Volcano evolution and eruptive fl ux on the thick crust of the Andean Central Volcanic Zone: 40Ar/39Ar constraints from Volcán Parinacota, Chile John M. Hora† Brad S. Singer Department of Geology and Geophysics, University of Wisconsin–Madison, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Gerhard Wörner Geowissenschaftliches Zentrum der Universität Göttingen, Abteilung Geochemie, Goldschmidtstrasse 1, 37077 Göttingen, Germany ABSTRACT the distributed basaltic volcanism prevalent to crust across their compositional spectrum. in those other arcs is virtually absent both However, other factors, including the presence The 163 k.y. history as well as the chemi- at Parinacota and elsewhere in the Central of the volcanic edifi ce itself (Pinel and Jaupart, cal and 46 km3 volumetric evolution of Vol- Volcanic Zone. This suggests that while the 2000), may also contribute to crustal “fi ltering” cán Parinacota are described in detail by hydrous, calc-alkaline magmas that make of ascending magmas, even on thick Andean new mapping, stratigraphy, and 57 40Ar/39Ar up the central volcanoes are not signifi cantly crust where edifi ce loading is <5% relative to ages determined from groundmass or sani- retarded by thick crust, primitive, dry basalts total crustal thickness. dine crystals in basaltic andesitic to rhyolitic might be. For those magmas that do erupt, there exists lavas. A more precise chronology of eruptions a broad spectrum of eruptive styles, indicating and associated eruptive volumes of this cen- Keywords: Andes, Central Volcanic Zone, Pari- a variety of storage and transport regimes in tral Andean volcano, which was built upon nacota, 40Ar/39Ar geochronology, eruptive rates. the upper crust. These range from numerous, 70-km-thick crust, provides a more com- small, and usually mafi c monogenetic centers, plete view of how quickly volcanic edifi ces INTRODUCTION which imply many ephemeral conduits through are built in this setting and how their mag- the upper crust, to large silicic calderas, which matic systems evolve during their lifetime. Subduction-zone magmas interact with the are evidence of long repose times of magma Development of the complex involved initial lithospheric mantle and crust of the overlying accumulation with correspondingly voluminous eruption of andesitic lava fl ows (163–117 ka) plate that must be traversed before they can magma chambers. Stratovolcanoes populate the followed by a rhyodacite dome plateau (47– erupt. Based on chemical and isotopic com- intermediate region of this spectrum, and they 40 ka) synchronous with the onset of the position of these magmas, such interaction is represent moderate-volume, multiply reacti- building of a stratocone (52–20 ka), which expected to be more extensive in regions of vated magmatic systems that have protracted was later destroyed by a debris avalanche ~3 thickened continental crust relative to oceanic histories and several episodes of activity. times larger than that at Mount St. Helens in arcs. This has been attributed to deeper stagna- This study examines in detail the volumes and 1980. Dome plateau emplacement occurred tion of primitive magmas (Hildreth and Moor- timing of eruption of arc magmas that have tran- faster and later than has previously been bath, 1988; Leeman, 1983) operating in addition sited the entire crust and erupted at Parinacota, published, implying a compressed duration to ubiquitous differentiation in shallow, subvol- a central Andean volcano built upon very thick of cone building and introducing a preced- canic plumbing systems. During ascent, magma crust. Accurate quantifi cation of time spans dur- ing 65 k.y. hiatus. Debris avalanche timing is progress through the crust is controlled both by ing which cone building took place allows us to refi ned here to be older than 10 but younger its rheology and density contrast between it and assess the nonuniformity of eruptive fl ux. Com- than 20 ka. Rapid postcollapse rebuilding the surrounding rock. Indeed, a large proportion parison of cone-building rates with volcanoes of the volcanic edifi ce is delineated by 16 of magmas crystallize within the crust, having built on thinner crust is a fi rst step toward mean- groundmass and whole-rock 40Ar/39Ar ages, never reached the surface. While global aver- ingful assessment of the role that crust plays as a which include some of the youngest lava age rates of volcanism are lower in continental rheological and density fi lter, i.e., its effect on the fl ows dated by this method. Increase in cone- versus oceanic settings (Crisp, 1984; White et probability that a given magma batch will erupt. building rate and a continued trend toward al., 2006), there is little evidence of signifi cantly The specifi c goals of the study are to (1) better more mafi c compositions following collapse increased intrusive:extrusive ratios in conti- understand the throughput of magma in a multi- imply an inter-relationship between the pres- nental crust (White et al., 2006). Calc-alkaline ply replenished subvolcanic system, (2) use that ence of the edifi ce and fl ux of magma from subduction-derived magmas typically con- information to integrate changes in volcano mor- the feeding reservoir. Cone-building rates at tain >4% dissolved water (Grove and Kinzler, phology with magma system confi guration, and Parinacota are similar to those at other well- 1986; Sisson and Grove, 1993), and they have (3) through comparison with cone-building rates dated volcanoes on thinner crust; however, signifi cantly decreased densities relative to dry of volcanoes on thinner crust, examine the role magmas of the same composition (Ochs and that the crustal column plays in modulating sur- †E-mail: [email protected] Lange, 1999), making them buoyant relative face fl ux and peak cone-building rates. Because GSA Bulletin; March/April 2007; v. 119; no. 3/4; p. 343–362; doi: 10.1130/B25954.1; 8 fi gures; 6 tables; Data Repository item 2007064. For permission to copy, contact [email protected] 343 © 2007 Geological Society of America Hora et al. time intervals between pulses of volcanic activ- long-lived, volcanic systems with adequate tem- volumes estimates are accurate. (2) Previous ity in a given region and associated magma vol- poral resolution to quantitatively constrain the geologic mapping of the complex has been pub- umes are highly variable, especially if disparate durations of cone-building events and interven- lished (Clavero et al., 2004; Wörner et al., 1988) eruption types occur, (i.e., cinder cones, strato- ing lacunae. The few extant geochronologic and has revealed an intricate history including: volcanoes, and caldera-sourced ignimbrites), studies of suffi cient detail for arc volcanoes a large variety of erupted compositions, shifting it is diffi cult to defi ne an appropriate time and indicate that cone building occurs in relatively vent locations, catastrophic destruction by sector area scale at which different arcs may be com- short-lived bursts of activity separated by peri- collapse with related debris avalanche processes, pared. By reducing the problem to simply com- ods of quiescence (Hildreth and Lanphere, and repeated episodes of cone growth. (3) Unlike paring cone-building rates of volcanoes that are 1994). Average eruption rates determined from many volcanoes that have undergone sector col- otherwise broadly similar in size and type and global compilations or integrated over large seg- lapse, Parinacota preserves enough precollapse by studying a time span that includes two such ments of an arc (Crisp, 1984; White et al., 2006) stratigraphy on its southern fl anks to allow broad cone-building periods, this study endeavors to are very useful for determining planetary-scale correlation of in situ units with those in the debris make meaningful comparisons without hav- energy budgets and long-term thermal evolution fi eld, based on age, chemistry, texture, and min- ing to tackle the nearly intractable problem of of Earth. However, histories of individual volca- eralogy. (4) Extensive geochemical and isotopic obtaining high-precision geochronology and noes determined on the basis of only a few age data (Clavero et al., 2004; Davidson et al., 1990; volumes for every volcano in the arc. determinations provide, at best an incomplete, Wörner et al., 1988) indicate that Parinacota has Composite stratovolcanoes are among the and at worst, a wholly inaccurate view of true erupted a high-K calc-alkaline magma series defi ning features of volcanic arcs. Due to their eruptive fl uxes at any given time in a particular (2.5–4.5 wt% K2O), thereby making even the longevity, they can be used to examine eruptive volcano’s history. youngest of its lavas good candidates for pre- fl ux at a single location through time. Subvolca- There are, however, less than a dozen subduc- cise 40Ar/39Ar dating. Finally, (5) a handful of nic magmatic processes have long been a focus tion-related volcanoes worldwide for which such published K-Ar, 40Ar/39Ar, 3He, and U-Th ages of petrology and geochemistry. However, inte- detailed geochronologic studies are closely tied indicate a life span of ~200 k.y. (Bourdon et al., grating the temporal variability and intercon- to stratigraphy, composition, and volume esti- 2000; Clavero et al., 2004; Wörner et al., 1988, nectivity of the magma reservoirs involved with mates, allowing a reasonably complete, quan- 2000a), which makes it amenable for a more time scales of crystallization and magma move- titative history to be translated into meaningful detailed 40Ar/39Ar study. ment (or stagnation) through parts of the system eruptive fl ux rates. Of these, Santorini (Druitt remains a frontier of igneous petrology (David- et al., 1999), Montserrat (Harford et al., 2002), TECTONIC AND GEOLOGIC SETTING son et al., 2005; Hawkesworth et al., 2004; and Seguam (Jicha and Singer, 2006; Jicha et Jicha et al., 2005). A complete appreciation of al., 2005) are stratovolcanoes built upon oceanic Parinacota (~46 km3 total erupted volume) is processes that modulate fl ux of magma through crust.