Detrital zircon and provenance analysis of Eocene–Oligocene strata in the South Sistan suture zone, southeast Iran: Implications for the tectonic setting

Abstract

The north-south–trending Sistan suture zone in east Iran results from the Paleogene collision of the Central Iran block to the west with the Afghan block to the east. We aim to document the tectonic context of the Sistan sedimentary basin and provide critical constraints on the closure time of this part of the Tethys Ocean. We determine the provenance of Eocene–Oligocene deep-marine turbiditic sandstones, describe the sandstone framework, and report on a geochemical and provenance study including laser ablation–inductively coupled plasma–mass spectrometry U-Pb zircon ages and 415 Hf isotopic analyses of 3015 in situ detrital zircons. Sandstone framework compositions reveal a magmatic arc provenance as the main source of detritus. Heavy mineral assemblages and Cr-spinel indicate ultramafic rocks, likely ophiolites, as a subsidiary source. The two main detrital zircon U-Pb age groups are dominated by (1) Late Cretaceous grains with Hf isotopic compositions typical of oceanic crust and depleted mantle, suggesting an intraoceanic island arc provenance, and (2) Eocene grains with Hf isotopic compositions typical of continental crust and nondepleted mantle, suggesting a transitional continental magmatic arc provenance. This change in provenance is attributed to the Paleocene (65–55 Ma) collision between the Afghan plate and an intraoceanic island arc not considered in previous tectonic reconstructions of the Sistan segment of the Alpine-Himalayan orogenic system.