formation of k-feldspar megacrysts in granodioritic plutons by
TRANSCRIPT
ORIGINAL PAPER
Formation of K-feldspar megacrysts in granodioritic plutonsby thermal cycling and late-stage textural coarsening
Breck R. Johnson • Allen F. Glazner
Received: 19 January 2009 / Accepted: 30 August 2009
� Springer-Verlag 2009
Abstract K-feldspar megacrysts in granite and granodi-
orite plutons are generally inferred to be early crystallizing
phases (grown to large sizes when the magma was mostly
liquid) owing to their large size, euhedral form, and fea-
tures that suggest deposition by magmatic sedimentation.
However, phase equilibrium experiments and natural
examples of crystallization and partial melting demonstrate
that K-feldspar is one of the last phases to nucleate and that
most crystal growth must occur after the magma has
exceeded 50% crystallization and is thus largely incapable
of flow and sedimentation. Megacryst size distributions,
compositions, and textural relationships from the Creta-
ceous Tuolumne Intrusive Suite, California, reveal that the
gradational transition from equigranular to megacrystic
granodiorite likely occurred via textural coarsening caused
by thermal cycling. Experimental and theoretical studies
demonstrate that rising temperature induces relatively more
melting in small crystals than in large ones, whereas linear
growth rates during cooling are similar. Thus, during
thermal cycling material is transferred from small crystals
to larger ones. Megacryst growth via thermal cycling
during incremental emplacement is consistent with the
required late growth of K-feldspar, explains the presence of
megacrysts in the inner parts of theTuolumne Intrusive
Suite and elsewhere, and may be a common process in
formation of megacrystic granitic rocks.
Keywords K-feldspar megacrysts � Textural coarsening �Tuolumne Intrusive Suite � Granodiorite � Thermal cycling
Introduction
Interpretation of textures in granitic plutons relies on dis-
tinguishing between primary magmatic crystallization
textures and later textural overprinting and readjustment. A
common assumption in igneous petrology is that large
euhedral megacrysts in granitic plutons represent mineral
phases that grew surrounded by melt, whereas anhedral and
subhedral crystals form later when existing crystals
obstruct their growth. However, this simple interpretation
is not always valid. For example, Means and Park (1994)
used multiphase crystallization experiments in a simple
ammonium solution to determine the relationship between
crystallization processes and the resulting textures. Their
work showed that early euhedral crystals can be over-
printed or obscured by younger ones during cooling, pro-
ducing textural relations that are at odds with the sequence
that would be inferred from classical petrography. Such
observations emphasize the need for caution when apply-
ing classic intuitive interpretations of igneous textures as
records of magmatic crystallizing sequences. Textural
differences in granodiorites, such as the presence or
absence of K-feldspar phenocrysts or megacrysts, are first-
order observations and a standard means typically used to
distinguish between distinct geologic events such as
intrusive pulses. However, if late-stage textural coarsening
Communicated by J. Blundy.
B. R. Johnson (&) � A. F. Glazner
Department of Geological Sciences, University of North
Carolina, Chapel Hill, NC 27599-3315, USA
e-mail: [email protected]; [email protected]
A. F. Glazner
e-mail: [email protected]
Present Address:B. R. Johnson
Anadarko Petroleum Corporation, 1201 Lake Robbins Dr,
The Woodlands, TX 77380, USA
123
Contrib Mineral Petrol
DOI 10.1007/s00410-009-0444-z
can produce K-feldspar megacrysts and overprint early-
formed primary textures, interpretations based on their
presence must account for such possibilities.
The origin of K-feldspar megacrysts (here defined as
crystals[5 cm in longest dimension; see below) in granites
and granodiorites has fascinated and perplexed petrologists
for over a century. Traditionally, megacrysts are inter-
preted as early-formed crystals that grew to significant size
while the host magma had low crystallinity and was
capable of flow. More recent hypotheses for megacryst
formation include late-stage textural coarsening, meta-
somatism, and changes in pressure, temperature, or mag-
matic liquid composition. The timing of formation, early or
late, is the most fundamental distinction between different
hypotheses for megacryst formation (Vernon 1986).
Whether K-feldspar megacrysts form early or late is
critical to geologic interpretations of their host plutons.
Rigid crystal frameworks form in crystallizing magmas at
*50% crystallinity (Vigneresse and Tikoff 1999); after the
magma reaches this threshold, self-segregation of crystals
(as by settling) is inhibited. If megacrysts are early-formed
crystals that grew to their present large sizes before magma
lockup, they can be used to track dynamic features such as
magma flow, crystal avalanching, and crystal sorting and
accumulation (e.g., Weinberg et al. 2001; Paterson et al.
2005; Vernon and Paterson 2008b). Conversely, if mega-
crysts form late, owing to late-stage textural coarsening,
they will overprint or obscure earlier textural relationships
and may signify a protracted cooling and crystallization
history.
Here we propose that K-feldspar megacrysts in grano-
diorite plutons form by late-stage textural coarsening dur-
ing thermal cycling. This result is based on mineralogical
and textural data from theTuolumne Intrusive Suite of
Yosemite National Park in California in conjunction with
results from experimental and natural petrologic studies of
granodioritic systems. During the early stages of crystal-
lization, textural coarsening is primarily a thermodynami-
cally driven process by which differences in surface
energies between smaller and larger crystals promote the
breakdown of smaller crystals to feed the more energeti-
cally favorable larger crystals. This process likely occurs in
all plutonic rocks; indeed, only by textural coarsening can
coarse-grained plutonic textures form from finer-grained
equivalents.
Textural coarsening and chemical reorganization can
account for many textural features of layered mafic intru-
sions that have otherwise been ascribed to crystal sedi-
mentation (Boudreau 1995; McBirney and Hunter 1995;
Boudreau and McBirney 1997). Similarly, interpretations
of K-feldspar megacrysts as early formed crystals (\50
vol% crystalline) that accumulate, diapirically rise, and
segregate through convection or filter pressing, as
suggested by Paterson et al. (2005), are invalidated if the
method of formation is late-stage textural coarsening. The
timing, amount, and type of coarsening depend on pressure,
temperature, and chemical conditions during and after
pluton construction.
Traditional models of plutons as voluminous blobs of
rising magma that undergo convection, crystal settling,
stoping, large-scale crystal fractionation and other pro-
cesses are under increasing scrutiny owing to recent geo-
chronologic data and interpretation of field data (Petford
et al. 2000; Mahan et al. 2003; Coleman et al. 2004;
Glazner et al. 2004; Cruden 2006; Glazner and Bartley
2006; Matzel et al. 2006; Bartley et al. 2008). Most
existing interpretations of textural relationships within
plutons are based on the large magma chamber model. If
the traditional models of magma chamber size and
emplacement processes are being reevaluated, then the
textural relationships that rely on them may need revision
as well.
Geologic setting and previous work
K-feldspar megacrysts characterize many granitic and
granodioritic plutons (e.g., Eggleton 1979; Vernon 1986;
Bateman 1992; Williamson 1999; McMurry 2001; Baxter
and Feely 2002; Dahlquist et al. 2006; Słaby et al. 2007).
Here we define ‘‘megacrysts’’ as K-feldspar crystals
[5 cm in longest dimension, realizing that the term is
sometimes used to refer to any crystals that are signifi-
cantly larger than others in a rock. Although this size
cutoff is arbitrary, it does correspond to the largest K-
feldspar crystals found in volcanic rocks. We are aware of
only one documented example of lava with K-feldspar
crystals larger than 5 cm, a dacite from Volcan Taapaca
in Chile (Clavero et al. 2004). In this paper we do not
distinguish between the different structural states of K-
feldspars.
Tuolumne Intrusive Suite
The Tuolumne Intrusive Suite (TIS; Bateman 1992) pro-
vides an excellent laboratory for the study of K-feldspar
megacrysts owing to exceptional glacially polished expo-
sures and a wealth of prior geochronologic, geochemical,
and mineralogical studies (e.g., Brodersen 1962; Kerrick
1969; Bateman and Chappell 1979; Kistler et al. 1986;
Bateman et al. 1988; Higgins 1999; Gray 2003; Coleman
et al. 2004; Gray et al. 2008). The TIS is one of several
large, zoned, Cretaceous intrusive suites in the Sierra
Nevada batholith (Bateman 1992). At the current level of
exposure, the TIS consists of five nested, texturally dis-
tinguishable map units that become younger and more
Contrib Mineral Petrol
123
felsic inward (Fig. 1). Thin outer units are the Kuna Crest
Granodiorite in the east and the tonalite of Glen Aulin and
equivalent rocks in the west, all of which are relatively
mafic, equigranular granodiorites, and tonalites. In suc-
cessive order inward are the equigranular facies of the Half
Dome Granodiorite, the porphyritic facies of the Half
Dome Granodiorite, the Cathedral Peak Granodiorite, and
the Johnson Granite Porphyry. Contacts between the dif-
ferent map units are generally gradational over 10–100 m,
although sharp contacts locally occur (Bateman 1992;
Coleman et al. 2006).
