Zoning in Igneous Plagioclase: Patchy Zoning

8/18/2019 Zoning in Igneous Plagioclase: Patchy Zoning

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Zoning in Igneous Plagioclase: Patchy ZoningAuthor(s): Joseph A. VanceSource: The Journal of Geology, Vol. 73, No. 4 (Jul., 1965), pp. 636-651Published by: The University of Chicago PressStable URL: http://www.jstor.org/stable/30069386 .Accessed: 01/11/2013 02:39

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ZONING INIGNEOUS

PLAGIOCLASE: PATCHY ZONING'

JOSEPH A. VANCEUniversity of Washington

ABSTRACT

Many igneous plagioclases exhibit irregular zoning consisting of corroded cores embayed and surroundedin crystallographic continuity by more sodic plagioclase. Inclusions of this sodic plagioclase within the cor-roded cores give the feldspar a mottled appearance and suggest the term patchy zoning for these textures.Textural evidence indicates that patchy zoning is the result of a two-stage replacement process involvingpartial resorption of early plagioclase crystals followed by crystallization of a more sodic plagioclase.Poikilitic inclusions commonly present within the patches of sodic plagioclase are believed to have crystal-lized from the melt trapped in the cores after corrosion.

Patchy zoning is interpreted here in terms of decrease in confining pressure on water-deficient magma

during its rise in the crust. The sequence of development is visualized as follows: (1) crystallization ofplagioclase at depth; (2) partial resorption of the plagioclase related to fall in pressure; (3) renewed crystal-lization at lower pressure, necessitating development of a more sodic plagioclase.

Zoning of this type is believed to be a diagnostic igneous textural feature indicating that the magmacontained a crystalline phase and was neither superheated nor saturated in its volatile components whenupward displacement initiated resorption.

I. INTRODUCTION

Plagioclase zoning comprises a uniquerecord of past changes in magmatic environ-ment which can provide critical evidence onseveral fundamental problems of magmatic

evolution. This paper is concerned with thesignificance of patchy zoning, a featurewhich, though widespread in igneous plagio-clases, has been ignored by many petrolo-gists. The first part of the paper is largelydescriptive and deals with evidence that thesetextures are related to magmatic resorption.The remainder of the paper is concernedwith the mechanics of the resorption process,the possible causes of the resorption, and thebroader petrogenetic implications of these

textures. The effect of decrease in pressureon plagioclase-melt equilibria and the gen-eral problem of magmatic resorption andcorrosion are considered in some detail.

II. DESCRIPTION

The plagioclase of many igneous rocksshows complex zoning consisting of corrodedcores filled and surrounded in crystallo-graphic continuity by plagioclase of more

1 Manuscript received July 20, 1964; revisedJanuary 12, 1965.

sodic composition (figs. 1-18). Isolated, ir-regular inclusions of plagioclase in opticalcontinuity with the rim impart a character-istic mottled appearance to the crystalsbetween corssed nicols. In the followingdiscussion these features are referred to bythe term patchy zoning. The sodic materialof the rims and of the patchy inclusions isdesignated respectively as the rim plagio-clase and the included plagioclase. Weaklycorroded material shows sparse develop-ment of isolated small inclusions and onlyslight embayment of the core, while in morehighly corroded plagioclases, characterizedby extensive enlargement and coalescence ofthe inclusions, the core survives only as anirregular, sievelike framework. The outer-most portions of the core tend to be less cor-roded than the interior and only in intenselycorroded material is the boundary betweencore and rim markedly anhedral (figs. 13and 16).

Typically the included plagioclase iseither homogeneous or has weak normal zon-ing, but exceptionally it shows delicateoscillatory zoning. The writer's observationssuggest that the development of patchyzoning, in particular the zonal compositionalvariation and to a lesser extent the intensity

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PATCHY ZONING

of corrosion, is commonly rather uniform inthe plagioclase of a given magmatic body.The plagioclase in some igneous bodiescharacteristically shows two or more distinct

stages of corrosion and patchyzoning

(seefigs. 7, 8, and 13).In sections cut parallel to (010) the in-

cluded plagioclase typically forms irregular

to subhedral patches elongate parallel to thea-crystallographic axis. In sections normalto a the inclusions tend to be subhedral,showing quasi-rectangular forms, bounded

by (001) and (010) and most commonlyelongate parallel to (010). In three dimen-sions, the inclusions form irregular platesparallel to (010) and elongate parallel to a.In some rocks a part of the included plagio-clase occupies narrow, irregular fractures inthe core (fig. 10).

Patchy zoning is exceedingly widespreadin magmatic plagioclase, especially in thecommon plagioclase-rich volcanic and plu-tonic rocks of calc-alkaline affinity. Numer-

ous references to this general group of tex-tures may be found in the literature, mostlyas brief notes in general petrographic de-scriptions. Among the many workers whohave specifically discussed this texture in

FIG. 1.-Quartz diorite, Black Peak batholith,near Rainy Pass, Washington. X22. Sec. II 010).Incipient patchy zoning, patches elongate || (001).An40o An34 Anas + An23. Parallel notation is usedfor all the sketches. An4o - An34 shows the range ofzoning of the core. An28 * An23 s the compositional

rangeof

therim

plagioclase. The difference betweenAn34 and An28 corresponds approximately to thecompositional gap between core and rim [rim andincluded plagioclase, stippled; core, without stippling;plagioclase with two stages of corrosion: intermedi-ate zone, heavily stippled; rim, lightly stippled;poikilitic inclusions, black].)

