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. All studied lithologies have a phenocryst assemblage of plagioclase>amphibole>biotite>Fe-Ti oxides. The crystal-rich pumice contains abundant microphenocrysts of orthopyroxene. Plagioclase (An₂₆-An₆₅) phenocrysts in all samples tend to have patchy, anhedral cores with thick oscillatory zoned rims. Plagioclase 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 are bimodal (Mg-Hastingsite, Mg-Hornblende) with rare tschermakites in both the lava and crystal-rich pumice. Amphibole thermobarometry suggests lower temperature (777-885 °C) and pressure (1.1-2.4 kbar) conditions for Mg-Hornblende with higher T (877-962 °C) and P (2.1-4.1 kbar ) for Mg-Hastingsite. 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. Amphiboles record a magmatic architecture that spans the majority of the upper crust, with two zones of magma storage and evolution: a deeper zone, associated with higher Sr/Y values, suggests magma evolution prior to the stabilization of plagioclase, and an upper zone where plagioclase textures and compositions record multiple episodes of assimilation and fractionation leading to a unique locus of recent silicic volcanism in southern Peru.