The effect of magnetic anisotropy on paleomagnetic directions in high-grade metamorphic rocks from the Juiz de Fora Complex, SE Brazil

Abstract

The Juiz de Fora Complex is mainly composed of granulites and granodioritic–migmatite gneisses and is a cratonic basement of the Ribeira fold belt, which was formed during the last stage of Brasiliano orogeny. Samples widely distributed over the studied region (SE Brazil, Rio de Janeiro State) were collected for paleomagnetic and magnetic anisotropy determinations. After measurement of anisotropy of low-field magnetic susceptibility (AMS) the same specimens were submitted to both alternating field (for anisotropy of remanent magnetization, ARM, determinations) and thermal demagnetization for paleomagnetic analyses. Demagnetization processes allowed isolating an eastern, steep, positive-inclination characteristic remanent magnetization (ChRM) direction from the rocks. The studied specimens are strongly heterogeneous at sample scale regarding the amount of magnetic minerals even for specimens from the same core. Rock magnetism indicates that there is a complex mixture of magnetic minerals with both low and high coercivity which are associated with coarse-grained (titano)magnetite and either fine-grained (titano)magnetite or ‘titanohematite’ grains, respectively. These magnetic minerals are responsible for both remanent direction and magnetic anisotropies, even though for some specimens paramagnetic minerals are the carriers of AMS fabric. The ChRM direction found for the rocks should represent the cooling phase of the Brasiliano event in the area, and it was acquired after the magnetic fabric was formed. The AMS and ARM fabrics are coaxial and tectonic in origin, and compare favorably with the mesoscopic-scale fabrics in the adjacent areas. However, ARM measurements have reached only low-coercivity grains and it could be determined in few specimens. Then only an estimate of ARM effect on ChRMs correction is shown. The rocks are strongly magnetically anisotropic (>50%) and foliated. The paleomagnetic directions from each specimen within a site tend to approach its AMS or ARM foliation ( K max– K int or ARM max–ARM min plane). These directions were corrected, specimen-by-specimen from the sites, taking into account the pole of AMS or ARM foliation ( K min or ARM min) and the value of the degree of anisotropy ( P). Results show that even for specimens with high P, the angular difference between corrected and uncorrected magnetization direction may be insignificant if the geomagnetic field at the time of acquired magnetization was approximately parallel to the magnetic foliation of the specimens. Our data set suggests that AMS is a powerful tool to correct the ChRM deviation from paleofields in rocks with complex magnetic mineralogy such as those from the Juiz de Fora Complex.