Journal of the Virtual Explorer

Although involved in the D2 and younger events, several kinds of Devonian magmatic rocks are recognized both in the North Gondwana margin and in the Central Armorican Domain (Figure 11). Conversely to the subsequent magmatic events, the Devonian one exhibits a significant mantle contribution (e. g. Pin and Duthou, 1990; Shaw et al. 1993; Pin and Paquette, 2002).

Figure 11. Synthetic map of the Devonian magmatism

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Synthetic map of the Devonian magmatism with diorite arc plutons, calc-alcaline volcanic-sedimentary series (Morvan and Vendée), and back-arc related rocks: Brévenne ophiolites, Bolazec rift, St-Jean-du-Doigt gabbro, Génis greenrocks, Ligne des Klippes. Available radiometric dates are also shown.

i) Calkalcaline volcanic, volcani-clastic and plutonic rocks form the Somme series in the NE Massif Central (Figure 11). Famennian-Frasnian terrigeneous siltstone, sandstone and conglomerate with volcanic clasts are interlayered with calc-alkaline andesite and trachyte, and contains massive sulfide deposits (Bebien and Gagny, 1981; Pin et al., 1982; Delfour, 1989). The very contact is not observed, however, regional geology suggests that the Somme series unconformably overlies D1 migmatites and metamorphic rocks. A similar volcaniclastic series is recognized in the Vosges (e. g. Ikene et al., 1991; Rizki et al., 1992). In Vendée, the Middle Late Devonian basalts of la Meilleraie (Figure 11), overlying Eifelian-Givetian terrigeneous rocks, crop-out in the core of the Chantonnay syncline (Wyns et al., 1989). The calc-alkaline mineralogy and the arc tholeiite geochemical signature of those volcanic rocks indicate that they belong to a magmatic arc related to subduction zone (Thiéblemont and Cabanis, 1986).

ii) Arc plutons such as diorite, tonalite and gabbro, crop-out as twenty-kilometer scale plutons widespread in southern Massif Armoricain and western Massif Central (Figure 11). Most of the plutons are laccoliths that exhibit the same solid-state flat-lying foliation and NW-SE trending lineation than their metamorphic host rocks showing that both rocks experienced the D2 event. U/Pb zircon ages as old as 370-380 Ma are measured in the Limousin (Bernard-Griffith et al., 1985, Pin and Paquette, 2002), in Vendée (Cuney et al., 1993, Bertrand et al., 2001) and Rouergue (Pin and Piboule, 1988). The youngest ages around 360-350 Ma attest for an isotopic resetting during the Early Carboniferous D2 tectonics. The calc-alkaline petrology and geochemistry of the diorites led many authors (e. g. Didier and Lameyre, 1971; Bernard-Griffith et al., 1985; Peiffer, 1986; Shaw et al., 1993) to interpret these rocks as markers of a north-dipping subduction zone. A comprehensive view in terms of arc-back-arc system related to a southward dipping subduction has been proposed (Faure et al., 1997, see section 5. 4).

Because of the same Late Devonian age and calc-alkaline geochemistry of the volcanic series and the diorites, we suggest that all these rocks formed in the same magmatic arc. The diorite-tonalite plutons and the volcanic-sedimentary rocks represent the deep and shallow levels of the arc respectively. An objection might arise due to the large spatial distribution of the dioritic plutons. However, it is worth noting that the present width does not correspond to the Devonian one since the diorite plutons are involved into D2 and D3 tectonics, thus they are not rooted in their present outcroping site. Moreover, the Late Carboniferous extensional tectonics (see section 5.7) also contributed to enlarge the distance between all the plutons.

iii) Ophiolites are recognized in eastern Massif Central in the Brévenne area. Oceanic rocks emplaced in Middle and Late Devonian as shown by the 365+/-10 Ma U/Pb age on zircon from acidic volcanics coeval with the mafic magmatic rocks (Pin and Paquette, 1998). Remnants of Devonian mafic magmatic rocks with a tholeiitic signature and deep sea sedimentary rocks (siliceous shales, radiolarian cherts, rare limestone) are found in South Vosges, in the "Ligne des Klippes" (Schneider et al., 1990). The Devonian mafic rocks and siliceous sedimentary rocks of the Génis Unit attest also for the existence of a Middle to Late Devonian oceanic basin. These Devonian rocks are distinct from those of the UGU which experienced the Silurian high pressure metamorphism. Devonian tholeitic and alkali basalts and dolerite crop out around the St-Jean-du-Doigt gabbro or in close association with siliceous sedimentary rocks in Bolazec rift (Figures 4, 11; Cabanis et al., 1982).

An aluminous magmatism, widespread in the north part of the Massif Central and more scattered in the Massif Armoricain took place before Late Visean (Figure 8). The biotite-cordierite Guéret monzogranite which is taken as the type lithology yields a Rb/Sr whole rock age of 356+/-10 Ma (Berthier et al., 1979). The Early Visean age (350-340 Ma) of these plutons complies with stratigraphic constraints since the Late Visean volcanic-terrigeneous "Tufs anthracifères" formation unconformably covers the Guéret massif. In the Limousin area, structural studies dealing with the Guéret-type granitoids show that they exhibit magmatic to solid state fabrics with a SE-NW trending mineral lineation (e. g. Bouchez and Jover, 1986; Roig et al., 1996, 1998). The deformation is dated around 346+/-4 Ma by 40Ar/39Ar method on biotite (Roig et al., 1996). The NW-SE trend consistency between the metamorphic lineation in the host rocks and the magmatic lineation in the plutons suggests that the same strain field was present when the Guéret-type plutons emplaced. Therefore, the pre-Late Visean magmatism can be considered as a late increment of the D2 event.

In the Massif Armoricain, Early Carboniferous granites crop out along the South Armorican Shear Zone (e. g. Pontivy, 344+/-8 Ma); in Central Brittany (Huelgoat, 336+/-13 Ma), or in Léon (St-Renan, 330-340 Ma; e. g. Le Corre et al., 1991). The crustal source of this pre-Late Visean magmatism is well acknowledged, but its structural setting and geodynamic significance remain poorly documented. Crustal thickening due to the D2 event appears as a likely mechanism to trigger the melting.

This magmatism, well developed in the northern and western parts of the Massif Central (Figures 2, 10), consists in aerial products with lava flows, ignimbrites, pyroclastic deposits, called “Tufs anthacifères series”, rhyolitic to dacitic dykes, hypovolcanic microgranites and coarse grained red granites. Geochemistry indicates that crustal melting was triggered by heat input from the mantle. Moreover, a mantle contribution to the source of magma is also likely (Pin and Duthou, 1990). The lower crust or upper mantle thermal input is responsible for a resetting of the isotopic systems. The 40Ar/39Ar ages of ca 335 Ma widespread in the pre-Visean metamorphic rocks of the northern Massif Central are due to such an overprint (Bruguier et al., 1998; Faure et al., 2002). The structural analysis of dykes indicates that NW-SE stretching of the crust controls their emplacement. This deformation, related to the early stage of orogenic collapse,is coeval with the D3 event (Faure, 1995; cf. section 5).

In the northern Cévennes, the Para-autochthonous Unit is underlain by migmatitic gneiss called the pre-Velay migmatites (Faure et al., 2001; Figure 10). The anatexis is dated between 333 and 324 Ma by the chemical U/Th/Pb method on monazite (Be Mezème et al., 2002). Similar ages are yielded by the migmatites and cordierite granites of the Montagne Noire Axial Zone (Figures 2, 3). The granitic-migmatitic dome that overprints southward overturned kilometer-scale Visean-Namurian recumbent folds, remains a controversial structure (cf. details in Soula et al., 2001). Gneiss, migmatites and granitoids exhibit a ENE-WSW (N50-70E) trending stretching lineation parallel to the dome long axis. This gneiss dome has been variously interpreted as : i) a transcurrent shear zone (Nicolas et al., 1977; Echtler and Malavieille, 1990); ii) an anticlinal stack (Mattauer et al. 1996, Matte et al., 1998); iii) a diapir (Schuilling, 1960; Faure and Cottereau, 1988); or iv) a metamorphic core complex (Van den Driessche and Brun, 1991-92). Even if ductile normal faulting coeval to the Late Carboniferous extension is clearly established along the northern side of the dome, an extensional setting for the crustal melting is not demonstrated yet. Pre-Velay migmatites are also developed but not well characterized in other parts of the Massif Central such as in South Millevaches. The migmatites widespread all along the south coast of the Massif Armoricain, in Vendée and Anticlinal de Cornouailles probably belong also to the same Late Visean event (Figures 4, 10).

This ca 325-310 Ma event corresponds to the main period of magma production. Porphyritic monzogranites and biotite-muscovite leucogranites form the two main types of plutons (Didier and Lameyre, 1971). The two types were derived from different magmas, but field relationships and geochronology show that they were emplaced coevally. Petro-structural and AMS studies of numerous Namurian-Westphalian plutons show that these bodies are characterized by a conspicuous NW-SE trending mineral, and stretching lineation. The same trend is also inferred from contact minerals in the pluton host rocks. In the Limousin, leucogranite plutons are often bound by ductile normal faults which also exhibits NW-SE trending hot slickenlines. This structural pattern is interpreted as the consequence of the syn-orogenic extensional tectonics of the Massif Central (Faure, 1995). In the Massif Armoricain, the Namurian-Westphalian plutonism is also widespread but dominantly located along hundred kilometers-scale dextral strike-slip faults (Figure 3). The South Armorican Shear Zone is worldwide famous since S-C structures have been first described there (Berthé et al., 1979).

In the lower crust, the 310-290 Ma time is that of emplacement of the cordierite granite-migmatite Velay dome (Figure 2, Malavieille et al., 1990; Ledru et al., 2002 and enclosed references). This huge thermal anomaly is probably the result of the interference between lithosphere-scale tectonics and asthenosphere-scale diapiric ascent. The Velay dome is bounded to the north by the Pilat detachment fault which is characterized by N-S to NNE-SSW slickenlines. Moreover, numerous dykes, acidic tuf, ash layers and more rarely alkaline basalts associated with terrigeneous formations crop out in most of the coal basins (cf. Faure, 1995 for details). It is now well accepted that the Late Carbonifeous magmatism occurred during the late orogenic extension of the Variscan Belt (cf. section 5.7).