The Variscan metamorphic evolution of the autochthonous domain of NW and Central Iberia is characterized by a Barrovian gradient followed by a high-temperature–low-pressure (HT/LP) event associated with voluminous granite magmatism. The structural, metamorphic and magmatic histories of the region are described briefly and the relations between them are explained. A coherent model for evolution of the continental crust is proposed using published radiometric ages, thermal models and seismic reflection profiles. The metamorphic evolution, including the high-temperature event, is explained by crustal thickening resulting from the Gondwana–Laurussia collision followed by a period of thermal relaxation and a long-lasting extensional stage. The fact that the highest temperatures were reached in the core of the Central Iberian arc, partly occupied by remnants of a huge allochthonous nappe stack, is discussed in relation to both the emplacement of the allochthon and subsequent oroclinal bending. The overburden provided by the allochthonous pile was decisive in triggering the high-temperature event. Orocline development mostly occurred later and had no significant effect on the metamorphic evolution, although it was important for the present localization of gneiss domes and granitoids. The possible role of the mantle in supplying additional heat to explain the HT/LP event is also discussed. It would seem that little mantle contribution was needed and there are no strong arguments for mantle delamination, although some kind of mantle–crust interaction is expected beneath the hot regions presently occupying the core of the Central Iberian arc.

Ophiolites of different Paleozoic ages occur in North-West (NW) Iberia in a rootless suture representing the remnants of the Rheic Ocean, Associated allochthonous terranes in the hanging-and foot-walls of the suture derive from the former margins.whereas the relative autochthon corresponds to the Paleozoic passive margin of northern Gondwana, The Paleozoic tectonic evolution of this part of the circum-Atlantic region is deduced from the stratigraphical. petrological. structural and metamorphic evolution of the different units and their ages. The tectonic reconstruction covers from Cambro-Ordovician continental rifting and the opening of the Rheic Ocean to its Middle to Upper Devonian closure. Then, the Variscan Laurussia-Gondwana convergence and collision is briefly described, from its onset to the late stages of collapse associated with the demise of the orogenic roots. ResumeUne suture sans racines et la perte des racines d'une cha'ine montagneuse : la cha'ine varisque du Nord-Ouest de I'Iberie. Des ophiolites d'ages differents affieurent dans le Nord-Ouest de l'Iberie dans une suture sans racines. temoin de l'ocean * Corresponding author.E-mail address: jrmc @usal.es (J.R. Martfnez Catahin).RhMque. Les terrains allochtones sur et sous la suture derivent de ses deux rnarges, tandis que I' autochtone relatif appartient a la marge passive du Nord de Gondwana. On peut deduire l'evolution des plaques dans cetle partie de la region circum-Atlantique it partir des dounees stratigraphiques, petrologiques, structurales, metarnorphiques et geochronologiques. Celte evolution inc1ut le developpernent d'un rift continental et l'ouverture de l'ocean Rhe'ique pendant le Carnbro-Ordovicien ainsi que sa ferrneture au Devonien rnoyen a superieur. On decrit aussi I' evolution de la convergence et collision varisque entre Laurussia et Gondwana, du debut jusqu'aux demiers stades d'un effondrernent associe a la perte des racines orogeniques.Keywords: Oceanic suture; Rheic Ocean; Variscan evolution; N\V IberiaMots des : Suture oceanique ; Ocean Rhelque ; Evolution varisque ; Nord-Ouest de l'Iberie IntrodnctionThe North-West (NW) of the Iberian Massif is located at the hinge zone of the Ibero-Armorican Arc ( Fig. 1) and preserves relicts of oceanic domains that once separated the Paleozoic continents [50,51]. A suture occurring in the hanging-wall of a large thrust system is rootless, which makes its interpretation difficult. However, the excellent exposure of the ophiolitic and associated allochthonous terranes permits the establishment of a sequence of emplacement, crosscutting relationships, and metamorphic gradients.In the absence of continental-scale strike-slip shear zones and faults, the Galician-northeru Portugal section is retrodeformable, permitting qualitative palinspastic reconstructions of the Gondwana-Laurussia conver gence. These characteristics make of NW Iberia a key site to uuravel the history of the Paleozoic plate evolution of the circum-Atlantic region, and specially that of the Rheic Ocean. This contrib...

A correlation between allochthonous units exposed in the NW Iberian Massif and the southern Armorican Massif is carried out based on lithological associations, structural position, age and geochemistry of protoliths and tectonometamorphic evolution. The units on both sides of the Bay of Biscay are grouped into Upper, Middle and Lower allochthons, whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the Parautochthon. The Lower Allochthon represents a fragment of the outermost edge of Gondwana that underwent continental subduction shortly after the closure of a Palaeozoic ocean which, in turn, is represented by the Middle Allochthon. The latter consists of supra-subduction ophiolites and metasedimentary sequences alternating with basic, mid-ocean ridge basalt (MORB)-type volcanics, with inheritances suggesting the proximity of a continental domain. Seafloor spreading began at the Cambro-Ordovician boundary and oceanic crust was still formed during the Late Devonian, covering the lifetime of the Rheic Ocean, which is possibly represented by the Middle Allochthon. The opening of the oceanic domain was related to pulling apart the peri-Gondwanan continental magmatic arc, which is represented by the Upper Allochthon.

The Careon ophiolite (Galicia, NW Iberian MassiO shows litho logical and geochemical features suggestive of an origin in a suprasubduction zone setting. As with other Devonian ophiolites in the European Variscan belt, it was generated within a contracting Rheic Ocean.This setting and the general absence of large Silurian-Devonian volcanic arcs on both of the Rheic Ocean margins strongly suggest that this ocean was closed by intraoceanic subduction directed to the north. This subduction removed the older normal (N) mid-oceanic-ridge basalt (MORB) oceanic lithosphere and gave rise to a limited volume of new suprasubduction zone oceanic lithosphere. The Careon ophiolite is a key element in understanding the evolution of the Rheic Ocean, which was the main oceanic domain that closed during the Paleozoic conver gence of Gondwana and Laurussia, preceding the assembly of Pangea.

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The tectonothennal evolution of a polyorogenic terrane in the Variscan belt of NW Spain has been constrained by 40 Ar/ 3 9 Ar laserprobe incremental heating experiments on mylonitic f � brics developed in major structures. Transitional levels between HP-HT and IP upper lUlits in the Ordenes Complex where metamorphic and structural records demonstrate two cycles of burial and exhlUllation were selected for dating. Two groups of ages have been defined: (1) Silurian-Early Devonian, obtained from mylonites of the Fomas extensional detachment, here considered as the minimlUll age for the start of tectonic exhlUllation of the HP HT lUlits and an upper age-limit for the HP-HT event itself; (2) Early to Mid-Devonian, from structures related to the Variscan convergence in the area, which include top-to-the-east thrusts and extensional detachments. A single, YOlUlger Carboniferous age obtained from the uppennost allochthonous sequences possibly reflects the final stages of emplacement of the allochthonous complexes. Our data indicate a polyorogenic character for a part of the Iberian allochthonous complexes, including Variscan (sensu stricto)and Early Variscan convergence, as well as an older, Early Palaeozoic cycle.

The allochthonous complexes of northwestern Iberia consist of a pile of units of Gondwanan and peri-Gondwanan provenance, and include oceanic lithosphere. The units are classiµed into upper, intermediate (ophiolitic), and basal. We present a detailed geological map and sections across the Ordenes Complex, together with a brief description of its units and a discussion of its structures.In the upper units, two complete cycles of burial and exhumation have been identiµed. The µrst cycle, of Early Ordovician age, records a convergent plate margin, possibly in a peri-Gondwanan domain. The second is Variscan, and the structural evolution of the three groups of allochthonous units re×ects progressive accretion to an active orogenic wedge. Continuous understacking of continental and oceanic fragments toward the west began with the upper units and ended with the basal units. The latter represent the outermost margin of Gondwana, and their subduction marked the closure of the intervening ocean, and the change from subduction to a collisional regime. Terrane accretion took place in the Devonian and, during the Late Devonian and the Carboniferous, the deformation progressed inboard of the Gondwana margin. Variscan emplacement of the allochthonous units occurred in two successive thrusting episodes. The µrst placed the basal units over the sedimentary cover of the Gondwana margin in what seems to follow a normal sequence of thrusting. The second carried the upper and ophiolitic units on top of the previous nappe pile and has an out-of-sequence character. A possible correlation of the Early Ordovician convergence, early Variscan accretion, subsequent oceanic closure, continent-continent collision, and renewed thrust activity during the late Carboniferous in northwestern Iberia is established with the Taconian, Acadian, and Alleghanian orogenies in the Appalachians.Figure 2. Geological map and composite cross section of northwestern Iberia, showing allochthonous complexes and their groups of units. For location, see Figure 1. IP, intermediate pressure; HP-HT, high-pressure-high temperature.

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