HR: 16:00h
AN: V54B-01 INVITED     [Abstracts]
TI: Metamorphism and exhumation of the youngest known HP/UHP terrane on Earth, eastern Papua New Guinea
AU: * Baldwin, S L
EM: sbaldwin@syr.edu
AF: Dept. of Earth Science, Syracuse University, Syracuse, NY 13244 United States
AU: Webb, L E
EM: lewebb@syr.edu
AF: Dept. of Earth Science, Syracuse University, Syracuse, NY 13244 United States
AU: Monteleone, B D
EM: bmontele@syr.edu
AF: Dept. of Earth Science, Syracuse University, Syracuse, NY 13244 United States
AU: Little, T A
EM: timothy.little@vuw.ac.nz
AF: School of Earth Sciences, Victoria University of Wellington, Wellington, 6000 New Zealand
AU: Fitzgerald, P G
EM: pgfitzge@syr.edu
AF: Dept. of Earth Science, Syracuse University, Syracuse, NY 13244 United States
AU: Chappell, J L
EM: jlchappe@syr.edu
AF: Dept. of Earth Science, Syracuse University, Syracuse, NY 13244 United States
AB: Variably retrogressed Late Miocene-Pliocene HP/UHP rocks occur in the exhumed lower plates of the D'Entrecasteaux Islands (DI) metamorphic core complexes (mccs) in eastern Papua New Guinea (PNG). There, HP/UHP exhumation occurred in a rapidly extending (>2 cm/yr) region where the Woodlark Basin seafloor spreading system (sfs) terminates into a zone of rifting. The evolution of this complex plate boundary zone involved northward subduction of the Australian continental margin beneath oceanic lithosphere, a collision that led to HP/UHP metamorphism of Jurassic-Cretaceous sediments and basalts, and southward obduction of oceanic crust and mantle (Papuan Ultramafic Belt). P-T-t-D data reveal the variability in paths followed by rocks as they were subducted and subsequently exhumed via top-to-the-north normal-sense motion on kilometer scale mylonitic shear zones. In situ ion probe analyses on zircons and garnets yielded trace and REE patterns indicative of coeval growth under eclogite facies conditions and ²⁰⁶Pb/²³⁸U zircon ages ranging from 7.9-2.8 Ma. In western Fergusson Island a 7.9 Ma eclogite contains coesite, recognized petrographically as an inclusion in omphacite, and confirmed by Raman spectroscopy. Garnet - cpx - phengite barometry (~26 kbar) underestimates peak pressures required by the presence of coesite. Thermochronology (⁴⁰Ar/³⁹Ar, fission track and (U-Th)/He) on host felsic gneisses record rapid cooling during exhumation at average rates of >1 cm/yr, and indicate lower plate rocks reached shallow crustal levels by ~1.5 Ma. On Goodenough Island, in situ ion probe analyses on zircon inclusions in garnet from retrogressed eclogite lenses within host felsic gneisses yielded ²⁰⁶Pb/²³⁸U ages of 2.8-2.9 Ma. Zircon/garnet REE patterns support an interpretation of coeval growth at (retrograde?) HP conditions. ²⁰⁶Pb/²³⁸U ages for the youngest zircon population from felsic host gneisses are concordant with in situ zircon analyses from retrogressed eclogite lenses suggesting that both were metamorphosed together. The easternmost DI, Normanby Island, located ~30 km from the active tip of the sfs system, exposes retrogressed blueschists in the deepest structural levels of a Plio-Pleistocene mcc. ⁴⁰Ar/³⁹Ar ages (3.0 Ma) on retrograde white mica provide a maximum age for rapid cooling and exhumation of this mcc footwall from depth. Based on regional patterns of stretching lineations and transfer fault systems, decreasing metamorphic grade from north to south, and the occurrence of the PUB in the upper plates of eastern PNG mccs, we infer that rifting has reactivated a former subduction thrust as a normal fault detachment system that is slipping in the direction of Woodlark-AUS plate motion. Extension in the DI mccs and the westward propagation of the sfs rift tip along the former plate boundary and into previously subducted crust may be kinematically linked to CCW rotation of the Woodlark microplate. Calculations based on tectonic reconstructions suggest that microplate rotation can drive subduction eversion and exhumation of rocks from >100 km depths.
DE: 1140 Thermochronology
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3652 Pressure-temperature-time paths
DE: 3654 Ultra-high pressure metamorphism
DE: 8109 Continental tectonics: extensional (0905)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005