Adakites from collision‐modified lithosphere

Abstract

Adakitic melts from Papua New Guinea (PNG) show adakitic geochemical characteristics, yet their geodynamic context is unclear. Modern adakites are associated with hot‐slab melting and/or remelting of orogenic mafic underplate at convergent margins. Rift‐propagation over collision‐modified lithosphere may explain the PNG adakite enigma, as PNG was influenced by rapid creation and subduction of oceanic microplates since Mesozoic times. In a new (rift) tectonic regime, decompressional rift melts encountered and melted remnant mafic eclogite and/or garnet‐amphibolite slab fragments in arc collisional‐modified mantle, and partially equilibrated with metasomatized mantle. Alternatively, hot‐slab melting in a proposed newborn subduction zone along the Trobriand Trough could generate adakitic melts, but recent seismic P‐wave tomographic models lack evidence for subducting oceanic lithosphere in the adakite melt region; however they do show deep subduction zone remnants as a number of high P‐wave anomalies at lithospheric depths, which supports our proposed scenario.