Geochemical Characterization And Petrogenesis Of The Scapolite–Bearing Gneisses In The Dahomeyide Suture Zone, South–Eastern Ghana

ABSTRACT

According to Knorring and Kennedy (1958), scapolite occurs in rock volume proportions of 20% at the Mampong Inselberg but other areas in the Dahomeyide suture zone have scapolite in the mafic gneiss in volume proportions of about 2%.Also, the amphibolite which occurs with the mafic gneiss in the study area is said to have undergone granulite facies metamorphism to produce the mafic gneiss (Fitches, 1970). The petrographical analyses of this research showed that the mafic gneiss has scapolite with 4 to 8 volume percent proportions (vol %).The scapolite observed at the Mampong Inselberg is same as the SO3 variety which is Ca-rich and CO2-bearing (Meionite) found by Knorring and Kennedy (1958), based on the fact that, the bulk geochemical data obtained for scapolite-bearing gneiss in both researches are similar. The Mampong Inselberg and surrounding areas have more amphibolite outcrops and fewer eclogite outcrops as compared to the areas north of the suture zone. This suggests that the areas at the southern part of the suture zone represents the region where the amphibolite facies metamorphism was intensive and this favours the development of the scapolite mineral in greater proportions. The (alkali, iron and magnesium) AFM diagram plotted for the rocks show that the amphibolite plot in the tholeiitic field while the mafic gneiss plot in the calc-alkaline field suggestng that they have two different evolution paths. This is supported by the fact that, on the field, some of the contacts observed between the mafic gneiss and the amphibolite is very sharp and amphibolite boudins also show relative movements with respect to the mafic gneiss. From the chondrite normalized rare earth element (REE) diagrams, the mafic gneiss and amphibolite show patterns which differ slightly from each other and can be likened to that of Lower crust (LC) materials. The amphibolite show a slight negative Europium (Eu) anomaly while the mafic gneiss

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does not. Spider diagrams also show patterns for the mafic gneiss and amphibolite which can be likened to that of the Lower crust (LC) materials. The patterns displayed by the rocks on the rare earth element diagrams and spider diagrams suggest that the rocks have originated from the Lower crust while other tectonic settings discrimination plots show that the rocks have Island arc tholeiite affinities and hence may have been formed in an Island arc setting.