ANNA HIETANEN U.S. Geological Survey, Menlo Park, California 94025

Origin of Andesitic and Granitic Magmas in the Northern Sierra Nevada, California

ABSTRACT Paleozoic and Mesozoic time below the west side of the northern Sierra Nevada. The meta- The early magmas of the northern Sierra morphic complex includes an andesitic suite of Nevada, calc-alkaline andesite of island-arc island-arc type that is tentatively correlated type and its derivatives, all low in potassium, with Devonian metavolcanic rocks in the were generated during the Devonian(?) Taylorsville area about 30 km to the northeast period, possibly along an eastward-dipping sub- (McMath, 1966) and with the suite of andesite duction zone. These magmas could have been in the Klamath Mountains (Davis, 1969; derived from mantle peridotite of the con- Burchfiel and Davis, 1972). This andesitic tinental plate by introduction of water from suite is a key to understanding the major the descending oceanic plate. Later, during the structural events and geochemical changes that Permian(?) period, the magmas became accompanied the formation of later granitic basaltic, with potassium-rich silicic derivatives magmas, which intruded the metamorphic indicating anhydrous conditions and a deeper complex in Late Jurassic and Early Cretaceous level of magma generation. Plutonism began in time. A second episode of andesitic volcanism Jurassic time, at the end of a period of intense was pyroclastic and occurred in the Tertiary; it deformation and metamorphism. The earliest was preceded and followed by basaltic eruptions intrusive rocks are gabbro and diorite. At the (Hietanen, 1972). end of the Jurassic period, large granitic plutons were emplaced. These grade from hornblende OUTLINE OF GENERAL GEOLOGY quartz diorite at the borders to monzotonalite The Feather River area in the northern at the centers. Trondhjemite occurs as the Sierra Nevada (Fig. 1) is in the inner part of an latest product of crystallization differentiation arcuate segment of the Nevadan orogenic belt of plutonic magmas. Exchange of elements be- (Hietanen, 1973). Here, northerly trends, tween plutonic and metamorphic rocks sug- typical of the Sierra Nevada south of the area gests that the plutonic magmas were composite. of Figure 1, turn to the northwest and in The partial melts of the downfolded volcanic places to the west, curving around small and sedimentary rocks were modified by partial plutons that were emplaced after the Paleozoic melts from the mantle and the subducted metasedimentary and metavolcanic rocks were oceanic lithosphere below. Relative amounts of deformed and recrystallized during the Jurassic. material contributed by each of the three Five major high-angle faults disrupt the struc- sources of plutonic magma changed with time, tural and lithologic continuity and divide the and these changes, along with differentiation metamorphic rocks into four belts, in which processes, were responsible for the diversity in rarely more than one formation is exposed. The composition of magmas. west sides of the faults were downdropped rela- tive to the east sides, and isoclinal folds typical INTRODUCTION of all local metamorphic rocks are overturned Petrologic and structural studies in the com- to the west (Fig. 2C). These structural features plex plutonic and metamorphic terrane of the are consistent with the idea that a subduction Feather River area in the northern Sierra zone lay beneath and west of this area during Nevada have revealed many features that bear the Paleozoic and Mesozoic and that the faults on the origin of andesitic and granitic magmas and overturned folds are surface expressions of in orogenic belts. The structures in this area the subduction process (Fig. 2). are consistent with the hypothesis that an east- The oldest rocks in the area are interbedded ward-dipping subduction zone existed during metachert and phyllite, and thus similar to

Geological Society of America Bulletin, v. 84, p. 2111-2118, 3 figs., June 1973

2111

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metamorphosed equivalents of sediments de- rocks and a few carbonate layers, was probably posited on ocean floors. These rocks, the Cala- deposited during intermit): sntly waning vol- veras Formation, of Paleozoic (Silurian?) age, canic activity in shallow v/ater between the are faulted against a belt of peridotite and island arc and the continent. serpentine in the east, and are overlain by meta- The large round plutons shown in Figure 1 andesite believed to be the lowest part of the are postkinematic. They were emplaced over a Franklin Canyon Formation (Hietanen, 1973). period of time at the end of Jurassic time. The Similar potassium-poor meta-andesite and in- oldest of these plutons, the Lumpkin pluton terlayered metadacite and metamorphosed and associated intrusive bodies, consists of sodarhyolite, all with pyroclastic structures and gabbro and quartz diorite that has more interbedded with metamorphosed tuffaceous hornblende and less quartz than the quartz layers, are exposed southwest of the Calaveras diorite in the other plutons. Most of the other Formation in a belt that is 4 to 9 km wide and plutons are monzotonalite grading outward to is bordered by faults. These rocks are also quartz dioritic border zones, 1- to 2-km wide. assigned to the Franklin Canyon Formation Exceptions are the Bucks Lake pluton, which and tentatively correlated with the Devonian includes a mass of pyroxene diorite in its center, metavolcanic rocks in the Taylorsville area and the Bald Rock pluton, which grades to about 30 km to the northeast (McMath, 1966). trondhjemite in its north-central part. A They are overlain by the Horseshoe Bene tonguelike body of trondhj emite extends to the Formation, a heterogeneous sequence of inter- northern part of the neighboring Cascade bedded metavolcanic and metasedimentary rocks including thin layers of limestone of pluton, proving that the Bald Rock pluton is probable Permian age. The oldest rocks in the the younger of the two. Trondhjemite occurs Horseshoe Bend Formation are metabasalt as a late differentiate in the Merrimac pluton with discontinuous thin layers of potassium- also and as dikelike bodies in the south-central feldspar-bearing metarhyolite, rhyolitic meta- part of the Cascade pluton. tuff, and quartzite. The middle part of the ORIGIN OF ANDESITIC MAGMA formation consists mainly of phyllite anc quartzite, with a little limestone and some Three major hypotheses to explain the pos- metavolcanic rocks. Another layer of basaltic sible origin of calc-alka!ic andesite magma have rocks with some quartzitic layers overlies the been advanced during recent years. Before the metasedimentary sequence and forms the upper advent of the plate-tector.ics concept, andesite part of the Horseshoe Bend Formation. Small was generally considered to be derived from bodies of metagabbro, metadiorite, and meta- basaltic magma either through fractionation or trondhjemite are most common in the Franklin by contamination by sialic material, even Canyon Formation and are considered to be though, in many areas such as island arcs, there deep-seated and hypabyssal equivalents of the is no evidence for either. In the light of plate metavolcanic rocks. These early intrusive rocks tectonics, two other possibilities have been were deformed and metamorphosed with the proposed for the generation of calc-alkalic metavolcanic rocks, which they resemble in andesite magma: partial melting of either their mineralogy and chemical composition, downgoing oceanic lithosphere (Dickinson, including the trace-element content. 1970) or of mantle perioditite in hydrous con- ditions (Yoder, 1969; McBirney, 1969a). The sequence of metamorphosed chert and Kuno (1968, 1969) has discussed the pos- shale (Calaveras Formation), overlain by sibility that andesitic magma may form by andesitic volcanic rocks (Franklin Canyon fractionation of basaltic rnagma. He advanced Formation) intruded by gabbroic and trondhje- this hypothesis on the basis of field associations mitic rocks, and in fault contact with a belt of and the frequency of eruption of basaltic ultramafic rocks of mantle origin, is similar andesite associated with basalt and andesite. In to rock assemblages in the oceanic lithosphere. the Cascade Range and in northern California, This similarity and the low potassium content large quantities of basaltic andesite were of the andesitic suite supports the contention erupted in Cenozoic time:. In the Feather River by Burchfiel and Davis (1972) that an island- area of northern Sierra Nevada, late Tertiary arc system extended from the Klamath Moun- hypersthene andesite was derived by crystal- tains to the northern Sierra Nevada in Devon- lization differentiation from a tholeiitic basalt ian time. The overlying Horseshoe Bend magma (Hietanen, 1972). There, two-pyrox- Formation, which includes metasedimentary ene basaltic andesite occurs as gradational

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Figure X. Sketch map of study area in the northern mian?). Figure 2C shows a schematic cross section Sierra Nevada: (1) Mainly rocks of the Calaveras Forma- along A-A'. U and D on faults indicate relative up- tion (Silurian?); (2) Franklin Canyon Formation thrown and downthrown sides. (Devonian?), (3) Horseshoe Bend Formation (Per-

members between gray basalt containing The hypothesis of contamination of basaltic numerous olivine and augite phenocrysts and magma by sialic material in the island-arc sys- silicic hypersthene andesite, rich in plagioclase. tems, where andesitic volcanism is common, is It is noteworthy that all these magmas erupted refuted by the fact that the crust there is very after the crust had been thickened by de- thin. Moreover, the trace-element content of formation and plutonism, suggesting that the andesites, including their low Sr87/Sr86 ratios, posttectonic andesite and possibly all con- speaks against the hypothesis of contamination tinental calc-alkalic andesite were formed by as discussed by Taylor (1969), Hedge (1969), fractionation or contamination of basaltic and Hedge and Peterman (1969). magmas. Calc-alkalic andesitic volcanism of island-arc

