Constraints on the Tectonic Setting of the Andaman Ophiolites, Bay of Bengal, India, from SHRIMP U-Pb Zircon Geochronology of Plagiogranite

The Andaman ophiolites are well exposed in the Andaman group of islands, which is part of the Sunda-Burmesedouble-chain arc system in the Bay of Bengal, India. Plagiogranites occurring on the eastern margin of the southernpart of South Andaman Island appear as interstitial vermicular and micrographic intergrowths of quartz and plagioclase.They are tonalitic to trondhjemitic in composition, and their Rb, Yb, Ta, and Y abundances are characteristicof a volcanic-arc affinity. Sensitive high-resolution ion microprobe U-Pb dating of zircons from a plagiogranite withinthe Andaman ophiolite has yielded a weighted mean 206Pb/238U age of 93.61.3 Ma, interpreted as the age of itscrystallization. The subduction-related plagiogranite has intruded a gabbro unit of the Andaman ophiolites as wellas extrusives of the East Coast Volcanics at this time. Since the Andaman ophiolitic rocks predate the plagiogranite,they cannot have been generated in the currently active Late Miocene Andaman-Java subduction zone and were mostlikely obducted onto the leading edge of the Eurasian continent at an earlier phase of subduction activity during earlyCretaceous time

Fine Euhedral Banding in Zircon - Metamorphic Enhacement of Igneous Structure

Fine euhedral banding of zircon is usually considered to be associated with magmatic crystallization processes. This is commonly true, but it can also be generated by solid-state replacement along bands in previously more homogeneous crystals. Zircons from the Neoproterozoic Cambaí Complex in southernmost Brazil show abundant evidence to support solid-state replacement. Investigations of more than 135 zircon crystals by Electron Probe Microanalysis and Electron Backscattering techniques show that the solid-state replacement process begins along fractures and expands to broad irregular patches, even forming younger nucleii in some crystals. The replacement continues along crystallographically-controlled bands, enhancing and resembling fine oscillatory igneous zoning. Microprobe analyses indicate that modified zones are enriched in Y, U and T in most BSE-bright bands, but in Hf in some others. The lighter (in BSE) parts in the core are texturally younger than the more homogeneous rims. The mechanisms involved include infiltration along fractures and defect sites and fluid-aided volume diffusion in less defective portions

Metamorphic evolution and U-Pb zircon SHRIMP geochronology of the Belizario ultramafic amphibolite, Encantadas Complex, southernmost Brazil.

The integrated investigation of metamorphism and zircon U-Pb SHRIMP geochronology of the Belizario ultramafic amphibolite from southernmost Brazil leads to a better understanding of the processes involved in the generation of the Encantadas Complex. Magmatic evidence of the magnesian basalt or pyroxenite protolith is only preserved in cores of zircon crystals, which are dated at 2257 ± 12 Ma. Amphibolite facies metamorphism M1 formed voluminous hornblende in the investigated rock possibly at 1989 ± 21 Ma. This ultramafic rock was re-metamorphosed at 702 ± 21 Ma during a greenschist facies event M2; the assemblage actinolite + oligoclase + microcline + epidote + titanite + monazite formed by alteration of hornblende. The metamorphic events are probably related to the Encantadas Orogeny (2257 ± 12 Ma) and Camboriu Orogeny (~ 1989 Ma) of the Trans-Amazonian Cycle, followed by an orogenic event (702 ± 21 Ma) of the Brasiliano Cycle. The intervening cratonic period (2000-700 Ma) corresponds to the existence of the Supercontinent Atlantica, known regionally as the Rio de la Plata Craton

The Proterozoic Guanhães banded iron formations, Southeastern border of the São Francisco Craton, Brazil: evidence of detrital contamination

A sequência supracrustal Guanhães, portadora de formações ferríferas bandadas (BIFs), ocorre como fatias tectônicas superpostas ao embasamento de terrenos granito-gnáissicos do tipo TTG de idade Arqueana, desenvolvidas no limite entre o período Proterozoico e o Paleozoico, durante a colagem Brasiliana. A idade máxima de deposição da sucessão é de ~2,18 Ga e foi determinada por datação de zircões detríticos em quartzitos. Além de quartzitos, a sucessão é composta por xistos, BIFs, gnaisses e anfibolitos, todos metamorfisados em condições de fácies anfibolito. A análise dos Elementos Terras Raras + Y (ETR+Y), normalizados ao PASS, para as BIFs de Guanhães, mostra enriquecimento em ETR pesados e anomalia positiva de Eu. Dois tipos de contaminação foram observados nas amostras. O primeiro é uma contaminação detrítica que resultou em baixos valores de Y/Ho (< 30) e Pr/Yb (SN). As evidências de contaminação desse tipo, combinadas à análise do empilhamento estratigráfico do pacote de rochas supracrustais, indicam que as BIFs de Guanhães foram depositadas em ambiente marinho raso. Um evento Cambriano expressivo está presente nos dados relativos à datação de zircões. O segundo tipo de contaminação resultou em maiores valores de anomalia de Eu, anomalia positiva de Ce e maiores concentrações de ETR, possivelmente devido à interação entre fluidos magmáticos posteriores e as BIFs de Guanhães. As amostras sem contaminação mostram distribuição de ETR semelhante a outras BIFs Précambrianas, particularmente às BIFs da Sequência do Morro Escuro e do Grupo Serra da Serpentina, com ausência de anomalia verdadeira de Ce e Y/Ho próximo aos valores da água do mar (45). Os dados geocronológicos e geoquímicos apresentados neste artigo sugerem correlação entre a sucessão supracrustal de Guanhães (GSSu) e os Grupos Serra da Serpentina e Serra de São José.The Guanhães banded iron formation (BIF) bearing succession occurs as tectonic slices, juxtaposed to Archean TTG granite-gneissic basement rock, developed during the Neoproterozoic-Cambrian Brasiliano collage. The succession has a maximum depositional age of ~2.18 Ga, from detrital zircons in quartzite, and consists of quartzites, schists, BIFs, gneiss and amphibolite, all metamorphosed under amphibolite facies conditions. The Guanhães BIF shows HREE enrichment and consistent positive Eu anomaly (PAAS-normalized REE+Y). Two types of contamination were observed in the samples. The first is contamination by an exotic detrital component, which resulted in low Y/Ho (< 30) and Pr/Yb (SN) ratios. Evidence of such contamination, combined with inferred stratigraphic stacking data, indicates that the Guanhães BIFs were deposited on a shallow marine environment. The second type of contamination resulted in higher Eu-anomalies, positive Ce-anomalies, and higher REE+Y concentrations, possibly due to the interaction between later magmatic fluids and the Guanhães BIF. A strong Cambrian event is recorded in zircon age data. The uncontaminated samples display REE+Y distribution similar to other Precambrian BIFs, particularly those from the Morro-Escuro Sequence and the Serra da Serpentina Group, without true Ce-anomalies and Y/Ho close to seawater values (45). Geochronological and geochemical data presented in this paper strongly suggest a correlation between the Guanhães supracrustal succession and the Serra da Serpentina and Serra de São José Groups.

The structure of western Serra do Curral, Quadrilátero Ferrífero, and tectonic control of the high-grade iron hard bodies

The structure of western Serra do Curral, Quadrilátero Ferrífero, represents the partial exposure of a syncline that verges to NNW - Piedade Syncline. This regional structure has parasitic folds denominated as B1A which are refolded by folds B1B that also have vergence to NNW. Both of them have ENE-WSW direction, are non-cylindrical folds and configure an interference pattern that is similar to Type 3 from Ramsay. This refolding occur during progressive deformation. The occurrence of highgrade magnetitic-martitic accumulation (> 64% Fe) of medium size (~ 100 Mt) is controlled by the combination of these folds and high angle thrust fault, both with the same direction. Above these structures, there is a development of folds with NNWSSE direction and WSW vergence (B2 folds). These folds are restricted to the B1 limbs and they configure an interference pattern named "abbuting fold". The age obtained by U/Pb SHRIMP on monazite grains is 2034 ± 11 Ma. The monazite grains occur on textural balance with the iron oxide from high-grade accumulations. This age corresponds to the formation timing of the high-grade bodies and suggests that the mineralization process is contemporary to the regional metamorphic peak of Quadrilátero Ferrífero

