serie NOVA TERRA nº 49

69 A peri-Gondwanan arc in NW Iberia. II: Assessment of the intra-arc tectonothermal evolution through U – Pb SHRIMP dating of ma fi c dykes F. Díaz García a , S. Sánchez Martínez b , P. Castiñeiras c , J.M. Fuenlabrada d , R. Arenas c , ⁎ a Departamento de Geología, Universidad de Oviedo, 33005 Oviedo, Spain b Institut für Geowissenschaften, Mineralogie – Petrologie und Geochemie, Goethe-Universität, Altenhöferallee 1, D-60438 Frankfurt am Main, Germany c Departamento de Petrología y Geoquímica e Instituto de Geología Económica (CSIC), Universidad Complutense de Madrid, 28040 Madrid, Spain d CAI de Geocronología y Geoquímica Isotópica, Facultad de Geología, Universidad Complutense de Madrid, 28040 Madrid, Spain a b s t r a c t a r t i c l e i n f o Article history: Received 23 April 2009 Received in revised form 17 September 2009 Accepted 23 September 2009 Available online 5 October 2009 Keywords: Arc dynamics Cambrian intra-arc deformation U – Pb SHRIMP dating Peri-Gondwanan arc Allochthonous complexes NW Iberia The arc-derived upper units of the Órdenes Complex, NW Iberia, are emplaced above the Variscan suture and contain a low-grade metasedimentary uppermost section, with a maximum depositional age of 510 – 530 Ma, intruded by a number of ma fi c dykes. Three deformational events affect the metasedimentary section. The youngest deformation event (D 3 ) is of undifferentiated Variscan age and consists of metre- to decametre-scale, close upright folds with axes plunging gently towards N20°E. The most important D 2 structure is a regional S 2 foliation axial planar of minor folds with dextral asymmetry. The presence of a stretching lineation parallel to the D 2 foldaxesisrelatedtoatop-to-the-northsenseofshearinginacontextofregionalextension.Theoldestmeter- scale D 1 folds are developedin suitable greywacke – pelite alternations and consist of tightfolds withchevron and similarmorphologies,axesplunginggentlytowardN20°E,andacontinuousS 1 axialplanarfoliation.Theessential characteristic of the D 1 deformation event is depicted by a set of west-vergent folds with reverse limbs less than 2 km in wavelength, that are affected in their lower part by the generalised presence of the regional S 2 foliation. The age of D 2 and D 1 structures is not well constrained. The diabase dykes intruding the low-grade turbidites cut the D 1 folds and their fi eld relationships suggest that they were emplaced at the end of the D 2 shearing event and prior to the upright D 3 Variscan folds. Zircon grains obtained from one of the diabase dykes were analysed for U – Pb at the SHRIMP-RG facility at Stanford University. An age of c. 510 Ma, based on the analysis of 31 individual zircon grains, is interpreted to date the crystallization of the Ares dyke. The tectonic and magmatic evolution of the top turbiditic series of the Órdenes Complex is tentatively related to the dynamics of a peri- Gondwanan arc developed during active subduction beneath Gondwana and suggests: (1) accretion beneath the arc during west-vergent (present coordinates) nappe development (D 1 ); (2) extension of the arc during top-to- the-north shearing (D 2 ); and (3) fi nal intrusion of the diabasic dykes into an intra-arc turbiditic series. This evolution spans the end of volcanic arc activity and the onset of the opening of the Rheic Ocean. © 2009 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. 1. Introduction The arc-derived upper units of the allochthonous complexes of NW Iberia include a thick turbiditic series that occupies the uppermost structural position. An earlier fi rst companion paper in this issue ( Fuenlabrada et al., 2010 ) describes the geochemistry of the turbidites and their provenance based on new Nd isotope data and a revision of previously published U – Pb ages for detrital zircon grains ( Fernández- Suárez et al., 2003 ). Fuenlabrada et al. (2010) also provide a general description of the tectonothermal evolution of the upper units based on fi eld evidence and U – Pb ages ( Abati et al., 1999, 2007; Fernández- Suarez et al., 2002, 2007; Ordoñez Casado et al., 2001 ). Present data suggest an intra-arc setting for the sedimentary basin in which the turbiditic series was deposited, located in a peri-Gondwanan realm peripheral to the margin of the West African Craton. The volcanic arc was probably built on the most external margin of Gondwana on thinned continental basement. The Nd isotopic data suggest an af fi nity with West Avalonia, Florida and the Carolina terrane, and a more westerly provenance than that accepted for the Early Paleozoic autochthonous sequence in NW Iberia and its equivalents in the Bohemian Massif ( Linnemann and Romer, 2002 ), largely con fi rming earlier data on the provenance of this allochthonous terrane ( Gómez Barreiro et al., 2007 ). This paper presents a detailed structural and microstructural anal- ysis of the turbiditic series from the coastal section of the Ares – Sada region, east of A Coruña city ( Fig.1 ),and U – Pb age data fordiabase dykes intruding the sedimentary succession. Structural analysis of the low- grade uppermost section of the upper units complements the history recorded in the rest of the upper units for two reasons. First, minor recrystallization and grain growth favour the preservation of old Gondwana Research 17 (2010) 352 – 362 ⁎ 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.2009.09.010 Contents lists available at ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr