The magmatic and tectono-metamorphic history of the Sistan suture zone, Iran: New insights into a key region for the convergence between the Lut and Afghan blocks

Abstract

The Sistan suture zone, eastern Iran, exposes a well-preserved accretionary complex that formed by the closure of a Neotethyan ocean basin between the Central Iranian Lut block and the Afghan microcontinent. Mélange sequences include tectonic slices of adakitic plagiogranitic rocks and rare jadeitite. Adakitic whole rock characteristics, compositional zoning of phengites, and matching U–Pb zircon and Rb–Sr isochron ages (c. 85–87 Ma) suggest that the plagiogranites formed before or during the high-pressure/low-temperature (HP/LT) metamorphism that affected other parts of the subduction zone complex. Meta-plagiogranites, eclogites, and blueschists only shared the same P–T evolution after blocks of the felsic intrusions had been tectonically incorporated into the mélange. The suture zone has also been intruded by felsic melts that have largely escaped metamorphic overprinting. The weighted mean U–Pb zircon age of 81.0 ± 1.2 Ma for a newly dated sample of this group is consistent with the 86–71 Ma range previously reported for such rocks. Melt formation is not limited to the forearc region and can be reconciled with a model suggesting subduction of a spreading ridge. A newly dated gabbro yielded a weighted mean U–Pb zircon age of 104.2 ± 0.5 Ma, which is interpreted to represent crystallization and is the youngest age reported so far for the Birjand ophiolite. The geological relevance of an even younger intercept age (91.0 ± 2.7 Ma) of a small zircon subset remains elusive.