Low–Ti Continental Tholeiite Origin of Magmas With Calc-Alkaline Signature in Transcurrent Settings: The Mississippian Matachel Volcanic Field (SW Iberian Massif)
Abstract
Financial support was provided by the Spanish Ministry of Economy, Industry and Competitiveness and the European Regional Development Fund (MINECO/FEDER CGL2015-63530-P), and by the UPV/EHU (GIU20/010). Technical support provided by the Geochronology and Isotope Geochemistry of the SGIker facility of the University of the Basque Country (UPV/EHU) is acknowledged.
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1. Introduction Continental tholeiites are usually associated to extraordinary flood basalt emissions and formation of large igneous provinces (Hawkesworth etal.,1999; Peate & Hawkesworth,1996, and references therein). They represent the most widespread type of intra-plate basalts (Stepanova etal.,2014). Such large magma emissions generally developed in relation to either anorogenic (hot spot) or rift (plate divergence) geodynamic settings (Ingle etal.,2004; Keppie etal.,2008; Peate etal.,1992; Pouclet etal.,2017; Shellnutt etal.,2014; Stepanova etal.,2014; White & McKenzie,1989), examples of their relationship with tectonic settings that do not associate deep mantle Abstract In the Mississippian Matachel small volcanic field of the Ossa-Morena Zone (southern Iberian Massif) outpoured basic-intermediate lavas exhibit geochemical characteristics of Low-Ti continental tholeiites and calc-alkaline lavas. Low-Ti continental tholeiites integrate two contrasting groups of rocks: basalts (Mg#: 54 to 70; Ti/Zr: 61–79; LaN/LuN: 1.6–2.9; εNdi: +4.0–+6.6; “Group #1”), and basalts and basaltic andesites (Mg#: 43 to 66; Ti/Zr: 36–58; LaN/LuN: 2.5–5.9; εNdi: −0.2–+3.5; “Group #2”). Primitive Group #1 tholeiitic magmas were generated by partial melting of a garnet-free lherzolite from an enriched lithospheric mantle, near the lithosphere-asthenosphere thermal boundary layer (with a very limited asthenosphere melting input). Progressive interaction of these magmas with crustal alkali igneous rocks resulted in the formation of the petrological evolutionary trends observed, to a larger extent in the case of Group #2 Low-Ti tholeiites. Further assimilation of amphibole-rich calc-alkaline metaigneous rocks might have originated the basalts and basaltic andesites with calc-alkaline signature (Mg#: 33 to 56; Ti/Zr: 25–78; LaN/LuN: 2.0–5.6; εNdi: +2.8–+4.8). These exhibit a “Cordilleran-type” evolutionary trend, though are unrelated to plate convergence. The magmas with calc-alkaline signature attest to a closed-system differentiation process controlled by the fractionation of plagioclase, clinopyroxene, magnetite and ilmenite. It is proposed that Mississippian lithospheric-scale intra-continental wrenching, unrelated to coeval mantle plume upwelling, reworked complexly docked mantle domains and triggered mantle melting. Enduring mid-upper crustal processes (magma storage in mid-crustal chambers and crustal assimilation) likely shaped the latest petrologic and geochemical aspects of the Matachel Low-Ti tholeiites and related rocks with calc-alkaline signature. Plain Language Summary The Matachel area of the southern Iberian Massif was the scenario around 350 million years ago of intra-continental volcanism subsequent to a period of continental crust thickening (the Variscan Orogeny). The intra-plate volcanic products there are both acid and basic (bimodal) and outcrop at present along elongated basins directed NW-SE, bounded by strike-slip faults. In this study, we focused on the silica-poor basic units (basalts to basaltic andesites). The more basic units present characteristics of titanium-poor continental tholeiite magmas, that is, they are not related as usual to flood basalt emissions and formation of large igneous provinces. The less basic units correspond to magmas with calc-alkaline signature, richer in titanium and rare earth elements. Overall, the geochemical and mineralogical features of these two types of basic volcanic products within the same continental scenario attest to an evolution from purely mantle-derived magmas (tholeiite units) toward others that record contamination with an older crust (calc-alkaline units), either purely continental crust or, more likely, an Andean-type continental arc formed during the Lower Paleozoic Cadomian Orogeny. SARRIONANDIA ETAL. © 2023 The Authors. Geochemistry, Geophysics, Geosystems published by Wiley Periodicals LLC on behalf of American Geophysical Union. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. Low–Ti Continental Tholeiite Origin of Magmas With Calc-Alkaline Signature in Transcurrent Settings: The Mississippian Matachel Volcanic Field (SW Iberian Massif) F. Sarrionandia1 , J. Errandonea-Martin2 , E. Larrondo3 , M. Carracedo-Sánchez2, B. Ábalos2, and J. I. Gil Ibarguchi2 1Departamento de Geología, Facultad de Farmacia, Universidad del País Vasco UPV/EHU, Vitoria-Gasteiz, Spain, 2Departamento de Geología, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU, Bilbao, Spain, 3Team Ingeniería y Consultoría S.L., Parque Científico y Tecnológico de Bizkaia, Zamudio, Spain Key Points: • Lithospheric-scale transcurrent tectonics can promote low degrees of lithospheric mantle melting • Different Low-Ti continental tholeiites can derive from an enriched lithospheric mantle, by variable assimilation of alkali crustal rocks • Low-Ti continental tholeiites can behave as mafic precursors of magmas with calc-alkaline signature by crustal assimilation Supporting Information: Supporting Information may be found in the online version of this article. Correspondence to: F. Sarrionandia, fernando.sarr[email protected] Citation: Sarrionandia, F., Errandonea-Martin, J., Larrondo, E., Carracedo-Sánchez, M., Ábalos, B., & Gil Ibarguchi, J. I. (2023). Low–Ti continental tholeiite origin of magmas with calc-alkaline signature in transcurrent settings: The Mississippian Matachel volcanic field (SW Iberian Massif). Geochemistry, Geophysics, Geosystems, 24, e2023GC011139. https:// doi.org/10.1029/2023GC011139 Received 18 JUL 2023 Accepted 10 AUG 2023 Author Contributions: Conceptualization: F. Sarrionandia, B. Ábalos Data curation: J. Errandonea-Martin, E. Larrondo Formal analysis: J. Errandonea-Martin, E. Larrondo, M. Carracedo-Sánchez Funding acquisition: J. I. Gil Ibarguchi 10.1029/2023GC011139 RESEARCH ARTICLE 1 of 39
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 2 of 39 upwelling (Hawkesworth etal.,1999; Shellnutt etal.,2014; White & McKenzie,1989) or with lithospheric-scale shear zones (wrench and convergent; Dessureau etal.,2000) being scarce. Published petrotectonic models most often convey their generation directly or indirectly to mantle plumes (Dessureau etal.,2000; Hawkesworth etal.,1999; Ingle etal.,2004; Lassiter & DePaolo,1997; Lightfoot etal.,1993; Neal etal.,2002; Peate & Hawkesworth,1996; Peate etal.,1992; Sandeman etal.,2014; Stepanova etal.,2014; White & McKenzie,1989). In large-scale tectono-magmatic settings such as the latter, the generation of crust-derived magmas is usually conspicuous (Keppie etal.,2008; Shellnutt etal.,2014), which also discloses an actual high extent of crustal reworking in relation with continental tholeiitic magmatism. Continental tholeiites possess a prominent petrotectonic significance for the regions where they are found, the hypotheses that would explain their origin and the specific processes involved in their genesis being diverse (Dupuy & Dostal,1984). Yet, continental tholeiite basalts have generally evolved rather than primitive geochemical/isotopic compositions that hinder the accurate recognition of their sources (Cox,1980; Dessureau etal.,2000; Dostal & Dupuy,1984; Dupuy & Dostal,1984; Sandeman etal.,2014). Additionally, compositional variations observed in continental tholeiitic basalts may be conditioned by the interaction of magmas generated from different mantle and crustal sources (Ingle etal.,2004; Lightfoot etal.,1993; Pouclet etal.,2017; Sandeman etal.,2014) and by crustal contamination (Cox & Hawkesworth,1985; Dupuy & Dostal,1984; Hawkesworth etal.,1999; Ingle etal.,2004; Keppie etal.,2008; Sandeman etal.,2014; Shellnutt etal.,2014; Stepanova etal.,2014) all of which obscure their primary characteristics. Extensive tholeiitic emissions have contributed significantly to crustal growth along Earth's history (Puchtel etal.,1998). Their lack of primary compositions constitutes a strong limitation for identification of the mantle source regions involved and, subsequently, for reconstruction of the tectono-magmatic evolution of the lithospheric domains where they occur. Presumed mantle source regions for tholeiitic magma extraction include (a) mantle plume asthenosphere/lithosphere systems (Dessureau etal.,2000; Ingle etal.,2004; Lassiter & DePaolo,1997; Leat,2008; Neal etal.,2002; Peate & Hawkesworth,1996; Peate etal.,1992), (b) subcontinental mantle lithosphere (Hawkesworth etal.,1999; Lassiter & DePaolo,1997; Peate & Hawkesworth,1996; Pouclet etal.,2017; Shellnutt etal.,2014), and (c) depleted asthenosphere (Dostal & Dupuy,1984; Dupuy & Dostal,1984; Frey etal.,1996; Pouclet etal.,2017; Stepanova etal.,2014). The apparently concomitant occurrence of High-Ti and Low-Ti continental tholeiites, or emission of associated alkali basalts, has been used as supporting evidence to unravel joint contributions of different mantle sources (Ingle etal.,2004; Lightfoot etal.,1993; Neal etal.,2002; Peate,1997; Peate etal.,1992; Pouclet etal.,2017). Alternative petrogenetic models suggest that both High-Ti and Low-Ti continental tholeiites can be generated by different degrees of partial melting of a spinel–peridotite lithospheric mantle (i.e., EMII-like), followed by variable amounts of crustal assimilation (Shellnutt etal.,2014). Hence, comprehension of the mantle sources implied in the generation of primitive tholeiitic magmas is still puzzling. This article deals with Tournaisian–Lower Viséan (Mississippian) Low-Ti continental tholeiites recently identified in volcanic outpourings from the Ossa-Morena Zone of the southwestern Iberian Massif. These include principally pillow-lava flows that constructed a several km in areal extension (but volumetrically limited) lava field within an ephemeral Carboniferous continental marine basin regionally known as the Matachel basin. Its current tectonic context relates to a major lithospheric-scale shear zone active during the Variscan orogeny (Burg etal.,1981) that reworked an older suture (Ábalos etal.,2023; Eguíluz etal.,2000). Continental tholeiitic volcanism also involved the coeval emission of calc-alkaline basalts and basaltic andesites. As far as we know, few examples have been reported of outpourings of continental tholeiites and associated basalts with calc-alkaline signature (Benek etal.,1996; Mitjavila etal.,1997; Pouclet etal.,2017). Based on the petrologic data presented (Sarrionandia etal.,2023) and discussed here, a genetic relationship between those geochemical assemblages is disclosed and a new petrotectonic model is proposed. Identification of the geochemical association reported, coupled to the relatively small emission volumes, provides an opportunity to further improve the knowledge of lithospheric processes that operated during the Variscan evolution of the Late Paleozoic northern Gondwana edge. 2. Regional Setting 2.1. The Ossa-Morena Zone The Iberian Massif represents the most extensive and complete segment exposed of the European Variscan Belt (Lotze,1945; Figure1a) This orogen resulted from a largely diachronous oblique collision between Laurussia Investigation: F. Sarrionandia, E. Larrondo, M. Carracedo-Sánchez Methodology: E. Larrondo, M. Carracedo-Sánchez Supervision: J. I. Gil Ibarguchi Writing – original draft: F. Sarrionandia, B. Ábalos Writing – review & editing: J. Errandonea-Martin, B. Ábalos, J. I. Gil Ibarguchi 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. 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Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 3 of 39 and Gondwana, and the consumption of the Rheic ocean (Braid et al.,2018; Díez-Fernández et al., 2016; Gutiérrez-Alonso et al., 2011; Matte & Ribeiro, 1975; Pereira, Chichorro, Johnston, et al., 2012; Pereira etal.,2017). The Iberian Massif was subdivided into major tectono-stratigraphic domains (Julivert etal.,1972; Lotze,1945) bounded by tectonic contacts that sometimes correspond to complex crustal-scale suture zones (Díez-Fernández etal.,2016; Quesada,1991; Schulmann etal.,2022, and references therein). Among these areas, the Ossa-Morena Zone (hereafter OMZ) of the SW Iberian Massif (Figure1b) encloses the remnants of a Cadomian arc ascribable to Andean-type plate convergence and subduction (Bandrés etal.,2002,2004; Eguíluz etal.,2000; López-Guijarro etal.,2008; Pin etal.,2002; Sánchez-Lorda etal.,2014). The active margin involved Figure 1. (a) Geological sketch map of the Iberian Massif. (b) Expanded Geological sketch map of the southern Iberian Massif, which displays the Carboniferous basins and the relative location of the Matachel Basin. 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 4 of 39 Gondwana hyper-extended crust and was active between ca. 645Ma and ca. 534Ma (Sarrionandia etal.,2020). The Cadomian arc constitutes the basement onto which the sedimentary and volcanic sequences of the Variscan cycle were unconformably overlain in the OMZ. The Variscan cycle was initiated in the midto late Cambrian with the development of an intra-continental rift (Chichorro etal.,2008; Sánchez-García etal.,2010,2014; Sarrionandia etal.,2012) that evolved into a passive margin with Ordovician-Silurian platform siliciclastic sequences (Robardet & Gutiérrez-Marco,2004). Middle Devonian–Carboniferous syn-orogenic ensembles were later deposited unconformably onto the rift-to-drift ensemble (Oliveira & Quesada,2019), tracking the complex and protracted closure of a re-entrant of the Rheic ocean (Braid etal.,2018). So far, it is widely accepted that Variscan orogenic development was diachronous and incorporated outstanding orogen-parallel (transcurrent) tectonic displacements (Kroner & Romer,2013; Pereira etal.,2010; Stampfli etal.,2013). At the Variscan orogen scale, this transcurrent stage has been ascribed as well to escape tectonics (e.g., Arthaud & Matte,1977; Brun & Burg,1982; Shelley & Bossière,2000). All this enabled the formation and closure of intra-orogenic strike-slip basins bounded by crustal-scale faults (Ábalos & Eguíluz,1991). As occurs in similar tectonic settings worldwide, these basins usually associate an outstanding magmatism (Dessureau etal.,2000, and references therein). Variscan wrenching was bracketed between 360 and 320Ma by some authors (Ábalos etal.,1991; Pereira, Chichorro, Johnston, etal.,2012; Pereira, Chichorro, Silva, etal.,2012; Pereira etal.,2009), whereas others hypothesize Tournaisian extension stages (ca. 360–345Ma) were followed by Viséan (345–330Ma) transpression (Simancas etal.,2006). Whatever the case, the continental-scale strike-slip faults bounded ephemeral marine basins infilled with interbedded syn-orogenic detrital sediments and volcanic deposits that were later deformed in a sinistral transpressional context until the late Carboniferous (Ábalos & Eguíluz,1991; Díez Fernández etal.,2021; Silva & Pereira,2004; Simancas etal.,2006). 2.2. The Early Carboniferous Magmatism and Its Origin Early Carboniferous magmatism in SW Iberia (including plutonic and volcanic rocks) was voluminous (Castro,2019; Sánchez-Carretero etal.,1990; Figure1b). In the case of intrusive plutonic rocks, this magmatism is represented by meta-aluminous and peraluminous calc-alkaline granitoids s.l., with rare alkali terms (Errandonea-Martin etal.,2019; García Casquero,1991; Lima etal.,2012; Moita etal.,2009,2015; Pereira etal.,2009; Pin etal.,2008; Pons,1982; Santos etal.,1990; Sarrionandia etal.,2013). The volcanic counterparts include meta-aluminous high-K calc-alkaline, peraluminous, N-MORB-like, and alkali rocks (Armendáriz etal.,2008; Munhá,1983; Oliveira etal., 2013; Rosa et al., 2008; Sánchez Carretero etal., 1989; Tornos etal.,2005). Excluding N-MORB-like and crustal-derived peraluminous magmatic rocks, the mantle source of the early Carboniferous calc-alkaline/alkaline magmatism remains unexplained. Two principal tectono-magmatic hypotheses have been proposed that envisage different regions of the mantle for magma extraction: an active or immediately demised subduction setting and a mantle plume. On one hand, the subduction context considers melt extraction from an enriched lithospheric mantle and/or upwelling asthenospheric mantle underneath (Castro etal.,1996; Díaz Azpíroz etal.,2006; Jesus etal.,2007; Moita etal.,2009,2015; Pereira etal.,2009,2017; Pin etal.,2008; Santos etal.,1990; Sarrionandia,2006; Tornos etal.,2005). On the other hand, the mantle plume hypothesis involves plume activity at ca. 350–340Ma in order to explain extraction of mantle-derived alkali magmas (Cambeses etal.,2015). Apart from these two models, the possibility also exists that the suggested subduction context actually corresponds to a frozen, earlier subduction-related edifice (late Neoproterozoic-Early Cambrian) that underwent Cambrian rifting and Variscan strike-slip reactivation. This might be supported by a correlation of surface alignments of Variscan granitoid plutons, batholiths and volcanic complexes with the thermal effects of coeval lithospheric shear zones separating mantle pieces with distinct pre-Variscan fabrics (Ábalos & Díaz,1995; Díaz etal.,1996; Sarrionandia etal.,2012, and references therein). 2.3. The Synorogenic Carboniferous Basins During the late Devonian–early Carboniferous, two main shallow marine basins developed in the SW Iberian Massif, separated by an emerged area that roughly corresponds with the central OMZ. These are the Los Pedroches basin to the North (Armendáriz etal.,2019), and the South-Portuguese Zone (Colmenero etal.,2002; Gabaldón etal.,1985) to the South (Figure1b), which are characterized by turbiditic successions and by a significant 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. 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Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 5 of 39 volcanic input (Armendáriz etal.,2008; Oliveira & Quesada,2019; Pereira, Chichorro, Silva, etal.,2012). Gabaldón etal.