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

A geological, petrological and geochemical study involving mineral chemistry and whole-rock major and trace element data has been carried out on the microgranular enclaves hosted in the Sinai Massif I-type granites. Enclave specimens were collected from two separated plutons along the western margin of the Sinai Massif: the northern pluton (NP) and southern pluton (SP).

Enclaves consist of monzogabbros, monzodiorites and diorites, as well as, rare monzonites, syenogabbros and syenodiorites. They are characterized by wide compositional range (48–62% SiO₂). Mineral chemistry and whole-rock chemistries both suggest that enclaves from SP and NP were derived from two to three magmas. Enclaves provide microstructural evidence pertaining to magmatic source material. The origin of present microgranular enclaves hosted in the I-type metaluminous granitoids is best explained by mixing/mingling of hot basic magmas with relatively cooler acidic magma in a plutonic environment. Substantial changes in rheological properties of magmas occur due to thermal diffusion at the interfaces of these globules and increase the undercooling rate. Magma mingling occurs instead of through mixing and homogenization. The mafic globules are dispersed randomly or concentrated into swarms by flow of the hotter magma.

The c. 2. 1 Ga sediment-dominated Boundiali-Bagoé supracrustal belt is situated in the lower Proterozoic Baoulé-Mossi domain of the West African Craton. The sedimentary units are characterised by an abundance of clastic turbiditic wackes and quartz wackes of unknown stratigraphic thickness and a 3.5 km thick sequence of poorly sorted, mass-flow paraconglomerates and metasubarkose. Volcanic assemblages form elongate bodies comprising metabasalts, meta-andesites, meta-andesitic volcaniclastics and epiclastics with minor jasperoidal metarhyolites. Contacts between the metasediments and metavolcanics are not exposed, but from structural evidence most relationships are thought to be tectonic. The volcano-sedimentary terrane is enveloped, and rarely intruded, by syn-tectonic granitoids and basic bodies. Post-tectonic granitoid emplacement marks the youngest Birimian magmatic episodes in the region. The metavolcano-sedimentary rocks and earlier granitoids were variably deformed during the Eburmian orogeny which is dominated by a generally NNESSW structural trend.

Siliciclastic metasedimentary rock units in other parts of the Baoulé-Mossi domain, namely the young, goldbearing Tarkwaian conglomerates in Ghana, have been compared lithostratigraphically to the coarse-grained clastic metasediments in the Boundiali-Bagoé supracrustal belt. However, extrapolation of the term “Tarkwaian” outside the type locality, even to detrital rocks closely analogous in terms of their sedimentary facies with the Tarkwaian of Ghana, is dangerous when little or no consideration is paid to such factors as local stratigraphic positioning. The style and timing of tectonism are controlling factors determining the occurrence and nature of sediment deposition, in terms of both the local and regional Birimian stratigraphy. Across the lower Proterozoic of West Africa diachroneity in tectonism and sedimentation leads to the formation of pockets of coarse-grained clastic sediments that occur at different stratigraphic positions during the development of individual supracrustal assemblages.

Without geochronological data, the correlation of these metasediments across West Africa is problematical and of limited significance.

A fundamental question in the evolution of the Pan-African/Brazilian domain in West Africa and Latin America concerns the degree and extent to which its continental crust was affected by the Late Proterozoic event. There are only a few ages on rocks that constitute the older nucleii within the remobilised zone. We present UPb data on zircons from the reactivated migmatite-gneissic basement in North-eastern Nigeria that constitute the first evidence of Late Archaen basement. The data can be explained by an important imprint of the Pan-African thermotectonic event resulting in considerable loss of the accumulated radiogenic lead in zircons from a 2.7 Ga. basement to give minimum upper (2.5 Ga.) and lower (550-500 Ma.) intercept ages for the gneissic basement around Toro and a better defined Pan-African lower intercept age of 599 Ma. for the anatectic Jada granite.

The young lower intercept ages are similar to available KAr and RbSr metamorphic/cooling ages and support the long interval of at least 200 Ma (>650 to 450 Ma) for the Pan-African orogeny. This lends credence to the translational hypothesis of the deformational activity from the west (near the suture) towards the east with time. These results confirm the heterogenous nature of the Pan-African event and call for a combination of methods to more precisely determine the primary age of crystallisation of the old basement in multiply metamorphosed/deformed terrains. The Eburnean (2.2-2.0 Ga.) event is not identified by the present work nor by UPb data on single zircon grains and calls for further study to properly define its spatial significance in the region.

Sedimentological and geochemical analyses have been carried out on lacustrine deposits of East Africa, at Lake Magadi (2°S, 36°E, Kenya) and at Green Crater Lake (0°S, 36°E, Kenya), to determine the parameters controlling climatic and environmental dynamics during late Pleistocene and Holocene. These sedimentary sequences were collected with a stationary piston corer.

At Lake Magadi (Fig. 1), sedimentary and geochemical control show three phases of lake level variation which corresponds to climatic change occurring during the last 40 thousand years. These phases were defined by three lithostratigraphic units.

Laminated deposits of Lake Magadi were formed during a wet period. Analysis of these laminae define two microfacies: a dark lamina, characterised by lacustrine organic matter and a light lamina enriched in detritus, carbonates (CaCO₃) and magadiite (NaSi₇O₁₃(OH)₃, 3H₂O). The formation and preservation of each couplet was favoured by climatic contrast, lake stratification and various origin of the sediments (autochthon and allochthon) in the drainage basin. Therefore a relative chronology can be derived from laminae counting and the duration of deposition of each couplet.

Spectral analysis applied on variation of the laminae thickness, shows the existence of three main periods, 4–7 years, 8–14 years and 18–30 years, respectively (Fig. 2). These cyclicites of the lacustrine environment precise former determinations established on more recent lacustrine sequences from East Africa. They are related to the global climatic cycle (quasi-biannual oscillations, El Nino Southern Oscillations and the sun spot cycles).

