Tectonophysics 461 (2008) 395–413

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Genesis and evolution of a syn-orogenic basin in transpression: Insights frompetrography, geochemistry and Sm–Nd systematics in the Variscan Pedroches basin(Mississippian, SW Iberia)

Maider Armendáriz a,⁎, Rafael López-Guijarro a, Cecilio Quesada a, Christian Pin b, Félix Bellido a

a Instituto Geológico y Minero de España, Ríos Rosas 23, 28003 Madrid, Spainb Laboratoire de Géologie, CNRS & Université Blaise Pascal, 5 rue Kessler, 63038 Clermont-Ferrand, France

⁎ Corresponding author. Tel.: +34 91 7287288; fax: +3E-mail addresses: m.armendariz@igme.es, maiderad

0040-1951/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.tecto.2008.02.007

a b s t r a c t

a r t i c l e i n f o

Article history:

The Pedroches basin is an ou Received 24 May 2007Received in revised form 11 January 2008Accepted 14 February 2008Available online 25 February 2008

Keywords:SW IberiaVariscan orogenyPedroches basinTranstension/transpressionSm–Nd systematics

tstanding syn-orogenic Variscan (Mississippian) depocenter, located in the vicinityof, and overstepping onto the Neoproterozoic (Cadomian) suture between the OssaMorena and Central Iberianzones of the SW Iberian Massif. Its formation appears to be related to transtensional processes at a majorreleasing bend in the reactivated suture during sinistral escape of the Ossa Morena Zone from the zone offrontal Variscan collision between northern Gondwana and Laurussia. Subsequent basin inversion resulted incompartmentalization and internal deformation, and was probably related to transpression at a restrainingbend along the reactivated suture (Badajoz–Córdoba shear zone). Basin fill consists of both igneous andmetasedimentary rocks generally deposited in a shallow marine, storm-dominated platform environment.By using new petrographic, geochemical and Sm–Nd isotopic data, we attempt to characterize: 1) the sources andgeological evolution of the Pedroches basin fill, and 2) the processes involved in basin development and sedimentsupply and dispersal. Sampling was carried out in both sedimentary and igneous rocks belonging to three majorstructural unitswithin the central partof thebasin:Guadiatounit,Guadalbarbounit andSouthPedrochesValleyunit.The combined Nd isotopic and geochemical data from the PedrochesMississippianmetasedimentary rocks indicateanupper continental crust provenance and ahighdegree of sedimentary recyclingof thesematerials throughout thebasin; i.e., they are derived from old, recycled upper crust without any significant juvenile component despite thepresence of interbedded basaltic rocks. The igneous rocks in turn correspond to submarine basalts and trachy-andesitic basalts plus some hybrid rocks including crust-derived components. Concerning the mafic rocks, majorelement compositions reveal a tholeiitic and transitional tholeiitic character for the Guadalbarbo unit rocks (MORBtype) andanalkalineaffinity for thoseof theGuadiatounit. Traceelement analyses, alongwithSm–Nd isotopic ratios,suggest two kinds of provenance sources for the Pedroches basin syn-sedimentary igneous rocks: 1) an importantcrust-contaminated and/or old, enriched mantle source for the Guadiato unit and the Pedroches Batholith, and 2) atime-integrated depleted mantle source for the Guadalbarbo rocks, in which no significant interaction withcontinental crust components is discernible. These latter rocks suggest that a strong attenuation, or even almostcomplete tectonic and/or thermal erosion of the continental lithosphere was reached, at least locally, in this region.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

The Variscan orogeny developed as a result of the collision betweenalready amalgamated Laurussia and Gondwana during closure of thePaleozoic Rheic Ocean (e.g., Matte, 1986, 1991, 2001, 2002; Martínez-Catalán et al., 1997). In western Europe, the prominent Ibero-ArmoricanArc (inset in Fig. 1) has been interpreted in connectionwith collision of apromontory in northern Gondwana (Ibero-Aquitanian Indentor; BrunandBurg,1982; Burg et al.,1987)with Laurussia (Matte andRibeiro,1975;Lefort,1979; Burg et al.,1981,1987; Brun and Burg,1982). In our view, themain structural features and the overall evolution of, and around the arc,

4 91 7287202.@yahoo.es (M. Armendáriz).

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are clearly explained by this indentor model, even though much of thepresent curvature was acquired later, in the Permian, as suggested bypaleomagnetic data (VanderVoo et al.,1997;Weil et al., 2000, 2001;Weiland Van der Voo, 2002; Gutiérrez-Alonso et al., 2004; Weil, 2006). TheIberian Massif (Fig. 1) occupies the southern half of the Ibero-ArmoricanArc and shows extremely contrasting structural development betweenits northern and southern segments. In the southern Iberian Massif,considered a world-class example of a transpressional orogen (seeQuesada, 2006; Quesada et al., 2006 and references therein), left-lateraltranscurrent regimes characterized themost part of its tectonic evolutionduring the Variscan orogeny (see under Ribeiro et al., 1990; Quesada,1991, 2006; Quesada et al., 2006, for the overall tectonic scenario).

The Variscan collision started in the Late Devonian and waned bythe Early Permian. Once collision started, syn-orogenic basins began

Fig. 1. Zonal division of the Iberian Massif (after Julivert et al., 1974; Quesada, 1991).

396 M. Armendáriz et al. / Tectonophysics 461 (2008) 395–413

to develop at appropriate locations within the orogen. One such a caseof syn-orogenic basin formed in southwest Iberia in the vicinity andrelated to the evolution of the tectonic boundary between the CentralIberian and OssaMorena zones during the Early Carboniferous: the so-called Pedroches basin (Gabaldón et al., 2004; Fig. 2). Owing to syn-and post-sedimentary deformation, the basin fill mainly occurs atpresent along rather small and narrow, generally fault-boundedoutcrops, with the exception of those, much larger, in the PedrochesValley area. They often show different sedimentary and stratigraphiccharacteristics (Quesada and Garrote, 1983; Gabaldón et al., 1985). Mostrocks are Viséan in age, but late Tournaisian and earliest Namurian rocksalso occur locally.

The main objective of this paper is to characterize the sources andgeological evolution of the Pedroches basin fill, as well as the processesinvolved in basin development, sediment supply and dispersal andcoeval volcanic activity. In this study we present new geological,petrographic, geochemical and Sm–Nd isotopic data in order to provideinformation about the provenance of the basin fill as well as of theregional and local tectonic setting during its evolution. The data comefrom metasedimentary and metaigneous rocks that belong to threestructural units, which collectively provide a south to north, nearlycomplete section across the central part of the basin: Guadiato unit,Guadalbarbo unit, and South Pedroches Valley, including the areapresently occupied by the Pedroches Batholith (Fig. 2).

2. Geological setting

The Mississippian Pedroches basin is located in the vicinity of,and overstepping onto the Neoproterozoic (Cadomian) suturebetween the Ossa Morena (OMZ) and Central Iberian (CIZ) zonesof the SW Iberian Massif (Quesada, 1983, 1991) (Fig. 1). It developed

as a syn-orogenic depocenter in response to largely oblique(sinistral) convergence during the Late Paleozoic Variscan orogeny(Gabaldón et al., 2004) (Fig. 2). Its formation was related to trans-tensional processes at a major releasing bend in the reactivatedsuture during south-eastward sinistral escape of the OMZ from thezone of frontal Variscan collision between northern Gondwana andLaurussia (Quesada, 2006; Quesada et al., 2006). This escapeallowed for a very pronounced along-strike migration of the OMZfrom northwest to southeast, by reactivation of the pre-existingCadomian suture, thence becoming the present Badajoz–Córdobashear zone (Burg et al., 1981; Quesada and Dallmeyer, 1994). By theLate Viséan, deformation in the marginal part of Gondwanaoccupied by the OMZ was still transpressional but with a highercomponent of across-strike shortening, due to the collision with theSouth Portuguese Zone (Laurussia), along which closure of this partof the Rheic Ocean was completed. Subsequent inversion of thePedroches basin produced internal deformation of the fill andenhanced compartmentalization, and was probably related totranspression at a restraining bend along the reactivated suture(Gabaldón et al., 2004; Quesada et al., 2006), aided by the afore-mentioned change to a more orthogonal regional strain regime. As aresult the OMZ was thrust on top of the CIZ, turning the basin into aperipheral foredeep, though still with a significant strike-slipcomponent. Many horse sinistral boundary faults were reactivatedas oblique thrusts, progressively leading to uplifting the region,which became integrally emerged by the Early Westphalian. Thisnorthward thrusting of the OMZ is clearly visible in the field (Pérez-Lorente, 1979; Quesada and Garrote, 1983; Pereira, 1999) and alsoimaged in the 3–4 upper seconds of the Iberseis deep seismicsounding profile (Simancas et al., 2003). In this profile, presence ofnorth-dipping reflectors in the basal upper crust (4–7 sec depth)

Fig. 2. Simplified geological map of the SW Iberian Massif showing the location of Mississippian units (based on Mapa Geológico de España 1:1.000.000, I.G.M.E. 1995). A–A'approximate location of the sections depicted in Fig. 9.

397M. Armendáriz et al. / Tectonophysics 461 (2008) 395–413

above a prominent, gently south-dipping mid-crustal reflector, haveno direct correlation with surface structures and their interpretationremains open to further research.

According to an idealistic paleogeographic reconstruction by Gabal-dón et al. (1985), which still appears to be valid (Fig. 3), most part of thesoutheasterly escaping OMZ was emerged by the Mississippian. Withinit, terrestrial syn-orogenic sedimentation occurred in fault-bounded,fluviatile and lacustrine basins, being theValdeinfiernoCoalfield the onlypresent remain of such type of basins (Wagner, 1978; Gabaldón et al.,1983; Quesada and Garrote, 1983) (Fig. 2). The southern margin of thePedroches basin, roughly runningWNW–ESE; i.e., parallel to the overallorientationof thebasin (Fig. 2), is characterizedby transitional, terrestrialto coastal sedimentary sequences, which unconformably onlap ontometamorphic OMZ rocks, most commonly onto the Precambrian base-ment (Garrote and Broutin, 1979; Quesada, 1983; Quesada and Garrote,1983; Gabaldón et al., 1985). This southern margin lies very close andoverlaps onto the (up to 50 kmwide, if brittle deformation is considered)Badajoz–Córdoba shear zone, which defines the boundary between theOMZ and the CIZ (Figs. 1 and 2; see Burg et al., 1981; Quesada andDallmeyer, 1994; San José et al., 2004, for a historical review of the

significance and discrepant views around this complex lineament). Theaforementioned compartmentalization was preferentially developedin this part, proving that displacements along this major lithosphericlineament continued during the whole life of the basin and beyond.

The central and northern parts of the Pedroches basin fill sit througha variably large sedimentary hiatus and/or erosional unconformity ontoLate Devonian metasedimentary rocks of the CIZ and consist of distal,mainly tempestitic and turbiditic deposits (Gabaldón et al., 1985;Gabaldón, 1989). No evidence of the northern margin of the basin ispreserved, though some terrestrial to marine transitional facies occur inan isolated outcrop at the core of the Guadalmez syncline, some tens ofkilometres north of themain exposure of Mississippian rocks (Quesada,1983) (Fig. 2).

Very significantly, three roughly syn-sedimentary, sigmoidal shapeigneous belts discontinuosly split the Pedroches basin (Figs. 2 and 3); i.e.,from south to north, Villaviciosa de Córdoba–La Coronada, Varas–Guadalbarbo and Pedroches Batholith, across which severe facies,structural and paleogeographic changes took place. It must beemphasized, however, that with the only exception of the transitionalfacies at and near the southern margin, the basin fill consists in general

Fig. 3. Sketch paleogeographical model for the Ossa Morena Zone in Lower Carboniferous times (modified after Gabaldón et al., 1985).

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terms of both metaigneous and metasedimentary rocks deposited inrelatively shallow, open continental shelf environments, inwhich stormactivity was the principal mechanism of sediment supply and dispersal(Quesada and Garrote, 1983; Gabaldón et al., 1985; Gabaldón, 1989). Nosedimentaryevidence exists suggestingdevelopmentof adeep troughatany time and at any location, at least within the present basin exposure.

3. The Pedroches syn-orogenic basin

Every isolated exposure of Carboniferous rocks in SW Iberia wasthought to represent a different basin up to the early 1980s when forthe first time some integrated studies on sedimentology, litho- andbiostratigraphy, petrology and structural geology allowed establishinga coherent correlation among them all as different parts of a large,single marine basin: so-called Northern OssaMorena basin at the time(Quesada, 1983; Quesada and Garrote, 1983; Gabaldón et al., 1985).Subsequent studies have confirmed this interpretation and providedmany details on the overall constitution and evolution of the basin(e.g., Quesada et al., 1990; Rodríguez-Martínez et al., 2000; Cózar et al.,2004, 2006; Armendáriz et al., 2005, 2007a; Rodríguez-Martínez,2005; Armendáriz, 2006). The name Pedroches basin, which standsfor the Pedroches Valley, a wide region in northern Córdoba provincewhere most of the basin fill is exposed, was first published byGabaldón et al. (2004) and is preferred here since it makes reference toa clearly established geographic area, not alike the ambiguous formername Northern Ossa Morena basin.

3.1. Structural division

As it has been repeatedly stated, the Pedroches Basin is presentlycompartmentalized into many fault-bounded structural units. Thisstatement is particularly true for the southern part, in which thebasin developed within, and was affected by deformation along, the

Badajoz–Córdoba shear zone. The main structural elements with asignificant paleogeographic influence on basin evolution were thethree igneous belts referred to in the previous section (from south tonorth: Villaviciosa de Córdoba–La Coronada, Varas–Guadalbarbo andthe area now occupied by the Pedroches Batholith, the bulk ofwhich was emplaced ca. 310Ma, after the inversion of the basin). Theyallow for a first order division of the basin into several structural/paleogeographic domains shown in Fig. 2. These are in turn internallysubdivided into second order structural units, especially towards thesouthern margin. Therefore, the following major structural divisionsare to be considered (from south to north):

– Benajarafe–Berlanga southern marginal unit– Villaviciosa de Córdoba–La Coronada igneous belt– Guadiato unit– Guadalbarbo (volcanic) unit– South Pedroches Valley unit– Pedroches Batholith– North Pedroches Valley unit.

Volcanic rocks occur scattered throughout the basin towards thebottom of the respective stratigraphic columns but are massive andconstitute most of the stratigraphic record within the three igneousbelts, the sigmoidal shape of which (Fig. 2) is consistent with sinistralmovements after and during emplacement. Not all three igneous beltshad the same paleogeographic importance. The Villaviciosa deCórdoba–La Coronada igneous belt is presently formed of bothplutonic and volcanic rocks, the latter showing evidence of subaerialeruption locally (Pascual, 1981; Sánchez-Carretero et al., 1989). It musthave formed, therefore, a belt of volcanic islands isolating a marginal,partially disconnected lagoon area from the main part of the basin(Quesada and Garrote, 1983; Gabaldón et al., 1985).

The Guadalbarbo belt apparently was much less important in thepaleogeographic differentiation of the basin fill. It is exclusively formed

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of submarine,mainly basaltic volcanic rocks, interbeddedwith turbiditicmetasediments. TheirN- toE-MORBcomposition (see below) indicates asource in the mantle for the volcanic rocks and perhaps an attempt ofoceanization (the so-called Varas–Guadalbarbo ophiolite; Crousilleset al., 1976; Crousilles and Dixsaut,1977), or at least extreme thinning ofthe CIZ crust in this part of the basin. Some evidence exists (see underSection 4.1) suggesting that this part may have constituted a relativetrough with respect to the adjacent domains, at least temporarily.

Finally, the very large Pedroches Batholith (Fig. 2), which presentlyconsists almost exclusively of shallow plutonic rocks intruded into thealready deformed basin fill, has not been at first glance considered tohave had a profound influence on sedimentation. Towards theirmarginsor within roof-pendants, however, some relics of syn-sedimentary vol-canic rocks (Quesada, 1983; Gabaldón et al., 1985) and presence of veryshallow water quartzarenite interbeds in the tempestite/turbiditesuccession (Larrea et al., 1992) indicate that here also a significantpaleogeographic boundarymay have existed during basin development,though verymuch obscured by subsequent intrusion, uplift and erosionof the syn-sedimentary volcanic belt.

According to available geochronological data, igneous activitywithinthe two belts in which plutonic rocks are exposed (Villaviciosa deCórdoba–La Coronada and Pedroches Batholith) extended well beyondbasin development and inversion (up to c. 310Ma;Garrote and Sánchez-Carretero, 1983; Gabaldón et al., 1985; Fernández-Ruiz et al., 1990;Alonso-Olazábal et al.,1999; Donaire et al.,1999; Alonso-Olazábal, 2001;García de Madinabeitia et al., 2001; García de Madinabeitia, 2002). Thisfact probably implies an alternation of multiple transtension/transpres-sion events as well as persistence of the underlying mantle thermalanomaly responsible for magma generation.

In this paper we present new data, relevant for the understandingof the basin development and evolution, coming from metasedimen-tary andmetaigneous rocks that belong to three structural units in thecentral part of the basin, from south to north: the Guadiato unit, theGuadalbarbo unit and the South Pedroches Valley unit (including thearea presently occupied by the Pedroches Batholith (Fig. 2).

3.2. Stratigraphy of the basin fill

Amajor limitingproblemfor the studyof this basin lies in the scarcityof fossil remains in most of the sedimentary successions making up itsfill. Only near the southernmargin, where abundant plant remains existin the terrestrial and transitional sequences (Garrote and Broutin, 1979;Quesada, 1983; Quesada and Garrote, 1983; Wagner, 1983, 1999), and inthe sparse carbonate rocks (e.g.,Mamet andMartínez-Díaz,1981; Cózar-Maldonado, 1998; Cózar and Rodríguez, 1999; Cózar, 2000; Rodríguez-Martínez et al., 2000; Cózar et al., 2004, 2006; Rodríguez-Martínez,2005), which on average do not exceed a few percent of the strati-graphic record, there is some control on the age of the sedimentaryfill of the basin. Terrigenous rocks, making up more than 90% of thefill, barely contain any identifiable, stratigraphically valuable fossils.Variably altered palynomorphs appear from time to time but, whenidentifiable, they show evidence of strong reworking of non-coevalremains, making them extremely difficult to use for stratigraphicpurposes (Rodríguez, Unpubl. Rep., 2003). Despite all these limita-tions, it canbeestablished that the Pedrochesbasin started todevelopasa syn-orogenic foredeep not later than the late Tournaisian, and perhapsearlier (Garrote and Broutin, 1979; Quesada, 1983). Its life as a marinebasin extended for most of the remaining Mississippian, certainly up tothe Early Namurian (Serpukhovian) (e.g., Gabaldón et al., 1985; Cózaret al., 2004, 2006).

From a lithostratigraphic point of view, when exposed, the basalsequence in all the units starts with a variably thick fluviatile to deltaicpolimict conglomerate, which overlies the local basement eitherunconformably (in the south) or paraconformably. In the case of thethree units dealt with herein the local basement corresponds to eitherPrecambrian metamorphic rocks or shallow marine, Early to Late

Devonian quartzarenites, mudstones and rare limestones. The latterrepresent deposition during the latest stages of the evolution of theCIZ as an internal, proximal part of the Gondwanan passive margin ofthe Rheic Ocean prior to the arrival of the Variscan deformation front(Quesada, 1991), the first expression of which being the uplift anderosion of such passive margin. Subsequent subsidence, probablyrelated to transtension, initiated basin development.

Above the basal conglomerate the lower part of the basin fillsuccession, which may reach up to several hundred metres inthickness, shows the highest variability in terms of lithology andfacies. Most volcanic rocks are located within the lower few hundredmetres, within and in between the igneous belts, suggesting theimportant role of lithosphere-through, transtension-related faultingin the origin of the basin. Within the igneous belts, probably locatedatop major lithospheric lineaments, igneous activity extendedthroughout the whole life of the basin and even beyond. Sedimen-tary rocks within this lower part of the successions are representedby deltaic to shallow-marine siliciclastics, including some conglom-erates, which interfinger with the volcanic rocks.

The upper part of the successions in our three units, as well as inthe rest of the basin, is very similar and corresponds to flyschoid,greywacke-mudstone alternations (Culm facies); the differences amongunits residing in their variable thickness (up to several thousandmetres), local facies variations and vertical evolution. Conglomeraticdebris flows locally occur interbedded within this sequence, witnessingfor syn-sedimentary tectonic activity. This change is interpreted asevidence for the transition from an overall transtensional regime duringbasin generation to an overall transpressional one, during which thebasin was transformed into a sort of peripheral foreland basin. A verypeculiar rock unit delineates the transition from the lower to the upperpart of theMississippian succession in central and northern units (fromthe Guadalbarbo unit northwards). It consists of a few metres thickpackage of radiolarian, iron oxide-rich purple shale (Haematite Dust ofPérez-Lorente, 1979), which is interpreted as a condensed sequencerecording initial flexural subsidence in the newly transformed forelandbasin. This evolution may have started from the Late Viséan whencomplete closure of the remaining Rheic Ocean south of the OMZ,brought the South Portuguese part of Laurussia to collidewith this outermargin of Gondwana (Ribeiro et al., 1990; Quesada, 1991). This collisionforced the upthrusting of the OMZ onto the CIZ and the interveningPedroches basin that had been recently created by transtensionalprocesses along theboundary between the two. As a result, the southernbasin margin became an upthrusting active one but still with a signifi-cant strike-slip component throughout its evolution (Gabaldón et al,2004). Towards the north, the present boundary of the Carboniferousexposure is defined by an inverted, originally extensional fault (SantaEufemia Fault, Fig. 2). We interpret this fault to represent the southernboundary of a syn-sedimentary forebulge, which developed to accom-modate the flexural response of the CIZ crust subjected to loading by theupthrusting of the OMZ onto its southern margin. According tostructural vergence, inversion and deformation of the basin fillpropagated across strike from south to north, causing the progressiveuplift and emergence of the basin in the same direction.

According to Gabaldón et al. (1985) sedimentation during this finalmarine stage took place on an open, relatively shallow continental shelfdominated by storm activity. Apart from local tectonically drivencollapse and formation of debris flow deposits, sediment supply anddispersal were mostly controlled by storm surge ebb currents thatproduced turbiditic sand bodies of a storm sand layer type (tempestites).When deposited above storm wave base, they were reworked to formhummocky cross-bedding and wave ripples as storm activity waned.Locally and/or at periods of enhanced subsidence, some layers reachedareas below storm wave base, being then indicated by classical Boumasequence turbidites. Lateral and vertical alternation of both types offacies indicates that deposition took place on a continental shelf duringall the basin evolution. This also applies to the Guadalbarbo unit, where

Fig. 4. Carbonate olistoliths in the Central Guadiato unit near Adamuz.

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MORB-like basalts occur, suggesting that no real oceanic crust formedthere in Carboniferous times. Fair-weather deposition is representedeverywhere by mud settling and intense bioturbation. In general terms,the lack of molasse-type sediments in most units may be interpreted asevidence for an underfilled (starved) nature of the basin throughout itsevolution; i.e., subsidence equalled or exceeded sediment supply.

An exception to this general scenario occurs in the Guadiato unit.This,which is integrally locatedwithin the Badajoz–Córdoba shear zone,is by far the most complex unit in the whole basin. The first peculiarfeature is the fact that it sits onto the actual boundary between the OMZand the CIZ. Internally, it consists of three main second order structuraldivisions, presently separated by north-verging sinistral thrusts. Theintermediate thrust sheet (Central unit of Quesada and Garrote, 1983),whose northern margin is the actual zones’ boundary fault, contains avery unique stratigraphic record. The main singularity resides in thedevelopment of relatively important Late Viséan to Serpukhoviancarbonate platforms (Armendáriz et al., 2005, 2007a; Armendáriz,2006; Cózar et al., 2006) at the onset of the foreland basin stage. Theseare succeededby the “normal”flyschoid successionsbut in this case theyare punctuated by interbedded chaotic mélange deposits, locally verythick, in which up to hectometre-size carbonate platform olistolithsoccur in profusion (Fig. 4) (e.g., Moreno-Eiris et al., 1995; Sarmiento andGutiérrez-Marco, 1999; Cózar et al., 2004; Armendáriz et al., 2005;Armendáriz, 2006). Some of the mélange deposits extend into theadjacent Guadalbarbo unit to the north but, there, no carbonates otherthan theolistoliths are found. The implication of this unique stratigraphyis that, contrary to the rest of the basin, the Central Guadiato unit mayhave been temporarily subjected to uplift (transpressional?) while therest of units were already undergoing flexural subsidence. Subsequentincorporation of this unit into the overall subsiding foredeep, may havebeen responsible for the collapse of the uplifted block and its carbonatecover in particular, triggering the formation of the spectacular chaoticmélange. Also in this unit an upward transition to molasse typesedimentation (deltaic and eventually fluviatile facies, with some coalmeasures of Early Namurian age; Quesada, 1983; Wagner, 1983, 1999;Quesada andGarrote,1983;Wagner and Jurado,1988), is an exception tothegeneral evolution, andmay indicate thepreservationof itsfinal upliftduring inversion.

4. Provenance of the basin fill

In this study we present new petrographic information, Sm–Ndisotope and whole-rock geochemical data, in order to determine theprovenance of the basin fill in the three units under consideration,which may in turn help to constrain the geological evolution of thePedroches basin. By basin fill wemean both the metasedimentary andthe metaigneous rocks deposited in between its margins.

4.1. What does the petrography tell us?

When exposed, the basal conglomerate of the Pedroches basin onlyincludes clasts derived from the local basement. This is an importantpoint to prove an autochthonous relationship of the basin with itsunderlying basement, despite the significantly large displacements thatmay have experienced some parts of it with respect to others. Inconclusion, basin and basementmust have travelled in solidarity. Higherup in the stratigraphy,mostmetasedimentary rocks arefineor veryfine-grained; thus very difficult to use to characterize their sources bymeansof petrography. Conglomerates and debris flow deposits, though muchless common, are far more informative to this purpose. Most conglom-erates in the lower sequence and also in the uppermost part of theCentral Guadiato unit stratigraphy, correspond to fluviatile to deltaicfacies. They are thus representative of the adjacent emerged catchmentarea. Also, many conglomeratic debris flows in the tempestitic uppersequence represent the collapse of similar environments, thereforeproviding the same type of information. Very significantly and apartfrom obvious intrabasinal clasts (carbonates, volcanic rocks, slumpbreccia, etc.), the range of identifiable cobble lithologies in the Guadiatoand the Guadalbarbo units includes protoliths from both the OMZ(Neoproterozoic black cherts, various metamorphic lithologies, varioustypes of granitoids) and the CIZ (Ordovician, Silurian and Devonianquartzites, late Ordovician carbonates; Gutiérrez-Marco et al., 1987;Sarmiento andGutiérrez-Marco,1999). On the contrary, in the SouthandtheNorth Pedroches Valley units only cobbles derived from the CIZ havebeen so far identified. This difference allows making a first paleogeo-graphic division into two drainage systems within the basin, with abarrier impeding the transport across it of OMZ coarse-grainedsediments. It may not be fortuitous that the boundary be located atthe Guadalbarbo unit, where the typically oceanic volcanic rocks occur.The same applies to the dispersal of the carbonate-bearing mélangesthat do not trespass beyond the Guadalbarbo unit. Even if thesedimentary structures indicate relative shallow water environmentsalso in this unit, this area may have represented a relative trough (thereal basin centre?), at least temporarily.

A singular case deserving special attention is that of the spectacularmélange deposits appearing in the Guadiato and Guadalbarbo units.Obviously, presence of coeval carbonates as amajor constituent tells thatwe are dealing with collapsed intrabasinal elements. At least for thecarbonates the source is within the basin, probably in the CentralGuadiato unit, which is the only onewhere an in situ, Late Brigantian agecarbonate platform has been so far identified in the entire region(Armendáriz et al., 2005; Armendáriz, 2006; Cózar et al., 2006).According to the stratigraphic data and the age and facies similaritiesbetween both types of carbonates, it may be considered that theolistostromic deposits could have been generated by disruption of the

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identified platform, or others, presently unexposed,with similar charac-teristics (Armendáriz et al., 2005; Armendáriz, 2006). Apart from car-bonates themélange deposits contain numerous well-rounded, cobble-size clasts that include a mixture of lithologies easily attributable toprotoliths from both the OMZ and the CIZ. They may derive from theshallow marine deltaic deposits that immediately underlie the in situcarbonate platform described near the town of Adamuz (Armendárizet al., 2005; Armendáriz, 2006; Cózar et al., 2006). The dispersion ofthese chaotic deposits is restricted to the own Central Guadiato unit andto the adjacent Guadalbarbo unit to the north. Again it tells us about a

Table 1Distinctive macroscopic and petrographic characteristics of metasedimentary and metaigne

Sample Unit Macroscopic characteristics Petrog

MA-6 PV Very fine-grained purple shale alternating with coarser siliciclasticlayers. Interbeds of volcaniclastic rocks.

Abundargillaby orie

MA-7 PV Very homogeneous dark green shales with intercalated carbonatelayers.

Fine-gmetam

MA-52 PV Metapelite screens between “Cerro Bermejo” dikes affected bycontact metamorphism.

Foliatewhite

MA-33 PV Channel-fill sandstones with intercalations of conglomerates.Thinning and fining upward sequences, with markedly erosionalbases. Massive to large scale cross-bedded internal structure.

Mediutexturemetampluton

MA-56 Gd Medium-grained and dark grey colour sandstones.Intraformational microbrecciation is observed.

Fine-toblastopmatrixobserv

MA-4 Gd Very homogeneous and fine-grained dark shales. Intenselybioturbated.

Fine-gmatterdevelo

902-124 Gd Homogeneous black shales interbedded with medium-to-coarse-grained greywackes. Abundant plant fossil remains.

Fine-gremaincleavag

MA-8 Gb Very homogeneous and dark grey shales. Interbedded withsubmarine basaltic lava-flows.

Fine-ggrains.and no

MA-3 Gd Dark grey colour vesicular basalt. They belong to a c. 70 m thicksuccession of submarine lava-flows separated by thin lutiticinterbeds.

PartialPl, Ampof Pl, Osurrou

MA-5 PB Pedroches granodiorite, fresh sample collected in a quarry. Mediutexturein the

MA-58MA-59

PB Metadacite/meta-andesite in Esparragosa de La Serena (PedrochesBatholith).

Porphyrecrystpheno

MA-9880-1880-6902-89

Gb Dark green colour and masive submarine basalts with pillow-lavastructures. Mudstone layers are commonly intercalated throughoutthe volcanic pile.

Partiallphenocgroundaltered

MA-61902-93902-94

Gb Dark green colour volcanic breccia. Massive aphanitic vulcanitefragments in a scarce matrix.

VolcanmicrodTabulamicrop

MA-49MA-50MA-51

PB “Cerro Bermejo” metadolerite/microgabbro. Deformed andmetamorphosed basic dikes intruded in Pedroches culm and cut bythe Pedroches Batholith.

Metadsometicontac

PV: PedrochesValley unit, Gb: Guadalbarbounit, Gd:Guadiato unit, PB: Pedroches Batholith, Prinminerals. Mineral determination by X-Ray Difractometry (XRD). Minerals abbreviations: Qtz:Dm: detriticmica, Bt: biotite,Ms:muscovite, Chl: chlorite, Ab: albite, Cal: calcite,Mtm:montmorRt: rutile, Ap: apatite, Zrn: zircon, Op: opaque, Ep: epidote, Srp: serpentine, Prh: prehnite, Spl:

gradient in the basin floor towards the latter and not beyond, at leastduring this event.

