Abstract
ABSTRACTThe Wa-Lawra Belt which is situated in the northern part of Ghana consists of Paleoproterozoic Birimian fine metasedimentary rocks metamorphosed to greenschist facies, particularly, in the western part. A whole-rock geochemical study of these metasedimentary rocks was undertaken to unravel their source area weathering, provenance and tectonic setting. Geochemical characteristics of the studied shales show that they are immature in nature and first cycle in origin, with little or no recycled component. Compared to Post-Archaean Australian Shales (PAAS), the studied shales indicate reduction in Zr, Hf, La, Nb, Th and Ta being the high field strength elements and evidences of transition metal enrichments in V, Ni, Sc, Co, and Cr. Major element geochemistry indicates that the shales were subjected to slight potassium metasomatism after deposition. Pre-metasomatized Chemical Index of Alteration calculations indicates that weak to moderate degree of chemical weathering took place at the sediment source area. Co-Th-La-Sc systematics reveals a combination of mafic and felsic provenances for the shales. Eu/Eu* together with values of Th/U and some abundances of trace elements show that the shales were mainly derived from juvenile rocks. Average REE model calculations suggest that the source materials are composed of about 49% basalt, 16% TTG and 35% granite.
Highlights
The West African craton (WAC) is divided into the Reguibat shield and Leo-man shield to the north and south respectively, comprising Archaean rocks of Liberian age (3.0–2.5 Ga) to the west and the Birimian Paleoproterozoic age to the east of the LeoMan shield, respectively (Figure 1)
Workers such as Abouchami et al (1990) and Boher, Abouchami, Michard, Albarède, and Arndt (1992) have suggested that within the Birimian, a relationship does exist between the tholeiitic magmatism and an oceanic plateau environment whereas several others have proposed that the entire Birimian crust grew in an island arc environment (Ama Salah, Liégeois, & Pouclet, 1996; Beziat et al, 2000; Sylvester & Attoh, 1992)
This paper presents the outcome of a bulk-rock geochemical study of Birimian metasediments of the Lawra belt, northwestern Ghana; the scanty geochemical studies of the metasedimentary rocks of the Birimian Supergroup in Ghana have been restricted to southwestern Ghana (Asiedu et al, 2017, 2004)
Summary
The West African craton (WAC) is divided into the Reguibat shield and Leo-man shield to the north and south respectively, comprising Archaean rocks of Liberian age (3.0–2.5 Ga) to the west and the Birimian Paleoproterozoic age to the east of the LeoMan shield, respectively (Figure 1). The processes involved in the growth of the Paleoproterozoic continental crust have always aroused debate amongst various researchers Workers such as Abouchami et al (1990) and Boher, Abouchami, Michard, Albarède, and Arndt (1992) have suggested that within the Birimian, a relationship does exist between the tholeiitic magmatism and an oceanic plateau environment whereas several others have proposed that the entire Birimian crust grew in an island arc environment (Ama Salah, Liégeois, & Pouclet, 1996; Beziat et al, 2000; Sylvester & Attoh, 1992). It has further been indicated that the Birimian of the Haute-Comoé was deposited in an intracontinental trans-tensional back-arc basin (Vidal & Alric, 1994)
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