MICROSTRUCTURE AND MAGNETIC-SUSCEPTIBILITY APPLIED TO EMPLACEMENT KINEMATICS OF GRANITES - THE EXAMPLE OF THE FOIX PLUTON (FRENCH PYRENEES)

The Variscan Foix granite (40 km2; Central Pyrenees) was subjected to detailed microstructural, magnetic susceptibility and anisotropy studies. Magnetic susceptibility is shown to be of dominant paramagnetic origin. Its magnitudes clearly define a zoning which compares with the petrographic zoning and reveals that the pluton is the southern remnant of a normally zoned pluton truncated during Alpine tectonics. Magnetic susceptibility anisotropy characterizes the planar and linear preferred orientations of the main iron-bearing silicates, namely biotite and accessory amphibole. It is argued that these fabrics also reflect the flow pattern of the granite.Microstructural observations aid in distinguishing the magmatic and late magmatic fabric signatures (main body) from the post-magmatic, solid-state, signature (leucogranite). In order to account for the consistently subhorizontal and NE-SW directed magmatic lineations in the northern sector of the Foix granite, an oblique intrusion is proposed. It may be coeval with the early Variscan regional SW-trending shearing event. Steepening of the structures in at least part of the southern sector may be coeval with the late development of upright folds and vertical cleavage in the country rocks. The utility of combined microstructural and magnetic susceptibility studies is demonstrated as an aid in the structural interpretation of granite intrusions, which act as kinematic markers of their host terranes. © 1990.