KIBARAN-A-TYPE GRANITOIDS AND MAFIC ROCKS GENERATED BY 2 MANTLE SOURCES IN A LATE OROGENIC SETTING (BURUNDI)

In the Mesoproterozoic Northeastern Kibaran Belt of Burundi (Central A frica) two distinct late Kibaran magmatic suites coexist, both includi ng A-type granitoids. They are located along the Boundary Zone between the Kibaran mobile belt (Western Internal Domain) and the Archaean Ta nzanian craton overlain by Mepoproterozoic foreland deposits (Eastern External Domain). Intense deformation, high-temperature metamorphism a nd intrusion of abundant peraluminous anatectic crustal granites occur only in the former domain whereas the Mesoproterozoic sedimentary cov er of the latter is much less or even nearly undeformed nor metamorpho sed. The first late Kibaran magmatism (350 km long Kabanga-Musongati a lignment with an emplacement age of 1275(+11/-10) Ma; U-Pb on zircon) is mainly composed of mafic and ultramafic layered rocks with subordin ate A-type acidic differentiates moderately enriched in incompatible e lements. Initial isotopic ratios (SrIR = 0.708; epsilon(Nd) = - 8) ind icate an old continental lithospheric mantle origin. The emplacement o f these late Kibaran magmatic rocks was controlled by late lateral she ar, possibly contemporaneous with the latest intrusions of the Kibaran peraluminous synkinematic granites of the Western Internal Domain (ap proximately 1330-1260 Ma). The second late Kibaran magmatism (40 km lo ng Gitega-Makebuko and Bukirasazi alignment with an emplacement age of 1249(+8/-7) Ma; U-Pb on zircon) is limited in volume. It is mainly gr anitic in composition (A-type), can be strongly enriched in incompatib le elements, and comprises both syenites and mafic rocks. Initial isot opic ratios (SrIR = 0.702; epsilon(Nd) = + 4.5 to - 1.4) point to an O IB-type asthenospheric/lower continental lithospheric mantle origin, w ith only slight contamination by the lower crust during differentiatio n. This group was also intruded during the late lateral shear. In both groups liquid lines of descent can be reconstructed, although some of the rocks have been strongly albitized. This indicates that the grani tes are produced by differentiation of less evolved magmas and not by crustal anatexis.Upwelling of the asthenosphere along the Tanzanian cr aton can generate by adiabatic pressure release the OIB-type basic mel ts and provide the heat necessary to melt the continental lithospheric mantle sources. This mechanism assigns a major role to a lithosphere- scale late Kibaran shear event occurring at the end of the regional co mpressive deformation between two rheologically contrasted domains. As cent of the asthenosphere, continental lithospheric mantle delaminatio n and late orogenic extensional collapse of the Western Internal Domai n are suggested as a possible geodynamic model for the entire Northeas tern Kibaran Belt. Additional work is however necessary to test this m odel. Finally, our results indicate that in the Northeastern Kibaran B elt the Kibaran orogeny ended at approximately 1250 Ma, despite variou s reactivation events occurring later (e.g. at approximately 1137 Ma).