HR: 16:45h
AN: V34C-04    [Abstracts]
TI: Evidence from U-Pb geochronology, mineral rare-earth element chemistry and trace element thermometry for short-lived ultrahigh-temperature metamorphism in the Anápolis-Itau\c{c}u Complex, Southern Brasília Belt
AU: * Baldwin, J A
EM: jbaldwin@mso.umt.edu
AF: Department of Geosciences, University of Montana, Missoula, MT 59812, United States
AU: Brown, M
EM: mbrown@geol.umd.edu
AF: Department of Geology, Laboratory for Department of Geology, Laboratory for Crustal Petrology University of Maryland, College Park, MD 20742, United States
AB: Integrated petrologic, chemical, and geochronologic studies of zircon, garnet, and rutile from ultrahigh- temperature (UHT) granulites in the Anápolis-Itau\c{c}u Complex, central Brazil, are used to constrain the significance of U-Pb zircon dates. REE analysis of zircon cores indicate positive-sloping HREE patterns, commonly inferred to be magmatic, whereas unambiguous metamorphic grains and overgrowths have flat to slightly negatively-sloping HREE patterns. D_REE(zrc/grt) partition coefficients in two samples investigated indicate equilibrium of zircon with cores of garnet, and progressive enrichment of garnet in HREE towards the rim. Ti-in-zircon crystallization temperatures are consistent with growth during both the prograde and post-peak evolution, and not at UHT conditions. Maximum Zr-in-rutile crystallization temperatures of inclusions armored in garnet are around 980°C, indicating growth of this phase near peak conditions indicated by independent calculated phase equilibria. Other populations of rutile linked to reaction microstructures developed during retrograde metamorphism record crystallization temperatures of 850-820°C. Rutile that is intergrown with ilmenite included within orthopyroxene and associated with exsolved zircon records temperatures around 760°C, indicating equilibrium with exsolved zircon. ID-TIMS U-Pb geochronological data from two of the four UHT granulite samples investigated define upper intercept ages of 641.3 ± 8.4 Ma (MSWD = 0.91) and 638.8 ± 2.5 Ma (MSWD = 1.03) that correlate with distinct periods of zircon growth near peak to immediately post-peak. Individual zircon U-Pb dates retrieved from all samples range from 649 to 634 Ma, indicating a maximum duration of approximately 15 Myr for the prograde to post-peak evolution of this terrain. This event is interpreted to record modest thickening of a backarc located behind the Arenópolis Arc at the western edge of the São Francisco Craton consequent upon terminal collision of the Parána Block with the arc during the amalgamation of Gondwana.
DE: 1115 Radioisotope geochronology
DE: 3625 Petrography, microstructures, and textures
DE: 3652 Pressure-temperature-time paths
DE: 3656 Ultra-high temperature metamorphism
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting