Magnetic fabric vs. magnetite and biotite shape fabrics of the magnetite-bearing granite pluton of Gameleiras (Northeast Brazil)

Abstract

A well-defined orientation pattern has been evidenced by the magnetic fabric (lineation and foliation) of the granite pluton of Gameleiras, Northeast Brazil. This magnetic fabric is roughly parallel to the magmatic fabric defined by the elongate mafic enclaves, and by the planar arrangement of tabular K-feldspar phenocrysts. Low-field anisotropy of magnetic susceptibility properties, as well as magnetic mineralogy studies and optical microscopy observations, indicate that multidomian, iron-rich magnetite is the main carrier of the magnetic susceptibility and its anisotropy. To examine in more detail the relationships between the magnetic and magmatic fabrics, measurements of two-dimensional shape fabrics of grain populations of biotite and magnetite have been carried out on a total of 500 grains using an automatic shape fabric analysis procedure on carefully selected and oriented thin sections. Angular departure (α) has been calculated between the maximum elongation of the mineral shape fabric of the given grain population and the maximum susceptibility axis of the corresponding macroscopic specimen. We found that the fabric of biotite correlates strongly with the magnetic fabric (α = 7° ± 8°). The fabric of magnetite, although not as well defined, mainly because of its weak shape anisotropy, still correlates with the magnetic fabric (α = 12° ± 18°). Origin of low-field magnetic fabric in the pluton of Gameleiras is concluded to be controlled principally by the statistical alignment of the long axes of inequant magnetite grains. Magnetic interactions between the grains of magnetite, where they form clusters, may account for the observed scattering or abnormal magnitudes of the magnetic anisotropy. Concerning the orientation of the magnetic fabric, as the long dimensions of the magnetites preferentially align with the boundaries of the other minerals, particularly biotite and titanite, the magnetic fabric as defined by the grains of magnetite is concluded to approximate closely the magmatic fabric defined by the shape fabric of biotite. However, as no correlation exists between shapes, or clustering, of magnetites and strain undergone by the deforming magma, the strength of the magnetic fabric in ferromagnetic granites cannot be used in a simple way as a measure of strain intensity.