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ELSEVIER Earth and Planetary Science Letters 171 (1999) 425438 www.elsevier.com/locate/epsl

Summary: ELSEVIER Earth and Planetary Science Letters 171 (1999) 425­438
Rapid exhumation of the Zermatt­Saas ophiolite deduced from
high-precision Sm­Nd and Rb­Sr geochronology
Jeffrey M. Amato L
, Clark M. Johnson, Lukas P. Baumgartner 1
, Brian L. Beard
Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA
Received 26 August 1998; revised version received 10 May 1999; accepted 9 June 1999
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 Zermatt­Saas ophi-
olite complex. We report an exceptionally well-constrained Sm­Nd isochron age of 40:6s2:6 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 Zermatt­Saas rocks reached greenschist-facies conditions around 38 s 2 Ma,
on the basis of Rb­Sr whole rock­phengite 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


Source: Amato, Jeff - Department of Geological Sciences, New Mexico State University


Collections: Geosciences