Discussion on ‘Detrital zircon geochronology of the Carboniferous Baixo Alentejo Flysch Group (South Portugal); constraints on the provenance and geodynamic evolution of the South Portuguese Zone’, Journal of the Geological Society , 172, 294–308

Abstract

In their recent study, Rodrigues et al . (2014) attempted a provenance analysis of the Carboniferous Baixo Alentejo Flysch Group (SW Iberia), discussing variability over time in sources of South Portuguese Zone turbidites. The U–Pb ages presented by these authors are similar to those previously obtained by Pereira et al . (2012 a , 2013). A comparison of U–Pb age distributions obtained in turbidites from the two studies using the Kolmogorov–Smirnov test indicates that they are not significantly different at the 5% confidence level, increasing the statistical significance of the detrital zircon populations of each stratigraphic formation. However, the interpretations put forward in the two studies are considerably different. The most controversial of these concerns the sources of the Lower Carboniferous turbidites forming the Baixo Alentejo Flysch Group (Mertola and Mira formations). Rodrigues et al . (2014) argued that the detrital zircon ages of the Mertola and Mira greywackes indicate provenance from an extra-basinal source (Ossa–Morena Zone) with a minor intra-basinal contribution (South Portuguese Zone). Pereira et al . (2012 a , 2013) proposed that Early Carboniferous flysch deposition (Lower Carboniferous turbidites; immature greywackes of the Mertola Formation and the base of the Mira Formation) mainly derived from a source terrane dominated by zircon populations of Middle–Late Devonian ages ( c . 393–359 Ma) that are absent in the Ossa–Morena Zone (Fig. 1). The Middle–Late Devonian zircons (37.7%) found in the Lower Carboniferous turbidites display simple growth patterns attributable to a single magmatic event, and are idiomorphic, with no evidence of long transport and/or multiple recycling (Pereira et al . 2012 a ). The Lower Carboniferous turbidites were less markedly influenced by sedimentary recycling than the Upper Carboniferous turbidites, as indicated by the presence of a percentage of detrital zircons with ages greater than c . 393 Ma (Lower Carboniferous turbidites 32%; Upper Carboniferous turbidites 96%; Fig. 1). These Early Palaeozoic and Precambrian zircon-forming events are characteristic of the Gondwana (Ossa–Morena Zone; Pereira et al . 2008, 2012 b , 2013, and references therein) and/or Laurussia (Meguma–South Portuguese Zone, Braid et al . 2011; Pereira et al . 2012 a ; and Avalonia, Waldron et al . 2011, and references therein) basements.