MAGNETIC FABRIC VS MAGNETITE AND BIOTITE SHAPE FABRICS OF THE MAGNETITE-BEARING GRANITE PLUTON OF GAMELEIRAS (NORTHEAST BRAZIL)

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 magm atic 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 st udies and optical microscopy observations, indicate that multidomain, iron-rich magnetite is the main carrier of the magnetic susceptibility and its anisotropy. To examine in more detail the relationships betwe en 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 a nalysis procedure on carefully selected and oriented thin sections. An gular departure (alpha) has been calculated between the maximum elonga tion 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 mag netic fabric (alpha = 7 degrees +/- 8 degrees). The fabric of magnetit e, although not as well defined, mainly because of its weak shape anis otropy, still correlates with the magnetic fabric (alpha = 12 degrees +/- 18 degrees). Origin of low-field magnetic fabric in the pluton of Gameleiras is concluded to be controlled principally by the statistica l alignment of the long axes of inequant magnetite grains. Magnetic in teractions 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 t he boundaries of the other minerals, particularly biotite and titanite , the magnetic fabric as defined by the grains of magnetite is conclud ed to approximate closely the magmatic fabric defined by the shape fab ric of biotite. However, as no correlation exists between shapes, or c lustering of magnetites and strain undergone by the deforming magma, t he strength of the magnetic fabric in ferromagnetic granites cannot be used in a simple way as a measure of strain intensity.