Journal of African Earth Sciences (1983)最新文献

Mt Cameroon is a large volcanic horst which belongs to the Cameroon Line. The morphology of the massif depends on tectonic control. Mt Cameroon is built upon a Precambrian metamorphic basement covered with Cretaceous to Recent sediments of the Douala and Rio del Rey basins. The oldest lavas could be of Upper Miocene age. Mt Cameroon has erupted six times in the 20th Century. The 1982 eruption took place inside the crater of an ancient cone. Volcanic risks are actual for the human constructions, mostly along the axis of the horst.

The lavas are picrites (with forsteritic olivine phenocrysts), alkali basalts (with salitic augite phenocrysts), hawaiites (with labrador-bytownite plagioclase phenocrysts) and mugearites (with scarce kaersutite phenocrysts and microlitic phlogopite or nosean). FeTi oxides phenocrysts are ubiquitous.

All the lavas are Ne-normative. The distribution of major and trace elements in the lava series is well explained by fractionations of the minerals found as phenocrysts, but plagioclase does not fractionate (Sr and Eu contents in the lavas increase with the differentiation). Thus, olivine, augite and FeTi oxide fractionations are respectively correlated with MgNi, CaCrSc and TiV distributions. Primitive magmatic liquids contain about 1 ppm uranium.

Ta-Th-Hf correlation and REE distribution clearly indicate that the Cameroon lava series is typically alkaline with no tholeiitic or transitional trend. This fact asserts that the Cameroon Line is not a rift system but the result of tension gashes due to the Adamaoua sinistral strike-slip faulting zone.

The late Tertiary volcanism from the volcanic province of Cap Verde Peninsula includes only silica-undersaturated basic lavas: olivine nephelinites, basanites and olivine alkaline basalts. This undersaturated alkaline series is sodic and indifferentiated.

Lavas contain olivine Fo₈₈ to Fo₅₆, clinopyroxene Wo₅₄En₃₄Fs₁₂ (fassaïte) to Wo₄₀En₄₂Fs₁₈ (augite) present both in phenocrysts and in groundmass. Plagioclase An₆₇An₄₈ is only found in basanites and olivine alkaline basalts. The olivine nephelinite is characterized by a soda-rich nepheline (Ne₇₇Ks₁₆Q₇). Microphenocrysts of iron-titanium oxides may form within the more differentiated lavas of the series.

Lower amphibolite to granulite facies metamorphic rocks in Nimba County, Liberia have yielded 2.2–2.9 Ga RbSr whole rock ages, indicating that they are part of the Archean Liberian age province. We report a 2040 Ma⁴⁰⁴⁰Ar/³⁹Ar plateau date on hornblende from an amphibolite in this region, and suggest that these rocks were also severelyreworked during the Eburnean (∼2.0 Ga) metamorphic episode. ⁴⁰Ar/³⁹Ar analyses of biotite and feldspars from neighboring schists also indicate the presence of two mild thermal events, at 1.5 Ga and 0.6 Ga. Paleomagnetic analyses of samples from these same metamorphic rocks reveal three components of magnetization. The predominant and most stable component (273°E, 21°N) is considered to have been acquired as a result of pos Eburnean uplift and cooling at ∼ 2.0 Ga, whereas the two less stable components with poles at 235°E, 43°N and 16°E, 36°N, probably correlate with the 1.5 Ga and 0.6 Ga thermal pulses, respectively. Rock units from southern Liberia also yield two secondary magnetizations, one at 247°E, 37°N and the other at 104°E, 5°N, and a 1.5 Ga ⁴⁰Ar/³⁹Ar date on plagioclase. Comparison of the paleomagnetic poles corresponding to the ∼2.0 Ga Eburnean component with published paleomagnetic data for West Africa is not consistent with prior interpretations of the polar wander path for West Africa. Our paleomagnetic data, when compared to poles of comparable age from the Kalahari Shield, still suggest that some form of displacement has occurred between the Kalahari and West African Shields since 2.0 Ga.

Proterozoic quartz-pebble conglomerates and sandstones near the base of the Mporokoso Group in northern Zambia were deposited in a braided stream system and are known to be locally enriched in gold. A detailed sampling programme was carried out in small area of good exposure in order to identify empirical guides to gold enrichment and to test the hypothesis that the gold is of placer origin.

A total of 2350 rock samples were taken from nine vertical profiles across the main 50–100 m thick conglomeratic interval, spread over a horizontal distance of 4 km. These were analysed for gold by atomic absorption spectrophotometry. Detailed sedimentological logging was carried out and the gold analyses were correlated with sedimentary parameters. Gold enrichment preferentially occurs:

• 1.

  (1) in a 1–2 km longitudinal stretch of the 50–100 m thick conglomeratic interval;

• 2.

  (2) in massive conglomerates (facies Gm) and then in trough crossbedded conglomerates (facies Gt), and not in trough cross-bedded sandstones (facies St);

• 3.

  (3) in beds of facies Gm and Gt which have small pebbles and a low proportion of vein quartz pebbles;

• 4.

  (4) in sections with frequent vertical changes of facies.

These correlations are consistent with a placer origin for the gold and should help to identify prospective areas within the Mporokoso Group.

In the central part of the Adrar des Iforas (Mali), the 2 Ba Eburnean granulatic unit has been thrust above a high-grade gneissic unit, the so-called ‘Kidal assemblage’, during an early event of the Pan-African orogeny. The Kidal assemblage can be defined as a tectonic mixing of an Eburnean granulitic basement, its sedimentary cover of Middle to Upper Proterozoic age (quartzites, marbles, basalts and metavolcanics) and various pretectonic rocks: ultrabasic to basic rocks, diorites, tonalites. All these rocks have been deformed during at least four main events and metamorphosed together. Thrusting of the Iforas Granulitic Unit above the Kidal assemblage happened during the first event D1. The movement direction was roughly N–S, as shown by the stretching lineation. Some field criteria indicate a sense of displacement towards the north. The lattice preferred orientation of quartz c- and 〈a〉 axes indicate that the slip was dominantly on prismatic and probably pyramidal planes along an 〈a〉 direction; consequently D1 deformation was achieved at high temperature or low-strain rate. The quartz c- and 〈a〉 axes do not show any constant asymmetry, so they do not indicate a sense of shear. Two metamorphic stages have been found in the Kidal assemblage: the first one is characterized by kyanite in aluminous metasediments and by the occurrence of garnet-clinopyroxene-bearing boundis of basic rocks. The P–T range of this event is located at 700 ± 50°C and around 10 Kb. The second event is a syntectonic high temperature (600–650°C) low pressure (3.5 Kb) stage accompanied by migmatization. Such a tangential deformation in barrowian-type metamorphic conditions and with N–S transport direction is known along the entire Trans-Saharan belt and cannot be related in a simple way to the collision between West African Craton and the mobile belt.

The Bir Safsaf-Aswan uplift is an east-west striking major basement high in SW Egypt. It consists mainly of granitic gneisses intercalated with amphibolites, marbles and calc-silicates. It is intruded by syntectonic S-type granites and late-tectonic I-type granitoids and by alkaline basaltic plugs and different dyke generations. Data on the metamorphic evolution revealed an early high-T (∼800°C) event, followed by amphibolite-migmatite facies and then by greenschist facies conditions. The following preliminary geological evolution is proposed: the East African craton in SW Egypt was formed and metamorphosed under high T conditions in Pre-Pan-African times. The accretion of the Arabian-Nubian shield led to the ckening of the adjacent continental African crust which caused migmatization and formation of S-type granites. Late tectonic uplift resulted in a low-grade metamorphic overprint and in the formation of I-type granites at around 570 Ma. The end of the Pan-African development is documented by the intrusion of igneous dykes at 521 Ma. The Pan-African generated fracture system was frequently reactivated in the Phanerozoic, as indicated by the ages of fracture bound continental volcanic rocks (193 ± 5 Ma; 155 ± 4 Ma; 87−81 Ma).

The mafic to ultramafic rocks of the Barberton greenstone belt, South Africa, form a pseudostratigraphy comparable to that of Phanerozoic ophiolites. This Archaean complex, referred to here as the Jamestown Ophiolite Complex, consists of a high temperature tectono-metamorphic peridotite overlain by an intrusive extrusive igneous section, which in turn is capped by a chert-shale sequence. There is a complete range from komatiitic to tholeiitic compositions within single intrusive units. Crustal contamination and magma mixing is evident from field and geochemical data.

Pillow structures, ⁴⁰Ar/³⁹Ar ages and oxygen isotope analysis suggest that hydrothermal interaction with the Archaean ocean severely hydrated and chemically altered the entire simatic section during its formation. As a consequence, only a ‘ghost’ igneous geochemistry is preserved. This regional open-system alteration may have increased the MgO content of the igneous rocks by as much as 13%, and the most primitive liquids, from which the extrusive sequence evolved, were ‘picritic’ in character. Rocks with a komatiitic chemistry were derived during crystal accumulation from picritic-crystal mushes (predominantly olivine-clinopyroxene) and/or by metasomatism during one or more subsequent episodes of hydration-dehydration.

In contrast to Phanerozoic ophiolites, the Jamestown complex is relatively thin (≦3 km), which implies that locally at least the ca 3.5 Ga oceanic crust was also thin. This is consistent with the regionally extensive metasomatic alteration, and is compatible with theoretical and experimental models predicting higher Archaean heat transfer from the mantle concentrated within Archaean oceans.

A preliminary account of Albian and Cenomanian ostracods from Tocqueville (Rass el Oued), North Algeria, agrees well with the known faunal homogeneity between North Africa and the Middle East and, consequently, with the distinct heterogeneity between the latter two regions compared with southern Europe, especially southern France and Spain. The Albian and Cenomanian ammonite essemblages comprise species of wide geographical distribution, with a certain concentration to the central reaches of the Tethys.