Isotopic geochemistry (O,Sr, Pb) of the Golda Zuelva and Mboutou anorogenic complexes,North Cameroun: mantle origin with evidence for crustal contamination

The annular (6–8 km diameter) Golda Zuelva and Mboutou anorogenic complexes of North Cameroun are composed of a suite of alkaline plutonic rocks ranging from olivine gabbro to amphibole and biotite granite. For the Mboutou complex there are two overlapping centres. In the Golda Zuelva complex the plutonic rocks are associated with a later hawaiite to rhyolite volcanic suite. A Rb/Sr whole rock isochron gives an age of 66±3 Ma for the Golda Zuelva granites, with initial⁸⁷Sr/⁸⁶Sr ratio of 0.7020, and demonstrates that plutonism and volcanism were essentially contemporaneous and probably cogenetic. For Golda Zuelva and the north Mboutou centre¹⁸O/¹⁶O (5.6–6.2),⁸⁷Sr/⁸⁶Sr (0.7030–0.7045) and Pb isotopic ratios (²⁰⁷Pb/²⁰⁴Pb: 15.60–15.64) support a mantle origin for the initial magmas. Unlike Sr isotopes, the O isotopic ratios of the granitic end members at Golda Zuelva (～7.5) indicate crustal contamination. Post-magmatic alteration was not significant.For the younger south Mboutou centre the O-, Sr- and Pb-isotopic data indicate more extensive magma-crust interaction and in a different (higher level?) crustal environment with δ¹⁸O granite=3.3‰,⁸⁷Sr/⁸⁶Sr ratios up to 0.706 and Pb isotopic ratios more markedly displaced from the oceanic volcanic field. The low-¹⁸O granites probably record, at least in part, a magmatic process with subsequent minor post-magmatic alteration effects. The major and trace element systematics between the north and south Mboutou centres are directly comparable. The evolution of the magmas were dominated by fractional crystallisation and progressive crustal contamination processes.     

Paleocurrents and the problem of continental drift

The hypothesis of continental drift is examined on the light of paleocurrent analysis. A peripherical radial transportation pattern was obtained in South America and Africa from marine sandstones deposited during Ordovician to Devonian times. This pattern is suggestive of a former adjacent position between both continents, being the source area located somewhere in eastern Brazil and western Africa (Figs. 8 and 9). The paleocurrents in both continents flowed away from this common area. Evidences of Ordovician and/or Silurian glaciation indicated a radial movement for the ice-sheets, away from the same source area. The Caradocian ice movement in Sahara was to the north, while the ice-flow in the Gape Province was to the south, and in Northeastern Brazil it was possibly to the west.The Carboniferous paleocurrent and ice-movement trends are suggestive of a territorial contiguity between Africa and South America. However, several data must be revised for stratigraphic control.The Permian formations in the Paraná and Karroo basins had independent development without any direct connection as suggested by cross-bedding analysis. The occurrence ofMesosaurus in both basins is a strong argument of their close position at the time of sedimentation.Mesozoic wind laid deposits occur in both continents from Upper Triassic to Lower Cretaceous time. The Triassic eolian sandstones were deposited by westerly paleowinds in middle latitude region at the time of continental contiguity. The Upper Jurassic-Lower Cretaceous eolian sandstones were deposited after the separation between the continents had started. The paleowind pattern includes westerlies, trade-,and return tradewinds. This pattern is suggestive for the presence of the Southern Atlantic Ocean in Upper Jurassic-Lower Cretaceous time.

