MICHAEL W. HIGGINS U.S. Geological Survey, Beltsville, Maryland 20705

Petrology of Newberry Volcano, Central Oregon

Note: This paper is dedicated to Aaron and Elizabeth of the magma which, in turn, are related to the Waters on the occasion of Dr. Waters' retirement. presence or absence of large volumes of water in the caldera lake. The interpretation is sup- ABSTRACT ported by field, petrographic, petrologic, chem- The eastern flank of the central and southern ical, trace-element, and isotopic data. Plots of Cascade Mountains is bordered by a belt of existing data for the Medicine Lake Highland shield volcanoes that appears to be a subprov- Volcano, another large complex shield center in ince of the Oregon high-alumina plateau the belt, show the same type of two-trend re- basalt petrologic province. Most of the vol- lation as those of Newberry Volcano. canoes in this belt are low shields in which dif- ferentiation from the parent high-alumina INTRODUCTION basalt magma has been relatively slight, but From northern California to central Wash- several are large complex shield centers where ington, the eastern flank of the Cascade Moun- differentiation has been extreme. The location tains is bordered by a belt, about 40 mi (64 km) of these large centers, and of some of the smaller wide, characterized by numerous shield vol- volcanoes as well, was largely determined by canoes (Fig. 1). This belt appears to be a sub- intersecting concentrations of faults and fault- province of Waters' (1962) high-alumina fissures of three regional fault systems. plateau basalt petrologic province. Most of the One of the largest of the complex volcanic volcanic piles in the belt are indistinguishable centers is Newberry Volcano in central Oregon, from the high-alumina basalt piles that underlie a shield volcano with a big caldera at its sum- and are interspersed with the High Cascade mit. The stratigraphy of the caldera walls and stratovolcanoes; they are low shields composed of features on the caldera floor at Newberry chiefly of thin flows of plagioclase-rich, olivine- allows detailed interpretation of the history of bearing high-alumina basalts that apparently the younger parts of the volcano and caldera. differentiated from the same magma type as The formation of Newberry Caldera was ap- the high-alumina basalts that characterize the parently a slow process controlled largely by southeastern Oregon basalt plateaus (Waters, faulting along the three regional fault systems. 1962). However, a few of these shield volcanoes The magma conduits were probably a gridlike are unique; they are large and complicated plexus of intersecting dikes and fissures, with volcanic centers where differentiation of the larger "magma pockets" at the grid inter- high-alumina basalt magma has been extreme, sections. The magma was trapped in shallow producing slightly alkalic andesitic basalts, chambers and periodically released by faulting. olivine andesites, andesites, dacites, and rhyo- The entrapment of the magma allowed dif- lites. One of the largest of these centers is ferentiation in the shallow chambers. Newberry Volcano, a shield volcano about 40 mi (64 km) long and 25 mi (40 km) wide that The stratigraphy and petrology of the wall rises from the basalt plateaus about 40 mi (64 sequence also allows determination of the rela- km) east of the crest of the Cascade Mountains tive time at which the caldera had grown large south of Bend, Oregon (Fig. 1). At the summit enough to hold a caldera lake. of the Newberry shield is Newberry Caldera, a On differentiation plots, chemical analyses of large complex nested caldera with two large the Newberry rocks show two trends: rocks lakes on its floor and a variety of volcanic erupted before the presence of a lake in the features on its floor and walls. caldera trend toward slight iron enrichment, whereas rocks erupted after water was present The purpose of this paper is to document the in the caldera generally trend toward alkali differentiation of the high-alumina basalt enrichment. These different trends are at- magma at Newberry Volcano. Some aspects of tributed to differences in the oxygen fugacity the geology and petrology of Newberry have

Geological Society of America Bulletin, v. 84, p. 455-488, f7 figs., February 1973 455

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"H" 100 KILOMETERS

EXPLANATION

âïiK High Cascade lavas

High-alumina shield volcano belt

High-alumina basalt plateaus of eastern Oregon

Columbia River lavas

Western Cascade rocks

Large stratovolcano

Large shield volcano and shield complexes

Figure 1. Sketch map showing generalized dis- tribution of some of the petrologic provinces in part of the Pacific Northwest (modified from Waters, 1962; and Snavely and others, 1973). The high-alumina shield volcano belt is considered a subprovince of the high- alumina plateau basalt province (Waters, 1962). S, Simcoe volcanic complex; N, Newberry Volcano; M, Medicine Lake Highland Volcano. Columbia River lavas not shown in gorge of Columbia River where they are beneath high-aluncina lavas. L, Lassen Peak; S, Mount Shasta; C, Crater Lake; TS, Three Sisters; H, Mount Hood; A, Mount Adams; SH, St. Helens; R, Mount Rainier; G, Glacier Peak; B, Mount Baker.

long been known from Howel Williams' ex- and particularly the major shield complexes cellent reconnaissance study (1935) and from such as Newberry Volcano, the Medicine Lake numerous topical studies published during the Highland (Anderson., 1941), and the Simcoe past 30 years. This paper presents the petrologic volcanoes (Sheppard, 1960), are directly re- results of a detailed study of the geology anc. lated to regional fault systems. In the Pacific petrology of the volcano. The geology, sum- Northwest (including Idahc, western Montana, marized here, will be published in detail in a and northeastern California), faults, fault later report. The stratigraphie nomenclature blocks, volcanic fissures, topographic linea- used in this report is informal and not current ments, many fold axes, and lineaments of un- U.S. Geological Survey usage. known origin show strong parallel alignment in two main trends—one N. 25°-50° W., the TECTONIC SETTING OF other N. 40°-60° E. Some of the major faults, NEWBERRY VOLCANO fault systems, and lineaments with these trends Waters (1962, p. 164-166) pointed out that are shown on small-scale geologic and tectonic the southeastern Oregon high-alumina basalts, maps (U.S. Geol. Survey and Am. Assoc.

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Petroleum Geologists, 1961; U.S. Geol. Survey, west trends are readily seen on the U.S. Army 1932), but the parallelism and abundance of Map Service raised-relief maps (Fig. 2). For these features are best seen on aeromagnetic purposes of description, the northwest-trending maps (Zietz and others, 1971) and on U.S. features in the area of Figure 2 will be in- Army Map Service raised-relief topographic formally called the Brothers system, because maps (1:250,000 scale). These northeast and one part of this system has long been called the northwest trends are so dominant on aero- Brothers fault zone; similarly, the northeast- magnetic maps that they are major features trending features will be informally called the even in the north-south-trending Cascade Walker Rim system for the prominent Walker Mountains (Blank, 1968; Zietz and others, Rim fault that forms a bold scarp for about 30 1971). mi (48 km) from Little Walker Mountain (Fig. In addition to the two main regional trends, 3) to where it appears to disappear beneath the subordinate trends are locally well developed. latest flows at the southwestern edge of the The two main subordinate trends are (1) north- Newberry shield. south-trending features, principally the Cas- Faults of the Brothers system cross the large cade Mountains, but also many small features northeast-trending fault blocks of the Basin east of the Cascades and some major lineaments and Range province in south-central Oregon at the western edge of the Cascades (Peck and (Fig. 2); they are easily seen on the northwest others, 1964); and (2) east-west-trending fea- side of Steens Mountain and on the southeast tures, chiefly long but relatively narrow side of the Warner Mountain-Hart Mountain lineaments associated with faults and fold axes block. They bound the graben of Silver Lake (Waters, 1962; Zietz and others, 1971). basin, Swan Lake Valley, and many others, and In southeastern Oregon and northeastern prominent fault-block mountains such as California, the dominant northeast and north- Coglan Buttes, Diablo-Wildcat Mountain,

Figure 2. Photograph of a mosaic of 1:250,000- Crescent, Oregon, 1962; Burns, Oregon, 1963; scale raised-relief maps (U.S. Army Map Service, 1962) Klamath Falls, Oregon, 1962; and Adel, Oregon, 1962. showing some of the faults and lineaments. Sheets are:

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'i m i > u f I i P r>

Figure 3. Photograph of the Crescent, Oregon, Service, 1962) showing some of the faults and linea- l:250,000-scale raised-relief map (U.S. Army Map ments. Connley Hills, Pine Mountain, and others. tinents, 1964). East-west lineaments also occur Faults of the Walker Rim system appear to be along this approximate boundary (Waters, less numerous than those of the Brothers sys- 1962). The basalts of this province are chiefly tem (Fig. 2). The most prominent develop- high-alumina basalts, rich in olivine and ment of Walker Rim system faults, here in- plagioclase and poor in pyroxene (Waters, 1962, formally called the Walker Rim fault zone, is a p. 163-164), although evidence is now be- zone approximately 20 mi (38 km) wide that ginning to accumulate which suggests that the crosses Newberry Volcano (Fig. 2). basalts become progressively more alkaline to The Walker Rim system appears to be re- the east, so that the province actually straddles lated to the fault system of the Basin and the gradational boundary between high- Range province. The characteristic north- alumina and alkali basalts (G. W. Walker, 1970, northeast (about N. 15°-25° E.) faults of the oral commun.). Basin and Range gradually change direction in As Fuller (1931; Fuller and Waters, 1929) south-central and southeastern Oregon and and Waters (1962) recognized, the nature of northeastern California until they blend into the volcanic rocks and particularly the location the northeast trend of the Walker Rim system. of major volcanic features within this province The petrologic province of southeastern Ore- are closely related to the concentration of gon high-alumina plateau basalts (Waters, faults and the nature of the fault system. Where 1962) corresponds in a general way to the the concentration of faults is relatively low and transitional area where the faults trend from fault intersections sparse, volcanic activity has about N. 30° E. to about N. 60° E. The generally been limited to small flows and chains northern boundary of this area coincides of craters along fissures of one or the other of roughly with a large gravity gradient that the iault systems. Where strong concentrations trends about N. 50° E. across central Oregon of faults of two or more systems intersect, (Berg and others, 1967; Am. Geophys. Union, major volcanic complexes have formed. Even Spec. Comm. Geophys. and Geol. Study Con- more important, however, is the fact that the

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active block faulting probably helped to form GEOLOGY AND PETROGRAPHY shallow reservoirs where the high-alumina basalt magma differentiated. Composition of the Shield and the Rocks beneath It At the Medicine Lake Highland, prominent faults of the Brothers system that form the The Newberry shield is so recent in age that Mahogany Mountain and Big and Little Table- intermittent streams on its flanks have cut only land horsts and the intervening grabens north- insignificant canyons, and earlier canyons have west of the volcano (and farther northwest, been inundated and filled by recent lava flows. the Swan Lake and Upper Klamath Lake Only Paulina Creek, which drains the caldera, grabens) and the Big Valley Mountains block exposes a significant thickness of rocks along its southeast of the volcano cross prominent course. These rocks belong to the uppermost north-trending Cascade system faults, which part of the caldera wall sequence and have been form the Indian Springs Mountain block south downfaulted to their present position. of the volcano and the Crumes Lakes and Tule- The younger flows of the shield, representing lake grabens and intervening horsts north of the only the visible uppermost 10 to 50 ft (3 to 15 volcano. Walker Rim system faults are also m) of approximately 3,000 ft (914 m) of rock, prominent. They account for Fisk Ridge and are mostly olivine basalts and olivine andesites. other lines of volcanic cones on the flanks of Sporadic outcrops of more siliceous rocks do the shield. occur on the flanks of the shield, but these ap- Newberry Volcano is at the intersection of pear insignificant in volume compared with the the Brothers fault zone with the Walker Rim vast covering of mafic flows and cinder cones. fault zone, and numerous Cascade system faults The shape of the Newberry shield suggests that and lineaments are also found on and around it is composed chiefly of olivine basalt and the volcano (Fig. 3). olivine andesite as are most of the other shield Both Newberry and the Medicine Lake volcanoes east of the Cascade Mountains Highland shield volcanoes must have built up (Anderson, 1941; Waters, 1962), but evidence as coalescing flows from and countless cones concerning the rocks that underlie the New- along, first one and then another, or perhaps berry shield is scant, as Williams (1935, p. 258) simultaneously along two or three, of the recognized. What little evidence there is, how- fault-fissure trends. They grew in an active ever, suggests that the shield is probably under- tectonic environment, and their centers of lain by several thousand feet (several hundred activity probably changed repeatedly. Gradu- meters) of Pliocene-Pleistocene volcanic and ally, the greatest volumes of shallow magma volcaniclastic rocks, of which high-alumina were localized at places of high concentration basalts are most important. At greater depths, of fault-fissure intersections, and the central Eocene to Pliocene rocks, such as those that areas of the developing shields became most crop out over large areas northeast of New- active. The magma chambers or conduits of berry (Walker and others, 1967; A. C. Waters, these volcanoes were probably not round or unpub. data), are probably present. stocklike, as such features are commonly visualized. Instead, these conduits were prob- Caldera Wall Sequence ably netlike dike intersections in plan, with A regular sequence of stratigraphic units larger magma pools at intersections of the fault- (Fig. 4) exposed in the walls of Newberry controlled grid. Caldera and in the gorge of Paulina Creek (Fig. At Newberry, as soon as the volcano had 5) records an orderly succession of events in the grown to nearly its present size, magma in the building of the upper part of the volcano and large elongate pockets of the gridlike plexus of allows detailed interpretation of the history of dikes was periodically released by movements this part of the volcano and caldera. on the intersecting fault sets, causing sub- Older Rhyolite (Unit 1). The oldest rocks sidence of some fault-bounded blocks. Other in the caldera walls are rhyolites exposed along blocks were probably dropped without emis- the base of the north wall and locally along the sion of lavas or pyroclastic material. Gradually, base of the south wall. These rocks are not these processes produced a caldera, as the vari- exposed in the gorge of Paulina Creek, or in the ous subsidence blocks grew in number and east wall, where younger rocks crop out ad- coalesced. This led to further pooling of the jacent to the caldera floor (Figs. 4 and 5). The magma to give extreme differentiation. rhyolites dip into the walls 20° to 25°, and

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PAULINA CREEK Basalt and andesite flows SOUTH WALL NORTH WALL and scoria from fissures and cones (local) Talus and recent ejecta Newberry ash and Basalt and andesite flows pumice deposits Mazama ash 10-20" (25-50 cm) and scoria from fissures and cones (local) Newberry ash and pumice deposits Talus and recent ejecta West wall basalt 5-70' (1.5-21 m) Newberry ash and Talus and recent ejecta Mazama ash 10-20" (25-50 cm) pumice deposits

Mazama ash 10-20" (25-50 cm) Paulina Peak Rhyolites Talus and recent ejecta Paulina Falls andesite flow and breccia 20-90' (6-27 m) Welded tuff 20-120' (6-37 m) 0-1200+' (0-366+ m) Mazama ash 10-20" (25-50 cm)

Mafic tuffs with graded beddinding I Mafic tuffs with graded bedding Mafic tuffs with graded bedding Mafic tuffs with graded bedding 10-30' (3-9 m) 40-100' (12-30 m) 20-60' (6-18 m) 80-150' (24-46 m>

Red scoria 5-35' (1.5-11 m) Scoria 40-60' (12-18 m) Red scoria 2-10' (.6-3 m) > Red scoria 5 30' (1.5-9 m) ^UNIT Piaty andesitic basalt Platy and es i tic basalt 30-110'(9-34 m) Piaty andesitic basa ft 80-250' (24-76 m) 100-300+' (30-91+ m> * UNIT 2

Older rhyolite 300-450+' <91-137+ m) >

Older rhyolite 60-80 + ' (18-24+ m)

Figure 4. Stratigraphy of the caldera walls.

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their maximum exposed thickness is about 400 composed of elongated lapilli of long-tube ft (122 m). There are several types of rhyolite andesite pumice and shards of dark-brown in the unit, and a small percentage of rocks glass. It contains numerous inclusions of blocks seem, on field and petrographic criteria, to be and lapilli of olivine basalt and diabase, and dacites (Table 1). abundant fragments of rhyolite identical with In general, the rocks of the older rhyolite rocks of the underlying rhyolite unit. unit have the appearance and characteristics of The large size of the particles, the welding, viscous lavas which were probably erupted the lack of grading, and the abundance of from nearby sources and piled up near the foreign fragments, probably torn from the sides vents. They probably erupted from fissures of the vent, suggest that the andesite ag- associated with the beginnings of caldera glutinate is a near-vent deposit. formation. Platy Andesitic Basalt (Unit 2). Overlying Andesite Agglutinate in the South Wall. In the older rhyolites in the caldera's north wall part of the caldera's south wall (Fig. 5), the and in part of the south wall (Figs. 4 and 5) are older rhyolite unit is overlain by a dark-brown black to gunmetal-gray lava flows of aphanitic to black, ungraded, slightly welded deposit and mostly nonporphyritic hypersthene-augite

l HILOMCTCB

EAST LAKE

PMJltH* LAKEI

Figure 5A. Orthophoto of Newberry Caldera. 1. sheep's rump. 10. East Lake fissure. 11. East Lake re- Paulina Creek. 2. Paulina Peak. 3. Big obsidian flow. 4. sort. 12. Rhyolite ridge. 13. Little crater tuff ring. 14. North obsidian flow. 5. Pumice cone obsidian flow. 6. East Lake tuff rings. 15. Interlake basalt flow of Hig- Game Hut obsidian flow of Higgins and Waters (1967). gins and Waters (1967). 16. East Lake fissure. 17. 7. East Lake obsidian flows. 8. The red slide. 9. The Northeast rhyolite flow. 18. Paulina Lake ash flow.

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Figure 5B. Generalized geologic sketch map of Newberry 'Caldera.

andesitic basalt (Table 1) with markedly platy red and red-brown scoria (Figs. 4 and 5). jointing and contorted flow banding. These Bombs and cinders in this scoria were pasty platy rocks also form the base of the east wall enough to stick together, and in many out- and are present in the gorge of Paulina Creek crops the scoria is welded into agglutinate. west of the caldera (Figs. 4 and 5). Locally, they Some of the bombs in. the scoria are as much as crop out on the western flank of the shield. 1 ft (30 cm) long, and this, together with the These andesitic basalts vary greatly in thick- agglutination, suggests that the vents through ness (Fig. 4), suggesting that the western and which the scoria was expelled were not far eastern sides of the volcano were lower than from the present outcrops. the north and south sides when they were South Wall Pumice and Scoria Deposits. In erupted. part of the south wall of the caldera, near Phenocrysts, glomeroporphyritic clots, and Paulina Peak, the red scoria unit is overlain by xenoliths in the platy andesitic basalt are a well-sorted unstratified deposit, as much as identical with those that occur in many of the 40 ft (12 m) thick, of pink- to orange-tinged olivine basalts of the Newberry shield and of white pumice lapilli (Figs. 4 and 5, and Table the area south, east, and southeast of the 1). This deposit (not shown separately on Fig. shield—-Waters' (1962) high-alumina basalt 5B) probably represents a pumice flow from a province. This suggests that the andesitic nearby vent. The pumice deposit is overlain by basalt may be the first stage in differentiation 60 to 80 ft (18 to 24 m) of brown and yellow of the high-alumina olivine basalt magma. mafic scoria containing large bombs and small Red Scoria (Unit 3). The platy andesitic pasty cinders that suggest a nearby source. basalt is overlain by 30 to 50 ft (9 to 15 m) of Mafic Tuffs with Graded Bedding (Unit 4).

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EXPLANATION

AH units Holocene in age; stratigraphie sequence approximate

Areas covered by landslide material, talus, or colluvium, and areas deeply covered with pumice and (or) heavily East Lake andesite flow and cone forested; no outcrops visible. Age of deposits varies

Resort andesite flow (Resort Lava Flow of Higgins and Rhyolitic obsidian flows Waters, 1967)

Mafic to intermediate flows and cinder cones Interlake Basalt flpw of Higgins and Waters (1967)

Not shown on map 19 Newberry ash and pumice deposits; Mafic tuff rings l,720zt 250 years old

Rhyoiite flows, domes,and complexes, including Paulina Peak Game Hut Obsidian Flow of Higgins and Waters (1967) i na * ' Rhyolites of Williams (1935) and northeast rhyoiite flow ;i;B

Rhyolitic pumice cones and their aprons West wall basalt

Ash-flow and pumice avalanche deposits. Paulina Falls andesite flow and breccia About 2.050+ 230 years old

Mafic to intermediate flows and cinders East wall welded tuff

Red scoria and mafic tuff units (units 3 and 4); in south wall East Lake rhyolitic obsidian flows includes local scoria, local pumice deposits, and dike and sills of andesitic basalt

Not shown on map Platy andesitic basalt (unit 2); in south wall includes local Mazama ash. About 6,600 years old andesitic agglutinate

Mafic to intermediate flank flows and cinders; Older rhyoiite (unit 1) age varies

JD U Contact Fault Crater rim

Dashed where gradational or approximate Many fou|ts not shovvn D, downthrown side; U, upthrown side

A series of graded beds of buff to brown mafic that they fell into sticky water-soaked beds tuff overlies the red scoria in the north, east, capable of sagging and dewatering beneath load and part of the south wall, and in the gorge of and impact but coherent enough to retain their Paulina Creek, and overlies the local scoria bedding almost undisrupted, rather than into a deposit in the western part of the south wall fluffy accumulation of dry pyroclastic materi- (Figs. 4 and 5). More than 150 individual beds als. of tuff and tuff breccia were counted in some The distribution of the tuffs in all three walls outcrops; the beds dip outward, away from of the caldera and in the gorge of Paulina the center of the caldera. The base of each bed Creek, their even bedding, and the draping of is composed of andesitic basalt and basalt the beds over minor irregularities suggest an fragments as large as blocks or lapilli generally air-fall origin. However, the quenched par- overlain by lapilli-sized to ash-sized particles. ticles of sideromelane glass (Table 1), the type Above this is a fairly sharp break followed by of graded bedding, and particularly the super- fine to very fine granular ash. In addition, the imposed upward grading of the entire as- entire accumulation of tuffs is graded; coarse semblage of graded tuffs are reminiscent of parts of the lower beds are generally much pyroclastic materials erupted under water and coarser than those of analogous beds near the then distributed outward by gravity flow top of the sequence. Large blocks have fallen (Fiske, 1963; Fiske and Matsuda, 1964). The into some of the finer beds, producing promi- petrography of the tuffs (Table 1), particularly nent bomb sags (Fig. 6). The marked sagging the lack of fritting, melting, oxidation, or other without disruption of the underlying beds sug- high-temperature alteration, the alteration of gests that these projectiles were airborne and lithic fragments to clay minerals and the vari-

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Rhyolitic obsidian flows Mafic to intermediate flank f1o»s and cinders Rocks of all flows are quite similar; they consist of al- Cinders: Most are dark-tan to dark-brown glass (RI ranges ternating gray and nearly colorless clear bands of glass from 1.540 to 1.586) with tiny granules of magnetite and (95 to 98 percent of the rock; RI 1.490 to 1.495), with crystallites of augite and plagiiclase. tiny greenish rod-shaped crystallites. The crystallites Flotia: Most common textjres are intergranular, intersertal, are quite closely appressed in the gray bands and cause the hyaloophitic, diktytaxitic, and -.rachytic. Groundmass is gray color, but in the colorless bands they are evenly commonly made up of microlites ot: plagioclase (labradorite scattered and less abundant. Most thin sections also have to andesine), augite, and rarely olivine, and granules of a few small grains of magnetite, scattered microlites, rare nagnetite. fast cotrmon phenocrysts are plagioclase phenocrysts of plagioclase, and rare prisms of pyroxene. (bytownite to andesine), augite, and olivine (Fo6s-so). Pumice Cone obsidian flow has more feldspar phenocrysts l-'ypersthene (generally Er.60-70) is rare. Glass ranges (some are An2o-26) than the other flows, and north obsidian from about 2 to about 10 percent. flow rocks contain rare large phenocrysts of augite (2V a East Lake andasite 55° to 57°). Rare corroded phenocrysts of basaltic horn- blende are locally present in big obsidian flow. Pilotaxitic or trachytic, porphyritic textures with little glass. The groundmass consists o- polysynthetically Andesites from fissure on east and southeast walls twinned plagioclase microlites (An52-s6)» anhedral and Cinder8: Dark-brown glass (RI 1.538) charged with tiny subhedral crystals and tiny granules of augite, tiny granules of magnetite and crystallites and microlites of granules and euhedral crystals of magnetite, anhedral and plagioclase and augite. subhedral crystals of olivine, and light-tan to colorless Flows; Porphyritic trachytic to hyalopilitic texture. glass. Phenocrysts (2 to 8 percent of the rock), in order The groundmass (about 95 to 96 percent of the rock) con- of abundance, are euhedral and subhedral, uncorroded, sists of brown glass with microlites of plagioclase (about normally zoned plagioclase crystals (cores Ani^-^s; rims Anso-ss), augite, and very rarely olivine, and tiny specks as sodic as An?sl, euhedral to anhedral crystals of augite of magnetite. Phenocrysts, in order of abundance, are (2tf a 53° to 58°), subhedral to euhedral twinned large, zoned, euhedral, and subhedral crystals of plagio- hypersthene crystals (En66-7o), scattered euhedral crys- clase (cores An^a-57; rims as sodic as An37), subhedral to tals and irregularly shaped grains of magnetite intimately anhedral crystals of augite (2V = 54° to 57°), subhedral to associated with augite, and large partially corroded but euhedral crystals of hypersthene (En67-7<>), and subhedral unaltered subhedral crystals of olivine. Many of the crystals of olivine (about Fo6<>?)- plagioclase phenocrysts have poikilitic inclusions of augite and magnetite, or of glass, symmetrically disposed Mafic to intermediate flows and cinders about a euhedral core. GlOTieroporpiyritic clots composed Cinder cones: Cinders from the Red Slide and the Sheep's of large crystals of augite and plagioclase with small Rump cinder cones are dark-tan to dark-brown glass grains of magnetite and rara small olivine crystals also (RI 1.541 to 1.582) with numerous vesicles and tiny micro- occur. The crystals of these clots are intimately inter- lites of augite and plagioclase. grown in highly irregular shapes anc resemble textures of Red Slide flou; Hyaloophitic texture in which lath-shaped Plutonic rocks. The minera'.s of these clots have composi- anhedral to euhedral, zoned plagioclase crystals (cores tions far removed from those of the groundmass minerals, An6o-es; rims as sodic as An6S), subhedral and anhedral suggesting that they are prcbably not xenoliths. augite crystals (2V 3 55° to 60°), cracked and iddingsi- tized subhedral olivine crystals (Fo73-eo)» and a few Resort andes ite crystallites of augite and plagioclase are in a brown glass Hyalopilitic texture in which glass constitutes 15 to 35 crowded with small grains of magnetite. Small vesicles are percent of the rock. In addition to the glass, the ground- conmonly filled with clay or zeolites (or both). mass consists of tiny plagioclase la':hs (mostly oligoclase), East lake fissure flows: Hyalopilitic to trachytic tex- and augite and magnetite granules. Phenocrysts are lath- tures in which small rounded to euhedral augite phenocrysts shaped, corroded crystals of plagioclase (cores Anm.**; and broken and corroded laths of andesine (about AnM) are rims slightly more sodic) with abundant glass inclusions, set in a groundmass (98 to 99 percent of the rock) of dark- subhedral and euhedral crystcls of augite (2V « 54° to 56°), brown magnetite-rich glass containing tiny microlites of subhedral and euhedral crystals of hypersthene (about En67). plagioclase and crystallites of plagioclase and augite. and rare euhedral magnetite crystals. Newberry ash and pumice deposits Interlake basalt of Hiqqins aid Water; (1967) Deposit no. 1: Pumice is light-gray to colorless glass Intergranular tex-ure pocked with subround ragged-edged (avg. RI 1.495) with very rare greenish crystallites. vesicles. The groundmass consists of plagioclase micro- Tiny lithic fragments are chiefly black obsidian. Broken lites (Ani«2-j,e), tiny interstitial aucite blebs, short crystals of feldspar are chiefly oligoclase. lathi ike augite crystals packed between plagioclase micro- Deposit no. 2: Pumice is light-gray to colorless glass lites, small anhedral olivine grains, small blebs and octa- (most cotrmon RI 1.494 to 1.497); range 1.490 to 1.502) with hedral grains of magnetite, ar.d a dark-tan glass. Pheno- rare greenish crystallites. Crystals and crystal fragments crysts, in order of abundance, are corroded subhedral are plagioclase (An30-37), salite, or high-lime augite crystals of plagioclase (AnS2-57), slijhtly fractured un- (2V = 55° to 56°), ferrohypersthene (En32.37), and very altered subhedral crystals of olivine [F076-83), and rare basaltic hornblende (nz=l.670+0.003). Lithic frag- nearly colorless anhedral grails of clmopyroxene (2V = ments are chiefly black obsidian, but fragments of platy 44° to 50°). rhyolite, andesites, and basalt are locally present. Mafic tuff rings Rhyolitic pumice cones and ash flows Medium and fine parts of typic«.! tuff ring beds are vitric Light-gray to nearly colorless glass (RI between 1.490 and 75 to 80 percent, including palagonitized glass), lithic 1.495) with rare microlites and crystallites of feldspar 10 to 20 percent), crystal (5 to 10 percent) tuff. The tatrix (60 to 80 percent of the rock) is generally well and augite(?). sorted in any particular part of a bed .ind consists of East Lake obsidian partially to completely palagonitized shards, beads, and Alternating gray and nearly colorless (locally devitrified) bubbles of light-tan sideromelane, and iif crystals of glass (94 to 97 percent of the rock), with tiny greenish plagiocfase, olivine, pyroxenes, and magnetite. Oval rod-shaped crystallites. The crystallites are quite close- vesicles, partially filled with calcite or with calcite ly appressed in the gray bands and cause the gray color; and zeolites, constitute as mucf- as 10 percent of the rock. but in the clear bands they are evenly scattered and less Of the coarser lithic fragments, 20 to 30 percent closely abundant. Scattered small grains of magnetite and micro- resemble the platy andesitic basalt (unit 2), 5 to 10 per- lites of feldspar complete the groundmass. Phenocrysts csnt resemble rocks of the older rhyolite unit (unit 1), are rare prisms of pyroxene (augite??) and very rare crys- and 30 to 40 percent are medium- to coarse-grained olivine tals of basaltic hornblende up to 1 cm long. basalts like those of parts of the shield and of the pla-

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teaus. Glassy fragments with occasional crystals of fectly euhedral, normally zoned plagioclase crystals plagioclase and clinopyroxene account for 15 to 45 percent (cores An6s-8ii rims An30*36). ragged anhedral, fairly of the fragments and probably represent the juvenile magma calcic augite grains (2V = 57° to 59°), euhedral, exten- of the tuff rings. Tuffs of the lower part of Little sively iddingsitized olivine crystals (Foyw?)» and rare Crater tuff ring show more palagonitization than the tuffs euhedral crystals of orthopyroxene, probably enstatite. 1n the upper part of the ring and more palagonitization Many of the plagioclase phenocrysts occur as complexes of than the tuffs of South and East Lake tuff rings. two or more crystals with a synneusls relation. In some, a core and one or two zones are joined to the end of Northeast rhyolite another core and zones, and further zones have been added Similar to Paulina Peak rhyolites. The groundmass {85 to around the whole. Many of the plagioclase crystals have 95 percent of the rock) consists of a mat of tiny, poorly inclusions of the groundmass augite. Some of the augite oriented feldspar crystallites broken by bands and streaks phenocrysts are granular aggregates with a collective of tiny hematite grains that parallel the platy partings. euhedral form. Plagioclase microlites (Ani2-2o)» euhedral grains of magne- tite, and granules of hematite are scattered through the Paulina Falls andesite flow and breccia groundmass. Phenocrysts, in order of abundance, are cor- Vesicles constitute 60 to 70 percent of the andesite flow. roded and embayed plagioclase crystals (cores Anao-axi The glass is partly to completely devitrified and charged rims more sodic than An2o)» oxidized crystals of hyper- with tiny specks of magnetite and hematite, plagioclase sthene (En62-$7)» scattered, Irregularly shaped grains of microlites, and augite microlites. Phenocrysts, In order magnetite, and small blebs of alkali feldspar. of abundance, are zoned plagioclase crystals (cores An35- ; rims about An ), euhedral and subhedral crystals Rocks of rhyolite ridge and rhyolite domes and flows 9 7 32 of hypersthene (En6e-7obsidians: Clear, nearly colorless glass (avg. RI lites and rare microlites of augite and olivine. Many 1.494) with very tiny aligned crystallites and a few lath- lapilli also contain zoned (but generally untwinned) cor- shaped feldspar microlites with forked ends and glass in- roded plagioclase phenocrysts (cores Anaí-ji») with inclu- clusions. Patches of silica minerals and sparse grains of sions of glass; some phenocrysts have thin irregularly clinopyroxene are seen in some thin sections. Flow band- shaped rims of sodic plagioclase (An M5?). Some lapilli ing, distinguished by color differences or by slight dif- 8 contain glomeroporphyritic clots of plagioclase and ferences in crystallinity and mineralogy, varies from well olivine, olivine and magnetite, two or more crystals of developed to non-existent. plagioclase, or less commonly, plagioclase and hypersthene. Red, massive rhyolites: Pilotaxltic-trachytic textures in The olivine in the clots and as phenocrysts is generally which flow alignment of plagioclase microlites (An9-ie) is euhedral or subhedral and not highly fractured, but some interrupted by phenocrysts, clots, and small patches of is slightly iddingsitized. cryptocrystalUne devitrified glass. Small, irregularly Devitrified fine ash encloses the pumice lapilli. This shaped aggregates of tiny orange-red hematite crystallites material, like the lapilli, has crystals of plagioclase cause the red color. Tiny subhedral and euhedral crystals (An2a-aO, hypersthene (En6a-7o)» olivine (Fos^-es), and of magnetite are evenly scattered through the groundmass, some augite (2V = 53° to 57°). In contrast to the lapilli, and rare perfectly euhedral microlites of hypersthene also plagioclase microlites are rare, and the numerous plagio- occur. The groundmass constitutes 97 to 98 percent of the clase phenocrysts are generally broken euhedra. Ferro- rock. Phenocrysts, in order of abundance, are unzoned, magnesian minerals are also fractured and broken. The anhedral to euhedral, corroded and embayed plagioclase minerals 1n the lapilli and ash are the same except for crystals (An20-29), anhedral to euhedral augite crystals degree of fracturing, breaking, and alteration. (2V • 46° to 54°), many with borders of magnetite, relatively Approximately 15 percent of the foreign rock fragments in rare euhedral and subhedral crystals of hypersthene the welded tuff are identical to the platy andesitic basalt (En62-68). and rare euhedral and subhedral magnetite pheno- (unit 2); 10 percent resemble rocks of the older rhyolite crysts. Some of the hypersthene crystals show exsolution (unit 1); 70 to 75 percent appear transitional between lamellae of clinopyroxene. Glomeroporphyritic clots, with olivine basalts of the shield and plateaus and the platy plagioclase crystals radiating from central grains of andesitic basalt. Most of the remaining fragments are augite and magnetite, are probably cognate. either dark-brown to black tachylitlc glass sprinkled with West wall basalt plagioclase and augite microlites, or extensively altered, Porphyritic with a subophitic groundmass that consists of fine-grained feldspathic rocks. plagioclase laths (An50-72; most about An6s)» augite gran- Mafic tuff (unit 4) ules, euhedral and subhedral magnetite crystals, iddingsi- tized olivine grains, and oxidized grains of hypersthene. Lower, coarser parts of beds of mafic tuff consist of Phenocrysts, in order of abundance, are anhedral to per- lithic (45 to 70 percent) and crystal (1 to 4 percent) fragments in a matrix of birefrlngent clays, zeolites,

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carbonate minerals, and palagonite. Approximately 20 to Red scoria (unit 3) 25 percent of the lithic fragments are identical to trie Chiefly brown glass (RI 1.569) with scattered crystallites platy andesitic basalt (unit 2); 5 to 10 percent resemble of plagioclase (andeslne-labradorite?). rocks of the older rhyolite unit (unit 1); at least 20 percent are medium- to coarse-grained olivine basalts that Platy andesitic basalt (unit 2) resemble some of the cognate xenoliths in the platy an- Pilotaxitic-trachytic texture with strong platy structure. desitic basalt (unit 2). In this last group, olivine con- Groundmass (95 to 97 percent of the rock) is composed of tent varies from 1 to 20 percent. Rocks appearing transi- strongly aligned cligoclase and andesine microlites tional between the medium- to coarse-grained olivine (An26-38) and augite microlites with a subophitic relation, basalts and the platy andesitic basalt (unit 2) are a1 so with small rare patches of inconspicuous light-tan glass fairly comnon. These have a pilotaxitic matrix of essen- between the plagioclase and augite microlites, and occa- tially the same minerals as the andesitic basalt, but the sional small, reso*bed and partly to completely iddingsl- alignment of the matrix is much weaker than in the ande- tized grains of olivine. Host of the phenocrysts are sitic basalt, the platy shears are absent, and phenocrysts plagioclase (cores range from Ansa to An5I») ranging from and glomeroporphyritic clots are much more abundant. The perfectly euhedral crystals to anhedral and broken grains, lithic fragments in the lower, coarser parts of beds of some of which show fritting and corrosion. Most are un- mafic tuff are mostly angular and subangular. Evidencs zoned, but some 1n the range An58-65 are normally zoned. for melting, fritting, oxidation, or other high-temperature Some ha^e jagged mantles of andeslne. Most are free of alteration is uncommon. Many of the fragments have rins of inclusions, but some have iiclusions of the groundmass clays or palagonite, indicating alteration due to hydrous minerals and of a clear gla.>s, and some have altered cores. reactions. About 25 to 30 percent of the lithic fragments Other phenocrysts a-e euhedral and subhedral hypersthene are very glassy, vesicular rocks with microlites of plagio- crystals (Enes-es). some of which show zoning or exsolu- clase and auglte, and occasionally olivine. Some fragments tion la/rellae, and rare magretite phenocrysts. are mainly slderomelane glass; others are tachylite. The Glomeroporphyritic clots are as common as phenocrysts 1n slderomelane may be fresh or almost completely palagonl- the andesitic basalt. They consist of two or more pheno- tlzed. These glassy fragments probably represent the juve- crysts of plagioclase or hypjrsthene, plagioclase and nile magma of this particular eruptive phase, whereas all hypersthene, plagioclase or lypersthene and magnetite, or the other fragments are from older units. The crystal of all three minerals. Most common are clots composed of fragments and crystals, in order of abundance, are oligo- two or more crystals of plag'oclase 1n a parallel or syn- clase to bytownite, olivine, auglte, and hypersthene. neusis relation. Sone of the plagioclase crystals of the The upper, finer parts of beds of mafic tuff are composed clots have sodic mantles and some are corroded and partial- of lithic fragments (15 to 20 percent) and crystal frag- ly-dissolved by reaction with late liquids. Small xeno- ments (10 to 15 percent) in a matrix of fine shards of tan liths are also relatively cooon 1n the andesitic basalt. slderomelane glass (some with tiny plagioclase microlites) Most have essentially the same mineralogy as the ground- 1n various stages of alteration to clays, zeolites, car- mass but are coarser grained ind generally have subophitic bonates, and palagonite. The I1th1c fragments are the same textures; some have more olivine than does the groundmass, rocks as in the coarser parts of the beds, except that suggesting cognate Inclusions. glassy fragments are uncommon, their place probably taker Andes 1 te egplutlnate. south we. 11 by the abundant glass shards. Chiefly Ught-brown g.ass (RI T7503) as shards and dis- In the lower, coarser part of the series of tuff beds, many crete lapilH. Some tiny microlites of andesine(?). foreign fragments and Inclusions are present. Most are Older rhyolite unit (unit 1) feldspatMc basalts and diabases, altered to clays, proba- Grayt platj rhyolite: Hemicrystalline texture. Ground- bly by reaction with steam from the magmas or from phreatic mass (94 percent of the rock) consists of a dense mat of explosions. crystallites, mainly feldspar, in which are set small The nature and extent of alteration of the mafic tuff is plagioclase microlites (Ani0»m)» euhedral magnetite grains, not consistent; 1t varies vertically within a given out- slender rots of hematite, and occasional small, irregular crop and laterally from outcrop to outcrop. Despite their blebs of alkali feldspar. Pherocrysts, in order of abun- buff and yellow-brown colors and striking resemblance to dance, are plagioclase (An22-ss), hypersthene (En6S-6s)> palagonite tuffs, most of these tuffs show relatively magnetite, and rarely auglte. Most of the phenocrystic little paiagonitizatfon. Clay minerals and zeolites are plagioclase occurs as aggregates of broken crystals; many the main alteration products, although carbonate minerals are normally zoned, with rims 2-6 mol percent more sodic are prominent 1n about 10 percent of the rocks sectioned. than cores. Most hypersthene pienocrysts are altered to Dike and sills, south wall magnetite except for a small core. Intergranular texture in which tiny grains of aug1te(?) Whitet maoeive, aphanit-lo rhyolite: Groundmass (68 to 70 and granules of magnetite fill Interstices between plagio- percent of the rock) consists of clear glass crowded with clase microlites (An5n-62)> Plagioclase ml crol 1 tes are tiny greenish blebs and crystallites and flow-oriented randomly oriented 1n centers of bodies and arranged microlites of sodic placloclase. Small, irregularly parallel to walls near contacts. Tiny, rounded to sub- shaped blebs and patches of alkali feldspar and crlsto- hedral grains of olivine and irregularly shaped patches of balite(?) ar3 scattered 1n this groundmass. The only light-tan glass also occur 1n the groundmass. The glass phenocrysts are euhedral unzoned plagioclase crystals constitutes about 2 percent of the rock in the center of (An25-so)- the dike and sills and about 6 to 9 percent near the con- Pink, masaive rhyolite: Crystalline groundmass (90 to 92 tacts. Phenocrysts, in order of abundance, are zoned lath- percent of the rock), probably derived from devitrification shaped plagioclase crystals (cores An 59.6"»; rims as sodic of glass, consisting of a densely interwoven mesh of tiny as An52) and subhedral to perfectly euhedral composltional- plagioclase microlites. Regularlv dispersed through this 1y zoned augite crystals (2V » 52° to 57°). Many of the fresh are reddish-brown hematite(?) margarltes; these are plagioclase crystals have cores outlined by opaque minerals more abundant around coarser textured patches 1n the ground- and tiny green crystallites. mass and around vesicles. The arrangement of the margarites Scoria, south wall and other crystallites follows old flowlines in the glass. Light- to dark-brown glass (RI 1.559) with rare microlites Pienocrysts, 'n order of .nbundanco, are euhedral and sub- of plagioclase (labradorite?). hedral, normally zoned plagloclast! crystals (cores An2o-2i»; r'ms Anin-ia)» small, euhedral hypersthene crystals Pumice deposit, south wall (En«i,-66). sotre showing slight oxidation and alteration to Colorless to grayish glass (RI 1.493). magnetite, and subhedral and euhedral magnetite grains. Some of the plagioclase crystals show oscillatory zoning. Glomerocrysts of hypersthene and magnetite or hypersthene and plagioclase are fairly common.

