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

Audiomagnetotelluric (AMT) soundings have been made at 14 sites along a 7 km profile in the Tonkouto-Léoba area across Early Proterozoic formations of the Kedougou Inlier in Eastern Senegal. The purpose of the study was to gain information about the electrical properties of the Mako Supergroup. The results, shown in the form of apparent resistivity pseudosections, indicate marked anisotropies for orthogonal components recorded at all sites. Consequently, two-dimensional (2-D) modelling is required to establish the electrical characteristics of the major geological units. The modelling results indicate resistivities in the range 20 000– 100 000 Ωm for the Early Proterozoic units which extend to depths of 18 km. A major discontinuity characterized by a resistivity of 1000 Ωm may arise from a structural change associated with the southeastern boundary of the Kakadian batholith. Beneath the highly resistive formations, a sharp fall of resistivity has been detected, which may be due in part to the presence of trapped fluids.

Paleocene palynomorphs are hardly extracted from the uppermost Dakhla Formation succession and the lowest Esna Formation rocks in two surface sections, northeast Kharga Oasis, Egypt. The identified palynomorphs are mainly dominated by small-sized angiosperm pollen. Foraminiferal test linings are abundant. Smooth land-derived spores and marine dinoflagellate cysts are rare, the latter are always distorted and not easy to identify. Gymnosperm pollen are never observed. Due to poor and less diverse palynomorphs, the dating of the studied section is controlled by previous foraminiferal studies. However, such palynofloral assemblage confirms the already known open marine depositional environment.

Rare metal pegmatites of the Cape Cross-Uis pegmatite belt (Namibia) containing sub-economic to economic concentrations of Sn, Nb, Ta, Li, occur in three main pegmatite swarms. Unzoned rare metal pegmatites are characterised by disseminated cassiterite mineralisation and varying Li, Ta and Nb concentrations, whereas zoned and poorly zoned pegmatites of the LI-rich rare metal type, beside low grade cassiterite mineralisation, contain higher Nb, Ta and Li concentrations (500 ppm Nb; 800 ppm Ta average). Occurrences of both types are confined to areas of extensive shearing in Pan-African greenschist facies metasediments within the tensional environment of a half-graben structure.

Rb/Sr isotope studies of various mineral phases in zoned and unzoned pegmatites in the study area indicate a late Pan-African emplacement age of 490 Ma for the pegmatitic melts. Petrogenetically, high ⁸⁷Sr/⁸⁶Sr initial ratios reflect partial melting of crustal material (basement rocks). Rare metal pegmatites in the study area are believed to have originated at the peak of regional metamorphism. This is indicated by higher metamorphic grades towards the Atlantic Ocean, which is combined with an increase in Li, Nb and Ta concentrations.

The Debre Zeyt volcanic district is located about 40 km southeast of Addis Ababa, on the western shoulder of the Ethiopian Rift. Two main phases of volcanic activity are shown to have occurred in the area, both younger than the main episodes of rifting. Eruption of acid lavas and pyroclastics, between 4 and 1 Ma, brought to the formation of the central volcanoes of Yerer, Bede Gebabe and Zikwala. Younger activity generated basaltic cinder cones, maars and lava flows, all aligned along the main rift direction. A few intermediate and acid volcanics are associated with the younger basalts.

Petrological and geochemical investigations have shown that the rocks from central volcanoes consist of peralkaline rhyolites and trachytes with a few trachyandesitic lava flows. All these rocks have high concentrations of Rb, Zr, LREE and other incompatible elements, and variable Ba and Sr contents. Measured Nd isotopic ratios are close to the bulk earth value, whereas Sr isotopic ratios are very variable, due to the combined effects of the high to extreme Rb/Sr values, variable ages and, possibly, different initial isotopic signatures. Basalts range from transitional to weakly alkaline in composition and display relatively homogeneous incompatible element contents and Nd and Sr isotopic ratios. Incompatible element ratios such as Ba/Rb are very variable in the basalts. The younger acid rocks have a large range of incompatible elements concentration with some rhyolites displaying very low values of some hygromagmaphyle trace elements (HYGE), such as Zr, Nb and LREE. The younger intermediate rocks have comparable HYGE contents as the basalts and define linear trends between basalts and low-HYGE rhyolites on several interelement variation diagrams.

