Geochemistry of mafic volcanic rocks from the Lake Kivu (Zaire and Rwanda) section of the western branch of the African Rift

Marcelot, G.; Dupuy, C.; Dostál, J.; Rançon, Jean-Philippe; Pouclet, André

doi:10.1016/0377-0273(89)90022-x

Cited by 19 publications

(15 citation statements)

References 27 publications

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“…Both magmatisms (Triassic and post-Triassic) have (La/Ce)n > I indicating an origin from an enriched mantle source (Marcelot et al, 1989). But higher ThlNb and lower Zr/Sm ratios in the Triassic magmatism could evidence some differences in their original mantle source.…”

Section: Discussionmentioning

confidence: 99%

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Morata¹,

Puga²,

Demant³

et al. 1997

Estud. geol.

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Mesozoic basic magmatism in the External Zones of the Betic Cordilleras (S. Spain) is represented by small tectonic bodies (ophites) in Triassic formations, and submarine flows with abundant pillow-Iavas interbedded with Jurassic sediments. Both basic igneous manifestations suffered very low-to low-grade metamorphism, more intense in the case of the ophites.Two types of ophites are distinguished on the basis of their primary mineralogy. In the first type, orthopyroxene is present in the less differentiated products. Clinopyroxene and Ca-plagioclase are the main primary phases and quartz appears in the more evolved rocks. In the second type, olivine is present in the less differentiated products. Ti-rich augite and Ca-plagioclase are also important primary minerals, but quartz is absent.Whole-rock chemistry (major and trace elements, including REE) also allows us to discriminate between these two groups. The first group has higher SiO z and lower TiO z , PzOs and lower NazO/KzO ratios than the second. Normative quartz is almost invariably present in this first group, whereas normative nepheline (lower than 5%) is characteristic of the second group. Both groups are Sr, K, Ba, Rb, Th, Nb and Ce enriched with respect to normal MORB, but the first group has higher K, Rb, Ba, Th and lower Nb, Ce contents than the alkaline group. NblY and TiN ratios are also different and show a tholeiitic affinity for the first group and transitional to alkaline for the second. Chondrite-normalised REE patterns in both groups are similar and characterized by LREE enrichment with respect to HREE. LREE/HREE ratios are, however, slightly higher in the transitional to alkaline group.Various discriminant tectonic diagrams indicate a continental intraplate setting for both ophite groups. This magmatism is related to the first extensional period of the Betic Cordilleras, during the Triassic-Jurassic. Geochemical differences between the two groups could evidence different degrees of crustal contamination, and/or different mantle sources. Trace element ratios [(La/Ce)n> 1, La/Nb < 1.5] for both magmatism are indicative of an enriched mantle source. Differences in ThIYb, Zr/Nb, ZrIY and Ba/Zr ratios underline the greater influence of a lithospheric component in the case of the Triassic magmatism.Key wOl"ds: Ophites: Continental intraplate magmatism; Mesozoie; External Zones; Betie Cordilleras. RESUMENEl magmatismo basico mesozoico en las Zonas Externas de las Cordilleras Beticas (S. Espafia) esta representado por pequefios bloques tect6nicos (ofitas) en las formaciones triasicas, y coladas submarinas, con abundantes pillow-Iavas intercaladas en los sedimentos jurasicos. Ambos tipos de manifestaciones fgneas sufrieron transformaciones metam6rficas en condiciones de grado bajo a muy bajo, mas intensas en el caso de las ofitas.Sobre la base de la mineralogfa primaria se han distinguido dos tipos de ofitas. En el primer tipo, aparece ortopiroxeno en los terminos menos diferenciados, siendo los clino- piroxenos y plagioclasas calcicas las pr...

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Section: Discussionmentioning

confidence: 99%

Untitled

Morata¹,

Puga²,

Demant³

et al. 1997

Estud. geol.

