The Margeride granite massif (French Central Massif) ; Le massif granitique de la Margeride (Massif Central Français) ; The Margeride granite massif (French Central Massif): petrography, geochemistry, structural geology, examples of gravity differentiation in granitic rocks, regional crustal evolution during the Variscan Orogeny ; Le massif granitique de la Margeride (Massif Central Français): étude pétrographique, géochimique et structurale, exemples de différenciation par gravité dans les roches granitiques, évolution régionale de la croûte au cours de l'orogenèse hercynienne

The granite of the Margeride constitutes a very large pluton in the french Massif Central. The main rock is a monzogranite with large K feldspar megacrysts (MFK).Two noticeable examples of gravitational differentiation at different scales are exhibited, apparently for the first time with so great evidence in felsic rocks :The first, from a calco-alkaline granitic magma intruded in a huge laccolith about 5 km thick. Dark facies, in the lower part, very rich in biotite and MFK changes progressively, to the upper part, into light facies, poor in MFK, containing biotite (few), cordierite and muscovite.The other, from a leucogranitic and more alkaline magma, in the lower part of a vertical dike crosscutting the former unit, with formation of an orbicular structure (orbicular granite of the Signal de Randon).In both cases, a stratigraphic superposition of successive generations of crystals or orbicules is observed. The Stokes law is used to analyse the sedimentation mechanism of crystals and orbicules particularly influenced by the increase of viscosity of the magma in proportion as its temperature decrease.Numerous and coherent data, of petrography, geochemistry and mineralogy show a progressive evolution toward a leucocratic, siliceous and alkaline pole at the upper part of the two differentiated bodies.The gravitational differentiation has permitted to present the evidence of post magmatic deformations in the granitic pluton, with an original method : bathymetric study of the floor of the laccolith assumed initially horizontal, using a map on which the curves of isocontent of biotite have been drawn. Large folds, prolonging structures well known out of the pluton, have been evidenced and also the succession of vertical and horizontal movements along the Saugues fault.Other applications of the gravitational differentiation are indicated : interpretation of geochemical variations in eruptive rock dikes, metagranite identification, metallogenic research.The generality of laccolithic structure in granitic plutons, even ...