Petrology of Clintonite-Bearing Marbles in the Boulder Aureole, Montana

American Mineralogist, Volume 64, pages 519-526, 1979

Petrologyof clintonite-bearingmarbles in theBoulder aureole, Montana

Jecr M. Rlcr Departmentof Geology,Uniuersity of Oregon Eugene,Oregon97403

Abstract

The trioctahedralcalcium brittle mica, clintonite,occurs locally in aluminousmarbles near the intrusivecontact of the Boulderbatholith. Assemblages include clintonite-calcite-olivine- clinopyroxene-phlogopiteand clintonite-calcite-olivine-spinel-phlogopite.Microprobe analysesof coexistingphases show clintonite to lie withjn the phasevolume calcite-olivine- clinopyroxene-spinel,close to the MgSi-rich end of the synthesizedsolid solution. Phase relationsare characterizedby both continuousand discontinuousreactions. Discontinuous reactionsbetween the observedphases result in a topology in pCOz-pHzOspace which restrictsthe stability field of clintonite to relatively low pCOz and/or high pHrO. The formation of clintonite was not relatedto metasomatism,but rather to reaction between calcite,olivine, clinopyroxene, spinel, and HzO introducedfrom the intrusivebody. In the Boulderaureole the formation of clintonitecan be restrictedto temperaturesbetween 580o and 620'C with fluid compositionsbetween XCOz of 0.05and approximately0.2.

Introduction changedthrough metasomatism.Such an origin has Trioctahedralcalcium brittle micas,most appro- also been invoked to explain the observationsthat priately referredto by the generalname clintonite, clintonite-bearingrocks are typically restricted to the form a solid-solutionseries which can be represented immediatecontacts of intrusiveigneous rocks and by the generalformula oftenspatially associated with skarns(Knopf, 1953). In the contact-metamorphicaureole surrounding + + Ca [(M g, Fe'z ), *,(Al, Fe3 )2 - A\ _ 10(O H, F "f "Si"O ), the northernmostportion of the Boulderbatholith of Although suchmicas are rare, being found occasion- Montana, clintoniteis found locally in impure lime- ally in thermally-metamorphosedimpure carbonate stones near the contact with granodiorite. Knopf rocks, they are interestingfrom a crystal-chemical (1953) reported several occurrencesof clintonite point of viewbecause of their high ratio of aluminum south and southeastof Helena in rocks which he to silicon(>l) in the tetrahedralsites. Olesch (1975) called pyrometasomatictactites. This report de- has synthesizedhomogeneous clintonites over the scribesclintonite-bearing marbles in the area l0 km compositionalrange z : 0.6 to 1.4.This rangeof westof Helena(Rice, 1975;1977). For theserocks the solid solution encompassesand on the aluminum- rather specialbulk-compositional requirements were rich sideexceeds the observedvariation in naturally- met in the originalsediment, and, exceptfor changes occurring brittle micas. Olesch and Seifert (1976) in the chemicalpotentials of HzO and COz,the for- havedetermined the upper thermal stability limit of mation of clintonite was not relatedto metasomat- the solid-solutionseries, and have shown that such ism, but ratherto nearlyisochemical reactions taking micas have a rather large P-T field of stability. In placein HzO-richfluids. view of the large P-Z stability field, they conclude that the critical parametersfor clintonite formation Clintoniteparageneses are not fluid pressureor temperaturebut a rather In the areawest of Helena(Fig. I ), granodioriteof specialbulk compositioncharacterized by high AlzO, the CretaceousBoulder batholith and Paleozoicsedi- and low SiO, contents.The rarity of such composi- ments are in subverticalcontact over a distanceof tions has led them to further suggestthat clintonite about 20 km. The metamorphosedcarbonate rocks willnot form unlessthe originalbulk compositionis are impure dolomitic limestones,typically silica-un- 0003404x/ 79 /0506-05 I 9$02.00 519 520 RICE: CLINTONITE-BEARING MARBLES

