Introduction

The variety of features preserved in calc-silicate-bearing rocks that resulted from the metamorphic recrystallization of carbonate rocks (primarily limestones and dolostones) are ascribed to interactions with chemically active fluids under specific pressure and temperature conditions through time and space. Thermal metamorphic reactions on pure carbonate rocks, originally composed almost exclusively of calcite or dolomite consist mainly of recrystallization of these phases, promoting crystal growth in an equidimensional fabric. However, the presence of accessory minerals concomitant with decarbonization and input from external fluids leads to the formation of a wide range of new assemblages (e.g., Sharma and Sharma 2020; Winter 2014) resulting in calc-silicate lithotypes (i.e., rocks with mineralogy in which calcium silicates are dominant). Calc-silicate and carbonate–silicate rocks spatially associated with calcite and dolomite marbles have been largely documented as products of contact metamorphism of primary carbonate rocks located in the vicinity of intrusions, usually developing a radial zonation with an increase of recrystallization and metamorphic grade toward the igneous body in aureoles of varying widths. The contact metamorphic facies are well described for metasedimentary rocks of detrital origin: the hornfels-facies, low P/T series (e.g., Winter 2020). Despite the designation made for index minerals in detrital metasedimentary rocks, these terms are applied to other lithologies in contact aureoles such as carbonate rocks (e.g., Sharma and Sharma 2020). Prograde isograds include: (i) low–grade albite–epidote facies at ca. 300–400 °C, (ii) medium-grade hornblende facies at ca. 400–550 °C, (iii) high-grade pyroxene facies at ca. 550–700 °C, and (iv) very high grade sanidine facies at temperatures higher than 700 °C. In thermally metamorphosed carbonate rocks, from low- to high-grade the common mineral assemblages are calcite, dolomite, minor quartz and calc-silicate minerals, namely: talc, tremolite, and epidote at low-grade (albite–epidote facies), diopside and grossular at medium-grade (hornblende facies), and scapolite, vesuvianite, forsterite and wollastonite (pyroxene facies) at high-grade (e.g., Sharma and Sharma 2020).

The presence of rocks bearing calc-silicate mineral assemblages is very common in long-lived carbonate basins that experienced magmatic events and regional metamorphism, providing good indicators of the tectono-magmatic and metamorphic evolution of key sectors in orogens. Good examples of such cases can be found in SW Iberia (Fig. 1a), developed in several segments of the Ossa-Morena Zone (OMZ), during the lower Palaeozoic (Variscan) opening of the Rheic Ocean (e.g., Nance et al. 2010; Ribeiro et al. 2007, 2010; Sánchez-García et al. 2003, 2010). An episode of carbonate sedimentation occurred in the north Gondwana margin during the Cambrian Series 2 (Gozalo et al. 2003; Liñán et al. 2004), well represented in the OMZ (Fig. 1b), intruded by igneous bodies during rift-related magmatic events that persisted until Lower Ordovician (e.g., Chichorro, et al. 2008; Dias da Silva et al. 2023; Díez Fernández et al. 2015; Sánchez-García et al. 2003, 2008, 2010, 2014), and by the Carboniferous syncollisional plutonism (e.g., Cambeses et al. 2019, 2021; Jesus et al. 2007; Lains Amaral et al. 2022; Pereira et al. 2017, 2022; Pin et al. 2008). The emplacement of such igneous bodies produced contact metamorphism during distinct Palaeozoic tectono-magmatic episodes and pose the question: What are the distinctive features and implications of this metamorphic overprint in the OMZ carbonate rocks? In addition, criteria to characterize the regionally metamorphosed marbles and the effect of late dolomitization is necessary to clearly separate the types of metamorphism (contact vs regional) and understand the extent of the role of accessory minerals in the development of calc-silicate minerals. This article presents a systematic study on the mineralogical and Sr-isotopic signature variations in carbonates from the central-northern region the OMZ during contact metamorphism resulting from Lower Palaeozoic rifting and Upper Palaeozoic synorogenic intrusions, as well as the late dolomitization processes.

Fig. 1
figure 1

a Tectonostratigraphic zonation of the Iberian Massif with the shaded area of Palaeozoic outcrops (adapted from Lotze 1945 and Julivert et al. 1974), highlighting the location of the Ossa-Morena Zone (OMZ). b Limits of the OMZ with the Central-Iberian Zone (in the northeast) and with the South Portuguese Zone (in the southwest), as well as its main domains (adapted from Robardet and Gutiérrez-Marco 2004), with emphasis on the studied Alter do Chão—Elvas—Cumbres Mayores unit in the central domain (Fig. 2). The northern OMZ domain corresponds to the Tomar-Badajoz-Córdoba Shear Zone and the Sierra Albarrana unit, the Central zone includes the Zafra-Córdoba-Alanís Unit, the Olivenza-Monesterio Antiform and the Montemor-Ficalho-Barrancos Hinolages unit, and the southern OMZ corresponds to the Beja-Aracena domain (Robardet and Gutiérrez-Marco 2004)

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Geological Setting

Carbonate rocks of the Ossa-Morena Zone

Carbonate sedimentation in the OMZ (Iberian Massif) manifested sparsely during four marine carbonate-production episodes from the Cambrian to the Devonian (Moreira et al. 2019), as well as in mixed siliciclastic-carbonate sedimentary basins during the Carboniferous (Armendáriz 2006; Cózar et al. 2006; González et al. 1990). The first episode of carbonate sedimentation took place during the Cambrian Series 2 (regional Ovetian-Marianian ages) related to rifting characterized by lithosphere thinning, synchronous with rift-related igneous events (Álvaro et al. 2014; Araújo et al. 2013; Gozalo et al. 2003; Liñán et al. 2004; Oliveira et al. 1991). Rifting in conjunction with the eustatic sea-level of the time generated a large carbonate platform environment, with microbial and archaeocyathan-microbial reef-like complexes (Álvaro et al. 2014; Gozalo et al. 2003; Liñán et al. 2004), which were responsible for producing the largest succession of carbonate rocks in the OMZ. This succession, known as the Detrital-Carbonate Group, is characterized by a sequence of calcite and dolomite limestones and marlstones interbedded with siliciclastic strata, metarhyolites and metabasalts (e.g., Sánchez-García et al. 2019).

