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Título

GARNET-KYANITE-SILIMANITE BEARING METAPELITE OF THE SOUTHERN SECTOR OF THE PORONGOS COMPLEX, DOM FELICIANO BELT, BRAZIL

Texto do resumo

The Dom Feliciano Belt records the Neoproterozoic (Tonian to Cambrian) geodynamic evolution, from Rodinia break-up to the amalgamation of southwestern Gondwana in the southern region of Brazil. The Porongos Complex is a fold and thrust foreland belt that comprises metavolcanic and metasedimentary rocks, with complex deformational and metamorphism conditions from greenschist to upper amphibolite facies. The stratigraphy and metamorphic zoning of this complex require additional information related to petrography, structural mapping and P-T conditions, especially due to the strong influence of shear zones, thus it is necessary to better characterize these rocks. Therefore, the objective of this study is to characterize a metapelitic rock from the southern part of the Porongos Complex, specifically its petrological, mineral chemistry and microstructural evidences. The studied metapelite has well-developed schistosity marked by lepidoblastic, granoblastic, nematoblastic, and porphyroblastic textures oriented during the last shear ductile event. Its mineralogy is represented by muscovite, quartz, tourmaline, chlorite, garnet, kyanite, K-feldpar, monazite, rutile and silimanite. Muscovite crystals are up to 1.8mm in size, and presents sigmoid shapes. The quartz grains occur in ameboid shapes with dynamic recrystallization marked by grain boundary migration, forming quartz ribbons. The garnet is intensely fractured, with quartz inclusions and filled with iron oxide, and locally chlorite, making it difficult to obtain the compositional profile. Kyanite porphyroblasts have sizes of up to 2.5 mm, concordant with the foliation. Acicular to needle-like sillimanite crystals occurs locally, "cutting" through the kyanite, representing an unstable phase. Monazite grains represent an accessory metamorphic phase. The tourmaline (dravite) occurs predominantly euhedral, within aggregates concordant and discordant to foliation. The chlorite (chamosite) occurs with a strong association with the tourmaline phase and completely replaces the biotites, with precipitation/association of rutile, ilmenite, and locally Fe-gahnite. K-feldspar is frequently found in SEM analysis and occurs near chlorites, but its genesis is not yet understood. Muscovite + quartz + garnet + kyanite + sillimanite correspond to the paragenesis of progressive metamorphism (S2). Tourmaline and chlorite is apparently associated with a shear zone influence (Sm), and fluids are sourced by proximal intrusions of Chacará São Jerônimo Granite. The S2 foliation is concordant with the Sm foliation, or has a low angle between the porphyroblasts of kyanite and mica-fish, tourmaline and chlorite. One brittle event is recorded by fracturing with Fe-oxide percolation. Evidence of upper intermediate P-T conditions was registered through the presence of kyanite porphyroblasts, formed during colisional settings, needle-like crystals of sillimanite is related to decompression path, and by the use of relative geothermometer in quartz (GBM). The presence of tourmaline, sigmoidal muscovite, and chlorite indicates an overprint of foliation and greenschist facies metamorphism. Therefore, this study can help in better understanding the metamorphic conditions of the Porongos Complex.

Palavras Chave

Porongos Complex; Metapelite; shear zones