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Rapid exhumation of the Zermatt-Saas ophiolite deduced from high-precision SmNd and RbSr geochronology

1999; Elsevier BV; Volume: 171; Issue: 3 Linguagem: Inglês

10.1016/s0012-821x(99)00161-2

1385-013X

Jeffrey M. Amato, Clark M. Johnson, Lukas P. Baumgartner, Brian L. Beard,

Geology and Paleoclimatology Research

SmNd 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 SmNd 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.