Structural and Petrological Evolution of the Shai Hills Tectonic Suture Zone, Southeastern Ghana.

ABSTRACT

High Pressure (HP) mafic granulites (mafic garnet gneisses) in the Shai Hills suture zone occur as NE/SW trending isolated inselbergs in the Accra plains. They characterize the suture zone rocks of the Pan-African orogeny in Ghana. Various aspects of the Shai Hills suture rocks have been studied yet relatively little is known about its structural and deformational evolution. This work presents a new research into the structural, petrological and deformational evolution highlighting on metamorphism, petrogenesis and tectonic setting of the unique suture zone in Ghana by integrating field work, petrographic studies and whole rock major, minor and trace element (including REE) geochemical data. The high pressure granulites have undergone prograde metamorphism during subduction and followed by later retrograde metamorphism as a result of later exhumation. They record at least four distinct metamorphic episodes of which early prograde mineral assemblages are represented by mineral inclusions of hornblende + ilmenite within some porphyroblastic garnets and clinopyroxenes. The peak assemblage is characterized by granulite metamorphic assemblages of porphyroblastic garnet + clinopyroxene + quartz + plagioclase. The peak granulite stage was followed by three successive retrogressive facies stages during exhumation. The hornblende-granulite stage followed peak granulite stage and is characterized by the introduction of dark red hornblende and corona texture of clinopyroxene around garnet. This stage was then followed by amphibolite stage with the introduction of green hornblende rims of some clinopyroxene and garnet. The introduction of chlorite and epidote defines the greenschist metamorphic assemblage stage in the rock. The terrane has undergone three main deformational stages: D1, D2 and D3. The D1 and D2 are characterized by ductile deformation while the D3 is characterized by brittle deformation. Shear sense indicators in the terrane indicate an early dextral shearing and later sinistral shearing. Geochemical data suggest that, the high pressure granulitic rocks may have preserved the geochemical imprints of their magmatic basaltic protoliths where the rocks are mainly characterized by Island Arc Tholeiite (IAT) imprints with few with ocean crust (N-MORB) imprints. The HP granulite displays trace elements patterns similar to that of the mafic granulite lower crust (LC) composition. They show depletions in HREEs and have enrichment of LREEs. They are depleted in Cs, Rb and Th with enrichment in Ba and Sr. They also display averagely positive Eu anomaly which is similar to the lower crust (LC). The combination of petrography, mineral compositions, micro and macro structures and geochemical data suggest that the rocks may have undergone initial crustal thickening during subduction and collision related tectonic processes accompanied by prograde metamophism followed by exhumation, cooling and retrogression.