Elton Luiz Dantas

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)

A determinacao de idades atraves do decaimento dos isotopos radioativos 235U e 238U para os isotopos radiogenicos 207Pb e 206Pb, respectivamente, utilizando o mineral zircao (ZrSiO4), e amplamente aplicada para decifrar processos geologicos. Um novo metodo tem sido desenvolvido nos ultimos anos, a ablacao a laser multi-coletor espectrometria de massas com plasma indutivamente acoplado (LA-MC-ICP-MS), superando o laborioso trabalho anteriormente necessario em outros metodos, na preparacao de amostras, e permite obtencao de razoes isotopicas com alta resolucao espacial de micrometros. O presente estudo descreve os procedimentos analiticos e os metodos usados na reducao de dados que estao sendo aplicados no Laboratorio de Geocronologia da Universidade de Brasilia. Explora-se a precisao e exatidao do metodo atraves da analise de tres padroes internacionais de zircao. Observa-se uma precisao entre 1,9 a 3,7% (2σ desvio padrao) e uma exatidao de 0,6 a 3,8% (2σ desvio padrao) para as razoes isotopicas de Pb e U dos padroes. Tambem foram obtidas idades pelo metodo LA-ICP-MS de duas amostras de zircoes naturais, que ja foram datadas anteriormente por outros metodos analiticos. A comparacao dos resultados mostra uma boa conformidade das idades obtidas, dentro dos limites de erro.Os dados demonstram o grande potencial do metodo analitico para analises isotopicas rapidas, precisas e exatas de U-Pb, numa escala de micrometros.

Late Neoproterozoic–Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the eastern brazilian pegmatite province and related mineral resources.

Abstract The Araçuaí orogen extends from the eastern edge of the São Francisco craton to the Atlantic margin, in southeastern Brazil. Orogenic igneous rocks, formed from c. 630 to c. 480 Ma, cover one third of this huge area, building up the Eastern Brazilian Pegmatite Province and the most important dimension stone province of Brazil. G1 supersuite (630–585 Ma) mainly consists of tonalite to granodiorite, with mafic to dioritic facies and enclaves, representing a continental calc-alkaline magmatic arc. G2 supersuite mostly includes S-type granites formed during the syn-collisional stage (585–560 Ma), from relatively shallow two-mica granites and related gem-rich pegmatites to deep garnet-biotite granites that are the site of yellow dimension stone deposits. The typical G3 rocks (545–525 Ma) are non-foliated garnet-cordierite leucogranites, making up autochthonous patches and veins. At the post-collisional stage (530–480 Ma), G4 and G5 supersuites were generated. The S-type G4 supersuite mostly consists of garnet-bearing two-mica leucogranites that are the source of many pegmatites mined for tourmalines and many other gems, lithium (spodumene) ore and industrial feldspar. G5 supersuite, consisting of high-K–Fe calc-alkaline to alkaline granitic and/or charnockitic to dioritic/noritic intrusions, is the source of aquamarine-topaz-rich pegmatites but mainly of a large dimension stone production.

Geology of the northern Borborema Province, NE Brazil and its correlation with Nigeria, NW Africa

Abstract The Borborema and Benin–Nigeria provinces of NE Brazil and NW Africa, respectively, are key areas in the amalgamation of West Gondwana by continental collision during the Brasiliano/Pan-African orogenies. Both are underlain by complex basement: Nigeria has c. 3.05 Ga Archaean crust but no known Palaeoproterozoic rocks >2.0 Ga; in NE Brazil, 2.6–3.5 Ga Archaean rocks form small cores within Palaeoproterozoic gneiss terrains affected by plutonism at c. 2.17 Ga. Both regions exhibit Late Palaeoproterozoic (c. 1.8 Ga) rift-related magmatism and metasedimentary sequences overlying the basement. The Seridó Group of NE Brazil (<0.65 Ga) is similar to the Igarra Sequence in SW Nigeria. The Ceará Group, which may date back to c. 0.85 Ga, is a passive margin deposit on crust thinned during initiation of an oceanic domain. In both provinces, basement and sedimentary cover were involved in tangential tectonics that resulted in crust-thickening by nappe-stacking associated with closure of this ocean. Frontal collision between c. 0.66 and 0.60 Ga later evolved to an oblique collision, generating north–south continental strike-slip shear zones at c. 0.59 Ga. In NE Brazil, the main Pan-African suture is probably buried beneath the Parnaíba Basin. The Transbrasiliano Lineament, interpreted as the prolongation of the Kandi–4°50 Lineament in Hoggar, may represent a cryptic suture.

Combined U-Pb and Lu-Hf isotope analyses by laser ablation MC-ICP-MS: methodology and applications

The Lutetium-Hafnium isotopic system represents one of the most innovative and powerful tools for geochronology and isotopic studies. Combined U-Pb and Lu-Hf in situ analyses on zircon by LA-MC-ICP-MS permit to characterize isotopically the host magma from which it crystallized furnishing significant information for sediment provenance and crustal evolution studies. In this paper we describe the Lu-Hf systematic by LA-MC-ICP-MS developed in the laboratory of Geochronology of the University of Brasilia and report the results obtained by repeated analyses of 176 Hf/ 177 Hf isotopic ratio of three zircon standards: GJ-1 = 0.282022 ± 11 (n=56), Temora 2 = 0.282693 ± 14 (n=25) and UQZ = 0.282127 ± 33 (n=11). The 176 Hf/ 177 Hf ratio (0.282352 ± 22, n=14) of gem quality zircon used as in-house standard have been also characterized. As a geological application, we analyzed two complex zircons selected from a migmatitic rocks from the Borborema Province, NE Brazil. On the basis of U-Pb and Lu-Hf data, two main crystallization events have been identified in both studied zircons. An older event at ca. 2.05 Ga recognized in the inherited cores represents a well-characterized paleoproterozoic magmatic event that affected the whole Borborema Province. A second crystallization event at � 575 Ma, recognized at the rims, represents a Neoproterozoic (Brazilian) high grade metamorphic-magmatic event.

Long-lived Neoproterozoic high-K magmatism in the Pernambuco–Alagoas Domain, Borborema Province, northeast Brazil

The Pernambuco–Alagoas (PE–AL) Domain contains major granitic batholiths typified by a wide range of TDM model ages (Archaean to Neoproterozoic), reflecting the important role of quartzofeldspathic plutons attending the Brasiliano (Pan-African) Orogeny. U/Pb zircon data for eight syn- to post-collision to syn-transcurrent granitic intrusions of the PE–AL Domain allow the studied plutons to be divided into two groups: (1) granitoids with crystallization ages older than 600 Ma (Água Branca, Serra do Catú, Serra da Caiçara, and Mata Grande plutons) and (2) granitoids with ages of ca. 590 Ma (Correntes, Águas Belas, Viçosa, and Cachoeirinha plutons). The intrusions of group 1, except for the Mata Grande Pluton, all show Nd TDM model ages ranging from 1.5 to 1.2 Ga, whereas the granitoids from group 2 and the Mata Grande Pluton have Nd TDM model ages ranging from 2.2 to 1.7 Ga. The studied granitoids are in part high-K, calc-alkaline, shoshonitic, and in part transitional high-K calc-alkaline to alkaline in terms of their bulk chemistry. Volcanic arc signatures associated with the Palaeoproterozoic TDM model ages are interpreted as inherited from the source rocks. The oldest ages and higher Nd TDM model ages recorded in the granitoids intruded in the southwestern part of the PE–AL Domain suggest that these intrusions are associated with slab-tearing during convergence between the PE–AL and Sergipano domains. The investigated plutons are coeval with high-K granitoids intruded within the Transversal Zone Domain of Borborema Province and calc-alkaline granitoids of the Sergipano Domain. This suggests that these geologic realms belonged to the same crustal block during the Brasiliano Orogeny. However, such large volumes of high-K granitoids with crystallization ages older than 600 Ma are not recorded in the Sergipano and Transversal Zone domains, suggesting differences in the crustal evolution of these three areas.

The tectonic evolution of the Transbrasiliano Lineament in northern Paraná Basin, Brazil, as inferred from aeromagnetic data

Data from six airborne magnetic surveys were compiled and analyzed to develop a structural interpretation for the Transbrasiliano Lineament in northern Parana Basin, Brazil. Magnetic lineaments, interpreted to reflect geologic contacts and structures at different depths, were illuminated using the matched-filter technique applied to aeromagnetic anomalies. Field-based structural measurements generally support our magnetic analysis. We estimated three primary magnetic zones: (i) a zone of deep magnetic sources at 20 km depth, (ii) an intermediate basement zone at 6 km depth, and (iii) a shallow zone of near-surface geological features at 1.5 km depth. The deepest zone exhibits three major NE trending crustal discontinuities related to the Transbrasiliano Lineament, dividing the region into four geotectonic blocks. Anomalies associated with the intermediate zone indicate directional divergence of subsidiary structures away from the main Transbrasiliano Fault, which strikes N30°E. The shallow magnetic zone includes near-surface post-Brasiliano orogenic granites. Our analysis identified NE trending sigmoidal lineaments around these intrusions, indicating intense zones of deformation associated with probable shear structures. At the shallow depth zone, magnetic anomalies caused by Cretaceous alkaline intrusive bodies and basalts of the Serra Geral Formation are enhanced by the matched-filter method. These igneous bodies are related to extensional NW striking lineaments and seismicity aligned along these lineaments suggests that they currently are reactivated. Prior to flexural subsidence of the Parana Basin, reactivation processes along transcurrent elements of the Transbrasiliano Lineament promoted extensional processes and formed initial Parana Basin depocenters. Cretaceous and more recent sedimentation also correlate with reactivations of NE striking structures.

