ANISOTROPY OF MAGNETIC-SUSCEPTIBILITY OF ECLOGITES - MINERALOGICAL ORIGIN AND CORRELATION WITH THE TECTONIC FABRIC (CABO ORTEGAL, SPAIN)

The fabric and the anisotropy of magnetic susceptibility of the Cabo O rtegal eclogite (NW Spain) are studied. These mafic rocks were metamor phosed and deformed under high pressures and temperatures between 390 and 370 Ma in a subduction/collision tectonic setting. Massive eclogit e slices and deformed eclogite in shear zones have bulk magnetic susce ptibilities of 31 to 82.10(-5) S.I. and 28 to 75.10(-5) S.I., respecti vely. The paramagnetic mineral fraction is the principal magnetic susc eptibility carrier. This fraction includes notably garnet and clinopyr oxene as matrix minerals, and ilmenite and rutile as accessory constit uents. Though magnetic anisotropy degree varies between 3.1 % and 6.6 %, variations of this parameter in each rock type are marked. In the d eformed eclogite, magnetic lineation (K-max) and the pole to the magne tic foliation (K-min) are coaxial and coincident with macroscopic petr ofabric elements (foliation and lineation). In the massive eclogite, t he magnetic fabric is dispersed along the principal structural planes and inversions are associated with samples with small degrees of aniso tropy. The anisotropy of magnetic susceptibility is interpreted as bei ng due to the crystallographic preferred orientation and spatial organ isation of the polymineralic aggregate. Relating the evolution of the symmetry of magnetic fabric to the symmetry of petrofabric or deformat ion is rather precluded since susceptibility has multiple origins and bulk magnetic fabric is due to minerals of different symmetry. (C) Els evier, Paris.