Preliminary Pétrographie, Chemical, and Age Data on Some Intrusive and Associated Contact Metamorphic Rocks, Pioneer Mountains, Southwestern Montana

E-AN ZEN U.S. Geological Survey, Reston, Virginia 22092 R. F. MARVIN H. H. MEHNERT U.S. Geological Survey, Denver, Colorado 80225

ABSTRACT method. Table 1 gives the K-Ar ages, analytical data, and locations; Table 2 gives chemical, normative, and modal analyses of the Petrographic and chemical data on five types of intrusive rocks of rocks; Table 3 gives the refractive indices of the minerals used. A the so-called Pioneer in the northeastern Pioneer Moun- brief description of each sample is given in Appendix l1. The rock tains of southwest Montana show that the rocks range from names used accord with the classification recommended by the In- diorite to . The volumetrically most important body is a ternational Union of Geological Sciences (Streckeisen, 1973), al- coarse-grained biotite-hornblende "granite" that superficially re- though this system may not always be in agreement with common sembles the Butte Quartz of the Boulder batholith, usage of names for the Boulder batholith, to which comparison

some 60 km to the northeast. Their KzO content puts the rocks into must be made. Tilling's "sodic series" of the Boulder batholith. K-Ar dating on Large samples of the five intrusive rocks, 3 to 5 kg each, were biotite and hornblende shows that the date of intrusion of all but quartered, and aliquots were chemically analyzed by the rapid rock the is about 70 m.y. The hornblende age of the quartz diorite indicates that it was intruded 76.5 m.y. ago. Its bio- tite age of 70.5 m.y. agrees with those of the younger intrusions. High-grade contact metamorphic rocks of the Silver Hill Forma- tion (Cambrian) at one locality yielded the same 70-m.y. age on biotite porphyroblasts. These ages compare closely with those of the Boulder and Philipsburg and show the same ten- dency for the less mafic rocks to be younger. Key words: geo- chronology, igneous and metamorphic petrology, absolute age, igneous rocks.

INTRODUCTION

The batholith in the Pioneer Mountains of southwestern Mon- tana has been studied little. Because the Pioneer batholith (a term here used informally) is midway between the Boulder and batholiths (Fig. 1), its relation to either of these larger intrusive complexes is of considerable interest. This interest is enhanced by recent attempts to interpret batholithic intrusions in the space-time frame of plate tectonics and by economic potentials of mineral de- posits in or near the plutons of the Pioneer batholith.

The northeastern part of the Pioneer batholith in the Vipond 50 KILOMETERS Park 15' quadrangle (Fig. 1) is now being mapped by Zen; R.I. Til-

ling participated earlier. The southeastern part of the batholith ii: EXPLANATION the Twin Adams, Torrey Mountain, and Ermont 7V2' quadrangles • • Other rocks was mapped by reconnaissance by Myers (1952). Zen and Tilling Tertiary Upper Cretaceous found that the Pioneer batholith is a composite body containing no intrusive rocks intrusive rocks fewer than about eight recognizable types of rocks, showing, Figure 1. Index map of southwestern Montana and adjacent Idaho, among other features, different degrees of alteration. Petrographi- showing Pioneer batholith (PM), Philipsburg batholith (Pb), Boulder cally, these types range from coarse pyroxenite, gabbro, and quartz batholith (Bd), Tobacco Root batholith (TR), McCarthy Mountain stock diorite to granite. Volumetrically, the most important are several (MM), and part of Idaho batholith (Id). Tertiary intrusive rocks (T) are plutons that, according to the classification of Streckeisen (1973), shown separately. Based on Tectonic Map of the United States (U.S. are referred to as quartz monzonite to granite. Geological Survey and American Association of Petroleum Geologists, 1961). Quadrangles: 1, Vipond Park; 2, Torrey Mountain; 3, Twin Adams; 4, Ermont. DATA

Biotite and (or) hornblende separated from five intrusive rocks 1 Copies of GSA supplementary material 75-5 may be ordered from Documents Sec- and two contact metamorphic rocks were dated by the K-Ar retary, Geological Society of America, 3300 Penrose Place, Boulder, Colorado 80301.

Geological Society of America Bulletin, v. 86, p. 367-370, 3 figs., March 1975, Doc. no. 50312.

367

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method (Table 2). Table 2 also gives the results of point counting of quadrangle, the Kootenai Formation (Lower Cretaceous) has been thin sections; 2,000 points were counted for each rock. For sample contact metamorphosed to cordierite-andalusite-biotite gneiss by BHS, two sections were counted because of the large crystal size. the intrusive body sampled at Ivanhoe Pit. Thus, the high The normative minerals calculated from the chemical analyses are metamorphic grade of the Silver Hill Formation at Hecla is consis- also given. Figure 2 gives the plot of various oxide contents and tent with a single event of contact metamorphism. For the two oxide ratios against silica; these are the same coordinates used by dated samples from Hecla, the nearest exposure of intrusive rocks Tilling (1973) for the Boulder batholith and facilitate comparison is about 1.5 km to the west-southwest (sample DR), but the inter- with his results. vening rocks are locally of lower metamorphic grade. Samples The mineral separates used in the isotopic determinations are all better than 99 percent pure (probably better than 99.5 percent for the hornblende samples) and are free of visible alteration products. TABLE 2. DATA ON THE FIVE DATED INTRUSIVE ROCKS OF THE PIONEER BATHOLITH, MONTANA X-ray diffractograms give single mineral patterns only. Some opti- DR BH9850 BHS IVP BC cal data measured on these minerals are given in Table 3. Chenrical Data

S102 52.4 64.7 72.8 65.7 69.5 K-AR AGES A120, 18.2 16.4 15.1 16.2 16.2 Fe20j 5.1 2.1 0.86 2.8 1.6 FeO 5.4 2.9 0.76 2.1 1.3 Field mapping established that the quartz diorite (sample DR) MgO 3.9 1.8 0.43 1.5 0.62 occurs as inclusions in the porphyritic granodiorite (sample BH CaO 8.7 4.2 2.2 4.5 3.7 Na20 3.0 3,3 3.1 3.0 3.8 KiO 1.3 2.8 3.5 2.6 2.7 9850), which in turn is cut by apophyses of the granite (sample + H20 1.0 0.75 0.96 0.80 0.79 BHS). This intrusive order is not contradicted by the K-Ar ages, al- H20" 0.07 0.04 0.14 0.12 0.15 0.26 though the age difference between the granodiorite and granite is T102 1.2 0.59 0.20 0.42 P20s 0.35 0.14 0.18 0.21 0.16 not resolved by the isotopic ages. The biotite age of DR agrees, MnO 0.11 0.10 0.08 0.08 0.08 0.02 <0.05 0.02 0.02 0.04 within analytical error, with those for the other two rocks of the C02 Total 101 100 1ÖÖ 100 101 sequence, but the hornblende is significantly older. We think that the biotite of DR lost some radiogenic argon by reheating during Modal Minerals Quartz 4.3 22.8 34.1 26.0 28.5 intrusion of one or both of the younger rocks, but the hornblende Plagloclase 64.9 39.9 31.5 51.0 47.1 of DR retained all or most of its radiogenic argon so that its K-Ar K- 0.0 17.9 29.0 4.3 17.3 Hornblende 17.2 2.3 0.0 4.3 0.3 age more nearly indicates the time of intrusion of the quartz diorite. Biotite 7.7 15.4 4.3 7.8 4.5 Chlorite 0.0 0.0 0.3 1.2 0.9 This interpretation accords with the findings of Hart (1964) and Sphene 0.4 0.6 0.0 0.8 0.3 Kistler and others (1965) for similarly discordant K-Ar ages of Others* 5.6 1.4 0.8 4.7 1.1 hornblende-biotite pairs. The close agreement of ages given by Name"*" Quartz Granodiorite Granite Tonalité Granodiorite diorite coexisting hornblende and biotite for both BH 9850 and IVP sug- gests that these ages closely approximate the time of intrusion. Normative Minerals Q 6.5 22.3 36.3 26.6 28.2 The two biotite K-Ar ages for the Silver Hill Formation at Hecla Or 7.6 16.6 20.6 15.4 15.8 Ab 25.2 28.0 26.2 25.4 31.9 are nearly identical and agree with the ages of the granodiorite, An 32.1 20.0 9.6 20.8 16.9 Uo 3.5 0.0 0.0 0.0 0.0 granite, and tonalite. Rocks of the Silver Hill Formation at Hecla En 9.6 4.5 1.1 3.7 1.5 have been multiply deformed, and the pelitic members locally show Fs 3.9 2.8 0.50 1.0 0.80 Mt 7.3 3.1 1.2 4.1 2.3 a strong schistosity, suggesting, at first, a regional rather than a 11 2.3 1.1 0.38 0.8 0.49 Ap 0.82 0.33 0.43 0.5 0.38 contact deformational episode. Such a regional episode, if it occurred, Cc 0.045 0.0 0.045 0.46 0.09 cannot be proven by our data. Elsewhere in the Vipond Park C 0.0 0.64 2.7 0.82 0.77 Thornton and Tuttle differentiation TABLE 1. K-AR AGES, ANALYTICAL DATA, AND GEOGRAPHIC LOCATION Index 39 67 83 67 76 OF SAMPLES FROM VIPOND PARK QUADRANGLE, MONTANA * Indicates orthlte, opaques, ep1dote-d1nozo1s1te, apatite, zircon, white mica, Rock type. Location Analyzed K,0 Ar'"* Ar"* Age (m.y.) myrmeklte. t Names according to IUGS system (Streckelsen, 1973). field no. (lat N., minerals (%) (moles/g) total Ar« ±2a long W.)

11 Quartz diorite 45°35'36" Biotite 8.93 9.496 x 10" 0.95 71.0 ± 2.4 TABLE 3. REFRACTIVE INDICES OF BIOTITE AND HORNBLENDE DR 112°57'10" 8.87 USED IN K-AR ISOTOPIC DETERMINATIONS 10 Hornblende 0.887 1.021 x IO" 0.86 76.5 ± 2.1 0.887 1.028 x 10"'° 0.65 77.0 ± 2.2+ Sample Other observations Porphyritic 45°35'18" Biotite 9.40 9.888 X IO"1' 0.90 69.9 ± 2.4 granodiorite 112°56'55" 9.42 9.888 X IO"" a1 = deep bluish green K 1 = 1.679 + 0.002 1 s BH 9850 DR hornblende x = olive yellow n i 1.663 ± 0.001 ¡¡•he = 18° Hornblende 0.915 0.9243 x 10-" 0.81 67.2 ± 1.9 X • 2r(-) - 70° 0.915 Coarse granite 45°35'14" Biotite 9.30 9.886 x IO"11 0.90 70.6 ± 2.4 BHS 112°56'55" 9.32 DR biotite n 1.654 ± 0.002 1 1 s = bluish green Tonalite 45°31 15" Biotite 9.12 9.712 x 10-" 0.94 71.0 ± 2.7 „ 1.674 t 0.002 x' = olive green IVP 112°50'20" 9.08 3 = BH 9850 hornblende 1.663 ± 0.002 a'Ao = 22° Hornblende 0.627 0.6405 x 10"" 0.80 68.0 ± 1.9 V 2P(-) - 70° 0.627 BH 9850 biotite n 3 1.651 ± 0.001 Porphyritic 45°25'00" Biotite 8.77 9.049 X 10"" 0.93 68.8 ± 2.3 s BHS b1ot1it1tet n 1.650 ± 0.001 granodiorite 112°51'12" 8.73 1 z = olive green BC \ » 1.658 ± 0.002 x' = straw yellow Biotite gneiss 45°36'29" Biotite 9.45 9.962 x 10"'" 0.95 0 ± 2.4 IVP hornblende a + a'Ac » 18° s 1.654 0.001 287-1 112°55'20" 9.50 2V{-) ~ 80° Biotite gneiss Biotite 9.43 10.04 x 10"" 0.91 5 ± 2.4 45°36'29" IVP biotite n = 1.649 ± 0.001 9.51 s 287-2 112°55'20" BC b1ot1te n - 1.649 ± 0.001 287-1 biotite5 n = 1.624 ± 0.002 Note: Constants: K"'X = 0.585 x 10-"/yr 5 287-2 biotite n » 1.646 ± 0.003 Xg =• 4.72 x lO-'Vyr 1 0 K * = 1.19 x 10"' atomic abundance. * All Indices measured at 24°C with Na light. Analysts: R. F. Marvin and H. H. Mehnert; Violet Merritt (potassium ( t All hornblende indices measured on grains (1) lying on an excellent cleavage, (2) having inclined extinction, and (3) showing off-center optical axis figure. try on biotites); Lois Schlocker (potassium chemistry on hornblendes). These fragments therefore He on the [110] cleavage, and defined Indices are ob- * Radiogenic argon. tained. t Argon content redetermined and age recalculated. 5 Only the Indices 1n the cleavage planes are measured.

