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We present here a tectonic-geodynamic model for the generation and flow of partially molten rocks and for magmatism during the Variscan orogenic evolution from the Silurian to the late Carboniferous based on a synthesis of geological data from the French Massif Central. Eclogite facies metamorphism of mafic and ultramafic rocks records the subduction of the Gondwana hyperextended margin. Part of these eclogites are forming boudins-enclaves in felsic HP granulite facies migmatites partly retrogressed into amphibolite facies attesting for continental subduction followed by thermal relaxation and decompression. We propose that HP partial melting has triggered mechanical decoupling of the partially molten continental rocks from the subducting slab. This would have allowed buoyancy-driven exhumation and entrainment of pieces of oceanic lithosphere and subcontinental mantle. Geochronological data of the eclogite-bearing HP migmatites points to diachronous emplacement of distinct nappes from middle to late Devonian. These nappes were thrusted onto metapelites and orthogneisses affected by MP/MT greenschist to amphibolite facies metamorphism reaching partial melting attributed to the late Devonian to early Carboniferous thickening of the crust. The emplacement of laccoliths rooted into strike-slip transcurrent shear zones capped by low-angle detachments from c. 345 to c. 310 Ma is concomitant with the southward propagation of the Variscan deformation front marked by deposition of clastic sediments in foreland basins. We attribute these features to horizontal growth of the Variscan belt and formation of an orogenic plateau by gravity-driven lateral flow of the partially molten orogenic root. The diversity of the magmatic rocks points to various crustal sources with modest, but systematic mantle-derived input. In the eastern French Massif Central, the southward decrease in age of the mantle- and crustal-derived plutonic rocks from c. 345 Ma to c. 310 Ma suggests southward retreat of a northward subducting slab toward the Paleotethys free boundary. Late Carboniferous destruction of the Variscan belt is dominantly achieved by gravitational collapse accommodated by the activation of low-angle detachments and the exhumation-crystallization of the partially molten orogenic root forming crustal-scale LP migmatite domes from c. 305 Ma to c. 295 Ma, coeval with orogen-parallel flow in the external zone. Laccoliths emplaced along low-angle detachments and intrusive dykes with sharp contacts correspond to the segregation of the last melt fraction leaving behind a thick accumulation of refractory LP felsic and mafic granulites in the lower crust. This model points to the primordial role of partial melting and magmatism in the tectonic-geodynamic evolution of the Variscan orogenic belt. In particular, partial melting and magma transfer (i) triggers mechanical decoupling of subducted units from the downgoing slab and their syn-orogenic exhumation; (ii) the development of an orogenic plateau by lateral flow of the low-viscosity partially molten crust; and, (iii) the formation of metamorphic core complexes and domes that accommodate post-orogenic exhumation during gravitational collapse. All these processes contribute to differentiation and stabilisation of the orogenic crust. Nous présentons dans ce papier un modèle géodynamique-tectonique pour la genèse et le fluage des roches partiellement fondues et le magmatisme au cours de l’évolution orogénique Varisque du Silurien au Carbonifère supérieur basé sur une synthèse des données géologiques du Massif Central Français. La subduction de la marge du Gondwana hyper-étirée est enregistré par des roches mafiques et ultramafiques affectées par un métamorphisme en faciès éclogitique. Ces éclogites forment pour certaines des boudins-enclaves dans des migmatites felsiques avec des reliques de faciès granulitique de HP retrogradées en faciès amphibolitique, ce qui atteste de la subduction de la marge continentale suivie d’une relaxation thermique et d’une décompression. Nous proposons que la fusion partielle à HP ait déclenché le découplage mécanique entre la plaque plongeante et les unités continentales partiellement fondues. Ceci a permis l’exhumation de ces roches gravitairement instables qui ont entrainé sur leur passage des blocs de lithosphère océanique et de manteau sous-continental. Les données géochronologiques disponibles sur les migmatites de HP contenant des