The second and third episodes of carbonate sedimentation have a much smaller expression and were formed contemporaneously with the drifting phase of the Variscan Cycle (Upper Ordovician and uppermost Silurian, respectively, e.g., Ribeiro et al. 2007). They are preserved in two synclines of the southern OMZ: the Valle and Cerrón del Hornillo synclines (Robardet and Gutiérrez-Marco 2004). In these synclines, a small Upper Ordovician carbonate unit, containing significant, often dolomitised and karstified bioclastic components, is exposed (late Katian stage, regional Kralodvorian). These limestones were generated during a warm climate event that preceded the global glaciation known as the Boda Event (e.g., Gutiérrez-Marco et al. 2019; Robardet and Gutiérrez-Marco 2004; Sarmiento et al. 2008, 2011). During the uppermost Silurian, a unit of black limestones interbedded with calcareous shales was deposited. This unit is representative of carbonate formation during anoxic/euxinic conditions from the Silurian Pridoli to the Lower Devonian Lochkovian (Gutiérrez-Marco et al. 2019; Robardet and Gutiérrez-Marco 2004). Other carbonate rocks attributed to the Silurian are also exposed in Portugal (Bencatel, Ferrarias, and Barrancos, among others; Moreira et al. 2019), though their ages are not well constrained (Moreira 2017; Piçarra and Le Meen 1994; Piçarra and Sarmiento 2006; Silvério et al. 2021). The last episode of carbonate sedimentation is a calciturbiditic reef system sequence developed during the Devonian (Emsian to Givetian), which is mostly located in the southwestern domains of the OMZ (Machado et al. 2009, 2010, 2020; Oliveira et al. 2019), and in places associated with subduction-related basaltic rocks (Moreira et al. 2010; Silva et al. 2011). In some areas (i.e., Cabrela—Toca da Moura basins) these limestones are interpreted as olistoliths (Dias da Silva et al. 2024; Pereira et al. 2006), however this interpretation is not broadly recognized (Machado et al. 2020; Oliveira et al. 2019).

Most biostratigraphic records in the carbonate rocks from the four episodes have been partially obliterated by deformation and metamorphism, and their age constraints have only been attributed based on stratigraphic correlations (e.g., Araújo et al. 2013; Oliveira et al. 1991). This problem was addressed by Moreira et al. (2019) who applied strontium isotopic signatures to contextualize the marine carbonate events in time. They obtained 87Sr/86Sr ranges of 0.708299–0.708777 for Cambrian and 0.707680–0.707784 for Devonian limestones, congruent with worldwide seawater Sr isotopic record (e.g., Denison et al. 1997; Veizer et al. 1999). The 87Sr/86Sr ratios of the presumed Silurian limestones are inconclusive and partially overlap the Cambrian Series 2 (Ovetian–Marianian) signature (Moreira et al. 2019).

Tectono-magmatic and metamorphic events in the Ossa-Morena Zone

Two major superimposed periods of magmatic activity took place during the Variscan orogenic cycle, related to both the opening (rifting) of the Rheic Ocean (e.g., Sánchez-García et al. 2003, 2008, 2010, 2014) and later continental collision (e.g., Cambeses et al. 2019, 2021; Dias et al. 2002; Jesus et al. 2007, 2016; Pereira et al. 2022). Both rift-related and synorogenic magmatic stages are known to induce thermal metamorphism and consequently are responsible for the genesis of a set of contact calc-silicate rocks (s.l.) within the Series 2 Detrital-Carbonate Group throughout the OMZ (e.g., Carriedo et al. 2006; Chichorro 2006; Cruz 2013; Gonçalves 1971; Maia et al. 2022; Pereira 1999).

The rifting of northern Gondwana during the Cambro-Ordovician promoted the emplacement of numerous igneous suites throughout the Iberian Massif in three different phases: (i) an early event during the Terreneuvian (ca. 531–525 Ma; e.g., Chichorro, et al. 2008; Dias da Silva et al. 2023; Ochsner 1993; Pereira et al. 2011; Salman 2004; Sánchez-García et al. 2008, 2010, 2014) with the occurrence of calc-alkaline felsic to intermediate volcanic or volcanic-sedimentary rocks; (ii) a main bimodal event (Series 2 to Furongian, ca. 518–489 Ma, covering the first episode of carbonate production; e.g., Dias da Silva et al. 2023; Etxebarria et al. 2006; Ochsner 1993; Salman 2004; Sánchez-García et al. 2008, 2010; Sarrionandia et al. 2012), that comprises sets of sub-alkaline and alkaline mafic and felsic-intermediate volcanic and plutonic rocks, and iii) a later event that also includes bimodal igneous suites with alkaline and peralkaline volcanic rocks and plutons (Late Furongian to Floian, ca. 490–470 Ma; e.g., Ochsner 1993; Díez-Fernández et al. 2015; Sánchez-García et al. 2010).

Locally, syn-rift thermal metamorphism manifests as small aureoles around intrusions and migmatization in areas of extremely thinned crust, particularly in the central and northern regions of the OMZ, along the boundary with the Central-Iberian Zone, and in the Obejo-Valsequillo Domain. This is reflected by thermal domes associated with partial melting of Ediacaran metasedimentary rocks in the Valuengo, Monesterio and Lora del Rio metamorphic areas of the Olivenza-Monesterio Anticline (e.g., Expósito et al. 2003; Montero et al. 1999; Ordóñez-Casado 1998; Simancas et al. 2004), in the Mina Afurtunada region (Ordóñez-Casado 1998; Sánchez-García et al. 2008), in the Sierra Albarrana Domain (e.g., Solís-Alulima et al. 2020, 2023) and in the La Serena Massif (e.g., Moreno-Martín et al. 2023). The development of such thermal domes is concomitant with the emplacement of several rift-related magmatic rocks (Díez-Fernández et al. 2015; Sánchez-García et al. 2008; Simancas et al. 2004; Solís-Alulima et al. 2020).

Mafic volcanism associated with Devonian strata (Moreira et al. 2010; Ribeiro et al. 2010; Santos et al. 1990; Silva et al. 2011) and subsequent syncollisional (Tournasian-Visean) and post-collisional (Upper Pensilvanian-Cisuralian) intermediate-mafic and felsic intrusions (e.g., Cambeses et al. 2019, 2021; Dias et al. 2002; Jesus et al. 2007, 2016; Pereira et al. 2022) mark the convergent phase of the Variscan orogen. These intrusions are coeval with widespread progressive Barrovian metamorphism and were followed by Buchan metamorphism (e.g., Dias da Silva et al. 2018, 2023; Ribeiro et al. 2019, and references therein). Thermal domes were also developed syncollisionally, e.g., in the Évora and Aracena Massifs (Dias da Silva et al. 2018; Pereira et al. 2009) and in Ponte de Sor-Seda (Dias da Silva et al. 2023).

