ELSEVIER Earth and Planetary Science Letters 165 (1999) 213–228

Early Ordovician orogenic event in Galicia (NW Spain): evidence fromU–Pb ages in the uppermost unit of the Ordenes Complex

J. Abati a,Ł, G.R. Dunning b, R. Arenas a, F. Dıaz Garcıa c, P. Gonzalez Cuadra d,J.R. Martınez Catalan d, P. Andonaegui a

a Departamento de Petrologıa y Geoquımica, Universidad Complutense, 28040 Madrid, Spainb Department of Earth Sciences, Memorial University, St. John’s, NF A1B 3X5, Canada

c Departamento de Geologıa, Universidad de Oviedo, 33005 Oviedo, Spaind Departamento de Geologıa, Universidad de Salamanca, 37008 Salamanca, Spain

Received 28 May 1998; revised version received 4 November 1998; accepted 11 November 1998

Abstract

New U–Pb data (zircon, monazite and rutile) obtained from rocks of the uppermost allochthon in the Variscan belt ofNW Spain indicate that the hangingwall to the suture includes an allochthonous unit with a pre-Variscan tectonothermalevolution. This evolution is characterised by an Early Ordovician (498–500 Ma) bimodal magmatism followed almost im-mediately (493–498 Ma) by a Barrovian style metamorphism up to the granulite facies. The metamorphism subsequent tothe igneous intrusions requires convergence and crustal thickening in order to generate the Barrovian facies pattern shownby the lithologies of the uppermost allochthon. The almost coeval magmatism and metamorphism, and the chemistry of themetabasites suggest an accretionary complex, probably related to a volcanic arc, as the most probable setting for the originof the uppermost unit. The implications of the existence of Early Ordovician convergent plate boundaries are discussed inthe context of Gondwana–Laurentia–Avalon interactions. 1999 Published by Elsevier Science B.V. All rights reserved.

Keywords: U=Pb; geochronology; Ordovician; thermal metamorphism; Hercynian orogeny; Galicia Spain

1. Introduction

The geology of the Northwest Iberian Massif rep-resents a complete transition from the external to theaxial zones of the Variscan belt. Five allochthonouscomplexes, located in Galicia (Spain) and Tras-os-Montes (Portugal), occupy the uppermost structuralposition in the axial zone. They consist of a pileof units stacked at the onset of the Variscan de-formation, and include a suture zone marked by

Ł Corresponding author. Tel.: C34-91-394-4898; Fax: C34-91-544-2535; E-mail: abati@eucmax.sim.ucm.es

several slices of oceanic lithosphere [1,2]. From bot-tom to top, the allochthonous units are grouped intobasal, ophiolitic and upper units (Fig. 1). During themain Variscan deformation, these units were thrustover the Iberian autochthon [1], which represents thelarger part of the Iberian Massif.

The occurrence of a suture points to a collisionalprocess and there is general agreement that oneof the colliding elements was the continental mar-gin of Gondwana, represented by the units in thefootwall to the suture: the Iberian autochthon andthe allochthonous basal units [2–4]. The origin andsignificance of the units in the hangingwall to the

0012-821X/99/$ – see front matter c 1999 Published by Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 9 8 ) 0 0 2 6 8 - 4

214 J. Abati et al. / Earth and Planetary Science Letters 165 (1999) 213–228

J. Abati et al. / Earth and Planetary Science Letters 165 (1999) 213–228 215

suture, representing the other colliding element, ismore problematic, and has been the source of a pro-longed controversy due to the lack of unambiguousdating of deformation and metamorphism. As can beseen in Fig. 1, two different types of units can bedistinguished above the ophiolites: the lower high-pressure and high-temperature (HP–HT) units andan intermediate-pressure (IP) uppermost unit. Iso-topic dating has been carried out previously in bothunits, and results are interpreted as either protolithor metamorphic ages. However, data relevant to thetiming of metamorphism have been reported only forthe HP–HT units, and the most recent results havebeen interpreted as indicative of an early Variscanage for the initial, high-pressure event [5–7].

The aim of this study is to date accurately thebimodal magmatism and the metamorphism of theuppermost unit of the Ordenes Complex, in order toconstrain the origin and tectonothermal evolution ofthe problematic colliding element, presently abovethe suture. The magmatism has been dated by the U–Pb method, analysing magmatic zircon populationsfrom gabbros and orthogneisses. To determine thetiming of high-grade metamorphism, U–Pb analyseshave been carried out on monazite included in biotitebelonging to the regional tectonic foliation of parag-neisses of the sillimanite zone. Due to the elevatedclosing temperature of monazite, above 700–725ºC[8–10], it is very likely that the mineral ages providethe age of the metamorphic thermal peak.

2. Geological setting

The Ordenes Complex is the largest of the al-lochthonous complexes preserved as mega-klippenin the axial part of the Iberian Massif. It consistsof a stack of thrust sheets, overprinted by exten-sional detachments, upright folds and faults. Thecomplex is underlain and surrounded by relativelyautochthonous sequences.

The autochthon, together with other comparableterranes of southern Europe, formed part of the con-tinental margin of Gondwana during the Neoprotero-

Fig. 1. Geological map and cross-section of the NW Iberian Massif, showing the allochthonous complexes and their units. The locationof the mapped area is shown in the upper part of the figure, in a reconstruction of Pangea based on Lefort [49].

zoic and Paleozoic. This margin registered the Cado-mian (Pan-African) orogeny, as well as Cambrian–Ordovician continental rifting processes, which re-sulted in the pulling away of the Avalon microconti-nent (or microcontinents) from the Gondwana main-land and the opening of the Rheic oceanic domain[11,12].

The allochthonous tectonostratigraphic units,which also occur in the other Iberian allochthonouscomplexes, can be classified according to their litho-logical associations and tectonothermal evolution,and also according to their relative position [2,13,14].What follows is a brief description of these units.

2.1. Basal units

The basal units consist of schists, paragneisses,and alternating igneous felsic and mafic rocks.Granitic and peralkaline orthogneisses have yieldedU–Pb and Rb–Sr ages of 480–460 Ma [15,16]. Themagmatism is interpreted to reflect an Ordovicianrifting episode [17]. Since there are no ophiolitesseparating them from the underlying parautochthon,the basal units are considered part of the continentalmargin of Gondwana formed after the opening of anocean, subsequent to the Ordovician rifting.

However, the basal units are considered al-lochthonous because there is a pressure gap in themetamorphic evolution between them and the pa-rautochthon. The basal units underwent an initialsubduction-related HP metamorphic event, whichreached 14–16 kbar and 650–700ºC [14,18]. Thefinal stages of subduction have probably occurred374 m.y. ago, the age of post-eclogitic white micas[15]. The polarity of subduction was roughly to thewest in present coordinates [2].

2.2. Ophiolitic units

The ophiolitic units appear repeated in severalthrust sheets and include metapelites and cherts,basalts, pillow breccias and hyaloclastites, diabases,metagabbros, plagiogranites, amphibolites and ultra-mafic rocks. In the Ordenes Complex, the upper

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thrust sheet represents the basal parts of an ophi-olitic sequence, characterised by the abundance ofpegmatitic gabbros. A protolith age of 395 š 2 hasbeen obtained for a gabbro differentiate in this unit[19]. Other ophiolitic units seem to represent theupper parts of the oceanic lithosphere, because ofthe abundance of pelitic sediments and greenstonespartially derived from basalts.

The ophiolites show a wide variety of metamor-phic conditions, ranging from intermediate-pressuregranulite facies to amphibolite and greenschist fa-cies. A high-pressure metamorphic gradient has beenidentified in some of the thrust sheets. The ophioliticnappes show eastward vergence, and were stackedduring the closure of a previously opened ocean. Thecoeval amphibolite-facies foliation in the medium-grade units was formed 390–380 m.y. ago [20],closely following oceanic crust generation.

2.3. Upper units

The upper units structurally overlie the ophiolitesand can be subdivided into HP–HT units, below,and an IP unit occupying the uppermost structuralposition.

2.3.1. High-pressure and high-temperature unitsThe HP–HT units consist of paragneisses, metab-

asites, and ultramafic rocks. The metabasites aregarnet–clinopyroxene granulites and eclogites, retro-graded to the amphibolite facies. The REE patternsof eclogites and some of the mafic units in the CaboOrtegal Complex are characteristic of the oceaniccrust [21]. However, the abundance of sedimentspoints to a different origin for many of the units.Gabbros occur in several stages of transformation,from virtually undeformed and scarcely affected bythe metamorphism, to coronitic metagabbros and HPgranulites. In the less deformed gabbros, subophiticand diabase textures have been preserved, indicat-ing an emplacement at relatively shallow levels. Thechemical characteristics of the gabbros are compara-ble to modern continental tholeiites, and compatiblewith a continental rift provenance [22,23].

The peak metamorphic conditions recorded by theHP–HT units are variable, reaching at least 18 kbar,and 700–850ºC [21]. Several early Paleozoic U–Pbages, between 490 and 480 Ma [21], were currently

interpreted as dating the HP–HT metamorphism, butSchafer et al. [5] consider these the ages of the maficprotoliths. Schafer et al. [5], Santos Zalduegui et al.[6] and Ordonez Casado et al. [7] have obtained U–Pb ages on zircon, monazite and titanite indicating ametamorphic event between 405 and 390 m.y. ago.According to the authors, these ages correspond tothe HP–HT event, which was followed by a subse-quent retrograde amphibolite-facies metamorphismat 390–380 Ma [20,24].

2.3.2. Uppermost unit with intermediate-pressuremetamorphism

The unit consists of a thick sequence ofterrigenous metasediments, and large bodies ofaugengneisses, amphibolites and gabbros. Themetasedimentary sequence is known as the OrdenesSeries. In the epizonal upper part, the metasedimentsare a succession, 2000–3000 m thick, of pelitesand greywackes [25]. Generalised graded beddingand other abundant sedimentary structures indicatea flyschoid character. The augengneisses show in-trusive relationships with the metasediments, andhave yielded ages of 496–460 Ma (U–Pb on zir-cons: [26,27]). The gabbros are also intrusive, anddeformed diabase dikes are common in the au-gengneisses and metasediments.

The metamorphism ranges from high-grade at thebasal part of the unit, to epizonal in the upper part.Several metamorphic zones have been mapped, andtheir limits are commonly extensional detachments,a feature also present in other allochthonous units,that indicates a strong thinning of the original nappepile [2,13,14,18].

The deepest part of the uppermost IP unit isoccupied by a large, rounded metagabbroic body,the Monte Castelo Gabbro (MCG), outcropping inthe western part of the Ordenes Complex (Fig. 2).This is a massive two-pyroxene gabbro, fine- tomedium-grained, with textures variable from gran-ular to intergranular and ophitic. The igneous min-eral assemblage consists of clinopyroxene, orthopy-roxene, plagioclase, occasional olivine, biotite andhornblende, and ilmenite, titanite, and zircon as ac-cessory minerals. The presence of olivine and thecommon ophitic textures point to a relatively shallowemplacement. The MCG is tholeiitic in character.A detailed geochemistry study is in progress, and

J. Abati et al. / Earth and Planetary Science Letters 165 (1999) 213–228 217

Fig. 2. Geological map of the Ordenes Complex. Outlined are the different units of the complex, the metamorphic zones of the uppermostunit, and the location of the samples collected for U–Pb geochronology.

includes major and trace elements, and REE data.When plotted in multi-element normalised diagramsfor comparison with basaltic compositions and tec-tonic discrimination diagrams, the analyses usuallyfall in fields related to volcanic arcs.

Near its bottom, the gabbro is cross-cut by gran-ulite-facies prograde shear zones, which produced acomplete recrystallisation of the igneous paragenesis

and display a new metamorphic assemblage con-sisting of garnet, orthopyroxene, biotite, plagioclase,quartz, ilmenite, and rutile, arranged in a granoblas-tic to grano-lepidoblastic texture.

Scarce metapelitic enclaves in the MCG showalso a granulite-facies mineral assemblage, compati-ble with that of the host rock, that includes garnet, or-thopyroxene, biotite, plagioclase, K-feldspar, quartz,

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sillimanite, ilmenite, and rutile. At the outcrop scaleor in hand specimen, a high-grade foliation is evi-dent in these granulites, which excludes the possi-bility of an origin by contact metamorphism. Theless retrogressed samples contain rounded, coarsegarnet (up to 1 cm) in a granoblastic polygonal ma-trix formed by plagioclase, quartz, K-feldspar, andsubeuhedral elongated orthopyroxene and reddish bi-otite. The peak mineral associations from the gabbroand the metasedimentary enclaves suggest tempera-tures above 750–800ºC and intermediate pressures.

Overlying and surrounding the MCG, a thick pileof monotonous plagioclase-rich paragneisses occurs.In some places, the contact between the gabbro andthe metasediments of the Ordenes Series containsnarrow granitic bands (50–100 m thick), probablygenerated by partial melting of the latter. In someoutcrops, the granites appear deformed and show aplano-linear fabric and abundant enclaves (restites)of paragneisses from the surrounding metasediments.These fine- to medium-grained granitoids containquartz, plagioclase, biotite, muscovite, clinozoisiteand small amounts of ilmenite, chlorite, and zircon.To the north of the MCG, the paragneisses containthe peak mineral assemblage garnet, biotite, mus-covite, plagioclase, quartz, sillimanite, and ilmenite,belonging to the first sillimanite zone. Generally,they are strongly retrogressed to the greenschist fa-cies, showing the breakdown of garnet and biotite tochlorite and the sillimanite being limited to needlesincluded in muscovite.

In the eastern part of the complex, the metamor-phic conditions of the Ordenes Series are slightlyhigher, reaching the sillimanite–K-feldspar zone andshowing abundant evidence of partial melting. Alarge outcrop of granitic augengneiss (the CorredoirasOrthogneiss) is located at the base of the unit, ina position equivalent to that of the MCG, and isseparated from the underlying HP–HT units by aductile shear zone, the Corredoiras Detachment [2](Fig. 2). The orthogneiss also shows evidence of thehigh-grade metamorphic event, such as garnet coro-nas around biotite and recrystallised muscovite-freedomains composed of garnet, biotite, plagioclase,K-feldspar, quartz, sillimanite, ilmenite, and rutile.The orthogneiss includes large bodies of migmatiticparagneisses with high-grade mineral assemblageswith garnet, biotite, K-feldspar, plagioclase, quartz,

sillimanite, and minor cordierite. The relationshipsbetween augengneisses and migmatites at the out-crop scale, suggest that the high-grade metasedimentswere initially large xenoliths in the granitic body(probably originating in a crustal region subjected topartial melting), and were subsequently deformed andmetamorphosed together with the host granitic rocks.

The central part of the Ordenes Complex is essen-tially a pelitic and semipelitic sequence of rocks inwhich Barrovian kyanite, staurolite, and garnet zonescan be recognised. In the upper part of the sequence,separated from the central part by an extensionaldetachment (Fig. 2), the metamorphism is of lowgrade, the igneous rocks are scarce, and the domi-nant lithologies are schists in the garnet and biotitezones, and slates, phyllites and metagreywackes inthe chlorite zone.

