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Protolith ages and exhumation histories of (ultra)high-pressure rocks across the Western Gneiss Region, Norway

Emily O. Walsh^,1, Bradley R. Hacker¹, Phillip B. Gans¹, Marty Grove² and George Gehrels³

¹ Department of Geological Sciences, University of California, Santa Barbara, California 93106-9630, USA
² Department of Earth and Space Sciences, University of California, Los Angeles, 595 Charles Young Drive East, Box 951567, Los Angeles, California 90095-1567, USA
³ Department of Geosciences, University of Arizona, Gould-Simpson Building No. 77, 1040 East 4th Street, Tucson, Arizona 85721, USA

The timing of protolith formation, ultra-high-pressure (UHP) subduction, and subsequent exhumation for the ultrahigh-pressure to high-pressure units across the eastern part of the Western Gneiss Region, Norway, were assessed using U/Pb zircon, Th/Pb monazite, and ⁴⁰Ar/³⁹Ar white mica ages. U/Pb zircon ages from eclogites demonstrate that oceanic and continental allochthons were emplaced onto the Baltica basement before the entire mass was subducted to (ultra)high pressure. Eclogites within the allochthons across the entire Western Gneiss Region are Caledonian and show a degree of zircon (re)crystallization that increases with peak pressure, permitting the interpretation that the entire region underwent synchronous subduction. ⁴⁰Ar/³⁹Ar white mica ages of 399 Ma indicate that the eastern part of the Western Gneiss Region had been exhumed to shallow crustal levels while UHP metamorphism was ongoing farther west, indicating a westward dip to the slab. The ⁴⁰Ar/³⁹Ar white mica ages also show a clear east-to-west gradient across the entire Western Gneiss Region, indicating that the Western Gneiss Region rose diachronously to crustal levels from east to west between 399 and 390 Ma.

Key Words: ultrahigh pressure • Western Gneiss Region • secondary ion mass spectrometry • ⁴⁰Ar/³⁹Ar • exhumation • laser ablation inductively coupled plasma–mass spectrometry

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				S. Johnston, B. R. Hacker, and M. N. Ducea Exhumation of ultrahigh-pressure rocks beneath the Hornelen segment of the Nordfjord-Sogn Detachment Zone, western Norway Geological Society of America Bulletin, September 1, 2007; 119(9-10): 1232 - 1248. [Abstract] [Full Text] [PDF]