Arc-magmatism and subduction history beneath the Zagros Mountains, Iran: A new report of adakites and geodynamic consequences

Abstract

The Zagros Mountains were formed by convergence between Eurasia and Africa/Arabia. Compared to the wealth of recent studies on the external Zagros fold and thrust belt, the internal Sanandaj–Sirjan and Urumieh–Dokhtar magmatic arcs (SSMA, UDMA) remain poorly studied, despite being some of the best-preserved active margins within the greater Alpine–Himalayan convergent system. We focus on the key geodynamic constraints provided by the subduction-related magmatism of the upper plate, which shifted ~ 300 km inward from the SSMA to the UDMA at the end of the Mesozoic. Major and trace element data show that all studied magmatic rocks display the characteristics of subduction-related calc-alkaline magmas. Rare-earth element (REE) systematics for Mesozoic (SSMA) and Eocene (UDMA) volcanic and plutonic rocks suggest a similar mantle wedge source. In contrast, major and trace element data for volcanic rocks postdating the Late Miocene reveal a typical adakitic signature along part of the UDMA. The amphibole and SiO 2-rich (59–72 wt.%) Zagros adakites have very low Y and HREE contents. Adakitic magmas are likely to result from the melting of mafic material at depth (i.e., subducted oceanic crust from the slab and/or earlier obducted ophiolites) under unusually high temperature thermal conditions, in response to the regional-scale thermal re-equilibration accompanying collision or, to slab break-off. The fact that the distribution of these adakites is spatially restricted to the central parts of the UDMA (i.e. 200–300 km along strike) supports the slab break-off hypothesis, as do preliminary tomographic images. The timing of this event is coeval with slab-break-off below southern Turkey, which supports the view that slab detachment propagated laterally in the Neotethyan slab, both to the west (Turkey) and to the south (Iran), during the last 10–5 Ma.