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Magma–Tectonic Interaction and the Eruption of Silicic Batholiths

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Magma–Tectonic Interaction and the Eruption of Silicic Batholiths Earth and Planetary Science Letters 284 (2009) 426–434 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl Magma–tectonic interaction and the eruption of silicic batholiths J. Gottsmann a,⁎, Y. Lavallée b, J. Martí c, G. Aguirre-Díaz d a Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ, United Kingdom b Department of Earth and Environmental Sciences, Ludwig-Maximilian University, Theresienstr. 41, 80333 Munich, Germany c Institute of Earth Sciences “Jaume Almera,” CSIC, Luis Sole i Sabaris is/n, Barcelona, 08028, Spain d Centro de Geociencias, UNAM, Campus Juriquilla, CP 76230, Juriquilla, Querétaro, Mexico article info abstract Article history: Due to its unfavorable rheology, magma with crystallinity exceeding about 50 vol.% and effective viscosity > Received 5 May 2008 106 Pa s is generally perceived to stall in the Earth's crust rather than to erupt. There is, however, irrefutable Received in revised form 20 April 2009 evidence for colossal eruption of batholithic magma bodies and here we analyze four examples from Spain, Accepted 6 May 2009 Mexico, USA and the Central Andes. These silicic caldera-forming eruptions generated deposits characterized Available online 30 May 2009 by i) ignimbrites containing crystal-rich pumice, ii) co-ignimbritic lag breccias and iii) the absence of initial Editor: C.P. Jaupart fall-out. The field evidence is inconsistent with most caldera-forming deposits, which are underlain by initial fall-out indicating deposition from a sustained eruption column before the actual collapse sequence. In Keywords: contrast, the documented examples suggest early deep-level fragmentation at the onset of eruption and magma repeated column collapse generating eruption volumes on the order of hundreds of cubic kilometers almost crystal-liquid mush exclusively in the form of ignimbrites. These examples challenge our understanding of magma eruptability relaxation time and eruption initiation processes. In this paper, we present an analysis of eruption promoters from geologic, strain rate theoretical and experimental considerations. Assessing relevant dynamics and timescales for failure of volcano–tectonics crystal-melt mush we propose a framework to explain eruption of batholithic magma bodies that primarily caldera involves an external trigger by near-field seismicity and crustal failure. Strain rate analysis for dynamic and static stressing, chamber roof collapse and rapid decompression indicates that large “solid-like” silicic reservoirs may undergo catastrophic failure leading to deep-level fragmentation of batholithic magma at approximately 2 orders of magnitude lower strain rates than those characteristic for failure of crystal-poor magmas or pure melt. Eruption triggers can thus include either amplified pressure transients in the liquid phase during seismic shaking of a crystal-melt mush, decompression by block subsidence or a combination of both. We find that the window of opportunity for the eruption of large silicic bodies may thus extent to crystallinities beyond 50 vol.% for strain rates on the order of >10− 3 to 10− 4 s− 1. © 2009 Elsevier B.V. All rights reserved. 1. Introduction magma tends to pond rather than erupt irrespective of the magma composition (Scaillet et al., 1998). With increasing crystallinity, a mush Magma stalled in an upper-level crustal reservoir consists of molten develops towards a rigid percolation threshold and by reaching a silicate fluid and various proportions of crystals and bubbles. According crystallinity exceeding 0.5, magma is believed to be uneruptable (Marsh, to Marsh (1989), increasing crystallinity (ϕ) due to the propagation of 1989; Vigneresse et al., 1996). Of course, there is evidence from effusive the solidification front transforms magma from a crystal suspension eruptions that produce dome lavas with effective viscosities of well in (0≤ϕ≤0.25) to a crystal-melt mush (0.25bϕb0.55) and finally to a excess of 1010 Pa s, yet high crystallinity is attributed to late stage rigid crust (0.55bϕ≤1). The eruptability of magma is generally seen to decompression-driven crystallisation upon degassing within the con- be directly dependent on its crystallinity and thus on its rheology. duit and does not reflect chamber conditions upon the onset of eruption Increasing crystal content has two important consequences for magma (Sparks et al., 2000). In the case of colossal silicic explosive eruptions 3 rheology. Firstly, it dramatically increases effective viscosity and hence (≥100 km of magma; Volcanic Explosivity Index (VEI)≥7; Newhall affects its flow behaviour and secondly, it strongly