A carregar...
6 resultados
Resultados da pesquisa
A mostrar 1 - 6 de 6
- Different meanings for cummingtonite-hornblende association in plutonic rocks, Iberian Massif, Portugal [Poster]Publication . Solá, A. Rita; Moita, Patrícia; Santos, José Francisco; Neiva, A.M.R.; Ribeiro, Maria Luísa
- Assinaturas isotópicas de Sr em águas minerais de Portugal e suas relações com o ambiente geológicoPublication . Ribeiro, Sara; Azevedo, Maria do Rosário; Santos, José Francisco; Medina, Jorge; Costa, Augusto
- 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.
- 87Sr/86Sr dating of the Alcácer do Sal Formation (upper Miocene, mainland Portugal)Publication . Ressurreição, Ricardo; Legoinha, Paulo; Dias, Ruben Pereira; Santos, José Francisco; Ribeiro, Sara; Patinha, Carla; Miranda, RuiThe Lower Tagus Basin was subjected to several marine transgressions along the Neogene, related to positive eustatic oscillations and also controlled by the regional tectonic activity. In the Alcácer do SalSines region, previous studies identified an episode of marine sedimentation interpreted as having occurred during the late Serravalian to early Tortonian interval, based on biostratigraphic data. This episode is represented in the Alcácer do Sal Formation. The study of the SMS-12- 01B borehole, drilled for mineral prospection purposes, located about 10 km NNE of Melides, allowed the identification of a 3 m thick layer of sediments containing marine fossils. 87Sr/86Sr determinations of oyster shells point to deposition about 11.5 Ma ago, validating previous proposals for the age of the formation. These data correspond to the first numerical ages obtained for the Alcácer do Sal Formation, allowing a solid correlation with other known marine units in the Lower Tagus and Algarve basins related to the same transgression episode
- The multistage crystallization of zircon in calc-alkaline granitoids : U-Pb age constraints on the timing of Variscan tectonic activity in SW IberiaPublication . Pereira, M. F.; Chichorro, Martim; Moita, Patrícia; Santos, José Francisco; Solá, A. Rita; Williams, Ian S.; Silva, J. B.; Armstrong, R. A.CL imaging and U–Th–Pb data for a population of zircons from two of the Évora Massif granitoids (Ossa-Morena Zone, SW Iberia) show that both calc-alkaline granitoids have zircon populations dominated by grains with cores and rims either showing or not showing differences in Th/U ratio, and having ages in the range ca. 350–335 Ma (Early Carboniferous). Multistage crystallization of zircon is revealed in two main growth stages (ca. 344–342 Ma and ca. 336–335 Ma), well represented by morphologically complex zircons with cores and rims with different ages and different Th/U ratios that can be explained by: (1) crystallization from melts with different compositions (felsic peraluminous to felsic-intermediate metaluminous; 0.001 Th/U ratio < 0.5) and (2) transient temperature fluctuations in a system where anatectic felsic melts periodically underwent injection of more mafic magmas at higher temperatures. The two studied calc-alkaline granitoids do not include inherited zircons (pre-Carboniferous), probably because they were formed at the highest grade of metamorphism (T 837 °C; granulite facies) and/or because they were derived from inheritance-poor felsic and mafic rocks from a previous cycle, as suggested by the internal structures of zircon cores. These Variscan magmatic rocks with crystallization ages estimated at ca. 336–335 Ma are spatially and temporally related to high-temperature metamorphism, anatexis, processes of interaction between crustal- and mantle-derived magmas and intra-orogenic extension that acted in SW Iberia during the Early Carboniferous.
- New 87Sr/86Sr constraints on the marine Miocene of the Lower Tagus Basin and the Alentejo Coastal Plain (Portugal) [Resumo]Publication . Ressurreição, Ricardo; Legoinha, Paulo; Dias, Ruben; Horta Pacheco dos Santos, José Francisco; Monteiro Ribeiro, Sara; Patinha, CarlaABSTRACT: Sedimentation in the Lower Tagus Basin was strongly controlled by eustatic oscillations, with several marine incursions during the Miocene. Previous studies, based on biostratigraphic and isotopic data, identified ten depositional sequences in the southwestern sector of the basin that correlate with thirdorder eustatic cycles. Some of these transgressions reached considerable amplitudes, with marine influence extending up to ~80 km inland under the present-day configuration of the territory. Preliminary results of 87Sr/86Sr isotopic dating from oyster shells collected in the Lower Tagus Basin (LTB) and the Alentejo Coastal Plain (ACP) are presented (figure 1). At Vila Nova da Rainha, a 15.69 Ma age (correlating with Langhian, V-b unit at Lisbon) anchors the marine-influenced base of the local succession, exposing a ~5 Ma hiatus between these levels and the overlying sands with Hipparion (Vallesian, MN 9). This age corroborates previous bio-chronostratigraphic assignments. Marine and marginal-marine sediments crop out along the eastern margin of the LTB and in the ACP. In the Alcácer do Sal region and along the Melides–Sines coast, these deposits are assigned to the Alcácer do Sal Formation (upper Serravallian–lower Tortonian). However, further north, as far as Lavre (Montemor-o-Novo), geological mapping attributes them to a broader interval corresponding to the Vindobonian (Langhian–Tortonian). Oyster shells collected at two sites along the Marateca stream (Monte da Moinhola and Monte Novo) and at Melides yielded ages of 11.13 Ma, 11.03–7.32 Ma and 8.58 Ma. These values indicate Tortonian deposition and are consistent with the Alcácer do Sal Formation, suggesting that marine Miocene deposits cropping out as far north as Lavre may belong to this unit, representing one of the most landward-penetrating Miocene transgressions. At the base of the sea cliff at Galé beach (Melides), calcareous sandstones and sands with marine fauna have been mapped as part of the Alcácer do Sal Formation. However, 87Sr/86Sr dating of oyster shells yielded ages between 6.79 and 5.80 Ma, indicating Messinian deposition. These sediments can therefore be correlated with the Esbarrondadoiro Formation, a Messinian marine unit of the Alvalade Basin whose connection with the coastal area had not previously been unequivocally identified. This study contributes to the understanding of the marine Miocene of southwestern Portugal and has implications for paleogeographic reconstruction and geological mapping.