Chemistry and petrological evolution of the Lastarria volcanic complex in the north Chilean Andes

DOI: https://doi.org/10.1017/S0016756800008451

The Lastarria volcanic complex, along the northern Chile-Argentina border, includes three morphostructural components: Southern Spur, Lastarria (the highest cone, 5697 m) and Negriales (a geographically associated lava field, 5.4 km3). Petrographically, the Lastarria complex consists of pyroxene andesites and pyroxene-amphibole dacites. The whole-rock geochemistry shows a bimodial silica variation between 57 and 68%, with peaks at 59-60% and 61.62% SiO2. Petrographie and chemical data indicate different magmatic sources for Lastarria and Negriales. Whole-rock geochemistry can be explained by crustal contamination and crystal-liquid fractionation, with differences in storage times in magma chambers being a major controlling factor. Strong textural, mineralogical and chemical evidence for magma mixing, shortly before explosive eruptions at Lastarria, suggests that this process may have triggered the violent eruptive volcanic activity which characterizes the latest stages of the main cone. Abundant bombs of banded clear pumice and dark scoria in pyroclastic flow deposits are the texturally heterogeneous products resulting from incomplete mixing homogenization.