Age of the metamorphic sole of the Papuan Ultramafic Belt ophiolite, Papua New Guinea

The Papuan Ultramafic Belt (PUB) ophiolite is former oceanic crust and upper mantle emplaced onto continental crust in Papua New Guinea (PNG) in a zone of general convergence between the Pacific and Australian plates. The metamorphic sole beneath the ophiolite is best exposed in the Musa-Kumusi divide and comprises a 40- to 300-m-thick body of granulite and amphibolite facies rocks. Geochronological studies on the metamorphic sole, using amphiboles from the granulites and amphibolites, yield measured K-Ar ages ranging from 65.0±0.7 to 57.2±0.6 Ma and average ⁴⁰Ar-³⁹Ar direct total fusion ages ranging from 67.0±0.7 to 59.5±0.2 Ma. Five of the six ⁴⁰Ar-³⁹Ar plateau ages, derived from age spectra, lie between 58.6±0.2 and 57.8±0.2 Ma, with an overall mean age of 58.3±0.4 Ma. The large spread in measured K-Ar and ⁴⁰Ar-³⁹Ar total fusion ages is thought to be caused by the presence of variable amounts of excess argon. The mean plateau age for five samples of 58.3±0.4 Ma is interpreted to mark the time of cooling of the metamorphic sole following peak metamorphism. We suggest that the development of the metamorphic sole and emplacement of the PUB ophiolite onto the PNG crust occurred in a relatively short time interval in the Paleocene.