éclogites indique une mise en place diachronique de nappes du Dévonien moyen au Dévonien Supérieur. Ces nappes ont chevauché un assemblage de métapélites et d’orthogneiss affectées par un métamorphisme de MP/MT allant du faciès schistes verts à amphibolite atteignant localement la fusion partielle et attribué à l’épaississement crustal du Dévonien supérieur au Carbonifère moyen. La mise en place de laccolithes enracinés dans des zones de cisaillement décrochantes et surmontés de détachements à faible pendage de c. 345 à c. 310 Ma est synchrone de la propagation vers le Sud du front de déformation Varisque marqué par le dépôt de sédiments détritiques dans les bassins d’avant-pays. Nous attribuons ces éléments à la croissance horizontale de la ceinture Varisque associée à la formation d’un plateau orogénique par fluage latéral de la racine orogénique partiellement fondue sous l’effet de la force gravitaire. La diversité des roches magmatiques témoigne d’une variété des sources crustales avec une contribution relativement modeste mais systématique de magmas issus du manteau. Dans la partie Est du Massif Central Français, la décroissance vers le Sud des âges de mise en place des magmas dérivés à la fois du manteau et de la croûte suggère le retrait d’un panneau plongeant vers le Nord vers la bordure libre constituée par la Paléotethys et située au Sud de la ceinture Varisque. La destruction de la chaine Varisque à la fin du Carbonifère est principalement le résultat de l’effondrement gravitaire accommodé par l’activation de détachements à faible pendage et l’exhumation-cristallisation de la racine orogénique partiellement fondue formant des dômes d’échelle crustale à cœur de migmatites de BP entre c. 305 et 295 Ma, concomitante au fluage latéral des unités de la zone externe de la chaine. Les derniers magmas extraits de la zone de fusion partielle forment des dykes et des laccolithes mis en place dans des détachements à faible pendage laissant derrière eux une croûte inférieure constituée de l’accumulation de granulites réfractaires de composition felsique à mafique. Ce modèle met en valeur le rôle primordial de la fusion partielle et du magmatisme sur l’évolution tectonique-géodynamique de la ceinture orogénique Varisque. En particulier, la fusion partielle et le transfert de magma (i) déclenchent le découplage mécanique entre le panneau plongeant et les unités subductées, permettant ainsi l’exhumation de ces dernières, (ii) favorisent le développement d’un plateau orogénique par fluage latéral de la croûte partiellement fondue de faible viscosité sous l’effet de la gravité, (iii) conduisent à la formation de metamorphic core complex et de domes qui accommodent l’exhumation post-orogénique au cours de l’effondrement gravitaire de la chaine. Tous ces processus contribuent à la différenciation et à la stabilisation de la croûte orogénique.

The global-scale crustal structure of Mars is shaped by impact basins, volcanic provinces, and a hemispheric dichotomy with a thin crust beneath the northern lowlands and a thick crust beneath the southern highlands. The southern highlands are commonly treated as a coherent terrain of ancient crust with a common origin and shared geologic history, plausibly originating from a giant impact(s) or a hemispheric-scale mantle upwelling. Previous studies have quantified the contribution of volcanism to this crustal structure; however, the influence of large impacts remains unclear. Here we present reconstructions of the past crustal thickness of Mars (about 4.2 Gyr ago) where the four largest impact basins (Hellas, Argyre, Isidis and Utopia) are removed, assuming mass conservation, as well as the main volcanic provinces of Tharsis and Elysium. Our reconstruction shows more subdued crustal thickness variations than at present, although the crustal dichotomy persists. However, our reconstruction reveals a region of discontinuous patches of thick crust in the southern highlands associated with magnetic and geochemical anomalies. This region, corresponding to Terra Cimmeria–Sirenum, is interpreted as a discrete crustal block. Our findings suggest that the southern highlands are composed of several crustal blocks with different geological histories. Such a complex architecture of the southern highlands is not explained by existing scenarios for crustal formation and evolution.A discrete block of thick ancient crust revealed by a crustal reconstruction suggests a complex geologic history for the southern highlands of Mars.

The southern part of the West African Craton includes the Baoulé-Mossi Domain, the world’s premier Paleoproterozoic gold province (~10,000 metric ton gold endowment). Structural, metamorphic, and geochronological data suggest gold mineralisation occurred during three episodes that span much of the Eoeburnean and Eburnean orogenic cycles. Eoeburnean orogenic and rare skarn-hosted gold deposits formed between ca. 2200 and 2135 Ma during repeated episodes of volcanism, plutonism, and shortening, which thickened the Paleoproterozoic crust. Early Eburnean orogenic and placer gold deposits formed between ca. 2110 and 2095 Ma during inversion, metamorphism, and subsequent oblique shortening of intra-orogenic basins filled after ca. 2135 Ma. This episode of mineralisation terminated when the Baoulé-Mossi Domain docked with the Archean Kénéma-Man Domain at ca. 2095 Ma. Late Eburnean orogenic and less common intrusion-related gold deposits formed between ca. 2095 and 2060 Ma during strike-slip to oblique-slip tectonics, post-collisional high-K plutonism and crustal reworking across the western and southern Baoulé-Mossi Domain. Eoeburnean gold deposits include ca. 10 % of the gold endowment of the Baoulé-Mossi Domain, whereas the Early Eburnean and Late Eburnean deposits include ca. 50–70% and 20–40%, respectively. Here, we highlight the favourable confluence of accretion-collision tectonics, involving juvenile crust formation as well as protracted magmatic, metamorphic, and deformation histories that resulted in diachronous gold events spread over at least 100 myr throughout the Baoulé-Mossi Domain.

The Naxos dome, in the middle of the Aegean domain, exposes the former root of the Alpine orogenic belt and represents a key natural example to investigate the development of gravitational instabilities during orogenic evolution and their impact on crustal differentiation. The Naxos dome is cored by migmatites with structures depicting second order domes with a diameter of 1-2 km nested in the first order deca-kilometer scale dome that formed at the onset of orogenic collapse. Zircon grains from the migmatites record a succession of crystallization-dissolution cycles with a period of 1-2 Myr. These features have been attributed to the development of convective and diapiric gravitational instabilities, related to thermally induced and compositional buoyancy. In this paper, we test the pertinence of this model with a thermal-mechanical numerical experiment performed with a volume of fluid method (VOF) known to preserve material phase interfaces during large deformation of viscous layers. Partial melting of the crust is modeled by strain-rate and temperature dependent viscosity and temperature dependent density. Moreover, horizontal layers with density, viscosity and heat production variations mimic more felsic or more mafic lithologies in a crust of intermediate composition. With basal heating, gravitational instabilities initiate with local segregation of the buoyant versus heavier layers, followed by diapiric upwelling of buoyant pockets of aggregated less dense material. Convection starts after 5 Myr, approximately when half of the crust has a viscosity lower than 1019 Pa s. The size of the convection cells increases as the temperature rises in the crust and reaches ∼25 km in diameter after ca. 20 Myr, which defines the size of first order domes. Some of the heterogeneous material is entrained in the convection cells with a revolution period of 1 to 3 Myr. However, most of the denser material accumulates in the lower crust, while the buoyant material segregates at the top of the convection cells and forms diapirs that correspond to second order domes, of several kilometers in diameter and nested within the first order domes. This model, which reproduces the first order characteristic dimensions of the Naxos nested domes and the periodicity of their zircon geochronological record, demonstrates the efficiency of gravitational instabilities in the formation of migmatite domes and, more generally, in the multi-scale dynamics of crustal differentiation leading to a felsic upper crust, an intermediate middle crust and a mafic lower crust.

The Mako Belt in the Kédougou-Kéniéba Inlier (eastern Senegal) preserves Paleoproterozoic (2.3–1.9 Ga) mafic and ultramafic rocks that record early crustal growth processes within the southern West African Craton (WAC). Basalt bulk rock compositions preserve primary melt signatures, whereas the associated ultramafic cumulates are variably serpentinized and are better assessed through mineral chemistry. Basalts occur as massive and pillow lavas, with MgO contents of 5.9–9.1 wt.% and flat to slightly LREE-depleted patterns (La/Smₙ = 0.73–0.88). Primitive mantle-normalized diagrams show subduction-related signatures, including enrichment in Ba, Pb, and Rb and depletion in Nb and Ta. Most basalts and all ultramafic rocks display (Nb/La)PM > 1, consistent with enriched mantle melting in a back-arc setting. Harzburgites and lherzolites have cumulate textures, high Cr and Ni contents, and spinel with chromian cores (Cr# > 0.6) zoned sharply to Cr-rich magnetite rims that overlap basalt spinel compositions. Integration of the petrographic, mineralogical, and whole-rock geochemical data indicates the presence of mafic melts derived from a subduction-modified mantle wedge and likely formed in a back-arc basin above a subducting slab, rather than from a plume or mid-ocean ridge setting. Regional comparisons with other greenstone belts across the WAC suggest that the Mako Belt was part of a broader arc–back-arc system accreted during the Eburnean orogeny (~2.20–2.00 Ga). This study supports the view that modern-style plate tectonics—including subduction and back-arc magmatism—was already active by the Paleoproterozoic, and highlights the Mako Belt as a key archive of early lithospheric evolution in the WAC.

The Mayo-Kebbi massif (Chad) exposes a Neoproterozoic juvenile crustal segment that has been tectonically accreted in the Central African Orogenic Belt and reworked during the Pan-African orogeny. It comprises a syntectonic high-K magmatic suite including the Zabili A-type granitic pluton. The Zabili pluton is made of a highly differentiated granite generated by fractional crystallization of a magma formed by partial melting of a Neoproterozoic juvenile protolith. Syn- to post-magmatic ductile to brittle deformation of the Zabili pluton is associated with metasomatism and deposition of uranium. Primary magmatic U-bearing minerals are zircon, monazite, and uranothorite. Late-magmatic deformation and Na-metasomatism are marked by the development of R’ antithetic shear zones with high-temperature dynamic recrystallization of K-feldspar and Ca-plagioclase phenocrysts coeval with crystallization of albite along deformation bands and grain boundaries, and crystallization of interstitial amphibole, calcite epidote and albite also affected by intracrystalline deformation. At this stage, U-bearing minerals are monazite, uraninite, brannerite (pseudomorphosed in ekanite). This late-magmatic event is dated by U–Th–Pb on monazite at 599 ± 4 Ma. Brittle deformation and Ca-metasomatism are marked by cataclastic zones and veins containing albite, epidote, calcite, chlorite, apatite, metamict zircon, pitchblende, U-silicates, and iron oxides. U-bearing minerals are altered and/or remobilized in ekanite, kasolite, and uranophane. These data suggest that the uranium mineralization hosted by the Zabili pluton records a superposition of processes and traces extreme crustal differentiation of a Neoproterozoic juvenile crustal segment reworked during the Pan-African orogeny.

The sub-continental lithospheric mantle (SCLM) beneath the Cameroon Volcanic Line (CVL) is vertically and laterally heterogeneous, consisting of a complex mixing of DMM, HIMU, and EM1, affected by modal or cryptic metasomatism, depending on the area. The petrography, whole-rock geochemical data, and minerals’ chemical composition of lavas and mantle xenoliths from the Bini Warack area, combined with Sr isotope compositions, provide constraints on the origin and thermochemical evolution of the SCLM beneath this sector of the CVL. The host lavas are basanite, basalt, and latite with OIB affinity, characterized by moderate to high silica and alkali contents (SiO 2 = 42.35–56.56 wt%, K 2 O+Na 2 O = 2.34–7.07 wt%), a high Ba/Rb ranging from 12.2 to 26.1, a low Rb/Sr from 0.03 to 0.08, strong enrichment in LREE relative to HREE (La N /Yb N : 9.3–30), and moderate enrichment in radiogenic isotopes ( e.g. , 0.702987 < 87 Sr/ 86 Sr initial < 0.703206; 0.512854 < 143 Nd/ 144 Nd initial < 0.512918) with positive εNd initial (+4.84 to +6.09). These features are consistent with an origin of the lavas by a low degree of partial melting (<2%) of a lherzolitic mantle source containing 2% to 6% garnet. These lavas have then evolved by fractional crystallization without any evidence of crustal contamination. The studied mantle xenoliths are spinel-bearing lherzolites, characterized by U/Th ratios typically lower than 1, a slight enrichment in LILE relative to HFSE, and mainly consist of minerals with fertile composition (Fo 84-91 ; spinel Cr#: 0.1–0.22; Al-rich pyroxenes). They are consistent with refractory mantle peridotite that evidences low partial-melting degrees. Trace element concentrations of host lavas (high Ba/Rb: 12.2–26.1 and low Rb/Sr: 0.03–0.08), together with olivine’s crystals chemical features (high Ca/Fe and 100*Mn/Fe ratios; low 100*Ni/Mg ratios) and low Ca/Al ratios (<5) of clinopyroxenes in spinel-bearing lherzolite xenoliths suggest that the SCLM beneath the Bini Warack area is likely a juvenile lithospheric mantle which that undergone a carbonate-rich metasomatism. Le manteau