HR: 15:10h
AN: T23C-06    [Abstracts]
TI: A Kinetic Model for Scattered Radiometric Ages in HP/UHP Terrains Using the Western Gneiss Region of the Scandinavian Caledonides as an Example
AU: * Brueckner, H K
EM: hannes@ldeo.columbia.edu
AF: Queens College and The Graduate Center of the City University of New York, Kessina Blvd, Flushing, NY 11367 United States
AU: * Brueckner, H K
EM: hannes@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, Rt. 9W, Palisades, NY 10964 United States
AB: Attempts to date the "peak" of metamorphism in HP/UHP terrains are complicated by radiometric dates that scatter over significant time periods: on the order of tens of millions of years in some terrains. For example, eclogites dated by Sm-Nd mineral isochron and U-Pb zircon techniques in the Western Gneiss Region (WGR) of the Norwegian Caledonides give ages that range from 402 to 422 Ma (the oldest age is a re-determination of an eclogite previously dated at 447 Ma). Some recent studies have proposed that the older ages should be discarded and the youngest ages should taken to date the HP/UHP metamorphism that accompanied the collision of Baltica and Laurentia during the Scandian Orogeny. This interpretation implies that HP/UHP metamorphism was a discrete, short-lived event that occurred during or shortly after the achievement of peak temperatures. However, if HP/UHP conditions are achieved through the subduction of continental crust into the mantle, eclogite-facies assemblages have the potential to form throughout the interval that the crust is within the eclogite stability field, during both subduction and eduction. Recent evidence suggests that eclogitization requires the introduction of fluids or the application of strain or the action of some other catalytic process and that where these processes do not occur the rocks can persist metastably as non-eclogite facies assemblages. This means that eclogitization can occur locally, wherever and whenever fluids are introduced or strain is localized, rather than occurring coherently within a short interval throughout the entire terrene. If so, all determined ages from the WGR could be correct with each one dating the formation of eclogite-facies assemblages in a particular place at a particular time. If so, the WGR resided at depths within the eclogite facies stability field for ca. 20 m.y. A testable consequence of this model is that eclogites from the part of the slab that was subducted to the deepest levels should have both the oldest and youngest eclogites with the ages dating the entry into, and the departure from, respectively, the eclogite stability field.
DE: 8102 Continental contractional orogenic belts
DE: 8159 Rheology--crust and lithosphere
DE: 3660 Metamorphic petrology
DE: 1035 Geochronology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting