Formation of K-feldspar megacrysts in granodioritic plutons by

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


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)


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


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%


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


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


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


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


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)


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


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


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)


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


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


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


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


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


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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