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 approximately 35.5 S in the Southern Volcanic Zone (SVZ) of the Chilean Andes, in a region that is compositionally transitional between more primitive compositions in the south and more evolved compositions in the north. The two main edifices, Cerro Azul and Descabezado Grande are early Holocene and latest Pleistocene in age and overlie the more mafic Casitas Shield. The sides of these edifices are deeply incised by Pleistocene glaciers, exposing stacks of lavas, which were sampled in seven vertical stratigraphic sections during two field seasons. At least ten different eruptive episodes, comprising 3-15 individual lava flows, have been identified primarily by various field criteria. 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.

The goal of this project is to use whole rock geochemistry, petrography and modal analysis, and mineral compositional data to determine the petrogenetic history of an eruptive episode in the approximate middle of the sections. Lavas sampled are primarily low-MgO (4.4-4.8 wt%), high Al2O3 (19.1-19.4 wt%) basalts (51.7-52.7 wt% SiO2), with rather low abundances of both incompatible elements (Zr, La, Rb, Ba) and compatible elements (Ni, Cr) for basalts of similar compositions elsewhere along the arc.

Geochemical data suggests fractional crystallization as the dominant process responsible for the lavas, followed by a period of recharge late in the episode. Lavas of this episode mark the end of a trend towards progressively lower Sr content (801-848 ppm), from earlier lavas characterized by some of the highest Sr abundances (1080 ppm) from the SVZ. Magma mixing dominates both very early and very late eruptive episodes of the shield, presaging the edifice-building events.

Similar comprehensive sampling at the nearby Tatara-San Pedro complex resulted in the development of a detailed composite volcanic stratigraphy. This study will compare lavas of the same age at the two neighboring volcanic centers with the goal of identifying regional/tectonic controls on magma generation and modification.