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 lead isotopes in high-alumina basalt and andesite at particular volcanos, although ABSTRACT the lead data are more variable than the New isotopic analyses of lead are reported strontium isotope 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 basalts, 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/2/431/3428578/i0016-7606-84-2-431.pdf by guest on 01 October 2021 432 CHURCH AND TILTON 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 period 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-block 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 second 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/2/431/3428578/i0016-7606-84-2-431.pdf by guest on 01 October 2021 HIGH CASCADES VOLCANIC ROCKS QUATERNARY WESTERN CASCADES VOLCANIC ROCKS TERTIARY COLUMBIA RIVER BASALT MIOCENE PRE-TERTIARY CRYSTALLINE ROCKS 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/2/431/3428578/i0016-7606-84-2-431.pdf by guest on 01 October 2021 434 CHURCH AND TILTON 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.