Microstructure Features in Paleo and Neoproterozoic Granitic Rocks, Southeastern Region of Brazil

Abstract

In this section we present the characteristic microstructures of granitic rocks and mylonites exposed in the southeastern region of Brazil, belonging to the geological context of the Neoproterozoic Araçuaí orogen and the Paleoproterozoic Mineiro belt. The studied rocks include most biotite and biotite hornblende tonalite and granodiorite and minor granite showing similar signatures to magnesian, calc-alkalic, metaluminous to peraluminous magmas. As a representative of continental or oceanic magmatic arcs, these granitoids are composed of plagioclase, K-feldspar, quartz, biotite, hornblende, garnet, muscovite, and sillimanite as major phases and pyroxene, apatite, epidote, allanite, titanite, zircon, monazite, rutile, magnetite, ilmenite, hematite, Fe-Ti oxides, pyrite, chalcopyrite, pyrrhotite, and graphite as accessory minerals. Typical magmatic textures include concentric and oscillatory zoning and exsolution, while isolated deformation microstructures and biotite replacing hornblende represent late-magmatic features. On the other hand, widespread undulose extinction, mechanical twinning, deformation bands, and recrystallized grains register crystal-plastic deformation under a variable temperature condition. Pyroxene and garnet fish and quartz ribbons are microstructures that record the temperature peak, under granulite facies in mylonites. Indeed, unshaped quartz grains and extremely lobated phase boundaries indicate the partial melting process.