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This study uses petrographical and whole-rock geochemical data to determine the provenance, paleo-weathering conditions, tectonic settings, paleo-climate, and mixing proportions of the conglomerate and greywacke in the Chagupana area within the Paleoproterozoic Birimian in North-West Ghana. The conglomerate is foliated and composed of dominant metamorphosed clasts which are matrix-supported, poorly sorted, and polymictic. The greywacke is immature, medium to coarse-grained, poorly sorted, and rich in quartz (53%) and feldspar (46%). The major element contents of 30 greywacke samples were determined by X-ray fluorescence, while the trace element contents including rare earth elements (REEs) were analyzed by inductively coupled plasma mass spectrometer. SiO2, Na2O, and Fe2O3 are slightly enriched while TiO2, MnO, MgO, CaO, K2O, and P2O5 are depleted, compared with average upper continental crust values. The greywacke is sodic with K2O/Na2O ratio of 0.63. Average chemical index of alteration (53%) and chemical index of weathering (58%) values indicate weak chemical weathering, while Plagioclase index of alteration (53%) value suggests weak-moderate alteration of plagioclase. The greywacke is from intermediate-felsic source rocks with minimum recycling, incipient alteration, and underwent K-metasomatism during diagenesis. Tectonically, the rocks belong to an active continental plate margin of a rift basin with slight positive Eu anomaly suggesting derivation from young undifferentiated volcanic arc. Very low TiO2 (< 2.0 wt.%) content suggests the influence of igneous rocks from magmatic arcs. Positive Ba-Th anomaly, negative Nb–Ta, and Zr-Hf anomalies suggest an arc setting associated with subduction. The detritus making up the Chagupana greywacke is best modeled with mixtures having 59% granite and 41% basalt. [ABSTRACT FROM AUTHOR]

This study presents the petrology, geochemistry, U–Pb ages, Lu–Hf and oxygen isotope compositions of Adaklu mafic granulites (ADMGs), from the Pan-African Dahomeyide suture zone in southeastern Ghana. The ADMGs show mafic precursor with low-K tholeiitic affinity. They display convex rare earth and trace elements characteristics without any obvious anomalies of Eu, Ti, Nb and Ta. The geochemical characteristics of ADMGs mimic those of N-MORB. Zircon U–Pb dating on the ADMGs reveals granulite facies metamorphic ages of ca. 595–602 Ma. However, a few zircons yield relatively older apparent 206Pb/238U ages of ca. 620 Ma, representing prograde metamorphic age. ADMGs preserve mean zircon εHf (t) values of +7.0 to +9.7 and δ18O values of 6.1–8.0‰. Based on petrographic observations, geothermobarometric calculations using conventional thermobarometry, mineral equilibria modeling, Ti-in-zircon and Zr-in-rutile thermometers reveal peak granulite facies P–T conditions of 0.95–1.2 GPa/940–1000°C, and retrograde amphibolite facies conditions of 0.83–0.93 GPa/575–710°C. The prograde metamorphic stage is inferred to be amphibolite or eclogite facies metamorphism. Hence, a clockwise a P–T–t path is proposed for the ADMGs. The overall results indicate that ADMGs are ultra-high temperature (UHT) granulites, and the protolith is altered oceanic crust consumed during the Pan-African collisional events. Asthenospheric upwelling induced by lithospheric delamination in the earliest extension of the thickened orogen, or by slab break-off in the background of collision may have provided the additional heat for UHT granulite facies metamorphism. [ABSTRACT FROM AUTHOR]

The volcanic rocks of the Buem Structural Unit in the Dahomeyide orogenic belt of southeastern Ghana, constitute a unique assemblage among the monocyclic sedimentary formations of this structural unit. Representative volcanic rock samples were collected from the Asukawkaw, Bowiri-Odumase and Nkonya areas which form a roughly north-south trend. The volcanic rocks comprise spherulitic, amygdaloidal, vesicular, phyric and aphyric varieties. Whole rock geochemistry shows that these volcanic rocks exhibit both alkaline and subalkaline characteristics. The alkaline varieties are relatively enriched in REE and incompatible trace element concentrations, similar to OIB; the subalkaline varieties show E-MORB and N-MORB REE and incompatible element characteristics. The rocks have low La/Nb (<1), low K/Nb (<450) and high Nb/U (averagely 47.3) values, suggesting no significant effect of crustal contamination. The key characteristics of these volcanic rocks are the distinct petrography and geochemistry, shown from the three separate localities, which may suggest source fractionation at different depths or modes of emplacement. The association of volcanic rocks of OIB, E-MORB and N-MORB affinities, with no significant crustal contamination, may suggest mantle derived magma that may have been related to rifting event and eventual emplacement at the eastern passive margin of the West African Craton. [ABSTRACT FROM AUTHOR]

