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‣ Dating minerals by ID-TIMS geochronology at times of in situ analysis: selected case studies from the CPGeo-IGc-USP laboratory

PASSARELLI, Cláudia R.; BASEI, Miguel A.S.; SIGA JR., Oswaldo; SATO, Kei; SPROESSER, Walter M.; LOIOS, Vasco A.P.
Fonte: Academia Brasileira de Ciências Publicador: Academia Brasileira de Ciências
Tipo: Artigo de Revista Científica
Português
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Since 1964, the Center for Geochronological Research - CPGeo, one of the interdepartmental centers of the Instituto de Geociências (IG) of São Paulo University, has developed studies related to several geological processes associated with different rock types. Thermal Ionization Mass Spectrometry Isotopic Dilution (ID-TIMS) has been the technique widely used in the CPGeo U-Pb Laboratory. It provides reliable and accurate results in age determination of superposed events. However, the open-system behavior such as Pb-loss, the inheritance problem and metamictization processes allow and impel us to a much richer understanding of the power and limitations of U-Pb geochronology and thermochronology. In this article, we present the current methodology used at the CPGeo-IGc-USP U-Pb laboratory, the improvements on ID-TIMS method, and report high-precision U-Pb data from zircon, monazite, epidote, titanite, baddeleyite and rutile from different rock types of several domains of the Brazilian south-southeast area, Argentina and Uruguay.; O Centro de Pesquisas Geocronológicas (CPGeo), um dos centros interdepartamentais do Instituto de Geociências (IG) da Universidade de São Paulo (USP), desde 1964 desenvolve estudos relacionados a diversos processos geológicos que se associam a diferentes tipos de rochas. A técnica amplamente utilizada no Laboratório U-Pb é a diluição isotópica por espectrometria de massa termo ionizada (ID-TIMS). Esta sistemática proporciona resultados bastante confiáveis e precisos na determinação das idades de eventos geológicos superpostos. Entretanto...

‣ Quimioestratigrafia isotópica (C, O, S e Sr), geocronologia (Pb-Pb e K-Ar) e proveniência (Sm-Nd) das rochas da Faixa Paraguai Norte, Mato Grosso; Isotope (C, O, S And Sr) chemostratigraphy, geochronology (Pb-Pb And K-Ar) and provenance (Sm-Nd) of rocks from the North Paraguay Belt, Mato Grosso

Figueiredo, Milene Freitas
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 03/11/2010 Português
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Anomalias biogeoquímicas expressivas têm sido reconhecidas em rochas marinhas neoproterozoicas depositadas em todos os continentes, não havendo similares ao longo do Fanerozoico. O interesse nesta Era aumentou nos últimos anos, especialmente em relação ao Ediacarano, período que precede a grande Explosão de Vida Cambriana e que abriga os primeiros organismos macroscópicos, conhecidos como Fauna de Ediacara. Assim, o entendimento das condições paleoambientais e paleoclimáticas, bem como o estabelecimento da cronologia dos eventos, é fundamental no entendimento da evolução da biota no planeta durante este período. As rochas da Faixa Paraguai Norte, na região de estudo, registram a ocorrência de duas glaciações, uma criogeniana e outra ediacarana (formações Puga e Serra Azul), separadas por uma espessa plataforma carbonática (Grupo Araras), que registra anomalias biogeoquímicas na água do mar ediacarano. Estas rochas foram depositadas na margem sudeste do Craton Amazônico, no intervalo de tempo entre a quebra do paleocontinente Rodínia e a colagem do Gondwana. O presente trabalho é apresentado na forma de artigos e apresenta estudos de: i) quimioestratigrafia isotópica (C, O, S e Sr) de detalhe nos carbonatos do Grupo Araras e da Formação Serra Azul; ii) proveniência (Sm-Nd) das rochas terrígenas das formações Puga e Serra Azul...

‣ Metamorphic evolution and U-Pb zircon SHRIMP geochronology of the Belizário ultramafic amphibolite, Encantadas Complex, southernmost Brazil

Hartmann, Leo Afraneo; Santos, João Orestes Schneider; Leite, Jayme Alfredo Dexheimer; Porcher, Carla Cristine; McNaughton, Neal Jesse
Fonte: Universidade Federal do Rio Grande do Sul Publicador: Universidade Federal do Rio Grande do Sul
Tipo: Artigo de Revista Científica Formato: application/pdf
Português
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O entendimento dos processos evolutivos do Complexo Encantadas no sul do Brasil foi aperfeiçoado através do estudo integrado do metamorfismo de um anfibolito ultramáfico e da geocronologia U-Pb SHRIMP de zircão. Os núcleos herdados de alguns cristais de zircão tem idades em torno de 2257 ± 12 Ma e constituem a única evidência preservada do protólito ígneo, que pode ter sido um basalto magnesiano ou um piroxenito. O metamorfismo M1 de fácies anfibolito formou abundante hornblenda na amostra investigada, possivelmente há 1989±21 Ma. Esta rocha ultramáfica foi re-metamorfizada talvez há cerca de 702±21 Ma durante um eventoM2 de fácies xistos verdes do metamorfismo regional. Durante o evento M2, a hornblenda foi recristalizada e formou a assembléia actinolita + oligoclásio + microclínio + epidoto + titanita + monazita. Estes eventos foram a manifestação da Orogênese Encantadas (2257 ± 12 Ma) e da Orogênese Camboriú (~1989 Ma) do Ciclo Transamazônico, seguidos por um evento orogênico (702 ± 21 Ma) do Ciclo Brasiliano. O período intra-cratônico entre 2000-900 Ma corresponde à estabilidade do Supercontinente Atlântica, cujos remanescentes são conhecidos como Cráton Rio de la Plata.; The integrated investigation of metamorphism and zircon U-Pb SHRIMP geochronology of the Belizário ultramafic amphibolite from southernmost Brazil leads to a better understanding of the processes involved in the generation of the Encantadas Complex. Magmatic evidence of the magnesian basalt or pyroxenite protolith is only preserved in cores of zircon crystals...

‣ Geocronologia e geoquímica isotópica de fácies félsicas marginais do maciço de Reguengos de Monsaraz (ZOM); Geochronology and Isotope Geochemistry of Felsic Marginal Facies of the Reguengos de Monsaraz Pluton (OMZ)

Antunes, A.; Santos, J.F.; Azevedo, M.R.; Corfu, F.
Fonte: Instituto Politécnico de Castelo Branco Publicador: Instituto Politécnico de Castelo Branco
Tipo: Conferência ou Objeto de Conferência
Português
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No sector sueste do maciço de Reguengos de Monsaraz (Zona de Ossa-Morena), ocorrem rochas félsicas quer em continuidade espacial com as litologias principais do plutonito quer em pequenos corpos aparentemente separados. Neste trabalho, estudou-se o granito marginal da zona de Arraieiras, bem como o tonalito da zona do ribeiro dos Gagos. Datações U-Pb (em zircão e monazite) revelam que estas litologias são, no essencial, contemporâneas entre si, bem como relativamente às fácies mais comuns do maciço de Reguengos, e que os processos de instalação magmática deverão ter decorrido há 337-338 Ma. Do ponto de vista geoquímico, o granito de Arraieiras parece ter uma relação genética forte com o resto do plutonito, através de processos de mistura de líquidos (de origens mantélica e de anatexia de metassedimentos) e diferenciação magmática, a que se terão seguido fenómenos locais de assimilação do encaixante no nível de instalação final. O paradoxo de haver rochas mais máficas do que o granito com assinaturas isotópicas mais “crustais” poderá ser explicado pela intervenção final destes processos de assimilação. Quanto ao tonalito de Gagos, as suas características geoquímicas, e em particular as razões isotópicas de Sr e Nd...

‣ 40Ar/39Ar geochronology at the Instituto de Geociências, USP: instrumentation, analytical procedures, and calibration

VASCONCELOS,PAULO M.; ONOE,ARTUR T.; KAWASHITA,KOJI; SOARES,ADALBERTO J.; TEIXEIRA,WILSON
Fonte: Academia Brasileira de Ciências Publicador: Academia Brasileira de Ciências
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/06/2002 Português
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Laser heating 40Ar/39Ar geochronology provides high analytical precision and accuracy, mum-scale spatial resolution, and statistically significant data sets for the study of geological and planetary processes. A newly commissioned 40Ar/39Ar laboratory at CPGeo/USP, São Paulo, Brazil, equips the Brazilian scientific community with a new powerful tool applicable to the study of geological and cosmochemical processes. Detailed information about laboratory layout, environmental conditions, and instrumentation provides the necessary parameters for the evaluation of the CPGeo/USP 40Ar/39Ar suitability to a diverse range of applications. Details about analytical procedures, including mineral separation, irradiation at the IPEN/CNEN reactor at USP, and mass spectrometric analysis enable potential researchers to design the necessary sampling and sample preparation program suitable to the objectives of their study. Finally, the results of calibration tests using Ca and K salts and glasses, international mineral standards, and in-house mineral standards show that the accuracy and precision obtained at the 40Ar/39Ar laboratory at CPGeo/USP are comparable to results obtained in the most respected laboratories internationally. The extensive calibration and standardization procedures undertaken ensure that the results of analytical studies carried out in our laboratories will gain immediate international credibility...

‣ Metamorphic evolution and U-Pb zircon SHRIMP geochronology of the Belizário ultramafic amphibolite, Encantadas Complex, southernmost Brazil

Hartmann,Léo A.; Santos,João O.S.; Leite,Jayme A.D.; Porcher,Carla C.; Mcnaughton,Neal J.
Fonte: Academia Brasileira de Ciências Publicador: Academia Brasileira de Ciências
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/09/2003 Português
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The integrated investigation of metamorphism and zircon U-Pb SHRIMP geochronology of the Belizário ultramafic amphibolite from southernmost Brazil leads to a better understanding of the processes involved in the generation of the Encantadas Complex. Magmatic evidence of the magnesian basalt or pyroxenite protolith is only preserved in cores of zircon crystals, which are dated at 2257 ± 12 Ma. Amphibolite facies metamorphism M1 formed voluminous hornblende in the investigated rock possibly at 1989 ± 21 Ma. This ultramafic rock was re-metamorphosed at 702±21 Ma during a greenschist facies eventM2; the assemblage actinolite + oligoclase + microcline + epidote + titanite + monazite formed by alteration of hornblende. The metamorphic events are probably related to the Encantadas Orogeny (2257±12 Ma) and Camboriú Orogeny (~ 1989 Ma) of the Trans-Amazonian Cycle, followed by an orogenic event (702±21 Ma) of the Brasiliano Cycle. The intervening cratonic period (2000-700 Ma) corresponds to the existence of the Supercontinent Atlantica, known regionally as the Rio de la Plata Craton.

‣ Dating minerals by ID-TIMS geochronology at times of in situ analysis: selected case studies from the CPGeo-IGc-USP laboratory

Passarelli,Cláudia R.; Basei,Miguel A.S.; Siga Jr.,Oswaldo; Sato,Kei; Sproesser,Walter M.; Loios,Vasco A.P.
Fonte: Academia Brasileira de Ciências Publicador: Academia Brasileira de Ciências
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/03/2009 Português
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Since 1964, the Center for Geochronological Research - CPGeo, one of the interdepartmental centers of the Instituto de Geociências (IG) of São Paulo University, has developed studies related to several geological processes associated with different rock types. Thermal Ionization Mass Spectrometry Isotopic Dilution (ID-TIMS) has been the technique widely used in the CPGeo U-Pb Laboratory. It provides reliable and accurate results in age determination of superposed events. However, the open-system behavior such as Pb-loss, the inheritance problem and metamictization processes allow and impel us to a much richer understanding of the power and limitations of U-Pb geochronology and thermochronology. In this article, we present the current methodology used at the CPGeo-IGc-USP U-Pb laboratory, the improvements on ID-TIMS method, and report high-precision U-Pb data from zircon, monazite, epidote, titanite, baddeleyite and rutile from different rock types of several domains of the Brazilian south-southeast area, Argentina and Uruguay.

‣ High spatial resolution analysis of Pb and U isotopes for geochronology by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS)

Bühn,Bernhard; Pimentel,Márcio M.; Matteini,Massimo; Dantas,Elton L.
Fonte: Academia Brasileira de Ciências Publicador: Academia Brasileira de Ciências
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/03/2009 Português
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Age determinations using the 235U and 238U radioactive decay series to the daughter isotopes 207Pb and 206Pb, respectively, using the mineral zircon (ZrSiO4), are widely used to decipher geological processes. A new method developed in the last couple of years, the laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), overcomes previous laborious sample preparation, and yields isotopic ratios and age data with a high spatial resolution of ten of microns. The present study describes the analytical set-up and data reduction process as presently applied at the Laboratory for Geochronology of the University of Brasília. It explores the precision and accuracy of the method by cross-analysing three international zircon standards. We arrive at a precision of 1.9 to 3.7% (2σ SD) and an accuracy of 0.6 to 3.8% (2σ SD) for and U isotopic ratios of the standards. We also apply the method to two natural zircon samples, which have previously been dated by other analytical methods. A comparison of the results show a good conformity of the age data,being whitin the error limits. The data demonstrate the great analytical potential of the method for rapid, precise and accurate U-Pb isotopic analyses on the micron scale.

