A Detailed Magmatic and Tectono-thermal Study of an Orogenic System in a Transitional Earth: The Eburnean Orogeny

The Sefwi Greenstone Belt of southwest Ghana offers a unique opportunity to study the enigmatic geodynamic settings responsible for the growth and reworking of the West African Craton during the Palaeoproterozoic. This multi-disciplinary study examines early magmatic accretion of the Sefwi Greenstone Belt, subsequent crustal thickening during craton maturation and stabilization and late-stage mid- to lower-crust exhumation. New whole-rock geochemical analysis of mafic and calc-alkaline lavas and tonalite-trondjhemite-granodiorite granitoids indicate mixed melt sources. These rocks yield ca. 2.19 to 2.13 Ga U-Pb zircons ages. Inherited cores in zircon grains indicate recycling of 2.23 Ga crustal material, with no evidence of Archean crustal input. Regional-scale field mapping and detailed metamorphic studies reveal an early crustal thickening ( 35km) event (D1) associated with low apparent geothermal gradients and N-S shortening, with late-D1 partial melting producing migmatitic paragneisses at the amphibolite-granulite facies transition. In-situ U-Pb SHRIMP analysis of monazite constrains the timing of peak temperature along clockwise P-T-t-D paths to ca. 2.07 Ga. This date is spatially and temporally linked to leucogranite emplacement along the north-western margin of the Sefwi belt. WSW-ENE D2 transtension is expressed as constrictional E-W folds, with coaxial L2 stretching lineations and sinistral displacement along major shear zones. D2 deformation resulted in differential exhumation of high grade, mid- to lower-crustal material along regional-scale NE-SW shear zones and NNE-striking crustal-scale detachments after 2.07 Ga. A tectonic mode switch to E-W shortening during D3 is indicated by dextral reactivation of regional shear zones associated with localized retrograde greenschist facies metamorphism. Building on recent work in NW Ghana, this study provides insights into the thermal regime of juvenile Palaeoproterozoic crustal lithosphere in the WAC. It illustrates the diversity of exhumation mechanisms active over >60 m.y. of tectonic accretion and deformation during the Eburnean Orogeny. Furthermore, it suggests greater complexity of transcurrent tectonism associated with changing crustal rheology during the Proterozoic.