Jean-Michel Lafon

Geology, geochemistry, and Pb–Pb zircon geochronology of the Paleoproterozoic magmatism of Vila Riozinho, Tapajós Gold Province, Amazonian craton, Brazil

Abstract Pb–Pb zircon geochronologic data define two different periods of intense igneous activity in the Vila Riozinho region, Tapajos Gold Province (TGP), south-central Amazonian craton. At ∼2.00–1.97 Ga, the Vila Riozinho volcanic sequence (2000±4 Ma, 1998±3 Ma) and the older Sao Jorge granite (1981±2 Ma, 1983±8 Ma) were formed. At ∼1.89–1.87 Ga, the younger Sao Jorge (1891±3 Ma), Jardim do Ouro (1880±3 Ma), and Maloquinha (1880±9 Ma) granites and the Moraes Almeida volcanic sequence (1890±6 Ma, 1881±4 Ma, 1875±4 Ma) were emplaced. Similar age intervals are registered throughout the TGP that is thus a little younger than the Paleoproterozoic terranes of the Maroni-Itacaiunas Province in the Guiana Shield. Geochemical and geochronologic data demonstrate that the Sao Jorge pluton is composed of two different granitoids, the older and younger Sao Jorge granites. These, as well as the Jardim do Ouro granite, are I-type and magnetite series. The Sao Jorge granites are high-K calc-alkaline and show significant geochemical variation, the Jardim do Ouro granite is more iron-rich and less oxidized. The Vila Riozinho Formation is calc-alkaline to shoshonitic and geochemically similar to the Sao Jorge granites. The Maloquinha granite and the Moraes Almeida Formation differ from the other studied rocks in petrographic and geochemical characteristics, have aluminous A-type affinities, and were probably derived by low-temperature crustal melting. The volcanic sequences of the studied area, formerly included into the Iriri Group of the Uatuma Supergroup, are divided into the Vila Riozinho and Moraes Almeida sequences. The presence of these two sequences in the same tectonic domain demonstrates the heterogeneity of the Uatuma Supergroup. The TGP registers accretionary processes related to the formation of the Atlantica supercontinent at ∼2.00 Ga. This was followed (at ∼1.88 Ga) by an intracontinental taphrogenic event that lasted throughout the Mesoproterozoic. The tectonic setting of the TGP was thus transitional between a subduction-related magmatic arc and a stable continental block undergoing extension.

Comparison of single filament Pb evaporation/ionization zircon ages with conventional U-Pb results: examples from the Precambrian of Brazil

Abstract This paper documents a systematic and detailed study comparing the 207 Pb/ 206 Pb apparent ages obtained by single zircon Pb evaporation/ionization method, in rock samples previously dated by conventional U-Pb zircon dissolution procedures. The technique applied here uses a single rhenium center filament to evaporate, ionize and measure the Pb isotopic ratios, in a thermal ionization mass spectrometer, by direct evaporation/ionization of a single zircon grain. The single zircon Pb evaporation/ionization method was applied in igneous and metamorphic rocks of Archean to Neoproterozoic age. The 207 Pb/ 206 Pb obtained for the igneous rocks with little or no evidence of deformation are, within the error, similar to the published U-Pb ages and may be considered as good indicator of the crystallization age of these rocks. However, uncertainties on the 207 Pb/ 206 Pb ratios measured on zircons may constitute a limitation of the Pb evaporation/ionization method, particularly for the precise determination of the age of crystallization of Neoproterozoic rocks or younger. For granulite facies rocks, where a Pb loss discordancy has been imprinted upon the zircons, the 207 Pb/ 206 Pb ages obtained by Pb evaporation/ionization method must be considered as minimum ages. The results obtained here along with the relative simplicity of the analytical procedures used in the single zircon Pb evaporation/ionization method show great potential for the application of this technique in the reconnaissance evaluation of the ages of rocks units of Precambrian crust where a geochronological framework is not well established.

Nd, Pb and Sr isotopes in the Identidade Belt, an Archaean greenstone belt of the Rio Maria region (Carajás Province, Brazil): implications for the Archaean geodynamic evolution of the Amazonian Craton

The Rio Maria granite–greenstone terrain (RMGGT), located in the Amazonian craton at about 250 km south of the Carajas Ridge, is a well preserved Mesoarchaean domain composed of metavolcanic rocks and several suites of granitoids ranging in age from ca. 2.96 to 2.87 Ga. The metavolcanics comprise a basal unit of komatiites and low-K tholeiites overlain by sodic metadacites. A Sm–Nd whole-rock reference isochron for the metabasalts, metagabbros (tholeiites) and metadacites gives an age of 3046±32 Ma (MSWD of 1.6, 1 σ ). Nd model ages ( T DM ) of the metadacites range between 3.04 and 3.11 Ga. The e Nd calculated at 3.0 Ga varies from +1.70 to +2.60 for the tholeiites and +0.30 to +3.22 for the metadacites. A Pb–Pb whole-rock linear array for the metabasalts yielded an anomalous older age of 3.41 Ga. The Sm–Nd age is coincident, within analytical error, with a Pb–Pb whole-rock errorchron age of 2944±88 Ma (1 σ ) obtained for the metadacites. Rb–Sr systematic for the metadacites yielded an isochron age that is ca. 400 Ma youngest. The isotopic, geochemical and geological data suggest that oceanic lithosphere was the probable source of the ultramafic and mafic rocks in such a way that partial melting of the depleted mantle generated komatiites and low-K tholeiites. Partial melting of oceanic crust transformed into garnet amphibolite or eclogite generated the parental magmas of the metadacites. Isotopic and trace element characteristics of both metavolcanic rocks and associated plutonics suggest intra-oceanic island arcs as the most probable tectonic setting of the RMGGT. Two main events of crustal accretion are recognised at about 3.04–2.96 Ga and 2.87 Ga. Geological correlation indicates that the oceanic island arcs were completely amalgamated at around 2.76 Ga and acted as sources for the overlying sediments of the Rio Fresco Group.

Evolution of Brasiliano-age granitoid types in a shear-zone environment, Umarizal-Caraubas region, Rio Grande do Norte, northeast Brazil

A sequence of Brasiliano-age granitoid types is exposed in a small area near the cities of Umarizal and Caraubas in Rio Grande do Norte State, Northeast Brazil. Porphyritic K-alkali-calcic monzogranite is an important facies of the oldest Caraubas intrusion (RbSr whole rock isochron age of ca. 630 Ma), which suffered solid-state deformation due to movements on a major NE-trending shear zone. The intrusion of the Prado and part of the Quixaba bodies was probably controlled by the shear zone. These two bodies include mafic/intermediate rocks, some of which contain two pyroxenes, and have hybrid, partly alkaline and partly shoshonitic geochemical characteristics. Rock types and ages are similar to those of some Pan-African occurrences in southwestern Nigeria. The Tourao body, intruded at ca. 590 Ma, presents preferred mineral orientations which are probably largely magmatic, since little evidence is found for widespread solid-state deformation. On the other hand, its intrusion may have been facilitated by the presence of the shear-zone faults. The rocks form a monomodal felsic K-alkali-calcic suite. With the exception of the Quixaba body, all these earlier granitoids are magmatic epidote- and magnetite-bearing porphyritic monzogranites with trace element geochemical characteristics of modern syn-collisional granites. The latest intrusion at ca. 545 Ma is mainly represented by potassic quartz syenites and related rocks, some of which contain fayalite or ferrohypersthene. These rocks possess neither well developed mineral orientations of magmatic origin nor signs of solid-state deformation. They are mineralogically similar to, but younger than some of the “bauchites” of central Nigeria. Geochemical signatures are comparable with those of modern within-plate granites. All granitoids present high (87Sr/86Sr)i ratios which range from 0.708 to 0.712, and increase with decreasing age. Such ratios are compatible with important or dominant crustal contributions. On the other hand, the more mafic rocks are likely to have formed from enriched mantle.

Uranium-lead dating method at the Pará-Iso isotope geology laboratory, UFPA, Belém - Brazil

Analytical procedures for U-Pb isotope dilution analyses at the Pará-Iso isotope geology laboratory of the Federal University of Pará (UFPA) are described in detail. The procedures are applied to zircon, titanite, rutile, apatite, columbite-tantalite and whole rock. Reagent preparation and chemical processing are done in clean-room conditions. Samples are dissolved using Teflon microcapsules in steel jacket Teflon Parr Instrument bomb or Teflon screw cap containers. U and Pb are separated using anion exchange AG 1 x 8 resin columns. Typical blanks for mineral sample amounts of 0.01 to 1.0 mg are less than 1 pg U and 20-30 pg Pb. Isotope analysis of the U and Pb from the same filament are carried out using a Finnigan MAT 262 mass-spectrometer in static and dynamic modes. The current analytical level is demonstrated on analyses of international standard zircon 91500 with three different 235U-205Pb and 235U-208Pb isotope tracers and whole rock standards. Results of analyses of two zircon samples are also presented.

