Zircon (Re)crystallization During Metamorphism: Zircon Paragenesis in the Papua New Guinea Coesite Eclogite Revisited

Abstract

The youngest known ultrahigh-pressure (UHP) eclogite from Tumagabuna Island, a small island of the D'Entrecasteaux Islands chain in Papua New Guinea, exhibits zircon U–Pb age variations spanning a few million years. This age variation complicates the interpretation of the evolution of this high and ultrahigh-pressure ([U]HP) terrane. We investigate the discrepancies in zircon ages using SHRIMP U–Pb dating and same-spot REE abundance data. Petrographic observations reveal a main mineral assemblage of garnet, omphacite, amphibole, coesite, phengite, rutile and zircon in the eclogite. The formation of the main mineral assemblage is constrained by P–T pseudosection modelling. The main mineral assemblage likely crystallized from a melt at UHP conditions of p = 3.5 ± 0.2 GPa and T = 690°C ± 40°C. Two distinct generations of zircon are identified: an older group with ages ranging between 7.0 ± 0.2 and 7.9 ± 0.3 Ma and a younger group with ages ranging between 4.4 ± 0.3 Ma and 5.5 ± 0.4 Ma. These zircon populations differ in internal zoning and MREE and HREE contents. The initial zircon growth phase corresponds to the formation of the main mineral assemblage at UHP conditions, whereas the younger zircon generation likely resulted from Zr release during exhumation-related breakdown of the Zr-bearing UHP phases garnet, rutile and possibly omphacite.