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Stratigraphy and nomenclature
The Palaeoproterozoic West African Craton is characterised by narrow volcanic greenstone belts, wide meta-sedimentary “basins” and vast granite-gneiss terranes, bounded and separated by crustal-scale transcurrent shear zones that formed during the Eburnean Orogeny. Sporadic exposures of high met-amorphic grade gneissic to migmatitic rocks occur throughout the Baoulé-Mossi domain, juxtaposed with low-grade rocks. Questions that continue to arise include: What is the tectono-stratigraphic ar-chitecture of the craton? Can we correlate stratigraphic units and deformation events at a craton-scale? Was the Eburnean Orogeny monocyclic or polycyclic? What do the strain patterns and metamorphic record of the craton tell us about the tectonic style during the Eburnean?
Previous studies have employed geochemical, geochronological and structural investigations in order to determine the stratigraphic framework, the tectonic environment and the phases and styles of tectonism recorded within the craton. Persisting controversies and questions regarding the Palae-oproterozoic evolution of the craton are mainly due to the limited geochemical, metamorphic and geochronological data, and the integrations of such datasets at a regional or craton scale, as well as complex geo-political relationships. This has produced a variety of evolutionary models for the West African Craton (Abouchami, et al., 1990; Baratoux, et al., 2011; Block, et al., 2016b; Boher, et al., 1992; de Kock, et al., 2012; Egal, et al., 2002; Feybesse, et al., 2006; Feybesse & Milési, 1994; Hein, 2010; Ledru, et al., 1994; Liégeois, et al., 1991; Milesi, et al., 1989; Tshibubudze, et al., 2009; Vidal & Alric, 1994; Vidal, et al., 2009).
Some of the earliest controversies generated in literature regarding the West African Craton, arose from conflicting interpretation of the stratigraphy of the craton between French and English geolo-gists in the 1940 – 1970s. Early literature, predominantly proposed by Anglophone geologists, divided the Birimian Supergroup into a lower series of greywackes, phyllites, shales and minor lavas, and an upper series of mafic to intermediate lavas and subordinate volcaniclastic rocks, greywacke and phyllite (Junner, 1935; Junner, 1940; Kesse, 1985; Ledru, et al., 1989; Lemoine, et al., 1985; Milési, et al., 1986). Conversely, francophone geologists proposed a basal Birimian volcanic and volcano-sedi-mentary unit, overlain by shallow basins filled with flysche-type and molasse sedimentary sequences (Aubouin, 1961; Bessoles, 1977b; Tagini, 1971b). In addition, structural, geochemical and isotopic studies by Eisenlohr and Hirdes (1992) and Leube, et al. (1990) suggest that the Birimian metavol-canic rocks and metasedimentary formations are contemporaneous distal, lateral facies equivalents. Most recently, Adadey, et al. (2009) and Perrouty, et al. (2012) proposed a stratigraphy for the Birim-ian Supergroup in southern Ghana using a compilation of the growing geochronological data for the region. The stratigraphy comprises the basal Sefwi Group metavolcanics and mica schists (>2174 Ma), constrained by the syn-tectonic emplacement of the Sekondi granitoid (Oberthür, et al., 1998). The overlying Kumasi Group phyllites and volcanoclastic rocks were deposited between ca. 2154–2125 Ma, based on detrital zircon ages, syn-depositional volcanism and late-depositional granitoid intru-sion (Adadey, et al., 2009; Oberthür, et al., 1998; Perrouty, et al., 2012).
