The geochemistry and evolution of early precambrian mantle

Seven high-purity cumulate clinopyroxenes from 2.7 b.y. maficultramafic rock associations from the Abitibi belt, Superior Province, Canada, have been analyzed for major elements and K, Rb, Cs, Ba, Sr and ⁸⁷Sr/⁸⁶Sr ratio. Attempts to reconstruct the trace element patterns of the original parent magmas were partially successful; Sr contents (140 ppm), K/Rb (470) and K/Ba (16) ratios are similar to those of modern low-K island arc tholeiites. K/Cs ratios (2700) are significantly lower than island arc tholeiites (17,000) or oceanic island and oceanic ridge basalts (> 30,000); the presentday mantle seems to be more depleted in Cs than in Archean times. Initial Sr isotope ratios of the 7 Archean clinopyroxenes average 0.70114±13(2σ) with relatively little variation; this value is in good agreement with initial ratios published for felsic and mafic rocks of the same age, though the latter show much larger variations and uncertainties. The pyroxene Sr isotope data, in conjunction with data for rocks of other ages, defines the following simple model for mantle evolution:

1. starting with primordial Sr, a short period of relatively rapid ⁸⁷Sr/⁸⁶Sr growth, followed by Rb depletion;
2. a period between ≧ 3.5 b.y. and ～ 1.7 b.y. when closed-system Sr isotope evolution occurred (with Rb/Sr ～ 0.023);
3. development of large-scale Rb/Sr heterogeneities in the mantle at ～ 1.7 b.y., leading to a present-day mantle with ⁸⁷Sr/⁸⁶Sr ranging from 0.7023 to 0.7065 and Rb/Sr ranging from ～ 0 to 0.065.

     

Supercontinent evolution and the Proterozoic metallogeny of South America

The cratonic blocks of South America have been accreted from 2.2 to 1.9 Ga, and all of these blocks have been previously involved in the assembly and breakup of the Paleoproterozoic Atlantica, the Mesoproterozoic to Neoproterozoic Rodinia, and the Neoproterozoic to Phanerozoic West Gondwana continents. Several mineralization phases have sequentially taken place during Atlantica evolution, involving Au, U, Cr, W, and Sn. During Rodinia assembly and breakup and Gondwana formation, the crust-dominated metallogenic processes have been overriding, responsible for several mineral deposits, including Au, Pd, Sn, Ni, Cu, Zn, Mn, Fe, Pb, U, P₂O₅, Ta, W, Li, Be and precious stones. During Rodinia breakup, epicontinental carbonate-siliciclastic basins were deposited, which host important non-ferrous base metal deposits of Cu–Co and Pb–Zn–Ag in Africa and South America. Isotope Pb–Pb analyses of sulfides from the non-ferrous deposits unambiguously indicate an upper crustal source for the metals. A genetic model for these deposits involves extensional faults driving the circulation of hydrothermal mineralizing fluids from the Archean/Paleoproterozoic basement to the Neoproterozoic sedimentary cover. These relations demonstrate the individuality of metal associations of every sediment-hosted Neoproterozoic base-metal deposit of West Gondwana has been highly influenced by the mineralogical and chemical composition of the underlying igneous and metaigneous rocks.     

The crustal evolution of South America from a zircon Hf‐isotope perspective

Hf‐isotope data of >1100 detrital zircon grains from the Palaeozoic, south‐central Andean Gondwana margin record the complete crustal evolution of South America, which was the predominant source. The oldest grains, with crustal residence ages of 3.8–4.0 Ga, are consistent with complete recycling of existing continental crust around 4 Ga. We confirm three major Archaean, Palaeoproterozoic (Transamazonian) and late Mesoproterozoic to early Neoproterozoic crust‐addition phases as well as six igneous phases during Proterozoic to Palaeozoic time involving mixing of juvenile and crustally reworked material. A late Mesoproterozoic to early Neoproterozoic, Grenville‐age igneous belt can be postulated along the palaeo‐margin of South America. This belt was the basement for later magmatic arcs and accreted allochthonous microcontinents as recorded by similar crustal residence ages. Crustal reworking likely dominated over juvenile addition during the Palaeozoic era, and Proterozoic and Archaean zircon was mainly crustally reworked from the eroding, thickened Ordovician Famatinian arc.     

Paleoclimate changes in the late precambrian: Evidence from the upper precambrian section of the South Urals

The climatic impact on the formation of fine-grained rocks from the Riphean stratotype and Vendian Asha Group on the western slope of the South Urals during the time interval lasting approximately 1200 Ma is considered. It is shown that these rocks are largely represented by “tectonosilicate-dominated” shales. This feature combined with changes in the average K₂O/Al₂O₃ values disavows the hypothesis in (Kennedy et al., 2006), according to which the growth of free oxygen concentration in the Late Riphean and Vendian atmosphere was determined by gradual intensification of the organic carbon extraction from the biosphere by clays. The average values of the hydrolyzate module, chemical index of alteration (CIA), and several lithogeochemical parameters calculated for the Riphean and Vendian clayey rocks provide grounds for the conclusion that intensity of weathering in paleodrainage areas during the accumulation of the Upper Precambrian sedimentary successions was low. The curve reflecting changes of the average CIA values in the Upper Precambrian fine-grained siliciclastic rocks of the South Urals is similar to some extent with the “standard” CIA_correct. curve (GonzalezAlvarez and Kerrich, 2012). It is assumed that changes in microand macrobiotic communities during the Late Precambrian were controlled to a variable extent by climate fluctuations as well. At the same time, these fluctuations most likely left the chemical composition of water in the ocean virtually unchanged, which is evident from analysis of the redox conditions in the ocean and the distribution of primary producers with the average CIA_correct. and CIA values.     

Geological comparison of precambrian and early palaeozoic terrains between the Southern West Coast of Africa and the South-East Coast of South America

This paper describes a geological comparison of similar lithostratigraphic sequences of Proterozoic and early Palaeozoic ages between the southern west coast of Africa (Namibia and South Africa) and the south-east coast of South America (Brazil and Uruguay). The oldest terrains present within the compared regions are those comprising the Rio de la Plata and the Richtersveld Provinces of ~ 2000 Ma. Both provinces appear to have evolved from earlier Archaean terrains, i.e., the Guapore Craton in South America and the Kalahari Craton in southern Africa, during the Transamazonic and Eburnian orogenic cycles, respectively. Both the Namaqua Province, affected by the Kibaran orogenesis at ~ 1200 Ma, and the Sinclair Group of 1300 Ma seem to have no comparable geological counterparts in southern Brazil and Uruguay.The late Precambrian—early Palaeozoic successions of the Gariep and Ribeira belts investigated here seem to be remarkably similar and it is suggested that they were parts of the same geological unit, i.e., a mobile belt, developed during the Pan-African-Brasil- iano orogenesis. Geological comparisons are particularly good between the Gariep, Porongos and Lavalleja Groups, and stratigraphic correlations can be successfully at- tempted even between geological formations, i.e., Marica, Piedra de Afilar and Stink- fontein Formations.The lack of detailed geological maps for South America makes it difficult to separate different geological formations from broad lithostratigraphic successions and, consequent- ly, comparisons can only be drawn between subgroups and several geological formations, i.e., Vacacai and Minas Subgroups with Grootderm Holgat and Oranjemund Formations. The extensive magmatic activity generated during the Brasiliano orogenesis and only present in the south-east coast of South America has not affected the southern west coast of Africa. However, two post-tectonic complexes, i.e., the Kuboos—Bremen line of intrusives and the Sierra de Animas Complex appear to be geochronologically analogous and petrologically they seem to have certain similarities. It is also suggested that the Camaqua Group in southern Brazil and the Nama Group could perhaps have certain similarities within their lithological successions and their ages may be also com- parable.The late stages of the geological evolution of the basin, represented by molassic sediments, are only present in south-east Brazil. Finally an interpretation is given of the geological evolution of the Proterozoic—early Palaeozoic terrains compared in this work.     

The precambrian of West Africa

Consequent upon continued active field investigations and the advent of radioactive dating of samples from the area, the time is thought opportune to attempt a synthesis of Precambrian correlations throughout West Africa. Some isotope age determinations would, at first glance, appear to necessitate radically new concepts of stratigraphic successions and correlations. For example, the Ahaggar Suggarian and the Anti-Atlas Precambrian I suggest age values consonant with Upper or then Middle Precambrian times; the Older and Younger Granites of Nigeria appear to have been emplaced in Lower Palaeozoic and Jurassic times respectively. These and other examples trend towards younger ages than had hitherto been surmised for many stratigraphic units. However, it is thought premature at this time to place too much emphasis on isotope age values obtained for these ancient rocks of West Africa.Field studies to date have been more successful in establishing successions rather than correlations, whereas isotope age determinations have achieved little in either respect.Several well-known series, once placed in the Precambrian, e.g. Falemian, Akjoujt, Bakel, are now thought to be rather Cambro-Ordovician in age. On the other hand, for other familiar units, e.g. Ahnet Purple Series, Buem Formation, the stratigraphic position remains problematical.

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Zusammenfassung Nach fortgesetzten aktiven Feldforschungen und mit dem Erscheinen von radioaktiven Datumsbestimmungen von Proben aus dem Gebiet, scheint es an der Zeit, eine Synthese der präkambrischen Verhältnisse in ganz Westafrika zu unternehmen.Einige isotopische Altersbestimmungen scheinen auf den ersten Blick eine vollkommen neue Auffassung der stratigraphischen Sukzessionen und Korrelationen nötig zu machen. So lassen zum Beispiel das Ahaggar Suggarien und das Anti-Atlas Präkambrium I Alterswerte vermuten, die mit dem oberen und mittleren Präkambrium übereinstimmen; die älteren und jüngeren Granite in Nigeria scheinen im unteren Paläozoikum bzw. im Jura übergelagert worden zu sein. Diese und andere Beispiele zeigen einen Trend zu jüngerem Alter, als bisher für viele stratigraphische Einheiten angenommen wurde. Jedoch wird es für verfrüht gehalten, den isotopischen Alterswerten für diese alten Gesteine in Westafrika eine zu große Bedeutung beizumessen.Feldstudien waren bisher erfolgreicher bei der Herstellung von Sukzessionen als bei der Herstellung von Korrelationen, während die isotopischen Altersbestimmungen für beides wenig erbracht haben.Einige wohlbekannte Reihen, z. B. Falemien, Akjoujt und Bakel, die zuvor ins Präkambrium gelegt wurden, werden nun dem Alter nach eher dem Kambro-Ordovicium zugeordnet. Andererseits bleibt für andere bekannte Reihen, z. B. die Ahnet Purple Series und de Buem Formation, die stratigraphische Position problematisch.

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Résumé Par suite des recherches actives et prolongées sur le terrain et de la venue d'échantillons radio-actifs datés de cette région, le temps semble opportun pour tenter une synthèse de corrélations précambriennes de toute l'Afrique Occidentale.A première vue, quelques déterminations d'âge absolu semblent nécessiter de nouvelles idées de successions et de corrélations stratigraphiques. Par example, l'Ahagar Suggarien et l'Anti-Atlas Précambrien I suggèrent des valeurs d'âge conforme au temps Précambrien supérieur ou moyen; les Vieux Granites (Older Granites) et les Granites Jeunes (Younger Granites) du Nigéria semblent avoir été placés respectivement dans le Paléozoique inférieur et le Jurassique. Ces examples-ci, et encore d'autres, tendent à indiquer pour beaucoup d'unités stratigraphiques des âges plus jeunes qu'on n'a supposés jusqu'à présent. Mais il est encore prématuré d'appuyer trop sur les valeurs d'âge absolu obtenus pour ces anciens terrains de l'Afrique Occidentale.Jusqu'à présent, des recherches sur le terrain ont réussi à établir des successions plutôt que des corrélations, pendant que les déterminations d'âge absolu n'ont eu que peu de succès des deux côtés.Quelques séries bien connues, qui jadis étaient placées dans le Précambrien, p. ex. le Falmien, la Série de l'Akjoujt et les Quartzites de Bakel sont à présent considérés d'être plutôt de l'âge Cambro-Ordovicien. Par contre, la position stratigraphique reste problématique pour d'autres unités bien connues, comme pour la Série Pourprée de l'Ahnet, la formation de Buem, et d'autres.

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The antiquity of man in South America

The initial peopling of South America is reviewed in terms of geochronological and archaeological data. The problem is put in historical perspective and a systematic evaluation is made of the latest typologically oriented arguments for the presence of preprojectile point complexes. Field evidence is very weak for early Biface and Chopper traditions. There is better support for an early Flake tradition, but it is not likely to precede the use of stone projectile points.Radiocarbon dates and “terrace dating” should be used only with great caution and careful attention to the nature of cultural and stratigraphic associations. Uncritical acceptance of determinations on bone and soil samples has led to extremely problematic cultural reconstructions. Emphasis on stratified sites with finished artifacts, faunal assocations, and charcoal dates leads to a reconstruction at odds with those presented in recent textbooks. It is concluded that the bulk of solid evidence favors the entry of man as a big game hunter using stone projectile points and having an adaptation much like that of the Paleo-Indians of North America.     

Principal stages in evolution of precambrian organic world: Communication 2. The late proterozoic

A new suggested model outlining the evolution of the organic world from the mid-Early Proterozoic (∼2.0 Ga) to the Early Cambrian is based on data characterizing the relevant chert-embedded and compression-preserved organic-walled microbiotas, impressions of soft-bodied multicellular organisms, and biomarkers. Critical analysis of overall paleontological data resulted in the distinguishing of seven successive assemblages of Proterozoic micro- and macrofossils. Being of global geographic range, the assemblages correspond to the major stages in evolution of the organic world and typify global units which are termed the Labradorian (∼2.0–1.65 Ga), Anabarian (1.65–1.2 Ga), Turukhanian (1.2–1.03 Ga), Uchuromayan (1.03–0.85 Ga), Yuzhnouralian (0.85–0.635 Ga), Amadeusian (0.635–0.56 Ga), and Belomorian (0.56–0.535 Ga). Characteristic of the Labradorian unit are microfossil assemblages of the Gunflint type including remains of morphologically bizarre prokaryotic microorganisms: star-like Eoastrion, umbrella-shaped Kakabekia, dumbbell-shaped Xenothrix, and some others. Fine-grained siliciclastic deposits of the same age yield the oldest remains of millimeter-sized eukaryotes: spherical to ribbon-like Chuaria and Tawuia. Microfossils prevailing in shallow-water carbonate facies of the Anabarian unit are akinetes of nostocalean cyanophyceae Archaeoellipsoides and entophysalidacean cyanobacteria Eoentophysalis, whereas acanthomorphic acritarchs Tappania and Shuiyousphaeridium dominate the assemblages of open-shelf facies, where they are associated with the first-found rare macroscopic multicellular fossils Horodyskia. The distinguishing feature of the next Turukhanian unit is the first occurrence of filamentous red alga Bangiomorpha and the stalked cyanobacterium Polybessurus. The Uchuromayan unit is characterized by the appearance and worldwide radiation of structurally complicated eukaryotic microorganisms, primarily of acanthomorphic acritarchs Trachyhystrichosphaera and Prolatoforma, branching thalli of green algae Aimophyton, Palaeosiphonella, Palaeovaucheria and Proterocladus, and of spiral-cylindrical cyanobacteria Obruchevella. In the Yuzhnouralian unit is recorded the first occurrence of vase-shaped testate amoebas Melanocyrillium, Cycliocyrillium, and others, and of scale microfossils Characodictyon, Paleohexadictyon, etc. As distinct from the others, the Amadeusian unit characterizes the global expansion of acanthomorphic acritarchs of complex structure (the Pertatataka-like assemblage of Tanarium, Cavaspina, Appendisphaera, and others) and associated remains of red algae and cyanobacteria Obruchevella. The terminal Belomorian unit marks the extinction of Pertatataka-type microfossils, the appearance of soft-bodied multicellular organisms on different continents, and the origin of diverse skeletal fossils in the terminal phase.     

Excess europium content in precambrian sedimentary rocks and continental evolution

We propose that the europium excess in Precambrian sedimentary rocks, relative to those of younger age is derived from volcanic rocks of ancient island arcs, which were the source materials for the sediments. Precambrian sedimentary rocks and present-day volcanic rocks of island arcs have similar REE patterns, total REE abundances and excess Eu, relative to the North American shale composite. The present upper crustal REE pattern, as exemplified by that of sediments, is depleted in Eu, relative to chondrites. This depletion is considered to be a consequence of development of a grandioritic upper crust by partial melting in the lower crust, which selectively retains europium.     

华南板块前寒武纪的构造演化史及成矿事件

华南板块由扬子板块和华夏褶皱系在四堡期碰撞形成,与罗迪尼亚大陆形成时间一致,碰撞形成了变质岩、岩浆岩及蛇绿岩等地质特征。华南板块形成之后的860~740Ma期间,经历了激烈的岩浆岩活动,其成因主要有三种解释:地幔说、俯冲说、陆内裂谷说,至今没有一个比较统一的认识。前寒武纪的岩浆活动使华南板块发生重要的成矿事件,在扬子板块周边形成了康滇地轴、九万大山-元宝、钦杭等成矿带。     

The precambrian of north-western Queensland,Australia

Precambrian rocks, ranging in age from probable Archaean to Upper Proterozoic, crop out over an area of 25,000 square miles in north-western Queensland. They fall into four large divisions, separated by major unconformities. Little is known of the history of the Archaean rocks, which are believed to include altered acid lavas, schists, gneisses and migmatite. The Lower Proterozoic strata form part of an orogenic belt. The Lower Proterozoic lower sequence is rich in altered acid and basic lavas. It has a maximum thickness of not less than 40,000 feet and was strongly deformed by east-west pressure and probably intruded by granite before the Lower Proterozoic upper sequence was deposited. Sediments form the bulk of the Lower Proterozoic upper sequence; these accumulated most thickly to the west and north-west of the core of lower sequence belt, but overlie lower sequence strata. The greatest thickness of sediments is at least 40,000 feet. The sequence was deformed by a renewal of east-west compressive stresses, accompanied by granite emplacement. Folding is strong to moderate, but lacks well-defined linearity in the west of the outcrop area.Upper Proterozoic sediments and lavas accumulated mainly in the west and north-west of the region. They are gently to moderately folded on west to north-west axes.The Precambrian in north-western Queensland, excluding the Camooweal Dolomite, crops out over about 25,000 square miles. Systematic field work was started by joint teams of the Commonwealth Bureau of Mineral Resources and the Geological Survey of Queensland in 1950.Reconnaissance mapping of the region was completed in 1954 but check work and more detailed mapping of areas of economic interest are still in progress. Maps are being prepared at a scale of 1 inch to 4 miles (1: 253,440) and a report on the area is being written (Carter andBrooks, in preparation). The Camooweal Dolomite, of Upper Proterozoic or Lower Cambrian age, is not included. Its stratigraphic and palaeographic position is discussed byNoakes (1956).The reconstruction of the geological history of the region presented below is an interpretation of the data obtained. The evidence is not sufficiently complete to enable a unique interpretation to be made of all aspects of the region's history. In particular the history of granite intrusion has not yet been completely unravelled.Paper published by permission of the Secretary, Department of National Development.     

The composition of the precambrian ocean waters

The composition of the Precambrian oceans was different from the present composition; this is a direct consequence of a CO₂-atmosphere depleted in oxygen. At the beginning the mineralization was lower. The oceans waters were chloro-dicarbonated with a lower tenor of chlorine, richer in phosphates and they were depleted in sulphates; sulphur was in the reduced state of sulphides. The abundance of phosphates supported the expansion of life. The cations Na⁺ and Mg²⁺ coexisted with Ca²⁺, Fe²⁺ and Ba²⁺ whose levels were higher than they are today. The main trends of the evolution are schematized in the figure.     

The ecology of hazardousness,the experience of South America

In many parts of South America, the socio-economic condition of the population is such that in many respects the people, because of their vulnerable state, are a disaster waiting to occur. The triggering action of the disaster will be an extreme physical or biophysical event. If any advance is to be made in natural hazard management, then the focus must be upon people first. Halt the process of underdevelopment in South American societies and you have taken a major step forward in natural hazard management. The major concern of this paper is not so much with the competition for first place in hazard proneness in South America, but rather the elaboration of the total ecology of what we commonly designate as disasters, which ordinarily occur at the interface of extreme natural phenomena and vulnerable settlement patterns, and which should be seen, as the extreme situation which is implicit in the everyday condition of the population. Vulnerability is the key concept in this relationship. South American nations vary greatly in their hazard proneness, in the vulnerability of sectors in their societies, in the losses and general repercussions of hazardous events, and in their ability to cope effectively with the post-disaster situation, either alone or with international aid.     

Dynamic topography in South America

Supracrustal tectonics and mantle flow interact to create Earth's topography. While tectonics is associated with the isostatic components of topography, the deflections caused by mantle dynamics, or dynamic topography, represent the non-isostatic components. South America is an ideal natural laboratory to analyze these two contrasting components from the high Andes to the distal plains. Both regions are active and affected by complex geodynamic processes like the subduction of oceanic ridges, geometry and age of slabs, etc. These subducting anomalies affect not only the convergence dynamics and stresses along the entire margin, but also the distribution of mass anomalies in the mantle, which are the main cause of sublithospheric flow and dynamic topography. Here we revisited five examples from north to south, which demonstrate that, the Andes and the distal forelands have been uncompensated since the beginning of the Cenozoic and that additional forces, such as mantle downwellings and upwellings, are required to account for the observed topographies in basins and elevations.     

Metallogenic provinces in South America

This paper discusses the broadscale distribution of some ore deposit types in South America, trying to discern possible relations with major geotectonic features. It does not cover all ore deposits but rather a selection based on available information, genetic types, geographic distribution, and a representation of deposit types which are either abundant or very rare in South America. Included are porphyry copper, zoned copper-zinc-lead-silver, contact-metasomatic, red-bed copper-uranium-vanadium, lead-zinc (including Mississippi Valley type), mercury, tin, pegmatite, carbonatite and alkalic complex, diamond, nickelcobalt-chromite-asbestos-talc, iron, bauxite, tungsten, titanium, uranium, gold, potash and phosphate deposits. Comparisons are made with the distribution of these deposit types in other continents, especially in North America and Africa. This leads to some speculations bearing on theories of ore genesis. The evidence suggests that upper crustal processes predominate in the formation of the majority of ore deposits considered (excepting carbonatites, alkalic complexes and diamond pipes). Also, the relative level of an orogenic belt exposed by erosion seems to be an important factor bearing on the presently known distribution of many ore deposit types. Furthermore, the fact that ore deposits of given types tend to be aligned along the Cordillera in more or less defined belts with changing chemical composition accross the Andes, which partake in all its inflections, and in patterns which are similar in North and South America, indicates that they are either dependent on the distribution of eugeosynclinal vs miogeosynclinal sediments in the original geosynclinal prism, or that their character is related to distance from the major batholitic masses or to shallow stocks and volcanic activity. The indicated correlation with sediment character and evaporites speaks in favor of modem concepts viewing hydrothermal fluids as mainly meteoric but heated and circulated largely by igneous activity. This study raises more questions than it can answer and calls attention to a number of problems requiring further research. From a practical point of view, this investigation reveals several possibilities for future exploration, as well as opportunities for further development of mineral resources in South America. Several countries could probably improve or diversify their mineral production significantly, thus raising the standard of living of their increasing population. Whereas in North America both Canada and the U.S.A. span from east to west the whole range of geological environments, from cratonic to geosynclinal and orogenic, in South America the distribución of countries is essentially either along the Cordillera or over shield areas, a fact of obvious geopolitical implications with regard to mineral resources.     

