HR: 1340h
AN: V23C-1552    [Abstracts]
TI: Unraveling Ages of Discrete Cryptic Events During Polyphase Orogenesis in SE Brazil
AU: * Brown, M
EM: mbrown@umd.edu
AF: Univ. of Maryland, Lab. for Crustal Petrology, Dept. of Geology, College Park, MD 20742, United States
AU: Reno, B L
EM: reno@umd.edu
AF: Univ. of Maryland, Lab. for Crustal Petrology, Dept. of Geology, College Park, MD 20742, United States
AU: Piccoli, P M
EM: piccoli@umd.edu
AF: Univ. of Maryland, Lab. for Crustal Petrology, Dept. of Geology, College Park, MD 20742, United States
AU: Trouw, R A
EM: rajtrouw@hotmail.com
AF: UFRJ, Dept. de Geologia, Cidade Universitaria, Rio de Janeiro, RJ, CEP 21949, Brazil
AB: The Neoproterozoic assembly of cratons in eastern Brazil is recorded by the successive Brasília, Ribeira and Buzios subduction-to-collision orogenic events in the intervals ca. 660-605 Ma (Reno et al., Frontiers in Mineral Sciences, abstract, 2007), ca. 580-550 Ma (Heilbron & Machado, Precambrian Res., 2003) and ca. 525- 505 Ma (Schmitt et al., Precambrian Res., 2004), respectively. The southern Brasília Belt displays characteristics of a passive margin in the east and an accretionary orogen in the west prior to terminal collision and final emplacement of the ENE-verging nappes. In the southern Brasília Belt, peak temperatures of 800-950 C are recorded, whereas in the Ribeira and Buzios Belts peak temperatures register around 780 C. Cryptic thermal overprinting by the younger events on the older orogenic belts is demonstrated by monazite age data from these Brasiliano-age belts in southeast Brazil. In the southern-most portion of the southern Brasília Belt, a SSE-plunging mineral elongation lineation and the occurrence of sillimanite represent macroscopic structural and petrologic evidence, respectively, of tectonic overprinting imposed during the Ribeira Orogeny. Comprehensive dating of monazite using the electron probe microanalysis method from throughout the southern-most Brasília Belt indicates widespread Ribeira-age (580-530 Ma) monazite growth, even tens of km north of the sillimanite-out isograd. The Buzios Belt lies outboard of the Ribera Belt; monazite from Ribera Belt migmatitic metapelites records Buzios-age growth. Monazite dates from the Ribeira belt determined using the electron probe microanalysis method were culled by chemical zone within individual grains. Exploratory data analysis (see Reno et al., Fall AGU, 2007) reveals distinct Ribeira-age (553-533 Ma) cores and Buzios-age (530-505 Ma) rims in monazite from rocks with no petrologic evidence of overprinting. Monazite grains likely grew during the Ribeira Orogeny, and were either partially recrystallized by dissolution-reprecipitation during overprinting by the Buzios Orogeny, or new monazite overgrew existing grains. In both the southern Brasília Belt and the Ribera Belt, monazite geochronology provides a record of cryptic overprinting by younger thermal events on older metamorphic terranes; this overprinting is neither recorded by the more robust U-Pb system in zircon, nor, in general is there a macroscopic record of new rock- forming mineral growth.
DE: 1040 Radiogenic isotope geochemistry
DE: 1115 Radioisotope geochronology
DE: 1140 Thermochronology
DE: 3660 Metamorphic petrology
DE: 8100 TECTONOPHYSICS
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting