Magmatic architecture across tectonic settings – the case of calc-alkaline vs. potassic volcanoes in the Sunda arc, Indonesia

Magmatic plumbing systems are vertically extensive, complex, and dominated by crystal mush (Cashman et al., 2017). Across tectonic settings, distinct lithospheric architecture, magma flux and magma composition modulate the anatomy and dynamics of magma plumbing systems (Ubide et al., 2023). Understanding magmatic architecture in convergent margins is of particular interest, because volcanic arcs host explosive eruptions, build the continental crust, and can accumulate critical metals, such as in porphyry copper deposits. Magma plumbing systems in volcanic arcs are commonly transcrustal; however, differences in magma flux (Best et al., 2016) and tectonic context, which modulates magma composition (Ward et al., 2024; Bennett et al., 2025), impose notable differences that have remained underexplored.

 

Here, we focus on the Sunda arc (Indonesia) as a natural laboratory to constrain differences in magmatic architecture from typical calc-alkaline volcanoes in the main arc to anomalous volcanoes located off-axis, where the slab reaches depths >200 km and erupted magmas become strongly alkaline and silica-undersaturated (potassic; Ward et al., 2024; Bennett et al., 2025). By applying novel high-resolution petrology to the crystal cargo (Davidson et al., 2005; Ubide & Kamber, 2018; MacDonald et al., 2023) and the carrier magmatic liquids (rock groundmass; Ubide et al., 2023), we resolve differences in storage depths and temperatures, magma dynamics and eruption triggers across the arc. We find that relative to typical calc-alkaline volcanoes, the anomalous alkaline volcanoes are characterized by deeper storage and more mafic compositions, which can trigger eruptions rapidly filtering eruptible liquids, similar to observations in ocean island settings (Ubide et al., 2022). Our new insights aim to assist monitoring of future eruptions and improve understanding of the architecture of magmatic systems that become fertile for mineralization in critical metals.

 

Best et al., 2016 Geosphere

Cashman et al., 2017. Science

Davidson et al., 2005 JVGR

Edmonds et al., 2019. Philosophical Transactions A

MacDonald et al., 2023 J Petrol

Ubide and Kamber, 2018 Nature Commun

Ubide et al., 2022 Geology

Ubide et al., 2023. The Encyclopedia of Volcanoes (pre-print in EarthArXiv)

Ward et al., 2024 EPSL