L. Tack et al., KIBARAN-A-TYPE GRANITOIDS AND MAFIC ROCKS GENERATED BY 2 MANTLE SOURCES IN A LATE OROGENIC SETTING (BURUNDI), Precambrian research, 68(3-4), 1994, pp. 323-356
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).