Lithos 105 (2008) 42–50

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How extensive are subsolidus grain-shape changes in cooling granites?

R.H. Vernon a,b,⁎, S.R. Paterson b

a Department of Geology and ARC National Key Centre for GEMOC, Macquarie University, Sydney, NSW 2109, Australiab Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA

a r t i c l e i n f o

⁎ Corresponding author. Department of Geology andGEMOC, Macquarie University, Sydney, NSW 2109, Aust

E-mail address: rhvernon@laurel.ocs.mq.edu.au (R.H

0024-4937/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.lithos.2008.02.004

a b s t r a c t

Article history:Received 13 August 2007Accepted 14 February 2008Available online 4 March 2008

The hypothesis that granites (sensu lato) commonly undergo extensive grain-shape changes (“recrystallization”)during cooling is not supported by microstructural evidence, especially the abundance of non-truncated zoningpatterns in mineral grains. Regional metamorphic felsic gneisses that have undergone extensiveneocrystallization/recrystallization at amphibolite facies conditions (which some have suggested areequivalent to those in slowly cooling granites) typically lose igneous microstructures, including oscillatoryzoning, and are characterized by polygonal to irregularly xenoblastic grains of quartz and feldspar, as well as byrounded inclusions of quartz in feldspar and vice versa. In contrast, granites typically show oscillatory zoning(especially in plagioclase), elongate, euhedral to subhedral feldspar grains, and euhedral plagioclase inclusionsin quartz and K-feldspar. The contrast between these two situations is probably the result of felsic gneissesundergoing chemical reactions and regional deformation, especially in the presence of water releasedpervasively from dehydration reactions (which are strong driving forces for the production of new grains),whereas the minerals in cooling granites are chemically compatible, without strong deformation, and so do notreact with each other or recrystallize. Therefore, massive granites without solid-state foliations generally do notundergo grain-shape changes, owing to inadequate driving forces, as well as to decreasing temperature. Animportant implication of this conclusion is that internal structures formed during chamber construction andmagmatic flow in granite plutons are not likely to be removed by subsolidus intergranular microstructuralchanges.

© 2008 Elsevier B.V. All rights reserved.

Keywords:GranitesK-feldspar megacrystsGrain shapesMicrostructureMineral zoningPluton

1. Introduction

This paper concerns the nature and extent of subsolidus grain-shape changes during slow cooling of granites (sensu lato). These daysmost petrologists probably regard granitic microstructures (Figs. 1, 2)as being essentially magmatic, in contrast to compositionally equiva-lent rocks showing clear evidence of solid-state grain-shape changes(Fig. 3). However, several petrologists have asserted that recrystalliza-tion and solid-state grain growth are extensive in granites generally, orare at least common in certain granite varieties (e.g., Drescher-Kaden,1948; Augustithis, 1973; Pitcher, 1979; Hughes, 1982, p. 162; Hibbard,1995, p. 228; Pitcher, 1997 p. 69). For example, Luth (1976, p. 336)stated that “most granitic rocks exhibit textural and mineralogicalfeatures which are less related to the ultimate magmatic origin andearly history than to subsequent sub-solidus recrystallisation.” Thisviewpoint received influential impetus from Tuttle (1952) and Tuttleand Bowen (1958), who inferred that granites consisting of K-feldsparand plagioclase in separate grains were formed by unmixing andrecrystallization of hypersolvus (quartz–alkali feldspar) granite duringslow subsolidus cooling, the process being assisted by water.

ARC National Key Centre forralia.. Vernon).

l rights reserved.

In addition, many petrologists have inferred that K-feldsparmegacrysts in granites and microgranitoid enclaves (“mafic enclaves”)grow in the solid state, though arguments against this hypothesishave been presented in detail by Vernon (1986) and Vernon andPaterson (2002, in press-b). Additional evidence for a magmatic originof K-feldspar megacrysts in granites is presented in Fig. 4.

