Idea Transcript
American Mineralogist, Volume 73, pages261-273, 1988
Pyroxene exsolution:An indicator of high-pressureigneouscrystallization of pyroxene-bearingqv rtz syenite gneissfrom the High Peaks region of the Adirondack Mountains Plur- W. Or,r,rr..q. Department of Geology and Geography,VassarCollege,Poughkeepsie,New York 12601, U.S.A.
How.q.nr W. Jlrrn, Er.rzlnnrs B. J.q.rrn Department of Geology and Geography,University of Massachusetts,Amherst, Massachusetts01003, U.S-A-
Ansrn-q.cr Relict, coarse,high-temperature inverted pigeonite lamellae in host augite and augite lamellae in host inverted pigeonite are locally preservedin metamorphosedigneousrocks from the Adirondack Mountains of New York State. The exsolution history of these pyroxenesconsistsof (l) crystallization of high-temperatureaugite or pigeonite, (2) exsolution of coarse "00l" lamellae of augite in host pigeonite and pigeonite in host augite, (3) fautting of lamellae and then of host along (100) as lattice parameterschangeduring cooling, (4) inversion of pigeonite to orthopyroxene,and (5) decomposition of the lamellae to produce intergrowths ofaugite and orthopyroxene along (100) ofhost grain. Thesetextureshave been observedin pyroxenesthat spanthe full rangeofcompositions found in Adirondack anorthositic and syenitic rocks. An inverted pigeonite from a pyroxene-bearingquartz syenite gneisscollected in the Mount Marcy quadrangleyields a reintegratedcomposition of Wo,r,EnorFsro, indicating that igneouscrystallization of this rock took place at pressuresgreaterthan or equal to approximately 9 kbar.
gioclase,pyroxene, hornblende, and garnet, occurs at the margins of the massif and in localized shearzoneswithin This paperdescribespyroxeneexsolutiontexturesfound the core of the massif. Syenite gneissestypically overlie in metamorphosed igneous rocks from the Adirondack the gabbroic anorthosite gneissfound at the margins of Mountains of New York State.Theseexsolution textures the anorthosite massif. Valley and O'Neil (1982)and Valley (1985)presented are petrologically significantin that they allow one to "see through" the effects of regional metamorphism and to evidencethat anorthosite in the Adirondacks was intrudstudy the igneoushistory of the rocks that contain them. ed at relatively shallow levels (>,,,F r
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Fig. 1. Generalizedgeologicmap of the Mount Marcy anorthosite massif. Locations for samples9-17, AL-4, BA-5, GB-2, and PO-17 are indicatedon the map.
PynoxnNn ExsoLUTroN Optimal phaseboundary theory was used by Robinson etal. (1971,1977),Robinson(1980), andJaffeeral.(1975) to explain why pigeonite lamellae in an augite host and arrgitelamellaein a pigeonitehost are oriented along nonrational planes.The theory statesthat monoclinic pyroxeneswith identical D dimensions and differing a, c, and B will have two planesof exactdimensionalfit. One plane, "001," is closeto (001), its orientation being controlled by the diference between the c dimensions of the host and lamellaepyroxenes.Another plane,"100," lies close to (100) and is controlled by the differencebetweenthe c dimensionsof the pyroxenes.Sincethe lattice parameters
are functions of pressure, temperature, and chemical composition, the orientation of the best-fit planes is also a function ofthese variables.The rapid expansionofpigeonite as it goes through the P2,/c = C2/c reversible, nonquenchabletransition is of particular importance in interpreting exsolution temperatures. At temperatures above the stability of P2,/ c pigeonite,a and c ofpigeonite are greater than a and c of augite.At temperatureswhere P2,/c pigeonite is stable, the reverse is true. Becauseof theserelationships,high-temperaturelamellae of pigeonite in host augite or augite lamellae in host pigeonite orient in obtuseanglep [c rr "001" < c A (001)];whereas low-temperaturelamellae orient in acute angle0 Fig.2). High-temperature lamellae are much coarser (approxi-
OLLILA ET AL.: PYROXENE EXSOLUTION
mately 10-100 pm thick) and spacedfarther apart than lamellae formed at lower temperatures, which are less than I pm thick. Examples of high-temperature exsolution in low-Ca pyroxenes (inverted pigeonite) are relatively common in plutonic igneousrocks and have been described from many localities. This type of exsolution was originally describedby Hess (1941, 1960) and has been described in Adirondack rocks by Davis (1971), Ashwal (1982), and Jaffe et al. (1983). Robinson et al. ( I 977) describedhigh-temperature( - 1000'C) exsolution in host augitesfrom the Bushveld complex in South Africa and the Nain complex in Labrador, and more recently Livi (1987) has describedsimilar exsolution features in pyroxenesfrom the Laramie anorthosite massif. Beforethe work of Ollila et al. (1983, 1984),this type of exsolution had not been recognizedin the Adirondacks. In contrast to the exsolution textures visible in monoclinic pyroxenes, exsolution between orthorhombic and monoclinic pyroxenesis much simpler. The only plane that is at all similar between orthopyroxene and clinopyroxeneis (100), and lamellaeof clinopyroxenein orthopyroxenehost or orthopyroxenein clinopyroxenehost orient along this plane. As pointed out by Robinson (1980),this type of exsolutionis common in metamorphic pyroxenesand in magnesianigneouspyroxenesthat crystallizedat temperaturesbelow the stability of pigeonite. This type of exsolution also occurs during the late stagesof exsolution of igneouspyroxenes. Exsolution textures in Adirondack rocks are complicatedby the fact that igneousrocks have been affectedby a regional metamorphism. This metamorphism has resulted in complex exsolution textures that have been poorly understood. In a few locations, however, igneous rocks were only slightly deformed during regional metamorphism, and pyroxenesin rocks from these localities retain recognizablehigh-temperatureexsolution textures that have proved to be the key in unraveling the exsolution history of Adirondack pyroxenes. Figure 2 is a schematic diagram showing a model for the exsolution history of metamorphosedigneousaugite. StagesI through 3 are basedon descriptions ofunmetamorphosed igneouspyroxenes(Robinson et al., 1977; Robinson, 1980).Stages4 through 6 are basedon observations of pyroxenes from Adirondack rocks. The diagram is applicable to both augite and pigeonite hosts,but inverted pigeonite that retains coarse "001" augite lamellae is much more common than host augite with relict, high-temperature "001" inverted pigeonite lamellae. Figures 3 through 5 are photomicrographs ofgrains that illustrate the exsolution processesoutlined in Figure 2. Relict, high-temperature exsolution lamellae in host augite were found initially in a coarse-grainedgabbroic anorthosite from the central part ofthe Santanoni quadrangle(sample9-17, Fig. 3). This rock is subophiticwith both augite and inverted pigeonite interstitial to plagioclase.Subsequently,similar exsolutiontextureswerefound in rocks from the central part of the Mount Marcy quadrangle(GB-2 and BA-5, Figs. 4 and 5). Theserocks are
263
onol."*u,roo, o, orro*Ln"textures Fig.2. L .oa.r based
for the exsolutionhistoryofigneousaugitein the Adirondacks. augite;(2) is (l) crystallizationof homogeneous The sequence exsolutionof coarse"001" pigeonitelamellae,which orient in obtuseangleB of the host augite;(3) faultingof pigeonitelamellaeduringcooling(Robinsonetal., 1977);(4) faultingof the hostaugitealong(100),inversionofpigeoniteand decomposialong(100)ofthe host lamellae tion of"001" invertedpigeonite augite;(5)formationof pyroxene"mesoperthite"-alltheorthopyroxenethat originallywasin "001" lamellaenow forms(100) and lamellaecoarsen, lamellaein hostaugite;(6) orthopyroxene fine pigeonitelamellae,which orient in acuteangleB of host (100) augite,exsolvefrom hostaugite.Notethatlow-temperature lamellaemay exsolveso as to protrudebeyond orthopyroxene the confinesofthe augitehost. weakly foliated and consist of medium- to fine-grained granoblasticaggregatesof garnet, pyroxene, and feldspar, but also contain coarser (>2 mm) pyroxene grains that retain relict igneous exsolution textures. Augite from sample9-17 showstexturesequivalentto stage4 in Figure 2. Augite in samplesBA-5 and GB-2 show textures intermediate betweenstages4 and 5 of Figure 2.It should be emphasizedthat these sorts ofexsolution textures are quite rare. Medium- to coarse-grainedaugite in metamorphosed igneous rocks from the Adirondacks most commonly shows exsolution textures corresponding to stage6 of Figure 2. Augite in gabbroic anorthosite gneiss typically occurs in granoblasticaggregatesand only contains fine, low-temperature"001" and "100" pigeonite lamellae such as illustrated betweenthe coarserlamellae in part 6 ofFigure 2. Inverted pigeonite containing coarse "001" augite exsolution lamellae is common in Adirondack anorthositic rocks (Davis, 1971;Ashwal, 1982;Jafe et al., 1983). Suchis not the casefor syenitic rocks. Davis ( I 97 I ) stated that inverted pigeonite was never observed in syenitic
264
OLLILA ET AL.: PYROXENE EXSOLUTION
Fig. 3. Photomicrograph of host augite from sample 9-17. Host augite is at extinction, and relict high-temperaturepigeonite lamellae now inverted to orthopyroxeneare oriented from lower left to upper right. These lamellae are faulted and have partially broken down to produce an intergrowth ofaugite and orthopyroxene along (100) ofhost augite (upper left to lower right). This grain correspondsto stage4 tnFig.2.
gneissesfrom the Saint Regisquadrangle.Bohlen and Essene (1978) describedinverted pigeonitefrom Adirondack pyroxene-bearingquartz syenite gneiss,but the pyroxenesthey describedlacked the distinctive exsolution texturesofinverted pigeonite.Bohlen and Essene'sconclusions were based on the reintegratedcompositions of exsolved grains that contain abundant augite exsolution lamellaeorientedalong (100) of host orthopyroxene.As can be seenin Figure2, the assumptionthat thesegrains were originally pigeonite is quite reasonablegiven the effectthat regional metamorphism has had on exsolution textures. There is one potential problem, however, in interpreting grains that only consist of augite and orthopyroxene intergrown along (100). Rietmeijer (1979), in an extensive study ofpyroxene exsolutiontexturesin syeniticrocks from the South Rogaland complex in Norway, pointed out that quite similar texturescould be produced by both exsolution processesand by epitaxial overgrowths. Some of the criteria Rietmeijer used to distinguish between exsolution lamellae and epitaxial overgrowths included the width of the lamellae, the shapeof the lamellae, and the presenceor absenceof inclusions along hostJamellae boundaries.Thesekinds of observationscannot be made if all high-temperature lamellae have broken down to produce augite-orthopyroxene"mesoperthite." It is uncertain how prevalent this phenomena is in Adirondack rocks, but this possibility decreasesthe certainty of igneous crystallization temperatures based on integrated compositionsof grainsthat do not show relict high-temperature-typeexsolution lamellae. Syenitic gneissesthat contain pyroxenes with relict,
high-temperature exsolution lamellae were found in the northeasterncorner ofthe Santanoni quadrangleand the northern part of the Mount Marcy quadrangle (Fig. l). Inverted pigeonitewith coarse"001" augite exsolution lamellae was found in sample AL-4 (Fig. 6), a mediumto coarse-grainedpyroxene-plagioclase-quartz-microperthite gneiss.Compositionally similar, but finer-grained and more strongly foliated rocks located in the southern part of the Santanoni quadrangle do not contain relict high-temperature exsolution lamellae. Exsolution textures are not as well preservedin syenitic rocks from the Mount Marcy quadrangleas in sample AL-4, but sample SC-6 (Fig. 7) contains what are interpreted as epitaxial intergrowths ofpigeonite (now inverted) and augite, and both samplesPO-17 (Fig. 8) and SC-6 contain orthopyroxene grains with abundant augite exsolution lamellae orientedalong (100) ofthe host orthopyroxene.Samples SC-6and PO-17 also contain host augitewith relict coarse "001" lamellae. CrrBnrrsrnv Mineral analyseswere obtained on an ETECAutoprobe at the University of Massachusetts.Microprobe analyses were performed at 15-kV acceleratingpotential and an aperture current of 0.03 prA.Standardsconsistedof both synthetic and natural silicate and oxide minerals, and corrections were made following the procedure of Bence and Albee (1968)and using the correctionfactorsof Albee and Ray (1970).Analytical resultsare listed in Table l. Reintegratedanalysesofexsolved pyroxeneswere obtained by a number of techniques. Most reintegrated analyseswere obtained by combining microprobe anal-
OLLILA ET AL.: PYROXENE EXSOLUTION
265
tioning of the electronbeam. Lamellaecompositionswere determinedby comparisonwith host spectra.Resultsfrom theseanalysesare listed in Table 1. Optical reintegration techniquesofthe grain shown in Figure 8 were evaluated by scanningcrack-freeareasas large as possibleand analyzing for Fe, Mg, Ca, and Si. Results of these analyses are shown in Figure 9. These analysesindicate that the optical reintegration techniquesprovide very reasonable limits on pigeonite compositions in this rock.
Fig.4. Photomicrograph of hostaugitein sampleGB-2.Fine (100)orthopyroxene exsolutionlamellaearenearlyvertical.Relict high-temperature pigeonitelamellaearehorizontal.This grain is intermediatebetweenstages4 and 5 in Fig. 2. The thick orthopyroxene lamellain the bottomthird ofthe grainis thought to represent an epitaxialovergrowthofpigeonite(nowinverted) on the augitegrain. Note how the augitehost is depletedof orthopyroxene lamellaenearthe coarseorthopyroxene lamella. In othergrains,epitaxialovergrowths suchasthis onehavebrokendownto produce(100)"mesoperthite."
yses of host and lamellae with relative volumes determined from point counts of photomicrographs.Approximately 1000points per grain wereusedfor this technique. In one case(BA-5), the microprobe beam was widened to about 20 pm, and approximately 100 analyseswere collected on a number of traverses across the grain. In this way a significant proportion of the grain was analyzed.For samplePO-17, mineral analysesof arrgiteand orthopyroxenehost determinedby Jaffeet al. (1978)were used in conjunction with point-count data from photomicrographs. The assumption that augite and orthopyroxene host and lamellae compositions are similar in this rock was checkedat Vassar Collegeby means of energydispersive analysesof host and lamellae on a scanning electron microscope equipped with an energy-dispersive analyzer. Use of an sEMprovided advantagesin that a backscattered-electronimage allowed more preciseposi-
Tnrvrpnru.ruREs AND pRESSUREoF CRySTALLIZATION Igneous crystallization temperatures for pyroxenes in anorthositicand syeniticrocks from the Adirondackshave been determinedpreviouslyby Ashwal (1982)and Bohlen and Essene(1978).As first pointed out by Bohlen and Essene(1978), reintegratedpyroxene temperaturesfrom a variety of metamorphosedigneousrocks from the Adirondacks yield temperatureswell above regional metamorphic temperatures.Bohlen et al. (1985) estimated maximum regional metamorphic temperaturesto be approximately 800 "C. Figure l0 showsintegratedpyroxene compositionsfrom Bohlen and Essene(1978) and this report. Crystallization temperaturesrange from approximately I150 "C for gabbroicanorthosite(9-17) to I100 'C for metagabbro(GB-2) and 950 to 850 for syenitic "C rocks (BA-5, PO-17, P-10). Pyroxenecrystallizedafter plagioclase in anorthositic rocks, so liquidus temperatures must have been somewhathigher than the pyroxene temperatureswould indicate. The depth at which anorthositic rocks crystallized is difficult to determine in the Adirondacks, becauseregional metamorphism has overprinted any contact aureole assemblagesthat might have been useful in this respect. Martignole and Schrijver (1971, 1973) and Martignole (1979), however, presentedargumentsin favor ofa deepseatedemplacementof anorthosite-charnockitesuites in the Grenville province and the Adirondacks. Martignole and Schrijver's(1971, 1973)and Martignole's(1979) arguments, however, are largely based on garnet-bearing Martignole (1979) left open the possibility assemblages. that the garnet-bearingassemblagesreflect metamorphic recrystallizationof a shallowly intruded igneousrock, but suggestedthat the preservationof ophitic texturesin mafic charnockitesis most easily explained by intrusion during high-grade regional metamorphism. More recently, Martignole (1986)has presenteda model that allows for both the shallowemplacementof anorthositeand the deep emplacementof charnockitic rocks. Valley and O'Neil (1982) concluded,on the basis of O-isotope studies of the wollastonite deposit at Willsboro. New York. that anorthositewas intruded at shallow levels(< l0 km) and then was metamorphosedat higher pressuresduring regional metamorphism. Valley (1985) strengthenedthis argument with a combined phase-equilibria and O-isotope study of Cascadeslide, a monticellite marble locality found in the Mount Marcy quadrangle. According to shallow-intrusion models, the orthoferrosilite origrnally described by Jaffe et al. (1978) from
OLLILA ET AL.: PYROXENE EXSOLUTION
266
Fig. 5. Coexistingaugite (right) and inverted pigeonite (left) from sample BA-5. The thick "001" augite lamellae in the inverted pigeonite and c of host augite are nearly vertical. The augite grain is intermediate between stages4 and 5 of Fig. 2, whereasthe inverted pigeonite correspondsto stage3. This differencein textural development is typical in theserocks. Relict igneoustextures are usually better preservedin inverted pigeonite than in host augite.
pyroxene-bearing quartz syenite gneiss in the Mount Marcy quadrangleis indicative only of minimum metamorphic pressuresof approximately 8 kbar (Jaffe et al., 1978;Bohlen and Boettcher,1981).As describedabove, however, re-examination of some of the specimenscollectedby Jaffeet al. (1978)has shownthat Fe-richorthopyroxene in these gneissesis, in fact, inverted pigeonite (Figs. 7 and 8). The composition of inverted pigeonite from sample PO-17, when compared to the "forbidden zone" boundariesin the solvi oflindsley (1983),suggests that igneouscrystallization of this rock must have taken place at a minimum of approximately 9 kbar (Fig. 10). DrscussroN Valley (1985)haspointedout that thereis a high degree of uncertainty associatedwith the position of "forbidden zone" boundariesin Lindsley's (1983) phase diagrams when applied to natural pyroxenesthat contain nonquadthat PO-17 could rilateralcomponentsand has suggested have crystallized at pressuresas low as 3 kbar. It should be pointed out in this regard,however, that the argument in favor of deep crystallization for Adirondack pyroxenebearing quartz syenitesis not based solely on the composition of samplePO-I7. Although samplePO-I7 is the most Fe-rich sample yet studied, numerous other samples such as AL-4 and SC-6 show textures indicative of igneous pigeonite and are sufrciently Fe rich to require high-pressurecrystallization.SamplesP-10, IN-11, and SR-29 from the study of Bohlen and Essene(1978)likewise require crystallization pressuresin excessof 3 kbar. These pyroxenescontain low percentagesof nonquadrilateral componentsand are significantlymore Fe rich than inverted pigeonitesdescribedby Ranson (1986) from the
Nain anorthositemassifor by Frost and Lindsley (1981) from the Laramie anorthosite massif. SamplesAL-4, SC6, PO-17, and SR-29 (Bohlenand Essene,1978)are also more Fe-rich than inverted pigeonitesdescribedby Smith (1974) from a portion of the Nain anorthosite massif that crystallized at higher pressure(Betg, 1977) than the area describedby Ranson(1986).Ranson(1986)reintegrated pyroxenesin a pyroxene-bearingquartz monzonite from a part of the Nain complex that crystallized at approximately 3 to 4 kbar. This rock contains the assemblage augite + invertedpigeonite + olivine + quartz. Pyroxene compositions from this rock (Fig. 10) are consistentwith Lindsley's(1983) "forbidden zone" boundariesand represent the compositions at which low-Ca pyroxene is replaced by olivine * quarrzat the pressure(3-4 kbar) and temperatureat which this pyroxene-bearingquartz monzonite crystallized.The relatively common occurrenceof inverted pigeonite more Fe rich than that found in sample PL-187 (Fie. l0) in Adirondack syenitic rocks indicatescrystallizatior' al pressureswell in excessof 3 kbar. If crystallization pressuresfor syenitic rocks are applicableto anorthosite,then the conclusionspresentedabove conflict with the shallow-intrusion hypothesis of Valley and O'Neil (1982)and with the modelsof Whitnev (1983) and Mclelland (1986). Although it is possible,given the uncertainties of metamorphic geothermometry and of phase relationships for natural pyroxenes, that Fe-rich pigeonite could have been produced during regional metamorphism, this is not thought likely. In order to evaluate this possibility, phaserelations in the pyroxene quadrilateral have to be examined. Figure 1l is a schematicrepresentation-based on natural assemblagesdescribedby Smith (1974) and the ex-
OLLILA ET AL.: PYROXENE EXSOLUTION
267
Fig. 6. Invertedpigeonitefrom sampleAL-4. Coarse"001" augitelamellaeareorienteddiagonallyfrom lowerleft to upper right and c of host orthopyroxene is vertical.This grain illustratesthe pigeoniteequivalentof stage4 in Fig. 2.
Fig. 7. Photomicrograph of intergrownaugite(top) and orthopyroxene Oottom)from sampleSC-6.The coarseaugitelamellain orthopyroxene is interpretedasan epitaxialintergrowth ofaugiteandpigeonite(nowinverted)and suggests that pigeonite wasthe initial low-Capyroxeneto crystallizein this rock.
perimental work of Huebner and Turnock (1980) and Turnock and Lindsley (198l)-of the phaserelationsbetween olivine (ol), quartz (q), orthopyroxene (opx), and clinopyroxene(cpx) at three different temperatures(Zl < T2 < T3). As can be seen in this diagram, Ca clearly increasesthe stability of orthopyroxene over olivine for the assemblageopx + ol + q. Ca, however,has a different effect on the assemblageopx + cpx + ol + q. In this assemblage,orthopyroxene is saturatedwith Ca, and any increasein Ca content oforthopyroxene or pigeonite reflects higher temperatures.These higher temperaturesin turn favor the stability of olivine + quafiz over orthopyroxene or pigeonite. These effects are evident in the solvi of Lindsley (1983) in which the "forbidden zone" for pyroxenesexpands to more magnesiancompositions as the Ca content of either orthopyroxene or pigeonite increases.This can be seen in Figure 10, which shows pyroxene compositions plotted on the l0-kbar solvus of Lindsley(1983). The shallow-intrusion model as stated by Whitney (l 983)requiresthat pyroxene-bearingquartz syenitegneiss
be metamorphosed fayalite-rich olivine granite. If one uses the l0-km maximum depth of intrusion suggested by Valley and O'Neil (1982)for theserocks,then olivine should have replacedorthopyroxenein rocks with l00Fe/ (Fe + Mg) ratios of >75 (Bohlen and Boettcher,l98l) and should have replaced pigeonite with l00Fe/(Fe + Mg) ratios of > 78 at 825 "C (Lindsley and Grover, I 980). At higher temperatures,olivine should replacepigeonite at more magnesiancompositions. According to the shallow-intrusion model, low-Ca pyroxeneswith 100Fe/(Fe+ Mg) ratios of >78 should record metamorphic rather than igneous temperatures. Bohlen and Essene(1978) reintegratedcompositionsof exsolved, Fe-rich, low-Ca pyroxenes in Adirondack syenitic gneissesand concluded that these rocks had igneous precursorsthat crystallized between 900 and 1000 "C (Fig. l0). Any hypothesis of shallow intrusion, however, requires that these rocks were originally olivinebearing granitesand would require that the temperatures determined by Bohlen and Esseneare metamorphic. The requirement that metamorphic temperatures be
OLLILA ET AL.:PYROXENE EXSOLUTION
268
basedon six oxygens andformulae analyses of pyroxenes 1. Electron-microprobe Tner-e AL-4
9-'t7 Sample: Cpx H
sio, Alros Tio, CrrO. l\4gO ZnO FeO. MnO BaO Naro Total Si AI Fe3* Total AI Ti Cr Pgs+** Mg Zn Fe2* Mn Ba Na Total
Wo.. En Fs
Cpx L
Opx H
Opx L
A u gI
Pig I
Cpx H
Cpx L
Opx H
OPx L
Aug I
Pig I
Cpx H
49.10 o.73 0 09
48.57 0.53 0.12
50.03 143 0.18
49.44 0.95 ojz
48.28 123 0.23 0.00 3.21 0.09 28.01 0.58 18.79
50.38 s0.39 2 43 2.86 015 0.23
CU.UC
1.90 0.07
49.26 50.01 1 65 2.18 0.03 0.11
50.15 2.20 0j2
50.80 191 o.21
50.82 187 o.24
1 16 3
11.21
16.23
16.46 13.20
14.70
0.21
0.24
0.09
0.12
0.18
0.12
12.47 0.27 21.65
13.05 0.22 2 1. 1 5
31.56 0.54 0.39
31.40 0.57 0.60
18.64 o37 14.79
25.91 0 44 6.72
17.09 0.20 20.40
16.75 0.18 20.79
38 01 0.45 0.52
3776 0.45 0.62
24 24 0.29
33.84 0.40 4.49
0 58 99.56
0.53 99.66
1.925 0 075
1A RA
't01 0.68 0.21 0.68 0.06 105 0.01 0.40 0.19 0.03 0.04 10078 100.00 99.70 100.43 100.44 100.61 100.40 99.97 100.28 100.43 101.09 1.949 1.957 1.954 1.971 1.961 1.958 1.968 1.937 1 926 1.925 1.932 1.923 0.051 0.029 0.025 0.042 0.032 0 046 0.043 0074 0.075 0.068 0 063 0.077
2.000 2.000 0 034 0.053 0 004 0 007
2.000 0 023 0.002
2.000 2 000 0 002 0.024 0.001 0.003
2 000 0.032 0.003
2.000 0.044 0.006
2.000 0.039 0 007
2.000 2.000 0.005 0.000 0.003 0.004
2.000 0.024 0.005
2.000 0.012 0 004
0.075 0.050 0.662 0.638
0 039 0.936
0.072 0 074 0 960 0.757
0.042 0.844
0.062 0.507
0.072 0.511
0.019 0.687
0.023 0.714
0.058 0.576
0.029 0.651
0.323 0.367 0.009 0.007 0.886 0 865
0.982 0.018 0016
0.955 0.526 0.019 0012 0.025 0.610
0.793 0014 0.277
0.488 0.007 0.842
0.467 0.006 0.857
1 256 0 015
1 252 0.015 o.022 0.027
0.735 0.010 0.569
1.097 0.013 0.191
2.000 0.008 0.008 0.000 0.083 0.193 0.003 0.862 0 020 0.81 3
0 043 0.039 2.036 2.026
0.003 2.019
0.002 0.030 2.036 2.036
0 014 2019
0.07s 2.031
0.078 2.037
0 001 2 008
0 005 2.040
0.052 2.029
0.016 2.013
0.053 2.042
43.1 38 2 18.7
42 0 36.8 21.2
09 48.4 50.8
1.3 49.4 49.2
28.6 42.2 29.3
15.8 43.4 40.8
43.7 287 27.6
44.8 28.8 26.4
1.1 35.0 63.9
1.4 35.8 62.8
zu.5 310 39.6
12.1 32.7 55.1
41.4 '10.7 47.9
Notei H : host, L : lamellae,| : retntegratedbulk composition . Total Fe reoorted as FeO. "" Fe3*and Wo, En, and Fs calculatedaccordingto the methodof Lindsley(1983). published by Jatfe et al. (1978). Lamellae t pigeonite compositions are based on opticai-reintegrationof exsolved grains and host compositions analysison a sianning electron microscope.These analyses are not as accurate as the cornpoiitions *ere determinedby means of energy-dispeirsive wavelength-dispersiveanalyses[ublished by Jaffe-etal. out do establishthat there is no significantdifferencebetween host and lamellaecompositions.
greater than 900 oC for large areas of the Adirondacks leadsto a number of inconsistencies.One problem is with the pyroxenesthemselves.If metamorphic temperatures were greater than 900 oC, pyroxenes with l00Fe/(Fe + Mg) ratios of > 92 should be hypersolvus(Lindsley, I 983). This is clearly not the case in Fe-rich rocks from the Mount Marcy quadrangle(Fig. l0). A secondproblem is that numerous mineral assemblagesand geothermometers that limit metamorphic temperaturesto lessthan approximately 800 "C (Bohlenet al., 1985)are common in the areas where a 900 'C metamorphic temperature is required by the shallow intrusion model. A third problem is that integated pyroxene compositions in these rocks indicate a range of temperatures.Although this is relatively easily explained by igneous processes,it is much more difficult to explain if the pyroxenesare the product of regionalmetamorphism.A fourth problem is that there is no evidence of olivine pseudomorphs in any of the rocks studied so far. Similar orthoferrosilite-bearingrocks from the Lofoten Islands of Norway retain clear-cut evidencethat the orthoferrosilite replacedfayalite-rich olivine (Ormaase4 1977).There is no textural evidencethat this has taken place in the Adirondacks.
Another argument against a metamorphic origin for the Fe-rich pigeonite in these rocks is the nature of the exsolution textures. Pyroxenesfrom similar rocks in the Nain anorthosite massif (Ranson, 1981; Huntington, 1980)and the Laramie massif(Frostand Lindsley, 1981; Livi, 1987)and pyroxenesfrom many other plutonic suites (Robinson et al., 1977) show exsolution textures similar to the relict textures observed in the Adirondacks. The difference between Adirondack pyroxenes and unmetamorphosed plutonic pyroxenes is in the breakdown of high-temperature pigeonite or augite lamellae (stage 3, Fig. 2) to intergrowths of augite and orthopl'roxene (stages 5 and 6, Fig. 2). Although it is diftcult to assessthe relative importance of thermal-induced versus stress-induced changes,the breakdown is most easily explained as resulting from deformation associatedwith regional metamorphism becauseof the correlation between rock fabric and degreeof preservationof relict lamellae.Rocks with igneous textures or weak metamorphic fabrics locally contain pyroxenes with relict igneous exsolution textures,whereaspyroxenesfrom granoblasticrocks only contain low-temperature, fine exsolution lamellae. Exsolution textures in relatively undeformed rocks from the
269
OLLILA ET AL.: PYROXENE EXSOLUTION TABLE1-Continued PO-171 Pig I
Cpx H
Cpx L
Opx H
Opx L
Aug I
46.74 0.25 0.09 000 395 0.24 47.30 u-oz 0.69
5036 2.04 0 27
50.51 2.06 0.26
48.68 0.92 0.09
49.18 0.86 0.06
49.92 49.31 1.91 1.31 0.18 0.15
9.2s
I36
12.06
1 24 9
1 0 . 1 5 1 11 3
1 58 7 o 17 20.64
16.03 0.18 20.11
JO.dC
JO.OJ
000 99.88 1.990 0.010
110 99.70 1.948 0.052
Opx H
2 000 0 002 0.002 0.000 0.002 0 251 0 008 1.682 0.022 0.031 0 000 2.001 1.6 12I 85.6
0.50 0.87
0.47 0.44
22.05 0.32 '1518
113 o.20 0 09 0.68 100.17 100.22 100.39 99 64 1.965 1.942 1.953 1.9s4 0.035 0 058 0.044 0.047 0 002 2.000 2.000 2 000 2.000 2 000 0.005 0.029 0.041 0.046 0.000 0.002 0.005 0.003 0.008 0.008
29.71 0.39 7.47 0 51 99.98 1.954 0.046
Cpx H
49.72 48.69 0.91 0.65 0.015 0.06 0.06 0 06 1.2'l 1.17 019 0.19 29.18 2963 0.30 0.43 19.s3 19.07 0.00 0.7'l 0.73 100.65 10241 1.990 1 984 6 0.010 0.01
0.078 0.533
0.071 0.539
0 052 0.722
0.033 0 744
0 070 0.589
0.061 0 657
0.435 0.006 0.855
0.448 0.006 0.833
1. 1 8 3 0.017 0.016
1. 1 9 1 0.01 6 0.019
0.648 0.011 0.633
0.923 0.013 0.317
2.000 0 032 0.004 0 002 0.029 0.071 0.006 0.965 0 015 0 833
0.083 0 085 2 039 2.036 43.6 44.9 30.4 30.8 24I 25.6
0.016 2.032 1.9 37 1 60 9
0.007 2.017 1.0 38.1 60.9
0.051 2.036
0 039 2.029 21.1 32.8 46.1
0.057 2.014 42.8 3.9 53.3
31.8 325
2.000 0.015 0.004
Santanoni quadrangle most closely resemble exsolution textures in pyroxenes from unmetamorphosed igneous rocks. Regardlessof their 100Fe/(Fe + MC) ratios, all pyroxenes that contain relict high-temperature lamellae have partially broken down to intergrowths of augite and orthopyroxeneorientedalong (100) ofthe host grain. This breakdown suggeststhat regional metamorphism took placeat temperaturesbelow the stability of pigeonite.This conclusion is entirely consistent with metamorphic geothermometry(Bohlenet al., 1985)and with pyroxene phaserelations(Lindsley, 1983). If initial exsolution and inversion took place during coolingofthese rocks from igneousconditions,as is consistentwith the histories of pyroxenesfrom both the Nain and Laramie anorthositemassifs(Huntington, 1980;Frost and Lindsley,1981;Ranson,1986;Livi, 1987),then orthopyroxene as well as pigeonite equilibria apply to the igneouspart ofthese rocks'histories.The orthopyroxene host in sample AL-4 (Fig. 6 and Table l) requires a minimum pressure in excessof 4.6 kbar if inversion took place at approximately 800 'C, according to the geobarometer of Bohlen and Boettcher(1981).An ideal ionic model as describedby Bohlen et al. (198l) was used to correct for comoonentsother than Ms and Mn. The min-
Cpx L
Opx H
Opx L
Aug I
45.50 0.36 011 0.06 1.36 0.37 49.81 1.10 0.84 0.06 0.03 99.60 1.983 0.017
44.67 0.36 012 0.02 1.03 0.41 49.92 1. 3 8 0.66
47.86 0.83 0.13 0.06 1.22 o.24 34 52 0 60 14.35
0.54 0.03 98.60 10037 1.984 1 975 0.019 0.016 0.006 2.000 2.000 0 025 0.000 0.004 0.004 0.002 0 001 0.025 0.007 0.075 0.068 0.013 0.007 1.833 1j72 0.021 0.0s2 0.637 0.045 0.000 0.043 0.003 2.013 2.011
2 000 0.001 0.004 0.002 0.009 0.088 0.012 1.806 0.041 0.039 0.001 0.055 0 003 2.012 2.006 1.7 2.O 42I 4.6 3.5 4.0 94.8 93.4 53.2 2.000 0 033 0.005 0.002 0.022 0.072 0.006 0.970 0.010 0.830
32.7 4.1 63.2
Pig I
Pig I
45.97 46 18 0.45 0.50 0.12 0 . 11 0.06 0.06 1.32 1.33 0.33 0.34 46.78 45 44 1.00 0.96 4.70 3.52 0 13 9974 1.983 0.017
0.18 99.84 1 983 0.017
2.000 0.006 0.004 0.002 2 0.01 0 086 0.011 1.676 0.037 0.163 0.001 0.011 2.009 12.0 4.3 83.7
