The working area comprises the southern part of the CVZ and the adyacent back arc region of Salar de Antofalla. Systematic sampling of Miocene-recently active volcanoes shows that the composition of the erupted magmas is subject to systematic changes in space and time. The volcanic rocks are from limited time periods before, during and after the presumed peak in Miocene crustal thickening (Quechua tectonic phase ~10 Ma). They show distinct geochemical properties. The volcanics of pre-Quechua age (16 - 20 Ma) comprise MgO-poor high-alumina-basalts (HAB) to rhyolites. Their differentiation sequence can be modeled with aid of major and trace element data as upper crustal processes with small amounts of assimilation. These melts dominated by mantle processes contrast with calcalkaline crustal dominated series of post-Quechua age which consist of basaltic andesites to dacites. Generally they have higher contents of MgO, Cr and Ni (7,7 wt.-%, 413 ppm, 141 ppm respectively) than the pre-Quechua rocks (MgO 4.1 wt.%, Cr 22 ppm, Ni 4 ppm), so there does not exist any genetic link between the HAB and the basaltic andesites. Most distinctive are differences in the LIL elements Th, Rb and Cs. The recent volcanic rocks reach comparatively the highest values (Th up to 60 ppm, Rb up to 220 ppm, Cs up to 18 ppm), whereas the most evolved pre-Quechua rocks present lower values (Th 7 ppm, Rb 93 ppm, Cs 2 ppm). A striking feature is furthermore, that in Pleistocene rocks the switch to higher contents occur between basaltic and acid andesites, but the Sr and Nd istopic ratios remain almost constant (87Sr/86Sr 0,70624 - 0,70714, eNd -3,14 to -3,84). We conclude that isotopically already altered basaltic andesites (Davidson et al., 1991) receive significant contributions of crustal melts during their ascent within the middle crust. These crustal melts have compositions similar to typical Central Andean ignimbrites (de Silva, 1987). This process is followed by upper crustal fractional crystallization.
Thereby the attributes of the contaminant are changing between arc and back arc, because Sr and Pb isotopic data show an obvious zonation between the Andean Cordillera and the Antofalla region. (All data fall in the typical range of the southern CVZ: 87Sr/86Sr initial 0,70505-0,7089, 208Pb/204Pb 38,446-39,167, 207Pb/204Pb 15,594-15,703, 206Pb/204Pb 18,551-18,988.) Within the Antofalla region 87Sr/86Sr initial ratios correlate with SiO2, and andesites (60 wt.% SiO2) of post-Quechua age show the highest values (0,70858 - 0,70894). Within the arc this trend is only weakly developed and the highest value reached is 0,70714 regardless of age or rock composition. The Pb isotopic data show the same pattern and 208Pb/204Pb and 206Pb/204Pb are zoned to higher values in the back arc (38,82-39,17, 18,858-18,988 resp.) and lower values in the arc (38,45-38,81, 18,551-18,867 resp.). 207Pb/204Pb do not show clearly a regional trend. Furthermore 208Pb/204Pb and 207Pb/204Pb correlate with Pb and SiO2 values. With given SiO2 content volcanic rocks of the Antofalla region are displaced to higher Pb isotopic composition compared to arc rocks. We interprete these correlations of isotopes with Pb and SiO2 contents as a consequence of open system behavior. Whether this is due to magma mixing or AFC processes, has yet to be studied in detail at each individual volcanic center. The enriched isotopic ratios of the back arc region can be explained by higher Th/Pb-, U/Pb- and 87Sr/86Sr ratios of contaminant(s). Therefore we infer a heterogenous structure in E-W direction of the Andean crust in our working area.
Davidson, J.P., Harmon, R.S. & Woerner, G., Geol. Soc. Am. Spec. Pap. 265 (1991).
de Silva, S.L., unpubl. Ph.D. thesis, Milton Keynes, UK (1987).