PUMICE COMPOSITIONS AND MINERAL CHEMISTRY OF BIMODAL PUMICE ERUPTED FROM LASCAR VOLCANO, CENTRAL VOLCANIC ZONE, CHILE

Lascar volcano is the most active volcano in the Central Volcanic Zone of the Andean Cordilleran, with 36 Holocene eruptions and a VEI 4 event in April, 1993. The volcanic evolution was described by Gardeweg et al. (1998) who suggested four stages of volcanism dating back to approximately 46 ka. Four major eruptive products define the phases including the Piedras Grandes unit (>26.5 ka), the Soncor ignimbrite (26.5 ka), the Tumbres-Talabre unit (7.1-9.3 ka) and the historic pyroclastic deposits (1848-present). Here we present whole rock geochemistry and plagioclase and pyroxene mineral chemistry from pumice from the Soncor ignimbrite, the 7.1 ka Tumbres lava flow, and the 1993 pyroclastic deposit. The Soncor pumice is a two-pyroxene andesite containing compositionally zoned plagioclase phenocrysts ranging from An_32-84. Calcic compositions are commonly rounded and sieved with more sodic rims. Tumbres lava flow and pumice are andesites of similar composition, but the lava flow contains olivine. Historic pyroclastic flows from the 1993 eruption consist of two different pumices: a light-colored dacitic pumice and a darker andesitic composition. The dacitic pumice contains two-pyroxenes, biotite, zoned plagioclase and minor apatite. The andesitic pumice contains two-pyroxenes, zoned and unzoned plagioclase and minor hornblende. Plagioclase from both pumices are more restricted in composition than the Soncor or Tumbres units, ranging from An_58-78.

The seismically imaged southern border of the Altiplano-Puna Magma Body (APMB) is approximately 10 km north of Lascar and is hypothesized to be the source of the dacitic material. Influence of this body is suggested to be observed through homogenous whole rock chemistry and plagioclase core compositions. We suggest that the banded pumices and bimodal pumices of the Soncor ignimbrite through the 1993 eruption are influenced from injection of homogeneous dacite to andesite from the APMB into upper crustal dacite shallow magma chamber which directly feeds eruptions at Lascar. To further constrain the magmatic evolution of Lascar’s reservoir and the connection of the APMB, this study will use Sr in plagioclase geospeedometry to constrain magma residence time between the Lascar eruptive phases and determine the relationship of the bimodal pumice compositions.