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A. Devonian

Generation o': calc-alkalic Water from the oceanic plate andesitic magma low in ascends to the peridotitic potassium in hydrous mantle of the upper plate and condi tions. lowers the melting point of peridotite. The first melt is andesite.

B. Permian

Thickening of the continental Generation of basaltic, plate by accumulation of andesitic and rhyolitic volcanic and sedimentary magmas containing more material and by folding and potassium in anhydrous faulti ng. condi t ions

Generation of plutonic magmas: Partial melts from Intermittently con- subducted oceanic litho- tinued deformation and sphère and overlying mantle thickening of the upper join the aiatectic magma plate. The folds are o' formed at the roots of to the west. The fault: downfolded metamorphic side down are surface e; complex to produce large the continued subduction of the quantities of monzotona- oceanic plate. 1 itic magma.

Figure 2. Sketches suggesting structural and mag- the northern Sierra Nevada. ma tic evolution of the postulated subduction zone in

type and subduction of the oceanic lithosphere posed hypotheses for the origin of calc-alkalic along inclined seismic zones are now known to andesite magmas, both of which are supported be intimately associated (Isacks and others, by experimental evidence. Green and Ring- 1969; Barazangi and others, 1970; Dickinson wood (1969) suggested, on the basis of their and Hatherton, 1967; Dickinson, 1970). This high-pressure experimental studies, that calc- association has led to the two recently pro- alkalic andesites were formed at depths of 80 to

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150 km, by partial melting of quartz eclogite, Yoder's experimental work, McBirney (1969a) or at depths of 30 to 40 km, by partial melting has suggested that the Cenozoic calc-alkaline of amphibolite and gabbro or fractional crys- lavas in Central America were formed at tallization of hydrous basalt. On the other elevated water pressures and that the necessary hand, on the basis of his experimental work, water was derived from a descending oceanic Yoder (1969) has pointed out that calc-alkalic plate underneath. andesite magma can be derived directly from In the Feather River area, the prevalence of the mantle peridotite in hydrous conditions. pyroclastic textures in the metamorphosed Adding water to peridotite has two important andesitic suite indicates explosive eruptions, effects: (1) it lowers the melting point by which commonly result from a high content of 250°C, and (2) the composition of the first water and other volatiles. The eruption of melt is andesitic, not basaltic as it is in anhy- potassium-poor andesitic rocks was followed by drous conditions (Yoder and Tilley, 1962). eruption of basalt and potassium-rich rhyolite The association of andesitic volcanism with in Permian time. This change in magma chem- inclined seismic zones and experimental deriva- istry may have resulted from the exhaustion of tion of calc-alkalic andesite liquid from eclogite, available water and from a higher temperature gabbro, and amphibolite at high temperatures at deeper levels (still above the Benioff zone), and pressures has led to the hypothesis of where magma was now generated from the partial melting of downgoing oceanic litho- mantle by partial melting in anhydrous con- sphere to produce andesite (Green and Ring- ditions (see Fig. 2B), in a manner suggested by wood, 1969; Dickinson, 1970). Recent seismo- Yoder and Tilley's experimental work (1962). logic and thermal studies on the modern island- A deeper level could have been attained, for arc systems, however, show that the descending example, by thickening of the crust above by oceanic plates under the andesitic volcanoes accumulation of new material and by deforma- remain cool and rigid for long periods of time tion. The high potassium content of the (Raleigh and Lee, 1969). For instance, in the metarhyolite associated with the Permian Tonga arc, the downgoing plate has been basalt is noteworthy in this regard. Dickinson traced as a rigid body down to 350 and 700 km and Hatherton (1967) came to the conclusion (Isacks and others, 1969; Barazangi and others, that the Quaternary potassium-rich andesitic 1970). Therefore it does not seem possible that magmas generally formed at deeper levels than the andesitic magma erupting from the vol- the potassium-poor magmas, and they extended canoes above such seismic zones (see Fig. 2A) correlation of the potassium content and depth was formed by melting of the downgoing of the seismic zone, near which magma was oceanic lithosphere. Moreover, Gorshkov's generated, to the Tertiary (Hatherton, 1969; (1969) studies suggest that the andesitic Dickinson, 1970). magma is generated in the upper mantle (asthenosphere) at a level considerably higher ORIGIN OF GRANITIC MAGMA than the focal zone of earthquakes. Generation of large quantities of granitic From the evidence provided by contact magma that intruded the metamorphic rocks metamorphism in general, it seems clear that during the Late Jurassic and Early Cretaceous the oceanic slab that descends into the hot can be considered as a part of a long chain of mantle must first undergo chemical changes events connected with the subduction of the comparable to those undergone by sedi- oceanic lithosphere as postulated above. Ex- mentary and igneous rocks in contact aureoles periments by Tuttle and Bowen (1958) showed of plutons. Partial melting could follow only that the ideal granitic magma is a eutectic after temperatures in the slab rose to certain melt that would form—at depths of 15 to 20 levels. The first and most important change in km and at a temperature of about 640°C— the descending slab would therefore be loss of from any rock that contains the necessary water from the oceanic sediments and from the elements. In the Feather River area, the hydrous minerals in the oceanic crust. This average composition of the plutonic rocks is water could then ascend to the overlying con- not that of true granite, but of monzotonalite tinental plate and hydrate the peridotite there, (Hietanen, 1961) with the following percent- thereby creating the conditions required for ages of major oxides: 66, SiC>2; 16, AI2O3; 2, andesitic melt to form directly from peridotite, FeO; 1.5, MgO; 4, CaO; 4.5, Na20; 1.8, as demonstrated by Yoder's (1969) experi- K2O; and with an average mode, in percent, of mental work (Fig. 2A). In accordance with 55 plagioclase (An30), 20 quartz, 8 orthoclase,

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and 17 dark constituents, hornblende, and approximately monzotonalitic composition. biotite. This plutonic rock thus contains less Rather, the geochemical changes in the meta- Si02 and K20 and more CaO, FeO, and MgC volcanic rocks indicate that the metamorphic than normal granite. Magma of monzotonalitic environment of the plutons contributed composition could form by partial melting at actively to generation of monzotonalitic about 700°C (Piwinskii, 1968) at depths of 20 magma. It was bled of elements needed for the to 30 km, assuming geothermal gradients of 30° formation of the eutectic rnelt and the remain- to 25°C per km. The oldest plutonic rocks, ing part was thus enriched in elements that however, are hornblende gabbro and diorite occurred in excess. (Lumpkin pluton in Fig. 1), contrary to the The 200-m.y. interval between Devonian sequence that would be expected in differ- andesitic volcanism and metamorphism that ential anatexis. It seems, therefore, that culminated in the Jurassic should have been anatexis of metamorphic rocks, although per- long enough for temperatures in the subducted haps the most important process in generation, oceanic plate to rise to where partial melting of plutonic magmas, was not the only process. would occur. According to the high-pressure Comparison of the chemical composition of experimental work by Green and Ringwood metavolcanic rocks in the Feather River area (1969), the liquids, formed by partial melting (Hietanen, 1973) with the average composition of quartz eclogite, amphibolite, and gabbro, of their probable unmetamorphosed equiv- constitute a typical calc-alkaline suite. The alents (using data presented by Daly, 1933; partial melts from these rocks in the subducted McBirney, 1969b; and Chayes, 1969) indicates oceanic lithosphere could have risen to magmas that all the metavolcanic rocks have undergone formed in the mantle above, and both magmas considerable chemical change during meta- could then have risen to the lower crust, where morphism. They became impoverished in large quantities of downfolded metamorphic silicon and alkalies and enriched in calcium, rocks would be digested to produce the pre- iron, and magnesium. For example, metadacite, dominantly monzotonalitic magma (Fig. 3). containing clusters of quartz with outlines of The quantities of melt from any one source original euhedral phenocrysts, consists now of could reasonably change vath time and loca- epidote, amphiboles, albite, and quartz. The tion, however, and thus a diversity of com- metadacite has no potassium-bearing minerals, position of plutonic magmas could result that and large amounts of epidote and amphiboles cannot be explained by simple fractional raise the CaO, FeO, and MgO contents to 9, melting or by differentiation. For instance, the 7, and 6.5 percent, respectively; thus 3 to 5 early plutonic rocks are commonly gabbroic, percent above amounts in an average dacite. contrary to the sequence that partial melting The Na 0 content is about 2 percent and the 2 should produce; that is, silicic melts first. Si02 content is about 54 percent. The average K20 content in the Franklin Canyon Forma- SUMMARY tion is less than 0.1 percent. Only the Al Os 2 The following three major stages are recog- content (17 percent) remained unchanged. On nized in the generation of magmas during the other hand, comparison of the composition subduction of the oceanic lithosphere into the of monzotonalite with the average composition mantle along the continental margin (Figs. 2 of unaltered andesite shows 1 to 2 percent less and 3): CaO, FeO, and MgO, and more Si02 and Na20 in the monzotonalite. The differences in com- 1. Island-src-type andesitic volcanism (De- position between the monzotonalite and basalt vonian in the study area). The oceanic plate are similar but even greater. Thus silicon and descending to the hot mantle would ultimately alkalies should be added and calcium, iron, and lose its water. This water could ascend to the magnesium subtracted, if the monzotonalitic peridotitic mantle of the continental plate magma were to form from these volcanic rocks above the Benioff zone, where it would lower by partial melting. It cannot be a mere coin- the melting point of peridc rite. The first melt cidence that the elements lost (Si, Na, K) and formed would be calc-alkalic andesite (Yoder, gained (Ca, Fe, Mg) by the metavolcanic rocks 1969). during their metamorphism should be re- 2. Basaltic and rhyolitic volcanism (Permian spectively added to and subtracted from the in the study area). Continued partial melting of average andesite and basalt to yield a rock of peridotite of the upper plate would produce basaltic liquid and it? derivatives in anhydrous

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Peridotite of the mantle above the Benioff zone

H2O from subducted oceanic plate Anhydrous partial melting

1. Calc-alkalic andesite. Devonian volcanism 2. Basalt and rhyolite. Permian volcanism and sedimentation

3a. Deformation and recrystal- 3b. Partial melting of sub- lization followed by ducted oceanic plate partial melting of the and the mantle above. downfolded part. Jurassic Jurassic and Cretaceous

Gabbroic magma

Monzotonalitic magma. Late Jurassic and Early Cretaceous plutonism

Figure 3. Sequence of generation of magmas in the northern Sierra Nevada. conditions. Potassium-rich rhyolite associated tural framework and evolution of the southern with the basalt may indicate depth for the part of the Cordilleran orogen, western United magma chamber greater than that indicated in States: Am. Jour. Sci., v. 272, p. 97-118. paragraph 1 above (Dickinson, 1970). Chayes, F., 1969, The chemical composition of 3. Deformation and recrystallization of the Cenozoic andesite, in McBirney, A. R., ed., Proceedings of the andesite conference: volcanic rocks of paragraphs 1 and 2. Genera- Oregon Dept. Geology and Mineral In- tion of monzotonalitic magmas in batholithic dustries Bull., v. 65, p. 1-12. dimensions. The exchange of elements between Daly, R. A., 1933, Igneous rocks and the depths of the magma and its environment suggests that the earth: New York, McGraw-Hill, 598 p. partial melting of the metamorphic rocks was Davis, G. A., 1969, Tectonic correlations, Klamath important in the generation of magmas. This Mountains and western Sierra Nevada, Cali- anatectic magma probably joined magmas fornia: Geol. Soc. America Bull., v. 80, p. formed by partial melting of the mantle and the 1095-1108. subducted oceanic crust below. Dickinson, W. R., 1970, Relations of andesites, granites, and derivative sandstones to arc- trench tectonics: Rev. Geophysics Space REFERENCES CITED Physics, v. 8, no. 4, p. 813-860. Barazangi, M., Isacks, B., and Oliver, J., 1970, Dickinson, W. R., and Hatherton, T., 1967, Propagation of seismic waves through and Andesitic volcanism and seismicity around the beneath the lithosphere that descends under Pacific: Science, v. 157, p. 801-803. the Tonga Island arc: Geol. Soc. America, Gorshkov, G. S., 1969, Geophysics and petro- Abs. with Programs (Ann. Mtg.), v. 2, no. 7, chemistry of andesite volcanism of the circum- p. 488-489. Pacific belt, in McBirney, A. R., ed., Pro- Burchfiel, B. C., and Davis, G. A., 1972, Struc- ceedings of the andesite conference: Oregon

