GCG - Resumos em livros de actas
Permanent URI for this collection
Browse
Browsing GCG - Resumos em livros de actas by Issue Date
Now showing 1 - 10 of 124
Results Per Page
Sort Options
- Petrology and Geochemistry of Recent Lavas from Santo Antão (Cape Verde Islands)Publication . Henriques, S.B.A.; Munhá, José; Silva, L. Celestino da; Mendes, M. H.ABSTRACT: Santo Antão Island is the northernmost island of the Cape Verde Archipelago that is located in the Atlantic Ocean between 17º 13’N-14º 48’N and 22º 42’W-25º 22’W. The genesis of this Archipelago is explained by the activity of a hot-spot, which began 25 My ago. The main stratigraphic sequences of Santo Antão island (Silva et al., 1994; 2004) have been dated by Plesner et al. (2003) and comprise, from base to top, the “Complexo Eruptivo Antigo” (7.57 ± 0.56 My) the “Formação Conglomerático-Brechóide”, the “Formação Eruptiva Principal” (3.24 ± 0.89 to 0.22 ± 0.03 My) (which is overlain by a white phonolitic deposit pumice) and the “Formação Eruptiva do Tope de Coroa” (0.17 ± 0.02 My to 0.09 ± 0.03 My). The “Complexo Eruptivo Antigo” includes chiefly basaltic flows. The “Formação Conglomerático-Brechóide” is a sedimentary unit formed during an intensive erosion period and was followed by an important volcanic event, represented by the “Formação Eruptiva Principal”, comprising nephelinitic, phonolitic, tephritic and basanitic lava flows. The “Formação Eruptiva do Tope de Coroa”, the most recent unit of the island, was divided (Silva et al. 2004) into three sub-units (figure 1): “Sequência Antiga”, “Sequência Intermédia” and “Sequência Superior”. The “Sequência Antiga” is composed by phonolitic and nephelinitic flows and scorias, the “Sequência Intermédia” comprises basaltic flows and scorias and the “Sequência Superior” includes basaltic, phonolitic and nephelinitic flows and scorias. Santo Antão recent lavas are silica undersaturated (carrying abundant feldspathoids, particularly hauyne), alkaline and considerably evolved (Mg#=13-53 wt%, Ni=0-252 ppm, Cr=0-434 ppm). They display a strong enrichment in incompatible trace elements, suggesting that their primary magmas were produced by low degrees of partial melting, from a heterogeneous, metassomatized, source (including residual garnet and phlogopite), with a significant HIMU component.
- Petrological and thermochronological constraintson the geodynamic evolution of the Ribeira Fold BeltPublication . Tassinari, Colombo C. G.; Munhá, José; Neto, Coriolano Dias; Santos, Teresa; Cordani, Umberto G.; Nutman, Allen P.; Fonseca, Paulo E.
- Oxygen fugacity and CO2 - N2 fluid inclusions as remnants of fluid and geodynamic evolution of Ribeira Fold Belt, SE BrazilPublication . Bento dos Santos, Telmo; Munhá, José; Tassinari, Colombo C. G.; Noronha, Fernando; Guedes, Alexandra; Fonseca, Paulo E.; Neto, Coriolano Dias; Dória, Armanda
- Thermochronological evidence for long-term elevated geothermal gradients in Ribeira Belt, SE BrazilPublication . Bento dos Santos, Telmo; Munhá, José; Tassinari, Colombo C. G.; Fonseca, Paulo E.; Neto, Coriolano Dias
- The parautochthonous thrust complex in Trás-os-Montes : essay of tectonostratigraphic correlation between Bragança and V. P. Aguiar sectorsPublication . Pinto de Meireles, Carlos Augusto; Ribeiro, Maria dos Anjos; Couto, Helena
- The relationships between charnockites and associated granitoids of Ribeira Belt : their ages and originsPublication . Tassinari, Colombo C. G.; Munhá, José; Bento dos Santos, Telmo; Neto, Coriolano Dias; Palácios, Teresa; Fonseca, Paulo E.