Concerning the petrography of the volcanic rocks (Table 1) littlecan be said about their provenance, other than the obvious mantlesource of the mafic rocks. Nevertheless, their presence across most ofthe basin area, and its duration even after basin closure, provide clearindication of the existence of a very large, long-lasting thermalanomaly underlying its lithospheric basement, which we feel mayhave played a major, not fully understood yet, role in its generationand evolution. Presence of cm to dm-thick acid volcanic ash layers

ous rocks from the Pedroches Mississippian basin

raphic characteristics Mineralogy

ant subangular quartz grains and radiolarians in anceous matrix rich Fe and Mn oxides. Main cleavage definedntation of siliceous particles.

Princ.: Qtz, Pl, Ms Sub.:Chl, Mtm, Ms, Hem, Ab

rained lepidoblastic texture. Affected by low grade regionalorphism. Little developed slaty cleavage.

Princ.: Qtz, Ill, Ms Sub.:Chl, Pl, Zrn, Op

d slate with retrogressed Crd porphyroblasts transformed tomica. Small late to post-kinematic Grt porphyroblasts.

Princ.: Bt, Ms, Crd Sub.:Grt

m-to-coarse-grained litarenite with blastopsammitic. Moderate sorting. Lithic clasts constituted by:orphic rock fragments (quartzite, phyllite and shale), cherts,ic and volcanic igneous rock fragments.

Princ.: Qtz, Pl, Kfs Sub.:Chl, Ms, Bt, Zrn, Mz, Tur,Rt, Op

-medium-grained greywacke with inequigranularsammitic texture. Qtz and Pl clasts in a very fine-grainedconstituted by Qtz, Pl, S and Chl. Kfs remobilization ised in microfissures.

Princ.: Qtz, Pl Sub.: Kfs,Chl, Ms, S, Cal, Zrn, Tur,Ap, Op

rained granolepidoblastic texture. Microscopic organicremains. Affected by low grade metamorphism. Wellped slaty cleavage.

Princ.: Qtz, Chl, Dm Sub.:Ab, Ms, S, Ilm

rained blastopelitic texture. Very abundant organic matters. Affected by low grade regional metamorphism. Fainte.

Princ.: Qtz, Pl, Dm, MsSub.: Chl, Ab, Kfs, Op

rained granoblastic texture. Rare volcanic origin quartzCarbonized organic matter remains. Very low deformationcleavage development.

Princ.: Qtz Sub.: Ill, Chl,Op

ly spillitized basaltic trachy-andesite with porphyritic texture.andBt phenocrysts inmersed in a hypocrystalline groundmassp, altered glass and mafic minerals. Amp phenocrysts arended by reaction rims. Vesicles filled with Ep±Cal.

Princ.: Pl, Amp, Bt Acc.:Ap, Op Sec.: Chl, S, Cal,Ep, Ab, Kfs

m-to-coarse-grained granodiorite with hypidiomorphic. Idiomorphic to subidiomorphic Bt crystals. Pertitic textureKfs crystals.

Princ.: Q, Pl, Kfs, Bt Acc.:Ap, Zrn, Op, Mz Sec.: Chl,S

ritic metadacite/meta-andesite. Very fine-grainedallized matrix with Pl phenocrysts and Amp and Pxcrysts retrogressed to Bt.

Princ.: Pl, Bt Sub.: Op,Qtz

yspillitizedbasaltswithhypocrystalline to subophitic textures. Plrysts and scarce and altered Cpx and Ol phenocrysts in amasscomposedof interstitial glass, Pl andCpxmicrolites,Opandglass. Cracks filled up by Chl, Prh, Cal and Srp.

Princ.: Pl, Cpx, Ol Acc.:Ap, Op, Spl Sec.: Chl, S,Ep, Cal, Srp, Prh

ic breccia of basaltic andesite composition . Subophitic-iabasic, hypocrystalline and intersectal-subophitic textures.r and altered Pl phenocrysts and very altered Px and Olhenocrysts.

Princ.: Pl, Cpx, Ol Acc.:Op, Ap, Spl Sec.: Chl, S,Qtz, Ab, Cal, Ep, Srp

olerites mainly constituted of green Amp and Pl. The Ampmes come from transformation of Px. Locally affected byt metamorphism creating foliated blastodoleritic textures.

Princ.: Amp, Pl Acc.: Ap,Op Sec.: Prh, Spn

c.: principalminerals, Sub.: subordinateminerals, Acc.: accessoryminerals, Sec.: secondaryquartz, Pl: plagioclase, Cpx: clinopyroxene, Amp: amphibole, Ol: olivine, Kfs: K-feldspar,illonite, S: sericite, Hem: haematite, Ill: illite, Ilm: illmenite, Tur: tourmaline,Mz:monazite,spinel, Spn: sphene, Crd: cordierite, Grt: garnet.

Table 2Chemical composition of studied rocks from the Pedroches basin

Samplelithologyunit

MA-6Sh PV

MA-7Sh PV

MA-52Sh PV

MA-33Sd PV

MA-56Sd Gd

MA-4Sh Gd

902-124Sh Gd

MA-8Sh Gb

MA-3BTA Gd

MA-5Grd PB

MA-58Mad PB

MA-59Mad PB

880-1B Gb

880-6B Gb

MA-9B Gb

902-89B Gb

MA-61BA Gb

902-93BA Gb

902-94BA Gb

MA-49Md PB

MA-50Md PB

MA-51Md PB

(wt.%)SiO2 66.97 60.81 55.18 86.07 62.51 57.81 58.82 55.26 52.11 67.38 61.07 62.41 49.47 47.23 48.31 47.79 51.5 51.22 54.01 49.47 49.04 48.03TiO2 0.60 0.78 0.87 0.44 0.74 0.92 0.93 0.80 1.26 0.53 1.02 0.94 1.17 1.44 1.49 0.97 1.01 0.86 1.02 2.06 2.16 1.77Al2O3 14.01 19.30 22.04 7.09 15.88 20.03 19.17 19.80 17.21 15.72 17.55 15.86 15.59 15.27 16.13 16.74 15.8 16.2 14.93 13.67 13.07 15.72FeO 7.76 5.89 7.96 1.89 6.07 7.11 6.69 5.91 7.36 3.05 5.21 5.09 9.24 8.71 9.45 7.81 7.96 7.82 7.33 12.37 12.70 11.23MgO 1.53 1.68 1.45 0.26 3.31 2.11 2.06 1.42 3.39 1.93 2.60 3.29 7.17 5.78 7.12 7.08 5.89 7.02 4.53 6.95 6.75 7.02MnO 0.38 0.03 0.35 0.02 0.10 0.09 0.08 0.07 0.10 0.05 0.08 0.09 0.16 0.14 0.16 0.14 0.14 0.14 0.12 0.22 0.25 0.26CaO 0.19 0.64 0.31 0.10 1.58 0.38 0.59 3.69 5.91 2.77 4.27 3.91 10.39 13.65 10.64 11.56 8.36 7.10 9.06 9.30 11.39 10.99Na2O 1.11 0.18 0.53 1.25 4.18 1.20 1.32 0.34 4.80 3.48 3.40 3.68 3.22 2.64 2.39 2.67 0.09 0.03 4.68 3.21 2.39 2.37K2O 2.77 4.82 3.94 1.25 2.23 3.70 3.47 4.46 2.35 3.92 3.09 2.83 0.46 0.05 0.36 0.63 0.58 2.06 0.40 0.21 0.07 0.28P2O5 0.05 0.07 0.05 0.05 0.17 0.17 0.16 0.08 0.65 0.17 0.21 0.28 0.09 0.17 0.16 0.06 4.66 3.57 0.09 0.16 0.17 0.14Loi 3.78 5.15 6.44 1.38 2.56 5.68 5.97 7.51 4.05 0.67 0.92 1.06 2.01 3.95 2.75 3.68 3.12 3.13 3.01 1.01 0.59 0.94Total 99.14 99.35 99.11 99.81 99.33 99.21 99.26 99.35 99.19 99.66 99.41 99.43 98.97 99.03 98.95 99.13 99.12 99.15 99.18 98.62 98.59 98.75CIA 74.2 76.5 80.6 67.7 60.6 77.2 75.9 69.9CIW 88.2 96.5 95.5 77.7 66.8 91.3 89.2 84.2(ppm)Ba 589 726 411.4 266.9 803.9 569 471 621 1323 516 715.3 480.4 104.5 8.6 42 41 80 123 65.9 86.1 25.1 36.9Cr 70 84 115 77 96 114 110 86 56 72 78 209 561 315 196 411 200 587 47 89 108 179Cs – 7 17 – 7.4 10 11.7 6 – 9.2 – – – – – – – – – – – –

Cu 106 44 121.2 – 31.9 40 32.3 39 19 36 8.4 15.7 72 68.9 58 52.9 65.9 77 30.9 135.4 142.6 141.4Ge 1.6 1.1 1.6 4.2 1 1.5 – 1.4 1.1 3.1 1.2 1.5 – – 1.1 – – – – – 1.7 1.5Hf 3.2 5.5 3.9 4.8 4.5 3.9 5.5 3.6 4.9 – 5.2 8.1 1.1 1.6 4.2 1.1 3.6 0.6 1.2 0.2 2 2.8Mo – – 0.2 – 0.4 – – – – – 0.6 0.5 1.1 1.2 – 0.1 – – 0.4 – 0.7 0.3Nb 12.6 17.7 19.5 6.6 10.9 15.6 15.8 18.2 9.4 7.7 12.7 11 1.7 5.9 4.8 1.1 2.3 1.1 2.5 6.6 7.7 5.6Ni 43 34 79.8 21 36.3 51 46.6 39 14 19 66.7 57.1 100 80 64 123.1 83.5 182 32 53.9 53.2 65.1Pb 28 17 24.4 12.2 13.3 29 27.9 11 12 39 29.6 30.9 10.4 3.7 3 6.6 2.7 2.7 3.4 5.9 3.7 2.7Rb 117 194 166.6 47.7 55.2 160 149.1 196 40 173 115.6 111.4 18 5 9 15.3 15.2 48 8.7 9.6 4.5 11.8Sc 14 15 21 4 13 19 20 19 23 10 19 20 52 37 35 38 34 34 29 50 56 51Sn 3.4 5.4 5.7 3.6 3 4.3 7.4 5.8 2.2 9.4 6.1 5.4 2.9 5.2 2.7 – 3 – 3.8 0.1 0.9 2Sr 140 41 41.7 33.6 160.6 107 106 102 384 381 327.8 373.1 110 128 112 101.1 88.5 42 79.7 202.6 189.8 250.1Ta – – 0.1 0.3 1.1 – – – – – – 1.6 – – 0.7 0.2 – – 0.1 – 0.8 –

