Tectonic significance of carbonatite and ultrahigh-pressure rocks in the Pan-African Dahomeyide suture zone, southeastern Ghana

Abstract

Abstract The association of carbonatite and ultrahigh-pressure (UHP) metamorphic rocks in the Dahomeyide suture zone of southeastern Ghana is unique among the Neoproterozoic orogens that surround the West African craton (WAC). Carbonatite occurs in an alkaline complex that decorates the sole thrust of the suture zone and is characterized by high concentrations of incompatible trace elements such as light rare earth elements (LREE), Sr and Ba. Within the suture zone deformed alkaline rocks, including carbonatite, together with mafic granulites form an imbricate stack of thrust panels that involve 2.1 Ga rocks of the WAC basement. The dominant rock unit of the suture zone is composed of mafic granulites in which garnet megacrysts preserve a diagnostic microstructure of UHP metamorphism; it consists of a crystallographically controlled array of exsolved rutile rods in garnet. Metamorphic pressures estimated from Ti concentrations in the inferred precursor garnet indicate P >3 GPa, which requires subduction (and exhumation) of the suture zone rocks to and from mantle depths during collisional orogeny on the WAC margin. Available age constraints on carbonatite magmatism suggest that continental rifting, leading to the formation of the passive WAC margin c . 700 Ma, occurred c . 100 Ma before intrusion of carbonatite, which was preceded by HP and UHP metamorphism at 610±5 Ma.