Latest Oligocene adakitic rocks in western Iran: implications for early crustal thickening and tectonic evolution of the Iran Block

Adakitic rocks occur in a variety of tectonic settings and are key to understanding the tectonic evolution and geodynamics of orogenic belts. We investigated latest Oligocene (23.5-22.5Ma) quartz monzonites and granites from the western segment of the Urumieh-Dokhtar magmatic belt in Iran, which are likely to have formed in response to the early stages of Arabia-Eurasia collision. The studied rocks have the geochemical characteristics of typical adakites, such as high SiO2 (60.18-68.82wt%) and Sr (499-793ppm) contents, low Y (8.90-17.1ppm) and Yb (0.88-1.58ppm) contents, and high Sr/Y (26.167.8) and (La/Yb)(N) (21.932.9) ratios. They have variable K2O (3.88-5.09wt%), MgO (0.44-2.74wt%; Mg#=33.7-52.5), Cr (4.27-40.59ppm), Ni (4.28-35.68ppm) and Th (9.56-59.59ppm) contents, and relatively depleted Sr-Nd isotopic compositions [(Sr-87/Sr-86)(i)=0.70450-0.70516; epsilon(Nd)(t)=2.1-2.7]. These characteristics indicate that the quartz monzonites were derived from the partial melting of delaminated lower crust that interacted with mantle peridotite with high MgO, Cr and Ni contents and depleted Sr-Nd isotopic compositions and suggest that the granites were formed by the fractional crystallization of quartz monzonitic magma. The geochemical features of the studied adakitic rocks could therefore have been affected by magmatic processes (e.g. fractional crystallization), which might be misleading in interpretations of their petrogenesis and related tectonic settings. The geochemical features of the studied rocks indicate that the crust of the western segment of the Urumieh-Dokhtar magmatic belt was thickened to c. 504.43km during the latest Oligocene (c. 23.5Ma) as a result of Arabia-Eurasia collision.