Late Kibaran magmatism in Burundi

Abstract The Late Kibaran magmatism in Burundi is represented by 2 magmatic associations. Small alkaline granitoid plutons constitute the first one. In Central Burundi, they form a N-S alignment of 40 km. A new RbSr 7 WR age of 1137 ± 39 M.a. (Ro = 0.7027 ± 0.0011; MSWD = 2.0), considered to date the emplacement of the granitoids, confirms a preliminary age of 1125 M.a. In Burundi and NW Tanzania, the second association is represented by an alignment of mafic and ultramafic intrusive bodies which form a prominent structure 350 km long and at least 25 km wide. The plutons have developed thermal aureoles. In Central Burundi, the granitoids, as well as the mafic and ultramafic rocks, occur on both sides of a major N-S trending Late Kibaran shear zone. This suggests a genetic relationship, not only between the shear and the magmatism, but also between both magmatic associations. Recent fieldwork confirms this hypothesis as both mafic rocks and granitoids have been observed in continuity within a single massif. More field observations and laboratory research, however, are needed before we can propose a coherent geodynamic model.