Magnetic fabric and its significance in the Florianópolis dyke swarm, southern Brazil |
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| Authors: | Maria Irene Bartolomeu Raposo |
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| Institution: | Instituto Astronômico e Geofisico, University of São Paulo, CP 9638, 01065-970, São Paulo, Brazil. E-mail: irene@iag.usp.br. |
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| Abstract: | Magnetic fabric was determined by applying the anisotropy from the low-field magnetic susceptibility (AMS) technique in 62 mafic dykes from the Mesozoic Florianópolis (Santa Catarina Island) dyke swarm, southern Brazil. These dykes cut the crystalline basement rocks, which are mainly Proterozoic. They are vertical or subvertical in dip and trend mainly NE, although NW-trending dykes are also found. Dykes are tholeiitic in composition and are geochemically similar to those from the Ponta Grossa swarm. Thicknesses vary from 0.3 to 60 m. Polished sections show that titanomagnetites carry the AMS in these dykes. Hysteresis parameters show that the magnetic minerals fall in the PSD range. Two types of magnetic fabric are recognized. Type I is characterized by K 1- K 2 parallel to the dyke wall, representing magma flow within the dykes; type II, with K 1- K 3 parallel to the dyke wall, was found in four dykes. Type I is found in 94 per cent of the dykes, and approximately 20 per cent of these have K 1 inclinations of less than 30°, suggesting horizontal or subhorizontal flow. About 80 per cent have K 1 inclinations of greater than 30°, due to inclined to vertical flow. The comparison of AMS studies from both the Florianópolis and the Ponta Grossa dykes suggests a source position closer to Santa Catarina Island than the Ponta Grossa arch. |
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| Keywords: | AMS dyke swarm magnetic fabric magnetic susceptibility |
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