Tectonometamorphic evolution of the Anicuns-Itaberaí sequence, southern Brasília Orogen: evidence of deep burial and syn-collisional exhumation

Abstract

Metamorphic pressure–temperature (P–T) conditions in the Southern Brasília Orogen are relatively well con- strained, particularly in its southernmost segments. However, the central and northern portions need further attention, especially regarding the metamorphism affecting the volcano-sedimentary sequences of the Arenopolis ́ Magmatic Arc (AMA). The Anicuns–Itaberaí Sequence (AIS), located in the southern segment of the AMA, comprises a well-exposed metavolcano-sedimentary succession of mafic to ultramafic metavolcanic rocks, met- achert, metarhythmite, marble lenses and quartzite. Well-preserved outcrops in the lowlands surrounding the Serra da Jiboia region, located in the central portion of the study area, were used to investigate the structural features and deformation recorded in metasedimentary rocks of the AIS and two samples of metagreywackes were selected for metamorphic modelling. The analyzed rocks correspond to garnet–chlorite–biotite–muscovite schists and were selected for microstructural analysis, quantitative mineral mapping, and thermodynamic modelling based on electron microprobe analyses. The results indicate that the peak metamorphic assemblage (garnet + biotite + plagioclase + muscovite + rutile + ilmenite) was stable at temperatures above 600 ◦C and pressures exceeding 0.95 GPa. The metamorphic evolution, inferred from compositional zoning in garnet, defines a clockwise P–T path, with initial garnet growth at 525–555 ◦C and 0.67–0.77 GPa, a metamorphic peak at 660 ◦C and 1.16–1.30 GPa, followed by decompression between 550 and 585 ◦C and 0.65–0.79 GPa. These results indicate that the AIS schists reached high-pressure amphibolite-facies conditions, exceeding those typical of the Barrovian metamorphism, and point to a collisional tectonic regime characterized by deep crustal burial and subsequent exhumation. Deformation structures were chronologically classified into four distinct phases (D1 to D4). The main deformation event (D2) is characterized by asymmetric folding, while subsequent reverse faulting and shearing (D3) are associated with post-burial exhumation during syn-collisional processes. Micro- structural evidence suggests that the metamorphic peak likely occurred between the end of D1 and the onset of D2, indicating that peak mineral growth was broadly synchronous with early stages of the main deformation phase and burial.