lithosphérique sous-continental (MLSC) sous la ligne volcanique du Cameroun (LVC) est verticalement et latéralement hétérogène, constitué d'un mélange complexe de DMM, HIMU et EM1, affecté par un métasomatisme modal ou cryptique selon la zone. La pétrographie, les données géochimiques des roches totales ainsi que la composition chimique des minéraux des laves et des xénolithes mantelliques de la région de Bini Warack, combinées aux compositions isotopiques Sr, fournissent des informations sur l'origine et l'évolution thermochimique du MLSC sous ce secteur de la LVC. Les laves hôtes ont des compositions de basanite, basalte et de latite présentant des affinités avec les OIB, et caractérisées par i) des teneurs modérées à élevées en silice et en alcalins (SiO 2 = 42.35–56.56%, K 2 O+Na 2 O = 2.34–7.07%), ii) de forts rapports Ba/Rb (12.2–26.1), iii) de faibles rapports Rb/Sr (0.03–0.08), iv) un fort enrichissement en terres rares légères par rapport aux terres rares lourdes (La N /Yb N : 9.3–30), et v) un enrichissement modéré en isotopes radiogéniques (0.702987 < 87 Sr/ 86 Sr initial < 0.703206; 0.512854 < 143 Nd/ 144 Nd initial < 0.512918) marqué par des εNd initiaux positifs (+4.84 – +6.09). Ces caractéristiques sont compatibles avec une origine des laves par un faible degré de fusion partielle (moins de 2%) d'un manteau à lherzolite contenant 2 à 6% de grenat. Les laves de Bini Warack ont ensuite évolué par cristallisation fractionnée sans trace de contamination crustale. Les xénolithes quant à eux sont des lherzolites à spinelle, caractérisés par des rapports U/Th généralement inférieurs à 1, un léger enrichissement en LILE par rapport aux HFSE et principalement constitués de minéraux de composition fertile (Fo 84-91; spinelle Cr# : 0.1–0.22; pyroxènes alumineux). Ces xénolithes sont compatibles avec un manteau péridotitique réfractaire ayant subi de faibles degrés de fusion partielle. Les teneurs en éléments traces des laves hôtes (Ba/Rb élevé : 12.2–26.1 et Rb/Sr faible : 0.03–0.08), les caractéristiques chimiques des cristaux d'olivine (rapports Ca/Fe et 100*Mn/Fe élevés; ainsi que les faibles rapports 100*Ni/Mg et Ca/Al <5) des clinopyroxènes des lherzolites étudiées, suggèrent que le MLSC sous la zone de Bini Warack est sans doute un manteau lithosphérique juvenile ayant subi un métasomatisme carbonaté.

The gold-rich Ketté formations belong to the Adamawa-Yadé block of the Neoproterozoic Central Africa Orogenic Belt. They consist of a complex association of tonalite, granodiorite, and heterogeneous granite including rafts of metapyroxenite, amphibolite, paragneiss and migmatite. Metapyroxenite and amphibolite are metaluminous (A/CNK: 0.65–0.95), calc-alkaline, enriched in LILE and LREE, with εNd 610 < 0 and T DM ages of 1.7 Ga, highlighting their enriched mantle source. Paragneiss are rich in LREE, Cr, V, Sc and Ni and display A/CNK (1.06–1.08) and A/NK (1.5–3.49) ratios consistent with a sedimentary protolith made of poorly sorted greywackes and litharenites with a contribution from mafic rocks of the juvenile crust. Granitoids display a variety of signatures, ranging from metaluminous to peraluminous (A/CNK: 0.94–1.04), calc-alkaline to alkaline. The heterogeneous granite in diffuse contact with migmatite have εNd 654 values of –9.61 and –8.11, and are interpreted to reflect local magma collection within the zone of dominant partial melting of the paragneiss. The tonalite and granodiorite, characterized by magmatic textures, display Mg# of 39.9–48.8, low HFSE contents, and enrichment in LILE and LREE. They were likely formed by the fractional crystallisation of a dioritic magma derived from the partial melting of amphibolite. The main structure of the migmatite is a shallow- to moderately dipping NE–SW trending syn-migmatitic foliation S mgm , marked by alternating leucosome and mesosome layers, with relics of an S 0 /S 1 foliation