Geochemical and Isotopic Evolution of Late Oligocene Magmatism in Quchan, NE Iran

Abstract Magmatic activity that accompanied the collision between Arabia and Eurasia at ∼27 Ma, provides unique opportunities for understanding the triggers and magma reservoirs for collisional magmatism and its different styles in magmatic fronts and back‐arcs. We present new ages and geochemical‐isotopic results for magmatic rocks that formed during the collision between Arabia and Eurasia in NE Iran, which was a back‐arc region to the main magmatic arcs of Iran. Our new zircon U‐Pb ages indicate that collisional magmatism began at ∼24 Ma in the NE Iran back‐arc, although magmatism in this area started in the Late Cretaceous time and continued until the Pleistocene. The collisional igneous rocks are characteristically bimodal, and basaltic‐andesitic and dacitic‐rhyolitic components show significant isotopic differences; εNd(t) = +4.4 to +7.4 and εHf(t) = +5.4 to +9.5 for mafic rocks and εNd(t) = +0.2 to +8.4 and εHf(t) = +3.4 to +12.3 for silicic rocks. The isotopic values and modeling suggest that fractional crystallization and assimilation‐fractional crystallization played important roles in the genesis of felsic rocks in the NE Iran collisional zone. Trace element and isotopic modeling further emphasize that the main triggers of the magmatism in NE Iran comprise a depleted to the enriched mantle and the Cadomian continental crust of Iran. Our results also emphasize the temporal magmatic variations in the NE Iran back‐arc from Late Cretaceous to Pleistocene.