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Th 17.3 19.3 17.6 7.5 8.3 16.2 15.2 19.3 10.4 18.8 15.9 17.9 1.3 1 2.4 0.2 1.2 1.5 1.6 1.9 1.6 –

U 3 2.6 2.9 1.5 2.2 4.5 2 4.2 4.1 5.6 5.1 6.6 1.2 1.7 1.2 1.5 1.2 1.1 0.2 1.6 1.4 0.4V 114 113 225 42 102 155 135 125 189 45 53 84 139 122 244 229 198 226 190 411 426 354Y 27.4 27.7 24.8 18.6 17.7 29.4 30.3 24 26.4 14.8 25.8 21.1 26 34.6 33.7 23.8 30.4 20.4 30.9 39.9 43 33Zn 78 55 139.6 30.9 70.6 105 98.1 48 76 40 71.3 63.7 111 98.6 70 59.6 65 65.3 52.9 93.6 99.3 81.7Zr 149 167 149 246.7 152 175 187.5 168 202 144 266 256.6 61 105 120 53.4 80.2 42 87.8 117.2 143.2 109.5Th/Sc 1.24 1.29 0.84 1.88 0.64 0.85 0.76 1.02 0.45 1.88 0.84 0.90 0.03 0.03 0.07 0.01 0.04 0.04 0.06 0.04 0.03 0.03Th/Nb 1.37 1.09 0.90 1.14 0.76 1.04 0.96 1.06 1.11 2.44 1.25 1.63 0.76 0.17 0.50 0.18 0.52 1.36 0.64 0.29 0.21 0.27Th/U 5.77 7.42 6.07 5.00 3.77 3.60 7.60 4.60 2.54 3.36 3.12 2.71 1.08 0.59 2.00 0.13 1.00 1.36 3.20 1.19 1.14 3.75La/Sc 2.62 2.71 2.35 5.88 2.08 1.67 2.11 2.18 1.23 3.30 1.77 1.44 0.02 0.09 0.13 0.03 0.19 0.04 0.19 0.18 0.16 0.15Y/Nb 2.17 1.56 1.27 2.82 1.62 1.88 1.92 1.32 2.81 1.92 2.03 1.92 15.29 5.86 7.02 21.64 13.22 18.55 12.36 6.05 5.58 5.89Zr/Nb 11.83 9.44 7.64 37.38 13.94 11.22 11.87 9.23 21.49 18.70 20.94 23.33 35.88 17.80 25.00 48.55 34.87 38.18 35.12 17.76 18.60 19.55La 36.70 40.70 49.40 23.50 27.00 31.70 42.10 41.40 28.40 33.00 33.60 28.80 1.10 3.30 4.60 1.01 6.30 1.20 5.57 8.80 8.80 7.60Ce 87.40 96.20 104 46.00 47.20 73.40 89.80 101.80 68.3 81.10 61.50 54.20 4.80 11.40 16.00 4.72 11.40 3.60 13.8 20.50 22.20 18.40Pr 11.20 12.10 10.10 5.90 5.60 10.40 10.50 12.40 8.90 9.60 7.70 6.90 1.10 2.10 2.70 0.96 1.80 0.71 2.12 2.90 3.20 2.70Nd 36.00 48.60 45.10 22.20 28.90 39.30 38.7 45.10 28.4 33.30 41.80 37.80 6.60 11.50 11.70 5.19 8.90 4.30 9.80 14.00 15.80 12.50Sm 7.24 9.26 7.76 4.35 5.46 7.89 7.52 8.79 6.32 5.70 8.25 7.40 2.60 4.00 3.96 2.03 2.80 1.70 2.92 4.35 4.88 3.86Eu 1.60 1.50 1.30 1.00 1.30 1.80 1.60 1.50 1.90 1.20 1.60 1.50 1.00 1.30 1.50 0.81 0.91 0.57 1.03 1.60 1.80 1.50Gd 6.70 6.70 5.10 4.00 4.30 7.20 7.06 6.00 6.30 3.90 6.20 5.50 3.80 5.50 5.40 3.32 3.80 2.56 3.93 4.40 5.00 4.00Tb 1.00 0.80 0.78 0.68 0.60 1.10 1.01 0.80 0.80 0.40 0.90 0.77 0.73 1.00 0.90 0.59 0.67 0.47 0.73 0.91 1.10 0.81Dy 5.90 4.80 4.10 3.60 3.40 5.60 6.17 4.50 5.00 2.10 4.70 4.10 5.09 6.94 6.50 4.01 4.30 3.27 4.71 5.70 6.60 5.00Ho 1.10 0.90 0.82 0.66 0.65 1.20 1.21 0.90 1.00 0.40 0.99 0.80 1.10 1.50 1.40 0.88 0.94 0.69 1.02 1.10 1.30 1.00Er 3.60 2.90 2.50 1.90 1.90 3.50 3.52 2.80 3.10 1.50 2.60 2.30 3.40 4.50 4.30 2.55 3.10 2.03 2.98 3.30 3.80 2.80Tm 0.31 0.21 0.36 0.32 0.41 0.32 0.49 0.20 0.47 0.12 0.41 0.32 0.50 0.66 0.70 0.38 0.46 0.29 0.44 0.51 0.64 0.41Yb 3.00 2.30 2.32 1.90 2.70 3.10 3.34 2.30 2.40 1.00 2.59 2.10 3.10 4.10 3.80 2.33 2.90 1.99 2.93 3.11 3.95 2.61Lu 0.39 0.29 0.34 0.33 0.40 0.51 0.49 0.29 0.31 0.10 0.40 0.32 0.51 0.68 0.52 0.35 0.44 0.29 0.43 0.48 0.61 0.37ΣREE 202.1 227.3 233.9 116.3 129.8 187 213.5 228.8 161.6 173.4 173.2 152.8 35.4 58.5 64 29.1 48.7 23.7 52.4 71.7 79.7 63.6(La/Yb)N 8.27 11.96 14.39 8.36 6.76 6.91 8.52 12.16 8.00 22.30 8.77 9.27 0.24 0.54 0.82 0.29 1.47 0.41 1.28 1.91 1.51 1.97(La/Sm)N 3.19 2.77 4.01 3.40 3.11 2.53 3.52 2.96 2.83 3.64 2.56 2.45 0.27 0.52 0.73 0.31 1.42 0.44 1.20 1.27 1.14 1.24(Gd/Yb)N 1.81 2.36 1.78 1.71 1.29 1.88 1.71 2.11 2.13 3.16 1.94 2.12 0.99 1.09 1.15 1.15 1.06 1.04 1.09 1.15 1.03 1.24Eu/Eu* 0.70 0.58 0.63 0.73 0.82 0.73 0.67 0.63 0.92 0.78 0.68 0.72 0.97 0.85 0.99 0.95 0.85 0.83 0.93 1.12 1.11 1.17

PV: Pedroches Valley unit, Gb: Guadalbarbo unit, Gd: Guadiato unit, PB: Pedroches Batholith, Sh: shales, Sd: sandstones, BTA: basaltic trachyandesites, B: basalts, BA: basaltic andesites, Md: Metadolerite, Mda: metadacite/meta-andesite,Grd: granodiorite, – not measured.

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interbedded in the sedimentary sequences witnesses for subaerialvolcanic eruptions either within the basin or at its margins. However,adscription to specific sources is difficult owing to their fine grain sizeand the overall lack of petrographic distinguishing features.

4.2. What does the geochemistry tell us?

The combineduseofmajor and trace element geochemical data fromboth metasedimentary and metaigneous protoliths, together with Sm–

Nd systematics, provide a powerful tool to constrain the provenance ofold rocks and to identify ancient tectonic settings and their evolution(e.g., Bhatia, 1985; Taylor and McLennan, 1985; Bhatia and Crook, 1986;McLennan, 1989; Condie, 1993; Hemming et al., 1995). Numerousstudies have demonstrated that some elemental abundances, such asthe rare earth elements (REE), Th, Sc, Hf, Ta, Y, Nb, Cr, Zr and Co, veryclosely reflect source compositions since they are least modified duringweathering, sedimentation, hydrothermalism, diagenesis and meta-morphism (e.g.,McLennanet al.,1980,1990; TaylorandMcLennan,1985;Bhatia and Crook,1986; Cullers et al., 1988; McLennan,1989; McLennanand Taylor, 1991; McLennan and Hemming, 1992). On this foundation,we analysed metasedimentary and metaigneous rocks from the threeunits accounted for in this study; i.e., Guadiato, Guadalbarbo, and SouthPedroches Valley units. The macroscopic, petrographic and mineralogi-cal characteristics of the studied rocks are summarized in Table 1 andtheir whole-rock geochemical compositions are listed in Table 2. Majorand trace elements have been analysed at the IGME laboratories(Madrid) by using XRF-spectrometry, ICP-AES and ICP-MS techniques.

4.2.1. Metasedimentary rocksMajor elements from fine-grained detrital rocks (shales) reveal

relatively evolved andmature compositions, with low SiO2/Al2O3 (2.5–

Fig. 5. A) Th vs. Th/U and B) Th/Sc vs. εNd(0) diagrams of Pedroches basin metasedimentaMcLennan, 1985) and D) PAAS-normalized (McLennan, 1989) REE distribution patterns ofsandstones.

4.8) and high K2O/Na2O (2.5–27.5) ratios, which reflect clay mineralsenrichment probably due to intense weathering. They also show highChemical Index of Alteration (CIA=[Al2O3/(Al2O3+Na2O+K2O+CaO*)]X100; Nesbitt and Young, 1982) and Chemical Index of Weathering(CIW=[Al2O3/(Al2O3+Na2O+CaO*)]X100; Harnois, 1988) ratios (oxidescalculation are in mol %, CIA=70–81 and CIW=84–96) (Table 2), whichindicate a relatively intense degree of chemical alteration or weatheringof the source area, or multiple reworking. The sandstones show higherSiO2/Al2O3 ratios, probably reflecting Qz enrichment due to intenseweathering, and slightly lower CIA and CIW values (see Table 2). SampleMA-56 (Table 1), amatrix-richmetagreywacke interbeddedwith basalticrocks, departs from this general behaviour and has yielded results closerto the shales but higher in Na2O, CaO andMgO, probably reflecting somekind of contamination with the adjacent volcanic rocks.

