Timing of UHP exhumation and rock fabric development in gneiss domes containing the world's youngest eclogite Facies rocks, southeastern Papua New Guinea

The youngest known ultrahigh-pressure (UHP) rocks in the world occur in the Woodlark Rift of southeastern Papua New Guinea. Since their crystallization in the Late Miocene to Early Pliocene, these eclogite facies rocks have been rapidly exhumed from mantle depths to the surface and today they remain in the still-active geodynamic setting that caused this exhumation. For this reason, the rocks provide an excellent opportunity to study rates and processes of (U)HP exhumation. We present New Rb-Sr results from 12 rock samples from eclogite-bearing gneiss domes in the D'Entrecasteaux Islands, and use those results to examine the time lag between (U)HP metamorphism and later ductile thinning, penetrative fabric development and accompanying metamorphic retrogression at amphibolite facies conditions during their exhumation. A Rb-Sr age for a sample of mafic eclogite (with no preserved coesite) from the core zone of the Mailolo gneiss dome (Fergusson Island) provides a new estimate of the timing of HP metamorphism (5.6 ± 1.6 Ma). The strongly deformed quartzofeldspathic and granitic gneisses (90-95% by volume) that enclose variably retrogressed relict blocks of mafic eclogite (5-10% by volume) yield Rb-Sr isochron ages from 4.4 to 2.4 Ma. For the UHP-bearing gneisses of Mailolo dome, previously published U-Pb ages on zircon and our Rb-Sr isochron ages are consistent with a mean time lag of 2.2 ± 1.5 Ma (~95% c.i.) for passage of the rock between eclogite and amphibolite facies conditions. New thermobarometric data indicate that the main syn-exhumational foliation developed at amphibolite facies conditions of 630-665 °C and 12.1-14.4 kbar. These pressure estimates indicate that the lower crust of the Woodlark Rift was unusually thick (>40 km) at the time of the amphibolite facies overprint, possibly as a result of accumulation and underplating of UHP-derived material from below. Our data imply a minimum unroofing rate of 10 ± 7 mm year^-1 (~95% c.i.) for the (U)HP body from minimum HP depths (73 ± 7 km) to lower crustal depths. This minimum unroofing rate reinforces previous inferences that the exhumation from the mantle to the surface of the gneiss domes in the D'Entrecasteaux Islands took place at plate tectonic rates. On the basis of previous structural studies and the new thermobarometry, we attribute the high (cm year^-1) exhumation to diapiric ascent of the partially molten terrane from mantle depths, with a secondary contribution from pure shear thinning of the terrane after its arrival in the crust. Copyright