THERMOCHRONOLOGICAL CONSTRAINTS ON THE TECTONIC EVOLUTION OF ACTIVE METAMORPHIC CORE COMPLEXES, DENTRECASTEAUX ISLANDS, PAPUA-NEW-GUINEA

Metamorphic core complexes in the D'Entrecasteaux Islands, Papua New G uinea, formed as the result of active extension at the western end of the propagating Woodlark Basin spreading center. Domes of high-grade m etamorphic rocks (i.e., amphibolites, eclogites, and migmatites), intr uded by large granodiorite bodies, comprise the lower plate of the D'E ntrecasteaux metamorphic core complexes. The domes are transected by k ilometer-scale shear zones. A thermochronologic study of the D'Entreca steaux Islands utilizing K/Ar, Ar-40/Ar-39, and fission track techniqu es has documented the unroofing history of these active metamorphic co re complexes. Gneisses in the cores of the domes cooled rapidly (great er-than-or-equal-to 100-degrees-C/m.y.) as indicated by hornblende and biotite Ar-40/Ar-39 apparent ages of approximately 2.7 to 3.0 Ma and approximately 1.6 to 1.7 Ma, respectively, and apatite fission track a ges of approximately 0.4 to 0.9 Ma. Ar-40/Ar-39 apparent ages on white mica, biotite, and potassium feldspar and fission track ages on apati tes from shear zone gneisses indicate extremely rapid cooling (in some cases > 500-degrees-C/m.y.) and suggest shear zones were active from 4.0 to 3.5 Ma and 1.9 to 1.4 Ma. In general, Ar-40/Ar-39 mineral ages for retrogressed core zone gneisses, shear zone gneisses, and granodio rites 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 fiss ion track ages from core zone gneisses, shear zone gneisses and granod iorites range from 0.4 to 1.0 Ma. Thermochronologic results indicate t hat emplacement of granodiorites closely coincided with retrogression of the metamorphic basement and movement on the outer shear zones boun ding the gneiss domes. The granodiorite bodies associated with the D'E ntrecasteaux Islands domes represent syn-kinematically emplaced granit oids intruded into an area of active continental extension.