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‣ Análise dos Estromatólitos do Grupo Itaiacoca (Proterozóico), ao Sul de Itapeva, SP; Analysis of the stromatolites from the Itaiacoca Group (Proterozoic) from south Itapeva, State of São Paulo, Brazil

Sallun Filho, William
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 09/11/1999 Português
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Estromatólitos foram estudados em nove localidades ao sul de Itapeva (SP), principalmente em metacalcários dolomíticos cinza-claro e, secundariamente, em metacalcários calcíticos cinza-escuro, do Grupo Itaiacoca, uma unidade vulcanossedimentar mesoproterozóica da Faixa Ribeira. Foram diferenciados em cinco formas colunares, a mais comum consistindo de colunas coniformes, não ramificadas, de diâmetros e alturas centimétricas a decimétricas, atribuídas a Conophyton. As outras quatro formas, com laminação convexa mas não coniforme, diferem em tamanho, silhueta e estilo/freqüência de ramificação. As diferenças na preservação dos estromatólitos são relacionadas ao comportamentos tectônicos distintos entre o metacalcário dolomítico (mais puro), com comportamento competente, e o calcítico (mais argiloso) que atuou de forma mais plástica. Nas melhores exposições desta área os estromatólitos estão agrupados em bioermas de Conophyton, sem indícios de exposição ou retrabalhamento sub-aéreo ou por ondas, evidenciando um ambiente calmo e relativamente profundo, provavelmente abaixo do nível de base de ondas, de talvez até algumas dezenas de metros de profundidade. Conophyton de Itapeva é semelhante a estromatólitos coniformes próximo a Abapã (PR)...

‣ Middle Proterozoic vein-hosted gold deposits in the Pontes e Lacerda region, southwestern Amazonian craton, Brazil

Geraldes, Mauro C.; Figueiredo, Bernardino R.; Tassinari, Colombo C. G.; Ebert, Hans D.
Fonte: Universidade Estadual Paulista Publicador: Universidade Estadual Paulista
Tipo: Artigo de Revista Científica Formato: 438-448
Português
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Structural, geochemical, and isotope studies were carried out on the gold deposits of the Pontes e Lacerda region (Mato Grosso state, Brazil), where rocks of the Aguapei and Rondoniano mobile belts (southwestern Amazonian craton) occur. The orebodies are hosted in metavolcanic, gneiss-granite, quartzite, tonalite, and granite units. Tectonics involve oblique overthrusting (from northeast to southwest), which led to the formation of recumbent folds and thrusts (pathways for the mineralizing fluids), upright folds, and faults with dominant strike-slip component. These unconformities represent potential sites for mineralization. During geological mapping, it was observed that the orebodies consist of quartz, pyrite, and gold, and that the hydrothermal alteration zone contains quartz, sericite, pyrite (altered to limonite), and magnetite (altered to hematite). Chalcopyrite, galena, and sphalerite occur only in the Onça deposit. Chemical analysis of sulfides indicates high contents of Bi, Se, and Te in sulfides and gold, suggesting plutonic involvement in the origin of hydrothermal solutions. K-Ar dating of hydrothermal sericites from gold veins yielded ages in the range from 960 to 840 Ma, which may indicate the age of original crystallization of sericite. Pb-Pb dating in galenas yielded model ages in the range from 1000 to 800 Ma for the Onça deposit...

‣ Isotopic Compositions of Carbonates and Organic Carbon from Upper Proterozoic Successions in Namibia: Stratigraphic Variation and the Effects of Diagenesis and Metamorphism

