Eocene subvolcanics with slab failure magmatism affinity in the Samakhon area, NE Iran: Evidence of Pre-Lutecian collision of the Central Iran and the Eurasian Plates

Abstract

Porphyritic monzogranite and diorite considered as subvolcanic bodies are exposed in northeastern Iran as stocks and dikes in the Samakhon region. Zircon U–Pb dating yields ages ranging from 48.2 to 42.9 Ma for these rocks. Whole rock chemistry shows large variation of SiO₂ (54.6–65.4 wt %), MgO (0.19–5.49 wt %), and the alkaline elements contents (Na₂O + K₂O = 5.54 to 7.53 wt %), and display a positive Eu anomaly (Eu/Eu∗ = 0.95–1.2). High contents of the large ion lithophile elements (e.g., Sr, Rb, Cs, and Ba), high LREE/HREE ratios (La_N/Yb_N = 0.9–1.4), and negative Ta, Nb, and Ti anomalies are consistent with the calc-alkaline magmatic series emplaced along active margins. The initial ⁸⁷Sr/⁸⁶Sr ratios and εNd_i values change from 0.7052 to 0.7063, and from −3.0 to −0.4, respectively suggesting the involvement of crustal components in the sources of these rocks. Nb/Y (1.5), Gd/Yb (2.9), La/Yb (24.8), Sm/Yb ratios (3.3), and Nb abundances (15.1–19.9) are high compared to typical arc magmatism, suggesting that these rocks are likely associated with slab failure. The metasomatic mantle above the subduction zone in a pre-to post-collisional regime is the most likely candidate for the sources of these rocks. The development of the Sabzevar Cretaceous arc-related magmatism and the situation of the Samakhon subvolcanic rocks with younger age (48-42 Ma) in the northern Sabzevar suture zone, can be attributed to slab failure magmatic activity after the collision between the Central Iran Block and the Eurasian plate during the Early Eocene. The current model suggests that the closure of the Sabzevar ocean and the collision in northeastern Iran happened earlier than formerly assumed, which was previously thought to have occurred in the Late Eocene.