Anisotropia magnética e gravimetria do Complexo Intrusivo Santa Angélica, Faixa Araçuaí

Abstract

The Santa Angélica Intrusive Complex (SAIC) is located in the Araçuai Belt, in Southeast Brazil. It is a Cambrian, NE elongated plutonic complex, composed of two twin plutons (lobes Northeast and Southwest) with a bull's eye shape and a concentric fabric ("onion skin"). It was formed during the post-collisional stage of the Araçuaí Orogen (AO), an example of Neoproterozoic orogen formed during the Brasiliano cycle. In the southern portion of the AO, the post-collisional plutons intrude into a deeper crust and have an inversely zoned concentric pattern, while in the northern portion of the AO they form larger granitic/charnockitic batholiths with sub-horizontal lineations. Although the SAIC is widely recognized as free of regional deformation due to its tectonic context and dating, important solid-state structures have been reported at its borders, and along an internal shear zone. In this work, the mechanisms responsible for the generation of these features was studied in order to get a better understand of the causes of the different architectures presented by post-collisional bodies when placed at different crustal levels. We applied 2D direct modeling of gravimetric data at CISA and its country rocks, together with a complete structural analysis by means of anisotropy of magnetic susceptibility (AMS), anisotropy of remanent magnetization (ARM) and oriented microstructural data from thin sections. The gravimetric data obtained with the field survey reflect the inversely zoned distributions, with a gradation from the granitic borders to the mafic cores. Magnetic analyses indicate that multidomain magnetite controls the magnetic fabric of the SAIC's country rocks. 2D gravimetric modeling shows that in the Northeast lobe represents the pluton's roots and its thickness is much lower than that of the Southwest lobe. Using microstructural analysis and magnetic mineral fabric, three zones were identified where solid-state microstructures occur: (i) country rocks with foliation parallel to contact with SAIC, (ii) the granitic edge and (iii) the internal shear zone. In areas (i) and (ii), higher temperature solid state microstructures are overlapped by lower temperature structures, while area (iii) has only high temperature solid state structures. Two hypotheses might explain this configuration.First, the structures represent the partitioning of deformation during the pluton's cooling and follow the regional stress field also expressed in the shear zones that cur the host rocks. Alternatively, they may represent a reverse diapirism, caused by the negative buoyancy of the mafic core after crystallization.