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

The Central Andean Volcanic Zone in South America contain many active volcanoes and volcano complexes due to constant subduction producing extrusive rocks varying in composition. The Lazufre Complex, along the 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, with younger activity at Lastarria between ~0.5-0.1 Ma. Typical phenocryst assemblage is plagioclase, orthopyroxene (opx), clinopyroxene (cpx) and minor amphibole.

Here, we present new mineral major and trace-element data combined with whole rock geochemistry and pressure and temperature conditions of two-pyroxene andesite and dacite lavas from Lazufre to better understand the magma plumbing system and relationship to the Lazufre magma body. Plagioclase phenocrsysts were analyzed for major elements by electron microprobe and for trace elements by LA-ICP-MS. Major- and trace-element data of plagioclase show heterogeneous compositions between the centers, which is different from whole rock major- and trace-element compositions. Plagioclase compositions range from An_35-78, with no difference in compositional range between centers. Light and middle REEs, such as Eu and La, are enriched, and heavy REEs, such as Yb, are depleted. Plagioclase range in FeO from 0.23-0.68% and 0.01-0.09% MgO, with no correlation between core and rim or between centers. Oscillatory Sr compositions are present in core to rim transects with ranges from 354-1041 ppm. The Ba concentrations range from 137-468 ppm, and the Eu concentrations range from 0.78-2.21 ppm. The overall trend of trace elements in feldspars is linear, which suggests magma mixing.

Detailed investigation of plagioclase compositions and 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 shared deeper zone at mid crustal levels (>20 km depth), a second zone at shallow-crustal levels (10-15 km depth) and several isolated storage zones distributed throughout the uppermost crust at <10 km depth.