(1985) and Oliveira and Quesada(2019) suggested that, excluding the South-Portuguese Zone, the disconnected Mississippian outcrops bounded by regional-scale faults in the OMZ (localities from 2 to 15 in Figure1b) would conform a single marine basin referred to as the North Marine Basin or the Los Pedroches Basin. Sánchez Carretero etal.(1989,1990) considered that their volcanic deposits were genetically related to the so-called Villaviciosa de Córdoba–La Coronada magmatic alignment that hosts a complex of bimodal igneous rocks. To the North of this volcanic alignment a shallow marine, storm-dominated, mixed (siliciclastic-carbonate) platform exists (Armendáriz etal.,2008; Colmenero etal.,2002; Gabaldón etal.,1985). Distal platform marine sequences include pillow MORB-type and alkali basalts interbedded with rhyolitic volcaniclastic deposits and slates (Armendáriz etal.,2008; Larrondo,2014; Pérez-Lorente,1979). The Tournaisian–Viséan (350–335Ma) volcanic record in the South-Portuguese Zone (the South Marine Basin) and neighbor domains was concomitant with sedimentation in a similar shallow-mixed marine platform, and it has been related to a geochemically complex bimodal magmatism too (Chichorro,2006; Pereira etal.,2009, Oliveira & Quesada,2019, and references therein). It consists of basic-intermediate arc-tholeiites and moderately alkaline lavas, basic to acid I-type, low-K to high-K calc-alkaline rocks (Chichorro,2006; Pereira etal.,2009; Ribeiro,1983), and acid igneous rocks of crustal derivation (Munhá,1983). The world-class massive sulphide deposits of the Iberian Pyrite Belt (Oliveira etal.,2013; Quesada,1998; Tornos etal.,2005) are genetically related with the acid magmatic rocks. 3. The Basic Volcanism of the Matachel Basin The Matachel basin currently forms a ∼55km long and >2km wide, NW-SE trending, tight, vertical and symmetric syncline. It is bounded by two regional-scale strike-slip faults converging down-dip, subsidiary with the Hornachos fault to the Northeast (Ábalos & Eguíluz,1991; Larrondo,2014). These faults separate the Carboniferous record of this syncline from Late Ediacaran and earliest Cambrian units of the Badajoz-Córdoba (or Coimbra-Córdoba; e.g., Burg etal.,1981) ductile shear belt (Figure2). The shear belt represents a reworked suture zone with a complex and protracted polyphase tectonothermal evolution (Ábalos etal.,1991,2023). Geological maps of the Matachel basin (Apalategui etal.,1983; Arriola etal.,1983; Sánchez Cela etal.,1977) disclose a ∼2km thick Mississippian volcano-sedimentary succession made of conglomerates, slates, greywackes and limestones with interbedded volcanic rocks. Three distinct volcanic episodes can be recognized that, according to their stratigraphic position from base to top, are as follows: an older “acid volcanism I,” a “basic volcanism”, and a younger “acid volcanism II” (Apalategui etal.,1983; Larrondo,2014). Sánchez Carretero etal.(1989) merged the igneous record of the Matachel basin with that of the Benajarafe basin, located at its SE prolongation within the regional-scale Villaviciosa de Córdoba–La Coronada magmatic alignment. Two main lithostratigraphic groups resulted from this correlation: the Campana (consisting of andesites, dacites and basalts) and Erillas (made of dacites and rhyolites) volcanic complexes. The Tournaisian–Lower Viséan Erillas group encloses the “basic volcanism” episode reported above. Ábalos and Eguíluz(1991) identified the left-lateral strike-slip character of the Matachel basin NW-SE bounding faults and reconstructed its traverse geometry as an inverted flower structure. According to these authors, the stair-stepped, dextral NNW–SSE faults that offset NW-SE linear basin and fault segments allowed accommodation of N–S stretching components that first drove basin subsidence and then structural inversion into a tight syncline (Apalategui etal.,1983). Localized sinistral transpression was recorded both by the basin infilling and by neighbor major faults (Ábalos,1992; Díez Fernández etal.,2021). Transpression gave rise in the volcano-sedimentary basin infilling to the development of a pervasive N130-140E foliation bearing gently dipping intersection and stretching lineations and to the generation of transected folds (Ábalos,1992). A syn-deformational very low-grade metamorphic overprint (chlorite zone) accompanied structural development. The “basic volcanism” of the Matachel basin can be petrologically characterized and geologically mapped (e.g., in the synthesis after the 1:25.000 survey conducted by Larrondo(2014); Figure2). According to the field relationships, it is represented mostly by pillow-lava flows (∼75% in volume) and to a lower extent by pahoe-hoe-like sheet lava flows, agglomerates, lapilli-tuffs, and tuffs (Figure3). The volcanic package also contains minor slate, greywacke, and micro-conglomerate beds. Gabbro and diorite syn-volcanic shallow intrusions were emplaced in the volcano-sedimentary package, too. These intrusives usually conform isolated or interconnected small-sized sills (<50m thick and <1.5km long), though occasionally form larger (400m thick and 3.5km long) irregular-shaped intrusions (Figure2). Field relationships also reveal that the volcanic centers were unevenly 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. 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Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 6 of 39 distributed, being more common in the central and eastern areas of the Matachel basin (Figure2). In this regard, only three lava flows (20–45m thick) have been recognized in the central area of the Matachel basin. By contrast, in the southern limb of the far-east area 30 lava flows (some of them up to 55m thick) were identified by Larrondo(2014). At the outcrops, the basic volcanic rocks studied appear as massive, apparently isotropic and dark colored. A general aphyric and poorly vesicular character of the lava flows is outstanding, whereas aphanitic textures prevail in the intrusives. Only in a few cases plagioclase phenocrystals (up to 4mm in size) are discernible to the naked eye in the lava flows. Pyroxene oikocrystals (up to 15mm in size) and plagioclase phenocrystals (up to 12mm in size) can be observed in the intrusives. 4. Methods A collection of 23 samples from different Matachel basic lava flows (10 samples) and intrusive bodies (13 samples) was selected for this study (see sample location in Table S1 in Supporting InformationS1). Analytical studies included a detailed petrographic characterization of each sample, as well as mineral chemistry and wholerock elemental and Sr, Sm-Nd isotopic determinations presented herein or available in Sarrionandia etal.(2023). Geological mapping was improved with Light Detection And Ranging (LIDAR) images at 5m resolution and with ortophotographs of the Spanish National Plan of Aerial Ortophotography (PNOA, 2009, 2011, and 2013). Figure 2. Geological sketch map of the Tournaisian–Lower Viséan Matachel Basin. Pole diagrams refer to the volcano-sedimentary sequence bedding of the west and east areas of the basin. These diagrams also includestretching lineations and lineations of bedding and foliation intersections. See Table S1 in Supporting InformationS1 for the location UTM coordinates of the studied samples (#1 to #23). 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 7 of 39 Subsequent image analysis allowed planning of field itineraries and improved delineation of the main contacts in the field at the scale 1:18.000. Detailed outcrop descriptions in terms of rock structure and texture followed the procedure of McPhie etal.(1993). The preliminary map design was made at the 1:25.000 scale (Larrondo,2014) on the basis of the Villafranca de los Barros (0829-II and 0829-IV), Hornachos (0830-I, 0830-III, and 0830-IV), Usagre (0855-I, 0855-II, and 0855-IV) and Maguilla (0856-III) sheets of the Spanish Topographic Map (MTN). For the petrographic characterization, rock thin sections (48mm×28mm in size, 30μm thick) were prepared by the Technical Service of the University of the Basque Country (UPV/EHU). Various thin sections were stained following the method of Marsaglia and Tazaki(1992) and exposed to concentrated hydrofluoric acid before immersion in a supersaturated solution of sodium cobaltinitrite and, subsequently, in a supersaturated solution of barium chloride. Microscopic surveys were performed using a Leica DM LP device fitted with a CCD camera for microphotographic image acquisition. Modal determinations of rock-forming constituents were obtained by the point-count method (1,500 points/sample; Tables S2 and S3 in Supporting InformationS1). Mineral element concentrations were determined by Electron Microprobe using a Cameca SX 100 instrument at the University of Oviedo. Operating conditions of the microprobe were as follows: 10s counting time (peak), ∼10nA beam current and 15kV accelerating voltage. Calibration was done against French Geological Survey (BRGM) standard minerals and matrix correction factors (PAP) were used. Fe 3+/Fe 2+ was estimated by charge balance criteria (Droop,1987) and mineral formulae were normalized to 6 O for pyroxene, 23 O for amphibole, 8 O for plagioclase and alkali feldspar, and 4 O for titanomagnetite and ilmenite (Sarrionandia etal.,2023). Before the whole rock major, trace and isotope geochemical characterization at the Geochronology and Isotope Geochemistry–SGIker Facility of the University of the Basque Country (UPV/EHU), rock samples were crushed and 2–4cm in size pieces free of exotic components (e.g., hydrothermal infills, enclaves, weathered edges, etc.) were picked. These were reduced to small fragments (<0.8mm) with a tungsten jaw crusher and quartered, Figure 3. Synthetic stratigraphic logs of the West and East areas of the Matachel Basin, outstanding the main Tournaisian–Lower Viséan volcanic events defined by Apalategui etal.(1983). For correlation purposes it has been also included a synthetic stratigraphic log of a central sector of this basin. 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 8 of 39 then milled in a tungsten ring mill to a homogenized powder. Major, trace and rare earth element concentrations were obtained by ICP–MS using the same mass spectrometer after sample fusion at ∼1100°C with LiBO2 and dissolution in diluted HNO3:HF acid mixture. Precision of all analyses was in general <2% (always <4%; cf. García de Madinabeitia etal.,2008, for additional details). Whole-rock Sr and Sm–Nd isotopic composition determinations involved purification of Sr, Sm and Nd following the procedures of Pin etal.(1994) and Pin and Santos Zalduegui(1997). Analyses were conducted on a multicollector ICP-MS (Neptune, Thermo Fisher Scientific). Mass fractionation was corrected following Balcaen etal.(2005) for Sr, Hofmann(1971) for Sm, and considering exponential laws of instrumental fractionation and specific algorithms for Nd. Elemental Sm and Nd concentrations were determined by isotope dilution using a mixed 149Sm/ 150Nd tracer. 5. Petrography and Mineral Chemistry The “basic volcanism” episode generated rocks made of variable amounts of plagioclase, clinopyroxene and accessory magnetite and ilmenite. Some of the associated intrusives include also amphibole and accessory alkali feldspar, apatite and zircon (Tables S2 and S3 in Supporting InformationS1). In general, these rocks appear variably altered to a secondary mineral assemblage including chlorite, opaque minerals, titanite, epidote, calcite, quartz, biotite, muscovite, actinolite-tremolite and alkali feldspar. This secondary assemblage is prevalent in the walls or infillings of rock micro-cracks and vesicles (amygdales). Its origin might be related to a mild regional metamorphic and/or to hydrothermal overprint. Petrography of the basic lava flows studied reveals a broad textural spectrum, particularly in the case of pillow-lavas. In them, undercooling textural evidence are widespread. Although secondary alteration puts limits to a detailed petrographic characterization of the primary mesostasis, the microtextures observed can be grouped into the following four holocrystalline textural types: microlithic with a cryptocrystalline groundmass, (micro)-porphyritic with an intergranular groundmass, intergranular, and subophitic. Textures of the associated intrusive rocks vary from diabasic (interstitial) to subophitic, with only the coarsest-grained terms showing panidiomorphic seriated microtextures. 5.1. Plagioclase In the basic lava flows, plagioclase generally appears as euhedral microliths. Plagioclase (micro)-phenocrystals and microliths exhibit undercooling textures such as skeletal, swallowtail, belt-buckle and bow-tie. Radiating microliths of plagioclase are also observed forming varioles in some textural varieties. In the intrusive basic rocks, plagioclase appears as euhedral crystals up to 5mm long with negligible compositional zoning between cores and rims (An60-52; Sarrionandia etal.,2023). By contrast, plagioclase in lava flows and in certain gabbros depicts extensive zoning (An58-01 and An66-01, respectively). Such strong compositional variations might be related to the secondary processes that affected these rocks. Compositional variations are markedly narrower in plagioclases from dioritic intrusives (ranging from An22-39 at cores to An20-35 at rims). 5.2. Clinopyroxene Clinopyroxene crystals exhibit a variety of undercooling morphologies (skeletal, dendritic, fibrous, radial) and, occasionally, constitute glomero-porphyritic or cumulophyric aggregates in the groundmass of the basic volcanic rocks. In subophitic textural varieties, they can form oikocrystals up to 8mm in size. In the diorites, clinopyroxene appears transformed to ferropargasitic amphibole. Compositional zoning has not been observed (Sarrionandia etal.,2023). Clinopyroxene structural formulae correspond to the Ca–Mg–Fe type of Morimoto(1988), with similar augite compositions in the gabbros (Wo37–41En36–46Fs8–15) and in the lava-flows (Wo31–39En32–49Fs7–22; Figure4a). The analyzed augites possess relatively high Mg# values (0.61–0.88 in lava-flows, 0.71–0.85 in gabbros) and exhibit wide variations in TiO2 (0.47–3.54 wt.%) and Cr2O3 (0–0.72 wt.%) contents (Sarrionandia etal.,2023). 5.3. Amphibole Amphibole variably replaces the clinopyroxene in the intrusives (fully in the diorites), where it forms coronitic textures. It appears as up to 0.02mm long, hypidiomorphic crystals with short prismatic shapes. Occasionally, it may also form isolated, up to 2mm long twinned xenomorphic to hypidiomorphic crystals with patchy zoning. 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 9 of 39 Figure 4. (a) Projection of analyzed pyroxenes in the Ca–Mg–Fe clinopyroxenes classification diagram of Morimoto(1988). (b, c) Projection of analyzed amphiboles in the classification diagram for calcic amphiboles of Leake etal.(1997). 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 16 of 39 diagram (Figure6). Normalized patterns are in general almost flat-shaped and less fractionated in the case of Group #1 (LaN/LuN: 1.6–2.9) than in Groups #3 (LaN/LuN: 2.0–5.6) and #2 (LaN/LuN: 2.5–5.9). From this viewpoint, fractionation in the Group #1 is similar for LREE (LaN/SmN: 1.1–1.8) and HREE (GdN/LuN: 1.0–1.5), whereas in the Groups #2 and #3 the fractionation of LREE is more pronounced, as reflected by their slightly segmented normalization profiles (LaN/SmN: 1.7–2.8; GdN/LuN: 1.0–1.6 and LaN/SmN: 1.4–2.5; GdN/LuN: 1.1–1.4, respectively; Figure8). N-MORB–normalized multi-element diagrams (Sun & McDonough,1989) show in general similar segmented patterns for the three geochemical groups, with marked enrichments in Th, particularly in Groups #2 and #3 (Figure8). These diagrams show an overall slight enrichment in LREE for the three groups, clearly less pronounced in Group #1, with roughly similar HREE contents respect to N-MORB and a slight depletion in P that is not always observable in Groups #2 and #3 (Figure8). E-MORB–normalized multi-element diagrams show near horizontal profiles with marked enrichments in Th. In detail, the Group #1 profiles fit overall to the E-MORB (albeit with marked depletions in Nb and, to a lesser extent, in P) whereas Group #2 and, particularly, Group #3 profiles display more enriched patterns (Figure8). Moreover, depletion in Nb and P respect to E-MORB are not always present in the Group #2 and are practically absent in Group #3 samples (Figure8). 6.2. Isotope Geochemistry Hydrothermal alteration and/or low-grade metamorphism can affect the Rb and Sr elemental contents (Pin & Waldhausrová,2007, and references therein) and, therefore, the Rb–Sr isotope systematics. In the rocks studied, this is reflected by the wide ranges of 87Rb/ 86Sr and 87Sr/86Sr isotopic ratios in each geochemical group (Table2). Those ratios appear uncorrelated with SiO2 contents and, thus, prevent the consideration of this isotopic system for further petrogenetic considerations. By contrast, the linear correlation observed in the elemental Sm–Nd diagram (Figure9a) reflects the stability of Sm and Nd during secondary alteration (DePaolo,1988). In Figure9a a rough clustering of the samples analyzed into the three geochemical groups already identified is outstanding. The Group #3 rocks appear as the most enriched in Nd (24.5–38.4ppm) and Sm (6.35–9.04ppm), the less enriched being the Group #1 basalts (Nd: 10.1–18.8ppm; Sm: 2.92–5.18ppm). Basalts and basaltic andesites of Group #2 (Nd: 15.2–20.3ppm; Sm: 4.02–5.06ppm) overlap the field of the most enriched samples of Group #1 (Figure9a; Table2). Geochemical differences among the three compositional groups identified are also reflected in their 147Sm/ 144Nd and 143Nd/ 144Nd ratios (Figure9b; Table2), that also enable us to discern a rough positive general trend in the 143Nd/ 144Nd– 147Sm/ 144Nd diagram (Figure9b). In general, Group #1 basalts are the most radiogenic and exhibit the highest isotopic ratios, whereas some Group #2 rocks exhibit the least radiogenic ratios. Sub-alkali basalts and basaltic andesites of the Group #3 lie in between both groups, though partially overlapping the projection area of Group #2 rocks (Figure9b; Table2). It may be noted in this diagram the nearly vertical trends defined by the Group #1 and, to a lesser extent, the Group #3 samples, while those of Group #2 show a marked positive correlation (Figure9b; Table2). As regards the εNd335Ma values (Table2), these also support the geochemical group distinction. The Group #1 basalts include the most radiogenic samples, which exhibit moderately positive εNd335Ma values comprised in a relatively narrow range (+4.0–+6.6). The Group #2 rocks include the less radiogenic samples, with εNd335Ma values mainly positive but encompassing a wider range (−0.2–+3.5). Finally, the Group #3 rocks (with the exception of the particularly unradiogenic sample MTC274B, with εNd335Ma=+0.9) present and the narrowest εNd335Ma range (+2.8–+4.8) and positive values comprised between those of the Groups #1 and #2. The projection of the whole set of εNd335Ma values versus SiO2 and Th contents shows two general evolutionary trends (Figures9c and9d). Samples of the Groups #1 and #2 can be integrated in a single array showing a rough negative correlation (Figures9c and9d). Samples of the Group #3 (excluding the deviating sample MTC274B) Figure 7. FeO t/MgO–SiO2 diagram for the discrimination of tholeiitic and calc-alkaline series evolution (adapted from Miyashiro,1974). 