Paleoproterozoic late-orogenic and anorogenic alkaline granitic magmatism from northeast Brazil

Abstract During the Transamazonian cycle (2.0±0.2 Ga), two silica-saturated alkaline granite suites were intruded along the border of the Sao Francisco craton, northeastern Brazil. The Couro de Onca intrusive suite (COIS) is late tectonic relative to major deformational events, and the Serra do Meio suite (SMS) postdates this event, being interpreted as anorogenic type, deformed during the Brasiliano cycle (0.65–0.52 Ga). The COIS encompasses alkali-feldspar granites, syenogranites, monzogranites, with dykes of similar composition and age (2.157±5 Ma). They are metaluminous rocks of alkaline affinity, whose evolution was probably controlled by mineral fractionation processes involving mainly plagioclase, amphibole, and Fe–Ti oxide. The SMS, whose probable age is 2.01 Ga, is composed of metaluminous and peralkaline granites, with associated quartz syenites, with Ti-aegirine transformed during the Brasiliano deformation to riebeckite+titanite+aenigmatite. The temperatures of two stages of metamorphic transformation, overprinted during the Brasiliano event, are estimated at about 550 and 400–450°C. Both magmatic suites are derived from enriched mantle sources. The metaluminous and peralkaline trends observed in the SMS, characterized by high concentrations of HFS and RE elements controlled mainly by F-activity, reflect the evolution of different primary magmas produced by successive melting of the same mantle sources, in an anorogenic setting.

Petrogenesis and U-Pb and Sm-Nd geochronology of the Taquaral granite: record of an orosirian continental magmatic arc in the region of Corumba - MS

O Granito Taquaral situa-se no sul do Craton Amazonico, na regiao de Corumba, extremo ocidente do estado de Mato Grosso do Sul (MS), proximo a fronteira Brasil-Bolivia. Ocorre como um batolito, sendo parcialmente recoberto pelas rochas sedimentares das formacoes Urucum, Tamengo, Bocaina e Pantanal e pelas aluvioes atuais. Seus litotipos sao classificados como quartzo monzodioritos, granodioritos, quartzo-monzonitos, monzogranitos e sienogranitos. Dois tipos de enclaves de natureza e origens diferentes sao encontrados, um de composicao mafica correspondente a xenolito e outro como enclave microgranular felsico. Observam-se duas fases deformacionais, uma de natureza ductil (F1) e outra ruptil (F2). Os dados geoquimicos indicam composicao intermediaria a acida para essas rochas e um magmatismo calcio-alcalino de medio a alto-K, metaluminoso a peraluminoso e sugerem uma colocacao em ambiente de arco. A analise geocronologica pelo metodo U-Pb (SHRIMP) em zircao foi realizada em um granodiorito aponta para uma idade de 1861 ± 5,3 Ma para sua cristalizacao. Analises Sm-Nd em rocha total fornecem valores de eNd(1,86 Ga) de -1,48 e -1,28 e TDM de 2,32 e 2,25 Ga indicando uma provavel fonte crustal riaciana. Admite-se que o Granito Taquaral corresponda a um magmatismo desenvolvido no final do Orosiriano, constituinte do Arco Magmatico Amoguija.

Petrogenesis, U-Pb and Sm-Nd geochronology of the Furna Azul Migmatite: partial melting evidence during the San Ignácio Orogeny, Paraguá Terrane, SW Amazon Craton

O Migmatito Furna Azul e um complexo de ~10 km2 localizado em Pontes e Lacerda, Mato Grosso, Brasil. Pertence ao Terreno Paragua, proximo ao limite com o Terreno Rio Alegre, sudeste da Provincia Rondoniana - San Ignacio - Craton Amazonico. O migmatito consiste de metatexitos transicionais com enclaves anfiboliticos e injecoes dioriticas. Os metatexitos sao distinguidos em ricos em residuo e ricos em leucossoma e exibem tres fases deformacionais marcadas pelo bandamento estromatico dobrado afetado por uma xistosidade espacada e metamorfisados na facies anfibolito alto, representada por granada, biotita e sillimanita, bem como pela formacao de clinopiroxenio nos enclaves. O retrometamorfismo para a facies xisto verde e marcado pela formacao de clorita, muscovita/sericita e prehnita. O metatexito rico em residuo apresenta maiores teores de CaO, Na 2O, separando-os do metatexito transicional enriquecidos em K2O, Ba e Rb. Comparando com produtos de anatexia, nota-se uma afinidade com produtos de protolitos tonaliticos e/ou anfiboliticos. Os dados geocronologicos (U-Pb SHRIMP em zircao e Sm-Nd em rocha total) mostraram que o metatexito rico em residuo teve sua cristalizacao em 1436 ± 11 Ma, com idade modelo TDM de 1,90 Ga e e Nd(1,43) de -0,54, enquanto a injecao dioritica cristalizou em 1341.7 ± 17 Ma com idade modelo TDM de 1,47 Ga e e Nd(1,34) de 3,39. Esses resultados evidenciam que o protolito do Migmatito Furna Azul teria sido formado durante a Orogenia San Ignacio (1.43 Ga) posteriormente retrabalhado, servindo de embasamento para o magmatismo tardi a pos-colisional representado pela Suite Intrusiva Pensamiento.