Rift-related volcanism predating the birth of the Rheic Ocean (Ossa-Morena zone, SW Iberia

Gondwana Research 17 (2010) 392–407

Contents lists available at ScienceDirect

Gondwana Research

j ourna l homepage: www.e lsev ie r.com/ locate /gr

Rift-related volcanism predating the birth of the Rheic Ocean(Ossa-Morena zone, SW Iberia)

T. Sánchez-García a,⁎, F. Bellido a, M.F. Pereira b, M. Chichorro c, C. Quesada a, Ch. Pin d, J.B. Silva e

a IGME, c/Ríos Rosas, 23, 28003-Madrid, Spainb Dpto. Geociências, Centro de Geofísica de Évora, Univ. Évora, Apt. 94, 7001-554 Évora, Portugalc CICEGe - Centro de Investigação em Ciência e Engenharia Geológica, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2829-516 Caparica, Portugald Dpt. Géologie, CNRS, Univ. Blaise Pascal, rue Kessler, 63038 Clermont-Ferrand, Francee Dpto. Geologia, Facultade Ciências, Univ. Lisboa, Portugal

⁎ Corresponding author.E-mail address: [email protected] (T. Sánchez-Garc

1342-937X/$ – see front matter © 2009 International Adoi:10.1016/j.gr.2009.10.005

a b s t r a c t
a r t i c l e i n f o
Article history:Received 6 May 2009Received in revised form 26 September 2009Accepted 13 October 2009Available online 25 October 2009

Keywords:Rift magmatismCambrian–OrdovicianGeochemistryRheic OceanOssa-Morena zoneSm–Nd isotopes

Two very different periods of magma emplacement in the crust of the Ossa-Morena zone (early and mainevents) in SW Iberia have been previously interpreted to record a Cambrian/Early Ordovician rifting eventthat is thought to have culminated in the opening of the Rheic Ocean during the Early Ordovician. Newstratigraphic, petrographic, geochemical and Sm–Nd isotope data from Cambrian volcanic rocks included insix key low-grade sections in both Portugal and Spain considerably improve our understanding of theseevents. These data: (1) confirm the existence of two rift-related magmatic events in the Cambrian of theOssa-Morena zone, (2) demonstrate that the early rift-related event was associated with migmatite andcore-complex formation in the mid-upper crust and is represented by felsic peraluminous rocks, the parentmagmas of which were derived mainly from crustal sources, and (3) show the main rift-related event to berepresented by a bimodal association of felsic and mafic rocks with minor amounts of intermediate rocks.Some of the mafic rocks show N-MORB affinity, whereas others have OIB or E-MORB affinities, suggestingdifferent heterogeneous mantle sources (depleted and enriched, asthenospheric and lithospheric, plume-likeand non-plume-like). The acid and intermediate rocks appear to represent hybrid mixtures of crust andmantle-derived magmas.This new data supports the hypothesis that the onset of rifting was associated with a process of oblique ridge-trench collision. We interpret the significant differences between the early and main events as reflecting theevolution fromawide rift stagewithpassive extensionmainly accommodatedby lower-crustflow in ahighheat-flow setting, to a narrow rift stage with active extension characterized by extension rates that outpaced thermaldiffusion rates.

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

1. Introduction

Two events played crucial roles in the evolution of the IberianMassifduring the Paleozoic. The first of thesewas a Cambrian/Early Ordovicianrifting event, which was largely responsible for the compartmentaliza-tion of the Paleozoic Iberian autochthonous margin of Gondwana andconsequent paleogeographic and lithotectonic differences between theCantabrian, West Asturian-Leonese, Central Iberian, and Ossa-Morenazones (Fig. 1) (Liñán and Quesada, 1990; Quesada et al., 1991; Quesada,1991; Sánchez-García, 2001; Sánchez-García et al., 2003; Quesada,2006; Quesada et al., 2006; Sánchez-García et al., 2008a). This riftingevent is interpreted as having culminated in the sequential opening ofthe Rheic Ocean by the Early Ordovician (Quesada, 1991; Sánchez-García et al., 2003, 2008a; Fernández et al., 2008). The second eventwasthe Variscan orogeny, which was caused by continental collisionfollowing the closure of the Rheic Ocean and is responsible for the

ía).

ssociation for Gondwana Research.

deformation of the Paleozoic Iberian margin of Gondwana and thepresent geometrical arrangement of units (Fig. 1) (Ribeiro et al., 1990;Quesada et al., 1991;Quesada, 1991;Quesadaet al., 2006;Quesada, 2006).

Previous to these Paleozoic events, the geodynamic evolution ofnorthern Gondwana was characterized by a period of arc growth andaccretion to the continental margin during the Ediacaran (Cadomianorogeny). This process had come to a fairly abrupt halt by the EarlyCambrian,when themargin, including parts of the previously accretedCadomian-Avalonian arc (Quesada, 1991; Nance et al., 2002; Sánchez-García et al., 2003; Murphy et al., 2006; Pereira and Quesada, 2006;Pereira et al., 2006; Sánchez-García et al., 2008a), started to undergodifferential uplift and erosion together with a cessation of subduction-related magmatism. This was followed by an onset of extensionaldeformation,whichwas responsible for crustal thinningwith compart-mentalization into subsiding (graben) andmore stable (horst) domains(Liñán and Quesada, 1990). At the same time mantle upwellingtriggered rift-related igneous activity, which reached its maximumexpression in Iberiawithin theOssa-Morenazone (Sánchez-García et al.,2003, 2008a). This evolution from subduction/arc growth to continental

Published by Elsevier B.V. All rights reserved.

mailto:[email protected]

http://dx.doi.org/10.1016/j.gr.2009.10.005

http://www.sciencedirect.com/science/journal/1342937X

Fig. 1. Zonal division of the Iberian massif, modified according to Julivert et al. (1974) and Quesada (1991).

393T. Sánchez-García et al. / Gondwana Research 17 (2010) 392–407

rifting is believed to have been associatedwith a process of ridge-trenchcollision, followed by the development of a slab window, whicheventually led to the opening of a brand new oceanic tract (the RheicOcean) and drifting of a terrane (Avalonia?) consisting mainly of a partof the previously accreted Ediacaran arc (Nance et al., 2002; Sánchez-García et al., 2003; Pereira et al., 2006; Pereira and Quesada, 2006;Murphy et al., 2006; Sánchez-García et al., 2008a; Nance et al., 2010-this volume). A model similar to the present collision of the East Pacificrise with the Pacific margin of North America, which is leading to theopening of theGulf of California and the separation of the Baja Californiapeninsula, would be broadly applicable (Sánchez-García et al., 2003;Lizarralde et al., 2007; Sánchez-García et al., 2008a).

Our research grouphas already published general descriptions of thesedimentary, structural andmagmatic expression of theCambrian/EarlyOrdovician rifting in theOssa-Morena zone (Sánchez-García et al., 2003;Quesada, 2006; Pereira et al., 2006; Pereira and Quesada, 2006;Chichorro et al., 2008; Sánchez-García et al., 2008a,b). However, aprecise understanding of the various elements has still not been arrivedat and this is particularly true of the igneous rocks. The main factorscontributing to this gap in our knowledge relate, on the one hand, to theheterogeneity of rock types (plutonic, subvolcanic and volcanic), rockcompositions (from acid to basic; calc-alkaline, tholeiitic, alkaline andperalkaline) and apparentmagma sources (asthenospheric, lithosphericand crustal) and, on the other hand, to the complex and varied Variscandeformation and metamorphic history of the different fault-boundstructural units that currently make up the Ossa-Morena zone. Withinthis context, making correlations between low-grade and high-gradeunits (locally migmatitic) becomes particularly difficult.

Outcropping low-grade crustal segments in the Ossa-Morena zonecontain both volcanic and shallow plutonic rocks, which, according totheir relationship with coeval sedimentary successions, can be assigned

to one of two periods of magma emplacement: (1) an early rift-relatedigneous event comprising felsic peraluminous rocks and associatedwithmigmatite formation during the development of core-complex struc-tures in mid- to upper-crust environments, and (2) a main rift-relatedigneous event, which produced predominantly basaltic and felsic(rhyolite) rocks and minor amounts of intermediate (trachyte) rocks(Sánchez-García et al., 2003, 2008a). Tholeiites and alkaline rockspredominate in this suite but minor calc-alkaline peraluminouscompositions are also present (Sánchez-García et al., 2008a).

We concentrate here on the stratigraphic, petrographic, geochemicaland isotopic correlation of themainly Cambrian volcanic components ofthe two rift-related igneous sequences that cut through some keystructural units in theOssa-Morena zone in both Portugal and Spain.Wechose six low-grade key sections to characterize the Cambrian volcanicrecord of the northeastern Alentejo and the southern flank of theOlivenza-Monesterio antiform: the Alter do Chão-Elvas, Assumar andOuguela sections in Portugal, and the Alconchel, Jerez and Segura deLeón sections in Spain (Figs. 2 and 3).

2. Cambrian volcanic rocks of the Ossa-Morena zone

2.1. Stratigraphy

TheCambrian succession in the various (low-grade) structural units ofthe Ossa-Morena zone rests unconformably upon a previously deformedEdiacaran basement [mainly the so-called Serie Negra (Alía, 1963)]. Itconsists of both sedimentary and volcanic rocks, and includes consider-able variation in both the facies and their thickness from unit to unit.Despite these variations, the sedimentary successions in most of thestructural units are made up of four components, in ascendingstratigraphic order (Fig. 3): a lower detrital formation (LDF), a detrital-

Fig. 2. Geological sketch map of the Ossa-Morena zone (OMZ in insert) adapted from Gonçalves (1970, 1971), Gonçalves and Fernandes (1973), Gonçalves et al. (1972, 1975, 1978),Oliveira (1984, 1992), Pereira (1999), Pereira and Silva (2002) and the geological map compilation of the Ossa-Morena zone in Spain (Quesada and Sánchez-García, 2002).

394 T. Sánchez-García et al. / Gondwana Research 17 (2010) 392–407

carbonate formation (DCF), an upper detrital formation (UDF) and amainrift-related, volcanic-sedimentary complex (MRV). The three lowestformations belong to the Early Cambrian, whereas the MRV is largelyMiddle Cambrian (Gonçalves, 1971, 1978; Liñán, 1978; Oliveira et al.,1991; Gil Cid, 1991; Liñán et al., 1993, 1995, 1996, 2002; Sánchez-Garcíaet al., 2003). In some units, a topmost siliciclastic sedimentary successionoverlies the MRV, which may locally extend into the Late Cambrian(Palacios, 1993). However, a significant characteristic of all the Cambriansuccessions in theOssa-Morena zone is the existenceof a sedimentary gapof variablewidth, extending inplaces as far downas the LDF, but generallycovering the entire Late Cambrian and the basal strata of the EarlyOrdovician (Liñán and Quesada, 1990). This gap represents a periodduringwhich thewholeOssa-Morena zonewas subjected to uplift, tilting,erosion and/or non-deposition.

Within this sedimentary framework there are volcanic rocksinterbedded at two specific intervals, although scattered volcanichorizons occur locally from the base to the top. The stratigraphicallylower volcanic package is coeval with the siliciclastic sedimentation ofthe LDF in the Early Cambrian (Fig. 3). It corresponds to the early rift-related igneous event described by Sánchez-García et al. (2003, 2008a)and contains only felsic volcanic and volcaniclastic rocks of peralumi-nous affinity. The stratigraphically higher volcanic package [main rift-

related igneous event of Sánchez-García et al. (2003, 2008a)] is recordedby the Middle (-Late?) Cambrian MRV (Fig. 3), which is typicallybimodal. The initial volcanic activity of this main event started duringthe deposition of the late Early Cambrian UDF, but became massiveduring the MRV (Sánchez-García et al., 2003).

As previously mentioned, an important stratigraphic character-istic of the Cambrian of the Ossa-Morena zone is the significantvariability in the thickness, facies and age of the various formationsfrom unit to unit (Liñán and Quesada, 1990; Sánchez-García andQuesada, 2001; Sánchez-García, 2001; Sánchez-García et al., 2003).The successions vary from a few hundred metres to almost 6 km inthickness (see Fig. 4 in Sánchez-García et al., 2003). These differencesare interpreted as evidence for synsedimentary tectonism and thedevelopment of an overall horst and graben structure (Liñán andQuesada, 1990; Sánchez-García et al., 2003).

2.2. Field relationships and macroscopic characteristics

2.2.1. Volcanic rocks of the early rift-related event (ER)In this group we include the Bodonal-Cala beds (Hernández Enrile,

1971) (Figs. 3 and4) andminor volcanic intercalations in the TorreárbolesFormation (Liñán, 1978) in Spain, and the Freixo-Segovia, Nave de Grou-

Fig. 3. Stratigraphic correlation chart of the Cambrian in the Ossa-Morena zone (northeast Alentejo and southern Olivenza-Monesterio tiform) (not to scale).

395T.Sánchez-G

arcíaet

al./Gondw

anaResearch

17(2010)

392–407

an

Fig. 4. Generalized stratigraphic column of the rift sequence in the Ossa-Morena zone.


Azeiteiros and São Vicente volcanic–sedimentary complexes in Portugal(Pereira and Silva, 2002, 2006).

The Portuguese Early Cambrian volcanic–sedimentary complexescorrespond to a felsic-dominated magmatic association composed offelsic tuffs, rhyolites and dacites coeval with the deposition of arkosicpelites and sandstones with interbedded conglomerates. Theseconglomerates have an arkosic matrix and contain clasts of blackcherts and slates deriving from the underlying Serie Negra, as well asfragments of rhyolite, dacite and arkose products cannibalized fromthe Early Cambrian volcanic–sedimentary rocks. The felsic volcanismand coeval detrital sedimentation extends laterally and graduallyupwards into the detrital–carbonate complexes of the DCF [Ouguelaand Assumar detrital–carbonate complexes of Pereira and Silva (2002,2006) and Pereira et al. (2006)].

Coeval sequences in Spain correspond to the sediment-dominatedLDF, withinwhich the Torreárboles Formation (Liñán, 1978) is themostrepresentative. Lying unconformably upon deformed Ediacaran rocks,the LDF consists mainly of an upward-fining-and-thinning siliciclasticsequence with some carbonate horizons towards the top (Liñán, 1978;Liñán and Quesada, 1990). It startswith fluvial and deltaic basal conglo-merates and arkosic sandstones (with abundant trace fossils) gradingupwards into shallow-marine sandstones andmudstones that graduallygive rise to shelf-carbonate deposits (Liñán and Quesada, 1990; Liñánet al., 1996; Sánchez-García et al., 2003). The fossil content of thisformation dates it to the Cordubian stage (Early Cambrian; Liñán et al.,2002, and references therein).

Coeval with this sedimentary formation, the Bodonal-Cala beds(Hernández Enrile, 1971) constitute the most representative exampleof volcanism belonging to the early rift-related igneous event ofSánchez-García et al. (2003, 2008a). Above a basal arkosic-matrixconglomerate containing pebbles from the Serie Negra of black chert,rhyolites, rhyolitic tuffs, porphyroids, sandstones and pelites, thesebeds consist of a heterogeneous alternation of felsic volcanic andvolcano-sedimentary rocks (crystal-rich and hyaloclastic rhyolitictuffs, glassy andporphyritic rhyolites and cinerites, in decreasing orderof abundance). Mafic dikes locally cut through the predominantlyfelsic lithologies, but their age and genetic correlation are difficultto determine. Towards the top, the beds include some carbonateintercalations and progressively extend upwards into the detrital–carbonate formation. Recent U–Pb dating (TIMS) of this igneous eventyielded an age of ca. 530 Ma (Romeo et al., 2006; Sánchez-García et al.,2008a), coevalwith similar igneous rocks in other parts of the VariscanBelt (Melleton et al., 2010).

2.2.2. Volcanic rocks of the main rift-related event (RR and MRV)Volcanic rocks start to become abundant oncemore towards the topof

the late Early CambrianUDF. This initial volcanismof themain rift-relatedigneous event (RR) is particularly important in the Jerez section (Fig. 3)(Dupont, 1979), where its composition is bimodal and associated withshallow plutonic rocks (Sánchez-García et al., 2003, 2008a). Recent U–Pbzircon dating (TIMS) (Sánchez-García et al., 2008a) of the Remediosgranite yielded an age of 517+/−2Ma (Fig. 3). The volcanic succession


includes keratophydic rhyolites, porphyry tuffs, ignimbrites, hyaloclas-tites, basalts and mafic tuffs, and interbedded sediments such as green,grey and violet slates, siltstones, arkose, greywacke and quartzitesandstones.

The fact that this initial volcanism of the main igneous event isonly substantial in the area of the Jerez section suggests that activity inthe Ossa-Morena zone was localized at this stage, although it was theprecursor of much more widespread activity during the subsequentMiddle Cambrian. Lack of material due to subsequent erosion isunlikely in most areas since no significant stratigraphic gaps existwithin the Early Cambrian (Gonçalves, 1971, 1978; Liñán, 1978;Oliveira et al., 1991; Gil Cid, 1991; Liñán et al., 1993, 1995, 1996, 2002;Sánchez-García et al., 2003).

The bulk of the main rift-related volcanism in the Ossa-Morenazone took place during the Middle Cambrian, and perhaps the lowerpart of the Late Cambrian (MRV). In our study area it includes theTerrugem Formation in Portugal (Gonçalves and Fernandes, 1973;Gonçalves, 1978; Oliveira et al., 1991), and the Alconchel-San Benitovolcanic–sedimentary complex and Umbría-Pipeta basalts in Spain.TheMRV is correlated with the Playón beds (Liñán and Perejón, 1981)in the Zafra section of the northern limb of the Olivenza-Monesterioantiform (which are not included in this study).

The volcanic rocks make up a bimodal association that includeslavas, porphyry tuffs, peperitic and hyaloclastic rocks of mainlyrhyolitic composition in the felsic end-member, and basalts, basalticandesites andmafic tuffs in the mafic end-member. Minor amounts ofintermediate (trachyte) rocks also occur. These volcanic and volca-niclastic rocks are intercalated with slates, siltstones, sandstones andconglomerates. Usually the sediments include varying amounts ofvolcanic components (such as volcanic crystals, volcanic fragmentsand glass shards).

The stratigraphy is similar in the Alconchel and Zafra sections (seeFig. 4 in Sánchez-García et al., 2003), where felsic rocks predominate atthe bottom, whereas mafic rocks predominate at the top. In the othersections, mafic rocks predominate throughout with only local appea-rances of felsic rocks.

2.3. Petrographical characteristics

In this study, we refer to a total of 39 new samples of volcanic rocks.Four samples belong to the early rift event and 35 to themain rift event.Among the latter, one sample belongs to a volcanic intercalation withinthe detrital–carbonate formation, 11 to the RR rocks interbeddedwithinthe UDF, and 23 to the MRV.

2.3.1. Volcanic rocks of the early rift-related event (ER)Detailed petrographic analyses of the volcanic rocks show that this

group includes rhyolites, quartz- and feldspar-rich rhyolitic crystal tuffsand rhyolitic ignimbrites with deformed glass shards, K-feldspar andquartz fragments. Exploded crystals are commonly found within thetuffs and ignimbrites. Thematrix in the lavas and ignimbrites consists ofdevitrified glasswith frequent spherulitic textures. The subvolcanic Calaporphyry is characterized by a coarse porphyritic texture containingK-feldspar, plagioclase and quartz phenocrysts (up to 10 cm long) ina dark-grey, fine-grained matrix.

2.3.2. Volcanic rocks of the main rift-related event (RR)The only sample of volcanic rock in the DCF comes from a sill

interbedded within the carbonates. This rock is a basaltic andesite andincludes apatite and opaqueminerals with skeletal textures caused bytheir rapid cooling.

The felsic rocks of the RR group interbedded within the UDFvary between potassic and sodic rhyolites and include lavas, tuffsand ignimbrites with glass shards, K-feldspar and quartz crystals,and lithic fragments in a devitrified fine-grained groundmass.The mafic lavas vary between plagioclase and amphibole-bearing

basalts and frequently show evidence of alteration processes, mainlyspilitization.

Finally, the MRV felsic rocks have a high alkali content, varyingbetween potassic and sodic end-members, and include lavas, auto-breccias, tuffs and some peperites. The mafic rocks in turn vary betweenplagioclase and amphibole-rich basalts, and some of them are vesicular.There are also some hyaloclastite mafic tuffs. Significantly, all the rocksbelonging to themain rift-related event show signs of spilite-keratophyrealteration, which is characteristic of volcanic rocks deposited insubaqueous environments. The mafic rocks show albitization of the Ca-plagioclase accompanied by the formation of chlorite, calcite, epidote,prehnite and other low-temperature hydrous minerals typical of thegreenschist facies. The felsic rocks also show evidence of the same sort ofalteration with the paragenesis of albite or albite-oligoclase and epidote,chlorite and calcite.

3. Geochemistry

In this study, we present new geochemical data (Tables 1 and 2)from volcanic rocks which complement those published in previouspapers (Sánchez-García et al., 2003; BellidoMulas et al., 2007; Sánchez-García et al., 2008a,b) and which will be used for comparison.

Whole-rock analyses of the major and most trace elements weremade by XRF at IGME Laboratories (Tres Cantos, Madrid) using aMagiX PAN analytical spectrometer equipped with a Rh tube. Themajor elements were analysed on glass discs of powdered rocks fusedwith lithium tetraborate. Trace elements were analysed in pressedpellets using PROTRACE software. Na contents were determined byAAS spectrometry in samples fused with lithium metaborate anddiluted with an acid solution. REE concentrations were determined byICP-MS with an Agilent 7500 CE spectrometer from samples fusedwith lithium metaborate and diluted with an acid solution. LOI weredetermined by ignition at 950 °C. The RMS (root mean square)relative to concentration varies between 1.65% (Fe) and 52% (P) forthe major elements, between 1.75% (Rb) and 35.28% (Hf) for thetrace elements, and is lower than 5% for the REE. The principalgeochemical characteristics of the volcanic rock groups studied areoutlined below:

3.1. Volcanic rocks of the early rift-related event (ER)

Only acidic rocks occur in this group, and these are overwhelm-ingly dominated by rhyolitic compositions (Table 1). All the samplesplot in the subalkaline field (Fig. 5A) and vary betweenmetaluminousand peraluminous types. In general, the samples described here aremore Na-rich than the samples reported in previous papers, with K2O/Na2O=0.18.

