The Life and Times of Silicic
Volcanic Systems
Ash plume erupting
from the Chaitén
volcano (Chile) in
2009. IMAGE CREDIT:
NASA
Colin J.N. Wilson1, and Bruce L.A. Charlier1,2
1811-5209/16/0012-0103$2.50
S
DOI: 10.2113/gselements.12.2.103
also, to a greater or lesser extent,
the product of interaction with
evolved (i.e. fractionated) crustal
materials through melting and
assimilation. For rhyolites, such
as the early erupted parts of the
Bishop Tuff (Rb/Sr ratios >100),
there needs to have been a substantially greater volume of crystals
grown and lodged at depth to form
a pluton than the volume of the
magma that was erupted at the
surface. This concept is the basis
behind the mush model for silicic
KEYWORDS : Caldera, rhyolite, magma chamber, Quaternary, silicic volcanism
magmatism, whereby substantial
plutons are considered to be the
INTRODUCTION
required counterpart to silicic
volcanism
(Bachmann
and
Bergantz 2004; Hildreth 2004).
The timescales and processes associated with silicic volcanic
rocks are often compared and contrasted with the plutonic Here, we summarise evidence from three Quaternary silicic
record and can be inferred from many types of informa- volcanic systems that bears on the timescales and processes
tion, from field-focussed studies to theoretical models. leading to the generation of silicic magmas and their release
These sources of information can, however, deliver quite
in volcanic events. We consider how the volcanic record
different messages, depending on what kind of evidence
can illuminate aspects of the large-scale magmatic processes
is used. Here, we consider what field-focussed studies tell
associated with modern batholith-scale pluton growth. We
us of the nature and behaviour of silicic volcanic systems.
can only draw the reader’s attention to a fraction of the
The simplest method of tracing volcanic timescales and relevant papers, and have, thus, mostly focussed on recent
processes is to date and study the products of successive
publications as an entry point to the literature.
eruptions from a particular volcano. In such cases, dating
the eruptions and determining the compositions of the
DATING THE VOLCANIC RECORD
products help track the changing nature of the hidden
subsurface magmatic system, including where magma is Two Approaches
generated and where melt accumulation occurs.
There are two complementary approaches with which to
ilicic volcanic systems provide timed snapshots at the Earth’s surface of
the magmatic processes that also build complementary plutons in the
crust. Links between these two realms are considered here using three
Quaternary (<2.6 Ma) examples from New Zealand and the USA. In these
systems, magmatic processes can be timed and the changes in magmatic
conditions can be followed through the sequence of quenched volcanic
eruption products. Before an eruption, magma accumulation processes can
occur on timescales as short as decades, and whole magma systems can be
rebuilt in millennia. Silicic volcanic processes, in general, act on timescales
that are too rapid to be effectively measured in the exposed plutonic record.
Even given a timed sequence of eruptions and their
deposits, however, linking and comparing volcanic records
to the inferred complementary plutonic record is not
simple. From the start, we draw attention to two aspects.
First, most fine-scale knowledge of volcanic processes comes
from studies of Quaternary rocks (produced during the
last 2.6 My), whereas nearly all accessible in situ plutonic
materials are typically tens of millions of years old, or
older. The resulting contrasts in the precision with which
volcanic and plutonic processes can be timed are one aspect
of why contrasts in interpretation have arisen. Second, a
key aspect of silicic volcanic rocks is that, with few exceptions, their isotopic compositions (particularly Sr, Pb, Nd
and O) and trace element data indicate that they are the
end product of large amounts of crystal fractionation and
1 SGEES, Victoria University
PO Box 600, Wellington 6140, New Zealand
E-mail:[email protected]
2 CEPSAR, The Open University
Walton Hall, Milton Keynes MK7 6AA, UK
E-mail:[email protected]
E LEMENTS , V OL . 12,
PP.
103–108
measure the tempo of Quaternary (and older) volcanic
records and their parental magmatic systems.
First, date the volcanic events and then use the changing
eruptive compositions to follow the detailed evolution
of the magma system through time. Young sequences
(<40–50 ka) are generally dated through radiocarbon
methods; deposits with overlapping and older ages are
typically dated by 40Ar/ 39Ar techniques. This approach
works well in areas like the Taupo Volcanic Zone (TVZ;
New Zealand), or Long Valley (California, USA), where
tens or more eruptions are available with which to follow
magmatic processes. It is less easy to apply at localities
like Yellowstone (western USA) where eruptions are widely
spaced in time, or if the eruption of interest happens to be
the fi rst one in a sequence.
