Metamorphic modeling and geothermobarometry of trace elements in metapelites and quartzites: example of Luminárias Nappe-MG

The Luminárias Nappe is a 40 km long, NNE-SSW elongated structure, located in the southern portion of the Neoproterozoic Brasília Orogen, which borders the São Francisco Craton (Minas Gerais, Brazil). It is composed of high aluminium metapelites and quartzites from the Carrancas Group. The present work focuses on the metamorphic characterization of the metapelites and the quartzites of the Luminárias Nappe by means of pseudosection modelling, mineral chemistry and the Zr-in-rutile thermometer. In the northern portion, the paragenesis is Cld + Chl + Ky + Rt + Qtz + Ms. In the center-north portion, the paragenesis is St + Grt + Rt + Qtz + Ms, with retro-metamorphic biotite, chlorite and ilmenite. The metamorphic peak assembly recorded in rocks from the southern portion is Grt + Ky + St + Rt + Qtz + Ms with retro-metamorphic biotite, chlorite and ilmenite. Results indicate the presence of a metamorphic gradient with conditions increasing from green-schist facies in the northern portion (560˚C and 10kbar) and center-north (610˚C and 12,5kbar) to amphibolite / eclogite facies in the southern portion (630˚C and 15kbar). Metapelitic rocks of the Luminárias Nappe followed a clockwise P-T-t path, characterised by an initial heating stage that is followed by strong decompression. Analyses of trace elements in rutile grains derived from quartzite indicate that they can be used for temperature calculation using the geothermometer Zr in rutile. However, the data shows that the homogenisation, or reequilibration, of the Zr content detrital rutile in quartzites occurs at higher temperatures than in the metapelites, at about 580˚C. No sillimanite has not been described in the studied rocks. Therefore, it suggests that the Interference Zone between the Brasília and Ribeira belts does not extent to the Luminárias Nappe region. In addition, our modelling shows that it would be require an isothermal decompression of approximately 8 kbar for the crystallization of sillimanite, which is incompatible with the overlapping of the Ribeira Belt over the Brasilia Belt, which would lead to an even greater burial of the units. (AU)