TIMESCALES OF (U)HP METAMORPHISM: U-PB AND TRACE-ELEMENT CONSTRAINTS FROM ZIRCON, D’ENTRECASTEAUX ISLANDS, PAPUA NEW GUINEA

Ultrahigh-pressure (UHP) terranes represent crustal material that has been subducted to great depths and subsequently exhumed to the surface. In order to decipher the mechanisms involved, the relative timing and relationship of the (U)HP eclogites to their host rocks must be well characterized. In southeastern Papua New Guinea (PNG), domal structures found on three islands, including from west to east, Goodenough, Fergusson, and Normanby Islands, expose (U)HP eclogites within migmatitic host gneiss. The PNG (U)HP terrane is unique in that it lies within a rift basin, and is not exposed as a result of later tectonic activity, thus preserving its original geodynamic setting. Studies of zircon from coesite-eclogite within Mailolo Dome of western Fergusson Island, suggest that UHP metamorphism began by ca. 5.6 Ma and final extension-driven exhumation occurred at ca. 1.8 Ma. To further constrain the evolution of the PNG (U)HP terrane, zircons were analyzed by U-Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS) geochronology from eclogites exposed in the Mailolo and Oitabu Domes (Fergusson Island) and the Goodenough Dome (Goodenough Island). Eclogite collected to the northeast of the only known coesite-locality, within Mailolo Dome, yield zircon dates of ca. 4.3 Ma. On eastern Fergusson Island in Oitabu Dome, another eclogite yields zircons that crystallized at ca. 4.6 Ma. On Goodenough Dome, eclogites yield much younger dates of ca. 2.8–2.6 Ma. Trace-elements were measured in dissolved zircon from Oitabu and Mailolo eclogites and have flat HREE slopes (Yb/Gd = 1.11–0.51) and no negative Eu anomalies (Eu/Eu* = 0.74–0.93), consistent with metamorphic zircon that (re-) crystallized under eclogite-facies conditions. Goodenough Dome zircon REE patterns are more variable and may reflect zircon growth during retrograde amphibolite-facies conditions. Across the PNG (U)HP terrane, the oldest zircon ID-TIMS dates for eclogite are recorded within Mailolo and Oitabu Domes at ca. 5.6–4.3 Ma, whereas the youngest dates occur within the Goodenough Dome at ca. 2.8–2.6 Ma. Exhumation of the Mailolo and Oitabu Dome eclogites from mantle depths to the surface occurred over ~3–4 m.y., whereas Goodenough Dome eclogites may record equilibration at lower crustal conditions prior to being exhumed over a ~1 m.y. period.