Magmatism as an essential driving force for formation of active metamorphic core complexes in eastern Papua New Guinea

Abstract

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. It 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.

Exhumation of (ultra-)high-pressure terranes: concepts and mechanisms

“…Many continental UHP terranes show evidence for migmatization in the mafic eclogite bodies as well as their host rocks, suggesting bulk weakening on a regional scale: see, for example, data from the Western Gneiss Region in Norway (Labrousse et al, 2002(Labrousse et al, , 2011, the Woodlark Basin (Hill et al, 1995;Gordon et al, 2012), the Dabie Sulu region (Wallis et al, 2005), the Kokchetav Massif (Ragozin et al, 2009) and the Greenland Caledonides (Lang and Gilotti, 2007). These studies suggest that conditions suitable for partial melting during the attainment of peak conditions or during exhumation are commonly experienced by UHP terranes.…”

Exhumation processes

“…Hill et al 1995)? Many of the arguments that concern the sustainability of topography and the maintenance of steady-state geotherms, critically depend on the time constants of the processes involved.…”

Woodlark Basin

“…Metamorphic core complexes on the D'Entrecasteaux Islands and in the Suckling-Dayman massif on the Papuan Peninsula were also exhumed in the Pliocene/Pleistocene (Davies, 1980;Davies and Warren, 1988;Hill, 1990). The best structural studies, geothermometry, geobarometry, and age dating of these complexes have been done on Goodenough and Fergusson Islands (Davies and Warren, 1992;Hill et al, 1992Hill et al, , 1995Hill and Baldwin, 1993;Baldwin et al, 1993;Lister and Baldwin, 1993;Hill, 1994;Baldwin and Ireland, 1995). There, normal movement along a 0.3-to 1.5-km-thick ductile mylonitic shear zone resulted in the uplift of deep metamorphic rocks and the juxtaposition of unmetamorphosed cover rocks.…”

“…2) show that (A) they are associated with Pliocene/Pleistocene granodiorite intrusions and amphibolite-facies ductile shear zones, (B) they have been rapidly exhumed from ~30 km depth (7-11 kilobar [kb]) in the last 4 m.y., (C) uplift continues (forming topography up to 2.5 km), and (D) they are very three dimensional and regionally discontinuous (or varying in grade) along strike (Davies andWarren, 1988, 1992;Hill, 1987Hill, , 1990Hill, , 1994Hill, , 1995Hill et al, 1992Hill et al, , 1995Hill and Baldwin, 1993;Baldwin et al, 1993;Lister and Baldwin, 1993;Baldwin and Ireland, 1995 Duncan, pers. comm., 1993;Walker and McDougall, 1982) and P4 (late Paleocene) foraminifer-bearing micrites overlying the basalts with tonalite-diorite-dacite intrusions K/Ar dated at 57-47 Ma (Rogerson et al, 1993).…”