Suprasubduction Zone Setting for the Youngest Rheic Ocean Floor

Care6n ophiolite, NW Spain: Suprasubduction zone setting for the youngest Rheic Ocean floor

Sonia Sanchez Martfnez "" Departamento de Petrologla y Geoqulmlca, Unlversldad Complutense, 28040 Madrid, Spain R·Icar d 0 Arenas } Florentino Dfaz Garcfa Departamento de Geologia, Unlversldad de OVledo, 33005 OVledo, Spain Jose Ram6n Martfnez Catalan Departamento de Geologia, Unlversldad de Salamanca, 37008 Salamanca, Spain Juan G6mez-Barreiro Department of Earth & Planetary SCiences, University of California at Berkeley, Berkeley, California 94720, USA Julian A. Pearce School of Earth, Ocean and Planetary SCiences, Cardiff University, CFto 3YE Cardiff, UK

ABSTRACT included in the GSA Data Repository.l The The Careon ophiolite (Galicia, NW Iberian MassiO shows lithological and geochemical geochemical features of these samples (Fig. 2; features suggestive of an origin in a suprasubduction zone setting. As with other Devonian Fig. DRI [see footnote I]) show that most of the ophiolites in the European Variscan belt, it was generated within a contracting Rheic Ocean. studied metabasites have compositions equiva­ This setting and the general absence of large Silurian-Devonian volcanic arcs on both of the lent to tholeiitic basalts (Fig. DRIA). Average Rheic Ocean margins strongly suggest that this ocean was closed by intraoceanic subduction rare earth element (REE) contents (Rg. DRl B) directed to the north. This subduction removed the older normal (N) mid-oceanic-ridge basalt of most rock types have concentrations around (MORB) oceanic lithosphere and gave rise to a limited volume of new suprasubduction zone ten times the chondrite abundances and ahnost oceanic lithosphere. The Careon ophiolite is a key element in understanding the evolution of flat chondrite-nonnalized REE patterns without the Rheic Ocean, which was the main oceanic domain that closed during the Paleozoic conver­ significant Eu anomalies. Only one group of gence of Gondwana and Laurussia, preceding the assembly of Pangea. metagabbro samples from the Care6n slice dis­ plays a more depleted pattern with a positive Eu

Keywords: Devonian ophiolite, suprasubduction, Galida, NW Spain, Rheic Ocean. anomaly (Ell/Ell*", = 1.38). According to their irrnnobile trace element INTRODUCTION beeninterpreted as having evolved in a suprasub­ characteristics, the metabasites show composi­ The Variscan suture exposed in Galida (NW duction zone setting (Dfaz Garcfa et al., 1999; tions transitional between normal (N)-MORB Iberian Massif, Spain) includes a variety of Pin et al., 2(02). and island-arc tholeiites in samples from the ophiolitic units emplaced between an upper ter­ V1hileprevious research identified the Care6n Orosa slice, and compositions closer to island­ rane with arc affinity (upper units) and a lower ophiolite as a suprasubduction type, it has not arc tholeiites in samples from the Care6n slice terrane interpreted to be the most external mar­ until now been possible to understand the mean­ (Rg. 2A; Rg. DRl C). Their immobile trace ele­ gin of Gondwana (basal units) (Fig. lA; Arenas ing of this mafic-ultramaficunit in plate tectonic ment patterns normalized to the average compo­ et al., 2007, and references therein). Two types evolution during the late Paleowic. The Rheic sition of N-MORB (Rg. 2B) vary from qllite fiat of ophiolites with different ages and represent­ Ocean was the largest oceanic domain that and close to one, in the samples from the Orosa ing distinct stages in the evolution of the Rheic closed during the convergence of Laurussia and slice and the dikes of the Care6n slice, to the Ocean have been described. The first group of Gondwana, preceding the assembly of Pangea slightly fractionated patterns of some gabbros ophiolitic units was formed in early Paleozoic (Mllrpby et al., 2006; Starnpfiiand Borel, 2002). from the Care6n slice, or those more depleted time, and it provides important information The continental margins originally oordering corresponding to the other set of gabbros from concerning the opening of the Rheic Ocean and this ocean can still be identified in the variably the same slice. Their most diagnostic feature is its early evolution. The second group (upper deformed Variscan belt, with disjointed outcrops the presence of a negative Nb anomaly, most ophiolitic units) is characterized by younger throughout Europe from SW Portugal to Bohe­ marked in the gabbros of the Care6n slice, ages and preserves a record of the final stages mia. The evolution of the Rheic Ocean reflects a which indicates an origin in a suprasubduction in the evolution of the Rheic Ocean. The Care6n complete Wilson's cycle, and it probably repre­ zone setting (Pearce, 1996). ophiolite is part of this second group. Locatedin sents one of the best examples of ocean dynam­ Relative abundances of isotopes,such as those the SE part of the 6rdenes Complex (Fig. lA), it ics in the framework of plate tectonics. of Srn and Nd, can also yield information aoout

includes gabbroic protoliths dated at 395 ± 2 Ma the original source material fromwhich the ophio­ (U-Pb in zircons by thermal ionization mass GEOCHEMICAL FEATURES litic rocks were likely derived. From the Sm-Nd s!",ctrornetry [TIMS]; Dfaz Garcia et al., 1999). OF THE MAFIC ROCKS isotopic data of Pin et al. (2002), initial �d values The ophiolite consists of three superimposed New samples from the Orosa slice (8 sam­ ranging between +7.1 and +9.2 (Fig. DRID, see tectonic slices (Orosa, Care6n, and Vilouriz, ples of strongly deformed amphioolite-fades footnote I) can be obtained for a crystallization respectively), of which the middle one is the metagabbros) and Care6n slice (24 samples of age for Care6n samples of 395 Ma. These val- thickest and shows the best-preservedophiolitic variably deformed dikes and metagabbros, all section (Fig. IB). The Care6n slice contains an of which have generally well-preserved igne­ lGSA Data Repository item 2007019, Tables ensemble of ultramafic rocks and ooth isotropic ous textures) were selected. They were first DRI-DR4 (whole rock major and trace element and pegmatoid gabbros, all intruded by dolerite examined petrographically and geochemically data of the Care6n metabasites) and Figure DRl dikes. This lithological sequence differs from to reject those displaying characteristics typi­ (additional geochemical diagrams), is available online at www.geosociety.org/pubs/ft2007.htm. or those considered characteristic of corrnnon mid­ cal of cumulates, which do not aid geochemi­ on request from [email protected] or Docu­ oceanic-ridge basalt (MORB)-type ophiolites cal identification of the tectonic setting (Pearce, ments Secretary, GSA, PO. Box 9140, Boulder, CO generated in divergent plate settings, and it has 1996). The analyses of these samples have been 80301, USA. CABO CABOORTEGAL ORTEOAL COMPLEX

A

I­ Z ::> z -0 w 0:: 43" �

ALLOCHTHONOU8 COMPLEXES: Textures _ Ultramallcrock8 c=J IP upperunn. _ Dlabase Coarse Q IlOIropicmetagabbro � .quant _ L..atediabase � PegmalDid metagabbro k�l�l�ll HP-HT uppar unilll � Serpentinite mylonil8 �=·- � LatepegmalDid metagabbro IS:l Metamorphicsole ii::: .. Upperophlolltleunits EillI Wehrlite silla • Corundum E3Serpentinite mylonile [Z] Felsicrocks � GarnetamphiboliIB � Lowerophlolltlcunits r i El Orosaslice B ���as :" .1 o 25 50 km amphibolites [=:J Basalunn. Foldlldand sheared = n r atoblastic - AmphiboliIB :a �:r � ultramaficlenses p l8 1[- Epidalll�bdile -+--- D 8omozasm6lange £1 I.J... - Gi'lenchill m:;:::,* PARAUTOCHTHON AND RELA11VE AUTOCHTHON: :::lE ___ ' IfTfoIaIict1 C:=l Schlstosedomain and Central Iberianzone � �=a�':!,eformed VARIICAN GRANROIDI:

II1II PosIkInemallc + + SynklnernaHc l + +1

Figure 1. A: Schematic geological map 01 allochthonous complexes 01 Galicia showing location of Care6n ophiolite. B: Detailed section 01 Care6n ophiolite: three tectonic slices (Orosa, Careon, and Vilouriz) repeat parts 01 an original oceanic lithosphere. lP-intermediate pres­ sure; HP-HT -high pressure-high temperature.

ues are similar to those of the depleted mantle, the Sltyza (Bohemian Massif; Dubiriska et al., European Variscan belt, but we believe they which implies that these rocks were derived from 20(4), Lizard (Cornwall; Nutman et al., 2001) are best interpreted as arc-related units. They the depleted mantle reservoir at or alxmt the time and, probably, Beja-Acebuches (SW Iberia; include mafic and ultramafic rocks alternating of their formation, and are therefore juvenile. Castro et al., 1996) ophiolites. The oceanic sec­ with abundant rnetasedirnents and even with 1be ENd values for the Care6n samples are com­ tions of these ophiolites developed around the large bodies of acid igneous rocks. Most, or all, parable to those of other Devonian ophiolites of Silurian-Devonian boundary. Therefore, it can of these units have protolith ages in the Cam­ the Variscan suture (e.g., Sltyza, Beja-Acebuches), be concluded that the more common ophiolites brian or at the Cambrian-Ordovician boundary, indicating a possible common origin. preserved in the Variscan belt were generated and they have been subjected to high-pressure shortly before the closure of the Rheic Ocean, (P) and high-temperature (T) metamorphism OPHIOLITESAND OTHER KEY and, hence, they are key markers also elucidat­ dated as Late Silurian (425-410 Ma by U-Pb and EL EMENTS IN THE EUROPEAN ing the tectonic setting of this closure and the 40AIP9 AI; Fernandez Suarez et al., 2007; G6rnez VARISCANBELT stages leading to the collision between Laurus­ Barreiro et al., 2006). Apart from the upper Ophiolites with a true oceanic origin are sia and Gondwana. allochthonous units in NW Iberia (Fig. lA), the uncommon in the European Variscan belt.Apart Other allochthonous units including mafic high-P and high-T units described in the French from the Care6n ophiolite, this group includes rocks have been described as ophiolites in the Massif Central, and other units in the Bohemian ca. 410 Ma by U-Pb and 4°Arj39 Ar (Fernandez-Suarez et al., 2007; G6mez-Barreiro et al., 2006), which prob­ ably records its accretion to the southern margin of Laurussia (Fig. 2C). Presently, questions remain as to whether a single (the Rheic Ocean; Linnemann et al., 2004; Murphy et al., 2006) or multiple oce­ anic domains (Franke, 2000; Winchester et al., 2oo2b) existed to the south of Avalonia in the Silurian and Devonian. In any case, it seems '" clear that the southern continental margin .00 .---_-_-_-_-_-_--. originally oordering the Rheic Ocean is pres­ ently represented by the Saxo-Thuringia and Ossa-Morena zones of the European Variscan belt. Important arc-related magrnatism ca. 360- 335 Ma has been studied in the Mid-German Crystalline Rise (Saxo Thuringia zone), where

Contln.nt. it has been attributed to subduction toward the ----�::;-�---- south (Altherr et al., 1999). The same age and t/Z////ZZlArce and �_ tectonic setting have been suggested for the ! Undllhrwnllm.d -.,Iccruet !����I Sup�uctJon_""'c cruet Late Devonian-Dinantian volcanism described in the French Massif Central (Pin and Paquette, Figure 2. A: Th-Hf-Ta diagram (Wood, 1980) for most representative metabasites 01 Care6n 2002). This subduction directed to the south and ophiolite. B: Normal-mid-oceanic-ridge-basalt-normalized trace element patterns 01 the the associated magrnatism are younger than the average composition 01 each type of metabasite; normalizing values are from Pearce (1996). C: Paleogeographic reconstruction 01 Rheic domain at Silurian-Devonian boundary (modi­ first deformation and coeval high-P metamor­ fied after Stampfli and Borel, 2002), showing generation of new oceanic lithosphere associ­ phism affecting the Gondwanan margin (dated ated with intra-oceanic subduction directed to the north. at 365-370 Ma in NV1 Iberia; Rodriguez et al., 2003). Therefore, they mainly occurred after the closure of the Rheic Ocean and have been inter­ Massif, such as the G6ry Sowie and Marianske Catalan et al., 1996). The glaucophane-bearing preted in relation to the opening and later clo­ Lame Massifs (Winchester et al., 2002a, and metamorphism can be recognized from the Ibe­ sure of a foredeep basin (Martinez Catalan et al., references therein), can be also included in this rian Massif to the Annorican and Bohemian 1997). The opening and closure of this basin group. All these arc-related units have been Massifs, suggesting a long continuity of the may explain the double vergence of the Variscan interpreted as fonning part of an individual ter­ Paleowic subduction affecting the Gondwanan belt (Matte, 1991). Tbe south-facing part of the rane distinguishable from Avalon, which rifted margin. This fact requires that the main oceanic belt shows the oldest tectonotherrnal evolution from Gondwana in the Early Ordovician dur­ domain, the Rheic Ocean, was closed at around and preserves information about the closure of ing the opening of the Rheic Ocean and finally 365-370 Ma (Late Devonian). It is important to the Rheic Ocean, which would have been coeval accreted to the southern margin of Laurussia stress the existence in the NW Iberian Massif with north-directed subduction (Matte, 1991; (G6mez Barreiro et al., 2(07). Accordingly, of two separate high-Pmetamorphicevents with Martinez Catalan et al., 1997). On the other this accretion can be dated as Silurian; it was different ages and characteristics, although with hand, the north-facingpart of the beltis younger, coeval with the high-P and high-T metamor­ a similar tectonic origin, as ooth are related to and its development was probably preceded by phism characteristic of these units, and probably subduction events below Laurussia. The older is subduction toward the south and probably also occurred just before the beginning of contrac­ a Late Silurian subduction event associated with by an important extensional event. The general tion in the Rheic Ocean. Other ophiolitic units the accretion of an arc-related terrane, and the absence of large Silurian-Devonianvolcanic arcs described in the European Variscan belt, such younger is a Late Devonian subduction affect­ associated with the closure of the Rheic Ocean as the Letovice-Rehberg ophiolites (HOCk et al., ing the Gondwanan margin. is also a characteristic of the European margin 1997), have uncertain chronology, but they also of Laurussia-Avalon. In this way, the Devonian show clear arc affinities, with abundant acid and DISCUSSION volcanic rocks described in southern Avalonia intermediate rocks that allow their distinction It is generally accepted that the Rheic Ocean in England have been interpreted as originating from the true oceanic ophiolites that developed beganto close during Ludlow times (ca. 420 Ma), in an extensional setting (Floyd, 1982), as have in the Silurian-Devonian limit. after the accretion of Avalonia to Laurussia and voluminous latest Devonian--Visean volcanism His pertinentto point out that the first Variscan the closure of the Iapetus Ocean (Stampfii and described in the continuation of Avalonia in the deformation in the most external margin of Borel, 2002). At the same time, another peri­ South Portuguese zone of SW Iberia. So, it can Gondwanahas been dated at 365-370 Main the Gondwanan arc-derived terrane is thought to be concluded that neither on the southern mar­ basal units of the NW Iberian Massif (Rodri­ have accreted to the southernmargin of Baltica­ gin of Avalonia, nor in the accreted arc-derived guez et al., 2003). This event was contempo­ Avalonia (G6mez-Barreiro et al., 2007). This terrane is there evidence of a large Late Silurian rary with high-P and low- to intennediate-T arc-derived terrane forms the upper allochtho­ or Devonian volcanic arc developed from sub­ metamorphism, frequently into the blueschist nous units in the NW Iberian Massif, and can be duction to the north. The same general absence facies with glaucophane-bearing mineral followed across the French Massif Central to the of large Silurian-Devonian volcanic arcs can be assemblages, which was likely generated dur­ Bohemian Massif (G6ry Sowie and Marianske concluded for the terranes located in the north­ ing subduction toward the north (i.e., below Lame Massifs). It is characterized by a high-P, ern margin of Gondwana, which were finally Laurussia and other accreted terranes; Martinez bigh-T metamorphic event dated at 425-410 Ma involved in the Variscan orogeny in Europe (Fig. 2C). However, our data require significant Journal of Geology, v. 107, p. 587-605, doi: Murphy, J.B., Gutierrez-Alonso, G., Nance, RD., generation of oceaniclithosphere in the Early to 1O.1OS6/31436S. Fernandez-Suarez, J., Keppie, J.D., Quesada, Middle Devonian, while the Rheic Ocean was Dubifiska, E., Bylina, P, Kozlowski, A, Dorr, W., C, Strachan, RA, and Dostal, J., 2006, Origin and Nejbert, K., 2004, U-Pb dating of serpen­ ofthe Rheic Ocean: Rifting along a Neoprotero­ evidently contracting. tinization: Hydrothermal zircon from a meta­ zoic suture?: Geology, v. 34, p. 325-328, doi: Considering the lithological section of the somatic rodingite shell (Sudetic ophiolite, SW 1O.1130/02206S.1. Care6n ophiolite, its suprasubduction zone geo­ Poland): Chemical Geology, v. 203, p. 183- Nutman, AP, Green, D.H., Cook, CA, Styles, chemical affinity, and the general characteristics 203, doi: 1O.1016/j.chemgeo.2003.1O.005. MT., and Holdsworth, RE., 2001, SHRIMP Fernandez-Suarez, J., Arenas, R, Abati, J., Martinez U-Pb zircon dating of the exhumation of the of the European Variscan belt, we su ggest that Catalan, JR, Whitehouse, M.J., and Jeffries, Lizard Peridotite and its emplacement over the Rheic Ocean was closed mainly by intra­ TE., 2007, U-Pb chronometry of polymeta­ crustal rocks: Constraints for tectonic models: oceanic subduction (Fig. 2C). This subduction morphic high-pressure granulites: An example Journal of the Geological Society of London, was probably located near its northern mar­ from the allochthonous terranes of the N'W v. 158, p. 809-820. in gin and involved consumption of old and cold Iberian Variscan belt, Hatcher, RD., ed., Pearce, J.A, 1996, A users' guide to basalt discrimi­ 4-D framework of continental crnst-Integrat­ nation diagrams, in Wyman, D.A, ed., Trace N-MORB-type oceanic lithosphere and the ing crustal processes through time: Geological element geochemistry of volcanic rocks: Appli­ generation of limited volumes of new oceanic Society of America Memoir (in press). cations for massive sulphide exploration: Geo­ lithosphere of suprasubduction zone type. This FIoyd, PA, 1982, Chemical variation in Hercyn­ logical Association of Canada, Short Course interpretation is compatible with the rarity or ian basalts relative to plate tectonics: Journal Notes 12, p. 79-113. of the Geological Society of London, v. 139, Pin, C, and Paquette, J.L., 2002, Le magmatisme absence of corrnnon N-MORB ophiolites, like p. 507-520. basique calcoalcalin d'age Devono-Dinantien those associated with divergent tectonic settings Franke, W., 2000, The mid-European segment of du nord du Massif Central, temoin d'une (Boudier and Nicolas, 1985), in the European the Variscides: Tectonostratigraphic units, ter­ marge active Hercynienen: Arguments geo­ Variscan belt.The model also explains the scar­ rane boundaries and plate tectonic evolution, in chimiques et isotopiques SrlNd: Geodinamica Franke, W., Haak, v.,Oncken, 0., and Tanner, Acta, v. 15, p. 63-77, doi: 1O.1016IS0985- city of older (pre-Silurian) ophiolites that could D., eds., Orogenic processes: Quantification 3111(01)01079-S. berelated to early stages of the Rheic evolution. and modelling in the Variscan Belt: Geologi­ Pin, C., Paquette, J.L., Santos Zalduegui, J.F., and Gil In addition, it seems that the absence of large cal Society of London Special Publication 179, Ibarguchi, J.I., 2002, An Early Devonian supra­ Silurian-Devonian volcanic arcs clearly requires p. 35-61. subduction zone ophiolite related to incipient the closure of the Rheic Ocean with an intra­ G6mez-Barreiro, J., Wijbrans, JR, Castiiieiras, P, collisional processes in the western Variscan Martinez Catalan, JR, Arenas, R, Diaz Gar­ belt: The Sierra de Care6n Unit, 6rdenes oceanic subduction zone. cia, F., and Abati, J., 2006, 4°ArJ39Ar laserprobe Complex, Galicia, in Martinez Catalan, JR, dating of mylonitic fabrics in a polyorogenic Hatcher, RD., Jr., Arenas, R, and Diaz Garcia, ACKNOWLEDGMENTS terrane of the N'W Iberia: Journal of the Geo­ F., eds., Variscan-Appalachian dynamics: The Financial support for this research has been pro­ logical Society of London, v. 163, p. 61-73, building of the late Paleozoic basement: Geo­ vided by Spanish projects B1E2001-0963-C02 and doi: 10.1144/0016-764905-012. logical Society of America Special Paper 364, CGL2004-0463-C02IBTE (Ministerio de Educaci6n G6mez-Barreiro, J., Martinez Catalan, JR, Arenas, p. 57-71. y Ciencia, Direcci6n General de Investigaci6n). It is R, Castiiieiras, P, Abati, J., Diaz Garcia, F., Rodriguez, J., Cosca, M.A, Gil lbarguchi, 1.1., and also a contribution to the International Geological and Wijbrans, J.R, 2007, Tectonic evolution Dallmeyer, RD., 2003, Strain partitioning and Correlation Programme 497, "The Rheic Ocean: Ori­ of the upper allochthon of the 6rdenes Com­ preservation of 4°ArJ39Ar ages during Variscan gin, Evolution and Correlatives." 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