Abstract
Paleoproterozoic rocks of the Birimian Supergroup occupy the south-eastern portion of the West
African Craton (WAC). The Bui belt in Ghana is one of the NE-SW trending volcano-sedimentary
belts of the Birimian Supergroup. Geochemical and geochronological constraints on the
petrogenesis and geodynamic setting of the rocks in the Bui belt are generally lacking. The
geochemistry of lavas, gabbro and volcaniclastic rocks as well as argillites and greywackes of the
Birimian Supergroup and siliciclastic sedimentary rocks of the Tarkwaian Group from the Bui belt
and adjoining Maluwe and Sunyani basins have been studied to constrain their source, ages and
tectonic setting of emplacement. The volcanic rocks are mostly aphanitic or aphyric and composed
of clinopyroxene and plagioclase microlites, mostly altered to chlorite and sericite respectively.
The plutonic rock consists of partially altered augite and albitic plagioclase. The rocks are subalkaline,
mostly low-K tholeiites with low Mg# (volcanics and volcaniclastics: 25-65; gabbro: 60),
which suggests they do not represent primary magmas but have evolved by fractionation following
partial melting. The samples show affinity to N-MORB and E-MORB on rare earth element (REE)
and multi-element diagrams, suggesting their derivation from variably depleted mantle sources.
On tectonic discriminant diagrams, the rocks appear to show geochemical affinity to mid-ocean
ridge basalts formed in a divergent environment. Sm-Nd whole rock isotope data for the volcanic
rocks define an errochron with an age of 2224+110/-130 Ma. All the samples have positive εNd2.2
Ga values between +2.3 and +3.7. Lu-Hf isotope data on the other hand gives an errochron with an
age of 1987 +87/-110 Ma and positive εHf2.2 Ga values ranging between +0.7 and +7.3. The
dominantly positive epsilon Nd and Hf values confirm magma generation from moderately
depleted mantle sources with no significant crustal contamination but emplaced during sea-floor
spreading. These data accordingly support a Paleoproterozoic age of juvenile magma extraction
from the depleted mantle and formation of new lithosphere in the Birimian terrane in the WAC
during the Eburnean orogeny.
The argillites are composed of quartz and sericite whereas the greywackes consist of quartz,
feldspar, sericite, and chlorite. The rocks exhibit enrichment in light rare earth element (LREE), a
slight negative Eu anomaly and flat heavy rare earth element (HREE) pattern on chondrite
normalised diagrams. Compared to upper continental crust (UCC) and Post Archean Australian
shale (PAAS), they are depleted in LREE, have a positive Eu anomaly and flat HREE pattern.
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Their geochemical signature indicates that the rocks were derived from the volcanic rocks and
granitoids in the surrounding volcanic belts and have been affected by moderate degrees of
chemical weathering. The metasedimentary rocks have whole rock εNd2.2 Ga that ranges between
+2.4 and +3.9 and εHf2.2 Ga between +3.2 and +5.9, which confirms sediment source from juvenile
source rocks. The rocks were deposited in an oceanic island arc in the Paleoproterozoic.
The Tarkwaian rocks are enriched in SiO2 more than the other oxide due to their dominant quartz
content. These rocks were deposited not later than 2125 Ma. They have predominantly positive
εHf values (+0.1 to +8.0), which together with their U-Pb zircon ages confirm sediments from the
Birimian volcanic belts, basins and intrusive rocks in the Bui belt area contributed detritus during
Tarkwaian sedimentation. The rocks were deposited in a passive margin setting developed during
intra-montane rifting within the volcanic belt.
Keywords: Birimian Supergroup, Bui belt Ghana, mafic rocks, metasedimentary rocks,
petrogenesis, geodynamic setting