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

Contact metamorphism surrounding the Alta stock Thermal constraints andevidence of advective heat transport from calcite * dolomite geothermometry

SrnpnrN J. Cooxr* JorrN R. BowtvrlNDepartment 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-TertiaryAlta stock has produced a series of isograds with increasing metamorphic grade: talc,tremolite, forsterite, clinohumite, and periclase. Calcite + dolomite geothermometry ap-plied to 30 samples of these dolomites indicates that the prograde metamorphism occurredover the approximate temperature range 410-575 "C for the tremolite-bearing throughpericlase-bearing zones. The calcite + dolomite temperatures follow a consistent trend ofincreasing temperature in the direction of the intrusive contact. Consistency between thistemperature regime and the positions of the Z-X"o, phase equilibria applicable to theprograde reactions requires a minimum fluid pressure of 75 MPa during the metamorphicevent. Comparisons between the peak temperature-distance profile defined by the calcite* dolomite data and temperature-distance profiles calculated from geologically reasonable,two-dimensional conductive cooling models of the stock suggest that the temperatureregime in the southern portion of the Alta aureole was affected significantly by advectiveheat transfer. Field, petrologic, and whole-rock stable isotopicdata, as well as the resultsof stable isotope transport modeling, are consistent with this possibility and require furtherthat the fluid flow responsible for this advection was lateral to the intrusive contact.

Ix'rnonucrroN

There are now several heuristic thermal modeling stud-ies that have shown that fluid infiltration can affect sig-nificantly the thermal regime surrounding a cooling plu-ton and consequently the spatial and temporal evolutionof its isotherms (e.g., Cathles, 1977; Norton and Knight,1977). This same fluid infiltration can also play an im-portant, and often interrelated, role in controlling theprogress of mixed-volatile reactions and hence the pro-grade T-Xro. path of progressive metamorphism of sili-ceous carbonate-bearing rocks in these aureoles (e.g.,Moore and Kerrick, 1976; Bowman and Essene, 1982;Ferry, 1983, 1989; Nabelek et al., 1984; Bebout and Carl-son, 1986). During progressive metamorphism of sili-ceous carbonate-bearing rocks, fluid infiltration competeswith the buffering capacity of the metamorphic reactionsfor control of pore fluid composition (Rice and Ferry,1982). Yet reaction progress and pore fluid compositionare also influenced by the rate and extent ofthe temper-ature increase experienced by a metamorphic rock. Forcontact metamorphism, this increase is controlled by thethermal budget of the aureole, which is a function of thesupply of heat available (from the size of the intrusion,heat of crystallization, and temperature of the magma),of the geometry of the intrusion, of the thermal properties

* Present address: Environ Corporation, 1980 Post Oak Bou-levard, Suite 2120, Houston, Texas 77056, U.S.A.

0003-{04xl94l0506-o5 I 3$02.00

of the magma and rocks (crystallized pluton and sur-rounding country rocks), and of the primary mechanismofheat transfer (e.g., conduction vs. advection) operatingin the stratigraphic section containing the reactive min-eral assemblages.

Hence any quantitative evaluation of the role either offluid infiltration in influencing the progress of metamor-phic reactions or of advection in the heating of contactaureoles requires independent definition of temperaturein the contact aureole. One method for establishing therequired temperature control in aureoles that contain si-liceous dolomites is calcite + dolomite solvus geother-mometry. Although applications of this geothermometerhave been frustrated by significant retrograde reequili-bration of calcite in regional metamorphic terranes (e.g.,Valley and Essene, 1980; Anovitz and Essene, 1987; Es-sene, 1989), this geothermometer has been successfullyapplied in contact metamorphic environments (e.g., Rice,1977:' Bowman and Essene, 1982:' Hover-Granath et al.,r 983).

The siliceous dolomites in the contact aureole of theAlta stock exhibit a well-developed progressive meta-morphic sequence that is well suited for study using cal-cite + dolomite geothermometry. This study focuses onthe application of calcite + dolomite geothermometry todocument the thermal regime of the metamorphism thatproduced this sequence and to determine whether thereis evidence of significant advectjve heat transport thataccompanied the prograde metamorphism.

5 1 3

5 t4 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 mineralassemblages in the massive dolomites (Moore and Kerrick, 1976). The preintrusive Alta-Gizzly thrust fault system repeats part ofthe Paleozoic carbonate section both north and south ofthe Alta stock. This study focuses on the metamorphism along the south-central margin of the Alta stock (box labeled study area).

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The Alta stock (granodiorite: Wilson, l96l) is one ofseveral mid-Tertiary plutons (38 Ma: Crittenden et al.,1973) in Utah's central Wasatch range (Fig. l). The stockhas intruded and contact metamorphosed a sequence ofPrecambrian and Paleozoic sedimentary rocks consistingprimarily of quartzites (Precambrian Big Cottonwood andCambrian Tintic Formations) and a single pelitic unit(Cambrian Ophir Formation) capped by carbonate rocks(Cambrian Maxfield and Mississippian Fitchville, Des-eret, and Gardison Formations). Within the study area,preintrusive thrusting (Alta-Gizzly thrust zone, Figs. Iand 2) has resulted in the repetition of a portion of thecarbonate section, placing Cambrian Maxfield over theupper Mississippian Deseret-Gardison Formations.

The south contact of the Alta stock is exposed over avertical interval of approximately 550 m; its outcrop pat-tern over this interval indicates that, with small-scale,local exceptions, it is subvertical (Fig. l). An additional500 m of vertical relief on this contact can be inferredfrom exposures in mine workings in the area (Calkins andButler. 1943). The bulk of the subsurface contact is withthe Precambrian section, but the exposed contact is pre-

dominantly with the Cambrian and Mississippian car-bonate rocks (Fig. l). The metamorphism associated withthe stock has affected the country rocks on the south(Maxfield, Deseret, and Gardison Formations, Figs. I and2) up to approximately 2 km from the intrusive contact.

Two additional mid-Tertiary plutons are exposed inthe vicinity ofthe Alta aureole. To the east, the Alta stockhas intruded the older (42 Ma) Clayton Peak stock(granodiorite). Several kilometers to the west is the youn-ger (32 Ma) Little Cottonwood stock (quartz monzonite).Moore and Kerrick (1976) reported overprinting of thewestern margin of the Alta aureole by the contact aureoleof the Little Cottonwood stock.