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 17 of 39 define a nearly horizontal array that intercepts the general trend integrated by Groups #1 and #2. Interestingly, the paths' intersection coincides with the transition from Group #1 to Group #2 samples (Figures9c and9d). Finally, it may be noted that Sm–Nd depleted mantle model ages (TDM; DePaolo,1988) are similar for the three geochemical groups (Table2; Figure9e). In detail, however, the calculated TDM values for Group #1 basalts (following Liew & Hofmann,1988) are slightly younger (0.77–1.22Ga) than those of the Group #2 (1.03–1.36Ga) and Group #3 rocks (0.84–1.34Ga). 7. Discussion In the following sections, we discuss the geochemical nature and linkage of the three magmatic associations identified in the Mississippian Matachel mid “basic volcanism” (and cogenetic subvolcanic intrusions) of its up to 1.000m thick volcanic basin infill. We bring to a focus their mantle sources, differentiation processes and petrotectonic significance in the regional geologic context of SW Iberia. 7.1. Geochemical Nature of the Magmatic Associations Whole-rock geochemical characteristics of basalts and basaltic andesites indicate that the “basic volcanism” of the Matachel Basin was nurtured by tholeiitic (Groups #1 and #2) and seemingly calc-alkaline (Group #3) magmas. Occurrence in the southern Iberian Massif of a Mississippian MORB-type tholeiitic magmatism was documented either in the distal areas of the Los Pedroches (North Marine) basin (Armendáriz etal.,2008) or in the South-Portuguese (South Marine) basin (Munhá,1983). In the latter domain Mitjavila etal.(1997) also Figure 8. Normalized multi-element diagrams of the geochemical groups identified in the “basic volcanism” of the Matachel Basin. Normalization values correspond to Chondrite (Sun & McDonough,1989), N–MORB (Sun & McDonough,1989), and E–MORB (Sun & McDonough,1989). 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 18 of 39 Sample Group SiO2 (wt.%) Rb (ppm) Sr (ppm) 87Rb/ 86Sr 87Sr/ 86Sr 2σ error (abs) 87Sr/ 86Sr335 Sm (ppm) Nd (ppm) 147Sm/ 144Nd 143Nd/ 144Nd 2σ error (abs) 143Nd/ 144Nd335 εNd335 TDM (Ga) TDM 2stg (Ga) MGA-1 #1 47.11 28.6 266 0.311 0.705792 0.000007 0.7043 12.2 3.72 0.1852 0.512949 0.000006 0.512543 6.6 0.91 0.53 MTC-84B #1 47.24 14.0 380 0.107 0.704568 0.000006 0.7041 12.7 3.33 0.1578 0.512844 0.000005 0.512498 5.7 0.77 0.60 MTC-83 #1 47.92 11.5 207 0.161 0.704455 0.000007 0.7037 18.8 5.18 0.1664 0.512841 0.000006 0.512476 5.3 0.90 0.64 MTC-42 #1 48.05 59.5 292 0.590 0.708690 0.000005 0.7059 10.1 2.92 0.1757 0.512853 0.000007 0.512468 5.1 1.05 0.65 HNC-3 #1 48.82 18.7 297 0.182 0.706050 0.000007 0.7052 12.6 3.63 0.1742 0.512793 0.000007 0.512411 4.0 1.22 0.74 VFB-12 #1 52.12 53.4 270 0.573 0.707910 0.000006 0.7052 14.9 4.24 0.1718 0.512847 0.000004 0.512470 5.1 0.98 0.64 MTC-48 #2 49.80 15.6 223 0.202 0.705146 0.000005 0.7042 18.4 4.82 0.1586 0.512725 0.000008 0.512377 3.3 1.08 0.79 MGA-20 #2 49.90 17.2 194 0.257 0.706400 0.000006 0.7052 15.2 4.02 0.1595 0.512620 0.000007 0.512270 1.2 1.36 0.95 MGA-22 #2 50.13 67.4 515 0.379 0.708335 0.000007 0.7065 18.7 4.26 0.1373 0.512600 0.000006 0.512299 1.8 1.03 0.91 MGA-12 #2 50.72 174 234 2.149 0.715514 0.000005 0.7053 19.1 5.06 0.1599 0.512737 0.000005 0.512386 3.5 1.07 0.77 USG-14 #2 54.51 51.9 387 0.388 0.706429 0.000007 0.7046 20.3 4.51 0.1345 0.512493 0.000005 0.512198 −0.2 1.19 1.06 MTC-275 #3 47.38 20.2 234 0.249 0.706489 0.000006 0.7053 31.3 8.49 0.1643 0.512728 0.000004 0.512368 3.1 1.18 0.80 MGA-2 #3 49.10 20.3 309 0.190 0.706278 0.000007 0.7054 38.4 9.04 0.1422 0.512661 0.000008 0.512349 2.8 0.97 0.83 MTC274B #3 50.25 25.9 270 0.278 0.706968 0.000008 0.7056 30.6 7.85 0.1552 0.512591 0.000006 0.512251 0.9 1.34 0.98 MTC-75 #3 50.45 15.7 237 0.192 0.704510 0.000007 0.7036 27.3 6.86 0.1519 0.512765 0.000007 0.512432 4.4 0.88 0.70 MTC-76 #3 51.36 15.1 226 0.193 0.704455 0.000006 0.7035 29.2 7.34 0.1520 0.512783 0.000007 0.512450 4.8 0.84 0.68 VFB-9 #3 54.04 318 226 4.072 0.724440 0.000005 0.7050 24.5 6.35 0.1567 0.512751 0.000004 0.512407 3.9 0.98 0.74 MGA-25 #3 54.75 18.5 218 0.245 0.705034 0.000007 0.7039 25.9 6.61 0.1546 0.512753 0.000007 0.512414 4.1 0.94 0.73 Table 2 Whole-Rock Isotope Data (Rb–Sr and Sm–Nd Systematics) of the Lava-Flows and Intrusives Integrated in the “Basic Volcanism” of the Matachel Basin (Measured and Age Corrected to 335Ma) 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 19 of 39 Figure 9. Isotopic and elemental relations relative to the Sm–Nd systematics (whole-rock) of the “basic volcanism” of the Matachel Basin. (a) Nd versus Sm elemental variation diagram. (b) 147Sm/ 144Nd versus 143Nd/ 144Nd isotopic ratios. (c) εNdi versus silica contents. (d) εNdi versus thorium contents. (e) Model ages of the analyzed samples determined from depleted mantle evolution following Liew and Hofmann(1988). The evolution of the depleted mantle of Goldstein etal.(1984)has also been projected. 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 20 of 39 identified continental tholeiites associated with alkali basalts and calc-alkaline dacites and rhyolites, although their genetic linkage remains loosely ascribed in terms of compositional criteria. The moderate radiogenic character (εNdi=−0.2–+6.6) of the Matachel Groups #1 and #2 tholeiites and the shapes of their multi-element normalization patterns are similar to those of enriched-MORB type basalts (Figure8). This points to a lithospheric contribution of source components that would also be supported by the characteristics of trace element systematics (Figure8; Table1). This is the case in particular of immobile incompatible elements sensitive to crustal contamination processes (e.g., Th and LREE), given that continental crust, particularly the upper crust, is enriched in La and Th relative to the primitive mantle (Puchtel etal.,1997; Rudnick & Fountain,1995; Rudnick & Gao,2003; Taylor & McLennan,1985). A crustal contribution to the tholeiitic magmas can be unraveled after various lines of geochemical evidence. Relative abundances in ThPM of the studied tholeiites (Group #1: 17.6–38.8; Group #2: 47.1–85.9) are an order of magnitude higher than those of the average N-MORB (Sun & McDonough,1989), whereas LaPM values (Group #1: 5.81–15.1; Group #2: 14.6–27.0) are only slightly higher. Conversely, Nb/ThPM values (Group #1: 0.195–0.556; Group #2: 0.132–0.247) are markedly low relative to those calculated for the average N-MORB, whereas Nb/ LaPM values (Group #1: 0.628–1.03; Group #2: 0.432–0.610) are fairly similar. The differences between La and Th enrichments are more pronounced in the Group #2 tholeiites, which would initially point to a higher upper crustal contribution in them. The prominent Nb negative anomaly in the primitive mantle-normalized diagram is also consistent with a generalized crustal contribution (Barth etal.,2000; Puchtel etal.,1997; Rudnick & Gao,2003; Weaver,1991). Its extent is reflected by the Nb/Nb* ratio, defined as NbPM/(ThPM x LaPM) 1/2 (Puchtel etal.,1997; Stepanova etal.,2014). Observed similar ranges of this parameter in the Groups #1 (0.35–0.70) and #2 (0.24–0.39) suggest significant albeit variable degrees of crustal contamination within each group, which is also consistent with the observed variability of Nb/ThPM and, to a lesser extent, Nb/LaPM values. The lower Nb/ Nb* ratio of Group #2 reflects again a higher crustal contribution in these tholeiites than in those of the Group #1. This effect of variable crustal contribution is also reflected in several tectono-magmatic diagrams. In the Zr/Y–Zr diagram of Pearce and Norry(1979), studied samples appear projected in the “within-plate basalts' field (Figure10a). Nevertheless, most of the Group #1 samples project clustered in the “MORB+within-plate basalts' field, slightly apart from the Groups #2 and #3 samples. Similarly occurs in the La/10–Y/15–Nb/8 diagram of Cabanis and Lecolle(1989; Figure10b). Though most samples appear projected in the “late to post-orogenic intra-continental” domain, the crustal input in Groups #2 and #3 rocks leads to their displaced projection from the Group #1 samples. It can, thus, be concluded that the “basic volcanism” of the Matachel Basin actually includes ordinary continental tholeiites among its different magmatic rock associations. Figure 10. Projection of the geochemical groups identified in the “basic volcanism” of the Matachel Basin in tectono-magmatic discrimination diagrams: (a) Zr/Y–Zr diagram (Pearce & Norry,1979), and (b) La/10–Y/15–Nb/8 diagram (Cabanis & Lecolle,1989). Employed symbols are those of Figure5. 