PARTIAL MELTING AND THE EXHUMATION OF A UHP TERRANE: CA-TIMS RESULTS FROM THE D'Entrecasteaux ISLANDS, PAPUA NEW GUINEA

Within ultrahigh-pressure (UHP) terranes, some eclogites are surrounded by migmatitic gneisses; in some cases, textures and cross-cutting relationships indicate multiple generations of crystallized partial melt. Although numerous investigations have focused on understanding how crustal material is subducted to mantle depths and returned to the surface, the timing and role of the melt generation in the context of UHP terrane evolution is incompletely understood. Of all UHP terranes, the D’Entrecasteaux Islands, Papua New Guinea (PNG), expose the youngest eclogites as well as the best evidence for partial melting in the form of migmatites and plutonic bodies. The (U)HP metamorphism occurred in PNG from ca. 8 to 2 Ma, as recorded in eclogites of two domal complexes that were exhumed and cooled through ~300 ºC by 3.5 to 1.6 Ma, respectively. The relatively young and short-lived history recorded in the PNG UHP terrane is ideal for U-Pb zircon geochronology. Zircon from five leucocratic samples and two eclogites were analyzed using single-crystal CA-TIMS analyses to deconvolve, with the highest precision possible, the timing of melting relative to (U)HP metamorphism and decompression/exhumation. The zircons were typically <150 µm in long dimension with low U and radiogenic Pb contents, ranging from 0.05 to 4.9 picograms. From Fergusson Island, the location of 8 Ma coesite-bearing eclogite, zircon ages from a strongly deformed granodioritic dike are interpreted to date partial melting during decompression ~3–4.5 Myr after UHP metamorphism. In comparison, exposures on nearby Goodenough Island, which contains 2 Ma eclogites, reveal multiple episodes of partial melting: 1) a strongly deformed granodioritic leucososome yields dates indistinguishable from the HP metamorphism; 2) a second strongly deformed granodioritic boudin yields dates ~100,000 yrs younger; and 3) a weakly deformed granodiorite body contains zircons that crystallized ~1.5 Myr after HP metamorphism. Overall, the CA-TIMS results indicate that melt was present at the time of HP metamorphism as well as during decompression and exhumation of the eclogites to the surface. Exhumation rates are at least 1.1 ± 0.1 cm/yr and were most likely driven by buoyancy generated first by melting at HP conditions and then by melting during decompression.