1. Introduction
Provenance of deposits in sedimentary basins used to be based on detrital modes of both modern and ancient sandstones, revealing crucial relationships primarily controlled by tectonic settings (Critelli, Reference Critelli1999, Reference Critelli2018). In the same time, extensive studies on geochemistry and mineralogy of the fine-grained sediments interlayered among coarse beds effectively integrate the provenance information inferred from detrital mode studies. Moreover, it is possible to explain and predict the sedimentary evolution and geological processes affecting fine-grained sediments and, thus, the relationship between source area and sedimentary basin. In fact, the chemical and mineralogical compositions of terrigenous sedimentary rocks preserve the signatures of the interaction of the source-area provenance, sorting and recycling, weathering and tectonic processes (Bhatia and Crook, Reference Bhatia and Crook1986; Roser and Korsch, Reference Roser and Korsch1988; McLennan et al. Reference McLennan, Hemming, McDaniel, Hanson, Johnsson and Basu1993; Fedo et al. Reference Fedo, Nesbitt and Young1995; Critelli and Nilsen, Reference Critelli and Nilsen1996; Cullers and Podkovyrov, Reference Cullers and Podkovyrov2000; Mongelli et al. Reference Mongelli, Critelli, Perri, Sonnino and Perrone2006; Cavalcante et al. Reference Cavalcante, Belviso, Bentivenga, Fiore and Prosser2011, Reference Cavalcante, Belviso, Piccaterra and Fiore2014, Reference Cavalcante, Perri, Belviso, Lettino, Prosser, La Bruna and Agosta2023; Madukwe et al. Reference Madukwe, Ogungbesan, Aturamu and Ajisafe2016; Critelli et al. Reference Critelli, Perri, Arribas, Herrero, Ingersoll, Lawton and Graham2018, Reference Critelli, Martín-Martín, Capobianco and Perri2021; Martín-Martín et al. Reference Martín-Martín, Perri and Critelli2023b; Perri et al. Reference Perri, Criniti, Sánchez-Navas, Cavalcante and Critelli2025). Furthermore, after deposition, clay minerals usually experiment transformation caused by the sedimentary and/or tectonic load, influencing the thermal maturity of clay sediments (e.g. Hoffman and Hower, Reference Hoffman, Hower, Scholle and Schluger1979; Corrado et al. Reference Corrado, Aldega, Di Leo, Giampaolo, Invernizzi, Mazzoli and Zattin2005; Cavalcante et al. Reference Cavalcante, Belviso, Laurita and Prosser2012, Reference Cavalcante, Prosser, Agosta, Belviso and Corrado2015, Reference Cavalcante, Perri, Belviso, Lettino, Prosser, La Bruna and Agosta2023). This is valuable when studies are driven to infer the provenance and weathering conditions in the source area and reveal the tectonic setting in which the sedimentary succession was deposited (Taylor and McClennan, Reference Taylor and McLennan1985; McLennan et al. Reference McLennan, Hemming, McDaniel, Hanson, Johnsson and Basu1993; Ramos-Vázquez et al. Reference Ramos-Vázquez, Armstrong-Altrin, Rosales-Hoz, Machain-Castillo and Carranza-Edwards2017; Wang et al. Reference Wang, Wang, Fu, Zhan, Armstrong-Altrin, Yu, Feng, Song and Zeng2018; Casse et al. Reference Casse, Montero-Serrano, St-Onge and Poirier2019; Ramírez-Montoya et al. Reference Ramirez-Montoya, Madhavaraju and Monreal2021). Several authors have established the tight correlation between trace-element geochemistry and sediment provenance and/or tectonic environment (Nesbitt and Young, Reference Nesbitt and Young1982; Bhatia and Crook, Reference Bhatia and Crook1986; Nelson and DePaolo, Reference Nelson and DePaolo1988; Condie and Wronkiewicz, Reference Condie and Wronkiewicz1990; Murray et al. Reference Murray, Buchholtz ten Brink, Jones, Gerlach and Russ1990; Floyd et al. Reference Floyd, Shail, Leveridge, Franke, Morton, Todd and Haughton1991; Cullers, Reference Cullers1994; Nagarajan et al. Reference Nagarajan, Armstrong-Altrin, Kessler, Jong and Mazumder2017; Hurst et al. Reference Hurst, Wilson, Grippa, Wilson, Palladino, Belviso and Cavalcante2021), as a matter of their relatively low mobility during sedimentary processes (e.g. McLennan et al. Reference McLennan, Hemming, McDaniel, Hanson, Johnsson and Basu1993; Cullers, Reference Cullers1994; Cárdenes et al. Reference Cárdenes, Rubio-Ordonez, Monterroso and Calleja2013 and references therein). Moreover, during mineral breakdown, the immobile trace elements are transported and deposited with clay minerals (Nesbitt et al. Reference Nesbitt, Young and McLennan1996), ending less influenced by diagenesis (Taylor and McLennan, Reference Taylor and McLennan1985; Totten and Blatt, Reference Totten and Blatt1993). The analysis of immobile trace elements and their elemental ratios are considered reliable indicators to characterize parent rocks, allowing to discriminate from felsic (high contents of Zr, Nb, Hf, Y and Th) or mafic (high values of Cr, V, Ni and Sc) sources (Cullers and Podkovyrov Reference Cullers and Podkovyrov2002; Tawfik et al. Reference Tawfik, Ghandour, Maejima, Armstrong-Altrin and Abdel-Hameed2017). Moreover, the lanthanide rare earth europium (Eu) anomaly is used as a proxy to identify input from either felsic or (ultra)mafic sources, discriminating the source of the sediments (Wang et al. Reference Wang, Wang, Fu, Zhan, Armstrong-Altrin, Yu, Feng, Song and Zeng2018; Lӧwen et al. Reference Lӧwen, Meinhold, Arslan, Güngӧr and Berndt2020). In addition, several major elements are useful to infer the weathering intensity and other paleoenvironmental conditions of the source area (Madhavaraju, Reference Madhavaraju and Ramkumar2015; Madhavaraju et al. Reference Madhavaraju, Tom, Lee, Balaram, Ramasamy, Carranza-Edwards and Ramachandran2016; Hernández-Hinosa et al. Reference Hernández-Hinojosa, Montiel-García, Armstrong-Altrin, Nagarajan and Kasper-Zubillaga2018; Barros dos Santos et al. Reference Barros dos Santos, Le Pera, Souza de Oliveira, Severino de Souza Júnior, Fabrício de Araújo, Corrêa and Carlos de Azevedo2019; Kasper-Zubillaga et al. Reference Kasper-Zubillaga, Martínez-Serrano, Arellano-Torres, Álvarez Sánchez, Patiño-Andrade, González-Bermúdez and Carlos-Delgado2021; Martizzi et al. Reference Martizzi, Chiyonobu, Hibi, Yamato and Arato2021). Major elements with different mobility are useful proxy of sedimentary provenance and paleoweathering. Several indices, such as the chemical index of alteration (CIA), may reflect the weathering trends of source material under variable climates (e.g., Nesbitt and Young, Reference Nesbitt and Young1982; McLennan, Reference McLennan1993; Huang et al. Reference Huang, Mei, Yang, Zhang, Li and Hohl2021; Demirel-Floyd et al. Reference Demirel-Floyd, Soreghan, Webb, Roche, Joo, Hall, Levy, Elwood Madden and Elwood Madden2023). This research focuses on the sedimentary succession of the Culm Facies deposited during the late Paleozoic around the Circum-Mediterranean area. In fact, the Circum-Mediterranean orogenic systems recorded the evidences of a complex geodynamic evolution during the Pangaea amalgamation, during the Carboniferous, and its subsequent fragmentation. The sedimentary record of coeval basins, formed along the European-Gondwanian margins, preserves crucial information about this period. The detrital modes of these basins, documenting the dismantling of the Variscan-Paleotethysian Orogen, and subsequent uplift and erosion, remain not very well known. This phase ended in the Late Carboniferous, when Gondwana-derived terranes were included in the Pangaea supercontinent and then lately formed the pre-Mesozoic basements of the Alpine-Mediterranean Belts (Stampfli, Reference Stampfli2000; Stampfli and Borel, Reference Stampfli and Borel2002, Reference Stampfli, Borel, Cavazza, Roure, Spakman, Stampfli and Ziegler2004; Stampfli et al. Reference Stampfli, Von Raumer, Borel, Martinez Catalán, Jr Hatcher, Arenas and Díaz García2002, Reference Stampfli, Vavassis, De Bono, Rosselet, Matti and Bellini2003, Reference Stampfli, Hochard, V´erard, Wilhem and vonRaumer2013; Stampfli and Kozur, Reference Stampfli, Kozur, Gee and Stephenson2006; Critelli et al. Reference Critelli, Mongelli, Perri, Martín-Algarra, Martín-Martín, Perrone, Dominici, Sonnino and Zaghloul2008; Spahić et al. Reference Spahić, Gaudenyi and Glavaš-Trbić2019; Criniti, Reference Criniti2023; Criniti et al. Reference Criniti, Martín-Martín and Martín-Algarra2023, Reference Criniti, Martín-Martín, Hlila, Maaté and Maaté2024, Reference Criniti, Talmat, Martín-Martín and Critelli2026). The Culm deposits are widely recognized as the product of a regional Carboniferous syn-orogenic deposits, preceding and following the Variscan Orogeny and final Pangaean assembling. They typically consist of turbiditic sequences that accumulated in basins distributed across the entire Gondwanan-Laurussian realm, parallel to the growing orogen. The event marks the climax of the Variscan (Hercynian) Orogeny, associated with the closure of the Rheic Ocean and the Paleotethysian collisions involving the northern Gondwana plates during the Late Paleozoic (Matte, Reference Matte2001; Nance et al., Reference Nance, Gutiérrez-Alonso, Keppie, Linnemann, Murphy, Quesada, Strachan and Woodcock2010; Criniti et al. Reference Criniti, Martín-Martín and Martín-Algarra2023, Reference Criniti, Martín-Martín, Hlila, Maaté and Maaté2024). Recent papers revealed the detrital modes of the Late Carboniferous Culm in the Malaguide Domain of the Betic Cordillera (Criniti et al. Reference Criniti, Martín-Martín and Martín-Algarra2023) and the Ghomaride Domain of the Rif Chain (Criniti et al. Reference Criniti, Martín-Martín, Hlila, Maaté and Maaté2024), giving important information on the source area of these deposits and proposing palaeogeographic and palaeotectonic reconstructions for the northern Gondwana continental margin. As such, a significant informational gap, regarding paleogeography, paleotectonics and source areas, in the western Paleotethys region located between the Iberian-French and African plates was filled. In this paper, we complete this recent information with the mineralogical and geochemical study of slates pertaining to the Late Carboniferous Culm Facies outcropping in the Internal Zone of the Betic Cordillera (Southern Spain) and Rif Chain (Northwestern Africa), in the northern Gondwana (Figure 1). Seven sectors (four in Spain: Murcia, Lorca, Granada and Malaga and three in Morocco: Ceuta, Tetouan and Chouen) comprising 16 stratigraphic sections (10 in Spain and 6 in Morocco) were studied. The study of these siliciclastic Carboniferous deposits is fundamental for better constrain and clarify the Paleotethysian-related pre-Alpine orogenic evolution.
