Late Miocene and Early Pliocene Basaltic Rocks and Their Implications for Crustal Structure, Northeastern California and South-Central Oregon

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Late Miocene and early Pliocene basaltic rocks and their implications for crustal structure, northeastern California and south-central Oregon

WENDELL A.CDUFFIELD ) U S' Geo,°S'cal Survey, Menlo Park, California 94025 ROBERT J. STERN University of Texas at Dallas, Richardson, Texas 75085

ABSTRACT and (4) to compare these lavas with basaltic (Iocs. 11, 16, 29, Fig. 2), and future work lavas from various tectonic settings else- may identify other outliers, but the bulk of A dominantly basaltic late Miocene and where in the world. Such comparisons may the lava field appears to lie north of the Pit early Pliocene (about 5 to 10 m.y. old) lava help to interpret the tectonic setting of the River. A similar interpretation of the distri- field lies directly east of the Cascade Range study area, which occupies a zone that is bution of volcanic rocks in this region has on the northwesternmost edge of the Basin transitional between the Cascade Range to been presented by Luedke and Smith and Range province. More than 12,400 km2 the west and the Basin and Range province (1981a, 1981b); the outline of a 5- to of Modoc County, California, and Klamath to the east. lO-m.y.-old basalt unit on their maps de- and Lake Counties, Oregon, termed the fines a volcanic field nearly identical to the Devils Garden, is underlain by basalt with a AREA AND VOLUME OF BASALT Devils Garden lava field as described here. 3 total volume of about 850 km . OF DEVILS GARDEN The east margin of the field is well defined The basalt of Devils Garden is diktytax- by older, upfaulted, and tilted rocks of the itic olivine tholeiite, characterized by high Basalt lava flows form a plateau north- Warner Range and Abert Rim; the west A1 content; low K, Rb, and Cs content; and west of Alturas, California (Ford and margin, although less well defined, is mark- high K/Rb and K/Cs ratios. In these others, 1963, p. 57). This plateau and adja- ed by nearly conformable onlap of younger respects, it resembles mid-oceanic-ridge cent areas to the north in Oregon are volcanic rocks in the Medicine Lake and basalt (MORB), although it differs in other known locally as the Devils Garden (see Klamath Falls areas. The area of the basalt respects, such as high concentration of Ba U.S. Geological Survey Klamath Falls, of Devils Garden that is buried by these and Sr, low K/Ba, and higher 87Sr/86Sr Oregon and California, 2° sheet, and younger lavas is unknown, but we estimate 2 (0.7036-0.7039) ratios. Chemical character- Figs. 1, 2), a name which we adopt here as that a minimum of 12,400 km is now istics indicate that little or no contamina- an informal name for the basalt. The best exposed. tion by sialic crustal material has occurred. exposures of this basalt are along the south- Along its south margin, the lava field is a The basalts originated in the upper mantle eastern part of the plateau north of Altu- fairly constant 30 m thick. In the more and were erupted through crust thinned by ras, and one of the thickest sections is about mountainous part of the lava field near the tectonic extension behind the Cascade 50 km north of Alturas and 4 km south of state line and in Oregon, however, the total Range volcanic arc. the Oregon-California border. Here, about thickness varies but commonly exceeds 350 m of basalt forms a series of cliffs that 70 m and may be much greater. We esti- INTRODUCTION are the thickest known continuous expo- mate the volume of the basalt of Devils sures in the study area. Garden to be at least 850 km3. A poorly known sequence of late Mio- The south margin of the plateau under- cene and early Pliocene basaltic lavas lain by the basalt of Devils Garden is traced GEOLOGIC SETTING underlies part of northeastern California by the course of the Pit River near Alturas and south-central Oregon. Reconnaissance (Figs. 1,2). The lavas here are nearly flat- The Devils Garden lava field (Figs. 1, 2) field studies and K-Ar dating suggest that lying and are cut by north-northwest- lies between the Basin and Range and Cas- these basalt flows were emplaced during a trending normal faults with as much as a cade Range physiographic provinces and relatively brief period and thus constitute a few tens of metres of offset. Northward, at makes up much of a poorly defined region coherent age and compositional group of about the Oregon-California border and called the Modoc plateau (Macdonald, lavas. We report here the results of our beyond, the terrane is increasingly moun- 1966). The physiography of the Modoc pla- efforts (1) to determine the areal extent and tainous and eroded. The basaltic lavas of teau is transitional between that of the to calculate the volume of these basaltic the entire region between Alturas and Basin and Range and the Cascade Range. lavas; (2) to establish the age of the lavas by Summer Lake in Oregon are similar in Part of the Modoc plateau is cut by north- K-Ar geochronology; (3) to determine the petrology and age and thus constitute a northwest-trending normal faults (Gay and petrology and origin of the lavas, using coherent lava field. Outliers of correlative Aune, 1958) that are roughly parallel to major and trace elements and Sr isotopes; basalt are known to the south of Alturas faults in a4jacent parts of the Basin and

Geological Society of America Bulletin, v. 94, p. 292-304, 6 figs., 2 tables, February 1983.

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Range (Lawrence, 1976). Figure 1 shows the positions of the major faults in southeastern Oregon, northeastern California, and northwestern Nevada. The basalt of Devils Garden unconfor- Figure 1. Northeastern California, mably overlies various tuffaceous continen- northwestern Nevada, aft'd southeast- tal and lacustrine sedimentary rocks that ern Oregon, showing major late crop out in several localities south of Cenozoic faults and Devils Garden Summer Lake, Oregon, and at the south lava field. Hachures on faults show margin of the volcanic field near Alturas, downdropped side. California. These sedimentary rocks range in age from early Miocene into earliest Plio- Range. Vertical offset is distributed across completely bury faults in older rocks. cene and may be progressively younger the Modoc plateau on many small faults Northwest-trending faults that may be prin- from north to south, based on scattered rather than being concentrated at the mar- cipally strike-slip (Gay, 1959) also cut the paleontological evidence. Near Summer gins of large horsts and grabens, as is typical Modoc plateau (Lawrence, 1976; Hannah, Lake, the sedimentary sequence is unnamed in the Basin and Range. This pattern of dis- 1977; Duffield and Fournier, 1974). These but appears to be lithologically similar to tributed fault offset extends westward from faults are parallel to the Brothers fault zone units of the John Day Formation of middle the Warner Range, without apparent inter- of central Oregon, which has been inter- Miocene to early Oligocene age in central ruption, to the east edge of the Cascade preted as a transcurrent structure that Oregon. Mammalian fossils from tufface- Range, where Quaternary lavas largely or bounds the northwest edge of the Basin and ous sedimentary rocks of the Alturas For-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/2/292/3434543/i0016-7606-94-2-292.pdf by guest on 01 October 2021 Figure 2. Northeastern California and 0 10 20 30 40 50 MILES south-central Oregon, showing sample localities, K-Ar ages, and outline of Devils Garden lava field. 0 10 20 30 40 50 60 70 80 KILOMETERS

mation of Dorf (1933) beneath the basalt of in the Medicine Lake Highlands area, but ally thinner than 10 m. Dikes that may have Devils Garden near Alturas are of early Pli- because there is no striking field evidence of fed some of the flows crop out locally; lava ocene (early Blancan) age (Repenning, a significant difference in age between these shields, interpreted to mark vent areas 1980). two groups of lavas, previous reconnais- (Duffield and Fournier, 1974), and cinder Basaltic to andesitic lava flows exposed in sance studies resulted in assignment of both cones occur unevenly throughout the field. fault scarps west of the Devils Garden lava groups to the Quaternary (Gay and Aune, Andesite flows, locally intercalated with field near Klamath Falls, Oregon, yield 1958). Radiometric dating done as part of basalt, account for perhaps 3% to 4% of the K-Ar ages of about 2 m.y. (Iocs. 31,32, Fig. this study, however, indicates that the basalt volume of the lava field. Late Miocene rhy- 2; Table I). These scarps are largely or of Devils Garden was emplaced during late olitic domes, flows, and associated pyro- wholly buried along strike by younger lavas Miocene and earliest Pliocene time (Table clastic deposits are fairly abundant in the associated with the Crater Lake volcanic 1) and is clearly older than the Quaternary northern part of the lava field but are nearly center to the north and the Medicine Lake lavas of the Medicine Lake area. Additional absent in the southern part (Fig. 2). We Highlands volcanic center to the south. The field work is needed to locate a contact estimate that about 150 km3 of rhyolitic contact between these Quaternary lavas and accurately between the Tertiary and Qua- rocks is present in the Devils Garden lava the basalt of Devils Garden is generally not ternary lavas. field (Fig. 2). The rugged topography that is shown on existing reconnaissance geologic By far the most voluminous rock type in characteristic of the northern part of the maps (Gay and Aune, 1958; Jennings and the Devils Garden lava field is olivine basalt field reflects the presence of partly eroded others, 1977). For example, Pleistocene with a diktytaxitic texture, almost all of domes. Locally, erosion has exposed rhyo- basalt laps over the basalt of Devils Garden which occurs as lava flows, each flow gener- lite, andesite, and basalt in interlayered or

20 tu H OC Q BABB Z o x

Z BACK-ARC BASIN BASALT FIELD(BABB) MORB FIELD

La Ce Nd Sn Eu Gd Dy Er Yb

Figure 3. Chondrite-normalized REE patterns for the basalt of Devils Garden. Note that patterns are approximately flat, whereas in detail Ce is depleted relative to Nd, and Nd is generally enriched relative to Sm. Note positive Eu anomalies; these are not due to accumulation of plagioclase in the lavas and must be intrinsic to the primary melts. Basalt of Devils Garden does not show the strong depletions in light REEs exhibited by MORB but is more like basalt from active back-arc spreading centers. Envelopes shown for MORB are from Schilling (1971), and for back-arc basin basalts (BABB) are for Scotia basin and Mariana Trough (Hart and others, 1972; Hawkesworth and others, 1977).

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intrusive bodies that indicate overlapping TABLE 1. POTASSIUM-ARGON DATES ON VOLCANIC ROCKS FROM THE periods of emplacement for the different DEVILS GARDEN LAVA FIELD AND NEARBY AREAS, CALIFORNIA AND OREGON rock types; K-Ar ages (Table 1) are consis- Field Map Latitude W. Longitude N. Rock type tent with these field relations. no.

AGE OF THE LAVAS Alt 20 1 41°47.3' 12ri4.2' Basalt DG 1 2 41°30.0' 120°37.9' do. We report 33 new K-Ar ages, mostly on Alt 4 3 41°37.9' 120°42.2' do. rocks from within the Devils Garden lava Alt 18 4 41°30.0' 120° 58.0' do. Alt 19 5 41°37.7' 121° 13.5' do. field (Fig. 2; Table 1). The localities of dated samples are distributed unevenly within the Alt 6 6 41°33.6' 120°45.4' do. lava field, but we believe that these samples Alt 12 7 41°51.5' 120° 45.3' do. do. provide reasonable control on the lower and PRM-2-70 8 43°03.0' 120°47.0' Alt 11 9 41°58.r 120° 06.3' do. upper age limits of the field. New K-Ar ages Alt 8 10 41°31.0' 120°28.7' do. on lavas in adjacent areas help to define the areal extent of the lava field where existing Alt 1 11 41°20.5' 120°55.4' do. 7-78 12 42° 20.2' 120°41.0' do. maps are inadequate and to identify outliers Alt 10 13 4I°58.0' 120°30.2' do. of the basalt of Devils Garden south and Alt 9 14 41°56.r 120°30.3' do. southwest of Alturas. Alt 26 29 41° 14.9' 120° 30.0' do. The basalt of Devils Garden ranges in Alt 21 15 41°45.2' 121°09.0' Andesite apparent age from 5.3 ± 0.7 to 10.5 ± 0.2 Alt 22 16 4I°24.2' 120°47.7' do. m.y.; most ages range between 6 and 9 m.y. Alt 13 17 41°50.4' 120° 46.7' do. These lavas form an intermediate group in 10-78 18 42°22.9' 120°31.7' do. Alt 15 19 4I°49.8' 120° 51.8' do. an apparent age progression of younger to older, west to east from the Cascade Range M-3-106 »20 42°35.6' 121° 13.4' Rhyolite across Devils Garden to the northeastern M-5-2 21 42°23.0' 121°25.5' do. M-3-35 *22 42° 19.7' 120° 51.9' do. part of the Basin and Range. Basalt and M-3-34 *23 42° 16.1' 120°43.8' do. andesite west of the Devils Garden lava S-5 30 41°46.3' 120° 17.8' do. field, within the Klamath Falls and Medi- M-3-37 *24 42°23.8' 120° 36.0' do. cine Lake Highlands area, are younger than M-3-36 *25 42° 18.3' 120° 37.9' do. 2 m.y. (Luedke and Lanphere, 1980; field M-3-40 »26 42°36.0' 120° 25.3' do. nos. 31, 32, Table 1). Basalt that crops out M-3-39 •28 42°34.6' 120°37.r do. along the crest of the Warner Range (the Warner Basalt of Russell, 1928) southeast Rocks from outside Devils Garden lava field of Alturas is about 14 m.y. old (Duffield and McKee, 1974); correlative basalt caps KFE-11 32 42° 20.8' 121°49.2' Rhyolite the northward extension of the Warner 39-78 33 42°28.0' 120°13.5' do. Range near Drake Peak in Oregon (Wells, KFW-6 31 42° 14.0' 121°48.8' Andesite 1980), and still farther north the basalt that Alt 23 27 41°25.8' 120°37.0' do. caps Abert Rim is about 15 m.y. old (field no. 33, Table 1). Farther east, in northwest- * McKee and others (1976). Note: All sample preparation and analytical work was done at the U.S. Geological Survey Isotope ern Nevada, widespread basalt in the Geology Laboratory in Menlo Park, California, using the techniques described in Dalrymple and Charles Sheldon Antelope Range yields Lanphere (1969). Whole-rock samples (except for obsidian) were crushed, sieved to 60- to JOO-mesh K-Ar ages of 9 to 10 m.y. old (E. H. size, washed, and treated for 30 min in 14% HNO3 and 1 min in 5% HF solutions before loading into a McKee, 1982, unpub. data). Adjacent to high-vacuum gas-extraction system. Argon analyses were performed by standard isotope-dilution the north, in Oregon, basalt of the Hart Mountain area is about 15-m.y.-old (Wat- kins and Baksi, 1974). Thus, the east-to- tion, crystals of labradorite are partly to (Table 2). The lavas are olivine-normative west age progression of basaltic lavas is completely enveloped in clinopyroxene. Py- tholeiite (Table 2), rich in A1 0 (16.7% to approximately characterized by>9-m.y.- 2 3 roxene also occurs as intersertal grains 17.7%), MgO (8.9% to 9.5%), CaO (10.5% old rocks east of Devils Garden, 9- to between feldspar laths. Phenocrysts are rare to 11.5%), and Ni (126-197 ppm) and 5-m.y.-old rocks within Devils Garden, and (< 1% of the rock), but scattered grains of depleted in Ti0 (0.75% to 1.1%), Na 0 late Pleistocene and younger rocks along 2 2 olivine, as large as 2 mm, occur locally, (2.3% to 2.5%), K 0 (0.13% to 0.43%), Rb the west side of Devils Garden. 2 commonly altered to iddingsite. Olivine is (0.89-3.55 ppm), Cs (0.01-0.036 ppm), and also present in the groundmass with pyrox- Zr (43-71 ppm). Incompatible-element PETROLOGY, CHEMISTRY, AND ene, magnetite, spinel, and dark glass. ratios are high (Na20/K20 = 5.8-19.2; Sr ISOTOPIC COMPOSITION Six samples of the basalt of Devils K/Rb = 772-1, 501; K/Cs = 43, 100-171, Garden were analyzed for major elements; 400), with the exception of K/Ba, which is The basalt of Devils Garden typically five of these samples were also analyzed for very low (10-15.1). displays a diktytaxitic texture. In thin sec- some trace elements and strontium isotopes Table 2 lists the concentrations of rare-

