American Mineralogist, Volume 79, pages 513-525, 1994

Contact metamorphismsurrounding the Alta stock Thermal constraintsand evidenceof advectiveheat transport from calcite * dolomite geothermometry

SrnpnrN J. Cooxr* JorrN R. BowtvrlN Department of Geology and Geophysics,University of Utah, Salt Lake City, Utah 841 12, U.S.A.

Ansrnc,cr Metamorphism of the siliceous dolomites in the contact aureole of the mid-Tertiary Alta stock has produced a series of isograds with increasing metamorphic grade: talc, tremolite, forsterite, clinohumite, and periclase.Calcite + dolomite geothermometry ap- plied to 30 samplesof thesedolomites indicatesthat the progrademetamorphism occurred over the approximatetemperature range 410-575 "C for the tremolite-bearingthrough periclase-bearingzones. The calcite + dolomite temperaturesfollow a consistenttrend of increasingtemperature in the direction of the intrusive contact. Consistencybetween this temperature regime and the positions of the Z-X"o, phase equilibria applicable to the progradereactions requires a minimum fluid pressureof 75 MPa during the metamorphic event. Comparisons between the peak temperature-distanceprofile defined by the calcite * dolomite data and temperature-distanceprofiles calculatedfrom geologicallyreasonable, two-dimensionalconductive cooling models of the stock suggestthat the temperature regime in the southern portion of the Alta aureole was affectedsignificantly by advective heat transfer.Field, petrologic,and whole-rockstable isotopicdata, as well as the results of stableisotope transport modeling, are consistentwith this possibility and require further that the fluid flow responsiblefor this advection was lateral to the intrusive contact.

Ix'rnonucrroN of the magma and rocks (crystallized pluton and sur- primary Thereare now severalheuristic thermal modelingstud- rounding country rocks), and of the mechanism (e.g., ies that have shown that fluid infiltration can affect sig- ofheat transfer conductionvs. advection)operating nificantly the thermal regime surrounding a cooling plu- in the stratigraphicsection containing the reactivemin- ton and consequentlythe spatialand temporal evolution eral assemblages. of its isotherms(e.g., Cathles, 1977; Norton and Knight, Hence any quantitative evaluation of the role either of progress 1977).This same fluid infiltration can also play an im- fluid infiltration in influencingthe of metamor- portant, and often interrelated,role in controlling the phic reactionsor of advectionin the heatingof contact progressof mixed-volatilereactions and hencethe pro- aureoles requires independent definition of temperature grade T-Xro. path of progressivemetamorphism of sili- in the contact aureole.One method for establishingthe ceous carbonate-bearingrocks in these aureoles(e.g., requiredtemperature control in aureolesthat contain si- Moore and Kerrick, 1976; Bowman and Essene,1982; liceousdolomites is calcite + dolomite solvus geother- geothermometer Ferry,1983, 1989; Nabelek et al., 1984;Bebout and Carl- mometry. Although applicationsof this son, 1986). During progressivemetamorphism of sili- have been frustrated by significant retrograde reequili- ceouscarbonate-bearing rocks, fluid infiltration competes bration of calcitein regionalmetamorphic terranes (e.g., with the bufferingcapacity of the metamorphicreactions Valleyand Essene,1980; Anovitz and Essene,1987; Es- for control of pore fluid composition (Rice and Ferry, sene, 1989),this geothermometerhas been successfully (e.g., 1982).Yet reactionprogress and pore fluid composition appliedin contactmetamorphic environments Rice, are also influencedby the rate and extent ofthe temper- 1977:'Bowman and Essene,1982:' Hover-Granath et al., ature increaseexperienced by a metamorphicrock. For r983). contactmetamorphism, this increaseis controlledby the The siliceousdolomites in the contact aureoleof the progressive thermal budgetof the aureole,which is a function of the Alta stock exhibit a well-developed meta- supply of heat available(from the size of the intrusion, morphic sequencethat is well suitedfor study usingcal- heat of crystallization, and temperature of the magma), cite + dolomite geothermometry.This study focuseson of the geometryof the intrusion,of the thermalproperties the applicationof calcite + dolomite geothermometryto documentthe thermal regimeof the metamorphismthat produced this sequenceand to determine whether there * Presentaddress: Environ Corporation, 1980 Post Oak Bou- is evidenceof significantadvectjve heat transport that levard,Suite 2120, Houston,Texas 77056, U.S.A. accompaniedthe progrademetamorphism. 0003-{04xl94l0506-o5 I 3$02.00 513 5t4 COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM

Fig. l. Geologic map ofthe Alta contact aureole (Baker et al.,1966; Crittenden, 1965). Isograd locations based on mineral assemblagesin the massivedolomites (Moore and Kerrick, 1976).The preintrusive Alta-Gizzly thrust fault systemrepeats part of the Paleozoiccarbonate section both north and south ofthe Alta stock. This study focuseson the metamorphism along the south- central margin of the Alta stock (box labeled study area).

Gnor,ocv dominantly with the Cambrian and Mississippiancar- rocks(Fig. l). The metamorphismassociated with The Alta stock (granodiorite:Wilson, l96l) is one of bonate has affected the country rocks on the south severalmid-Tertiary plutons (38 Ma: Crittenden et al., the stock (Maxfield, Deseret, Gardison Formations, Figs. I and 1973)in Utah's centralWasatch range (Fig. l). The stock and 2 km from the intrusive contact. has intruded and contact metamorphosed a sequenceof 2) up to approximately mid-Tertiary plutons are exposed in Precambrianand Paleozoicsedimentary rocks consisting Two additional vicinity Alta aureole.To the east,the Alta stock primarily of quartzites(Precambrian Big Cottonwood and the ofthe has intruded the older (42 Ma) Clayton Peak stock Cambrian Tintic Formations) and a single pelitic unit (granodiorite). kilometers to the west is the youn- (Cambrian Ophir Formation) capped by carbonaterocks Several ger(32 (quartzmonzonite). (Cambrian Maxfield and MississippianFitchville, Des- Ma) Little Cottonwoodstock Moore Kerrick (1976) reported overprinting of the eret, and Gardison Formations).Within the study area, and Alta by the contactaureole preintrusive thrusting (Alta-Gizzly thrust zone, Figs. I westernmargin of the aureole Little stock. and 2) has resultedin the repetition of a portion of the of the Cottonwood carbonatesection, placing Cambrian Maxfield over the upper MississippianDeseret-Gardison Formations. Coxucr METAMoRPHTSM The south contact of the Alta stock is exposedover a This study focuseson the metamorphism along the vertical interval of approximately550 m; its outcroppat- south-centralmargin of the Alta stock (Fig. l). This por- tern over this interval indicates that, with small-scale, tion ofthe contactaureole was selectedfor study because local exceptions,it is subvertical(Fig. l). An additional of its well-developed prograde metamorphic sequence 500 m of vertical relief on this contact can be inferred within the dolomitic marbles(Moore and Kerrick, 1976), from exposuresin mine workingsin the area(Calkins and its simple structureand excellentexposures of the car- Butler. 1943).The bulk of the subsurfacecontact is with bonate units, which allow reliable tracing of individual the Precambriansection, but the exposedcontact is pre- rock units up metamorphic grade, and a position unaf- COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM 515

