CHEMOSTRATIGRAPHIC ANALYSIS AND PETROGENETIC MODELLING: MAGMA CHAMBER EVOLUTION IN VOLCAN CERRO AZUL, CHILEAN ANDES

Volcan Cerro Azul is one edifice in a large-volume Holocene volcanic complex, located in the Transitional Southern Volcanic Zone of the Chilean Andes. Glacial activity and mass wasting have dissected the slopes of the volcano, exposing numerous stratigraphic sections. Every lava flow was sampled in four of these vertical stratigraphic sections, and the samples were analyzed for complete major and trace element abundances. This sampling strategy facilitates the construction of chemical stratigraphic columns revealing both the long-term geochemical evolution of the complex and short-term trends related to individual eruptive episodes. Seven different eruptive episodes, and associated eruptive hiatuses, were identified based on field observations such as soil development, erosion, mingled flow margins, and flow extent and morphology. Petrogenetic models could then be constructed for individual eruptive episodes, instead of one model for the entire complex. This study examined the oldest episode exposed in these sampled sections, containing lava flows CSN.2-10.

Petrographic data (e.g. point counts, textural analysis) and phenocryst compositions (using SEM-EDS and microprobe) were collected for all the flows. These data were then compared to the whole rock geochemical characteristics in order to assess various petrogenetic models. Geochemical trends encompassing all the flows in a composite stratigraphic section are evidence of the accumulated effects of different petrogenetic processes on the entire magmatic plumbing system. Short-term trends as revealed in changes within a single eruptive episode may reflect the dominance of the final process occurring in, perhaps, rather shallow chambers. For example, strontium (Sr) abundances for all fifty-two basalt and basaltic andesite flows sampled generally decrease upsection (1081 to 638 ppm) whereas Sr abundances during several episodes (including flows CSN.2-10) show a marked increase. Petrographic evidence for disequilibrium (zoned phenocrysts, rims, etc.) and size of phenocrysts decreases upsection, coupled with an increase in percent groundmass.