HR: 0800h
AN: T41C-1211    [Abstracts]
TI: Thermochronologic and Geochemical Investigations of High Pressure Metamorphism and Pliocene Exhumation of the D'Entrecasteaux Islands, Southeastern Papua New Guinea
AU: * Monteleone, B D
EM: bmontele@syr.edu
AF: Dept. of Earth Sciences, Syracuse University, Syracuse, NY 13244 United States
AU: Baldwin, S L
AF: Dept. of Earth Sciences, Syracuse University, Syracuse, NY 13244 United States
AU: Fitzgerald, P G
AF: Dept. of Earth Sciences, Syracuse University, Syracuse, NY 13244 United States
AU: Little, T A
AF: School of Earth Sciences, Victoria University of Wellington, Wellington, 6000 New Zealand
AU: Webb, L E
AF: Dept. of Earth Sciences, Syracuse University, Syracuse, NY 13244 United States
AB: Evidence for high pressure (HP) metamorphism in eastern PNG exists as variably retrogressed eclogites and blueschists in the lower plates of the D'Entrecasteaux Island (DI) metamorphic core complexes (MCCs). Eclogites from Fergusson and Goodenough Islands record P-T conditions up to 24 kbar, 930$\deg$C and 21 kbar, 890$\deg$C, respectively. In situ U-Pb analyses on zircons from these variably retrogressed eclogites yielded ages from $\sim$7.9 to 4.3 Ma on Fergusson Island, and from 2.9 to 2.1 Ma on Goodenough Island. Zircon analyses from each sample yielded what is interpreted to be a single age population containing no inherited zircons. In-situ trace and rare earth element (REE) analyses on adjacent zircon and garnet from one Fergusson Island sample suggest that zircons grew at 4.3 $\pm$ 0.4 Ma at depths of $\sim$75 km, documenting zircon growth under HP metamorphic conditions and exhumation at plate tectonic rates (cm/yr). Additional trace and REE analyses will determine if 7.9-2.1 Ma zircons grew prior to, during, or subsequent to eclogite facies metamorphism and will further constrain P-T-t-D paths followed by these eclogites. The Prevost Range of eastern Normanby Island, the easternmost of the DI, is a Pliocene metamorphic core complex exposing greenschist and blueschist facies metamorphic rocks from shallower structural levels than Goodenough and Fergusson Islands. Rocks exhumed from the deepest structural levels, located in the NE Prevost Range, are retrogressed blueschists which record pressures from 6 to 6.5 kbar. $^{40}$Ar/$^{39}$Ar step heat experiments on white mica from the overprinting retrograde fabric on a blueschist sample yield a weighted mean age of 3.03 $\pm$ 0.08 Ma, giving a minimum age for blueschist facies metamorphism and documenting the timing of white mica growth and/or cooling below closure temperature during MCC exhumation. Step heat analyses on white mica from a calc-silicate in the NW Prevost Range yield a complex spectrum with apparent ages from 7 to 4 Ma, reflecting either partial thermal resetting of white mica or multiple white mica growth stages during metamorphism. $^{40}$Ar/$^{39}$Ar analyses on plagioclase within greenschists from shallower structural levels give complex age spectra with apparent ages from $\sim$100 to 2.4 Ma, suggesting that protolith ages were preserved or only partially reset during metamorphism preceding Pliocene exhumation.
DE: 8109 Continental tectonics--extensional (0905)
DE: 8150 Plate boundary--general (3040)
DE: 3660 Metamorphic petrology
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting