OREGON STATE UNIVERSITY

Stop 25: Aspero

Cerro Aspero

S23 04.836 W67 42.166

 

Leaving Cerro Purico we headed West to Cerro Aspero. Cerro Aspero is a post-glacial (<0.5 My) dome erupted after the Purico ignimbrites (1.3-1.0 My). Hawkesworth et al. (1982) found that mafic enclaves at Cerro Aspero were basaltic-andesite in composition, and therefore  represent the most primitive compositions found in the Purico Complex.

Aspero Dome

Image of the Cerro Aspero dome.

The host rock at Cerro Aspero, similar to the other domes of the Purico Complex is a crystal-rich dacite. Multiple mafic enclaves were collected at this location. The enclaves are fine grained, dark gray, and porphyritic. Crystals include quartz, plagioclase, and hornblende. Quartz crystals are relatively large (~10 mm) and have green reaction rims of clinopyroxene. The quartz crystals along with some of the plagioclase may not be primary. The crystals are sitting in a groundmass of glass, plagioclase, and hornblende.

Mafic enclave in Cerro Aspero dacite.

Rocks from Aspero also have large megacrysts of plagioclase that may represent the granitic basement beneath the Purico Complex.