UNDERSTANDING THE PETROLOGY AND GEOCHEMISTRY OF BIOTITE-BEARING DACITIC/RHYOLITIC TEPHRA-FALL DEPOSITS FROM MISTI VOLCANO, PERU

Misti, a major stratovolcano in the Central Volcanic Zone of southern Peru, has produced frequent explosive eruptions and emplaced wide-spread tephra-fall deposits throughout its history. Misti is largely an amphibole-bearing andesitic volcano but sporadic dacites and rhyolites have erupted. A biotite-bearing dacite-rhyolite sequence of at least four tephra-fall deposits emplaced between ~40 ka and ~25 ka is particularly noteworthy. From oldest to youngest the deposits are informally named Cogollo, Anchi, Sacarosa, and Conchito. We characterize the whole-rock geochemistry, petrography, and crystal and glass compositions of the Cogollo, Anchi, and Conchito to understand the unique succession of Misti’s biotite-bearing felsic units.

Petrographic analyses of two pumice each from the Cogollo and Conchito show that despite both being biotite-plagioclase bearing units, there are notable differences in crystal size and abundance. Crystals in the Conchito pumice are largely anhedral, whereas those in the Cogollo are euhedral. In general, crystals in the Cogollo are larger, but less abundant than those in the Conchito. Specifically, the Cogollo pumice contain ~7% crystals (1–2% biotite and 4–5% plagioclase), whereas the Conchito pumice contain 10–17% (3–7% biotite and 7–10% plagioclase). The Conchito’s biotite average 0.4 mm in long dimension and plagioclase average 0.6 mm, whereas the Cogollo’s biotite average 0.7 mm and plagioclase average 1.1 mm. A distinct flow texture is also present throughout most Cogollo pumice but is less common in the Conchito. Additionally, sparse small amphibole are present in the Conchito, but not the Cogollo.

Whole-rock x-ray fluorescence data demonstrate that the Cogollo and Anchi are high-silica rhyolites (av. ~72–73 wt.% SiO2), while the Conchito, like the Sacarosa, is dacite (~65 wt.% SiO2). All are high-K calc-alkaline units like Misti’s other deposits but such a contiguous sequence of biotite-bearing rhyolite and dacite is anomalous in the volcano’s andesite-dominated history.

Glass and crystals from the Cogollo, Anchi, and Conchito will be analyzed by electron microprobe. The resulting data will be used in conjunction with textural evidence to determine the pre-eruption magma storage and ascent conditions that led to eruption of this unique group of deposits.