Tectonic controls on the transition from homogeneous crystal-rich to zoned crystal-poor ignimbrites

Silicic calderas exhibit cyclic magmatic progression, with some systems transitioning from homogeneous crystal-rich to subsequent zoned crystal-poor caldera-forming eruptions—yet mechanisms governing this shift remain elusive. Here we contrast two caldera-forming eruptions from the Furongshan caldera complex in Southeast China. Our integrated zircon oxygen-hafnium isotopes, trace elements and bulk-rock data revealing a distinct shift from homogeneous crystal-rich tuffs to compositionally zoned crystal-poor tuffs, accompanied by a decrease in water content and oxygen fugacity. We attribute this temporal shift to a tectonic transition from water-fluxed melting linked to slab dehydration, a hallmark of classical arc magmatism, to adiabatic decompression melting driven by asthenospheric upwelling associated with the progressively rollback subduction of the paleo-Pacific plate. Our findings reveal that large magmatic systems record plate-scale tectonic reorganization via marked shifts in magma water contents and the petrological traits of volcanic deposits. Editorial summary: Caldera-forming eruptions record large-scale tectonic changes, reflected in a shift from water-rich to water-poor magmatic systems, according to geochemical and isotope analysis of a polycyclic caldera system in southeast China.