PETROGENETIC EVOLUTION OF LAVAS FROM THE CASITAS SHIELD, DESCABEZADO GRANDE- CERRO AZUL VOLCANIC COMPLEX, CHILEAN ANDES

The Descabezado Grande Cerro Azul Volcanic Complex is located in the Southern Volcanic Zone (SVZ) of the Chilean Andes. It is characterized by the glacially incised Casitas Shield, a plateau comprised of multiple stacks of lava flows, and two major volcanic edifices, Descabezado Grande and Cerro Azul. Samples were collected in stratigraphic order from eight vertical sections from the southern flank of the Casitas Shield and analyzed for complete major and trace element concentrations. This study focuses on samples from three eruptive episodes exposed in the CDCS section in order to determine the petrogenetic processes responsible for modifying the magma composition from the original source. These flows are predominantly basalts to basaltic andesites with low abundances of most incompatible elements (e.g. Ba, La, Ce, Rb, Zr) relative to other SVZ lavas. The oldest eruptive episode of the three (CDCS 8-15) is characterized by primarily aphanitic lava flows that show little chemical variability. Field evidence (e.g. lack of soil horizons, mingled rubble zones) strongly suggests that flows of this episode erupted over a short period of time. Lavas (CDCS 16-20) of the next eruptive episode decrease in incompatible elements while increasing in compatible elements with time. Abundant plagioclase glomerocrysts are present in a gray aphanitic matrix. Lavas (CDCS 21-24) of the youngest eruptive episode are characterized by sharp increases in Ca and Fe⁺³ and sharp decreases in K and Na suggesting replenishment of these elements from magma mixing. In the field these lava flows have a greater abundance of vesicles and modal plagioclase with some zoning and a sugary matrix. These three eruptive episodes follow the trends of the entire complex as they increase in compatible elements, like Ca and Mg, and decrease in incompatible elements such as K, Zr, and Sr. They are also more primitive compared to the rest of the complex as shown by low Ba/La ratios and generally low concentrations of LILE.