Author(s)

Victor Graham, George Mensah Tetteh

Department of Geological Engineering, University of Mines and Technology, Tarkwa, Ghana.

Granitoids at Butre area, south-western part of the Birimian of Ghana are gabbro to granodiorite which intruded comagmatic basalt and andesite of volcanic island arc setting. Differentiation was from probably mantle fractionates on tholeiitic basalt trend through gabbro-diorite and granodiorite. Yet, the diorite intruded into the volcanic rocks prior to plate collision. Plagioclase feldspars of labradorite to andesine compositions (An₅₂ - An₄₈), amphibole and pyroxene are in association with accessory tourmaline, actinolite, apatite and garnet. Relict amphibolite facies metamorphism is preserved in xenoliths in gabbro to diorite which show greenschist facies. Generally, plagioclase is partially altered to fine quartz, sericite and carbonates whereas chlorite and epidote are of amphibole alteration. These alterations are typical of carbonatisation but diorite shows partial carbonate-sericite alteration. Alterations followed granodiorite emplacement. Arsenopyrite and pyrite accompanied emplacement of diorite and granodiorite respectively while amphibolite facies metamorphism introduced magnetite; haematite and pyrrhotite accompanied greenschist facies metamorphism. Earlier opaque minerals possibly were consumed to form later minerals. All the rocks are LREE depleted (La/Sm chondrite normalised ratios of basalt = 0.8 to 2.5; andesite = 2.03; diorite = 1.95 to 3.1; gabbro-diorite = 0.9 to 2.05; granodiorite = 1.66); diorite, basalt and andesite are fairly enriched in HREE. Enrichment of FeO_T, MgO, SiO₂, CaO and depletion of TiO₂, K₂O and Zr might be linked with carbonitisation, sericitisation, chloritisation, silicification and sulphidation linked to gold mineralisations in the Birimian of Ghana.

Granitoids, Petrography, Alteration, Metamorphism, Geochemistry, Butre

Graham, V. and Tetteh, G. (2024) Petrology and Geochemical Relationship of Birimian Gabbro-Diorite-Granodiorite Dykes in Basaltic Rocks at Butre Area, SW Ghana. Journal of Geoscience and Environment Protection, 12, 107-130. doi: 10.4236/gep.2024.122007.