TITLE:
Petrology and Geochemical Relationship of Birimian Gabbro-Diorite-Granodiorite Dykes in Basaltic Rocks at Butre Area, SW Ghana
AUTHORS:
Victor Graham, George Mensah Tetteh
KEYWORDS:
Granitoids, Petrography, Alteration, Metamorphism, Geochemistry, Butre
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.12 No.2,
February
29,
2024
ABSTRACT: 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
FeOT, 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.