Coxucr METAMoRPHTSM

This study focuses on the metamorphism along thesouth-central margin of the Alta stock (Fig. l). This por-tion ofthe contact aureole was selected for study becauseof its well-developed prograde metamorphic sequencewithin the dolomitic marbles (Moore and Kerrick, 1976),its simple structure and excellent exposures of the car-bonate units, which allow reliable tracing of individualrock units up metamorphic grade, and a position unaf-

COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM 5 1 5

TABLE 1. Prograde reactions in dolomitic marbles

3 Do + 4Oz + H ,O :Tc + 3Cc + 3CO"2Tc + 3 Cc :T r + Do + H ,O + CO,

5 Do + 8Oz + H rO :T r + 3Cc + 7CO,Tr + 11 Do:8 Fo + 13 Cc + gCO, + H,O

4 Fo + Do + H,O : Chm + Cc + CO,Do: Per + Cc + CO"

fected by metamorphism associated with the Clal.ton Peakstock to the east (Smith, 1972) and the Little Cottonwoodstock to the west (Moore and Kerrick, 1976).

In the study area (Figs. I and 2), prograde metamor-phism of the SiOr-undersaturated siliceous dolomites hasresulted in the successive appearance of talc (Tc), trem-olite (Tr), forsterite (Fo), clinohumite (Chm), and peri-clase (Per) (Moore and Kerrick, 1976). The periclase hasnow been replaced by retrograde brucite (Br). These indexminerals, with in most cases extensive amounts of calcite,are produced by a series of prograde metamorphic reac-tions that are summarized in Table l, Reactions l-6, andwhich have been discussed previously by Moore and Ker-rick (1976). The unmetamorphosed equivalents of themarbles in the aureole consist predominantly of dolomite(Do) with minor quartz (Qz) and calcite (Cc) (both typi-cally < 10 modal0/o). Hence prograde metamorphism hasproduced abundant coexisting calcite and dolomite pairsthroughout most of the thermal aureole. Exceptions arethe innermost periclase zone, where dolomite is virtuallyexhausted, and in the talc zone, where only small amountsof generally fine-grained calcite exist at the margins ofchert nodules.

Ax.lr.yrrc.ll, pRocEDUREs

Sample selection

Ubiquitous textural relationships document the coex-istence of equilibrium calcite + dolomite pairs at allmetamorphic grades. Samples for geothermometry wereselected from unweathered field specimens that lackedobvious evidence of alteration of primary metamorphicminerals. Thin sections of these samples were then stainedwith alizarin red to verify the existence ofcoexisting cal-cite and dolomite. Samples containing suitable calcite +dolomite pairs were then examined optically (400 x mag-nification) for evidence of dolomite exsolution from cal-clte.

Exsolved dolomite was not observed in any of thetremolite zone calcite grains nor in most of the samplesfrom the forsterite zone; however, several samples fromthe highest grade forsterite zone and virtually all periclasezone samples contain individual calcite grains that exhib-it minor exsolved dolomite. For geothermometry mea-surements, the least affected samples from the inner for-sterite and periclase zones were chosen for analysis.

Analytical methods

Calcite analyses were performed on the Cameca SX-50electron microprobe at the University of Utah. A l5-keV

0€o

t90a Sonp le l oco l l on t l l h

tenocrolure ln oC Cc-00

Fig.2. Geologic map of the study area with sample locationsand calcite + dolomite geothermometry results. Peak tempera-tures ('C) recorded by coexisting calcite + dolomite pairs (datafrom Table 2), calculated from the equation ofAnovitz and Es-sene (1987). Metamorphic isograds labeled by index mineral:talc (Tc); tremolite (Tr); forsterite (Fo); and periclase (Per). Geo-logic units are Ct : Tintic; Co : Ophir; Cm : Maxfield; Mf :Fitchville; Mdg : p.t"t"t and Gardison; Tind : intermediatedikes; Qal : alluvium. Alta stock shown by random bar pattem.The preintrusive Altz-Gizzly thrust (labeled) has repeated partof the carbonate section.

beam current, which is required to detect Fe, and a 20-nA sample current, which was chosen to avoid volatil-izalion of Mg, were used to obtain the analyses. Multipleanalyses of single points were performed on many sam-ples by reducing the beam diameter from 30 to 10 rrm inlO-pm steps to confirm the absence of Mg volatilization.For most analyses, a 30-pm beam diameter was used toreintegrate potential submicroscopic-scale exsolution.Various smaller beam sizes were used on the tremolitezone samples because the calcite grains were too small touse a 30-gm beam. Element concentrations were calcu-lated from relative peak intensities using the 6bO algo-rithm ofPouchou and Pichoir (1991).

Analytical results

The compositions of calcite grains coexisting with do-lomite were determined on 30 samples (Table 2). Rep-

0 )(2)(3)(4)(5)(6)

5 1 6 COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM

Tle|-e 3, Calcite analysesTABLE 2. Geothermometrv data

Zone SampleMean

N MgCO. Sdfr

89.24aPer Per Fo Fo88.3 90 4b 88.8 88.33

std,erf.

Max. Irc) rfo) ZoneM9CO. mean max. Sample

Tr89.16b

5705554905504805005705555605655755455304905355505354504904904754855 1 5535530535s404504304 1 0440

Fo

Note: maximum, mean, standard deviation, and standard error of weightpercent MgCO3 in calcite. Number of individual grains analyzed per sampleis given by N-

' Results of f-test comparisons for sample pairs from the same outcrop:A5 vs . A7 , t : 0 . 18 ; C2 vs . C5 , t : 0 . 07 ; D2 vs . D8 , f : 0 90 ; t o1 > 1 .282required to reiect.

'- Sample 88.60 exhibits a bimodal distribution. Values reported for twodistributions.

resentative analyses of calcite are listed in Table 3. Aminimum of 20 calcite grains were analyzed in each sam-ple. The data in Table 2 summarize the results of thesesingle point analyses on individual calcite grains fromeach sample.

Multiple point analyses were performed on a numberof calcite grains in each sample to test their submicro-scopic homogeneity and chemical zonation. The intra-granular compositional variation of the individual grainsexamined was <0.3 wto/o Mg in all samples and was typ-ically <0.2 wt0/0. No systematic chemical zoning was ob-served in any sample. However, because of the relativelylarge beam diameter chosen for our analytical procedure,any chemical zoning that might exist in the outermostpoftions of the calcite grains (s50 pm from grain edge)could not be identified.

Almost all samples exhibit significant grain to grainvariations in Mg content that exceed significantly thevariation in Mg content within individual grains (at leastin grain interiors). Such variations are usually attributedto retrograde equilibration (Rice, 1977; Valley and Es-

CaO 53.13 53.15 52.78MgO 2 50 2.41 2.57FeO 0 03 0.00 0 09MnO 0.00 0.02 0 03co, 44.44 44.35 44.30

Torat 100.10 99 93 99.76CaCO. 93.81 94.04 93.50MgCO" 6.15 5.93 6.33FeCO3 0.04 0.00 0.12MnCO. 0 00 0.03 0.04

Note; oxides reported in weight percent, carbonate end-members in moleoercent normalized to 100.

sene. 1980: Bowman and Essene, 1982; Hover-Granathet al.. 1983: Anovitz and Essene. 1987; Essene, 1989).Retrograde equilibration produces calcite grains with Mgcontents lower than those defined at peak metamorphictemperatures. Averaging all data incorporates such ret-rograde effects and therefore underestimates significantlythe actual peak temperature experienced by a sample.This is a serious problem in slower-cooled regional meta-morphic terranes. At Alta, retrogradation has also clearlyaffected many of the samples from the periclase zone be-cause even maximum temperatures preserved in thesesamples (Table 2) are lower than temperatures defined inthe inner forsterite zone still farther away from the intru-sion (the heat source) and are significantly lower thanminimum temperatures (575 "C) required by phase equi-librium limits for periclase stability (see below). Becausethe signal of interest is peak temperature, we utilize themaximum Mg content (maximum I) recorded in eachsample as the most valid indicator of the peak metamor-phic temperature for the following discussions. Becauseof the potential effects of retrogradation, even the maxi-mum temperatures preserved in a sample must be viewedas minimum estimates of actual peak metamorphic tem-peratures.

Reported temperatures (Table 2) were calculated usingthe calcite + dolomite geothermometry expression ofAnovitz and Essene (1987). The applicable range of thisexpression is 250-800 "C. Uncertainty in these temper-atures due to the calibration of the geothermometry ex-pression and to analytical uncertainty is estimated to be+50 qC for temperatures below 500'C and -r25 "C above500 "C. Temperatures in Table 2 are rounded to the near-est 5 0C.

To evaluate outcrop-scale equilibration, sample pairsfrom three outcrops were tested for consistency betweentheir compositional distributions (Table 2). The meancalcite compositions of the samples were compared sta-tistically using a Student's I distribution. In all three cases,the sample means were found to be not statistically dif-ferent at a confidence level of0.9. These results are con-sistent with the attainment of thermal equilibrium by cal-cite + dolomite pairs on an outcrop scale.

88.3 2488-14 2688.40 2888.51 3088.A5. 2088.A7" 3088.86 2690.2 2590.4b 2688.7 2688.8 2288.16 2888.18 2488.20 2388.33 3388.55 2788.60" 11

3288.C2. 2288.C5. 3488.D2- 2288.D8' 3090.7 2590.13b 2790.18c 3090.19 2590.21 3089.10 2589.16b 2889.20 2689.24a 24

5 48 0.594 99 0.493.28 0.304.29 0.693.49 0.273.47 0.455.08 0.574.60 0.605.54 0.365.62 0.225.10 0.724.80 0.434.23 0.663.16 0.414.08 0.425j2 0.234.50 0.402.54 0.583.17 0.443.16 0.553.05 0.392.94 0.453.75 0.444.23 0.453.02 0.804. ' t5 0.523.57 0.812.89 0.451.54 0.681.39 0.772j6 0.74

012 6.20 5450.10 5.72 5250.06 4.14 4450.13 5.59 4950.06 3.97 4600.08 4.38 4550.11 6.12 530o.12 5.68 5100.08 5.93 5500.04 5.98 5s00.15 6.40 5300.08 5.48 5200.13 s.06 4950.09 4.17 4400.07 5.16 4900.04 5.59 530o12 5.25 5050.10 3.35 3950.09 4.17 4400.09 4.14 4400.08 3.85 4300.08 4.00 4250.08 4.75 4700.09 5.19 4950.1s 5.08 4300.10 5.14 4900.15 5.31 4600.09 3.32 4200.13 3.03 2900.15 2.69 2650.1s 3.23 365

53.34 54.75 54.542.09 ' t .21 1.310.o2 0.05 0.030.0s 0.00 0.01

44.17 44.32 44.2699.65 100.33 100.1594.76 96.95 96 715.16 2.98 3 23o.02 0.07 0.040.05 0.00 0.02

COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM 5t'l

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Thermal constraints on metamorphism

The maximum temperatures calculated for each sam-ple are shown on a map of the study area in Figure 2 andas a function of horizontal distance from the intrusivecontact in Figure 3. The temperatures range from 410 to450 "C for the outer tremolite zone, are 490 'C for theouter forsterite zone, and reach a high of 575 'C in theinner forsterite zone near the periclase isograd (Fig. 3).These temperatures are interpreted to be minimum esti-mates of peak metamorphic temperatures and follow aconsistent trend of increasing temperature in the direc-tion ofthe intrusive contact (Fig. 3).

The maximum calculated temperatures (570-575 'C)

occur on both sides of the periclase isograd in both thepericlase and forsterite zones. Temperatures of 570-575'C for the periclase zone are slightly less than those re-corded for the periclase zone associated with the northernBoulder batholith (590-625 'C: Rice, 1977) and the BlackButte stock (585-600 oC: Bowman and Essene, 1982),both of which formed at depths similar to the Alta stock(Pr: 100-150 MPa).

Several periclase zone samples, particularly those closeto the intrusive contact, yield temperatures considerablylower than the maximum observed temperatures. Severalof these temperatures are < 500 'C, which is well belowthe temperatures required by phase equilibria for the sta-bility of periclase (see below) and below those (600-625'C) typically attained in periclase-bearing zones near ig-neous contacts (Rice, 1977; Bowman and Essene, 1982).These large discrepancies indicate that these periclase-zone calcite grains have undergone significant retrogradereequilibration.

A second-order polynomial curve provides a good fitto the temperature-distance trend defined by the calcite+ dolomite geothermometry (Fig. 3). This curve allowsthe contact temperature (2.) to be estimated. The result-ing value, 626 "C, represents the likely peak temperatureat the igneous contact; this temperature is not a great dealhigher than that at the periclase isograd (570-575 'C).

Constraints on fluid pressure during metamorphism

The thermal regime indicated by the results of calcite+ dolomite geothermometry also places limits on fluidpressures (Pr) during the metamorphic event. Because theresults of calcite + dolomite geothermometry are nearlyindependent ofpressure (Anovitz and Essene, 1987), thefluid pressure regime during the metamorphic event musthave been such that the Z-X.o, locations of the mixed-volatile phase equilibria that define the stability field ofpericlase are consistent with the measured calcite + do-lomite geothermometry results.

