1 Introduction

In the latest years, many works have focused on understanding the magmatic and tectonic events that took place during the Neoproterozoic and Early Paleozoic in the peri-Gondwanan magmatic arc-systems related to the Cadomian Orogeny (e.g., Murphy and Nance 1989; Ballèvre et al. 2001; Linnemann and Romer 2002; von Raumer et al. 2002; Pereira et al. 2006;Linnemann et al. 2014; Andonaegui et al. 2016; Arenas et al. 2018; Díez Fernández et al. 2019). This effort has gone hand by hand with an aim to deciphering the relative position of Variscan terranes along the paleomargin of Gondwana before its Devonian collision against Laurussia, figuring out in the way the paleogeography of the Ediacaran paleomargin of Gondwana (e.g., Díez Fernández et al. 2010; Ábalos et al. 2012; Avigad et al. 2012; Pereira et al. 2012a, 2020; Fernádez Suárez et al. 2014; Stephan et al. 2019a, b; Fuenlabrada et al. 2020). Geochemical, isotopic and geochronological approaches have become highly important to identify the provenance of the Iberian terranes. U–Pb–Hf geochronology and isotope geochemistry in detrital zircons have grown into a useful mean in providing paleogeographic models for their Gondwanan provenance (e.g., Fernández-Suárez et al. 2000, 2003, 2014; Ugidos et al. 2003a; Pereira et al. 2008, 2012a; Bea et al. 2010; Díez Fernández et al. 2010; Ábalos et al. 2012; Talavera et al. 2012; Albert et al. 2015; Orejana et al. 2015; Hernderson et al. 2016) and for constraining the tectonothermal history and the particular provenance of correlative European terranes (e.g., Linnemann et al. 2007; Drost et al. 2011; Avigad et al. 2012, 2018; Abbo et al. 2015; Arenas et al. 2016a; Collet et al. 2020; Martínez Catalán et al. 2020).

Together with the development of U–Pb–Hf geochronological and isotope geochemistry tools, whole-rock geochemistry and Nd-isotope composition studies in metasedimentary rock sequences have provided new data to improve the understanding about the provenance and the probable depositional environment of the Iberian metasedimentary record (e.g., Nägler et al. 1995; Ugidos et al. 2003b; López-Guijarro et al. 2008; Bea et al. 2010; Fuenlabrada et al. 2010, 2012, 2016; Pastor-Galán et al. 2013; Fernández-Suárez et al. 2014; Villaseca et al. 2014; Díez Fernández et al. 2017; Rojo-Pérez et al. 2019). An extensive sedimentary record in the Iberian Massif has allowed the study of the late Ediacaran evolution of a peri-Gondwanan active margin and its subsequent transition to a passive margin during Cambrian–Ordovician times (Rodríguez-Alonso et al. 2004b; Pereira et al. 2006; Linnemann et al. 2008; Sánchez-García et al. 2008, 2014; Fuenlabrada et al. 2020). Geochemical and Nd-isotope data from Iberian terranes have provided evidence for changes in the depositional settings by that time. The filling of a peri-Gondwanan back-arc basin, strongly influenced by ongoing magmatic activity, occurred during Ediacaran times (Fuenlabrada et al. 2012, 2016; Pereira et al. 2012a; Fernández-Suárez et al. 2014). The subsequent widening of that basin during the Early Cambrian period led to a change in the siliciclastic contributions to more recycled and older detritus (Fuenlabrada et al. 2020). The Nd-isotope information from siliciclastic series of the Iberian Massif represents an effective and comparative method for paleoreconstructions of the current Iberian terranes during the Neoproterozic and Paleozoic times (Fernández-Suárez et al. 2014a; Albert et al. 2015; Cambeses et al. 2017; Fuenlabrada et al. 2020), allowing a comparison between tectonostratigraphic units, members of the same terrane, but currently dispersed over the Iberian Massif due to superimposed deformation.

The current disposition of domains in the Iberian Massif was the result of the Pangea assembly during the Devonian through to the Carboniferous period after the closure of the Rheic and other oceanic basins (Franke 1989; Matte 1991; Winchester et al. 2002; von Raumer et al. 2003; Nance et al. 2010; Stampfli et al. 2013). The collision between Gondwana and Laurussia raised the Variscan Orogen (Fig. 1a), which is featured by suture zones and far-traveled (allochthonous) terranes that spread over the Iberian Massif and onto autochthonous sections of the margin of Gondwana (Fig. 1b) (Martínez Catalán et al. 1997, 2007; Ribeiro et al. 2007; Simancas et al. 2009; Arenas et al. 2014, 2016a; Díez Fernández and Arenas 2015; Díez Fernández et al. 2016). The generally accepted criterion for defining the autochthonous character of Iberian terranes has been their lower (plate) structural position relative to Variscan suture zone exposures (e.g., Ries and Shackleton 1971; Arenas et al. 1986, 2016a; Simancas et al. 2009; Martínez Catalán et al. 2020 and references therein). This is many times accompanied by a particular metamorphic evolution during the Variscan accretionary history, which makes them different from the Iberian allochthonous sections, i.e., a Barrovian-type evolution and a usual lack of high-P metamorphism (Dallmeyer et al. 1997; Martínez Catalán et al. 2009; Díez Fernández et al. 2016). The autochthonous sections represent inboard domains of the North African margin of Gondwana and include pre-Variscan metasedimentary sequences that range between Ediacaran and Devonian in age (Alvarez Nava et al. 1988; Gutierrez-Marco et al. 1990; San José et al. 1990; Valladares et al. 2000, 2002; Rodríguez Alonso et al. 2004a).

Fig. 1
figure 1

a Zonation of the Variscan Orogen (Díez Fernández and Arenas 2015; Arenas et al. 2016b). b Geological map of the Iberian Massif showing the distribution of autochthonous and allochthonous terranes in NW and SW Iberia (Díez Fernández and Arenas 2015). Location of the geological map presented in Fig. 2 is shown. AF Azuaga Fault, BAO Beja-Acebuches Ophiolite, CA Carvalhal Amphibolites, CF Canaleja Fault, CMU Cubito-Moura Unit, CO Calzadilla Ophiolite, CU Central Unit, ET Espina Thrust, HF Hornachos Fault, IOMZO Internal Ossa-Morena Zone Ophiolites, LLF Llanos Fault, MLF Malpica-Lamego Fault, OF Onza Fault, OVD Obejo-Valsequillo Domain, PG-CVD Puente Génave-Castelo de Vide Detachment, PRF Palas de Rei Fault, PTF Porto-Tomar Fault, RF Riás Fault, VF Viveiro Fault

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Here we present a new comprehensive whole-rock and Nd-isotope geochemical data set from the metasedimentary rocks cropping out in an autochthonous section of the SW Iberian Massif, locally referenced to as the Sierra Albarrana Group. Major and trace elements showing low-mobility characteristics during weathering, sedimentary transport, and post-depositional processes, would provide clues to track the influence of the geodynamic frame over the development and evolution of the sedimentary basins in which the sedimentary protoliths of the Sierra Albarrana Group were deposited. Similarly, this geochemical and isotope approach would contribute to constrain the provenance and relative paleoposition of this autochthonous section of the Variscan Orogen relative to other peri-Gondwanan terranes of the Iberian Massif, with particular focus on other autochthonous sequences of the Iberian Variscides that are now exposed in the southernmost section of the Central Iberian Zone (Fuenlabrada et al. 2016), i.e., the closest ones to the study case.

2 Geological setting

The first division of the Iberian Massif made by Lotze (1945) and Julivert et al. (1972) resulted in several geotectonic zones (Cantabrian, West Asturian-Leonese, Central Iberian, Ossa-Morena, and South-Portuguese zones). They show particular stratigraphic, structural, magmatic and metamorphic features, defining external or more internal domains within the southernmost section of the Variscan Orogen (Fig. 1a) (Martínez Catalán et al. 2009; Simancas et al. 2009; Arenas et al. 2014; Díez Fernández et al. 2016). After that initial division, several authors have contributed with new data and perspectives towards a further discrimination of geotectonic zones (e.g., Galicia-Trás-os-Montes Zone; Farias et al. 1987). Certain domains that constitute these geotectonic zones preserve particular characteristics from the earlier Variscan orogenic imprint, tightly connected to continental subduction (Fernández-Suárez et al. 2002a; Stampfli et al. 2002; Martínez Catalán et al. 2009; Kroner and Romer 2013; Rubio Pascual et al. 2013; Albert et al. 2015; Arenas et al. 2016a). Devonian high-P metamorphism and/or peri-Gondwanan paleoposition are typical features for the terranes that are thrust onto inboard sections of Gondwana in Iberia, the latter being the autochthon to a suture zone and its upper plate that can now be observed as allochthonous dismembered pieces over the Iberian Massif (Díez Fernández and Arenas 2015). This allochthonous–autochthonous pair is the essence to the distinction of allochthonous complexes in NW Iberia, and can be also observed in the SW of the Iberian Massif. As a result, most of the former Ossa-Morena Zone emerged as a new allochthonous complex, composed by terranes with different origin and tectonothermal evolution, while the sections underlying this complex became part of the Iberian Autochthon (Fig. 1b; Díez Fernández and Arenas 2015).

The Ossa-Morena Complex preserves typical features for a North Gondwanan margin provenance (López-Guijarro et al. 2008, 2012b; Pereira et al. 2008; Rojo-Pérez et al. 2019), with a pre-Variscan (Ediacaran to Devonian) metasedimentary and metaigneous record (Fig. 2). The Cadomian tectonothermal history is linked to the geodynamic activity of a peri-Gondwanan continental arc during Ediacaran and Cambrian times (Eguíluz et al. 2000; Pereira et al. 2006; Linnemann et al. 2008; Díez Fernández et al. 2017, 2019; Arenas et al. 2018; Rojo-Pérez et al. 2019). In that context, the Ediacaran metasedimentary rocks constituting the so-called Serie Negra Group (Carvalhosa 1965) define the basal part of an upper allochthonous terrane (Díez Fernández and Arenas 2015; Arenas et al. 2016b; Díez Fernández et al. 2016), and although it appears widely throughout the complex, it is especially important in the central area of the Olivenza-Monesterio antiform (Eguiluz 1987; Quesada et al. 1990) (Fig. 2). The Serie Negra Group has been traditionally divided into the Montemolín and Tentudía formations (Eguiluz 1987). Occupying the lower part of the series, the Montemolín Formation is mainly composed by metasandstones (including black quartzites), mica-schists, and abundant intercalations of metabasites. The maximum depositional age for this formation has been estimated at 591 ± 11 Ma (U–Pb in detrital zircons; Ordoñez Casado et al. 2009). The protoliths of this formation have been interpreted as related to a passive margin setting (Quesada 1990; Cambeses et al. 2017), although their geochemical composition suggests the influence of a Cadomian volcanic arc during their sedimentation (López Guijarro 2006; Rojo-Pérez et al. submitted). The Tentudía Formation, as well as the Don Alvaro and Oliva de Mérida Successions (Bandrés et al. 2001), rest on top and includes metagreywackes, black metapelites, black quartzites and felsic metaigneous rocks and minor metabasites. The study of U–Pb in detrital zircons from the sedimentary protoliths of this formation yielded a maximum depositional age of 560–544 Ma (Schäfer et al. 1993; Ordóñez-Casado et al. 1998; Fernández Suárez et al. 2002b; Linnemann et al. 2008), with a probable deposition linked to the evolution of a back-arc basin (Bandrés 2001; Cambeses et al. 2017; Rojo-Pérez et al. 2019, submitted). Overlying discordantly, the volcanoclastic Malcocinado Formation (Fricke 1941), with igneous intrusions (El Escribano, El Mosquil, La Bomba; Sánchez Carretero et al. 1989), probably contains the boundary between Ediacaran and Cambrian times (Pereira 2015), while the discordant sequences of the Torreárboles Formation, mainly composed by metasandstones and metaconglomerates, have been attributed to the Early Cambrian (Liñan and Palacios 1983; Liñan 1984; Liñan et al. 1984). The Central, Cubito-Moura, and Escoural units (Fig. 1b) are members of a terrane that is located at the base of the allochthonous nappe pile (Basal Allochthonous Units), i.e., underlying the upper allochthonous terrane cited above (Uppermost Allochthonous Units) and on top of the Autochthon to the Ossa-Morena Complex (Fig. 1b). The three of them, especially the Central Unit, have continental crust affinity, peri-Gondwanan provenance (Díez Fernández et al. 2017) and experienced Devonian high-P metamorphism (Moita et al. 2005; Abati et al. 2018).

Fig. 2
figure 2

Geological map of the Central Iberian Zone–Ossa-Morena Complex boundary (Rojo-Pérez et al. 2019; Based on the 1:1.000.000 geological map of Spain and Portugal, IGME 2015). AF Azuaga Fault, PG-CVD Puente Génave-Castelo de Vide Detachment. Location of the geological map and cross-section presented in Fig. 3 is shown

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The Sierra Albarrana Group (Delgado Quesada 1971; Garrote 1976; Azor 1994; González del Tánago 1995) is exposed immediately to the SW of the Badajoz-Córdoba Shear Zone, classically considered as the boundary between the Ossa-Morena Zone and the southernmost section of the Central Iberian Zone (Robardet 1976; Martínez Poyatos 1997; Pérez Estaún et al. 2004) (Fig. 2), where a dip-slip Variscan extensional fault is presented (Martín Parra et al. 2006). The best exposures of the Sierra Albarrana Group are located to the south of the Azuaga Fault (Chacón et al. 1974; Delgado Quesada et al. 1977; Apalategui et al. 1985; Ábalos et al. 1990), and resting under the Central Unit as an autochthonous section of the Variscan Orogen (Azor 1994; and references therein). According to González del Tánago and Peinado (1990), the tectonometamorphic evolution of this group defines it as a NW–SE elongated thermal dome, dominated by metasedimentary rocks and minor metabasites, folded into an antiformal structure. The Sierra Albarrana Group is composed mainly of four different lithostratigraphic successions, with a dominant siliciclastic character, from bottom to top: (i) Albarrana Succession (AS), (ii) Cabril-Peña Grajera Succession (CGS), (iii) Albariza-Bembézar Succession (ABS), and (iv) Azuaga Formation (AzF) (Fig. 3).

