Bolivia
Synonyms: None
Location: 22º30'S; 66º45'W
Average/Max Elevation: 4700m/5400m
Caldera Diameter: 10km x 10km
Volume: ~1000 cub. km
Age Range: 6 to 9 Ma
First recognised as an unusual volcanic construct by Friedman and Heiken (1977) on Skylab orbital photography, and independantly as an ignimbrite shield on MSS images by Baker (1981), Cerro Panizos straddles the frontier between Bolivia and Argentina and is the most easterly of the large silicic complexes in the APVC. Dating (Ort, 1992) indicates that Panizos is Late Miocene in age with no Holocene activity.
Panizos is a roughly circular feature about 40 km in diameter with an average elevation of about 4,700 m. It is a radially symmetrical shield of ignimbrites more than 300 m thick with a central complex of dacitic coulées and lava domes reaching up to 5,400 m in elevation. In a detailed stratigraphy study Ort et al. (1987) recognized three ignimbrites closely spaced in time with total a volume exceeding 500 km3 . The later flows and domes were erupted in the ignimbrite vent area. Because the whole structure has radial dips of about 3° from the center of the complex, Baker (1981) interpreted it as an ignimbrite shield which had undergone little syn- or post eruptive collapse. However, Ort et al. (1989) have found evidence that the rocks in the center of the shield had subsided (down-sagged), with local inward dips of up to 6º, and that the ignimbrite center had several vents. They interpret Panizos as either a collapse caldera, filled by late-stage ignimbrites and post caldera collapse lavas, or a nested caldera. Notwithstanding this, it remains an ignimbrite center of unusual character and appearance.
Trace element and isotopic compositions indicate significant crustal contamination of the high-K dacitic ignimbrites and lavas of Panizos. Open system magmatic processes with replenishment in the magma chamber may best account for the geochemical characteristics of the Panizos magmas (Ort et al., 1988). Crustal influence on ignimbrite compositions decreases up-section in general, but a period of replenishment and subsequent mixing is recorded in the middle of the sequence resulting in a 'hybrid' composition of the late ignimbrites and the platform lavas.
References
Baker, M.C.W., 1981. The nature and distirbution of upper Cenozoic ignimbrite centres in the Central Andes. J. Volcanol. Geotherm. Res. v11, p293-315.
Friedman, J.D., and Heiken, G. 1977. Report In: Skylab explores the earth. N.A.S.A. Publication 380, p137 - 170.
Ort, M.H., Coira, B.L., and Mazzoni, M.M., 1989. Eruptive behaviour, vent locations, and caldera development of Cerro Panizos, Central Andes. IAVCEI, New Mex. Bur. Mining & Min. Res. Bull. 131 p208.
Ort, M., 1992. Orbicular volcanic rocks of Cerro Panizos; their origin and implications for orb formation. Geol Soc Am Bull, 104: 1048-1058.