PHASE DIAGRAM CALCULATIONS OF KYANITE-BEARING ECLOGITES FROM OIATABU DOME, D’ENTRECASTEAUX ISLANDS, PAPUA NEW GUINEA

The deep burial of crustal material and subsequent exhumation to the surface results in the formation of ultrahigh-pressure (UHP) terranes. In southeastern Papua New Guinea (PNG), Pliocene (U)HP eclogites and migmatitic host gneiss are exposed in domal structures across the D’Entrecasteaux Islands. In comparison to older (U)HP terranes, PNG is unique in that it resides within an active rift basin, and is not overprinted by later tectonic activity, thus preserving a record of its formative geodynamic setting. A detailed understanding of the pressure-temperature (P-T) conditions at which these (U)HP eclogites equilibrated is crucial for determining the tectonic evolution of this unique (U)HP terrane. Prior classical thermobarometry studies of eclogites from three of the domes, Goodenough, Mailolo, and Oiatabu, reveal a wide range of P-T estimates from ~12–27 kbar and ~530–930 ºC, including estimates from the only known coesite-bearing eclogite from Mailolo Dome (Davies and Warren, 1992; S. Baldwin et al., 2004, 2008). Preliminary phase diagrams have been calculated with the program PerpleX for Oiatabu eclogites, using the peak-pressure assemblages garnet + omphacite + kyanite ± phengite + quartz + rutile. A kyanite-phengite eclogite yields modal proportions and compositional isopleths for garnet, omphacite, and phengite that constrain near-UHP conditions at 24–26 kbar and 590–610 ºC, in good agreement with classical thermobarometry results (26–27 kbar and 625–650 ºC; Ravna and Terry, 2004). An additional kyanite eclogite sampled ~3 km to the south yields lower P-T estimates of ~16–22 kbar and ~600–650 ºC; this eclogite contains amphibole, some of which may be interpreted as part of the peak assemblage. The eclogites likely followed a near-isothermal decompression path (e.g., Davies and Warren, 1992). Replacement of kyanite in both eclogites by sapphirine– and spinel–plagioclase symplectites broadly constrains later retrogression to conditions of 6–11 kbar and ~650 ºC (i.e., J. Baldwin et al., 2015). In comparison to P-T results from (U)HP eclogites exposed ~30 km to the northwest in Mailolo Dome (18–27 kbar and 650–930 ºC), phase diagram calculations and classical thermobarometry suggest similar high pressures, but a lower temperature (≤650 ºC) history for the Oiatabu kyanite-bearing eclogites.