TIMESCALES OF PARTIAL MELTING AND UHP EXHUMATION: CA-TIMS RESULTS FROM MELT FRACTIONS EXPOSED IN THE D’ENTRECASTEAUX ISLANDS, PAPUA NEW GUINEA

Studies of ultrahigh-pressure (UHP) terranes reveal eclogite hosted within migmatitic gneiss. When present, melt will have significant rheological implications for the deformation and exhumation of UHP terranes. The UHP terrane of southeastern Papua New Guinea (PNG) exposes Pliocene eclogites and abundant evidence for partial melting within domal structures across Goodenough, Fergusson, and Normanby Islands. Zircons from eclogite suggest UHP metamorphism began by ca. 5 Ma, and Ar-Ar thermochronology indicates exhumation of the rocks to the surface by ca. 2 Ma. Determining the relative timing and relationship of the migmatitic gneisses to (U)HP eclogites is crucial for resolving how crustal rocks are subducted to mantle depths and subsequently exhumed; therefore, zircon was extracted from a variety of eclogites, leucosomes, and dikes from all three islands for U-Pb CA-TIMS analyses to resolve the timing of HP metamorphism and melt crystallization. To the east of the UHP locality, eclogites from Mailolo and Oitabu domes yield ages of ca. 4.3 Ma and ca. 4.6 Ma, respectively. Eclogite from Goodenough dome appears to be an outlier yielding an age of ca. 2.7 Ma. Results from the UHP locality and other parts of the western Fergusson Island dome reveal melt-present deformation in the form of foliation-parallel leucosomes with crenulated margins that formed from ca. 3.5–3.0 Ma and weakly-deformed cross-cutting dikes at ca. 2.4 Ma. In comparison, zircons from a similarly deformed foliation-parallel leucosome within Goodenough dome to the west reveal melt crystallization from ca. 4.7–2.7 Ma. The youngest igneous rocks are felsic non-deformed intrusions which contain zircons as young as ca. 1.8 Ma. Finally, in the south, Normanby dome records host orthogneiss development by ca. 5.9 Ma and emplacement of deformed granodiorite sills by ca. 4.1 Ma. Zircons from non-deformed felsic and mafic dikes yield similar crystallization ages to Goodenough Island, with zircons as young as 1.8 Ma. Domal history shows peak metamorphism of eclogites occurred at ca. 5 Ma followed by emplacement within the upper crust across the entire PNG (U)HP terrane by ca. 1.8 Ma. The results indicate that exhumation of the PNG (U)HP terrane was assisted by multiple stages of partial melting that increased the overall buoyancy of the subducted crustal body.