F u n d a d a e n 1 9 6 2 S O C I E D A D G E O L O G I C A D E C H I L E la serena octubre 2015 472 Isotopic evolution at San Pedro - Linzor volcanic chain, Central Andes. Benigno Godoy 1* , Paula Martínez 1 , Gerhard Wörner 2 , Petrus Le Roux 3 , Shoji Kojima 4 , Shan de Silva 5 , Diego Morata 1 , Miguel Angel Parada 1 1 Departamento de Geología, Centro de Excelencia en Geotermia de los Andes (CEGA), Facultad de Ciencas Físicas y Matemáticas, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile 2 Abteilung Geochemie, GZG, Göttingen Universität, Goldschmidtstraße 1, Göttingen 37077, Germany 3 Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa 4 Departamento de Ciencias Geológicas, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile 5 College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA *Contact e-mail: bgodoy@ing.uchile.cl Abstract. The San Pedro – Linzor volcanic chain located in the Central Andean volcanic zone runs along the western border of the Altiplano-Puna magma body (APMB). The APMB corresponds to a partially molten upper crustal (<25 km depth) MASH-type zone, now thought to be a crystal- mush, related to the eruption of ignimbrites and dacitic domes of the Altiplano-Puna Volcanic Complex. Eruption of the San Pedro – Linzor volcanic chain started 2 Ma ago, generating the NW-SE trending volcanic edifices observed within the chain. This volcanic chain shows a decrease in 87 Sr/ 86 Sr isotope ratios in the orientation of the volcanic chain, from Toconce (>0.7075) to San Pedro (<0.7070) volcanoes. Changes in the isotopic ratios would be associated with different extends of interaction between mantle-derived magmas and the APMB. Thus, erupted lavas in the SE would have assimilated more crustal material than those that evolved in the NW-most part of the volcanic chain. Keywords. Isotopic variations, Geochronology; Central Andean magmatism; Altiplano-Puna magma body Introduction During its evolution, Central Andean volcanic front has migrated in an eastwards direction since the Jurassic to reach its present position (Coira et al., 1982). This eastwards migration has been accompanied by an increase in the Sr-isotope ratios of erupted lavas caused, mainly, by the thickening of the continental crust in this magmatic arc (Haschke, 2002; Mamani et al., 2010). On the other hand, the Altiplano-Puna magma body represents the remnants of an upper crustal MASH-type zone related to the eruption of ignimbrites and domes in the Central Andes (Zandt et al., 2003; Burns et al., 2015). This generated a volcano-tectonic province which was denominated the Altiplano-Puna volcanic complex (de Silva, 1989). In this study, new isotopic data for the San Pedro – Linzor volcanic chain are presented. This chain runs along an offset and is oblique to the general N-S trend of the active volcanic front just north of the Atacama Basin and is related to a system of NW-SE transcurrent faults. This new dataset shows a shift in isotopic characteristics of Central Andean magmatism along the volcanic chain. Geology The San Pedro – Linzor volcanic chain is part of the Plio- Pleistocene evolution of the Central Andean magmatic arc. This volcanic chain is located at the western border of the Altiplano-Puna magma body (Figure 1). The volcanic chain includes the San Pedro – San Pablo complex, and the Paniri, Cerro del Leon, Toconce and Linzor volcanoes. The Chao Dacite and Chillahuita domes and the La Poruña scoria cone are also included in this volcanic chain (Figure 1). Figure 1. Satellite image showing the location and components of the San Pedro – Linzor volcanic chain. Locations of sampled lavas with 87 Sr/ 86 Sr are indicated in the figure. Petrographically, the lava flows of the volcanoes vary from basaltic-andesite to hornblende-dacite, with pyroxene andesite as the main lithological type (Godoy et al., 2014