GEOCHEMISTRY AND PETROGENESIS OF LAVAS FROM THE CASITAS SHIELD, VOLCAN CERRO AZUL, SOUTHERN VOLCANIC ZONE, CHILE

The Cerro Azul/Descabezado Grande (DGCA) volcanic complex is located at 35.5o S in the Southern Volcanic Zone (SVZ) of the Chilean Andes. The two main edifices, Cerro Azul and Descabezado Grande are early Holocene and latest Pleistocene in age and overlie the more mafic Casitas Shield. Pleistocene glaciers deeply incised the sides of these edifices exposing vertical stacks of lavas. Lavas were sampled in seven vertical stratigraphic sections during two field seasons in order to construct chemical stratigraphic columns. All samples were analyzed for complete major and trace element abundances. At least ten different eruptive episodes, comprising 3-15 individual flows, have been identified primarily by various field criteria. The petrogenesis of each episode may be modeled separately, revealing the influence of short-term, shallow processes superimposed over the longer-term behavior of the volcanic complex with time. Geochemical changes with time (upsection) include increases in Zr and Ba, La, Ce, and Rb for the first four episodes (flows 1-18), then sharp decreases during the next episodes (flows 19-34), while Y and TiO2 both increase steadily with time (12-19 ppm and .8-1.1 wt% respectively); Sr decreases steadily from 1100 to 801 ppm. Demonstrably mixed magmas were sampled during most of the eruptive events. Samples from two physically correlative stratigraphic sections (flows CSN.25-33 and CSS.17-24) were examined in this study. Flows studied were compositionally similar, relatively high-Al basaltic andesites (Al2O3=19.1-19.2 wt%; SiO2=51.7- 52.7 wt%) that are high in Sr, Al2O3 and Na2O and low in Zr, Ni, Cr, MgO, and CaO relative to other lavas of similar SiO2 content in the SVZ. Data from whole rock geochemistry, petrography and modal analysis, and mineral compositional data were combined to suggest that high rates of magma supply prompted mixing of two magmas, one slightly more evolved than the recharge composition. Mineral zoning patterns and compositions suggest that the chamber magma had undergone limited fractional crystallization prior to recharge and subsequent eruption. Within-flow geochemical variation was evaluated by comparing samples taken from the same flows exposed in section CSN and the more distal section CSS, approximately 1.5 km apart.