Magnetic fabric data on partially molten granitic rocks from the Florianópolis Batholith, Southern Brazil

Abstract

Contact thermal aureoles around mafic intrusions provide information about the emplacement and segregation of melts. The Garopaba-Silveira region is characterised by intrusions with large heat input responsible for the formation of a thermal aureole in the granitoids of the Paulo Lopes Suite (PLS), which extends along the coast to more distant locations in this region. Such melting processes are observed from meso-to microscale, along the granitoid-mafic dike interface and in areas as distant as 500 m from the mafic dikes. It is suggested that these melting processes have been aided by the breakdown of hydrated minerals, such as biotite, through dehydration-melting processes, as commonly found in migmatites and partially molten rocks. On the other hand, the contacts between the granitoids and the mafic dikes are predominantly sharp and straight, as well as, in some dikes, they are sinuous, diffuse, and present back veining, attesting to mutual intrusion. Given the complexity of these melting processes along the granitoid-mafic dike interface, this study applies magnetic susceptibility anisotropy (AMS), anisotropy of anhysteretic remanent magnetisation (AARM) and rock magnetism techniques to melting processes in a thermal aureole context. The magnetic mineralogy of the mafic dikes comprises ferrimagnetic minerals, magnetite, and Ti-poor titanomagnetite. In the granitoids, the ferrimagnetic contribution is dominant, although there are paramagnetic contributions, allowing for direct lateral variations of size of the magnetic grains at different distances (profiles), which may indicate crystallisation time as well as thermal effects in these rocks due to the mafic dike intrusions.