As lithospheric plates are subducted, rocks are metamorphosed under high-pressure and ultrahigh-pressure conditions to produce eclogites and eclogite facies metamorphic rocks. Because chemical equilibrium is rarely fully achieved, eclogites may preserve in their distinctive mineral assemblages and textures a record of the pressures, temperatures and deformation the rock was subjected to during subduction and subsequent exhumation. Radioactive parent-daughter isotopic variations within minerals reveal the timing of these events. Here we present in situ zircon U/Pb ion microprobe data that dates the timing of eclogite facies metamorphism in eastern Papua New Guinea at 4.3 +/- 0.4 Myr ago, making this the youngest documented eclogite exposed at the Earth's surface. Eclogite exhumation from depths of approximately 75 km was extremely rapid and occurred at plate tectonic rates (cm yr(-1)). The eclogite was exhumed within a portion of the obliquely convergent Australian-Pacific plate boundary zone, in an extending region located west of the Woodlark basin sea floor spreading centre. Such rapid exhumation (> 1 cm yr(-1)) of high-pressure and, we infer, ultrahigh-pressure rocks is facilitated by extension within transient plate boundary zones associated with rapid oblique plate convergence.

Metamorphic core complexes in the D'Entrecasteaux Islands, Papua New Guinea, formed as the result of active extension at the western end of the propagating Woodlark Basin spreading center. Domes of high-grade metamorphic rocks (i.e., amphibolites, eclogites, and migmatites), intruded by large granodiorite bodies, comprise the lower plate of the D'Entrecasteaux metamorphic core complexes. The domes are transected by kilometer-scale shear zones. A thermochronologic study of the D'Entrecasteaux Islands utilizing K/Ar, 40Ar/39Ar, and fission track techniques has documented the unroofing history of these active metamorphic core complexes. Gneisses in the cores of the domes cooled rapidly (>100øC/m.y.) as indicated by hornblende and biotite 40Ar/39Ar apparent ages of-2.7 to 3.0 Ma and -1.6 to 1.7 Ma, respectively, and apatite fission track ages of-0.4 to 0.9 Ma. 40Ar/39Ar apparent ages on white mica, biotite, and potassium feldspar and fission track ages on apatites from shear zone gneisses indicate extremely rapid cooling (in some cases >500øC/m.y.) and suggest shear zones were active from 4.0 to 3.5 Ma and 1.9 to 1.4 Ma. In general, 40Ar/39Ar mineral ages for retrogressed core zone gneisses, shear zone gneisses, and granodiorites are 2.0 to 3.0 Ma (amphiboles), 1.5 to 1.7 Ma (muscovites), and 1.4 to 1.8 Ma (biotites) and 1.0 to 2.0 Ma (K-feldspars). Apatite fission track ages from core zone gneisses, shear zone gneisses and granodiorites range from 0.4 to 1.0 Ma. Thermochronologic results indicate that emplacement of granodiorites closely coincided with retrogression of the metamorphic basement and movement on the outer shear zones bounding the gneiss domes. The granodiorite bodies associated with the D•Entrecasteaux Islands domes represent syn-kinematically emplaced granitoids intruded into

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"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' 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 7W 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.

The D'Entrecasteaux Islands in eastern Papua New Guinea are composed of a number of active metamorphic core complexes which have been intruded by granodiorite plutons during their formation. The plutons do not appear to have been intruded by diapiric processes as previously suggested. Late, relatively undeformed plutons form flat‐lying bodies which crosscut structural boundaries and are strongly discordant to core complex shear zones. Granodiorite magmatism and the development of the metamorphic core complexes have occurred in a linear zone which coincides with a zone of thick crust and rugged topography. It is proposed that plutonism facilitated deformation in ductile extensional shear zones which resulted in tectonic exhumation of deep crustal rocks and formation of the metamorphic core complexes. The source of the plutons is thought to be related to a linear zone of mantle upwelling beneath the islands related to the propagation of the Woodlark seafloor spreading center into continental crust. It is suggested that a localized heat source of this type, which can provide heat and magmatic material to the crust, is essential for the development of metamorphic core complexes.