EXHUMATION OF HIGH-PRESSURE METAMORPHIC ROCKS DURING CRUSTAL EXTENSION IN THE DENTRECASTEAUX REGION, PAPUA-NEW-GUINEA

The D'Entrecasteaux Islands of eastern Papua New Guinea consist of a number of active metamorphic core complexes formed under an extensional tectonic setting related to sea-floor spreading in the west Woodlark Basin. The complexes are defined by mountainous domes (>2500 m high) of fault-bounded, high-grade metamorphic rocks (including eclogite facies) intruded by 2-4-Ma granodiorite plutons. Garnet-clinopyroxene exchange thermometers indicate that the temperature of equilibration of the eclogites was 730-900-degrees-C. The jadeite component of omphacite indicates minimum pressure of 21 kbar, suggesting depths of >70 km. The metamorphic rocks have undergone widespread retrogression to amphibolite facies. Retrogression of the metamorphic basement is associated with shearing and formation of the metamorphic core complexes. P-T conditions in the early stages of shear zone activity, determined using the garnet-biotite exchange thermometer and the GASP and GRIPS barometers, were 570-730-degrees-C and 7-11 kbar. A second phase of re-equilibration at much lower pressures appears to be related to the widespread intrusion of granodiorite plutons. One re-equilibrated gneiss indicated maximum temperature of 730-degrees-C at estimated pressures of approximately 4 kbar. This late, high-temperature metamorphism is also indicated by reactions involving the production of hercynite and corundum in aluminous gneisses and formation of sillimanite at the expense of kyanite. Two major episodes of granodiorite intrusion occurred during uplift and exhumation of the core complexes. Both closely coincide spatially with high-temperature metamorphic rocks, the onset of deformation in extensional shear zones and subsequent uplift of the metamorphic basement. These observations indicate a fundamental link between uplift and granodiorite intrusion during continental extension and the formation of the D'Entrecasteaux Islands metamorphic core complexes.