Abstract
Recent geological mapping in Burundi defined several generations of granite intrusions in the Burundi Group (middle Proterozoic) according to their structural and geochemical characteristics. Early (Gr 1) and synkinematic (Gr 2) peraluminous granites are related to an extensional tectonic phase D 1 with horizontal deformation and consequent foliation which occurred from ∼ 1400 to ∼ 1260 Ma ago. Peraluminous Gr 3 granites (1185 Ma old) are syntectonic to a compressional phase D 2 which is characterized by upright folds in NNE and NNW directions. Subsequent alkalic Gr 4 granites (∼ 1100 Ma) are associated with transcurrent faults (D′ 2) of lithospheric scale. The Gr 1–3 granitoids display variable 87 Sr 86 Sr initial ratios (0.707–0.735) reflecting the effects of high-level contamination and, to a lesser extent, the composition of their source-region. From isotopic and rare-earth element (REE) data, a large-ion lithophile (LIL)-enriched source of possible Eburnean age, which produced silicic magmas mainly through plagioclase fractionation, is inferred for some granites; however, for most of them the contribution of supracrustal rocks was important. The Gr 4 granites have low Sr initial ratios, suggesting a mantle origin.
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