FLUIDS RELATED TO ENDOSKARN FORMATION PROCESSES IN THE GUANHÃES AREA, MINAS GERAIS STATE, BRAZIL

MARIA LOURDES SOUZA FERNANDES, KAZUO FUZIKAWA, JOSÉ MARQUES CORREIA NEVES, ESSAID BILAL, CRISTINA MARIA WIEDEMANN LEONARDOS

Resumo


The Borrachudos Suite consists of a group of A-type granitic bodies intruding Archean and Paleoproterozoic rocks outcropping in the central part of Minas Gerais State. Their age range from 1.77 to 1.67 Ga and the emplacement is related to the opening of the Espinhaço rift. On the eastern border of Morro do Urubu plutonite which is part of the Suite and crops out for an extension of 2 km to the southeast of Guanhães town, occurs a greenish crystalline rock. A continuous transition from Borrachudos granite to these rocks can be observed. Five different alteration stages of transformation are defined on the basis of mineral suites, mineral and total rock chemistry. These features allowed the interpretation of these rocks as endoskarnites. Their microscopy showed the presence of areas with abundant fluid inclusions (FI) in quartz. FI distribution, their morphological and compositional characteristics are the same in all five stages, except that the frequency drops rapidly from stage I to V. Compositionally the inclusions are carbonic, aqueo-carbonic, and aqueous and they are all associated in parallel planes. Either in the first or second types of FI the carbonic phases present similar microthermometric behavior and values: Th(CO2)=+27.0 to +29.5ºC and Tm(CO2)=-56.7 to -57.6ºC. The latter indicates an almost pure CO2 which was confirmed by micro-Raman spectroscopy. The former defines the 0.7 to 0.75g/ cc isochores, which at 800ºC temperature, calculated from amphibole crystallization at magmatic stage as the upper limit for the metassomatic process, indicate a pressure of 3.0 to 3.5 kbar. This range of pressure points to the process having occurred at maximum depth of roughly 10 km. The monophase aqueous inclusions presented Tm(ice)=-1.3ºC indicating a solution of low salinity (~2.2wt% NaCl equivalent) which might be even lower as this Tm is dependent on the pressure developed when the fluid is frozen. The three types of inclusions are interpreted as products of post-formational changes of a homogeneous aqueo-carbonic fluid trapped above the solvus of H2O-CO2-NaCl(<~2.6wt%) system. The occurrence of deuteric carbonates and their dissolution by fluids from granites may have been the source of CO2 in metasomatized rocks.

Palavras-chave


Late Paleoproterozoic anorogenic granites; Borrachudos granite; Endoskarn formation; Fluid inclusions.

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