40Ar/39Ar dating of white micas from the epidote to the omphacite zones, northern New Caledonia: tectonic implications

An 40Ar/39Ar study of white micas from high-pressure metamorphic rocks of northern New Caledonia yielded cooling ages of 37 ± 1 Ma for both epidote and omphacite zone samples. Whole-rock samples from the lawsonite zone yielded ages in the range 44–51 Ma with complicated age spectra, probably reflecting both detrital and newly grown micas. The areal extent of the mica samples, over 300 km2, suggests that the epidote and omphacite zone rocks cooled through the muscovite closure temperature, about 350 °C, as a coherent cooling unit. Simple thermal modeling suggests that these rocks could have closed at similar times if the unroofing rate were greater than 2–10 mm∙a−1. Lawsonite zone rocks occur structurally within about 0.5 km of garnet–omphacite rocks, suggesting the possibility of major postmetamorphic tectonic displacement.