The Cambrian carbonate formation in Alter do Chão—Elvas—Cumbres Maiores unit of OMZ

Eustatic oscillations in the Cambrian during the opening of the Rheic Ocean had the utmost role in the development of the reef complexes that increased carbonate productivity, leading to the thick impure limestone sequences (e.g., Álvaro et al. 2014; Liñán et al. 2004; Oliveira et al. 1991). The Carbonate Formation (also referred to as Detrital-Carbonate Formation in Sánchez-García et al. 2019) outcrops in the Alter do Chão—Elvas—Cumbres Maiores unit of the OMZ (ACECMu; Robardet and Gutiérrez-Marco 2004) and is the largest exposed carbonate succession (variably metamorphosed) in the OMZ. It is composed of Cambrian late Series 2 impure limestones, dolostones, marls, marbles, and calc-silicate rocks (s.l.), locally intruded by a variety of igneous bodies emplaced during the Cambro-Ordovician and Carboniferous (Fig. 2; Gonçalves 1971; Gonçalves and Assunção 1970; Gonçalves and Fernandes 1973; Gozalo et al. 2003; Liñán et al. 2004; Oliveira et al. 1991; Palacios et al. 2013). In the ACECMu, the bedding strikes from WNW-ESE to NNW-SSE, is moderately dipping to the NE and is often folded. To the southeast of the ACECMu, it follows the geometry of the Monesterio Antiform, dipping moderately to steeply to the NE in the northern flank and to SW in the southern flank. The metamorphic grade varies from anchizone to greenschist facies (chlorite zone) with an increasing metamorphic grade towards the NW (Gonçalves and Assunção 1970; Gonçalves and Fernandes 1973; Mata and Munhá 1990; Moreira et al. 2019). This is congruent with the recrystallization features and the presence of accessory minerals identified in marbles unrelated to the intrusions, namely muscovite, biotite, chlorite, and albite (Moreira et al. 2019). Marbles that underwent contact metamorphism described in the literature contain epidote, tremolite, diopside, vesuvianite, and garnet (Andrade 2022; Cruz 2013; Gonçalves and Fernandes 1973).

Fig. 2
figure 2

Geologic map of the Alter do Chão—Elvas—Cumbres Mayores unit of Ossa-Morena Zone (with sample locations and areas of detailed maps with contact metamorphism), showing a the Alter do Chão—Elvas Sector (Portugal) and b part of the Elvas—Cumbres Maiores Domain (Spain) with the Olivenza-Monesterio Anticline, Partially adapted from Gonçalves (1971) and Palacios et al. (2013). The “alk” rift-related felsic bodies are alkaline or peralkaline granitoid rocks, while other felsic bodies are calc-alkaline granites (peraluminous), mentioned in the text. Mafic rocks are gabbros and diorites. Ages from Díez Fernández et al. (2015) and Ochsner (1993) [1], Cambeses et al. (2019), Dallmeyer et al. (1995) and Errandonea-Martin et al. (2024) [2] and Cruz et al. (2022) and Pereira et al. (2017) [3]. Coordinate system: WGS84, UTM 29N

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The Furongian-Lower Ordovician magmatic rocks that intruded the Carbonate Formation and promoted the occurrence of metamorphic calc-silicate assemblages include the Alter do Chão and Elvas gabbro-diorite suites, the Alter Pedroso, Vaiamonte, Outeirão, Pombal-Varche, Falcato, Alcamins (Fig. 2a; Carrilho Lopes 2020; Díez Fernández et al. 2015; Gonçalves 1971), Almendral, Barcarrota and Jerez de los Caballeros peralkaline granitoids and mafic dykes (Fig. 2b; Etxebarria et al. 2006; Palacios et al. 2013; Sarrionandia et al. 2012). These peralkaline bodies were emplaced during extreme crustal extension (e.g., Díez-Fernández et al. 2015), also responsible for the development of the Cambro-Ordovician thermal domes exposed elsewhere in the OMZ (e.g., Expósito et al. 2003; Simancas et al. 2004). The Carboniferous syn- to post-collisional igneous bodies that intruded the Carbonate Formation are 1) the Visean Brovales, Burguillos del Cerro, and Valencia del Ventoso plutonic complexes near Jerez de los Caballeros, in the core of the Monesterio Antiform (Fig. 2b; e.g., Cambeses et al. 2015, 2019, 2021; Dallmeyer et al. 1995); and 2) the large Santa Eulália Plutonic Complex (SEPC; Fig. 2a) from Upper Pensilvanian-Cisuralian (Cruz et al. 2022; Pereira et al. 2017) between Alter do Chão and Elvas. In this region, The mineral assemblages in carbonate rocks with no spatial relation to igneous bodies are assumed to only represent the effect of regional metamorphism and later weathering and dolomitization processes. Rocks located near the intrusions that have developed calc-silicate assemblages and recrystallization textures are assumed to have been affected by contact metamorphism.

Sampling and methods

Carbonate rocks affected by contact metamorphism and metasomatism surrounding the Alter do Chão gabbro (Fig. 3a), the alkaline-peralkaline rocks of Falcato (Fig. 3b), Almendral (Fig. 3c) and Vaiamonte (Fig. 4), and in the vicinity of the SEPC (Figs. 4b, 5) are exposed. Marbles, dolostones and calc-silicate rocks (s.l.) of the Carbonate Formation outcropping throughout the ACECMu were collected for petrographic, mineralogic and 87Sr/86Sr analyses to constrain primary and modification features (Table 1). The data were compared with previously studied low-grade calcite marbles interpreted as geochemically representative of unaltered rocks in the central-northern OMZ, as well as with rocks that were affected by secondary dolomitization (samples VB-2, VB-12, VB-18 and ALT-1 from Moreira et al. 2019).

Fig. 3
figure 3

Detailed geologicl maps with location of the studied samples associated with igneous intrusions from ACES and ECMD, including structural features. a Samples collected near the Alter do Chão gabbro. b Samples associated with the Falcato peralkaline massif, in the northern enclave and in the eastern contact. c Samples from the roof pendant from the Almendral body. Maps partially adapted from Gonçalves and Assunção (1970), Gonçalves and Fernandes (1973) and Muelas Peña et al. (1976)

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Fig. 4
figure 4

Detailed geologic maps with the location of the studied samples related to igneous intrusions associated with the Vaiamonte body and the SEPC, including structural features a Sample localities near the northern border of the Vaiamonte igneous rocks. b Map of the Monforte area, with sample localities close to the southern border of the Vaiamonte body, and samples associated with the SEPC located on the country rocks and in the roof pendant. Partially adapted from Gonçalves and Fernandes (1973)

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Fig. 5
figure 5

Adapted from Gonçalves and Assunção (1970)

Map of the rocks collected in the southeastern limit of the SEPC, with samples from intrusion-unrelated marbles, and rocks associated with the Santa Eulália Plutonic Complex.

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Table 1 Location and field classification of studied rocks
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Polished thin sections were produced according to standard techniques in the laboratories of the Department of Geosciences of the University of Évora. Samples with carbonate minerals were stained using alizarin red solution, imparting a reddish hue to calcite crystals while other carbonate components (e.g., dolomite) maintained the original color (Ayan 1965).

Sample powder (grain size <63 µm) were analyzed by XRD in the HERCULES Laboratory (University of Évora) using ruker D8 Discover diffractometer with DaVinci geometry and Lynxeye linear detector to determine mineral compositions. Diffractograms were collected with 2θ ranging 3°–75°, with incremental steps of 0.05° (1 s/step). Mineral identification and semi-quantification were facilitated using the software DIFFRAC.SUITE EVA from Bruker (using the ICDD PDF-2 database), applying the Reference Intensity Ratio method (RIR; Hubbard et al. 1976; Hubbard and Snyder 1988). The mineralogical data obtained by XRD are available in Supplementary Information 1 (S1).

Strontium isotopic compositions were determined for the carbonate samples in the Laboratory of Isotope Geology of the University of Aveiro. Samples underwent leaching with 1 M CH3COOH for 2 h at room temperature to facilitate 87Sr/86Sr isotopic ratio determination from only the carbonate phases. Approximately 0.1–0.2 g of each sample was dissolved in 10 mL of 1N acetic acid in a closed Teflon cup (Savillex®) on a heating plate at 150 °C. The resulting suspension was transferred to a centrifuge tube to isolate the undissolved residue and the supernatant solution was recovered, dried, and dissolved in heated 6 M HCL solution and subsequently in 7 M HNO3 for strontium extraction by ion chromatography. The segregated strontium residue was then deposited on a central Ta filament with 0.5N H3PO4. Isotopic ratios were measured on a VG Sector 54 thermal ionization mass spectrometer (TIMS) in dynamic mode with beam intensities of 1–2 V for 88Sr. The isotopic ratios were corrected according to the exponential fractionation law for the value 88Sr/86Sr = 0.1194. The SRM-987 standard was analyzed along with the collected samples and the result provided a confidence interval of over 95%. All reagents used in the sample preparation were purified by double distillation. Ultra-pure water (18.2 MW cm) was obtained using a Milli-Q Element system (Millipore). This procedure follows that of Moreira et al. (2019) who also conducted carbonate rock analysis for the OMZ.

Mineralogical and petrographic features of the ACECMu

Rocks that underwent regional metamorphism

The carbonate rocks located far from the intrusions are assumed to represent the effects of regional metamorphism (samples BM-1, MONF-6, BARB-13, BARB-19) and commonly preserve bedding features. However, in thin section they appear massive, with no visible internal primary structures. Petrographically, these marbles show very fine to medium grain size in a granoblastic polygonal texture and do not have calc-silicate phases in their composition (Figs. 6a–b and 8). Calcite is the dominant phase (>50% in the mineral assemblage), preserved as subhedral-anhedral crystals with evidence of bulging and ductile recrystallization of calcite (e.g., Fig. 6a–d). The modal abundance of accessory silicate mineral assemblage and mode varies (Fig. 8), and consists mainly of quartz, feldspars (plagioclase and minor K-feldspar) and muscovite with minor chlorite, clays, and opaque minerals. Sample BM-1 exhibits twinned calcite and rare twinned dolomite crystals (Fig. 6c), and rare late dolomite rhombs (Fig. 6d). Calcite twins appear only in coarse grain samples and vary between type I and type II, with crystals showing narrow and wider twin thickness randomly through the rock (Fig. 6a–b).

Fig. 6
figure 6

Photomicrographs of petrographic aspects of the samples assumed unaffected by the intrusin. a Generic aspects of pure calcite marble (regionally metamorphosed), displaying the inequigranular granoblastic polygonal texture and type I and II calcite twins (sample MONF-19). b Confirmation of the calcite fraction with the Alizarin Red solution in sample MONF-6. c Subhedral calcite and dolomite crystals in polygonal fabric, in which dolomite presents twinning (sample BM-1). d Coeval calcite and dolomite crystals in medium-fine mass, with growth of a dolomite rhomb with a cloudy appearance in sample BM-1. e Rhomb and anhedral dolomite crystals next to large dissolution holes from JC-1. f Euhedral dolomite rhomb next to dissolution hole in sample BARB-11

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Rocks that underwent late dolomitization

Dolomite-rich rocks (JC-1, BARB-10, BARB-11 and BARB-12) preserve fine-grained cloudy late dolomite, occasionally displaying subhedral to euhedral rhomb contours growing over the primary carbonate and accessory minerals (Fig. 6d–f). Dissolution patches may be associated with euhedral-subhedral crystals (Fig. 6e–f). The dolomite totally or partly obliterates the original textures, indicating secondary growth. The occurrence of secondary dolomite, in carbonate rocks not affected by contact metamorphism, was reported by Moreira et al. (2018, 2019).

Contact metamorphism associated with rift-related gabbros

Samples BAT-1 and BAT-2 were collected near the Alter do Chão gabbroic massif (Fig. 3a). The rocks are composed of K-feldspar (25.3–39.7%), diopside (12.2–18.7%), scapolite (4.2–23.3%) and calcite (10.4–13.2%). Scapolite crystals occur as large irregular porphyroblasts in a groundmass of diopside, quartz, feldspar, and calcite.

Contact metamorphism associated with rift-related alkaline-peralkaline granitoids

Samples FALC-3A and FALC-F3 were collected from two different locations in the Falcato peralkaline massif (Fig. 3b). In the northern enclave (FALC-3A), rocks adjacent to the intrusion are substantially composed of silicates, namely garnet (38.4%), plagioclase (24.1%), quartz (17.7%), and diopside (17%), with minor calcite (2.9%, with patchy type IV twin; Fig. 7e–f). Garnet occurs as blasts surrounding carbonates, while epidote, calcite and quartz appear within garnet cores, filling dissolution holes and cracks (Fig. 7e). On the eastern contact, sample FALC-F3 contains dominant calcite (70.4%) forming irregular-shaped crystals in a mosaic texture frequently with tenuous type II twins, and subordinate amounts of pargasite (19.4%) and feldspars (plagioclase = 3.5%; K-feldspar = 6.7%) (Fig. 7g).

Fig. 7
figure 7

Distinct textural and mineralogical aspects of rocks bearing calc-silicate minerals. a Outcrop of rock with calc-silicate bands in Vaiamonte body (VCS-1). b Photomicrograph of contact between calc-silicate layers, showing scapolite + feldspar + quartz rich band and scapolite + plagioclase + epidote retrogressed band. c Photomicrograph of an example of the scapolite and plagioclase relationship, both with epidote inclusions (VCS-3). d Photomicrograph of scapolite porphyroblast in a sample from the S limit of Vaiamonte body (MONF-8). e Photomicrograph showing euhedral garnet around calcite. Garnets are very fractured and contain epidote, diopside, calcite and quartz growing from dissolution zones and cracks in the core (FALC-3A). f Crossed-polar image of the same sample (FALC-3A), emphasizing the calcite thick type-IV twins, patchy or interrupted. g Photomicrograph showing polygonal calcite grains with irregular shapes and low-relief pargasite near calcite boundaries (Falc-F3). h Crossed-polar photomicrograph of anisotropic garnet fragment and atoll filled with groundmass minerals (calcite and epidote) (VLA-2). i Calcite dispersion highlighted by the Alizarin Red solution on the calc-silicate arrangement (BARB-16). j Photomicrograph of vesuvianite marble sample from the SEPC, showing inequigranular mosaic of irregular calcite (with type I and II twins) and vesuvianite aggregates (MONF-3). k Calcite-diopside vein on fine-grained calc-silicate (BARB-16)

