The Vila de Cruces Ophiolite: A Remnant of the Early …de Cruces ophiolite attains 3500–4000 m in thick-ness, although its internal structure is imbricated. This article describes

The Vila de Cruces Ophiolite: A Remnant of the Early Rheic Ocean in the Variscan Suture ofGalicia (Northwest Iberian Massif)Author(s): Ricardo Arenas, José R. Martínez Catalán, Sonia Sánchez Martínez,Javier Fernández‐Suárez, Pilar Andonaegui, Julian A. Pearce, and Fernando CorfuReviewed work(s):Source: The Journal of Geology, Vol. 115, No. 2 (March 2007), pp. 129-148Published by: The University of Chicago PressStable URL: http://www.jstor.org/stable/10.1086/510645 .

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[The Journal of Geology, 2007, volume 115, p. 129–148] � 2007 by The University of Chicago. All rights reserved. 0022-1376/2007/11502-0001$15.00

129

ARTICLES

The Vila de Cruces Ophiolite: A Remnant of the Early Rheic Oceanin the Variscan Suture of Galicia (Northwest Iberian Massif)

Ricardo Arenas,1 Jose R. Martınez Catalan,2 Sonia Sanchez Martınez,Javier Fernandez-Suarez, Pilar Andonaegui, Julian A. Pearce,3

and Fernando Corfu4

Departamento de Petrologıa y Geoquımica e Instituto de Geologıa Economica (CSIC),Universidad Complutense, 28040 Madrid, Spain

(e-mail: [email protected])

A B S T R A C T

The Vila de Cruces ophiolite is one of the ophiolitic units involved in the Variscan suture of the northwest IberianMassif. This ophiolite consists of a tectonically repeated succession of greenschist facies volcanic rocks, commonalternations of metasediments of pelitic or siliceous character, and scarce orthogneisses, metagabbros, and serpen-tinites. The protolith age of a granitic orthogneiss that intruded the mafic rocks is dated at Ma (U-Pb in497 � 4zircons). This age can be considered a reference for the generation of the ophiolite. According to their contents ofsome of the most immobile trace elements, the greenschist and the metagabbros are derived from basaltic magmaswith compositions similar to those of island-arc tholeiites. The influence of a subduction zone in the generation ofthe original basaltic magmas can be deduced from the marked negative Nb anomaly observed in all the metabasicrocks of this ophiolite. The granitic orthogneisses can also be genetically related to the basic rocks because they aresimilar to granitic rocks generated in volcanic arcs. The Vila de Cruces ophiolite is interpreted as a suprasubductionzone ophiolite generated in Late Cambrian times, during the early stages of the opening of the Rheic Ocean. Theophiolite was probably generated in a back-arc basin developed during the first stages of the pulling apart and laterdrift of one or more peri-Gondwanan terranes, one of them represented by the upper allochthon of the northwestIberian Massif.

Online enhancements: data tables.

Introduction

Early Paleozoic paleogeographic reconstructionsfor the northern peri-Gondwanan realm show thatthe rift of the Avalon microcontinent and its laterdrift to the north gave rise to an oceanic domainknown as the Rheic Ocean (Stampfli and Borel2002; Winchester et al. 2002). It is conceivable thatother smaller terranes were also detached from

Manuscript received December 20, 2005; accepted October8, 2006.

1 Author for correspondence.2 Departamento de Geologıa, Universidad de Salamanca,

37008 Salamanca, Spain.3 School of Earth, Ocean and Planetary Sciences, Cardiff Uni-

versity, CF10 3YE Cardiff, United Kingdom.4 Department of Geosciences, University of Oslo, N-0316

Oslo, Norway.

Gondwana during this time. This could be the caseof the allochthonous terrane currently located tothe south of Avalon and occupying the uppermoststructural position in the European Variscan Belt.This terrane has been preserved in the allochtho-nous complexes of northwest Iberia forming theupper units, or upper allochthon (Abati et al. 1999;Fernandez-Suarez et al. 2003; Gomez-Barreiro et al.2006), and can be followed across western Europe,forming part of a nappe stack with high-pressureunits and ophiolites within the Variscan alloch-thonous complexes (fig. 1a; Arenas et al. 1986; Mar-tınez Catalan et al. 2002).

In spite of the possible existence of a young RheicOcean already in the Late Cambrian, and consid-



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Journal of Geology V I L A D E C R U C E S O P H I O L I T E 131

ering the subsequent opening of a wide oceanicdomain during the Ordovician and part of the Si-lurian, the ophiolites preserved in the Variscanallochthonous complexes are too young to datethose early stages because their ages are Late Si-lurian to Early Devonian. This is the case for theSleza ophiolite in the Bohemian Massif (Dubin-ska et al. 2004), the Lizard ophiolite in Cornwall(Clark et al. 1998), and the Careon ophiolite inGalicia (Dıaz Garcıa et al. 1999). The scarcity ofolder oceanic lithosphere in the European Var-iscan Belt, or even its frequent absence, can bereadily explained by consumption of the older,colder, and denser oceanic lithosphere by sub-duction (e.g., Molnar and Atwater 1978). In thisway, only the youngest lithosphere of the RheicOcean would have escaped subduction, and itwould have been eventually obducted over thecontinental margin of Gondwana during the Var-iscan orogeny (Sanchez Martınez et al. 2007).

It is possible, however, that the paleogeographicmodels so far postulated for the Lower Paleozoic areincomplete, because old oceanic lithosphere repre-senting the Late Cambrian Rheic Ocean may in factexist in the European Variscan Belt, although it isapparently scarce and has not received much atten-tion. This is the case for some low-grade allochtho-nous units designated as the lower ophiolitic unitsin the northwest Iberian Massif (Arenas et al. 2007).These ophiolites consist of thick successions ofgreenschist facies volcanic rocks, with common in-terbedded pelitic or siliceous metasedimentaryrocksand scarce orthogneisses, metagabbros, and serpen-tinites. They can be found associated with othermore common ophiolitic sequences that appear inan upper structural position and have been dated asEarly Devonian (Dıaz Garcıa et al. 1999; Arenas etal. 2007). Located in the Ordenes Complex, the Vilade Cruces ophiolite attains 3500–4000 m in thick-ness, although its internal structure is imbricated.This article describes the structure, lithology, age,and geochemistry of the Vila de Cruces ophiolite.We suggest that the ophiolite can be interpreted asa remnant of the oldest Rheic Ocean.

