The Sanandaj-Sirjan Zone (SaSiZ) is a magmatic terrane within the Zagros Orogen, western Iran, marking the Tethyan suture zone between the Afro-Arabian Plate and the Central Iran Micro-Continent. Mafic-intermediate dyke swarms with Middle Jurassic (Group-1: hornblende gabbro and diorite) and Late Eocene (Group-2: hornblende-pyroxene gabbro) ages are recognized in the Malayer-Boroujerd Plutonic Complex of the northern SaSiZ. Group-1 dykes have elemental and isotopic signatures consistent with melting of a mantle source modified during Neo-Tethyan subduction. Some Group-1 magmas evolved to intermediate compositions through assimilation and fractional crystallization. Group-2 dykes have within-plate trace element geochemical signatures, modelled as deriving from low-degree melting of asthenospheric mantle without a subduction influence. Published models postulate either a Cretaceous-Eocene Neo-Tethyan flat-slab scenario, or a Latest Cretaceous-Palaeogene Neo-Tethyan break-off event beneath the SaSiZ. Such models do not reconcile with the Late Eocene presence of within-plate magmatism in westernmost Iran, very close to the Zagros Suture. We argue that a period of flat-slab subduction concluded with sub-parallel subduction of a Neo-Tethyan ridge to the trench. The resulting slab break-off event in the Late Eocene is responsible for generation of the distinct Mesopotamia and Zagros slabs in mantle tomography models. Break-off was followed by small volume within-plate type magmatism before short-lived re-establishment of Tethyan subduction prior to the final Arabia-Eurasia collision.

Supplementary material: Field photographs, photomicrographs, additional geochemical plots, descriptions of analytical methods and tables of geochemical modelling parameters are available at https://doi.org/10.6084/m9.figshare.c.4126196.

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