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Frailes Plateau |
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| Synonyms: Los Frailes | ||
| Location: 19º30'S; 66º15'W | ||
| Country: Bolivia | ||
| Average/Max Elevation: 4800m/5300m | ||
| ~Area: >8500 sq. km | ||
| ~Volume: ~3000 cub. km | ||
| Age Range: 8 to 0 Ma |
4.1 Los Frailes (19° 30'S 66° 15'W)
The Los Frailes complex is a vast ~8,500 km2 ignimbrite plateau on the western edge of the Eastern Cordillera of Bolivia. It has an average elevation of about 4,500 m and is the most easterly of any of the large silicic systems in the CVZ. It is the product of a prolonged period of volcanic activity ranging from ~25 Ma ago to recent times. Ignimbrites forming the plateau were erupted on to a highly dissected topography of Palaeozoic and Cretaceous basement and early Miocene volcanic rocks. At its thickest, the ignimbrite plateau is ~1 km thick. Many of the eruptive centres are associated with a variety of Sn-Ag-polymetallic ore deposits, making the Cordillera de Los Frailes an exceptionally interesting metallogenic province (Grant et al., 1977).
Several individual calderas are present within the Frailes plateau, but the most obvious is the Kari-Kari caldera situated on the southeastern edge of the complex (Francis et al., 1981; Schneider, 1987). Previously thought to be an intrusive igneous complex, this 20 Ma old resurgent caldera was the source of the crystal-rich, densely-welded garnet-bearing Kari Kari ignimbrite. The outflow sheet of the ignimbrite is largely eroded and is only found north-west of the caldera, but ~500 km3 of ignimbrite is ponded within the caldera (Schneider, 1987). Cerro Rico, a rhyolitic stock, was intruded along the caldera ring fractures about 13 Ma, and represents the last activity of the caldera. It is a classic locality to the economic geology community because of its historic importance as an epithermal tin-silver deposit, around which the city of Potosi grew up. Potosi was at one time the largest city in the Americas. Much of the silver that financed the Spanish rule of South America was mined from the Cerro Rico stock (Francis et al. 1981).
Radiometric ages of ignimbrites in the Frailes plateau range in age from more than 20 Ma to7 Ma (Grant et al., 1977,1979; Francis & Baker, 1978). Baker (1981) interpreted the plateau as being formed of two interfingering ignimbrite shields. Sources postulated for these ignimbrites are Cerro Villacolo, an eroded composite cone with a large summit crater; and Huanapa Pampa (Cerro Pascual Canaviri), a large dome complex in the centre of the eastern part of the plateau (Baker, 1981; Crown et al., 1989). Post-ignimbrite activity is represented by several domes on the central part of the plateau; these may or may not have accompanied explosive eruptions. Other eruptive centres associated with the Frailes complex include Cerro Wila Kkolu (Livicucho) and Cerro Condor Nasa to the north, which appear to be sources of locally distributed ignimbrites. Some of these ignimbrites show excellent rheomorphic textures in the field
Holocene activity from the Frailes complex is confined to the Nuevo Mundo complex q.v. on the southern part of the plateau. This silicic flow/dome complex was the site of an explosive eruption which showered air fall tephra over much of the countryside to the east as far as Potosi (section 3.1.2).
Schneider (1987) argued that while the pyroclastic rocks associated with centres on the eastern part of the complex (Kari Kari, Wila Kkolu, Condor Nasa) are markedly peraluminous (~ peralkaline) , containing Al-rich phases such as garnet, cordierite and tourmaline, the ignimbrites of the Frailes plateau itself are more typically metaluminous (~ calcalkaline), and are therefore more typical of CVZ ignimbrites. Initial 87Sr/86Sr for the silicic rocks (63-69% SiO2) range from 0.710-0.711, 143Nd/144Nd ranges from -5.45 to -6.22. Schneider (op. cit.) went on to argue that the combined petrological and Sr, Nd and Pb isotopic characteristics are best interpreted in terms of the ignimbrites being dominantly melts of upper amphibolite to lower granulite facies crustal rocks of pre-Tertiary age.
Approximately 70 km to the north of the Frailes plateau is another smaller ignimbrite complex, the Morococala ignimbrite plateau. Little is known about this complex which covers an area some 60 x 30 km, but it is similar in many respects to the Frailes plateau. It has a late Miocene age of ~7.3 Ma (Evernden et al., 1977), it appears to be a single ignimbrite shield with slope angles of about 2-3° (Baker, 1981), and a peraluminous dacitic chemistry (Koeppen et al., 1987). The vent area is marked by two overlapping domes, Cerro Macho Tankha Tankha and Cerro Huayna Tankha Tankha, about 4-5 km in diameter and about 500 m high.