The Notch Peak Contact Metamorphic Aureole, Utah: Petrology of the Big Horse Limestone Member of the Orr Formation

V. C. HOVER GRANATH Conoco, inc., Exploration Research Division, Ponca City, Oklahoma 74603 J. J. PAPIK.E Institute for the Study of Mineral Deposits, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701 T. C. LABOTK.A Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37916

ABSTRACT INTRODUCTION

The Upper Cambrian Big Horse Limestone Member of the Orr This study is part of a project examining the extent of ther- Formation is contact metamorphosed by the Jurassic Notch Peak mally induced element migration during the contact metamorphism quartz monzonite in the central portion of the House Range, Utah. of Cambrian argillaceous limestones intruded by the Jurassic Notch Two lithologic types were sampled from specific stratigraphic Peak pluton in west-central Utah. It focuses on a suite of samples horizons within the member over a lateral distance of 6 km, at collected from the Big Horse Limestone Member of the Orr Forma- locations reflecting a range in metamorphic grades. The rocks were tion which can be traced along strike for a distance of 6 km from the metamorphosed at ~2 kbar, on the basis of estimates of strati- pluton. The mineralogy and petrology of these samples have been graphic overburden at the time of intrusion. The lithologies are rela- studied in order to determine the temperature gradient and fluid- tively pure dolomitic limestones and impure argillaceous limestones phase compositions during metamorphism. (argillites). The progressive metamorphism has resulted in the suc- The Notch Peak pluton discordantly intrudes Middle and cessive appearance of talc, tremolite, scapolite, diopside, and for- Upper Cambrian limestones in the central portion of the House sterite in the meta-limestones and the appearance of biotite, Range (Fig. 1). The Cambrian stratigraphy of the region is summa- tremolite, diopside, plagioclase, scapolite, vesuvianite, grossular, rized by Palmer (1971) and Hintze and Palmer (1976). The stratig- and wollastonite in the argillites. Mineral assemblages of the lime- raphy of the Big Horse Limestone Member (Fig. 2) is well stones compared with isobaric 2 kbar-phase equilibria in the documented by Lohmann (1977). In general, the member consists Ca0-Mg0-Si02-H20-C02 system suggest that: (1) fluid buffering of six upward-shallowing asymmetric cycles deposited at the Cam- by metamorphic reactions has occurred; (2) domains of equilibrium brian platform edge. The base of each cycle consists of mudstones are small; and (3) the limestones behaved as relatively closed sys- and wackestones interbedded with terrigenous quartz siltstones. tems during metamorphism. Maximum temperatures and X(C02) The proportion of carbonate increases upward in these rocks, and composition consistent with assemblages in the limestone are: each cycle is capped by algal boundstones or oolitic grainstones containing minor terrigenous material. The boundary between the Grade T °C Max X(C02) top of one cycle and the base of the overlying cycle is sharp. Indi- vidual cycles can be traced over a distance of 6 km from Little low 450-475 0.50-0.60 Horse Canyon directly into contact with the pluton along Contact medium 475-575 0.75 Canyon (Fig. 1). At the western end of Contact Canyon, the pluton high 575-600 0.80 discordantly intersects the top of the section. Eastward along the Temperatures estimated by the calcite-dolomite geothermometer canyon, it intersects progressively lower horizons until at the junc- are consistent with these temperature estimates. tion of Contact and Side Canyons (Fig. 1) the pluton dips shallowly Temperatures estimated from limestone mineral assemblages beneath the base of the member. Consequently, samples collected are used to establish the sequence of prograde reactions in the from northeast to southwest around the margin of the pluton reflect multi-component argillite system. Compositions of fluids in equilib- successively higher grades of metamorphism. rium with low-grade argillites are poorly constrained but may Gehman (1958) presented a preliminary map of the Notch

have reached a maximum X(C02) of ~ 0.75. Fluid compositions in Peak pluton and described the mineralogy of the metamorphosed equilibrium with medium- to high-grade argillites were more H20- Marjum and Weeks Formations (Fig. 1). Stratigraphic data do not rich than X(C02) = 0.20 indicated primarily by the presence of constrain the time of intrusion, but two radiometric ages have been wollastonite at temperatures between 475 and 600 °C. Therefore, reported: 193 m.y. (Triassic) Pb-a date (zircon) (Whelan, 1970); and the argillites were either initially more water-rich than limestones at 143 m.y. (Jurassic) K-Ar date (biotite) (Armstrong and Suppe,

the same grade, or they were more open to H20-rich fluids during 1973). This younger age may associate the pluton with the Sevier metamorphism. Orogeny (Hintze, 1973), which did not strongly deform the Cam-

Geological Society of America Bulletin, v. 94, p. 889-906, 8 figs., 3 tables, July 1983.

889

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 890 HOVER GRANATH AND OTHERS

SAMPLE STATIONS IN THE BIG HORSE LIMESTONE MEMBER

113025' 113°20'

39°10

Figure 1. Geologic map of a portion of the Notch Peak 15' Quadrangle (after Hintze, 1974) showing the locations of sampling stations in the Big Horse Limestone Member. Each station represents several samples collected from several stratigraphie horizons within the member. The approximate location of samples in the section is shown in Figure 2. Stations shown as connected circles represent locations where a vertical sampling traverse was made through several stratigraphie horizons.

brian strata in the House Range. The present relief is the result of thereby establishing the thermal gradient during metamorphism late Tertiary block faulting. The regional dip of the sedimentary and (2) to compare mineral assemblages resulting from differences rocks is ~ 10° to the southwest except in the immediate vicinity of in initial bulk composition between the two contrasting lithologies. the pluton. where the sediments are gently domed. To this end, the mineralogy and bulk compositions of each litho- The pressure during the metamorphism of the Big Horse logic type will be described. The sequence of mineral assemblages Limestone Member can be estimated from the amount of strati- will then be compared with experimentally determined phase equi- graphic overburden at the time of intrusion of the Notch Peak libria in chemical systems appropriate for each lithology in order to pluton. The thickness of Paleozoic and Triassic strata overlying the constrain the physical parameters of metamorphism. member at: the time of intrusion ranges from 6.2 to 6.5 km. This estimate is based on the stratigraphic section through the Silurian METHODS from Hintze (1974) and isopach maps for Devonian through Trias- sic strata from L. F. Hintze (1973, and 1980, personal commun.). The Big Horse Limestone Member is well exposed along ~ 300- The thickness of overburden corresponds to a pressure of 1.6 to 1.9 m-high cliffs around the margin of the Notch Peak pluton. The kbar during intrusion. This estimate is consistent with water pres- locations of sampling stations are shown in Figure 1. At each sta- sures calculated from compositions of biotites in the pluton (P. I. tion, the oolitic cycle 2 limestone and the cycle 1 argillite and the Nabelek, 1980, personal commun.). For the purposes of this study, cycle 2 argillite occurring on either side were sampled (Fig. 2). To a total pressure of 2 kbar will be assumed. assess vertical variations, additional limestone and argillite samples Two lithologies were sampled in the Big Horse Limestone from lower stratigraphic horizons were collected at several stations. Member: relatively pure limestones, some of which are now mar- The rocks were examined in thin section; identification of bles, and impure argillites, some of which have been metamor- some phases was aided by X-ray diffractometry. Selected samples phosed to calc-silicate rocks. The primary goals of this study are: were analyzed with an ARL-EMX-SM electron microprobe, using (1) to establish the lateral variation in mineral assemblages resulting simple silicate and carbonate standards. Data were reduced on-line from the prograde contact metamorphism within each lithology, by the method of Bence and Albee (1968) with the correction fac-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 NOTCH PEAK. CONTACT METAMORPHIC AUREOLE, UTAH 891

Figure 2. (Left) Schematic stratigraphic column giving approximate thicknesses of Upper Cambrian strata (after Hintze, 1974). Patterns for each interval are the same as in Figure 1. Strata overlying the Big Horse Limestone Member are not shown in Figure 1. (Right). Enlargement of the Big Horse Limestone Member of the Orr Formation (after Lohmann, 1977), showing the approximate location of samples collected at stations shown in Figure 1. The following samples were collected at each station: (1) a quartz-rich silt horizon near the base of the cycle 1 argillites (~2m above the top of cycle 2); (2) an oolitic carbonate from the top of the cycle 2 limestones; and (3) a quartz-rich silt horizon near the base of the cycle 2 argillites (14 to 20 m below the top of cycle 2).

tors of Albee and Ray (1970). Complete mineral analyses and ana- "medium grade"); and (4) are marked by the appearance of forster- lytical procedures are given in Hover (1981). Mineral compositions ite (termed "high grade"). Isograds in Figure 3 refer to these subdi- described in this paper are based on those data. visions and are based on field and petrographic observations. Most mineral and bulk compositions of these limestone sam- MINERAL ASSEMBLAGES IN THE LIMESTONES ples can be adequately represented by the Ca0-Mg0-SiC>2-H20- CO2 system. If H2O and CO2 behaved as boundary value Mineral assemblages in the analyzed limestone samples are components whose chemical potentials are independent variables given in Table 1 and their distribution in Figure 3. The progressive externally controlled during metamorphism (Zen, 1963), then min- contact metamorphism of the limestone has resulted in the succes- eral and bulk compositions may be projected from H2O and CO2 sive appearance of talc, tremolite, scapolite, diopside, and forster- onto the CaO-MgO-SiC>2 plane of this system (Fig. 4). At constant ite. The limestone samples have been divided into four categories on PT (= Pf), divariant assemblages would then be characterized by the the basis of the following criteria: those that (1) retain original presence of three phases in this five-component system. In the low-, sedimentary and diagenetic textures and mineralogy (termed medium-, and high-grade subdivisions above, four or more phases "unmetamorphosed"); (2) contain relict sedimentary textures and commonly occur in the assemblages. This suggests that the mineral are marked by the appearance of talc + tremolite (+ scapolite) assemblages are related to univariant reactions or invariant points (termed "low grade"); (3) have recrystallized to a hornfelsic texture, in this system; in which case, temperature and the chemical poten- do not contain talc, but contain tremolite or diopside (termed tials of H2O and CO2 are not independent variables.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 892 HOVER GRANATH AND OTHERS

TABLE 1. MINERAL ASSEMBLAGES IN LIMESTONE SAMPLES

Cycle* Cc Do Qtz Tc Tr Di Fo Phi Cte Plag Ksp Scap Other

Unmetamorphosed

I2H-I22 2 X X X X An4_6 X Goet,Py 11H-116 2 X X X X X AnAn4 Goet Low-grade 7H-55 2 X,Y Y (X)(Y) X,Y X X,Y Py,Gr

5H-46 2 X,Y X,Y (X)(Y) Y X,Y (Y) (Y) Y(An20-25) X Py,Gr 23H-136 2 X X (X) X X X(An2.4) Py,Gr

20H-99 2 X X (X) X X X(Ani2-i3) Py,Gr Medium-grade 8H-130 2 X X X Ans X Py,Gr 8H-112t 4 X (X) X X X Py,Gr 8H-113 5 X X X X Py,Gr 8H-114 6 X X X X Ans X Gr J OH- Hit 4 X,Y X X,Y X,Y X Py,Gr 18H-90 2 X X (X) R (X) X X Py,Gr 16H-85 2 X X (X) R X X Ani-8 Po,Gr High-grade 3H-38A 2 X X (X) R X X (X) X An4 Po,Gr 14H-81 2 X X X X X X Po,Gr 10H-107t 6 X (X) X X X Po,Gr

*See right side of Figure 2 for approximate location of cycle in stratigraphie section, t Optical identification only; feldspars are present but compositions not determined. Parentheses indicate phase is isolated or minor. X,Y notation refers to presence of two assemblages in different areas of a thin section. R = retrograde phase. An„ = anorthite mole percent (n); Cc = calcite; Cte = chlorite; Di = diopside; Do = dolomite; Fo = forsterite; Goet = geothite; Gr = graphite; Ksp = K-feldspar; Plag = plagioclase; Po = pyrrhotite; Py = pyrite; Qtz = quartz; Scap = scapolite; Tc = talc; Tr = tremolite.

Unmetamorphosed Limestones (Miller Canyon), and 20H (Contact Canyon, Fig. 3). Mineral assemblages in these rocks include: calcite + dolomite + quartz + The limestones collected at stations 12H and 11H (Little Horse talc, calcite + quartz + talc + tremolite, and calcite + dolomite + Canyon, Fig. 1) are oolitic, peloidal, and skeletal grainstones domi- quartz + talc + tremolite (Fig. 4b). Additional minor phases are nated by sedimentary and diagenetic textures and mineralogy. The shown in Table 1. Because the total abundance of silicate phases is carbonate clasts in these samples have been partially replaced by low, it is difficult to find minerals in a given sample in mutual fine-grained rhombohedral dolomite (~50 to 400 /um). Silicate contact. Mineral assemblages listed in Table 1 describe phases pres- phases make up less than ~ 5% of the rocks and consist of angular to ent within ~ 1 mm2 areas. In general, these samples exhibit relict subrounded fine-grained (— 10 to 30 /um) detrital quartz, albite sedimentary textures; however, the over-all grain size is increased (Aii4_6), and K-feldspar disseminated in peloids. In sample 11 H-l 16, relative to the unmetamorphosed limestones. Relict ooids and chlorite and phlogopite flakes (~ 100 ;um) are present in some peloids occur as ovoid areas of fine-grained (~ 30 to 50 /¿m) calcite peloids. and/or dolomite set in a matrix of coarse-grained 600 /xm) horns- The minerals present in sample 12H-122 are calcite, dolomite, felsic calcite. In samples that contain dolomite, it occurs as subhe- quartz, albite, K-feldspar, and pyrite; in sample 11 H-l 16, the min- dral to anhedral grains that no longer exhibit euhedral replacement erals present are calcite, dolomite, quartz, chlorite, phlogopite, and textures observed in the unmetamorphosed samples. Silicate phases albite (Table 1; Fig. 4a). Goethite is present as pseudomorphs after are present in minor amounts (less than ~ 5% to 10%). Fine-grained pyrite and as staining along stylolites and between grains. Fe2+ + (10 to 30 um) quartz and plagioclase (50 to 100 nm), present in (Fe2+ + Mg) ratios of chlorite and phlogopite in sample 11 H-l 16 are some samples, are disseminated within relect peloids and ooids. low (~0.03) and similar to ratios obtained for these phases in higher- Talc occurs as individual flakes (100 to 200 ium) and tremolite as grade samples, suggesting that they may be diagenetic or very low prismatic blades (-500 ;um) with rhombic cross sections (50 to 100 grade metamorphic, not detrital in origin. Fe2+/(Fe2+ + Mg) ratios /im). Scapolite occurs as stubby euhedral grains 100 to 200 /im) of calcite + dolomite show that calcite is generally more iron-rich in samples from Side Canyon. Minor phlogopite and/or chlorite than is coexisting dolomite. occurs as flakes isolated from talc and/or tremolite within or adjacent to relict peloids and ooids. Low-Grade Limestones Compositions of talc, tremolite, chlorite and phlogopite, dolomite and calcite approximate endmember compositions within Limestone samples characterized by the appearance of talc + their respective solution series. Fe2+/(Fe2+ + Mg) ratios are almost tremolite were collected at stations 7H and 5H (Side Canyon), 23H the same for all coexisting phases, but it appears that Mg is pre-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 NOTCH PEAK. CONTACT METAMORPHIC AUREOLE, UTAH 893

Figure 3. Distribution of metamorphic mineral assemblages in limestone samples of the Big Horse Limestone Member. Sym- bols are explained in inset of the figure. Isograds shown are based on field and pét- rographie observations and do not correspond to specific metamorphic reaction.

ferred over Fe in the sequence dolomite >tremolite >talc = phlogo- Canyon) and at stations 10H (sample 10H-111) and 8H (Side pite = chlorite >calcite in samples where these phases coexist. Canyon, Fig. 3). Assemblages include calcite + dolomite + tremolite, Scapolite compositions are sodium- and chlorine-rich. A typical calcite + dolomite + quartz + tremolite, calcite + dolomite + diop- chemical formula is Cai 4Na2.5K.01 Al4Si8024Clo.6(C03)o.4- side, calcite + quartz + tremolite + diopside, and calcite + quartz + More phases are present in each sample than are predicted for diopside (Fig. 4c). Minor phases are summarized in Table 1. Calcite the simple CaO-MgO-SiC>2 system if H2O and CO2 behaved as and dolomite in these samples are coarser grained (300 to 600 ^m) boundary value components. For example, in calcite-rich areas of than in the low-grade samples, and textures are hornfelsic, although sample 7H-55, the assemblage talc + tremolite + calcite + quartz some samples show relict sedimentary textures. Tremolite is usually (+ scapolite) occurs. In dolomite-rich areas of the same thin section, bladed, and grain sizes range from 20 to 600 /im. Diopside occurs as the assemblage talc + dolomite + calcite + quartz (+ scapolite) poikilitic anhedral grains locally reaching 1 mm and as stubby sub- occurs. The quartz grains are mantled by calcite and separated from hedral grains 100 to 150 pim in size. In sample 18H-90, it occcurs as the other phases. The presence of the two mineral assemblages in fine-grained inclusions (< 10 jum) in some calcite grains. Talc in this sample may reflect local variation in the initial bulk composi- samples 16H-85 and 18H-90 is texturally distinct from its occur- tion with domains smaller than a thin section. Other examples are rence in the low-grade samples. It occurs as pseudomorphs after summarized in Table 1. tremolite or as fibrous clots disseminated throughout the samples. Fine-grained quartz (< 20 ¿¿m) is often associated with this talc, Medium-Grade Limestones suggesting that these phases may have formed during retrograde alteration of tremolite. Chlorite and phlogopite also appear as clots Limestones characterized by tremolite or diopside, talc-absent disseminated throughout some samples. Plagioclase (Ani-s) and assemblages were collected at stations 18H and 16H (Contact K-feldspar occur as fine grains (10 to 30 ¿¿m), locally appearing

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 TABLE 2. MINERAL ASSEMBLAGES IN THE ARGILLITE SAMPLES

Sample Cycle* Layer Typet Qtz Cc Ksp Plag Mica Tr Di Wo Gr Ves Scap Other

Unmetamorphosed 12H-123 2 X X X An2-i2, An2o Muse, Biot Clays and Hem

I1H-115 1 X X An3-7 Muse, Biot Clays and Hem 11H-117 2 X X X An3-5 Muse Clays and Hem Low-grade 23H-137 1 X X X AnI5 Biot X Sp, Po 23H-135 2 calc-silicate X X X An67-78 X X Cte, Po, Hem biotite-rich X X An67 Biot Po 7H-56 1 calcite-rich X X X An70-78 X X Po, Hem calc-silicate (X) X X An68-71 X X Sp, Po 7H-54A 1 calc-silicate X (X) X X X Cte, Po, Hem biotite-rich X (X) X An74 Biot X X Po 5H-47 I calc-silicate X X X X X X Sp, Po 5H-44 2 calc-silicate X X X Ango Biot X Cte, Sp, Po, Hem

•Cycle refers to location in stratigraphie horizon in the Big Horse Canyon Member (refer to Figure 2). t Layer type = dominated by particular phase, abbreviation given below, calc-silicate = layer not dominated by particular phases. * Border on calcite layers. S Rim on vesuvianite. "Borders on wollastonite-rich layer. tt Borders on feldspar-rich layer. «Rim on garnet. Act = actinolite; An = anorthite; Biot = biotite; Cc = calcite; Cte = chlorite; Di = diopside; Ep = epidote; Gr = grossular; Hem = hematite; Ksp = K-feldspar; Musc = muscovite; Plag = plagioclase; Pr = prehnite; Po = pyrrhotite; Py = pyrite; Qtz = quartz; Scap = scapolite; Sch = scheelite; Sp = Sphene; Tr = tremolite; Ves = vesuvianite; Wo = wollastonite. Parentheses indicate mineral is minor or isolated as an inclusion.

FACIES SERIES DIAGRAMS FOR CYCLE 2 LIMESTONES

S ¡02 Si02 Quartz Quartz a. Unmetamorphosed c.Medium grade and very low grade Figure 4. Fades se- Stations T< ~ 400°C Stations: 450°C < T <~550°C ries diagrams for analyzed 12H P, = 2kb 18H 16H cycle 2 limestone sam- 11H ples considered in the pure Ca0-Mg0-Si02- H2O-CO2 system. Min- erals appropriate for each metamorphic grade are CaO MgO CaO MgO Calcite Dolomite Calcite Dolomite plotted at their endmember compositions. Assemblages were not observed for the stippled parts of the diagrams. Temperatures to the right Si02 SiOz Quartz d. Medium ta Quartz of each diagram are taken b. Low grade Hicjh grade from the 2 kbar phase ~500°C< T< ~600°C diagrams of Slaughter T< ~ 450°C Pf =2kb > = 2kb and others (1975) and Eg- Talc gert and Kerrick (1981). A total pressure of 2 kbar was estimated based Olivine on the amount of stratigraphic overburden.

CaO MgO CaO MgO Calcite Dolomite Calcite Dolomite

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 NOTCH PEAK. CONTACT METAMORPHIC AUREOLE, UTAH 895

TABLE 2. (continued)

Sample Cycle* Layer Typet Qtz Cc Ksp Plag Mica Tr Di Wo Gr Ves Scap Other

Medium- to high-grade 8H-129 1 calcite-rich X X X X* X Pr, Py, Hem feldspar-rich X X An»3 X Pr, Sp, Py 8H-59 2 calcite-rich X X An67-79 X X* Po, Hem

feldspar-rich (X) X An73-8i, An2 (Biot) X Cte, Sp, Po 20H-98 1 wollastonite-rich (X) X Ans-11 X X X Sp, Py, Hem feldspar-rich (X) X An3 X X Sp, Py 20H-100 2 wollastonite-rich (X) X X X Sp, Py, Hem feldspar-rich (X) An62-65, Ang3.89 X X Sp, Py 18H-89 1 calcite-rich X X X X Gr85 Sp, Py, Hem § Gr + Ves-rich (X) X X X Gr77-80 ** X Sp, Py, Hem wollastonite-rich (X) X Am-10 X X Sp 17H-87 2 calcite-rich (X) X X X X Gr84 X Sp, Py, Hem vesuvianite-rich X X X X X** Sp :t grossular-rich X X X X Gr87-88 Sp feldspar-rich (X) X An4 X X Sp, Py § 16H-84 1 Gr + Ves-rich (X) (X) X X Gr72-78 ** X Sp wollastonite-rich X An47 X X Gr7|_78 Sp calc-silicate X X An28-40 X X X Sp, Py §t 16H-86 2 Gr + Ves-rich (X) X An2-7 X X Gr72-84 X Pr, Sp, Hem vesuvianite-rich (X) (X) An2 X X X Pr, Sp feldspar-rich (X) X X (X) Pr, Sp, Py 14H-80 1 vesuvianite-rich (X) X Ani X X xtt Sp grossular-rich (X) X An<5 X X Gr55 Sp

feldspar-rich (X) X An3o-45 X X Sp, Py 3H-36 1 wollastonite-rich (X) (X) An43 X X Gr8o Sp feldspar-rich (X) X An2, An43_66 X (X) Sp 3H-37 1 vesuvianite-rich (X) X An2 X X X*» Pr, Sp Gr + Ves-rich (X) X X X Gr80«" X Pr, Sp feldspar-rich (X) X An 17-22 X X Sp, Po 3H-39 2 Wo + Ves-rich X X An<5 X X X** Pr, Sp feldspar-rich X X An24-30, An4o X X Pr, Sp 3H-40 2 wollastonite-rich (X) X X X Pr, Sp Station I0H-traverse 10H-108 6 calcite-rich X X X X X* Pr, Sp feldspar-rich X X Ans X X xtt Ap, Sp 10H-106A 6 calcite-rich X X X X Sp Gr + Ves-rich (X) X An2-6 X X Gr69-73 X"tt Sp feldspar-rich X X X X 10H-105 6 calcite-rich X X An35 X X Sp § Gr + Ves-rich (X) An<5 X X Gr80* X Sp, Pr feldspar-rich (X) X An2-5, An34_35 X X Sp, Po 10H-104 6 vesuvianite-rich X X X X X Gr57-70 x**tt Pr, Sp, Sch

grossular-rich X X X Gr55 Pr, Sp 10H-103 6 calcite-rich X X X Gr80-86Ì X Sp vesuvianite-rich (X) X X Gr76-78 X»« Sp, Sch vesuvianite-rich X X X X Sch, Sp

10H-102 6 Gr + Ves-rich X An3, An37-44 X Gr82 X Pr grossular-rich X X An2,Anio-i5,An39, 57 Act X Gri9_32, Gr59-60 Pr, Cte, Sp, Ep, Sch

murky to opaque. Caleite, dolomite, talc, tremolite, diopside, chlo- are marked by the appearance of forsterite. The mineral assemblage rite, and phlogopite compositions are near endmembers. calcite + dolomite + tremolite + forsterite characterizes these rocks Like the lower-grade samples, the occurrence of assemblages of (Fig. 4d). Minor phases are summarized in Table 1. In these rocks, four phases or more suggests that H2O and CO2 did not behave as calcite and dolomite occur in coarse-grained (300 to 600 nm) boundary-value components in all cases. For example, in 8H-114, granoblastic textures. Fine-grained (< 10 yum) microdolomite grains the four-phase assemblage calcite + quartz + tremolite + diopside occur within calcite grains in sample 14H-81. Forsterite is present in occurs. Tremolite is intergrown with or rimmed by diopside. fine-grained (~ 30 ¡j.m) inclusions rimmed by calcite within dolomite Anhedral quartz is disseminated throughout the sample and is in grains in sample 3H-38A and as subhedral to euhedral grains (~ 100 contact with some tremolite and diopside intergrowths. to 200 |um) disseminated between calcite grains in samples 14H-81 and 1 OH-107. Tremolite occurs as bladed inclusions in dolomite High-Grade Limestones (3H-38A) and as individual grains (~50 to 150 ¿im) in all samples. Talc, quartz, chlorite, phologopite, and plagioclase (Ans) grains The highest grade limestones, collected at stations 14H and 3H have the same textural relationships as in the medium-grade sam- (Contact Canyon) and 10H (sample 10H-107, Side Canyon, Fig. 3) ples. Forsterite compositions are near end member (Fo97_96).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 896 HOVER GRANATH AND OTHERS

Figure 5. Distribu- tion of mineral assemblages in argillitic samples. Key to symbols is explained in inset of the diagram. Isograds shown are based on petrogra- phically determined assemblages and do not refer to specific reactions.

Fe2+/(Fe2+ + Mg) ratios of coexisting ferromagnesian phases show member along a short traverse away from the pluton. This traverse that forsterite is the most iron-rich silicate phase. includes a sample of the pluton and adjacent skarn. Again, more than three phases are present in these samples. The argillite samples can be categorized into three mineralogi- The four phases calcite + dolomite + forsterite + tremolite in sample cal/textural groups roughly corresponding to subdivisions dis- 14H-81 occur within an area less than -0.50 mm2, suggesting that cussed for the limestones. Argillites collected at stations repre- this is an equilibrium assemblage. In sample 3H-38A, forsterite + senting unmetamorphosed conditions in interbedded limestones calcite + dolomite are found in mutual contact but not with tremo- retain their original sedimentary or diagenetic textures and miner- lite. In addition, talc occurs as intergrowths with fine-grained alogy. Those collected at stations corresponding to low-grade quartz, adjacent to or rimming tremolite grains and may be the metamorphism generally contain quartz and calcite. These samples result of retrograde alteration. In sample 1 OH-107, calcite + forster- are marked by the appearance of calcic plagioclase (An^o-75), bio- ite + tremolite are observed in mutual contact. Dolomite grains are tite, tremolite, diopside, and scapolite. Argillites collected at sta- separated from this assemblage by calcite mantles. tions representing medium- to high-grade metamorphism are characterized by the appearance of wollastonite, grossular, vesuv- MINERAL ASSEMBLAGES IN THE ARGILLITES ianite, a change in plagioclase to more albitic compositions, and the disappearance of tremolite, biotite, scapolite, and quartz. Mineral assemblages in argillites from the Big Horse Lime- stone Member are given in Table 2 and their distribution in Figure Unmetamorphosed and Very Low Grade Argillites 5. The argillites were collected at the same stations as limestones discussed in the previous section. Argillites from station 10H were Unmetamorphosed argillites from station 12H (Little Horse collected from several stratigraphic horizons at the base of the Canyon) and 11H (North Canyon, Fig. 1) consist of interlaminated

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 NOTCH PEAK. CONTACT METAMORPHIC AUREOLE, UTAH 897

(1 to 10 mm) carbonate-rich and detrital silicate silt-rich layers. The (for example, 23H-135). Scapolite compositions are more calcic carbonate-rich layers are of two types. The first is texturally similar than in the limestone samples; a typical chemical formula is

to the unmetamorphosed limestones discussed previously and con- Ca2.2Nai.6K0.2(Si7.5Ak5)O24Cl0.3(CO3)0.7. sists of carbonate clasts with minor, disseminated, silt-sized (50 to 100 Mm)> detrital quartz, K-feldspar, and albitic plagioclase grains. Medium- to High-Grade Argillites Muscovite and minor biotite flakes 200 jum) are also present. The second type is predominantly micrite, also containing minor detrital The argillite samples collected at stations 8H and 10H (Side material. Silt-rich layers contain a much greater proportion of sil- Canyon) and stations 20H, 18H, 16H, 14H, and 3H (Contact icate phases, set in either a fine-grained micrite + clay-mineral Canyon, Fig. 5) contain grossular, vesuvianite, ± wollastonite. matrix or a blocky sparite matrix. The ratio of detrital-silicate to Except for those from station 20H, samples in this group corre- carbonate material varies from layer to layer. spond to talc-free, medium- to high-grade limestones. Relict sedi- Minerals in these rocks include calcite, quartz, albite, mentary compositional layering on the scale of ~ 1 to 10 mm persists K-feldspar, muscovite, biotite, and clay minerals (Table 2). Dolo- even in the highest-grade samples adjacent to the igneous contact. mite is not present. The compositions of plagioclase range from An2 The over-all grain size within individual layers is increased relative to Anio, and K-feldspar ranges from Or93 to Or89. Qualitative to the low-grade argillites, and textures are hornfelsic. energy dispersive microprobe analyses of the clay-mineral matrix Mineral assemblages in these rocks are strongly dependent on show that it contains MgO-, FeO-, and TiCh-bearing phases. The the initial bulk composition of the individual layers in which they Fe2+/(Fe2+ + Mg) ratio of biotite in sample 11H-115 is 0.14, more occur. In feldspar-rich calc-silicate layers, the diagnostic mineral iron-rich than biotite present in limestone collected at the same assemblage is diopside + K-feldspar + plagioclase + wollastonite + station. Detrital muscovite compositions are nearly endmember calcite (Table 2). Calcite is usually a minor constituent in these with vlAl/(total octahedral cations) ratios of 0.76-0.89. layers. Diopside occurs as stubby subhedral grains (~ 50 to 300 /¿m) set in a groundmass of anhedral K-feldspar and plagioclase. Wol- Low-Grade Metamorphosed Argillites lastonite occurs as disseminated fine-grained (~50 to 100 /jm) laths or needles. Mineral assemblages in calcite-rich layers and nodules Argillite samples collected at stations 7H, 5H (Side Canyon) contain calcite + diopside + wollastonite with the additional phases and 23H (Miller Canyon) that correspond to low-grade conditions K-feldspar + plagioclase, K-feldspar + grossular, vesuvianite, in limestones are marked by the disappearance of muscovite, the K-feldspar + grossular + vesuvianite. Calcite is coarse grained (300 appearance of calcic plagioclase (An67-8o), biotite, chlorite, tremo- to 600 /urn) and granoblastic. Grossular and vesuvianite in these lite, diopside, scapolite, and sphene. In all samples, relict sedimen- layers are fine grained (< 300 fim) and anhedral. Wollastonite-rich tary compositional layering is present on the scale of 1 to 5 mm. calc-silicate layers contain the assemblage wollastonite + calcite + Mineral assemblages and textures are dependent on the layer type. diopside with the additional phases: K-feldspar + plagioclase, K- All assemblages contain quartz + K-feldspar. Additional minerals in feldspar + plagioclase + grossular, plagioclase + grossular, or vesu- silicate-rich layers are calcite + biotite + plagioclase, calcite + tremo- vianite. In these layers, wollastonite occurs as coarse-grained lite + biotite + plagioclase, calcite + tremolite + diopside + biotite + intergrowths with diopside, feldspars, and anhedral grossular or plagioclase, and calcite + diopside + biotite + plagioclase. Plagio- vesuvianite. clase and K-feldspar are fine-grained (< 30 (im). Calcite is a rela- Grossular and vesuvianite porphyroblasts (often reaching tively minor phase. Tremolite and/or diopside in these layers occurs ~ 2 cm) generally occur at the boundary between calc-silicate layers as poikilitic anhedral grains which can only be distinguished from and calcite-rich layers. In several samples, grossular may occur as the groundmass microscopically as areas of higher birefringence partial rims or replacements of vesuvianites, separating it from and uniform extinction. Biotite occurs as flakes ~ 200 to 300 /im in adjacent calcite-rich or wollastonite-rich layers (for example, sam- size. In calcite-rich calc-silicate layers, additional minerals in the ples 3H-37, 16H-84, 16H-86, 18H-89, and 10H-105; Table 2). In assemblage are: calcite + diopside + tremolite + plagioclase, calcite + some samples, vesuvianite appears as rims on euhedral grossular tremolite + diopside + scapolite, and calcite + diopside + scapolite. grains (samples 10H-103, 10H-104, and 10H-106; Table 2). In other Tremolite and diopside grains in these layers are better formed and layers of the same samples, individual grossular or vesuvianite por- exhibit typical bladed habits locally approaching 1 mm in length. phyroblasts occur without rimming relationships. Calcite is coarse-grained (~ 100 to 500 ^m), and its texture is gra- Epidote and actinolite were found only in the skarn sample noblastic. Scapolite occurs as poikilitic anhedral grains locally 10H-102 in the assemblage: calcite + plagioclase + diopside + garnet approaching ~ 1 mm. Pyrrhotite is disseminated in all samples and + epidote + actinolite. Scheelite (CaW04), a mineral typical of is locally altered to hematite. Sphene occurs as fine (< 10 to 20 ¡xm) tungsten-molybdenum skarns, is present near the igneous contacts anhedral grains. Quartz grains in both layer types are more rounded at station 10H. Sphene is present in all medium- to high-grade than in unmetamorphosed samples. Chlorite occurs in some sam- argillite samples. In samples closest to the igneous contact, wollas- ples intergrown with tremolite and diopside and may be a retro- tonite also occurs along fractures and between bedding planes. grade alteration of these phases. Prehnite occurs as a replacement of vesuvianite and grossular, and Plagioclase compositions range from An^7 to Anso, with the as in-fillings along fractures. exception of sample 23H-137 in which it is Anis. The more anor- Plagioclase compositions in medium- to high-grade argillites thitic compositions in each sample occur in calcite-rich layers. can generally be divided into two groups. Plagioclase that occurs in K-feldspar compositions range from Orgo to Orgs. Fe2+/(Fe2++ Mg) calc-silicate layers associated with diopside and K-feldspar range in

ratios in diopside, tremolite, biotite, and chlorite are greater than composition from An2o to A1140. Plagioclase that occurs as inclu- ~0.15. The preference of Mg over Fe in these phases is diopside sions in garnet or vesuvianite porphyroblasts or is nearby has more > chlorite > biotite > tremolite in samples where these phases coexist albitic (An < 10) compositions. These latter grains are clouded and

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 898 HOVER GRANATH AND OTHERS

may have been hydrothermally altered. Compositions of plagio- valid. The fluid inclusions do, however, contain dissolved salts, and clase in samples from stations 8H and 20H are transitional between the presence of graphite and pyrite indicates that fluids also contain the low-grade and high-grade compositions. K-feldspars in the other carbon- and sulphur-bearing species. Therefore, assuming Pf

medium- to high-grade samples contain up to ~ 10 mole percent Ab. = P (H2O) + P(C02) is approximate. Rice (1977) has shown that for Fe2+/(Fe2+ + Mg) ratios of the pyroxenes in these rocks show natural mineral compositions similar to those in the present study, considerable variation between samples and also between analyses the displacement of equilibria due to solid solution in constituent from the same sample. This variation appears to be temperature phases is within the error of experimental studies. Inferred reactions independent, because both high-grade and low-grade compositions can therefore be directly compared with the experimentally deter- span the range from 0.10 to 0.45. This variation may reflect differ- mined equilibria. The presence of dissolved salts and non-binary ences in bulk composition between stations and differences in bulk CO2-H2O species in the fluid phase will lower the reaction equili- compositions of individual layers within a given sample. bration temperatures in T-X(CC>2) space. Garnet compositions in the medium- to high-grade argillites Figure 6 is a summary of the 2-kbar T-X (CO2) phase diagram fall principally along the grossular-andradite (CajAl2SÍ30i2- for the Ca0-Mg0-Si02-H20-C02 system after Eggert and Kerrick Ca3Fe23+S¡30i2) solid-solution series. In all samples except the (1981). Evidence of fluid buffering would be found in either the skarn, the molecular proportion of grossular endmember ranges common occurrence of an equilibrium assemblage of four phases from 55 to 85 mole percent. The most andraditic compositions in related by univariant reactions or by five phases related by invariant this group are found in highest-grade samples from stations 14H points (Greenwood, 1975). In the ternary CaO-MgO-SiC>2 diagram, and 3H. The most grossular-rich garnets (Grso) are those that occur this would appear as a crossing of tie lines between reactants and in calcite-rich layers or immediately adjacent to vesuvianite where products of the fluid-buffering reaction. As discussed above, min- textural evidence suggests that they are replacing the vesuvianite. eral assemblages and textural relations between phases in the lime- Garnet compositions in the skarn sample (10H-102) are Fe3+ and stone samples shows abundant evidence that fluid buffering by Mn-rich and show a range over a thin section. In one area, the metamorphic reactions has occurred and also that domains of equi- garnet compositions are andraditic, and individual grains are zoned librium are small. In the following discussion, possible prograde from Goo at the core to ~Gri9 at the rim. In another area, two paths of metamorphism will be proposed by comparing the garnets having distinct compositions, G05 and Gréo, are present in observed sequence of assemblages in the limestones with the loca- contact with one another. Elsewhere in the same thin section, garnet tion of these reactions in isobaric T-X (CO2) space. The prograde of composition Gr¡¡2 appears as a partial rim on vesuvianite. paths illustrated in Figure 6 can be used to constrain peak meta- Vesuvianite analyses were reduced according to the general morphic temperatures and fluid compositions at each grade of mineral formula proposed by Rucklidge and others (1975): metamorphism.

Cai9Fe(Fe, Mg, Ti, Mn, Al)8Al4Si|gO70(OH, F)8, assuming that Samples 12H-122 and 11H-116 contain the assemblage calcite iron is Fe3+. Ti and Fe3+ contents vary inversely with Al. The most + dolomite + quartz which is stable in the low-temperature divariant Fe3+- and Ti-rich compositions (> 1.9 and 1.4 cations/formula unit, field below reactions shown in Figure 6. It is assumed that all respectively) occur in samples from stations 14H and 3H and in the metamorphosed limestones were composed of this assemblage prior skarn sample 10H-102. The lowest Ti compositions (~ 0.25 cations/ to metamorphism. Minor chlorite and phlogopite in sample 11H- formula unit) are from vesuvianites in calcite-rich layers. Individual 116 presumably recrystallized from clay minerals during diagenesis vesuvianite porphyroblasts from a given sample are slightly zoned or very low grade metamorphism. 3+ with enrichments of both Ti and Fe at the rim relative to the Limestone samples from stations 7H, 5H, 23H, and 20H that interior of the grains. represent low-grade metamorphism contain talc + tremolite- bearing assemblages. Talc is stable in siliceous limestones between METAMORPHIC REACTIONS AND PHASE EQUILIBRIA reactions 2 and 4 shown in Figure 6. Thus, low-grade assemblages formed at temperatures lower than -475 °C, corresponding to the Limestone Phase Equilibria temperature maximum of reaction 4, and under fluid compositions more hhO-rich than Xco2 ""0.65, corresponding to the invariant To explain observed mineral assemblages in the limestone point A in Figure 6. Because of the large areal distribution of talc samples and to estimate temperature and fluid compositions, min- (Fig. 3), the initial fluid composition was probably fairly H20-rich. eral reactions relating assemblages can be compared with experi- The value of X(C02) -0.25 was arbitrarily chosen for the initial mentally determined phase equilibria in isobaric T-X (CO2) space, fluid composition in Figure 6. assuming: Samples 7H-55, 5H-46, and 20H-99 contain within a single 1. Total pressure (Pr) is constant. thin section the five phases: calcite, dolomite, talc, tremolite, and

2. PT = Huid pressure (Pf) = PH2O + Pco2- quartz. These five phases are stable at invariant point A. In detail, 3. Substitution of components not in this system does not sig- however, only three or four phases are observed in contact or within nificantly displace the location of experimentally determined local domains of presumably different bulk compositions. Each of equilibria in the pure system. these four low-grade samples exhibits slightly different textural The thermodynamic equations describing equilibrium reac- relationships between the five phases, suggesting several possible tions in T-X (CO2) space are summarized in Greenwood (1967a) T-X(CC>2) trajectories during low-grade metamorphism. and Kerrick (1974). A total pressure of 2 kbar has been estimated For example, in sample 7H-55, dolomite-rich areas contain the based on the amount of stratigraphic overburden. The presence of assemblage calcite + dolomite + talc + quartz. In dolomite-absent fluid inclusions in garnets from high-grade argillites (Feldman and areas of the sample, the assemblage calcite + talc + tremolite + Papike, 1981) suggests that a fluid phase was present during quartz is observed. The quartz grains in both assemblages are metamorphism, and therefore assuming that Pj = Pf is probably embayed and mantled by calcite. These observations suggest that in

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 NOTCH PEAK CONTACT METAMORPHIC AUREOLE, UTAH 899

750

l C02

Figure 6. Isobaric 2-kbar T-X (CO2) phase diagram (after Eggert and Kerrick, 1981), summarizing reactions in the CaO-MgO- S1O2-H2O-CO2 system (solid curves). Reaction 14 is from Greenwood (1976b), reaction 16 is from Hoschek (1973), and reaction 17 is from Rice (1977). Numbered reactions are the same as used in the text. Abbreviations used are the same as in Table 1. Stars and arrows

represent possible T-Xco2 paths during metamorphism as described in the text.

dolomite-rich areas of the sample, the fluid composition may have maximum of-475°. The presence of the two different assemblages been buffered along the path of reaction 2 (circled stars, Fig. 6) until suggests that local equilibrium existed in areas of slightly different quartz in the immediate vicinity was isolated from the assemblage. bulk compositions on a scale smaller than a thin section. At this point, the assemblage became divariant. With continued In another sample, 5H-46, the assemblage calcite + dolomite + heating, tremolite would not be produced by reaction 3 because talc + tremolite occurs in areas less than 0.20 mm. In sample 20H- quartz was essentially unavailable to react with talc. In the 99, this assemblage occurs within a single 600-/um ooid. In both dolomite-absent areas of the sample, presumably more quartz was samples, fine-grained quartz grains mantled by calcite are dissemi- initially available to continue fluid buffering along reaction 2. If nated throughout the slide but not in contact with either talc or dolomite is consumed prior to reaching invariant point A, the tremolite. If quartz is part of the calcite + dolomite + talc + tremolite assemblage would follow the path shown as long arrows in Figure assemblage, then these samples indicate temperatures and fluid 6. Tremolite is produced by reaction 3, and the fluid phase is buf- compositions of invariant point A, X(CC>2) = 0.65 and T ~ 450 °C. If fered until quartz is isolated from the calcite + talc + tremolite however, quartz was isolated from the remaining phases prior to assemblage. The two assemblages calcite + dolomite + talc and cal- reaching invariant point A, then the four-phase assemblage calcite + cite + talc + tremolite are both stable in T-X(CC>2) space within the dolomite + talc + tremolite suggests fluid buffering along reaction 4 divariant field between reactions 3 and 4 at temperatures up to a toward the temperature maximum of 475 °C and X(CC>2) = 0.50

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 900 HOVER GRANATH AND OTHERS

(along the path of circled stars). If quartz was isolated from the assemblage at invariant point A, the reaction proceeds along the path of filled stars toward more HC>2-rich compositions (Fig. 6). Limestone samples representing medium-grade metamorphic conditions are marked by the disappearance of talc. The Si02-poor rocks contain the assemblage tremolite + calcite + dolomite (8H- 113, 10H-111, and 16H-85). This divariant assemblage is stable over a wide range of fluid compositions at temperatures above reactions 4 and 5 and below the forsterite- and diopside-forming reactions 7 and 8 (Fig. 6). Minor quartz mantled by calcite in some samples suggests that it is isolated from the assemblage. Because bulk compositions of these samples were SiC^-poor, reaction 6 (Fig. 6) did not occur in these samples. Continuation along the prograde path shown by circled and filled stars is consistent with the observed assemblage in these rocks. Sample 18H-90 contains very fine-grained diopside inclusions within a calcite grain; elsewhere in this sample, calcite grains contain inclusions of tremolite. The diopside may have been locally produced by reaction of tremolite inclusions in calcite by reaction 8. Although dolomite is not observed in contact with the diopside, it is present throughout the sample, suggesting that diopside + calcite + 0.80 1.00 1.20 1.40 1.60 dolomite may have been stable. This assemblage is stable in the 1 divariant field between reactions 8, 9, and 11 at temperatures T°K xlO3 greater than ~ 525 °C and fluid compositions more CC>2-rich than invariant points C and B (Fig. 6). Thus, the fluid phase had a Figure 7. The experimental data points of Goldsmith and significant range in composition over the distance of a thin section. Newton (1969) illustrating the change in log (XMgCOj) (mole Samples 8H-130, 8H-112, and 8H-114 contain diopside and fraction) of calcite coexisting with dolomite as a function of could have followed the prograde path shown by the short arrows temperature (1/T °K). At constant 1/T, the highest log (XMgC03) in Figure 6. Samples 8H-114 and 8H-112 contain the univariant corresponds to highest pressure runs (up to 20 kbar), and the lowest assemblage calcite + quartz + diopside + tremolite related by reac- log (XMgCOj) corresponds to low pressure runs (1.7 to 2 kbar). tion 6. This fluid-buffering assemblage constrains the maximum Three linear-least-squares fits to the data are explained in the text. fluid composition to be X(CC>2) = 0.75, corresponding to the temperature maximum of the reaction curve at 550 °C. Sample 8H-130 contains the divariant assemblage calcite + quartz + diopside, which is stable over a wide range in T-X(CC>2) space at quartz-rich medium-grade limestones constrain the maximum temperatures above reaction 6 and below reaction 14. temperatures to be -550 °C, corresponding to the maximum in Samples from stations 3H, 14H, and 10H, representing the reaction 6 at X(C02) = 0.75. The lack of wollastonite in these rocks highest grades of metamorphism, contain forsterite. Sample 14H-81 is consistent with the possible high X(CC>2) fluid composition at this contains the univariant assemblage calcite + dolomite + tremolite + temperature. Assemblages in the high-grade limestones are stable at forsterite, related by reaction 7, which suggests that metamorphism temperatures between ~ 550 and 600 °C with a maximum X(CC>2) < occurred along the prograde path shown as circled and filled stars 0.85. Evidence of retrogression of tremolite to talc + quartz in sam- in Figure 6. This fluid-buffering assemblage is consistent with a ples closest to the igneous contact suggests that an influx of H2O- temperature maximum of ~ 600 °C. Because diopside is not present, rich fluids occurred after peak metamorphism. the fluid composition is required to be more F^O-rich than X(CC>2) = 0.85, corresponding to invariant point C. Sample 3H-38A contains Calcite-Dolomiite Geothermometry the divariant assemblage calcite + dolomite + forsterite. This assemblage is stable in the area above reaction 7 and therefore does The experimental data of Graf and Goldsmith (1955), Gold- not constrain fluid compositions. Sample 10H-107 contains calcite + smith and Heard (1961), and Goldsmith and Newton (1969) show forsterite + tremolite. This assemblage is stable in the divariant field that the MgCO ) content of calcite coexisting with dolomite is fixed between reactions 7 and 10, suggesting that maximum metamorphic as a function of temperature (and to a small degree, pressure) by the temperatures were below ~ 610 °C. calcite-dolomite miscibility gap. Therefore, temperatures of equilib- In summary, mineral assemblages in the low-, medium- and rium in rocks that contain coexisting calcite and dolomite can be high-grade limestones suggest that fluid buffering by metamorphic estimated. The experimental data points of Goldsmith and Newton reactions occurred until one of the reactants was consumed. (1969) are plotted in Figure 7. The variation of MgCC>3 content Assemblages in the metamorphosed limestones may therefore be with temperature follows a relation of the form: used to constrain maximum temperatures and composition of the

coexisting fluid at each grade of metamorphism. Assemblages in the log X (MgC03) = -A/T + B. low-grade rocks constrain maximum temperatures to be ~475 °C and a maximum X(CC>2) = 0.50-0.65. Assemblages in quartz- The constants A and B can be calculated through a least-squares deficient medium-grade rocks are stable at temperatures between regression analysis of the experimental data. Three such least- ~475 and —575 °C, over a wide range in fluid compositions. Some squares fits are also shown in Figure 7. In this study, temperatures

