Origine et developpement de litages dans des roches de composition granitique

Anisotropies are very common in granitic intrusions and they can be divided in two types: mineral orientations (e.g. foliation or lineation) or heterogeneities. The heterogeneities generally are either due to change in mineral proportion or due to variation in composition compare to the host rock. The layering is probably the most spectacular and most interesting form of heterogeneities, because it forms during the formation of the magmatic body. The layering is generally defined in sedimentary rock but it is difficult to transpose the definition to the magmatic rock. In contrast to sedimentary rocks, the layering in igneous rocks can vary in appearance, texture and chemistry within the same intrusion. Two different areas have been chosen to study the heterogeneities and particularly the layering in igneous rocks and are described in details in the present thesis.;The second study area has been chosen in the central part of the Grenville province in the Tadoussac region. This granitic body is coeval to the intrusion of mafic magmas within a shear zone. The layers observed in this area have a mineral composition and microstructure which are intermediate compare to the granite and to the mafic rock. These structures are probably generated by the mixing between these different types of magma. The petrography and geochemistry of the rocks are compatible with metasomatic reactions between the magmatic liquid and the crystals.;These different occurrences of layers in granitic rocks are not exhaustive and numerous processes of formation may result in the formation of layers. This implies that the study of layers be carried out using various techniques to decipher between the different processes which lead to their formation. Hence, multi-disciplinary studies are needed to refine their model of formation.;The first study area corresponds to the Tuolumne intrusive suite located in the Sierra Nevada in California. The different field, petrography, and microstructural studies have demonstrated that the layerings are similar to sedimentary structures. The whole rock and mineral geochemistry indicate that the layers are not formed by either a process of fractional crystallisation or in situ crystallisation. In contrast, the layers are interpreted to be the result of dynamic processes and crystal separation (unmingling).