ELSEVIER Earth and Planetary Science Letters 165 (1999) 213–228 Early Ordovician orogenic event in Galicia (NW Spain): evidence from U–Pb ages in the uppermost unit of the Ordenes Complex J. Abati a,Ł , G.R. Dunning b , R. Arenas a , F. Dı ´az Garcı ´a c , P. Gonza ´lez Cuadra d , J.R. Martı ´nez Catala ´n d , P. Andonaegui a a Departamento de Petrologı ´a y Geoquı ´mica, Universidad Complutense, 28040 Madrid, Spain b Department of Earth Sciences, Memorial University, St. John’s, NF A1B 3X5, Canada c Departamento de Geologı ´a, Universidad de Oviedo, 33005 Oviedo, Spain d Departamento de Geologı ´a, Universidad de Salamanca, 37008 Salamanca, Spain Received 28 May 1998; revised version received 4 November 1998; accepted 11 November 1998 Abstract New U–Pb data (zircon, monazite and rutile) obtained from rocks of the uppermost allochthon in the Variscan belt of NW Spain indicate that the hangingwall to the suture includes an allochthonous unit with a pre-Variscan tectonothermal evolution. This evolution is characterised by an Early Ordovician (498–500 Ma) bimodal magmatism followed almost im- mediately (493–498 Ma) by a Barrovian style metamorphism up to the granulite facies. The metamorphism subsequent to the igneous intrusions requires convergence and crustal thickening in order to generate the Barrovian facies pattern shown by the lithologies of the uppermost allochthon. The almost coeval magmatism and metamorphism, and the chemistry of the metabasites suggest an accretionary complex, probably related to a volcanic arc, as the most probable setting for the origin of the uppermost unit. The implications of the existence of Early Ordovician convergent plate boundaries are discussed in the context of Gondwana–Laurentia–Avalon interactions.  1999 Published by Elsevier Science B.V. All rights reserved. Keywords: U=Pb; geochronology; Ordovician; thermal metamorphism; Hercynian orogeny; Galicia Spain 1. Introduction The geology of the Northwest Iberian Massif rep- resents a complete transition from the external to the axial zones of the Variscan belt. Five allochthonous complexes, located in Galicia (Spain) and Tra ´s-os- Montes (Portugal), occupy the uppermost structural position in the axial zone. They consist of a pile of units stacked at the onset of the Variscan de- formation, and include a suture zone marked by Ł Corresponding author. Tel.: C34-91-394-4898; Fax: C34-91- 544-2535; E-mail: abati@eucmax.sim.ucm.es several slices of oceanic lithosphere [1,2]. From bot- tom to top, the allochthonous units are grouped into basal, ophiolitic and upper units (Fig. 1). During the main Variscan deformation, these units were thrust over the Iberian autochthon [1], which represents the larger part of the Iberian Massif. The occurrence of a suture points to a collisional process and there is general agreement that one of the colliding elements was the continental mar- gin of Gondwana, represented by the units in the footwall to the suture: the Iberian autochthon and the allochthonous basal units [2–4]. The origin and significance of the units in the hangingwall to the 0012-821X/99/$ – see front matter c  1999 Published by Elsevier Science B.V. All rights reserved. PII:S0012-821X(98)00268-4