The REE patterns are similar, with more fractionated values for theLREE [(La/Sm)n=4.40] than the HREE [(Gd/Yb)n=1.29]. However,the (La/Yb)n ratio is somewhat lower, with values close to 5. Inaddition, the average Eu-anomaly (0.79) is smaller than that found inthe previously published analyses (Fig. 5C). The felsic rocks of the ERgroup have Nb contents (average=9.3 ppm; max=12.3 ppm;min=6.40 ppm) close to continental crust values (average=8 ppm;Rudnick and Gao, 2004). Primitive mantle-normalized spider dia-grams show moderate to negligible negative Nb anomalies (averageLa/Nb=1.37) typical of the continental crust, with values closer tothose of the primitive mantle (1.17) as reported by Palme and O'Neill(2004) (Fig. 5D). The average Nb/Th ratio (1.16) is also close to that ofthe upper continental crust (1.43) of Rudnick and Gao (2004). Thesystematically negative εNd values (see below) likewise point to aprobable crustal origin for these rocks. The notable Zr positiveanomaly (Fig. 5D) may be related to a high zircon content in thepredominantly metasedimentary source of this group of rocks. All thegeochemical characteristics outlined above suggest that thesematerials derive mainly from a continental crustal source, although

Table 1Major and trace element data for the ER and RR (UDF and MRV) felsic and intermediate rocks: oxides as wt.%; trace elements in ppm.

Sample S1 S2 S3 S4 S5 S6 S7 S8

Group ER ER ER ER MRV MRV MRV MRV

SiO2 68.00 80.75 77.56 76.24 71.37 77.74 59.82 77.64Al2O3 14.84 11.53 11.26 12.29 13.44 10.29 15.63 10.99FeO 5.15 0.27 1.74 1.23 4.18 3.85 10.19 3.06MnO 0.10 0.02 0.04 0.03 0.05 0.08 0.09 0.02MgO 1.93 0.10 0.38 0.10 0.99 0.34 2.67 0.10CaO 0.61 0.39 2.13 0.67 0.21 0.48 1.66 0.03Na2O 2.79 5.70 4.80 7.33 3.73 3.46 2.00 4.23K2O 2.69 0.25 0.36 0.31 4.08 1.67 3.75 3.07TiO2 0.78 0.44 0.36 0.70 0.31 0.31 1.01 0.17P2O5 0.10 0.10 0.11 0.16 0.05 0.05 0.11 0.05PPC 2.44 0.54 1.08 0.90 1.19 1.37 2.02 0.48Total 99.43 100.09 99.82 99.96 99.59 99.63 98.94 99.83Ba 955.20 435.00 56.10 55.30 390.70 502.50 311.50 507.90Cs 0.00 3.60 2.90 0.00 5.60 9.30 6.70 16.00Ga 18.60 9.20 7.80 12.10 36.90 44.30 37.20 22.20Hf 5.50 5.00 2.00 3.30 32.50 44.90 22.90 19.00Nb 12.30 7.00 6.40 11.50 131.50 150.60 86.70 103.80Rb 74.90 6.10 8.50 8.70 59.90 48.20 86.80 37.50Sr 135.90 116.30 94.50 77.20 107.10 59.40 136.70 29.90Ta 0.00 2.10 0.00 1.00 8.70 8.50 2.40 8.90Th 9.00 5.90 7.20 9.50 23.00 22.70 12.50 16.90U 1.30 0.50 1.00 1.40 5.10 5.30 1.30 3.10Zr 219.20 195.90 144.50 291.90 1241.50 1662.50 941.10 564.10La 28.20 6.35 4.17 18.90 100.00 127.00 51.10 88.20Ce 60.00 16.50 8.63 36.20 225.00 285.00 138.00 183.00Pr 6.58 1.93 1.12 4.30 26.80 34.40 14.20 19.10Nd 26.70 7.95 4.56 17.00 108.00 141.00 64.40 66.50Sm 5.09 1.92 1.20 3.59 19.70 29.80 14.30 8.85Eu 1.23 0.41 0.37 1.12 2.08 3.14 3.75 0.74Gd 4.94 2.20 1.49 3.54 17.00 28.60 13.30 7.32Tb 0.67 0.37 0.28 0.53 2.66 4.41 2.04 1.14Dy 4.05 2.37 1.90 3.13 16.40 27.70 13.00 7.33Ho 0.78 0.52 0.41 0.63 3.10 5.30 2.57 1.58Er 2.35 1.61 1.28 1.97 9.15 15.60 8.02 5.12Tm 0.34 0.24 0.20 0.28 1.34 2.30 1.22 0.82Yb 2.23 1.70 1.47 1.93 8.37 14.60 8.06 5.61Lu 0.33 0.23 0.24 0.31 1.22 2.16 1.23 0.84Y 19.40 13.50 10.00 14.20 77.80 144.00 72.50 51.60HFS 5388.87 3290.65 2808.94 5236.26 3659.38 4237.90 7709.70 2028.53LILE 23,577.59 2636.62 3175.94 2779.26 34,792.37 14,916.58 31,833.97 26,357.26REE 143.49 44.30 27.32 93.43 540.82 721.01 335.19 396.15LREE 126.57 34.65 19.68 79.99 479.50 617.20 282.00 365.65HREE 15.69 9.24 7.27 12.32 59.24 100.67 49.44 29.76(La/Yb)n 8.52 2.52 1.91 6.60 8.05 5.86 4.27 10.59UTMX 144,218 144,635 660,970 672,147 656,343 663,197 662,194 666,888UTMY 4,335,088 4,321,368 4,314,676 4,267,366 4,290,540 4,281,091 4,280,206 4,264,851


they do have show features that may indicate greater subcrustalparticipation than is the case for the previously published set ofsamples [e.g. lack of clearly defined Nd negative anomalies andpresence of rocks with weakly negative εNd values (see below)].

When the compositions are plotted on the Y–Nb tectonicdiscrimination diagram (Pearce et al., 1984), all the samples fall inthe orogenic field, which is consistent with our previous data (Fig. 5B),but somewhat at odds with the interpreted extensional environment.In line with other researchers (e.g., Linnemann et al., 2008; Nanceet al., 2010-this volume), our interpretation of this apparentcontradiction envisages inheritance of the orogenic signature fromthe subduction-related calc-alkaline volcano-sedimentary rocks(Cadomian arc) that should have existed at their crustal source.

3.2. Volcanic rocks of the main rift-related event (RR and MRV)

This group is typically bimodal with respect to SiO2 andpredominantly includes basaltic and rhyolitic types, with a minorproportion of intermediate compositions (Fig. 6). Subalkaline typespredominate and most (both felsic and mafic types) have a higherZr-content than published standards for similar volcanic associa-tions [OIB, E-MORB and N-MORB of Sun and McDonough (1989)and BCC of Rudnick and Gao (2004) for the acid and intermediaterocks].

3.2.1. Mafic rocksWithin the basic group (SiO2<52%), geochemical characteristics

allow us to define two different subgroups (Table 2). These twosubgroups are indistinguishable both visually and petrographicallyand are intimately associated with each other in the same outcrops,at least on the southern flank of the Olivenza-Monesterio antiform,which is reported here.

3.2.1.1. Mafic rocks with OIB and E-MORB affinities. The vast majority ofthe samples belonging to this group correspond to volcanic rocks ofthe Middle Cambrian MRV, although a few also come from volcanichorizons (RR) within the Early Cambrian UDF. Noticeably, the alkalicontent is highly modified by the alteration processes, mentionedabove. These processes may also cause a small relative increase inSiO2.

REE normalization to the primitivemantle of the OIB-affinity basalts(Palme and O'Neill, 2004) shows moderate REE fractionation (Fig. 7A)and negligible europium anomalies. The average (La/Yb)n ratio has avalue of 6.55,which liesmidwaybetween theOIB and E-MORB values ofSun and McDonough (1989). The average Nb/Th ratio (8.64) in oursamples is lower than theOIB values of Sun andMcDonough (1989) andcloser to those of the primitive mantle of Palme and O'Neill (2004).These values are also lower than those of our previously published datafor this group, pointing in this case to a less enriched source.

Table 1Major and trace element data for the ER and RR (UDF and MRV) felsic and intermediate rocks: oxides as wt.%; trace elements in ppm.

S9 S10 S11 S12 S13 S14 S15 S16 S17

MRV MRV MRV MRV UDF UDF UDF UDF UDF

52.63 54.44 74.88 58.35 77.42 67.80 76.11 72.70 77.1415.64 16.75 12.74 15.47 12.05 15.88 12.77 13.74 11.4910.20 12.34 1.99 10.99 2.52 3.02 1.46 2.25 2.440.23 0.07 0.04 0.27 0.02 0.01 0.03 0.05 0.052.64 1.58 0.22 0.42 0.10 0.21 0.09 1.22 1.284.89 1.48 1.26 0.74 0.25 0.18 0.39 0.13 0.023.87 7.84 2.14 3.92 5.03 2.17 3.36 0.26 0.384.01 0.50 4.61 5.96 1.08 9.41 4.56 7.26 4.681.76 1.89 0.19 1.12 0.21 0.16 0.12 0.20 0.160.87 0.91 0.05 0.33 0.10 0.01 0.01 0.05 0.052.14 0.84 1.65 1.16 1.00 0.81 0.97 2.01 2.15

98.88 98.64 99.77 98.73 99.79 99.66 99.87 99.86 99.831064.30 365.90 341.80 3662.60 166.70 1921.20 498.80 606.20 537.40

7.00 9.50 1.00 1.90 3.3029.20 24.60 29.30 27.10 28.70 32.40 26.90 29.20 25.2011.60 9.90 8.90 6.20 20.00 16.40 7.50 10.90 10.1068.30 42.10 66.80 40.50 25.60 80.90 82.20 14.30 13.0054.40 8.50 114.40 65.80 47.70 156.30 121.50 70.30 64.50

437.40 119.70 42.10 250.80 83.70 146.00 58.50 22.70 20.402.30 2.80 3.40 4.10 2.20 4.60 6.80 0.708.70 5.60 17.60 4.50 23.90 18.90 18.20 25.10 23.702.00 1.40 2.50 0.20 8.20 1.20 2.90 4.80 3.40

538.70 312.80 291.40 259.60 586.60 506.60 304.70 361.00 305.0068.10 47.20 74.10 28.10 40.00 86.10 117.00 29.00 28.20

159.00 91.50 129.00 66.70 114.00 183.00 51.50 62.2017.90 9.76 16.40 8.65 14.00 21.70 28.20 6.89 7.3482.50 38.40 58.80 35.80 60.10 86.80 112.00 29.20 31.6016.70 8.10 12.60 7.45 15.50 18.40 26.10 6.99 8.045.05 3.55 1.08 5.62 1.44 3.21 2.61 0.34 0.47

15.47 7.99 13.60 7.35 18.20 18.20 27.10 8.09 9.432.24 1.16 2.16 1.05 3.57 2.97 4.40 1.61 1.91

12.90 6.70 13.00 5.97 25.40 18.00 23.30 12.10 14.002.33 1.31 2.62 1.13 5.65 3.54 4.30 2.72 3.016.49 3.63 7.69 3.18 17.90 10.50 11.80 9.08 9.660.90 0.48 1.10 0.44 2.89 1.55 1.58 1.37 1.355.44 3.09 7.03 2.86 18.60 10.70 9.95 9.46 9.070.81 0.45 1.02 0.47 2.77 1.59 1.38 1.35 1.29

66.10 33.90 78.40 29.00 166.60 88.30 113.00 72.70 80.7015,126.94 15,749.93 1850.98 8552.93 2557.93 1803.11 1406.04 1911.14 1611.0435,047.65 4780.84 39,022.06 53,580.26 9474.81 80,628.49 38,668.78 61,048.69 39,595.66

395.83 223.32 340.20 174.77 340.02 466.26 503.72 169.70 187.57344.20 194.96 290.90 146.70 243.60 396.00 283.30 123.58 137.3846.58 24.81 48.22 22.45 94.98 67.05 83.81 45.78 49.728.43 10.29 7.10 6.62 1.45 5.42 7.92 2.06 2.09

701,885 665,314 661,837 662,618 629,164 683,791 683,496 690,504 690,4454,217,040 4,263,815 4,277,603 4,277,399 4,324,758 4,248,256 4,246,268 4,248,217 4,248,195


The basalts plot mainly in the within-plate field in the tectonicdiscrimination diagrams of Pearce and Cann (1973), in accordancewith our interpretation of their anorogenic origin (Fig. 8A). In thediscrimination diagram of Condie (2005), however, the samplesoverlap the boundary between the plume and non-plume sourcefields, and lie close to the Nb-Line (Fig. 8C). This may reflect theinfluence of a recycled component in the mantle in the origin of thesebasalts. A similar interpretation has been proposed for Neoproterozoicmafic magmatism associated with the breakup of Rodinia in thesouthern Yangtze Block (Zhou et al., 2007).

