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by Ricardo Arenas and Sonia Sánchez Martínez

Variscan ophiolites in NW Iberia: Tracking lostPaleozoic oceans and the assembly of PangeaDepartamento de Petrología y Geoquímica e Instituto de Geociencias (UCM, CSIC), Universidad Complutense, 28040 Madrid, Spain.E-mail: arenas@geo.ucm.es

DOI:10.18814/epiiugs/2015/v38i4/82427

In the Galicia Region of the NW Iberian Massifseveral allochthonous complexes (Cabo Ortegal,Órdenes and Malpica-Tui) contain a rootless Variscansuture that can be traced along the belt, from Iberiato the Bohemian Massif in Central Europe. Withinthese allochthonous complexes are several ophiolitezones bounded by two different continental terranes.There exist in NW Iberia two different ophiolite groupswith different chemical compositions, isotopic signaturesand structural positions. The Bazar and Vila deCruces ophiolites, characterized by c. 500 Ma protolithages, represent the Lower Group, whereas the Careón,Purrido and Moeche ophiolites containing 395 Mamaficultramafic sequences represent the Upper Group.This younger group constitutes the most widespreadophiolites in the Variscan Belt. A thick serpentinitemélange (Somozas Mélange) occurring at the baseof the Cabo Ortegal Complex also belongs to theophiolite zones of the Variscan suture. In thispaper we describe the Galician ophiolites of theVariscan suture and discuss their tectonic setting offormation. We interpret the generation of the Galicianophiolites within the geodynamic and paleogeographicevolution of the Rheic Ocean and the Pangeasupercontinent.

IntroductionIn the Variscan Belt, a long suture zone is outlined by thick

sequences of mafic and ultramafic rocks that are interpreted asophiolites. These formed during the closure of at least one oceanicdomain during the main stage of Pangea assembly. They are includedin allochthonous complexes of several exotic terranes that were, insome cases, affected by high pressure metamorphic events (MartínezCatalán et al., 2007; Fig. 1). It is accepted that the Rheic Ocean wasthe main oceanic domain closed during the collision of Gondwanaand Laurussia in Late Devonian and Carboniferous times (Matte,

2001; Stampfli and Borel, 2002; Winchester et al., 2002) but other,minor, oceanic domains also participated in the formation of Pangea(Arenas et al., 2014a). The Rheic Ocean was the largest ocean duringthe Paleozoic, in Middle Cambrian-Early Ordovician times, formedduring the rifting and later drifting of Avalonia and other minor terranesaway from Gondwana (Murphy et al., 2006; Nance et al., 2010; vanStaal et al., 2012).

Several recent papers have described the lithologies, chemicalcompositions and isotopic geochronology of the ophiolites of NWIberia (Arenas et al., 2007a; Sánchez Martínez et al., 2009). It is nowwell-known that their igneous protoliths correspond to two age groupswhich cannot be linked to one stage in the evolution of a single oceanicdomain. These ophiolites provide information about the oceanicdomains located along the periphery of Gondwana in pre-Ordoviciantimes as well as the oceanic domains involved in the final assemblyof Pangea. Two groups of ophiolites can be distinguished: an oldergroup containing Cambrian meta-igneous rocks while a secondyounger group includes Devonian gabbroic rocks. Moreover, a thickserpentinite mélange forms part of these groups of mafic-ultramaficunits.

The long Variscan-Appalachian-Alleghenian Orogen, whichextends through parts of Europe and eastern North America, containskey information for reconstructing the amalgamation history of thePangea supercontinent. In the Variscan Belt, the oldest tectono-thermalevents are preserved in the complex suture zone that can be tracedfrom the Iberian Peninsula to the Bohemian Massif (Fig. 1). Twodifferent events of high pressure metamorphism appear to haveoccurred, relatively close in time although separated by thedevelopment of oceanic basins. This evolution is unusual in largecollisional belts in which tectono-thermal evolution is commonlyinterpreted as reflecting a single high-P (HP) or ultra-high-P (UHP)metamorphic event associated with subduction of one of the collidingcontinental margins (Platt, 1986; Beaumont et al., 2009). In theVariscan Belt, both HP events and the development of some of theoceanic domains occurred after the earliest Devonian and are thusbroadly coeval with the initial stages of the assembly of Pangea.

This paper presents a short overview of the features and origin ofthe ophiolites along the Variscan suture of NW Iberia, and provides aconceptual model within the context of Pangea assembly. Theseophiolites are interpreted in relation to the origin and tectonothermalevolution of the other allochthonous terranes in the region. Thegeological section exposed in the NW Iberian Massif is taken asan example, but allochthonous terranes are fairly continuous along

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the suture and are largely comparable throughout the EuropeanVariscan Belt (Faryad and Kachlik, 2013; Kroner and Romer, 2013;Ballèvre et al., 2014; Von Raumer et al., 2015). Recent isotopicand geochronological data on the origin of these ophiolites andU-Pb geochronological constraints on the HP events provide newinsights into the early events involved in the formation of Pangea.We infer that the history of convergence and collision was probablylonger and more complex than previously described, and thatthe origin and evolution of the most common ophiolites in theVariscan belt were related to the amalgamation of the supercontinentPangea.

Terranes incorporated into the Variscansuture of NW Iberia

The NW Iberian section of the Variscan Belt contains terraneswith contrasting origins and tectono-thermal evolutions (Arenas etal., 1986; Martínez Catalán et al., 2009). The Central Iberian Zone isthe lowest sequence and, together with a parautochthonous domain(Parautochthon or Schistose Domain), defines the main section ofthe Gondwanan margin involved in the Variscan Orogen (MartínezCatalán et al., 2009) (Figs. 1 and 2). Above that sequence, a set ofterranes of allochthonous and alleged exotic origin forms a nappe

stack representating the rootless suture zone (Figs. 1 and 2). Theorigins of the allochthonous complexes have been extensivelydiscussed (Martínez Catalán et al., 2009). Three main groups ofterranes have been identified, two of which show continental crustalaffinities (Basal and Upper Units). These are separated by ophiolitesrepresenting the suture itself (ophiolitic units; Fig. 2).

Located immediately below the suture, the Basal Units containmetasedimentary rocks (comprised of a thick pile of metagreywackeswith minor metapelites, graphitic schist, calc-silicate lenses,metacherts and quartzites), calc-alkaline to alkaline-peralkalinemetagranitoids, and some mafic rocks. Maximum depositional agesfor the metasedimentary series range between Ediacaran and EarlyOrdovician (Díez Fernández et al., 2010, 2012a) with Nd model agesof between 1.78 and 2.22 Ga (Fuenlabrada et al., 2012). Major andtrace element geochemistry of the metagreywackes suggests depositionin association with a peri-Gondwanan arc system built upon a thinnedcontinental margin. Calc-alkaline (c. 493 Ma; Abati et al., 2010) andalkaline-peralkaline (c. 475-470 Ma; Díez Fernández et al., 2012b)granitoids were generated within this arc, suggesting a change overtime from convergence to continental rifting. The Basal Units areconsidered to represent a section of the most external margin ofGondwana originally located somewhere between the West Africanand Saharan cratons (Díez Fernández et al., 2010). The first tectono-thermal event recorded in these units is a HP and low- to intermediate-

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Fig.1. Terranes and oroclines of the Variscan belt (Martínez Catalán, 2011). Arcs: BA, Bohemian; CIA, Central Iberian; IAA, Ibero-Armorican; MCA, Massif Central. Zones of the Iberian Massif: CIZ, Central Iberian; CZ, Cantabrian; GTMZ, Galicia-Trás-os-Montes;OMZ, Ossa-Morena; SPZ, South Portuguese; WALZ, West Asturian-Leonese. Shear zones and faults: BCSZ, Badajoz-Córdoba; JPSZ,Juzbado-Penalva; LPSZ, Los Pedroches; NPF, North Pyrenean; PTSZ, Porto-Tomar; SISZ, Southern Iberian. Location of the geologicalmap and section presented in Fig. 2 is also shown.

