Petrography and Geochemistry of Granitoid Gneisses and Associated Rocks in Parts of the Suhum Basin, Ghana: Implications for Petro genesis and Tectonic Setting

ABSTRACT

The Paleoproterozoic Birimian rocks within the Nsawam, Suhum and Koforidua enclave of the Suhum basin have been analyzed for their major and trace element concentrations and mineral chemistry. Twenty-nine (29) thin sections were prepared for petrographic studies. Twenty-five (29) samples were analysed for their major and trace elements contents and twenty-four (24) samples were analyzed for the mineral chemistry of selected phases. Based on the petrography, the geology of the area is characterized predominantly by hornblendebearing migmatitic-granitoid gneisses which are intruded by leucocratic granites and pegmatites. Gabbros occur sporadically throughout the area with gneissic biotite granite restricted to the north-eastern part of the study area. The high LaN/YbN values and moderate to high (Tb/Yb)N indicate the presence of garnet and amphibole at the site of partial melting. High Mg# (39.13-52.56), lower (Ta/La)N (0.05–0.58) and higher (Hf/Sm)N (0.98–3.53) values suggest that the parental magmas of the granitoid gneisses might be derived from partial melting of basaltic source (lower crust) metasomatized chiefly by slab-derived melts at differential depths. The migmatites are characterized by poikiloblastic amphiboles with inclusions of biotite, feldspar and quartz. The melanosome and leucosome show similar trace element patterns but large variations in trace element concentrations pointing to in-situ partial melting and crystallization of the leucosome with the melanosome representing the residuum.

Combined petrographic and geochemical evidences points towards water-assisted biotitebreakdown melting lowering of solidus temperatures so that migmatization can take place at amphibolite facies conditions. The leucocratic granites have high Sr/Y (15.58-549.14), lower Rb/Sr (0.02-0.91) and Nb/U (0.65-6.45) which implies their derivation from felsic igneous protoliths in the upper crustal portions in a compressive environment. The mafic composition of the biotite-hornblende restites, their middle to lower crust affinities, co-linear harker trends and similarities in their primitive mantle normalized spider diagrams suggests that the restites iii have a co-genetic relationship with their host granitoid-gneisses and thus, may represent remnants of the protolith from which the granitoid gneisses were derived. Their high but variable SiO2 contents (58.3–75.1 wt.%), K2O/Na2O values (0.14-2.2) and (FeO+Fe2O3)/MgO ratios (1.5-6.6), low Ce and negative Nb-Ta-Ti anomalies exhibited by all the rocks are akin to volcanic arc granitoids and suggests subduction-related signatures or melting of crustal rocks formed along subduction zone setting. Correspondingly, the slightly variable Sr- and Eu-anomalies indicate varying degrees of plagioclase fractionation. Generally, all the rocks exhibit medium-, high-K to shoshonitic characteristics coupled with calc-alkaline affinities which are akin to magmas which originated in an active continental margin. These rocks show trace element characteristics which suggests syn- to late-tectonic emplacement. Numerical calculations from mineral chemistry data based on amphiboleplagioclase thermobarometry indicates that the gneisses and migmatites formed at P-T conditions of 632-712°C and 5.2-7.2 kbar, signifying deeper sources. The overall petrographic and geochemical data suggest that these rocks are high temperature, I-type, calcalkaline rocks which formed during a single orogenic event related to a continental arc environment where subduction zone components played a role in the generation of their parental melts.