Airborne geophysics and geochemistry provide new insights in the intracontinental evolution of the Mesoproterozoic Kibaran belt (Central Africa)

Abstract

The Mesoproterozoic Kibaran belt in southwest Rwanda (Central Africa) consists of two contrasting metamorphic sequences. The first is essentially composed of weakly deformed, low-grade pelitic rocks with many quartzitic intercalations and some volcano-sedimentary sequences. The second consists of medium-to high-grade metamorphic metasediments and gneisses, intruded by sheared granitoids. Existing geological maps are of limited use in understanding the evolution of this part of the Kibaran belt. A combination of airborne γ-spectrometry data with Landsat TM imagery readily distinguishes known lithologies, and in particular detects two distinct granite types. Trace element data for one granite type does not correspond with known petrochemical trends of Kibaran granites, and may belong to a pre-Kibaran basement. The combining of these data with a recently published schematic geological map of the northeast Kibaran belt and re-interpreted field structural data, suggests a model for the Kibaran orogenic evolution in terms of extensional detachment tectonics and associated metamorphic core complexes.

Palaeoproterozoic to Neoproterozoic growth and evolution of the eastern Congo Craton: Its role in the Rodinia puzzle

“…Both sections of the Kibaran Belt contain at least two metasedimentary successions, the oldest of which was intruded by S-type granitoids in the north and by intermediate-felsic I-type plutons in the south both of which are dated between 1.38 and 1.37 Ga (Tack et al, 2002;Kokonyangi et al, 2004a). In the Kibaran Belt s.s. this magmatic phase has been linked to supra-subduction magmatism (Kokonyangi et al, 2004a(Kokonyangi et al, ,b, 2005(Kokonyangi et al, , 2006 while in the NE Kibaran Belt this magmatic/thermal event has been linked to extensional detachment/collapse based on the noted absence of a compressional phase in the structural record (Klerkx, 1987;Klerkx et al, 1993;Fernandez-Alonso and Theunissen, 1998). A second tectonic event affected both sedimentary successions, and is recorded in the Kibaran Belt s.s., with P-T conditions of 740-780 • C and 6.0-6.5 kbar (Kokonyangi et al, 2004a).…”

Late Neoproterozoic overprinting of the cassiterite and columbite-tantalite bearing pegmatites of the Gatumba area, Rwanda (Central Africa)

“…These belts have been interpreted as: a collisional orogeny (Kampunzu et al, 1986;Rumvegeri, 1991); as an intracratonic orogen with different periods of extension and compression (Klerkx et al, 1984(Klerkx et al, , 1987 and as an intracratonic extensional detachment structure, conditioned by transtensional strike-slip reactivation of NW-trending shear zones in the Palaeoproterozoic basement (Fernandez Alonso and Theunissen, 1998). These models implicitly consider that the Kibaran orogeny occurred in (Central) Africa in late Mesoproterozoic times (1.4-1.0 Ga) and had a protracted character with culmination from before 1370 Ma to 1310 Ma (Cahen et al, 1984).…”

The Mesoproterozoic Kibaride belt (Katanga, SE D.R. Congo)

“…This model assumes that the early Kibaran magmatic rocks were emplaced during plate convergence, possibly along an active continental margin resulting from convergence between the Congo craton (NW) and the Tanzania-Bangweulu craton (SE). The second model was proposed by Klerkx et al (1987) and supported by Fernandez-Alonso and Theunissen (1998). It supposes that the Kibaran belt is intracontinental and links the early Kibaran magmatic rocks to an extensional rift setting.…”

“…Furthermore, the structural data indicate that the early Kibaran granitoids were emplaced during the compressional (D 1 ) deformation event, which is marked by asymmetric folds and reverse faults/thrusts (Kampunzu et al, 1986;Rumvegeri, 1991;Kokonyangi et al, 2004Kokonyangi et al, , 2005. Thus, the structural evidence is in direct conflict with the interpretation of Klerkx et al (1987) and Fernandez-Alonso and Theunissen (1998). An intracontinental, rift setting for the Kibaran belt also implies that the Congo and Tanzania-Bangweulu cratons existed as a single continental block before the Kibaran orogeny.…”

“…In Rwanda and Burundi, the mafic and felsic rock associations have been interpreted as representing extension-related bimodal magmatism (e.g. Klerkx et al, 1987;Fernandez-Alonso and Theunissen, 1998). Nevertheless, it should be noted that felsic igneous rocks occurring in the so-called ''bimodal suites'' from extensional zones around the world are never SP felsic magmas, but rather metaluminous to peralkaline magmas.…”