Fig. 3
figure 3

Geological map and cross-section of the Sierra Albarrana Group (based on Insúa et al. 1990 and own data), showing location for samples ALBA-01 to ALBA-31

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González del Tánago y Peinado (1990) included the first two aforementioned units into a single one (Albarrana Gneisses Unit) that occupies the core of the antiformal structure, and consisting of rocks fairly different from the dominant schistose or phyllitic nature of the rocks in the rest of the units. The Albarrana Succession [also named as Albarrana Quartzites or Albarrana Formation by Delgado Quesada (1971) and Garrote et al. (1980), respectively] occupies the central area of the dome structure and it is mainly composed of quartzites (mainly feldspathic), paragneisses, with minor schists, migmatites, pegmatitic bodies and lenses of amphibolite. The paragneisses are essentially meta-arkoses with a predominant monotonous quartz-feldspathic composition (González del Tánago 1995). The Cabril-Peña Grajera Succession rests on top of the Albarrana Succession and consists mainly of paragneisses, migmatites, fine-medium grain sized metasandstones, feldspathic quartzites and minor schists, most of which derive from semipelitic and pelitic protoliths. This unit corresponds to the Blastomilonitic Formation and Cabril-Peña Grajera Formation defined by Delgado Quesada (1971) and Garrote et al. (1980), respectively, and Insúa et al. (1990) distinguished an additional member made of paragneisses and schists referred to as the Cerro Pavillos Succession (Fig. 3). The gneisses and migmatites of this unit are characterized by penetrative tectonic banding defined by quartz-feldspathic and biotite-rich layers (Azor 1994). The metasedimentary rocks so far described preserve primary sedimentary structures such as ripples and cross-bedding, and were considered as intertidal deposits with an important contribution from felsic volcanic sources by González del Tánago (1995; and references therein).

The Albariza-Bembézar Succession (Albariza-Bembézar Schist Unit; González del Tánago 1995) rests on top of the Cabril-Peña Grajera Succession (Fig. 3). Despite being divided into two formations according to their position in the NE or SW limb of the antiformal structure of the region (La Albariza and Bembézar formations; Delgado Quesada 1971; Garrote et al. 1980), their lithostratigraphic similarity, lateral structural continuity and metamorphic homogeneity were taken as solid grounds to merge them into a single succession, which may show some local minor differences regarding the abundance in amphibolite layers, pegmatites, or in the size of staurolite and garnet porphyroblasts (Insúa et al. 1990; González del Tánago 1995). This unit is mainly composed by a metapelitic succession of schists, amphibolite lenses, garnet-bearing amphibolites (Gónzalez del Tánago and Arenas 1991), interbedded with minor metasandstones and white quartzites, and dykes of diabase. Pegmatitic bodies can be found close to the contact with the Albarrana Succession, where it shows higher strain and incipient migmatization.

The Azuaga Formation mantles the antiform structure of the study area and occupies the upper structural position within the Sierra Albarrana Group (Fig. 3). The Azuaga Formation is separated from the so-called Central Unit (Azor et al. 1994) (Fig. 1b) by the Azuaga Fault, and from the rest of the Ossa-Morena Complex to the SW by the Onza and Malcocinado faults (Azor 1994). However, materials that can be correlated with those found in the Albariza-Bembézar Succession, and mainly in Azuaga Formation are found in a thrust sheet located to the north of the Central Unit (Fig. 2). The current juxtaposition of this unit onto the rest of the Sierra Albarrana Group was interpreted to have been carried out by the Casa del Café Fault (Azor 1994). The Azuaga Formation shows low-grade metamorphism, and consists of fine-grained metasedimentary sequences with turbiditic origin, mainly composed of characteristic laminated slates, as well as phyllites, minor quartzite intercalations, post-metamorphic dykes of diabase and some mica-schists towards the lower part of the series (González del Tánago 1995). The Cardenchosa pluton intruded this unit and produced contact metamorphism in its host (Insúa et al. 1990). This pluton is affected by Variscan deformation (Simancas et al. 2000), has been dated at Early Ordovician and interpreted as part of a late-rift episode, related to the Cambrian–Ordovician rift stage of the Iberian Massif (Azor et al. 2016; Sánchez García et al. 2019).

The successions here described were affected by several phases of deformation (Dallmeyer and Quesada 1992,2012; Azor et al. 2004). Earlier deformation was accompanied by intermediate-P metamorphism, which evolved to lower-P conditions in the whole set and to higher-T conditions towards the lower structural section (González del Tánago and Peinado, 1990). This gave rise to a normal metamorphic zonation from the biotite zone on top up to partial melting conditions towards the bottom (Garrote 1976; González del Tánago 1995). The penetrative metamorphic fabrics formed in this area were considered as formed in relation to a ductile shear zone (Azor and Ballèvre 1997), while the upright folds that affect the series were interpreted as Variscan and related to strike-slip shear zones (Simancas et al. 2000).

The age of the successions in Sierra Albarrana has been traditionally attributed to Precambrian times (Delgado Quesada 1971; Garrote 1976; Garrote et al. 1980; Quesada et al. 1990; López-Guijarro et al. 2008), although a correlation between the quartzites in the Albarrana Succession and the Ordovician Armorican Quartzite of the Central Iberian Zone was made by Apalategui et al. (1985). In the quartzites of the Albarrana Succession, ichnofossils, such as Monocraterion, suggest sedimentation in a shallow marine environment from Early Cambrian times onward (Azor et al. 1991; Marcos et al. 1991). López-Guijarro et al. (2008) ruled out a deposition age younger than Ordovician for the sedimentary series in Sierra Albarrana according to the deformation, metamorphism and intrusion of granitoids in Early Ordovician times, thus being a record prior to the Ordovician rifting of the Iberian Massif. The fossil content in the Azuaga Formation has provided an Early-Middle Cambrian age to this series (Azor 1994; Liñán 1978; Liñán and Quesada 1990; Jensen et al. 2004). Therefore, an Early Cambrian age can be attributed to the units of the Sierra Albarrana Group from the Azuaga Formation downwards (Azor 1994; Azor and Ballèvre 1997; Jensen et al. 2004).