3. U–Pb geochronology

3.1. Selection of samples

Eight samples were selected for U–Pb geochro-nology in order to establish the age of magmatismand metamorphism in the uppermost unit of the Or-denes Complex. Five of the samples are high-gradeparagneisses and the other three are meta-igneousrocks. Sample localities are shown in Fig. 2. Fel-sic and mafic igneous rock types were selected toconfirm the contemporaneity of the bimodal mag-matism, suggested by the field relationships. Themetasediments were selected from different high-grade metamorphic conditions (from sillimanite toorthopyroxene zones) to investigate possible age dif-ferences relevant to their tectonothermal evolution.For that purpose, zircon from the igneous rocks andmonazite fractions from the metasedimentary rocksamples were recovered; in addition, rutile was sepa-rated from G97-4 in order to obtain cooling ages inan attempt to constrain the timing of retrogression.

G97-1 is a gabbro from the central part of theMCG, sampled in a zone with abundant pegmatoids,where quartz was visible in the field, to ensure thepresence of enough zircon. The texture is intergranu-lar and scarce olivine is present in the non-pegmatiticzones.

G97-2 is a granitoid rock located in the contact

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between the MCG and the Ordenes Series, sampledin a zone free of enclaves and slightly deformed.The rock is fine- to medium-grained, and has abun-dant mica-fishes of biotite, recrystallised quartz mo-saics and porphyroclasts of plagioclase and clino-zoisite. Accessory minerals are ilmenite, generallysurrounded by titanite, and zircons included in bi-otite with a mean size of 0.2 mm.

G97-3 is an augengneiss from the CorredoirasMassif. This coarse- to very coarse-grained meta-granite shows large K-feldspar megacrysts (up to 7cm), surrounded by a moderately recrystallised ma-trix with plagioclase, quartz, biotite, garnet, ilmeniteand titanite.

G97-4 is a metapelitic granulite-facies xenolithcollected in the southern part of the MCG. Theparagneiss exhibits a well-preserved high-tempera-ture mineral assemblage with garnet, orthopyroxene,biotite, sillimanite, rutile, ilmenite, K-feldspar, pla-gioclase and quartz. Light yellow monazite withpleochroic halos can easily be identified as inclu-sions in biotite.

G97-5 and G97-6 are semi-pelitic paragneisses ofthe sillimanite zone from the western part of the up-permost unit. They are fine- to medium-grained andshow a gneissic foliation defined by domains richin biotite, muscovite and scarce sillimanite, alter-nating with quartz–plagioclase domains. Garnet andilmenite are present in both domains, and numerouszircon and monazite grains can be seen included inthe biotite defining the fabric.

G97-7 and G97-8 are fine- to medium-grainedmigmatites from the eastern part of the uppermostunit, outcropping as regional scale mega-enclaves inthe Corredoiras Orthogneiss. The gneisses are grano-lepidoblastic and contain the peak mineral assem-blage garnet, biotite, plagioclase, K-feldspar and sil-limanite. Cordierite is also present as a secondaryphase.

3.2. Analytical techniques

All the steps of U–Pb method were carried out inthe Department of Earth Sciences, Memorial Univer-sity of Newfoundland (Canada). The laboratory pro-cedures resemble, in general terms, those describedby Dube et al. [28].

The samples were powdered and mineral fractions

were obtained by density separation on a Wilfley ta-ble and heavy liquids (Methylene iodide), and bymagnetic separation using a Frantz isodynamic sep-arator. After sieving with a 70 mesh sieve, finalmineral fractions for analyses were hand-picked un-der a binocular microscope selecting the crystalsaccording to criteria of morphology, size, colour andclarity. The abrasion technique after Krogh [29] wasused to remove the outer surfaces of the minerals tominimise Pb loss, except in the smallest or delicatefractions which are indicated in Table 1. Then, theminerals were cleaned in an ultrasonic bath withlow Pb blank 4 N HNO3, doubly distilled H2O, anddistilled acetone, and subsequently weighed in a mi-crobalance. The mineral fractions were mixed with205Pb=235U isotopic tracer. Zircon and rutile fractionswere dissolved for 5 days at 210ºC in 8 N HNO3 andconcentrated HF using Teflon bombs sealed by stain-less steel jackets. Monazite fractions were digestedin 6 N HCl using Savillex Teflon vials placed on ahot plate at 150ºC.

The U and Pb were separated by ion exchangechemistry in columns with DOWEX AG1-X8 resinin chloride form, following a modified method ofKrogh [30] for zircon, and HBr chemistry after Man-hes et al. [31] for monazite and rutile. Purified Pband U were loaded together on single Re filamentswith silica gel and H3PO4. Isotope ratios have beenmeasured using a Finnigan-MAT 262 multicollec-tor thermal ionisation mass spectrometer in staticmode, with faraday cups calibrated against NBS 981and with 204Pb measured in the secondary electronmultiplier (SEM) in ion counting mode. The onlyexceptions were the rutile fractions, because theirlow U and radiogenic Pb contents made it necessaryto measure all the Pb ratios by peak jumping usingthe SEM-ion counter.

Uncertainties on ages and isotope ratios are re-ported at 2¦ , considering the propagation of errorsfrom the measurement by mass spectrometry, theisotope fractionation, the Pb and U blanks (2–12pg Pb, 1 pg U), and the uncertainty on the iso-tope composition of initial common Pb, according tothe model of Stacey and Kramers [32]. The abovecalculation of uncertainties was made using an un-published program of the Royal Ontario Museum,Canada, and the uncertainty on the ages are reportedat 95% confidence level.

