PETROLOGY, MINERAL CHEMISTRY, AND GEOTHERMOBAROMETRY OF ANDESITES FROM THE LAZUFRE COMPLEX, CENTRAL ANDES, CHILE: INSIGHTS INTO THE PRE-ERUPTIVE MAGMA PLUMBING SYSTEM

The Central Andes in South America contain many active volcanoes and volcano complexes due to constant subduction producing extrusive rocks varying in composition. The Lazufre Complex, Chile-Argentina border, consists of two Pleistocene volcanic centers, Lastarria and Cordon del Azufre, erupting medium- to high-K, calc-alkaline andesite and dacite lava flows and domes. Whole-rock K-Ar dates of lavas from Cordon del Azufre place the most recent eruptions at 0.6-0.3 Ma ±0.3 Ma. The most recent eruptive activity at Lastarria has been dated at ~0.5-0.1 Ma. Typical phenocryst assemblage is plagioclase, orthopyroxene (opx), clinopyroxene (cpx) and minor amphibole.

Here we determine the pressure and temperature conditions of two-pyroxene andesites and dacites from Cordon del Azufre combined with temperatures and pressures from Strechern et al. (2017) to better understand the magma plumbing system and relationship to the Lazufre magma body. Plagioclase, pyroxene, and Fe-Ti oxides were analyzed using LA-ICPMS and electron microprobe analysis (EMPA). Major- and trace- element data of plagioclase and pyroxene show homogeneous compositions between the centers, which is consistent with whole rock major and trace element compositions. The primary composition of the pyroxenes are augite and diopsides for cpx and hypersphene for opx and the primary composition of the plagioclases are An_35-78. REE diagrams show the samples enriched in Eu and La, and depleted in Yb. The overall trend of trace elements in feldspars is linear, which suggests magma mixing. Whole-rock geochemistry shows 61-62% silica, 5.2-5.5% FeO, 2.6-3.3% MgO, and 5.2-5.5% CaO. The Fe-Ti oxide thermo-oxybarometric, and cpx and cpx-liquid thermobarometric models were used. Initial temperatures and pressure calculations of both centers range from 970-1070°C and 60-700 MPa, respectively.

Detailed investigation of plagioclase textures alongside crystal size distribution analyses provide evidence for magma mixing as a major pre-eruptive process that blends multiple crystal cargoes together. Distinct magma storage zones are suggested, with a deeper zone at mid crustal levels (>20 km depth), a second zone at shallow-crustal levels (10-15 km depth) and several magma storage zones distributed throughout the uppermost crust at <10 km depth.