A missing Cretaceous magmatic arc of Neo-Tethys in Iran

Abstract

Magmatic arcs are generally considered to be the direct record of subduction zone. Magmatic activity can start with subduction initiation until the end of oceanic subduction. In the Neo-Tethys tectonic domain, arc magma gaps in Alps and Iran prove that arc magmatism and oceanic subduction are not always coupled. Unlike the Alps, where arc magmatism is absent, or the Gangdese, where arc magmatism is continuous, Iran exhibits an intermittent arc magma record. Since the subduction of the Neo-Tethys Ocean in the Jurassic, Iran has recorded two phases of magmatic activities: the Middle Jurassic (200-140 Ma, with a peak at ~170 Ma) and the Eocene (55-25 Ma, with a peak at ~40 Ma), which are attested by the age peaks of detrital zircons from Mesozoic-Cenozoic clastic rocks. The Cretaceous magma record is sparse, but Cretaceous detrital zircons are abundant (120-65 Ma, with a peak at ~90 Ma). Regarding this mismatched age record of detrital zircons and magmatic rocks, we choose the Makran forearc basin deposits as the target because it receives thick detritus from Eurasia to form a tens of kilometer thick sedimentary sequence. We conducted a detrital zircon study from the Makran to explore the magmatic evolution of the Iranian Tethys zone. Euhedral zircon grains, obvious oscillatory zoning, low zircon Th/U>0.1, and trace element geochemistry indicate Cretaceous magmatic zircons sourced from the continental magmatic arcs rather than ophiolites. Positive zircon Hf isotopes excludes the source region of the Gangdese arc which is more depleted. We further used machine learning to confirm our provenance results, that reveal Cretaceous (120-65 Ma) magmatism by the Neo-Tethys Ocean subduction in Iran. The decreasing trend of Cretaceous zircons U-Pb in Late Cretaceous to Pliocene strata indicates gradual denudation of the Cretaceous magmatic arc from deep to shallow. Cretaceous zircon peaks disappears abruptly in the Pliocene rocks, implying that the Cretaceous magmatic arc was completely denuded. Thus, we confirm that since the subduction initiation, magmatic activity was continuous in Iran but the “missing” was due to the denudation process. This works also highlights the importance of comprehensive analysis before discussing subduction geodynamics based on the record of magmatic outcrops.