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T (LIT) event dated at c. 370 Ma (Rodríguez et al., 2003; Abati et al.,2010). A variety of HP mica schists and orthogneisses, C-type eclogites(Coleman et al., 1965) and some blueschists were formed at that time(Arenas et al., 1995, 1997; Rodríguez et al., 2003; López Carmona etal., 2013, 2014).

Resting above this suture zone, the Upper Units consist of a 10-12 km thick pile of: metasedimentary rocks (mainly metagreywackes)and large massifs of calc-alkaline orthogneisses, and gabbros, withcompositions of island-arc tholeiites, together with medium to high-grade mafic rocks, including B-type eclogites (Coleman et al., 1965),HP granulites, and some ultramafic massifs. The structurally highestlow grade metagreywackes have a Middle Cambrian maximumdepositional age (Fernández-Suárez et al., 2003), with Nd model agesranging between 0.72 and 1.22 Ga, and major and trace elementcompositions typical of active margin settings (Fuenlabrada et al.,2010). Protolith ages for the gabbros and granitoids range between490 and 520 Ma (Fernández-Suárez et al., 2007; Andonaegui et al.,2012). These units were probably part of a Cambrian peri-Gondwananmagmatic arc located in the periphery of the West African Craton, tothe West of the external margin section represented by the Basal Units(Díez Fernández et al., 2010; Fuenlabrada et al., 2010; Albert et al.,2015). The Upper Units may be divided into two groups based onmetamorphic criteria. An uppermost section shows intermediate-P(IP) metamorphism ranging from chlorite to granulite facies. The maintectono-thermal events recorded in this uppermost section areCambrian in age and were probably developed in response to theaccretionary dynamics of the peri-Gondwanan arc system (Abati etal., 1999, 2007; Díaz García et al., 2010). A lower section ischaracterized by HP and high-T (HT) metamorphism dated at c. 400-390 Ma (Ordóñez Casado et al., 2001; Fernández-Suárez et al., 2007).The latter metamorphic event reached UHP conditions in otherdomains of the Variscan belt (Lardeaux et al., 2001).

The ophiolites of NW Iberia consist of a diverse group of mafic-ultramafic sequence making up five different ophiolites and aserpentinite mélange (Fig. 2). Two groups of ophiolitic units havebeen distinguished (Fig. 2): an older group (Lower Ophiolitic Units)containing meta-igneous rocks of Late Cambrian age (c. 497-495Ma), and a younger group (Upper Ophiolitic Units) including gabbroicrocks of Devonian age (Emsian-Eifelian; c. 395 Ma). The LowerOphiolitic Units are interpreted as a series of mafic complexes linkedto the dynamics affecting the most external margin of Gondwana inCambrian-Early Ordovician times. The Middle Devonian ophiolitesare the most abundant group found in the Variscan suture (Díaz Garcíaet al., 1999; Murphy et al., 2011; Arenas et al., 2014b). The origin ofthis group of ophiolites has been the subject of different interpretations.The most relevant features of the Galician ophiolites are described inthe following sections.

Ophiolitic unitsAccording to a typical Wilson Cycle, a collisional orogen should

contain a single ophiolitic belt, representative of the consumed oceanicdomain. However, it is known that preservation of ancient N-MORBtype oceanic lithosphere is extremely difficult, since its thermalstructure favors complete elimination by subduction. In contrast, manyof the ophiolites emplaced in orogenic belts are sequences typicallygenerated in supra-subduction zone settings, either during the laststages of the closure of the ocean or during the opening of more-or-less ephemeral basins (Leitch, 1984; Pearce et al., 1984; Dilek and

Furnes, 2011). Ophiolitic belts generated in connection with pull-apart ephemeral basins have also been described (Murphy et al., 2011).These buoyant oceanic lithospheres show greater resistance tosubduction and can be easily thrust over continental margins instead(Murphy et al., 2011). Moreover, the dynamics of the collision andthe eventual development of a high rate of lateral convergence mayallow the incorporation of oceanic sections of different ages andcharacteristics into the orogen. Hence, the real dynamic context maygenerate situations that tend to increase the variety of ophiolitic beltsincorporated into an orogen which rarely contains a simple ophiolitegenerated at one particular time. In the Variscan Belt recent studieshave described a variety of ophiolites with significant differences inlithological constitution and age. Such diversity responds to a varietyof dynamic settings of generation. Proper understanding of thesewould improve knowledge of paleogeographic evolution in pre-Variscan and Variscan times.

In the NW Iberian Massif, a set of tectono-stratigraphic unitscomprising mafic and ultramafic lithologies, and almost devoid ofmetasedimentary rocks, were interpreted as ophiolites included inthe allochthonous complexes (Arenas et al., 1986). Two groups ofophiolitic units are distinguished in Galicia (Arenas et al., 2007a)(Fig. 2), which can be generally correlated with equivalent groupsalong the entire belt. The Upper Ophiolitic Units are Devonian in ageand according to their chronology they represent the most commontype of ophiolites in the Variscan suture (Díaz García et al., 1999;Murphy et al., 2011; Arenas et al., 2014b). Ophiolites of this agehave also been described in Cornwall (Lizard Ophiolite; Clark et al.,1998; Nutman et al., 2001), Armorican Massif (Drain Ophiolite;Ballèvre et al., 2009, 2014) and the Bohemian Massif (Sleza Ophiolite;Dubinska et al. 2004; Kryza and Pin 2010). These ophiolites havebeen traditionally considered to be related to the evolution of theRheic Ocean, their origin coinciding with the final stages of itsdevelopment (Díaz García et al., 1999; Sánchez Martínez et al.,2007a). However, new geochronology and isotope geochemistry dataquestion the relationship of the Devonian ophiolites with this ocean,suggesting that they are more likely to be related to the opening ofminor ephemeral oceanic domains (Arenas et al., 2014a). The LowerOphiolitic Units include Cambrian mafic-ultramafic sequences whichdeveloped in different dynamic contexts. Their origin may be relatedeither to the opening of the Rheic Ocean (Arenas et al., 2007b) or toa geodynamic regime that occurred prior to the opening of this ocean(Sánchez Martínez et al., 2012).

In the NW Iberian Massif in Galicia, five ophiolitic units havebeen delineated. The Lower Ophiolitic Units comprise the Vila deCruces and Bazar ophiolites while the Upper Ophiolitic Units arerepresented by the Careón, Purrido and Moeche ophiolites. A thickmélange of serpentinite, the Somozas Mélange, can be included amongthe ophiolitic and related sequences of NW Iberian Massif.. Otherdifferent ophiolitic units of similar age and setting have also beendescribed in the neighboring region of Trás-os-Montes, Portugal (Pinet al., 2006). The ophiolitic units of Galicia will be describedseparately, as they show different lithologic constitutions and tectono-thermal evolutions.