TABLE1. Prograde reactions in dolomitic marbles

3 Do + 4Oz + H,O:Tc + 3Cc + 3CO" 0) 2Tc + 3 Cc:Tr + Do + H,O + CO, (2) 5 Do + 8Oz + HrO:Tr + 3Cc + 7CO, (3) Tr + 11 Do:8 Fo + 13 Cc + gCO, + H,O (4) 4 Fo + Do + H,O : Chm + Cc + CO, (5) Do: Per + Cc + CO" (6) fectedby metamorphism associatedwith the Clal.ton Peak stockto the east(Smith, 1972)and the Little Cottonwood stock to the west (Moore and Kerrick, 1976). In the study area (Figs. I and 2), prograde metamor- phism of the SiOr-undersaturatedsiliceous dolomites has resultedin the successiveappearance of talc (Tc), trem- 0 €o olite (Tr), forsterite (Fo), clinohumite (Chm), and peri- clase(Per) (Moore and Kerrick, 1976).The periclasehas now beenreplaced by retrogradebrucite (Br). Theseindex minerals,with in most casesextensive amounts of calcite, are produced by a seriesof prograde metamorphic reac- tions that are summarizedin Table l, Reactionsl-6, and which havebeen discussed previously by Moore and Ker- rick (1976). The unmetamorphosedequivalents of the marblesin the aureoleconsist predominantly of dolomite (Do) with minor quartz (Qz) and calcite (Cc) (both typi- cally < 10 modal0/o).Hence prograde metamorphism has producedabundant coexisting calcite and dolomite pairs t90 throughout most of the thermal aureole.Exceptions a Sonplelocollon tllh are tenocrolureln oCCc-00 the innermostpericlase zone, where dolomite is virtually exhausted,and in the talc zone,where only small amounts of generallyfine-grained calcite exist at the margins of Fig.2. Geologicmap of thestudy area with samplelocations chert nodules. and calcite+ dolomitegeothermometry results. Peak tempera- tures('C) recordedby coexistingcalcite + dolomitepairs (data Ax.lr.yrrc.ll, pRocEDUREs from Table2), calculatedfrom the equationofAnovitz and Es- Sample selection sene(1987). Metamorphic isograds labeled by indexmineral: talc (Tc);tremolite (Tr); forsterite(Fo); and periclase (Per). Geo- Ubiquitous textural relationshipsdocument the coex- logicunits areCt : Tintic; Co : Ophir; Cm : Maxfield;Mf : istence of equilibrium calcite + dolomite pairs at all Fitchville;Mdg : p.t"t"t and Gardison;Tind : intermediate metamorphic grades.Samples for geothermometry were dikes;Qal : alluvium.Alta stockshown by randombar pattem. selectedfrom unweatheredfield specimensthat lacked The preintrusiveAltz-Gizzly thrust(labeled) has repeated part obvious evidenceof alteration of primary metamorphic of the carbonatesection. minerals.Thin sectionsof thesesamples were then stained with alizarin red to verify the existenceofcoexisting cal- beam current, which is required to detect Fe, and a 20- cite and dolomite. Samplescontaining suitable calcite + nA sample current, which was chosento avoid volatil- dolomite pairs werethen examinedoptically (400 x mag- izalion of Mg, were used to obtain the analyses.Multiple nification) for evidenceof dolomite exsolutionfrom cal- analysesof singlepoints were performedon many sam- clte. ples by reducingthe beam diameterfrom 30 to 10 rrm in Exsolved dolomite was not observedin any of the lO-pm stepsto confirm the absenceof Mg volatilization. tremolite zone calcitegrains nor in most of the samples For most analyses,a 30-pm beam diameterwas usedto from the forsterite zone; however, several samplesfrom reintegrate potential submicroscopic-scaleexsolution. the highestgrade forsterite zone and virtually all periclase Various smaller beam sizeswere used on the tremolite zonesamples contain individual calcitegrains that exhib- zone samplesbecause the calcitegrains were too small to it minor exsolveddolomite. For geothermometrymea- use a 30-gm beam. Element concentrationswere calcu- surements,the leastaffected samples from the inner for- lated from relative peak intensities using the 6bO algo- steriteand periclasezones were chosenfor analysis. rithm ofPouchouand Pichoir(1991).

Analytical methods Analytical results Calciteanalyses were performed on the CamecaSX-50 The compositions of calcite grains coexisting with do- electronmicroprobe at the University of Utah. A l5-keV lomite were determinedon 30 samples(Table 2). Rep- 516 COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM

TABLE2. Geothermometrvdata Tle|-e 3, Calcite analyses

Mean std, Max. Irc) rfo) Zone Per Per Fo Fo Tr fr Zone Sample N MgCO. Sd erf. M9CO. mean max. Sample 88.3 90 4b 88.8 88.33 89.16b 89.24a 52.78 53.34 54.75 54.54 88.3 24 5 48 0.59 012 6.20 545 570 CaO 53.13 53.15 't.21 88-14 26 4 99 0.49 0.10 5.72 525 555 MgO 2 50 2.41 2.57 2.09 1.31 88.40 28 3.28 0.30 0.06 4.14 445 490 FeO 0 03 0.00 0 09 0.o2 0.05 0.03 88.51 30 4.29 0.69 0.13 5.59 495 550 MnO 0.00 0.02 0 03 0.0s 0.00 0.01 88.A5. 20 3.49 0.27 0.06 3.97 460 480 co, 44.44 44.35 44.30 44.17 44.32 44.26 88.A7" 30 3.47 0.45 0.08 4.38 455 500 Torat 100.10 99 93 99.76 99.65 100.33 100.15 88.86 26 5.08 0.57 0.11 6.12 530 570 CaCO. 93.81 94.04 93.50 94.76 96.95 9671 90.2 25 4.60 0.60 o.12 5.68 510 555 MgCO" 6.15 5.93 6.33 5.16 2.98 3 23 90.4b 26 5.54 0.36 0.08 5.93 550 560 FeCO3 0.04 0.00 0.12 o.02 0.07 0.04 Fo 88.7 26 5.62 0.22 0.04 5.98 5s0 565 MnCO. 0 00 0.03 0.04 0.05 0.00 0.02 88.8 22 5.10 0.72 0.15 6.40 530 575 88.16 28 4.80 0.43 0.08 5.48 520 545 Note; oxides reportedin weight percent,carbonate end-members in mole 88.18 24 4.23 0.66 0.13 s.06 495 530 oercent normalizedto 100. 88.20 23 3.16 0.41 0.09 4.17 440 490 88.33 33 4.08 0.42 0.07 5.16 490 535 88.55 27 5j2 0.23 0.04 5.59 530 550 88.60" 11 4.50 0.40 o12 5.25 505 535 sene. 1980:Bowman and Essene,1982; Hover-Granath 32 2.54 0.58 0.10 3.35 395 450 et al.. 1983:Anovitz and Essene.1987; Essene, 1989). 88.C2. 22 3.17 0.44 0.09 4.17 440 490 grains 88.C5. 34 3.16 0.55 0.09 4.14 440 490 Retrogradeequilibration producescalcite with Mg 88.D2- 22 3.05 0.39 0.08 3.85 430 475 contentslower than those definedat peak metamorphic 88.D8' 30 2.94 0.45 0.08 4.00 425 485 temperatures.Averaging all data incorporatessuch ret- 90.7 25 3.75 0.44 0.08 4.75 470 515 90.13b 27 4.23 0.45 0.09 5.19 495 535 rograde effectsand therefore underestimatessignificantly 90.18c 30 3.02 0.80 0.1s 5.08 430 530 the actual peak temperature experienced by a sample. 90.19 25 4.'t5 0.52 0.10 5.14 490 535 problem in slower-cooledregional meta- 90.21 30 3.57 0.81 0.15 5.31 460 s40 This is a serious 89.10 25 2.89 0.45 0.09 3.32 420 450 morphic terranes.At Alta, retrogradation has also clearly 89.16b 28 1.54 0.68 0.13 3.03 290 430 affectedmany of the samplesfrom the periclasezone be- 89.20 26 1.39 0.77 0.15 2.69 265 410 89.24a 24 2j6 0.74 0.1s 3.23 365 440 causeeven maximum temperaturespreserved in these samples(Table 2) are lower than temperaturesdefined in Note: maximum,mean, standard deviation, and standarderror of weight percent MgCO3in calcite.Number of individualgrains analyzedper sample the inner forsterite zone still farther away from the intru- is given by N- (the heat source)and are significantlylower than ' sion Resultsof f-test comparisonsfor samplepairs from the same outcrop: minimum temperatures(575 "C) requiredby phaseequi- A5 vs.A7, t:0.18; C2vs. C5, t:0.07; D2vs. D8, f:0 90; to1> 1.282 required to reiect. librium limits for periclasestability (seebelow). Because '- Sample 88.60 exhibits a bimodaldistribution. Values reportedfor two the signal of interest is peak temperature, we utilize the distributions. maximum Mg content (maximum I) recorded in each sample as the most valid indicator of the peak metamor- phic temperature for the following discussions.Because of the potential effectsof retrogradation, even the maxi- resentativeanalyses of calcite are listed in Table 3. A mum temperaturespreserved in a samplemust be viewed minimum of 20 calcitegrains were analyzed in eachsam- as minimum estimatesof actual peak metamorphic tem- ple. The data in Table 2 summarizethe resultsof these peratures. single point analyseson individual calcite grains from Reported temperatures(Table 2) were calculatedusing eachsample. the calcite + dolomite geothermometry expression of Multiple point analyseswere performed on a number Anovitz and Essene(1987). The applicablerange of this of calcite grains in each sample to test their submicro- expressionis 250-800 "C. Uncertainty in thesetemper- scopic homogeneityand chemical zonation. The intra- atures due to the calibration of the geothermometry ex- granularcompositional variation of the individual grains pression and to analytical uncertainty is estimated to be -r25 examinedwas <0.3 wto/oMg in all samplesand was typ- +50 qCfor temperaturesbelow 500'C and "C above ically <0.2 wt0/0.No systematicchemical zoning was ob- 500 "C. Temperaturesin Table 2 are rounded to the near- servedin any sample.However, because of the relatively est 5 0C. largebeam diameterchosen for our analyticalprocedure, To evaluateoutcrop-scale equilibration, sample pairs any chemical zoning that might exist in the outermost from three outcropswere testedfor consistencybetween poftions of the calcitegrains (s50 pm from grain edge) their compositional distributions (Table 2). The mean could not be identified. calcitecompositions of the sampleswere comparedsta- Almost all samplesexhibit significantgrain to grain tistically using a Student'sI distribution. In all three cases, variations in Mg content that exceed significantly the the sample means were found to be not statistically dif- variation in Mg contentwithin individual grains(at least ferent at a confidencelevel of0.9. Theseresults are con- in grain interiors).Such variations are usuallyattributed sistentwith the attainmentof thermal equilibrium by cal- to retrogradeequilibration (Rice, 1977; Valley and Es- cite + dolomite pairs on an outcrop scale. COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM 5t'l

