Abstract: Mantle-derived peridotitic garnet xenocrysts from kimberlites in the Mbuji Mayi and Kundelungu areas and from heavy-mineral concentrates collected in the Luebo area, D.R. Congo, have been analysed for major- and trace-element compositions in order to understand the structure and composition of the subcontinental lithospheric mantle (SCLM) and the diamond potential of the kimberlites. The lithosphere beneath the Kundelungu Plateau is ca 175 km thick and has been affected by pronounced melt metasomatism. Garnets from the Kundelungu Plateau indicate an initially cool geotherm (~ 35 mW/m2), which was disturbed by asthenospheric melts that penetrated the SCLM shortly before kimberlite intrusion ca 32 Ma ago. Harzburgitic garnets are very rare, but some lherzolitic garnets display compositions similar to garnets included in diamond. Garnets from the Mbuji Mayi region indicate a cool geotherm (35 mW/m2); the SCLM is ~ 210 km thick and was affected by melt-related and phlogopite-related metasomatisms. Harzburgitic garnets form about 33% of the analysed population. The garnets from the Luebo region indicate a cool lithospheric geotherm (35 mW/m2) typical of cratonic areas. The SCLM from which the garnets were derived was relatively thick (205 km), affected by melt-related and phlogopite-related metasomatisms and characterised by the presence of a ~ 80-km thick harzburgite-rich layer. In terms of peridotitic diamond potential, Mbuji Mayi and Luebo are more prospective than Kundelungu. The initially cool conductive geotherm, the presence of some garnets with compositions similar to garnets included in diamond and the presence of sporadic diamond in the Kundelungu Plateau suggest that diamond initially was present in the lithosphere and the observed paucity of diamond may be due to the melt-related metasomatism that affected the lithosphere in the region. We suggest that the lithospheric mantle beneath Kundelungu is a strongly modified Archean cratonic lithosphere that has survived beneath the area during Proterozoic tectonism.
Abstract: Analysis of UâPb ages, Hf isotopes and trace-element compositions in zircon collected from modern drainages is a robust methodology for tracking crustal and magmatic evolution. The application of the technique to zircons from the Luebo area in the central part of the Congo-Kasai Craton indicates the presence of a not, vert, similar3.6 Ga crust that underwent late Archean (2.9â2.5 Ga) reworking. Very low initial Hf-isotope ratios in zircons with UâPb ages between 1.9 and 2.6 Ga suggest the presence of crust even older than 3.6 Ga at depth. The Archean crust underwent further reworking during Paleoproterozoic, Mesoproterozoic and Neoproterozoic thermal events. Post-Archean generation of juvenile crust appears to have occurred only sporadically during MeoproterozoicâNeoproterozoic time. The magmatic activity and tectonic events observed in the Neoproterozoic Katangan Belt had a regional extent as they also impacted the Congo-Kasai Craton where alkaline magmatism has occurred from late Archean to early Cambrian time. The ages of zircons with low HREE and U contents suggest three separate episodes of kimberlite magmatism in the central part of the Congo-Kasai Craton, in late Archean (2.8â2.6 Ga), Neoproteorozoic (850â582 Ma) and Cretaceous (116â70 Ma) time.
Abstract: Kimberlites are small-volume melts whose eruption is related to weak extension of the lithosphere, and their ages can carry important tectonic information. We have carried out in situ LAM-ICPMS UâPb dating of groundmass perovskite in two kimberlitic pipes (Kambeli and Msipashi) in Kundelungu Plateau (SE Congo). The method is validated by the analysis of perovskites from four previously dated South African Group I kimberlites. Intercepts of regression lines on inverse-Concordia plots yield precise UâPb ages (2Ï = 2â6 Ma) and a well-constrained value for the 207Pb/206Pb of the common-Pb component. The ages of Group I kimberlites in Kimberley District cluster around 87 Ma, similar to those determined by other techniques demonstrating the robustness of the technique. The perovskites from the Kundelungu kimberlites that are intrusive into the Biano Subgroup (Neoproterozoic Katangan Supergroup) yield an age of 32.3 ± 2.2 Ma. The EoceneâOligocene age of the Kundelungu kimberlites corresponds to the opening of the northern part of the East African Rift and implies the southward prolongation of this extension. Considering the position of the Kundelungu Plateau and gravity data for the region, the kimberlite intrusion may reflect extension on the Lake Mweru-Luapula graben. The Luizi impact structure in the Kundelungu Plateau, previously described as Pleistocene, pre-dates the kimberlitic event.
Abstract: The Gungwania and Talala kimberlitic pipes on the Kundelungu Plateau (Katanga, SE Congo) have been used as drillholes, to obtain crustal zircons for a study of crustal evolution in the region and to constrain the age of the basement and the sedimentary
provenance of the Katangan Supergroup. Two hundred and twenty-nine zircon grains were analysed for UâPb ages and Hf-isotope compositions. Juvenile Mesoarchean (â¼3.4â3.3 Ga) crust in the region underwent recycling during Neoarchean and Paleoproterozoic time. The Paleoproterozoic event involved little production of juvenile crust, and this Paleoproterozoic crust was recycled during Mesoproterozoic and Neoproterozoic time. Both the Mesoproterozoic and Neoproterozoic were characterised by bimodal magmatism reflecting extension, linked with the break up of Rodinia and the opening of the Katangan basin, and the development of juvenile crust. Detailed studies of the external morphology of the zircons in different age groups suggest that the basement beneath the Kundelungu region is predominantly Paleoproterozoic in age; other zircon populations may be derived from the Katangan sediments. The absence of any zircons younger than 560 Ma supports deposition of the Biano Subgroup, which hosts the kimberlites, during the Pan-African Lufilian orogeny. The Archean Congo and Zimbabwe cratons, the Paleoproterozoic Ubendian Belt (Bangweulu Block), the Paleo- to-Mesoproterozoic Irumide Belt, the Mesoproterozoic Kibaran Belt and the Choma Kalomo Block may all have contributed material to the sediments of the Katangan Supergroup.
Abstract: The Nguba and Kundelungu Groups constitute the middle and upper parts of the Neoproterozoic Katangan Supergroup, respectively, and consist of conglomerates, sandstones, mudrocks and carbonates. During deposition, the Katangan basin received sediments originating from both northern and southern sources. The Nguba and Kundelungu Groups siliciclastic rocks have elemental abundances and ratios suggestive of a relatively felsic TTG source, although slightly more mafic compositions occur in the Nguba Group and the overlying ââPetit Conglome´ratââ Formation at the base of the Kundelungu Group. Modal compositions of the Nguba Group rocks indicate a basement uplift provenance, and geochemical parameters indicate the source of both the Nguba and Kundelungu Groups had an active continental margin character. Source area weathering was moderate in the Nguba Group. Low Chemical Index of Alteration (CIA) and Plagioclase Index of Alteration (PIA) indices and relatively uniform chemical compositions of the ââGrand Conglome´ratââ and the ââPetit Conglome´ratââ Formations
lying respectively at the bases of the Nguba and Kundelungu Groups are compatible with deposition in a cool or frigid climate, and support their presumed petrographic based glaciogenic origin. High CIA and PIA indices in Upper Kalule rocks in the middle part
of the Kundelungu Group point to the intensification of source weathering, possibly under tropical to subtropical climate under steady state conditions. Geochemical similarities between the Nguba Group and the ââPetit Conglome´ratââ are compatible with a change from an extensional setting to compression, with derivation of the ââPetit Conglome´ratââ by reworking of the underlying units during basin inversion. Change in provenance signatures and weathering indices in the Upper Kalule Formation may reflect reduced tectonism and resumption of supply of more weathered extrabasinal detritus, similar to that which fed the basal Roan Group. Overall the data suggest derivation mainly from pre-Katangan Proterozoic sources with continental arc characteristics. The adjacent
Paleoproterozoic Ubendian Belt, particularly the Bangweulu block calcalkaline plutonic and volcanic province, is a suitable candidate as the source for the Nguba and Kundelungu Group sedimentary rocks. However, Mesoproterozoic and Archaean terrains have also contributed a minor component to the basin.
Abstract: The Neoproterozoic Katangan R.A.T. (ââRoches Argilo-Talqueusesââ) Subgroup is a sedimentary sequence composed of red massive to irregularly bedded terrigenous dolomitic rocks occurring at the base of the Katangan succession in Congo. Red R.A.T. is
rarely exposed in a continuous section because it was affected by a major layer-parallel decollement during the Lufilian thrusting. However, in a number of thrust sheets, Red R.A.T. is in conformable sedimentary contact with Grey R.A.T which forms the base of
the Mines Subgroup. Apart from the colour difference reflecting distinct depositional redox conditions, lithological, petrographical and geochemical features of Red and Grey R.A.T. are similar. A continuous sedimentary transition between these two lithological
units is shown by the occurrence of variegated to yellowish R.A.T. The D. Strat. ââDolomies Stratifie´esââ formation of the Mines Subgroup conformably overlies the Grey R.A.T. In addition, a transitional gradation between Grey R.A.T. and D. Strat. occurs
in most CuâCo mines in Katanga and is marked by interbedding of Grey R.A.T.-type and D. Strat.-type layers or by a progressive petrographic and lithologic transition from R.A.T. to D. Strat. Thus, there is an unquestionable sedimentary transition between Grey R.A.T. and D. Strat. and between Grey R.A.T. and Red R.A.T. The R.A.T. Subgroup stratigraphically underlies the Mines Subgroup and therefore R.A.T. cannot be comprised of syn-orogenic sediments deposited upon the Kundelungu (formerly ââUpper Kundelunguââ) Group as suggested by Wendorff (2000). As a consequence, the Grey R.A.T. CuâCo mineralisation definitely is part of the Mines Subgroup Lower Orebody, and does not represent a distinct generation of stratiform CuâCo sulphide mineralisation younger than the Roan orebodies.
Abstract: The lithosphere beneath the Luebo, Mbuji Mayi and Kundelungu regions of the Democratic Republic of the Congo has been mapped and evaluated for diamond potential using peridotitic garnets and xenoliths from the kimberlites. Crustal zircons from the Luebo region and Kundelungu kimberlites were analysed to gain an understanding of the crustal and magmatic evolution of the southern Congo.
U-Pb analysis of perovskites indicates that the Kundelungu kimberlites intruded at 32.3 ± 2.2 Ma, and are contemporaneous with the initiation of the East African Rift. This implies the southern extension of this initial stage of the rifting, formerly only recognised in the eastern part of Africa (Ethiopia and Kenya). The Lake Mweru-Luapula graben in the northern part of Kundelungu Plateau is thus related to the initial stages of the East Africa Rift and the Luizi impact structure in eastern part of the Kundelungu Plateau is older than 32 Ma.
The Kundelungu Plateau located off-craton, and the Mbuji Mayi and Luebo areas located on-craton, are both characterised by low geotherms typical of cratonic areas. However, in Kundelungu this initially-low geotherm was disturbed by upwelling of hot asthenospheric melts, perhaps during the initial stages of the East African Rift. The Mbuji Mayi and Luebo regions have more diamond potential, as suggested by the presence of abundant harzburgitic garnets and chromites compared with Kundelungu. However, the originally low geotherm in Kundelungu, which is similar to that of cratonic areas where diamond is found, the presence of some harzburgites in the lithospheric section of Kundelungu and the presence of lherzolitic garnets with sinuous REE patterns similar to those of diamond inclusions indicate the presence of diamond. The scarcity of diamond in Kundelungu could be due to the pronounced metasomatism that affected the lithosphere. Similar studies on more kimberlite pipes may provide insights in terms of the diamond potential of the whole Kundelungu region.
Kundelungu kimberlites show two different textural types typical of hypabyssal and crater facies. The Kundelungu kimberlites are typically Group I kimberlites. Major- and trace-element analysis of the Kundelungu kimberlites and the Sr-Nd isotopic composition of their groundmass perovskite indicate a source similar to that of OIB-like melts related to the plumes associated with the East African Rift beneath the subcontinental lithospheric mantle.
The zircon data reveal the existence of previously unrecognised ancient crust in southern Congo with ages ~3.6 Ga, possibly inside the Congo-Kasai Craton. Tectonic and magmatic events observed inside the Katangan Belt also affected the Congo-Kasai Craton. The bimodal magmatism observed during Mesoproterozic-Neoproterozoic time in the Kundelungu region involved the production of juvenile crust. In the Congo-Kasai Craton (Luebo region) the magmatism during the same period was produced by remelting of older crust. The basement beneath the Kundelungu region is mostly Proterozoic. There were three kimberlitic episodes inside the Congo-Kasai Craton, at 2.8-2.6 Ga (late Archean), 800-600 Ma (Neoproterozoic) and 114-90 Ma (Cretaceous).
Limited Re-Os data on sulfides in peridotitic xenoliths from the Kundelungu kimberlites confirm the presence of an Archean lithospheric mantle in the region. This observation, the low geotherm and the Archean Hf-isotopic signature of Neoproterozoic zircons from Kundelungu suggest that the lithosphere beneath the Kundelungu region is a strongly modified cratonic lithosphere, overlain by Archean crust that was strongly modified in later tectonic episodes. The Katangan sedimentary basin probably was formed in response to the refertilisation of the lithospheric mantle, inducing a higher density that produced sudsidence of the crust.
