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Age Determinations on Some Rocks from the Boulder Batholith and Other Batholiths of Western Montana

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Age Determinations on Some Rocks from the Boulder Batholith and Other Batholiths of Western Montana BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. ee. PP. 607-610 MAY 1955 Short Notes AGE DETERMINATIONS ON SOME ROCKS FROM THE BOULDER BATHOLITH AND OTHER BATHOLITHS OF WESTERN MONTANA BY RANDOLPH W. CHAPMAN, DAVID GOTTFRIED, AND CLAUDE L. WARING Introduction monzonite, granite, alaskite, aplite, and pegma- tite; quartz monzonite is by far the most This paper presents some new age data abundant rock type. The relative ages of all recently obtained on rocks from the batholiths these types have not been determined, although in western Montana. The Boulder batholith, the alaskite, aplite, and pegmatite cut the a large composite intrusion composed mainly others. of quartz monzonite, is on the Continental The role of the Boulder batholith in the Divide and covers an area of about 1200 square geologic history of western Montana is fairly miles. The Philipsburg batholith is a much clear. In Late Cretaceous time a thick series of smaller and simpler intrusion of quartz mon- andesitic pyroclastics and flows was extruded zonite about 50 miles west of the center of over a wide area. Plant remains of probable the Boulder batholith. The Idaho batholith Judith River age (Roland Brown, Personal is a huge composite body of granitic rocks, the communication), just slightly older than middle eastern edge of which is about 100 miles west Late Cretaceous, have been found in pyroclastic of the center of the Boulder batholith. Most of beds high in this thick series. Near the close of the age data presented here were obtained from the Cretaceous the pyroclastics and flows were the Boulder batholith, and the age of this body folded and faulted by the Laramide deforma- is now reasonably evident. The data from the tion. Following this, either in very Late Cre- Philipsburg and Idaho batholiths suggest taceous (Knopf, 1913, p. 34) or very early certain time relationships between these bodies Tertiary time, the plutonic rocks of the Boulder and the Boulder batholith, but more informa- batholith formed by one or more processes of tion is needed before positive conclusions can uncertain nature. These plutonic rocks in- be drawn. truded volcanics and older rocks and exten- This study is part of a program undertaken sively contact metamorphosed them. Consider- by the U. S. Geological Survey on behalf of ably later, a variety of fairly siliceous volcanics the Division of Research of the Atomic Energy was extruded. The oldest of these are basal Commission. tuffs with early Oligocene fossils; these tuffs Montis R. Klepper supplied much of the rest with marked unconformity on the eroded geologic information used in dating the Boulder plutonic rocks of the batholith. In part con- batholith. George H. Hayfield helped to sepa- temporaneous with the tuffs and in part some- rate the minerals and to concentrate and purify what later, perhaps in Oligocene or Miocene the radioactive zircon and monazite. Esper S. time, flows and tuffs of trachyte, rhyolite, and Larsen, Jr., Howard W. Jaffe, Lorin R. Stieff, dacite were extruded onto a topography that and Thomas W. Stern offered valuable assist- had been cut on the batholith, probably during ance and advice. Eocene time. From the above stratigraphic and structural Nature and History of the Boulder Batholith relations, it can be concluded that the Boulder The Boulder batholith is a composite pluton batholith is later than middle Late Cretaceous consisting of quartz diorite, granodiorite, quartz and pre-early Oligocene. 607 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/66/5/607/3431734/i0016-7606-66-5-607.pdf by guest on 25 September 2021 608 CHAPMAN ET A L.—BOULDER BATHOLITH Method of Age Determination The accuracy of the alpha-activity measure- ments is believed to be ±5 per cent, and the The method described by Larsen et al. accuracy of the lead analyses 6-10 per cent. (1952) was used to determine the age of the rocks. This method is based on lead-alpha Preparation and Processing of Samples activity ratios in suitably radioactive accessory minerals, principally zircon and monazite. Chapman collected and processed the rock In both zircon and monazite the lead present types. Fifty-pound samples of each were TABLE 1.