nanc,

rtugal

d Dept. Geocincias, Univ. vora, Portugale Instituto de Geocincias, Univ. S. Paulo, Brazilf GeoBioTec. Dept. Geocincias, Univ. Aveiro, Portugal

a r t i c l e i n f o

Article history:Received 31 May 2009Received in revised form 14 September 2009Accepted 21 September 2009

Gondwana Research 17 (2010) 408421

Contents lists available at ScienceDirect

Gondwana

w.sense towards the NNE. The rst stage, which involved IOMZOS cold obduction, exhumed HP-rocks (ca.370 Ma) and created a foreland bulge towards the Iberian Terrane's SW ank; this mechanism controlled thedevelopment of the volcanic-related carbonate shelves and the contemporaneous Terena ysch trough to theNE. The second stage corresponds to BAOC hot obduction, carrying the IOMZOS klippen in piggy-back style andaffecting the southern margin of the LGS at ca. 350340 Ma.

2009 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

The Variscan orogeny affects the Palaeozoic basement of Central andSW Europe, NW Africa and NE America, being the result of a majorcontinental collision at the end of the Palaeozoic that involvedLaurentia, Baltica and Gondwana and contributed to the formation of

Ribeiro et al., 1995) and the SW IberianMorocco Arc (Simancas et al.,2005). A complex Wilson cycle sustained the growth of this belt,comprising the opening and closure of two main oceans, the Rheic andPalaeotethys (or Middle European Ocean). These oceans are nowrecorded as sutures, dened by tectonically dismembered/imbricateterranes comprising HP-rock assemblages, rootless obducted ophiolitesPangaea (Ribeiro et al., 2007; Dias et al., 2006;2002; Melleton et al., 2010; Casini and Oggi2010this issue). This orogenic belt includes twcontribute to its curved plan-shape: the Iberia

Corresponding author. CeGUL, Faculdade de CiFax: +351 21 750 00 64.

E-mail address: pefonseca@fc.ul.pt (P.E. Fonseca).

1342-937X/$ see front matter 2009 International Adoi:10.1016/j.gr.2009.09.005Terrane) to oceanic rifting. The BAOC displays geochemical features compatible with derivation from a short-lived back-arc basin (?LowerMiddle Devonian; TDM (SmNd) ~380440 Ma). The Variscan ophiolite beltspreserved along the SW Variscides were emplaced by antithetic obduction in two stages, both with shearingAvailable online 4 October 2009

Keywords:Ophiolite beltsGeodynamic evolutionSW Iberian Variscidesa b s t r a c t

The boundary between the Iberian and South-Portuguese terranes in SW Iberia is traced by a suture recordingthe closure of Rheic and related oceans through NNE-directed subduction. The western segment of the suturecomprises imbrications of the Iberian Terrane Relative Autochthon (Neoproterozoic Lower Palaeozoic) andAllochthonous Complexes. The Iberian Terrane Relative Autochthon is overlain by LowerMiddle Devonianmetasediments and an unconformable cover of Lower Carboniferous (calc-alkaline) VolcanosedimentaryComplexes, both preserving evidence of very-low grade metamorphism. The Allochthonous Complexesinclude disrupted slivers of eclogite (ca. 370 Ma), klippen and slices of an internal ophiolite sequence(IOMZOS), a basal tectonic mlange (Moura Phyllonitic Complex) and an external ophiolite (Beja-AcebuchesComplex, BAOC). A large layered gabbroic sequence (LGS), whereas part of the Beja Igneous Complex, intruded(ca. 350340 Ma) the SW suture domains. The IOMZOS represents obducted remnants of the Rheic Ocean, forwhich available SHRIMP UPb data indicate an age of 4795 Ma. This ocean, which separated Iberia from apromontory of Gondwana and Avalonia (now represented by the South-Portuguese Terrane), opened near theCambrianOrdovician boundary through a rift-jump from an intra-cratonic setting (within the Iberianb Dept. Geologia, Faculdade de Cincias, Univ. Lisboa, Portugalc Centro de Geofsica de vora, Univ. vora, Portugala CeGUL, Faculdade de Cincias, Univ. Lisboa, PoG. Machado f, A. Jesus aVariscan ophiolite belts in the Ossa-Morecharacterization and geodynamic signica

A. Ribeiro a,b, J. Munh a,b, P.E. Fonseca a,b,, A. Arajo

j ourna l homepage: wwVera, 2004; Matte, 2001,ano, 2008; Keppie et al.,omajor oroclines,whichnArmorican Arc (Fig. 1;

ncias, Univ. Lisboa, Portugal.

ssociation for Gondwana Research.Zone (Southwest Iberia): Geologicalced, J.C. Pedro a,d, A. Mateus a,b, C. Tassinari e,

Research

e lsev ie r.com/ locate /grand continental allochthonous units (Ribeiro et al., 1990; Simancaset al., 2002).

The SW Iberian traverse (e.g. Ribeiro et al., 2007), which is at a high-angle to the main NWSW Variscan structures of transpressive origin,runs from NE to SW (Fig. 2) across: (1) the Iberian Terrane (includingthe Cantabrian (CZ), West-AsturianLeonese (WALZ) and Central-Iberian (CIZ) Zones); (2) the Ossa-Morena Zone (OMZ), a distinctlithospheric block accreted to the Iberian Terrane during the Cadomian

Published by Elsevier B.V. All rights reserved.

409A. Ribeiro et al. / Gondwana Research 17 (2010) 408421(e.g. Ribeiro et al., 2007) or Variscan Cycle (Simancas et al., 2003); and(3) the South-Portuguese Terrane (SPT). In this tectonic framework, theOMZSPZ and OMZCIZ boundaries represent two major sutures: theformer corresponding to the SW Iberian Suture, whereas the latter,coincides with the TomarBadajozCordoba Shear Zone (TBCSZ) andeither forms a Cadomian suture reactivated as a Variscan intra-plate

Fig. 1. (A) Subdivision of Iberia and correlation of Variscan sutures in Western Europe

Fig. 2. Interpretative cross section of the SW Iberianower structure, or more simply, a Variscan suture, depending on theinterpretation of the OMZ mentioned above (Ribeiro et al., 2007;Simancas et al., 2003). The western limit of the Iberian and OMZ blockscorresponds to the PortoTomarFerreira do Alentejo Shear Zone(PTFASZ), a dextral palaeo-transform that connects the SW and NWIberian sutures and cuts the TBCSZ (Fernndez et al., 2003; Chamin

. (B) Schematic representation of the main terranes in the SW Iberian Variscides.

Variscides (adapted from Ribeiro et al., 2007).

structural trend of the Variscan orogen (Fig. 3). The SW limit of thisregion corresponds to the Ferreira do Alentejo Thrust Zone (FATZ),

410 A. Ribeiro et al. / Gondwana Research 17 (2010) 408421which places the OMZ sequences on top of those belonging to the SPT.The western boundary of the FATZ coincides with the PTFASZ, whereasits NE border (located within the Beja Sub-zone) follows the OriolaPedrogoSerpa tectonic alignment. This latter alignment represents anearly N-directed thrust (strongly displaced by late shear zones) thatplaces allochthonous units on top of the OMZ relative autochthon.Towards the N, calc-alkaline diorite/granite bodies of the vora Massifbecome increasingly abundant. The rocks of the study area comprisestronglydeformedunits preservingevidenceof a complexmetamorphicregime, as well as the diachronous emplacement of igneous bodies ofvariable composition (gabbros to granites). Scattered exposures of very-low grade, weakly deformed, Devonian and Lower Carboniferousmetasediments are also present (Boogaard, 1972; Conde and Andrade,1974; Oliveira et al., 1991, 2006; Pereira and Oliveira, 2006;Machado etal., 2009). Previous studies (Conde and Andrade, 1974; Andrade et al.,1976) and a recent monograph (Machado et al., 2009), allow theexposed lithostratigraphical sequences to be grouped into four mainunits: (1) OMZ Relative Autochthon; (2) Devonian Metasediments andLower Carboniferous Volcanosedimentary Complexes; (3) Allochtho-nousComplexes, integrating slices of the IOMZOS; and (4) theBAOC, thefootwall of which stands on the Pulo do Lobo Complex, an accretionaryprism developed during Variscan subduction (Quesada et al., 1994;Fonseca and Ribeiro, 1993).

The main igneous intrusion found in the study area runs along theSW border of the OMZ and is known as the Beja Igneous Complex(BIC). It comprises the LGS, rimmed by a horneblende-rich gabbro to(grano-)diorite facies (Cuba-Alvito Complex), alongside a late anddiscontinuous envelope (Baleizo Porphyry Complex) made up ofepizonal igneous rock-assemblages (Silva et al., 1970; Andrade, 1974,et al., 2003, 2007; Ribeiro et al., 2007; Romo et al., 2008, submitted forpublication). Within the OMZ two major tectonic units are differenti-ated, from SW to NE (Fig. 2): the Beja and the EstremozBarrancos Sub-zones. The former display a complex structure due to NNE thrusting ofallochthonous complexes over the OMZ relative autochthon ofNeoproterozoic to Middle Devonian age; the preserved metamorphicrecord varies from Early-Variscan (eclogite and blueschist faciesconditions) to Late-Variscan (amphibolite/greenschist facies condi-tions) (Fonseca et al., 1999; Jesus et al., 2007;Rosas et al., 2008; Pin et al.,2008). The EstremozBarrancos Sub-zone has a simple structurecharacterised by SW-facing structures developed in Lower Palaeozoicvolcanosedimentary sequences recrystallised under greenshist faciesconditions (Arajo et al., 2006). These sub-zones are separated by theTerena syncline, which displays steep NWSE axial plane and dextrallytransected slaty cleavagewithin a Lower Devonianysh succession thathas undergone very-low grade metamorphism (Pereira et al., 1998,1999; Piarra et al., 1998; Piarra, 1997; Arajo et al., 2006).

The SW Iberian suture is the main goal of the present work,particularly the rootless and dismemberedVariscanophiolite sequences(e.g. Munh et al., 1986; Fonseca et al., 1999; Ribeiro et al., 2008) thatform the Internal Ossa-Morena Zone Ophiolite Sequences (IOMZOS)and the BejaAcebuches Ophiolite Complex (BAOC). These sequencesare developed in twobelts that differ in age, evolution and emplacementmechanisms, and record distinct geodynamic histories, the signicanceof which is critical to ongoing discussion of the evolution of the SWEurope Variscides. Additionally, BAOC is differentiated from the BejaLayered Gabbroic Sequence (LGS) (Figueiras et al., 2002; Jesus et al.,2007; Pin et al., 2008; Ribeiro et al., 2008), and this provides usefulinsights to the geodynamic evolution of the SW Iberian suture.

2. Geological setting

The study area denes a belt about 35 km wide across the NWSE1983; Santos et al., 1990; Jesus et al., 2007).3. Key geological units

3.1. OMZ relative autochthon

This unit includes, from structural bottom to top (Arajo et al.,2005; Fonseca et al., 1999; Rosas et al., 2008): 1) a metavolcanicmetasedimentary sequence of Neoproterozoic age; 2) marbles withinterngered metavolcanics followed by a thick and monotonousmetadolomitic succession assumed to be of Lower Cambrian age; and3) a phyllitic series with intercalated marbles and bimodal metavol-canics formed during the Cambrian/Ordovician-Silurian.

Over large areas of the OMZ Autochthon, the mild Variscanoverprint preserves evidence of older (Cadomian) magmatic andmetamorphic features (e.g. Salman, 2004). The Neoproterozoicsequence crops out along the axis of D1 W to SW-verging anticlines(sometimes with inverted long-limbs containing the Palaeozoicseries), comprising units of the Srie Negra Group (black phyllites,lidites, and interbedded bimodal volcanics; e.g. Pascual, 1981;Apalategui and Higueras, 1983; Palcios, 1983; Lin and Palcios,1983; Gonalves and Palcios, 1984; Oliveira et al., 1991; Beetsma,1995) occasionally overlain by felsic gneisses or leptinites. Compre-hensive mapping of typical structures, such as those of Serpa-Brinches, Vidigueira-Vila de Frades and Alvito-Viana do Alentejo andEscoural (Fonseca, 1995; Arajo, 1995), revealed a tectonic contactbetween the Srie Negra Group and the felsic gneisses or leptinites,marking an early imbrication event towards the NNE. The latterrocks are part of an extensive migmatitic complex that comprisesmostly ortho-gneisses (often preserving blastomylonitic features)believed to represent Cadomian crustal basement (Fonseca, 1995).Leptinites (LPHT granulites) sporadically show quartz porphyroclastswith -type asymmetrical tails indicating shearing towards the N,consistent with kinematical criteria observed in BAOC units (Quesadaet al., 1994; Fonseca et al., 1999). The HT metamorphic re-crystal-lisation involved in the genesis of the leptinites suggests localtemperature increase, possibly marking the onset of the thermaldoming that led to subsequent heterogeneous migmatite development.Cambrian olivine-bearing marbles (resting on units belonging to themigmatitic complex; Oliveira et al., 1991) and scattered occurrences ofspinelgarnetcordierite granulites are also related to the thermaldoming, dated of ca. 300 Ma (geochronological work in progress).

The Lower Cambrian succession records the development of anextensive carbonate platform covering a signicant part of the OMZpalaeogeographic domain. The succession preserves deformationaland metamorphic features acquired during the Variscan undergreenschist/amphibolite transitional facies conditions. The overlyingseries, which record similar Variscan characteristics, includes minormarbles (mostly at the base of the sequence) and abundant metavol-canic rocks. The eastern upper part of the dominantly phyllitic sequencecontains fossil assemblages of Silurian ages. In the PortelFicalhoantiform of the Beja Sub-zone, the Lower Palaeozoic lithostratigraphicsuccession occurs as a condensed sequence with probable unconfor-mities (Arajo et al., 2006).

