A 100-million-year-old volcanic arc in Iran holds clues to the history of a vanished ocean whose closure helped shape the Middle East.
An ancient volcanic system in northwestern Iran became difficult to recognize as tectonic deformation, erosion, and burial altered its geological record. Younger magmatic activity during the Eocene further obscured the evidence. Researchers have now identified surviving rocks from a previously unrecognized continental magmatic arc that was active about 101 million to 97 million years ago.
The international team, led by Istanbul Technical University (ITU), calls the system the Azerbaijan Continental Magmatic Arc. Described in Geoscience Frontiers, the arc dates to the Late Cretaceous evolution of the Neo-Tethys Ocean. Its remnants lie in a region that is now part of the Arabia-Eurasia collision zone.
Reconstructions of Iran’s magmatic history have largely focused on the Sananda-Sirjan Magmatic Arc and the Urumieh-Dokhtar Magmatic Arc. Recognizing a distinct Late Cretaceous system in the northwest extends that framework and adds evidence for reconstructing the region’s history before continental collision. The wider Neo-Tethyan system connects this history to the Eastern Mediterranean, the Middle East, and Western Asia, whose geology was profoundly shaped by the ocean’s closure.
Rock chemistry points to ancient subduction
To investigate the arc, the researchers examined volcanic and plutonic rocks from several localities in northwestern Iran, particularly along the Siah Cheshmeh-Khoy-Misho-Tabriz Fault. Volcanic rocks form from material erupted at the surface, while plutonic rocks form when magma cools underground. Both can preserve evidence of continental arc magmatism, in which an oceanic plate sinks beneath a continent and helps generate magma above the descending plate.
The team used uranium-lead dating of zircon crystals, known as zircon U-Pb geochronology, to establish the rocks’ ages. Hafnium (Hf) isotope analyses helped trace the sources of the magma, while whole-rock geochemistry revealed the rocks’ chemical composition. Together, the results showed characteristics consistent with a continental magmatic arc formed through subduction, the process in which one tectonic plate descends beneath another.
The collaboration brought together ITU researchers and colleagues from the Institute of Tibetan Plateau Research and the Xinjiang Institute of Ecology and Geography, both part of the Chinese Academy of Sciences, along with ETH Zurich in Switzerland, Karadeniz Technical University in Türkiye, and Damghan University in Iran.
Dr. Ali Mohammadi, the study’s corresponding author at Istanbul Technical University, explained: “This study shows that the magmatic history of northwestern Iran is more complex than previously recognized. By integrating geochronological, isotopic, and geochemical evidence, we identified a Late Cretaceous continental arc that was subsequently obscured by later tectonic processes.”
A fault may mark an ancient boundary
The arc’s fragmented record provides additional constraints for reconstructing subduction, the migration of magmatic arcs, continental growth, and the closure of ancient oceans. It also raises a question about the fault along which many of the rocks were investigated. The authors suggest that the Siah Cheshmeh-Khoy-Misho-Tabriz Fault may mark an important tectonic boundary associated with the reorganization of the Neo-Tethyan system.
Changes in subduction may help explain why this magmatic episode was relatively short-lived. Possible contributors include slab rollback, in which the descending plate retreats as it sinks, and arc migration, a shift in the location of magmatic activity. These remain proposed explanations for the development of the Azerbaijan arc as researchers refine models of northwestern Iran’s Late Cretaceous tectonic history.
Reference: “The missing late Cretaceous magmatic arc in the Arabia-Eurasia collision zone (NW Iran): constraints from zircon geochronology, Hf isotopes, and geochemistry” by Ali Mohammadi, Hadi Shafaii Moghadam, Amaneh Kaveh-Firouz, Anna Lechmann, Fulong Cai and Lin Ding, 3 November 2025, Geoscience Frontiers.
DOI: 10.1016/j.gsf.2025.102205
This work was financially supported by the State Key Laboratory of Tibetan Plateau Earth System, Resources and Environment, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, China; the Swiss National Science Foundation, Switzerland; under Grants 2-77644-09 and 2-77979-14; and the Forschungsreserven/Burg J.-P. fund under Grant 2-78159-99.
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