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 NOTCH PEAK. CONTACT METAMORPHIC AUREOLE, UTAH 901

TABLE 3. X(MgC03) OF CALCITE AND CALCULATED below 600 °C. The dashed curve (Fig. 7) is the least-squares fit for TEMPERATURES OF CYCLE 2 LIMESTONES all of the data of Goldsmith and Newton (1969) and thus neglects the small pressure dependence of MgCC>3 solubility. The dash-dot Sample No. X(MgC03)* (± W)t T°C(± W) curve is the least-squares fit calculated by Rice (1977), using the combined data of Goldsmith and Newton (1969) and Graf and High-grade Goldsmith (1955). The latter data are for temperatures exceeding 14H-81 4-1 0.0235(0.0042) 400(22) 548 °C and constrain his curve to deviate from the lower tempera- 4-2 0.0457(0.0344) 502(114) ture data points of Goldsmith and Newton (1969). The X(MgC03) 4-3 0.0441(0.0304) 496(104) contents of calcite given in Table 3 represent an average of several 3H-38A individual analyses collected from a single calcite grain. This proce- 4-2 0.0438(0.0045) 495(17) dure was followed to minimize errors resulting from X-ray counting 5-1 0.0431(0.0049) 492(19) statistics and to evaluate the homogeneity of an individual grain. 6-2 0.0447(0.0057) 498(22) For each average analysis, the range in homogeneity (± W in Table Medium-grade 3) was calculated at the 99% confidence level through use of the I6H-85 statistical equation of Goldstein (1976). For each sample, several 2-1 0.0499(0.0017) 518(6) grains were analyzed in this way to evaluate the attainment of equi- 2-2 0.0519(0.0033) 525(12) 4-1 0.0502(0.0025) 519(9) librium throughout the sample. 4-2 0.0487(0.0021) 513(8) Samples 16H-85, 18H-90, 20H-99, 5H-46, and 7H-55, which 18H-90 show no evidence of retrograde dolomite exsolution, yield tempera- 2-1A 0.0417(0.0034) 486(14) ture estimates with small or overlapping errors. The calculated 2-2 0.0392(0.0022) 476(9) temperatures are in close agreement with maximum temperatures Low-grade determined through considerations of phase equilibria, despite the 20 H-99 evidence of rétrogradation of silicate phases in some samples (for 1-1 0.0287(0.0033) 428(16) example, 16H-65 and 18H-90). The highest grade samples (14H-81 1-2 0.0392(0.0041) 476(17) 1-4 0.0320(0.0030) 444(14) and 3H-38A) contain calcite grains with microdolomite inclusions, 3-1 0.0298(0.0022) 433(11) which may be the result of exsolution from an originally high- 5 H-46 magnesium calcite. Temperatures calculated for sample 14H-81 2-1 0.0338(0.0007) 452(3) show large errors and variation between grains. The calculated 2-2 0.0341(0.0014) 453(7) temperatures for both samples are lower than predicted by compar- 2-3 0.0311(0.0026) 439(13) ison with phase equilibria. The X(MgCOj) contents of calcite grains 4-1 0.0308(0.0012) 438(6) in the unmetamorphosed samples 11H-116 and 12H-122 probably 4-3 0.0319(0.0025) 443(12) 4-5 0.0315(0.0014) 458(6) represent original sedimentary and/or diagenetic compositions; therefore, temperatures calculated for these samples are unreliable. 7H-55 1-1 0.0237(0.0011) 401(6) 3-1 0.0247(0.0009) 407(4) Argillite Phase Equilibria 5-2 0.0318(0.0011) 443(5) 5-4 0.0295(0.0011) 431(6) 6-1 0.0313(0.0014) 440(7) Mineral assemblages, mineral compositions, and bulk compo- 6-2 0.0319(0.0015) 443(7) sitions of argillitic samples must be considered in terms of the com- Unmetamorphosed^ ponents K20, Na20, CaO, A1203, MgO, FeO, Si02, H20, and C02. 11H-116 The first step in understanding the complex phase relations 3-1 0.0063(0.0013) 260(17) observed in the argillite samples is to place the mineral assemblages 4-1 0.0095(0.0015) 297(15) within the temperature constraints determined through analyses of 0.0023(0.0002) 187(4) 4-2 mineral assemblages and calcite-dolomite geothermometry of 12H-I22 limestone samples collected at the same stations. Because mineral 262(69) 2-1 0.0064(0.0068) assemblages in the argillites are complicated by solid solution, 2-2 0.0084(0.0034) 286(33) inferred reactions cannot be directly compared with experimentally Note: Complete analyses are given in Hover (1981). determined T-X(C02) diagrams; however, the assumed temperature

*X(MgC03) = Mg/S cations. conditions of metamorphism can be used to determine the sequence t± W = range in homogeneity calculated by equation 4 of Goldstein ( 1976). of prograde reactions in the natural system. Comparison of mineral *X(MgC03) contents probably reflect sedimentary and/or diagenetic assemblages between argillites from different stations that appear to compositions, and temperatures are probably unreliable. have formed under essentially identical temperatures can provide information regarding differences in composition of coexisting metamorphic fluids at locations throughout the contact aureole. of equilibration (Table 3), were calculated by the following equa- Low-Grade Argillites. Reactions that have produced the tion (solid line, Fig. 7) based on the 1.7 to 2-kbar data of Goldsmith observed mineral assemblages between argillites from the unmeta- and Newton (1969): morphosed station 11H and the low-grade stations 7H, 5H, and 23H can only be inferred, because samples representing interme-

log (XMgC03) = -1473/T + 0.5063. diate stages were not collected. The samples experienced maximum metamorphic temperature conditions of between ~ 450 to 475 ° C, as These data are most appropriate for the Big Horse Limestone which estimated from limestone mineral assemblages and calcite-dolomite was subjected to metamorphism at about 2 kbar and temperature geothermometry.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 902 HOVER GRANATH AND OTHERS

The disappearance of muscovite and increased An content in 3 tremolite + 6 calcite + K-feldspar =

plagioclase between the unmetamorphosed and low-grade argillite 12 diopside + phlogopite + 6 C02 + 2 H20. (17) samples might be the result of the reaction This reaction occurs at temperatures greater than reaction 6 (Fig. 6) muscovite + calcite + 2 quartz = (Rice, 1977).

K-feldspar + anorthite + H20 + C02. (15) Mineral assemblages in the low-grade argillite samples suggest that fluid buffering by reactions has occurred. For example, the At 2 kbar, the reaction has a temperature maximum at 475 °C (± 20 assemblages biotite + tremolite + calcite + quartz + K-feldspar and

°C) and X.(C02) = 0.50 (Hewitt, 1973a). Solid solution of albite in diopside + tremolite + quartz + calcite + K-feldspar are each related plagioclase reduces the temperature of equilibration, and therefore, by isobarically univariant reactions in the pure KA102-Ca0-Mg0- the occurrence of this reaction is consistent with temperatures esti- Si02-H20-C02 system. Fluid compositions could therefore have mated from assemblages in interbedded limestones. been driven by the progression of these reactions to X(C02) = 0.75. Biotite is presumed to have been initially formed by reactions This composition corresponds to the temperature maximum of the between iron and magnesian clay minerals, and either muscovite or reactions in T-X(C02) space. In the pure system, temperature max- K-feldspar, or both. Sample 23H-137 contains the assemblage bio- ima of the reactions are greater than ~ 450-475 °C estimated from tite + tremolite + K-feldspar + quartz + calcite, suggesting that the assemblages and calcite-dolomite geothermometry of limestones reaction collected at the same stations. Therefore, if fluid compositions were driven to this X(C02) composition by the progression of the reac- 5 phlogopite + 6 calcite + 24 quartz = tions, reactions 15 and 6 must be displaced to lower temperatures by solid solution in the phases participating in the reaction. If the 3 tremolite + 5 K-feldspar + 2 H20 + 6 C02 (16) position of reaction curves in T-X(C02) space is not significantly has occurred. This reaction marks the first appearance of tremolite affected by solid solution, then reactions 15 and 6 must be inter- in the argillites. The reaction has been described by Carmichael sected at low X(C02) values corresponding to temperatures of (1970), Hewitt (1973b), Thompson (1973), Ferry (1976), and Rice ~ 450-475 °C. The presence of the six-phase invariant assemblage (1977) in application to regionally metamorphosed and contact- calcite + quartz + diopside + tremolite + biotite + K-feldspar requires metamorphosed argillaceous carbonates. Reaction 16 has been that reaction 6 and 16 intersect. The location of this invariant point experimentally determined by Hoschek (1973) and Hewitt (1975). in T-X(C02) space has not been determined experimentally.

At 2 kbar, this equilibrium has a temperature maximum of 523 ± 10 Minimum X(C02) compositions are constrained by the °C at X(C02) = 0.75 (Hoschek, 1973). The data of Hewitt (1975), absence of zoisite in the low-grade argillites. The maximum stability

however, suggest a lower maximum temperature of 475 ±20 °C. of zoisite in mixed H20-C02 fluids is governed by the reaction The higher temperature location is more consistent with the T-

XiCC^) phase diagram shown in Figure 6 (Rice, 1977). However, in 3 anort hite + calcite + H20 = 2 zoisite + C02. (21) light of limestone temperature estimates (450 to 475 °C), the 523 °C temperature maximum seems too high if fluid compositions were At 2 kbar, zoisite is stable at fluid compositions more H20-rich than buffered to X (C0 ) = 0.75 at the maximum. Either (1) fluid compo- 2 X(C02) = 0.05-0.10 in the temperature range of 450-475 °C based sitions were more H20-rich in the argillites at equivalent tempera- on experimental and calculated locations of reaction 21 in isobaric tures, permitting reaction 16 to be intersected at low temperatures; T-X(C02) space (Storre and Nitsch, 1972; Gordon and Greenwood, (2) substitution of additional components displaced the location of 1971; Kerrick, 1974). The presence of calcite + anorthite (+ quartz) the equilibrium to lower tempertures; or (3) limestone temperature assemblages in the low-grade argillites requires fluid compositions estimates may be too low. to be more C02-rich than this minimum. The presence of scapolite Samples 23 H-135, 7H-56, and 5H-47 contain the assemblage + calcite (+ quartz) in some assemblages is also consistent with calcite + quartz + tremolite + diopside + K-feldspar + (plagioclase or possible high X(C02) compositions based on the T-X(C02) topol- scapolite), suggesting that the reaction ogy calculated by Ellis (1978). Medium- to High-Grade Argillites. The medium- to high-grade

tremolite + 3 calcite + 2 quartz = 5 diopside + C02 + H20 (6) argillites experienced temperatures between -475 and - 600 °C as estimated from limestone mineral assemblages and calcite-dolomite has occurred in these samples. Reaction 6 occurs at higher tempera- geothermometry. Mineral assemblages appear to be controlled by tures than reaction 16 in the pure system (Fig. 6) (Rice, 1977). The initial bulk compositions of individual layers. The presence of large presence of diopside in argillites from stations where the limestones garnet and vesuvianite porphyroblasts at the boundaries between indicate temperatures of 475 to 450 °C indicates again that either layers suggests that gradients across layers have existed. fluid compositions were more water-rich in the argillites at equiva- Metamorphic reactions pertaining to stability of grossular, lent temperatures or that solid solution of additional components in vesuvianite, wollastonite, anorthite, calcite, and quartz can be con-

diopside and tremolite displaced the location of the reaction to sidered within the framework of phase equilibria in the Ca0-Al203- lower temperatures. Mg0-Si02-H20-C02 system. A schematic T-X(C02) topology for Biotite + diopside-bearing assemblages are common in low- diopside-bearing assemblages is shown in Figure 8 and is based on grade argillites. For example, sample 7H-54A contains the assem- the diagram of Kerrick and others (1973). The diagram considers blage calcite + quartz + diopside + tremolite + biotite + K-feldspar + only reactions that involve grossular, zoisite, anorthite, calcite, plagioclase within a single layer and sample 5H-47 contains the quartz, wollastonite, and diopside. The coefficients of the reactions assemblage calcite + quartz + diopside + biotite + K-feldspar + scap- have been calculated using the ideal mineral formula Ca^MgjAlio olite. These assemblages suggest that reactions 6 and 16 intersect to Si|8069(0H)8 for vesuvianite. make reaction 17: This topology includes the vesuvianite-absent invariant points

