Abstract Supported by the mush model, the origin of crystal-poor, high-silica rhyolitic magmas (≥ 75 SiO₂ wt.%) is commonly linked to melt extraction in shallow, crystalline mushes, yet their complementary cumulates remain elusive in the upper crust. Here we present the “dynamic mush model” for the Campo Alegre-Corupá system (Brazil), combining textural analysis and thermodynamic modeling to show that high-silica melts formed through upper-crustal fractionation, leaving a granitic residue and feeding a caldera-forming eruption. Syenites and melasyenites represent their silicic-mafic cumulates, the syenites being formed at relatively low crystallinities (~16-33 of bulk vol.%) after extraction of large amounts of interstitial melts (~47-80 of liquid vol.%). In contrast to static models (extraction window ~50-70 vol.%), our results indicate early melt segregation in a dynamic reservoir. Alkali-feldspar crystals were size-selectively and hydraulically sorted by upward melt flow; aggregates were then formed at low crystallinities, supposedly via synneusis, enabling rapid sinking, and were repacked by recharge. This process, enhanced by relatively high magma fluxes, efficiently separated crystals from melt, explaining the origin of large-volume eruptible magmas in the upper crust. Our findings redefine mush evolution while underscoring the role of flow-driven crystal sorting (elutriation) in caldera-forming systems.
Lino et al. (Sat,) studied this question.