Geochemical evidences for quantifying crustal thickness over time in the Urumieh-Dokhtar magmatic arc (Iran)

Geochemical data on igneous rocks of the Urumieh-Dokhtar magmatic arc (UDMA) are used to reconstruct temporal variations in crustal thickness during the Tertiary. We use two approaches, (1) whole rock Sr/Y and La/Yb ratios, and (2) Nd isotopes and an assimilation model for magma genesis. Our results indicate that the crust was likely of normal thickness (~35 km) and remained constant during Eocene to Oligocene, corresponding to slab roll back and back arc extension along the Eurasian active margin. Crustal thickening began during the early Miocene and culminated in the middle Miocene (~50 km). This thickening is marked by a distinct increase in Sr/Y and La/Yb ratios (changing the stability of certain mineral phases) and decrease in ɛNd (increasing crustal assimilation). Thickening was likely due to intrusion of magma as well as crustal shortening and was a consequence of the final roll back of the Neo-Tethys slab and subsequent continental collision.