F. K. MILLER ] E. H. McKEE | U.S. Geological Survey, Menlo Park, California 94025 R. G. YATES J

Age and Correlation of the Windermere Group in Northeastern Washington

ABSTRACT and altered. The determination of a radio- metric age for the group is important, how- Greenstone of basaltic composition forms ever, for a number of reasons: (1) the Winder- the middle part of the Windermere System in mere unconformably overlies the Belt Super- southern British Columbia and the correlative group, or correlatives of the Belt Supergroup, Windermere Group in northeastern Washing- and therefore establishes an upper age limit ton. The greenstone, together with the rest of for Belt sedimentation; (2) because the the Windermere in this region, is highly Windermere rocks are indicators of the East sheared, altered, and metamorphosed, except Kootenay orogeny (White, 1959), they provide for a small exposed mass of relatively unaffected an upper age limit for at least part of that rocks near the community of Chewelah, period of crustal unrest; (3) conglomerate Washington. K-Ar ages on whole rocks and (diamictite) that forms the base of the Winder- mineral separates from the Chewelah area mere has been proposed by Aalto (1971) to be indicate the greenstone was probably extruded of glacial origin. The age obtained for the between 827 and 918 m.y. ago. overlying greenstone establishes an upper age limit for that proposed glaciation, or if the INTRODUCTION greenstone was extruded during the proposed Greenstone (basalt) of latest Precambrian glacial period, an age for the glaciation; and Y age (1,600 to 800 m.y.; James, 1973) crops (4) the Windermere was deposited during an out extensively in northeastern Washington. It interval near the end of Precambrian time that overlies a thick diamictite (Aalto, 1971) and is represented by gaps in the geologic record in conglomerate unit and is overlain by the Monk many sections around the world. A precise Formation, also of Precambrian age. Collec- date, therefore, provides another reference tively, these three units make up the United point for part of the radiometric and paleo- States equivalent of the Canadian Windermere magnetic time scales where few data are System, which, in southern British Columbia, available. occupies the stratigraphic position between In this paper, the distinction between con- the Precambrian Purcell System (Belt Super- glomerate and diamictite will not be made. It group in the U.S.) and the earliest Cambrian is important to note that much of the con- rocks (Reesor, 1957). Canadian geologists have glomerate in the Windermere is diamictite of referred to the Windermere rocks as the glacial origin, and that rocks of Precambrian Windermere Series (Walker, 1926, p. 13-20) age at other localities in western North America and the Windermere System (Reesor, 1957). and in other parts of the world contain In this report, the Windermere is extended into diamictite of demonstrated glacial origin Washington and Idaho, where it will be called (Harland, 1964). However, not all of the the Windermere Group. conglomerate in the Windermere is diamictite, Radiometric dates on the Windermere in and the sedimentology of these rocks is beyond the United States have not been attempted the scope of this paper. Therefore, these rocks before, because the rock generally is both will be called only conglomerate in the dynamically and thermally metamorphosed remainder of this paper.

Geological Society of America Bulletin, v. 84, p. 3723-3730, 4 figs., November 1973 3723

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WINDERMERE GROUP IN THE Irene Volcanics in Canada (Little, 1960). The UNITED STATES upper Windermere unit, the Monk Formation, retains its name throughout the length of the In the United States, the Windermere belt within the United States but is called Group is exposed in a narrow belt about 130 either Monk Formation or the Horsethief km (80 mi) long that strikes about N. 35° E. Creek Group in Canada. and is nowhere more than about 13 km (8 mi) This belt of Precambrian rocks and the wide (Fig. 1). The southeastern edge of the Paleozoic rocks that overlie it have been belt is limited by erosion, and the northwestern sheared, complexly faulted, and locally tightly limit is buried beneath younger rocks. folded. They have also been regionally meta- morphosed to the greenschist fades and thermally metamorphosed at many places by Mesozoic and Cenozoic plutons. As a result, much of the conglomerate that makes up the lower part of the Huckleberry Formation has been metamorphased almost to a phyllite, and clasts in the Shedroof Con- glomerate are stretched and boudined. The volcanic rocks throughout the Windermere belt are highly chloritized and s.t many places are cut by numerous closely spaced shear zones. About 13 km (8 mi) north of the community of Chewelah, however, an exposed mass of Windermere several square kilometers in area appears to have escaped much of the deforma- tion and alteration that affects these rocks at other localities. Six samples of greenstone collected near the center of this mass yield whole-rock K-Ar ages ranging from 233 to 851 m.y.; plagioclase and pyroxene mineral separates from these rocks yield ages ranging from 603 to 918 m.y. The Huckleberry Formation in the vicinity of rhe collected samples is made up entirely of greenstone; the conglomerate, which may Figure 1. Geologic sketch map showing location of dated samples and relation of the Windermere Group be as much as 1,000 m thick about 16 km (10 in northeastern Washington to surrounding rocks. mi) to the southwest, thins stratigraphically to zero or near zero thickness here. Because the lower contact of the greenstone is a fault, it is Different names have been applied to cor- also possible that some conglomerate has been relative formations within the Windermere tectonically removed. Group at several locations along the belt (Fig. Flows, flow breccia, and minor beds of light- 2). In the southwestern part of the belt, con- colored tuff make up the greenstone in this glomerate and volcanic rocks are included in area. Petrographically, much of the rock a single unit, the Huckleberry Formation. The resembles the volcanic breccia and aquagene conglomerate makes up an informally de- tuff described by Carlisle (1963) in British signated lower member, and the volcanic Columbia. Volcanic breccia makes up about rocks an upper member (Campbell and 80 percent of the section where the samples Loofbourow, 1962). In the northeast corner were collected, and fine-grained to aphanitic of Washington, the conglomerate is known as dark-green flows of basalt make up about 20 the Shedroof Conglomerate and the volcanic percent. Although the racks are midlly rocks are called the Leola Volcanics (Park and chloritized and locally epidotized, the primary Cannon, 1943). These two formations con- igneous textures in many places are perfectly tinue north of the international boundary but preserved. In several of the dated specimens, are known as the Toby Conglomerate and the most of the pyroxene crystals (augite, average

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sw NE

WASHINGTON BRITISH COLUMBIA

Horsethief Creek Group -EOu T TobyConglomerate^

Purcell System

Toby Conglomerate

Figure 2. Diagrammatic cross section along strike ceal the relations between the Toby, Monk-Horsethief of the Windermere belt showing stratigraphic relations Creek, and Shedroof Conglomerates. No horizontal or within the Windermere. Mesozoic plutonic rocks con- vertical scale.

Ny = 1.686, average 2V = 54°) are completely jacent pieces of a small block (about 5 g) of unaltered, although plagioclase reflects a low basalt. Good reproducibility in replicate grade of metamorphism in that it is much potassium analyses on each sample indicates more sodic (Ans±2) than would be expected that the analyzed pieces are representative in a normal basalt. parts of a homogeneous sample. The same Individual flows and flow-breccia units are reproducibility was obtained for replicate difficult to trace laterally owing to an almost analyses on mineral separates. Argon analyses 100 percent cover of lichen on the rocks and to were made using standard isotope-dilution the irregular contacts between flows and techniques in a Nier-type, 6-in.-radius, 60°- flow-breccia units. The few attitudes obtained sector mass spectrometer. The potassium indicate that the flows are nearly horizontal analyses were by flame photometer using a or dip shallowly to the northwest. The green- lithium internal standard. The ± values were stone here is unconformably overlain by con- assigned on the basis of experience with glomerate, argillite, and dolomite of the Monk duplicate analyses, and they represent the Formation, and is locally intruded by the additive effects of uncertainties in the argon 98-m.y.-old Starvation Flat Quartz Monzonite and potassium analyses, in the isotopic com- (Miller and Clark, 1973). The dated specimens position and concentration of the Ar38 tracers, were collected about 1.6 km (1 mi) from sur- and in the concentration of the flame pho- face exposures of this pluton (see Fig. 3). tometer standards. The variation in the calculated ages (Table K-Ar Age of the Greenstone from the 2) appears to be the result primarily of dif- Windermere Group ferences in the degree and type of alteration of The dated samples (Table 1) were selected the samples and of the grain size. because of their relatively low degree of altera- Sample 1. Plagioclase is highly altered; tion, and because they were representative of almost every grain contains large amounts of the flow rocks found in the formation. In thin fine-grained chlorite. Most pyroxene shows section, the texture of the samples resembles slight to moderate darkening or discoloration, the texture of "fresh" basalt of late Tertiary especially around the edges of crystals. This is age. In all of them, however, the plagioclase is the coarsest grained sample dated; the average albite, chlorite is present, and the hand groundmass grain size is about 0.2 mm. The specimens have a definite green cast. Table 1 rock contains a few plagioclase and pyroxene gives the calculated ages and the analytical phenocrysts that average about 0.5 mm in data from which the calculations were made. length. For each whole-rock sample, analyses for Sample 2. Plagioclase and pyroxene in this both potassium and argon were made on ad- sample are relatively unaltered. Many of the

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EXPLANATION II7°45'

Figure 3. Geologic sketch map showing locations and regional setting of dated samples.

plagioclase crystals are almost free of any size is about 0.1, and the rock contains no fine-grained chlorite. A small amount of phenocrysts. pyroxene shows some discoloration, but most Sample 4. Both plagioclase and pyroxene is fresh and clear. The grain size is only slightly are slightly more altered than in samples 2 and less than that of sample 1, with average ground- 3. There is a fair amount of intergranular, fine- mass size just under 0.2 mm. This sample also grained, semi-opaque altered material that contains minor plagioclase and pyroxene may have been glass originally. Groundmass phenocrysts that average about 0.4 mm in grain size averages about 0.2 mm, and sparse length. plagioclase and pyroxene phenocrysts average Sample 3. Both plagioclase and pyroxene about 0.4 mm. are relatively unaltered. The average grain Sample 5. Plagioclase is moderately altered

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TABLE 1. ANALYTICAL DATA FOR DATED SAMPLES FROM THE WINDERMERE GROUP*

lH) Sample Material K20 Ar rad Radiogenic Ar Calculated no.t (») (mol/g) (%) age (m.y.)

( Basalt 0.060 9.53 x 10-10 67 851 + 45 I Plagioclase 0.133 1.60 x ÎO-1" 92 678 + 34

2 Basalt 0.104 1.59 x 10-11 63 827 ± 34

3 Basalt 0.114 1.45 x 10-" 75 724 ± 29

( Basalt 0.112 8.87 x 10-11 69 472 ± 24 4 < Plagioclase 0.119 2.08 x 10"11 51 918 ± 37 t Pyroxene 0.040 4.24 x 10"" 66 603 ± 30

5 Basalt 0.093 6.23 x TO"11 30 407 ± 20

6 Basalt 0.753 2.76 x 10"10 92 233 ± 12

* See text for detailed explanation. t Sample numbers are the same as those shown in Figure 3. Constants used: AB = 4.72 x 10~la/yr, Xe - 0.585 x 10~l yr, and K^^/K total *> 1.19 x 10u moles/mole.

and contains a fair amount of chlorite. Pyrox- parison of modal analyses suggest that the ene is only slightly altered. The texture is alternation of the rocks is probably the cause. uniform and fine grained. Average grain size Three types of alteration are recognized: is 0.03 to 0.08 mm. Sparse phenocrysts averrge (1) albitization of the plagioclase, (2) chloritiza- between 0.2 and 0.7 mm. This is the finest tion involving plagioclase and other parts of grained sample dated. the rock, and (3) alteration of intergranular Sample 6. Plagioclase is very altered and material producing an unidentified very fine the pyroxene is almost completely unaltered. grained semi-opaque product probably made Over-all, the sample is by far the most altered up of several different minerals. of those dated. The grain size averages about The anomalous albitic composition of the 0.2 mm. The rock is nonporphyritic. plagioclase appears to be due to low-grade meta- The geologic framework of the samples is morphism and is probably not indicative of a well established, and the bracket in the geologic spilitic suite. Analyses of ten basaltic rocks time scale into which their age must fall is from the Huckleberry Formation have an reasonably clear cut. The youngest dates on average Na20 content of about 2 percent, less Belt Supergroup sedimentation are <900 m.y. than half that of an average spilite. Sample (Obradovich and Peterman, 1968). The Monk 2 of the dated rocks is an exception, however, Formation in the vicinity of the dated samples and has a Na2Û content of 4.7 percent. The is unconformably overlain by the Lower Cam- significance of this high Na20 in a single brian Addy Quartzite; the currently accepted specimen is not known, but it appears to be age for the base of the Cambrian is 570 m.y. an anomaly and is not representative of the (Holmes, 1964). The age of the basalt should, suite. therefore, fall between 570 and about 900 m.y. Because determinations on three whole Three of the whole-rock dates and three on rocks and a plagioclase separate have yielded mineral separates are within this bracket, and three whole-rock dates are obviously too young. TABLE 2. MODAL COMPOSITION OF GREENSTONE SAMPLES Although there is a considerable spread in the FROM THE WINDERMERE GROUP ages, even among the dates falling in the Sample number acceptable bracket, several of the lower 1 2 3 4 5 6

values were obtained early in the study. We Plagioclase 11 17 18 17 7 4 are now confident that, with careful selection Pyroxene 3 26 28 15 25 22 Opaque minerals 2 7 6 2 5 11 of specimens having a minimum of alteration, Chlorite 3 12 9 11 6 12 Chlorite within plagioclase crystals 28 18 21 23 30 45 we can consistently obtain ages between 800 Carbonate minerals tr. 4 4 1 1 tr. and 900 m.y. on basalt from the Huckleberry Epidote 9 1 tr. 4 3 tr. Slightly altered pyroxene 32 4 5 6 5 3 Formation. We consider the original cooling Unidentified alteration products 12 11 9 21 18 3 age of the rock to be probably between 825 Number of points counted 1,200 900 900 900 900 900 and 900 m.y. Apparent age (m.y.) 851 827 724 472 407 233 We do not fully understand why some rocks Note: See text for descriptions of individual samples have yielded ages that are obviously too young, that emphasize the relative degree of alteration and the grain size. although thin section examination and com-

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Precambrian ages, either the albitization of 900 the plagioclase occurred not long after the basalt was extruded or the albitization process o did not appreciably affect the K/Ar ratios in o the rocks. Whereas the albitization process is 800 pervasive throughout all of the basalt, the development of the other two types of altera- tion is considerably variable on a local scale. 700 This observation, in combination with the fact that samples 2, 3, and 4 were collected within 50 m of one another and all yielded different ages, suggests that the argon loss 600 causing the younger ages is not caused by the albitization, but by the other types of altera- tion. The variability in argon retention on ^ 500 such a local scale also appears to rule out the later heating by the Starvation Flat Quartz Monzonite as a cause for the argon loss. Figure ^ 400 k 4 indicates the rather inexact, though sug- gestive, correlation between calculated age and degree to which the two later types of alteration are developed. ^ 300

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ACKNOWLEDGMENTS Little, H., 1960, Nelson map area, west half, British Columbia: Canada Geol. Survey Mem. We are grateful to M. D. Crittenden and 308, 205 p. Z. E. Peterman for many enlightening dis- Miller, F. K., and Clark, L. D., 1973, Geology of cussions on the Windermere and its equivalents the Chewelah-Loon Lake area, Stevens and in other parts of North America. Spokane Counties, Washington: U.S. Geol. Survey Prof. Paper (in press). Obradovich, J. D., and Peterman, Z. E., 1968, REFERENCES CITED Geochronology of the Belt Series, Montana: Aalto, K. R., 1971, Glacial marine sedimentation Canadian Jour. Earth Sci., v. 5, p. 737-747. and stratigraphy of the Toby Conglomerate Park, C. F., and Cannon, R. S., 1943, Geology and (upper Proterozoic) southeastern British ore deposits of the Metaline quadrangle, Columbia, northwestern Idaho, and north- Washington: U.S. Geol. Survey Prof. Paper eastern Washington: Canadian Jour. Earth 202, 81 p. Sci., v. 8, p. 753-787. Reesor, J. E., 1957, The Proterozoic of the cordil- Campbell, Ian, and Loofbourow, J. S., 1962, leran in southeastern British Columbia and Geology of the magnesite belt of Stevens southwestern Alberta: Royal Soc. Canada County, Washington: U.S. Geol. Survey Bull. Spec. Pub., no. 2, p. 150-177. 1142—F, p. F1-F53. Walker, J. F., 1926, Geology and mineral deposits Carlisle, D., 1963, Pillow breccias and their of Windermere map-area, British Columbia: aquagene tuffs, Quadra Island, British Colum- Canada, Geol. Survey Mem. 148. bia: Jour. Geology, v. 71, no. 1, p. 48-71. White, W. H., 1959, Cordilleran tectonics in Harland, W. B., 1964, Evidence of Late Precam- British Columbia: Am. Assoc. Petroleum brian glaciation and its significance, in Nairn, Geologists Bull., v. 43, no. 1, p. 60-100. A.E.M., Problems in paleoclimatology: Lon- don, Interscience Pubs., Inc., p. 119-149. Holmes, A., 1964, Principles of geology (2d ed.): New York, Ronald Press, p. 360-361. James, H. L., 1973, Subdivision of the Precambrian: MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY An interim scheme to be used by the U.S. 9, 1973 Geological Survey: Am. Assoc. Petroleum PUBLICATION AUTHORIZED BY THE DIRECTOR, U.S. Geologists Bull, (in press). GEOLOGICAL SURVEY

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