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470 International Journal of Earth Sciences (2021) 110:467–485 13 the Saxo-Thuringian Zone (SXTZ, Kossmat 1927) contains several Cadomian stratigraphic sequences with scarce Vari- scan overprint (Linnemann and Buschmann 1995), and probably equivalent to the Serie Negra Group (Pereira et al. 2006; Linnemann et al. 2008). The main outcrops are dis- tributed along the northern margin of the SXTZ (Fig. 3), constituting a total of six siliciclastic series composed essentially by turbiditic metagreywackes, slates, phyllites, schists, metamafic rocks, black metacherts and some layers of quartzites and metaconglomerates toward the upper part of some formations. The oldest maximum depositional age for these formations is considered at c. 629 Ma, according to the U–Pb zircon dating of a granite pebble (Frohnberg Formation; Linneman et al. 2000). Moreover, a minimum depositional age at c. 543 Ma has been estimated using detri- tal zircon grains extracted from greywacke turbidites and a glaciomarine tillite (Linnemann et al. 2008, 2014, 2018). All these SXTZ formations are intruded by Ediacaran and Cambrian granitic rocks, with ages ranging between c. 540 and 537 Ma (Linnemann et al. 2000, 2010a, 2014, 2018). The Paleozoic sequence overlaying these Neoproterozoic series contains incomplete Cambro-Ordovician series and a more complete stratigraphic succession reaching the Lower Carboniferous (Brause 1968; Linnemann et al. 2010b). Sample selection and methodology Sample selection Twenty-two samples of siliciclastic rocks belonging to the Serie Negra Group were collected in the Olivenza–Mones- terio antiform. Ten samples belong to the Montemolín For- mation (MO-1 to MO-11) and were collected in the northern Fig. 2 a Geological map of the Ossa–Morena Complex including the south part of the Central Iberian Zone. Based on the 1:1.000.000 geological map of Spain and Portugal (IGME 2014; Rojo-Pérez et al. 2019). b Geological map of the Iberian Massif showing the distribu- tion of allochthonous terranes in NW and SW Iberia (Díez Fernández and Arenas 2015). BAO Beja-Acebuches Ophiolite, CMU Cubito- Moura Unit, CU central unit, IOMCO Internal Ossa-Morena Com- plex Ophiolites, OVD Obejo-Valsequillo Domain, PG–CVD Puente Génave-Castelo de Vide Detachment, PTSZ Porto-Tomar Shear Zone. c Schematic stratigraphic column of the Serie Negra Group. TE and MO stars represent the sampling locations within the Olivenza–Mon- esterio Antiform &KDSWHU