Abstract
The Sistan Suture Zone (SSZ) in eastern Iran extends as a N–S trending belt over more than 700km 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. 116–139Ma) 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. 120km 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. 87Ma for eight samples representing different metamorphic grade and widely separated locations (weighted mean=85.7±0.8Ma). 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±1.3Ma is about 3myr younger than the corresponding RbSr age. A still younger RbSr age of 78.9±0.5Ma 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±1.4Ma, 86.1±1.5Ma, 88.7±1.4Ma and 87.2±1.2Ma, respectively. The results of this study do not support previous interpretations suggesting >125Ma 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.
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