Sm-Nd isotope data from garnets in ultrahigh-pressure (coesite-bearing)
eclogite from the Western Alps were obtained to determine the age of
peak metamorphism and exhumation rates of deeply buried oceanic crust in
the Zermatt-Saas ophiolite complex. We report an exceptionally
well-constrained Sm-Nd isochron age of 40.6 +/- 2.6 Ma from the Lago di
Cignana eclogite. Difficulties in dating Alpine eclogites using this
method were overcome by using an HF acid-leaching procedure on garnet to
remove mineral inclusions. The Zermatt-Saas rocks reached
greenschist-facies conditions around 38 +/- 2 Ma, on the basis of Rb-Sr
whole rock-phengite isochrons. These data, combined with existing
estimates of the pressure and temperature conditions of the eclogite,
give an estimate for the initial exhumation rate of 10 to 26 km/m.y.,
depending on the inferred pressure for the recrystallization of
phengite, followed by slow exhumation of 0.3 km/m.y. from 34 to 14 Ma.
The initial exhumation rates at Lago di Cignana are among the highest
yet determined for high-pressure terranes, and are well constrained
because the age has been determined directly on the high-pressure
assemblage. The rapid exhumation rates presented hen place critical
constraints on collisional tectonic models. (C) 1999 Elsevier Science
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