Paper No. 9
Presentation Time: 11:00 AM


GARBER, Joshua M., Department of Earth Science, University of California Santa Barbara, 1006 Webb Hall, University of California, Santa Barbara, CA 93117, SMIT, Matthijs A., Department of Geosciences and Natural Resource Management, Section of Geology, University of Copenhagen, Øster Voldgade 10, 1350 København K, Copenhagen, Denmark, KYLANDER-CLARK, Andrew, Department of Earth Science, University of California, Santa Barbara, CA 93106 and HACKER, Bradley R., Department of Earth Science, University of California, Santa Barbara, Santa Barbara, CA 93106,

New lutetium-hafnium (Lu-Hf) dates of eclogites from the Western Gneiss Region (WGR) of Norway indicate a component of Hf isotopic inheritance in texturally simple metamorphic garnet, suggesting that fully-recrystallized garnet can preserve isotopic evidence of multiple orogenic cycles. The WGR records the Caledonian subduction and collision of the Baltica plate under Laurentia. High-P metamorphism (2.0-3.5 GPa, 650-800 oC) is recorded by eclogite pods within a larger body of quartzofeldspathic orthogneiss. The eclogite-facies recrystallization has been constrained to a 15-20 Myr period, from ~420-400 Ma, by a combination of Lu-Hf/Sm-Nd garnet and U-Pb zircon ages (Kylander-Clark et al., 2009; Krogh et al., 2011). All of these Lu-Hf dates were obtained from mineral separates that were subjected to magnetic separation and high-pressure autoclave dissolution, which can introduce an age bias if inherited Hf-rich inclusions (e.g., zircon) are dissolved along with the host garnet. The most-precise Lu-Hf dates were obtained on inclusion-free garnet and clinopyroxene separates, such that the reported age is that of the inclusion-free rims. Additionally, eclogites with garnets lacking rare-earth element (REE) zoning produce dates that post-date eclogite-facies metamorphism (e.g., 369±11 Ma: Kylander-Clark et al., 2007). To better constrain the age components present in eclogite-facies garnet, we performed Lu-Hf geochronology using a tabletop dissolution method that digests the mineral of interest but typically leaves behind Hf-rich inclusions. Our first experiments were done on whole grains picked from crushed rock ground down to the size of single grains. Garnet-clinopyroxene pairs from these samples yield scattered Lu-Hf data with apparent ages from ~440-530 Ma, interpreted to represent mixing between dominant Caledonian and lesser Sveconorwegian (~950 Ma) garnets. While further Lu-Hf work is ongoing, the following conclusions can be drawn from the existing data: (1) Norwegian eclogite garnets preserve evidence of multiple events, similar to zircons. (2) Lu-Hf geochronology on bulk garnet crystals reveals inherited components that may have been previously filtered out by pre-concentration and magnetic separation.