RAPID EXHUMATION HISTORY OF THE (U)HP PAPUA NEW GUINEA TERRANE: INSIGHT FROM ZIRCON AND PSEUDOSECTION ANALYSIS

Ultrahigh-pressure (UHP) terranes document the subduction of crustal material to mantle depths and their exhumation back to shallow levels. 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 found with an actively rifting margin. To understand how the subduction and exhumation history compares among four of the gneiss domes, eclogites and multiple generations of melt have been dated using ID-TIMS geochronology, and preliminary pseudosection analysis has been applied to the eclogites. To date, coesite has only been documented in one PNG eclogite sample (Baldwin et al., 2008), whereas non coesite-bearing (HP) eclogite-facies assemblages are widespread. A pseudosection from an eastern dome suggests that rocks in its core preserve mineral assemblages that were stable at ~600 º C and ~26 kbar, just below the coesite-in reaction line. Uranium-Pb and trace-element results from many eclogite samples and two orthogneiss samples indicate that eclogite-facies conditions lasted until ca. 4.3 Ma. The crustal rocks then likely ascended rapidly as diapirs from mantle depths until they reached the base of the crust, where approximate neutral buoyancy was achieved. The near-isothermal decompression path followed by the diapirs drove extensive melting that likely began at high pressure and continued through amphibolite-facies retrogression and strong ductile extensional deformation at crustal depths. Strongly deformed leucosomes and sills record the earliest melt crystallization at ca. 3.8–2.8 Ma in the westernmost dome, 3.5–3.0 Ma in the central dome, and ca. 4.1 Ma in the easternmost dome. Weakly deformed dikes suggest that ponding and flattening of the partially exhumed terranes at the base of the crust where the rocks had stalled lasted until ca. 2.3 Ma in the west and until ca. 1.9 Ma in the east. By ca. 1.8 Ma, all of the rocks were emplaced as gneiss domes in the shallow crust due to upper-crustal extension. The buoyancy of melt along much of the exhumation path combined with upper-crustal and mantle extension likely aided in the rapid (ca. 4 Ma eclogite-facies conditions to ca. 2 Ma shallow-crustal conditions) exhumation of this extreme UHP terrane.