Colachi & Acamarachi
Structure and Evolution
Colachi and Acamarachi are two distinct cones about 6 km apart which are the most conspicuous elements of a small volcanic complex (Figure 25.1 and 25.2). Acamarachi (A; 6046 m) is the highest peak in the region and is a simple symmetrical cone conspicuous from the ground for the extremely steep slopes near its summit; angles of ~45o have been measured on steeply dipping lava flow remnants. A poorly preserved summit crater is present and the lack of any morphologically distinct lava flows on the flanks suggest that it was largely constructed in pre-Holocene times, but the summit lava flows may be young. A large dome is present on its northern side, upsetting the symmetry of the volcano. Colachi is a similar but smaller symmetrical cone, with a degraded summit crater and some recognisable flow features, especially on its eastern flank and in the summit region. Gardeweg (2003) notes that these volcanoes have separate magmatic systems.
Although these two cones are two of the loftiest peaks in the area, they are actually relatively small edifices with heights of ~800 -1,2000 m above their base level of ~4,800 m. Preliminary mapping indicates that they are built on a large uplifted block of welded ignimbrites, possibly the resurgent center to an older (late Miocene ?) caldera.
The most recent activity in the Colachi-Acamarachi complex has been the eruption of a pair of small silicic flows or coulées (LD). The largest of these covers ~7 km and occupies the saddle between the two volcanoes, and there is a smaller one (~3 sq km) at the western foot of Colachi. These are typically glassy and have the conspicuous flow ridged morphology of other silicic flows of the region. They appear pristine and are probably post-glacial. There no records of historic eruptions.
There was no evidence of activity on recent visits in 1988 and 1989 to the region.
Lavas from Acamarachi and Colachi are predominantly porphyritic hornblende/biotite or pyroxene dacites and andesites (Ramirez & Gardeweg, 1982). Plagioclase (An34-42) dominates the mineral assemblage with augite and hypersthene, or hornblende and biotite - the proportions of these two groups vary according to rock type. Rare olivine and quartz are also found. Plagioclase in the rocks with olivine have more calcic plagioclases with An45-48. Dacites, from the two recent flows and from Acamarachi, are porphyritic with predominantly plagioclase (An32-44) with hornblende, biotite and large resorbed quartz set in a glassy groundmass. Rare small augites are also found.
Gardeweg, M.C. (pers. comm.), 1993, as cited in Smithsonian Global Volcanism Program: Colchani. Accessed July 2008, http://www.volcano.si.edu/world/volcano.cfm?vnum=1505-095.
Ramirez, C.F., & Gardeweg, M.,1982. Hoja Toconao. Carta Geologica de Chile No. 54. S.N.G.M. Santiago, Chile.