Structural And Petrologic Evolution Of The Internal Lithotectonic Units Of The Dahomeyide Orogen In Southeastern Ghana

ABSTRACT

Mafi-Tsawla and surroundings geologically belongs to the internal zone of the Pan-African Dahomeyide orogenic belt, in southeastern Ghana. The geology of these internal units requires a total understanding of the structures and lithological types and their sequences. However, the structural and petrologic evolution of these internal units is unresolved, and this results in limited understanding of the geology of the terrain. The objectives of this study are to conduct a systematic field mapping on the internal units and produce a composite geological map and appropriate cross section(s); and to conduct an integrated study of structures, field relations, petrology and major and trace element geochemistry of these units in order to deduce their structural and petrologic evolution. The internal lithotectonic units have been studied using the method of systematic field mapping and rock sampling in the field with petrographic, microstructural, and limited geochemical analysis of selected samples in the laboratory together with the interpretation of aeromagnetic data. The results show that the terrain is dominated by highly heterogeneous stromatic biotite migmatites, felsic augen gneisses and biotite gneisses with lenses of remnant amphibolite, biotite schist and hornblende-biotite gneiss all intruded by granites and granodiorites; pegmatoid and aplite. The amphibolite evolved from a mafic protolith through metamorphism. The felsic gneisses however, either evolved from the amphibolite or some different rock rich in continental materials. The migmatites then formed from the felsic gneisses and finally the pegmatoids and aplites evolved. The terrane is characterised by geological structures such as; foliations; isoclinal, drag, infrafolial, ptygmatic folds and crenulations; dextral and sinistral shear faults; joints; boudins and porphyroblasts with kinematic indicators revealing a coupled dextral-sinistral sense of shear. Analyses of all structural features reveal that the Dahomeyide internal units

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have undergone five deformations (D1-D5) of which D2 was most intense. Petrographic and major and trace element geochemistry show that the biotite migmatites and granitoids are felsic in composition. Major and trace element geochemistry shows that migmatites and granitoids show sub-alkaline (calc-alkaline) signatures and formed in volcanic arc, syn-collisional and within plate settings. Negative Ti and Nb anomalies are indications of crustal contamination during subduction and exhumation. Negative Eu anomalies are observed in all rocks except the pegmatoid, a migmatite and a biotite gneiss with positive Eu anomalies. The investigated rocks maybe genetically unrelated and hence might have evolved from different protoliths. The granitoid, gneiss and migmatitic terrane of the Pan-African Dahomeyide orogenic belt in Southeastern Ghana is poly deformed and poly metamorphosed and shows a high degree of lithologic heterogeneity.