Geological Setting

The allochthonous complexes of the northwest Ibe-rian Massif, and their equivalents in the rest of Eu-rope, are located in the more internal part of theVariscan Belt (fig. 1a). They appear as megaklippenwith synformal structure, including a number ofallochthonous units with the characteristics of far-traveled terranes (Arenas et al. 1986; Martınez Cat-

alan et al. 1999). Five different allochthonous com-plexes exist in Galicia and in the Portuguese regionof Tras-os-Montes. According to their lithologicalconstitution and structure, the allochthonous com-plexes of Ordenes and Cabo Ortegal are represen-tative of the ensemble (fig. 1b). They include threedifferent terranes, which, from bottom to top, havebeen named basal units (or lower allochthon), ophi-olitic units (or middle allochthon), and upper units(or upper allochthon).

The middle allochthon has a composite charac-ter, including lithological assemblages and ophio-lites with contrasting ages (Arenas et al. 2007). Twogroups can be distinguished according to theirstructural relative position, the upper and lowerophiolitic units. Both occur to the southeast of theOrdenes Complex, where the Careon ophioliteoverlies the Vila de Cruces ophiolite (fig. 2). Thegabbroic protoliths of the Careon ophiolite havebeen dated at 395 Ma (U-Pb in zircon; Dıaz Garcıaet al. 1999). To date, these are the only protolithage data in the ophiolites of northwest Iberia.

The upper units (or upper allochthon) structurallyoverlie the ophiolites. The upper units have beensubdivided in two assemblages with contrasting tec-tonothermal evolutions: the lower was affected byhigh-pressure, high-temperature (HP-HT) metamor-phism, whereas the upper assemblage is character-ized by intermediate-pressure (IP) metamorphism(fig. 1b). In spite of their different tectonothermalevolutions, both assemblages have been consideredparts of a single peri-Gondwanan terrane (MartınezCatalan et al. 2002). This terrane was rifted fromGondwana during the Lower Paleozoic, and its driftto the north, probably coeval with the described mo-tion of Avalonia (Murphy et al. 2006), is the currentlyaccepted scenario for the opening of the Rheic Ocean(Abati et al. 1999; Fernandez-Suarez et al. 2003; Are-nas et al. 2007). This drifted terrane was subse-quently accreted to Laurussia, roughly coincidingwith the moments of largest width of the RheicOcean (Gomez-Barreiro et al. 2006).

Underlying the ophiolites, the basal units consistof a lithological assemblage with continental affin-ity (schists and paragneisses, orthogneisses, andmetabasites), which is considered to represent themost external part of the Gondwanan margin (Mar-tınez Catalan et al. 1996). This margin was sub-ducted below the ophiolitic units during the initialstages on the Variscan Orogeny in Gondwana andaffected by high-pressure and low- to intermediate-temperature metamorphism (Arenas et al. 1995).



Figure 2. Geological map and cross section of the Vila de Cruces and surrounding allochthonous units. Abbreviationsas in figure 1.




Structure

The Vila de Cruces ophiolite is typical of the modeof occurrence of units in the allochthonous com-plexes of northwest Iberia. Instead of being a con-tinuous thrust sheet, it has a lensoidal outcropshape and is isolated from the comparable Moecheophiolitic unit in the Cabo Ortegal Complex (fig.1b). This type of occurrence is a consequence of thedismembering of the units that took place throughrepeated thrusting and extensional events, produc-ing normal detachments. The lens is about 26 kmlong and up to 4 km wide, elongated east-west, andpinched in the middle, continuing another 24 kmto the northeast by a narrow strip less than 500 mwide (fig. 2). Its upper boundary is a thrust fault,which is commonly regarded as separating the Vilade Cruces Unit from the overlying Careon ophio-litic unit. In the hanging wall to the thrust, draggingof the Corredoiras Unit during eastward motion lefta tail of orthogneisses more than 15 km long behindthe main body, cropping out around Melide. Fur-thermore, the thrust left several isolated horses oforthogneisses, HP-HT amphibolites, and Careonflasergabbros, amphibolites, and ultramafics. Thelower boundary of the Vila de Cruces Unit is acurved normal fault of late Variscan age known asthe Pico Sacro detachment (PSD). South and south-east of the PSD, the Lalın Unit, representative ofthe basal units, is exposed and carried to its presentposition by the Lalın-Forcarei thrust, below whichcrop out the parautochthon and abundant synki-nematic Variscan granitoids (fig. 2).

The Vila de Cruces Unit consists of an alternationof metabasites and metapelitic phyllites and schists,with scarce and thin layers of granitic orthogneisses,serpentinites, and metacherts. The dominant li-thology is greenschist facies metabasites exhibitingan intense deformation, though the occasional pres-ervation of igneous textures suggests a metabasalticorigin with minor presence of coarse- to middle-grained gabbros. Metapelites dominate in the upperpart, whereas the main body of orthogneiss occursin the central part of the unit, to the east of Vila deCruces, and that of ultramafics in the Campo Marzoslice, to the south of the PSD (figs. 2, 3).

In the upper part of the unit, greenschists andmetapelites alternate, with the latter commonlyshowing phyllonitic character at the contacts withunderlying greenschists. Thin layers of intensivelysheared serpentinite and talc schists also occur atsome of these boundaries and in the greenschists.Shear bands are common in the phyllonites, butclear and consistent kinematic criteria are rare, asoften occurs in very low-temperature fault rocks.