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Samples related to the Almendral body were collected from a roof pendant (Fig. 3c). In the inner part of the roof pendant, sample VLA-1 show higher carbonate mineral abundance (calcite = 76.6%, with type I and type II twins) and low percentage of calc-silicate phases (tremolite = 5%), while sample VLA-3, collected closer to the intrusion, contains a smaller proportion of calcite (55.3%, with irregular untwinned crystals) and greater amount of calc-silicate minerals (diopside = 13.9%, scapolite = 9.8%). In the intermediate area, sample VLA-2 only contains diopside (12.3%) and garnet (18.7%) within a very retrogressed epidote-rich matrix. The garnet, anisotropic under polarized light (also described in unusual cases, e.g., Rossman and Aines 1991), occurs as aggregates or very fractured in a calcite-epidote retrogression matrix (Fig. 7h) or as atolls around calcite and epidote crystals. Rare fluorite occurs as an accessory in an epidote-calcite vein. Samples VLA-3A and VLA-3B, collected near the intrusion are mostly composed of carbonate minerals with few calc-silicate phases (carbonate fraction >90% and calc-silicates <3.5%).

The silicates consist of accessory chlorite, K-feldspar, tremolite, dolomite, mica (muscovite) and epidote and, in these two samples, calcite often shows intersecting type I and type II twins, sometimes curved (type III).

Samples VCS-1, VCS-3, VCS-4 (northern limit, Fig. 4a) and MONF-7, MONF-8, MONF-9 (southern limit; Fig. 4b) were collected from the vicinity of the Vaiamonte peralkaline body, are characterized by various assemblages, often developing a calc-silicate layering (Fig. 7a–b). The samples from the northern limit display a decrease of the carbonate fraction with an increment of the calc-silicate phases (scapolite and tremolite) from the outer zone (VCS-4, calcite + dolomite = 82.2%; tremolite = 6.1%) to the proximity of the intrusion (VCS-3, carbonates = 12.3%; scapolite + tremolite = 46.6%; VCS-1, carbonates = 2.4%, scapolite + tremolite = 74.7%). The southern border also shows a similar pattern, with a decrease in the carbonate abundance towards the intrusion, compensated by the growth of calc-silicate phases. The main difference between these segments lies in the nature and abundance of calc-silicate minerals: abundant tremolite (northern limit), and talc, chlorite, and albite (southern limit) (Fig. 8). Epidote and plagioclase are common phases but are not always detected by XRD. Scapolite often occurs as irregular porphyroblasts with low relief, associated with plagioclase with epidote inclusions (Fig. 7c) or as large blasts growing over the fine epidote matrix (Fig. 7d). The geometry of calcite twinning is dominated by type I and II twins, more often observed in coarse-grained samples. In both areas, samples with an assemblage typical of albite–epidote facies (VCS-4 and MONF-9) show a larger percentage of dolomite (10.7 and 25.5%), while samples with scapolite contain no dolomite.

Fig. 8
figure 8

a Results of x-ray diffraction (XRD) analysis for studied samples showing (a) the total abundance of calcite and dolomite in the rock and b the silicate mineralogy relative to the whole non-carbonate fraction, with fraction of silicates common to all groups and calc-silicate phases. Table of XDR data in Supplementary Information 1 (S1)

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Contact metamorphism related to the Santa Eulália Plutonic Complex

The samples related to the Santa Eulália Plutonic Complex were collected from the northwestern (MONF-1, MONF-4 and MONF-5; Fig. 4b) and southeastern (BARB-14 and BARB-18; Fig. 5) part of the metamorphic aureole, as well as from the roof pendants (MON-1, MONF-2, MONF-3, BARB-15, BARB-16 and BARB-17; Figs. 4b and 5). Samples MONF-1, MONF-4 and MONF-5 have dominant silicate mineral composition, constituted by mica (46.3–58.6%), with subordinate amounts of calc-silicate minerals (scapolite >3.7%, and epidote >0.9%); the carbonate fraction varies between 20.1–38.3%. In the southeastern part, sample BARB-14 is composed mostly of dolomite (85.8%, out of 99.6% carbonate fraction) and remnant chlorite (0.2%) and plagioclase (0.3%), while BARB-18 is a calc-silicate rock, containing a predominance of the calc-silicate minerals diopside (26.6%), scapolite (24.1%) and vesuvianite (3.6%), with less calcite (20%). The samples collected from the roof pendants show substantial mineralogical variability, from carbonate rocks (MON-1, MONF-2, MONF-3 and BARB-17) formed mostly by calcite (65.8–81.8%), to rocks almost solely consisting of calc-silicate phases and feldspars (BARB-15 and BARB-16; Fig. 7i). Calc-silicate mineral assemblage on roof-pendant samples contain common diopside (2.1–25.5%, in all samples), scapolite (2.1–45.9%, MON-1, BARB-15 and BARB-16) and vesuvianite (3.6–16.7%, on MONF-2 and MONF-3), as well as rare wollastonite (10.1%, on BARB-15, though not identified petrographically) (Fig. 7j). Veinlets with coarse-grained carbonate and diopside crystals usually cut the altered marbles (Fig. 7k).

Carbonate and calc-silicate nomenclature for the carbonate formation rocks

The analyzed mineralogical composition of the Carbonate Fm is presented in Fig. 6, which shows significant heterogeneity in the carbonate fraction (Fig. 8a), calc-silicate assemblages and other non-carbonate minerals (Fig. 8b). The samples are grouped according to their mineralogical features in typical OMZ regionally metamorphosed marbles, marbles with late dolomite and rocks with calc-silicate mineralogy. Some Ca-poor silicate phases, namely quartz, mica (muscovite or biotite), albite and K-feldspar are common in all types of samples, while calc-silicates only appear in samples spatial associated with intrusions. The ternary classification plots show that rocks distant from igneous bodies with calcite dominant over dolomite and silicate fraction composed only of quartz, mica and feldspars correspond to rocks derived from impure limestones, while the rocks with dominant cloudy dolomite plot in the varying fields of impure and pure dolostones (Fig. 9a). Overall, the samples with no calc-silicate minerals consist of impure marbles with variable mineralogy (Fig. 9b). Samples associated with the intrusions have a wide range of composition, varying from high-carbonate fractions (derived from calcitic dolostone or dolomitic limestone, Fig. 9a), to rocks mostly embodied by silicate minerals (from calc-silicate-bearing impure marbles to calc-silicate s.s. rocks, Fig. 9b). Most of these samples fall within the ‘carbonate–silicate rock’ field, with modal silicate fraction between 50–95%, and only three samples (VCS-1, BARB-15, and FALC-3) are classified as calc-silicate rocks s.s., with <5% carbonates and >50% calc-silicates over the common mineralogy (Fig. 9b).

