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

Abstract

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.