Microprobe and whole-rock chemical analyses indicatethat phase equilibria in the CaO-MgO-SiOr-HrO-CO,system, after adjustment for the effects of F, can be rea-sonably applied to the observed mineral assemblages in

a PerL F o. T r

700

600

5@

r("c )lo0

JW

2so' I 250 r 500

Dis tonce . m

Fig. 3. Results of calcite + dolomite geothermometry plottedas a function of sample distance from the stock contact. Thesolid curve represents a second-order polynomial frt to the cal-cite + dolomite geothermometry results, excluding the lower,retrograded temperatures in the periclase zone. This fitted curveis used to represent the peak temperature-distance profile for theaureole in the text. Because the fit is good, extrapolation of thecurve to the igneous contact provides a good estimate ofcontacttemperature QJ : 626 "C.

the Alta contact aureole (Moore and Kerrick, 1976; Cook,1992). The most restrictive equilibrium is Reaction 6,which was responsible for the appearance of periclase.Figure 4 illustrates T-Xco. equilibria governing the for-mation of periclase calculated at a constant lithostaticpressure of 150 MPa and various fluid pressures betweenhydrostatic (approximately 50 MPa) and lithostatic (150MPa) pressure conditions. The lithostatic pressure (P' )estimate, 100-200 MPa, was made by Wilson (1961) us-ing stratigraphic measurements. However, his recon-struction is complicated by possible stratigraphic thin-ning over an ancestral Uinta arch or possible thickeningfrom preintrusive thrusting. Multiple preintrusive thrustshave been mapped by Baker et al. (1966) in both thenorth and south sides of the Alta stock (Fig. l). Mineralassemblages from the Ophir Formation (biotite * anda-lusite + potassium feldspar + quartz + cordierite) in theinnermost aureole also constrain pressures to 150 + 50MPa (Kemp, 1985).

The maximum temperatures recorded by calcite + do-lomite geothermometry at the periclase isograd, whichlies a maximum of 200 m from the intrusive contact, are5'70-575'C (Figs. 2 and 3). These temperatures requirea minimum fluid pressure of approximately' 75 MPa tostabilize the assemblage Per + Cc relative to Dol (Fig.4). As most of the periclase zone samples have been af-fected by retrograde reequilibration, 575 'C must be con-sidered the minimum temperature for the formation ofthis zone. Higher temperatures would permit correspond-ingly higher fluid pressures. If575 "C is close to the actualmaximum temperature achieved at the periclase isograd,

. 1 6-@- -l'Fo Pr, tr

a/tt9)

./

a*-')P)

Pr=150 MPo--- P;l50 MPo

100 MPoI5 Mpo50 MPo

5 1 8 COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM

Xco,

Fig. 4. Z-X.o. phase equilibria governing the formation ofclinohumite- and periclase-bearing mineral assemblages in thepericlase zone al a constanl lithostatic pressure of I 50 MPa andfluid pressures of 50,75, 100, and 150 MPa. For the estimatedlithostatic pressure of 100-200 MPa for the Alta aureole (Wil-son, 1961; Kemp, 1985), these fluid pressures correspond ap-proximately to hydrostatic (50), intermediate (75- 100), and lith-ostatic (150) pressure conditions. Reactions in addition toReactions 5 and 6 listed in Table I are (7) brucite (Br) : periclase(Per) + H.O; (8) dolomite (Do) + H,O : calcite (Cc) + Br +COr. These equilibria restrict the stabiliry of periclase toT-X.o. conditions above invariant point I and Reactions 6 and7. Calcite-dolomite geothermometry defines a minimum tem-perature of 575 'C for the formation of periclase (marked 7"".on the temperature axis). This minimum temperature, combinedwith these phase equilibria, yields a minimum estimate of fluidpressure of about 75 MPa for contact metamorphism in the au-reole. Equilibria were calculated using thermodynamic data fromHelgeson et al. (1978), except for clinohumite, which was ex-tracted from data of Duffy and Greenwood (1979). Unit activi-ties were assumed for all solids except clinohumite (a.n- : 0.35).Values of /.o. and /",o were calculated using the modified Red-lich-Kwong equation of Bowers and Helgeson (1983).

then metamorphism in the periclase zone must have oc-curred at fluid pressures below the estimated lithostaticpressure of 150 MPa.

Evidence of advective heat transfer

Moore and Kerrick (1976) have shown that the pro-grade metamorphic sequence exhibited by the dolomitesrequires infiltration by an HrO-rich fluid with increasingmetamorphic grade. In addition to driving the progradereactions, this fluid flow may have also influenced thethermal regime surrounding the stock. Under the properconditions, advective heat transport can significantly al-ter thermal regimes around cooling plutons (e.g., Cathles,1977; Norton and Knight, 1977;Parmentier and Schedl,

GEOTOGIC MODEL

4 C z V O L C A N I C S

e LATE Pz , Mz , Cz CLAST ICS

3 Pz CARBONATES

5

ALTA4 p € , C Q U A F I T Z T T E S

> I U U F

-8000 6 2000 4000 6000 8000

DISTAI,ICE (n)

Fig. 5. Geologic model of the Alta stock and surroundingunits used in the finite element simulations. Unit 1, Tertiaryvolcanics; Unit 2, l-ate Paleozoic, Mesozoic, and Cenozoic sed-iments; Unit 3, Paleozoic carbonate rocksl Unit 4, Precambrianand Cambrian quartzites and shales; Unit 5, Alta stock. Stockgeometry, size, and location within stratigraphic section con-strained by available geological field evidence. The dashed linedenotes the stratigraphic position ofthe first Alta-Grizzly thrustfault (Fig. 2) within the study area. The bracket along the rightmargin indicates the stratigraphic section of carbonate rockssampled for geothermometry ( 5.4 to -5.6 km).

l98l; Furlong et al., l99l). If significant advective heattransfer occurred in conjunction with the prograde meta-morphism at Alta, then evidence of this advective trans-port should be preserved in the peak thermal profile de-fined by the calcite + dolomite results (Fig. 3), whichrecords the cooling history of the Alta stock as a functionof time and distance from the margin of the stock.

To test the observed thermal profile for evidence ofadvective heat transfer, a conductive cooling model ofthe Alta stock was constmcted using the finite-elementmethod (Cook, 1992). The evolution of the thermal re-gime surrounding the stock is governed by the heat-trans-port equatron:

*g(^,*ur) : t 'p,c,+ (r - i lo,ct{

in which 7 represents the temperature, C, and C" the spe-cific heat capacities ofthe fluid and solid, respectively, p"the solid density, @ the porosity, and If the thermal con-ductivity tensor of the solid-fluid composite

trr: @trf + (l _ d)I;.

The governing partial differential Equation 9 was solvednumerically using the Galerkin finite-element method(Huyakorn and Pinder, 1983) applied over triangular el-ements.

-2000

s=ai -+ooos'{J

{l{J

-6000

9- | p",l--

0 0 40 0 2

(e)

COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM 5 1 9

The geologic model used in the simulations is illus-trated in Figure 5. The model is divided into five distinctrock units, based on the observed stratigraphic and struc-tural relationships surrounding the stock, reported sub-surface geology (Calkins and Butler, 1943; Baker et al.,1966), and late Paleozoic, Mesozoic, and Cenozoic stra-tigraphy from elsewhere in the central Wasatch range(Wilson, 1961; Hintze, 1973).

Units I through 4 constitute the stratigraphic sectioninto which the stock was emplaced. Unit I consists ofvolcanic rocks, roughly equivalent in age to the stock;Unit 2, a package of clastic sediments, predominantly ofMesozoic and Cenozoic age; Unit 3, the Paleozoic sili-ceous dolomites exposed in the contact aureole; and Unit4, the Precambrian and Cambrian quartzites and shalesthat make up the bulk ofsubsurface contact length oftheAlta stock. The section of the model that corresponds tothe stratigraphic section that was sampled in the contactaureole for calcite + dolomite geothermometry is indi-cated by the bracket along the right margin in Figure 5.The dashed line within Unit 3 denotes the location of theAlta-Grizzly thrust zone (Fig. 2).

Unit 5 represents the Alta stock. The mapped exposureof the Alta stock (Fig. 1) has an approximate aspect ratioof 2:l and a half-width of approximately 1.5 km. As thisgeometry more closely approximates a cylinder than aninfinite slab, the axisymmetric form of Equation I wasused in the models, rather than the planar coordinatesthat have been typically used in heuristic model studiesof cooling plutons (Cathles, 1977; Norton and Knight,1977). Outcrop patterns indicate that the southern por-tion of the igneous contact is nearly vertical and has nomajor irregularities. There is no geologic evidence for anoutward-dipping igneous contact nor for the presence ofsignificant amounts of igneous material in the form ofsills or dikes extending any great distance into the aureolebeneath or within the marble section. The emplacementdepth (5 km) is based on the stratigraphic reconstructionby Wilson ( I 96 I ) and is consistent with more recent pet-rologic and fluid inclusion studies of the skarns and meta-pelites in the aureole (Kemp, 1985) and of the Alta stock(John, l99 l ) .

In the model, mean values were used for thermal con-ductivities (I") and specific heat capacities (C") for eachstratigraphic unit (Table 4). Thermal conductivities arebased on compiled values for rocks of analogous com-position (Carslaw and Jaeger, 1959; Woodside and Mess-mer, 1961); heat capacities are based on heat capacitiesfor minerals (Robie et al., 1978) and mineral modes forthe rock units. The values oftr. and C" were also chosenfor the individual units on the basis of their average tem-perature under a conductive steady-state geothermal gra-dient with a basal heat flux of 100 mW/m'zand a meansurface temperature of l0'C. This approach is consistentwith the results of numerical experiments conducted byGiberti et al. (1984), which indicate that mean values canbe used as reasonable approximations for the propertiesof the solids (i.e., heat capacities, thermal diffusivities)

Note: O : porosity (in percent), p" : density (kg/m3), Ii : thermal con-ductivity try(m "C)1, C": specific heat capacity [J/(kg''C)].

. C, for Unit 5 (intrusion) : 1150 J/(kg "C) + SO0 J/(kg'"C) to accountfor latent heat of crystallization.

because they are relatively insensitive to changes in pres-sure and temperature. For the stock (Unit 5), an addi-tional 300 J/(kg''C) has been added to the heat capacityof the rocks to account for the latent heat of crystalliza-tion of the magma.

In contrast to the solid properties, parameter sensitiv-ity studies conducted by Straus and Schubert ( I 977) dem-onstrate that explicit account must be taken ofthe pres-sure-temperature dependence of the properties of HrO,as variations in these parameters affect significantly thethermal regimes in both conductively and advectivelycooled systems.

Accordingly, the model accounts for the changes in thethermal properties of the pore fluids. The calculationsassume that the pore fluid is pure HrO. This neglectspossible effects of a potentially significant CO' compo-nent in the fluid and of salinity on fluid properties. Thisassumption is however unavoidable, as a comprehensiveset ofequations describing the transport properties ofbothhigh-salinity and mixed-volatile fluids has not yet beendeveloped.

Fluid densities for liquid HrO were calculated using theequations of Meyer et al. (1967) for temperatures belowthe critical temperature and those of Keenan et al. (1978)elsewhere. Specific heat capacities were computed usingthe expressions ofKeenan et al. (1978) and thermal con-ductivities from the formulation of Kestin (1978).

Initial and boundary conditions are required to obtainspecific solutions to the governing differential equation.These boundary conditions are

A TI , , - : 0 mWm' : f o r - 8 km '< z < 0 km a t x

dx : 0 km and x : 8 km

^,.# : loo mw/m'z

Z : l 0 " C

TABLE 4. Thermal parameters

Unit 6 p,

'I

234c

10.010.05.0

0 5

27002700270027002700

2.O2.22 03.01 . 8

9501 0001 0501 1001 450.

f o r 0 k m < x = 8 k m a t z :-8 km

f o r 0 k m < r c < S k m a t z :0 k m

corresponding to insulating conditions along the verticalsubsurface boundaries; a constant conductive heat flux of100 mWm'zapplied along the subsurface boundary, rep-resenting an elevated regional preintrusive heat flow,

520 COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM

6m

500T / O ^ \

@

300

m

E

=aF-

sl{J

!UJ

-2000

-4000

-"""0 2000 4000 6000 8000DISTANCE (n)

Fig. 6. Conductive cooling model at 5000 yr following em-placemenl of the Alta stock. The emplacement temperature ofthe stock was 825'C. The bracket along the right margin denotesthe stratigraphic section corresponding to the exposed contactaureoie (-5.1 to -5.6 km). The carbonate section sampled forgeothermometry corresponds to the deeper portion of this sec-tion, the interval -5.4 to -5.6 km.

which is consistent with a region experiencing periodicintrusive eventsl and a constant temperature of l0 .C atthe surface boundary, representing a rnean annual tem-perature. The assumption ofconstant heat flux across theentire base of the model leads to some overestimate ofwall-rock temperatures near the igneous contact after in-truslon.

The model treats the stock as if it were injected instant-ly at 825'C into hydrostatically pressured, HrO-saturatedcountry rocks under a steady-state conductive geothermalgradient. Although instantaneous injection is geologicallyunrealistic, this approach is considered acceptable, as up-ward migration rates of thermal maxima are significantlyless than the rates of igneous intrusion (Norton andKnight, 1977).

The emplacement temperature estimated for the stock(825 "C) is based on the results of zirconium geother-mometry (Watson and Harrison, 1983) applied to whole-rock analyses of the Alta stock (maximum whole-rockzirconium content 220 ppm) and is consistent with itsgranodiorite bulk composition (Cook, 1992). This geo-thermometer assumes that l00o/o of the whole-rock zir-conium was initially present in the liquid portion of themagma. Thus this approach yields a maximum temper-ature estimate for the stock because of the possibility ofinherited zircon from the source region. In addition, John(1991) has applied empirical biotite crystallization geo-thermometry to biotite compositions from the Alta stock.For all samples from buffered assemblages, biotite crys-tallization temperatures ranged from 650 to 840.C, with

D i s t o n c e , m

Fig.7. Comparison between geothermometry results (Fig. 3)and peak temperature profiles (curves A and B), correspondingto the model illustrated in Fig. 6. Curves A and B define maxi-mum model temperatures achieved along a horizontal profile at- 5.5 km during 100 000 yr of cooling for initial intrusion tem-peratures of 825 (curve A) and 925 'C (curve B). Maximumtemperature at the igneous contact corresponds to those (825and925 'C) obtained instantaneously upon intrusion. The 5.5-km level corresponds to the average depth of the stratigraphicinterval sampled for geothermometry in the contact aureole. Alsoshown is the second-order polynomial fit to the geothermometrydata (solid line) and a stippled band centered on this fitted curvebased on estimated uncertainties in the geothermometry resultsof +25'C above 500 "C and +50'C below 500 "C. Bracketsdenote temperature limits at the positions of the periclase (Per)and tremolite (Tr) isograds based on geothermometry results andphase equilibria (Cook, 1992; this study).

most results in the range 700-800 "C. These results areconsistent with the emplacement temperature estimatebased on zirconium geothermometry.

