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- Inspecting zircon populations of the Iberian Pyrite Belt: tracking the Cadomian record of the South Portuguese ZonePublication . Lains Amaral, João; Solá, A. Rita; M. Bento dos Santos, TelmoABSTRACT: The palaeogeographic location of the southernmost zone of the Iberian Massif, the South Portuguese Zone (SPZ), prior to the amalgamation of Pangaea is still a matter of debate. In this work, we attempt to track its palaeogeographic setting during the final stages of the Cadomian Cycle.
- Exhumation of an anatectic complex by channel flow and extrusion tectonics: structural and metamorphic evidence from the Porto–Viseu Metamorphic Belt, Central-Iberian ZonePublication . M. Bento dos Santos, Telmo; Rodrigues, José Feliciano; Castro, Paulo; Cotrim, Beatriz; Pereira, Inês; Ferreira, Joana; Pinto De Meireles, Carlos Augusto; Ferreira, Narciso; Ferreira, Priscila; Ribeiro, António; Guimarães, FernandaAbstract: Structural and metamorphic analyses of the Porto–Viseu Metamorphic Belt (PVMB), an anatectic complex located in the Central-Iberian Zone (CIZ), Iberian Variscan Massif, constrain the kinematic features of the polyphase Variscan deformation, as well as the boundary conditions associated with the exhumation and emplacement of the belt onto the CIZ’s lower-grade metasediments. A sharp metamorphic contrast is evident between the lowest grade rocks of the anatectic complex—garnet-staurolite mylonitic schists with a minimum metamorphic peak at P = 5.7 kbar and T = 635 °C—and the host chlorite-biotite metasediments of the CIZ. Additional P–T estimates for metatexites in the PVMB provided P = 7.6–7.9 kbar and T = 770–810 °C, confirming high-grade metamorphic peak conditions. The temperature jump between the anatectic complex and the low-grade host rocks exceeds 400 °C if the anatectic core of the belt (metatexites, diatexites, and S-type granites) is considered, implying a vertical difference of 10–17 km. The PVMB is limited in the NE by a D2 shear zone displaying normal kinematics, which has been explained so far by extensional tectonics. However, the SW boundary is limited by a D2 shear zone showing reverse kinematics. This pair of coeval and antithetical ductile shear zones bordering the PVMB, associated with the increase of the metamorphic grade towards the axial domain of the belt, is consistent with anatectic channel flow, followed by extrusion tectonics of the PVMB within an overall D2 compressive setting.
- 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.
- Geophysical characterization of the Cercal Paleozoic structure, Iberian Pyrite Belt, from a mineral exploration perspectivePublication . Feitoza, L. M.; Carvalho, João; Represas, Patricia; Ramalho, Elsa; Sousa, Pedro; Matos, João Xavier; Albardeiro, Luís; Morais, Igor; Santos, Telmo M. Bento DosABSTRACT: The Cercal Volcano-Sedimentary Complex (Cercal VSC) structure is the westernmost sector of the Iberian Pyrite Belt (IPB) giant metallogenic province and was explored since Roman times for volcanogenic massive sulphides (VMS) and Fe–Mn (Ba) deposits. However, presently only a single deposit is known, the Salgadinho Cu-Ag-Au stockwork/disseminated deposit. Nevertheless, according to several mining companies that operated in the area during the last two decades, the Cercal area keeps a high potential for the discovery of new massive and stockwork mineralization deposits hosted in VSC felsic volcanic rocks. The lack of seismic data and deep drill holes (> 800 m), has been limiting the discovery of possible deep seated massive/stockwork deposits. The goal of this work is to provide new insights into the Cercal deep and near surface structure through the integrated interpretation of geophysical and geological data and contribute to the discovery of new stockwork or massive sulphide deposits. For this purpose, we reprocessed ground gravity, airborne magnetic/radiometric and electrical resistivity/induced polarization data which was interpreted and integrated with geological data. Data interpretation included 2.5D forward gravimetric modeling constrained by i) a new rock density database (from outcrops and drill-holes) built under the scope of this work, ii) the scarce available drill-holes and iii) geological information. A small magnetic susceptibility/conductivity database based on drill-hole core samples was also built to assist the magnetic and electrical/chargeability data interpretation. The integrated interpretation agrees with some of the expected geological scenarios predicted by surface mapping and exploration drill-hole logs in the IPB and provides further details on the deep structure of the Cercal Anticline, a NNW-SSE oriented VSC unrooted structure, controlled by variscan SW verging thrust faults and discordant strike-slip Late-Variscan faults. This geological scenario favors the possibility of new findings at relatively shallow depths (200–900 m), and a few sites for further exploration are indicated based on the integrated interpretation. Therefore, this work contributes to the understanding of the Cercal deep structure from a mineral exploration perspective and confirms the proposed structural models for the IPB’s westernmost region.
- Cadomian/Pan-African consolidation of the Iberian Massif assessed by its detrital and inherited zircon populations: is the similar to 610Ma age peak a persistent Cadomian magmatic inheritance or the key to unravel its Pan-African basement?Publication . Chichorro, Martim; Solá, A. Rita; Santos, Telmo M. Bento Dos; Amaral, Joao LainsABSTRACT: This work assessed the age distribution of Cadomian/Pan-African orogenic events (550-590 and 605-790Ma, respectively) in several zones of Iberian Massif by means of detrital and inherited zircon analysis compilation. Detrital zircon age spectra show that throughout the late Neoproterozoic-to-Early Ordovician era (similar to 120Ma sedimentary record), the main systematic peak occurs at similar to 610Ma, followed by peaks at typical Cadomian ages (similar to 590-550Ma). Inherited zircons incorporated in Cambrian-to-Lower Ordovician igneous rocks show typical Cadomian ages (similar to 590-550Ma) but, once again, a remarkably consistent Pan-African similar to 610Ma peak occurs. In accordance with compiled zircon data and taking into account the evidence of North African peri-cratonic inliers, Ediacaran (similar to 610Ma) zircons incorporated in Paleozoic magmas provide indirect evidence of Pan-African magmatism, suggesting that these magmas and synorogenic sediments are likely to constitute the cryptic stratigraphic infrastructure of most of the Iberian Massif. The main source of similar to 610Ma inherited zircons may be the lateral chrono-equivalents of the Saghro and Bou Salda-M'Gouna Groups (Anti-Atlas, Morocco) and/or coeval igneous rocks from West African Craton or Trans-Sahara Belt, emplaced at a stratigraphic level below the late-Ediacaran sediments of the Ossa Morena Zone and the Central Iberian Zone. Assuming that the Iberian crust is a fragment of the Pan-African orogen, a relative paleoposition situated between the West African Craton and the Trans-Saharan Belt during the Late Neoproterozoic is proposed. The closed-system behaviour of Stenian-Tonian detrital zircon ages in the Trans-Sahara Belt suggests that this mega-cordillera acted as a barrier, in paleogeographic terms, to separating the Sahara Metacraton from Iberia. In Iberia, the opening of the system to Stenian-Tonian detrital zircon during the Ordovician indicates that, at that time, the Trans-Saharan Belt had already become a vast peneplain, which favoured a large drainage system with a long-distance transport mechanism that fed the passive continental margins.
- U-Pb detrital zircon ages during tectonic inversion: provenance analysis of siliciclastic sequences of the Ossa-Morena ZonePublication . Chichorro, Martim; Santos, Telmo M. Bento Dos; Solá, A. Rita; Lopes, C.; Cachapuz, P.SUMÁRIO: Análise de proveniência de sucessões detríticas da Zona de Ossa-Morena é reinterpretada a fim de demarcar os estádios de inversão tectónica do Ciclo Varisco. Zircões detríticos de unidades de Rift do Câmbrico médio-superior (Fm de Ossa, Fm de Fatuquedo) mostram não ter existido variações significativas das áreas fonte quando comparadas com os sedimentos ediacarianos, que refletem erosão/reciclagem da crosta Pan-Africana e do Arco Cadomiano, espelhando a herança típica do Norte do Gondwana. O padrão da Fm da Colorada grava o início da subducção, pois mostra que o soco da ZOM já está em inversão, embora ainda com exumação limitada dos depocentros em Rift do Câmbrico. Por sua vez, as curvas da Fm de Terena já são consistentes com exumação ativa e erosão/reciclagem subaérea das sequências vulcano-sedimentares sin-Rift, refletindo as primeiras fases colisionais variscas.
