The UM series consists of Hbl, olivine (Ol), orthopyroxene (Opx), and spinel (Spl), along with magnetite (Mag) and ilmenite (Ilm) as accessory components (abbreviations according to Whitney and Evans, 2010). Depending on the modal composition (17 point-counting analyses) and using the classification of Streckeisen (1974), several petrographic types can be defined: dunites, harzburgites, Hbl harzburgites, orthopyroxenites, and Hbl orthopyroxenites (Figure 3A). Among the UM rocks, some appear very homogeneous (dunites), whereas others, essentially the harzburgites, consist of alternating centimeter-thick layers of various UM types (see below).

Dunites (two investigated samples) occur as metric layers at the bottom of the series and alternate with harzburgite (Figure 4A). They are nearly entirely composed of fine-grained crystals (0.3–0.5 mm), defining a polygonal texture (adcumulate, Figures 4B,C). They consist of 90%–95% Ol (Fo_88–90), approximately 3% Opx (Wo_0.2–1.6 En_88.7–92.3 Fs_7.4–10.6), and rare interstitial amphibole (Amp) (magnesio-hornblende 0.8 < Mg# < 0.92). Small grains of Cr–Al Spl with an Mg# of 56–59 and a Cr# of 26–32, along with Mag and Ilm, are included within the olivine (Kilzi, 2014). The analyzed dunite (Supplementary Table S1) displays relatively low SiO₂ (36.61 wt%) and Al₂O₃ (1.28 wt%) contents, a high Mg# (88.4), and high Ni (2445 ppm) and Cr (4450 ppm) contents (Supplementary Table S1). On the chondrite-normalized REE diagram (Figure 5), dunite shows a slightly depleted flat REE pattern [total REE = 0.4 ppm; (La/Yb)_N = 0.94]. The mantle-normalized trace element pattern shows Cs, Th, U, and Nb enrichment, positive Pb and Hf anomalies, and negative Ba and Zr anomalies (Figure 5).

Harzburgites, fine- to medium-grained (0.5–2 mm) rocks, are the dominant rock types. They comprise alternating centimeter-thick layers differing in their modal compositions (Figure 3A): 1) Hbl-poor harzburgite layers consisting of 55%–90% Ol, 10%–45% Opx, and less than 5% Amp; 2) Hbl harzburgite layers consisting of 70%–85% Ol, 10%–20% Opx, and 15%–20% Amp; and 3) Hbl-rich harzburgite layers consisting of 40%–50% Ol, 20%–30% Opx, and more than 30% Amp (Figure 3A). All harzburgite types contain 1%–5% accessory minerals, Spl grains with an Mg# of 28–76 and a Cr# of 39–81, Ilm with high FeO and TiO₂ contents (44 wt% and 52 wt%, respectively), hematites (Hem), and Fe–Ni–Cu sulfides. Serpentine (Srp) and talc (Tlc) occur as common alteration products in the harzburgite samples. Others display secondary anthophyllite (Ath) fibers, 0.1–1 mm long, with a mean Mg# of 88. Additionally, the harzburgites are layered (Figure 6), including millimetric- to centimetric-thick layers of 1) orthopyroxenite consisting of (up to 90%) prismatic Opx, approx. 4% Ol, approx. 3% Amp, approx. 2% Spl (Al₂O₃: 37.8–58.8 wt%; Cr₂O₃: 8.4–23.6 wt%; FeO(T): 14–23 wt%), and Ilm crystals; and 2) 2–3-cm-thick Hbl layers consisting of large prismatic Hbl (>90% of the modal content). In the harzburgites, the chemical composition of Ol and Opx is homogeneous and close to the dunite mineral composition, that is, Fo_88–90 for Ol and Wo_0.2–1.6 En_88.7–92.3 Fs_7.4–10.6 for Opx. Amphiboles are magnesio-hornblendes [Leake et al., 1997, in Kilzi (2014)], having a composition that varies slightly, with Mg# decreasing from 92 in the Hbl-poor harzburgites to 81 in the Hbl-rich harzburgites.

Whole-rock analyses (Supplementary Table S1) of the Hbl-poor harzburgites (three investigated samples) display relatively low SiO₂ (40–41.2 wt%) and Al₂O₃ (average 1.4 wt%) contents and a high Mg# (89–92), along with high Ni (1837–2641 ppm) and Cr (900–2260 ppm) contents. They exhibit (samples 09CA09 and 09CA02, Figure 7) 1) weakly REE-enriched patterns, with ∑REE = 1.6–2.8 ppm; 2) moderately LREE (Light Rare Earth Elements)-enriched patterns of (La/Yb)_N = 1.1–2.7 and (La/Sm)_N = 0.5–1.9; 3) flat to moderately depleted HREE (Heavy Rare Earth Elements) of (Gd/Yb)_N = 1.38–1.78; and 4) a slightly positive or negative Eu anomaly (Eu/Eu* = 0.97–1.15). The mantle-normalized trace element patterns exhibit Cs, Th, U, and Nb enrichments; positive Cs, Pb, and Hf anomalies; and negative Ba, Sr, and Zr anomalies (Figure 7).

Hbl harzburgites (Supplementary Table S1; seven investigated samples) display slightly higher Al₂O₃ and CaO contents (up to 1.46 wt% and 1.59 wt%, respectively) and a high Mg# (90–91). They exhibit REE-enriched patterns with total REE = 1.85–3.54 ppm, (La/Yb)_N = 1.89–4.76, and Eu/Eu* = 0.94–1.75 (Figure 7).

Hbl-rich harzburgites (five investigated samples) have higher SiO₂ (up to 44.79 wt%), Al₂O₃, and CaO contents (1.5–5.62 wt% and 1.27–5.63 wt%, respectively) and lower MgO (average 37.3 wt%) contents than dunites and other harzburgites with an Mg# close to 89 (Supplementary Table S1). They also display high Ni (1550–2330 ppm) and Cr (1300–2600 ppm) contents. Hbl-rich harzburgites have the highest REE content (total REE up to 40 ppm) among all harzburgitic rock types. They are moderately LREE-enriched and HREE-depleted, with (La/Yb)_N = 7.71–14.3, (La/Sm)_N = 2.15–3.16, and (Gd/Yb)_N = 2.14–4.47 (Figure 7), and are characterized by slightly negative Eu anomalies (Eu/Eu* = 0.94–1.62). Their mantle-normalized trace element patterns show enriched Cs, Th, La, and Ce; positive Ba, Nb, Pb, and Nd anomalies; and negative Rb, U, Sr, and Zr anomalies (Figure 7).

Orthopyroxenite and Hbl-bearing orthopyroxenites (three investigated samples) occur as centimetric layers alternating with dunites and harzburgites. They are almost entirely composed of fine-grained crystals (0.3–0.5 mm) defining an adcumulate texture. They consist of 85%–95% euhedral Opx associated with 5%–10% Amp, and minor Ilm and Spl. Opx and Hbl have compositions comparable to those found in harzburgites. Orthopyroxenite and Hbl orthopyroxenites have a major element composition close to that of Hbl-rich harzburgites with SiO₂ (44–47.51 wt%), Al₂O₃ (2.62–5.54 wt%), CaO (2.78–5.78 wt%), and MgO (30–33.05 wt%). Their Mg# value is also high, approaching 90. In contrast, orthopyroxenite displays weak REE enrichment (∑REE = 5.7 ppm), moderately depleted to slightly enriched LREE patterns (La/Yb)_N = 1.22 and (La/Sm)_N = 0.7, and no Eu anomaly (Figure 5). Finally, its mantle-normalized trace element pattern is characterized by Cs enrichment, positive Nb and Nd anomalies, and negative Ba, Sr, and Zr anomalies (Figure 5). Hbl orthopyroxenites differ (Supplementary Table S1) by their REE total contents (up to 60 ppm) with (La/Yb)_N = 8.25 to 10.13 and (La/Sm)_N = 2, related to the Hbl modal abundance.

To summarize, the Castillon ultramafic rock types correspond to a layered sequence with the alternation of mainly dunites, orthopyroxenites, and harzburgites with minor hornblendite layers. REE contents in the ultramafic rocks reveal a strong correlation between their Amp modal composition and REE patterns, where an increase in the Amp content is commonly associated with higher total REE concentrations and higher La/Yb ratios (Supplementary Table S1).

Located above the UM series, the Px-bearing mafic series occurs as layers, 1 dm to 1.5 m thick, sometimes strongly boudinaged within the felsic granulites (Figure 2B). The series is composed of norites [plagioclase (Pl)+Opx + Hbl], gabbro–norites [Pl + Opx + clinopyroxene (Cpx)+Hbl], and gabbros “stricto senso (s.s.)” (Pl + Cpx + Hbl; Figure 8). Layers of diverse types of Px-bearing mafic rocks alternate with layers of Hbl gabbros (M2, see below). Moreover, contacts between the two types of mafic rocks appear to have been parallelized by deformation; however, in detail, they are irregular, sinuous, and often fingered, leading to the inclusion of fragments of norite and gabbro–norite within the Hbl gabbro. Pyroxene-bearing mafic rocks show two scales of igneous layering: 1) layering at the outcrop scale with alternating decametric to metric layers of norites and gabbro–norites, and 2) layering at the hand specimen scale with alternating thin, centimeter-thick layers of gabbro–norite and norite (Figure 8).

Norites (four investigated samples), located at the bottom of the Px-bearing mafic series, occur as boudinaged layers, varying between 10 and 50 cm in thickness within the gneisses. They consist of Pl + Opx + Amp + Opq ± biotite (Bt). The variations in Hbl, Opx, and Pl modal proportions (Hbl: 5%–30%, Opx: 5%–40%, and Pl: 20%–60%) allow distinguishing several types of norite, spanning from Hbl leuconorites to Hbl melanorites (Figure 3B). Norites consist of rounded and subhedral Opx (hypersthene ranging in composition from Wo_0.8–1.2 En_60–67 Fs_32–38 to Wo_1.1–1.9 En_54–56 Fs_43–45), Pl (labradorite Ab_41–47 An_53–59 Or_0.2–0.51), and fine-grained Ilm with high FeO (44 wt%) and TiO₂ (52 wt%) contents. These minerals are cumulus and are associated with subhedral to anhedral intercumulus edenite (6.5 < Si < 6.85 and an Mg# ranging from 68 to 71); together they define a mesocumulate texture (Figure 8C). Norites display moderate variations in SiO₂ (45.5–48.8 wt%), Al₂O₃ (15–16.6 wt%), MgO (8.5–10 wt%), FeO(T) (9.9–12.3 wt%), CaO (6–11 wt%), and Mg# (55–64). They have significant Cr (230–470 ppm) and Ni (131–256 ppm) contents (Supplementary Table S2). On chondrite-normalized REE- and mantle-normalized trace element diagrams (Figure 9), norites are characterized by 1) moderately enriched REE patterns, with ∑REE = 61–80 ppm; 2) moderately enriched LREE patterns, with (La/Yb)_N = 1.8–2.4 and (La/Sm)_N = 1.3–2; 3) moderately depleted HREE patterns, with (Gd/Yb)_N = 1.2–1.6; 4) a weak negative Eu anomaly, with Eu/Eu* = 0.7–1; 5) Cs and Rb enrichments; 6) positive U and Pb anomalies; and 7) a slightly negative Nb anomaly.

Gabbro–norites (six investigated samples) occur as 1–4-m-thick layers located at the top of the noritic series. They are medium- to coarse-grained (1–3 mm) characterized by alternating Hbl-rich and Hbl-poor thin layers (0.5–1 cm thick). Typical samples consist of approx. 35%–40% Opx, approx. 30%–40% Cpx, approx. 20%–25% Pl, approx. 5%–20% Amp, and <3% accessory minerals, mainly Ilm. Opxs are hypersthenes (Wo_0.8–1.4 En_52–56 Fs_42–48; Kilzi, 2014) and are slightly more ferriferous than those of norites. Cpxs are diopsides (Wo_44.5–48 En_37.5–39 Fs_12.5–19), with an Mg# grading from 65 to 73. They occur as anhedral and locally poikilitic crystals, including Pl, Opx, and Ilm grains (Figure 8D). Poikilitic Amps are edenites and magnesio-hastingsites, following the classification of Leake et al. (1997), with an Mg# of 53–62 (Kilzi, 2014). Pls are labradorites (Ab_50–52 An_47–50 Or_0.1–0.9). Gabbro–norites display notable variations in SiO₂ (39–48 wt%), Al₂O₃ (14.5–18.6 wt%), and CaO (9–14 wt%) contents, with a low Mg# (43–60) (Supplementary Table S2). These variations are related to the heterogeneous repartition (microlayering) of some constituent mineral phases, such as Cpx and Amp. They have moderate Cr (23–334 ppm), Ni (45–85 ppm), and Zr (10–2 ppm) contents (Supplementary Table S2). On chondrite-normalized diagrams (Figure 10), gabbro–norites appear moderately REE-rich (∑REE = 46–85 ppm), with (La/Yb)_N = 0.5–2, (La/Sm)_N = 0.5–1.5, and (Gd/Yb)_N = 1.3–2.1. They show minor negative or positive Eu anomalies (Eu/Eu* = 0.9–1.3). Mantle-normalized trace element patterns are characterized by Cs enrichment and negative anomalies in Rb, U, Zr, and Pb (Figure 9). Gabbros s.s. (Le Maitre et al., 2004) occur within the gabbro–norite as centimetric Cpx-rich layers. In these gabbro layers, anhedral Cpxs are diopsides (Wo_47.8–49.7 En_36.7–39.8 Fs_9.5–15.5), with a high CaO content (approx. 24 wt%) and Mg#, ranging from 70 to 81 (Kilzi, 2014). Anhedral poikilitic patches of Amp (up to 1 cm in size) are mainly magnesio-hastingsites, with an Mg# ranging from 52 to 60 and Si varying from 5.4 to 6.6 apfu (Kilzi, 2014).

Gabbro layers (two investigated samples) have a low SiO₂ content (approx. 39 wt%) and high CaO (approx. 17 wt%) and TiO₂ (approx. 3.4 wt%) contents. Their Mg# is relatively low (approx. 62–63). They have moderate Cr (250–280 ppm), Ni (144–156 ppm), and Co (70–75 ppm) contents, along with high Zr (253–319 ppm) contents (Supplementary Table S2). They show strong REE enrichment (∑REE = 239–245 ppm) and very strong REE fractionation, with (La/Yb)_N = 10, (La/Sm)_N = 3, and (Gd/Yb)_N = 2.7, but no Eu anomalies (Figure 9). Mantle-normalized trace element patterns are characterized by Cs enrichment; negative Rb, Ba, U, and Sr anomalies; and positive Th, Nb, Pb, and Nd anomalies (Figure 9).

Hornblende gabbros (M2 series; 16 samples) occur as layers alternating with Px-bearing UMM series (M1). Locally, Hbl gabbros crosscut the Px-bearing rocks and include them in the form of decimetric fragments. Hbl gabbros are composed of Hbl, An-rich Pl (bytownite–anorthite), and low amounts of opaque minerals (<2%). Their modal composition (Amp/Pl) evolves along the cross section and allows us to define, from the bottom to the top, three Hbl gabbro types (Le Maitre et al., 2004; Figure 3B): Hbl melagabbros (75%–82% Hbl vs. 18%–20% Pl), Hbl mesogabbros (almost 55%–60% Hbl vs. 40%–45% Pl), and Hbl leucogabbros (15%–35% Hbl vs. 65%–85% Pl). Hbl leucogabbros are characterized by a specific mineral association: Pl + Hbl + corundum (Crn)+Spl + sapphirine (Spr).

