Browsing by Author "Carvalho, D."
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- Dados geocronológicos para o soco pré-Mesozoico localizado a Oeste da Falha Porto-Tomar: implicações geodinâmicasPublication . Santos, Telmo M. Bento Dos; Chichorro, Martim; Carvalho, D.; Cachapuz, P.; Solá, A. Rita; Dantas, E.; Romão, José; Moreira, Noel; Silva, I. Dias da; Clavijo, E.; Díez-Montes, Alejandro; Valverde-Vaquero, Pablo; Santos, R. V.; Linnemann, UlfSUMMARY: Geochronological data from the pre-Mesozoic basement of the Lusitanian Basement (West of the Porto-Tomar Fault) was obtained. Results show a predominant Cadomian/Pan-African and Eburnean inheritance/source, placing this sector as a peri-Gondwanan domain. However, two major differences can be seen in the data: samples with a clear absence in Mesoproterozoic ages vs. samples with a clear presence of these ages and Silurian-Devonian ages. If the former show clear signs of affinity with the Ossa-Morena Zone, the latter are similar to the South Portuguese Zone, being one of the samples, probably deposited in the Middle-Upper Devonian, interpreted as having a mixed source.
- Ediacaran and early-palaeozoic bimodal volcanism in the Ossa-Morena Zone, SW Iberian Massif: New clues for intraplate rifting shortly after the Cadomian OrogenyPublication . Cachapuz, P.; Chichorro, Martim Afonso; Bento dos Santos, Telmo; Carvalho, D.; Linnemann, Ulf; Zieger-Hofmann, M.; Zieger, J.; Dantas, E.; Santos, R. V.; Moita, Patrícia; Beltrame, M.; Solá, A. Rita; Díaz-Azpíroz, M.; Fernández, C.ABSTRACT: This study focuses on bimodal meta-volcanic rocks present in the Iberian Massif, in the Evora-Aracena Metamorphic Belt of the Ossa-Morena Zone (OMZ), near the boundary with the South Portuguese Zone. New petrological, geochemical (whole-rock and Sr-Nd isotopes) and LA-ICP-MS U-Pb zircon geochronology data are presented to track the magmatic evolution of both felsic and mafic rocks and their respective geodynamic setting during the transition between the Cadomian Orogeny and subsequent Palaeozoic rifting events. Our research shows that the Evora-Aracena Metamorphic Belt (EAMB) in the southwestern OMZ possesses a significant record of the transition between the Cadomian and Variscan cycles. U-Pb geochronological data confirm the existence of both felsic and mafic Ediacaran igneous rocks (546 and 556 Ma, respectively), a novelty in the region. The Cadomian-related felsic rocks show geochemical similarities to meta-felsic rocks formed during the initial phases of the Cambrian rifting event, namely very negative epsilon Nd values (-10.1 to -11.8), depleted HREE patterns and orogenic signatures. Such similarities imply that meta-felsic rocks formed in the earliest stages of the Cambrian Rift-to-Drift event resulted from a shallower melting. Moreover, meta-felsic rocks formed in the early stages of the Cambrian rifting (525 Ma) present a significant amount of Ediacaran aged zircons, whereas the Cadomian-related counterparts display a few discordant Cambrian ages, likely a consequence of Pb-loss. This indicates that the convergent, subduction-related regime that prevailed during most of the Cryogenian-Ediacaran times along the boundaries of NW Gondwana was swiftly supplanted by a rift-dominated regime during the Cambrian, with no substantial gap between these two thermal events. Meanwhile, meta-felsic rocks formed during the Rift-to-Drift (525-480 Ma) event exhibit progressively higher epsilon Nd values (-5.0 to +3.0), HREE-enrichment, as well as anorogenic signatures, denoting an increasing contribution of mantle-derived melts. This increasing mantle contribution eventually led to the occurrence of bimodal magmatism in the Middle Cambrian Main Rift event. Such Cambrian mafic rocks, unlike the Ediacaran mafic rocks that present flat REE patterns and continental arc signatures, display varying degrees of LREE enrichment and N-MORB to E-MORB signatures.
- First evidence of Cadomian felsic magmatism in the Évora-Aracena Domain, Ossa-Morena Zone, SW Iberian MassifPublication . Cachapuz, P.; Chichorro, Martim; Santos, Telmo M. Bento Dos; Carvalho, D.; Benoit, M.; Dantas, E.; Solá, A. RitaSUMÁRIO: Dados geoquímicos obtidos em meta-riólitos da região de Viana do Alentejo incluídos na formação Xistos de Moura permitem diferenciar 3 grupos distintos: Grupo I – rico em HREE com assinatura orogénica; Grupo II – também enriquecido em HREE, mas apresentando natureza anorogénica; e Grupo III - anorogénico, mais enriquecido em REE que os anteriores. Tais informações, quando combinadas com dados geocronológicos U-Pb, obtidos em amostras dos grupos I e III, permitem afirmar que estes riólitos resultaram de um magmatismo contínuo entre a Orogenia Cadomiana e o evento rift-to-drift do Câmbrico.
- Tales from xenoliths: the link between the Papôa Volcanic Breccia and the Berlengas Archipelago (West Portugal): geodynamic implicationsPublication . Rosa, Ana Rita; Cachapuz, P.; Santos, Telmo M. Bento Dos; Pereira, Inês; Solá, A. Rita; Carvalho, D.; Romão, José; Valverde-Vaquero, PabloABSTRACT: The Papoa Volcanic Breccia (Peniche, West Portugal) is a Mesozoic pyroclastic deposit that includes numerous igneous and metamorphic xenoliths belonging to the Variscan basement of the Lusitanian Basin, in which this unit outcrops. These rocks correspond mostly to syenogranites and diatexites affected by intense pyrometamorphism and, despite their strong secondary alteration by weathering, they still preserve several original petrographic features that are traceable to the lithologies that comprise the Berlengas Archipelago, located 15 km offshore the Peniche coast. New geochemical and mineral chemistry data now indicate a genetic relationship between these basement xenoliths and the Late Devonian migmatites outcropping in the Farilhoes Anatectic Complex. Such evidence strongly suggests the physical continuity of these high-grade metamorphic terrains at least to the Peniche region, thus constraining the nature and composition of the crustal block beneath the sedimentary sequences of the Lusitanian Basin in the studied location. In addition, in situ LA-ICP-MS U-Pb dating of rutile formed during the pyrometamorphic event endured by the xenoliths provided an age of 92.3 +/- 1.4 Ma for the Papoa volcanic eruption, placing this occurrence within the third magmatic cycle related to the opening and spreading of the North Atlantic Ocean in the West Iberian Margin.