Maurício Antônio Carneiro

Pb, Sr and Nd isotope constraints on the Archaean evolution of gneissic-granitoid complexes in the southern São Francisco Craton, Brazil

Abstract Three Archaean gneissic complexes (Bonfim, Belo Horizonte and Campo Belo) in the southern part of the Sao Francisco craton were studied by means of UPb (zircon and titanite, monazite) and SmNd, RbSr and PbPb (whole rock) methods. In the Bonfim metamorphic complex the period 2780-2700 Ma is marked by in situ anatexis of gneisses, intrusion of tonalites, mafic dikes and late granites, and deposition of the Rio das Velhas greenstone belt. SmNd T DM model ages on its gneisses, amphibolites and granites range from 2800 to 3000 Ma, indicating involvement of older sialic crust in the 2780-2700 Ma event. In the Belo Horizonte metamorphic complex UPb zircon geochronology has yielded ages of 2860, 2776 and 2712 Ma and RbSr whole rock isochrons on its gneisses and migmatites yielded ages in the range 2800-2750 Ma (with large uncertainties) and 87 Sr/ 86 Sr initial ratios of 0.700-0.710. The highest initial ratio (0.710) obtained for the migmatites suggests that ensialic episodes participated in the tectonomagmatic evolution. Existence of older crust is suggested by UPb zircon ages of ∼ 3030, 2920 and 2880 Ma from inherited grains in metavolcanics and orthogneisses. RbSr ages of 2250-2130 Ma on granitoids and UPb (titanite, monazite) ages from 2320 to 2030 Ma on granitoids, felsic veins and amphibolite enclaves indicate reworking of this complex in the Palaeoproterozoic. In the Campo Belo metamorphic complex granite and greenstone lithologies have yielded ages of ∼ 3380-3000, 2900 and 2650 Ma by UPb zircon and RbSr and PbPb whole rock methods. T DM ages on its granites, gneisses and granulites have yielded ages from 3070 to 2780 Ma indicating involvement of older sialic material during the 2650 Ma event. These complexes indicate major events during the period 2860-2700 Ma, including reworking of established sialic crust with components up to 3380 Ma old. Late granites were injected between 2700-2600 Ma, preceding final assembly and stabilization of the Archaean crust.

U-Pb evidence for late Neoarchean crustal reworking in the Southern São Francisco Craton (Minas Gerais, Brazil)

The Passa Tempo Metamorphic Complex is one of several metamorphic complexes that form the Archean sialic crust of the southern Sao Francisco Craton. It encompasses hypersthene-bearing gneissic rocks, with subordinate NW- or EW-trending mafic-ultramafic bodies and granodioritic to alkali-granitic, weakly foliated, and light-colored granitoids. These granitoids are the product of generalized migmatization that followed granulite-facies metamorphism. To determine the ages of the granulite-facies metamorphism and granitoid genesis, we obtained U-Pb ages on zircon extracted from the mesosome and leucosome of the migmatitic gneisses. For the mesosome, a discordia that intercepts Concordia at 2622 ± 18 Ma is interpreted as a minimum age for granulite-facies metamorphism. For the leucosome, the upper intercept of discordia at 2599 ± 45 Ma corresponds to migmatization and granitoid genesis. Contemporaneous metamorphism and magmatism have been documented elsewhere in the Sao Francisco Craton, especially in the southern portion, demonstrating vast and vigorous reworking of sialic crust by the end of the Neoarchean.

Campo Belo Metamorphic Complex: tectonic evolution of an Archean sialic crust of the southern São Francisco Craton in Minas Gerais (Brazil)

Systematic geological studies performed in the study area allowed the characterization of six lithodemic units: three gneissic, one amphibolitic, one supracrustal and one fissure mafic. The mineral assemblage and the structural record of these lithodemic units indicate that the study area was affected by five tectonothermal events. The structural pattern of the first and oldest event occurred under granulite facies conditions and reveals essentially a sinistral kinematic pattern. The second event, showing dominant extensional characteristics, is related to the generation of an ensialic basin filled by the volcano-sedimentary sequence of the supracrustal lithodemic unit. The third event, which is the most expressive in the study region, is characterized by a vigorous regional migmatization process and by the generation of the Claudio Shear Zone, presenting dextral kinematic movement. The fourth event is represented by a fissure mafic magmatism (probably two different mafic dike swarms) and finally, the fifth event is a regional metamorphic re-equilibration that reached the greenschist facies, closing the main processes of the tectonic evolution of the Campo Belo Metamorphic Complex.

Proterozoic crustal evolution at the southern margin of the amazonian craton in the state of mato grosso, Brazil: evidence from RbSr and K:Ar data

Abstract Geologic mapping on a 1:100,000 scale in the Sao Jose dos Quatro Marcos (SJQM) area, southwest of the state of Mato Grosso, central-western Brazil, showed a remarkable variety of Precambrian lithologic units. Predominant are grey tonalitic gneisses with a RbSr isochron age of 1971±770 Ma, an initial 87 Sr 86 Sr ratio of 0.7071, and a KAr biotite cooling age of 1500 Ma. The gneisses gneisses show interlayered “first-generation” amphibolite lenses and bands (KAr amphibole age of 1500 Ma), and some rare relicts of grey gneissic granulites, together with associated pink augen gneisses. Rare outcrops of deformed banded calc-silicate rocks are recrystallized by contact metamorphism. These units are intruded by a granitoid suite constituted by early, usually somewhat deformed tonalites and granodiorites, and late massive pink granodiorites and monzo- and syegogranites (RbSr isochron age=1472 ± 19 Ma, i.r.=0.7037). They show enclaves of grey gneisses and “second-generation” amphibolites. A large amphibolite body to the west is also considered part of this second-generation basic sequence. Scattered rhyolite outcrops to the east represent the southernmost remnants of the volcanic Rio Branco Group. The grey gneisses, here formally identified as the ”Sao Jose dos Quatro Marcos Gneiss“ and probably derived from Lower Proterozoic tonalites, were complexly deformed and recrystallized, at ∼ 2.0 Ga, into predominantly amphibolite-facies rocks. The first- and second-generation amphibolites document two distinct commingling episodes, separated by at least 0.5 Ga, each directly associated with the corresponding host-rock granitoid magmatism. The porphyritic granites were converted into pink augen gneisses before 1500 Ma and are thus not contemporaneous to the 1380 Ma syn-kinematic San Ignacio granitoids from eastern Bolivia. Calc-silicate rocks are interpreted as remnants of supracrustal sequences, perhaps related to the Quatro Meninas Group. The 1500 Ma granitoids, formally named the “Jaboti Intrusive Suite”, invaded under a syn- to late-kinematic regime, determining a unique thermal-magmatic event in this region.

Comportamento dos sistemas Sm-Nd e Rb-Sr da seqüência acamadada máfico-ultramáfica Ribeirão dos Motas (arqueano), cráton São Francisco Meridional: evidências de enriquecimento mantélico e fracionamento isotópico

The Ribeirao dos Motas layered sequence (SARM) crops out in the southern part of the Sao Francisco Craton, Brazil. This sequence comprises phaneritic metaultramafic and metamafic rocks, which, although slightly deformed and metamorphosed, retain primary igneous layers. Porphyritic rocks with idiomorphic pyroxene crystals and heteradcumulate and adcumulate textures are also present. Eighteen isotopic analyses were performed in the SARM, comprising rocks with primary (relict) textures, as well as rocks in amphibolite facies and retro-metamorphosed to green-schist facies. Seven samples yield a Sm/Nd isochron age of 2.79 ± 0.30 Ga (MSWD = 1.2 e µNdt = + 0.48), constraining the accretion time of the SARM rocks. The positive µNdt value coupled with the Rb/Sr evidence is consistent with mantle source relatively enriched in Nd and Sr isotopes. Nevertheless, some SARM samples display isotopic fractionation and disturbance, which can be ascribed to the following processes or their combinations: a) mobilization of the incompatible elements due to regional high grade metamorphism; b) isotopic changes during upper amphibolite facies overprint; c) isotopic resetting by low-grade fluids associated to the Claudio Shear zone, which is located nearby the SARM.

Evolution of arc magmatism in the Carmópolis de Minas Layered Suite, Minas Gerais, Brazil: Sm-Nd and Rb-Sr isotope geochemistry

The Carmopolis de Minas Layered Suite (CMLS) is a Neoarchean Unit metamorphosed at amphibolite- to granulite-facies conditions, comprised of metaultramafic rocks, amphibolites and metarhyolites. The CMLS is marked by two distinct phases of tholeiitic and calc-alkaline magmatism. Previous geochemical data and regional constraints suggest that the CMLS is possibly a metamorphosed arc-tholeiitic suite of the type boninite-basalt-andesite-rhyolite, interpreted in this study as a dismembered remnant of the juvenile oceanic arc. The rocks of tholeiitic affinity record a signature of depleted to enriched mantle with positive to weakly negative eNd(t) and predominantly positive eSr(t) values, indicating some degree of crustal assimilation. The rocks of calc-alkaline affinity present a signature of juvenile crust with eNd(t) close to the CHUR and weakly positive eSr(t) values. The Sm-Nd isochron age of 2736 ± 300 Ma obtained with eNd(i) = +0.4, suggests the edification of an intra-oceanic arc during the late phases of the Rio das Velhas Tectonothermal Event. The arc evolution involved tholeiitic magmatism in the early stages in association with calc-alkaline magmatism, of probably anatectic character, in the tardy-orogenic stages.