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Table 1U=Pb data

Fraction Weight Model Concentration Measured Corrected atomic ratios a Apparent ages (Ma)(mg) Th=U U Pb rad. Pb com. 206Pb

204Pb

208Pb206Pb

206Pb238U

207Pb235U

207Pb206Pb

206Pb238U

207Pb235U

207Pb206Pb(ppm) (ppm) (pg)

G97-1 (Monte Castelo Gabbro)Z1 fragmented prisms 0.529 0.513 220 18.6 9 64327 0.1644 0.08046 š 28 0.6342 š 24 0.05716 š 6 499 499 498Z2 fragmented prisms 0.130 0.495 227 19.1 22 6768 0.1591 0.08023 š 30 0.6332 š 22 0.05724 š 10 497 498 501Z3 fragmented prisms 0.130 0.547 222 18.9 20 7144 0.1756 0.08028 š 26 0.6336 š 22 0.05724 š 8 498 498 501

G97-2 (granitoid)M1 small grains 0.085 35.887 1188 1023.5 37 13673 11.4881 0.07921 š 28 0.6206 š 22 0.05683 š 10 491 490 485M2 coarse grains 0.081 45.455 904 975.2 25 15028 14.4444 0.08028 š 30 0.6301 š 24 0.05692 š 8 498 496 489Z1 prismatic, clear, small grains 0.054 0.357 211 17.1 18 3249 0.1146 0.08038 š 34 0.6349 š 26 0.05729 š 14 498 499 503Z2 prismatic, clear, coarse grains 0.060 0.389 228 18.6 20 3406 0.1249 0.08032 š 30 0.6346 š 24 0.05730 š 12 498 499 503

G97-3 (Corredoiras Orthogneiss)Z1 clear, euhedral, small 0.211 205 16.3 17 12500 0.0957 0.07982 š 24 0.6297 š 20 0.05721 š 6 495 496 500

G97-4 (Metapelitic enclave in MCG)M1 pale yellow, euhedral 0.099 36.824 1127 1012.0 64 8851 11.831 0.08039 š 28 0.6325 š 24 0.05707 š 8 498 498 494M2 pale yellow, euhedral 0.108 42.403 1034 1039.3 66 8573 13.381 0.08035 š 34 0.6319 š 28 0.05704 š 8 498 497 493R1 red 0.130 1.150 5 0.4 43 75 0.3702 0.06256 š 34 0.4825 š 206 0.05594 š 222 391 400 450R2 reddish brown 0.220 0.706 4 0.3 50 82 0.2282 0.06101 š 30 0.4658 š 92 0.05538 š 100 382 388 427

G97-5 (Ordenes Series paragneiss)M1 small grains, turbid 0.061 5.463 7074 1363.7 70 30979 1.733 0.07994 š 32 0.6297 š 26 0.05714 š 4 496 496 497M2 small grains, turbid 0.018 4.783 6677 1199.3 69 8710 1.541 0.07999 š 46 0.6306 š 38 0.05718 š 6 496 496 498

G97-6 (Ordenes Series paragneiss)M1 rounded, clear, yellow, not abraded 0.046 3.385 8446 1242.1 31 62253 1.087 0.07943 š 28 0.6247 š 24 0.05704 š 4 493 493 493M2 8 grains, not abraded 0.008 4.568 6497 1108.5 21 12459 1.454 0.07850 š 24 0.6168 š 21 0.05699 š 6 487 488 491

G97-7 (Ordenes Series migmatite)M1 small grains, turbid, not abraded 0.020 6.958 4650 1054.9 32 14306 2.242 0.07951 š 24 0.6254 š 22 0.05705 š 6 493 493 493M2 small, clear, not abraded 0.036 8.564 5130 1103.9 46 20007 2.072 0.07945 š 28 0.6248 š 24 0.05703 š 4 493 493 493

G97-8 (Ordenes Series migmatite)M1 yellow, not abraded 0.024 7.985 3181 984.7 29 12656 3.557 0.07744 š 22 0.6065 š 20 0.0568 š 6 481 481 484

a Corrected for fractionation, spike, laboratory blank (2 to 12 pg common lead; 1 pg uranium), and initial common lead from the model of Stacey and Kramers [32].

J. Abati et al. / Earth and Planetary Science Letters 165 (1999) 213–228 221

3.3. Results

The U–Pb results are shown in Table 1. The sam-ple from the Monte Castelo Gabbro (G97-1) yieldedabundant fragments of coarse zircon prisms (0.3mm mean size), very clear, with rare distinguishablecrystal faces and edges (Fig. 5b,c). The homogeneityof grain size and morphology of the zircon grainssuggest the presence of one single igneous popula-tion. Three analyses carried out on mineral fractionswith different degrees of abrasion are concordantand overlap, giving a precise age of 499 š 2 Ma(Fig. 3a), which is interpreted as the age of igneouscrystallisation.