- U-Pb zircon SHRIMP dating of a protracted magmatic setting and its volcanic emplacementPublication . Lains Amaral, João; Solá, A. Rita; Santos, Telmo M. Bento Dos; Tassinari, Colombo C. G.; Gonçalves, JoãoA geochronological study using SHRIMP U-Pb analysis of zircon grains has been conducted to date felsic volcanic rocks hosting the six massive sulphide deposits of the giant Aljustrel mining district in the Iberian Pyrite Belt. A multiple method age calculation approach was used to validate and ponder calculated Concordia ages (emplacement and inherited), which included weighted average, probability density peak(s), Tuff Zirc and Unmix functions. This approach was particularly useful to interpret the wide continuous single U-Pb ages (320–405 Ma) recorded in the Aljustrel volcanic rocks. The volcanic pile (>250 m) that hosts the Aljustrel deposits was emplaced between 359 and 353 Ma. Upper Devonian inheritance, representing subvolcanic activity, is well-represented in the volcanic rocks of Aljustrel (373–365 Ma). Older Devonian inherited zircon ages at 405 Ma, 388 Ma and 380 Ma were retrieved, hypothetically representing deep plutonism or other melting episodes, which suggests a long-lasting (~50 Ma) magmatic activity in the Aljustrel district. Older pre-Devonian inherited ages, uppermost Silurian and early to late Cambrian, and post-emplacement ages (~330–345 Ma) were also detected, with the latter reflecting Pb loss most likely driven by the main Variscan orogenic event. Maximum ages obtained for the volcanic rocks in the different deposits open the possibility that the last pulses of volcanic activity and subsequent deposition of the massive sulphides were diachronic in the different Aljustrel sub-basins. Additionally, results imply that, contrary to previously assumed, Gavião and São João-Moinho deposits are probably not the same ore lens disrupted by tardi-Variscan faults. This opens new opportunities for mining exploration and targeting in the Aljustrel district and points out the importance of high-resolution geochronological studies in mining and brownfield areas.
- The bimodal Fii-A2-type and calc-alkaline volcanic sequence of the Aljustrel brownfield region, Iberian Pyrite Belt, SW Iberian MassifPublication . Lains Amaral, João; Solá, A. Rita; Santos, Telmo M. Bento Dos; Feitoza, L. M.; Tassinari, Colombo C. G.; Crispim, Lourenço; Chichorro, Martim; Hofmann, Mandy; Gãertner, Jessica; Linnemann, Ulf; Gonçalves, JoãoABSTRACT: The Iberian Pyrite Belt (IPB) is a late Devonian- Early Carboniferous world-class polymetallic VMS province that includes significant Cu-(Sn)-Pb-Zn-(Ag) deposits of massive sulphides and feeder zones. The Aljustrel brownfield region contains one of the highest concentrations of ore in the IPB in 6 known deposits (Gaviao, Sao Joao, Moinho, Algares, Estacao and Feitais). To delve into the petrogenesis of the Aljustrel early Carboniferous (similar to 355 Ma) felsic-dominated bimodal volcanism, new whole-rock trace elements and Sm-Nd isotopes, and U-Pb in zircon were obtained. Based on Ga/Al and Y/Nb ratios, it is shown that Aljustrel felsic magmatism has the geochemical features of A2-type melts, typical of post-collisional and back-arc settings. U-Pb in zircon for a juvenile felsic volcanic rock point to antecrysts ages spanning from 387.9 to 366.6 Ma and a maximum emplacement age of 354.3 +/- 2.6 Ma. These long-lasting melting events, present in both juvenile (epsilon Ndi = +1.79) and evolved felsic rocks (epsilon Ndi =-5.07), imply heterogeneous sources dominated by zircon-bearing igneous rocks. The Sm-Nd model ages are in accordance with previous Lu-Hf model ages in zircon, reinforcing that the isotopic variability is related to the same petrogenetic process. Subordinated Aljustrel mafic rocks, coeval with the abundant felsic volcanism, show orogenic signatures, namely Nb-Ta-Ti negative anomalies and calc-alkaline affinities, whereas Sm-Nd isotopic data (epsilon Ndi = +1.54 to +5.48) points to variable to no contamination with crustal material. These geochemical results suggest derivation from an enriched mantle source modified by subduction metasomatism. In addition, the mafic rocks did not provide zircons for geochronological analysis, with the exception of one sample, in which a Concordia age of 402.1 +/- 15.5 Ma was obtained from a single grain. The combined geochemical signatures of mafic and felsic volcanic rocks suggest asthenospheric rise, but this solely does not explain the abundance of zircon antecrysts in the felsic rocks. Therefore, a geodynamic model that includes a continuous evolution from Devonian to Carboniferous times is inferred. This more complex and broader geodynamic model for the Iberian Pyrite Belt in which successive metal remobilization occurred after successive melting events, fits the present geochemical data and is more likely to explain why the Iberian Pyrite Belt is a unique metallogenetic province.
- Novos dados U-Pb em zircão do Maciço de Évora e correlação com as unidades basais da Zona de Galiza-Trás-os-MontesPublication . Solá, A. Rita; Chichorro, Martim; Cachapuz, P.; Santos, Telmo M. Bento DosSUMMARY: We present new U-Pb zircon data from the Évora Massif (SW Ossa–Morena Zone). Série Negra paragneiss of Escoural Fm. yields a maximum depositional age of ~580 Ma. Felsic orthogneiss “leptinites” included in Escoural Fm., commonly attributed to the Precambrian, yield a concordant age of 513 ± 3 Ma. Felsic volcanics interbedded in the Monfurado Fm. and the Safira Orthogneiss show very discordant zircon ages and two systematic populations at ~510 Ma and ~460 Ma. Given the stratigraphic constraint of Monfurado Fm., both magmatic rocks are considered coeval and emplaced at ~510 Ma during the main rift-related magmatism. The ~460 Ma age is not fully understood yet. Given the differences in the ages of magmatism in the Évora Massif, recently correlated with the Lower Allochthon of Galiza-Trás-os-Montes Zone, where only late rift-related magmatism is recorded, we suggest that they represent different sedimentary basins positioned in the same paleogeographic contexts.
- Remobilização elementar em rochas magmáticas e metamórficas de alto grau: análise in situ e condicionantes geológicos no contexto do afloramento crítico do Rio Almansor, Zona de Ossa Morena (ZOM)Publication . Pedrosa, Lucas Guilherme G. O.; Santos, Telmo M. Bento Dos; Silva, TeresaABSTRACT: O afloramento do Rio Almansor (Montemor-o-Novo), Setor Montemor-Ficalho da Zona de Ossa Morena (ZOM), é parte de um complexo anatético exumado por uma zona de cisalhamento varisca de orientação NW-SE e cinemática esquerda. É composto por diatexitos (encaixante), leucossomas e encraves básicos-intermédios, estes últimos distribuídos em dois grupos: dioritos gabroicos e quartzo-dioritos/tonalitos. É um alvo interessante para o objetivo deste trabalho em quantificar a remobilização elementar nas interfaces entre diferentes litologias de alto grau metamórfico e, em particular, o seu potencial em concentrar metais. Apresentam-se os resultados de análises químicas in situ com FRX portátil nas unidades presentes, após realização de cartografia de detalhe, bem como nas interfaces entre o corpo diatexítico principal e os encraves.