Magmatic architecture of a potassic arc volcano: Batu Tara, eastern Sunda arc, Indonesia

Abstract Subduction zones produce arc volcanism that is typically calc-alkaline. However, unusual subduction geodynamics may lead to anomalously alkaline volcanoes. Despite potentially hosting explosive eruptions and ore deposits, the magmatic architecture of spatially and geochemically anomalous arc volcanoes remains underexplored. Batu Tara volcano (eastern Sunda arc, Indonesia) is located off-axis to the main arc, where the slab reaches depths of ~230 km. The volcano produces ultrapotassic mafic magmas with abundant phenocrysts of clinopyroxene and leucite. To explore magma transport and storage prior to volcanism at Batu Tara, we combine high-resolution mineral and groundmass petrology and apply a range of thermobarometry models on lava flows and cross-cutting dykes. The microcrystalline groundmass is typically phonotephrite that defines a Si-undersaturated alkaline trend with broader variations in the dykes than in the lavas. Clinopyroxene, the main phenocryst phase, shows complex growth zoning with a vast range of compositions from Mg# 55-91. Alternating low-Mg# and high-Mg# zones have distinctive green and white colour, respectively, in plane polarised light. All clinopyroxene compositions are cogenetic and record pre-eruptive histories within a main region of magma stagnation in the middle crust (green zones; 1062 ± 26 °C; ~300 MPa, ~10 km depth), commonly replenished by hot, mantle-derived mafic magmas (white zones; 1105 ± 46 °C). Repeated replenishment and mixing are consistent with sector zoning across clinopyroxene populations, with mafic rejuvenation acting as an eruption initiation mechanism that results in compositionally homogeneous liquids (groundmass) that carry complexly zoned phenocrysts. The lavas are plagioclase-free and fed directly from the main reservoir. In contrast, the dykes contain plagioclase phenocrysts, which suggest cannibalism of degassing mushes at shallower crustal depths. Our data suggest that Batu Tara volcano, and possibly other anomalous arc volcanoes dominated by mafic compositions, are fed by a relatively direct transfer of mafic melts from depth to the surface, with shallow mushes developed during maturation of the magma feeder system.