Magnetic anisotropy of the Redencao granite, eastern Amazonian craton (Brazil): Implications for the emplacement of A-type plutons

A magnetic fabric study was performed on the Redencao pluton in an attempt to understand its emplacement history. The Redencao pluton is part of the 1.88 Ga, anorogenic, A-type Jamon suite that intruded 2.97-2.86 Ga-old Archean granitoids of the Rio Maria Granite-Greenstone Terrane in the eastern Amazonian craton (northern Brazil). Previous gravity survey indicates that the pluton is a 6 km-thick, tabular intrusion. It is characterized by a concentric distribution of facies, with rings of seriated and porphyritic granite that cut across the main facies of even-grained monzogranites. The whole set is intruded by leucogranites that occupy the center of the pluton. Petrographic examination, magnetic susceptibilities, coercivity-spectra and thermomagnetic curves indicate that the magnetic fabric is primarily carried by coarse-grained multidomain magnetite. This is reinforced by the coincidence of magnetic susceptibility and remanence anisotropy principal axes. The absence of solid-state deformation features and the low anisotropy degrees indicate that the magnetic fabric is magmatic in origin. The magnetic fabric displays a systematic pattern, with all fades, including the rings of porphyritic granite, being characterized by concentric, gently dipping foliations associated with gently plunging lineations. Only the central leucogranitic facies shows a slightly discordant pattern with steeply dipping fabrics at its northeastern sector. An emplacement model by vertical stacking of successive magma batches is proposed for the construction of the Redencao pluton, which reconciles the tabular shape of the intrusion, the petrographic and geochemical zoning, and the magnetic fabric pattern. Initially, two magma batches were emplaced as sills. First the even-grained monzogranite, then the sedated and porphyritic granites, which formed by mingling of a leucogranitic melt with the host biotite-monzogranitic magma as attested by geochemical data and field evidence. The final shape of the pluton was acquired after the intrusion and inflation of the central leucogranite giving raise to the concentric pattern of facies in map view. (C) 2010 Elsevier B.V. All rights reserved.