serie NOVA TERRA nº 49

131 Geochemistry and tectonostratigraphy of the basal allochthonous units of SW Iberia (Évora Massif, Portugal): Keys to the reconstruction of pre-Pangean paleogeography in southern Europe Rubén Díez Fernández a , b , ⁎ , José Manuel Fuenlabrada c , Martim Chichorro d , M. Francisco Pereira b , Sonia Sánchez-Martínez a , José B. Silva e , Ricardo Arenas a a Departamento de Petrología y Geoquímica e Instituto de Geociencias (UCM, CSIC), Universidad Complutense de Madrid, 28040 Madrid, Spain b IDL/Departamento de Geociências, ECT, Universidade de Évora, Évora, Portugal c CAI de Geocronología y Geoquímica Isotópica, Universidad Complutense, 28040 Madrid, Spain d CiCEGE/Departamento de Ciências da Terra, Universidade Nova de Lisboa, Lisbon, Portugal e IDL/Departamento de Geologia, Faculdade de Ciências, Universidade de Lisboa, Lisbon, Portugal a b s t r a c t a r t i c l e i n f o Article history: Received 20 July 2016 Accepted 21 October 2016 Available online 16 November 2016 The basal allochthonous units of NWand SW Iberia are members of an intra-Gondwana suture zone that spreads across the Iberian Massif and was formed during the collision of Gondwana and Laurussia in the late Paleozoic. This suture zone is made of allochthonous terranes and is currently preserved as a tectonically dismembered ensemble. A multi-proxy analysis is applied to the basal allochthonous units of Iberia to test their af fi nity and po- tential usage for tracing a suture zone. A comparison of the lithostratigraphy, tectonometamorphic evolution, geochronology, and geochemical characteristics of the Ediacaran series of these units reveals striking af fi nities. They derive from rather similar immature sedimentary successions, deposited along the same continental mar- gin, and in relation to a Cadomian magmatic arc. Sm – Nd systematics indicates that the isotopic sources are among the oldest of the Iberian Massif (ca. 2.15 – 1.5 Ga), suggesting a very strong contribution from the West AfricanCraton. These Ediacaranserieswere affectedby high-P and low- tomedium-T metamorphism (blueschist to eclogite facies) during the Late Devonian (ca. 370 Ma). They occur below allochthonous ophiolitic sequences, and on top of autochthonous or parautochthonous domains lacking of high-P and low- to medium-T Devonian metamorphism, i.e., tectonically sandwiched between lithosphere-scale thrusts. The combination of all these characteristics makes theseparticular Ediacaranseriesdifferent fromtherest oftheterranesof the IberianMassif. Such singularity could be useful for tracing more occurrences of the same suture zone along the Variscan orogen, particularly in cases where its preservation and recognition may be cryptic. It also contributes to improve the paleogeographic reconstruction of the margin of Gondwana during the Ediacaran. © 2016 Elsevier B.V. All rights reserved. Keywords: Whole-rock geochemistry Sm – Nd systematics Allochthonous terranes Gondwana Suture zone Variscan orogen 1. Introduction Among the Iberian basement areas, today part of the European Variscan basement, several allochthonous terranes are assembled in a huge pile of Variscan nappes ( Arenas et al., 1986; Díez Fernández and Arenas, 2015; Martínez Catalán et al., 2007; Ribeiro et al., 1990; Ries and Shackleton, 1971 ). At the base of these nappes, a set of high-P units has been identi fi ed, meaning that the upper nappes were formerly sepa- rated from their relative autochthons by a continental subduction zone. The various exhumation mechanisms of such high-P assemblages imply contrasted structural relationships and severe internal structural distortion after exhumation (e.g., Beaumont et al., 2009; Chemenda et al., 1996; Faure et al., 2003 ). The quantity of crustal material returning from mantle depths or being eroded/obliterated during its return to shallower depths (e.g., Beaumont et al., 2009; Gerya et al., 2008 ) remain a source of uncertainty for paleogeographic reconstructions. The assess- ment of a pre-orogenic position and the paleogeography of subducted continental crust, consequently, cannot only be based on classical stratigraphy and metamorphic-structural criteria, but would need also restoration of tectonic events, using proxies able to resist severe deformation and recrystallization. If considering highly deformed basement areas, the chemical bulk composition of metasedimentary rocks does not change considerably, be- sides loss of water and carbon dioxide or in the case of metasomatism. The ratios of trace-elements are best preserved due to their low mobility Lithos 268 – 271 (2017) 285 – 301 ⁎ Corresponding author at: Departamento de Petrología y Geoquímica e Instituto de Geociencias (UCM, CSIC), Universidad Complutense de Madrid, 28040 Madrid, Spain. E-mail addresses: rudiez@ucm.es (R.D. Fernández), jmfuenla@pdi.ucm.es (J.M. Fuenlabrada), ma.chichorro@fct.unl.pt (M. Chichorro), mpereira@uevora.pt (M.F. Pereira), s.sanchez@geo.ucm.es (S. Sánchez-Martínez), jbsilva@fc.ul.pt (J.B. Silva), arenas@geo.ucm.es (R. Arenas). http://dx.doi.org/10.1016/j.lithos.2016.10.031 0024-4937/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos