⁴⁰Ar/³⁹Ar constraints on obduction of the Seram ultramafic complex: Consequences for the evolution of the southern Banda Sea

On Kaibobo (W Seram), obduction of hot oceanic lithosphere produced high-grade metamorphism and granite in overthrust continental crust. ⁴⁰Ar/³⁹Ar plateau ages of 5.65-6.0 Ma and 5.4 Ma of muscovite and biotite from the sole, among the youngest so far measured on single crystals, indicate complete resetting of the Ar-isotope system. Chloritized biotite crystals gave disturbed spectra and younger ages. Single fusion on biotite from the granite gave an age of 5.5 Ma. Time limits for the obduction-exhumation history of the Kaibobo complex and sole are approximated by simple 1D forward P-T-t modelling, constrained by PT data and ⁴⁰Ar/³⁹Ar thermochronology (c. 400°C at 6 Ma; c. 320°C at 5.5 Ma). Best fit results indicate that the decelerating post-emplacement exhumation of ophiolite and sole began ≤ 8 Ma ago. The rate during the first 1 Ma of this uplift exceeded 7.5 mm a^-1. Undoing 8 Ma of migration in the SW Pacific and Australian regimes of motion back-tracks Kaibobo to the site where obduction-emplacement ended: near the southeastern corner of the Banda Sea plate. Similar compositions and time-settings of the Kaibobo and N Timor ophiolites lead us to postulate that, during the Early Miocene, slow-rate spreading occurred in the oceanic lithosphere of the southern Banda Sea, south of the current volcanic arc.