PRESSURES AND TEMPERATURES OF HP AND UHP METAMORPHISM IN THE D'ENTRECASTEAUX ISLANDS, SOUTHEASTERN PAPUA NEW GUINEA

The youngest known eclogites (8 – 2 Ma) at the Earth's surface have been exhumed within the lower plates of the D'Entrecasteaux Island metamorphic core complexes in southeastern Papua New Guinea. While ages of eclogite-facies metamorphism require cm/yr exhumation rates, the absence of phengite + kyanite within these variably-retrogressed eclogites has frustrated efforts to obtain precise and accurate P-T estimates for HP/UHP metamorphism. Variable pressure estimates are obtained using albite – jadeite – quartz barometry (20 – 24 kbar), garnet – cpx – phengite barometry (18 – 26 kbar) (Ravna and Terry, 2004), and the jadeite component of pyroxene (> 13 kbar for T > 600ºC). However, the occurrence of coesite in one sample and TiO₂ exsolution in garnet rims suggest UHP conditions were attained in some samples. Temperature estimates based on Fe – Mg partitioning in garnet and pyroxene vary by up to 300ºC within individual samples depending on the assumption of Fe³⁺/Fe²⁺ in omphacite, and are thus not reliable. In light of these difficulties, we applied new thermometers based on [Ti] in zircon and [Zr] in rutile (Watson et al., 2006; Zack et al., 2004) to seven samples, three of which had zircon of suitable size for ion probe analyses. Zircon and rutile thermometry (Watson et al., 2006) yielded temperatures ranging from 611 – 880ºC, with temperature estimates from multiple analyses within individual samples varying by < 150ºC. Zircon – rutile temperature estimates overlap for two samples, and are discordant by < 30ºC for the third. Rutile analyses for seven samples yielded systematically higher (up to 130ºC) temperature estimates using Zack et al. (2004), but both thermometers yielded increasing temperatures with decreasing sample age from 8 – 3 Ma, consistent with a documented increase in geothermal gradient during that time. Zircon and rutile thermometry may be particularly useful for many eclogites lacking phengite, kyanite, and constraints for Fe³⁺/Fe²⁺ in pyroxene.