granulites may have preserved the geochemical imprints of their magmatic protoliths where: (i) rocks characterized by island arc tholeiitic (IAT) imprints predominate, and (ii) those with oceanic crust (N-MORB) imprints occur but are subordinate. Rocks with eclogitic mineral assemblages tend to have MORB imprints. We interpret the association of IAT rocks with those having MORB compositions as evidence for a Pan-African subduction zone where arc-related oceanic crust formed either in back-arc or intra-arc basins. The HP granulites display trace element patterns that are very similar to mafic lower crust (LC) composition such as: (i) nearly identical concentrations of Ti, Mn and P, (ii) strong depletions in Cs, Rb, K and Th, and (iii) enrichment in Ba, Ta and Sr. But the HP granulites also differ from LC in that (i) rocks with IAT imprints show peculiar LREE enrichment with flat (La/Sm) N, (ii) those with oceanic crust geochemical signatures are slightly depleted in LREE and have low (La/Yb) N relative to LC, and (iii) they show no Eu anomaly. The data suggest that the Dahomeyide suture zone rocks may represent accessible LC formed by HP metamorphism of subduction zone basalts.