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287-1 and 287-2 were collected on the flank of a structural dome ple BHS, but the K-Ar age, 49 m.y., is much younger. This age that coincides with the metamorphic high; the data suggest that the could conceivably be a hybrid age resulting from some Eocene high-grade contact metamorphism reflects a near-surface but un- thermal event. The ages do not contradict the possibility that part exposed pluton. of the Idaho batholith is of the same age as the Boulder and the Tilling and others (1968) made a geochronologic study of the Pioneer batholiths. plutonic rocks forming the Boulder batholith. These intrusions Giletti (1966) determined two biotite K-Ar ages for quartz mon- cover a time span from about 78 m.y. to about 68 m.y. ago. The zonite of the Tobacco Root batholith, Montana. These are 75 and mafic rocks have the oldest average age (76 m.y.), the granodiorites 52 m.y.; the former age is closely comparable to that on similar average about 75 m.y., the Butte Quartz Monzonite averages about rocks of the Boulder batholith. Brumbaugh (1974) reported an age 73 m.y., and the siliceous and leucocratic rocks have the youngest of 71 m.y. for a quartz-monzonite stock at McCarthy Mountain ages. Our much more limited data show the same general trend, (Fig. 1). Hyndman and others (1972) published K-Ar ages on two and within the limits of uncertainties of the data, our ages for pet- pairs of coexisting biotite and hornblende from the Philipsburg rographically comparable rocks correspond closely with or are batholith, Montana. The Philipsburg area is structurally on trend slightly younger than those of the Boulder batholith; Tilling and with the Pioneer Mountains, and the sedimentary rocks are of simi- others (1968) showed that similar intrusive rocks in the southern lar facies. The biotite ages (73.4 and 74.0 m.y.) and hornblende part of the Boulder batholith tend to be younger than the main ages (72.0 and 76.7 m.y.) are comparable with but slightly older bodies. than our results. Whether the difference is significant is as yet un- The Idaho batholith is a complex intrusive body whose internal certain. geologic relations remain to be determined. McDowell and Kulp (1969) showed that available ages, mainly K-Ar on biotite, range CHEMICAL DATA from 156 to 38 m.y., and that these ages fall into distinct groups. In hand specimen, the "coarse quartz monzonite" represented by Tilling (1973) suggested that the rocks of the Boulder batholith sample L-1121 (McDowell and Kulp, 1969) is very similar to sam- represent two magma series, the "main series" and the "sodic series"; the latter series is found predominantly in the southwestern part of the composite batholith (Tilling, 1973, Fig. 17), toward the

60 64 68 Plag K-feldspar Si0 ,IN WEIGHT PERCENT 2 Figure 3. Plots of selected chemical, normative, and modal data for five

Figure 2. Alkali versus silica plots of five intrusive rocks of Pioneer intrusive rocks of Pioneer batholith. A, K20-Na20-Ca0 plot and compari- batholith, and comparison with Boulder batholith (from Tilling, 1973). son with Boulder batholith (from Tilling, 1973); B, normative An-Ab-Or Rocks belong to sodic series only on plots involving K20; on Na20 plot plot based on chemical data of Table 2; C, modal -potassium they more nearly belong to main series. feldspar-quartz plot based on 2,000 points for each rock.

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maverick and has no counterpart, insofar as the chemical data go, were made in laboratories of the U.S. Geological Survey under the in the Boulder batholith. On a Na20-Si02 plot (Fig. 2A), our data, supervision of L. F. Shapiro. We thank J. S. Huebner, Priestley except for BC, fall in the field of Tilling's main series; BC falls in the Toulmin III, D. R. Wones, and especially R. I. Tilling for discus- area of his sodic series. However, on K20-Si02 and K20/(K20+ sion of the petrology of the rocks and for help in packing out sam- Na20)-Si02 plots, which are the primary bases of Tilling's ples. R. I. Tilling, John P. Wehrenberg, and R. E. Zartman re- classification (1973; 1974, written commun.), our analyses fall in viewed the manuscript. We are grateful for their helpful comments. the field of his sodic series. Thus the rocks of the Pioneer batholith, similar to those of the Boulder batholith, are "sodic" by virtue of REFERENCES CITED their very low K20 content. Sample BHS is of a coarse-grained granite that, according to field Brumbaugh, D. S., 1974, Geometry and movement of the McCarthy Moun- evidence, makes up the largest pluton in the nothern Pioneer tain structural salient, southwestern Montana: Geol. Soc. America, Mountains. Despite the hand specimen resemblance between this Abs. with Programs (Rocky Mountain Sec.), v. 6, no. 5, p. 430. Giletti, B. J., 1966, Isotopic ages from southwestern Montana: Jour. rock and the Butte Quartz Monzonite, a comparison of the chemical Geophys. Research, v. 71, p. 4029-4036. analyses for BHS with individual analyses for the Boulder batholith Hart, S. R., 1964, The petrology and isotopic-mineral age relations of a (R.I. Tilling, 1974, personal commun.) shows that chemically they contact zone in the Front Range, Colorado: Jour. Geology, v. 72, p. are not very close. The analysis of BHS shows more silica (for the 493-525. Butte Quartz Monzonite, Si02 content is about 65 percent) but less Hyndman, D. W., Obradovich, J. D., and Ehinger, Robert, 1972, CaO, MgO, Ti02 and iron oxide. BHS resembles rocks of the sodic Potassium-argon age determinations of the Philipsburg batholith: series, such as the Moose Creek and Moosetown plutons, most Geol. Soc. America Bull., v. 83, p. 473-474. closely. These plutons are also spatially closer to the Pioneer Karlstrom, T.N.V., 1948, Geology and ore deposits of the Hecla mining batholith than are most of the other plutons of the Boulder district, Beaverhead County, Montana: Montana Bur. Mines and batholith. Geology Mem. 25, 87 p. Kistler, R. W., Bateman, P. C., Brannock, W. W., 1965, Isotopic ages of The ternary diagram (Fig. 3A) compares the KsO, Na20, and minerals from granitic rocks of the central Sierra Nevada and Inyo CaO contents of our rocks with Tilling's data. The points, with the Mountains, California: Geol. Soc. America Bull., v. 76, p. 155-164. exception of DR, fall nicely in the field of the sodic series; Tilling McDowell, F. W., and Kulp, J. L., 1969, Potassium-argon dating of the (1973) pointed out that this diagram best brings out the distinction Idaho batholith: Geol. Soc. America Bull., v. 80, p. 2379-2382. between his two series. Figure 3B gives the normative An-Ab-Or Moore, J. G., 1959, The quartz diorite boundary line in the western United plots, and Figure 3C gives the modal plagioclase-potassium States: Jour. Geology, v. 67, p. 198-210. feldspar-quartz contents. The apparent lack of correlation be- Myers, W. B., 1952, Geology and mineral deposits of the northwest quarter tween the modal data and those from the Boulder batholith proba- Willis quadrangle and adjacent Brown's Lake area, Beaverhead bly merely reflects the need for many more analyses before any County, Montana: U.S. Geol. Survey Open-file Rept. 147, 46 p. Streckeisen, A. L., 1973, Plutonic rocks: Classification and nomenclature trend can be established. On the whole, the modal data do show recommended by the IUGS Subcommission on the Systematics of that our rocks have greater affinity to the sodic series than to the Igneous Rocks: Geotimes, v. 18, no. 10, p. 26-30. main series. Sample DR, which has no modal potassium feldspar, Tilling, R. I., 1973, Boulder batholith, Montana: A product of two con- again is an exception because Tilling's plot (1973, Fig. 16) indicates temporaneous but chemically distinct magma series: Geol. Soc. that all the rocks of the Boulder batholith contain some potassium America Bull., v. 84, p. 3879-3900. feldspar. Tilling, R. I., Klepper, M. R., and Obradovich, J. D., 1968, K-Ar ages and Occurrence of the quartz diorite DR is interesting also because it time span of emplacement of the Boulder batholith, Montana: Am. Jour. Sci., v. 266, p. 671-689. is east of the "quartz diorite line" proposed by Moore (1959). U.S. Geological Survey and American Association of Petroleum Geologists, However, on the basis of field evidence, the intrusive body is small, 1961, Tectonic map of the United States, exclusive of Alaska and so its occurrence does not contradict Moore's location of the line. Hawaii: Washington, D.C., U.S. Govt. Printing Office, 2 sheets, scale 1:2,500,000 [1962], ACKNOWLEDGMENTS

We are indebted to Cristina S. Zen for the excellent mineral MANUSCRIPT RECEIVED BY THE SOCIETY MAY 20, 1974 separations used in isotopic determinations. The chemical analyses REVISED MANUSCRIPT RECEIVED AUGUST 19, 1974

Primed in U.S.A.

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