Structure and Evolution
Located on the Chile/Bolivia frontier, Volcan Parinacota (P) is the southernmost and youngest of a pair known jointly as the Nevados Payachata (Figure 10.1); the oldest is Volcan Pomerape. These are unusual amongst Central Andean volcanoes in that they have been previously studied, and useful descriptions and data are available on their structural evolution, volcanology and geochemistry (Katsui and Gonzalez, 1968; Wörner et al., 1988; Francis and Wells, 1988; Clavero et al., 2004). Pomerape is extensively glaciated and therefore pre-Holocene (Wörner et al., 1988; Francis & Wells, 1988). Parinacota consists of an older dome and flow complex capped by a younger andesitic cone. About 13,500 years ago (14C date; Francis & Wells, 1988) Parinacota suffered a major cone collapse which destroyed the SW sector of the volcano. Emplacement of the resulting débris avalanche deposit (DF) altered the regional hydrology by damming the earlier westward drainages and led to the formation of Lake Chungara (4,550 m). Water seeping through the débris avalanche deposit supplies numerous smaller lakes trapped within the hummocky terrain. The largest of these is Lake Cotocotani. Subsequent lava eruptions have concealed the avalanche detachment scarp. A well-preserved summit crater ~300 m diameter (Figure 10.2) and morphologically pristine lava flows (e.g. L; Figure 10.3 ) on the western flanks show that this activity has continued up until recent times (Francis & Wells, 1988).
Although there have been no reports of historic activity from Parinacota, Wörner et al. (pers. comm, 1989) suggest that a summit fumarole may be present on the basis of sulphurous gases near the summit. According to Wörner et al. (1988) the most recent activity is manifested by a number of small, relatively mafic, satellite vents and associated flows. These are the "Ajata" flows and occur on the south and southwestern flanks of the volcano.
Thick permanent ice cover on Parinacota presents a clear mudflow hazard in the event of renewed activity. The main Arica-La Paz highway traverses the southern margin of the débris flow deposit and Lake Cotocatani. Hazards from any future activity, particularly mudflows, are plainly manifest, although there are few large communities nearby.
The lavas of the Nevados de Payachata volcanoes range in composition from high K2O andesite through to rhyolites (53%-76% SiO2;Wörner et al.,1988). However, there appears to be a difference between pre- and post-collapse volcanism because whereas the débris deposit contains blocks of hornblende bearing andesite, dacite, and rhyolite (see also Francis & Wells, 1988), the post-collapse volcanism appears to have been exclusively of pyroxene andesite (Katsui & Gonzalez, 1968; Wörner et al.,1988). A detailed analysis of the geochemical and isotopic evolution of the Nevados de Payachata complex as a whole was made by Wörner et al. (1988). They suggested that the magmas of the complex evolved through both assimilation-fractional crystallization processes as well as through magma-mixing. Isotope compositions show minor variations: 206Pb/204Pb=17.95-18.20; 208Pb/204Pb= 38.2-38.5; 87Sr/86Sr= 0.70612-0.70707; 143Nd/144Nd = 0.51238-0.51230; dO18SMOW=+6.8% to +7.6% SMOW. These characteristics, together with their high Ba and Sr contents relative to other Andean magmas, have been used to argue for a complex petrogenetic evolution involving at least three different components: mantle derived primitive magmas, lower crust and upper crust.
Clavero R., J. E., Sparks, S. J., Polanco, E., & Pringle, M. S., 2004. Evolution of Parinacota volcano, Central Andes, Northern Chile. Rev. geol. Chile, dic. 2004, vol.31, no.2, p.317-347.
Francis, P.W., & Wells G.L., 1988 Landsat Thematic Mapper Observations of débris avalanche deposits in the Central Andes. Bull Volcanol. 50:258-278.
Katsui, Y, & Gonzalez-Ferran, O., 1968 Geologia del area neovolcanica de los Nevados de Payachata. Univ. de Chile, Fac. Ciencias Fis.y Mat., Dept. Geologia Publ. 29 p.1-61.
Wörner, G., Harmon, R.S., Davidson,.J., Moorbath, S., Turner, D.L., McMillan, N., Nye, C., Lopez-Escobar, & Moreno, H., 1988. The Nevados de Payachata volcanic region (18°S/69°W, N. Chile). I. Geological, geochemical and isotopic observations. Bull. Volcanol. 50:287-303.