serie NOVA TERRA nº 49

143 crust as the dominant source for sediment (continental arc), (3) minor contrast among the location of source areas (indicated by Pb and Nd systematics), and (4) derivation from an old craton inferred from Nd isotopic data and U – Pb detrital zircon age populations ( Linnemann and Romer, 2002; Linnemann et al., 2004, 2008 ). Previous correlations established between Ediacaran series of the Bohemian Massif and SW Iberian Massif suggested a diachronic evolution along the northern margin of Gondwana during the late Neoproterozoic, including the development of a continental magmatic arc system and the opening of back-arc basins at the periphery of the West African Craton ( Linnemann et al., 2008; M.F. Pereira et al., 2006 ). Given the correlation of Ediacaran rocks from NW Iberia and SW Iberia suggested in this work, similar subduction-related geotectonic process- es may have also affected the metasedimentary rock series exposed in NW Iberia during the Neoproterozoic (e.g., Díez Fernández et al., 2010 ). The similarities shown by the Ediacaran series involved in the Variscan orogeny, both in SW Iberia, NW Iberia, and Central Europe, strengthen the idea of a rather continuous peripheral orogen that devel- oped along the margin of Gondwana in Ediacaran times ( Linnemann et al., 2007; Nance and Murphy, 1994 ). Some authors believe that this orogen and the terranes that derived from it can be followed up to base- ment rocks in China (in the so-called Asian or Chinese terranes; Stamp fl i et al., 2013; von Raumer et al., 2015 ), meaning that comparable units might be found along thousands of kilometers along the paleomargin of Gondwana. 5.2. A single Precambrian terrane? The multi-proxy evaluation of the Ediacaran series of the basal alloch- thonous units of the Iberian Massif reveals striking similarities in their tectonostratigraphy, metamorphic and microstructural evolution, geo- chronology of main tectonic events, protolith ages, provenance of their metasedimentary rocks, and whole-rock and isotope geochemistry. They reinforce recently proposed structural correlations between high-P allochthonous terranes of the Iberian Massif ( Fig. 1 ; Díez Fernández and Arenas, 2015 ), and put forward the existence of a rather distinctive Ediacaran series as the base to a far-traveled tectonic pile made of continental and ophiolitic terranes with Gondwanan derivation. These particular series, the Ediacaran basal allochthonous units, would consist of metagreywackes and albite-bearing schists and paragneisses (showing clear Bouma cycles in the less-deformed terms), layers of black (organic- and heavy minerals-rich) metasedimentary rocks, lenses of ma fi c rocks (eclogites, variably retrogressed eclogites, and amphibolites), and bodies of orthogneisses of Cambrian-Ordovician age. Sm – Nd model ages of its metasedimentary rocks would approach signi fi cantly a Paleoproterozoic range ( Fig. 6 ; 2.15 – 1.5 Ga), and their deposition would be connected to a peri-Gondwanan arc system built close to, and probably at the expense of the West African Craton. Some differences in the lithostratigraphy of the previously described basal allochthonous units, such as the absence of an Early Cambrian sequence containing marbles in NW Iberia, or the presence of black quartzites in the Évora Massif instead of black metapelites like in NW Iberia, may be explained by tectonic removal via Variscan faults and/ or by lateral variations within a single paleogeographic domain, respec- tively. The latter hypothesis could also provide an explanation for the slight variation observed in the isotopic contributions of the metasedimentary series in NW and SW Iberia. Note that we are dealing with a quite large piece of continental crust, which according to present-day structures extends for at least 400 km in both, cross- section and along strike ( Fig. 1 ), and is still rather homogeneous regard- ing most of its distinctive features. Building on this thought, the basal allochthonous units exposed in the Central Unit, located in between NW Iberia and the Évora Massif ( Fig. 1 ), show an intermediate compo- sition in terms of isotopic sources ( Fig. 6 ). Thus, there is a regional trend suggesting more (yet limited) contributions of juvenile material to- wards the occurrences located to the SW. The absence of sharp