Jurassic Igneous Rocks of the Central Sanandaj–Sirjan Zone (Iran) Mark a Propagating Continental Rift, Not a Magmatic Arc

Abstract

Understanding the Mesozoic and younger geological evolution of Iran is key for understanding the Alpine‐Himalayan orogenic belt be‐ cause it captures the transition from normal subduction to advanced continental collision like that now occurring between India and Tibet. The Sanandaj–Sirjan Zone (SaSZ) is a key part of the Iran convergent margin and is situated between Cadomian (500–600 Ma) crust of the Iranian plateau and off‐scraped sediments of the Zagros fold‐and‐ thrust belt. The SaSZ also bisects the Late Cretaceous Zagros ophi‐ olites into inner and outer belts (Shafaii Moghadam & Stern, 2015). Correct interpretation of Jurassic igneous rocks in the SaSZ is essen‐ tial for reconstructing the early evolution of the Iranian convergent margin and motivates our study. Most studies of Jurassic SaSZ igneous rocks consider that these formed at a convergent plate margin (Berberian & Berberian, 1981; Deevsalar et al., 2017; Esna‐Ashari, Tiepolo, Valizadeh, Hassanzadeh, & Sepahi, 2012; Khalaji, Esmaeily, Valizadeh, & Rahimpour‐Bonab, 2007; Maanijou, Aliani, Miri, & Lentz, 2013; Sepahi, Salami, Lentz, McFarlane, & Maanijou, 2018; Shahbazi et al., 2010). Recent works have increasingly questioned this interpretation (Azizi, Lucci, Stern, Hasannejad, & Asahara, 2018b; Hunziker, Burg, Bouilhol, & Quadt, 2015; Zhang, Chen, Yang, Hou, & Aghazadeh, 2018). In this paper, we compiled published data for SaSZ Jurassic igneous rocks, including 70 U–Pb zircon ages and 1 Rb–Sr whole‐rock ages, 170 whole‐rock geochemical analyses and 97 Sr–Nd isotope ratios and used these to infer magma sources and tectonic setting. The compilation reveals that these igneous rocks are commonly contaminated by continental crust and show an age progression from SE to NW from 177 to 144 Ma that is best explained by a propagating continental rift on the SW margin of Eurasia.