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

Field study of the granitic rocks in the Wadi Risasa area, Southeastern Sinai, Egypt, shows that they comprise three granitic groups namely the older granite, younger granites (phases II & III) and alkali granites. Petrographically, the older granites are mainly granodiorite, while the younger granite and alkali granites are granite. Major and trace element data reveal that the older granites are mainly calcalkaline whereas the younger granites show a gradual increase in alkalinity from phase II to phase III. The granites of phase III are alkaline while the alkali granites are peralkaline. The older granites are formed at high to intermediate vapour pressure, while the younger granites are formed at intermediate to low vapour pressure. Tectonically, the older granites and younger granites are classified as I-type granites which are related to orogenic granite evolved in a volcanic arc. The alkali granites are emplaced in anorogenic belt which reflects continental epeirogenic uplift. They are classified also as A-type granite, originated in a within-plate environment.

Heavy mineral concentrates from stream sediments in the Anka schist belt of NW Nigeria were examined for their ilmenite, magnetite and gold contents. A spatial relationship was found between samples with high ilmenite: magnetite ratios and gold anomalies. The ilmenite may have formed at relatively low temperatures from ulvo-spinel-magnetite exsolution at the same stage of gold mobilization.

Birimian units of the Beposo/Bomfa district of Ghana lie across the western boundary of the Ashanti volcanic belt. The established Upper/Lower Birimian stratigraphic contact (or volcanic belt/sedimentary basin boundary), has been redefined as a fault zone, part of a steep, NE trending, brittle-ductile shear zone. Siliciclastic lithofacies predominate, with meta-argillites spanning the boundary. Volcanics are absent. Along-strike discontinuity of the Upper Birimian volcanic belt as a stratigraphic unit is emphasised. Arkosic basin-fill sediments were derived from a granitic source to the west. An extensional, fault controlled intracratonic rift is inferred as the setting for the deposition of Birimian units.

Rift closure was enacted by the Eburnian Orogeny, ∼2000 Ma.NW-SE directed shortening produced regional F₁, N40°E trending folds. Subsequently, cleavage parallel, lateral slip along pre-existing crustal weaknesses produced dextral shearing and NW trending, F₂, flexures and folds. Repeated reactivation and flushing of faults by mineralised fluids produced quartzitic tectonite units in which mylonite textures predominate. Late stage F₃ folding about WSW-ENE axes was post-dated by late orogenic, low angle thrusts (D₄). Tarkwaian sericite quartz schists and granodiorite plutons have been affected by all phases of deformation.

Three “younger granite” plutons from the Eastern Desert of Egypt are studied: petrographic and geochemical characteristics of the barren pink granites at Wadi Sikait and Wadi Nugrus are similar, of alkaline, mildly peraluminous nature and are enriched in LIL-elements and LREE with moderate negative Eu anomalies. In contrast, the Sn-Ta-W-bearing albite granite of Abu Dabbab is alkaline, peraluminous muscovite granite; its chemical specialization is manifested by the pronounced enrichment in Ta, Sn, W, F, Rb and Li coupled with marked depletion in Ca, Ti, Mg, Sr and Ba. Elemental ratios (e.g., K/Rb, Rb/Sr, Ba/Rb) discriminate the albite granite and the pink granites into “mineralized and barren granites”, respectively.

The albite granite is derived from Na-rich magma of within-plate characteristics. Fluorine was an important complexing anion during magmatic evolution history. The albite granite is emplaced at shallow depth (<100 MPa) and at the intersection of structural weaknesses. The pink granites might have a crustal and/or LIL-element enriched mantle sources, in which the subduction-related fingerprints are partly obliterated. For both types, reactivation of regional structures played a significant role in magma generation.

Acid metasomatism is mainly manifested by the development of thin greisen veins along fracture systems in the albite granite. The chemistry of greisenization using mass balance approach reveals that the process is accompanied by dramatic increase in SiO₂, Fe₂O₃, MnO, F, Sn and Li as well as significant loss in Na₂O, K₂O, Ba, Nb and Zn. The process causes a significant increase in volume (30%). Changes in chemical components are consistent with the observed mineralogical changes. Microprobe results reveal that the wolframite crystals are typically huebnerite with Fe-rich cores and Mn-rich rims. Compositional variations in wolframite crystals are attributed to the physicochemical conditions (pH, T, etc.) and chemistry of the ore-bearing fluids.

A mapping and geochemical study of granitic rocks which are exposed in the Aouli Paleozoic window, Haute Moulouya area, Morocco, has revealed the presence of four distinct facies: (1) a granodiorite, which is the most mafic member of the complex; (2) a medium-grained grey granite; (3) a pinkish, medium-grained homogeneous granite; (4) a leucocratic muscovite granite. The four facies are calcalkaline to subalkaline in nature - none of them is truly alkaline as had been suggested previously. The available data show that the granodiorite and the muscovite granite are geochemically distinct. They also suggest that the pinkish facies evolved from the grey facies by fractional crystallization. The horizontal to subhorizontal spatial distribution of SiO₂ contours and observed field relationships both suggest that the grey phase is related to the pink phase by multiple injection supporting the hypothesis that the whole complex was emplaced into a pre-existing horizontal shear zone. A late-orogenic to post-orogenic tectonic environment of emplacement is suggested based on a number of discriminant diagrams.

