Thermochronologic constraints on the tectonic evolution of active metamorphic core complexes, D'entrecasteaux Islands, Papua New Guinea

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 (≥100°C/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 >500°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, 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 an area of active continental extension.