Blueschist emplacement in the Sepik Headwaters region, Papua New Guinea: field relations, petrology, isotopic dating and tectonic setting

The Tau Blueschist in the headwaters of the Sepik River in north-central New Guinea occurs in an allochthonous east–west lens (55 × 8 km) known as the Tau body. The well-foliated Tau mafic schists contain abundant blue amphibole with some intimately associated pelitic–calcareous–graphitic metasediments. Within the Tau body, the metamorphic grade increases towards the north, from lawsonite rocks to epidote blueschists. Pumpellyitic lawsonitic metabasites are known only from river float. Isolated occurrences of coarsely crystalline mafic tectonic blocks (knockers) occur within and just north of the Tau body and comprise a complete spectrum from epidote blueschist to eclogite that includes amphibolite. The blueschists occur within the upper Mesozoic to Eocene Salumei Formation of prehnite–pumpellyite to low-greenschist facies grade. The Salumei Formation comprises a tectonic mixture of Upper Cretaceous mostly pelitic sediments derived from the Australian continent to the south and Eocene ophiolite fragments and arc-related volcanogenic rocks from the north. The region is dominated by west-to-northwest-trending folds, observed in most outcrops, and high-angle faults. The mesoscopic structures and steep dips of bedding and foliation in pre-Oligocene rocks suggest regional-scale isoclinal folding. Regionally, the Frieda and Fiak-Leonard Schultze faults separate the Salumei Formation (and Tau body) to the south from amphibolite and greenschist grade Ambunti Metamorphic rocks to the north; both may have the same protolith. The Ambunti Metamorphics, the Oligocene diorites intruding them and the Tau blueschists all rapidly cooled from ∼500 °C in the late Oligocene (<i>ca</i> 27–23 Ma). The Ambunti Metamorphics and Oligocene diorites are unconformably overlain by unmetamorphosed middle Miocene Wogamush Formation. Subsequent quartz diorite intrusions into the southernmost Ambunti and Salumei Formation metamorphics and the lower Wogamush Formation volcanics was part of the middle Miocene (<i>ca</i> 15 Ma) Maramuni Arc. We propose that the Sepik Terrane was separated from New Guinea in the Late Jurassic by a narrow backarc basin that was starved of sediment until the Late Cretaceous when the Salumei delta prograded across it. Paleogene subduction dipping northwards beneath the Sepik Terrane led to late Eocene to early Oligocene suturing to the New Guinea margin and partial exhumation of an accretionary wedge including the Tau Blueschist and serpentinised mantle slivers. Late Oligocene extension and diorite intrusion were associated with rapid cooling and exhumation of the metamorphic rocks. Subduction flipped and the middle Miocene Maramuni Arc and Wogamush Formation were emplaced followed by collisional tectonism with the Melanesian Arc. This placed the margin into compression and created the modern orogen.