Second, date the minerals in eruption products to look
at the evolution of the magma system as captured in a
single eruption. All methods exploit as internal clocks the
radioactive decay of certain isotopes, the ratios of which are
considered to be unaffected by residence at high temperatures in the magma system. Early approaches, specifically
103
A PR IL 2016
in very evolved silicic systems, exploited the Rb–Sr system
in feldspars. More recently, however, most dating of
magmatic systems has been done through the analysis
of U- and Th-bearing minerals (particularly zircon). We
outline below the methods and results from zircon dating.
Zircon Dating
Zircon as a dating tool has been used for many decades
(Hanchar and Hoskin 2003). All methods revolve around
one or more of the decay chains of 238U (to 206Pb), 235U (to
207 Pb) and 232 Th (to 208Pb), and exploit the fact that zircon
incorporates significant amounts of U and Th and effectively excludes Pb from its structure as it crystallises. Thus,
ingrowth of ppb levels of radiogenic Pb can be measured
in the crystals without it being swamped by the tens of
ppm levels of ‘common’ Pb that are typically present in the
host melt. For the Quaternary examples considered here,
the most used tool is the 238U–206Pb system, in part because
of the relative abundance of 238U and in part because of
a characteristic of its decay chain. Within this chain, one
intermediate product is 230 Th, which has a half-life of
~75 ky. At secular equilibrium, the activity of 238U is equal
to that of 230 Th. As zircon grows, however, U is preferentially taken up over Th in the crystal (typically by a factor
of five to ten), and the 238U/230 Th activity shifts to values
>1. Subsequent decay of 238U then serves to accumulate
230 Th at a rate faster than the 230 Th can decay until the
decay and production rates are matched and a new secular
equilibrium is achieved after about 350 ky (Schmitt 2011).
There are, thus, two ways of dating the zircons in young
volcanic rocks. The fi rst, using U/Th disequilibrium, can
in principle be applied back to 350 ka (i.e. ~5 half-lives of
230 Th), although the precisions of calculated ages decrease
as the decay system approaches secular equilibrium.
Once a data point falls within error of secular equilibrium, no meaningful age information can be derived, and
the technique is therefore best suited to rocks that are
<100 ka in age. The second method, applied to rocks that
are >~150 ka, measures the 238U and 206Pb abundances in
the crystals and so derives a direct age estimate, but this
has to then incorporate a correction for the initial 230 Th
disequilibrium. Typically, this correction adds 60–100 ky
to the age estimate, so it can be neglected for Neogene
and older rocks, but is significant in Quaternary examples.
At present, measurements of isotopes for dating purposes
in zircons revolve around three spectrometry techniques
(Hanchar and Hoskin 2003; Schaltegger et al. 2015) (FIG. 1).
Each technique has its advantages and disadvantages.
Isotopic dilution thermal ionization mass spectrometry
(ID-TIMS) is highly precise, but requires specialist chemical
separation, clean laboratory techniques and is generally applied to single whole grains. Secondary ion mass
spectrometry (SIMS) and laser ablation inductively coupled
mass spectrometry (LA-ICP-MS) methods are less precise
but have the advantages of the rapidity of analysis, the
ability to analyse growth zones within crystals in order to
place age determinations into a textural context, and not
requiring specialist skills. In situ analysis of cross-sectioned
grains allows for core–rim relationships to be explored, but
the spot size (20–30 μm) does not permit the outermost
growth zone to be analysed. Surface profiling can be used to
analyse the outermost surfaces of grains but is then ‘blind’
with respect to textural information until after analysis.
CASE STUDIES IN THE VOLCANIC RECORD
Taupo Volcanic Zone (TVZ)
The central Taupo Volcanic Zone (New Zealand) has been
an exceptionally active area of silicic volcanism for the past
~2 My (Wilson and Rowland 2016) and is comparable, in
overall terms, to the modern Yellowstone system. Both are
exceptionally dynamic areas that have produced multiple
supereruptions, although associated with contrasting
crustal settings: the TVZ with a rifting continental arc;
Yellowstone with hotspot magmatism on thick continental
crust. We use two scales here to highlight the volcanic–
magmatic history of the TVZ: fi rst, its shorter-term record
(<60 ka), as expressed at the two highly active volcanoes
of Taupo and Okataina; second, its overall 2 My record of
waxing and waning.
