L. Tack

Early Neoproterozoic magmatism (1000–910 Ma) of the Zadinian and Mayumbian Groups (Bas-Congo): onset of Rodinia rifting at the western edge of the Congo craton

Abstract New ion microprobe U–Pb zircon ages, as well as some geochemical and isotopic analyses, for key igneous units within the central part of the West Congo belt are integrated with geological information to provide an updated geological map (1:1 000 000 scale) and a synthetic type cross-section of the belt, as well as an updated lithostratigraphic chart of the ‘West Congo Supergroup’. Three Neoproterozoic units are recognised, from oldest to youngest, the Zadinian, Mayumbian and West Congolian ‘Groups’. Emplacement of early Zadinian peralkaline granites (Noqui massif, 999±7 Ma) and rhyolites (Palabala) was accompanied by incipient rift sedimentation, corresponding to the onset of transtensional rifting, preferentially in a transverse mega-shear setting along the margin of the Congo craton. Subsequent upper Zadinian magmatism produced a thick (1600–2400 m) basaltic sequence (Gangila), which has geochemical characteristics typical of continental flood basalts (CFBs). The Gangila basalts, associated with major pull-apart rifting, were followed rapidly by the 3000–4000 m thick Mayumbian rhyolitic lavas, dated at 920±8 Ma at the base and 912±7 Ma at the top. The felsic lavas are intruded by coeval high-level (micro)granites, whose emplacement is dated at 924±25 Ma (Mativa body) and at 917±14 Ma (Bata Kimenga body) in the Lufu massif. This voluminous bimodal magmatic province is similar to the Parana and Deccan provinces, and shares similar lithospheric sources. It corresponds to the initial, transtensional rifting stage along the western edge of the Congo craton before Rodinia breakup. The early Neoproterozoic rocks of the West Congo Supergroup rest unconformably on a ca. 2.1 Ga Palaeoproterozoic polycyclic basement (Kimezian Supergroup). No Mesoproterozoic events are recorded in the area. Following the initial, transtensional early Neoproterozoic (ca. 1000–910 Ma) rifting stage, Bas-Congo behaved as a passive margin of the Congo craton, as indicated by deposition of ca. 4000 m of Neoproterozoic (pre-Pan-African) platform sediments (lower part of West Congolian Group) preceding ca. 2000 m of Pan-African molasse-type sediments (upper part of West Congolian Group). In the late Neoproterozoic, during Pan-African assembly of Gondwanaland, the Bas-Congo passive margin, which was largely protected by thick lithosphere of the Congo craton, collided with a western active margin to form the Brasiliano-Aracuai belt, now preserved adjacent to the Sao Francisco craton of Brazil. This collision, which ended in Bas-Congo at ca. 566 Ma, induced relatively limited effects in the West Congo belt, which experienced no late Neoproterozoic magmatic activity.

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

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).

The Malagarazi supergroup of southeast Burundi and its correlative Bukoba supergroup of northwest Tanzania: neo- and mesoproterozoic chronostratigraphic constraints from Ar-Ar ages on mafic intrusive rocks

Abstract Neo- and Mesoproterozoic subtabular sedimentary units with associated mafic volcanic and shallow depth intrusive rocks are exposed in southeastern Burundi and northwestern Tanzania along the border of the Tanzania Craton. This paper presents the first attempts to date these mafic intrusive rocks by the Ar-Ar technique. The 40Ar-39Ar step-wise heating results obtained on mineral separates of five (micro)gabbros provide temporal constraints for the two episodes of magmatism previously considered as Neo- and Mesoproterozoic on the basis of lithostratigraphic arguments. The Ar-Ar results reveal the presence of excess Ar in samples formerly attributed to the Neoproterozoic magmatic episode (intrusive mafic rocks within the Musindozi and Kavumwe Groups in Burundi and their correlatives, the ‘Kabuye-Gagwe amygdaloidal lavas’ exposed in Burundi and Tanzania) and yield a better lower age estimate of 795 ± 7 Ma compared to the formerly reported K-Ar ages. Two distinct Mesoproterozoic magmatic episodes characterised by the emplacement of shallow depth intrusive mafic rocks have been recognised. One yields a range of comparable cooling ages of 1340 ± 9 Ma in the Kavumwe Group (Burundi) and of 1379 ± 10 Ma and 1355 ± 10 Ma in the Bukoba Sandstone Group (Tanzania). The second is recorded in the Nkoma Group (Burundi) with the emplacement of a dyke at 1282 ± 5 Ma. The Mesoproterozoic ages correspond to minimum ages for the deposition of the sedimentary country rocks belonging to these groups and elucidate former uncertainties regarding their chronostratigraphic position. These data confirm a previously proposed Kibaran foreland setting for the Nkoma, Kavumwe and Bukoba Sandstone Groups and establish that the extent of the Neoproterozoic Malagarazi (Burundi) and Bukoba (Tanzania) Supergroups is restricted to a more confined area than previously admitted.

Post-collisional granite magmatism in the central Damaran (Pan-African) Orogenic Belt, western Namibia

The central Damaran (Pan-African) Orogenic Belt in western Namibia is dominated by elongated granite-gneiss domes, surrounded by Neoproterozoic Damaran cover as periclinal synclinoria, mainly composed of amphibolite-facies metasedimentary rocks. The domes consist of remnants of pre-Damaran gneissic basement and/or Damaran granitoids. There is a high strain zone along the margin between the granite-gneiss domes and the Damaran cover, interpreted in terms of a late extensional mid-crustal detachment, following oblique north-south collision of the Kalahari and Congo Cratons (ca 550 Ma). The present paper focusses on two granite-gneiss domes (Khan and Ida) in western-central Namibia, where a previously unrecognised generation of early post-collisional Damaran granitoids (ca 530 Ma old), have invaded the pre-Damaran basement and cross-cut the detachment zone. These granitoids are classified as syeno-monzogranites with peraluminous high-K calc-alkaline compositions and I/S-type signatures. Geochemical data suggest that the syeno-monzogranites were derived from a lower crustal source, tentatively ascribed to the pre-Damaran gneissic basement. The granitoids also show evidence for magma contamination through partial assimilation with dark, amphibolitic cover rocks. As a result of this hybridisation of the syeno-monzogranites, granitoids with more dioritic compositions were developed. The syeno-monzogranites were then cross-cut by Damaran pegmatitic leucogranites (ca 510 Ma). Further evidence of episodic post-collisional magmatic activity is provided by late-stage mineralisation, and by a ca 465 Ma (40Ar-39Ar) thermal overprint in the cover rocks. It is proposed that the post-collisional magmatic activity emanated from the endogenic part of a mid-crustal Damaran granitic batholith, whose cupola-like emplacement in dome structures coincided with regional uplift, and subsequent cooling before ca 465 Ma.

Chapter 13 Neoproterozoic sequences of the West Congo and Lindi/Ubangi Supergroups in the Congo Craton, Central Africa

Abstract The focus of this chapter is the West Congo Supergroup in the West Congo Belt (WCB), which extends along the western margin of the Congo Craton from Gabon in the north to northern Angola in the south, and the Lindi/Ubangi Supergroup of the Lindian and Fouroumbala – Bakouma Basins exposed on the northern margin of the craton. In both regions, up to two distinct diamictite horizons have been recognized, the younger of which is often associated with carbonate rocks. Geochronological constraints are generally rather poor, many of the deposits lack modern sedimentological analysis, and the glacial versus non-glacial genesis of the diamictites is a matter of debate in the literature. However, recent studies suggest a periglacial influence of diamictite deposition, particularly for the sequences in the WCB. The stratigraphy of the various basins is described, available geochemical and geochronological information collated, and recent work regarding the periglacial nature of the diamictites discussed. Finally, an updated chronostratigraphic correlation between the basins is presented. However, much more work is required, particularly in the Neoproterozoic basins on the northern margin of the Congo Craton, and more accurate geochronological constraints are required before the Neoproterozoic palaeogeography and depositional environments of the western and northern Congo Craton can be fully understood.

Main characteristics and review of mineral resources of the Kabanga-Musongati mafic-ultramafic alignment in Burundi

Abstract The first part of this paper gives the main characteristics of the Kabanga-Musongati (KM) mafic-ultramafic alignment in Burundi. This 350 km long alignment, with an emplacement age of 1275±11 Ma (UPb on zircon), extends from Burundi to Uganda. In Burundi, where the KM alignment consists of nine main massifs, a petrological study has defined stratigraphical units comparable to those identified in other layered igneous massifs, such as the Great Dyke of Zimbabwe and the Bushveld Complex in South Africa. The second part of this paper deals with the metallogeny of the KM alignment in Burundi. A review of the well-known deposits is given. Their stratigraphical position and likely origin are discussed in the light of petrological results. New data on platinum group elements (PGE) are provided. Finally, pathways for future mineral exploration are proposed, in particular for PGE-rich sulphides.