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 -0.04 -0.05 -0.20

X C02 *

Figure 8. Schematic isobaric T-X(C(>2) phase diagram for diopside-bearing calc- Other abbreviations are Ves (vesuvianite), Gr (grossular), Zo (zoisite), and Wo (wol- aluminous silicate rocks modified after Kerrick and others (1973), illustrating the lastonite). Reaction numbers are those used in the text. Temperature and fluid com- prograde metamorphism of medium- to high-grade argillites. Mineral compositions positions of invariant points A and B are taken from the 2-kbar isobaric T-X(COj)

are projected onto the plane anorthite (An)-calcite (Ca)-quartz (Qtz) from diopside. phase diagram of Kerrick (1974) for the Ca0-AI203-Si02-H20-C02 system.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 904 HOVER GRANATH AND OTHERS

A and B. The T-X(CC>2) locations of these points are based on the Samples 20H-100, 3H-39, 3H-40, and 10H-108 do not contain experimental studies summarized by Kerrick (1974) and Winkler grossular. The mineral assemblage present in these samples is cal- (1976). Invariant point B in Figure 8 is defined by the intersection of cite + wollastonite + vesuvianite. This assemblage is stable at

reactions 21 and 23 and occurs at T =490 °C and X(C02) =0.05- temperatures greater than reaction 14 and at X(CC>2) fluid composi- 0.10 (Gordon and Greenwood, 1971; Kerrick, 1974). Point B is tions between reactions 21 and 27 and is consistent with the pro- accessible only to diopside-absent systems. Point A is defined by the grade continuation along the path shown by the arrows (Fig. 8). intersection of reactions 14, 22, 23, and 24 and occurs at T = 600 Reactions 28, 25, and 22 would not occur in these rocks, because and X(CC»2)-0.20 (Gordon and Greenwood, 1971; Kerrick, 1974; bulk compositions fall below the vesuvianite-quartz join. Winkler, 1976). Solid solution in both grossular and anorthite par- In argillites that contain grossular in addition to wollastonite + ticipating in the reactions above will displace the location of these vesuvianite, vesuvianite porphyroblasts contain only inclusions of equilibria relative to the pure system. This displacement can be diopside, calcite, plagioclase, or quartz (for example, samples 3H- calculated through the equilibrium constant for the reaction as out- 37, 16H-84,16H-86, 18H-89, 10H-105). This suggests that the vesu- lined by Kerrick and others (1973) and Kerrick (1974). For grossu- vianite in these samples was produced by reaction 26, and that lar compositions containing 20 mole percent andradite, typical of wollastonite and grossular joined the assemblage later with increas- compositions in the medium- to high-grade argillites, solid solution ing temperature when the prograde path crossed reactions 14 and of albite in plagioclase causes the location of reaction A to be 25 (Fig. 8). displaced to temperatures below 600 °C and X(CC>2) compositions In other samples, however, grossular rims or replaces vesuvian- less than 0.20. ite, separating it from adjacent calcite-rich or wollastonite-rich lay- Mineral assemblages in the medium- to high-grade argillites ers (for example, samples 3H-37, 18H-89, 16H-86, 10H-I05). The containing the phases related by equilibria shown in Figure 8 will be grossular in these samples contains wollastonite inclusions, suggest- interpreted with respect to possible prograde paths also illustrated ing that the breakdown or replacement of vesuvianite occurred in the diagram. Textural evidence suggests that mineral assem- within the stability field of wollastonite. The assemblage grossular + blages in these argillites must be considered within small domains calcite + wollastonite, occurring at the grossular-calcite layer boun- defined by individual layers of presumably differing bulk composi- dary, is stable on the C02-rich side of reaction 27. This suggests that tions, or within small areas at the boundaries between layers. Thus, fluid compositions had a local range in XCOÎ- for a given sample, each individual layer may contain a different A prograde path, more C02-rich than the arrowed path, is assemblage appropriate for the bulk composition of that layer. shown as filled stars on Figure 8 and is indicated by samples 10H- Mineral assemblages within these small domains contain only three 103, 10H-104, and 10H-106. In these samples, vesuvianite occurs as phases (+ diopside); therefore, T-X(C02) paths do not follow reac- rims replacing grossular. The vesuvianite grains either contain wol- tion curves. lastonite inclusions or are intergrown with wollastonite blades sug- The low-grade argillites contain the assemblage calcite + quartz gesting the vesuvianite may have formed at the expense of grossular + plagioclase. This assemblage is stable in the divariant field to the within the stability field of wollastonite by reaction 27. C02-rich side of vesuvianite- and grossular-forming reactions 23 Samples 20H-98 and 3H-36 do not contain vesuvianite. Gros- and 26 and temperatures below the wollastonite in reaction 14 sular in these samples contains wollastonite inclusions, suggesting (Fig. 8). It is assumed that medium- and high-grade argillites that the quartz-absent reaction 24 may be responsible for the for- contained this assemblage prior to metamorphism. The presence of mation of grossular + wollastonite + plagioclase assemblages in vesuvianite and grossular in the medium- to high-grade argillites these rocks. If this reaction occurred, then the prograde path is that requires that initial fluid compositions were more H20-rich than illustrated by circled stars in Figure 8. This implies that the fluid X(CC>2) =0.20, the compositions of invariant point A. The lack of composition in equilibrium with these rocks was more C02-rich zoisite in these samples requires fluid compositions to have been than fluids in equilibrium with vesuvianite-bearing rocks collected more C02-rich than ~ 0.05-0.10 defined by the maximum stability at the same stations (for example, 20H-100 and 3H-37, 39, 40). of zoisite, reaction 21 (Fig. 8). Mineral assemblages in the medium- to high-grade argillites Prograde metamorphism of most samples appears to have fol- cannot be used to define a unique temperature or composition of lowed the T-Xco2 Path indicated by arrows on Figure 7. For exam- the coexisting fluid because: (1) limited data are available with ple, vesuvianite in the wollastonite-absent sample 8H-59 occurs at respect to vesuvianite stability in mixed volatile fluids, and (2) solid the boundary between a calcite-rich layer containing the assemblage solution in natural phases can displace location of equilibria rela- calcite + diopside and feldspar-rich layer containing the assemblage tive to the pure system. But, in the temperature range of 475 to 600 plagioclase (An^j-jg) + diopside + K-feldspar + calcite. If quartz was °C estimated from limestone mineral assemblages, the presence of initially present in either layer, vesuvianite may have formed by wollastonite in the argillite assemblages constrains fluid composi- reaction 26: tions to be between XCOÎ ~0.03-0.20. This estimate is determined by the lower stability limit of wollastonite. 3 diopside + 11 calcite + 5 anorthite + In some medium- to high-grade argillites, prehnite occurs as a 2 quartz + 4 H2O = vesuvianite (26) replacement of vesuvianite and garnet porphyroblasts and as infill- ings in fractures. Prehnite is stable only in H20-rich fluid when the prograde path illustrated by arrows intersected this compositions with X(CC>2) less than =0.02, and temperatures less reaction (Fig. 8). than ~380 °C based on the 2 kbar T-X(CÛ2) diagram compiled by