Knoll, Andrew; Germs, Gerard J. B.; Hayes, J. M.; Kaufman, Alan J.
Fonte: Elsevier Publicador: Elsevier
Português
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The carbon isotope geochemistry of carbonates and organic carbon in the late Proterozoic Damara Supergroup of Namibia, including the Nama, Witvlei, and Gariep groups on the Kalahari Craton and the Mulden and Otavi groups on the Congo Craton, has been investigated as an extension of previous studies of secular variations in the isotopic composition of late Proterozoic seawater. Subsamples of microspar and dolomicrospar were determined, through petrographic and cathodoluminescence examination, to represent the "least-altered" portions of the rock. Carbon-isotopic abundances in these phases are nearly equal to those in total carbonate, suggesting that C-13 abundances of late Proterozoic fine-grained carbonates have not been significantly altered by meteoric diagenesis, although O-18 abundances often differ significantly. Reduced and variable carbon-isotopic differences between carbonates and organic carbon in these sediments indicate that isotopic compositions of organic carbon have been altered significantly by thermal and deformational processes, likely associated with the Pan-African Orogeny. Distinctive stratigraphic patterns of secular variation, similar to those noted in other, widely separated late Proterozoic basins, are found in carbon-isotopic compositions of carbonates from the Nama and Otavi groups. For example...

‣ Eukaryotic Organisms in Proterozoic Oceans

Knoll, Andrew Herbert; Javaux, Emmanuelle J.; Cohen, Phoebe A; Hewitt, David
Fonte: Royal Society, The Publicador: Royal Society, The
Tipo: Artigo de Revista Científica
Português
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The geological record of protists begins well before the Ediacaran and Cambrian diversification of animals, but the antiquity of that history, its reliability as a chronicle of evolution and the causal inferences that can be drawn from it remain subjects of debate. Well-preserved protists are known from a relatively small number of Proterozoic formations, but taphonomic considerations suggest that they capture at least broad aspects of early eukaryotic evolution. A modest diversity of problematic, possibly stem group protists occurs in ca 1800–1300 Myr old rocks. 1300–720 Myr fossils document the divergence of major eukaryotic clades, but only with the Ediacaran–Cambrian radiation of animals did diversity increase within most clades with fossilizable members. While taxonomic placement of many Proterozoic eukaryotes may be arguable, the presence of characters used for that placement is not. Focus on character evolution permits inferences about the innovations in cell biology and development that underpin the taxonomic and morphological diversification of eukaryotic organisms.; Organismic and Evolutionary Biology

‣ A synthesis of the Proterozoic evolution of the Mount Isa Inlier

Betts, P.; Giles, D.; Mark, G.; Lister, G.; Goleby, B.; Ailleres, L.
Fonte: Taylor & Francis Ltd. Publicador: Taylor & Francis Ltd.
Tipo: Artigo de Revista Científica
Publicado em //2006 Português
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By virtue of its large area of exposure of different crustal levels, and preservation of a protracted (400 million years) Palaeoproterozoic to Mesoproterozoic tectonic evolution, the Mt Isa Inlier is an excellent natural laboratory to study Proterozoic tectonic processes. The inlier preserves evidence of intracontinental basin development, plutonism, low-pressure metamorphism, orogenesis at different crustal levels, and crustal-scale metasomatism. In addition, the Mt Isa Inlier is endowed with a variety of ore deposits, including the Mt Isa Pb - Zn - Ag and Cu deposits, Century Zn - Pb - Ag deposit, Cannington Ag - Pb - Zn deposit, and the Osborne and Ernest Henry iron oxide Cu - Au deposits. Basement rocks were deformed and metamorphosed during the ca 1900 - 1870 Ma Barramundi Orogeny and intruded by the granitic rocks of the ca 1850 Ma Kalkadoon and Ewen Batholiths and their coeval Leichhardt Volcanics. Three stacked and superimposed superbasins evolved between ca 1800 and ca 1595 Ma. These basins evolved in an environment characterised by elevated heat flow and transient episodes of magmatism and basin inversion in an inferred continental backarc setting. The ca 1600 - 1500 Ma Isan Orogeny probably records two phases of orogenesis. The first phase (ca 1600 - 1570 Ma) involved approximately north - south to northwest - southeast shortening in which a northwest-vergent fold-thrust belt evolved in the Eastern Fold Belt and localised basin inversion occurred in the Western Fold Belt. The second phase (ca 1550 - 1500) involved thick-skinned deformation in the Eastern and Western Fold Belts...

‣ The hot southern continent: heat flow and heat production in Australian Proterozoic terranes

McLaren, S.; Sandiford, M.; Hand, M.; Neumann, N.; Wyborn, L.; Bastrakova, I.
Fonte: Geological Society of Australia Publicador: Geological Society of Australia
Tipo: Artigo de Revista Científica
Publicado em //2003 Português
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Available surface heat-flow measurements from Australian Proterozoic terranes (83 ± 18 mWm−2) are significantly higher than the global Proterozoic average of ∼50 mWm−2. Seismic evidence for the presence of relatively cool mantle together with the lack of evidence for neotectonic processes normally associated with thermal transients suggests that anomalous heat flow must reflect crustal radiogenic sources (U, Th and K). This is supported by a compilation of more than 6000 analyses from 455 individual granites, granitic gneisses and felsic volcanics which shows that the present-day average heat production of these rock types is 4.6 µWm−3 when normalised by area of outcrop (over more than 100 000 km2) ; roughly twice that of ‘average’ granite. At the time of this felsic magmatism (ca 1850–1500 Ma) heat production rates were some 25–30% greater than the present day such that the total complement of U, Th and K in many parts of the Australian Proterozoic crust may have contributed as much as 60–85 mWm−2 to the surface heat flow, or 2 to 3 times the present-day continental average. This extraordinary enrichment has played a key role in the tectonothermal evolution of the Australian Proterozoic crust, and has important implications for our understanding of the thermal budget of ancient continental crust.; S. McLaren...

‣ Temporal constraints on the timing of high-grade metamorphism in the northern Gawler Craton: implications for assembly of the Australian Proterozoic

Payne, J.; Hand, M.; Hatch, K.; Wade, B.
Fonte: Taylor & Francis Ltd. Publicador: Taylor & Francis Ltd.
Tipo: Artigo de Revista Científica
Publicado em //2008 Português
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LA-ICPMS U-Pb data from metamorphic monazite in upper amphibolite and granulite-grade metasedimentary rocks indicate that the Nawa Domain of the northern Gawler Craton in southern Australia underwent multiple high-grade metamorphic events in the Late Paleoproterozoic and Early Mesoproterozoic. Five of the six samples investigated here record metamorphic monazite growth during the period 1730-1690 Ma, coincident with the Kimban Orogeny, which shaped the crustal architecture of the southeastern Gawler Craton. Combined with existing detrital zircon U-Pb data, the metamorphic monazite ages constrain deposition of the northern Gawler metasedimentary protoliths to the interval ca 1750-1720 Ma. The new age data highlight the craton-wide nature of the 1730-1690 Ma Kimban Orogeny in the Gawler Craton. In the Mabel Creek Ridge region of the Nawa Domain, rocks metamorphosed during the Kimban Orogeny were reworked during the Kararan Orogeny (1570-1555 Ma). The obtained Kararan Orogeny monazite ages are within uncertainty of ca 1590-1575 Ma zircon U-Pb metamorphic ages from the Mt Woods Domain in the central-eastern Gawler Craton, which indicate that high-grade metamorphism and associated deformation were coeval with the craton-scale Hiltaba magmatic event. The timing of this deformation...

‣ Unravelling the tectonic framework of the Musgrave Province, Central Australia.

Wade, Benjamin P.
Fonte: Universidade de Adelaide Publicador: Universidade de Adelaide
Tipo: Tese de Doutorado
Publicado em //2006 Português
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The importance of the Musgrave Province in continental reconstructions of Proterozoic Australia is only beginning to be appreciated. The Mesoproterozoic Musgrave Province sits in a geographically central location within Australia and is bounded by older and more isotopically evolved regions including the Gawler Craton of South Australia and Arunta Region of the Northern Territory. Understanding the crustal growth and deformation mechanisms involved in the formation of the Musgrave Province, and also the nature of the basement that separates these tectonic elements, allows for greater insight into defining the timing and processes responsible for the amalgamation of Proterozoic Australia. The ca. 1.60-1.54 Ga Musgravian Gneiss preserves geochemical and isotopic signatures related to ongoing arc-magmatism in an active margin between the North Australian and South Australian Cratons (NAC and SAC). Characteristic geochemical patterns of the Musgravian Gneiss include negative anomalies in Nb, Ti, and Y, and are accompanied by steep LREE patterns. Also characteristic of the Musgravian Gneiss is its juvenile Nd isotopic composition (ɛNd1.55 values from -1.2 to +0.9). The juvenile isotopic signature of the Musgravian Gneiss separates it from the bounding comparitively isotopically evolved terranes of the Arunta Region and Gawler Craton. The geochemical and isotopic signatures of these early Mesoproterozoic felsic rocks have similarities with island arc systems involving residual Ti-bearing minerals and garnet. Circa 1.40 Ga metasedimentary rocks of the eastern Musgrave Province also record vital evidence for determining Australia.s location and fit within a global plate reconstruction context during the late Mesoproterozoic. U-Pb detrital zircon and Sm-Nd isotopic data from these metasedimentary rocks suggests a component of derivation from sources outside of the presently exposed Australian crust. Best fit matches come from rocks originating from eastern Laurentia. Detrital zircon ages range from Palaeoproterozoic to late Mesoproterozoic...

‣ Basin analysis and the geochemical signature of Paleoproterozoic sedimentary successions in northern Australia : constraints on basin development in respect to mineralisation and paleoreconstruction models.

Lambeck, Alexis
Fonte: Universidade de Adelaide Publicador: Universidade de Adelaide
Tipo: Tese de Doutorado
Publicado em //2012 Português
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Secular changes in the characteristics of sedimentary basins and their associated mineral deposits in Proterozoic Australia are directly related to the evolving global tectonic regimes and global changes in atmospheric and oceanic redox states. Identifying these secular changes provides critical information to assist in applying first pass techniques for regional exploration in Australia. The break-up and formation of the Nuna supercontinent is recorded within sedimentary basins within Proterozoic Australia. Sedimentary basins deposited between 1910 Ma and 1810 Ma formed during the Nuna supercontinent amalgamation and host orogenic gold mineralisation, whereas those deposited between 1710 Ma ─ 1575 Ma are directly associated with the break-up of Nuna and host lead-zinc mineralisation. Sediments in northern Australia deposited during the Nuna amalgamation, and before the Great Oxidation Event, consist of fine-grained iron-rich/mafic mudstones and siltstones which are geochemically characterised by high FeO contents, high Cr/Th and low Th/Sc values. This sedimentary assemblage includes the gold-bearing succession of the Dead Bullock Formation, Tanami region and Koolpin Formation, Pine Creek Orogen. This contrasts with the regionally overlying stratigraphy...

‣ Geotectonics in the Gawler craton : constraints from geochemistry, U-Pb geochronology and Sm-Nd and Lu-Hf isotopes.

Howard, Katherine E.
Fonte: Universidade de Adelaide Publicador: Universidade de Adelaide
Tipo: Tese de Doutorado
Publicado em //2012 Português
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The southern Australian Mesoarchean to early Mesoproterozoic Gawler Craton holds a pivotal place in the architecture of Proterozoic Australia. Although in recent years a growing body of work has signifi cantly improved our understanding of the tectonic evoluti on of the Gawler Craton, the lack of outcrop across large areas is an impediment to determining the tectonic framework. This study uses geochemical, geochronological (U-Pb zircon and monazite) and isotopic (Whole rock Sm-Nd and zircon Lu-Hf) data on samples mostly obtained from drill holes in regions of limited to non- existent outcrop to better delineate the tectonic setting of Proterozoic metasedimentary and igneous units in the western, central and northern Gawler Craton and the orogenic events which have affected them. It is common practice in sedimentary provenance studies to use similarities in the detrital zircon age histograms from sedimentary systems to identify potential source regions, and therefore to make interpretations about paleogeographic settings. However, this method is limiting as the timing of zircon growth events is not a unique criterion of specific terrains. Nevertheless, these limitations can be overcome by employing additional isotopic data sets such as Sm - Nd and Lu - Hf that provide information on the crustal evolution of the source region. As an example...