3 Whole rock geochemistry

3.1 Rock sampling and petrography

Thirty one samples of siliciclastic rocks (3 feldspathic quartzites and 28 meta-sandstones/meta-shales) were collected from the four successions of the Sierra Albarrana Group (Fig. 3). Table 1 shows sample details, including their location, while Fig. 4 illustrates their main petrographic features. The three quartz-feldspathic samples of the Albarrana Succession (ALBA-01 to ALBA-03) are essentially composed of Qz and Pl, with Ms, Bt, and Kfs, and Rt as main accessory (mineral abbreviations after Whitney and Evans 2010) (Fig. 4a). Among the meta-sandstones and meta-shales, eight samples were collected from the Cabril-Peña Grajera Succession (ALBA-04 to ALBA-11). They are fine-grained, grey-greenish, low strained rocks that change to medium-grained textures near quartzite layers (ALBA-09 to ALBA-11). The mineral assemblage observed in these rocks is composed of Qz, Pl, Kfs, Bt, Ms and Sil, with Tur and opaque as main accessory minerals (Fig. 4b, c). Fourteen metasedimentary rocks were collected in the Albariza-Bembézar Succession (ALBA-12 to ALBA-25). They consist of fine to medium-grained phyllites, mica-schists, and metagreywackes which may contain Grt, St, And, Sil, Qz, Pl, Bt, Ms, Chl, with Tur and opaque as the main accessory minerals. They define a metamorphic sequence with rapid decreasing P–T conditions from bottom to top (Fig. 4d–f). Finally, six fine-grained phyllites and metagreywackes were collected in the Azuaga Formation (ALBA-26 to ALBA-31). They present a lepidoblastic fabric with Qz, Pl, Bt, Ms, and Chl as the most frequently observed minerals (Fig. 4g, h).

Table 1 Information of the studied samples (metasedimentary rocks from Sierra Albarrana Group—SW Iberian Massif)
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Fig. 4
figure 4

Petrographic microphotographs showing textural features of the most representative rocks in the Sierra Albarrana Group. a Fine-grained feldspathic metaquartzite from Albarrana Succession; b Paragneiss from Cabril-Peña Grajera Succession; c Fine-grained metagreywacke from Cabril-Peña Grajera Succession; d Fine-grained micaschist from Albariza-Bembézar Succession; e Fine-grained garnet bearing phyllite from Albariza-Bembézar Succession; f Fine-grained metagreywacke from Albariza-Bembézar Succession; g Fine-grained phyllite from Azuaga Formation; and h Metagreywacke from Azuaga Formation

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3.2 Analytical methods

Geochemical and isotope analyses were performed on firstly crushed and powdered rock samples at Universidad Complutense de Madrid (Spain). Major and trace elements were analyzed at Activation Laboratories Ltd. (Actlabs, Canada) following a fusion sample digestion with lithium metaborate or tetraborate, and ICP-OES (for major elements and Sc, Be, V, Zr, Ba, and Sr) and ICP-MS (for the rest of trace elements) analytical procedures. Along with the samples, two sample duplicates, two procedural method blanks, and fourteen international standards materials were analyzed in order to verify the quality of the analyses. The complete list of these certified and measured results are included in Supplementary Quality Control Table. The detection limits for the analytical procedure range from 0.01% for the majority of the major elements to 0.001% for TiO2 and MnO; and for the trace elements, from 0.002 ppm for the Lu to 30 ppm for the Zn contents. Resulting data for the major and trace elements geochemical analysis are given in Table 2, and in Table 3 for rare earth elements (REE).

Table 2 Whole rock major and trace element data of metasedimentary rocks from SW Iberian Autochthonous Domain (Sierra Albarrana Group)
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Table 3 Whole rock rare earth element data of metasedimentary rocks from SW Iberian Autochthonous Domain (Sierra Albarrana Group)
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Sm–Nd isotope analyses (Table 4) were performed at Geochronological Service of Universidad Complutense de Madrid, using Isotope Dilution Mass Spectrometry (ID-TIMS). Samples were spiked with a mixed 149Sm–150Nd tracer before performing a sample dissolution with ultraclean reagents (HF-HNO3-HCl). The resulting solutions were passed through a double step chromatographic separation, by using DOWEX AG®50 × 8, in order to separate the complete REE group from the bulk matrix of the sample and in a second stage, LnResin® (Eichrom Technologies, Inc., USA) for a complete separation of Sm and Nd. Both purified fractions were loaded, together with 1 µl of ultrapure 0.05 M H3PO4, on a pre-cleaned Re triple-filament arrangement and analyzed on an IsotopX-Phoenix® mass spectrometer (TIMS), following a single and a multi-dynamic collection procedure for Sm and Nd analysis, respectively. A normalized value of 0.7219 for the 146Nd/144Nd ratio (O’Nions et al. 1979) was considered for a continuous correction of the Nd-isotope fractionation. Likewise, corrections for the presence of 142Ce and 144Sm isotopes were continuously performed to prevent isobaric interferences during the analysis. Seven replicas of the Nd-reference standard JNdi-1 (Tanaka et al. 2000) were run along with the samples, yielding an average value of 143Nd/144Nd = 0.512107, with an internal precision of ± 0.000005 (2σ). Analytical errors on 147Sm/144Nd and 143Nd/144Nd ratios were estimated at less than 0.1% and 0.006%, respectively, with a total Sm and Nd procedural method blank under 0.1 ng.

Table 4 Whole rock Nd isotope data of metasedimentary rocks from SW Iberian Autochthonous Domain (Sierra Albarrana Group)
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3.3 Results

3.3.1 Geochemical classification

Major and trace elemental concentrations are given in Table 2. When plotting the major elements on the Herron (1988) geochemical classification diagram (Fig. 5a), the majority of the fine and medium-grained schistose samples plot in the shale field, closer to the Post Archean Australian Shale (PAAS; Taylor and MacLennan 1985), some of them closer to the limit with the wacke field, matching stronger sandy features observed in the petrographic study (Table 1), and higher SiO2/Al2O3 ratios (Table 2). Five samples fall within the wacke field (ALBA-04, -06, -07, -23, and -31), which confirms field and petrographic observations. Meanwhile, quartzites samples from the Albarrana Succession (AS) plot in the arkose field, displaying the lowest values for the SiO2/Al2O3 ratios and very low K-contents (Fig. 5b). From the arkosic and relative immature character of the Albarrana quartzites, an increase of the sedimentary maturity can be observed from the Cabril-Peña Grajera Succession (CGS) to the Albariza-Bembézar Succession (ABS) and Azuaga Formation (AzF) samples. Samples from these two latter successions share a similar geochemical composition. AS and CGS samples show higher SiO2/Al2O3 (8.30 and 4.17, respectively) and lower K2O/Na2O ratios (0.12 and 1.86, respectively) than those observed in the PAAS (3.32 and 3.08, respectively). The latter implies a relative immature character, with a predominance of plagioclase over K-feldspar, compatible with very low values of K2O in the quartzite samples (avg. 0.37 wt.%) (Fig. 5b). High SiO2 contents (82.25 wt.% and 63.75 wt.%, avg. values for AS and CGS, respectively) and particularly a relative lower Al2O3 abundances (10.14 wt.% and 15.81 wt.%, avg. values for AS and CGS, respectively) are probably related to a limited presence of micas, as confirmed by a negative or very poor correlation between Al, Fe, and K in the samples from these two units. On the contrary, the metasedimentary samples from the ABS and AzF show lower SiO2 and higher Al2O3 abundances (60.10 and 63.91 wt.%, and 20.60 and 18.18 wt.%, respectively), with SiO2/Al2O3 (3.10 and 3.57) and K2O/Na2O ratios (2.85 and 2.24) closer to those of the PAAS, indicating a strong pelitic character, in agreement with an increase in sedimentary maturity (Fig. 5a). This is also confirmed by Al2O3/TiO2 (20.85 and 22.89) and Al2O3/Na2O (13.13 and 10.86) ratios within the range of those of the PAAS (18.90 and 15.75, respectively).

Fig. 5
figure 5

source area weathering and recycling in Sierra Albarrana metasedimentary rocks. Post Archean Australian Shale (PAAS; Taylor and McLennan 1985) has been included for comparison

a Chemical classification diagram for siliciclastic sediments (Herron 1988). b K2O vs. Chemical Index of Alteration (CIA; after Nesbitt and Young 1982) for comparison of the

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3.3.2 Source area weathering and recycling

The average CIA (Chemical Index of Alteration; Nesbitt and Young 1982) and PIA (Plagioclase Index of Alteration; Fedo et al. 1995) values for the metasedimentary rocks from Albarrana Succession (53 and 53, respectively) and Cabril-Peña Grajera Succession (56 and 60, respectively) indicate relative fresh rocks and low weathering alteration (Table 2) (Fig. 5b). Both indexes are far below those of the PAAS (CIA = 70; PIA = 79), and imply a limited secondary conversion of plagioclase/K-feldspars to clay minerals. Significant K-addition can be excluded in view of the slightly higher K2O contents in the CGS metasedimentary samples (avg. 4.57 wt.%) compared to those of the PAAS (3.7 wt.%), and the very low K2O contents of the quartzites from AS (avg. 0.37 wt.%) (Fig. 5b). As expected, this very low K abundance is in agreement with low contents in other LILE elements (Ba, Rb, and Cs). Low Rb (avg. 11.7 ppm) together with high Sr contents (avg. 245.3 ppm) in these samples provide very low Rb/Sr ratios (0.05), representing a low redistribution of the Sr2+ and Ca2+, considering also the high CaO abundances (avg. 1.98 wt.%), and typical Sr abundances for quartz-feldspathic rocks (Mielke 1979). However, the slightly high CaO contents suggest that hydrothermal alteration and CaO gain could be ruled out after considering the low LOI values (0.47 and 1.99 for AS and CGS, respectively).

An increase in the chemical maturity is consistent with a higher influence of post-depositional processes over the terrigenous sediments, as confirmed by PIA values for Albariza-Bembézar Succession and Azuaga Formation samples (82 and 80, respectively) (Table 2), which are similar to PAAS (79). This implies certain transformation of plagioclase/feldspar to clay minerals, in agreement with high Al (20.60 and 18.18 wt.%, respectively), and low Ca contents (0.48 and 0.58 wt.% CaO, respectively). Values below 1 of the Index of Compositional Variability (ICV; Cox et al. 1995) calculated for the samples from ABS and AzF (0.7 and 0.8, respectively) imply the presence of clay minerals dominantly with a recycling character. The relative weathering and post-sedimentary alteration of these samples are confirmed by average CIA values of 71 for both units, matching those of the PAAS (70) and indicating a moderate weathering (Fig. 5b), as also deduced from high Rb/Sr ratios (1.27 and 1.03, respectively), well beyond the 0.5 value that McLennan et al. (1993) suggest for a limited chemical alteration.

3.3.3 Rare earth elements composition (REE)

Quartz-feldspathic Albarrana Succession samples show very low bulk REE contents (avg. 8.3; Table 3), as expected from their high quartz contents, the relative lack of REE-bearing heavy minerals, and the aforementioned small fraction of clay minerals (Taylor and McLennan 1988). Their chondrite-normalized REE patterns (Nakamura 1974; Fig. 6a) are highly influenced by the significant abundance in plagioclase and feldspar (Taylor and McLennan 1988; Condie et al. 1995), where the most distinctive features are a depletion in LREE, a positive trend in HREE (GdN/YbN = 0.4), and a highly positive Eu-anomaly (avg. 4.49), compatible with their high plagioclase fraction and CaO abundances. Sample ALBA-03 presents higher HREE contents, which combined with a very high Zr (1113 ppm) and Hf (31.4 ppm) abundances (Fig. 6c), suggest a likely greater presence of zircon and rutile with low fractionation of the TiO2, probably accumulated in the coarser fraction of the sediment, since these heavy minerals commonly remain inert through sorting processes (Garcia et al. 1991). By contrast, the bulk REE-abundances in the metasedimentary samples Cabril-Peña Grajera Succession (290.5), Albariza-Bembézar Succession (331.7), and Azuaga Formation (231.9) (Table 3) are above those of the PAAS (184.8), although their chondrite normalized REE patterns are similar (Fig. 6a, b), sharing typical features with felsic igneous sources with an upper crust provenance: (i) a significant LREE fractionation over their HREE abundances, with high LaN/YbN ratios (avg. 12.5, 14.5, and 8.5, respectively), (ii) a negative Eu anomaly (0.59, 0.67, and 0.68, respectively) comparable with that of the PAAS (0.65), and finally (iii) almost flat HREE patterns, with GdN/YbN values close to unity (1.8, 2.2, and 1.5, respectively), and slightly higher than those of the PAAS (1.3). The large variation in LREE for CGS samples (ranging from 74 to 542) (Fig. 6a) is probably because of variable LREE-bearing mineral content (e.g. monazite), as indicated by the wide range in GdN/YbN ratios (from 0.8 to 3.5) (McLennan 1989; McLennan and Taylor 1991), with an average value typical for igneous rocks with an upper crust affinity. While HREE contents in ABS are controlled by the garnet fraction, zircon is a major factor in HREE abundances for AzF samples, in agreement with a positive correlation between Zr, Hf and HREE contents.

Fig. 6
figure 6

Chondrite normalized rare earth element plots: a Metasedimentary rocks from Albarrana (AS) and Cabril-Peña Grajera (CGS) successions (Samples ALBA-01 to ALBA-11); b Metasedimentary rocks from Albariza-Bembézar Succession (ABS) and Azuaga Formation (AzF) (samples ALBA-12 to ALBA-31). Normalizing values are from Nakamura (1974). The dotted line corresponds to the PAAS (Post Archean Australian Shale; Taylor and McLennan 1985). c PAAS-normalized trace elements multi-variation diagrams of the Early Cambrian quartzites from Albarrana Succession (AS—Sierra Albarrana Group)