Fig. 9
figure 9

Ternary diagrams of mineralogical composition plotting obtained XRD results. a Applying the sedimentary rock nomenclature according to carbonate fraction (from Giannini 2003). b Distinguishing impure marbles and rocks bearing calc-silicate minerals (adapted from Davis and Ferry 1993; Mathieu et al. 2015). Carbonate fraction represents the percentage of calcite (%Cal) and dolomite (%Dol) in each sample. Minerals comprising the modal abundance of other minerals and calc-silicate minerals as in Fig. 8

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Sr-isotope values of carbonates from the ACECMu

The 87Sr/86Sr analyses for the carbonate samples are presented in Table 2, and plotted according to mineralogical abundance and nearby intrusion in Fig. 8. Samples with very low-carbonate fraction (FALC-3A, VCS-1, BARB-15; carbonate minerals <5%) were not analyzed.

Table 2 Results of 87Sr/86Sr analysis for the studied samples
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Intrusion-unrelated calcite marbles show a low 87Sr/86Sr ratio (0.70832–0.70877), while dolostones show higher values (0.70887–0.70955). The samples collected near the intrusions yielded more variable results:

  • The two low-carbonate samples collected near the Alter do Chão gabbro (BAT-1 and BAT-2), yield high 87Sr/86Sr values (0.70906–0.70939).

  • The carbonate sample next to the Falcato massif (FALC-F3) yielded 87Sr/86Sr = 0.70876.

  • The rocks collected from the roof pendant of the Almendral massif yielded 87Sr/86Sr values that appear unrelated to the sample location: 87Sr/86Sr = 0.70876 in the centre (VLA-1), 87Sr/86Sr = 0.70953 in the intermediate zone (VLA-2), and 87Sr/86Sr = 0.70869 near the contact (VLA-3). Rocks near the contact composed of dominant calcite (VLA-3A) or coeval calcite–dolomite (VLA-3B) with very little calc-silicate phases yield consistently lower ratios of 0.70838 and 0.70841, respectively.

  • In the northern limit with the Vaiamonte intrusive, 87Sr/86Sr vary from 0.70815 (VCS-1) to 0.70911 (VCS-3), while in the southern border the ratios are slightly higher, ranging from 0.70854 (MONF-9) to 0.70959 (MONF-8).

  • In the low-carbonate samples (<50% carbonates) collected from the NW contact of the SEPC granite, 87Sr/86Sr range from 0.70853 (MONF-4) to 0.70869 (MONF-1) and 0.70876 (MONF-5). On the SE border, samples yielded more variable 87Sr/86Sr results, which vary from 0.70836 (BARB-14, lowest value of the group) to 0.70943 (BARB-18, with the highest value). A somewhat coherent span of values was determined in samples from the roof pendants, ranging 0.70850 (MON-1), 0.70852 (MONF-2) and 0.70857 (MONF-3) in the western roof pendant, and a broader range of 0.70846–0.70878 (BARB-16) in the eastern roof pendant.

Discussion

Regional metamorphism and late dolomitization overprint

The Cambrian impure marbles commonly contain dominant calcite or coexisting calcite–dolomite with accessory quartz, feldspar and mica and no trace of calc-silicate minerals. The recrystallization of the calcite during regional metamorphism is assumed to have obliterated most of the primary or sedimentary structures. In some samples, the strong secondary dolomitization of the rocks from the Carbonate Formation is interpreted to have taken place after burial, i.e., after diagenesis and the regional Variscan metamorphic stage, by the action of meteoric or high salinity waters (e.g., Moreira et al. 2019). The dolomite rhomb that has grown over a dolomite-rich marble (sample BM-1; Fig. 6d), suggests that the dolomitizing event occurred after regional metamorphism. The significative amounts of quartz and mica in both the calcite-rich and dolomitized marbles confirm that the regional metamorphism in this sector did not exceed the chlorite zone/lower greenschist facies (as described by Moreira et al. 2019). Additionally, the dominant calcite type I and II twins (Fig. 6a–b) and rarer unaltered dolomite twin (sample BM-1; Fig. 6c) suggest maximum temperatures of around 300 °C (e.g., Passchier and Trouw 2005).

The near total replacement of primary calcite by dolomite (often rhombohedral and with rare cloudy patches; Fig. 6d–f) appears to have occurred while quartz, muscovite and to a lesser extent chlorite and feldspars were preserved (see also Moreira et al. 2019). Typical regionally metamorphosed marbles with no evidence of dolomitization yield carbonate fraction 87Sr/86Sr similar to those expected from the 87Sr/86Sr isotopic curve of Cambrian Series 2 seawater. In contrast, when secondary dolomite is the dominant phase, the 87Sr/86Sr are noticeably higher, with an apparent boundary at around 87Sr/86Sr = 0.7088 (Figs. 10, 11b). Dolomitization is interpreted to be the result of the post-metamorphic interaction with meteoric, saline or other shallow fluids, and the 87Sr/86Sr ratio is dependent on the composition of the surrounding rocks that compose the drainage basin, the climatic conditions, and the evaporation rates. The significant increase of the 87Sr/86Sr ratio coud also have been caused if the dolomitizing fluids resulted from the mixing of waters derived from high 87Sr/86Sr sources like upper crustal rocks and pelagic sediments, and/or achieved higher values by evaporative concentration (e.g., Faure and Mensing 2005; Moreira et al. 2018, 2019; Rollinson and Pease 2021). Therefore, 87Sr/86Sr values between 0.7083 and 0.7088 may represent the primary/diagenetic signature for Cambrian limestones of the Carbonate Formation (Fig. 10), while the higher values are likely explained by secondary late dolomitization.

Fig. 10
figure 10

87Sr/86Sr from marbles and dolostones collected distally from and proximally from igneous bodies. 87Sr/86Sr from carbonate rocks from other OMZ locations are plotted for comparison (from Moreira et al. 2019)

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Fig. 11
figure 11

Binary plots variation of mineralogical and 87Sr/86Sr values for all samples analyzed. a %Carb/(%Carb + %Com) vs %Carb/(%Carb + %Cs). b 87Sr/86Sr values vs the carbonates ratio. c 87Sr/86Sr values vs %Carb/(%Carb + %Com). d 87Sr/86Sr values vs %Carb/(%Carb + %Cs). %Carb refers to the modal abundance of calcite and dolomite, %Cal and %Dol refer to calcite and dolomite respectively, %Com encompasses the common mineralogy excluding the calc-silicate phases and %Cs the amount of calc-silicate mineralogy

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The calcite marbles throughout the region have no proximity to igneous bodies, and therefore the mineralogical and isotopic results are interpreted as representative of the composition of the protolith. However, as the dolomitization appears to have been “late”, occurring after diagenesis and metamorphism, rocks exhibiting late dolomitization will not be considered further here.

