The distinct core-to-rim zonation of different REEs in garnet in
metamorphic rocks, specifically Sm relative to Lu, suggests that Sm-Nd
and Lu-Hf isochron ages will record different times along a prograde
garnet growth history. Therefore, REE zonations in garnet must be
measured in order to correctly interpret the isochron ages in terms of
the garnet growth interval, which could span several m.y. New REE
profiles, garnet crystal size distributions, and garnet growth modeling,
combined with previously published Sm-Nd and Lu-Hf geochronology on a
UHP eclogite of the Zermatt-Saas Fee (ZSF) ophiolite, Lago di Cignana
(Italy), demonstrate that prograde garnet growth of this sample occurred
over a similar to 30 to 40 m.y. interval. Relative to peak metamorphism
at 38 to 40 Ma, garnet growth is estimated to have begun at similar to
11 to 14 kbar pressure at similar to 70 to 80 Ma. Although such a
protracted garnet growth interval is surprising, this is supported by
plate tectonic reconstructions which suggest that subduction of the
Liguro-Piemont ocean occurred through slow and oblique convergence.
These results demonstrate that REE zonations in garnet, coupled to
crystal size distributions, provide a powerful means for understanding
prograde metamorphic paths when combined with Sm-Nd and Lu-Hf
geochronology. (C) 2009 Elsevier B.V. All rights reserved.