Journal of African Earth Sciences (1983)最新文献

A cross section of the Namama Thrust Belt (NTB) in Mozambique is described in some detail. The NTB is a ‘Lurian’ age (circa 1000 Ma) tectono-metamorphic feature of orogen rank. Its structure, on all scales, is dominated by a rotation of the fold axes towards the E-W transport direction, at an angle to the NNE overall trend of the belt. In connection with a high-T metamorphic development, dry assemblages with clinopyroxene and garnet grew and granite was emplaced in the belt and its foreland. The post-kinematic thermal event appears to have encompassed the time span from the Lurian orogeny to the emplacement of early-Paleozoic (‘Pan-African’) granites some 500 Ma ago. The overall geologic evolution of the NTB is discussed, and a possible sequence of events proposed.

The small isolated peaks of Kubi Algi and Derati on the periphery of the Koobi Fora basin, to the north-east of Lake Turkana in northern Kenya, are remnants of silicic peralkaline volcanic centres. Detailed geological maps of the areas of the two mountains are presented. Both are massive bodies of generally aphyric, microgranular pantellerite sharing similar petrography and chemistry. Kubi Algi shows evidence of having formed as an extrusive dome and is considered the source of local pantellerite lava flows, here designated the Il Burrka Formation. Derati mountain can best be interpreted as a denuded plug of a second extrusive centre. The volcanoes were active in the middle Miocene towards the end of a period of regional magmatism extending from late-Oligocene times. The pantellerites are holocrystalline and thus contrast with the normally glassy over-saturated peralkaline rocks from the East African rifts, including older pyroclastic pantellerites of the northern Lake Turkana region. Despite being very finely crystalline, they show mineralogical features seen elsewhere in more slowly cooled, deep-seated, peralkaline granites. A very broad range of feldspar compositions present in the rocks is explained by the interaction of groundwater with the rapidly cooling magma. Of additional interest is the abundance of aegirine, present as a product of primary magmatic crystallization and, in the Derati rock, as a hydrothermal mineral. It contains significant but highly variable amounts of titanium and zirconium, the latter broadly equivalent to typical maximum concentrations reported from peralkaline intrusive complexes.

Soil morphology, clay and primary mineral composition, soil chemistry and soil microflora were studied in a prominent soil catena on a late glacial moraine of Liki III age in the upper Afroalpine zone on Mount Kenya. Variations in the physical, mineral and chemical components of this catena are used to determine the degree to which material moves through the toposequence, and as a basis for discussion of the interrelationships involved. Soil profile expression is largely a function of changes imposed on each soil member by position in the catena and movement downslope.

The soil microflora appear impoverished in both numbers and species through all members of the catena. From the analysis of soil chemical characteristics we deduce that mineral deficiencies, particularly phosphorus, and possibly pH, serve to limit the activities of bacteria and fungi, and hence their capacity to contribute to organic matter breakdown and soil forming processes.

The geochemistry of mafic and ultramafic rocks in the Wami River granulite complex of the central part of the coastal region, Tanzania, suggests that their protolith consisted of basaltic rocks with a calc-alkaline differentiation trend. Some of the ultramafic rocks show primitive chemical characteristics including high (Mg/Mg + Fe_tot) ratios and high Ni contents. These primary magmas later evolved mainly by olivine fractionation to yield the parent magmas which cooled to form the protolith of the bulk of the mafic granulites. The effect of the granulite facies metamorphism on the geochemistry of the rocks is restricted to depletion of Nb and Rb and possibly the enrichment of Ba.

Study of the structural domains in Morocco leads to the recognition of several structural lines which represent, for the most part, limits between the different Tertiary folded belts. Some of these limits are permanent zones of weakness which acted more or less continuously from Precambrian to Present. Inherited from Precambrian orogenies, represented in the West-African shield, they were reactivated repeatedly in its epicratonic margin and mostly in the northern domains where they controlled basin sedimentation and belt deformation during the Hercynian and Atlasic orogenies.

The Miocene foraminiferal species Brizalina mandoroveensis (Graham, deKlasz, Rérat) from the Early to Middle Miocene of the Gulf of Guinea, West Africa, displays polymorphism in its pattern of ribbing, such that the normal form is ornamented with longitudinal riblets and the subordinate morph with a ladderlike pattern of struts joined in parallel to riblets. Multivariate statistical analyses based on size variables disclose subtle differences in the variability of the tests, which may be correlated with the frequencies of the two morphs in a particular sample. The relationship between sets constituted by morphological size variables, on the one hand, weighed against measures of shape, on the other, shows a significant relaxation in the level of morphological integration between sets over time.

Large deposits of good quality glass sands of the Quaternary age occur in ancient and modern river channels of the Niger Delta and constitute a major proportion of the lithologic sequence in the low-lying, swampy coastal region.

Texturally, the sands are well graded coarse to fine sand with negligible amounts (less than 5%) of gravel and fine (< 0.06 mm fractions. The granulometric, chemical and mineralogical analyses of the sands show that they are suitable for the manufacture of glass bottles and can also be used for the production of sheet glass after simple and inexpensive benefication processes. The estimated reverves are immense and can serve several large-scale glass manufacturing industries for many decades.

A plutonic complex containing both charnockitic and non-charnockitic granite rocks (Older Granites) occurs within the amplhibolite facies rocks of gneisses and migmatites in the Ado Ekiti-Akure region of southwestern Nigeria. This complex comprises three petrographic types of charnockitic rocks and three of granitic rocks. These are the coarse-grained charnockitic variety, massive fine-grained and the gneissic fine-grained types, while the granitic rocks consist of the fine-grained biotite granite, medium- to coarse-grained and the porphyritic biotite-hornblende granites.

Field observation shows remarkable close association of these charnockitic and non-charnockitic components of the complex, and also geochemical evidence provides indications for a petrogenetic link between the rocks. The coarse charnockitic rock type and the granitic rocks in the area have high K₂O levels relative to SiO₂, high K₂O/Na₂O and high FeO/MgO ratios. There are striking chemical similarities which also characterize rocks of the rapakivi suite. Comparable petrogenetic processes are therefore thought to have been in operation, and the granites of the area are linked to rapakivi granite types which could be a product of charnockitic plutonism. All the above features are reminiscent of the rapakivi granite-massive anorthosite-charnockitic rock series, the close association of whic is well documented in some parts of the world.

On the basis of trace element geochemistry, the charnockitic rocks are divided into two groups which are the ‘normal’ -LIL (large-ion-lithophile) and LIL-deficient types. The ‘normal’ -LIL type is represented by the coarse-grained charnockitic type while the LIL-deficient type is the massive fine-grained variety. The LIL-deficient variety has low REE while the ‘normal’ -LIL charnockitic rock type and the granites are enriched in REE and exhibit fractionated patterns.

Both the LIL and REE patterns are consistent with fractionation processes involving separation of LIL-deficient phases from a basic magma emplaced under high grade conditions, and the ‘normal’ -LIL rock type with the granites represent equivalents of rapakivi granites that crystallized from the residual melt at higher structural levels.

The Chilga lacustrine deposit from a small graben in the heart of the Northwestern Ethiopian Highlands (37°E, 12°N) is one of the ubiquitous intertrappean continental sedimentations on the Plateau. The basalt layer which makes the bottom of the basin has been dated as 8 Ma. The lacustrine sedimentation occurred contemporaneously with active volcanic phases in the region. Silicic aggregates are common in the cements and as important mineralogic constitutents of these phases whereas periods of calm sedimentation are characterized by thick lignite seams and the presence of authigenic minerals such as pyrite and vivianite. The sequence shows a general upward fining and evolution from shallow fluviatile to reduced lacustrine basin. The palynoflora from the sequence has a unique palaeofloral assemblage where the abundance of Guineo-Congolian-like pollen taxa, pteridophytes and absence of conifers imply a regional palaeoaltitude much lower than at present. The uplift of the Plateau at a rate of 0.1 mm yr⁻¹ similarly suggests palaeotitudes of ca 900–1000 m.

Metallurgical grade chromite ore in Zimbabwe is mined from two underground mines, Peak Mine and Railway Block Mine, in Shurugwi. Peak Mine is at present just over 800 m deep. In the search for new chromite ore bodies, such a depth limits the application of the conventional geophysical exploration tools. Exploration diamond drilling is becoming more and more an expensive resort. Alternative and effective geophysical techniques are therefore being actively sought after. The high resolution seismic reflection technique, carried out right within Peak Mine, has the potential to become a useful exploration tool.