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Variscan accretionary complex of northwest Iberia: Terrane correlation and succession of tectonothermal events
Jose R. Martlnez Catalan
Departamento de Geologia, Unlversldad de Salamanca, 37008 Salamanca, Spain Ricardo Arenas
Departamento de Petroiogia y Geoquimlca, Unlversldad Complutense, 28040 Madrid, Spain Florentino Diaz Garcia
Departamento de Geologia, Unlversldad de OVledo, 33005 OVledo, Spain Jacobo Abati
Departamento de Petroiogia y Geoquimlca, Unlversldad Complutense, 28040 Madrid, Spain
ABSTRACT The allochthonous terranes of northwest Iberia can be correlated with specific pale ogeo graphic realms of the continental masses and intervening oceans involved in the Variscan colli sion. Assuming that the existing ophiolites represent the suture formed by the closure of the Rheic ocean, the units in the fo otwall to the suture correspond to the outer edge of the Good waDa continentalmargin, and the units in the hanging waD are interpreted as fragments of the conjugate margin, represented by the Me guma terrane. This correlation establishes a precise link betweencircum-Atlantic terranes, and makes it possible to draw a relatively simple sce nario of the successive tectonothermal events recorded. Following the amalgamation of Avalon to LaUl'entia, the remaining outboard terranes underwent a progressive accretion to this conti nent that ended with the collision between Laurentia and Gondwana.
INTRODUCTION position in the nappe pile: basal, intermediate, granulite, amphibolite and greenschist facies. The allochthonousterranes of northwest Iberia and upper units. Because the intermediate units The ophiolitic nappes were stacked during the outcrop in five synforms or structural basins as show clear oceanicaffinities, they are referred to closure of the Rheic ocean. The thrustsshow east megaklippen (Fig. 1), and consist of a pile of as ophioliticand are interpreted as thesuture sep vergence, and the coeval amphibolite facies foli units, stacked in the first stages of theVariscan aratingtwo paleogeographic realms. ationwas formed�390--380m.y.ago (Dalhneyer orogeny, duringthe Lower and :MiddleDevonian. The basal unitsconsist of schists, paragneisses, et al., 1991, 40Ar/39 Ar on hornblende concen The nappe pile was subsequently thrust, during and alternations of igneous felsic and mafic trates), closely following oceanic crust genera the Upper Devonian-Lower Carboniferous, over rocks. Granitic and peralkaline orthogneisses tion as young as 395 Ma (Dunning et al., 1997, a Paleozoicsedimentary sequence that forms the have yielded ages of 480-460 Ma (Van Calsteren U-Pb on zircons). upper part of its relative autochthon (Martinez et al., 1979, Rb-Sr whole rock; Santos Zalduegui The upper units structurally overlie the ophio Catallmet al., 1996; Dal1meyer et al., 1997). et al., 1995, U-Pb on zircons). The magmatism lites and can be subdivided into high-pressure We outline a correlation of the allochthonous reflects an Ordovician rifting episode (Ribeiro and intermediate-pressure units. The high units ofnorthwest Iberia with specific paleogeo and Floor, 1987). Becausethere are no ophiolites pressure units occupy the lower relativestruc graphic realms of the continental masses in separating them from their relative autochthon, tural position.They consist of paragneisses and volved in the Variscan collision, as well as an in the basal units are considered part of the conti mafic and ultramafic rocks. The characteristic tervening ocean, and describe their accretionary nental margin of Gondwana. However, these rocks are metabasites, commonly garnet history, based mainly on the tectonothermal units are considered to be allochthonousbecause clinopyroxene granulites and eclogites, retro eventsrecognized in thedifferent unitsand on the there is a dramatic change in the metamorphic graded to the amphibolite facies. Gabbros occur isotopic age dataavailable. evolutionbetween them and their relative autoch in several stages of transformation, from virtu