The TIS displays a variation in U-Pb zircon ages of
*8 Ma from the 93.5 Ma granodiorite of Kuna Crest in
the east to the 85.4 Ma Johnson Granite Porphyry in the
center of the suite (Coleman et al. 2004; Fig. 1). The Half
Dome Granodiorite (including the porphyritic facies) dis-
plays a U-Pb age range of nearly 4 m.y. from outer to inner
contact. Calculated times for a single magma chamber of
this size to cool below zircon closure temperature are
significantly less than 1 m.y. (Glazner et al. 2004), a
timeframe inconsistent with observed age data; these data
require incremental emplacement of magmas derived
sequentially from a source lower in the crust (Coleman
et al. 2004; Glazner et al. 2004; Gray et al. 2008). In the
principal area studied here, between May Lake and Tenaya
Lake (Fig. 1), the transition from the outer tonalite of Glen
Aulin to the inner Cathedral Peak Granodiorite, units that
differ in age by [5 m.y., does not display any sharp con-
tacts. The scale and geometry of individual injections
remain unresolved.
CA
USA
Johnson Granite Porphyry
porphyritic Half Dome Granodiorite
equigranular Half Dome Granodiorite
Cathedral Peak Granodiorite
tonalite of Glen Aulin (west)granodiorite of Kuna Crest (east)
TUOLUMNE INTRUSIVE SUITE38o 11'
119o 15'119o 30'37o 38'
37o 45'
37o 56'
38o 0'
38o 11'119o 10'119o 34'
37o 38'119o 45'
N
(92.8 ± 0.1- 88.8 ± 0.8 Ma)
}(93.5 ± 0.7-93.1 ± 0.1 Ma)
U-Pb zircon dates are from Coleman et al.,2004
Greater Sierra Nevada Batholith(Mesozoic Plutons)
Sentinel Granodiorite (95.1 ± 1Ma)Yosemite Creek Granodiorite
OLDER IGNEOUS UNITS
(88.8 ± 0.8- 88.1 ± 0.2 Ma)
(85.4 ± 0.1 Ma)
0 5 10km
Olmsted Point
Tenaya Lake
Tuolumne MeadowsRanger Station
Tuolumne River
Lyell Fork
May Lake
Fig. 1 (Top) Geologic map of
theTuolumne Intrusive Suite,
California (after Huber et al.
1989). Central units (redshades) contain megacrysts,
outer blue units lack
megacrysts. The TIS becomes
both younger and more felsic
from the outer Glen Aulin and
Kuna Crest units to the central
Johnson Granite Porphyry.
Black box is the area shown in
the close-up. (Bottom) Map
showing measurement locations
(green triangles) for data in
(Fig. 5)
Contrib Mineral Petrol
123
Megacrysts: early- versus late-growth interpretations
K-feldspar megacrysts have been the focus of petrologic
studies for over a century (Pirsson 1899; Pitcher 1997).
Geochemical observations, such as sawtooth zoning of Ba
and internal isotopic zonation, are viewed as evidence for
an igneous origin (Kerrick 1969; Mehnert and Buesch
1981; Long and Luth 1986; Vernon 1986; Cox et al. 1996;
Gagnevin et al. 2005; Moore and Sisson 2007; Vernon and
Paterson 2008a). Hypotheses for the origin of Ba variation
in K-feldspar megacrysts include mafic magma recharge
(Cox et al. 1996), changes in pressure-temperature condi-
tions (Mehnert and Buesch 1981; Dickson 1996), and
growth of different phases that compete for residual Ba
(Long and Luth 1986). Long and Luth (1986) and Worner
et al. (2004) argued that correlation of Ba zoning among K-
feldspar megacrysts of different sizes points toward a
common origin for all megacrysts within a crystallizing
magma chamber. Cox et al. (1996) viewed megacrysts as
early, fast-growing phases and inferred that oscillatory
zoning, resorption, and mineral inclusions are produced by
episodes of mafic magma recharge that supply additional
Ba and cause remelting followed by regrowth upon cool-
ing. Gagnevin et al. (2005) interpreted chemical zonation
in megacrysts as growth zones representing migration of
megacrysts through a chemically stratified and convecting
magma chamber.
A tenet of igneous petrology is that phenocrysts are
crystals that grew early in the crystallization sequence,
when sufficient space and material were available to
accommodate such growth (Pirsson 1908; Winter 2001).
This classic viewpoint is one of the principal lines of evi-
dence in favor of early growth for K-feldspar megacrysts.
The small sizes of included phases such as plagioclase and
biotite relative to groundmass grains of the same minerals,
and their parallel arrangement along crystallographic
growth faces, are viewed as representing the sizes of other
mineral phases during K-feldspar growth and therefore an
indication of early growth (Kerrick 1969; Mehnert and
Buesch 1981; Vernon and Paterson 2008a). Proponents of
an early igneous origin for megacrysts cite textural rela-
tionships such as megacryst-rich clusters and their sys-
tematic distribution and orientation within plutonic suites,
features they attribute to movement and segregation while
the magma was still liquid enough to flow (Vernon 1986;
Paterson et al. 2005; Weinberg 2006; Vernon and Paterson
2008a).
An alternative view is that K-feldspar megacrysts are
porphyroblasts that grew under subsolidus conditions.
Methods of growth involve potassic metasomatism by
which potassium-rich fluids replace existing mineral phases
(Stone and Austin 1961; Collins and Collins 2002). Dick-
son (1996) viewed oscillatory Ba zonation as representing
discrete oscillations of temperature and pressure and
therefore as evidence for such a history.
A relatively new hypothesis is that K-feldspar mega-
crysts form by textural coarsening (Higgins 1999). Using
crystal size distributions, Higgins (1999) suggested that K-
feldspar megacrysts owe their size to slow cooling near
mineral liquidus temperatures. During early nucleation and
growth, the growth of crystals above a critical size is
thermodynamically favored (Ratke and Voorhees 2002).
With sufficient time, the system achieves a lower energy
state by dissolving or resorbing smaller grains and the
larger, more thermodynamically favorable crystals grow
from the existing melt and from material provided by the
dissolved crystals. Variations to the textural coarsening
hypothesis evoke various methods of temperature cycling
as the driving force for crystal growth (Higgins and
Roberge 2003; Simakin and Bindeman 2008).
Evidence from experimental and natural examples of
crystallization
Most interpretations of the origin of K-feldspar megacrysts
require that they be early crystallizing phases. However,
natural and experimental studies of rocks of appropriate
composition consistently indicate that K-feldspar is one of
the last phases to nucleate and crystallize. These data come
from experimental petrology, phenocryst assemblages in
volcanic rocks, and natural melting experiments.
Experimental studies on natural and synthetic grano-
dioritic compositions consistently show that K-feldspar
nucleates and grows late in the crystallization sequence
(Piwinskii and Wyllie 1968; Winkler and Schultes 1982;
Whitney 1988; Fig. 2). In granodiorites, maximum esti-
mates of the amount of melt present when K-feldspar
nucleates during cooling are between 65 and 70 vol%
(Winkler and Schultes 1982; Whitney 1988). Winkler and
Schultes (1982) determined that K-feldspar melt-out tem-
peratures are only 6–11�C greater than the bulk-rock soli-
dus temperatures for three different compositions near the
granodiorite-granite transition on a quartz-alkali feldspar-
plagioclase diagram. They estimated the temperature range
of cotectic crystallization of K-feldspar, quartz, and pla-
gioclase (35–65% of the total crystallizing granodioritic
magma) to be only 6–10�C.
Natural lavas confirm that K-feldspar grows late; for
example, dacite lavas similar in bulk rock composition to
the TIS granodiorites (65–72 wt% SiO2 and 2–4 wt% K2O)
commonly lack K-feldspar crystals even though phenocryst
abundances are 20–30 vol% (Singer et al. 1995; Nakada
and Motomura 1999; Costa and Singer 2002; Costa et al.
2004; Miller 2004).
Swanson (1977) demonstrated experimentally that low
nucleation rates coincide with fast growth rates for
Contrib Mineral Petrol
123
K-feldspar in the granite system. He concluded that with
fewer nuclei and faster growth than the other phases, K-
feldspar megacrysts might be expected. However, Swanson
also noted that a majority of K-feldspar growth occurs
when the rock is largely crystalline, and that this would
limit the ability of megacrysts to grow into the surrounding
crystal mush.