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Zusammenfassung Die Hypothese der Kontinentaldrift der Südkontinente Afrika und Südamerika wird mit Hilfe der Analyse von PalÄoströmungen untersucht.Marine Sandsteinablagerungen vom Ordoviz bis zum Devon zeigen in Südamerika und auch in Afrika peripher-radiales Transportnetz.Dies weist auf eine früher zusammenhÄngende Position zwischen beiden Kontinenten hin, deren Ursprung in Ost-Brasilien und West-Afrika gelegen haben mu\, da die Strömungsrichtungen in beiden Kontinenten von diesem Gebiet ausgehen (Fig. 8 und 9).Die Vereisung im Ordoviz und/oder Silur zeigt eine sternförmige Bewegungsrichtung des Eises vom gleichen Ursprungsgebiet aus. Die Strömungsrichtung zur Caradoc-Zeit war in der Sahara nach N gerichtet, wÄhrend sich das Eis in der Kap-Provinz nach S und in NE-Brasilien möglicherweise nach W bewegte.Die Strömungsrichtungen und Eisbewegungen im Karbon deuten auf einen territorialen Zusammenhang zwischen Afrika und Südamerika hin, jedoch bedürfen einige Daten einer stratigraphischen überprüfung.Das Perm im Paraná- und Karroo-Becken zeigt eine unabhÄngige Entwicklung ohne direkten Zusammenhang, wie er aus SchrÄgschichtungsmessungen angedeutet wurde. Das Vorkommen vonMesosaurus in beiden Becken ist ein gewichtiges Argument für ihre benachbarte Lage wÄhrend der Sedimentation.Im Mesozoikum finden wir von Obertrias bis Unterkreide in beiden Kontinenten Äolische Ablagerungen. Die der Trias wurden durch westliche Winde in mittleren geographischen Breiten sedimentiert, zu einer Zeit, als die beiden Kontinente noch zusammenhingen. Die Sandsteine des Oberen Juras und der Unteren Kreide wurden abgelagert, nachdem die Trennung der Kontinente begonnen hatte.Das PalÄowindsystem zeigt westliche und Passatwinde. Dieses System spricht bereits für das Vorhandensein des Südatlantiks wÄhrend dem Oberen Jura und der Unteren Kreide.

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Sumário No presente trabalho o problema da deriva continental é examinada do ponto de vista da análise das paleocorrentes. MediÇÕes de estratos cruzados em arenitos marinhos do Ordoviciano ao Devoniano, realizadas na áfrica e na América do Sul indicaram urn padrÃo de transporte periférico-radial. Êste padrÃo é sugestivo de uma justaposiÇÃo pretérita entre ambos continentes. A área fonte estaria localizada na regiÃo oriental da América do Sul e Ocidental da áfrica (figs. 8 e 9). Em ambos continentes as paleocorrentes fluiram a partir desta área comum. EvidÊncias de glaciaÇÃo Ordoviciana e/ou Siluriana indicam um movimento radial das massas de gÊlo a partir da mesma área fonte. No Sahara, o movimento do gÊlo no Caradociano fazia-se para o norte, enquanto que, na Provincia do Cabo era para o Sul, e no Nordeste do Brasil Êle provàvelmente dirigia-se para o oeste.No Carbonifero, as paleocorrentes e tendÊncia do movimento das geleiras sÃo sugestivas de uma contiguidade territorial entre a América do Sul e áfrica. Contudo, várlos dados devem ser revisados para controle estratigráfico.As formaÇÕes Permianas nas bacias do Paraná e do Karroo apresentam desenvolvimento independente. O estudo das paleocorrentes nÃo sugere qualquer ligaÇÃo direta. Entretanto, a ocorrÊncia deMesosaurus em ambas bacias, constitui forte argumenta a favor de maior proximidade entre elas na época de sedimentaÇÃo.Os depósitos eólicos mesozóicos ocorrem em ambos continentes desde o Triássico Superior até o Cretáceo Inferior. Os arenitos eólicos Triássicos foram depositados em regiÃo de latitude média pelos paleoventos de oeste. Nésta época, ambos os continentes encontravam-se unidos. Os arenitos eólicos do Jurássico Superior-Cretáceo Inferior foram depositados após o início da separaÇÃo. O sistema de paleoventos inclui ventos de oeste, aliseos e aliseos de retÔrno. Êste padrÃo de circulaÇÃo aérea é sugestivo da presenÇa do Atlântico sul durante o Jurássico Superior — Cretáceo Inferior.

