Age of the Basement Rocks of Southwest Montana

Age of the basement rocks of southwest Montana

H. L. JAMES U.S. Geological Survey, Port Townsend, Washington 98368 C. E. HEDGE U.S. Geological Survey, Denver, Colorado 80225

ABSTRACT 113 m.y. Inclusion of other published data in Wyoming and Montana. The exposed for the Tobacco Root Range yields a best-fit rocks have complex histories, and the Rb-Sr analyses of a suite of quartzo- value of 2,730 ± 85 m.y. This age corre- questions of true ages and age relations are feldspathic gneisses that are interlayered sponds closely to that of the principal only gradually yielding to attack by isotopic with beds of marble, quartzite, and am- metamorphic-plutonic epoch of the Bear- dating. This paper is concerned mainly with phibolite in the Ruby and Tobacco Root tooth Mountains, to which the term "Bear- the results and implications of Rb-Sr Ranges and the Gallatin River canyon of tooth orogeny" has been applied. It also analyses of a suite of samples of quartzo- southwest Montana show that the age of demonstrates that the major Precambrian feldspathic gneiss from the Ruby Range, the metamorphism of these strata occurred metasedimentary sequences of the region Tobacco Root Range, and the Gallatin about 2,750 m.y. ago. The 13 samples are of Archean age. River canyon area (Fig. 1). analyzed are from rock units that have in the past been assigned stratigraphically to INTRODUCTION GEOLOGY the Pony Group, Cherry Creek Group, and Dillon Granite Gneiss. Except for two Precambrian rocks of pre-Belt age — that General Outline samples of anomalous composition, the is, older than about 1,400 m.y. — are data define a linear array on an isochron widely distributed throughout the northern The geology of southwest Montana, east diagram that has a best-fit value of 2,762 ± and central Rocky Mountains, principally of the longitude of Dillon (Fig. 1), is charac-

Bozeman

Figure 1. Index map of part of southwest Montana showing areas of older Precambrian (hachured) and sample locations.

Geological Society of America Bulletin, Part I, v. 91, p. 11-15, 2 figs., 1 table, January 1980, Doc. no. 00104.

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terized by a series of uplifted blocks that had been made for the dolomite-bearing se- product of isochemical metamorphism of typically are cored by Precambrian igneous quence in the Tobacco Root Range by Win- arkosic beds that were interleaved with car- and metamorphic rocks, flanked and partly chell (1914). These correlations and exten- bonates and pelites of the Cherry Creek capped by Paleozoic and Mesozoic strata, sions may well be valid, but their specula- Group; in a later paper, however, Garihan and separated by discontinuous basins of tive basis should be recognized. (1979) reopens the possibility of an igneous Tertiary and Quaternary deposits. Each of The term "Pony Series" (now "Pony derivation. The most likely explanation, in the various blocks, which include the Gal- Group") was introduced by Tansley and our opinion, is that the granite gneiss has latin, Madison, Tobacco Root, Gravelly, others (1933) for the various gneisses and multiple origins: some of it metamorphosed and Ruby Ranges (as well as the Beartooth schists exposed near Pony, in the northeast- sedimentary rock, as suggested by Garihan Mountains, about 80 km east of the Galla- ern part of the Tobacco Root Range. This and Okuma; some of it truly igneous, as tin Range), has distinctive geology and sequence was assumed to be older than proposed by Heinrich; and some of it re- structure, but the Precambrian cores do rocks assigned to the Cherry Creek Group. mobilized pre-existing basement rock. In have a number of features in common. The Most workers, however, now consider the general, we see little justification for most abundant rock types are quartzo- relative ages of the two groups — if in fact separating the Dillon Granite Gneiss from feldspathic gneisses of varied composition. they are two separate groups — to be inde- other similar quartzo-feldspathic gneisses of The gneisses are interlayered on a scale of terminate, although Reid (1957) concluded the region, unless the term is restricted to less than a metre to a thousand metres with tentatively that the Pony Group was the the single body at the crest of the Ruby amphibolite, probably mostly diabase sills younger of the two groups. In general, the Range. originally, and metasedimentary rocks, rocks assigned to the Pony Group contain among which dolomitic marble, quartzite, more dark-colored gneisses and less mate- GEOLOGY OF THE and iron-formation are the most distinctive. rial of obvious sedimentary origin; but SAMPLE LOCALITIES The metamorphic grade ranges from green- McThenia (1960, p. 155), in his report on schist facies (rare) to granulite, but upper the Pony gneisses of the Madison River The general locations of the 13 analyzed amphibolite facies is dominant. Relatively canyon, noted that "all of the major rock samples, all of which are quartzo-feld- unmetamorphosed rocks assigned a later types of the Pony also occur in the Cherry spathic gneiss, are shown on the index map Precambrian (Precambrian Y) age occur lo- Creek." Beds of quartzite and of iron- (Fig. 1). More specific location data and cally at the north margin of the Tobacco formation are present in both. petrographic details are given in the Ap- Root Range, but in most of the area under The preserved metasedimentary rocks of pendix. discussion the older Precambrian rocks are the region undoubtedly reflect the earlier Of the samples from the Ruby Range, overlain directly by strata of Paleozoic and existence of widespread deposits of impres- two (1-69 and 12-69) are from the type area younger age. sive dimensions. Whether these remnants of the Dillon Granite Gneiss. In outcrop, represent one or several sequences cannot this rock is strongly foliated but relatively Stratigraphy be determined with the data at hand. The homogeneous on a scale of a few metres. simplest and most conservative interpreta- Two samples (13-69 and 14-69) are from a The mutual relations within and between tion is that the strata assigned to the Pony moderately thick (>50 m) sheet of granite the rocks of pre-Belt age in the several struc- and Cherry Creek Groups are parts of a gneiss interlayered with, and presumably tural blocks are far from being understood. single complex sequence. intrusive into, quartzite and dolomitic Formal stratigraphic names are few, and The third formal name in common use in marble on the western margin of the Ruby most of the general terms that have been in- the area is "Dillon Granite Gneiss," intro- Range. One sample (2-69) is from a thin sill troduced are of dubious validity and use- duced by Heinrich (1960, p. 16) as a desig- (< 25 m thick) in dolomitic marble of the fulness. In the Ruby, Tobacco Root, nation for a "thick intrusive mass of granite Cherry Creek Group. The structure and Gravelly, and Madison Ranges, three names gneiss" that occurs on the crest of the Ruby texture of this body is that of a tectonite. are in continued use: "Cherry Creek Range, separating rocks of the Cherry Creek Two samples (15-69 and 16-69) are from Group," "Pony Group," and "Dillon Group from those assigned to a "pre— an incompletely mapped area in the south- Granite Gneiss." The confusion surround- Cherry Creek group." Many smaller bodies western part of the range. We have not had ing use of these terms warrants a brief re- of similar granitic gneiss believed to be in- the opportunity to re-examine this area view. trusive into the Cherry Creek sequence were and, at present, are unable to advance a The term "Cherry Creek Series" (now also labeled "Dillon Granite Gneiss" by good explanation for the seemingly anoma- "Cherry Creek Group") was first applied by Heinrich. Subsequent, more detailed map- lous isotopic results. Petrographically the Peale (1896) to the predominantly meta- ping in the Ruby Range has shown that the rock is not significantly different from some sedimentary rocks exposed on the east flank relationships between the granitic gneiss facies of the Dillon Granite Gneiss. of the Gravelly Range in the vicinity of and the metasedimentary rocks of the area Three samples (43-72, 44-72, and 45-72) Cherry Creek. The sequence is charac- are complex (James and others, 1969; are from a single quarried outcrop in the terized by thick beds of dolomite. Perry Garihan, 1979). Even the main mass of Horse Creek area on the western flank of (1948) extended the term to the dolomite- granitic gneiss, designated by Heinrich, the Tobacco Root Range (Vitaliano and bearing terrane in the western part of the contains structurally concordant sheets of Cordua, 1979). The rock is a foliated Ruby Range, some 55 km distant and struc- dolomitic marble and other recognizable gneiss, layered on a scale of a metre or so, turally separate, on the intuitive assumption metasedimentary rock. Garihan and with compositions ranging from am- that the presence of dolomite marble was Okuma (1974) concluded that the granite phibolitic to granitic. As shown in the map definitive. A similar intuitive correlation gneiss is in fact a metamorphic rock, the published by Burger (1967), which covers

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the area immediately to the north, the ter- TABLE 1. Rb-Sr ANALYTICAL RESULTS rane consists of various gneisses that are in- 87 86t 87 86 +t terlayered and complexly folded with mar- Sample no. Rb Sr Rb /Sr Sr /Sr * Apparent age (ppm) (ppm) ble and other metasedimentary rocks. In older reports, these rocks were assigned to 1-69 159 81.5 5.779 0.9236 2,662 m.y. the Cherry Creek Group. Unpublished 2-69 121 143 2.469 0.7999 2,766 mapping in the Horse Creek area by 12-69 1.7* 203* 0.024* 0.7052 indeterminate 13-69 140* 67.1* 6.130* 0.9503 2,807 Cordua (1973) has shown that the layered 14-69 169* 58.6* 8.642* 1.0539 2,819 gneisses are cut by relatively undeformed 15-69 73.9 87.5 2.488 0.8883 5,114 mafic dikes that have been metamorphosed 16-69 79.1* 95.3* 2.335* 0.8620 4,696 to amphibolite. A complex thermal history 43-72 176 76.5 6.822 0.9606 2,630 for the sampled gneiss therefore is certain. 44-72 163 143 3.349 0.8331 2,724 45-72 152 73.2 6.165 0.9636 2,938 Mueller and Cordua (1976) concluded that 78-72 118 121 2.856 0.8205 2,887 the rocks have been affected by two princi- 79-72 126 150 2.451 0.8053 2,935 pal periods of metamorphism, the first to 80-72 124 210 1.720 0.7808 3,193 granulite facies and the second to upper + Determined by isotope dilution and have a precision of ± 1%, except those indicated by an amphibolite facies. asterisk which are by X-ray fluorescence and are precise only to ±2.5%. 87 88 Samples 78-72, 79-72, 80-72 are from * Normalized to a Sr /Sr of 0.1194. Precision is ±0.0003. tf 8T 88 1 the Gallatin River area, between the Galla- Assuming an initial Sr /Sr of 0.701. Decay constant: Xs = 1.42 x 10~" yr" . tin and Madison Ranges. The area has been studied by Spencer and Kozak (1975), who Mueller and Cordua (1976). Sixteen of the of sequences with an appreciable compo- showed the gneiss, with infolded quartzite, 18 samples make a reasonably linear array nent of original sediments, presents some to be continuous with that of the Madison on the isochron diagram. Two others special problems. Material derived from River canyon about 45 km to the northwest (sample nos. 15-69 and 16-69) plot far to different geologic terranes may be of differ- where McThenia (1960) referred the as- the left. ent ages and may have different initial semblage to the Pony Group. In general, the The dating of metasedimentary rocks, or ratios of Sr^/Sr86. Sediments generally con- gneisses range from leucocratic to am- phibolitic, and they are layered on a scale of metres or less. The sampled rocks span the transition reported by Giletti (1966, 1971) between apparent ages of about 1,600 m.y. on the west and those considerably older on the east, as determined by K-Ar and Rb-Sr analyses of biotite and muscovite. Our results confirm Giletti's conclusion that the younger mica ages are the result of a later thermal event. As is evident from Figure 1, the Pre- cambrian terrane that has been sampled is physically continuous, or nearly so, from the Ruby Range in the southwest, through the Tobacco Root and Madison Ranges, to the Gallatin Range on the east. Although the lithologic character of this terrane does differ from one part of the area to another, notably in the amount and nature of material of recognizable sedimentary origin (such as quartzite and dolomite), there re- mains a general homogeneity in structure, degree of metamorphism, and presence of quartzo-feldspathic gneiss as a major com- ponent.

DISCUSSION

The Rb-Sr data are given in Table 1 and presented on an isochron plot in Figure 2. Also shown on the isochron diagram are data from five samples from the Tobacco 87Rb/86Sr Root Mountains, recently published by Figure 2. Rb-Sr isochron diagram.

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tain appreciable contents of water which, The Precambrian record subsequent to boudins of amphibolite and hornblende during metamorphism, is an effective the Archean metamorphic event also is very gneiss. medium of diffusion of elements. The incompletely known. It is reflected only by 3. Intense deformation and metamor- water, however, undoubtedly moves and the presence of younger pegmatite and phism, in a multiphase event that included results in an open system; locally elements mafic intrusions, few of which have been as a minimum the following elements: (a) a may be added or removed, developing firmly dated. The pegmatites probably are first generation of tight folds, generally complex chemical patterns. For the pres- of several ages; most show some deforma- isoclinal and locally recumbent; (b) ent, we are considering the analyzed tion and probably were emplaced at a late metamorphism to granulite or upper am- samples to represent a single population de- tectonic or immediately post-tectonic stage phibolite facies, accompanied by syntec- spite probable differences in ultimate ori- of the major orogeny. However, scattered tonic emplacement of igneous bodies rang- gin. small bodies of undeformed pegmatite, ing in composition from tonalitic to grani- A best-fit line for the data (Table 1), with commonly tourmaline bearing, may be of tic; (c) a second generation of folding; (d) the exception of samples 15-69 and 16-69, much younger age. Muscovite from one local emplacement of mafic dikes, generally gives an age of 2,760 ±115 m.y. If the five such pegmatite has yielded a K-Ar age of as thin sheets; and (e) a second metamorph- samples from the Tobacco Root Mountains 1,660 m.y. (Giletti, 1966, sample 8), and it ism, to amphibolite facies. The ages ob- (Mueller and Cordua, 1976) are included in is possible that the emplacement of this tained for quartzo-feldspathic gneisses in the regression, an age of 2,730 ± 85 m.y. is pegmatite suite accompanied the pervasive this present study (about 2,750 m.y.) prob- obtained with an initial Sr87/Sr8a of 0.704. thermal event that has been shown by ably record the late or final stages in this These ages are considered to date the ap- Giletti to have affected the region about orogenic epoch. proximate time of metamorphism. 1,600 m.y. ago. The mafic dikes represent 4. A regional thermal event with an age The data show conclusively (1) that the an even more complex problem. One of about 1,600 m.y., which reset many of major stratigraphic sequences of the region group, known only from a few occurrences the isotopic clocks, notably that of K-Ar, in (Pony Group, Cherry Creek Group) are of in the Ruby and Tobacco Root Ranges, the region west of the Gallatin River. In Archean age, and (2) that these rocks were sharply truncates the structure of the gneis- general, this does not appear to have been a affected by dynamothermal metamorphism ses and metasedimentary strata, but the rock-forming event, although some small about 2,750 m.y. ago. This age corresponds rock itself is completely metamorphosed to bodies of undeformed pegmatite may have to the time of the major metamorphic and amphibolite. The age of this dike suite re- been emplaced at that time. structural event in the Beartooth orogeny mains unknown. Dikes of unmetamorph- 5. Long-continued regional uplift, with (Reid and others, 1975). osed (or only slightly metamorphosed) periodic emplacement of sharp-walled diabase are much more abundant, and ac- The data at hand do not provide a basis diabase dikes at intervals between 1,455 cording to Wooden and others (1978) three for discrimination of events prior to the m.y. and 1,120 m.y. ago. During this sets of dikes can be distinguished. The old- metamorphism about 2,750 m.y. ago. Re- period, clastic sediments were deposited on est set is believed to have been intruded gression of the Sii7/Sr86 ratios to mantle- the northern and western flanks of the about 1,455 m.y. ago, and the other sets type values (about 0.700 for this time broad regional uplift, presumably as a about 1,120 to 1,130 m.y. ago. period) suggests that the metasedimentary marginal facies in the depositional basin of rocks of the Pony and Cherry Creek Groups In summary, the Precambrian history of the Belt Supergroup of later Precambrian could not have existed earlier than 300 to the region is known only in fragmentary age. 400 m.y. prior to the metamorphic event. fashion, and that of the area under im- Such an interpretation requires that the mediate consideration (essentially from the APPENDIX. LOCATION AND samples remained closed systems during the Gallatin River west) cannot as yet be fully PETROGRAPHY OF SAMPLES metamorphism, and this has probably not integrated with the history of the more been the case. completely studied Beartooth Mountains to Ruby Range the east. In outline form, the known elements of the Precambrian record for the 1-69 Christensen Ranch Quadrangle (1:24,000). GEOLOGIC HISTORY From main mass of Dillon Granite Gneiss in Ruby, Tobacco Root, Gravelly, Madison, SWV. sec. 6, T. 8 S., R. 6 W., Madison The early history of southwestern Mon- and Gallatin Ranges are as follows: County, Montana. tana involved the deposition of one or more 1. Deposition of one or more thick se- Foliated gneiss. Irregular texture. Com- posed of perthitic K-feldspar (some grains thick sedimentary sequences. Further, in quences of sediments, now represented by showing microcline twinning), quartz (un- contrast to the greenstone-graywacke as- units of quartzite, dolomite, and iron- dulatory extinction), zoned oligoclase, sociation that typifies much of the Archean formation, and less certainly by micaceous minor muscovite, and accessory minerals. of the Canadian Shield, the deposits of this schists and various layered gneisses. The 2-69 Christensen Ranch quadrangle (1:24,000). region were, in considerable part, of shelf or age of this epoch of sedimentation is not Sill-like body of sheared gray gneiss in dolomitic marble, SWV» sec. 1, T. 8 S., R. 7 miogeosynclinal character, and recogniz- definitely known, but on the basis of Sr W., Madison County, Montana. able metavolcanic rock is rather scarce. Do- isotopic ratios and analogy with similar Strongly deformed rock. Long stringers lomitic marble and orthoquartzite form strata in the Beartooth Mountains, it is of quartz, granulated oligoclase, lesser units as much as several hundred metres probably 3,000 m.y. or more. microcline, minor muscovite, and pink garnet. Some alteration ot sericite and thick, and most of the thin but widespread 2. Widespread emplacement of mafic in- clinozoisite. beds of iron-formation lack a volcanic as- trusives, mainly as sills and dikes, now rep- 12-69 Christensen Ranch quadrangle sociation (Bayley and James, 1973). resented by sheets, lenticular bodies, and (1:24,000). From main body of Dillon

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Granite Gneiss, Hill 7092 in NEVi sec. 18, SE'A sec. 13, T. 6 S., R. 4 E., Gallatin adjacent quadrangles, Madison and T. 8 S., R. 6 W., Madison County, County, Montana. Beaverhead Counties, Montana: U.S. Montana. Light-colored gneiss. Fine-grained (0.1 to Geological Survey Open-File Map, scale Foliated rock, inequigranular texture. 0.3 mm), granular texture. Mostly nonper- 1:20,000. Composed dominantly of microcline (larger thitic microcline and quartz; rare un- McThenia, A. W., Jr., 1960, Geology of the grains are perthitic), lesser quartz and twinned plagioclase and perthite. Minor Madison River Canyon area north of Ennis, oligoclase, rare small flakes of muscovite. biotite, muscovite, clinozoisite. Montana, in Campan, D. E., and Anisgard, 13-69 Ashbough Canyon quadrangle 79-72 Portal Creek road, Garnet Mountain H. W., eds., West Yellowstone — Earth- (1:24,000). Sheet-like body of Dillon Gran- quadrangle (1:62,500). Outcrop in NW'A quake area: Billings Geological Society ite Gneiss on north side of Axes Canyon, in- sec. 13, T. 6 S., R. 4 E., Gallatin County, Guidebook, 11th Annual Field Conference, terlayered with quartzite and dolomitic Montana. p. 155-164. marble in SW'A sec. 24, T. 8 S., R. 8 W., Fine-grained gray gneiss. Granular tex- Mueller, R. A., and Cordua, W. S., 1976, Rb-Sr Beaverhead County, Montana. ture. Composed mainly of nonperthitic mi- whole rock age of gneisses from the Horse Foliated gneiss. Allotriomorphic granular crocline, quartz, and greenish-brown biot- Creek area, Tobacco Root Mountains, texture. Composed of andesine (marginally ite; minor oligoclase; rare muscovite. Ac- Montana: Isochron/West, no. 16, p. 33-36. zoned to albite), quartz, K-feldspar (un- cessory sphene, zircon, magnetite, apatite, Peale, A. C., 1896, Three Forks Folio, Montana: twinned, finely perthitic), green hornblende, 80-72 Garnet Mountain quadrangle (1:62,500). U.S. Geological Survey Folio 24. and dark-brown biotite; accessory garnet, Outcrop on U.S. Highway 191, near bridge Perry, E. S., 1948, Talc, graphite, vermiculite, magnetite, apatite, and allanite(?). over Gallatin River, in NW'A sec. 15, T. 5 and asbestos in Montana: Montana Bureau 14-69 Lighter-colored gneiss from same general S., R. 4 E., Gallatin County, Montana. of Mines and Geology Memoir 27, 44 p. location as sample 13-69. Foliated gray gneiss. Fine- to coarse- Reid, R. R., 1957, Bedrock geology of the north Strongly foliated, irregular texture. Com- grained, irregular texture. Mainly nonper- end of the Tobacco Root Mountains, Madi- posed of finely perthitic microcline, quartz, thitic microcline, quartz, biotite, and lesser son County, Montana: Montana Bureau of sericitized andesine, minor dark-brown oligoclase, accessory garnet, apatite, epi- Mines and Geology Memoir 36, 25 p. biotite, accessory apatite, magnetite, zircon, dote. Reid, R. R., McMannis, W. J., and Palmquist, and allanite(P). J. C., 1975, Precambrian geology of North 15-69 and 16-69 Timber Creek road, Ashbough Snowy Block, Beartooth Mountains, Mon- Canyon quadrangle (1:24,000). NEVi sec. REFERENCES CITED tana: Geological Society of America Special 6, T. 9 S., R. 7 W., Beaverhead County, Paper 157, 135 p. Montana. Bayley, R. W., and James, H. L., 1973, Pre- Spencer, E. W., and Kozak, S. J., 1975, Pre- Two samples from same outcrop of cambrian iron-formations of the United cambrian evolution of the Spanish Peaks layered coarse-grained granitic gneiss; States: Economic Geology, v. 68, p. 934- area, Montana: Geological Society of layers differ mainly in content of biotite. 