Variation of Ti-in-quartz in gneiss domes exposing the world's youngest ultrahigh-pressure rocks, D'Entrecasteaux Islands, Papua New Guinea

Authors

  • Megan Korchinski,

    1. School of Geography, Environment, and Earth Sciences, Victoria University of Wellington, Wellington 6040, New Zealand
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  • Timothy A. Little,

    1. School of Geography, Environment, and Earth Sciences, Victoria University of Wellington, Wellington 6040, New Zealand
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  • Euan Smith,

    1. School of Geography, Environment, and Earth Sciences, Victoria University of Wellington, Wellington 6040, New Zealand
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  • Marc-Alban Millet

    1. School of Geography, Environment, and Earth Sciences, Victoria University of Wellington, Wellington 6040, New Zealand
    2. Now at Origins Laboratory, Department of the Geophysical Sciences, Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
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Abstract

[1] The D'Entrecasteaux Island gneiss domes, Papua New Guinea, expose the world's youngest ultrahigh-pressure rocks (5–8 Ma), which were exhumed nearly isothermally to the mid-crust at rates of cm yr−1. Because quartz grains are internally distorted by deformation bands and have strongly interlobate grain boundary microstructures indicative of rapid grain-boundary migration, we infer that Ti content in the quartz was frozen in by dynamic recrystallization that accompanied the continued deformation of the gneisses as they were exhumed through the crust toward the surface. We infer that average Ti content of an assemblage of quartz grains in a sample depends on the ratio of cooling rate to dynamic recrystallization rate. An increase in the former preserves and captures a larger fraction of relatively high-Ti, high-temperature grains, whereas an increase in the latter removes such relict grains and replaces them with a newer set of lower-Ti, low-temperature recrystallized grains. Based on this concept, we use Ti-in-quartz as a tool to infer domains of differing relative cooling rate across four D'Entrecasteaux Island domes. Ti concentrations in quartz grains from 89 samples of quartzofeldspathic gneiss and eclogite increase inwardly away from the dome margins (2.5–20 ppm) toward the center of the domes (20–>100 ppm). From this, we infer that the most rapidly cooled rocks were exhumed near the center of the domes. Our data reinforces structurally based arguments that the symmetric domes were emplaced vertically into the crust as diapirs, rather than laterally exhumed as a result of large-magnitude slip on dome-bounding detachment faults.

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