Hornblende melagabbros (two investigated samples; Figure 10B) are mainly associated with norites and gabbro−norites (Figure 2B), and rarely with ultramafic rocks. In these rocks, Amp consists mainly of magnesio-hastingsites, with Si varying from 6 to 6.5 apfu, FeO(T) ranging from 10.30 to 13.02 wt%, and Mg# ranging from 62 to 72 (Kilzi, 2014). Pls are An-rich bytownites (An_82–88). Locally, Hbl melagabbros include centimetric to decimetric fragments of edenite labrador-bearing norites of the M1 group. Hbl melagabbros display SiO₂ contents ranging from 39 to 47.8 wt% and an Mg# ranging from 55 to 71. They are moderately REE-enriched (∑REE = 22–26 ppm), with (La/Yb)_N = 0.8–1, (La/Sm)_N = 0.85–1, and (Gd/Yb)_N = 1–1.3, have positive Eu anomalies (Eu/Eu* = 1.6–3.5), and display a relatively flat REE pattern (Supplementary Table S3; Figure 11). Their mantle-normalized trace element patterns have positive Pb and Sr anomalies and negative Zr anomalies (Figure 11).

Hbl mesogabbros (six investigated samples; Figure 10C) occur in the middle part of the UMM series. In these rocks, Pl is anorthite (An_91–96), and Amp has an intermediate magnesio-hastingsite to pargasite composition, with FeO(T) ranging from 8 to 11.7 wt% and an Mg# ranging from 69 to 78.7 (Kilzi, 2014), being slightly more Mg-enriched than Hbl melagabbros. Hbl mesogabbros have an SiO₂ content of approx. 44 wt%, except for one sample with an SiO₂ content of 48 wt%, and an Mg# ranging from 73 to 77 (Kilzi, 2014). In the chondrite-normalized plots (Supplementary Table S3; Figure 11), Hbl mesogabbros appear less REE-enriched than Hbl melagabbros, with ∑REE = 5–10 ppm, a positive slope of LREE (La/Yb)_N = < 1, and (La/Sm)_N < 1. They show comparable HREE patterns (Gd/Yb)_N = 0.7–0.8) and Eu anomalies (Eu/Eu* = 3.5–3.8). Normalized trace element patterns show the same positive (Pb and Sr) and negative (Zr and P) anomalies as Hbl melagabbros, with the latter exhibiting a slightly greater enrichment of trace elements (Nb to Lu; Figure 11).

Hbl leucogabbros (eight investigated samples) occur as 1–20-m-thick layers at the top of the M2 mafic series (Figure 2). They mostly consist of a Pl–Amp association defining a mosaic texture (Figure 10D). Pls are anorthites (An_93–96), and Amps are pargasites, with Si contents varying from 6.15 to 6.45 apfu, FeO(T) ranging from 5 to 6 wt%, and an Mg# of 86–89 (Kilzi, 2014). Locally, leucogabbros exhibit Crn, Spl, and Spr paragenesis associated with granulitic metamorphism (Monchoux and Roux, 1975; Abraham et al., 1977). On the chondrite-normalized REE diagrams, Hbl leucogabbro displays weak REE enrichment (∑REE = 2–6 ppm), with (La/Yb)_N = 1–12, (La/Sm)_N = 1–6, and (Gd/Yb)N = 1–2.4, along with a strongly positive Eu anomaly (Eu/Eu* = 3–8.2). Mantle-normalized trace element patterns show negative Th, Nb, Ce, Pr, Zr, Sm, and Nd anomalies and positive Sr, Pb, U, and Eu anomalies (Figure 11).

In summary, Hbl gabbros show a remarkable modal and chemical evolution from Hbl-rich melagabbros at the bottom of the series to Pl-rich leucogabbros at the top. This evolution (Supplementary Table S3; Figure 12) is underlined by 1) a decrease in the whole-rock SiO₂ content from 47.80% to 38% and an increase in Al₂O₃ from 16.40% to 29.4%; 2) an increase in whole-rock Mg# grading from 55 to 86.2 and Mg# Hbl grading on average from 62 to 89 (Kilzi, 2014). This evolution is coeval with a decrease in FeO both in Hbl and whole rocks (12%–5% for Hbl and 11%–3% for whole rock) (Supplementary Table S3), whereas MgO remains nearly constant; 3) an increase in Cr from 600 to 1280 ppm and Ni from 59 to 652 ppm (Supplementary Table S3); 4) a progressive depletion in whole-rock REE content, grading on average from 26 to 2 ppm (Supplementary Table S3); and 5) an increased Eu anomaly (Eu/Eu*), grading from 1.6 to 8.2 (Supplementary Table S3). This evolution is singular and remarkable because the less evolved leucogabbros are at the top of the Hbl gabbro pile, whereas the more evolved melagabbros are at the bottom (Figure 2B). It is consistent with an increase in the Pl/Amp ratio from the bottom to the top of the series (Supplementary Table S1).

