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 Proterozoic supracrustal rocks are considered as part of the Buem Structural Unit (BSU) which is the most external tectonic unit of the Pan-African Dahomeyide fold belt. The geodynamic significance of the BSU is still debated: it could represent either a portion of the West-African passive margin during the rifting and the drifting of the Pan-African ocean (Affaton, 1990), or a continental rift-zone (Jones, 1990) or the Pan-African oceanic crust itself (Burke and Dewey, 1972 and Burke and Dewey, 1973; Shackleton, 1976). The mafic lavas derived from tholeiitic magmas, which originated by partial melting of the lithospheric mantle with a possible asthenospheric contribution. They are quite similar to MORBs and free of any crustal contamination. They are related to 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 grade metamorphism of the prehnite-pumpellyite facies developed under temperatures of 200 to 300 degrees C. Such a thermal imprint clearly marks an extensional tectonic stage. Later, the early parageneses were re-equilibrated under slightly higher pressures during the Pan-African collision. Both magmatic features and early metamorphic characters of these metavolcanic rocks are in good agreement with a context of lithospheric thinning characterized by mantle-derived magmatism and high thermal-gradient. These results fit with previous interpretations mainly based on sedimentary and structural features: the BSU represents a portion of a Proterozoic passive margin which suffered strong lithospheric stretching, with deep seated normal faults and mantle derived magmatism, and a subsequent severe subsidence evidenced by the prominent detrital sedimentation. Such a tectonic setting developed during the Pan-African oceanic drifting. (C) 1997 Elsevier Science B.V.