RUTILE CRYSTALLIZATION TEMPERATURE IN A GARNET-KYANITE GNEISS FROM THE SOUTHERN BRASÍLIA OROGEN

Zirconium-in-rutile thermometry was applied to a garnet-kyanite gneiss from the Southern Brasilia Orogen to retrieve its metamorphic temperature conditions. The gneiss sample BR5-25 was collected from the Andrelândia nappe system in a quarry near Três Pontas, State of Minas Gerais, Brazil. The specimen contains the mineral assemblage of garnet, quartz, biotite, and kyanite along with minor rutile and accessory monazite and zircon. Rutile occurs both as inclusions in garnet and as matrix mineral. The concentration of zirconium in rutile was determined using the Cameca SX5 Tactis electron microprobe at the University of Massachusetts Amherst. The determined Zr concentration in rutile varies from 74 to 2476 ppm, with a single matrix rutile showing exceptionally high Zr contents up to 9930 ppm. Five calibrations of the Zr-in-rutile thermometry by Zack et al. (2004; Z04), Watson et al. (2006; W06), Ferry & Watson (2007; FW07), Tomkins et al. (2007; T07), and Kohn (2020; K20) were utilized to calculate the crystallization temperatures of rutile. The result shows that the estimated temperatures are consistent for three calculations FW07, T07 and K20, with the calculated average temperature for rutile ranging from 765 – 785 °C assuming 13 kbar pressure if necessary. The calibration W06 yields a lower average temperature at 570 °C and Z04 a higher average at 895 °C. The average temperature for rutile in garnet is calculated to be 770 – 790 °C using the three consistent calibrations and Zr concentration of 638 – 1593 ppm. The same calibrations yielded average calculated temperatures of 760 – 780 °C for matrix rutile, but display a large temperature span from 520 to 1100 °C. A further investigation of core and rim domains of rutile shows that temperatures from core to rim fall mostly within the range of errors (± 30 °C). These estimated temperatures are consistent with previous calculated temperatures for samples from the region. As demonstrated in previous studies, our study shows that Zr-in-rutile is a robust thermometry for estimating metamorphic temperatures. When applying, rutile from different texture settings should be examined. We also recommend using multiple Zr-in-rutile calibrations for calculating temperatures, but the results of calculations with FW07, T07 and K20 are consistent.