Method
Typical precision
Advantages/
disadvantages
(U–Pb and U–Th)
U–Pb: very high precision, <0.1%
U–Th: very high precision
2
3
(10 to 10 yr on ages <300 ka)
ID-TIMS
(U–Th, U–Pb)
Multiple grains
~500 ȝP
Single grain
(plus chemical
abrasion - CA)
LA-ICP-MS pit
5HTXLUHVVSHFLDOLVHGORZEODQN
VHSDUDWLRQDQGORDGLQJWHFKQLTXHV
6ORZDQGWLPHFRQVXPLQJ
ID-TIMS
(U–Pb)
Very high precision, <0.1%
6XUIDFHSUR¿OLQJ
(SIMS or LA-ICP-MS)
(U–Th or U–Pb)
SIMS: 10–PLFURQVSRWVL]H
PLFURQGHSWK
20–PLQVSHUDQDO\VLV
WRVLJPDSUHFLVLRQ
SIMS: Very high spatial resolution,
in particular ZLWK VXUIDFHSUR¿OLQJ
Fast analysis
Very high ionisation ef¿FLHQF\
Cross section
(SIMS or LA-ICP-MS)
(U–Th or U–Pb)
LA-ICP-MS: 30–PLFURQVSRWVL]H
10–PLFURQVGHSWK
~PLQVSHUDQDO\VLV
WRVLJPDSUHFLVLRQ
LA-ICP-MS*RRGVSDWLDOUHVROXWLRQ
Very fast analysis
Variable ionisation ef¿FLHQF\
CA SRWHQWLDOO\UHPRYHVSDUWRIWKH
grain’s history
SIMS pit
Single grain
Indium
LA-ICP-MS pit
Single grain
3RRUVSDWLDOUHVROXWLRQPL[LQJ
RIGLIIHUHQWDJHGRPDLQVZLWKLQ
VLQJOHFU\VWDOVRUGLIferent crystal
SRSXODWLRQVZLWKLQPXOWLJUDLQ
DOLTXRWVLVXQDYRLGDEOH
SIMS pit
epoxy
Epoxy
~100 ȝP
The main spectrometry techniques that are applied to
dating young volcanic rocks, with examples of their
typical precision, and their advantages and disadvantages.
Technique acronyms are as follows: ID-TIMS (isotopic dilution
FIGURE 1
E LEMENTS
thermal ionization mass spectrometry); LA-ICP-MS (laser ablation
inductively coupled plasma mass spectrometry); SIMS (secondary
ion mass spectrometry); CA (chemical abrasion).
104
A PR IL 2016
1. Crystals in the eruption products are a mixture (just
like in andesites). Some crystals (or their outer parts)
are phenocrystic, grown in the evolving melt in which
they were erupted; others are antecrystic, grown in
forerunner parental melts; while others are xenocrystic,
derived from unrelated sources. Age data and compositional and/or isotopic contrasts in zircon or other
minerals can distinguish between the different crystal
origins (Charlier et al. 2005). Mineral populations
include variety at the whole-grain scale due to mixing
(Storm et al. 2014), or a contrast between diverse, older
cores extracted from the deeper magmatic roots and
uniform rims grown in the melt-dominant body (a
magmatic E Pluribus Unum) (Allan et al. 2013). Evidence
for the existence of melt-dominant bodies may be muted
or absent in the crystals that are left behind and that go
to form any associated growing plutonic body.
2. The (geo-)chemical processes involved in the development of a magma act on separate, longer timescales
than the physical processes which assemble the erupted
melt-dominant body. For example, the magma system
for the 25.4 ka Oruanui eruption has a source history
going back to ~100 ka, but the 530 km3 erupted magma
body was assembled in <3,000 years (Allan et al. 2013),
and the magma system was largely reset with respect
to compositions and zircon age spectra in <5,000 years
after the Oruanui eruption (Barker et al. 2014; FIG. 2).
The TVZ rhyolites emphasise that eruptible meltdominant bodies are ephemeral entities associated with
longer-lived crystal-dominant plutonic sources.
Over the past 350 ky, within the central TVZ about 50%
of the area has been occupied by calderas. If depths and
volumes of the parental magma systems are similar to
those of younger examples studied in detail, then there
is a composite pluton being developed at depths of ~4–15
km below the surface (FIGS. 3 AND 4). Outside the caldera
areas, the intense geothermal fluxes require that there are
abundant and voluminous crustal intrusions, but these
have reached levels accessible by drilling (~3.2 km) at only
one location. If the activity from 2.0 Ma to 350 ka was
similar in extent and intensity to that at <350 ka, then
over ~6,000 km 2 and a vertical depth of >10 km within
the quartzo-feldspathic crust, the TVZ composite pluton
has been constructed, and is continuing to grow as you
100
Medium- to Longer-Term Volcanic–Plutonic
Record of the TVZ
On timescales of 105 –106 years, the TVZ volcanic record
is notably punctuated, with shifting caldera sources for
large-scale volcanism. The zircon record implies that plutoE LEMENTS
10
Number of grains analysed
ll ns
ma tio
e s up
re e er
h
t
T ci
da
0
Zircons from
rhyolite subgroup 2 pumices
erupted 7–2.8 ka
(99 analyses)
20
3. There may be multiple melt-dominant bodies present
simultaneously in the shallow crust, each with independent crystallization histories (from zircon age spectra)
and geochemical origins (e.g. Shane et al. 2008; Allan
et al. 2012; Storm et al. 2014). Despite their eruptive
vigour, neither Taupo nor Okataina volcanoes developed
a single, all-encompassing magmatic system; instead,
they have multiple heterogeneous magmatic domains
developed over vertical and horizontal length scales of
kilometres to tens of kilometres.
4. There is strong evidence for external, tectonic controls
on whether eruptions occur or not, and how they
progress (Allan et al. 2012). Overall, there are complex
interactions between magmatic and tectonic processes
that (a) generate magma in crystal-rich crustal bodies,
(b) cause melt-dominant magma bodies to accumulate,
and (c) control whether such bodies erupt or cool back
into crystal-rich mush (Rowland et al. 2010). The record
at Taupo (Charlier et al. 2005; Barker et al. 2014) shows
periods of enhanced zircon growth with cooling (and
growth of other minerals) and a lack of eruptive activity,
alternating with periods of disturbance (heating and/
or rifting) with numerous eruptions (e.g. FIG. 2). These
periods recur on timescales of 103 –10 4 years, which is
too short a scale to be easily seen in the Long Valley
and Yellowstone systems where eruptions are spaced at
10 4 –105 year intervals.
Age (ka)
50
25
0
40
Zircons from
rhyolite subgroup 1 pumices
erupted 12–10 ka
(203 analyses)
30
37–41 ka peak
Four key points arise about volcanic processes and timings
from studies of young TVZ silicic volcanic rocks at Taupo
and Okataina.
nism is equally episodic. Sometimes the two records are
closely linked such that the peak of zircon crystallisation
occurs just prior to eruption. However, in the larger systems
(Oruanui, Whakamaru, Kidnappers, Ongatiti), there are
peaks in the zircon age spectra sometimes >100 ky prior
to eruption, suggesting that the plutonic and volcanic
rhythms are not always synchronised on that timescale.
In contrast, the deeper magmatic input into the TVZ, as
reflected in the location and vigour (total ~4.2 gigawatt)
of high-temperature (>250 °C) geothermal systems, is
often inferred to have remained more uniform (Wilson
and Rowland 2016).
86–96 ka peak
Shorter-term Histories at Taupo
and Okataina Volcanoes
20
10
0
20
Zircons from
Oruanui pumices
(207 analyses)
Oruanui
eruption
age (25.4 ka)
10
0
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Isochron slope
Compilation of zircon U–Th model ages from Taupo
volcano (New Zealand), where ages are calculated
from the slope of an isochron constructed between a whole-rock
value and each individual zircon analysis on an isotope evolution
diagram (see Charlier et al. 2005). Histograms represent the
individual model ages; the red curves are fitted probability distribution function (pdf) lines that reflect the values and uncertainties
associated with the age data. These data highlight the disconnect
between peaks in zircon crystallization (shown by the peaks in the
red pdf curves) and the timing of single eruptions (Oruanui) or
clusters of eruptions (rhyolite subgroups 1 and 2), the products of
which were sampled. The grey vertical line shows the time span
(20.5–17 ka) when three other small dacite eruptions occurred at
Taupo – no data are presented here from these zircon-poor
eruption products. A FTER BARKER ET AL. (2014).
105
FIGURE 2
A PR IL 2016
read this article, at a modern rate of ~50 km3 per ky, with
about 10–20% of its volume having been periodically spat
out as eruptions.
This pluton is compositionally zoned, with only the
shallower parts being quartz-bearing (Allan et al. 2013),
and probably shows strong lateral compositional diversity at
any given depth. Isotopically, it reflects a mixture of ~25%
recycled metasedimentary crustal rocks and ~75% fractionates from mantle melts, which volumetrically means net
crustal growth. Zircon age data from different portions of
this pluton will show clustering into periods of enhanced
crystallisation, but only longer-term periodicities would be
discernible in a Mesozoic analogue. The TVZ pluton is only
one of several composite silicic plutons developed as part of
the New Zealand convergent plate boundary. Other plutons
underlie the Coromandel area north of the TVZ, and their
volcanic products are represented by voluminous silicic ash
beds in deep-ocean cores (Carter et al. 2003); the earliest
of the corresponding plutons – the 16.4 ± 0.1 Ma Paritu
pluton (zircon U–Pb age: pers comm TR Ireland 2012) – has
already been uplifted to the surface.
Long Valley, California
Long Valley and adjacent areas in eastern California are
best known for the 0.77 Ma Bishop Tuff eruption, but the
silicic volcanic record extends over ~3.5 My, up to as recent
as ~1,350 CE (Hildreth 2004). Aspects of the volcanic record
at Long Valley that are relevant here include the following.
1. There have been six magmatic foci active since 3.5 Ma,
with their surficial volcanic footprints migrating
between adjacent areas, with minimal overlap and
distinct compositional characteristics (Hildreth 2004).
At depth, however, it is likely that all six foci are contributing towards the construction of a composite pluton
(FIGS. 3 AND 4).
2. Rhyolitic volcanic products of the Glass Mountain
(~2.8–0.86 Ma) and Bishop systems have compositions indicative of extreme fractionation, consistent
with co-generation of thousands of cubic kilometres
of intermediate to silicic mush at depth (Hildreth 2004).
Overall, the relative contributions of older pre-existing
crustal lithologies have diminished with time as the
proportions of mantle-derived material have increased
(Simon et al. 2014). However, the Bishop Tuff zircon
record shows that recycling of older Glass Mountain
material was minimal: the Bishop magma body accumulated with its own zircon age signature over ~80 ky,
as shown by age contrasts between zircon cores and
rims (Chamberlain et al. 2014). The average of multiple
rim ages from SIMS analyses replicates within uncertainty the average ID-TIMS age estimate from Bishop
zircons and show that the ID-TIMS method masks the
histories inherent in the grains. Any recycling of older
plutonic material suggested from geochemical evidence
(Simon et al. 2014) is concealed by the stripping out of
older zircons and resetting of the magma chronological
record (cf. Taupo: Barker et al. 2014).
3. In the recent record there is a close link between volcanism, dike emplacement and tectonism (Bursik et al.
2003). Similar links are suspected to have occurred
in the past, but are hard to demonstrate. The overall
position and shape of the Bishop Tuff caldera reflects a
pre-existing jog in the Sierra Nevada Range’s front (Riley
et al. 2012). During the Bishop eruption, the initial vent
area in the south-central part of the caldera lay along
the line of the Laurel Creek Fault. The extension of
this fault connects through the post-caldera resurgent
dome structure to reach the northern caldera rim where
E LEMENTS
50 km
modern shallow
felsic pluton
Okataina
caldera
Yellowstone
caldera
Lake
Taupo
Taupo
caldera
Long Valley
caldera
deeper ma
pluton
older shallow
felsic pluton
Map outlines for the three caldera systems discussed
in this article. (LEFT) The Taupo Volcanic Zone (TVZ).
Green area is the post-350 ka pluton outline superimposed on the
larger, pink, 2 My TVZ pluton outline. Black lines are caldera
outlines. A FTER WILSON AND ROWLAND (2016). (CENTER) The Long Valley
system. Blue areas are volcanic foci; Long Valley caldera is indicated
in black; purple is the outline of the Long Valley pluton. A FTER
H ILDRETH (2004). (R IGHT) The Yellowstone system. Caldera outlines in
black, and outlines of plutons are in brown, orange and red. A FTER
CHRISTIANSEN (2001) AND H UANG ET AL. (2015). The respective calderas
and associated inferred plutonic bodies give an idea of their
comparative scale. The three blue lines on each caldera system
indicate the cross sections in FIGURE 4.
FIGURE 3
activity fi rst broke out in that sector. A tectonic connection, since exploited by the resurgent dome activity, is
suspected to be present.
There is a still-growing ‘Long Valley pluton’ that extends
(from the volcanic footprints) over ~1,500 km 2, elongate
parallel to the eastern Sierra front, and extending from
~6 km depth to the base of the crust (FIGS. 3 AND 4). It
has been growing over ~3.5 My, with a record that can be
investigated through zircon studies of ~2.8 My. As with
the TVZ pluton, there is clear evidence for vertical and
lateral heterogeneity (Hildreth 2004) and a long-term role
for tectonic processes (Riley et al. 2012). In contrast with
the TVZ, there is evidence for systematic changes in the
evolution of the pluton (Simon et al. 2014). Note that the
time gap between growth of the Long Valley pluton and the
youngest of the adjacent Sierra Nevada plutons (i.e. 2.8 to
87 Ma) is less than the time gap between adjacent plutons
within the Sierra Nevada batholith. If translated back into
the Precambrian, would the Long Valley pluton be treated,
for example, as another contributor to the greater Sierra
Nevada batholith?
Yellowstone
Yellowstone is the latest of at least seven large silicic
caldera-related volcanic systems associated with migration of the North American plate over a deep-seated
hotspot (Morgan et al. 2009). These systems have developed over 16.5 My, stretching for 700 km along the Snake
River Plain and marking the volcanic expression of major
silicic magmatism, although the earlier examples are now
buried by voluminous basalts. The Yellowstone system itself
dates back to 2.1 Ma, has generated three caldera-forming
events and last erupted with voluminous rhyolite lava
around 70 ka (Christiansen 2001). Zircon age (and other
crystal-specific) data on the caldera-forming eruptions and
their aftermaths paint diverse pictures of the Yellowstone
system. Contrasting views are held regarding the timing
and origins of the younger rhyolites: these views centre
on the role of melting and recycling of crustal lithologies versus the extraction of melts from a long-lived mush
system (Watts et al. 2012; Stelten et al. 2015; Wotzlaw et
al. 2015).
