From Magmatic to Phreatic: A Two-stage Eruption History of Chihsing Volcano, Northern Taiwan

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Abstract
Chihsing Volcano represents the youngest and highest edifice within the Tatun Volcano Group (TVG) of northern Taiwan, yet the timing and character of its recent magmatic activity have remained poorly constrained. This study integrates 40Ar/39Ar and 14C geochronology, mineral chemistry, whole-rock geochemistry, and Sr–Nd isotopes from drill cores to reconstruct late-stage evolution of the volcano. Our results establish a two-stage eruptive model for the Late Pleistocene to Early Holocene transition. The first stage (16–8 ka) featured magmatic eruptions producing calc-alkaline andesitic lava flows. 40Ar/39Ar ages (14.0 ± 2.2 to 11.6 ± 3.6ka) mark these as TVG's youngest reliably dated magmatic units, demonstrating persistence far later than previously recognized. Mineral thermobarometry and geochemistry indicate origin from a homogeneous 7–15 km deep crustal reservoir (1135–1184°C) with repeated recharge events producing thorough magma homogenization evidenced by narrow isotopic (εNd = +2 to +3) and geochemical ranges, and negligible crustal contamination. The second stage (~8 ka) transitioned to phreatic activity. 14C dating of crater-fill sediments confirms sedimentation initiation around 7.4 ka, with rates increasing from 0.02 to 0.17 cm/yr after 3.6 ka. Ongoing phreatic activity continues through residual magmatic heat driving pyroclastic density currents, tephra fallout, and landslides. These findings reveal a persistent magma supply system beneath the TVG and suggest that the potential for magmatic transport remains active. Consequently, future hazard assessments for the Taipei metropolitan area should integrate both phreatic and potential magmatic eruption scenarios.

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