serie NOVA TERRA nº 49

51 A peri-Gondwanan arc in NW Iberia I: Isotopic and geochemical constraints on the origin of the arc — A sedimentary approach J.M. Fuenlabrada a , R. Arenas b , ⁎ , S. Sánchez Martínez c , F. Díaz García d , P. Castiñeiras b a CAI de Geocronología y Geoquímica Isotópica, Facultad de Geología, Universidad Complutense de Madrid, 28040 Madrid, Spain b Departamento de Petrología y Geoquímica e Instituto de Geología Económica (CSIC), Universidad Complutense de Madrid, 28040 Madrid, Spain c Institut für Geowissenschaften, Mineralogie-Petrologie und Geochemie, Goethe-Universität, Altenhöferallee 1, D-60438 Frankfurt am Main, Germany d Departamento de Geología, Universidad de Oviedo, 33005 Oviedo, Spain a b s t r a c t a r t i c l e i n f o Article history: Received 22 April 2009 Received in revised form 17 September 2009 Accepted 23 September 2009 Available online 6 October 2009 Keywords: Cambrian turbidites Sediment geochemistry Nd isotope composition Provenance Peri-Gondwanan arc Allochthonous complexes NW Iberia The arc-derived upper terrane in the NW Iberia Variscan belt contains a 3000 m thick turbiditic formation at its structural top. Most of the sandstones are feldspathic greywackes with a framework of quartz and weakly altered plagioclase. Rock fragments of vitric and microgranular texture are common in polymictic conglomerates and coarse-grained greywackes, together with slates, cherts and bipyramidal volcanic quartz fragments. Although recrystallization under greenschists facies conditions (chlorite and biotite zones) and the presence of two cleavages hinder detailed textural analysis, the sandstones appear to be typically immature, fi rst-cycle sandstones. The metagreywackes have average major and trace element compositions similar to PAAS (Post Archean Australian Shale), which is considered to re fl ect the composition of the upper continental crust. Their trace element composition is very consistent and records deposition within a convergent tectonic setting, probably in an intra-arc basin located in a volcanic arc built on thinned continental margin. Detrital zircon populations suggest a Middle Cambrian maximum depositional age (530 – 500 Ma) and a Gondwanan provenance located at the periphery of the West African Craton. Nd isotope data suggest mixing Ediacaran and Paleoproterozoic sources for the provenance of the greywackes, with T DM ranging between 720 and 1215 Ma with an average of 995 Ma ( n =20) — an age range unrepresented in the detrital zircon population. The Nd model ages are similar to those exhibited by West Avalonia, Florida or the Carolina terrane, but younger than those of Cambrian and Ordovician sandstones and shales from the autochthonous realm. These data suggest a westernmost location along the Gondwanan margin for the upper terrane of NW Iberia relative to other terranes located in the footwall of the Variscan suture, consistent with several previously proposed paleogeographic models for the NW Iberia terranes. © 2009 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. 1. Introduction The European Variscan Belt and its continuation through the Appalachian Orogen is a major orogenic belt developed during the fi nal stages of the assembly of Pangea as a result of the closure of the Rheic Ocean ( Casini and Oggiano, 2008; Nance et al., 2010-this issue; Keppie et al., 2010-this issue; Melleton et al., 2010 ) and the collision between Gondwana and Laurussia ( Matte, 1991; Martínez Catalán et al., 2007 ), which was probably oblique ( Arenas et al., 2009 ). In Europe, the most internal part of this belt includes a succession of allochthonous complexes that de fi ne the main suture zone and are considered to be remant klippen of a large nappe pile ( Fig. 1 ). The allochthonous complexes in the NW Iberian Massif include three main terranes designated, from bottom to top, the basal units, ophiolitic units and upper units ( Fig. 2 ). The basal units are considered to be the most external margin of Gondwana subducted beneath Laurussia at the onset of Variscan deformation (c. 370 Ma; Arenas et al., 1995; Martínez Catalán et al., 1996; Rodríguez et al., 2003; Abati et al., 2010 ), whereas the upper units are interpreted to be an arc-derived terrane. This arc also has a peri-Gondwanan provenance ( Fernández- Suárez et al., 2003 ), but left the main continent during the Middle Cambrian – Early Ordovician and drifted north contemporaneously with the opening of the Rheic Ocean, which is represented within the stack of allochthonous units by different types of ophiolites ( Díaz García et al., 1999; Pin et al., 2002; Arenas et al., 2007; Sánchez Martínez et al., 2007a,b ). This rifted arc was fi nally accreted to the southern margin of Laurussia during the Lower or Middle Devonian ( Gómez Barreiro et al., 2007 ). Gondwana Research 17 (2010) 338 – 351 ⁎ Corresponding author. Departamento de Petrología y Geoquímica, Facultad de Geología, Universidad Complutense de Madrid, C/José Antonio Novais, no. 2, 28040 Madrid, Spain. E-mail address: arenas@geo.ucm.es (R. Arenas). 1342-937X/$ – see front matter © 2009 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi: 10.1016/j.gr.2009.09.007 Contents lists available at ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr