MAGMATISM AS AN ESSENTIAL DRIVING-FORCE FOR FORMATION OF ACTIVE METAMORPHIC CORE COMPLEXES IN EASTERN PAPUA-NEW-GUINEA

The D'Entrecasteaux Islands in eastern Papua New Guinea are composed of a number of active metamorphic core complexes which have been intruded by granodiorite plutons during their formation. The plutons do not appear to have been intruded by diapiric processes as previously suggested. Late, relatively undeformed plutons form flat-lying bodies which crosscut structural boundaries and are strongly discordant to core complex shear zones. Granodiorite magmatism and the development of the metamorphic core complexes have occurred in a linear zone which coincides with a zone of thick crust and rugged topography. St is proposed that plutonism facilitated deformation in ductile extensional shear zones which resulted in tectonic exhumation of deep crustal rocks and formation of the metamorphic core complexes. The source of the plutons is thought to be related to a linear zone of mantle upwelling beneath the islands related to the propagation of the Woodlark seafloor spreading center into continental crust. It is suggested that a localized heat source of this type, which can provide heat and magmatic material to the crust, is essential for the development of metamorphic core complexes.