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

摘要

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 here place critical constraints on collisional tectonic models.