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3.3.4 Provenance

In addition to the above mentioned characteristic REE-patterns similar to the PAAS and typical in felsic igneous sources with an upper continental crust affinity, numerous elemental abundances and their ratios suggest dominant felsic contributions for the metasedimentary rocks of the Sierra Albarrana Group. The Al2O3/TiO2 ratios (on average for Albarrana, Cabril-Peña Grajera, Albariza-Bembezar Successions and Azuaga Formation: 30.2, 20.5, 20.9, 22.9, respectively; Table 2) of these two insoluble elements and usually unaffected by weathering processes show values above those of the PAAS (18.9), and support the felsic nature of source rocks. Low Cr and Ni contents are even below the detection limits in the Albarrana Succession quartz-feldspathic samples, and rule out mafic/ultramafic contributions (Armstrong-Altrin et al. 2004). Very low Cr/V ratios (0.88, 0.84, and 0.80 for Cabril-Peña Grajera, Albariza-Bembezar Successions and Azuaga Formation, respectively) (Table 2) and similar to those of the PAAS (0.79) exclude a mafic/ultramafic source, confirmed by high Y/Ni values, displaying a trend to granitic end-members compositions, with an absence of mafic/ultramafic detritus (Hiscott 1984; McLennan et al. 1993) (Fig. 7a). Relatively high La (avg. 64.2, 71.8, and 46.4 ppm, respectively) and Th (avg. 18.9, 22.3, and 18.8 ppm, respectively) abundances found in CGS, ABS, and AzF samples, compared to those of the PAAS (38.0 and 14.6 ppm, respectively) are in agreement with a felsic affinity. Therefore, the comparison between these two latter immobile elements with those commonly associated with mafic/ultramafic source rocks (Sc, Cr and Co), yields very low Sc/Th (avg. 0.78, 0.74, and 0.84 for CGS, ABS, and AzF, respectively) and Cr/Th (avg. 4.19, 4.01, and 4.30, respectively) ratios, lower than those of the PAAS (1.10, and 7.53, respectively), and within the range of felsic rocks (Cox et al. 1995; Cullers, 2002; McLennan et al. 2006) (Fig. 7b). Likewise Cr/Th and Eu/Eu* values (Tables 2, 3) fit those proposed by Cullers (1994) (2.5–17.5 and 0.48–0.78, respectively) for sediments sourced from felsic rocks. The La/Sc vs. Co/Th diagram (Fig. 8a) places the Sierra Albarrana metasedimentary rocks close to the PAAS and to typical values for felsic volcanic rocks, and far from the common Co/Th and La/Sc values in basaltic and andesitic sources. Felsic volcanic rocks represent an important terrigenous contribution in post-Archean continental arc systems (Condie et al. 1993). In differentiating specific felsic volcanic compositions, the Nb/Y vs Zr/Ti diagram in Fig. 8b shows that most of the samples plot in the rhyodacite/dacite composition field, following the Rhyolitic/dacitic trend (Winchester and Floyd 1977; Fralick and Kronberg 1997).

Fig. 7
figure 7

Provenance discrimination diagrams. a Cr/V vs. Y/Ni diagram. The curved line represents the model mixing between ultramafic and granitic end-members (after Hiscott 1984; McLennan et al. 1993). b Cr/V vs. Sc/Th showing the affinity of the metasedimentary rocks from Sierra Albarrana Group for a felsic source

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Fig. 8
figure 8

a Co/Th vs. La/Sc diagram for provenance discrimination of siliciclastic rocks (average compositions of igneous rocks from Condie 1993). b Zr/Ti vs. Nb/Y diagram (fields after Winchester and Floyd 1977; Fralick and Kronberg 1997) showing affinity for felsic volcanic rhyolitic/dacitic sources

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3.3.5 Tectonic setting

A set of PAAS-normalised major and trace elements diagrams from three units described in Sierra Albarrana Group are presented in Fig. 9, with the exception of the three quartz-feldspathic samples from the Albarrana Succession, whose abundances represent an almost widespread compositional depletion related to PAAS (Fig. 6c). The patterns displayed by the three units match those proposed by Winchester and Max (1989) for passive continental margins (Fig. 9), and also match those found in Early Cambrian shales (slates) from the autochthonous section in the southern Central Iberian Zone (Pusa Gp; Fuenlabrada et al. 2016) and in Middle Cambrian metapelitic rocks from the NW Basal Allochthonous Units (Fuenlabrada et al. 2012). Both are indicative of an upper continental crust provenance, whose sedimentation was influenced by the earlier stages of a Cambrian–Ordovician passive margin (Pieren Pidal 2000; Díez Fernández et al. 2010; Fuenlabrada et al. 2020). They show an average geochemical composition similar to PAAS, with the majority of the normalized major and trace elements close to unity, with no evident depletion in LILE elements, and negative anomalies in different proportions in the case of Sr, P, and Ti abundances (Fig. 9). They all suggest an important influence of the sedimentary processes over the terrigenous sediments, as a result of distant source areas and effective recycling processes. This is confirmed by Th/U ratios (avg. 7.6, 4.4, 4.8, and 4.7 for AS, CGU, ABU, and AzU, respectively) around those of PAAS (4.7), and above typical values for sedimentary rocks sourced from crustal igneous rocks with cratonic provenance (3.5–4.0; McLennan et al. 1993).

Fig. 9
figure 9

PAAS-normalized trace elements multi-variation diagrams of the Early Cambrian metasedimentary series of the Sierra Albarrana Group (SW Iberian Autochthonous Domain). a Metasedimentary rocks from Cabril-Peña Grajera Succession (CGS); b Metasedimentary rocks from Albariza-Bembézar Succession (ABS); c Metasedimentary rocks from Azuaga Formation (AzF). PAAS after Taylor and McLennan (1985). Passive margin shales (after Winchester and Max 1989), Early Cambrian shales from Central Iberian Zone (Fuenlabrada et al. 2016), and Middle Cambrian metapelitic rocks from NW Basal Allochthonous Units (Fuenlabrada et al. 2012) fields have been included for comparison (see the text for a further explanation)

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The geochemistry of the metasedimentary rocks from Sierra Albarrana Group is compatible with deposition in a peri-continental section influenced by a continental active margin that evolved to a more stable environment (passive margin?) during Early Cambrian times. The La/Th versus Hf diagram (Fig. 10a) shows some affinity for acidic arc sources, some samples having similar values to those of the PAAS (average value for upper continental crust). However, the majority of the samples displays an evident trend to higher Hf contents, which according to Floyd and Leveridge (1987) could be consequence of the erosion effect of a continental basement with increasingly older metasedimentary rocks. Such trend in the denudation process could explain the increase in maturity observed in the Sierra Albarrana Group, with a progressive release of zircon and other accessory, but highly stable minerals and the subsequent enrichment in Zr and Hf that is typical for passive margin sediments (Bhatia and Crook 1986; McLennan et al. 1993). Supporting the latter idea, the Th/Sc and Zr/Sc ratios (Table 2) (Fig. 10b) show values that suggest sorting and recycling to some extent, what fits with passive margin turbidites better than with sediments related to an active arc, which would follow normal geochemical compositional variations and limited recycling (McLennan et al. 1993; Cox et al. 1995) (Fig. 10b).

