Cordons Puntas Negras & Chalviri
Structure and Evolution
This confusing collection of numerous vents, lava flows, domes, and small cones forms two main chains (Figure 29.1); Cordon Puntas Negras (CPN), a 20 km long volcanic complex trending roughly southeast from Chiliques towards Volcan Punta Negra (VPN), and Cordon Chalviri extending15 km southwards from Volcan Puntas Negras to Cerro Tuyajto (T). At least 25 different vents are present within the complex, distributed over an area of about 500 km2. Although this complex broadly resembles the smaller Escalante-Sairecabur complex (q.v), Cordon Puntas Negras is the result of a several different styles of activity which have produced constructs of differing character. Typical structures are generally small cones a few hundred meters in height with well preserved summit craters (for example Cerros Cenizas (CC), Aguas Calientes Laguna Escondida (E) and Chinchilla(C)) and short (5 km) lava flows, although it is difficult to locate the source of many of the individual flows, or to relate them to a particular cone. One distinct maar-type vent can be seen (M).
Much of the activity has been post-glacial as evidenced by the pristine morphology of the features. The dominant style of activity appears to have been the eruption of andesitic lavas. A 13 km2 silicic flow/dome complex with a roughly circular outline is another major component. The thick dacite lavas, which are distributed radially around the single vent, show typical large flow ridges and are characterised by a high albedo. The location of the vents along two main directions suggests tectonic control although it is difficult to relate this to the regional structure. The CPN effectively delimits the southern limit of the huge La Pacana Complex - a major caldera complex (see section 4.2.3). It is possible that the CPN developed along one of the regional fault systems which controlled the formation of the La Pacana Complex. The preservation and morphology of the centre immediately to the southeast of Co. Laguna Escondida suggests that it is the most recent event in this complex.
A spectrum of lava types ranging from porphyritic pyroxene andesites, hornblende and biotite andesites, to hornblende and biotite dacites is found associated with these centres. The dominant plagioclase varies from An45-48 in olivine bearing andesites, through An34-48 in most of the other andesites and An32-44 in the dacites (Ramirez & Gardeweg, 1982). Augite and hypersthene, and hornblende and biotite are common in their respective rock types, but the latter two are the most common in the dacites. Flow banding and spherulitic devitrification textures are common. Rare occurrences of coexisting olivine and quartz in the dacites are accompanied by evidence for magma mixing. Deruelle (1982) presents 4 analyses of major and trace elements which show that the lavas range from 58.18% to 65.29% SiO2, Sr varies from 348 to 526 while Rb ranges from 76 to 136. Harmon et al. (1984) report dO18 of 7.8, 87Sr/86Sr of 0.70708 - 0.70714, and Pb-isotope ratios of 18.88, 15.63, 38.84 (206/204, 207/204, 208/204 respectively) for two andesites from these centres. In general, the magmas are have typical calc-alkaline geochemistry and Sr-Nd-Pb isotopic signatures for the arc.
Deruelle, B., 1982., Petrology of the Plio-Quaternary volcanism of the south-central and meridional Andes. Jour. Volcanol. Geotherm. Res. 14:77-124.
Harmon, R.S., Barreiro, B.A., Moorbath, S., Hoefs, J., Francis, P.W., Thorpe, R.S., Deruelle, B., McHugh, J., and Viglino, J.A., 1984. Regional O-, Sr- and Pb-isotope relationships in late Cenozoic calc-alkaline lavas of the Andean Cordillera. J. Geol. Soc. Lond., 141:803-822
Matteini, M., Mazzuoli, R., Omarini, R., Cas, R., Maas, R., 2002. Geodynamical evolution of Central Andes at 24°S as inferred by magma composition along the Calama-Olacapato-El Toro transversal volcanic belt. Journal of Volcanology and Geothermal Research, 118(1-2):205-228.
Ramierez-C.F. and Gardeweg-M. (1982) Hoja Toconao. Carta Geologica de Chile, 54. SNGM, Santiago.