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

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. 125 Ma 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 me ́lange, document peak-metamorphic conditions in eclogite or blueschist facies followed by near-isothermal decompression resulting in an epidote–amphibolite-facies overprint. 40 Ar/ 39 Ar step heating experiments were performed on a phengite þ 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 (2s 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.