More recently, Bartley et al. (2004) and Coleman et al. (2005)suggested that recrystallization in granites is capable of obscuring oreven obliterating major intrusion structures, such as internal (e.g.,sheet) contacts. For example, Coleman et al. (2005, p. 1) stated that“thermal models indicate that slow incremental growth can maintainmuch of a pluton at temperatures corresponding to the amphibolitefacies of metamorphism for extended time periods. Prolongedannealing at such temperatures will promote subsolidus texturalmodification that is likely to further obscure any primary intrusivecontacts between intrusive increments.” Similarly, Glazner and Bartley(2006) stated that “recrystallization is a well-established mechanismfor rearranging crystal boundaries and obliterating contacts betweenlithologically identical rocks” in granitic intrusions. In addition,Coleman et al. (2005, p. 18) stated that the structures shown byK-feldspar in the Tuolumne Batholith, California, USA, “are notclearly igneous (and certainly not clearly sedimentary) and havebeen significantly modified by late-stage processes.”

We presume that by “recrystallization”, these authors mean graingrowth, rather than strain-induced recrystallization, as the granites

Fig. 1. (a) Granodiorite from the Half Dome granodiorite, Tenaya Lake, Tuolumne Batholith, California, USA, showing some crystal faces in plagioclase (Pl) and hornblende (Hbl), aswell as oscillatory growth zoning in plagioclase. However, many Qtz/Pl and Pl/Pl grain boundaries are irregular, owing to mutual interference and probably also to contact meltingduring growth (Park and Means, 1996; Vernon et al., 2004). Crossed polars; base of photo 4.5 mm. (b) Aggregate of hornblende grains, showing many crystal faces, in a granodioritefrom the Bathurst Batholith, Dunkeld, New South Wales, Australia. The aggregate shows no evidence of grain-shape changes during cooling. Plane-polarized light; base of photo4 mm. (c) Granite showing evidence of extensive exsolution of albite in K-feldspar (with microcline twinning), with preservation of original magmatic crystal shapes and simplegrowth twinning. Crossed polars; base of photo 4 mm. (d) Aggregate of recrystallized quartz (Qtz), K-feldspar (Kfs) and minor biotite (Bt) in a lower granulite facies metapelite fromBroken Hill, western New SouthWales, Australia. The quartz/K-feldspar boundaries are smoothly curved to lobate, and the quartz inclusions in K-feldspar tend to be rounded. Crossedpolars; base of photo 2.5 mm.

43R.H. Vernon, S.R. Paterson / Lithos 105 (2008) 42–50

under discussion do not show evidence of strong or extensive solid-state deformation, though evidence of minor strain accumulation; forexample, subgrains in quartz (Fig. 2c), minor recrystallization ofquartz, and minor grain-boundary migration in feldspar may bepresent locally. Presumably the driving force for the putative graingrowth is meant to be the minimization of interfacial free energy,rather than strain energy.

To test the hypothesis of extensive subsolidus grain-shape changesduring cooling, we discuss microstructures of granites and felsicgneisses (Figs. 1, 2, 3) from several areas, with emphasis on theTuolumne Batholith, Sierra Nevada Batholith, California, USA, which hasbeen used as a specific example by recent proponents of the hypothesis.Because the hypothesis refers to granites that are essentially unde-formed, it excludes solid-state grain-shape changes (neocrystallizationand recrystallization) that occur during intrusion-promoted carapacedeformation (Vernonet al., 2004; Johnsonet al., 2004) andduring strongregional deformation, such as in the production of augen gneisses andmylonites. We also assume that the hypothesis excludes intragranularsolid-state changes in granites, such as exsolution (e.g., Lee et al., 1995;Parsons and Lee, 2005), as well as features promoted by deformationinvolving small strain accumulations, namely microcline twinning,myrmekite and subgrains in quartz, discussions about the origins ofwhichhavebeen summarizedbyVernon (2004). Also excluded are rocksshowing evidence ofmetasomatism, such as A-type granites affected byaddition of Na, B and F (Pitcher, 1997, pp. 263–4), and rocks affected by

deuteric alteration processes, such as greisening and tourmalinization(e.g., Huang, 1962, p. 97).

2. Testing the hypothesis that major grain-shape changes occurduring slow cooling

2.1. Grain shapes

Solid-state grain growth and grain-boundary adjustment typicallyproduce polygonal quartz and feldspar grain shapes in metamorphicrocks (Voll, 1960; Kretz, 1966; Vernon, 1968, 1976, 1999, 2004), and sosimilar grain shapes would be expected if granites recrystallize duringcooling. However, though hypersolvus granites and some syenitesmay show anhedral, roughly polygonal shapes of mutually imping-ing K-feldspar grains, and aplites commonly show anhedral grainshapes (owing to simultaneous crystallization and consequentcompetition for space involving quartz, K-feldspar and plagioclase),granite/granodiorite microstructures do not resemble those of meta-morphic rocks. Instead, they are characterized by elongate crystals ofplagioclase with euhedral growth zoning and primary twins, com-monly with interstitial quartz and K-feldspar, though the grainboundaries are commonly irregular (Figs. 1a, 2c). These irregularitiescould be due to (1) mutual interference of simultaneously growingminerals, (2) contact melting during growth in the magma (Park andMeans, 1996; Vernon et al., 2004), and/or (3) mild solid-state

Fig. 2. (a) K-feldspar porphyroblast in amphibolite facies regional metamorphic granofels, Baja California, México, showing predominantly rounded inclusions of quartz andplagioclase distributed at random, together with elongate inclusions of biotite in dispersed grains and small aggregates. These inclusion shapes and distributions do not resemble theregular concentric crystallographic arrangement of euhedral inclusions in igneous K-feldspar shown in (d). Crossed polars; base of photo 4 mm. (b) Monzonite from the Tilba area,south-eastern Australia, showing euhedral crystals of plagioclase and hornblende enclosed in a large grain of K-feldspar (Kfs), with no sign of subsolidus grain-shape changes. Crossedpolars; base of photo 1.75 mm. (c) Euhedral inclusions of plagioclase (Pl) in hornblende (Hbl), and crystal faces of plagioclase against K-feldspar (Kfs) in a granodiorite from the KunaCrest lobe of the Tuolumne Batholith, showing no evidence of grain-shape changes during cooling. The quartz shows subgrains, resulting frommild solid-state deformation. Crossedpolars; base of photo 1.5 mm. (d) Euhedral inclusions of plagioclase aligned parallel to oscillatory zoning in K-feldspar megacryst in porphyritic quartz monzonite form theSimilkameen area, Alaska. This microstructure is characteristic of K-feldspar phenocrysts that have grown in a magma (Vernon, 1986). Sample by courtesy of Andrew Buddington.Crossed polars; base of photo 4 mm.

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deformation during cooling. Sorby (1877) suggested that theirregular outlines of quartz grains in many granites are “due to themutual interface of the imperfectly grown crystals, which could notdevelop their true crystalline planes of any considerable extent.” Thisstill appears to be a reasonable interpretation.

All minerals in granitic magmas (apart from those previouslyeliminated or armoured by reaction relationships, such as orthopyr-oxene in S-type granites) crystallize to the solidus and so impinge oneach other, which means that grain-boundary configurations dependon the relative interfacial free energies of the minerals and theirorientations. Minerals characterized by low-energy crystal faces (e.g.,hornblende and biotite) may retain crystal faces after impingement(Fig. 1b), whereas quartz and feldspar commonly develop high-energyboundaries thatmay be irregular, at least on themicroscope scale, eventhough zoning patterns commonly indicate euhedral growth from theliquid (e.g. Vernon, 2004, p. 102), as shown in Figs. 1a, 2d and 4a.

The suggestion of Tuttle (1952) and Tuttle and Bowen (1958) thatseparate grains of K-feldspar and plagioclase in granites may be due toextreme exsolution from alkali feldspar conceivably might apply toCa-poor alkali granites and syenites, especially in the later stages ofcrystallization, but is unlikely to apply to more calcic granites, whichcan precipitate plagioclase and K-feldspar simultaneously (Tuttle,1952; Tuttle and Bowen, 1958). Moreover, the common presence ofmicroperthite in alkali granites is an indication of the limited extent ofsolid-state diffusion in granites (Tuttle, 1952), and even whereunmixing is extensive, original crystal shapes are typically preserved

(Fig. 1c). Therefore, feldspar unmixing is unlikely to cause majormicrostructural changes. Moreover, shallowly-emplaced, relativelyrapidly cooled granites have feldspar microstructures similar to thoseof larger or deeper batholiths.

