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- Caracterização preliminar da deformação dos ortognaisses alcalinos e hiperalcalinos da Faixa Blastomilonítica (Zona de Ossa-Morena)Publication . Roseiro, José; Moreira, Noel; Nogueira, Pedro; de Oliveira, Daniel Pipa SoaresSUMMARY: In the Blastomylonitic Belt (northernmost Ossa-Morena Zone), alkaline and peralkaline orthogneisses are found as elongated bodies with WNW-ESE to NW-SE orientation hosted within a strongly deformed Neoproterozoic to Lower Cambrian metasedimentary strata. Despite some similarities between them (ascribed to the very similar nature of their igneous protolith) the deformational features displayed in all of them are different, for example in the mineral segregation and recrystallization (including the development of augens) observed, and in different strike-dip orientation of foliations and lineation plunge. The dissimilarities are presumably due not only from different primary features, but also to the type of ductile shearing and metamorphic conditions they underwent. This way, field data will be complemented with petrographic studies for mineralogical and microdeformation characterization, and crossed to the regional structural information, thus constraining the deformation patterns of the alkaline-peralkaline orthogneisses on the Blastomylonitic Belt.
- Contact metamorphism and dolomitization overprint on Cambrian carbonates from the Ossa‑Morena Zone (SW Iberian Massif): implications to Sr‑chronology of carbonate rocksPublication . Roseiro, José; Moreira, Noel; Andrade, Laura; Nogueira, Pedro; de Oliveira, Daniel Pipa Soares; Eguiluz, Luis; Mirao, Jose; Moita, Patrícia; Santos, José Francisco; Ribeiro, Sara; Pedro, J.ABSTRACT: he Cambrian Series 2 Carbonate Formation from the Alter do Chão Elvas-Cumbres Mayores unit (Ossa-Morena Zone, SW Iberian Massif) is composed of regionally metamorphosed marbles and marlstones that underwent chlorite zone metamorphism and preserve the primaeval limestone 87Sr/86Sr ratios (0.7083–0.7088). These are consistent with the established Lower Cambrian seawater curve, and therefore used for age constraints in formations lacking fossil contents. The regional mineralogical and Sr-isotopic features of the carbonate rocks are frequently overprinted by the effects of contact metamorphism induced by magmatic bodies emplaced during rift-related and synorogenic events of the Palaeozoic, as well as by post-metamorphic dolomitization processes. The development of calc-silicate minerals due to contact metamorphism is common in the rocks of the Carbonate Formation and apparently results from the interaction of the protolith with fluids of different origin: (i) internally produced fluids released by conductive heating (observed in external contact aureoles) and (ii) external intrusion-expelled fluids that, besides leading to the appearance of distinctive assemblages, also promote an influx of strontium content (observed in roof pendants). Calc-silicate mineralogy varies substantially throughout the region, likely due to the heterogeneous distribution of silicate minerals of the protolith, progression of intrusion-driven fluids, and the irregular effect of thermal gradients. Results suggest that high-grade contact metamorphism (hornblende facies or higher) and dolomitization processes imposed on the Carbonate Formation significantly influence the isotopic signatures of the carbonates, providing limitations in applying Sr-isotopic chronology.
- Transição Ordovícico-Silúrico na Zona Centro-Ibérica: apenas um contacto estratigráfico ou um elemento estruturante do Varisco Ibérico?Publication . Tereso, V.; Dias, R.; Romão, José; Roseiro, José; Coke, Carlos; Fonseca, Rita; Ribeiro, A.SUMMARY: Although the Ordovician-Silurian transition in the Central-Iberian Zone is usually considered a normal stratigraphic one, recent studies shows that often the black-shales of the Lower Silurian control the development of shear zones subparallel to the main layers. This behavior has been observed, not only in the Marão-Alvão region, but also in other sectors like, the synclines of S. Mamede, Mação and Buçaco. This seems to show that the Lower Silurian weak layers have been a major factor in controlling the evolution of the Iberian Variscides. Ongoing studies in the Celorico de Basto region will try to characterize and understand the evolution of these shear zones.