35 research outputs found
Estudo Geológico-Estrutural do Complexo de Granja (NW Ceará)
O Complexo de Granja (CG), situado no extremo NW do Estado do Ceará, é constituído
por uma sucessão de gnaisses orto- e paraderivados correspondendo a um cinturão metamórfico de médio a alto grau e é composto pelas seguintes unidades principais: (1) ortognaisses de afinidade
TTG (Tonalito-Trondhjemito-Granodiorito), mostrando evidências de migmatização; (2) paragnaisses com granada e silimanita, intensamente migmatizados e; (3) granulitos máficos e félsicos aos quais
frequentemente se associam corpos lenticulares de anfibolitos e de quartzitos. Estas litologias estão parcialmente cobertas por formações detrítico-lateríticas e sedimentares clásticas do Terciário e
depósitos elúvio-coluvionares e aluvionares do Quaternário. De acordo com os dados
geocronológicos atualmente disponíveis, estas unidades representam fragmentos do embasamento Paleoproterozóico (2.3-2.1 Ga), profundamente afetados pela orogenia Brasiliana durante o Neoproterozóico (600 Ma). A partir da recente informação adquirida nos levantamentos de campo
e dos estudos petrográficos realizados, foi possível elaborar um novo mapa geológico para uma área adjacente ao município de Granja no âmbito do Programa Geologia do Brasil (Folha Cartográfica Granja SA.24-Y-C-III; 1:100.000), extraindo as seguintes conclusões principais: (1) os
litótipos mostram evidências de terem sido afetados por metamorfismo regional de alto grau, que terá atingido as condições de fusão parcial, com consequente produção de uma grande variedade de estruturas migmatíticas (e.g. estromáticas, dictioníticas, “schlieren”, “augen”); (2) o bandado migmatítico está intensamente dobrado, sugerindo que os processos de migmatização se iniciaram antes do último evento de deformação dúctil (D3 regional, D2 Brasiliana) que afetou a região; (3) em
todas as unidades do CG, a anisotropia planar mais penetrativa é a foliação S3, com direção N 50º E e inclinação variável (35º a 80º para SE), encontrando-se diretamente relacionada com a atuação de um importante acidente dextro, de escala crustal e orientação NE-SW, que atravessa a porção
sul e sudeste da área cartografada – a Zona de Cisalhamento de Granja (ZCG) e; (4) nas zonas em que a influência da ZCG foi mais acentuada, a foliação S3 adquire um carácter milonítico, obliterando frequentemente as anisotropias anteriores. As análises petrográficas confirmam a classificação adotada em trabalhos anteriores para cada um dos tipos litológicos presentes no CG.
Com efeito, os dados obtidos neste trabalho para os ortognaisses TTG e para os paragnaisses mostram que o metamorfismo regional prógrado (tipo Barroviano) que os afetou se situam na transição entre o topo da fácies anfibolítica e a fácies granulítica, tendo atingido as condições de
fusão parcial, enquanto os granulitos félsicos e máficos registam evidências de terem sido sujeitos a pressões mais elevadas
Características petrográficas y geoquímicas de los paragneises de Granja (Dominio Medio Coreaú, NW Ceará, Brasil)
The Granja Granulite Complex (GGC) exposed in the Médio Coreaú Domain (NW Ceará, Brasil) consists mainly of garnet and sillimanite migmatitic paragneisses enclosing discontinuous lenses of mafic granulites and enderbites. According to the published geochronological data, this high-grade metamorphic belt represents a segment of the Paleoproterozoic basement intensely reworked during the Brasiliano/Pan-African Orogeny in the Neoproterozoic (600 Ma). The Granja paragneisses are strongly foliated rocks characterized by the alternance of dark garnet-biotite-sillimanite-rich layers and millimeter-thick leucocratic quartz-feldspathic bands, interpreted as indicative of incipient melting. As melt contents increase, layer-parallel leucosomes become thicker and a well-developed stromatic layering is defined. Both the gneissic and stromatic fabrics are strongly overprinted by a penetrative mylonitic foliation correlated to the last reactivation of the dextral NE-SW trending Granja Shear Zone (GCZ) that cuts across the studied area. Mineral assemblages and microstructures indicate that these rocks were affected by granulite-facies metamorphism and anatexis followed by decompression and cooling. In order to constrain the protolith composition of the Granja paragneisses, twelve whole-rock samples from the parts of the migmatitic paragneisses that appear to have undergone little or no melt extraction were analysed for major and trace elements. In the classification diagram of Herron (1988), the samples plot in the transition between the greywacke and the pelite fields, suggesting that the pre-metamorphic sequence was dominantly composed by shales and immature clastic sediments (greywackes). Their chondrite normalized REE patterns show a moderate LREE enrichment (LaN/YbN = 9.46-15.50), flat HREE profiles and negative Eu anomalies (Eu/Eu∗= 0.63-0.82), closely resembling those of PAAS (Post-Archean average Australian Shale) and Early Proterozoic Greywackes. Geochemical data also suggest that the precursor sediments of the Granja paragneisses derived from source areas of felsic to intermediate composition and were deposited in a tectonically active continental margin/continental island arc setting.El Complejo Granulítico de Granja (GGC), expuesto en el Dominio Medio Coreaú (NW del Estado de Ceará,
Brasil), está constituido predominantemente por paragneises granatíferos con silimanita en cuyo seno se intercalan
cuerpos lenticulares, discontinuos, de granulitas máficas y enderbitas. De acuerdo con los datos geocronológicos
publicados, este terreno metamórfico de alto grado representa un segmento de un basamento Paleoproterozoico
que fue intensamente retrabajado en el Neoproterozoico, durante la Orogénesis Brasiliense / Panafricana (600 Ma).