Morphological and anatomical observations on some precambrian plants from the Witwatersrand,South Africa

The columnar form of carbonaceous layers in some Precambrian Witwatersrand conglomerates after detailed SEM examination has been shown to be a primary plant structure. By oxidation of the organic components at about 500° C a skeleton of length-orientated interconnected filaments is obtained which consist mainly of uranium oxide, lead oxide and gold. Some of the filaments preserved the original microbiological structures in detail.Brief heating of the columns causes a separation into different types of tissue thus making the reconstruction of the original plant possible. A comparison of the inner and outer structure and the probable metabolism with recent plants showed a definite resemblance to certain types of lichens. Following the presently used term of thucholite for the carbonaceous material the name ofThuchomyces lichenoides is suggested for the fossilized plant. A probably parasitic, filamentous fossilized microorganism which occurs in the Basal Reef together with Thuchomyces was namedWitwatersrand conidiophorus. Both fossils are described and their possible environment discussed.

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Zusammenfassung Die säulige Form von kohligen Lagen in den präkambrischen Witwatersrand Konglomeraten wurde durch detaillierte Untersuchungen im REM als primäre pflanzliche Struktur erkannt. Durch Oxydation der organischen Komponenten bei etwa 500° C erhält man ein Skelett aus längsorientierten und einander verknüpften mineralischen Filamenten (meist Uranoxyd, Bleioxyd und Gold), die zum Teil die ursprünglichen mikrobiologischen Formen bewahrt haben. Kurzes Erhitzen der Säulen bewirkt eine Aufspaltung in verschiedene Gewebearten wodurch eine Rekonstruktion der ursprünglichen Pflanze möglich wurde. Vergleiche des inneren und äußeren Aufbaus mit rezenten Pflanzen zeigt deutliche Ähnlichkeiten mit bestimmten Flechtenarten. In Anlehnung an die bisher gebräuchliche Bezeichnung Thucholith für die kohlige Substanz wirdThuchomyces lichenoides als Name für die fossile Pflanze vorgeschlagen. Ein wahrscheinlich parasitärer, fadenförmiger, fossiler Mikroorganismus, der im Basal Reef zusammen mit Thuchomyces vorkommt, wurdeWitwateromyces conidiophorus benannt.Die beiden Fossilien werden beschrieben und ihre mögliche Umwelt diskutiert.

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Résumé L'étude détaillée, au microscope électronique à balayage, de la forme colonnaire de couches charbonneuses de certains conglomérats Précambriens du Witwatersrand, a permis d'y reconnaître une structure végétale primaire. Par l'oxydation des composants organiques a environ 500°, on obtient un squelette de filaments reliés entre eux et orientés dans le sens de la longueur, principalement à base d'oxyde d'uranium, d'oxyde de plomb, et d'or. Quelques-uns de ces filaments ont conservé leurs structures microbiologiques originelles dans tous leurs détails.Un bref réchauffement des colonnes cause la séparation des différentes sortes de tissus à partir desquels il est possible de reconstituer la plante originelle. Une comparaison de la structure interne et externe et du métabolisme probable avec des plantes récentes indique une ressemblance indéniable avec certaines sortes de lichens. Compte tenu de la désignation de thucholite pour la substance charbonneuse, le nom deThuchomyces lichenoides est suggéré pour cette plante fossile.Un autre micro-organisme filamenteux, probablement parasitaire, découvert dans le Basal Reef, avec Thuchomyces, a reçu le nom deWitwateromyces conidiophorus. Les deux fossiles sont décrits et leur environnement probable, discuté.

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. 600° C — . . , —, . , . . «Thucholit» «Thuchomyces lichenoides», , , — «Witwateremyces conidiophorus». .