Geological sketch map of the Betic-Rif Cordillera with location of the sampled sectors.
Figure 1. Long description
The map displays the Betic-Rif Cordillera with various sectors labeled. Key locations include Murcia, Lorca, Almeria, Granada, Malaga, Gibraltar, Ceuta, Tetouan, and Chaouen. The map is color-coded to represent different geological domains such as Neogene-Quaternary, MFB Domain, External Domain, Foreland, Malaguide and Ghomaride, Frontal Units, Alpujarride and Sebtide, and Nevado-Filabride. The inset map shows the broader context of the Iberia and Eurasia regions. The map also includes the Mediterranean Sea and the Atlantic Ocean.
2. Geological setting
The Betic Cordillera and Rif Chain, which form the Gibraltar Arc (Figure 1), constitute the westernmost segment of the Western Alpine Peri-Mediterranean Orogen (Critelli, Reference Critelli2018; Martín-Martín et al. Reference Martín-Martín, Guerrera and Tramontana2020a,b; Guerrera et al. Reference Guerrera, Martín-Martín and Tramontana2021), being part of the broader Maghrebian Chain that also includes the African Tell (Belayouni et al. Reference Belayouni, Guerrera, Martín-Martín, Tramontana and Bullejos2023a), the Calabria-Peloritani Arc, and the southern Apennines (Critelli and Martín-Martín, Reference Critelli and Martín-Martín2022, Reference Critelli and Martín-Martín2024), all these Alpine branches sharing similarities in stratigraphic successions, tectonic units and structural style. The Betic Cordillera and Rif Chain encompass several domains: the Internal Domain, originating from the Meso-Mediterranean Microplate and flanked by various Tethyan oceanic branches (Vera, Reference Vera2004; Chalouan et al. Reference Chalouan, Michard, El Kadiri, Negro, Frizon de Lamotte, Soto, Saddiqi, Michard, Saddiqi, Chalouan and Frizon de Lamotte2008; Michard et al. Reference Michard, Hoepffner, Soulaimani, Baidder, Michard, Saddiqi, Chalouan and Lamotte2008; Jabaloy-Sánchez et al. Reference Jabaloy Sánchez, Martín-Algarra, Padrón-Navarta, Martín-Martín, Gómez-Pugnaire, López Sánchez-Vizcaíno, Garrido, Quesada and Oliveira2019; Guerrera et al. Reference Guerrera, Martín-Martín and Tramontana2021); the Maghrebian Flysch Basin (MFB) Domain (Figure 1), representing the Meso-Cenozoic southwestern oceanic branch of the Tethys (Critelli, Reference Critelli2018; Martín-Martín et al. Reference Martín-Martín, Guerrera and Tramontana2020a,b; Guerrera et al. Reference Guerrera, Martín-Martín and Tramontana2021; Belayouni et al. Reference Belayouni, Guerrera, Martín-Martín, Le Breton and Tramontana2023b); the External Domain (Figure 1), composed of Meso-Cenozoic units from the Betic and Riffian sub-domains, including the South Iberian Margin (Vera, Reference Vera2004) and the North African Margin (Chalouan et al. Reference Chalouan, Michard, El Kadiri, Negro, Frizon de Lamotte, Soto, Saddiqi, Michard, Saddiqi, Chalouan and Frizon de Lamotte2008; Michard et al. Reference Michard, Hoepffner, Soulaimani, Baidder, Michard, Saddiqi, Chalouan and Lamotte2008; Martín-Martín et al. Reference Martín-Martín, Perri and Critelli2023b, 2024) and the Variscan forelands of the Iberian and African Atlas-Mesetas regions (Chalouan et al. Reference Chalouan, Michard, El Kadiri, Negro, Frizon de Lamotte, Soto, Saddiqi, Michard, Saddiqi, Chalouan and Frizon de Lamotte2008; Michard et al. Reference Michard, Hoepffner, Soulaimani, Baidder, Michard, Saddiqi, Chalouan and Lamotte2008, and references therein). Of particular interest in this study is the Internal Betic-Rif Domain, which is subdivided, from base to top, into four main tectonic complexes (Martín-Martín et al. Reference Martín-Martín, Guerrera and Tramontana2020a,b, Reference Martín-Martín, Guerrera, Cañaveras, Alcalá, Serrano, Maaté, Hlila, Maaté, Tramontana, Sánchez-Navas and Le Breton2023a,b,c, Reference Martín-Martín, Guerrera, Cañaveras, Javier Alcalá, Serrano, Maaté, Hlila, Maaté, Sánchez-Navas, Miclăus, Tent-Manclús and Bullejos2024): the Nevado-Filábride Complex (exclusively in the Betic branch), the Betic Alpujarride and its Rifian equivalent Sebtide, the Betic Malaguide and its Rifian counterpart Ghomaride and the Frontal Units (‘Dorsal’) present in both branches. The two upper tectonic complexes contain Paleozoic sedimentary successions, the Malaguide and Ghomaride (and Frontal) units consisting of Ordovician to Carboniferous rocks (Herbig, Reference Herbig1984, Reference Herbig1985, Reference Herbig1989; Martín-Algarra, Reference Martín-Algarra1987; Chalouan and Michard, Reference Chalouan and Michard1990; Rodríguez-Cañero et al. Reference Rodríguez-Cañero, Maaté and Martín-Algarra1990, Reference Rodríguez-Cañero, Martín-Algarra, Sarmiento and Navas-Parejo2010; Navas-Parejo et al. Reference Navas-Parejo, Martín-Algarra, Rodríguez-Cañero, Martínez-Pérez, Furió, Santos-Cubedo and Poza2012a,b, Reference Navas-Parejo, Rodríguez-Cañero and Martín-Algarra2015; Rodríguez-Cañero and Martín-Algarra, Reference Rodríguez-Cañero and Martín-Algarra2014; Cabrera-Porras et al. Reference Cabrera-Porras, Rodríguez-Cañero and Martínalgarra2019, Reference Cabrera-Porras, Rodríguez-Cañero and Martín-Algarra2020; Martín-Algarra et al. Reference Martín-Algarra, García-Casco, Gómez-Pugnaire, Jabaloy-Sánchez, Laborda-López, López Sánchez-Vizcaíno, Mazzoli, Navas-Parejo, Perrone, Rodríguez-Cañero, Sánchez-Navas, Quesada and Oliveira2019), with minor lithological and age variations between units (Figure 2). From base to top, the Lower-Middle Paleozoic sequence begins with Ordovician-Silurian slates interbedded with siliceous sandstones and quartzites, overlain by a flysch-like alternation of sandstones and graded slates (Morales Fm; Martín-Algarra et al. Reference Martín-Algarra, García-Casco, Gómez-Pugnaire, Jabaloy-Sánchez, Laborda-López, López Sánchez-Vizcaíno, Mazzoli, Navas-Parejo, Perrone, Rodríguez-Cañero, Sánchez-Navas, Quesada and Oliveira2019). This is followed by a thin, silica-rich lydite layer unit often associated with a cineritic horizon and then by condensed pelagic sediments consisting of bluish-pinkish, thinly bedded tentaculites-bearing limestones and marls. Above this lies the characteristic ‘trilogy’ of rocks attributed to the Silurian-Devonian transition (Chalouan, Reference Chalouan1986, Reference Chalouan, Flügel, Sassi and Grecula1987), succeeded by a hemipelagic, relatively condensed Upper Silurian to Upper Devonian series (Santi Petri Fm and the pre-Falcoña succession; Rodríguez-Cañero and Martín-Algarra, Reference Rodríguez-Cañero and Martín-Algarra2014; Cabrera-Porras et al. Reference Cabrera-Porras, Rodríguez-Cañero and Martínalgarra2019, Reference Cabrera-Porras, Rodríguez-Cañero and Martín-Algarra2020; Martín-Algarra et al. Reference Martín-Algarra, García-Casco, Gómez-Pugnaire, Jabaloy-Sánchez, Laborda-López, López Sánchez-Vizcaíno, Mazzoli, Navas-Parejo, Perrone, Rodríguez-Cañero, Sánchez-Navas, Quesada and Oliveira2019) composed of Lower Devonian nodular limestones with tentaculites and brachiopods, a ferruginous lyditic interval, and