2.2. Inclusion shapes

Solid-state grain growth typically produces rounded shapes ofinclusions of quartz in feldspar, feldspar in quartz, and feldspar infeldspar (Kretz, 1966; Vernon, 1968, 1976, 1999, 2004), as shown inFigs.1d and2a. However, granites typically showeuhedral inclusions ofplagioclase in K-feldspar (Fig. 2b, c, d), plagioclase in quartz (Fig. 2c),plagioclase in hornblende (Fig. 2c), plagioclase in biotite, hornblende inK-feldspar (Fig. 2b) and quartz in K-feldspar. Evenwhere irregularitiesin the outlines of plagioclase inclusions in K-feldspar are produced byexsolution of albite from the host K-feldspar (e.g., Voll, 1960; Hibbard,1965; Vernon, 1986), the internal zoning shows that the plagioclaseinclusions grew as euhedral crystals, and that their general elongate,euhedral shapes have not been changed by intergranular solid-stateprocesses.

2.3. Truncation of zoning

Agood test of the hypothesis that extensive grain-shape changes occurduring cooling is to examine growth zoning in plagioclase or K-feldspar.If recrystallization is so extensive as to obliterate major structures,

Fig. 3. (a) Granodiorite from theWyangala Batholith, New SouthWales, Australia, showing extensively recrystallized quartz (Qtz) and deformed biotite (Bt), caused by strong regionaldeformation, probably during cooling. Despite the strong solid-state deformation, the general igneous shapes of plagioclase grains (Pl) are preserved, and wholesale grain-boundary“sweeping” has not taken place. Crossed polars; base of photo 7.5. mm. (b) Elongate aggregate of recrystallized biotite (Bt), forming an incipient foliation, in a weakly deformedgranodiorite from the Bathurst Batholith, Dunkeld, New South Wales, Australia. Partly crossed polars; base of photo 2.5 mm. (c) Upper amphibolite facies felsic gneiss, Broken Hillarea, New SouthWales, Australia, showing polygonal grains of quartz (Qtz), plagioclase (Pl) and garnet (Grt) with smoothly curved to broadly lobate boundaries, together with minorbiotite (Bt) and sillimanite needles (top-right). Crossed polars; base of photo 4.4 mm. (d) Amphibolite facies felsic gneiss, Grenville Province, Ontario, Canada, consisting of polygonalgrains of quartz (Qtz) and plagioclase (Pl), with elongate grains of biotite. From Vernon (2004, Fig. 4.28). Crossed polars; base of photo 1 mm.

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such as internal granite contacts, a mechanism for removing primarymagmatic grains and replacing themwith newgrainswhilemaintaininga coarse grainsize is required. For example, “diffusion creep in thepresence of melt involves dissolution and re-precipitation, whichdestroy old mineral zoning and favour equilibration of grains with aninterstitial melt” (Berger and Rosenberg, 2002, p. 416). A suitable high-temperature solid-state process would be grain-boundary migrationrecrystallization (e.g., Urai et al., 1986; Vernon, 2004, pp. 338–342), butthis requires very high temperatures in dry rocks — around 1000 °C forfeldspar, judging from observations of some recrystallized anorthosites(Lafrance et al., 1995,1998), which, for quartz at least, can be lowered toamphibolite facies temperatures if water is present (Mancktelow andPennacchioni, 2004). Perhaps this could be a major recrystallizationprocess at granite near-subsolidus temperatures, especially in thepresence of water. The process could produce major microstructuralchanges, as grain boundaries sweep across and transform adjacentgrains, formingstrongly lobate grainboundaries, suchas those shownbythe metamorphic rock illustrated in Fig. 1d.

However, in such a process, compositional zoning would beremoved from grains that are completely replaced by new grains,and would be truncated by new grain boundaries in partly replacedgrains. This grain-boundary “sweeping”would have to be extensive oreven complete (leaving no zoned remnants) if recrystallization wasadvanced enough to obliterate major internal pluton structures, asinferred by Coleman et al. (2005). For example, delicate oscillatoryzoning in quartz (revealed in cathodoluminescence images) andplagioclase would be removed by this process.

Even in the absence of high-temperature grain-boundary “sweep-ing”, less spectacular solid-state grain-boundary adjustment would still

truncate zoning patterns. However, oscillatory zoning (especially inplagioclase) is common in granites, and evidence of zoning truncation ingranites is generally unobserved, apart from (1) internal zoningtruncations caused by remelting in response to changed magma con-ditions during a crystal's growth, as opposed to truncation by a currentgrain boundary, and (2) local examples of truncation by adjacentplagioclase grains, inferred to be caused by contact melting duringmagmatic crystallization (e.g., Vernon et al., 2004, Fig. 30); the presenceof cores in both plagioclase grains is required before a confidentinterpretation involving this process canbemade (Vernonet al., 2004). Infact, compositional zoning in plagioclase is typically preserved almost tothe edges of crystals, only the outer zones being indented (Fig. 1a).

The common preservation of complete oscillatory growth zoning(Fig. 1a), even in plagioclase phenocrysts adjacent to groundmass withlargely anhedral quartz and feldspar in porphyritic granites, indicatesthat plagioclase crystals in granitic magma grow in contact with liquidfor nearly all their history, and that little or no subsolidus grain-shapeadjustment occurs. This interpretation also applies to zoned K-feldsparin granites (e.g., Vernon, 1986). Oscillatory zoning would not occur atall as a result of the improbable exsolution–recrystallization processpostulated by Tuttle (1952) and Tuttle and Bowen (1958), becauseexsolution would produce a single plagioclase composition.

In addition, euhedral inclusions of plagioclase arranged concen-trically in crystallographically related zoning patterns are commonlypreserved in K-feldspar megacrysts in granites (Hibbard, 1965;Kerrick, 1969; Vernon, 1986, 2004), as shown in Fig. 2d, confirmingnot only that the megacrysts grow in a liquid, but also that no solid-state grain-boundary “sweeping” takes place. Even where strongsolid-state deformation of granites occurs, producing recrystallization

Fig. 4. Field photos of K-feldspar megacrysts in the Tuolumne Batholith. (a) Zoned K-feldspar megacrysts in the Cathedral Peak granodiorite unit in the northern Toulumne Batholith.6.5 cm-long “Chapstick” tube for scale. (b) Zoned, euhedral K-feldspar megacrysts in the Cathedral Peak unit cut by late aplitic veins, indicating that the megacrysts grew to theirpresent sizes and shapes prior to late melts migrating into veins. 6.5 cm-long “Chapstick” tube for scale. (c) Clusters of K-feldspar megacrysts in the Cathedral Peak unit near a block(cognate enclave) of schlieren banding and a few K-feldspar megacrysts. Euhedral K-feldspar megacrysts are aligned in the schlieren layers and cut by a late aplite vein. Theseobservations suggest that euhedral megacrysts grew and were physically aligned parallel to the schlieren layers, after which the entire block broke off, was engulfed by thegranodiorite magma, and was finally cut by veins of late melt. The K-feldspar megacrysts show no evidence of late growth. (d) Rounded xenolith of the 88–86 Ma Cathedral Peakgranodiorite in the 85 Ma Johnson granite porphyry. The xenolith contains a large K-feldspar megacryst, indicating that this megacryst grew in the Cathedral Peak unit, after which afragment of the unit was remelted, up to but not including the megacryst, in the slightly younger Johnson granite magma. 6.5 cm-long “Chapstick” tube for scale.

46 R.H. Vernon, S.R. Paterson / Lithos 105 (2008) 42–50

of quartz and biotite, plagioclase grains commonly retain their igneousgeneral shapes (Fig. 3a).

Other primary magmatic structures typically preserved in granitesare growth twins in K-feldspar (Fig. 1c), plagioclase and hornblende(Fig. 1b), patchy zoning, graphic intergrowths, synneusis aggregates,and evidence of discontinuous reactions (e.g., clinopyroxene rimmedby hornblende).

2.4. Mildly deformed granite

Marked solid-state grain-shape changes in granites occur inresponse to deformation (e.g., Voll, 1960; Vernon, 1976; Berthé et al.,1979; Hanmer, 1982; Vernon et al., 1983; Choukroune and Gapais,1983; Vernon and Williams, 1988; Simpson and Wintsch, 1989;Tobisch et al.,1991; Schulmann et al., 1996; Passchier and Trouw,1996;Vernon, 2000; Vassallo and Vernon, 2000; Vernon and Paterson,2002; Vernon, 2004; Vernon et al., 2004). Microstructural changes instrongly deformed granites include recrystallization/neocrystalliza-tion of plagioclase, biotite and quartz, marked grainsize reduction,development of foliations, and truncation of zoning patterns adjacentto developing folia (e.g., Vernon et al., 2004, Fig. 11).

To evaluate the effects of weak solid-state deformation inessentially “massive” intrusive granites (i.e., those without a solid-state foliation), we have examined the mildly deformed DunkeldGranodiorite, Bathurst Batholith, New South Wales, Australia. Thisgranite shows evidence of solid-state deformation in the form ofsubgrains in quartz, strain-induced migration of quartz–quartz and

quartz–feldspar grain boundaries, patchy formation of microscopicmicrocline twinning, local development of myrmekite, and evenmorelocal development of discontinuous folia of recrystallized biotite(Fig. 3b) that are similar to those described by Vernon et al. (2004) forintrusion-related carapace deformation in the San José pluton,México.Nevertheless, oscillatory zoning in plagioclase, euhedral shapes ofhornblende (Fig. 1b) and titanite crystals, and euhedral shapes ofplagioclase, quartz and biotite inclusions in K-feldspar have all beenpreserved, indicating that the degree of grain-boundarymigrationwaslimited.

2.5. Structure of felsic gneisses

Coleman et al. (2005) suggested that their inferred recrystallizationof coolinggranite occurs atpressures and temperaturesequivalent to theamphibolite metamorphic facies, and so it is worth examining felsicgneisses that have been recrystallized in amphibolite facies terranes, tosee what structural features are indicative of recrystallization at theseconditions. Amphibolite facies felsic gneisses are characterized by thefollowing features (Figs. 1d, 2a, 3c, d): (1) xenoblastic, polygonal toirregularly shaped grains of quartz and feldspar, commonly withoutappreciable grain elongation (Figs. 1d, 3c, d); (2) smoothly curved tobroadly lobate grain boundaries (Figs. 1d, 3c, d); (3) rounded inclusionsof quartz in feldspar (Figs. 1d, 2a) and vice versa; (4) absence or rarity ofidioblastic quartzor feldspar grains, and (5) absenceof oscillatory zoningin feldspar. Thesemicrostructural features are completely different fromthose shownby typical “massive” granites (i.e., thosewithout solid-state

Fig. 5. Field photos of contacts in the Tuolumne Batholith. (a) Contact between the Cathedral Peak (CP) and Half Dome (HD) granodiorites. This contact can be mapped for more than100 km. We interpret this contact as representing the juxtaposition of two different pulses of magma in the chamber. 15 cm ruler for scale. (b) Graded schlieren layers that can befollowed laterally for no more than tens of metres and do not separate distinct magmas. We interpret these contacts as forming by flow sorting during internal magma movementwithin a single batch of magma. 15 cm ruler for scale. (c) Schlieren-bound troughs in the Half Dome granodiorite near the eastern margin of the Tuolumne Batholith. These troughstruncate one another (indicating episodes of erosion), show mineral grading, are cut by magmatic faults, are deformed by magmatic folding, and commonly have K-feldsparmegacrysts aligned along the trough boundaries. We interpret these structures as reflecting local flow channels formed by magma movement along boundaries between fairlyviscous magmas. A number of subtle features are preserved in this outcrop, and so have not been obscured by grain-shape changes. Map board for scale. (d) Local schlieren-boundtroughs re-intruded by Cathedral Peak granodiorite. K-feldspar megacrysts are aligned in the troughs and concentrated in a funnel-shaped accumulation (see arrow), which weinterpret as a K-feldspar megacryst batch that rose from the trough region into the Cathedral Peak granodiorite magma. Photo ca. 3 m wide.

47R.H. Vernon, S.R. Paterson / Lithos 105 (2008) 42–50

foliations), including the ones specifically referred to by Coleman et al.(2005).

The reason for the microstructural contrast between granites andfelsic gneisses is probably connectedwith thedeformation andprogrademetamorphic reactions that accompany regional metamorphism, boththese factors contributing to nucleation and growth of new grains. Incontrast, the minerals remain chemically compatible as granites cool(apart from exsolution and deuteric or later hydrothermal changes,evidence of which is easily discernable), and deformation is not a majorfactor in the “massive” granites under discussion.

2.6. Internal contacts in plutons

As noted previously, Coleman et al. (2005, p. 1) suggested thatrecrystallization in granites is capable of obliterating major intrusionstructures, such as internal contacts, which can provide informationabout chamber construction and/or internal chamber processes (e.g.,Zák and Paterson, 2005), as shown in Figs. 5 and 6. Four possible typesof internal boundaries in granite plutons are defined by juxtaposedunits with: (1) distinct compositions and microstructures (Fig. 5),(2) the same compositions, but different microstructures (Fig. 6a),(3) similar microstructures, but distinct compositions; and (4) iden-tical compositions and microstructures. In our experience, type 4contacts are uncommon, since even contacts between essentiallyidentically batches of magma are marked by (a) slight microstructuralvariations if the juxtaposed magmas were at two different tempera-

tures, (b) slight modal changes in minerals, possibly due to flowsorting (Barrière, 1981), as shown in Fig. 5a, and/or (c) the presence ofmicrogranitoid enclaves or xenoliths trapped along the contact(Fig. 6d). The four types of internal contact may form by addition ofnew pulses into a chamber (Fig. 5a), by localized flowofmagmawithinan existing chamber (Fig. 5b, c, d), and by processes during crystal-lization, such as crystal–liquid fractionation (Fig. 5b, c, d), the last twoprocesses implying that even recognized internal contacts do notnecessarily imply multiple pulses (e.g., Clarke and Clarke, 1998).

In view of the evidence of grain shapes, inclusion shapes andoscillatory zoning discussed previously, intergranular changes would beunlikely to be capable of removing the compositional ormicrostructuralevidence of the type 1 to type 3 contacts. Only in the unusual situation,where internal contacts juxtapose compositionally identical phases andare not marked by other objects or microstructural changes mightintergranular changes modify the contact enough to make it evenmorecryptic, though evidence of such a process has not yet been presented.

One test of the contact-obliteration hypothesis is to determine(1) whether internal contacts, or gradations in composition or mag-matic microstructures between contacts are preserved in a pluton(implying that insufficient recrystallization occurred to remove thesefeatures), and (2) whether these features are laterally continuous(increasing the likelihood that the contact separates two magmabatches) or gradually disappear along strike (increasing the likelihoodthat these contacts reflect processes in one magma batch). Anothertest would be to see whether magmatic foliations and lineations

Fig. 6. Field photos of internal magma contacts in Sierran and Cascades plutons, USA. (a) Contact between two compositionally similar but structurally different granodiorites in theJackass Lakes pluton, Sierra Nevada, California, USA. 15 cm ruler for scale. (b) Internal layering in the ∼9 kb Tenpeak pluton, Cascades, Washington. Person for scale. (c) Internallayering in the ∼7 kbar Entiat pluton, Cascades, Washington, USA. Rock hammer for scale. (d) Internal layering in the Entiat pluton, showing a host-rock xenolith along an internalcontact. 15 cm ruler for scale. The layering in (c) and (d) are sharp andwell preserved, despite the fact that these plutons remained at amphibolite facies conditions for several millionyears.

48 R.H. Vernon, S.R. Paterson / Lithos 105 (2008) 42–50

defined by the alignment of minerals with magmatic microstructuresoverprint internal contacts, implying that magmatic processes, in-cluding crystal growth and alignment, continued to occur after forma-tion of the contacts, but without obliterating them. The followingobservations on granites of the Sierra Nevada Batholith, California,USA, and the Cascades core, Washington, USA, are relevant.

(1) Bateman and Chappell (1979), Bateman (1992) and Zák andPaterson (2005) noted (a) the preservation of distinct contactsbetween the four main composition units of the Tuolumne Batholith(Fig. 5a), (b) that these contacts can be followed for tens of kilometres(Fig. 5a), and (c) that these contacts can be continuously mappedthroughout the batholith. The contacts locally preserve evidence ofmingling/mixing and local flow of magma, resulting in accumulationsof minerals or enclaves (Fig. 5b, c, d). Some knife-sharp contacts can bemapped, at the mineral scale, for many kilometres. None of thesefeatures has been masked by intergranular grain-shape changes.

