Abstract

Metamorphic core complexes in the D'Entrecasteaux Islands are forming as a result of active extension at the western end of the Woodlark Basin spreading center. High-grade metamorphic rocks exist in the cores of gneissic domes, including eclogites formed at minimum depths of 45-75 km (i.e., 13-21 kbar). High temperatures were maintained during uplift of basement rocks from 13-21 kbar (T ≈ 730-900 °C) through 7-11 kbar (T ≈ 570-730 °C) to ∼5-6 kbar (T ≈ 730 °C). Subsequent rapid cooling (>>100 °C/m.y.) of the basement occurred between 1 and 2 Ma (on the basis of 40Ar/39Ar geochronology). The core-zone rocks were transected by kilometre-scale, amphibolite-facies ductile shear zones active at depths of 25-35 km. Extensive intrusion of granodiorite plutons occurred, and the age of granodiorite intrusions constrains late-stage shear-zone activity to ∼2 Ma and younger. Unroofing was accomplished by faults and shear zones bounding the gneiss domes, juxtaposing them against ultramafic rocks and sedimentary deposits. Surface uplift continues today and has led to topographic elevations of up to 2.5 km.

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