The zircons from G97-2, the granitoid rock atthe northern contact of MCG, are euhedral elon-

Fig. 3. U–Pb concordia diagrams for the igneous samples from the uppermost unit of the Ordenes Complex. (a) Monte Castelo Gabbro(zircon). (b) Granite located at the north limit of Monte Castelo Gabbro (zircon and monazite). (c) Corredoiras Orthogneiss (zircon). (d)combined data.

gated prisms ranging in size from 0.1 to 0.5 mm(Fig. 5a). They are clear, with sharp edges andfaces and without overgrown rims. Two abradedfractions containing small and large crystals (Z1 andZ2, respectively), gave an almost concordant age of500š2 Ma (0.9–1% discordant) (Fig. 3b). The gran-itoid also contains monazites, which are rounded,pale yellow and rather turbid. Some monazites fromthis and the following samples have an orange sur-face coating, probably of apatite [33], which wascompletely removed by air abrasion or HNO3 clean-ing. The two analysed fractions are not duplicated,and plot slightly above the concordia, defining areverse discordance (Fig. 3b). Fraction M2, formedby the coarse monazite grains (¾0.15 mm) provides206Pb=238U and 207Pb=235U ages of 498 and 496 Ma,

222 J. Abati et al. / Earth and Planetary Science Letters 165 (1999) 213–228

respectively, which corroborates the data from thezircon, whereas M1, formed by the small grains(0.05–0.1 mm), gives younger ages (206Pb=238U D491 Ma; 207Pb=235U D 490 Ma). The age differenceof the monazite can be attributed to two factors: (1)the smaller grain size of minerals of M1 fractioncould favour a lower closing temperature of the U–Pb system; or (2) Pb loss after crystallisation. In thecases of reverse discordance in these analyses, the207Pb=235U age is considered the best estimate ofcrystallisation of the mineral, although consideringthe mean of the three apparent U–Pb ages does notmodify significantly this age.

One fraction of euhedral, clear zircon prisms fromCorredoiras Orthogneiss (G97-3) gives a slightlydiscordant analysis with a 207Pb=206Pb age of 500š2Ma, very similar to the previous ones (Fig. 3c).

Monazite from the metapelitic granulite G97-4is euhedral, pale yellow and clear (Fig. 5d,e), andforms a homogeneous population with a mean sizeof 0.15 mm. Two almost concordant fractions give a207Pb=235U age of 498 š 2 Ma (duplicated analyses,Fig. 4a) with 207Pb=206Pb ages slightly lower (494and 493 Ma), defining a reverse discordance (�0.9%discordant). Two reddish brown rutile fractions donot yield precise ages due to the large common Pbcorrection and because the low U and radiogenic Pbcontents (5 ppm U; 0.4 ppm radiogenic Pb) increasethe uncertainties of the measured isotopic ratios. The206Pb=238U ratio is the most precise and less affectedby the common Pb correction, and 206Pb=238U agesare probably the best estimates of the closing of theU–Pb system in these rutiles. 206Pb=238U ages of391 š 3 and 382 š 3 Ma are about 100 Ma youngerthan the ages yielded by the monazites in the samesample.

Monazite from the two paragneisses G97-5 andG97-6, from the sillimanite zone, yield similar ages:a concordant age of 496š 3 for G97-5 (Fig. 4b), andan age of 493š 1 Ma given by a regression line withthe lower intercept pinned at 10 š 10 Ma (G97-6;Fig. 4c). Monazite grains from the former are verytiny (0.02–0.05 mm), lentil-shaped, yellow and tur-bid (Fig. 5f), whereas monazites from G97-6 arevery sparse high-quality grains, subrounded, clearand of 0.1 mm mean size.

The migmatites included in Corredoiras Or-thogneiss (G97-7 and G97-8) contain monazites very

similar to those of G97-5, and G97-7 also yielded aconcordant age of 493 š 3 (Fig. 4d), whereas G97-8seems to be slightly younger with a 207Pb=206Pb ageof 484š 2 Ma (Fig. 4e). This last age is less reliablebecause only one single analysis is available, and itonly can be considered as a minimum age.

3.4. Discussion

3.4.1. Age of magmatismThe absence of any inherited component, the very

limited Pb loss, and the igneous morphology of zir-cons, indicate that the obtained ages are the ages ofcrystallisation of the magmatic bodies. The possi-bility of some resetting of the U–Pb system duringthe granulitic metamorphic event subsequent to thecrystallisation of the magmas (see above) can beruled out for several reasons: (1) the characteristicigneous morphology of the zircons without showingany evidence of overgrowths or recrystallisation; (2)the concordant character of the ages suggests that theU–Pb system behaved as a closed system (metamor-phosed zircon is generally highly discordant, unlessnewly crystallised) [34]; (3) Pb diffusion in pris-tine zircon is insignificant until temperatures above1000ºC [34]; and (4) the literature reports severalexamples of igneous rocks that have undergone agranulitic metamorphism and the U–Pb system ofthe zircon remained unaltered [35,36].

