RENDlCONn OELlA SOCIET). ITAUANA DI MINERALOGIA E PETROLOGlA, 19aa, \bI. 0·2, pp. H)·)H

The K.feldspar mineralogy of granites and rhyolites:a generalized case of pseudomorphism of the magmatic phase

ROBERT F. MARTINDcpanmcnt of Geological Sciences, McGiD Univoersity, J4SQ University s~, Monue.J, Quaxc, Canada HJA lA?

ABSTIACT. - Fdsic igneous rocks Ihat preserve trulyrna.glJllltic fddspus probably do IJO( exist on Emh; whatis found is a psazJomorphic auembl8gc. Rhyolitt$senerally consist of $&tlidine cryploperthitc pbcnocrystsin which exsoIudon-rcLlted lamd18e formed by spinocWdecomposition.~ devitrified, the lJllIuiJ: likdyconsists cl slnin-fRle, man- ordered a1ka1i fddspars. Ion­exchangc rcactions may dislurb original bulkcompositions. In pristine hyptl'5OlvUJ rocks, cxsolutionand ordcring have evolved from thc s.nidincC1'yptoptrthite SUlfIC, but rC'Crysulliulion is vcrylocalized, possibly owing to .. hydrogen·hopping .. to pryoptn tetrahcd... and induce AI·Si ordering. Laterintroduction of WIlter complett$ the ordering, coarsensthc assemblage, and removes sll'uctural strain, but tell·laIc domains of the precursor may remain. Subsolvusrocks probAbly do not go through • $Iage of coherentpcnhitic intergrowth; success of the orthocl.asc _microdine inversion depends on the availability of Wllttt,

but also is affected by many other flClon. The dfecti~grain.size in coarse-grained, inversion-twinnedmicrocline, on the scale of ten mic:romettn, tJ:plains itspropensity to open-system behavior, which generallyleads 10 isotopic disturbances aod gcochcmical anomalit$.

Key woTds: K.fddspar, Al-Si order, exsolution, hydrogenhopping, role of WIllcr.

With l'1ll'e exceplions, granitic rocks areignrous in the sense that they crystallized athigh tem~ratures from a magma. But is themineralogy of fresh-looking granitic rocks stilligneous? The answer is no, again with minorexceplions. In other words, granitic rockscontain a suite of fe1sic and mafic mineralsthat have re-equilibrated (partly or completdy)

with an aqueous fluid that circulated throughthe granite body as it contracted and cooled.These in situ recrystallization-ty~ reactionsmay well leave the overall texturalrelationships among the feWc and maficminerals intact, leading to the widely heldbelief that the fresh-looking minerals also areintact. This brief review focuses on the natureof the potassium-rich feldspar in felsic rocks,volcanic as well as plutonic, and explores someof the consequences of the fact that theobserved K·fddspar, sanidine, orthodase ormicrocline, is a pseudomorph of the magmaticphase, which was disordered sanidine oranorthoclase (or an assemblage of both) in thepresence of the silicate melt.

Magmatic alkali-rich feldspar(s) Ut near­surface magmas

The distribution of tetrahedrally co­ordinated AI and Si exhibits long-rangedisorder in a felsic magma, by definition. Thesame applies to quenched samples of suchmagmas (e.g., glass in obsidian). Should suchmagmas be allowed to begin to crystallize priorto eruption, one could expect a singledisordered alkali-rich feldspar (sanidine solidsolution) to form ·near the Iiquidus.Vitrophyric quartz·feldspar porphyries, whichare commonly encountered in areas of recentfelsic volcanism, provide good examples ofsuch interrupted hy~rsolvus crystallization.

344 R.F. MAJmN

Yet, the subsolidus modifications that are thesubject of this paper can begin to take placewithin hours or days of eruption;investigations of the feldspar mineralogy ofsuch virrophyric porphyries generally indicatethat the matrix has begun to devitrify, andthe feldspar has~n to unmix and to order.

Are there examples in nature of «bcU[a1ly.quenched systems? 1be answer is yes, but theyare very rare. Unexsolved and disordered (Na,K)-feldspar occurs in partially fused granitexenoliths transported to the surface in me1ilitenephelinite lava in the volcano Nyiragongo,in eastern Zaire (LarnNEN & SAHAMA, 1981).MOROGAN & MARTIN (1985) characterized thefeldspar mineralogy of a suite of partiallymelted fenitized granitic rocks similarlytransported to the surface in thenatrocarbonatite lava at Oldoinyo Lengaivolcano, Tanzania. Electron-microprobe dataand refined cell parameters indicate thatsingle-phase disordered anorthodase anddisordered sanidine (2f, 0.52-0.54; formaximum disorder, a 2f t value of 0.50 isexpected) both occur in this suite. Such low­calcium (Na, KJ.feldspars are easy tosynthesize, but are so reactive that they canonly be expected in crustal xenoliths broughtto the surface and quenched in a pyroclasticeruption.

O'BRIENT (1986) has described «anextraordinary example of arrested plutonicdevelopment.. in a volcanic and shallowsubvolcanic complex of calc-alkaline affinityin Rabb Park, New Mexico. In addition torhyolite porphyry, O'BRIEr-rT describedsanidine granite, sanidine apUte, and sanidinepegmatite. The sanidine, which has begun tounmix in all cases (as revealed by a schillerphenomenon), coexists with oligoclase;average compositions of pairs considered tohave formed at equilibrium give temperatureestimates of 650-750°C. The average bulkcomposition of the sanidine in the pegmatiticfacies is Or"Ab4,An2' KEEFER & BROWN(1978) found a cryptoperthite of bulkcomposition Or,IAb48An/ from the sameunit to consist of a argely coherentintergrowth of untwinned, monoclinicsanidine (Or6,Abl~' 2t

b0.56) and exsolved

anorthoclase lamellae ( rnAb78) up to 50

nm across, in which the extreme strain in thelattice predudes a calculation of the degreeof AI-Si order. The compositions inferred by1CEEFER and BROWN (1978) areconsistem withattempted equilibration at 465 :f:: 20°C. Thisexample is thus more evolved than the twOcases from East Africa discussed above, butthe extent of the changes are not so great thatthe original assemblage cannot be inferredclosely.

The early stages of ordering and exsolutionare generally considered to be diffusion­controlled. Cryptoperthitic intergrowths ofsanidine and disordered a1bite form byspinodal decomposition, as the magmaticsanidine solid-solution .bypasses .. the strain­free solvus and crosses the coherent solvus(path H, Fig. 1). The extent of coarsening ofthe lamellae in the cryptoperthite and ofadjustments in their composition varies as afunction of cooling rate. In a study of sanidinephenocrysts in a section of the Bishop Tuff,SNOW & YUND (1985) found a bulkcomposition of OrMlAAbn.JAnl.); as resultof the cooling of this blanket of tgnimbriticmaterial, the single-phase sanidine unmixedto largely coherent lamellae of approximatecomposition Or 7 and Orn (inferredcompositions corrected for the influence ofelastic strain due to coherence) as it crossedthe coherent salvus, at approximately 475°C.The lamellar spacing ranges from 78 (top andbottom of the 60·m section) to 155 run 43 mbelow the top. The degree of AI-Si orderincreasal from a value 2t t of 0.50 (maximumdisorder) to 0.62 as a result of annealing inthe ignimbritic deposit. SNOW & YUND (1985)did not mention whether or not these samplesof rhyolitic ignimbrite still possess a glassymatrix. On the assumption that they do not,one must question the hypothesis that whatwent on within the sanidine crystal is entirelydue to a diffusion-related process.Devitrification of the groundmass wasaccompanied by a significant development insecondary porosity (- 8%) and permeability,thus increasing the possibility that hydrogenor water (or both) became involved in AI-Siordering and, perhaps, unmixing in some way(see below). The involvement of hydrogencould explain why lamellar coarsening in

THE K.FELDSPAR MINERALOGY OF GRANITES AND RHYOUTES, A GENERAUZEO CASE ETC. 34:5

sanidine from the Bishop Tuff seems to bemuch faster than the extrapolated rate fromexperimemal data. Whereas the presence ofwater does not seem to affect the rate ofdiffusion of Na and K (e.g., HOKANSON &YUND, 1986), it is conceivable that a film ofwater in the feldspar grain or, on an even

smaller scale, ~hydrolytic weakening», canenhance the mobility of atoms by inducinglocal mobilization of (T04) groups. Theextent of mobility at this stage clearly wasrestricted, however, in view of the persistenceof 1) strain in the two struct~ that makeup sanidine cryptoperthite, and 2)

-----'--->

,,:S,•

M /I/1

/ I/ I T

,,,,,S:,,,,,,,'\.•

\

---- ,,S\\

,,,,,,

H

,,,H:,

T('C)

700

400

two feldspars300

o 0.2 0.8 10

Fig. 1. - One interpretation of the subsolidus phase relationships in the system NaAlSi)O!. KAlSi,Oa.t lowconfining pressure. The strain·free solvus shows a discontinuity on the K·rich limb that reUects the first·orderbreak expec;ted between a relatively disordered monodinic feldspar (M) Ind tridinic K·rich feldspar (n. Ahomogeneous (Na, Kl-feldspar typical of a volcanic assemblage or a hypersolvus epizonal plutonic rock (i.e., bulkcomposition between AbwOrJO Ind AbJOOr~ see text) may fail 10 nucleate exsolution lamellae upon crossing theurain-free solvus. As an example, A~Or'll will follow path H as it cools further as a metastable phase, .ndevencually will encwmer the coherent solvus, with which is associated the coherent spinodal. Once across, sucha homogeneous phase will undergo spinodal decomposition to I lamellar intergrowth of two strained, metasrablefeldspars whose final composition is defined by the two limbs of the coherent soh'Us al the closure temper.ture.A fluid phase, such as is expected to be released during the cooling of ignimbritic bodies (sec text), will efficientlyconvert the mell.$lable K-rich feldspar to. bulk composition on the strain-free solvus (path l). 1he more potassicbu1k-eompolition of the primary K.fddspar expected 10 cryslallize from. magl'Dll subject 10 subsolvus aysu.Iliution(subsolklus path S, AbllOr,. chosen IS In example) ckarly precludes the same subsolidus pIilth as in the first c;ue.Here, two CI$C$ are possible: either the orthoclue will invat to coarsely albi(C. and pericline-twinned microcline,or it will bep~ metl.$tably 1$ «onhoclase_, though M·twinned on a \'tt}' fine scale (Iweed u~J:tun) uponcrossi"lthe inversion. lbe strain·free solvus WI$ determined experimenla1ly by Mu.n."I (1974); the IWO-phaseloop (dashed in) is consistent with the aperimental d.t. reponed in th.t study. lbe coherenl $ONus and coheremspinodal are those of YUND (1984, Fig. I), rdevam to disordered structures .t low confining preuura.