3.2.1.2. Mafic rocks with N-MORB affinities. We have described inprevious papers (Sánchez-García et al., 2003, 2008a) how this groupcomprises only subalkaline rocks, as is shown in the Zr/TiO2 vs. silicadiagram (Winchester and Floyd, 1977) (Fig. 6). It is composed of maficsubvolcanic and volcanic rocks and minor contemporaneous plagio-granite bodies and albite tuffs. In this study, only volcanic rock sampleswere analysed. Most samples of this group belong to volcanic bedsintercalated within the UDF, although there are other samples fromthis formation that haveOIB affinities (see above). Field relations showthat the mafic and felsic rocks are contemporaneous, as demonstratedby mutually crosscutting relationships. One of these plagiogranitebodies was recently dated by U/Pb zircon (TIMS) at 517+/−2 Ma(Sánchez-García et al., 2008a).

Tholeiitic rocks account for 67% of the samples, although in generalthey have higher SiO2 contents than the samples studied previously(Sánchez-García et al., 2008a).

REE normalization to the primitive mantle (Palme and O'Neill,2004) shows an almost flat pattern or one slightly depleted in LREE(Fig. 7B), with an average (La/Yb)n=1.13 and no significant Euanomalies. As reported in previous papers, the average Nb/Th value ofour samples (2.66) is lower than those of the N-MORB standards(19.42) published by Sun and McDonough (1989), our average Nbcontents being similar to theirs (2.27 vs. 2.33), whereas our Thcontents are considerably higher (1.13 vs. 0.12).

In the tectonic discrimination diagrams of Pearce and Cann (1973),the samples show a clear N-MORB affinity (Fig. 8B). This characteristicis confirmed in the Zr/Y–Nb/Y diagram of Condie (2005) (Fig. 8D),which may indicate a depleted mantle source for these basalts.

3.2.2. Intermediate and felsic rocksFelsic rocks form the bulk of this group, which contain only

subordinate intermediate types. Co-genetic shallow plutonic andsubvolcanic rocks also occur in the region, but only the volcanic rocksare considered here. All of these show characteristics that lead us tointerpret them as being primary magmatic rocks sensu Coleman andDonato (1979). Across the spread of rock compositions, which rangefrom trachytes/andesites to rhyolites (Table 1), SiO2>52%, although

Table 2Major and trace element data for the mafic rocks (RRb-OIB and RRb-N-MORB samples): oxides as wt.%; trace elements in ppm.

Sample S18 S19 S20 S21 S22 S23 S24 S25 S26 S27 S28

Group RRb-OIB/MRV

RRb-OIB/MRV

RRb-OIB/MRV

RRb-OIB/MRV

RRb-OIB/MRV

RRb-OIB/MRV

RRb-OIB/MRV

RRb-OIB/MRV

RRb-OIB/MRV

RRb-OIB/MRV

RRb-OIB/MRV

SiO2 46.88 43.71 46.20 47.19 42.45 45.72 47.92 45.07 45.86 48.24 47.82Al2O3 14.54 14.16 17.21 10.06 15.17 16.88 12.73 16.25 14.21 15.06 15.75FeO 11.86 12.73 9.53 10.16 11.43 12.04 12.40 12.36 10.15 12.61 12.04MnO 0.33 0.21 0.13 0.16 0.10 0.20 0.19 0.18 0.12 0.23 0.19MgO 6.45 5.53 3.71 6.83 1.64 5.86 2.18 7.40 7.27 4.34 6.72CaO 10.14 6.97 8.43 10.60 11.50 7.31 7.67 8.09 7.30 4.82 8.55Na2O 3.20 2.43 4.28 2.90 5.42 3.94 3.40 3.42 2.05 5.48 3.38K2O 0.39 0.02 0.72 0.61 1.26 0.22 0.72 0.24 1.34 0.04 0.39TiO2 2.93 3.30 2.38 1.98 3.13 2.68 3.09 2.54 2.95 3.44 2.35P2O5 0.42 0.49 0.28 0.21 0.58 0.54 0.47 0.33 0.56 0.80 0.26PPC 1.58 9.02 6.06 4.08 6.09 3.50 7.85 2.77 7.07 3.55 1.19Total 98.72 98.57 98.91 94.77 98.76 98.88 98.63 98.63 98.89 98.61 98.63Ba 268.30 102.00 200.40 187.80 298.60 87.80 851.20 94.90 204.80 11.70 80.20CsGa 25.00 24.00 22.20 19.10 22.10 23.20 23.00 23.40 20.00 27.00 21.40Hf 2.90 5.20 4.70 3.50 5.00 2.60 8.30 1.50 6.30 8.80Nb 31.40 31.90 24.00 16.80 23.90 23.30 27.40 21.60 33.10 53.50 22.40Rb 10.60 3.80 17.60 7.90 31.70 5.40 34.30 6.90 22.00 1.10 12.30Sr 358.10 269.10 526.20 219.20 172.40 343.70 78.00 648.40 213.70 136.00 444.10Ta 5.00 1.30 1.80 1.40 0.30 3.40 1.50 1.30Th 1.70 3.60 2.90 3.80 2.80 4.30 4.60 4.60 4.50 5.00 1.30U 0.90 2.80 2.80 0.70 0.90 1.70 0.10 3.40 1.10 2.10Zr 211.40 267.30 223.00 169.20 182.50 201.40 280.40 173.60 236.60 392.30 171.20La 23.70 31.80 24.60 23.00 28.50 24.50 31.70 24.00 30.90 32.90 14.20Ce 53.20 72.30 54.10 49.50 58.20 56.50 70.90 48.60 70.70 79.40 30.70Pr 7.40 8.75 7.24 6.13 7.08 6.98 8.45 5.92 8.34 9.87 4.49Nd 33.50 37.90 31.30 26.40 30.80 33.70 39.40 28.10 38.60 43.60 19.30Sm 8.30 7.94 6.75 5.46 6.46 7.58 8.84 6.24 8.02 9.22 4.67Eu 2.98 2.57 2.16 1.86 2.27 2.73 2.60 2.15 2.41 2.92 1.63Gd 8.96 7.71 6.51 5.59 6.33 7.32 8.56 5.96 7.40 8.63 5.04Tb 1.37 1.14 0.97 0.82 0.95 1.04 1.29 0.89 1.07 1.21 0.79Dy 7.66 6.64 5.51 4.87 5.44 5.89 7.89 5.20 5.90 6.77 4.62Ho 1.45 1.25 0.97 0.93 0.96 1.04 1.51 0.95 1.08 1.22 0.87Er 3.84 3.48 2.72 2.46 2.56 2.77 4.31 2.59 2.87 3.27 2.42Tm 0.50 0.50 0.36 0.35 0.34 0.37 0.63 0.36 0.41 0.43 0.32Yb 3.19 2.99 2.18 2.18 2.01 2.10 3.81 2.12 2.28 2.68 2.01Lu 0.46 0.48 0.32 0.34 0.31 0.34 0.60 0.34 0.37 0.37 0.29Y 35.60 31.60 23.70 23.50 23.80 28.70 42.90 26.90 30.90 31.80 22.70HFS 19,721.68 22,266.67 15,748.91 13,025.77 21,565.74 18,685.95 21,004.99 16,877.86 20,510.57 24,643.03 15,453.31LILE 3953.18 640.33 6836.08 5530.20 11,035.47 2373.43 7072.67 2803.15 11,670.59 631.37 3828.68REE 156.51 185.45 145.69 129.89 152.21 152.86 190.49 133.42 180.35 202.49 91.35LREE 126.10 158.69 123.99 110.49 131.04 129.26 159.29 112.86 156.56 174.99 73.36HREE 27.43 24.19 19.54 17.54 18.90 20.87 28.60 18.41 21.38 24.58 16.36(La/Yb)n 5.00 7.16 7.60 7.11 9.55 7.86 5.60 7.62 9.13 8.27 4.76UTMX 687,337 625,973 644,245 644,153 646,842 656,104 664,502 663,182 701,662 685,450 661,839UTMY 4,244,520 4,320,917 4,301,388 4,301,326 4,292,834 4,290,237 4,282,660 4,281,100 4,216,262 4,230,873 4,277,612


there is a significant gap between 59%<SiO2<69%, as we havepointed out in previous papers (Sánchez-García et al., 2003, 2008a).This set of samples includes alkaline as well as predominantlysubalkaline compositions (Fig. 6).

Primitivemantle-normalized REE patterns (Palme andO'Neill, 2004)showmoderate LREE fractionation [(La/Sm)n=2.99] and, althoughmostof the patterns are flat [(Gd/Yb)n=1.50], some show slightly fraction-ated values for the HREE. Negative Eu anomalies are common, but somefeldspar-rich samples displaypositive ones (Fig. 9A). These Eu anomaliesare probably related to different degrees of plagioclase fractionation.

In primitive-mantle-normalized spider diagrams for incompatibleelements (Palme and O'Neill, 2004), the samples show a generalenrichment in LILE elements, with a strong negative anomaly in Sr andTi (Fig. 9C). The Zr contents are higher than average continental crustand basalt compositions, but many felsic alkaline rocks contain highquantities of these elements. In the Tertiary and Quaternary volcanicsof the Cañadas Formation in the Canary Islands, for example, averageZr values reach as high as 822 ppm (Brändle and Bellido, 2000;Thirlwall et al., 2000 and others).

In the Ga/Al–Zr tectonic discrimination diagram (Whalen et al.,1987), an anorogenic affinity is shown for all but three samples, whichplot in the Oceanic Ridge granite (ORG) field (Fig. 9B). The Y–Nbdiagram (Pearce et al., 1984) shows the same characterization, withsome samples plotting near the boundarywith the ORG field (Fig. 9D).On the basis of their low Nb contents, we interpret the three“anomalous” samples as showing a certain degree of crustalcontamination in their evolution.

