Correlations between Dikes of the Monument Swarm, Central , and Picture Gorge Basalt Flows

JONATHAN S. FRUCHTER* Center for Volcanology, University of Oregon, Eugene, Oregon 97403 STUART F. BALDWIN

ABSTRACT INTRODUCTION

Samples from 39 dikes and 11 other intrusive bodies of the Monument Swarm Monument dike swarm in central Oregon were analyzed by in- strumental neutron activation analysis. All but two of the dikes This study of the Monument dike swarm (Waters, 1961; Wilcox have distinctive trace element compositions that clearly identify and Fisher, 1966; Figs. 1 and 2 here) is based on intensive sampling them as possible sources for the basalt flows at the Picture Gorge of the dikes and irregular sills mapped by Wilcox and Fisher (1966) type section and equivalent or stratigraphically lower flows else- near Kimberly and Monument as well as of scattered specimens ob- where in the region. The correlations are supported by tained from dikes in other areas now considered part of the paleomagnetic polarity measurements and by major element Monument swarm. Thayer (1957) included dikes located in the Al- analyses of nine of the samples. Key words: geochemistry, stratig- drich Mountains, and other dikes have been located by Thayer, raphy, trace element analyses, lava, intrusion, igneous rocks, chem- P. T. Robinson, and us (a list of sample locations is in the Society's ical analysis, volcanology. repository1). The dike swarm was considered a source for basalt of the Co- lumbia River Group as early as 1901 (Merriam, 1901). Because of its proximity, it has generally been regarded as the source of the Picture Gorge Basalt, although little evidence has been provided to clearly exclude other units of the Columbia River Group. To clarify this problem, the composition and natural remanent magnetism (NRM) of many of these dikes and other related intrusive bodies were studied in order to correlate them with known flow se- quences. Abundances of certain trace elements prove especially use- ful in providing characteristic "fingerprints" for the main chemical types (Osawa and Goles, 1970; Nathan and Fruchter, 1974).

Chemical Types in the Columbia River Group

The chemical and informal stratigraphic nomenclature proposed by Wright and others (1973) and amplified by Nathan and Fruch- ter (1974) is used in this paper. Wright and others (1973) defined several chemical types of Columbia River Basalt on the basis of major element composition and related them in a general way to several informal stratigraphic units they devised. Nathan and Fruchter (1974) showed that chemical types princi- pally associated with the lower, middle, and upper Yakima strati- graphic units of Wright and others (1973) can be clearly distin- guished from Picture Gorge types, using the elements La, Sm, Th, Fe, Sc, and Cr. High-Mg and low-Mg Picture Gorge chemical types are distinguished by well-defined differences in Cr and La. Sm and Fe are also useful in distinguishing high- and low-Mg Picture Gorge types, but the smaller separation of their ranges renders them slightly less diagnostic.

Results

Compositions of 50 samples from dikes and other intrusive bodies are given in Table 1. Table 2 lists major element analyses of nine representative dikes. In Figure 3, assignments are illustrated by comparing La and Cr contents of the dikes with those of the known chemical types re-

1 Copies of GSA supplementary material 75-7 may be ordered from Documents Sec- * Present address: Battelle, Pacific Northwest Laboratories, Richland, retary, Geological Society of America, 3300 Penrose Place, Boulder, 80301, 99352. USA.

Geological Society of America Bulletin, v. 86, p. 514-516, 3 figs., April 1975, Doc. no. 50409.

514

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/4/514/3443944/i0016-7606-86-4-514.pdf by guest on 26 September 2021 CORRELATIONS BETWEEN DIKES OF MONUMENT SWARM AND PICTURE GORGE BASALT FLOWS 515

Figure 2. Map of area of comprehensive sampling in Monument dike swarm between towns of Kimberly and Monument, Oregon. (R) indicates reverse polarity.

TABLE 1. ANALYSES OF SAMPLES FROM MONUMENT DIKE SWARM, CENTRAL OREGON* lated to the Picture Gorge stratigraphic sequence. These two trace

Sample* Fe Sc Cr Co La Sm Lu Na Hf Th La/Sm elements show the greatest differences in abundances between the («) (*) Picture Gorge and Yakima types. All other compositional data confirm those assignments (see Table 1 and Nathan and Fruchter, D-1 8.5 45 173 40 8.2 4.3 0.52 1.9 D-2 8.1 46 172 39 8.6 4.5 0.45 2.22 1.9 1974). D-3A 8.2 43 173 40 7.5 3.6 0.47 2.20 2.1 D-3B 8.2 45 171 40 7.3 3.7 0.48 2.0 As a precaution against the possibility that trace element data D-4 7.8 37 179 43 5.5 2.9 0.36 2.01 1.9 were not sufficient to make valid identifications, major element D-5 10.4 44 56 40 10.2 4.4 0.70 2.25 2.3 D-6A 10.1 43 28 35 11.4 4.6 0.71 2.26 2.3 analyses were done on nine of the samples (Table 2). The analyses D-6B 10.1 43 30 35 11.1 4.0 0.66 2.23 2.5 D-7A 8.4 42 198 44 6.6 3.6 0.44 2.18 2.1 confirmed the assignments made on the basis of trace elements; D-7B 8.2 40 191 44 6.5 3.4 0.40 2.09 1.9 they fit well within the compositional boundaries for Picture Gorge D-8 8.0 39 176 44 6.3 3.2 0.41 1.98 2.0 D-9 8.4 36 127 35 7.2 3.9 1.8 chemical types defined by Wright and others (1973). D-10 10.0 44 33 39 10.5 4.4 0.67 2.23 2.4 D-12 7.9 41 163 39 7.5 3.0 0.43 1.99 2.5 D-13 10.1 42 58 40 10.3 4.4 0.62 2.22 2.3 DISCUSSION OF CHEMICAL RESULTS D-14 10.8 45 <30 46 11.2 4.6 0.69 2.21 2.4 D-15 8.8 40 164 42 6.9 3.6 0.51 1.9 D-16 6.5 34 241 46 2.5 1.6 0.33 1.50 1.6 D-17 6.3 34 231 44 2.5 1.5 0.29 1.45 1.7 All but two dikes have compositions identifying them as belong- D-18 8.1 43 116 37 9.5 4.0 0.56 2.3 ing to either high- or low-Mg Picture Gorge chemical types. The D-19 7.6 41 155 42 6.1 2.8 0.45 1.80 2.2 D-20 8.3 43 86 38 8.3 3.8 0.51 2.19 2.2 possibility that a few Yakima-type dikes may exist in the swarm is 0-21 8.8 45 179 42 8.3 4.0 0.51 2.24 2.1 D-23 8.4 40 181 43 6.3 3.5 0.39 1.8 not precluded, but such dikes certainly constitute no more than a D-24 8.3 42 171 41 8.5 4.0 0.47 1.9 minor component of the swarm. D-25 8.3 43 77 40 8.6 3.7 0.49 2.06 2 8 1.0 2.3 D-26 9.2 43 105 42 9.5 4.1 0.57 2.13 3 0 1.3 2.3 The total range of composition of flows at and near the type sec- D-27 8.0 41 80 8.1 3.6 0.45 2.07 2 6 2.3 D-28 11.0 46 30 36 11.6 0.70 1.6 tion of the Picture Gorge Basalt is found in the dikes, suggesting D-29 8.6 42 168 46 7.5 3.6 0.44 2.07 2 5 0.6 2.1 that these Picture Gorge flows could all have