At Green Crater Lake, the study of the sedimentary sequence was completed by physico-chemical analysis of the waters and interface sediments which demonstrate the carbonate, sodium, bicarbonate composition and the thermal and chemical stratification of the modern lake. The sedimentary sequence is characterized by volcanic deposits overlain by physico-chemical analysis of the lake waters and interface sediments which demonstrate the carbonate, sodium, bicarbonate composition and the thermal and chemical stratification of the modern lake. The sedimentary sequence is characterized by volcanic deposits overlain by silt and clays deposited before 7400 years B.P., followed by loweing of the lake level at 3000 years B. P.

Results from lake Magadi document the occurrence of a wet period starting at about 12,000 years B. P. The methodology applied on modern Green Crater lake provides base of interpretative models for other Holocene sequence lacustrine systems of intertropical zones.

A study of planktonic and benthic foraminifera from 17 marl samples of the upper Late Miocene to the Early Pliocene from the Eastern Rabat area (Morocco) was carried out. The benthic fauna is analyzed quantitatively and sorted by a cluster analysis. The resulting faunal assemblages, together with the Planktonic/Benthic-ratio and the species richness, are attached to three different environments: an Ammonia beccarii-assemblage of the inner shelf (inner neritic); a Hanzawaia boueanum-assemblage of the middle shelf (middle neritic); and a Planulina ariminensis-assemblage of the upper slope (upper bathyal). In their temporal succession they represent a transgression-regression sequence.

The onset of tectonism in the northern Kenya Rift is believed to be recorded by the deposition of immature, arkosic sandstones generally referred to as Turkana Grits. These sandstones have been interpreted by various workers as either Cretaceous or Miocene in age. Detailed re-mapping of the Turkana Grits in the Mount Porr (Kajong) area and a new mapping in the Lariu Range, both on the shores of Lake Turkana, coupled with interpretation of newly acquired seismic reflection profiles, have resulted in an integrated interpretation of the age and deposition of the Turkana Grits in these remote areas.

In both areas the grits are predominantly coarse-grained, basement derrived sandstones deposited on a dissected basement topography by aggrading fluvial systems. Locally, syndepositional faulting has displaced and rotated some of the sandstones. Although the sediments are unfossiliferous in both areas, from their relationships with the basement and overlying volcanics, and their place within the regional tectonic context, it appears that the Turkana Grits in the southern Lake Turkana area are probably Oligo-Miocene deposits.

El-Gluf area is located in the north Eastern Desert of Egypt. It is underlain mainly by Precambrian basement rocks and Phanerozoic sediments. It has been systematically surveyed using high-sensitivity airborne gamma-ray spectrometric and magnetic methods. To identify and outline significant radiometric zones of anomalously high uranium and thorium concentrations in El Gluf biotite granite. These zones could be favourable for potential economic radioactive and/or metallic mineralization. This study could serve as a model for investigating the relationship between the geological structure and radioactive mineralization.

Statistical analysis of the gamma-ray spectrometric data were carried out to delineate anomalies. A two-dimensional trend analysis of faults as tracedfrom the geological map, the radiometric gradients, magnetic lineations and the courses of wadis (valleys) were carried out to delineate major and minor trends in the area.

Seven spectrometric anomalies (two pure uranium, two pure thorium, and three mixed) could be related to the prevailing faulting directions: N-S, N-W and NE. The NW and NE fault trends proved to have a significant impact on the localization of radiometric anomalies, which may point to potential and structurally controlled uranium and thorium mineralizations whose structures are considered to have acted as channel ways for the mineralized solutions.

The Namibian Erathem in South Africa is defined by lower and upper age limits of 900 Ma and 570 Ma and therefore includes most of the Neoproterozoic Erathem. This time bracket incorporates the Pan-African Gariep and Saldania mobile belts as well as some preceding intrusions in the Richtersveld. The upper age limit is surpassed by continuous post-orogenic granite intrusion up to about 500 Ma and deposition in the Nama foreland basin up to about 530 Ma. These rocks are therefore also discussed. An analysis of all existing age data on these rocks reflect polyorogenic and polymagmatic histories related to collisional events for both the Gariep and Saldania Belts that are comparable to that of the Damara Belt in Namibia and other Pan-African belts in Africa and South America. The Pan-African tectonic cycle in South Africa (±650 Ma – 500 Ma) also led to partial resetting of older basement rocks along the so-called West Coast Belt.

The granitic plutonism in the area around Wadi: El-Sheikh, Southwestern Sinai, Egypt is mainly represented by the Older and Younger Granitoid Rocks. The younger granitic rocks comprise in turn, two granitic phases, distinguished on a mineralogical and chemical basis.

Biotites separated from the studied granitoid rocks have been examined. The chemical data of 14 new analyzed biotites, from both the Younger and the Older Granitoids show that the biotites are ferrous iron varieties.

The significance of the Fe_t/(Fe_t+Mg) ratio as a relative measure of biotite crystallization suggests that the biotites may be formed under a temperature range of 685° to 742°C which indicate that the more iron rich biotite crystallizing with fall of temperature and with decrease of oxygen fugacity. Relations between the values of iron and magnesium of biotite and the solidification index of the host granitoid rocks show that the values of iron are approximately higher in biotites than in the host granitic rocks and that both granitoid rocks and biotites become gradually iron-rich magnesium poor with decreasing solidification index and as differentiation proceeds. The significance of the oxidation ratio of the granitoid rocks is elucidating the possible conditions of evolution of these rocks.

The biotites in the two granitic cycles can be discriminated by trace elements and show sequential change through the presumed differentiation series.