Early Eocene high-Sr/Y magmas from the Urumieh-Dokhtar paleo-arc, Iran: Implications for the origin of high-flux events in magmatic arcs

Abstract

High-flux magmatic events, or flare-ups, are seen in many continental and transitional arcs but the underlying cause is not fully understood. Throughout Iran, voluminous calc-alkaline rocks emplaced between ~53 Ma and ~37 Ma are manifestations of a high-flux event. This high-flux event has been attributed to extension in the upper plate of one or more Neo-Tethyan subduction zones. Here, we present new findings in the Urumieh-Dokhtar paleo-arc, Iran where emplacement of Early Eocene granitic intrusions (ca. 55–52 Ma) immediately preceded the high-flux event. Geochemically, the studied rocks display high-K calc-alkaline compositions, continental crust-like trace element patterns, variable depletion of Y and heavy rare-earth elements, and SrNd isotopes that are close to or slightly more enriched than the Bulk Silicate Earth. The SrNd isotopic compositions indicate that the intrusions derived from upper plate materials. Relatively high Sr/Y and La/Yb of the studied rocks indicate that the residue of melt extraction, the crystalline assemblage that was left behind during magma differentiation, or both, to have existed in the deep arc crust. Following a ~ 20 Myr magmatic lull, the Early Eocene crustal thickening was most likely related to underthrusting of rear-arc crust towards the main arc, a process that not only promoted rapid root growth but also triggered the Eocene high-flux event. In the Iranian case example in particular, magmas formed during the high-flux event do not display elevated Sr/Y, implying that dense, locally thickened root might have foundered from the base of the arc lithosphere to the convecting mantle soon after their formation.