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‣ Precambrian geotectonic units of the Rio de La Plata craton

BETTUCCI, Leda Sanchez; PEEL, Elena; OYHANTCABAL, Pedro
Fonte: TAYLOR & FRANCIS INC Publicador: TAYLOR & FRANCIS INC
Tipo: Artigo de Revista Científica
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The main Precambrian tectonic units of Uruguay include the Piedra Alta tectonostratigraphic terrane (PATT) and Nico Perez tectonostratigraphic terrane (NPTT), separated by the Sarandi del Yi high-strain zone. Both terranes are well exposed in the Rio de La Plata craton (RPC). Although these tectonic units are geographically small, they record a wide span of geologic time. Therefore improved geological knowledge of this area provides a fuller understanding of the evolution of the core of South America. The PATT is constituted by low-to medium-grade metamorphic belts (ca. 2.1 Ga); its petrotectonic associations such as metavolcanic units, conglomerates, banded iron formations, and turbiditic deposits suggest a back-arc or a trench-basin setting. Also in the PATT, a late to post-orogenic, arc-related layered mafic complex (2.3-1.9 Ga), followed by A-type granites (2.08 Ga), and finally a taphrogenic mafic dike swarm (1.78 Ga) occur. The less thoroughly studied NPTT consists of Palaeoproterozoic high-grade metamorphic sequences (ca. 2.2 Ga), mylonites and postorogenic and rapakivi granites (1.75 Ga). The Brasiliano-Pan African orogeny affected this terrane. Neoproterozoic cover occurs in both tectonostratigraphic terranes, but is more developed in the NPTT. Over the past 15 years...

‣ The geotectonic environments of Early Precambrian granulites in Brazil

Wernick, Eberhard; De Almeida, Fernando F.M.
Fonte: Universidade Estadual Paulista Publicador: Universidade Estadual Paulista
Tipo: Artigo de Revista Científica Formato: 1-17
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A review is presented concerning Archaean granulites occurring in some old domains of the South American Platform, which was consolidated at the end of the Brazilian Cycle (900-500 Ma). The rocks occur in different geotectonic environments and show variable ages, structures and lithological associations. The most important complexes are the Atlantic Granulite Belt in the São Francisco Craton and the Goias Granulite Belt in the Central Goias Massif, both several hundred kilometres long. The former is composed of the Caraibas Complex, the Jequié Complex, the Salvador Complex and several minor granulite occurrences along the Brazilian coast in the States of Espírito Santo and Rio de Janeiro. The latter includes the large basic-ultrabasic complexes of Barro Alto, Tocantins and Canabrava. Both belts consist of massive or foliated rocks, banded or homogeneous and varying from acidic to ultrabasic in composition. They are the result of metamorphism affecting diversified supra- and infracrustal material. The Atlantic Granulite Belt lies between greenstone/granite terrains which show ovoid and boomerang-type dome structures. The contacts between both are either tectonic or transitional. Another occurrence of Archaean granulites comprises intercalations of palaeosomes and melanosomes within migmatites and anatectic rocks. These vary in size from small lenses to irregular complexes which may attain sizes of several hundred square kilometres. Apart from migmatites...

‣ Recognition of Cretaceous, Paleocene, and Neogene tectonic reactivation through apatite fission-track analysis in Precambrian areas of southeast Brazil: Association with the opening of the south Atlantic Ocean

Tello Saenz, C. A.; Hackspacher, P. C.; Hadler Neto, J. C.; Iunes, P. J.; Guedes, S.; Ribeiro, L. F B; Paulo, S. R.
Fonte: Universidade Estadual Paulista Publicador: Universidade Estadual Paulista
Tipo: Artigo de Revista Científica Formato: 765-774
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Apatite fission-track analysis was used for the determination of thermal histories and ages in Precambrian areas of southeast Brazil. Together with geological and geomorphologic information, these ages enable us to quantify the thermal histories and timing of Mesozoic and Cenozoic epirogenic and tectonic processes. The collected samples are from different geomorphologic blocks: the high Mantiqueira mountain range (HMMR) with altitude above 1000 m, the low Mantiqueira mountain range (LMMR) under 1000 m, the Serra do Mar mountain range (SMMR), the Jundiá and Atlantic Plateaus, and the coastline, all of which have distinct thermal histories. During the Aptian (∼120 Ma), there was an uplift of the HMMR, coincident with opening of the south Atlantic Ocean. Its thermal history indicates heating (from ∼60 to∼80 °C) until the Paleocene, when rocks currently exposed in the LMMR reached temperatures of ∼100 °C. In this period, the Serra do Mar rift system and the Japi erosion surface were formed. The relief records the latter. During the Late Cretaceous, the SMMR was uplifted and probably linked to its origin; in the Tertiary, it experienced heating from ∼60 to ∼90 °C, then cooling that extends to the present. The SMMR, LMMR...

‣ Ambient Pyrite in Precambrian Chert: New Evidence and a Theory

Knoll, Andrew H.; Barghoorn, Elso S.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /06/1974 Português
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Ambient pyrites of two distinct types were described from middle Precambrian rocks of the Lake Superior area. A new class of this phenomenon is here described from middle Precambrian chert from western Australia. The newly found ambient pyrites are quite minute and characteristically occur in groups forming a “starburst” pattern. All three types of ambient pyrite may be explained in terms of pressure solution initiated by gas evolution from organic material attached to the pyrite. Thermal degradation of the kerogen produces the gases which, due to the impermeability of the encompassing chert, build up the pressures necessary to initiate solution. Pyrite appendages bear a striking resemblance to micro-organisms and, thus, constitute the smallest pseudofossils known.

‣ Secular Change in the Precambrian Silica Cycle: Insights from Chert Petrology

Simonson, Bruce M.; Maliva, Robert G.; Knoll, Andrew
Fonte: Geological Society of America Publicador: Geological Society of America
Tipo: Artigo de Revista Científica
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Chert deposits preserve a record of secular change in the oceanic silica cycle. The evolutionary radiation of silica-secreting organisms resulted in a transition from abiological silica deposition, characteristic of the Archean and Proterozoic eons, to the predominantly biologically controlled silica deposition of the Phanerozoic. Comparative petrography of Phanerozoic and Precambrian cherts indicates that an earlier change in chert deposition occurred toward the end of the Paleoproterozoic era (ca. 1.8 Ga). In Neoproterozoic and Mesoproterozoic strata, early diagenetic chertification is largely restricted to peritidal environments. These early diagenetic cherts typically occur as nodules or discontinuous beds within carbonate deposits that have similar depositional textures. The cherts formed primarily by carbonate replacement with subsidiary direct silica precipitation. Some of the Paleoproterozoic cherts associated with iron formations, however, are distinctly different from younger cherts and appear to have formed largely by direct silica precipitation at or just below the seabed. These primary cherts lack ghosts or inclusions of carbonate precursors, have fine-scale grain fracturing (possibly from syneresis), exhibit low grain-packing densities...

‣ Anomalous Carbonate Precipitates: Is the Precambrian the Key to the Permian?

Grotzinger, John P.; Knoll, Andrew
Fonte: Society for Sedimentary Geology Publicador: Society for Sedimentary Geology
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Late Permian reefs of the Capitan complex, west Texas; the Magnesian Limestone, England; Chuenmuping reef, south China; and elsewhere contain anomalously large volumes of aragonite and calcite marine cements and seafloor crusts, as well as abundant microbial precipitates. These components strongly influenced reef growth and may have been responsible for the construction of rigid, open. reefal frames in which bryozoans and sponges became encrusted and structurally reinforced. In some cases, such as the upper biostrome of the Magnesian Limestone, precipitated microbialites and inorganic crusts were the primary constituents of the reef core. These microbial and inorganic reefs do not have modern marine counterparts; on the contrary, their textures and genesis are best understood through comparison with the older rock record, particularly that of the early Precambrian. Early Precambrian reefal facies are interpreted to have formed in a stratified ocean with anoxic deep waters enriched in carbonate alkalinity. Upwelling mixed deep and surface waters, resulting in. massive seafloor precipitation of aragonite and calcite. Luring Mesoproterozoic and early Neoproterozoic time, the ocean became more fully oxidized, and seafloor carbonate precipitation was significantly reduced. However...

‣ Microfossils from the Late Precambrian Draken Conglomerate, Ny Friesland, Svalbard

Knoll, Andrew
Fonte: The Paleontological Society Publicador: The Paleontological Society
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An intraformational "flake" conglomerate within the Late Precambrian (700-800 m.y.) Draken Conglomerate of Ny Friesland, Spitsbergen, contains abundant, well preserved microfossils in silicified shards of microbial mats, as well as in associated silicified clasts of non-stromatolitic mud. Sedimentation apparently took place in a lagoonal environment subjected to occasional storms. Within one local area, three distinct microbial mat associations can be recognized, each dominated by a single filamentous cyanobacterial mat builder occurring in densely interwoven patterns oriented parallel to bedding. One mat association often contains an associate mat building filament, and two of the three associations harbor mat-specific stromatolite dwelling blue-greens. Three taxa, all probable cyanobacteria, apparently lived as microbenthos in the non-stromatolitic mud. Approximately a dozen planktonic taxa have been recognized in this microbiota; these occur as randomly scattered individuals and clusters in all three mat associations and in the carbonate mud. The Draken microbiota provides a case study of the ecological heterogeity and microbial distribution within a single late Precambrian environment, and suggests the importance of paleoecology in analyses of Proterozoic evolution and biostratigraphy. Twenty-eight taxa are recognized of which eight are formally described as new: Salome svalbardensis n. gen. et ap....

‣ Precambrian Eukaryotic Organisms: A Reassessment of the Evidence

Barghoorn, Elso S.; Knoll, Andrew Herbert
Fonte: American Association for the Advancement of Science Publicador: American Association for the Advancement of Science
Tipo: Artigo de Revista Científica
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Comparison of partially degraded unialgal cultures of Chroococcus turgidus with coccoid microfossils from the Late Precambrian Bitter Springs formation, Australia, suggests that the Precambrian fossil record has been seriously misinterpreted. Use of degradational features as taxonomic characters has resulted in unrealistically high estimates of Precambrian algal diversity. There is at present no compelling evidence for the presence of eukaryotic microfossils in rocks from the Bitter Springs formation or any older sedimentary sequences.; Organismic and Evolutionary Biology

‣ The biostratigraphy and palaeoecology of South Australian Precambrian stromatolites

Preiss, Wolfgang Victor
Fonte: Universidade de Adelaide Publicador: Universidade de Adelaide
Tipo: Tese de Doutorado Formato: 4467662 bytes; 5102768 bytes; 4307202 bytes; 1288658 bytes; 848266 bytes; 3586933 bytes; 5051465 bytes; 2952314 bytes; 230082 bytes; application/pdf; application/pdf; application/pdf; application/pdf; application/pdf; application/pdf; application/pdf; app
Publicado em //1971 Português
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Precambrian stromatolites in South Australia are almost entirely restricted to the folded rock sequence of the Adelaide Geosyncline, a large, deeply subsiding basin with predominantly shallow - water sediments. The history of research into the age and fossils of the Precambrian rocks is reviewed, and a possible time - framework is suggested on the basis of available radiometric data. Stromatolites, laminated structures formed by trapping of detritus and precipitation of chemical sediment by algae and bacteria, have been studied by other workers from at least two points of views : most Western authors regard stromatolite morphology to be purely environmentally determined, while one Russian school maintains that it is largely controlled by the algae present, and that stromatolites evolve as a consequence of the evolution of the algae forming them. They concluded this from an empirical study of widespread stromatolites of different ages, which made possible the biostratigraphic subdivision and correlation of many Late Precambrian sections. The Russian methods of study and taxonomy have now been applied to South Australian stromatolites for the first time. Of the eighteen forms of columnar stromatolites described, five are identical or nearly identical to Russian forms. Nine forms are new...

‣ Sedimentology of the late Precambrian Mundallio Subgroup : a clastic - carbonate ( Dolomite, Magnesite ) sequence in the Mt. Lofty and Flinders Ranges, South Australia

Uppill, Robin K
Fonte: Universidade de Adelaide Publicador: Universidade de Adelaide
Tipo: Tese de Doutorado Formato: 950745 bytes; 5355009 bytes; 5720446 bytes; 1437941 bytes; 2175186 bytes; 1767412 bytes; 1573173 bytes; 3484808 bytes; 2945863 bytes; 428823 bytes; application/pdf; application/pdf; application/pdf; application/pdf; application/pdf; application/pdf; appli
Publicado em //1980 Português
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During deposition of the mixed carbonate - clastic sequence of the Mundallio Subgroup, the " Adelaide Geosyncline " was a very shallow, elongate sedimentary basin, flanked to the west and east by older Precambrian basement. In much of the southern and northern Flinders Ranges, clastic deposition predominated in the lower Mundallio Subgroup. In the north, alternating development of shallow mudflats and sandflats ( Nankabunyana Formation ) depended on the interplay between the sediment supply and winnowing processes, while dolomite mudstones were locally deposited in the shallowest areas. In the eastern half of the Willouran Ranges, massive shales were deposited as the environment remained persistently below wave base ( Camel Flat Shale ), but a renewed sand influx led to deposition of the Tilterana Sandstone. In the southern Flinders Ranges, terrigenous clay and silt were deposited on submergent mudflats which shallowed into intermittently exposed dolomite mudflats ( Nathaltee Formation ). Dolomite mudflats were a more persistent feature in areas more distal from the terrigenous source, and sometimes contained isolated, ephemeral lakes which were sites of magnesite deposition ( Yadlamalka Formation ). Dolomite and magnesite mudstone deposition of the Yadlamalka Formation became wide spread in the northern and southern Flinders Ranges in the upper Mundallio Subgroup...

‣ Dating of Precambrian metasedimentary rocks and timing of their metamorphism in the Soursat Metamorphic Complex (NW IRAN): Using LA-ICP-MS, U-Pb dating of zircon and monazite

Badr, M.; Masoudi, F.; Collins, A.; Cox, G.
Fonte: National Center for Scientific Research Publicador: National Center for Scientific Research
Tipo: Artigo de Revista Científica
Publicado em //2010 Português
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Soursat Metamorphic Complex (SMC) in west of Takab city is one of the polyphase metamorphic terrenes in northern Sanandaj-Sirjan metamorphic belt of Iran. The SMC composed mainly of metasedimentary rocks associated with granitic intrusions. Based on metamorphic rocks, two metamorphic phases could be separate in the complex. M1 is representative of regional metamorphisms which varies from greenschist to amphibolite facies and consists of mica-schist, garnet-schist, staurolite-schist, kyanite- schist, fibrolite-schist, garnet amphibo‌lites, marble, gneiss. M2 is contact metamorphism with clear outcrop just in the central of SMC and overprinted on M1. This phase consists of andalusite-schist, cordierite-schist and Actinolite- schist. SMC is in tectonic contact with Precamb‌rian to Paleozoic sedimentary rocks which make it difficult to date it based on stratigraphy. In this study, U-Pb dating of zircons and Monazites used in order to find the ages of deposition and metamorphism of metasediments in SMC. U-Pb dating of zircons from a staurolite-schist in the complex by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yielded a maximum Precambrian depositional age of 605±43. Monazites were also dated from a garnet-schist using the same technique and yielded a 238U/206Pb age of 61±8 which is interpreted as the age of the regional metamorphism. Based on these ages...

‣ Evolution of Precambrian life in the Brazilian geological record

Fairchild, Thomas Rich; Sanchez, Evelyn A. M.; Pacheco, Mirian Liza A. F.; Leme, Juliana de Moraes
Fonte: CAMBRIDGE UNIV PRESS; NEW YORK Publicador: CAMBRIDGE UNIV PRESS; NEW YORK
Tipo: Artigo de Revista Científica
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Precambrian rocks comprise nearly one-quarter of the surface of Brazil and range from Paleoarchean (ca. 3.6 Ga) to the latest Ediacaran (0.542 Ga) in age. Except for controversial phosphatized 'embryo-like' microfossils like those from the lower Ediacaran Doushantuo Formation, China and complex rangeomorphs, Brazilian research has revealed all major categories of Precambrian life forms described elsewhere - microbialites, biomarkers, silicified microfossils, palynomorphs, vase-shaped microfossils, macroalgae, metazoans, vendobionts and ichnofossils - but the paleobiological significance of this record has been little explored. At least four occurrences of these fossils offer promise for increased understanding of the following aspects of Precambrian biospheric evolution: (i) the relationship of microbialites in 2.1-2.4 Ga old carbonates of the Minas Supergroup in the Quadrilatero Ferrifero, Minas Gerais (the oldest Brazilian fossils) to the development of the early oxygenic atmosphere and penecontemporaneous global tectonic and climatic events; (ii) the evolutionary and biostratigraphic significance of Mesoproterozoic to Ediacaran organic-walled microfossils in central-western Brazil; (iii) diversity and paleoecological significance of vase-shaped heterotrophic protistan microfossils in the Urucum Formation (Jacadigo Group) and possibly the Bocaina Formation (Corumba Group)...

‣ Disparate rates, differing fates: tempo and mode of evolution changed from the Precambrian to the Phanerozoic.

Schopf, J W
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 19/07/1994 Português
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Over the past quarter century, detailed genus- and species-level similarities in cellular morphology between described taxa of Precambrian microfossils and extant cyanobacteria have been noted and regarded as biologically and taxonomically significant by numerous workers world-wide. Such similarities are particularly well documented for members of the Oscillatoriaceae and Chroococcaceae, the two most abundant and widespread Precambrian cyanobacterial families. For species of two additional families, the Entophysalidaceae and Pleurocapsaceae, species-level morphologic similarities are supported by in-depth fossil-modern comparisons of environment, taphonomy, development, and behavior. Morphologically and probably physiologically as well, such cyanobacterial "living fossils" have exhibited an extraordinarily slow (hypobradytelic) rate of evolutionary change, evidently a result of the broad ecologic tolerance characteristic of many members of the group and a striking example of G. G. Simpson's [Simpson, G.G. (1944) Tempo and Mode in Evolution (Columbia Univ. Press, New York)] "rule of the survival of the relatively unspecialized." In both tempo and mode of evolution, much of the Precambrian history of life--that dominated by microscopic cyanobacteria and related prokaryotes--appears to have differed markedly from the more recent Phanerozoic evolution megascopic...

‣ An overview of Precambrian rocks in Sonora

Anderson, Thomas H.; Silver, Leon T.
Fonte: Universidad Nacional Autónoma de Mexico Publicador: Universidad Nacional Autónoma de Mexico
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em //1981 Português
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The oldest stratified rocks recognized in NW Sonora (and in Mexico) are deformed muscovite-quartz schists, quartzites, and biotite-quartzofeldspathic gneisses near Caborca, which are cut by calcalkaline intrusives ranging from 1,710 to 1,750 m.y. in age. Southwest of Caborca, upper amphibolite facies layered quattzofeldspathic and amphibolitic gneisses were apparently deformed and metamorphosed at about 1,660 ± 15 m.y. ago, concealing original lithologies and ages. In northeastern Sonora, a younger belt of eugeosynolinal strata, about 1,680 ± 20 m. y. old was tightly folded and metamorphosed to greenschist facies about 1,650 m.y. ago. Numerous granitic plutons intruded into the older Precambrian crust about 1,410 to 1,440 m.y. ago. These major intrusive masses are not known to have been accompanied by regional sedimentation or deformation. Rare, small plutons of micrographic granite added to the Precambrian crystalline complexes about 1,100 m.y. ago, are the youngest Precambrian igneous rocks recognized. They limit the age of a thick miogeoclinal sequence of unmetamorphosed quartzose sandstones, carbonates with numerous stromatolite horizons, and shales which rest nonconformably on them. The sequence is overlain without unconformity by a fossiliferous Lower Cambrian section. The northwestern and northeastern Precambrian suits appear to be separated by a Jurassic magmatic arc and a postulated shear structure of large lateral displacement. Both suites correlate northward into related belts in the SW United States. To the east they are concealed by Phanerozoic cover. Abrupt termination of Precambrian exposures south and west suggests major younger tectonic features which we suspect played important but undefined roles in the apparent absence of Precambrian basement under much of northern and west-central Mexico.

‣ Age measurements of the Precambrian rocks of the Death Valley-Mojave Desert region, California

Lanphere, Marvin; Wasserburg, G. J.
Fonte: American Geophysical Union Publicador: American Geophysical Union
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em //1962 Português
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Ar^(40)-K^(40) and Sr^(87)-R^(87) age measurements in the eastern Mojave Desert indicate two separate early Precambrian events (see table). The older event is approximately 1650 m.y. old and is evidenced by pegmatites and associated metamorphic rocks in the Mountain Pass district. Ages were measured on coarse muscovite and potassium feldspar, MP-1 and MP-2, from a pegmatite which cuts across biotite-bearing gneisses, MP-7 and MP-9. These data confirm the widespread areal extent of this ancient metamorphic terrane. Ages of biotite, MP-21 and MP-22, from the shonkinite, which intrudes the metamorphic rocks, at Mountain Pass and the Rb-Sr age of potassium feldspar, MM-3f, from granite in the Marble Mountains suggest a period of igneous intrusion in the 1350 to 1410 m.y. interval. Metamorphic rocks in the central Panamint Range have been mapped and are shown to be stratigraphically early Precambrian. K-Ar ages of approximately 80 m.y. have been measured on biotite, muscovite, and hornblende. The minerals show no memory of a Precambrian age. The early Precambrian rocks show no evidence of a younger period of metamorphism. However, a younger metamorphism can be recognized in the overlying Precambrian(?) Noonday dolomite and Johnnie formation.

‣ The Upper Precambrian of South America

Almeida, Fernando F M de; Hasui, Yociteru; Neves, Benjamim Bley de Brito
Fonte: Universidade de São Paulo, Instituto de Geociências Publicador: Universidade de São Paulo, Instituto de Geociências
Tipo: info:eu-repo/semantics/article; info:eu-repo/semantics/publishedVersion; ; ; ; ; ; Formato: application/pdf
Publicado em 01/12/1976 Português
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This paper deals with the structural organization and tectonic evolution of South American Continent basement during the Upper Precambrian. The South American Platform is the old platform of South America. It has more than half of this extension covered by sediments and volcanic rocks of Phanerozoic age; the basement is exposed in three vast shields and several little massifs. In the exposed basement some cratonic nuclei have been distinguished, with structures developed in the Middle Precambrian (Trans-Amazonic) and Lower Precambrian (Jequié and Guriense). The Lower Precambrian structures are described in small scattered nuclei, all the rest seeming to have been remobilized by tectonic, magmatic and thermal processes of Trans-Amazonic age. These processes affected large areas but are still insufficiently understood. In the Upper Precambrian, these cratonic nuclei underwent intense process of reactivation, in large areas, with formation of volcano-sedimentary covers, acid, basic and alkaline intrusive rocks, cataclastic zones and thermally affected zones. During the Upper Precambrian, geosynclinal evolution processes developed at the borders and between the cratons, generating fold belts and regions. The firstly developed belt is located in Central Brazil...

‣ Search for shock-metamorphosed grains in Precambrian spherule layers

Smith, Frank C.
Fonte: University of Delaware Publicador: University of Delaware
Tipo: Tese de Doutorado
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Glass, Billy P.; Martin, Ronald E.; There is minimal physical evidence in only a few of the ???17 Precambrian spherule layers to support an impact origin. A search was done for shock-metamorphosed grains in the following spherule layers: Carawine, Jeerinah, and Bee Gorge (formerly Wittenoom) in Western Australia, Monteville in South Africa, and Gr??nses?? in South-West Greenland. Samples went through acid digestion, and the residues were wet sieved. The 63-125 ??m (?? 125-250 ??m) size fractions went through heavy liquid separation. For most samples, the heavy mineral assemblages consist predominantly of anatase, rutile, tourmaline, and zircon (?? chrome spinel) grains. Using micro-Raman spectroscopy, the high-pressure, ??-PbO2 -structured polymorph of TiO2 (TiO2 II) was identified in 27 buff rutile grains from the Carawine, Jeerinah, Bee Gorge, and Monteville spherule layers. For three of the layers, rutile + TiO2 II grains were found only in their upper parts. For a sample or stratigraphic subdivision within a sample, rutile + TiO2 II grains comprise ???1-5% of the rutile population. The TiO2 II polymorph is interpreted as a shock-induced phase that is syngenetic with respect to its host spherule layer. The rutile + TiO2 II grains provide physical evidence to support an impact origin for these four spherule layers. Using a universal stage microscope...

‣ The tectonostratigraphy, granitoid geochronology and geological evolution of the Precambrian of southern Ethiopia

Yibas, B; Reimold, W U; Armstrong, Richard; Koeberl, Christian; Anhaeusser, C.R.; Phillips, David
Fonte: Pergamon-Elsevier Ltd Publicador: Pergamon-Elsevier Ltd
Tipo: Artigo de Revista Científica
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Two distinct tectonostratigraphic terranes, separated by repeatedly reactivated deformation zones, are recognised in the Precambrian of southern Ethiopia: (1) granite-gneiss terrane, which is classified into sub-terranes and complexes, and (2) ophiolitic

‣ Late Precambrian paradoxal glaciation and obliquity of the Earth - A Discussion of dynamical constraints

Pais, M; Le Mouel, J; Lambeck, Kurt; Poirier, J
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
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G.E. Williams' observations on Late Precambrian glaciogenic sequences pose some major challenges to the understanding of the dynamical evolution of the Earth's orbit and spin that have not yet been adequately addressed. In this paper, we investigate whether dissipative core-mantle coupling is an effective mechanism for reducing the obliquity by such a large amount as suggested by Williams [G.E. Williams, Earth Sci. Rev. 112 (1993) 1-45], and we examine the relationship between the obliquity and spin rate in the presence of such coupling. We show that this mechanism not only cannot explain the rapid and substantial decrease in obliquity, but also that such a decrease is in contradiction with existing paleorotation evidence for the same period, thus confirming the previous findings of Neron de Surgy and Laskar [O. Neron de Surgy, J. Laskar, Astron. Astrophys. 318 (1997) 975-989].

‣ FOSSIL CONTENT AND STRUCTURAL RELATIONSHIPS OF THE SAN LUIS ZONE and THE CABORCA ZONE OF NW SONORA, MEXICO. SUPPRESSION OF THE PRECAMBRIAN Z OF CABORCA

Radelli,Luigi; Weiss,Vivianne Solis; Dórame-Navarro,Miguel; De La Cruz-Ortega,Lissette del Carmen; Urrutia,Jesé
Fonte: Boletin de Geología Publicador: Boletin de Geología
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/12/2008 Português
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In the Caborca region of western Sonora a Precambrian Z does not cover a unique Precambrian socle as previously believed. Two tectonic zones occur there instead : the San Luis Zone and the Caborca Zone. The first is comprised of the Precambrian San Luis socle of gneiss and granite, crossed by 1.1 Ga old anorthosites, the San Luis sedimentary cover, and, above it, a Lower Jurassic volcano-sedimentary sequence. The Caborca Zone consists of the Precambrian Bamori socle of parametamorphic rocks crossed by 1.1 Ga old Aibo granite, and of the Gamuza sedimentary cover. 1.1 Ga ago the two zones were far away from each other. They have been brought together by the Nevadian orogeny. Both zones are allochthonous, and the Caborca Zone is a nappe upon the San Luis Zone. The lowermost units of the Gamuza cover furnished psammocorals and a possible Pterophyllum jageri (?). Accordingly, its geological age is either Palaeozoic or Triassic.The San Luis cover furnished Nematophites [Prototaxites (?) and Nematothallus] from its lower part; Calcispongiae, Cardaicarpus' seeds, and Artisia from its upper part. Thus, it is a Devono-Carboniferous unit. The study area belongs in the « Baja-Borderland » block, which underwent, an Eocene northwards drifting of about 900 - 1000 km...