thon. The basal units registered an initial high ally undeformed and unmetamorphosed, to DESCRIPTION OF UNITS pressure metamorphic event, and are viewed as coronitic metagabbros and high-pressure gran The relativeautochthon formed part of the con part of the external edge of Gondwana after the ulites. In the less-deformedgabbros, subophitic tinentalmargin ofGondwanaduring the Late Prot opening of the Rheic ocean, having undergone a and diabase textures have been preserved, indi erozoicand Paleozoic.TIris marginpreserves evi westward subduction (in present coordinates) cating an emplacement at relatively shallow denceof the Cacbmian (Pan-African) orogeny as 380--370 m.y. ago, under an accretionary wedge levels. The chemical characteristics of the well as of a Cambrian-Ordoviciancontinental rift consisting of a pile formed by theophiolitic and gabbros, comparable to modem continental ing process that resulted in thepulling apart of the upper units (Arenas et al., 1995; Martinez tholeiites, are compatible with a continental rift Avaloo microcontinentfrom the Goodwana main Catallmet al., 1996). provenance (Van Calsteren and Den Tex, 1978). land and the opening of theRheic ocean (Cocks The ophiolitic units include basalts, pillow For the mafic rocks, Lower Ordovician ages, and Fortey, 1988). The extensional magmatic breccias, diabases, metagabbros, plagiogranites, between 490 and 480 Ma (Peucat et aL, 1990, activity has been dated at 490-465 Ma (U-Pb on amphibolites, and ultramafics. They occur in U-Pb on zircons), are considered to be protolith zircons: Lancelotet al., 1985; Gebauer, 1993). severalthrust sheets, and depict a wide variety of agesbased on differentiationby cathodolumines For the allochthon, it is useful to classifytheir metamorphic grades, ranging from low-grade, cence of magmatic and metamorphic zircon units into threegroups, according to theirrelative high-pressureconditions to intermediate-pressure domains (SchaJer et al., 1993, ion-microprobe CABO ORTEGAL
INTERMEDIATE -PRESSURE UPPER HIGH UNITS -PRESSURE } OPHIOLlTIC UNITS Figure 1. Geologic map of northwest Iberia outlining BASAL UNITS allochthonous units and the l I t ! RELATIVE AUTOCHTHON megaklippenwhere theycrop PALEOZOIC out: Cabo Ortegal, Ordenes, D and Malpica-Tui in Spain, � UPPER � PROTEROZOIC and Braganlfa and Morais in � PRE-VARISCAN Portugal. For location, see � ORTHOGNEISSES Figure 2.
o 50 km Is .H.I
SPANISH-PORTUGUESE BORDER:
U-Pb). Based on the age of the YOlmgest detrital Grand Banks of Newfoundland (Fig. 2). The of therelative autochthon (Lancelot et al., 1985; zircon grain (507 Ma), Schafer et al. (1993) sug outermost terraneidentified in the Appalachians Gebauer, 1993), are similar to those of the west gested that at least part of the metasediments are of Maritime Canada is Meguma, the main com African craton, and point to a common Gond younger than the Cambrian. The high-pressure ponent of which, the Meguma Group, consists wanan basement for Meguma, the upper and metamorphism has been dated as between 405 of a thick sequence of as much as 10 000 m of basal units, and the relative autochthon of Iberia. and 390 Ma, and the subsequent retrograde am Cambrian-Ordovician detrital rocks, often tur However, while the upper units are separated phibolite facies metamorphism is dated as biditic (Keppie and Dallmeyer, 1987). A Late from the basal units and their autochthon by 390-380 Ma (Schafer et aI., 1993; Santos Ordovician-Early Devonian bimodal vo1canism ophiolites, the Meguma terrane is in fault contact Zalduegui et aI., 1996, U-Pb on zircons, mon was reported by Keppie and Dalhneyer (1987), with the larger Avalon terrane, and no ophiolites azites, and titanites). and Dallmeyer and Keppie (1987) cited a 450 occur between them. The thick detritic sequence The overlying intermediate-pressure units m.y. old granite (Rb-Sr whole rock). that characterizes Meguma should be linked to a include a very thick sequence of terrigenous A correlation can be established between the large emerged area. We suggest that Meguma