At present, geophysical data outline the pluton-scale
volumes of inferred partially molten material below the
youngest caldera (Huang et al. 2015). There is an upper
106
A PR IL 2016
FIGURE 4 Scaled cross-sections (see the respective three blue lines
on FIG. 3) through the three volcanic–plutonic systems
discussed in this article. (LEFT) The Taupo Volcanic Zone depicted at
the time of the 25.4 ka Oruanui eruption. A FTER WILSON ET AL. (2006)
AND A LLAN ET AL . (2012). (C ENTER ) The Long Valley system depicted at
the time of the 0.77 Ma Bishop Tuff eruption. A FTER CHAMBERLAIN ET AL.
(2015). (R IGHT) The present-day Yellowstone system. A FTER H UANG ET AL.
(2015). The relative sizes of the plutonic roots to these three volcanic
systems are evident. Vertical and horizontal scales are the same in
each case. Mafic magmas are shown in red; dacite to rhyolite in bright
blue; and high-silica rhyolite in pink (crystal-rich) through mauve
(crystal-poor).
crustal (<20 km depth) volume of 10,000 km3 inferred to
be rhyolitic with an average 9% melt, and a lower crustal
(25–45 km depth) volume of 46,000 km3 inferred to be
basaltic and with ~2% melt. These volumes of partially
molten material are the drivers of exceptional fluxes of
geothermal heat (about 5.3 GW) and of volatiles from
magma and heated country rock (Hurwitz and Lowenstern
2014). None of the other silicic systems considered here
has been geophysically imaged to this level of detail, or
to such depths.
the Basin-and-Range province, and attributed there to
contrasts in the volatile abundances and temperatures
of the magmatic systems (Christiansen and McCurry
2008).
3. The plutonic bodies associated with the rhyolitic volcanism at Taupo and Yellowstone both appear to have
distinct felsic upper parts (10–12 km thick at Taupo,
~15 km at Yellowstone) and mafic lower parts. At Taupo,
such stratification cannot be distinguished geophysically from mantle lithologies. At Long Valley, the roots
of the pluton merge upwards through the crust over a
thickness of ~30 km, and a distinct ‘mafic versus felsic’
separation is not thought to be present.
DISCUSSION
Yellowstone, when compared to the TVZ and Long Valley,
shows some contrasts, despite the generally comparable
timespans of activity.
This brief overview of three Quaternary silicic volcanic
systems reveals several factors that should be considered
when making any comparison with the plutonic record.
1. In the Snake River Plain, silicic volcanism and plutonism
are followed by voluminous mafic volcanism, burying
the silicic source calderas and modifying the crustal
signature of the silicic magmatism. At Yellowstone,
minor amounts of basalt predate and are interspersed
with rhyolitic products of the older two calderas. Then,
as the volcanic focus has migrated east, the western
parts of the two earlier calderas have begun to be buried
under basalt lavas. The appearance of abundant basalt
volcanism has been taken as the sign (as at Long Valley)
that the upper crustal magmatic system has waned and
solidified and allowed basalt to pass through, rather than
acting as a density trap (Hildreth 2004). This systematic
progression contrasts with the great diversity of behaviour in the TVZ where the crustal magmatic systems wax
and wane multiple times within a timescale equivalent
to the lifespan of one caldera cycle at Yellowstone and
where there is no overall progression to basaltic volcanism at the surface.
1. Although large-scale crustal magmatism and growth
of plutonic bodies need not be accompanied by largescale volcanism (e.g. Bacon et al. 1981), large-scale silicic
volcanism demands the presence of complementary greater
volumes of intrusive material (cf. Streck 2014). There
is a central role for mafic magmatism in providing
heat, volatiles and differentiated melts, but these are
expressed in different guises: as a sharply defi ned mafic
underplate to the TVZ (Wilson and Rowland 2016); as
diffuse ‘distributed intrusions’ at Long Valley (Hildreth
2004); and as a vast, vertically extensive, thermal root
at Yellowstone (Huang et al. 2015) (FIG. 4).
