Issamu Endo

The tectonic evolution of the Quadrilátero Ferrífero, Minas Gerais, Brazil

Abstract The structural framework of the Quadrilatero Ferrifero is the result of two main deformational events. The first is an extensional event of Palaeoproterozoic age (between 2 100 and 1700 Ma, the Transamazonian Event), which resulted in the formation of Archaean granite-gneiss domes and the nucleation of regional synclines in the overlying Rio das Velhas and Minas supergroups strata. Such regional synclines are continuous with each other and are bordered by structural granite-gneiss highs. At the contact between these units, ductile-brittle to ductile extensional shear zones are developed, representing the dislocation surfaces of Archaean blocks. This tectonism is probably related to the evolution of a metamorphic core complex, formed in the hinterland to the west of a Transamazonian collision zone, which structured the Paramirim Craton. The second event is compressive and associated with the closure of the Pan-African/Brasiliano proto-ocean (650–500 Ma), situated to the east of Quadrilatero Ferrffero (QF). During this event, a west-verging fold-and-thrust belt (FTB) developed, causing inversion, amplification, translation and rotation of the basinal synclines. The FTB affected mainly the eastern portion of QF and obliterated many of the tectonic features of the extensional event.

New evidence of Neoarchean crustal growth in southern São Francisco Craton: the Carmópolis de Minas Layered Suite, Minas Gerais, Brazil

A Suite Acamadada Carmopolis de Minas esta situada no dominio do Complexo Metamorfico Campo Belo, Minas Gerais, SE-Brasil, que junto a outros complexos metamorficos nos arredores do Quadrilatero Ferrifero, compoem a crosta sialica arqueana da parte meridional do Craton Sao Francisco. Esta suite compreende um segmento remanescente de arco magmatico, deformado e metamorfisado em facies anfibolito-superior a granulito. Exibe estratos acamadados e macicos, constituidos por litotipos com assinatura litogeoquimica equivalente a uma serie toleitica de baixo-K. Uma fase de magmatismo tholeiitico e outra de magmatismo calcio-alcalino foram identificadas. Uma amostra de anfibolito e duas de metarriolito foram utilizadas para datacao U-Pb em zircoes usando MC-LA-ICP-MS. O anfibolito apresentou zircoes com razoes 232Th/238U igneas e idade concordia de 2752 ± 18 Ma. Esse resultado foi interpretado como idade minima da fase tholeiitica. As amostras de metarriolito apresentaram zircoes com razoes 232Th/238U magmaticas e foram datadas em 2713 ± 9.9 e 2710 ± 31 Ma. Essa variacao foi interpretada como intervalo minimo de idades da fase calcio-alcalina. Uma das amostras de metarriolito apresentou duas populacoes de zircoes herdados. Estas populacoes foram datadas em 3374 ± 30 e 2859 ± 23 Ma, sugerindo a contribuicao de crostas mais antigas na genese da suite. Os resultados demonstram que a genese desta suite representa um episodio de formacao de crosta juvenil durante o Evento Tectonotermal Rio das Velhas (2780 Ma). Este episodio sin a tardi-orogenico envolveu desenvolvimento de arco, magmatismo tholeiitico a calcio-alcalino, metamorfismo e reciclagem crustal.

Geochemistry of hematitite and itabirite, Quadrilátero Ferrífero, Brazil

Twenty-one samples of hematitite and twelve samples of itabirite were collected from different deposits of the Quadrilatero Ferrifero (QF) area and were analyzed for trace and rare earth elements. The purpose of the study is to understand the element distribution in the QF in comparison with other iron formations (IF) around the world. Trace element contents are relatively low with considerable variability, being lower than the contents in the Algoma IF, Anamikie IF, Maru IF in Nigeria and Orissa IF in India. REEs abundance is relatively low, but higher than REEs of Hamersley IF of Western Australia, Surgur belt in India and lower than Kuruman IF in South Africa. Chondrite normalized patterns show slight degrees of fractionation for LREE to HREE and slightly positive Eu anomalies coupled with positive values of (La/Yb)CN and (La/Sm)CN ratios.

Reavaliação e novos dados geocronológicos (Pb/Pb e K/Ar) da região do Quadrilátero Ferrífero e adjacências