The structure and kinematics showed by the OMZ Autochthon inthe Beja Sub-zone (Arajo et al., 2006) can be interpreted as aconsequence of three main phases of deformation acting successively(Table 1). Recumbent folding with axial plane cleavage coupled withtop-to-the-W thrusting dominates the D1 phase. N or NE-vergentback-thrusting and folding of D1 structures are the dominant effects ofa D2a event, followed progressively by refolding and thrusting to theSW during D2b (all these structures developed under greenschistfacies conditions). Open, NWSE, upright folding with slaty cleavagein Lower Carboniferous volcanosedimentary complexes and (mild)crenulation cleavage in older metasedimentary sequences can beattributed to D3, indicating that these units were deposited at a timeof relative tectonic quiescence between the D2 and D3 deformation

phases.

411A. Ribeiro et al. / Gondwana Research 17 (2010) 4084213.2. Devonian metasediments and lower carboniferousvolcanosedimentary complexes

Scattered exposures of Middle Devonian sediments recording very-low grade metamorphism (limestones, calci-turbidites and shales;Conde and Andrade, 1974; Oliveira et al., 1991) occur in severallocations in western OMZ. Some of these carbonate rocks were studiedin detail, namely at Pedreira de Engenharia (Beja Sub-zone) where theycontain Eifelian conodonts (Boogaard, 1972), but their relationship tothe surrounding siliciclastic sediments remains controversial (Boo-gaard, 1972; Pereira and Oliveira, 2006). At a re-examined location inthe Odivelas dam area, calci-turbidites surround bioherm limestone's,

Fig. 3. Schematic geological map and interpretative cross secwhich contain a relatively diversied reef fauna that indicate astratigraphic position near the Givetian/Eifelian boundary (Machadoet al., 2009). Field relationships and available mineralogical/geochem-ical data suggest the reef system developed on a volcanic structurewithsynchronous peripheral calciturbidite sedimentation and volcanism(Machado et al., 2009). Other (meta-) sedimentary and (calc-alkaline)volcanic sequences in the samearea (near theOMZ-SPZ boundary) formtheLowerCarboniferousCabrela andTocadaMouraComplexes (Pereiraand Oliveira, 2006), and rest with angular unconformity on MiddleDevonian (meta-) sediments (Ribeiro, 1983). Hence, the available datasuggest that suitable conditions for the development of volcanic-relatedcarbonate shelves existed in the Beja Sub-zone during Lower to Middle

tion of the IOMZOS klippen (adapted from Pedro, 2004).

cal u

412 A. Ribeiro et al. / Gondwana Research 17 (2010) 408421Table 1Synoptic correlation between the tectono-metamorphic events recorded in key geologiDevonian times. This contrasts with the sedimentary palaeoenviron-ment recorded by the ysh sequence presently preserved along theTerena syncline to the E, which was fed simultaneously from the SW(Beja Sub-zone) and NE (EstremozBarrancos Sub-zone).

3.3. Allochthonous complexes

Above the OMZ Relative Autochthon are two AllochthonousComplexes. The uppermost is composed of: (1) intricate imbricationsof different rock types that include phyllites possibly correlative withthe Upper Proterozoic units of the OMZ; (2) marbles akin of thoseincluded in the Lower Cambrian series of the OMZ; (3) gneisses ofvariable composition and age; and (4) duplexes of dismemberedophiolite sequences (comprising metamorphosed peridotites, gab-bros and dolerites) that form the IOMOZOS. Imbrications within thisAllochthonous Complex preserve rock types (eclogites and blueschists)that resulted from an early HP-metamorphic event (Fonseca et al.,1999; Booth-Rea et al., 2006) dated at ca. 370 Ma (Moita et al., 2005).This Sm/Ndwhole rockgarnet age (37117Ma) is consistent (within2 error) with 40Ar/39Ar plateau ages obtained for (retrograde)eclogitic amphibole aliquots (3604 Ma), suggesting that decompres-sion and exhumation (cooling) took place shortly after eclogite peakmetamorphism. The lower allochthonous unit corresponds to theMoura Phyllonitic Complex (Arajo et al., 2005), an accretionarysequence of ophiolitic slices, mac alkaline and per-alkaline metavol-nits in the SW Iberian Variscides.canics, as well as disrupted fragments of various metasedimentsderived from the OMZ Relative Autochthon and containing Siluriangraptolites (Piarra, 1991). Hence, the Moura Phyllonitic Complexcorresponds to an assortment of rock types of Neoproterozoic to LowerPalaeozoic age, such that the development of the tectonic mlangemust be Silurian or younger.

According to the recent review of Arajo et al. (2006), the structureand kinematics displayed by the Allochthonous Complexes can beinterpreted as the result of fourmain deformation phases (Table 1). Rockunits preserving the effects of HP-metamorphism show evidence of anearly phase recorded by recumbent folding and foliation development.Theassociatedkinematicsare unknowndue to strongdeformation in theseries with unknown sedimentary polarity. Nevertheless, Rosas et al.(2008) proposed a top-to-the-S shear sense based on syntheticboudinage of metamorphosed layered mac bodies (mostly amphibo-lites and, sometimes, blueschists). N to NE shearing and a at-lyingfoliation, and early (370360 Ma; Moita et al., 2005) partial retrogres-sion of the HP-mineral assemblages (eclogites and blueschists) can beassigned to the D1 phase. These structures are well preserved in far-travelled klippen and some restricted domains of the thrust-units neartheir root zone, which corresponds to themain suture between theOMZand SPT (Fonseca et al., 1999; Booth-Rea et al., 2006). Superimposednorthward shearing along at-lying shear zones associated withpervasive metamorphic retrogression to amphibolite/greenschist faciesconditions are themain effects of the D2 phase. Since this also affects the

413A. Ribeiro et al. / Gondwana Research 17 (2010) 408421intrusive contact of the (BIC) LGS, the D2 deformation phase must beyounger than the crystallisation/emplacement age of the LGS, i.e. ca.350 Ma (Jesus et al., 2007; Rosas et al., 2008). Late WNW-ESE folds thatare slightly vergent to the S and contain an axial plane cleavagedeveloped under low grade greenschist facies conditions record the D3phase.

3.3.1. Internal OMZ Ophiolite Sequences (IOMZOS)The IOMZOS is made up of metre- to kilometre-scale allochtho-

nous fragments of oceanic lithosphere that outcrop as klippenoverlying, and/or imbrications within, the Moura Phyllonitic Complex(~50 km up to the NE of the SW Iberian suture; Pedro, 2004; Arajoet al., 2005; Pedro et al., 2005, 2006), in a discontinuous WNWESEalignment. These ophiolite fragments preserve evidence of metamor-phic re-crystallisation under greenshist/amphibolite transition faciesconditions, heterogeneous strain accommodation, and shear zonesand related structures responsible for its early dismemberment thatdisplay micro-, macro- and mesoscopic kinematic criteria supportingtransport and emplacement from SW to NE (Pedro, 2004; Arajo et al.,2005). A reconstruction of IOMZOS architecture that removes theeffects of shearing and takes into account the available mineralogicaland petrological/geochemical data (Fonseca et al., 1999; Pedro, 2004),shows that it includes, from bottom to top: (1) strongly serpentiniseddunites and wehrlites; (2) amphibolitised pyroxenite cumulates; (3)variably altered metagabbros and aser-gabbros; (4) metagabbrosintruded by metadolerite dykes; and (5) metabasalts (greenschists/amphibolites) with intercalations of radiolarian metacherts. The mostcomplete sequences preserved in IOMZOS klippen usually correspondto a thick, discontinuous crustal section (including sporadicallystratiform pyroxenite cumulates) together with a mantle sequence(serpentinised peridotites) that is often cut by metagabbroicpegmatoids and metadolerite/metabasalt dykes. These features,especially the structure of the crustal component and the petro-graphic nature of residual mantle, classify the IOMZOS as a LherzoliticOphliolite Type (LOT), commonly related to slow spreading centers(Juteau and Maury, 1999).

Geochemical and geochronological features of the IOMZOS werereported by Pedro et al. (submitted for publication). In summary, theIOMZOS displays geochemical signatures that are transitional betweenthose of N-MORB and E-MORB, indicating a wide ocean origin for theprotoliths of the observed units. There are no obvious indications ofeither orogenic inuence and/or crustal contamination in the genesis ofthe IOMZOS rocks. Zircons from an IOMZOS metagabbro sampleanalysed by SHRIMP UPb dating methods (see also Ribeiro et al.,2008) were anhedral, optically homogeneous, with high Th/U values(1.03 to 2.55) reecting a magmatic afliation. Five of the seven spotsanalysed dene a concordia age of 4795 Ma (2, MSWD=1.19). Thisage data, together with the reconstructed IOMZOS pseudo-stratigra-phy, suggest that the allochthonous IOMZOS fragments are remnants ofanobducted portion of the RheicOcean in SW Iberia,whichopenednearthe Cambrian/Ordovician boundary and closed in the Middle/UpperDevonian (Ribeiro et al., 2007).

3.3.2. BejaAcebuches Ophiolite Complex (BAOC)The BAOC forms a narrow (less than 1.5 km wide) belt emplaced

between the OMZ and SPT, the N and S limits correspond of which to(re-)activated reverse-sinistral WNW-ESE shear zones dipping to theSSW (Munh et al., 1989; Quesada et al., 1994; Fonseca, 1995; Mateuset al., 1999). The reconstructed section of this macultramac beltstrongly suggests that it represents a highly fragmented, but stillrecognizable, Variscan obducted ophiolite (e.g. Munh et al., 1989;Quesada et al., 1994; Fonseca andRibeiro, 1993; Fonseca, 1995;Castro etal., 1996, 1999; Daz Aspiroz et al., 2004). Metacumulate rocks includedin the lower section of the ophiolite sequence are strongly serpenti-nized. Nevertheless, former harzburgites (and/or clinopyroxene-rich

peridotites) appear to represent the main rock type. However, duniteand relatively well preserved wehrlite/troctolite rocks were alsoidentied in several of the studied sections of the Guadiana Valley andFerreira do AlentejoMombeja areas. Metagabbroic rocks (mostlyderived from gabbros and gabbro-norites) record evidence of earlydynamic re-crystallisation under very high temperatures followed byVariscan development of amphibolite facies assemblages, locallyaccompanied by retrogression to greenschist metamorphic conditions(Quesada et al., 1994; Fonseca et al., 1999; Figueiras et al., 2002). Theupper units of the BAOC are represented by poorly preserved remnantsof sheeted dikes and pillowed metabasalts with radiolarian cherts(Fonseca, 1995; Eden, 1991).

The BAOC was emplaced by antithetic obduction towards the N, asevidenced by early-developed anisotropic fabrics that are wellpreserved in the metagabbroic section (Quesada et al., 1994; Fonseca,1995). Based on textural/mineral relics of original magmatic struc-tures/assemblages, delamination and obduction of this oceanic crustalfragment must have occurred under residual magmatic temperatures(Figueiras et al., 2002). Rapid dissipation of the residual magmaticheat, coupled with strong strain partitioning towards the lowersections of the obducted sequence, likely prevented dynamic re-crystallisation of both the OMZ autochthon and the upper BAOCsection (Gonalves et al., 1998). The evidence is well preserved on themetagabbroic section, but not in the lower peridotitic section due topervasive serpentinisation following emplacement (Fonseca et al.,1999; Figueiras et al., 2002).

Any synthesis of the structure and kinematics of the BAOC needs toconsider the effects of three main phases of deformation (Fonseca andRibeiro, 1993; Fonseca et al., 1999; Figueiras et al., 2002): (1) N-directedshearing on sub-horizontal planes sometimes obliterated by stretchingmineral lineations developed under amphibolite facies conditionsdene the D1 phase; (2) shearing to the WNW on sub-horizontalplanes along with stretching mineral lineations developed undergreenshist facies conditions dene D2 (this was coeval with thenucleation/propagation of sinistral WNWESE shear zones usuallysubject to intense and long-lived hydrothermal activity; Mateus et al.,1999); and (3) shearing towards the S along steep brittle reverse faultsthat reactivated D2 shear zones (subparallel to the Ferreira do Alentejothrust zone) dene D3 (Table 1).

Until recently, geochronological data for the BAOC were restrictedto cooling 40Ar/39Ar ages obtained for amphiboles (Dallmeyer et al.,1993; Castro et al., 1999), which dene a trend of increasing age fromE to W along the Spanish part of the belt (from 3373 Ma inCortegana to 3281 Ma in Aracena), consistent with the slightlyolder ages reported for the Portuguese counterpart (3411 Ma). Theseresults date regional cooling through the 550500 C isotherm(s) andthe regional trend is interpreted to be the result of the metamorphicevolution experienced by the suture (Castro et al., 1999; Daz Aspirozet al., 2004). Azor et al. (2008) reported UPb SHRIMP data on zirconsincluded in amphibole-rich rocks sampledalong the entirebelt. Thedatarange from 3323 Ma in Spain to 3404 Ma in Portugal. According tothese authors, the ages, which overlap the 40Ar/39Ar metamorphic ages,date the crystallisation of the basaltic/gabbroic protoliths. However,some of the samples dated by Azor et al. (2008) may have beenunsuitable for characterizing the BAOC and the interpretation ofthe reported isotopic data is controversial. Indeed, according to themost recent geological mapping revisions of the SerpaGuadianaValley (Portugal), the belt corresponding to the BAOC is narrowerthan previously thought, such that the samples collected in Portugaland dated by Azor et al. (2008) may correspond to hetero-geneously deformed rocks belonging to the southern border of theLGS. Furthermore, as comprehensively discussed by Pin and Rodriguez(in press), the UPb ages reported by Azor et al. (2008) may be biasedtoward ca. 1020 Ma to youngvalues, because of the failure to recognizethatmost of their spot analysesmight have been affected byweak, butsignicant, radiogenic lead loss, along with possible analytical