FIG. 3.-Quartz diorite, Squire Creek pluton,near Darrington, Washington. X 14. Ana4 - An331An24 - An1s. Sec. || (010). Moderate corrosion,patches elongate || (001).

FIG. 2.-Granodiorite, Chilliwack complex,Washington. X21. An49 An34 Ani8 - Anl2. Sec. 1(010). Carlsbad synneusis twins. Incipient patchyzoning, patches elongate 11 001).

FIG. 4.-Granodiorite, Cathedral batholith,Okanogan County, Washington. X 10. An27An24 An17 An3. Sec. 1 (010). Two grains in paral-lel synneusis relationship. Moderate corrosion,patches elongate || (001).

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JOSEPH A. VANCE

plutonic rocks are Becke (1892), Holmquist(1906, p. 157), Gillson (1932, p. 33), Larsen(1948), Callegari (1958, 1963), Yeats (1958,p. 205), Karl (1959, p. 172), Snelling (1960,p.

194), and Vance(1962).

References tosimilar textures in hypabyssal and volcanicrocks include those of Graeff and Brauns(1893, p. 129), Homma (1936, p. 138), Kuno(1936, p. 126), Larsen, Irving, Gonyer, andLarsen, (1938, p. 229), Fuster (1954), Fusterand Ibarrola (1956), Ogniben (1956, 1964),and Scharbert (1957, p. 155).

In most rocks showing well-developedpatchy zoning some of the plagioclase crys-tals contain tiny poikilitic inclusions of

other minerals (figs. 6, 11-18). These in-clusions are generally rounded or irregular

FIG. 5.-Granodiorite, Grotto batholith, nearMonte Cristo, Washington. X12. An57 An461An38 - An31si. ec. 11 010). Carlsbad synneusis twins.Moderately strong corrosion, inclusions elongate I(001).

FIG. 6.-Granodiorite, Caulfeild pluton, nearVancouver, British Columbia. X>18. An34 An27An20-+ An14. Sec. jj (010). Moderate corrosion.Poikilitic inclusions of quartz.

in shape and are restricted to the interior ofthe corroded cores where they are in contactwith, and commonly are surrounded by, thesodic included plagioclase. Some vitrophyres

show analogous textures in which small,irregular-to-subrectangular inclusions ofglass occur within the patchy filling of thecorroded plagioclase cores (figs. 15-16).These various types of inclusions are re-ferred to here as poikilitic inclusions.

III. ORIGIN OF PATCHY ZONING

A. MECHANISM OF REPLACEMENT

The literature reveals little agreement asto the origin of patchy zoning, except on twobroad points. First, it is generally believedthat the rocks showing these textures are ofmagmatic origin. This conclusion appears

FIG. 7.--Granodiorite, Dead Duck pluton, WhiteChuck River, Washington. X23. An77 * An74Ans4 - An5 IAn23 An18. Sec. _La. Several broadalbite lamellae. Complex corrosion with two genera-tions of patchy zoning. Patches elongate 11 010).

FIG. 8.--From same specimen as fig. 7. X32.

Sec. || (010). Complex corrosion with two generationsof patchy zoning.

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PATCHY ZONING

justified, since patchy zoning in plutonicrocks does not appear to differ essentiallyfrom similar textures in volcanic rocks ofunquestioned igneous origin. Second, it is

agreed that the sodic plagioclase inclusionshave replaced the more calcic feldspar ofthe corroded core. There is, however, noconsensus as to the mechanism of the re-placement process.

Patchy zoning might develop throughany of three fundamentally different re-placement mechanisms. The first possi-bility is that replacement is the resultof metamorphism (including autometamor-phism) of the rock in the solid state. This

origin is rejected as a general explanation,however, or the following reasons: 1) patchyzoning is prominently developed in manyvolcanic rocks lacking any indication ofmetamorphic or autometamorphic altera-tion; (2) the rather uniform development ofpatchy zoning in certain igneous bodiesseems inconsistent with metasomatic re-placement; and (3) on the basis of texturalevidence considered below.

The second possible mechanism of direct

replacement is by reaction between a mag-matic liquid and earlier plagioclase crystals.This process would involve mutual exchangeof material between the melt and the plagio-clase crystals by diffusion through the crys-tal lattice. This explanation deserves con-sideration because it is in harmony with themagmatic origin of the rocks and becausethe sodic composition of the included plagio-clase and rim plagioclase relative to thecores fits the sequence of Bowen's reaction

series. Formation of patchy zoning by somesuch form of diffusive reaction has beenadvocated by Fuster (1954) and by Fusterand Ibarrola (1956). Such an origin, how-ever, is incompatible with certain texturalfeatures of the patchy zoning. As notedabove, the included plagioclase is in opticalcontinuity with and has the same composi-tion as the inner zones of the rim plagio-clase. The rim plagioclase characteristicallyshows euhedral normal or oscillatory zoning,

features traditionally considered to form bydirect magmatic crystallization. If the rim

plagioclase is magmatic, it seems unreason-able to suppose that the compositionallyidentical included plagioclase is of a differentorigin.

In many plagioclases, moreover, there isclear evidence of the former presence of cor-rosional cavities, ruling out formation ofpatchy zoning by any mechanism of re-placement in the solid state. First, the in-

FIG. 9.-Leucocratic hornblende norite, SanMarcos gabbro, southern California. X 14. Ans4Anso [An67-* An59. Simple albite twinning. Moder-ate corrosion, patches elongate |1 (010).