Diagnóstico dos processos de licenciamento e fiscalização das pedreiras de rochas ornamentais na regiãocentro-sul de Minas Gerais

Apresenta-se o diagnostico da analise de 130 processos de licenciamento e fiscalizacao ambiental oriundos de 81 pedreiras de rochas ornamentais, localizadas em cinco municipios da regiao centro-sul de Minas Gerais (Campo Belo, Candeias, Oliveira, Sao Francisco de Paula, Claudio). Tais processos estao arquivados na Fundacao Estadual do Meio Ambiente de Minas Gerais - FEAM. Da analise desses processos, juntamente com as visitas ao campo, constatou-se o desrespeito dos empreendedores aos autos de infracao e as medidas mitigadoras propostas e exigidas pela FEAM. Consequentemente, muitos processos sao arquivados ou ficam pendentes. O resultado mais visivel desse imbroglio e que, na regiao em analise, nenhuma area de extracao de rochas ornamentais foi submetida a processo de recuperacao ambiental, evidenciando a externalizacao de impactos e correspondentes custos sociais para desconforto da populacao da regiao. Os efeitos recentes da atividade de extracao de rochas ornamentais tornam-se ainda mais severos na medida em que, sobretudo nos municipios de Candeias e Sao Francisco de Paula, diversas areas foram paralisadas por razoes de mercado, com consequente reducao da oferta de trabalho para a populacao local.

Hydrogeochemical characteristics of pit lakes formed in abandoned ornamental rocks quarries of Campo Belo Metamorphic Complex, Minas Gerais, Brazil

Purpose – The purpose of this research is to determine the major environmental impacts and, especially, to evaluate the geochemical characteristics of water of the lakes formed in abandoned quarries of ornamental rock of the Campo Belo Metamorphic Complex.Design/methodology/approach – To do so, 12 quarries were chosen to be studied. They were mapped in a detail scale and their rocks were described and sampled for petrography analysis. Whenever present, the lakes were evaluated too. In situ measurements included pH, Eh, conductivity, resistivity, total dissolved solids and temperature. Water samples were collected to determine alkalinity, sulfates, chlorates, turbidity, suspended sediments and the concentrations of major and trace elements using the ICP‐OES.Findings – Abandoned open‐pit mining operations have resulted in the creation of numerous pit lakes. About 90 per cent of the quarries visited and studied were abandoned or interdicted by environmental organs, what will probably lead to its subsequent a...

A suíte metatholeiítica da seqüência metavulcanossedimentar Rio Manso, Quadrilátero Ferrífero (MG)

A sequencia metavulcanossedimentar Rio Manso compreende rochas metaultramaficas com intercalacoes de metamafitos e rochas metassedimentares psamo-peliticas. Localiza-se no dominio geologico do Quadrilatero Ferrifero em Minas Gerais, porcao meridional do Craton Sao Francisco. O mapeamento litoestrutural (escala 1:10.000) assim como os estudos petrograficos, mineralogicos e litogeoquimicos permitiram identificar, no âmbito dessa sequencia, uma suite metatholeiitica. Em termos geoquimicos, se comparada com outras ocorrencias mundiais, a sequencia metavulcanossedimentar Rio Manso se assemelha a sequencia mesoarqueana de Barbeton. Essa semelhanca sugere que a sequencia estudada corresponde a um terreno granito-greenstone que poderia representar a continuidade nordeste do Complexo Metamorfico Campo Belo de idade mesoarqueana.

Implicações geológicas e tectônicas da interpretação magnetométricada região de Oliveira, Minas Gerais

This work presents the interpretation of magnetometric data obtained for the Oliveira region (1:100,000 scale), located into the southern portion of the Sao Francisco Craton (Minas Gerais, Brazil). The geology of this region encompasses Archean igneous and metamorphic rocks of the Campo Belo Metamorphic Complex, (Archean and Proterozoic) supracrustal sequences and a Proterozoic mafic dike swarm. Due to advanced weathering, the region lacks rock outcrops and in this particular case magnetometry plays an important role. Thus, the interpretation of geophysical data allowed the characterization of five major structural lineament families, named L, M, N, O and P, at the regional sialic substrate. Field Geologic mapping confirmed these lineaments which are predominantly related to two (gabbronoritic and gabbroic) mafic dike families, metaultramafites, iron formations and faults.