The effect of the endothermic metamorphic reactionsthat occurred in the marbles and underlying pelitic rocks(Figs. I and 2) on the thermal evolution of the aureolehas not been incorporated into the model. Ferry (1980)has noted that metamorphic reactions can, depending onbulk composition and mineral assemblage, consume asignificant quantity ofheat. The effect ofthese endother-mic reactions is to lower temperature throughout the au-reole. Numerical experiments by Bowers et al. (1990) sug-gested that temperatures may be overestimated by asmuch as 40-50 "C if the effect of endothermic reactionsis ignored. Thus the calculated temperature regimes pre-sented in this study should be regarded as maximum es-tlmates.

Figure 6 illustrates the results of the cooling model at5000 yr following emplacement of the stock. A peak tem-perature-distance profile, corresponding to this model overa span of 100000 yr, is shown as curve A in Figure 7.This model profile was taken at a depth of -5.5 km,which corresponds to the average depth of the strati-graphic section sampled for geothermometry in the con-tact aureole (i.e., data ofFig. 3).

COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM 521

A comparison in Figure 7 between the model profile(curve A) and the observed peak temperature-distanceprofile based on a second-order polynomial fit to the cal-cite + dolomite data from Figure 3 indicates that thisconductive model fails to achieve the observed peak tem-peratures at virtually all distances; observed temperaturesare significantly greater than model temperatures (typi-cally > 100 'C beyond the first 100 m from the igneouscontact) at all distances. This is a result ofthe rapid cool-ing of the upper portion of the stock that occurs withinthe first few hundred years following its emplacement. Inthe model, the minimum temperature required by geo-thermometry and phase equilibria for the formation ofpericlase (575 'C) was achieved only within l5 m of theintrusive contact, a distance much less than the observedmaximum distance of 200 m (bracket labeled Per). Thediscrepancies between model results and the geother-mometry data exceed the uncertainties in the geother-mometry data (stippled band centered on the regressioncurve in Fig. 7).

One possible explanation for these discrepancies is thatthe actual emplacement temperature of the stock wasgreater than the temperature estimated by zirconium geo-thermometry (825 'C). Curve B in Figure 7 represents ananalogous peak temperature profile for a conductive modelin which the emplacement temperature was set a|925 "C.The increased intrusion temperature has relatively littleeffect on the model thermal profile, and there remain largediscrepancies between observed and model temperaturesthroughout the aureole. The predicted width ofthe peri-clase zone does increase slightly to approximately 25 m.This modest improvement implies that to achieve theobserved width of the periclase zote (T > 575 'C up to200 m from the intrusive contact) using strictly conduc-tive heat transfer would require a model with an initialintrusion temperature well in excess of 925 "C. Such aninitial intrusion temperature is inconsistent with theavailable geothermometry results from both the marblesand the stock and probably exceeds the maximum pos-sible liquidus temperature for a granodiorite magma aswell. The presence of stable, euhedral hornblende in thestock (Wilson, 196 1) at the low pressures indicated forthe Alta aureole (<200 MPa) would require a HrO con-tent in the magma in excess of 5 wto/o (Merzbacher andEggler, 1984; Rutherford and Devine, 1988). This impliesthat the magma was at, or very near, HrO saturation whenit was intruded. The maximum possible temperature forintrusion of the Alta stock then corresponds to the HrO-saturated liquidus temperature for a granodiorite magmaat low pressures (<150 MPa), which is approximately960 "C (Rutherford and Devine, 1988). Such a high in-trusion temperature directly contradicts the geother-mometry evidence from both the stock and the aureole.

If an initially hotter stock cannot account for the ob-served discrepancy, then an alternative mechanism to el-evate the model thermal profile is an increase in thepreintrusive wall-rock temperature (f.). At a minimum,?"^ would have to be increased to >375-400'C to elim-

inate the discrepancies between model results and thelower limit of uncertainty in the geothermometry data(Fig. 7) throughout the aureole, not just in the outer au-reole where the effects of increasing ?|" are the greatest.

However, there are geologic, petrologic, and thermalfactors that constrain I" to much less than 375 "C. Thepreintrusive geothermal gradient of 5 5 'Clkm selected forthe model and the resulting T-": 275'C at the depth ofthe sampled section of the contact aureole (-5.2 to -5.5

km) are already significantly elevated values comparedwith those (30 "C/km and 7".: 165 "C) appropriate forthe regional tectonic setting of the Wasatch range and forthe estimated depth of emplacement (3-6 km) of the Altastock (Wilson, l96l; Cook, 1992). These elevated valueswere selected to accommodate the possible thermal ef-fects ofprevious intrusive activity in the area (Crittendenet a l . , 1973).

Initial wall-rock temperatures of = 375-400 "C are alsoinconsistent with the absence of the regional develop-ment of either talc or tremolite in these siliceous dolo-mites. At the very HrO-rich pore fluid compositions(X.o, << 0.03) characteristic of deep ground waters insedimentary sequences prior to the onset of metamorphicreactions (for example, Hitchon and Friedman, 1969),thermodynamic calculations show that the first appear-ance of talc would be below 325 "C. If the actual initialtemperatures at Alta were in the vicinity of 375-400 'C

required to eliminate most of the discrepancy betweenmodel results and the lower limits to geothermometry,then there should be widespread evidence of regional low-grade metamorphism (ubiquitous presence of talc andeven tremolite) in the siliceous dolomites stratigraphical-ly equivalent to those in the aureole. No such regionalmetamorphism is observed in this area of the Wasatchrange, nor has it been reported in the literature; on thecontrary, the spatial distribution of both tremolite andtalc in these protoliths is clearly related to the Alta, LittleCottonwood, and Clayton Peak stocks and therefore iscontact melamorphic in origin.

