serie NOVA TERRA nº 49

78 foliation (S 2 ) that affects the entire intermediate pressure upper units. In addition, the chronology of these two deformational events has been constrained by U – Pb dating of the dyke swarm and reinterpretation of previously reported detrital zircon ages ( Fernández-Suárez et al., 2003 ). As a result, a more complete knowledge of the structural evolution of the upper units has emerged that contributes to a better understanding of the Cambrian subduction processes affecting the margin of Gondwana, and thesubsequentriftingassociatedwiththeopeningoftheRheicOcean(see ( Murphy et al., 2009; Santosh et al., 2009 ), and references therein). The vergence of the D 1 folds has remained an open question since the visionary paper of Matte and Capdevila (1978) , but detailed study of the low-grade sector has revealed the existence of a fold nappe, the basic geometry of which consists of two antiform – synform pairs of west- vergent recumbent folds later folded into an upright Variscan antiform. This D 1 evolution suggests accretion below the volcanic arc, consistent with the development of a subduction zone below the external thinned margin of Gondwana, which has been previously proposed to account for the origin of the volcanic arc itself ( Arenas et al., 2007; Martínez Catalán et al., 2007; Abati et al., 2009 ). Subduction eventually led to the accretion at the base of the arc of oceanic slices, as is suggested by evidence from the western part of the Órdenes Complex, where a thick Cambrian ophiolitic sequence (the Bazar ophiolitic unit) has been emplaced immediately below the arc-related plutonic and metasedimentary successions ( Sán- chez Martínez, 2009 ). The development of the D 1 fold nappes was followed by a general top- to-the-north shearing event (D 2 ) that, in the study area, is only weakly developed and disappears upwards through a gradual deformation front. Field observations indicate that ma fi c dykes were emplaced at the end of the D 2 shearing. The 510 Ma protolith age obtained for the ma fi c dykes and the maximum depositional age obtained for the turbiditic succession (510 – 530 Ma), restrict the earliest stages of the tectonic evolution of the upper units to a short time interval. The regional S 2 foliation of the uppermost series can be traced downwards into the intermediate pressure upper units, where it developed under increasing temperature conditions with depth, and shows a metatexitic appearance dated at c. 490 – 500 Ma using U – Pb geochronology in monazites ( Abati et al., 1999 ). This tectonic change is best interpreted in the more general framework of accretion and break up along the northern Gondwana margin. Recently, Nance et al. (2002) , Murphy et al. (2006) and Linnemann et al. (2008) proposed a geodynamic model for this margin, in which latest Neo- proterozoic to Early Cambrian ridge – trench collision, leads diachronously to the termination of subduction and the generation of a continental transform during the Cambrian, along which the Rheic Ocean later opened. Our data for the arc-derived upper terrane in NW Iberia indicate that accretionary processes, recorded by the D1west-vergent fold nappes, continued until 510 Ma and that this age marks the change to a period of north-directed extension, anatexis, intrusion of arc plutonics, and emplacement of a ma fi c dyke swarm, eventually linked to ridge subduction. A similar family of ma fi c dykes with a comparable age has been described in the tectonic blocks of the Somozas mélange, the basal tectonic mélange of the allochthonous complexes of NW Iberia ( Arenas et al., 2009 ). This mélange includes highly dismembered remnants of a volcanic arc similar to that represented in the upper units. In this case, the diabasic dykes show compositions typical of island-arc tholeiites and intrude submarine volcanic successions with calc-alkaline composition. It has been suggested that the intrusion of the tholeiitic swarm de fi nes the transition to an extensional regime in a mature calc-alkaline arc, which would favour the development of intra-arc basins and marks the onset of the opening of the Rheic Ocean. Acknowledgements Financial support for this research was provided by Spanish project CGL2007-65338-CO2/BTE (Ministerio de Ciencia e Innovación). We would like to thank the SUMAC staff at Stanford University, especially Joe Wooden and Ariel Strickland, for their help in operating the SHRIMP instrument and in interpreting the results. P. Castiñeiras's stay at the SUMAC facility was fi nanced with a “ Profesores UCM en el extranjero ” travel aid. The authors also thank to two anonymous referees for insightful review of the manuscript. This study is a contribution to the IGCP Project 497: “ The Rheic Ocean: Origin, evolution and correlatives ” . References Abati, J., Dunning, G.R., Arenas, R., Díaz García, F., González Cuadra, P., Martínez Catalán, J.R., Andonaegui, P., 1999. Early Ordovician orogenic event in Galicia (NW Spain): evidences from U – Pb ages in the uppermost unit of the Órdenes Complex. Earth and Planetary Science Letters 165, 213 – 228. Abati, J., Castiñeiras, P., Arenas, R., Fernández-Suárez, J., Gómez-Barreiro, J., Wooden, J., 2007. Using SHRIMP zircon dating to unravel