Os estudos geocronologicos efetuados no Quadrilatero Ferrifero e regioes adjacentes tem sido fundamentais para a elucidacao dos processos geodinâmicos relacionados a formacao e consolidacao deste segmento crustal. Sao reconhecidos dois conjuntos estratigraficos maiores: o Supergrupo Rio das Velhas e unidades correlatas, e o Supergrupo Minas. O evento geologico melhor caracterizado (U/Pb, em zircao) esta associado a deposicao, metamorfismo e magmatismo do Supergrupo Rio das Velhas. O magmatismo associado ao evento metamorfico principal desta unidade se deu no intervalo entre 2.780 Ma e 2.730 Ma. Rochas vulcânicas do Grupo Nova Lima (base do Supergrupo Rio das Velhas) apresentaram idade (U/Pb, em zircao) de 2.770 Ma. Datacoes (Pb/Pb) em rochas carbonaticas do Supergrupo Minas forneceram idades de cerca de 2.420 Ma. Granitoides intrusivos na sequencia basal do Supergrupo Minas forneceram idades de 2.612 Ma (U/Pb, em zircao) e 2.608 Ma (Pb/Pb, em zircao por evaporacao). A idade do magmatismo Transamazonico esta bem registrado em granitos alojados em descontinuidades crustais ocorridos em torno de 2.120 Ma. O ciclo Brasiliano foi responsavel pelo reequilibrio isotopico diferenciado do sistema K/Ar. Sao controversos os parâmetros tectonicos como vergencia e polaridade metamorfica dos eventos arqueanos e paleoproterozoicos.

A structural model of the Fábrica Nova region, Santa Rita syncline, Quadrilátero Ferrífero: flanking folds as a folding mechanism

This study focuses on the eastern flank of the Santa Rita syncline (Dorr 1969), with specific emphasis on the region known as Fabrica Nova. Important iron ore deposits are located on the flanks of this structure, such as Timbopeba, Alegria, Sao Luiz, Tamandua, Almas and Fabrica Nova. The Santa Rita syncline is a fold with N-S axial direction and of subregional scale, with roots in the adjacent basement of the Santa Barbara Complex and sectioned by the Agua Quente thrust fault. The hypothesis of this study is that the structural framework of the region resulted from the superposition of at least three deformation phases on the Ouro Preto nappe. The Fabrica Nova mine, located in the central portion of the study area, is embedded in a synformal structure with a 100/20 trending axis named Fabrica Nova synform. The proposed model to explain the particular structural geometry of this region is based on the flanking folding mechanism (Passchier 2001). This mechanism may have been developed by E-W crustal shortening during the F4 tectonic deformation phase.

Geometria do Sinclinal Gandarela baseada na deconvolução Euler 2D e 3D: quadrilátero ferrífero (MG)

Euler deconvolution, based on the Eulers equation, is a powerful tool that can be applied in the estimative of mean depths of magnetic sources or magnetic bodies. This procedure, combined with geological and structural data, showed the definition of the geometry in depth of the Gandarela syncline region, located in the center-north portion of the Quadrilatero Ferrifero, Minas Gerais. Its structural framework is defined as a reclined fold with NE-SW trending axial trace and hinge zone plunging to the southeast. The Gandarela syncline is composed of thick metasedimentary units of Minas Supergroup and of greenstone sequence of the Rio das Velhas Supergroup. The results based on 2D and 3D Euler deconvolution applied in magnetic data from Brasil-Alemanha aerogeophysical project allowed to estimate the mean depths of the magnetic sources probably, associated to iron formations (Caue Formation and Nova Lima Group). Additionally, the geometry of the structure was confirmed by 2D magnetic modeling.

Litoestratigrafia e processos geológicos regionais estruturais de formação da bacia do Rio Paracatu (Lithostratigraphy and regional structural geological processes molding Paracatu River...)

Este artigo tem como objetivo apresentar as grandes feicoes litoestratigraficas e tectonicas regionais de formacao da bacia do Rio Paracatu. Pelo vies da abordagem regional, elucidam-se as questoes referentes aos resultados de expressao superficial da bacia pela interacao dos processos exogenos com as estruturas profundas. Sao elaborados mapas de litoestratigrafia, estruturas rupteis, estruturas ducteis, estereogramas e modelos visuais de geologia estrutural. Demonstra-se como a formacao do Domo de Cristalina gerou dobras de descolamento anteparadas pela Serra de Sao Domingos, dando origem as esturutras de sinclinais e anticlinais das Cristas de Unai. A B S T R A C T This paper presents the lithostratigraphy and major regional tectonic processes that molded Paracatu River Basin. Through the regional analysis approach, an investigation is undertaken on how the exogenous processes and the in-depth structures interacted, resulting in the superficial expression of the basin. The results are maps for lithostratigraphy, brittle and ductile structures, stereograms and visual models for structural geology. The paper demonstrates how the Cristalina dome emerged and generated detachment folds, shielded by Sao Domingos Ridge, originating the synclinal and anticlinal structures of Unai Crests. Keywords: Paracatu, lithostratigraphy, structural geology, cristalina dome

Analog Models of Flanking Structures and a Natural Example in the Quadrilátero Ferrífero, Minas Gerais

The aim of this study is to present analog models of flanking structures and to analyze the Fábrica Nova synform, Quadrilátero Ferrífero, Minas Gerais, from a geometric point of view. We set up seven models using a linear viscous silicone and produced flanking structures with a shear velocity of 2 cm h-1. At different initial orientations with respect to the shear zone boundary, a rigid cross-cutting element with lubricated boundaries was deformed via sinistral bulk flow at a shear strain of γ = 1.28. The most interesting features of our experiments are the geometries of the different marker lines, which are heterogeneous and resulted from thickening and thinning of the silicone at the cross-cutting element terminations. To compare our analog models and the Fábrica Nova synform, we analyzed the outermost marker line of the analog models and the top surface of the Cauê Formation in the Paleoproterozoic metasediments. The best comparisons between the experiments and the natural example were obtained by our CIS90 model in terms of the flexure shape near the cross-cutting element and the cross-cutting element orientation. Thus, we suggest that the cross-cutting elements in both situations act as obstacles and consequently produce local perturbations in laminar flow.