originated in the D-30 9.0 47 163 43 7.7 3.7 0.50 2.17 2 4 0.9 2.1 D-31 8.9 44 187 44 8.4 3.6 0.47 2.01 2 6 0.8 2.1 Monument swarm. D-32 10.1 44 30 41 10.4 4.3 0.71 2.00 2 7 2.4 Dike samples D-16 and D-17 (Table 1) are compositionally un- D-33 10.2 44 30 40 10.6 4.4 0.67 1.92 3 0 1.4 2.4 D-34 9.7 44 75 43 11.3 4.5 0.62 2.15 3 2 1.7 2.5 like any of the chemical types associated with the Columbia River D-35 8.9 44 198 47 6.7 3.7 0.41 2.11 2 3 1.8 D-36 8.5 44 152 43 7.2 3.4 0.45 2.05 2 4 0.9 2.1 D-37 11.0 45 <30 39 11.7 4.8 0.74 2.17 3 9 1.6 2.4 TABLE 2. XRF MAJOR ELEMENT ANALYSES OF SAMPLES FROM MONUMENT D-38 10.9 44 <30 40 10.8 4.7 0.73 1.85 2 8 1.2 2.3 DIKE SWARM, CENTRAL OREGON D-39 9.7 42 <30 41 13.8 5.3 0.66 2.27 4 4 1.9 2.6 1-1 8.7 42 158 41 8.5 2.0 4.3 0.42 D-3 D-6 0-8 D-14 D-16 D-18 D-26 D-35 D-37 1-2 8.6 39 156 45 6.2 3.4 0.37 1.8 1-3 8.9 42 163 43 7.6 4.0 0.38 1.9 1-4 8.7 41 173 44 7.5 3.8 0.46 2.0 Si02 50.85 51.89 48.10 50.89 47.67 50.74 50.71 47.90 50.36 1-5 8.3 41 193 45 6.3 3.3 0.38 1.9 TiO; 1.54 2.16 1.53 2.23 0.79 1.48 1.58 1.55 1.90 1-7 8.4 42 181 43 6.7 3.7 0.40 1.8 Al 2O3 14.93 13.66 15.78 13.39 17.57 15.34 14.53 15.62 13.37 1-8 8.2 39 187 43 6.0 3.4 0.40 1.8 MgO 6.12 4.33 7.17 4.41 9.12 6.06 5.46 6.74 4.59 1-9 8.2 39 196 45 5.7 3.1 0.37 1.8 FeiOs* 11.94 15.20 12.04 15.90 9.46 11.92 13.03 11.90 16.87 I-10A 8.4 41 186 44 7.1 3.7 0.41 1.9 MnO 0.22 0.25 0.19 0.25 0.15 0.19 0.30 0.19 0.24 r-ios 8.7 42 188 45 7.1 3.8 0.42 1.9 CaO 10.72 8.92 10.89 9.05 12.57 10.60 9.80 10.46 8.08 i-n 8.2 46 116 39 8.1 3.8 0.47 2.08 2 0 0.8 2.1 Na20 2.96 3.05 2.67 2.98 2.02 2.79 2.87 2.84 2.92 K20 0.66 0.86 0.48 0.85 0.15 0.65 0.78 0.48 0.72 Note: Relative errors: 1 a = 1 percent Na; 3 percent Fe, Sc, Cr, Co, La, Sm, Lu, P20< 0.24 0.38 0.22 0.35 0.06 0.20 0.23 0.24 0.33 Hf; 5 percent Th. L.I.+ 0.86 0.80 1.79 0.87 1.43 2.01 1.55 0.91 1.13 Determined by instrumental neutron activation analysis using a modified version Total 101.04 161.50 100.36 101.18 101.00 101.98 100.84 H3 100.63 of the technique of Gordon and others (1968). * In ppm, except as noted. * All Fe as Fe20j. t D denotes dike samples; I denotes samples from other intrusive bodies. f Loss on ignition.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/4/514/3443944/i0016-7606-86-4-514.pdf by guest on 26 September 2021 516 FRUCHTER AND BALDWIN

Cr (ppm) Sample La (ppm) type section, this section provides a reasonable representation of 100 150 200 0-3 the magnetostratigraphy of the Picture Gorge sequence. D-B 0-10 Figure 3 shows that the dike polarities are in good agreement D-13 with the stratigraphie assignments. Dike samples with high-Mg Pic- 0-14 0-28 ture Gorge affinities are normal in polarity, whereas some of those 0-32 0-33 with low-Mg Picture Gorge affinities are normal and some are re- D-34 versed. This is the pattern at the type section. The reversal in the |itj| 0-37 0-38 sequence further narrows the stratigraphie assignments within the 0-39 lip low-Mg Picture Gorge group. It is likely that because of their nor- mal polarity, D-10 and D-14 are older than D-5, D-6, and D-13. mm * 0-2 D-3 0-4 CONCLUSIONS D-7 0-8 0-12 D-13 The chemical and paleomagnetic data reported here confirm cor- 0-19 0-21 relations made previously between dikes of the Monument swarm ••PM1k D-23 and Picture Gorge flows. The previous correlations were made on D-24 0-29 the bases of stratigraphie relations, geographic proximity, pétro- 0-30 Jil D-31 graphie characteristics, and a few chemical analyses (Thayer, 1957; ••BsBp D-33 Waters, 1961; Wright and others, 1973). It is probable that the 0-36 t-t Monument swarm is the major and possibly the only source of the JBS 1-2 Picture Gorge basalt. 1-3 1-4 m 1-5 mi 1-7 1-8 ACKNOWLEDGMENTS 1-9 •B I-10 WÊÊÈÊÊÊ I- II This work was supported by National Science Foundation Grant •f D-9 GA-33181X, G. G. Goles, principal investigator. S. Nathan pro- • • D-18 D-20 vided invaluable suggestions in the field and laboratory; P. T. 0-25 D-26 Robinson pointed out the location of several dikes; and D. A. 0-27 Swanson, T. L. Wright, J. A. Vance, S. Nathan, G. G. Goles, R. V. • Fisher, and V. R. Uppuluri reviewed the manuscript. REFERENCES CITED Figure 3. Graphic representation of Cr and La abundances in Monu- ment dike samples together with ranges of these elements in low-Mg (light Gordon, G. E., Randle, K., Goles, G. G., Corliss, J. B., Beeson, M. H., and stipple) and high-Mg (heavy stipple) Picture Gorge Basalt. R denotes re- Oxley, S. S., 1968, Instrumental activation analysis of standard rocks versed remanent magnetism as determined by a Model 70 portable fluxgate with high resolution x-ray detectors: Geochim. et Cosmochim. Acta, magnetometer manufactured by California Electronic Manufacturing v. 32, p. 369-396. Company. Merriam, J. C., 1901, A contribution to the of the John Day basin: California Univ. Dept. Geol. Sci. Bull., v. 2, p. 269-314. Group. Two samples from the Flat Creek section (Thayer and Nathan, S., and Fruchter, J. S., 1974, Geochemical and paleomagnetic Brown, 1966) have a major element composition somewhat similar stratigraphy of the Columbia River Group in central Oregon with par- to these two dikes (D. Swanson, 1973, personal commun.), and a ticular reference to the relationship between the Picture Gorge and pair of flows about 120 km to the southwest on Oregon State Yakima basalts: Geol. Soc. America Bull., v. 85, p. 63-76. Osawa, M., and Goles, G. G., 1970, Trace element abundances in Colum- Highway 27, 16 km north of the junction with U.S. Highway 20, bia River basalts: Columbia River Basalt Symposium, 2d, Cheney, have a composition almost identical to D-16 and D-17 (Osawa and Washington, 1969, Proc.: Cheney, Eastern Washington State College Goles, 1970; Uppuluri, 1973). Press, p. 173-175. Dike samples D-9, D-18, D-20, D-25, D-26, and D-27 are clearly Thayer, T. P., 1957, Some relations of later Tertiary volcanology and struc- Picture Gorge type in overall chemical composition, but their trace ture in eastern Oregon: Internat. Geol. Cong., 20th, Mexico, D. F. element compositions do not allow easy identification as to high- or 1956, Sec. I: Vulcanologia del Cenozoico, v. 1, p. 231-245. low-Mg types. Major element analyses of D-18, D-25, D-26, and Thayer, T. P., and Brown, C. E., 1966, Local thickening of basalts and late D-27 helped in assigning them to the high-Mg type and presumably Tertiary silicic volcanism in the Canyon City quadrangle, northeastern would do so for D-9 and D-20 as well. Oregon: U.S. Geol. Survey Prof. Paper 550-C, p. C73-C78. Uppuluri, V. R., 1973, A stratigraphie and compositional study of basalts of the Columbia River Group near Prineville, central Oregon [M.S. PALEOMAGNETIC POLARITIES thesis]: Eugene, Univ. Oregon, 87 p. Waters, A. C., 1961, Stratigraphie and lithologie variations in the Colum- Valid assignment of the dikes to various positions in the strati- bia River basalt: Am. Jour. Sci., v. 259, p. 283-311. graphic sequence on the basis of chemical composition must not Watkins, N. D., and Baksi, A. K., 1974, Magnetostratigraphy of the Co- conflict with the paleomagnetic stratigraphy observed in the flows. lumbia Plateau basalts in Oregon, Washington, and Idaho: Am. Jour. Watkins and Baksi (1974) and Nathan and Fruchter (1974) deter- Sci., v. 274, p. 148-189. mined the paleomagnetic stratigraphy at the type section and dem- Wilcox, R. E., and Fisher, R. V., 1966, Geologic map of the Monument quadrangle, Grant County, Oregon: U.S. Geol. Survey Geol. Quad. onstrated that the top four flows (all of low-Mg Picture Gorge Map GQ-541. composition) are reversed and that flows in the lower part of the sec- Wright, T. L., Grolier, M. J., and Swanson, D. A., 1973, Chemical varia- tion (all of the high-Mg flows and the lowermost flows of low-Mg tion related to the stratigraphy of the Columbia River basalt: Geol. Picture Gorge composition) are normal. Soc. America Bull., v. 84, p. 371-386. Nathan and Fruchter (1974) informally extended the paleomagnetic stratigraphy to several nearby sections, using a portable fluxgate magnetometer. Since the only reversal found so MANUSCRIPT RECEIVED BY THE SOCIETY MAY 28, 1974 far in the Picture Gorge sequence is that equivalent to the one at the REVISED MANUSCRIPT RECEIVED AUGUST 30, 1974

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/4/514/3443944/i0016-7606-86-4-514.pdf by guest on 26 September 2021