4. Sm–Nd isotopes

All analytical procedures, including mass spectrometer analyses,were undertaken at the Laboratoire deGéologie, CNRS-Université BlaisePascal, Clermont-Ferrand, France. Whole-rock samples were powderedand spiked with 149Sm/150Nd in the dissolution process. The separationwas performed in three steps: (1) cation-exchange columns with HClchemistry, (2) transuranide extraction chromatography columns withHNO3 chemistry, and (3) chromatographic extraction columns forlanthanides (Pin et al., 1994; Le Fèvre and Pin, 2002, 2005). Sample

Table 2Major and trace element data for the mafic rocks (RRb-OIB and RRb-N-MORB samples): oxides as wt.%; trace elements in ppm.

S29 S30 S31 S32 S34 S35 S36 S37 S38 S39 S40

RRb-OIB/MRV

RRb-OIB/MRV

RRb-OIB/MRV

RRb-OIB/MRV

RRb-OIB/UDF

RRb-OIB/UDF

RRb-OIB/UDF

RRb-OIB/UDF

RRb-N-MORB/UDF

RRb-N-MORB/UDF

RRb-N-MORBUDF

50.80 48.93 43.73 39.13 44.85 52.01 49.26 50.88 48.61 46.91 47.1518.49 15.84 14.46 13.53 12.97 16.22 14.09 14.02 13.45 16.89 17.048.86 12.43 14.29 15.13 9.90 11.22 11.56 15.06 12.05 7.75 9.220.18 0.05 0.16 0.15 0.28 0.15 0.34 0.13 0.22 0.14 0.152.01 2.57 5.20 3.74 14.31 3.03 5.56 4.22 6.03 8.75 8.494.19 4.18 6.16 8.04 10.05 3.10 6.18 2.41 9.83 12.15 9.186.75 7.21 4.54 3.07 0.71 6.32 2.54 3.90 3.02 2.63 2.511.33 0.93 0.29 1.07 0.66 0.35 0.23 0.19 0.23 0.24 0.713.01 2.26 4.36 4.54 1.22 2.14 2.24 2.76 2.24 1.22 1.230.69 1.01 0.54 0.61 0.10 1.18 0.26 0.67 0.21 0.08 0.082.68 3.65 4.48 9.18 3.84 3.05 6.46 4.12 2.80 2.38 3.23

98.98 99.07 98.20 98.17 98.89 98.76 98.71 98.35 98.68 99.15 98.99625.70 212.80 114.50 131.30 61.50 91.00 44.70 102.70 954.10 27.60 338.30

0.9025.10 35.00 27.30 24.30 14.90 31.40 20.40 27.40 20.30 16.10 17.003.80 15.60 7.60 5.10 0.40 13.60 4.30 11.10 1.20 3.90

36.30 78.50 46.10 45.70 5.30 65.90 7.40 13.10 3.70 1.10 2.0028.00 9.00 5.30 23.90 23.20 7.90 15.00 7.50 4.00 3.50 21.70

347.90 197.20 237.30 176.70 281.60 101.60 274.10 971.90 214.70 265.80 421.703.30 4.80 6.90 6.00 4.40 2.50 2.603.40 9.50 4.70 3.10 1.40 7.00 1.90 3.60 1.80 1.20 0.401.60 3.00 1.20 1.10 2.20 1.50 3.10 1.10 0.70

249.80 650.70 301.40 249.60 83.80 509.20 203.90 304.00 145.70 80.20 80.2019.00 77.20 25.70 24.60 8.40 46.90 10.10 20.70 8.70 2.79 4.7149.70 159.00 61.20 58.70 19.00 114.00 25.90 50.00 20.40 8.75 11.807.21 19.90 8.23 8.00 2.52 14.20 4.18 7.30 3.27 1.55 1.89

32.10 81.30 35.80 34.90 12.00 64.80 20.00 35.10 17.60 8.65 9.567.72 16.80 8.26 8.01 2.79 14.30 5.84 9.91 5.70 2.90 3.082.82 4.50 2.71 2.50 0.96 4.55 1.72 2.74 1.95 1.12 1.197.75 16.40 8.50 7.90 3.13 14.40 7.14 11.80 7.23 3.89 3.881.13 2.26 1.22 1.15 0.50 2.01 1.24 2.02 1.22 0.66 0.696.33 12.40 6.67 6.38 3.07 11.30 8.03 12.50 8.21 4.42 4.401.16 2.28 1.22 1.19 0.58 2.11 1.68 2.63 1.73 0.90 0.883.03 6.20 3.27 3.20 1.64 5.62 4.98 7.60 5.09 2.69 2.590.39 0.84 0.42 0.42 0.23 0.73 0.71 1.05 0.73 0.37 0.362.37 5.18 2.54 2.52 1.45 4.51 4.69 6.70 4.66 2.40 2.340.33 0.76 0.36 0.37 0.23 0.64 0.71 1.01 0.68 0.35 0.35

29.50 63.40 30.90 29.50 15.40 55.80 45.70 61.00 45.90 23.20 21.5021,407.11 18,870.70 28,881.12 30,265.55 7867.42 18,674.71 14,855.20 19,918.26 14,558.78 7778.78 7842.7612,147.88 8385.08 2864.43 9259.38 5898.99 3282.22 2289.34 2700.48 3096.44 2318.60 6692.67

141.04 405.02 166.10 159.84 56.50 300.07 96.92 171.06 87.17 41.44 47.72115.73 354.20 139.19 134.21 44.71 254.20 66.02 123.01 55.67 24.64 31.0422.49 46.32 24.20 23.13 10.83 41.32 29.18 45.31 29.55 15.68 15.495.40 10.04 6.81 6.57 3.90 7.00 1.45 2.08 1.26 0.78 1.36

661,410 716,548 714,239 715,823 626,770 685,701 671,908 648,246 664,501 692,480 692,1154,262,532 4,217,473 4,216,408 4,215,526 4,324,073 4,230,408 4,261,880 4,306,292 4,284,207 4,248,384 4,249,065


decomposition was done by fusion with a LiBO2 flux in an inductionfurnace at about 1150 °C, as described by Le Fèvre and Pin (2005). Smand Nd isolation was then carried out using cation exchange andextraction chromatography methods adapted from Pin and SantosZalduegui (1997). Sm andNd concentrationsweremeasured by isotopedilution using a mixed 149Sm/150Nd tracer and thermalionization massspectrometry (TIMS), which allow 147Sm/144Nd ratios to be determinedto a precision of 0.2% (Le Fèvre and Pin, 2002). Smwasmeasured in thesingle-collectionmodeon an automated VG54Emass spectrometer, andNd isotope ratios were measured in the static multi-collection modewith a Thermo Finnigan Triton TI instrument with normalization to146Nd/144Nd=0.7219. The JNdi-1 isotope standard measured duringthese analyses gave 143Nd/144Nd=0.512105±6 (2 measurements),corresponding to a value of 0.511848 for the La Jolla standard (Tanakaet al., 2000).

In this study, we report the Sm–Nd data (Table 3) for 14 Cambrianvolcanic rock samples, 4 of which have already been published(Sánchez-García et al., 2008b), while 10 are derived from newsamples. In total, 8 samples belong to the RR andMRV and 6 to the ER.

In general terms, we can find no clear correlation trend for the εNdvalues in relation to the stratigraphic position of the samples. Neitherarewe able to correlate the εNd valueswith the SiO2 content or the La/Yb, La/Sm, La/Nb or Th/Nb ratios.

All of the samples belonging to the ER display negative εNdt values,ranging from −0.6 to −7.9 (Table 3 and Fig. 10). The correspondingTDM ages vary between 1.14 and 1.80 Ga. On the whole, the ER rockshave negative εNdt values and Th/Nb ratios close to upper continentalcrust values, suggesting a significant crustal component and aderivation dominantly from ancient continental crust.

Within the RR and MRV groups almost all of the samples displaypositive εNdt values (Table 3 and Fig. 10), the only exception beingsample S17, which shows εNdt=−0.6. The rest vary from +2.2 to+5.6, which is quite typical of mantle-derived magmas with somedegree of crustal contamination. TDMmodel ages fall between 1.27 and0.74 Ga, and the isotopic behaviour of the volcanic rocks situated nearthe top of the RR succession is noticeably more homogeneous. Thehighest value (1.27 Ga) corresponds to the anomalous sample withnegative εNdt, suggesting a higher degree of crustal contamination.

Fig. 5. Geochemistry of the early rift-related igneous suite. (A) Zr/TiO2 vs. silica diagram (Winchester and Floyd, 1977). (B) Y–Nb tectonic setting discrimination diagram of Pearceet al., 1984. (C) Primitive mantle-normalized REE patterns, normalizing values from Palme and O'Neill, 2004. (D) Simplified spider-diagram normalized to primitive mantle values ofPalme and O'Neill, 2004. Shaded area: plot area of previously published data from the same rock unit.


The wide range of εNdt values and TDM model ages reinforces the ideathat the mantle-derived primitive magmas were contaminatedto varying degrees by continental crust during the main rift-relatedevent. In our opinion, the most likely way in which mantle-derivedmagmasmight become contaminated in thismanner is bymixingwithcrustal melts in the lower crust.

Fig. 6. Zr/TiO2 vs. silica diagram (Winchester and Floyd, 1977) for the main rift-relatedigneous rocks. Symbols: filled circles: Middle Cambrian main rift-volcanism (MRV);crosses: RR volcanic rocks in the upper detrital formation (UDF).

5. Discussion and conclusions

Before discussing the geological and geodynamic significance ofthe data presented here, it is necessary to describe the geodynamicsetting of the Ossa-Morena zone prior to the onset of rifting. The zonewas part of a continental magmatic arc that became accreted to theouter continental margin of Gondwana (Iberian Autochthon) near theWest African craton (Murphy and Nance, 1989; Quesada, 1990a,b,1991; Murphy and Nance, 1991; Quesada, 1997; Linnemann et al.,2008; Pereira et al., 2008) during the Neoproterozoic Cadomianorogeny (see Figs. 8-A and 9-A, B, C in Sánchez-García et al., 2008a). Ittherefore lay in an active margin setting at the beginning of theCambrian (Quesada, 1990a,b, 1991, 1997, 2006). We have suggestedin the past that a feasible process capable of bringing about thecessation of subduction, while at the same time triggering the onset ofextensional deformation with its associated magmatic evolution, isthe oblique collision of a mid-ocean ridge with the trench that wouldhave existed outboard of the Ossa-Morena zone during the growth ofthe Neoproterozoic arc (Sánchez-García et al., 2003). An alternativehypothesis may be the impingement of a mantle-plume beneath thiszone.

