MAGMA PETROGENESIS OF STRATIFGRAPHICALLY COLLECTED FLOWS FROM 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. Several authors have attributed this change in composition to differences in the crust to mantle ratio. The goal of this project is to use whole rock geochemistry, modal analysis and mineral compositional data to determine the petrogenetic history of a sequence of lavas sampled in stratigraphic order from the early shield stage of DGCA. Lavas sampled are primarily basaltic and have compositions ranging from SiO2=50.6-52.9 wt%, MgO=5.9-7.1 wt%, Ni=45-106 ppm, Cr=77-226ppm. Incompatible element (e.g. Zr, Rb, La) abundances are among the lowest reported for basalts in the SVZ.

The two main edifices, Cerro Azul and Descabezado Grande are early Holocene and latest Pleistocene in age and overlie the Casitas Shield. The sides of these edifices are deeply incised by Pleistocene glaciers, exposing complete stratigraphic sections composed of basement rocks and stacks of lava flows that overrode them. Basement rocks include the 7Ma granodioritic Invernada Pluton and metasedimentary rocks. Each flow in several of these vertical stratigraphic sections was sampled, facilitating the identification of separate eruptive events. The youngest flow of these sections has a preliminary date of .34Ma (Drake, unpublished data). 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 thee two neighboring volcanic centers with the goal of identifying regional/tectonic controls on magma generation and modification.

Processes potentially affecting the compositional variability include magma mixing, assimilation and fractional crystallization. Multiple parents and sources, polybaric fractionation histories, and differing thickness of continental rock would allow for differing amounts of modification from parental compositions.