problems during Pb/U calibration and/or common Pb correction. As

discussed below, this points to a maximum age of ~350 Ma, consistentwith other determinations for LGS rocks. Despite these uncertainties,opening of the BejaAcebuches back-arc basin can be bracketedbetween the Upper Devonian mlange of the Pera-Mora, NW Huelva(Eden and Andrews, 1990), and the age of the oceanic tholeiite dykesthat intrude the Pulo do Lobo Unit (Devonian) at Trindade (Munh,1983). SmNdmodel ages for the BAOC of 380440 Ma (Mullane, 1998)are compatible with these constraints and suggest a time-span of 400370 Ma for the opening/closure of the narrow back-arc basin.

3.4. Pulo do lobo complex

The lower unit(s) of the Pulo do Lobo accretionary complex,comprising Early Frasnian Ribeira de Limas Formation (Pereira andOliveira, 2006) phyllites and Trindade MORB-type tholeiites (Munh,1983), were affected by an early (greenschistlower amphibolite facies)tectonometamorphic event (locally D1; see Table 1)with top-to-the-SWshear sense. Anunconformity separates the Pulo do Lobo (lower) unit(s)from the very-low grade Late Famenian Santa Iria and Horta da TorreFormations (Fonseca and Ribeiro, 1993; Pereira and Oliveira, 2006),which represent post-D1 ysch deposits exclusively affected by D2(Table 1). These stratigraphic relationships indicate anorogenic hiatus ofabout 14 Ma (between D1 and D2) that corresponds to two distinct SW-directed tectonic pulses (involving thrusting and folding) synthetic tocoeval SW-directed subduction. Based on tectonostratigraphic evolu-tionary patterns of modern accretionary complexes (e.g., Dickinson,

1977; Moore et al., 1980), the Santa Iria and Horta da Torre Formationsrepresent upper trench slope basins of the accretionary prism.

3.5. Layered Gabbroic Sequence (LGS) of Beja Igneous Complex (BIC)

The LGS can be divided in two major segments separated by theMessejana strike-slip fault zone. The western segment, fromWTorroto Beringel, is much wider and comprises different successions oflayered gabbroic rocks, the lower suites showing the most primitiveSrNd isotopic signature (Pin, et al., 1999, 2008). In the easternsegment, from Beringel to Serpa, magmatic layering is only observedat small scales and evidence of crustal assimilation is quite common,as recorded by evolved SrNd isotopic compositions (Pin, et al., 1999,2008) and a higher abundance of amphibole-rich rocks. The strike ofthe magmatic layering ranges from NWSE to WNWESE and it dipsgently (2535) to the SSW, although signicant deviations mayoccur locally as a result of disturbances induced either by magmaticuxes or mechanical deformation adjoining major shear zones.

Recent studies undertaken in the western domains of the LGS(Jesus et al., 2005a,b, 2006a,b) clearly demonstrate that successivemagma replenishments were involved in its development, enablingthe recognition of six main series. The available data also show thatvariations in layering (type, orientation and associated minerallineation) and other structural features reect the prevailing stresseld at the time each series was emplaced and consolidated. Thedistinct series consequently record dissimilar structural featuresbecause, over the same time interval they exhibited different

414 A. Ribeiro et al. / Gondwana Research 17 (2010) 408421Fig. 4. Synchronous interference D2aD2b as observed in the NESW Guadiana River cross section (adapted from Arajo, 1995).

rheological behaviour, from suspension/viscous ow that generatednon-dynamic features, to plastic ow (with increasing solidication)leading to dynamic recrystallization and the development of mineralsegregation bands and lineations (e.g. McBirney and Nicolas, 1997;Boudreau and McBirney, 1997; Petford, 2003). Dissipation of thelatent heat of crystallisation induced strain partitioning, allowing thedevelopment of different shear zones that disrupt the LGS and itsenvelop (Mateus et al., 1999; Figueiras et al., 2002). Accordingly, earlystructural features reect dynamic deformation in the rst series toconsolidate are subparallel to layering (varying from NWSE toWNWESE and dipping 306 to the SW) and develop C/S fabricswith top-to-the-N shear. These structural features are compatiblewith those of D2 in the Allochthnous Complexes and to D2a in the OMZAutochthon (Table 1). Subsequent deformation in the LGS is

characterised by (semi-)ductile shear zones, mainly striking WNWESE and dipping steeply (N65) towards the SW. Kinematical criteria,when preserved, point to a predominantly left-lateral component ofmovement, coupled with thrusting to the NNE. These early shearzones are commonly reactivated under (semi-)brittle conditions, therespective movement planes being subparallel but dipping to theNNE.

The available geochronological data (Jesus et al., 2007, andreferences therein, Azor et al., 2008; Pin and Rodriguez, in press.)suggests that the LGS records the early stages of collision magmatismalong the southern border of the OMZ (from ca. 355 Ma to ca. 345 Ma).Taking this evidence together with present knowledge concerning thegeodynamic evolution of the SW Iberian Variscan suture (Ribeiro et al.,2007), it is plausible to correlate thesemagmatic stageswith the onset of

415A. Ribeiro et al. / Gondwana Research 17 (2010) 408421Fig. 5. (A) Palinspatic reconstruction at the onset (~390 Ma) and end (~370 Ma) of the coldobduction stage. (B) Palinspatic reconstruction during the hot obduction stage (~360345 Ma).

themechanism thatmost likely triggered the rise of LGSmagmas in thistectonic background is slab break-off at the subduction zone, as

ont

416 A. Ribeiro et al. / Gondwana Research 17 (2010) 408421discussed in Jesus et al. (2007). This would have allowed the localincursion of asthenosphericmantle, thus detaching the dense, relativelycold, mantle root, and perhaps even the mac lower crust of OMZsouthern border. The result would be the juxtaposition of hot mantleand subducted rock, coupled with sudden added buoyancy (as thedown-dragging force of thedense rootwas removed) and themelting ofmetasomatized (low melting-T) components of the mantle.

4. Discussionoblique continental collision between the OMZ and SPZ. Consequently,

Fig. 5 (cAn integrative discussion of the data summarized requires aninterpretation of the structure and kinematics of the different unitswithin the study area. Hence, the deformational phases (events) ineach unit must be rst correlated using: (1) evidence from superposeddeformation; (2) the associated tectono-metamorphic regime; (3) theirradiometric ages; and (4) their relationship to magmatic episodes. It isoften emphasised that (D1,Dn) nomenclature used in routinestructural work does not exclude the possibility of continuity betweendifferent events in a process of plate convergence that can last for sometens of Ma. Indeed, when the available data is plotted on a synopticcorrelation table (Table 1), the critical evidence for a sequence oftectono-metamorphic events emerges naturally from superpositioncriteria in the OMZ Autochthon of the Beja Sub-zone (Arajo et al.,2006).On this basis,we infer (Fig. 4): (1) thepresence of a confrontationzone between dominant early NE-facing vergences in domains situatedto the SW, and SW-facing vergences in domains situated to theNE; (2) atransient period of back-thrusting to the NE (D2a) followed by forwardthrusting to the SW(D2b), corresponding to the space-timemigration inthat zone; (3) the prevalence of thrusting to the NNE in SW domains;and (4) the predominance of thrusting/folding to the WSW in most ofthe NE domains of the OMZ Autochthon. Additionally, thrusting of theAllochthonous Complexes to N or NE clearly expresses the obductionregime that generated the Variscan ophiolite belts in two contrastingthermal regimes with the same kinematics (Fig. 5).

The older regime is of cold obduction type, generating the IOMZOSand the associated HP-rocks dated at ca. 370 Ma (Moita et al., 2005).This date corresponds roughly to the exhumation/late emplacementof HP-rocks near the MiddleUpper Devonian boundary, onto theLowerMiddle Devonian volcanic-related carbonate shelves of theBeja Sub-zone. It is possible that this subduction/obduction regimestarted earlier, near the Silurian/Devonian boundary, because theforeland (?) exural-bulge of the ophiolite nappes (progressingtowards the N or NE) would explain the uplift of this platform and thesubsequent subsidence of the more distal Terena ysch trough. Theyounger regime is of hot obduction type, leading to BAOC emplace-ment and causing deformation of the LGS SW-margin at ca. 350 Ma(Pin, et al., 1999, 2008; Jesus et al., 2007). Importantly, BAOCemplacement associated with the development of HT-LP mineralassemblages. During the hot obduction stage, a rapid evolution took

inued).place from crustal thickening under ductile conditions of a retro-wedge facing N to NE (Jesus et al., 2007), to the generation of surgezones (Coward, 1982) that locally cut down into pre-existingstructures as extensional faults under brittle conditions. These eldrelationships are consistent with rapid exhumation of the LGS and itscountry-rocks, followed by the development of Lower Carboniferousvolcanosedimentary complexes recrystallized under very-low grademetamorphic conditions. This evolutionary pattern is compatiblewith continued top-to-the-NE subduction of the SPT under the OMZand the mechanism of slab break-off detailed below. Such anextended period of obduction/subduction (lasting at least 20 Ma,but possibly 50 Ma; see above) coherently accounts for all theavailable stratigraphical, structural, kinematical, petrological/geo-chemical and geochronological data. It is also compatible with theidea that short orogenic pulses are the local response to changingboundary conditions in a continuous geodynamic regime of obliqueconvergence; in this case, in the transpressive belt of the SW IberianVariscides.

4.1. Geodynamic evolution

A general geodynamic scheme for the evolution of the SW Iberiansuture in the framework of the Variscan Fold Belt is proposed in Figs. 5and 6. At the end of the Neoproterozoic, Iberia was part of thesupercontinent Pannotia, and comprised a collage of terranesassembled during the Cadomian/Avalonian orogeny, mostly between

620 and 540 Ma (Fernndez-Suarz et al., 2000). This craton laterevolved to conditions that favoured the development of a carbonateplatform during most of the Cambrian period. Subsequent riftingresulted in the establishment of two intra-cratonic rifts; onepositioned along the axis of the CIZ and the other, further SW, alongthe TBCSZ, presumably by reactivation of a former Cadomian suture(Ribeiro et al., 1990, 2007). Rifting conditions along the length of theTBCSZ is indicated by the nature and extent of the regional Cambrianmagmatism (Mata and Munh, 1990). Both rifts were separated by aCadomian basement high that fed sediments to the CIZ axis-troughfrom the SW.

During Upper Cambrian and Lower Ordovician times, the IberianTerrane (WALZ, CIZ and OMZ) experienced a pervasive episode ofextensional tectonics related to transient inversion (Sardic phasesensu latu) caused by the migration of both an (aborted) intra-cratonic rift inside the CIZ/OMZ and a rift at the OMZ/SPT boundary.The latter evolved to thewide Rheic Ocean, inducing a later opening ofthe narrow PalaeotethysMassif Central Ocean between Iberia andArmorica (Romo et al., 2005; Ribeiro et al., 2007 and referencestherein). Throughout this time interval, the Cadomian basement wasthinned through extensional detachments at mid- and upper-crustallevels (Ribeiro et al., 2009). Concurrently, signicant bimodalcontinental magmatism and localised LPHT metamorphism tookplace, coupled with underplating and partial melting of Cadomianbasement that allowed the emplacement of batholiths (that cut thePalaeozoic cover) and laccoliths near the Cambrian/Ordovician

were followed by obduction of Rheic oceanic lithosphere over theFinisterra microcontinent, bringing the BAOC over the OMZ autoch-thon. The inherent development of high relief induced the initiation(in the Lower Devonian) of sedimentation into the Terena Basin fromthe SW. Antithetic cold obduction (Ribeiro, 2002) was presumably setoff by intra-oceanic thrusting within the Rheic Ocean. Continuedsubsidence in the Terena Basin during the Lower and MiddleDevonian probably reects the development of a foreland bulge dueto the progressive thickening of an orogenic wedge to the SW, withIOMZOS cold obduction followed by BAOC hot obduction.

The late collisional stage of the SW Iberia suture canbe summarizedas follows (Jesus et al., 2007). A complex wedge systemwithin the SWIberian Variscides developed during this collision, involving an OMZupper plate to the north and the SPT passivemargin on the lower plateto the south. The vora-Beja Sub-zone, located in the upper plate abovethe N-dipping subduction zone, is reinterpreted as a retro-wedgedomain that was kinematically coupled to the SPT pro-wedge and thesubduction system. Retro-wedge growth is linked to upper plate uplift(early collision) and late-orogenic wedge thickening. The early stagesof magmatism in the retro-wedge are related to asthenosphericmantle upwelling induced by slab break-off. Concurrently, crustalthinning in the SPZ, caused by subduction blocking and subsequentslab break-off, led to underplating by basaltic magmas and theconsequent development of signicantmagmatism in the SPZ (namelyalong the Iberian Pyrite Belt). Regional LPHT metamorphism andsubsequent magmatic events in the retro-wedge domain were caused

417A. Ribeiro et al. / Gondwana Research 17 (2010) 408421boundary. The protracted CambrianOrdovician rifting is thought tobe related to an ascending mantle plume (operating under high heatow mode) and consequent thermal erosion. Evidence for thiswidespread extensional episode can also be inferred from the PTpaths recorded by poly-metamorphic rocks in the ContinentalAllochthonous Terrane (CAT) of NW Iberia and in the autochthon ofCIZ and OMZ (Ribeiro et al., 2007, 2009).

The subsequent evolution of the SW Iberia transect involved thefollowing main stages (Figs. 5 and 6). The onset of Rheic subductionoccurred near the SilurianOrdovician boundary (ca. 450430 Ma Ribeiro et al., 2007). This clearly preceded the opening of the BejaAcebuches back-arc basin (BAB) between OMZ-Iberia and Finisterra(ca. 390370?Ma) and its rapid closure (ca. 370350 Ma), events thatFig. 6. Interpretative cross sections at the end of (A) the cold obductiby long-term high heat ow sustained by (1) mac magma under-plating, (2) stacking of high-heat producing upper-crustal lithologies,and (3) (moderate to) rapid crustal uplift. Mass advection andorogenic architecture were strongly inuenced by asymmetricremoval towards the foreland and by transient mechanical propertiesof the wedge system associated with the anomalous thermal regime.

4.2. The Rheic ocean

To reconstruct the history of RheicOcean, the general context of theEuropean Variscides must be considered (McKerrow et al., 2000;Robardet, 2003; von Raumer and Stampi, 2008, Nance et al., 2010this issue). During the Early Ordovician, Avalonia drifted away fromon stage (~370 Ma) and (B) the hot obduction stage (~345 Ma).

418 A. Ribeiro et al. / Gondwana Research 17 (2010) 408421the Armoricain Terrane assemblage, opening the Rheic. This eventwasdiachronous along the Variscides as a result of a zipping mechanism.We favour a model in which drifting progressed from E to W alongpalaeogeographic zones that were less curved in the Lower Palaeozoicthan in Upper Palaeozoic, thereby explaining why the onset of Rheicclosure is older in the east than in the west. This interpretation isconsistent with data reported for both Mid-European (Franke, 2000)and SW European transects (Ribeiro et al., 2007), and the fact thatsubduction usually starts in the older segments of any ocean andpropagates towards the younger segments. Indeed, remnants of aSilurianDevonian arc are preserved on the southernmost margin ofthe RhenishMassif (Franke, 2000); a situation that contrasts with thatof the Iberian transect, where bimodal magmatism extends from theLower Ordovician to the end of the Silurian in the Iberian Terrane andOssa-Morena Zone. Closure of the Rheic Ocean induced the opening ofsmall ocean basins within the Armoricain Terrane assemblage, such asthe Saxo-Thuringian Ocean (Franke, 2000) and the Mid-European,Massif Central-Galicia (Matte, 2002) or Palaeotethys (von Raumeret al., 2002) Ocean between Armorica to the north and the IberiaAquitaine to the south (present geographical coordinates). Thelocation and widths of these small oceans varies along strike in theVariscides, due to the interplay of transform faults between differentmicro-continents and magmatic arcs. In the case of Iberia/Armorica,Palaeotethys abutted against the PTFASZ, a leaky intra-continentaltransform that had been active since ca. 500 Ma and separated easternIberia fromwestern Finisterra, a ribbon continent (Murphy et al., 2008,2010this issue) at the margin of Gondwana (Ribeiro et al., 2007).

The inferred kinematic evolution of the plates involved in RheicOcean closure (Iberia, Finisterra and the South-Portuguese Terrane equivalent to Avalonia; Ribeiro et al., 2007) is schematicallyillustrated in Figs. 4 and 5. During the subduction/obduction stage,the PTFASZ (a NS dextral transform, as described above) played acrucial role controlling the kinematics. Shear towards the NE in thecold obduction stage suggests that the orientation of the PTFASZchanged fromNS to NESW at the southern tip of the Finisterra Plate.This was due to a rheological change from intra-oceanic transformwithin Rheic lithosphere to intra-continental transform along theFinisterraIberia boundary. The development of a triple junctionbetween Finisterra, Iberia and Avalonia may be inferred by analogywith the San Andreas (to NNW, dextral) and Mendocino (to W,dextral) transforms at the Mendocino triple junction (e.g. Dickinsonand Snyder, 1979).

As subduction of the Rheic Ocean proceeded beneath Iberia, thehot obduction regime became oriented towards NS convergence,because it was dominated by dextral movement on the PTFASZwithinthe continental domain. The PTFA transform was consumed bysubduction until complete closure of the Rheic Ocean and relatedback-arc basins, which was followed by collision of Finisterra, Iberiaand Avalonia. The advance of further deformation through intra-continental collision and continued NS dextral displacement becamecompatible with NESW convergence in the main subduction/collision zone to the SW (presently traced by the Ferreira doAlentejoSerpa thrust zone) and sinistral transpression in the intra-plate TBCSZ near the OMZ-CIZ boundary (Romo et al., submitted forpublication). The progress of intra-continental deformation bysinistral transpression in the Iberian branch of the IberoArmoricanArc (Ribeiro et al., 2007, and references therein) and movement onleft-lateral shear zones, changed from WNWESE to EW in the SWIberia domain as the arc was gradually bent during orogenic episodesin the interval ca. 350300 Ma.