TABLE 1. (Continued) Rb/Sr ratio in the basalt of Devils Garden, however, is too low to account for the observed strontium isotopic composition. 40Arrad 40Arrad Material k2o Age The Rb/Sr ratio in the samples should 12 dated (wt %) IO" mol/g (%) (m.y.) exceed that in the source rock because partial melting and any subsequent fractiona- Whole rock 0.384 2.9375 . 4.2 5.3 ±0.7 do. 0.270 2.2460 9.0 5.6 ± 0.6 tion leads to increased Rb/Sr. Thus, the do. 0.190 1.6125 4.2 5.9 ± 0.7 lowest observed ratio in the basalt of Devils do. 0.230 2.0816 1.4 6.0 ± 4.2 Garden (Rb/Sr = 0.003) should be greater do. 0.167 1.5047 2.0 6.3 ± 4.0 than that in the source rock. Assuming an 87 86 do. ' 0.431 4.0722 6.2 6.6 ± 1.2 initial Sr/ Sr ratio of 0.69898 for the do. 0.328 3.1527 3.5 6.7 ±2.5 Earth, a rock with an Rb/Sr ratio of 0.003 do. 0.471 4.7156 13.1 6.8 + 0.7 would have developed 87Sr/86Sr = 0.6996 do. 0.268 2.7490 3.3 7.1 ± 2.5 over the duration of the Earth's history, do. 0.201 2.2713 2.7 7.8 ±4.0 considerably different from the strontium do. 0.180 2.0308 2.2 7.8 ± 4.3 isotopic composition of the basalt of Devils do. 0.126 1.5492 5.9 8.5 ± 1.3 Garden. do. 0.257 3.1609 6.9 8.5 ± 1.3 do. 0.178 2.4498 4.7 9.5 ± 2.0 do. 0.226 0.9204 1.1 2.9 ± 2.5 TECTONIC AFFINITIES OF do. 1.305 10.6575 12.9 5.7 ± 0.6 BASALT OF DEVILS GARDEN do. 1.367 16.2239 39.5 8.2 ±0.2 do. 1.276 15.9581 19.9 8.7 ± 0.4 The chemical composition of the basalt of do. 2.780 42.1973 52.4 10.5 ± 0.2 Devils Garden is unusual in that basalt do. 5.4 ± 0.9 3.29 25.6355 4.5 erupted on the continents is rarely so Plagioclase 0.930 7.4074 8.1 5.5 ± 0.5 depleted in incompatible elements. With the do. 0.320 2.6574 1.2 5.8 ±4.0 exception of slightly higher MgO and con- Biotite 6.55 68.9568 75.2 7.3 ±0.9 siderably lower Ti02 contents, the major- Obsidian 4.42 46.7598 31.9 7.3 ±0.3 do. 3.47 52.2647 60.4 10.2 ± 0.3 element composition of the basalt of Devils Garden is very similar to that of basalt Sanidine 11.69 124.4914 40.0 7.4 ±0.3 erupted in and around modern ocean ba- Biotite 7.99 86.0475 25.7 7.5 ±0.5 sins. This includes basalts from along Obsidian 4.42 48.5396 54.9 7.6 ±0.2 do. 4.51 51.5156 76.0 7.9 ±0.1 mid-ocean ridges (MORB) (Engel and others, 1965; S. R. Hart, 1971; Schilling, 1971), Rocks from outside Devils Garden lava field basalts of back-arc basins (BABB) (Hart and others, 1972; Hawkins, 1977; Hawkes- worth and others, 1977; Saunders and Tar- Obsidian 0.490 1.3067 2.2 1.9 ±0.9 do. 0.741 16.1934 31.4 15.1 ±0.8 ney, 1979), and tholeiitic basalts from do. 1.234 4.0690 6.6 2.3 ± 0.4 primitive island arcs (IATH) (Jakes and Plagioclase 0.360 3.9934 2.5 7.7 ±4.0 White, 1972). It is important for an under- standing of the tectonic evolution of the Devils Garden area to determine which of procedures. Potassium analysis was performed by a lithium metaborate flux fusion flame photometer these depleted basalt types provides the technique, the lithium serving as internal standard (Ingamells, 1970). Errors, shown as the ± values, are estimates of the standard deviation of analytical precision. K-Ar ages were calculated using the con- most appropriate analog for the basalt of stants for the radioactive decay and abundance of 40K recommended by the International Union of Devils Garden. For comparison, Table 2 Geological Sciences Subcommission on Geochronology (Steiger and Jager, 1977). These constants are: lists the average values for MORB, BABB, 10 vr 10yr 40 4 At = 0.581 X io - A/j = 4.962 x IO" "', and K/Klolal - 1.167 x I0' mol/mol. and IATH; both the similarities and differences between the basalt of Devils Garden and these three types of depleted basalt are earth elements (REEs) in the basalt of Dev- the envelope defined by modern mid- outlined below. ils Garden. Normalized to chondritic values oceanic-ridge basalt (MORB) (Fig. 3), and plotted against atomic number, these although the tendency for relatively flat There are no significant differences in data yield relatively flat curves (Fig. 3). curves with a relatively weak LREE deple- major-element chemistry between the basalt Chondrite-normalized Ce/Yb ratios, (Ce/ tion is more like that of back-arc-basin of Devils Garden and MORB, BABB, and Yb)r\[, range from 0.96 to 1.31 and reflect a basalt (BABB) (group 11 MORB; Hart and IATH. Trace-element abundances and ra- slight over-all enrichment in the light-rare- others, 1972; Bryan and others, 1976; tios may provide a more sensitive discrimi- earth elements (LREEs). In detail, the Hawkesworth and others, 1977). The REE nant among the different types of tholeiite. LREE contents show a slight depletion in patterns of the basalt of Devils Garden also K and Rb are as depleted in the basalt of

Ce, with (Ce/Nd)N = 0.75-0.97. Nd is not resemble that of island-arc tholeiitic (IAT) Devils Garden as in MORB and more depleted relative to the heavier REEs, with series (Jakes and Gill, 1970). depleted than in either BABB or IATH. The 87 86 (Nd/Sm)N = 1.07-1.27. The REE patterns The Sr/ Sr ratio in the basalt of Devils K./ Rb ratio in the basalt of Devils Garden is for the basalt of Devils Garden fall within Garden ranges from 0.7036 to 0.7039; the very high and is also more like that of

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Field no. (map no.) Field no. (map no.) Island-arc

Alt-1 (11) Alt-9 (14) Alt-10 (13) Alt-11 (9) Alt-19 (5) Alt-26 (29) MORB* BABBt tholeiitet

MAJOR ELEMENTS (wt %) MAJOR ELEMENTS (wt %)

Si02 49.1 48.3 47.0 48.8 48.6 47.7 49.34 50.7 51.6

ai2o3 16.8 16.9 16.7 16.9 17.7 17.0 17.04 16.6 15.9

Fe203 2.3 2.7 3.0 4.9 1.8 0.95 1.99 2.74 FeO 7.0 6.8 6.8 4.6 7.3 7.8 6.82 7.84 7.04

