Western Kentucky University
Petrogenetic History of Lavas from the Casitas Shield
Institution
Western Kentucky University
Faculty Advisor/ Mentor
Andrew Wulff
Abstract
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 focused 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. Three eruptive episodes were identified and 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. PLM and SEM analysis revealed sieve-textures and zoning in feldspar to support magma mixing. Using the geochemical analyses and chemostratigraphy strong evidence is provided for magma mixing as the dominant petrogenetic process affecting the compositions of the lavas.
Petrogenetic History of Lavas from the Casitas Shield
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 focused 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. Three eruptive episodes were identified and 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. PLM and SEM analysis revealed sieve-textures and zoning in feldspar to support magma mixing. Using the geochemical analyses and chemostratigraphy strong evidence is provided for magma mixing as the dominant petrogenetic process affecting the compositions of the lavas.