Petrological Forensics and Magma Dynamics at Ticsani Volcano in Southern Peru

Volcán Ticsani is a Holocene andesitic-dacitic dome complex in the Central Volcanic Zone (CVZ) of the Andes. This volcano represents a recent episode of explosive silicic volcanism in a region dominated by andesitic composite cones. Despite being designated as a "high-risk" volcano, Ticsani's magmatic history remains poorly understood. We present here a preliminary petrological investigation of Ticsani using whole-rock geochemistry, petrography, and electron microprobe analyses.

Our analyses focused on a crystal-rich andesitic pumice, crystal-rich dacite lava, and three crystal-poor dacitic pumice. Ticsani rocks have bulk-rock characteristics typical of CVZ silicic rocks but are distinct in their trace element characteristics. All studied lithologies have a phenocryst assemblage of plagioclase>amphibole>biotite>Fe-Ti oxides. Plagioclase (An25-An65) phenocrysts in the crystal-rich pumice are normally zoned and non-sieved, whereas plagioclase in the dacitic lava, also normally zoned, occasionally exhibits sieve texture with thin euhedral rims. In contrast, plagioclase in the crystal-poor pumice tend to be fractured, sieved, and reversely zoned. Amphiboles in all samples are bimodal in composition (Mg-Hastingsite, Mg-Hornblende). Amphibole thermobarometry suggests crystallization temperatures of ~750-975 °C at pressures ranging from 1-4 kbar. T and P uncertainties are ±22°C and ±12% respectively.

Trace elements in Ticsani rocks indicate a deep magmatic origin. Amphibole thermobarometry reveals a shallow zone of magma storage that spans the upper crust from ~15 to 4 km. Plagioclase zoning and textures record crystallization in the shallow upper crust under variable magmatic conditions. These findings offer valuable insights into the magmatic conditions preceding explosive silicic eruptions.