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
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ctive continental margins are shaped by subduction-related magmatism, and the Central Andes of South America are a prime example. The Central Andean orogen has evolved over the past 25 My via magmas ascending from the mantle and interacting with increasingly thickened continental crust. This process is reflected in the volumes and compositional variations of the magmas that erupt at the surface. These compositional variations can be traced in time and space, and, herein, we provide explanations for their cause and explore the nature of the Central Andes transcrustal magma systems that feed the iconic stratovolcanoes today.
flat-slab subduction, increased plate coupling and, as a consequence, plate shortening, uplift, erosion, and sedimentation. Second, deposition of plateauforming ignimbrites (Fig. 1), which represent large volumes of mixed mantle- and crust-derived silicic magmas containing 70 wt% and 78 wt% SiO2 . Third, the ignimbrites are locally overlain by flatlying, 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 iconic andesitic and dacitic stratoVOLCANISM IN THE CENTRAL ANDES volcanoes of the Central Andes, which are characterized by AND ITS GEOLOGICAL CONTEXT a composition of 55–68 wt% SiO2 (Fig. 2). True rhyolites Magmas form in subduction zones by partial melting in (>69 wt% SiO2 ) are exceedingly rare in stratovolcanoes, yet the mantle wedge in response to the addition of fluids it is such rocks that dominate the compositional spectrum from the down-going oceanic lithosphere. In the Central of the ignimbrites. Modern (2,000 m in height and many have summit elevatimes; 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 accelerlarge 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 individual lava flows are rare and concentrated in a few ignimbrite deposits are extremely well-preserved, and their 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, were erupted in the Andagua/Huambo monogenetic field changed through time, the Central Andes are an excellent (Mg# = 65.3; SiO2 = 51.8 wt%) [Mg# = MgO/(FeOt + MgO) natural laboratory to study the interaction between crustal × 100, molar] with a few occurrences of shoshonites in the evolution and magma genesis. Peruvian back-arc (Mg# = 69.6; SiO2 = 51.6 wt%) (Mamani The link between tectonic evolution and magmatism et al. 2010) (Fig. 2). is conveniently documented by a typical stratigraphic At the other end of the compositional spectrum, the sequence of deposits throughout the western slope of the Central Andes boast one of the largest ignimbrite provinces Central Andes (Wörner et al. 2002). Here, we observe four 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 and to a zone of anomalously low seismic velocities in Goldschmidtstr. 1 the middle crust of the southern Central Andes (Ward et 37077 Göttingen, Germany E-mail:
[email protected] al. 2014). 2 Instituto Geológico Minero Metalúrgico (INGEMMET) Av. Canada 1470 San Borja, Lima 41, Peru 3 2° Investing Initiative, Schönhauser Allee 188, 10119 Berlin, Germany
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A simple SiO2 wt% histogram (Fig. 2) is instructive and summarises the major-element characteristics of magmas erupted in the Central Andes since Miocene times. Compositions more mafic than andesite are rare because such primitive magmas are too dense and will stagnate, 237
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Distribution of stratovolcanoes (Miocene to Holocene) and monogenetic volcanic centers (Pliocene to Holocene) in the Central Andes. Active volcanoes mentioned in the text are marked in red; the volcanoes of Parinacota (P), Taapaca (T) and Aucanquilcha (A) are marked in large blue letters. Largevolume ignimbrites are color-coded according to age (age column
bottom right); caldera structures are outlined in yellow. The location of the mid-crustal Altiplano–Puna Magmatic Body (red dashes) is based on geophysical data (Zandt et al. 2003). Ignimbrites from the Southern Peruvian Volcanic Complex are mostly between 20 Ma and 5 Ma. Figure modified from Freymuth et (2015).
cool, and crystallize during ascent to the surface. This highlights the effective crustal density filter in processing mantle-derived magmas through magmatic differentiation and crustal assimilation in this thick-crust continental arc. Subsequent assimilation and compositional differentiation leads to magmas of more evolved compositions. Appropriately, andesites with a range from 55–68 wt% SiO2 and those formed by differentiation, assimilation, and mixing in (trans-)crustal magma systems represent the most abundant magma types. Magmas having 68–72 wt% SiO2 rarely erupt in the Central Andes, but where they do they typically form crystal-rich domes (or “tortas”) indicative of high magma viscosities. These domes may represent the crystal mushes from which more silicic and voluminous ignimbrites are derived by melt extraction. However, the maximum of the SiO2 distribution between 64–67 wt% SiO2 for (older) intrusive rocks falls close to this minimum in composition for erupted lavas. Further differentiation and mixing with crustal melts produce large volumes of silicic magmas that can feed large-volume ignimbrite eruptions (Fig. 2).
address these questions separately for andesite magmas and for magmas that form large-volume ignimbrites because the flux of andesites is continuous and evenly distributed in space whereas ignimbrites erupt from major calderas and caldera clusters during discrete flare-up episodes (de Silva and Kay 2018 this issue).
Figure 1
There are three main questions with respect to Andean magmatism. First, how do magmas form beneath the Central Andes? Second, how do magmatic trace-element and isotopic compositions reflect changing conditions of magma evolution during the past 35 My of Andean orogeny and increasingly thickened continental crust? Third, what is the role of the Andean crust in explaining the variation in isotopic ratios in lavas that are spatially distinct? We E lements
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COMPOSITIONAL CHANGES IN TIME AND SPACE
Andesites through Time Investigating how the chemical signatures in andesites change through time is addressed by compiling traceelement and isotopic data for volcanic rocks erupted over the last 180 My of active continental margin evolution (Figs . 3 and 4). Such an analysis shows that the traceelement signatures of andesites change systematically through time and suggests that this change is related to the shortening and thickening of the continental crust. In igneous geochemistry, certain trace-element ratios are indicative of a prominent role for particular igneous minerals in magma genesis. For example, the Sm/Yb and Sr/Y ratios for garnet can be characteristic because garnet is known to fractionate middle from heavy rare-earth elements (HREEs). Thus, these ratios increase during highpressure magma evolution. Amphibole fractionation will decrease the Dy/Yb ratio in andesites, whereas titanite suppresses this ratio in silicic magmas. At low pressures, plagioclase and clinopyroxene will be the dominant
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fractionating phases and Sr/Y ratios will remain low. These general principles can be used to link trace-element signatures in magmas to the pressure (i.e. depth within the crust) where magma evolution takes place. In thin-crust settings, where plagioclase, pyroxene, and amphibole dominate the equilibrium assemblage in andesites, the ratios of Sr/Y, Sm/Yb, and Dy/Yb will be low. Increasing pressure of fractional crystallization and/or assimilation processes tends to increase Sr/Y, Sm/Yb and Dy/Yb (Kay et al. 1988, 1999; Coira et al. 1993; Mamani et al. 2010). Figure 3 shows these three trace-element ratios and their evolution through time. Figures 3A and 3C clearly indicate that – with few exceptions – the maximum Sr/Y and Dy/Yb ratio are found only in intermediate andesites (55–68 wt% SiO2 ) that are younger than 5 Ma (Pliocene, blue symbols in Fig. 3). Basalts (