‣ Early Mesoproterozoic metamorphism in the Barossa Complex, South Australia : links with the eastern margin of Proterozoic Australia

Morrissey, L.; Hand, M.; Wade, B.; Szpunar, M.
Fonte: Taylor & Francis Ltd. Publicador: Taylor & Francis Ltd.
Tipo: Artigo de Revista Científica
Publicado em //2013 Português
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LA-ICP-MS U–Pb geochronological data from metamorphic monazite in granulite-facies metapelites in the Barossa Complex, southern Australia, yield ages in the range 1580–1550 Ma. Metapelitic rocks from the Myponga and Houghton Inliers contain early biotite–sillimanite-bearing assemblages that underwent partial melting to produce peak metamorphic garnet–sillimanite-bearing anatectic assemblages. Phase equilibrium modelling suggests a clockwise P–T evolution with peak temperatures between 800 and 870C and peak pressures of 8–9 kbar, followed by decompression to pressures of 6 kbar. In combination with existing age data, the monazite U–Pb ages indicate that the early Mesoproterozoic evolution of the Barossa Complex is contemporaneous with other high geothermal gradient metamorphic terranes in eastern Proterozoic Australia. The areal extent of early Mesoproterozoic metamorphism in eastern Australia suggests that any proposed continental reconstructions involving eastern Proterozoic Australia should share a similar tectonothermal history.; L. J. Morrissey, M. Hand, B. P. Wade and M. Szpunar

‣ Constraints on Proterozoic crustal evolution from an isotopic and geochemical study of clastic sediments of the Gawler Craton, South Australia

Simpson, C. A.
Fonte: Universidade de Adelaide Publicador: Universidade de Adelaide
Tipo: Tese de Doutorado
Publicado em //1994 Português
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The Gawler Craton comprises tocks varying in age from Archaean to more recent Phanerozoic sediments. The rocks of greatest interest in defining processes of early crustal formation and evolution in the Australian continent, are the basement material older than approximately 1400 Ma (pre-cratonisation), comprising deformed and metamorphosed rocks suites of Archaean and Proterozoic metasediments and gneisses. These suites span an immense period of intense geological history, and as such are a topic of much past and present study. Detailed mapping in the Tumby Bay region of eastern Eyre Peninsula outlines stratigraphic and structural evolution of a sequence of Proterozoic rock suites, these are proposed to be related to other recognised deformation episodes elsewhere within the Gawler Craton, thus regional correlation is inferred. A new theory for development of two lineations within the map region is postulated by two movement directions along the Kalinjala Mylonite Zone. Geochemically the Proterozoic sediments of the Gawler Craton are similar to upper crustal average values of Taylor & McClennan (1985). However, characteristic depletions in Nb and Sr are recognised. Consistency in trace element compositions for Archaean and Proterozoic samples would suggest recycling of older Archaean crust into Proterozoic sediments and granitoids. Analysis of representative trace element ratios and indices of alteration and weathering suggest some change in geochemistry throughout the Proterozoic period. Selected Proterozoic elastic sedimentary suites were geochemicaly and isotopically (Sm-Nd) analysed...

‣ Anoxygenic Photosynthesis Modulated Proterozoic Oxygen and Sustained Earth’s Middle Age

Johnston, David T; Wolfe-Simon, Felisa Lauren; Pearson, Ann; Knoll, Andrew Herbert
Fonte: National Academy of Sciences Publicador: National Academy of Sciences
Tipo: Artigo de Revista Científica
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Molecular oxygen (O2) began to accumulate in the atmosphere and surface ocean ca. 2,400 million years ago (Ma), but the persistent oxygenation of water masses throughout the oceans developed much later, perhaps beginning as recently as 580–550 Ma. For much of the intervening interval, moderately oxic surface waters lay above an oxygen minimum zone (OMZ) that tended toward euxinia (anoxic and sulfidic). Here we illustrate how contributions to primary production by anoxygenic photoautotrophs (including physiologically versatile cyanobacteria) influenced biogeochemical cycling during Earth's middle age, helping to perpetuate our planet's intermediate redox state by tempering O2 production. Specifically, the ability to generate organic matter (OM) using sulfide as an electron donor enabled a positive biogeochemical feedback that sustained euxinia in the OMZ. On a geologic time scale, pyrite precipitation and burial governed a second feedback that moderated sulfide availability and water column oxygenation. Thus, we argue that the proportional contribution of anoxygenic photosynthesis to overall primary production would have influenced oceanic redox and the Proterozoic O2 budget. Later Neoproterozoic collapse of widespread euxinia and a concomitant return to ferruginous (anoxic and Fe2+ rich) subsurface waters set in motion Earth's transition from its prokaryote-dominated middle age...

‣ Synthesis of the Proterozoic evolution of the Mt Isa Inlier

Betts, P; Giles, D; Mark, G; Lister, Gordon; Goleby, B; Ailleres, L
Fonte: Blackwell Publishing Ltd Publicador: Blackwell Publishing Ltd
Tipo: Artigo de Revista Científica
Português
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By virtue of its large area of exposure of different crustal levels, and preservation of a protracted (∼400 million years) Palaeoproterozoic to Mesoproterozoic tectonic evolution, the Mt Isa Inlier is an excellent natural laboratory to study Proterozoic

‣ The 1.8 - 1.5 links between the north and south Australian cratons and the palaeo- to meso-Proterozoic configuration of Australia

Giles, D; Betts, P; Lister, Gordon
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Português
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The Archaean and Early-Middle Proterozoic (1.8-1.5 Ga) terranes of the North Australian Craton and the South Australian Craton are separated by ∼400 km of ca. 1.33-1.10-Ga orogenic belts and Phanerozoic sediments. However, there is a diverse range of ge

‣ Anomalous lithosphere beneath the Proterozoic of western and central Australia: A record of continental collision and intraplate deformation?

Fishwick, Stewart; Reading, Anya
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Português
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A new surface wave seismic tomography model of Australia is presented which provides a means of investigating the lithospheric structure beneath the Proterozoic regions in the west, north and centre of the continent with improved resolution and reliabilit

‣ SHRIMP U-Pb detrital zircon ages from Proterozoic and Early Palaeozoic sandstones and their bearing on the early geological evolution of Tasmania

Black, Lance; Calver, Clive; Seymour, David B; Reed, A.
Fonte: Blackwell Publishing Ltd Publicador: Blackwell Publishing Ltd
Tipo: Artigo de Revista Científica
Português
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Detrital zircons from 13 Late Mesoproterozoic to Early Neoproterozoic sandstones and two Palaeozoic sandstones from Tasmania were dated in order to improve constraints on depositional ages, to test correlation between Proterozoic inliers, and to characterise source regions. These include successions considered to be the oldest presently exposed in Tasmania. Typical features of the age distributions of the Proterozoic rocks are prominent data concentrations at 1800-1650 Ma and 1450-1400 Ma, and a minor spread of Archaean ages. Statistical testing of the similarity of the age profiles shows that widespread quartzarenaceous samples from the Detention Subgroup, Needles Quartzite and from the Tyennan region are strongly similar, consistent with broad correlation. Relatively large differences are seen between the Detention Subgroup and the conformable, stratigraphically higher Jacob Quartzite, which contains an additional spread of 1300-1000 Ma zircons suggestive of a Grenvillian source. Age profiles of the quartzarenites and quartzwacke turbidites (Oonah Formation and correlatives) cannot be readily differentiated. The Oonah Formation likewise includes samples with and without Grenvillian ages, and there is no 750 Ma zircon population that would be expected if the turbidites were genetically related to the Wickham Orogeny...

‣ Contrasting styles of Proterozoic crustal evolution: A hot-plate tectonic model for Australian terranes

McLaren, Sandra; Sandiford, Michael. A; Powell, Roger
Fonte: Geological Society of America Inc Publicador: Geological Society of America Inc
Tipo: Artigo de Revista Científica
Português
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Proterozoic terranes in Australia record complex tectonic histories in the interval 1900-1400 Ma that have previously been interpreted by means of simple intracratonic or plate-tectonic models. However, these models do not fully account for (1) repeated tectonic reactivation (both orogenesis and rifting), (2) mainly high-temperature-low-pressure metamorphism, (3) rifting and sag creating thick sedimentary basins, (4) the nature and timing of voluminous felsic magmatism, (5) relatively large aspect ratio orogenic belts, and (6) a general paucity of diagnostic plate-boundary features. A key to understanding these histories is the observation that Australian Proterozoic terranes are characterized by an extraordinary, but heterogeneous, enrichment of the heat-producing elements. This enrichment must contribute to long-term lithospheric weakening, and thus we advocate a hybrid lithospheric evolution model with two tectonic switches: plate-boundary-derived stresses and heat-producing-element-related lithospheric weakening. The Australian Proterozoic crustal growth record is therefore a function of the magnitude of these stresses, the way in which the heat-producing elements are distributed, and how both of these change with time.

‣ Iridium anomalies and fractionated siderophile element patterns in impact ejecta, Brockman Iron Formation, Hamersley Basin, Western Australia: evidence for a major asteroid impact in simatic crustal regions of the early Proterozoic earth

Glikson, Andrew; Allen, Charlotte M
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Português
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A stratigraphically consistent <20-cm-thick unit of microkrystite spherule and microtektite-bearing impact fallout ejecta overlying volcanic tuff of the 4th Shale Macroband (DGS4) of the Dales Gorge Member (2.47-2.50 Ga), Brockman Iron Formation, Hamersley Group, Western Australia, displays anomalous platinum group element (PGE) and other trace metal patterns. The unit has high Ir (13 ppb) and Pt (35 ppb), and low Pd (2.7 ppb) and Au (1.55-1.88 ppb). The low Pd/Ir ratios and low Cr/V suggest depletion in volatile PGE and metals relative to refractory PGE and V, contrasted to the ubiquitous high Pd/Ir of most terrestrial rocks. Marked depletion in the volatile Rare Earth Element (REE) abundances in stilpnomelane spherule cores is consistent with this model. The loss of volatile PGE, analogous to relations in 3.24 Ga impact fallout units of the Barberton greenstone belt (S3 and S4), suggests fractionation related to atmospheric spherule condensation. The microkrystite spherule unit locally incorporate fragments and up to meter-scale boulders of banded chert and stromatolite carbonate, suggesting tsunami transport postdating spherule deposition. DGS4 microkrystite spherules are dominated by stilpnomelane mantled by K-feldspar shells, which consist of inward-radiating fibrous feldspar aggregates suggestive of devitrification. The K and REE enrichment of spherule margins are contrasted to flat REE patterns of the stilpnomelane cores...

‣ U-Pb evidence of ~1.7 Ga crustal tectonism during the Nimrod Orogeny in the Transantarctic Mountains, Antarctica: implications for Proterozoic plate reconstructions

Goodge, John W; Fanning, Christopher; Bennett, Victoria
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Português
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The Pacific margin of East Antarctica records a long tectonic history of crustal growth and breakup, culminating in the early Paleozoic Ross Orogeny associated with Gondwanaland amalgamation. Periods of older tectonism have been proposed (e.g. Precambrian Nimrod and Beardmore Orogenies), but the veracity of these events is difficult to document because of poor petrologic preservation, geochronologic uncertainty due to isotopic resetting, and debated geological field relationships. Of these, the Nimrod Orogeny was originally proposed as a period of Neoproterozoic metamorphism and deformation within crystalline basement rocks of the Nimrod Group, based on ∼1000 Ma K-Ar mineral ages. Later structural and thermochronologic study attributed major deformation features in the Nimrod Group to Ross-age basement reactivation. Yet, new SHRIMP ion microprobe U-Pb zircon age data for gneissic and metaigneous rocks of the Nimrod Group indicate a period of deep-crustal metamorphism and magmatism between ∼1730-1720 Ma. Igneous zircons from gneissic Archean protoliths show metamorphic overgrowths of ∼1730-1720 Ma, and an eclogitic block preserved within the gneisses contains zircons yielding an average metamorphic crystallization age of ∼1720 Ma. Deformed granodiorite that intrudes the gneisses and associated metasedimentary rocks yields a concordant zircon crystallization age of ∼1730 Ma. Despite scant petrologic evidence for these metamorphic and igneous events...