Journal of African Earth Sciences (1983)最新文献

{"title":"The sediments of Wadi Qena (Eastern Desert, Egypt)","authors":"Klaus Bandel,&nbsp;Jochen Kuss,&nbsp;Nikolaus Malchus","doi":"10.1016/0899-5362(87)90086-8","DOIUrl":"10.1016/0899-5362(87)90086-8","url":null,"abstract":"<div><p>The first deposition of near-shore sandstones on basement rock in the northern Wadi Qena area occurred during Lower Carboniferous times. Paleozoic deposits are overlain by similar sandstones of Cretaceous age with a large but inconspicuous hiatus between them representing erosion and non-deposition. A Cenomanian transgression inundated sandstones in the north as well as basement rock in the south. The Cenomanian and Turonian sea deposited marine and near-shore material in the Wadi Qena area, and only to the south of it were fluviatile beds laid down. The extant Red Sea Hills, at that time, represented a high and formed a peninsula extending in northerly direction into the shelf sea of the Tethys ocean. The sea withdrew in or after Coniacian times and the following erosion removed almost all Coniacian marine deposits. During Campanian times, the sea returned and also flooded the Red Sea Hill peninsula. Phosphoritic marls, at times of unrest, were redeposited as phosphorite sands in the south. Carbonate deposition followed, ending in Maastrichtian times. An erosional phase during the Cretaceous-Tertiary transition removed most of these chalks and limestones from the Wadi Qena area, and Paleocene and Eocene seas deposited limestone and mmarly chalk before a final regression marked the closure of the Tethys ocean.</p></div>","PeriodicalId":100749,"journal":{"name":"Journal of African Earth Sciences (1983)","volume":"6 4","pages":"Pages 427-455"},"PeriodicalIF":0.0,"publicationDate":"1987-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/0899-5362(87)90086-8","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"53877421","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"A cross section of the Namama Thrust Belt (Mozambique)","authors":"Paola Cadoppi,&nbsp;Mario Costa,&nbsp;Rosalino Sacchi","doi":"10.1016/0899-5362(87)90090-X","DOIUrl":"10.1016/0899-5362(87)90090-X","url":null,"abstract":"<div><p>A cross section of the Namama Thrust Belt (NTB) in Mozambique is described in some detail. The NTB is a ‘Lurian’ age (<em>circa</em> 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.</p></div>","PeriodicalId":100749,"journal":{"name":"Journal of African Earth Sciences (1983)","volume":"6 4","pages":"Pages 493-500, IN7-IN11, 501-504"},"PeriodicalIF":0.0,"publicationDate":"1987-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/0899-5362(87)90090-X","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"53877473","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Geology of Kubi Algi and Derati mountains, pantellerite bodies of Miocene age from the northern part of the Kenyan Rift Valley","authors":"R.T. Watkins","doi":"10.1016/0899-5362(87)90100-X","DOIUrl":"10.1016/0899-5362(87)90100-X","url":null,"abstract":"<div><p>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.</p></div>","PeriodicalId":100749,"journal":{"name":"Journal of African Earth Sciences (1983)","volume":"6 4","pages":"Pages 603-616"},"PeriodicalIF":0.0,"publicationDate":"1987-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/0899-5362(87)90100-X","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"53877617","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Charnockite-granite association in SW Nigeria: rapakivi granite type and charnockitic plutonism in Nigeria?","authors":"V.O. Olarewaju","doi":"10.1016/0899-5362(87)90108-4","DOIUrl":"10.1016/0899-5362(87)90108-4","url":null,"abstract":"<div><p>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.</p><p>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.</p><p>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.</p><p>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.</p></div>","PeriodicalId":100749,"journal":{"name":"Journal of African Earth Sciences (1983)","volume":"6 1","pages":"Pages 67-77"},"PeriodicalIF":0.0,"publicationDate":"1987-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/0899-5362(87)90108-4","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"53877696","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The occurrence and economic potential of clean sand deposits of the Niger Delta","authors":"Enuvie G. Akpokodje,&nbsp;J.O. Etu-Efeotor","doi":"10.1016/0899-5362(87)90107-2","DOIUrl":"10.1016/0899-5362(87)90107-2","url":null,"abstract":"<div><p>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.</p><p>Texturally, the sands are well graded coarse to fine sand with negligible amounts (less than 5%) of gravel and fine (&lt; 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.</p></div>","PeriodicalId":100749,"journal":{"name":"Journal of African Earth Sciences (1983)","volume":"6 1","pages":"Pages 61-65"},"PeriodicalIF":0.0,"publicationDate":"1987-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/0899-5362(87)90107-2","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"53877684","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"RbSr and KAr geochronometry of southeastern Ghana","authors":"E.K. Agyei ,&nbsp;J.E.J.M. van Landewijk ,&nbsp;Richard Lee Armstrong ,&nbsp;J.E. Harakal ,&nbsp;K.L. Scott","doi":"10.1016/0899-5362(87)90057-1","DOIUrl":"10.1016/0899-5362(87)90057-1","url":null,"abstract":"<div><p>The Cape Coast granite complex of the West African Craton in southern Ghana has been dated as 2051 ±45 Ma by whole rock RbSr;. Mineral dates are ∼ 20006 for hornblende by KAr, 1974 ± 28 Ma for biotite by RbSr and ∼ 1900 Ma for biotite by KAr. A few biotite dates are somewhat younger. The low whole-rock ⁸⁷Sr/⁸⁶Sr initial ratio, 0.7015 ± 0.0002, indicates that the granites were created shortly before 2050 Ma. The successively younger mineral dates result from uplift, erosion, and cooling following the Eburnian cratonization event.</p><p>Dahomeyan metamorphic rocks of southeastern Ghana give biotite RbAr dates of 330–542 Ma, mostly 508–542 Ma, due to the Pan-African orogeny. Hornblende KAr dates are somewhat older, at 561–611 Ma.</p><p>Gneisses from Teshie, Numgua, and Tema Beach are parts of a reworked older basement at least 2841 ± 468 Ma old with an initial ratio of 0.7041 ± 0.0014. Gneisses from Ho and Dzigakope also appear to be pre-Eburnian with RbSr dates of 2176 ± 44 Ma and about 2800 Ma, respectively, but this is not as well established. Nine whole-rock samples of Dahomeyan gneiss scatter about an isochron of 515 ± 16 Ma with an initial ratio of 0.7056 ± 0.0002.</p><p>An intrusive carbonatite from Kpong gives concordant whole-rock RbSr, biotite RbSr, and biotite KAr dates of 593 ± 250, 545 ± 11 and 572 ± 15 Ma, respectively, but it also gives discordant and geologically contradictory plagioclase KAr and Rb dates of 665 ± 20 and 975 ± 167 Ma, respectively. Excess Ar and radiogenic Sr in xenocrysts unrelated to the groundmass are suspected.</p></div>","PeriodicalId":100749,"journal":{"name":"Journal of African Earth Sciences (1983)","volume":"6 2","pages":"Pages 153-161"},"PeriodicalIF":0.0,"publicationDate":"1987-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/0899-5362(87)90057-1","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"53876949","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"High resolution seismic reflection, an exploration tool within an underground environment (example from Zimbabwe)","authors":"J.J. Mutyorauta","doi":"10.1016/0899-5362(87)90112-6","DOIUrl":"10.1016/0899-5362(87)90112-6","url":null,"abstract":"<div><p>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.</p></div>","PeriodicalId":100749,"journal":{"name":"Journal of African Earth Sciences (1983)","volume":"6 1","pages":"Pages 109-115"},"PeriodicalIF":0.0,"publicationDate":"1987-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/0899-5362(87)90112-6","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"53877755","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Limnogeologic studies on an intertrappean continental deposit from the northern Ethiopian Plateau (37°03′E, 12°25′N)","authors":"Kedamawit Yemane ,&nbsp;Maurice Taieb,&nbsp;Hugues Faure","doi":"10.1016/0899-5362(87)90110-2","DOIUrl":"10.1016/0899-5362(87)90110-2","url":null,"abstract":"<div><p>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 <em>ca</em> 900–1000 m.</p></div>","PeriodicalId":100749,"journal":{"name":"Journal of African Earth Sciences (1983)","volume":"6 1","pages":"Pages 91-101"},"PeriodicalIF":0.0,"publicationDate":"1987-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/0899-5362(87)90110-2","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"53877726","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Permanence of structural lines in Morocco from Precambrian to Present","authors":"A. Pique ,&nbsp;M. Dahmani ,&nbsp;D. Jeannette ,&nbsp;L. Bahi","doi":"10.1016/0899-5362(87)90066-2","DOIUrl":"10.1016/0899-5362(87)90066-2","url":null,"abstract":"<div><p>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.</p></div>","PeriodicalId":100749,"journal":{"name":"Journal of African Earth Sciences (1983)","volume":"6 3","pages":"Pages 247-256"},"PeriodicalIF":0.0,"publicationDate":"1987-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/0899-5362(87)90066-2","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"53877042","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The geochemistry of mafic and ultramafic rocks in the Wami River granulite complex, central coastal Tanzania","authors":"M.A.H. Maboko,&nbsp;N.K. Basu","doi":"10.1016/0899-5362(87)90042-X","DOIUrl":"10.1016/0899-5362(87)90042-X","url":null,"abstract":"<div><p>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.</p></div>","PeriodicalId":100749,"journal":{"name":"Journal of African Earth Sciences (1983)","volume":"6 6","pages":"Pages 845-850"},"PeriodicalIF":0.0,"publicationDate":"1987-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/0899-5362(87)90042-X","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"53876646","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}