Magmatismo há ca. 660 - 640 Ma no Domínio Socorro: registros de convergência pré-colisional na aglutinação do Gondwana Ocidental

Uranium-lead zircon ages between 660 and 640 Ma, obtained from a series of calc-alkaline orthogneisses and plutons in southeast Brazils Central Mantiqueira Province, suggest that a significant period of magmatism occurred in this region prior to the collisional assembly of West Gondwana (presently constrained in the region between ca. 625 and 580 Ma). While the nature of this earlier magmatism is presently unclear, some preliminary Sm-Nd data suggest that these magmas were not solely derived from the Paleoproterozoic lithosphere, but appear to represent hybrid products of Paleoproterozoic and Neoproterozoic sources. As such hybrid mixtures have been most commonly observed in continental arc settings, it is possible that the 660 to 640 Ma magmatism represents arc magmatism that resulted from subduction of Neoproterozoic oceanic crust during early precollisional convergence and closure of a branch of either the Adamastor or Goianides oceans.

U-Pb and Sm-Nd constraints on the nature of the Campinorte sequence and related Palaeoproterozoic juvenile orthogneisses, Tocantins Province, central Brazil

Abstract The Palaeoproterozoic era was the most important stage of crustal accretion in the South American Platform, being responsible for the development of several magmatic arcs, which represent approximately 35% of the present-day continental crust. The recently mapped Campinorte volcano-sedimentary sequence and associated plutonic rocks represent this Palaeoproterozoic history in the northern Brasilia Belt, central Brazil. The sequence consists of metapsammites and metapelites, with interbedded lenses of gondites and metacherts, as well as rhyolite and pyroclastic deposits. Tonalite, granodiorite and granite crystallized between c. 2.18 and 2.16 Ga, as indicated by U–Pb zircon analyses. Sm–Nd TDM model ages range between c. 2.1 and 2.7 Ga, with ϵNd values ranging from −2.14 to +3.36, indicating the dominantly juvenile nature of the original magmas. A LA-ICPMS provenance study of zircon grains from a quartzite sample reveals a single sediment source with Palaeoproterozoic age. The data presented here provide new information on the Palaeoproterozoic juvenile crust of central Brazil and suggest correlation with other Palaeoproterozoic provinces, especially the Birimian Belt in West African Craton and those of the Guiana Shield, thus contributing to reconstruction of the Columbia supercontinent.

The Northern Brasília Belt

The Brasilia belt is a well-preserved Neoproterozoic orogenic belt within the Tocantins Province, central Brazil. Its northern segment strikes N–S, and verges eastwards to the Sao Francisco craton. The northern Brasilia belt external zone is a fold-thrust belt of low-grade passive margin metasedimentary rocks as well as syn-orogenic sedimentary sequences related to a magmatic arc. The internal zone includes deep sea sediments associated to an ophiolite melange, arc-type calc-alkaline volcanics and intrusives, and S-type collisional granites. Metamorphic grade increases westwards from non-metamorphic and low-grade rocks, in the east, to high-temperature amphibolite facies rocks, culminating in ultrahigh-T granulites in the metamorphic core. The belt is the result of convergence of the Amazonian, Sao Francisco and Paranapanema paleocontinents and involved the subduction of a wide oceanic lithosphere and development of primitive island arc systems. Convergence also entrapped the Goias massif, an exotic continental fragment, comprising Archean granite-greenstone terrains, Paleoproterozoic granite-gneiss, Neoproterozoic layered complexes, and Proterozoic cover rocks, exposed between the magmatic arc and the metamorphic core in the central part of the belt, and the external zone farther north.

Ion Exchange Chromatography and Mass Bias Correction for Accurate and Precise Zn Isotope Ratio Measurements in Environmental Reference Materials by MC-ICP-MS

Precise and accurate d66/64Zn data for environmental reference materials (RMs) including rocks, sediments, soils and plants are presented in order to improve the metrological traceability and analytical control of Zn isotope ratio determinations in future environmental studies. Previously developed ion exchange chromatography protocols were adjusted to enable faster sample throughput and instrumental mass bias processes were investigated. The improved chromatographic protocol yielded precise and quantitative recoveries (99 ± 7%, σ, n = 16), while the mass bias correction using Cu as external dopant provided precisions better than 0.02‰, 2σ, n = 7. Investigations into spectral and non-spectral interferences identified significant formation of Cr and Ti oxides and hydroxide ionic species. Analysis of six RMs (BHVO-2 basalt (USGS), BCR-2 basalt (USGS), AGV-2 andesite (USGS), 2709 San Joaquin soil (NIST), 1646a estuarine sediment (NIST) and 1573a tomato leaves (NIST)) showed good reproducibility (< 0.01‰, 2s, 5 ≤ n ≥ 1).