Finally, if the thermal profile recorded in the Alta au-reole is ascribed solely to conductive heat transport fromthe Alta stock, then basic heat flow-balance considera-tions for conduction require specific relationships amongintrusion (f-), contact (f), and initial wall-rock (7".)temperatures. These basic considerations (Turcotte andSchubert, 1982, p. 168-172) can be summarized by thefollowing generalization:

T.: x(T^ - T") + T" (10)

where the specific value of the coefficient x depends onthe thermal properties of the intrusion and wall rocks,the amount of latent heat of crystallization assigned tothe intrusion, and the initial difference in temperaturebetween magma and wall rock. For many common rockand magma types intruded into the shallow crust, valuesof x are close to 0.6 (Turcotte and Schubert, 1982, p.172).The value of the coefficient x increases with an in-creasing difference between Z- and Z" and with increas-

s22 COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM

ing latent heat of crystallization. For the specific situationat Alta, x > 0.61 even for the geologically unrealisticsituation of f. : 0 "C. For the likely minimum possibleT"at Alta, 100'C (based on a minimum estimated depthof emplacement: 3 km and an unperturbed geothermalgradient of 30 'Clkm), x : 0.623. Equation l0 is rou-tinely invoked to make estimates of 2., on the basis ofknown or assumed values of Z- and 7"". However, if both?"- and Z. are known from applications of geothermom-etry, then these same heat balance relationships establishrather firm upper limits to Z" for any model of the ther-mal evolution of an aureole that invokes strictly conduc-tive heat transport.

The second-order polynomial fit to the geothermom-etry data in the Alta aureole defines T,: 626'C (Fig. 3).With f. : 626 "C, T^: 825'C on the basis of the resultsof zirconium geothermometry, and x : 0.61, Equationl0 yields an upper limit to T-": 325'C (for the morerealistic value of x : 0.623, the maximum 7". would be300'C). This upper limit to I" is only somewhat higherthan that (7": 275'C) used in the previous model cal-culations (curves A and B: Fig. 7). Use of this upper limitto T" (325 'C) would yield model temperatures for theouter aureole that are still 60'C below the measured ther-mal profile (but close to the lower limits of uncertainty).Such an increase is also inadequate to remove significantdiscrepancies (up to 75 "C) between model results andmeasured temperatures throughout much of the rest ofthe contact aureole. Thus these geologic, petrologic, andthermal considerations suggest that a geologically realisticincrease in Z. alone is a very unlikely mechanism bywhich to resolve the large discrepancy between the con-ductive model results and geothermometry data.

If the large discrepancy between the thermal profilespredicted by conductive cooling models for the stock andthe observed thermal profile defined by geothermometryand phase equilibria cannot be resolved by geologicallyreasonable increases in intrusion and wall-rock temper-atures alone, then these discrepancies must be due to oneor more of the following possibilities: (1) increase in theheight of the stock, (2) thermal effects of magma convec-tion, (3) hidden intrusions or dipping igneous contact be-neath the aureole, (4) advective heat effects.

lncrease in the vertical extent of the stock. One possi-bility is that the top of the stock is significantly higherthan the top of the stock in the thermal model. The two-dimensional results indicate that the largest increases inwall-rock temperatures occur in the region below themidpoint of the stock's vertical margin (Fig. 6). If thestock had had a greater vertical extent in the model, thenthe carbonate stratigraphic section exposed in the contactaureole would have been located lower relative to themidpoint of the stock's vertical margin and would havebeen placed in a higher temperature regime (Fig. 6). Asconstructed, the model already has a postulated 0.3 kmofadditional vertical extent beyond the present erosionalexposure. The top of the stock would still have to beraised by >> I km to account for the discrepancy of > 100

"C in temperature between observed and model thermalprofiles. The occurrence of several roof pendants withinthe stock (Fig. l) suggests that the present erosional ex-posure lies near the original upper margin of the stock.However, a significantly greater vertical extent than thatalready included in the model cannot be completelyruled out.

Thermal effects of magma convection. Magma convec-tion can affect maximum temperatures in a contact au-reole (Jaeger, 1964). In attempting to simulate vigorousmagma convection, Bowers et al. (1990) found that theeffects of magma convection on maximum temperature-distance profiles normal to the igneous contacts of planaror regular geometry (analogous to the south contact ofthe Alta stock) were minor, except very near the igneouscontact. However, they did find that the effects are moresignificant locally near geometric irregularities in an ig-neous contact (reentrants and apophyses). Some magmaconvection may have occurred in the Alta stock, on thebasis of local occurrences of flow banding. We cannotevaluate quantitatively the effect of magma convectionbecause of the lack of information with which to defineactual convection parameters appropriate for the Altastock. On the basis of the numerical experiments of Bow-ers et al. (1990), it is likely that the effects of magmaconvection on the temperature-distance profiles for thesouth Alta aureole would be relatively small, given theregular, vertical geometry of the south contact (Figs. Iand 2).

Hidden intrusions. The occurrence of hidden intrusivemasses or dipping igneous contacts beneath wall rockscan exert significant effects on temperature. There is nogeological evidence in either outcrop or mine workingsfor intrusions underlying or near the south part of theAlta aureole in the areas sampled for geothermometry.Outcrop patterns indicate that for the level of the aureoleexposed, the contact of the Alta stock is nearly verticaland regular. In addition, isograds parallel this igneouscontact (Moore and Kerrick, 1976;-Fig. I of this paper)and show no evidence of perturbation by hidden intru-sions. As there is no geological evidence for hidden in-trusions, this possibility is speculation and is not a test-able hypothesis.

Advective heat effects. Advective heat transport mayalso increase temperatures in the aureole (Cathles, 1977;Norton and Knight, 1977; Parmentier and Schedl, l98l;Furlong et al., l99l). The possibility that advective heattransfer contributed significantly to the thermal evolutionofthis section ofthe aureole is particularly attractive be-cause ofindependent field, petrologic, and stable isotopicevidence for the occurrence of infiltration-driven meta-morphism.

Geologic, petrologic, and isotopic eyidence for fluid flow

Consider the formation of periclase by the reaction

Do: Per * Cc * CO,. (6)

In a closed system, a prograde reaction can only advance

as long as temperature rises, and, correspondingly, a pro-grade reaction can only buffer the pore fluid compositionto higher X.o, values as long as temperature continues torise. Once a rock reaches a maximum temperature, noadditional reaction can occur. At the pressure conditionsestimated for the Alta aureole (P.: 75-100 MPa), verylittle periclase can be produced under closed system con-ditions unless temperatures exceed the steep-sloped por-tion of the T-X.n. equilibrium surface for Reaction 6, orabove approximately 700 'C. Thus the formation of asignificant quantity of periclase without infiltration by aHrO-rich fluid requires temperatures well in excess of700 'C. Such high temperatures near the igneous contactare precluded by the conductive cooling models of thestock, on the basis of the estimated intrusion temperatureof the magma (approximately 825 "C) and the results ofcalcite + dolomite geothermometry.

Yet many of the marbles contain abundant periclaseand are nearly devoid of dolomite, indicating that Re-action 6 has gone nearly to completion in these samples.This discrepancy between observed reaction progress andthe limited abundance of periclase predicted for closed-system conditions requires infiltration of a significantvolume of H,O-rich fluid (Cook, 1992) to permit the for-mation of periclase at more reasonable temperatures(-600 'C) by displacing and diluting CO' generated bythe reaction.

