American Mineralogist, Volume 73, pages 1302-1324, 1988

Fluid infiltration through the Big Horse Limestone Member in the Notch Peak contact-metamorphicaureole, Utah TnBooonnC. LA.norx-c. Department of Geological Sciences,University of Tennessee,Knoxville, Tennessee37996, U.S.A. Pnrnn I. Nnnnr,rx Department of Geology, University of Missouri, Columbia, Missouri 6521I, U.S.A. J. J. P.lpmr Institute for the Study of Mineral Deposits, South Dakota School of Mines and Technology,Rapid City, SouthDakota 57701.U.S.A.

Ansrucr Calcareousargillites in the Upper Cambrian Big Horse Limestone Member of the Orr Formation, west-central Utah, have undergonecontact metamorphism where they were intruded by the JurassicNotch Peak stock. Metamorphism of the rocks resulted in nearly complete decarbonation, yet the mineral assemblagesindicate that the fluid phase was nearly free of COr, indicating interaction with a substantial volume of water. This study is designedto determine the volume of externally derived water with which the Big Horse Limestone Member interacted and the mode of interaction. The rocks in the aureole are characterizedby three gradesof metamorphism, separatedby the diopside and the wol- lastonite isograds.Assemblages in incipiently metamorphosedsamples are calcite + quarrz + muscovite + biotite + chlorite * plagioclase.Diopside-zone samples contain calcite 4 quartz + diopside * tremolite + phlogopite + K-feldspar + plagioclaseor a subsetof this assemblage.Wollastonite-zone assemblagesare typified by calcite + wollastonite * diopside * vesuvianite * grossular + K-feldspar * plagioclase.Conditions within the diopside zone, determined from calcite + dolomite temperaturesof interbeddeddolomitic marbles and from the phase equilibria in the system CaO-MgO-Al,O3-SiOr-KAlSi3O8- CO2-H2O,were about 434 "C and )r.o, : 0.024, at a pressureof 2 kbar. These are the inferred conditions for the invariant assemblagecalcite + quartz + tremolite + diopside + phlogopite + K-feldspar, which occurs locally in the diopside zone. Conditions in the wollastonite zone appearto have been 7 > 450'C, x.o, < 0.002. The amounts of volatiles releasedby the rocks were calculatedby the mass balanceprotolith - metamo{phic rock, balancedby appropriate amounts of HrO and COr. Diopside-zone rocks evolved 45 + 23 g of CO, and 16 + 7 g of HzO per kilogram of protolith. Wollastonite-zonerocks released 158 + 45 g of CO, and l0 + 3 g of HrO per kilogram of protolith. The amountsof HrO required to maintain the equilibrium fluid compositions arc 0.73 + 0.36 kg of HrO per kilogram of rock in the diopside zone and ll.2 + 5.0 kg of H'O per kilogram of rock in the wollastonite zone. This water/rock ratio in the wollastonite zone is about 25 times that indicated by stable-isotopestudies on the same rocks. The T-x.o, path of metamor- phism of this rock was (l) at the diopside isograd, the conditions were near, but below, the invariant point calcite + qtrartz * tremolite + diopside + phlogopite + K-feldspar; (2) minor reaction buffered the conditions to those of the invariant point, 434 "C, x"or: 0.024, which representsthe megascopicallyrecognizable diopside isograd; (3) infiltration of 0.3 rock massesof HrO drove the reaction phlogopite * 3 calcite + 6 quarlz : 3 diopside + K-feldspar + 3CO, + HrO to completion; (4) the reaction tremolite + 3 calcite * 2 quartz + 5 diopside + 3CO, + HrO occurred throughout the diopside zone until tremolite was consumed,when T -- 464 oC and x.o, : 0.04; and (5) infiltration of at least 13.3 rock massesof HrO at the wollastonite isograd was required to drive the reaction calcite+ euartz: wollastonite+ CO, to completionatT:464"C, x"o, :0.004. Infil- tration was abetted by the reduction in solid volume of 25o/o.The high water/rock ratio in the wollastonite zone combined with the low water/rock ratio in the diopside zone indicates that the high ratio does not representthe fluid flux, but rather the volume of the fluid reservoir with which the wollastonite-zone rocks equilibrated. Mass-balancecalcu- lations show that there was more than sufficient HrO in the pore spacein the Notch Peak 0003404x/88/ | r r2-r302s02.00 r302 LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHICAUREOLE 1303

stock to have acted as the reservoir. The reaction front representedby the wollastonite isograd was the final outermost locus of effective mixing between the reservoir and the evolved fluid, and the position of the front was governed by the rates of ditrusion of CO, through the fluid and of heat transfer through the rock. Simple calculations of the tem- perature and fluid flux through the aureole,based on heat- and fluid-transport equations, indicate that heat transport on the margins of the stock occurred primarily by conduction, particularly outside the wollastonite zone, and that the major fluid flux through the aureole occurredwithin the first year of intrusion, long before the temperaturehad risen sufficiently for metamorphism to have occurred. Fluid fluxes through the aureoleafter 1000 yr, when significant metamorphism occurred, dropped to the order of I kgl(m,.yr), or less. This amount is sufficient to have interacted with the diopside-zone rocks, giving water/rock ratios (by mass)of the order 0.1. The fluid flux through the wollastonite zone is given by the rate of advancement of the isograd, which gives fluxes ranging from >42kg/(m,.yr) at 50 m from the contact to 0 kg/(m'z.yr)at the maximum distance of 410 m. The results of this study indicate that in those metamorphic terranesin which there is a changefrom rock-dominated to fluid-dominated systemsacross an isograd, the water/rock ratio in the fluid-dominated system representsnot the fluid flux, but the mass of fluid in a reservoir with which the rocks equilibrated.

INrnonuc:ttoN convecting, infiltrating fluid phase recycles through the The fluid phase in metamorphic terranes is the major metamorphic terrane, mixes with fluids evolved from the agent for the transport of mass and, in some cases,heat metamorphic rock, escapesto the surface, and is re- through the crust. Knowledge of the composition and flux chargedwith fluid added to the geothermal system. of the fluid phaseis vital to the determination ofchemical Previous studiesof metamorphosedcarbonate rocks in differentiation processesoccurring in the middle and up- contact and regional metamorphic terranes have docu- per parts of the crust. Yet, like the fugitive components mented both rock-dominated(e.g., Rice, 1977; Hover- that make up the fluid, the fluid phasehas fled the ancient Granath et al., 1983;Labotka, l98l) and fluid-dominat- geothermal systems,now exposed at Earth's surface as ed (e.g.,Ferry, 1987; Bucher-Nurminen,1981; Rumble metamorphicterranes. Minerals in the metamorphicrocks et al., 1982; Hover-Granath et al., 1983; Bebout and and their fluid inclusions, if any, are the only remaining Carlson, 1986)behavior. The behavior offluids in con- evidence of the fluid. The nature of the fluid-its com- tact-metamorphic environments depends on the rock position, flux, source,and destination-must be inferred types and the geometry of the aureole. Nowhere is this from the compositions and abundancesof the minerals more clearly evident than in the aureole of the Notch in the rock or, in some cases,from the fluid inclusions. Peakstock. The work of Hover-Granathet al. (1983)and It is clear from simple calculations (Walther and Or- Nabelek et al. (1984) has shown that relatively pure car- ville, 1982)that fluids within the crust must migrate, and bonate rocks were impervious to fluids, whereas in- this migration can result in the dissipation of heat and terbedded argillaceouscarbonate rocks acted as aquifers dissolved ions through the crust. Becausemost meta- for fluids emigrating from the stock. morphic rocks have porosities much lessthan l0l0,fluids The purposeof this paper is to determine the nature of generatedby metamorphic reactions must have escaped the physical processof infiltration in the contact-meta- into a hydrothermal system consisting of a mixture of morphic aureoleof the Notch Peak stock and to establish evolved and infiltrating fluids. By means of mineral re- the meaning of the water/rock ratio in the permeablecal- actions, the rock maintains equilibrium with the fluid careous argillites. The procedure used to determine during changesin intensive variables. Thus, the compo- amount of fluid that interacted with the argillites is to sitions and abundancesof the minerals monitor the fluid's calculate the mass balance between metamorphic rocks composition and abundance.There can be a complex in- and their unmetamorphosedprotoliths. A companion pa- terplay betweenthe compositionsand volumes of evolved per, Labotka et al. (1988), examinesthe systematicsof and infiltrating fluids. At one extreme, equilibrium fluid the bulk rock chemistry to establish the validity of the compositions can reflect the stoichiometric ratios of the mass-balancecalculations presentedhere. The method is volatile componentsevolved by the rock if there is little describedby Labotka et al. (1984) and is similar to the or no infiltration. At the other extreme,during infiltration reaction-progressmethod of Ferry (1983).The result is of substantial amounts of externally derived fluid, the the amount and composition of the fluid evolved by the buffer capacity ofthe rock can be exceeded,and the equi- rock during metamorphism. The amount of externally librium fluid composition will reflect the composition of derived fluid is determined by comparison of the evolved the infiltrating fluid. Reality lies between the extremes, fluid with the equilibrium ,rco2value determined from particularly in contact-metamorphicaureoles, in which a the mineral assemblages.The water/rock ratios calculat- I 304 LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE

: NotchPeokStock

Fig. l. Generalized geologic map of a portion of the Notch Peak quadrangle,Utah, modified from Hintze (1974), showing samplelocalities and the diopside and wollastonite isogradsbased on assemblagesin calcareousargillites in the Big Horse Limestone Member of the Orr Formation and in the Weeks Limestone. ed by this method vary considerably over the metamor- al. (1983).At low grades,the limestonemarbles contain phic aureole, and this variation is examined for the re- assemblagescharacterized by calcite + dolomite + talc lation between the ratio and the amount of fluid that and calcite * talc * tremolite. At medium grade, talc is infiltrated the aureole. Rate of infiltration, that is, the replacedby diopside in the assemblage.The highestgrade fluid flux, is estimated from a simple model for heat and marbles contain forsterite. Although the marbles consist mass transport in the aureole. The integrated flux from predominantly of calcite and dolomite, most samples this model is then compared with the water/rock ratios contain low-variance assemblages;divariant assemblages calculated from the mass balance. The results indicate in the systemCaO-MgO-SiOr-HrO-CO, are common, and that fluid fluxes are consistentwith water/rock ratios for univariant assemblagesoccur. These assemblagesindi- rock-dominated, or buffered, systems.Water/rock ratios cate that the equilibrium values of Pco, and Prro were in fluid-dominated systemscan overestimatethe flux by buflered by the mineral reactions. In addition, the do- several orders of magnitude. In these cases,the water/ mains of equilibrium were found to be smaller than the rock ratio indicates the amount of fluid that interacted area sampled by a thin section, indicating local control with the rock; however, this fluid did not infiltrate the of p.o, and p"ro by the rock. At a pressureof 2 kbar, an rock, but occupied a reservoir that effectively communi- estimate based on the amount of stratigraphic overbur- cated with the rock. den during Jurassictime, the mineral assemblagesappear to have buffered ,rco2to values as high as 0.8 at temper- pETRoLocy Gnolocy AND oF THE NorcH PEAK aturesas high as 575 "C. AUREOLE The calcareousargillites contain radically different as- The Notch Peak stock, located in west-central Utah, semblagesfrom those in the marbles, reflecting the dif- intruded the Upper Cambrian Weeks Formation and the ferencesin rock composition and in fluid composition Big Horse Limestone Member of the Orr Formation dur- during metamorphism of the two rock types. Unmeta- ing Jurassic time (Fig. l). The Notch Peak stock is an morphosed or very low grade argillites contain calcite + ovoid composite of granite and quartz monzonite intru- qtJaftz + albite + chlorite, phlogopite, muscovite, or sions; the petrogenesisof the granitic rocks is described K-feldspar. Low-grade rocks contain the assemblagecal- by Nabeleket al. (1986).The stockdiscordantly intrudes cite * quartz + diopside + K-feldspar * calcic plagio- the gently dipping Big Horse Limestone Member, which clase. Many samples contain tremolite in addition. At can be traced along strike for more than 6 km. The stra- high metamorphic grade, the argillites are characterized tigraphy of the Big Horse Limestone Member consistsof by the assemblagecalcite * wollastonite + diopside + six upward-shallowingcycles, each comprising thinly in- grossular + vesuvianite + K-feldspar + albite. These terbedded calcareous mudstones and quartz-rich silt- assemblagesare stable in very HrO-rich fluids. Two iso- stones,capped by algal or oolitic limestone. The individ- gradsare shown in Figure l, basedon theseassemblages. ual cycles can be traced directly to the contact with the The lower-grade isograd is the first appearanceof diop- Notch Peak stock. side. Rocks at lower grade than the diopside isograd are The petrologyof the contact-metamorphosedBig Horse only incipiently metamorphosed or are unmetamor- Limestone Member was describedbv Hover-Granath et phosed.The higher-gradeisograd marks the first appear- LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE l 305

TABLE1, Samplesof the Big Horse LimestoneMember of the Orr Formation

Map distance Meta- Ftuid from contact morphic Temperaturet composition+ Sample (m) grade' Mineral assemblage** fc) (x"o.) Cycle 1 argillites 14H-80 72 + 488 High Cal + Wo + Diop + Ves + Gross + Ksp + Plag 466 + 156 0.0014(0.0054) 16H-84 120 High Cal + Wo + Diop + Ves + Gross + Ksp + Plag 519+ 18 0.0642(0.0183) 3H-37 240 High Cal + Wo + Diop + Ves + Gross + Ksp + Plag 495 + 34 0.0015(0.0108) JN-JO 240 High Cal + Wo + Diop + Gross + Ksp + Plag 495 + 34 0.0500(0.0108) 18H-89 240 High Cal + Wo + Diop + Ves + Gross + Ksp 481 + 17 0.0056(0.0075) 20H-98 360 High Cal + Wo + Diop + Gross + Ksp + Plag 445 + 29 (0.0031) 5H-47 720 LOW Cal + Qtz + Trem + Diop + Scap + Ksp 447 + 21 0.0299 23H-137 720 LOW Cal + Otz + Bio + Trem + Ksp + Plag 8H-129 960 LOW Cal + Otz + Diop + Gross + Scap + Ksp + Plag / n-co 1440 LOW Cal + Qtz + Trem + Diop + Ksp + Plag 428 + 14 0.0176 11H-115 2400 Unmet. Cal + Qtz + Musc + Bio + Plag 248+2 Cycle 2 argillites 16H-86 120 High Cal + Wo + Diop + Ves + Gross + Ksp + Plag 519+ 18 0.0032(0.0183) 3H-40 204 High Cal+Wo+Diop+Ves 495 + 34 (0.0108) 3H-39 240 High Cal + Wo + Diop + Ves + Ksp + Plag 495 + 34 (0.0108) 17H-87 240 High Cal + Wo + Diop + Ves + Gross + Ksp + Plag 20H-100 JOU High Cal+Wo+Diop+Ves+Plag 445 + 29 (0.0031) 5H-44 720 Low Cal + Otz + Bio + Diop + Ksp + Plag 447 + 21 0.0388 23H-135 720 LOW Cal + Otz + Bio + Trem + Diop + Ksp + Plag 8H-s9 960 LOW Cal + Bio + Diop + Ves + Ksp + Plag 7H-544 1440 Low Cal + Qtz + Bio + Trem + Diop + Scap + Ksp + Plag 428 + 14 0.0176,0.0193 11H-117 2400 Unmet. Cal + Qtz + Musc + Ksp + Plag 248 + 23 12H-123 5280 Unmet. Cal + Qtz + Musc + Bio + Ksp + Plag 274 + 76 ' Graderepresented by mineralassemblages in argillites.High = wollastonitezone. Low = diopsidezone. Unmet. = unmetamorphosedor incipiently metamorohosed .. Completemineral assemblages, including minor and trace minerals,are givenin Hover-Granathet al. (1983). t Temperaturesare from calcite + dolomite equilibriain adjacent limestones.Errors represent homogeneityof calcite. From Hover-Granathet al. (1983) + Calculatedfrom the equilibrialisted in Table 4 at the given temperature and 2000 bars. Multiple values represent separate equilibria.Values in parenthesesare upperlimits imposed by wollastonitestability. $ Error representsuncertainty in sample location.

ance of wollastonite. Isograds based on reactions in the wollastonite isograd, which separatesthe diopside zone marblesare describedby Hover-Granathet al. (1983). from the wollastonite zone occurs at a distance ofabout 500 + 150 m from the contact.The isogradis closerto Satvrpr,ns the contact on the south and southeastside ofthe stock Twenty-two argillite samplesfrom the top two cycles than it is on the east and northeast side. The difference of the Big Horse Limestone Member were analyzed for may be related to the geometryof the stock; the southeast major and trace elementsby X-ray fluorescenceand in- wall is nearly vertical, whereasthe northeastcontact dips strumental neutron-activation analysis. The methods of gently outward. analysisare describedin Labotka et al. (1988). O-, C-, The temperature of metamorphism of the samples, and H-isotope analyseswere determined on these sam- shown in Table l, is taken from the calcite-dolomite tem- plesby Nabeleket al. (1984).Microprobe analyses of the perature recorded by the cycle 2 limestone between the minerals in thesesamples were obtained by Hover (198l). argillitesamples and is from Hover-Granathet al. (1983). These data are integrated here to determine the balance Most argillite samples lack mineral assemblagesappro- between the amount and composition of fluid evolved priate for calculating the metamorphic temperature, so from the argillite and the amount and composition de- the temperaturesof the adjacent limestoneswere used in rived externalto the rock. The samplelocations are shown phase-equilibrium calculations and as constraints on the in Figure 1. Table I lists the samplesand their mineral calculatedtemperature from heat flow. Hover-Granath et assemblages.The samplesare divided into three groups al. ( 1983) showedthat thesetemperatures were consistent accordingto their mineral assemblages.Wollastonite-zone with the mineral assemblagesin the limestone samples. rocks (high grade)contain calcite + wollastonite + gros- The major-element bulk compositions of the argillites sular + vesuvianite + diopside, diopside-zonerocks (low are given in Table 2 ofLabotka et al. (1988).Changes in grade)contain calcite + qvafiz + diopside and generally the bulk composition with metamorphic grade are dis- lack grossular and vesuvianite, and unmetamorphosed cussedby Labotka et al. (1988) who found no systematic rocks lack tremolite or diopside although some have al- changein the composition with metamorphic grade, ex- most certainly undergonephlogopite-forming reactions. cept for the loss of COr. There is some evidencefor non- The approximate distances of the samples from the systematicredistribution of feldspar components,partic- contact with the stock are also eiven in Table l. The ularly Sr, in some high-grade samples. However, there l 306 LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT.METAMORPHIC AUREOLE

TABLE2. Representativemineral analyses from argillitesof the Big Horse LimestoneMember

Mineral: Chlo- Musco- Biotite Biotite Tremo- Diop- Diop- Gros- Vesuvi- K-feld- Plagio- Plagio- Plagio- rite vite lite side side sular anite spar clase clase clase Grade: LOW Unmet. Unmet Low Low Low High High High High Unmet. Low High Sample: 5H- 12H- 11H- 5H- 5H- 5H- 14H- 14H- 14H- 14H- 11H- 5H- 14H- 44 123 115 44 47 44 80 80 80 80 115 44 80 Reference: 5-164 1-33 1-9 1-54 3-53 3-27 3-4 2-22 1-4 1-21 1-5 4-23 1-22 sio, 30.28 45.59 40.79 4157 55.48 53.18 52.27 3766 35.97 65.59 67.88 44 47 69.90 Tio, 0.00 0.80 0.41 0.19 0.01 0.24 0.02 1 13 3.57 Alros 19.25 30.59 17.34 12.29 2.57 1.54 0.70 11.75 15.14 1846 20.03 35.36 19.47 Cr.O" 0.10 0.03 0.02 0.00 0 00 0.00 0.01 0.00 0.00 FeO 13.27 5.40 5.80 8.44 5.71 5.87 11 .15 1217 4.60 0.00 0.05 0.08 0.09 MnO 0.20 0 03 0.02 0.08 0.00 0.10 0.17 0.18 0.04 Mgo 23.23 1 39 20.56 22.04 2103 14.38 11 .15 0.00 t.tJ 0.19 0 66 0.49 0.10 13.03 24.41 23.36 35.11 34.97 0.12 0.67 18.35 0.29 Na.O 0.00 0.19 0.06 0.00 0.28 0.11 014 0.29 0.51 1138 1.07 11.6s KrO 0.26 10.06 9.34 10.69 0.32 15.98 o 12 0.47 0.05 Total 86.78 94.74 94.83 95.40 98.43 99.83 99.97 98.00 95.71 100.66 100.13 99.80 101.45 Cationproportions Si 3.01 e 1e 2.90 3.01 7.66 1.97 2.00 3.00 17.97 3.00 2.97 2.06 3.01 Ti 0.00 004 0.o2 0.01 0 00 0.01 0.00 0.07 1.34 AI 2.25 2.47 1.45 1.05 0.42 0.06 0.03 1.10 8S2 1.00 1 03 1.93 0.99 Cr 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0 00 0.00 Fe3** 0j2 0.00 0.00 0.81 1.92 Fe 1.10 0.31 0.34 0.51 0 54 0 18 0.36 0.00 0.00 000 000 Mn 0.02 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.02 Mg 3.44 0.14 2.18 2.38 4.33 0.79 0.64 0 00 0.84 0.02 0.05 0.04 0.01 1.93 0.97 0.96 2 99 18.72 0.01 0.03 0.91 0.01 Na 0 002 0.01 0 0.08 0 01 0.01 o.28 0.04 0.96 0.10 0.97 K 0.03 0.88 0.85 0.99 0.06 0.93 0.01 0.03 0.00 - Calculatedvia the methodof Papikeet al. (1974)

appearsto have been no change in the ratios of major tion, the fluid compositions are estimated quantitatively elements during metamorphism. The mass balance be- from the thermodynamicdata of Helgesonet al. (1978), tween unmetamorphosed and high-gade samplescan then as tabulatedby Rice (1983), and the H,O-CO, mixing be calculatedby preserving major-element ratios. propertiesgiven by Kerrick and Jacobs(1981). The Ker- The representativecompositions of minerals from un- rick and Jacobsfugacities give x.orthat is about 90lomore metamorphosed, diopside-zone, and wollastonite-zone H,O rich than those of Holloway (1977) for HrO-rich samplesare given in Table 2. A complete set of analyses fluids. This error is small comparedwith the uncertainties have been deposited.' These were determined by micro- in the data themselves,as discussedbelow. probe analysis using an automated ARLEMX microprobe at the StateUniversity of New York at Stony Brook. An- Diopside zone alytical conditions are completely described by Hover The diopside isogradrepresents the changefrom phlog- (1981).These mineral compositionsare used in the mass- opite + calcite + quartz assemblagesin unmetamor- balance calculations in which the major cations are dis- phosedrocks to diopside + K-feldsparassemblages. Many tributed among the coexisting minerals. diopside-zonerocks contain all theseminerals plus trem- Fr,urn coMposlTroN olite. The six-phaseassemblage is invariant in the system SiOr-KAlSi3Or-MgO-CaO-COr-HrOat a constant pres- The first step in determining the amount of fluid infil- sure and with a binary CO,-H,O fluid. Rice (1977) de- tration that occurred during metamorphism of the argil- scribed two reactionswithin this system, 5 phlogopite + lites is to determine the composition of the fluid in equi- 6 calcite * 24 quartz: 3 tremolite + 5 K-feldspar + librium with the mineral assemblages.Hover-Granath et 6CO, + 2HrO and 3 tremolite * 6 calcite * K-feldspar al. (1983) outlined the general composition of the fluid : 12 diopside + phlogopite + 6CO, + 2H2O. A third in equilibrium with argillites using T-x.o, diagrams reaction, which is a linear combination of these two, is showing the reaction paths based on the textures of the phlogopite * 3 calcite + 6 qvartz: K-feldspar + 3 diop- rocks. Here the equilibria and compositions are calculat- side * 3CO, + HrO. This reaction best representsthe ed to establish the range in conditions that prevailed diopside isograd in the aureole of the Notch Peak stock within the diopside and wollastonite zones. In this sec- becausethe products representthe diopside-zoneassem- blage, whereas the reactants occur in the unmetamor- 'A copy of the analysesmay be ordered Document as AM- phosed The intersection of the two reactions de- 88-393 from the BusinessOfrce, Mineralogical Societyof Amer- rocks. ica, 1625 I Street,N.W., Suite 414, Washington,D.C. 20006, scribed by Rice (1977) occurs at the ?"-x.o, conditions U.S.A. Pleaseremit $5.00 in advancefor the microfiche. 360 "C and 0.002, based on the fugacity coefficients of LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE r30'7

600

(_) + o

o o ^r . Phl c@ i'emry -4: +eI\L Phl + "-

P = 2000bors 350

0,0s xCOz 0.10

Fig. 2. The HrO-rich portion of the 2-kbar Z-x.o, diagram showing reactions that affectedthe Big Horse Limestone Member. Heavy lines were calculatedfrom available thermodynamic data, and light lines are reactionsinvolving vesuvianite with positions constrainedby experimental data. The details around the invariant points [Wo, An] and [An, Czo] can be seenin Hover-Granath etal. (1983).Invariantpoint [I] representstheassemblagecalcite+ quartz + phlogopite+ K-feldspar+ diopside+ tremolite.

Kerrick and Jacobs(1981). The calculated equilibria are ant point. No attempt is made here to correct the values shown in Figure 2, and the stoichiometry of the reactions used to calculate Figure 2. and their equilibrium constantsare given in Table 3; the The assemblagewithin the diopside zone is strictly uni- equilibrium-constant expressionswere taken from com- variant becauseof Fe substitution for Mg. The diopside pilations of Rice (1977, 1983). In the vicinity of the in- in sample5H-44 (Table 2)hasa value of Mg/(Mg + Fe) variant point, all the reactions nearly coincide and have :0.81, and the biotite has a value of 0.82. In sample steep, nearly vertical slopes. For example, on the low- 23H-135, the Mgl(Mg * Fe) value of diopside ranges temperature side of the invariant point, the reactions 3 from 0.61 to 0.67, the tremolite has a value of 0.64 to calcite + tremolite * 2quartz:5 diopside + 3CO, + 0.65, and the biotite has a value of 0.64. The partitioning H,O and 3 calcite + phlogopite * 6 quartz: 3 diopside of Fe and Mg among the phasesis almost equal, and, + K-feldspar + 3CO, + HrO are separated by only therefore, the substitution of Fe for Mg does not signifi- 0.00001in x.o, at 330 'C-an amount strictly unresolv- cantly affect the T-x.orconditions of the equilibria. able within the errors in the thermodynamic quantities- and .x.o, increasesby a value of 0.0004 over the temper- Wollastonite zone ature increasefrom 330 to 360 "C. The wollastonite-zoneassemblage was formed by the Calcite-dolomite temperatures recorded by the diop- reaction calcite + euartz : wollastonite + CO, and by side-zonerocks range from 428 + 14'Cto 447 + 2l "C, reactionsthat formed grossularand vesuvianite. The net indicating that the temperatureof 360 'C for the invariant result of these reactionswas nearly complete decarbona- point is probably too low. The fluid compositions cal- tion of the argillites. Hover-Granath et al. (1983) showed culated from the recorded temperaturesand the equilib- schematic ln-x.o, relations among the minerals in the ria in Table 3 are listed in Table I and range from Jc.o, wollastonite-zoneassemblage. They assumedthat the in- : 0.018to 0.034.The midpoint of the overlappingerrors variant point [Wo, Czo], which involves reactionsamong in temperature is 434 oC, and the averagevalue of x.o, anorthite, calcite, qtrarrz, diopside, grossular,and vesu- is 0.024;these values are taken to be thoseofthe invari- vianite,was stable. Recent work by Hochellaet al. (1982) 1308 LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE

TleLe 3, Equilibriain calcareousargillites

Mineral formulas Ouartz(Qtz) sio, Calcite(Cal) CaC03 Phlogopite(Phl) KMg3AlSisOjo(OH), KJeldspar(Ksp) KAlSi3Os Tremolite(Trem) Ca,MgsSLOdOH), Diopside(Diop) CaMgSi,O6 Wollastonite(Wo) CaSiO" Anorthite (Ano0 CaAlrSi20s Grossular(Gross) Ca"AlrSi"O,. Vesuvianite(Ves) Ca,.Mg"Al,,Si,rO".(OH)" Clinozoisite(Czo) Ca2Al3SioOlrOH Reaction In ,C (2000bars)

(Diop) 5 Phl + 6 Cal + 24 Qtz : 3 Trem + 5 Ksp + 2H,O + 6CO, -65368/r+ 141.14 (Atz) 3 Trem + 6 Cal + Ksp: Phl + 12 Diop + 2H,O + 6CO, -55$6/r + 126.26 (Trem) Phl + 3 Cal + 6Otz : Ksp + 3 Diop + H,O + 3CO, -303261T+ 66.850 (Phl,Ksp) Trem + SCal + 2Qtz : 5 Diop + HrO + 3CO, 287541T+ 64.369 (Czo,Ves, Wo) Anor + 2 Cal + Qtz : Gross + 2COz -126091T+ 27.198 (Czo,Ves, Cal) Gross+Qtz:Anor+2Wo -3361.7/r+ 3.9507 (Czo,Ves, Otz) Anor + Wo + Cal : Gross + CO, -2420tr + 9.034 (Czo,Ves, Anor, Gross) Cal + Otz: Wo + CO, -10189/r+18.164 (Otz,Ves, Gross,Wo) 2czo + COr:3 Anor+ Cal+ HrO -8074.217+ 11.615 (Czo,Anor, Cal) 2 Ves + 6 Qtz:11 Gross+ 4 Diop+ Wo + gHrO (Czo,Anor, Qtz) 2 Ves + 5 Wo + 6CO,: 11 Gross +4 Diop + 6 Cal + gH,O (Czo,Anor, Wo) 2 Ves + 5 Qtz + COr:1'l Gross+ 4 Diop+ Cal +gHrO (Anor,Wo, Ves) 2 Czo + 5 Cal + 3 Otz : 3 Gross + HrO + sCO, -45939/r + 93.258 (Anor,Wo, Cal) 5 Ves + Czo + 4 Qtz : 29 Gross + 10 Diop + 23H,O (Anor,Wo, Otz) 3Ves + 14CO2: 9 Gross+ 6 Diop+ 5Czo + 14Cal + 11HrO (Anor,Wo, Gross) 3 Ves + 87CO2: 11 Czo + 87 Cal + I Qtz + 6 Diop + 8H,O (Czo,Qtz, Wo) 2 Ves + 11COr: 6Gross + 4 Diop+ 5 Anor + 11 Cal + gHrO

and Valley et al. (1985) has shown that the stableinvari- The reaction among these minerals, listed as (Czo, Anor, ant point is [Wo, Anor]; that is, vesuvianite forms at low Qtz) in Table 3, is constrainedto lie at a value of x.o, of temperaturesfrom the assemblageclinozoisite + calcite about 0.0015 by the intersectionof the wollastonite + + qvarlz rather than anorthite + calcite + quartz. The calcite + quartz equilibrium and the reaction (Czo, Anor, reactions among diopside-bearing calc-aluminum sili- Cal), investigatedby Hochella et al. (1982). The reaction cates are shown in Figure 2. All vesuvianite-absentre- is probably nearly vertical in Figure 2 becausethe high- actions were calculated from the equilibrium-constant temperatureside of the reaction, the side that releasesthe expressionsofRice (1983)and the fugacitycoemcients of greaternumber of moles of a volatile component, is gros- Kerrick and Jacobs(1981). The positionsofreactions in- sular * diopside * calcite. Thus, the reaction curve must volving vesuvianite in Figure 2 are constrained by the have a negativeslope and must be asymptotic to the tem- reaction 2 vesuvianite -t 6 quartz: ll grossular+ 4 perature axis. An absoluteerror on the jr.o, value cannot diopside + wollastonite * 9HrO (Czo, Anor, Cal), which be determined, but even a 100/oerror inlnf.o, does not occurs at 400 'C, 2 kbar, and xco2: 0 (Hochella et al., increasethe value to more than 0.002. This value is used 1982),and the topology ofthe vesuvianite-absentreac- as the estimate of x.o, in the wollastonite zone. tions. The formula Ca,rMgrAl,, Si I sO6e(OH)e, determined The actual wollastonite-zone assemblagehas a higher by Valley et al. (1985),was usedto balancethe reactions. variance than the simplified system. Both grossular and All the reactions are listed in Table 3. vesuvianite contain substantialFerO, and TiO, (Table 2). The fluid compositions listed in Table I for wollaston- Fe3*is stronglypartitioned into the garnet;Ihe Ko: (Xo"t-/ ite-zone assemblages,calculated from the equilibrium Xor)"'"""/(Xr"r*/Xo)"'"is about 3.5. The effect of Fe3+ in (Czo, Ves, Qtz) in Table 3, are generallylow in x.or. The theseminerals is to extend the stability field of garnet to values in many casesare, however, inconsistent with the even more water-rich conditions than those shown in stability of wollastonite becausethe plagioclaseis very Figure 2. Ti is more strongly partitioned into vesuvianite; albitic, requiring a long extrapolation from the end-mem- the K" : (Xr,/Xo)c'"'"/(X-ri/X^)"'is about 0.4. This par- ber reaction. The value of rco2 is estimated instead by titioning extendsthe stability field of vesuvianite to more the stability of the mineral assemblage.The wollastonite- COr-rich conditions than shown in Figure 2. The effects zone assemblage,wollastonite * vesuvianite + grossular of the partitioning of TiO, and FerO. between grossular + diopside + calcite, is univariant in the CaO-AlrO.- and vesuvianite on x.o, tend to cancel. MgO-SiOr-COr-HrO system at Po,. : P"",ro: 2000 bars. The transition from the diopside zone to the wollas- LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE r 309

y : tonite zone is abrupt. There is little evidence to indicate metamorphic rock. The mass-balancerelation is Ax, which reactionsresulted in the formation of grossularand in which y is the composition of a rock' A is a matrix of vesuvianite. The textural evidence for the sequenceof the compositions of the constituent minerals, and x is the mineral gowth, describedby Hover-Granath et al. (1983), abundanceof the minerals. The solution of this equation is complex. The plagioclasein the diopside-zoneassem- is exact if the number of minerals equals the number of blagesis generallyvery anorthitic, whereasthe wollaston- components or if the abundances(x) are known; other- ite-zone plagioclaseis albite. There is, however, no evi- wise, the solution gives the least-squaressolution. Com- dence for the formation of clinozoisite as predicted by plete details about this calculation are given in l-abotka the T-x.o, diagram. The transition appearsto represent et al. (1984).The results,given in Table 4, are expressed per a distinct changein x.o, from that of the diopside zone as a mass transfer of minerals in grams kilogram of (0.024) to tbat of the wollastonitezone (<0.002). The the protolith. Table 4 indicates the grams of minerals re- transition also representsa substantial amount ofdecar- consumed and produced during metamorphism. The - bonation. Thus, the wollastonite isograd is consideredto sulting relation, protolith metamorphic rock' is bal- be a discontinuity, an infiltration front, at which the ar- anced for HrO and COr. These are the net amounts of gillites were totally recrystallized. HrO and CO, releasedby the rock. This mass-balance method calculatesthe total amounts of volatiles released Trm rvr.lss BALANCE and was used in preferenceto measuringthe progressof The fluids that were in equilibrium with the argillites individual reactions becausethe thin sectionsof coarse- and be- in the Big Horse Limestone Member were nearly uni- grained rocks are unrepresentativeof the bulk formly HrO-rich, particularly in the wollastonite zone causethe mass balance is indicative of the total amount period where the mineral assemblageindicates that the fluid of evolved fluid, integrated over the entire of composition was near xcoz: 0.002. The metamorphic metamorphism. reactions, however, were generally decarbonation reac- Table 4 showsthe amount of CO, and HrO evolved by av- tions. Labotka et al. (1988) have shown that the CO,/ wollastonite-zone and diopside-zone rocks. On the a5 gQ'6 CaO wt0/oratio decreasedfrom the value for calcite (0.78) erage,wollastonite-zone rocks evolved 158 t + + of in unmetamorphosedrocks to nearly 0.00 in wollaston- + 1.0mol) of CO, and l0.l 2.7 8 (0.6 0.2 mol) ite-zone rocks. The decreasein this ratio occurred dis- HrO per kilogram of protolith' The changein mineralogy in continuously, with major changesat the diopside and and loss of volatiles were accompaniedby a reduction wollastonite isograds and little change between the iso- solid volume of 24o/o+ 60/o.The volume of the released grads.Labotka et al. (1988) have also shown that, with volatiles was about 73o/o+ 200/oof the starting rock vol- the exception of COr, there was no changein the major- ume. This was calculated assuming specific volumes of element chemistry during metamorphism. It is possible, HrO and CO, equal to 1.5 and 1.6 cm3/9,respectively, then, to calculatethe amount of CO, evolved by the rock which are appropriate for a temperature of about 500 "C becausethe mineralogy of the protolith as well as of the and a pressureof 2 kbar (Burnham et al., 1969; Shmonov metamorphic rock is known. and Shmulovich, 1974). The diopside-zone rocks had + g The unmetamorphosedargillites contain calcite, quarlz, evolved45 + 23 g(1.0 + 0.5mol)of CO, and16 7 plagioclase,and some combination of K-feldspar, biotite, (0.9 + 0.4 mol) of H,O per kilogram of protolith with a -7o/o muscovite, and chlorite. In somerocks, the minerals were AZ"ora: + 4o/o. probably produced by very low grade metamorphic re- Metamorphism resulted in a substantial loss in solid actions; in others, they are detrital minerals. The proto- volume; some rocks had lost nearly one-third their orig- quantity lith assemblageis taken to be either calcite * quatlz + inal volume! The rocks also evolved a large of : muscovite + chlorite + phlogopite or calcite + qtrartz + CO, and HrO, having an averagecomposition of x.o, : muscovite + phlogopite + K-feldspar. These are com- 0.86 for wollastonite-zonerocks and x.o, 0.53 for diop- mon assemblagesin very low grade,"biotite-zone" rocks side-zonerocks. There is no systematicloss of volatiles (Labotkaand Nabelek,1986; Ferry, 1984).This assump- within either zone. The amount of releasedvolatiles is pro- tion eliminates from consideration the biotite-forming generally correlated to the amount of calcite in the reactions,about which little is known in these rocks. tolith (Labotka et al., 1988).The principal loss of CO, The diopside-zoneassemblage is consideredto be cal- and HrO appearsto have occurred at the isograds. cite * quartz + diopside * K-feldspar + calcic plagio- Although metamorphism resulted in a net loss of both clase. Many diopside-zone rocks contain phlogopite or CO, and HrO, the wollastonite-zonerocks appearto have tremolite in addition, indicating that the diopside- and lost less HrO than the diopside-zonerocks. This implies K-feldspar-forming reactions did not go to completion. that a small amount of HrO was added to wollastonite- phlogopite In these cases,the mass-balancecalculations determine zone rocks. At the diopside isograd, where the maximum amounts of CO, and HrO that could have broke down to diopside + K-feldspar, an averageof 1.0 per been releasedby the samples.The plagioclaseis repre- mol of CO, and 0.9 mol of HrO were evolved kilo- sentedby the separatecomponents albite and anorthite. gram of protolith, resulting in a solid-volume loss of 7ol0. The mass balance is calculated by conserving the At the wollastonite isogad, where calcite and quattz rc- amount of cations, except H and C, per kilogram of the acted to form wollastonite and where grossularand ve- 1310 LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE

TABLE4. Mass balance(grams per kilogramof protolith)

sample otz cal chl Musc Phl Ksp Anor Diop wo Gross ves co. Hro Av*,o* %""* Wollastonitezone -236 -286 80 0 -69 154 151 0 225 134 61 39 126 9 -19 58 -276 84 -400 -61 -68 -56 85 0 198 249 116 25 176 13 -26 83 -213 -240 JO 0 -71 -165 160 0 241 69 57 48 106 I -17 -254 50 -405 -114 -117 0 82 -13 191 158 107 172 178 15 -28 85 -248 -390 89 -71 -51 -3 38 0 133 282 83 45 171 10 -25 -233 80 98 320 0 -4s -99 98 0 145 243 37 29 141 6 -20 63 -306 -472 oo -89 -51 -'17 47 0 185 335 84 63 208 13 -30 96 -304 40 -505 -71 -16 0 11 -34 119 420 48 104 222 7 -30 100 -242 -256 '112 39 0 -162 -179 241 0 251 10 61 139 11 -19 54 -194 -205 87 0 -66 173 161 0 250 25 31 72 90 8 -15 43 -285 -473 100 -75 -17 0 12 -64 140 351 170 23 208 10 -31 95 -254 -359 \x) -44 -67 -77 99 -10 189 207 78 69 158 10 -24 73 o 36 103 44 42 78 73 21 49 138 41 49 45 3620 Diopsidezone -59 -71 -78 47 -102 5s 35 111 67 00 031 12 -5 19 -47 -51 -43 137 -124 37 62 108 26 00 o22 10 -3 14 -41 co 54 72 -87 64 18 96 41 00 024 10 -4 15 -179 -182 44 125 167 -7 121 186 249 00 080 24 -13 46 -141 -128 -43 135 - 184 -50 162 155 156 00 056 16 -9 32 -112 -126 -120 -296 54 109 134 266 67 00 056 24 -8 36 -97 -102 -80 - (x) tou 35 89 1s3 101 00 o45 16 -7 27 d 57 52 36 /o 56 58 65 86 00 023 7 4 13

. Note. Grams lost (-) or gained (+) per kilogram ot protolith during the mass transfer protolith - metamorphic rock. Mean : (x) and standard deviation: o. . Percentof the protolith solid volume lost '- Percentof the solidvolume. Calculated with specificvolumes of 1.5 and 1.6 ml/g for H2Oand COr, respectively.

suvianite formed by reactions among anorthite, calcite, actions, thereby maintaining a constant fluid composi- wollastonite, and diopside, an additional 2.6 mol of CO, tion. per kilogram of protolith was releasedand 0.3 mol of The amount ofdecarbonation and dehydration that the HrO per kilogram of protolith was added to the rock on diopside-zonesamples have undergone(Table 4) at a fluid average,resulting in a further volume reduction of l7o/o compositionof x.o, : 0.024 (w.or:0.057) requiresthat of the original volume. 730 + 360 g of HrO per kilogram of rock have infiltrated this zone. The amount that infiltrated the wollastonite INrnrnarroN isograd dependson the:r.o, that prevailed at the isograd. The composition of the volatile fraction evolved by the The value ofx.o, within the zone was 0.002, but the value argillites during metamorphism was significantly ditrer- at which the reaction occurred must have been higher. ent from the equilibrium fluid composition indicated by As described in a following section, a value of 0.004 is the mineral assemblages.A water-rich fluid must have adopted as the value at the isograd. The nearly complete infiltrated the argillites to dilute the CO,-rich fluid evolved decarbonation at the wollastonite isograd, where an ad- by the rocks. The amount of fluid that infiltrated the ar- ditional I l3 + 50 g of CO. per kilogram of protolith was gillites can be estimated by balancing the evolved fluid lost and 6 + 8 g of HrO per kilogram of protolith was with sufficient HrO to obtain the equilibrium composi- gained at a fluid composition of x.o, > 0.002 (ilr"o, 2 tion. This procedure,described by Ferry (l 980, I 986) for 0.005) requires an infiltration of as much as 22.5 + 9 kC rocks undergoing fluid-composition-buffering reactions, of HrO per kilogram of protolith. If the value of x.o, was can be written as 0.004at the isograd,then the infiltratedamount was I1.2 t 5 kg per kilogram of protolith. These values, repre- ffico, senting water/rock ratios (by weight) of 0.73 in the diop- wcoz : " (l) lllcoz,, I /71u2o,. I ff|n2o,i side zone and 11.9 to 23.2 in the wollastonitezone in- tegrated over the entire time of metamorphism, are in which w"o, is the weight fraction of COr, zn is the mass phenomenal. Allowing for the greatestpossible error in of volatile component in grams per kilogram of protolith, the equilibrium fluid composition within the wollastonite the subscript e refers to evolved volatiles (or, ifnegative, zone-xcoz: 0.01 based on wollastonitestability-the added), and the subscript i refers to infiltrated volatiles. water/rock ratio by weight still had to be 4.6. Equation I is independent of the amount of initial pore The calculated water,/rockratios are so large that it is fluid because the composition remains constant. The worth consideringother evidencefor massiveinfiltration equation is accurateif every increment of infiltrating HrO of HrO. One implication of the mass balanceis the large causesan incremental advance in the volatilization re- loss in solid volume during metamorphism, -250lo in the LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE 13rl

NaCl

3 d field

Hzo CO Phl+Cc+ (400o) Di + Ksp (500o) (400o) Wo Fig. 3. Isothermal, isobaric phase diagram for NaCl-HrO- asymptotic to the NaCl-HrO side of the diagram and intersect CO, showing the solvus at 500 (black line) and 400 "C (gray the solvusat halite saturation.The enlargementshows the nature line), 2000 bars. The critical point is indicated on the 400 qC ofthis intersection for the calcite + qnafiz: wollastonite reac- solvus as CP. Curves showing the stability limits of calcite + tion schematically. The fugacity coefrcients of Bowers and quartz and phlogopite + calcite + q\artz for the two tempera- Helgeson(1983a) were used to calculatethe solvi and the posi- tures are also projectedonto the diagram. The curves for 500'C tions of the reactions. are black; those for 400'C are gray. These reaction curves are wollastonite zone.We have beenunable to document this the water/rock ratio measuredby the mineral assemblage large AZby mapping individual layers; however, several and the integratedfluid flux. Labotka and Nabelek (1987) deformation featuresindicating a changein volume have showedthat the degreeofisotope exchangewas aided by been noted. Near the contact with the stock, the country the amount of volatilization. The discrepancyin the water/ rocks are cut by numerous veins, now filled with grossular rock ratios determined by the isotope study and by this and vesuvianite, and sills ofgranite, which indicate that study probably indicates differencesin the processesre- the rock was fractured during metamorphism. In addi- corded by the rocks. tion, the country rocks along the margins of the stock are The values of infiltrated HrO are calculatedby assum- locally ductilely deformed, indicating that some of the ing that local equilibrium is maintained during infiltra- solid volume loss createdduring reaction may have been tion. Nonequilibrium processes,for example, exsolution occupiedby the intrusion. and escapeofa COr-rich vapor from the fluid phase,can The d'3Cdata of Nabeleket al. (1984)indicate that, in have significant consequenceson the calculated water/ general,decarbonation took placefractionally; that is, the rock ratios. A small amount of NaCl disolved in the fluid evolved CO, was removed from the rock as it was being significantly expands the immiscibility region between generated.The implication of this processis that x"o, in H.O-rich and COr-rich phases (Bowers and Helgeson, the infiltrating fluid phase was 0.0. The mineral assem- 1983a). The two-phase fluid can be fractionated effica- blagesare consistentwith a nearly pure HrO fluid phase, ciously by escape("boiling off') of the less-denseCOr- and if a large volume of HrO were infiltrating the wollas- rich phase.Trommsdorffand Skippen (1986) argued that tonite zone, the evolved CO, would have readily been this processcan lead to halite saturation in the small pro- removed. The water/rock ratio indicated by the O-iso- portion of residual HrO-rich liquid. Bowersand Helgeson tope data for wollastonite-zonerocks is only l0o/oto 300/o (1983b) showed that many mixed-volatile reaction sur- of that indicated by the mineral assemblages.The differ- facesintersect the HrO-CO, solvus at geologicallysignif- encereflects either incomplete isotope exchangebetween icant temperatureswith the addition of very little NaCl the infiltrating H.O and the rock or a differencebetween to the fluid. Figure 3 shows the H,O-COr-NaCl phase r3t2 LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE

TeeLe5. Progressivemetamorphism of NP 18H-89,a modelfor the BigHorse Limestone Member

Nascent Wollastonite-zone Highdiopside-zone Lowdiopside-zone diopside-zone Unmetamorphosed assemblage assemblage assemblage assemblage assemblage

Quartz 0.00 0.0 15.98 16.5 16.68 17.1 21.56 22.2 24.78 25.5 Calcite 20.78 20.8 JU.C / 50.7 52.11 52.2 56.17 56.3 59.76 59.9 Muscovite 0.00 0.0 0.00 0.0 0.00 0.0 0.00 0.0 5.13 5.1 Chlorite 000 0.0 0.00 0.0 0.00 0.0 0.00 00 7.12 6.5 Phlogopite 0.00 00 0.00 0.0 0.00 0.0 5.65 5.5 0.28 0 3 Anorthite 000 0.0 7.52 7.4 7.14 71 7.14 7.1 0.00 0.0 Tremolite 0.00 0.0 0.00 00 4.13 5.1 4.13 3.7 0.00 0.0 Diopside 13.33 113 14.01 11.9 8.79 0.00 0.0 0.00 0.0 Wollastonite 28.22 265 000 0.0 000 0.0 0.00 0.0 0.00 0.0 Grossular 8.28 o.J 000 0.0 000 0.0 0.00 0.0 0.00 0.0 Vesuvianite 451 ea 0.00 0.0 0.00 0.0 0.00 0.0 0.00 0.0 KJeldspar 3.77 4.0 3.77 4.0 3.77 4.0 0.00 0.0 0.00 0 0 Albite 159 t.o | .cY 16 1.59 t.o 1.59 1.6 1 59 1.6 Sphene 0.58 05 0.58 0.58 u.c 0.58 0.5 0.58 0.5 Apatite 0.75 06 075 0.6 075 UO u /3 0.6 o.75 0.6 Sum 81.82 75.4 94 78 93.2 95 54 94.4 97.s8 97.6 100.00 100.0 Soliddensity 2 96 2.78 2.74 2.73 Cumulativeloss of volatiles(grams per kilogramof protolith) CO, loss 171.4 40.4 33.6 15.8 HrO loss 10.4 11.7 10.8 8.3 Cumulativediffe.ence in specitic enthalpy (ioules per kilogram of protolith) 332544 105317 91022 54196 Nofe; Wt% indicatesweight percent of protolith (i e., unmetamorphosedassemblage)

diagram at 2 kbar,400 and 500 "C, calculated from the The results are impressive, but they might not be rea- data of Bowersand Helgeson(1983a), and the positions sonable.It should be possibleto place quantitative limits of the equilibria calcite + qtrafiz : wollastonite * CO, on the fluid and heat fluxes over the lifetime of the au- and phlogopite * 3 calcite * 6 quartz: K-feldspar + 3 reole,based on a simple physical model for heat and mass diopside + 3CO, + H2O, calculated with the fugacity transport. The physical model must be simple because coefficientsof Bowersand Helgeson(1983a). Both equi- many parameters-like the amount ofreleased COr, fluid libria, which causedmajor decarbonationof the Big Horse composition,LV"ot;a, and VZ-depend on the rock com- LimestoneMember, are asymptoticto the HrO-NaCl side position and the geometry of the aureole. The carbonate ofthe triangle and fail to intersect the solvus betweenthe rocks have a rangein composition, and the geometriesof two fluid phases.Although infiltration of a saline solution the stock and country rock are irregular. For simplicity, could have causedseparation of an initially COr-rich pore one rock composition,that of 18H-89, is chosento rep- fluid, this would have occurred before decarbonation. resentthe carbonaterock composition. The composition Even under NaCl-saturatedconditions, the major decar- of this sample(Table 2 in Labotka et al., 1988)is close bonation reactions occurred in the one-phase,HrO-rich to the average,and the mass balance (Table 4) indicates region, and no separationofa COr-rich phasecould have that the amounts of evolved CO, and HrO were also near occurred. If there was a substantial amount of dissolved the averageof the analyzedsamples. salts in the infiltrating fluid, then the calculated water/ Table 5 shows the changesin the mode of this sample rock ratios are minimal values becausethe equilibrium with progressivemetamorphism. The protolith contained value of Jc.o, decreasesto zero and substantially more the assemblagecalcite + qvartz + chlorite * muscovite HrO had to have infiltrated the rock. + biotite + albite. Breakdown of chlorite and muscovite reactionslike muscovite * quartz T-x"or-m^ro.i PATH oF METAMORPHISM by chlorite + calcite + : phlogopite * anorthite + CO, + H2O, and chlorite + representative A sample calcite + q\artz: tremolite + anorthite + CO, + HrO The conclusion is inescapable:the wollastonite zone in produced the first recognizablemetamorphic assemblage. the Big Horse Limestone Member interacted with vast There was little loss in solid volume and minor produc- quantities of an HrO-rich fluid. The percolation was, no tion of CO. and HrO at this point. Diopside and K-feld- doubt, abetted by the substantial loss in solid volume, spar joined the assemblageby the fluid-composition- which certainly enhancedpermeability. The large waler/ buffering reactionsamong all theseminerals, as described rock ratio resulted from the effect ofconfining the water in the previous section on fluid composition. These re- in the reactive layers. The intervening limestone layers actions proceeded until phlogopite and tremolite were were essentiallyimpermeable. consumedand accountedfor a small loss in solid volume LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE 1313 and the releaseof about 30 g of CO, and HrO per kilo- tolith released15.8 e of CO, and 8.3 g of HrO (Table 5)' gram of protolith in the molar ratio of x.o, : 0.75. Field requiring infiltration of 234 g of HrO per kilogram of : evidenceis insufficient to resolve the first appearanceof protolith to maintain the fluid composition at xcoz tremolite + phlogopite from the first appearanceof diop- 0.024. side + K-feldspar. In this simplified model, the first ap- After thesereactions went to completion, the rock con- pearanceof tremolite + phlogopite coincides with the sisted of calcite + quartz + tremolite + phlogopite + inception of reactionsproducing diopside + K-feldspar. calcic plagioclase,and continued reaction among these After phlogopite and tremolite were consumed,wollas- minerals produced diopside + K-feldspar. Under equi- tonite, vesuvianite, and grossular were produced by re- librium conditions, an invariant assemblagecan only oc- actions like calcite * quartz : wollastonite * COr, an- cur at an isograd,not within a wide zone. This is because orthite + calcite + quafiz : grossular * COr, and there can be no heat or fluid flux acrossthe isograd until anorthite + diopside * calcite + q\arrz * HrO - ve- one of the minerals is consumed. The diopside isograd, suvianite + COr. This step resulted in a quartz-free and then, is representedby the invariant assemblage. nearly calcite-freeassemblage with a volume loss ofabout The invariant-point reactionscan be representedby the 250/0,the releaseof 13 1 g of CO, per kilogram of protolith two linearly independentreactions phlogopite + 3 calcite and absorptionof 1.3g of HrO per kilogram of protolith. -l 6 quartz: K-feldspar + 3 diopside + 3CO, + HrO : Although the final assemblageappears to be isobarically (Reaction I) and tremolite + K-feldspar phlogopite + univariant, the substantial amounts of Fe and Ti in the 2 diopside l- 4 quarrz (Reaction II). Figure 4 shows the grossularand vesuvianite increasethe variance.The wol- possibleprogress of thesetwo reactionsfor the bulk com- lastonite-zonereactions are presumed to go to comple- positionof l8H-89 on a {,-{" plot (Thompsonet al., 1982; tion at the wollastonite isograd. Rice and Ferry, 1982).The origin representsthe incipient isogradassemblage calcite + quartz + tremolite + phlog- The diopside zone opite in the proportions shown in Table 5. The diopside The most appropriate T-x.o, path of metamorphism zone representsa traverse from the origin in Figure 4 to for the model is difficult to choose.As describedabove, the farthest corner. For the two reactions (I and II) rep- the diopside-zonesamples contain assemblagesthat fall resentingthe invariant point, water/rock ratio (w/R) is- on either side of the isobaric invariant point calcite * opleths are nearly parallel to isenthalps.Gradients in the q\artz + tremolite + diopside + phlogopite + K-feld- heat and fluid fluxes are, therefore, nearly parallel, and spar, and some samplescontain the invariant assemblage regardlessof the relative proportion of the fluxes, reac- (Table l). The range in temperature within the diopside tions must proceedin a nearly horizontal direction on the zone is small-zero, within the errors-indicating near- {,-{,, plot. The diopside isogradis representedby the hor- invariant-point conditions. Rice and Ferry (1982) have izontal traverseacross the plot from the origin to the limit suggestedthat many isogradsrepresent invariant assem- (phlogopite).This traverserequired an infiltration of294 blages,particularly in those terranesthat are "rock-dom- g of HrO and an addition of 36800 J of heat per kilogram inated." In the Big Horse Limestone Member, the entire of protolith. diopside zone seemsto representinvariance. The occur- The diopside zone is representedby the univariant as- rence of invariant assemblagesimplies that the initial semblage calcite + quartz + tremolite * diopside + temperature at the isograd was lower than that of the K-feldspar. The diopside zone persisted until tremolite invariant point, but very little reaction was required to was consumed,when the assemblageno longer buffered buffer the fluid composition to that of the invariant point the fluid composition. Within this zone, temperatureand (seeabove). It is only at the invariant point that signifi- fluid composition are related by the equilibrium constant -t : cant reaction can occur, as describedby Rice and Ferry for the reaction tremolite + 3 calcite 2 qtattz 5 (1982), resulting in a noticeable change in mineralogy. diopside + 3CO, + HrO (Reaction III): However, the invariant assemblageitself is incapable of -28754/T 3ln f"o. + ln/',o : + 64.369, (2) buffering the fluid composition at that of the invariant point. All the reactionsrelease CO, and HrO in the ratio where 3:1. The only reactionthat can occur,in the absenceof fcoz: xcoz'Ycorf\o, (3) infiltration, is the fluid-absent reaction tremolite * K-feldspar : phlogopite + 2 diopside * 4 quartz. Infil- and fluid, however, drives decarbon- tration of an HrO-rich xa6r: 3{ur/(4fr' I mn o,,/Mt o), (4) ation reactionsto maintain the fluid composition at that ofthe invariant point. wheremrro,,: massof infiltrated HrO and Mnro: mo- The nascentdiopside-zone assemblage formed from the lecular weight of HrO. breakdown of muscovite and chlorite in the incipiently This set ofequations can be representedin generalform -- metamorphosed (or unmetamorphosed) rocks. The re- as f(T) g(x, m'r,J; therefore, only if the heat or mass actions responsible for the disappearanceof these min- flux is known can the reaction progressbe calculated.A erals must have occurred at x.o, conditions tear 0.024, simple constraint and an additional assumption, how- the value at the invariant point. Each kilogram of pro- ever, allow Ihe T-x"o, path to be determined. The con- 1314 LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE

o.o75 Itremolite]

0.050

i tJr !rl !lo io sl R Ht O i O.O5 .c. !> !6 - 0.025 Reaction I: Phlogopite + 3 Calcite + 6 Quartz = - o.o50 3 Diopside + K-feldspar + 3 CO2 + H2O Reaction II: Tremolite + K-feldspar = Phlogopite + 2 Diopside + 4 guartz

Fig. 4. The reaction spacefor the diopside-isogradinvariant are given by assemblage.A fluid-present (I) and a fluid-absent (II) reaction are necessaryand sufficient to describethe abundancesofmin- Jtr x6;or:0.024: erals within the given bulk composition of sample NP 18H-89. 4{, I mrro.lMnro The diagram is limited within the bulk composition by the dis- appearanceof one phase.The phasethat disappearsat a bound- L : aHftt+ aHl,tr: 272792tt+ l2g72tr. ary is indicated in brackets. The only reaction that can occur along a boundary is the one indicated by the absentphase. These Mrro iE the molecular weight of HrO , mrro.,is the mass of infil- reactionsare given in Table 3. The contours indicate the amount trated HrO, and Al1is the enthalpy of reaction. Theseare nearly ofheat (Z) required by the reaction progress(shaded lines) and parallel, indicating that in the presenceof thermal and fluid- the amount of HrO required to maintain constant fluid com- pressuregradients, advancement through the diagram is nearly position (broken lines) per kilogram ofprotolith. The contours horizontal, i.e., in the direction ofincreasing {,.

straint is to consider the mineral assemblageto be in pore space.Labotka et al. (1984) showed that for infini- equilibrium with only the fluid that was in the pore space tesimal processes,Equation 5 can be integrated if Avrro,, created at the diopside isograd. Any fluid that was gen- : v"ro'At is is the secondassumption. The result is eratedby Reaction III or that infiltrated the diopside zone displaced and consequently diluted pore V the fluid. The t: volume of the pore spaceis presumedto have been con- 3v"or*uH2o+yH2o,i stant; that is, significant deformation did not occur until - (3uto' * '"'o + u",,o''D'o). after metamorphism when fluid generation ceasedand .1n(1"o' (6) - yH2o pore pressureswere reducedto lithostatic or lower values. \ 3r.o, (3y.o, + + vrro.)!/' The additional assumption is that fluid infiltration was a The volume fraction is related to the mole fraction, as- uniform process so that ratio the between mH2o,iand t suming ideal mixing, by was constant, as it is in strictly isothermal infiltration in which x"o, is constant. xco, lco2: - (7) The constant-volume constraint can be representedby xco, * (l x.or)(vr rJv.or)'

y'"or : l3v.orAfi t + (V - 3v.orA{,,, - v"roA{r, The initial value ygo, that of the invariant point, was - Av^ro.)y"orf/V (5) 0.061, which correspondsto rco2 :0.024. Specificvol- umes of CO, and HrO were taken to be 1.63 and 1.50 in which ./co, is the volume fraction of COr, ./6o, is the mllg, respectively(Shmonov and Shmulovich, 1974; volume fraction after incremental reaction progressAErr Burnham et al., 1969). The porosity was 5.60lo(22 mL and incremental influx of a volume of HrO Av"ro,,,v is per kilogram of protolith), which representsthe solid- the molar volume, and V is the constant volume of the volume loss after the conversion of the protolith to the LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE 1315 incipient diopside-isogradassemblage and after the con- are involved, major recrystallization of the rock occurred sumption of phlogopite in the invariant assemblage(Ta- by the reaction calcite + quartz : wollastonite + CO, ble 5). The diopside zone terminated wheretremolite was (Reaction IV). The fluid composition along this reaction oC, consumedby Reaction III. The value of {,,, at that point at a temperatureof 464 the high-temperaturelimit of was0.051. the diopsidezone, is x.o, : 0.004.This compositionwill The composition of the fluid and, therefore, the tem- be taken as the composition at the wollastonite isograd. perature at the limit of the diopside zone can be calcu- Formation of the wollastonite-zoneassemblage, resulting lated from Equation 5 with the assumption of a value for in the lossof l3l g of CO, per kilogram of protolith and vrro.,.Limits on the volume of infiltratingfluid are 3267 absorption of 1.3 g of HrO per kilogram of protolith (Ta- mll{ for isothermal infiltration (calculated from Eq. 4 ble 5), required the infiltration of 13300 g of HrO and with x.o, : 0.024) and 0 for a closed system. The cal- 227 000 J ofheat per kilogram ofprotolith' Thesevalues culatedfluid composition at the limit of the diopsidezone, are consistentwith the rangesof values observed for all then, must fall within the range xcoz : 0.024 to xcoz: wollastonite-zonerocks, as describedin the previous sec- 0.212. The only way of determining the proper value of tion. v"ro,,is by knowing the value of x.o, at the high-temper- The Z-x.o, path ature limit of the diopside zone. This value must be near 0.024,certainly not greaterthan 0.05 (Table l), but it is Figure 5 summarizes the T-x"o, conditions and the not known precisely. The diopside isograd required 234 mineral assemblagesthat are believed to be representa- g of infiltrated HrO per kilogram of protolith to break tive of the metamorphism of the Big Horse Limestone down chlorite and muscovite plus 294 g per kilogram of Member. The protolith contained the calcite- and quartz- protolith to break down phlogopite. Becausethe amount bearing assemblagemuscovite + chlorite + phlogopite. ofdecarbonation that occurred within the diopside zone Within this unmetamorphosed or incipiently metamor- was only about 150/oof the total decarbonation at this phosed zone, the temperature was less than 434 "C, and : point, the amount of infiltrating water was arbitrarily the fluid composition was near xcoz 0.02' The inter- chosen as 100 mL per kilogram of protolith (equivalent granular porosity was essentially0. Near the diopside iso- to 1915 mL/[). If this amount of water interactedwith grad, chlorite and muscovite broke down under the infil- the rocks in the diopside zone, a fluid would have been tration of 234 g of HrO per kilogram of protolith to form produced having a composition of x.o, : 0.04. the assemblagephlogopite + tremolite + calcic plagio- The temperature at this composition would have been clase,the T-x.o, conditions intersected the reaction (Ksp), 464 "C (from Eq. 2). Theseresults are consistentwith the and, with a trivial amount of reaction, the T-x.o, con- observed temperature and fluid composition. At the ditions reachedthose ofthe invariant point. The diopside maximum reasonablevalue for x.o, of 0.05, indicated by isograd representedthe persistenceof the invariant as- valuesin Table 1, the error in the temperatureat the limit semblageuntil the reaction (Trem) consumed the avail- zone is 2 and the error in the able phlogopite. At this point, the porosity was about 6010, of the diopside only "C, 'C, amount of infiltrated fluid is only 25 mL per kilogram nco2was 0.024, the temperature was 434 and 528 g protolith. of HrO per kilogram of protolith had infiltrated the Big The integrated water/rock ratio in the diopside zone Horse Limestone Member. The diopside zone represent- for this representativerock would have been about 0.59, ed the univariant assemblagetremolite + diopside + derived from the amount of HrO required to break down K-feldspar * calcic plagioclase. The high-temperature chlorite * muscovite, the amount required by the invari- limit of the diopside zone was reached when tremolite ant assemblage,and the amount required by the contin- was consumed,x.o, was 0.046,temperature was 464 "C, uous breakdown of tremolite within the zone. This value an additional 67 g of H,O per kilogram of protolith in- is consistent with the values required by the constant fluid filtrated the rock, and the porosity had increasedto 7o/o. composition describedin the previous section on infiltra- Major recrystallization occurred at the wollastonite iso- t10n. grad. The rocks were nearly completely decarbonated resulting in the assemblagewollastonite + diopside + The wollastonite zone K-feldspar * vesuvianite * grossular, with only minor occurred at this The wollastonite isograd, although considerably more amounts of calcite. Massive infiltration 464 'C and x.o, : 0.004, the complex than the diopside isogradbecause of the number isograd;at a temperatureof 13.3, more than twenty of reactionsrepresented, is much simpler to model. The water/rock mass ratio was about rocks (0.59). isograd is consideredto be a discontinuity acrosswhich times greaterthan the ratio for diopside-zone representsan infiltration several reactions occurred, resulting in the wollastonite- The wollastonite isograd truly from the stock' zone assemblagewollastonite + calcite + vesuvianite * front of water-rich fluid that emanated grossular + * K-feldspar. The reason for this diopside Fr-urn FLow AND HEAT ADvEcrIoN discontinuity appearsto be the large quantities of HrO that must have interacted with the Big Horse Limestone A fluid reservoir Member, as indicated by the mass-balancecalculations. There is a substantialdifference between the wollaston- Although vesuvianite- and grossular-producingreactions ite and diopside zonesin the amount of interaction with 1316 LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE

NotchPeak Sfock

Wollo.stonite hne t A A t 8OOOm fwo

DiopsirTe Wollqstonite S O

F N a'!l + c)

o'oo4 o.o24 0.040 x coz Fig. 5. Conceptualization of the T-x.or-m.o, path of meta- + tremolite + phlogopite * calcic plagioclase.Diopside and morphism of the calcareousargillites. The top portion shows a K-feldspar joined the assemblageat the isograd. On the high- radial section ofthe aureole,divided into three zones,separated temperature side of the isograd, phlogopite disappeared. by the diopside and wollastonite isograds.I is the radius ofthe Throughout the diopside zone,tremolite + calcite + quartz con- stock, and r is the radial distance in the model calculations,as tinously broke down to form diopside. The wollastonite isograd indicated in Table 6. The assemblagesin thesezones are shown representsthe reaction front where wollastonite * vesuvianite on an AlrOr-KAlSirOr-MgO diagram, projected from calcite * + grossularformed. The T-x.ordiagram showsthe path of meta- qvartz + HrO + COr. Wollastonite-zonerocks contain wollas- morphism. The path changedfrom that of a fluid-composition tonite instead of quartz. At the diopside isograd, the protolith buffer (a rock-dominated system)in the diopside zone to that of formed the nascent diopside-zoneassemblage calcite + quartz an unbuffered,fluid-dominated systemin the wollastonite zone. externally derived fluid. The fluid that infiltrated the wol- gins. The apparent infiltration of the wollastonite zone lastonite zone would have attained a composition of x.o, can be realistically imagined to have occurred by the : 0.004. If the fluid that interacted with wollastonite- equilibration ofthe wollastonite-zonerocks with the large zone rocks infiltrated the diopside zone, then only 842 g reservoir of water issuing from the crystallizing stock. As of fluid per kilogram of protolith out of the more than l3 the wollastonite isograd advanced, the large change in kg of fluid per kilogram of protolith actually interacted porosity allowed the fluid near the isograd, which con- with the diopside-zonerocks. How can rocks on one side sisted mostly of CO, evolved from the Big Horse Lime- of the wollastonite isograd have interacted with a large stone Member, to mix thoroughly with the water-rich volume of water, whereasthe rocks on the other side have fluid in the reservoir of the Notch Peak stock. In this case, not? Clearly, little infiltrating fluid crossed the isograd, the large water/rock ratio representsthe size of the res- and there is no evidencefor large-scaletransport ofwater ervoir that equilibrated with the wollastonite-zonerocks, parallel to the isograd becausethe intervening limestone but does not representthe integrated fluid flux through shows no effects. Vertical streaming of water probably the wollastonite zone. occurred,but the most significant vertical transport prob- The size of the reservoir required by a 500-m-wide bly occurred at the roof of the stock, not along the mar- wollastonite zone can be calculated from the model, as- LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE l317 suming sphericalgeometry of the stock. The dips on the advection of the fluid issuing from the crystallizing mag- contact between the stock and the carbonate rock range ma. The following discussionis more thoroughly devel- from 0 to 25, and the apparent radius of the stock is oped by Bird et al. (1960), Carslawand Jaeger(1959), 3360 m. Using the maximum dip of 25", which gives the Muskat (1937), and other texts on transport phenomena. most conservativeestimate, the radius of the stock must The heat flux, J, is given by be 8 km. With this radius, the ratio of the volume of the J : -K.VT * c"ro7Q, (8) wollastonite zone to surface area of the stock is 532 m. The use of the ratio minimizes the uncertainty of the in which { is the thermal conductivity, c"ro is the specific shapeofthe unexposedpart ofthe stock. The amount of heat of water, and Q is the mass flux of water. The first CO, evolved from the Big Horse Limestone Member in term on the right-hand side ofEquation 8 is the conduc- the wollastonite zone was 310 m3/m2, calculated from tive flux, and the second is the advective flux. Conser- Table 5; the total amount is determined by multiplying vation of energyrequires number The this by the surfacearea ofthe contact. amount -boc)(07/0t): V'J, (9) of HrO required for a fluid composition with x.o, : 0.004 was 34300 m3/m2.The intergranular porosity of the pro- which, from Equation 8, is equivalent to wollastonite zone was 25o/o, tolith of the about cone- : v'(vT -V'c",oIQ. (10) spondingto a volume of 133 m3lm'?.The total volume of boc)(67/0t) the reservoir relative to the surfacearea was 34500 m3/ The quantity (poco)is the product ofthe density and spe- m2. The Big Horse Limestone Member is 220 m thick cific heat of the bulk rock-and-pore-fluid composite; it and is in contact with the stock for a distance of 9 km, can be representedby but only half the unit is aryillite. The afected surfacearea - Pul: QPszocsro* (l Q)P,c,, (l l) is 9.9 x 105m2. The required reservoir size was, there- fore, 3.4 x lQto nt:. This value representsa volumetric in which @is the porosity, and the subscript r refers to water/rock ratio of 64.8 for the argillite, and a weight the rock. ratio of 13.3,as requiredby Table 5. The volume of the Fluid flow is governed by Darcy's law in which the stock, again assumingspherical geometry, is 2. 14 x 10" mass flux is proportional to the fluid-pressure gradient m3, 100 times the required reservoir size.Total porosities (vp): ofgranitic rocks, reported by Norton and Knapp (1977), -K"vp, (r2) are about 4ol0.If the Notch Peak stock has a similar po- Q: rosity, the volume of the pore spacewas more than suf- in which the constant of proportionality, Ko,the hydrau- ficient to act as the reservoir for the fluid that infiltrated lic conductivity, depends on the nature of the rock and the Big Horse Limestone Member and, perhaps,was suf- the fluid. Ko can be written as pk/q, in which p is the fluid ficient for the entire aureole. density, 4 is the fluid viscosity, and k is the rock perrne- The amount of water expelledfrom the granitic magma ability. Conservation of fluid mass requires crystallization can also be estimated. Nabelek et during -(0ps,6Q/0t): V'Q. (13) al. (1983) determined that the magma contained about 3 wto/oHrO and that about l0o/oof that water was incor- The equationsgoverning fluid flow are strictly analogous porated into biotite and hornblende.The amount of HrO to those governing heat flow. The time derivative in releasedduring crystallization, therefore, was about 1.6 Equation 13 can be expandedto give x l0'akg,or2.9 x 10" mi (atl.77 x l0-3 m3lkg).This is about 3 to l0 times the amount of pore spaceavailable. The expelledfluid must have been under significantpres- ooofr-opo#, (14) sure, and most must have escapedthrough the roof. The ry:,#* amount that interacted with the Big Horse Limestone isobaric thermal expansion of the fluid Member was only about l0o/oof the total. in which a is the of the fluid. The water/rock ratio calculatedfrom the massbalance and B is the isothermal compressibility for flow ofheat and fluid (8 and l2) can does not, apparently, represent the integrated fluid flux The equations equations of continuity (9 and I 3) during metamorphism, but rather the volume of the fluid be combined with the give the temperature and reservoir with which the rocks equilibrated during meta- to two equations that describe pressure of position and time. For con- morphism. What, then, was the fluid flux during meta- fluid as functions morphism? The flux of fluid and heat through the Big stant conductivities, these are - Horse Limestone Member can be estimated from the lflP'rocrro+(l $P,c,l(07:/dt) simplified model for metamorphism and the consider- : KjV2T * crroKo(VT'Vp + TV'p) (15) ation ofheat and fluid transport from the stock. and - : Heat and fluid flow p(06/0t)+ 6p0@p/0t) 6pa(07/0t) K,Y'F. (16) The heat from the Notch Peak stock is consideredto In general,there are no solutions to this set ofnonlinear have been transported primarily by conduction and by equations in closed form. Analytic solutions have been l3r8 LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE

TABLE6. Constantsand fieldequations for fluid and heat transportin the Big Horse LimestoneMember

Symbol Description

A Radius of the Notch Peak stock (m) 8000 q Thermalexpansion of water (K ') 1.51x 10-3 2 p Compressibilityof water (MPa 1) 1.45x 10-n 2 Specificheat, diopside-zoneand unmetamorphosedrocks 1191 1 Specificheat, water 4421 2 'I c*" Specific heat, wollastonite-zonerock U/(kg K)l 1084 n Viscosity ot water (Pa s) 6.6x10s 4 AH Enthalpyof tremolite-consumingreaction (J/€) 285752 3 Thermal diffusivity(m'!ls) 7.72 x 10-'1 4 l(*. Permeability,wollastonite zone (m2) 2 x 1o-t" c K Permeability(m'?) 1 x 10-16 5 4 Thermal conductivity [W/(m K)l 2.51 4 4.. Hydraulicconductivity, wollastonite-zone rock [kg/(m Pa s)] 2.03 * 10* 1 Kq Hydraulicconductivity, diopside-zone and unmetamorphosedrocks [kg/(m.Pas)] 1'ol x 10-" 1 I Heat of reaction at diopside isograd (J/kg) 57 888 1 Heat of reaction at wollastonite isograd (J/kg) 227227 I Po Initialpressure above ambient in the stock (MPa) 100 4 a Porosity outside the wollastonitezone 0.01 1 d*. Porosity inside the wollastonitezone 0.25 I fat Location of the diopside isograd (m) Location of the wollastoniteisograd (m) p Density,diopside-zone and unmetamorphosedrocks (kg/m3) 2730 1 Density,water (kg/m3) 666.67 2 Density,wollastonite-zone rock (kg/ms) 2960 1 To Initialtemperature above ambient of the stock ("C) 963 o t Time (s) Extent of reaction (kg 1) E 'I Hydraulicdiffusivity, wollastonite zone (m,/s) 836.6 Hydraulicdiffusivity, diopside-zone and unmetamorphosedrocks (m'./s) 1.04 1 fnitiafconditions: f : Ioandp: potorr< A T:0andp:0lor r> A. Boundaryconditions: f : 0 and p: 0 tor r: a. -'** ^(#).("qI) I n[(o).(fl ]:^,* Heat flow: +(t - .4#-' r rdi rdp*c*o\pct{at:,,,W.?(:r]-' "r{(9(9 ?(#)]} - rwr* ,"(,*).'-,"u(,,t) -'*,*"(fl : -*l#. ;(#)] A

determined for the conditions of steady fluid flux-that of equations must be solved numerically (e.g., Cathles, is, when Vp is constant-by McKenzie (1984) and by l98l; and Norton and Knight, 1977). Bickleand McKenzie (1987)and for the conditionswhen In the case of the metamorphism of the Big Horse advection of heat is minor in comparison with conduc- Limestone Member, there is no evidence for substantial tion-that is, when the nonlinear term in Equation 15 thermogravitational flow of fluid. The existenceof hori- can be neglected-by Delaney(1982). In most cases,es- zontally stratified, relatively impervious marbles proba- pecially those concerningthe thermogravitational flow of bly impeded vertically convectingfluid. The depth of the fluid or those with complex boundary conditions, the set intrusion, about 6 km, lies within the realm of single-pass LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE 1319

700 3xI05

Calcite-Dolomite Fl

Temperature H o

2xl05 t{ Ft +J X art Time L C) H p I lxlO5 g rr Temperature q)

(Dv *5 \-/ 5E

500 too0 1500 2000 Distance from Contact (m)