Glass compositions representing late-stage magmatic
liquids and natural examples of partial melts in granodio-
ritic and granitic rocks are consistently high-silica rhyolite
with *1/3 normative orthoclase (Al-Rawi and Carmichael
1967; Rutherford et al. 1985; Bachmann et al. 2002;
Nakada and Motomura 1999). Data from the Fish Canyon
Tuff (Bachmann et al. 2002) are particularly instructive, as
this rock has a bulk composition that is comparable to
many megacryst-bearing granodiorites (e.g., 65–68 wt%
SiO2; 4 wt% K2O). The tuff contains 45–55 vol% pheno-
crysts, and thus was near the rheologic lock-up point upon
eruption. About 6 wt% of the rock consists of phenocrysts
of K-feldspar (Or*70), but the rock contains 25 wt% nor-
mative orthoclase, indicating that over 80 wt% of the K-
feldspar at the point of eruption was dissolved in the
magmatic liquid (assuming that the remainder crystallized
as Or70 feldspar). This is consistent with the composition
and proportion of glass in the rock; 50 wt% of high-silica
rhyolite glass that is 32 wt% normative orthoclase would
yield about 23 wt% Or70 K-feldspar.
Methods
Sampling strategy
Sampling strategy focused on traverses at high angles to
mapped contacts from two different areas of the TIS. Most
interpretations given below are based on a set of 30 sam-
ples collected across a *3-km transition from equigranular
to megacrystic granodiorite on the western half of the suite
north of Tenaya Lake (Fig. 1). Phenocryst size measure-
ments were made on glacially polished outcrops, and rock
samples (1–10 kg) were collected from blasted road cuts
whenever possible. Owing to the extremely large sizes of
K-feldspar megacrysts (up to 15 cm locally) in the
Cathedral Peak Granodiorite, larger samples (up to 20 kg)
were collected from this unit.
Measurement of K-feldspar crystals
K-feldspar dimensions were quantified by measuring the
length (longest dimension parallel to a crystal axis) and
perpendicular width of the ten largest K-feldspar crystals
within a 1-m2 outcrop area along three transects. This
method of crystal size quantification enabled efficient and
objective quantification of crystal size. The main transect
consists of measurements from 92 stations spanning the
central portion of the suite from the western equigranular
Half Dome Granodiorite at Olmsted Point into and across
the megacrystic Cathedral Peak Granodiorite, ending near
the eastern porphyritic-equigranular Half Dome Granodi-
orite contact on the Lyell Fork of the Tuolumne River
(Fig. 1). The third transect traversed the equigranular Half
Dome Granodiorite to the Johnson Granite Porphyry from
northwest to southeast along the Tuolumne River. Clean
areas that appeared representative of the units as a whole
(without K-feldspar clusters or mafic features such as
schlieren or enclaves) at *50 m intervals were targeted.
Double measurements within *10–20 m of each other at
five random sites demonstrated the reproducibility of the
technique (Table 1).
Matrix minerals (plagioclase and quartz) were measured
with a ruler on stained and unstained samples collected
from different map units. Measurement of several of the
visibly largest hornblende crystals were made with a ruler
at most K-feldspar measurement stations.
Analytical methods
Backscattered electron and cathodoluminescence imagery
Backscattered electron (BSE) and grayscale cathodolumi-
nescence (CL) images were produced at UNC-Chapel Hill
10
50
70
90
10 50 70 9030
30
Tem
pera
ture
o C
800
750
700
Plag
Qtz
K-spar
Wt.% H2O Added 1 2 3 4 5 6 7
Bi+Ox+L
K-spar+Qtz+Plag+Bi+Ox
V
V L
Fig. 2 Experimentally derived crystallization sequence for the
Mount Airy granodiorite at 2 kb confining pressure after Whitney
(1988). Numbered contours indicate the amount of liquid (vol% melt)
remaining. Note that K-feldspar crystallizes relatively late and the
minimum amount of crystallization to occur before nucleation of
K-feldspar is *35%
Contrib Mineral Petrol
123
on a Leica 440 scanning electron microscope. Typical
conditions for analysis utilized an acceleration voltage of
15 kV and beam currents of 5–10 nA. Images of entire
probe sections are mosaics of 70–90 grayscale images.
Color CL images were obtained at the Smithsonian Insti-
tution Department of Mineral Sciences using a CCD-based
image acquisition system using the methods of Sorensen
et al. (2006).
Electron microprobe
Microprobe analyses were conducted at Duke University
on a Cameca Camebax electron microprobe utilizing
Cameca PAP correction software for data reduction. An
accelerating voltage of 15 kV and beam current of 15 nA
was used for all analyses. Typical spot sizes ranged from
3–5 lm. All standards consisted of natural silicate miner-
als. Locations for the analyses were based on previously
constructed BSE and CL images of each sample that show
internal zone boundaries.
X-ray mapping and energy dispersive X-ray spectroscopy
Production of X-ray maps employed a silicon drift detector
and 4pi Revolution software on the scanning electron
microscope at UNC-Chapel Hill. Typical conditions for
analysis utilized an acceleration voltage of 15 kV and
beam currents of 10–20 nA depending on sample proper-
ties and required spectral resolution. X-ray maps were
gathered using 1.2 ls time constants for fast acquisition.
Count rates were 120,000–140,000 c/s with dead times
ranging from 50 to 60%. Semi-quantitative energy dis-
persive X-ray spectroscopy (EDS) area analyses were
obtained using natural standards and the 4pi Revolution
analysis software. Identical measurement conditions were
used on standards and unknown samples.
Stained rock slabs
Rock slabs were cut from the larger samples collected
and polished to remove saw marks. Each polished face
Table 1 Representative K-feldspar size measurements
Map unit Easting Northing Crystal area (cm2)
1 2 3 4 5 6 7 8 9 10 Avg
Khd 282831 4189455 0.03 0.14 0.21 0.21 0.24 0.35 0.36 0.54 0.60 0.63 0.33
Khdp 283961 4190208 2.52 2.52 2.85 2.99 3.06 3.22 3.22 3.23 3.61 4.20 3.14
Khdp 284152 4190473 5.13 5.20 5.51 5.76 5.94 6.00 6.30 6.40 6.65 9.50 6.24
Khdp 284927 4191522 7.80 8.40 9.12 9.60 9.84 9.88 10.00 11.25 11.96 12.00 9.99
Khdp 284972 4191578 10.50 11.00 11.40 11.70 12.15 12.42 12.65 14.04 14.50 21.60 13.20
Kcp 285667 4193072 17.55 17.60 21.60 24.00 24.75 27.00 37.74 39.20 45.00 45.88 30.03
Kcp 286583 4194454 10.00 12.48 12.90 13.00 13.05 16.50 18.40 23.20 28.12 33.00 18.07
Kcp 287282 4194983 11.10 11.76 12.19 13.05 13.20 13.26 13.44 14.56 16.65 29.90 14.91
Kcp 289554 4196858 5.40 6.24 6.80 7.04 7.79 9.20 10.50 18.15 21.09 29.70 12.19
Kcp 290198 4196209 4.80 5.10 5.60 5.75 6.15 6.40 8.36 9.00 15.00 17.69 8.39
Kcp 291456 4194776 1.30 2.55 3.60 3.60 3.78 4.05 4.08 4.80 8.84 12.25 4.89
Double measurement locations
Khdp 284046 4190270 2.64 2.64 2.76 3.15 3.38 3.75 4.00 4.08 5.80 7.00 3.92
Khdp 284069 4190288 2.34 2.48 2.50 2.80 2.99 3.25 3.84 4.35 5.28 5.94 3.58
Khdp 284336 4190567 3.30 5.80 5.94 6.08 6.80 7.00 7.13 9.90 12.00 12.75 7.67
Khdp 284341 4190571 4.80 5.44 5.60 6.88 7.00 7.74 7.92 7.92 9.36 11.20 7.39
Khdp 285081 4191652 10.40 10.80 11.00 11.27 11.50 13.16 13.50 15.00 16.80 18.72 13.22
Khdp 285085 4191650 10.12 11.70 12.50 12.74 13.00 13.20 14.25 14.85 16.50 20.40 13.93
Kcp 288849 4195372 6.00 7.25 7.74 9.20 9.20 10.25 11.50 11.70 13.76 19.25 10.59
Kcp 288849 4195365 8.00 8.64 10.07 10.44 11.76 12.00 12.40 12.54 12.92 21.00 11.98
Kcp 291627 4194851 2.86 3.10 3.64 4.18 4.18 4.65 4.76 6.00 6.30 16.00 5.57
Kcp 291628 4194825 3.85 5.28 5.40 6.30 8.00 9.00 9.20 9.25 12.00 16.24 8.45
Representative K-feldspar size measurements from selected stations across the traverse in (Figs. 1, 5)
Double measurement locations (within 20 m) demonstrate the reproducibility of the technique from five locations
Error in measurements is ±2 mm
All locations are UTM NAD83
Contrib Mineral Petrol
123
was immersed in 29 M hydrofluoric acid for *1 min and
then rinsed in water. A 5-s bath in a 20% amaranth
solution for plagioclase and a 10-s bath in a 50% sodium
cobaltinitrite solution for alkali feldspar provided the best
color results. Samples were rinsed between and after each
stain with water. Each slab was scanned into 600 dpi
images and filtered for K-feldspar using image analysis
software.
Results
General observations
K-feldspar megacrysts in the TIS are pink to white, euhe-
dral, 5–15 cm crystals and exist only in the younger, more
central units of the suite. The largest megacrysts, K-feld-
spar clusters, and K-feldspar crystals in modal layers are
Fig. 3 General characteristics of K-feldspar megacrysts. a Megacryst
at high angle to mafic modal layering. b Small K-feldspar megacryst
displaying the characteristic zone parallel arrangement of included
mineral grains. c K-feldspar phenocryst from the porphyritic Half
Dome Granodiorite. Reflected light on cleavage surface illuminates
the interstitial tendrils radiating *2 cm into the adjacent matrix. d K-
feldspar megacryst (6.5 cm long) in Cathedral Peak Granodiorite
illustrating Carlsbad twinning, a common feature in TIS megacrysts.
Reflected light reveals that the K-feldspar is intergrown into the small
offset aplite, and anhedral, gray blebs of quartz cross the megacryst
along the trace of the former crack, which was refracted across the
crystal. The most likely explanation for this relationship is that the
megacryst and surrounding granodiorite were cracked and invaded by
aplite, and then the megacryst regrew K-feldspar as it healed and
some of this new feldspar grew into the dike (Pitcher 1997, p. 85).
e K-feldspar megacryst from Cathedral Peak Granodiorite containing
a 1 cm hornblende near the core of the crystal. f K-feldspar megacryst
(11 cm long) representative of largest crystal sizes near the outer
margins of the Cathedral Peak Granodiorite (crystal border outlined
for clarity). g Typical K-feldspar (highlighted white) distribution in
the Cathedral Peak Granodiorite. h Half Dome Granodiorite (left)—Cathedral Peak Granodiorite (right) contact along the Tuolumne
River (traverse in Fig. 1). Note the presence of K-feldspar megacrysts
within the mostly equigranular Half Dome. The transition from
equigranular to megacrystic textures is more pronounced along this
traverse relative to the Tenaya Lake traverse. i K-feldspar megacrysts
in close proximity within Cathedral Peak Granodiorite. j Cathedral
Peak K-feldspar megacrysts on two sides of a subtle internal contact.
Note the greater abundance of K-feldspar on the right side of the
contact
Contrib Mineral Petrol
123
limited primarily to the outer margins of the Cathedral
Peak Granodiorite (Fig. 1). In megacrystic units, smaller
matrix K-feldspar crystals are interstitial and mostly
irregular and anhedral. In contrast, K-feldspar in the
equigranular Half Dome Granodiorite is subhedral and
rarely forms anhedral interstitial crystals. The euhedral
appearance of megacrysts and phenocrysts in outcrop can
be misleading, because in many areas thin tendrils of K-
feldspar reach as much as 2 cm from megacryst rims into
the surrounding matrix (Fig. 3c).
Megacrysts locally weather out as raised knobs and
whole crystals owing to accumulation of biotite along their
outer margins, a characteristic not observed in equigranular
K-feldspar crystals. Zone-parallel arrangement of matrix
minerals (primarily \2 mm plagioclase, hornblende,
quartz, biotite, and titanite) is common within phenocrysts
and megacrysts (Fig. 3). Rare larger (5–10 mm) grains of
these matrix minerals also occur in megacrysts.
Maximum K-feldspar sizes
Megacryst size relationships are systematic throughout the
TIS, and interpreted contacts between the equigranular
Half Dome Granodiorite, porphyritic Half Dome Grano-
diorite, and Cathedral Peak Granodiorite were based upon
them. K-feldspar sizes gradually change through the central
portion of the TIS, illustrating the progressive change from
small (2–5 mm) equigranular to megacrystic textures as
noted by Bateman and Chappell (1979; their Fig. 4).
Measurements across three different equigranular to meg-
acrystic transitions show consistent and steady increases in
the average area of each of the 10 largest K-feldspars in a
1 m2 area from 0.2 to 30 cm2, followed by a more gradual
decrease to *7 cm2 in the central portion of the suite
(Table 1; Fig. 5). In contrast, bulk rock K2O and K-feld-
spar mode remain constant across these same transects
(Fig. 5; Bateman et al. 1988). These data require that there
are far fewer small crystals of K-feldspar in areas where
megacrysts are present. Stained slabs (Fig. 4; see below)
and CSD studies (Higgins 1999) confirm the mass deficit of
matrix K-feldspar in areas containing megacrysts.
Feldspar compositions
K-feldspar crystals are potassic (Or85–95) in all units of the
TIS (Table 2; Fig. 6). Plagioclase core compositions are
Fig. 3 continued
Contrib Mineral Petrol
123
consistent with the results of Gray (2003), becoming more
albitic from margin (An25–40) to center (An15–25) of the
suite. Albite (generally An\7) is locally present in small
amounts in the marginal and equigranular units of the TIS
and as up to a few percent cryptoperthite in some K-feld-
spar crystals.
Three-feldspar assemblage
Three-feldspar assemblages exist in units containing K-
feldspar phenocrysts and megacrysts (Fig. 6). K-feldspar
exists in three forms: megacrysts and phenocrysts, matrix
grains, and interstitial tendrils. Plagioclase (An15–25) exists
as euhedral to subhedral matrix grains and as irregular
cores of included grains within K-feldspar megacrysts
(Fig. 7). Albite (An2–5) is present in minor amounts as
exsolved micro-, crypto-, and patch perthite in megacrysts,
and commonly along borders between K-feldspar and
plagioclase. Small crystals of plagioclase included in
megacrysts typically display irregular albite (An2–7) rims
that are 1–10 lm thick (Fig. 7).
Megacryst zoning
Backscattered electron and CL images of megacrysts show
concentric compositional zones (Fig. 8). Correlation of
backscattered intensity with electron microprobe analyses
indicates that concentrations of BaO range from *0.05 to
2.8 wt% with the greatest celsian components being on the
core sides of the growth zones (Figs. 9, 10). Individual
zones range in width from 50 to 500 lm and many
cut through multiple inner zones (Fig. 10). Parallel
Equigranular Half Dome Megacrystic Cathedral PeakPorphyritic Half Dome
10 cm
Interstitial K-feldspar
a
b
d e
PlagKfs
Q
c
4 cm
Fig. 4 a Rock slabs stained for
K-feldspar (black). Samples
highlight the gradational
transition across the western
change from equigranular to
megacrystic textures in (Fig. 5).
Close-ups of equigranular (b)
and megacrystic (c) samples
illustrate the lack of smaller K-
feldspar crystals and interstitial
nature of K-feldspar crystals in
megacrystic textures relative to
equigranular textures. d, eUnfiltered color images from
the same areas as b, c help to
distinguish different mineral
phases. The interstitial nature of
K-feldspar (yellow) and coarser
textures of matrix minerals in
megacrystic samples relative to
equigranular textures
(plagioclase red, quartz gray,
mafic minerals black) are easily
observed
Contrib Mineral Petrol
123
arrangement of small included grains of plagioclase,
titanite, hornblende, biotite, and rare apatite accent differ-
ent growth zones (Figs. 3, 8).
Discussion
Late crystallization of K-feldspar
Phase equilibrium data summarized above clearly demon-
strate that growth of K-feldspar to megacrystic sizes in
magmas of granodioritic composition must occur after
the system is mostly crystalline. The only way in which
this could be violated is if K-feldspar crystallized at
temperatures significantly above its equilibrium crystalli-
zation temperature. However, whereas delayed nucle-
ation (supercooling) is a well-established phenomenon in
crystallizing systems, premature nucleation is not. Thus, all
available experimental and compositional data require that
K-feldspar is a late-crystallizing phase in granodioritic
systems. Vernon and Paterson 2008a suggested that higher
Ba concentrations would facilitate earlier nucleation of
K-feldspar, but TIS magmas show no difference in Ba
concentrations between equigranular and megacrystic tex-
tures (Bateman et al. 1988; Gray 2003; Gray et al. 2008),
and data summarized above show that Ba-bearing natural
dacite lavas also precipitate K-feldspar late in the crystal-
lization sequence.