(2) The same authors also noted a remarkably consistent and fairlysymmetrical pattern of internal compositional, microstructural andchemical gradations between these visible contacts in the TuolumneBatholith, an observation easily explained by magmatic processes, butrequiring a unique pattern of intergranular changes or recrystallization,if formed by subsolidus processes. This symmetric pattern of zonationand preservation of laterally continuous internal contacts occurs notonly in themain batholith, but also in four lobes extending out from themain batholith (Economos et al., 2005; Memeti et al., 2005, 2006).

(3) Wahrhaftig (1979), Bateman (1992), Reid et al. (1993), Patersonet al. (2004), Zák and Paterson (2005) and Vernon and Paterson (inpress-a) have described numerous, schlieren-bound troughs, tubes,pipes local diapirs, mineral clusters and local layered zones, all of

which preserve distinct contacts with the surrounding granites (Fig.5b, c, d). Our recent mapping indicates that the Tuolumne Batholithcontains thousands of these features, and that they are widelydistributed, indicating that intergranular changes were not importantenough to mask any of these contacts throughout the batholith.

(4) Teruya and Miller (2000), Paterson et al. (2003) and Zák et al.(2007) have observed that both the main internal contacts (Fig. 5a) andthe local features described above (Fig. 5b, c, d) are commonlyoverprinted by one or more magmatic foliations. These foliations aredefined by grains showing typical magmatic microstructures (euhedralto subhedral crystal shapes, synneusis, internal zoning, growth twins,euhedral inclusions), indicating little or no grain-shape changes of theminerals defining these fabrics. Thus, these observations indicate thatboth internal contacts and overprinting magmatic structures wereformed above the solidus, and have not been appreciably altered byyounger intergranular changes.

(5) The slightly older (97–98 Ma) Jackass Lakes pluton, immedi-ately south of the Tuolumne Batholith (McNulty et al., 1996; Pignottaet al., 2005), was emplaced at the same crustal level as the TuolumneBatholith, and preserves widespread internal contacts at the metre tokilometre scales (Fig. 6a). These internal contacts are sharp and easilyfollowed, indicating that intergranular changes did not alter them.This pluton is an excellent example of the formation and preservationof many internal contacts.

(6) Paterson andMiller (1998) described extensively sheeted, deeper(c. 6–7 kbar) tonalitic plutons in the Cascades core,Washington, USA. Inthese plutons, (a) internal contacts are readily apparent, (b) disaggre-gated and locally rotated enclaves (xenoliths and microgranitoidenclaves) occur along the contacts, and (c) observations show very little

49R.H. Vernon, S.R. Paterson / Lithos 105 (2008) 42–50

or no evidence of grain-shape changes (Fig. 6b, c, d). Thus, even in thesedeeper (20–40 km) and hotter (tonalite versus granodiorite) plutons,intergranular changes are notwidespread, and internal contacts arewellpreserved, being continuous over hundreds of metres to kilometres,even between sheets with similar compositions. These intrusions showthat igneous microstructures can be preserved even in granites em-placed at amphibolite facies conditions.

3. Conclusions

The common preservation of euhedral inclusions and especiallyoscillatory mineral zoning in the minerals of undeformed granites isstrong evidence that extensive subsolidus grain-shape changes do notoccur during slow cooling of granitic magmas. This is true even for thegranites that are emplaced at amphibolite facies conditions. Conse-quently, magmatic structures, such as internal boundaries, schlierentroughs and magmatic fabrics, are unlikely to be obliterated byintergranular changes, unless subsolidus tectonic deformation occurs.In the plutons we have examined, including the Tuolumne Batholith,these internal structures, formed during both the construction of theplutons and subsequent internal flow, are well preserved, and showno evidence of removal by microstructural changes.

Acknowledgements

We thank Cal Barnes, Barrie Clarke and Sheila Seaman for criticallyreviewing the typescript, and Valbone Memeti for helpful commentsand for providing some figures. SRP acknowledges financial supportfrom NSF grants EAR-0073943 and EAR-0537892.

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