Browsing by Author "Anum, S."
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Item Geochemical and Sr‐Nd isotopic records of Paleoproterozoic metavolcanics and mafic intrusive rocks from the West African Craton: Evidence for petrogenesis and tectonic setting(Geological Journal, 2018-03) Sakyi, P.A.; Anum, S.; Su, B.X.; Nude, P.M.; Su, B.C.; Asiedu, D.K.; Nyame, F.; Kwayisi, D.Metavolcanics and mafic intrusive rocks of the Paleoproterozoic Birimian terrane in the southeastern part of the West African Craton, Ghana, were analyzed for major and trace elements and Nd and Sr isotopic data to constrain the geodynamic evolution of the Birimian Supergroup. The metavolcanic rocks consist of metabasalts, meta-andesites, and amphibolites, whereas the mafic intrusions are mainly gabbros, hornblendites, and dolerites. The rocks are tholeiitic in composition and show the classic features of arc magmatism. The metavolcanics display significant enrichments in large ion lithophile elements (LILE) and light rare earth elements, relative to high field strength elements (HFSE) and heavy rare earth elements. The multielement patterns of the rocks also show positive Pb, Ba, Th, and Sr and negative Nb, Ta, and Ce anomalies that are typical characteristics of subduction-related magmas. They also have La/Nb ratios <3 and La/Ta ratios <43 that are similar to other Archean and Birimian greenstone belts in West Africa. The rocks have εNd (2.1 Ga) values of -0.96 to +2.60, and Nd model ages of 2.24-2.51 Ga (TDM1) and 2.16-2.45 Ga (TDM2), indicating their juvenile character with possible contributions from pre-Birimian crustal materials in their sources. The εNd values suggest a depleted source and further indicate that they were probably produced in an almost entirely oceanic environment with minor influence from the continental crust. The Nd isotopic results are consistent with the island arc model, which views Paleoproterozoic terranes of the West African Craton in the context of subduction-accretion processes. Accordingly, these processes may have played a role in the formation of the Columbia supercontinent during the Paleoproterozoic (2.1-1.8 Ga) orogenic events.Item Geochemistry and geochronology of granitoids in the Kibi-Asamankese area of the Kibi-Winneba volcanic belt, southern Ghana(Journal of African Earth Sciences, 2015-02) Anum, S.; Sakyi, P.A.; Su, B.-X.; Nude, P.M.; Nyame, F.; Asiedu, D.; Kwayisi, D.In Ghana the West African Craton is represented by Birimian and Tarkwaian rocks with extensive granitoid bodies. Granitoids from Asamankese area of the Kibi-Winneba volcanic belt, southern Ghana were analysed for major and trace element contents and found to be characterised by highly-fractionated REE, enrichments, in LILE, and depletion in Nb, Ta and Sr. The LILE enrichment relative to strong Nb-Ta depression, indicates that these granitoids were emplaced in an active margin. Based on field relations, geochemical composition and geochronological data, the granitoids from the Kibi-Asamankese area can be divided into three types, namely; the Eburnean biotite granodiorite (2133-2127. Ma) and hornblende granodiorite (2147. Ma), and the Pre-Eburnean gneissic biotite granite (2193. Ma). The geochemical data of the studied rocks plot in the tholeiitic field, whereas on the A/CNK-A/NK diagram, they generally fall within the metaluminous field, with A/CNK values between 0.69 and 0.88. U-Pb dating of zircons in the granitoids yielded ages ranging from 2193 to 2127. Ma, which are among the oldest ages obtained from the granitoid plutons in Ghana. Such high-precision geochronological data indicate that magmatism occurred over a time-span of about 70. Ma. This provides further evidence that the period 2.1-2.2. Ga was one of the important stages of Birimian magmatism that led to the generation of the granitoids. From the above-mentioned ages, it is possible to link the geological activities to crustal processes and establish the cyclic geotectonic evolution in the West African Craton over time as part of an arc-back-arc basin system. © 2014 Elsevier Ltd.Item New zircon U–Pb ages for erratic emplacement of 2213–2130 MaPaleoproterozoic calc-alkaline I-type granitoid rocks in the LawraVolcanic Belt of Northwestern Ghana, West Africa(Precambrian Research, 2014) Sakyi, P.A.; Su, Ben-Xun; Anum, S.; Kwayisi, D.; Dampare, S.B.; Anani, C.Y.; Nude, P.M.tPaleoproterozoic Birimian granitoid rocks from the Lawra volcanic belt of northwestern Ghana havebeen analyzed for their major, trace, REE concentrations and their zircon U–Pb age. Based on the mineralassemblages, the calc-alkaline, metaluminous to weakly peraluminous I-type granitoids were classifiedas: (1) gneissic biotite granite, (2) hornblende granodiorite, (3) biotite granite, (4) two-mica granite and(5) pyroxene hornblende gneiss. The high LaN/YbN(∼4–146) values of the granitoids, characterized byenriched LREE pattern and flat to depleted HREE pattern ((Dy/Yb)N= 0.95–3.47), is typical of rocks derivedfrom an evolved magma or partial melting of the depleted mantle. Similarly, the slightly negative Sr- andEu-anomalies indicate varying degrees of plagioclase fractionation. Their high but varying SiO2contents(58.1–76.1 wt.%), K2O/Na2O ratios (0.17–1.2) and (FeO + Fe2O3)/MgO ratios (0.98–4.02), and low Ce andTi suggest that they are volcanic arc granites (VAG). The granitoids have zircon U–Pb ages between2130.8 ± 9.5 Ma and 2213 ± 76 Ma, and preclude the contamination of the juvenile Birimian crust bynotable amounts of reworked Archaean crustal material. However, these ages spanning ∼81 Ma wererecorded by all the rock types in the study area, with no distinct age difference among them. Accordingly,the area could be described as an undifferentiated terrain that experienced erratic emplacement of pulsesof granitic magma during the period. Furthermore, the oldest ages of 2211 Ma and 2213 Ma recorded inthis study suggest that the emplacement of Birimian granitoids in Ghana may have commenced muchearlier than previously reported in the literature. The results demonstrate the juvenile character of thegranitoid rocks. We therefore infer that the Birimian granitoids of the Lawra volcanic belt formed fromlarge-scale crustal growth process with large amounts of juvenile crust formed from the depleted mantlein an island arc environment.