Journal of African Earth Sciences (and the Middle East)最新文献

Ferromanganese nodules from Lake Malawi have been analysed for 13 rare earth elements by ICP mass spectrometry. Total REE levels are considerably higher than previously reported for lacustrine or continental margin nodules and shale-normalised profiles display many characteristics commonly associated with deep ocean concretions. While shale-normalised REE values for the nodules exceed 1.0 across the entire REE spectrum, the surrounding lake muds are enriched only in LREE. Significant fractionation, involving the preferential assimilation of HREE during nodule growth is thus inferred. The nodules are also characterised by positive Ce and negative Tb anomalies, generally consistent with REE incorporation through redox-mediated exchange processes.

The study of lava and pyroclastic formations from Odienné showed the calco-alkaline character of the volcanism in that region and revealed the structural context of its origin. It appears that, contrary to the general idea which links the calco-alkaline volcanism to the subduction zones, the Odienné volcanism is associated with the shearing constraints along Sassandra fault. The intensity of those constraints was such that it met the temperature and pressure conditions of a subduction zone.

The lithofacies and planktic foraminiferal content of the Oligo-Miocene interval penetrated by four wells (El-Temsah-2, Boughaz-1, Mallaha-1X and San El-Hagar-1) drilled in the Isthmus of Suez and in the eastern reach of the Nile Delta enable to clarify the chronostratigraphy and sedimentological and tectonic history of the studied sequence.

The Oligocene is represented by the Qantara Formation which is conformably overlain by the Aquitanian-Burdigalian (early Miocene) Rudeis Formation. The early Miocene is separated from the middle Miocene (Langhian-early Serravallian Kareem Formation) by a hiatus, probably reflecting the mid-Clysmic tectonic event that affected the Gulf of Suez region.

A stratigraphic gap between the early and late Serravallian Kareem and Serravallian to Tortonian Sidi Salim Formations may have been caused by the uplift of the isthmus of Suez which induced the beginning of evaporite sedimentation in restricted basins in the southern part of the Gulf of Suez. The Messinian is either represented by a regressive sequence of coarse siliciclastics known as the Qawasim Formation or by evaporites.

First heat flow density determinations from southeastern Zaïre are presented. Sites are located in the late Proterozoic metasedimentary cover of the Pan-African belt (600 Ma.). For each individual boreholes, heat flow ranges between 48 and 72 mWm⁻². The average value of 62 mW m⁻² for the sites is similar to that of 66 mW m⁻² observed in Zambia. Both values are higher than what is expected for Pan-African terranes. These heat flow determinations in Shaba province of southeastern Zaïre, together with gravity and seismological observations, support the hypothesis of lithospheric thinning for this area. As already suggested for Zambia, this lithospheric thinning may be associated with a southwestern extension of the East African Rift System from Tanganyika across the central African plateau.

Cassiterite from four different alluvial mining localities on the Jos Plateau were investigated from the point of view of their mineralogy, colour variety and chemistry. The optical properties of each of the colour varieties as revealed by the incident light microscopic studies coupled with the variation in their chemical composition as revealed by the electron microprobe analyses indicate that the cassiterites crystallized under different physicochemical conditions. Some (black varieties) are high temperature forms, whereas others, i.e., the red and white varieties are low temperature forms. Furthermore, the chemistry of these cassiterites tends to support the age migration of the Younger Granites whereby the northern cassiterites tend to be older, richer in SnO₂ and poorer in the elements Fe, Nb and Ta than the southern cassiterites.

A whole-rock PbPb date of 3021 + 47–49 Ma, found for granodioritic granitoid occurring within biotite gneiss in the Sandoa-Kapanga region of the Kasaï Craton, southern Zaïre, in reflected in single zircon evaporation data (centered around 3014 ± 2.3 Ma) obtained from the biotite gneiss itself. Other zircon evaporation dates found in the biotite gneiss range from ca. 3.0 Ga to 3.09 Ga and are interpreted as minimum ages of discordant zircon phases overprinted during one or more metamorphic events. Anomalously high Pb initial ratios characterise the granitoid and consequently its ca. 3.02 Ga age, taken together with the corresponding zircon date, is considered to reflect a metamorphic event during which anatectic melting took place to produce the granodioritic magma. During the same event, overgrowths formed on older zircon cores. The ca. 3.02 Ga date is the oldest yet recognised in the southern Kasaï Craton and a correlation is suggested with the D₂ metamorphic event of this region. Additionally a prehistory back to at least 3.09 Ga is indicated by the other zircon dates in the biotite gneiss.

A model of metallogenesis for the carbonate-hosted base metal deposits of the Otavi Mountain Land (Namibia) is proposed based on stratigraphic, mineralogical, metal association, Pb and S isotope systematics and fluid inclusion data. Three mineralising episodes are recognised. The first involved low-temperature and high-salinity basinal fluids, which formed the Berg Aukas-type deposits of Mississippi Valley-Type (MVT) affinity. These are characterised by a Zn-Pb metal association. The second episode appears to have involved Cu-rich fluids having higher temperatures and lower salinities of possible metamorphic origin. These formed Tsumeb-type deposi ts, characterised by a metal association of Cu-Pb-Zn-Ag, locally accompanied by Fe-Mn oxides and silicates. The third and last episode is related to recent weathering, which resulted in the precipitation of V-rich minerals.

Published U-Pb and Pb-Pb zircon dates from pre- to late-tectonic granitoids of the Kibaran (Grenvillian) Natal Metamorphic Province fall within the range ≈1200 to ≈1040 Ma. A major problem regarding the duration and timing of magmatism in the Natal belt has been the age of the youngest intrusive events. This is important within a Gondwana context, especially with respect to the nature and distribution of the Pan-African event in this area. It is known that the youngest magmatic rocks in Natal are certain undeformed, cross-cutting microgranite dykes (Mbizana Microgranite) from the Margate Terrane. These dykes have previously given a Rb-Sr whole-rock date of ≈950 Ma, showing them to be broadly late Kibaran in age. This paper presents new U-Pb data from four zircon fractions from the Mbizana Microgranite, which give a date of emplacement of 1026 ± 3 Ma. Consequently, this date is considered to define the latest magmatic event in the Natal Metamorphic Province and points to the timing of the cessation of Kibaran magmatism in this area. The data supports recent suggestions that there was no Pan-African magmatic activity in Natal. The ≈950 Ma Rb-Sr dates previously obtained for these rocks are re-interpreted as representing the time at which the dykes cooled through the Rb-Sr biotite and K-feldspar blocking temperatures.