Ediacaran–Palaeozoic tectonic evolution of the Ossa Morena and Central Iberian zones (SW Iberia) as revealed by Sm–Nd isotope systematics

Abstract

Sm-Nd isotopic analyses of Palaeozoic sedimentary and igneous rocks in the southwest Iberian Massif (western end of the European Variscan Belt) are presented in order to unravel its complex poly-orogenic evolution during the closure of the Rheic Ocean and the amalgamation of Pangea. The Gondwanan margin in southwest Iberia SW Iberia is subdivided into the Ossa Morena and Central Iberian zones, separated by the Badajoz-Córdoba Shear Zone which represents a cryptic suture zone between these terranes. The relationships between these terranes, and between units preserved within the suture zone (e.g. the Sierra Albarrana Group) during the Palaeozoic and Neoproterozoic are controversial. Sm-Nd isotopic studies of representative sedimentary sequences covering the entire pre-Variscan record of the Ossa Morena and Central Iberian zones show very similar characteristics from the uppermost Ediacaran onwards. These data indicate that their accretion to one another must have been completed by the Late Neoproterozoic-Ediacarin that time (an event assigned to Cadomian orogeny) and that they never separated substantially from each other since that time. The Sm-Nd isotopic composition of the Sierra Albarrana Group metasedimentary rocks is similar to that of the pre-Cadomian sequences of the Ossa Morena Zone (Serie Negra), suggesting derivation from a common source. The common provenance of the Palaeozoic sequences in the two zones is identical to that of the pre-Cadomian Serie Negra of the Ossa Morena Zone, which in accordance with the data presented herein and published U-Pb zircon data indicates a West African affinity.

Potential sources of Ediacaran strata of Iberia: a review

“…The T DM ages estimated for all Edicaran strata of Iberia ages are significantly older than their depositional ages, indicating a source mainly composed of old crustal rocks, with minor contribution of different proportions of juvenile material. The Malcocinado and San Jerónimo formations volcaniclastic rocks show εNd (i = 550) values ranging from +2.1 to −5.1 and T DM ages vary from 1.1 to 1.4 Ga, suggesting a mixed provenance consisting of juvenile and old, recycled crustal components (Lopez-Guijarro et al, 2008;Pin et al, 2002). The San Jerónimo Formation calc-alkaline volcanics (Cordoba andesites) yield εNd(i = 550) positive values ranging from +2.9 to +7.6 and T DM ages vary from 0.6 to 0.9 Ga (Pin et al, 2002).…”

“…Narcea Slates show εNd(550 < i < 600) values ranging from −2.0 to −3.4 and T DM ages of 1.3-1.5 Ga [27,87,88] not very different from the Schist-Greywacke Complex (Lower Series) yielding εNd(545 < i < 560) values from -0.4 to −5.3 and T DM ages of 1.1-1.7 Ga (Lopez-Guijarro et al, 2008;Nägler, Schäfer, & Gebauer, 1995;Sola et al, 2011;Tassinari, Medina, & Pinto, 1996;Ugidos et al, 2003). Serie Negra εNd(550 < i < 570) values are more negative varying from −5.5 to −12.4 and T DM ages from 1.5 to 1.8 Ga Lopez-Guijarro et al, 2008;Pereira, Medina, et al, 2006;Sola et al, 2011). The T DM ages estimated for all Edicaran strata of Iberia ages are significantly older than their depositional ages, indicating a source mainly composed of old crustal rocks, with minor contribution of different proportions of juvenile material.…”

“…The San Jerónimo Formation calc-alkaline volcanics (Cordoba andesites) yield εNd(i = 550) positive values ranging from +2.9 to +7.6 and T DM ages vary from 0.6 to 0.9 Ga (Pin et al, 2002). Chichorro et al, 2008;Fernandez-Suarez et al, 2013;Lopez-Guijarro et al, 2008;Nägler et al, 1995;Pereira, Medina, et al, 2006;Sola et al, 2011;Tassinari et al, 1996;Ugidos et al, 2003;2010): A -Chondrite normalised REE plot; B -εNd vs. time plots; εNd(i) calculated for the stratigraphic ages in the interval c. 600-545 Ma.…”

“…The analysis of the distribution of detrital zircons from SchistGreywacke Complex (Lower Series) greywackes sampled near the OMZ-CIZ boundary shows a predominance of Ediacaran ages following the distribution of zircon ages of Serie Negra and overlying calc-alkaline volcanism. Turbidites in the Schist-Greywacke Complex (Lower Series) that are more immature contain clasts of volcanic and plutonic rocks that most probably come from the late Ediacaran calc-alkaline magmatism of the OMZ (Lopez-Guijarro et al, 2008;Medina, 1996). Moreover, the distribution of the zircon (U-Pb) ages of Série Negra locally resembles that observed with regard to the Schist-Greywacke Complex (Lower Series), with Cryogenian-Ediacaran ages predominant (Figure 6).…”

“…Another possibility to explain the Tonian and Mesoproterozoic detrital zircons findings in the Ediacaran strata of Iberia may be the existence of a basement consisting of rocks with these zircon-forming events that is not known in the Cadomian belt. This speculative hypothesis admits that an 'exotic' terrain was probably transported from east (Saharan Metacraton and Arabian Nubian shield) through strike-slip movements along the NW margin of Gondwana (Fernandez-Suarez et al, 2013;Lopez-Guijarro et al, 2008), getting later exposed to denudation and thus contributing to a limited extent with detritus to the Ediacaran basins of Iberia.…”