Structure and Evolution
Cerro Escorial (E) is an obscure volcano, which appears to represent the most recently active vent of a more extensive northwest/southeast trending chain of older vents (~8 Ma; Naranjo & Cornejo, in prep.) known collectively as the Corrida de Cori, which defines the Argentina/Chile frontier (Figure 35.1; 35.2). A large active sulphur mine, La Casualidad (LC) is worked in the southernmost part of this chain, which is extensively fumarolically altered. The young vent of Escorial is located 4 km north-west of the mine, and consists of a 1 km diameter summit crater, and some young lava flows which extend westwards 3-4 km into Chile. These blocky andesite lava flows (L) have yielded a K-Ar age of 0.4 + 0.4 Ma (Naranjo & Cornejo, in prep.). On the TM image, they appear youthful and contrast strongly with the surrounding completely exposed surface of a ~1.2 Ma ignimbrite (Naranjo & Cornejo, in prep.). Ground photos suggest that they have a thin mantle of aeolian material, perhaps derived from the underlying ignimbrite. Richards and Villeneuve (2002) dated a summit lava flow at 0.342 Ma that overlies an ignimbrite with an age of 0.457 Ma.
The ignimbrite surface slopes away from the volcanic cordillera on both sides, suggesting an uplift of some hundreds of meters. A prominent set of northwest-southeast trending fractures is apparently related to the same regional tectonic stress field that was responsible for controlling the development of the Corrida de Cori. The Escorial ignimbrite originated in the collapse of an eruption column, leaving a deposit only on the flanks and not near the summit, where the material would move very quickly. The lava flows (L) on the south west flank appear to be the most recent activity. Fumarolic deposits on the north east of the summit indicate extensive hydrothermal alteration there. The hydrothermal system at Cerro Escorial is high-temperature and active at depth.
No activity has been observed in visits to the area and the volcano is apparently dormant (J.A. Naranjo, personal communication, 1989).
Naranjo and Cornejo (in preparation) report that the young lava from Escorial is a dark andesite, with a trachytic, porpyritic texture. Sieve-textured plagioclase, hypersthene, augite, resorbed quartz, resorbed olivines, and oxides are the main phenocryst phases. The disequilibrium mineral assemblage and textural features suggest that magma mizing was important in the genesis of these andesites. Major (SiO2, 60.31%; K2O, 2.30%) and trace elements are similar to most other CVZ andesites (J.A. Naranjo and P. Cornejo, in preparation, 1990).
J.A. Naranjo & P. Cornejo, in preparation. Hoja Salar de Isla. Carta Geologica de Chile S.N.G.M. Santiago, Chile.
J.P. Richards & M. Villeneuve, 2002. Characteristics of late Cenozoic volcanism along the Archibarca lineament from Cerro Llullaillaco to Corrida de Cori, northwest Argentina. Journal of Volcanology and Geothermal Research, 116(3-4):161-200.