Abstract: Nepheline syenite and carbonate-rich rocks known as the Kpong complex occur as deformed layers within the Pan-African Dahomeyide suture zone, in southeastern Ghana. The dominant phases in the nepheline syenite are nepheline and feldspar, whereas the carbonate-rich rock is dominated modally by Sr-rich calcite and annitic biotite. Biotite, calcite and nepheline have similar compositions in the nepheline syenite and the carbonate-rich rock. The rocks of the Kpong complex are enriched in alkalis, Ba, Sr, and Nb, have high LREE/HREE ratios, and show large depletions in Zr and Hf relative to primitive mantle. These features are characteristic of carbonatitic and related alkaline rocks worldwide. Whole-rock major elements in the Kpong complex with respect to SiO2 in bivariate plots show coherent variations with linear trends. However, based on the low Mg number, low Cr and Ni contents in whole rocks, and on overall high TiO2, FeO, rocks of the Kpong complex appear to have crystallized from an evolved parent, rather than primary mantle-derived magma. The ubiquitous disaggregated enclaves within the carbonate-rich horizons, the linear compositional arrays defined in binary variation diagrams, and overlaps in major modal mineral compositions all suggest magma assimilation of country rock was involved in the evolution of the Kpong complex. [Copyright &y& Elsevier]

The Suhum Basin granitoid complex is an important granitoid complex of the Birimian terrane of Ghana for unravelling the crustal growth and evolution of the West African Craton (WAC) during the Rhyacian Eburnean orogeny. Almost the entire Suhum Basin is occupied by an extensive granitoid complex, which contains useful information for constraining debatable plate tectonic issues, especially during the Archean-Paleoproterozoic transition period. We present petrography, whole-rock geochemistry, and mineral chemistry data of biotite, amphibole, and plagioclase to constrain the temperature-pressure conditions of emplacement, petrogenesis, tectonic setting, the evolution of the granitoids complex of the Suhum Basin, and its implications for the crustal growth and evolution of the WAC. Four lithological types; granite gneiss, migmatites, leucogranites, and mafic enclaves, characterise the granitoid complex of the Suhum Basin. Biotites from the granitoid complex have an annite-siderophyllite composition, and that, coupled with their calc-alkaline and I-type signatures, indicates crystallisation of the granitoid complex of the Suhum Basin under oxidised conditions. The medium-to high-K character of the rocks, together with the calc-alkaline nature, may be a reflection of the generation of magma in regions where the mantle wedge might have interacted with enriched fluids from the underlying plate during dehydration. The enrichment of LILE and LREE relative to HREE and HFSE and the negative Eu, Nb-Ta, and Ti anomalies of the granitoids complex may indicate derivation from enriched magma sources with varying degrees of fractionation in an arc environment. Amphibole-plagioclase thermobarometry indicates that the granitoid complex formed at P-T conditions of 600–712 °C and 5.2–7.2 kbar, signifying a deeper depth (19–27 km) of emplacement. The overall geochemical data suggest that the rocks formed during a single orogenic event related to a volcanic arc environment where subduction zone components played a role in the generation of their parental magmas. This finding is therefore consistent with the onset of "modern-style" subduction-related processes during the Archean-Paleoproterozoic transitional period. • The Suhum Basin granitoid complex formed in an arc environment. • Evidence of "modern-style" subduction-related processes in magma generation. • The complex is composed of granite gneiss, migmatites, leucogranites, and mafic enclaves. • The complex provides valuable insights into the crustal growth and evolution of the West African Craton. [ABSTRACT FROM AUTHOR]