Lower Ophiolitic Units

Bazar Ophiolite

The Bazar Ophiolite, located in the westernmost part of the

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Fig.2. (a) Geological map and cross section of the allochthonous complexes of the NW Iberian Massif (Galicia region). These show the distribution and general structure of the terranesinvolved in the Variscan suture. The locations and names of the ophiolites are also indicated. (b) Schematic columns showing the lithological constitution of the ophiolites from Galicia.

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Órdenes Complex (Fig. 2), consists of an imbricate of tectonic slices,c. 5000 m thick, containing metagabbroic rocks and a minor proportionof ultramafic rocks at the base of the ophiolite (Díaz García, 1990).The main tectonic slice (Carballo-Bazar; Sánchez Martínez et al.,2009, 2012; Fig. 2) is c. 4000 m thick and is composed of amphibolitesand foliated metagabbros with an HT foliation, which evolved froman initial granulite-facies tectono-thermal event (Fig. 3). Scarce metre-sized boudins of mafic granoblastic granulites are preserved withinthe metagabbros (Fig. 3). They are wrapped by the HT foliation andtheir mineral association is transitional between low and intermediatepressure conditions (plagioclase-clinopyroxene-orthopyroxene-hornblende-ilmenite±garnet±olivine). The lower part of the main sliceconsists of relatively well-preserved gabbros and ultramafic rocks,with minor leucogabbros and tonalites (Fig. 2). The geochemicalfeatures of the most representative lithologies of the Bazar Ophioliteare variable. The common amphibolites, which are the most abundantlithological type, and the metagabbros, show compositions equivalentto island-arc tholeiites or N-MORB (Fig. 4a). However, the maficgranulites seem to be transitional between MOR (mid-ocean ridge)and WP (within-plate) basalts, with normalized trace element patternssimilar to those of T-MORB generated by plume-ridge interactions(Pearce, 1996) (Fig. 4a).

Zircon grains obtained from an amphibolite sample have aninternal structure characterized by the presence of a magmatic coresurrounded by a very irregular overgrowth rim (Fig. 5a). Their analysis

yielded two different groups of U-Pb concordant ages (Fig. 5b) whichare significant according to their contrasting 176Yb/177Hf ratios (Fig.5c). The first group corresponds to an average age of 495 ± 2 Mainterpreted as the intrusive age of the gabbroic protoliths. The secondgroup yields an average age of 475 ± 2 Ma and is interpreted to datethe high-T metamorphic event (Sánchez Martínez et al., 2012). TheHf isotope composition of the zircons from this amphibolite samplereveals the juvenile signature of the mafic protoliths of this unit anda lack of interaction with old crustal components (Fig. 5d). Thesefeatures, combined with the trace element composition of the maficrocks, suggest that the protoliths of the Bazar Ophiolite were generatedin an oceanic setting, although the mafic rocks are compositionallyheterogeneous.

The compositional features, age and tectono-thermal evolutionof the Bazar Ophiolite indicate that it may represent a lithosphericsection of the Cambrian peri-Gondwanan ocean. The specific dynamicsetting in which the mafic rocks originated is uncertain, but thepresence of metagabbros with N-MORB composition suggests that itinvolved the participation of a mid-ocean ridge. The oceaniclithosphere represented by the Bazar Ophiolite should have beenbuoyant, given the small time difference (c. 20 Ma) between the agesof protoliths and HT metamorphism, and was accreted under theperi-Gondwana system of magmatic arcs. The development of a low-medium-P granulitic metamorphism is compatible with overheatingassociated with the base of the magmatic arc, but could also be caused

a b

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Fig. 3. Field and petrographic aspects of the Bazar Ophiolite. (a) Coarse-grained gabbro. (b) Strongly sheared metagabbro. (c) Low tointermediate-P mafic granulite composed of clinopyroxene, orthopyroxene, plagioclase, brown hornblende, biotite and very thin garnetcoronas, occasional olivine crystals can also appear. (d) HT amphibolite developed after the granulites. It shows a first generation of brownhornblende subsequently replaced by blue-green amphibole.

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A = N-MORB - B = E-MORB and within-plate tholeiites - C = alkaline within-plate basaltsD = destructive plate-margin basalts: island-arc tholeiites (Hf/Th>3.0) and calc-alkaline basalts (Hf/Th<3.0)

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Fig. 4. Th-Hf-Ta diagrams (Wood, 1980) and N-MORB-normalized trace-element patterns (selected elements and normalizing valuesaccording to the criteria of Pearce, 1996) for the most representative lithologies of the ophiolites included in the allochthonous complexesof Galicia. (a) Bazar Ophiolite. (b) Vila de Cruces Ophiolite. (c) Somozas Mélange. (d) Careón Ophiolite. (e) Purrido Ophiolite. (f) MoecheOphiolite.

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by the subduction of a section of a mid-ocean ridge and the openingof an ephemeral asthenospheric window (Fig. 6).

Vila de Cruces Ophiolite

Located SE of the Órdenes Complex, the Vila de Cruces Ophiolitehas a complex structure characterized by the presence of at least twosuperimposed tectonic slices with a total thickness of c. 4000 m (Fig.2). The main lithologies are greenschist rocks of possible metavolcanicorigin, alternating with abundant layers of phyllites, mica schists andschists with albite and garnet porphyroblasts, and scarce metachertbands. Lenticular intercalations of metagabbros and two bodies oftonalitic orthogneisses, transitional to metagabbroic types, can alsobe identified. Thin serpentinite bands occur along the contact betweenthe main slices. Immobile trace element abundances of both the mafictypes and the tonalitic orthogneisses are typical of magmas generatedat destructive plate margins (Fig. 4b). Greenschists and metagabbroshave compositions characteristic of island-arc tholeiites. The presenceof a strong negative Nb anomaly compared to N-MORB (Fig. 4b) isalso compatible with the supra-subduction zone setting proposed forthis ophiolite (Arenas et al., 2007b).

The orthogneisses from this ophiolite are dated at 497 ± 4 Ma(U-Pb in zircon; Arenas et al., 2007b). However this unit wasinterpreted as a composite terrane instead of a simple ophiolite(Sánchez Martínez, 2009; Sánchez Martínez et al., 2009) based onthe discovery of zircon populations of imprecise Mesoproterozoicages (1168–1176 Ma) in two samples of metagabbros. New U-Pbzircon dating has provided very consistent ages of c. 500 Ma in asample of metagabbros and three samples of orthogneiss (SánchezMartínez et al., 2013). That is now considered to be the igneousprotolith age of the Vila de Cruces Ophiolite. As in the UpperOphiolitic Units, discussed below, the presence of Mesoproterozoiczircons is considered to be evidence of interaction between the igneousprotoliths and the continental crust.

The Vila de Cruces Ophiolite has a penetrative regional foliation,which transects a previous schistose fabric. Both schistosities areassociated with different generations of mappable-scale recumbentfolds (Arenas et al., 2007b). The regional metamorphic conditionscorrespond to the greenschist facies, but they can reach amphibolitefacies or transitional conditions to that facies in the upper levels.There, the micaschists contain a mineral assemblage with garnet-chlorite-white mica but lack biotite, suggesting that the unit underwent

Fig. 5. U-Pb dating and Lu-Yb-Hf isotope data of a metagabbroic amphibolite sample from the Bazar Ophiolite. (a) Cathodo-luminiscenceimages of selected zircons, circles representing the spot size of U-Th-Pb analyses. (b) Concordia diagram showing the results of the U-Pbanalyses, with two statistically coherent groups of data. (c) 176Yb/177Hf versus apparent age diagram, the different trends of each group ofanalyses appearing encircled. (d) εεεεεHf versus apparent age plot; the depleted mantle array (DM) is extrapolated from average modern-dayvalues of mid-ocean ridge basalts (Chauvel and Blichert-Toft, 2001), assuming a linear behaviour from εεεεεHf = 2 at 4000 Ma (Vervoort andBlichert-Toft, 1999). Red symbols = zircon cores, blue symbols = zircon overgrowths-recrystallized domains. Modified after Sánchez Martínezet al. (2012).

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an initial HP metamorphic event. The age of the regional foliation isestimated at 363-367 Ma, according to 40Ar/39Ar dating of white micaconcentrates from two phyllite samples (Dallmeyer et al., 1997).

Because the chemical composition of the mafic and felsic rocksis typical of magmatic arcs, and the abundance of sedimentary rocks,the Vila de Cruces Ophiolite is considered to be a supra-subductionzone sequence generated in a Cambrian back-arc basin opened at theperiphery of Gondwana (Arenas et al., 2007b) (Fig. 6). It thereforerepresents a transitional crust domain where magmas of juvenilemantle provenance interacted with a thinned continental crust.

Upper Ophiolitic Units

Careón Ophiolite

This ophiolitic unit contains the best preserved meta-igneoussuccession in the NW Iberian Massif (Fig. 2). It consists of threeimbricated sheets that repeat the mantle-crust transition with a totalthickness of c. 1500 m (Figs. 7 and 8). The thickest slice is the c.1000 m thick Careón Sheet, which consists mainly of serpentinizedultramafic rocks and isotropic metagabbros, with abundant stocks of

pegmatoid metagabbros emplaced at all levels as well as scarcewehrlite sills. Diabase and late diabase dykes cut all exposedlevels, from the deepest mantle section to the most superficial crustalsections.

The lithological constitution of the Careón Ophiolite differssignificantly from the classical HOT and LOT ophiolitic types definedby Nicolas (1989, 1995) (Fig. 8). Therefore, it was interpreted as anophiolite generated in a supra-subduction zone setting by DíazGarcía et al. (1999). The intrusion of abundant diabase dikes at alllevels of the Careón Sheet suggests the development of this ophiolitevia extension and thinning, in the upper plate of a subductionzone. Immobile trace element abundances (Wood, 1980) (Fig. 4d)are typical of island-arc tholeiitites, and the youngest diabase dykeshave compositions transitional to N-MORB. All lithologies presentin the Careón Ophiolite display a negative Nb anomaly comparedto N-MORB, which is compatible with generation in an activesupra-subduction zone setting (Sánchez Martínez et al., 2007a)(Fig. 4d).

The Careón Ophiolite was the first ophiolite to be dated by isotopegeochronology in the NW Iberian Massif. U-Pb zircon dating of asample of pegmatoid metagabbro from the Careón Sheet yielded a

CAMBRO - ORDOVICIAN(500-480 Ma)

FUTURE UPPER UNITS

FUTURE BASAL UNITS AND

AUTOCHTHONUS SERIES

CONTINENTS

OCEANS

Iapetus Ocean

LAURENTIA

SouthAmerica

Africa

BALTICA

SIBERIATaconic Arc

Avalonia

GONDWANA

Gander Arc

Ocean

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ASTHENOSPHERIC MANTLE

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Future RheicOcean

Back-arc basin:Vila de Cruces

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500-490 Ma

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Continental rifting

Ridge subduction(asthenospheric window)

Bazar Ophiolite(high-T metamorphism)

490-480 Ma

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(a)

(c)

Fig. 6. A paleogeographic reconstruction (a) and interpretive tectonic cross sections of the Iapetus realm during the Cambro-Ordovician (band c). These diagrams show the tectonic setting for a time immediately prior to the opening of the Rheic Ocean. The Bazar Ophioliteprobably represents a remnant of the peri-Gondwanan oceanic lithosphere accreted below a volcanic-arc system, whereas the Vila deCruces Ophiolite contains the remnants of an incipient backarc basin. Based on Winchester et al. (2002), Arenas et al. (2007b), GómezBarreiro et al. (2007) and Sánchez Martínez et al. (2012).

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protolith age of c. 395 Ma (Díaz García et al., 1999). Similar U-Pbages were obtained later by analyzing zircons from a similarmetagabbro sample from the same sheet (Pin et al., 2002). The threeimbricated sheets have a penetrative foliation with evidence ofincreasing temperature towards the top of each sheet, reaching thehighest temperatures close to the contact with the ultramafic rocks ofeach overlying sheet. There, the gabbroic rocks were transformedinto garnet amphibolites. There are also higher temperaturemetamorphic soles with very local development of metre-scalecorundum-rich layers (Fig. 8). The P-T conditions reached duringthe deep imbrication of the ophiolitic sheets are estimated as 11.5Kbar and 650 °C in the garnet amphibolites from the upper part ofthe Careón Sheet (Fig. 8). The mineral lineation in the amphiboliteshas a consistent E-W orientation, with an associated top-to-the-eastsense of shearing (Gómez Barreiro et al., 2010). The imbricationoccurred at c. 377 Ma, according to the 40Ar/39Ar dating of hornblendesfrom the amphibolitic foliation (Dallmeyer et al., 1997).

The Careón Ophiolite was initially interpreted as having beengenerated in an intra-Rheic Ocean supra-subduction zone (Díaz Garcíaet al., 1999; Sánchez Martínez et al., 2007a), beneath which wassubducted almost all of the old dense oceanic lithosphere of the RheicOcean. Thus, common N-MORB compositional types have not beenclearly described so far in the Variscan Belt. The youngest lithospheregenerated in this intra-Rheic subduction zone at c. 395 Ma wouldhave been buoyant, and was accreted shortly after its formation at c.377 Ma, below the southern margin of Laurussia. These are now themost common ophiolites in the Variscan suture. This initialinterpretation will be discussed in the following sections in the lightof new data obtained from the other Upper Ophiolitic Units.

Purrido Ophiolite

Located at the westernmost limit of the CaboOrtegal Complex, the Purrido Ophiolite consists of c.300 m of monotonous amphibolites and garnet-bearingamphibolites of metagabbroic origin (Figs. 2 and 9).Unlike the Careón Ophiolite, well preserved igneousfeatures are not seen but the amphibolite types thatappear within both units are very similar frommineralogical and compositional points of view, soboth of these ophiolites have been traditionallycorrelated (Sánchez Martínez et al., 2007b). The maficrocks have compositions typical of island-arctholeiites, with slight negative Nb anomalies (SánchezMartínez et al., 2007b) (Fig. 4e).

U-Pb dating of an amphibolite with a dominantzircon population of c. 1160 Ma was originallyinterpreted as the age of the gabbroic protolith butwith minor populations ranging between 1.2-1.6 Gaconsidered to be inherited grains (Sánchez Martínezet al., 2006). However, subsequent U-Pb zircon datingof two amphibolite samples provided a singleconsistent age at c. 395 Ma, interpreted as the actualprotolith age thus coinciding with the age previouslyobtained in the Careón Ophiolite (Sánchez Martínezet al., 2011). The Lu-Hf isotope geochemistry inzircons from this ophiolite demonstrate complexevolution, clearly indicating participation of Paleo-Proterozoic isotopic sources and mixing with juvenilematerial at c. 395 Ma (Fig. 10). Whole rock Sm-Nd

a b

c

Fig. 7. Field aspects of the Careón Ophiolite. (a) Layered ultramafic rocks. (b, c)Complex intrusive relationships with local development of mingling processes.

isotope geochemistry of the amphibolitic rocks also indicatesimportant isotopic heterogeneity of the mafic material (Murphy andGutierrez-Alonso, 2008; Sánchez Martínez et al., 2011). The Purridoamphibolites show intense plano-linear foliation development inthe amphibolite facies which has been dated at 391.3 ± 6.6 Ma(40Ar/39Ar in hornblendes; Dallmeyer et al., 1997).

