serie NOVA TERRA nº 49

89 Sm – Nd isotope geochemistry and tectonic setting of the metasedimentary rocks from the basal allochthonous units of NW Iberia (Variscan suture, Galicia) José Manuel Fuenlabrada a , Ricardo Arenas b , ⁎ , Rubén Díez Fernández c , Sonia Sánchez Martínez b , Jacobo Abati b , Alicia López Carmona b a CAI de Geocronología y Geoquímica Isotópica, Facultad de Geología, Universidad Complutense de Madrid, 28040 Madrid, Spain b Departamento de Petrología y Geoquímica e Instituto de Geociencias (UCM-CSIC), Universidad Complutense de Madrid. 28040 Madrid, Spain c Departamento de Geología, Universidad de Salamanca, 37008 Salamanca, Spain a b s t r a c t a r t i c l e i n f o Article history: Received 12 November 2011 Accepted 2 June 2012 Available online 9 June 2012 Keywords: Metasedimentary rocks Tectonic setting Sm – Nd isotope geochemistry Allochthonous complexes Variscan belt NW Iberia The basal units of the allochthonous complexes of NW Iberia are formed by thick metasedimentary rock se- quences intruded by granitoids, ranging in composition from calc-alkaline (c. 493 Ma) to minor alkaline – peralkaline massifs (c. 475 – 470 Ma), and ma fi c rocks. The granitoids were transformed into variably de- formed othogneisses and the associated ma fi c rocks were transformed into amphibolites, blueschists and eclogites during eo-Variscan high-P metamorphism dated at c. 370 Ma. Two different superimposed metasedimentary rock sequences can be distinguished. The lower sequence (maximum depositional age at c. 560 Ma) is mainly composed of metagreywackes, while the upper sequence (maximum depositional age at c. 500 Ma) consists of mica schists and other minor types. Major and trace element geochemistry of the metagreywackes of the lower sequence suggests that they were generated in relation to a peri-Gondwanan arc system built on the thinned continental margin, although some chemical transition to passive margin greywackes is also observed. This sedimentary sequence was probably deposited in an Ediacaran – Early Cam- brian back-arc setting or retro-arc setting, closer to the thinned platform of the continental margin. The geo- chemical features of the sedimentary rocks of the upper sequence suggest some af fi nity with passive margin sediments; they were probably deposited closer to the continental domain and to certain distance from the most active zones of the magmatic arc. The Nd model ages of 23 analysed samples are Paleoproterozoic and range between 1782 Ma and 2223 Ma (average value 1919 Ma). The Nd model ages are slightly younger in the upper sequence than in the lower sequence, but altogether they de fi ne a single population, and therefore the two metasedimentary rock sequences can be clearly related. Sedimentation probably took place within the same basin located in the continental platform of Gondwana, the main source areas of these sedimentary rocks did not change during the Late Neoproterozoic and Cambrian times. The Nd model ages are very old and they seem to be compatible with Paleoproterozoic or Archean source areas, with only minor participation of younger sources probably represented by intrusive Cadomian – Pan-African granitoids. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The interpretation of metasedimentary series involved in the suture zone of mountain belts is a key issue to unravel the origin of the most relevant terranes, frequently with different tectonic settings and some- times characterized by an important exoticism ( Hatcher et al., 2007; Merschat and Hatcher, 2007; Searle, 2007 ). These are generally highly deformed and variably metamorphosed azoic metasedimentary series; consequently little information can be obtained from traditional studies based in paleontology or stratigraphy. In these cases, provenance anal- ysis based in U – Pb dating of detrital zircons can be a complement to major and trace element geochemistry and Sm – Nd systematics ( Díez Fernández et al., 2010; Drost et al., 2004; Linnemann and Romer, 2002; Linnemann et al., 2004 ). In favourable situations, these data allow to determine the tectonic setting and location of the sedimentary paleo-basins. Sedimentary series formed by greywackes are particularly useful for these studies since the analysis of immobile elements during post-sedimentary and orogenic processes has been proven to be suit- able for constraining the tectonic setting ( Bathia and Crook, 1986 ). This paper presents a case-study of the metasedimentary rocks from the basal units of the allochthonous complexes of NW Iberia, one of the far-travelled terranes involved in the Variscan suture that are exposed in southern Europe ( Martínez Catalán et al., 2009 ). The structural and metamorphic characteristics of this crustal-derived terrane indicate Lithos 148 (2012) 196 – 208 ⁎ Corresponding author at: Departamento de Petrología y Geoquímica e Instituto de Geociencias (UCM-CSIC). Facultad de Geología, Universidad Complutense de Madrid, C/ José Antonio Novais, no 2, 28040 Madrid, Spain. Tel.: +34 639601919; fax: +34 915442535. E-mail addresses: jmfuenla@pas.ucm.es (J.M. Fuenlabrada), arenas@geo.ucm.es (R. Arenas), georuben@usal.es (R. Díez Fernández), s.sanchez@geo.ucm.es (S. Sánchez Martínez), abati@geo.ucm.es (J. Abati), alcarmona@geo.ucm.es (A. López Carmona). 0024-4937/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi: 10.1016/j.lithos.2012.06.002 Contents lists available at SciVerse ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos