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

Discrete ash horizons in Holocene sediments from northern Lake Malawi provide evidence of six eruptive episodes within the nearby Rungwe Volcanic Field between c.9000-360 BP. Rare earth element (REE) analyses show the ash layers to be strongly enriched in La, Ce, Pr, Nd, Sm, Tb, Dy, Er, Tm, Yb and Lu, with low Eu/Eu∗ and high La_N/Sm_N values, relative to the surrounding muds. Mixing calculations suggest possible affinities between the Rungwe ash emissions and silicic volcanics from other important Quaternary centres (e.g. Naivasha) with respect to HREE geochemistry. The LREE spectra are less comparable and may indicate a less fractionated ash assemblage for Rungwe Field. In the absence of clear in situ evidence regarding the timing and frequency of Holocene eruptions at Rungwe, the Lake Malawi sediments may prove a valuable reconstructive tool. However, the direction and extent of ash dispersal is strongly controlled by wind/climatic factors and the retention of a complete record at any single location is unlikely.

The Upper Proterozoic lagoon of the Schisto-calcaire Cycle (West Congolian Supergroup) of the Nyanga-Niari basin (Congo, Gaboon) and the western end of the Comba basin (Congo), corresponds to internal shelf deposits isolated from the open marine environment by an oolitic and locally stromatolitic continuous barrier. In the East, this confined environment communicates with the open marine environment by a high followed by shelf deepening to the North East, towards the Congolian basin.

The interpretation of the outcrops in the basin from drilling data leads to interpret very peculiar mixed facies of dolomites and limestones as lagoon of deposits with evaporites. Sequence of the internal lagoon show calcareous or marly, detritic, bioclastic or stromatolitic basal unit bearing copper mineralizations.

The proliferation of algal mats, dolomite and evaporite deposits must be compared with the present limestone sedimentation in confined lagoons.

Gebel Ghorabi is located at the extreme northern end of Bahariya Oasis and the mineralized area covers an area of about 2 km². Geologically, the iron ore is composed of random alternations of three main yellow, brown and dark brown colored bands. The former band is relatively thicker than the others. The iron particles range in size from a (pisolitic ⪢) fraction > 2 mm to earthy (the so-called pisolites may grade down from coarse to about 0.25 mm in size).

A bulk sample corresponding more or less to the yellow iron ore band contains 54.16% Fe₂O₃, 26.13% SiO₂ and 5.39% Al₂O₃ as major components.

Technologically, the combined effect of a thermal reduction and of a quenching shock on the crushed iron sample (-12 mm) has been investigated. This technique has been found to sufficiently enhance the magnetic properties of the iron minerals which could be easily separated by using a low intensity magnetic separator.

The maximum severance of quartz grains from the reduced iron oxides was reached for samples subjected to a slow heating at 700°C for 60 min. and to a fast cooling by quenching in water.

A flow sheet for handling the yellow iron ore is here proposed to produce a magnetic iron concentrate with 87.63% Fe₃O₃ and 1.40% SiO₂, and with a recovery of 97.21%.

Major and trace element behaviour during initial and advanced weathering stages in a weathering profile and saprolith developed on two subalkaline granites respectively indicated that Ca, Na and Sr are preferentially removed. Fe²⁺, K and Y are also lost, followed by P, Mg, Rb and Si. Zr and Nb remained constant relative to Ti. Mn, Fe³⁺, Al, Ga and Th initially increase eventually decreasing during advanced stages of weathering.

U, Th and the REE behaviour could be related to the primary and secondary mineralogy of the granites. The HREE are removed in solution, while the concentrations of LREE are significantly increased relative to HREE during early weathering stages. Ce is initially slightly depleted relative to other LREE, but positive Ce animalies develop during advanced stages of weathering in the profile. Previous hydrothermal alteration of the granite is largely responsible for the behaviour of REE, U and Th during weathering. U and Th values increase with increasing oxidation. U correlates with Fe³⁺ adn Th with Mn and Fe³⁺.

The cartographic description, based on new observations, shows that the Nekor major fault is constituted of several segments which had an independent evolution in space and time. The stratigraphic, structural and tectonometamorphic correlations recently made in the central and eastern Rif show a perfect correspondence between the two Subrif blocks, separated by the Nekor fault. Therefore, the Nekor fault cannot be considered as a typical transform zone, nor as a mobile zone of major paleogeographic importance. The attempt of correlation between the seismicity and the brittle structures related to the Nekor fault, shows that the remobilization of the north-south second order faults is responsible of the present-day seismicity in the central and oriental Rif. This remobilization is consistent with the North-South compressional stress field linked to the convergence between Africa and Europe plates.

Microgranites and rhyolites of the Edough complex, Annaba, NE Algeria are described. Their petrology and geochemistry indicate that these calc-alkaline, probable Tertiary rocks, evolved by feldspar-dominated fractional crystallization of a basic magma. Trace elements and isotopic ratios of Sr (⁸⁷Sr/⁸⁶Sr_16.0 = 0.71947−0.72404) and detailed petrogenetic modelling exclude the granites from being produced solely by simple crystal fractionation of a basaltic magma and the similarities of these ratios to those of the Edough biotite gneisses (⁸⁷Sr/⁸⁶Sr_16.0 = 0.71840−0.73008) confirm a genetic link. We conclude that the Edough Tertiary magmatic rocks and perhaps many of the Tertiary igneous rocks of NE Algeria were produced by crystal fractionation of mantle-derived basaltic magma which subsequently assimilated substantial amounts of crustal material of the type Edough biotite gneisses.

Contrasting structural, textural, fabric and mineralogical features between an Early Proterozoic plutonic migmatite suite and granitoids of the Dixcove-type from southern Ghana have been attributed to their contrasting geological histories, despite the fact that they are coeval, cogenetic and have similar geochemical properties. The migmatites occur as strongly foliated, banded, medium- to coarse-grained rocks and display shear and swirled structures. In contrast, the Dixcove granitoids are weakly foliated, homogeneous, fine- to medium-grained porphyritic rocks associated with volcanic rocks and show extensive hydrothermal alteration.

There is no observable variation in P-T regime within the migmatite terrane exposed in southern Ghana, yet different geothermobarometers applied to constrain their conditions of formation consistently indicate that the migmatites were emplaced at relatively deep crustal levels, in excess of 5 kbars, compared to the Dixcove granitoids which crystallised under relatively shallow conditions, generally less than 5 kbars.

These results suggest that the differences between the migmatites and Dixcove granitoids might have been caused by their different depths of emplacement, rather than deformational or bulk compositional differences as previously thought. These data provide significant constraints to any model for the evolution of these rocks and, indeed, the early Proterozoic terrane of the West African craton.