Concerning trace elements, the Pedroches basin shales show high Thabundances (15–19 ppm; Table 2 and Fig. 5A) and Th/Sc ratios that arerelatively uniform and close to 1.0 (0.8–1.3; Table 2 and Fig. 5B), closelyresembling the values typical for the upper continental crust (McCullochandWasserburg,1978; Taylor andMcLennan,1985;McLennan et al.,1993;Rudnick and Gao, 2003). These shales also present high Nb contents (13–20 ppm) and very uniform Th/Nb ratios (0.9–1.37) comparable with theupper continental crust (ca. 1; Taylor and McLennan, 1985; Rudnick andGao, 2003) (seeTable2). Theyalso showhighTh/U (3.6–7.6; Fig. 5A) andLa/Sc ratios (1.7–2.7), as is typical for upper crust sediments (Taylor andMcLennan,1985;McLennanetal.,1993;RudnickandGao,2003).Asa resultof dilution by quartz, the sandstones display lower Th and Nb concentra-tions (ranging from 7.5 to 8.3 ppm and from 6.6 to 10.9 ppm, respectively),but they exhibit high Th/U (3.8–5; Fig. 5A) and La/Sc (2.1–5.9) ratios.

As far as fine-grained clastic deposits are concerned, the REE areconsidered to be unfractionated during the sedimentary cycle (e.g.,McLennan et al., 1980; Taylor and McLennan, 1985) and, consequently,

ry rocks (modified from McLennan et al., 1993), C) chondrite-normalized (Taylor andmetasedimentary rocks in the Pedroches basin. Legend: Square: shales, Black rhomb:

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they constitute consistent provenance indicators, which reflect theaverage composition of the source area (e.g., McLennan, 1989). TotalREE contents (ΣREE) for the Pedroches basin shales vary from 187 to234 ppm and from 116 to 130 ppm in the sandstones (Table 2).Chondrite-normalized REE patterns (Taylor and McLennan, 1985)(Fig. 5C) in the studied shales and sandstones are very homogeneous;they display REE fractionationwith Lan/Ybn ratios ranging from 6.8 to14.4, LREE enrichmentwith Lan/Smn ratios of 2.5–4.0, and lower HREEfractionation (Gdn/Ybn=1.3–2.4). A clear negative Europium anomaly(Eu/Eu*=0.58–0.82) is also observed, and most likely represents a featureinherited from source materials, such as upper crustal granitoids orsediments derived therefrom (Table 2 and Fig. 5C). The PAAS-normalized(PAAS — Post-Archean Average Australian Shales; McLennan, 1989) REEdistributionpatternsof thestudiedshales andsandstones showflatprofiles(Fig. 5D), indicating that from a REE perspective they are closely similar tothe post-Archean upper continental crust (Nance and Taylor, 1976). Insummary, no significant geochemical differences were observed for themetasedimentary rocks of the different units studied (see Table 2).

In general terms and according to various tectonic environmentsand provenance criteria established by several authors (Taylor andMcLennan, 1985; McLennan, 1989; McLennan et al., 1993), all the geo-chemical characteristics of the studied rocks reflect the predominanceof upper crustal sources and a high degree of sedimentary recycling.

4.2.2. Igneous and metaigneous rocksVolcanic rocks in the three units under consideration (Guadiato,

Guadalbarbo and South Pedroches Valley units) mostly correspond to

Fig. 6. Pedroches basin igneous rocks plotted on: A) AFM diagram (Tilley, 1960; calcalka(Winchester and Floyd, 1977); C) Ti–Zr–Y discrimination diagram for basalts (after Pearce andBlack triangle: Guadiato basaltic trachyandesites, Black square: Guadalbarbo basalts, BlaEsparragosa de La Serena metadacite/meta-andesite, +: Pedroches Batholith granodiorite.

mafic, basaltic compositions, with very minor representation of felsic,pyroclastic andepiclastic rocks,whichoccur in theSouthPedrochesValleyunit immediately below the radiolarian purple shale. These probablyderived from reworking of subaerial or shallow-submarine explosiveeruptions sourced in one of the coeval volcanic belts. The lack of this typeof felsic tuffs in the Guadalbarbo unit makes very unlikely the possibilitythat they originated in the Villaviciosa de Córdoba–La Coronada belt, tothe south. On the contrary, the option that the almost entirely dismantledvolcanic belt atop the present Pedroches Batholith could have been thesource is strengthened. We have not analysed these felsic rocks but if thecorrelationwith the Pedroches Batholith was correct, the composition ofthe granitoid rocks there is hybrid calcalkaline (peraluminous), being theresult of mixing a mantle-derived mafic (tholeiitic) pole and a crust-derived felsic granitic pole (Fernández-Ruiz et al., 1990; Carracedo, 1991;Larrea et al., 1996; Larrea, 1998; Castro et al. 1999).

Concerning the far more abundant mafic rocks they show very sig-nificant geochemical variations among units. According to IUGS criteria(Le Maitre, 1989), the analysed igneous rocks correspond to submarinebasalts (B, samples 880-1, 880-6, MA-9, 902-89) and basaltic andesites(BA, samples MA-61, 902-93, 902-94) for the Guadalbarbo unit, basaltictrachyandesites (BTA, sample MA-3) for the Guadiato unit, andPedroches granodiorite (Grd, sample MA-5), Cerro Bermejo metadoler-ite dikes (Md, samples MA-49, MA-50, MA-51) and Esparragosa de LaSerena metadacites/meta-andesites (Mad, samples MA-58, MA-59) forthe Pedroches Batholith (see Tables 1 and 2).

The Cerro Bermejo metadolerites show tholeiitic affinity while theGuadalbarbo samples correspond to tholeiitic and transitional tholeiitic

line-tholeiitic boundary after Irvine and Baragar, 1971); B) Zr/TiO2 vs. SiO2 diagramCann, 1973); D) Zr/Y–Zr discrimination diagram (after Pearce and Norry, 1979). Legend:ck circle: Guadalbarbo basaltic andesites, Rhomb: Cerro Bermejo metadolerites, x:

Fig. 7. A, B, C and D) Chondrite-normalized REE patterns for Pedroches basin rocks. Normalization values after Taylor andMcLennan (1985); E, F, G and H) N-MORB-normalizedmulti-elemental diagrams for Pedroches basin rocks. Normalization values after Sun and McDonough (1989). Legend as in Fig. 6.

406 M. Armendáriz et al. / Tectonophysics 461 (2008) 395–413

407M. Armendáriz et al. / Tectonophysics 461 (2008) 395–413

basalts as indicated by the AFM diagram (Tilley, 1960) (Fig. 6A), whereasthe Guadiato basaltic trachyandesites and Pedroches Batholith granodior-ite andmeta-andesite/metadacites (Esparragosa de La Serena) show calc-alkaline affinity (Fig. 6A). In the SiO2 versus Zr/TiO2 diagram (Winchesterand Floyd, 1977) (Fig. 6B) the Guadalbarbo basalts and the Cerro Bermejometadolerites plot in the vicinity of the limit between the subalkaline andthe alkaline fields. The Esparragosa de La Serena samples overlap thedacite–andesite boundary, whereas the Guadiato basaltic trachyandesitemay corresponds to evolved alkaline terms (Fig. 6B). The latter probablyrepresents a broadly alkaline differentiated liquid rich in Zr and K2O.

The tectonic settingof our rockswasfirst inferredbyusingPearceandCann (1973) diagram, based on the distribution of selected incompatibletrace elements (Ti, Zr, Y, Nb) that show very limitedmobility during sea-water alteration and metamorphism. In this diagram, the Guadalbarbobasalts and the Cerro Bermejo metadolerites plot in the ocean-floorbasalts (MORB) field (Fig. 6C). The same observation can be made inanother discrimination diagram for basalts (Zr/Y–Zr diagram; Pearce andNorry, 1979), where the Guadalbarbo and Cerro Bermejo samples plot intheMORB type basalt field, while theGuadiato sample displays affinitieswith within-plate, continental basalts (Fig. 6D).

The Guadalbarbo unit basalts and Cerro Bermejo metadoleritescontain lowabundances of incompatible elements (e.g., Nb, Th, U, Zr, Hf).They have Th contents belowor close to the detection limits and low Th/Sc ratios (0.01–0.07 for the Guadalbarbo and 0.03–0.04 for CerroBermejodikes) (see Table 2). Since Th is an incompatible element,whichis strongly enriched in the upper continental crust, whereas Sc is onlyweakly fractionated during magma genesis, the Th/Sc ratio will betypically low in juvenile magmas (bca. 0.1) (McLennan et al., 1993).They also exhibit high values of Y/Nb (6–22 for the Guadalbarbo flowsand ∼6 for Cerro Bermejo dikes) and Zr/Nb (18–49 and 18–20, respec-tively) ratios (see Table 2), suggesting strongly depletedmantle sources.In contrast, the Guadiato basaltic trachyandesite, the Pedrochesgranodiorite and Esparragosa de La Serena metadacites/meta-andesitesshow strong enrichment in incompatible elements (see Table 2). Theserocks present higher Th/Nb ratios (1.11 for the Guadiato and 1.25–1.63for Esparragosa de La Serena) and distinctly greater Th/Sc ratios (0.45

Table 3Sm–Nd isotope data together with εNd and TDM values from the Pedroches basin metasedim

Sample Unit Lithology Age (Ma) Sm (ppm) Nd (ppm) 14

MA-6 PV Sh 340 7.24 36.0 0.MA-7 PV Sh 330 9.26 48.6 0.MA-52 PV Sh 340 7.76 45.1 0.MA-33 PV Sd 330 4.35 22.2 0.MA-56 Gd Sd 340 5.46 28.9 0.MA-4 Gd Sh 330 7.89 39.3 0.902-124 Gd Sh 320 7.52 38.7 0.MA-8 Gb Sh 340 8.79 45.1 0.MA-3 Gd BTA 330 6.32 28.4 0.ZS-1* PB To 312 6.89 35.6 0.BL-25* PB Grd 312 4.56 26.1 0.R-10* PB Grd 312 4.64 23.5 0.PZ-1* PB Grd 312 4.63 23.7 0.TAV-1* PB Lgrd 312 5.03 27.5 0.EC-2* PB Mgr 312 6.44 30.1 0.CD-1* PB Mgr 312 5.48 24.5 0.MA-58 PB Mad 320 8.25 41.8 0.MA-59 PB Mad 320 7.4 37.8 0.MA-9 Gb B 340 3.96 11.7 0.902-93 Gb BA 340 1.7 4.3 0.902-94 Gb BA 340 2.92 9.8 0.MA-49 PB Md 320 4.35 14.0 0.MA-50 PB Md 320 4.88 15.8 0.MA-51 PB Md 320 3.86 12.5 0.

