A recipe for making potassium-rich magmas in collisional orogens: New insights from K and Fe isotopes

Abstract

Ultrapotassic volcanism commonly occurs following calc-alkaline magmatism in continental collisional tectonic zones, but some key aspects of this transition remain ambiguous. In particular, there are uncertainties regarding the changing nature of the mantle source and the contribution of recycled continental material during the petrogenesis of ultrapotassic igneous rocks. Here, we show that the calc-alkaline to ultrapotassic transition in magmatism in Western Anatolia (Türkiye) during the past 55 Ma was associated with conspicuous shifts in the K and Fe isotope compositions of the bulk eruptive rocks. The ∼52–17 Ma transitional (tholeiitic to calc-alkaline) and calc-alkaline rocks show low δ56Fe (0.05 ± 0.07 ‰, 1SD) and high δ41K values (from –0.41 ‰ to 0.00 ‰), relative to the overall higher δ56Fe (0.16 ± 0.05 ‰, 1SD) and lower δ41K values (from –0.32 ‰ to –0.55 ‰) of the ∼19–15 Ma shoshonitic-ultrapotassic rocks in this area. The low δ41K values observed in the shoshonitic-ultrapotassic rocks are interpreted to reflect incorporation of potassium from deeply subducted recycled continental crust and the heavy Fe isotope signatures of these rocks imply a pyroxenite source. The rapid onset and short duration of eruption of the shoshonitic-ultrapotassic rocks is consistent with formation of phlogopite-pyroxenite veins within the peridotite mantle during slab rollback and/or breakoff. These veins were then rapidly and completely consumed by upwelling hot asthenospheric mantle.