thinly bedded tentaculites-bearing micritic limestones that grade upward into laminated sandy limestones and slates ranging from Givetian (or Emsian?) to Famennian (Rodríguez-Cañero and Martín-Algarra, Reference Rodríguez-Cañero and Martín-Algarra2014). The Carboniferous is divided into four lithostratigraphic units (Figure 2): the Falcoña Formation (radiolarian chert, slates and limestone beds), the Retamares Member of the Almogía Formation (a thin turbiditic sequence of sandy-pelitic alternations with coarse polygenic conglomerates sourced from underlying units or a metamorphic/granitic basement), the Greywacke-Slate Member of the Almogía Formation (arenitic-turbiditic and microconglomeratic at the base, fining upward into black micritic limestone) and the Marbella Conglomerate Formation (coarse polygenic conglomerates and breccias with olistostromic blocks, often derived from Upper Silurian-Devonian reef platforms; Blumenthal, Reference Blumenthal1930, Reference Blumenthal1949; Geel, Reference Geel1973). The upper three units (Almogía and Marbella Conglomerate formations) belong to the Culm Facies (Martín-Algarra et al. Reference Martín-Algarra, García-Casco, Gómez-Pugnaire, Jabaloy-Sánchez, Laborda-López, López Sánchez-Vizcaíno, Mazzoli, Navas-Parejo, Perrone, Rodríguez-Cañero, Sánchez-Navas, Quesada and Oliveira2019 and references therein).
Paleozoic stratigraphy of the Ghomaride Complex (modified from Blumenthal, Reference Blumenthal1930; Hermes, Reference Hermes1966; Geel, Reference Geel1973; Bourrouilh and Jaeger, Reference Bourrouilh and Jaeger1976; Bourrouilh, Reference Bourrouilh1977; Bourrouilh and Lys, Reference Bourrouilh and Lys1977; Bourrouilh and Gorsline, Reference Bourrouilh and Gorsline1979, Reference Bourrouilh and Gorsline1980; Herbig, Reference Herbig1984, Reference Herbig1985, Reference Herbig1989; Martín-Algarra, Reference Martín-Algarra1987; Chalouan and Michard, Reference Chalouan and Michard1990; Rodríguez-Cañero et al. Reference Rodríguez-Cañero, Maaté and Martín-Algarra1990, Reference Rodríguez-Cañero, Martín-Algarra, Sarmiento and Navas-Parejo2010; Navas-Parejo et al. Reference Navas-Parejo, Martín-Algarra, Rodríguez-Cañero, Martínez-Pérez, Furió, Santos-Cubedo and Poza2012a,b, Reference Navas-Parejo, Rodríguez-Cañero and Martín-Algarra2015; Rodríguez-Cañero and Martín-Algarra, Reference Rodríguez-Cañero and Martín-Algarra2014; Cabrera-Porras et al. Reference Cabrera-Porras, Rodríguez-Cañero and Martínalgarra2019, Reference Cabrera-Porras, Rodríguez-Cañero and Martín-Algarra2020; Martín-Algarra et al. Reference Martín-Algarra, García-Casco, Gómez-Pugnaire, Jabaloy-Sánchez, Laborda-López, López Sánchez-Vizcaíno, Mazzoli, Navas-Parejo, Perrone, Rodríguez-Cañero, Sánchez-Navas, Quesada and Oliveira2019).
3. Studied sectors and sections
Sixteen stratigraphic sections were sampled from seven sectors from Betic Cordillera (S Spain) and Rif Chain (N Morocco), respectively (Figure 3): Murcia (Campix-1,-2 and-3 sections), Lorca (Barranco Hondo and Barranco Coroneles sections), Granada (El Molinillo section) and Malaga (Río Campanillas-1,-2, Venta Cadenas and Arroyo de Coche sections); and Ceuta (W Fnideq section), Tetouan (Oued el Lile, Mouklata-1,-2,-3, and Jbel Tora sections) and Chaouen (Akchour section). The medium-coarse detrital suites of the same sampled sectors and sections were recently studied by Criniti et al. (Reference Criniti, Martín-Martín and Martín-Algarra2023, Reference Criniti, Martín-Martín, Hlila, Maaté and Maaté2024). In the Murcia Sector, three monotonous sections of 40, 60 and 30 m thick, respectively, were sampled in the Campix area of Sierra Espuña (UTM coordinates: 30S 626962E, 4187413N; 627832E, 4187980N; 628841E and 4188353N). These sections show olive-green slates with fine intercalations of fine-sandstones (Figure 4a). Samples 22A-45 to-61 were collected in this sector (Figure 3).
Stratigraphic columns of the sampled sectors and sections with location of the collected samples.
Figure 3. Long description
The table compares the stratigraphic columns of sampled sectors and sections in the Maláguide (Betic Cordillera, Spain) and Ghomaride (Rif Chain, Morocco) regions. It is divided into two main sections: Maláguide and Ghomaride, each with multiple sectors. The Maláguide section includes sectors Murcia, Lorca, Granada, and Málaga, while the Ghomaride section includes sectors Ceuta, Tetouan, and Chaouen. Each sector lists specific formations and their corresponding stratigraphic sections. The table details the formations and sections with specific labels and values indicating the presence of different geological features such as sandstones, conglomerates, and basal polygenic conglomerates. The table is organized into columns representing different sectors and rows representing different stratigraphic sections and formations. Each cell contains specific labels and values that describe the geological features present in each section.
Field facies of the sampled sections: (a) Campix-2 section (sample 22A-58); (b) Rambla Coroneles section (sample 22A-70); (c) Molinillo section (sample 22A-101); (d) Río Campanillas-2 section (sample Cu-4); (e) W Fnideq section (sample 23A-128); (f) Oued el Lile section (sample 23A-39); (g) Mouklata-2 section (sample 23A-16); and (h) Akchour section (sample 23A-89).
Two sections were sampled in the Lorca Sector: the Rambla Coroneles section (UTM coordinates: 30S 602460E, 4175240N), with thickness of 130 m, and the Barranco Hondeo section (UTM coordinates: 30S 587336E, 4168278N), with a thickness of 110 m. They consist of greenish-brownish sandstones and quartz-rich pebble conglomerates (Retamares Mb) in the lower part and olive-green slates with minor intercalations of sandstones (Greywacke-Slate Mb) towards the top (Figure 4b). Samples 22A-62 to-97 were collected in this sector (Figure 3).
In the Granada Sector, the 40 m thick Molinillo section (UTM coordinates: 30S 461752E, 4129407N) was sampled. It consists of greenish-brownish sandstones and quartz-rich conglomerates (Retamares Mb) evolving upwardly to olive-green slates (Greywacke-Slate Mb) (Figure 4c). Samples 22A-98 to-101 were collected in this sector (Figure 3).
In the Malaga Sector, four sections were sampled: the 125 m thick Río Campanillas-1 (UTM coordinates: 30S, 366161E, 4075267N) and the 175 m thick Río Campanillas-2 (UTM coordinates: 30S 366214E, 4076046) sections consist of polygenic conglomerate with abundant quartzite rounded pebbles (Retamares conglomerates) and greenish-brownish sandstones and olive-green slates (Greywacke-Slate Mb). The 50 m thick Venta de Cadenas (UTM coordinates: 30S, 364536E, 4078580N) and the 100 m thick Arroyo de Coche (UTM coordinates: 30S, 364995E, 4083212N) sections made of olive-green slates (Greywacke-Slate Member of the Almogía Fm), followed by a polygenic conglomerate with quartzite rounded pebbles and less rounded carbonate clasts (Marbella Conglomerate Fm) were also sampled (Figure 4d). Samples 22A-1 to-26 and Cu-1 to-9 were collected in this sector (Figure 3).