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Figure 6. A bomb sag in tuff beds of the mafic graded tuff (unit 4).

ability of the alteration, suggests low-tempera- ture-type explosions connected with water. The nature of the bomb sags and the fact that the tuffs have not been visibly affected by wind or running water suggests that they were not Figure 7. Microdrawings (crossed polarizers) show- deposited dry. ing (A) basalt inclusion from the mafic tuff unit, (B) The evidence suggests that the mafic tuffs rock appearing transitional between the basalts and the record the appearance of a shallow lake in the platy andesitic basalt, (C) platy andesitic basalt (unit 2). slowly developing caldera. This allowed entry The difference in magnification between the three drawings is necessary because of grain size. The matrix of water into the vents that had previously in all three rocks consists of plagioclase, magnetite, and erupted the underlying red scoria and ag- tiny granules of augite. Phenocrysts are plagioclase glutinate. Underwater eruptions from these (pi), olivine (ol), augite (ag), and hypersthene (hy). same vents then projected chilled and granu- Note the transitional texture from (A) to (B) to (C). The lated bits of volcanic glass and fragments of the basalt (A) is identical to basalts of the shield and of underlying rocks; the more violent steam ex- areas to the east and southeast of Newberry Volcano, in plosions probably drove eruption clouds into the "plateaus." the air far above the water surface. Debris Some of the lithic fragments in the tuffs falling back into the water gradually built up (Table 1) are petrologically significant because one or more tuff rings, which may have filled they represent a complete transition from the the shallow lake. Much of the debris, blown basalts of the Oregon plateaus to the platy high in the air by the more violent rhythmic andesitic basalts (Fig. 7). explosions, was deposited on land around the The mafic tuff unit is of particular impor- caldera as wet, sticky air-fall mud similar to tance because it records the time when the that described by Moore (1967) and Waters caldera had grown large enough to hold a and Fisher (1971a). The beheaded flanks of this lake. Prior to this, there is no evidence for tuff blanket are the mafic-graded tuffs in the any appreciable body of water associated caldera walls. with the volcano. After this, on the other hand,

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there is much evidence that water intermit- welded tuff units commonly grade from a tently filled the developing caldera; in fact, welded basal or middle zone into unwelded water was probably present more often than upper zones (Smith, 1960; Ross and Smith, not. 1960), but this body of rock appears to be Dike and Sills in the South Wall. In the equally welded throughout. Other uncon- south caldera wall, near Paulina Peak, the solidated deposits in the caldera walls were es- stratigraphic sequence is interrupted by a dike sentially unaffected by erosion, however, and it and sills of andesitic basalt (Fig. 4 and Table 1). seems unlikely that a thick unit, as required to The dike cuts the older rhyolite, platy andesitic give such a thoroughly welded base, would be basalt, red scoria, local pumice deposit, and entirely removed. Therefore, it seems more part of the local scoria deposit (Fig. 4). The likely that the welded tuff had a very high dike and sills must be older than the present emplacement temperature. caldera wall faults, and younger than the local The large percentage and large size of the scoria deposit, but there is no direct evidence pumice fragments (Table 1), the abundance of to indicate whether they were intruded before fragments of older rocks that must have been or after the mafic tuffs (unit 4) were deposited. ripped from the sides of the vent, the un- East Wall Welded Tuff. In most of the east oxidized crystal fragments, the probable high caldera wall, the mafic tuff unit is overlain by emplacement temperature, and the field rela- 20 to 120 ft (6 to 35.5 m) of dacitic welded tuff tions suggest that the welded tuff originated (Figs. 4 and 5; Table 4). Large collapsed pumice from pumice flow (Ross and Smith, 1960, p. 7, lapilli and fragments in the tuff resemble 33). Its source was probably near the present streaks and bands of lustrous black obsidian deposits, and it may have boiled out from enclosed in a gray aphanitic matrix. These fractures that accompanied downfaulting of the pumice streaks drape over foreign inclusions. eastern part of the volcano's summit. The The deep-brown color of the glass in this channel-like shape of the deposit suggests that tuff (Table 1), the complete collapse and elon- it was almost certainly confined to low sections gation of the pumice lapilli, and the lack of of the summit, and to shallow "valleys." It pore space suggest that the tuff is intensely probably originated from an initial violent welded (Smith, 1960, p. 824). This suggests explosion, as suggested by the lithic fragments, either that it had a very high emplacement followed by rapid flowirig-out of incandescent temperature or that the body of rock now pumice and ash away from the vent area. visible represents only the basal part of a once Paulina Falls Andeske Flow and Breccia. much thicker welded tuff unit (Smith, 1960, p. In the basal part of the low west wall of the 823-824; Ross and Smith, 1960, p. 24). Thick caldera, at Paulina Falls, and in the gorge of

Iddingsitized olivine

Aggregate of granular augite with collective euhedral shape

Zoned plagioclase

Ophitic groundmass

Oxidized orthopyroxene

1.0 mm Figure 8. Microdrawing of west wall basalt, showing shape. Crossed polarizer!-., an aggregate of granular augite with collective euhedral

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Paulina Creek, the mafic tuffs (unit 4) are platy rhyolites (85 to 90 percent); (2) black overlain by an andesite unit that consists of a obsidians (5 to 7 percent); and (3) red massive basal breccia as much as 30 ft (9 m) thick that rhyolites (less than 2 percent), a few of which grades up into a flow as much as 70 ft (21 m) are porphyritic (Table 1). Paulina Peak thick (Figs. 4 and 5; Table 1). As Williams rhyolites unconformably overlie the mafic (1935, p. 263) recognized, the granulation and tuff unit (unit 4) just east of Paulina Peak oxidation in the basal part of the andesite unit (Figs. 4 and 5). There, the rhyolite is only is probably autobrecciation, possibly due to the about 150 ft (46 m) thick, and it thins and dis- hot andesite flowing across wet mafic tuffs. appears a few hundred feet to the east. It West Wall Basalt. A gray to brick-red thickens rapidly to the west, and at Paulina porphyritic basalt overlies the Paulina Falls Peak more than 1,200 ft (366 m) is exposed in andesite in the low west wall of the caldera the caldera wall. The structure of these rhyolite (Figs. 4 and 5; Table 1). Plagioclase phenocrysts flows suggests that they spread away from fault- with calcic cores and sodic rims, peculiar ag- fissures on which caldera collapse was taking gregates of augite grains with collective place. euhedral shapes (Fig. 8), end-to-end synneusis Although the Paulina Peak rhyolites are cut of plagioclase crystals with further zones around by the caldera wall faults, they are probably the aggregations (Fig. 9), the presence of "syn-caldera" in age, rather than strictly olivine both in the groundmass and as pheno- "post-caldera." crysts, the iddingsitization of the olivine, and the oxidation of orthopyroxene suggest that the Rocks on the Caldera Floor and Walls west wall basalt has undergone at least two After the foundering of Newberry Caldera to phases of crystallization under very different approximately its present depth and shape, physical and chemical conditions. The unusual volcanic activity and faulting brought about rezoned end-to-end plagioclase synneusis is a many changes on the shield and in the caldera. common feature in the olivine basalts of the The "post-caldera" history of the volcano is plateaus east and southeast of Newberry Vol- divisible into three parts by the presence of the cano (A. C. Waters, 1971, oral commun.). Mazama ash (Powers and Wilcox, 1964) and a Probably olivine basalt magma moved into the younger pumice-fall deposit from a vent inside volcano during one period of faulting but did the caldera that has been called "Newberry not reach the surface. It partially crystallized ash and pumice deposit no. 2" (Higgins, 1969). and differentiated during a period of relative Thus, some features are older than Mazama quiescence and was released to the surface ash; others are younger than Mazama ash, but during a later period of faulting. older than the pumice-fall deposit; and still Paulina Peak Rhyolites. The most spectac- others are younger than the pumice-fall ular of the caldera wall units is the thick pile of deposit. These relations aid in recognition of an rhyolites that underlies Paulina Peak and the approximate "post-caldera" stratigraphic se- area downslope to the southwest for about 3.2 quence. mi (5.1 km). The three main types of rock in Rhyolite Ridge and Rhyolite Domes and the Paulina Peak unit are (1) gray and reddened Flows. During and shortly after the closing stages of caldera formation, a long, domical ridge of rhyolite (Rhyolite Ridge) issued from the same fault-fissures that had produced the Paulina Peak rhyolites, and domes and flows of similar rhyolite were erupted on the floor of the caldera and from the fault-fissures associated with the collapse (Figs. 4 and 5; Table 1). Field relations suggest that all these features are Élpvjs® about the same age. Mafic Tuff Rings. Among the earliest "post-caldera" events was the growth of three Figure 9. Microdrawing of synneusis relations of mafic tuff rings (Fig. 5) on the floor of the plagioclase in the west wall basalt. 1 and 2 are separate caldera. The two smaller rings, south of East crystals, with foreign inclusions in the cores, joined end Lake, are composed of hundreds of thin beds of to end by zones that enclose both. partially palagonitic lithic tuff. Most beds are

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graded and at least poorly sorted, and the size Paulina Lake and project into the lake, in- of most lithic fragments ranges from about 1.5 dicating that the direction of flow was toward in. (3 cm) to lapilli and fine ash, although the lake. At the lakeshore, the flow is highly larger fragments as much as 4.5 ft (1.4 m) brecciated, oxidized, and altered, especially at across are sporadically distributed through the its bottom. Locally, the flow overlies water-laid beds. The matrix is a fine partially palagonitic mafic tuff beds of Little Crater tuff ring, and basaltic ash (Table 1). The bedding and altera- in places it appears :o have ploughed into these tion are typical of tuff rings that have formed beds. All these features indicate that the flow subaqueously and grown above the surface of entered the lake. the water (Waters and Fisher, 1971a, 1971b; Resort Andesite. The Resort andesite flow Heiken, 1971). is exposed in roadcuts of the Interlake Road at The rocks of Little Crater tuff ring, the East Lake Resort (Fig. 5). It is a fine-grained, largest and best preserved of the three tuff nonporphyritic rock with contorted layers of rings, can be divided into two gradational parts. platy jointing (Table 1). It is reddened and In the lower part of the ring, the tuffs are autobrecciated and r anges from highly vesicular extremely palagonitized (Table 1); juvenile to dense. These textures were probably pro- fragments are glassy and not vesicular, and duced by the flow moving over wet ground even the foreign lithic fragments commonly near the old lakeshore. have thin glassy rinds. These beds contain logs, East Lake Andesite. The East Lake pine needles, plants, and leaves in various stages andesite flow issued from fissure vents near the of silicification. Some of these lower beds are southeast wall, coincident with a cone of East graded and sorted like most of the beds of the Lake andesite (Fig. 5). The rock is a light other two tuff rings, but most are poorly sorted reddish-gray andesite (Table 1) with numerous and ungraded and probably represent material blocky phenocrysts of zoned plagioclase that erupted under water and distributed outward are as much an 6 mrr. long. Glomeroporphyritic as a slurry in a manner described by Waters and clots in the andesite, poikilitic plagioclase Fisher (1971a, p. 169; 1971b). In contrast to phenocrysts, and the corroded olivine crystals the lower part of the ring, the beds of the upper are interpreted as crystal accumulates, formed part are well graded and sorted, and vesicular while the magma was held in shallow chambers. fragments seem to increase toward the top. East Lake Obsidian Flows. The East Lake Organic material is very rare and palagonitiza- obsidian flows erupted from a northeast-trend- tion less extreme in these upper beds. This part ing fissure zone southeast of East Lake (Fig. 5); of the tuff ring was not formed in water but late faulting has reopened this zone of fissures resulted from wet air-falls of material thrown into the air by phreatic explosions. The across the flows. Beth flows extend as lobate division between the two parts of the tuff ring masses from the fissure zone toward the lake. probably represents the approximate water Except for their greater age and devitrification, level at which the growing submarine cone these flows aire similar to the later obsidian emerged and started building above the lake. flows on the caldera floor (Table 1). Rhyolitic Pumice Cones and Ash Flows. Field relations suggest that beds of Little There are eight rhyolitic pumice cones, or Crater tuff extend north beneath younger remnants of pumice cones, and two ash flows on rocks, and that the tuffs divided the caldera the floor and walls cf Newberry Caldera (Fig. lake. 5). All consist of rhyolitic pumice (Table 1), Interlake Basalt Flow of Higgins and Waters with varying amounts of black and gray (1967). The Interlake Basalt Flow (Fig. 5 and obsidian chips, and varying amounts of platy Table 1) was erupted from a vent now covered rhyolite fragments that are probably from the by Central Pumice Cone near the center of the underlying rhyolite domes, flows, and com- strip of land that divides the caldera lakes. The plexes. These features vary only slightly in age, vent was probably a northwest-trending fissure, and probably represent a single "phase" of and, at the time of eruption, the land between activity. the lakes was probably a low ridge composed of Game Hut Obsidian Flow of Higgins and mafic tuff from the vent of Little Crater tuff Waters (1967). Game Hut Obsidian Flow ring and from fissures extending northwest emerged from a lateral vent at the southeast across the lake. Numerous lobes of the Inter- base of Central Pumice Cone and spread for lake Basalt Flow extend to the eastern shore of about one-quarter of a mile (400 m) to the

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southeast (Fig. 5). It is mantled by Newberry these cones, the Red Slide, was also the vent for ash and pumice deposits; thus, it must be a basalt flow, the Red Slide flow (Fig. 5 and younger than the "main building phase" of Table 1). Central Pumice Cone, but older than its final One of the most spectacular features of New- eruptions. It is petrographically similar to the berry Caldera is the East Lake fissure, an open other obsidian flows in the caldera (Table 1). scar in the north wall above East Lake (Fig. 5), Newberry Ash and Pumice Deposits. There that was described in detail by Higgins and are two rhyolitic air-fall ash and pumice lapilli Waters (1970). Andesite scoria, agglutinate, deposits (Table 1) that were erupted from and thin glassy vesicular flows (Table 1) form Central Pumice Cone. The older deposit, mounded spatter banks at the edges of the "Newberry ash deposit no. 1" (Higgins, 1969), fissure. These deposits are not covered by the is poorly exposed, and its extent is unknown. Newberry ash and pumice deposits. The younger deposit, "Newberry ash and Rhyolitic Obsidian Flows. Three young pumice deposit no. 2" (Higgins, 1969), is flows of rhyolitic obsidian are found within distinctive, widespread, and valuable for rela- Newberry Caldera (Fig. 5). All three are com- tive dating. This deposit is divided into: (1) a posed of fresh, undevitrified obsidian and main pumice fall, consisting chiefly of un- graded pumice lapilli which extends east 20 to TABLE 2. BASALTS OF THE PLATEAU 40 mi (32 to 64 km) from the caldera; and (2) an overlying unit of as many as five graded ash Oxides 1 2* 3 49.79 falls, consisting of pumice and lithic fragments, S102 48,0 49.15 A120j 17.2 17.73 17.47 confined to within about 3 mi (about 5 km) of Fe20j 1.6 2.76 5.94 FeO 7.7 7.20 4.08 the caldera. Wood beneath the main pumice MgO 8.7 6.91 6.70 fall has a carbon-14 age of 1,720 ± 250 yrs. CaO 10.3 9.91 9.84 Na20 3.0 2.88 3.08 Mafic to Intermediate Flows and Cinder M 0.66 0.72 0.64 H20+ 0.76 0.40 0.19 Cones. The Red Slide and the Sheep's Rump H20- 0.07 0.25 0.06 T102 1.6 1.52 1.31 (Fig. 5) are two large cinder cones on the walls P20i 0.21 0.26 0.29 of Newberry Caldera that are younger than the MnO 0.18 0.14 0.17 CO 2 <0.05 0.06 0.01 Newberry ash and pumice deposits. Both are Other composed of basaltic cinders (Table 1). One of Total 100.0 99.89 99.57

4. FLANK ROCKS Recalculated water free

3. POSTCALDERA ROCKS Oxides 1 2 3 2. SYNCALDERA ROCKS S102 48.4 49.52 50.13 Al 20s 17.3 17.86 17.58 Rhyolite flows, domes, and complexes Fe20j 1.6 2.78 5.98 FeO 7.8 7.25 4.10 West wall basalt MgO 8.8 6.96 6.74 CaO 10.4 9.98 9.90 Paulina Falls andesite flow and breccia Na20 3.0 2.90 3.10 K20 0.66 0.72 0.64 East wall welded tuff T102 1.6 1.53 1.31 0.29 P20s 0.21 0.26 Mafic tuffs with graded bedding MnO 0.18 0.14 0.17 C02 0.0 0.06 0.01 (Water in early caldera) Molecular norma 1. PRECALDERA ROCKS 1 2 3 Scoria in south wall -. . ... , Dike and sills? oz 0.00 0.00 1.95 Pumice deposit ¡n south wall OR 3.89 4.29 3.82 AB 26.06 26.06 27.91 Red scoria AN 31.48 33.59 32.26 CO 0.00 0.00 0.00 DI 14.55 11.10 12.04 Platy andesitic basalt EN 0.00 9.88 12.85 FS 0.00 3.80 0.43 Andesite agglutinate in south wall F0 14.01 4.01 0.00 FA 5.17 1.54 0.00 Older rhyolite MT 1.67 2.91 6.27 IL 2.23 2.13 1.84 BASALTS OF THE PLATEAU AP 0.44 0.55 0.61 Other NE-0.51 CC-0.15 CC-0.03 Figure 10. Division of the rocks of Newberry DI 30.48 30.37 33.70 Volcano, based on interpretations of the volcano's history. * Averageof 21 analyses.

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pumiceous obsidian (Table 1). These flows are are aligned parallel to t.ae regional fault sys- among the youngest features of Newberry tems. Volcano (Peterson and Groh, 1969). The cinder cones vary in age. The oldest are partly buried beneath the uppermost lava Rocks on the Flanks of Newberry Volcano flows of the shield. Other well-shaped cones are Mafic to Intermediate Flank Flows arid covered by Mazama ash, and many of these Cinders. There are more than 200 mafic have soil layers that developed before the ash cinder cones on the Newberry shield. As fell. Still other cinder cones are not covered Williams (1935, p. 278-279) noted, most of the with Mazama ash but are covered with pumice large cinder cones range in height from 200 to from the Newberry ash and pumice deposit 400 ft (61 to 122 m); a few reach a height of eruptions. Some cinder cones are even younger about 500 ft (152 m). Most are % to % ™ than the Newberry ash and pumice deposits. (0.4 to 0.5 km) in diameter and have craters. Between and around the cinder cones, the Almost all of the cones consist of scoriaceous Newberry shield is covered with basaltic to bombs, lapilli, and cinders of red, red-brown, or andesitic lava flows of various ages; these flows black basalt, basaltic andesite, or andesite vary in size from less than l/L mi (0.4 km) to (Table 1). Many of the cones and spatter ridges flows more than 5 mi (8 km) long and 2 mi

TABLE 3. PRECALDERA ROCKS

Older rhyolite Andesite P 1 a t y a n d (; s i t i c b a salt Red agglutinate scoria

Oxides 4 5 6 7 8 9 10 11 12 13 14 15 16

Si02 71.5 72.38 66.9 51.9 52.30 52.50 52.57 52.9 52.9 53.2 53.2 53.50 52.9 Al 20j 14.5 14.21 14.4 15.6 15.93 16.50 16.02 15.2 15.4 .15.6 15.6 17.05 16.4 9.1 Fe203 2.3 1.97 1.8 4.2 3.57 4.00 5.01 6.3 3.8 3.8 2.4 2.41 FeO 0.56 0.51 2.6 7.5 7.82 6.88 6.29 5.3 7.2 7.8 8.6 8.50 0.88 MgO 0.34 0.18 0.55 4.1 4.02 3.92 3.88 3.5 3.9 3.7 3.6 3.72 4.3 CaO 1.1 0.80 2.3 7.6 7.90 8.30 7.86 7.5 7.9 7.4 7.5 7.40 7.6 3.5 Na20 4.8 5.31 4.9 3.7 4.37 3.55 4.41 3.8 3.9 4.1 4.2 3.90 0.85 K20 3.4 3.83 2.8 0.84 0.87 0.81 0.76 1.1 0.92 0.8C 0.97 0.73 0.48 0.90 h2o+ 1.0 0.19 2.4 1.5 0.06 0.10 0.18 1.1 0.77 0.59 0.10 h2o- 0.09 0.01 0.66 0.10 0.03 0.20 0.02 0.29 0.53 0.02 0.15 0.25 1.1 1.5 T102 0.28 0.23 0.47 2.2 2.29 2.45 2.16 2.2 2.1 2.3 2.5 1.80 0.54 0.35 P20s 0.04 0.04 0.09 0.46 0.42 0.30 0.39 0.49 0.40 0.50 0.30 MnO 0.09 0.07 0.14 0.20 0.20 0.40 0.19 0.23 0.20 0.22 0.20 0.20 0.17 <0.05 Nil <0.05 C02 <0.05 0.01 <0.05 <0.05 0.01 Nil 0.01 <0.05 <0.05 <0.05 Other S=tr S=tr Total 100.0 39. n 100.0 99.9 99.79 99.91 99.75 99.3 99.9 ldo.i 100.0 99.86 99.6

Recalculated water free Oxides 4 5 6 7 8 9 10 11 12 13 14 15 16 52.81 53.7 53.6 53.5 53.6 53.76 54.2 Si02 72.3 72.71 69.0 52.8 52.45 52.70 15.6 15.7 15.7 17.13 16.8 Al 20s 14.7 14.27 14.9 15.9 15.97 16.56 16.09 15.4 4.01 5.03 6.4 3.9 3.8 2.4 2.42 9.3 Fe20a 2.3 1.97 1.9 4.3 3.58 FeO 0.56 0.51 2.7 7.6 7.84 6.90 6.31 5.4 7.3 7.8 8.7 8.54 0.90 MgO 0.34 0.18 0.56 4.2 4.03 3.93 3.89 3.6 4.0 3.7 3.6 3.73 4.4 CaO 1.1 0.80 2.4 7.7 7.92 8.33 7.89 7.6 8.0 7.4 7.6 7.43 7.8 3.56 4.43 3.9 4.0 4.1 4.2 3.91 3.6 Na20 4.9 5.33 5.1 3.8 4.38 0.76 0.93 0.80 0.97 0.73 0.87 K20 3.4 3.84 2.9 0.85 0.87 0.81 1.1 2.16 2.2 2.1 2.3 2.5 1.80 1.5 Ti02 0.28 0.23 0.48 2.2 2.29 2.45 0.30 0.39 0.49 0.40 0.50 0.54 0.30 0.35 P20s 0.04 0.04 0.09 0.46 0.42 MnO 0.09 0.07 0.14 0.20 0.20 0.40 0.19 0.23 0.20 0.22 0.20 0.20 0.17 0.00 0.0 0.0 0.0 0.0 0.00 0.0 C02 0.0 0.01 0.0 0.0 0.01 0.00

Molecular norme 4 5 6 7 8 0 10 11 12 13 14 15 16 QZ 26.13 23.23 20.43 5.06 1.37 5.60 3.54 8.49 4.85 5.01 3.15 3.91 9.15 OR 20.37 22.70 17.12 5.13 5.21 4.88 4.56 6.73 5.59 4.83 5.84 4.38 5.22 AB 43.72 47.83 45.54 34.35 39.74 32.53 <0.21 35.33 36.00 37.60 38.46 35.58 32.69 AN 5.27 3.64 9.35 24.28 21.56 27.25 22.01 21.92 22.41 22.27 21.27 27.30 27.60 CO 1.13 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 DI 0.00 0.01 1.49 9.28 12.24 10.22 12.00 10.63 12.16 9.51 10.47 6.41 6.44 EN 0.95 0.49 1.28 8.66 7.44 7.57 6.37 5.24 7.14 7.54 7.31 8.75 9.13 FS 0.00 0.00 1.97 4.54 4.52 3.56 2.11 0.60 3.91 4.85 6.24 7.92 0.00 F0 0.00 0.00 0.00 0.00 0.00 O.CO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 FA 0.00 0.00 0.00 0.00 0.00 O.CO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MT 0.94 0.87 1.95 4.54 3.78 4.27 5.32 6.82 4.08 4.06 2.56 2.56 0.00 IL 0.40 0.32 0.68 3.17 3.23 3.48 3.06 3.17 3.01 3.27 3.55 2.55 1.70 AP 0.08 0.08 0.19 0.99 0.89 0.64 0.83 1.06 0.86 1.07 1.15 0.64 0.76 Other HT-1.00 HT-0.80 0.00 0.00 CC-0.03 O.CO 0.00 0.0D 0.00 0.00 0.00 0.00 HT-6.60 TN-0.72 DI 90.25 93.78 83.12 44.56 46.34 43.04 48.33 50.57 46.46 47.46 47.48 43.90 47.09

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«elded tuff in Paulina Falls West wall basalt Paulina Peak rhyolites and rhyolite ridge rocks North rhyolite flow Paulina east wall andesite Lake dome

Si02 67.92 68.6 60.43 60.85 50.7 51.65 52.2 69.7 69.80 69.88 71.07 71.27 71.45 70.4 70.5 70.7 69.7 A120J 15.22 15.8 16.06 17.10 18.2 17.05 17.7 14.9 14.85 14.96 14.92 14.91 15.12 14.9 15.0 14.7 15.2 Fe203 1.50 1.6 3.48 2.18 2.6 7.56 6.9 2.4 1.07 1.62 1.09 0.96 0.95 1.8 0.82 1.8 0.66 FeO 2.84 2.5 3.03 4.30 5.3 3.79 3.8 0.92 2.37 1.37 1.72 1.80 1.78 0.88 1.8 0.80 2.4 MgO 0.60 0.40 1.71 2.21 6.0 3.87 3.6 0.39 0.36 0.53 0.22 0.24 0.34 0.42 0.45 0.32 0.39 CaO 2.20 1.8 4.58 4.35 9.4 8.26 8.2 1.7 2.00 1.69 1.08 1.11 1.70 1.6 1.6 1.4 1.7 Na20 6.08 5.6 5.71 5.20 3.2 4.10 3.6 5.5 5.10 5.55 6.04 6.04 4.43 5.1 5.1 5.0 5.4 K20 2.37 2.2 1.54 1.30 1.1 0.66 0.70 3.2 3.18 3.12 3.03 3.02 3.49 3.4 3.4 3.6 3.1 H O+ 2 0.26 0.50 0.53 0.65 1.4 0.27 0.76 0.67 0.50 0.19 0.17 0.03 0.15 0.62 0.63 1.0 0.61 H2O- 0.02 0.00 0.07 0.15 0.09 0.07 0.05 0.18 0.30 0.02 0.01 0.00 0.15 0.11 0.04 0.13 0.07 Ti0 2 2.16 0.44 1.30 0.90 1.3 2.09 1.9 0.41 0.30 0.36 0.29 0.28 0.30 0.38 0.34 0.37 0.36 P2O5 0.39 0.12 0.59 0.45 0.44 0.32 0.32 0.00 tr 0.09 0.05 0.03 tr 0.20 0.12 0.00 0.12 MnO 0.19 0.14 0.15 0.20 0.16 0.18 0.18 0.11 tr 0.09 0.09 0.09 Nil 0.18 0.07 0.08 0.09 C0 2 0.01 <0.05 0.61 Nil <0.05 0.01 0.05 <0.05 Nil 0.06 0.00 0.00 Nil <0.05 <0.05 <0.05 <0.05 Other S=tr S=tr Total 101.76 99.7 99.79 99.84 99.9 99.88 99.96 100.1 99.83 99.53 99.78 99.78 99.86 100.6 99.9 99.9 99.8 Recalculated water free Oxides 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 Si0 2 68.26 69.2 60.92 61.43 51.5 51.87 52.6 70.2 70.48 70.35 71.35 71.44 71.76 70.9 71.1 71.6 70.3 A120J 15.29 15.9 16.19 17.26 18.5 17.14 17.9 15.0 14.99 15.06 14.97 14.94 15.18 15.0 15.1 14.9 15.3 Fe20, 1.50 1.6 3.50 2.20 2.6 7.59 7.0 2.4 1.08 1.63 1.09 0.96 0.95 1.8 0.82 1.8 0.66 FeO 2.85 2.5 3.05 4.34 5.4 3.80 3.8 0.92 2.39 1.37 1.72 1.80 1.78 0.88 1.8 0.80 2.4 MgO 0.60 0.40 1.72 2.23 6.1 3.88 3.6 0.39 0.36 0.53 0.22 0.24 0.34 0.42 0.45 0.32 0.39 CaO 2.21 1.8 4.61 4.39 9.6 8.29 8.3 1.7 2.01 1.70 1.08 1.11 1.70 1.6 1.6 1.4 1.7 6.11 5.75 5.25 Naz0 5.6 3.3 4.11 3.6 5.5 5.14 5.58 6.06 6.05 4.44 5.1 5.1 5.1 5.4 K20 2.38 2.2 1.55 1.31 1.1 0.66 0.70 3.2 3.21 3.14 3.04 3.02 3.50 3.4 3.4 3.6 3.1 TÌO2 0.50 0.44 1.31 0.90 1.3 2.09 1.9 0.41 0.30 0.36 0.29 0.28 0.30 0.38 0.34 0.37 0.36 0.11 0.59 0.45 0.44 0.32 P20s 0.12 0.32 0.00 0.00 0.09 0.05 0.03 0.00 0.20 0.12 0.00 0.12 MnO 0.14 0.14 0.15 0.20 0.16 0.18 0.18 0.11 0.00 0.09 0.09 0.09 0.00 0.18 0.07 0.08 0.09 0.01 0.61 0.00 0.0 0.01 C02 0.0 0.05 0.0 0.00 0.06 0.00 0.00 0.00 0.0 0.0 0.0 0.0 Molecular norms 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 OZ 16.00 20.71 11.37 11.21 0.00 5.15 8.02 20.16 20.72 19.94 19.42 19.41 25.47 22.60 21.49 22.84 19.53 OR 13.99 13.08 9.14 7.74 6.58 3.97 4.24 19.01 18.97 18.47 17.84 17.75 20.79 20.23 20.21 21.53 18.41 AB 54.56 50.62 51.52 47.04 29.09 37.48 33.12 49.65 46.23 49.95 54.06 53.97 40.11 46.11 46.06 45.45 48.74 AN 7.24 8.20 13.71 18.78 32.45 26.70 30.81 6.55 8.32 6.71 4.64 4.63 8.51 6.68 7.20 7.03 7.69 CO 0.00 1.34 0.00 0.34 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.06 0.44 0.34 0.04 0.17 DI 2.31 0.00 1.10 0.00 9.49 10.26 6.72 1.54 1.36 0.58 0.30 0.52 0.00 0.00 0.00 0.00 0.00 EN 1.23 1.11 4.27 6.15 12.96 5.75 6.82 0.31 0.81 1.26 0.56 0.59 0.95 1.17 1.25 0.89 1.08 FS 2.14 2.38 0.63 4.23 3.78 0.00 0.00 0.00 2.04 0.55 1.53 1.67 1.69 0.00 1.86 0.00 2.91 FO 0.00 0.00 0.00 0.00 0.00 0.11 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 FA 0.00 0.00 0.00 0.00 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MT 1.57 1.68 3.66 2.30 1.00 1.00 1.69 2.75 4.95 5.41 1.55 1.13 1.70 1.14 0.86 1.29 0.69 IL 0.70 0.62 1.82 0.39 0.42 0.53 0.48 0.52 1.26 1.83 2.96 2.71 0.57 0.42 0.50 0.40 0.50 AP 0.23 0.25 1.24 0.06 O.OO 0.42 0.25 0.00 0.95 0.93 0.68 0.69 0.00 0.00 0.19 0.10 0.25 Other CC-0.03 0.00 CC-1.55 0.00 O.OO HT-0.14 0.00 HT-0.41 0.00 0.00 2.09 1.45 HT-0.65 0.00 CC-0.15 0.00 0.00 [CC-0.03, [CC-0.13, HT-2.06] HT-1.32] DI 84.58 84.44 72.06 66.02 35.69 46.62 45.40 88.85 85.94 88.38 41.35 86.40

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(3.2 km) wide. Most flows are sparsely porphy- rocks outside the caldera is a small post-New- ritic, vesicular, high-alumina olivine basalts and berry ash and pumice deposit no. 2 obsidian basaltic olivine andesites (Table 1). Flow types flow on the southeast flank of the shield about range from pahoehoe to aa, both types com- 4 mi (6.4 km) from the caldera. monly being present in the same flow. CHEMISTRY AND PETROLOGY Siliceous Rocks. Outside the caldera, these rocks are mostly confined to the extreme flanks The rocks of Newberry Volcano can be of the volcano. The most prominent of these divided into four groups on the basis of are three small domes of spherulitic platy geographic and geologic position and on inter- rhyolite on the western flank and two large pretations of the geologic history of the vol- rhyolite and obsidian domes, East Butte and cano and its caldera: (1) precaldera roc\s—all China Hat, on the extreme eastern flank. All of units of the caldera-wall sequence that predate these domes are older than the Mazama ash. formation of an early caldera large enough to The only other known occurrence of siliceous contain a lake; ('.'.) syncaldera rocks—all units

TABLE 5. POSTCALDERA ROCKS

»0 IO O >— -a "na Ol •i- y u aj X T- Interlake to *-> •M East Lakä

Oxides 34 35 36 37 38 39 40 41 42 ¿3 44 45 46 47

Si02 53.10 54.0 55.4 63.41 52.7 71.5 72.5 70.6 72.8 72.35 49.0 52.84 55.29 56.71 A120S 17.68 17.8 16.4 16.63 17.9 14.3 14.3 13.9 13.9 13.98 17.8 17.63 17.48 16.45 Fe20a 2.13 3.0 2.7 2.08 3.2 0.87 0.33 0.53 0.67 0.60 S.2 2.12 1.05 1.17 FeO 5.83 4.9 6.0 2.99 5.1 1.3 1.7 1.7 1.3 1.78 3.1 6.12 6.39 6.41 MgO 5.41 5.4 3.7 1.39 5.4 0.30 0.33 0.26 0.21 0.30 '.2 5.17 4.39 3.89 CaO 8.99 8.5 7.0 3.51 8.5 1.1 1.1 1.4 1.4 1.30 1 1.2 8.70 7.79 7.14 4.19 Na20 3.68 3.1 4.3 6.04 3.7 4.5 4.6 4.5 4.5 5.04 3.1 3.68 3.93 K20 1.09 1.1 1.5 1.78 1.1 4.1 4.1 4.0 4.1 3.92 0.46 0.98 1.24 1.49 h2o+ 0.25 0.56 0.10 0.27 0.36 1.2 0.43 2.2 0.45 0.45 (1.88 0.42 0.44 0.52 0.17 0.03 h2o- 0.03 0.06 0.15 0.11 0.09 0.30 0.03 0.24 0.05 0.05 (1.02 0.03 TiOj 1.18 1.1 1.5 1.04 1.3 0.25 0.23 0.38 0.22 0.25 1.2 1.34 1.20 1.25 P205 0.35 0.33 0.35 0.43 0.34 0.13 0.10 0.09 0.02 tr C.16 0.37 0.30 0.35 MnO 0.14 0.15 0.18 0.15 0.17 0.05 0.05 0.03 0.06 Nil C. 18 0.15 0.15 0.14 C02 0.01 0.05 <0.05 0.00 <0.05 <0.05 <0.05 <0.05 <0.05 Nil <0.05 0.04 0.00 0.01 Other S=tr Total 99.87 100.0 55.3 99.83 93.3 99.9 99.8 33.3 39.7 100.02 99.6 99.59 99.82 99.75

Recalculated Jaier free Oxides 34 35 36 37 38 39 40 41 42 43 44 45 46 47 72.4 73.4 72.69 49.7 53.29 55.73 57.16 Si02 53.31 54.3 55.9 63.76 53.0 72.6 73.0 14.0 14.04 18.1 17.78 17.61 16.58 A120Î 17.75 17.9 16.6 16.72 18.0 14.5 14.4 14.3 0.54 0.60 6.3 2.13 1.05 1.17 Fe20a 2.13 3.0 2.7 2.09 3.2 0.88 0.33 0.7 FeO 5.85 4.9 6.1 3.00 5.1 1.3 1.7 1.7 1.3 1.78 3.1 6.17 6.44 6.46 MgO 5.43 5.4 3.7 1.39 5.4 0.30 0.33 0.26 0.21 o.:o 7.3 5.21 4.42 3.92 CaO 9.02 8.5 7.1 3.52 8.6 1.1 1.1 1.4 1.4 1.30 10.3 8.77 7.85 7.19 3.1 3.71 3.96 4.22 Na20 3.69 3.1 4.3 6.07 3.7 4.6 4.6 4.6 4.5 5.06 4.1 4.1 0.46 0.98 1.24 1.50 K20 1.09 1.1 1.5 1.78 1.1 4.2 4.1 3.93 1.35 1.20 1.26 Ti02 1.18 1.1 1.5 1.04 1.3 0.25 0.23 0.38 0.22 0.25 1..Î 0.09 0.02 0.03 0. 16 0.37 0.30 0.35 P20s 0.35 0.33 0.35 0.43 0.34 0.13 0.10 MnO 0.14 0.15 0.18 0.15 0.17 0.05 0.05 0.09 0.06 0.0) 0.18 0.15 0.15 0.14 0.0 0.04 0.00 0.01 C02 0.01 0.05 0.0 0.00 0.0 0.0 0.0 0.0 0.0 0.00

Molecular norms 34 35 36 37 38 39 40 41~ 42 43~ 44~ 45 46 47 0Z 0.81 5.68 4.28 11.43 1.68 25.07 24.56 23.88 25.49 22.56 0.55 1.49 3.11 4.59 OR 6 44 6 54 8.97 10.50 6.52 24.67 24.41 24.31 24.49 23.24 2.75 5.83 7.36 8.87 AB 33.05 28.03 39.10 54.13 33.33 41.15 41.62 41.56 43.86 45.42 28.17 33.26 35.47 37.90 AN 28.51 31 63 21.29 12.99 29.09 4.69 4.84 6.09 5:68 3.96 33.7) 28.89 26.54 21.84 CO 0.00 0 00 0.00 0.00 0.00 0.86 3.59 0.00 0.00 0.00 0.0.) 0.00 0.00 0.00 DI 11.00 6.68 9.25 1.27 8.80 0.00 0.00 0.36 0.97 2.01 13.17 9.50 8.28 9.18 EN 11.04 12 41 7.36 3.41 11.51 0.84 0.92 0.69 0.44 0.54 13.54 11.09 9.66 8.13 FS 4.52 3.50 4.04 1.75 3.19 1.16 2.18 1.80 1.01 1.29 O.Otl 4.95 6.16 5.75 F0 0.00 0 00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 O.OO 0.00 0.00 0.00 FA 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 C.00 0.00 0.0C 0.00 0.00 0.00 WT 2.23 3.16 2.86 2.17 3.36 0.93 0.35 0.57 0.71 0.63 5.IE 2.23 1.10 1.23 IL 1.64 1.54 2.12 1.45 1.82 0.35 0.32 0.54 0.31 0.35 1.69 1.88 1.68 1.75 AP 0.73 0.69 0.74 0.90 0.71 0.28 0.21 0.19 0.04 0.00 0.34 0.78 0.63 0.74

Other

DI 40.32 40.28 52.37 76.09 41.55 90.91 90.62 89.78 90.86 91.25 31.49 40.61 45.98 51.38

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younger than the precaldera rocks that fit (Table 1). Mineralogically, these basalts appear into the transitional period when the caldera to be of alkalic affinity, showing no reaction was being enlarged to approximately its present between olivine and pyroxene (Kuno, 1950, size and shape; (3) postcaldera roc\s—all units 1968; Tilley, 1950; Poldervaart, 1964), and in and around the caldera that are younger than generally having olivine and a Ca-rich clinopy- the completion of the caldera; and (4) flan\ roxene (Kuno, 1968) in the groundmass. Most rocks—all rocks on the flanks of the volcano. In of the andesites are porphyritic, plagioclase- addition to these four groups of rocks, there are rich, hypersthene-augite andesites that also the basalts of the Oregon plateau around the contain small amounts of olivine (Table 1). edges of the Newberry shield (Tables 2 through Clots and cognate inclusions are prevalent in 7, and Fig. 10). most of the rocks, and many of the rocks show In general, most of the Newberry basalts are evidence of two stages of crystallization. olivine-augite basalts with traces of hypersthene Mineralogically and texturally, the rocks ap-

TABLE 5 (continued)

Andesites from Game Hut Pumice cone North fissure on east and obsidian flow obsidian flow obsidian flow Big obsidian flow southeast wall

Oxides 48 49 50 51 52 53 54 55 56 57 58 59 60

Si02 72.8 72.9 72.8 72.9 72.9 73.1 72.22 72.4 72.6 73.40 58.3 58.35 61.9 A120S 14.6 14.0 14.6 14.1 13.9 14.0 14.41 14.4 14.4 14.20 15.8 16.27 16.1 Fe203 0.77 0.77 0.44 0.66 0.56 0.33 0.50 0.64 0.64 0.24 2.7 1.01 1.9 FeO 1.3 1.3 1.6 1.4 1.4 1.7 1.62 1.6 1.6 1.76 5.2 7.38 4.2 MgO 0.34 0.28 0.25 0.27 0.24 0.28 0.18 0.18 0.24 0.18 2.9 3.07 2.1 CaO 1.1 1.0 1.0 0.98 1.6 0.99 0.84 0.87 0.92 1.35 6.0 6.30 6.0 Na20 4.3 4.6 4.2 4.6 4.4 4.6 5.28 4.9 4.7 4.15 4.3 4.24 4.6 K20 4.0 4.2 3.8 4.2 4.1 4.2 4.01 4.1 3.9 4.10 1.8 1.75 2.4 H2O+ 0.43 0.51 0.76 0.58 0.42 0.45 0.12 0.50 0.63 0.40 0.74 0.10 0.73 H2O- 0.00 0.02 0.14 0.02 0.06 0.00 0.01 0.06 0.03 0.10 0.14 0.10 0.08 Ti02 0.24 0.21 0.21 0.22 0.23 0.2) 0.23 0.22 0.20 0.20 1.4 1.15 1.0 P2O5 0.10 0.09 0.09 0.10 0.02 0.09 0.03 0.09 0.09 tr 0.43 0.18 0.84 MnO 0.04 0.06 0.05 0.05 0.12 0.05 0.06 0.06 0.05 tr 0.14 0.15 0.14 C02 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 0.00 <0.05 <0.05 Nil <0.05 Nil 0.07 Other S=tr Total 100.0 $9.9 59.9 lOÛ.l 1ÛÛ.0 100.0 99. SI 100.1 100.0 100.08 99.9 100.05 102.Ì

Recalculated water free Oxides 48 49 50 51 52 53 54 55 56 57 58 59 60 Si02 73.1 73.3 73.5 73.3 73.3 73.4 72.67 72.8 73.1 73.70 58.9 58.43 61.1 AI 20ä 14.7 14.1 14.7 14.2 14.0 14.1 14.49 14.5 14.5 14.25 16.0 16.29 15.9 Fe203 0.77 0.77 0.44 0.66 0.56 0.33 0.50 0.64 0.64 0.24 2.7 1.01 1.9 Feo 1.3 1.3 1.6 1.4 1.4 1.7 1.63 1.6 1.6 1.76 5.3 7.39 4.1 MgO 0.34 0.28 0.25 0.27 0.24 0.28 0.18 0.18 0.24 0.18 2.9 3.07 2.1 CaO 1.1 1.0 1.0 0.98 1.6 0.99 0.84 0.87 0.92 1.35 6.1 6.30 5.9 Na20 4.3 4.6 4.2 4.6 4.4 4.6 5.31 4.9 4.7 4.16 4.3 4.24 4.5 K20 4.0 4.2 3.8 4.2 4.1 4.2 4.03 4.1 3.9 4.11 1.8 1.75 2.4 TÌO2 0.24 0.21 0.21 0.22 0.23 0.21 0.23 0.22 0.20 0.20 1.4 1.15 1.0 P20s 0.10 0.09 0.10 0.10 0.02 0.09 0.03 0.09 0.09 0.00 0.43 0.18 0.82 MnO 0.04 0.06 0.05 0.05 0.12 0.05 0.06 0.06 0.05 0.00 0.14 0.15 0.13 C02 0.0 0.0 0.0 0.0 0.0 0.0 0.00 0.0 0.0 0.00 0.0 0.00 0.06

Molecular norms

QZ 27.31 25.27 28.70 25.23 25.58 24.98 21.52 23.60 25.47 27.29 9.29 6.06 11.02 OR 23.82 25.01 22.78 24.99 24.44 24.97 23.75 24.35 23.23 24.46 10.81 10.40 14.04 AB 38.92 41.63 38.27 41.60 39.86 41.56 47.52 44.22 42.55 37.62 39.24 38.28 40.90 AN 4.84 4.41 4.37 4.24 6.12 4.35 3.79 3.75 4.01 6.76 18.80 20.31 16.04 CO 1.58 0.31 2.21 0.49 0.00 0.33 0.00 0.59 1.09 0.53 0.00 0.00 0.00 DI 0.00 0.00 0.00 0.00 1.40 0.00 0.16 0.00 0.00 0.00 6.88 7.95 6.05 EN 0.95 0.78 0.70 0.75 0.47 0.78 0.48 0.50 0.67 0.50 5.93 6.63 3.95 FS 1.22 1.29 1.99 1.49 1.16 2.20 1.88 1.83 1.85 2.30 3.29 7.33 2.73 F0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 FA 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MT 0.81 0.81 0.47 0.69 0.59 0.35 0.52 0.67 0.67 0.25 2.87 1.06 1.97 IL 0.34 0.29 0.30 0.31 0.32 0.29 0.32 0.31 0.28 0.28 1.98 1.61 1.38 AP 0.21 0.19 0.21 0.21 0.04 0.19 0.06 0.19 0.19 0.00 0.91 0.38 1.74 CC- Other 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.18 DI 90.08 91.94 89.78 91.84 89.91 91.53 92.82 92.19 91.27 89.39 59.36 54.75

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pear to be a series of differentiates from a berry analyses trenc from the high-alumina common parent, the high-alumina plateau field into the alkali field with increasing dif- basalts. ferentiation. Thus, the chemical analyses sup- Chemical analyses and norms of the high- port the field and petrographic evidence that alumina plateau basalts, rocks from the four the Newberry rocks are differentiates of the Newberry groups, and certain other rocks such slightly alkalic, high-alumina plateau basalt as inclusions, are given in Tables 2 through 7; magma (Waters, 1962). analysis 2 from Waters, 1962; analyses 9, 15, 20, None of the Newberry rocks contains 25, 29, 43, 57, 59, 64, and 73 from Williams, nepheline in the norm (Tables 2 through 7); 1935. Detailed locations and pétrographie few of the rocks contain olivine in the norm, descriptions of these samples will be published despite the fact that many have modal olivine. in a later report. The analyses show that the Most of the rocks contain quartz in the mode Newberry rocks taken as a whole are calc-alkalic and hypersthene in both the norm and mode. (Peacock, 1931), with an alkali-lime index of Differentiation at Newberry has been ex- about 58 (Fig. 11). On Kuno's (1960, 1968) treme, as shown by :he high differentiation alumina-alkali diagrams, the Newberry anal- index compared with silica content of the more yses plot partly in the high-alumina basalt silicic rocks (Tables 2 through 7), when com- field and partly in the alkali-basalt field. On pared with these parameters of other rock Kuno's (1968) alkali-silica diagram, the New- suites (Thornton and Tuttle, 1960). This sup-

TAELE 6. FLANK ROCKS

Oxides 61 62 63 64 65 66 67 68 69 70 71 72

S10a 48.2 49.1 50.1 50.70 54.6 55.32 55.5 56.4 58.1 69.5 70.0 72.4 AljOj 17.5 17.6 17.5 18.05 15.9 16.94 18.2 15.3 16.2 15.8 15.6 14.2 Fe20s 3.4 1.0 2.0 1.62 3.4 1.22 1.6 6.3 2.1 1.7 2.3 2.2 FeO 6.1 8.0 7.1 6.96 6.6 6.36 4.7 3.8 4.9 0.72 0.64 0.24 MgO 7.9 8.9 7.8 7.60 3.5 4.85 4.9 3.0 3.5 0.13 0.24 0.17 CaO 9.8 10.0 9.8 9.70 7.1 7.98 8.4 6.1 6.6 1.0 0.96 1.1 Na20 2.4 2.6 2.8 2.70 3.6 3.88 3.3 4.1 3.8 3.9 4.0 4.3 M 0.31 0.43 0.42 0.68 1.2 •;.29 1.4 1.4 1.9 3.2 3.2 3.5 HjO+ 1.7 0.71 0.60 Nil 0.88 0.51 0.43 0.34 1.0 2.1 1.6 0.75 HjO- 0.82 0.10 0.13 0.10 0.22 0.02 o:o7 0.20 0.11 1.2 1.1 0.75 TiOj 1.4 1.0 1.3 1.30 2.1 V6 0.90 1.8 1.2 0.17 0.22 0.17 P20s 0.27 0.31 0.23 0.23 0.43 0.26 0.32 0.44 0.41 0.10 0.08 0.10 MnO 0.17 0.16 0.17 0.40 0.20 0.15 0.12 0.16 0.14 0.07 0.07 0.08 CO 2 <0.05 <0.05 <0.05 Nil 0.05 C.01 <0.05 <0.05 0.05 <0.05 <0.05 <0.05 Other S=tr

Total 100.0 99.9 100.0 100.04 99.8 99.95 99.9 99.4 100.0 99.6 100.0 100.0

Recaliulatsd water free Oxides 61 62 63 64 65 66 67 68 69 70 71 72 71.9 73.5 Si02 49.5 49.5 50.5 50.73 55.3 55.64 55.9 57.1 58.7 72.2 AI2O3 18.0 17.8 17.6 18.06 16.1 17.03 18.3 15.5 16.4 16.4 16.0 14.4 Fe20s 3.5 1.0 2.0 1.62 3.4 1.22 1.6 6.4 2.1 1.8 2.4 2.2 FeO 6.3 8.1 7.2 6.96 6.7 6.39 4.7 3.8 5.0 0.74 0.65 0.24 MgO 8.1 9.0 7.9 7.60 3.5 4.87 4.9 3.0 3.5 0.13 0.24 0.17 CaO 10.1 10.1 9.9 9.70 7.2 8.02 8.5 6.2 6.7 1.0 1.0 1.1 4.1 Na20 2.5 2.6 2.8 2.70 3.6 3.90 3.3 4.1 3.8 4.1 4.4 K20 0.31 0.43 0.42 0.68 1.2 1.29 1.4 1.4 1.9 3.3 3.3 3.6 Ti0z 1.4 1.0 1.3 1.30 2.1 1.16 0.90 1.8 1.2 0.17 0.22 0.17 0.44 0.41 0.10 0.08 0.10 P20s 0.27 0.31 0.23 0.23 0.43 0.26 0.32 MnO 0.17 0.16 0.17 0.40 0.20 0.H 0.12 0.16 0.14 0.07 0.07 0.08 C02 0.0 0.0 0.0 0.00 0.05 0.01 0.0 0.0 0.05 0.0 0.0 0.0

Molecular norme 61 62 63 64 65 ~~66~ 67 68 69 ' 70 71 72

QZ 0.44 0.00 0.00 0.00 8.55 2.70 5.30 12.06 9.31 31.51 31.01 30.01 OR 1.88 2.54 2.49 4.00 7.29 7.54 8.30 8.51 11.39 19.78 19.59 21.16 AB 22.11 23.34 25.23 24.16 33.26 34.92 29.73 37.87 34.63 36.64 37.23 39.51 AN 37.00 35.08 34.07 35.01 24.37 25.06 30.82 19.76 21.86 4.51 4.40 4.92 CO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 4.96 4.53 1.76 DI 8.86 10.00 10.58 9.16 6.93 10.29 7.12 6.73 6.67 0.00 0.00 0.00 EN 18.83 9.45 14.31 14.59 7.70 10.15 10.99 5.16 7.56 0.38 0.69 0.48 FS 4.66 4.06 5.36 5.89 4.18 5.77 4.14 0.00 3.66 0.00 0.Q0 0.00 F0 0.00 8.72 2.59 2.29 0.00 o.co 0.00 0.00 0.00 0.00 0.00 0.00 FA 0.00 3.74 0.97 0.92 0.00 o.co 0.00 0.00 0.00 0.00 0.00 0.00 MT 3.65 1.05 2.10 1.69 3.66 1.28 1.68 5.60 2.23 1.55 1.24 0.40 IL 2.00 1.39 1.82 1.80 3.01 1.62 1.26 2.58 1.70 0.25 0.32 0.24 AP 0.58 0.65 0.48 0.48 0.93 0.55 0.67 0.95 0.87 0.22 0.17 0.21 Other 0.00 0.00 0.00 0.00 CC-0.13 CC-0.03 0.00 HT-0.78 CC-0.13 •IT-0.21 HT-0.84 HT-1.30 01 24.45 25.90 27.75 28.19 49.13 45.23 43.35 58.46 55.36 87.96 87.85 90.71

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ports the contention that Newberry magmas suite (Williams, 1935; Smith and Carmichael, were trapped in shallow reservoirs and allowed 1968, p. 213, 234), whereas the volcano con- to differentiate. sidered as a whole has approximately as much An estimate of the composition of Newberry andesite as dacite and rhyolite. Considered Volcano is about 98 percent basalt, andesitic chronologically, the rocks of the caldera area basalt, and basaltic andesite; about 1 percent an- define a series of compositional cycles (Fig. 12) desite; and about 1 percent rhyolite and dacite. in which silicic rocks are commonly followed by In the immediate area of the caldera, an estimate intermediate and then mafic rocks. This is about 65 percent basalt, andesitic basalt, further supports the view that shallow magma and basaltic andesite; about 5 percent andesite; reservoirs held and periodically released and about 30 percent rhyolite and dacite. Thus, material by faulting after in situ differentiation. the caldera gives the impression of a bimodal TABLE 7. OTHER ROCKS Controls of Differentiation Most FMA plots (Wager and Deer, 1939) of o >— v- m> volcanic rock suites show a scatter of points, 3 Ji which nevertheless are interpreted as defining fairly smooth curves either toward iron enrich- tu o ment (Skaergaard or Fenner trend) or toward Inclusions Mixed scoria 2 U- G -5 alkali enrichment (Cascade, calc-alkaline, or Oxides 73 74 75 76 77 78 79 Bowen trend). Most curves, however, lie some- S102 48.60 51.6 59.75 64.10 66.55 61.8 53.2 where between the extreme Skaergaard and A1203 17.84 19.8 15.27 15.02 14.80 13.4 17.1 Fe203 1.84 4.8 5.19 3.09 2.57 3.8 4.5 Cascade trends. Previous FMA and F'MA FeO 7.20 3.3 2.81 2.30 1.53 0.32 4.1 MgO 7.90 3.3 2.33 1.40 0.88 0.98 5.4 (where F' is FeO + 0.9 Fe203) plots for CaO 11.65 10.8 4.76 3.14 2.22 5.3 8.4 Newberry Volcano used a relatively small Na20 2.47 2.9 4.62 4.87 4.94 4.6 3.2 K20 0.25 0.55 1.98 2.77 3.19 2.3 1.1 number of analyses, and they too showed a H2O+ 0.25 1.1 1.16 1.78 2.09 4.5 0.77 scatter of points interpreted as curves toward H2O- 0.30 1.2 0.07 0.03 0.06 0.51 0.12 T102 1.30 1.2 1.31 0.88 0.64 0.67 1.3 moderate iron enrichment (Higgins, 1968, p. P20s tr 0.26 0.42 0.19 0.13 0.80 0.39 MnO 0.30 0.14 0.15 0.11 0.09 0.13 0.15 305; McBirney, 1968, p. 105; Taylor, 1968, p. C02 Nil <0.05 0.00 0.01 0.05 0.06 47; Best, 1969, p. 65). Figure 13A is an F'MA Other S=tr plot for Newberry Volcano using all the Total 99.90 101.0 99.82 99.69 99.69 99.2 99.8 analyses in Tables 2 through 7. Taken as a

Recalculated water free whole, the points could be interpreted as de- fining a differentiation curve from the general Oxides 73 74 75 76 77 78 79 vicinity of the high-alumina plateau basalts, up S102 48.91 52.3 60.60 65.48 68.22 65.6 53.8 toward slight iron enrichment, ending with A120S 17.95 20.1 15.48 15.34 15.17 14.2 17.3 Fe20s 1.85 4.9 5.26 3.15 2.63 4.0 4.6 alkali-rich rhyolites. However, when the points FeO 7.24 3.3 2.85 2.34 1.56 0.33 4.1 MgO 7.95 3.3 2.36 1.43 0.90 1.0 5.5 are distinguished as belonging to the various CaO 11.72 10.9 4.82 3.20 2.27 5.6 8.5 groups of rocks, on the basis of the history of Na20 2.48 2.9 4.68 4.97 5.06 4.9 3.2 K20 0.25 0.55 2.00 2.82 3.27 2.4 1.1 the volcano, there appear to be two main curves Tf02 1.30 1.2 1.32 0.89 0.65 0.71 1.3 P20s 0.00 0.26 0.42 0.19 0.13 0.84 0.39 or trends. The precaldera rocks and plateau MnO 0.30 0.14 0.15 0.11 0.09 0.13 0.15 basalts erupted before a water-holding caldera C0 0.00 0.0 0.00 0.01 0.00 0.05 0.06 2 was present, trend up toward slight iron en-

Molecular norme richment; whereas the postcaldera rocks, erupted after water was present in the caldera, 73 74 75 76 77 78 79 trend more toward alkali enrichment (Fig. 0Z 0.00 7.43 13.33 16.55 19.20 18.13 6.40 OR 1.48 3.33 11.97 16.76 19.34 14.51 6.60 13B). Most syncaldera rocks (Fig. 13A) plot AB 22.25 26.68 42.44 44.79 45.52 44.12 29.17 with the postcaldera rocks, but a few plot with AN 36.98 40.37 15.43 11.21 9.02 9.75 29.50 CO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 the precaldera rocks. When these are con- DI 16.81 10.24 4.74 2.73 1.13 5.78 7.88 sidered individually, however, all the points EN 6.37 4.39 4.21 2.71 1.93 0.00 11.59 FS 2.49 0.15 0.00 0.25 0.00 0.00 1.27 that plot with the postcaldera rocks are for F0 7.10 0.00 0.00 0.00 0.00 0.00 0.00 FA 2.78 0.00 0.00 0.00 0.00 0.00 0.00 rocks that field and petrographic evidence in- MT 1.93 5.14 4.24 3.31 2.49 0.00 4.78 IL 1.82 1.71 1.87 1.26 0.91 0.75 1.84 dicate were erupted while water filled the AP 0.41 0.00 0.56 0.90 0.28 1.79 0.83 0.00 HT-0.87 CC-0.03 HT-0.18 forming caldera, and all the points that plot Other 0.00 5.17*CC-0.15 37.47 67.76 78.13 84.08 DI 23.75 76.80 42.19 with the precaldera rocks are for rocks that were apparently erupted while the forming * CC-0.14/ HT-2.83; WO-1.93/ TN-0.37.