‣ Geochronology and heavy metal flux to Guanabara bay, Rio de Janeiro state: a preliminary study

Baptista Neto,Jose A.; Peixoto,Tomas C.S.; Smith,Bernard J.; Mcalister,John J.; Patchineelam,Soraya M.; Patchineelam,Sambasiva R.; Fonseca,Estefan M.
Fonte: Academia Brasileira de Ciências Publicador: Academia Brasileira de Ciências
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2013 Português
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Guanabara Bay, located in Rio de Janeiro state. It is surrounded by the second most important metropolitan area of the country. Over recent decades, land disturbance and urbanization in the surrounding area has significantly increased sediment input to the bay and had a negative effect on its overall environmental. This is especially related to high volumes of untreated sewage and industrial effluents. This study evaluates the history of this human impact through detailed examination of a sediment core taken from the northern portion of Guanabara Bay. A geochronology is established using 210Pb dating and related to organic carbon and heavy metal fluxes to the sediments. This gave a calculated net average sedimentation rate for the core of 0.67 cm.year-1. The organic carbon and heavy metals flux started to increase at the beginning of the last century and the highest values was observed in the top of the cores.

‣ Relative geochronology of the mineralization, alteration, and brecciation at the Galena Hill deposit, Navidad Project, Chubut, Argentina

Wafforn, Stephanie
Fonte: Quens University Publicador: Quens University
Tipo: Tese de Doutorado
Português
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The Galena Hill Deposit, located in the province of Chubut, Argentina, is a breccia-hosted, intermediate-sulphidation epithermal deposit, which was emplaced during the Late Jurassic following a period of intense volcanism. Mineralization is hosted in two autobrecciated latite flows, which belong to the Middle to Late Jurassic Cañadón Asfalto Formation. The purpose of this study is to document the relative geochronology of the mineralization, alteration, and brecciation events in order to understand the genesis of the deposit. Mineralization at Galena Hill is composed of galena and pyrite, with minor sphalerite, chalcopyrite, and tennantite. Silver is also occurs in the deposit, hosted in solid solution with pyrite or as minor silver sulfosalts. This study documents four phases of hydrothermal activity. The Pre-Ore phase consists of non-mineralized fine-grained calcite that has no associated alteration, and is found in the matrix of the Stage 1 hydrothermal breccia. The Main Ore phase comprises coarse-grained galena, pyrite, and minor sphalerite, chalcopyrite, tennantite and silver sulfosalts. This phase is associated with moderate to intense argillic alteration, chloritization and oxidation, and it is found in the matrix of the Stage 2 hydrothermal breccia. The Post Ore phase consists of coarse-grained calcite with minor disseminated barite...

‣ Geochronology in South Australia

Cooper, J.; Foden, J.; Prescott, J.; Veeh, H.; Webb, A.
Fonte: Taylor & Francis Ltd. Publicador: Taylor & Francis Ltd.
Tipo: Artigo de Revista Científica
Publicado em //2008 Português
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Geochronology studies in South Australia were developed in three Adelaide institutions. (i) University of Adelaide: (ia) Department of Geology and Mineralogy, where four single chemical measurements were reported in 1923-1955 and 14 lead-alpha measurements in 1961; MSID Rb-Sr work began in 1972 followed by zircon U-Pb about 1980 and Sm-Nd later; (ib) luminescence equipment was developed in the Department of Physics and is used to date recent sediments. (ii) Flinders University, where alpha spectrometry disequilibrium dating of recent sediments was active during 1972-2000; and (iii) AMDEL, which supplied a variable commercial service of K-Ar, Rb-Sr and zircon dating during period 1972-1999.; J. A. Cooper, J. D. Foden, J. R. Prescott, H. H. Veeh & A. W. Webb; © 2008 Informa plc

‣ 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...

‣ Timing of the magmatism of the paleo-Pacific border of Gondwana: U-Pb geochronology of Late Paleozoic to Early Mesozoic igneous rocks of the north Chilean Andes between 20° and 31°S

Maksaev, Víctor; Munizaga, Francisco; Tassinari, Colombo
Fonte: Servicio Nacional de Geología y Minería Publicador: Servicio Nacional de Geología y Minería
Tipo: Artigo de Revista Científica
Português
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Indexación: Scielo; ABSTRACT. U-Pb zircon geochronological data provide record of about 130 Ma of igneous activity in the Andes of northern Chile, which extended episodically from the latest Early Carboniferous to Early Jurassic (328-194 Ma). The overall U-Pb data show that volcanism and plutonism were essentially synchronous and major episodes of igneous activity developed during the Late Carboniferous to Mid-Permian (310 to 260 Ma) and from Late Permian to Late Triassic (255-205 Ma), with less prominent episodes in the mid-Carboniferous (330 to 320 Ma), and Early Jurassic (200-190 Ma). Thus, from the Carboniferous to the Early Triassic dominantly silicic magmatism developed along the Chilean segment of the southwestern border of Gondwana supercontinent. Further magmatism developed during the Mid-Late Triassic (250-194 Ma) was bimodal and synchronous with rift-related, continental and/or marine sedimentary strata related to the early stages of break-up of Gondwana. Most of the silicic volcanic rocks of the Precordillera and Domeyko Cordillera of northern Chile (21°30' to 25°30'S) are older than the silicic rocks assigned to the Choiyoi succession in Argentina, being instead equivalent in age to Carboniferous to Early Permian marine sedimentary sequences present in the eastern Argentinean foreland. On the other hand...

‣ Testing long-term patterns of basin sedimentation by detrital zircon geochronology, Centralian Superbasin, Australia

Maidment, David; Williams, Ian; Hand, Martin
Fonte: Blackwell Publishing Ltd Publicador: Blackwell Publishing Ltd
Tipo: Artigo de Revista Científica
Português
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Detrital zircon geochronology of Neoproterozoic to Devonian sedimentary rocks from the Georgina and Amadeus basins has been used to track changes in provenance that reflect the development and inversion of the former Australian Superbasin. Through much of

‣ Isotope geochemistry and geochronology of the Nico Perez Terrane, Rio de la Plata Craton, Uruguay

Mallmann, Guilherme; Chemale, Farid Jr.; Avila, Janaina N.; Kawashita, K.; Armstrong, Richard
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Português
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U-Pb SHRIMP zircon geochronology, bulk-rock geochemistry, and Sr-Nd isotopic data are herein presented in order to better constrain the tectonic evolution of the Nico Pérez Terrane (NPT), Uruguay. The studies are focused on two key geological units locat

‣ The contribution of geochronology to understanding the paleozoic geological history of Australia

Williams, Ian; Pulford, Anna
Fonte: Blackwell Publishing Ltd Publicador: Blackwell Publishing Ltd
Tipo: Artigo de Revista Científica
Português
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From the viewpoint of isotope geochronology, studies of the Paleozoic geological history of Australia can be divided into three main eras: (i) the era of stratigraphy, when the ages of igneous and metamorphic events were inferred primarily from stratigrap

‣ Combined U-Pb geochronology and Hf isotope geochemistry of detrital zircons from early Paleozoic sedimentary rocks, Ellsworth-Whitmore Mountains block, Antarctica

Flowerdew, M J; Millar, Ian L; Curtis, Michael L; Vaughan, A P M; Horstwood, M S A; Whitehouse, M.J.; Fanning, Christopher
Fonte: Association of Engineering Geologists Publicador: Association of Engineering Geologists
Tipo: Artigo de Revista Científica
Português
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U-Pb detrital zircon geochronology from the upper Cambrian to Devonian part of the Ellsworth Mountains succession, Antarctica, yields dominant late Mesoproterozoic and late Neoproterozoic-Cambrian age populations that are consistent with a provenance from

‣ ELA-ICP-MS U-Pb zircon geochronology of regional volcanism hosting the Bajo de la Alumbrera Cu-Au deposit: implications for porphyry-related mineralization

Harris, Anthony; Allen, Charlotte M; Bryan, Scott; Campbell, Ian; Holcombe, Rodney; Palin, J M
Fonte: Springer Publicador: Springer
Tipo: Artigo de Revista Científica
Português
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ELA-ICP-MS U-Pb zircon geochronology has been used to show that the porphyritic intrusions related to the formation of the Bajo de la Alumbrera porphyry Cu-Au deposit, NW Argentina, are cogenetic with stratigraphically well-constrained volcanic and volcaniclastic rocks of the Late Miocene Farallón Negro Volcanic Complex. Zircon geochronology for intrusions in this deposit and the host volcanic sequence show that multiple mineralized porphyries were emplaced in a volcanic complex that developed over 1.5 million years. Volcanism occurred in a multivent volcanic complex in a siliciclastic intermontane basin. The complex evolved from early mafic-intermediate effusive phases to a later silicic explosive phase associated with mafic intrusions. Zircons from the basal mafic-intermediate lavas have ages that range from 8.46 ± 0.14 to 7.94 ± 0.27 Ma. Regionally extensive silicic explosive volcanism occurred at ∼8.0 Ma (8.05 ± 0.13 and 7.96 ± 0.11 Ma), which is co-temporal with intrusion of the earliest mineralized porphyries at Bajo de la Alumbrera (8.02 ± 0.14 and 7.98 ± 0.14 Ma). Regional uplift and erosion followed during which the magmatic-hydrothermal system was probably unroofed. Shortly thereafter, dacitic lava domes were extruded (7.95 ± 0.17 Ma) and rhyolitic diatremes (7.79 ± 0.13 Ma) deposited thick tuff blankets across the region. Emplacement of large intermediate composition stocks occurred at 7.37 ± 0.22 Ma...

‣ U-Pb LA-ICP-MS GEOCHRONOLOGY AND REGIONAL CORRELATION OF MIDDLE JURASSIC INTRUSIVE ROCKS FROM THE GARZON MASSIF, UPPER MAGDALENA VALLEY AND CENTRAL CORDILLERA, SOUTHERN COLOMBIA

Bustamante,Camilo; Cardona,Agustín; Bayona,Germán; Mora,Andrés; Valencia,Víctor; Gehrels,George; Vervoort,Jeff
Fonte: Boletin de Geología Publicador: Boletin de Geología
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/12/2010 Português
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New U-Pb zircon geochronology from four granitic units sampled along a southeast-northwest transect between the Garzón Massif and the Serranía de las Minas (Central Cordillera), records a Middle Jurassic magmatic activity with two different spatio-temporal domains at ca. 189 Ma and 180-173 Ma. Reconnaissance data suggest that the four granitoids are characterized by mineralogical and geochemical characteristics akin to a continental magmatic arc setting. The new results suggest that the southern Colombian continental margin includes remnants of tectono-magmatic elements formed by the subduction of the Farallon plate under the South American continental margin. This Middle Jurassic arc magmatism is part of the broader Andean scale arc province, and is significant for understanding the tectonic and paleogeographic scenario that characterized the Mesozoic tectonic evolution of the Northern Andes.

‣ Geology, geochronology, and geochemistry of Isla María Madre, Nayarit, Mexico

Pompa-Mera,Valerie; Schaaf,Peter; Hernández-Treviño,Teodoro; Weber,Bodo; Solís-Pichardo,Gabriela; Villanueva-Lascurain,Daniel; Layer,Paul
Fonte: Instituto de Geología, UNAM Publicador: Instituto de Geología, UNAM
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/04/2013 Português
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Islas Marías archipelago is located 110 km NW of San Blas, Nayarit in the mouth of the Gulf of California. The archipelago is formed by San Juanito, María Madre, María Magdalena, and María Cleofas islands. The position of these islands represents a key pointfor paleogeographic and paleotectonic reconstructions of northwestern Mexico and of the tectonic evolution of Baja California Peninsula. María Madre is the largest island and covers an area of 145 km². This study presents the first detailed geological map of the island together with geochemical and geochronological data of its lithological units. Isolated basement rocks are exposed along the western coast in the form of migmatites and orthogneisses of granodioritic to granitic compositions and middle Jurassic ages (163 -170 Ma). In the west-central part, a metasedimentary sequence, with biotite ± garnet paragneisses and folded calcsilicate rocks with or without garnet bands of unknown ages, is exposed as a roof pendant of the underlying Cretaceous intrusions (80.8 - 83.4 Ma) of tonalitic to granitic compositions. These granitoids are cut by mafic and pegmatitic dikes. We named the overall assemblage of metamorphic and plutonic rocks as "Papelillo Complex" due to the predominant outcrops in the homonymous canyon. The Papelillo Complex is overlain by ignimbrites...