The Ceuta Sector was studied in the 30 m thick W Fnideq section (UTM coordinates: 30 S, 366161 E, 4075267 N) made of greyish to greenish slates with fine sandstone intercalations (Figure 4e). Samples 23A-126 to-128 were collected (Figure 3).
Four sections were sampled in the Tetouan Sector: the 300 m thick Oued el Lile (UTM coordinates: 30 S, 366161 E, 4075267 N), the 100 m thick Mouklata-1 (UTM coordinates: 30 S, 366161 E, 4075267 N), the 40 m thick Mouklata-2 (UTM coordinates: 30 S 366214 E, 407604 6) and the 120 m thick Jbel Thora (UTM coordinates: 30 S, 366161 E, 4075267 N). They show coarse grained graywackes alternating with microconglomerates and sandstones at the bottom (Retamares Member of the Almogía Formation) evolving upwardly to slates and pelitic levels (Greywacke-Slate Member of the Almogía Fm) (Figure 4f and g). Samples 23A-10 to-40 were collected (Figure 3).
The Chaouen Sector, studied in the 70 m thick Akchour section (UTM coordinates: 30 S, 366161 E, 4075267 N), consists of polygenic conglomerate and breccia (Marbella Conglomerate Fm) with local intercalations of greenish sandstones and slates (Figure 4h). Samples 23A-55 to-89 were collected (Figure 3).
4. Methods
The mineralogical and chemical composition of 25 slate samples was determined at the University of Granada Laboratory by X-ray diffraction (XRPD) analysis conducted on both whole sample and <2 μm fraction and by X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) for major and trace elements of whole-rock samples. XRPD data were determined using a PANalytical X’Pert Pro diffractometer (Cu-Kα radiation, 45 kV, 40 mA) equipped with an X’Celerator solid-state linear detector. Diffraction patterns were obtained by continuous scanning between 3 and 50°2θ, with a resolution of 0.01°2θ, with a total scan time of ∼15 min (equivalent to 20 s of counting time for 0.01°2θ). X-ray analysis was performed using powders from whole rock samples after manual grinding with an agate mortar. The clay fraction (<2 μm) was obtained after removing carbonates with diluted acetic acid. It was then collected by centrifugation at 3000 rpm and analysed using an air-dried, solvated with ethylene glycol and heated (350°C for 2 h) sample (Moore and Reynolds, Reference Moore and Reynolds1997). Qualitative analysis of both whole samples and clay fractions was performed according to Brown & Brindley (Reference Brown, Brindley, Brindley and Brown1980), Moore and Reynolds (Reference Moore and Reynolds1997), and Leoni et al. (Reference Leoni, Lezzerini, Battaglia and Cavalcante2010).
The percentage of the mineral phases on both bulk rocks and clay fractions was obtained, following the procedure proposed by Cavalcante et al. (Reference Cavalcante, Fiore, Lettino, Piccarreta and Tateo2007) and thus measuring the integrated intensities of the peaks using WINFIT computer program (Krumm, Reference Krumm1999). Furthermore, the mineralogical composition of the bulk rocks was refined using wbAffina software combining the major element chemical composition (whole rock XRF analysis) with XRPD mineralogical results (Leoni et al. Reference Leoni, Lezzerini and Saitta2008; Cesarano et al. Reference Cesarano, Bish, Cappelletti, Cavalcante, Belviso and Fiore2018). The illite crystallinity was determined using the international standards proposed by Warr and Rice (Reference Warr and Rice1994). The values obtained in our laboratory were transformed into Crystallinity Index Standard (CIS) values using the equation CIS = 0.972x + 0.1096, calibrated using the Warr and Rice (Reference Warr and Rice1994) standards. Anchizone limits for CIS calibrated values are 0.32–0.52º2θ (Warr and Ferreiro Mählmann, Reference Warr and Ferreiro Mählmann2015). Temperature estimates were determined after transforming the CIS values into their equivalent KI (Warr and Ferreiro Mählmann, Reference Warr and Ferreiro Mählmann2015), using the (Kubler Index (KI) vs T) diagram reported in Cavalcante et al. (Reference Cavalcante, Belviso, Laurita and Prosser2012). This diagram was reconstructed considering the KI value and the temperature data reported in the literature (see Cavalcante et al. Reference Cavalcante, Belviso, Laurita and Prosser2012 and references therein). Illite-Muscovite b parameter was determined on the peak d (060,-331) position considering the 211quartz peak as internal standard (Wang et al. Reference Wang, Frey and Stern1996).
Major-element composition of samples was determined by using a wavelength-dispersive XRF sequential spectrometer S4 Pioneer (BRUKER). Lithium tetraborate fused beads were employed for bulk sample homogenization. Typical precision was better than ±1.5% for a concentration of 10 wt.%. Zirconium was determined by XRF with a precision better than ±4% for 100 ppm Zr.
REE and other trace elements were analyzed by ICP-MS, after HNO3 + HF digestion of 0.1 g of sample powders in a Teflon-lined vessel at ∼180 °C and 200 psi for 30 min, evaporation to dryness and subsequent dissolution in 100 ml of 4 vol.% HNO3. Trace element measurements were performed in triplicate with a PE SCIEX ELAN-5000 spectrometer using rhodium as an internal standard. Precision, determined by well-know international Geological Certified Reference Material WSE (Whin Sill Dolerite/Basalt Standard), BR (Basalt Standard), and AGV (Andesite Standard), analysed as unknowns, was better than ±2% and ±5% for concentrations of 50 and 5 ppm, respectively.
The petrographic study of slates was performed on 4 thin section samples (2 in the Malaguide Complex from the Internal Betic Cordillera in Spain and 2 in the Ghomaride Complex from the Internal Rif Chain from Morocco) by using polarizing optical microscope where metamorphic texture and microstructure, mineralogical assemblage of the protolith and metamorphic mineral assemblage are qualitatively described in order to assess the metamorphic grade.
5. Results
5.a. Bulk mineralogy of the slates
The mineralogical composition of the studied samples is characterized by the presence of quartz (Qtz), feldspars (k-feldspars and plagioclase), goethite (Gth), calcite (Cal), hematite (Hem), anatase (Anat) and by phyllosilicates consisting of illite-muscovite (Il-Ms), mixed layer illite-smectite (I/S), chlorite (Chl), trioctahedral mixed-layer (TML; chlorite/vermiculite and/or chlorite/smectite and/or chlorite/vermiculite/smectite and biotite/chlorite), paragonite (Pg) and dickite-kaolinite (Kao) (Figure 5). In detail, Qtz ranges from 12.9 to 34.7 wt%, feldspars average 10 wt%, with K-feldspar less than plagioclase (Table 1). Gth is present in almost all samples with low percentages (5 wt%); Cal and Hem are present in only a few samples and in very small quantities. Anatase (Anat) is detected in all samples with very low percentage. Phyllosilicates are mainly represented by illite/muscovite (Il/Ms), ranging from 18 to 54 wt%, with the highest value reached in the Greywacke-Slate Mb (54 wt%) of the Malaguide Complex. Moreover, the highest value of the Retamares Mb is evidenced in the Malaga Sector (34–52 wt%) where itself reveal an increasing trend upwardly both in the Retamares Mb and in the Marbella Conglomerate Fm (18–32 wt%). An increasing content is evidenced southward in the Ghomaride Complex with the highest value reached in the Marbella Conglomerate Fm (43 wt%). Chl varies from 2 to 19 wt% with the highest value reached in the Retamares Mb of the Malaguide Complex in the easternmost sectors. Lower values of this phase are present in the Greywacke-Slate Mb in the Murcia Sector, corresponding to the upper part of the Spanish succession. In general, it deeply decreases upwardly the succession, with a sharp increase upwardly in the Marbella Conglomerate Fm (13–18 wt%). In the Ghomaride Complex, it is remarkable in the Greywacke-Slate Mb (12.7–14 wt%), while it is absent in the Retamares Mb and moderate in the Marbella Conglomerate Fm (5 wt%). Most samples are characterized by the presence of mixed layers of illite/smectite (I/S) with variable percentages (5–20 wt%). In particular, in the Malaguide Complex, it is generally more evident in the Greywacke-Slate Mb (0–20 wt%), with a decreasing trend upwardly, than in the Retamares Mb (0–10 wt%) and Marbella Conglomerate Fm (0–5 wt%). TML ranges from 0 up to 20 wt%. Lower values of TML were measured in the Greywacke-Slate Mb (1–4 wt%) in the Murcia Sector (upper part of the Malaguide succession), while the highest are in the Retamares Mb in the easternmost sectors. Similarly, lower values are detected in the Ghomaride succession in Morocco (0–8 wt%). However, the sum of Chl and TML in the Malaguide succession shows an increasing trend upwardly in the Retamares Mb, where the highest values are evidenced in the easternmost sectors, a sharp decreasing trend in the Greywacke-Slate Mb, and an increase in the Marbella Conglomerate Fm (Table 1; Figure 6). The Ghomaride succession shows the highest content of Chl and TML in the Greywacke-Slate Mb, with a decreasing trend upwardly the succession in the Marbella Conglomerate Fm, while it is absent in the Retamares Mb. Dickite-kaolinite is present in only four samples: three of them are from the Greywacke-Slate Mb in the Murcia Sector and show a percentage lower than 7 wt%. One sample of the Retamares Mb, collected from the lower part of the Ghomaride succession, shows a content around 10 wt%. Paragonite is present only in one sample collected from the upper part of the Ghomaride succession belonging to the Marbella Conglomerate Fm.