Fig. 6. Distribution of the maximum temperaturein the au- and diopside isogradsare determinedby the temperaturesof464 reole,calculated from a simple conductive heat-flow model. Cal- "C and 434 "C, respectively.The positive-slopingcurve indicates cite-dolomite temperaturesare plotted accordingto the map dis- the time after intrusion when the maximum was reached.The tance from the contact. The true distancesare probably not as wollastonite isograd ceasedadvancing at -50000 yr, and the great, indicating good agreementbetween the heat-flow model diopside isograd stopped at -70000 yr. and the actual temperature. The distances of the wollastonite fluid flow strggestedby Wood and Walther (1986).Ther- porosity of 0.01 and a permeabilityof l0-o darcies.The mogravitational convection, no doubt, played a major permeabilities were taken from those of sandstoneand role in heat transport after magmatic fluids were expelled siltstone having similar porosities listed by De Wiest from the stock, but during metamorphism of the rocks (1965).The large porosity in the wollastonitezone was on the margins of the stock, the significant interaction surely eliminated by deformation, but as a limiting case, between fluid and rock occurred in the absenceofgravi- the porosity is assumedto have been 25o/o.As described tational recirculation. below, the only effect of the large porosity is to make the Calculation ofthe heat and fluid transport through the porosity outside the wollastonite zone the limiting factor Big Horse Limestone Member requiresknowledge of sev- for the fluid flux. The porosity of 0.01 is used as a lower eral parameters:the size and geometryof the Notch Peak limit for the unmetamorphosedrocks, which have essen- stock, the initial temperatures of the magma and the tially no intergranular porosity, and for the stock (Norton country rock, the fluid pressurein the stock, the physical and Knapp, 1977). All the values used in the calculations propertiesof the fluid phase,and the diffusivities, perme- are given in Table 6. The stock is assumedto be a sphere abilities, and porosities of the rock types. Most of these with a radius of 8 km, intrusion is assumedto have oc- simply are not known. Those that are reasonably well curred over a very short time, and the fluid flow is as- known are the temperatureof the magma (Nabelek et al., sumed to be radial. The fluid pressure in the stock is 1986) and the intergranular porosity of the Big Horse completely unknown. A value of 100 MPa, representing Limestone Member, basedon the presentwork. The ther- the estimatedbursting strengthof the granitic rock based mal diffusivities of all rock types are presumed to be the on values reported by Clark (1966), is used in the calcu- same. The fluid is considered to be pure HrO, and its lations. Iffluid pressuresexceeded the strength, the flow thermal expansionand bulk modulus are calculatedfrom of fluid through the fractured rock would very quickly values of specificvolume given in Burnham et al. (1969). have relieved this overpressure.Even in the absenceof The viscosity of water was determined by extrapolation fracture development,the high fluid pressurewould have from values given by Clark (1966). Bulk porosity, i.e., been quickly dissipatedby flow through rocks having the intergranular porosity plus fracture porosity, and per- modest porosity and permeability used in these calcula- meability are the most difficult to estimate. Two sets of tions, as describedbelow. valuesare used.The first correspondsto those ofthe wol- The boundary conditions imposed by the metamorphic lastonite zone with a porosity of 0.25 and a permeability zonesare related to the heatsofreaction. At the diopside of 2 darcies, and the second to all other rocks with a isograd, 57888 J/kg are consumed by the formation of r320 LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE

porosity is assumedto have been constant over the time Calculated Flux scaleofthe diffirsion ofpressure through the fluid. In the Notch Peak Aureole caseof the wollastonite-zone conditions, this time scale is seconds,and the assumption is not unrealistic. E Within these limitations, the equations for heat and N fluid flow are now described.The initial conditions are Z b.|) rooo : To,p : po, r < A; T : 0, p : 0, r > A. The boundary X conditionsarc T : 0, p : 0, r : (n. Heat flow is

ldp*c*+ (r - dNt#:.,1#.?(#)),(,7)

ol or lo lo t@ 1000 and fluid flow is Time $rears) - Fig. 7. Flux of water through the aureoleat three distances r,-u(X)r,*(T):."1#. ?(#)l (l8) from the contact.Even though the peakflux occurswithin the firstyear, the major contribution to the integratedflux occursat These equations have the solution longtimes because the flux goesto zerovery slowly. ,:*r"l"tffi*",r(ryAf the assemblagecalcite + quartz + tremolite + diopside + K-feldspar * plagioclase.Thus, the differencein heat _+\r flux through this isograd is Lp(dxo,/dt) (Carslaw and Jae- ger, 1959).Within the diopsidezone, heat is continuously ([email protected], -*'(#o)] (re) absorbedby the reaction tremolite * 3 calcite -l 2 quartz [".' ) : 5 diopside + 3CO, + HrO. The heat flow through the (Crank,1975). diopside zone is thuslAH,(d{/dD + pcl (dT/At) (Labotka et al., 1984).At the wollastonite isograd,heat is absorbed p:v,r";;\l-,(4-.A.*r(4jrJ) atarate of L*"p(dx""/dt),whereL*"is227 000 J/kg. The porosity permeability and also ditrer within the wollas- To ot tonite zone and outside of it. The complete set of field r Arc \F equations is given in Table 6. v7l Clearly, an analytic solution to the equations in Table -".,(+#)] 6 does not exist. The equations must be integrated nu- l"-"(#) merically. A thorough development of the numerical so- 'hlpolr - ^ lution of the equations is not attempted here. Rather, + r": - ) \ 0 K @l limits on the fluid flux can be determined from two cases: f / \ /, * t)l\-l the first using the parameters for the wollastonite zone .l"rr{'l) + ..r{' and the secondusing those for rocks outside the wollas- | \zVotl \2Vot/l tonite zone. The heatsofreaction are neglected.The larg- / -!(r^-7,,1 est of theseis that of the wollastonite isograd and is only "Bx-.1" )\/'t half the heat liberated by the crystallization of the magma r\'" V , - in the stock. The boundary conditions are simply that the .lexptI l-U O'\ /-t,l + '),\l (20) pressure go great --l- e*pl-#ll. and temperature to zero at distances L'\ 4at / \ Tw, tl from the contact. The initial conditions remain as listed in Table 6. The constantsare defined in Table 6. The relative contributions ofconduction and advection The maximum temperature distribution in the Notch to the heat flow are seenfrom the coefficientsx and cSo/ Peak aureole, calculated from Equation 19 at the time (pc),, in which (pc)orefers to the bulk rock plus pore water. when 0T/0t: 0, is shown in Figure 6; the calcite + do- For the conditions of the wollastonite zone, the ratio of lomite temperaturesfrom Table I are also shown. There the coefficientsis 27, and, for the conditions outside the is good agreementbetween calculated and observedtem- wollastonite zone, the ratio is 5.4 x 105.Thus, advection peraturesfor sampleswithin about 500 m from the con- makes a small to infinitesimal contribution to the heat tact, but the two samplesat a Ereaterdistance were ap- flow. For the purposeshere, advection can be neglected. parently 20 to 50 oC hotter than the calculated In addition, the porosity is assumed to be constant in temperatures.This apparent discrepancyis the result of time. Certainly, the large porosity in the wollastonite zone the differencebetween the map distancefrom the contact is eliminated by deformation, but as a limiting case,the and the true distance. The error between the map and LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE t32l true distanceincreases at greatdistances from the contact. The calculated temperature distribution gives the dis- Fluid Flux through tanceofthe wollastoniteisograd as 410 m and that ofthe Notch Peak Aureole diopside isograd as 595 m. The wollastonite-isogradpo- sition is consistent with the field evidence, but the ap- N 2OOO parent width of the diopside zone is about 1000 m, much - greater than the calculated 185 m. A dip of 25o on the bo stock contact indicates that the true radial distanceofthe diopside isograd is 634 m, in close agreementwith the ; F looo calculateddistance. lrr The period during prograde metamorphism was less than about 70000 yr. At that time, the diopside isograd had advanced to its greatestdistance from the contact (Fig. 6). The wollastonite isograd had ceasedto advance at 50000 yr after intrusion. r 0 10 roo 1000 10,ooo The fluid flux, given by -K"(0p/0r), depends on the Time $ears) fluid pressurewithin the stock, the permeability, and po- Fig. 8. Flux at 500 m for initial fluid pressuresof 50, 100, rosity. The fluxesat threelocations, 500, 1000,and 1500 and 200 MPa in the stock.The flux is approximatelypropor- m from the contact, are shown in Figure 7 for a perme- tional to the pressure. ability of 10-'6 m2,a porosity of lol0,and an initial fluid pressurewithin the stock of 100 MPa. The most impres- and 70000 yr for the diopside isograd. The integral in sive feature of Figure 7 is that even though the maximum Equation 2l was evaluated numerically for the two iso- flux was substantial, about 1500 kgl(m'z.yr) at 500 m, grads, and the results are shown in Figure 9. The posi- pressuredissipated extremely rapidly. The pressurepulse tions of the two isograds and the instantaneous fluxes was nearly completely dissipatedwithin the first year af- through the isogradsover the time of metamorphism are ter rntmslon. shown in Figure 9. The flux through the isotherms is nearly The value ofthe calculatedflux dependson the choice independent of position becausethe isograds are never ofthe initial fluid pressure.The effectsofthis choice on farther apart than 200 m and the compressibility and the calculated flux at a distance of 500 m are illustrated thermal expansion of water make small contributions to in Figure8 for initial pressuresof 50, 100,and 200 MPa. the pressure.The total flux is shown as the water/rock The shapesof the curves are similar, with only a slight ratio, which is given by Q,oo,/p,o"uri""g"a.The water/rock shift in the time of peak flux. The time scalefor pressure ratios given in Figure 9, which reach the maximum val- relaxation is about 0. I yr in all cases,and the magnitude ues of0.12 for the wollastoniteisograd at 50000 yr and of the flux is approximately proportional to the initial 0.1 I for the diopside isograd at 70000 yr, can be com- pressure.For the casein which the permeability and po- pared with those calculated from mineral equilibria if it rosity are appropriate for the wollastonite zone, the pres- is assumedthat all the fluid that passedthrough the iso- sure completely dissipatesin seconds,and, therefore,the grad equilibrated with the rock; otherwise, the values in fluid flux through the wollastonite zone was no different Figure 9 must be regardedas maximum values. from that through the diopside zone. Even though the major flux occurred almost immedi- ately after intrusion, long before the temperaturerose suf- DrscussroN ficiently for metamorphism to have occurred,the low flux The model results depend on several simplifications that persistedfor long times made the major contribution and assumptions. First, the stock is assumed to have to the time-integrated water/rock ratio. The tails on the sphericalgeometry with a radius of 8 km. The geometry fluid-flux curves in Figures 8 and 9 approach zero only is based on the surfaceexposure, but the shapemay not at very long times, much longer than the time period of be spherical.The effect of a smaller radius on the model metamorphism of <105 yr. The total water/rock ratio, is to reduce the calculated width of the aureole and the attributable to fluid infiltration, was calculated for the amount of evolved HrO. The simple geometry,however, diopside- and wollastonite-zonerocks for the conditions resultsin a calculatedaureole width and temperaturefield of Figure 7. The total flux through the 434 and,464'C consistentwith observations.Second, the water evolved isotherms,representing the diopside and wollastonite iso- from the crystallizing stock is assumedto present an in- grads,is given by ternal pressure of 100 MPa instantaneously with em- placement. The pressure is an assumed value, and the : - *, (2r) initial condition is a simplification. Volatiles were prob- e,oer f (*),:,*^* ably releasedover the period of crystallization, which must "'^"- have been comparable to that of metamorphism. In this The value of l-," is 50000 yr for the wollastonite isograd case,higher fluid pressurescould have been sustainedfor 1322 LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE

A CO2 Production c

a |r $ roo N o b0

0.1 2345 234 log ltime (years)] log [time (years)]

rM@h o D lntuia ntux F Water/Rock Ratio Xro N j( Wollastontte nr lsogmd Diopside & b0 b0r Wollastonite Diopside Isograds Isograd

o.t 00t 123456 123456 log [time (years)] log [time 6'ears)l

Fig. 9. Fluid flux through the diopside and wollastonite iso- sufficiently great early in the processthat the system probably grads.(A) Position of the isogradswith time. At the maximum was rock-dominated in the initial stagesof metamorphism. (D) distances,the temperaturereached a maximum, the rocks began The integrated fluid flux was similar for both isogradsand reached to cool, and the isogradsceased to advance.(B) The flux through the maximum value at the temperaturemaximum when the re- the isogradsat any time after intrusion was nearly constant be- actions ceasedto advance.The water/rock massratio was ofthe causeof the small distance separatingthe isograds (cf. Fig. 7). order of0.1, similar to that ofthe diopsidezone, but at least (C) The rate of CO, produced at the wollastonite isograd was two orders of magnitude lower than that of the wollastonite zone.

