Download Silicic Magmatism and the Volcanic–Plutonic Connection

Silicic Magmatism and the
Volcanic–Plutonic Connection
Cerro Galan ignimbrite, Argentina, a
~1,000 km3 dacite
deposit from a supereruption 2 million
years ago.
Craig C. Lundstrom1 and Allen F. Glazner2
1811-5209/16/0012-0091$2.50
DOI: 10.2113/gselements.12.2.91
T
he relationships between silicic volcanic and plutonic rocks have long
puzzled geologists because the rich set of observations from petrology,
geochronology, thermal modeling, geophysical techniques, and
geochemistry have led to contradictory interpretations. Although compositional evolutionary trends leading to granite and rhyolite are congruent, it is
not clear if rhyolites are formed by the extraction of melt from shallow crystal
mushes that otherwise solidify to form granite plutons, or are derived from
a greater depth in parallel with granite plutons, or are formed by processes
separate from those which form granite plutons. Finding a consistent explanation for the silicic volcanic–plutonic relationship bears on important Earth
science questions, including, “How is silicic continental crust formed?” and,
“Can we predict supereruptions?”
Bowen (1928) popularized the
hypothesis that magmas more
silicic than basalt are produced
by fractional crystallization, but
partial melting of hydrous rocks
also produces silicic magma (e.g.
Sisson et al. 2005). Among the
terrestrial planets, regardless of
how they form, silicic rocks are
abundant only on Earth.
Silicic magmatism contributes to
society in more ways than simply
providing us with dry land. To
take a few examples: Many of the
metals critical to modern civilization (e.g. Cu, Mo, Au, W, Sn, Pb,
KEYWORDS : granite, rhyolite, continental crust, igneous petrology, magma
Zn, In, Li, Nb, Ta, Be) occur in
deposits formed by silicic magma“The block of granite which was an obstacle in the pathway
tism; much commercially viable
of the weak becomes a stepping-stone in the pathway of
geothermal power production is sourced in shallow silicic
systems; topographic highs on thick silicic crust (e.g. the
the strong” – THOMAS CARLYLE, THEOSOPHICAL FORUM
Cordillera of North and South America) trap moisture,
which provides water to hundreds of millions of people;
INTRODUCTION
elements of biological relevance (e.g. K, P) are enriched in
silicic continental crust over basaltic crust. In addition,
Granites, Rhyolites, and Earth Evolution
some of the world’s most spectacular scenery is carved in
The Earth is unique among the terrestrial planets in having
granitic rocks, whose coarse grain size and great strength
an abundance of both liquid water and silicic igneous
make them particularly resistant to erosion (FIG. 2).
rocks at its surface. These seemingly unrelated features
are linked by the phase equilibria that govern melting At the same time, silicic magmatism has the potential to
end civilization (Self and Blake 2008). Past silicic super(Campbell and Taylor 1983). Earth’s continents result from
(ignimthe accretion of low-density, relatively silica-rich igneous eruptions have produced pyroclastic flow deposits
3
rocks that have formed by melting and differentiation in brites) with volumes in excess of 1,000 km (Miller and
Wark 2008), emitting enough magma in a matter of days
the presence of water, giving the Earth its characteristic
to bury an area the size of England more than 10 meters
bimodal distribution of topography (FIG. 1). Continental
deep. The volume of a supereruption can be 5,000 times
crust sits several kilometers above basaltic oceanic crust
that erupted from the 2010 event at Eyjafjallajökull volcano
with the topographic offset fi lled with water. One can
(Iceland), which, though relatively small, shut down air
plausibly argue that the development of complex landbased life ultimately depends on the formation of conti- traffic across Europe and cost the airline industry ~US$1.7
billion. Deposits from such eruptions cover huge areas of
nents—and the origin of the continents is largely a story
of magma evolution from basalt to silicic rocks, such as the world, including extensive parts of North and South
America, Africa, and Australia.
granite and rhyolite.
Rocks formed from basaltic magma are abundant in the Many aspects of silicic magmatism remain poorly understood, including the question raised in this thematic issue
terrestrial planets, and the recipe for making them is well
of Elements: What is the relationship between erupted silicic
known: just partially melt the planet’s mantle (Bowen 1928).
The recipe for making silicic magma is less well- established. volcanic rocks, such as ignimbrites, and their intruded,
slowly cooled counterparts, such as granites? This question
builds from questions about silicic magmatism that have
1 Department of Geology
persisted for over a century. For example, the so-called
University of Illinois Urbana Champaign
“room problem” arises when 1,000s of km3 of granite are
Champaign, IL 61820, USA
intruded into preexisting rocks; this problem was viewed
E-mail: [email protected]
by many during the mid-part of the 20 th century as fatal
2 Department of Geological Sciences
to an intrusive origin for granitoids. As a result, an in
University of North Carolina
situ metamorphic (“transformist”) origin was offered
Chapel Hill, NC 27599, USA
E-mail: [email protected]
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0.08
Relative
Area, % 0.04
0
-8
Oceans
-4
AA LVC
BB
CC
DD
Continents
0
Elevation, km
4
0.16
Oceans
n=16,294
0.12
Relative
0.08
Probability
Continental
North America
0.04
n=26,313
0
40
Representative silicic rocks from western North
America. (A) Aerial view to the south of the Mono
Craters, a series of rhyolite and dacite domes erupted mostly from
about 40,000 to 600 years ago. These domes were erupted onto a
surface largely composed of Bishop Tuff, a >600 km3 rhyolite ignimbrite erupted 760,000 years ago from the Long Valley caldera (LVC;
dashed line marks the caldera’s topographic wall). The Mono chain
continues to the south as the Inyo Craters, which terminate north
of Mammoth Mountain (marked by star); the caldera, Mammoth
Mountain, and Mono–Inyo chain all represent spatially separate foci
of silicic volcanism (Hildreth 2004). (B) Cliffs of leucogranite,
2.5 km tall, are exposed in the east face of Great Trango (6,452 m)
and Nameless (6,650 m) Spires, Karakoram Range (Pakistan).
PHOTO COURTESY OF M IKE SEARLE. (C) The 1 km tall southeast face of
El Capitan in Yosemite Valley, California (USA) exposes a complex
interaction zone between several plutonic units including multiple
granites (light tan and light gray) and two sets of lower-silica dikes
(dark gray), all emplaced between about 107 Ma and 103 Ma
(Putnam et al. 2015). It is commonly assumed that complex
volcanic systems like those in the upper panel have complex
plutonic systems like those in the lower panel forming below them
(and vice versa), but this is a difficult assertion to test.
(D) Sediments deposited in the continental interior of the western
United States contain abundant volcanic ash that was presumably
derived from arc volcanoes that erupted above plutonic rocks now
represented by Cordilleran batholiths such as the Sierra Nevada.
Here, in central Utah, the Upper Cretaceous Mancos Shale contains
numerous volcanic ash layers (light bands). PHOTO COURTESY OF L AUREN
B IRGENHEIER.
FIGURE 2
50
60
wt % SiO2
70
80
(TOP) Distribution of elevation for oceans and continents on the Earth. Most oceanic topography lies at
3–6 km depth, whereas half of the dry land on the Earth is below
370 m elevation. ( BOTTOM ) Distribution of silica (SiO2) in igneous
rocks from the ocean basins and western North America. The
occurrence of abundant igneous rocks with SiO2 >55 wt% on the
continents, and particularly the almost complete restriction of silicic
igneous rocks (SiO2 >66 wt%) there, contributes to the relatively
high elevations of the continents. ELEVATION DATA FROM E AKINS AND
SHARMAN (2012); COMPOSITIONAL DATA FROM PETDB (OCEANS) AND NAVDAT
(NORTH A MERICA), BOTH ACCESSIBLE THROUGH WWW.EARTHCHEM.ORG.
