THERMAL AND GEODYNAMIC SETTING OF THE BUEM VOLCANIC-ROCKS NEAR TIELE,NORTHWEST BENIN, WEST-AFRICA

The mafic volcanic rocks of Tiele occur as intercalations within very low-grade metasediments, mainly metapelites and meta-arenites. These P roterozoic supracrustal rocks are considered as part of the Buem Struc tural Unit (BSU) which is the most external tectonic unit of the Pan-A frican Dahomeyide fold belt. The geodynamic significance of the BSU is still debated: it could represent either a portion of the West-Africa n passive margin during the rifting and the drifting of the Pan-Africa n ocean (Affaton, 1990), or a continental rift-zone (Jones, 1990) or t he Pan-African oceanic crust itself (Burke and Dewey, 1972 and Burke a nd Dewey, 1973; Shackleton, 1976). The mafic lavas derived from tholei itic magmas, which originated by partial melting of the lithospheric m antle with a possible asthenospheric contribution. They are quite simi lar to MORBs and free of any crustal contamination. They are related t o a strongly attenuated continental lithosphere, and a passive margin would be the most likely tectonic environment for the coeval volcanic and sedimentary activities. On the other hand, these rocks demonstrate effects of early static recrystallizations related to a very low grad e metamorphism of the prehnite-pumpellyite facies developed under temp eratures of 200 to 300 degrees C. Such a thermal imprint clearly marks an extensional tectonic stage. Later, the early parageneses were re-e quilibrated under slightly higher pressures during the Pan-African col lision. Both magmatic features and early metamorphic characters of the se metavolcanic rocks are in good agreement with a context of lithosph eric thinning characterized by mantle-derived magmatism and high therm al-gradient. These results fit with previous interpretations mainly ba sed on sedimentary and structural features: the BSU represents a porti on of a Proterozoic passive margin which suffered strong lithospheric stretching, with deep seated normal faults and mantle derived magmatis m, and a subsequent severe subsidence evidenced by the prominent detri tal sedimentation. Such a tectonic setting developed during the Pan-Af rican oceanic drifting. (C) 1997 Elsevier Science B.V.