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 21 of 39 The wide geochemical diversity of continental tholeiites worldwide led to a preliminary subdivision into the Low– Ti (<2 wt.% TiO2) and High–Ti (>2 wt.% TiO2) tholeiite types (Bellieni etal.,1984; Mantovani etal.,1985). Peate etal.(1992) proposed a more comprehensive classification scheme based on high-field strength element contents such as Ti, Zr, Y and compiled Sr-, Nd-, and Pb-isotope data of the Paraná Gondwanan continental tholeiites. According to these authors, High–Ti tholeiites exhibit higher ratios in Ti/Y (>350), Zr/Y (>4.0) and Sr/Y (>4.5) than those of Low–Ti tholeiites (Ti/Y<330; Zr/Y<7.0; Sr/Y<13). In the Matachel tholeiitic basalts and basaltic andesites, the observed TiO2 contents (<1.94 wt.% TiO2) and Ti/Y (238–338) and Zr/Y (3.96–6.87) ratios compare to those of Low–Ti continental tholeiites. Only one sample (MTC-84B) departs of this ascription, due to a relatively high Ti/Y ratio (Ti/Y: 377). Peate and Hawkesworth(1996) revisited the classification of Low–Ti tholeiites, and conducted a more detailed geochemical and isotopic characterization. They proposed distinction between two end-members with contrasting Ti/Zr ratios and Nd-, Sr-, and Pb-isotope characteristics, termed “Esmeralda-type” and “Gramado-type.” “Esmeralda-type” tholeiites have lower 87Sr/ 86Sri (0.7045–0.7080) and higher Ti/Zr (>60) and εNdi (−4 to +3) than “Gramado-type” rocks ( 87Sr/ 86Sri: 0.7070–0.7150; Ti/Zr<60; εNdi: −8 to −3). Regardless of their higher radiogenic character, the composition of the Matachel Low–Ti continental tholeiites unravels the presence of both types. The composition of Group #1 basalts (Ti/Zr: 61–79; εNdi: +4.0 to +6.6) concurs with the “Esmeralda-type”, whilst Group #2 basalts and basaltic andesites (Ti/Zr: 36–58; εNdi: −0.2 to +3.5) are closer to the “Gramado-type” (Figure S1 in Supporting InformationS1). Nevertheless, considering that emitted lava volumes in the Matachel Basin are extremely far from those of Paraná continental flood basalts, the coined “Esmeralda” and “Gramado” terms to differentiate both Low-Ti sub-types have been loosely applicated in the studied case. Apart from the Low-Ti tholeiites, a calc-alkaline association has also been identified in the Matachel basic volcanic rocks. The Group #3 sub-alkali basalts and basaltic andesites integrate it. Notwithstanding, basalts of this association display low CaO/Al2O3 ratios (0.4–0.6), comparable to those exhibited by “high-Mg” calc-alkaline series rocks such as boninites (0.5–0.6; Pearce & Reagan,2019, and references therein), adakites (0.3–0.5; Martin etal.,2005) and sanukitoids (∼0.3; Martin etal.,2009). Specifically, they are slightly enriched in TiO2 (2.26– 3.19 wt.%), Zr (192–294ppm) and Y (46.3–58.7ppm) with respect to common values in boninites (TiO2<0.5 wt.%; Zr<58ppm; Y<19ppm; Pearce & Reagan,2019, and references therein) and adakites (TiO2<1.49 wt.%; Zr<188ppm; Y<13ppm; Martin etal.,2005). On their part, basaltic andesites of this association are also slightly enriched in TiO2 (1.08–2.37 wt.%), Zr (240–268ppm) and Y (34.4–49.4ppm) with respect to common values in sanukitoids (TiO2<1.21 wt.%; Zr<316ppm; Yb<35.0ppm). The markedly low MgO (2.8–5.1 wt.%), Ni (<22ppm) and Cr (<100ppm) contents of the Group #3 rocks in comparison with those exhibited by boninites (MgO>8 wt.%; Ni>59ppm; Cr>300ppm; Pearce & Reagan,2019), adakites (MgO∼5.0 wt.%; Ni: 103ppm; Cr: 157ppm; Martin etal.,2005) and sanukitoids (MgO: ∼4.0 wt.%; Ni>36ppm; Cr>58ppm; Martin etal.,2009) elsewhere, and the near flat-shaped REE normalization-patterns (La/YbPM=2.63–6.48), altogether lead us to discard their consideration as genuine “high-Mg” calc-alkaline series rocks. This conclusion would be reinforced by the exhibited relatively low K2O contents of analyzed samples with respect to their SiO2 contents (Table1). The Matachel Group #3 (“calc-alkaline”) association could be considered tentatively as made of basalts and basaltic andesites as those of “normal” arc settings. If the plate convergence setting were correct, their Nb (up to 16.2 wt.%), P2O5 (0.23–0.59 wt.%), La (up to 29.2 wt.%), Nd (up to 39.8 wt.%) and Th contents (up to 12.5 wt.%) would be closer to those of calc-alkaline andesites of active continental margins than to those of intra-oceanic and island arcs (Kelemen etal.,2007). Nevertheless, their relatively low Mg# values (33–56; Table1) and 143Nd/ 144Nd ratios (0.512591–0.512783; Table2), and their markedly high TiO2 (1.08–3.19 wt.%; Table1) and Fe2O3 (7.43–13.34 wt.%; Table1) are at variance with those of average subduction-related magmatic rock equivalents (Kelemen etal.,2007). It is outstanding that Al2O3 contents of the Group #3 basalts are akin to those of tholeiitic basalts. Additionally, the normative mineralogy of the less altered basalts (LOI<2.5 wt.%) corresponds to oversaturated tholeiites (Yoder & Tilley,1962) and the same can be concluded after their εNdi values (+2.8–+4.8; excluding sample MTC274B). All the above permits us to conclude that, despite the Group #3 rocks exhibit in appearance a calc-alkaline evolutionary trend, a considerable uncertainty remains regarding the actual nature of this geochemical association. Consequently, the inference of a plate-convergence tectonic setting on geochemical grounds can be seriously challenged based upon the detailed geochemical data presented here. 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 22 of 39 7.2. Inferences on the Magma Sources On the basis of isotope and element abundances/ratios, two distinct Low-Ti continental tholeiite types and a (not necessarily so) “Cordilleran-type” calc-alkaline association may be distinguished in the Matachel Mississippian volcanic sequences. A close temporal and spatial occurrence of Highand Low-Ti tholeiitic basalts, commonly associated also with alkali basalts and/or peraluminous rhyolites, is a well-documented phenomenon worldwide (Dessureau etal.,2000; LaFléche etal.,1998; Lightfoot etal.,1993; Neal etal.,2002; Peate etal.,1992; Pouclet etal.,2017; Shellnutt & Jahn,2011). Nevertheless, as far as we know, few examples have been reported of outpourings of continental tholeiites with associated calc-alkaline basalts and basaltic andesites (Benek etal.,1996; Mitjavila etal.,1997; Pouclet etal.,2017) unrelated to plate-convergence geodynamic scenarios. In a first approximation, the existence of tholeiitic and “calc-alkaline” basaltic lava flows in the Matachel basic magmatic association might point to a possible derivation from separate mantle sources. Indeed, compositional variations in continental tholeiites of other igneous provinces have sometimes been related to simultaneous, or temporally close, magma contributions from distinct mantle sources (Ingle etal.,2004; Lightfoot etal.,1993; Neal etal.,2002; Peate,1997; Peate etal.,1992; Pouclet etal.,2017). The whole-rock geochemistry of the three Matachel geochemical groups distinguished puts constraints to a straightforward interpretation of primary compositional magmas and their source (e.g., MgO≥13 wt.%; Ni=300–500ppm; e.g., Green etal.,2001; Sato,1977). Conservative incompatible elemental contents and ratios of Nb/ThPM, Th/YbPM, and Nb/LaPM reveal as well a variable crustal contribution. Notwithstanding, coupling Nd isotopic compositions with indicators of crustal contamination (such as the εNdi–Th/ZrPM and εNdi–La/NbPM graphs) unravels a rough single negative array that integrates the tholeiitic (Groups #1 and #2) and calc-alkaline (Group #3) associations that had been originally clustered according to their whole-rock geochemistry (Figures11a and11b). This transition cannot be explained simply in terms of an increasing “en route” crustal contamination of mantle-derived melts. Instead, it appears that those associations already evolved before traversing the crust. Primary magma source identification for the Matachel magmatic rocks can be traced from the relations of the usually conservative elements during crystal fractionation (Nb/Yb, Th/Yb, La/Sc, Zr/Y, Ti/Y, Ti/V, and La/Yb). Their variations might also reflect changes in the nature of the source, its depth and/or melting rates (Condie,2005; Fitton etal.,1997; Pearce,2008). Projection in the Th/Nb–Nb/Yb diagram (Pearce,2008) of the data available shows a departure from the “MORB–OIB array” and unravels the interaction of mantle-derived magmas with crustal components (Figure11c). Some elemental ratios (e.g., Th/Yb and La/Y) of the source mantle material could have been thus modified by crustal assimilation. Metabasic crustal rocks are the best candidates as sources of crustal contamination, since a significant contribution of sediments or ancient continental crust would not be supported by the εNdi values determined herein (DePaolo,1988). Identification of the samples with a smaller “crustal” input, and therefore a presumed stronger mantle source signature, might be guided by consideration of only those bearing Nb/La ratios >0.85 times the chondritic value (Lassiter & DePaolo,1997). Surprisingly, among the Matachel magmatic rocks, the less evolved tholeiitic samples exhibit Nb/La ratios higher than 0.85 (Group #1: 0.92–1.07), being the samples of the association with calc-alkaline signature those displaying sub-chondritic and markedly low Nb/La ratios (<0.56). Therefore, the less evolved Matachel tholeiitic samples (considering alternative criteria) could be the geochemically closer candidates to track the “primitive” tholeiitic magmas. Those specific samples plot clustered in the domain for enriched-MORB sources in the Th/Nb–Nb/Yb diagram (Pearce,2008; Figure11c). This is in accordance with the E-MORB character inferred from their multi-element normalization-patterns (Figure8) and with their trace element systematics departure from OIB parameters (Sun & McDonough,1989). All this permits us to discard a deep mantle-source for the tholeiitic magmas. In order to constrain further the depth of the mantle source of the studied Low-Ti continental tholeiites, a discussion on the presence or absence of stable garnet in the magma source mantle rocks is required (Green & Ringwood,1970; Lassiter & DePaolo,1997). In this regard, it can be highlighted that the La/YbPM, La/YPM, Sm/YbPM, Nb/YPM, and Zr/YPM ratios are directly related to the degree and depth of partial melting, since in the presence of stable garnet, HREE are preferentially retained by this mineral over the light and middle REE during melting (Class etal.,1998; Lassiter & DePaolo,1997; Shaw etal.,2003). In the case studied here, normalization with respect to primitive-mantle compositions (Taylor & McLennan,1985) results in relatively low Dy/ Yb(N) (1.15–1.28) and La/Yb(N) (1.53–2.87) values in the presumed “primitive” samples of Group #1 rocks. This (together with low values of the Sm/YbPM or La/YbPM) might have resulted from partial melting of a garnet-free mantle source (Shaw etal.,2003). Specifically, the aforementioned low ratios, together with the relatively flat 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 23 of 39 HREE patterns and the moderate Nd radiogenic character of the “primitive” samples (εNdi=+5.1–+6.6), likely point to a (garnet-free) lherzolitic mantle. The relatively low Nb/Nb* values of the most “primitive” tholeiitic samples (Group #1: 0.457–0.701) with respect to mantle values (<1ppm; Taylor & McLennan,1985) is also congruent with the presence of Ti-bearing phases both in the upper mantle (e.g., rutile, FeTiO3 ilmenite, garnet, pyroxene, spinel and post-spinel phases; Matrosova etal.,2021) and underneath (Al, Ca, and Ti oxide and silicate refractory constituents of perovskite; O'Neill & Palme,1998; Matrosova etal.,2020), and/or amphibole in the mantle source (Foley etal.,2000). Additionally, if amphibole was present in the source rocks, the E-MORB fingerprints that exhibit the studied tholeiites (Figure8) might be explained, too. From the perspective of the two end-member Low–Ti tholeiites types of Peate and Hawkesworth's(1996) classification, the Matachel Group #1 basalts fit the “Esmeralda-type”, whereas basalts and basaltic andesites of the Group #2 are closer to the “Gramado-type.” The cited authors suggested that “Gramado-type” tholeiites might result from a prime contribution of heterogeneous continental mantle lithosphere, whereas “Esmeralda-type” counterparts might result from the contamination of asthenosphere-derived melts by subcontinental lithospheric mantle/magmas. Stepanova etal.(2014) have shown that primary magmas of “Esmeralda-type” tholeiites might result from relatively high melting degrees (c. 15%) of a depleted mantle (DM-type) source in the spinel peridotite stability field. However, the debate persists so far as to whether Low-Ti continental tholeiites result from the assimilation of enriched lithosphere by proportionally larger asthenospheric melts or the most significant melt generation occurred within the lithospheric mantle (Dessureau etal.,2000). Figure 11. εNdi variation trends with respect to (a) Th/ZrPM ratio, and (b) La/NbPM ratio of the identified geochemical groups in the “basic volcanism” of the Matachel Basin; normalization values with respect to the primitive mantle (Sun & McDonough,1989). Projection of the geochemical groups identified in the “basic volcanism” of the Matachel Basin in the (c) Th/Yb–Nb/Yb diagram of “MORB–OIB array” (Pearce,2008), and (d) K2O–SiO2 diagram. 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 24 of 39 The Variscan orogeny triggered an abrupt geochemical change in mafic magmatism in most of the central-Europe terranes (Dostal etal.,2019). In the Iberian Massif most of the Variscan mafic magmatism implied melting of lithospheric mantle sources, variably enriched due to crustal material subducted during the Late Cadomian and the Variscan orogenic cycles (Bea etal.,2021; Orejana etal.,2020; Villaseca etal.,2004). Nevertheless, the moderately high initial εNd values (from +5.4 to +5.8) and younger model ages (TDM=735–620Ma) of the tholeiitic rocks of Central Iberian (López-Moro etal.,2007), coupled with their low Th/Yb and relatively high Nb/U (mostly from 20 to 47) values, suggests a scarce continental crustal imprint, and therefore, the lack of supra-subduction signatures and the existence of an old (pre-Neoproterozoic) heterogeneous enriched subcontinental mantle beneath Central Iberia (Villaseca etal.,2022). In the regional setting of Matachel magmatic province, provenance of the Group #1 “primitive” samples from an enriched MORB-type source beneath the Ossa-Morena Zone places additional constraints on magma source identification. The assumed source after the above discussion points to a “metasomatically” enriched lithospheric mantle source (e.g.,: Donnelly etal.,2004; Workman etal.,2004). The enriched MORB signature, on its part, is usually related to intra-oceanic subduction environments (Schilling etal.,1983). However, such a plate-convergence geodynamic setting is at odds with the intra-continental transcurrent setting currently observed (and interpreted to have persisted since the early Cambrian, well before the Mississippian). The mantle source enrichment should, thus, be inherited. Moreover, the existence in the Iberian Massif of sanukites and, therefore, the participation of metasomatized mantle sources by a subducting basaltic slab and sediments (e.g., Gómez-Frutos & Castro,2022; Gómez-Frutos etal.,2023) reinforces the heterogeneous character of the inferred lithospheric mantle, which led to the near coeval generation of tholeiitic and sanukitoid magmas during the Variscan orogeny. Bearing in mind the regional tectonic evolution of the region, the Cadomian subduction setting is the best candidate to explain the enriched mantle source (see Sarrionandia etal.,2020, and references therein, for further details). Although this would need to be confirmed with more robust evidence, the principal mantle source region of the Matachel Low-Ti continental tholeiites would correspond to a (rutile-) amphibole-bearing, garnet-free lherzolite of the subcontinental mantle lithosphere conformed as a supra-subduction mantle wedge beneath the northern margin of Gondwana during the late Neoproterozoic-early Cambrian. The presence of “Esmeralda-type” Low-Ti continental tholeiites in the Matachel basin likely discloses a limited contribution of asthenospheric melts (Peate & Hawkesworth,1996; Stepanova etal.,2014) from neighbor mantle domains underneath, which is also possible in supra-subduction inherited settings. Slight variations in the degree of melting and the heterogeneous character of the mantle sources frozen after Cadomian lithospheric stabilization would be consistent with the variations of conservative element ratios ascribed to the most “primitive” tholeiitic batches (Condie,2005; Pearce,2008; Pearce & Norry,1979). Interaction of these mantle-derived melts with a metabasic continental lower and middle crust would explain the rest of the geochemical characteristics of the Matachel continental tholeiites. The previous discussion departs from the usually assumed relationship (direct or indirect) between Low–Ti continental tholeiites and mantle-plumes (Stepanova etal.,2014, and references therein), in spite of the fact that the distinctive geochemical signature of mantle-plume sources is not evident in some Low-Ti rocks (Peate & Hawkesworth,1996). Indeed, MORB-type tholeiites (i.e., the “Esmeralda-type”) could indicate a transition from mantle-plume melting to decompression-induced melting of ambient mantle during continental breakup and ocean basin opening (Stepanova etal.,2014). Decompression and melting of a tectonically complex enriched lithospheric mantle can be achieved by alternative mechanisms, too. As discussed below in further detail (Section7.5), generation of the Low-Ti continental tholeiites studied might have taken place in relation to wrench shear zone reworking across the lithosphere. 7.3. Origin of the Association With Calc-Alkaline Signature The Matachel “basic volcanism” association with calc-alkaline signature (Group #3 rocks) exhibits no discernible time and/or spatial gaps with respect to the of Low-Ti continental tholeiite outpourings (Groups #1 and #2). The observed calc-alkaline basalt Al2O3 contents are similar to or within the range of those of tholeiitic basalts. The same stands for Fe0.8 values calculated after Klein and Langmuir (1,987; Fe0.8=5.7 in calc-alkaline basalts, Fe0.8=3.8–8.7 in Group #1, Fe0.8=5.4–7.3 in Group #2 rocks) for CaO/Al2O3 ratios (CaO/Al2O3=0.6, 0.55–0.64, and 0.34–0.55, respectively), and for the Ti/Y ratios (326–268 in the calc-alkaline basalts). The main difference observed relates to the more enriched character of the LREE content of Group #3 rocks (Figure8). 