T40 Creative Contributions in Volcanology and Petrology: In Honor of Katharine V. Cashman, Recipient of the 2023 Distinguished Geological Career Award from the Mineralogy, Geochemistry, Petrology and Volcanology Division (Posters)

Sunday, 15 October 2023
8:00 AM - 5:30 PM
Hall B

PETROLOGICAL FORENSICS AND MAGMA DYNAMICS AT TICSANI VOLCANO IN SOUTHERN PERU

Paper number: T40-20
Program: Technical Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-393432

© Copyright 2023 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.