The geochemical and geochronological data provided by thePurrido Ophiolite are interpreted as reflecting interaction betweenDevonian gabbroic magmas and continental crust. This interpretationcalls into question the initially proposed origin for the Upper OphioliticUnits in an intra-Rheic Ocean subduction zone, seeming to indicate adifferent tectonic and paleogeographic setting. This new interpretationwill be discussed below in the context of a new model for the originand tectono-thermal evolution of the terranes incorporated in theVariscan suture.

Moeche Ophiolite

The Moeche Ophiolite crops out widely in the eastern part ofCabo Ortegal Complex (Fig. 2). The Moeche and Purrido ophiolitesare in contact in a small coastal section to the West of Cedeira village,where the Moeche Ophiolite occupies the lower structural position.This ophiolitic unit consists of c. 500 m of greenschists with abundantintercalations of phyllites and mica schist and scarce metagabbrosand serpentinites (Fig. 9). Th-Hf-Ta plots for the metabasites (Wood,1980) indicate chemical compositions transitional between N-MORBand island-arc tholeiites. They show a slight enrichment of traceelement abundances relative to N-MORB (Sánchez Martínez et al.,2007b) (Fig. 4f).

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U-Pb dating of zircons from a mylonitic greenschist yield an ageof 400 ± 3 Ma (Arenas et al., 2014b) which is comparable to the agesobtained in Careón and Purrido ophiolites. Lu-Hf isotope signaturesof the zircons clearly indicate contributions from a continental source.εHf values in the analyzed zircons are negative (generally belowεHf = -5), and hence they are not compatible with crystallizationfrom a juvenile mantle source (Fig. 10). Isotopic Hf data from thezircons of the Moeche Ophiolite are similar to those from the PurridoOphiolite. These data are again incompatible with formation in anoceanic setting far from continental domains.

The internal structure of the Moeche Ophiolite is poorly known.It displays a regional foliation in greenschist facies conditions whichgenerally has mylonitic-ultramylonitic character. This hinders the

recognition of the original nature of the mafic igneous protoliths whichhave been regarded as both basaltic and gabbroic. The regionalfoliation is axial planar to tight outcrop-scale recumbent folds. 40Ar/39Ar dating of the mylonitic foliation in the phyllite intercalationshas provided an age of c. 364 Ma (whole rock analysis; Dallmeyer etal., 1997), which is slightly younger than the other regional fabricsdated in the Upper Ophiolitic Units.

Somozas Mélange

The lowest part of Cabo Ortegal Complex includes a thick tectonicmélange which is only represented in this part of the allochthonouscomplexes of the NW Iberian Massif (Fig. 2). This tectonic mélange

CAREÓN UNIT

Ultramafic rocks

Pegmatoid metagabbro

Felsic rocks

Wehrlite sills

Diabase

Serpentinite mylonite

Late diabase

Metamorphic sole

Orosa sheet amphibolites

Garnet amphibolite

Corundum

Isotropic metagabbro

Late pegmatoid metagabbro

Detachment fault

Folded and shearedultramafic lenses

Amphibolite

Epidote amphiboliteGreenschist

High-T foliation

Porphyroclasticperidotite

Serpentinite mylonite

Mylonitic greenschistfacies metabasite

Granonematoblasticamphibolite

Nematoblasticamphibolite

Coarse equant

Mainly undeformedmetagabbro

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THy

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0

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01000

oC

oC

oC

600

oC

Ar/Ar376.8Ma

U-Pb395 Ma

1.15 GPa650 ºC

Fig. 8. Synthetic column of the Careón Ophiolite showing the main lithologies and imbricates, distribution of metamorphic facies, textures,levels where dynamic recrystallization of main components occurred, and the saw-tooth thermal gradients developed during the earlieststages of thrusting. The U-Pb age of the gabbroic protoliths, 40Ar/39Ar age of hornblende in metamorphic soles and the peak P-T conditionsreached during the accretion are also shown. Modified from Díaz García et al. (1999).

CAREÓN UNIT

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is located in the advancing front of the large thrust sheet representedby the allochthonous complexes. The Somozas Mélange was initiallydescribed by Arenas et al. (1986), Arenas (1988) and Marcos et al.(2002). More recently, Arenas et al. (2009) presented a detailed mapof the mélange in addition to whole-rock geochemical data of theigneous rocks and some U-Pb zircon ages.

The Somozas Mélange is better exposed in the southern part ofCabo Ortegal Complex, where it comprises a c. 1800 m thick unitcomposed of two contrasting members (Fig. 2). The upper member isc. 800 m thick and is a mélange with a strongly sheared serpentinite

matrix. This matrix surrounds tectonic blocks ranging from metre-hectometre to kilometre sizes. The tectonic blocks consist ofgabbro, diabase, granitoid, metabasalt, andesitic basalt, pillow breccia,pillow lava, hyaloclastite, marble, phyllite, sandstone andconglomerate, together with HT metamorphic rocks includingorthogneisses, common amphibolite and zoisite-rutile rich metabasites(Fig. 11). Many of these tectonic blocks are exotic: their source rockshave not been identified in the allochthonous complexes or theParautochthon. The lower member is c. 1000 m thick and it iscomposed of a mélange with a matrix of ochre colored phyllites or

a b

c d

e f

Fig. 9. Field aspects of the Purrido and Moeche ophiolites. (a) Exposures of the Purrido Ophiolite in the western coastal sections of theCabo Ortegal Complex (foreground). (b) Banded Purrido amphibolites in Baleo beach; these show thick intra-foliation folds. (c) MoecheOphiolite exposed in San Vicente Island, in the eastern section of the Cabo Ortegal Complex. (d, e, f) Typical mylonitic greenschists of theMoeche Ophiolite in San Vicente island. Observe the large recumbent folding present in this unit, well shown in photograph (e).

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blue colored phyllonites surrounding tectonic blocks of the lithologiesincorporated into the serpentinite mélange. No HP rocks have beendescribed so far in the Somozas Mélange, although some of the high-T tectonic blocks reached at least the highest pressure conditions of

the amphibolite facies. However, a detailed study of all mineralassemblages has not yet been made.

U-Pb zircon dating was undertaken on a tonalitic orthogneissfrom a large high-T tectonic block and on two few deformed granitoidsaffected by low-grade recrystallization (Arenas et al., 2009). The HTorthogneiss yields an age of c. 485 Ma (SHRIMP), and the twogranitoids yield ages of c. 499 Ma (SHRIMP) and c. 527 Ma (LA-ICP-MS). Whole rock geochemistry of all the igneous rocks in themélange, whether volcanic, plutonic or dykes, points to a volcanicarc setting (Fig. 4c). A negative Nb anomaly is exhibited by alllithologies, a feature considered an indication of a supra-subductionzone setting (Fig. 4c). Furthermore, the U-Pb ages of detrital zirconpopulations present in a block of conglomerate indicate a maximumage of sedimentation at about the Early-Middle Ordovician boundaryand deposition at the paleo-margin of Gondwana with a typical West-African provenance (Arenas et al., 2009).