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Dept. Geology and Mineral Industries Bull., of the andesite conference: Oregon Dept. v. 65, p. 91-98. Geology and Mineral Industries Bull., v. 65, p. Green, T. H., and Ringwood, A. E., 1969, The high 185-189. pressure experimental studies on the origin of 1969b, Andesitic and rhyolitic volcanism of andesites, in McBirney, A. R., ed., Proceed- orogenic belts, in The earth's crust and upper ings of the andesite conference: Oregon Dept. mantle: Am. Geophys. Union Geophys. Mon. Geology and Mineral Industries Bull., v. 65, 13, p. 501-507. p. 21-32. McMath, V. E., 1966, Geology of the Taylorsville Hatherton, T., 1969, The geophysical significance area, northern Sierra Nevada: California Div. of calc-alkaline andesites in New Zealand: Mines and Geology Bull., v. 190, p. 173-183. New Zealand Jour. Geology and Geophysics, Piwinskii, A. J., 1968, Experimental studies of v. 12, p. 436-459. igneous rock series, central Sierra Nevada Hedge, C. E., 1969, The sources of calc-alkaline batholith, California: Jour. Geology, v. 76, no. magmas as indicated by Sr isotopes [abs.], in 5, p. 548-570. McBirney, A. R., ed., Proceedings of the Raleigh, C. B„ and Lee, W.H.K., 1969, Sea-floor andesite conference: Oregon Dept. Geology spreading and island-arc tectonics, in Mc- and Mineral Industries Bull., v. 65, p. 191. Birney, A. R., ed., Proceedings of the andesite Hedge, C. E., and Peterman, Z. E„ 1969, Sr87/ conference: Oregon Dept. Geology and Min- Sr86 of circum-Pacific andesites: Geol. Soc. eral Industries Bull., v. 65, p. 99-110. America, Abs. with Programs for 1969, pt. 7 Taylor, S. R., 1969, Trace element chemistry of (Ann. Mtg.), p. 96. andesites and associated calc-alkalic rocks, in Hietanen, Anna, 1961, A proposal for clarifying the McBirney, A. R., ed., Proceedings of the use of plutonic calc-alkalic rock names: U.S. andesite conference: Oregon Dept. Geology Geol. Survey Prof. Paper 424-D, p. 340-343. and Mineral Industries Eiull., v. 65, p. 43-63. 1972, Tertiary basalts in the Feather River Tuttle, O. F„ and Bowen, N. L., 1958, Origin of area, California: U.S. Geol. Survey Prof. granite in the light of experimental studies in Paper 800-B, p. B85-B94. the system NaAlSi3Os KAlSijOs-SiOj-HIO: 1973, Geology of the Pulga and Bucks Lake Geol. Soc. America Mem. 74, 153 p. quadrangles, Butte and Plumas Counties, Yoder, H. S., Jr., 1969, Calc-alkalic andesites, ex- California: U.S. Geol. Survey Prof. Paper 731. perimental data bearing on the origin of their Isacks, Bryan, Sykes, Lynn R., and Oliver, Jack, assumed characteristics, in McBirney, A. R., 1969, Focal mechanisms of deep and shallow ed., Proceedings of the andesite conference: earthquakes in the Tonga-Kermadec region Oregon Dept. Geology and Mineral Industries and the tectonics of island arcs: Geol. Soc. Bull., v. 65, p. 77-90. America Bull., v. 80, p. 1443-1469. Yoder, H. S., Ir„ and Tilley, C. E., 1962, Origin of Kuno, H., 1968, Origin of andesite and its bearing basalt magmas: An experimental study of on the island arc structure: Bull. Volcanol., natural and synthetic rock systems: Jour. v. 32, p. 141-176. Petrology, v. 3, no. 3, p. 342-532. 1969, Andesite in time and space, in Mc- Birney, A. R., ed., Proceedings of the andesite conference: Oregon Dept. Geology and MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER Mineral Industries Bull., v. 65, p. 13-20. 9, 1972 McBirney, A. R., 1969a, Compositional variations REVISED MANUSCRIPT RECEIVED FEBRUARY 5, 1973 in Cenozoic calc-alkaline suites of Central PUBLICATION AUTHORIZED BY THE DIRECTOR, U.S. America, in McBirney, A. R., ed., Proceedings GEOLOGICAL SURVEY

PRINIEO IN U.S.A.

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