A couple of outcrops yielded a top-to-the-east andeast-southeast sense of movement, consistent withcriteria found in thrust faults in the surroundingunits (Martınez Catalan et al. 1996, 2002). Con-versely, low-temperature fault rocks are rare in thelower half inside the unit. There, to the east of Vilade Cruces, metapelites form rather continuous andthin bands, some of which join each other (fig. 3).

Petrographic studies show the pervasive presenceof a regional greenschist facies cleavage and one ormore generations of crenulation or poorly devel-oped crenulation cleavage. The regional cleavageseems to be the first one in many of the metaba-sites, where it shows a mylonitic character, but insome of them, mainly in the metapelites, a formercleavage strongly microfolded can be identified.The regional cleavage is axial planar to overturnedfolds recognizable in thin sections, outcrops and tomap scale (fig. 3). The folds show east vergence andaxial surfaces dipping between 20� and 70� and par-allel to the main cleavage. Fold axes and intersec-tion lineations associated with the main cleavageplunge to the north-northwest–south-southeast ornorth-south except when close to the PSD (DivarRodrıguez and Iglesias P. de Leon 1982). Minerallineations can be seen mostly in the orthogneisses,with an attitude similar to fold axes and intersec-tion lineations (fig. 3), suggesting that they also rep-resent a composite fabric (intersection lineation)rather than a stretching lineation. Often, the maincleavage has a low-dipping attitude to the west,north, or east because of late open folds that over-printed the unit. The crenulations are related tothese late folds (roughly north-south, steeply dip-ping) and to the PSD (low dips, often to the east orsoutheast).

According to these data, and based on the geolog-ical map, the Vila de Cruces Unit is interpreted asan imbricated thrust sheet (see cross section in fig.2). The unit includes an upper thrust sheet to thenorth, where imbricates are abundant, and a lowerthrust sheet in the southern part, more continuousand where only two imbricates exist. Furthermore,it includes the Campo Marzo slice, entirely madeup of ultramafics and probably representing a vestigeof the mantle that once underlay the supracrustals.

In the upper thrust sheet, every horse consists ofmetapelites (phyllites) at the base and greenschistfacies metabasites on top, although in the western-most horse, metabasites occur both below andabove the phyllites, suggesting that the metapeliteswere intercalated. Phyllonitization occurs system-atically at the bottom of the metapelites, adjacentto the thrust faults.

In the lower thrust sheet, the metapelites clearly



134 R . A R E N A S E T A L .

Figure 3. Detailed geological map of the Vila de Cruces area showing the axial traces of recumbent and overturnedfolds and several of the thrust imbricates. The sampling localities and ages obtained by the 40Ar/39Ar methodin phyllites by Dallmeyer et al. (1997) and by the U-Pb method in an orthogneiss (this article) are also shown.

ın-Forcarei thrust; Sacro detachment.LFT p Lal PSD p Pico

alternate with the metabasites. Two thin strips ofpelitic schists run nearly parallel for about 5 km tothe southeast of Vila de Cruces, until they join eachother to the east in an area where the metapelitesturn thicker (figs. 2, 3). This suggests the presenceof large recumbent isoclinal folds with extraordi-narily thickened hinges and thinned limbs. Their

presence would not be surprising because a hugerecumbent anticline has been identified in the un-derlying Lalın Unit (see cross section in fig. 2; Mar-tınez Catalan et al. 1996). In addition to the recum-bent folds, several kilometer-scale overturned foldshave been identified (fig. 3). Both the main cleavageand the axial surface of the overturned folds cross-




cut the hinge and limbs of the proposed recumbentfolds. Therefore, the huge recumbent folds wouldbe previous to the main cleavage and possibly as-sociated with the earlier cleavage identified in themetapelites.

The imbricate structure of the Vila de CrucesUnit forms the base of a spectacular duplex alsoinvolving the ophiolites and the upper units (figs.2, 4). In the uppermost part of the duplex, south ofBelmil, horses of the ophiolitic Careon Unit andthe HP-HT Belmil Unit alternate, locally disturbingthe typical order of stacking (fig. 2). In some cases,thrusts are responsible for the absence of units suchas (i) the Careon Unit southwest of Belmil, (ii) theHP-HT upper units along most of the contact be-tween the IP upper units and the ophiolites, and(iii) the Vila de Cruces Unit to the west of theOrdenes Complex. This is partly due to superpo-sition of two thrust generations, as can be seen inthe Careon Unit, where young, low-temperaturethrusts cut an older imbricate stack. This resultsin an out-of-sequence character of the thrusts thatdeveloped the Vila de Cruces–Belmil duplex (Mar-tınez Catalan et al. 2002).

Figure 4 shows the proposed evolution of the Vilade Cruces–Belmil duplex. It includes horses forwhich displacement is less than (hinterland-dippingduplexes), equals (antiformal stacks), or exceeds(foreland-dipping duplexes) fault spacing (Mitra andBoyer 1986). If length of the greenschist/metapelitecontact (38 km) is compared with its sectional length(30 km), a shortening of 8 km due to imbricationcan be estimated only for the upper thrust sheet.The two imbricates in the lower thrust sheet rep-resent a shortening of 18 km, predominantly accom-modated by the horse where Vila de Cruces is lo-cated (fig. 2). Tectonic superposition of the upperthrust sheet over the lower thrust sheet requires 23km of shortening, which gives a minimum of 49 kmof total shortening within the Vila de Cruces Unit,without considering previous folding.

Tectonothermal Evolution

Using the map and cross section, microscopic anal-ysis of the cleavages and mineral assemblages, andavailable geochronological data of tectonothermalevents, we propose a structural evolution. Serpen-tinites and talc schists occur often in the thrustfaults, but they can be seen also at folded meta-basites/metapelite contacts, suggesting that theywere there before recumbent/overturned folding.These can be explained by an early phase of im-brication of mafic rocks and the overlying pelites.These thrusts could have rooted in the mantle, ex-

plaining the presence of serpentinites at the con-tacts between metapelites and metabasites. Alter-natively, the early phase would have emplacedportions of the oceanic mantle that would have de-tached and glided over the pelitic sediments. Sub-sequently, the Vila de Cruces Unit underwent duc-tile deformation, giving rise to the first cleavageand possibly to large recumbent folds. This eventexplains the thickening of the unit and its burialand pressurization and can be related to the high-pressure/low- to intermediate-temperature eventidentified in the upper imbricates (Martınez Cat-alan et al. 2002).

The first cleavage was followed by the develop-ment of the main one. Its low-dipping attitude andthe asymmetry of the associated overturned foldssuggest a general ductile shear mechanism with atop-to-the-east movement. This cleavage has beendated by Dallmeyer et al. (1997) at 367–363 Ma(40Ar/39Ar, muscovite concentrates in phyllites; seefig. 3 for sampling localities) and at 364 Ma in sim-ilar rocks of the equivalent Moeche ophiolite in theCabo Ortegal Complex.

The early imbricates and the recumbent folds areinterpreted to be a consequence of accretion andsubduction of the Vila de Cruces Unit beneath anactive orogenic wedge being developed to the northor west of the ocean represented by the ophioliticunits (Martınez Catalan et al. 1996). The maincleavage would then correspond to a subsequentphase of exhumation, and its age, 367–363 Ma, isconsistent with the diachronous character of de-formation established for the allochthonous units(Dallmeyer et al. 1997). The migration of defor-mation with time, younging toward the lower unitsof the nappe stack, is interpreted in terms of theirprogressive accretion to the orogenic wedge. Themain phase of thrusting and imbrication is not pre-cisely dated, but 40Ar/39Ar ages around 330 Ma inmetapelites of the upper IP unit (Dallmeyer et al.1997; Gomez Barreiro et al. 2006) could reflect itsactivity or represent a minimum age limit for thisevent.

The PSD, with a top-to-the-northwest move-ment, represents the final extensional collapse ofthis part of the orogen. It is younger than 323 �

Ma, the age of the Negreira granodiorite (Rb-Sr11method; Bellido et al. 1992), and roughly synchro-nous with the late upright folds, dated at 314 � 6Ma (Rb-Sr and K-Ar methods; Capdevila and Via-lette 1970; corrected by Ries 1979). The PSD par-tially reactivated the out-of-sequence sole thrust atthe base of Vila de Cruces. Coeval with this de-tachment, high-angle normal faults formed in twoconjugate families in the central area of the Vila de



136 R . A R E N A S E T A L .

Figure 4. Proposed structural evolution of the Vila de Cruces–Belmil duplex.

Cruces Unit, where they contributed to form theneck between two megaboudins where the unit ex-hibits its maximum thickness (fig. 2).

Ophiolite Constitution

The Vila de Cruces Unit exhibits an imbricate in-ternal structure, where the original ophiolite sec-tion is not preserved. A composite schematic sec-tion has been compiled from the geological mapand field observations (fig. 5). The estimation of theoriginal ophiolite thickness is hampered by its per-vasive imbrication and by the possible existence oflarge recumbent folds.

The ophiolite may represent an oceanic sequenceof thick basalts and interbedded pelitic and sili-ceous sediments, intruded by stocks of gabbros andgranitoids. The complex internal structure of theophiolite hinders an accurate estimation for thethickness of the original oceanic crust. The maficand sedimentary components of the ophiolite restover the serpentinized ultramafic rocks of theCampo Marzo slice, which can be interpreted as a

piece of suboceanic upper mantle. This contact istectonic (PSD) and hides the original basal contactof the basaltic-sedimentary sequence. Therefore, itis not known whether a basal plutonic sequenceoccurred in the ophiolite. As a result of the struc-tural complexities, the present section of theophiolite cannot be compared with typical ophio-lites generated in divergent (mid-ocean ridge basalt[MORB] type; Hawkins 2003; Pearce 2003) or con-vergent (suprasubduction type; Pearce et al. 1984b)settings. Accordingly, the tectonic setting of theVila de Cruces ophiolite may be more appropriatelyinterpreted by considering the geochemical char-acteristics of the mafic and granitic metaigneousrocks.

U-Pb Dating: Analytical Methods and Results

One sample of the largest body of granitic ortho-gneisses (G-126) was selected for U-Pb dating, andits location is shown in figure 3 and in table A1,available in the online edition and from the Journalof Geology office. The metagranitic rocks are as-




Figure 5. Schematic section showing the internal constitution of the Vila de Cruces ophiolite. It contains threemain tectonic slices, the lowest one representing the mantle section.

sociated with gabbros and leucogabbros showingmutual intrusive relationships. The contact be-tween the plutonic rocks and the greenschists, sup-posedly of metabasaltic origin, is pervasivelysheared. However, an original intrusive relation-ship seems reasonable, suggesting that the age ofthe granitic body represents a minimum age for theophiolitic ensemble. In any case, if a simple sce-nario is considered (i.e., a context related to theevolution of a single oceanic domain), the granit-oids and the mafic rocks should be coeval. More-over, as will be shown below, the chemistry of thegranitic and mafic rocks is in agreement with acommon dynamic setting.