FIGURE 1
instead (Read 1948). However, the strong correspondence
between observed granites and melt compositions dictated
by quartz–feldspar equilibria ended the magmatic versus
metamorphic debate (Tuttle and Bowen 1958), but still left
the room problem unsolved. Similarly, the identification
of ignimbrites as coming from giant eruptions of silicic
magma also occurred relatively recently (Smith 1960). In
many ways, the enigmatic relationship of silicic volcanic
to plutonic rocks presented here is simply an extension of
ongoing questions about the origin and mechanism for
forming Earth’s silicic magmas.
with volumes of several thousand km3. Batholiths around
the world contain much greater volumes of plutonic rocks
of comparable composition (granite and granodiorite), and
individual mappable bodies of such rock (plutons) can have
volumes comparable to, or greater than, those of the largest
supereruptions (Frazer et al. 2014).
A Brief Introduction to Silicic Rocks
Field identification of a rock as volcanic or plutonic is
based mainly on texture, i.e. on the size and arrangement
of crystals in the rock and the proportion of crystals to
glass. On Earth, low-silica basalts dominate the volcanic
record, whereas plutonic rocks, or at least those exposed
at the surface by tectonics and erosion, are significantly
more silica-rich (FIG. 3; Daly 1914). Plutonic and volcanic
rocks span virtually identical ranges in composition and
mineralogy; the compositions of the vast majority fall on
coincident scattered linear trends in major-element composition space and range from ~45 wt% SiO2 to ~75 wt% SiO2.
The historical view of plutons is that they reflect now-frozen
magma chambers that were as large as the pluton itself, a
view that has been promoted in countless publications over
the past century. This view led to a comfortable duality
between volcanic and plutonic rocks, with plutons serving
as the staging areas from which volcanic rocks are erupted.
Nearly 200 years ago, Lyell (1838) drew red blobs labeled
“Plutonic (Granite, & cc.)” feeding an erupting volcano
(FIG. 4), and this image persists today.
Silicic volcanic rocks include rhyolite and dacite and may
be emplaced on the surface as lava, pyroclastic fall deposits
(“volcanic ash”), and pyroclastic flow deposits (ignimbrites
or welded tuffs), the latter resulting from catastrophic
ground-hugging “ash hurricanes” that comprise most
of the output of silicic supereruptions. Silicic eruptions
range from quiet extrusion (e.g. the 2004–2008 eruptions
of Mount St. Helens, Washington, USA) to supereruptions
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Studies of silicic magmatism have benefitted greatly from
advances in geochemical analysis. The ability to analyze
large swaths of the periodic table down to the µg/g (or
better) level reveals secrets that have led to a revolution
in understanding of the genesis of rhyolite. The development of in situ methods (e.g. laser-ablation inductively
coupled plasma mass spectrometry; ion probe) has provided
precise geochemical and isotopic analyses of spots with
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dimensions on the order of 10 µm, producing spatially
resolved U–Pb ages for zircon and titanite and putting the
spotlight on silicic magmatism. Improvements in ultraclean lab procedures, chemical and thermal pretreatment,
and precision of thermal ionization mass spectrometry now
allow U–Pb dating of zircon with a precision of 0.1%.
ENIGMAS
Overview
Here is a short list of current enigmas.
ƒ Supereruptions show that huge magma bodies must exist,
at least transiently, but geophysical surveys have yet to
locate any present-day huge, eruptible magma bodies.
Not surprisingly, this cornucopia of new data has generated
far more questions than answers, with many questions
surrounding the relationship between silicic volcanic and
plutonic rocks. Observations from active volcanic systems
lead to views that differ from those based on plutons or
geophysical observations. Thermal modeling indicates
that building a large body of magma requires accumulating magma at rates that are inconsistent with geophysical and geodetic data. These and other issues constitute
inconsistencies that arise from studying different parts of
a magmatic system with different techniques.
ƒ Phase equilibria suggest that rhyolite magmas last equilibrate with a quartz–feldspar residue at shallow levels in
the crust, but the plutons that should be the residue after
rhyolite extraction do not have the expected complementary geochemical signature.
ƒ Precise geochronology of silicic plutons requires that
they grew in increments, but the boundaries between
such increments can rarely be seen in the field.
ƒ Thermal models indicate that magma must accumulate
extremely rapidly to form a large eruptible magma body,
but such accumulation rates are inconsistent with measured rates of geodetic movement.
In his well-known poem “The Blind Men and the Elephant,”
John Godfrey Saxe wrote:
And so these men of Indostan
Disputed loud and long,
Each in his own opinion
Exceeding stiff and strong,
Though each was partly in the right,
And all were in the wrong!
ƒ Many plutons show systematic geochemical variations
consistent with outside-in crystallization of a large
magma body, but this cannot happen if the plutons are
emplaced incrementally.
ƒ Estimates of the proportion of partial melt in various
magmatic areas derived from seismic and magnetotelluric (electrical conductivity) methods commonly show
significant disagreement.
In this issue of Elements, five articles cover some of the
significant enigmas that confound our understanding of
the Earth’s silicic rocks. Our hope is that bringing together
disparate methods and viewpoints will move us all closer
to a whole-elephant view of how silicic magmatic systems
work.
Geophysical Imaging
and the Elusive Magma Chamber
Magma chambers like those in FIGURE 4 must exist at some
level in the crust before large eruptions. However, they
may be ephemeral and, thus, difficult to capture in the
present-time snapshots that geophysical methods provide.
Seismology has undergone rapid advancements, including
increases in computing power and the development of large
organized efforts such as ORFEUS and IRIS (using the US
National Science Foundation’s EarthScope USArray). New
geodetic techniques, such as continuous global positioning
system (GPS) and interferometric synthetic aperture radar,
provide real-time data on deforming magmatic systems,
thereby providing a method to examine dynamic processes.
Pritchard and Gregg (2016 this issue) discuss the wide range
of geophysical methods available for observing magmatic
systems.
0.03
Volcanic
n = 29,019
0.02
0.01
0
0.03
0.02
Plutonic
n = 11,156
Although seismic images of magma chambers are commonly
reported, none to date have revealed a true magma body
with a high percentage of melt. In order to be mobile and
capable of intrusion and extrusion, a magma must be
at least 50% molten. Many seismically imaged “magma
chambers” are perhaps only a few percent molten and are
better called “blobby zone[s] where the velocity is low and
attenuation is high” (Lees 2007). Mobile magma bodies
may exist in unstudied areas, of course, but it is curious
that none of the well-studied areas on Earth, including
Yellowstone (Huang et al. 2015), seem to be underlain by
large, high-melt fraction bodies of magma.
0.01
0
40
45
50
55
60
wt% SiO2
65
70
75
80
Relative probability histograms of silica for volcanic
and plutonic rocks from western North America, using
all samples for which SiO2 was reported. These arrays support field
observations by showing strong biases of volcanic rocks toward
low-silica compositions and plutonic rocks toward high-silica
compositions. Volcanic peak at ~54 wt% SiO2 represents the
Columbia River basalts (western USA), which are both widespread
and oversampled relative to many other areas in the database. In
spite of this compositional bias in exposure, the ranges of and
correlations among compositional variables are virtually identical
between volcanic and plutonic rocks (Glazner et al. 2015).
DATA FROM NAVDAT (AT WWW.EARTHCHEM.ORG).
FIGURE 3
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Another important part of the Earthscope program has
been the implementation of a moving array for magnetotelluric imaging. This tomographic method provides a
complementary technique to seismology and is particularly
sensitive to the presence of conductive liquid, both melt
and hydrothermal fluids. However, seismic and magneto-
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for every 10 km 3 of granite. Such
residues have not been geophysically
observed under batholiths, though
it is possible that they have been
removed by delamination.
Understanding how differentiation
from mafic to silicic magmas occurs is
critical to understanding the volcanic–
plutonic connection. Silicic intrusive
suites are commonly petrologically
zoned, with compositions grading
from lower-silica rocks on their rims
to higher-silica rocks in their interiors.
For many years, this was interpreted
as inward solidification of a plutonsized magma chamber (Bateman and
Chappell 1979); comparable zoning
of some ignimbrite deposits has been
interpreted to reflect an inverted
record of zoning in the volcanic
magma chamber prior to eruption,
with the top of the chamber being
cooler and more evolved (McBirney
1968). Where these chemical zonings develop must be
intimately interwoven with questions of the differentiation
mechanisms, the heat flow, and the timescales of cooling.
The classic view of the plutonic–volcanic connection
goes back to Charles Lyell’s 1838 classic Elements of
Geology and continues to be popular. In this model, a magma
chamber is connected to the surface via a planar or pipe-like
conduit, and therefore active volcanoes should overlie significant
volumes of molten material where the proportion of liquid is >50%.
The inability of seismic surveys to reveal such magma bodies calls
this simple view into question.
FIGURE 4
Because mineral–melt separation may play an important
role in forming silicic magmas, the physical properties of
silicic melts are critically important. As magmas become
increasingly silicic, their viscosities increase dramatically
because silicate tetrahedra link up and polymerize the
melt. Viscosity increases from the relatively low values of
a basaltic melt (<103 Pa s) to high values when a melt has
a SiO2 content exceeding 60–65 wt% (>106 Pa s). Rhyolitic
melt can be 4 to 10 (or more) orders of magnitude more
viscous than basaltic melt, depending on the water content
of the magma (FIG. 5). Because rates of mechanical separation processes (e.g. crystal settling or buoyant melt percolation) scale with viscosity, these processes must operate
many orders of magnitude more slowly in rhyolitic melts
than in basaltic magmas.
telluric estimates of the percent partial melt in a given
area commonly disagree significantly (Pritchard and Gregg
2016 this issue).
Finally, advancements in geodetic monitoring and data
processing have provided unprecedented images of surface
deformation related to magmatic activity. The development of remote sensing methods and improvements in GPS
have led to images of surface expansion and contraction of
volcanoes reflecting either magma injection, hydrothermal
fluctuation, or both. These signals commonly indicate
activity in the system related to pulses of heat that must
ultimately reflect addition of magma. How these deformation signals translate into amounts of magma added per
time (flux) will always be difficult to ascertain given the
short time period over which deformation is measured.
Magmatic water plays a critical role in silicic systems, but
details about how water accumulates, moves, and affects
the various processes remain unclear. For instance, whereas
eruptions unambiguously involve the generation of large
amounts of gases (dominated by water), whether vapor
formation occurs via decompression due to ascent or via
crystallization-induced water saturation is unknown.
Indeed, preeruptive water contents of melts are poorly
known, and eruption triggering and decompression instead
may reflect external control, such as roof collapse. Volatile
component behavior remains a significant wild card in
silicic rock petrogenesis.
Experimental Petrology and Phase Equilibria
Igneous rock suites show compositional trends that follow
expectations of mineral–melt equilibria as determined
by laboratory experiments. Tuttle and Bowen (1958)
showed that Earth’s silicic rocks (both granitoids and
silicic volcanic rocks) have bulk compositions that closely
resemble hydrous melts in equilibrium with quartz and
feldspar. Subsequent work has examined the effects of other
major components on compositional evolution. Scaillet
et al. (2016 this volume) summarize these fi ndings and
delineate the change in phase assemblages that occur as
a result of particular magma compositions (in this case,
peraluminous, metaluminous, and peralkaline).
If rhyolites represent a silicic melt extracted from a granite’s
crystal residue (crystal fractionation), then granite and
rhyolite compositions should be complementary—i.e.
elements that are compatible in the granite crystal assemblage will be enriched in the granite and depleted in the
rhyolite. However, global, regional, and local comparisons
of trace element patterns fail to support this. Elements
such as Sr and Ba, which are sensitive to the removal
of feldspar phases, indicate no significant difference in
behavior between the evolution of the two rock types
(Glazner et al. 2015).
Regardless of whether silicic melts are produced by extended
fractional crystallization or wet partial melting of basalt,
the volume ratio of crystal residue to silicic melt has to
be high, generally around 10:1. Thus, the observation of
large amounts of silicic plutonic rocks (FIG. 3), particularly in convergent margin settings, provides particular
challenges because vast volumes of crystal residue should
be left behind: for example, ~100 km3 of gabbroic residue
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10
small magma bodies presents yet another contradiction:
such bodies will cool even faster than a single large body,
thus making constraints based on cooling time even more
stringent.
10
nh
yd
ro
u
Viscosity, Pa s
s
108
106
A
104
102
s
Ba
1
50
alt
wt%
+5
R
h
l
yo
Other studies from volcanic rocks document widely
varying timescales based on fi ne-scale crystal observations. In situ U–Pb and U-series dates within a zircon can
vary by hundreds of thousands of years. Thus, the small
volume secondary ion mass spectrometry (SIMS) analyses
contrast sharply with the single grain isotope dilution
thermal ionization mass spectrometry (ID-TIMS) method
in terms of the obtained temporal information; this difference is attributed to the time interval that each method
samples. These observations point to longer timescales and
have led to suggestions that volcanic systems may spend
significant time as a mostly crystal mush that becomes
rejuvenated by mafic magma underplating at the time of
eruption. Finally, at the same time, diffusion profi les and
modeling can indicate day to week timescales of magma
residence: for instance, trace element profi les suggest that
the Bishop Tuff underwent a significant heating event just
days before its eruption (Wark et al. 2007).
ite
O
H2
60
wt% SiO2
70
80
Another intriguing age-related observation is that large
caldera systems erupt in a periodic fashion. For instance,
Yellowstone has had three major eruptions occurring
at ~650 ky intervals. Compositional data through time
should show the effects of the erupted magma repeatedly
sampling an evolving (and depleting) source if melt were
extracted from residual crystals during each cycle. But no
depletion signal is seen. If these series of rhyolites reflect
melt extraction from a crystal mush, then stirring and
homogenization with new magma must occur to hide this
depletion process.
Viscosity calculated along a basalt–rhyolite mixing
line. Viscosity of anhydrous rhyolite is 9 orders of
magnitude greater than anhydrous basalt; adding 5 wt% water to
the system decreases this ratio to about 4.5 orders of magnitude.
BASALT AND RHYOLITE COMPOSITIONS FROM USSLER AND G LAZNER (1989);
VISCOSITY MODEL OF G IORDANO ET AL . (2008).
FIGURE 5
Geochronologic Timescales
Recent U–Pb geochronology observations have led to a
paradigm shift about how silicic intrusions form. Whereas
compositional zoning of the Tuolumne Intrusive Suite in
Yosemite (California, USA) led Bateman and Chappell
(1979) to suggest inward crystallization of a pluton-sized
magma chamber, precise dating has shown that this set of
intrusions crystallized over 8–10 My, greatly exceeding the
thermal cooling time (~105 y) of such a magma chamber
(Coleman et al. 2004). This led to the idea that plutons
may form by small increments over timescales on the order
of 105 –10 6 years. Coleman et al. (2016 this issue) show
that through the use of multiple geochronology systems,
detailed time–temperature histories of plutons can be
determined.
GLOSSARY
Dacite: Relatively high-silica volcanic rock containing
~65–70 wt% SiO2 .
Granite, granitoids: Plutonic rocks composed predominantly
of quartz, alkali feldspar, and plagioclase feldspar,
typically in subequal amounts.
Ignimbrite: A deposit formed from a pyroclastic flow.
Magma: Although the defi nition of this fundamental
geologic substance is disturbingly vague, the American
Geosciences Institute Glossary of Geology defi nition
is appropriate here: “naturally occurring molten or
partially molten rock material, generated within the
Earth and capable of intrusion and extrusion.”
Comparing the geochronology of volcanic and plutonic
systems shows important similarities and differences.
On the one hand, the million-year timescales of pluton
emplacement contrast sharply with the much shorter
timescale for volcanic processes occurring in similar-sized
magma bodies. For instance, ages for single zircons from
the large supereruption that produced the Kilgore Tuff
(1,800 km3 from Yellowstone-track caldera; Wotzlaw et al.
2014) show a tight range: ~30 zircons vary by only ~0.1%.
Thus, whereas ~10% age variation can occur in intrusions,
the age range for the eruptible portion of a similar sized
volcanic system appears to be much smaller.