Geochemical modelling indicates that the major and trace element composition of the acid rocks from central volcanoes can be satisfactorily explained by a derivation from basaltic parents by fractional crystallization. Nd-isotopic ratios which, for the largest part, fall within the range of younger basalts suggest that, except for the Yerer rhyolites, the assimilation of the upper continental crust did not play a major role during magma evolution.

The relatively constant HYGE contents of basaltic and intermediate younger volcanics exclude an evolution by fractional crystallization for this suite. The linear trends on inter-element diagrams suggest that mixing processes between basaltic magmas and an acid rock or liquid may be responsible for the generation of the intermediate rocks. This hypothesis is supported by Ba/Rb vs. Rb relationships revealing hyperbolic mixing trends between acid and basaltic end-members. This process also generated important geochemical variations within the basalts.

In Lake Malawi, ferromanganese nodules and oolites occur in oxic areas at water depths of 80 to 160 m at the facies boundary between sands and muds. In most cases, the nodules have been found at the water/sediment interface. The nodules and micronodules range in colour from dark brown to rusty or yellowish. Some tend to be of metallic lustre on the outside. The size of the nodules ranges from about a millimeter to about a centimeter in diameter while the average is 3 mm.

Three main mineral species have been identified in Lake Malawi nodules: 1) manganite (λ-MnOOH); 2) geothite [∝-FeO (OH)] and 3) vivianite [Fe₃ (PO₄)₂. 8H₂O].

A likely origin of Fe and Mn that form the nodules in Lake Malawi is from tthe sediments themselves. Another possible source can be through the movement of deep waters from the geothermally active areas of the lake to the shallow oxic areas where nodules are presently forming.

The vivianite in the vivianite-rich nodules originates from dissolution of apatite (from fish debris) within the sediment and redeposition as Fe-phosphate under reducing conditions. Studies of the vivianite-rich micronodules using a scanning electron microscope (S.E.M.) and an electron microprobe have shown that in some samples, vivianite is associated with nontronite [(Ca Mg).5Fe₂(Si Al)₄O₁₀]. The nontronite occurs as a coating around the vivianite.

Small-scale horsts and grabens, formed prior to the deposition of the Lower Cretaceous Kurnub Sandstone, are described for the first time from remote exposures near the eastern shore of the Dead Sea, Jordan. The grabens preserve a sequence of Permo-Triassic and Triassic rocks (Umm Irna and Ma'in Formations), flanked by horsts of Cambrian sandstone (Umm Ishrin Formation). The pre-Cretaceous faults are mostly orientated NNE or ENE, with a maximum vertical displacement of 130 m. Rotation of a joint system, formed during the late Palaeozoic, in the Cambrian sandstone indicates that the horst blocks were rotated anti-clockwise during block faulting. Local and regional stratigraphical evidence suggest that faulting occurred during an extensional tectonic regime in late Jurassic to early Cretaceous times. The horsts and grabens did not affect sedimentation of the basal Kurnub Sandstone and they are preserved without relief below the unconformity, suggesting a period of erosional peneplanation prior to deposition of the fluvial Kurnub Sandstone. The structures probably represent an extension of the Central Naqab-Sinai fault zone which was displaced by sinistral shear along the Dead Sea-Gulf of Aqaba Rift in Tertiary (Neogene) times.

Lithofacies sequences in the Umm Irna Formation exposed in the grabens comprise, in ascending sequence, braided to low-sinuosity or meandering fluvial siliciclastic facies, derived from the Arabo-Nubian Shield to the south and south-east. Lithological characteristics, bedforms and pedogenic features suggest a fluctuating, seasonal, semi-arid climate. Transgression of the Tethys Ocean in early Triassic times resulted in deposition of shallow-marine siliciclastics and carbonates (Ma'in Formation) in subtidal to intertidal environments.

The sequence stratigraphic concepts are applied to the Neogene “post-nappes” basins of the Cap des Trois Fourches area. It allows to correlate sequences and unconformities through this key area and reconstruct the development of the Atlantic-Mediterranean gateway through the South Riffian strait for the past 10 Ma. During this period of time, the sediment accumulation which was controlled essentially by sea level changes, have recorded tectonics and volcanics pulses. The Atlantic-Mediterranean gateways were cut off during two short period of times of 200 to 500 kyr that occurred during the Upper Tortonian and the Upper Messinian, respectively. The present day situation began during Upper Pliocene time.

Drill hole CY-4 in the Cyprus Crustal Study Project of the International Crustal Research Drilling Group is the deepest hole at present in the intrusive sequence of the Troodos opiolhite. It intersects diabase dykes, isotopic and layered gabbros and cumulate ultramafic rocks before terminating in serpentinized periodotites at a depth of 2263 m. Studies of the alteration of the sequence shows that the diabase dykes have been transformed to greenschist-facies grade spilites characterized by albite, actinolite and chlorite. Amphibolite facies grade metamorphic rocks are encountered in the plutonic complex, while the cumulate ultramafic rocks have been extensively serpentinized. Alteration throughout the complex appears to be strongly influenced by fracture porosity. Fluid inclusions in quartz from veins and alteration zones indicates the presence of two distinct fluid types. In the sheeted dykes, fluids of seawater and near seawater salinity dominate, while in the plutonic sequence hypersaline fluid inclusions with daughter crystals of halite and calcium chloride are dominant. Mineralogical and spatial evidence suggests that the hypersaline fluids are a by-product of the extensive serpentinization in the ultramafics of the ophiolites and depletions of ¹³C suggestive of the influence of biogenic carbon. The study suggests that the ophiolite cooled by the convective circulation of seawater through it, with the circulation cells eventually evolving off- axis into two distinct systems, an upper system dominated by the circulation of seawater, and a lower cell strongly influenced by the serpentinization of the deep level peridotites.

The Zwartwater Suite is intrusive into the De Kraalen Formation, a suite of early Archaean supracrustal rocks, which consists of a sequence of predominantly meta-sedimentary rocks, comprising quartz-magnetite banded iron-formations, calc-silicate gneiss and quartzite, interlayered with subordinate meta-volcanic rocks. The sill-like nature of the Zwartwater Suite results in a concordant interlayering of the Zwartwater Suite and the De Kraalen Formation.

Sills in the Zwartwater Suite have compositions which range from amphibolite, hornblendite and pyroxenebearing hornblendite to olivine-bearing hornblendite and serpentinite. Relic olivine crystals are presenved locally in serpentinite. The rocks are typically equigranular and medium- to coarse-grained. Relic textures indicate that they has a cumulate origin. The Zwartwater Suite, De Kraalen Formation and sorrounding granitoid rocks are disrupted by numerous closely spaced NW trending faults which prevent correlation of individual units for more than 100 m along strike.

Geochemical data indicate that compositional variations in the Zwartwater Suite result from differentiation controlled by the crystallisation of olivine and clinopyroxene, with minor orthopyroxene and plagioclase crystallisation. Reversals in chemical trends result from either a repeated influx of primitive magma and/or convective overturns. Incompatible element ratios are similar to those of layered complexes developed in the Barberton Sequence.

The Okavango alluvial fan occurs in an extension of the East African Rift system. Changes in water dispersal patterns on the fan have been attributed both to neo-tectonic activity and to sedimentation. Analysis of SPOT satellite imagery in an area where major changes in water distribution are currently taking place indicates that these changes are initiated by neo-tectonic activity, which creates interconnected graben systems which divert water flow. The process of redistribution is accentuated by sedimentation in distributary channels. The graben systems arise due to East-West extension which has produced intersecting, distributary channels. The graben systems arise due to East-West extension which has produced intersecting, conjugate fault sets. Analysis of associated seismic activity indicates that seasonal flooding has no influence on seismicity, which is due entirely to crustal extension.