Antecryst acquisition when eruptible and non-eruptible silicic magmas collide in the upper crust, illustrated by the ~ 14 ka Belfond event, Saint Lucia (Lesser Antilles)

Abstract Antecrysts are relic or co-magmatic phenocrysts that are common in volcanic rocks of arc settings. Their occurrence is a foundation of the crystal mush paradigm for crustal magmatism, although the putative mush is rarely erupted. The 13.6 ka Belfond eruption (Saint Lucia) produced dacite lava with glass-bearing diorite enclaves, previously shown to have synchronous zircon crystallisation extending to eruption. The cummingtonite-bearing dacite and Ca-amphibole-bearing diorite are porphyritic, and the diorite lacks cumulate features. Most of the dacite plagioclase phenocrysts record late-stage partial resorption and a high 87 Sr/ 86 Sr overgrowth not unlike the 87 Sr/ 86 Sr value of the groundmass. Their low 87 Sr/ 86 Sr interior zone indicates they are antecrystic. Furthermore, the textures and melt inclusion compositions of dacite plagioclase resemble those in the diorite enclaves. Conversely, relatively high 87 Sr/ 86 Sr overgrowths on plagioclase and cummingtonite overgrowths on Ca-amphiboles in the diorite record late-stage melt percolation. Thus, we envisage a more radiogenic dacite magma intruded a crystallising mushy diorite magma during ascent. Cummingtonite formation (P H2O = 200–300 MPa; ~ 7–8 km) indicates this occurred in the upper crust. The Ca-amphibole and plagioclase populations in the enclaves both display disparate inter-crystal zonation patterns and, in the latter, interior zone Sr-isotope discordance. These features indicate the diorite was incrementally built by earlier magma inputs, concordant with its long zircon history. Rather than the diorite and dacite having a parent-product connection, the Belfond rocks show that magmas stalled in the crust may become mineralogically heterogeneous from prolonged inputs and that eruptible magmas gain their antecrystic cargo from interacting with such sources.