Representative X-ray diffraction patterns of bulk samples. For abbreviations, see the text.
Figure 5. Long description
The image contains multiple X-ray diffraction patterns displayed as line graphs. Each graph plots intensity in counts per second (cps) on the vertical axis against the angle in degrees (2θ) on the horizontal axis, ranging from 4 to 34 2θ degrees. The patterns are labeled with sample identifiers such as 22A-19, 22A-58, 22A-96, CU-7, 22A-6, 22A-101, 22A-92, 23A-21, and 23A-13. Key mineral peaks are annotated with labels like Chl (Chlorite), Trioct-ML (Trioctahedral mixed-layer), Il-Ms (Illite-Muscovite), Kao (Kaolinite), Qlz (Quartz), Pl (Plagioclase), Cal (Calcite), and Hem (Hematite). Each pattern shows distinct peaks corresponding to these minerals, indicating their presence in the samples. The patterns are aligned vertically, allowing for comparison of mineral compositions across different samples.
Mineralogical composition (w%) of the studied samples
Table 1 Long description
The table presents the mineralogical composition in weight percentages of various geological samples. It is organized into columns labeled Formation/Member, Sector, Sample, Qtz, K-fel, Pl, Anat, Cal, Gth, Hem, Il/Ms, I/S, Pg, Chl, Trioct ML, and Kao. The rows are grouped by Formation/Member, Sector, and Sample, with specific values for each mineral type. Notable trends include the highest value of illite-muscovite (Il/Ms) in the Greywacke-Slate Mb of the Malaguide Complex and varying percentages of chlorite (Chl) and trioctahedral mixed-layer (TML) across different formations and sectors.
Qtz, quartz; Pl, plagioclase; K-feld, k-feldspars; Anat, anatase; Cal, calcite; Gth, goethite; Hem, hematite; Il-Ms, illite-muscovite; I/S, mixed layer illite/smectite; Pg, paragonite; Chl, chlorite; Trioct-ML, trioctahedral mixed layer; Kao, Kaolinite-Dickite. Min V, Minimum Value; Max V, Maximum Value; %, percentage mean value; DvSt, Deviation Standard; C%, (DvSt/ χ)x100; χ, middle value.
Mineralogical composition of the bulk sample slates.
5.b. Clay fraction (<2 μm) of the slates
The semi-quantitative analysis of <2 μm fraction shows the presence of Il/Ms, Pg, mixed layers I/S, Chl and TML (Figure 7). In particular, a prevailing content of Il/Ms is observed in all samples (from 59 wt% to 91 wt%; Table 2; Figure 8): in the Malaguide succession, it shows a decreasing trend in the Retamares Mb, increasing in the Greywacke-Slate Mb and Marbella Conglomerate Fm, while in the Ghomaride succession, the highest values are in the Greywacke-Slate Mb. Pg is present in one sample (23A-89) of the Ghomaride succession belonging to the Marbella Conglomerate Fm up to 6 wt%. Mixed-layer I/S ranges from 2.5 al 20.5 wt% and shows different ordering (from random R0 to long-range R3 ordering). The randomly R0 phase is mainly present in the samples collected from the Greywacke-Slate Mb in the Murcia Sector (upper part of the Malaguide Complex); I/S R1 is recognized only in two samples from Retamares Mb in the Granada Sector (Malaguide succession) and from the Greywacke-Slate Mb in the Tetouan Sector (Ghomaride succession). Finally, I/S R3 is present in most samples except in the Greywacke-Slate Mb in the Murcia Sector. Chl is detected in almost all samples with percentages ranging from 2 to 17 wt%, assuming the lowest contents (from 1 to 7 wt%) in the upper part of the Malaguide succession belonging to the Greywacke-Slate Mb (Murcia and Lorca Sectors). As for TMLs, they are present in almost all samples in percentages ranging from 2 to 36 wt% (Table 2). Higher values are observed in the Retamares Mb in the Malaga Sector. The sum of Mg-bearing minerals, as Chl and TML, ranges from 0 to about 40 wt%. Again, as can be seen in Figure 8, the highest values are detected in the Retamares Mb in the Malaga Sector (lower part of the Malaguide succession), while the lowest ones are in the Greywacke-Slate Mb in the Murcia Sector (upper part of the Malaguide succession).
Representative X-ray diffraction patterns of clay fraction. For abbreviations see text.
Figure 7. Long description
A line graph showing X-ray diffraction patterns of clay fractions. The horizontal axis represents the angle in degrees ranging from 4 to 31 2θ degrees. The vertical axis represents intensity in counts per second. Multiple data lines are labeled with identifiers such as 22A-19, 22A-58, 22A-96, CU-7, 22A-6, 22A-101, 22A-92, 23A-21, and 23A-13. Peaks are labeled with identifiers like Chl, Trioct-ML, Il-Ms, and Kaol. Each data line shows distinct peaks at various angles, indicating different mineral compositions.
Mineralogical composition (w%) of phyllosilicates in the <2 μm fraction
Table 2 Long description
The table presents the mineralogical composition of phyllosilicates in the <2 m fraction across different formations and members. It includes data for various sectors such as Malaga, Murcia, Lorca, Chaouen, and Tetouan. The table is organized into columns for Formation/Member, Sector, Sample, Il/Ms, Pg, Chl, Kao, I/S R0, I/S R1, I/S R3, Trioct ML, CIS, and b mica. Each row provides specific values for these categories. Notable trends include varying percentages of Il/Ms, Pg, Chl, and mixed-layer I/S across different samples and sectors. The data highlights the prevalence of Il/Ms in all samples, with percentages ranging from 59 to 91 percent. Mixed-layer I/S shows different ordering patterns, and Chl is detected in almost all samples with percentages ranging from 2 to 17 percent. Trioct ML values range from 2 to 36 percent, with higher values observed in the Retamares Mb in the Malaga Sector.
Il-Ms, illite-muscovite; Pg, paragonite; Chl, chlorite; Kao, Kaolinite-Dickite; I/S-R0,-R1,-R3, mixed layer illite/smectite from random R0 to long-range R3 ordering; Trioct-ML, trioctahedral mixed layer; CIS, Crystallinity Index, standardized according to Warr and Rice (Reference Warr and Rice1994); b mica, Illite-Muscovite b parameter. Min V, Minimum Value; Max V, Maximum Value; %, percentage mean value; DvSt, Deviation Standard; C%, (DvSt/ χ)x100; χ, middle value.
Illustration of the mineralogical composition of the < 2 μm fraction.
CIS values ranges from 0.32 to 0.52 °2Ɵ, and the b parameter varies from 8.993 to 9.016 Å (Table 2), except for 22A-97 sample showing the higher value (9.026 Å). These data indicate metamorphic grades corresponding from lower anchizone to upper anchizone (Warr and Ferreiro Mählmann, Reference Warr and Ferreiro Mählmann2015) and low-pressure conditions (from 2 to 5 kbar; Guidotti and Sassi, Reference Guidotti and Sassi1986; Potel et al. Reference Potel, Ferreiro Mählmann, Stern, Mullis and Frey2006; Cavalcante et al. Reference Cavalcante, Belviso, Laurita and Prosser2012). The lowest KI values are measured in the Retamares Mb in the Malaga Sector, belonging to the lower part of the Malaguide succession (Almogía Fm). Such values indicate that this sector experienced the highest temperatures (around 280–300°C). In this sector, the Marbella Conglomerate (upper part of the Malaguide succession) shows higher KI (0.30–0.32 °2Ɵ) in comparison with Retamares corresponding to lower temperatures (250°C). KI values in the Greywacke-Slate Mb of Lorca Sector indicate lowest temperatures, 230–250°C, however, linked to very low-grade metamorphism. The highest KI values and the lowest b parameter are recorded in the Greywacke-Slate Mb in the Murcia Sector, corresponding to a temperature of about 200°C and a pressure of about 3 kbar, estimates consistent with the lower anchizone. This sector is also characterized by the presence of I/S R0. Moreover, temperatures of about 250°C are inferred for the whole Ghomaride succession (Morocco region) on the basis of KI values ranging from 0.28 to 0.36.