DrscussroN 700 Thermal constraints on metamorphism a Per LFo The maximum temperaturescalculated for each sam- 600 .Tr ple are shownon a map of the study areain Figure2 and as a function of horizontal distancefrom the intrusive contactin Figure 3. The temperaturesrange from 410 to 5@ 450 "C for the outer tremolite zone, are 490 'C for the r("c) 'C outer forsteritezone, and reach a high of 575 in the lo0 inner forsterite zone near the periclaseisograd (Fig. 3). Thesetemperatures are interpretedto be minimum esti- mates of peak metamorphic temperaturesand follow a JW consistenttrend of increasingtemperature in the direc- tion ofthe intrusive contact(Fig. 3). 2so' The maximum calculatedtemperatures (570-575 'C) I 250 r 500 occur on both sidesof the periclaseisograd in both the Distonce.m periclaseand forsteritezones. Temperatures of 570-575 Fig.3. Resultsof calcite+ dolomitegeothermometry plotted 'C for the periclasezone are slightly lessthan those re- as a functionof sampledistance from the stockcontact. The corded for the periclasezone associatedwith the northern polynomialfrt to the cal- 'C: solid curverepresents a second-order Boulderbatholith (590-625 Rice, 1977)and the Black cite + dolomitegeothermometry results, excluding the lower, oC: Butte stock (585-600 Bowman and Essene,1982), retrogradedtemperatures in the periclasezone. This fittedcurve both of which formed at depthssimilar to the Alta stock is usedto representthe peak temperature-distance profile for the (Pr: 100-150MPa). aureolein the text. Becausethe fit is good,extrapolation of the Severalpericlase zone samples,particularly those close curveto the igneouscontact provides a goodestimate ofcontact : to the intrusive contact,yield temperaturesconsiderably temperatureQJ 626"C. lower than the maximum observedtemperatures. Several are < 500 'C, which is well below of thesetemperatures the Alta contactaureole (Moore and Kerrick, 1976;Cook, phaseequilibria for the sta- the temperaturesrequired by 1992). The most restrictive equilibrium is Reaction 6, periclase(see and below those(600-625 bility of below) which was responsiblefor the appearanceof periclase. 'C) in periclase-bearingzones near ig- typically attained Figure 4 illustrates T-Xco. equilibria governing the for- (Rice, Bowman Essene,1982). neouscontacts 1977; and mation of periclase calculated at a constant lithostatic indicate periclase- These large discrepancies that these pressureof 150 MPa and various fluid pressuresbetween grains retrograde zone calcite have undergonesignificant hydrostatic(approximately 50 MPa) and lithostatic(150 reequilibration. MPa) pressureconditions. The lithostatic pressure(P' ) polynomial curve provides a good fit A second-order estimate,100-200 MPa, wasmade by Wilson(1961) us- by the calcite to the temperature-distancetrend defined ing stratigraphic measurements.However, his recon- geothermometry(Fig. 3). This curve allows + dolomite struction is complicatedby possiblestratigraphic thin- (2.) to be estimated.The result- the contacttemperature ning over an ancestralUinta arch or possiblethickening peak ing value, 626 "C, representsthe likely temperature from preintrusive thrusting. Multiple preintrusive thrusts is not a greatdeal at the igneouscontact; this temperature have been mapped by Baker et al. (1966) in both the periclaseisograd (570-575 'C). higher than that at the north and south sidesof the Alta stock (Fig. l). Mineral assemblagesfrom the Ophir Formation (biotite * anda- Constraints on fluid pressureduring metamorphism lusite + potassium feldspar + quartz + cordierite) in the The thermal regime indicatedby the resultsof calcite innermost aureolealso constrainpressures to 150 + 50 + dolomite geothermometryalso placeslimits on fluid MPa (Kemp, 1985). pressures(Pr) during the metamorphicevent. Because the The maximum temperaturesrecorded by calcite + do- resultsof calcite + dolomite geothermometryare nearly lomite geothermometry at the periclase isograd, which independentofpressure (Anovitz and Essene,1987), the lies a maximum of 200 m from the intrusive contact,are fluid pressureregime during the metamorphicevent must 5'70-575'C (Figs.2 and 3). Thesetemperatures require have been such that the Z-X.o, locations of the mixed- a minimum fluid pressureof approximately'75 MPa to volatile phase equilibria that define the stability field of stabilize the assemblagePer + Cc relative to Dol (Fig. periclaseare consistentwith the measuredcalcite + do- 4). As most of the periclasezone sampleshave beenaf- 'C lomite geothermometryresults. fectedby retrogradereequilibration, 575 must be con- Microprobeand whole-rockchemical analyses indicate sideredthe minimum temperaturefor the formation of that phase equilibria in the CaO-MgO-SiOr-HrO-CO, this zone.Higher temperatureswould permit correspond- system,after adjustmentfor the effectsof F, can be rea- ingly higherfluid pressures.If575 "C is closeto the actual sonably applied to the observed mineral assemblagesin maximum temperatureachieved at the periclaseisograd, 518 COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM

GEOTOGIC MODEL .16 -@- -l'Fo Pr, tr 4 Cz VOLCANICS

-2000 a/ tt9) ./ s e LATE Pz, Mz, Cz CLASTICS = a 9- | p", a*-' i -+ooo l-- )P) s 3 Pz CARBONATES {'{J Pr=150MPo l{J --- P;l50 MPo -6000 5 100MPo ALTA I5 4 p€, C QUAFITZTTES Mpo >IUUF 50MPo -8000 002 004 6 2000 4000 6000 8000 Xco, DISTAI,ICE(n) Fig. 5. Geologicmodel of the Alta stockand surrounding Fig. 4. Z-X.o. phase equilibria governing the formation of units usedin the finite elementsimulations. Unit 1, Tertiary clinohumite- and periclase-bearingmineral assemblagesin the volcanics;Unit 2, l-atePaleozoic, Mesozoic, and Cenozoicsed- periclasezone al a constanl lithostatic pressureof I 50 MPa and iments;Unit 3, Paleozoiccarbonate rocksl Unit 4, Precambrian fluid pressuresof 50,75, 100, and 150 MPa. For the estimated and Cambrianquartzites and shales;Unit 5, Alta stock.Stock lithostatic pressureof 100-200 MPa for the Alta aureole (Wil- geometry,size, and locationwithin stratigraphicsection con- son, 1961; Kemp, 1985),these fluid pressurescorrespond ap- strainedby availablegeological field evidence.The dashedline proximatelyto hydrostatic(50), intermediate (75- 100),and lith- denotesthe stratigraphicposition ofthe first Alta-Grizzlythrust ostatic (150) pressureconditions. Reactions in addition to fault (Fig.2) within the studyarea. The bracketalong the right : Reactions5 and 6 listed in Table I are (7) brucite (Br) periclase margin indicatesthe stratigraphicsection of carbonaterocks : (Per) + H.O; (8) dolomite (Do) + H,O calcite(Cc) + Br + sampledfor geothermometry( 5.4 to -5.6 km). COr. These equilibria restrict the stabiliry of periclase to T-X.o. conditions above invariant point I and Reactions 6 and 7. Calcite-dolomite geothermometry defines a minimum tem- l98l; Furlong et al., l99l). If significantadvective heat perature of 575 'C for the formation of periclase(marked 7"". transfer occurred in conjunction with the progrademeta- on the temperatureaxis). This minimum temperature,combined morphism at Alta, then evidenceof this advectivetrans- with thesephase equilibria, yields a minimum estimateof fluid port should be preservedin the peak thermal profile de- pressureof about 75 MPa for contact metamorphism in the au- fined by the calcite + dolomite results (Fig. 3), which reole. Equilibria were calculatedusing thermodynamicdata from recordsthe cooling history of the Alta stock as a function Helgesonet al. (1978), for except clinohumite, which was ex- of time and distancefrom the margin of the stock. tracted from data of Duffy and Greenwood (1979). Unit activi- To test the observed thermal profile for evidence of tieswere assumed for all solidsexcept clinohumite (a.n- : 0.35). advective heat transfer, a conductive cooling model of Values of /.o. and /",o were calculatedusing the modified Red- lich-Kwong equationof Bowersand Helgeson(1983). the Alta stock was constmctedusing the finite-element method (Cook, 1992).The evolution of the thermal re- gime surroundingthe stockis governedby the heat-trans- then metamorphism in the periclase zone must have oc- port equatron: curred at fluid pressures below the estimated lithostatic pressure of 150 MPa. :t'p,c,+(r - (e) *g(^,*ur) ilo,ct{ Evidence of advective heat transfer in which 7 representsthe temperature, C, and C" the spe- Moore and Kerrick (1976) have shown that the pro- cific heat capacitiesofthe fluid and solid, respectively,p" grade metamorphic sequence exhibited by the dolomites the solid density,@ the porosity,and If the thermal con- requires infiltration by an HrO-rich fluid with increasing ductivity tensorof the solid-fluid composite metamorphic grade. In addition to driving the prograde trr: @trf+ (l _ reactions, this fluid flow may have also influenced the d)I;. thermal regime surrounding the stock. Under the proper The governing partial differential Equation 9 was solved conditions, advective heat transport can significantly al- numerically using the Galerkin finite-element method ter thermal regimes around cooling plutons (e.g., Cathles, (Huyakorn and Pinder, 1983)applied over triangularel- 1977; Norton and Knight, 1977;Parmentier and Schedl, ements. COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM 519

The geologicmodel used in the simulations is illus- TABLE4. Thermalparameters trated in Figure5. The model is divided into five distinct Unit 6 p, rock units, basedon the observedstratigraphic and struc- 'I tural relationshipssurrounding the stock, reported sub- 10.0 2700 2.O 950 2 10.0 2700 2.2 1000 surfacegeology (Calkins and Butler, 1943; Baker et al., 3 5.0 2700 20 1050 1966),and late Paleozoic,Mesozoic, and Cenozoicstra- 4 2700 3.0 1100 c 2700 1.8 1450. tigraphy from elsewhere in the central Wasatch range 05 : (Wilson,1961; Hintze, 1973). Note: O : porosity (in percent),p" : density (kg/m3),Ii thermal con- I 4 constitutethe stratigraphicsection ductivitytry(m "C)1,C": specificheat capacity[J/(kg''C)]. Units through . C, for Unit 5 (intrusion): 1150J/(kg "C) + SO0J/(kg'"C) to account into which the stock was emplaced.Unit I consistsof for latent heat of crystallization. volcanic rocks, roughly equivalent in age to the stock; Unit 2, a packageof clasticsediments, predominantly of Mesozoicand Cenozoicage; Unit 3, the Paleozoicsili- are relativelyinsensitive to changesin pres- ceousdolomites exposed in the contactaureole; and Unit becausethey For the stock (Unit 5), an addi- 4, the Precambrianand Cambrian quartzitesand shales sure and temperature. has beenadded to the heat capacity that make up the bulk ofsubsurfacecontact length ofthe tional 300 J/(kg''C) for the latent heat of crystalliza- Alta stock.The sectionof the model that correspondsto of the rocks to account magma. the stratigraphicsection that was sampledin the contact tion of the to the solid properties,parameter sensitiv- aureolefor calcite + dolomite geothermometryis indi- In contrast by Strausand Schubert( I 977)dem- cated by the bracket along the right margin in Figure 5. ity studiesconducted accountmust be taken ofthe pres- The dashedline within Unit 3 denotesthe locationof the onstratethat explicit of the propertiesof HrO, Alta-Grizzly thrust zone (Fig. 2). sure-temperaturedependence in theseparameters affect significantlythe Unit 5 representsthe Alta stock.The mappedexposure as variations regimes in both conductively and advectively of the Alta stock(Fig. 1) hasan approximateaspect ratio thermal of 2:l and a half-width of approximately1.5 km. As this cooledsystems. accountsfor the changesin the geometrymore closelyapproximates a cylinder than an Accordingly,the model properties the pore fluids. The calculations infinite slab, the axisymmetric form of Equation I was thermal of pore fluid is pure HrO. This neglects used in the models, rather than the planar coordinates assumethat the possible of a potentially significantCO' compo- that have been typically used in heuristicmodel studies effects fluid and of salinity on fluid properties.This of cooling plutons (Cathles,1977; Norton and Knight, nent in the unavoidable,as a comprehensive 1977).Outcrop patternsindicate that the southernpor- assumptionis however the transport propertiesofboth tion of the igneouscontact is nearly vertical and has no set ofequations describing mixed-volatile fluids has not yet been major irregularities.There is no geologicevidence for an high-salinity and outward-dippingigneous contact nor for the presenceof developed. for liquid HrO werecalculated using the significantamounts of igneousmaterial in the form of Fluid densities Meyer et al. (1967) for temperaturesbelow sills or dikesextending any greatdistance into the aureole equationsof and thoseof Keenanet al. (1978) beneathor within the marble section.The emplacement the critical temperature heat capacitieswere computed using depth (5 km) is basedon the stratigraphicreconstruction elsewhere.Specific the expressionsofKeenan et al. (1978)and thermal con- by Wilson ( I 96 I ) and is consistentwith more recentpet- formulation of Kestin (1978). rologicand fluid inclusionstudies of the skarnsand meta- ductivities from the are requiredto obtain pelitesin the aureole(Kemp, 1985)and of the Alta stock Initial and boundaryconditions governingdifferential equation. (John,l99l). specificsolutions to the conditionsare In the model, mean valueswere usedfor thermal con- Theseboundary ductivities (I") and specificheat capacities(C") for each AT -8 < km at x stratigraphicunit (Table 4). Thermal conductivitiesare I,,-:0 mWm': for km'< z 0 dx :0 km basedon compiled values for rocks of analogouscom- km andx: 8 position (Carslawand Jaeger,1959; Woodside and Mess- mer, 1961);heat capacitiesare basedon heat capacities for minerals (Robie et al., 1978)and mineral modesfor ^,.# : loomw/m'z for0km

6m -2000 E 500 T/O^\ = a -4000 @ sF- l{J ! UJ 300

m

Distonce,m -"""0 Fig.7. Comparisonbetween geothermometry results (Fig. 3) 2000 4000 6000 8000 and peaktemperature profiles (curves A and B), corresponding DISTANCE(n) to the modelillustrated in Fig. 6. CurvesA and B definemaxi- mum modeltemperatures achieved along a horizontalprofile at Fig.6. Conductivecooling model at 5000yr followingem- - 5.5km during100 000 yr of coolingfor initialintrusion tem- placemenl of the Alta stock.The emplacementtemperature of peraturesof 825 (curveA) and 925 'C (curveB). Maximum thestock was 825'C. Thebracket along the right margindenotes temperatureat the igneouscontact corresponds to those(825 the stratigraphicsection corresponding to the exposedcontact 'C) -5.6 and925 obtainedinstantaneously upon intrusion. The 5.5- aureoie(-5.1 to km).The carbonatesection sampled for km level correspondsto the averagedepth of the stratigraphic geothermometrycorresponds to the deeperportion of this sec- geothermometry -5.4 -5.6 intervalsampled for in thecontact aureole. Also tion,the interval to km. shownis the second-orderpolynomial fit to thegeothermometry data(solid line) and a stippledband centered on this fittedcurve basedon estimateduncertainties in the geothermometryresults which is consistentwith a region experiencingperiodic of +25'C above500 "C and +50'C below500 "C. Brackets intrusive eventsland a constanttemperature of l0 .C at denotetemperature limits at the positionsof the periclase(Per) andtremolite (Tr) isogradsbased on geothermometryresults and the surfaceboundary, representinga rneanannual tem- phaseequilibria (Cook, 1992; this study). perature.The assumptionofconstant heat flux acrossthe entire baseof the model leadsto some overestimateof wall-rock temperaturesnear the igneouscontact after in- most resultsin the range 700-800 "C. Theseresults are truslon. consistentwith the emplacementtemperature estimate The model treatsthe stockas if it wereinjected instant- basedon zirconium geothermometry. ly at 825'C into hydrostaticallypressured, HrO-saturated The effectof the endothermicmetamorphic reactions country rocks under a steady-stateconductive geothermal that occurredin the marblesand underlyingpelitic rocks gradient.Although instantaneousinjection is geologically (Figs. I and 2) on the thermal evolution of the aureole unrealistic,this approachis consideredacceptable, as up- has not been incorporatedinto the model. Ferry (1980) ward migration rates of thermal maxima are significantly has noted that metamorphic reactionscan, dependingon less than the rates of igneous intrusion (Norton and bulk composition and mineral assemblage,consume a Knight, 1977). significantquantity ofheat. The effectofthese endother- The emplacementtemperature estimated for the stock mic reactionsis to lower temperature throughout the au- (825 "C) is based on the results of zirconium geother- reole.Numerical experiments by Bowerset al. (1990)sug- mometry (Watsonand Harrison, 1983)applied to whole- gestedthat temperaturesmay be overestimatedby as rock analysesof the Alta stock (maximum whole-rock much as 40-50 "C if the effectof endothermicreactions zirconium content 220 ppm) and is consistentwith its is ignored. Thus the calculated temperature regimespre- granodiorite bulk composition (Cook, 1992).This geo- sentedin this study should be regardedas maximum es- thermometerassumes that l00o/oof the whole-rockzir- tlmates. conium was initially present in the liquid portion of the Figure 6 illustrates the results of the cooling model at magma.Thus this approachyields a maximum temper- 5000 yr following emplacementof the stock.A peaktem- ature estimatefor the stock becauseof the possibility of perature-distanceprofile, correspondingto this model over inheritedzircon from the sourceregion. In addition,John a span of 100000 yr, is shown as curve A in Figure 7. (1991) has applied empirical biotite crystallizationgeo- This model profile was taken at a depth of -5.5 km, thermometryto biotite compositionsfrom the Alta stock. which corresponds to the average depth of the strati- For all samplesfrom bufferedassemblages, biotite crys- graphic section sampled for geothermometry in the con- tallization temperaturesranged from 650 to 840.C, with tact aureole(i.e., data ofFig. 3). COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM 521

A comparisonin Figure 7 betweenthe model profile inate the discrepanciesbetween model results and the (curve A) and the observedpeak temperature-distance lower limit of uncertainty in the geothermometrydata profile basedon a second-orderpolynomial fit to the cal- (Fig. 7) throughoutthe aureole,not just in the outer au- cite + dolomite data from Figure 3 indicates that this reole where the effectsof increasing ?|" are the greatest. conductivemodel fails to achievethe observedpeak tem- However, there are geologic,petrologic, and thermal peraturesat virtually all distances;observed temperatures factorsthat constrain I" to much lessthan 375 "C. The 'Clkm are significantly greater than model temperatures (typi- preintrusive geothermalgradient of 55 selectedfor cally > 100 'C beyond the first 100 m from the igneous the model and the resultingT-": 275'C at the depthof -5.5 contact)at all distances.This is a resultofthe rapid cool- the sampledsection of the contactaureole (-5.2 to ing of the upper portion of the stock that occurswithin km) are already significantlyelevated values compared the first few hundredyears following its emplacement.In with those (30 "C/km and 7".: 165 "C) appropriatefor the model, the minimum temperaturerequired by geo- the regional tectonic setting of the Wasatch rangeand for thermometry and phaseequilibria for the formation of the estimateddepth of emplacement(3-6 km) of the Alta periclase(575 'C) was achievedonly within l5 m of the stock(Wilson, l96l; Cook, 1992).These elevated values intrusive contact,a distancemuch lessthan the observed were selectedto accommodatethe possiblethermal ef- maximum distanceof 200 m (bracketlabeled Per). The fects ofprevious intrusive activity in the area (Crittenden discrepanciesbetween model results and the geother- et al., 1973). mometry data exceed the uncertainties in the geother- Initial wall-rocktemperatures of = 375-400 "C arealso mometry data (stippledband centeredon the regression inconsistentwith the absenceof the regional develop- curve in Fig. 7). ment of either talc or tremolite in thesesiliceous dolo- One possibleexplanation for thesediscrepancies is that mites. At the very HrO-rich pore fluid compositions the actual emplacementtemperature of the stock was (X.o, << 0.03) characteristicof deep ground waters in greaterthan the temperatureestimated by zirconium geo- sedimentarysequences prior to the onsetof metamorphic thermometry(825 'C). Curve B in Figure7 representsan reactions(for example, Hitchon and Friedman, 1969), analogouspeak temperature profile for a conductivemodel thermodynamiccalculations show that the first appear- in which the emplacementtemperature was set a|925 "C. ance of talc would be below 325 "C. If the actual initial 'C The increasedintrusion temperaturehas relatively little temperaturesat Alta were in the vicinity of 375-400 effecton the model thermal profile, and there remain large required to eliminate most of the discrepancybetween discrepanciesbetween observedand model temperatures model resultsand the lower limits to geothermometry, throughoutthe aureole.The predictedwidth ofthe peri- then there should be widespreadevidence of regionallow- clasezone doesincrease slightly to approximately25 m. grade metamorphism (ubiquitous presenceof talc and This modest improvement implies that to achievethe eventremolite) in the siliceousdolomites stratigraphical- observedwidth of the periclasezote (T > 575 'C up to ly equivalentto those in the aureole.No such regional 200 m from the intrusive contact) using strictly conduc- metamorphism is observed in this area of the Wasatch tive heat transfer would require a model with an initial range,nor has it been reported in the literature;on the intrusion temperaturewell in excessof 925 "C. Such an contrary, the spatial distribution of both tremolite and initial intrusion temperature is inconsistent with the talc in theseprotoliths is clearly related to the Alta, Little available geothermometry results from both the marbles Cottonwood, and Clayton Peak stocksand thereforeis and the stock and probably exceedsthe maximum pos- contactmelamorphic in origin. sible liquidus temperature for a granodiorite magma as Finally, if the thermal profile recorded in the Alta au- well. The presenceof stable,euhedral hornblende in the reole is ascribedsolely to conductive heat transport from stock (Wilson, 1961) at the low pressuresindicated for the Alta stock, then basic heat flow-balanceconsidera- the Alta aureole(<200 MPa) would require a HrO con- tions for conduction require specificrelationships among tent in the magma in excessof 5 wto/o(Merzbacher and intrusion (f-), contact (f), and initial wall-rock (7".) Eggler,1984; Rutherford and Devine, 1988).This implies temperatures. These basic considerations (Turcotte and that the magma was at, or very near, HrO saturationwhen Schubert,1982, p. 168-172) can be summarizedby the it was intruded. The maximum possibletemperature for following generalization: intrusion of the Alta stockthen correspondsto the HrO- T.: x(T^ - T") + T" (10) saturatedliquidus temperaturefor a granodiorite magma at low pressures(<150 MPa), which is approximately where the specific value of the coefficient x depends on 960 "C (Rutherford and Devine, 1988).Such a high in- the thermal properties of the intrusion and wall rocks, trusion temperature directly contradicts the geother- the amount of latent heat of crystallizationassigned to mometry evidence from both the stock and the aureole. the intrusion, and the initial differencein temperature If an initially hotter stock cannot account for the ob- betweenmagma and wall rock. For many common rock serveddiscrepancy, then an alternativemechanism to el- and magmatypes intruded into the shallowcrust, values evate the model thermal profile is an increase in the of x are close to 0.6 (Turcotte and Schubert, 1982, p. preintrusivewall-rock temperature(f.). At a minimum, 172).The value of the coefficientx increaseswith an in- ?"^would have to be increasedto >375-400'C to elim- creasingdifference between Z- and Z" and with increas- s22 COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM

ing latent heatof crystallization.For the specificsituation "C in temperature between observed and model thermal at Alta, x > 0.61 even for the geologicallyunrealistic profiles.The occurrenceof severalroof pendantswithin situation of f. : 0 "C. For the likely minimum possible the stock (Fig. l) suggeststhat the presenterosional ex- T"at Alta, 100'C (basedon a minimum estimateddepth posurelies near the original upper margin of the stock. of emplacement: 3 km and an unperturbedgeothermal However, a significantly greater vertical extent than that gradient of 30 'Clkm), x : 0.623. Equation l0 is rou- already included in the model cannot be completely tinely invoked to make estimatesof 2., on the basis of ruled out. known or assumedvalues of Z- and 7"".However, if both Thermal effectsof magmaconvection. Magma convec- ?"- and Z. are known from applications of geothermom- tion can affect maximum temperaturesin a contact au- etry, then thesesame heat balancerelationships establish reole (Jaeger,1964). In attemptingto simulatevigorous rather firm upper limits to Z" for any model of the ther- magma convection,Bowers et al. (1990) found that the mal evolution of an aureolethat invokesstrictly conduc- effectsof magmaconvection on maximum temperature- tive heat transport. distanceprofiles normal to the igneouscontacts of planar The second-orderpolynomial fit to the geothermom- or regular geometry (analogousto the south contact of etry data in the Alta aureoledefines T,: 626'C (Fig. 3). the Alta stock)were minor, exceptvery nearthe igneous With f. : 626 "C, T^: 825'C on the basisof the results contact. However, they did find that the effectsare more of zirconium geothermometry,and x : 0.61, Equation significantlocally near geometricirregularities in an ig- l0 yields an upper limit to T-": 325'C (for the more neous contact (reentrants and apophyses).Some magma realisticvalue of x : 0.623, the maximum 7".would be convection may have occurred in the Alta stock, on the 300'C). This upper limit to I" is only somewhathigher basis of local occurrencesof flow banding. We cannot than that (7": 275'C) usedin the previousmodel cal- evaluatequantitatively the effect of magma convection culations(curves A and B: Fig. 7). Use of this upper limit becauseof the lack of information with which to define to T" (325 'C) would yield model temperatures for the actual convection parameters appropriate for the Alta outer aureolethat are still 60'C below the measuredther- stock.On the basisof the numericalexperiments of Bow- mal profile (but closeto the lower limits of uncertainty). ers et al. (1990), it is likely that the effectsof magma Suchan increaseis also inadequateto remove significant convection on the temperature-distanceprofiles for the discrepancies(up to 75 "C) betweenmodel results and south Alta aureolewould be relatively small, given the measuredtemperatures throughout much of the rest of regular, vertical geometry of the south contact (Figs. I the contact aureole.Thus thesegeologic, petrologic, and and 2). thermalconsiderations suggest that a geologicallyrealistic Hidden intrusions.The occurrenceof hidden intrusive increasein Z. alone is a very unlikely mechanismby massesor dipping igneouscontacts beneath wall rocks which to resolve the large discrepancybetween the con- can exert significant effectson temperature. There is no ductive model resultsand geothermometrydata. geologicalevidence in either outcrop or mine workings If the large discrepancy between the thermal profiles for intrusions underlying or near the south part of the predictedby conductivecooling models for the stockand Alta aureole in the areas sampled for geothermometry. the observedthermal profile definedby geothermometry Outcrop patternsindicate that for the level of the aureole and phaseequilibria cannot be resolvedby geologically exposed,the contact of the Alta stock is nearly vertical reasonableincreases in intrusion and wall-rock temper- and regular. In addition, isograds parallel this igneous aturesalone, then thesediscrepancies must be due to one contact (Moore and Kerrick, 1976;-Fig.I of this paper) or more of the following possibilities:(1) increasein the and show no evidenceof perturbationby hidden intru- heightof the stock,(2) thermal effectsof magmaconvec- sions.As there is no geologicalevidence for hidden in- tion, (3) hidden intrusionsor dipping igneouscontact be- trusions,this possibility is speculationand is not a test- neath the aureole, (4) advective heat effects. able hypothesis. lncrease in the vertical extent of the stock. One possi- Advective heat effects. Advective heat transport may bility is that the top of the stock is significantlyhigher also increasetemperatures in the aureole(Cathles, 1977; than the top of the stockin the thermal model. The two- Norton and Knight, 1977;Parmentier and Schedl,l98l; dimensionalresults indicate that the largestincreases in Furlong et al., l99l). The possibilitythat advectiveheat wall-rock temperaturesoccur in the region below the transfercontributed significantly to the thermalevolution midpoint of the stock's vertical margin (Fig. 6). If the ofthis sectionofthe aureoleis particularlyattractive be- stockhad had a greatervertical extentin the model, then causeofindependent field, petrologic,and stableisotopic the carbonatestratigraphic section exposedin the contact evidence for the occurrence of infiltration-driven meta- aureole would have been located lower relative to the morphism. midpoint of the stock'svertical margin and would have been placedin a higher temperatureregime (Fig. 6). As Geologic, petrologic, and isotopic eyidencefor fluid flow constructed,the model alreadyhas a postulated0.3 km Consider the formation of periclaseby the reaction ofadditional vertical extentbeyond the presenterosional Do: Per * Cc * CO,. (6) exposure.The top of the stock would still have to be raisedby >>I km to accountfor the discrepancyof > 100 In a closedsystem, a progradereaction can only advance COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM 523 as long as temperaturerises, and, correspondingly,a pro- Application of one-dimensionalisotope transport '8O gradereaction can only bufferthe pore fluid composition models indicate that the observedpattern of deple- to higherX.o, valuesas long as temperaturecontinues to tion in the south aureole (Cook, 1992) is inconsistent rise. Once a rock reachesa maximum temperature,no with fluid flow toward the igneouscontact (i.e., in the additional reactioncan occur.At the pressureconditions direction of increasing temperature) (Bowman et al., estimatedfor the Alta aureole(P.: 75-100 MPa), very 1994).Rather, transport modeling demonstrates that the little periclasecan be producedunder closedsystem con- observedisotopic patterns are consistentwith the flow of ditions unlesstemperatures exceed the steep-slopedpor- significantamounts of '8O-depletedfluid laterally away tion of the T-X.n. equilibrium surfacefor Reaction6, or from the igneouscontact (i.e., in the direction ofdecreas- above approximately 700 'C. Thus the formation of a ing temperature)(Bowman et al., 1994), a conclusion significant quantity of periclasewithout infiltration by a consistent with the geothermometry data and thermal HrO-rich fluid requires temperatureswell in excessof modeling resultspresented here. Application of one-di- 700 'C. Suchhigh temperaturesnear the igneouscontact mensionalflow modelsis justified by the field,petrologic, are precluded by the conductive cooling models of the and isotopic evidenceindicating one-dimensional(i.e., stock, on the basisof the estimatedintrusion temperature bedding-controlled)flow in the marbles. of the magma (approximately825 "C) and the resultsof Numerical models(two-D, axisymmetric)of advective calcite + dolomite geothermometry. heat transportrequire cumulativefluid flux approximat- Yet many of the marbles contain abundant periclase ing 103to 3 x 103m3lm'? and time scalesof approxi- and are nearly devoid of dolomite, indicating that Re- mately 5 x 103yr to match the observedcalcite + do- action 6 has gonenearly to completionin thesesamples. lomite thermometry profile and to reproducethe observed This discrepancybetween observed reaction progress and width of the periclasezone (Cook, 1992).One-dimen- the limited abundanceof periclasepredicted for closed- sionalmodels of O isotopetransporl require a minimum system conditions requires infiltration of a significant cumulative fluid flux of approximately8 x 102m3/m2 to volume of H,O-rich fluid (Cook, 1992)to permit the for- reproducethe observedpatterns of'8O depletionin the mation of periclaseat more reasonabletemperatures dolomitic marblesof the south aureole(Bowman et al., (-600 'C) by displacingand diluting CO' generatedby 1994).This flux estimateis a minimum one becausein the reaction. an axisymmetric aureole such as the Alta aureole flow- The spatial distribution of periclase within the peri- lines are radial, not parallel.The generalconsistency of clasezone indicatesthat this infiltration was largelylat- these flux estimateslends credenceto the possibility that eral to the intrusive contact.Within the periclasezone fluid flow has systematicallyaffected the thermal regime the original bedding in the marblesis almost horizontal ofthe southernpart ofAlta aureoleat the level ofcurrent (0-10"W dip) when correctedfor postintrusionrotation exposufe. on the Wasatch fault and is nearly perpendicular to the intrusivecontact. Throughout the zone,periclase-bearing Sulrprlnv bedsalternate with periclase-absentbeds. This pattern is The main points of this study can be summarizedas inconsistent with vertical flow through this zone, requir- follows: ing that fluid flow was bedding-controlledand largelylat- l. Calcite + dolomite geothermometryapplied to si- eral to the vertical intrusive contact. A lateral flow pat- liceousdolomites in the contactaureole of the Alta stock tern is further supportedby the metasomaticintroduction constrainsthe temperatureof metamorphism to the range of bedding-concordantludwigite and thin garnet-pyrox- 410-575 "C. These temperaturesare compatible with eneskarn layers within individual periclase-bearingbeds. temperaturelimits from phaseequilibria for the observed Lateral infiltration by a significant amount of '8O-de- mineral assemblages. pleted fluid is also required by stable isotope data. Much 2. The minimum temperaturerecorded by calcite * of the periclasezone is significantlydepleted in '8O (up dolomite geothermometryfor the formation of periclase to l6Vm:Cook, 1992).These depletions are far too large (575 'C) requiresa minimum fluid pressureof approxi- to result from isochemicaldecarbonation alone (Valley, mately 75 MPa during the metamorphicevent. However, 1986)and requirethe infiltration ofa significantquantity if 575 'C is closeto the actualpeak temperature achieved of '8O-depletedfluid. Within the '8O-depletedpericlase at the periclase isograd, the fluid pressuresduring the zone there are also significantbed to bed variations in event must have been considerablyless than the esti- 6'80 values (Cook, 1992). As with the distribution of mated lithostaticpressure of | 50 MPa. periclase,this patternof'8O depletionis inconsistentwith 3. The temperature-distanceprofile recordedby cal- vertical flow in the periclasezone and requires that fluid cite + dolomite pairs for the exposedlevel of the south flow was bedding-controlledand largely lateral to a nearly aureoleis significantlyhotter at all distancesthan maxi- vertical igneouscontact. The most '8O-depletedcalcites mum temperature profiles computed from geologically have d'8O values (8-107m)equivalent to those in ex- reasonable,conductive cooling models of the Alta stock changeequilibrium with the Alta stock(8-9V*), implying that utilize initial magma and wall-rock temperatures that the fluids equilibratedisotopically with the adjacent consistent with available petrologic and geologic evi- Alta stock prior to infiltrating the periclasezone. dence. 524 COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM

4. This significant and systematic discrepancy Cook, S.J. (1992) Contact metamorphism surrounding the Alta stock, throughoutthe aureolesuggests one or more possibilities: Little Cottonwood Canyon, Utah. Ph.D. thesis, University of Utah, (1) this sectionof the aureoleformed greater Salt Lake City, Utah. at a much Cnttenden, M D. (1965) Geology of the Dromedary Peak quadrangle, depth relative to the top of the intrusion than estimated; Utah. U S. Geological Survey Map GQ-378. (2) magma convection significantly affected the thermal Crittenden,M.D., Stuckless,J.S., Jr., Kistler, R.W., and Stern,T.W. (1973) profile; or (3) the observed thermal profile was affected Radiometric dating of intrusive rocks in the Cottonwood area, Utah. by advective heat transfer. U.S. GeologicalSurvey Joumal ofResearch,I, 173-178. (1979) possibility Dufl, C.J., and Greenwood, H.J. Phaseequilibria in the system 5. The ofadvective heat transportis partic- MgO-MgF.-SiO.-H,O. American Mineralogist, 64, ll56-l17 4. ularly attractive, with the field observations,petrological Essene,E.J. (1989) The current status of themobarometry in metamor- constraintson the formation ofpericlase,spatial patterns phic rocks In Geological Society ofAmerica SpecialPaper,43, l-44. of D'8Ovalues (Cook,1992), and isotopetransport mod- Ferry, J.M. (1980) A casestudy of the amount and distribution of heat eling results(Bowman eI al., 1994),which independently and fluid during metamorphism Contnbutions to Mineralogy and Pe- trology,7l,373-385. indicate extensive,bedding-controlled fluid infiltration -(1983) On the control of temperature,fluid composition, and re- lateral to the intrusive contact during prograde meta- action progressduring metamorphism. American Journal of Science, morphism. The similarity of flux estimatesfrom inde- 283-4,2rO-232. -(1989) pendent heat transfer and mass (O isotope) transport Contact metamorphism of roof pendantsat Hope Valley, Alpine County, Califomia, USA: A record of the hydrothermal system models(Cook, 1992;Bowman et al., 1994)required to of the Sierra Nevada batholith. Contnbutions to Mineralogy and Pe- reproducethe observedthermal and isotopic character- trology, 101, 402-417 isticsof the marbleslends credence to the possibilitythat Furlong, K.P., Hanson, R.B., and Bowers,J.R. (1991) Modeling thermal fluid flow has systematicallyaffected the thermal regime regimes:In Mineralogical Society of America Reviews in Mineralogy, 26,437-506. ofthe southernpart ofAlta aureoleat level the ofcurrent Giberti, G., Moreno, S., and Sartoris, G (1984) Evaluation of approxi- exposure. mations in modeling the cooling of magmatic bodies. Journal of Vol- canologyand Geothermal Research,20, 297-31O. Acrxowr,nncMENTs Helgeson,H.C, Delaney,J.M., Nesbitt, H.W., and Bird, D.K. (1978) Summary and critique of the thermodynamic propertiesof rock-form- We thank Lukas Baumgartner and an anonymous reviewer for their ing minerals.American Journal of Science,278A,l-228. constructive reviews. This study representsa portion of the senior au- Hintze, L F. (1973) Geologic history of Utah Brigham Young University thor's Ph D. dissertationat the University ofUtah. Financial support for GeologyStudies, 20, l-181. this study was provided by NSF grantsEAR-8904948 and EAR-9205085 Hitchon, B., and Friedman, I. (1969) Geochemistry and origin of for- to J.R.B. Additional support was provided by the University of Utah mation waters in the westem Canadasedimentary basin. I. Stablelso- Mining and Mineral ResourcesResearch Institute in the form of a re- topes ofhydrogen and oxygen.Geochimica et CosmochimicaActa, 33, searchfellowship to the senior author. t32t-1349 Hover-Granath,V.C., Papike,J.J , and Labotka,T.C. (1983)The Notch RrrunrNcns crrED Peak contact metamorphic aureole,Utah: Petrology ofthe Big Horse Limestone member of the Orr Formation. GeologicalSociety of Amer- Anovitz, L.M., and Essene,E.J (1987) Phase equilibria in the system ica Bulletin.94. 889-906. CaCO.-MgCO.-FeCO.. Journal of Petrology, 28, 389-414. Huyakorn, P S., and Pinder, G.F. (1983) Computational methods in sub- Baker,A A , Calkins,F.C., Crittenden, M.D., and Bromfield,C.S. (1966) surfaceflow, p.39-42. Academic, Orlando, Florida. Geologic map of the Brighton quadrangle,Utah. U.S. GeologrcalSur- Jaeger,J.C. (1964) Thermal effectsofintrusions. Reviews in Geophysics vey Map GQ-535 and SpacePhysics,2, 443-466 Bebout, G E., and Carlson, W.D. (1986) Fluid evolution and transport John, D.A. (1991) Evolution ofhydrothermal fluids in the Alta stock, during metamorphism: Evidence from the Llano Uplift, Texas. Con- Central Wasatch Mountains, Utah. U.S. Geological Survey Bulletin, tnbutions to Mineralogy and Petrology, 92,518-529 1977,5l p. Bowers,J.R., Kerick, D.M., and Furlong, K P (1990) Conduction model Keenan,J.H., Keyes,FG, Hill, P.G, and Moore, J.G. (1978) Steam for the thermal evolution of the Cupsupic aureole, Maine American tables. Wiley, New York. Joumal of Science.290. 644-665. Kemp, W.M., III (1985) A stableisotope and fluid inclusion study of the Bowers,T.S., and Helgeson,H.C. (1983)Calculation of the thermody- contact Al(Fe)-Ca-Mg-Si skarns in the Alta Stock aureole,Alta, Utah, namic and geochemicalconsequences ofnonideal mixing in the system 65 p M S. thesis,University of Utah, Salt Lake City, Utah. H.O-CO,-NaCI on phase relations in geologic systems:Equation of Kestin, J. (1978) Thermal conductivity of water and steam. Mechanical state for H.O-CO.-NaCI fluids at high pressuresand temperatures. Engineering,100, 1255-l 258. Geochimica et Cosmochimica AcIa. 47. 1247-1275. Merzbacher, C., and Eggler, D.H. (1984) A magmatic I,eohydrometer, Bowman, J.R., and Essene,E J. (1982) P-T-X(CO,) conditions ofcontact applications to Mount St. Helens and other dacitic magmas Geology, metamorphism in the Black Butte aureole,Elkhom, Montana. Amer- r2,587-590. ican Journal of Science,282, 3 | l-34O. Meyer, C.A., McClintoch, R B , Silvestri, G.J , and Spencer,R.C. (1967) Bowman, J.R., Willett, S.D., and Cook, S.J.(1994) Oxygenisotopic trans- ASME steam tables: Thermodynamic and transport properties of steam. port and exchangeduring fluid flow: One-dimensionalmodels and ap- American Societyof Mechanical Engineers,New York. plications. American Journal of Science. 294, | - 55. Moore, J.N., and Kerrick, D.M. (1976)Equilibria in siliceousdolomrtes Calkins,FC., and Butler, BS. (1943)Geology and ore depositsof the ofthe Alta aureole,Utah. AmericanJournal ofScience, 276,502-524. Cottonwood-American Fork district, Utah. U S. Geological Survey Nabelek,P I., Labotka,T.C., O'Neil, J.R., and Papike,J.J (1984)Con- ProfessionalPaper, 201, 154 p. trasting fluid/rock interaction between Notch Peak granitic intrusion Carslaw, H S., and Jaeger,J.C (1959) Conduction of heat in solids, p. and argillites and limestones in western Utah: Evidence from stable 510. Clarendon, Oxford, U K isotopesand phaseassemblages. Contributions to Mineralogy and Pe- Cathles, L.M. (1977) An analysisof the cooling of intrusives by ground- trology,86, 25-34. water convection which includes boiling Economic Geology, j2,804- Norton, D., and Knight, J. (1977) Transport phenomenain hydrothermal 826. systems:Cooling plutons. American Journal of Science,277,937-981. COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM 525

