HR: 14:50h
AN: V33E-05 INVITED    [Abstracts]
TI: Late Miocene coesite-eclogite exhumed in the Woodlark Rift, Papua New Guinea
AU: * Baldwin, S L
EM: sbaldwin@syr.edu
AF: Dept. of Earth Science, Syracuse University, 204 Heroy Geology Laboratory, Syracuse, NY 13244, United States
AU: Webb, L E
EM: lewebb@syr.edu
AF: Dept. of Earth Science, Syracuse University, 204 Heroy Geology Laboratory, Syracuse, NY 13244, United States
AU: Monteleone, B D
EM: Brian.Monteleone@asu.edu
AF: Dept. of Earth Science, Syracuse University, 204 Heroy Geology Laboratory, Syracuse, NY 13244, United States
AB: One of the most exciting frontiers in the field of continental dynamics concerns the formation and exhumation of ultrahigh-pressure (UHP) rocks. The youngest exhumed UHP rock known on Earth was recently discovered in the lower plate of the D'Entrecasteaux Islands metamorphic core complexes within the Woodlark Rift of Papua New Guinea. This coesite-eclogite preserves a record following subduction to mantle depths (> 90 km). Unraveling its subduction and exhumation history requires establishing direct links between mineral assemblages and their isotopic ages, and "seeing through" the effects of possible overprinting thermal events caused by rifting, and propagation of the Woodlark Basin seafloor spreading system. A range of thermochronologic and petrologic techniques was used to unravel the P-T-t history of coesite-eclogite, including in situ ion microprobe techniques, and new thermometers based on [Zr] in rutile and [Ti] in zircon. Results indicate zircon crystallized at 650-675° C, below the closure temperature for Pb diffusion in zircon, at 7.9 ± 1.9 Ma. Rutile thermometry, using the calibration of Tomkins et al. (2007) that accounts for the pressure effect on the [Zr] in rutile, yielded temperatures of 695-743° C. These new thermometers provide more precise temperatures than those obtained previously using Grt-Cpx Fe-Mg exchange thermometers. Results suggest that zircon grew at a lower temperature than rutile, or that a pressure effect, not yet documented, exists for the [Ti] in zircon thermometer. During exhumation from depths > 90 km, the coesite-eclogite was partially retrogressed under amphibolite- facies conditions, pegmatite formed in strain shadows surrounding its amphibolite rind, and its quartzofeldspathic host underwent partial melting. ⁴⁰Ar/³⁹Ar muscovite ages from the pegmatite indicate that by 3.5 Ma temperatures had decreased to < 500° C. Apatite fission track data indicate that the eclogite host had cooled to below 120° C by 0.6 Ma. The integrated data set suggests an increase in apparent cooling rate (i.e., ~35° C/m.y. from 8 Ma to 3.5 Ma, ~135° C/m.y. from 3.5 Ma to present) during rapid (cm/yr) exhumation from UHP depths to the surface. Exhumation of Late Miocene coesite-eclogite within the Woodlark Rift requires reconsideration of the geological and tectonic evolution of the entire region to assess whether rifting has reactivated a former subduction thrust as a normal fault detachment system that is slipping in the direction of Woodlark-Australian plate motion.
DE: 1100 GEOCHRONOLOGY
DE: 1140 Thermochronology
DE: 3652 Pressure-temperature-time paths
DE: 3654 Ultra-high pressure metamorphism
DE: 8110 Continental tectonics: general (0905)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting