Les granites de la Margeride, de Chambon-le-Château et de Saint-Christophe-d'Allier (Massif central): des laccolites synchrones des derniers stades de l'épaississement varisque

Abstract

Introduction - Geological setting. The French Massif Central is a segment of the Variscan Belt that was built up during the Devonian and Carboniferous during the collision between Gondwana and Larussia (Autrun and Cogné, 1980; Bard et al., 1980; Latte, 1986). The orogenic development of the Massif Central can be broken down into two major stages: 1) A stage of mountain building marked by the development of nappe tectonism, at first in the inner zones (between 400 and 340 Ma) and then in the outer zones (between 340 and 325 Ma), accompanied by migmatization and early granite emplacement. 2) A stage of erosion accompaniend by extensional tectonism and the emplacement of migmatite domes and granite. The southeastern part of the Massif Central was involved in all or part of this evolution. Between the major crustal thickening of Carboniferous age (345 Ma) and the Iate-orogenic anatectic and extensional events, however, one notes that considerable amounts of porphyritic potassium granite were emplaced between 340 and 325 Ma in the Lyonnais, Vivarais, Sénouire, Châtaigneraie and Margeride areas (fïgs. 1 and 5). A review is given of what is currently known concerning the structure of the eastern part of the Margeride granite complex (Margeride, Saint-Christophe-d'Allier, Chambon-le-Château granites). It is shown that these instrusions were contemporaneous with Visean-Numurian thrust faulting and the place of this tectonic activity within the Variscan orogenic history is discussed. Geometry and chronology of the intrusion. Lithostructural and gravimetric data confirm the lamellar shape of the granites (laccoliths) and an analysis of the magmatic contacts between the three granite bodies shows that they were contemporaneous. Similarly, it appears that the enstern part of the Margeride granite complex consists of three super- posed " granitic sheets " that are geometrically concordant with the foliation, which was subhorizontal during the granite emplacement (figs. 2, 3, and 4). Structural analysis. Study of the internal structure of the granites and of the structures developed in the country rock shows thrust faults with a SE to SSE vergence localized in the footwall of the Iaccoliths. These faults are marked by mylonites within which the geometry and deformation kinematics are defined by a stretching lineation contained within a foliation, by asymmetric pressure shadows and by shear bands (photos 2 to 7). An important deformational gradient at laccolith scale is marked by a progressive decrease in the intensity of plastic deformation of the minerals and the fabric away from the faults. The deformation nevertheless retains tho same geometry, and analysis of this geometry at the scale of the Margeride laccolith (figs. 1C and 4) shows that all the internal structures of this massif are related to a single tectonic event. Relationships between metamorphism and deformation in the host rocks to the granites. Mylonitization of the metamorphic country rock caused a recrystallization of the MP/HT metamorphic paragenesis that marks the regional S1-2 foliation which developed around 345 Ma (Carboniferous), at the height of crustal thickening. This recrystallization took place in two phases : at first a dynamic phase during which the biotite, sillimanite and/or white mica were stable; then the development of a late-tectonic blastic growth of andalusite and cordierite (developed at the expense of biotite and garnet). Origin of the eastern Margeride faults, their place in the regional sequence and the Variscan orogeny. The metamorphic conditions of the mylonitization, as well as the constancy of the axes of deformation and kinematics developed during the pre-subsolidus magmatic fabric with globular quartz; photo 1) and subsolidus (mylonitic texture with drawn-out quartz ; photos 2 to 6) stages of the granite, reflect the syntectonic character of the Margeride -complex. Consequently, in view of the age of he Margeride granite, this tectonism would have developed during the Visean-Namurian: around 335 Ma at the earliest and around 325 Ma (thermic closure of the system) at the latest. In addition, the establishment of a slative chronology (photos 8 to 13) and a geochronology with respect to the sequence of regional tectonic, metamorphic and magmatic events shows that the thrust faulting in the eastern Margeride was: (a) later than the peak Variscan nappe tectonism (i.e. the major crustal thickening generally assigned to the Early Variscan (around 345 Ma] but forming, with this latter, a continuum during which the shear kinematics remained much the same; (b) synchronous with the regional D3 deformation, which was contemporaneous with the granite emplacements of the Margeride complex; (c) earlier than the Velay " dome ", and also earlier than the nor mal faulting and Namurian-Westphalian folding that affected the " dome " environment. This regional end-Visean tectonic event (between 335 and 325 Ma) therefore contributed to the regional crustal thickening, representing its final stages. The tectonism is also recognized in western Châtaigneraie and in the Sénouire and Lyonnais series where it was of régional extent (fig. -5). It marks the last increments of the Varisaun accretionary tectonism, which was sealed, after 320 Ma, by the Pilat detachment fault and the Velay magmatism. lt therefore ended, as at Montagne Noire, during the Namurian (between 325 and 320 Ma). Conclusion. It appears that for the eastern, southeastern and southern parts of the Massif Central the process of Variscan mountain building ended in the Visean-Namurian (around 325 Ma). In the eastern part of the Massif Central,this final stage controlled the emplacement and geometry of a major suite of intrusions. It was then sealed very rapidly by the post-collisional evolution marked by a tectonic thinning of the thickened domain (after 320 Ma; Pilat; Malavieille et al., 1.990: Montagne Noire; Echtler, 1990; Echtler and Malavieille, 1990; Maluski et al., 1991) or by the " Velay magmatism and tectonism ".