147Sm/144Nd ratios are determinedwith a precision of 0.2%. Nd isotopic ratios are normalized tmean at the 95% confidence level (2 S.E.). The 143Nd/144Nd ratios are expressed using the epsicalculated relative to a chondritic uniform reservoir (CHUR) with the following present-day1980, 1984) and λ(147Sm)=6.54×10–12 y−1 (Steiger and Jäger, 1977). Neodymiummodel age1988). Legend. PV: Pedroches Valley unit, Gb: Guadalbarbo unit, Gd: Guadiato unit, PB: Pedrobasaltic andesites, Md: Metadolerite, Mda: metadacite/meta-andesite, To: tonalite, Grd: gran

Guadiato and 0.8–0.9 Esparragosa de La Serena), closer to the valuetypical for materials of the upper continental crust (about 1.0 or a littlesmaller;McLennanet al.,1993), thus indicatingcrustal contamination. Inthe same line, Y/Nb ratios (2.8-Guadiato, 1.9-Pedroches granodiorite,∼2-Esparragosa de La Serena) are lower (see Table 2).

The total average contents (ΣREE) for the Guadalbarbo and CerroBermejo rocks is very low (ranging from 24 to 64 ppm and from 64 to80 ppm, respectively). The samples from the Guadalbarbo unit displayalmost flat or slightly convex-upwards chondrite-normalized REEpatterns (Fig. 7A and B); i.e., flat or LREE-depleted patterns, withunfractionated HREE, typical of normal mid-ocean ridge basalts.Samples 902-89 and 880-1 are poorer in REE and have a greaterdepletion in LREE, thereby showing a greater N-MORB affinity. The Eunegative anomalies are low or negligible (0.83–0.99), implying thatplagioclase fractionation was subordinate, or that Eu occurred as thetrivalent species. The Cerro Bermejo rocks display similar REE patterns(Fig. 7C). On the other hand, for the Guadiato basaltic trachyandesite,Pedroches Batholith granodiorite and Esparragosa de La Serena metada-cites/meta-andesites ΣREE contents are much higher (162 ppm,173 ppmand 153–173 ppm, respectively), in line with their derivation from dif-ferent sources. All these rocks display LREE enrichment (Lan/Smn values of2.8-Guadiato, 3.6-Pedroches granodiorite and 2.5-2.6-Esparragosa de LaSerena; and Lan/Ybn values of 8, 22.3 and 8.8-9.3, respectively) and HREEfractionation (Gdn/Ybn values of 2.1-Guadiato, 3.2-Pedroches granodioriteand 1.9-2.1-Esparragosa de La Serena) (Fig. 7C and D). Europium negativeanomalies (Eu/Eu*) are faint; 0.92-Guadiato, 0.78-Pedroches granodioriteand 0.68-0.72-Esparragosa de La Serena. The Guadiato basaltic trachyan-desites could have been derived from the evolution of an enrichedmantlemelt close to an E-MORB (alkaline affinity).

In the spider diagrams normalized toN-MORB (Fig. 7E, F, G andH) itcan be observed that for Guadalbarbo and Cerro Bermejo rocks (Fig. 7E,F and G), the HREE and HFSE elements group show flat configurationsand similar concentrations to those typical for N-MORBs. They areenriched in LILE elements (Rb, Ba, K and U), which could reflect themobility of these elements during post-igneous alteration processes.On the contrary, the Guadiato, Pedroches and Esparragosa de La Serena

entary and metaigneous rocks

7Sm/144Nd 143Nd/144Nd ±2S.E. εNd(0) εNd(T) TDM (Ga)

1215 0.512078 ±0.000006 −10.9 −7.6 1.571151 0.511906 ±0.000006 −14.2 −10.8 1.721039 0.511938 ±0.000006 −13.6 −9.6 1.521182 0.512135 ±0.000006 −9.8 −6.5 1.441140 0.511881 ±0.000006 −14.7 −11.1 1.741213 0.512147 ±0.000007 −9.5 −6.4 1.461173 0.512096 ±0.000008 −10.5 −7.4 1.481179 0.511931 ±0.000005 −13.8 −10.3 1.731344 0.512268 ±0.000007 −7.2 −4.6 1.471171 0.512264 ±0.000008 −7.3 −4.1 1.231055 0.512329 ±0.000009 −6.0 −2.4 1.031197 0.512309 ±0.000006 −6.4 −3.4 1.191180 0.512307 ±0.000007 −6.4 −3.3 1.181106 0.512273 ±0.000006 −7.1 −3.7 1.151293 0.512326 ±0.0000013 −6.1 −3.4 1.291351 0.512376 ±0.000008 −5.1 −2.7 1.291194 0.512254 ±0.000008 −7.5 −4.3 1.281184 0.512297 ±0.000008 −6.6 −3.4 1.202041 0.513158 ±0.000006 10.2 9.9 –

2396 0.513104 ±0.000003 9.1 7.3 –

1804 0.512869 ±0.000005 4.5 5.3 0.91877 0.512995 ±0.000006 7.0 7.4 –

1874 0.513004 ±0.000007 7.2 7.6 –

1863 0.513009 ±0.000004 7.3 7.7 –

o 146Nd/144Nd=0.7219 and theirwithin-runprecision is given as the standard error on thelon notation, as deviations in parts per 104 from the contemporaneous chondritic value,characteristics: 143Nd/144Nd=0.512638, 147Sm/144Nd=0.1966 (Jacobsen and Wasserburg,s (TDM) were calculated by using the depletedmantle model described by DePaolo (1981,ches Batholith, Sh: shales, Sd: sandstones, BTA: basaltic trachyandesites, B: basalts, BA:odiorite, Lgrd: leucogranodiorite, Mgr: monzogranite, * data from Donaire et al., 1999.

408 M. Armendáriz et al. / Tectonophysics 461 (2008) 395–413

rocks show a rather fractionated configurationwith a distinct negativeNb anomaly (Fig. 7G and H).

4.3. What do the Sm–Nd isotope data tell us?

Sm–Nd isotope data were determined in the Geology Laboratory atthe Blaise Pascal University (Clermont-Ferrand, France). Sample decom-positionwas achievedby fusionwith a LiBO2flux in an induction furnaceat ca. 1150 °C, as described by Le Fèvre and Pin (2005). Then, Sm and Ndisolation was carried out by cation exchange and extraction chromato-graphy methods adaptated from Pin and Santos-Zalduegui (1997). SmandNd concentrationsweremeasured by isotopedilution using amixed149Sm-150Nd tracer and thermal-ionization mass spectrometry (TIMS),allowing determining 147Sm/144Nd ratios with a precision of 0.2%. TheJNdi-1 isotopic standard measured during the analyses gave a 143Nd/144Nd=0.512105+/−6 (2 measurements), corresponding to a value of0.511848 for the La Jolla standard (Tanaka et al., 2000). Smwasmeasuredin the single collection mode on an automated VG54E mass spectro-meter andNd isotopic ratiosweremeasured in the staticmulticollectionmode with a Thermo Finnigan Triton TI instrument with normalizationto 146Nd/144Nd=0.7219. The results are reported in Table 3.

4.3.1. Metasedimentary rocksThe whole-rock geochemical features of the metasediments shown

inTable 2 are consistentwith theNd isotopic data (Table 3). Neodymiumcontents of the metasedimentary rocks fall into a range of 22–49 ppm

Fig. 8. Plot of εNd vs. time (Ga) for: A) metasediments and, B) metaigneous rocks; and f Sm/N

setting fields modified from McLennan and Hemming (1992). + data from Donaire et al., 19

and 147Sm/144Nd ratios vary from 0.1039 to 0.1215, as is typical formaterials from the upper continental crust (∼0.12, McCulloch andWasserburg, 1978). All the samples, ranging in stratigraphic age from340 to 320 Ma, display strongly negative εNd(T) (from −6.4 to −11.1)values (Table 3), which do not show any discernible correlation witheither stratigraphy or geographic location. Such markedly negativevalues imply that the Pedroches metasediments were derived fromreservoirs with low time-integrated Sm/Nd ratios, that is, source-rockscharacterized by LREE-enrichment during a long time span. The uppercontinental crust (specifically, most granitoids and sediments derivedtherefrom), bestfits this requirement. Depletedmantlemodel ages (TDM)vary from 1.44 to 1.74 Ga, with an average value of ca.1.6 Ga, interpretedto reflect the mean Nd crustal residence age of the ultimate sources ofthe studied sediments. This crustal residence age is much older than thedepositional ages and demonstrates that these metasediments arebasically the result of crustal reworking processes (Fig. 8A and A’).

In summary, the combined Nd isotopic and geochemical data fromthe Pedroches Mississippian metasedimentary rocks clearly demon-strate an upper continental crust provenance and a high degree ofsedimentary recycling for thesematerials throughout thebasin; i.e., theyare derived from old, reworked upper crust without any significantjuvenile component despite the presence of interbedded basaltic rocks.

4.3.2. Igneous and metaigneous rocksAsalreadyobserved for theirelemental geochemistry, themetaigneous

rocks reveal fairly contrasted isotopic characteristics. The Guadalbarbo

d vs. εNd(T) diagram for: A’) metasedimentary rocks and B’) metaigneous rocks. Tectonic99. Legend as in Figs. 5 and 6.

409M. Armendáriz et al. / Tectonophysics 461 (2008) 395–413

and Cerro Bermejo rocks show high 147Sm/144Nd ratios (from 0.1804 to0.2396 and from0.1863 to 0.1877, respectively; see Table 3). These sampleshave strongly positive εNd(T) values (Fig. 8B and B’). Such highly radiogenicNd isotope signature implies that these basalts were extracted frommantle sourceswith strongLREE-depletionona time-integratedbasis, anddid not suffer any significant interaction with typical continental crustcomponents, characterized by negative εNd values. On the contrary, theGuadiato, Pedroches granitoids (data from Donaire et al., 1999) andEsparragosa de La Serena rocks present lower 147Sm/144Nd ratios (seeTable 3) and markedly negative ε Nd(T) values (see Table 3 and Fig. 8B andB’),withamodel age (TDM)of1.5Ga for theGuadiatoandameanNdcrustalresidence age of 1.2 for the Pedroches granitoids and Esparragosa de LaSerenametadacites/meta-andesites (see Table 3 and Fig. 8B and B’). Theseisotopic features demonstrate an important crustal contamination and/orold enriched mantle source for the Guadiato mafic sample, and a majorcrustal contribution for the other rocks.

5. Discussion and conclusions: a model of basin development

The data and primary interpretations presented in precedingsections along with previously published data on both local details ofthe Mississippian Pedroches basin and its regional context provide avast reference framework, from which a new model of basingeneration and evolution can be derived. We have concentrated ourefforts in characterizing the central parts of the basin where theextremely complicated noise of the more sensible marginal parts isdiluted, and only themore important, long term changes are recorded.At this stage it is worthwhile reminding that the Pedroches basinformed and evolved within a classical example of transpressionalorogen (Ribeiro et al., 1990; Quesada,1991, 2006; Quesada et al., 2006)and that part of the basin developed atop a major sinistral wrenchlineament, the Badajoz–Córdoba shear zone (Figs. 1 and 2; Burg et al.,1981; Quesada and Dallmeyer, 1994).

