MONAZITE PETROCHRONOLOGY OF A GARNET-BEARING GNEISS FROM SOUTHERN BRASÍLIA OROGEN

Monazite petrochronology has become an important tool to infer metamorphic and tectonic evolution of orogenic processes. In this study, we examined a garnet-kyanite gneiss from the Tres Pontas area, southern Brasília Orogen using phase equilibrium modeling and monazite petrochronology to determine its metamorphic pressure-temperature-time (P-T-t) path and infer tectonic processes. The garnet-kyanite gneiss consists of garnet, kyanite, biotite, plagioclase, quartz, rutile and ilmenite, with accessory monazite and carbonate. Garnet is millimeter to centimeter in size, subhedral to anhedral in shape and commonly contains quartz, rutile, biotite, and kyanite inclusions. Garnet displays a flat compositional zoning, with the Mg# (=Mg/(Mg+Fe)) of 0.23 and X_Ca (=Ca/(Ca+Fe+Mg+Mn)) of 0.03 in the core, and Mg# of 0.23 and XCa of 0.04 in the rim. Biotite is brown and subhedral to anhedral and defines the foliation of the specimen. Kyanite occurs as large and smaller anhedral porphyroblast, oriented parallel to the foliation. Plagioclase and rutile occur both as a mineral both in the matrix and inclusion in garnet. Isochemical phase equilibrium modeling using PERPLE_X along with garnet isopleth thermobarometry yielded a prograde P-T path, from 1.4 GPa, 600 °C in the field with plagioclase, garnet, white mica, biotite, clinopyroxene, kyanite, quartz, ilmenite, and rutile, to 1.0 GPa, 780 °C in the field of plagioclase, garnet, white mica, biotite, kyanite, quartz, ilmenite, with minor melt. The determined peak temperature is consistent with the temperature estimates using Zr-in-rutile thermometers by Sanders et al. (2024). Wavelength dispersive spectrometer mapping of monazite indicates at least three distinctive domains, low-Y and high-Th, high-Y and low-Th, and low-Y and higher Th, which is consistent with garnet breakdown during heating and exhumation. Microprobe monazite dating of these domains extends from 647 ± 1, to 624 ± 2 and 618 ± 2 Ma. These results suggest that the sample and thus the thrust nappe was heated as they exhumed to shallower crust level in the southern Brasília orogen, with likely heat from radiogenic and conducting heating.