- New data on the late Famennian miospore assemblage of the Cercal Anticline, westernmost Iberian Pyrite Belt area, PortugalPublication . Pereira, Zélia; Matos, João Xavier; Fernandes, Paulo; Oliveira, José TomásThe Cercal Anticline (CA) is located in the westernmost region of the Iberian Pyrite Belt (IPB). The exposed stratigraphic sequence includes, in ascending order, the Volcano-Sedimentary Complex (VSC) followed by the Xistos das Abertas Fm, which passes gradually to the flysch sequence of the Mira Formation (Carvalho, 1976). The VSC is composed from base to top by: Felsic Volcanics (Vα), that comprehends lavas, pumice and volcaniclastic beccias with intercalations of volcanogenic shales; diabases and intermediate/basic subvolcanics (Vβ); jaspers and the S. Luís Formation (SL), a unit composed of shales, phyllites, siliceous shales and volcanogenic sediments. About 500m NW of São Luís village, at the base of this last unit, brachiopods ascribed to the late Strunian were found (Quiring, 1936). This determination led Carvalho (1976) to assume that the volcanics of the VSC are older than the Strunian, meaning so that this volcanism is the oldest in the Portuguese IPB. The stratigraphic succession of the CA is still poorly constrained, in terms of lithostratigraphy and age. In fact, three boreholes carried out in the hinge zone of the anticline by the Elf Aquitane company, about 1km NW of São Luis village, showed that below the felsic volcanics a thick (>250m) succession of dark shales and siltstones occur. These shales yielded very well preserved specimens of Grandispora echinata together with Ancyrospora spp., Apiculiretusispora sp., Auroraspora macra, Cristicavatispora dispersa, Diducites poljessicus, D. mucronatus, D. versabilis, Emphanisporites annulatus, Grandispora cornuta, Punctatisporites spp., Retispora cf. macroreticulata, Retusotriletes phillipsii, R. planus, R. triangulatus, R. rugulatus, Rugospora explicata and R. radiata. This assemblage indicates the VH Biozone of late Famennian age. It is similar to those found in shales interbedded in felsic volcanics in several regions of the Portuguese Pyrite Belt (Neves Corvo Mine, Albernoa Anticline, São Domingos Mine). Palynostratigraphic research in the CA is currently still in progress. However, these preliminary results show that the age of the CA felsic volcanics is similar to that obtained in the northeastern branches of the Portuguese Pyrite Belt and no prove that the volcanism migrated in time to the NW, as suggested by Carvalho (1976), could be detected.
- Volcanic facies architecture, hydrothermal alteration and subsea-floor replacement at the Neves Corvo deposit, Iberian Pyrite BeltPublication . Rosa, Carlos J. P.; Relvas, Jorge M.R.S.; McPhie, Jocelyn; Pereira, Zélia; Oliveira, José Tomás; Pacheco, Nelson; Barriga, FernandoThree felsic volcanic sequences constitute the host succession to the Neves Corvo VHMS deposit. The lower volcanic sequence (late Famennian) consists of a rhyolitic fiamme-rich facies association that comprises polymictic and overall graded quartzphyric fiamme breccia units (up to 60 m thick). These units have pyroclastic origin and constitute the substrate to the rhyolite facies association (intermediate volcanic sequence). The rhyolite facies association (late Strunian) comprises intervals of coherent quartz-feldspar-phyric rhyolite (up to 10 m thick) that are enclosed by much thicker intervals (up to 250 m) of jigsaw-fit and clast-rotated monomictic rhyolite breccia. Laterally these breccias grade to beds of monomictic rhyolite breccia that alternate with crystal-rich sandstone. The units defined by the rhyolite facies association are rhyolitic lavas. The massive sulfide orebodies (late Strunian) directly overly the lavas or are interleaved with relatively thin (up to 50 m) intervals of mudstone. The upper volcanic sequence (early Visean) consists of a thin interval of monomictic dacite breccia. The host succession to the Neves Corvo orebodies thus comprises proximal to source vent deposits from submarine explosive