O EOL GIC G A D D A E D C E I H C I L E O S F u n 2 d 6 la serena octubre 2015 ada en 19 Isotopic evolution at San Pedro - Linzor volcanic chain, Central Andes. Benigno Godoy1*, Paula Martínez1, Gerhard Wörner2, Petrus Le Roux3, Shoji Kojima4, Shan de Silva5, Diego Morata1, Miguel Angel Parada1 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: [email protected] Abstract. The San Pedro – Linzor volcanic chain located in volcanic front just north of the Atacama Basin and is the Central Andean volcanic zone runs along the western related to a system of NW-SE transcurrent faults. This new border of the Altiplano-Puna magma body (APMB). The dataset shows a shift in isotopic characteristics of Central APMB corresponds to a partially molten upper crustal (<25 Andean magmatism along the volcanic chain. 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 Geology the San Pedro – Linzor volcanic chain started 2 Ma ago, generating the NW-SE trending volcanic edifices observed The San Pedro – Linzor volcanic chain is part of the Plio- within the chain. This volcanic chain shows a decrease in Pleistocene evolution of the Central Andean magmatic arc. 87Sr/86Sr isotope ratios in the orientation of the volcanic This volcanic chain is located at the western Border of the chain, from Toconce (>0.7075) to San Pedro (<0.7070) Altiplano-Puna magma Body (Figure 1). The volcanic volcanoes. Changes in the isotopic ratios would be chain includes the San Pedro – San Pablo complex, and the associated with different extends of interaction between Paniri, Cerro del Leon, Toconce and Linzor volcanoes. mantle-derived magmas and the APMB. Thus, erupted lavas in the SE would have assimilated more crustal The Chao Dacite and Chillahuita domes and the La Poruña material than those that evolved in the NW-most part of the scoria cone are also included in this volcanic chain (Figure volcanic chain. 1). 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). Figure 1. Satellite image showing the location and components This generated a volcano-tectonic province which was of the San Pedro – Linzor volcanic chain. Locations of sampled denominated the Altiplano-Puna volcanic complex (de lavas with 87Sr/86Sr are indicated in the figure. Silva, 1989). In this study, new isotopic data for the San Pedro – Linzor Petrographically, the lava flows of the volcanoes vary from volcanic chain are presented. This chain runs along an Basaltic-andesite to hornBlende-dacite, with pyroxene offset and is oBlique to the general N-S trend of the active andesite as the main lithological type (Godoy et al., 2014 472 AT 1 GeoloGía ReGional y Geodinámica andina and references therein; Martínez, 2014). Small-volume Paniri, while even lower ratios (<0.7070) were obtained for pyroclastic flows related to the volcanoes, and the Chao San Pedro volcano (FiGures 1 and 3). On the other hand, Dacite and Chillahuita domes have the most evolved Toconce and Cerro del Leon lavas show significantly dacitic composition, while the scoria flows of La Poruña higher (>0.7075) 87Sr/86Sr signatures (FiGures 1 and 3), cone are the most mafic, varyinG from basaltic-andesite to amonGst the hiGhest in the arc front of the Central Andean andesite (Godoy et al., 2014). All lavas show consistently Volcanic Zone for mafic andesites (Godoy et al., 2014). low LREE/HREE ratios compared to maGmas that erupted further North. StronG HREE depletion is Generally considered to be the result of maGmatic evolution (differentiation and assimilation) at hiGh pressures in thickened crust (e.G. Mamani et al., 2010). Thus, the Geochemical characteristics of maGmas erupted alonG this volcanic chain is related to an AFC-type evolution of mantle-derived maGmas, with contamination by crustal material that most-probably occurred at shallow upper crustal levels (Godoy et al., 2014). Geochronology of the volcanic chain New 40Ar/39Ar datinG, and published K/Ar Figure 2. 40Ar/39Ar aGes (circles) vs. position of obtained aGes GeochronoloGical data of sampled lavas from Paniri, Cerro from lava samples from the volcanic chain. Also, K/Ar (squares), del Leon and Toconce volcanoes are Given in FiGure 2. and 3He (diamond) aGes are presented. 40 39 Ar/ Ar datinG has been carried out by us and EnerGía 40 39 Andina S.A. (pers. comm.). Ar/ Ar data were obtained 87 86 from Groundmass and amphibole of selected samples. Figure 3 plots Sr/ Sr isotope ratios against SiO2 content Also, a 3He age from the La Poruña scoria cone (Wörner et of selected lavas. It is clear from this fiGure that Toconce al., 2000) is presented in Figure 2. No age data has been and Cerro del Leon volcanoes, located at the SE extreme obtained for San Pedro volcano, however, accordinG to of the chain, have hiGher 87Sr/86Sr values than lavas O’Callaghan and Francis (1986) this volcano is younger erupted at Paniri and San Pedro volcanoes at similar SiO2 than San Pablo which pre-dated the last Glacial episode. content. FiGure 3 also shows that Toconce and Cerro del Also, historical and fumarolic activity has been reported Leon lavas have similar, and even higher, 87Sr/86Sr values for San Pedro volcano (Global Volcanism ProGram, 2013). than the domes recoGnized in the area. San Pedro and Thus a Pre-Holocene to Recent age is proposed for this Paniri volcanoes however have 87Sr/86Sr ratios similar to volcano (Figure 2). Literature and new aGe data indicate those obtained by Mamani et al. (2010) for La Poruña that the San Pedro – Linzor volcanic chain has evolved in scoria cone (Figure 2) the last 2 Ma (FiGure 2). This is consistent with the aGe of Toconce IGnimbrite (6.52 Ma, Salisbury et al., 2011) which constitutes the basement of this volcanic chain. Thus, construction of the main stratovolcanoes of the chain occurred between 1.9 and 0.1 Ma. On the other hand, eruption of both the most and the least differentiated magmas in the volcanic chain occurred at 100 ka (Chao Dacite and Chillahuita domes, and La Poruña scoria cone, respectively) (FiGure 2). Isotopic variations at San Pedro-Linzor volcanic chain 87Sr/86Sr isotope ratios for selected lavas from the San Pedro – Linzor volcanic chain are plotted in Figure 3, toGether with published values. Also, a previously sampled 87 86 87 86 lava from La Poruña scoria cone has a Sr/ Sr ratio of Figure 3. Sr/ Sr ratios vs. SiO2 (wt. %) diagram for selected 0.7066 (Mamani et al., 2010), while dacitic domes in the samples. Also included are data from Mamani et al. (2010) for SE yielded 87Sr/86Sr ratios of 0.70806 (Chao Dacite) and San Pedro (black squares) and La Poruña scoria cone (black 0.70805 (Chillahuita) (de Silva et al., 1994) (Figure 3). trianGle), as well as data for Chao Dacite (grey diamond) and The data show an increase in 87Sr/86Sr isotopic values for Chillahuita (white diamond) from de Silva et al. (1994). Grey 87 86 these volcanoes from NW to SE (FiGures 1 and 3). Low area indicates variation of Sr/ Sr vs. SiO2 for younG lavas (< 5 Ma) erupted in the Central Volcanic Zone (Mamani et al., 2010). 87Sr/86Sr ratios (<0.7080) are recoGnized in the NW at 473 ST 3 METAMORFISMO Y MAGMATISMO EN ZONAS DE SUBDUCCIÓN The 87Sr/86Sr compositions of lavas from the San Pedro – Therefore, the decrease in the 87Sr/86Sr composition of Linzor volcanic chain define a NW-SE trend (Figures 1 magmas erupted along the San Pedro – Linzor volcanic and 3). This westwards lateral migration of the 87Sr/86Sr chain can be associated with a progressive change in the ratios could be closely associated with the presence of the extent of the interaction of mantle-derived magmas with Altiplano-Puna magma body (Figure 4). Due its partially molten crustal material during stagnation at upper rheological characteristic, the APMB would act as a barrier crustal levels (< 25 km) (Figure 4). for mantle-derived magmas during their ascent causing mantle-derived magmas to pond below this partially Conclusions molten zone (25-30 km depth; de Silva et al., 2006) (Figure 4). The San Pedro – Linzor volcanic chain was erupted in the last 2 Ma in a NW-SE trending orientation over a Ponding of magmas rising from deeper sources are likely ignimbritic basement. During its construction, the to lead to interaction with the partially molten zone, evolution of this volcanic chain have been highly resulting in more radiogenic Sr in the eruptive products influenced by the presence of the Altiplano-Puna magma, (Figure 4).