Effects of contact metamorphism on the protolith

Mineralogical changes

The textural and mineralogical transformation of the Carbonate Formation during contact metamorphism from the two different tectono-magmatic events during the Variscan Cycle are noteworthy from outcrop to microscopic scale. The overprint of calc-silicate minerals is expressed in the external contact aureoles of igneous bodies and enclaves/roof pendants, with notable growth of assemblages as fracture-filling veins, texture overgrowth, and replacement of previous fabrics. The metamorphic effects related to the time-separated Cambro-Ordovician and Carboniferous intrusions may have developed in overlapping aureoles. However, no definitive evidence of a rock affected by more than one metamorphic episode has been identified in the studied samples. The amount and mineralogy of the calc-silicate minerals that formed depend on the mineralogical composition of the protolith, especially regarding dolomite and silicate accessories (Fig. 11a, c). Orogenic compaction and deformation have obscured the width and shape configuration of the thermal metamorphic aureoles. However, some metamorphic isograds based on prograde calc-silicate mineral assemblages can be partially inferred (Figs. 3, 4, 5).

Calcite consistently appears as the predominant carbonate phase, with exceptions found in samples VCS-1 and BARB-15, where only minor dolomite is present. In the case of BARB-14, dolomite constitutes around 85% of the modal abundance and the samples VCS-4, MONF-9 and VLA-3B exhibit significant dolomite within the overall carbonate fraction.

Rocks containing calc-silicate minerals that were sampled far from the intrusions exhibit the typical mineral assemblages of the albite–epidote facies and appear to represent the lowest metamorphic grade. These include samples MONF-9 (with talc and epidote) and MONF-4, VCS-4 and VLA-1 (with tremolite), likely the product of low-grade contact metamorphism, translated by the first prograde metamorphic reactions in impure carbonates: dolomite (Dol) + quartz (Qz) + water (H2O) = talc (Tlc) + calcite (Cal) + carbon dioxide (CO2) followed by Tlc + Cal + Qz = tremolite (Trem) + H2O + CO2 when reaching the ca. 300–400 °C interval (Bucher 2023; Sharma and Sharma 2020; Winter 2014). The regular absence of talc in the most external regions of the metamorphic aureole may be indicative of low water contents in the interstices or pore spaces of the protolith, leading to the direct formation of tremolite through Dol + Qz + H2O = Trem + Cal + CO2 (e.g., Sharma and Sharma 2020). The common presence of epidote can be explained by the reaction: plagioclase (Plg) + Cal + H2O = epidote (Ep) + CO2 (Winter 2014), implying the presence of detrital plagioclase in the protoliths. The prevalent calcite type I and type II twins in crystals from the albite–epidote samples seem compatible with the low-grade metamorphic conditions.

The occurrence of diopside may signify the establishment of medium-grade conditions (hornblende facies), resulting from temperatures reaching approximately 400–550 °C, at the expense of lower-grade assemblages, i.e., Trem + Cal + Qz = diopside (Di) + CO2 + H2O (Bucher 2023; Sharma and Sharma 2020). Diopside is found associated with all intrusions except the Vaiamonte massif. In the Falcato metamorphic aureole (sample FALC-F3) pargasite appears along with calcic amphiboles that show some replacement of calcium by sodium. These likely formed from lower temperature tremolites (e.g., Bucher 2023).

Medium grade rocks from roof pendants and enclaves exhibit diopside, vesuvianite (in the SEPC, samples MONF-3 and MONF-2) and garnet (predominantly in northern Falcato and Almendral, in samples FALC-3A and VLA-2). Vesuvianite can occur at lower temperatures (e.g., Winkler 1976) however, the typical association with diopside in studied samples suggest it was formed at least at medium grade conditions (samples MONF-2, MONF-3 and BARB-18). In the Almendral roof pendant, anisotropic garnet is assumed to be hydrogrossular (OH−bearing garnet, Rossman and Aines 1991), characterized by concentric zonation or atolls surrounding calcite or epidote, implying retrogression (Fig. 7g).

Garnets can be produced either at the expense of common protolith minerals via Cal + Plg + Qz = garnet (Grt) + CO2, or through the prograde reaction Ep + Cal + Qz = Grt + H2O (Bucher 2023; Sharma and Sharma 2020). Fluorite was only found in a calcite-epidote vein in the medium-grade Almendral roof pendant (near VLA-2) possibly resulting from the action of fluids expelled from the alkaline-peralkaline intrusion.

At medium-grade conditions, calcite becomes less common (expected since most metamorphic reactions imply decarbonization). Calcite in these samples is usually not twinned or shows type IV twins (Fig. 7j) which develop at higher temperatures than calcite with type I and type II twins.

In the inner belts of the thermal aureoles, higher grade samples (ca. 550–700 °C) consistently contain scapolite, presumably formed through the reaction Plg + Cal = scapolite (Scp) (Bucher 2023; Sharma and Sharma 2020). Scapolite was observed near all intrusions except near the Falcato massif. Even though wollastonite has not been confirmed in thin section, its presence in a SEPC roof pendant (BARB-15) is not unreasonable, as the simple reaction Cal + Qz = wollastonite (Wo) + CO2 can happen at high temperatures (e.g., Bucher 2023).

No forsterite was present in any of the collected semples. This could be due to the protolith composition, as the presence of forsterite in metamorphosed silicious dolostones is favored if the protolith contains high Mg/Fe and low SiO2, whereas low Mg/Fe and high SiO2 favors the appearance of diopside (Winter 2014). Similar reasons could explain the absence of periclase, since this mineral is only expected in rocks derived from SiO2-poor and (Al2O3-poor) dolostones or dolomitic limestones (e.g., Bucher 2023).

The occurrence of garnet, vesuvianite, fluorite and wollastonite appear to be restricted to the roof pendants, which suggests that the external fluid flow and thermal dispersion is heterogenous around the intrusions. Jamtveit et al. (1992) and Winter (2014) suggest that the occurrence of wollastonite- and garnet-in reactions are triggered by H2O-rich intrusion-driven reaction fronts near magmatic bodies, as they require a CO2-poor fluid. In addition, Winter (2014) also suggested that the formation of vesuvianite is associated with water-rich fluids. In this way, the presence of such mineralogy in roof pendants and enclaves could be explained by the preference for intrusion-derived fluid pathways, leading to high-H2O ingress into the country rocks above the intrusion, since the upwards route through hydraulic fractures and faults would be the most likely path, due to the low density of the aqueous fluids. In contrast, the sideways movements of intrusion-related fluids depend largely on the porosity, permeability and structure (e.g., bedding planes or foliation). This could explain the mineralogy typical of internally buffered reactions (precursor minerals reacting with pore fluids to maintain a relatively stable chemical composition despite temperature changes) found around the intrusive bodies since the massive character of the original marls and limestones of the Carbonate Formation (e.g., Araújo et al. 2013) would have prevented sideways fluid circulation. Table 3 summarizes the sample distribution regarding the thermal metamorphic grades of the studied samples.

