DECIPHERING THE MULTI-STAGE EXHUMATION OF A UHP TERRANE FROM MANTLE DEPTHS TO THE SURFACE IN ~3 MYR: THE IMPORTANCE OF COMBINING DETAILED METAMORPHIC PETROLOGY WITH HIGH RESOLUTION ISOTOPIC ANALYSES (Invited Presentation)

Ultrahigh-pressure (UHP) terranes represent a rare opportunity to investigate how pressure-temperature-deformation conditions evolve within subduction zones, the interactions of the subducting plate with the overlying mantle wedge, and the transfer of mass and fluids from the shallow to the deep Earth. The Papua New Guinea UHP terrane is unique for two main reasons: 1) it is the youngest (Pliocene) known UHP terrane on Earth; and 2) it is being exhumed within multiple east–west domal structures along an actively rifting margin. Combining metamorphic petrology with detailed, high-resolution isotopic and geochemical analyses allows us to document the exhumation of this small UHP terrane from mantle depths to Earth’s surface. Zircon from fresh eclogite records peak metamorphism until ca. 5.2 Ma at UHP conditions of ~27–31 kbar and ~715 ºC; the zircon contain inclusions of the peak assemblage (omphacite, garnet, rutile, phengite) with the same composition as the matrix phases used in P-T pseudosection modeling. Partially overprinted eclogite contain zircon associated with retrogression phases (symplectite, amphibole) that record initial exhumation from ca. 4.6–4.3 Ma; these zircon host inclusions of crystallized hydrous melts that indicate partial melting during this exhumation phase. Melt crystallization within lower-grade eclogite–amphibolite-facies rocks occurred from ca. 2.9–2.6 Ma. REE analyses from zircon, garnet, and plagioclase suggest that the zircon (re)crystallized in the presence of garnet and melt in all the retrogressed samples. Non-deformed dikes and plutons record emplacement of the domes within the upper crust by ca. 1.8 Ma. Similar oxygen- and hafnium-isotope zircon values, δ¹⁸O = 6.7–6.3 ‰ and ε_Hfi= 12.3–6.6, from the fresh and retrogressed eclogites indicate that the fluid composition remained unchanged throughout the exhumation path from the mantle to crustal depths. Overall, detailed measurements and observations of zircon inclusions and zircon textural location within the bulk rock combined with high-precision geochronology, high-resolution isotopic analyses, and metamorphic petrology extensively document the entire pressure-temperature-time-deformation path and the fluid and melt history of mafic crustal rocks during their ~3 Myr exhumation to the surface.