MAGNETIC ANALYSIS OF ABAETÉ DIAMONDIFEROUS DISTRICT (MG) AND ITS POSSIBLE PRIMARY SOURCES

ABSTRACT. This work studies the origin of the diamonds from the important mining region, Abaete Diamondiferous District (Minas Gerais, Brazil). The discovery of pipes with micro diamonds and/or crater facies kimberlites in the entire Alto Paranaiba Igneous Province (macro region of Abaete District) enhanced the interest about understanding the diamonds origins. The diamonds from this district usually show few cleavages, present more than 100 carats (ct) and are associated to numerous indicator minerals for kimberlite, suggesting proximity between their primary and current sources. In order to corroborate with the area geophysics knowledge, high resolution maps were prepared using airborne magnetic data integrated with geology and field studies. The products were used to: suggest the continental fracture zones which allowed the ascent of alkaline magma; correlate magnetic responses with stratigraphic and tectonic features; indicate the magnetic responses of the lineaments that govern the diamantiferous drainages; suggest possible kimberlite clan rocks; and estimate the basement depth by Euler Deconvolution. The crystalline basement is concave and the sedimentary rocks thickness may achieve about 3,000m. The magnetic anomalies selected as possible kimberlite clan rocks were detailed using ground magnetic surveys and their Analytic Signal was adopted to estimate the magnetic source geometries by Euler Deconvolution. These possible kimberlitic pipes look like each other and presenting depths of up to 200 m and diameters of up to 300 m. Keywords : mineral prospecting, Euler Deconvolution, Alto Paranaiba Igneous Province. RESUMO. Este trabalho estuda a origem dos diamantes de uma importante regiao mineradora, o Distrito Diamantifero Abaete (MG). A descoberta de kimberlitos com microdiamantes e/ou facies de cratera em toda Provinciaignea do Alto Paranaiba (macrorregiao do distrito) aumentou o interesse de compreender a origem destes diamantes. Os diamantes da area geralmente apresentam poucas feicoes de clivagem, mais que 100 quilates (ct) e estao associados aos minerais indicadores de rochas kimberliticas, sugerindo proximidade entre suas fontes primarias e atuais. A fim de contribuir com o conhecimento geofisico da area, mapas geofisicos de alta resolucao foramconfeccionados usando dados aeromagneticos integrados a geologia e aos estudos de campo da regiao. Os produtos desta integracao foramutilizados para: sugerir as zonas de fraturas continentais que permitiram a ascensao magmatica alcalina; correlacionar a magnetometria com a estratigrafia e as feicoes tectonicas; indicar as feicoes magneticas dos lineamentos que regem as drenagens diamantiferas; sugerir possiveis rochas do clan kimberlitico; e estimar a profundidade do embasamento atraves da Deconvolucao de Euler. O embasamento cristalino e concavo e a espessura das camadas de rochas sedimentares atinge cerca 3.000 m. As anomalias magneticas, selecionadas como possiveis intrusoes kimberliticas, foram detalhadas em levantamentos magneticos terrestres e a Deconvolucao de Euler foi aplicada em dados de Sinal Analitico destes levantamentos. As possiveis intrusoes kimberliticas sao semelhantes entre si, apresentando nao mais que 200 m de profundidade e 300 m de diâmetro. Palavras-chave : prospeccao mineral, Deconvolucao de Euler, Provinciaignea do Alto Paranaiba.

Estimativa da espessura elástica efetiva da litosfera do sul do cráton são francisco usando dados da missão Grace

This paper presents the estimates of effective elastic thickness (T e) from free-air anomalies/geoid undulations and topography data in the southern Sao Francisco Craton, by using admittance function technique. The gravity field elements have been calculated from geopotential model GGM02C (GRACE mission). The observed admittance was computed by dividing the cross-spectra of the free-air anomalies/geoid undulations by the power of topography. Theoretical admittances were estimated based on two hypothesis of flexural compensation mechanism: surface topographic loads or loads beneath of lithosphere. The best fit was obtained by using the second hypothesis, were anegative density contrast (located at depth of 150 km) suggested 45 km for T e value. This low valiue of elastic thickness, combined with high heat flow and positive geoid undulations, can be explained by the presence of density perturbations in the mantle, which are associated with the chemical depletion and thermal anomalies, both are correlated with the complex geodynamic evolution of this important cratonic area in the South American plate.