The Buem Structural Unit (BSU) of the Pan-African Dahomeyide orogenic belt, southeastern Ghana, is characterized by the occurrence of clastic and chemical sedimentary rocks, volcanic rocks and mafic-ultramafic rocks. The mafic rocks, comprising mainly gabbros occur in smaller proportion within the BSU. Representative samples of the gabbros have been analyzed petrographically and geochemically to infer their mode of emplacement, petrogenesis and tectonic setting. Two distinct gabbro types have been identified from petrography and whole-rock major and trace elements geochemistry, namely the B1 and B2 gabbros. The gabbros are generally phaneritic, holocrystalline and composed of plagioclase and clinopyronexe which have been replaced either completely or partially by sericite, chlorite and epidote. On the whole, B1 gabbros are deformed, altered, alkaline in nature, show no significant effect of crustal contamination and has affinity to OIB/E-MORB. The B2 gabbros, are relatively less deformed, subalkaline, akin to N-MORB and show arc signatures with minimal crustal contamination. The geochemical characteristics of the B1 gabbros are similar to the Buem volcanic rocks. The similarity of the B1 gabbros to the volcanic rocks may suggest a similar source and tectonic setting. Thus, the B1 gabbros may be related to rifting and emplacement at the eastern margin of the West African Craton (WAC). The effect of minimal crustal contamination and the arc nature of the B2 gabbros may suggest subduction related magmatism. Taken together, the occurrence of the gabbros may be related to rifting and subduction-collision at the eastern margin of the WAC. [ABSTRACT FROM AUTHOR]

• This study presents first-time petrography, geochemistry, geochronology and Lu-Hf isotope compositions of the Dahomeyide Paleoproterozoic basement. • Previous studies suggested that the terrane represents part of the juvenile 2.1 Ga Birimian crust. • The results obtained from this study suggest that the terrane could not part of the juvenile 2.1Ga Birimian crust; rather it represents Neoarchean crust reworked during the Paleoproterozoic Eburnean Orogen. • This study provides first comprehensive data on evidence of reworked Archaean crust in the Dahomeyide Belt along the southeastern border of the West African Craton. • This information is crucial because it provides further evidence supporting older crustal contribution in the evolution of the Birimian crust which has been promoted as a sole juvenile crust formed in a remote environment without any Archaean crustal contribution. This study presents petrography, geochemistry, geochronology, and Lu-Hf isotope compositions of Paleoproterozoic orthogneisses from the Dahomeyide Belt, southeastern Ghana. Traditionally, it is indicated that these rocks represent part of the Birimian of the West African Craton, remobilized during the Pan-African Orogen. The results obtained from this study indicate that the Dahomeyide basement are mainly amphibole and biotite orthogneisses of medium-K, tonalitic composition, and biotite and muscovite bearing orthogneisses of high-K granodioritic composition. Both suites are mainly magnesian, calc-alkaline, metaluminous to weak peraluminous with A/CNK values of 0.86 – 1.03. They are rich in LREEs, LILEs and elements such as Ba, K, Pb, and Sr. They are moderately rich in V and Zn and show variable (La/Yb) N and Sr/Y ratios with geochemical characteristics akin to the Neoarchaean sanukitoids. However, they are depleted in Ni and Cr relative to the sanukitoids. Zircon U-Pb ages determined on two amphibole-biotite and four biotite-muscovite orthogneisses yield 2145 ± 12–2122 ± 27 Ma and 2105 ± 18–2125 ± 25 Ma respectively. The amphibole-biotite orthogneisses show ε Hf (~2.1 Ga) values of −9.5 to −3.7, pointing to mean model ages of (T DM1) of 2721 ± 24–2761 ± 14 Ma and T (DM2) of 3053 ± 26–3125 ± 25 Ma. Similarly, the biotite and muscovite orthogneisses present ε Hf (~2.1 Ga) values of −10.6 to −1.7, corresponding to mean T (DM1) of 2743 ± 14–2733 ± 20 Ma and mean T (DM2) of 2949 ± 26–3111 ± 36 Ma. The data suggest that the Dahomeyide basement rocks are derived mainly from partial melting of Neoarchaean lower crust and sediments with little or no mantle contribution. A collision between Birimian arc and an unknown Archaean crust (possibly, now the Dahomeyide Belt) accounted for the formation of these rocks. This study provides first data on the evidence of reworked Archaean crust in the Pan-African Dahomeyide Belt along the eastern border of the West African Craton. Our data serves as comprehensive evidence supporting older crustal contribution in the geodynamic evolution of the juvenile Eburnean/Birimian crust of the West African Craton. [ABSTRACT FROM AUTHOR]