Descrição e interpretação de migmatitos: metodologia para abordagem em campo, exemplo na Faixa Brasília Meridional

Abstract

Migmatites occur in contact-metamorphic aureoles or areas of medium to high metamorphic grade within collisional orogens. In collisional orogens, migmatites correspond to the crustal belt that has recorded overlapping of regional metamorphism, deformation, anatexis, segregation, drainage, and crystallization of trap melt (i.e., melt that did not ascend to form plutons). This superimposition occurs over large time intervals (tens of millions of years), and the detailed study of a single outcrop can provide petrogenetic data for intervals greater than 40 Ma. However, given the anisotropy and heterogeneity of these rocks, it is necessary to develop methods that facilitate the field approach and, at the same time, are effective in acquiring petrogenetic data. The purpose of this dissertation is to present the petrogenetic methods and data obtained from a single outcrop of garnet-biotite diatexite located in the Southern Brasília Orogen, in the Interference Zone with the Ribeira Orogen, using a methodology developed by the author. The detailed mapping, the detailed petrographic study and the geochronological study via U-Pb (LA-ICP-MS) indicate that the garnet-biotite diatexite is a local diatexite whose formation period lasted approximately 64Ma and result from a fold rupture. During this period, there were two episodes of partial melting (which appear to be of the hydrate-breakdown type) and changes in the melt fraction, strain, pressure, and temperature. These characteristics, together with the probability of genesis from partial melting of different protoliths intercalated, provided conditions for the formation of eleven migmatitic facies and allowed the separation of the period of evolution into five anatectic-deformational phases: DA1, DA2, DA3, DA4, and DA5. The partial fusion, crystallization, and deformation recorded in the first three phases (DA1, DA2, DA3) were associated with the formation of the Southern Brasília Orogen. The phase DA4, however, recorded a metamorphic and/or deformational episode and partial melting associated with the formation of the Ribeira Orogen. The processes identified in phases DA1, DA2, DA3, DA4, and DA5 occurred in geotectonic contexts already described in the literature. Therefore, migmatite outcrops can be used as local geotectonic tracers in areas of long evolutionary history and/or high geological complexity (e.g., orogen interference; orogen overlap) to facilitate the identification of overlapping processes and the establishment of a relative chronology between them.