2.000 0.008 0.004 0.002 0.014 0.084 0.010 1.618 0.035 0.216 0.001 0.015 2.007 15.1 42 80.7
imum pressureof 4.6 kbar is a conservativeestimatesince it assumesthat nonquadrilateral components are completely partitioned into orthopyroxene,that inversion took placeat a temperature25 'C below the minimum stability ofpigeonite (Lindsley, I 983), and that the orthopyroxene 'C. Bohlen and Boettcher contained 30/oCaSiO, at 800 have suggestedthat this is the maximum CaSiO, component for Fe-rich orthopyroxene at temperaturesbelow approximately 850 "C. A minimum crystallization pressureof approximately6.3 kbar for sampleAL-4 is a much more reasonableestimate according to Lindsley's (1983) forbidden-zoneboundaries,but even ifone usesthe conservative pressureestimate basedon orthopyroxene, the minimum pressurerequiredfor the crystallizationof AL-4 is well in excessof the 3-kbar maximum suggestedby Valley and O'Neil (1982)for anorthositicrocks at Willsboro, New York. Likewise, the metamorphic pressuredetermined for sample PO-17 (approximately 8 kbar) by Jafe et al. (1978) and Bohlen and Boettcher(1981) also representsa minimum igneous pressureif inverted pigeonite in this rock inverted as it cooled from igneous conditions. The association of anorthosites with granitic and syenitic rocks is a worldwide phenomenon, and although geochemicalevidencehas ruled out a comagmatic origin
2',10
OLLILA ET AL.: PYROXENE EXSOLUTION
Hd o lamellae . host, from J a f f e e t a l . ( 19 7 8 ) r o p t i c E l l gr e i n t e g r E t e d b u l kc o m p o s i t i o n " S E Mr e i n t e g r E t e d b u l kc o m p o s i t i o n
t
Fig. 8. Photomicrograph of invertedpigeonitefrom sample PO-17.Orthopyroxene hostis at partialextinction,and (100) augitelamellaearevertical.Thisgrainis thepigeoniteequivalent of stage6 in Fig. 2. No traceof originalhigh-temperature lamellaearevisible,andaugitelamellaehavecoalesced andcoarsened.This grainwaschosenfor opticalreintegration because it is not in contactwith an augitegrain.Bulk compositlons were determinedboth includingand not includingthe largerareasof augite.Both compositions are listedin Table I and arewithin the pigeoniterange.Opticalreintegrationvalueswerechecked by meansof energy-dispersive analysison a scanningelectron microscope.Four areasthat were as largeas possiblewhile avoidingcrackswerescanned. Resultsareplottedin Fig. 9.
Fs
ofexsolutionlamellaeand Fig.9. Energy-dispersive analyses invertedpigeonitein samplePO-17plottedon the Fe-richpart Analyseswereobtainedby useof of the pyroxenequadrilateral. from matchroutineusinghostcompositions a Kevexquantitative wereanalyzedfor Ca, Pyroxenes Jaffeet al. (1978)asstandards. Wo [Cal(Ca+ Mg + Mg, Fe, and Si and are plottedas molo/o Fe)1,Fs [Fe/(Ca+ Ms + Fe)],andEn [Mgl(Ca+ Mg + Fe)].
of Valley and O'Neil (1982) and Valley (1985), the assumption that pyroxene-bearingquartz syenitesintruded at the same time as anorthosite may have to be re-examined in more detail. There is some isotopic evidenceto support the contention that pyroxene-bearingquartz syeniteswere intruded at a differenttime than anorthosite.Silver (1969),on the basis of a U-Pb study of zircons from Adirondack syenitic rocks, concluded that these rocks had an igneous crystallizationage of approximately 1130 Ma. In contrast,Ashwal and Wooden(1983)concluded,on the basis of a Nd-Sm study, that anorthosite in the Mount Marcy for these rocks, it is commonly thought that they are co- massif in the Adirondacks has a crystallization age of geneticor at least roughly coeval (Emslie, 1978; Morse, approximately1288Ma. Critical evaluationof theseages, l98l; Philpotts, 198l; Mclelland, 1986).Furthermore, basedupon differentdecaysystems,must await more data, it has commonly been assumedin the Adirondacks that but it at least appearspossible that anorthositic and sythe gradational contactsbetweenanorthositic and syenit- enitic rocks were intruded at different times and under ic rocks (de Waard and Romey, 1969;Davis, 1971;Jaffe different conditions. A better understanding of the relaet al., 1983)imply that theserocks were magmasat the tionship between anorthositic and syenitic rocks awaits same time and that mixing occurred between the two more detailed isotopic studies, a more complete underrock types.This sort of relationship has beendocumented standing of the wollastonite deposit at Willsboro, New for the Nain anorthositemassifby Wiebe (1980),and if York, and an understandingof the relationship between such is the casefor the Adirondacks, then crystallization the anorthosite found at Willsboro and that found in the pressuresfor pyroxene-bearingquartz syenitesare appli- Marcy massif. If, however, conclusionsregardingshallow cableto anorthosite.Given the discrepancy,however,be- emplacementof anorthosite and the deep emplacement tween the conclusionsDresentedhere and the conclusions of syenitic rocks are both correct, then a model such as
271
OLLILA ET AL.: PYROXENE EXSOLUTION
I O kbar mole 5 2es-rroo:10:o:g
75
50
25
En
tat
Fig. 10. Compositions ofpyroxenes from anorthositic and syenitic rocks from the Adirondacks and Labrador (PL-187) plotted on the 10-kbarsolvusofLindsley (1983).P-10 is from Bohlen and Essene(1978)and PL-187 is from Ranson(1986).The dashed line is the l0-kbar boundary ofthe "forbidden zone" (Lindsley, 1983).Pyroxenecompositionsto the right ofthis line are metastable with respectto the assemblageaugite + olivine + q:uartz.The 5-kbar "forbidden-zone" boundary (dotted line) is also shown. Reintegratedcompositionsof exsolvedgrains are plotted exceptfor sampleAL-4. Alteration made reintegrationof inverted pigeonite impossible in sampleAL-4; however, analysesof host augite and orthopyroxenewere obtained. Exsolution textures (Fig. 8) clearly indicate that this samplecontainsinverted pigeonite.SamplePO- I 7 suggeststhat crystallization of theserocks took placeat pressures greaterthan or equal to approximately 9 kbar.