Los paragneises de Granja son rocas con una fuerte foliación, caracterizadas por la alternancia de niveles
oscuros, ricos en granate-biotita-silimanita, y bandas milimétricas leucocráticas cuarzo-feldespáticas interpretadas
como resultado de fusión incipiente. Con el incremento del componente fundido, aumenta el espesor de los
leucosomas concordantes y las rocas adquieren un aspecto estromático. A estas estructuras se superpone una
foliación milonítica asociada a la última reactivación de la Zona de Cizalla de Granja, un accidente con dirección
NE-SW y movimiento dextral que atraviesa la zona estudiada. Las asociaciones minerales y las texturas de reacción
indican que después de alcanzar las condiciones de fusión parcial en la facies granulítica, las rocas sufrieron
descompresión y enfriamiento.
Con el fin de caracterizar la composición de los protolitos de los paragneises de Granja, se analizaron los
elementos mayores y tierras raras de doce muestras de aquellas partes de los paragneises migmatíticos en los
que la extracción de fundido parece haber sido nula o muy limitada. En el diagrama de clasificación de Herron
(1988), las muestras se sitúan en la transición entre los campos de las grauvacas y pelitas, sugiriendo que en
la secuencia pre-metamórfica dominaban sedimentos arcillosos y sedimentos clásticos poco maduros (grauvacas).
Los patrones de tierras raras normalizados a condrita muestran un moderado enriquecimiento en las tierras
raras ligeras (LaN/YbN=9.46–15.50), perfiles planos en tierras raras pesadas y anomalías negativas de Eu
(Eu/Eu*=0.63–0.82), con pautas muy próximas tanto a las del PAAS (Post-Archean average Australian Shale)
como a las del EP GREY (Early Proterozoic Greywackes). Los datos geoquímicos sugieren también que los sedimentos
precursores de los paragneises de Granja tuvieron su origen en un área fuente con composición félsica a
intermedia y fueron acumulados en un ambiente de margen continental activo / arco insular continental
Petrographical and geochemical signatures of the Granja paragneisses (NW Ceará, Brasil)
The Granja Granulitic Complex
(GCC) constitutes a NE–SW
trending high-grade
metamorphic belt occupying the
western sector of the Médio
Coreaú Domain (NW Ceará,
Brasil) (Figs. 1 and 2).
According to the published data,
the GCC represents a segment
of the Paleoproterozoic
basement intensely reworked
during the Brasiliano Orogeny.
Lithologically, the GCC is mainly
composed of garnet-sillimanite
migmatitic paragneisses,
enclosing discontinuous lenses
of mafic granulites and
enderbites.
At outcrop scale, the GCC
gneisses are well-foliated fineto
medium-grained grey rocks
showing a stromatic layering
parallel to the main regional
fabric (Fig. 3). The occurrence of
lenses of leucocratic quartzfeldspathic
material interlayered
with the host paragneisses
indicates that these rocks
reached partial melting
conditions during the Brasiliano
orogenic event (≈ 600 Ma).
Petrographically, the gneisses
are characterized by the
presence of mineral
assemblages dominated by
garnet, sillimanite and biotite
with moderate amounts of
quartz, plagioclase, cordierite
and K-feldspar (Figs. 4a-b).
Twelve whole-rock samples
were analyzed for major and
trace elements. As shown in
Graph 1, all the analyzed
samples plot in the sedimentary
field. Their REE patterns closely
resemble those of PAAS (PostArchean
average Australian
Shale) confirming the
sedimentary origin of the
paragneiss protoliths (Graph 2).
