Estudio de las fábricas magnéticas y del magnetismo de las rocas en los plutones de Sotillo y Ribadelago (Sanabria, Zamora)

Abstract

Magnetic fabric and rock magnetism studies were performed in 14 samples of the Ribadelago and Sotillo plutons (Iberian Variscan belt, NW Spain). These heterogeneous granitoids are composed by granodiorites, tonalités, quarz-diorites and diorites, mainly organized into dikes and sills. The magnetic fabric, revealed by the anisotropy of low-field magnetic susceptibility (AMS), is dominantly carried by paramagnetic iron-bearing silicates (biotite and amphibole), as infered from the thermomagnetic curves. However, a small contribution of magnetite, titanomagnetite, and perhaps pyrrhotite can be evidenced. In some cases, the magnetite seems to be restricted to the feldspar fraction. Remanent coercitivity spectra, determined from implementation of anhysteretic remanent magnetization during alternating field (AF) tumbling reveal that two magnetite families with different coercitivity distributions are present: A main one within 13-35 mT and a minor fraction within 35-60 mT. The anisotropy of anhisteretic remanence has been studied within these two coercivity windows (pAAR 13-35 mT and pAAR 35-60 mT). Comparison between the pAAR fabrics and the AMS fabric, helps to distinguish between primary (magmatic) and secondary fabrics (post-magmatic) of the ferromagnetic phases