and effusive eruptions. However, the ore-forming process relates both in time and space with the rhyolitic lavas, which are coeval with the mineralization. Neves Corvo is well known for its high-grade Cu ores and unique cassiterite mineralization. Ore-related hydrothermal activity overprints an early metasomatic stage and relates with a multi-sourced hydrothermal system, responsible for early stringer and massive cassiterite deposition and subsequent massive sulfide oregeneration. In the Corvo orebody, the early deposition of massive cassiterite ores was fed by an independent stockwork in a tectonically-bounded alignment. Textural and petrographic analyses, geochemistry and oxygen-isotope data indicate brusque flushing of the tin-bearing fluid into seawater after minimal fluid-rock interaction during up flow. Massive sulfide-related hydrothermal alteration is essentially stratabound and controlled by permeability contrasts. Alteration zonation is classical, consisting of an inner chlorite/donbassite-quartz-sulfides-(sericite) core that grades into sericitequartz- sulfides-(chlorite) and paragonite-quartz-sulfides-(chlorite) peripheral envelopes. The aluminous hydrothermal alteration mineralogy coupled with elemental and stable isotope geochemistry indicates very low pH, unusually high maximum interaction temperature and predominant low-sulfidation alteration/mineralization conditions. Textural and mass-balance analyses show extensive silicate-sulfide replacement in the coherent volcanic rocks of the footwall sequence, and disseminated replacement mineralization in the volcaniclatic/sedimentary units.
- Short-lived episodes of carbonate productivity associated with the effusion of submarine lava flows in the Middle Cambrian of the Coastal Meseta, MaroccoPublication . Javier Álvaro, J.; Ezzouhairi, Hassan; Ayad, N. Ait; Charif, Abdel; Popov, Leonid; Ribeiro, Maria Luísa
- Zircon dating and inheritance of a pre-Variscan granite, SW IberiaPublication . Solá, A. Rita; Ribeiro, Maria Luísa; Neiva, A.M.R.The Portalegre pluton is an aligned series of Pre-Variscan granitoids located along the boundary of the Central Iberian/ Ossa Morena Zones (SW Iberian Massif, Portugal). Singlezircon geochronology (U-Pb SHRIMP and Pb/Pb stepwise evaporation) yield an Upper Cambrian/Lower Ordovician age (492.7 ± 3.5 Ma) interpreted as the magmatic crystallization age. The inherited zircon cores indicate the involvement of sources with a wide range age-components: an important Late Neoproterozoic (548 ± 7 Ma and c. 611-681 Ma) population and a Mesoproterozoic (c. 0.95-1.1 Ga) and older (c. 2.6 Ga) components. Younger zircon ages of 358 ± 36 and 387 ± 7 Ma were also found and interpreted as the record of a Variscan metamorphic event. The presence of Grenvillian zirconforming events in the protholiths of Portalegre granites is significant in the regional geodynamic context of the Iberian Massif (Central Iberian affinity?). At present, the Grenvillian ages are not noticeable in the Late Neoproterozoic/Early Cambrian record of the Ossa-Morena Zone, that has been correlated with West African Craton [1]. These data suggests that the Central Iberian Zone and Ossa-Morena Zone were independent peri-gondwana terrains with diferent paleogeographic affinities before the Ordovician times. The overall chemistry for the Portalegre granites shows they are very differentiated (SiO2=74-76 wt %), peraluminous (A/CNK=1.1–1.4); have low Zr=36-125ppm, Th/Ta=2-10, ΣREE= 22-134 and 1000Ga/Al >3. Their isotopic signatures (87Sr/86Sr)493=0.7050-0.7065, εNd493(-2.88 to -0.85) and δ18O=10.5-10.8‰, are compatible with partial melting of relatively young recycled metaigneous ± enriched mantle sources. The age pattern from the inherited zircon cores in the Portalegre granites shows that the late Neoproterozoic age (Cadomian) basement was actively involved in their magma generation. The Grenvillian and Archaean zircons can be accounted for by that source component but they do not imply the presence of an older pre-Neoproterozoic basement rocks beneath SW Iberia.