Whilst the advent of precise geochronology has largely settled the stratigraphic debate, there are con-trasting interpretations of deformation relationships. This includes suggestions of an early deforma-tion event recorded by the lower meta-volcanic units that is not recognised in upper meta-sedimen-tary units, which suggests a polycyclic evolution for the Eburnean Orogeny (de Kock, et al., 2012; Feybesse, et al., 1990; Ledru, et al., 1991; Perrouty, et al., 2012). This model suggests that there was an early orogenic event prior to the emplacement of the Birimian meta-sedimentary rocks. Authors sug-gest that high-grade terranes and some greenstone belts record elevated metamorphic conditions and polyphase deformation, interpreted as an older basement upon which meta-sedimentary units were deposited and subsequently deformed during the Eburnean Orogeny sensu stricto (s.s.) (e.g. Arnould, 1961; Lemoine, et al., 1990; Perrouty, et al., 2012). The age and terminology of the early deformation event differs throughout the Baoulé-Mossi Domain, including the “Burkinian” orogenic cycle in the Ivory Coast (2400 – 2150 Ma) (Lemoine, et al., 1990), the “Tangaean” event of Burkina Faso (2170 – 2130 Ma) (Hein, 2010; Tshibubudze, et al., 2009), or, more recently, “Eburnean I” (2266 – 2150 Ma) (Allibone, et al., 2002) or “Eoeburnean” event in Ghana (2195 – 2150 Ma) (Baratoux, et al., 2011; de Kock, et al., 2011; de Kock, et al., 2012; Perrouty, et al., 2012). Early deformation often corresponds with an initial phase of magmatic accretion and crustal thickening (e.g. Feybesse, et al., 2006; Milési, et al., 1992; Vidal, et al., 2009). Each of these events are interpreted to precede the tectonic assembly of the Paleoproterozoic and Archaean domains and transcurrent tectonism and greenschist facies meta-morphism attributed to the Eburnean Orogeny s.s. (2130 – 1980 Ma) (Feybesse, et al., 2006; Jessell, et al., 2012; Ledru, et al., 1991; Lompo, 2010; Oberthür, et al., 1998; Pitra, et al., 2010).
Monocyclic interpretations for the Eburnean Orogeny deem volcano-plutonic belts and metasedi-mentary basins to be contemporaneous, lateral facies equivalents or representative of crustal segments of varying depths. This interpretation requires greenschist facies metasedimentary provinces to be coeval, supracrustal equivalents to high grade metamorphic terranes, which were juxtaposed during later deformation (Block, et al., 2015; Block, et al., 2016b; Eisenlohr & Hirdes, 1992; Hirdes, et al., 1996; Hirdes, et al., 2007; Leube, et al., 1990; Opare-Addo, et al., 1993). These models suggest that deformation and metamorphism across the craton reflect a long-lived, progressive orogenic event (Block, et al., 2016b; Eisenlohr & Hirdes, 1992; Hirdes, et al., 2003). Given the variety of interpreta-tions and discrepancies between the terranes, additional geochronologically-constrained structural and metamorphic studies are required in order to elucidate the spatial and temporal variations of deformation events across the craton.
Metamorphic record and tectonic style of the West African Craton
Metamorphism of the meta-volcanic and meta-sedimentary rocks within greenstone belts reveal pre-dominantly low-grade metamorphism from greenschist to amphibolite facies (Galipp, et al., 2003; John, et al., 1999; Kříbek, et al., 2008). In contrast, metamorphism and migmatization of orthogneiss-es and paragneisses in high-grade terranes is associated with upper amphibolite to granulite facies conditions (Bessoles, 1977a; Block, et al., 2015; de Kock, et al., 2011; John, et al., 1999; Opare-Addo, et al., 1993), with high-pressure granulites predominantly restricted to the tectonic contact between the Archaean Kénéma-Man and Palaeoproterozoic Baoulé-Mossi domains (Pitra, et al., 2010). Whilst geochronological data is relatively limited, the timing of Eburnean metamorphism is constrained by U-Pb ages of metamorphic monazite, zircon and titanite in between 2110 and 2080 Ma (e.g. de Kock, et al., 2011; Oberthür, et al., 1998), with older paroxysmal metamorphism at ca. 2130 Ma proposed by Block, et al. (2015; 2016b) based on metamorphic monazite and zircon ages. Sm-Nd garnet-whole rock isochron ages from central Ivory Coast are older still at ca. 2150 Ma (Boher, et al., 1992).
