Anisotropy of magnetic susceptibility of the Pyrenean granites

Manuel Porquet,Dennis Leblanc,Jean Baptiste Auréjac,Borja Antolín,José María Tubía ,Esther Izquierdo‐Llavall ,Luis Alberto Longares ,Belén Oliva‐Urcia ,A Casas ,A Aranguren ,Jean‐Luc Bouchez ,Gérard Gleizes ,Teresa Román‐Berdiel ,Javier Ramajo,Julia Cuevas,Emilio L Pueyo,Yoann Denèle,Philippe Olivier,Vicente Santana,Asier Hilario,Néstor Vegas

doi:10.1080/17445647.2017.1302364

Abstract

ABSTRACTIn this paper, we report on a compilation of more than 2200 sites (more than 10,000 individual measurements) where anisotropy of magnetic susceptibility (AMS) was studied in granites from the Variscan Pyrenees. The standardization and homogenization of this information has allowed us to produce three Main Maps that synthesize all the information related with the AMS of the Pyrenean granites. We also describe the problems found during the construction of the database (variable geo-positioning, different published information, etc.). The information derived from 21 granite bodies, the database, and the synthesis maps (magnetic susceptibility, Km, and the orientation of the magnetic foliation, plane perpendicular to k3, and of the magnetic lineation, k1) allow us to see for the first time a complete image of this important kinematic and petrographic indicator.

Highlights

Anisotropy of magnetic susceptibility (AMS) is a sound and proven technique to determine the mineral-preferred orientation of rock volumes (Borradaile & Henry, 1997; Graham, 1954; Parés, 2015; Tarling & Hrouda, 1993)
In this paper, when discussing AMS spherical information, we had to face three main problems: (1) data were represented in scalar format, they had to be converted to directional format with hundreds of measurements; (2) in some cases magnetic foliation had to be converted into k3; (3) most of the direction data needed to be changed into stereographic space (360°) instead of the 180° space used by some authors, which meant adding the N–S–E–W
Measures below 100 × 10−6 SI correspond to leucogranites; measures from 100 to × 10−6 SI correspond to monzogranites; measures from to 300 × 10−6 SI correspond to granodiorites; and measures above 300 × 10−6 SI correspond to quartz diorites

Summary

Introduction

Anisotropy of magnetic susceptibility (AMS) is a sound and proven technique to determine the mineral-preferred orientation of rock volumes (Borradaile & Henry, 1997; Graham, 1954; Parés, 2015; Tarling & Hrouda, 1993) It is founded on the parallelism between the crystallographic and magnetic fabrics of some paramagnetic minerals, especially phyllosilicates (Martín-Hernández & Hirt, 2003). The reason lies in the total content in iron and its mineral fractioning, a fact that promoted a classification of granites as either magnetic or non-magnetic (Ellwood & Wenner, 1981; Ishihara, 1977) 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 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)

Methods

Discussion

Conclusion