346 R.E MARTIN

compositions consistent with attemptedequilibration in the region 500-400°C (Fig.I), rather than at a lower [~perature.

Using as points of comparison theinterlamellar spacing and the inferred degr«:of AI-Si order, the cryptopel'[hitic sanidinephenocrysts in the Bishop Tuff seem to befurther removed from their starting point, ahomogeneous disordered sanidine, than thematerial from the Rabb Park subvolcaniccomplex. In both cases, and others that showstraight, coherent lamellae in bright-Heldtransmission electron microscopy (e.g.,HOKANSON & YUND, 1986, Fig. la), theevolution is co be considered a result of anisochemical process; within the boundaries ofthe original single crysul, the definition ofa closed system must have been closelysatisfied.

How widespread and representative iscoherent sanidine cryptoperthite in the 150km) of Bishop Tuff, to focus on oneexample? The mineralogical and geochemicalstudy of HlLORETH (1979) and the TEMinvestigation of SNOW & YUNO (1985) arebased on samples that are relatively pristine.Does such a metastable phase persist in areaswhere hydrothenna.l circu.lation has been moreactive, and where vapor·phase deposits areprominent (e.g., SHERIDAN, 1970; FISHER &SCHMINCKE, 1984; chapter 8P Follow-upinvestigations to characterize the microtextureand degret: of AI-Si order of the feldspars insuch areas will be required to investigate thenature of the transition from a magmatic toa hydrothermal assemblage of feldsparminerals. Also, how common is suchcryptoperthite in Paleozoic or Precambrianvolcanic complexes? One would expect thatthe older the volcanic suite, the greater thelikelihood that it has been reheated regionalIy.Such a metamorphic overprint wouldeffectively promote the transformation of thecryptoperthitic assemblage of strained K-richand Na-rich feldspars into a strain-fret:.noncoherent pair of feldspars that mayor maynot preserve the original bulk composition.In view of the complications of open-systembehavior possible during the cooling ofignimbritic rocks and during subsequentmetamorphic overprints, careful TEM and

X-ray-diffraction investigations of theearly stages of destruction of coherentcryptoperthitic intergrowths would set:m tobe most welcome at this point.

Cryptoperthitic sanidine in the context ofopen-system behavior in ignimbriticcomplexes

Felsic magmas that attain water saturationnear the earth's surface erupt explosively. Theresulting ash-flow tuffs contain magmaticwater in two forms: 1) dissolved in glass(which makes up the bulk of the ignimbriticdeposit). on account of the sluggish responseof the system to decompression as the magmarose, and 2) in vesicles in the vitrophyricpumice lapilli, which represent «dasts» ofvesiculating magma. As welding occurs deepin the interior of an ignimbritic cooling unit,the watet that was trapped in vesicles is forcedto the rim of the flattened pumice fragment.This water can then tise through the coolingunit, migrating down the temperaturegradient. It can indu~ devitriHcation of glass,and will become saturated in a host of phases,including the feldspars, which may appear invapor-ph~deposits that are characteristic ofthe more vesicular upper parts of the coolingunit (e.g., SMITH, 1960; FISHER &ScHM:INCKE, 1984). As the ignimbritic depositcools, joints (typically columnar) form andprovide meteoric water an access to the deepinterior of the sheet. The front ofdevitrification can be thought of as migratingefficiently away from any crack; because ofthe attendant decreases in volume of the rockas it cools (due to thermal contraction) andas it devitrifies (owing to the more efficientpacking in crystalline matter than in glass),the process is self-propagating, unless of coursethe deposition of the assemblage ofhydrothermal minerals reduces the secondaryporosity and seals the channdways.

In terms of feldspar mineralogy, one mustquestion the fate of sanidine cryptoperthitein this setting. In view of the widespreadavailability of water, be it magmatic ormeteoric or, more likely, a mixture, it seemsclear that an assemblage of strained fddspanthat constitute the phenocrysts in the

THE K·FElDSPAR MINERAlOGY OF GRANITES AND RHYOUTES, A GENERALIZED CASE ETC. }4 7

ignimbrite will quickly be converted todiscrete domains of strain-free Na- and K-richfeldspars that have the composition anddegree of AI-Si order dictated by thethermodynamically stable assemblage ofph~s (path I, Fig. 1)_ As the groundmassresulted from devitrification induced by anaqueous fluid (i.e., no aqueous fluid, nodevitrification). the Na-rkh and K-richfeldspars in !he felsitic groundmass most likelyare strain-free, and also have the compositionand degree of AI-Si order dictated by thethermodynamically stable assemblage at thetime of formation. Of course, further coolingwill typically lead to compositionally well­equilibrated, but structurally met astable,assemblages. In addition, younger orogeniescan reheat an area regionally, leading tosuperimposed effects that can further modifyan assemblage of feldspars texturally,compositionally (including isotopically) andstructurally. The older the volcanic rock, themore likely are these younger modifications.A general role of thumb pm::licts that the finerthe grain size, the more likely is theassemblage to approach a fully equilibratedone, compositionally as well as structurally.In these settings, isochemical behavior is notto be expected. Preferential removal of thealkalis and oxygen isotopic exchange can beexpected to lead to discrepant KfAr ages indevitrified rhyolite (CERLlNG et al., 1985).

Detailed X-ray-diffraction studies of thefeldspar mineralogy of older ignimbriticdeposits are rare, in spite of the fact thatmobility of economically important elements(e.g., U, Th, Pb, Zn) commonly accompaniesthe hydrothermal events that modified thefeldspars. PAYE'ITE & MARTIN (1986) foundthat in a uraniferous high-fluorine rhyoliticignimbrite at Harvey, in New Brunswick(eastern Canada), which was not laterreheated, most samples contain orthoclase(2t t - 0.88; nomenclature of RmBE 1983) anda1bne. Some samples, which presumably haveinteracted with water at a lower temperature.contain intermediate microcline and a1bite.However, the proportion of K-rich feldsparto a1bite has been disturbed by Na-for-Kexchange. In two samples in which theexchange is extreme, the phenocrysts now

consist of an albite pseudomorph of theoriginal assemblage. whereas the matrixcontains orthoclase and a mere trace of albite.Single-crystal studies (X-ray or electrondiffraction) have yet to be carried out toconfirm that all traces of coherence have beenremoved during recrystallization. However.the absence of anomalies in the a parameterof the orthodase and intermediate microclineare consistent with this proposal.

A similar chemical and powder-diHractioninvestigation has been carried out on thefeldspar mineralogy of ignimbriric rocks fromthe Early Proterozoic Upper Aillik Group, ineastern Labrador. The ignimbritk rhyolitksequence. which locally is uranium­mineralized and metasomatized (WHITE &MARTIN. 1980), was folded and reheatedduring the Elsonian orogeny, then again,perhaps, by the Grenville orogeny, in viewof the area's proximity to the Grenville Front.During the hydrothermal stage of evolutionof the ignimbrites. there is evidence for thedevelopment of convection cells. Thiscirculation led to rocks that containa1bite + quartz in the high-temperature partsof the cell, others that contain lowmicrocline + quartz in the low-temperatureportions. There is still material in which theproportion of K and Na has remained moreor less intact. Although slight departures fromcomplete AI-Si order in the microcline (t 100.91-0.93) are encountered in some samples,very well-ordered microcline is the norm.Cases of arrested (incomplete) conversionshow that the matrix may be monomineralic(albite or microcline), whereas the phenocrystsmay still contain a second feldspar of theoriginal perthitic assemblage; this is a resultof the rule of thumb stated earlier concerningthe greater reactivity of the fine-grainedassemblages. As a result of these metasomaricmodifications in the alkali feldspar, the bulkcomposition of the metaignimbrites can varywidely in K/Na value. These modificationshave relevance in mineral explorntion, becausethe mobility of the alkalis is linked with thatof uranium (WHITE & MARTIN, 1980).

Compared to the relatively pristine casesdiscussed earlier. the ignimbrites of Harveyshow the effects of limited open-system

348 R.F. MARTIN

~havior in the mineralogy of the alkali£ddspar(s). whereas the Aillik suite generaUyillustrates a complete equilibration of thefeldspar assemblage to relatively pure, well­ordered microdine that coexists with pureordered albite. In the latter instance, theprofound metasomatic changes associatedwith mineralization likdy are imposed duringthe original cooling and devitrificarion of theignimbritic S«Iuence, the conversion oforthoda~+ slightly disordered aIbite (like atHarvey) to microdine + aIbite may haveoccurred as a result of the Hudsonian andGrenvillian otogenics.

Magmatic assemblages in plutonic bodies?

Are chances of preserving the magmaticalkali feldspar bener in plutonic bodies?Intuitively, one should expect the answer tothis question to be no, in view of the muchslower rate of cooling in a truly plutonic body.In fact, the true answer depends almostentirely on the importance of fluxes ofhydrogen and wa[er (be it magmatic orexternally derived) during the cooling of thepiu ton. Some piutons. especially thoseresulting from the intrusion of relativelyunevolved magmas. seem to have crystallizedfrom relatively dry magmas. at least comparedto those that I'CIChed satunltion and that wereemplaced as ignimbrites in a volcanic setting.The most detailed case-study. which focuseson the Precarnbrian Klokken syenite, inGret:n1and. is that of PARSONS (1978). BROWNet al., (1983). BROWN & PARSONS (1984a) andPARSONS & BROWN (1984). Many horizons ofthe layered body of hypersolvus syenitecontain cryptaperthite featuring coherentlamellae of K- and Na-rich feldspars exsolvedfrom a ternary solid-solution (bulkcompositions generally close toOr)~bS0nJ:PARSONS, 1978). On the otherhana. some horizons are pegmatitic, andpresumably formed from a ~we[[er. batch ofmagma. The cryptoperthite grains in thesehorizons are traversed by whitened cracksalong which there has been «catastrophic. lossof coherency and very striking coarsening ofthe perthitic intergrowth. PARSONS (1978)made the point that locally derived water

played an essential role in the conversion ofthe relatively strained. cohermtly intergrownassemblage in the cryptoperthite to a coarser.noncoherent integrowth of turbid microcline+ albite.

In intrusive complexes like the Klokken.the relative dryness of the magma dictatesthat at the subsolidus stage of the pluton'shistory, hydrothermal m:rystallization willbe localized. and will approach the isochemicalcase. Studies by transmission electronmicroscopy (BROWN et al., 1983; BROWN& PARSONS, 1984a. b; see also PARSONS& BROWN, 1984) illustrate dearly thesequence of steps followed in the perthiticassemblages of the Klokken syenite. Theauthors related these steps to the relativelysimple microtextures of the orthodasecryptoperthites from the pristine ignimbritesand fdsic dyke-rocks mentioned earlier. Suchstrictly planar lamellae as in the orthodasecryptoperthire were dearly a precursor stage.but are not preserved at Klokken. They havegiven way to .wavy. lamellae of low albiteand diagonally associated microcline in a braidmicroperthite that can still be found preservedin domains in even the most rurbid grains (theturbid parts contain the .catastrophicallycoarsened., patchy assemblage of lowmicrocline and low albite, and were notinvestigated by the TEM approach).Surprisingly, the lamellar spacings measuredperpendicular to b * in the Klokken case aresimilar to those found in the more quicklyquenched systems. Many factors mayinfluence the kinetics of coarsening of thelamellae, as evaluated by the authors, not theleast of which is the influence of the anorthitecomponent (which is more important in theKlokken samples. and which may counteractthe eHects of longer time and presence ofstructurally sited water).

A review of available TEM data on thedevelopment of microtextures incryptoperthitk alkali feldspar (BROWN &PARSONS. 1984b) dearly shows that thecoverage of the Klokken pluton was uniquethen; not much has happened in theintervening years to change the situation.Much more TEM work will be required ongeologically wen·understood felsic complexes

llIE K·FELDSPAR MINERALOGY OF GRANITES AND RHYOUn:S: AGENERAUZED CASE ETC. 349

to test the sequence of development ofmicrotextures proposed (BROWN & PARSONS,1984a, Fig. 8). Even in ancient igneouscomplexes containing coarsdy perthitic alkalifeldspar, vestiges of the early stages of thefeldspar's history may well be preservedlocally. However, the very fine scale andapparently high degree of structural coherencyof cryptopenhitic intergrowths make a TEMapproach mandatory.

A possible role for H" in the orderingreaction in cryptoperthite

In a summary of their findings concerningthe nature of the K-feldspar co be expectedin cryptopenhite, BROWN & PARSONS (l984b)pointed out the general lack of dislocationsand othet signs of structural strain at theinterfaces. They appealed to the minimizationof coherent elastic energy and to intetfaceorientation as the factors that determinewhether the K·rich member of the associationwill be orthoclase, so-called «high microcline»,or low microcline. They viewed the orderingreaction of the K-rich phase as due to thediffusion of AI and Si. Most authors haveviewed this phase transformation, whichoccurs at a temperature below 500°C, to betruly reconstructive, and therefore to involvethe hydrogen ion, clearly required to «pryopen» the T - 0 - T linkages (e.g., DoNNAYet al., 1959). AI and Si simply do not diffusethrough crystal structures like Na, K and 0do; they are mobilized as tetrahedralcomplexes [nOH)4' perhaps). Furthermore,at temperatures in the stability field of lowmicrocline, perhaps closer to 400°C than'OO°C, can true incracrystalline diffusion besufficiently effective that some domains ahundred or more nanometers across grow atthe expense of others, especially in the contextof a cooling epizonal pluton?

GOLDSMITH (I986, 1987, 1988)demonstrated in high-P (15-20 kbars),«essentially dry» experiments that changes indegree of AJ-Si order take place orders ofmagnitude faster than at low pressures in thepresence of waer. He deduced that hydrogen,a very mobile species formed by dissociationof water in the solid pressure-transmitting

medium surrounding the encapsulated sample,diffused into the sample holder in the piston­cylinder assembly (molecules of water couldnot have diffused through the noble-metalcapsule). The dissociation constant of wateris known to increase with increasing confiningpressure, such that the phenomenon of«hydrogen hopping» can lead to measurablemobilization of 104 groups in the relativelyshort duration of Goldsmith's experiments(maximum 44 days). Dissociation of magmaticwater in the Klokken complex, althoughinherently not as efficient as at high pressures,could have catalyzed the in situ conversion ofa disordered monoclinic (Na, K)-feldspar tomicrocline in an as-yet poorly understood waydistinct from solution and redeposition, thesteps that occur in the presence of molecularwater. The proposed role of hydrogen, whichwould permeate the feldspar structure withease, may account for the scarcity ofdislocations in the strained assemblages, asnoted by BROWN & PARSONS (l984b), and forenhanced changes in degree of AJ-Si orderwhile a feldspar is being defonned (YUND &TULUS, 1980). Although alkali feldspar isnominally anhydrous, detailed spectroscopicinvestigations (e.g., HoFMEISTEJt & ROSSMAN,198') do prove the presence of structurallysited OH and Hp at ppm-type levds. Asthe H + would be repeatedly consumed andreleased, the absolute concentrations ofhydrogen required in this closed·systemordering would be comparably small.

The influx of water in hypersolvus rocks

As the strained, very finely perthiticmagmatic feldspar cools, it will contract.Cleavages become prominent at this stage,because thermal contraction is highlyanisotropic. On a larger scale, the network ofcooling joints in the ignequs lxxIy allows theentry of water (recirculated magmatic water,meteoric water. or a mixture), whichcontributes to a more rapid cooling than bymere conduction. The water propagates to theinterior of the crystals via the cleavage planes,and reacts with the strained assemblage,proclucing what PARSONS (1978) caUed«catastrophic coarsening» of the perthitic

3'0 R.F. MARTIN

texture. At this stage, the K-rich feldsparbecomes turbid, as a result of a dusting offinely crystalline pigments deposited on thewalls of fluid inclusions (FOLK, 1955) andmicrofissures. A very common pigment in thevacuoles is hematite; its presence can beconfirmed by electron-microprobe analysis(e.g., LALONDE & MARTIN, 1983). In a near­surface body, hematite pigmentation may wellreflect the overall loss of hydrogen or influxof oxygen (or both) from outside the pluton.The pink coloration and «catastrophiccoarsening» of the perthitic texture arecommonly accompanied by an increase in thedegree of AI·Si order and the removal of strainin the intergrowth. The new perthiticassemblage consists of strain-free K-richfeldspar (onhoclase, intermediate microcline,low microcline, or a mixture), usually in ahighly modified exsolution texture. TAYLOR& FORESTER (971) and FORESTER & TAYLOR(1977) have demonstrated that in thedominantly hypersolvus granites of theScottish Tertiary complexes, the inception ofturbidity is accompanied by exchange ofoxygen in the feldspar structure with that inthe fluid phase. Where the fluid is of meteoricorigin, the 0180 value of the recrystaIlized K­rich feldspar may be significantly reducedfrom its magmatic value, but where the fluidphase is of magmatic derivation, the 0180will be affected only in a very subtle way. Itshould be clear to the reader, however, thatif solution and redeposition have occurred, notonly the eight oxygen atoms (our of thirteenin a formula unit) have exchanged with thefluid, but all thirteen must have. In otherwords, Na and K, and, by implication, Rb,Sr, Ar, Ca, Ba, Sr and Pb, may well have beeninvolved in the exchange. Part of the iron thatforms the hematite pigment in the ubiquitouspink granites may have been rejected from thestructure of the high-temperature feldsparupon its recrystallization, but it is clear thatsome of the iron is contributed from adjacentmafic phases, which generally are oxidized andaltered in such isotopically disturbed granites(TAYLOR & FORESTER, 1971; FERRY, 1985).

The K.feldspar m subsolvus granitic rocks

Unlike hypersolvus rocks, mwhich the bulk

composition of the magmatic feldspargenerally falls between Or30Ab7o andOr70Ab 0' and whose evolution wasdiscussed in the previous section, subsolvusgranitic rocks contain two primary feldspars,sodic plagioclase and a K-rich feldspar. Thelatter probably first formed as a disordered,monoclinic phase in the case of most magmas,but the reader should be aware of theexistence of fluorine-rich leucogranitic meltswhose solidus is so low that crystallization ofprimary microcline becomes a possibility. Forexample, WEBSTER et a1. (1987) reported asolidus of 500°C for the felsic magma thatwas emplaced at Spor Mountain, Utah (1.2wt.% F). Leucogranitic magmas with morethan twice the amount of fluorine do exist,such that an even lower solidus could perhapsbe expected in those rare instances. A detailedinvestigation of the feldspar mineralogy ofsuch complexes has yet to be undertaken.

BROWN & PARSONS (1984b) proposed thatfor an Or-rich primary feldspar typical of asubsolvus granite, orthoclase is expected.TEM investigations show the M-type arrayof finely twinned domains, characteristicallyarranged in a «tweed texture» (EGGLETON &BUSECK, 1980; FITZ GERALD & McLAREN,1982). Exsolved albite is usually present, butis invariably less prominent than in thehypersolvus case because of the richness ofthe original bulk composition in potassium.Also, and for the same reason, exsolution isexpected to occur at a lower temperature thanin the hypersolvus case. The tweed textureforms as the orthoclase crosses into the fieldof stability of microcline, below 500°C. Thevery sluggish kinetics of the ripening of thistexture prevent its efficient recrystaIlizationto coarsely twinned low microcline in manyplutonic rocks, even of Archean age. In fact,the persistence of orchoclase in a plutonicbody can be taken as an indication that thearea in question escaped deformation, regionalreheating and circulation of an aqueous fluidsince the pluton cooled. These factors wouldhave induced the conversion of orthoclase tolow microcline. The persistence of orthoclaseor intermediate microcline provides anindication that the rocks probably havebehaved as a closed system since the original

THE K-FEWSPAR MINERALOGY OF GRANITES AND RHYOLlTES: A GENERALIZED CASE ETC, 351

cooling; as a result, geochronological workbased on Rb-Sr and K-Ar systems probablywill be closely concordant with results of U­Pb dating using zircon.

In the micro- to macroperthitic texturetypical of more coarsely twinned lowmicrocline from subsolvus granites andpegmatites, the width of the albite lamellaemay exceed 1 mm, and the presence of thepolysynthetic twins in these lamellae may evenbe visible in a hand specimen, as in perthitefrom the type locality, Perth, Ontario. Theboundary zones between albite and microclinedomains generally are considered noncoherentin such coarse intergrowths; the cellparameters of the two feldspars show noevidence of strain. Did such coarse perthitego through a coherent cryptoperthitic stage?Or was cooling so slow that nucleation didoccur upon crossing of the K-limb of thestrain~free solvus (path S, Fig. I)? If so, whatwas the role of water in this nucleation stepand in the coarsening, so as to enhance (and,apparently, overtake) the diffusional process?These challenging questions, stated by YUND

(1984), may not have unique answers;furthermore, an experimental approachcannot be used to provide answers. As anillustration, the interlamellar spacing expectedby intracrystalline diffusion after 10,000years, as extrapolated from laboratory resultson cryptoperthite initially held at 570°C is«only» 4600 A at 300°C (cooling r~teO.OOOroC/day, initial spacing 349 A)!

It seems clear that to get an interlamellarspacing orders of magnitude greater, watermust be involved. If this reasoning is sound,then petrologists and geochemists mustaddress the next level of obvious questions:what of the original chemistry of the feldsparhas survived intact? An isochemical case­history in the context of a process involvingsolution and redeposition, probably repeated,would seem to involve special pleading. Wasthe water in isotopic equilibrium with thefeldspar? In view of the relative reactivity ofthe alkali feldspar among the rock-formingminerals, its oxygen isotope geochemistrycannot be expected to have survivedunmodified, as mentioned earlier.

What about the argon content of orthoclase

or microcline? Why are the conventional K-Arages they define commonly aberrant?HARRlSON & McDouGALL (1982) showed thatthree microcHne separates from a Cretaceousgranitic batholith in New Zealand (age ofemplacement: 114 Ma) yield plateau ages of103,99 and 93 Ma for the last 35% of 39Arreleased, which are consistent with a closuretemperature of a~proximately 130°C. Thefirst 65% of the 3 Ar released define a linearincrease in age from 80 Ma to the abovevalues, and represent a temperature intervalover which an accumulation of radiogenic40Ar took place, below 100°C. The inferreddiffusion gradients show that the microclinegrains continued to respond to theirenvironment at these low subsolidustemperatures. ZEITLER & Frrz GERALD (1986)and ZEITLER (1987) showed that K-richfeldspar concentrates than have a saddle­shaped age spectrum have incorporated excess40Ar near the grain margins at lowtemperatures. A TEM examination of twosamples shows that noncoherent andsemicoherent interfaces and dislocations serveto greatly reduce the effective size of grains,to create a large range in grain sizes, and toallow fast «diffusion» to occur along thesepathways. Their analysis assumes thatdiffusion in fact is the dominant process toexplain the mobility of argon isotopes; analternative is that a fluid medium is acontributing factor, just as it seems to be inthe coarsening of the exsolution texture (Le.,the mobility of Na and K). The movementof the excess 40Ar component is efficientunder water pressure, but negligible when thesamples are heated dry (ZElTU:R, 1987). Thismay indicate that water is (and was) directlyinvolved in the migration, rather than hintat the structural siting of the excess argon,in anionic positions. The sanidine sampleexamined by Zeitler, from an ignimbrite,presumably is much more retentive of itsargon because of rhe presence of continuous,coherent, and tightly spaced lamellae of K­rich and Na·rich feldspar. It seems clear thata thorough characterization of feldsparpopulations selected for investigation bystepwise 4°Arj39Ar analysis (X-ray or electrondiffraction, transmission electron microscopy)

R.F. MARTIN

will be required to fully model the behaviorof argon entrapment and later release.

FITZGERALO & McLAREN (1982) andMcLAREN (1984) described a bewilderingarray of microtextures in microcline fromgranites and pegmatites, and related these toobservations made by optical microscopy.They showed that the cross-hatched patternof twin-related domains is indeed due to setsof albite and pe:ridine twin lamellae that we~formed at the time of inversion frommonoclinic to triclinic symmetry, close to400°(. Lower-temperatuu: reoystallization,presumably promoted by a fluid medium,preferentially replaces che peridine twinlamellae by a much finer sec of serrated albitetwins. This step greatly increases the interfaceboundaries and discontinuities and providespossible channdways for .fast diffusion» ofwater and various solutes like argon. Theeventual disappearance of pericline twinlamellae in microcline would Sttm to providea means to .. rank» different samples of lowmicrocline in an evolutionary sequence.

Much has been written concerning scatterin Rb·Sr isochrons. Although a thoroughreview is beyond the scope of this essay andmay be very difficult for want of pertinentdata, there probably is a close relationshipbetween degr~ of AI·Si order in the K·richfeldspar (the main Rh.bearing phase in agranite), the miaotextuu: defined by twinneddomains, and the spacing of its exsolutionlamellae on tbe one hand, and open-systembehavior leading to scatter about an isochronor to an anomalously young age on the other.The clearest evidence of the great mobilicyof 87S r in K-fddspar comes from studies ofrubidium-enriched, geologically evolvedgranitic pegmatites (e.g., CLARK & CERNY,1987). Attempts to correct the total Srcontent of Rb-rich microcline for radiogenic87S r commonly result in negativeconcentrations of Sr! Even in plutonic rocksthat are not so extremely enriched in Rh,open-system behavior involving Rb and 87Sris widespread and subtle. It is very commonto find a 200-Ma (or more) discrepancybetween an Archean Rb-Sr age and a well­constrained V-Pb age determination usingabraded zircon grains. The problem of

resetting of the Rb-Sr system is particularlysevere in orogenic belts subjected to episodicregional reheating (e.g., EASTON. 1986). Thesusceptibility of a plutonic suite to this typeof resetting likely is a function of the((porosity» of the K-feldspar, which resultsfrom the density of noncoherent internalboundaries associated with twinned domains,and lamellae and. patches of albite.

Concluding remarks

In a benchmark contribution on themineralogical distinctions betw~n volcanicand plutonic felsic rocks, TUTTLE (1952)concluded that the feldspars found in plutonicrocks «show much evidence of change froman initial character identical with thecorresponding minerals of the extrusiveequivalents». The intervening years have seenmajor advances in the characterization ofthese changes, firstly by X-ray-diffraetion andelectron-microprobe analyses, and morere<:ently by transmission electron microscopy.

STEWART & WRJGHT (1974) reviewed theoccurrence of sanidine, orthoclase andmicrocline in common igneous andmetamorphic rocks. They stressed that thereis not a unique path of AI-Si ordering in K­rich feldspars, and that natural assemblagesseem most consistent with a continuoustransformation between sanidine andorthoclase, then a discontinuoustransformation between orthoclase andmicrocline. The occurrence of domains oftweed-textured orthoclase within microcline(Frrz GERALD & Md.AREN, 1982; ZEITLER &FITZ GERALD, 1986) is consistent with thediscontinuity between orthoc1ase andmicrocline. If the transformation did notinvolve major recrystallization needed tocoarsen the twin-related domains and toincrease the degree of long-range orderinvolving AI and Si, such metastable remnantswould not be expected to occur. The easydiffusion of the hydrogen ion throughcrystalline matter may explain the very local,in situ early AI-Si ordering that occurs incryptoperthite. An Archean piuton thatcrystallized from a relatively dry magma andthat has not been subjected to later

THE K·FELDSPl'r..R MINERALOGY OF GRANITES AND RHY'OUlES: 11 GENERAlJZED CASE ETC. 3'3

deformation and (hydrolthermaI disturbancesshould still contain such cryptoperthitic alkalifeldspar. The recrystallization of such fineimergrowths to micro- and macroperthiteinvolves the influx of water. Even with anavailable source of water, the transformationis difficult, and in many cases, a coolingepizonal gf'anite ends up with orthodasemicroperthite or imermediate microclinemicroperthite (or a mixture oforthoclase + intermediate mkrocline + albite).The appearance of low microcline in suchepizonal complexes may have to await anepisode of local or regional reheating. Ofcourse, the transformation is much more likelyto go to completion in mesozonal andcatazonal complexes. Broad genef'alizationsare difficult, as OOlk composition of the parentfeldspar, that of the host rock, overall grain­size, effective grain-size, thermal history,deformation-related external stresses, fugacityof hydrogen, and pore-fluid composition willall exert an influence of the magmatic productto yield the observed pseudomorphicassemblage in a rhyolitic or granitic rock.Open-system behavior in K-rich feldsparintroduces a serious «can of wormSlt that hasto be properly evaluated in investigations ofigneous petrogenesis and radiometric dating.

Acknowledgements. - I am very grateful for theinvitation to panicipate in the SIMP oonferen~ onGl"anitf:$ and their Surroundings. Research costs areddl'1lyed by an operating grant (A772l) from the NatunlScieoc" and Engineering Roearch Council of Canada.

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