Special Research Center 267
DEFORMATION PROCESSES IN THE ANDES
Freie Universitaet Berlin · Technische Universitaet Berlin · GeoForschungsZentrum Potsdam · Universitaet Potsdam
Sedimentation, tectonics and volcanism in the Salar de Antofalla area, southern Puna (NW Argentina) - Project D1B
Geochemistry of the ignimbrites of the Salar de Antofalla region, NW Argentina
Wolfgang Schnurr and Konrad Görler
Memorias del X Congreso Latinoamericano de Geología, Buenos Aires, Noviembre 1998
New geochemical and age data were obtained from different felsic ignimbrites and lavas of the southern Puna. Several small-volume (<10km3, mainly valley-fill type) and few large-volume (>10km3, mainly plateau-type) ignimbrites and felsic lava-flows of Miocene - Quaternary age are located in the Salar de Antofalla (SAF) region and in the adjacent Salar de Incahuasi (SIH) area. The best ignimbrite exposures are located to the S and W of the SAF (eg. Vallecito, Tambería, Antofalla and Archibarca ignimbrites) and to the S of SIH (eg. Incahuasi and Campo de la Piedra Pómez ignimbrites). The lavas are located to the N of the Antofalla village (Antofallita), to the S and SE of the Antofalla volcano (Colada Ácida Las Cuevas, after González 1992) and to the SW of the SIH (Cerro Cueros del Purulla).
Age data from the ignimbrites reflects continuous magmatic activity. The oldest age currently obtained is 11,7 ± 0,3 Ma (Ar/Ar) for the ignimbrite located in the Antofalla valley (Antofalla ignimbrite). The youngest age is represented by the non-welded ignimbrite at Campo de la Piedra Pómez with 0,2 ± 0,1 Ma (K/Ar).
The ignimbrites are generally non to poorly welded but vitrophyre horizons are also present (Tambería and Incahuasi ignimbrites). The mineral assemblage comprises plagioclase, quartz and biotite as main phenocrysts and variable amounts of Fe-Ti oxides in a glass matrix. Sanidine, hornblende and pyroxen are restricted to some units.
The ignimbrites and lavas are predominantly rhyolites and subordinate dacites of the high-K calc-alkaline suite. Preliminary geochemical results permit several conclusions:
- fractionation of plagioclase, magnetite/ilmenite and subordinate K-feldspar was important in magma genesis.
- variation in Zr, Ba, Rb, REE and Y concentrations between the ignimbrites for a given SiO2 range indicate local differences in the crustal source or different fractionation histories.
- La/Yb ratios are between 4 and 40. The ignimbrites from the adjacent Cerro Galán (E of SAF) and Cerro Tuzgle (NE of SAF) also have La/Yb ratios <30 (Francis et al. 1989) and <35 (Coira and Kay 1993) respectively. The variable La/Yb ratios in our samples are explained by variations in the La/Sm ratios (4-11) at constant, low Sm/Yb ratios (1-3). This suggest garnet was not involved in the genesis of these felsic magmas.
The ignimbrites and lavas from SAF and SIH coexist with contemporaneous mafic to intermediate lavas from stratovolcanoes and monogenetic centers. The relationship between the felsic centers and these more mafic centers is not clear at this point. However, major and trace element behavior (MgO, Zr, chondrite normalized REE-patterns) suggest that the ignimbrites are not related to the basalts and basaltic andesites by fractionation. Additional work on trace elements and Sr and Nd isotopes is in progress in order to obtain a more detailed petrogenetic information for the felsic ignimbrites and lavas of the Antofalla region.
Coira B. and S.M. Kay, 1993. Implications of Quaternary volcanism at Cerro Tuzgle for crustal and mantle evolution of the Puna Plateau, Central Andes, Argentina. Contributions to Mineralogy and Petrology 113: 40-58.
Francis P.W., R.S.J. Sparks, C.J. Hawkesworth, R.S. Thorpe, D.M. Pyle, S.R. Tait, M.S. Mantovani and F. McDermott, 1989. Petrology and geochemistry of volcanic rocks of the Cerro Galán caldera, northwest Argentina. Geological Magazine 126: 515-547.
González O.E., 1992. Geología de la Puna austral entre los 25°15´ a 26°30´ de latitud sur y los 66°25´ a 68°00´ de longitud oeste, provincias de Catamarca y Salta, Argentina. Acta Geológica Lilloana XVII (2): 63-88.
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