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 ultramafi c, mafi c 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 signifi cance of previously published U\Pb Sistan ocean Neotethys Iran 40 39 Ar/ Ar 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 Rb\Sr 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 40 39 Ar/ Ar dating for fi ve of these samples yielded in most cases ages that are identical to the Rb\Sr results within analytical uncertainty. For one sample the 40 39 Ar/ Ar age of 81.3 ± 1.3 Ma is about 3 myr younger than the corresponding Rb\Sr age. A still younger Rb\Sr age of 78.9 ± 0.5 Ma was determined for a retrograde epidote-biotite assemblage in an overprinted domain of an eclogite. Ion probe U\Pb zircon ages for two eclogites and two meta-acidic rocks from the mélange yielded weighted mean 206 238 Pb/ U 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.