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The Late Proterozoic Katangan basic rocks (dolerites and basalts) outcropping at Kibambale in central Shaba (Zaire) are classified as extension-related tholeiites. The concentrations of immobile major and trace elements resemble those encountered in Phanerozoic enriched oceanic ridge basalts (E-MORB) emplaced in incipient oceanic rifts such as Red Sea and Tadjura Gulf troughs. This conclusion is also sustained by: elemental ratios which are MORB-like; multielement chondrite-and MORB-normalized patterns; sedimentological data, especially the shallow water conditions which prevailed during the deposition of lower Kundelungu tillite in which these basic rocks are interstratified; and the geological and structural framework, notably the emplacement of these igneous rocks in a basin not distant from the Kibaran continental land mass.In this area the geological units containing the igneous rocks are not affected by significant tangential tectonism and so the present structural relationships are relatively undisturbed compared with those which prevailed during the emplacement of the igneous rocks.

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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...