In magmas similar in composition to typical grano-
diorites of the TIS, K-feldspar does not begin crystallizing
until the magma is near 50% crystalline, a crystal content
coincident with the sharp rheological change that causes
magmas to behave essentially as solids (Marsh 1981;
Vigneresse et al. 1996; Costa 2005). For magmas of
granodiorite composition, the liquid composition at this
point is minimum-melt rhyolite (e.g. Bachmann et al. 2002;
Nishimura et al. 2005), and the final *50% of the mag-
ma’s life consists of cotectic crystallization of quartz,
plagioclase, and K-feldspar. Thus, most ([75%, as sum-
marized above) K-feldspar crystallizes after the magma is
so crystal-rich that it behaves as a solid with melt-filled
pore spaces rather than as a liquid, and the K-feldspar
crystals cannot attain final megacrystic sizes until crystal-
lization is complete. These relationships rule out processes
Kcp
Khdp
Khde
Cathedral Peak Granodiorite
porphyritic Half Dome Granodiorite
equigranular Half Dome Granodiorite
avg megacryst size (cm2)
rock K2O (wt. %)
K-feldspar mode (vol. %)
Khd
e
Khd
e
Khd
p
Khd
p
Kcp
Kcp
Easting
0
5
10
15
20
25
30
35
280000 284000 286000 288000 290000 292000 294000 296000 298000 300000
cm2
- vo
l. %
- w
t.%
282000
Fig. 5 K-feldspar size measurements along with whole-rock K2O
and K-feldspar mode from across the central portion of the TIS,
measurement locations marked on the map in (Fig. 1). Easting is used
for the abscissa because mapped contacts strike approximately north–
south. Data highlight the non-megacrystic–porphyritic–megacrystic
transitions. Both modal K-feldspar (red circles) and whole-rock K2O
(blue squares) show little variation across the contacts. The average
area (cm2) of the ten largest K-feldspars in a 1-m2 area (green circles)
shows a steep increase starting at the contact of the mapped
porphyritic Half Dome Granodiorite and then decreasing to the
center of the megacrystic Cathedral Peak Granodiorite. These data
require that fewer small crystals exist in the areas with megacrysts
(see text). Color variation in background indicates inferred thermal
history of the system. Red areas are thermally mature or cycled more
times than blue areas. (K-feldspar mode and K2O data from Bateman
et al. 1988)
Contrib Mineral Petrol
123
Ta
ble
2F
eld
spar
com
po
siti
on
s
Map
unit
K-f
eldsp
ar
Kga
Khde
Khdp
Kcp
Avg
(12)
Min
2r
Max
2r
Avg
(13)
Min
2r
Max
2r
Avg
(70)§
Min
§2r
Max
2r
Avg
(11)*
Min
*2r
Max
2r
wt
%
SiO
264.0
864.4
40.9
064.6
40.9
164.5
063.8
20.8
964.3
10.9
063.5
864.7
60.9
163.9
90.9
064.1
764.4
50.9
063.8
70.8
9
Al 2
O3
18.8
819.0
40.2
718.7
90.2
618.9
619.2
90.2
718.8
40.2
619.0
819.2
30.2
718.7
70.2
618.9
918.8
80.2
618.8
70.2
6
FeO
0.0
40.0
40.0
60.0
00.0
00.0
80.0
10.0
30.0
70.0
70.1
00.1
20.0
70.0
60.0
60.1
00.1
70.0
70.0
80.0
6
CaO
0.0
50.1
90.3
80.0
20.0
20.0
20.0
50.0
20.0
10.0
10.0
50.0
70.0
20.0
00.0
00.0
30.0
50.0
20.0
10.0
2
Na 2
O0.9
52.0
00.0
80.7
10.0
50.8
81.2
90.0
60.6
70.0
51.3
22.3
40.0
80.4
80.0
40.9
71.3
40.0
60.5
90.0
4
K2O
15.1
613.6
50.2
215.6
60.2
215.6
814.7
50.2
116.0
00.2
214.6
713.3
20.2
116.4
90.2
315.2
214.8
80.2
115.7
00.2
2
BaO
0.9
81.1
80.2
80.8
70.2
40.4
71.4
00.2
70.2
30.1
71.3
81.2
40.2
80.0
00.0
00.8
90.6
30.2
21.1
30.2
6
Tota
l100.1
3100.5
5100.6
9100.5
9100.6
0100.1
3100.3
2101.0
799.7
8100.3
8100.4
0100.2
4
mol
%
Ab
8.5
17.7
6.3
7.7
911.3
85.9
911.7
020.5
34.2
38.6
511.8
85.2
6
An
0.2
0.9
0.1
0.0
80.2
40.0
20.2
30.3
50.0
00.1
30.2
30.0
5
Or
89.5
79.3
92.1
91.3
085.8
993.5
785.6
076.9
395.7
789.6
086.7
792.6
4
Cn
1.8
2.1
1.6
0.8
32.5
00.4
22.4
82.1
90.0
01.6
11.1
32.0
5
Map
unit
Pla
gio
clas
e
Kga
Khde
Khdp
Kcp
Avg
(6)
Min
2r
Max
2r
Avg
(10)
Min
2r
Max
2r
Avg
(14)
Min
2r
Max
2r
Avg
(50)
Min
2r
Max
2r
wt
%
SiO
257.7
959.0
30.8
355.6
60.7
864.9
368.3
40.9
659.8
10.8
461.5
766.9
00.9
459.4
60.8
364.5
368.1
70.9
553.8
50.7
5
Al 2
O3
26.7
826.4
50.3
728.1
60.3
922.6
120.2
00.2
826.2
30.3
723.1
819.3
60.2
724.6
20.3
421.7
820.0
90.2
828.9
80.4
1
FeO
0.1
40.1
20.0
60.1
50.0
60.1
10.1
10.0
60.1
70.0
70.1
40.0
70.0
60.1
80.0
70.1
10.0
30.0
50.1
80.0
7
CaO
8.2
27.5
00.1
410.0
70.1
62.9
90.0
50.0
27.0
50.1
44.2
60.5
80.0
46.8
60.1
42.8
50.2
60.0
311.5
10.1
8
Na 2
O6.8
07.1
50.1
76.0
30.1
610.2
511.8
90.2
47.9
60.1
89.1
411.6
50.2
37.7
30.1
79.7
911.8
90.2
44.9
01.2
8
K2O
0.2
10.1
90.0
20.1
90.0
20.1
90.1
20.0
20.2
10.0
20.1
80.1
20.0
20.1
80.0
20.1
90.1
40.0
20.1
00.0
2
BaO
0.0
30.0
50.1
10.0
00.0
00.0
10.0
40.0
80.0
00.0
00.0
20.0
00.0
00.0
00.0
00.0
30.0
00.0
00.0
00.0
0
Tota
l99.9
6100.4
9100.2
7101.0
8100.7
5101.4
398.4
898.6
899.0
399.2
9100.5
799.5
2
mol
%
Ab
59.2
62.5
51.4
85.1
699.0
366.3
478.4
896.6
966.4
085.0
098.0
443.2
3
An
39.6
36.3
47.5
13.8
00.2
432.5
220.4
82.6
732.5
813.8
51.1
856.1
8
Or
1.2
1.1
1.0
1.0
30.6
61.1
41.0
10.6
41.0
21.0
90.7
80.5
9
Cn
0.1
0.1
0.0
0.0
20.0
70.0
00.0
30.0
00.0
00.0
60.0
00.0
0
Dat
are
pre
sent
the
aver
age,
min
imum
,an
dm
axim
um
K-f
eldsp
ar(O
r)an
dpla
gio
clas
e(A
n)
anal
yse
sfo
rea
chm
apunit
.E
nti
redat
aset
isplo
tted
in(F
ig.
6)
*D
ata
only
giv
enfo
rpro
be
dat
aw
ith
erro
rs.
Fig
ure
9in
cludes
dat
anot
use
dto
calc
ula
teav
erag
e,m
inim
um
,or
max
imum
com
posi
tions
§A
nal
yse
sin
clude
pro
be
loca
tions
atth
eed
ge
of
incl
uded
pla
gio
clas
egra
ins
whic
hm
ayac
count
for
the
slig
htl
ym
ore
sodic
anal
yse
s
Contrib Mineral Petrol
123
that involve physical accumulation and segregation of
already-formed megacrysts in a dynamic, flowing, fluid
magma.
The near-absence of megacrystic dacite lavas on Earth
further substantiates the late development of K-feldspar
megacrysts. We cannot explain the one exception to this
(Taapaca). However, in the Johnson Granite Porphyry of
the TIS, highly rounded blobs of Cathedral Peak-type
granite, cored by megacrysts, are common (Titus et al.
2005). Field relations suggest that these blobs were par-
tially crystallized Cathedral Peak magma that was caught
up in the Johnson magma, and if they had become more
disaggregated they could have introduced megacrysts into
the otherwise non-megacrystic magma.
porphyritc Half Dome0
1
Half Dome0
0.51
Glen Aulin0
1
70
012
012
012
0 10 20 30 40 50
Cathedral Peak0
1
50 60 70 80 90 100012
Plagioclase K-Feldspar
Vo
lcan
ic C
om
po
siti
on
cm
An Or
Or
mo
l %
An
m
ol%
450
oC
Per
iste
rite
Gap
e
10 10 20 30 40 50
0 10 20 30 40 50
0 10 20 30 40 50
50 60 70 80 90 100
50 60 70 80 90 100
50 60 80 90 10070
0.5
0.5
0.5
Fig. 6 Electron microprobe analyses of plagioclase and K-feldspar
from different map units of the TIS (outer to inner = top to bottom;
Glen Aulin, equigranular Half Dome, porphyritic Half Dome,
Cathedral Peak). Analyses show K-feldspar compositions much more
potassic than volcanic rocks of similar composition (Carmichael
1960; Bachmann et al. 2002; Zellmer and Clavero 2006). Plagioclase
compositions are consistent with evolving whole rock compositions
but rims of plagioclase are quite sodic, another indication of low-
temperature equilibrium (peristerite gap after Carpenter 1981)
Fig. 7 Color
cathodoluminescence image
(green Ca-plag, red Na-plag,
blue K-feldspar, black quartz)
with locations of electron
microprobe analyses. K-feldspar
and included plagioclase in this
megacrystic sample show a
range of compositions.
Cathodoluminescence color
clearly distinguishes included
plagioclase calcic cores from
sodic rims. K-feldspar adjacent
to plagioclase gives slightly
more potassic compositions
Contrib Mineral Petrol
123
Growth of megacrysts by late-stage selective textural
coarsening
If K-feldspar nucleates and grows late in the crystallization
history of a magma, then megacrysts must grow late.
Megacryst distributions, chemical compositions, and tex-
tural observations in the TIS are consistent with late growth
of K-feldspar megacrysts by textural coarsening, as sug-
gested by Higgins’ (1999) crystal-size distribution data.
We propose that K-feldspar megacrysts grow to large sizes
during the later stages of crystallization owing to cycling of
temperature during incremental emplacement, in the pres-
ence of a late-stage magmatic liquid. In Higgins’ hypo-
thesis and in ours, small crystals dissolve to feed growing
megacrysts. In Higgins’ hypothesis, crystals below a cri-
tical size dissolve with time when the system is buffered
near the K-feldspar liquidus for an extended period owing
to latent heat of crystallization. In our hypothesis, multiple
episodes of reheating and fluid fluxing owing to arrivals of
later magma batches provide the required thermal stimulus
for textural coarsening.
Selective coarsening of K-feldspar relative to other
phases is governed by the idiomorphic tendencies of K-
feldspar in conjunction with its low enthalpy of fusion,
which makes it a late-crystallizing phase in granodiorite
magmas. In contrast, quartz also crystallizes late but rarely
forms euhedral crystals in granites, presumably because it
is less idiomorphic. In this regard K-feldspar might be
compared to garnet, which typically forms relatively large,
idiomorphic crystals in metamorphic rocks, rather than
being dispersed throughout the rock in small crystals as is
quartz.
We propose that megacryst growth occurs by remelting
and recrystallization of small, higher-energy K-feldspar
crystals during repeated pulses of granodiorite magma
emplacement during pluton construction (Fig. 11). Coars-
ening may occur during thermal fluctuations with small
amounts of melt remaining, during small-degree partial
remelting, or in the presence of a fluid phase. Analogous
coarsening of crystals by thermal cycling in ice cream and
other systems is a well-studied phenomenon (Vacek et al.
1975; Horsak et al. 1975; Donhowe and Hartel 1996;
Hartel 1998; Flores and Goff 1999), and Simakin and
Bindeman (2008) recently showed theoretically that ther-
mal cycling should move mass from smaller to larger
crystals. Small K-feldspar crystals may partially or com-
pletely dissolve, along with the edges of larger crystals,
which after multiple reheating and subsequent cooling
events texturally coarsen to become the megacrysts
(Fig. 11).
Granodiorite solidus temperatures are typically assumed
to be *650–700�C (e.g., Winkler and Schultes 1982;
Whitney 1988). This would seem to limit megacryst
growth to these temperatures or above, and Ti-in-titanite
thermometry of megacrysts in the Sierra Nevada are lar-
gely consistent with this (Moore and Sisson 2007),
although they also measured nominally subsolidus tem-
peratures. However, components such as B and Li, which
are present to some degree in any granodioritic system, can
depress solidus temperatures down to *400�C (Sirbescu
Fig. 8 Mosaic of 65 backscattered electron images of porphyritic Half
Dome Granodiorite K-feldspar displaying characteristic concentric Ba
zoning and zonal arrangement of included mineral phases (Bt biotite,
Hb hornblende, Kfs K-feldspar, Plag plagioclase, Ti titanite, Q quartz).
Dashed ovals illustrate areas where tendrils of K-feldspar are
crystallographically continuous into adjacent matrix. Dashed rectanglefrom core to rim is the area shown for the analyses in (Fig. 9). Smaller
sizes of included minerals relative to matrix grains are due to the
inability of the growing K-feldspar megacryst to incorporate or grow
around the larger grains with each new growth zone (50–500 lm)
Contrib Mineral Petrol
123
and Nabelek 2003; Nabelek and Sirbescu 2006). Thus, it is
likely there was at least a trace of liquid present down to
*400�, the reequilibration temperatures suggested by
feldspar mineralogy (see below). Lower viscosities and
improved diffusivity also typify fluxed systems with very
small amounts of melt remaining (Nabelek and Sirbescu
2006), a characteristic that would further facilitate late-
stage textural coarsening at extremely low melt fractions.
Physical requirements of textural coarsening
Evidence of textural coarsening of TIS megacrysts is
illustrated by the CSD studies of Higgins (1999): stained
slabs from TIS samples transecting the equigranular to
megacrystic transition (Fig. 4), and measurements of K-
feldspar maximum sizes throughout the TIS (Fig. 5).
Higgins measured K-feldspar sizes from five outcrops
100
110
120
130
140
150
160
170
180
190
200
0 1 2 3 4 5 6 7 8
0
0.5
1
1.5
2
2.5
3
Gra
ysca
le (5
pix
el a
vg)
BaO
wt %
Distance (mm)
Grayscale Correlation with BaO wt %
Core Rim
Bt
Plag
> 5 %Na2O
11109
Fig. 9 Backscattered electron
image from area outlined in
(Fig. 8) illustrating Ba zoning
from core to rim of K-feldspar
phenocryst. Electron
microprobe data (diamonds) in
different BSE grayscale zones
verify the sawtooth nature of
zoning and correlation to BSE
intensity. A five-pixel moving
average of grayscale (black linethrough probe locations)
displays multiple sharp
increases followed by more
gradual decreases (gray arrows)
in Ba concentration with each
new growth zone
Fig. 10 a, c Backscattered
electron images illustrating the
sawtooth nature and resorption
(c) of internal megacryst Ba
zoning with higher Ba
concentrations at the core side
(bottom of a and right of c) of
new growth zones. b, d Barium
X-ray maps from the same area
as (a, c) verify increased Ba
with BSE grayscale brightness
Contrib Mineral Petrol
123
across the Cathedral Peak Granodiorite and interpreted the
shape of the CSD data as being indicative of textural
coarsening. Stained rock slabs from samples collected
along the transition from equigranular Half Dome Grano-
diorite to megacrystic Cathedral Peak Granodiorite illus-
trate this coarsening (Fig. 4). The largest K-feldspar
crystals increase from\1 cm to[5 cm in length along the
transect (Fig. 5), whereas the remaining matrix K-feldspar
crystals decrease in size and become less abundant (Fig. 4).
K-feldspar modes are relatively constant across this tran-
sition, ranging from 23 to 29 vol% (Bateman et al. 1988).
Samples that contain phenocrysts and megacrysts lack
small K-feldspar crystals and contain large tonalitic areas
(2–3 cm2) that are essentially free of K-feldspar; such K-
poor areas are not observed in non-coarsened rocks
(Fig. 4).
Because volume scales as the cube of length, a crystal
whose axes are larger than another by a factor x has a
volume x3 times larger; i.e., for crystals with the same
shape, a crystal whose longest edge is 8 cm has 64 times
the volume of one whose longest edge is 2 cm. Given that
megacrysts get dramatically larger across the transition
zone whereas K-feldspar modes remain relatively constant
(Fig. 5), there must be a significant deficit of smaller K-
feldspar crystals in areas with megacrystic textures. This is
evident in the textures observed from stained slabs (Fig. 4).
K-feldspar textural relationships display systematic
variations during megacryst development. Small euhedral
to subhedral grains in equigranular samples become diffuse
and intergrown around other groundmass grains in meg-
acrystic samples (Fig. 4). The edges of growing pheno-
crysts and eventual megacrysts display long (up to 2 cm
from the crystal) thin tendrils of K-feldspar material that
radiate away from the otherwise euhedral crystal (Bateman
1992; Higgins 1999). Interstitial tendrils are crystallo-
graphically continuous with the megacrysts (Figs. 3, 8).
Swanson (1977) suggested that fast growth rates of K-
feldspar during conditions of high crystal contents (late in
the crystallization sequence) would produce this inter-
grown texture. Higgins (1999) interpreted tendrils as
+
Progression through time
Idealized Schematic Model
Wal
l Roc
k (o
uter
con
tact
)
Cen
ter
of th
e su
ite
K-f
elds
par
Cry
stal
Siz
e
Minimum T required for coarsening Tem
p
Time/Space
coarsening window
+
-
Half Dome Cathedral PeakPorphyriticHalf Dome
JohnsonGranite
Fig. 11 Schematic model illustrating the size distribution of K-
feldspar (gray rectangles, not to scale) from margin (left) to core
(right) of the suite. Multiple individual emplacement pulses (marked
by vertical gray lines) are not to scale and orientation is not specified.
Black lines mark the approximate boundaries between map units.
Temperature curve represents the development of the thermal status
upon pluton emplacement. Sharp increases mark the emplacement of
new injections (not to scale). Coarsening occurs when the system
achieves enough thermal inertia to sustain temperatures within the
‘‘coarsening window’’ and effectively cycle temperatures within that
temperature interval through subsequent emplacement pulses. The
greatest coarsening is achieved in areas where thermal cycling of
previous pulses is maximized (cycled the most times) through
resorption/crystallization events. Gray lines on crystal size graph are
snapshots in time representing the crystal sizes as the suite is being
emplaced
Contrib Mineral Petrol
123
evidence of late growth during the last phases of cooling.
We view the presence of interstitial tendrils only in phe-
nocrystic and megacrystic areas as preservation of the
coarsening process. The tendrils may represent remnant
pathways by which interstitial K-feldspar migrated toward
the megacrysts.
Measurement of crystal sizes shows overall coarsening
of bulk-rock textures in areas where megacrysts exist; this
is evident in Fig. 4. The long axes of the largest quartz
and plagioclase crystals in stained samples average 0.3 and
0.4 cm, respectively, in equigranular samples and 1.0 and
0.7 cm in samples that contain the largest megacrysts.
Coarsening of these other phases could be produced by
partial melting and resorption of material that is primarily
made up of a three-phase assemblage (K-feldspar, quartz,
and plagioclase). Coarsened textures in areas containing
megacrysts support the hypothesis that areas displaying
megacrysts are thermally mature.
Implications of late-stage textural coarsening
The timing of K-feldspar growth places restrictions on
crystal sizes during different stages of pluton growth.
Estimates of the amount of K-feldspar crystallized at
*50% bulk-rock crystal contents in rocks of granodioritic
composition is \25% of the total K-feldspar in the rock.
Once again, for comparison we refer to the chemically
similar Pagosa Peak Dacite of the Fish Canyon Tuff sys-
tem, which has 45–55% phenocrysts with K-feldspar only
representing 5–10% of the rock (Bachmann et al. 2002).
Granodioritic xenoliths, understood to be intrusive equiv-
alents, collected from the tuff record 20–30% modal K-
feldspar (a characteristic of most granodiorites) suggesting
that only 25–30% of the total K-feldspar had crystallized at
the point where *50% of the magma had crystallized.
Using this estimate and the relationship between crystal
length and volume, the calculated size that a 10-cm
megacryst would be as the system crossed the 50% crys-
talline threshold is *6.3 cm (a factor of 0.251/3). As noted
earlier, K-feldspar phenocrysts[5 cm in length are almost
unknown in volcanic rocks; yet, granite petrologists often
interpret larger megacrysts as large crystals at the sizes we
find them today that moved about and accumulated in
clusters during times of magma mobility (Vernon 1986;
Weinberg et al. 2001; Paterson et al. 2005; Vernon and
Paterson 2008a, b).
Phenocrysts \5 cm in length certainly may exist in
volcanic and plutonic systems early in a magma’s lifetime,
when it is liquid enough to flow, but such crystals must be
coarsened significantly after magma lockup to produce the
extreme crystal sizes and other textural relationships
observed in megacrystic plutonic suites. The fundamental
inconsistency between experimental and observational data
indicating late K-feldspar growth and geologic interpreta-
tions requiring existence of megacrysts when the magma
was highly fluid is the main motivation for our hypothesis
of late-stage coarsening.
Significance of megacryst sizes and distribution
K-feldspar megacrysts become increasingly larger from the
equigranular Half Dome Granodiorite toward the outer
margin of the Cathedral Peak Granodiorite, then smaller
across the central portion of the Cathedral Peak (Fig. 5).
Gradational megacryst size relationships and distribution
patterns are common characteristics of megacrystic plutons
(Lockwood 1975; Bateman 1992) and suggest that textural
development from different map units is related. Size
transitions occur across map units that are very long-lived
([3 m.y.) and have been shown to be made up of discrete
pulses of magma that did not mix or homogenize on the
scales necessary to explain the gradational nature of
megacryst size distributions throughout the suite (Coleman
et al. 2004, Glazner et al. 2004; Gray et al. 2008). Size
variations occur with no change in the modal amount of K-
feldspar or significant changes to bulk rock compositions
and are consistent with the CSD data of Higgins (1999)
(Fig. 5). Therefore, interplutonic megacryst size distribu-
tions develop independently of a large magma chamber
that is capable of internal flow that mechanically concen-
trates larger megacrysts in certain areas of the suite. Rather,
megacrysts formed during conditions of magma immobi-
lity, requiring that variations of megacryst sizes reflect
different degrees of coarsening and are an indication of the
thermal history of the suite. The exceptionally coarse outer
margins of the Cathedral Peak must be the locus of optimal
conditions for coarsening. A possible explanation of this is
given in Fig. 11.
Several mapped pluton contacts within the TIS are
coincident with the textural development of K-feldspar.
Thermal maturation of the system in areas where mega-
crysts exist also provides enhanced annealing of internal
contacts (Coleman et al. 2006). The older equigranular
Half Dome Granodiorite preserves multiple diffuse internal
contacts whose interpretation is subject to debate. The
relative scarcity of such internal contacts in the inner
megacrystic Cathedral Peak Granodiorite is likely a result
of thermal annealing after the system had sufficient thermal
inertia to reheat, remelt, and redistribute material locally.
Rare internal contacts within the Cathedral Peak are
characterized by K-feldspar concentration (Fig. 3j). Such
contacts may represent late-stage localized coarsening
owing to concentrated fluid flow. The degree to which
previous pulses of magma crystallized is unknown, but
sufficient crystallization must have occurred to prohibit
large-scale fractionation or redistribution of crystallized
Contrib Mineral Petrol
123
phases (Gray et al. 2008). Preserved in the internal com-
positional characteristics of K-feldspar megacrysts is evi-
dence for such a cooling history.
The presence of K-feldspar megacrysts in the TIS and
other zoned intrusive suites may reflect a link between
megacrysts and long-lived, incremental emplacement.
Several other plutonic suites show similar megacryst size
relationships and distributions to those observed in the
Cathedral Peak Granodiorite. Examples of megacrysts in
the younger more silicic units include the quartz monzonite
of Mono Recesses (Lockwood 1975) and the Wheeler Crest
Granodiorite (Bateman 1992) of the Sierra Nevada Bath-
olith, the Endako Batholith, north-central British Columbia
(Villeneuve et al. 2001), and several examples of Caledo-
nian granites from the British Isles (Le Bas 1982; Pank-
hurst and Sutherland 1982). Many authors also note that
megacryst size distributions tend to be gradational in nat-
ure, like the TIS, with a decrease in size or abundance
inward (Wagener 1965; Lockwood 1975; Chappell and
White 1976; Bateman and Chappell 1979; Pankhurst and
Sutherland 1982). Additional high-precision geochrono-
logy and mapping of plutons containing megacrysts will
allow for a better understanding of the relationships
between megacryst distributions and temperature, space,
and time.
Internal megacryst zoning
Although individual megacrysts show no consistent zona-
tion of K and Na, owing presumably to the rapidity with
which K and Na exchange, compositional zoning is marked
by variations in Ba concentration (Figs. 8, 9, 10). Partition
coefficients for Ba between K-feldspar and silicate melt
range from[1.5 to 10 depending on temperature, pressure,
H2O, and K-feldspar composition (Long et al. 1978; Guo
and Green 1989). Therefore, early crystallizing K-feldspar
will preferentially take up Ba.
Barium zones in TIS megacrysts show distinctive saw-
tooth truncation of inner zones (Figs. 8, 9, 10). These
relationships are seen in other examples of K-feldspar
megacrysts (Mehnert and Buesch 1981; Vernon 1986; Cox
et al. 1996; Gagnevin et al. 2005; Zellmer and Clavero
2006). Many zones are wavy and truncate one or more
inner zones, an indication of resorption (Fig. 10). The
presence of multiple resorption periods and the morphol-
ogy of Ba growth zones are evidence for multiple reheating
events and support the interpretation that megacrysts grew
from melt or a fluid phase. Smaller matrix K-feldspar
grains show normal Ba zoning with increased Ba concen-
trations near the core of the grain. Megacryst zones from
different crystals within the same map unit display differ-
ent concentrations of Ba and different zone widths (Long
and Luth 1986). We interpret such differences as
representing similar thermal histories, but due to differ-
ences in proximity to thermal input, localized coarsening
conditions develop. Crystallization of multiple zones by
several separate growth events creates the observed saw-
tooth Ba zonation.
