CHEMICAL STRATIGRAPHY AND PETROGENESIS OF THE CASITAS SHIELD, DESCABEZADO GRANDE-CERRO AZUL VOLCANIC COMPLEX, CHILEAN ANDES

The Descabezado Grande-Cerro Azul (DGCA) volcanic complex is located at approximately 35.4 degrees S in 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 the Casitas Shield. This plateau has been deeply encised, revealing stacks of lava flows exposed along the valley walls. More than 140 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, with at least twelve eruptive episodes identified by field observations and confirmed by nine 40Ar/39Ar dates. The second lowest flow in the composite section yielded an age of 0.51+-0.05 Ma, and the majority of flows in the composite section erupted in the range .47-.43 Ma. These flows are primarily basalts/basaltic andesites characterized by generally high incompatible element abundances, particularly Sr (800-1080 ppm), and rather low Ni and Cr. The composite chemical stratigraphy shows changes in lava compositions reflecting both different parental magmas and the secular dominance of different petrogenetic processes. In general, lavas become more evolved through episodes 1-5 during periods of seemingly low eruptive rates. Lavas are more compositionally similar during episodes 6-8, and sparsely olivine-phyric basalts erupted during episode 9 (.2 Ma younger), including lavas that are among the most primitive in the SVZ with high Cr (254-267 ppm) and MgO (8-8.3%), and low LILE, P2O5, Al2O3, Na2O, and K2O. Lavas erupted during episodes 10 and 11 become progressively more evolved. The shield is capped by aphanitic basalts, with the youngest age of 0.10 +- 0.09 Ma.

Strontium (87Sr/86Sr) ratios increase from 0.703578 near the bottom of the composite section to 0.704033 in the upper flows, paralleling a general trend towards decreasing Sr abundances with time. Nd and Pb isotope data are less systematic in variation, with 143Nd/144Nd ratios ranging from .512762 - 0.512811. Several cross-cutting basaltic dikes are characterized by very high MgO (11.7%), Ni (226 ppm) and Cr (776 ppm) contents, very low Nb, Zr, Y, Rb, and Ba, and the lowest 87Sr/86Sr ratios (.703533).