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Lead and Strontium Isotopic Studies in the Cascade Mountains: Bearing on Andesite Genesis

Lead and Strontium Isotopic Studies in the Cascade Mountains: Bearing on Andesite Genesis

S. E. CHURCH 1 Department of Geological Sciences, University of California, Santa Barbara, California G. R. TILTON J 93106

Lead and Strontium Isotopic Studies in the Cascade Mountains: Bearing on Andesite Genesis

Note: This paper is dedicated to Aaron and Elizabeth three samples of Oregon coastal sedimentary Waters on the occasion of Dr. Waters' retirement. rocks. There is no demonstrable difference between in high-alumina and andesite at particular volcanos, although ABSTRACT the lead data are more variable than the New isotopic analyses of lead are reported strontium data. Strontium in crystalline for 40 volcanic rocks from the Cascade Moun- basement rocks from northern Washington is tains of Washington, Oregon, and California. substantially more radiogenic than that in the Strontium isotopic compositions were also calcic and calc-alkalic volcanic rocks of the determined in 33 volcanic rocks from the same Cascade Mountains. The same is true of lead area. In addition, lead and strontium isotopic in some, though not all, of the basement rocks. data are given for feldspar and whole-rock The isotopic data are utilized, along with samples of prominent varieties of crystalline other trace-element data to test models of basement rocks from northern Washington. andesite genesis. The isotopic compositions of The Sr87/Sr86 values of the volcanic rocks lead and strontium in andesite and high- average 0.7037, with no significant difference alumina basalt from the Cascade Mountains between andesite and high-alumina basalt. are consistent with derivation of the two lava The ratios exhibit no measurable correlation types from a common source, or with derivation with strontium concentrations over a range of of andesite by differentiation of high-alumina 200 to 1,500 ppm. Strontium in the Cascade basalt magma. The data do not support models rocks is slightly more radiogenic than that in in which the Cascade calcic and calc-alkalic oceanic ridge , but less radiogenic than magmas were formed by melting of Oregon in most continental basalt, including the coastal eugeosynclinal sedimentary rocks, or Columbia River basalt. Comparisons with by anatexis of basement rocks similar to those published data for strontium in Pacific Ocean now exposed near volcanic centers in northern sediments and coastal graywackes in Oregon Washington. Various subduction zone melting indicate much higher Sr8'/Sr86 values in the models are also considered. Mixing models, sedimentary rocks when compared to the whereby radiogenic strontium and lead are Cascade volcanic rocks, thus ruling out anatec- added to magmas containing lead and strontium tic models involving large sediment contribu- with isotopic compositions similar to those in tions. oceanic ridge tholeiite, do not fit the observed 206 Lead isotopic compositions from the vol- data satisfactorily because values of Pb / Pb204 in Pacific Ocean sediments are too low canic rocks are variable but tend to be rather 206 204 constant at a single volcanic center. A note- to account for the Pb /Pb values in the worthy feature of the lead data are the higher Cascade lavas. Mixtures of eugeosynclinal Pb207/Pb204 values (or larger fj. values in a sedimentary rock and oceanic ridge tholeiite model age diagram) for the Cascade volcanic can probably account for the Cascade volcanic rocks compared to published Pb207/Pb204 rock isotopic data in a general way, but the values and fi values for Pacific Ocean tholeiitic uniform isotopic composition of strontium in basalt. The Pb206/Pb204 values from the the lavas over a wide range of concentrations is Cascade volcanic rocks are higher than in hard to understand in terms of such a model. oceanic sediments from the northeast Pacific The same is true for strontium in the oceanic Ocean but are lower than the average ratio for sediment mixing model. The uniform isotopic

Geological Society of America Bulletin, v. 84, p. 431-454, 12 figs., February 1973 431

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composition of strontium in the Cascade lavas, and therefore are a favorable area for studying irrespective of strontium concentration and the andesite problem. Figure 1 is a simplified geologic setting, argues against contamination geological map of Washington, Oregon, and of magma with radiogenic strontium during northern California showing the extent of the ascent. A similar argument can be made for outcrop area of volcanic rocks in this region. lead. Large volumes of pyroxene andesite have The best explanation of the isotopic and played an important role in the geologic trace-element data appears to be a multistage development of the western United States. model in which orogenic andesite is derived Along the Cascade crest line, from northern from three or more stages of partial melting California to southern Washington, orogenic of mantle material in which crustal mate- andesite is closely associated with large volumes rials play an insignificant role. This model best of contemporaneous high-alumina basalt. Calcic explains the uniform Sr87/Sr86 values in the and calc-alkalic rocks were also being extruded lavas over a wide range of strontium con- during the Tertiary and are exposed in the centrations. It can also explain why strontium western Cascade Mountains. Thus, it is evi- in lavas extruded at continental margins, dent, from the geological data, that the process such as in the Cascade Mountains and in that formed these volcanic rocks has been Japan, has the same isotopic composition as active over a of several million years. strontium in lavas from the Mariana arc system in the Pacific Ocean. PREVIOUS WORK pb206/pb204 and pb208/pb204 yalues m Geological Investigations Cascade lavas may show a correlation with crustal thickness, but this does not prove that Numerous geological investigations have crustal contamination is responsible for the been carried out in the Cascade Mountains. trend. The lead and strontium data exhibit no Williams (1934, 1935, 1942, 1944), Thayer correlation with the Quartz Diorite Line of (1936), Verhoogen (1937), Coombs (1939), Moore. Anderson (1941), Waters (1961), Fiske and others (1963), Hopson and others (1965, 1967), INTRODUCTION Sheppard (1967), Higgins (1968), Wise (1969), The origin of orogenic andesite is currently Tabor and Crowder (1969), and others have one of the more controversial problems in made detailed field, petrographic, and chemical igneous geology. Workers have proposed studies of individual volcanic centers. Many of numerous mechanisms for the generation of these workers concluded that andesites dif- andesitic liquids. These include differentiation ferentiated from the high-alumina basaltic from parent basaltic magma, derivation from magma associated with the volcanic centers the upper mantle by one or more stages of they studied. Waters (1962) pointed out that partial melting, contamination of basaltic this differentiation would occur at shallow magma by crustal rocks, melting of geosynclinal levels because contemporaneous high-alumina sediments, and anatexis of crustal rocks. The basalt was commonly being extruded nearby recently formulated mechanism of subduction and because structure in this region would of segments of ocean floor along Benioff zones form shallow fault- reservoirs. Waters in island-arc environments has led to still other also suggested that if andesite were produced theories of petrogenesis of calcic and ca.c- by crustal contamination of basaltic magma, alkalic lava. The various theories are being that process must also take place at shallow tested from many standpoints. These include levels for the same reasons. major- and trace-element chemistry, experi- Wise (1969) pointed out the apparent exis- mental petrology, field work, and isotopic tence of two andesitic magma series at Mt. investigations, including both light stable and Hood; one is a high-alumina basalt differentia- radiogenic isotopes. This study will attempt tion sequence, and the is the pyroxene to place some limits on the origin of andesitic andesite sequence of Mt. Hood which differed magma for the Cascade volcanic province, from the first in having a lower K2O concentra- mainly by comparing the isotopic composition tion for any given SÍO2 value. Wise proposed of lead and strontium in Cascade andesitic that the andesitic magma of Mt. Hood formed lava with those from possible source materials. as a primary magma rather than as a product The Cascade Mountains are typical of of differentiation from a high-alumina basalt continental andesitic chains in most respects series and that the K.2O-SÍO2 number of these

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Figure 1. Geologic sketch map of the western United Adams, Washington; H, Mt. Hood, Oregon; J, Mt. States modified from the U.S.G.S. Tectonic Map of the Jefferson, Oregon; T, Three Sisters volcanos, Oregon; United States (1962b). The major volcanic centers of the N, Newberry Caldera, Oregon ; C, Crater Lake, Oregon Cascade Mountains, which extruded large volumes of (which is all that remains of Mt. Mazama); P, Mt. pyroxene andesite during the Quaternary, are located McLoughlin (formerly Mt. Pitt), Oregon; M, Medicine by the following symbols: B, Mt. Baker, Washington; Lake volcanic center, California; S, Mt. Shasta, G, Glacier Peak, Washington; R, Mt. Rainier, Wash- California; and L, Lassen Peak, California. ington; SH, Mt. St. Helens, Washington; A, Mt.

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two rock series was a primary feature of the element abundances in andesite from several partial melting processes responsible for their places in the world (but none from the Cascade formation. Range). Their data, particularly on the rare Work by Fiske and others (1963) and by earth elements, can be used to place limits on Hopson and others (1965, 1967) pointed out the genesis of andesitic magma associated with that Mt. Rainier, Mt. Baker, and Glacier Pacific Ocean island arcs. The lack of a positive Peak differ from the other major Cascade anomaly indicates that the high- volcanic centers to the south. These northern alumina character of the andesitic magma andesitic volcanic centers show a consistent cannot be produced by concentrating plagio- major-element chemistry, predominantly sili- clase crystals (this argument assumes that ceous pyroxene andesite, whereas those from the europium is reduced in the magma as was the southern part of the chain show a much true for the San Marcos Gabbro [Towell and greater diversity, ranging from olivine basalt others, 1965]). Rare-earth abundances in to dacite or rhyodacite. These data are sum- andesite are quite variable, usually about one- marized in part by McBirney (1968). Hopson third to one half that of shale, and their and others (1967) have proposed an anatectic patterns are subparallel to that of shale, but origin for the andesitic and dacitic lavas of the the variation is about the same as that observed Northern Cascade Mountains. in the volcanic rocks from Hawaii (Schilling and Winchester, 1969). Mixtures of basalt- Geophysical Investigations granite and basalt-granodiorite do not yield Geophysical data from the continents! area the appropriate trace-element abundances shown in Figure 1 are lacking. Bouguer gravity found in andesites in the southwest Pacific maps (U.S. Geological Survey, 1962a) show Ocean (Taylor, 1969) or in the Cascade that the gravity anomaly dips steeply to the Mountains (Church, unpub. data). Crystal- east, striking parallel to the coast, and reaching lization of large percentages (that is, greater a local maximum under the Northern Cascade than 2 percent) of magnetite or olivine from Mountains. a high-alumina basalt magma is also unlikely A limited number of seismic refraction to produce andesite (Duncan and Taylor, profiles exists for this region. White and 1968) although this conclusion has been chal- Savage (1965) published a seismic profile shot lenged by Hedge (1971). Precise knowledge of along the 50° N. line of latitude, crossing the the distribution coefficients for nickel and Cascade Mountains trend north of Mt. Baker. magnesium between various and Additional crustal sections can be constructed liquids would help to solve this enigma. Taylor from the data of Dehlinger (1969), Dehlinger (1969) has proposed that andesites are primary and others (1968), and Shor and others (1968). magmas from the upper mantle derived via However, the best seismic data for this region the two-stage partial melting process proposed form a northeast-southwest composite profile by Green and Ringwood (1968) and that the which crosses the Cascade trend at a 40° angle alkalies and other large mobile cations needed (Johnson and Couch, 1970). This section is a to make andesitic magma, have come from composite line from Revelstoke, British oceanic sediments which have been carried Columbia, to the Cascadia Plain off the coast down into the mantle along active Benioff of Oregon. All of these data yield valuable zones. information on crustal thickness and the nature of the oceanic-continental crust transition zone Isotopic Geochemical Tracers and suggest that the Cascade Mountains root The isotopic systems of -strontium zone extends to about 33 km depth. and uranium-thorium-lead have a unique potential as geochemical tracers and, as such, Geochemical Studies are particularly useful in attempting to solve Little trace element work has been done on problems of magma genesis because isotopic the calcic and calc-alkalic rocks from the ratios, unlike element concentrations, are not Cascade Mountains. Trace-element abundances affected by chemical processes, such as crystal have been determined for rocks from Glacier fractionation and partial melting. Several Peak (Tabor and Crowder, 1969) and from Mt. isotope studies of the Cascade Ranges and Lassen and Crater Lake (Nockolds and Allen, adjoining Pacific Ocean floor have shown the 1953). Taylor (1969) and Taylor and others potential value of isotopic tracers in decipher- (1969a, 1969b, 1971) have studied trace- ing genetic relations among the rocks. Stron-

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tium in oceanic tholeiite from the Juan de Fuca and Gorda Ridges (Hedge and Peterman, 1970) is less radiogenic than strontium in Cascade andesite (Hedge and others, 1970; Peterman and others, 1970a; Church, 1970); strontium in the Cascade andesite is, in turn, less radio- genic than strontium in coastal geosynclinal sedimentary rocks of Oregon (Peterman and others, 1967) or in recent Pacific Ocean A0=?.34i M = 8.9 sediments off Washington and Oregon (Church, Bo = 10.213 K = 3.B

1971). Strontium in the andesite is indistin- Cg s 28.96 T0= 4.W. Id'ïEARS guishable from that in Cascade high-alumina basalt (Hedge and others, 1970; Church, 1970). The fewer lead data show similar patterns. Lead in the Cascade andesite (Church, 1970, Figure 2. Lead isotope growth curves. The lower and this paper) is more radiogenic than lead curve is for U236 and U238 decay products (Pb207 and in oceanic ridge basalt (East Pacific Rise, Pb206) whereas the upper curve is for Th232 and U238 Tatsumoto, 1966), but less radiogenic th'an lead decay products (Pb208 and Pb206) plotted relative to the 204 in the Oregon coastal sedimentary rocks stable isotope Pb . (Tatsumoto and Snavely, 1969). Lead data from siliceous igneous rocks (Doe, 1967) for this region appear to be slightly higher than is, they have model ages which are younger than that from the calcic and calc-alkaline rocks of their geologic age. Conversely, samples whose the Cascade Mountains, but we shall not com- lead isotopic ratios plot to the left of this line ment on them until more data have been ob- have model ages which are older than their tained. Data are also available for lead in geologic ages. All the lead data in this paper will sediments from the northeastern Pacific Ocean be presented on this type of growth-curve basin (Chow and Patterson, 1959, 1962). The diagram. However, since this study deals only present study will utilize these and other with volcanic rocks which are, at most, a previously unpublished data to study rock million years old, all of the data will be con- relations in the Cascade Ranges, placing centrated in the two small squares outlined particular emphasis on the problem of the in Figure 2 which represent the maximum origin of the andesitic lava. bounds of all the lead-isotope diagrams to follow. The new isotopic and chemical data ISOTOPIC DETERMINATIONS determined during this study on high-alumina The most convenient and geologically mean- basalt, andesite, and dacite from the area of ingful method of comparing lead-isotope data study are presented in Table 1. All isotopic utilizes the growth curve concepts and graphic data have been corrected for fractionation methods. Figure 2 shows a particular set of based on replicate analyses of CIT reference curves based on a simple closed model system lead and of a basalt from the same locale as that indicate how Pb206/Pb204, Pb207/Pb204, BCR-1 (see Appendix for details). 208 204 and Pb /Pb values change through time In order to test the isotopic homogeneity of relative to one another. The values used for a single volcano, nine samples from the Mt. initial lead ratios are those determined from Hood area were analyzed for lead and stron- meteoritic troilite (Oversby, 1970). The tium. Plagioclase phenocrysts and groundmass geochron is the line which passes through the separates from sample H-68-14 were also 206 204 primordial lead and intersects the Pb /Pb - analyzed. The lead and strontium data indicate 207 204 Pb /Pb growth curve at the present, no isotopic discrepancy between phenocrysts assuming a 4.55 b.y. old earth. All recent and groundmass, in contrast to the observations samples whose lead isotopic ratios fall on that on separates from an alkali-basalt dike line appear to have approximated this simple from West Texas (Dasch, 1969a) and lead data closed model system for the entire duration of on plagioclase and host basalts from the their history, and are said to have model ages southern Rockies (Doe and others, 1969). The of zero million years. Recent samples whose andesite-dacite isotopic data from Mt. Hood lead isotopic ratios plot to the right of this samples are all identical within the range of line have future (or negative) model ages; that experimental error and indicate that the lavas

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STATE Pb*!i Pbiü Pbîîl Sr" Percent Percent Pb Rb Sr Locality Rock type 5 -pgîôT •pgTôi •spsr Tb " ± 0 <102 K PPm ppm ppm Sample

WASHINGTON Mt. Baker B-68-2 andesite 18.71 15.53 38.21 0.7037 0.0002 55.2 1.2 4.0 13 725 B-68-13* andesite 18.90 15.55 38.43 0.7031 0.0004 6}.5 1.95 4.5 37 430

Glacier Peak GP-1 andesite 18.89 15.61 38.62 0.7042 0.0006 6" .5 1.6 6.5 26 350 GP-26b-62 dad te 18.85 15.62 38.62 61.4 1.54 4.0 21 630 DFC-65-61# basalt 18.75 15.60 38.45 0.7032 0.0001 50.8 0.32 4 613

Mt. Rainier RR-4 andesite 18.98 15.61 38.66 0.7040 0.001 61.2 1.58 7.0 27 375 R2-1 andesite 18.91 15.58 38.55 0.7034 0.001 56.4 1.23 3.0 22 350 R-Doe+ obsidian 19.04 15.62 38.83

Mt. St. Helens SH-28-1 dacite 18.86 15.65 38.73 0.704C 0.0004 64.5 1.3 4.5 28 325 SH-29-1 andesite 18.83 15.60 38.60 0.7037 0.001 59.8 1.1 7.5 26 440 SH-34-1 basalt 18.87 15.62 38.72 0.7027 0.0006 48.3 0.39 2.0 6 225

Mt. Adams A-68-18 andesite 18.99 15.60 38.56 0.7039 0.0007 55.9 1.2 4.4 26 420 A-68-23 andesite 18.99 15.65 38.62 0.7035 0.0004 59.2 2.16 6.0 62 395

S1mco Mts. CMI-1 basalt 19.26 15.63 38.71 1.5 SM-3 dacite 19.17 15.71 39.01 8.0

OREGON Mt. Hood H-68-1 daclte 18.72 15.51 38.38 0.7035 0.0003 61.65 1.59 5.4 18 385 H-68-4B andésite 18.80 15.56 38.38 0.7046 0.0008 59.4 1.7 4.3 27 565 H-68-9A* basalt 18.81 15.54 38.41 0.7042 0.0005 49.8 0.84 3.0 6 490 H-68-9B* xenolith 18.76 15.58 38.27 0.7055 0.0005 49.8 0.1 1.6 <1 850 H-68-13 basalt 0.7046 0.0004 56. S 0.92 8 540 H-68-14 andesite 18.75 15.56 38.32 0.7039 0.0006 62.4 1.17 5.4 16 565 H-68-14* 18.78 15.54 38.3C H-68-14p plagioclase 18.75 15.55 38.39 0.7039 0.0005 2.5 H-68-14g groundmass 18.73 15.53 38.33 0.7035 0.0007 6.7 H-68-15 andesite 18.77 15.54 38.41 0.7040 0.001 61.3 0.92 3.6 14 325 H-E-117-4 dacite 0.7038 0.0006 CR-2 basalt 18.92 15.55 38.42 1.0 CR-3 andesite 18.77 15.54 38.42 5.5 CR-7 andesite 18.80 15.59 38.49 3.0 CR-68-9 basalt 18.87 15.56 38.50 0.7045 0.0007 48.5 0.21 0.4 5 190 5B37 basalt 0.7034 0.0009 52.5 1.29 13 800

