Kibaran A-type granitoids and mafic rocks generated by two mantle sources in a late orogenic setting (Burundi)

Abstract

In the Mesoproterozoic Northeastern Kibaran Belt of Burundi (Central Africa) two distinct late Kibaran magmatic suites coexist, both including A-type granitoids. They are located along the Boundary Zone between the Kibaran mobile belt (Western Internal Domain) and the Archaean Tanzanian craton overlain by Mesoproterozoic foreland deposits (Eastern External Domain). Intense deformation, high-temperature metamorphism and intrusion of abundant peraluminous anatectic crustal granites occur only in the former domain whereas the Mesoproterozoic sedimentary cover of the latter is much less or even nearly undeformed nor metamorphosed. The first late Kibaran magmatism (350 km long Kabanga-Musongati with an emplacement age of 1275 _+ ~ Ma; U-Pb on zircon) is mainly composed of mafic and ultramafic layered rocks with subordinate A-type acidic differentiates moderately enriched in incompatible elements. Initial isotopic ratios (SrlR = 0.708; ~Nd = --8 ) indicate an old continental lithospheric mantle origin. The emplacement of these late Kibaran magmatic rocks was controlled by late lateral shear, possibly contemporaneous with the latest intrusions of the Kibaran peraluminous synkinematic granites of the Western Internal Domain ( ~ 1330-1260 Ma). The second late Kibaran magmatism (40 km long Gitega-Makebuko and Bukirasazi alignment with an emplacement age of 1249_+ s Ma; U-Pb on zircon) is limited in volume. It is mainly granitic in composition (A-type), can be strongly enriched in incompatible elements, and comprises both syenites and mafic rocks. Initial isotopic ratios (SrlR=0.702; ~Na= +4.5 to -1.4) point to an OIB-type asthenospheric/lower continental lithospheric mantle origin, with only slight contamination by the lower crust during differentiation. 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 granites are produced by differentiation of less evolved magmas and not by crustal anatexis. Upwelling of the asthenosphere along the Tanzanian craton can generate by adiabatic pressure release the OIBtype basic melts 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 compressive deformation between two rheologically contrasted domains. Ascent of the asthenosphere, continental lithospheric mantle delamination and late orogenic extensional collapse of the Western Internal Domain are suggested as a possible geodynamic model for the entire Northeastern Kibaran Belt. Additional work is however necessary to test this model. Finally, our results indicate that in the Northeastern Kibaran Belt the Kibaran orogeny ended at ~ 1250 Ma, despite various reactivation events occurring later (e.g. at ~ 1137 Ma).