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Central Andes: Mountains, Magmas, and Minerals

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Central Andes: Mountains, Magmas, and Minerals See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/326723311 Magmatism in the Central Andes Article in Elements · August 2018 DOI: 10.2138/gselements.14.4.237 CITATIONS READS 43 1,427 3 authors: Gerhard Wörner Mirian Mamani Georg-August-Universität Göttingen Universidad Peruana de Ciencias Aplicadas (UPC) 265 PUBLICATIONS 9,054 CITATIONS 41 PUBLICATIONS 619 CITATIONS SEE PROFILE SEE PROFILE Magdalena Blum-Oeste Data Science Retreat Berlin 21 PUBLICATIONS 102 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: The Central Andean Pica Gap. Processes and signals leading to the Late Pleistocene volcanic shutdown. View project Reliability and Validityof the Duruoz Hand Index in an Argentinian Population with Scleroderma View project All content following this page was uploaded by Gerhard Wörner on 27 November 2018. The user has requested enhancement of the downloaded file. August 2018 Volume 14, Number 4 ISSN 1811-5209 Central Andes: Mountains, Magmas, and Minerals GERHARD WÖRNER, TAYLOR F. SCHILDGEN, and MARTIN REICH, Guest Editors Elements of an Extreme Land Topographic Evolution Magmatism Volcanism and Flare-ups Nitrate Deposits and Hyperaridity Mineral Resources Magmatism in the Central Andes Gerhard Wörner1, Mirian Mamani1,2, and Magdalena Blum-Oeste1,3 1811-5209/18/0014-0237$2.50 DOI: 10.2138/gselements.14.4.237 ctive continental margins are shaped by subduction-related magma- flat-slab subduction, increased tism, and the Central Andes of South America are a prime example. The plate coupling and, as a conse- quence, plate shortening, uplift, ACentral Andean orogen has evolved over the past 25 My via magmas erosion, and sedimentation. ascending from the mantle and interacting with increasingly thickened conti- Second, deposition of plateau- nental crust. This process is reflected in the volumes and compositional varia- forming ignimbrites (FIG. 1), which represent large volumes of mixed tions of the magmas that erupt at the surface. These compositional variations mantle- and crust-derived silicic can be traced in time and space, and, herein, we provide explanations for magmas containing 70 wt% and their cause and explore the nature of the Central Andes transcrustal magma 78 wt% SiO2. Third, the ignim- systems that feed the iconic stratovolcanoes today. brites are locally overlain by flat- lying, phenocryst-poor andesite KEYWORDS: Central Andean magmatism, isotopes, ignimbrite, magma mixing, shield lavas that may indicate assimilation, magmatic regimes, transcrustal magma systems hotter and dryer parent magmas. Fourth, the development of the VOLCANISM IN THE CENTRAL ANDES iconic andesitic and dacitic strato- AND ITS GEOLOGICAL CONTEXT volcanoes of the Central Andes, which are characterized by a composition of 55–68 wt% SiO (FIG. 2). True rhyolites Magmas form in subduction zones by partial melting in 2 (>69 wt% SiO ) are exceedingly rare in stratovolcanoes, yet the mantle wedge in response to the addition of fluids 2 it is such rocks that dominate the compositional spectrum from the down-going oceanic lithosphere. In the Central of the ignimbrites. Modern (<3 Ma) andesitic edifices can Andes (FIG. 1), subduction has been active since Jurassic reach >2,000 m in height and many have summit eleva- times; however, significant shortening of the crust, crustal tions well over 6,000 m. Ojos del Salado in northern Chile thickening, and formation of the Altiplano–Puna Plateau is the world’s highest active volcano at 6,887 m. These began only at about 35 Ma (Late Eocene), with acceler- large clustered volcanoes are the products of intracrustal ated shortening during the last 10 My. Consequently, magmatic systems that have typical lifetimes from between mantle-derived magmas must now traverse the thickest a few 100 ka to several My (e.g. Hora et al. 2007; Walker crust (>70 km) of any subduction zone on Earth (Beck et et al. 2013). al. 1996). Because of the increasingly arid climate on the western margin of the Central Andes, volcanic edifices and Fields of smaller, monogenetic volcanoes and related ignimbrite deposits are extremely well-preserved, and their individual lava flows are rare and concentrated in a few composition and distribution can be studied back in time. regions, e.g. the Andagua Valley, at Negrillar, as well as Because the chemical and isotopic composition of magmas in the back-arc region. Out of more than 1,500 analysed are strongly affected by interaction with crustal material samples, the most mafic magma in the Central Andes during ascent, and because the thickness of the crust has during Holocene times, and the only true basalt lavas, changed through time, the Central Andes are an excellent were erupted in the Andagua/Huambo monogenetic field natural laboratory to study the interaction between crustal (Mg# = 65.3; SiO2 = 51.8 wt%) [Mg# = MgO/(FeOt + MgO) evolution and magma genesis. × 100, molar] with a few occurrences of shoshonites in the Peruvian back-arc (Mg# = 69.6; SiO = 51.6 wt%) (Mamani The link between tectonic evolution and magmatism 2 et al. 2010) (FIG. 2). is conveniently documented by a typical stratigraphic sequence of deposits throughout the western slope of the At the other end of the compositional spectrum, the Central Andes (Wörner et al. 2002). Here, we observe four Central Andes boast one of the largest ignimbrite provinces general events. First, molasse-type sedimentation that on Earth (de Silva and Kay 2018 this issue). Monotonous started ~35–25 Ma during a magmatic lull, indicating or crystal-rich dacites to rhyolites of Miocene age contain individual flows of thousands of cubic kilometres. In this issue, de Silva and Kay (2018 this issue) discuss how these 1 Abt. Geochemie, Geowissenschaftliches Zentrum “ignimbrite flare-ups” are related to increased mantle input Universität Göttingen Goldschmidtstr. 1 and to a zone of anomalously low seismic velocities in 37077 Göttingen, Germany the middle crust of the southern Central Andes (Ward et E-mail: [email protected] al. 2014). 2 Instituto Geológico Minero Metalúrgico (INGEMMET) A simple SiO wt% histogram (FIG. 2) is instructive and Av. Canada 1470 2 San Borja, Lima 41, Peru summarises the major-element characteristics of magmas erupted in the Central Andes since Miocene times. 3 2° Investing Initiative, Compositions more mafic than andesite are rare because Schönhauser Allee 188, 10119 Berlin, Germany such primitive magmas are too dense and will stagnate, ELEMENTS, VOL. 14, PP. 237–244 237 AUGUST 2018 Distribution of stratovolcanoes (Miocene to Holocene) bottom right); caldera structures are outlined in yellow. The FIGURE 1 and monogenetic volcanic centers (Pliocene to location of the mid-crustal Altiplano–Puna Magmatic Body (red Holocene) in the Central Andes. Active volcanoes mentioned in the dashes) is based on geophysical data (Zandt et al. 2003). text are marked in red; the volcanoes of Parinacota (P), Taapaca (T) Ignimbrites from the Southern Peruvian Volcanic Complex are and Aucanquilcha (A) are marked in large blue letters. Large- mostly between 20 Ma and 5 Ma. FIGURE MODIFIED FROM FREYMUTH ET AL. volume ignimbrites are color-coded according to age (age column (2015). cool, and crystallize during ascent to the surface. This address these questions separately for andesite magmas and highlights the effective crustal density filter in processing for magmas that form large-volume ignimbrites because the mantle-derived magmas through magmatic differentiation flux of andesites is continuous and evenly distributed in and crustal assimilation in this thick-crust continental space whereas ignimbrites erupt from major calderas and arc. Subsequent assimilation and compositional differen- caldera clusters during discrete flare-up episodes (de Silva tiation leads to magmas of more evolved compositions. and Kay 2018 this issue). Appropriately, andesites with a range from 55–68 wt% SiO2 and those formed by differentiation, assimilation, COMPOSITIONAL CHANGES IN TIME and mixing in (trans-)crustal magma systems represent the AND SPACE most abundant magma types. Magmas having 68–72 wt% SiO2 rarely erupt in the Central Andes, but where they do Andesites through Time they typically form crystal-rich domes (or “tortas”) indica- Investigating how the chemical signatures in andesites tive of high magma viscosities. These domes may represent change through time is addressed by compiling trace- the crystal mushes from which more silicic and voluminous element and isotopic data for volcanic rocks erupted over ignimbrites are derived by melt extraction. However, the the last 180 My of active continental margin evolution maximum of the SiO2 distribution between 64–67 wt% (FIGS. 3 AND 4). Such an analysis shows that the trace- SiO2 for (older) intrusive rocks falls close to this minimum element signatures of andesites change systematically in composition for erupted lavas. Further differentiation through time and suggests that this change is related to and mixing with crustal melts produce large volumes the shortening and thickening of the continental crust. of silicic magmas that can feed large-volume ignimbrite In igneous geochemistry, certain trace-element ratios eruptions (FIG. 2). are indicative of a prominent role for particular igneous There are three main questions with respect to Andean minerals in magma genesis. For example, the Sm/Yb and magmatism. First, how do magmas form beneath the Sr/Y ratios for garnet can be characteristic because garnet
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