MAGNETIC-SUSCEPTIBILITY OF THE MONT-LOUIS ANDORRA ILMENITE-TYPE GRANITE (PYRENEES) - A NEW TOOL FOR THE PETROGRAPHIC CHARACTERIZATION AND REGIONAL MAPPING OF ZONED GRANITE PLUTONS

We have observed remarkably consistent patterns of concentric zonation in the values of low-field magnetic susceptibility measurements over the Variscan age Mont-Louis Andorra granite pluton of the eastern Pyrenees of Andorra, Spain and France. This zonation is a function of the rock petrology since a close correlation is shown between the petrographic nature (as defined by chemical analysis) and the magnetic susceptibility magnitude of the rocks. It reflects the dominantly paramagnetic nature of the granite, carried by Fe-bearing silicates, and this is demonstrated by the fact that low-field susceptibilities of representative specimens are almost equal to (1) their high-field susceptibilities and (2) their calculated susceptibilities using Fe contents, assuming a null ferromagnetic contribution. We conclude that this technique accurately reflects the modal abundances of ferromagnesian phases in rocks of the ilmenite series and that it represents a powerful and efficient tool for the reconnaissance surveying of petrological variations in granitoid plutons.