959. America Bulletin, v. 86, p. 785-792. Texture irregular. General composition: Burger, H. R., Ill, 1967, Bedrock geology of the Tansley, W., Schafer, F. A., and Hart, L. H., oligoclase, quartz, slightly perthitic microc- Sheridan District, Madison County, Mon- 1933, A geological reconnaissance of the line, and red-brown biotite; minor garnet tana: Montana Bureau of Mines and Geol- Tobacco Root Mountains, Madison Coun- and muscovite; accessory zircon. ogy Memoir 41, 22 p. ty, Montana: Montana Bureau of Mines Cordua, W. S., 1973, Precambrian geology of the and Geology Memoir 9, 57 p. Tobacco Root Range southern Tobacco Root Mountains, Madi- Vitaliano, C. J., and Cordua, W. S., 1979, son County, Montana [Ph.D. dissert.]: Geologic map of the southern Tobacco 43-72, 44-72, 45-72 Horse Creek road, Sheri- Bloomington, Indiana University, 300 p. Root Mountains, Madison County, dan quadrangle (1:24,000). Quarried Garihan, J. M., 1979, Geology and structure of Montana: Geological Society of America roadside outcrop of layered gneiss in NW'A the central Ruby Range, Madison County, Map and Chart Series MC-31, scale sec. 7, T. 5 S., R. 4 W., Madison County, Montana: Geological Society of America 1:62,500. Montana. Bulletin, Part 1, v. 90, p. 323-326. Winchell, A. N., 1914, Mining districts of the 43-72 Light-colored layer. Irregular tex- Garihan, J. M., and Okuma, A. F., 1974, Field Dillon quadrangle, Montana: U.S. Geologi- ture. Dominantly microcline (larger grains evidence suggesting a non-igneous origin cal Survey Bulletin 574, 191 p. heavily perthitic) and quartz, minor oligo- for the Dillon quartzo-feldspathic gneiss, Wooden, J. L., and others, 1978, The late Pre- clase, scarce muscovite. Ruby Range, southwestern Montana: cambrian mafic dikes of the southern To- 44-72 Dark-colored layer. Irregular tex- Geological Society of America Abstracts bacco Root Mountains, Montana — ture. Dominantly oligoclase (70%), red- with Programs, v. 6. p. 510. Geochemistry, Rb-Sr geochronology and re- dish-brown biotite and quartz, minor Giletti, B. J., 1966, Isotopic ages from south- lationship to Belt tectonics: Canadian Jour- K-feldspar and garnet. western Montana: Journal of Geophysical nal of Earth Sciences, v. 15, p. 467—479. 45-72 Light-colored garnetiferous layer. Research, v. 71, p. 4029-4036. Texture extremely irregular. Composed 1971, Discordant isotopic ages and excess principally of K-feldspar (variably perthitic, argon in biotites: Earth and Planetary Sci- some grains with microcline twinning), ence Letters, v. 10, p. 157-164. quartz, pink garnet. Rare separate grains of Heinrich, E. W., 1960, Geology of the Ruby plagioclase; accessory magnetite and Mountains and nearby areas in southwest- sphene. ern Montana: Montana Bureau of Mines Geology Memoir 38, pt. 2, p. 16, 28, 38, Gallatin Canyon Area PI. 1. MANUSCRIPT RECEIVED BY THE SOCIETY APRIL James, H. L., Wier, K. L., and Shaw, K. W., 6, 1979 78-72 Portal Creek road, Garnet Mountain 1969, Map showing lithology of Precam- REVISED MANUSCRIPT RECEIVED JUNE 25, 1979 quadrangle (1:62,500). Layered gneiss in brian rocks in the Christensen Ranch and MANUSCRIPT ACCEPTED JUNE 27, 1979

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