PETROLOGY AND GEOCHEMISTRY OF DEFORMED CARBONATITE AND NEPHELINE-SYENITE GNEISS IN THE PAN-AFRICAN DAHOMEYIDE OF SOUTHEASTERN GHANA, WEST AFRICA

Deformed carbonatite and nepheline-syenite gneiss occur at the suture zone between the autochthonous West African Craton with its cover rocks and craton-verging nappes of the Pan-African Dahomeyide orogen in southeastern Ghana, West Africa. The Dahomeyide rocks are Neoproterozoic high-pressure granulite gneisses associated with eclogites, and bounded by NE-SW trending shear zones. The suture zone is characterized by distinctive high-pressure mafic and rocks (Gt-Hb gneiss). The carbonatites are plutonic with disaggregated xenoliths of the quartzo-feldspathic basement gneisses. The carbonitite-syenite assemblage is ~50 m thick and traceable for 60 km along strike. These rocks are enriched alkalis, Ba, Sr, Nb and LREE/HREE ratios compared to common alkaline rocks. Melting models based on 22 incompatible trace elements imply 0.3% to <1% partial melting an enriched, carbonated garnet lherzolite source.

The occurrence of carbonitite and alkaline syenite is inconsistent with convergent plate margins and collisional orogenesis. Their occurrence as deformed layers that separate rocks of the West African Craton from the Dahomeyide nappes implies that they pre-existed both convergence and collision. We infer that these rocks formed in response to older (mid-Proterozoic?) extension and rifting that formed the trailing edge margin of the West African Craton. They were deformed during the collision and exhumed during the decompression and extension phases.