Metamorphism of the Palaeozoic Cinco Villas Massif (Basque Pyrenees): Illite Crystallinity and Graphitization Degree

Abstract

AbstractThe degree of metamorphism affecting the Devonian-Carboniferous rocks of the Palaeozoic Cinco Villas massif has been studied by (a) petrographic techniques, (b) illite crystallinity, and (c) degree of graphitization of the carbonaceous material. Some mineralogical differences have been found between the Devonian and Carboniferous rocks; paragonite and mixed-layer paragonite/muscovite, typical of anchimetamorphic areas, appear in the Devonian but are not found in the Carboniferous rocks. These are characterized by the local appearance of chloritoid, garnet, amphibole, epidote, andalusite and biotite, and the generalized presence of muscovite/chlorite. Illite crystallinity shows a metamorphic zonation (anchizone epizone) towards the granitic Aya massif, and a concentric pattern around the intrusive body.The optical analysis shows that the first effects of the regional metamorphism began before the D2 deformation, reaching its paroxysm during this phase and continuing afterwards. The degree of graphitization evolves progressively with metamorphism, and this fact is reflected in an exponential relationship between d(002) and crystallinity Lc(002). The similarity in the degree of graphitization between homologous materials within and outside the Aya aureole suggests a similar kinetic factor for both the thermal and regional metamorphisms, or a similar duration time. This fact, as well as the distribution and relationship between the deformation and recrystallization of the minerals, suggests a syn-plutonic regional metamorphism developed at low pressures, in accordance with the value of the b0 parameter for the white micas, and probably a maximum temperature of about 500°C.