The onset of collision between already amalgamated Laurussia andthe so-called Ibero-Aquitanian Indentor, a promontory in northernGondwana (Brun and Burg,1982; Burg et al.,1987), promoted initiationof southeasterly tectonic escape from the zone of collision of the OMZ,which occupied the outer margin of Gondwana (Quesada, 1991;Sánchez-García et al., 2003). This displacement was accommodated byoverriding of the OMZ onto the relict of the Rheic Ocean that stillexisted southwest of the promontory and,most important for our case,by reactivation under a sinistral strike-slip regime of a preexistinglithosphericweakness: the Cadomian suture between theOMZand theIberian Massif, which thence became the so-called Badajoz–Córdobashear zone (Abalos, 1990; Abalos et al., 1991; Quesada, 1991, 2006;Quesada et al., 2006). The escape of the OMZ resulted in southeasterlypropagating transpressional uplift, not only of the zone itself but also ofthe adjacent, more internal passive margin of the CIZ, which by theearly Mississippian was entirely emerged and subjected to erosion.

These were the conditions prior to the formation of the syn-orogenic Pedroches basin. What changes in this scenario contributedfor it to happen? In our view, two interrelated elements were respon-sible for it (a cross-section sketch of the proposed model is depictedon Fig. 9): 1) a change to transtensional conditions, allowing for ex-tension, lithospheric thinning and facilitating magma ascent andemplacement in the upper crust, and 2) the overriding of a deepmantlethermal anomaly or enhanced decompression melting of the mantleduring lithospheric thinning, responsible for the generation of magmas.These two elements find support on, and help explaining the variedstratigraphic record in the first part of the basin history and itscompartmentalization into different units separated by volcanic belts(Fig. 9A and B). According to petrographic data two drainage systems,one in the south (OMZ) and another in the north (CIZ), appear to havebeen the sources of detrital sediments at that stage. A physiographic lowcan be identified at the location of theGuadalbarbounit, as suggested bythe fact that no detrital elements sourced in the OMZ trespass this unit

northwards. Geochemical andNd isotopedata confirm that the sourceofthe sediments involved largely recycled old-continental crust, as is thecase of the two flanking zones of the Iberian Massif, but cannotdiscriminate amongunits.On theotherhand, theyalso tell that themaficigneous rocks had a source in the asthenospheric mantle, thoughcontamination by continental lithosphere rocks or magmas is indicatedlocally; e.g., in the Guadiato unit and the Pedroches Batholith.

Whether magmatismwas related solely to decompression meltingduring transtension, to the overriding at that time of an anomaloushot mantle or a combination of both possibilities is still a matter ofspeculation. A major recent finding is the recognition of a significantsea water temperature drop of c. 5 °C, recorded within the in-situcarbonate platform preserved in the Central Guadiato unit nearAdamuz by means of oxygen isotopic data from brachiopod shells(Armendáriz et al., 2007a,b). This severe cooling recorded in nearlyequatorial areas (Scotese and McKerrow, 1990; Witzke, 1990; Scoteseet al., 1994; Scotese, 2000; Torsvik and Cocks, 2004), which mayrepresent the expression of the coeval glaciations in southernGondwana (e.g., Smith, 1963; Charrier, 1986; Rocha-Campos et al.,2000; Isbell et al., 2001, 2003; Stone and Thomson, 2005; Trosdtorf etal., 2005), implies the existence of a connection of the Pedroches basinwith open ocean areas. According to the overall plate configuration inthe Mississippian (Scotese, 2000; Stampfli and Borel, 2004; Blakey,2006) such a seawaywas only possible towards the present southeast;i.e., in the direction where a Paleotethys MOR was spreading close tothe northern margin of Gondwana. In the opposite direction theVariscan collisional mountain belt was then being formed (Matte,1986, 1991, 2001, 2002; Martínez-Catalán et al., 1997).

A possibility opened to further research, which is here outlined as aprovocative hypothesis, is that during collision between Laurussia andGondwana the latter might have been pushed southeastwards tocollide with such Paleotethys spreading ridge or, perhaps, a branchdeparting from it. In this context, the first part of the evolution of thePedroches basin referred to herein as a result of transtension inconnection with a major releasing bend in the overall left-laterallithosphere-through Badajoz–Córdoba shear zone, could in fact be theconsequence of a combination of both transtension and the overridingof a slabwindow developed as a result of ridge-trench encounter, itselfinducing rifting in the upper (Iberian) plate. Final collision of thesouthern margin of the OMZ with the South Portuguese Zone by thelate Viséan (Fig. 1), which culminated the closure of this part of theRheic Ocean, imposed a change to more orthogonal strain conditionsthat triggered basin inversion and may have also caused the abortionof the propagating rift. Obviously, there is much speculation in thishypothesis since no evidence of an active margin towards the eastremains in Iberia, owing to subsequent breakup of Pangea during thePermian and theMesozoic, but most of the characteristics described inthis paper could be better explained if it proved right. Could theCorsica batholith be a part of such an active margin?

Whatever the case, a major finding in this study lies in therecognition of the far more homogeneous character of the upper partof the stratigraphic record across the basin, with only few exceptions.It consists in most cases of tempestite/turbidite sequences of the so-called Culm flysch, which are interpreted as the classical type of syn-orogenic sedimentation once the basin was transformed into aperipheral foreland basin in the footwall, Central Iberian Zone. Thischange implied returning to the prevailing regional transpressionalconditions that apparently persisted until the complete inversion ofthe basin, its emergence and the deformation of the basin fill.Explaining this turn back is not a problem since transpressionwas thepermanent strain regime during the whole Variscan orogeny in SWIberia (Ribeiro et al. 1990; Quesada, 1991; Silva and Pereira, 2004), theproblem lies in understanding the previous switch to a transtensionalregime, which could be related to either presence of a releasing bend,to rifting triggered by the collision with the speculated MOR, or to acombination of both. The final evolution of the basin as a peripheral

Fig. 9. Cross-section tectonic model for the formation and evolution of the Pedroches basin. A) Nucleation stage, B) Transtensional stage, C) Onset of inversion (foreland basin) stage;D) Detail explaining the evolution of the Guadiato unit and the formation of chaotic mélange deposits.

410 M. Armendáriz et al. / Tectonophysics 461 (2008) 395–413

foredeep from the Late Viséan onwards was coeval to, and probablyconnected with the final closure of the remaining part of the RheicOcean that existed between the OMZ (Gondwana) and the SouthPortuguese Zone (Laurussia) (Fig. 1; Quesada, 2006; Quesada et al.,2006, and references therein). This closure resulted in obliquecollision between the two continents and pushed the OMZ north-wards on top of the adjacent CIZ and the recently created Pedrochesbasin (Fig. 9C). The first expression in our basin of the oblique

(sinistral) overthrusting of the OMZ onto the leading edge of the CIZwas a sudden deepening, beautifully expressed by the condensedradiolarian purple shale that separates the two sequences in centraland northern parts. Strong facies similarities in the stratigraphy fromthis purple shale upwards suggest that no obvious compartmentaliza-tion remained in these parts of the basin at this stage. The subsequentevolution was related to massive detrital sedimentation (the Culmflysch) that dispersed across the basin from the southern active

411M. Armendáriz et al. / Tectonophysics 461 (2008) 395–413

margin subjected to uplift and erosion. According to Gabaldón et al.(1985) and Gabaldón (1989) sediment supply and dispersal from thecoastal and transitional areas was mainly controlled by an intensestorm activity, something easy to understand given the subequatorialposition of this part of theworld during the Carboniferous assumed bymost researchers (Scotese and McKerrow, 1990;Witzke, 1990; Scoteseet al., 1994; Scotese, 2000; Torsvik and Cocks, 2004). This fundamentalmechanism interoperated with other tectonically driven processessuch as slumps, gravitational slides and debris flows as is typical inmost syn-orogenic basins. Petrographic, geochemical and Nd isotopedata from rocks belonging to this upper sequence are very similar inall units. They collectively indicate derivation from recycled uppercontinental crust sources.

As it has been described in Section 3.2, an exception to this generalscenario occurs in the Guadiato unit. Its main singularity resides in thedevelopment of relatively important Late Viséan to Serpukhoviancarbonate platforms at the onset of the foreland basin stage (Fig. 9C)(Armendáriz et al., 2005, 2007a; Armendáriz, 2006; Cózar et al., 2006).These are succeeded by the “normal” flyschoid successions but in thiscase they include interbedded chaoticmélange deposits, inwhichup tohectometre-size carbonate platform olistoliths occur (Fig. 4) (e.g.,Moreno-Eiris et al., 1995; Sarmiento and Gutiérrez-Marco,1999; Cózaret al., 2004; Armendáriz et al., 2005; Armendáriz, 2006). Some of themélange deposits extend into the adjacent Guadalbarbo unit to thenorth but not beyond, perhaps suggesting that the relative troughreferred to above on the location of this unit still persisted at this stage.The implication of this unique stratigraphy is that, contrary to the restof the basin, the Central Guadiato unit may have been temporarilysubjected to uplift (transpressional?) while the rest of units werealready undergoing flexural subsidence. Subsequent incorporation ofthis unit into the overall subsiding foredeep, may have beenresponsible for the collapse of the uplifted block and its carbonatecover in particular, triggering the formation of the spectacular chaoticmélanges (Fig. 9C and D).

Eventual returning to transtensional conditions is demonstrated bypost-inversionmagma emplacement along the Villaviciosa de Córdoba–La Coronada igneous belt and especially the Pedroches Batholith, whosemain phase of magma emplacement took place around 310 Ma ago(upper Westphalian–Stephanian) (Fernández-Ruiz et al., 1990; Alonso-Olazábal et al., 1999; Donaire et al., 1999, Alonso-Olazábal, 2001;García de Madinabeitia et al., 2001; García de Madinabeitia, 2002), aswell as by formation of terrestrial, intermontane successor basins; e.g.,the Westphalian B, Peñarroya–Belmez–Espiel basin (Quesada, 1983;Gabaldón et al., 2004). The fact that igneous activity lasted until thewaning of the Variscan orogeny implies the persistence at depth of thethermal anomaly responsible for it.Was it the sameone that contributedto the opening of the Tethys Ocean during the subsequent breakup ofPangea?

Acknowledgements

This work has been funded by the SpanishMinistry of Education andScience in the frame of the FEDER-CICYT (Ref. BTE2002-03819) project.Contribution to IGCP project 497 (The Rheic Ocean). We are grateful tothe laboratory staff at IGME for elaboration of thin sections and whole-rock chemical analyses. Finally, we thank journal reviewers, Dr. JoséBrandãoSilva andDr. KerstinDrost, for their constructive comments thatsignificantly improved the original manuscript.

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