Magmatic water variation in the Neo-Tethys: Controlled by convergence rate and slab geometry

Water content in magmatic systems exerts a pivotal control on processes from melt generation to eruption, yet the magmatic water variation from subduction to collision and its drivers remain enigmatic. The Gangdese belt in southern Tibet, which archives magmatism from Neo-Tethyan oceanic lithosphere subduction to collision, provides a unique temporal record to address this gap. We apply a fractional crystallization model to both major and trace elements of whole-rock geochemistry data in the Gangdese belt, and we use the difference between amphibole and plagioclase proportions to indicate the variation in magmatic water. Combined with numerical simulations, we link the variation in magmatic water of the Neo-Tethys to the change of convergence rate and slab geometry, including normal subduction, slab rollback, and flat-slab subduction. Low convergence rates and slab rollback elevate temperatures, enhancing water release and magmatic hydration, while high convergence rates and flat-slab subduction cool the system, limiting slab dehydration. From 215 Ma to 160 Ma, shifting slab geometry from normal to rollback to flat-slab matched with magmatic water variability, whereas 160−15 Ma variations correlated with convergence rate changes. Using the fractional crystallization model, we obtain the variation in magmatic water of the Neo-Tethys subduction zone. We further explore its connection to the change of subduction parameters, advancing understanding of subduction zone water cycles and their broader implications for arc magmatism.