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 25 of 39 Geochemical similarities are also observed in the moderate Nd radiogenic character of the samples with calc-alkaline signature (εNdi=+2.8–+4.8; excluding sample MTC274B). The positive εNdi values and the observed trends in diagrams coupling εNdi values with conservative element ratios (Th/ZrPM, Nb/ThPM, La/ NbPM), alike the Groups #1 and #2 tholeiites, point to an addition of a metabasic crustal component to the mantle source prior to magma diversification (DePaolo,1988; Pearce,2008). The higher relative abundances in ThPM (34.1–102) and LaPM (27.0–42.4) of the calc-alkaline basalts further support a crustal component in them (Puchtel etal.,1997; Rudnick & Fountain,1995; Rudnick & Gao,2003; Taylor & McLennan,1985) higher than in the tholeiites. Also remarkable in this regard are the loose or absent troughs in P, Ti and Nb in the E-MORB multielemental normalization-diagrams (Figure 8) usually observed in calc-alkaline rocks (Foley etal.,2000, and references therein). The Nb/NbPM* (0.32–0.41), Nb/ThPM (0.22–0.39) and Nb/LaPM (0.43–0.54) ratios in the calc-alkaline basalts are only slightly smaller than those of the Group #2 tholeiites. Actually, the Group #3 samples studied overlap the “late to post-orogenic intra-continental” domain of the La/10–Y/15–Nb/8 diagram of Cabanis and Lecolle(1989; Figure10b). As was the case for the tholeiites, a “within-plate” character of the calc-alkaline rocks is outstanding. This can be inferred also from their near clustered projection in the “within-plate basalts' field of the Zr/Y–Zr diagram (Pearce & Norry,1979; Figure10a). It is therefore apparent that both the calc-alkaline and tholeiitic associations might have formed contemporaneously and that the genetic relationships of their parent magmas are plausible. Explaining the difference between calc-alkaline and tholeiitic magmatic series has been one of the central topics of Igneous Petrology, a general consensus far from being reached so far (Kelemen etal.,2007). Low-Ti tholeiitic basalts with arc-like trace-element characteristics were described in the Ferrar large igneous province (LIP; e.g., Hergt & Brauns,2001, and references therein). According to these authors, a small proportion (∼3%) of subducted terrigenous sediments contaminating a highly depleted mantle might explain geochemical features as the dealt with here. The mantle wedge (involving the asthenosphere and subcontinental lithosphere above a subduction channel) is the most likely tectonic scenario for such a mantle source (Hacker etal.,2003). Nevertheless, such enrichment would promote high Al2O3 and K2O contents in primitive magmas, contrary to those observed in the less evolved samples from Group #3 (Figures6 and11d). An alternative hypothesis would involve contamination of MORB-type magmas by an enriched-MORB source, followed by the addition of calc-alkaline rocks/magmas extracted from a depleted mantle reservoir (Pereira etal.,2007, and references therein). Such a complex model was intended to explain specifically the origin of metabasites with convergent plate margin geochemical signatures formed during the Cadomian orogeny in the OMZ. Thus, unless it is regarded as a possible inherited scenario, the time gap with Carboniferous Low-Ti continental tholeiites makes it an insufficient explanation by itself. Despite the low Mg# values of the Group #3 rocks with calc-alkaline signature (32.55–55.67; Table1), their derivation from partial melting of subducted basalt without major-element equilibration with the overlying mantle might be discarded, as this process remains controversial (Kelemen etal.,2007, and references therein). The fact that the εNdi values of Group #3 rocks with calc-alkaline signature fall within the range of those of Groups #1 and #2 tholeiitic rocks could point to a single primitive magma (e.g., Figures9c and9d) that later experienced two distinct evolutionary paths (i.e., tholeiitic and calc-alkaline). In this regard, interactions with a heterogeneous crust would be of prime importance. The Group #3 basalts with calc-alkaline signature bear very high TiO2 and Fe2O3 contents compared with the tholeiites (Figure6; Table1) that support magma crustal contamination. Though to some extent speculative, further contamination of a Low-Ti tholeiitic primary magma with H2O-rich (e.g., amphibole-rich) crustal metabasites would led to an increase of the oxygen fugacity leading to a calc-alkaline evolutionary trend (Waters etal.,2020). Alternatively, interaction with less ferriferous and titaniferous dry rocks (olivineand, notably, pyroxene-rich metabasites) might lead to generation of the parental magmas from which evolved the Group #2 Low-Ti continental tholeiites. Whatever the case, and considering also that LOI values of most samples of the Group #3 are markedly low (see Table1), the relative abundance of H2O in the calc-alkaline parental magma would be below ∼2 wt.%, since geochemical modeling (see Section7.4) indicates that plagioclase saturation was reached (Kelemen etal.,1990; Müntener etal.,2001). Note that these hypotheses discard the existence of primary Carboniferous (Variscan) arc basalts. The presence of pre-Variscan metaigneous rocks in the OMZ is well documented after decades of geological exploration (see Section2.1 for further details). Igneous activity in the OMZ was widespread between ca. 645Ma and ca. 534Ma in relation with an active subduction setting (Bandrés etal.,2002; López-Guijarro etal.,2008; Pin etal.,2002; Sánchez-Lorda etal.,2014; Sarrionandia etal.,2020, and references therein). Further magmatic 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 32 of 39 Figure 15. (a) Tectonic model of the SW Iberian lithosphere and underlying upper mantle showing its envisaged complex (inherited) structural geology. See text for further details. Drawing of crustal features, subcontinental lithospheric mantle reflectors and tectonic province arrangements were inspired in the IBERSEIS deep seismic reflection profile (SSW crustal segment; Carbonell etal.,2004), the ALCUDIA profile (NNE crustal segment; Martínez-Poyatos etal.,2012) and Ábalos and Díaz(1995). Upper mantle high/low seismic velocity zone boundaries (in cases the lithosphere/asthenosphere boundary) and Hales transition after Palomeras etal.(2011,2017), respectively. The “baby” mantle plume and the sinking slabs are speculative interpretations proposed in this study. (b) Close view of the rectangle area in (a) with details of the magma source inherited tectonic realms and their relationship with the three rock groups studied. 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Geochemistry, Geophysics, Geosystems SARRIONANDIA ETAL. 10.1029/2023GC011139 33 of 39 amphibole-bearing, garnet-free lherzolite. This would have given rise to formation of primitive Group #1 Low-Ti tholeiitic magmas (possibly with a very limited asthenosphere melting input; Figure15b). Progressive interaction of these magmas with crustal (likely Cambrian) alkali igneous rocks resulted in formation of the petrological evolutionary trends observed, to a larger extent in the case of Group #2 Low-Ti tholeiites. Further assimilation of amphibole-rich calc-alkaline metaigneous rocks (likely Cadomian) and subsequent plagioclase, clinopyroxene, and Ti-bearing oxide fractionation can explain the nature of parent melts of calc-alkaline basaltic rocks (seemingly “Cordilleran-type” calc-alkaline, though unrelated to plate convergence) after the Low-Ti tholeiites. Enduring mid-upper crustal processes (magma storage in mid-crustal chambers) likely shaped the latest petrologic and geochemical aspects of the Matachel Low-Ti tholeiites and related rocks with calc-alkaline signature. Data Availability Statement Mineral chemical data are available (Open Access) in Sarrionandia etal.(2023). Studied rock samples are available in the Faculty of Science and Technology of Campus de Lejona (fernando.sarr[email protected]). Information about field location coordinates of the studied samples is in Supporting InformationS1 of this article. Regarding the reproducibility of results, no specific fee-based software is needed; as for whole-rock data processing, the open-source software Geochemical Data Toolkit (GCDkit; Janoušek etal.,2016) was used. References Ábalos, B. (1992). Variscan shear-zone deformation of late Precambrian basement in SW Iberia, implications for circum-Atlantic pre-Mesozoic tectonics. 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We also greatly appreciate the comments and suggestions made by Antonio Castro and an anonymous reviewer during the revision of a first version of the manuscript that greatly improved the quality of this study. 15252027, 2023, 11, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GC011139 by Universidad Del Pais Vasco, Wiley Online Library on [15/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
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