U-Pb geochronology and whole rock geochemistry data indicatethat the Somozas Mélange contains abundant materials derived froma peri-Gondwanan magmatic arc. This observation suggests that themargin of Gondwana itself might have participated in the formationof the mélange, although the exact time of mélange formation isunknown. However, the position of the mélange also suggests arelationship with the subduction and accretion that affected the BasalUnits. The upper section of the Somozas Mélange is a typicalserpentinite mélange. Its formation implies the existence of activesubduction for a protracted period, with generation of a low viscositychannel in the overlying mantle (Gerya et al., 2002; Hebert et al.,2009). The lower section of the mélange was developed later andrepresents an imbrication zone of the upper serpentinite melange withthe higher levels of the Parautochthon. Thus the time of formation ofthe Somozas Mélange can be roughly constrained between the age ofthe subduction, which affected the Basal Units (c. 370 Ma; Abati etal., 2010), and the age of the imbrication with the Parautochthon.That date is, at present, unknown, but logically should be similar toor slightly earlier than the age estimated for the basal thrust of theParautochthon above the Central Iberian Zone (c. 343 Ma; Dallmeyeret al., 1997).

Paleozoic oceans and the assembly ofPangea

Based on the U-Pb chronology obtained for their mostrepresentative lithologies, the ophiolites of NW Iberian Massif are oftwo main age groups: Cambrian (Lower Ophiolitic Units) andDevonian (Upper Ophiolitic Units). The Somozas Mélange is aseparate unit, the age of which is not known precisely. The mainOphiolitic Units (excepting the Bazar Ophiolite) show chemicalcompositions related to supra-subduction zones or pull-apart basins.This interpretation is also compatible with the lithological features ofthe best preserved ophiolitic sections, such as the Careón Ophiolite.Thus, the age of the ophiolites involved in the Variscan suture spansmuch of the Paleozoic; significantly, the time range between theopening and closure of the main Paleozoic ocean generated in theperi-Gondwana context (the Rheic Ocean). Moreover, the youngestophiolites are approximately coeval with the first tectono-thermalevents related to the assembly of Pangea. The Variscan suture thereforehas specific features that throw light on the relationship between theopening and closure of a large oceanic domain, and complex tectono-

-20

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T DM m

odel age

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Fig. 11. Field aspects of tectonic blocks in the Somozas Mélange.(a) Massive basaltic andesites. (b) Broken pillow-breccia. (c, d)Sheared pillow breccias with local presence of intrusive diabasedykes. (e) Strongly sheared pillow breccias. (f) Undeformed pillowbreccia with preserved large whole pillows.

Fig. 10. εεεεεHf versus age diagram combining the U-Pb and Lu-Hfisotope data of zircons from the Purrido and Moeche ophiolites.Crustal evolution trends (176Lu/177Hf = 0.0113) for the Archean,Paleoproterozoic and Mesoproterozoic crusts are also shown. Basedin Sánchez Martínez et al. (2011) and Arenas et al. (2014b).

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thermal episodes culminating in the finalassembly of a supercontinent.

The Upper Units of the allochthonouscomplexes have been interpreted as a section ofa peri-Gondwanan arc, rifted from the mainlandin Cambrian-Early Ordovician times during theopening of the Rheic Ocean (Fig. 12). Thatrifting should be contemporary with that of theAvalonia microcontinent (Murphy et al., 2006),although the terrane represented by the UpperUnits had a distinct identity and provenance fromfurther east along the paleomargin of Gondwana(Abati et al., 2007; Gómez Barreiro et al., 2007)(Fig. 12). The HP-UHP metamorphism affectingthe lower part of the Upper Units at c. 400-390Ma would have developed during the accretionof this terrane to the southern margin ofLaurussia and would have defined the changeto a convergent regime in the Rheic Oceandomain (Gómez Barreiro et al., 2007; SánchezMartínez et al., 2007a). The evolution proposedfor the Upper Units implies that the Variscansuture ophiolites were generated within theRheic Ocean realm. The Cambrian ophiolites,excepting the Bazar Ophiolite, would be linkedto the initial stages of the opening of this oceanreminiscent of the Western Alps Tethys (Dilekand Furnes, 2014; Balestro et al. 2015; Saccaniet al. 2015), whereas the Devonian ophioliteswould have originated during the final stages ofits closure (Arenas et al., 2007a) analogous tomany supra-subduction zone ophiolites (Dilek,2003; Dilek and Furnes, 2011; Yang et al. 2015).The Devonian ophiolites, the commonest in theVariscan suture, have been interpreted as supra-subduction zone ophiolites generated at the intra-Rheic subduction zone located near the Southernmargin of Laurussia (Díaz García et al., 1999;Sánchez Martínez et al., 2007a). This subductionzone would have generated buoyant oceaniclithosphere that would have accreted soon after,below the Upper Units then constituting theSouthern margin of Laurussia, and later thrustabove the most external margin of Gondwana

MIDDLE ORDOVICIAN465 Ma

CONTINENTS UPPER UNITS

OCEANS EUROPEAN VARISCAN BELT

TOR

NQ

UIS

TO

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N

G

ON

DWANA

IAPETUS OCEAN

Gander Arc

Taconic Arc

Laurentia

SouthAmerica

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Siberia

Avalonia

CAMBRO-ORDOVICIAN500-490 Ma

(a)

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IAPETUS OCEAN

Gander Arc

Taconic Arc

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OCEAN

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IA

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AN

Laurentia

Africa

Baltica

Siberia

Avalonia

EARLY SILURIAN440 Ma

(c)LAURUSSIA

GONDWANA

Laurentia

SouthAmerica

Africa

Baltica

Siberia

Anta

rctica

India

Ara

bia

Carolina

EARLY CARBONIFEROUS350 Ma

Avalonia

(d)

Fig. 12. Paleozoic paleogeography showing the distribution of terranes in the peri-Gondwanan realm and the main stages involved in the assembly of Pangea. This modelinterpreted the Upper Units of NW Iberia as a far traveled terrane drifted from Gondwanaand finally accreted to the southern margin of Laurussia later than Avalonia. The RheicOcean opened in Late Cambrian-Early Ordovician times and widened during drifting tothe North of Avalonia and the NW Iberia Upper Units. Based on Winchester et al. (2002),Arenas et al. (2007b), Gómez Barreiro et al. (2007)

it is not possible to assert that the suture zone they define was aconsequence of its closure. If the suture zone of NW Iberia is notlinked to the evolution of the Rheic Ocean, the main argument forconsidering the Upper Units as a terrane drifted to the North fromGondwana during the opening of this oceanic domain is not supported.However, there are also problems for the possible development of anepisode of HP-UHP metamorphism in a small peri-Gondwana terranecolliding with Laurussia. That type of tectono-thermal evolution isgenerally associated with deep subduction of the thinned margin of alarge continental landmass (Warren et al., 2008; Beaumont et al.,2009).

Regarding the HP-UHP event of the Upper Units, the existing U-Pb geochronological data indicate that it took place at c. 400-390 Ma(Ordóñez Casado et al. 2001; Fernández Suárez et al. 2007)approximately coincident with the age of the mafic rocks of the

(Basal Units), at the onset of the Variscan deformation (c. 370 Ma).The activity of the intra-Rheic Ocean subduction zone also wouldexplain the widespread absence in the Variscan Belt of N-MORB-type oceanic lithosphere typical of the Rheic Ocean, which wouldhave been completely eliminated by subduction.