Table 3 Summary of thermal grades according to the calc-silicate phases and calcite (Cc) twin types found on samples from the Carbonate Formation of the OMZ, with information on their location within the aureole configuration (near the outer, middle, or inner isograds) and position relative to the intrusion (on the sideways of the magmatic bodies or in enclaves as roof pendants)
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The origin of mica, plagioclase and K-feldspar in the studied calc-silicate rocks (s.l.) is still uncertain, as these minerals can be either inherited (detrital) and therefore stable under a wide thermal range (as discussed in Moreira et al. 2019) or a product of metamorphic recrystallization. Major retrogression features consist of garnet breakdown to epidote, quartz and calcite (Fig. 7g, i–j) and, less commonly, scapolite to plagioclase, calcite and epidote. These observed retrogression features could suggest the breakdown of high-grade minerals during later regional greenschist facies metamorphism, thermal relaxation during exhumation or decompression.

Implications of mineralogical modifications on the Sr-Isotopic signature

The 87Sr/86Sr values obtained for the carbonate fraction of the calc-silicate rocks with low-grade metamorphic assemblages are compatible with the signature of regional marbles not related with intrusions (87Sr/86Sr < 0.7088). Dolomite is unstable at the temperatures of the higher albite–epidote facies and the calc-silicate mineral assemblages are mainly produced with total consumption of dolomite, usually forming residual hydrothermal calcite (Sharma and Sharma 2020). This could be due to internal buffering being able to preserve the previous 87Sr/86Sr composition. The heat transferred by conduction from the igneous body would vaporize interstitial waters in massive limestones, which inhibit pervasive fluid flow. Vaporization would promote local reactions without substantial Sr influx, thus maintaining the precursor Sr isotopic signature. Despite that, high-grade rocks located near the intrusion with significant amounts of calc-silicates (>25%) show an increase in 87Sr/86Sr values that slightly contrast with the typical signature of unaltered Cambrian marbles (Figs. 9, 11d). Although the lack of detailed results for each individual area of sample collection, a plot of the regional data shows a wide fan-shaped pattern, suggesting a non-linear relationship between the development of calc-silicate minerals and the increase of the radiogenic Sr isotopic signature in carbonates (Fig. 11d). Samples with higher grade assemblages (BAT-1, BAT-2 and FALC-F3) fall within the pattern (Fig. 11d).

The non-carbonate mineral fraction in the protolith appears to exert a strongcontrol on the formation of the thermal metamorphic mineral assemblages. Sample BARB-14, located in the vicinity of the SEPC calc-alkaline granitoid, is composed almost exclusively of carbonates and displays the established Cambrian 87Sr/86Sr signature. This observation provides further evidence of carbonate recrystallization buffering the Sr values in samples that did not develop calc-silicate minerals. Samples VLA-3A and VLA-3B (also collected near an intrusion, in the Almendral roof pendant) similarly confirm such inferences, with poor development of calc-silicate phases and maintenance of the protolith 87Sr/86Sr values. This could be due to no quartz being present in the assemblages, hindering the progression of the thermal reactions. Rocks in the outer thermal aureole (VCS-4, MONF-4, MONF-9, VLA-1) that did not pass albite–epidote conditions, did not undergo the further metamorphic reactions that promote decarbonation, and therefore were also able to preserve low 87Sr/86Sr values (Fig. 11d). Moreover, rocks with medium- to high-grade minerals fall within the fan-dispersion area, suggesting a link between the Sr signature and a decrease in the carbonate fraction (due to decarbonation reactions). The exact nature of the trends vary with the type of intrusion (Fig. 11d). The increase in 87Sr/86Sr with increasing grade could be due to external input of magmatic-derived Sr or to the successive decarbonation reactions of the rocks through the contact metamorphic stages. If both processes contributed to the thermal metamorphic overprint on the studied rocks, they would show significantly different trends.

Overall, the rocks metamorphosed at hornblende facies or higher that underwent mineralogical, or those that experienced dolomitization, pose a challenge for using Sr-based chronology (c.f. Moreira et al. 2019). However, Sr chronology systematics still appear to hold in samples where the metamorphic conditions did not pass the albite–epidote facies or in samples where the protolith is only composed of carbonate minerals. These types of samples generally preserve the initial 87Sr/86Sr ratios, thus maintaining their effectiveness for chronologic assessments.

Conclusions

The regionally metamorphosed Series 2 carbonate rocks of the OMZ include calcite or calcite–dolomite impure marbles and marlstones with accessory quartz, feldspars, mica and chlorite, and display 87Sr/86Sr values of 0.7083–0.7088, typical of lower Cambrian seawaters. Late dolomitization is characterized in samples that were unaffected by contact metamorphism by the growth of rhombohedral dolomite, with total replacement of calcite, and an increase in 87Sr/86Sr values >0.7088. Contact metamorphism in the ACECMu is characterized by a mineralogical and 87Sr/86Sr overprint in aureoles around intrusions. The main observations may be summarized as:

  • The typical prograde assemblage preserved in the outer aureole is epidote + amphibole (usually tremolite), followed by the appearance of diopside in the middle aureole and by scapolite in the inner aureole. A fluid-driven transformation is suggested within the roof pendants and enclaves based on the almost exclusive presence of garnet (±fluorite), vesuvianite and wollastonite in rocks from these settings. In rocks with a high modal proportion of silicate minerals located near the igneous bodies (in the roof pendants, enclaves or adjacent to the intrusion), the primary carbonate minerals are substantially substituted by calc-silicate assemblages.

  • The extent to which 87Sr/86Sr values in the carbonate fraction are modified by thermal metamorphism or retain the protolith signature is still unclear. Our results suggest that distance from the intrusion does not directly correlate to the carbonate 87Sr/86Sr ratio. Samples with a higher abundance of silicate minerals were likely more reactive during thermal perturbations, leading to changes in mineralogy and hence isotopic signatures with increasing decarbonization at higher temperature. The 87Sr/86Sr ratio in carbonate phases tends to increase with the growth of calc-silicate mineral assemblages and decreasing carbonate modal proportion) above hornblende-facies conditions. These conditions therefore provide a limit to carbonate Sr-isotope chronology. The exception provided by marbles with a very high carbonate fraction located adjacent to the intrusions. These remained unreactive during contact metamorphism and consequently they maintain their Cambrian strontium isotope fingerprint.