Nova Terra 52

469 International Journal of Earth Sciences (2021) 110:467–485 13 isotopic sources. This methodology has been scarcely used to investigate the provenance of the azoic series involved in the Variscan Orogen, unlike the many published papers focused on U–Pb geochronology of detrital zircon grains (e.g., Fernández-Suárez et al. 2007; Abati et al. 2010). Only combined U–Pb–Hf data on detrital zircon can provide com- parable results in this sense (Avigad et al. 2012; Morag et al. 2012). In the Iberian Massif, whole-rock major and trace element geochemistry and Nd isotopic geochemistry of siliciclas- tic rocks have been used in recent studies to investigate the tectonic setting and provenance of the sedimentary series deposited during the Ediacaran–Cambrian transition (Ugi- dos et al. 2003; Pereira et al. 2006; Guijarro et al. 2008; Fernández-Suárez et al. 2014; Fuenlabrada et al. 2016, 2020). However, the possible correlations of these Ediacaran series along the Variscan Orogen, between the Iberian and Bohemian massifs, and based on geochemical grounds that analyse the rock as a whole have not been explored in detail. In this sense, this work will improve the knowledge of the oldest siliciclastic series described in the Iberian Massif, the so-called Serie Negra Group (Carvalhosa 1965), including a new whole-rock geochemical and Sm–Nd isotopic database. These data are compared with the available isotopic (Nd) data from equivalent series described in the Bohemian Mas- sif (Linnemann and Romer 2002), with the aim of testing the affinity of their isotopic sources between the Ediacaran series from both currently distant regions. Such comparison may help to refine reconstructions for the paleobasins once located along the (North African) margin of Gondwana that are now (dismembered and) spread as disconnected indi- vidual blocks of Cadomian basement throughout central and western Europe (Stephan et al. 2019a, b). Geological setting In the SW Iberian Massif, the lower part of the stratigraphic record of the continental allochthons of the Ossa–Morena Complex (Díez Fernández and Arenas 2015; Arenas et al. 2016b; Fig. 2) is part of a Cadomian basement that encom- passes rocks from Neoproterozoic to Early Cambrian age. The Serie Negra Group (Carvalhosa 1965) is composed by a succession of metapelites and metagreywackes, interbed- ded with metamafic rocks and some layers of calc-silicate rocks and black quartzites. The thickest exposures of the Serie Negra Group occur in three key areas along NW–SE trending regional structures. From north to south these areas are: the Obejo-Valsequillo Domain (Ordóñez-Casado 1998; Bandrés 2001), the Olivenza–Monesterio antiform (Egui- luz and Quesada 1980; Montero et al. 1999) (Fig. 2) and the Aracena-Almadén de la Plata region (Ábalos 1987). Metamorphism in the Serie Negra Group ranges between greenschists facies conditions and pervasive migmatization. Among these key areas, the Olivenza–Monesterio antiform contains the most complete and well-preserved cross section to the Serie Negra Group, which shows a thickness exceed- ing c. 3000 m (Eguíluz 1988). The core of this antiform contains three stratigraphic formations, which from older to younger are referred to as the Montemolín, Tentudía and Malcocinado formations. Montemolín and Tentudía for- mations are traditionally considered to be two members of the Serie Negra Group. The c. 1000 m thick Montemolín Formation consists of metapelites, mica schists, quartz-rich schists, metagreywackes, metasandstones, abundant amphi- bolites (Sánchez-Lorda et al. 2013, 2016) toward the top and interbedded black quartzites. This formation may show extensive migmatisation towards the bottom (Montero et al. 1999). The Tentudía Formation is composed of metasand- stones, metagreywackes, slates, phyllites, metacherts, black quartzites, layers of marble and micaschists. The overly- ing unconformable volcaniclastic Malcocinado Formation (Fricke 1941; Eguíluz et al. 2000) is composed by meta- conglomerates, metasandstones, metapelites, meta-andesites and meta-rhyolites. The Serie Negra Group together with the Malcocinado Formation are unconformably covered by the Early Cambrian Torreárboles Formation (Liñán and Fernández-Carrasco 1984), composed by fluvial to shallow marine deposits with conglomerates, metasandstones and slates (Fig. 2). A maximum depositional age of c. 565–541 Ma has been obtained for the Tentudía Formation using detrital zircon grains (Schäfer et al. 1993; Casado 1998; Linnemann et al. 2008). The Malcocinado Formation probably straddles the Ediacaran–Cambrian boundary (Pereira 2015). The Tor- reárboles Formation is Early Cambrian (Liñán and Fernán- dez-Carrasco 1984), with a maximum depositional age of c. 540–532 Ma (Eguíluz 1988; Perejón et al. 2004; Pereira et al. 2011). Large massifs of igneous rocks, mainly of gra- nitic–tonalitic composition, intruded into the Serie Negra Group between c. 600 and 540 Ma (Ordóñez-Casado 1998; Sánchez-García et al. 2003; Linnemann et al. 2008; Alvaro et al. 2014). This Ediacaran igneous suite was generated in relation to the prominent Avalonian–Cadomian peri-Gond- wanan magmatic arc (Sánchez-García et al. 2013; Albert et al. 2015; Andonaegui et al. 2016). The chronology of this igneous suite suggests an age of c. 600 Ma for the undated oldest levels of the Montemolín Formation. This chronology is also compatible with the relationship of the Serie Negra Group with the opening of a fore-arc or back-arc basin in the peri-Gondwanan realm and the generation of associated supra-subduction zone type oceanic lithosphere (Arenas et al. 2018). A similar Cadomian basement is widely exposed in other sections of the Variscan Orogen. In the eastern branch of the orogen, in the northern part of the Bohemian Massif (Fig. 3), Iberian–Bohemian correlations