Journal of African Earth Sciences (1983)最新文献

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 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.

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 Nigerian basement consists of Eburnean granitic and metamorphic rocks into which are folded Upper Proterozoic supra-crustal low grade metasedimentary and metavolcanic rocks forming N-S elongate belts. Pan-African granitoids mark the last major event and they have intrusive and/or tectonic relationships with the earlier units. Structural features are often complex. The Basement can be divided into the Eastern and Western Provinces, where the latter is marked by the occurrence of a variety of metallogenetic types and greater development of the supracrustal N-S belts, while the Eastern province is dominated by Pan-African granitoid rocks and a paucity of metallic mineral occurrences.

The principle metallogenetic types that are (or have been) exploited are iron deposits, gold vein mineralization and SnTaNb-bearing pegmatites. Small occurrences are known of chromite, asbestos, CuNi sulphides, manganese and a variety of industrial minerals including talc, kyanite and magnesite. Metallogenetic types found in other Pan-African terrains that are markedly absent in Nigeria include basemetal deposits of plate accretion type (e.g. Saudi Arabia) and uranium (e.g. of Rossing type). It is suggested that Nigeria's relatively low metal production from basement deposits is attributable to a regional low metallogenetic inheritance which in turn has discouraged intensive prospecting but there is good potential especially for industrial mineral deposits.

New geochemical and RbSr isotopic age data are presented for three suites of plutonic rocks from the central and western parts of the Wadi Shuqub quadrangle. The syn-tectonic diorite-tonalite are the products of early arc magmatism and the post-tectonic biotite-granite is the result of intra-arc plutonism. These plutonic suites have an intrunsive relationship with the layered metavolcanic and metasedimentary rocks of the Baish and Bahah groups, which have contributed to the development of an oceanic crust.

The syn-tectonic tonalite, which forms a large batholith of tonalite-quartz diorite in the western part of the Wadi Shuqub quadrangle, yields an age of 854 ± 10 Ma (2σ) with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70159 ± 0.00004 and an MSWD value of 1.97. The late-tectonic diorite-tonalite stock yields an age of 815 ± 13 Ma (2σ) with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7029 ± 0.00005 (2σ) and an MSWD value of 1.81. The post-tectonic suite of biotite-granite from the Gabalat area defines a poorly fitted Rb-Sr isochron age of 552 ± 20 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.71327 ± 0.003 (2σ) and an MSWD value of 4.47.

The syn- to late-tectonic tonalite and diorite-tonalite are of low-K calc-alkaline affinity and are characterized by low abundances of K, Rb/Sr, and high Sr, CaO, MgO, TiO₂, and FeO^t (total Fe as FeO is denoted as FeO_t. These two suites with low initial ⁸⁷Sr/⁸⁶Sr ratios are likely to be derived from a mantle differentiation or partial melting of the basic rocks above the subduction zone. The post-tectonic biotite-granite is enriched in SiO₂, K, Rb, Rb/Sr and depleted in Sr, CaO, MgO and FeO_t and is of dominantly high-K calc-alkaline affinity. This suite of biotite-granite with high initial ⁸⁷Sr/⁸⁶Sr ratio appears to have been derived from lower crustal material. Limited trace element data on diorite-tonalite and biotite-granite show that the biotite-granite is enriched in Rb, Pb, Zn, Sn, U, Th, Zr, and Y and depleted in Sr, Ba, Cu and Ni compared with diorite-tonalite.

Only specific desert environments are significant suppliers of the millions of tons of dust blown out annually from the Sahara to its fringes and across the seas. The major dustsource is weathered debris detrained by fluvial transport and redeposited on alluvial fans, in wadis and terminal basins. Rocky gravel and wet sebha surfaces are the only effective dust traps in the desert. Thick loess mantles accumulate on the fringes of the desert after medium or long distance transport where the rate of accretion exceeds 50 g m⁻² per year over long a period. At lower deposition rates the dust becomes incorporated and assimilated by the local ground soil (or ocean sediment). Such additions have affected distant areas in southern Europe and Africa and have contributed to their fertility. More desert loess-derived or affected soils are likely to be recognized in the future.