changes within the basal allochthon, such as that between the autochthon and the whole basal allochthon, suggests that the different Ediacaran series making the basal allochthon belonged to the same paleogeographic do- main, a section of a continental margin with little, though progressive variations in its sources. The age of post-Precambrian and pre-Variscan magmatism that affected the series of the basal allochthonous units is also slightly differ- ent. Its age range is Cambrian to Ordovician in NW Iberia (e.g., Díez Fernández et al., 2012a ) and in the Central Unit (e.g., Ordóñez Casado, 1998 ), whereas it is essentially Cambrian in SW Iberia (e.g., Chichorro et al., 2008 ). Such small variations in the age of early Paleozoic magmat- ic events may be due to slightly different positions of these series along the margin of Gondwana in Cambrian-Ordovician times. However, the synchrony of high-P Variscan metamorphism affecting those same se- ries suggests that they were spread laterally along the same continental margin in Late Devonian times, when they all were simultaneously incorporated to a continental subduction system. The Ediacaran series of the basal allochthonous units of the Iberian Massif are overlain by pieces of transitional and oceanic crust representing a Variscan intra-Gondwana suture zone ( Díez Fernández and Arenas, 2015 ). This suture zone probably resulted from the closure of a Devonian peri-Gondwanan basin opened in the course of the multi- stage collision between Gondwana and Laurussia in late Paleozoic times ( Arenas et al., 2014a, 2014b ). This suture zone is traceable at a regional scale by means of the Late Devonian high-P metamorphism that affects the upper part of its lower plate, i.e., the basal allochthonous units them- selves. The lithological and geochemical data of these high-P units, along with the regional tectonostratigraphic frame, provide solid grounds on which future research could identify more occurrences of that suture zone throughout the Variscan orogen. We are aware that the geological features here suggested to be used as an additional guide for tracing a suture zone might not be applicable to other sections of the Variscan orogen. Changes in the Precambrian paleogeography along-strike of the Gondwanamargin are likely, although pending proper description. However, these proxies seem to work quite well for the case of the Iberian Massif and thence, they should be consid- ered as a new approach to understand, evaluate, and acknowledge the fundamental role of the Precambrian geology in the reconstruction of Paleozoic geodynamics at a much larger scale. The correlation and geochemical af fi nity between the Ediacaran se- ries of the basal allochthonous units of the Iberian Massif pointed out here leads to the question whether various Precambrian continental terranes participated in the Late Devonian subduction system formed during the collision of Gondwana and Laurussia. We rather suggest that it was a single, yet slightly heterogeneous continental margin whose lower section consisted of Ediacaran immature sedimentary series deposited in relation to a Cadomian magmatic arc. 6. Conclusions We conclude that the lithostratigraphical, tectonometamorphic, geo- chronological, and geochemical af fi nities between the basal allochtho- nous units of NW and SW Iberia suggest that their high-P rocks de fi ne the base of an intra-Gondwana suture zone of the Iberian Massif, derived from a rather similar Ediacaran immature sedimentary series, formerly deposited along the same continental active margin related to a Cadomian magmatic arc. The isotopic sources of these series, as constrained by Sm – Nd systematics, are among the oldest of the Iberian Massif (2.15 – 1.5 Ga), indicating a very strong contribution of sources lo- cated in the West African Craton. The age of the fi rst deformation and metamorphism recorded in these series is Late Devonian (ca. 370 Ma) and is related to continental subduction (blueschist to eclogite facies metamorphism). Remnants of the basal allochthonous units are overlain by allochthonous ophiolitic units of different age and nature, and are found on top of sections of continental crust lacking of pervasive high- P and low- to medium-T Devonian metamorphism, i.e., separated by 297 R.D. Fernández et al. / Lithos 268 – 271 (2017) 285 – 301