The stratigraphic distribution of the rift-related volcanism demon-strates that therewas an initial magmatic flooding of the Ossa-Morenacrust (early rift-related event) composed of peraluminous acidicvolcanic and shallow plutonic rocks coeval with, and probably sourcedin, migmatites that were forming in mid-upper crust environments inassociation with core-complex style deformation (Sánchez-García

Fig. 7. Geochemistry of the main rift-related mafic igneous rocks. (A) OIB affinity subgroup normalized to primitive mantle (REE values from Palme and O'Neill, 2004). (B) N-MORBaffinity subgroup normalized to primitive mantle (REE values from Palme and O'Neill, 2004). (C) Simplified spider-diagram normalized to primitive mantle of Palme and O'Neill(2004) for the OIB affinity subgroup. (D) Simplified spider-diagram normalized to primitivemantle of Palme and O'Neill (2004) for the N-MORB affinity subgroup. Symbols as in Fig. 6.


et al., 2003, 2008a,b). The new petrological, geochemical and Nd-isotope data presented in this paper support the hypothesis that thisearly volcanism, represented by the Bodonal-Cala, Freixo-Segovia,Nave de Grou-Azeiteiros and São Vicente volcano-sedimentarycomplexes, contained a noticeable mantle input superimposed uponthe predominant participation of continental crust in the genesis ofthe magmas (negative εNd values), despite the absence of mantle-derived rock in the crustal section of this stage outcropping today. Themoderate-to-high negative εNdt values and TDM ages in excess of 1 Ga(average c. 1.6 Ga) obtained for the ER rocks are very similar to thesignatures obtained for sediments and crust-derived magmas of anyage in the Ossa-Morena zone (López-Guijarro et al., 2008; Armendárizet al., 2008), again emphasising the predominant role played bycrustal partial-melting processes in the genesis of the parent magmasof this group of rocks. These values are also similar to those obtainedfor felsic orthogneisses in other high-grade units of the Ossa-Morenazone (Chichorro et al., 2008).

The Early Cambrian high-grade metamorphic and anatecticprocesses recorded in high-grade units of the Ossa-Morena zone(not included in this study) were related to an Early Cambrianextensional tectonic event coupled with extreme heating of thecontinental crust. Both may have been a response to a strong thermalanomaly results from the impingement at the base of the Ossa-Morena lithosphere of either a mantle plume or a slab windowfollowing ridge-trench collision. We prefer the latter (Sánchez-Garcíaet al., 2003, 2008a,b). In our view, the rapid shift from crustalcompression (Ediacaran Cadomian orogeny) to crustal extension(Cambrian rifting) is best explained by a process of progressivethermal expansion of the upper (Ossa-Morena) plate leading topassive gravitational collapse, perhaps still within an overall conver-gent regime at the plate scale.We envisage a scenario similar to recent

situations at many points on the Pacific margin of North America, suchas the Basin and Range.

For the second period of magmatic flooding of the Ossa-Morenacrust (main rift-related event), the new petrological, geochemical andisotope data confirm the presence of magmas derived from differentsources (asthenospheric, lithospheric and crustal), as we havepreviously reported (Sánchez-García et al., 2003, 2008a,b; Chichorroet al., 2008). This represents a significant change from the early riftingevent and probably reflects an evolution from passive to activeextensional deformation, which affected the entire lithosphere andfacilitated the upwelling of magmas from sources as deep as theasthenosphere. A recent model that may be applicable is that of theLate Miocene evolution of the Baja California-Sonora-Sinaloa area,which culminated in the opening of the oceanic Gulf of California(Lizarralde et al., 2007, and references therein).

Within this main event we have distinguished two subgroups ofmafic rocks (Table 3 and Figs. 7 and 8) with geochemical character-istics that imply different sources and geodynamic settings, althoughneither subgroup strictly correspond to a given stratigraphic posi-tions. On a statistical basis, however, it would appear that the alkalinecharacter of the basalt samples increases from bottom to top, as Mataand Munhá (1990) and Ribeiro et al. (1997) have pointed out, butboth alkaline and tholeiitic rocks coexist in the UDF as well as in theMRV.

Rocks of the stratigraphically lower levels of the main event (theRR group within the UDF) contain, on average, more SiO2, MgO, Cr, Yand have higher Th/Nb, Zr/Nb and Zr/Th ratios, and less P2O5, TiO2,K2O, Nb, Th, Zr with lower K/Rb, K/U, Nb/La, Zr/Y and (La/Yb)n ratios(Table 2 and Fig. 7C) than the volcanic rocks of the upper levels (MRVgroup). All these data are compatible with a predominance ofdepleted mantle sources for the UDF magmas (N-MORB affinity),

Fig. 8. Geochemistry of the main rift-related mafic igneous rocks. (A) OIB affinity subgroup plotted on the Zr–Ti–Y tectonic discrimination diagram of Pearce and Cann (1973). (B) N-MORB affinity subgroup plotted on the Zr–Ti–Y tectonic discrimination diagram of Pearce and Cann (1973). (C) OIB affinity subgroup plotted on the Zr/Y–Nb/Y tectonicdiscrimination diagram of Condie (2005). (D) N-MORB affinity subgroup plotted on the Zr/Y–Nb/Y tectonic discrimination diagram of Condie, 2005. Shaded area: plot area ofpreviously published data from the same rock units. Symbols as in Fig. 6.


although the presence in this group of OIB and E-MORB affinity rocksindicates the possible coexistence of more than one mantle source forthe magmas. The geochemical characteristics of the volcanic rocks ofthe uppermost Cambrian stratigraphic levels (MRV) also favour thecoexistence of asthenospheric and enriched-mantle sources duringthis stage.

εNdt values show little variation within the MRV rocks (+2.9to +3.2), but a wider range within the RR rocks in the UDF (−0.6to +5.6). The narrow range of εNd values in the MRV group implies arelatively homogeneous source. The wider range in the UDF group, onthe other hand, can be interpreted as evidence of a variable contributionof recycled continental materials, as suggested by negative (−0.6) topositive (+5.6) εNdt values. A similar interpretation can be inferredfrom the TDMmodel ages, the variations of which are smaller and closerto the stratigraphic age in the MRV (0.74 to 0.88 Ga) than in the UDF(0.81 to 1.27 Ga). The fact that the model ages are older than thestratigraphic–chronological ages suggests that the parent magmas ofthese rockswere variably contaminatedwith components derived fromancient continental crust.

The geochemical and isotopic data set suggests that themagmas thatgave rise to the OIB-affinity basalt subgroup were probably extractedfrom aheterogeneousmantle reservoirwithmixed plume-like andnon-plume-like characteristics (Condie, 2005); i.e. a source with differentdegrees of enrichment in recycled components. The subgroup shows(Table 2 and Fig. 7A) higher average HREE contents (26 ppm) than thestandard OIB (21 ppm) reported by Sun and McDonough (1989), andlower HFSE contents (19 ppm), LILE (5 ppm), ∑REE (169 ppm) andLREE (141 ppm) than this standardOIB(HFSE=20 ppm, LILE=13 ppm,

∑REE=200 ppm and LREE=176 ppm). These differences may be areflection of a contribution frommixed sources in the OIB-affinity Ossa-Morena zone rocks, although the existence of contamination and/oralteration effects may have also played a role.

The N-MORB affinity basalt subgroup, has a higher Th content thanthe standard N-MORB (Sun andMcDonough, 1989), whichwe attributeto mild contamination with Th-rich crustal materials during the rise ofthe parent magmas to the surface. As can be seen in Table 2 and Fig. 7Band D, this subgroup exhibits enrichment in LILE (4 ppm), HFSE(10 ppm), ∑REE (59 ppm), LREE (37 ppm) and HREE (21 ppm)compared to the standard N-MORB (Sun and McDonough, 1989;LILE=707 ppm, HFSE=8 ppm, ∑REE=40 ppm, LREE=22 ppm, andHREE=17 ppm), which points to the existence of heterogeneity in themantle source. Within this subgroup, we have previously reported thepresence of volumetrically minor Mg-rich basalts, the formation ofwhich requires high melting temperatures (Sánchez-García et al.,2008a). The presence of these types of basalt favours the hypothesisof non-plume-like source magmas, as can be seen in Fig. 8D (Condie,2005).

Finally, the geochemical characteristics of the felsic and interme-diate rocks of the main rift-related igneous event suggest a derivationfrom variable amounts of mixing between crust- and mantle-derivedmagmas, as described by Sánchez-García et al. (2003, 2008a,b). Thenew data, especially the neodymium isotopic data, corroborate thisinterpretation. The εNdt variation between mildly negative (−0.6)and mildly positive (+3.2) values suggests that no purely crustal ormantle-derived rock exists in this group, but rather a generalized setof hybrid rocks.

Fig. 9. Geochemistry of themain rift-related intermediate and felsic igneous rocks. (A) Primitive mantle-normalized REE patterns, normalizing values from Palme and O'Neill (2004).(B) Tectonic setting discrimination diagram of Whalen et al. (1987). (C) Simplified spider-diagram normalized to primitive mantle values of Palme and O'Neill (2004). (D) Y–Nbtectonic setting discrimination diagram of Pearce et al. (1984). Shaded area: plot area of previously published data from the same rock unit. Symbols as in Fig. 6.


In summary, the main conclusions that we can draw from the datapresented in this study are as follows:

– The existence in the Ossa-Morena zone of two major, but different,rift-relatedmagmatic events separated in time during the Cambrian.

– The older, early rift-related event was associated with migmatiteand core-complex formation in the mid-upper crust and isrepresented by felsic peraluminous rocks, the parent magmas ofwhich were predominantly derived from crustal sources. An inputof mantle-derived components is, however, discernible, despitethe absence of coeval mafic rocks at the present level of erosion.

Table 3Sm–Nd isotope data.

Sample Sm Nd 143N

(µg g−1) (µg g−1) 147Sm/144Nd (2 s.

S20 6.34 29.2 0.1313 0.51S8 18.8 139 0.0818 0.51S9 16.5 78.3 0.1273 0.51S17 8.39 37.0 0.1369 0.51S13 17.4 68.1 0.1546 0.51S34 2.77 12.2 0.1368 0.51S39 3.16 9.74 0.1958 0.51S35 15.2 67.4 0.1364 0.51S4 3.56 18.3 0.1175 0.51S3 1.27 5.54 0.1389 0.51M1 8.69 44.1 0.1192 0.51M2 6.71 31.0 0.1308 0.51M3 5.42 25.8 0.1268 0.51M4 9.02 45.9 0.1187 0.51

– The later, main rift-related event is represented by a bimodalassociation of felsic and mafic rocks with minor amounts ofintermediate rocks. The mafic rocks belong to two distinct groups;one with N-MORB affinity and the other with OIB or E-MORBaffinities. This reflects the contribution of partial melts derivedfrom different, heterogeneous, depleted and enriched, astheno-spheric and lithospheric, plume-like and non-plume-like mantlesources. The acid and intermediate rocks appear to representhybrid mixtures of crustal- and mantle-derived magmas.

– The new data support our previous interpretation that the onset ofrifting was connected to a process of oblique ridge-trench collision

d/144Nd εNd(0) Age εNd(t) TDM (Ga)

e.) (t, Ma)

2577 (6) −1.2 500 3.1 0.882413 (6) −4.4 500 2.9 0.742578 (5) −1.2 500 3.2 0.832406 (7) −4.6 520 −0.6 1.272608 (5) −0.6 520 2.2 1.132566 (7) −1.4 520 2.5 0.962922 (4) 5.5 520 5.6 1.062642 (5) 0.0 520 4.0 0.811960 (3) −13.3 530 −7.9 1.722163 (3) −9.3 530 −5.4 1.802342 (8) −5.8 530 −0.6 1.142262 (7) −7.4 530 −2.9 1.442171 (4) −9.1 530 −4.4 1.532153 (6) −9.5 530 −4.3 1.43

Fig. 10. εNd(t) vs. age diagram (DePaolo and Wasserburg, 1976; DePaolo, 1981). Symbols as in Fig. 6 for RR rocks and as in Fig. 5 for ER rocks.


(Sánchez-García et al., 2003, 2008a,b) similar to the ongoing collisionof the East Pacific risewith the active Pacificmargin ofNorthAmerica.

– Following Lizarralde et al. (2007), we interpret the significantdifferences between the early and main rift-related events as beingthe result of an evolution from an initial “wide rift” stage, withpassive extension mainly accommodated by lower-crustal flow in ahigh heat-flow setting, to a “narrow rift” or “localized rift” stage,characterized by extension rates that outpaced thermal diffusionrates (active extension). Extension consequently affected the entirelithosphere, allowing for horst and graben style deformation of theupper crust and for the upwellingofmagmas from sources located asdeep as the asthenosphere.

– A combination of thermal expansion associated with massivemagma emplacement during the main event, and rift shoulderuplift, is thought to be responsible for the overall uplift of the Ossa-Morena zone, which resulted in erosion/non deposition during theLate Cambrian-earliest Ordovician.

– The progression of rifting eventually culminated in the opening ofthe Rheic Ocean by the Early Ordovician, evidence for which is tobe found in a widespread marine transgression (breakup uncon-formity; Quesada, 1991) that restored continental shelf sedimen-tary conditions in the Ossa-Morena zone for the remainder of thepre-Variscan Palaeozoic.

Acknowledgements

The authors acknowledge financial support from the IGME (projectno. 348) and the Spanish Ministry of Education and Science (grant no.CGL2006-12245BTE). Thorough and positive reviews by the editor ofthe special issue, R.D. Nance, and two anonymous referees haveimproved the paper and are also appreciated. Thanks also go to A.L.Tate for revising the English text. Contribution to IGCP Project no. 497:The Rheic Ocean, Origin, Evolution and Correlatives.

References

Alía, M., 1963. Rasgos estructurales de la Baja Extremadura. Boletín Real SociedadEspañola Historia Natural (Geología) 20, 247–262.

Armendáriz, M., López-Guijarro, R., Quesada, C., Pin, Ch., Bellido, F., 2008. Genesis andevolution of a syn-orogenic basin in transpression: insights from petrography,geochemistry and Sm–Nd systematics in the Variscan Pedroches basin (Mississip-pian, SW Iberia). Tectonophysics 461, 395–413.

Bellido Mulas, F., Sánchez-García, T., Díez Montes, A., 2007. Caracterización petrológicay geoquímica de los materiales volcánicos y volcanosedimentarios de las capas deBodonal-Cala (Zona de Ossa-Morena, SW de España). VI Congresso Ibérico deGeoquímica, Vila Real (Portugal): Extended abstract, pp. 93–96.

Brändle, J.L., Bellido, F., 2000. Aplicación de una base de datos geoquímicos a lacaracterización y estudio de los edificios y ciclos volcánicos de la isla de Tenerife(Islas Canarias, España). Boletín Geológico Minero 111 (6), 63–92.

Chichorro, M., Pereira, M.F., Díaz-Azpíroz, M., Williams, I.S., Fernández, C., Pin, Ch., Silva,J.B., 2008. Cambrian ensialic rift-related magmatism in the Ossa-Morena zone(Évora–Aracena metamorphic belt, SW Iberian Massif): Sm–Nd isotopes andSHRIMP zircon U–Th–Pb geochronology. Tectonophysics 461, 91–113.

Coleman, R.G., Donato, M.M., 1979. Oceanic plagiogranite revisited. In: Barker, F. (Ed.),Trondhjemites, dacites and related rocks. Developments in Petrology, 6. Elsevier,Amsterdam, pp. 149–168.

Condie, K.C., 2005. High field strength element ratios in Archean basalts: a window toevolving sources of mantle plumes? Lithos 79, 491–504.

DePaolo, D.J., 1981. Neodymiun isotopes in the Colorado Front Range and crustal–mantle evolution in the Proterozoic. Nature 291, 193–196.

DePaolo, D.J., Wasserburg, G.J., 1976. Nd isotopic variations and petrogenetic models.Geophysical Research Letters 3, 249–252.

Dupont, R., 1979. Cadre géologique et métallogenèse des gisements de fer du sud de laprovince de Badajoz (Sierra Morena occidentale-Espagne). Thèse Institut NationalPolytechnique de Lorraine.

Fernández, C., Becchio, R., Castro, A., Viramonte, J.M., Moreno-Ventas, I., Corretgé, L.G.,2008. Massive generation of atypical ferrosilic magmas along the Gondwana activemargin: implications for cold plumes and back-arc magma generation. GondwanaResearch 14, 451–473.

Gil Cid, M.D., 1991. Caracterización paleontológica del Cámbrico de la zona de OssaMorena (Z.O.M.) y sus paleorrelaciones con Marruecos y Centroeuropa. BoletínGeológico y Minero 102 (1), 3–18.

Gonçalves, F., 1970. Carta Geológica de Portugal 1/50000: Noticia Explicativa da folha37-A. Serviços Geológicos de Portugal, Elvas, pp. 1–50.

Gonçalves, F., 1971. Subsídios para o Conhecimento Geológico do Nordeste Alentejano.Memórias dos Serviços Geológicos de Portugal Nº18 (Nova Série), 1–62.

Gonçalves, F., 1978. Estado Actual do Conhecimento Geológico do Nordeste Alentejano.Separata IV Curso Extensão Universitária de Ciências Geológicas. Facultade deCiências de Lisboa 1975 (76), 1–23.

Gonçalves, F., Fernandes, A.P., 1973. Carta Geológica de Portugal 1/50000: NoticiaExplicativa da folha 32-B. Serviços Geológicos de Portugal, Portalegre, pp. 1–45.

Gonçalves, F., Torre de Assunção, C., Coelho, A.V.P., 1972. Carta Geológica de Portugal 1/50000: Noticia Explicativa da folha 33-C. Serviços Geológicos de Portugal, CampoMaior, pp. 1–41.

Gonçalves, F., Zbyszewsky, G., Coelho, A.V.P., 1975. Carta Geológica de Portugal 1/50000: Noticia Explicativa da folha 32-D. Serviços Geológicos de Portugal, Sousel,pp. 1–49.

Gonçalves, F., Perdigão, J.C., Coelho, A.V.P., Munhá, J.M., 1978. Carta Geológica dePortugal 1/50000: Noticia Explicativa da folha 33-A. Serviços Geológicos dePortugal, Assumar, pp. 1–49.

Hernández Enrile, J.L., 1971. Las rocas porfiroides del límite Cámbrico-Precámbrico delflanco meridional del anticlinorio Olivenza-Monesterio (Badajoz). Boletín Geoló-gico y Minero 82, 143–154.

Julivert, M., Fontboté, J.M., Ribeiro, A., Conde, L.N., 1974. Mapa Tectónico de la PenínsulaIbérica, Baleares y Canarias. Instituto Geológico y Minero de España, pp. 1–101.

Le Fèvre, B., Pin, C., 2002. Isotope dilution with matrix element removal: a key for high-precision, high-accuracy trace analysis of geological samples using ICP-MS.Geostandards Newsletter 26, 135–148.

Le Fèvre, B., Pin, C., 2005. A straightforward separation scheme for concomitant Lu–Hfand Sm–Nd isotope ratio and isotope dilution analysis. Analytica Chimica Acta 543,209–221.

Liñán, E., 1978. Bioestratigrafía de la Sierra de Córdoba. Tesis doctorales de laUniversidad de Granada 191, 1–212.

Liñán, E., Perejón, A., 1981. El Cámbrico inferior de la Unidad de Alconera, Badajoz (SWde España). Boletín. Real Sociedad Española de Historia Natural 79, 125–148.


Liñán, E., Quesada, C., 1990. Rift Phase (Cambrian) stratigraphy of the Ossa-Morenazone. In: Dallmeyer, R.D., Martínez García, E. (Eds.), Pre-Mesozoic Geology of Iberia.Springer-Verlag, Berlin, pp. 259–266.

Liñán, E., Perejón, A., Sdzuy, K., 1993. The Lower-Middle Cambrian stages andstratotypes from the Iberian Peninsula: a revision. Geological Magazine 130 (6),817–833.

Liñán, E., Gámez-Vintaned, J.A., Palacios, T., Alvaro, J., Gozalo, R., Mayoral, E., MorenoEiris, E., Perejón, A., Quesada Ochoa, C., Sánchez García, T., 1995. The Cambrian ofthe Alconera Unit. Pre-Conference Field Guide: XIII Reunión Geología del OestePeninsular/IGCP 319–320, pp. 1–10.

Liñán, E., Gonçalves, F., Gámez-Vintaned, J.A., Gozalo, R., 1996. Evolución paleogeo-gráfica del Cámbrico de la Zona de Ossa-Morena basada en el registro fósil. In:Araújo, A., Pereira, M.F. (Eds.), Estudos sobre a geología da Zona de Ossa-Morena(Maciço Ibérico. Livro de homenagem ao Prof. Francisco Gonçalves, Universidadesde Evora, pp. 1–26.

Liñán, E., Gozalo, R., Palacios, T., Gámez-Vintaned, J.A., Ugidos, J.M., Mayoral, E., 2002.Cambrian. In: Gibbons,W.,Moreno, T. (Eds.), TheGeology of Spain. Geological Society,London, pp. 17–29.

Linnemann, U., Pereira, M.F., Jeffries, T., Drost, K., Gerdes, A., 2008. Cadomian Orogenyand the opening of the Rheic Ocean: new insights in the diachrony of geotectonicprocesses constrained by LA-ICP-MS U-Pb zircon dating (Ossa-Morena and Saxo-Thuringian Zones, Iberian and Bohemian Massifs. Tectonophysics 461, 21–43.

Lizarralde, D., Axen, G.J., Brown, H.E., Fletcher, J.M., González-Fernández, A., Harding, A.J.,Holbrook,W.S., Kent, G.M., Páramo, P., Sutherland, F., Umhoefer, P.J., 2007. Variation instyles of rifting in the Gulf of California. Nature 448, 466–469.

López-Guijarro, R., Armendáriz, M., Quesada, C., Fernández-Suárez, J., Murphy, J.B., Pin,Ch., Bellido, F., 2008. Ediacaran-Palaeozoic tectonic evolution of the Ossa Morenaand Central Iberian zones (SW Iberia) as revealed by Sm–Nd isotope systematics.Tectonophysics 461, 202–214.

Mata, J., Munhá, J., 1990. Magmatogénese de metavulcanitos Câmbricos do NordesteAlentejano: os estádios iniciais de «rifting» continental. Comunicaçōes ServiçosGeológicos de Portugal 76, 61–89.

Melleton, J., Cocherie, A., Faure, M., Rossi, Ph., 2010. Precambrian protoliths and EarlyPaleozoic magmatism in the French Massif Central: U–Pb data and the NorthGondwana connection in the west European Variscan belt. Gondwana Research 17,13–25.

Murphy, J.B., Nance, R.D., 1989. A model for the evolution of the Avalonian–Cadomianbelt. Geology 17, 735–738.

Murphy, J.B., Nance, R.D., 1991. A supercontinent model for the contrasting character ofLate Proterozoic Orogenic belts. Geology 19, 469–472.

Murphy, J.B., Gutiérrez-Alonso, G., Nance, R.D., Fernández-Suárez, J., Keppie, J.D.,Quesada, C., Strachan, R.A., Dostal, J., 2006. Origin of the Rheic Ocean: rifting along aNeoproterozoic suture? Geology 34, 325–328.

