U-Pb SIMS zircon ages for Cambro-Ordovician rocks, Valongo Anticline, northwestern Portugal
DOI:
https://doi.org/10.13133/2280-6148/17274Keywords:
zircon, U/Pb, dates, Valongo, Ordovician, PortugalAbstract
Zircons in three samples of the Cambrian Montalto Formation and “Tremadoc-Arenig Santa Justa Formation of the Valongo Anticline in northwest Portugal were dated by U-Pb secondary ion mass spectrometry (SIMS). A rhyolite sample near the top of the Montalto Formation gave clusters of Lower Paleozoic (600 to 450 Ma) and Upper Carboniferous zircon ages (360 to 300 Ma), with one Paleoproterozoic age (~1750 Ma). The middle black chert sample, underlying the massive “Arenig” quartzites of the Santa Justa Formation, gave only two zircon ages of 827 Ma and 492 Ma. The uppermost volcanic sandstone, overlying the massive “Arenig” quartzites of the Santa Justa Formation, gave similar zircon ages to the lowermost sample, but with mostly Neoproterozoic ages (933 to 560 Ma). The youngest Upper Carboniferous age clusters are caused by zircon alteration or hydrothermal recrystallization during Variscan metamorphism. The oldest Neoproterozoic ages are detrital and represent erosion of regional basement. The Lower Paleozoic ages are also mostly detrital but are from zircons eroded from underlying lavas and volcaniclastics associated with the rifting that preceded the opening of the Rheic ocean. The youngest concordant age of 516±16 Ma of the rhyolite may be the best age for the top of the Montalto Formation, which puts it as equivalent to Middle Cambrian Series 2. The other early Paleozoic zircon dates are all earlier Cambrian. The overlying samples give older ages suggesting progressive erosion of underlying rocks during rift shoulder uplift. A major period of volcanic activity occurred in the latest Cambrian to earliest Ordovician of northern Portugal, though this is poorly constrained by biostratigraphy. Our results, thoughlimited in number,indicate that only the upper quartzite part of the Santa Justa Formation can be equivalent to the so-called “Arenig” quartzites (also poorly constrained) in NW France and Iberia. The Upper Cambrian and Tremadocian seems to be missing in the studied area where a major unconformity probably occurs below the lower Santa Justa Formation.
References
Catalán, J.R.M.,2011. The Central Iberian arc: implications for the Iberian Massif. Geogaceta 50-1, 7-10.
Couto, H., 1993. As mineralizações de Sb-Au da região Dúrico-Beirã. Phd thesis, Porto, Faculty of Sciences, University of Porto. 2 volumes, pp. 606.
Couto, H., 2013. The Ordovician of Valongo Anticline (Northern Portugal): State of Art. Geology, Exploration and Mining, SGEM2013 Conference Proceedings, 1, 203–208.
Couto, H., Knight, J., 2014. The Montalto Formation, a pre- to basal Ordovician succession in the Durico-Beirẫ area (northern Portugal). In: Rogerio R., Pais, J., Kullberg, J.V., Finney, S.C.(Eds.), STRATI 2013: First International Congress on Stratigraphy at the Cutting Edge of Stratigraphy, Springer, New York. 381-384..
Couto, H., Roger, G., 2017. Palaeozoic Magmatism Associated with Gold-Antimony-Tin-Tungsten-Lead-Zinc and Silver Mineralization in the Neighboring of Porto, Northern Portugal. IOP Conference Series: Earth and Environmental Science 95, 1-11. http://doi/10.1088/1755-1315/95/2/022054.
Couto, H., Knight, J., Lourenço, A., 2014. Rifting at the Cambrian– Ordovician transition in northwestern Portugal. Comunicacoes Geologicas 101, 251-254.
Gonçalves, E. J. dos S., 2013. Hidrogeologia das areas de Valongo, de Paredes, et de Arouca, no contexto do Anticlinal de Valongo. Thesis, Univeristad de Porto, Portugal. pp.355.
Gutiérrez-Marco, J. C., Sá, A.A., Rábano, I., Sarmiento, G.N., García-Bellido, D.C., Bernárdez, E., Lorenzo, S., Villas, E., Jiménez-Sánchez, A., Colmenar, J., Zamora, S., 2015. Iberian Ordovician and its international correlation. Stratigraphy 12 (3–4), 257–263.
Gutiérrez-Marco, J.C., Sá, A.A., García-Bellido, D.C., Rabano, I., 2017. The Bohemo-Iberian regional chronostratigraphical scale for the Ordovician System and palaeontological correlations within South Gondwana. Lethaia 50, 258–295
Liñán, E., Perejón, A., Sdzuy, K., 1993. The Lower-Middle Cambrian stages and stratotypes from the Iberian Peninsula: a revision. Geological Magazine 130 (6), 817–833.
Linnemann, U., Pereira, M. F., Jeffries, T., Drost, K., Gerdes, A., 2008. The Cadomian Orogeny and the opening of the Rheic Ocean: The diachrony of geotectonic processes constrained by LA-ICP-MS U–Pb zircon dating (Ossa-Morena and Saxo-Thuringian Zones, Iberian and Bohemian Massifs). Tectonophysics 61. 21-43.
Ludwig, K. R., 2012. Isoplot 3.75: A geochronological toolkit for Microsoft Excel. Berkeley Geochronology Center Special Publication, 5, 1-75.