It is implausivle, however, to interpret in this model the Devonianophiolites as remnants of oceanic lithosphere formed in open oceanicdomains. The new isotope geochemistry data indicate that the gabbroicprotolith interacted with ancient continental crust. Many existingzircons in mafic rocks of the Purrido and Moeche ophiolites show aLu-Hf isotope composition that is only compatible with a continentalorigin. Therefore, they must be interpreted as inherited zircons in themafic magma (Sánchez Martínez et al., 2011; Arenas et al., 2014b).However, there is no conclusive data to connect the generation of theDevonian ophiolites with the evolution of the Rheic Ocean, therefore

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Upper Ophiolitic Units, which has been repeatedly dated by U-Pbgeochronology at c. 400-395 (Díaz García et al., 1999; Pin et al.,2002, 2006; Sánchez Martínez et al., 2011; Arenas et al., 2014b). Forthe HP metamorphic event, the U-Pb geochronology may reflect theage of zircon growth that took place at high temperatures and thereforewould have occurred later than the onset of deformation and themetamorphism itself. The subduction affecting this section of thevolcanic arc would logically have started before 400-390 Ma, althoughit is not possible to know the precise age and, therefore, must haveoccurred prior to the generation of the Devonian ophiolites.

The recently obtained data, especially the Lu-Hf isotope datafrom the zircons of the Devonian ophiolites and detailed U-Pbgeochronology of the HP metamorphic events, favor an alternativeinterpretation to that initially proposed for the allochthonous terranesof the Variscan suture of the NW Iberian Massif. These data suggesttwo successive stages of collision between Gondwana and Laurussia,which took place in a context of dextral convergence, separated intime by the opening of a relatively wide, probably a pull-apart, oceanicbasin (Arenas et al., 2014a).

The Upper Units are considered the most external part of thecontinental platform of Gondwana. That platform was probably verywide and composed of a thick turbidite series intruded by large bodiesof gabbros and granitoids. Its origin was related to the activity of avolcanic arc during Cambrian-Early Ordovician times that later

underwent strong extension and subsequent thinning. As evidence ofrenewed igneous or deformational activity was not preserved in theplatform until the beginning of the Devonian HP-UHP metamorphicevent, this must have been a typical passive margin during much ofOrdovician and Silurian times, with no evidence for its separationfrom the mainland. Convergence between Gondwana and Laurussiafavored the development of a first collision and subduction to thenorth of the most external section of the platform which underwentthe HP-UHP event prior to c. 400-390 Ma. The interaction betweenthe two large continents allowed the deep subduction of the thinnedprograde margin of Gondwana. The southern margin of Laurussiaacted as a retro-continent and the most important collision probablytook place along the eastern part of Avalonia and, perhaps, along themargin of Baltica (Fig. 13).

Continued dextral convergence favored the fast development ofa wide pull-apart basin during the Lower Devonian, in which maficrocks would have been generated at c. 395 Ma constituting, later,the typical ophiolites of the Variscan suture (Fig. 13). A modernequivalent for this setting can be found in the pull-apart basingenerated between the North American Plate and the Caribbean Plate,although in that case the continental convergence shows sinistralmotion. There, the Gonâve microplate, occupying the pull-apart basin,is comprised of oceanic-type lithosphere, generated by the activity ofthe Mid-Caiman Spreading Centre, and scarce or nonexistent

UPPER UNITS OPHIOLITIC UNITS BASAL UNITS

c.420 MaContracting Rheic Ocean

BALTICALAURENTIA

AVALONIA

ARCTIDA

URALIAN

ASIATICGONDWANA

RHEIC OCEAN

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URALIAN

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URALIAN

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c.380-370 MaSecond collision: Ophiolite obduction and second HP - event

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AVALONIA

ARCTIDA

URALIA

N

ASIATIC

RHEICOCEAN

PALEOTETHYS

c. 395 MaOPHIOLITE

HP BELTRhenohercynian

(d)

Fig. 13. Reconstruction of the Rheic Ocean realm at the Silurian-Devonian boundary (a), and the initial collision between Gondwana andLaurussia at c. 410-400 Ma following the complete closure of the Rheic Ocean (b). This collision caused subduction of a section of themargin of Gondwana and generated the HP-UHP metamorphic belt preserved in the allochthonous Upper Units exposed in the Variscansuture. The true Rheic Ocean suture is not represented in NW Iberia. Dextral motion between Gondwana and Laurussia favored theopening of a rather ephemeral pull-apart basin at c. 395 Ma with generation of new oceanic lithosphere (c). The second and final collisionat c. 380-370 Ma caused the accretion of buoyant oceanic lithosphere followed by new subduction affecting the margin of Gondwana,thereby developing a second HP-LIT metamorphic belt (d). The two different HP belts and the ophiolitic units dated at c. 395 Ma can beidentified in the allochthonous terranes which outline the Variscan suture from Iberia to the Bohemian Massif. Modified after Arenas et al.(2014a).

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sedimentary cover (Brink et al., 2002). In this modern example, asapparently occurred in the inception of Pangea, the pull-apart basinwas generated after an initial collision event between the two platesthat currently limit the basin (García Casco et al., 2008; Sommeret al., 2011).

The dextral convergence of Gondwana finally facilitated theclosure of the basin and the initial accretion of buoyant oceaniclithosphere below the northern continent after c. 380 Ma; the age ofthe prograde foliation in the Careón and Purrido ophiolites (Dallmeyeret al., 1997). This lithosphere escaped the renewed subduction whileundergoing amphibolite facies regional metamorphism, although somemetamorphic soles with local growth of corundum indicate the localoccurrence of high thermal gradients (Díaz García et al., 1999). Theimbrication of new mafic sheets took place under a lower greenschistfacies thermal regime, favoring the accretion of the other sheets ofDevonian ophiolites (Moeche Ophiolite) and other older maficsequences generated by the activity of the peri-Gondwanan magmaticarc (Vila de Cruces Ophiolite). The result was a complex suture zonedeveloped during protracted dextral convergence and containing twoophiolitic belts of different ages, the Cambrian Lower Ophiolitic Unitsand Devonian Upper Ophiolitic Units. The existence of a thickserpentinitic mélange at the base of the allochthonous pile, theSomozas Mélange, has also been interpreted in the context of dextralconvergence (Arenas et al., 2009).

The final collision between Laurussia and Gondwana, as aconsequence of oblique convergence, took place at c. 370Myr (DíezFernández et al., 2012c). It initiated a renewed north-directedsubduction of the outer section of the most external thinned marginof Gondwana, this time with easternmost provenance (Basal Units;Díez Fernández et al., 2010; Fuenlabrada et al., 2012). Thus a secondHP metamorphic event was generated, in this instance of LIT,originating C-Type eclogites, blueschists and HP metapelites(Fig. 13). The convergence went on during a long period of progressiveintra-continental deformation, which propagated to the south, therebyaffecting the margin of Gondwana and reaching the external Variscanareas about 40-50 Ma later (Dallmeyer et al., 1997).