Analyses of strain patterns and metamorphic relationships have drawn a range of interpretations on the tectonic styles and geodynamic mechanisms responsible for the tectonic evolution of the West African Craton. Some authors interpret dome and basin geometries within the craton, which they suggest was associated with Archaean-like diapirism-related vertical tectonics, with late horizontal shortening purely accommodated by transcurrent shear systems (e.g. Lompo, 2010; Pouclet, et al., 2006; Pouclet, et al., 1996; Vidal, et al., 1996). In such studies, amphibolite facies metamorphism is at-tributed to contact metamorphism during granitoid emplacement (Debat, et al., 2003; Gasquet, et al., 2003; Pons, et al., 1995; Soumaila & Garba, 2006). In the case of the Sefwi Greenstone Belt, large tracts of regional amphibolite facies gneisses are peculiarly attributed to an underlying, unexposed K-rich pluton (Hirdes, et al., 1993). Alternative proposals for orogenic styles in West Africa include “mod-ern-type” thrust-related tectonics and nappe stacking (Feybesse, et al., 2006; Feybesse & Milési, 1994; Milési, et al., 1992). Indeed, the cold (~15°C/km) apparent geothermal gradients deduced for rocks metamorphosed at the greenschist-blueschist facies transition are interpreted as evidence of mod-ern-style subduction during the formation and assembly of the craton (Ganne, et al., 2012). Preserva-tion of cold geothermal gradients is inconsistent with many studies which interpret deformation and metamorphism of the West African Craton as a weak, hot orogen (Caby, et al., 2000; Lompo, 2009; Pons, et al., 1995; Vidal, et al., 2009). The weak, hot orogen model consists of extensive magmatism, often diapirically emplaced, with subsequent lateral shortening associated with homogeneous thick-ening and late-orogenic development of large-scale shear zones. The high-pressure granulite facies metamorphism at the Archaean-Palaeoproterozoic boundary along the Sassandra Shear Zone in west-ern Ivory Coast may be elucidated by either crustal thickening by unit stacking in a collisional setting or homogeneous thickening of a warm, weak crust (Pitra, et al., 2010). In NW Ghana, late-orogenic exhumation of amphibolite-granulite facies rocks occurs during orogenic collapse following crustal thickening, with the formation of anatectic domes resembling modern collisional orogens (Block, et al., 2016b). Given the diversity of tectonic models, deformation styles and distribution of metamor-phic terranes proposed for the West African Craton, it is possible that the craton preserved spatial and temporal variations in the crust. Elucidation of such variations requires continued integration of structural, metamorphic and geochronological studies.
Geodynamic setting and crustal formation
Based on εNd(t) and εHf(t) data, the Palaeoproterozoic crust of the West African Craton displays a dom-inantly juvenile character (Abouchami, et al., 1990; Block, et al., 2016a; Boher, et al., 1992; Taylor, et al., 1992), with short crustal residence times. Recent studies, however, show a greater Archaean influence in southern Mali and far southeast Ghana (Parra-Avila, et al., 2016; Petersson, et al., 2017; Petersson, et al., 2016). The magmatic processes and geodynamic setting of juvenile crust formation are attributed to a number of scenarios including: an oceanic plateau setting (Abouchami, et al., 1990; Boher, et al., 1992); the accretion of multiple volcanic island arcs (Baratoux, et al., 2011; Dampare, et al., 2008; Pawlig, et al., 2006; Senyah, et al., 2016; Sylvester & Attoh, 1992); the development of a volcanic island arc system within an oceanic plateau setting (Boher, et al., 1992); or intra-cratonic rift-ing leading to the opening of oceanic domains that were subsequently inverted (Leube, et al., 1990). Alternatively, Begg, et al. (2009) suggest that the West African Craton represents the amalgamation of Archaean-aged lower-lithospheric domains.
Geochemical studies reveal the bimodal nature of early volcanism across the craton, with a basal se-quence of tholeiitic basalts overlain by calc-alkaline volcanic rocks noted in a number of greenstone belts across the domain (Abouchami, et al., 1990; Baratoux, et al., 2011; Dampare, et al., 2008; Hirdes, et al., 1996; Pouclet, et al., 2006; Senyah, et al., 2016). Lompo (2009) interpret tholeiitic volcanism as a mantle plume event, similar to the differentiated oceanic flood basalt analogy proposed by Abouch-ami, et al. (1990). Intrusive suites in the southern portion of the West African Craton are frequently described as tonalite-trondhjemite-granodiorite (TTG) series (Doumbia, et al., 1998; Vidal, et al., 2009), based on normative feldspar discrimination plots after Barker (1979). These intrusions have historically been divided into three main groups, as defined by the primary ferro-magnesian mineral and the host domain: 1) variably foliated, amphibole-bearing granitoids hosted in volcanic belts; 2) biotite-bearing granites hosted in meta-sedimentary “basins;” and, 3) potassic, alkaline biotite-mus-covite granites and muscovite leucogranites (Baratoux, et al., 2011; Block, et al., 2016a; Egal, et al., 2002; Gueye, et al., 2008; Hirdes, et al., 1992; Leube, et al., 1990; Lompo, 2009; Oberthür, et al., 1998; Tapsoba, et al., 2013; Vegas, et al., 2008). Extensive geochemical characterisation of these intrusions reveal calc-alkaline affinities, negative Nb-Ta, Ti and P anomalies coupled with positive Pb anomalies, with diverse Eu and Sr anomalies and REE fractionation patterns (Baratoux, et al., 2011; Block, et al., 2016a; Egal, et al., 2002; Eglinger, et al., 2017; Feybesse, et al., 2006; Gueye, et al., 2008; Hein, 2010; Leube, et al., 1990; Petersson, et al., 2016; Tapsoba, et al., 2013; Vegas, et al., 2008), bearing striking similarities to classical arc geochemical signatures (Arculus, et al., 1999). TTG magmas and associ-ated lavas are interpreted as the production of partial melting of the down-going slab in a subduction setting (Baratoux, et al., 2011; Peucat, et al., 2005; Pouclet, et al., 2006; Sylvester & Attoh, 1992), with high-pressure, low-HREE TTGs and LILE-enriched diorites in NW Ghana interpreted as the product of delamination of the lower crust and lithospheric mantle (Block, et al., 2016a).
Whilst these lines of evidence suggest that crustal accretion occurred primarily in a subduction set-ting, a number of characteristics of the craton do not conform with those of modern accretionary orogens. The major site of juvenile crustal growth on the modern Earth is associated with extensive magmatism in magmatic arcs forming along convergent plate margins (Arculus, 1994). Lateral accre-tion of juvenile terranes or mobile belts against older craton margins results in lateral crystallisation age gradients and isotopic boundaries (Gastil, 1960; Hoffman, 1988; Zeh, et al., 2009). In the West African Craton, significant volumes of juvenile crust form in an oceanic domain, distal to Archaean crustal influences. Furthermore, there are no clear age gradients preserved in sub-parallel greenstone belts emplaced between 2300 and 2180 Ma. The youngest juvenile crust is located in Kedougou-Kénié-ba Inlier of Senegal, suggesting a westward gradient (Hirdes & Davis, 2002). Hirdes and Davis (2002) propose a major suture in central Ivory Coast, joining an older eastern sub-province with a 50- 100 M.y. younger western sub-province. A ca. 2130 Ma suture between two crustal blocks is hypothesised by Parra-Avila, et al. (2017) for a similar location, based on a comprehensive geochronological tran-sect across Burkina Faso, northern Ghana, southern Mali and eastern Guinea; however, the studies are not in exact agreement, making the extent of the suture somewhat ambiguous. Finally, the meta-morphic record and litho-structural architecture do not preserve the same asymmetries, exotic crustal fragments, accretionary prism and metamorphic conditions as modern accretionary systems, thus convoluting the application of a subduction collision model for the West African Craton.
The greenstone-granite-gneiss terranes of the Palaeoproterozoic West African Craton bear a number of similarities to Archaean provinces. The lithological assemblage, craton-scale shear zones and dom-inant low- to medium-grade metamorphism are explained by a variety of geodynamic and tectonic models, reminiscent of both Archean and Phanerozoic processes. As such a number of questions arise when attempting to elucidate the geological evolution of the region. These include: What was the geo-dynamic setting in which the juvenile crust was generated and does it conform to the modern plate tectonic regime? What were the orogenic processes responsible for the amalgamation and stabilisation of such large areas of crust? What are the implications of these finding in understanding the evolution of geodynamic processes and lithospheric properties in the early Earth?
Table of contents :
1.1 General Introduction
1.2 West African Craton Geology
1.2.2 Geological Setting
2 Research aims and rationale
2.1 Research aims
2.2 Thesis outline
2.2.1 Chapter 3: Transtension-related lower crust exhumation in the late stages of the Palaeoproterozoic Eburnean Orogeny, SW Ghana: Evidence for diachronous assembly of the São Luís-West African Craton
2.2.2 Chapter 4: Petrological and geochronological evidence of collisional orogenesis and lower crust exhumation during the Palaeoproterozoic Eburnean Orogeny, SW Ghana, West African Craton
2.2.3 Chapter 5: Palaeoproterozoic juvenile crust formation in southern Ghana, West Africa: New insights from igneous geochemistry and U-Pb-Hf zircon data
2.2.4 Chapter 6: Discussion and conclusions
3 The geology and tectonic evolution of southwest Ghana
Chapter 3: Transtension-related lower crust exhumation in the late stages of the Palaeoproterozoic Eburnean Orogeny, SW Ghana: Evidence for diachronous assembly of the São Luís-West African Craton
3.2 Geological Setting
3.3. Study area
3.3.1 Litho-structural domains
3.3.2. Major Shear zones
3.4 Methodology and data
3.4.2 Processing and interpretation of geophysical data
3.5 Lithological associations and geological map
3.5.2 Geophysical response of tectono-metamorphic domains
3.6 Tectono-metamorphic history
3.6.1 Deformation sequence
3.6.2 Metamorphic history
3.6.3 Structural-metamorphic map
3.7.1 Timing of Eburnean deformation in the Sefwi Belt
3.7.2 Rheology, tectonic style and exhumation
3.7.3 Orogenic model, regional correlations and Transamazonian ties
3.7.4 Implications for Paleoproterozoic geodynamics
4 The metamorphic evolution of SW Ghana
Chapter 4: Petrological and geochronological evidence of collisional orogenesis and lower crust exhumation during the Palaeoproterozoic Eburnean Orogeny, SW Ghana, WAC
4.2 Geological Setting
4.2.1 West African Craton geology
4.2.2 Geology of southwest Ghana
4.3.1 Petrographic analysis and mineral chemistry
4.3.2 P-T calculations
4.4 Petrography and mineral chemistry
4.4.1 High-grade rocks
4.4.2 H2O-saturated melting
4.5.1 P-T conditions and P-T paths
4.6.1 Metamorphic evolution of the study area
4.6.2 Regional context
4.6.3 Burial and exhumation
4.6.4 Implications for Palaeoproterozoic tectonics
5 Crustal evolution of the West African Craton
5. 0 Introduction
Chapter 5: Palaeoproterozoic juvenile crust formation in southern Ghana, West
Africa: New insights from igneous geochemistry and U-Pb-Hf zircon data
5.2 Palaeoproterozoic geology of West Africa
5.3 Main lithologies and stratigraphy
5.4 Sampling and analytical methods
5.4.1 Whole rock geochemistry
5.4.2 Zircon U-Pb dating
5.4.3 In-situ Lu–Hf analyses
5.5.1 Lithological and petrological descriptions
5.5.2 Whole rock geochemistry
5.6.1 Petrogenesis of magmatic suites
5.6.2 Timing of juvenile crust formation, recycling and basin deposition
5.6.3 Implications for the crustal architecture of the West African Craton
5.6.4 A geodynamic model for the southern West African Craton
6 Discussion and conclusion
6.1.2 Evolution of the West African Craton
6.1.3 Implications for Palaeoproterozoic geodynamics and orogenesis
6.1.4 Recommendations for future research