Early Cretaceous exhumation of high-pressure metamorphic rocks of the Sistan Suture Zone, eastern Iran

Abstract

The Sistan Suture Zone (SSZ) of eastern Iran is part of the Neo-Tethyan orogenic system and formed by convergence of the Central Iranian and Afghan microcontinents. Ar-Ar ages of ca. 125Ma have been obtained from white micas and amphibole from variably overprinted high-pressure metabasites within the Ratuk Complex of the SSZ. The metabasites, which occur as fault-bounded lenses within a subduction melange, document peak-metamorphic conditions in eclogite or blueschist facies followed by near-isothermal decompression resulting in an epidote-amphibolite-facies overprint. 40Ar/39Ar step heating experiments were performed on a phengite + paragonite mixture from an eclogite, phengites from two amphibolites, and paragonite from a blueschist; 'best-fit' ages from these micas are, respectively, 122.8±2.2, 124±13, 116±19 and 139±19 Ma (2σ error). Barroisite from an amphibolite yielded an age of 124±10 Ma. The ages are interpreted as cooling ages that record the post-epidote-amphibolite stage in the exhumation of the rocks. Our results imply that both the high-pressure metamorphism and the epidote-amphibolite-facies overprint occurred prior to 125 Ma. Subduction of oceanic lithosphere along the eastern margin of the Sistan Ocean had therefore begun by Barremian (Early Cretaceous) times. Copyright © 2008 John Wiley & Sons, Ltd.