Petrology and geodynamic significance of the Tertiary alkaline lavas from the Kahuzi–Biega region, Western Rift, Kivu, Zaire

Western Uganda is a key region for understanding the development of the western branch of the East African rift system and its interaction with pre-existing cratonic lithosphere. It is also the site of the topographically anomalous Rwenzori Mountains, which attain altitudes of [5000 m within the rift. New structural and geochronological data indicate that western Uganda south and east of the Rwenzori Mountains consists of a WSW to ENE trending fold and thrust belt emplaced by thickskinned tectonics that thrust several slices of Proterozoic and Archaean units onto the craton from the south. The presence of Archaean units within the thrust stack is supported by new Laser-ICP-MS U-Pb age determinations (2637-2584 Ma) on zircons from the Rwenzori foothills. Repetition of the Paleoproterozoic units is confirmed by mapping the internal stratigraphy where a basal quartzite can be used as marker layer, and discrete thrust units show distinct metamorphic grades. The thrust belt is partially unconformably covered by a Neoproterozoic nappe correlated with the Kibaran orogenic belt. Even though conglomerates mark the bottom of the Kibaran unit, intensive brittle fault zones and pseudotachylites disprove an autochthonous position. The composition of volcanics in the Toro-Ankole field of western Uganda can be explained by the persistence of a cratonic lithosphere root beneath the northwardly thrusted Archaean and Palaeoproterozoic rocks of westernmost Uganda. Volcanic geochemistry indicates thinning of the lithosphere from [140 km beneath Toro-Ankole to ca. 80 km beneath the Virunga volcanic field about 150 km to the south. We conclude that the western branch of the East African rift system was initiated in an area of thinner lithosphere with Palaeoproterozoic cover in the Virunga area and has propagated northwards where it now abuts against thick cratonic lithosphere covered by a thrust belt consisting of gneisses, metasediments and metavolcanics of Neoarchaean to Proterozoic age.

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“…Data from GEOROC database (http://georoc. mpch-mainz.gwdg.de/georoc/), Pouclet et al (1981), Lubala et al (1987) and Rosenthal et al (2009) …”

Section: Geochronological Informationmentioning

confidence: 94%

Continuous cratonic crust between the Congo and Tanzania blocks in western Uganda

Link¹,

Koehn²,

Barth³

et al. 2010

Int J Earth Sci (Geol Rundsch)

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“…However, it should be noted that these are Pliocene-Pleistocene grabens situated between the Miocene basins of Lake Tanganyika, to the south, and Mwutan Nzige (Lake Albert) to the north (Lubala et al 1987). Thus the tholeiitic/transitional basalts of Virunga, Bukavu and Mwenga-Kamituga can be shown to have preceded the individualization of their respective grabens.…”

Section: Magmatism and Modality Of Evolution Of Rift Systemsmentioning

confidence: 94%

“…The Western Rift comprises a string of grabens whose definition became clear about 7 Ma ago (Lubala et al 1987), as in Kenya. However, the Western grabens are less evolved than their Eastern counterparts (Ebinger 1989), attested to in:…”

Section: Western Riftmentioning

confidence: 99%

Magmatism in Extensional Structural Settings

Kampunzu¹,

Lubala²

1991

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“…7 -Qtz-Ab+Or-Hy-Ol-Ne combined ternary diagrams of the Kivu basaltic lavas. Data after Meyer (1953), Meyer and Burette (1957), Denaeyer et al (1965), Denaeyer (1972), Pouclet (1976), Guibert (1977), Villeneuve (1978), Kampunzu et al (1979Kampunzu et al ( , 1983Kampunzu et al ( , 1998b, Bellon and Pouclet (1980), De Paepe and Fernandez-Alonso (1981), Kanika et al (1981), Lubala (1981), Lubala et al (1982Lubala et al ( , 1984Lubala et al ( , 1987, De Paepe (1983), Auchapt (1987), Auchapt et al (1987), Marcelot et al (1989), , Furman and Graham (1999), and new analyses (Table 1). Normative nomenclature: T, tholeiite (Qtz); Ol-T, olivine-tholeiite (Ol and > 15% Hy); Ol-B, olivine basalt (Ol and < 15% Hy); Alk-B, alkali basalt (0.01% < Ne < 5%); Bs, basanite (5% < Ne < 15%); Neph, nephelinite (Ne > 15%).…”

