MONOGENETIC VOLCANOES AS WINDOWS INTO TRANSCRUSTAL MUSH: A CASE STUDY OF SLAMET AND LOYANG VOLCANOES, CENTRAL JAVA

Monogenetic volcanic fields (MVFs) are commonly associated with long-lived polygenetic composite volcanoes in many tectonic settings. The association between polygenetic and monogenetic volcanoes has raised questions as to the source of monogenetic melts – specifically, whether they are derived from the transcrustal mush underlying a nearby polygenetic volcano, or from a distinct batch of parental magma. Answering such questions provides insight into the magmatic drivers of eruptions, particularly where, among several possible eruptive events, magmas will erupt in hybrid polygenetic-monogenetic systems.

Gunung Slamet and a member of its extensive MVF, Gunung Loyang, were chosen for this purpose. Found in Central Java, Indonesia, the Slamet system experiences frequent VEI < 2 eruptions, while also being one of the most historically active volcanoes in the region. In this study, we analyzed the textural and chemical features of scoria and bombs from both vents.

We present analyses of the crystal cargo contained in the erupted lavas of both volcanoes, including olivine- and clinopyroxene-hosted melt inclusions. These melt inclusions are the first such analyzed for volatile element contents (H₂O and CO₂) in the Slamet-Loyang system, filling a crucial gap in our understanding of volatile budgets in the Sunda Arc. We then applied hierarchical clustering and mass balance models to constrain the features of the olivine populations in both systems.

The olivines in both volcanoes cluster into five distinct textural groups, defined by the presence, or lack of, zoning and resorption, and according to the type and degree of zoning they exhibit. Olivines show a range of moderate to evolved Fo%, and significant variation in nickel contents (50–1600 ppm). By estimating the saturation pressures of entrapped melt inclusions, we observe that Loyang olivines are drawn from a similar petrological system as Slamet, but from a deeper (12 km.) and more mafic source. Our work suggests that monogenetic volcanoes like Loyang can be used as “petrological windows” into the deeper storage of transcrustal mush at composite stratovolcanoes and their associated MVFs.