5.c. Petrography and Geochemistry of the slates
The mineral assemblage of the slate protoliths consists of quartz, plagioclase, K-feldspar, biotite, muscovite, Fe-oxide and rare calcite; the metamorphic mineral assemblage of very-low grade metamorphic conditions can be observed only in the fine-grained matrix, and it is composed by chlorite, sericite, epidote and fine-recrystallized quartz grains (Figure 9). The slates show a pervasive foliation in the fine-grained types defined by weakly deformed micaceous levels and quartz-feldspathic laminae in which anastomosing lens-shaped quartz-feldspar aggregates can be observed (Figure 9a); recrystallized matrix is shown by aligned flakes of fine-mica, chlorite derived by a nearly complete substitution of biotite and quartz with lobate and thin recrystallization grain boundaries (Figure 9b). In the samples with medium grain size, the relict sedimentary granular structures can be well observed both in the quartz-feldspar portion and between micaceous layers where biotite does not show destabilization signs (Figure 9c, d).
Microphotographs of slates from Spain and Morocco under cross polarized light. Spain: (a) thin foliation marked by white micas (Ms) in quartz-feldspathic matrix bounding lithic porphyroclast (Qtz-Kfeld lithic); (b) detrital structures and fine-grained recrystallization of quartz (Qtz) and white micas (Ms). Morocco: (c) penetrative recrystallization in quartz-feldspathic layers alternating with decolourised biotite (Bt) relicts; (d) quartz porphyroclasts (Qtz) with recrystallised edges in a fine-grained anastomosed silico-clastic matrix.
Figure 9. Long description
Panel A: A microscopic image of a slate from Spain showing thin foliation marked by white micas in a quartz-feldspathic matrix bounding a lithic porphyroclast. Panel B: A microscopic image of a slate from Spain displaying detrital structures and fine-grained recrystallization of quartz and white micas. Panel C: A microscopic image of a slate from Morocco showing penetrative recrystallization in quartz-feldspathic layers alternating with decolorized biotite relicts. Panel D: A microscopic image of a slate from Morocco featuring quartz porphyroclasts with recrystallized edges in a fine-grained anastomosed silico-clastic matrix.
Chemical composition based on major and trace elements including rare earth element (REE)of the studied slates is listed in Table 3. Nineteen samples derive from the Malaguide succession (Spain) and six samples from the Ghomaride succession (Morocco). Only the fine-grained intervals of each succession have been considered here. Two samples were excluded for their outlier compositions: 22A-59 from Almogía Formation in the Malaguide succession containing high Fe2O3 value (35.81 wt%) and 23A-13 sample from Almogía Formation in the Ghomaride succession for its high SiO2 content (77.78 wt%).
Description: Chemical composition and calculated ratios of elements for the Maláguide and Gomaride slates
Table 3 Long description
The table presents the chemical composition of slates based on major and trace elements, including rare earth elements (REE). It includes data for nineteen samples from the Malaguide succession in Spain and six samples from the Ghomaride succession in Morocco. The table is organized into several columns: Sample, Location, Compass Lat N, Compass Long W, Compass Elev, SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, P2O5, LOI, Total, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Sc, Th, Cr, Ni, Cu, Zn, Ga, Rb, Sr, Zr, Nb, Cs, Ba, Hf, Ta, Pb, U, V, As, Sb, W, Bi, Be, B, Mo, Sn, Ge, Ag, Cd, In, H2O, CO2, S, F, Cl, Br, I, N, O, C, H, He, Ne, Ar, Kr, Xe, Rn, At, Ra, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr, Rf, Db, Sg, Bh, Hs, Mt, Ds, Rg, Cn, Nh, Fl, Mc, Lv, Ts, Og, D, T, P, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Kr, Rb, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, I, Xe, Cs, Ba, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Po, At, Rn, Fr, Ra, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr. Each row provides the specific values for each element in the samples. The table also includes calculated ratios of elements for the Malaguide and Ghomaride slates.
Note: LOI, Loss of Ignition; LREE, Light Rare Earth Elelement; HREE, Heavy are Earth Elelement; REE, Rare Earth Elelement; CIW, Chemical Index of Weathering; CIA, Chemical Index of Alteration; CIA’, Chemical Index of Alteration calculated calculated without the CaO content.
Whole-rock chemistry, on the basis of SiO2/Al2O3 and Fe2O3/K2O ratios, indicates an origin of protoliths from shale sediments with a strong overlap of the Malaguide and Ghomaride samples (Figure 10). Considering the overall chemical compositions in detail, it is possible to highlight some differences. The Malaguide slate samples from the lower part of the succession (Almogía Formation, Retamares Mb) have an average content of SiO2, Al2O3 and K2O corresponding to 55.55 wt%, 20.93 wt% and 4.58 wt%, respectively while their CaO content is low, averaging 0.44 wt% due to the lack of calcite. Fe2O3 and MgO have average values of 8.34 wt% and 2.91 wt%, respectively, in line with Cr/Th (19.76) and Zr/Sc (10.54) ratios; the Rb/Sr ratio averages 1.64 and the ∑ REE is 108.76 ppm showing a fractionated pattern with La/Yb ratio of 10.35.
Herron’s diagram (Herron, Reference Herron1988) showing a homogeneous origin of Maláguide and Ghomaride slates. The protoliths of slates fall in the field of shales.
The Malaguide slate samples (Spain) belonging to the upper part of the succession (Greywacke-Slate Mb and Marbella Conglomerate Fm) show a SiO2 contents ranging from 53.7 wt% to 65.3 wt% (average 57.1 wt%), Al2O3 between 17.3 to 24.21 wt% (average 20.12 wt%) and K2O averaging 4.31 wt%; the CaO content is approximately 1.22 wt% based on two samples with some calcite grains. The Fe2O3 and MgO contents are, on average, 7.72 wt% and 1.96 wt%, respectively. The Cr/Th and Zr/Sc ratios are around 13.25 and 13.15, respectively, and the Rb/Sr ratio is 1.1, their ∑REE being 118.89 ppm with La/Yb of 10.47.
The Ghomaride slate samples (Morocco) show SiO2, Al2O3 and K2O contents averaging 58.77 wt%, 19.85 wt% and 3.95 wt%, respectively. The Cr/Th, Zr/Sc and Rb/Sr ratios assume 22.50, 28.76 and 1.22 on average, respectively; the ∑REE is around 93.59 with La/Yb ratio of 9.71.
6. Discussion
6.a. Relationships between chemistry, mineralogy and metamorphic features
Distinct differences in quartz abundance can be observed between the lower and upper parts of the Malaguide succession (Spain): the lower part (Retamares Mb) contains an average of 23.29 wt% of quartz, while the upper part (Greywacke-Slate Mb) has a higher quartz content, around 25.4 wt%, although two samples show very low contents (Table 1). The quartz content is also higher in the Ghomaride slate samples, with an average of 27.4 wt% (Table 1).
Strong relationships between Il/Ms vs K2O and Qtz vs SiO2 are shown in Figure 11, in fact, the relevant Il/Ms content is corroborated by the strong correlation between K2O and Al2O3 and negative Al2O3 vs SiO2 relationship (Figure 11); in addition, Cr, Co, Ni and V show a positive correlation with Al2O3 suggesting a linkage to Il/Ms contents due to their electric charges adsorbed on the clay surfaces or in the clay interlayers (Figure 11). On the contrary, the absence of correlation between Cr, Co, Ni and V and the abundances of Chl+TML suggest that these elements were lost from these minerals and acquired by newly formed illite and muscovite (Figure 11).
Binary diagrams showing the relationships between chemical and mineralogical compositions. SiO2, Al2O3, K2O and (Cr+Co+V+Ni) contents depend on Il/Ms.
Figure 11. Long description
The image contains six scatter plots arranged in a 2x3 grid. Each plot shows relationships between different chemical and mineralogical compositions. Panel A: Scatter plot with Il/Ms on the y-axis and K2O on the x-axis. Panel B: Scatter plot with Qz on the y-axis and SiO2 on the x-axis. Panel C: Scatter plot with K2O on the y-axis and Al2O3 on the x-axis. Panel D: Scatter plot with Al2O3 on the y-axis and SiO2 on the x-axis. Panel E: Scatter plot with Al2O3 on the y-axis and Cr+Co+V+Ni on the x-axis. Panel F: Scatter plot with Il/Ms on the y-axis and Cr+Co+V on the x-axis. Each plot includes data points represented by different symbols and colors, indicating different types of conglomerates and formations. The data points show various trends and patterns across the plots.
The MgO, Fe2O3 and Heavy Rare Earth Element (HREE) contents are, instead, bound to Chl and TML as shown in Figure 12, while are unrelated to the Il/Ms; furthermore, the Chl+TML content decreases upwards in relation to the increase in acidic detritus in the Malaguide succession. In fact, the Qtz appears to increase towards the upper part from 23.3 to 25.4w%.
Binary diagrams of MgO vs Chl+TML (a), Chl+TML vs HREE (b), HREE vs Il/Ms (c) and Zr/Sc vs HREE (d). A: the more mafic compositions of the Maláguide lower part slates are shown; B: correlation between HREE and Chlorite+TML is highlighted; C: non-correlation between Il/Ms and HREE; D: on average, the Zr/Sc ratio of the Maláguide slates in the bottom part is lower due to the slightly higher Sc values.