metasedimentaryrocks, large bodiesof amphibo upper unitsin the allochthonous terranesof Iberia may represent the southeastern continental lites, and intrusive augengneisses and gabbros. and Meguma. Both terranesinclude thick terrige margin of Avalon because of its position and also The augengneisses have yielded Lower Ordovi nous successions with commonflyschoid charac the age of its sediments, which fit the separation cian ages of 460-496 Ma (U-Pb on zircons: teristics and ages, and registered a lower Paleo of Avalon from Gondwana in the Ordovician Kuijper, 1980; Dallmeyer and Tucker, 1993). The zoic bimodal magmatism. Furthermore, the (Cocks and Fortey, 1988). Meguma is viewed gabbros commonly have subophitic and diabase similarity in the oldest ages obtained from upper as the conjugate rift pair of the Gondwana mar textures. The gradeof metamorphism varies from intercepts and inherited zircons from Meguma gin represented by the basal units and their the greenschist to the intermediate-pressure and the Iberian upper units, between2.7 and l.8 autochthon in Iberia. granulitefacies. Ga, reflects the provenance from a common source (Kuijper, 1980; Peucat et al., 1990; Krogh ACCRETIONARY HISTORY CORRELATION OF THE UPPER UNITS and Keppie, 1990; Dalhneyer and Tucker, 1993; The accretion of Avalon to Laurentia occurred In a late Paleozoic reconstruction of Pangea Schafer et aI., 1993; Santos Zalduegui et aI., probably duringthe early Silurian (Murphyet al., (Lefort, 1989), western Iberia lies close to the 1995). These ages, also found in the orthogneisses 1995), and the first cleavage in Meguma is dated as 415-390 Ma (Dalhneyer and Keppie, 1987). AVALONIAN PLATE VARISCAN BELT Sutface geology and deep seismic reflectionpro AVALON TERRANE la ALLOCHTHONOUS TERRANES filesshow that Avalon was underthrust westward (EMERGED/SUBMERGED) RELATIVE AUTOCHTHON beneath the Laurentian plate, perhaps as much as � MEGUMA TERRANE 200 km (Hatcher, 1989; Quinlan et aI., 1992). (EMERGED/SUBMERGED) EXTERNAL THRUST BELT AND FOREDEEP BASIN In this context, the deformation in the upper POSSIBLE EXTENSION unitsof the Iberian allochthonous terranesand, in mum OF MEGUMA TERRANE particular, the subduction of some of them, prob ably reflects the continuous understacking of LAURENTIA Avalonian and Meguma units toward the west following their accretion to Laurentia (Fig. 3). Because of their structural position, overlying the ophiolites, the high-pressure upper units are viewed as fragments of the outermost edge of Meguma(Fig. 3A). Thesubduction-related high pressureevent occurred between 405 and 390 Ma (Schtifer et aI., 1993; Santos Zalduegui et aI., 1996), and was coeval with deformation of the overlyingaccretionary wedge, of which Meguma was a part (Fig. 3B). The high-pressure upper unitsunderwent a de compressive episode during their emplacement onto the ophiolitic units. Synchronous amphibo lite facies metamorphism,dated as 390--380 Ma, was retrogressive in the upper unitsand prograde in the underlying ophiolites. In the latter, this 2. event reflects the westward subduction and Figure Reconstruction of Pangea around Iberia, based largely on Lefort (1989). RHZ-Rheno hercynian zone, SPZ-South Portuguese zone. Depths in meters refer to present-day isobaths. understacking of the oceanic lithosphere related to the closure of the Rheic ocean (Fig. 3C). This closure was immediately followed by the sub duction of the outer edge of Gondwana (Fig. 3D) because the end of the high-pressure event in the basal units has been dated as 374 Ma (Van Calsteren et al., 1979, Rb-Sr on phengites). The westward polarity of this subduction has been - �:R E�:� !�fo�� f!::!Uf!:: - deduced from the metamorphic gradient across - - - - I- - I- -u-p- - --�:�"'4- -:"- "':I- - - -I --- �- �: AMERICANORTH TERRANEAV jI TERRANEMAI PERUN IT 1 UNITS I UNITSASAL I REL the basal units (MartfnezCatalan et aI., 1996). I The continued underthrusting of continental I crust induced the exhumation of the basal units, and was accompaniedby the thinningof the over lying orogenic wedge, partly accomplishedby the development of normal detachments (Fig. 3E). This stage, ca. 375-365 Ma, gave way to a regime of intracontinentaldeformation that progressedto ward themore externalparts of theorogen (Fig. 3, F and G) and lasted until 290 Ma (Martfnez Catalan et al., 1996; Dalhneyer et al., 1997). This relatively simple scenario is complicated by transcurrent movements and escape tectonics, not considered in this essentially cross-sectional model. However, the existence of Devonian Carboniferous foredeep basins on both sides of the Ibero-Armorican arc must be taken into account. Known as the Rhenohercynian and OCEANIC CRUST AND OPHIOLlTES South Portuguese zones (Fig.2), these realmsare CONTINENTAL CRUST INVOLVED IN THE ALLOCHTHONOUS probablycontinuous all over the outer parts of the TERRANES OF NW IBERIA arc, and were at least partially flooredby oceanic lithosphere, represented by the Lizard and Beja LlTHOSPHERIC MANTLE Acebuches ophiolites. The Lizard ophiolite has been dated as Early Devonian (Davies, 1984, o 200 400 600 800 1000 km cl I.. . m cl L.. +.. 1 Sm-Nd in gabbro), and the Beja-Acebuches ophiolite is probably also Devonian (Oliveira, Figure 3. Accretionary evolution of the northwest Iberian allochthonous terranes across a trans 1990). According to Franke (1989), the basins verse from Iberia to Newfoundland. cannot be explained by early Paleozoic rifting Dalhneyer, RD., Ribeiro, A, and Marques, F., 1991, OrdenesComplex, Galicia, NW Spain: Tecton because extension-related volcanism continued Polyphase Variscan emplacement of exotic ics, v. 15, p. 106-121. lllltilthe mid-Visean (see also Oliveira, 1990),a terranes (Morais and Bragan<;a Massifs) onto M1J1"thy,J. B., Nance, RD., Keppie,J. D., Dostal,J., and Iberian successions: Evidence from 4OAr/39Ar Cousens, B. L., 1995, OdysseyofWest Avalonia: time when the Rheic ocean was apparently mineralages: Lithos, v. 27, p. 133-144. Isotopic constraints forLate Proterozoic Ill-Early alreadyclosed. We support the interpretationthat Dalhneyer, RD., MartinezCatalan, J. R, Arenas, R, Silurian Paleogeography, in Hibbard,J. P., van they representa back-arc extension related to the OilIba-guchi, J. I., Gutierrez-Alonso, 0., Faria:;, Staal, C R, mdCavvood, P. 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Subsequently.they were MaiinezCatalan,J. R, 1997, A LowerDevonian Peucat, J. J., Bernard-Griffiths, J., Oil Ibarguchi,J. I., penetratively deformed by tlrrustsduring the Car ophiolite in the allochthonous complexes of the Dallmeyer, RD., Menot, R P., Comichet, J., and boniferous (Bames and Andrews, 1986; Fonseca Iberian Massif (\hrriscan Belt): V-Pb zircon Iglesias Ponce de Leon, M., 1990, Geochemical and Ribeiro. 1993). protolith age from the Careon ophiolite: Terra and geochronological cross section of the deep Abstracts, v. 9, p. lOO. Variscan crust: The Cabo Ortegal high-pressure Floyd, P., 1984, Chemical variation in Hercynian nappe (northwestern Spain): Tectonophysics, CONCLUSIONS basalts relative to plate tectonics: Geological v. 177, p. 263-292. The upper units of the northwest Iberian Society of LondonJournal, v. 139, p. 507-522. Quinlan,G M., Hall, J., W:tlliams, H., Wright, J. 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L., and Floor, P., 1987, Magmatismo to the suture, represent the conjugate outer mar RD., ed., Terranes inthe Circrnn-AtlanticPaleo peralcalino no Ma:i<;oHesperico: sua distribu.:;ao zoic orogens: Geological Society of America e significado geodin3mico, in Bea, F., Camicero, gin of Gondwana. The ensemble llllderwent a Special Paper 230, p. 67-90. A, Gonzalo, J. C, Lopez-Plaza, M., and progressive accretion tothe continental margin of Gebauer, D., 1993, Intra-grain zircon d::t:ing within the RodriguezAlonso, M. D., eels., Geologia de los Laurentiathat endedby the collision between this Iberian Massif: 0110 de Sapo augengneisses, granitoides y rocas asociadas del Macizo continentand Gondwana. The accretionalhis tory bimodal gneissesfrom the Massif de Guilleries Hesperico: M