The spatial distribution of periclase within the peri-clase zone indicates that this infiltration was largely lat-eral to the intrusive contact. Within the periclase zonethe original bedding in the marbles is almost horizontal(0-10"W dip) when corrected for postintrusion rotationon the Wasatch fault and is nearly perpendicular to theintrusive contact. Throughout the zone, periclase-bearingbeds alternate with periclase-absent beds. This pattern isinconsistent with vertical flow through this zone, requir-ing that fluid flow was bedding-controlled and largely lat-eral to the vertical intrusive contact. A lateral flow pat-tern is further supported by the metasomatic introductionof bedding-concordant ludwigite and thin garnet-pyrox-ene skarn layers within individual periclase-bearing beds.

Lateral infiltration by a significant amount of '8O-de-

pleted fluid is also required by stable isotope data. Muchof the periclase zone is significantly depleted in '8O (upto l6Vm: Cook, 1992). These depletions are far too largeto result from isochemical decarbonation alone (Valley,1986) and require the infiltration ofa significant quantityof '8O-depleted fluid. Within the '8O-depleted periclasezone there are also significant bed to bed variations in6'80 values (Cook, 1992). As with the distribution ofpericlase, this pattern of'8O depletion is inconsistent withvertical flow in the periclase zone and requires that fluidflow was bedding-controlled and largely lateral to a nearlyvertical igneous contact. The most '8O-depleted calciteshave d'8O values (8-107m) equivalent to those in ex-change equilibrium with the Alta stock (8-9V*), implyingthat the fluids equilibrated isotopically with the adjacentAlta stock prior to infiltrating the periclase zone.

523

Application of one-dimensional isotope transportmodels indicate that the observed pattern of '8O deple-tion in the south aureole (Cook, 1992) is inconsistentwith fluid flow toward the igneous contact (i.e., in thedirection of increasing temperature) (Bowman et al.,1994). Rather, transport modeling demonstrates that theobserved isotopic patterns are consistent with the flow ofsignificant amounts of '8O-depleted fluid laterally awayfrom the igneous contact (i.e., in the direction ofdecreas-ing temperature) (Bowman et al., 1994), a conclusionconsistent with the geothermometry data and thermalmodeling results presented here. Application of one-di-mensional flow models is justified by the field, petrologic,and isotopic evidence indicating one-dimensional (i.e.,bedding-controlled) flow in the marbles.

Numerical models (two-D, axisymmetric) of advectiveheat transport require cumulative fluid flux approximat-ing 103 to 3 x 103 m3lm'? and time scales of approxi-mately 5 x 103 yr to match the observed calcite + do-lomite thermometry profile and to reproduce the observedwidth of the periclase zone (Cook, 1992). One-dimen-sional models of O isotope transporl require a minimumcumulative fluid flux of approximately 8 x 102 m3/m2 toreproduce the observed patterns of'8O depletion in thedolomitic marbles of the south aureole (Bowman et al.,1994). This flux estimate is a minimum one because inan axisymmetric aureole such as the Alta aureole flow-lines are radial, not parallel. The general consistency ofthese flux estimates lends credence to the possibility thatfluid flow has systematically affected the thermal regimeofthe southern part ofAlta aureole at the level ofcurrentexposufe.

Sulrprlnv

The main points of this study can be summarized asfollows:

l. Calcite + dolomite geothermometry applied to si-liceous dolomites in the contact aureole of the Alta stockconstrains the temperature of metamorphism to the range410-575 "C. These temperatures are compatible withtemperature limits from phase equilibria for the observedmineral assemblages.

2. The minimum temperature recorded by calcite *dolomite geothermometry for the formation of periclase(575 'C) requires a minimum fluid pressure of approxi-mately 75 MPa during the metamorphic event. However,if 575 'C is close to the actual peak temperature achievedat the periclase isograd, the fluid pressures during theevent must have been considerably less than the esti-mated lithostatic pressure of | 50 MPa.

3. The temperature-distance profile recorded by cal-cite + dolomite pairs for the exposed level of the southaureole is significantly hotter at all distances than maxi-mum temperature profiles computed from geologicallyreasonable, conductive cooling models of the Alta stockthat utilize initial magma and wall-rock temperaturesconsistent with available petrologic and geologic evi-dence.

COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM

524

4. This significant and systematic discrepancythroughout the aureole suggests one or more possibilities:(1) this section of the aureole formed at a much greaterdepth relative to the top of the intrusion than estimated;(2) magma convection significantly affected the thermalprofile; or (3) the observed thermal profile was affectedby advective heat transfer.

5. The possibility ofadvective heat transport is partic-ularly attractive, with the field observations, petrologicalconstraints on the formation ofpericlase, spatial patternsof D'8O values (Cook,1992), and isotope transport mod-eling results (Bowman eI al., 1994), which independentlyindicate extensive, bedding-controlled fluid infiltrationlateral to the intrusive contact during prograde meta-morphism. The similarity of flux estimates from inde-pendent heat transfer and mass (O isotope) transportmodels (Cook, 1992; Bowman et al., 1994) required toreproduce the observed thermal and isotopic character-istics of the marbles lends credence to the possibility thatfluid flow has systematically affected the thermal regimeofthe southern part ofAlta aureole at the level ofcurrentexposure.

Acrxowr,nncMENTs

We thank Lukas Baumgartner and an anonymous reviewer for theirconstructive reviews. This study represents a portion of the senior au-thor's Ph D. dissertation at the University ofUtah. Financial support forthis study was provided by NSF grants EAR-8904948 and EAR-9205085to J.R.B. Additional support was provided by the University of UtahMining and Mineral Resources Research Institute in the form of a re-search fellowship to the senior author.

RrrunrNcns crrEDAnovitz, L.M., and Essene, E.J (1987) Phase equilibria in the system

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Geologic map of the Brighton quadrangle, Utah. U.S. Geologrcal Sur-vey Map GQ-535

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Bowman, J.R., and Essene, E J. (1982) P-T-X(CO,) conditions ofcontactmetamorphism in the Black Butte aureole, Elkhom, Montana. Amer-ican Journal of Science, 282, 3 | l-34O.

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COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM

Cook, S.J. (1992) Contact metamorphism surrounding the Alta stock,Little Cottonwood Canyon, Utah. Ph.D. thesis, University of Utah,Salt Lake City, Utah.

Cnttenden, M D. (1965) Geology of the Dromedary Peak quadrangle,Utah. U S. Geological Survey Map GQ-378.

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Mnm-rscrrrr REcErwD Mercu 29, 1993MeNuscrrm ACCEFTED J.cNUnnv 19, 1994

COOK AND BOWMAN: ALTA STOCK CONTACT METAMORPHISM