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Título

MINERAL CHEMISTRY OF MAGNETITE GRANITE IN THE PELOTAS TERRAIN, DOM FELICIANO BELT, RS

Texto do resumo

The Magnetite Granite occurs in Morro Redondo and Monte Bonito region, part of the Pelotas Terrain, southeastern of Dom Feliciano Belt. Its occurrence is associated to monzogranites with singular characteristics compared to adjacent lithologies, marked by presence of magnetite mega crystals. It is recognized as a lithology component of Pinheiro Machado Complex (PMC), older unity of the Pelotas Terrain, related to the Pinheiro Machado Orogeny. However, the Magnetite Granite (MG), at ~610 Ma, probably represents a PMC (~635-610Ma) younger crystallization event. In this sense, this search seeks to analyze the mineral chemistry from the MG to investigate crystallization parameters in the geologic context. The methodology to it was performed electron microprobe analyzes on two representative samples at UFRGS laboratory and, the mineral data were processed using the software GCDKit mineral. It was analyzing minerals both in MG matrix and associated to magnetite mega crystals assembly. The matrix mineralogy is composed by quartz, feldspar, biotite, chlorite, magnetite, epidote, titanite, calcite and ilmenite. The magnetite mega crystals are bordered for the most part by titanite, muscovite, biotite and plagioclase. The matrix essential elements chemical analyzes, classify feldspar in sanidine, orthoclase, albite and oligoclase. Primary biotite exhibits a chemical composition typically of the annite. These biotite are interpreted as derived from crustal material, crystallized from a calc-alkaline magma, type-I granites with temperatures around of 528º C. Oxides mega crystals analyses, indicated pure magnetite bordered by ilmenite, which also occurs as small inclusions. The ilmenite was destabilized and partially replaced by titanite in the magmatic stage. The silicate mineralogy associated to mega crystals, here denominated as post-magmatic, is composed by albite, oligoclase predominantly and rare orthoclase. The biotite in the magnetite borders displays Fe/(Fe + Mg) ratios ranging from 0.5 to 0.65, along with an Al_IV content of 1.2, indicating a tendency towards annite composition. The chlorite composition was characterized as Fe-chlorite. The presence of titanite both in the border and in equilibrium with magnetite, of the same composition, demonstrates that the prevailing conditions during the crystallization are relatively oxidizing. These characteristics suggest that the post-magmatic/hydrothermal assembly of the MG crystallized under moderately oxidizing conditions close to Nickel-Nickel-Oxide (NNO) buffer with average temperature around of 436º C. Based on this information, the mineral chemistry indicates oxygen fugacity changing conditions, probably caused by magnetite mega crystal crystallization associated to hydrothermal activity. Under these conditions, the Magnetite Granite suggests a metallogenic potential in the studied area.

Palavras Chave

Magnetite Granite; mineral chemistry; crystallization parameters; type-I granite.