Across-arc geochemical variations in the Paleogene magmatic belt of Iran

Abstract

Across-arc geochemical variations between igneous rocks from the magmatic front (MF) to rear-arc (RA) are commonly observed for convergent margins, but the processes that are responsible for these variations are unclear. To address these questions, we studied a well-exposed Cenozoic continental arc, the Urumieh-Dokhtar Magmatic Belt (UDMB) of Iran and its rear-arc counterpart to the north.North Iran RA magmatism is bimodal, characterized by mafic to felsic igneous rocks of calc-alkaline to shoshonitic affinity. The new zircon U -Pb ages show that plutonic rocks are ~ 42–38 Ma whereas volcanic rocks are older at ~52 Ma. 87Sr/86Sr(t) and εNd(t) values of UDMB RA igneous rocks range from 0.70483 and 0.70783 and –2.5 and + 3.6, respectively. Zircons from plutonic rocks show εHf(t) values ranging from +1.8 to +9.8, higher than volcanic rocks with εHf (t) between +2.6 and –9.2. Isotopic data agrees with inferences from incompatible trace element ratios (e.g., Th/Yb, Zr/Nb and La/Yb) that mafic plutonic rocks originated from an enriched mantle source, whereas felsic volcanic rocks show more interaction with continental crust. High magmatic fluxes did not occur at the same time in the MF and RA; UDMB MF flared-up at ~ 54–37 Ma and 20–5 Ma, overlapping RA magmatic flare-ups at 45–40 Ma and 30–25 Ma. The available major and trace element data from the UDMB show that most magmatic rocks experienced strong fractional crystallization and assimilation-fractional crystallization processes, making it difficult to identify across-arc geochemical variations. Moreover, the bulk rock εNd(t) and zircon εHf(t) values are highest at the MF, but the across-arc isotopic variability seems to be mainly controlled by crustal influence. High-flux magmatism in the UDMB was related to protracted heating of the crust as a result of subduction of Neotethys oceanic lithosphere. This weakened the continental lithosphere, leading to strong extension in Paleogene time accompanied by decompression melting and magmatic flare-ups.