U-Pb analytical work was performed at theMineralogical-Geological Museum, University of

Oslo. The orthogneiss was crushed with a jawcrusher and pulverized with a hammer mill. Heavy-fraction enrichment on a Wilfley table, magneticseparation in a Frantz isodynamic separator, anddensity separation with di-iodomethane (CH2I2)were used to separate the zircons. Grains to be an-alyzed were hand-picked in alcohol under a bin-ocular microscope, and all the fractions were sub-sequently air-abraded following the method ofKrogh (1982). The selected zircon fractions werewashed in 4N HNO3 on a hot plate and rinsed re-peatedly with H2O and acetone. A mixed 205Pb/235Uspike was added to the sample after weighing andtransfer to the dissolution vessel. Zircon was dis-solved in HF (�HNO3) in Teflon minibombs at ca.185�C. The solutions were subsequently evapo-



138 R . A R E N A S E T A L .

Table 1. Results of ID-TIMS U-Pb Dating for Sample G-126

Parameter

Analysis

Z1 Z2 Z3 Z4

Weighta (mg) 2 3 12 5U (ppm) 417 343 594 164Th/Ub .39 .26 .34 .15Pbcomc (pg) 1.6 .5 3.6 34.7206Pbd/204Pb 2588 8699 9627 175206Pbe/238U .07996 .07098 .07761 .10610

2jf (abs) .00037 .00025 .00023 .00038207Pb/238U .6313 .5609 .6108 1.1863

2j (abs) .0034 .0020 .0021 .0162207Pb/206Pb .05726 .05731 .05708 .08109

2j (abs) .00020 .00014 .00008 .00106Apparent age (Ma):

206Pb/238U 495.9 442.0 481.8 650.1207Pb/235U 496.9 452.1 484.1 794.2207Pb/206Pb 501.6 503.6 494.7 1223.6

a Weights better than 10% when sample weight is more than 10 mg.b Model Th/U ratio estimated from 208Pb/206Pb ratio and age of the sample.c Total common Pb in sample, including initial and blank Pb.d Measured ratio, corrected for fractionation and spike contribution.e Corrected for spike, fractionation, and blank and initial common Pb (Stacey and Kramers 1975).f 2j uncertainty calculated by error propagation procedure that takes into account internal measurement statistics and externalreproducibility as well as uncertainties in blank and common Pb correction.

rated, redissolved in 3.1N HCl and passed throughanion exchange columns in HCl medium to purifyU and Pb. U and Pb were finally collected togetherin the bombs used for dissolution and loaded to-gether on outgassed Re filaments with H3PO4 andsilica gel. Isotopic ratios were measured on a Fin-nigan-MAT 262 mass spectrometer using up to fourFaraday detectors in multicollection mode. Verysmall fractions were measured by peak jumping ona secondary electron multiplier (ion countingmode). Total procedural blanks were less than 2–5pg Pb and 0.1–0.3 pg U. The Stacey and Kramers(1975) model was used to subtract initial commonPb in excess of the laboratory blank. Regressionlines were calculated using the model 1 algorithmof Ludwig (1989) with intercept errors quoted at95% confidence level. Decay constants are thoseof Jaffey et al. (1971).

Four zircon fractions were analyzed; the resultsare given in table 1 and presented in the concordiaplots of figure 6. The general features of analyticaldata and the results are reported below.

Fraction Z1 consisted of four equant prisms withmilky appearance. The error ellipse of this analysisoverlaps the concordia curve at the 2j confidencelevel and has a concordia age of Ma496.5 � 2.1with 23% probability of concordance (Ludwig1998). However, this fraction is 1.2% discordantand has a 207Pb/206Pb age of 501.6 Ma. Fraction Z2consisted of three slightly turbid prism fragmentsfrom the magnetic split at ca. 1 A. The analysis is12.6% discordant and has a 207Pb/206Pb age of 503.6

Ma. Fraction Z3 consisted of seven clear prismsslightly longer than those of Z1. The analysis is2.7% discordant and yielded a 207Pb/206Pb age of494.7 Ma. Finally, fraction Z4 consisted of threeclear long prisms. The analysis is highly discordant,with a 207Pb/206Pb age of 1223.6 Ma, implying thepresence of an inherited lead component. This frac-tion is also different from the other three fractionswith regard to U content, model U/Th, and com-mon Pb content (table 1). Fraction Z1 is the mostconcordant and constrains the crystallization ageof the rock to between ca. 496 and 502 Ma. Giventhat inherited zircon is present in the rock (fractionZ4), fraction Z1 could be slightly displaced to theright owing to the presence of minor amounts ofinherited lead, in which case the concordia agewould best constrain the crystallization age of thezircons. If no inherited lead component is present,then the 207Pb/206Pb age is the best estimate for thecrystallization age. A discordia forced through

Ma using fractions Z1, Z2, and Z3 yields an0 � 50upper intercept age of Ma, and a discordia497 � 12using fractions Z1 and Z4 yields a lower interceptage of Ma.495.1 � 2.8/ � 3

Considering the above and taking into accountthat fraction Z3 has the youngest 207Pb/206Pb ageand therefore is the least likely to contain inheritedlead, whereas fractions Z1 and Z2 may be affectedby minor inheritance, the crystallization age of thismetagranitoid would be best constrained as

Ma, which includes the uncertainty as-497 � 4sociated with analyses Z1, Z2, and Z3.




Figure 6. U-Pb concordia diagrams showing the results of U-Pb dating of zircon in a sample (G-126) of graniticorthogneisses of the Vila de Cruces ophiolite. Errors are given at the 2j level.

It is very important to note that fraction Z4proves the participation of a Precambrian crustalcomponent (1.2 Ga or older) in the genesis of thisorthogneiss. This datum is also considered signif-icant in order to determine the tectonic setting ofthe Vila de Cruces ophiolite because it rules out alarge open oceanic basin. In this way, the presenceof a Precambrian crustal component suggests thatthe ophiolite was probably generated in the areasurrounding a continental domain rather than aMORB-type environment.

Chemical Characteristics of the Igneous Suite

A set of 20 samples of the most representativemetaigneous lithologies in the Vila de Crucesophiolite was selected for study of their geochem-ical characteristics. The location and classificationof these samples appear in table A1. Twelve sam-ples are variably deformed greenschists, four aremetagabbros, and the remaining four samples aregranitic orthogneisses similar to that used for U-Pb dating (including the dated sample; table A1).Crushing and powdering of the samples were per-formed at the Universidad Complutense of Madrid,and the subsequent chemical analyses were carriedout at the Activation Laboratories (Actlab) in Can-ada. The digestion procedure was the lithiummetaborate/tetraborate fusion, and the analyticaltechnique used to measure the elemental concen-trations was inductively coupled plasma mass spec-trometry (ICP-MS). The results are given in tablesA2 and A3, available in the online edition and from

the Journal of Geology office. The quality of thedata was tested in the laboratories of the Earth,Ocean and Planetary Sciences department at Car-diff University (acid digestion, ICP-MS). Analysesare generally within 10% of accepted values forstandards, except in cases of concentrations closeto the detection level, where errors can be larger.The metabasic rocks of the Vila de Cruces ophiolitegenerally show very low concentrations in some ofthe key elements used to infer the tectonic settingof the ophiolite, Cr, Nb, and Ta. For these elements,the detection levels reported by Actlab are 20, 0.2,and 0.01 ppm, respectively.

Classification of the Igneous Suite. Accordingto their SiO2 content, most of the greenschistsand metagabbros have basic compositions, thoughsome of them exhibit intermediate compositions( wt%). In relation to the rest ofSiO p 47.65–54.032

the major elements, the compositional range is rel-atively low for Al2O3 (12.49–16.98 wt%), MnO (0.15–0.33 wt%), Na2O (1.70–6.22 wt%), K2O (0.01–0.34wt%), TiO2 (0.71–1.84 wt%), and P2O5 (0.05–0.17wt%), and slightly higher for Fe2O3(T) (9.56–16.11wt%), MgO (4.51–11.26 wt%), and CaO (3.14–11.39 wt%). The orthogneisses are acid rocks( wt%) with a rather restrictedSiO p 73.08–77.492

compositional range for the rest of the major ele-ments ( wt%;Al O p 12.52–13.86 Fe O (T) p2 3 2 3

wt%; wt%;1.03–3.28 MnO p 0.01–0.05 MgO pwt%; wt%;0.49–1.40 CaO p 1.90–2.47 Na O p2

wt%; wt%;4.51–5.15 K O p 0.20–0.48 TiO p2 2

wt%; wt%).0.14–0.26 P O p 0.03–0.052 5

It is known that processes such as hydrothermal



140 R . A R E N A S E T A L .

Figure 7. a, Total alkalis versus SiO2 diagram (Le Maitre et al. 1989). b, SiO2–Zr/TiO2 diagram of Winchester andFloyd (1977). c–e, Chondrite-normalized rare earth elements plots; normalizing values are from Nakamura (1974).

alteration, metamorphism, and ductile deforma-tion can cause chemical variation in the concen-tration of many elements, especially most of themajor elements and the large-ion lithophile traceelements. Accordingly, problems may arise in ob-taining an accurate chemical classification of thealtered igneous lithologies. Considering this prob-lem and using a combination of mobile and im-mobile elements (fig. 7a, 7b), we can see that mostmetabasites of the Vila de Cruces ophiolite show

compositions characteristic of subalkaline basalts,though a few samples plot in the field of basalticandesites (fig. 7a, 7b). According to their overallgeochemical characteristics (tables A2, A3; see alsofigs. 7–10), and considering the rather limited com-positional variation in the set of samples, the meta-basic rocks can be identified as members of a tho-leiitic suite. Regarding the granitic orthogneisses,they show chemical characteristics typical of sub-alkaline acid rocks, and in figure 7, they plot in the




Figure 8. Variation diagrams of selected trace elements (Ti, Y, Hf, Ce vs. Zr) for the greenschists and metagabbrosof the Vila de Cruces ophiolite.

rhyolite fields. The general absence of intermediatemembers between the mafic and acid rocks, sinceit is considered characteristic of normal calc-alkaline series, suggests that a single igneous suitewith tholeiitic affinity is represented in the ophio-lite. Although it should be used very carefully, ow-ing to its high mobility, the very low content ofK2O in the orthogneisses (!0.50%) is also in agree-ment with the suggested tholeiitic affinity.

The greenschists have total rare earth element(REE) contents ranging between 20.58 and 64.91ppm, with concentrations between four and 26 timesthe chondritic abundances (Nakamura 1974). Theirchondrite-normalized REE patterns (fig. 7c) are al-most flat for the heavy REEs (HREEs; [Gd/Yb] pN

), with the light REEs (LREEs) variably de-0.88–1.06pleted in relation to the HREEs ([La/Sm] pN

) and slightly positive or negative Eu0.34–0.76



142 R . A R E N A S E T A L .

Figure 9. Trace elements tectonic discrimination diagrams for the samples of the Vila de Cruces ophiolite. a, Ti-Zr-Y (Pearce and Cann 1973); b, Ti-Cr (Pearce 1975); c, Th-Hf-Ta (Wood 1980); d, Ta-Yb (Pearce et al. 1984a).

anomalies ( , calculated accord-∗Eu/Eu p 0.84–1.15ing to Taylor and McLennan 1985). The metagabbroshave lower REE concentrations than the green-schists, with total contents ranging between 18.16and 35.18 ppm. They show abundances between twoand 15 times the chondritic values, and their nor-malized REE patterns (fig. 7d) are very similar tothose of the greenschist, almost flat for the HREEs( ), with variable depletion in[Gd/Yb] p 0.93–1.04N

LREEs ( ) and slightly positive[La/Sm] p 0.27–0.42N

or negative Eu anomalies ( ). Ac-∗Eu/Eu p 0.94–1.32cording to their REE contents, the metagabbros canbe considered less evolved lithologies than the green-schists. However, the two patterns are parallel,which probably suggests a genetic relationship be-

tween the protoliths of greenschists and metagab-bros. The granitic orthogneisses show the highestREE contents, ranging between 39.18 and 105.29ppm. They show abundances of six to 52 times thechondritic values and show fractionated REE pat-terns (fig. 7e) characterized by moderate enrichmentof the LREEs ( ), flat HREE pat-[La/Sm] p 1.36–1.97N

terns ( ), and a marked negative[Gd/Yb] p 0.83–1.18N

Eu anomaly ( ).∗Eu/Eu p 0.30–0.74Tectonic Setting of the Ophiolite. Chemical mo-

bility can be evaluated by studying the correlationbetween the most significant incompatible ele-ments (fig. 8). TiO2, Y, Hf, and Ce show a systematicpositive correlation with increasing Zr, which sug-gests that their contents were not affected by sig-




Figure 10. Immobile trace element plots of the most representative metaigneous lithologies of Vila de Crucesophiolite. a, b, Greenschists and metagabbros plots corresponding to the compositional range for each set of samplesand their average composition, respectively. Normalizing values corresponding to N-MORB average compositionaccording to Pearce (1996). c, d, Orthogneisses plots corresponding to their compositional range and average com-position respectively. Normalizing values corresponding to ORG average composition according to Pearce et al.(1984a).

nificant modification during alteration. Themetagabbroic rocks appear as the less evolved li-thologies, with the lowest contents in Zr; they plotin the origin of the evolving line defined by theigneous suite. These plots confirm that the im-mobile trace elements can be used to investigatethe tectonic setting of the Vila de Cruces ophiolite.

The geochemical diagrams that have proved tobe most useful in identifying the tectonic settingof the Vila de Cruces ophiolite are Ti-Zr-Y (Pearceand Cann 1973), Ti-Cr (Pearce 1975), Hf-Th-Ta

(Wood 1980), and Ta-Yb (Pearce et al. 1984a). Thelast two diagrams can also be used for acid rocks,especially the Ta-Yb diagram, which is specific forgranitoids. In the Ti-Zr-Y diagram, most of the ba-sic rocks plot in the field of the island-arc tholeiites(fig. 9a), while in the Ti-Cr diagram the same rocksplot in the region characteristic for suprasubduc-tion zone basalts (fig. 9b). The Hf-Th-Ta projection(fig. 9c) is probably the most significant to identifysubduction-related basaltic rocks. Most of thegreenschists and metagabbros show very low Ta



144 R . A R E N A S E T A L .

contents, which explains their plotting in the fieldof the destructive plate-margin basalts. Moreover,the ratios shown by these rocks areHf/Th ≥ 4.8characteristic of island-arc tholeiites. In this samediagram, the granitic orthogneisses are also iden-tified as lithologies generated in destructive plate-margin settings, which is confirmed by the Ta-Ybdiagram, where they plot in the field of volcanic-arc granitoids (fig. 9d).

Trace element abundances normalized to the av-erage composition of a rock of known origin are analternative approach to deciphering the tectonicsetting of an igneous suite. This kind of represen-tation is even more significant using just a fewelements with the most immobile behavior andhighest discriminating power (Pearce 1996). Thenormalizing factor used to plot the metabasic rockswas the average N-MORB composition (Pearce1996), whereas the orthogneisses have been nor-malized to the average oceanic ridge granite com-position (Pearce et al. 1984a). As can be observedin figure 10a, the compositional range is more re-stricted in metagabbros than in greenschists, al-though this is probably influenced by the fewersamples analyzed of the first lithology. As a whole,the metabasic rocks show a pattern depleted in allthe selected trace elements in relation to the N-MORB. This depletion is more marked in the me-tagabbros. The normalized trace element abun-dance pattern corresponding to the greenschists isrelatively similar to that of the N-MORB (fig. 10b),but it shows slight depletions in Th, Ce, Zr, andTi. The most outstanding characteristic in this pat-tern is the marked negative anomaly of Nb. Thistype of anomaly is considered characteristic of mag-mas generated in a subduction zone, in which thepreferential retention of Nb into some mineralphases during dehydration of the subducting slabcauses depletion of Nb relative to Th and Ce, inthe subduction-related magmas (Pearce and Peate1995; Pearce 1996). The pattern of the metagabbrosis parallel to that of the greenschists, thoughslightly more depleted, which suggests a similartectonic setting.

The range observed in the normalized trace el-ement abundance pattern of the orthogneisses ismarkedly narrow (fig. 10c). Except in the case ofTh content, they show a general depletion in re-lation to a typical ORG. The average normalizedpattern shows an important negative anomaly inTa and Nb (fig. 10d), which together with the lowcontents of Y and Yb and the slight fractionationin Th are typical of granitoids generated in volcanicarcs or suprasubduction zones (Pearce et al. 1984a).

Given the information obtained from the trace

elements with the most immobile behavior andwith the highest discriminant ability, the green-schists and metagabbros of the Vila de Crucesophiolite show compositions characteristic ofisland-arc tholeiites. Similarly, the granitic ortho-gneisses can be classified as granitoids generated ina volcanic arc. Therefore, it can be concluded thatthe geochemical characteristics of the igneous li-thologies of the ophiolite are consistently differentfrom those typical of mid-ocean ridge ophiolitesgenerated in divergent settings and are compatiblewith a convergent plate tectonic setting. We there-fore conclude that the Vila de Cruces ophioliteformed in a suprasubduction zone environment.

Discussion: Implications for the Openingof the Rheic Ocean

Evidence based on stratigraphic (Linnemann et al.2000), paleontologic (McKerrow et al. 2000; Forteyand Cocks 2003), paleomagnetic (Tait et al. 2000),igneous (Sanchez-Garcıa et al. 2003), and tectono-thermal (Martınez Catalan et al. 2002) data suggestthat the opening of the Rheic Ocean occurred in theLate Cambrian to Early Ordovician, while its closuretook place in Devonian to Carboniferous times dur-ing the long collision between Gondwana and Lau-russia that generated Pangaea. Recently, it has beenproposed that the opening of the Rheic Ocean oc-curred by rifting along a Neoproterozoic suture(Murphy et al. 2006). In spite of the large amount ofavailable data concerning the history of this ocean,which is currently included in plate tectonics re-constructions for Paleozoic times (Stampfli andBorel 2002; Winchester et al. 2002), ophiolitic en-sembles representing its oldest oceanic crust are elu-sive. Many uncertainties exist regarding the openingitself, which may have been preceded by an EarlyCambrian rifting event described in the paleo marginof Gondwana and marked by abundant alkaline mag-matism (Floyd et al. 2000; Linnemann et al. 2000;Sanchez-Garcıa et al. 2003). The ophiolites currentlyconsidered related to the Rheic in the Variscan Belthave ages ranging between Late Silurian and EarlyDevonian (Clark et al. 1998; Dıaz Garcıa et al. 1999;Dubinska et al. 2004). Hence, these ophiolites canbe considered representative of only the youngestoceanic lithosphere originated in the Rheic Ocean,shortly before its closure, and they were probablygenerated during intraoceanic subduction associatedwith the destruction of older oceanic lithosphere(Sanchez Martınez et al. 2007). The common elim-ination by subduction of the oldest cold and denseoceanic lithosphere could explain the infrequentpreservation of oceanic lithosphere generated in the




Figure 11. Schematic cartoon showing three significant steps in the evolution of the Vila de Cruces ophiolite. a,Tornquist Ocean begins to close in a destructive plate margin in the periphery of Gondwana. b, Individualization ofa peri-Gondwanan terrane by slab rollback related to the subduction of the Tornquist lithosphere. This terrane willbe emplaced on top of the allochthonous nappe stack of northwest Iberia. c, Closure of the Tornquist Ocean, accretionof the future upper allochthon to Laurussia, and maximum width reached by the Rheic Ocean. Plate tectonicsreconstruction for the Cambro-Ordovician boundary based on Winchester et al. (2002).

first stages of oceanic opening (Molnar and Atwater1978).

The Vila de Cruces ophiolite can be interpretedas a remnant of the oceanic lithosphere generatedduring the first stages in the opening of the RheicOcean (fig. 11). The lithologies, characterized bygreenschist series of basaltic origin interbeddedwith sediments and minor metagabbros and ortho-

gneisses, suggest that it is not representative ofcommon oceanic lithosphere generated in mid-oceanic spreading centers (Anonymous 1972; Bou-dier and Nicolas 1985). Its geochemical character-istics suggest generation in a suprasubduction zoneenvironment—more precisely, in a back-arc basincaused by subduction directed to Gondwana (fig.11). The U-Pb age of 497 Ma obtained in the meta-



146 R . A R E N A S E T A L .

granitoids represents a minimum age for the open-ing of the back-arc basin, which was probablyfollowed by separation from Gondwana and sub-sequent drift of the terrane currently forming theupper allochthon of the northwest Iberian Massif.This terrane includes huge massifs of gabbros andgranitoids dated at 500 Ma (Abati et al. 1999), withgeochemical characteristics typical of volcanicarcs, and its accretion to Laurussia occurred at ap-proximately 430–415 Ma (Gomez Barreiro et al.2006; Fernandez-Suarez et al. 2007), when theRheic Ocean reached its maximum width.

According to the model of figure 11, the Vila deCruces ophiolite represents an incipient stage inthe development of the marginal basin that even-tually spread to the Rheic Ocean. The geochemicalarc affinity of the basic rocks and the presence ofsedimentary rocks favor this interpretation. Theophiolite probably represents a piece of the oceaniclithosphere located close to the Gondwana margin.The abundance of detrital sediments in this sectionprobably accounts for its final preservation becausethey are less prone to being subducted. As youngeroceanic lithosphere developed later and was pro-gressively more separated from the continentalmargin, it was sediment starved and was elimi-nated by intraoceanic subduction later than 430–415 Ma (Sanchez Martınez et al. 2007). This intra-oceanic subduction, in turn, gave way to the mostcommon arc-related ophiolites preserved in the Eu-ropean Variscan Belt, generated around 400 Ma.

Conclusions

The evolution of the Rheic Ocean included a firststage of continued opening that extends from 500Ma, when subduction-related arcs and back-arc ba-sins began to develop in the periphery of Gond-wana, until around 430–415 Ma, the time when theisland arcs originally rifted from Gondwana wereaccreted to Laurentia-Baltica. This arc-continent

collision immediately preceded the beginning ofconvergence between Laurussia and Gondwana andthe progressive closure of the ocean. If the relativeabundance of Early Devonian ophiolites in theEuropean Variscan Belt is taken into account, andconsidering that most of these ophiolites are char-acterized by lithological sequences typical of su-prasubduction contexts, it could be suggested thatmost of the Rheic lithosphere was consumed veryquickly, probably by intraoceanic subduction, de-veloped between 415 and 390 Ma. Closure occurredjust before the onset of subduction of the outermostmargin of Gondwana, which is currently consid-ered to mark the beginning of the Variscan Orogenyin Europe. The high-pressure metamorphism de-veloped during this event has been dated in north-west Iberia at around 370–365 Ma (Rodrıguez et al.2003). Consequently, it can be considered that theRheic Ocean existed during some 135 m.yr., andits evolution can be regarded as a model in platetectonics. The Vila de Cruces ophiolite is a remnantof the oldest oceanic lithosphere generated in thisocean, and its preservation in the most internal partof the northwest Iberian Massif allows the study ofthe first stages of its evolution.

A C K N O W L E D G M E N T S

Financial support for this research has been pro-vided by Spanish projects BTE2001-0963-CO2 andCGL2004-0463-CO2/BTE from the Ministerio deEducacion y Ciencia, Direccion General de Inves-tigacion. This is a contribution of the project Inter-national Geological Correlation Program 497, “TheRheic Ocean.” Stimulating discussions with mem-bers of this project during 2004–2005 strongly helpedus to clarify the evolution of this lost Paleozoicocean. The criticisms, comments, and suggestionsof three anonymous reviewers helped to improve themanuscript and are kindly acknowledged.

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The Vila de Cruces Ophiolite: A Remnant of the Early …de Cruces ophiolite attains 3500–4000 m in thick-ness, although its internal structure is imbricated. This article describes