Nance, R.D., Murphy, J.B., Keppie, J.D., 2002. Cordilleran model for the evolution ofAvalonia. Tectonophysics 352, 11–31.

Nance, R.D., Gutiérrez-Alonso, G., Keppie, J.D., Linnemann, U., Murphy, J.B., Quesada, C.,Strachan, R.A., 2010. Evolution of the Rheic Ocean. Gondwana Research 17 (2-3),194–222 (this volume).

Oliveira, V.M., 1984. Contribuição para o conhecimento geológico-mineiro da região doAlandroal-Juromenha (Alto Alentejo). Estudos, Notas e Trabalhos do Serviço deFomento Mineiro XXVI (1-4), 103–126.

Oliveira, J.T. (coord.), 1992. Carta Geológica de Portugal 1/500000: Noticia, Folha Sul,Serviços Geológicos de Portugal.

Oliveira, J.T., Oliveira, V., Piçarra, J.M., 1991. Traços gerais da evolução tectono-estratigráfica da Zona de Ossa-Morena. Cadernos Laboratorio Xeolóxico Laxe,Coruña 16, 221–250.

Palacios, T., 1993. Acritarchs from the Volcanosedimentary Group Playón beds. Lower–Upper Cambrian, Sierra Morena, southern Spain. Terra Nova Abstracts 6, 3–4.

Palme, H., O'Neill, H.S.C., 2004. Cosmochemical estimates of mantle composition. In:Holland, H.D., Turekian, K.K. (Eds.), TheMantle and Core. Treatise on Geochemistry,vol. 2. Elsevier-Pergamon, Oxford, pp. 1–38.

Pearce, J.A., Cann, J.R., 1973. Tectonic setting of basic volcanic rocks determined usingtrace element analyses. Earth Planetary Science Letters 19, 290–300.

Pearce, J.A., Harris, N.B.W., Tindle, A.G., 1984. Trace element discrimination diagrams forthe tectonic interpretation of granitic rocks. Journal of Petrology 25, 956–983.

Pereira, M.F., 1999. Caracterização da estructura dos domínios setentrionais da Zona deOssa-Morena e seu limite com a Zona Centro-Ibérica, no Nordeste Alentejano.Unpublished Tese de Doutoramento, Universidade de Évora, Portugal, 1–115.

Pereira, M.F., Quesada, C., 2006. Ediacaran to Visean crustal growth processes in theOssa-Morena zone (SW Iberia). IGCP 497 Evora Meeting 2006: ConferenceAbstracts and Field Trip Guide. Publicaciones del Instituto Geológico y Minero deEspaña, Madrid, pp. 1–115.

Pereira, M.F., Silva, J.B., 2002. Neoproterozoic–Paleozoic tectonic evolution of theCoimbra-Cordoba shear zone and related areas of the Ossa-Morena and Central-Iberian zones (Northeast Alentejo, Portugal). Comunicações Instituto Geológico eMineiro 89, 47–62.

Pereira, M.F., Silva, J.B., 2006. Nordeste Alentejano. In: Araújo, A., Dias, R. (Eds.),Geología de Portugal. Universidade de Évora.

Pereira, M.F., Chichorro, M., Linnemann, U., Eguiluz, L., Silva, J.B., 2006. Inherited arcsignature in Ediacaran and Early Cambrian basins of the Ossa-Morena zone (IberianMassif, Portugal): paleogeographic link with European and North AfricanCadomian correlatives. Precambrian Research 144, 297–315.

Pereira, M.F., Chichorro, M., Williams, I.S., Silva, J.B., 2008. Zircon U–Pb geochronology ofparagneisses and biotite granites from the SW IberianMassif (Portugal): evidence for

a paleogeographic link between the Ossa-Morena Ediacaran basins and the WestAfrican craton. In: Ennih, N., Liegeois, J.P. (Eds.), The Boundaries of the West AfricanCraton: Geological Society of London Special Publication, vol. 297, pp. 385–408.

Pin, Ch., Santos Zalduegui, J.F., 1997. Sequential separation of light rare-earth elements,thorium and uranium by miniaturized extraction chromatography: application toisotopic analyses of silicate rocks. Analytica Chimica Acta 339, 79–89.

Pin, Ch., Briot, D., Bassin, Ch., Poitrasson, D., 1994. Concomitant separationof Sr and Sm–Ndfor isotopic analysis in silicate samples, based on specific extraction chromatography.Analytica Chimica Acta 298, 209–217.

Quesada, C., 1990a. Precambrian terranes in the Iberian Variscan Foldbelt. In: Strachan,R.A., Taylor, G.K. (Eds.), Avalonian and Cadomian Geology of the North Atlantic.Blackie, New York, pp. 109–133.

Quesada, C., 1990b. Precambrian successions in SW Iberia: their relationship to “Cadomian”orogenic events. Geological Society (London) Special Publication 51, 353–362.

Quesada, C., 1991. Geological constraints on the Paleozoic tectonic evolution oftectonostratigraphic terranes in the Iberian Massif. Tectonophysics 185, 225–245.

Quesada, C., 1997. Evolución geodinámica de la Zona de Ossa-Morena durante el cicloCadomiense. In: Araújo, A., Pereira, M.F. (Eds.), Estudos sobre a geología da Zona deOssa-Morena (Maciço Ibérico). Universidade de Évora, pp. 205–230.

Quesada, C., 2006. The Ossa-Morena zone of the Iberian Massif: a tectonostratigraphicapproach to its evolution. Zeitschrift der Deutschen Gesellschaft für Geowis-senschaften 157 (4), 585–595.

Quesada, C., Sánchez-García, T., 2002. Cartografía geológica continua de la Zona Ossa-Morena, Escala 1:50.000. Instituto Geológico y Minero de España.

Quesada, C., Bellido, F., Dallmeyer, R.D., Gil Ibarguchi, J.I., Oliveira, J.T., Pérez Estaún, A.,Ribeiro, A., Robardet, M., Silva, J.B., 1991. Terranes within the Iberian Massif:correlations with West African sequences. In: Dallmeyer, R.D., Lecorché, J.P. (Eds.),The West African orogens and Circum-Atlantic correlations. Springer-Verlag, Berlin,pp. 267–294.

Quesada, C., Sánchez-García, T., Bellido, F., López-Guijarro, R., Armendáriz, M., Braid, J.,2006. Introduction: the Ossa-Morena zone — from Neoproterozoic arc throughEarly Palaeozoic rifting to late Palaeozoic orogeny. In: Pereira, M.F., Quesada, C.(Eds.), Ediacaran to Visean Crustal Growth Processes in the Ossa-Morena Zone (SWIberia). IGCP 497 Evora Meeting 2006: Conference Abstracts and Field Trip Guide.Publicaciones del Instituto Geológico y Minero de España, Madrid, pp. 51–73.

Ribeiro, A., Quesada, C., Dallmeyer, R.D., 1990. Geodynamic evolution of the IberianMassif. In: Dallmeyer, R.D., Martínez-García, E. (Eds.), Pre-Mesozoic Geology ofIberia. Springer Verlag, Berlin, pp. 399–409.

Ribeiro, M.L., Munhá, J., Mata, J., Palácios, T., 1997. Vulcanismo na Zona de Ossa-Morenae seu enquadramento geodinâmico. In: Araújo, A., Pereira, M.F. (Eds.), Estudossobre a geologia da Zona de Ossa-Morena (Maciço Ibérico). Livro de homenagem aoProf. Francisco Gonçalves. Universidade de Évora, pp. 37–56.

Romeo, I., Lunar, R., Capote, R., Quesada, C., Piña, R., Dunning, G.R., Ortega, L., 2006. U/Pbage constraints on Variscan magmatism and Ni-Cu-PGE metallogeny in the Ossa-Morena zone (SW Iberia). Journal of the Geological Society of London 163, 1–9.

Rudnick, R.L., Gao, S., 2004. Composition of the continental crust. In: Rudnick, R.L. (Ed.),The Crust: In: Holland, H.D., Turekian, K.K. (Eds.), Treatise on Geochemistry, vol. 3.Elsevier-Pergamon, Oxford, pp. 1–64.

Sánchez-García, T., 2001. Volcanismo cámbrico en la Unidad de Alconera (sector deZafra-ZOM-Provincia de Badajoz). Boletín Geológico y Minero 112 (3), 89–102.

Sánchez-García, T., Quesada, C., 2001. Variscan transpressional inversion of a LowerPaleozoic rifted margin (Ossa-Morena zone, SW Iberia. IGCP project 453, meetingon Collisional orogens, Sion. Extended Abstract 95–97.

Sánchez-García, T., Bellido, F., Quesada, C., 2003. Geodynamic setting and geochemicalsignatures of Cambrian–Ordovician rift-related igneous rocks (Ossa-Morena zone,SW Iberia). Tectonophysics 365, 233–255.

Sánchez-García, T., Quesada, C., Bellido, F., Dunning, G., González de Tanago, J., 2008a.Two-step magma flooding of the upper crust during rifting: the Early Paleozoic ofthe Ossa-Morena zone (SW Iberia). Tectonophysics 461, 72–90.

Sánchez-García, T., Bellido, F., Pereira, M.F., López-Guijarro, R., Quesada, C., Chichorro,M., Silva, J.B., Pin, Ch., 2008b. Expresión magmática temprana de un riftintracontinental en el margen de Gondwana durante el Cámbrico Inferior: Zonade Ossa-Morena (SWMacizo Ibérico, Portugal, España). Geo-Temas 10, 1567–1572.

Sun, S.S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts:implications for mantle composition and processes. In: Saunders, A.D., Norry, M.J.(Eds.), Magmatism in Ocean Basins: Geological Society, London, Special Publica-tion, vol. 42, pp. 429–448.

Tanaka, T., Togashi, S., Kamioka, H., Amakawa, H., Kagami, H., Hamamoto, T., Yuhara, M.,Orihashi, Y., Yoneda, S., Shimizu, H., Kunimaru, T., Takahashi, K., Yanagi, T., Nakano,T., Fujimaki, H., Shinjo, R., Asahara, Y., Tanimizu, M., Dragusanu, C., 2000. JNdi-1: aneodymium isotopic reference in consistency with LaJolla neodymium. ChemicalGeology 168, 279–281.

Thirlwall, M.F., Singer, B.S., Marriner, G.F., 2000. 39Ar–40Ar ages and geochemistry ofthe basaltic shield stage of Tenerife, Canary Islands, Spain. Journal of Volcanologyand Geothermal Research 103, 247–297.

Whalen, J.B., Currie, K.L., Chappell, B., 1987. A-type granites: geochemical character-istics, discrimination and petrogenesis. Contributions to Mineralogy and Petrology95 (4), 407–419.

Winchester, J.A., Floyd, P.A., 1977. Geochemical discrimination of different magmaseries and their differentiation products using immobile elements. ChemicalGeology 20 (4), 325–343.

Zhou, J., Li, X.H., Ge, W., Li, Z.X., 2007. Age and origin of middle Neoproterozoic maficmagmatism in southern Yangtze Block and relevance to the break-up of Rodinia.Gondwana Research 12, 184–197.

Rift-related volcanism predating the birth of the Rheic Ocean (Ossa-Morena zone, SW Iberia

Documents

Transcript of Rift-related volcanism predating the birth of the Rheic Ocean (Ossa-Morena zone, SW Iberia