HAFNIUM ISOTOPIC INHERITANCE IN ECLOGITES OF THE WESTERN GNEISS REGION, NORWAY

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.