—AGE DETERMINATIONS ON MINERALS FROM BOULDER, PHILIPSBURG, AND IDAHO BATHOLITHS, MONTANA Counts in Lead Age Number Rock Mineral a/mg/hr (ppm) (MY) BOULDER BATHOLITH S2-C-45-Z Quartz monzonite Zircon 227 8 69 S2-C-60-Z Quartz monzonite Zircon 203 6 71 52-C-10a-Z Quartz monzonite Zircon 160 4.6 69 S2-C-8-Z Alaskite Zircon 4990 127 61 S2-C-8-M Alaskite Monazite 6S45 231 72 PHTLIPSBUKG BATHOLITH S3-C-198-Z Quartz monzonite Zircon 858 18 50 IDAHO BATHOLITH 53-C-210-Z Quartz monzonite Zircon 275 6.2 54 53-C-210-M Quartz monzonite Monazite 3123 79 51 52-C-45 Medium-grained porphyritic hornblende-biotite quartz monzonite with large phenocrysts of pink potash feldspar, from the quarry 1J£ miles west of Boulder, Montana 52-C-60 Medium- to fine-grained hornblende-biotite quartz monzonite with more ferromagnesian minerals than in 52-C-45, from a road cut about 3 miles northeast of Elk Park, Montana 52-C-10a Medium-grained quartz monzonite with about 30 per cent biotite and hornblende, from the border of the batholith about 7 miles southeast of Helena, Montana 52-C-8 Medium-grained alaskite taken half a mile southwest of the summit of Elkhorn Peak, Montana 53-C-198 Medium-grained biotite-hornblende quartz monzonite from the Philipsburg batholith, 1 mile east of Philipsburg, Montana 53-C-210 Medium-grained white gneissoid quartz monzonite from the border zone of the Idaho batholith in Lost Horse Creek, Bitterroot Mountains, Montana is believed to be essentially radiogenic. This crushed and sized, and the radioactive acces- has been convincingly borne out by geologic sories were concentrated and purified by evidence and, where possible, by comparison heavy liquids, the Frantz isodynamic mag- with isotopically determined ages (unpublished netic separator, and hand picking. Gottfried data by E. S. Larsen, Jr.). made an alpha count of each purified concen- The alpha activity was first determined for trate, and Waring determined spectrographi- each radioactive accessory by a thick^source cally the amount of lead in each concentrate. alpha-counting technique, and the lead content The age of each mineral was then calculated was then measured by a spectrographic method from the lead-alpha activity ratio. The results developed by Waring and Worthing (1953). are shown in Table 1. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/66/5/607/3431734/i0016-7606-66-5-607.pdf by guest on 25 September 2021 SHORT NOTES 609 Age Relationships of the Batholiths total lead would give an age of 84 million years, or an error of approximately 22 per cent. The ages of five rock types from the Boulder This indicates that the presence of original batholith were determined. These gave an lead in the sample is negligible; otherwise a average of 68 million years and suggest that considerably greater age would have been the batholith was emplaced at or near the close obtained. Monazite is known to contain small of the Cretaceous. All five determinations fall amounts of original lead, approximately 1 to within ±10 per cent of the average, a very 3 per cent of the total lead. In the monazite close conformance for rocks so young. However, sample containing 231 ppm of lead (52-C-8-M), as the results are all within the limit of error of the effect of original lead is negligible. The very the lead-alpha activity method, no conclusions close agreement of the monazite and zircon can be drawn as to the relative ages of the five ages in one case and the rather close agreement rock types. in the other indicate the reliability of age de- The age of 50 million years determined for terminations of monazite by this method. the Philipsburg batholith suggests that this Monazite determinations seem particularly pluton is somewhat younger than the Boulder suitable for rocks containing zircon with less batholith. However, great significance should than 10 ppm lead and alpha activity of less not be attached to this one determination. than 100 a/mg/hr. A white gneissoid quartz monzonite from A worthwhile analogy can be drawn between the Idaho batholith yielded both zircon and the Boulder batholith and the pitchblende monazite that gave ages of 54 and 51 million deposits of the Colorado Front Range. The years, respectively, a remarkably close agree- pitchblende deposits are believed to have ment for two minerals with such different lead formed during the Laramide orogeny. Although contents. Another determination, previously generally assigned to the end of the Cretaceous, made by Howard W. Jaffe on a monazite from these deposits are believed by many geologists the same gneissoid quartz monzonite taken to be late Paleocene (Holmes, 1947, p. 140). farther west along Lost Horse Creek, gave an On the basis of isotopically corrected lead and age of 72 million years. These data suggest uranium ratios, Holmes (1947) has assigned that this rock is at least as young as, if not an age of 58 million years to these deposits. somewhat younger than, the rocks of the The results obtained by the lead-alpha activity Boulder batholith—i.e., very Late Cretaceous method on zircon and monazite from these or even early Tertiary. Age determinations batholiths show close agreement with age previously made on certain other rocks from determinations by the lead-uranium method the Idaho batholith, however, gave an average on bodies of about the same geologic age. age of 100 million years (unpublished data by Larsen, Gottfried, Jaffe), and it seems reason- References Cited ably clear that the bulk of this enormous body Holmes, Arthur, 1947, The construction of a is probably mid-Cretaceous.
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