Discrete ultrahigh-pressure domains in the Western Gneiss Region, Norway: Implications for formation and exhumation

D. B. Root, B. R. Hacker, P. B. Gans, M. N. Ducea, E. A. Eide, J. L. Mosenfelder

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134 Scopus citations


New eclogite localities and new 40Ar/39Ar ages within the Western Gneiss Region of Norway define three discrete ultrahigh-pressure (UHP) domains that are separated by distinctly lower pressure, eclogite facies rocks. The sizes of the UHP domains range from c. 2500 to 100 km2 ; if the UHP culminations are part of a continuous sheet at depth, the Western Gneiss Region UHP terrane has minimum dimensions of c. 165 × 50 × 5 km. 40Ar/ 39Ar mica and K-feldspar ages show that this outcrop pattern is the result of gentle regional-scale folding younger than 380 Ma, and possibly 335 Ma.The UHP and intervening high-pressure (HP) domains are composed of eclogite-bearing orthogneiss basement overlain by eclogite-bearing allochthons. The allochthons are dominated by garnet amphibolite and pelitic schist with minor quartzite, carbonate, cale-silicate, peridotite, and eclogite. Sm/Nd core and rim ages of 992 and 894 Ma from a 15-cm garnet indicate local preservation of Precambrian metamorphism within the allochthons. Metapelites within the allochthons indicate near-isothermal decompression following (U)HP metamorphism: they record upper amphibolite facies recrystallization at 12-17 kbar and c. 750 °C during exhumation from mantle depths, followed by a low-pressure sillimanite + cordierite overprint at c. 5 kbar and c. 750 °C. New 40Ar/39Ar hornblende ages of 402 Ma document that this decompression from eclogite-facies conditions at 410-405 Ma to mid-crustal depths occurred in a few million years. The short timescale and consistently high temperatures imply adiabatic exhumation of a UHP body with minimum dimensions of 20-30 km. 40Ar/39Ar muscovite ages of 397-380 Ma show that this extreme heat advection was followed by rapid cooling (c. 30 °C Myr-1), perhaps because of continued tectonic unroofing.

Original languageEnglish (US)
Pages (from-to)45-61
Number of pages17
JournalJournal of Metamorphic Geology
Issue number1
StatePublished - Jan 2005


  • Ar/Ar
  • Cordierite
  • Eclogite
  • Norway
  • Thermobarometry
  • Ultrahigh pressure

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology


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