Three Sisters S-68-7* andesite 18.88 15.59 38.63 0.7036 0.0006 58.0 1.29 3.3 20 355 Rb-68-3B basalt 18.85 15.61 38.54 0.7040 0.001 54.4 0.92 1.0 14 330

Newberry Caldera NC-68-3* basalt 18.96 15.57 38.69 0.7028 0.0009 53.2 1.22 3.2 20 365 NC-PC-6* andesite 18.97 15.63 38.68 0.7037 0.0004 54.2 0.77 2.0 10 265 HAB-68-12 basalt 0.7037 0.0005

Crater Lake CL-68-6* dacite 18.83 15.57 38.47 0.7042 0.001 67.2 1.58 1.5 19 775 CL-68-10 dacite 0.7048 0.001 68.26 2.12 34 167 715 CL-68-11* basalt 18.93 15.62 38.59 0.7036 0.0005 51.4 1.06 4.5 9 10 735 CL-68-12 basalt 0.7040' 0.0008 54.5 1.2

CALIFORNIA Mt. Shasta MS-68-6 basalt 18.76 15.54 38.25 55.7 0.93 1.5 13 1050 MS-68-7* dacite 18.95 15.66 38.70 70.0 2.34 7.8 44 103 MS-68-12* andesite 18.92 15.65 38.72 61.0 1.14 15 435

Medicine Lake ML-68-6 obsidian 18.95 15.58 38.63 71.0 3.18 5.0 106 105 ML-68-11B basalt 18.93 15.60 33.53 49.2 0.56 1.2 12 305 15.63 38.72 ML-Doet obsidian 18.93

Mt. Lassen L-68-1 andesite 18.84 15.55 38.42 0.7040 0.0006 61.8 1.65 4.6 32 450 L-68-4 dacite 63.8 1.91 57 455 L-68-10* basalt 18.98 15.66 38.81 3.3 L-68-11* dacite 18.91 15.60 38.63 69.4 2.41 3.0 57 308 L-68-14 basalt 18.95 15.64 38.78 0.7039 0.0007 56.5 1.83 12.0 47 420

*Lead data by silica gel method; Shields (1970). tLead data froa Doe (1967). ttSr data from Hedge and others (1970); chemical data from Tabor and Crowder (1969, p. 31, no. 21). 1The lead data are normalized to the absolute values of CIT reference lead (Catanzaro, 1967). 2Strontium ratios are relative to a value of Sr87/Sr86 " 0.7081 for tha Simer and .imend SrCO$ reference samples.

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are isotopically homogeneous with respect to lead and strontium. The lead from highly silicic rocks (Mt. Rainier and Medicine Lake; Doe, 1967) is slightly more radiogenic than the lead from basalt and andesite from these volcanic centers. Data from ML-68-6 should compare directly with Doe's obsidian data (corrected to ClT absolute reference values) as both samples are from Little Glass Mountain. The differences between these analyses are near the maximum range of uncertainty expected for the ratios given. Alternatively, the obsidian flow was contaminated near the surface and the lead did not come to isotopic equilibrium before extrusion. These data are included in the "Cascade volcanic suite" lead field, although we will not discuss the meaning of the lead data from these silicic volcanic rocks until more data are available and until these analytical discrepancies have been resolved. The lead from the Simco Mountains, which Figure 3. Lead isotope data from volcanic rocks lie 30 mi southeast of Mt. Adams and 60 mi east from the Cascade Mountains and from Pacific Ocean of the trend of the Cascade Mountains in ridge basalt. The error box is shown in the upper left southern Washington, is much more radiogenic of each diagram, (x) East Pacific Rise basalt lead data than the bulk of the lead from the Cascade (Tatsumoto, 1966); ( O) Calc-alkaline andesite, (•) dacite; (•) obsidian (Doe, 1967 and this study); and volcanic rocks, and the lead contents are some- (+) high-alumina basalt lead data from the Cascade what higher than those of corresponding rock Mountains volcanic rocks. types from the Cascade Mountains proper. Therefore, the Simco Mountain lead data will cones, with lead data from andesite and dacite, be excluded from the "Cascade volcanic suite" indicates that from the basalt are es- lead field. sentially identical with lead from andesite as Following the suggestion of Doe and others seen in Figure 3. All of the available strontium (1969) that continental contamination of isotope data, including those of Hedge and recent volcanic rocks may be controlled by the others (1970) and of Peterman and others lead isotopic composition of basement rock (1970a) for Cascade andesite and high-alumina feldspars, lead and strontium analyses have basalt are presented in Figure 4. The majority been done on leucocratic separates of "- of the analyses fall between 0.7035 and 0.7042, ment" rocks from northern Washington. These a range of only 0.1 percent for all types of separates contain the least radiogenic- com- lava. The range is only slightly outside the ponents of lead and strontium in the rocks error limits of the measurements. There is no because these minerals have low parent/ significant difference between the isotopic daughter ratios, except where the potassium composition of strontium from andesite and feldspar content is relatively high. The isotopic basalt treated as groups. From the lead and data for this suite of rocks are presented in strontium results we conclude that the high- Table 2. alumina basalt and the andesitic and dacitic lava could be derived from common sources. DISCUSSION There is nothing in the isotope data to preclude derivation of andesite .from differentiation of Basalt-Andesite Comparisons high-alumina basalt magma, although as noted A comparison of the high-alumina basalt lead above, nickel distribution may preclude such data, which include data from high-alumina an origin. We shall return to this topic below. basalt from the older flows underlying the It is also instructive to compare the Cascade andesitic volcanic centers as well as from high- volcanic rock lead data with those of oceanic alumina basalt from nearby satellitic cinder tholeiite because oceanic ridge basalt is

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Rock unit sample Whole- rock data Leucoc-atic separate data

Percent Percent Rb Sr Pb5> Pbjii- Pb Rb Sr SiOj K ppm ppm "Pb*" Pb2°* -Pb"* V6 ppm ppm ppm

Pre-Tertiary rocks Chelan rocks Jm-68-4 60.0 1.35 37 532 18.83 15.60 38.55 0.7051 0.0008 6.0 10 300 Jm-68-10« 57.8 0.33 6 400 18.65 15.59 38.35 0,7053 (1.0006 1.7 <1 295 Jm-68-11* 71.3 0.83 20 303 18.83 15.49 38.32 0.7071 0.0008 1.3 2 205 C-514-1* 18.88 15.60 38.43 0.7059 0.0008 1.0 8 138 C-845-1 18.88 15.58 38.37 0.7058 0.0007 4.6 16 298 CQ-10-1 W 58.0 0.46 10 885 18.88 15.66 38.65 0.7049 0.0007 10.0 C-340-3 VI 62.7 0.48 13 865 18.80 15.59 38.41 0.7043 0.0007 9.0 Swakane Gneiss JM-68-8* 72.2 1.82 55 334 19.68 15.66 39.28 0.7097 0.0010 3.2 13 168 C-883-2 19.19 15.63 38.86 0.7082 0.0007 30.0 66 185 CQ-12-1+W 6 400 19.45 15.60 39.10 0.712 0.002 10.0 Skagit Gneiss JM-69-10* 64.2 1.50 32 245 18.92 15.61 38.63 0.7048 0.0008 2.0 <1 185 JM-69-12* 70.2 0.01 0 512 18.82 15.61 38.35 0.7066 0.0010 2.5 17 240 Turtleback complex JM-68-21 63.2 0.58 8 80 0.7043 0.0007 29 130 Yellow Astor complex JM-69-7* 59.5 1.64 43 135 18.60 15.58 38.49 0.7114 0.0308 1.5 25 97 JM-69-9* 63.8 2.48 57 175 21.72 15.70 40.19 1.3 67 110 Marblemount belt JM-68-16* 58.8 0.6 16 560 18.87 15.56 38.49 0.7056 0.0007 3.5 4 430 Tertiary rocks Cloudy Pass pluton CR-20 W 19.04 15.52 38.46 5.5 CR-21 W 18.99 15.55 38.77 5.5 Sitkum stock RWT-212-62 W 19.04 15.58 38.59 8.0 Duncan Hill pluton C-541 i* 74.3 4.80 156 85 18.99 15.62 38.68 0.7086 0.0009 7.0 208 95 C-671 70.5 3.62 103 330 102 195 C-685 i* 67.7 2.87 69 330 18.93 15.62 38.60 0.7055 0.0007 3.5 68 240 C-747 61.5 1.73 44 500 11 310

C-753 64.8 2.60 63 430 0.7051 0.0004 36 265 C-795* 62.7 1.97 55 375 18.94 15.59 38.50 0.7050 0.0007 2.8 8.5 195

W - All analyses made on whole rocks. + = Strontium isotopic analysis by L. T. Aldrich (from Tilton cjid others, 1965). # = KzO and S1O2 data from unpublished data of C. A. Hopson. * = Lead isotopic analyses done by silica gel method of Shields (1970, p. 71-75).

