U-Pb in Zircon Chronochemistry of the Altiplano-P
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Research Paper THEMED ISSUE: PLUTONS: Investigating the Relationship between Pluton Growth and Volcanism in the Central Andes GEOSPHERE Geochronological imaging of an episodically constructed GEOSPHERE; v. 12, no. 4 subvolcanic batholith: U-Pb in zircon chronochemistry of the doi:10.1130/GES01258.1 Altiplano-Puna Volcanic Complex of the Central Andes 1 1 2, 1, 1, 2, 11 figures; 3 tables; 4 supplemental files Jamie M. Kern , Shanaka L. de Silva , Axel K. Schmitt *, Jason F. Kaiser *, A. Rodrigo Iriarte *, and Rita Economos * 1College of Earth, Ocean, and Atmospheric Science, Oregon State University, Corvallis, Oregon 97331, USA 2 CORRESPONDENCE: desilvas@ geo .oregonstate Department of Earth and Space Sciences, University of California, Los Angeles, California 90021, USA .edu CITATION: Kern, J.M., de Silva, S.L., Schmitt, A.K., Kaiser, J.F., Iriarte, A.R., and Economos, R., ABSTRACT magmatic history at distinct upper crustal magmatic foci implicates a shared 2016, Geochronological imaging of an episodically connection deeper within the APMB. constructed subvolcanic batholith: U-Pb in zircon Zircons from 15 crystal-rich monotonous intermediate ignimbrites and 1 Each ignimbrite records the development of a discrete magma. Zircon chrono chemistry of the Altiplano-Puna Volcanic Complex of the Central Andes: Geosphere, v. 12, crystal-poor rhyolite ignimbrite erupted during the 11–1 Ma Altiplano-Puna age distributions of individual ignimbrites become more complex with time, no. 4, p. 1054–1077, doi:10.1130/GES01258.1. Volcanic Complex (APVC) ignimbrite flare-up record multiscale episodicity in reflecting the carryover of antecrysts in successively younger magmas and the magmatic history of the shallowest levels (5–10 km beneath the surface) of attesting to upper crustal assimilation in the APVC. Although present, xeno- Received 1 September 2015 the Altiplano-Puna Magma Body (APMB). This record reveals the construction crysts are rare, suggesting that inheritance is limited. This is attributed to Revision received 4 April 2016 of a subvolcanic batholith and its magmatic and eruptive tempo. basement assimilation under zircon-undersaturated conditions deeper in the Accepted 4 May 2016 Published online 27 May 2016 More than 750 U-Pb ages of zircon rims and interiors of polished grains APMB than the pre-eruptive levels, where antecrysts were incorporated in zir- determined by secondary ion mass spectrometry define complex age spec- con-saturated conditions. tra for each ignimbrite with a dominant peak of autocrysts and subsidiary Magmatic ages for individual ignimbrites are older than the 40Ar/39Ar erup- antecryst peaks. Xenocrysts are rare. Weighted averages obtained by pooling tion ages. This difference is interpreted as the average minimum Zr-saturated the youngest analytically indistinguishable zircon ages mostly correspond to melt-present lifetime for APVC magmas, the magmatic duration or Δ age. The the dominant crystallization ages for zircons in the magma. These magmatic average Δ age of ca. 0.4 Ma indicates that thermochemical conditions for ages are consistent with eruptive stratigraphy, and fall into four groups defin- zircon saturation were maintained for several hundreds of thousands of years ing distinct pulses (from older to younger, pulses 1 through 4) of magmatism prior to eruption of APVC magmas. This is consistent with a narrow range of that correlate with eruptive pulses, but indicate that magmatic construction zircon saturation temperatures of 730–815 °C that record upper crustal condi- in each pulse initiated at least 1 m.y. before eruptions began. Magmatism was tions and Zr/Hf, Th/U, Eu/Eu*, and Ti that reveal protracted magma differen- initially distributed diffusely on the eastern and western flanks of the APVC, tiation under secular cooling rates an order of magnitude slower than typical but spread out over much of the APVC as activity waxed before focusing in pluton cooling rates. In concert, these data all suggest that the pre-eruptive the central part during the peak of the flare-up. Each pulse consists of spatially magma reservoirs were perched in a thermally and chemically buffered state OLD G distinct but temporally sequenced subpulses of magma that represent the during their long pre-eruptive lifetimes. Trace element variations suggest sub- construction of pre-eruptive magma reservoirs. Three nested calderas were tle differences in crystallinity, melt fraction, and melt composition within dif- the main eruptive loci during the peak of the flare-up from ca. 6 to 2.5 Ma. ferent zones of individual magma reservoirs. Significant volumes of plutonic These show broadly synchronous magmatic development but some discor- rocks associated