Sa. Bowtell et al., SM-ND ISOTOPIC EVIDENCE ON THE AGE OF ECLOGITIZATION IN THE ZERMATT-SAAS OPHIOLITE, Journal of metamorphic geology, 12(2), 1994, pp. 187-196
Rocks within the Zermatt-Saas ophiolite of the western Alps have under
gone eclogite facies metamorphism during subduction prior to the Alpin
e collision. The metamorphic history of these rocks is well defined, w
ith eclogitic assemblages being followed by the limited growth of blue
schist assemblages of glaucophane and paragonite. Subsequent greenschi
st alteration occurs adjacent to faults, veins and metasediments. Away
from such sources of water, retrogression is very limited. Sm-Nd isot
opic analyses of an essentially unretrogressed eclogitic metabasalt su
ggest that eclogite facies metamorphism occurred at 52 +/- 18 Ma. The
large uncertainty is due to the presence of very small amounts of Nd-r
ich epidote present as inclusions within garnet. As the closure temper
ature of garnet to Sm & Nd is thought to be >600-degrees-C, resetting
due to post-high-pressure diffusion is thought to be insignificant. Gi
ven the fine-grained protolith to the sample analysed, and its extensi
ve deformation under eclogite facies conditions, incomplete homogeniza
tion of pre-metamorphic isotopic variations is also considered unlikel
y to be responsible for the young age. A Tertiary age of eclogitizatio
n means that models of early Alpine evolution based on the cessation o
f high-pressure metamorphism in the Cretaceous need to be revised.