Timing the structural events in the Palaeoproterozoic Bole-Nangodi belt terrane and adjacent Maluwe basin, West African craton, in central-west Ghana
Résumé
The Maluwe basin, north-adjacent to the Sunyani basin, is the northernmost of the northeast-trending Eburnean volcaniclastic depositories in Ghana. These basins are separated from one another by remnants of Eoeburnean crust, all formed during the evolution of an arc-backarc basins complex in a Palaeoproterozoic intraoceanic environment. The Bole-Nangodi belt terrane to the northwest, of mostly Eoeburnean crust is fault bounded with the Maluwe basin along the northeast-trending Bole-Navrongo fault zone. The stratigraphic sequence, which was the key to unravelling the structural evolution of the study area, was established by means of field observations aided by precision SHRIMP geochronology. The quartzitic, pelitic, quartzofeldspathic and granitic gneisses of the Eoeburnean crust (>2150 Ma) experienced complex metamorphic mineral growth and migmatitization, mostly under static crustal conditions and were subjected to several deformation episodes. The foliated mafic and metasedimentary enclaves within the lfanteyire granite establish deformation to have taken place prior to similar to 2195 Ma, while the tectonically emplaced Kuri amphibolites within the 2187-Ma gneissic Gondo granite indicate a stage of rifting followed by collision. Deformation of granite dykes in the Condo granites at similar to 2150 Ma concluded the development of the Eoeburnean orogenic cycle (D-EE). The Sawla Suite, contemporaneous with the deposition of the Maluwe Group, intruded the tectonic exhumed Bole-Nangodi terrane during extension between similar to 2137 and 2125 Ma. The rifting separated the Abulembire fragment from the Bole-Nangodi terrane. During subsequent northwestward subduction of young back-arc basin oceanic crust the volcaniclastic strata of the Maluwe Group and Sawla granitoids were deformed (D-E1) under chlorite/sericite greenschist-grade conditions. The NE-trending folds had subhorizontal axes and subvertical axial planes. Simultaneous to the D-E1 orogenesis the molasses of the Banda Group was deposited concordantly on the submerged Sunyani strata after a hiatus of 20 million years. After cessation of the NW-SE-directed compression the early Tanina Suite intruded as batholiths, dykes and sheets and produced garnet, staurolite, sillimanite and kyanite in their thermal aureoles. Docking of the Sunyani basin produced the D-E2 thrust related folding and stacking in the deformed and granitoid invaded Maluwe basin as well as the single stage sin- and anticlinoria in the Sunyani and Banda Groups. In the Maluwe basin the Abulembire fragment acted as a resistor and the approaching front rotated anticlockwise and clockwise around the barrier to form west- and north-directed piggy-back thrust-stacking and deformation of the Tanina Suite granitoids. Due to the low metamorphic conditions the D-E2 fabric is limited to crenulation cleavages in the more psammitic and pelitic units. The fold axes are double plunging (N-S and E-W) up to 60 degrees with the axial planar fabric subvertical. Post-D2 tectonic relaxation has allowed the emplacement of the last Tanina Suite calc-alkaline melts and was succeeded by N-S extension fracturing (DE3) along which mantle derived Wakawaka gabbroids and syenite intruded. The D-E1 folding occurred between 2125 and 2122 Ma and D-E2 before 2119 Ma. The tectonic relaxation occurred at 2118 Ma. Around 2100 Ma, NE-SW directed strike-slip shearing (D-E4), fractured the Bole-Nangodi terrane and enhanced the basin-belt boundary. Along the boundary, the displacement was dextral along vertical faults but, southward, it became more east-over-west thrust related. Associated tension gashes are filled with vein quartz and pegmatite and typical of the brittle sector of the crust. Tectonism in this part of the intraoceanic accretionary arc back-arc complex was concluded by limited, right-lateral strike-slip (D-E5) movement which formed some breccias.