Ti02 0.88 0.88 1.1 0.75 0.84 0.89 1.49 1.23 0.80 MnO 0.14 0.14 0.15 0.15 0.15 0.16 0.17 0.16 0.17 MgO 8.9 9.3 8.6 9.3 9.3 9.5 7.19 7.40 6.73 CaO 10.7 10.5 11.1 11.5 10.8 10.5 11.72 10.89 11.74 Na20 2.5 2.5 2.5 2.3 2.5 2.3 2.73 3.06 2.41 K20 0.28 0.23 0.43 0.31 0.13 0.35 0.16 0.39 0.44

P2O5 0.18 0.19 0.19 0.16 0.14 0.19 0.16 0.17 0.11 + h2o 0.60 0.57 0.73 0.55 0.48 2.3 0.69

h2o- 0.34 0.31 0.28 0.29 0.04 0.11 0.58 0.45 co2 0.00 0.01 0.02 0.02 0.00 0.00

Total 99.72 99.33 98.60 100.53 99.78 99.75 100.08 98.44 100.13

NORMATIVE MINERALS (wt %) NORMATIVE MINERALS (wt ?o)

Q Or 1.665 1.373 2.611 1.827 0.770 2.076 Ab 21.286 21.364 21.735 19.415 21.209 19.532 An 34.002 34.551 33.983 34.790 36.786 35.155 Ne

Di 14.708 13.464 17.129 16.553 12.942 12.872 Hy 15.117 13.520 4.219 16.007 11.408 10.344 Ol 7.163 9.046 14.393 1.936 11.864 14.195 Mt 3.356 3.953 4.469 7.088 2.617 1.382 Hm

II 1.682 1.688 0.215 1.421 1.599 1.696 Ap 0.429 0.454 0.462 0.378 0.332 0.452

Total 99.408 99.2282 97.7040

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K 1,874 1,657 2,931 1,336 2,742 1,260 3,237 3,608 Rb 2.39 1.19 2.78 0.89 3.55 1.16 5 5.0 Sr 399 273 324 259 234 140 208 200 Cs 0.036 0.010 0.020 0.031 0.016 0.018 Ba 166.2 165.8 194.0 89.2 197.0 7.8 65 75 Ce 11.03 8.41 9.77 9.60 10.19 9.99 13.7 2.6 Nd 10.76 7.23 7.39 8.34 7.68 8.79 9.4 Sm 2.72 2.01 2.06 2.51 2.18 3.09 2.86 Eu 1.056 0.972 0.845 0.999 0.896 1.15 1.06 Gd 3.39§ 2.71§ 2.68 3.26§ 2.80§ 3.45 Dy 3.91 3.34 2.94 3.91 3.27 4.05 Er 2.68 2.21 1.93 2.57 2.31 2.57 Yb 2.52 2.23 1.89 2.51 2.18 2.94 2.48 1.4 Zr 52 51 43 71 61 95 106 70 Ni 126 197 169 130 153 97 66 30 R7Sr/86Sr 0.70362 ± 8 0.70371 ± 6 0.70387 ± 6 0.70367 ± 9 0.70389 ± 7 0.7028 0.7030 0.7037 0.70363 ± 6 Na20/K20 8.9 10.9 5.8 7.4 19.2 6.6 17.1 7.8 K./Rb 784 1,392 1,054 1,501 772 1,086 647 720 K/Cs 52,100 165,700 146,600 43,100 171,400 70,000

K/Ba 11.3 10.0 15.1 15.0 13.9 161.5 49.8 48.1 Rb/Sr 0.006 0.004 0.009 0.003 0.015 0.008 0.024 0.025 Ba/Sr 10.417 0.607 0.599 0.344 0.842 0.056 0.313 0.375

Note: Whole-rock major elements analyzed by the methods of Shapiro (1975); K, R, Cs, Ba, and REEs after the methods of Hart (1971) and Shimizu (1974), and Sr, Zr, and Ni by X-ray fluorescence 87 86 86 88 87 86 after the methods of Stern (1979). Sr/ Sr analyzed on 6-in.-radius mass spectrometer at DTM, normalized to Sr/ Sr = 0.1194 and E + A SC03 Sr/ Sr = 0.70800. Total procedural blank for Sr is < 0.3 x 10"9 g. *Data for abyssal tholeiite are from the following sources: major elements, Zr and Ni (Engel and others, 1965); K, Rb, Sr, Cs, Ba, and 87Sr/86Sr (slow-spreading ridges; Hart, 1971); REE (Schilling, 1971). fMean composition for basalt from the East Scotia Sea back-arc basin (Saunders and Tarney, 1979). Total iron expressed as FeO. t Jakes and Gill (1970); Jakes and White (1972). § Interpolated data.

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2O00 MID-OCEAN RIDGE BASALT (MORB) Figure 4. K versus K/Rb correlation between 1790 basalt of Devils Garden and oceanic basalt types, modified from Gast (196S) and Chow and others (1980). Field of typical BABB is defined by data of 1500 O Average I AT H A Average MORB Hart and others (1972) and Saunders and Tarney 1250 (1979). Note that K/Rb ratio of basalt of Devils BASALT Of DEVILS GARDEN JO Garden is as low as that for extremely depleted a. MORB. 1000

750 tive concentrations of K, Rb, and Cs, as ALKALI OLIVINE well as Ni abundances, in the basalt of Dev- 500 BACK-ARC BASALT BASIN BASALT(BABB)' ils Garden are most like those of MORB and least like those of IATH. 250 In contrast, contents of the alkaline-earth trace elements Ba and Sr, as well as the _L _L _L _L 87 86 Sr/ Sr ratio, are more like those of vol- 0.01 0J02 OJOS 0.1 0.2 as 10 canic rocks of island arcs and back-arc ba- %K sins. Sr abundance in the basalt of Devils MORB than of either BABB or IATH (Fig. 92 x 103 (Hart and others, 1972). The con- Garden is two to three times enriched rela- 4). The K/Cs ratio is extremely high, com- centration of Ni in the basalt of Devils tive to MORB and resembles that in BABB parable to that of MORB. In comparison, Garden is higher, although generally com- or IATH. The difference is even greater for K/Cs in IATH is < 30,000 (Hart and oth- parable to that of MORB; by comparison, Ba; the basalt of Devils Garden contains ers, 1970; Dixon and Batiza, 1979) and both BABB and IATH typically contain about 90-200 ppm Ba, more than ten times Mariana Trough BABB has K/Cs = 35- much less Ni. Thus, both absolute and rela- more than in MORB. As is the case for Sr, the Ba concentration in the basalt of Devils Garden is more like that in BABB and

