Journal of African Earth Sciences (1983)最新文献_第2页

A gravity and magnetic traverse from Port Sudan to Abu Hamad, NE Sudan. A model of rifting 从苏丹港到苏丹东北部阿布哈马德的重力和磁场导线。裂谷的一个模型

Journal of African Earth Sciences (1983) Pub Date : 1987-01-01 DOI: 10.1016/0899-5362(87)90040-6

Abdel Ati Sadig

引用次数: 0

Mount Cameroon: an active volcano of the Cameroon Line 喀麦隆山:喀麦隆线的一座活火山

Journal of African Earth Sciences (1983) Pub Date : 1987-01-01 DOI: 10.1016/0899-5362(87)90061-3

Bernard Déruelle , Jean N'ni, Robert Kambou

{"title":"Mount Cameroon: an active volcano of the Cameroon Line","authors":"Bernard Déruelle , Jean N'ni, Robert Kambou","doi":"10.1016/0899-5362(87)90061-3","DOIUrl":"10.1016/0899-5362(87)90061-3","url":null,"abstract":"<div>Mt Cameroon is a large volcanic horst which belongs to the Cameroon Line. The morphology of the massif depends on tectonic control. Mt Cameroon is built upon a Precambrian metamorphic basement covered with Cretaceous to Recent sediments of the Douala and Rio del Rey basins. The oldest lavas could be of Upper Miocene age. Mt Cameroon has erupted six times in the 20th Century. The 1982 eruption took place inside the crater of an ancient cone. Volcanic risks are actual for the human constructions, mostly along the axis of the horst.The lavas are picrites (with forsteritic olivine phenocrysts), alkali basalts (with salitic augite phenocrysts), hawaiites (with labrador-bytownite plagioclase phenocrysts) and mugearites (with scarce kaersutite phenocrysts and microlitic phlogopite or nosean). FeTi oxides phenocrysts are ubiquitous.All the lavas are Ne-normative. The distribution of major and trace elements in the lava series is well explained by fractionations of the minerals found as phenocrysts, but plagioclase does not fractionate (Sr and Eu contents in the lavas increase with the differentiation). Thus, olivine, augite and FeTi oxide fractionations are respectively correlated with MgNi, CaCrSc and TiV distributions. Primitive magmatic liquids contain about 1 ppm uranium.Ta-Th-Hf correlation and REE distribution clearly indicate that the Cameroon lava series is typically alkaline with no tholeiitic or transitional trend. This fact asserts that the Cameroon Line is not a rift system but the result of tension gashes due to the Adamaoua sinistral strike-slip faulting zone.</div>","PeriodicalId":100749,"journal":{"name":"Journal of African Earth Sciences (1983)","volume":"6 2","pages":"Pages 197-214"},"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)90061-3","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"53876999","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}

引用次数: 92

Quelques precisions sur la composition mineralogique des laves tertiaires de la province volcanique de la Presqu'île du Cap-Vert (Senegal) 佛得角半岛(塞内加尔)火山省第三纪熔岩矿物学组成的一些细节

Journal of African Earth Sciences (1983) Pub Date : 1987-01-01 DOI: 10.1016/0899-5362(87)90067-4

Abdoulaye Dia , Jean Babkine , Michel Rollet

引用次数: 2

40Ar/39Ar and paleomagnetic results from Liberia and the Precambrian APW data base for the West African Shield 利比里亚40Ar/39Ar和古地磁结果及西非地盾前寒武纪APW数据库

Journal of African Earth Sciences (1983) Pub Date : 1987-01-01 DOI: 10.1016/0899-5362(87)90094-7

T.C. Onstott, J. Dorbor

{"title":"40Ar/39Ar and paleomagnetic results from Liberia and the Precambrian APW data base for the West African Shield","authors":"T.C. Onstott, J. Dorbor","doi":"10.1016/0899-5362(87)90094-7","DOIUrl":"10.1016/0899-5362(87)90094-7","url":null,"abstract":"<div>Lower amphibolite to granulite facies metamorphic rocks in Nimba County, Liberia have yielded 2.2–2.9 Ga RbSr whole rock ages, indicating that they are part of the Archean Liberian age province. We report a 2040 Ma4040Ar/39Ar plateau date on hornblende from an amphibolite in this region, and suggest that these rocks were also severelyreworked during the Eburnean (∼2.0 Ga) metamorphic episode. 40Ar/39Ar analyses of biotite and feldspars from neighboring schists also indicate the presence of two mild thermal events, at 1.5 Ga and 0.6 Ga. Paleomagnetic analyses of samples from these same metamorphic rocks reveal three components of magnetization. The predominant and most stable component (273°E, 21°N) is considered to have been acquired as a result of pos Eburnean uplift and cooling at ∼ 2.0 Ga, whereas the two less stable components with poles at 235°E, 43°N and 16°E, 36°N, probably correlate with the 1.5 Ga and 0.6 Ga thermal pulses, respectively. Rock units from southern Liberia also yield two secondary magnetizations, one at 247°E, 37°N and the other at 104°E, 5°N, and a 1.5 Ga 40Ar/39Ar date on plagioclase. Comparison of the paleomagnetic poles corresponding to the ∼2.0 Ga Eburnean component with published paleomagnetic data for West Africa is not consistent with prior interpretations of the polar wander path for West Africa. Our paleomagnetic data, when compared to poles of comparable age from the Kalahari Shield, still suggest that some form of displacement has occurred between the Kalahari and West African Shields since 2.0 Ga.</div>","PeriodicalId":100749,"journal":{"name":"Journal of African Earth Sciences (1983)","volume":"6 4","pages":"Pages 537-552"},"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)90094-7","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"53877526","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}

引用次数: 33

Empirical guides to gold enrichment in Proterozoic quartz-pebble conglomerates in northern Zambia 赞比亚北部元古代石英-卵石砾岩中金富集的经验指导

Journal of African Earth Sciences (1983) Pub Date : 1987-01-01 DOI: 10.1016/0899-5362(87)90099-6

C.P. Andrews-Speed , B.P. Mulumba

{"title":"Empirical guides to gold enrichment in Proterozoic quartz-pebble conglomerates in northern Zambia","authors":"C.P. Andrews-Speed , B.P. Mulumba","doi":"10.1016/0899-5362(87)90099-6","DOIUrl":"10.1016/0899-5362(87)90099-6","url":null,"abstract":"<div>Proterozoic quartz-pebble conglomerates and sandstones near the base of the Mporokoso Group in northern Zambia were deposited in a braided stream system and are known to be locally enriched in gold. A detailed sampling programme was carried out in small area of good exposure in order to identify empirical guides to gold enrichment and to test the hypothesis that the gold is of placer origin.A total of 2350 rock samples were taken from nine vertical profiles across the main 50–100 m thick conglomeratic interval, spread over a horizontal distance of 4 km. These were analysed for gold by atomic absorption spectrophotometry. Detailed sedimentological logging was carried out and the gold analyses were correlated with sedimentary parameters. Gold enrichment preferentially occurs: <ul><li>1.(1) in a 1–2 km longitudinal stretch of the 50–100 m thick conglomeratic interval;</li><li>2.(2) in massive conglomerates (facies Gm) and then in trough crossbedded conglomerates (facies Gt), and not in trough cross-bedded sandstones (facies St);</li><li>3.(3) in beds of facies Gm and Gt which have small pebbles and a low proportion of vein quartz pebbles;</li><li>4.(4) in sections with frequent vertical changes of facies.</li></ul>These correlations are consistent with a placer origin for the gold and should help to identify prospective areas within the Mporokoso Group.</div>","PeriodicalId":100749,"journal":{"name":"Journal of African Earth Sciences (1983)","volume":"6 4","pages":"Pages 595-601"},"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)90099-6","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"53877607","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}

引用次数: 3

Tectonometamorphic evolution of the gneissic Kidal assemblage related to the Pan-African thrust tectonics (Adrar des Iforas, Mali) 泛非洲逆冲构造相关的片麻岩基达尔组合构造变质演化(马里Adrar des Iforas)

Journal of African Earth Sciences (1983) Pub Date : 1987-01-01 DOI: 10.1016/0899-5362(87)90104-7

M. Champenois , A.M. Boullier , V. Sautter , L.I. Wright , P. Barbey

{"title":"Tectonometamorphic evolution of the gneissic Kidal assemblage related to the Pan-African thrust tectonics (Adrar des Iforas, Mali)","authors":"M. Champenois , A.M. Boullier , V. Sautter , L.I. Wright , P. Barbey","doi":"10.1016/0899-5362(87)90104-7","DOIUrl":"10.1016/0899-5362(87)90104-7","url":null,"abstract":"<div>In the central part of the Adrar des Iforas (Mali), the 2 Ba Eburnean granulatic unit has been thrust above a high-grade gneissic unit, the so-called ‘Kidal assemblage’, during an early event of the Pan-African orogeny. The Kidal assemblage can be defined as a tectonic mixing of an Eburnean granulitic basement, its sedimentary cover of Middle to Upper Proterozoic age (quartzites, marbles, basalts and metavolcanics) and various pretectonic rocks: ultrabasic to basic rocks, diorites, tonalites. All these rocks have been deformed during at least four main events and metamorphosed together. Thrusting of the Iforas Granulitic Unit above the Kidal assemblage happened during the first event D1. The movement direction was roughly N–S, as shown by the stretching lineation. Some field criteria indicate a sense of displacement towards the north. The lattice preferred orientation of quartz c- and 〈a〉 axes indicate that the slip was dominantly on prismatic and probably pyramidal planes along an 〈a〉 direction; consequently D1 deformation was achieved at high temperature or low-strain rate. The quartz c- and 〈a〉 axes do not show any constant asymmetry, so they do not indicate a sense of shear. Two metamorphic stages have been found in the Kidal assemblage: the first one is characterized by kyanite in aluminous metasediments and by the occurrence of garnet-clinopyroxene-bearing boundis of basic rocks. The P–T range of this event is located at 700 ± 50°C and around 10 Kb. The second event is a syntectonic high temperature (600–650°C) low pressure (3.5 Kb) stage accompanied by migmatization. Such a tangential deformation in barrowian-type metamorphic conditions and with N–S transport direction is known along the entire Trans-Saharan belt and cannot be related in a simple way to the collision between West African Craton and the mobile belt.</div>","PeriodicalId":100749,"journal":{"name":"Journal of African Earth Sciences (1983)","volume":"6 1","pages":"Pages 19-27"},"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)90104-7","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"53877654","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}

引用次数: 19

Petrology, geochemistry and structural development of the Bir Safsaf-Aswan uplift, Southern Egypt 埃及南部Bir safaf - aswan隆起的岩石学、地球化学和构造发育

Journal of African Earth Sciences (1983) Pub Date : 1987-01-01 DOI: 10.1016/0899-5362(87)90109-6

R. Bernau , D.P.F. Darbyshire , G. Franz , U. Harms , A. Huth , N. Mansour , P. Pasteels , H. Schandelmeier

{"title":"Petrology, geochemistry and structural development of the Bir Safsaf-Aswan uplift, Southern Egypt","authors":"R. Bernau , D.P.F. Darbyshire , G. Franz , U. Harms , A. Huth , N. Mansour , P. Pasteels , H. Schandelmeier","doi":"10.1016/0899-5362(87)90109-6","DOIUrl":"10.1016/0899-5362(87)90109-6","url":null,"abstract":"<div>The Bir Safsaf-Aswan uplift is an east-west striking major basement high in SW Egypt. It consists mainly of granitic gneisses intercalated with amphibolites, marbles and calc-silicates. It is intruded by syntectonic S-type granites and late-tectonic I-type granitoids and by alkaline basaltic plugs and different dyke generations. Data on the metamorphic evolution revealed an early high-T (∼800°C) event, followed by amphibolite-migmatite facies and then by greenschist facies conditions. The following preliminary geological evolution is proposed: the East African craton in SW Egypt was formed and metamorphosed under high T conditions in Pre-Pan-African times. The accretion of the Arabian-Nubian shield led to the ckening of the adjacent continental African crust which caused migmatization and formation of S-type granites. Late tectonic uplift resulted in a low-grade metamorphic overprint and in the formation of I-type granites at around 570 Ma. The end of the Pan-African development is documented by the intrusion of igneous dykes at 521 Ma. The Pan-African generated fracture system was frequently reactivated in the Phanerozoic, as indicated by the ages of fracture bound continental volcanic rocks (193 ± 5 Ma; 155 ± 4 Ma; 87−81 Ma).</div>","PeriodicalId":100749,"journal":{"name":"Journal of African Earth Sciences (1983)","volume":"6 1","pages":"Pages 79-90"},"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)90109-6","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"53877711","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}

引用次数: 41

Geology and mineral resources of West Africa 西非的地质和矿产资源

Journal of African Earth Sciences (1983) Pub Date : 1987-01-01 DOI: 10.1016/0899-5362(87)90065-0

Michael Weakes

引用次数: 1

The Jamestown Ophiolite Complex, Barberton mountain belt: a section through 3.5 Ga oceanic crust 巴伯顿山带詹姆斯敦蛇绿岩杂岩:3.5 Ga洋壳剖面

Journal of African Earth Sciences (1983) Pub Date : 1987-01-01 DOI: 10.1016/0899-5362(87)90007-8

Maarten J. de Wit , Roger A. Hart , Rodger J. Hart

{"title":"The Jamestown Ophiolite Complex, Barberton mountain belt: a section through 3.5 Ga oceanic crust","authors":"Maarten J. de Wit , Roger A. Hart , Rodger J. Hart","doi":"10.1016/0899-5362(87)90007-8","DOIUrl":"10.1016/0899-5362(87)90007-8","url":null,"abstract":"<div>The mafic to ultramafic rocks of the Barberton greenstone belt, South Africa, form a pseudostratigraphy comparable to that of Phanerozoic ophiolites. This Archaean complex, referred to here as the Jamestown Ophiolite Complex, consists of a high temperature tectono-metamorphic peridotite overlain by an intrusive extrusive igneous section, which in turn is capped by a chert-shale sequence. There is a complete range from komatiitic to tholeiitic compositions within single intrusive units. Crustal contamination and magma mixing is evident from field and geochemical data.Pillow structures, 40Ar/39Ar ages and oxygen isotope analysis suggest that hydrothermal interaction with the Archaean ocean severely hydrated and chemically altered the entire simatic section during its formation. As a consequence, only a ‘ghost’ igneous geochemistry is preserved. This regional open-system alteration may have increased the MgO content of the igneous rocks by as much as 13%, and the most primitive liquids, from which the extrusive sequence evolved, were ‘picritic’ in character. Rocks with a komatiitic chemistry were derived during crystal accumulation from picritic-crystal mushes (predominantly olivine-clinopyroxene) and/or by metasomatism during one or more subsequent episodes of hydration-dehydration.In contrast to Phanerozoic ophiolites, the Jamestown complex is relatively thin (≦3 km), which implies that locally at least the ca 3.5 Ga oceanic crust was also thin. This is consistent with the regionally extensive metasomatic alteration, and is compatible with theoretical and experimental models predicting higher Archaean heat transfer from the mantle concentrated within Archaean oceans.</div>","PeriodicalId":100749,"journal":{"name":"Journal of African Earth Sciences (1983)","volume":"6 5","pages":"Pages 681-730"},"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)90007-8","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"53876422","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}

引用次数: 241

Notes on mid-Cretaceous biostratigraphy of Algeria 阿尔及利亚中白垩世生物地层学注释

Journal of African Earth Sciences (1983) Pub Date : 1987-01-01 DOI: 10.1016/0899-5362(87)90034-0

Stefan Majoran

引用次数: 4