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Those particular levels are usually characterized by low strain and lower metamorphic grade than those in the Mérida Massif. Overlying these metasedimentary rocks, the unconformable Malcocinado Forma- tion (Fricke, 1941) consists of andesitic tuffs, rhyolites, phyllites, me- tasandstones, metagreywackes and polygenic conglomerates with peb- bles of the Serie Negra (Pereira et al., 2006; Cambeses et al., 2017). The age of the Malcocinado Formation probably crosses the Ediacaran- Cambrian boundary (Pereira, 2015). The Torreárboles Formation is considered to be the lowest unconformable Cambrian series (Liñán, 1984). It is constituted by fluvial to shallow marine shelf deposits with slates, metasandstones, metaconglomerates and metabreccias, with a maximum depositional age within the range c. 567–532 Ma (Linnemann et al., 2008; Pereira et al., 2011). The Ediacaran and Early-Middle Cambrian series of the Ossa- Morena Complex were intruded by extensive igneous rocks ranging in age from c. 600 to 470 Ma. This magmatism affected the Gondwanan margin allegedly during a transition from a subduction-related setting (Ediacaran-Early Cambrian) to a rifting context (Middle Cambrian- Early Ordovician) (Sánchez-García et al., 2003, 2013; Linnemann et al., 2008; Pereira et al., 2012; Díez Fernández et al., 2015). The oldest ig- neous rocks of the Serie Negra are mafic in composition and are dated at c. 610–585 Ma (U-Pb in zircons; Sánchez-Lorda et al., 2016). Boni- nitic mafic rocks of the Calzadilla Ophiolite were dated at c. 600 Ma (U- Pb in zircons; Arenas et al., 2018), and appear imbricated with lower members of the Serie Negra. The geochemistry of those mafic rocks suggests a fore-arc setting for their genesis, a context that can also be extended to at least part of the basin where the Serie Negra was de- posited (Arenas et al., 2018). Palinspastic restoration of Variscan structures in the Iberian Massif places the Central Iberian Zone in a paleogeographic position further inland into Gondwana relative to the Ossa-Morena Complex (Figs. 1 and 2; Díez Fernández et al., 2016), thus suggesting that the deposition of the Lower Alcudian Series took place in a back-arc setting (e.g., Rodríguez-Alonso et al., 2004; Orejana et al., 2015). 3. Major and trace elements geochemistry 3.1. Methodology Sixteen new samples of siliciclastic rocks (mainly metagreywackes) from the Serie Negra were collected in the Obejo-Valsequillo Domain outside the Mérida Massif (Fig. 3). All of them show low to moderate strain, low-grade metamorphism, and correspond to typical facies of the upper levels of the Tentudía Formation (Bandrés, 2001). Three of the metagreywacke samples (OV-1 to OV-3) were collected near Valsequillo village (UTM 30/301.796/4.250.980), in a series composed by thin layers of metagreywackes alternating with slates and intruded by the Ediacaran Valsequillo metagranitoid (Bandrés, 2001). Seven samples of rhythmic metagreywackes (OV-6 to OV-12) were collected in a section where sedimentary structures (Bouma sequences) are well preserved, near Palomas village (UTM 30/750.540/4.285.214). The last six sam- ples (OV-13 to OV-16) were collected near Villar del Rey village (UTM 30/685.463/4.333.499), in a section formed by layers of homogeneous fine-grained metagreywackes variably strained. All these samples have been compared with the 12 samples of metagreywackes from the Lower Alcudian, which were previously studied by Fuenlabrada et al. (2016). The later would be the samples CIA-13 to CIA-20 (UTM 30/273.270/ 4.316.456) and CIA-21 to CIA-24 (UTM 30/327.349/4.315.193) (Fig. 3). The new 16 samples were analyzed for their whole-rock geochem- ical composition of major and trace elements and for their Sm-Nd iso- topes. Major and trace element analyses were performed in the ActLabs laboratories, Ontario (Canada). Crushing and powdering of the samples were performed at Universidad Complutense de Madrid. Lithium me- taborate/tetraborate was used for fusion and the elements were mea- sured by inductively coupled plasma mass spectrometry (ICP-MS). The Fig. 3. (A) Geological map of the Central Iberian Zone – Ossa-Morena Complex boundary. (B) Sketch showing the distribution of the main units around this boundary. Based on the 1:1.000.000 geological map of Spain and Portugal (IGME, 2014). AF, Azuaga Fault; EsT, Espiel thrust; PG-CVD, Puente Génave – Castelo de Vide detachment. OV and CIA stars represent the sampling locations in the Obejo-Valsequillo Domain (Serie Negra) and Central Iberian Zone (Lower Alcudian Series), respectively. E. Rojo-Pérez et al. 3UHFDPEULDQ 5HVHDUFK ² &KDSWHU