Fig. 10
figure 10

a La/Th vs. Hf diagram for active/passive margin source discrimination of sandstones (fields from Floyd and Leveridge 1987; and references therein). b Th/Sc vs. Zr/Sc diagram (after McLennan et al. 1993). Samples plot away from igneous compositional variations, suggestive of an increase in sedimentary sorting and recycling

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3.3.6 Sm–Nd isotopes

The Sm–Nd isotope data for the Sierra Albarrana metasedimentary rocks are given in Table 4 and Fig. 11. An Early Cambrian maximum depositional age (ca. 530 Ma) has been selected for εNd(i) calculations. No significant variation is observed in the whole group of samples, which defines a single Nd-isotope population. Relatively similar 143Nd/144Nd initial ratios, ranging from 0.51138 to 0.51172, suggest a likely common source for all of them. Their 147Sm/144Nd ratios (from 0.1092 to 0.1287) are similar to those of continental crust (~ 0.12; DePaolo 1983), and always below the limit (147Sm/144Nd = 0.165), defined by Stern (2002) for reliable Nd model age calculations, sharing typical values among clastic sediments (0.1–0.13; Zhao et al. 1992). They show relatively uniform εNd(0) values (from − 17.1 to − 10.3; Table 4) and negative εNd(530) values, varying between − 11.3 and − 4.5 (Fig. 11). Nd TDM model ages were calculated according to DePaolo (1981), yielding Late Paleoproterozoic to Early Mesoproterozoic ages (1388–1897 Ma), with an average value at 1721 Ma, similar to those already published by López-Guijarro et al. (2008).

Fig. 11
figure 11

TDM model ages (after DePaolo 1981) of the Early Cambrian metasedimentary series from the Sierra Albarrana Group (SW Iberian Autochthonous Domain). TDM model ages of Early Cambrian metasedimentary sequences from Central Iberian Zone (Fuenlabrada et al. 2016), and TDM model ages of Sierra Albarrana Group (López-Guijarro et al. 2008) have been included for comparison

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4 Discussion: type of basin model in which the Sierra Albarrana Group was deposited

The quite negative εNd(530) values (from − 11.3 to − 4.5; Table 4) calculated for the metasedimentary series of the Sierra Albarrana Group indicate a dominant input of terrigenous materials with an old continental crust affinity and limited contributions from juvenile sources. The geochemical compositions place the deposition in a continental active margin. Given the significant and increasing influence of the sedimentary processes and transport over the pelitic protoliths, and the recycled character of the terrigenous sediments, the conditions would have probably evolved to those of a passive margin. Both εNd(530) and fSm/Nd values (from − 0.445 to − 0.346; Table 4) are compatible with those proposed by McLennan and Hemming (1992) for sedimentary rocks deposited in tectonic settings sharing continental active margin and passive margin/cratonic features. The geochemical composition of our samples (Figs. 9, 10) points to the same direction. The negative εNd(530) and geochemical features such as Th/U, Th/Sc, and Eu/Eu*, account for a significant sedimentary maturity and recycled character (McLennan et al. 1993), favouring a provenance from old and reworked continental upper crust materials with an evident felsic contribution. The metasedimentary rocks from Sierra Albarrana Group display uniform Sm/Nd ratios, which suggests an intense recycling history and a limited contribution from differentiated sources. These samples show Nd-isotope features quite different from those found in typical arc-related rocks, with variable Sm/Nd ratios and positive εNd (e.g., metasedimentary series in the NW Upper Allochthonous Units; Fuenlabrada et al. 2010, 2020). As mentioned before, the rest of geochemical proxies indicate a reworking of detrital materials with a high influence of the sedimentary transport, and likely linked to a continental active margin or an incipient passive margin. This is also confirmed by the comparison with a wide range of geochemical indicators used by Bhatia and Crook (1986) to constrain the affinity with certain geodynamic scenarios. Considering Nd model ages in siliciclastic rocks as a weighted average of contributions with a different age and provenance, potentially derived from nearby surrounding source areas and/or, according to some authors areas located thousands of kilometres away (e.g., Avigad et al. 2003; Morag et al. 2011), the here presented very old Nd model ages (avg. 1721 Ma; Table 4; Fig. 11) account for a limited and variable mixing with juvenile detrital materials coming from the outermost part of the active margin and magmatic arc sources, and suggest a dominant input from older basement sections. The closeness of the paleobasins of the Ediacaran sedimentary sequences from the Serie Negra Group to the Cadomian magmatic arc define specific and identifiable geochemical features (Rojo-Pérez et al. 2019 and submitted), although their Nd model ages do not mirror significant juvenile contributions from the magmatic activity, with constant and old TDM ages with an old crustal affinity from a Cadomian basement provenance (Rojo-Pérez et al. submitted). The geochemical and isotope data here presented for the Sierra Albarrana Group suggest that their sedimentary protoliths were not laid down coevally with ongoing magmatism, at least not close to a magmatic axis, since there is no geochemical evidence for a significant influence of the arc-system over the geochemical features in the whole group of rocks from Sierra Albarrana. In this regard, the recycling of an old (Paleoproterozoic) basement into newly-formed igneous rocks that were eroded from the arc-system should be considered a rather minor contributor, unlike what might have happened with the coeval Ediacaran–Cambrian Malcocinado volcanoclastic series, with Nd-isotope features more in line with a greater influence of the magmatic activity of the Cadomian arc (López-Guijarro et al. 2008). All these geochemical and isotopic characteristics match those found in the Early Cambrian sedimentary sequences of the autochthonous section of the Central Iberian Zone (Pusa shales Formation—Upper Unit), which show, in average values, slightly less negative εNd(530) (from − 5.2 to − 4.0) and younger TDM model ages (1444–1657 Ma; Fuenlabrada et al. 2016), although overlapping some individual values obtained in the autochthonous section of the Sierra Albarrana Group (Fig. 11). Homogeneous geochemical data of the Early Cambrian sedimentary sequences in Central Iberian Zone also support a high textural and mineralogical maturity compatible with old crustal materials, probably coming from a reworked continental source in an emerged Gondwanan hinterland location (Fuenlabrada et al. 2016). The Paleoproterozoic and Mesoproterozoic TDM model ages (Fig. 11) probably result from a mixing of limited Neoproterozoic and dominant Paleoproterozoic/Archean components, pointing to the denudation of an old continental crust (Paleoproterozoic basement), or a crust built at the expense of it, that was exposed during Early Cambrian times. This is compatible with an exposure of a Paleoproterozoic basement (e.g., in the Moroccan Meseta; Pereira et al. 2015) and/or erosion of old recycled continental crust. The increase in weathering indexes (CIA and PIA values; Table 2) in the metasedimentary rocks of the Albarrana Succession through to the Azuaga Formation is compatible with both possibilities.

Although our results are in agreement with those obtained by Lopez-Guijarro et al. (2008), who suggested common Archean/Paleoproterozoic old continental crust sources for the Serie Negra Group and the metasedimentary rocks of Sierra Albarrana during the latest Neoproterozoic, a Cambrian age for the sedimentary sequences of the Sierra Albarrana Group precludes a direct comparison with the Ediacaran Serie Negra Group. The most probable contributor for the detrital material was a Paleoproterozoic basement and/or an old recycled continental crust currently located in more eastern sections of the Gondwanan margin than those of the Serie Negra. The Tuareg Shield (Fig. 12b) shows abundant Paleoproterozoic and Archean source rocks with scarce Neoproterozoic and Mesoproterozoic ages (Brahimi et al. 2018), being probably the dominant contributor, through the Saharan platform, as previously suggested by Cambeses et al. (2017). These old recycled materials present Nd-isotope features comparable with those of the southern Central Iberian Zone (Fuenlabrada et al. 2016, 2020), what suggests easternmost locations for the Variscan autochthonous sections of the Iberian Massif (Bea et al. 2010; Talavera et al. 2013; Fernández-Suárez et al. 2014) (Fig. 12b).

