Investigating the connection between superhydrous melts, magma reservoir nucleation and the scarcity of mafic eruptions in post-collisional settings

Magma reservoir emplacement in the brittle upper-crust typically requires prolonged heating to modify host-rock rheology, allowing magma to accumulate rather than erupt. In thermally immature crust, early eruptions are expected to be mafic, yet many volcanic provinces, particularly those with water-rich melts, lack significant initial mafic volcanism, suggesting an alternative trapping mechanism. We investigate this using Caraci volcano (Romania's Apuseni Mountains), a water-rich system that started with relatively differentiated, andesitic eruptions. The first eruptions at Caraci extruded water-rich (5–6 ± 0.35 wt% dissolved H 2 O) amphibole-plagioclase andesites (890–920 ± 22 °C) stored at water-saturated conditions in the upper-crust (~2 ± 0.5 kbar). These magmas were associated with superhydrous primitive melts (>6 wt% H₂O), which evolved in a polybaric system (crystallization pressure varying from 2 to 8 ± 0.5 kbar), evidenced by high-Mg amphibole and delayed plagioclase crystallization. These deeper storage melts likely reached water saturation during ascent towards the upper crust, inducing degassing and crystallization, which increased viscosity and enabled stalling in the cold crust, explaining the lack of initial mafic eruptions and providing the efficient trapping mechanism in a thermally immature crust. Progressive drying of the source and generation of less hydrous magmas initiated the eruptions and construction of the stratovolcano, and ultimately terminated the system. As a result, the final eruption extruded somewhat drier (4–4.5 ± 0.35 wt% H 2 O), hotter (930–960 ± 45 °C) pyroxene-bearing andesites erupted from water-undersaturated storage. This trapping mechanism of water-rich magmas may explain the typical scarcity of mafic volcanism in arc and post-collisional settings. • Degassing-driven crystallization enables magma trapping during ascent. • Superhydrous melts stall in the cold upper-crust, suppressing early mafic eruptions. • This mechanism explains the scarcity of early calc-alkaline mafic eruptions in arcs. • High-Mg amphibole evidences deep, primitive superhydrous magmas. • Progressive source drying controls volcano construction and shutdown.