Metodologia U-PB em zircão com o uso de LAM-MC-ICPMS

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Idade U-Pb e Geoquímica do Traqui-Andesito Toleítico de Gouveia (MG) Comparadas com Rochas Básicas Toleíticas Regionais Pedro Lessa e Formiga: Fragmentos de uma Large Igneous Province (LIP) Toniana

Em Gouveia (MG), uma rocha subvulcânica traqui-andesítica toleítica de alto-K, guardando fenocristais de plagioclásio e sanidina e matriz também rica em feldspatos, com biotita, epidoto magmático e quartzo, aflora na forma de corpos irregulares e diques subverticalizados. A idade U-Pb de 938,5 +/- 4,5 Ma encontrada para o traqui-andesito de Gouveia é praticamente a mesma das rochas metabásicas da suíte Pedro Lessa (933 +/- 20 Ma) e dos diques Formiga (940 +/- 50 Ma), apontando para o agrupamento de todas essas rochas em um mesmo evento magmático. O tratamento conjunto da geoquímica da rocha traqui-andesítica com a das rochas regionais toleíticas Pedro Lessa e Formiga, revela cogeneticidade entre estes litotipos. O magma basáltico gerador destas rochas, com assinatura OIB (típica de pluma), é de origem provável da fusão parcial de um peridotito portador de granada, teria passado por contaminação litosférica e se diferenciado por meio do processo de cristalização fracionada até o traqui-andesito investigado. Esse magmatismo básico/intermediário encontra-se inserido no contexto magmático de uma large igneous province (LIP – província ígnea gigante) toniana reconstruída nos cratons São Francisco e Congo

The Barreiro suite in the central Ribeira Belt (SE-Brazil) : a late tonian tholeiitic intraplate magmatic event in the distal passive margin of the São Francisco Paleocontinent

New geochemical, U-Pb, Lu-Hf and Sr-Nd data from the Barreiro Suite metabasites in comparison with enclaves within the distal Andrelândia Group and the orthogranulites of the Juiz de Fora Complex are presented. Geochemical data suggest intraplate setting, with high and low-TiO 2 , TDM Nd ages between 1.80 and 1.41 Ga, negative ΕNd t and ( 87 Sr/ 86 Sr)i between 0.714 and 0.747. Results contrast with part of the Juiz de Fora Complex enclaves, with island arc tholeiites- calcalkaline basalts (IAT-CAB) geochemical signatures, TDM Nd ages between 2.58 and 2.16 Ga, positive ΕNd t values and (87Sr/86Sr) i between 0.700 and 0.712. U-Pb data for the Barreiro Suite yielded a crystallization age of 766 ± 13 Ma and a metamorphic overprint of 619 ± 6 Ma. The results indicate three episodes of mafic magmatism in the Occidental terrane of the Ribeira Belt. The two older episodes are related to Rhyacian arc evolution (ca. 2.2 to 2.1 Ga) and to the Statherian (ca. 1.7 Ga) tectonics, and occur only within the Juiz de Fora Complex, while the younger ca. 766 Ma episode constrains the timing of distal passive margin evolution. An important implication is that these late Tonian metabasic rocks could have been a source of detrital zircons for the sedimentation along the distal Andrelândia basin

Contrasting provenance and timing of metamorphism fromparagneisses of the Ara?ua?-Ribeira orogenic system, Brazil : hints for Western Gondwana assembly.

The Ara?ua? orogen and the Ribeira belt make up a complex Neoproterozoic-Cambrian orogenic system, the Ara?ua?-Ribeira orogenic system(AROS) located fromthe eastern to southeastern Brazil. Along the AROS, the Ediacaran Rio Doce magmatic arc represents a geotectonic connection between the Ara?ua? and the Ribeira orogenic domains. Although the nature and evolution of the Rio Doce plutonic rocks is regionally well established, it lacks detailed studies on the paragneisses found along the western and central regions of this magmatic arc. Besides information on the nature and provenance of their sedimentary protoliths, the paragneisses provide data to unravel the palaeogeographic scenario fromthe precursor to arc-related basins. Six samples of Al-rich gneisses covering a large AROS region were selected for electron microprobe (EMP) mineral analyses in order to obtain geothermobarometric data and monazite ages, as well as for Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) isotopic analyses on zircon (U-Pb, Lu-Hf) and monazite (U-Pb). The different age spectra fromdetrital zircon grains and contrasting Hf isotopic signatures suggest a complex sedimentary history. Located in thewestern sector of the study region, the samples RC-02 and RC-34, with an 80% age peak of detrital zircon grains from 2158 Ma to 1830 Ma, ?Hf(t) from ?2.2 to ?22.7, and Hf TDM model ages from 3530 Ma to 2440 Ma, suggest sediment sources located in the S?o Francisco craton basement. The samples RC-03, also fromthewestern sector, and RC-46 fromthe southern sector, have a more complex assemblage of detrital zircon grains with an 87% age peak from 987 Ma to 592 Ma, ?Hf(t) from +14.9 to ?2.9, and Hf TDM model ages from 2220Ma to 720 Ma, indicating provenance from mainly juvenile sources of distinct ages. Candidates to be juvenile sources for RC-03 and RC-46 sedimentary protoliths are the Rhyacian Juiz de Fora and Pocrane complexes in the basement of the Rio Doce arc, the Neoproterozoic Rio Negro arc systemof the Ribeira belt, and AROS ophiolite complexes. Samples RC-30 and RC-38 from the eastern sector of the study region, with most detrital zircon ages between 650 Ma and 552 Ma and very negative ?Hf(t) (?25.3 to ?16.5), suggest main sediment sources in the Rio Doce arc. By extending U-Pb analyses on metamorphic zircon and monazite, we have identified a complex timing of metamorphism, represented by metamorphic ages ranging from 621 Ma to 480 Ma, with the main collisional activity between 580Ma and 540 Ma. Geothermobarometric studies on garnet porphyroblasts, syn-kinematic to the D2 regional foliation, show a retrograde metamorphic path typical of continental collision belts, starting with P-T conditions of Tmax = 733 ?C and Pmax = 6.43 kbar. Our data also suggest: i) the studied paragneisses represent distinct Neoproterozoic basin stages, shifting from passive to active margin settings; ii) if the Rio Negro arc system really provided sediments for the basin stage represented by the RC-03 and RC-46 paragneisses, it would have amalgamated with the AROS before 614 Ma; iii) the final amalgamation of Western Gondwana took place around 540 Ma in the focused region; iv) an important re-heating period (520?480 Ma) can be related to the AROS gravitational collapse, afterWestern Gondwana assembly

Granites of the intracontinental termination of a magmatic arc : an example from the Ediacaran Ara?ua? orogen, southeastern Brazil.

The Ara?ua? orogen of southeastern Brazil together with the West Congo belt of central West Africa form the Ara?ua??West Congo orogen generated during closure of a terminal segment of the Neoproterozoic Adamastor Ocean. Corresponding to an embayment in the S?o Francisco?Congo Craton, this portion of the Adamastor was only partially floored by oceanic crust. The convergence of its margins led to the development of the Rio Doce magmatic arc between 630 Ma and 580 Ma. The Rio Doce magmatic arc terminates in the northern portion of the Ara?ua? orogen. Granitic plutons exposed in the northern extremity of the arc provide a rare opportunity to studymagmatismat arc terminations, and to understand the interplay between calc-alkalinemagma production and crustal recycling. The plutons forming the terminus of the arc consist of granodiorites, tonalites and monzogranites similar to a magnesian, slightly peraluminous, calcic- (68%) to calc-alkaline (24%), with minor alkali-calcic (8%) facies, medium- to high-K magmatic series. Although marked by negative Nb?Ta, Sr and Ti anomalies, typically associatedwith subduction-relatedmagmas, the combined Sr, Nd and Hf isotopic data characterize a crustal signature related to anatexis of metamorphosed igneous and sedimentary rocks, rather than fractional crystallization of mantle-derived magmas. Zircon U?Pb ages characterizes two groups of granitoids. The older group, crystallized between 630 and 590 Ma, experienced a migmatization event at ca. 585 Ma. The younger granitoids, emplaced between 570 and 590 Ma, do not show any evidence for migmatization. Most of the investigated samples show good correlation with the experimental compositional field of amphibolite dehydration-melting, with some samples plotting into the field of greywacke dehydration-melting. The studied rocks are not typical I-type or S-type granites, being particularly similar to transitional I/S-type granitoids described in the Ordovician Famatinian arc (NW Argentina). We suggest a hybrid model involving dehydrationmelting of meta-igneous (amphibolites) and metasedimentary (greywackes) rocks for magma production in the northern termination of the Rio Doce arc. The real contribution of each end-member is, however, a challenging work still to be done

Análise estrutural e estratigráfica das unidades basais da Bacia Espinhaço

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Análise estrutural e estratigráfica das unidades basais da Bacia Espinhaço

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Metodologia U-PB em zircão com o uso de LAM-MC-ICPMS

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Método de separação mineral para datação U/PB de amostras da Bacia de Rincón Blanco, Argentina

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Método de separação mineral para datação U/PB de amostras da Bacia de Rincón Blanco, Argentina

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