presentedby Martignole (1986) in which anorthositeintrusion is followed almost immediately by crustal thickening and in which syenitesare intruded into thickened crust provides at least one solution to this problem. CoNcr,usroNs The complex exsolution textures described above are interpreted to result from the combined effectsof cooling from igneous conditions and regional metamorphism. Adirondack pyroxenesdiffer from typical igneouspyroxenesin that relict coarseexsolution lamellae have broken down to intergrowths of augite and orthopyroxene oriented along (100) of the host grain. Reintegrated compositions of magnesianto moderately Fe-rich pyroxenes (samples9-17, GB-2, and BA-5) showingrelict igneous exsolution textures record temperatures well above regional metamorphic temperaturesand confirm that these are igneous rather than metamorphic pyroxenes.We interpret the mobilization of exsolution lamellae to be causedby regional metamorphism at temperaturesbelow
En
-J
Fig. I l. Schematicphaserelations among pyroxenes,and o1ivine * quartz (ol) at different temperatures(Tl < T2 < T3) and constant pressure.The three-phasetriangle opx-aug-ol (Zl) moves to the left, and the Ca content of opx increasesas temperature increases.At 72, pigeonite becomesstable and forms via the reaction augite + orthopyroxene * olivine + qtf,artz: pigeonite.As temperaturefurther increases,the three-phasetriangle pig-aug-ol moves to the left, and the gap in Ca content between pigeonite and augite narrows (73). These trends are evident in the boundaries ofthe "forbidden zones" in the solvi ofLindsley (1983)and Fig. l0 ofthis paper.IncreasedCa con-
tent stabilizesboth orthopyroxeneand pigeoniterelative to olivine + quartz when the assemblageis not saturatedwith respect to augite. If the assemblageis saturated with augite, then increasedCa content of either orthopyroxeneor pigeonite reflects an increasein temperature that in turn stabilizes olivine + quartz relative to low-Ca pyroxene.
272
OLLILA ET AL.: PYROXENE EXSOLUTION
those necessaryto stabilize pigeonite during the Grenvillian metamorphic event. This conclusion is basedon the differencesbetweenAdirondack pyroxenesand typical igneouspyroxenes,the ubiquitous occurrenceof (100) intergrowths ofaugite and orthopyroxene regardlessofthe Fe/Mg ratio of the pyroxene,and the correlation between the degreeofpreservation ofrelict igneousexsolution textures and metamorphic fabric in the host rock. Fe-rich inverted pigeonite from sample AL-4 contains coarse"001" augite lamellae that are most reasonably interpreted as resulting from cooling from igneous conditions, and these lamellae have partially broken down to intergrowths of augite and orthopyroxene.Low-Ca pyroxenefrom samplePO-I7 has a bulk compositionthat clearly indicates original crystallization as pigeonite even though low-Ca pyroxenesin this rock now consist ofintergrowths of orthopyroxene and augite with augite lamellaeorientedalong(100)ofthe host.The compositions of these pyroxenes, as well as pyroxenes described by Bohlen and Essene(1978), require crystallizationpressures well in excessof 3 kbar, and the composition of low-Ca pyroxenein samplePO-17 (Fig. l0 and Table l) suggestsa minimum igneous crystallization pressure of greater than approximately 9 kbar. Fe-rich igneous pyroxenesindicate that models requiring shallow emplacement of Adirondack anorthosite cannot be applied to the syenitic rocks that surround the Marcy anorthosite massif.
application of olivine-quartz-orthopyroxene barometry. Earth and PlanetaryScienceLetters,47, l-10. Bohlen,S.R , Valley,J W , and Essene, E.J.(1985)Metamorphismin the Adirondacks I. Petrology, pressureand temperature.Joumal of Petrology,26, 971-992 Buddington,A.F. (1939)Adirondackigneousrocks and their metamorphism GeologicalSociely of America Memoir 7, 354 p -(1969) Adirondack anorthositicseries New York StateMuseum and ScienceServiceMemoir 18,215-231. Davis, B.T.C. (1971)Bedrockgeologyofthe St Regisquadrangle,New York. New York State Museum and ScienceService Map and Chan S e r i e s1 6 ,3 4 p . relationshipsin the de Waard,D , and Romey,W D. (1969)Pelrogenetic anorthosite-chamockiteseriesof Snowy Mountain dome, south-central Adirondacks New York State Museum and ScienceService Memoir l 8, 307-3l 5 Emslie,R.F. (1978)Anorthositemassifs,rapakivi granites,and late Proterozoic rifting of North America. PrecambrianResearch,7, 6l-98. Frost, R, and Lindsley,D H (1981)Crystallizationconditionsofferrosyeniteassociatedwith the l,aramie anorthosite,Wyoming. Geological Societyof America Abstractswith Programs,13, 455. Hess,H H (1941)Pyroxenesof common mafic magmas Par-t2 American Mineralogist,26, 573-594. -(1960) Stillwaterigneouscomplex,Montana,a quantitativemineralogicalstudy GeologicalSocietyof America Memoir 80, 230 p. Huebner,J S, and Tumock, A.C (1980)The melting relationsat 1 bar of pyroxenescomposed largely of Ca-, Mg-, and Fe-bearing components.AmericanMineralogist,65, 225-2'71. Huntington, H.D. (1980) Anorthositic and related rocks from Nukasorsuktokh Island, Labrador. Ph.D. thesis, University of Massachusetts, Amherst, Massachusetts Jaffe,H.W., Robinson,Peter,Tracy, RJ., and Ross, Malcolm. (1975) Orientation of pigeonite exsolution lamellae in metamorphic augite; correlation with composition and calculatedoptimal phaseboundaries. AmericanMineralogist,60, 9-28. JaFe,H W , Robinson, Peter, and Tracy, R.J. (1978) Orthoferrosilite and AcxNowr,BlcMENTS other iron-rich pyroxenesin microperthite gneissof the Mount Marcy area,AdirondackMountains AmericanMineralogist,63, I I l6-1136. This paper, in part, is derived from Paul Ollila's Ph.D researchat the Jaffe,H W., Jafe, E.B.,Ollila, P.W., and Hall, L.M. (1983)BedrockgeUniversity of MassachusettsThe authors would like to thank David ology of the High Peaksregion,Marcy massii Adirondacks,New York. Leonard of the University of Massachuseltsand Jerry Calvin of Vassar Contribution 46, Department of Geology and Geography,University Collegefor their assistancewith electron microprobe and sprra-roaanalAmherst,Massachusetts, 78 p of Massachusetts, ysis An early version of this manuscript was reviewed by William RanLindsley, D H (1983) Pyroxene thermometry. American Mineralogist, son, and his comments and suggestionswere most helpful. A critical re68,477-493 view by JacquesMartignole also significantlyimproved the quality ofthis Lindsley,D.H., and Grover, J.E. (1980)Fe-richpigeonite:A geobarompaper. eter.GeologicalSocietyofAmerica Abstractswith Programs,12,472. Livi, K.J.T. (1987)Geothermometryof exsolvedaugitesfrom the Laramie anorthositecomplex, Wyoming. Contnbutions to Mineralogy and RsrrnpNcns crrno Petrology,96, 37 l-380. Albee, A.L., and Ray, L. (1970) Correction factors for electron microMartignole, J. (1979) Charnockite genesisand the Proterozoiccrust. Preprobe microanalysis of silicates,oxides, carbonates,phosphates,and cambrianResearch.9. 303-310 -(1986) sulfatesAnalyticalChemistry,42, 1408-1414. Someproblemson crustalthickeningin the centralpart of Ashwal, L.D. (1982) Mineralogy of mafic and Fe-Ti oxide rich differenthe Grenville Province GeologicalAssociation ofCanada SpecialPatiatesof the Marcy anorthositemassif,Adirondacks, New York Amerp e r3 1 . i c a nM i n e r a l o g i s 6t ,7 , | 4 - 2 1 . Martignole,J, and Schrijver,K. (1971)Associationof(hornblende)-garAshwal,L.D., and Wooden,J.L (1983)Sr and Nd isoropegeochronology, net-clinopyroxene"subfacies"of metamorphismand anorthositemasses. geologichistory, and origin ofthe Adirondack anorthosite.Geochimica 8. 698-704. CanadianJournalofEarth Sciences, et CosmochimicaActa,47, I 875-1887. (1973) Effect of rock composition on appearanceof gamet in anBence,A E., and Albee, A.L. (1968) Empirical correction factors for the orthosite-charnockitesuites. Canadian Journal of Earth Sciences.10, electron microprobe analysisof silicatesand oxides Journal of Geoltt32-1139 ogy,16,382403. Mclelland, J.M. (1986)Pre-Grenvillianhistoryof the Adirondacksas an Berg,J.H. (1977)Regionalgeobarometryin the contactaureolesof the anorogenic,bimodal calderacomplex of mid-Proterozoicage.Geology, anorthositic Nain complex, Labrador Journal of Petrology, 18, 399t4,229-233 430 Morse, S A. (l 98 l) A partisan review of Proterozoicanorthosites.AmerBohlen, S.R., and Boettcher,A L (l 98 1) Experimentalinvestigationsand ican Mineralogist,67, 1087-l 100 geological applications of orthopyroxene geobarometry. American Ollila, Pw. (1984)Bedrockgeologyofthe Santanoniquadrangle,New Mineralogist,66, 951-964. Amherst, MassachuYork. Ph D. thesis,University of Massachusetts, Bohlen,S.R.,and Essene,E.J.(1978)Igneouspyroxenesfrom metamorsetts phosed anorthosite massifs Contributions to Mineralogy and PetrolOllila, P.W., Jafle,E.B.,and Jaffe,H.W. (1983)Pyroxeneexsolutionhiso g y , 6 5 ,4 3 3 4 4 2 . tory in the Adirondack anorthositemassif.GeologicalSocietyof AmerBohlen,S R, Essene, E J, and Boettcher,A.L. ( I 98 I ) Reinvestigation and ica Abstractswith Programs,14,69-70.
273
OLLILA ET AL.: PYROXENE EXSOLUTION Ollila, P.W, Jaffe,H.W., and Jaffe,E.B. (1984) Iron-rich inverted pigeonite:Evidence for the deep emplacementof the Adirondack anorthosite massif.GeologicalSocietyof America Abstractswith Programs, 15.54 Ormaasen, D E (1977) Petrology of the Hopen mangerite-charnockite intrusion,Lofoten,north Nonvay Lithos, 10, 291-310. Philpotts,A.R. (1981)A model for the generationof massif-typeanorthosites Canadian Mineralogist, 19, 233-254. Ranson,WA. (1981)Anorthositesof diversemagma typesin the Puttusaluk Lake area,Nain complex, Labrador CanadianJournal ofEarth S c i e n c e s1,8 , 2 6 4 1 . -(1986) Complex exsolution in inverted pigeonite: Exsolution mechanismsand temperaturesof crystallizationand exsolution.American MineralogisL, T l, 1322-1336 Rietmeijer,F J.M. (1979)Pyroxenesfrom iron-rich igneousrocksin Rogaland,S.W. Norway Ph D. thesis,University of Utrecht, Netherlands. Robinson,Peter.(1980)The compositionspaceofterrestrialpyroxenesIntemal and extemal limits Mineralogical Societyof America Reviews in Mineralogy,7, 419-476 Robinson,Peter,Jaffe,HW., Ross,M., and Klein, Cornelis,Jr (1971) Orientation of exsolution lamellae in clinopyroxene and clinoamphiboles: Consideration of optimal phase boundaries.American Mineralogist,56, 909-939. Robinson,Peter,Ross,M., Nord, G L, Jr., Smyth,J R, and Jaffe,H W (1977)Exsolutionlamellaein augiteand pigeonite:Fossilindicatorsof lattice parametersat high temperature and pressure American Mineralogist,62, 857-873. Silver, LT (1969) A geochronologic investigationof the Adirondack complex, Adirondack Mountains, New York. New York StateMuseum and ScienceServiceMemoir 18,233-251 Smith, D. (1974)Pyroxene-olivine-quartzassemblages in rocks associated with the Nain anorthositemassif,Labrador JournalofPetrology, 15, 58-78 Turnock,A.C., and Lindsley,D.H. (1981)Experimenraldeterminationof pyroxenesolvi for P < I kb, 900 and 1000'C. CanadianMineralogist, t9, 255-261. Valley,J.W (1985)Polymetamorphism in the Adirondacks:Wollastonite at contactsofshallowly intruded anorthosite.In A.C. Tobi and J L R Touret, Eds, The deep Proterozoic crust in the North Atlantic provinces.D Reidel Publishing Company, Dordrecht, Netherlands NATO AdvancedScienceInstituteSeries.ser c. 158.217-236
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1. S,tnnpm DESCRIPTIoNS
9-17. Coarse-grainedsubophitic gabbroic anorthosite consistinverted pigeonite I 0/0, ing of plagioclase(An46)800/0, augite I 6010, ilmenite was collected near the summit of Donand 30/0.Sample aldson Mountain, Santanoniquadrangle. pyroxene-plagioclase-quartzAI-4. Medium- to coarse-grained, microperthite 610/0, microperthitegneissconsistingof qttartz 160/0, oligoclase 150/0,augite 2.50/0,inverted pigeonite l.4o/o, homand zirblende 3.00/o,gamet0.3o/o,opaques0.30/0,apatite O.4o/o, con 0.10/0.Sample was collected in the northeastern corner of the Santanoni quadrangle on the top of a hill I km south of CamerasPond. BA-5. Fine-grained pyroxene monzonite granulite consisting plagioclase25010, inverted pigeonite 130/0, of microperthite 190/o, avgite l4o/0,garnet2jo/o,apalite 4o/0,and opaques50/0.Medium to coarsegrains ofplagioclase and pyroxene,which retain relict igneoustextures,are enclosedin a fine-grainedgranoblasticmatrix consisting of microperthite, plagioclase,and garnet. The sample was collected on Basin Mountain in the Mount Marcy quadrangle. GB-2. Medium- to fine-grainedgabbro ganulite consistingof plagioclase(Anruto Anrr) 33ol0,orthopyroxene(includesinverted pigeonite) 150/0,augite l9o/0,gamet 160/0,ilmenite 110/0,magnePyroxenesoccur both as granoblastic, tite 2o/o,and apatite 4010. fine-grainedaggregatesand as coarser(up to 2 mm) grains that retain relict igneous exsolution textures. The sample was collected in Guideboard Brook in the Mount Marcy quadrangle.