longer periods, resulting in higher integrated fluxes. The zone. The processthat occurred at the infiltration front magnitude of this effect can be estimated by considering was the mixing of CO, generatedby reaction with the that crystallization of the stock occurred on the time scale reservoirof HrO. The mixing, or diffirsion,of CO, through of 50000 yr and that the crystallization of the stock pro- the HrO-rich fluid must have occurred at a grealer rate ceededwith \r. If the porosity was l0l0,the rate of growth than the diffusion of heat in order for the wollastonite of pore spacewas 3.9 x lOt m3/yr''.If HrO was evolved isograd to have advanced isothermally. This relation is over the same time scale, a fluid pressure of 2.8 MPa required by the change from a rock-dominated meta- could have been sustained for the period of metamor- morphic systemin the diopside zone to a fluid-dominat- phism within the growing pore space,assuming a total ed wollastonite zone. The rate at which CO, was released amount of water of l0'4 kg in the magma. In the steady- at the wollastonite isograd is given by m"orp,..*(dt*"/dt), state limit, which gives the greatestintegrated flux, if the in which m"o, is the mass of CO, released(131 g per pressurewas dissipatedover a distanceof l0 km or great- kilogram of protolith) and the derivative is the rate of er, the water/rock ratio for metamorphic rocks would have advancementof the wollastonite isograd. The rate of ad- been 0.3, or less. This value is similar to that presented vance of the wollastonite isograd depends on the time in the model and to that for the diopside zone, but is only and distance, as can be seen in Figure 9. At small dis- 3oloor less of the apparent flux through the wollastonite tances and times, the rate is very high, and the rate ap- zone. Thus, the difference between the observed diop- proaches0 at 50000 yr. The rate dropped to lessthan 0.1 side-zoneflux,0.59, and the model value,0.ll, can be m/yr at 100 yr after intrusion, and the rate of CO, pro- accountedfor in the simplified initial and boundary con- duction was less than 36 kgl(m'?'yr) throughout most of ditions of the model. The observedhigh water/rock ratio the period of metamorphism. After 10000 yr, the rate in the wollastonite zone, however, cannot be explained dropped to less than 3 kgl(m'z'yr). The rate of diffirsion by inaccuraciesin the model. of CO, through the fluid must have been fast enough to The water/rock ratio in Figure 9 is much too low for eliminate the concentration gradient between the site of the value indicated by mineral equilibria in wollastonite- generation of CO, and the HrO-rich reservoir. A rough zone rocks (13.3). Even allowing for a 25o/oerror in the estimate of the diffusion coefficientis given by a/p, which value ofx.o, becauseofthe choice offugacity coefficients, is the self-diffusioncoefficient in a dilute gas(Reif, 1965). the water/rock ratio had to be at least 10.7 in the wollas- For water under the conditions given in Table 6, the coef- tonite zone. The wollastonite-zonerocks interacted with ficient has a value of 2.97 m'?lyr.If CO, behavessimilar- a far-greaterquantity of HrO than actually infiltrated the ly, the root-mean-square displacement \,47T over the LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT.METAMORPHIC AUREOLE r323 time scaleof 50000 yr, the time of formation of the wol- Bird, R B., Stewan,W.E., and Lightfoot,E.N. (1960)Transport phenom- lastonite zone, is 545 m. This distance is only slightly ena,780 p. Wiley, New York. H.C. (1983a)Calculation of the thermody- greater Bowers,T.S., and Helgeson, than the width of the wollastonite zone,indicating namic and geochemicalconsequences ofnonideal mixing in the system that, unless turbulence aided the mixing, the character- H,O-COr-NaCl on phase relations in geologic systems:Equation of istic times for diffusion of heat and for diffusion of CO, state for HrO-COlNaCl fluids at high pressuresand temperatures. through the fluid were similar. The actual T-x.or-t path Geochimicaet CosmochimicaActa, 47, 1247-1275. -(1983b) and geochemicalcon- at the wollastonite isograd was probably much more Calculation of the thermodynamic sequencesof nonideal mixing in the system HrO-COlNaCl on phase complicated than the simple steadyvalue adopted for the relationsin geologicsystems: Metamorphic equilibria at high pressures calculations.Initial rapid diffusion ofheat at the contact and temperaturesAmerican Mineralogist, 68, 1059-1075. causedthe generationof CO, by the wollastonite-isograd Bucher-Nurminen, K (1981) The formation of metasomatic reaction reactions, which buffered the fluid composition and re- veins in dolomitic marble roofpendants in the Bergellintrusion (Prov- ince Sondrio, northern Italy). American Joumal ofScience,281, | 197- tarded the advance of the isograd until the CO, mixed t222. with the reservoir and the local value of x.o, was reduced. Burnham, C.W., Holloway, J R., and Davis, N.F. (1969)Thermodynamic This close interplay probably occurred throughout the properties ofwater to 1000 .C and 10,000bars. GeologrcalSociety of lifetime of metamorphism in the wollastonite zone and AmericaSpecial Paper 132. (1959) ofheat in solids,510 may be the cause of the complex mineral textures de- Carslaw,H.S, and Jaeger,J.C. Conduction p. Oxford University Press,New York scribedby Hover-Granathet al. (1983)for theserocks. Cathles,L M. (l 98 1) Fluid flow and genesisofhydrothermal ore deposits. Water/rock ratios are commonly calculatedfrom min- Economic Geology 75th Anniversary Volume, 424457. eral assemblagesand rock compositions, either in the Clark, S P., Jr. (1966)Handbook ofphysical constants GeologicalSociety manner usedhere or by point-counting techniques.These of AmericaMemoir 97, 587 p. Crank.J (1975)The mathematicsofdiffusion,4l4 p. Oxford University calculatedwater/rock ratios rigorously indicate the Press.New York amount of fluid that interacted, or equilibrated, with the Delaney, P.T. (1982) Rapid intrusion of magma into wet rock: Ground- rock, but do not necessarilyrepresent the amount of fluid water flow due to pore pressureincreases Joumal ofGeophysical Re- that flowed through the rock. There can be a great differ- search.87. 7739-7756 p. Wiley, New York. encebetween the water/rock ratio and the time-integrat- De Wiest,R.J.M (1965)Geohydrology, 366 Ferry, J.M. (1980) A casestudy of the amount and distribution of heat ed fluid flux. It is not easy,in most cases.to recognize and fluid during metamorphism. Contributions to Mineralogy and Pe- the difference.The difference is quite evident at a place trology,7l,373-385. like Notch Peak where an isograd separatesrocks with -(1983) On the control of temperature,fluid composition, and re- low-variance assemblagesfrom rocks with high-variance action progressduring metamorphism. American Journal of Science, 283-A,201-232 assemblages.Here there was a change in the metamor- -(1984) A biotite isogradin south-centralMaine, U S.A.:Mineral phic system from rock-dominated to fluid-dominated. reactions, fluid transfer, and heat transfer. Journal of Petrology, 25, This is reflected in the water/rock ratio, which changed 87r-893. from -0.6 in the rock-dominatedsystem to - 13 in the - (l 986) Reactionprogress: A monitor of fluid-rock interaction dur- fluid-dominated system. The large water/rock ratio in- ing metamorphic and hydrothermal events. In J.V. Walther and B J. Wood, Eds., Fluid-rock interactions during metamorphism, p 60-88. dicates the size of a reservoir that interacted with the Springer-Verlag,New York. rocks. The ability of this reservoir to communicate with -(1987) Metamorphichydrology at l3-km depth and 400-550'C. the rocks was greatly enhanced by the creation of sub- American Mineralogist, 72, 39-58. stantial porosity by the decarbonation reactions that oc- Helgeson,H.C., Delany,J M., Nesbitt,H.W , and Bird, D.K. (1978)Sum- the thermodynamic properties of rock-forming isograd. The wollastonite isograd Notch mary and critique of curred at the at minerals. American Journal of Science,2'78-4, l-299 Peak is, then, a reaction front governed by the diffusion Hintze, L.F. (1974) Preliminary geologicmap of the Notch Peak quad- of heat from the stock and of CO, through the HrO-rich rangle Millard County, Utah. U.S Geological Survey Miscellaneous fluid issuing from the stock. Field StudiesMap MF-636. Hochella,M F., Liou, J G., Keskinen,M.J., and Kim, H.S. (1982)Syn- AcrNowr,srcMENTS thesis and stabiliry relations of magnesiumidocrase. Economic Geol- ogy,77, 798-808 Researchon contact metamorphism by the Notch Peak stock has been Holloway, J.R (1977) Fugacity and activity of molecular speciesin su- fundedby the Departmentof EnergyContract DE-ACOl-82ER-12050- percritical fluids In D.G. Fraser,Ed., Thermodynamics in geology,p. A00l and Grant DE-FG0I-85ER-13407to J. Papikeand the National 161-181Reidel, Boston. gants ScienceFoundation EAR 85I I 347 to T. I-abotkaand EAR 8512081 Hover. V.C. (1981)The Notch Peakmetamorphic aureole, Utah: Min- go to P. Nabelek. Our thanks to V. Hover-Granath, who first described eralogy,petrology, and geochemistryofthe Big Horse Canyon Member the fascinatingbehavior of the Big Horse Limestone Member, and to Neal of the Orr Formation. M.S. thesis,State University of New York, Stony who White, ably assistedus in the field. Careful critical reviews of the Brook, New York. manuscript by John Ferry, Simon Peacock,and Frank Spear helped us Hover-Granath,V.C., Papike,J.J., and Labotka,T.C (1983)The Notch strengthenour argumentsand clarify the presentation. Peak contact metamorphic aureole,Utah: Petrology of the Big'Horse LimestoneMember of the Orr Formation. GeologicalSociety of Amer- RnrnnrNcrs crlnn ica Bulletin,94, 889-906. Bebout,G.E., and Carlson,W.D (1986)Fluid evolution and transport Kerrick,D.M., andJacobs, G.K. (1981)A modifiedRedlich-Kwong equa- during metamorphism: Evidencefrom the Llano uplift, Texas.Contri- tion for HrO, CO:, and HrO-CO, mixtures at elevated pressuresand butions to Mineralogy and Petrology,92,518-529 temperatures American Journal of Science,28 l, 735-7 68. Bickle,M.J., and McKenzie,D (1987)The transportof heatand matter Labotka, T.C. (1981) Petrologyofan andalusite-type,regional metamor- by fluids during metamorphism. Contributions to Mineralogy and Pe- phic terrain, Panamint Mountains, California. Journal ofPetrology, 22, trology,95, 384-392. 26t-296. t324 LABOTKA ET AL.: FLUID INFILTRATION IN CONTACT-METAMORPHIC AUREOLE

Labotka, T C, and Nabelek, P.I (1986) Petrology of the contact-meta- Reif, F. (1965)Fundamentals ofstatistical and thermal physics,651p. morphosed Weeks Formation, Notch Peak, Utah Intemational Min- McGraw-Hill. New York. eralogicalAssociation Abstractswith Program, 147-148. Rice, J.M. (1977)Progressive metamorphism of the impure dolomitic -(1987) Isotopicexchange and the fluid/rockratio in contact-meta- limestone in the Marysville aureole, Montana. American Joumal of morphosed calc-argillitesfrom Notch Peak, Utah Geological Society Science,277,l-24 of American Abstracts with Programs, 19, 737. -(1983) Metamorphismof rodingites:Part I Phaserelations in a Labotka,T C , White,C.E., and Papike,J J. ( I 984)The evolutionofwater portion of the systemCaO-MgO-Al,O3-SiOr-COr-HrO. American Jour- in the contact-metamorphicaureole of the Duluth Complex, nonh- nal of Science,283-A, l2l-150. easternMinnesota. Geological SocietyofAmerican Bulletin, 95, 788- Rice, J M., and Ferry, J M. (1982) Buffering, infiltration, and the control 804. of intensive variables during metamorphism. Mineralogical Society of Labotka,T C, Nabelek,P I., Papike,J.J, Hover-Granath,V.C., and laul, AmericaReviews in Mineralogy,10,263-326 J C (1988)Effects ofcontact metamorphismon the chemistryofcal- Rumble,D.,III, Ferry,J M., Hoering,T C , and Boucot,A.J. (1982)Fluid careousrocks in the Big Horse Limestone Member, Notch Peak,Utah. flow during metamorphism at the Beaver Brook fossil locality, New AmericanMineralogist, 73, 1095-1110. Hampshire. American Journal of Science,282, 886-919 McKenzie, D. (1984) The generationand compaction of partially molten Shmonov, V M , and Shmulovich, K I. ( I 974) Molal volumes and equa- qC rock. Joumal of Petrology,25, 7 13-765 tion ofstate ofCO, at temperaturesfrom 100 to 1000 and pressures Muskal, M (1937)The flow ofhomogeneousfluids throughporous me- from 2000to 10,000bars. Akademiya Nauk SSSRDoklady, 217 ,206- dia, 763 p McGraw-Hill, New York 209. Nabelek,P I., O'Neil, J R., and Papike,J.J (1983)Vapor phaseexsolu- Thompson,J B., Jr., Laird, J., and Thompson,A.B (1982)Reactions in tion as a controlling factor in hydrogen isotope variation in granitic amphibolite,greenschist, and blueschist.Joumal of Petrolosy,23, l- rocks: The Notch Peak granitic stock, Utah. Earth and Planetary Sci- enceLetters, 66, 137-150. Trommsdorff, V., and Skippen, G. (1986) Vapour loss ("boiling") as a Nabelek,PI., Labotka,TC, O'Neil,JR., and Papike,JJ (1984)Con- mechanismfor fluid evolution rn metamorphic rocks. Contributions to trasting fluid/rock interaction betweenthe Notch Peak granitic intru- Mineralogy and Petrology, 94, 317-322. sion and argillites and limestonesin westem Utah: Evidencefrom sta- Valley,J.W., Peacor,D.R., Bowman,J.R , Essene,E J., and Allard, M.J ble isotopesand phaseassemblages. Contributions to Mineralogy and (1985)Crystal chemistry ofa Mg-vesuvianiteand implications ofphase Petrology,86,25-34. equilibria in the system CaO-MgO-AlO,-SiOr-HrO-CO'. Journal of Nabelek,P I., Papike,J.J., and Laul, J C. (1986)The Notch Peakgranitic MetamorphicGeology, 3, 137-153. stock, Utah: Origin ofreverse zoning and petrogenesis.Journal ofPe- Walther, J.V , and Orville, P.M. (1982) Volatile production and transport trology,27, I 035-1069. in regional metamorphism. Contributions to Mineralogy and Petrolo- Norton, D , and Knapp, R. (1977)Transport phenomenain hydrothermal gy,79,252-257 systems:The nature of porosity. American Joumal of Science,277, Wood, B J., and Walther,J.V (1986)Fluid flow during metamorphism 913-936. and its implicationsfor fluid-rock ratios In J.V. Walther and B.J Wood, Norton, D., and Knight, J. (1977)Transport phenomena in hydrothermal Eds , Fluid-rock interactionsduring metamorphism, p. 89-l 08. Spring- systems:Cooling plutons. American Joumal of Science,27 7, 937-981. er-Verlag,New York Papike,J.J, Cameron,KL., and Baldwin, K. (1974)Amphiboles and pyroxenes:Characterization of other than quadnlateral components and estimatesof ferric iron from microprobe data GeologicalSociety MeNuscnrsr REcETvEDSnr"rprnrsen 14, 1987 of AmericaAbstracts with Programs,6, 1053-1054. M.rxuscnrpr AccEPTEDJurv 29, 1988