2. The overall bimodal (basalt and rhyolite) nature of the
Yellowstone eruptive products has fuelled much debate
over the relative roles of crustal melting versus fractionation and the apparent absence of intermediate compositions (e.g. Streck 2014). Petrological contrasts between
Yellowstone and areas like the TVZ are mirrored in the
western USA in older Cenozoic silicic magmatism of
E LEMENTS
2. Zircon chronologies for silicic volcanic and plutonic rocks are
open to different interpretations. Age estimates for single
crystals or parts of crystals only date those crystals or
parts of crystals – they do not, in themselves, provide
direct information on the onset of any particular
magmatic system or constrain the magma residence
time. Context is paramount in interpreting age information. Timescales visible through zircon records are
also limited by the associated precisions. Analytical
precisions of ±0.2% at 95% confidence for ID-TIMS
are readily achievable, but, even so, the uncertainties
on ages of grains in Mesozoic Sierra Nevada batholiths
cover the lifespan of entire caldera-forming supereruptive magma systems in the TVZ. The techniques
107
A PR IL 2016
associated with ID-TIMS age dating of zircons (in particular the use of chemical abrasion and the analysis of
whole or half grains) serve to mask the very diversity of
age data that give insights into the rhythm and tempo
of Quaternary volcanic systems (e.g. Chamberlain et
al. 2014). The demonstrable rapidity with which supersized volcanic systems can grow, erupt, then change
into new systems is invisible in the plutonic record,
not only because of the prolonged cooling histories
of plutons but also because the dating methodologies
cannot keep up with the pace. A fundamental implication from studies of Quaternary systems is that largescale volcanic processes can act on timescales that are
too rapid for most plutonic records to see.
3. Fluctuations in volcanic output do not always directly reflect
magmatic input at depth. This is demonstrably the case on
the ‘short’ timescales of thousands to tens-of-thousands
of years in the TVZ, where periods of enhanced crystallization (i.e. magma cooling) recorded in the zircon
REFERENCES
Allan ASR, Wilson CJN, Millet M-A,
Wysoczanski RJ (2012) The invisible
hand: tectonic triggering and modulation of a rhyolitic supereruption.
Geology 40: 563-566
Allan ASR, Morgan DJ, Wilson CJN,
Millet M-A (2013) From mush to
eruption in centuries: assembly of the
super-sized Oruanui magma body.
Contributions to Mineralogy and
Petrology 166: 143-164
Bachmann O, Bergantz GW (2004) On
the origin of crystal-poor rhyolites:
extracted from batholithic crystal
mushes. Journal of Petrology 45:
1565-1582
Bacon CR, Macdonald R, Smith RL,
Baedecker PA (1981) Pleistocene highsilica rhyolites of the Coso volcanic
field, Inyo County, California. Journal
of Geophysical Research: Solid Earth 86:
10223-10241
Barker SJ and 6 coauthors (2014)
Post-supereruption magmatic reconstruction of Taupo volcano (New
Zealand), as reflected in zircon ages and
trace elements. Journal of Petrology 55:
1511-1533
age spectra occur during lulls in surface volcanism.
The processes associated with physical accumulation
of eruptible bodies of magma operate on timescales
that may be wholly disconnected from the processes of
assimilation and fractionation that control the chemical
compositions of silicic volcanic rocks. As a result, the
physical accumulation of super-sized melt-dominant
bodies can occur much more rapidly than gradualistic
models would suggest.
ACKNOWLEDGMENTS
We thank Joe Wooden (Stanford University, USA) and
Trevor Ireland (Australian National University) for their
help and collaboration in our zircon ion-probe studies. We
also thank Aidan Allan, Simon Barker, Katy Chamberlain,
George Cooper and Wes Hildreth for many valuable discussions, and Craig Lundstrom, Axel Schmitt and Justin Simon
for illuminating reviews.
Christiansen EH, McCurry M (2008)
Contrasting origins of Cenozoic
silicic volcanic rocks from the western
Cordillera of the United States. Bulletin
of Volcanology 70: 251-267
Shane P and 5 coauthors (2008) Silicic
recharge of multiple rhyolite magmas
by basaltic intrusion during the 22.6 ka
Okareka eruption episode, New Zealand.
Lithos 103: 527-549
Christiansen RL (2001) The Quaternary
and Pliocene Yellowstone Plateau
volcanic field of Wyoming, Idaho,
and Montana. U.S. Geological Survey
Professional Paper 729-G: 1-143
Simon JI and 5 coauthors (2014)
Assimilation of preexisting Pleistocene
intrusions at Long Valley by periodic
magma recharge accelerates rhyolite
generation: rethinking the melting
model. Contributions to Mineralogy
and Petrology 167, doi: 10.1007/
s00410-013-0955-5
Hanchar JM, Hoskin PWO (eds) (2003)
Zircon. Reviews in Mineralogy
and Geochemistry, Volume 53.
