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Age of the Basement Rocks of Southwest Montana

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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. 11 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/91/1/11/3433957/i0016-7606-91-1-11.pdf by guest on 02 October 2021 12 JAMES AND HEDGE 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/91/1/11/3433957/i0016-7606-91-1-11.pdf by guest on 02 October 2021 AGE OF BASEMENT ROCKS, SOUTHWEST MONTANA 13 the area immediately to the north, the ter- TABLE 1.
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