thought to represent the largest partial melt isotope fractionation or differences between of the upper mantle (Gast, 1968; Kay and methods or investigators based on data for the others, 1970) and because oceanic ridge basalt California Institute of Technology reference figures prominently in subduction zone models lead sample. for the origin of volcanic rocks in island-arc Strontium isotopic data are compared in environments (Armstrong and Cooper, 1971; Figure 4. Sr87/Sr86 values for oceanic tholeiite Tatsumoto, 1969). Figure 3 shows data for from the East Pacific, Juan de Fuca, Gorda, three tholeiites from the East Pacific Rise and Explorer Rises, which average 0.7026, are (Tatsumoto, 1966). Tatsumoto (1970, written lower than Sr87/Sr86 values from the Cascade commun.) has recently found similar isotopic volcanic rocks. Still another comparison can compositions for lead in oceanic tholeiite from be made with tholeiitic basalt from the Francis- the Juan de Fuca and Gorda rises. Comparison can Formation in California (Sinha and Davis, of the Cascade volcanic rock lead data shows 1970). The chemical composition of these that the East Pacific rise tholeiite has lower basalts is similar to that of the oceanic tholeiite, Pb206/Pb204, Pb207/Pb204, and Pb208/Pb204 even though the rocks have experienced values than the Cascade volcanic rocks. The low-temperature metamorphism (Ernst, differences are far outside errors due to possible 1970). The lead ratios from these rocks cover

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15 r- tium in these rocks be controlled by strontium like that in alkali basalt. Lead data, although fewer than those for strontium, closely parallel the strontium data. Pb206/Pb204, Pb207/Pb204, 10 - and Pb208/Pb204 values are higher in the Cascade lavas than in oceanic tholeiite (Fig. NO. OF 3) but are well within the broad range of lead OBS. isotopic ratios from alkali basalt from ocean 5 |- islands (Oversby and Gast, 1970). Again, the isotopic composition of lead in the Cascade calcic and calc-alkalic lavas would have to be controlled in the mantle by the lead like that in alkali basalt. It should be noted that the .702 .704 .706 concentration of lead (and strontium) in Figure 4. Strontium isotopic composition of high- alkali basalt is about five times that in oceanic alumina basalt (hashured) and andesite and dacite tholeiite, and that the abundance of alkali (open) from the Cascade volcanic chain; includes data from Hedge and others (1970) and Peterman and others basalt in ocean basins seems to be much smaller (1970a). Data from Pacific Ocean ridge basalt (x) from than that of oceanic tholeiite (Engel and Engel, summary by Peterman and others (1970a), and from 1964). Juan de Fuca, Gorda, and Explorer rises (stippled), When isotopic data for calcic and calc- Hedge and Peterman (1970), are included for compar- ison. alkaline lavas from the entire circum-Pacific belt are compared, a mantle origin gains added 87 86 a large range, and include some samples that plausibility. The Sr /Sr data for these are even more radiogenic than the Cascade rocks (see Table 3) show that there is little leads and they appear to have developed in a variation except in rocks from New Zealand. slightly lower fi environment. The Sr87/Sr86 There seems to be little effect by continental values from the lavas are rather uniform at a material unless xenoliths are actually included value of around 0.707, indicating a much more in the rocks themselves (see the data of Push- radiogenic strontium than that in any of the kar, 1968). A simple explanation for the 87 86 other rock groups discussed above. If these uniformity of the Sr /Sr in these chains, Sr^/Sr86 values are the result of equilibration regardless of the crustal structure, is that there with water, then "weathering" or meta- is very little crustal contamination during the morphic process affecting oceanic ridge basalt ascent of the magmas except in the case of New source materials could be a very important Zealand. The isotopic composition of process in the genesis question (Church, 1972). in andesite is indistinguishable from that in basalt and is also consistent with a mantle Andesite Genesis origin (Taylor, 1968). Mantle Origins. The isotopic data do not Coats (1962) and Gorshkov (1969) show exclude a mantle origin for the Cascade vol- that major-element concentrations of calcic canic rocks; to some extent they support such and calc-alkalic rocks in the Aleutian and an origin. Oceanic basalt probably provides Kurile arcs are quite similar in the segments of the best index of the isotopic composition of the arcs close to the continents and in segments lead and strontium in the outer mantle. far removed from land. A plot of K/Rb versus Peterman and Hedge (1971) report a range of weight percent K for the Cascade rocks shows a Sr87/Sr86 from 0.7012 to 0.7057 with a weighted trend identical to that found by Jake§ and average of 0.7035 for 90 oceanic basalt samples. White (1970) for other circum-Pacific island The average Sr87/Sr86 value is remarkably arcs. At low potassium concentrations (that is, close to that of the Cascade volcanic rocks. The basalt), the K/Rb ratio is near 1,000. With oceanic average does depend strongly on increasing potassium content, the K/Rb ratio weighting of various basalt types, however. decreases regularly to about 350 at about 2.0 Whereas the average ratio for tholeiites is weight percent K. No contamination effect can 0.7026 (Hedge and Peterman, 1970), the be observed using K/Rb as an indicator for Cas- "oceanic average" depends heavily on the cade Mountain lavas which have obviously weighting of alkali basalt. A mantle origin for been extruded through the continental plate. the Cascade calcic and calc-alkalic lavas would Minor- and trace-element data argue against require that the isotopic composition of stron- derivation of andesite directly from primitive

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TABLE 3. Sr87/Sr06 DATA mantle in a single episode of partial me.ting FOR ISLAND-ARC VOLCANIC ROCKS (Taylor and others, 1969b; Taylor, 1969). For

example, the rare-earth data from some Locale Roc» type Sr"/Sr,e* Reference andesites do not show the depletion of heavy rare-earth elements with respect to light rare- Marianas B, D 0.7037 Pushkar (1968) earth elements that such a model predicts. K/ 0.7042 Hedge (1966) Rb and K/Cs are high while Ba/Sr, Rb/Sr, Salpan A, C 0.7036 Pushkar (1968) Izu Islands, and Th/U are low compared to those expected Japan B, A 0.7038 Pushkar (1968) for a single-stage derivation from a primitive N. Honshu, Hedge and mantle. There are also problems with the Ni, Japan B, A 0.7037 Knight (1969) Co, and V concentrations as well. Central Anerica B, A, D 0.7041 Pushkar (1968) New Britain B. A, D 0.7036 Peterman and A modification of a mantle-origin modsl is others (1970c) one in which andesite is derived by differentia- Cascade Petennan and Mountains B, A, D 0.7037 others (1970a) tion of high-alumina basalt magma. We have Hedge and others (1970) already noted that the isotopic composition Church (1970) of lead and strontium is indistinguishable New Zealand B 0.7043 Ewart and from that in Cascade high-alumina basalt and Stipp (1968) andesite, and that these data argue strongly for A 0.7056 some genetic relation between the two rock Aleutians D 0.7042 Hedge (1966) Lesser Antilles B, A 0.7043 Hedge and types. Many trace-element data from calcic Lewis (1971) and calc-alkaline rocks from other circum- Pacific island arcs have very similar concentra- * = Sr"7/SrBt «IT SzCOa - 0.70S1. tions including Co, V, Sc, Cr, and Zr (Taylor, B ~ basalti A = andesitei D - dacites. 1969); their potassium contents also cover similar ranges (Jakes and White, 1970). mantle origin by direct partial melting can be Most of the strontium concentration data solved with a multistage melting model. Data indicate similar ranges of concentrations for from rare earths and other trace elements sug- andesite and high-alumina basalt; however, gest that oceanic tholeiite is derived from in some cases, high strontium concentrations mantle regions that have experienced previous present a problem for basalt differentiation partial melting episodes (Tatsumoto, 1966; models. Peterman and others (1970a) found Gast, 1968; Kay and others, 1970) because strontium concentrations of 1,200 to 1,500 ppm of their depleted trace-element characteristics. in andesite from Mt. Shasta and Mt. Lassen If orogenic calcic and calc-alkalic rocks are and much lower concentrations of around 400 derived by partial melting of oceanic tholeiite ppm for nearby basalt. They pointed out that as proposed by Green and Ringwood (1968), these data oppose derivation of andesite from the trace-element data can be accounted for the basalt because of the severe requirement by multiple episodes of partial melting. Stage placed on the plagioclase content of the source one in our model is the formation of a volume materials. Hedge and others (1970) have also of mantle depleted in large cations, which found a high (1,544 ppm) strontium con- have been removed by a partial melting process. centration in a basalt from the Mt. Hood area. Stage two is the formation of oceanic tholeiite It seems unlikely that so many trace-element by a large degree of partial melting of the concentrations would agree so closely for both depleted mantle—10 to 15 percent according rock types if all andesites formed by differentia- to the models of Gast (1968) and Kay and tion from high-alumina basalt magmas. Cer- others (1970). Stage three involves subduction tainly, some sequences such as the Gunsight of the oceanic crust followed by partial melting Butte sequence from Mt. Hood (Wise, 1969) in the descending slab to form calcic and calc- are related by differentiation, but those rocks alkaline rocks as proposed by Green and Ring- which form large andesitic volcanos are another wood (1968). This model is similar to, but not problem. We therefore believe that the bad the same as, Taylor's two-stage model (Taylor and strontium isotope data indicate that island- and others, 1969b; Taylor, 1969) in which the arc basalts and andesites are derived from initial stage of melting is assumed to take place similar source materials, but that they are not in an undepleted mantle. Since our model in- necessarily related by magmatic differentiation. volves more than two episodes of partial melt- ing, we w ill call it a multistage model. However, Multistage Model. The trace element the isotcpic data require that material more problems discussed above in connection with a

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compare the lead isotopic data for northeast Pacific Ocean sediments (Chow and Patterson, 1959, 1962) with the Cascade volcanic lead data. The Pb207/Pb204 and Pb207/Pb204 values in the sediments are high enough to satisfy the Cascade lead data; however, the Pb206/ Pb204 values in the sediments are lower than PI,* those in the lavas. The overall lead data thus indicate that mixtures of tholeiite and oceanic sediment lead cannot account for the isotopic composition in the Cascade volcanic rocks. Of course this conclusion assumes that the lead data from the oceanic tholeiite and open-ocean sediments are representative. It should be fv; noted, however, that none of the sediment lead PI,* data is from the immediate area off the Wash- ington-Oregon coast, and that all data are from shallow cores. From a qualitative standpoint, sediments could be used to supply radiogenic strontium to raise the Sr87/Sr86 ratio from 0.7026 to Figure 5. Lead isotopic data for northeast Pacific 0.7036 in the Cascade lavas as can be seen in open-ocean sediments (•) from Chow and Patterson Figure 9. These clay data (Church, 1971) are (1959, 1962). Lead data for the Cascade volcanic rocks from the vicinity of the Juan de Fuca and (shown as a field) and for the East Pacific Ocean rise Gorda rises and are thus pertinent to this study. basalts (x) have been included for comparison. If world-wide oceanic clay data were used (Dasch, 1969b), the same conclusion would radiogenic than that which produces oceanic result. However, when strontium concentra- tholeiite be involved in the formation of tions are considered, the tholeiite-clay mixture andesitic lava; that is, that the source materials model encounters difficulties. For example, it is similar to those which are melted to produce not obvious that one can produce the high alkali basalt are needed to supply the radiogenic concentrations (1,000 to 1,400 ppm) of stron- isotopes if crustal sources are not involved. tium found in northern California andesites 87 86 Crustal Contamination Models. Many with a Sr /Sr value of 0.7036 (Peterman and models for the formation of orogenic andesites others, 1970a) by adding sediments that con- involve subduction of ocean-floor tholeiitic tain about 100 ppm of more radiogenic stron- basalt (and oceanic sediments) along a Beniolf tium (~.712) to oceanic tholeiite (~.7026). zone, followed by partial melting at depth in The uniformity of isotopic composition of the descending limb. Green and Ringwood strontium in the Cascade volcanic rocks over a (1968) have presented evidence that partial wide range of concentrations from 100 to melting of tholeiitic basalt under these condi- 1,400 ppm argues strongly against the above tions would produce a spectrum of compositions mixing model if small batch partial melting is ranging from basalt to dacite, depending on the rule. From present information we con- pressure. Taylor (1969) and Taylor and others clude that tholeiite-sediment mixing does not (1969a, 1969b) have pointed out the need to explain the strontium data satisfactorily. add elements with large ionic radii, namely Another possible source of strontium and alkali and alkaline earth elements and to de- lead is eugeosynclinal sediments. In New plete some of the transition in the source Zealand, Ewart and Stipp (1968) have found materials for andesitic liquids. Since the that graywacke sediments from the marginal isotopic composition of lead and strontium (though not from the axial) facies of the North in oceanic tholeiite does not agree with that in Island geosyncline and neighboring andesites the Cascade volcanic rocks, it seems likely that contain strontium with nearly identical isotopic the necessary radiogenic lead and strontium, compositions. The isotopic compositions of as well as the large cations, could be supplied lead in the sediments and andesite also agree from the oceanic sediments (Tatsumoto, 1969; closely (Armstrong and Cooper, 1971), indi- Taylor and others, 1969a). In Figure 5, we cating that for New Zealand, graywackes

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Sr 87 St86 Figure 6. Isotopic composition of strontium in Pacific Ocean (gj Church (1971). Also included for Oregon coastal graywackes (•) Peterman and others reference are the strontium data from the Cascade (1967), Franciscan volcanics (•) Sinha and Davis volcanic rock suite and the Pacific Ocean ridge basalt. (1970), and open-ocean sediments from the northeast Other symbols are the same as in Figure 4. could play a dominant role in andesite genesis. basalt with lead from the Cascade volcanic Such a model does not fit the Cascade data. rocks. It seems possible that the Franciscan Peterman and others (1967) have found Sr87/ rocks could be representative of the tholeiitic Sr86 values near 0.708 in graywackes and other rocks actually found in subduction zones, eugeosynclinal sedimentary rocks of Oregon in which case the radiogenic lead needed for and northern California, which have Sr87/Sr86 the calcic and calc-alkalic magma is available. values similar to that of the marginal géosyn- The Sr87/Sr86 value in the Franciscan basalt clinal faciès rocks in New Zealand. Peterman averages 0.707 (Sinha and Davis, 1970), a value and others (1970a) and Church (1970) have considerably higher than that for the Cascade noted that strontium in the sediments is much lavas. However, Sinha and Davis concluded too radiogenic to be the main source of stron- that the high Sr87/Sr86 values were due to tium for the andesites (see Figure 6). The lead equilibration with sea-water strontium. Oceanic data yield the same result if the few measure- tholeiitic basalt, somewhat less altered by ments of Tatsumoto and Snavely (1969), shown exchange with sea water, might provide source in Figure 7, are representative of the isotopic material with major-element concentrations character of lead in early Tertiary sediments and isotopic compositions of lead and strontium deposited off the Washington-Oregon coast. appropriate for the Cascade lavas or for all Lead in the Cascade volcanic rocks would calc-alkaline lavas. Mixing would have to be apparently have higher Pb206/Pb204 values if on a large scale and very complete to account the isotopic composition were derived largely for the uniformity of isotopic composition of from lead similar to that in the early Tertiary lead and strontium in the Cascade lavas com- sedimentary rocks. pared to the Franciscan volcanics, and the basaltic lavas containing 1,000 to 1,500 ppm Mixtures of the eugeosynclinal sedimentary strontium would still have to be accounted for rocks and oceanic tholeiite could produce in some special way. Alteration, however, strontium with the isotopic composition found would satisfy that criteria and may provide for the Cascade rocks, and probably lead as the correct chemical isotopic characteristics of well. This model encounters the same difficulty the source material. noted above for the model involving mixtures of oceanic tholeiite and open-ocean sediments; A more quantitative treatment of various namely, explaining the uniform strontium mixing models for the isotopic composition of isotopic composition in the lavas over a wide strontium is shown in Figure 9, which con- range of concentrations. siders strontium from three sources—mantle, Still another possible source of radiogenic coastal graywackes, and Pacific Ocean sedi- lead and strontium is indicated by data from ments. We assume that mantle strontium is Franciscan tholeiitic basalt from California mainly in clinopyroxen:: as suggested by the (Sinha and Davis, 1970). The Franciscan rocks data from a lherzolite nodule from Dish Hill, are altered basalt whose chemical composition California (Peterman and others, 1970b). The indicates that they were probably once clinopyroxene from the Dish Hill nodule con- oceanic tholeiite. Figure 8 compares the tains 85 percent of the strontium in the sample isotopic composition of lead in Franciscan on a mineral basis and has a strontium isotopic

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Pb ' Pb!

Figure 7. Lead isotope data from Oregon coastal graywackes (Tatsumoto and Snavely, 1969). (A) indicate measured isotopic ratios and (•) indicate Figure 8. Lead isotope data for the Franciscan isotopic compositions corrected for the growth of radio- volcanic rocks (Sinha and Davis, 1970). The open field genic lead. The stippled field is that defined by lead is that of the Cascade volcanic suite. isotopic data for northeast Pacific open-ocean sediments (from Fig. 5), and the open field is the Cascade volcanic we have not exceeded the range of Sr87/Sr86 suite. values from Cascade lavas, but we greatly composition of 0.7016. Pacific Ocean tholeiite exceed the physical limits of the crustal con- averages 0.7026, but basalts have been found taminant allowed by major-element chemistry, with Sr87/Sr86 values as low as 0.7012 (Hedge and the isotopic composition of lead. Figure 9 and Peterman, 1970). If we further assume, for also shows that a very wide range of strontium the sake of argument, that the higher Sr87/Sr86 concentrations would be required in the mantle values found in andesite from around the world material in order to account for the Cascade are not the result of partial melting and mixing data by pure mixing models. Partial melting of various mantle mineral phases but are caused models would help to meet this difficulty, by the addition of crustal material to mantle although strontium is not highly enriched by material along subduction zones, we can then partial melting (Gast, 1968). calculate the effect of this addition. The average It is also apparent from Figure 9 that the andesite and dacite from the Cascade Moun- Cascade lavas do not fit quantitative mixing tains have about 700 ppm of strontium with models involving sediments and oceanic 87 86 Sr /Sr = 0.7037. Assuming that this is the tholeiite, which contain approximately 100 result of mixing of nine parts mantle material ppm strontium with Sr87/Sr86 = 0.7026, if we 87 86 with Sr /Sr = 0.7016 and an unknown assume that strontium is not strongly concen- strontium concentration with one part crustal trated in the liquid during partial melt- contaminant having the isotopic composition ing. Mixing diagrams for nine parts of oceanic and range of concentration for strontium shown tholeiite and one part of graywacke or oceanic in Figure 9, we can calculate a concentration sediment can explain the lead isotopic com- value for the mantle (M in Fig. 9). This num- position in the Cascade lavas reasonably well, ber has no rigorous physical significance; but on the basis of the strontium data we be- however, it does demonstrate several points. lieve no significance can be attached to the The dashed curves in Figure 9 represent lead calculations. mixing curves for mantle liquids of this cal- Crustal Anatexis. Another mechanism that culated composition and the crustal contami- has been proposed for the origin of Cascade nant. By adding 50 percent crustal material, andesite is partial or complete melting of

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• basement rocks (Fiske and others, 1963; .712 - " PACIFIC OCEAN SEDIMENTS Hopson and others, 1967). The similarity of major-element chemistry of the volcanic rocks CONTAMINANT to that of underlying batholiths and basement rocks, the scarcity of basalts of either type associated with the northern andesitic volcanic centers and the presence of an extensive depth spectrum of igneous rocks, from volcanic rocks through epizonal and mesozonal plutons to the migmatitic rocks of the Chelan basement _l I J I ''''I complex, led these authors to postulate crustal 100 500 1000 2000 anatexis as a possible source for andesitic ipm Sr magma. In subsequent work on Glacier Peak Figure 9. Strontium mixing diagram. Shown on volcano, Tabor and Crowder (1969) point out this diagram are the average Pacific oceanic sediment that satellitic basaltic lava was extruded near data (Church, 1971); the average isotopic composition Glacier Peak soon after the dacitic lava of and range of concentration for oceanic ridge basalt Glacier Peak volcano, as is observed in many (Peterman and Hedge, 1971), and the Oregon Coast of the volcanic centers in the southern Cascade Range graywacke data from Peterman and others Mountains. The presence of this basaltic (1967). The average strontium concentration for the magma in this vicinity suggests that the same Cascade andesite was determined by averaging data partial melting process probably took place from Hedge and others (1970), Peterman and others (1970a), and data from this study. M is the calculated in the upper mantle all along the Cascade mantle concentration (see text fcr details). Dashed lines chain. Tabor and Crowder (1969, p. 52) point represent mixtures of this mantle composition with the out in evaluating the hypothesis of Hopson contaminant strontium; 50% line indicates the range and others (1967) that: of composition of Sr87/Srs6 and strontium concentration values for a 1:1 mixture of mantle liquid and crustal The similarity between the chemical composition contaminant. of most of the Glacier Peak flows and that of the Cloudy Pass batholith suggests that these rocks are ratios are significantly different and cast comagmatic, but the similarity is not unique; most serious doubts on the crustal melting model, other "hypersthene andesites" of the western assuming that the samples are isotopically Cordillera and nearby granodiorite plutons are representative of the proposed source material. also similar. The similarity in bulk chemical com- The strontium result on the Turtleback com- position could reflect a common magma for all plex, which is of the same age and thought to these rocks, not just a common magma for a single be identical to rocks in the Yellow Astor com- pluton and a nearby volcano. Or, more likely, it plex (Misch, 1966), is suspect. Examination of may indicate that all were derived from magmas this sample in thin section indicates that patchy formed through similar processes from diverse albitization has taken place and the exact materials, for example, through crystallization differentiation or anatexis. effect of this process on strontium isotopes is unknown. The concentrations of rubidium and We have also tested the crustal anatexis strontium in the whole rock indicate that if model with isotope data. The basement-rock this system had remained closed throughout its 87 86 data (Table 2) were collected from a selected history, the initial Sr /Sr value of the rock suite of pre-Tertiary rocks furnished by C. A. would have been 0.7002, a highly unlikely Hopson and J. M. Mattinson. A general value, indicating that the closed system 87 86 description of most of these units, as well as assumption is invalid. The Sr /Sr value of detailed isotopic age determinations, can be the Yellow Astor sample (.711.4) is thought to found in Mattinson (1970). The strontium be a more representative one for this type of isotope data (Fig. 10), give the most conclusive basement rock. Strontium from the Swakane evidence concerning the anatectic origin (.708 to .712) and Skagit Gneisses (.7048 to hypothesis. The strontium isotopic composition .7066) is more radiogenic than that in the of rocks from the Chelan complex, excluding Cascade andesite. Neither of these rock types the analyses for the leucosomes, which are more can be involved in any major way in the forma- radiogenic, averages 0.7051 + .0008. The aver- tion of andesite. To summarize, in all the rock 87 86 age isotopic composition of strontium from the units studied, the Sr /Sr data are higher Glacier Peak lava, including the data of Hedge than those of the Glacier Peak lava. We and others (1970) is 0.7036 ± .0003. These emphasize that the basement-rock strontium

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.706 .707 .708 .709 .710 .711 .712 St87 Sr»6 Figure 10. Strontium isotopie composition of Cas- Marblemount Belt ©5 Yellow Astor complex (fy|); cade andesite (•) and basalt and for selected Ter- and a xenolith from the Mt. Hood lava (•). Samples tiary and pre-Tertiary crystalline rocks from the north- which are predominantely leucocratic minerals have a ern Cascade Mountains of Washington : Chelan Ortho- triangle in the upper right-hand corner of the gneiss (¡cj; Swakane Gneiss {x}; Skagit Gneiss ([J) ; symbol. Duncan Hill pluton ; Turtleback complex ([t]);

data are from plagioclase-rich leucocratic few samples are so radiogenic that they do not fractions of the rocks and that the whole-rock plot in the diagram of Figure 11. This observa- Sr87/Sr86 values would be more radiogenic. tion may indicate that the lead and strontium The strontium data indicate that if the crustal of the volcanic rocks remain unaffected by melting hypothesis is to be preserved it is their passage through the continental plate, necessary to assume that the magma formed but that shallow plutonic rocks pick up radio- by melting of lower crustal rocks whose Sr87/ genic components of lead and strontium during Sr86 value is less radiogenic and more uniform shallow emplacement and differentiation. than that in the exposed crystalline basement Figure 12 shows the range of variation of rocks whose field relations and major-element K2O and SiOa for the Cascade Mountains chemistry have been used as the basis for pro- lavas in comparison with other rock groups posing the crustal anatexis model. We have no mentioned in this study. Particularly notice data on material from the "lower crust" from that the majority of the pre-Tertiary "base- this region; thus attempting to evaluate this ment" rock samples, especially those of the source material would be conjecture. Chelan complex (data from this study and from The lead isotopic data, presented in Figure C. A. Hopson, 1969, written commun.), has 11, show that some of the rocks from the markedly less potassium for a given SiOi con- Chelan complex have lead with isotopic tent than do the Cascade lavas. Trace-element compositions which are identical with that of data on these rocks show that the rubidium some of the Cascade andesitic rocks, and in content of these samples is low, and that the particular nearly identical with that of the K/Rb of the pre-Tertiary rocks are somewhat volcanic rocks from nearby Glacier Peak. higher than the K/Rb of andesite from the However, lead from the other pre-Tertiary Cascade Mountain volcanic suite. rocks from this area, and from the Miocene The presence of radiogenic strontium and Cloudy Pass batholith, whose major-element lead in the crystalline basement rocks raises chemistry closely matches that of the Glacier the question whether the magmas were con- peak lava (see Fig. 12), is significantly more taminated during ascent so that the lead and radiogenic than lead from the lava of Glacier strontium (in the volcanic rocks) are not Peak and all of the other Cascade lava. As in accurate measures of these elements in the the case of the strontium data, the lead from source materials. Several points argue strongly the leucocratic separates should be less radio- against this possibility. First, the data of Hedge genic than the whole-rock lead would be. A and others (1970) and of this study show that

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indicators of crustal thickness to look for cor- relations. We find no correlation of isotopic composition with crustal structure as indicated by the quartz diorite line of Moore (1962). However, we do find some increase in Pb206/ Pb20» and Pb208/Pb204 values with increasing crustal thickness using the calculated Bouguer gravity anomalies (U.S. Geol. Survey, 1962a) as indicators of crustal thickness. Sr87/Sr86 data again show no correlation with crustal thick- ness. Close scrutiny of isotopic data from vol- éanos which lie along a line transverse to the general trend of the Cascade Mountain chain, such as Mt. St. Helens-Mt. Adams-Simco Mountains, Three Sisters-Newberry Caldera, and Mt. Shasta-Medicine Lake, generally show increases in Pb2C6/Pb204 and Pb208/Pb204 values with increase in distance from the con- tinental margin. The one exception is the lower Pb208/Pb204 values at Mt. Adams com- pared to those from Mt. St. Helens. This is the Pb'M reverse of the trend noted by Tatsumoto and Pb2" Knight (1969) for lead isotopic data from Figure 11. The isotopic composition of lead in Japan and the reverse of what was noted by Tertiary and pre-Tertiary crystalline rocks from north- Doe and others (1969) for crustal contamina- ern Washington. Lead data for the pre-Tertiary rocks tion effects in recent basalt from the southern (•), for Cloudy Pass pluton and Sitkim stock ( O) which underlie the Glacier Peak volcano (•) and for lead Rocky Mountains. If crustal contamination data from the Duncan Hill pluton ( A). Sample JM-69-9 has occurred, the magma has picked up radio- would plot far to the right off the diagram. genic lead rather than less radiogenic feldspar lead. the isotopic composition of strontium in andesite extruded in areas of known basement Review and Comparison of Models of rocks (Glacier Peak and Mt. Baker) is not Andesite Genesis. We, in common with different from that of strontium in andesite many other authors, favor a model of andesite where basement rocks are not found (Mt. St. genesis in island-arc environments in which a Helens, Mt. Hood). On a broader scale, the descending slab of oceanic crustal material is isotopic composition of strontium in andes'te partially melted along a subduction zone. In from Saipan (Hedge, 1966) where continental such a model, oceanic tholeiite should play a rocks are absent is the same as that found in prominent role as a source material; however, Cascade andesite. The uniformity of the K/Rb it is clear that the tholeiites alone cannot supply in the Cascade volcanic rocks when compared all of the necessary constituents for andesites. with oceanic volcanic rocks (Jakes and White, Radiogenic strontium and lead, as well as 1970), also argues against substantial crustal additional large cations are required, as noted contamination. And, finally, the very uniform by several authors (for example, Taylor, 1969; strontium isotopic composition found in the Tatsumoto, 1969; Armstrong, 1971). The main Cascade andesite would not be expected if point of divergence in the various theories contamination were an important factor. concerns the source of these additional mate- A correlation of the potassium content of rials. We consider these next. andesitic lava with depth to the Benioff zone Ewart and Stipp '1968) suggest that eugeo- has been noted for circum-Pacific lava (Dickin- synclinal sediments are the source of these son and Hatherton, 1967), but this counters materials in New Z.ealand. Although such a the argument that the potassium contents of model appears to work there, it encounters lavas from the Cascade volcanic rocks are con- great difficulties in the Cascade Mountains, trolled by crustal contamination (Condie and as noted above. The uniformity of isotopic Potts, 1969); conversely, it probably indicates composition of strontium in the Cascade lavas a deeper source area. We have plotted the lead over the wide range of concentrations is hard and strontium isotopic data against various to understand in terms of a eugeosynclinal

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where reasonably close control on the isotopic composition of lead in calc-alkaline rocks and neighboring oceanic sediments exists— the Tonga Arc (Sinha and Hart, 1971) and the Cascade Mountains (Church, in prep.). In the Tonga Arc, Sinha and Hart (1971) found that the sediments and volcanic rocks plot on dis- tinctly different lines in a Pb207/Pb204- Pb206/Pb204 diagram in such a way as to indicate little or no mixing of sediment lead with lead in the volcanic rocks. Rather, the rock lead plots on a line in the diagram that includes oceanic tholeiite and alkali basalt. Figure 12. K20-Si02 diagram for various rock series Church (in prep.) was able to show that mix- discussed in this study. The Cascade calcic and calc- tures of lead from oceanic tholeiite from the alkalic volcanic rock suite all plot within the limits Juan de Fuca and the Gorda Rises with lead defined by the dashed lines. Data from Mt. Adams from oceanic sediments in the same general ( this study) define a very steep slope whereas the area of the Pacific Ocean cannot yield lead that data from Mt. St. Helens ( Q, this study and J. Ver- is as radiogenic as those of the Cascade calc- hoogen, 1937) define the shallowest slope. Data from alkaline volcanic rocks. (2) The high Th/U Glacier Peak volcanic rocks (•) and from the Cloudy ratios (Sinha and Hart, 1971; Church, in prep.) Pass pluton ( o) are from Tabor and Crowder (1969), and Sr87/Sr86 ratios (Dasch, 1969b; Church, Tables 3 and 4). Notice that the Glacier Peak volcanic 1971) of oceanic sediments place limits of 2 to rocks have slightly less K20 than do the rocks from the Cloudy Pass pluton for any given Si02 value. Data on 5 percent on the amount of sediment that can additional pre-Tertiary crystalline rocks from northern be mixed with tholeiitic basalt to obtain calc- Washington are also included. The Chelan Orthogneiss alkaline lava. If this is so, the concentrations of ( •) has been separated from all other rocks ( A) of this K, Rb, U, Th, Pb, Sr, and Ba in andesite type because this of rocks is thought to represent require enrichments of these elements by the most probable parent rock in the anatexis model factors of 10 to 20 over their concentrations of Hopson and others (1967). However, notice that the in the source material at the time of magma K 0 content of the Chelan Orthogneiss is outside the 2 formation. However, Armstrong's model re- entire field of data for the Cascade volcanic rocks. These data were kindly provided by C. A. Hopson. Data on quires melting of approximately 20 percent of the Duncan Hill pluton ( +), thought to be of anatectic the oceanic slab to account for the rate of origin, were also provided by C. A. Hopson and F. W. andesite generation. Our calculations indicate Cater. Data on the Juan de Fuca and Gorda rise basalts that the volume of andesite present in the (hachured field) are from Kay and others (1970); and Cascade Mountains (Waters, 1962) would be data from the Columbia River basalts (shown as only 1 to 3 percent of the minimum volume of stippled fields for the Picture Gorge, Ellensburg, and the subducted slab over the last 5 m.y. (3) Yakima series) are from Waters (1961). Using oceanic sediments for a source of large cations in andesite may lead to a model that is in conflict with the observations of Dickinson sediment contamination model; also the and Hatherton (1967) concerning the cor- uniform strontium isotopic composition and relation of the potassium content of the chemical composition of andesite from con- magma with depth to the Benioff zone. If a tinental margins, such as Japan and the Cascade sediment-basalt mixture is carried down a sub- area, compared with lava from oceanic island duction zone to a region where partial melting arcs far removed from the continents, is not begins, the sediments will be the first material easy to explain with the model. to melt and should contribute large quantities Armstrong and Cooper (1971) and Arm- of elements with large cationic radii, such as strong (1971) argue persuasively for deriving potassium, to the early magmas. In such a case, the materials from ocean-basin sediments. the normalized potassium content of the Armstrong (1971) showed that the sedimentary magmas might be expected to decrease rather layer could supply the necessary quantities of than increase as a function of depth to the large cations to account for andesite production Benioff zone. at the present time. Nevertheless, this theory has difficulties that cast doubts on its validity. These problems lead us to propose a model Some of these are: (1) There are now two areas in which oceanic sediments or other crustal

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materials play an insignificant role in the genesis consistent either with derivation of the of andesitic magma. We assume that the addi- magma from a common source, or derivation tional materials are derived from rock types of andesite by differentiation of high-alumina other than oceanic tholeiite that might be basalt magma. On tiie basis of published trace present in the subducted slab of oceanic crust. element data from other areas, we favor deriva- We must assume that the great abundance of tion from a common source for the formation of tholeiite seen at the surface of the slab does not the large volcanos of predominantly pyroxene adequately characterize the composition of the andesite. entire slab. For example, an average composi- 3. Strontium in the Cascade volcanic rocks tion of 95 percent tholeiite and 5 psrcent (Sr87/Sr86 = .7037) is more radiogenic than alkali basalt might be capable of supplying the that in oceanic tholeiite from the East Pacific, large cations for andesitic lava. An alternate, Juan de Fuca, and Gorda Ridges (Sr87/Sr86 = or supplemental source might be altered basalt 0.7026). The Pb207Pb204, Pb207/Pb204 and of the kind found in the Franciscan terrain, Pb208/Pb204 values are likewise higher in the of California. Still another alternative would be andesite than in oceanic tholeiite (Church, to add material at depth from rocks that bound unpub. data). the volcanic slab, especially on the continental 4. Various mixing models, whereby radio- side which should be much hotter than the rock genic lead and strontium are added to magma on the oceanward side. If these rocks are dunite containing lead and strontium with isotopic or peridotite, as commonly assumed, it would compositions similar to those in oceanic tho- be difficult to obtain substantial quantities of leiite, do not fit the isotopic data satisfactorily. elements with large cationic radii from such Pb206/Pb204 values in Pacific Ocean sediments rocks. are apparently too low to account for the Pb206/ 204 207 We realize that our model has unsatisfactory Pb data in the Cascade lava and the Pb / 204 features. The source of the radiogenic lead and Pb values in Pacific Ocean sediments are strontium and of the elements with large higher than those in the Cascade lava. Mixtures cationic radii is not precisely specified. In of Tertiary eugeosynclinal sediments (inferred common with other models involving mixed from Oregon coastal graywacke data) and source materials, it does not offer an obvious oceanic tholeiite can probably account for the explanation of the rather uniform strontium Cascade volcanic rock isotopic data in a gen- and lead isotopic composition in circum- eral way, but the uniform isotopic composition Pacific andesite in general. A better under- of strontium in the Cascade lavas over a wide standing of the tectonic and chemical processes range of concentrations is hard to understand in occurring in the mantle, particularly in sub- terms of such a model. The same is true for strontium in the oceanic sediment-tholeiite duction zones, and determination of the mixing model. isotopic composition of strontium and lead in volcanic and sedimentary rocks with the 5. According to strontium and lead isotopic higher precisions now attainable will hopefully data, andesite from the Cascade Mountains clarify these questions. which has Sr87/Sr86 of .7037 cannot be made by partially or wholly melting the coastal SUMMARY AND CONCLUSIONS graywacke sedimentary rocks from the Coast 1. The isotopic composition of strontium in Range of Oregon or similar eugeosynclinal andesitic lava from the Cascade Mountains is rocks of the same age from the same source remarkably constant with an average Sr87/Sr86 areas. Thus, the Cascade results do not fit the = 0.7037 over a range of concentrations from model proposed for the origin of andesite in 200 to 1,500 ppm. Other circum-Pacific New Zealand (Ewart and Stipp, 1968) which 87 86 andesites, except those from New Zealand, has Sr /Sr of .7056. have nearly identical strontium isotopic 6. The uniform isotopic composition of compositions. The isotopic composition of lead strontium in the Cascade lavas, irrespective in the Cascade volcanic rocks is variable and of strontium concentration and geologic plots to the right of the geochron indicating setting, argues against large-scale contamina- future single-stage model ages. tion with crustal radiogenic strontium during 2. High-alumina basalt and andesite from the ascent of the magma. the Cascade Mountains Range have lead isotop- 7. None of the pre-Tertiary crystalline rock ic composition which are essentially identical; units now exposed in the Northern Cascade the same is true of strontium. These data are Mountains appears to have both lead and

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strontium with isotopic compositions that Patent Fund, Graduate School, University of match those of andesite. These observations California, Santa Barbara. Laboratory work argue against formation of the Cascade andesitic was supported by National Science Foundation lava by partial or complete melting of basement Grant GA-12478. rocks of the kinds now at the surface. The heat flow and seismic data available indicate a APPENDIX: ANALYTICAL maximum temperature at the base of the crust PROCEDURES in this region of 800°C, a much lower tempera- ture than that of andesitic magma. Sample Collection and Preparation 8. The best explanation of the isotopic and trace element data appears to be a multistage Suites of representative rocks were collected from model in which calcic and calc-alkaline magma the following volcanic centers: Mt. Baker, Mt. Rainier, Glacier Peak, Mt. St. Helens, Mt. Adams, is derived from mantle material by three or Mt. Hood, Mt. Jefferson, Three Sisters, Newberry possibly more stages of partial melting, and in Caldera, Crater Lake, Mt. McLoughlin, Mt. which crustal materials play an insignificant Shasta, Medicine Lake, and Mt. Lassen. All avail- role. Such a model best explains the constancy able literature and field work on particular volcanic of Sr87/Sr86 values in the Cascade lavas over a centers were used as a basis for sample collection. wide range of strontium concentrations, Where previous detailed work had been done at a particularly if sea-floor "weathering" is an particular volcanic center and different rock groups affective agent in increasing alkalies and alkaline defined on the basis of chemistry, petrography, or earths, in the source material. It also explains age, a special attempt was made to collect a representative suite of samples from each group. why strontium in andesite extruded at con- C. A. Hopson, W. S. Wise, M. W. Higgins, and tinental margins, such as in the Cascade R. W. Tabor provided additional samples from Mountains and in Japan, has the same isotopic volcanic centers which they have studied. J. M. composition as strontium in andesite from the Mattinson and C. A. Hopson provided samples of Mariana arc system in the Pacific Ocean, which the basement rock groups from northern Washing- appears to be far removed from continental ton. Samples were selected from these suites for material. isotopic analysis on the basis of their age and chem- 206 204 208 204 istry. A brief description and the exact location of 9. Pb /Pb and Pb /Pb values in the samples can be found in Church (1970) and Cascade lavas may show an increase with crustal Mattinson (1970). thickness, but this does not prove that crustal The samples selected for isotopic analysis were contamination is responsible for the trend. The prepared as follows. The volcanic rocks were broken lead and strontium isotopic data exhibit no into centimeter-sized chips. Approximately 200 correlation with the quartz diorite line of grams of each sample were washed successively in Moore (1962). singly distilled water, then warm (80°C) 6N 10. Lead data from the Glacier Peak lava hydrochloric acid, and finally in doubly distilled and the underlying pluton suggest that the water. This procedure is necessary to insure that no intrusive rocks have been contaminated by weathering products are included in the analysis. These samples were either processed in chip form radiogenic lead during shallow cooling and or ground in a tungsten carbide ball mill when emplacement. finer sized fractions were needed. Basement rock samples were crushed and leucocratic separates ACKNOWLEDGMENTS obtained using conventional heavy liquid separation Special thanks are due to C. A. Hopson and techniques. The crushed mineral separates were A. C. Waters for their stimulating discussions, washed successively in singly distilled water, warm for their valuable field assistance and for access 8N nitric acid, warm 6N hydrochloric acid, and to unpublished data (CAH). M. Tatsumoto finally in doubly distilled water. The samples were crushed in a tungsten carbide ball mill. generously provided information on his vola- tilization apparatus and techniques which Rubidium and Strontium Procedures enabled us to analyze lead from several high- alumina basalts. Mark Stein provided tech- Approximately 100 mg of sample were digested in hydrofluoric and perchloric acids in covered nical assistance with the electronic equipment. teflon beakers. The residue was leached in 2.5N J. L. Aronson, M. N. Bass, B. R. Doe, C. A. hydrochloric acid and the Rb and Sr isolated on a Hopson, and W. I. Ridley read the original Dowex 50 cation exchange column using 2.5N HC1 manuscript and made valuable improvements. as the elutant. Blank determinations indicate .003 Field work was carried out with grant support fig of Sr and .008 fig of Rb per analysis; the from the Department of Geology and the effect of the blanks is negligible. Strontium was

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oxidized and run on single Re filaments. Several which were run using the lead technique sets of data were taken and each observation was have been corrected for isotopic fractionation by normalized to a SR86/Sr88 value of .1194. An direct comparison of our average CIT values with average of ten analyses of the Eimer and Amend that of Catanzaio (1967), that is, about .3 percent SrCOj standard, which was run periodically during per mass unit. Comparison of these normalized the study, gave a Sr8VSr86 value of .7081 ± .0006. values with those done by double spiking shows Rubidium was determined on a few of the leuco- that this correction is adequate within the limits cratic separates as calibration standards for x-ray of error stated. Isotopic fractionation varied about fluorescence work. Rubidium was also run on single ±.1 percent per mass unit for the samples com- Re filaments; no attempt was made to correct for pared (that is, from .2 to .4 percent per mass unit). fractionation in the Rb analyses. Replicate analysis of a basalt sample (ACW-1) collected from the same quarry as BCR-1 was also Lead Procedures run periodically. We obtained the following best average value for ACW-1 during the course of this Samples were processed for lead by one of three study: Pb206/Pb"4 = 18.79 ± .03, Pb207Pb204 = methods: conventional wet dissolution in hydro- 15.60 ± .05, and Pb2°8/Pb204 = 38.75 ± .12, Pb = fluoric and perchloric acids in covered teflon con- 8 ppm. This is in excellent agreement within error tainers; a volatilization technique in which finely with the double-spiked cata on BCR-1 (Oversby, powdered sample mixed with carbon was heated in 1971, written commun.) and with the concentration a vacuum at 1,000 to 1,100°C for two to three data for rock chips of BCR-1 (Tatsumoto, 1971, hours (Masuda, 1962); and a volatilization pro- written commun.). Subsequent work by Church cedure in which rock chips were heated to 1,300°C shows that the values obtained by the silica gel for 20 min in outgassed carbon crucibles using method are much more precise: ACW-1, Pb206/ induction heating. The apparatus for the last Pb2»4 = 18.78 ± .02, Pb207/Pb204 = 15.60 ± .03, technique was very similar to that used by M. and Pb208/Pb204 = 38.73 ± .05. In view of the Tatsumoto 1969, oral comm.). Lead was chemically likelihood that BCR-1 has been contaminated isolated using conventional dithizone techniques during processing (Tatsumoto, 1971, oral com- (Tilton and others, 1955). The lead concentration mun.), we find the agreement with the absolute data herein reported have been determined during values for BCR-1 (Oversby, 1971, written this step and are accurate to ± 10 percent assuming commun.) remarkable. The following values were 100 percent yield during volatilization. obtained for a single analysis of AGV —1: Pb206/ Lead blanks for wet chemical dissolution are Pb2«4 = 18.91 ± .02, Pb20VPb204 = 15.59 ± .03, about .3 tig per analysis. Total lead blanks for :he and Pb20s/Pb204 = 38.46 ± .05. Other samples Masuda volatilization process range from .15 to 1.0 were run using the silica gel method (Shields, ng depending on the cleanliness of the quartz 1970, p. 71-75). This technique gives a much sample vial. Most samples had a blank of .3 to .5 ng more predictable fractionation pattern. All analyses of common lead per analysis. Blanks for the done using this method have been corrected for Tatsumoto volatilization apparatus are somewhat .13 percent per mass unit discrimination. All higher ranging from .45 to .6 fig; .5 ^g of lead per analyses were done at the same temperature. analysis is typical. No corrections have been applied to the lead data to remove blank lead since both are modern leads and would differ one from another Concentration Data Determinations by only a few percent. Blanks would never supply The silica, potassium, rubidium, and strontium more than 3 percent of the total lead and are usually data in Tables 1 and 2 w:re determined using less than 1 or 2 percent of the total lead analyzed. standard x-ray spectrometric techniques (Jenkins Three samples were analyzed by all three and DeVries, 1967). Standard curves were made methods. The differences between methods is on using the data available on the U.S. Geological the same order of magnitude as the differences be- Survey standard rocks (Gordon and others, 1968; tween successive analyses of the same lead and the Fleischer, 1969; and Flanagan, 1969) and on chem- range of differences in fractionation between runs. ically analyzed samples from Mt. Hood (Wise, Statistical error limits are .1 percent per mass unit. 1969). Working curves for rubidium and strontium It therefore seems justifiable to compare lead determinations were made using isotope dilution isotopic data collected using these three methods. data from additional samples available from Davis For more details on these procedures and tests, see (1969). Standards for the trace element determina- Church (1970). tions in feldspar were analyzed by isotope dilution Two different sample loading techniques were during the course of this study. For further details used for the lead isotopic analyses. Most of the on analytical procedures, see Church (1970). samples were run on single Re filaments and the lead was loaded as a sulfide. Isotopic fractionation REFERENCES CITED was monitored by running several samples using a double spike technique (Compston and Oversby, Anderson, C. A., 1941, Volcanoes of the Medicine 1969; Dallwitz, 1970) and by running the CIT lead Lake Highlands: California Univ. Pubs. Geol. standard periodically. All data in Tables 1 and 2 Sci., v. 25, p. 347-422.

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