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

Abstract

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 susceptibilifies 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. ]NTRODU•ONModeling the origin and emplacement of granite plutons is usually based on a structured study, beginning with the field characterization of the principal rock types and their spatial distribution and followed by detailed structural and/or petrological investigations. The pertinence of any petrogenetic or emplacement model is indeed dependent on the accuracy of the original field observations and laboratory characterizations of the rocks. Important field constraints include knowledge of whether the pluton is compositionally zoned and whether any such zoning is composite or gradational. Acquisition of such information is highly dependent on the quality of outcrop and the collection of spatially distributed samples for modal analysis. This stage of the investigation is often the most laborious part of the study, although essential data are usually generated. Unlike every other aspect of petrology and structural geology this facet of petrostructural research in granite plutons has not yet benefited significantly from the development of modem techniques. We contend that low field magnetic susceptibility measurements can contribute to the rapid and effective field mapping and petrographic characterization of certain types of granitoid plutons, particularly those of the ilmenite series, rather than the magnetite series granitoids [Ishihara, 1977; Takahashi et INow at G6oscience de l'Environement, URA 132 CNRSPaper number 9ZIB01590. 0148-0227/93/9ZIB-01590505.00.al., 1980]. The present approach, which therefore constitutes a basis for more detailed tectonophysical studies of graniteiris, naturally has strong potential to be a tool for geophysical exploration of a large fraction of the continental crust.Magnetic susceptibility measurement in low magnetic field (a few 10 -4 T) offers the promise of a simple and inexpensive technique. Two principal aspects can be explored with this technique, namely, structural aspects using the anisotropy of magnetic susceptib...

A preliminary study of the anisotropy of magnetic susceptibility (AMS) of Boroujerd granitoids, Sanandaj-Sirjan Zone, West Iran

“…The lineations are the best indicators of the stretching direction to which the magma was subjected while the mineral fabric was forming [4,45,46]. This interpretation is strongly supported by the mafic dykes that crosscut the pluton in the very northwest of Boroujerd perpendicular to the magmatic lineations (Figure 8; [47]). From a structural point of view, it is concluded that the plutons were subjected to an overall protracted N-S to NNW-SSE stretching regime during emplacement and cooling.…”

“…In the low susceptibility granite plutons, such as the Boroujerd, the magnetic susceptibility is regarded as a tool for petrographic characterization and regional mapping [47]. The study of Boroujerd shows that there is good correlation between magnetic susceptibility and the modal classification of Streckeisen (1976) through the iron-content of the rock.…”

Anisotropy of magnetic susceptibility of the Pyrenean granites

“…For all these reasons, the application of AMS in calc-alkaline plutons (non-magnetic; iron is fractioned mostly in biotite) has represented a turning point in the interpretation of the kinematics of their emplacement modes (Bouchez, 1997). Apart from being able to precisely determine the preferred orientation of biotites in apparently isotropic rocks (a main marker of the rock fabric), AMS also yields a control of the deformation intensity (relative differences among the AMS ellipsoids) and can be used as a petrological mapping variable, being correlated with iron content (Gleizes, Nédélec, Bouchez, Autran, & Rochette, 1993).…”

Tectonic Insight in the Southwest Gondwana Boundary Based on Anisotropy of Magnetic Susceptibility

“…The antiferromagnetic contribution, typically provided by hematite and goethite, is negligible in practice. The ferromagnetic contribution is commonly a major component and it is associated with magnetite and occasionally with sulfides such as pyrrhotite [3,5].…”

“…The base of the method is the measure of the orientation and intensity of the magnetic susceptibility (K) of a rock sample whose intensity is the sum of the diamagnetic, paramagnetic, antiferromagnetic and ferromagnetic responses of constituent minerals [3]. The diamagnetic response is typically very weak, and opposes the applied field; this response is typically contributed by iron-free silicates such as quartz and feldspar.…”