with ignimbrites are supported by geophysical data, the lim- OPEN ACCESS dance in their later eruptive histories. These relations are interpreted to indi- ited compositional range over 10 m.y., the thermal inertia of the magmatic cate that eruptive tempo is controlled locally from the top down, while mag- systems, and the evidence of resurgent magmatism and uplift at the calderas matic tempo is a more systemic, deeper, bottom-up feature. Synchroneity in and eruptive centers, the distribution of which defines a composite, episodi- cally constructed subvolcanic batholith. *Current addresses: (Schmitt) Institut für Geowissenschaften, Universität Heidelberg, Im Neuen- The multiscale episodicity revealed by the zircon U-Pb ages of the APVC heimer Feld 234-236, 69120 Heidelberg, Germany; (Kaiser) Geosciences, Southern Utah Univer- flare-up can be interpreted in the context of continental arc magmatic sys- sity, 351 W. University Blvd., Cedar City, Utah 84720, USA; (Iriarte) Facultad de Ciencas, Universi- This paper is published under the terms of the dad Major San Andres, La Paz, Bolivia; (Economos) Roy M. Huffington Department of Earth tems in general. The APVC ignimbrite flare-up as a whole is a secondary CC-BY license. Sciences, Southern Methodist University, PO Box 750395, Dallas, Texas, USA. pulse of ~10 m.y., with magmatic pulses 1 through 4 reflecting tertiary pulses © 2016 The Authors GEOSPHERE | Volume 12 | Number 4 Kern et al. | U-Pb in zircon chronochemistry of the Altiplano-Puna Volcanic Complex Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/4/1054/4178070/1054.pdf 1054 by guest on 24 October 2019 Research Paper of ~2 m.y., and the individual ignimbrite zircon spectra defining quaternary Andes (de Silva, 1989a; de Silva et al., 2006a). The APVC is the result of one of pulses of <1 m.y. This hierarchy of pulses is thought to reflect how a magmatic the youngest ignimbrite flare-ups on Earth, erupting >15,000 km3 of magma front, driven by the primary mantle power input, propagates through the crust during multiple supereruptions from 11 to 1 Ma that define an episodic vol- with individual magmatic events occurring over sequentially smaller spatial canic history (Fig. 1). Here we present an extensive data set of high-spatial- and faster temporal scales in the upper crust of the Central Andes from ~30 km reso lution U-Pb zircon ages that define zircon crystallization histories over the to the surface. ~10 m.y. spatial and temporal span of the APVC ignimbrite flare-up and use this to complement and extend our understanding of the development of con- tinental magmatic systems. The U-Pb data are complemented by reconnais- INTRODUCTION sance zircon trace element data that targeted sample subsets representative of the diversity of composition and eruptive style in the APVC. In particular, The eruptive histories of ignimbrite flare-ups are useful as a proxy for plu- we explore the implications of these data for the development of the plutonic ton formation at depth (Elston, 1984; Lipman, 2007; Best et al., 2013; Lipman system underlying large silicic volcanic fields. and Bachmann, 2015; Christiansen et al., 2016). However, such eruptive his- tories provide an incomplete picture as magma could be emplaced but not erupted. Identifying the pre-eruptive history of the magmas is critical to ef- GEOLOGIC BACKGROUND forts to better understand the volcanic-plutonic connection. At the forefront of these efforts are recent studies that take advantage of advancements in The APVC in the Central Andes occupies an arid, high-elevation plateau at high-spatial-resolution U-Pb and U-Th zircon dating in the past 15 years that the political triple junction between Argentina, Chile, and Bolivia (Fig. 1A). The allow for identification of zircon crystal populations that significantly predate dry climate preserves multiple large caldera centers and their accompanying eruption ages (Reid et al., 1997; Brown and Fletcher, 1999; Reid and Coath, ignimbrites, providing an unparalleled view of some of the largest explosive 2000; Vazquez and Reid, 2002; Schmitt et al., 2002, 2003; Charlier et al., 2005; silicic eruptions in the world. The ignimbrite plateau of the APVC developed Bryan et al., 2007). These studies have successfully used pre-eruption zircon as part of a regional ignimbrite flare-up during the Neogene development of crystallization histories to infer the behavior of magmas prior to eruption. In a the Central Volcanic Zone of the Andes (de Silva et al., 2006; Salisbury et al., silicic magma, zircon begins to crystallize once a magma of appropriate com- 2011; Freymuth et al., 2015). The APVC is the most intense and youngest locus position cools below the zircon saturation temperature determined by the melt of the central Andean flare-up, where over a 10 m.y. period