tectonothermal events in arc environments. The early Palaeozoic arc of NW Iberia revisited. Terra Nova 19, 432 – 439. Abati, J., Gerdes, A., Fernández-Suárez, J., Arenas, R., Whitehouse, M.J., Díez Fernández, R., 2009. Magmatism and early-Variscan continental subduction in the northern Gondwana margin recorded in zircons from the basal units of Galicia, NW Spain. Geological Society of America Bulletin 122 (1 – 2), 219 – 235. Anders, E., Grevesse, N., 1989. Abundances of the elements: meteoritic and solar. Geochimica et Cosmochimica Acta 53, 197 – 204. Arenas, R., Martínez Catalán, J.R., Sánchez Martínez, S., Fernández-Suárez, J., Andonaegui, P., Pearce, J.A., Corfu, F., 2007. The Vila de Cruces Ophiolite: a remnant of the Early Rheic Ocean in the Variscan suture of Galicia (Northwest Iberian Massif). Journal of Geology 115, 129 – 148. Arenas, R., Sánchez Martínez, S., Castiñeiras, P., Jeffries, T.E., Díez Fernández, R., Andonaegui, P., 2009. The basal tectonic mélange of the Cabo Ortegal Complex (NW Iberian Massif): a key unit in the suture of Pangea. Journal of Iberian Geology 35, 85 – 125. Berthé, D., Choukroune, P., Jeghouzo, P., 1979. Orthogneiss, mylonite and non coaxial deformation of granites: the example of the South Armorican Shear Zone. Journal of Structural Geology 1, 31 – 42. Black, L.P., Kamo, S.L., Allen, C.M., Davis, D.W., Aleinikoff, J.N., Valley, J.W., Mundil, R., Campbell, I.H., Korsch, R.J., Williams, I.S., Foudoulis, C., 2004. Improved 206 Pb/ 238 U microprobe geochronology by the monitoring of a trace-element-related matrix effect, SHRIMP, ID-TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards. Chemical Geology 205, 115 – 140. Castiñeiras, P., 2005. Origen y evolución tectonotermal de las unidades de O Pino y Cariño (Complejos Alóctonos de Galicia): Nova Terra, vol. 28. 279 pp. Condie, K.C., Belousova, E., Grif fi n, W.L., Sircombe, K.N., 2009. Granitoid events in space and time: constraints from igneous and detrital zircon age spectra. Gondwana Research 15, 228 – 242. Corfu, F., Hanchar, J.M., Hoskin, P.W.O., Kinny, P., 2003. Atlas of zircon textures. In: Hanchar, J.M., Hoskin, P.W.O. (Eds.), Zircon. Mineralogical Society of America, Washington: Reviews in Mineralogy and Geochemistry, vol. 53, pp. 468 – 500. Fernández Suárez, J., Arenas, R., Abati, J., Martínez Catalán, J.R., Whitehouse, M.J., Jeffries, T.E., 2007. U – Pb chronometry of polymetamorphic high-pressure granulites: an example from the allochthonous terranes of the NW Iberian Variscan belt. In: HatcherJr. Jr., R.D., Carlson, M.P., McBride, J.H., Martínez Catalán, J.R. (Eds.), 4-D framework of continental crust: Geological Society of America Memoir, vol. 200, pp. 469 – 488. Fernández-Suarez, J., Corfu, F., Arenas, R., Marcos, A., Martínez Catalán, J.R., Díaz García, F., Abati, J., Fernández, F.J., 2002. U – Pb evidence for a polymetamorphic evolution of the HP – HT units of the NW Iberia Massif. Contributions to Mineralogyand Petrology 143, 236 – 253. Fernández-Suárez, J., Díaz García, F., Jeffries, T.E., Arenas, R., Abati, J., 2003. Constraints on the provenance of the uppermost allochthonous terrane of the NW Iberian Massif: inferences from detrital zircon U – Pb ages. Terra Nova 15, 138 – 144. Fuenlabrada, J.M., Arenas, R., Sánchez Martínez, S., Díaz García, F., Castiñeiras, P., 2010. A peri-Gondwanan arc in NW Iberia. I: isotopic and geochemical constraints to the origin of the arc — the sedimentary approach. Gondwana Research 17 (2 – 3), 338 – 351. Gomez Barreiro, J., 2007. La Unidad de Fornas: Evolución tectonometamór fi ca del SO del Complejo de Órdenes: Nova Terra, vol. 32. 334 pp. Gómez Barreiro, J., Martínez Catalán, J.R., Arenas, R., Castiñeiras, P., Abati, J., Díaz García, F., Wijbrans, J.R., 2007. Tectonic evolution of the upper allochthon of the Órdenes complex (northwesternIberianMassif):structuralconstraintstoapolyorogenicperi-Gondwanan terrane.In:Linneman,U.,Nance,R.D.,Kraft,P.,Zulauf,G.(Eds.),TheevolutionoftheRheic Ocean: from Avalonian – Cadomian active margin to Alleghenian – Variscan collision: Geological Society of America Special Paper, vol. 423, pp. 315 – 332. Hoskin, P.W.O., Schaltegger, U., 2003. The composition of zircon and igneous and metamorphic petrogenesis. In: Hanchar, J.M., Hoskin, P.W.O. (Eds.), Zircon. Mineral- ogicalSocietyofAmerica,Washington:ReviewsinMineralogyandGeochemistry,vol. 53, pp. 27 – 62. Ireland, T.R., Williams, I.S., 2003. Considerations in zircon geochronology by SIMS. In: Hanchar, J.M., Hoskin, P.W.O. (Eds.), Zircon. Mineralogical Society of America, Washington: Reviews in Mineralogy and Geochemistry, vol. 53, pp. 215 – 241. Korotev, R.L., 1996. A self-consistent compilation of elemental concentration data for 93 geochemical reference samples. Geostandards Newsletter 20, 217 – 245. Linnemann, U., Romer, R.L., 2002. The Cadomian Orogeny in Saxo-Thuringia, Germany: geochemicalandNd – Sr – Pbisotopiccharacterizationofmarginalbasinswithconstraints to geotectonic setting and provenance. Tectonophysics 352, 33 – 64. Linnemann, U., Pereira, F., Jeffries, T.E., Drost, K., Gerdes, A., 2008. The Cadomian Orogeny and the opening of the Rheic Ocean: The diacrony of geotectonic processes 361 F. Díaz García et al. / Gondwana Research 17 (2010) 352 – 362