The Rubim Pluton (Minas Gerais, Brazil): a petrostructural and magnetic fabric study

The Rubim pluton (160 km2), a suite of diorite and porphyritic granodiorite bodies, was subject to magnetic susceptibility and anisotropy studies. The pluton intrudes high-grade to migmatitic gneisses of the Jequitinhonha Complex situated in the Neoproterozoic Araçuaı́fold belt, Southeast Brazil. The magnetic susceptibility is shown to be of paramagnetic to ferromagnetic origin. The magnetic fabric is remarkably homogeneous in orientation throughout the pluton and this fabric reflects the flow pattern of the pluton. Almost all of the investigated microstructures correspond to the magmatic state of the pluton, so measured magnetic lineations and foliations may be interpreted, respectively, as directions and planes of magmatic flow. Magnetic-magmatic structural mapping of the Rubim pluton shows a good parallelism between the magmatic fabric in the pluton and the solid-state fabric in the country rocks. The geometry between the pluton and country rocks suggest that the magma was emplaced as a sill during progressive deformation of the country rocks. That deformation is to southwest, with a thrust component. The country rocks and the Rubim pluton represent a cross-section of the middle crust deformed under low P (P=4–5 Kb)–high T (T=715–790°C) conditions. The data suggest that the emplacement of the pluton was coeval with an orogenic stage in the Brasiliano Araçuaı́fold belt.