Structural and metamorphic evolution of the Gabon Orogenic Belt: Collision tectonics in the lower proterozoic?

Abstract

Abstract The Archaean granite-gneiss terrain of central Gabon comprises medium- to high-grade gneisses metamorphosed at 3.15 Ga, forming the Monts Cristal and underlying the Mitzic region, intruded by and enclosed as enclaves in granitoids 3.0–2.6 Ga old that form the extensive Chaillu and North Gabon blocks. These rocks are unconformably overlain in the east of Gabon by Early Proterozoic Francevillian sedimentary rocks, between 2.3 and 2.0 Ga old, forming the Franceville and Booue Basins. The Francevillian is overthrust on its west side by the Ogooue Metamorphics, themselves considered to be of Early Proterozoic age. These rocks crop out in a broad synclinorium consisting essentially of paragneisses, with orthogneisses appearing locally at the base. The overall tectonic setting is discussed in terms of the evolution of the various domains from west to east. 1. (1) On the west flank of the synclinorium is a migmatitic-granulitic dome, the Abamie Dome, in which structures diverge away from the core. Metamorphism is of high-temperature, intermediate- to low-pressure type. 2. (2) On the east flank of the Ogooue synclinorium the medium-grade Ogooue Metamorphics (sillimanite-biotite, kyanite-staurolite, staurolite-muscovite-biotite) have been thrust eastwards on to the low-grade (kyanite-sericite) Francevillian. 3. (3) The Francevillian is parautochthonous, being widely detached from its Archaean basement owing to the effects of the same eastward-directed tangential tectonics. 4. (4) Locally, slices of Archaean basement have been tectonically incorporated. The structural cross-section illustrates a history of diapiric uprise passing eastward into tangential tectonics that is typical of classical collision belts. The suture marking the collision is interpreted to be located in the Abamie migmatite dome, where the nappes are rooted. The Gabon Orogenic Belt is correlated with the collision belts of about the same age on the eastern margin of the Congo craton and with the Trans-Amazonian Orogenic Belt of the Guyana Shield.