Fig. 12
figure 12

a Simplified model (not to scale) of an Early Cambrian retro-arc basin for the SW Iberian Autochthonous Domain (Sierra Albarrana Group). b Schematic paleogeographic reconstruction model (not to scale) for the Gondwanan margin during the approximate Ediacaran–Cambrian transition (c. 540 Ma) (after Albert et al. 2015; Brahimi et al. 2018). Iberian sedimentary basins locations are shown in relation to the main North African cratonic masses. CIZ—Central Iberian Zone. OMC (Ossa-Morena Complex), AM (Armorican Massif), and SXTZ (Saxo-Thuringian Zone) locations according to Rojo-Pérez et al. submitted. Arrows show the probable provenance for the detrital materials (see text for further explanation)

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Recent paleogeographic reconstructions of the peri-Gondwanan domain acknowledge that the autochthonous sections of the Variscan Orogen occupied inward positions across the margin of Gondwana relative to their respective allochthons (Díez Fernández et al. 2016). This reconstruction fits well with the orogen-scale distribution of paleogeographic domains of the Cadomian arc-system during the latest Ediacaran and Cambrian. The most external sections of the Cadomian arc-system, such as the fore-arc and arc, are preserved in the Variscan Upper Allochthon (Andonaegui et al. 2002; Díaz García et al. 2010; Fuenlabrada et al. 2010; Albert et al. 2015). The Variscan Ophiolitic and Basal allochthons include remnants of a Cadomian back-arc (Abati et al. 2010; Díez Fernández et al. 2010, 2016; Fuenlabrada et al. 2012; Andonaegui et al. 2017; Arenas et al. 2018, 2020; Sánchez Martínez et al. 2020), while in the autochthon were accumulated thick sedimentary series in the flank of that back-arc basin closer to the mainland in a differentiated lateral position (Rodríguez Alonso et al. 2004b, 2020; Pereira et al. 2012a; Fuenlabrada et al. 2016). Variscan subduction/accretion polarity for the peri-Gondwanan terranes of Iberia was directed toward Laurussia (i.e., to the W and SW in present-day coordinates; Díez Fernández et al. 2016). Therefore, the current juxtaposition of Variscan allochthons on top of autochthons could be roughly translated in paleogeographic terms into a more internal position across the margin as we move down through the Variscan tectonic pile, and also as we move E or NE through the same allochthonous or autochthonous terrane. Accordingly, the position of the Sierra Albarrana Group to the SW of the Variscan autochthon widely exposed in the Central Iberian Zone would imply a position closer to the mainland for the latter, in a more eastern lateral location along the Gondwanan margin, closer to the Sahara Metacraton (Fig. 12b) (Bea et al. 2010; Talavera et al. 2012; Kromer and Romer 2013; Fernández-Suárez et al. 2014; Albert et al. 2015; Stephan et al. 2019b; Fuenlabrada et al. 2020).

Given the widespread angular unconformity between Ediacaran and Early Cambrian rocks that can be recognized across the Iberian Massif, topographic uplift must have played a role in the onset of Early Cambrian basins across the margin. Ediacaran thrusting (obduction) directed toward mainland Gondwana was responsible for the emplacement of peripheral (fore-arc) sections of the arc-system onto inboard sections of the Gondwana margin (Díez Fernández et al. 2019), which came along with coeval inversion (inland-verging folds; Díaz García et al. 2010) and across-margin segmentation (Villaseca et al. 2014) of back-arc Cadomian basins. Inland-directed obduction of peripheral sections of an arc-system together with inland-verging folds in the back-arc are strongly compatible with the development of a retro-arc (retro-foreland) basin over the back of the arc-system (Fig. 12a). In a convergent scenario, the interaction between the peri-Gondwanan trench and the external part of the continent during Neoproterozoic times resulted in widespread contraction (Díez Fernández et al. 2019), thus generating significant relief, either at the expense of the fore-arc, arc, or even the back-arc, which would have been the main detritus suppliers for a retro-arc basin. The more external Ediacaran–Cambrian paleoposition of the Variscan autochthonous section that is preserved in Sierra Albarrana relative to the rest of the Variscan autochthon that is preserved elsewhere to the N and NW, makes the Sierra Albarrana Group a good candidate to represent the infilling of a Cambrian retro-arc basin fed by detritus coming from tectonically uplifted sections of the arc and fore-arc, as this would be a domain of the Ediacaran back-arc of the autochthon rather close to the uplifted sections. All the geochemical features of those sequences presented in this work are fully compatible with this interpretation.

The distribution of siliciclastic materials within the Sierra Albarrana sedimentary basin follows predictable models for the evolution in foreland basins, where the sedimentary sequences record long-term climatic and tectonic changes (Armitage et al. 2011). The presence of Skolithos, Monocraterion, and Arenicolites in the Albarrana Succession indicates a shallow marine environment for the deposition of this series (Azor et al. 1991), possibly in a bulge section. In the Cabril-Peña Grajera and Albariza-Bembézar successions, the flysch-type deposits, with relatively immature sediments and turbiditic character, could be the result of deepening in the retro-arc basin, possibly related to tectonically-induced subsidence in a fore-deep section. The greater influence of the sedimentary transport, as suggested by the increase in the geochemical maturity and recycling character of the metasedimentary record in the Albariza-Bembézar Succession and Azuaga Formation could be related to a widening of the basin, what would have heralded the terminal stages of contraction in the upper plate, where the continuous input of recycled sediments from old basement sources filled the basin in a more stable environment such as is the case of the intertidal zone deposits in a marine platform suggested for the Azuaga Formation (Insúa et al. 1990; González del Tánago 1995; and references therein).

5 Main conclusions

Nd-isotope data from metasedimentary series of the Sierra Albarrana Group (SW Iberian Autochthonous Domain) are compatible with old reworked materials with continental crust affinity and limited contributions from juvenile sources. This, together with a high recycling character of the terrigenous sediments place the basins within an active margin sharing geochemical features with the initial stages of a passive margin. The geochemical and isotopic features of the Sierra Albarrana Group are similar to those found in the sedimentary sequences of the autochthonous section of the Central Iberian Zone, which implies a comparable depositional setting during their sedimentation in Early Paleozoic times and a close paleolocation of their sedimentary basins, but in an easternmost location along the Gondwanan margin (Tuareg Shield—Sahara Metacraton). The progressive denudation of a Paleoproterozoic basement or a recycled continental crust was the dominant source for the recycled materials with old Nd model ages found in the metasedimentary sequences in Sierra Albarrana. The exposure of this old crustal section after topographic uplift was caused by an Ediacaran inland-directed thrusting, which emplaced fore-arc sections of the arc-system onto inboard sections of the Gondwanan margin. This contractive scenario during latest Neoproterozoic times caused that these uplifted arc and fore-arc sections were the dominant detrital contributors for the filling of a retro-arc basin, which is considered as the most probable setting for the deposit of the Sierra Albarrana sedimentary rocks in Early Cambrian times.