HIGH-PRESSURE GRANULITES AND ECLOGITES OF THE DAHOMEYIDE SUTURE ZONE (ADAKLU, SOUTHEASTERN GHANA): PETROGENETIC EVOLUTION AND TECTONIC IMPLICATIONS

ABSTRACT The Dahomeyide suture zone marks continental amalgamation between the Paleoproterozoic West African Craton (WAC) and the Sahara Metacraton (SMC). This suture zone of the PanAfrican orogenic belt is encrusted by UHP/HP metamorphic rock bodies and associated ultramafic igneous plutonic rocks in SE Ghana through to Togo and Benin. The main identified rock types consist of mafic granulites (hornblende-garnet-pyroxene gneiss and pyroxenegarnet-hornblende gneiss), felsic gneiss, eclogite and pyroxenite. The petrography, structures and geochemistry of these rocks have been studied with the objective of inferring their petrogenesis and tectonic setting. Forty (40) thin sections were prepared and twenty-six samples were analyzed for major and trace elements using the Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Some garnet porphyroblasts observed in the mafic granulite of the suture zone have preserved an indicative microstructure of UHP and prograde-retrograde metamorphism. These were respectively attributed to the exsolved rutile rods in garnet and also disequilibrium textures (corona textures) of incomplete reactions observed within the granulites and eclogites. The area has been affected by three deformational events: D1, D2 and D3. The D1 event (characterized by dextral sense of deformation) was a transpressional event which produced the penetrative S1 foliations which are sub-parallel to the axial plane and axis of fold F1. D2 is marked by a dominant extensional event which resulted in the normal faults (Ft1 and Ft2) of the area and the stretched and rotated boudins in the pyroxene-garnethornblende gneiss. D2 is marked by sinistral sense of deformation and a subordinate compressional event which is evident by the thrusting of the boudins. D3 which is the last of the main phases of deformation (dextral sense) was a compressional event which is evident in the garnet-hornblende gneiss at Torkor. D3 resulted in the formation of the recumbent folds F2 which have over-printed F1. These three deformation events confirm with a typical convergent boundary deformation which starts with collision, extension and a final collision which are all represented by the D1, D2 and D3 respectively. The suture zone rocks at Adaklu consist largely of rocks with IAT affinities and also include suites of rocks with N-MORB (oceanic crust) and subordinate calc-alkaline geochemical signatures. These signatures are typical characteristics of an arc environment setting. There is also some crust contamination in the mafic rocks and hence the crust (lower crust) played an active role during the evolution of these rocks. In contrast the mafic/ultramafic rocks show very little crustal contamination as these rocks show negative Th, moderate U and almost flat Zr-Hf anomalies. Low Th/Yb and high Nb/Th ratios in the mafic/ultrmafic rocks are inconsistent with crustal contamination. However, comparably, the felsic gneiss has higher Th and Pb contents, higher Th/Yb and low Nb/Th ratios. Based on the geochemical data represented on the pyroxenites (high Ba/Nb, ~36-262; Ba/Zr, ~1.44-4.03; Ba/Th, ~144-524; and U/Th, ~0.5-1 relative to the primitive mantle and Mg# and Ni content), it is proposed that the primary melts of these ultramafic plutonic rocks in the area were derived from the normal lithospheric mantle wedge which had not been depleted by previous melt extractions and strongly modified by hydrous fluids.