Parmentier,E.M., and Schedl,A. (1981) Thermal aureolesof igneous porous medium: Effectsof temperature-and pressure-dependentther- intrusions: Somepossible indications of hydrothermal convectivecool- modynamic and transport properties.Journal of GeophysicalResearch, ing. JournalofGeology, 89, l-22. 82,325-333 Pouchou,J-L, and Pichoir, F (1991)Quantitative analysis ofhomoge- Turcotte,D.L., and Schuben,G. (1982)Geodynamics: Applications of neousor stratified microvolumes applying the model "PAP" In K.F.J. continuum physics to geologicalproblems, p. 168-172. Wiley, New Heinrich and D E. Newberry, Eds , Electron probe quantitation, p 3 I - York. 75. Plenum,New York. Valley, J W. (1986) Stable isotope geochemistryof metamorphic rocks Rice, J.M. (1977) Contact metamorphism of impure dolomitic limestone In Mineralogical Societyof America Reviews in Mineralogy, 16,445- in the Boulder aureole, Montana. Contributions to Mineralogy and 489 Petrology, 59, 237-259 Valley, J.W , and Essene,E J. (l 980) Calc-silicatereactions in Adirondack Rice, J M., and Ferry, J.M. (1982) Buffering, infiltration and control of marbles: The role of fluids and solid solutions. Geological Society of intensive variables during metamorphism. In MineralogrcalSociety of America Bulletin,91, I l4-l 17, 720-815 America Reviews in Mineralogy, 10,263-326 watson, E.B., and Harrison, TM. (1983) Zircon saturationrevisited: Robie,R.A., Hemingway,8.S., and Fisher,J R (1978)Thermodynamic Temperature and composition effects in a variety of crustal magma propertiesofminerals and relatedsubstances at 298.I 5K and I bar ( I 0' types. Earth and PlanetaryScience l-etters, 64,295-304. pascals)pressure and at higher temperatures U.S. Geological Survey Wilson, J.W. (1961) Geology of the Alta stock. Ph D. thesis, California Bulletin, 1452,456p. Institute of Technology,Pasadena, California. Rutherford,M.J., and Devine, J.D. (1988)The May 18, 1980eruption Woodside,W.H, and Messmer,J.H. (1961)Thermal conductivitiesof of Mount St. Helens. III. Stability and chemistry of amphibole in the porous media. II. Consolidatedrocks Journal ofApplied Physics,32, magma chamber. Journal of GeophysicalResearch, 93, 119 49-1 1959 1699-1706 Smith, R K- (1972) The mineralogy and petrology of the contact meta- morphic aureolearound the Alta stock, Utah Ph.D thesis,University of Iowa, Iowa City, Iowa Mnm-rscrrrr REcErwDMercu 29, 1993 Straus,J M., and Schubert,G. (1977\ Thermal convection of water in a MeNuscrrm ACCEFTEDJ.cNUnnv 19, 1994