AGE AND PETROGENESIS OF LAVAS FROM THE CASITAS SHIELD, DESCABEZADO GRANDE-CERRO AZUL VOLCANIC COMPLEX, CHILEAN ANDES

The Descabezado Grande-Cerro Azul (DGCA) volcanic complex is located at approximately 35.5 degrees S in the transitional Southern Volcanic Zone (SVZ) of the Chilean Andes. The complex is characterized by two large latest Pliocene to early Holocene volcanic edifices and many smaller vents rising above a plateau comprising lavas of an earlier shield-building stage, the Casitas Shield. This plateau has been deeply encised, revealing stacks of lava flows exposed along the valley walls. More than 100 lava flows were sampled from eight vertical stratigraphic sections and have been analyzed for complete major and trace element abundances. These were compiled in a composite chemical stratigraphy, and at least twelve eruptive episodes were identified based on field observations (soil development, erosional features, mingled top and bottom rubble zones, etc.). Several long flows served as the basis in the field for physical correlations between sections, and were also used to evaluate within-flow compositional variation.

Nine new 40Ar/39Ar dates have refined this composite stratigraphy. The second lowest flow in the composite stratigraphic section (CSN.2) yielded an age of 0.51+-0.05 Ma. Approximately 32 more flows from the CSN section and 20 more from other sections erupted in the range .47-.43 Ma. These flows are primarily basalts and basaltic andesites characterized by generally high incompatible element abundances, particularly Sr (800-1080 ppm), and rather low Ni and Cr. Unconformably overlying these lavas is a sequence of compositionally monotonous, sparsely olivine-phyric basalts more than .2 Ma younger. These lavas are characterized by higher Cr (254-267 ppm) and MgO (8-8.3%), and low LILE, P2O5, Al2O3, Na2O, and K2O, and are the closest to possible parental compositions. The shield is capped by a set of eleven aphanitic basalt flows from Volcan Cerro Azul, with the youngest flow yielding an age of 0.10 +- 0.09 Ma. A nearby vitrophyric dacite dome represents the youngest sampled lavas with an age of 0.10 +- 0.03 Ma. The new dates also provide a basis for comparing the DGCA lavas to flows of similar age at the neighboring Tatara-San Pedro complex. The excellent composite stratigraphy developed at each complex affords a rare opportunity to compare coeval petrogenetic processes at two neighboring large volcanic complexes.