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“…Also plotted in Fig. 10 are mafic lavas from RungweTanzania and Lake Kivu, which are interpreted to derive from a source peridotite that has suffered metasomatism (Marcelot et al 1989;Furman 1995), carbonatite-metasomatised peridotite Fig. 10 Variation of fractionation-corrected Ti/Eu versus Zr/Hf ratios in Lake Nyos basalts compared to other mafic lavas from Rungwe and Lake Kivu (East African Rift System) which are considered to have suffered metasomatism, carbonatite-metasomatised peridotite xenoliths from Ethiopia, Tanzania, Australia and Italy.…”

Section: Trace Element Evidencementioning

confidence: 99%

Depth of Melt Segregation Below the Nyos Maar-Diatreme Volcano (Cameroon, West Africa): Major-Trace Element Evidence and Their Bearing on the Origin of CO2 in Lake Nyos

Aka

2015

Advances in Volcanology

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The Nyos maar-diatreme volcano on the Oku Volcanic Group (OVG) in NW Cameroon carries yet the most infamous maar lake in the world because the lake exploded in 1986 releasing CO 2 that killed *1,750 people and over 3,000 livestock. A process of safely getting rid of accumulated gas from the lake started in 2001. Even though *33 % of it has been removed, gas continues to seep into the lake from the mantle, so the lake still poses a thread. Available data on basaltic lava from the maar-diatreme volcano and other volcanoes of the OVG are used here to determine the depth and location where the magmas are produced, and to make inferences on the generation of CO 2 in the Nyos mantle. Fractionation-corrected major element data agree well with experimental data on mantle peridotite and suggest that Lake Nyos magmas formed at pressures of 2-3 GPa in the garnet stability field. This inference is corroborated by trace element models that indicate small degree (1-2 %) partial melting in the presence of residual garnet (2-3 %). The basalts have elevated High Field Strength Element (HFSE) ratios (Zr/Hf = 48.5 ± 1.2 and Ti/Eu = 5,606 ± 224) which cannot be explained by any reasonable fractional crystallization model. A viable mechanism would be melting of a mantle that was previously spiked by percolating carbonatitic melts. It is suggested that small degree partial melting of this metasomatised mantle produces the lavas with super chondritic HFSE ratios, and is generating the CO 2 that seeps into and accumulates in the lake, and which asphyxiated people and animals during the 1986 gas disaster. This finding requires that current efforts to degas Lake

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“…Numerous geochemical analyses of South-Kivu and Virunga lavas are available from the literature, though their accuracy is highly variable. Trace element and isotopic data are provided by Mitchell and Bell (1976), Vollmer and Norry (1983), Hertogen et al (1985), Vollmer et al (1985), , Auchapt (1987), Auchapt et al (1987), Marcelot et al (1989), Toscani et al (1990), Demant et al (1994), Rogers et al (1998), Furman and Graham (1999), Platz et al (2004) and Chakrabarti et al (2009a). These data are reviewed and, when necessary, completed by new analyses ( Table 1).…”

Section: Geochemical Composition Of Lavasmentioning

confidence: 99%

“…To document this question, it is necessary to constrain the volcano-tectonic evolution, to comfort the geochemical data, and to obtain numerous age markers. Many years ago, we sampled and studied all the volcanic rocks of the South-Kivu and Virunga areas (Pouclet, 1973(Pouclet, , 1976(Pouclet, , 1980Pouclet et al, 1981Pouclet et al, , 1983Pouclet et al, , 1984Marcelot et al, 1989). We provided the first significant age data set and discovered the North Idjwi nephelinites, the oldest lavas of the rift (Bellon and Pouclet, 1980).…”

Section: Introductionmentioning

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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Geochemistry of mafic volcanic rocks from the Lake Kivu (Zaire and Rwanda) section of the western branch of the African Rift

Cited by 19 publications

References 27 publications

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Depth of Melt Segregation Below the Nyos Maar-Diatreme Volcano (Cameroon, West Africa): Major-Trace Element Evidence and Their Bearing on the Origin of CO2 in Lake Nyos

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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