Three Hbl orthopyroxenites, one Hbl-poor harzburgite, and one Hbl-rich harzburgite were processed for isotopic measurements. They display relatively low measured Nd values and intermediate Sr isotopic signatures (¹⁴³Nd/¹⁴⁴Nd: 0.512455–0.512690 and ⁸⁷Sr/⁸⁶Sr: 0.704448–0.708936).

These samples display a wide range of isotopic signatures, both within individual sample types and across the entire sample set. Norites (five samples) are characterized by high ⁸⁷Sr/⁸⁶Sr values (0.70741–0.71196) and variable ¹⁴³Nd/¹⁴⁴Nd values (0.512306–0.512673), gabbro−norites (six samples) show relatively restricted ⁸⁷Sr/⁸⁶Sr values (0.70357–0.70577) and variable ¹⁴³Nd/¹⁴⁴Nd values (0.512617–0.513032), and the two gabbros display very similar isotopic compositions (⁸⁷Sr/⁸⁶Sr: 0.70864 and 0.70865; ¹⁴³Nd/¹⁴⁴Nd: 0.512676 and 0.512689).

The three analyzed isotopic signatures of Hbl melagabbros vary from 0.704624 to 0.706722 for ⁸⁷Sr/8⁶Sr and from 0.512683 to 0.512983 for ¹⁴³Nd/¹⁴⁴Nd. Hbl mesogabbros (nine samples) present ⁸⁷Sr/⁸⁶Sr values of 0.703644–0.705087 and ¹⁴³Nd/¹⁴⁴Nd values of 0.512455–0.513160, whereas Hbl leucogabbros (seven samples) are characterized by consistent ⁸⁷Sr/⁸⁶Sr (0.703655–0.704460) and slightly variable ¹⁴³Nd/¹⁴⁴Nd (0.512632–0.513164) values. It is noteworthy that prior to age correction, M2 samples exhibit the highest non-radiogenic ⁸⁷Sr/⁸⁶Sr and radiogenic ¹⁴³Nd/¹⁴⁴Nd values among the entire sample collection.

After age correction, the ultramafic samples are characterized by low, crustal-contaminated isotopic signatures. The harzburgites fall in the middle of the dataset, having slightly positive εNd₍₄₆₀₎ (+0.45 to +0.76) for variable radiogenic ⁸⁷Sr/⁸⁶Sr₍₄₆₀₎ (0.70396–0.70842). The orthopyroxenites display an intermediate but still contaminated isotopic signature (+1.21 to +3.48; 0.70396–0.70419).

The two analyzed gabbros (M1 series) exhibit homogeneous, juvenile εNd₍₄₆₀₎ signatures (+5.70 and +5.90) but highly radiogenic ⁸⁷Sr/⁸⁶Sr₍₄₆₀₎ (0.70749 and 0.70797). The gabbro−norites (M1) display highly variable age-corrected isotopic signatures, with their εNd₍₄₆₀₎ varying from +1.66 to +6.59 and ⁸⁷Sr/⁸⁶Sr₍₄₆₀₎ from 0.70353 to 0.70572. Their initial Nd and Sr isotopic ratios remain roughly anti-correlated, suggesting that both radio-chronometers have been affected by mixing/contamination that may have resulted in this variability (Depaolo, 1981). Norites (M1) are also characterized by highly variable initial isotopic signatures. Their εNd₍₄₆₀₎ values vary from −1.62 to +3.95 and ⁸⁷Sr/⁸⁶Sr₍₄₆₀₎ from 0.70322 to 0.7064. However, in contrast to gabbro−norites, their signatures are positively correlated in the Nd–Sr diagram (Figure 15), which has no clear petrologic or geochemical significance.