affects its mechanical and Self,1982), the evacuation of a subsurface reservoir generally results properties (Dingwell, 1997). Most explosive volcanic eruptions tap in caldera collapse. Most of these eruptions are dominated by crystal suspensions with bulk properties favorable for viscous flow, scavenging crystal-poor magma suspensions and their eruption is ascent and thus eruption (Woods,1995). The threshold magma viscosity controlled by processes internal to the magmatic system, including 6 for eruption is on the order of 10 Pa s (Takeuchi, 2004). Above this limit overpressurisation, ring fault initiation and subsequent roof collapse (Gudmundsson, 2006). However, there are also examples of caldera- ⁎ Corresponding author. forming eruptions involving crystal-rich (ϕ around and exceeding 0.5) E-mail address: [email protected] (J. Gottsmann). magmas throughout the geological record, among which are the most 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.05.008 J. Gottsmann et al. / Earth and Planetary Science Letters 284 (2009) 426–434 427 Table 1 a flow deposit blurs the information on the original magma crystal List of case examples and characteristics of deposits. content (Cas and Wright, 1993) and these deposits should not be used Case examples Description of deposits as evidence for crystal-rich magmas. All our examples expose primary Permo-carboniferous Prats d'Aguilo >>50 km3 (likely >100 km3) of crystal- magmatic crystal contents in pumices and thus interpretations are not dacites (Spain) (Martí, 1996) rich dacitic ignimbrites and lavas; crystal based on crystal concentrations in the related ignimbrites (Fig. 3). content of up to 60 vol.% in pumice; lithic Primary clast vesicularities are low (up to 0.3) in the case examples and pumice-rich ignimbrite; primary (Fig. 3). Welding in some deposits is a concern for assessing primary vesicularity of pumice uncertain due to welding, but evidence for poor initial clast vesicularity and thus extra care was taken in selecting inflation; stratigraphy suggests dacites uncollapsed fragments for the assessment of primary vesicle content. correspond to intracaldera deposits Although vesicularities can vary significantly in explosive eruptive during basin development. products (Houghton and Wilson, 1989) it is frequently assumed Eocene–Oligocene ignimbrites of Durango >>200 km3 possibly >1000 km3 of (particularly in numerical consideration of explosive volcanism) that State, central Sierra Madre Occidental, ignimbrites; crystal content exceeding Mexico (Aguirre-Díaz and 40 vol.% in pumice; association with fragmentation occurs at a vesicularity of about 77 vol.% (Sparks et al., Labarthe-Hernañdez, 2003) several graben systems; fissure type 1997) and “fragmentation vesicularities” of pumices from Plinian eruption vents; graben formation eruptions are in broad agreement with the threshold value (Klug and intimately related to large-scale eruption Cashman, 1994) for “classic” explosive eruption scenarios. However, as of ignimbrites (Fig. 1c); liquefaction structures in sediments immediately explained above, the case examples show low degrees of primary clast underlying ignimbrites along caldera vesicularity. The absence of pumice-fall deposit and hence lack of margin (Fig. 1d). evidence for a substained (Plinian) eruption column indicates that 3 Cerro Panizos volcanic centre, 6.7 Ma >600 km DRE of two crystal-rich dacitic vesiculation-induced fragmentation (see also next section) upon (Central Andes) (Ort, 1993) ignimbrites in area of normal faulting; system decompression was of second order. crystal content of up to 50 vol.% in pumice; vesicularity of pumice in the The question is then as to how to tap and erupt magma, which lower cooling unit is less than 20 vol.%; defies the concept of eruptability. formation of ignimbrite sheets related to onset and formation of caldera collapse 3. Magma rheology and fragmentation evidenced by increased lithic contents in the lower unit. Pagosa Peak Dacite ca. 28 Ma (San Juan >200 km3 ignimbrite immediately 3.1. Model magma Volcanic field, Basin and Range province predate eruption of Fish Canyon Tuff 3 U.S.A. (Bachmann et al., 2000) (~5000 km ); crystal content of up to In quantifying rheology and fragmentation of magma relevant for the 50 vol.% in pumice. vesicularity of pumice case examples, we assume hereforth a model dacite magma represent- in PPD is around 25 vol.% (at least 60% lower than pumice from FCT); angular and ing an averaged analogue to the investigated eruptions at the following equant glass shards dominant; conditions: temperature of 750 °C, water activity of 1 wt.% (below unity concentration of lithic fragments at base at 150 MPa or 5 km depth), a calc-alkaline metaluminous bulk com- of unit indicating conduit enlargement
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