HR: 0800h
AN: T21A-0509    [Abstracts]
TI: Ancient Protolith Ages and Scandian Metamorphism Across the Western Gneiss Region, Norway
AU: * Walsh, E O
EM: walshe@union.edu
AF: Geology Department, Union College, Schenectady, NY 12308 United States
AU: Hacker, B R
EM: hacker@geol.ucsb.edu
AF: Department of Geological Sciences, University of California, Santa Barbara, CA 93106-9630 United States
AU: Gans, P B
EM: gans@geol.ucsb.edu
AF: Department of Geological Sciences, University of California, Santa Barbara, CA 93106-9630 United States
AU: Grove, M
EM: marty@argon.ess.ucla.edu
AF: Department of Earth and Space Sciences, University of California, Los Angeles 595 Charles Young Drive East, Box 951567, Los Angeles, CA 90095-1567 United States
AU: Gehrels, G
EM: ggehrels@geo.arizona.edu
AF: Department of Geosciences, University of Arizona Gould-Simpson Building #77 1040 E 4th St., Tucson, AZ 85721 United States
AB: Recent work shows the ultrahigh-pressure (UHP) eclogite-facies event in the Western Gneiss Region (WGR) occurred $\sim$415-400 Ma. However, other (U)HP eclogite-facies events in the Scandinavian Caledonides make it necessary to determine eclogite ages across the WGR before constructing tectonic models. Also, insufficient data exist to constrain the ages of two amphibolite-facies events (at $650-750\deg$C, $\sim$11 kbar and at 6-7 kbar) characteristic to the WGR, which provide important pins on the exhumation process. To these ends, we dated zircon from eclogites, monazite and muscovite from pelites in a 220 x 100 km E-W band across the WGR. Zircon and monazite were dated by secondary ion mass spectrometry and laser-ablation inductively coupled plasma mass spectrometry to extract igneous core and metamorphic rim ages. Most eclogites record Gothian ($\sim$1.7-1.5 Ga) or Sveconorwegian ($\sim$1.2-0.9 Ga) igneous protolith ages similar to those of the Baltica orthogneiss. The easternmost eclogite records a Svecofennian ($\sim$1.8-2.0 Ga) upper intercept age; zircon and monazite from the Trollstigen area record upper intercept and concordia ages $>$ 2 Ga, not previously known in the WGR. While Upper Allochthon eclogites and pelitic gneisses retain the Early to Middle Ordovician and Early Silurian ages of their Iapetan protoliths, the age of the last metamorphism in all the eclogites is Scandian. Basement eclogites were least affected by this Scandian (re)crystallization, and the westernmost eclogites were most affected. Precambrian monazite ages occur only within basement gneisses; Upper Allochthon gneisses yield peak monazite ages of $\sim$424 Ma and $\sim$374 Ma. The most robust geochronological signature of the Scandian metamorphism is a steady westward decrease in muscovite $^{40}$Ar/$^{39}Ar ages from $\sim$394 Ma to $\sim$384 Ma. The entire WGR was involved in the Scandian phase of the Caledonian Orogeny, including the allochthons, which were emplaced onto the Baltica basement before subduction to UHP. Allochthon monazite record this nappe emplacement as well as a late episode of fluid intrusion. The predominance of Precambrian ages in basement rocks may reflect a paucity of fluids. The muscovite age gradient is compatible with near-coeval closure at $\sim$$400\deg$C across the WGR, followed by eastward tilting. If the UHP metamorphism occurred at $\sim$415-400 Ma, complete exhumation to upper crustal levels in the eastern part of the study area may have taken $<$ 5-10 Ma, with the rocks cooling at rates $>$ $50-100\deg$C/Ma. Such rapid cooling indicates tectonic exhumation, likely aided by significant erosion.
DE: 9335 Europe
DE: 8102 Continental contractional orogenic belts
DE: 1035 Geochronology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting