Multigrain concentrates of hornblende and muscovite together with whole-rock slate/phyllite samples have been dated Variscan deformation systematically prograded diachronously eastward across the orogen as new crustal material was added along the front of the developing orogenic wedge. However, the entire orogen remained tectonically active with different structural features forming at different times and at different places. An average propagation rate of ca. 5 kmlm.y. is suggested by consideration of a 20-25 Ma difference in correlative fabric ages and present separations.

A structural and metamorphic study was carried out in the basal units of the Ordenes Complex in Spain, thought to represent a subducted part of the Paleozoic margin of Gondwana. According to their metamorphic evolution, this part of the margin was subducted at the onset of the Variscan Orogeny, becoming part of an accretionary complex developed below a colliding element built previously. Variations in the PT conditions of the first high‐pressure metamorphic event along the units indicate a polarity of the subduction to the west. Subsequent underthrusting of more continental material blocked the subduction and triggered the ascent and exhumation of the basal units, whereas the convergence continued. Recumbent folds and thrusts developed along with successive normal detachments. Compressional and extensional structures were synchronous or alternated in time and together induced the thinning and tapering of the orogenic wedge and its lateral spreading. The unroofing took place locally under an inverted temperature gradient caused by a detachment which carried a part of the hot mantle wedge above the subduction zone over the subducted units.

The allochthonous terranes of northwest Iberia can be correlated with specific pale ogeo graphic realms of the continental masses and intervening oceans involved in the Variscan colli sion. Assuming that the existing ophiolites represent the suture formed by the closure of the Rheic ocean, the units in the fo otwall to the suture correspond to the outer edge of the Good waDa continental margin, and the units in the hanging waD are interpre ted as fragments of the conjugate margin, represented by the Me guma terrane. This correlation establishes a precise link between circum-Atlantic terranes, and makes it possible to draw a relatively simple sce nario of the successive tectonothermal events recorded. Following the amalgamation of Avalon to LaUl'entia, the remaining outboard terranes underwent a progressive accretion to this conti nent that ended with the collision between Laurentia and Gondwana.

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...

In situ uranium-lead dating (LA-SF-ICP-MS and SIMS) and Lu-Hf isotope analyses (LA-MC-ICP-MS) of zircon from eclogite facies rocks from the basal units of the Variscan Belt in Galicia constrain their magmatic and metamorphic evolution andgive some clues about the nature and origin of the involved basement. The samples studied are two felsic gneisses, two eclogites, and one eclogitic gneiss of intermediate composition (metatonalite). Oscillatory-zoned zircon cores from the felsic samples gave a main clustering of U-Pb ages at 493 ± 2 and 494 ± 2 Ma, and some older ages that represent inherited cores. Zircon grains from the intermediate and one of the mafi c rocks show no inherited cores and yielded ages of 494 ± 3 and 498 ± 6 Ma, respectively, interpreted as time of protolith crystallization. Variably developed homogeneous zircon rims in one felsic gneiss yielded an age of 372 ± 3 Ma, and very tiny zircons of one eclogite gave 350 ± 2 Ma, both of which we interpret as metamorphic ages.The new age data demonstrate that the calc-alkaline magmatic suite described in the basal unit is ca. 20 Ma older than the alkaline to peralkaline plutonic suite of the same unit (dated at 472 ± 2 Ma; Rodríguez et al., 2007), and thus probably represents a distinct geologic event. Overgrowth rims are interpreted as metamorphic on the basis of their Lu/Hf and Th/U ratios. The 372 ± 3 age is considered as dating the high-pressure (high-P) metamorphism, and is essentially in agreement with previous Ar-Ar and Rb-Sr data. This high-P metamorphism marks the initial early-Variscan subduction of the Gondwana margin. The inherited zircon ages and Hf isotopic composition of zircons point to a considerable input of crustal material with West African Craton provenance to the felsic magma.

No abstract

The Santiago Schists are located in the Basal Unit of the Ordenes Complex, one of the allochthonous complexes outcropping in the inner part of the Hercynian Belt in the north-west of the Iberian Peninsula. Their tectonothermal evolution is characterized by the development of an eo-Hercynian metamorphic episode (c. 374 Ma) of high-P, low-to intermediate-T. The mineral assemblage of the high-P episode is preserved as a very thin S i = S 1 foliation included in albite porphyroblasts, being composed of: albite + garnet4 + white mica-1 + chlorite-1 + epidote + quartz + rutile i ilmenite. The equilibrium conditions for this mineral assemblage have been estimated by means of different thermobarometers at 495 i 10 "C and 14.7 f 0.7 kbar (probably minimum pressure). The later evolution (syn-D2) of the schists defines a decompressive and slightly prograde P-T path which reached its thermal peak at c. 525 f 10 "C and 7 kbar. Decompression of the unit occurred contemporaneously with an inversion of the metamorphic gradient, so that the zones of garnet-11, biotite (with an upper subzone with chloritoid) and staurolite developed from bottom to top of the formation.The estimated P-T path for the Santiago Schists suggests that the Basal Unit, probably a fragment of the Gondwana continental margin, was uplifted immediately after its subduction at the beginning of the Hercynian Orogeny. It also suggests that the greater part of the unroofing history of the unit took place in a context of ductile extension, probably related to the continued subduction of the Gondwana continental margin and the contemporaneous development of compensatory extension above it. The inverted metamorphic gradient seems related to conductive heat transferred from a zone of the mantle wedge above the subducted continental margin, when it came into contact with the upper parts of the schists along a detachment, probably of extensional character.The general metamorphic evolution of the Santiago Schists, with the development of h i g h 2 assemblages with garnet prior to decompressive and prograde parageneses with biotite, is unusual in the context of the European Hercynian Belt, and shows a close similarity to the tectonothermal evolution of several high?, low-to intermediate-T circum-Pacific belts.

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.