Microstructures and quartz lattice‐preferred orientations in the eclogite‐bearing migmatitic gneisses of the D'Entrecasteaux Islands, Papua New Guinea

Abstract

[1] Lattice preferred orientations (LPO) of quartz in gneiss domes of the D'Entrecasteaux Islands, Woodlark Rift shed insight into exhumation of the world's youngest (~5-7 Ma) eclogite-bearing terrane at cm/yr rates. We focus on deformation that affected the terrane as it transited between lower crustal depths and the surface, including: (1) grain-scale deformation mechanisms; and (2) style of flow and mode of emplacement of the domes. Electron-backscatter diffraction was used to analyze microstructure and LPOs of 37 quartzofeldspathic gneiss samples that enclose meter-scale mafic blocks preserving original eclogite-facies assemblages. During exhumation of the ultrahigh-pressure (UHP) terrane, gneisses were retrogressed in the amphibolite facies at lower crustal depths. The LPOs change from dome cores to carapaces, consistent with decreasing deformational temperatures. In the relatively chilled outer carapaces of the domes, the quartz LPOs consist of mostly crossed-girdle [c]-axis patterns, with some cleft-girdle and small-circle LPOs, and record dislocation creep accommodated by mixed-< a > slip. In the cores of the migmatitic domes, a chessboard pattern of subgrains is common, and quartz LPOs primarily record prism-[c] slip, probably at >630 C. Other microstructures indicate recovery by high-temperature grain-boundary migration. Grain-boundary mobility was anisotropic, leading to strong grain-shape fabrics oblique to foliation, but not obviously relatable to shear sense. Evidence for melt-present deformation is abundant, and microstructures (including partially dissolved feldspar grains) indicate some deformation by fluid-assisted grain-boundary diffusion creep. LPOs in carapace rocks are symmetrical, recording flow that was dominantly coaxial. We interpret the gneiss domes to have been emplaced into the rift as partially molten diapirs.Components: 18,942 words, 13 figures, 1 table.

Continental breakup and UHP rock exhumation in action: GPS results from the Woodlark Rift, Papua New Guinea

“…In the diapir model, buoyant rise of a previously subducted fragment of continental crust occurs at the onset of rifting. Results from geodynamic modeling of the diapir process [ Ellis et al ., ] agree well with structural geological evidence for vertical ductile thinning (pure shear) in the gneiss domes and ductile flow perpendicular to the plate motion direction [ Little et al ., ]. The geodynamic models also produce localization of extension both off the axis of the rising UHP bodies and within the area of UHP exhumation and gneiss dome emplacement, which agrees well with our GPS results requiring extension in the area of UHP exhumation, as well as on the off‐axis Ma'aiu Fault.…”

Tracking the exhumation of a Pliocene (U)HP terrane: U-Pb and trace-element constraints from zircon, D'Entrecasteaux Islands, Papua New Guinea

“…Seismic velocity measurements from the D'Entrecasteaux Islands reveal anomalously low‐density mantle supporting the thinned crust, a relationship that suggests that extension has taken place in both the crust and lithospheric mantle within the Woodlark Rift [ Abers et al ., ]. In addition, crustal extension is expressed by active normal faults within the Woodlark basin [ Abers , ] and by the exposure of the east‐west series of gneiss domes in the D'Entrecasteaux Islands [ Hill et al ., ; Little et al ., ].…”

“…Evidence for melting occurs in both zones and is ubiquitous throughout all structural levels of the domes; however, the percentage of leucosomes and dikes increases toward the dome centers from ∼15% in the carapace to as much as ∼70% in the structurally deepest exposures of the core zone [ Gordon et al ., ]. In most cases, partial melting, and intrusion of granitoid dikes are interpreted to have been coeval with ductile deformation in the core of the domes [ Little et al ., ]. Microstructures include prism‐[c] slip lattice‐preferred orientations, chessboard subgrain structure in quartz, melt‐filled strain shadows, and shape‐preferred orientations of euhedral, magmatic feldspars.…”

“…Microstructures include prism‐[c] slip lattice‐preferred orientations, chessboard subgrain structure in quartz, melt‐filled strain shadows, and shape‐preferred orientations of euhedral, magmatic feldspars. All these microstructures suggest high‐temperature deformation in the presence of intergranular melt [ Little et al ., ].…”

Paleomagnetic evidence for vertical‐axis rotations of crustal blocks in the Woodlark Rift, SEPapua New Guinea: Miocene to present‐day kinematics in one of the world's most rapidly extending plate boundary zones

“…The Woodlark Rift presents an opportunity to investigate rift migration, localization, and continental breakup, in a rapidly extending, active setting that hosts examples of geologically significant and exciting phenomena, such as continental breakup ahead of a propagating oceanic spreading center [ Taylor et al ., ], and exhumation of Neogene‐aged UHP rocks [ Little et al ., ]. New analyses of high‐resolution multichannel seismic reflection data [ Fitz and Mann , ]; new structural and geochronologic data [ Daczko et al ., ; Gordon et al ., ; Little et al ., ]; and new high‐quality GPS data for the rift [ Wallace et al ., ] allow us to integrate our results into a robust, interdisciplinary tectonic framework that is emerging for this rift.…”