O peridotito cromitífero de Santa Luz e diques máficos associados, Bahia-Brasil: remanescentes de um ofiolito transicional relacionado ao greenstone belt paleoproterozóico (>2.1Ga) do rio Itapicuru?

The Santa Luz chromite rich peridotite complex crops out in the Serrinha Block, NE Sao Francisco Craton, aproximately between grey gneisses of the Archean basament and supracrustal rocks of Palaeoproterozoic rocks of the Rio Itapicuru greenstone belt. It comprise serpentinites, serpentinized harzburgites, massive chromitites, gabbroic lenses and amphibolite dykes/veins with epidote rich margins this rocks association is interpreted as melt percolation of mantle peridotites followed by ocean floor metasomatism. Massive chromitites show major elements chemistry and platinum group elements signature similar than ophiolite chromitites.Aplites and pegmatitic dikes intrude the peridotite complex. The complex is older than 2085 Ma and younger than 2983 Ma based on U-Pb ages obtained respectively on a pegmatite dyke and a host banded gneiss. The Santa Luz peridotite complex and associated mafic dykes may have its origin related to rifting of a magma-poor continental margin that later evolved into an ocean and eventually formed the Palaeoproterozoic Rio Itapicuru Greenstone belt.O complexo peridotítico de Santa Luz ocorre no Bloco Serrinha (NE do Cráton do São Francisco), aproximadamente entre gnaisses do embasamento arqueano e rochas supracrustais do greenstone belt do Rio Itapicuru. Ele contém serpentinitos, harzburgitos serpentinizados, cromititos maciços, faixas gabróicas e diques/veios de anfibolitos. Todo o conjunto é cortado por pegmatitos e aplitos. A associação máfica e ultramáfica é interpretada como percolação de magma basáltico em peridotitos do manto. Os cromititos maciços apresentam química mineral e padrões de elementos do grupo da platina semelhantes aos de cromititos de ofiolitos. O complexo é mais antigo que 2085 ± 12 Ma e mais novo que 2983 ± 8 Ma, idades obtidas respectivamente em dique aplítico e gnaisse bandado encaixante. O complexo peridotítico de Santa Luz e os diques máficos associados podem ser relacionados à fase rifte de uma margem continental pobre em magma, a qual evoluiu para a abertura de um oceano e posteriormente se fechou para finalmente formar o greenstone belt do Rio Itapicuru durante colisão continental no Paleoproterozóico.Fil: Olivera, Elson Paiva. Instituto de Geociências, UNICAMP; BrasilFil: Escayola, Monica Patricia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Souza, Zorano Sergio. Depart. de Geologia e Pós-Graduação em Geodinâmica e Geofísica, UFRN; BrasilFil: Finoto Bueno, Juliana. Instituto de Geociências, UNICAMP; BrasilFil: de Souza Araújo, Marcelus Glaucus. Instituto de Geociências, UNICAMP; BrasilFil: Mcnaughton, Neal. Department of Geology and Geophysics, UWA; Australi

Nucleon-nucleon elastic scattering analysis to 2.5 GeV

A partial-wave analysis of NN elastic scattering data has been completed. This analysis covers an expanded energy range, from threshold to a laboratory kinetic energy of 2.5 GeV, in order to include recent elastic pp scattering data from the EDDA collaboration. The results of both single-energy and energy-dependent analyses are described.Comment: 23 pages of text. Postscript files for the figures are available from ftp://clsaid.phys.vt.edu/pub/said/n