Rapid exhumation of the Zermatt-Saas ophiolite deduced from high-precisionSm-Nd and Rb-Sr geochronology

Sm-Nd isotope data from garnets in ultrahigh-pressure (coesite-bearing) ecl ogite from the Western Alps were obtained to determine the age of peak meta morphism 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. Difficul ties in dating Alpine eclogites using this method were overcome by using an HF acid-leaching procedure on garnet to remove mineral inclusions. The Zer matt-Saas rocks reached greenschist-facies conditions around 38 +/- 2 Ma, o n the basis of Rb-Sr whole rock-phengite isochrons. These data, combined wi th existing estimates of the pressure and temperature conditions of the ecl ogite, 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 e xhumation 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 r ates presented hen place critical constraints on collisional tectonic model s. (C) 1999 Elsevier Science B.V. All rights reserved.