PETROLOGICAL FORENSICS AND MAGMA DYNAMICS AT TICSANI VOLCANO IN SOUTHERN PERU

Volcán Ticsani is a Holocene andesitic-dacitic dome complex located 30 km SE of Volcán Huaynaputina in the southern Peruvian Andes. These volcanoes represent a recent episode of explosive silicic volcanism in a region dominated by andesitic composite cones. Despite being closely related to Huaynaputina and designated as a "high-risk" volcano by the Geophysics Institute of Peru, Ticsani's magmatic history remains poorly understood. We present here a preliminary petrological investigation of Ticsani.

Ticsani rocks define a high-K andesite-dacite with K2O/Na2O typical of Central Volcanic Zone silicic rocks, but are distinct in their high Sr/Y. Our analyses focused on a crystal-rich andesitic pumice, crystal-rich dacite lava, and three crystal-poor dacitic pumice. The crystal-rich pumice and lava samples have more evolved glass compositions (~75-76% SiO₂), whereas the crystal-poor pumice samples have less evolved glass compositions (~73% SiO₂). All studied lithologies have a phenocryst assemblage of plagioclase>amphibole>biotite>Fe-Ti oxides. The crystal-rich pumice contains abundant microphenocrysts of orthopyroxene. Plagioclase (An₂₅-An₆₅) phenocrysts in all samples tend to have patchy, anhedral cores with thick oscillatory zoned rims with varying degrees of sieve texture. Amphiboles are bimodal (Mg-Hastingsite, Mg-Hornblende) with rare tschermakites in both the lava and crystal-rich pumice. Amphibole thermobarometry suggests crystallization temperatures of 777-962 °C and pressures of 1.1-4.1 kbar. T and P uncertainties are ±22°C and ±12% respectively.

Whereas Ticsani's K₂O/Na₂O is higher than that of Huaynaputina, both share high Sr/Y, indicating a shared deeper magmatic origin. Amphibole compositions indicate a shallow reservoir of magma storage that spans the upper crust from ~15 to 4 km while isotopic characteristics indicate a deeper magmatic origin, leading to a unique locus of recent silicic volcanism in southern Peru.