The high abundance of Il/Ms in both bulk samples and in the <2 μm fraction (Tables 1 and 2), together with the control of these mineral phases over the whole chemistry, suggests similar metamorphic conditions corresponding to a very-low metamorphic grade for all studied slates. In addition, the composition of protoliths and the metamorphic grade in the Malaguide and Ghomaride slates are comparable, although some structural differences depend on the grain size of protoliths (Figure 9). A more evident metamorphic foliation can be observed in the clayey types, whereas relict sedimentary and granular structures can be seen in the sandstone-siltstone types. Overall, the bulk composition of protoliths is similar and referable to a clastic clayey source. An initial comparison among the three groups reveals that (i) Ghomaride slates (5 samples of the Almogía Formation) have chemical composition in the same range as the whole Spanish succession; consequently, the Ghomaride slates can be considered chemically comparable with the Malaguide ones; (ii) the felsitic (quartz and SiO2 contents) components increase upwards in the Malaguide slates, while the mafic contents decrease as showed by Fe2O3, MgO contents, Zr/Sc ratio and by the abundance of TML+Chl vs MgO (Figure 12).
The regular relationships between Il/Ms contents with K2O, Al2O3 and SiO2 could indicate that these phases were formed during the incipient metamorphism, while part of chlorite and TML (prevalent biotite) represents detrital components derived from a more mafic source, especially in the lower part of the Malaguide succession. This can be deduced from Figure 13a in which a shift in chemical compositions towards medium-high Co values is related to an increase of a mafic source input (oceanic crust or ophiolites?). Similar deductions can be made for the distribution of Th/Sc and Zr/Sc ratios (Figure 13b) in which the slates of the Malaguide lower part show a composition shifted towards basalt. However, these inferences come from the comparation of our data with Post-Archaean Australian Shales (PAAS) composition (Taylor and McLennan, Reference Taylor and McLennan1985). In fact, the slates from lower part in the Malaguide succession show an enrichment of Fe2O3+MgO, Cr+Co and V with respect to PAAS and minor content of Zr and Th (Table 3).
Th-Hf-Co triangular plot and binary diagram (Zr/Sc vs Th/Sc). The slates of the Maláguide lower part have a more mafic composition than those of the upper part and the Ghomaride slates. UCC: Upper Continental Crust; TC: Transitional Crust; OC: Oceanic Crust; PAAS: Th-Hf-Co triangular plot.
Figure 13. Long description
Panel A: A triangular plot displays the relationships between Th, Hf, and Co contents. The plot includes data points for Marbella Conglomerate, Greywacke-Slate Mb, and Retamares Mb from both Malaguide and Ghomaride formations. The axes are labeled Th, Hf, and Co. The plot shows the distribution of these elements within the upper continental crust (UCC), total continental crust (TC), and oceanic crust (OC). Panel B: A binary diagram plots Zr/Sc against Th/Sc. It includes data points for the same geological formations as Panel A. The axes are labeled Zr/Sc on the x-axis and Th/Sc on the y-axis. The diagram shows a compositional trend line with markers for basalt, andesite, PAAS, and granite.
A certain variability of the slate compositions in the comparison with REE patterns of PAAS was observed. The chondrite normalized REE patterns are characterized by the typical shale-like trend with enriched-LREEs ((La/Yb)ch on average 10.57), flat distribution of HREEs ((Gd/Yb)ch 1.68 on average), positive Ce anomalies (Ce/Ce* 1.28) and negative Eu anomalies (Eu/Eu* on average 0.71). Some samples of the lower part of the Malaguide show a REE depletion (Figure 14).
Diagram of the rare earth element (REE) patterns normalized to Chondrite (McDonough and Sun, Reference McDonough and Sun1995) showing the range values of the studied formations. Some samples of slates from the Maláguide lower part (Retamares Mb) are REE depleted, while the most of them show PAAS (Post-Archaean Australian Shales from Taylor and McLennan, Reference Taylor and McLennan1985) like compositions (black line).
With regard to HREE, there is a positive correlation with the abundance of chlorite and TML (Figure 12b) and a negative correlation with Il/Ms (Figure 12c) and Zr/Sc (Figure 12d), which can be interpreted as indicating a detrital provenance for some chlorites and TML and a metamorphic origin for Il/Ms. The absence of relationships between LREE and micaceous minerals could indicate that LREE are related to scattered accessory phases such as apatite and monazite.
6.b. Estimated temperatures (metamorphism)
Using KI value (Jaboyedoff and Thellin, Reference Jaboyedoff and Thelin1996; Merriman and Frey; Reference Merriman, Frey, Frey and Robinson1999; Cavalcante et al. Reference Cavalcante, Belviso, Laurita and Prosser2012 and reference therein) and b parameter (Guidotti and Sassi, Reference Guidotti and Sassi1986; Potel et al. Reference Potel, Ferreiro Mählmann, Stern, Mullis and Frey2006; Cavalcante et al. Reference Cavalcante, Belviso, Laurita and Prosser2012), the P-T of the analysed slates was estimated. Temperatures around 250°C have been calculated for Ghomaride succession (Morocco). The samples collected in the Malaguide successions recorded different temperatures. In particular, in the Greywacke-Slate Mb of the Almogía Formation, from the Murcia Sector, the lowest P-T values (2–3 kbar; around 200°C) were recorded, slightly higher temperatures (230–250°C) being deduced in the Lorca Sector. In the Malaga Sector Retamares Mb of the Almogía Formation, the highest temperatures (280–300°C) have been calculated for the Retamares Mb of the Almogía Formation and the lowest temperatures (around 250°C) in the Marbella Conglomerates. The calculated temperatures indicate that the studied slates, in both Malaguide and Ghomaride successions, were affected by a very low-grade metamorphism (T ranges from 200–300°C).
The occurrence of typically diagenetic minerals, as kaolinite-dickite and illite-smectite or chlorite-smectite mixed-layers, which has been also widely described in the Malaguide rocks of Permian-Triassic ages, is related to the very-low grade metamorphism, in diagenetic and low-anchizonal conditions, which affected the Malaguide Complex during the Alpine Orogeny (see Gómez-Pugnaire et al. Reference Gómez-Pugnaire, Nieto, Abad, Velilla, Garrido, Acosta-Vigil, Barich, Hidas, López Sánchez-Vizcaíno, Quesada and Oliveira2019, and references therein). Smectite has been described as a final retrograde-diagenetic alteration product in the Malaguide rocks of Sierra Espuña (Nieto et al., Reference Nieto, Velilla, Peacor and Ortega-Huertas1994) and in general in all the internal zones of the Betics (Nieto et al. Reference Nieto, Pilar Mata, Bauluz, Giorgetti, Árkai and Peacor2005).
6.c. Paleo-depositional environment, paleoweathering and paleoclimate
Geochemical data (Ni/Co vs V/(V+Ni)) show that the studied deposits sedimented in oxidizing conditions (Figure 15). In the analysed samples, CIA values (Figure 15b) of the three subsets fall in the range from 67% to 77% suggesting that the clays were formed through prevailing moderate alteration in the source area. The same indications result from the presence of a positive Ce anomaly and from the relationships between K2O/Al2O3 and Ga/Rb ratios (Figure 16), suggesting a prevalence of medium to long-lasting weathering under oxidizing environmental conditions (Figure 15 and Table 3) (Mongelli et al. Reference Mongelli, Critelli, Perri, Sonnino and Perrone2006).
(a) V/(V+Ni) vs Ni/Co diagram showing an oxidizing environment of deposition; (b) Ternary A–CN–K (Nesbitt and Young, Reference Nesbitt and Young1982) and A–C–N (Harnois, Reference Harnois1988) plots indicating a moderate weathering for Spanish and Morocco slates. Key: CIA, chemical index of alteration (left scale); CIA’, Chemical Index of Alteranion without CaO content (right scale); A, Al2O3; CN, CaO+Na2O; K, K2O; Ab, Albite; Smt, smectite; Kln, kaolinite; Gbs, gibbsite; Ilt, illite; Ms, muscovite; Bt, biotite; Kfs, K-feldspar.
Figure 15. Long description
Panel A: A scatter plot showing the relationship between V/(V+Ni) and Ni/Co, indicating an oxidizing environment of deposition. The x-axis represents Ni/Co, and the y-axis represents V/(V+Ni). Different colored markers represent various formations: Marbella Conglomerate, Greywacke-Slate Member, and Retamares Member from both Malaguide and Ghomaride complexes. Panel B: Two ternary plots, A-CN-K and A-C-N, indicating moderate weathering for Spanish and Moroccan slates. The axes represent Al2O3 (A), CaO+Na2O (CN), and K2O (K). The plots show the distribution of different minerals such as Albite, Smectite, Kaolinite, Gibbsite, Illite, Muscovite, Biotite, and K-feldspar. The data points are color-coded to represent different formations.
Binary diagram of the Ga/Rb–K2O/Al2O3 ratios indicating the paleoclimate of the studied mudrocks (Roy and Roser, Reference Roy and Roser2013).
6.d. Unroofing and paleotectonic implications
Mineralogical and geochemical proxies highlighted differences in quartz abundance in the two studied sectors (Table 1, Figure 11). There is an upward increase from Retamares Mb to Greywacke-Slate Mb in the Malaguide succession (Spain), while, in general, the quartz content is also higher in the Ghomaride slates. This is also testified by binary diagrams Chl+TML vs HREE, HREE vs Il/Ms and Zr/Sc vs HREE and Zr/Sc ratio (Figure 12). In fact, they indicate an increasing mineralogical maturity of sediments from bottom to top that could be correlated to a progressive erosion and unroofing after a major tectonic phase and the recycling of previous sedimentary cover.
A post sedimentary metamorphism is testified by the high abundance of Il/Ms in both bulk samples and in the <2 μm fraction (Tables 1 and 2). The bulk composition of protoliths is similar and referable to a clastic clayey source. A metamorphic foliation has been observed in the clayey types, whereas relict sedimentary and granular structures are seen in the sandstone-siltstone successions. The calculated temperatures indicate that the studied slates in both the Malaguide and Ghomaride successions were affected by a very low-grade metamorphism (T ranges from 170°C to 250°C), that can be attributed to both late-Variscan s.l. or Alpine metmorphism.
A possible magmatic supply is evidenced by the Fe2O3 and MgO contents, the Zr/Sc ratio, the trend of the TML+Chl vs MgO diagram (Figure 12), the Th-Hf-Co triangular plot and the binary diagram Zr/Sc vs Th/Sc (Figure 13). This inference is also supported by the comparison with PAAS showing an enrichment of Fe2O3+MgO, Cr+Co and V with respect to PAAS and minor content of Zr and Th (Table 3). In conclusion, from bottom to top (Retamares Mb to Greywacke-Slate Mb) of the Malaguide succession the felsic (quartz and SiO2 contents) components increase, while mafic contents (oceanic crust or ophiolites) decrease. This could indicate the erosion of a magmatic arc and/or relict oceanic suture in the hinterland, clearer recorded in the lower part of the succession.
The variability of the slate compositions observed in comparison with REE patterns of PAAS (Figure 14) suggests a detrital provenance for some chlorites and TML and a metamorphic origin for Il/Ms. This could indicate that a tectonic phase with associated low-grade metamorphism took place before the deposition of the Culm Facies, testifying a continental collision with subduction associated to the closure of an oceanic branch.
Similar interpretations were proposed by Criniti et al. (Reference Criniti, Martín-Martín and Martín-Algarra2023, Reference Criniti, Martín-Martín, Hlila, Maaté and Maaté2024, Reference Criniti, Talmat, Martín-Martín and Critelli2026) by means of sandstones in the same lithostratigraphic units. According to our findings and of the above-mentioned authors, in the north Gondwana, several oceanic branches of the Paleotethys Ocean separated a terrane called Paleo-Mesomediterranean microplate (PMM) from the Iberian-Central French Domain, to the north, and from the Paleo-Atlas-Saharian Domain, to the south, during the Carboniferous (Figure 17). On this microplate, the Malaguide and Ghomaride domains, where the Culm Facies sedimented, were placed on the margin towards the southern oceanic branch of the Paleotethys Ocean. A possible oceanic suture, resulted from the closure of a Palaeotethys branch, with its sedimentary cover, affected by metamorphism, must have been the source area which supplied the Malaguide-Ghomaride Basin during the Culm Facies sedimentation. The oceanic branch supposedly separated the Paleo-Mesomediteranean terrane from Iberian-Central French Domain, its suture being located in the hinterland of the basin. In turn, the Malaguide-Ghomaride Basin must have been shortened and closed towards the end of Carboniferous, during the Paleotethysian tectonic phase of Pangea supercontinent construction. Subsequently, the Malaguide-Ghomaride domains were affected by the Alpine tectonics during the development of Alpine Perimediterranean Chains, as well as, by the opening of the western Mediterranean Sea din Cenozoic times.
Paleogeographic-paleotectonic maps modified from (Pérez-Estaún et al. Reference Pérez-Estaún, Bea, Bastida, Marcos, Catalán, Martínez-Poyatos, Arenas, García, Azor, Simancas, González and Vera2004; Vera, Reference Vera2004; Michard et al. Reference Michard, Hoepffner, Soulaimani, Baidder, Michard, Saddiqi, Chalouan and Lamotte2008; Arenas et al. Reference Arenas, Díez-Fernández, Rubio-Pascual, Sánchez-Martínez, Martín-Parra, Matas, González del Tánago, Jímenez-Díaz, Fuenlabrada, Andonaegui and García-Casco2016; Criniti et al. Reference Criniti, Martín-Martín and Martín-Algarra2023, Reference Criniti, Martín-Martín, Hlila, Maaté and Maaté2024, Reference Criniti, Talmat, Martín-Martín and Critelli2026; and references therein). (a) Laurentia and Gondwana configuration at 350 Ma (Early Carboniferous); (b) Pangaea configuration at 310 Ma (Late Carboniferous); (c) detail of the northern Gondwana configuration in the Mediterranean area at 310 Ma (Late Carboniferous) with location of the geological cross-section (Figure 17d); (d) paleogeographic-paleotectonic cross-section of the northern Gondwana from the Iberian plate to the Paleo-Atlas-Saharian Domain passing through the Maláguide-Ghomaride Subdomains pertaining to the Paleo-Mesomediterranean microplate.
Figure 17. Long description
Panel A: A global map showing the configuration of Laurentia and Gondwana at 350 million years ago (Early Carboniferous). Various landmasses such as Siberia, China, and Arabia are labeled. Panel B: A global map depicting the configuration of Pangaea at 310 million years ago (Late Carboniferous). Landmasses like North America, South America, Africa, and Eurasia are labeled. Panel C: A detailed map of the northern Gondwana configuration in the Mediterranean area at 310 million years ago (Late Carboniferous). It includes the location of a geological cross-section. Panel D: A paleogeographic-paleotectonic cross-section from the Iberian plate to the Paleo-Atlas-Saharian Domain, passing through the Malaguide-Ghomaride Subdomains of the Paleo-Mesomediterranean microplate. The cross-section includes various geological domains and features such as the Armorica Domain, Avalonia Domain, and the Central French Massif.
7. Conclusions
-
• The mineralogical and chemical composition of 25 slate samples and the petrography of 4 coarse-grained slates were determined from seven sectors from the Carboniferous Culm Facies from the Malaguide (S Spain) and the Ghomaride (N Morocco) domains.
-
• These domains consist of the Almogía Fm (including the Retamares and the Greywacke-Slate mbs), followed by the Marbella Conglomerate Fm.
-
• The mineral assemblage of the slate protoliths consists of quartz, plagioclase, K-feldspar, biotite, muscovite, Fe-oxide and rare calcite. Whole-rock chemistry indicates a shale deposit origin of protoliths.
-
• There is an upward increasing content in quartz in the succession, but also from the Malaguide to the Ghomaride domains. This indicates a certain maturity of sediments from bottom to top and from one domain to the other, which could be correlated with the progression of erosion after a previous tectonic phase and the recycling of previous sediments.
-
• The calculated temperatures indicate that the studied deposits of Culm Facies were affected by post-sedimentary very low-grade metamorphism in anchizonal conditions becoming slates.
-
• A possible magmatic supply is evidenced by the presence of mafic contents which decrease upward. This could indicate the erosion of a volcanic arc and/or relict oceanic suture in the hinterland, more active during the sedimentation of the lower lithostratigraphic units.
-
• Detrital provenance for some chlorites and TML indicates a metamorphic origin for Il/Ms. This could indicate that a tectonic phase associated with low-grade metamorphism took place before the deposition of the Culm Facies.
-
• The geochemical features suggest a prevalence of medium-to long-lasting weathering under oxidizing environmental conditions in the hinterland during the deposition of the Culm deposits.
-
• According to our findings and of from recent published papers, several oceanic branches of the Paleotethys Ocean in the north Gondwana separated a terrane called Paleo-Mesomediterranean (PMM) microplate from the Iberian-Central French Domain, to the north, and from the Paleo-Atlas-Saharian Domain, to the south, during the Carboniferous.
-
• On the PMM, the Malaguide and Ghomaride domains, where the Culm Facies sedimented, were placed on the margin towards the southern oceanic branch of Paleotethys Ocean.
-
• A low grade metamorphism must have been taking place during the closure of the northern oceanic branch, separating PMM from Iberian-Central French Domain, the magmatic arc/oceanic suture becoming the source area to supply the sedimentary material for the Culm Facies deposits.
-
• This area must have been a main part of the hinterland for the Malaguide-Ghomaride Culm basin. The Culm basin closed and structured at the end of the Pangea supercontinent construction during the Paleotethysian tectonic phase during the latest Carboniferous.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0016756826101071.
Acknowledgements
The authors are thankful for the critical and constructive comments of the reviewers. This research was funded by the Spanish Ministry of Science and Innovation, research project number PID2020-114381GB-I00. S. Criniti and S. Critelli were supported by MUR-PRIN 2022 (P2022WNCEC_002, H53D23011410001).
Competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Open access
