Multi-Tiered Magmatic Plumbing System of the Tatun Volcano Group, Taiwan: Insights from Mineral Thermobarometry and Reservoir Evolution

The potential reactivation of the Tatun Volcano Group (TVG) in northern Taiwan has raised significant public and geohazard concerns. To constrain magma stagnation depth and reconstruct the plumbing system architecture of the TVG, this study applies mineral thermobarometry to 33 core samples from four andesitic lava units (Lv1–Lv4) retrieved from a deep borehole at Mt. Tatun. Mineral chemistry and petrography reveal polybaric crystallization, stagnation, and mixing across multiple crustal levels. Magma evolution progressed from deep-crustal orthopyroxene crystallization to mid-to-shallow-crustal amphibole/clinopyroxene crystallization, depicting a crystal-stagnation ceiling at 4km. This evolution was accompanied by mixing of ascending mafic magmas and stagnated, amphibole-bearing intermediate magma batches. Prior to eruption, the stagnated magmas, initially in a highly crystallized lock-up state, were reactivated by replenished magma recharge, triggering multi-stage causing stepped amphibole decomposition (decompression rims and thermal oxidation) and promoting dominant feldspar microlite accumulation over fractionation. Amphibole decompression rims yield slow magma ascent rates of 0.004 to 0.011 m/s, consistent with the widespread distribution of effusive lava flows across the TVG. Comparing these petrological results with modern seismological data reveals that the paleo-reservoir spanned a wider range of magma-ponding depths (4–22 km). The dimensional retreat of the main reservoir from the late Pleistocene to the present (8–20 km) suggests a significant reduction of the TVG’s magma volume, with the reservoir ceiling descending from 4 km to 8 km depth over 110 kys. The observed low-velocity magma ascent is linked to post-collisional crustal rebound and asthenosphere upwelling within the extensional regime of the Northern Taiwan Volcanic Zone. Although the modern, stable stress field implies a recessed magma supply relative to the Pleistocene, low-explosivity effusive activity remains a potential long-term threat. A known hydrothermal reservoir at 2–4 km depth may serve as the final staging area priming future magma extrusion.