Spatial and Temporal Variations in Basalt Geochemistry in the N. Kenya Rift

Lippard, Stephen J.; Truckle, P. H.

doi:10.1007/978-94-009-9803-2_11

Cited by 12 publications

(6 citation statements)

References 14 publications

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“…The feasibility of deriving plateau phonolites by partial melting of an alkali basaltic lower crust was evaluated with least squares major element modeling (Tables 4 and 5 The input data and results of three calculations to derive phonolite Ke85-25 from alkaline basaltic compositions are shown in Table 4. There are few published analyses of Miocene basalts erupted within the Kenya rift, and they have been characterized as highly evolved [e.g., Lippard and Truckle, 1978]. Source A is the average of some rift Miocene basalts [Norry et al, 1980] 6, 10 b 0.32 6, 10' 2.5, 3.3…”

Section: Results Of Major Element Modelingmentioning

confidence: 99%

“…The volume of basaltic magma required to produce 50,000 km 3 of erupted If the lower crust beneath Kenya was, instead, pervasively injected and/or underplated by basaltic magmas, synchronous with the voluminous eruption of alkalic basalt in northern Kenya during the early and middle Miocene, the concept of subsequent partial remelting of this material in the late Miocene avoids the problems of the enormous magma chambers and the large percentage of liquid at lower crustal pressures. Crystallization of these basalts in the lower crust may also explain the eruption of the characteristically highly evolved Miocene basaltic lavas [e.g., Lippard and Truckle, 1978].…”

Section: Discussionmentioning

confidence: 99%

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The origin of Kenya rift plateau‐type flood phonolites: Evidence from geochemical studies for fusion of lower crust modified by alkali basaltic magmatism

Hay¹,

Wendlandt²,

Wendlandt³

1995

J. Geophys. Res.

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Geochemical investigations support the petrogenesis of Kenya rift plateau‐type flood phonolites (14–11 Ma) by partial melting of an alkali basaltic material at lower crustal pressures. High‐pressure/high‐temperature experiments on a natural plateau phonolite (Hay and Wendlandt, this issue) document multiple saturation of augite, andesine, titanomagnetite, and phlogopite at 0.7 GPa, 1000°C, XCO2 = 0.42, with amphibole appearing at 975°C. A least squares solution to major element modeling, involving subtraction of the compositions of near‐liquidus augite, andesine, titanomagnetite, and olivine (Fo67; hypothesized product of incongruent melting of hydrous phases) from a Kenya Miocene alkali basalt composition, indicates that plateau phonolites can be derived by 15 wt % fusion of this hypothetical parental material (∑R2 = 0.07). Alkali basaltic magmas may have injected and/or underplated the lower crust in southern Kenya during prior rift‐related basaltic volcanism (23–14 Ma). Bulk Earth values of (87Sr/86Sr)i and εNd near zero for four plateau phonolite samples are consistent with a mantle‐derived parental composition. Three of these four samples reflect little, if any, postmelting modification; one sample may have evolved by fractional crystallization (high Rb/Sr, low Ba, Sr and Mg #). A fifth sample may show evidence of assimilation and fractional crystallization processes (elevated radiogenic Sr and Pb, large negative Eu anomaly, and low Ba, Sr, and Mg #). Much of the geochemical variation among plateau phonolite lavas, however, can be ascribed to melting of a predominantly alkali basaltic source with contributions from a lower crustal protolith. A mantle‐derived source is also supported by Sr‐Nd‐Pb isotope data for the phonolites, which indicate that the alkali basaltic source can be described in terms of high U/Pb (HIMU) and enriched mantle (EM1 and EM2) components.