FIG. 10.-From same specimen as fig. 9. X22.Sec. [I (010). Note the occurrence of included pla-gioclase as fracture fillings. The detail of the includ-ed plagioclase shows oscillatory zoning, which in-dicates crystallization around two centers on the

interior walls of the corroded grain.

cluded plagioclase of many rocks (e.g., fig.10) shows zoning which indicates progres-sive filling of internal cavities by crystal-lization from the walls inward. Second,direct replacement does not explain thecommon association of poikilitic inclusionswith the included plagioclase. Finally, thepresence of a melt phase within the cor-roded cores is indisputable for those volcanic

plagioclases exhibiting glassy poikilitic in-clusions.

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JOSEPH A. VANCE

Once direct replacement is ruled out, it isseen that a two-stage mechanism must beinvolved in the formation of patchy zoning.This process entails partial resorption of theplagioclase followed by crystallization ofmore sodic plagioclase as inclusions withinthe corroded cores and as marginal over-growths. The poikilitic inclusions are simply

FIG. 11.-Quartz monzonite, Mt. Pilchuck, Sno-homish County, Washington. X 28. An41 + An37|An29 - An13. Two grains in synneusis relationship.The section of the larger grain is || (010). Moderatelystrong patchy zoning. Poikilitic inclusions (quartz,black; biotite, hatched). Note the preferential corro-sion along the coalescent boundary of the two grains.

FIG. 12.-Quartz diorite. Toat's Coulee intrusion,Okanogan County, Washington. X 13. An61 An55|An39 * An28. Sec. || (010). Strong corrosion. Poiki-

litic inclusions (quartz, black; biotite and minorhornblende, hatched).

explained by this hypothesis as crystalliza-tion products of the melt trapped within thecorroded cores after resorption (see Homma[1936, p. 153]). A similar origin for patchyzoning appears to be either endorsed or im-plied by several of the investigators citedabove, many of whom speak of corrosion inreference to the origin of patchy zoning. The

two-stage replacement process proposedhere is believed to be compatible, not only

with the textures of patchy zoning, but withthe compositional contrast between coreplagioclase and included plagioclase andwith the geological evidence. To show thisit is necessary to consider in detail the originof the two major textural elements of patchyzoning, the corroded cores, and the moresodic plagioclase of the inclusions and rims.

B. ORIGIN OF THE CORRODED CORES

1. CRYSTALLIZATION OF THE CORES

The cores of most of the plagioclasesstudied show weak normal zoning or regularand delicate euhedral oscillatory zoningsuperimposed on a weak normal trend.

These features and the typical idiomorphicoutlines of the cores suggest crystallizationfrom a melt. Following the interpretation ofHills (1936) and of Vance (1962), the writerbelieves the oscillations to be the result ofrecurrent supersaturation and crystalliza-tion in a water-deficient melt.

The rather weak range of normal zoningdeveloped in the plagioclase cores requirescomment. Inspection of Bowen's diagramfor the system albite-anorthite (1913) would

lead one to expect fractional crystallizationto produce a much wider compositionalvariation than is actually observed in mostnormal zoning. One might, therefore, sup-pose that this zoning is the result, not offractional crystallization, but of incomplete,though close, approach to equilibrium crys-tallization. However, regular and delicateoscillatory zoning is sharply preserved inmany of these same plagioclases, a relationinconsistent with the extensive diffusion in

the solid state, which approach to equilibri-um would require. The weak normal zoningthus appears to reflect fractional crystal-lization after all, but it is clear that someadditional factor has largely counteractedthe strong fractionation anticipated fromconsideration of Bowen's data alone.

One possible explanation has recentlybeen suggested by Wyllie (1963), who citesexperimental data indicating markedly dif-ferent liquidus and solidus slopes and

changes in slope in polycomponent plagio-clase systems than in the simple system

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PATCHY ZONING

albite-anorthite. Another possible factor,although it is difficult to evaluate quantita-tively, is the influence of increasing pressureof volatiles on the form of the liquidus and

solidus during progressive crystallization.Whatever the true explanation, it is clearthat Bowen's data for the simple dry systemat low pressure give little more than a crudeapproximation of the behavior of complexnatural plagioclase systems under the condi-tions prevailing in nature.

FIG. 13.-Granodiorite, Grotto batholith. Fromsame specimen as figs. 5 and 17. X80. Ansi -*Ano An,3 -* An53 An36 + An25. Sec. _la. Severalbroad albite lamellae. Complex corrosion with twogenerations of patchy zoning. Highly corroded innercore preserved only as isolated scraps and as an ir-regular shell. Poikilitic inclusions of augite and somemagnetite.

FIG. 14.-Dacite porphyry, from a Tertiary dike,Suiattle River, Washington. X23. An49 An43An3s * An33. Section of a phenocryst _la. Some

albite twinning. Moderate corrosion with complexchanneling. Poikilitic inclusions of quartz.

2. CAUSE OF RESORPTION

Introduction.-The resorption involvedin the formation of patchy zoning could arisein any of four principal ways: (1) through

equilibrium crystallization, as is possible incertain two-feldspar systems; (2) throughthe influence of water on the plagioclaseliquidus; (3) through increase in tempera-ture; or (4) through fall in pressure in awater-deficient melt. To be acceptable as anexplanation of the resorption in patchyzoning the hypothesis must both agree with

FIG. 15.-Andesite vitrophyre, Ross Pass, east-ern Snohomish County, Washington. X40. Anso +

An481An37--* An3s. Section of a phenocryst la.Simple albite and multiple pericline twinning. Mod-erately strong corrosion. The poikilitic inclusions areglass.

FIG. 16.-Andesite vitrophyre from the previousspecimen. X37. An56 An521 n43 -* Anas. Com-posite phenocryst of two coalescent grains in Carls-bad synneusis relationship. The section of the largergrain is _la. Strong corrosion. In the large individual

the core is preserved as a discontinuous shell withcross-septa 1[ 001). Vitric poikilitic inclusions.

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JOSEPH A. VANCE

the other geologic evidence and account forthe break in composition between core andrim in reasonable terms. Although any oneof these processes could bring about resorp-tion, detailed consideration suggests thatfall in pressure is probably the factor of mostgeneral significance.

Equilibrium crystallization and resorption.-Recent work (Tuttle and Bowen, 1958,p. 134: Stewart and Roseboom, 1962, p.308) has shown that certain liquids in two-feldspar systems will crystallize early plagio-clase, which, under equilibrium conditions,may be partially or completely resorbedduring later crystallization of alkali feldspar.

This process, however, cannot be responsiblefor the resorption associated with patchyzoning. It is inapplicable to those manyrocks that show patchy zoning but do notcontain alkali feldspar. The mechanismexplains neither the renewed crystallizationof plagioclase characteristic of all patchyzoning nor the multiple corrosion found incertain plagioclases. Finally, in many por-phyritic rocks with patchy zoning all thephyric minerals (e.g., plagioclase, alkali feld-

spar, and quartz), not just the plagioclase,show indications of resorption. Resorptionof all phenocryst phases implies disturbanceof equilibrium in the magma as a whole andcan scarcely be the result of progressive andgradual consolidation of the melt underequilibrium conditions.

Influence of water pressure on liquidustemperatures.-The influence of increasingwater pressure in depressing the liquidustemperatures of water-saturated systems is

well known through experimental studies.One implication of this relation is thatsimultaneous increase of pressure and watercontent of a water-saturated magma wouldfavor resorption of any crystals present.Resorption might also occur without neces-sary introduction of water where a magmawith excess water experiences an increasein total pressure. Increases in total pressure,however, would seem to require that themagma be depressed to deeper crustal levels,

an assumption that seems at variance withgeological experience. A further objection

is based on the experimental data of Yoder,Stewart, and Smith (1957, p. 207) on thesystem NaAlSi3O8-CaAl2Si2O8-H20 whichindicate that resorption of plagioclase due to

increasein

water pressure (at essentiallyconstant temperature) would be followedby crystallization of a more calcic, not amore sodic, plagioclase. Analogous reasoningapplies to water-bearing but undersaturatedmagmas, although experimental data arelacking. Here, introduction of water (atconstant total pressure) would tend to bringabout resorption. As in the water-saturatedmagma just considered, however, renewedcrystallization would necessitate formation

of a more calcic plagioclase, a relation con-trary to that found in patchy zoning.Temperature increase and resorption.-

Increase in temperature is perhaps the mostobvious of the several possible factors thatmight bring about magmatic resorption. Itis doubtful, however, that most patchyzoning can have formed in this way, for it isunlikely that rise in temperature is a suffi-ciently common event in magmatic con-solidation to be related to so widespread a

feature. Moreover, consideration of theseveral possible mechanisms of increase intemperature shows that most of these areincompatible with certain of the features ofpatchy zoning. In particular, they fail toaccount for the abrupt break in compositionbetween the more calcic cores and the sodicrims.

Mixing of magmas is one possible way ofincreasing temperature. This would tend tobring about resorption of any phenocrysts

present in the cooler magma. Two differentsituations must be considered; one in whichneither magma is superheated, the other inwhich the hotter magma is superheated. Thefirst case, probably the more common one,is complex and will not be discussed in de-tail here. It is sufficient to note that suchmixing commonly produces distinctive min-eralogical complexities, notably the associa-tion of plagioclase phenocrysts with strik-ingly different composition and zoning and

the association of strongly corroded crystals(those of the cooler magma fraction) with

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PATCHY ZONING

others (from the hotter magma fraction)which are not corroded. This general processcould involve magmas of entirely differentcomposition or magmas of the same com-

position but in different stages of consolida-tion and at different temperatures, as wheredifferent portions of a single magma bodyare mixed during emplacement (Milch,1905). Rocks with such a history have beendescribed by Kuno (1936), Larsen et al.(1938), and Ogniben (1956) and apparentlyare not rare in certain volcanic environ-ments. However, inasmuch as the distinc-tive characteristics of this process are absentin most rocks showing typical patchy zoning

and simple patchy zoning is not consistentlydeveloped in rocks with a history of mixing,the process must be rejected as a generalexplanation.

If we turn instead to mixing of magmas,

one of which is superheated, there is a pos-sible combination of factors through whichpatchy zoning might arise. A superheated,more albitic melt mixed with a cooler, more

anorthitic magma could lead to partial re-sorption of plagioclase crystals in the latterand, upon cooling, to precipitation of moresodic plagioclase. Although this process ac-counts for all the features of patchy zoning,it can scarcely be the usual mode of forma-tion of these textures. As a general explana-tion, this hypothesis would imply that eachrock showing patchy zoning is the product oftwo or more initial magmas-one super-heated and of more albitic composition, the

other more anorthitic and without super-heat-which somehow had been broughttogether and mixed at precisely the rightmoment in their cooling histories. There is,however, no independent evidence to sug-

FIG. 17.-Granodiorite, Grotto batholith. From the same specimen as figs. 5 and 14. X23. An58 + AnsAn43 An27. Sec. Ia. Strong corrosion. Patches elongate 11 010). Detail of the interior of the grain showspoikilitic inclusions of antiperthitic orthoclase.

FIG. 18.-Granophyre dike, Middle Fork Snoqualmie River, Washington. X44. Anso -* An28 An24Anis. Section of a plagioclase phenocryst _la. Albite twinning. Complex corrosional channeling. The detailedsketches show the association of quartz (black) and orthoclase (no pattern) as poikilitic inclusions in theincluded plagioclase (stippled).

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JOSEPH A. VANCE

gest that most rocks showing patchy zoninghave any such complex history. The im-probability of this explanation is compound-ed by those rocks showing two or morestages of patchy zoning.

The temperature of phenocrysts mightalso be raised by sinking of the crystals intosubjacent hotter melt (Homma, 1936, p.138). Snelling (1960, p. 206) advocates sucha mechanism to account for patchy zoningin the plagioclase of certain Australiangranites. Resorption could occur by thisprocess, provided that temperature increasewith depth were enough to offset elevationof the melting point with pressure and that

sinking were rapid enough that simultane-ous loss of heat to the environment did notlead to crystallization. The writer, however,is not aware of any case in which sinking ofeither quartz or feldspar phenocrysts in agranitic melt has been convincingly demon-strated. Presumably, sinking of crystals isunlikely in normal water-deficient (under-saturated) silicic melts, which are knownfrom experimental studies (e.g., Burnham,1964) to be highly viscous. If such gravita-

tive settling were common, accumulationsof quartz and feldspar crystals would beconspicuous in many gently dipping silicicsills and dikes.

Rise in temperature may also result fromrapid crystallization associated with loss ofvolatiles and diminishing pressure (cf. Tut-tle and Bowen, 1958, p. 54). In these water-saturated magmas, however, separation ofvolatiles necessitates continuous crystal-lization in spite of increasing temperature.

Larsen (1948) expressed the view that thecorroded plagioclase cores in certain quartzdiorites of the southern California batholithare xenocrysts. A related hypothesis is thatof Rittmann (1960, p. 223), who regardscorroded crystals in most igneous rocks asrelics of older rocks that have survived in-complete melting and anatexis. Conceiv-ably, patchy zoning could develop by eitherof these processes. The evidence suggests,however, that these hypotheses must be

rejected for most patchy zoning. In manymagmatic bodies the corroded cores appear

to be rather uniformly distributed through-out the mass as an essential element of thefabric. The cores tend to be of uniform com-position and show in their zoning and idio-morphic outlines the imprint of magmaticcrystallization. In general, they are notgrossly incompatible with the rest of themineral assemblage. These relations suggestthat the cores crystallized from a relativelyhomogeneous magma body and are cognaterather than accidental. Features that mightsupport either the idea of incorporation ofxenocrysts or the presence of anatecticrelics (e.g., variation in the composition,zoning, and distribution of the corroded

cores; association with other minerals ofexotic origin; and the presence of xenolithscontaining plagioclase identical to that ofthe cores) are typically absent. It is also tobe noted that partial melting of the sup-posed xenocrysts requires that the magmathat incorporated them be superheated.In some cases this assumption contradictsother evidence. As discussed above, reactionin the solid state cannot be used to explainthe corrosion of the cores, since this process

is incompatible with the textural features ofpatchy zoning.Decrease in confining pressure and resorp-

tion.-The writer considers fall in hydro-static pressure related to upward displace-ment of the magma to be the most impor-tant single cause of magmatic resorption.This concept, although largely ignored inthe current literature, is a venerable one(e.g., Lagorio, 1887, p. 510), based on thefact that in most water-deficient systems the

melting point decreases with falling pres-sure. In magmas containing crystals at theirliquidus temperature rapid decrease in pres-sure at approximately constant temperatureprovokes a displacement of equilibrium thatmust tend to be restored by partial or com-plete resorption.

Carr's thermodynamic data (1954) forthe plagioclases indicate that with increas-ing pressure both the liquidus and the soli-dus are raised relative to Bowen's (1913)

curves at atmospheric pressure. The meltingpoint of albite is displaced more than that of

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PATCHY ZONING

anorthite, so that the slopes of the liquidusand solidus are appreciably lessened. Iso-thermal fall in pressure would thus shiftequilibrium so that both liquid and crystals

tend to becomemore

calcic, a conditionrequiring at least partial solution (resorp-tion) of the crystals.

Geologic evidence appears to supportthe role of falling pressure as a major factorin resorption. Especially significant is thefact that corrosion is prominent in thephyric minerals of many volcanic rocks butis generally lacking in the ground masscrystals. There is wide agreement that suchphenocrysts are the products of intratelluric

crystallization and that the hiatal por-phyritic texture of these rocks reflectsabrupt displacement of the magma to highercrustal levels or to the surface. Although lesseasily demonstrated, a similar history in-volving initial crystallization at depth fol-lowed by vertical movement to the site offinal emplacement seems clear for certainplutonic rocks as well. This conclusion isbased on the observation that at least asmall percentage of phenocrysts is present

in the early dikes and chilled phases of manyplutonic bodies and that fusion and othermore extreme effects indicative of superheatare lacking at most plutonic contacts. Inplutonic rocks, however, corrosional featuresin these early crystals are commonly maskedby later crystallization.

In any case, it is clear that many magmashave experienced at least two phases ofcrystallization-one at a deeper and one at ahigher level-and that corrosion is often

conspicuous in the products of the firstphase. These relations strongly suggest thatresorption is related to rise of the magmaduring emplacement, and fall in pressure isthe one physical change immediately andinevitably associated with this rise.

The presence of narrow fractures in thecores of many plagioclases showing patchyzoning (e.g., fig. 10) supports this interpre-tation. This fracturing is older than the rim

plagioclase which fills the narrow cracks andis believed to have occurred in response to

sudden decrease in pressure at the time ofresorption.

3. MECHANICS OF RESORPTION

Preservation of zoning inthe corrodedcrystals indicates that equilibrium was not

maintained during the resorption process.Equilibrium melting would require that thecrystals, during resorption, be continuouslymade over by diffusive reaction to a homo-geneous, more calcic plagioclase. The ma-terial studied, however, gives no indicationof reaction or homogenization, for delicateprimary oscillatory zoning is present inmany of the resorbed cores.

In addition, most of the corroded plagio-clase studied shows weak normal zoning.The intensive corrosion of the inner, mostcalcic, parts of these crystals indicates asufficiently great displacement of equilibri-um that the entire plagioclase phase (whichtypically represents a zonal variation inexcess of 2-3 per cent An) was subject toresorption. This incomplete resorption isfurther evidence of disequilibrium. In somecases resorption might have been retarded

by a shift in equilibrium so rapid that dif-fusion of the resorbed material away fromthe crystals was unable to maintain a homo-geneous melt composition. Crystallizationmay, therefore, have been renewed beforeresorption was complete.

Intense corrosional channeling of the in-terior of the cores and relatively weak attackon their outer portions is characteristic ofthe resorption process. The controlling fac-tor in this selective corrosion appears to be

the surface-energy relations on the corro-sional interface. An irregularity or imperfec-tion on a crystal face is a site of higherenergy and looser bonding than is an intactsite. Once the energy barrier presented bythe crystal's surface has been breached atsuch an irregularity, resorption may pro-gress much more readily inside the crystal.It is of interest that corrosion in plagioclaseoccurs most readily along the plane (010),

and especially along the a-axis within thisplane; it thus tends to follow the directions

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along which the plagioclase experiencedmost rapid growth.

C. ORIGIN OF THE RIM PLAGIOCLASE

AND INCLUDED PLAGIOCLASE

If the explanation of resorption in termsof fall in pressure suggested here is correct,the abrupt break in composition betweenthe corroded cores and the rim plagioclasemust be related to this same factor. If thezonal change in composition can be ex-plained by this same relation, the proposedmechanism of resorption gains strong sup-port; if it cannot be so explained, the theorymust be abandoned. In order to analyze the

relation of plagioclase composition to pres-sure, it is necessary to construct hypotheti-cal liquidus and solidus curves for the sys-tem albite-anorthite at high confining pres-sure. Through the calculations of Carr(1954), the relative displacement of themelting points of the end members albiteand anorthite with pressure are known withsufficient accuracy to serve as a basis fordiscussion. Unfortunately, however, the ex-act form of the curves to be drawn between

these points is conjectural and will remainso until established experimentally or untilthe heats of mixing of albite and anorthiteat high confining pressure are determinedso that the curves may be calculated (cf.Bowen, 1913). Carr drew one possible set ofcurves by taking Bowen's curves for the drysystem at atmospheric pressure as a startingpoint, flattening both curves greatly, andpivoting them to fit the calculated meltingpoints of albite and anorthite at higher

pressure. Although his curves permit fall inpressure as a mechanism of resorption, theyindicate that a more calcic, rather than amore sodic, plagioclase should form whencooling leads to renewed crystallization atlower pressure. The experimental work ofYoder et al. on the system NaAlSi3-Os-CaAl2Si208-H20 (1957, p. 207) at 5,000 barswater pressure indicates a stronger curva-ture of the plagioclase solidus at higher pres-sure than shown by Bowen's curves at low

pressure. If, as assumed here, this relationcarries over into the dry system at high pres-

sure, hypothetical curves such as those infigure 19 may be suggested. When the high-pressure curves are drawn in this or in anyof several other ways so that either liquidus

or solidus,or

both,have

appreciable curva-ture, the relations are reversed from thoseshown by Carr; although pressure fall willstill lead to resorption, a more sodic plagio-clase crystallizes from the melt at lowerpressure (and temperature).

The development of patchy zoning maybe traced in figure 19. With fall in pressureat nearly constant temperature, both liq-uidus and solidus are depressed. To re-establish equilibrium, the melt must become

richer in anorthite, a condition which willtend to be reached by resorption of theplagioclase crystals already present. Theexact course of resorption depends on thedegree of departure from equilibrium. Threedifferent possibilities must be examined, thelimiting cases of perfect equilibrium andperfect disequilibrium and the actual casewhich is probably a close approach to com-plete disequilibrium. To bring this discus-sion in closest possible harmony with the

observed features of patchy zoning and topermit easiest visualization, it is assumed ineach case that (1) fractional crystallizationof a melt L has given rise to normally zonedcrystals X-Xi, the outermost zone of whichis in equilibrium with melt Li; and (2) fall inpressure would, under equilibrium condi-tions, be just sufficient to completely resorbthe crystals.

In one limiting case equilibrium is main-tained. This would lead to complete resorp-

tion giving a liquid L2 (having the samecomposition as the initial melt) in equilib-rium with vanishing crystals made over byreaction to the composition X2. If crystal-lization were renewed at this time, or, in-deed, at any time during equilibrium re-sorption, the new plagioclase would havethe same composition as the crystals andthere would be no break in compositionbetween core and rim.

The other limiting case is that of com-

plete disequilibrium. Here crystallizationceases with fall in pressure, but the crystals

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do not react with the melt and are neitherresorbed nor changed in composition. Whentemperature falls so that the liquidus isreached at the lower pressure, a distinctly

more sodic plagioclase X3 (in equilibriumwith melt L3) will crystallize as rims aroundthe more calcic cores. With perfect disequi-librium the gap in composition between coreand rim will be at its maximum. Zoningsequences with a sharp break in compositionbetween core and rim and little or no re-sorption of the core are quite common in

would appear to demand complete resorp-tion, normally only a small fraction of thecores, commonly less than one-third byvolume, has been resorbed. Accordingly, a

small, but typical increment of resorptionhas been used in the diagram to illustratethis general case. Through partial resorptionthe melt will become slightly more calcic andwill move with falling temperature towardL4, the exact path being dependent uponthe timing of heat loss relative to pressurefall. When L4 is reached, a more sodic

AB AN

FIG. 19.-Phase diagram of the system albite-anorthite showing the relations of the liquidus and soli-dus at low pressure (the lower pair of curves) with the hypothetical curves at high confining pressure (upperpair of curves). See explanation in text.

natural plagioclase. In most cases this dis-

equilibrium behavior can probably be as-cribed to extremely slow reaction rates.Most of the plagioclase crystals studied

show wide departure rom equilibrium. Thisis clear from the abrupt break in composi-tion between rim and core, from the absenceof any notable compositional change orhomogenization of the corroded plagioclase,and from the fact that even the most calcicplagioclase of the cores usually shows evi-dence of corrosion, a relation implying that

the entire plagioclase phase was potentiallysubject to corrosion. Although equilibrium

plagioclase X4 will crystallize as rims and

inclusions. The compositional gap betweenthe core and rim, although still marked, isnot as great as in the case of complete dis-equilibrium. As already noted, however, thefact that even the calcic cores are corrodedindicates a displacement of equilibriumin excess of that needed to resorb to entireplagioclase phase. This could allow develop-ment of a much wider compositional gapthan that indicated on the diagram.

The preceding discussion was concerned

with renewed crystallization resulting fromfall in temperature. Renewed crystalliza-

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tion, however, may also come about throughattainment of saturation in volatiles duringrise of magma in the crust (Vance, 1962, p.753). The general relations here probably donot differ

significantlyfrom those just

discussed. The experimental work of Yoderet al. (1957, p. 207) indicates that theequilibrium curves for water-saturated pla-gioclase melts at high pressure, thoughlowered, are very nearly parallel to and,except for stronger curvature of the solidus,do not differ greatly in shape from, thecurves for anhydrous melts at low pres-sures.

The contrast in composition between

cores and rims in plagioclase showing patchyzoning has been explained differently byother workers. These other hypotheses, how-ever, depend upon accidental introductionof material and can scarcely account for sogeneral a phenomenon as patchy zoning.The present theory alone explains these fea-tures without change of magma composi-tion.

D. SIGNIFICANCE OF THE

POIKILITIC INCLUSIONS

1. GENERAL REMARKS

In many plagioclases with patchy zoning,poikilitic inclusions have developed wherecrystallization of the included plagioclasehas sealed off magmatic liquid in the in-terior of the corroded crystals. Certainvitrophyres give an especially clear pictureof this process (figs. 15, 16). The plagioclaseof these rocks typically contains small sub-rectangular inclusions of glass surroundedby a thin zone of

includedplagioclase.

Crystallization of the included plagioclasetook place from the walls inward, tending toproduce negative crystals with simple pina-coidal forms. Rapid chilling terminatedcrystallization preserving small glass in-clusions within the included plagioclase. Inplutonic environments the melt includedwithin the crystals after corrosion eventual-ly must crystallize, partly as sodic plagio-clase and partly as poikilitic inclusions of

other minerals. This origin is confirmed bythe restriction of the poikilitic inclusions to

the inner portions of the corroded crystals,where they exist only in contact with orsurrounded by the included plagioclase.Poikilitic inclusions are absent in the outerportions of cores where components of theincluded melt not required for plagioclasecrystallization were able to diffuse outwardthrough channels connecting with the ex-ternal melt.

The poikilitic inclusions in the plagio-clase of most rocks comprise several mineralspecies. Pyroxene, hornblende, alkali feld-spar, quartz, biotite, and magnetite are themost common included phases in normalcalc-alkaline rocks. There is considerable

variation in the form of the included miner-als. Pyroxene, hornblende, and magnetiteare generally more or less oval in shape,while inclusions of quartz are often moreirregular. Alkali feldspar, like quartz, may beirregular, but it commonly forms somewhatrectangular antiperthitic inclusions moldedon subhedral to euhedral internal crystalfaces of the included plagioclase. In somerocks, quartz develops these subrectangularforms also.

Poikilitic inclusions are restricted to min-erals that crystallized later than the plagio-clase cores, and in most cases their crystal-lization appears to be essentially contem-poraneous with that of the included plagio-clase. This relation is extremely useful indetermining the paragenetic sequence. Poi-kilitic inclusions of this type are unique inthat, although they are the products ofdirect magmatic crystallization, they areyounger than the corroded core plagioclase

which surrounds them as a host (cf. Shand,1951, p. 105).

2. ANTIPERTHITIC TEXTURES

Potassium feldspar is common as antiper-thitic poikilitic inclusions in certain quartzmonzonites, granodiorites, and quartz dio-rites with patchy zoning (figs. 17, 18).Ordinarily these inclusions are quite smalland are easily overlooked, although at high-er magnification they can be distinguished

from the surrounding included plagioclaseby their negative relief and somewhat dusty

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appearance. Most such inclusions show acommon orientation with their host, inwhich their a-crystallographic axes are verynearly parallel. In sections cut normal to

the a-axis, the alkali feldspar typically ap-pears as small, almost rectangular, crosssections which give a nearly centered acutebisectrix interference figure.