Margalef, A., Castiñeiras, P., Casas, J. M, Navidad, M., Liesa, M., Linnemann, U., Hofmann, I., Gartner, A., 2016. Detrital zircons from the Ordovician rocks of the Pyrenees: Geochronological constraints and provenance. Tectonophysics 681, 124–134.
Mattinson, J.M., Graubard, C.M., Parkinson, D.L., McClelland, W.C., 1996. U‐Pb reverse discordance in Zircons: the role of fine‐scale oscillatory zoning and sub‐micron transport of Pb. AGU Geophysical Monograph Series 95, 355-370.
Murphy, J. B., Gutiérrez-Alonso, G., Nance, R. D., Fernández-Suárez, J., Keppie, J. D., Quesada, C., Strachan, R. A., Dostal, J., 2006. Origin of the Rheic Ocean: Rifting along a Neoproterozoic suture? Geology, 34, 325–328.
Pochon, A., Gloaguen, E., Branquet, Y., Poujol, M., Ruffet, G., Boiron, M-C., Boulvais, P., Gumiaux, C., Cagnard, F., Gouazou, F., Gapais, D.. 2018. Variscan Sb-Au mineralization in Central Brittany (France): A new metallogenic model derived from the Le Semnon district. Ore Geology Reviews 97. 109-142.
Pouclet, A., Álvaro, J. J., Bardintzeff, JM, Imaz, A.G., Monceret, E., Vizcaïno, D., 2017. Cambrian–early Ordovician volcanism across the South Armorican and Occitan domains of the Variscan Belt in France: Continental break-up and rifting of the northern Gondwana margin. Geo- science Frontiers 8, 25-64.
Reyes−Abril, J., Villas, E., Gutiérrez−Marco, J.C., 2010. Orthid brachiopods from the Middle Ordovician of the Central Iberian Zone, Spain. Acta Palaeontologica Polonica 55(2), 285–308.
Ribeiro, A., Munhá, J., Dias, R., Mateus, A., Peirera, E., Ribeiro, L., Fonseca, P., Araújo, A., Oliveira, T., Romão, J., Chaminé, H., Coke, C., Pedro, J., 2007. Geo- dynamic evolution of the SW Europe Variscides. Tectonics 26, 1-24.
Rodríguez-Tovar, F.J., Stachacz, M., Uchman, A., Reolid, M., 2014. Lower/Middle Ordovician (Arenigian) shallow-marine trace fossils of the Pochico Formation, southern Spain: palaeoenvironmental and palaeogeographic implications at the Gondwanan and peri-Gondwanan realm. Journal of Iberian Geology 40(3), 539-555.
Romano, M., 1982. The Ordovician biostratigraphy of Portugal – a review with new data and re-appraisal. Geological Journal 17, 89-110.
Romano, M., & Diggens, J.N. (1974). The stratigraphy and structure of Ordovician and associated rocks around Valongo, north Portugal. Comunicações dos Serviços Geológicos de Portugal, 57, 23–50.
Romano, M., Brenchley, P.J., McDougall, N.D., 1986. New information concerning the age of the beds immediately overlying the Armorican Quartzite in Central Portugal. Geobios 19(4), 421-433.
Sá, A.A., Gutiérrez-Marco, J.C., Piçarra, J.M., García-Bellido, D.C., Vaz1, N., Aceñolaza, G.F., 2011. Ordovician vs. “Cambrian” Ichnofossils in the Armorican Quartzite of Central Portugal. In J.C. Gutiérrez-Marco, I. Rábano & D. García-Bellido (Eds.), Ordovician of the World. Cuadernos del Museo Geominero, 14. Instituto Geológico y Minero de España, Madrid. 483-492
Schmitz, M. D., Bowring, S., A., Ireland, T., 2003. Evaluation of Duluth Complex anorthositic series (AS3) zircon as a U-Pb geochronological standard: New high-precision isotope dilution thermal ionization mass spectrometry results. Geochimica et Cosmochimica Acta 67(19), 3665-3672.
Schulmann, K., Martínez Catalán, J.R., Lardeaux, J.M., Janoušek. V., Oggiano, G., 2014. The Variscan orogeny: extent, timescale and the formation of the European crust. Geological Society, London Special Publication 405, 1-6.
Shaw, J., Johnson, S.T., 2016. Terrane wrecks (coupled oroclines) and paleomagnetic inclination anomalies. Earth-Science Reviews 154, 191–209.
Shaw, J., Gutiérrez-Alonso, G., Johnston, S.T., Pastor Galán, D., 2014. Provenance variability along the Early Ordovician north Gondwana margin: Paleogeographic and tectonic implications of U-Pb detrital zircon ages from the Armorican Quartzite of the Iberian Variscan belt. Geological Society of America Bulletin 126 (5-6), 702–719.
Stampfli, G.M., Borel, G.D., 2002. A plate tectonic model for the Paleozoic and Mesozoic constrained by dynamic plate boundaries and restored synthetic oceanic isochrones. Earth and Planetary Science Letters 196, 17-33.
Wiedenbeck, M., Alle, P., Corfu, F., Griffin, W.L., Meier, M., Oberli, F., Von Quadt, A., Roddick, J.C., Spiegel, W., 1995. Three natural zircon standards for U-Th-Pb, Lu-Hf, trace element and REE analyses: Geostandards Newsletter 19, 1-23.
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