locally preserved within the mesosome. Granulite-facies relics with a peak at 8–10 kbar/≥800 °C are preserved in metapyroxenite. Networks of texturally continuous leucosome veins concordant to discordant to the syn-migmatitic foliation, localization of granitic veins in shear zones and fold axial planes, attest for deformation in the presence of melt. Granitoids form kilometre-scale plutons with gently dipping magmatic fabrics (S m -S m /C 2 ) concordant to the syn-migmatitic foliation. These fabrics are transposed into a steeply dipping (47°–74°) E–W to NW–SE trending mylonitic fabric (S myl -S 3 /C 3 ), bearing a moderately plunging (15°–49°) stretching lineation L 3 , marking the folded Mama Shear Zone (MaSZ). These data indicate that the Ketté region has recorded magmatic accretion of a mafic crust originating from an enriched mantle source. This crust and its associated sediments were subsequently tectonically thickened during the Pan-African orogeny and affected by partial melting. Migration of dioritic to granitic magmas from the partially molten orogenic root led to syntectonic crustal differentiation. Les formations aurifères de Ketté, dans le bloc Adamawa-Yadé de la Ceinture Orogénique d'Afrique Centrale, sont constituées d'une association complexe de tonalite, granodiorite et granites hétérogènes, renfermant des enclaves de metapyroxénite, d'amphibolite, de paragneiss et de migmatite. Les metapyroxénites et amphibolites sont métalumineuses, enrichies en LILE et LREE, avec des valeurs εNd 610 < 0 et des âges modèles T DM de 1.7 Ga, indiquant une source mantellique enrichie. Les paragneiss sont riches en LREE, Cr, Sc et Ni et présentent des rapports A/CNK et A/NK caractéristiques de protolithes riches en greywackes et litharénites, avec une contribution de roches mafiques issues de la croûte juvénile. Les granitoïdes ont des signatures métalumineuses à peralumineuses, calco-alcalines à alcalines. Les granites hétérogènes présentent des contacts diffus avec les migmatites, des valeurs εNd 654 de −9.61 et −8.11, et reflètent une accumulation locale de magma au sein de zones de fusion partielle dominante des paragneiss. Les tonalites et granodiorites présentent des textures magmatiques, des valeurs Mg# de 39.9–48.8, de faibles teneurs en HFSE et des enrichissements en LILE et LREE. Elles résulteraient de la cristallisation fractionnée d'un magma dioritique issu de la fusion partielle des amphibolites. Les migmatites présentent une foliation syn-migmatitique S mgm orientée NE-SW, à pendage faible à modéré, marquée par l'alternance leucosomes-mésosomes, avec des reliques de la foliation S 0 /S 1 localement préservées. Des reliques du faciès granulitique témoin d'un pic métamorphique à 8–10 kbar/≥800 °C sont conservées dans les metapyroxénites. Les réseaux continus de leucosome, concordants à discordants à la foliation syn-migmatitique, ainsi que la localisation des veines granitiques dans des zones de cisaillement et dans les axes des plis, témoignent d'une déformation en présence de magma. Les plutons kilométriques de granitoïdes présentent des fabriques magmatiques (S m -S m /C 2 ) à faible pendage, concordantes avec la foliation syn-migmatitique. Ces fabriques sont transposées en des fabriques mylonitiques (S myl -S 3 /C 3 ) orientées E-W à NW-SE à fort pendage (47°–74°), portant une linéation d'étirement Le 3 à plongement modéré (15°–49°), dans la zone de cisaillement plissée de Mama. Ces données indiquent que le socle de Ketté a enregistré l'accrétion d'une croute mafique dérivée de la fusion du manteau enrichi. Cette croûte et les sédiments y associés ont ensuite été tectoniquement épaissis durant l'orogénèse Pan-Africaine, favorisant la fusion partielle. La migration de magmas de composition dioritique à granitique issus de la racine orogénique partiellement fondue a conduit à une différenciation crustale syntectonique.