Y/J\ INTRAOCEANIC ISLAND ARCS IATH than that in MORB. Comparison of Ba/ Sr ratios, which range from 0.34 to 0.84 | BACK-ARC BASIN BASALTS (BABB) in the basalt of Devils Garden, further j BASALT OF DEVILS GARDEN emphasizes this relation. This large relative I I MORB enrichment in Ba is especially apparent for the K/ Ba ratio, which in the basalt of Devils 100- Garden (10-15) is among the lowest known for terrestrial basalt, ten times lower than in MORB, and three to five times lower than in IATH or BABB. On a plot of K/Rb versus K/Ba ratio (Fig. 5), the basalt of Devils Garden is distinct from both MORB and BABB but partially overlaps with basalt of intra-oceanic island arcs. The 87Sr/86Sr ratio in the basalt of Devils Garden is signif- icantly greater than in MORB and most 50 BABB (Hofmann and Hart, 1978, Table 2). The strontium isotopic composition of the basalt of Devils Garden resembles that of basalt of intra-oceanic island arcs as well as of some BABB recovered from the Lau basin (Stern, in press).

10- REE data do not help to discriminate between the basalt analogs being considered 0 + here. REE patterns for the basalt of Devils 500 1000 1500 Garden fall within this field occupied by both MORB and BABB (Fig. 3). Although K/Rb not plotted in Figure 3, the basalt of Devils Figure S. K/Ba versus K/Rb for various oceanic magma types. Field for intraoceanic Garden would also fall within the field of island arcs is taken from Chow and others (1980), and that of BABB is defined by data from IATH. Consideration of Ti and Zr contents the East Scotia basin (Hawkesworth and others, 1977; Saunders and Tarney, 1979), Lau of the basalt of Devils Garden is also equiv- basin (Gill, 1976), and Mariana Trough (Hart and others, 1972). Field for MORB is ocal; plotted on a Ti-Zr discriminant dia- outlined by data of Philpotts and others (1969) and Hart (1975). Note unusual combination gram (Fig. 6), these occupy the field of high K/Rb and low K/Ba in the basalt of Devils Garden. common to MORB, IATH, and BABB.

A convincing case that the basalt of Dev- the average Sierra Nevada granitic rock, a and Sr (234-399 ppm) and positive Eu ils Garden belongs uniquely to one of the typical crustal material, contains 68% Si02, anomalies do not indicate the removal of

major classes of low-K tholeiite cannot be 3.6% K2, 1,000 ppm Ba, and 118 ppm Rb plagioclase. made. Strong similarities to MORB in (Dodge, 1972). No more than 15.5% of this 3. Addition of Sr- and Ba-rich material to alkali-metal trace-element and Ni contents rock could be added to 84.5% of MORB to MORB parental magma to produce the are recognizable, as well as strong similari- generate 168 ppm Ba in the basalt of Devils observed 87Sr/86Sr and K/Ba ratios seems ties to IATH in the concentrations of Garden. This mixture of Sierran granite and unlikely. Strontium isotopic compositions 87 alkaline-earth trace elements and in Sr/ MORB would also contain 52% Si02) 0.7% that are unsupported by observed Rb/Sr 86 Sr ratio. Basalt of Devils Garden seems K20, and 19.2 ppm Rb, with K/Ba = 36 ratios, as well as a low K/Ba ratio, are char- most like BABB, which represents a class of and K/Rb = 301. Assuming the Sierran acteristic of much Miocene and younger low-K tholeiite spanning the geochemical granite to be 100 m.y. old, to contain 500 basalt in northeastern California and adja- and isotopic range between MORB and ppm Sr, and to have an initial 87Sr/86Sr cent parts of Oregon and Nevada. For IATH. ratio of 0.7030, the modeled mixture of example, basalt for which the 87Sr/86Sr MORB and granite would contain 196 ppm ratio cannot have been formed by single- 87 86 PETROGENESIS Sr and have Sr/ Sr = 0.7034. Although stage evolution of sources with the Rb/Sr the 87Sr/86Sr values for the basalt of Devils ratio determined in the basalt was reported Many geologists have noted low-K tho- Garden are satisfied by this mixing model, from southern Nevada (Hedge and Noble, leiitic basalt in northeastern California. the Si02, K, and Rb values are not. Fur- 1971), northern Nevada (McKee and Mark, Anderson (1941) reported a low-K (0.09% thermore, a mixture of 15.5% Sierran 1971; Mark and others, 1975), southeastern K2O) olivine tholeiite from the Medicine granite and 84.5% MORB would contain Oregon (the Steens Basalt, Noble and oth- Lake Highlands area, a few kilometres west about 52% Si02, considerably more than ers, 1973; Hart, 1981), and the Cascade vol- of the Devils Garden lava field. Yoder and that in the basalt of Devils Garden. canoes, Mounts Shasta and Lassen (Peter- Tilley (1962) reported a similar low-K 2. Petrogenetic models involving signifi- man and others, 1970). Basalt with low (0.12% K2O) olivine tholeiite several kilo- cant admixtures of crustal material to basal- K/Ba ratio was reported for Pleistocene metres southwest of the locality studied by tic melts require the mix to occur in crustal lava of the Medicine Lake Highlands Anderson. Waters (1962), Smith and Car- magma chambers, where the latent heat of (K/Ba = 10.6-27.7; Mertzmann, 1977); the michael (1968), Leeman and Rogers (1970), fusion necessary to cause wall-rock assimi- 14- to 15-m.y.-old Picture Gorge Basalt Mertzmann (1977), and Carlson (1980) de- lation is furnished by the crystallization of (K/Ba = 10.2-18.6); the lower part of the scribed low-K olivine tholeiite in northeast- liquidus and near-liquidus phases (see Yakima Basalt Subgroup (K/Ba =15.8- ern California. James, 1982). In the case of the basalt of 23.7) of the Columbia River Basalt Group (McDougall, 1976); the 15-m.y.-old Steens The geochemistry of the basalt of Devils Devils Garden, these phases would be oli- Basalt (K/Ba = 14-18; Gunn and Watkins, Garden is of special petrologic and tectonic vine, pyroxene, and plagioclase. The pe- 1970); Tertiary basalt of the Feather River interest. The low concentrations of alkali trology and chemistry of the basalt of area, California (K/Ba = 7.3-19.2; Hieta- metals, light REE and L1L (large ion litho- Devils Garden rules this out as a mechanism nen, 1972); and 8- to 11-m.y.-old basalt of phile), contrast with more common enrich- in the formation of the basalt. Specifically, northern Nevada (K/Ba = 2-17; McKee ments of these elements in other continental the high MgO (8.9% to 9.5%), and Mark, 1971; Mark and others, 1975; basalt suites. The basalt of Devils Garden / 100 Mg \ Hart, 1981). It is unlikely that crustal rocks and some other basalts east of this volcanic Mg# ( >68), of appropriate Ba and Sr contents are of field studied by Hart (1981) are clearly V Mg + Fe / sufficiently great geographic extent to have derived from magmas that have had no sig- and Ni content (126-197 ppm) do not indi- contaminated basaltic magma over such a nificant amounts of upper crustal contami- cate the fractional crystallization of signifi- widespread region and over such a long nation. Many continental basalt flows were cant olivine or pyroxene, whereas the high period. contaminated by continental crust as the concentration of A1203 (16.8% to 17.7%) magmas moved from their mantle source regions to the surface. We see no evidence that the basalt of Devils Garden has been contaminated by the introduction of upper crustal material into a more primitive 8000 magma. The arguments for this are: 1. Addition of sialic crustal material to provide the required amount of Ba to a 6000 E MORB-like parental magma results in too o. CL much SiOj, K, and Rb in the mix relative to the basalt of Devils Garden. For example, 4000

Figure 6. Ti-Zr discriminant diagram (Pearce and Cann, 1973). 2000 MORB plots in fields B and D, IAT (low-potassium tholeiite of Pearce and Cann, 1973) in fields A and B, and calc-alkaline arc basalt in fields C and B. This diagram does not discriminate BABB; field defined by basalt dredged from spreading center of 60 East Scotia back-arc basin is included for comparison. Zr (ppm)

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4. The relation between Nd content and Ni content of 126-197 ppm are consistent derivation analogous to that of circum- 87Sr/86Sr ratio suggests no crustal compo- with the derivation of this magma from a Pacific back-arc basin basalt. Thus, we infer nent in the basalt of Devils Garden. Carlson peridotitic mantle source. In this respect, that Devils Garden magma was derived and others (1981a) reported 87Sr/86Sr = the basalt of Devils Garden may be re- from a mantle source and was erupted 0.70347 eNd = + 6.5 from a similar low-K garded as a primary magma. The major- through either oceanic crust or normal sialic tholeiite from northeastern California; these element chemistry of the basalt of Devils crust, and that passage through the crust values fall within the "mantle array" as Garden (that is, high-alumina olivine tho- was too rapid to permit contamination or defined by oceanic volcanic rocks. On a plot leiite) is consistent with an origin by 20% was erupted through unusually thin sialic of 87Sr/86Sr versus 143Nd/ l44Nd, the basalt melting of anhydrous mantle peridotite crust, with little or no contamination. Crus- of Devils Garden lies within a field outlined within 25 to 70 km of the Earth's surface tal thinning by extension is suggested by the by basalt of oceanic islands that shows no (Green and Ringwood, 1967; Presnall and abundant normal faults that cut the study evidence of contamination by sedimentary others, 1979). If the high concentrations of area. Although no individual fault appears rocks. Sr, AI2O3, and Eu indicate total breakdown to record major normal offset, total hori- 5. Sialic crust may not exist beneath the of plagioclase during melting, then melting zontal extension associated with normal Modoc plateau in northeastern California. may have occurred within the stability field faults may be considerable across the Devils The principal rock types in the region are of plagioclase peridotite. Experimental re- Garden region, especially if many relatively intermediate and mafic lavas. Xenoliths of sults showing that plagioclase peridotite older faults are buried by the youngest sialic crustal rocks have not been reported transforms to spinel lherzolite at pressures lavas. If the amount of crustal extension is from the basalt of Devils Garden. Published between 8 and 14 kbar indicate that the comparable to that in the Basin and Range maps show no pre-Tertiary basement rocks basalt of Devils Garden was generated at a province to the east (about 10%, according for at least 100 km in all directions from the depth of no more than 50 km (Ito and to Thompson and Burke, 1974; as great as center of the Devils Garden lava field. Kennedy, 1968; Green and Hibberson, 50%, according to Hamilton and Myers, 1970). If the crust beneath the Modoc pla- 1966), then 10 to 50 km of east-west crustal We emphasize that our data are most teau is as thick as that in the adjacent Basin extension may have taken place across the readily explained as an uncontaminated and Range province (40 km; see Thompson Modoc plateau since middle Miocene time. primary melt of the mantle, and that basalt and Talwani, 1964; Dehlinger and others, with these characteristics can form in other The idea that thin or no sialic crust exists 1965; Hill, 1978), then the magma must ways. For example, if the contaminant is in the Modoc plateau as a result of crustal have been generated within the uppermost itself depleted in LIL elements, it is possible extension is consistent with the Cenozoic 10 to 30 km of the mantle. Leeman and to produce the observed geochemical varia- history indicated by paleomagnetic studies. Rogers (1970) suggested a similar depth for tion. Hart (1981) has used this approach to The paleomagnetic data suggest as much as the formation of Cenozoic basaltic magmas explain the composition of some basalt 27° of post-Eocene clockwise rotation of throughout the Basin and Range province. from northern Nevada. In his view, these the Klamath Mountains, the Coast Ranges basalts resulted from partial melting of a Several inferences regarding the evolu- in Oregon and Washington, and the Cas- depleted lower crust. Carlson and others tion of the mantle beneath northeastern cade Range about a pole in the Coast (1981b) noted that even where the evidence California can be made from the data pre- Ranges of southern Washington (Magill for crustal contamination is strong, as is the sented here. First, the mantle was severely and Cox, 1981). In effect, the Modoc pla- case with some of the basalts from the depleted in K, Rb, and Cs before generation teau lies in the wake of a huge, relatively 87 Columbia River Basalt Group, eNd- Sr/ of the basalt of Devils Garden. Second, this coherent terrane that has shifted about 86 Sr isotopic values are insensitive to the depletion (melting?) did not eliminate the 340 km westward during rotation in the effects of small amounts of crustal contam- alkaline-earth metals Ba and Sr so effec- past 20 m.y. (Magill and Cox, 1981, Fig. 4). ination. tively as it separated K, Rb, and Cs, possi- Hamilton (1978, Fig. 4) also proposed It is unlikely that the basalt of Devils bly owing to sequestering in plagioclase. westward displacement of the Klamath Garden was produced by melting of sub- Third, the depletion of Rb relative to Sr Mountains during late Cenozoic time, an ducted oceanic crust of the Farallon plate. resulted in a very low Rb/Sr ratio and thus idea consistent with rifting and crustal Nearly total melting of the subducted Faral- effectively arrested any increase in 87Sr/ extension in the Modoc plateau. The Kla- lon plate (MORB) would be necessary to 86Sr ratio. This depletion of alkali relative math block may have moved westward generate magma with the same major- to alkaline-earth elements may also explain along an east-west-trending, left-lateral element composition as the basalt of Devils the unusually low K/Ba ratio of the basalt fault zone lying just south of the Cascade Garden, and even if this were possible, the of Devils Garden. Range. The Battle Creek fault zone (Helley composition of the magma so produced and others, 1981) may represent part of would be inconsistent with the Ba and Sr IMPLICATIONS FOR THE LATE such a fault system; it flattens as it passes contents and the 87Sr/86Sr ratio of the CENOZOIC EVOLUTION OF south of the Klamath Mountains and basalt of Devils Garden. NORTHEASTERN CALIFORNIA merges with large north-south-trending thrust faults on the west of the Klamaths. The chemical data suggest an origin in the By this arrangement of faults, the region mantle for the basalt of Devils Garden. The chemical characteristics of the basalt west of the Modoc plateau could have High MgO (8.9% to 9.5%), Mg# >68, and of Devils Garden are most consistent with a

moved westward to accommodate crustal geologic implications for the Pacific North- Mineralogists, Pacific Section, p. 38-70. extension in the Devils Garden region. west states: Seismological Society of Amer- Hamilton, W., and Myers, W. B„ 1966, ica Bulletin, v. 55, p. 587-608. Cenozoic tectonics of the western United Hannah (1977) suggested that rifting and Dixon, T. H„ and Batiza, R„ 1979, Petrology States: Review of Geophysics, v. 4, no. 4, contemporaneous eruption in the Modoc and chemistry of Recent lavas in the north- p. 509-547. plateau may be related to crustal spreading ern Marianas: Implications for the origin of Hannah, J. L., 1977, Tectonic setting of the behind a volcanic arc; Uyeda and Kanamori island-arc basalts: Contributions to Miner- Modac region, northeastern California, in (1979) implied a similar explanation. The alogy and Petrology, v. 70, p. 167-181. Short contributions to California geology: Dodge, F.C.W., 1972, Trace-element contents of California Division of Mines and Geology position of the Modoc plateau relative to some plutonic rocks of the Sierra Nevada Special Report 129, p. 35-39. the Cascade Range provides the geometry batholith: U.S. Geological Survey Bulletin Hart, S. R., 1971, K, Rb, Cs, Sr, and Ba contents required for such a plate-tectonic interpre- 1314-F, p. F1-F13. and Sr isotope ratios of ocean floor basalts: tation. Whether crustal extension and at- Dorf, Erling, 1933, Pliocene floras of California: Royal Society of London Philosophical tendant volcanism in the Modoc plateau are Carnegie Institution of Washington Publica- Transactions, v. 268, p. 573-587. tion 412, p. 1-112. 1975, Deep Sea Drilling Project Leg 34 genetically linked to the Cascade volcanic Duffield, W. A., and Fournier, R. O., 1974, basalts and ocean ridge basalts: A geochemi- arc in the same manner as back-arc spread- Reconnaissance study of the geothermal cal comparison: Carnegie Institution of ing is linked to active oceanic island arcs is resources of Modoc County, California: Washington Year Book 74, p. 218-224. unknown, but the analogy with back-arc U.S. Geological Survey Open-File Report, Hart, S. R., Brooks, C„ Krogh, T. E., Davis, G. components and a convergent plate margin 19 p. L., and Hava, D., 1970, Ancient and Duffield, W. A., and McKee, E. H„ 1974, Ter- modern volcanic rocks: A trace element is striking. tiary stratigraphy and timing of Basin and model: Earth and Planetary Science Letters, Range faulting of the Warner Mountains, v. 10, p. 17-28. ACKNOWLEDGMENTS northeast California [abs.]: Geological So- Hart, S. R„ Glassley, W. E., and Karig, D. 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