Short-lived subduction and exhumation in Western Papua (Wandamen peninsula): Co-existence of HP and HT metamorphic rocks in a young geodynamic setting
Résumé
Understanding processes of formation of accretionary wedges requires studying young and well preserved examples. The Lengguru wedge (West Papua) is younger than 10 Myr and is a result of oblique and fast subduction (11 cm/yr) of the Bird's Head (a part of the Australian margin beneath the Melanesian Arc). Thus, the rapid formation of this wedge (actually a Fold and Thrust Belt) may be attributed to a single tectonic event. High pressure (HP) metabasic and metasedimentary rocks have been discovered in the core of the wedge in the Wandamen peninsula. Locally, these rocks are overprinted by migmatization during the decompression. Field relationships indicate that migmatites and anatectic leucogranite crosscut the HP metamorphic rocks. This paper reports characterization of petrology and thermobarometry of metasediments and metabasic samples, all included in a former mélange-type rock association, as well as geochronology of metasediments. Pressure–Temperature (P–T) conditions highlight two stages of metamorphism within a small time bracket. In metasediments, a first stage of relatively HP (~ 13–17 kbar) is observed as attested by a first paragenesis of garnet–kyanite–phengite. Garnet, kyanite and phengite are then destabilized in favor of biotite during the decompression (5–12 kbar and > 550 °C). Late fractures filled with chlorite and a second generation of white mica crosscut the foliation (< 7 kbar and 500–600 °C). In metabasic rocks, the peak of pressure is reached from 17 to 23 kbar and from 700 to 800 °C. Geochronological study was conducted using U–Pb dating (LA-MC-ICPMS) on zircons from the metasedimentary and migmatitic rocks. Some zircon cores often present high Th/U ratio typical for magmatic origin, as confirmed by trace element signature of metasediments, and recorded ages older than 300 Ma, witness of the activity of a volcanic arc on the former margin of the Australian craton. Most other zircons present a low Th/U ratio attesting their metamorphic origin, confirmed by inclusions of kyanite. The metamorphic zircons are dated between 5.61 ± 0.04 Ma and 8.1 ± 1.1 Ma. These very young metamorphic ages attest for a rapid subduction and exhumation event.