New age constraints for the geodynamic evolution of the Sistan Suture Zone, eastern Iran

Abstract The Sistan Suture Zone (SSZ) in eastern Iran extends as a N–S trending belt over more than 700 km along the border area between Iran and Afghanistan. The SSZ formed as a result of eastward-directed subduction of a Neotethyan ocean basin beneath the Afghan block and includes a tectonic mélange consisting of disrupted meta-ophiolitic rocks within a low-grade matrix of ultramafic, mafic and pelitic schists. Some mélange blocks were affected by eclogite-, blueschist- and/or epidote amphibolite-facies P–T conditions. Understanding of the petrological and geochronological record of these rocks plays a key role in unravelling the geodynamic evolution of the SSZ. The main aim of the present study was to assess the geological significance of previously published 40Ar/39Ar ages (c. 116–139 Ma) which have not provided robust age constraints for geodynamic reconstructions on a regional scale. For this purpose, samples were collected within a NNW–SSE trending belt spanning a distance of c. 120 km that exposes the major occurrences of high-pressure/low temperature rocks and epidote amphibolites. Multi-point RbSr mineral isochrons indicate a regional consistent pattern with ages ranging between c. 83 and c. 87 Ma for eight samples representing different metamorphic grade and widely separated locations (weighted mean = 85.7 ± 0.8 Ma). Additional 40Ar/39Ar dating for five of these samples yielded in most cases ages that are identical to the RbSr results within analytical uncertainty. For one sample the 40Ar/39Ar age of 81.3 ± 1.3 Ma is about 3 myr younger than the corresponding RbSr age. A still younger RbSr age of 78.9 ± 0.5 Ma was determined for a retrograde epidote-biotite assemblage in an overprinted domain of an eclogite. Ion probe UPb zircon ages for two eclogites and two meta-acidic rocks from the mélange yielded weighted mean 206Pb/238U ages of 87.3 ± 1.4 Ma, 86.1 ± 1.5 Ma, 88.7 ± 1.4 Ma and 87.2 ± 1.2 Ma, respectively. The results of this study do not support previous interpretations suggesting > 125 Ma high-pressure/low-temperature metamorphism and amphibolite-facies overprinting, but instead document the importance of Late Cretaceous subduction zone processes for the geodynamic evolution of the SSZ.