Eocene magmatic flare-up and crustal thickening in NW Iran: quantitative constraints on the subduction-to-collision transition

How continental crust forms during subduction remain debated: does it grow through steady-state magmatism or brief, high-flux events (flare-ups)? The drivers and crustal growth potential of such flare-ups are poorly constrained. The Alborz–Azerbaijan magmatic belt in NW Iran preserves a significant Eocene flare-up, offering an ideal natural laboratory to address this question. We investigate the late Eocene Tarom plutonic bodies (TPBs) using geochemical and isotopic data to constrain their petrogenesis, quantify crustal growth, and reconstruct regional tectonic evolution. The TPBs were emplaced rapidly at ca. 40 Ma, originating from low-degree partial melts of metasomatized mantle, followed by fractional crystallization with minimal crustal contamination. This flare-up yielded a magmatic addition rate of ca. 73 km3 km−1 Myr−1, comparable to major cordilleran systems, demonstrating significant net crustal growth. Using chemical mohometry, we track crustal thickness through time: from ca. 28 km during Cretaceous rifting, to ca. 38 km during the Eocene flare-up, increasing to ca. 50 km in the Oligocene, and peaking at ca. 56–62 km in the Miocene. This trajectory reflects the transition from subduction to collision. Subsequent geochemical shifts signal slab break-off at ca. 10–12 Ma and lithospheric delamination at ca. 6 Ma, triggering renewed mantle-dominated magmatism. Our results demonstrate that the Eocene flare-up was a fundamental crust-building episode, not merely a surficial volcanic event. By quantifying magma production and crustal thickening, we provide a robust framework linking plate convergence, deep mantle processes, and crustal evolution, with broad implications for understanding collisional orogens worldwide.