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. - . , - , , . . (. 8 9). / . Caradoc , , — , , , . , , . , . . . , . . . .

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Under the auspices of the Conselho Nacional de Pesquisas (Brasil), Geological Survey of South Africa, and Institute Algerien du Pétrole.     

Some major concepts of metallogeny

These notes, which summarize part of over ten years of laboratory work, aim to clarify and inter-relate three fundamental concepts of current Metallogeny: consanguinity, heritage, province. — 1. The concept of consanguinity: its criteria, limits and graduations, are first presented; 2. They are illustrated by the analysis of a complex example: the lead-zinc mineralisations in Trias around the Cévennes (South of the French Massif Central); 3. The likelihood of consanguinity for various strata-bound mineralisations, particularly misappreciated and misinterpreted, is then shown: — tungsten in the Cévennes — gold at Passagem (Brazil), about which microstructural and geochemical arguments are especially employed — cobalt at Bou Azzer (Marocco); 4. From consanguinity in a sedimentary environment, the authors turn to the crustal phenomena cycle, during which the heritages of the cations succeed one another. Examples of heritages, linked with sedimentation-volcanism: Sierra de Carthagena (Spain) — metamorphism — granitisation, are rapidly examined or referred to in literature. Particular attention has been paid to heritage by secretion, illustrated by the antimony lodes of the French Massif Central. Consequently, it may be understood how the successive heritages involve geochemical permanences and determine the geochemical provinces going back to ancient times in which the mineral provinces were built up. 5. In a last section the transformist guide line is compared with other concepts and tools of Metallogeny: lineaments and isotopic ratios. — In conclusion, the intra-crustal heritages still remain under-estimated and badly deciphered and are not sufficiently taken into consideration in mineral exploration.

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Zusammenfassung Diese Veröffentlichung, die einen Teil der während eines Jahrzehnts in einem Laboratorium ausgeführten Arbeiten kurz behandelt, hat die Aufgabe, drei Grundbegriffe der gegenwärtigen Metallogenese klarzustellen und miteinander zu verknüpfen: Familiarität (Verwandtschaft), Vererbung und Provinz. — 1. Der Begriff der Verwandtschaft sowie seine Kriterien, Grenzen und Nuancen werden zuerst auseinandergesetzt. 2. Dann wird dieser Begriff anhand der Analyse eines komplizierten Beispiels veranschaulicht, und zwar der Blei-Zink-Mineralisationen in der Trias am Rand der Cevennen, im Süden des französischen Zentralmassivs gelegen; 3. Anschließend wird die Wahrscheinlichkeit der Verwandtschaft für verschiedene schichtgebundene, besonders verkannte und falsch interpretierte Mineralisationen aufgezeigt. Es handelt sich einerseits um das Wolfram der Cevennen sowie das Kobalt von Bou Azzer (Marokko) und andererseits um das Gold von Passagem (Brasilien), zu dessen Erklärung insbesonders mikrostrukturelle und geochemische Argumente herangezogen werden; 4. So gelangt man in den Kreislauf der Krustalerscheinungen, währenddessen Vererbungen von Kationen aufeinanderfolgen. Beispiele von Vererbungen, die an Sedimentation, Vulkanismus (Sierra von Cartagena, Spanien), Metamorphose und Granitisierung gebunden sind, werden kurz untersucht oder anhand von Literaturangaben erwähnt. Besondere Erwähnung verdient die Sekretionsvererbung, die am Beispiel der Antimonerzgänge des französischen Zentralmassivs veranschaulicht wird. Somit wird verständlich, wie die aufeinanderfolgenden Vererbungen geochemische Beständigkeiten bedingen. Sie bestimmen ebenfalls geochemische, auf das hohe Altertum zurückgehende Provinzen, auf denen sich die Mineralprovinzen aufbauen; 5. Zum Schluss wird diese transformistische Leitrichtung mit anderen Konzepten und Arbeitsmitteln der Metallogenese verglichen: den Lineamenten und Isotopenverhältnissen. — Es wird der Schluß gefolgert, daß die intrakrustalen Vererbungen immer noch unterschätzt, schlecht entziffert und bei der Lagerstättenforschung ungenügend in Betracht gezogen werden.