4.3. Emplacement of allochthonous complexes and variscan ophiolitebelts

The geodynamic evolution proposed here, with antithetic obduc-

tion and ake generation (Ribeiro et al., 2007), is consistent with: (1)the general geometry and kinematics of structures in the Allochtho-nous Complexes and OMZ Relative Autochthon to the west of theTerena syncline (Fonseca et al., 1999; Arajo et al., 2005); (2) thepalaeogeographic setting of the Terena Basin, which was fed fromthe west on its western side and from the east on its eastern side;(3) evidence for continued NEdirected subduction beneath the OMZfrom Upper Devonian to Lower Carboniferous times (Jesus et al.,2007); and (4) geophysical imaging of lithospheric structure(Carbonell et al., 2004; Vieira da Silva et al., 2007; Muoz et al.,2008). However, the geodynamic model requires the maintenance ofa high temperature thermal regime over a broad time interval,probably caused by long-lasting oblique ridge subduction and,subsequently, by slab beak-off following the onset of obliquecontinental collision between the OMZ and SPZ (as a consequenceof subduction blocking). This scheme may also explain differences inthe age, evolution and emplacement mechanism of the IOMZOS andBAOC. Their protoliths differ in age, being ca. 480 Ma (LowerOrdovician) for the IOMZOS and Middle/Upper Devonian for theBAOC. The geochemical signature of the IOMZOS is transitionalbetween N-MORB and E-MORB, favouring an afliation with Rheicoceanic lithosphere and contrasting with that of the BAOC, whichpoints to a suprasubduction and a narrow back-arc basin. Onset ofobduction took place near the Silurian/Devonian boundary forIOMZOS and in the late Middle Devonian for the BAOC. Emplacementis governed by cold obduction for the IOMZOS, inducing a HPLTregime in the lower plate at ca. 370 Ma (Moita et al., 2005; Arajoet al., 2005), and by hot obduction for the BAOC, triggering a HTLPregime at ca. 355345 Ma (Jesus et al., 2007). The N- to NE-directedtransport direction for both ophiolites is, nevertheless, similar, andemplaced the IOMZOS onto gneisses of unknown ages that areconsidered part of Finisterra, and the BAOC onto the OMZ Allochthon.The HPLT regime related to IOMZOS emplacement is slightly older(ca. 370 Ma) than the HTLP regime related to BAOC emplacement(ca. 350 Ma). This favours piggy-back emplacement of the IOMZOS bylater thrusting of the BAOC, which structurally underlies the Mouramlange (locally recording blueschist metamorphic conditions)related to emplacement of the IOMZOS (Arajo et al., 2005).Nonetheless, the transpressive nature of the SW Iberia suture at theintersection with the dextral PTFA transform suggests obliquesubduction/obduction with migration of obduction from N to S. Thisreasoning is consistent with several important observations: (1) theIOMZOS extends further N than the BAOC; (2) the IOMZOS standsgeometrically above the BAOC; and (3) IOMZOS klippen extend atleast 50 km to NE of the OMZ/SPT suture, whereas the BAOC is anarrow band no more than 1.5 km wide along the suture. Hence, theake geometry of antithetic obduction propagates from N to S withcontinued top-to-the-NE thrusting continuing for at least 20 Ma (andpossibly 50 Ma).

Nevertheless, many problems presently remain open. One is theage of the opening of the BejaAcebuches back-arc basin. Fieldrelationships indicate that the basin must be older than the UpperDevonian mlange of PeraMora (NW Huelva; Eden and Andrews,1990) and close to the age of oceanic-tholeiite dykes that intrude thePulo do Lobo Unit (Devonian) at Trindade (Munh, 1983). SmNdmodel ages of 380440 Ma (Mullane, 1998) are compatible with theseconstraints and suggest a time-span of ~3050 Ma for the openingand closure of a slow and diffuse spreading back-arc basin that shouldhave been quite narrow in comparison with the Rheic Ocean. Anotherproblem involves the connection between the Beja and Aracenatraverses. These are separated by the Ficalho WSWENE sinistralstrike-slip fault, which is conjugate to the PTFASZ and displaying anoffset of ca. 30 km. In the Aracena traverse (Daz Aspiroz et al., 2004and references therein) the BAOC was thrust to the SW over the SPTwithout evidence for either antithetic obduction or klippen, like thoseof the IOMZOS. This favours a model of synthetic obduction with

opposite polarity to that of the Beja traverse. An additional issue

419A. Ribeiro et al. / Gondwana Research 17 (2010) 408421waiting for a satisfactory solution concerns the exhumation process ofthe HPLT metamorphic assemblages.

5. Conclusions

The SW Iberia Variscan suture corresponds to the Iberian/South-Portuguese terrane boundary, which traces the closure of the Rheic andrelated oceans byNNE subduction of the latter terrane under the former.The western segment of this suture displays the imbrication of variousunits belonging to: (1) the NeoproterozoicLower Palaeozoic OMZRelative Autochthon, which passes up into LowerMiddle Devonianvery-low grade metasediments; (2) an unconformable cover of LowerCarboniferous calc-alkaline Volcanosedimentary Complexes that re-cord very-low grade metamorphism; and (3) Allochthonous Com-plexes. The latter include disrupted slivers of eclogite (ca. 370 Ma)klippen and slices of an internal ophiolite sequence (IOMZOS), a basaltectonic mlange (Moura Phyllonitic Complex), and an externalophiolite (BejaAcebuches Complex, BAOC). A large layered gabbroicsequence (LGS) intruded (at ca. 350340 Ma) the SW suture domains.

According to the available data, the IOMZOS comprises obducted(dismembered and highly incomplete) remnants of a wide ocean.Preliminary geochronological information dates the IOMZOS at 4795.1 Ma, indicating a source thatwas contemporaneouswithper-alkalinemagmatic activity in the northern OMZ continental autochthon (at ca.480500 Ma), and favouring the interpretation that their protoliths arerepresentative of the Rheic Ocean. This ocean, which separated Iberiafroma promontory of Gondwana andAvalonia (now represented by theSPT), opened near the CambrianOrdovician boundary through a rift-jump from an intra-cratonic setting (within the Iberian Terrane) to anintra-oceanic setting. Conversely, rocks of the BAOC outline the OMZ/SPT suture and preserve evidence of early obduction to the N. SmNdmodel ages (380440 Ma) and geochemical characteristics of the BAOCare compatible with its derivation from a short-lived (LowerMiddleDevonian?) back-arc basin.

The Variscan ophiolite belts preserved within the SW Variscideswere emplaced by antithetic obduction in two stages, both withshearing senses toward the NNE. The rst stage involved coldobduction of the IOMZOS, exhumation of HP-rocks (at ca. 370 Ma),and the creation of a foreland bulge towards the SW ank of the OMZ.This mechanism controlled the development of the volcanic-relatedcarbonate shelves and the contemporaneous Terena ysch troughfurther NE. The second stage corresponded to hot obduction of theBAOC, which carried the IOMZOS klippen in piggy-back style andaffected the southern margin of LGS at ca. 350340 Ma. Late-orogenicwedge thickening affected the retro-wedge domain. Early magmaticpulses were related to astenospheric mantle upwelling induced byslab break-off and resultant thermal regime sustained long-term highheat ow, resulting in regional LPHT metamorphism and successivemagmatic activity (until ca. 300 Ma).

Acknowledgements

The authors acknowledge funding from FCT (MCTES, Portugal)awarded through the research project POCA-PETROLOG (UI: 263;POCTI/FEDERCeGUL), and on-going one-year grants from the researchunit Centro de Geofsica de vora. Paul Ryan and an anonymous refereeare thanked for their careful reviews of the manuscript and helpfulcomments. The authors are also grateful to DamianNance for the carefulediting of the manuscript.

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421A. Ribeiro et al. / Gondwana Research 17 (2010) 408421

Variscan ophiolite belts in the Ossa-Morena Zone (Southwest Iberia): Geological characterizatio.....IntroductionGeological settingKey geological unitsOMZ relative autochthonDevonian metasediments and lower carboniferous volcanosedimentary complexesAllochthonous complexesInternal OMZ Ophiolite Sequences (IOMZOS)BejaAcebuches Ophiolite Complex (BAOC)

Pulo do lobo complexLayered Gabbroic Sequence (LGS) of Beja Igneous Complex (BIC)

DiscussionGeodynamic evolutionThe Rheic oceanEmplacement of allochthonous complexes and variscan ophiolite belts

ConclusionsAcknowledgementsReferences