Mineralogical Society of America,
Chantilly, 500 pp
Hildreth W (2004) Volcanological
perspectives on Long Valley, Mammoth
Mountain, and Mono Craters: several
contiguous but discrete systems.
Journal of Volcanology and Geothermal
Research 136: 169-198
Huang H-H and 5 coauthors (2015) The
Yellowstone magmatic system from
the mantle plume to the upper crust.
Science 348: 773-776
Hurwitz S, Lowenstern JB (2014)
Dynamics of the Yellowstone hydrothermal system. Reviews of Geophysics
52: 375-411
Bursik MI, Renshaw C, McCalpin J, Berry
M (2003) A volcanotectonic cascade:
activation of range front faulting and
eruptions by dike intrusion, Mono
Basin-Long Valley caldera, California.
Journal of Geophysical Research: Solid
Earth 108, doi: 10.1029/2002JB002032
Morgan LA, Cathey HE, Pierce KL (eds)
(2009) The track of the Yellowstone
hotspot: multi-disciplinary perspectives on the origin of the YellowstoneSnake River Plain volcanic province.
Journal of Volcanology and Geothermal
Research 188: 1-304
Carter L and 5 coauthors (2003) Demise
of one volcanic zone and birth of
another - a 12 m.y. marine record of
major rhyolitic eruptions from New
Zealand. Geology 31: 493-496
Riley P, Tikoff B, Hildreth W (2012)
Transtensional deformation and structural control of contiguous but independent magmatic systems: Mono-Inyo
Craters, Mammoth Mountain, and Long
Valley caldera, California. Geosphere 8:
740-751
Chamberlain KJ, Wilson CJN, Wooden
JL, Charlier BLA, Ireland TR (2014) New
perspectives on the Bishop Tuff from
zircon textures, ages and trace elements.
Journal of Petrology 55: 395-426
Chamberlain KJ, Wilson CJN, Wallace
PJ, Millet M-A (2015) Micro-analytical
perspectives on the Bishop Tuff and its
magma chamber. Journal of Petrology
56: 605-640
Charlier BLA and 5 coauthors (2005)
Magma generation at a large, hyperactive silicic volcano (Taupo, New
Zealand) revealed by U-Th and U-Pb
systematics in zircons. Journal of
Petrology 46: 3-32
E LEMENTS
Rowland JV, Wilson CJN, Gravley DM
(2010) Spatial and temporal variations in magma-assisted rifting, Taupo
Volcanic Zone, New Zealand. Journal of
Volcanology and Geothermal Research
190: 89-108
Schaltegger U, Schmitt AK, Horstwood
MSA (2015) U–Th–Pb zircon geochronology by ID-TIMS, SIMS, and laser
ablation ICP-MS: recipes, interpretations, and opportunities. Chemical
Geology 402: 89-110
Schmitt AK (2011) Uranium series
accessory crystal dating of magmatic
processes. Annual Review of Earth and
Planetary Sciences 39: 321-349
108
Stelten ME, Cooper KM, Vazquez
JA, Calvert AT, Glessner JJG (2015)
Mechanisms and timescales of generating eruptible rhyolitic magmas at
Yellowstone caldera from zircon and
sanidine geochronology and geochemistry. Journal of Petrology 56: 1607-1642
Storm S, Schmitt AK, Shane P, Lindsay JM
(2014) Zircon trace element chemistry
at sub-micrometer resolution for
Tarawera volcano, New Zealand, and
implications for rhyolite magma evolution. Contributions to Mineralogy
and Petrology 167, doi: 10.1007/
s00410-014-1000-z
Streck MJ (2014) Evaluation of crystal
mush extraction models to explain
crystal-poor rhyolites. Journal of
Volcanology and Geothermal Research
284: 79-94
Watts KE, Bindeman IN, Schmitt AK
(2012) Crystal scale anatomy of a dying
supervolcano: an isotope and geochronology study of individual phenocrysts
from voluminous rhyolites of the
Yellowstone caldera. Contributions to
Mineralogy and Petrology 164: 45-67
Wilson CJN, Rowland JV (2016) The
volcanic, magmatic and tectonic setting
of the Taupo Volcanic Zone, New
Zealand, reviewed from a geothermal
perspective. Geothermics 59: 168-187
Wilson CJN, Blake S, Charlier BLA,
Sutton AN (2006) The 26.5 ka Oruanui
eruption, Taupo volcano, New Zealand:
development, characteristics and evacuation of a large rhyolitic magma body.
Journal of Petrology 47: 35-69
Wotzlaw J-F, Bindeman IN, Stern RA,
D’Abzac F-X, Schaltegger U (2015) Rapid
heterogeneous assembly of multiple
magma reservoirs prior to Yellowstone
supereruptions. Nature Scientific
Reports 5, doi: 10.1038/srep14026
A PR IL 2016