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WEIGHT PERCENT Si02 EXPLANATION

INCLUSION CINDER HILL DIKE SYNCALDERA ROCK A USGS analysis © USGS analysis O USGS analysis Williams (1935) e Williams (1935) FLANK ROCK ® USGS analysis MIXED SCORIA PRECALDERA ROCK x USGS analysis O Williams (1935) • USGS analysis 4 Williams (1935) POSTCALDERA ROCK WELDED TUFF, FORT ROCK VALLEY O USGS analysis BASALT OF THE PLATEAU <(> Williams (1935) • USGS analysis • USGS analysis • Writers (1962)

Figure 11. CaO and Na20 + K20 versus Si02 for about 38 would make the rocks calc-alkalic (Peacock, Newberry Volcano rocks. The alkali-lime index of 1931). caldera was filled with other lavas or volcani- Thus, there appears to be a direct correlation clastic rocks and thus relatively dry (see between differentiation trends at Newberry descriptions of individual units). Flank rocks Volcano and the presence or absence of a water- also plot with both curves. Matching the rela- holding caldera. tive ages of these flank rocks precisely with the Cause of the Double Trends. The experi- ages of the caldera rocks is impossible. How- ments of Kennedy (1955) and Osborn and his ever, those flank rocks relatively near the coworkers (Muan and Osborn, 1956; Osborn, caldera and younger than Mazama ash (hence, 1959, 1962; Osborn and Roeder, 1960) on the probably postcaldera) plot with the postcaldera effects of oxygen fugacity on the behavior of rocks, whereas flank rocks older than Mazama iron oxide during crystallization of basaltic ash and far from the caldera plot with the magma have shown the possibility of different precaldera rocks. oxygen conditions producing two different re- The same two-trend relation seen on the action series for igneous rocks. When oxygen F'MA plots for Newberry Volcano also holds fugacity remains high (or constant) during true for iron/iron + magnesia versus silica crystallization of a basaltic liquid, magnetite plots (Fig. 14), for alkalis versus iron/iron + coprecipitates with other ferromagnesian sili- magnesia plots, for a plot of magnesia versus cates, and the residual liquid is silica rich. total iron, for magnesia versus silica plots, and However, when oxygen fugacity is much lower for a plot of titania versus silica (Fig. 15). (or fixed), iron olivine rather than magnetite Moreover, on an alkali versus silica plot, the separates, and the residual liquid becomes iron precaldera rocks plot higher (more alkalic) than rich. The first condition could account for the postcaldera rocks—the postcaldera rocks Cascade-type trends, the second for Skaergaard- start low and trend high. Most of the flank type trends. rocks plot within the high-alumina field of Carmichael (1964) in a detailed study of the Kuno (1968). rocks of Thingmuli Volcano, Iceland, showed

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DECREASING AGE Figure 12. Chronologic (stratigraphic) arrangement rhyolites, rhyolite ridge rocks, and northeast rhyolite of some of the Newberry units plotted against their flow; 9. Interlake basalt; 10. Resort andesite; 11. East average differentiation index (DI) to illustrate the cyclic Lake andesite; 12. Cinder Hill basalt; 13. East Lake eruptive history of the younger part of the volcano. obsidian flows; 14. Paulina Lake ash flow; 15. obsidian Numbers are: 1. older rhyolite unit; 2. andesite ag- layer in wall of central pumice cone; 16. mafic inter- glutinate in south wall; 3. platy andesitic basalt unit; 4. mediate flows and cinders; 17. obsidian flows; 18. red scoria unit; 5. welded tuif of east wall; 6. Paulina fissure flows on caldera walls. Falls andesite; 7. west wall basalt; 8. Paulina Peak

that differences in oxygen fugacity had affected rocks have the appearance and characteristics not only minerals directly related to the iron of a continuous differentiation series from the systems but salic constituents as well. He was some type of magma that erupted as the high- thus first to show field, petrographic, and chem- alumina plateau basalts. Distinct inclusions and ical data supporting Osborn's experimentally mineral clots, characteristic of the Oregon derived hypothesis. high-alumina plateau basalts, occur throughout Oxygen fugacity is intimately related to the Newberry sequence, even in the rhyolites. water content and pressure, especially in vol- Trace elements (Table 8 and Fig. 16) deter- canic rocks. The presence of large volumes of mined on rocks that are important in defining water over the shallow magma chambers of the two trends on the chemical plots are virtu- Newberry Caldera must have had a profound ally the same for the precaldera and postcaldera effect on the differentiating magma. That dif- rocks with similar silica contents. Sr87/Sr86 ferences in oxygen fugacity were the cause of ratios (A. K. Sinha and Higgins, unpub. data) the double trends, and not some coincidence, for these same rocks are essentially identical such as two separate magmas whose mingling (also see Church and Tilton, 1973). Differentia- was fortuitously timed to coincide with the tion trends of constituents which should not presence of a water-holding caldera, is sup- be affected by changes in the iron systems show ported by field, petrographic, chemical, trace- no regular differences between the precaldera element, and isotopic data. and postcaldera rocks. As noted by Waters (1962), the Newberry Mineralogically, the Newberry rocks also fit

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Figure 13B. An F'MA plot of the Newberry pre- trend toward alkali enrichment. The general area of the caldera and postcaldera rocks, and the basalts of the "plateau basalts" represents the immediate "parent" plateau. The precaldera rocks define a trend toward magma, iron enrichment, whereas the postcaldera rocks define a

well with the hypothesis of oxygen-fugacity and Katsura, 1965). The petrography of this control of differentiation. Mafic minerals are, in banded scoria is strikingly similar to that of the general, richer in iron in the precaldera rocks "mixed lavas" from Lassen Peak, described than in the postcaldera rocks (Table 1), and by Macdonald and Katsura (1965). The dark basaltic hornblende occurs only in the post- bands are andesite, and the light bands are caldera rhyolites (Table 1). dacite (Table 7). The light bands (no. 77), the The only known occurrence of' 'mixed lavas" dark bands (no. 75), ar.d the mixed scoria as a at Newberry Volcano is in Mixture Butte, an whole rock (no. 76) all plot with the same trend elongate, pre-Mazama ash cinder cone on the on plots affected by the inferred presence of western slope of the shield, about 100 yds south water. They generally plot with the pre- of the road that connects the caldera with U.S. Mazama ash flank rocks where they belong Highway 97 (not shown on Fig. 5). The cone stratigraphically. Sr87/Sr86 ratios (Higgins, is composed chiefly of red and brown cinders in Waters, and Sinha, unpub. data) are the same layers that dip away from the fissure vent, but for the light, dark, and whole mixed-scoria about 2 percent of the scoria are ' 'mixed lavas," samples. This is in contrast to the Katmai such as those at Katmai (Fenner, 1923, p. 38; (Novarupta) mixed scoria whose light and dark Curtis, 1968) and at Lassen Peak (Macdor.ald layers are significantly different in Sr^'/Sr86

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EXPLANATION

POSTCALDERA ROCK PRECALDERA ROCK BASALT OF THE PLATEAU

O USGS analysis • USGS analysis • USGS analysis

4> Williams (1935) • Williams (1935) • Waters (1962)

ratios (Higgins, Waters, and Sinha, unpub. Medicine Lake Caldera formed relatively data). slowly, as Anderson's conclusions (1941, p. 358-362) suggest, water probably filled parts of Medicine Lake Highland it at various stages of development. Anderson's Division of units into precaldera and post- study also suggested (1941, p. 384) that caldera sequences is much more complicated at Medicine Lake was probably once much larger the Medicine Lake Highland Volcano than at than it is now. Newberry Volcano because the Medicine Lake On an F'MA plot (Fig. 17), the rocks of the Caldera was subsequently filled by lava flows Medicine Lake Highland Volcano appear to and cinders from rim volcanoes and by the show two trends when divided according to products of vents within the caldera (Ander- Anderson's (1941) precaldera and postcaldera son, 1941). Moreover, Anderson lacked the groupings. The two trends are not as distinct as benefits of carbon-14 dating to help establish a those on the Newberry F'MA plots, possibly chronology. Nevertheless, he did divide the because syncaldera rocks are grouped with the rocks at Medicine Lake Highland into pre- postcaldera rocks, but the precaldera rocks do caldera and postcaldera sequences (Anderson, plot higher toward the F' corner. A two-trend 1941, especially the geologic map). If the relation is also probable on an alkali versus

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WEIGHT PERCENT Si02 Figure 14. A. Iron/iron + magnesia versus silica plot of the precaldera and postcaldera rocks of New- plot for all of the rocks of Newberry Volcano. Symbols berry Volcano and the basalts of the plateau. Symbols as in Figure 11. B. Iron/iron + magnesia versus silica as in Figure 13B.

iron/iron + magnesia plot, and on plots of The two-trend relation shown on the chemi- titania, magnesia, and iron/iron + magnesia cal plots for the Medicine Lake Highland versus silica. Thus, the differentiation at the Volcano is supported by oxygen-isotope data Medicine Lake Highland Volcano also appears reported by Taylor (1968). Unfortunately, to have been controlled to some extent by Taylor sampled only postcaldera and syncaldera oxygen-fugacity differences due to the forma- rocks at Newberry Volcano (1968, p. 5); but at tion of a caldera large enough to hold a Medicine Lake Highland, his samples included significant amount of water. The control was both precaldera and postcaldera rocks. Taylor's less than at Newberry Volcano. Figure 12 (1968, p. 46) shows how the Medicine

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3

O tr

45 50 55 60 65 70 75

WEIGHT PERCENT Si02 Figure 15. Titania versus silica plot for the pre- the basalts of the plateau. Two trends are seen. Symbols caldera and postcaldera rocks of Newberry Volcano and as in Figure 13B.

Lake Highland rocks vary in 018/016 ratios northern California to central Washington. with decreasing age. When comparison of Most of the volcanoes in this belt are low samples with about the same silica contents is shields, in which differentiation from the parent made (that is, comparison of the basalts only), high-alumina basalt magma has been slight, but distinct differences in 018/016 ratios are seen several are large complex centers, in which between the precaldera and postcaldera rocks. differentiation has been extreme, producing andesites, dacites, and rhyolites. One of the SUMMARY largest of these is Newberry Volcano. Evidence A belt of high-alumina shield volcanoes, a has been presented that the origin of the New- subprovince of Waters' (1962) high-alumina berry rocks involved differentiation of the plateau basalt petrologie province, lies at the parent slightly alkalic high-alumina basalt eastern edge of the Cascade Mountains from magma in shallow magma chambers or pockets,

TABLE 8. TRACE ELEMENTS IN PPM BY QUANTITATIVE SPECTROGRAPHS ANALYSIS

Syncaldera Postcaldera rocks Flank rocks rocks

3 I— O

r- 3 I— -O a» E io

Ba 280 1,000 400 320 360 280 1,000 460 520 740 920 870 900 720 Cr 190 5 70 15 13 14 4 3 130 2 2 2 71 3 Cu 110 14 120 100 120 6 8 120 8 16 9 100 93 Ni 130 <4 20 15 8 <4 10 43 <4 <4 <4 10 <4 Pb <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 Sc 61 11 60 59 62 48 17 28 48 7 6 10 32 46 Sr 550 79 840 530 880 560 180 500 880 70 70 64 870 300 V 230 9 320 380 280 340 <10 80 200 <10 <10 <10 140 190 Y 50 80 80 60 80 70 90 70 60 60 60 70 60 70 Zr 100 320 150 140 160 140 300 160 120 300 280 390 170 220 Cs 2 5 5 1 4 1 5 1 4 5 2 2 2 2 Rb 7 150 18 26 17 92 33 42 180 140 150 51 54

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Figure 16. Plots of selected trace elements for some of the rocks of Newberry Volcano. The rocks are from the precaldera, postcaldera, syncaldera, flank rocks, - "other rocks," and "plateau basalts" suites. The _ samples from the precaldera and postcaldera suites are - the same as those that define two trends on the preced- _ ing chemical plots; but on the trace elements plots, the - only significant distinctions are between basalts and rhyolitic rocks (inside dashed lines) on some diagrams. Symbols as in Figure 11.

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Figure 17. An F'MA plot of all available analyses for platy andésites which may represent syncaldera rocks the rocks of the Medicine Lake Highland Volcano. The are shown as circled dots; and a sample of an iron- rocks are divided according to Anderson's (1941) pre- enriched segregation vein from the Warner Basalt caldera and postcaldera groupings. Solid circles = pre- (Kuno, 1965) is shown as a diamond. caldera rocks; open circles - postcaldera rocks; two

where it was channeled and trapped by block Special appreciation is due George Riddle, faulting on three regional fault systems. Late U.S. Geological Survey, Denver, Colorado; stages of this differentiation were greatly in- P. Elmore, G. Chloe, J. Glenn, L. Artis, H. fluenced by oxygen-fugacity differences related Smith, J. Kelsey, and S. Botts, U.S. Geological to water in the caldera. A similar history and Survey, Washington, D.C., for the chemical course of magma differentiation and control is analyses on which much of this study is based; probable for the Medicine Lake Highland Vol- and Janet D. Fletcher, U.S. Geological Survey, cano, another of the large complex centers in Washington, D.C., for the quantitative spec- the shield volcano belt. trographic analyses. Special thanks are due V. M. Seiders, D. S. Harwood, N. S. MacLeod, ACKNOWLEDGMENTS and R. A. Sheppard for particularly construc- Aaron Waters suggested this study and par- tive reviews of the manuscript. ticipated in much of it. He also read and greatly improved early versions of the manu- REFERENCES CITED script. I am also indebted to William S. Wise American Geophysical Union, Special Committee for providing computer norm calculations. for the Geophysical and Geological Study of Both Waters and Wise contributed numerous the Continents, 1964, Bouguer gravity helpful suggestions during informal discussions. anomaly map of the United States (exclusive

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