Characteristics and mode of emplacement of gneiss domes and plutonic domes in central-eastern Pyrenees

Gneiss domes and plutonic granitoid domes make up almost 50% of the pre-Hercynian terrains in the Central and Eastern Pyrenees. From a structural study of the shape and internal structure of the domes and of their relationships with the enclosing rocks, it can be shown that both types of domes were emplaced diapirically during the major regional deformation phase and the peak of regional metamorphism.The study also shows that the internal structure, the overall shape and general behaviour relative to the host rocks are similar for plutonic domes and for gneiss domes. This appears to be in good agreement with H. Ramberg's (1967, Gravity Deformation and the Earth's Crust. Academic Press, London; 1970, Model studies in relation to intrusion of plutonic bodies. In: Mechanisms of Igneous Intrusion (edited by Newall, G. & Rast, N.) Geol. J. Spec. Issue2, 261–286.) model studies showing that dome or mushroom-like structures, similar to those observed, develop when there is a small viscosity ratio between the rising body and its enclosing medium.This implies a high crystal content for the granitoid magma. This crystal content has been estimated by (i) calculating the viscosity and density in natural conditions from petrological data for the magma considered as a suspension, using the model and program of J. P. Carron et al. (1978 Bull Soc. géol. Fr.20, 739–744.); (ii) using the recent results of experimental deformation of partially melted granites of I. van der Molen & M. S. Paterson (1979, Contr. Miner. Petrol.70, 299–318.) and (ii) comparing the preceding results with the data obtained by deformation experiments on rocks similar to those enclosing the domes. The minimum crystal content for the development of a dome-like structure has been, thus, estimated to about 70%, i.e. a value very close to that estimated by van der Molen & Paterson (1979) to be the critical value separating the granular framework flow from suspension-like behaviour.The effect of small variations in the viscosity of the rising body are then simulated by centrifuge experiments. These small variations appear to exert a strong control on the shape and rate of rise of the domes. They are thought to be sufficient to account for the variations in shape and structure and the level of emplacement of the different types of gneissic and plutonic domes.Finally, more complex experiments, with models built in order to simulate as closely as possible the natural structural evolution of the region as deduced from petrological and structural data, are reported. Their implications for the regional interpretation of the relationships between gneissic and plutonic domes is then discussed.