Cerro Galan Caldera
Located in a remote part of the Catamarca province of northwestern Argentina, Cerro Galan is the best exposed large caldera in the world. Investigations have been carried out by Francis & Baker (1978), Francis et al. (1978, 1983,1989), and Sparks et al. (1985).
Cerro Galan caldera is the most recent component of a large silicic system which developed along two prominent north-south faults about 20 km apart. Like most large silicic systems, it has had a long and complex history. Volcanism in the area began about 15 Ma ago with the eruption of several andesite and dacite composite volcanoes. Subsequently, activity has been dominantly explosive and at least 9 major ignimbrite eruptions occurred between 7 and 4 Ma ago.
About 2.2 Ma ago eruption of more than 1000 km3 of crystal-rich, homogeneous, dacitic magma formed the present 35 x 20 km caldera and the resulting Cerro Galan ignimbrite which extends for up to 100 km radially from the caldera rim. Caldera formation was followed by resurgence of the central part of the caldera to the present elevation of 6,100 m. Approximately 1.2 km thickness of densely welded ignimbrite is exposed within the resurgent block. Laguna Diamante, a small lake occupying the south-west part of the caldera floor, is a relic of a much larger lake which occupied the floor of the caldera prior to resurgence. In common with other Valles-type calderas, the youngest event in the evolution of the caldera was eruption of dacite lavas from vents on the caldera ring fractures 2.1 Ma ago. Eruption of the small mafic monogenetic scoria cones and lavas of the Antofagasta de la Sierra valley (q.v.) along regional north-south faults west of caldera preceded and followed formation of the caldera. It is likely that the youngest of these satellite vents is much less than 10,000 years old.
Francis et al. (1989) studied the petrology and petrogenesis of the volcanic rocks associated with the Cerro Galan complex, summarised here. The ignimbrites typically contain a phenocryst assemblage of plagioclase, biotite, quartz, and oxides. Sanidine is abundant only in the Cerro Galan ignimbrite. In geochemical terms, the ignimbrites are rather homogeneous, high-K dacites to rhyodacites (66 - 69% SiO2), suggesting either that the source magmas were thoroughly mixed prior to eruption, or that if any compositional zonation were present, it was very weak. The ignimbrites have ?O18 = +10 - +12.5, 87Sr/86Sr =0.7108-0.7181, 144Nd/143Nd= 0.51215-0.51225, and 206Pb/204Pb, and 207Pb/204Pb, and and 208Pb/204Pb = 18.87-18.92, 15.65-15.69, and 39.06-39.16 respectively. The mafic lavas of the satellite vents are basaltic andesites and andesites (51-57% SiO2), with 87Sr/86Sr = 0.7055-0.7094, and 44Nd/143Nd= 0.51250-0.51290. These characteristics suggest that the ignimbrites represent melts from a deep and uniform region of the crust below Cerro Galan, some of which may have been Proterozoic of age. Some of the mafic magmas which provided the heat source for the melting may have been mixed into the silicic melts; these mafic magmas may be represented by those of the Antofagasta de la Sierra valley (Francis et al., 1989). Incompatible elements may have been enriched in the crustal melts by fractional crystallization or by metasomatic processes in their source region. Crystallization is thought to have occurred dominantly in the source region, but re-equilibration at shallower levels also occurred.
Francis, P.W., & Baker, M.C.W., 1978. Sources of two large-volume ignimbrites in the Central Andes: Some Landsat evidence. J. Volcanol. Geotherm. Res. v.4, p81-87.
Francis, P.W., Hammill, M., Kretschmar, G.A., & Thorpe, R.S., 1978. The Cerro Galan caldera, northwest Argentina and its tectonic setting. Nature v274, p748-751.
Francis, P.W., O'Callaghan, L.J., Kretschmar, G.A., Thorpe, R.S., Sparks, R.S.J., Page, R.N., De Barrio, R.E., Gillou, G, and Gonzalez, O.E., 1983. The Cerro Galan ignimbrite. Nature. v301, p51-53.
Francis, P.W., R.S.J. Sparks, Hawkesworth, C.J., Thorpe, R.S., Pyle, D.M., Tail, S.R., Mantonvani, M.S., McDermott, F., 1989. Petrology and petrogenesis of volcanic rocks of the Cerro Galan Caldera, northwest Argentina. Geol. Mag. v5, p515-547.
Sparks, R.S.J., Francis, P.W., Hamer, R.D., Pankhurst, R.J., O'Callaghan, L.J., Thorpe, R.S., and Page R.N., 1985. Ignimbrites of the Cerro Galan caldera, northwest Argentina. J. Volcanol. Geotherm. Res. v25, p205-248.