Clintonite-bearinglimestones have been found at four localitieswithin 100meters of the intrusivecontact.Knopf (1953)reported an additionaloccurrence at the contactof the smallsatellite stock in the north- ern portion of Figure l. The assemblagesin these samplesare eithercalcite-clintonite-olivine-clinopyroxene-phlogopiteor calcite-clintonite-olivine-col- orlessspinel-phlogopite. As illustratedin Figure 3, clintonitetypically occurs as randomly-oriented elon- gate blades I to 2 mm in length. When the brittle mica is abundant(ca. 20 modal percent)the rock takeson a microscopicjackstraw texture. The clintonite is characterizedby a smalloptic angle(2V" : 5"; and pleochroismranging from colorlessalong a to very pale brown alongB and 7. Olivine and clinopy- roxeneoccur as prismaticidiomorphic poikiloblasts composedof numeroussubhedral grains with common extinction.The observedtextural relationships indicate that clintonite formed by prograde meta- morphismin equilibriumwith its coexistingphases. Basedon the calculationsof Jaeger(1957), the temperatureat the immediatecontact of the granodiorite is believedto have been between600 and 700"C.Structural and stratigraphicrelations summa- Fig. l. Outline geologic map of the northernmost portion of the rizedby Rice(1975) suggest that the total lithostatic Boulder batholith showing the location of clintonite-bearing pressureduring metamorphismwas approximately samples. Numbered isograds described in detail by Rice (1977) include: (l) first appearance of tremolite and phlogopite; (2) first 1000bars. The absenceof eitherpericlase or brucite appearance of diopside in silica-saturated bulk compositions; (3) appearanceof olivine * spinel and simultaneous disappearanceof calcic amphibole * dolomite; (4) appearanceof diopside * calcite * spinel. dersaturatedand highly aluminous.In the central and outer portionsof the contactaureole, rocks of this bulk compositioncontain the assemblagecalcite- dolomite-calcicamphibole-chlorite-phlogopite (see Fig. 2 for chemographicrelations). Isograd 3 marks the first appearanceof the assemblageolivine-calcite-spineland the simultaneousdisappearance of calcicamphibole * dolomite.Between isograd 3 and the intrusivecontact, the predominantmineral as- semblageis calcite-dolomite-olivine-spinel-phlogopite. Less common bulk compositionsare slightly more silica-richand containthe assemblagecalcic amphibole(pargasite) * calciteinward from isograd 3. Rocks of this bulk compositionreact along Fig. 2. Compositionof modelphases in the systemCaO-MgO- isograd 4 to produce clinopyroxene,resulting in SiOr-AlrOr-COr-HrOprojected from calcite,COz, and HrO onto the assemblagecalcite-olivine-clinopyroxene-spinel- the plane dolomite-quartz-corundum. Abbreviations and phlogopiteinward from isograd4. Overmost of the compositions:Qtz = quartz;Co = corundum;Do : dolomite;Fo : : : : area, the assemblagecalcite-olivine-clinopyroxene- forsterite;Tr tremolite; Di diopside;Cats calcium Tschermak'spyroxene; Sp = MgAl-spinel; Cte : chlorite spinelcontinues inward to the contact;locally, how- [Mg"ALSLO,.(OH)'];Mi"" : rangeof clintonitesolid solution; ever,rocks of this bulk compositioncontain clinto- Mi' : 6u14tz.Alo6Si,.Al2"O.(OH)r;ML : CaMgrAlSiAl, nite. O'o(OH>. RICE: CLINTONITE.BEARING MARBLES 521

Table 2. Microprobe analysesof clintonite and phlogopite

CLINTONIIE PHLOCOPIIE 2 SiO2 79.2 18.8 18.4 39.8 40.0 TiO2 0.40 o.47 0.46 0.61 0.62 A1203 39.6 39.8 41.01 17.0 t6.4 FeO* L.69 1.38 L.46 1.60 r .55 Mco 2L.6 2t.6 20.9 25.5 25.6 CaO la ) 13.0 r2.9 <0.02 <0.02 Kzo <0.01 <0.01 0.01 10.7 LO.7 Na2O 0.72 0.18 0.16 0.26 0.22 F 0.1 0.3 0.2 0.5 0.4 95.86 95.4s 95.42 95.76 95.32

ForrruLa mtts per I eations Fig 3. Photomicrograph of the assemblage calcite (Cc)- si L.345 r.321 L.296 2.796 2-823 olivine (Ol )-diopside (Di)-clintonite (Mi) Alrv 2.655 2.679 2.704 I.204 L.rt1 pseudomorphsafter periclaseimplies a temperature Atvr 0.615 0.626 0.699 0.207 0.190 of lessthan 650oC(Greenwood, 1967' 1975). This Ti 0.021 0.025 0.024 0.032 0.033 3* rangeof contacttemperatures (600-650"C) is sup- Fe 0.018 0.046 0.002 0.001 0.001 ported by calcite-dolomitegeothermometry, which Fe2* 0.081 0.035 0.084 0.094 0.092 2.256 2.264 2.L94 2.671 2.692 yields temperaturesslightly greater than 600oC for Ca 0.991 0.979 0.973 <0.001 <0.001 samplesnear the contact(Rice, 1977). K <0.001 <0.001 0.001 0. 959 0.953 Na 0.016 0.024 0.022 0.035 0.031 Table L Microprobe analysesof clinopyroxene and olivine crrnoplFdVEfrE x Total iron as FeO, x* Totals less oxygen equivalency of fluorine. 723 si02 52.I 50.3 48.8 41.0 40.9 Mineral chemistry Tio2 0.80 1.93 2.s8 0.01 <0.01 A12O3 3.39 4.63 6.00 <0.01 <0.01 Electronmicroprobe analyses of coexistingphases FeO* 2.30 2.02 L.96 4 . s8 4.88 in three samplescontaining the assemblagecalcite- 16.5 16.0 15.7 s5.1 54.r olivine-clinopyroxene-clintonite-phlogopiteare Ca0 25.0 25.5 24.7 0.01 0.10 givenin TablesI and2. Standardsincluded a suiteof 100.1 100.4 99. 8 100.7 99.98 naturaland syntheticolivine, clinopyroxene, phlogopite, and anorthite, as well as a seriesof synthetic Cations per 6 otAgens 4 orAgens oxides.Data reduction was performed with a Bence- Si 1.898 1. 834 r.790 0.976 0.982 Albee-typecorrection procedure. Ti o.022 0.053 0.071 0.0002 < .0002 Olivineis typicalof that in metamorphosedimpure A1 0.r45 0.198 0.259 <. 0001 < ,0001 dolomitesin beingclose to its idealend-member com- Fe 0.070 0.062 0.060 0.091 0.o41 position (mole percentforsterite >95). In contrast, Mg 0.896 0.871 0.858 1.955 1.936 clinopyroxenecontains significant amounts of alumi- o.976 0.996 0.969 0.0002 0.002 num and titanium.Below the analysesof clinopyrox- Fomula units per 4 cations eneare listedcalculated cations per six oxygens.All S1 r.894 7.828 1.785 three analysesyield a cation sum which is slightly .-IV 0.106 0.r72 0.2r5 greater than 4.0, suggestingthe presenceof ferric ..vI 0.039 0.026 0.o44 iron. The data havetherefore been recalculated into Ti 0.o22 0.053 0.071 4.0 In pro- 1+ formula units normalizedto cations. this te 0. 014 0.034 0.027 1L cedure,additional oxygen is addedto bring its sum !e 0.056 0.o27 0.033 to 6.0, and for each amount of oxygenadded twice 0.894 0.867 0.856 that amount of Fe2+is convertedto Fe3+(Finger, 0.97 4 0.993 0.968 1972).The resultingformulae have ferrousto ferric ratiosranging from 4.0 to 1.0.Titanium is considered * Total iron as FeO to be entirelyTio+. Combining Fd+ with Mg, and 522 RICE: CLINTONITE-BEARING MARBLES

Figure 4 showsthe observedcompositions of coexistingclinopyroxene and clintonite projectedonto a portion of the plane SiO2-AtOr-MgO. Note that the compositionof a given clintonitelies slightly to the left of a tie line connectingthe compositionsof coexistingclinopyroxene and spinel.The variationof clinopyroxeneand brittle mica compositionsimplies attainmentof chemicalequilibrium and movementof the 3-phasetriangle Fo-Cpx-Mi as a function of intensivevariables. similar to that observedwith 3- phase triangles on the familiar AFM projection (Thompson,1957). Origin of clintonite Aside from the proximity of clintonite-bearing rocks to the intrusivecontact, there is little evidence in eithertheir mode of occurrenceor compositionto suggestthat theserocks are of metasomaticorigin. In particular,the presenceof relativelylow-variance assemblagesis not indicative of mass transfer down externally-controlledchemical potential gradients. On the contrary, the occurrenceof the isobaric

Fig. 4. Compositionsof coexistingclinopyroxene and clintonite projectedonto a portion of the planeSiOz-MgO-ALOg.

Fe3+ with Al, calcium Tschermak'scomponent (CaAlSiAlO.)ranges from 5.3 to 7.1 mole percent, while the titanium component(CaTiAlzO.) ranges from 2.3 to 7.1 mole percent. As with clinopyroxene,the mica analyseshave beenrecast into formula units by normalizationto a constantsum of cations.However, for thesemicas it B makes very little differenceif one normalizesto 8 cationsor 22 oxygens.Phlogopite is similar in composition to that found in clintonite-free rocks throughout the contact aureole. There is no in- dicationof calciumsubstitution in phlogopite,nor of potassiumsubstitution in clintonite. The observed clintonitecompositions (Si : 1.29to 1.34atoms) lie closeto the MgSi-richend of the solid-solutionseries synthesizedby Olesch(1975) and are some of the most Al-poor natural clintonites. This feature in- dicateslittle or no diminutionof the MgSi-richend of the solid solution at temperaturesdown to about Fig. 5. (A) Configurationof 3-phasetriangles resulting from 600oC.The brittle micas appear to be strictly tri- reaction(a) Fo + Di + Sp t Cc * H,O = Mi + CO,. (B) octahedralin that thereis no substitutiontoward the Configurationof 3-phasetriangles resulting from reaction(b) Di + dioctahedralcalcium brittle mica margarite. Cc*Sp*HrO=FofMi*COz. RICE: CLINTONITE.BEARING M ARBLES 523 divariant assemblagecalcite-olivine-clinopyroxene- Table 3. Stoichiometricreaction coefficients of model reactions spinelinward from isograd4, and the observationof cc Do Fo 1!r DL Cte Sp Mi C)z Hz) clintonite compositionsplotting within this phase lHi, cte, spl volume, are strong evidence that the clintonite t-) +6 -4 +13 +10 +4 formed by nearlyisochemical reactions involving the 2-5 +6 -3 +11 +5 +3 observedphases and a COz-HrO vapor. 3-3+1 -1 +4 +1 +1 The lack of adequatethermodynamic data for the 4+4-3 +2 -1 -2 -1r -I clintonitesolid-solution series precludes calculation 5 +13 +8 +9 +1 Mil of mineralequilibria in termsof P, T and lDi, XCO,. It is 6-4 +23 +2 -13 +13 +8 +50 possible, however,to gain a qualitativeunderstand- t-4 +I7 +2 -11 +11 +4 +42 ing of clintonitephase equilibria by an examination 8 -23 +r7 +3 -8 +8 -Lr +29 of the possiblereactions between the phasesobserved 9+2-2 +3 -1 +1 +2 +4 in the Boulder aureole.The calculationof reactions [M1, Do] betweenthe observedphases is complicatedby solid 10 +7 +5 -r1 -2 +2 -7 +3 -3 solution in clinopyroxene,tremolite, chlorite, and +5 +2 +3 Trl clintonite.Since all thesephases exhibit solid solution lDo, L2 +1 +4 -1 +11 -5 -1 +9 of : phase the type MgSi 2Al, the relationsmust be 13 -1 +2 -1 -1 +1 +1 +1 characterizedby both continuousand discontinuous 14 +1 -1 +2 {-6 -3 -1 +3 reactions(see J. B. Thompson,1957'A. B. Thomp- 15 -5 +11 +2 -6 +3 +5 +21 son, 1976).Discontinuous reactions involve either the lDo, Fol appearanceor disappearanceof a phaseor phase 16 +9 +4 -5 +39 -r7 -9 +33 assemblage,whereas continuousreactions involve 17 +15 +11 -39 -4 +2 -15 +3 -1 -1 -2 +1 +3 only variationin the compositionof the phasesand 18 +5 +4 will, in general,only be observedpetrographically as a changein the relativeproportions of phaseswith aluminum-richclintonite (z : 0.6) coexistingwith changingmetamorphic grade. Depending on the for- spineland corundum(Fig. 5B) overa widerange of mula chosenfor the clintoniteend-member, it is pos- pressureand temperature.It thus seemslikely that sible to derive two differentsets of model reactions eventhough the analyzedclintonites lie to the left of betweenthe observedphases. To illustratethis point, the clinopyroxene-spineltie line, the original forma- considerthe possiblereactions between the phases tion of clintonitewas in fact not due to a reactionof calcite, forsterite,clinopyroxene, spinel, clintonite, type (a) but rather a reactionof type (b). andvapor. If thecomposition of theclintonite lies on In view of theseconsiderations, an intermediate the forsteriteside of the clinopyroxene-spineltie line clintonite of composition CaMgrAlSiAlsoro(OH) (e.9., Mi, of Fig. 2) the formation of clintonite is (Mi, of Fig. 2 with z : 1.0)was chosen as a model describedby the following generalreaction: end-member.Stoichiometric coefficients for possible Fo * Cpx * Cc t Sp * HrO a: Mi * COz (a) discontinuousreactions between this clintonite and the phasescalcite, dolomite, forsterite, tremolite, On the otherhand, if the compositionof clintoniteis diopside, clinochlore, spinel, CO2, and HrO have chosento lie to the right ofthe clinopyroxene-spinel beencalculated by the method of Finger and Burt tie line (e.9.,Mi, of Fig. 2) the correspondingreac- (re72). tion formingclintonite is: Although more than 70 reactionscan be written phases, Cpx f Cc * Sp + HrOeMi * Fo + COz(b) betweenthe above a Schreinemakersmulti- systemanalysis indicated that, for bulk compositions As shownin Figure 5, the chemographicrelations lying within the phasevolume calcite-dolomite-diop- and tie line flips resultingfrom thesetwo discontin- side-spinel,only l8 are stablein the presenceof a uousreactions are distinctly different. Although both COr-HrO fluid. Stoichiometricreaction coefficients reactionsproduce the 3-phasetriangles (Fo-Cpx-Mi for the stableequilibria are givenin Table3, and the and Fo-Mi-Sp) observedin the Boulder aureole, ttCOz-ttHrOtopology is shown in Figure 6. Of two reaction(a) producesthe 3-phasetriangle clinopyrox- possiblepCOz-pHzO topologies, Figure 6 is consid- ene-clintonite-spinel(Fig. 5A). The presenceof this eredto be stablebecause (l) naturalassemblages in 3-phasetriangle is not consistentwith the experimen- Al-free siliceousdolomites require the stable exis- tal data of Oleschand Seifert (1976),which show tence of invariant point [Mi,Cte,Sp](Trommsdorff, <11 RICE.' CLINTONITE-B EARING M ARBLES T\ Fo + Tr Sp + C", ,il

Fo*Gc fct",rti,sfl \ Fo*Gc I+

foo,t

+l DiI + Sp

n [o.,r.)

\ '.\ Di + --- -\ 5p :"1:

Fig. 6. pCO, us. pHzO diagram ofend-member discontinuous reactions. Invariant point notation is that ofphases absent. Shaded area shows the stability field for clintonite-bearing assemblagesobserved in the Boulder aureole. Chemographic diagrams are projections from CaCOs, CO,, and HrO. X indicates observed 3-phase(* calcite) assemblages. RICE: CLINTONITE.BEARING MARBLES 525

1972;Skippen and Trommsdorff,1975), and (2) the observation of the six-mineral assemblagecorre- spondingto invariantpoint [Di,Mi] in the Boulder aureole(Rice, 1977) requires it alsoto be stable. Figure6 can be usedto explainthe formationof clintonitein the Boulderaureole. Examination shows that the observedparageneses are stable only in fluids characterizedby a relativelylow chemicalpotential of COzand/or a high chemicalpotential of HzO.The distribution of low-variancemineral paragenesesin the outer and centralportions of the aureolerequires COr-rich fluids, and indicatesbuffering of the fluid compositionby the local mineralassemblages. Field T'C evidencesuggests that isograd3 is the resultof reaction at invariantpoint [Di,Mi] to which the composition of the fluid was driven by the bufferingprocess. At a slightlyhigher grade of metamorphism,clinopy- roxeneformed at the expenseof calcicamphibole and calcitealong isograd4. Inward from isograd4, the observedassemblages are restrictedto lie belowreac- tion 2 (Fig. 6). The innermostportion of the aureole is characterizedby the isobaricdivariant assemblages calcite-dolomite-forsterite-spineland calcite-forsterite-clinopyroxene-spinel,which by themselvesare not capableof bufferingthe fluid composition.The o.1.2.s.4 local occurrencesof clintonite-bearingparageneses are relatedto the latter assemblageby the following *co, reaction: Fig. 7. Temperature-fluidcomposition diagram for a total approximatelocations of stable 2Di + Cc t 6Sp + 3H,O : 3Mi * Fo * pressureof 1000bars, showing CO, reactionsinvolved in the formation of clintonite.Shaded area (14) shows the stability field for clintonite-bearingassemblages It is clearthat bulk compositionscapable of pro- observedin the Boulderaureole. ducing clintonite are presentin thesemarbles, and that the only specialconditions necessary for its for- lyzedclintonites lie in projectionto the left of the tie mation are appropriatetemperatures and the pres- line can beexplained by subsequentmovement of the enceof a fluid characterizedby a relatively low chem- 3-phasetriangles Fo-Cpx-Mi and Fo-Mi-Sp toward ical potentialof COzor a high chemicalpotential of more MgSi-rich compositionsby continuousreac- HrO. Theseconditions were met locally near the in- tions betweenthe phasesCc-Fo-Cpx-Mi and Cc- trusive contact,where initially COr-rich fluids were Fo-Mi-Sp, respectively.The observedcompositional diluted by the addition of HrO from the intrusive variation(Fig. a) is consistentwith suchmovement of body. The nearlyubiquitous presence of phlogopite the 3-phasetriangles. in apparenttextural equilibrium in marblesboth ad- At the presenttime, it is not possibleto accurately jacentto and far removedfrom the intrusivecontact determinethe location of the stableclintonite equi- doesnot supportBucher-Nurminen's (1976) sugges- libria in terms of P, T, and XCO'. Equilibrium (15) tion that brittle micasform from reactionsinvolving has been studied experimentallyin mixed volatile the breakdownof phlogopiteunder conditions of low (HrO-COr) fluids by Hoschek(1976). Unfortunately PKrO. Hoschek'sexperimental brackets, which covera 30o The formationof clintoniteby the discontinuous temperatureinterval at 4000bars pressure, are rather reaction(14) would requirethe compositionof the wide and do not placestrict limits on the thermody- initially-formed clintonite to lie to the right of the namic parameterswhich govern equilibrium(15). clinopyroxene-spineltie line. The fact that the ana- This fact, coupled with the lack of compositional RICE: CLINTONITE-BEARING MARBLES data for the solid-solutionphases in the experiments, thophyllite in roof pendantsof the Bergellgranite, Sondrio, makesit impossibleto extendthe reactionto other northernltaly. Schweiz.Mineral. Petrogr. Mitt., 56,413426. (1972) in the calculatedferric iron pressuresor to use it to calculatethe equilibrium Finger,L. W. The uncertainty content of a microprobe analysis.Carnegie Inst. Wash. Year conditionsfor other clintonitereactions with any de- Book.71.600-603. greeof confidence. - and D. M. Burt (1972)RrecrroN: A Fortran IV computer Realizingthe largeuncertainties involved with the program to balancechemical reactions. Carnegie Inst. Wash. clintoniteequilibria, and also for that matter with Year Book, 71, 616-620. (1967) in chlorite equilibria (seeRice, 1977,or Bowman and Greenwood,H. J. Mineral equilibria the systemMgO- SiOr-HrO-COr.In P. H. Abelson,F'd., Researches in Geochem- Essene,1977), an approximateisobaric T-XCO, dia- istry,2, p. 542-547.Wiley, New York. gram is presentedin Figure7. Although probablynot - (1975)Buffering of pore fluids by metamorphicreactions. correct in detail, this phase diagram describesthe Am. J. Sci..275. 573-594. occurrenceof clintonitein the Boulderaureole rather Hoschek,G. (1976)Zur Stabilitetvon Clintoniteim SystemCaO- well. The T-XCO2locationof equilibrium(15) and MgO-AlrO5-SiOr-HrO-COr. Fortschr. M ineral., 54, 39-40. Jaeger,J. C. (1957)The temperaturein the neighborhoodof a henceinvariant point [Do,Tr] is consistentwith, al- coolingintrusive sheel. Am. J. Sci.,255, 306-318. thoughnot demandedby, the experimentalbrackets Knopf, A. (1953)Clintonite as a contact-metasomaticproduct of of Hoschek(1976), and with calcite-dolomitetem- the Boulderbathylith, Montana. Am. Mineral.,38,ll13-lll7. peratures(600'C+20') determinedfor samplesin the Olesch,M. (1975)Synthesis and solid solubilityof trioctahedral innermostportion of the Boulderaureole. Figure 7 brittle micas in the systemCaO-MgO-ALOs-SiOz-HzO. lrn. Mineral.. 60. 188-199. indicatesthat the field of stability for the observed - xnd F. Seifert(1976) Stability and phaserelations of tri- assemblagesis rather small. The stabilityfield may be octahedralcalcium brittle micas(clintonite group). J. Petrol., further limited by noting that, underthe assumedP- r 7, 29t-314. Z conditions,the absenceof either periclaseor bru- Rice, J. M. (1975) Progressiuemetamorphism of impure dolomite cite requiresthe fluid phaseto have XCOz > 0.05 and mineral equilibria in the system CaO-MgO-SiOz-KzO- Al2Os-COz-H2O.Ph.D. Thesis,University of Washington,Se- (Greenwood,1975). The formationof clintonitein attle. the Boulder aureolecan thus be restrictedto the - (19'17)Contact metamorphismof impure dolomitic lime- temperaturerange of 580' to 620"C with fluid com- stonein the Boulderaureole, Montana. Contrib.Mineral. Pet- positionsbetween XCOz of 0.05 and approximately rol.. 59.237-259. phase 0.2. Skippen,G. B. and V. Trommsdortr(1975) Invariant rela- tions among mineralsin l-X fluid sections.Am. J. !ci.,275, -573. Acknowledgments 56l Thompson, A. B. (1976) Mineral reactionsin pelitic rocks: I. The support of NSF grant EAR77-23020 is gratefully acknowl- Predictionof P-T-X (Fe-Mg) phaserelations . Am. J. Sci.,276 , edged.B. W. Evans and M. Oleschimproved the manuscriptwith 40t-424. their constructive reviews. Thompson,J. 8., Jr. (1957)The graphicalanalysis of mineral assemblagesin pelitic schists.lrr. Mineral.,42, 841-858. (1972) of References Trommsdorff,V . Changein ?-X duringmetamorphism siliceousdolomitic rocks of the central Alps. Schweiz.Mineral. Bowman,J. R. andE. J. Essene(1977) Contact metamorphism of Perrog.Milt.,52,14. chlorite-bearing dolomites at Elkhorn, Montana (abstr.). Geol. Soc. Am. Abstracts with Programs,9,908. Manuscript receiued, April 4, 1978; Bucher-Nurminen, K. (1976) Occurrence and chemistry of xan- accepted for publication, July 10, 1978.