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

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Title: Discrete ultrahigh-pressure domains in the Western Gneiss Region, Norway: implications for formation and exhumation.
Authors: Root, D. B.1,2 rootd@si.edu, Hacker, B. R.1, Gans, P. B.1, Ducea, M. N.3, Eide, E. A.4, Mosenfelder, J. L.5
Source: Journal of Metamorphic Geology. Jan2005, Vol. 23 Issue 1, p45-61. 17p. 3 Diagrams, 5 Charts, 7 Graphs, 3 Maps.
Subjects: Eclogite, Metamorphic rocks, Facies, Cordierite, Atmospheric pressure measurement
Geographic Terms: Norway
Abstract: New eclogite localities and new40Ar/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 fromc.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 ofc.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, calc-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 andc.750 °C during exhumation from mantle depths, followed by a low-pressure sillimanite + cordierite overprint atc.5 kbar andc.750 °C. New40Ar/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. [ABSTRACT FROM AUTHOR]
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Abstract:New eclogite localities and new40Ar/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 fromc.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 ofc.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, calc-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 andc.750 °C during exhumation from mantle depths, followed by a low-pressure sillimanite + cordierite overprint atc.5 kbar andc.750 °C. New40Ar/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. [ABSTRACT FROM AUTHOR]
ISSN:02634929
DOI:10.1111/j.1525-1314.2005.00561.x