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Illustrated by Examples

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Laboratory associated with the C.N.R.S. (n^o 65). Contributors to this paper: P. Routhier (former Director of the Laboratory at present Director in the B.R.G.M.) and P. Brouder, R. Fleischer, (at present in the Penarroya C⁰), J. C. Macquar, Melle M. J. Pavillon, G. Roger and H. Rouvier.     

Considerations generales sur la distribution de l'uranium dans les roches granitoides

Using definite examples the authors show that it is possible to find, in a given granite, relations between uranium and the other elements, but these relations cannot be generalized. They cannot depend on the petrographical and mineralogical properties rather than on the chemical composition. One finds with the essential minerals some apparent relations but they are valid only for a given rock. The nature of a granite depends, mainly, on its formation and consequently on the original material from which it derived. The present data on radioactivity of granites cannot be generalized.     

La sédimentation vaseuse dans les estuaires de Bretagne occidentale

Sans résumé     

Cosmological Models with ‘Some’ Variable Constants

Various models are considered with metric type flat FRW i.e. with k = 0 whose energy-momentum tensor is described by a perfect fluid whose generic equation of state is p = ωρ and taking into account the conservation principle div(T _i ^j ) = 0, but considering some of the‘constants’ as variable. A set of solutions through dimensional analysis is trivially found. The numeric calculations carried out show that the results obtained are not discordant with those presently observed for cosmological parameters together with the electromagnetic and quantum quantities. This revised version was published online in July 2006 with corrections to the Cover Date.     

Environmental assessment of a large industrial marine complex based on a community of benthic filter-feeders

Biological quality in a bay affected by man's activities was evaluated by means of the composition of assemblages of sponges and ascidians. Our results showed that the structure of these two groups of filter-feeders aided in discriminating between undisturbed and disturbed areas, establishing different "environmental health categories" from moderately to strongly disturbed areas, and in ascertaining the extension of the area of each "health category". We were able to divide the bay into four zones based on type of disturbance or anthropogenic source: (1) stations free of any source of disturbance, (2) stations under moderate disturbance, located close to industrial ports, millworks, etc., (3) stations that are under the direct influence of industrial wastes such as a power station and oil refinery, and (4) stations near strongly disturbed areas, influenced directly by harmful steelworks activities. We differentiated clearly between four large species assemblages, and related the composition of these assemblages to different kinds of disturbances. Thus, these species could be used to manage the marine environment in this bay by comparing the observed fauna, with expected fauna in an unstressed site. Moreover, the joint presence of the sponge Cliona vastifica and tunicate Policitor adriaticum seems always to indicate a more or less pristine environmental situation, functioning as bioindicators of normal conditions. We think that the use of specific bioindicators for monitoring disturbance is a valid tool to establish baselines to predict impacts associated with industrial development in many marine ecosystems. The advantages to monitoring communities on hard rocks versus sandy or muddy bottoms are also commented upon.     

Landslide Susceptibility Models Utilising Spatial Data Analysis Techniques. A Case Study from the Lower Deba Valley,Guipuzcoa (Spain)

A quantitative methodology for landslide susceptibility zonationis described and its application to a study area in the lower part of the Deba Valley (Guipúzcoa,Spain) presented. Susceptibility models were obtained on the basis of statisticalrelationships between known mass movements and conditioning factors. A landslide rupturehypothesis was set and a digital database consisting of seventeen causal factors layers constructed.The modelling procedure was implemented utilising a GIS. The susceptibility analysis methodis based on the Favourability Functions approach, and two different mathematicalframeworks: probability theory and Zadehïs fuzzy set theory. Several landslidesusceptibility models were produced and validated using different sets of independent landslide data.The predictive capability of models was determined.     

Large-Scale Temperature Changes across the Southern Andes: 20th-Century Variations in the Context of the Past 400 Years

Long-term trends of temperature variations across the southern Andes (37–55° S) are examined using a combination of instrumental and tree-ring records. A critical appraisal of surface air temperature from station records is presented for southern South America during the 20th century. For the interval 1930–1990, three major patterns in temperature trends are identified. Stations along the Pacific coast between 37 and 43° S are characterized by negative trends in mean annual temperature with a marked cooling period from 1950 to the mid-1970s. A clear warming trend is observed in the southern stations (south of 46°S), which intensifies at higher latitudes. No temperature trends are detected for the stations on the Atlantic coast north of 45° S. In contrast to higher latitudes in the Northern Hemisphere where annual changes in temperature are dominated by winter trends, both positive and negative trends in southern South America are due to mostly changes in summer (December to February) temperatures. Changes in the Pacific Decadal Oscillation (PDO) around 1976 are felt in summer temperatures at most stations in the Pacific domain, starting a period with increased temperature across the southern Andes and at higher latitudes.Tree-ring records from upper-treeline were used to reconstruct past temperature fluctuations for the two dominant patterns over the southern Andes. These reconstructions extend back to 1640 and are based on composite tree-ring chronologies that were processed to retain as much low-frequency variance as possible. The resulting reconstructions for the northern and southern sectors of the southern Andes explain 55% and 45% ofthe temperature variance over the interval 1930–1989, respectively. Cross-spectral analysis of actual and reconstructed temperatures over the common interval 1930–1989, indicates that most of the explained varianceis at periods >10 years in length. At periods >15 years, the squaredcoherency between actual and reconstructed temperatures ranges between 0.6 and 0.95 for both reconstructions. Consequently, these reconstructions are especially useful for studying multi-decennial temperature variations in the South American sector of the Southern Hemisphere over the past 360 years. As a result, it is possible to show that the temperatures during the 20thcentury have been anomalously warm across the southern Andes. The mean annual temperatures for the northern and southern sectors during the interval 1900–1990 are 0.53 °C and 0.86 °C above the1640–1899 means, respectively. These findings placed the current warming in a longer historical perspective, and add new support for the existence of unprecedented 20th century warming over much of the globe. The rate of temperature increase from 1850 to 1920 was the highest over the past 360 years, a common feature observed in several proxy records from higher latitudes in the Northern Hemisphere.Local temperature regimes are affected by changes in planetary circulation, with in turn are linked to global sea surface temperature (SST) anomalies. Therefore, we explored how temperature variations in the southern Andes since 1856 are related to large-scale SSTs on the South Pacific and South Atlantic Oceans. Spatial correlation patterns between the reconstructions and SSTs show that temperature variations in the northern sector of the southern Andes are strongly connected with SST anomalies in the tropical and subtropical Pacific. This spatial correlation pattern resembles the spatial signature of the PDO mode of SST variability over the South Pacific and is connected with the Pacific-South American (PSA) atmospheric pattern in the Southern Hemisphere. In contrast, temperature variations in the southern sector of the southern Andes are significantly correlated with SST anomalies over most of the South Atlantic, and in less degree, over the subtropical Pacific. This spatial correlation field regressed against SST resembles the `Global Warming' mode of SST variability, which in turn, is linked to the leading mode of circulation in the Southern Hemisphere. Certainly, part of the temperature signal present in the reconstructions can be expressed as a linear combination of four orthogonal modes of SST variability. Rotated empirical orthogonal function analysis, performed on SST across the South Pacific and South Atlantic Oceans, indicate that four discrete modes of SST variability explain a third, approximately, of total variance in temperature fluctuations across the southern Andes.