Though antiperthite is generally attrib-uted to replacement or exsolution, neitherhypothesis can account for the presenttextures. The replacement theory fails toexplain why the poikilitic alkali feldspar isalways in contact with, and usually sur-rounded by, the sodic included plagioclase,

and why the inclusions are found only in theinterior of the corroded cores. The exsolu-tion theory likewise presents difficulties.Although the alkali feldspar and sodic pla-gioclase could conceivably be interpretedas complementary exsolution products, thishypothesis does not explain why the in-clusions are present only in the inner partof the crystal but are absent in plagioclaseof identical composition in the outer por-tions of the core and in the rim. In addition,

neither replacement nor exsolution is com-patible with the coexistence of other poi-kilitically included minerals with the alkalifeldspar in the same corrosional cavities(fig. 18).

IV. SUMMARY AND PETROGENETIC

IMPLICATIONS

Formation of most patchy zoning is in-terpreted here in terms of two distinctphases of crystallization separated by an

interval of resorption. The first stage ofcrystallization occurs at depth in a magmaundersaturated in its volatile components.Corrosion is thought to occur in response tofalling pressure related to rise of the magmain the crust. The compositional gap betweenthe cores and the rims reflects crystallizationat confining pressures corresponding to twodifferent crustal levels. Igneous plagioclaseslacking the sharp zonal compositional breakbetween core and rim may in most cases be

inferred to have crystallized in situ afteremplacement of the magma.

The degree of resorption appears to bedependent on two factors: the magnitudeof the decrease in confining pressure andtime, which largely controls how far resorp-

tion will go towardcompletion.

Texturalevidence suggests that equilibrium, in thesense of compositional remaking of the crys-tals by reaction with the melt, is seldomclosely approached during the corrosionprocess.

Because equilibrium curves have not yetbeen experimentally determined for water-deficient plagioclase melts at various con-fining pressures and, more especially, be-cause equilibrium melting is not attained

during the corrosion process, no precisesignificance can be assigned either to thedegree of corrosion or to the width of thecompositional gap between core and rim interms of absolute change in pressure. Thesefeatures can be applied usefully, however,because they commonly tend to be ratheruniform within a given igneous body. Thisuniformity provides a criterion, along withother textural and mineralogical data, forcorrelation of genetically related magmatic

bodies (e.g., isolated plutons) emplaced atnearly the same crustal level. A furtheruseful value is the modal volume of theplagioclase cores. The modal volume may beregarded as the minimum amount of crystal-line material present in the melt at the timeof emplacement. Similar values for nearbymagmatic bodies would tend to supporttheir correlation.

Decrease in confining pressure related torise of water-deficient magma in the crust

is here considered to be among the most im-portant causes of resorption in phenocrysts.If this interpretation is correct, two implica-tions may be drawn for magmatic rockscontaining plagioclase with patchy zoningor, indeed, any phenocrysts showing cor-rosion features related to pressure decrease.(Textures closely analogous to patchy zoningin plagioclase are common in certain phyricpotassium feldspars [cf. Savolahti, 1962, p.56]). First, it is clear that the magma con-

tained crystals and thus was not super-heated at the time of emplacement. Second,

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Sardegna): Memorie Accad. Patavania Sc. Lett.Arti, Classe Sc. Mat. e Nat., v. 57, p. 1-36.

RITTMANN, A., 1960, Vulkane und ihre Tatigkeit,2d ed.: Stuttgart, Ferdinand Enke.

SAVOLAHTI, ., 1962, The rapakivi problem and therules of idiomorphism of minerals: Comm.Geologique Finland Bull., v. 204, p. 33-111.

SCHARBERT, H., 1957, Uber Ganggesteine aus demoberisterreichischen Mitihlviertel (westlich derRodelstirung): Neues Jb. Min. Geol. Palaont.,v. 90, p. 135-202.

SHAND, S. J., 1951, Eruptive Rocks, 4th ed.: London,Thomas Murby & Co.

SNELLING, N. J., 1960, The geology and petrology ofthe Murrumbidgee batholith, Australia: Quart.Jour. Geol. Soc. London, v. 115, p. 187-217.

STEWART, D. B., and ROSEBOOM, E. H., 1962, Lowertemperature terminations of the three-phaseregion plagioclase-alkali feldspar-liquid: Jour.

Petrology, v. 3, p. 280-315.TUTTLE, O. F., and BOWEN, N. L., 1958, Origin of

granite in the light of experimental studies in thesystem NaASi3Os-KAlSiaOs-SiO2-HO0: eol. Soc.America Mem. 74.

VANCE, J. A., 1962, Zoning in igneous plagioclase:normal and oscillatory zoning: Am. Jour. Sci.,v. 260, p. 746-760.

WYLLIE, P. J., 1963, Effects of the changes in slopeoccurring on the liquidus and solidus paths in thesystem diopside-anorthite-albite: Min. Soc.America Spec. Paper 1, p. 204-212.

YEATS, R. S., 1958, Geology of the Skykomish areain the Cascade Mountains of Washington: Un-published Ph.D. thesis, Univ. Washington,Seattle, 243 p.

YODER, H. S., STEWART, D. B., and SMITH, J. R.,1957, Feldspars: Carnegie Inst. Wash. YearBook, 56, p. 206-214.

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Zoning in Igneous Plagioclase: Patchy Zoning

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