Classification of the NW Iberia ophiolitesDilek and Furnes (2011, 2014) presented a new classification of

ophiolites according to their geochemical signature and internalstructure. They distinguished two first order groups: subduction-unrelated and subduction-related ophiolites. The first type includescontinental margin (CM), mid-ocean ridge (MOR) and plume-type(P) ophiolites. CM ophiolites are formed during continental riftingand generation of an incipient ocean. In subsequent stages oflithospheric extension and seafloor spreading the resulting ophiolitetype is MOR, whose features may vary depending on the spreadingrate at the ridge (fast, intermediate or slow spreading ridges). P typeophiolites form at plume-proximal oceanic ridges or as part of oceanicplateaus. Subduction related ophiolites include supra-subduction zone(SSZ) and volcanic arc (VA) ophiolites. SSZ ophiolites representoceanic lithosphere formed in the extended upper plates of subductionzones (Dilek and Furnes, 2011). The tectonic setting for the oceaniccrust forming these ophiolites includes the fore-arc, the back arc andthe incipient arc. Within back-arc tectonic environments it is possibleto distinguish trench-proximal or trench-distal spreading centersdepending on the extent of subduction influence. Furthermore, itwould be possible to differentiate SSZ oceanic or continental back-

arc basin ophiolites whether they resulted from seafloor spreading in“ensimatic” or “ensialic” settings. VA ophiolites differ from SSZ in athicker and more fully developed arc crust with calc-alkalinegeochemical signatures and the presence of more varied igneouslithologies.

In the case of the NW Iberia ophiolites, the Bazar ophiolite is theonly one which does not record a clear influence of a subductionzone in the generation of its igneous protoliths (Fig. 4). Consequentlyit can be classified as an ophiolite unrelated to subduction, mostprobably of the MOR type (Fig. 6). As discussed before, it probablyrepresents Cambrian (c. 495 Ma) peri-Gondwanan oceaniclithosphere, accreted during Early Ordovician times (c. 475 Ma),shortly after its generation at the ridge. Given this particular evolutionmode and the geochemical features of its lithologies as transitionalbetween N-MORB and E-MORB similar to the trench-proximal Taitaoophiolite in Southern Chile (Le Moigne et al., 1996), it is not possibleto discard the possibility that part of the oceanic lithosphere wasgenerated while the ridge was subducted (ridge-subduction ophiolite,according to the classification of Pearce, 2014) (Fig. 6).

The igneous lithologies of the Vila de Cruces ophiolite (c. 500Ma) show geochemical features clearly indicative of an origin relatedwith the activity of a subduction zone (Fig. 4). It can be classified asa subduction-related SSZ type of ophiolite generated in a back-arcbasin (Fig. 6). The significant negative Nb anomalies suggest a trenchproximal position of the basin, and, given the important amount ofsediment contained in the ophiolite (Fig. 2), as well as the evidenceof interaction between the igneous protoliths and the continental crust,it would probably represent a continental backarc basin (Dilek andRobinson, 2003, and references therein).

The classification of the Devonian (c. 395 Ma) ophiolites (Careón,Purrido and Moeche) following the guidelines of Dilek and Furnes(2011, 2014) is more complicated. We have interpreted the tectonicsetting where the oceanic lithosphere formed as a pull-apart basinformed after a first collision between Gondwana and Laurussia(Fig. 13). Nevertheless, it is clear that subduction zone influence doesexist as it is recorded by the geochemistry of the associated maficrocks (Fig. 4). It is noticeable that such subduction components varyfrom one ophiolite to another, being more significant in Careón andprogressively attenuating in Purrido and even more in Moeche. Themost plausible interpretation of this observation is that this pull-apartbasin opened within a back-arc basin, which persisted after the initialcollision. In this context, variation of the influence of the subductionzone depended on the proximity of the spreading ridge of the pull-apart basin to the original trench. Thus, applying the new ophioliteclassification, it is clear that the Devonian ophiolites are in generalterms of subduction-related SSZ type.

Finally, to apply the new ophiolite classification to the SomozasMélange, we should first address the problem of the origin of thatunit. The development of the mélange itself was distinct from theorigin of the igneous lithologies forming part of the tectonic blocks.They should represent at least in part sections of an oceanic sequence,as the development of a low viscosity channel in the lithosphericmantle implies dehydration of its mafic components duringsubduction. The compositions of the mafic lithologies (c. 527-485Ma) present in the mélange show a clear participation of a subductioncomponent (Fig. 4). Therefore, the ophiolitic components includedin this serpentinite mélange are of SSZ type, and were developed inrelation to a Cambrian subduction zone active in the peri-Gondwananrealm.

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ConclusionsIn the NW Iberian Massif, two main groups of ophiolites are

preserved in the Variscan suture: Cambrian (c. 500 Ma; LowerOphiolitic Units) and Devonian (c. 395 Ma; Upper Ophiolitic Units).A thick serpentinitic mélange of uncertain age and origin is alsopresent. These ophiolites are composed of mafic sequences with minorstrongly deformed ultramafic rocks metamorphosed from greenschistto low-P granulites facies. Although the primary mineralogy is notpreserved, the plutonic, intrusive and volcanic textures and structuresare frequently observed. Sedimentary rocks are rather scarce and it isunclear whether their presence in the ophiolite has tectonicsignificance.

The lithological and geochemical features of most of theseophiolites show that they are not of common MOR types. Theyinstead formed in a supra-subduction zone setting (SSZ) or duringthe opening of pull-apart type ephemeral basins. Perhaps, the onlyexception is the Bazar Ophiolite, which has a more representativeoceanic composition that seems to reflect a Cambrian accretion zoneof peri-Gondwanan oceanic material below a magmatic arc system.This, together with the Vila de Cruces Ophiolite, also generated duringCambrian times in relation to the opening of a back-arc basin,reflects, to some extent, the earliest stages of the opening of the RheicOcean, the largest Paleozoic ocean generated in the peri-Gondwanansetting.

The Devonian ophiolites (Careón, Purrido and Moeche ophiolites)were previously interpreted to have been generated at an intra-RheicOcean supra-subduction zone. This subduction zone would have beenresponsible for the removal of almost all the lithosphere of the RheicOcean. However, recent isotopic data (Lu-Hf in zircon) clearlyindicates that the source of the mafic rocks of these ophiolitesinteracted with continental crust. Therefore they are better interpretedas remnants of an oceanic lithosphere generated in an ephemeral basin,probably of a pull-apart type, developed during dextral convergence.That basin probably opened at c. 400-390 Ma, apparently after a firstinteraction between the continental margins, which triggered the HP-UHP metamorphism in the subducted margin of Gondwana. Its totalclosure occurred at c. 370 Ma, when the Gondwana margin underwenta second event of subduction and HP metamorphism. The buoyantoceanic crust of this ephemeral basin was accreted at high-T, belowthe terrane which currently constitutes the Upper Units of NW IberianMassif.

The two ophiolitic belts in Galicia preserve evidence for thecomplex tectono-thermal events that occurred during the assemblyof Pangea. This collision was not simple, but occurred in at least twodistinct events separated by the opening of pull-apart basins. Inaddition, the tectono-thermal events related to the assembly of Pangeaseem to start by c. 390 Ma, considerably earlier than previouslyconsidered, and thus they lasted longer. Therefore, the conclusionspresented in this paper indicate that the early events in the assemblyof Pangea are more complex than previously described. Nevertheless,these findings are not surprising since the history of convergence andcollision between two large continental masses is commonly notsimple, and complexity is the norm rather than the exception, as canbe observed in some modern examples of continental convergence.

AcknowledgementsFinancial support has been provided by the Spanish project

CGL2012-34618 (Ministerio de Economía y Competitividad).Insightful reviews by Brendan Murphy and J.A. Winchester are kindlyacknowledged.

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