The Assembly of UHP Terranes: Was the Western Gneiss Region Built By Sequential or Repeated (Ultra)High-Pressure Events?

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Understanding the formation and exhumation of ultrahigh-pressure rocks continues to be one of the outstanding tectonic questions of our time because of the impact such processes have had on the exchange of material between the crust and mantle, the generation and collapse of mountain belts, the formation and processing of continental crust, and tectonic plate motions. A major step forward will be to understand how ultrahigh-pressure orogens are constructed: by subduction of continental margins, subduction of microcontinents, intracontinental subduction or another process?This project tests whether ultrahigh-pressure orogens form through repeated subduction of the same tectonic unit or through sequential subduction of different units, by examining a specific example, the Western Gneiss Region of Norway. Following careful petrology, zircon genesis is assessed by optical and back-scattered electron petrography, cathodoluminescence, trace-element abundances, and Raman spectroscopy. Single grains and grain fragments are analyzed by thermal-ionization mass spectrometry, select populations by chemical abrasion thermal-ionization mass spectrometry, and inherited cores by laser-ablation multiple-collector inductively-coupled-plasma mass spectrometry. Monazite genesis is determined by deconvolving the record of major- and trace-element zoning in silicate and phosphate minerals in different textural settings. Following reconnaissance secondary-ion mass spectrometry dating, single grains and grain fragments will be analyzed by thermal-ionization mass spectrometry.The strength of this approach lies in its use of state-of-the-art zircon and monazite geochronology and petrology, collaboration with researchers at cutting-edge facilities, and our familiarity with other ultrahigh-pressure orogens. The impact of this research should be considerable and broadly applicable because of the archetypal nature of the Norwegian ultrahigh-pressure terrane.