SUSCETIBILIDADE MAGNÉTICA: UM INDICADOR DA EVOLUÇÃO PETROLÓGICA DE GRANITÓIDES DA AMAZÔNIA

Abstract

Magnetic susceptibility (MS) and opaque mineral studies of granitic rocks from Amazonia has led to a better understanding of the relationship between different magnetic characteristics and the processes that occurred during the evolution of these rocks. Tin-mineralized granites such as the Antônio Vicente, Mocambo, Velho Guilherme, Água Boa, Madeira and Pedra Branca massifs have low MS, and their most altered facies, which are tin-specialized, have the lowest MS values. Relatively reducing oxygen fugacity conditions (fO2 ), and, to a lesser extent, the origin of these granites from highly differentiated magmatic melts are probably responsible for the lower MS values. Musa (Mu) and Jamon (J) granites have either high MS values or magnetite (Mt) modal contents, which decrease with magmatic differentiation (amphibole facies —> biotite facies —> leucogranites). High fO2 conditions, near the NNO and HITMQ buffers, dominated during the crystallization of these granites. The Cigano Granite and the Rio Maria Granodiorite show the larger distribution of MS values. Oxygen fugacity conditions between NNO and FMQ buffers prevailed during the formation of the Cigano Granite, explaining its magnetic behavior. High MS values in the Rio Maria Granodiorite occur only near the contact with Proterozoic granitic intrusions (Mu and J granites), due to the formation of metamorphic magnetite, while lower MS values occur in other parts of the batholith.