Geochemistry of Lava Gamalama Volcano, Indonesia: Implications for Magma Genesis and Tectonic Setting

Abstract

Gamalama Volcano (Ternate Islands, North Maluku, Indonesia) is an active arc stratovolcano. To investigate magma genesis and the tectonic setting of recent Gamalama lavas, we combined petrographic observations with whole-rock major- and trace-element analyses. Five fresh samples, representing the Young Gamalama phase, were examined petrographically and analysed for major elements by XRF and for trace and rare-earth elements by ICP-OES/ICP-MS. The lavas are basaltic–andesitic (SiO₂ = 52.1–55.7 wt.%) and plot in the calc-alkaline, medium-K field. Petrography shows abundant plagioclase (andesine–labradorite), clinopyroxene, and Fe–Ti oxides embedded in the groundmass; local plagioclase chloritization records limited secondary alteration. Major-element variation diagrams (Harker, AFM) indicate early clinopyroxene and Fe–Ti oxide fractionation followed by late plagioclase crystallization, with subordinate evidence for periodic magma mixing. Trace-element and rare earth element systematics display large-ion lithophile elements and light enrichment and pronounced negative Nb–Ta anomalies, consistent with slab-influenced arc magmatism. The Nb–Ta depletion may reflect rutile sequestration in slab residues and/or transfer by saline slab-derived fluids; both mechanisms are considered viable. Tectonic discrimination diagrams (La/Yb vs Th/Nb; La/Yb vs Nb/La) place the samples in the volcanic-arc field, consistent with eastward subduction of the Molucca Sea Plate beneath Halmahera. These findings provide new constraints on petrogenesis of the Young Gamalama lavas and underscores the necessity of future multidisciplinary studies to unravel the volcano’s longer-term magmatic evolution.

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