     

Glacial history of southernmost South America

In southernmost South America, an incomplete radiometrically dated glacial chronology has been obtained by KAr dating for the interval 3.5-1 MY ago, and a more detailed chronology by C-14 dating for the last 25,000 years, with some older minimal ages. The first major glaciation was about 3.5 MY ago during the middle Pliocene. Little is yet known about glacial fluctuations during the interval 3.5-2.1 MY ago. Between 2.1 and 1 MY ago many glaciations occurred, probably including the greatest of late Cenozoic time which took place after 1.2 MY and, according to inconclusive evidence, before 1 MY ago. The Patagonian Gravel in its type area is mid-Pliocene to early Pleistocene glacial outwash that accumulated from the first to the greatest glaciations. During the late Pleistocene several glaciations occurred, but only the most recent has been radiometrically dated. During the last glaciation the glaciers were most extensive before 56,000 BP. Successively smaller advances that culminated about 19,500 BP and, probably, about 13,000 BP were separated by an interstade when glaciers shrank by more than half. The glaciers receded rapidly after 13,000 BP and were within their present borders by 11,000 BP; they remained so during the European Younger Dryas Stade 11,000-10,000 BP. Neoglacial regional readvances culminated 4600-4200 BP, probably 2700-2000 BP, and during the last three centuries; most glaciers reached their Neoglacial maxima during the first episode. Between readvances, the glaciers shrank within their present borders.     

REE Geochemical Evolution and Its Significance of Early Precambrian Metamorphic Terrain,Wuyang,Henan

The supracrustal rocks of the Wuyang metamorphic terrain are divided into the Zhao anzhuang,Tieshanmiao and Yangshuwan Formations.These three Formations were dated at 3000-2550Ma,2550-2300Ma and 2300-2200Ma,respectively.∑REE and La/Yb)n of the Zhao anzhuang Formation volcanic rocks are obviously higher than those of the Tiesanmiao Formation equivalents,suggesting a sedimentary gap(2550 Ma boundary)between these two formations,The Zhao‘anzhuang Formation is older than the Tieshanmiao Formation.The sediments of these two Formations show no obvious differences in REE and are generally characterized by low ∑REE and positive Eu anomalies.On the contrary,the sediments of the Yangshuwan Formation are characterized by high ∑REE and negative Eu anomalies.Detailed discussions demonstrate that the Yangshuwan Formation was deposited in an oxidizing environment whereas the other two formations were formed in a reducing environment.At the end of the evolution of the Tieshanmiao Formation about 2300 Ma ago,the sedimentary environment was transformed from reducing to oxidizing .On the basis of the SHAB (soft/hard acid and base)theory,an oxidation-reduction model for sedimentary REE evolution has been established .It is proposed that the mantle tends to become gradually depleted in REE.especially in LREE,and the indices ∑REE and La/Yb) n of mantle-dervived volcanic rocks also tend to become lower and lower.     

南海成因及其演化模式探讨

尽管南海已进行深入的调查与研究,提出多种成因模型,包括挤出模型、弧后扩张模型、古南海俯冲拖曳模型等,但因其所处构造位置特殊,周边构造环境经历了复杂的改造,所有成因模式均未能得到广泛的认可。本文从三大板块相互作用入手,结合南海实测数据,提出南海形成的弧后扩张—左旋剪切模型。认为南海是古南海往北俯冲的弧后盆地,菲律宾海板块往北漂移形成的大规模左旋走滑是南海扩张的触发因素。印度—欧亚碰撞产生中南半岛挤出主要影响西南海盆扩张方向,使得扩张轴从近东西向转为北东向。南海及邻区晚中生代以来的演化可以分为以下阶段：1）早白垩世开始澳大利亚板块往北漂移,新特提斯洋往北俯冲消亡,导致弧后扩张,形成古南海;2）晚白垩世末—始新世,古南海往北俯冲,导致弧后拉张形成陆缘裂谷;3）早渐新世,受菲律宾海板块西缘大型左旋走滑影响,在原有裂谷的基础上从东往西海底扩张,形成南海;4）渐新世末,受俯冲后撤的影响,扩张中心往南跃迁,同时受西缘断裂左旋活动的影响,扩张轴从近东西西逐步转为北东向;5）早中新世晚期,南沙地块—北巴拉望地块与卡加延脊碰撞,南海扩张停止。