Geochronology of the Birim Supergroup of the West African craton in the Wa-Bolé region of west-central Ghana: Implications for the stratigraphic framework

The Birim rocks of the West African craton comprise belts of greenschist- to amphibolite-grade gneiss and schist, and subparallel basins of greenschist-grade phyllite of volcaniclastic and epiclastic origin, which were intruded by igneous rocks. The granitoids intruded between 2213 and 2060. Ma and overlap with the volcaniclastic units dated between 2211 and 2064. Ma. The simultaneous occurrence of the magmatic events and irregular distribution of the rock ages hamper the formulation of a stratigraphic succession. SHRIMP spot analyses were done on older cores, crystals and rims from 23 rocks from the Bolé-Wa region in west-central Ghana. The crystallization ages range from 2195 to 2118. Ma, the inherited ages from 2876 to 2130. Ma, and metamorphic ages from 2114 to 2090. Ma. Aided by metamorphic, structural and chemical studies an older geotectonic cycle (2195-2150. Ma), containing the Dole and Guropie Suite and Bolé Group, was established. These units were subjected to several orthogonal and shear deformation events. These events were followed by the contemporaneous Sawla calc-alkaline monzonitic plutonism (2132-2126. Ma) and deposition of the epiclastic Maluwe Group (2137-2125. Ma) of calc-alkaline felsic to tholeiitic volcanic origin. Deformation of the basin beds was succeeded by the intrusion of the Tanina Suite granitoids of 2122-2120. Ma, which, themselves, were deformed prior to 2119. Ma. At 2118. Ma syenite and gabbro intruded along conjugate extension fractures. The gabbro and syenite of the Wakawaka Suite were only affected by three events of brittle strike-slip faulting. The first had significant displacement along NNE- to NE-directed shear zones, while the latter only formed conjugate joint systems with limited transport. Palaeo- to Neoarchaean cores, the oldest yet reported in the Baoulé Mossi domain, are restricted to the gneissic Dole Suite biotite granites. The presence of Dole-, Guropie-, Sawla-, and Tanina-aged older cores and grains in younger rocks reflects continuous reworking of the developing crust during successive magmatic episodes. Zircon rim growth between 2105 and 2090. Ma indicates posttectonic crustal thickening. The low Rb/Sr R_o of ∼0.7032 of gabbro and monzonite, and the recycling of the Birim-age crust confirm the primary and juvenile nature of the West African craton after ∼2195. Ma. With the various ages obtained, it was possible to link deposition, magmatism and deformation to crustal processes, and establish the cyclic geotectonic evolution in the West Africa craton (individual basin opening and closure) over time as part of an intraoceanic arc-back-arc basin system.