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 NOTCH PEAK. CONTACT METAMORPHIC AUREOLE, UTAH 905

Winkler (1976). This suggests that either the assemblage ed in possibly contained significant amounts of dissolved salts and other an original H20-rich fluid or that a possible influx of H20-rich carbon and sulfur species. fluids occurred after peak metamorphism. Similar occurrences of Fluid compositions in equilibrium with limestones were CO2- talc in the highest grade limestones suggest that there was indeed a rich and similar to those produced by the devolatilization reactions. late influx of H2O. In addition, domains of equilibrium are smaller than the area of a thin section. The limestones appear to have been relatively imper- SUMMARY vious to externally derived fluids and to have acted as relatively closed systems during metamorphism. The higher-grade argillites, The analyzed limestones from the Big Horse Limestone on the other hand, equilibrated with H20-rich fluids. This differ- Member represent an approximately isochemical system through- ence in the composition of fluids in equilibrium with the two rock out the lateral extent of outcrop. Bulk compositions and mineral types suggests that if extensive fluid migration occurred, the flow assemblages can be adequately explained by the CaO-MgO-SiC>2- was channeled within the argillaceous layers that are bordered by H2O-CO2 system. The progressive metamorphism of this lithologic the limestone layers. The presence of talc replacing prograde miner- unit has resulted in a sequence of prograde mineral assemblages als in high-grade limestones and also the prehnite replacing vesuvi- that indicate fluid buffering by metamorphic reactions has anite and grossular in the high-grade argillites suggest that a occurred, and therefore, that H2O and CO2 behaved as initial-value possible influx of H20-rich fluids occurred after peak metamor- components (Zen, 1963). Mineral assemblages in the low-grade phism in samples closest to the igneous contact. limestones that contain talc + tremolite + calcite + dolomite + quartz constrain X(CC>2) to be ~0.60 and maximum temperatures to be ACKNOWLEDGMENTS ~450 to ~475 °C at PT = Pf = 2 kbar. Some assemblages in low- grade limestones suggest that domains of equilibrium may have This research was funded by Department of Energy contract been small. Fluid buffering by mineral reactions probably occurred number DE-AC02-79ER10412.A001 (to J. J. Papike, State Univer- within these domains until one of the reactants was consumed. In sity of New York at Stony Brook). Our thanks go to K. C. Loh- medium-grade limestones that contain the univariant assemblage mann for his assistance in field work and for providing analyses of calcite + tremolite + diopside + quartz, fluid compositions could carbonate working standards, to Walter Holzwarth for keeping the have become as C02-rich as X(C02) - 0.75, corresponding to the microprobe in tune, and to Neal White and Ken Baldwin for their temperature maximum of ~ 550 °C at PT = Pf + 2 kbar in T-X(C02) assistance with computer problems. Special thanks go to James W. space. The univariant, four-phase assemblage olivine + tremolite + Granath for his critical review of early drafts of this manuscript. calcite + dolomite present in the highest grade limestones is indica- Constructive reviews by Lehi F. Hintze, Dana Griffen, Samuel B. tive of fluid buffering. This assemblage constrains peak metamor- Romberger, John Ferry, Arden Albee, and Steve Simon were very phic temperatures to between ~575 and 600 °C. Temperatures helpful. obtained by calcite-dolomite geothermometry are consistent with maximum temperatures estimated from phase equilibria. REFERENCES CITED Calc-silicate-rich argillite lithologies collected from the Big Horse Limestone Member represent a complicated multicompo- Albee, A. L., and Ray, L., 1970, Correction factors for electron probe analysis microanalysis of silicates, oxides, carbonates, phosphates, and nent chemical system. Large lateral variations in bulk composition sulfates: Analytical Chemistry, v. 42, p. 1408-1414. between samples occur, principally the result of variations in the Armstrong, R. L., and Suppe, J., 1973, Potassium-argon geochronometry initial proportion of carbonate-rich to detrital silicate-rich sedimen- of Mesozoic igneous rocks in Nevada, Utah, and southern California: tary layers. The progressive metamorphism of this lithology has Geological Society of America Bulletin, v. 84, p. 1375-1392. resulted in a variety of mineral assemblages that occur within small Bence, A. E., and Albee, A. L., 1968, Empirical correction factors for electron micronanalysis of silicates and oxides: Journal of Geology, v. 76, domains defined by the initial bulk composition of individual sedi- p. 382-403. mentary layers. The presence of the univariant assemblages calcite + Carmichael, D. M., 1970, Intersecting isogrades in the Whetstone Lake quartz + biotite + tremolite + K-feldspar and calcite + quartz + area, Ontario: Journal of Petrology, v. 11, p. 147-181. tremolite + diopside within individual layers in the low-grade sam- Eggert, R. G., and Kerrick, D. M., 1981, Metamorphic equilibria in the ples suggest that fluid buffering by mineral reactions has occurred siliceous dolomite system: 6 kb experimental data and geologic implica- tions: Geochimica et Cosmochimica Acta, v. 45, p. 1039-1049. within these small domains. The fluid compositions are poorly con- Ellis, D. E., 1978, Stability and phase equilibria of chloride and carbonate strained but may have been as CC>2-rich as X(CC>2) = 0.75, corre- bearing scapolites at 750 °C and 4000 bar: Geochimica et Cosmochim- sponding to temperature maxima of univariant reactions in ica Acta, v. 42, p. 1271-1281. T-X(CC>2) space relating these assemblages. The presence of scapo- Feldman, M. D., and Papike, J. J., 1981, Metamorphic fluid compositions lite in some of these samples is compatible with this high X(CC>2) from the Notch Peak Aureole, Utah [abs.]: EOS (Geophysical Union American Transactions), v. 62, p. 435. fluid composition. In contrast with C02-rich fluids in the lime- Ferry, J. M., 1976, Metamorphism of calcareous sediments in the stones and low-grade argillites, the occurrence of grossular, Waterville-Vassalboro area, south-central Maine: Mineral reactions vesuvianite, and wollastonite in medium- to high-grade argillites and graphical analysis: American Journal of Science, v. 276, requires fluids to be more H20-rich than X(CC>2) = 0.20. The occur- p. 841-882. rence of scapolite-, graphite-, and pyrite-bearing assemblages indi- Gehman, H. M., Jr., 1958, Notch Peak Intrusive, Millard County, Utah, geology, petrogenesis and economic deposits: Utah Geological and cates that fluids were not binary mixtures of CO2 and H2O but Mineralogical Survey Bulletin, v. 62, p. 1-50.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021 906 HOVER GRANATH AND OTHERS

Goldsmith, J. R., and Heard, H. C., 1961, Subsolidus phase relations in the Kerrick, D. M., 1974, Review of metamorphic mixed volatile (H2O-CO2) system CaCOj-MgCX^: Journal of Geology, v. 69, p. 45-74. equilibria: American Mineralogist, v. 59, p. 729-762. Goldsmith,.). R., and Newton, R. C., 1969, P-T-X relations in the system Kerrick, D. M., Crawford, K. E„ and Randazzo, A. F., 1973, Metamor-

CaC03 -MgCOj at high temperatures and pressures: American Journal phism of calcareous rocks in three roof pendants in the Sierra Nevada, of Science, v. 267A, p. 160-190. California: Journal of Petrology, v. 114, p. 303-325. Goldstein, J. I., 1976, Statistics of X-ray analysis: Microbeam Analysis Lohmann, K. C., 1977, Causative factors of the outer detrital belt House Society Proceedings, 11th Annual Conference, p. 1-8. Embayment: A sedimentologic examination of a terrigenous-carbonate Gordon, T. M., and Greenwood, H. J., 1971, The stability of grossularite in depositional system, early Upper Cambrian (Dresbachian), east- H2O-CO2 mixtures: American Mineralogist, v. 56, p. 1674-1688. central Utah and west-central Nevada [Ph.D. thesis]: Stony Brook, Graf, D. L., and Goldsmith, J. R., 1955, Dolomite-magnesian calcite rela- New York, State University of New York at Stony Brook. tions at elevated temperatures and CO2 pressures: Geochimica et Cos- Palmer, A. R., 1971, Cambrian of the Great Basin and adjacent areas, mochimica Acta, v. 7, p. 109-128. western United States, in Cambrian of the New World: London, Wiley- Greenwood, H. J., 1967a, Mineral equilibria in the system Mg0-Si02-H20- Interscience, p. 1-78. CO2: Researches in Geochemistry, v. 2, p. 542-567. Rice, J. M., 1977, Progressive metamorphism of impure dolomitic lime- 1967b, Wollastonite: Stability in H2O-CO2 mixtures and occurrence in stone in the Marysville Aureole, Montana: American Journal of a contact-metamorphic aureole near Salmo, British Columbia, Canada: Science, v. 277, p. 1-24. American Mineralogist, v. 52, p. 1669-1680. Rucklidge, J. C., Kocman, V., Whitlow, S. H„ and Gabe, E. J., 1975, The 1975, Buffering of pore fluids by metamorphic reactions: American crystal structure of three Canadian vesuvianites: Canadian Mineral- Journal of Science, v. 275, p. 573-593. ogist, v. 13, p. 15-21. Hewitt, D. A., 1973a, Stability of the assemblage muscovite-calcite-quartz: Slaughter, J., Kerrick, D. M., and Wall, V. J., 1975, Experimental and American Mineralogist, v. 58, p. 785-791. thermodynamic study of equilibria in the system Ca0-Mg0-Si02- 1973b, The metamorphism of micaceous limestone from south-central H2O-CO2: American Journal of Science, v. 275, p. 143-162. Connecticut: American Journal of Science, v. 273a, p. 444-469. Storre, B„ and Nitsch, K. H., 1972, Die reaktion 2 zoisit + 1 C02 ~ 1975, Stability of the assemblage phlogopite-calcite-quartz: American 3 anorthit + I calcit + H2O: Contributions to Mineralogy and Mineralogist, v. 60, p. 391-397. Petrology, v. 35, p. I —10. Hintze, L. F., 1973, Geologic history of Utah: Brigham Young University Thompson, P. H., 1973, Mineral zones and isogrades in "impure" calcare- Geology Studies, v. 20, p. 1-181. ous rocks, an alternative means of evaluating metamorphic grade: Con- 1974, Preliminary geologic map of the Notch Peak Quadrangle, Mil- tributions to Mineralogy and Petrology, v. 42, p. 63-80. lard County, Utah: U.S. Geological Survey Miscellaneous Field Whelan, J. A., 1970, Radioactive and isotopic age determinations of Studies Map MF-636. Utah rocks: Utah Geological and Mineralogical Survey Bulletin, v. 81, Hintze, L. F., and Palmer, A. R., 1976, Upper Cambrian Orr Formation: Its p. 1-75. subdivisions and correlatives in western Utah: U.S. Geological Survey Winkler, H.G.F., 1976, Petrogenesis of metamorphic rocks (4th edition): Bulletin 1405-G, p. 1-25. New York, Springer-Verlag. Hoschek, G., 1973, Die reaktion phlogopit + calcit + quarz = tremolit + Zen, E-An, 1963, Components, phases, and criteria of chemical equilibrium

Kalifeldspat + H2O + C02: Contributions to Mineralogy and Petrol- in rocks: American Journal of Science, v. 261, p. 929-942. ogy, v. 39, p. 231-237. Hover, V. C., 1981, The Notch Peak metamorphic aureole; Utah: Mineral- ogy, petrology, and geochemistry of the Big Horse Canyon Member of MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 6, 1981 the Orr Formation [M.S. thesis]: Stony Brook, New York, State Uni- REVISED MANUSCRIPT RECIEVED AUGUST 25, 1982 versity of New York at Stony Brook. MANUSCRIPT ACCEPTED SEPTEMBER 1, 1982

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/7/889/3444721/i0016-7606-94-7-889.pdf by guest on 01 October 2021