Section: Captionmentioning

confidence: 99%

The Cenozoic volcanism in the Kivu rift: Assessment of the tectonic setting, geochemistry, and geochronology of the volcanic activity in the South-Kivu and Virunga regions

Pouclet¹,

Bellón

Bram

2016

Journal of African Earth Sciences

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International audienceThe Kivu rift is part of the western branch of the East African Rift system. From Lake Tanganyika to Lake Albert, the Kivu rift is set in a succession of Precambrian zones of weakness trending NW-SE, NNE-SSW and NE-SW. At the NW to NNE turn of the rift direction in the Lake Kivu area, the inherited faults are crosscut by newly born N-S fractures which developed during the late Cenozoic rifting and controlled the volcanic activity. From Lake Kivu to Lake Edward, the N-S faults show a right-lateral en echelon pattern. Development of tension gashes in the Virunga area indicates a clockwise rotation of the constraint linked to dextral oblique motion of crustal blocks. The extensional direction was W-E in the Mio-Pliocene and ENE-WSW in the Pleistocene to present time.The volcanic rocks are assigned to three groups: (1) tholeiites and sodic alkali basalts in the South-Kivu, (2) sodic basalts and nephelinites in the northern Lake Kivu and western Virunga, and (3) potassic basanites and potassic nephelinites in the Virunga area. South-Kivu magmas were generated by melting of spinel + garnet lherzolite from two sources: an enriched lithospheric source and a less enriched mixed lithospheric and asthenospheric source. The latter source was implied in the genesis of the tholeiitic lavas at the beginning of the South-Kivu tectono-volcanic activity, in relationships with asthenosphere upwelling. The ensuing outpouring of alkaline basaltic lavas from the lithospheric source attests for the abortion of the asthenorpheric contribution and a change of the rifting process. The sodic nephelinites of the northern Lake Kivu originated from low partial melting of garnet peridotite of the sub-continental mantle due to pressure release during swell initiation. The Virunga potassic magmas resulted from the melting of garnet peridotite with an increasing degree of melting from nephelinite to basanite. They originated from a lithospheric source enriched in both K and Rb, suggesting the presence of phlogopite and the local existence of a metasomatized mantle. A carbonatite contribution is evidenced in the Nyiragongo lavas.New K-Ar ages date around 21 Ma the earliest volcanic activity made of nephelinites. A sodic alkaline volcanism took place between 13 and 9 Ma at the western side of the Virunga during the doming stage of the rift and before the formation of the rift valley. In the South-Kivu area, the first lavas were tholeiitic and dated at 11 Ma. The rift valley subsidence began around 8 to 7 Ma. The tholeiitic lavas were progressively replaced by alkali basaltic lavas until to 2.6 Ma. Renewal of the basaltic volcanism happened at ca. 1.7 Ma on a western step of the rift. In the Virunga area, the potassic volcanism appeared ca. 2.6 Ma along a NE-SW fault zone and then migrated both to the east and west, in jumping to oblique tension gashes.The uncommon magmatic evolution and the high diversity of volcanic rocks of the Kivu rift are explained by varying transtensional constraints during the rift history

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Petrology and geodynamic significance of the Tertiary alkaline lavas from the Kahuzi–Biega region, Western Rift, Kivu, Zaire

Cited by 5 publications

References 20 publications

Continuous cratonic crust between the Congo and Tanzania blocks in western Uganda

Continuous cratonic crust between the Congo and Tanzania blocks in western Uganda

Magmatism in Extensional Structural Settings

The Cenozoic volcanism in the Kivu rift: Assessment of the tectonic setting, geochemistry, and geochronology of the volcanic activity in the South-Kivu and Virunga regions

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