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Section: Results Of Major Element Modelingmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

The origin of Kenya rift plateau‐type flood phonolites: Evidence from geochemical studies for fusion of lower crust modified by alkali basaltic magmatism

Hay¹,

Wendlandt²,

Wendlandt³

1995

J. Geophys. Res.

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“…Studies of the spatial and temporal variation in basalt composition along the rift have shown that (1) most sectors of the within-rift sequences show a decrease of silica-undersaturation of ma®c lavas with time; and (2) ma®c lavas W and E of the rift are normally more silica-undersaturated than are lavas within the rift (Lippard & Truckle 1978;Truckle 1980;Macdonald et al 2001). These trends are related to two effects: the disappearance of nephelinites from the sequences and an increasing importance of hy-normative basalts volumetrically.…”

Section: Range Of Basaltic Magmasmentioning

confidence: 99%

Magmatism of the Kenya Rift Valley: a review

MacDonald

2002

Transactions of the Royal Society of Edinburgh: Earth Sciences

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Tertiary±Recent magmatism in the Kenya Rift Valley was initiated c. 35 Ma, in the northern part of Kenya. Initiation of magmatism then migrated southwards, reaching northern Tanzania by 5±8 Ma. This progression was accompanied by a change in the nature of the lithosphere, from rocks of the Panafrican Mozambique mobile belt through reworked craton margin to rigid, Archaean craton. Magma volumes and the geochemistry of ma®c volcanic rocks indicate that magmatism has resulted from the interaction with the lithosphere of melts and/or¯uids from one or more mantle plumes. Whilst the plume(s) may have been characterised by an ocean island basalttype component, the chemical signature of this component has everywhere been heavily overprinted by heterogeneous lithospheric mantle. Primary ma®c melts have fractionated over a wide range of crustal pressures to generate suites resulting in trachytic (silica-saturated and -undersaturated) and phonolitic residua. Various Neogene trachytic and phonolitic¯ood sequences may alternatively have resulted from volatile-induced partial melting of underplated ma®c rocks. High-level partial melting has generated peralkaline rhyolites in the south±central rift. Kenyan magmatism may, at some future stage, show an increasing plume signature, perhaps associated ultimately with continental break-up. KEY WORDS: Plume±lithosphere interactionsThe Red Sea and the Gulf of Aden are two arms of a postulated triple junction centred on the Afar region (Fig. 1). Oceanic crust began to form under these arms c. 10 Ma. The third arm, the East African Rift System (EARS), extends southwards through Ethiopia, Kenya and Tanzania into Mozambique. Whilst lithospheric extension is occurring beneath the EARS, the rift has not developed to the stage of ocean crust formation. The EARS has come to be seen, therefore, as a prime location in which to investigate the early stages of continental break-up.This review focuses on the magmatism of the Kenya Rift section, used here to indicate that section of the EARS extending from northern Kenya to northern Tanzania. The primary aim is to demonstrate the complex polybaric evolutionary history of the magmatism, starting with the postulated plume-induced generation of primary magmas by variable degrees of partial melting of heterogeneous, metasomatised lithosphere, through to generation of salic magmas by assimilation±crystal fractionation processes over a range of crustal pressures and by partial melting of crustal protoliths, also over a range of pressures.

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“…Des magmas transitionnels d'~ge quaternaire ont 6t6 d~crits dans les rifts d'Ethiopie et du Kenya (ZANETTIN et al, 1974;LIPPARD et TRUCKLE, 1978;PIc-CIRILLO et al, 1979) off ils ont 6t6 interpr&~s comme des marqueurs d'un important amincissement crustal implicitement mis en liaison avec un devenir oc6ani-que potentiel (type 2) de ia branche orientale du rift Africain (BARBERI et SANTA-CROCE, 1980). La d~couverte de basaltes transitionnels dans le rift du Kivu sous lequel l'amincissement de la crofite est mod6r~ indique que ces laves ne sont pas limit~es fi la branche orientale du rift Africain.…”

Section: Interpretation Structurale Et Conclusionunclassified

Les basaltes transitionnels dans l'évolution des rifts continentaux: Exemple de la Haute-Ruzizi dans le Rift de l'Afrique Centrale (Kivu-Zaîre)

et al. 1984

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ZusammenfassungDie letzten vulkanischen Ergfisse des Hohen Ruzizi (Kivu-Rift, Za'ire) sind Basalte, die Mg-Olivin, Ca-und Ti-reiche Pyroxene, Plagioklas (An66_ 45), Titanomagnetit und Ilmenit enthalten. Einige dieser Basalte sind Cr-reich (bis 470 ppm) und enthalten Ti-Cr-Spinelle.Olivin-Spinell-, Olivin-Schmelze-und Ilmenit-Magnetit-Thermometer zeigen hohe Erstarrungstemperatur der Schmelze (>1300 ~ C) an und mittlere O2-Fugazit/it (leicht fiber NNO).Vom Chemismus her weisen die Basalte tholeitische Zfige auf, doch zugleich zeigt die mineralogische Zusammensetzung (Ca-und Ti-reiche Pyroxene und Ti-reiche Chromspinelle) und die Kristallisation unter m~iBigem O2-Partialdruck Charakteristika der Alkalireihe.Diese Ambivalenz ist typisch ftir Ubergangsmagmen in Gebieten mit Dehnungsstrukturen.Struktur-und geophysikalische Untersuchungen haben deutlich gemacht, dab die Entwicklung der Riftsysteme des Kivu und von Kenya im Quarter typisch gem~iBigte (Kivu) bis starke (Kenya) Dehnungsraten kontinentale Riftzonen besitzen. Diese Phasen der Entwicklung werden yon r angezeigt, die frtiher auftreten als die ~>Ozeani-sierung~ der Kruste. AbstractThe High Ruzizi last volcanic flows (Kivu rift, Zaire) are saturated basalts containing Mg-olivine, Ca and Ti-rich pyroxene, plagioclase (An66_ 45), titanomagnetites and ilmenite.Some of this basalts are Cr-rich (up to 470 ppm) and contain Ti-chromian spinels. Olivine-spinel, olivine-liquide and ilmenite-magnetite thermometers indicate high-temperature liquidus crystallization (> 1300 ~ C) and moderate fO2 conditions (slightly higher than NNO).Such basalts exhibit chemical tholeiitic affinities but their mineralogical composition (Ca and Ti rich clinopyroxene and Ti-rich chromian spinels) and their crystallization under moderate PO 2 are rather indicative of alkaline characteristics.This ambivalence is typical of distensive areas transitional magmas. Structural and geophysical studies established that the quaternary evolution of Kivu and Kenya rifts is typical of moderated (Kivu) to important (Kenya) extension rates of continental rifts. These evolutive phases are indicated by transitional basalts which appear earlier than crust ~ 1300~ C si l'on se ref6re aux temp6ratures d'6quilibres olivine-spindle et olivine-liquide) et ont cristallis~s sous des fugacit6s d'oxyg6ne 16g~rement sup6rieures au tampon NNO (6quilibre magn6tite-ilm6nite de la p~te).Ils pr6sentent des caract6res chimiques de laves thol6iitiques mais en m6me temps leur composition min6ralogique (clinopyrox6nes calciques et titanif6res, richesse en Ti de spinelles chromif6res) ainsi que leur 6volution sous des fugacit...

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Spatial and Temporal Variations in Basalt Geochemistry in the N. Kenya Rift

Cited by 12 publications

References 14 publications

The origin of Kenya rift plateau‐type flood phonolites: Evidence from geochemical studies for fusion of lower crust modified by alkali basaltic magmatism

The origin of Kenya rift plateau‐type flood phonolites: Evidence from geochemical studies for fusion of lower crust modified by alkali basaltic magmatism

Magmatism of the Kenya Rift Valley: a review

Les basaltes transitionnels dans l'évolution des rifts continentaux: Exemple de la Haute-Ruzizi dans le Rift de l'Afrique Centrale (Kivu-Zaîre)

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