3.4.2. Ages of metamorphismThe relatively high closing temperature of mon-

azite, above 700–725ºC [8–10], and the general lackof inherited components, makes this mineral spe-cially suitable for dating high-grade thermal events.The growth of monazite during amphibolite- andgranulite-facies prograde metamorphism of metased-imentary aluminous rocks is widely documented[9,33,37], and their U–Pb crystallisation ages arefrequently interpreted as close to the thermal peak[37–40]. All the ages obtained in this study fromupper amphibolite- and granulite-facies metapelitesare remarkably uniform and constrained in a narrowinterval between 493 and 498 Ma (except G97-8at 484 Ma). Most of the monazites are found in-cluded in biotite lepidoblasts and, therefore, theirages are considered as the maximum ages for thedevelopment of the gneissic fabric, probably very

J. Abati et al. / Earth and Planetary Science Letters 165 (1999) 213–228 223

Fig. 4. U–Pb concordia diagrams for the metasedimentary gneisses from the uppermost unit of the Ordenes Complex. (a) Metapeliticenclave in Monte Castelo Gabbro (monazite and rutile). (b) Ordenes Series paragneiss (monazite). (c) Ordenes Series paragneiss(monazite). (d) Ordenes Series migmatite included in Corredoiras Orthogneiss (monazite). (e) Ordenes Series migmatite included inCorredoiras Orthogneiss (monazite). (f) Combined data.

224 J. Abati et al. / Earth and Planetary Science Letters 165 (1999) 213–228

Fig. 5. Scanning electron microscope (SEM) images of zircon and monazite crystals from the samples analysed for U–Pb geochronology.(a) Euhedral zircon prism from granite G97-2. (b) Fragmented zircon prism from Monte Castelo Gabbro (G97-1). (c) Typical irregularmorphology (fragmented) of zircon from Monte Castelo Gabbro (G97-1). (d, e) Examples of euhedral monazite from the metapeliticgranulite G97-4. (f) Lentil-shape monazite from paragneiss G97-5.

close to the metamorphic peak. Monazite ages ofmetasediments are only slightly younger than themagmatic zircon ages and both can be considered

almost synchronous. This fact can be interpreted intwo ways.

(1) Monazites from metasediments could have

J. Abati et al. / Earth and Planetary Science Letters 165 (1999) 213–228 225

been formed by the thermal influence (contact meta-morphism) of the plutonic bodies during their intru-sion. Then, their ages would be dating magmatism,not metamorphism, and the younger ages of mon-azites could be explained by a lower closing tem-perature with respect to zircons. Monazites would bethen contact-metamorphism relicts that were over-grown by the biotite lepidoblasts during a later re-gional metamorphism.

(2) Monazites could reflect a metamorphic eventimmediately following the intrusion of the igneousbodies.

The former possibility has been ruled out forseveral reasons.

(1) The metasedimentary rocks were collectedover a wide area of the uppermost IP unit, andin some cases (e.g. samples G97-5 and G97-6) faraway from the Ordovician plutons (Fig. 2). It seemsunlikely that contact metamorphism affected all thesedimentary series, including zones distant from theintrusions.

(2) The metamorphism subsequent to the intru-sion of the igneous bodies reaches the granulitefacies in the deepest parts of the IP uppermost unit.This fact can be observed for example in the MCG,which is deformed by granulite-facies shear zones(see Section 2.3.2). The temperature conditions typ-ical of the granulite facies are clearly above theclosing temperature of the U–Pb system in mon-azites [8–10], and in consequence a metamorphismwith these characteristics should lead to the totalresetting, or at least a partial resetting of the system.Therefore, the age of the high-grade metamorphismcannot be later than the age of the monazites.

The previous arguments support the existence of ahigh-grade metamorphic event immediately follow-ing the intrusion of the igneous bodies. The rutileages of 380–390 Ma imply a great time span fromthe high-grade event if they were interpreted ascooling ages. The closure temperature of the U–Pbsystem in rutile is estimated around 450ºC [41] lead-ing to an unrealistically slow cooling rate. Therefore,we interpret this age as a new thermal event between450º and 650ºC, probably related to the accretionand stacking of the uppermost unit to the Variscanorogenic wedge [4].

4. Significance of the new ages in theinterpretation of the upper units

The U–Pb isotopic ages of the Monte CasteloGabbro, the granitoid rocks and the high-grademetasedimentary rocks, clustering between 493 and500 Ma, indicate that emplacement of the igneousbodies was immediately followed by their burialand by a regional Barrovian-type metamorphism thatreached granulite facies. Furthermore, we must keepin mind that the chemistry of the MCG points to avolcanic arc setting, but also that the chemistry ofthe gabbros of the HP–HT units [22,23], point toan intracontinental extensional regime of roughly thesame age [5].

A convergent scenario seems to fit the differ-ent data. We suggest that the uppermost IP unitrepresents an accretionary complex, built with sed-iments deposited adjacent to a volcanic arc or ina previously passive continental margin. The gab-bros resulted from the subduction of the oceaniclithosphere, whereas the granitoids developed bypartial melting of the thickened wedge, and werethemselves, together with the gabbros, buried andmetamorphosed shortly after their emplacement. Theextensional regime indicated by the chemistry of thegabbros of the HP–HT units [22,23], may reflectthe separation of the volcanic arc from the conti-nental realm where it was initially developed. TheMORB-type REE pattern of eclogites [21] in theHP–HT units may indicate that oceanic lithospherewas generated in a back-arc setting.