The Tabakoto gold deposit is part of the highly endowed west-Malian gold belt, which hosts several world-class deposits. Located in the Paleoproterozoic Kédougou-Kéniéba Inlier (KKI) of the West African Craton (WAC), the deposit is contained in Birimian metasedimentary rocks of the Kofi series that are intruded by magmatic dikes. The Tabakoto deposit is characterized by multiple overlapping stages of magmatic dike intrusions and hydrothermal alteration during distinct regional-scale deformation events. The metasedimentary rocks are characterized by a S0/1 foliation delineated by the alternation of metagreywacke and meta-argillite. This foliation is affected by upright folds with N-S trending axial planes marked by a S2 schistosity and cross-cut by conjugate steep-dipping dextral NE-SW and sinistral NW-SE trending faults. These structures record regional-scale E-W shortening, first associated with crustal thickening and then with N-S stretching, evolving from ductile to brittle deformation. The gold-bearing V2 quartz-pyrite veins developed in dikes and metasedimentary units, are parallel to the S2 schistosity of the DT2 deformation event. These veins contained reduced phases such as pyrrhotite, loellingite and scheelite as well as bismuth. The second gold-bearing V3a quartz-carbonate veins are located in the NE-SW and NW-SE trending faults of the DT3 deformation event. The latter veins are marked by sodic alteration followed by carbonate (i.e., dolomite-ankerite ± calcite ± siderite) and phyllic (i.e., chlorite-muscovite-sericite) alterations. In the quartz-pyrite veins, gold occurs as inclusions in pyrite, arsenopyrite, and pyrrhotite, whereas in quartz-carbonate shear veins, gold is present in microfractures cross-cutting pyrite and arsenopyrite and at the contact between grains of pyrite and arsenopyrite, and of arsenopyrite and pyrrhotite. The litho-structural and mineralogical features of the Tabakoto deposit portray a polyphase mineralization with (i) an intrusion-hosted orogenic gold system, where the metabaslatic to metarhyolitic dikes acted as suitable emplacement for fluids flow and (ii) a hydrothermal fluid circulation under ductile to brittle conditions.

The architecture of the crust in the French Massif Central (FMC) is described as a nappe stack composed, from top to bottom, of the Upper Gneiss Unit (UGU), the Lower Gneiss Unit (LGU), and the Para-Autochthonous Unit (PAU), which are intruded by Carboniferous granitic plutons. In the Pontgibaud region, the Upper Gneiss Unit/Lower Gneiss Unit (UGU/LGU) attribution and the relationship between metamorphic and magmatic rocks are uncertain. This hinders both our understanding of the geodynamic processes that led to the construction of the Variscan orogenic belt, and a precise assessment of the deep geothermal potential of this high-heat flow area. Our field investigations document a metamorphic gradient with, from South to North, micaschists and paragneiss with a gently, dominantly southward-dipping foliation grading into migmatites that display a steeper foliation. A maximum late Ediacaran deposition age of sedimentary protoliths is determined from LA-ICPMS U-Pb dates of the youngest detrital zircon populations that range from 588.9 ± 7.1 to 571.8 ± 8.2 Ma. Equilibrium phase diagrams calculated for selected samples document a metamorphic gradient from 2.3- 5.3 kbar, and 570°-660°C, characterizing a mid- to low pressure-high temperature event dated from 357.6 ± 1.5 to 342.2 ± 4.6 Ma by LA-ICP-MS U-Pb analyses of metamorphic monazite. The contact between the metasedimentary rocks and migmatites is crosscut by peraluminous, cordierite-biotite-bearing Claveix and Gelles granitic plutons dated at 338.0 ± 7.3 Ma and 323.3 ± 9.7 Ma, respectively by U-Pb on zircon. The distribution of U-Pb dates and Lu-Hf isotopic compositions of detrital zircon in metamorphic rocks; the absence of HP mineral relics; the age of HT metamorphism of ca. 357-345 Ma; and the lack of a major mylonitic tectonic contact, which would typify the base of UGU, collectively support that the exposed crust of the Pontgibaud area corresponds to LGU metamorphic rocks. Moreover, the mineralogy, bulk chemistry and zircon Lu-Hf isotopic composition of the granites and migmatites are consistent with an origin by partial melting of LGU metasedimentary rocks. These data show that the Pontgibaud area exposes a section along a Carboniferous MP-LP/HT metamorphic gradient from micaschists to migmatites. Genesis and extraction of a large volume of granitic magma require higher temperatures (>800°C) that were not reached by the exposed rocks. Accordingly, we propose that the local mid- and lower crustal levels are made of rocks with a similar protolith as those exposed at the surface, but affected by higher metamorphic grade and magma extraction.