To summarize, for the UM and M1 series, we identify facies having clearly heterogeneous isotopic signatures, ranging from partially crustal contaminated to clearly juvenile. Only the ultramafic facies remain systematically contaminated as their low initial Nd and Sr contents make them more sensitive to the crustal contamination process.

This series has clearly juvenile, depleted mantle-related, isotopic signatures that are some of the highest ever reported in southern Europe for Ordovician ultramafic/mafic lithologies (Lotout et al., 2017; Montero et al., 2009; Navidad et al., 2010; Navidad et al., 2018; Orejana et al., 2017; Pin and Marini, 1993; Casas et al., 2015; Solá et al., 2008; Andonaegui et al., 2012; Andonaegui et al., 2016). Their εNd₍₄₆₀₎ signatures are +5.13 to +7.11 (although one is −0.24) for Hb meso- and melagabbros and +4.59 to +8.31 (however two with +0.08 and −0.99) for Hb-leucogabbros. For these samples, ⁸⁷Sr/⁸⁶Sr₍₄₆₀₎ varies from 0.70334 to 0.70590 for Hb meso-/melagabbros and 0.70307 to 0.70417, with the exception of the three previously identified samples (10CA04B: 0.70473, 10CA01: 0.70590, and 10CA07C: 0.70376).

In summary, the M2 samples show very juvenile initial isotopic signatures in Nd, without any trace of continental crust contamination. For the discussion, we consider the isotopic signatures of this group as those of magmatism extracted from the underlying mantle. The other isotopic signatures are either partially or completely modified through crustal contamination or Variscan metamorphism.

Among the UMM series of the Castillon massif, the Hbl gabbro forms a singular series. These rocks are characterized by a high-modal content of An-rich plagioclase (up to 96) and a high Mg# (up to 86), but they lack Px. They display chemical similarities with anorthosites and sakenites; therefore, the formation and evolution of the Hbl gabbro can be analyzed based on these similarities. The origin of anorthosites has been widely discussed (e.g., Ashwall, 2010; Ashwal and Myers, 1994; Ashwal et al., 1998; Latypov et al., 2020; Takagi et al., 2005; Polat et al., 2017). Latypov et al. (2020) considered that the parental magmas of anorthosites are alumina-rich basaltic or basaltic andesitic residual melts derived from deep-seated magma chambers. The transfer of the differentiated melt from the magma chamber to shallower magmatic reservoirs implies a subsequent pressure decrease and a superheating process that leads to the crystallization of An-rich plagioclase by shifting the melt composition to the divariant domain of plagioclase as a liquidus phase before the crystallization of a second phase when the monovariant curve is reached (Latypov et al., 2020).

Given that the Hbl leucogabbros in this study share similarities with anorthosites, we propose that a similar process could be at the origin of the Castillon M2 series, in which Hbl corresponds to the second crystallizing mineral phase. The crystallization of An-rich plagioclase under high-temperature conditions (Latypov et al., 2020) would result from the rapid uplift of the hydrated melt from which the Mg-rich Hbl from the M2 series could crystallize as an intercumulus phase. In the Castillon massif, the anorthositic characteristic of the M2 Hbl gabbros is more pronounced in the leucogabbros located at the top of the series than in the underlying meso- and mela-Hbl gabbros. This could be associated with the increase in the modal proportion of the most An-rich plagioclase toward the top of the series through a density control of the plagioclase segregation during the crystallization toward the top of the pile (Higgins et al., 2015; Namur et al., 2011; Polat et al., 2017). Most rocks from the Hbl gabbro M2 series are characterized by very juvenile Nd isotopic signatures, ruling out any crustal contamination of their mantle-derived parental melt. However, almost all samples from the UM M1 series are affected by this crustal contamination process. The latter display εNd₍₄₆₀₎ values between +5.70 and +6.64, i.e., signatures similar to those from the Hbl-bearing series. Therefore, it is possible that the magmatic events that led to the genesis of both series are associated. This assumption implies that the crustal contamination by lower crust fluids, as indicated by the isotopic Nd–Sr characteristics, has heterogeneously affected the entire investigated Castillon massif, although preferentially its lower sections where UM and M1 rocks dominate (Figure 2B).