Plutonic: Referring to magma that crystallized at depth
and is, therefore, relatively coarse-grained. Granite is
a plutonic rock.
Pyroclastic: Said of material ejected into the air during
eruption, such as volcanic ash.
Pyroclastic fall: Fine material from an explosive eruption
that falls like snow. Pyroclastic fall material can circle
the globe if fi ne enough and thrown high enough.
Pyroclastic flow: A hot, fluidized, ground-hugging mass
of fragmental material from an explosive eruption
that moves across the ground surface at high speed.
Pyroclastic flows are dominated by fi ne (<2 mm)
particles of volcanic ash and are sometimes called “ash
flows.”
The tight age span of volcanic magma bodies becomes
important when put in the context of large eruptions
formed by amalgamation of many small discrete bodies
(Wilson and Charlier 2016 this issue). In essence, large
geochemical differences can be observed within distinct
parts of supereruptions that would be eliminated if convection and mixing in a large blob of magma occurred. For
instance, the same Kilgore zircons show oxygen isotope
ratios that vary by over 4 per mil (Wotzlaw et al. 2014) in
sharp contrast to uniformity expected for crystallization
from homogeneous melt. The idea of the amalgamation of
E LEMENTS
Rhyolite: High-silica volcanic rock containing >~70 wt%
SiO2 .
Silica: The chemical component SiO2 .
Silicic: Denoting a high concentration of silica (typically
>66 wt%). Silicic melts are far more viscous than
less-silicic ones.
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(Lundstrom et al. 1998). The origin of order-of-magnitude
magma flux variations, if they exist, presents yet another
enigma.
Thermal Timescales
The upper crust is a thermal boundary layer set by the
Earth’s surface temperature. Thus, magmas residing within
a few kilometers of the surface are rapidly cooled once
emplaced. This constraint is at the root of many of the
enigmas listed above, because, although observations may
suggest an upper-crustal differentiation process, cooling
models place strong temporal constraints on how long
these processes have to operate. For instance, because of
rapid upper-crustal cooling, thermal models require that
magma must be added at vertical accumulation rates >10
mm/y in order to make an eruptible magma body (Schöpa
and Annen 2013). Such high rates are difficult to reconcile with geodetic, geophysical, and petrological data from
volcanic areas. Blundy and Annen (2016 this issue) discuss
the constraints imposed by heat flow and cooling on both
pluton emplacement and volcanic magma chamber growth.
CONCLUDING REMARKS
The volcanic–plutonic connection of silicic rocks remains
mysterious, despite decades of work in a variety of fields.
Current observations from petrology, geochronology,
thermal modeling, geophysical techniques, and geochemistry lead to contradictory interpretations. The importance
and potential impacts of understanding this connection
range from discerning how Earth’s continental crust
formed to predicting volcanic eruptions. As with Saxe’s
blind men trying to “decipher” an elephant, deciphering
the message from silicic rocks will require integrating all
possible observations to fi nally reveal our silicic elephant’s
true form.
The magma accumulation rate required by thermal
modeling to develop a large magma body in the upper
crust is 1–2 orders of magnitude greater than the rate of
vertical magma accumulation recorded by granite plutons
determined by geochronology (Coleman et al. 2016 this
issue). These melt fluxes must ultimately reflect mantle
melt production rates, which depend on decompression,
on addition of volatiles (H 2O), or on addition of heat, and
are closely tied to plate convergence or divergence rates
REFERENCES
Bateman PC, Chappell BW (1979)
Crystallization, fractionation, and
solidification of the Tuolumne intrusive series, Yosemite National Park,
California. Geological Society of
America Bulletin 90: 465-482
Blundy JD, Annen CJ (2016) Crustal
magmatic systems from the perspective
of heat transfer. Elements 12: 115-120
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ACKNOWLEDGMENTS
We thank Adam Kent and Gerhard Wörner for thoughtful
reviews that significantly improved the paper, and Bernie
Wood for editorial handling. This work was supported by
National Science Foundation grants EAR-0312691, 0336070,
0538129, and 062210 to AFG and grants EAR-1019632,
0944169 and OCE-1060754 to CCL.
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