Geochemical data also suggest
that these sediments derived
from source areas of felsic to
intermediate composition and
were deposited in a tectonically
active continental margin
(Graphs 3 and 4)
Sr-Nd isotope signatures of surficial sediments from the Portuguese continental shelf
This study focus on the Sr-Nd isotope signatures of detrital and carbonate fractions of seven samples from surficial sediments deposited in the Portuguese continental shelf. 87Sr/86Sr and 143Nd/144Nd isotope compositions of the lithic component are used to constrain potential endmember contributions and determine sediment provenance. The 87Sr/86Sr ratios measured in the acid leachates of the seven sediment samples (carbonate fraction) lie within the range 0.7088 – 0.7092, close to the value for modern seawater (0.7091 – 0.7092), suggesting a dominant biogenic origin for this component. In contrast, the Sr isotope signatures for acid-leached silicate residues (lithic fraction) from the same samples are widely variable (87Sr/86Sr = 0.7179 to 0.7346) reflecting the input of terrigenous particles from three distinct sources (northern Iberian Variscan crust, southern Iberian Variscan crust and North African aerosols). The narrow range of ƐNd(0) values displayed by these samples (-8.5 to -10.6) does not allow effective discrimination between the different endmembers
Transformação de coordenadas cartográficas: o exemplo da região de Granja (NW do Ceará, Brasil)
Este estudo tem como principal objetivo mostrar a importância de se efetuar a análise metódica da cartografia de
base que suporta a produção de Cartografia Geológica, antes de iniciar o trabalho de campo. Em certas regiões, as cartas
base publicadas estão ainda associadas a redes geodésicas clássicas, o que obriga a proceder à transformação de coordenadas
para assegurar que todos os dados geo-espaciais são definidos no mesmo sistema de referência espacial. Neste
estudo, selecionaram-se 8 pontos localizados sobre as cartas publicadas na região de Granja (NW do Ceará, Brasil),
determinaram-se as coordenadas no Datum topocêntrico das cartas (CA 70-72) e procedeu-se à sua transformação para
os Data geocêntricos WGS84 e SIRGAS2000. As diferenças entre as coordenadas obtidas nos três sistemas são residuais,
indicando que erros introduzidos quando se desconsideram parâmetros essenciais como o Datum Geodésico são, neste
caso, mínimos para efeitos de Cartografia Geológica.This study draws attention to the importance of conducting a detailed analysis of cartographic products that sustain the Geological Mapping before
starting the field work. In some regions, the published base maps are still related to classical geodetic networks. Therefore, it is fundamental to perform the
adequate coordinate transformation to ensure that all spatial data are defined relatively to the same spatial reference system. This is particularly relevant
when GPS and GIS are used to obtain and plot spatial data. The present study involved the selection of 8 points located on the published maps for the
Granja region (NW Ceará, Brazil), the determination of their coordinates in the CA 70-72 topocentric map Datum and their subsequent transformation
to the geocentric Data WGS84 and SIRGAS2000. The differences between coordinates are residual, indicating that the error introduced when
essential parameters such as the Geodetic Datum are disregarded are, in this case, minimum
Distribution of soft bottom sediments in the Portuguese Continental Shelf
This work presents a simplified map
of the spatial distribution of surficial
sediments and rock outcrops along
the Portuguese continental shelf in
ArcGIS format.
Nearly 500 samples of soft bottom
shelf sediments were collected in
the scope of several research
projects and analyzed for grain-size.
Based on the raw grain-size data,
the samples were classified
according to a modified version of
the Folk classification system and
used to create a new sediment
distribution map and develop an
easy-to-use GIS interface
A Bottom Sediment Distribution Map for the Portuguese Continental Shelf in ArcGis Format: Mapa da Distribuição de Sedimentos Superficiais da Plataforma Continental Portuguesa em Formato ArcGis
This work presents a simplified map of the spatial distribution of surficial sediments and rock outcrops along the Portuguese continental shelf in ArcGIS format. Preliminary map preparation involved scanning, georeferencing and digitizing of pre-existing base maps from the Portuguese Hydrographic Institute and harmonization with other published and non-published cartographic information. Samples of soft bottom shelf sediments from nearly 500 sites, collected in the scope of the MeshAtlantic, RENSUB and ACOSHELF projects, were analyzed for grain-size and total organic matter. Grain-size analysis was performed using dry and wet sieving. The procedure includes chemical destruction of organic matter with H2O2, chemical dispersion with tetra-sodium pyrophosphate, separation of the mud fraction by wet sieving and dry sieving of both sand and gravel fractions. Raw grain-size data were expressed as weight percentages and classified according to a modified version of the Folk classification system. Grain size data for the whole set of samples were used to create a new sediment distribution map and develop an easy-to-use GIS interface, that will be available in the MeshAtlantic website, as part of a global broadscale habitat map for the Atlantic Area.
Overall, the bottom sediment distribution patterns show that: (a) the coarser deposits occur mainly in the inner and mid-shelf of the northwestern sector of the Portuguese shelf, to the south of the Nazaré and Setúbal canyons, and are sparsely represented in the southern shelf; (b) sand banks dominate in the near shore shelf, but can sporadically be found at greater depths; (c) muddy sands characterize the deeper shelf; (d) patches of sandy mud and mud reflecting a less energetic hydrodynamic regime cover a large sector of the southern shelf and areas located off the mouths of major rivers in the western shelf; (e) mixed sediments are residual; (f) rock outcrops are unevenly distributed throughout the entire shelf
A broad scale seabed substrate map of the Portuguese coast
The MeshAtlantic project aimed to generate a broad-scale habitat map for the
shelf and coastal area of the Atlantic Area. Till 2012, a detailed seabed
substrate map of the Portuguese coast, which covers a great extension of the
Western Iberian coast, was not known