At Leru-Okigwe, the Enugu-Port Harcourt expressway cuts through the Campanian Nkporo Shales which pass upwards into the cyclic, ripple laminated sandstones and shales of the Lower Maastrichtian Mamu Formation. The Mamu Formation at this locality consists of a 60 m thick shale-sandstone sequence with the basal and middle part of the section consisting of a total of 9 carbonate units. These carbonate units vary from 10 to 70 cm in thickness, cyclically interbedded with shales and are overlain by coarsening upwards sandstone bodies. Detailed mapping and petrographic studies indicate that the carbonate units are divisible into a lower finely laminated mudstone which passes upwards into oolitic packstone/grainstone in the middle and is overlain by an upper set of laminated mudstones. The lowest mudstone unit (dark grey to greenish rock) is finely laminated, pelleted, oncolitic and sparsely fossiliferous. The oolitic packstone/grainstone consists of oolites cemented together by siderite microspar. Identifiable bioclasts include tests of small size benthic foraminifera, pelecypods and rare ostracod carapace. This unit attains a maximum thickness of about 70 cm. The upper mudstone units consist essentially of uniformly recrystallised siderite microspar. Intraclasts include micritised pelecypod fragments and small foraminifera tests. Ovoid, flat bottomed and biconvex vugs developed good geopetal structures in the mudstone.

Petrographic, Xray diffraction and microprobe analyses indicate that the carbonate constituent in these units consists of solid solutions of FeCO₃MgCO₃CaCO₃ and minor MnCO₃. Sideritization, the dominant replacement process has led to the recrystallization of the micritic matrix and microcrystalline siderite is commonly associated with goethite, chamosite relics and quartz.

The carbonates with associated chamosite are thought to have formed in a shallow marine subtidal to intertidal environment developed during periods of rise and fall in sea level. Formation of chamosite-bearing oolites record periods of increasing wave energy corresponding to storm conditions between quiet shallow marine sedimentation. At least five diagenetic stages involving micritization, dissolution of the primary chamosite, replacements of chamosite by siderite cement, growth of blocky calcite and a continuing replacement of the preexisting minerals by goethite were establised from textural and compositional evidence.

The recognition of shallow marine subtidal to intertidal environments for carbonates at the Mamu/Nkporo Formation transition supports continuous marine influences with periodic subaerial exposure of sediments in the Mamu deltas after the deposition of the prodeltaic Nkporo Shales.

Audiomagnetotelluric (AMT) soundings were made on two profiles traversing a metamorphic basement-sedimentary transition zone on the eastern margin of the Douala basin in southern Cameroon.

The sounding curves and pseudosections of both profiles suggest the existence of two blocks of different lithologies.

Also the interpretation of the AMT data reveals a distinct electrical conductivity discontinuity on both profiles for each of the two orientations of the telluric line. This discontinuity marks the metamorphic basement-sedimentary contact.

On one profile, some stations correlate well with each other, but not with one of them which presents singular characteristics.

There is evidence of a conducting layer in the entire sedimentary section (5 Ohm.m resistivity) of thickness about 100 metres and the variability of its depth can help us to evaluate the dip of the sedimentary deposits.

A close examination of the transition zone suggests vertical tectonic movements of several hundred metres.

Using the base of the supposed basal sandstone layer as a reference horizon, we have estimated: the dip of the sedimentary syncline to be 5%; the top of sandstone layer to have a dip of 3%; the thickness of the basal sandstone to vary between 500 to 850 m and calculated the total thickness of the sedimentary deposits to be about 1100 m.

The high electrical conductivity in the sedimentary section demands further soundings in the lower frequency range (10⁻² to 10 Hz) required in deep MT exploration work.

Three lithostratigraphic units are mappable in the eastern Bida Basin. They include from top to bottom the Agbaja Ironstone, Patti Formation, and Lokoja Sandstone. Thirty one palynomorph from species are described from the Patti formation. The Palynomorph assemblage indicates a Campano-Maastrichtian age (Upper Cretaceous) age. The Patti Formation has therefore been assigned a campano-Maastrichtian age while the underlying Lokoja Sandstone and the overlying Agbaja Ironstone both of which are barren of fossils, are either Campano-Maastrichtian or older, and Campano-Maastrichtian or younger respectively.

An isolated outlier of red sand, in an incipient stage of diagenesis, is described from the northeastern Cape, South Africa. The deposit is of significance because it is unique in the locality, and is not a product of current geomorphic processes. The locality lies above the Great Escarpment, between 1800 m and 1900 m altitude, at latitude 31°23′S; longitude 26°41′E. The lithology has similarities with Kalahari type sands from the northwestern Cape. A thermoluminescence date of 20500 B.P. places the depositional event in the late Pleistocene. The deposit is attributed to strong aeolian activity during the late Pleistocene hypothermal.

Statistics on geophysical activities in Nigeria for the ten-year period (1975–1984) was compiled mainly from questionnaires distributed to government and private agencies, oil and mining companies, and universities which engage in groundwater, petroleum and mineral exploration, engineering and research work. Similar studies had been done worldwide by Epsey (1975, 1976, 1977) and Whitmire (1978). From the statistics, it was deduced that electrical resistivity, magnetic, seismic, radiometric, gravity, airborne and ground magnetic survey methods are the main geophysical techniques used which resulted in the discovery of some of the natural resources (oil, gas, minerals and groundwater) buried a few kilometers below the Nigerian soil. Airborne and ground magnetic surveys have been carried out by at least two government agencies, namely: the Geological Survey of Nigeria and Nigerian Steel Council. The compilation also reveals that a greater part of geophysical and drilling activities of operating oil companies is currently concentrated in the sedimentary basins, mainly the oil-rich Niger Delta and near offshore areas. From the available statistics, at least three companies, the National Steel Council, the Geological Survey of Nigeria, and Kano State Water Resources Engineering and Construction Agency have employed geophysical methods for engineering.