Ba concentrations across the TIS are variable but are
clearly not systematically higher in megacrystic units
(Bateman et al. 1988; Gray et al. 2008). The source for Ba
in each new growth zone could be provided from melting
of existing K-feldspars and need not result from input of
new Ba-rich magma, as required by Cox et al. (1996).
Melting of the outermost shell of a growing megacryst
along with melting significant proportions or entire smaller
K-feldspar crystals liberates Ba that may become redis-
tributed. In particular, early-formed small crystals should
contain significant concentrations of Ba that would be
released during melting of those crystals. Upon cooling, the
new melt would then be recrystallized onto existing K-
feldspar, plagioclase, and quartz crystals.
According to this hypothesis, Ba zoning results from
redistribution rather than by addition of Ba into the system
(Cox et al. 1996), partial melting of biotite (Williamson
1999; Collins and Collins 2002), or regional tectonic
stresses (Dickson 1996). Biotite mode shows only a slight
decrease of *1 vol% (from an average of 5.1% in the Half
Dome Granodiorite to 3.8% in the Cathedral Peak Grano-
diorite). Inner units of the TIS that contain megacrysts do
not show significant interaction with mafic magmas at the
level of emplacement. Mafic enclaves are rare in the
megacrystic Cathedral Peak Granodiorite although they
appear more common in the equigranular Half Dome
Granodiorite. Multiple events of mafic magma recharge
could create similar results by thermal cycling of the sys-
tem without the need for large-scale Ba addition or con-
tamination. Rather, simple reheating by subsequent pulses
of granodiorite magma provokes coarsening of K-feldspar
crystals without external Ba enrichment.
The slow nature of Ba diffusion in K-feldspar preserves
internal Ba zoning through prolonged cooling histories.
Cherniak (2002) determined diffusion rates for Ba in Or61
orthoclase of 4.54 9 10-23 to 1.08 9 10-19 m2/s for tem-
peratures of 828–1028�C. Diffusion rates for Na in K-feld-
spars are *6 orders of magnitude faster than those for Ba at
the same temperatures (Foland 1974). The time required to
change the midpoint composition of a 100-lm-wide Ba zone
held at 700�C by more than 10% of the starting composition
is [10 m.y. (Fig. 10 of Cherniak 2002). Destruction of
internal Ba zones in TIS megacrysts (50–500 lm) owing to
prolonged cyclic cooling is unlikely at these extremely slow
diffusion rates and near-solidus temperatures.
Concentric Ba zoning is also delineated by the parallel
arrangement of small matrix minerals along each growth
zone (Figs. 3, 8). The incorporation of all matrix phases is
Contrib Mineral Petrol
123
an indication that growth of megacrysts occurs late while
all other phases are present. The small size of the included
minerals has been interpreted to be a representative size of
that phase during megacryst formation (Kerrick 1969;
Mehnert and Buesch 1981; Vernon and Paterson 2008a).
However, we suggest the small sizes result from a sorting
process during megacryst growth and the incorporation of
the smaller grains. Only grains small enough to be sur-
rounded by a growth shell can be fully incorporated into
the growing megacryst. Crystals that are too large cannot
be included and accumulate at the edges as the megacryst
grows. Possible evidence of this is the observation that TIS
megacrysts often display mafic matrix grains surrounding
megacryst edges; these likely are crystals that were
excluded from the growing crystal.
Feldspar compositions and the origin of albite
Near end-member feldspar compositions and the presence
of a three-feldspar assemblage indicate that feldspars
equilibrated at temperatures below the nominal granodio-
rite solidus (Essene et al. 2005; Fig. 6; Table 2). Megacryst
compositions range from Or85–95 in the TIS and in other
megacrystic suites in the Sierra Nevada (Piwinskii 1968;
Kerrick 1969). Volcanic phenocrysts in rocks of similar
compositions generally range from Or60 to Or75 (e.g.,
Carmichael 1960; Hildreth 1977; Bachmann et al. 2002)
and fractionate toward more sodic conditions as tempera-
ture falls. The presence of Or85–95 compositions in the TIS
rocks requires that they exsolved albite at temperatures
below *500�C.
The lack of significant, optically visible perthite in most
megacrysts is consistent with prolonged exsolution. In
areas where perthite is visible, average compositions of
1–2 mm2 areas obtained by scanning are Or80–85. Thus,
even where perthite is visible, compositions are too potassic
to be the low-temperature exsolved equivalents of their
assumed starting compositions. This requires that signifi-
cant amounts of albite migrated out of the megacrysts.
Plagioclase compositions spanning the peristerite gap
(An2–7 and An15–35) also argue for low-temperature
equilibration (Fig. 6; Carpenter 1981). Albite compositions
only occur where plagioclase and K-feldspar are in contact
with one other and in minor patch perthite within mega-
crysts. Rogers (1961) proposed that such albite forms
during the last stages of granite crystallization because it is
the composition in equilibrium with oligoclase. However,
this explanation does not account for the lack of plagio-
clase in the An5–15 range. A likely origin is simply exso-
lution. Prolonged low-temperature cooling in both potassic
alkali feldspar and sodic plagioclase induces exsolution,
contributes mass to the sodic rims of plagioclase crystals,
and locally produces near end-member albite.
Dynamic versus static clusters of megacrysts
The presence of K-feldspar megacrysts in clusters and
within biotite-rich layers and schlieren has been cited as the
strongest evidence for growth of megacrysts before magma
lockup (Paterson et al. 2005; Vernon and Paterson 2008a),
because those authors interpret the megacrysts to have been
deposited in the clusters and within the layers. An alter-
native to early dynamic processes is that megacryst clusters
and accumulations represent zones of concentrated fluid
migration that allowed for and facilitated textural coars-
ening of K-feldspar (see discussion by Higgins 1999).
Higgins suggested that K-feldspar clusters and masses that
appear as both vertical and horizontal channels are actually
sections through tubes and sheets of concentrated fluid
migration. Repeated reheating and subsequent resorption
and remelting events would create greater porosity in areas
with greater proportions of the phase being dissolved
(K-feldspar). Therefore, the final appearance of such areas
would be expected to have higher concentrations of the
largest crystals. We reiterate that late growth of K-feldspar,
as required by phase equilibria, rules out the interpretation
that such features result from sedimentary accumulation of
already formed megacrysts.
Conclusions
The interpretation of granitic textures is fundamental to
understanding one of the most abundant constituents of the
Earth’s continental crust, granodioritic plutons. Many
geologic maps and interpretations rely on differences in
textural relationships (i.e. presence or absence of pheno-
crysts or megacrysts) to distinguish between different units
and the relationships between those units. If map units that
are based on textural differences do represent composites
of several related geologic events and the interplay of
temperature, space, and time rather than a single large
discrete geologic event, then geologic interpretations that
rely on such relationships may be flawed. The development
and significance of K-feldspar megacrysts within plutons
remain controversial. However, geochronologic and tex-
tural data from the TIS suggest that granitic plutons are
emplaced incrementally and that megacrystic textures are
evidence of such an emplacement process.
The observation that some compositionally indistin-
guishable granitic plutons contain K-feldspar megacrysts
whereas others do not is significant. The presence and
relationships of K-feldspar megacrysts in the other igneous
suites of the Sierra Nevada and many others globally is an
indication of protracted and probably cyclic cooling his-
tories indicative of incremental emplacement. The almost
complete lack of volcanic K-feldspar megacrysts is an
Contrib Mineral Petrol
123
indication that megacrystic textures develop late in the
crystallization sequence and therefore cannot be extruded.
Our model attempts to accommodate experimentally
observed late growth of K-feldspar along with observed
spatial, chemical, and textural observations of megacrysts
and incorporate recent ideas about incremental pluton
growth and petrogenesis.
Acknowledgments Support for this work was provided by grants
from the National Science Foundation (EAR-0336070 and EAR-
0538129) and student research grants from the Geological Society of
America, the University of California’s White Mountain Research
Station, and the University of North Carolina at Chapel Hill Martin
and Bartlett Funds. Jan van Wagtendonk, Peggy Moore, and Greg
Stock of Yosemite National Park Service graciously provided logis-
tical support this work. Thoughtful and thorough reviews by Tony
Kemp, an anonymous reviewer, and Editor Jon Blundy greatly
improved the manuscript. We thank Drew Coleman, John Bartley,
Ryan Mills, Jesse Davis, Rich Gashnig, John Gracely, Bryan Law,
Chris Glazner, and Susie Johnson, who assisted in the field and par-
ticipated in several spirited discussions, and Alan Boudreau and John
Rogers for additional discussion and insight. We appreciate the help
and use of the color CL at the Smithsonian Institution from Sorena
Sorensen. Use of Duke University’s electron microprobe was essen-
tial to this work and we thank Alan Boudreau for his help.
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