This Early Ordovician plate convergence posesa problem in the context of Gondwana–Laurentiainteractions. It is true that the new plutonic andmetamorphic ages correlate with the Taconic eventin the Appalachians and contemporaneous events(Finnmarkian, Grampian) in other parts of the Cale-donian belt. But these episodes are commonly as-cribed to convergence within the Iapetus ocean, andoccurred along the northern side of Avalon [12,42].By that time, the northern margin of Gondwanawas apparently undergoing extension, leading to theseparation of Avalon and the opening of the Rheicocean [11,12]. Avalon is the outermost emergentterrane in the margin of the North Atlantic Oceanopposite to the Iberian Peninsula, and was adjacentor very close to western Iberia before the break up

226 J. Abati et al. / Earth and Planetary Science Letters 165 (1999) 213–228

of Pangea (Fig. 1). Characterised by its Gondwananprovenance and Pan-African deformation and mag-matism ([43], and references therein), no Ordovicianorogenic events have been reported for Avalon.

A possible explanation is that oblique move-ments between Laurentia and Gondwana, and=or acomplex scenario involving several microcontinents,could have inserted an exotic volcanic arc or a con-vergent plate margin in the Rheic ocean, betweenGondwana and Avalon [44]. However, the devel-opment of an Ordovician convergent margin andassociated magmatism within Gondwana itself hasbeen documented at least for the central Andes inArgentina [45], and Early Ordovician metamorphicages have been found in the Variscan basement ofcentral Europe and the Alps, also in relation with themargin of Gondwana [46]. Furthermore, Sandemanet al. [47] have found rocks of volcanic-arc affin-ity and similar protolith age (499C8

�3 Ma) adjacentto the Lizard ophiolite, in the Variscan belt of SWEngland. Consequently, the possibility that the oro-genic episode identified in the uppermost units of theIberian allochthon could have occurred in either theGondwana or Rheic domains cannot be ruled out.

Martınez Catalan et al. [4] put forward an accre-tionary model for the Iberian allochthonous com-plexes. According to this model, the continentalmargin of Laurentia registered, after the early amal-gamations of volcanic arcs related to the Taconicevent, the successive incorporation of Avalon, piecesof the Rheic oceanic lithosphere, and the collision ofthe outermost margin of Gondwana. Now, we shouldadd an accretionary complex, probably related toan Early Ordovician island arc, which subsequentlybecame involved in the Variscan convergence andaccretion. An updated version of the Variscan accre-tionary history can be seen in Martınez Catalan et al.[48]. However, a version including the position andpolarity of subduction of the former Early Ordovi-cian accretionary complex cannot be drawn yet, dueto the lack of reliable data.

The Iberian allochthonous units were accretedduring the Early and Middle Devonian [2,4,20,24].The 390–380 Ma ages obtained from two rutile frac-tions of the metapelitic granulite included in MonteCastelo Gabbro, may reflect cooling following thefirst Variscan tectonothermal event in the uppermostunit. The HP–HT upper units registered an Early

Devonian metamorphic episode, between 405 and390 m.y. ago [5–7]. It is not clear if these ages cor-respond to the high-pressure and high-temperatureevent or not. Our discovery of a Lower OrdovicianBarrovian metamorphism in the IP uppermost unitsuggests that an older age is also possible for theHP–HT event, as was proposed by Kuijper [26] andPeucat et al. [21]. In any case, we suggest that the405–390 Ma ages reflect the accretion of the upperunits to the wedge under development at the southernmargin of Avalon. Their deformation and metamor-phism would be related either to subduction, if theHP event is Early Devonian, or to underthrusting andcrustal thickening, if it is Early Ordovician.

5. Conclusions

An Early Ordovician magmatic and metamor-phic event, constrained between 493 and 500 Ma,has been identified by U–Pb geochronology in theuppermost unit of the Ordenes Complex, in NWSpain. The Barrovian character of the metamor-phism clearly points to crustal thickening and, con-sequently, reveals a compressional tectonic event.Taking into account the chemical signature of thegabbros and the flyschoid character of the metased-iments, the uppermost units of the allochthonouscomplexes in the Iberian Peninsula, occurring abovethe ophiolites marking the suture, are consideredpieces of an Early Ordovician accretionary complex.This implies a relationship with a convergent platemargin, and suggests a volcanic arc as the moreprobable setting for these uppermost units. The un-derlying HP–HT upper units, also in the hangingwallto the suture, may be related to the same volcanicarc, and former interpretations of Early Ordovicianisotopic data as metamorphic ages should be consid-ered again, in the light of the new results from theuppermost unit of the Ordenes Complex.

Convergent plate boundaries seem to have ex-isted in other marginal areas of Gondwana duringthe Ordovician and, in particular, have been iden-tified in the Variscan basement of central Europeand the Alps. In spite of the dominantly extensionalregime that is often assumed for this period in theNE margin of the Paleozoic supercontinent (openingof the Rheic ocean), very little is known of the de-

J. Abati et al. / Earth and Planetary Science Letters 165 (1999) 213–228 227

tailed plate configuration and kinematics of the Rheicocean and the Avalon microcontinent(s). In this con-text, our data support the existence of short-livedconvergent plate boundaries in the realm betweenLaurentia and Gondwana during the early Paleozoic,but more data will be necessary to constrain theirextent, geometry and polarity of subduction.

Acknowledgements

We are grateful to K. Brueckner, Ph. Matte andK. Mezger for critical review of the manuscript thathelped us to improve its quality. Support for thiswork was provided by grants PB94-1396-CO2 andAPC1997-0011 of the Spanish DGICYT. [CL]

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