U.S. Department of the Interior

U.S. Geological Survey

US SG

Professional Paper 1692

Eruptive History and Chemical Evolution 

of the Precaldera and Postcaldera Basalt-Dacite 

Sequences, Long Valley, California: Implications 

for Magma Sources, Current Seismic Unrest, 

and Future Volcanism
This page intentionally left blank
Eruptive History and Chemical Evolution of the 

Precaldera and Postcaldera Basalt-Dacite Sequences, 

Long Valley, California: Implications for Magma Sources, 

Current Seismic Unrest, and Future Volcanism

Professional Paper 1692

U.S. Department of the Interior

U.S. Geological Survey

By Roy A. Bailey
ii 

U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

This report and any updates to it are available online at:

http://pubs.usgs.gov//pp/p1692/

Additional USGS publications can be found at:

http://geology.usgs.gov/products.html

For more information about the USGS and its products:

Telephone: 1–888–ASK–USGS (1–888–275–8747)

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement of the U.S. Government.

Although this report is in the public domain, it contains copyrighted materials that are noted in the text. Permission 

to reproduce those items must be secured from the individual copyright owners. 

Cataloging-in-publication data are on file with the Library of Congress (URL http://www.loc.gov/).

Produced in the Western Region, Menlo Park, California

Manuscript approved for publication, March 1, 2004

Text edited by George A. Havach

Layout and design by Sara Boore

Front Cover Long Valley Caldera from above Mammoth Rock on the southwest rim to Glass Mountain on the northeast rim (30 km). The resurgent dome is the forested ridge in the middle distance. 

The seismically and geothermally active south moat is the lowland in foreground. The White Mountains are snowcapped on the right horizon. View northeastward.
 iii

In Memoriam

Roy Alden Bailey (1929–2003)

Roy A. Bailey received his B.A. in geology from Brown University in 1951 and his M.Sc. from Cornell University in 

1953. He was a Fulbright Scholar in New Zealand in 1956, where he met his wife Patrice and began his career-long professional 

attachments. Roy began his career with the U.S. Geological Survey (USGS) as camp cook and geologic field assistant in New 

Mexico during the summers of 1950–52, and became a full-time geologist with the USGS in 1953 in Reston, Va. After working 16 years as a geologist, he returned to graduate school at the Johns Hopkins University from 1969 to 1971, completing his 

Ph.D. in 1978 under the USGS Employee Training Program. His research with the USGS focused primarily on the study of large 

silicic volcanic systems. His fruitful collaboration with Robert L. Smith and Clarence S. Ross led to the publication of classic 

papers on resurgent calderas and chemical zonation in magma reservoirs. Roy’s principal field efforts involved geologic mapping of Valles Caldera (Jemez Mountains, N.Mex.) and the Long Valley volcanic field in eastern California. Through careful 

mapping of the volcanic rocks in eastern California, he was able to identify the source of the widespread Bishop Tuff as a large 

resurgent caldera at the base of the eastern Sierra Nevada escarpment, now known as Long Valley Caldera.

During the 1960s, Roy was an instructor in the National Aeronautics & Space Administration (NASA)’s Apollo training 

program, tutoring astronauts in the recognition of lunar volcanic features by comparing them to similar features on the Earth. 

During 1980–83, he served as coordinator of the USGS Volcano Hazards Program, which was greatly increased in size and complexity after the 1980 eruption of Mount St. Helens, Wash. In this managerial role, he deftly guided the Survey’s expanded monitoring and research activities in Hawaii, Alaska, and the conterminous United States. In 1983, he rotated back into a research 

position, moved to Menlo Park, Calif., in 1984, and resumed his studies of the Long Valley volcanic field, where escalating 

seismicity and magmatic unrest raised concerns about a possible eruption. He retired in 1995, after 42 years of service, and 

continued working essentially full time as a Pecora Fellow (1995–97) and as Scientist Emeritus with the USGS Volcano Hazards 

Team (1997–2003).

Roy was a Fellow of both the Geological Society of America and the Mineralogical Society of America and held membership in the American Geophysical Union and the Commission on Explosive Volcanism of the International Association of Volcanology and Chemistry of the Earth’s Interior. Throughout his long and distinguished career, Roy was recognized by his peers 

for his perceptive geologic observations and exceptional mapping skills. His carefully documented studies of the products and 

processes of large explosive eruptions contributed to fundamental understanding of the evolution of caldera-forming magmatic 

systems.

—David P. Hill
This page intentionally left blank
 v

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Acknowledgments.........................................................................................................................................3

Geologic-Tectonic Setting............................................................................................................................4

Long Valley Volcanism ..................................................................................................................................7

Introduction............................................................................................................................................7

Precaldera Sequence ..........................................................................................................................7

Postcaldera Sequence ......................................................................................................................10

Caldera Suite ..............................................................................................................................13

Basalt Enclaves.................................................................................................................14

Early Intracaldera Basalt-Trachyandesite Lavas ........................................................14

Late Intracaldera Basalt-Trachyandesite Lavas .........................................................14

Extracaldera Basalt-Trachyandesite Lavas .................................................................14

Dacite Lavas ......................................................................................................................15

Mono-Inyo Suite ........................................................................................................................15

Basalt lavas........................................................................................................................16

Dacite lavas .......................................................................................................................16

Petrography and Chemistry........................................................................................................................17

General Relations................................................................................................................................17

Petrography .........................................................................................................................................18

Precaldera Sequence ...............................................................................................................18

Basalt ................................................................................................................................18

Trachyandesite..................................................................................................................18

Dacite ................................................................................................................................21

Postcaldera Sequence .............................................................................................................21

Quenched-Basalt Enclaves.............................................................................................21

Basalt ................................................................................................................................21

Trachyandesite..................................................................................................................22

Dacite-Rhyolite..................................................................................................................22

Summary.............................................................................................................................22

Major- and Trace-Element Chemistry..............................................................................................25

Previous Work ............................................................................................................................25

Methods.......................................................................................................................................25

Precaldera Sequence ...............................................................................................................25

General Relations .............................................................................................................25

Basalt-Trachyandesite.....................................................................................................29

Dacite ................................................................................................................................30

Summary.............................................................................................................................30

Postcaldera Sequence .............................................................................................................32

Basalt-Trachyandesite.....................................................................................................32

Dacite-Rhyolite..................................................................................................................33

Summary.............................................................................................................................42

Comparison of the Precaldera and Postcaldera Sequences.............................................42
vi 

Sr, Nd, and Pb Isotopes...............................................................................................................................42

Sampling...............................................................................................................................................42

Regional Relations: Sri=0.706 Isopleth.............................................................................................43

Long Valley General Isotopic Relations ..........................................................................................44

Sr and Nd Isotopes.............................................................................................................................46

Pb Isotopes ..........................................................................................................................................47

Petrogenesis.................................................................................................................................................47

General Introduction ..........................................................................................................................47

Long Valley Basalt Sources ..............................................................................................................53

Precaldera Basalts....................................................................................................................53

Postcaldera Basalts ..................................................................................................................54

Buttresses and Horseshoe Lake Basalts......................................................................54

Caldera Basalts.................................................................................................................54

Mono-Inyo Basalts ...........................................................................................................55

Evolution of the Precaldera Sequence ...........................................................................................56

Central and East Sierran Suites ..............................................................................................56

Basin and Range Suite..............................................................................................................59

Summary......................................................................................................................................59

Evolution of the Postcaldera Sequence..........................................................................................59

Caldera and Mono-Inyo Suites................................................................................................59

Basalt ................................................................................................................................59

Trachyandesite..................................................................................................................59

Basalt-Dacite Mixing .......................................................................................................61

Dacite ................................................................................................................................61

Summary......................................................................................................................................62

Tectonomagmatic Model............................................................................................................................63

Recent Reactivation of Basaltic Magmatism..........................................................................................66

Future Volcanism and Hazards..................................................................................................................66

References Cited..........................................................................................................................................67

Figures

1. Sketch map of the Owens Valley Rift, east-central California, showing location 

and extent of the Long Valley Volcanic Field, as well as the Big Pine and 

Coso Volcanic Fields ....................................................................................................................5

2. Sketch geologic map of central part of the Long Valley Volcanic Field, Calif., 

showing distribution of precaldera and postcaldera basalt-dacite sequences................6

3. Histograms of number of dated lava flows per 100 k.y. versus time....................................8

4. Sketch geologic map of west moat of Long Valley Caldera, Mammoth Mountain, 

and Devils Postpile area, Calif. (after Bailey, 1989)...............................................................11

5. Sketch geologic map of June Lake-Mono Craters area, Calif............................................12

6. Sketch geologic map of Mono Lake area, Calif.....................................................................13

7. Plots of K2O+Na2O versus SiO2 contents of precaldera and postcaldera 

basalt-dacite sequences...........................................................................................................17

8. SiO2-variation diagrams for the precaldera Central Sierran, East Sierran, and 

Basin and Range suites .............................................................................................................26

9. Plots of compositional characteristics of basaltic lavas versus distance from west 

edge of the Long Valley Volcanic Field, Calif., showing chemical transition through 
 vii

the Central Sierran, East Sierran, and Basin and Range suites, across the Sierra 

Nevada-Basin and Range transition .......................................................................................29

10. Plots of variations in primitive-mantle-normalized trace-element and chondritenormalized rare-earth-element contents of precaldera mafic lavas.................................31

11. Plots of variations in primitive-mantle-normalized trace-element contents of 

ocean-island-basalt-like lavas .................................................................................................32

12. Plots of variations in primitive-mantle-normalized trace-element and chondritenormalized rare-earth-element contents of precaldera trachyandesite-dacite lavas ......33

13. Comparative SiO2-variation diagrams for the Caldera and Mono-Inyo suites of 

postcaldera sequence ...............................................................................................................34

14. Plots of variations in primitive-mantle-normalized trace-element and chondritenormalized rare-earth-element contents of postcaldera basalts ......................................40

15. Plots of variations in primitive-mantle-normalized trace-element and chondritenormalized rare-earth-element contents of postcaldera dacites and rhyolites..............41

16. Plots of Ni, Cr, and Sc versus MgO contents of precaldera and postcaldera sequences ....43 

17. Plots of Zr/Ba ratio versus SiO2 content of precaldera and postcaldera sequences .....43

18. Plots of variations in primitive-mantle-normalized trace-element profiles of 

precaldera and postcaldera basalts .......................................................................................44

19. Sketch map of central part of the Long Valley Volcanic Field, Calif., showing location 

of Long Valley Caldera and distribution of Sr-isotopic samples of precaldera and

 postcaldera basalts...................................................................................................................45

20. Plots of Sr- versus Nd-isotopic ratios for Long Valley basalts............................................46

21. Plots of Sr-isotopic ratio versus SiO2 content of Long Valley basalts................................47

22. Plots of comparative Sr- and Pb-isotopic ratios of precaldera and postcaldera basalts....48

23. Mantle-normalized trace-element profiles of typical lithospheric- and 

asthenospheric-mantle-derived melts in the Basin and Range Province ........................53

24. Primitive-mantle-normalized trace-element profiles of postcaldera basalts in 

comparison with representative global basalt types ...........................................................56

25. Plots of Ta/La versus La/Sm ratios of precaldera and postcaldera basalts in relation 

to other basalts ...........................................................................................................................57

26. Plot of Th/Yb versus Ta/Yb ratios for precaldera and postcaldera lavas..........................58

27. SiO2-variation diagrams illustrating chemical evolution of Caldera basalts and 

trachyandesites ..........................................................................................................................60

28. Plot of Al2O3 versus SiO2 contents for East Sierran lavas and postcaldera rhyolites......62

29. West-to-east cross section from the southern Sierra Nevada across the Basin and 

Range Province to the Colorado Plateaus Province, showing inferred crust-lithosphere-asthenosphere relations..............................................................................................63

30. West-to-east cross sections of the Long Valley Volcanic Field, Calif., showing 

inferred subcrustal relations ....................................................................................................64

Tables

1. Stratigraphic units of the Long Valley Volcanic Field, Calif., and their salient features .......9

2–5. Chemical analyses of lavas of:

 2. The East Sierran suite.........................................................................................................19

 3. The Basin and Range suite ................................................................................................20

 4. The Caldera suite.................................................................................................................23

 5. The Mono-Inyo suite...........................................................................................................49

 6. Sr-, Nd-, and Pb-isotopic ratios for rock samples from the Long Valley 

Volcanic Field, Calif.............................................................................................................50
This page intentionally left blank
Eruptive History and Chemical Evolution of the 

Precaldera and Postcaldera Basalt-Dacite Sequences, 

Long Valley, California: Implications for Magma Sources, 

Current Seismic Unrest, and Future Volcanism

By Roy A. Bailey1

1

Deceased.

Abstract

The Long Valley Volcanic Field in east-central California straddles the East Sierran frontal fault zone, overlapping 

the Sierra Nevada and western Basin and Range Provinces. 

The volcanic field overlies a mature mid-Tertiary erosional 

surface that truncates a basement composed mainly of 

Mesozoic plutons and associated roof pendants of Mesozoic 

metavolcanic and Paleozoic metasedimentary rocks. Long 

Valley volcanism began about 4 Ma during Pliocene time 

and has continued intermittently through the Holocene. The 

volcanism is separable into two basalt-rhyolite episodes: (1) 

an earlier, precaldera episode related to Long Valley Caldera 

that climaxed with eruption of the Bishop Tuff and collapse 

of the caldera; and (2) a later, postcaldera episode structurally 

related to the north-south-trending Mono-Inyo Craters fissure 

system, which extends from the vicinity of Mammoth Mountain northward through the west moat of the caldera to Mono 

Lake. Eruption of the basalt-dacite sequence of the precaldera 

basalt-rhyolite episode peaked volumetrically between 3.8 and 

2.5 Ma; few basalts were erupted during the following 1.8 m.y. 

(2.5–0.7 Ma). Volcanism during this interval was dominated 

by eruption of the voluminous rhyolites of Glass Mountain 

(2.2–0.8 Ma) and formation of the Bishop Tuff magma chamber. Catastrophic rupture of the roof of this magma chamber 

caused eruption of the Bishop Tuff and collapse of Long Valley Caldera (760 ka), after which rhyolite eruptions resumed 

on the subsided caldera floor. The earliest postcaldera rhyolite 

flows (700–500 ka) contain quenched globular basalt enclaves 

(mafic magmatic inclusions), indicating that basaltic magma 

had reentered shallow parts of the magmatic system after a 

1.8-m.y. hiatus. Later, at about 400 ka, copious basalts, as well 

as dacites, began erupting from vents mainly in the west moat 

of the caldera. These later eruptions initiated the postcaldera

basalt-rhyolite episode related to the Mono-Inyo Craters fissure system, which has been active through late Pleistocene 

and Holocene time.

This report addresses only the basalt-dacite sequences of 

the two Long Valley basalt-rhyolite episodes. Contrasting mineralogy and chemistry of the precaldera and postcaldera basaltdacite sequences indicate that these two sequences had different 

sources and modes of origin. Precaldera basalts are predominantly olivine-augite phyric, with high MgO, K2O, P2O5, Ba, 

Rb, and Sr contents and low Al2O3, TiO2, and FeO* contents. 

Most precaldera basalts have modified island-arc-basalt 

(IAB)-like trace-element signatures and are depleted in highfield-strength elements (HFSEs: Ta, Nb, Ti) and enriched in 

large-ion-lithophile elements (LILEs: Rb, Ba, K, Sr) and lightrare-earth elements (LREEs). Such characteristics for basalts 

in the Basin and Range Province are generally attributed to a 

lithospheric-mantle source. Rare ocean-island-basalt (OIB)-like 

lavas, from a presumed asthenospheric-mantle source, also 

occur in the precaldera sequence, indicating that both asthenospheric- and lithospheric-mantle melts were erupting simultaneously and in close proximity during precaldera time.

In contrast, most postcaldera basalts are predominantly 

plagioclase phyric, with higher Al2O3, TiO2, and FeO* contents 

and lower MgO and P2O5 contents. Although they also have 

LILE-enriched, IAB-like trace-element signatures, they are 

less depleted in HFSEs and less enriched in LILEs and LREEs 

than are precaldera basalts. The trace-element signatures of 

postcaldera basalts closely resemble that of the mafic-lowercrust global average, suggesting that these basalts may have 

been generated by partial melting of a mafic-lower-crustal 

source or, alternatively, by mixing of lithospheric-mantle melt 

with a midocean-ridge-basalt (MORB) mantle component.

Superimposed on these temporal mineralogic and chemical differences are provincial differences related to contrasting tectonic regimes west and east of the East Sierran frontal 

fault zone, which locally separates the Sierra Nevada and 

Basin and Range Provinces. In the precaldera sequence, these 

provincial differences can be grouped into three magmatic 

suites: a Central Sierran suite, an East Sierran suite, and a 
2 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

Basin and Range suite. Central and East Sierran basalts have 

high Mg numbers (Mg#=67–80), Ce/Yb ratios, and Y contents, 

whereas Basin and Range basalts have lower Mg numbers 

(Mg#=58–67), Ce/Yb ratios, and Y contents. These differences 

are inferred to reflect a thick (40–100 km deep) lithospheric 

mantle and a garnet peridotite source beneath the Sierra 

Nevada, and a thinner (30–75 km deep) lithospheric mantle and 

a spinel peridotite source beneath the western Basin and Range 

Province. Provincial mineralogic and chemical contrasts also 

are evident in the more highly evolved precaldera dacites. East 

Sierran dacites are hornblende-biotite-plagioclase (and some 

quartz) phyric and relatively enriched in incompatible elements. Basin and Range dacites typically lack biotite phenocrysts; they are hornblende-plagioclase phyric and have lower 

incompatible-element contents. In addition, the East Sierran 

suite shows compositional gaps between basalts, trachyandesites, and dacites, wherein most trachyandesites are attributable 

to mixing of basaltic and dacitic end members of which the 

dacitic end members were derived by melting of midcrustal to 

upper-crustal Mesozoic granitoids. The Basin and Range suite, 

in contrast, shows a more nearly continuous secular trend, with 

increasing SiO2 contents, wherein basalt fractionated to trachyandesite and dacite, in turn, fractionated from trachyandesite.

In the postcaldera sequence, two temporally and geographically distinct magmatic suites are recognized: an older 

(50–700 ka) Caldera suite, situated in and near the west moat 

of Long Valley Caldera; and a younger (<30 ka) Mono-Inyo 

suite, confined to the axis of the north-trending Mono-Inyo 

Craters fissure system. Both suites are bimodal, with a conspicuous compositional gap between trachyandesite and dacite 

but not between basalt and trachyandesite. Compositional 

trends in postcaldera basalts and trachyandesites, particularly 

in the Caldera suite, suggest a two-stage evolution involving 

(1) contamination of parental basalt by assimilation of basement granitoids or by mixing with coeval dacite at midcrustal 

levels, followed by (2) selective diffusion of Ba and K into this 

contaminated basalt during intimate contact with sanidine- and 

biotite-rich residual Moat Rhyolite magma (or its congealed 

equivalent) at shallow crustal levels. Postcaldera dacites and 

associated low-silica rhyolites, which have significantly lower 

Sr- and higher Nd-isotopic ratios than precaldera dacites, were 

probably derived by partial fusion of mafic lower-crustal rocks 

and subsequently modified by crystal fractionation.

These relations suggest the following speculative tectonomagmatic model. Since about 4 Ma, an asthenosphericmantle welt has been rising through extending and thinning 

lithospheric mantle along the west edge of the Basin and 

Range Province, providing sufficient heat to melt and erode 

the lithospheric mantle to the base of the lower crust. The 

hiatus in Long Valley basaltic volcanism at 2.5–0.7 Ma, coincident with eruption of precaldera Glass Mountain rhyolites 

(2.1–0.8 Ma) and formation of the Bishop Tuff rhyoliticmagma chamber, suggests that the chamber formed during 

an episode of diminished regional extension, which caused 

ponding of basalt near the crust/lithospheric-mantle interface, resulting in crustal underplating, fusion, and magmatic 

differentiation. At about 800 ka, renewed regional extension 

allowed injection of basalt into the base of the Long Valley 

rhyolitic-magma chamber, possibly triggering eruption of the 

Bishop Tuff. Subsequent repeated brief episodes of crustal 

extension and basalt influx into the magma chamber probably 

account for its longevity (>1 m.y.), the frequent mixing and 

mingling of basaltic and rhyolitic magmas, and a history of 

concomitant basaltic and rhyolitic eruptions.

The current (post-1980) seismic unrest at Long Valley, 

accompanied by uplift of the resurgent dome, may signal the 

onset of a new episode of regional extension and basalt injection into the residual silicic-magma chamber. The post-1989 

seismic unrest at Mammoth Mountain, accompanied by 10- to 

30-km-deep, long-period earthquakes and copious emission of 

mantle-derived CO2 and He, is inferred to result from influx of 

basalt from a deep mantle source, possibly portending a small 

extracaldera basaltic eruption like that at nearby Red Cones 

about 5 ka. However, with intense seismic swarms continuing 

episodically in the south moat of the caldera, accompanied 

by resurgent dome inflation at short-term rates as high as 20 

cm/yr, basaltic or basalt-triggered rhyolitic eruptions within 

the caldera also are distinctly possible. Nevertheless, available 

worldwide eruption-frequency data for historically active restless calderas indicate that eruptions have occurred in only 10 

of 60 known instances of unrest. Eruptions along the MonoInyo Craters Volcanic Chain at 250- to 700-year intervals over 

the past 5 k.y. indicate that future eruptions there are likely but 

not necessarily imminent.

Introduction

Long Valley Caldera and its associated Pliocene to Holocene volcanic field, in central eastern California, straddle the 

East Sierran frontal fault zone, overlapping the Sierra Nevada 

and western Basin and Range Provinces. This volcanic field, 

therefore, is critically situated to provide information and 

insight into the magmatic and tectonic contrasts and the nature 

of the crustal transition between these two provinces. In addition, an understanding of the volcanic and magmatic evolution 

of the Long Valley Volcanic Field is crucial to determining 

the cause of the current seismic and magmatic unrest that has 

afflicted the area since 1980 (Bailey, 1983; Hill and others, 

1985a, b; Bailey and Hill, 1990). Acceleration of this unrest 

since 1989, as indicated by increasing uplift of the intracaldera resurgent dome and by sporadic, intense seismic swarms, 

together with evidence for the influx of mantle-derived mafic 

magma into the roots of the magmatic system, has prompted a 

more careful look at the eruptive history and chemical evolution of the entire volcanic field—particularly at the Pleistocene 

to Holocene sequence, which bears more directly on the type 

and course of future eruptions.

U.S. Geological Survey (USGS) studies of the Long Valley-Mono Basin area commenced in 1972 as part of the USGS 

Geothermal Research Program. Between 1972 and 1980, 
effort was focused on geologic, hydrologic, and geophysical 

studies to determine the origin and economic potential of the 

associated geothermal system (Muffler and Williams, 1976). 

Since 1980, however, at the onset of the current seismic and 

magmatic unrest, research and monitoring have focused on 

assessing the probability of future volcanic eruptions and on 

determining the associated potential hazards. The Long Valley 

Volcanic Field has a 4-m.y. history of volcanic activity. For 

the past 5 k.y., eruption intervals along the Mono-Inyo Craters 

Volcanic Chain, which traverses the west moat of the caldera, 

have ranged in duration from 250 to 700 years, indicating 

that future eruptions there are likely, though not necessarily 

imminent.

The current episode of volcanic unrest at Long Valley 

began in 1978 with an M=5.8 earthquake on Wheeler Crest, 

12 km southeast of the caldera. During the following 2 years, 

seismicity gradually migrated northwestward toward the 

caldera, culminating in May 1980 with an unprecedented 

2-day sequence of four M=6 earthquakes, accompanied and 

followed by hundreds of aftershocks that shook the resort town 

of Mammoth Lakes (Sherburne, 1980). Since then, the area 

has been subjected to varying, but nearly continuous, seismic 

unrest, accompanied by gradual uplift of the resurgent dome in 

the center of Long Valley Caldera at an average rate of about 

2 cm/yr, but with short-term rates as high as 20 cm/yr. This 

uplift was inferred to be caused by injection of new magma 

into the subjacent Long Valley magma chamber (Ryall and 

Ryall, 1983; Savage and Clark, 1983). Subsequent seismic, 

geodetic, and geophysical monitoring and research have 

substantiated that conclusion and raised concern for possible 

near-term volcanic eruptions. In 1982, after a swarm of rapidly 

shallowing earthquakes, the USGS issued a “Notice of Potential Volcanic Hazard” (Miller and others, 1982). 

More recent seismic events have intermittently heightened the concern for possible eruptions—in particular, three 

intense and unusually prolonged earthquake sequences that 

punctuated the numerous sporadic seismic swarms after 1980: 

one in 1983 in the south moat of the caldera; another in 1989 

beneath Mammoth Mountain; and a third in the south moat, 

lasting from June 1997 to January 1998. During each of these 

events, deformation studies (trilateration and leveling; Savage 

and Cockerham, 1984; Hill and others, 1990; Langbein and 

others, 1995; D.P. Hill, unpub. data, 1998) and seismic signatures (Julian, 1983; Aki, 1984; Savage and Cockerham, 1984) 

suggested that magma was being injected as dikes to depths 

as shallow as 2 km in the south moat and beneath Mammoth 

Mountain. The recognition of mantle-derived He in Mammoth 

Mountain fumaroles (Welhan and others, 1988) and of marked 

changes in 3

He/4He ratios accompanying regional seismic 

events (Sorey and others, 1993; Hilton, 1996), together 

with the 1994 discovery that since 1990 copious amounts 

of mantle-derived CO2 had been accumulating in soils and 

escaping from fumaroles around Mammoth Mountain (Farrar and others, 1995), suggested that this magma is basaltic in 

composition. Corroborating this suggestion was the detection 

of long-period earthquakes beneath Mammoth Mountain at 

10- to 30-km depth (Pitt and Hill, 1994; Hill, 1996); such 

earthquakes are generally understood to be caused by magma 

or fluid movement in confined channels (Chouet, 1996). 

After the 1989 Mammoth Mountain earthquake sequence and 

again during the 1997–98 sequence, uplift of the intracaldera 

resurgent dome increased significantly (Langbein and others, 

1993, 1995; D.P. Hill, written commun., 1998), strongly suggesting that new magma, probably basalt rising from a deep 

mantle source, was recharging and inflating the residual Long 

Valley rhyolitic-magma chamber, as well as being injected as 

dikes beneath Mammoth Mountain and the south moat. The 

likelihood and timing of possible eruptions related to this new 

influx of magma will depend on the frequency and longevity 

of its injection into the magmatic system—which, ultimately, 

may depend on the future course of regional extensional 

tectonism.

This report is based on long-term, continuing USGS 

geologic and petrologic studies of Long Valley Caldera and 

the Mono-Inyo Craters Volcanic Chain (Bailey and others, 

1976; Bailey, 1982, 1984, 1989). It describes field relations 

and the petrographic, chemical, and isotopic characteristics of 

the precaldera and postcaldera basalt-dacite sequences and discusses their petrogenesis. It outlines the volcanic and chemical 

evolution of the Long Valley Volcanic Field, relates the current 

magmatic unrest to past magmatic evolution, suggests possible 

models for both past volcanism and the current seismicity, and 

discusses the types of eruption likely to occur in the foreseeable future.

Acknowledgments

This report includes the results of geologic mapping 

conducted between 1972 and 1987, while I was ably assisted 

in the field by Rob Koeppen, Tom Holecek, Fred Wilson, 

Roy Chaudet, and Marybeth Hughett Price. Bob Sperandio 

and the late Kerry Dickson provided tireless and competent 

assistance in the preparation of samples for chemical analysis. 

All samples were analyzed in the U.S. Geological Survey’s 

chemical laboratories in Reston, Va., Lakewood, Colo., and 

Menlo Park, Calif.

I am grateful to N.W. Rogers for compiling chemical analyses of the Basin and Range basalts collected by his 

Newcastle-on-Tyne (U.K.) research group. I also gratefully 

acknowledge permission by Roy Chaudet and Krishna Sinha 

to present previously unpublished Sr-isotopic data on East 

Sierran lavas (table 6).

In preparing the manuscript, I benefited greatly from 

discussions with Dave Hill, Mike Sorey, Charlie Bacon, Tom 

Sisson, Art Lachenbruch, Bob Jachens, and George Thompson. Early reviews by Tom Sisson and Julie Donnelly-Nolan, 

subsequent reviews by Mike Clynne and Charlie Bacon, and 

later reviews of selected sections by Art Lachenbruch, Ron 

Kistler, Dave Hill, and Mike Sorey are greatly appreciated. 

All the reviewers provided constructive suggestions and 

Acknowledgements 3
4 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

comments that greatly improved the manuscript’s content 

and presentation; however, I accept full responsibility for the 

final content of this report and for any errors of fact or fancy 

contained therein. David Ramsey prepared digital versions 

of the maps and illustrations. Manuel Nathenson and Wes 

Hildreth provided editorial assistance after the author’s death 

in July 2003.

Geologic-Tectonic Setting

The Long Valley Volcanic Field (figs. 1, 2) covers an 

area of about 4,000 km2 at the north end of the Owens Valley 

Rift, overlapping the central and eastern Sierra Nevada and 

the western Basin and Range Provinces.2 The Long Valley Volcanic Field is the largest and northernmost of three 

approximately contemporaneous volcanic fields within the 

Owens Valley Rift, which includes the Big Pine and Coso 

Volcanic Fields, 70 and 180 km, respectively, to the south. 

Lavas of all three of these Pliocene/Pleistocene to Holocene 

volcanic fields have erupted through basement rocks that 

compose the Sierra Nevada batholith—predominantly Mesozoic granitic-granodioritic plutons and subordinate associated 

Paleozoic metasedimentary and Mesozoic metavolcanic roof 

pendants.

Before the onset of Pliocene volcanism, the Long Valley 

region and most of the Basin and Range Province to the east 

was an elevated upland with relatively low topographic relief 

that rose eastward from the Central Valley of California to 

about 2,100-m elevation. The upland surface was mantled by 

a Tertiary (Miocene) regolith (Matthes, 1960; Huber, 1981), 

remnants of which are still preserved locally in the High Sierra 

south and west of Long Valley. At that time, the East Sierran frontal fault scarp had little or no topographic expression 

(Matthes, 1939; Christensen, 1966; Huber, 1981), and streams 

east of the present scarp flowed generally westward down 

the now-oversteepened dip slope of the Sierra Nevada to the 

Central Valley.

Owens Valley rifting and volcanism began about 4.5 Ma, 

shortly after opening of the Gulf of California (Larsen and 

others, 1968; Ward, 1991) and uplift of the Sierra Nevada crest 

(Huber, 1981), which apparently coincided also with accelerated spreading of the East Pacific Rise (Atwater and Molnar, 

1973), as well as of the East Scotia Ridge spreading center 

(Barker, 1995). Thus, the initiation of Owens Valley rifting 

and volcanism is related to a global episode of accelerated 

plate motion. Complicating local tectonic relations include 

(1) westward encroachment on the Owens Valley region by 

classic Basin and Range extensional block faulting, which has 

been spreading laterally eastward and westward from central 

Nevada over the past 17 m.y. (Eaton, 1982); and (2) rightlateral shear stress sympathetic to the northwest-trending San 

Andreas Fault system that was initiated at the California coast 

about 29 Ma and has since migrated inland to the Walker Lane 

in eastern Nevada (Savage and others, 1995). Right-lateral 

offset was the dominant motion on the north-northwest-trending Owens Valley Fault during the 1872 Lone Pine earthquake 

(Beanland and Clark, 1994). Recent earthquakes on apparent 

conjugate north- to north-northeast-trending faults in the High 

Sierra south of Long Valley Caldera show complementary 

left-lateral displacement mechanisms (Savage and Cockerham, 

1984; Hill and others, 1985b).

The Long Valley Volcanic Field is near the intersection 

of two major structural trends: (1) the north-northwest-trending East Sierran frontal fault zone; and (2) the geophysically 

defined, nearly orthogonal, northeast-trending faults bounding 

Mono Basin (Pakiser, 1970) and the Excelsior Mountains, 

which constitute the Mono Basin-Excelsior Mountains Fault 

zone (Savage and others, 1995). This northeastward trend, 

which is dominant in the basins and ranges of northwestern 

Nevada, is relatively uncommon in the Long Valley region but 

is evident in the faults truncating the north end of the White 

Mountains, bounding Deep Springs Valley, and transecting 

the Volcanic Tableland. Long Valley Caldera also lies within 

a major left-stepping offset in the East Sierran frontal fault 

scarp, commonly referred to as the “Mammoth embayment.” 

This offset is just south of the northeast-trending Mono 

Basin-Excelsior Mountains Fault zone. The intersection 

between these northwest- and northeast-trending structures 

also marks the site of a major northeastward offset in the historically active north-trending Central California Shear Zone 

and its apparent northward extension, the Central Nevada 

Seismic Zone (Savage and others, 1995, fig. 1). The nearcoincidence of these structural and seismic elements probably constitutes a region of exceptional crustal weakness that 

has allowed passage of mafic magma from the mantle to the 

surface and provided space for the generation of the younger 

silicic-magma chambers of the Long Valley Volcanic Field. 

Further evidence for structural control of the volcanic field 

is suggested by the distribution of precaldera mafic lavas, 

particularly in the west half of the field. West of the present Sierra Nevada crest, most Long Valley mafic vents and 

flows are confined to a triangular topographic depression that 

is bounded by the range crest, west-trending Kaiser Ridge, 

and northeast-trending Chiquito Ridge (fig. 1). Although this 

depression, drained by the upper San Joaquin River headwaters, appears to be erosional in origin (Matthes, 1960), confinement of nearly all the Pliocene volcanic vents within this 

triangle suggests that the depression is structurally controlled. 

Although no bounding faults have been recognized, the basin, 

with its eastern apex at a major offset at the East Sierran 

frontal fault zone, suggests a depressed triangular fault block 

within the Sierra Nevada dip slope.

2

Leeman (1982), Menzies and others (1983), and Fitton and others (1988) 

applied the term “Sierra Nevada Province” or “Sierran subprovince” (in a 

magmatic sense) to the part of the Basin and Range Province immediately east

of the Sierra Nevada; Fitton and others (1991) referred to this region as the 

“Western Great Basin province.” In this report, the term “Sierra Nevada Province” is used in the traditional structural/geographic sense but is restricted to 

the uplifted Sierra Nevada range, while the term “Basin and Range Province” 

is applied locally to the region just east of the East Sierran frontal fault, as 

well as to the entire block-faulted region of the Western United States.
Mono 

Lake

Figure 2

FISH LAKE VALLEY

WHITE MOUNTAINS

EUREKA VALLEY

INYO MOUNTAINS

SIERRA

NEVADA

CHIQUITO RIDGE

KAISER RIDGE

LONG VALLEY

CALDERA

LONG VALLEY

VOLCANIC

FIELD

BIG PINE

VOLCANIC

FIELD

COSO

VOLCANIC

FIELD

0 10 20 KILOMETERS

Volcanic rocks

(<4 Ma)

sedimentary fill

Owens Valley Rift,

Normal fault;

ball and bar on 

downthrown side

119° 118°

38°

37°

36°

CALIFORNIA

Figure 1. Sketch map of the Owens Valley Rift, east-central California, showing location and extent of the Long Valley 

Volcanic Field, as well as the Big Pine and Coso Volcanic Fields. Dashed line encloses scattered high-K lavas of the central 

Sierra Nevada (Van Kooten, 1980), inferred to be part of the Long Valley Volcanic Field.

Geologic-Tectonic Setting 5
6 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

Postcaldera dacite-rhyolite

0 5 10 KILOMETERS

Postcaldera basalt-andesite

Precaldera dacite

Precaldera basalt-andesite

Normal fault; ball and bar on downthrown side

Mono

Lake

Lake 

Crowley

BP

MO

N

O

C

R

AT

E

RS

LVF

HSF

HCF

RVF

3

9

5

395

LONG

VALLEY

Resurgent

dome

Qer

Qmr

Glass Moun

Long ValleyCaldera margin

MM

tain

LV

BM

CV JM

AG

SJ DM

DP

RC

HL

PB

ML

S I E R R A

N E VA D A

B A S I N

A N D

R A N G E

119°00'

38°00'

37°45'

EXPLANATION

MG

OR

Qhrd CP

PI

JL

Figure 2. Sketch geologic map of central part of the Long Valley Volcanic Field, Calif. (fig. 1), showing distribution of precaldera and 

postcaldera basalt-dacite sequences. AG, Agnew Meadow; BM, Bald Mountain; BP, Black Point; CP, Cone Peak; CV, Crestview; DM, 

Deadman Pass; DP, Devils Postpile; HL, Horseshoe Lake; JL, June Lake; JM, June Mountain; LV, Lee Vining; MG, McGee Mountain; ML, 

Mammoth Lakes; MM, Mammoth Mountain; OR, Owens River Gorge; PI, Paoha Island; RC, Red Cones; SJ, San Joaquin Ridge. Sierra 

Nevada east-frontal faults: HCF, Hilton Creek Fault; HSF, Hartley Springs Fault; LVF, Lee Vining Fault; RVF, Round Valley Fault. Qer, basaltic 

enclave-bearing Early Rhyolite dome; Qmr, basaltic enclave-bearing Moat Rhyolite.
Tectonism and volcanism within the Owens Valley have 

continued episodically from Pliocene through Holocene time. 

Recent major seismic swarms on faults bounding the northern 

Owens Valley (the Round Valley Fault in 1984 and the White 

Mountain [Chalfant Valley] Fault in 1986), together with a 

general increase in regional seismicity (Ryall and Ryall, 1981) 

and in deformation rate since 1978 (Savage and others, 1995), 

suggest that the rift is undergoing the onset of a new episode 

of extension. According to Raleigh and others (1982), a coeval 

post-1978 increase in seismicity also has been evident along 

the San Andreas Rift zone.

Formation of the East Sierran frontal fault scarp and the 

Owens Valley Rift coincided approximately with the inception 

of mafic volcanism at Long Valley. Many of the major faults 

crossing the region, including the East Sierran frontal fault 

scarp, are growth faults, displacing early mafic lava flows as 

much as 1,000 m but younger units only a few tens of meters. 

Thus, faulting and volcanism along the west edge of the 

Basin and Range are contemporaneous and intimately related. 

Although the larger, older fault scarps within the volcanic field 

trend generally northwest, parallel to the East Sierran frontal 

fault zone, many younger, smaller and shorter faults trend 

north-northeast to northeast. The Mono-Inyo Craters Volcanic Chain exhibits this north-northeastward trend, as does a 

conspicuous set of joints and young faults in basement rocks 

of the region, some showing 2- to 3-m displacement of late 

Pleistocene glaciated bedrock surfaces. Many recent seismic 

epicenters in the Sierran block south of the caldera are aligned 

along such north-northeast-trending faults (see Hill and others, 

1985b, fig. 8). The age of the Mono-Inyo vents along these 

north-northeast-trending fissures and faults suggests a change 

in the local extensional-stress direction from northwestsoutheast to west-east in the past 30 k.y. However, although 

north-northeast-trending structures seem to be most recently 

active in the volcanic field, young Long Valley volcanic vents 

are not consistently aligned along them, nor are older vents 

consistently aligned northwest, indicating that both trends 

have been exploited by rising magma throughout the life of the 

volcanic field (see Bailey, 1989, supp. maps C, D).

Long Valley Volcanism

Introduction

Long Valley volcanism, which began about 4 Ma in the 

Pliocene and has continued through the Holocene (Bailey 

and others, 1976; Bailey, 1989), includes (1) an episode of 

a precaldera basalt-trachyandesite-dacite-rhyolite volcanism 

(4.0–0.8 Ma) (fig. 2) that climaxed with eruption of the Bishop 

Tuff and formation of Long Valley Caldera at 760 ka, and (2) 

an episode of a postcaldera basalt-trachyandesite-dacite-rhyolite volcanism (400 ka–Holocene) that is structurally related 

to the north-trending Mono-Inyo Craters fissure system (fig. 

2). The second episode temporally overlapped extrusion of 

the intracaldera Early and Moat Rhyolites (700–100 ka). This 

report focuses primarily on the petrologic and chemical differences within and between the precaldera and postcaldera 

basalt-dacite sequences of these basalt-rhyolite episodes and 

on their magmatic and tectonic implications.

Eruption of the precaldera basalt-dacite sequence climaxed between 3.8 and 2.5 Ma (fig. 3), with eruption of the 

more evolved dacitic lavas late in this interval. Few, if any, 

lavas of basaltic-dacitic composition were erupted during 

the following 1.8 m.y. (2.5–0.7 Ma), when volcanism was 

dominated by eruption of the Glass Mountain rhyolites and 

formation of the Long Valley (Bishop Tuff) magma chamber. 

Mafic lava did not reappear in Long Valley until immediately 

after eruption of the 760-ka Bishop Tuff and collapse of the 

caldera. Mafic lava first appeared as quenched basalt enclaves 

in the 700- to 500-ka intracaldera Early and Moat Rhyolites 

(units Qer, Qmr, fig. 2)3

 and later, about 400 ka, as basalt-trachyandesite flows in the west moat of the caldera. This hiatus 

in mafic volcanism between 2.5 and 0.7 Ma was clearly a 

regional rather than local phenomenon because coeval hiatuses 

in mafic volcanism also occurred in both the Coso and Cima 

Volcanic Fields far to the south (Novak and Bacon, 1986; 

Farmer and others, 1995), suggesting that volcanic activity along the Owens Valley Rift is linked to or influenced 

by regional tectonic stresses. As discussed below, the lavas 

preceding and succeeding this hiatus in Long Valley mafic 

volcanism have significantly different mineralogic and chemical signatures, suggesting that the precaldera and postcaldera 

sequences were derived from different magmatic sources and 

evolved by markedly different magmatic processes.

Precaldera Sequence

The precaldera sequence includes the widespread 

basalt-trachyandesite flows (unit Tba, fig. 2) extending from 

the Adobe Hills (Gilbert and others, 1968; Lange and others, 1993) in the northeast to the central Sierra Nevada in the 

southwest (fig. 1), encompassing the Central Sierran ultrapotassic basanite-tephrite lavas studied by Van Kooten (1980, 

1981). Peripheral to and within the present site of Long Valley 

Caldera, these basalt-trachyandesite lavas evolved to more 

silicic dacite domes, flows, and tuffs (units Tbm, Tsjd, Tsjt,

fig. 2), now preserved mainly on the north and northwest 

caldera rim in the vicinity of Bald Mountain and San Joaquin 

Ridge.

Notable differences in form and distribution of the more 

mafic lavas are evident west and east of the present Sierra 

Nevada crest: those to the west form small scattered vents with 

flows of limited volume and extent, whereas those to the east 

are more voluminous and commonly form broad lava shields 

and plateaus. This contrast also is evident farther south along 

the Sierra Nevada, where, near the Big Pine and Coso Volcanic 

3

Unit symbols in figures 4 through 6 correspond to those on the geologic 

map of Long Valley by Bailey (1989).

Long Valley Volcanism 7
8 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

Fields, vents and flows in the High Sierra are few and limited 

in extent, whereas those to the east in the Owens Valley are 

more voluminous and extensive (fig. 1). This contrast may be 

caused by differences in magma source depth related to the 

thickness of the lithosphere beneath the Sierra Nevada and 

Basin and Range Provinces—lavas in the west erupting from 

greater depth through the relatively thick Sierran basement, 

and those in the east erupting from shallower depth through 

the tectonically thinned and more fractured Basin and Range 

basement. Another factor may be that greater degrees of 

partial melting accompanying rapid thinning of the Basin and 

Range lithosphere produced greater volumes of magma. These 

morphologic and volumetric differences between lavas erupted 

in the Sierra Nevada and the Basin and Range Provinces are 

accompanied by mineralogic, chemical, and isotopic differences that further distinguish the precaldera lavas west and 

east of the East Sierran frontal fault zone. These provincial 

differences permit subdivision of the precaldera sequence into 

three geographic suites (from west to east): a Central Sierran suite, an East Sierran suite, and a Basin and Range suite 

(table 1), as discussed in detail below.

The precaldera lavas described in this study were collected mainly within 10 km of the Long Valley Caldera rim. As 

indicated above, however, chemically and temporally related 

lavas extend as far as 40 km southwestward into the central 

Sierra Nevada, as well as northeastward to Cowtrack Mountain 

and the Adobe Hills (fig. 1). Chemical and isotopic data on 

ultrapotassic lavas of the Central Sierran suite were reported 

by Van Kooten (1980, 1981) and Moore and Dodge (1980). 

Few data are available for the Adobe Hills lavas, which, 

though apparently similar in age, may constitute a separate 

volcanic subfield. These lavas compose a thick sequence of 

3- to 4-Ma basaltic-trachyandesitic flows that thin laterally and 

are interbedded with coarse-grained stream deposits, suggest4.54.4.34.24.143.93.83.73.63.53.43.3.23.132.92.82.72.62.5 2.32.2.121.91.81.71.61.51.41.31.21.110.90.80.70.60.50.40.30.20.10

0

5

10

15

Basalt enclaves

Basalt

Andesite

Dacite

4 . 5 4 . 0 3 . 5 3 . 0 2 . 5 2 . 0 1 . 5 1 . 0 0 . 5 0 . 0

A Precaldera Postcaldera

Q e r Q r m

sequence sequence

EXPLANATION

NUMBER OF FLOWS

4.54.4.34.24.143.93.83.73.63.53.43.3.23.132.92.82.72.62.52.42.32.2.121.91.81.71.61.51.41.31.21.1 0.90.80.70.60.50.40.30.20.10

0

5

10

15

Glass Mountain rhyolite

Early rhyolite

Moat rhyolite

4 . 5 4 . 0 3 . 5 3 . 0 2 . 5 2 . 0 1 . 5 0 . 0

B EXPLANATION Bishop Tuff

1.0 0.5

AGE, IN MILLIONS OF YEARS

Figure 3. Histograms of number of dated lava flows per 100 k.y. versus time. A, Basalt, trachyandesite, and dacite lavas. 

B, Rhyolite lavas. Note hiatus in mafic volcanism (fig. 3A) during eruption of the Glass Mountain rhyolite (fig. 3B). Qer, 

basaltic enclave-bearing Early Rhyolite; Qmr, basaltic enclave-bearing Moat Rhyolite. Note that number of radiometrically 

dated flows does not strictly correspond to volume of erupted flows within individual age spans.
Table 1. Stratigraphic units of the Long Valley Volcanic Field, Calif., and their salient features.

[Diagnostic phenocrysts (parentheses, present in minor amounts): aug, augite; bio, biotite; diop, diopside; hbl, hornblende; hyp, hypersthene; leuc, leucite; ol, olivine; phlog, phlpyroxene (hyp+aug); san, sanidine; qtz, quartz. Do., ditto]

Map unit/locality Age Diagnostic Phenocrysts Suite Rock type 

(figs. 4, 5, 6) (Ma) phenocrysts (vol pct)

Postcaldera sequence

Mono-Inyo------------------ Qd, Paoha Island --------------------------------------- 0.0002 Dacite -----------------------------------plag, hyp, aug 1–5 

Qry, Paoha Island -------------------------------------- .0002–0.0006 Rhyolite-------------------------------------------------------------------------------------------------------Qri, Inyo Craters rhyolite1 ---------------------------- .0006 do ---------------------------------------------------------------------------------------------------------

Qby, Red Cones ----------------------------------------- .005 Basalt----------------------------------------------------------------------------------------------------------

Qyc, Black Point --------------------------------------- .013 Trachybasalt ---------------------------plag, ol, aug 1–5 

Qa1, Qa2, June Lake ----------------------------------- .025?–0.030 do -----------------------------------plag, ol, aug 1–5 

Qro, Qry: Mono Craters rhyolite1------------------- .0002–0.035 Rhyolite---------------------------------plag, hyp 0–1 

Qpd, Mono Craters, Paoha Island------------------- .1? Porphyritic dacite --------------------plag, ol, aug 5–10 

 Caldera --------------------- Qya --------------------------------------------------------- <.025? Trachyandesite------------------------plag, ol 0–1 

Qqm2, northwest moat -------------------------------- <.05? Trachydacite---------------------------plag, hbl, bio, pyx (san, qtz) 20–30 

Qrm2, Mammoth Mountain, Lincoln Peak------- .05 Rhyolite---------------------------------plag, san 5–10 

Qqm, Mammoth Mountain, west moat ------------ >.05–<0.25? Trachydacite---------------------------bio, hbl, pyx, plag 25–50 

Qrm1, Mammoth Mountain -------------------------- .212–0.256? Rhyolite---------------------------------bio, aug 0–5 

Qaq, west moat, Devils Postpile -------------------- .1 Aphyric trachydacite ----------------(plag, hyp, aug) 5–10 

Qpb, Horseshoe Lake, west moat------------------- .1 Porphyritic basalt---------------------ol, aug (plag) 5–10 

Qob, Buttresses------------------------------------------ .2? do -----------------------------------ol, aug 5–10 

Qa, west moat, extracaldera -------------------------- .1–0.4 Trachyandesite------------------------plag, ol 0–1 

Qab, west moat, extracaldera ------------------------ .1–0.4 Trachybasalt ---------------------------plag, ol 0–1 

Qgb, west moat ----------------------------------------- .4? Glomeroporphyritic basalt ---------plag, ol 0–1 

Corehole Inyo-4, groups I–V ------------------------ .15–0.415? Basalt-trachyandesite--------------------------------------------------------------------------------------

Qb(mr), in Moat Rhyolite----------------------------- .5 Basalt enclaves------------------------microphyric ol, aug 0–1 

Qb(er), in Early Rhyolite------------------------------ .68 do -----------------------------------microphyric ol, aug 0–1 

Qhrd, northeast moat----------------------------------- .7? Dacite -----------------------------------hbl, plag 20–30 

Moat Rhyolite1------------------------------------------- .1–0.5 Rhyolite-------------------------------------------------------------------------------------------------------

Early Rhyolite1 ------------------------------------------ .65–0.75 do ---------------------------------------------------------------------------------------------------------

Bishop Tuff1---------------------------------------------- .76 do ---------------------------------------------------------------------------------------------------------

Glass Mountain rhyolite1------------------------------ .8–2.1 do ---------------------------------------------------------------------------------------------------------

Precaldera sequence

Basin and Range ---------- Tbm--------------------------------------------------------- ? Dacite -----------------------------------hbl, plag 10–15 

Tba---------------------------------------------------------- 3.1–3.2 Trachyandesite------------------------hbl, plag (ol, aug) 20–50 (5–15) 

Tba---------------------------------------------------------- 3.1–3.2 Alkali olivine basalt -----------------ol, aug 5–10 

East Sierran----------------- Tsjd/Tsjt--------------------------------------------------- 2.7–3.1 Dacite -----------------------------------bio, hbl, plag 30–50 

Tba---------------------------------------------------------- 3.1–3.2 Trachyandesite------------------------ol, aug, hyp (plag) 1–10 

Tba---------------------------------------------------------- 3.1–3.2 Alkali olivine basalt -----------------ol, aug (plag) 5–10 

Central Sierran ------------ Tba---------------------------------------------------------- 3.3–3.8 Trachyandesite------------------------ol, aug (plag) n.d. 

Tba---------------------------------------------------------- 3.3–3.8 Alkali olivine basalt -----------------ol, diop, plag (san) n.d. 

Tba---------------------------------------------------------- 3.3–3.8 Potassic olivine basalt---------------diop, ol, san (leuc, plag, phlog) n.d. 

Tba---------------------------------------------------------- 3.3–3.8 Ultrapotassic basanite ---------------san, diop, ol (phlog, leuc) n.d. 

1Unit not discussed in this report.
10 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

ing that they initially filled a Pliocene basin (Gilbert and others, 1968; Lange and others, 1993). Presently, however, they 

form an uplifted east-trending anticlinal ridge that is broken by 

a system of hinged northwest- and northeast-striking normal 

faults, which, in turn, are cut by numerous smaller, younger 

(possibly active), east-trending normal faults that suggest 

active gravitational sliding on the anticlinal flanks. The Adobe 

Hills are the locus of numerous scattered, small to moderate 

(M=1–5) earthquakes, with hypocenters localized within a 

gently east dipping tabular zone at 15- to 20-km depth (A.S. 

Ryall, Jr., oral commun., 1982). The Adobe Hills lavas possibly were fed initially by a Stillwater-like sill complex that is 

currently being reinflated by influx of new magma; however, 

owing to its remoteness and difficult access, little is known 

about heat flow or active deformation in the area.

In the proximity of Long Valley Caldera (fig. 2), the main 

precaldera basalt-trachyandesite eruption sites are (clockwise 

from the southwest) near Pumice Butte and at Deadman and 

Agnew Passes on San Joaquin Ridge, near Crestview and Bald 

Mountain on the north rim, and at Cone Peak on the east rim; 

on the southeast, vents occur in the upper Owens River Gorge 

and on the north shoulder of McGee Mountain. The thickest 

(450 m thick) of these sequences is at Agnew Pass on San 

Joaquin Ridge, where early mafic lavas spilled into steepwalled canyons, damming them and forming lakes into which 

subsequent lavas flowed, producing masses of pillow lava and 

palagonite breccia. Other examples of precaldera canyon-filling pillowed lavas occur at Clark Lakes and on the east wall of 

San Joaquin Canyon near Pumice Butte. The longest (10 km 

long) precaldera flows erupted from Cone Peak on the east rim 

of the caldera and flowed eastward across the site of the Black 

and Benton Ranges before their uplift, faulting, and tilting.

Typically, these proximal mafic lavas include a lower 

sequence of thin (1–3 m thick) basalt flows overlain by a few 

thicker (10 m thick) trachyandesite flows. At a few sites (near 

Pumice Butte, San Joaquin Ridge, June Mountain, and Bald 

Mountain), these mafic flows (unit Tba, fig. 2) are overlain by 

thick dacite domes, flows, and tuffs (units Tbm, Tsjd/t, fig. 2). 

Presumably, similar basalt-dacite flows are buried within the 

caldera, but only one of four deep intracaldera drill holes—

IDFU 44–16 in the west moat (Suemnicht and Varga, 1988)—

has intersected any significant thickness (475 m) of precaldera 

lavas before penetrating Sierra Nevada basement (Paleozoic 

metasedimentary rocks at the drillsite). This apparent paucity 

of precaldera lavas within the caldera may be due to precaldera 

erosion, dispersion of precaldera vents, or fortuitous intersection of topographic highs in the irregular precaldera Sierran 

basement by most of the drill holes.

The precaldera volcanic episode also includes the highSiO2 rhyolite dome-flow complex of Glass Mountain on the 

northeast rim of the caldera, as well as the Bishop Tuff, widely 

distributed around the caldera as welded pyroclastic flows 

(ignimbrite). The rhyolites of Glass Mountain were mapped by 

Metz and Bailey (1993), and their chemical and isotopic compositions were described by Metz and Mahood (1985, 1991), 

Halliday and others (1989), Davies and others (1994), and 

Davies and Halliday (1998). Evidence bearing on the longevity and depth of their source magma chamber was discussed 

by Sparks and others (1990), Halliday (1990), and Mahood 

(1990). The Bishop Tuff was first mapped and described by 

Gilbert (1938), Sheridan (1965), and Hildreth (1979), and Wilson and Hildreth (1997, 1998) later amplified its stratigraphy 

and chemistry. Neither the rhyolite of Glass Mountain nor the 

Bishop Tuff is described or discussed here except incidentally.

Postcaldera Sequence

After the collapse of Long Valley Caldera, rhyolite 

continued to erupt as domes, flows, and tephra on the caldera 

floor, creating the Early Rhyolite and Moat Rhyolite formations between 700 and 100 ka (Bailey and others, 1976; Bailey, 

1989). Heuman and Davies (1997) described the chemical 

evolution of these intracaldera rhyolites. The postcaldera 

basalt-dacite lavas coeval with the Early and Moat rhyolites 

were erupted mainly from vents within and adjacent to the 

west moat of the caldera. Most of these vents are aligned along 

fissures that constitute the north-south-trending Mono-Inyo 

Craters fissure system, which extends 80 km southward from 

Mono Lake through Mammoth Mountain (fig. 2) to Mono 

Hot Springs (35 km south of Mammoth Mountain, near Lake 

Thomas Edison). The basalt-dacite lavas of this postcaldera 

sequence are distinguishable on the basis of age, distribution, and chemistry as two distinct basalt-dacite suites (table 

1): an older (60–680 ka) Caldera suite and a younger (0.2–30 

ka) Mono-Inyo suite. The Caldera suite includes lavas within 

and immediately southwest of Long Valley Caldera, whereas 

the Mono-Inyo suite is more narrowly confined to the axis 

of the Mono-Inyo Craters Volcanic Chain. Both suites were 

erupted along the Mono-Inyo Craters fissure system. (Note the 

distinction between the “Mono-Inyo Craters fissure system,” 

the “Mono-Inyo Craters Volcanic Chain,” and the “MonoInyo suite,” the last of which is geographically and temporally 

more restricted and based on distinctive chemical criteria.) 

Miller (1985) and Sieh and Bursik (1986) outlined the eruptive history of the rhyolite domes, flows, and tephras of the 

Mono-Inyo Craters Volcanic Chain, and Sampson and Cameron (1987) and Kelleher and Cameron (1990) described the 

chemical evolution of these rocks.

The earliest postcaldera mafic magma appeared as 

quenched enclaves in one 680-ka Early Rhyolite lava and in 

one 500-ka Moat Rhyolite unit (units Qer, Qmr, fig. 2). Voluminous mafic lava flows first appeared about 415 ka in the 

west moat and persisted until about 60 ka. Between 250 and 

100 ka, these mafic eruptions spread southwestward outside 

the caldera to Horseshoe Lake (unit Qpb, fig. 4), Pumice Butte 

(unit Qa), Mammoth Pass (unit Qa), and Devils Postpile (units 

Qob, Qab, Qpb); interspersed with them were eruptions of 

more evolved lavas: (1) the dacites and low-SiO2 rhyolites 

of Mammoth Mountain (units Qqm, Qrm1, Qrm2) and two 

similar domes (unit Qqm) in the west moat, (2) several hybrid 

dacite dome flows (unit Qqm) in the northwest moat, and (3) 
Qaq

Qrm1

Qa

Qa Qpb

Horseshoe

 Lake

Qab

Qyb

Red Cones

Qa

Qya

Qa

Qa

Pumice

Butte

Qob Qaq

Qpb

Qab

Qpb

Qrm1 Qa

Qab

Qab

Qa

Qa

Qab Mammoth

 Lakes Lake Mary Rd

203 Qpb

Qgb

Qer

Qmr

Qrmp

395

Qrm1

Qpb

Qpb

Qab

Qab

Qab

Qri

Qab

Qab

Qa

Qpe

Qpe

Qmr

Qa

Qa

Qa

Qgb

Qer Qab

Qqm2

Qqm2

Qri

Qri H a r t l e y S p r i n g s F a u l t

203

Devils

Postpile

R

E

S

U

R

G E N T D

O

M

E

INYO - 4

37 40' o

37 35' o

119 00' o

0 1 2 3 4 KILOMETERS

Qqm

Qqm

Qrm2

Qqm

Qqm

Qrm2 Qqm

Qqm

Qrmp

Qri

Figure 4. Sketch geologic map of west moat of Long Valley Caldera, Mammoth Mountain, and Devils Postpile 

area, Calif. (after Bailey, 1989).

Long Valley Volcanism 11
12 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

37 55' o

Qa1 Qa1

Qa2

Qa2

120

395

Mono Lake

Panum Crater

C r a t e r s

M o n o

Qpd

Qr

Qr

Qr

Qr

Qr

June Lake

Qa1

A e o l i a n B u t t e s

37 50' o

119 00' o

0 1 2 3 4 KILOMETERS

Figure 5. Sketch geologic map of June Lake-Mono Craters 

area, Calif. (after Bailey, 1989).

EXPLANATION (Figs. 4 - 6)

Dacite and low-silica rhyolite

Qd Dacite of Negit and Paoha Islands (6000 to <250 B.P.)

Qry Low-silica rhyolite of Paoha Island (~250 B.P.)

Qri Low-silica rhyolite of Inyo Craters (600 B.P.)

Qr High-silica rhyolite of Mono Craters (30 ka - 600 B.P.)

Qqm2 Dacite of the northwest caldera moat (<50? ka)

Qpd Porphyritic dacite of Mono Craters and Paoha Island 

(~100? ka)

Qrm2 Low-silica rhyolite of Mammoth summit and Lincoln Peak 

(100? - 50 ka)

Qqm Dacite of Mammoth Mountain and Earthquake Domes

Qrm1 Basal low-silica rhyolite of Mammoth Mountain 

(255? - 100? ka)

Basalt and trachyandesite

Qyb Basalt of Red Cones (<10 ka)

Qyc Palagonitic basalt tephra of Black Point (13.3 ka)

Qya Younger trachyandesite of Pumice Butte (<100 - >25 ka, 

or possibly <10 ka)

Qa2 Basalt of June Lake secondary rootless cinder cone 

(30 - 25 ka)

Qa1 Basalt of June Lake main cinder cone (30 - 25 ka)

Qab Sparsely phyric basalt of the caldera moat (100? - 64 ka)

Qgb Varient of Qab with olivine-plagioclase clusters

Qpb Coarsely phyric basalt of the caldera moat and 

Devils Postpile (280? - <100 or >25 ka)

Qa Trachyandesite of Mammoth Pass, Pumice Butte, 

and the caldera moat (155 - 90 ka)

Qaq Sparsely phyric dacite of Devils Postpile and west 

caldera moat (undated)

Qob Olivine-augite basalt of the Buttresses; oldest flows in the 

Devils Postpile area, erupted through glacial ice of 

Tahoe or older age

Qmr Moat Rhyolite; locally contains mafic enclaves (~500 ka)

Qer Early Rhyolite; locally contains mafic enclaves (~650 ka)

Qhrd Hornblende-plagioclase phyric dacite, northeast caldera 

moat (~700 ka)

Symbols:

Volcanic vent

Normal fault; ball and bar on downthrown side

Direction of lava flowage from vent

Topographic outline of Long Valley Caldera floor

Main road
two crystal-poor dacite flows (unit Qaq) in the west moat and 

at Devils Postpile. After about 30 ka, mafic eruptions migrated 

northward along the Mono-Inyo Craters fissure system to June 

Lake (unit Qa [25–30 ka], fig. 5), then to Black Point (unit 

Qyc [13 ka], fig. 6) on Mono Lake, and finally southward to 

Red Cones (unit Qyb [5 ka], fig. 4). These mafic eruptions 

were followed by dacite and low-SiO2 rhyolite eruptions at 

Mono Lake (units Qd [0.6–2 ka] and Qry [200 B.P.], fig. 6) 

and at Inyo Craters (unit Qri [600 B.P.], fig. 4) and were overlapped by high-SiO2 rhyolite eruptions at Mono Craters (units 

Qro [1–35 ka], Qry [600 B.P.], figs. 5, 6). The most recent (≤5 

ka) eruptions along the Mono-Inyo Craters Volcanic Chain, of 

both basaltic and rhyolitic composition, have been confined to 

its northern and southern extremities, in and near Mono Lake 

and Inyo Craters and at Red Cones.

Because postcaldera Long Valley volcanism is particularly relevant to the current (post-1980) seismic unrest in 

the area and provides perspective on the type and characteristics of possible future eruptions, the postcaldera lavas are 

described in greater detail below.

Caldera Suite

Mafic to intermediate-composition lavas of the Caldera 

suite are localized mainly within the west half of Long Valley 

Caldera but also occur to the southwest outside the caldera 

to the vicinity of Devils Postpile. Their vents, except for the 

Early Rhyolite basalt enclaves, are aligned mainly along fissures parallel to the Mono-Inyo Craters Volcanic Chain and 

are clearly related to it. Thus, both the Caldera and Mono-Inyo 

Qyc

Black

Point

Qd

Qd

Qry

Qry

Qpd

Paoha

Island

Island

Negit

Islets

Negit

Qd Qd

Mono Lake

Lee

Vining

Lee

Vining

Fault

67

395

38 00' o

119 00' o

0 1 2 3 4 KILOMETERS

Figure 6. Sketch geologic map of Mono Lake area, Calif. (after Bailey, 1989).

Long Valley Volcanism 13
14 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

suites are here considered to be related to the Mono-Inyo Craters magmatic system.

Basalt Enclaves

The earliest postcaldera basalt occurs as mafic enclaves 

(magmatic inclusions) enclosed in a 680-ka Early Rhyolite 

dome (unit Qer, fig. 2) in the center of the resurgent dome and 

in a 500-ka Moat Rhyolite agglutinate ring and lava flow on 

the north flank of the resurgent dome (unit Qmr, fig. 2). The 

enclaves in the Early Rhyolite dome consist of vesiculated 

globular masses, as much as 10 cm in diameter, and slabs, as 

much as 50 cm across and 10 cm thick, showing thin, dark, 

chilled margins against the enclosing obsidian. The coarse 

vesicles at the outer margins of the enclaves are filled with 

finely vesicular white rhyolite pumice that merges with the 

enclosing obsidian, suggesting that both the basalt and obsidian were molten and vesiculating simultaneously—the dense 

rhyolite melt expanding to pumice as it oozed into the large 

vesicles forming at the enclave margins. The slablike enclaves 

apparently originated in the subsurface as vertical and horizontal sheets that intruded into and were marginally chilled by the 

molten host rhyolite, then were disrupted while their interiors 

were still molten, and finally were extruded with the rhyolite dome. The size distribution of the vesicles within these 

slabs suggests that they represent both vertical and horizontal 

sheets: in some slabs, vesicles formed symmetrically about 

a medial plane, as if the result of vertical flow, whereas in 

other slabs, vesicles are distributed asymmetrically, increasing in size and abundance from one side to the other, as if 

the result of gravitational control in a horizontal sheet. The 

extremely fine grain size and relatively high MgO content of 

the basalt indicates that it reacted little with its rhyolite host 

and probably was injected at relatively shallow depth only 

shortly before disruption and extrusion. The position of the 

enclave-rich Early Rhyolite dome at the center of the resurgent 

dome suggests that the basalt was fed into the rhyolitic-magma 

chamber from a subjacent central feeder, where the combination of injection pressure and buoyancy due to vesiculation 

quickly carried it to the chamber roof and facilitated its extrusion. The younger basalt enclaves in the 500-ka Moat Rhyolite 

(unit Qmr, fig. 2) on the north flank of the resurgent dome 

occur as 2- to 10-cm-diameter globular bombs in a pumiceous 

agglutinate ring and as denser globs in a flow issuing from it. 

Compositionally, these enclaves are more contaminated by 

their coarsely porphyritic (cooler) rhyolite host and probably 

were in contact with it longer than those in the aphyric (hotter) 

Early Rhyolite.

Early Intracaldera Basalt-Trachyandesite Lavas

The oldest and thickest complete section of postcaldera 

mafic lavas is a 319-m-thick sequence that was penetrated by 

slant corehole Inyo-4, drilled at the Inyo Craters (fig. 5) in the 

west moat of the caldera (Eichelberger and others, 1988). This 

flow sequence, described in detail by Vogel and others (1994), 

ranges in age from 151 to 415 ka and comprises five distinct 

compositional and stratigraphic groups (I through V, from 

top to bottom). Groups I through IV occur as a continuous 

sequence of superposed flows; group V, which occurs only as 

clasts in breccia dikes cutting Early Rhyolite at about 650-m 

depth, 300 m below the base of the flow sequence, is believed 

to be derived by downward raveling in fissures from group 

V flows at the base of the flow sequence, which fortuitously 

were not intersected by the slant corehole (Vogel and others, 

1994). Each group grades cyclically upward toward more 

evolved compositions from basalt to trachyandesite (50–58 

weight percent SiO2). Significantly, this flow sequence is not 

interrupted by other intracaldera eruptive or sedimentary units, 

even though rhyolitic eruptions were occurring elsewhere in 

the caldera during this interval. Nor does the sequence enclose 

any of the glacial deposits believed to have formed during this 

interval. These relations suggest that the lavas were erupted 

over a much shorter interval than their K-Ar ages indicate (see 

discussion below).

Late Intracaldera Basalt-Trachyandesite Lavas

The intracaldera surface mafic flows sampled in this 

study (units Qpb, Qab, Qa, figs. 2, 4) form outcrops in the 

north, west, and south sectors of the caldera moat; their 

exposed thickness seldom exceeds a few tens of meters. Their 

relation to the corehole Inyo-4 flows is problematic. K-Ar 

ages on these surface flows in the south moat, as determined 

by Curry (1971), are 440 ka for unit Qa and 192 ka for unit 

Qab, whereas ages determined by Mankinen and others (1986) 

on these same flows are 129 and 64 ka, respectively. This 

discrepancy remains unresolved; however, their relation to 

overlying Tioga till (10–25 ka) and interbedded Casa Diablo 

till (now recognized as probably correlative with Older Tahoe 

till [120–140 ka])4

 suggest that Mankinen and others’ ages are 

likely correct.

Extracaldera Basalt-Trachyandesite Lavas

Basalts and trachyandesites similar in composition and 

age to the intracaldera lavas also were erupted outside the caldera at Mammoth Pass (unit Qa [86 ka], fig. 4), Pumice Butte 

(units Qa, Qya [<10?–200 ka]), and Devils Postpile (units 

4

This report closely follows the mapping and classical nomenclature of 

Sharp and Birman (1963) for the glacial moraines along the East Sierran 

escarpment, for example, Tioga, Tenaya, Tahoe, and Mono Basin, in order of 

increasing age. The ages of glaciations referred in this report are based on the 

cosmogenic 26Al ages of Sierran glacial moraines determined by Phillips and 

others (1990, 1996) and on the analyses of Owens Lake sedimentary deposits 

in coreholes OL90–1 and OL90–2 by Benson and others (1996), Smith 

and others (1997), and Bischoff and others (1997). This research indicates 

approximate dates of 25–15 ka for the Tioga Glaciation, 32–30 ka or 60–50 

ka for the Tenaya Glaciation, and 140–120 ka for the Tahoe Glaciation. The 

Mono Basin/Casa Diablo Glaciations have not been dated but possibly fall 

in the range 155–145 ka. These ages, however, are still a subject of debate 

and continuing research. In addition, some confusion has been introduced 

concerning the naming of moraines; for example, in Bloody Canyon, the 

moraines originally named and mapped as Tenaya and Tahoe are referred to as 

“Younger Tahoe” and “Older Tahoe,” respectively, by recent workers.
Qob, Qab, Qpb [100?–200? ka]). The trachyandesite flows 

at Mammoth Pass (Mammoth Pass flows, unit Qa [86 ka]), 

which were erupted at the south base of Mammoth Mountain from a crater now partly buried by dacite flows, poured 

both westward and eastward into San Joaquin Canyon and 

Horseshoe Lake basin, respectively. In Horseshoe Lake basin, 

Mammoth Pass trachyandesite flows are overlain by porphyritic basalts (Horseshoe Lake flows, unit Qpb, fig. 4), which 

were erupted from a subaerial vent within the present lakesite 

and poured eastward into Twin Lakes basin. This Horseshoe 

Lake vent has since been obliterated by Tioga glacial erosion; 

however, at very low lake levels, remnant oxidized spindle 

bombs and vent agglomerate are preserved at the shores. These 

Horseshoe Lake flows overlie Mammoth Pass trachyandesite 

and are littered with Tioga glacial erratics, indicating an age of 

25–90 ka.

The trachyandesite flows in the vicinity of Pumice Butte 

(units Qa, Qya, fig. 4), which emerge from the base of two 

large cinder cones, have not been dated radiometrically. The 

older of these flows (unit Qa) are glaciated, most likely by 

Tahoe glaciers, but the younger flows (unit Qya) are not, suggesting that the younger flows postdate the Tioga Glaciation 

(25–10 ka); however, Tioga glaciers in San Joaquin Canyon 

may not have been thick enough to override these younger 

flows, and so, though younger than the Tahoe Glaciation 

(140–120 ka), they could predate the Tioga Glaciation.

At Devils Postpile, a sequence of lavas filling the bottom 

of San Joaquin Canyon includes, in stratigraphic succession, 

(1) the Buttresses porphyritic basalt flows (unit Qob, fig. 4), 

(2) the Rainbow Falls dacite flow (unit Qaq), (3) the Mammoth 

Pass trachyandesite flows (unit Qa), and (4) the Devils Postpile 

porphyritic basalt flow (unit Qbp) and its probable cindery vent 

complex (unit Qab). The main mass of the Buttresses flows, 

which is perched on the west wall of San Joaquin Canyon, was 

fed by dikes that originate in a dike- and sill-laced palagonite 

complex on the canyon floor. A similar, smaller dike-fed mass 

occurs 2 km south on the east wall of the canyon. The flows 

(numbering at least three) have pillowed, palagonitic bases, 

indicating eruption into shallow water. These flows are inferred 

to have been erupted through a glacier and to have ponded in 

meltwater between the glacier margin and the canyon wall. 

Although the flows have not been dated radiometrically, topographic and intraglacial stratigraphic relations suggest that they 

postdate the Bishop Tuff. Though designated “Older basalt” 

(Bailey, 1989) because of their moderate degree of alteration, 

they are not necessarily the oldest flows in the postcaldera 

sequence. They clearly predate the Tioga Glaciation but could 

have been extruded through Tahoe or older glaciers, suggesting an age possibly as young as 120 ka. The Devils Postpile 

flow was erupted from a cinder-cone complex (unit Qab) north 

of the Postpile, flowed downcanyon, and ponded behind a 

Tahoe(?) recessional moraine (now mostly removed by erosion). Slow cooling of this ponded flow under subaerial conditions accounts for its spectacular, coarse columnar jointing. The 

flow was later overridden by Tioga glaciers, which grooved and 

polished its upper surface.

Dacite Lavas

On the northeast edge of the Long Valley Caldera floor 

at the southwest foot of Glass Mountain is a small, distinctive 

hornblende-plagioclase-phyric dacite dome (unit Qhrd, fig. 

2). Its youthful morphology indicates that it was erupted on 

the caldera floor shortly after collapse, probably between 760 

and 500 ka. Exceptionally well formed, small (5–8 cm)-diameter columnar jointing on its lower flanks suggests that it was 

extruded into and chilled by Pleistocene Long Valley Lake 

water. This dome is petrographically and chemically similar to 

precaldera Basin and Range dacites on the north caldera rim; 

however, the dome is not a downfaulted precaldera remnant 

but appears to represent dacite lava from a vestigial precaldera Basin and Range magma source that leaked up along the 

caldera ring fracture shortly after caldera collapse. Though 

older and isolated from other Caldera and Mono-Inyo lavas, 

the dacite dome is included as a member of the postcaldera 

sequence (see table 1).

The more evolved dacitic lavas associated with intracaldera and extracaldera mafic lavas in the western part of 

the caldera include (1) Mammoth Mountain, a large domical 

dome-flow complex of coarsely porphyritic dacite (unit Qqm

[60?–180 ka], fig. 4) and low-silica rhyolite (units Qrm1 [225 

ka], Qrm2 [50 ka]); (2) two large dacite domes in the west 

moat (unit Qqm [142–148 ka]), petrographically similar to 

Mammoth Mountain; (3) a distinctive crystal-poor dacite flow 

(unit Qaq [103 ka]) in the west moat, similar to the Rainbow 

Falls dacite (unit Qaq) at Devils Postpile; and (4) a cluster of 

four hybrid porphyritic dacite domes and flows in the northwest moat (unit Qrm2 [<50? ka]).

On Mammoth Mountain, Koeppen (1983) recognized five 

eruptive sequences: (1) sparsely porphyritic biotite-augite rhyolite flows (unit Qrm1, fig. 4), exposed sporadically around the 

base of the mountain, and an associated locally thick pumice 

fall (unit Qrmp); (2–4) coarsely porphyritic hornblende-biotite 

and hypersthene-augite dacites (unit Qqm), forming the bulk of 

the edifice; and (5) porphyritic hornblende-biotite rhyolite (unit 

Qrm2), forming the summit dome. Noting the abundance of 

mafic inclusions in the dacites and the distribution of apparently coeval basalt vents within 4 to 5 km peripheral to the 

mountain, Koeppen suggested that the dacitic-magma chamber 

formed a shadow zone above a subjacent basalt sill complex.

Mono-Inyo Suite

Younger (<30 ka) Mono-Inyo lavas are limited to the axis 

of the Mono-Inyo Craters Volcanic Chain. Mono-Inyo vents 

are distributed sporadically along the axis between Black Point 

in the north and Red Cones in the south. As noted above, both 

the Caldera and Mono-Inyo suites are structurally related to 

the Mono-Inyo Craters fissure system but constitute only a 

part of the Mono-Inyo Craters Volcanic Chain. The distribution, age, and chemical and isotopic characteristics of the 

Caldera and Mono-Inyo suites differ sufficiently to justify 

distinguishing them.

Long Valley Volcanism 15
16 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

Basalt Lavas

June Lake.—At about 30 ka, trachyandesite eruptions 

(unit Qa1, fig. 5) formed a 90-m-high cinder cone northeast of

June Lake. The cone was breached on its north side by a flow 

that broke out at its base and rafted away large segments of 

the cone, distributing them as scattered reddish-brown cindery 

hillocks on the flow surface. This flow bifurcated into two 

lobes to the north and northeast around the south end of Aeolian Buttes. At the toe of the northeastern lobe, a secondary, 

apparently rootless, cindery agglutinate vent formed; this vent 

was breached on its northeast side and fed lava from the main 

flow to a thinner, narrower secondary flow (unit Qa2), which 

coursed another 7 km northward down the valley between 

Aeolian Buttes and Mono Craters. Outflow from this secondary vent caused deflation of the main northeastern flow surface 

as lava drained from beneath its crust, thereby lowering its 

surface about 10 m, forming a marginal “bathtub ring”-like 

lava ledge along the south side of Aeolian Buttes. The toe of 

this northeastern lobe and its associated rootless vent abut 

and bury the trace of a north-trending, west-facing fault scarp 

that displaces adjacent Tahoe and Tenaya(?) glacial moraines, 

suggesting that the position of the toe and rootless vent is 

related to ponding of the northeastern lobe against this scarp 

or, possibly, against the nearly coincident Tenaya(?) terminal 

moraine. The rootless vent agglutinate contains granitic blocks 

apparently derived from this moraine. The June Lake flow 

overlies Tahoe and Tenaya(?) terminal moraines and is partly 

overlapped by Tioga moraines. Bursik and Gillespie (1993), 

using a slightly different scenario, presented evidence suggesting that the June Lake eruptions occurred 30–25 ka.

Black Point.—Black Point, a flat-topped basaltic cinder 

cone on the north shore of Mono Lake (unit Qyc, fig. 6), is 

composed mainly of brown palagonitized scoriaceous ejecta 

that was erupted within Pleistocene Lake Russell (ancient 

Mono Lake) at 13.3 ka (Lajoie, 1968), shortly after the lake’s 

highest stand (2,155-m elevation), postdating the Tioga 

Glaciation (Benson and others, 1990). The initial Black Point 

eruptions were subaqueous, and interaction between the hot 

ejecta and lakewater caused pervasive palagonitization of the 

main cone. Its final eruptive pulse, however, occurred subaerially from a vent above lake level, because the youngest, black, 

cindery ejecta mantles the top and unconformably drapes the 

upper, terraced flanks of the brown, palagonitized cone, particularly on its west and southwest sides. These younger beds are 

composed of unaltered, poorly consolidated to unconsolidated, 

glassy, black basaltic cinder and ash, which, though probably 

also emplaced in lakewater, was sufficiently air cooled before 

immersion to prevent palagonitic alteration. The difference in 

emplacement temperature of the early brown palagonitized 

ejecta and the overlying black cinder and ash is indicated 

by the presence of botryoidal calcite fissure fillings (likely 

deposited by hydrothermal solutions) in the palagonitized 

beds and, in contrast, the presence of coarse “thinolite” crystal 

clusters and rosettes (Russell, 1886–87) in the vitric black 

ash and cinder. These thinolite crystals, now known to be 

pseudomorphs of ikaite (CaCO3·6H2O; Shearman and Smith, 

1985), form only at near-freezing temperatures (Bischoff and 

others, 1993a, b; Council and Bennett, 1993), indicating that 

the enclosing, air-cooled black cinder and ash was emplaced 

in very cold water. Apparently, enough time lapsed between 

eruption of the palagonitized beds and the unaltered black ash 

beds for the lake level to be lowered below the eruptive vent, 

which is on the south side of Black Point, 30 m below its flat 

summit. This lake-level change occurred over a sufficient 

duration to allow formation of at least two lake terraces below 

the summit terrace, indicating that the later eruption occurred 

long after the earlier eruptions, during the accelerated lakelevel decline due to rapid climate change after 13.3 ka (see 

Benson and others, 1990, fig. 4).

Red Cones.—Red Cones (unit Qyb, fig. 4), about 5 km 

south-southwest of Mammoth Mountain and on the east rim of 

the Middle Fork San Joaquin Canyon, consists of twin 80-mhigh reddish-brown basalt cinder cones from which spilled a 

2.5-km-long tongue of thin pahoehoe flows down the east wall 

of the canyon. These flows, which have not been glaciated and 

clearly postdate the Tioga Glaciation (28–15 ka), could be, on 

the basis of soil profiles, as young as 5 ka (C.D. Miller, oral 

commun., 1978). Though erupted amidst older Caldera basalts, 

Red Cones, on the basis of age, chemistry, and isotopic data, 

belongs to the younger Inyo-Mono suite.

Dacite Lavas

The youngest postcaldera dacites occur at the north end 

of the Mono-Inyo Craters Volcanic Chain, mainly in Mono 

Lake. Negit Island (unit Qd, fig. 6), the largest dacitic mass, 

comprises a 100-m-high, black, agglutinated cinder cone and 

a flow emerging from its south base. The island also includes 

four rubbly-surfaced flows that abut the north and east sides 

of the cinder cone and were erupted from three vents aligned 

approximately east-west. Several small islets east of Negit 

Island constitute the top of another, mostly submerged dacitic 

flow. On the northeast end of Paoha Island is a cluster of 

young black dacite cinder cones and an associated 1-km-long 

dacite flow (unit Qd). The main mass of Paoha Island consists 

of white to buff tuffaceous lake-bottom sedimentary deposits 

that were uplifted by a shallow rhyolitic intrusion or cryptodome (unit Qry), the upper carapace of which protrudes as a 

sediment-littered mass of blocky obsidian on the north third of 

the island (Lajoie, 1968). A similar, smaller mass of rhyolitic 

lava that forms a small islet east of the Negit islets appears to 

be the source of numerous buoyant rhyolitic pumice blocks, 

many as large as 1 m in diameter, that floated to the lake surface and drifted to the north and west shores, where they have 

since become encrusted with carbonate deposits (Stine, 1987). 

Using ash stratigraphy and 14C dating of tree trunks rooted 

on Mono Lake terraces and strandlines, Stine determined that 

Negit Island and the adjacent islets formed between 2,000 

and 270 B.P., that Paoha Island rose above lake level after 265 

B.P. (A.D. 1685), and that the Paoha dacite cones and flow 

were erupted after 227 B.P. (probably between A.D. 1723 and 
1855), shortly before arrival of the first exploratory expeditions scouting routes across the Sierra Nevada. An interesting 

historical note bearing on this eruption appears in the diary of 

Zenas Leonard, a member of Joseph Walker’s 1834 expedition through the region (Smith, 1993, p. 198): Camping on 

the shore of what was evidently Mono Lake (although exact 

details of the expedition’s route are uncertain), Leonard wrote, 

“There is also a great quantity of pumice stone floating on the 

surface of the water, and the shore is covered with them.” That 

pumice blocks were still floating in the lake in 1834 suggests 

that the eruption probably occurred only a few years, or possibly a decade or two, earlier.

Petrography and Chemistry

General Relations

On total-alkalis/SiO2-variation diagrams (fig. 7), both 

the precaldera and postcaldera sequences follow subparallel 

trends, evolving toward more silicic compositions over time, 

from basaltic though trachyandesitic to dacitic and rhyolitic. 

Although both sequences have alkali-lime indices (Peacock, 

1931) of about 57, suggesting an apparent calc-alkalic trend, 

most of the mafic lavas have a distinctly alkalic affinity, as 

reflected by their position above the alkalic/subalkalic line in 

figure 7 and as implied by International Union of Geological 

Sciences (IUGS) nomenclature (Le Maitre, 1989). The lavas 

span the fields of basanite-tephrophonolite, basalt, trachybasalt, basaltic trachyandesite, trachyandesite, trachydacite, 

dacite, and rhyolite. On K2O-SiO2 variation diagrams (not 

shown) most lavas fall in the high-K field, and some precaldera lavas would be considered absarokites and shoshonites. 

For simplicity in this report, however, the more general terms 

“basalt” (basalt, trachybasalt), “trachyandesite” (basaltic 

trachyandesite, trachyandesite), “dacite,” “trachydacite,” and 

“rhyolite” are used.

In spite of general similarities, significant mineralogic 

and chemical differences distinguish the precaldera and postcaldera sequences, as well as the chemical-geographic suites 

within them. Mineralogic differences, evident in hand samples 

during the early stages of mapping, are now attributable to 

40 45 50 55 60 65 70 75 2

3

4

5

6

7

8

9

10

11

12

basalt

basaltic

andesite

andesite

dacite

rhyolite

alkali

rhyolite

trachyte

trachybasalt

basaltic 

trachyandesite

tephriphonolite

basanite

tephrite

1 2

3

5

6

7

8

1 0

9

1 1

1 2

A

phonotephrite trachydacite

4

trachyandesite

East Sierran suite

Central Sierran suite

EXPLANATION

40 45 50 55 60 65 70 75

2

3

4

5

6

7

8

9

10

11

12

Basin and Range suite

basalt

basaltic

andesite

andesite

dacite

rhyolite

alkali

rhyolite

trachyte

trachybasalt

basaltic 

trachyandesite

tephriphonolite

basanite

tephrite

B

phonotephrite trachydacite

trachyandesite

EXPLANATION

alkali

rhyolite

40 45 50 55 60 65 70 75 2

3

4

5

6

7

8

9

10

11

12

Corehole Inyo-4 basalt-andesite

Dacite-rhyolite

Unit Qer-Qmr mafic enclaves

Basalt-andesite

Sample C-12 (table 4) 

basalt

basaltic

andesite

andesite

dacite

rhyolite

trachyte

basaltic

trachyandesite

tephriphonolite

basanite

tephrite

phonotephrite dacite

C

trachy--

basalt

trachytrachyandesite NE

Q e r

Q m r EXPLANATION

C-12

K 2 O + N a 2 O C O N T E N T, I N W E I G H T P E R C E N T

40 45 50 55 60 65 70 75

2

3

4

5

6

7

8

9

10

11

12

Mafic enclaves

Dacite-rhyolite

basalt Basalt-andesite

basaltic

andesite

andesite

dacite

rhyolite

trachyte

basaltic 

tephriphonolite

basanite

tephrite

phonotephrite trachytrachybasalt

D

trachyandesite

trachyandesite

alkali

rhyolite

dacite

EXPLANATION

SiO2 CONTENT, IN WEIGHT PERCENT

Figure 7. K2O+Na2O versus SiO2 contents of precaldera and 

postcaldera basalt-dacite sequences. A, Central and East Sierran 

suites. Dashed line outline encloses the potassic Central Sierran 

suite; gray shading encloses the East Sierran suite. Note compositional gaps in the East Sierran suite. B, Basin and Range suite. 

Note compositional continuity of the Basin and Range suite relative to gaps in the East Sierran suite. C, Caldera suite. Note compositional gap between 59 and 64 weight percent SiO2. D, MonoInyo suite. Note compositional gap between 54 and 63 weight 

percent SiO2, occupied only by small-volume, mixed-composition 

mafic enclaves. International Union of Geological Sciences (IUGS) 

classification after Le Bas and others (1986).

Petrography and Chemistry 17
18 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

fundamentally different magmatic sources and processes. As 

discussed below, differences between the precaldera and postcaldera sequences are related to their derivation from different 

sources within the crust-mantle substructure, whereas differences within the sequences reflect fundamentally different 

magmatic processes operative in the Sierra Nevada and Basin 

and Range Provinces, that is, west and east, respectively, of the 

East Sierran frontal fault zone. In this report, the Long Valley 

precaldera and postcaldera sequences are subdivided informally into the following temporally and geographically, as 

well as chemically, defined suites (see table 1): the precaldera 

sequence (0.8–∼4.0 Ma), comprising a Central Sierran suite, 

an East Sierran suite, and a Basin and Range suite; and the 

postcaldera sequence (0–700 ka), comprising a Caldera suite 

(50–∼700 ka) and a Mono-Inyo suite (0–30 ka).

Distinct mineralogic and chemical differences distinguish 

the lavas erupted east and west of the East Sierran frontal fault 

zone, defined locally by the Hilton Creek and Hartley Springs 

Faults (fig. 2), which trend northwest in left-stepping echelon 

fashion through the middle of the caldera. The precaldera 

Central Sierran and East Sierran suites include lavas erupted 

west of this fault zone, and the Basin and Range suite includes 

those erupted east of it. The transition between these three 

suites is gradational rather than abrupt; East Sierran lavas 

are intermediate in composition between Central Sierran and 

Basin and Range lavas. All three suites were coeval, spanning 

the interval 4.0–2.5 Ma. In the postcaldera sequence, basaltdacite lavas of the Caldera and Mono-Inyo suites constitute 

part of the Mono-Inyo Craters Volcanic Chain, which, as noted 

above, was erupted from the north-trending fissure system 

that extends from south of Mammoth Mountain, through the 

west moat of the caldera, to Mono Lake. The Caldera suite, 

limited to the southern part of the volcanic chain, includes 

64–415-ka lavas erupted within the caldera (intracaldera), 

as well as some erupted outside the caldera (extracaldera) 

in the vicinity of Mammoth Mountain and Devils Postpile. 

The Mono-Inyo suite includes only the youngest (0.2–30 ka) 

postcaldera lavas—those erupted mainly in the northern part 

of the chain—but also at least one of several cinder cones on 

the west wall of the caldera, as well as Red Cones, south of 

Mammoth Mountain. The petrographic and chemical characteristics of the precaldera and postcaldera sequences are 

described below, focusing first on the differences within the 

two sequences (that is, between the precaldera Central Sierran, 

East Sierran, and Basin and Range suites and between the 

postcaldera Caldera and Mono-Inyo suites) and then on the 

salient differences between the two sequences (see table 1).

Petrography

Precaldera Sequence

Basalt

The Central Sierran suite comprises the ultrapotassic 

lavas of the central Sierra Nevada studied by Van Kooten 

(1980, 1981), who described them as ultrapotassic basanites, 

potassic olivine basalts, and alkali olivine basalts, which correspond in the IUGS classification of figure 7 to basanite-tephrophonolite, basalt, trachybasalt, and basaltic trachyandesite. 

All the lavas are olivine-diopside phyric; the more potassic 

lavas also contain phenocrysts of sanidine, plagioclase, and 

phlogopite±leucite; matrices consist of olivine, diopside, 

plagioclase, apatite, magnetite, and pseudobrookite±sanidine

±phlogopite±leucite (Van Kooten, 1980).

East Sierran and Basin and Range basalts, mainly alkali 

olivine basalts, are, despite their chemical differences, mineralogically and texturally similar. In both suites, the basalts 

are predominantly olivine-augite phyric, although a few lavas 

contain subordinate plagioclase phenocrysts. Pyroxenes are 

less calcic than in the Central Sierran suite. Ferromagnesian 

phenocrysts, commonly as large as 3 mm in diameter, constitute 5 to 10 volume percent of the rock; rare picritic basalts 

contain as much as 20 volume percent olivine; and in the less 

common plagioclase-bearing basalts, total phenocrysts make 

up as much as 40 volume percent. The more mafic flows are 

nepheline normative (tables 2, 3). Olivine and augite phenocrysts are subhedral to euhedral; olivine is altered commonly 

to iddingsite and less commonly to chloritic minerals. Plagioclase phenocrysts are euhedral, weakly zoned, and finely 

albite twinned. Matrices typically consist of plagioclase, alkali 

feldspar, olivine, augite, Fe-Ti oxides, apatite, and, commonly, 

phlogopite, which also occurs as a vapor-phase mineral in 

vesicles; matrices range widely in texture from finely pilotaxitic to coarsely trachytic.

Various mantle and deep-crustal mafic xenoliths occur in 

some Central Sierran basalts (Van Kooten, 1980; Van Kooten 

and others, 1985; Ducea and Saleeby, 1996). Some East Sierran flows contain upper-crustal granitoid xenoliths, as well as 

partially resorbed xenocrysts of plagioclase and quartz, many 

of which are volcanic rather than plutonic in origin: relatively 

clear and lacking microscopic inclusions and fractures common in xenocrysts derived from the local Mesozoic granitoid 

basement, they appear to be phenocrysts derived by mixing 

with coeval dacitic magma.

Trachyandesite

Unlike the basalts, East Sierran and Basin and Range trachyandesites are distinct in mineralogy and texture. East Sierran trachyandesites are olivine-augite or olivine-augite-plagioclase phyric; the more silicic variants also contain hypersthene 

and (or) biotite phenocrysts. Most are relatively crystal poor 

(1–10 volume percent), but plagioclase- and biotite-bearing 

flows contain as much as 20 volume percent phenocrysts. 

Matrices consist of plagioclase, augite, apatite, and Fe-Ti 

oxides±hypersthene±biotite±sanidine. Olivine shows the same 

alteration as in the basalts. Phenocrysts are generally euhedral, 

but in some flows olivine and augite are partially resorbed. 

Basin and Range trachyandesites, in contrast, are typically 

hornblende-plagioclase phyric, although olivine-augite and 

olivine-augite-plagioclase phyric variants occur locally. Horn-
Table 2. Chemical analyses of lavas of the East Sierran suite.

[Major-element-oxide contents in weight percent; minor-element contents in parts per million. Mg#, magnesium number, Mg/(Mg+Fe); SiO2*, SiO2 content recalculated to 100-

analyzed]

Sample----------------------- S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 

Field No. -------------------- cDP83–85 cDP83–90 DP72–7 cDP83–100 cDP83–75 cDP83–74 cDP83–93 D–801e DP72–10 DP78–34A MC86–1 DP81–20 M72–19 cDP83–68 DP78–33A DP78–

Rock type ------------------- Basalt Basalt Basalt Basalt Basalt Basalt Basalt Andesite Andesite Andesite (xeno) Andesite Andesite Andesite Andesite Dacite Daci

Map unit (figs. 4–6) ------ Tba Tba Tba Tba Tba Tba Tba Tba Tba Tsjd Tba Qaq Tba Tba Tsjd Tsjd 

Locality---------------------- San Joaquin San Joaquin Deadman San Joaquin San Joaquin San Joaquin San Joaquin Agnew Deadman San Joaquin Hartley Springs Pumice Northwest San Joaquin San Joaquin San Joa

Ridge N. Ridge N. Pass Ridge N. Ridge W. Ridge W. Ridge N. Meadow Pass Mountain Fault Butte 9352 wall Mountain Mountain Ridg

Latitude N------------------- 37°44.5′ 37°44.5′ 37°40.5′ 37°44.1′ 37°43.4′ 37°43.4′ 37°44.1′ 37°40.7′ 37°40.6′ 37°42.5′ – – 37°34.1′ 37°44.8′ 37°43.1′ 37°42.5′ 37°42

Longitude W --------------- 119°8.8′ 119°8.8′ 119°3.9′ 119°8.1′ 119°7.4′ 119°7.4′ 119°8.1′ 119°3.9′ 119°3.9′ 119°6.5′ – – 119°3.8′ 118°59.9′ 119°6.6′ 119°6.5′ 119°5

Major-element oxides

SiO2* ------------------------- 50.05 50.07 51.51 51.69 52.17 52.34 52.35 55.45 56.17 57.28 57.61 58.59 59.26 60.17 66.24 67.3

SiO2--------------------------- 49.40 48.90 49.60 49.60 50.60 50.40 50.30 53.00 52.60 55.80 57.10 57.70 58.79 59.00 64.60 65.3

TiO2 -------------------------- 1.30 1.31 1.22 1.24 1.07 .94 .94 .94 .82 .85 .73 .59 .86 .73 .49 .4

Al2O3 ------------------------- 15.60 15.60 15.30 16.10 14.40 14.90 15.20 15.70 15.20 19.30 16.20 16.80 17.21 16.10 16.80 16.6

Fe2O3 ------------------------- 2.60 3.59 2.53 2.97 3.71 3.29 3.43 2.60 1.87 4.13 2.70 2.73 2.24 2.68 2.48 1.6

FeO --------------------------- 5.55 4.44 4.80 4.38 3.73 3.79 3.65 3.80 4.20 1.80 3.31 2.22 3.34 2.39 .68 1.3

MnO-------------------------- .14 .13 .13 .11 .11 .11 .11 .14 .12 .08 .10 .10 .09 .08 .07 .0

MgO-------------------------- 10.00 9.67 8.11 8.33 10.20 9.96 9.59 5.00 5.36 2.51 5.85 5.29 3.82 4.26 1.10 .9

CaO--------------------------- 8.46 8.58 7.91 7.34 7.43 7.40 7.30 7.40 6.95 5.60 6.42 5.73 5.32 5.17 3.06 2.7

Na2O-------------------------- 3.22 3.03 3.29 3.04 3.05 2.98 2.98 3.80 3.54 3.86 3.96 4.50 4.06 4.02 4.02 3.7

K2O --------------------------- 1.86 1.85 2.87 2.38 2.24 2.07 2.10 2.70 2.59 2.95 2.41 2.53 3.03 3.18 3.92 3.9

P2O5--------------------------- .57 .57 .53 .47 .45 .46 .48 .50 .39 .53 .33 .29 .45 .44 .30 .2

H2O+ -------------------------- .65 .84 .77 1.32 1.29 1.48 1.60 .92 .29 1.60 .15 .19 .29 .34 1.60 2.3

H2O− -------------------------- .29 .91 .95 1.25 1.18 1.76 1.77 n.a. .26 .46 .11 .17 .04 .59 .38 .2

CO2 --------------------------- <.02 .20 1.50 .98 .19 <.02 .10 2.80 6.20 .01 <.01 <.01 .00 .25 .01 .0

F------------------------------- n.a. n.a. .07 n.a. n.a. n.a. n.a. n.a. .06 .09 n.a. n.a. n.a. n.a. .05 .0

Cl------------------------------ n.a. n.a. <0.0035 n.a. n.a. n.a. n.a. n.a. .02 .03 n.a. n.a. n.a. n.a. .02 .0

 Total ---------------------- 99.64 99.62 99.58 99.51 99.65 99.54 99.55 99.30 100.47 99.60 99.38 98.85 99.54 99.23 99.58 99.6

Mg#--------------------------- 70 70 68 69 72 73 73 60 62 45 65 67 56 62 41 41 

Minor elements

Ba ----------------------------- 1,200 1,250 1,430 1,320 1,270 1,500 1,590 1,539 1,320 1,930 1,490 1,560 1,918 1,740 1,760 1,705 Co----------------------------- 38.6 36.4 33.8 31.5 36.9 35 33.1 28.9 29.3 16.65 25.6 23.8 16.6 17.7 7.6 5.6

Cr ----------------------------- 361 351 310 312 542 599 526 298 257 5.25 186 212 57.7 186 5.4 8.9

Cu----------------------------- 39 28 – – 47 32 52 43 35 – – – – – – – – 32 22 – – – – 

Hf ----------------------------- 4.13 4.15 4.9 4.57 4.95 4.69 4.76 4.5 4.5 4.35 3.85 3.73 4 5.61 4.9 4.7

Nb----------------------------- 14 15 20 19 10 14 12 – – 8 8 12 10 – – 13 10 9 

Ni ----------------------------- 194 205 143 236 196 198 – – – – – – 132 151 87 129 – – – – Rb----------------------------- 32.4 31 52 48.2 31.6 32 29 43 44 49 47.2 40.7 50 54.3 76 69 

Sb ----------------------------- .16 .146 .6 .061 <.19 .097 <0.088 <.5 <0.6 1.05 .398 .407 .3 .244 1.5 1.5

Sc ----------------------------- 21.5 21.8 20.3 22.9 21.9 22.3 20.7 14.45 16.8 9.9 14.7 13 9.59 10.4 5.285 4.9

Sr------------------------------ 1,300 1,250 1,020 790 890 1,050 1,000 1,000 1,090 1,820 1,210 1,720 1,820 1,150 1,010 879 

Ta ----------------------------- .773 .805 1 .83 .534 .47 .46 .73 .6 .5 .472 .255 .48 .681 .7 .6

Th ----------------------------- 4.76 4.8 5 2.63 2.66 3.29 3.31 4.1 4 6.1 3.5 3.08 3.8 5.77 9.2 8.7 

U ------------------------------ 1.43 1.63 1.3 .781 .816 1.08 1.02 1.3 1.3 1.8 1.07 1.13 1.4 1.88 2.4 2.3 

Y ------------------------------ 21 20 13 22 17 20 17 36 8 4 20 16 23 19 8 10 

Zn ----------------------------- 46.8 51.2 73 <38 48 53.4 44 104 68 74 83.6 74.2 88 54.8 55 50 

Zr------------------------------ 182 167 200 161 168 150 160 310 200 130 143 123 187 198 228 230 

La ----------------------------- 43.5 43.6 41 29 24.5 28.6 29.4 42 32 48 28.6 30.6 50 43.2 48 45 

Ce ----------------------------- 97.5 97.8 72 61.6 50.1 56.9 60.8 73 60 95 60.1 64.5 92 84.3 92 85 

Nd----------------------------- 41.9 47.6 40 27.8 23.5 28.5 29 31 31 46 25.5 28.3 47 32.9 38 35 

Sm ---------------------------- 7.24 7.47 4.7 5.35 4.84 5.19 5.33 5.9 3.9 5.5 4.28 4.39 7.6 5.26 4.6 4.3 

Eu ----------------------------- 1.97 2 1.46 1.48 1.35 1.4 1.41 1.43 1.65 1.77 1.26 1.22 1.73 1.35 1.25 1.1

Tb ----------------------------- .621 .683 .62 .589 .517 .492 .496 .53 .52 .6 .482 .436 .67 .494 .46 .4

Tm ---------------------------- .284 .302 .29 .285 .234 .22 .23 .24 .25 .2 .206 .32 .211 .22 .2

Yb----------------------------- 1.7 1.76 1.7 1.78 1.35 1.33 1.34 1.6 1.6 1.1 1.29 1.06 1.7 1.22 1.1 1.2 

Lu ----------------------------- .246 .258 .27 .271 .203 .194 .188 .23 .27 .18 .198 .148 .22 .168 .18 .1
Table 3. Chemical analyses of lavas of the Basin and Range suite. [Major-element-oxide contents in weight percent; minor-element contents in parts per million. Mg#, magnesium number, Mg/(Mg+Fe); SiO2*, SiO2 content recalculated to 100-

analyzed] Sample --------------------- B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 

Field No.------------------- GM78–1 CT72–3 CD73–8 CT85–7 CT72–1 CD85–6A GM85–4 CD85–5 CT85–6 CT78–7 CT85–30 GM85–3 CT85–9 CT78–8 CT72–4 CT85–28 

Rock type------------------ Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt Andesite Andesite Andesite Andesite Dacite Andesite Andesite Dacite 

Map unit (figs. 4–6)----- Tba Tba Tba Tba Tba Tba Tba Tba Tba Tba Tba Tba Tbm Tba Tbm Tbm 

Locality -------------------- Kelty Crestview 3 East wall Bald Mountain Crestview 1 Owens Dry Fork Owens NE. Bald Cowtrack Clark Dry Fork Bald Mountain, Cowtract Bald Mountain, Bald Mountain

Meadow Spring River 6 River 5 Mountain Mountain Canyon NE. 8874 summit S. summit E. Control 

Latitude N----------------- 37°46.0′ 37°45.7′ 37°41.2′ 37°47.7′ 37°45.7′ 37°34.1′ 37°47.5′ 37°42.9′ 37°48.4′ 37°57.9′ 37°46.1′ 37°48.6′ 37°47.4′ 37°58.9′ 37°46.4′ 37°46.4′ 

Longitude W-------------- 118°39.8′ 118°59.5′ 118°40.6′ 118°52.3′ 118°52.5′ 118°37.0′ 118°40.6′ 118°25.8′ 118°52.4′ 118°50.4′ 118°54.9′ 118°41.2′ 118°51.8′ 118°48.6′ 118°54.4′ 118°54.0′ 

Major-element oxides SiO2* ----------------------- 49.35 50.20 51.90 52.05 54.20 52.77 52.78 53.02 54.19 54.48 54.61 56.80 57.45 60.45 60.96 63.61 

SiO2 ------------------------- 48.60 49.60 51.40 51.30 53.70 51.40 52.20 52.40 53.70 53.00 52.70 56.20 56.20 59.10 60.14 62.70 

TiO2 ------------------------- 1.66 .90 1.42 1.32 1.03 1.40 1.32 1.42 1.36 1.70 .96 .87 .97 .89 .71 .61 

Al2O3 ----------------------- 14.10 15.70 15.40 14.90 16.20 15.00 17.20 15.20 17.40 16.40 15.40 16.30 15.00 16.50 17.30 16.80 

Fe2O3 ----------------------- 3.22 3.66 1.63 3.75 2.27 4.80 5.05 4.13 3.48 3.17 2.64 3.00 5.52 3.03 4.86 2.93 

FeO ------------------------- 6.20 6.50 6.00 4.39 5.00 3.31 2.88 3.92 3.95 4.60 4.20 3.54 .09 2.80 .45 1.16 

MnO ------------------------ .15 .17 .13 .13 .13 .12 .13 .13 .12 .12 .11 .11 .08 .10 .11 .05 

MgO ------------------------ 10.22 9.18 6.93 8.13 7.32 6.60 5.20 6.48 4.80 5.20 6.06 5.22 4.59 3.04 2.32 1.95 

CaO ------------------------- 8.67 9.09 7.91 7.69 7.39 6.85 8.70 7.08 7.11 6.44 7.95 6.25 6.09 5.17 5.30 4.60 Na2O------------------------ 3.98 2.79 3.90 3.00 3.77 2.73 3.65 2.87 4.22 3.77 3.61 3.94 3.51 3.22 4.19 4.12 

K2O ------------------------- .90 .96 3.53 3.19 1.93 4.17 2.04 4.19 2.42 2.29 2.40 3.03 4.96 3.61 2.91 3.38 

P2O5 ------------------------- .79 .25 .78 .76 .34 1.02 .53 1.02 .56 .59 .48 .48 .82 .31 .37 .27 

H2O+ ------------------------ 1.10 .64 .42 .29 .17 .44 .22 .13 .12 .77 .17 .20 .16 1.20 .71 .18 

H2O− ------------------------ .18 .32 .15 .16 .10 .70 .19 .12 .28 .31 .35 .16 .08 .34 .17 .27 

CO2-------------------------- .01 .01 .01 .01 .02 .01 .01 .01 .01 .01 2.29 .01 .01 .01 .01 .01 

F ----------------------------- .11 .02 .13 n.a. .04 n.a. n.a. n.a. n.a. .30 n.a. n.a. n.a. .07 n.a. n.a. 

Cl ---------------------------- .02 .00 .01 n.a. .01 n.a. n.a. n.a. n.a. .18 n.a. n.a. n.a. .04 n.a. n.a. 

 Total -------------------- 99.91 99.79 99.75 99.02 99.42 98.55 99.32 99.10 99.53 98.85 99.32 99.31 98.08 99.43 99.55 99.03 

Mg#------------------------- 67 63 63 66 65 61 56 60 55 56 63 60 62 50 46 48 

Minor elements Ba --------------------------- 1,680 753 1,970 2,440 1,060 2,330 1,090 2,380 1,330 1,300 1,350 1,640 3,410 1,270 1,803 1,980 

Co --------------------------- 42.5 46.4 31.6 34.6 31.5 29.2 28.8 28.2 24.9 27.7 29.1 29.2 19.6 13.6 18.6 10.3 

Cr---------------------------- 470 331 229 280 220 227 125 199 68.1 139 232 167 123 12.1 139.2 12.6 

Hf---------------------------- 5.6 2.3 5.6 6.13 3.5 8.38 4.14 8.96 4.58 5.4 4.4 5.3 8.18 6.6 5.3 4.28 

Nb --------------------------- 19 5 15 16 10 18 14 18 20 22 16 16 16 15 -- 10 Ni---------------------------- -- -- -- 219 -- 154 72.8 92.8 62.8 -- 184 126 124 -- -- 19.6 

Rb --------------------------- 97 36 76 46.6 36 68.1 35.8 68.2 40.7 37 35.8 53.6 65 142 61 62.9 

Sc---------------------------- 24.3 34 24.8 18.7 18.6 19.3 21.4 19.4 14.9 14.2 16.2 12.7 11.4 11 10.72 8.16 

Sr ---------------------------- 1,163 685 960 1,400 925 1,030 1,100 1,080 1,290 1,005 1,160 1,460 2,590 791 2,210 1,410 

Ta---------------------------- 1.23 .64 .89 .69 .7 1.05 .885 1.02 1.33 1.64 .889 .711 .575 1.08 .89 .471 

Th --------------------------- 3.9 2.3 2.7 3.61 2.8 2.98 4.73 3.28 5.34 3.6 4 5.12 8.89 18.7 4.6 4.85 

U----------------------------- 1.1 .6 .9 .959 .5 .935 1.23 1.33 1.21 1.3 1.08 1.31 .745 5.7 1.3 1.72 

Y----------------------------- 16 15 13 30 12 26 32 26 24 17 20 32 26 18 27 20 

Zn --------------------------- 95 89 111 116 74 105 81.8 105 82.9 93 93.1 97.9 102 76 95 78 

Zr ---------------------------- 160 160 230 225 150 268 185 325 193 230 168 226 296 304 273 138 La---------------------------- 42 19 32 44.8 26 33 44 34.1 42.3 42 40.1 52.4 63.7 50 49 32.2 

Ce --------------------------- 84 34 62 89.7 46 71 80.7 71.9 88.4 77 84.4 92.3 143 86 84 61.6 

Nd --------------------------- 44 23 35 43.4 27 31.5 38.1 31.8 38 41 35.8 48.6 67.7 40 39 27.2 

Sm--------------------------- 6.6 3 5.7 7.71 3.8 6.14 7.41 5.95 6.01 6.8 5.77 7.47 10.8 6.3 6.8 4.91 Eu --------------------------- 1.93 1.02 1.66 2.18 1.18 1.81 2.05 1.76 1.75 1.99 1.59 1.75 2.67 1.44 1.57 1.23 

Tb --------------------------- .87 .49 .59 .739 .57 .743 .977 .742 .685 .78 .628 .943 .74 .65 .73 .471 

Tm--------------------------- .33 .39 .39 .306 .29 .345 .436 .355 .404 .33 .299 .377 .215 .39 .21 .191 

Yb --------------------------- 1.9 2.3 1.8 1.75 1.6 2.04 2.62 1.98 1.73 1.9 1.75 2.17 1.2 1.8 1.8 1.1 

Lu --------------------------- .3 .35 .28 .249 .28 .294 .366 .292 .255 .3 .244 .3 .176 .29 .25 .167 
blende-plagioclase phyric flows are generally crystal rich (20 

volume percent), whereas the few olivine-augite-bearing flows 

are generally crystal poor (5–10 volume percent). Hornblende 

phenocrysts typically are larger and more abundant than 

plagioclase phenocrysts. Hornblende phenocrysts invariably 

are completely dehydrated to opaque granular intergrowths of 

Fe-Ti oxides, augite, and plagioclase but retain their diagnostic euhedral shape. Matrices typically consist of pilotaxitic to 

trachytic intergrowths of plagioclase, augite, apatite, and Fe-Ti 

oxides. Hypersthene and biotite are rare both as phenocrysts 

and matrix minerals, although one particularly alkalic flow 

northeast of Bald Mountain (sample B13, table 3) contains 25 

volume percent phenocrysts, including as much as 10 volume 

percent biotite.

Dacite

East Sierran and Basin and Range dacites also differ 

markedly. East Sierran dacites (Central Sierran dacites do 

not occur) are predominantly biotite-hornblende-plagioclase 

phyric; hypersthene, augite, sanidine, and, rarely, quartz may 

also be present; and phenocryst contents range from 30 to 50 

volume percent. In contrast, Basin and Range dacites typically 

lack biotite and are hornblende-plagioclase phyric; augite also 

is present sporadically; and phenocryst contents range from 10 

to 15 volume percent. Plagioclase phenocrysts in both suites 

are euhedral and complexly twinned, with weakly zoned, partially resorbed (sieved) interiors rimmed by more continuous, 

thin, oscillatory zoning. Hornblende phenocrysts in the two 

suites differ significantly: in Basin and Range dacites (as in the 

trachyandesites) hornblende invariably is altered to an opaque 

mixture of Fe-Ti oxides, augite, and plagioclase, whereas in 

Sierran dacites, hornblende, as well as biotite, is relatively 

unaltered—both only slightly reddened by oxidation. Sierran 

dacites possibly formed at shallower depths than Basin and 

Range dacites, so that their hornblende phenocrysts underwent 

less severe decompression and dehydration upon eruption. 

Mafic enclaves, consisting mainly of relatively coarse grained 

intergrown hornblende and plagioclase (and rare garnet), are 

common in East Sierran dacites but absent to sparse in Basin 

and Range dacites.

Postcaldera Sequence

Quenched-Basalt Enclaves

The globular and slablike basalt enclaves in the intracaldera Early and Moat rhyolites are nearly aphyric, containing 

only tiny (≤0.3 mm diam) microphenocrysts of olivine and 

augite set in a matrix of feathery, radiating, or sheathlike 

intergrowths of acicular plagioclase and green pyroxene. The 

enclaves generally are more coarsely vesicular at their margins 

than at their centers, whereas the matrices are microvesicular throughout, with polygonal interstices lined with Fe-Ti 

oxide-dusted brown glass. Most enclaves have conspicuously 

darker, finer grained margins than interiors. They resemble 

the mafic enclaves observed in the Inyo and Mono rhyolite 

domes (Varga and others, 1990; Kelleher and Cameron, 1990), 

which, though texturally similar, are trachyandesitic as a result 

of greater mixing with their host rocks. They also closely 

resemble chilled the mafic enclaves in the Coso Range rhyolites described by Bacon and Metz (1984) and Bacon (1986). 

These postcaldera mafic enclaves differ significantly from 

those that commonly occur in precaldera dacites, which consist of coarse-grained intergrowths of hornblende and plagioclase, are seldom vesicular, lack chilled margins, and generally 

are more evolved chemically. The coarser grain size and lower 

vesicularity of the precaldera enclaves suggest incorporation 

at greater depth, and the absence of chill rinds and their more 

evolved compositions suggest less thermal contrast between 

enclave and host rocks and longer contact with the host rocks 

before extrusion.

Basalt

Except for the above-described mafic enclaves and the 

Buttresses and Horseshoe Lake flows (described below), 

postcaldera basalt-trachyandesite lavas in both the Caldera 

and Mono-Inyo suites are mineralogically and texturally 

similar and do not warrant separate description. They are 

conspicuously plagioclase phyric, with subordinate olivine 

and augite phenocrysts. Even among the most crystal poor 

lavas, plagioclase is the dominant mineral phase and clearly 

was present on the liquidus during initial crystallization. 

These postcaldera basalts are of two distinct petrographic 

types, finely (sparsely) porphyritic and coarsely porphyritic; 

however, they do not differ significantly in chemical composition, suggesting that the coarsely porphyritic type, which 

is more prevalent, is simply a more completely crystallized 

variant of the finely porphyritic type, in turn implying that the 

more porphyritic basalts resided and cooled in the subsurface 

for longer periods before eruption.

Sparsely porphyritic basalts (units Qgb, Qab, fig. 4; 

corehole Inyo-4 groups I–III) are typically plagioclase-olivine 

phyric and contain less than 1 volume percent phenocrysts; 

plagioclase phenocrysts are euhedral, 2 to 6 mm long, and 

weakly zoned; olivine phenocrysts are smaller than 0.5 mm 

in diameter. Some of these flows (unit Qgb) contain sparse 

glomerocrysts of plagioclase (≤1 cm diam) and olivine (≤0.5 

cm diam), similar in composition to solitary phenocrysts. 

Coarsely porphyritic basalts (unit Qpb; corehole Inyo-4 groups 

IV, V) are typically plagioclase-olivine±augite phyric and 

contain 15 to 30 volume percent phenocrysts; the plagioclase 

phenocrysts are elongate laths or blocky crystals varying in 

size (max 5 mm diam). The larger plagioclase phenocrysts 

have weakly zoned, strongly sieved interiors, with partially 

resorbed rounded outlines surrounded by continuous, thinner, 

more strongly zoned rims. Olivine and augite phenocrysts are 

subhedral to euhedral and rarely exceed 1.5 mm in diameter. 

Matrices are typically a felty mesh of small plagioclase laths 

with interstitial olivine, augite, and Fe-Ti oxide granules and 

have a pilotaxitic to intergranular texture.

Petrography and Chemistry 21
22 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

The Buttresses (unit Qob, fig. 4) and Horseshoe Lake 

(unit Qpb) basalts are coarsely olivine-augite phyric and mineralogically, texturally, and chemically resemble precaldera 

alkali-olivine basalts. As noted above, however, they postdate 

the Bishop Tuff and also are not necessarily the oldest postcaldera flows.

Trachyandesite

Postcaldera trachyandesites, the greatest volume of 

which occur in the west moat (unit Qa, fig. 4; corehole Inyo-4 

groups I–III) and at Mammoth Pass (unit Qa) and Pumice 

Butte (units Qa, Qya), are petrographically akin to the abovedescribed sparsely porphyritic basalts in that they are plagioclase-olivine phyric, containing phenocrysts that range from 

0.5 to 1.5 mm in diameter and constitute less than 1 volume 

percent of the rock.

Dacite-Rhyolite

Postcaldera dacites are petrographically the most diverse 

group of lavas in the entire volcanic field. They range in texture from nearly aphyric to coarsely porphyritic and contain 

widely varying phenocryst assemblages; their textures reflect 

crystallization conditions ranging from near equilibrium to 

extreme disequilibrium. Such diversity, together with their 

relatively small volumes and scattered distribution, suggest 

that they formed as small, discrete magma batches in scattered chambers, probably at various depths. Intimately associated with them on Mammoth Mountain and at Mono Lake 

are smaller volumes of low-SiO2 (70 weight percent SiO2) 

rhyolite.

The oldest postcaldera dacite (unit Qhrd, fig. 2), an 

isolated lava dome on the northeast floor of the caldera at the 

base of Glass Mountain, is distinctly unlike and unrelated to 

dacites of the Mono-Inyo Craters Volcanic Chain. The lava 

contains small plagioclase and hornblende phenocrysts set in 

a microcrystalline to glassy matrix and petrographically and 

chemically resembles typical precaldera Basin and Range 

dacite. However, its matrix texture, together with its unusually 

small diameter columnar jointing—consequences of extrusion 

into and chilling by Pleistocene Long Valley Lake—indicates a 

postcaldera eruptive age, in spite of its precaldera mineralogic 

and chemical affinities.

Mammoth Mountain lavas, as well as two large domes 

in the west moat (Earthquake Domes), consist predominantly of coarsely porphyritic hornblende-biotite dacite (unit 

Qqm, fig. 4), with varying hypersthene, augite, accessory 

apatite, and Fe-Ti oxide contents. Plagioclase, the dominant 

phenocryst, occurs as multiple-size populations in crystals 

ranging from 1 to 5 mm in diameter. The larger phenocrysts 

have broadly zoned, sieved cores and finely zoned continuous rims; the rims and smaller phenocrysts commonly have 

euhedral outlines. Total phenocryst contents range from 25 

to 50 volume percent. The oldest Mammoth Mountain flows 

(unit Qrm1) are sparsely porphyritic plagioclase-biotite±augite 

rhyolite (70 weight percent SiO2); the youngest unit, the 

summit dome (unit Qrm2), also rhyolite (70.5 weight percent 

SiO2), is coarsely porphyritic, containing biotite, hornblende, 

plagioclase, and sparse sanidine phenocrysts, the sanidine 

commonly rimmed by plagioclase. This unit, as well as many 

of the intracaldera Moat and Inyo rhyolites, may be described 

as “rapakivi rhyolite,” analogous to rapakivi granite.

The hybrid dacite domes and flows clustered in the northwest caldera moat (unit Qqm2, fig. 4) are distinctive in that 

they contain abundant phenocrysts derived from other magma 

types. Three of these four flows, in addition to containing 

typical dacite phenocrysts (hornblende-biotite-augite-hypersthene-plagioclase), also contain large, resorbed plagioclase, 

sanidine, and quartz phenocrysts from a rhyolitic source, as 

well as numerous small (1–5 cm diam), ovoid, reddish-brown 

mafic enclaves of basaltic-trachyandesitic composition. These 

particular units, the youngest (probably <50 ka) of the intracaldera dacites, have apparently mixed and mingled with both 

basaltic and rhyolitic magmas and incorporated phenocrysts 

from rhyolitic magmas, either western Moat Rhyolite or Inyo 

Craters rhyolite. A fourth flow in this cluster, compositionally 

a 60-weight-percent-SiO2 trachyandesite (sample C12, table 

4; described as “olivine quartz latite” by Rinehart and Ross, 

1964, p. 56–60), contains, in addition to hornblende, biotite, 

and plagioclase phenocrysts, magnesian olivine, augite, and 

more calcic plagioclase crystals in varying degrees of resorption and reaction—apparently the result of thorough mixing 

of dacitic and basaltic magmas. Judging from the homogeneous distribution of incorporated basaltic phenocrysts and the 

homogeneity of the matrix, mingling in this flow, as well as in 

the other hybrid dacites, occurred within a subsurface chamber, well before extrusion.

The distinctive crystal-poor dacite variant in the west 

moat (unit Qaq, fig. 4) and at Devils Postpile (Rainbow 

Falls flow, unit Qaq) contains as much as 10 volume percent 

plagioclase, hypersthene, augite, and magnetite phenocrysts, 

0.5 to 1.0 mm across, commonly in glomerophyric clusters. A 

few larger plagioclase phenocrysts have blocky to vermicular 

sieved cores, but most phenocrysts are euhedral and lack disequilibrium textures. The groundmass is typically brown glass 

crowded with crystallites.

Most Mono-Inyo dacites (unit Qd, fig. 6) are sparsely 

porphyritic to nearly aphyric, containing 1 to 5 volume percent 

euhedral plagioclase±hornblende±biotite±hypersthene±augite 

phenocrysts, 0.5 to 2.0 mm long, set in a matrix of brown glass 

containing varying amounts of plagioclase microlites and crystallites. In contrast, the isolated dacite dome (unit Qpd, fig. 

5) at the north end of the Mono Craters rhyolite chain, partly 

buried by rhyolite flows, is coarsely porphyritic, with unusually large (>2 cm diam) plagioclase phenocrysts; it contains 

numerous mafic enclaves with exceptionally calcic (An90) 

plagioclase phenocrysts.

Summary

Excluding the distinctive, highly potassic Central Sierran lavas, precaldera basalts of both the East Sierran and 
Table 4. Chemical analyses of lavas of the Caldera suite.

[Major-element-oxide contents in weight percent; minor-element contents in parts per million. Mg#, magnesium number, Mg/(Mg+Fe); SiO2*, SiO2 content recalculated to 100-

on ignition at 925°C; n.a., not analyzed]

Mafic enclaves Intracaldera lavas

Sample ----------------------- C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 

Field No. -------------------- M92–1 M72–90 M72–37 M72–7 CT72–13 DP73–1 M72–15 DP73–12 M72–99 M72–5 DP72–13 M73– 5 DP72–1A M73–4 

Rock type ------------------- Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt Andesite Andesite Andesite Dacite Dacite 

Map unit (figs. 4–6) ------ Qer Qmr Qgb Qab Qpb Qpb Qa Qab Qpb Qab Qa Qqm Qqm Qqm 

Locality---------------------- Resurgent North NW. moat S. moat 3 N. moat W. moat 1 NW. moat W. moat 2 S. moat 2 S. moat 1 Inyo Ctr L. NW. moat NW. wall Earthquake 

dome flow dome 

Latitude N------------------- 37°40.6′ 37°43.4′ 37°42.5′ 37°38.7′ 37°45.2′ 37°39.9′ 37°43.5′ 37°40.7′ 37°38.3′ 37°39.5′ 37°41.5′ 37°40.1′ 37°44.2′ 37°40.0′ 

Longitude W --------------- 118°53.4′ 118°53.5′ 118°56.9′ 118°55.9′ 118°53.9′ 119°1.3′ 118°57.7′ 119°0.5′ 118°53.9′ 118°55.6′ 119°0.6′ 119°1.4′ 119°0.5′ 118°59.5′ 

Major-element oxides

SiO2* ------------------------- 49.17 53.57 48.99 50.48 50.83 50.92 51.73 51.86 52.19 54.66 56.72 60.41 65.99 66.41 

SiO2 --------------------------- 48.80 52.03 48.19 50.25 50.69 50.74 51.63 51.65 51.99 54.25 56.25 59.88 64.95 66.10 

TiO2--------------------------- 1.69 1.30 1.94 1.94 1.54 1.55 2.14 1.88 1.31 1.98 1.54 1.04 .56 .66 

Al2O3 ------------------------- 16.90 18.34 17.50 17.67 18.52 18.53 17.45 17.51 20.76 17.64 17.49 17.16 16.75 16.83 

Fe2O3 ------------------------- 4.24 17.28 19.99 3.13 1.43 3.19 3.19 2.93 1.65 3.53 2.49 1.35 1.42 3.06 

FeO --------------------------- 4.84 n.a. n.a. 6.81 7.00 5.05 6.88 6.14 5.19 5.23 5.41 4.14 1.91 .00 

MnO -------------------------- .14 .12 .16 .14 .14 .13 .15 .15 .10 .14 .12 .10 .07 .11 

MgO -------------------------- 7.11 4.64 6.37 5.07 5.56 5.55 4.16 4.70 4.34 2.97 2.85 2.69 .80 .83 

CaO--------------------------- 11.30 8.47 8.98 8.33 8.80 9.11 7.54 7.79 8.61 6.12 5.96 4.60 2.34 2.02 

Na2O-------------------------- 3.02 3.51 3.62 4.02 3.83 3.71 4.18 4.10 3.78 4.65 4.39 4.29 4.42 5.51 

K2O --------------------------- .88 1.75 1.20 1.65 1.64 1.54 1.87 2.02 1.57 2.21 2.26 3.61 5.04 4.23 

P2O5--------------------------- .32 .42 .59 .53 .58 .54 .61 .72 .32 .53 .42 .27 .16 .19 

H2O+ -------------------------- .3 1.24 (LOI) −.29 (LOI) .22 .01 .15 .01 .13 .29 .34 .46 .45 1.03 .06 

H2O− -------------------------- .29 – – – – .06 .02 .03 .01 0 .04 .1 .07 .05 .07 0 

CO2 --------------------------- <0.01 n.a. n.a. .01 n.a. .01 .01 n.a. n.a. 0 .01 .01 .01 n.a. 

 Total---------------------- 99.44 99.09 99.18 99.83 99.76 99.83 99.83 99.72 99.95 99.69 99.72 99.64 99.53 99.6 

Mg#--------------------------- 61 56 54 49 55 56 43 49 54 39 40 47 31 35 

Minor elements

Ba ----------------------------- 425 1,040 695 1,025 1,004 938 925 1,075 1,010 1,480 1,091 1,292 1,590 1,883 

Co ----------------------------- – – 22.0 41.9 34.8 27.3 30.7 36.7 27.8 27.5 18 20.6 15.1 4.4 1.7 Cr------------------------------ 215 35 22.5 57.2 13.6 89 35.1 24.4 53.1 <2 3.3 44 3 1.9 

Cu ----------------------------- 43 – – – – 49 23 25 30 24 26 19 16 16 2 <2 

Ga ----------------------------- – – – – – – 26 24 28 27 26 30 21 26 27 21 26 

Hf ----------------------------- – – 4.17 4.5 5.1 4.4 4.3 5 5.3 3.1 – – 4.4 6.8 8 9.2 

Nb----------------------------- 13 – – – – – – – – – – – – – – – – – – – – – – 20 – – 

Ni------------------------------ 60 14.4 52 23 29 44 10 17 40 <10 – – 27 – – – – 

Rb ----------------------------- 11 37.8 14 25 24 22 27 28 25 28 42 83 99 89 

Sc------------------------------ – – 20.40 24.30 22.73 19.32 20.75 21.84 19.44 13.79 31 16.53 10.89 5.2 4.54 

Sr ------------------------------ 660 1,190 1,080 1,120 1,920 1,200 1,820 820 890 1,160 1,200 480 430 410 

Ta ----------------------------- – – .94 1.00 1.01 .86 .89 .9 1.09 .67 – – .82 1.13 1.1 1.58 

Th ----------------------------- – – 2.7 2.7 4.4 2.9 3.9 2.8 4.7 2.2 – – 4.2 8.3 12.4 8.9 

U------------------------------- – – 1.66 .6 .9 .09 .8 2.1 1.3 26.3 – – 1.4 2.6 3 2.8 

V------------------------------- – – – – – – 321 329 250 373 230 200 258 268 140 46 <15 

Y------------------------------- 18 – – – – 41 35 27 41 27 16 49 27 22 26 28 

Zn ----------------------------- 59 82 87 104 105 102 114 114 75 – – 122 75 55 75 

Zr------------------------------ 178 200 202 213 270 305 320 327 405 257 273 369 377 481 

La ----------------------------- 30 177 38 59 41 52 43 61 30 – – 40 52 73 63 

Ce ----------------------------- 49 75.2 79 106 80 96 81 108 56 – – 72 87 113 113 

Nd----------------------------- – – 97 41 56 43 48 46 58 32 – – 39 38 41 49 Sm----------------------------- – – 14.8 7.5 9.7 8.3 8.4 8.4 7.8 5.1 – – 6.8 6.7 6.3 8.1 

Eu ----------------------------- – – 2.28 2.25 2.39 2.1 2.1 2.19 2.32 1.43 – – 1.65 1.62 1.42 1.67 

Gd----------------------------- – – 12.3 6.4 9.8 7.9 7 7 7.2 5.1 – – 6.7 4.6 7.7 7.7 

Tb ----------------------------- – – 1.63 .86 1.03 .99 .91 1.06 .94 .63 – – .75 .72 .65 .89 

Tm ---------------------------- – – .72 .42 .28 .20 .28 .30 .35 .19 – – .25 .30 .20 .39 

Yb----------------------------- – – 4.52 2.5 2.5 2.6 2.2 2.5 2.6 1.4 – – 2.1 2.4 2 3.2 

Lu ----------------------------- – – .64 .36 .33 .35 .32 .34 .36 .19 – – .28 .35 .32 .45 

1Total Fe as Fe2O3.
Table 4. Chemical analyses of lavas of the Caldera suite—Continued Mammoth Mountain flows Extracaldera lava

Sample ----------------------- C18 C19 C20 C21 C22 C23 C24 C25 C26 C27 C28 C29 C30 

Field No --------------------- kDP72–5 kDP72–9 kDP72–2 kDP73–115 kDP74–85 DP73–25 DP73–31 DP73–119B kDP73–140 DP73–29 DP78–26 DP75–4 DP87–8 

Rock type ------------------- Dacite Dacite Dacite Dacite Dacite Rhyolite Rhyolite Rhyolite Rhyolite Basalt Basalt Basalt Basalt 

Map unit (figs. 4–6) ------ Qqm Qqm Qqm Qqm Qqm Qrm Qrm Qrm Qrm Qpb Qob Qb Qpb 

Locality---------------------- Summit SE. ridge NW. ridge NE. ridge Lincoln Basal flow N. face Summit Summit Horseshoe Buttresses Mammoth Devils 

Peak Lake Crest Postpile 

Latitude N------------------- 37°37.9′ 37°37.5′ 37°38.2′ 37°38.5′ 37°38.3′ 37°37.5′ 37°38.1′ 37°37.5′ 37°37.9′ 37°36.6′ 37°36.5′ 37°35.5′ 37°37.5′ 

Longitude W --------------- 119°2.1′ 119°1.2′ 119°2.8′ 119°1.7′ 119°3.0′ 119°2.5′ 119°1.7′ 119°2.5′ 119°2.1′ 119°1.1′ 119°6.2′ 119°1.3′ 119°5.1′ 

Major-element oxides SiO2* ------------------------- 64.42 66.21 67.87 68.75 68.87 70.77 70.41 70.73 70.55 49.72 51.35 50.33 54.66 

SiO2 --------------------------- 63.23 65.71 67.36 67.96 67.86 69.89 68.27 69.21 70.37 49.20 50.50 49.69 53.69 

TiO2--------------------------- .88 .69 .60 .58 .54 .40 .50 .4 .33 1.30 .90 1.57 1.77 

Al2O3 ------------------------- 16.38 16.56 16.11 15.44 15.32 15.45 14.92 15.33 15.73 14.90 15.10 17.84 18.06 

Fe2O3 ------------------------- 3.72 2.87 1.94 1.58 1.59 11.78 12.41 11.75 1.67 2.30 2.67 19.55 17.99 

FeO --------------------------- .62 .72 .98 1.21 1.21 n.a. n.a. n.a. .00 5.60 4.60 n.a. n.a. 

MnO -------------------------- .09 .07 .08 .09 .08 .11 .07 .11 .12 .06 .13 .14 .12 

MgO -------------------------- 1.56 .98 .86 .77 .78 .27 .59 .27 .26 10.00 10.15 6.51 3.31 

CaO--------------------------- 3.14 2.70 1.93 1.73 1.80 .89 1.44 .88 .77 10.10 8.63 9.20 7.35 

Na2O-------------------------- 4.32 4.87 5.01 4.87 4.84 5.29 4.40 5.25 5.67 2.90 3.16 3.63 4.02 

K2O --------------------------- 3.91 3.87 4.21 4.49 4.37 4.78 4.45 4.76 4.78 1.80 1.93 1.16 2.11 

P2O5--------------------------- .30 .20 .16 .14 .14 .08 .15 .07 .04 .80 .58 .41 .61 

H2O+ -------------------------- 1.66 .28 .27 .4 1.17 .54 (LOI) 2.19 (LOI) .59 (LOI) .04 .3 .75 −.15 (LOI) .31(LOI) 

H2O− -------------------------- .19 .14 .11 .04 .1 – – – – – – .01 n.a. .36 – – – – 

CO2 --------------------------- .18 .2 .21 .13 .09 n.a. n.a. n.a. n.a. .08 .19 n.a. n.a. 

 Total---------------------- 100.182 99.86 99.83 99.43 99.89 99.47 99.40 98.62 99.79 99.34 99.74 99.53 99.34 

Mg#--------------------------- 42 35 36 35 35 – – – – – – 24 70 72 – – – – 

Minor elements Ba ----------------------------- 1,459 1,475 1,783 1,518 1,423 1,440 1,290 1,450 1,644 1,450 1,240 576 1,040 

Co ----------------------------- 7.8 5.3 3.3 3.6 3.7 .381 2.57 .37 .3 42 37 39.5 19.9 

Cr------------------------------ 4.4 2.9 2.8 3.2 2.2 .65 .47 .685 4 475 479 82.1 28.9 

Cu ----------------------------- 10 5 5 4 5 – – – – – – – – – – – – – – – – 

Ga ----------------------------- 20 20 20 20 20 – – – – – – 28 – – 3.9 – – – – 

Hf ----------------------------- 6.7 7.4 7.5 7.6 7.7 9.7 7.40 9.72 9.9 4.1 8 3.77 4.6 

Nb----------------------------- – – – – – – – – – – – – – – – – – – – – – – – – – – 

Ni------------------------------ – – – – – – – – – – 4.34 5.42 6.06 – – 212 26 100 9.92 

Rb ----------------------------- 91 81 98 112 102 105 114 107 116 25 .8 19.1 45.2 

Sc------------------------------ 6.54 4.66 3.55 4.54 3.33 3.91 3.93 3.91 3.84 24.7 21.5 21.1 17.50 

Sr ------------------------------ 290 310 260 390 200 144 251 133 140 2,140 1,480 924 961 

Ta ----------------------------- 1.29 1.35 1.34 1.41 1.43 1.84 1.60 1.83 1.91 .55 .53 .76 1.09 

Th ----------------------------- 6.7 7 7.2 8.5 8.1 10.6 10.0 10.7 10.7 5.6 5.4 2.44 3.8 

U------------------------------- 2.3 2.2 2.5 2.6 2.7 3.34 3.08 3.4 3.3 1.39 1.2 .68 1.32 

V------------------------------- 59 42 28 39 31 – – – – – – – – – – 9 – – – – 

Y------------------------------- 38 30 25 36 32 – – – – – – 29 – – 80 – – – – Zn ----------------------------- 78 62 64 66 56 52.8 47.30 53.3 58 74 130 73.5 77.8 

Zr------------------------------ 333 380 375 345 406 397 286 400 419 216 – – 188 207 

La ----------------------------- 50 46 48 56 49 60.9 46.7 61.1 72 69 52 25.9 36.1 

Ce ----------------------------- 85 81 78 94 82 120 92 118 128 154 98 53.2 73.3 

Nd----------------------------- 39 36 27 38 32 47.5 36.6 46.4 51 71 50 28.5 37.7 

Sm----------------------------- 6.9 6.3 5.3 6.8 5.6 7.59 6.12 7.39 8.7 11.2 4.9 5.88 6.78 

Eu ----------------------------- 1.5 1.33 1.31 1.22 1.06 1.39 1.09 1.38 1.4 2.81 1.64 1.73 1.99 

Gd----------------------------- – – – – – – – – – – 5.55 4.63 5.92 7.3 7.07 – – 4.77 5.79 

Tb ----------------------------- .76 .68 .48 .76 .63 .805 .638 .806 .86 .81 .68 .69 .78 

Tm ---------------------------- – – – – – – – – – – .498 .384 .504 .45 .27 .36 .32 .37 

Yb----------------------------- 2.7 2.2 2.1 2.5 2.2 3.15 2.38 3.17 3.6 1.63 1.8 2.0 2.23 

Lu ----------------------------- .39 .33 .32 .38 .35 .489 .359 .481 .48 .24 .26 .28 .33 
Basin and Range suites are typically olivine-augite phyric. 

East Sierran trachyandesites typically contain olivine, augite, 

hypersthene, and plagioclase phenocrysts, whereas Basin 

and Range trachyandesites are predominantly hornblendeplagioclase phyric. East Sierran dacites are hornblende-biotite-plagioclase±pyroxene phyric, whereas Basin and Range 

dacites, like their associated trachyandesites, are hornblendeplagioclase phyric, with hornblende thoroughly altered to 

opaque Fe-Ti oxides-pyroxene-plagioclase aggregates. East 

Sierran dacites commonly contain mafic enclaves, whereas 

Basin and Range dacite generally lack them.

In contrast, postcaldera basalts and trachyandesites 

of both the Caldera and Mono-Inyo suites are dominantly 

plagioclase-phyric, with lesser amounts of olivine and augite. 

Postcaldera dacites vary widely in composition, verging 

toward rhyolite (70 weight percent SiO2). Except for the older 

northeast-moat dacite, they petrographically resemble precaldera East Sierran dacites rather than Basin and Range dacites, 

typically are hornblende-biotite-plagioclase±pyroxene phyric, 

and commonly contain mafic enclaves and basaltic, as well as 

rhyolitic, xenocrysts.

Major- and Trace-Element Chemistry

Previous Work

Long Valley as a cohesive volcanic entity did not attract 

the attention of petrologists until 1968, when it was first recognized as a caldera with a central resurgent dome (Smith and 

Bailey, 1968). However, earlier work by Gilbert (1938, 1941) 

on the Bishop Tuff and Glass Mountain, and by Rinehart and 

Ross (1964) and Huber and Rinehart (1967) on the geology of 

the Mount Morrison and Devils Postpile quadrangles, contributed significantly toward recognition of the caldera, which, in 

turn, influenced the selection of Long Valley as a focus of the 

USGS’ Geothermal Research Program in 1972 (Muffler and 

Williams, 1976). In addition to the seminal geochemical study 

of the Bishop Tuff by Hildreth (1979) and the USGS-supported work reported here and elsewhere (Bailey and others, 

1976; Bailey, 1983, 1984), several additional petrologic and 

isotopic studies, partly stimulated by the current (post-1980) 

seismic unrest at Long Valley, have contributed to understanding the evolution of the volcanic field: the work of Metz and 

Mahood (1991) and Halliday and others (1989) on the Glass 

Mountain rhyolite complex, of Sampson and Cameron (1987) 

and Kelleher and Cameron (1990) on the Inyo Craters and 

Mono Craters rhyolites, respectively, of Cousens (1996) on the 

postcaldera basaltic lava sequence, and of Heuman and Davies 

(1997) on the intracaldera Early and Moat Rhyolites.

Methods

The rock samples described and the chemical analyses 

reported here were collected between 1972 and 1987 during mapping of the volcanic field (Bailey, 1989). The field 

numbers listed in tables 2 through 5 show the quadrangle and 

year in which the sample was collected, for example, M72–21: 

M=Mount Morrison quadrangle, 72=1972, 21=21st sample 

collected that year. The analyses were performed in the USGS 

laboratories in Lakewood, Colo., Reston, Va., and Menlo Park, 

Calif. Whole-rock major-element analyses of rocks collected 

between 1972 and 1976 were performed by classical wetchemical methods at the Lakewood laboratory; major-element 

analyses of rocks collected after 1976 were performed by 

wavelength-dispersive X-ray-fluorescence spectroscopy, also 

at Lakewood. Trace-element analyses before 1976 were done 

by various methods, including quantitative photospectroscopy, 

atomic-absorption spectroscopy, and fluorometric analysis. 

After 1976, trace-element and rare-earth-element analyses 

were performed by energy-dispersive X-ray-fluorescence and 

neutron-activation analysis at Lakewood and Reston, respectively. The methods and precision and accuracy of USGS 

chemical analyses were evaluated by Taggart and others 

(1987) and Baedecker and McKown (1987). The few previously unpublished Sr-isotopic analyses of Long Valley lavas 

reported here, which were by R.W. Kistler (USGS, Menlo 

Park) and R.E. Chaudet (Virginia Polytechnic Institute), are 

listed in table 6, along with all other available published Sr-, 

Nd-, and Pb-isotopic analyses.

Precaldera Sequence

General Relations

On total-alkalis- and K2O-SiO2-variation diagrams (figs. 

7A, 8A) and, to some degree, on P2O5-, Rb-, Ba-, and Zr-SiO2-

variation diagrams (fig. 8B), the Central Sierran and East 

Sierran suites constitute three distinct subparallel trends with 

increasing SiO2 content, each at successively lower alkali contents: (1) Central Sierran basanite-tephrophonolite lavas, (2) 

Central Sierran basalt-trachyandesite lavas, and (3) East Sierran basalt-trachyandesite-dacite lavas. Compositional trends 

1 and 2 comprise the ultrapotassic and potassic mafic lavas of 

Van Kooten (1980, 1981); trend 3 comprises the East Sierran 

basalts and associated more highly evolved trachyandesites 

and dacites. A small group of Basin and Range lavas, somewhat more potassic than average, also fall within trend 2. Significantly, the three Sierran compositional trends are distributed generally west to east across the Sierra Nevada crest, with 

alkalinity decreasing systematically eastward. In addition, a 

striking feature of East Sierran trend 3 is the conspicuous compositional gaps between basalts, trachyandesites, and dacites 

on major- and trace-element-variation diagrams (figs. 7A, 8). 

In contrast, the Basin and Range suite, though closely following the East Sierran trend, shows a more continuous range of 

compositions, with increasing SiO2 content. This difference 

between the East Sierran and Basin and Range suites does not 

appear to be the result of limited or selective sampling because 

analyses from two other complementary, though smaller, data 

sets (Moore and Dodge, 1980; Ormerod and others, 1988) 

show similar patterns and compositional gaps.

Petrography and Chemistry 25
26 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

4 5 5 0 5 5 6 0 6 5 7 0 7 5

Basin and Range suite

East Sierran suite

Central Sierran suite

4 0

10

12

14

16

18

20

22

Al2O3 CONTENT, IN WEIGHT PERCENT

A

EXPLANATION

4 0 4 5 5 0 5 5 6 0 6 5 7 0 7 5

0.0

1.0

2.0

3.0

TiO2 CONTENT, IN WEIGHT PERCENT

40 45 50 55 60 65 70 75 0.0

2.0

4.0

6.0

8.0

10.0

12.0

FeO* CONTENT, IN WEIGHT PERCENT

40 45 50 55 60 65 70 75

0

2

4

6

8

10

12

14

MgO CONTENT, IN WEIGHT PERCENT

4 0 4 5 5 0 5 5 6 0 6 5 7 0 7 5

0

2

4

6

8

10

12

CaO CONTENT, IN WEIGHT PERCENT

4 0 4 5 5 0 5 5 6 0 6 5 7 0 7 5

1

2

3

4

5

6

Na2O CONTENT, IN WEIGHT PERCENT

4 0 4 5 5 0 5 5 6 0 6 5 7 0 7 5

0

2

4

6

8

10

K2O CONTENT, IN WEIGHT PERCENT

4 0 4 5 5 0 5 5 6 0 6 5 7 0 7 5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

P2O5 CONTENT, IN WEIGHT PERCENT

SiO2 CONTENT, IN WEIGHT PERCENT

Figure 8. SiO2-variation diagrams for the precaldera Central Sierran, East Sierran, and Basin and Range suites. 

A, Major elements. B, Selected trace elements. C, Rare-earth elements. Gray shading defines distinct fields of East 

Sierran basalt, trachyandesite, and dacite, showing compositional gaps; thin dashed outlines define more nearly 

continuous trend of the Basin and Range suite. Dotted vertical line at 53 weight percent SiO2 marks transition 

between basalt and trachyandesite.
40 45 50 55 60 65 70 75

0

50

100

150

Central Sierran suite

Basin and Range suite

East Sierran suite

EXPLANATION

B

Rb CONTENT, IN PARTS PER MILLION

40 45 50 55 60 65 70 75

0

10

20

30

40

Nb CONTENT, IN PARTS PER MILLION

40 45 50 55 60 65 70 75

0

1000

2000

3000

4000

5000

6000

Ba CONTENT, IN PARTS PER MILLION

40 45 50 55 60 65 70 75

0

5

10

15

Th CONTENT, IN PARTS PER MILLION

40 45 50 55 60 65 70 75

0

1

2

3

 

Ta CONTENT, IN PARTS PER MILLION

40 45 50 55 60 65 70 75

0

500

1000

1500

2000

2500

3000

Sr CONTENT, IN PARTS PER MILLION

40 45 50 55 60 65 70 75

0

100

200

300

400

500

600

700

800

Zr CONTENT, IN PARTS PER MILLION

40 45 50 55 60 65 70 75

0

10

20

30

40

50

60

Y CONTENT, IN PARTS PER MILLION

SiO2 CONTENT, IN WEIGHT PERCENT

Figure 8—Continued

Petrography and Chemistry 27
28 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

4 0 4 5 5 0 5 5 6 0 6 5 7 0 7 5 0

20

40

60

80

100

120

Central Sierran suite 

Basin and Range suite

East Sierran suite

La CONTENT, IN PARTS PER MILLION

C

EXPLANATION

40 45 50 55 60 65 70 75

0

25

50

75

100

125

150

175

Ce CONTENT, IN PARTS PER MILLION

55 60 65 70 75 40 45 50

0

20

40

60

80

100

Nd CONTENT, IN PARTS PER MILLION

40 45 50 55 60 65 70 75

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Eu CONTENT, PARTS PER MILLION

4 0 45 50 55 60 65 70 75 0.0

0.5

1.0

1.5

2.0

Tb CONTENT, IN PARTS PER MILLION

40 45 50 6055 65 70 75

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Tm CONTENT, IN PARTS PER MILLION

40 45 50 55 60 65 70 75

0

1

2

3

4

5

Yb CONTENT, IN PARTS PER MILLION

40 45 50 55 60 65 70 75

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

SiO2 CONTENT, IN WEIGHT PERCENT Lu CONTENT, IN PARTS PER MILLION

Figure 8—Continued
Basalt-Trachyandesite

In spite of mineralogic and textural similarities, Sierran 

and Basin and Range basalts (≤53 weight percent SiO2) differ 

chemically in both obvious and subtle ways (fig. 8). Most Central and East Sierran basalts have higher (at equivalent SiO2

contents) MgO, K2O, and P2O5 contents and lower Al2O3, TiO2, 

FeO*, and heavy-rare-earth-element (HREE) contents than 

Basin and Range basalts (figs. 8A, 8C). Ultrapotassic Central 

Sierran basalts also have mostly higher trace-element contents 

(Rb, Ba, Sr, Zr, Y), whereas most East Sierran and Basin and 

Range basalts are indistinguishable on the basis of these trace 

elements, including light-rare-earth elements (LREEs) (fig. 

8B, 8C). Many of these chemical differences are reflected 

geographically by progressive changes from west to east 

across the Sierra Nevada-Basin and Range transition (fig. 9). 

Except for the three westernmost ultrapotassic Central Sierran lavas and the unusually potassic Basin and Range basalts 

(which probably represent very small degrees of partial melting; see below), Mg number (Mg#=100×MgO/(MgO+FeO*)) 

decreases from 71–80 in the Central Sierra, through 68–73 

in the East Sierra, to 56–67 in the Basin and Range, whereas 

- 1 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 44

46

48

50

52

54

56

Central Sierran suite

East Sierran suite

SiO

Basin and Range suite

 CONTENT, IN WEIGHT PERCENT

2

7

1

3

6

1 0

1 1

9

1 2

4

8

5

2

EXPLANATION

CaO/MgO RATIO Ce/Yb RATIO Y CONTENT, IN PARTS PER MILLION

- 1 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0

50

60

70

80

90

Mg NUMBER

- 1 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0

0

1

2

3

4

5

6

7

8

9

10

K O CONTENT, IN WEIGHT PER CENT 

7

2

1 2

3

6

9

5

4

8

1 1

1 0

1

2

WEST-TO-EAST DISTANCE, IN KILOMETERS

- 1 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

- 1 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0

0

10

20

30

40

50

60

70

80

90

100

- 1 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0

0

10

20

30

40

50

60

Figure 9. Compositional characteristics of basaltic lavas versus distance from west edge of the Long Valley Volcanic Field, Calif. 

(0 on abscissa), showing chemical transition through the Central Sierran, East Sierran, and Basin and Range suites, across Sierra 

Nevada-Basin and Range transition. Solid green lines in the Central Sierran suite connect lavas erupted from same vent.

Petrography and Chemistry 29
30 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

CaO/MgO ratio increases on average from 0.76 in the Central 

Sierra, through 0.80 in the East Sierra, to 1.10 in the Basin and 

Range. Average chondrite-normalized Ce/Yb ratios [(Ce/Yb)n] 

also show a west-to-east-decreasing progression from 14 in 

the Central Sierra, through 12 in the East Sierra, to 9 in the 

Basin and Range. K2O and Y contents also generally decrease 

eastward.

These regional trends also are evident in primitivemantle-normalized trace-element- and chondrite-normalized 

rare-earth-element (REE) patterns (fig. 10). Although nearly 

all precaldera basalts have generally similar normalized traceelement patterns—typically enriched in large-ion-lithophile 

elements (LILEs: Rb, Ba, K, Sr) and depleted in high-fieldstrength elements (HFSEs: Ta, Nb, Ti)—subtle geographic 

differences are evident: Central Sierra ultrapotassic lavas 

have generally higher Rb, Ba, K, Sr, P, Zr, Tb, and Y contents 

than East Sierran and Basin and Range basalts. The eastward 

change in relative Y content seems particularly significant, 

wherein Central Sierran potassic lavas are predominantly 

Y enriched, East Sierran basalts show little Y enrichment 

or depletion, and Basin and Range basalts show varying Y 

enrichment and depletion, in that those enriched in Y are more 

alkalic than average and compositionally similar to Central 

Sierran basalts (see fig. 7B). Central Sierran ultrapotassic 

basalts show slight negative Eu anomalies (fig. 10), whereas 

East Sierran and Basin and Range basalts do not. For most elements, Basin and Range basalts (fig. 10) generally vary more 

widely and change less systematically with SiO2 content than 

Central and East Sierran basalts.

Comparison of the compositional trends of basalts relative to trachyandesites reveals other significant differences 

between the East Sierran and Basin and Range suites. For 

many relatively compatible elements (TiO2, FeO*, CaO, P2O5, 

REEs), the clustered East Sierran basalt analyses (figs. 8A, 8C) 

show steep negative subtrends with increasing SiO2 content 

that do not project toward the clustered East Sierran trachyandesitic compositions. In contrast, Basin and Range basalts, 

though showing considerably more compositional scatter, 

have compatible-element trends verging toward their associated trachyandesites. This contrast is particularly evident for 

REE contents (figs. 8C, 10C): East Sierran basalts evolved by 

depletion of both LREEs and HREEs within a limited SiO2

range, whereas Basin and Range basalts show only slight, if 

any, REE depletion over a broader SiO2 range.

Most precaldera basalts have primitive-mantle-normalized trace-element signatures resembling those of island-arc 

basalts (IABs), which in the continental Western United 

States are generally considered indicative of derivation from a 

lithospheric-mantle source (see discussion below; Fitton and 

others, 1988). However, a single alkali-olivine basalt in the 

Central Sierran suite (sample 12 of Van Kooten, 1980) has a 

trace-element signature resembling that of ocean-island basalt 

(OIB). In figure 11, the trace-element signature of this sample 

is compared with the signature of average OIB (Sun and 

McDonough, 1989), as well as with the range of signatures for 

OIB-like basalts from the Coso and Lunar Craters Volcanic 

Fields. The sample differs from them only slightly in its higher 

Ba and Th contents and lower Yb and Lu contents, suggesting 

contamination by upper-crustal rocks. Such OIB-like basalts in 

the Basin and Range Province are generally considered to be 

derived from an asthenospheric-mantle source. The sample 12 

flow also is distinctive in containing mafic lithospheric-mantle-derived peridotite and pyroxenite xenoliths (Van Kooten, 

1981; Van Kooten and others, 1985), further suggesting a 

sublithospheric-mantle source.

Dacite

East Sierran and Basin and Range dacites also show 

contrasting major- and trace-element compositions and trends 

(fig. 8). East Sierran dacites have higher K2O, P2O5, Rb, Th, 

Ta, Zr, Y, and REE contents than Basin and Range dacites. East 

Sierran dacites are separated from their associated trachyandesites by a conspicuous compositional gap, whereas Basin and 

Range dacites form a continuous trend with trachyandesites. 

This difference between East Sierran and Basin and Range 

trachyandesite-dacite trends is also evident in chondrite-normalized REE diagrams (fig. 12). East Sierran trachyandesites 

and dacites (fig. 12A) have weakly sigmoidal patterns, with a 

relatively small change in REEs with increasing SiO2 content; LREE contents are slightly higher, and HREE contents 

slightly lower, resulting in a crossover pattern pivotal on Eu. In 

contrast, Basin and Range trachyandesites and dacites (fig 12B) 

show straighter patterns and a relatively large overall depletion 

of REEs with increasing SiO2 content, resulting in a descending parallel pattern. Neither East Sierran nor Basin and Range 

dacites have significant Eu anomalies. Similar patterns are evident on primitive-mantle-normalized trace-element diagrams 

(fig. 12): East Sierran dacites, relative to their associated trachyandesites, are enriched in elements more incompatible than 

Sr and depleted in elements less incompatible than Sr, whereas 

Basin and Range dacites, relative to their associated trachyandesites, are progressively depleted in all elements regardless of 

relative incompatibility. As noted above, these contrasts suggest fundamentally different magmatic processes or different 

sources for the generation of East Sierran and Basin and Range 

trachyandesites and dacites (see section below entitled “Evolution of the Precaldera Sequence”).

Summary

Most precaldera basalts have island-arc-like mantle-normalized trace-element signatures, although they are somewhat 

more enriched in LILEs than typical island-arc basalts. The 

most significant difference between the East Sierran and Basin 

and Range suites is that the East Sierran suite shows significant composition gaps between basalt, trachyandesite, and 

dacite, whereas the Basin and Range suite shows a relatively 

continuous evolutionary trend from basalt to dacite. Central 

and East Sierran basalts have higher MgO, K2O, and P2O5

contents and lower Al2O3, TiO2, FeO, and HREE contents than 

Basin and Range basalts. Geographically, these differences 

are reflected by progressive west-to-east decreases in Mg 
1 0 1

10

100

1000

1 (53.4)

3 (51.6)

5 (50.8)

8 (44.5)

Rb Ba Th Ta Nb K La Ce Nd S r P Z r T i Tb Y Yb Lu

A

M g # = 6 8 - 7 9

C e n t r a l S i e r r a n EXPLANATION

b a s a n i t e - t e p h r i t e

0 1 0 0

1

10

100

1000

(Ce/Yb)n=8-20

LaCe Nd SmEu Tb TmYbLu

10

1

10

100

1000

 6 (50.8)

 9 (52.2)

10 (51.8)

Rb Ba Th Ta Nb K La Ce Nd Sr P Zr Ti Tb Y Yb Lu

B

Mg#=74-80

Central Sierran EXPLANATION

absarokite-shoshonite

ROCK/PRIMITIVE-MANTLE RATIO

ROCK/CHONDRITE RATIO 1000

100

10

1

(Ce/Yb)n=11-18

LaCe Nd SmEu Tb TmYbLu

1 0

S1 (50.0)

S2 (50.1)

S3 (51.5)

S4 (51.7)

S5 (52.2)

S6 (52.3)

S7 (52.4)

Rb Ba Th T a Nb K La Ce Nd S r P Z r T i Tb Y Yb Lu

C

M g # = 6 8 - 7 3

EXPLANATION East Sierran basalts

1

10

100

1000

0

1000

100

10

1

LaCe Nd SmEu Tb TmYbLu

(Ce/Yb)n=9-14

LaCe Nd SmEu Tb TmYbLu 0 10

B1 (49.4)

B3 (51.9)

B5 (54.2)

B4 (52.0)

B6 (52.8)

B7 (52.8)

B8 (53.0)

Rb Ba Th Ta Nb K La Ce Nd Sr P Zr Ti Tb Y Yb Lu

D

Mg#=56-67

Basin and Range EXPLANATION

basalts

1000

100

10

1

1000

100

10

1

(Ce/Yb)n=4-13

Figure 10. Variations in primitive-mantle-normalized trace-element and chondrite-normalized 

rare-earth-element contents of 

precaldera mafic lavas. A, Central 

Sierran basanite-tephrite lavas 

(data from Van Kooten, 1980). B, 

Central and East Sierran alkali 

basalt (data from Van Kooten, 

1980). C, East Sierran basalt. D, 

Basin and Range basalt. Numbers 

in explanations refer to samples 

1 through 12 of Van Kooten (1980) 

and samples listed in tables 2 and 

3; numbers in parentheses are 

SiO2 contents (in weight percent). Mg#, magnesium number. 

(Ce/Yb)n, chondrite-normalized 

Ce/Yb ratio.

Petrography and Chemistry 31
32 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

number, K2O content, Ce/Yb ratio, and Y content across the 

Sierra Nevada-Basin and Range transition. Chondrite-normalized REE patterns of East Sierran trachyandesites and dacites 

indicate simultaneous enrichment of LREEs and depletion 

of HREEs, whereas chondrite-normalized patterns of Basin 

and Range trachyandesites and dacites indicate evolution by 

Figure 11. Variations in primitive-mantle-normalized trace-element contents of ocean-island-basalt (OIB)-like lavas. A, Long 

Valley: alkali-olivine basalt sample 12 (Van Kooten, 1980) in 

comparison with OIB global average (Sun and McDonough, 1989). 

B, Lunar Craters Volcanic Field (Lum and others, 1989). C, Coso 

Volcanic Field (Bacon and Metz, 1984). Higher-than-average Ba, 

Th, and Sr contents in sample 12 (fig. 11A) may be due to cryptic 

crustal contamination.

0 1 0

1

100

1000

 OIB global average 

 #12 Central Sierran suite 

Rb Ba Th T a Nb K La Ce Nd S r P Z r T i Tb Y Yb Lu

EXPLANATION

10

0 1 0

1

10

100

1000

Rb Ba Th T a Nb K La Ce Nd S r P Z r T i Tb Y Yb Lu

EXPLANATION

Lunar Craters basalt

B

0 1 0 1

10

100

1000

Rb Ba Th T a Nb K La Ce Nd S r P Z r T i Tb Y Yb Lu

EXPLANATION

R O C K / P R I M I T I V E - M A N T L E R A T I O

Coso basalt (Pleistocene

A

C

progressive depletion of all REEs. The contrasting chemistries 

of East Sierran and Basin and Range basalts suggest derivation 

from different sources, and the divergent trends of their associated trachyandesites and dacites suggest an origin involving 

different magmatic processes.

Postcaldera Sequence

Postcaldera lavas in both the Caldera and Mono-Inyo suites 

are bimodal, with compositional gaps between trachyandesites 

and dacites (figs. 7C, 7D, 13) but not between basalts and 

trachyandesites. In the Caldera suite, the single sample within 

the trachyandesite-dacite gap (59–64 weight percent SiO2) is 

the hybrid “olivine-quartz latite” flow (sample C12, table 4), 

which, on the basis of petrography and chemistry, clearly is 

a commingled basalt and dacite. In the Mono-Inyo suite, the 

trachyandesitic-dacitic compositional gap is broader at lower 

SiO2 contents (54–63 weight percent SiO2), and all the samples 

within the gap represent extremely small volume mafic enclaves 

in dacite and rhyolite lavas that clearly represent mixed basalthost hybrids. The Caldera suite has slightly higher total alkali 

contents (both K2O and Na2O) than the Mono-Inyo suite (figs. 

7C, 7D, 13A), as reflected in their Peacock alkali-lime indices of 

56.2 and 57.5, respectively. The Caldera suite also has generally 

higher P2O5 and REE contents (figs. 13A, 13C).

Basalt-Trachyandesite

Caldera and Mono-Inyo basalts differ in subtle but significant ways. Like the suite as a whole, Caldera basalts have 

slightly higher total alkali contents than Mono-Inyo basalts 

(fig. 7C, 7D). The most mafic and oldest Caldera basalts (≤50 

weight percent SiO2, Mg#>60) include the Early Rhyolite 

mafic enclaves (sample C1, table 4), corehole Inyo-4 group 

V breccias, and two older flows in the south and west moats 

(samples C3, C4, table 4). Caldera basalts and trachyandesites 

show a nearly continuous range in composition up to about 

58 weight percent SiO2 but generally cluster into five progressively more silicic compositional groups, particularly evident 

in the corehole Inyo-4 samples (Vogel and others, 1994). The 

uppermost moat flows (intracaldera basalt-trachyandesite, fig. 

13; USGS analyses) have slightly higher Na2O, K2O, P2O5, and 

REE contents than the corehole Inyo-4 flows (Michigan State 

University analyses), suggesting that these uppermost flows 

may constitute a slightly more evolved chemical group (see 

subsection above entitled “Late Intracaldera Basalt-Trachyandesite Lavas”), but this difference could be due to analytical 

differences between the two laboratories.

On mantle-normalized trace-element diagrams (fig. 

14), nearly all Caldera basalts, including the Buttresses and 

Horseshoe Lake flows (fig. 14A), the most mafic basalts (fig. 

14B), and the more evolved porphyritic basalts (fig. 14C), 

are characteristically depleted in K relative to La. On chondrite-normalized REE diagrams, the sparsely phyric, parental 

Caldera basalts (fig. 14B) have relatively low LREE contents 

and moderate (Ce/Yb)n ratios (5), whereas the more evolved 
porphyritic basalts (fig. 14C) have higher LREE contents and 

higher (Ce/Yb)n ratios (8–11). Normalized trace-element patterns of Mono-Inyo basalts resemble those of Caldera basalts, 

except that Mono-Inyo basalts have more coherent patterns 

and apparent higher K/La ratios, resulting in a distinct K peak; 

however, this difference is due to their lower REE contents 

(figs. 14D) rather than higher K contents. Mono-Inyo basalts 

have sigmoidal rather than straight chondrite-normalized REE 

patterns but, like the most mafic Caldera basalts, have low 

(Ce/Yb)n ratios (6–8).

The Buttresses and Horseshoe Lake basalts, though 

temporally in the postcaldera sequence, plot within the 

precaldera East Sierran basalt range on Al2O3-, TiO2-, FeO*-, 

MgO-, CaO-, K2O-, and Ba-SiO2-variation diagrams (figs. 

13A, 13B) and are significantly more mafic than all other 

postcaldera basalts: the Buttresses with an Mg number of 72 

and Horseshoe Lake with an Mg number of 70. Like other 

Caldera basalts, however, both the Buttresses and Horseshoe 

Lake basalts are depleted in K relative to La (fig. 14A), partly 

because of their unusually high LREE contents. Thus, they 

have attributes of both precaldera East Sierran basalts and 

postcaldera Caldera basalts.

S11 (57.6)

S14 (60.2)

S15 (66.2)

S16 (67.3)

S17 (68.0)

S19 (68.1)

A EXPLANATION

1000

100

10

1

B10 (54.5)

B12 (56.8)

B16 (63.6)

B17 (63.7)

B18 (66.6)

B19 (69.1)

B EXPLANATION

1000

100

10

1

R O C K / P R I M I T I V E - M A N T L E R A T I O

R O C K / C H O N D R I T E R A T I O

Rb Ba Th Ta Nb K La Ce Nd Sr P Zr Tl Tb Y Yb Lu

Rb Ba Th Ta Nb K La Ce Nd Sr P Zr Tl Tb Y Yb Lu

57

66

63

1000

100

10

1

69

67

64

56

54

1

10

100

1000

La Ce Nd SmEu Tb TmYbLu

La Ce Nd SmEu Tb TmYbLu

Dacite-Rhyolite

The dacite lavas of the Caldera and Mono-Inyo suites, like 

the basalts and trachyandesites, also differ in subtle ways. Caldera “dacites” have higher total alkali contents than Mono-Inyo 

dacites, owing mainly to their higher Na2O contents (figs 7C, 

7D); thus, in the IUGS classification, they plot as trachydacite 

rather than dacite. Caldera trachydacites also have higher Al2O3

contents (fig. 13A) and slightly lower FeO*, MgO, and CaO 

contents than Mono-Inyo dacites, as well as generally higher 

Ba, Zr, Y (fig. 13B), and most REE contents (fig. 13C). These 

differences are particularly evident in normalized trace-element 

and REE diagrams (fig. 15). Porphyritic dacites of both suites 

(figs. 15B, 15C) have generally similar profiles, but relative to 

crystal-poor dacites (fig. 15A) they are more depleted in Sr, P, 

and Ti and more enriched in Rb, K, and Zr, resulting in more 

sharply spiked patterns. The porphyritic dacites have small 

negative Eu anomalies, whereas the crystal-poor dacites do not, 

indicating that the porphyritic dacites underwent plagioclase 

fractionation but the crystal-poor dacites did not. The low-SiO2

rhyolites of both suites (fig. 15D) are generally similar; relative 

to their associated dacites, they are more strongly depleted in 

Figure 12. Variations in primitive-mantle-normalized trace-element and chondrite-normalized rare-earthelement contents of precaldera trachyandesite-dacite lavas. A, East Sierran lavas. B, Basin and Range 

lavas. Note contrasting crossover and parallel patterns of East Sierran and Basin and Range lavas, respectively, and greater compositional change in Basin and Range lavas. Numbers in explanations denote samples listed in tables 2 and 3; numbers in parentheses and on figures are SiO2 contents (in weight percent).

Petrography and Chemistry 33
34 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

Figure 13. Comparative SiO2-variation diagrams for the Caldera and Mono-Inyo suites of postcaldera sequence. A, A′, 

Major elements. B, B′, Selected trace elements. C, C′, Rare-earth elements. Light-gray shading and thin dashed outlines in 

figures 13A, 13B, and 13C define trends of the East Sierran and Basin and Range suites, respectively, for comparison.

 Corehole Inyo-4 basalt-andesite

Caldera dacite-rhyolite

Unit Qer-Qmr enclaves

Caldera basalt-andesite

Q e r

Q m r

C 1 2

EXPLANATION

Al2O3 CONTENT, IN WEIGHT PERCENT

12

14

16

18

20

22

45 50 55 60 65 70 75

4 5 5 0 5 5 6 0 6 5 7 0 7 5

0

1

2

3

Q e r

Q m r C 1 2

TiO2 CONTENT, IN WEIGHT PERCENT

4 5 5 0 5 5 6 0 6 5 7 0 7 5

0

2

4

6

8

10

12

Q e r

Q m r

C 1 2

FeO* CONTENT, IN WEIGHT PERCENT

45 50 55 60 65 70 75

0

2

4

6

8

10

12

14

MgO CONTENT, IN WEIGHT PERCENT 

Qer

Qmr

C12

45 50 55 60 65 70 75

0

2

4

6

8

10

12

CaO CONTENT, IN WEIGHT PERCENT 

Q e r

Q m r

C 1 2

45 50 55 60 65 70 75

1

2

3

4

5

6

Na2O CONTENT, IN WEIGHT PERCENT 

Qer

Qmr

C12 NE

45 50 55 60 65 70 75

0

1

2

3

4

5

6

7

8

9

10

Qer

Qmr

C12

NE

K2O CONTENT, IN WEIGHT PERCENT P2O5 CONTENT, IN WEIGHT PERCENT

45 50 55 60 65 70 75

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

C 1 2

Q e r

Q m r

SiO2 CONTENT, IN WEIGHT PERCENT

A
Mono-Inyo enclaves

Mono-Inyo dacite

Mono-Inyo basalt

22

20

18

16

14

12

45 50 55 60 65 70 75

Al2O3 CONTENT, IN WEIGHT PERCENT

EXPLANATION

3

2

1

0

TiO

45 50 55 60 65 70 75

2 CONTENT, IN WEIGHT PERCENT

0

2

4

6

8

10

12

FeO*

45 50 55 60 65 70 75

 CONTENT, IN WEIGHT PERCENT

 

0

2

4

6

8

10

12

14

45 50 55 60 65 70 75

SiO2 CONTENT, IN WEIGHT PERCENT

MgO CONTENT, IN WEIGHT PERCENT

P2O5 CONTENT, IN WEIGHT PERCENT

0

2

4

6

8

10

12

CaO CONTENT, IN WEIGHT PERCENT

45 50 55 60 65 70 75

45 50 55 60 65 70 75

1

2

3

4

5

6

Na2O CONTENT, IN WEIGHT PERCENT

0

2

4

6

8

10

45 50 55 60 65 70 75

K2O CONTENT, IN WEIGHT PERCENT

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

45 50 55 60 65 70 75

A'

Figure 13—Continued

Petrography and Chemistry 35
36 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

45 50 55 60 65 70 75

0

50

100

Corehole Inyo-4 basalt-andesite

Caldera basalt-andesite

Unit Qer-Qmr mafic enclaves

Caldera dacite-rhylite

NE

Q e r

C 1 2

EXPLANATION

150

Rb CONTENT, IN PARTS PER MILLION

45 50 55 60 65 70 75 0

1000

2000

Ba CONTENT, IN PARTS PER MILLION 

Q m r

Q e r

C 1 2

1500

500

45 50 55 60 65 70 75

0

5

10

15

Th CONTENT, IN PARTS PER MILLION

NE

C 1 2

45 50 55 60 65 70 75

0

1

2

3

NE

C12

Ta CONTENT, IN PARTS PER MILLION

45 50 55 60 65 70 75

0

10

20

30

40

Nb CONTENT, IN PARTS PER MILLION

Qer

45 50 55 60 65 70 75

0

500

1000

1500

2000

Q e r

Qmr

NE

C12

Sr CONTENT, IN PARTS PER MILLION

45 50 55 60 65 70 75

0

100

200

300

400

500

600

NE

Qer

C12

Zr CONTENT, IN PARTS PER MILLION

45 50 55 60 65 70 75

0

10

20

30

40

50

 

Qer

C12

SiO2 CONTENT, IN WEIGHT PERCENT

Y CONTENT, IN PARTS PER MILLION

B

Figure 13—Continued
Mono-Inyo basalt-andesite

Mono-Inyo mafic enclaves

Mono-Inyo dacite

EXPLANATION

0

50

100

150

Rb

45 50 55 60 65 70 75

 CONTENT, IN PARTS PER MILLION

0

500

1000

1500

2000

45 50 55 60 65 70 75

Ba CONTENT, IN PARTS PER MILLION

15

10

5

0

45 50 55 60 65 70 75

Th CONTENT, IN PARTS PER MILLION

 55

3

2

1

0

45 50 60 65 70 75

Ta CONTENT, IN PARTS PER MILLION

40

30

20

10

0

Nb

45 50 55 60 65 70 75

 CONTENT, IN PARTS PER MILLION

2000

1500

1000

500

0

Sr

45 50 55 60 65 70 75

 CONTENT, IN PARTS PER MILLION

600

500

400

300

200

100

0

45 50 55 60 65 70 75

Zr CONTENT, IN PARTS PER MILLION

50

40

30

20

10

0

45 50 55 60 65 70 75

SiO2 CONTENT, IN WEIGHT PERCENT

Y CONTENT, IN PARTS PER MILLION

B'

Figure 13—Continued

Petrography and Chemistry 37
38 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

45 50 55 60 65 70 75

0

20

40

60

80

Qer

NE

C12

La CONTENT, IN PARTS PER MILLION

4 5 5 0 5 5 6 0 6 5 7 0 7 5

0

25

50

75

100

125

150

175

Qer

Qmr

NE

C12

Ce CONTENT, IN PARTS PER MILLION

45 50 55 60 65 70 75

0

20

40

60

80

100

Qer

NE

Qmr

C12

Nd CONTENT, IN PARTS PER MILLION

45 50 55 60 65 70 75

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Qer

Qmr

NE

C12

Eu CONTENT, IN PARTS PER MILLION

45 50 55 60 65 70 75

0.0

0.5

1.0

1.5

2.0

Corehole Inyo-4 basalt-andesite

Unit Qer-Qmr mafic enclaves

Caldera dacite-rhyolite

Caldera basalt-andesite

Qer

Qmr

NE

C12

EXPLANATION

Tb CONTENT, IN PARTS PER MILLION

45 50 55 60 65 70 75

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Qer

Qmr

NE

C12

Tm CONTENT, IN PARTS PER MILLION

45 50 55 60 65 70 75

0

1

2

3

4

5

Qer

Qmr

NE

C12

Yb CONTENT, IN PARTS PER MILLION

45 50 55 60 65 70 75

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Qer

Qmr

NE

C12

SiO2 CONTENT, IN WEIGHT PERCENT Lu CONTENT, IN PARTS PER MILLION

C

Figure 13—Continued
0

20

40

60

80

45 50 55 60 65 70 75

La CONTENT, IN PARTS PER MILLION

45 50 55 60 65 70 75

0

25

50

75

100

125

150

175

Mono-Inyo mafic enclaves

Mono-Inyo dacite

Mono-Inyo basalt-andesite

EXPLANATION

Ce CONTENT, IN PARTS PER MILLION

4 5 5 0 5 5 6 0 6 5 7 0 7 5

0

20

40

60

80

100

Nd CONTENT, IN PARTS PER MILLION

4 5 5 0 5 5 6 0 6 5 7 0 7 5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

SiO2 CONTENT, IN WEIGHT PERCENT

Eu CONTENT, IN PARTS PER MILLION

45 50 55 60 65 70 75

0.0

0.5

1.0

1.5

2.0

45 50 55 60 65 70 75

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Tb CONTENT, IN PARTS PER MILLION Tm CONTENT, IN PARTS PER MILLION

45 50 55 60 65 70 75

0

1

2

3

4

5

Tb CONTENT, IN PARTS PER MILLION

45 50 55 60 65 70 75

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Lu CONTENT, IN PARTS PER MILLION

C'

Figure 13—Continued

Petrography and Chemistry 39
40 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

10

1

10

100

1000

C28 Buttresses

C27 Horseshoe Lake

Rb Ba Th Ta Nb K La Ce Nd Sr P Zr Ti Tb Y Yb Lu

R O C K / P R I M I T I V E - M A N T L E R AT I O

EXPLANATION

East Sierran basalts

A

Compositional range of

10

1

10

100

1000

LaCe Nd SmEu Tb Tm YbLu

R O C K / C H O N D R I T E R AT I O

(Ce/Yb)n= 18-24

CALDERA

porphyritic basalts

CALDERA

parental basalts

MONO-INYO

basalts

1 0

1

10

100

1000

C4 (50.5)

C5 (50.3)

C6 (50.2)

C7 (51.7)

Rb Ba Th Ta Nb K La Ce Nd S r P Z r T i Tb Y Yb Lu

B EXPLANATION

10

1

10

100

1000

LaCe Nd SmEu Tb TmYbLu

(Ce/Yb)n= 8-11

10 0

1

10

100

1000

C3 (49.0)

Corehole Inyo-4 V (50.8)

Corehole Inyo-4 V (50.5)

Corehole Inyo-4 V (50.3)

C1 (49.7)

Rb Ba Th Ta Nb K La Ce Nd Sr P Zr Ti Tb Y Yb Lu

C EXPLANATION

10

1

10

100

1000

La Ce Nd SmEu Tb Tm Yb Lu

(Ce/Yb)n= 5-8

10

1

10

100

1000

M1 Red Cones (50.2) 

M4 June Lake vent (53.3) 

M5 June Lake flow (53.4)

M2 Black Point (50.5) 

M3 West wall (53.0)

Rb Ba Th Ta Nb K La Ce Nd Sr P Zr Ti Tb Y Yb Lu

D EXPLANATION

10

1

10

100

1000

La Ce Nd Sm Eu Tb TmYbLu

(Ce/Yb)n= 3-6

Figure 14. Variations in primitive-mantle-normalized trace-element and chondrite-normalized 

rare-earth-element contents of postcaldera basalts. A, Buttresses and Horseshoe Lake basalts 

relative to compositional range of East Sierran basalts. B, Caldera porphyritic basalts. C, Caldera 

parental basalts. D, Mono-Inyo basalts. Numbers in explanations refer to samples in tables 4 and 

5; numbers in parentheses are SiO2 contents. Note progressively lower light-rare-earth-element 

(LREE) contents (lower (Ce/Yb)n ratios) from figures 14A through 14D, particularly low trace-element and LREE contents in the Black Point and Red Cones basalts in figure 14D.
1 0

1

10

100

1000

M15 (62.8)

M16 (63.5)

M17 (65.3)

Rb Ba Th Ta Nb K La Ce Nd Sr P Zr Ti Tb Y Yb Lu

A EXPLANATION

10

1

10

100

1000

La Ce Nd Sm Eu Tb Tm Yb Lu

1 0

1

10

100

1000

M14 (67.5)

M18 (67.6)

M19 (67.6)

Rb Ba Th Ta Nb K La Ce Nd Sr P Zr Ti Tb Y Yb Lu

B EXPLANATION

R O C K / P R I M I T I V E - M A N T L E R A T I O

MONO-INYO

crystal-poor dacites

MONO-INYO

porphyitic dacites

CALDERA

porphyritic trachydacites

Mammoth Mtn. - Paoha Is.

low-SiO2 rhyolites

10

1

10

100

1000

La Ce Nd SmEu Tb TmYb Lu

1 0

1

10

100

1000

C14 (66.4)

C16 (68.7)

C18 (64.4)

Rb Ba Th Ta Nb K La Ce Nd Sr P Zr Ti Tb Y Yb Lu

C EXPLANATION

10

1

10

100

1000

La Ce Nd Sm Eu Tb TmYb Lu

R O C K / C H O N D R I T E R A T I O

1 0

1

10

100

1000

M20 (70.0)

M21 (70.6)

C26 (70.8)

C23 (70.7)

Rb Ba Th Ta Nb K La Ce Nd Sr P Zr Ti Tb Y Yb Lu

D EXPLANATION

10

1

10

100

1000

La Ce Nd Sm Eu Tb TmYb Lu

Figure 15. Variations in primitive-mantle-normalized trace-element and chondrite-normalized rare-earth-element contents of postcaldera dacites and rhyolites. A, Mono-Inyo aphyric 

dacites. B, Mono-Inyo porphyritic dacites. C, Caldera porphyritic dacites. D, Caldera (Mammoth 

Mountain) and Mono-Inyo (Paoha Island) low-SiO2 rhyolites. Numbers in explanations refer to 

samples listed in tables 4 and 5; numbers in parentheses are SiO2 contents (in weight percent). 

Note progressive decrease in Sr, P, and Ti contents and increase in Eu anomaly from figures 

15A through 15D.

Petrography and Chemistry 41
42 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

Sr, P, and Ti. Mammoth Mountain rhyolites (Caldera suite) and 

Paoha Island rhyolites (Mono-Inyo suite) have moderate negative Eu anomalies. Mono-Inyo rhyolites are noticeably more 

depleted in REEs than Caldera rhyolites.

Summary

Both the Caldera and Mono-Inyo suites are bimodal, with 

compositional gaps between trachyandesite and trachydacite/

dacite. The few samples within these gaps represent small-volume mafic enclaves in dacite and rhyolite lavas produced by 

mixing of basaltic magma with the host rocks. The most mafic, 

presumed parental, Caldera and Mono-Inyo basalts have low 

LREE contents and low (Ce/Yb)n ratios, suggesting derivation 

from an LREE-depleted source. Caldera basalts, including the 

Buttresses and Horseshoe Lake basalts, typically are depleted 

in mantle-normalized K relative to La. Although both Caldera 

and Mono-Inyo basalts have similar K2O contents, Mono-Inyo 

basalts have lower LREE contents, giving them a distinctive 

K peak on mantle-normalized diagrams (K/La ratios >1). Significantly, all Mono-Inyo lavas (basalts through rhyolites) have 

lower REE contents than comparable Caldera lavas. Caldera 

basalts and trachyandesites fall into five cyclical groups, each 

at progressively higher ranges in SiO2 content.

Caldera trachydacites are generally more coarsely porphyritic than Mono-Inyo dacites. Caldera trachydacites have 

slightly higher Na contents and small negative Eu anomalies, 

whereas Mono-Inyo dacites lack such anomalies. Mammoth 

Mountain (Caldera suite) and Paoha Island (Mono-Inyo suite) 

low-SiO2 rhyolites have lower Sr, P, and Ti contents, higher Zr 

contents, and larger negative Eu anomalies than their associated dacites. Mono-Inyo rhyolites are more depleted in Ba, Ta, 

Nb, Ti, and REE contents than Caldera rhyolites.

Comparison of the Precaldera and Postcaldera 

Sequences

Most major-oxide trends of the precaldera and postcaldera 

sequences (fig. 13A) are subparallel, but postcaldera lavas have 

slightly higher TiO2, FeO*, and Na2O contents and lower MgO 

and P2O5 contents. In the precaldera sequence, however, Al2O3

content increases with increasing SiO2 content, whereas in the 

postcaldera sequence Al2O3 content decreases with increasing 

SiO2 content. These differences apparently reflect crystallization of mafic minerals (olivine, pyroxene, amphibole) in 

the precaldera sequence and of plagioclase in the postcaldera 

sequence. Many postcaldera trace-elements trends (Rb, Th, 

Ta, Zr, Y, REEs) also depart from precaldera trends (figs. 13B, 

13C), particularly at higher than 60 weight percent SiO2. For 

some elements (Th, LREEs), postcaldera dacites/trachydacites 

fall in the same elevated range as precaldera East Sierran dacites.

Except for the Buttresses and Horseshoe Lake flows, 

postcaldera basalts have higher Al2O3, TiO2, and FeO* contents and lower MgO, P2O5, Rb, and Ba contents than precaldera basalts (figs. 13A, 13B). Mg numbers for the most mafic 

postcaldera basalts are lower (53–67) than for precaldera 

East Sierran basalts (68–80) and similar to those for Basin 

and Range basalts (56–67). Most postcaldera basalts have 

distinctly lower Ni and Cr contents than precaldera basalts 

(fig. 16), but similar Sc contents; the Buttresses and Horseshoe 

Lake flows, however, have MgO, Ni, and Cr contents similar 

to those of precaldera basalts. The postcaldera sequence has 

significantly higher Zr/Ba ratios (0.3–0.7) than the precaldera sequence (fig. 17) by virtue of its lower Ba contents. On 

mantle-normalized trace-element diagrams (fig. 18), postcaldera basalts are less enriched in LILEs (Rb, Ba, K, Sr) and less 

depleted in HFSEs (Ta, Nb, Ti) than precaldera basalts and 

thus show less strongly spiked profiles. A distinctive feature 

of most Caldera basalts is their lower mantle-normalized K/La 

ratios, owing to their relatively higher LREE and lower K contents. Caldera and Mono-Inyo basalts have significantly lower 

chondrite-normalized Ce/Yb ratios ((Ce/Yb)n=8–11 and 6–8, 

respectively) than most precaldera basalts ((Ce/Yb)n=9–20), 

and show a progressive decrease in LREE contents over time.

Postcaldera dacites, relative to precaldera dacites, have 

higher SiO2 contents, approaching low-SiO2 (70 weight percent SiO2) rhyolites, and have higher total alkalis, Rb, Ta, and 

Nb contents and lower Ba and Sr contents. One exception is 

the intracaldera hornblende-plagioclase-phyric dacite dome 

at the base of Glass Mountain, which for most elements plots 

in the field of precaldera Basin and Range dacites (figs. 13B, 

13C); like Basin and Range dacites, this dome is particularly 

depleted in REEs relative to all other postcaldera dacites (fig. 

13C), supporting the inference that it represents a postcaldera 

vestige of precaldera Basin and Range dacitic magma.

The overall impression gained from these chemical 

relations is that the postcaldera sequence was derived from a 

different source than the precaldera sequence and that the two 

sequences evolved by different magmatic processes.

Sr, Nd, and Pb Isotopes

Sampling

The Long Valley Volcanic Field has not been studied 

comprehensively for Sr, Nd, and Pb isotopes; the precaldera 

sequence is particularly sparsely sampled, especially for Nd. 

Van Kooten (1981) reported Sr- and Pb-isotopic data for 

ultrapotassic Central Sierran lavas. R.W. Kistler (unpub. data, 

1987) measured the Sr-isotopic ratios of 19 representative 

lavas but included only two basalts each from the precaldera 

and postcaldera sequences. Chaudet (1986) analyzed six 

basalts and three dacites from the East Sierran suite for Sr 

isotopes, and Ormerod (1988) and Ormerod and others (1988) 

analyzed a few basalts for Sr and Nd isotopes. Cousens (1996) 

measured Sr, Nd, and Pb isotopes for 39 postcaldera mafic 

lavas, including 12 from corehole Inyo-4. A few additional 

Sr- and Nd-isotopic measurements on Mono-Inyo lavas 

were made by Sampson and Cameron (1987), Kelleher and 

Cameron (1990), and Christensen and DePaolo (1993). All 
10 2 3 4 5 6 7 8 9 10 11 12

0

50

100

150

200

250

300

350

400

Corehole Inyo-4 basalt-andesite

Mono-Inyo suite

East Sierran suite

Basin and Range suite

Caldera suite

Ni CONTENT, IN PARTS PER MILLION 

A

Q p b

EXPLANATION

10 2 3 4 5 6 7 8 9 10 11 12

0

100

200

300

400

500

600

700

800

Cr CONTENT, IN PARTS PER MILLION

B

Q p b

0 1 2 3 4 5 6 7 8 9 10 11 12

0

5

10

15

20

25

30

35

40

MgO CONTENT, IN WEIGHT PERCENT 

Sc CONTENT, IN PARTS PER MILLION 

C

45 5 0 5 5 6 0 6 5 7 0 7 5

0.0

0.2

0.4

0.6

0.8

Basin and Range suite

East Sierran suite

Caldera suite

Mono-Inyo suite

Mono-Inyo mafic enclaves

EXPLANATION

SiO2 CONTENT, IN WEIGHT PERCENT

Z r / B a R AT I O

Figure 16. Ni, Cr, and Sc versus MgO contents of precaldera and postcaldera sequences. Note low Ni and Cr contents and high Sc contents of postcaldera suites relative to 

precaldera suites, suggesting preeruption settling of olivine 

and late crystallization of augite in postcaldera suites. Note 

also exceptionally high Ni and Cr contents of postcaldera 

parental basalt of Horseshoe Lake (unit Qpb, figs. 4–6).

Figure 17. Zr/Ba ratio versus SiO2 content of precaldera and 

postcaldera sequences. Note high (>0.2) Zr/Ba ratios of postcaldera lavas and, particularly, of Mono-Inyo mafic magmatic 

enclaves, suggesting possible involvement of mafic lower crust.

available Sr-, Nd-, and Pb-isotopic data are listed in table 6. The 

locations of Sr-isotopic samples and their respective isotopic 

ratios are mapped in figure 19. 87Sr/86Sr, 143Nd/144Nd, 206Pb/ 204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios are plotted in figures 

20 and 21. Because of the complexity of regional relations and 

the paucity of isotopic data for the Long Valley Volcanic Field, 

the generalizations presented below are tentative and subject to 

revision as more data become available.

Regional Relations: Sri=0.706 Isopleth

The interpretation of isotopic data from the Long Valley 

Volcanic Field is complicated by the fact that in its vicinity, 

the Sri=0.706 isopleth of Kistler and Peterman (1973), and the 

west edge of North American crystalline basement inferred to 

coincide with it, are offset by major northwest-trending Late 

Cretaceous-early Tertiary transcurrent faults (IBB2, IBB3; 

inset, fig. 19A; Kistler, 1993). These faults coincide with 

strands of the present East Sierran frontal fault zone, which 

passes through the middle of Long Valley Caldera. The position 

and trend of this fault zone, which presently displays mainly 

Pliocene-Holocene dip-slip offset, are evidently controlled by 

these older transcurrent structures. The southwest-trending segment of the Sri

=0.706 isopleth in the middle of the caldera (fig. 

19) is displaced along the East Sierran frontal fault zone (IBB3; 

inset, fig. 19A) 175 km northwestward to the vicinity of Lake 

Tahoe. The westward continuation of the displaced isopleth 

arcs southwestward from Lake Tahoe through the western Sierran foothills and passes 30 km to the west of the caldera, just 

west of the ultrapotassic Central Sierran lavas at the southwest 

edge of the volcanic field, where it locally coincides with the 

contact between North American crystalline basement on the 

east and Mesozoic accreted terranes on the west. By inference, the northeast sector of the volcanic field is underlain by 

Paleozoic eugeosynclinal rocks, and the northwest, southwest, 

and southeast sectors by Paleozoic miogeosynclinal rocks and 

Sr, Nd, and Pb Isotopes 43
44 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

Precambrian North American crystalline basement (Kistler 

and Peterman, 1973). Kistler and Peterman’s (1973) Sri=0.706 

isopleth is defined by the Sr-isotopic ratios of midcrustal to 

upper-crustal Mesozoic plutonic rocks, which they inferred to 

be partly derived from mafic sources in the lithospheric mantle 

(Kistler and Peterman, 1978), citing as evidence that associated 

Mesozoic mafic volcanic rocks have isotopic ratios and distributions similar to those of the plutons. If Long Valley lavas 

are derived from this same lithospheric-mantle source, their 

Sr-isotopic ratios should mimic those of coextensive Mesozoic 

plutons and volcanic rocks and show similar changes across 

the Sri=0.706 isopleth, as appears to be so for most precaldera 

basalts but not for postcaldera basalts.

Long Valley General Isotopic Relations

Sr-isotopic ratios for most precaldera basalts conform 

approximately to the Sri

=0.706 isopleth (fig. 19A) as defined 

by Kistler and Peterman (1973), although Sr-isotopic ratios 

for the Basin and Range suite (87Sr/86Sr=0.70603–0.70613) 

east of the East Sierran frontal fault zone (IBB3; Kistler, 

1993) are slightly lower than those for the Central and East 

Sierran suites (87Sr/86Sr=0.70618–0.70670) west of the fault 

zone. A precaldera basalt north of the caldera and north of the 

Sri=0.706 isopleth has a seemingly anomalously high ratio 

(

87Sr/86Sr=0.70613) but probably falls within the plotting error 

of the isopleth as defined by Kistler and Peterman (1973). 

The low Sr-isotopic ratios of Central Sierran suite samples 

5 (87Sr/86Sr=0.70597), 9 (87Sr/86Sr=0.70598), and 12 (87Sr/ 86Sr=0.70430) (red numbers, fig. 19A) are consistent with an 

origin, based on chemistry, as OIB melts or crustally contaminated OIB melts from an asthenospheric mantle source.

Most postcaldera basalts (fig. 19B) were erupted 

from vents south and west of the Sri=0.706 isopleth and, 

like most precaldera basalts, have Sr-isotopic ratios higher 

than 0.706, consistent with derivation from a lithosphericmantle source. However, the most mafic postcaldera 

basalts of both the Caldera and Mono-Inyo suites have 

Sr-isotopic ratios less than 0.706, including the oldest Caldera basalts: corehole Inyo-4 groups IV and V 

(87Sr/86Sr=0.70516–0.70597) and the youngest Mono-Inyo 

basalts at Black Point (87Sr/86Sr=0.70538–0.70545) and Red 

10

1

10

100

1000

S1 (50.0)

S2 (50.1)

S3 (51.5)

S4 (51.7)

S5 (52.2)

S6 (52.3)

Rb Ba Th Ta Nb K La Ce Nd Sr P Zr Ti Tb Y Yb Lu

Mg#= 70-67

A EXPLANATION basalts

East Sierran

ROCK/PRIMITIVE-MANTLE RATIO

0 1 0 1

10

100

1000

B1 (49.4)

B3 (51.9)

B4 (52.0)

B6 (52.7)

B7 (52.8)

B8 (52.8)

Rb Ba Th Ta Nb K La Ce Nd S r P Z r T i Tb Y Yb Lu

Mg# 67-58

B EXPLANATION

basalts

Basin and Range

0 1 0 1

10

100

1000

C4 (50.5)

C6 (50.9)

C8 (51.9)

C3 (49.0)

C1 (49.2)

Rb Ba Th Ta Nb K La Ce Nd S r P Z r T i Tb Y Yb Lu

Mg# 61-43

C Caldera basalts EXPLANATION

0 1 0 1

10

100

1000

M1 Red Cones (50.2)

M4 June Lake (53.3)

M2 Black Point (50.5)

M3 West wall (53.0)

Rb Ba Th Ta Nb K La Ce Nd S r P Z r T i Tb Y Yb Lu

M g # = 6 3 - 4 8

D Mono-Inyo basalts EXPLANATION

Figure 18. Variations in primitive-mantle-normalized trace-element profiles of precaldera and postcaldera basalts. A, East 

Sierran basalts. B, Basin and Range basalts. C, Caldera basalts. 

D, Mono-Inyo basalts. Numbers in explanations refer to samples 

listed in tables 2 through 5; numbers in parentheses are SiO2

contents (in weight percent). Mg#, magnesium number. Note 

lower Rb, Ba, and K contents of postcaldera basalts (Caldera 

and Mono-Inyo suites) relative to precaldera basalts in the East 

Sierran and Basin and Range suites. Note also relatively high La, 

Ce, and Nd contents (K/La<1) of Caldera primitive basalt (C1, red) 

and lower La, Ce, and Nd contents (K/La>1) of primitive Mono-Inyo 

basalt (M1, red).
38°00'

45'

30'

30' 15' 119°00' 45'

0 5 10 15 KILOMETERS

MONO

LAKE 

B 0.70538

 0.70526

 0.70616

 0.70633

 0.70634

 0.70620

 0.70629

 0.70597

 0.70623

 0.70628 0.70620 0.70591

 0.70670

 0.70633

 0.70521

 0.70628

 0.70629

 0.70596

 0.70613

 0.70646

38°00'

45'

30'

30' 15" 119°00' 45'

 0.70608

 0.70594

 0.70613

 0.70603

 0.70608 0.70623

4 0.70674

10 0.70648

12 0.70430

11 0.70643

8 0.70653

9 0.70598

3 0.70606 5 0.70597

6 0.70643

1 0.70628

2 0.70624

7 0.70628

 0.70601

 0.70695

 0.70629

LON G VALLEY CALDE RA

MONO

LAKE 

A

CALIFORNIA

NEVADA Lake Tahoe

IBB2

IBB3

Sri > 0.706

Sri < 0.706

LV

Sri = 0.706

Mono Lake

Sri > 0.706

Sri < 0.706

Sri < 0.706

Sri > 0.706

HSF

HCF

HSF

HCF

Figure 19. Sketch map of central part of the Long Valley Volcanic Field, Calif., showing location 

of Long Valley Caldera (dashed outline) and distribution of Sr-isotopic samples of precaldera and 

postcaldera basalts. A, 87Sr/86Sr ratio of precaldera basalts. B, 87Sr/86Sr ratio of postcaldera basalts. 

Long- and short-dashed lines in inset are Sri

=0.706 isopleth and faults IBB2 and IBB3, respectively 

(after Kistler, 1993). Shaded dashed line in figures 19A and 19B is Long Valley segment of Sri

=0.706 

isopleth. Data from table 6. HCF, Hilton Creek Fault; HSF, Hartley Springs Fault.

Sr, Nd, and Pb Isotopes 45
46 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

Cones (87Sr/86Sr=0.70516–0.70531). Such relatively low 

Sr-isotopic ratios are inconsistent with derivation from a 

lithospheric-mantle source. These precaldera and postcaldera 

isotopic relations are discussed further in the following subsections and plotted in figures 20 and 21.

Sr and Nd Isotopes

Sr- and Nd-isotopic ratios of Long Valley basalts are 

plotted in figure 20A, together with the isotopic ranges of local 

Mesozoic Sierra Nevadan batholithic plutons and the global 

ranges for OIB and MORB. Sr-isotopic ratios of Long Valley 

basalts fall in the least radiogenic range for Sierran plutons, 

and their least radiogenic Sr-isotopic ratios (Mono-Inyo 

basalts) are skewed toward the ranges for MORB and Basin 

and Range OIB-like basalts represented by Lunar Craters, 

Nev., and Amboy and Pisgah Craters, Calif. (fig. 20B.).

As noted above, precaldera Central and East Sierran 

basalts (fig. 20A), with few exceptions, have higher 87Sr/86Sr 

ratios than Basin and Range basalts. The few 143Nd/144Nd ratios 

that are available for Central Sierran and East Sierran basalts 

appear to be lower than for Basin and Range basalts (Nielsen 

and others, 1991).

Postcaldera basalts, on average, have slightly lower 87Sr/86Sr and higher 143Nd/144Nd ratios than precaldera basalts 

(fig. 20B). In particular, the most mafic (“parental,” fig. 20B) 

Caldera basalts (table 6; corehole Inyo-4 group V breccias, 

intracaldera south-moat 100-ka Hot Creek basalt flow, and 

extracaldera Soda Springs [Devils Postpile] basalt) plot in a 

tight cluster at lower Sr- and slightly higher Nd-isotopic ratios 

than the majority of Caldera basalts. In contrast, the Buttresses 

and 100-ka Horseshoe Lake basalts have significantly higher 

Sr- and lower Nd-isotopic ratios than most Caldera basalts. 

Individually, however, they have quite distinct Sr- and Ndisotopic ratios: the Buttresses flows plot within the range of 

precaldera basalts, whereas the Horseshoe Lake flows have 

distinctly higher Sr- and Nd-isotopic ratios, near the trend for 

most other postcaldera basalts.

Some Mono-Inyo basalts have the lowest Sr- and highest Nd-isotopic ratios of the all Long Valley basalts; they also 

decrease in 87Sr/86Sr ratio and increase in 143Nd/144Nd ratio, as 

well as become more mafic in composition, over time from June 

Lake (30 ka) through Black Point (13 ka) to Red Cones (5 ka).

Few isotopic data are available for Long Valley dacites. 

Three precaldera East Sierran dacites (table 6; Chaudet, 1986) 

have 87Sr/86Sr ratios (0.70724–0.70949) significantly higher 

than those of their associated basalts and trachyandesites 

(0.70618–0.70702). In contrast, the few measured postcaldera 

dacites have Sr- and Nd-isotopic ratios similar to those of 

their associated basalts (table 6; Kelleher and Cameron, 1990; 

Cousens, 1996).

The contrasting Sr-isotopic ratios of basalts, trachyandesites, and dacites within and between the precaldera and 

postcaldera sequences are plotted in figure 21. At comparable 

SiO2 contents, most Central and East Sierran basalts (fig. 21A) 

0.702 0.704 0.706 0.708 0.710 0.712

0.5120

0.5122

0.5124

0.5126

0.5128

0.5130

0.5132

0.5134

Lunar Craters basalts

Postcaldera sequence basalts

Precaldera sequence basalts

Sierran granitoids 

Amboy-Pisgah Craters basalts

EXPLANATION

1 4 3 N d / 1 4 4 N d R AT I O

8 7 S r / 8 6 S r R AT I O

EXPLANATION

Fig. 20B

MORB

OIB

A

0.705 0.706 0.707

0.5122

0.5124

0.5126

0.5128

0.5130

Corehole Inyo-4 groups I-III 

Mono-Inyo basalt

Basin and Range basalt

Buttresses-Horseshoe Lake basalt

Corehole Inyo-4 groups IV-V

Caldera basalt

H

B

B P

RC

Parental J L

B

Figure 20. Sr- versus Nd-isotopic ratios for Long Valley basalts. A, 87Sr/86Sr versus 143Nd/144Nd ratios for Long Valley basalts in comparison with isotopic range of local Sierra Nevada Mesozoic granitoids 

(Kistler, 1993), as well as general isotopic range of ocean-island 

basalt (OIB) and midocean-ridge basalt (MORB) (after Menzies, 

1989) and isotopic composition of Lunar Craters and Amboy-Pisgah 

Craters OIB-like basalts (Lum and others, 1989; Glazner and others, 

1991) from the Basin and Range Province. B, Enlargement of figure 

20A, showing detailed relations among precaldera and postcaldera 

basalts. B, Buttresses basalts; BP, Black Point basalt; H, Horseshoe 

Lake basalt; JL, June Lake basalt; RC, Red Cones basalt. Note compositional shift of Caldera parental basalts and Mono-Inyo basalts 

(BP, RC) toward OIB-like Lunar Craters and Amboy-Pisgah Craters 

basalts. Three “parental” units are discussed in text.

have perceptibly higher 87Sr/86Sr ratios than Basin and Range 

basalts. Central Sierran OIB-like basalt (sample 12) plots well 

below the range for all other precaldera basalts. East Sierran 

dacites have widely scattered, high 87Sr/86Sr ratios in the same 

general range as local Mesozoic plutons, whereas Basin and 

Range dacites, in contrast, have lower 87Sr/86Sr ratios similar to 

those of their associated basalts. In the postcaldera sequence 

(fig. 21B), Caldera basalts and trachyandesites show a perceptible increase in 87Sr/86Sr ratio with increasing SiO2 content, 
consistent with progressive contamination by a more radiogenic component. 87Sr/86Sr ratios of some Mono-Inyo basalts 

cluster well below those of Caldera basalts. Caldera dacites 

have 87Sr/86Sr ratios only slightly higher than their associated 

basalts and trachyandesites. Mono-Inyo dacites have 87Sr/86Sr 

ratios similar to those of Caldera dacites; however, these ratios 

are considerably higher than those of their associated basalts, 

suggesting that the Mono-Inyo dacites are isotopically unrelated to the basalts.

Pb Isotopes

Pb-isotopic ratios (fig. 22) for postcaldera basalts are 

generally higher than for precaldera basalts; postcaldera 

basalts have significantly higher 206Pb/204Pb ratios than precaldera basalts, but overlapping 207Pb/204Pb and 208Pb/204Pb 

ratios. Precaldera potassic Central Sierran basalts (fig. 22) 

form four distinct groups, mostly distinguishable on the basis 

of 206Pb/204Pb ratios: I (sample 8, basanite), II (samples 1–7), 

III (samples 9–11), and IV (sample 12, alkali olivine basalt, 

which has a higher 206Pb/204Pb ratio and a lower 87Sr/86Sr ratio). 

OIB-like basalt (sample 12; fig. 22), plots within the Pb-Srisotopic range for other OIB-like basalts in the Basin and 

Range Province, as typified in figure 22 by Coso basalts. The 

few Pb-isotopic data for precaldera Basin and Range basalts 

overlap group II (Central Sierran samples 1–7), as do those for 

the chemically similar postcaldera Buttresses basalts.

Postcaldera basalts as a group are isotopically more 

radiogenic in Pb than precaldera basalts, but Caldera and 

Mono-Inyo basalts are indistinguishable on the basis of Pb isotopes, as previously noted by Cousens (1996). Note, however, 

that the Horseshoe Lake basalts plot in the range for postcaldera basalts, as expected, whereas the postcaldera Buttresses 

basalts plot in the range for precaldera basalts.

Petrogenesis

General Introduction

The origin and source of Basin and Range basalts have 

been controversial and much-debated subjects (Menzies and 

others, 1983; Fitton and others, 1988; Menzies, 1989; Kempton and others, 1991). Cenozoic Basin and Range basalts 

typically exhibit two distinctive mantle-normalized trace-element signatures (fig. 23; Fitton and others, 1988; Leeman 

and Harry, 1993): (1) those with simple, upwardly convex 

normalized trace-element profiles (fig. 23B) and low Sr- and 

high Nd-isotopic ratios typical of OIB basalts; and (2) those 

depleted in HFSEs (Ta, Nb, Ti) and enriched in LILEs (Rb, 

Ba, K, Sr) (fig. 23A), with generally higher Sr- and lower 

Nd-isotopic ratios, that typically show subduction-zone IABlike normalized trace-element signatures, strongly depleted 

in Ta and Nb and with a distinct decrease in elements less 

incompatible than Zr. In the Basin and Range Province, 

basalts with the first signature are sometimes considered 

to represent subcontinental-asthenospheric-mantle melts, 

while basalts with the second signature are considered to be 

derived by partial melting of continental-lithospheric mantle, 

which has acquired its IAB-like pattern from ancient (Proterozoic) subduction processes, subsequently modified by LILE 

enrichment (Kempton and others, 1991). The low Ta, Nb, 

and Ti contents are believed to be due to retention in such 

phases as ilmenite and sphene in the subducting slab, and 

the high LILE contents to be due to metasomatic enrichment 

40 45 50 55 60 65 70 75 0.704

0.705

0.706

0.707

0.708

0.709

0.710

Central Sierran suite

Basin and Range suite

East Sierran suite

Sierran granitoids

1 2

A

8

EXPLANATION

40 45 50 55 60 65 70 75

0.704

0.705

0.706

0.707

0.708

0.709

0.710

Caldera basalts

Caldera dacite

Sierran granitoids

Mono-Inyo dacite

Mono-Inyo basalt

Corehole Inyo-4 groups IV, V

Corehole Inyo-4 groups I-III

B

B

H

Qer parental

RC

B P

J L

EXPLANATION

8 7 S r / 8 6 S r R AT I O

SiO2 CONTENT, IN WEIGHT PERCENT

Figure 21. Sr-isotopic ratio versus SiO2 content of Long Valley 

basalts. Data from table 6. A, Precaldera sequence. Note higher 87Sr/86Sr ratios of the Central and East Sierran suites relative to 

the Basin and Range suite, as well as much higher and more 

diverse 87Sr/86Sr ratios of East Sierran trachyandesite and dacite, 

similar to Sierra Nevada granitoids. B, Postcaldera sequence. 

Note low 87Sr/86Sr ratios of Mono-Inyo basalts (BP, RC) relative 

to Caldera basalts. as well as low 87Sr/86Sr ratios of postcaldera 

dacite relative to precaldera dacite (fig. 21A). Note also gradual 

increase in isotopic ratios with SiO2 content for Caldera basalttrachyandesite lavas, suggesting contamination by Sierran 

granitoids or dacites.

Petrogenesis 47
48 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

by hydrous fluids released from the slab (Fitton and others, 

1988; Saunders and others, 1988). Although some workers 

(Thompson and others, 1983, 1984; Wenrich and others, 

1995) attribute the IAB-like signature of continental basalts 

to contamination of asthenospheric-mantle melts by crustal 

rocks, most workers (Fitton and others 1988, 1991; Lum 

and others, 1989; Kempton and others, 1991; Ormerod and 

others, 1991; Bradshaw and others, 1993; Leeman and Harry, 

1993; Asmerom and others, 1994; Farmer and others, 1995; 

Turner and Hawkesworth, 1995; Reid and Ramos, 1996; 

Yogodzinski and others, 1996) consider crustal contamination to be of minor importance in the province, although 

examples of crustal contamination are common (Bacon and 

Metz, 1984; Glazner and others, 1991; Glazner and Farmer, 

1992).

Kempton and others (1991) reviewed the arguments 

against significant crustal contamination in basin and Range 

IAB-like basalts, noting that (1) chemical modeling by mixing of asthenospheric mantle (OIB) and upper-crustal end 

members requires incorporation of 150 to 400 percent crustal 

component, producing model basaltic compositions with 

unrealistically high SiO2 contents; (2) modeling of Sr-, Nd-, 

and Pb-isotopic compositions also yields basalts with unrealistically high SiO2 contents; and (3) Sm/Nd- and Pb-isotopic 

model ages indicate a source with an age in the range 1.5–2.0 

b.y., similar to that of the lithosphere beneath most of the 

Western United States. Additionally, Mukasa and Wilshire 

(1997) analyzed undisputed lithospheric-mantle xenoliths in 

Mojave Desert basalts, showing that they have typical IABlike signatures depleted in Nb, Ta, and Ti. Also, Ormerod and 

others (1991) argued that the depleted-Nb-Ta signatures of 

Late Cenozoic Big Pine basalts, considered to be partial melts 

of lithospheric-mantle spinel peridotites, reflect low source 

abundances rather than mineralogic effects produced during 

Cenozoic partial melting.

Spatial and temporal relations of OIB- and enriched 

IAB-like Cenozoic lavas in the Basin and Range Province also 

indirectly support their respective asthenospheric- and lithospheric-mantle sources. In several volcanic fields—for example, Lunar Craters, Nev. (Lum and others, 1989), the Colorado 

River Trough, Nev.-Ariz. (Bradshaw and others, 1993; Feuerbach and others, 1993), and the Coso Volcanic Field (Bacon 

and Metz, 1984; Novak and Bacon, 1986)—OIB-like basalts 

typically postdate enriched IAB-like basalts. OIB-like basalts 

also are generally localized along narrow north-trending zones 

of extreme extension (>200 percent) where heat flow is high 

and geophysical and seismic studies indicate a relatively thin 

lithosphere and a shallow asthenosphere. Leeman and Harry 

(1993) provide an explanation based on theoretical considerations for these temporal and spatial relations, suggesting that 

they are the natural consequence of decompression melting in 

regions of extending and thinning lithosphere. They theorize, 

on the basis of experimental data, that melting occurs first 

in the lower lithospheric mantle (mainly in its lowermost 25 

km) and then in the subjacent asthenosphere when it has risen 

above 80-km depth because of lithospheric thinning. Thus, 

18.6 18.7 18.8 18.9 19.0 19.1 19.2 19.3 19.4

0.703

0.704

0.705

0.706

0.707

0.708

Corehole Inyo-4 groups IV, V

Mono-Inyo basalt

Caldera basalt

Buttresses-Horseshoe Lake basalt

Central Sierran basalt

Basin and Range basalt

Coso basalt (Pleistocene)

H

12

11

10

9

8

A

1-7

EXPLANATION

B

parental

8 7S r / 8 6S r R AT I O

JL

BP

RC

1-7

18.6 18.7 18.8 18.9 19.0 19.1 19.2 19.3 19.4 15.55

15.60

15.65

15.70

15.75

15.80

15.85

12 B

H

8

9

10

11

B

2 0 7P b / 2 0 4P b R AT I O

18.6 18.7 18.8 18.9 19.0 19.1 19.2 19.3 19.4 38.2

38.3

38.4

38.5

38.6

38.7

38.8

38.9

39.0

39.1

39.2

8

12

9

10

11

B

H

C

1-7

2 0 6P b / 2 0 4P b R AT I O

2 0 8P b / 2 0 4P b R AT I O

Figure 22. Comparative Sr- and Pb-isotopic ratios of precaldera 

and postcaldera basalts. Data from table 6. A, 87Sr/86Sr versus 206Pb/204Pb ratios; B, 207Pb/204Pb versus 206Pb/204Pb ratios; C, 208Pb/ 204Pb versus 206Pb/204Pb ratios. B, Buttresses basalt; BP, Black Point 

basalt; H, Horseshoe Lake basalt; JL, June Lake basalt; RC, Red 

Cones basalt. “Parental,” Caldera parental basalts. Dashed green 

lines enclose and connect precaldera Central Sierran basalts; 

dashed red line encloses Caldera parental basalts; dotted blue 

connects Coso basalts. Note similar isotopic ratios of sample 12 

(OIB-like alkali olivine basalt) and Coso basalt (figs. 22A, 22B). 

Note that in spite of similarity in age and chemical and mineralogic composition, Buttresses basalt plots with precaldera basalts, 

whereas Horseshoe Lake basalt plots with postcaldera basalts.
Table 5. Chemical analyses of lavas of the Mono-Inyo suite.

[Major-element-oxide contents in weight percent; minor-element contents in parts per million. Mg#, magnesium number, Mg/(Mg+Fe); SiO2*, SiO2 content recalculated to 100-

on ignition at 925°C; n.a., not analyzed]

Mono-Inyo basalts Inyo Domes enclaves Mono Domes enclaves M

Sample----------------------- M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15 M16 M17 

Field No--------------------- DP81–2 BO81–1 DP86–3 MC74–8 MC74–7 1VGC–1 1LV87–1 183083–1 DP87–6 2M12–2B 2M12–1B 2M14–1B 2M18–1B MC74–11 BO75–2 BO74–4 BO75–4 

Rock type ------------------ Basalt Basalt Basalt Basalt Basalt Andesite Andesite Andesite Andesite Basalt Andesite Andesite Andesite Dacite Andesite Andesite Dacite 

Map unit (figs. 4–6) ------ Qyb Qyc Qa Qa Qa Enclave Enclave Enclave Enclave Enclave Enclave Enclave Enclave Qrd Qd Qd Qd 

Locality---------------------- Red Cones Black W. wall June N. June NE. Glass Deadman Deadman Deadman Dome (12) Dome (12) Dome (14) Dome (18) Dome (12) Negit Paoha Negit 

Point Creek Creek Creek Creek Island Island Island 

Latitude N ------------------ 37°25.4′ 38°1.5′ 37°41.9′ 37°49.7′ 37°53.1′ 37°44.8′ 37°42.8′ 37°42.8′ 37°42.8′ 37°54.1′ 37°54.1′ 37°53.4′ 37°52.4′ 37°54.1′ 38°1.2′ 38°0.7′ 38°1.5′ 

Long W---------------------- 119°3.5′ 119°6.0′ 119°3.3′ 119°4.1′ 119°3.4′ 119°1.3′ 119°1.1′ 119°1.1′ 119°1.1′ 119°2.0′ 119°2.0′ 119°0.2′ 119°0.4′ 119°2.0′ 119°3.0′ 119°1.5′ 119°3.0′ 

Major-element oxides

SiO2*------------------------- 50.20 50.54 52.99 53.29 53.40 59.61 60.14 60.74 61.10 52.60 54.60 57.90 61.70 67.49 62.81 63.45 65.28 

SiO2 -------------------------- 50.10 50.10 52.90 52.90 53.00 57.30 59.70 58.50 60.80 53.20 54.56 58.82 63.03 67.12 62.50 62.90 64.70 

TiO2 -------------------------- 1.33 1.46 1.39 1.54 1.57 .92 .98 .91 .96 2.11 .94 1.51 1.23 .79 1.04 1.01 .76 

Al2O3 ------------------------- 17.00 18.40 18.90 18.30 18.20 17.00 16.30 15.40 16.60 17.70 17.5 16.15 15.63 14.60 16.50 16.60 16.60 

Fe2O3 ------------------------- 2.40 2.14 2.60 3.11 2.53 3.06 2.79 2.90 2.14 10.28 9.22 8.15 6.77 1.40 1.69 1.52 1.59 

FeO--------------------------- 6.60 6.52 5.24 5.07 5.63 2.60 3.30 2.90 3.42 n.a. n.a. n.a. n.a. 2.76 3.69 3.40 2.60 

MnO-------------------------- .14 .13 .11 .11 .11 .11 .17 .18 .17 .10 .10 .10 .10 .09 .09 .09 .07 

MgO-------------------------- 8.31 5.96 4.25 4.07 4.02 2.74 3.38 3.13 3.30 4.31 4.47 4.10 3.08 1.64 1.91 1.49 1.23 

CaO--------------------------- 9.70 9.07 9.02 8.48 8.43 5.03 5.31 4.99 5.30 7.74 7.75 5.93 4.92 3.30 4.17 3.52 3.27 

Na2O ------------------------- 3.24 3.77 3.57 3.64 3.65 4.11 4.50 4.65 4.10 3.84 3.38 3.90 4.08 4.00 4.34 4.63 4.49 

K2O--------------------------- .76 1.25 1.48 1.62 1.69 2.90 2.61 2.46 2.52 1.60 1.66 2.59 3.02 3.60 3.23 3.59 3.52 

P2O5 -------------------------- .23 .34 .38 .43 .43 .35 .23 .30 .20 .31 .38 .38 .28 .15 .34 .39 .28 

H2O+-------------------------- .11 .08 .08 .35 .38 3.37 (LOI) .68 (LOI) 3.08 (LOI) .81 (LOI) .13 (LOI) .87 (LOI) .51 (LOI) .14 (LOI) .36 .24 .09 .16 

H2O- -------------------------- .11 .10 .05 .16 .15 – – – – – – .3 – – – – – – – – .02 .08 .11 .12 

CO2 --------------------------- .01 .42 .01 .01 .01 .02 .02 0 .17 n.a. n.a. n.a. n.a. .12 .01 .01 .01 

 Total --------------------- 100.04 99.74 99.98 99.79 99.80 99.51 99.97 99.40 100.79 101.3 100.8 102.1 102.3 99.95 99.83 99.35 99.40 

Mg# -------------------------- 63 50 50 48 48 48 71 68 53 45 49 50 47 – – 40 36 36 

Minor elements

Ba----------------------------- 364 698 773 853 829 937 477 464 478 444 352 303 298 303 1,250 1,390 1,440 

Co----------------------------- 41.5 35.1 28 27.6 26.3 – – – – – – – – – – – – – – – – 10.9 11.9 6.76 7.18 

Cr ----------------------------- 257 19.9 36.8 29.4 26.9 5.9 49 12 10 – – – – – – – – 9.8 9.61 .75 1.4 

Cs ----------------------------- .254 .201 – – .577 .561 – – – – – – – – – – – – – – – – 3.3 1.58 1.99 1.79 

Cu----------------------------- – – – – – – – – – – 8.5 9.5 15 12 – – – – – – – – 12 – – – – – – Hf ----------------------------- 2.83 3.43 3.44 3.78 3.74 – – – – – – – – – – – – – – – – 3.7 5.83 6.85 6.37 Nb----------------------------- 10 16 16 16 12 26 29 27 13 19 18 22 21 – – 16 20 14 

Ni ----------------------------- 122 61 34.4 16.8 25.2 14 31 22 26 – – – – – – – – – – 9.9 9.03 9.15 

Rb----------------------------- 11 16 27.2 29.7 30.5 89 80 77 69 35 45 71 87 117 69.5 90.7 79.9 

Sb ----------------------------- .114 .125 – – .175 .288 – – – – – – – – – – – – – – – – .8 .413 .372 .52 

Sc ----------------------------- 27.6 18.8 20.9 22.2 20.7 – – – – – – – – – – – – – – – – 7.51 10.2 9.17 6.87 

Sr------------------------------ 532 728 1,130 1,100 1,040 873 483 492 422 643 592 374 308 230 527 586 518 Ta ----------------------------- .373 .747 .67 .755 .72 – – – – – – – – – – – – – – – – 1.72 .953 1.37 .953 

Th----------------------------- 1.09 1.62 2.39 2.62 2.66 – – – – – – – – – – – – – – – – 11.2 6.01 8.8 6.61 

U ------------------------------ .324 .525 .865 .611 .749 – – – – – – – – – – – – – – – – 3.62 1.96 2.8 2.18 

V ------------------------------ – – – – – – – – – – – – – – – – – – – – – – – – – – 120 – – – – – – 

Y ------------------------------ 16 18 18 20 18 10 14 11 26 26 – – – – 30 34 22 26 22 

Zn----------------------------- 100 101 82 83.6 79.8 83 100 110 106 – – – – – – – – 64 81.4 84.6 77.3 

Zr ----------------------------- 93.2 146 128 179 158 167 182 187 190 171 159 212 198 202 241 279 248 

La ----------------------------- 12.8 18.6 22.8 25.8 24.9 – – – – – – – – – – – – – – – – 23 31.4 41 33.4 

Ce----------------------------- 29 40.3 50.3 58.8 55.8 – – – – – – – – – – – – 60.6 – – 42.5 68.6 88.5 70.1 

Nd----------------------------- 15.6 20 23 27.6 22.8 – – – – – – – – – – – – 28.8 – – 24 28.2 36 28.6 

Sm ---------------------------- 3.7 4.44 4.46 5.07 4.75 – – – – – – – – – – – – 6.03 – – 4.6 5.19 6.7 5.16 

Eu----------------------------- 1.26 1.48 1.46 1.65 1.55 – – – – – – – – – – – – 1.44 – – .78 1.46 1.86 1.35 

Gd----------------------------- 3.93 4.37 4.25 5.06 4.66 – – – – – – – – – – – – 5.16 – – – – 4.1 5.67 4.48 

Tb----------------------------- .604 .617 .579 .648 .64 – – – – – – – – – – – – – – – – .71 .657 .851 .637 

Tm ---------------------------- .319 .303 .241 .301 – – – – – – – – – – – – – – – – – – – – .368 .381 .332 

Yb----------------------------- 2.04 1.78 1.51 1.77 1.67 – – – – – – – – – – – – 2.79 – – 2.5 1.87 2.35 1.81 

Lu----------------------------- .299 .251 .236 .254 .239 – – – – – – – – – – – – – – – – .33 .272 .357 .261 

1Data from Varga and others (1990). 2Data from Kelleher and Cameron (1991).
Age SiO2 87Sr/86Sr 143Nd/144Nd 206Pb/204

Sample Reference Rock type Locality (Ma) (wt pct) ratio ratio ratio 

PRECALDERA SEQUENCE Central Sierran suite 8 7 Basanite-----------------------------------------Hill 8888, Merced Peak quadrangle ---------------- – – 44.45 0.70744 – – 18.70

4 7 Phonotephrite ---------------------------------Hill 7885, Marion Peak quadrangle----------------- – – 48.35 .70675 – – 18.89

12 7 Alkali-olivine basalt-------------------------Hill 8056, Merced Peak quadrangle ---------------- – – 48.52 .70431 – – 19.11

7 7 Mugearite --------------------------------------Near BM 4980, Bass Lake quadrangle------------- – – 50.47 .70628 – – 18.88

6 7 do--------------------------------------------Near hill 9841, Merced Peak quadrangle---------- – – 50.75 .70644 – – 18.89

5 7 Phonotephrite ---------------------------------Hill 10298, Merced Peak quadrangle--------------- – – 50.82 .70598 – – 18.86

11 7 Hawaiite----------------------------------------Near hill 8888, Merced Peak quadrangle---------- – – 51.44 .70653 – – 19.01

3 7 Phonotephrite ---------------------------------Near hill 9841, Merced Peak quadrangle---------- 3.59 51.59 .70607 – – 18.91

10 7 Mugearite --------------------------------------Miller Meadow, Merced Peak quadrangle -------- – – 51.77 .70648 – – 19.00

9 7 do--------------------------------------------Cora Lake, Merced Peak quadrangle --------------- – – 52.15 .70599 – – 19.00

1 7 Tephriphonolite ------------------------------Near hill 8675, Merced Peak quadrangle---------- 3.58 53.39 .70631 – – 18.872 7 do--------------------------------------------Near BM 4980, Bass Lake quadrangle------------- 3.45 54.91 .70635 – – 18.91

East Sierran suite cDP83–85 4 Basalt--------------------------------------------Agnew Pass------------------------------------------------ 3.1 50.05 0.70622 – – – – 

cDP83–90 4 do-------------------------------------------- do-------------------------------------------------------- 3.1 50.07 .70629 – – – – 

cDP83–100 4 do--------------------------------------------N. San Joaquin Ridge----------------------------------- 3.1 51.69 .70618 – – – – 

cDP83–94 [93] 4 do-------------------------------------------- do-------------------------------------------------------- 3.1 – – .70670 – – – – 

cDP83–68 4 Andesite ----------------------------------------C. San Joaquin Ridge ----------------------------------- 3.1 60.17 .70702 – – – – 

DP78–34 4 Mafic enclave in sample DP78–33------San Joaquin Mountain ---------------------------------- 3.0 57.28 .70739 – – – – 

DP78–33 4 Dacite ------------------------------------------- do-------------------------------------------------------- 3.0 66.24 .70724 – – – – 

DP78–35 4 Dacite, South Dome-------------------------San Joaquin Ridge --------------------------------------- 3.0 67.32 .70949 – – – – 

cDP83–81 4 Dacite, East Dome--------------------------- do-------------------------------------------------------- 3.0 – – .70732 – – – – 

Basin and Range suite CT 72–1 1 Trachybasalt-----------------------------------Crestview, N. caldera wall----------------------------- 3.50 50.20 0.70601 – – – – 

AOB–1 5 Basalt--------------------------------------------NW. caldera wall----------------------------------------- 2.95 59.26 .70613 – – – – 

M 72–19 (2) 1 Trachyandesite-------------------------------- do-------------------------------------------------------- 2.95 59.26 .70633 – – – – 

CT 74–2 (3) 1 Hornblende-augite dacite ------------------N. caldera wall, BM 8325 ----------------------------- 3.00 60.96 .70596 – – – – 

D85–55 2 Basalt--------------------------------------------Owens River Gorge ------------------------------------- 3.20 52.81 .70609 .51240 – – D85–191 2 do--------------------------------------------W. caldera rim -------------------------------------------- 3.10 50.06 .70595 – – – – 

D85–204 2 do--------------------------------------------Sagehen Meadow ---------------------------------------- 3.00 48.70 .70594 – – – – 

D85–235 2 do--------------------------------------------McGee Mountain ---------------------------------------- 2.60 48.98 .70623 .51250 – – MSTba 6 Basalt, Pliocene series----------------------McLaughlin Spring-------------------------------------- – – 51.95 .70613 .51245 18.93 

WCTba 6 do--------------------------------------------Wilfred Creek--------------------------------------------- – – 48.28 .70603 .51247 – – 

GSTba 6 do--------------------------------------------Cowtrack Mountain quadrangle --------------------- – – 49.32 .70608 .51249 18.90 

Table 6. Sr-, Nd-, and Pb-isotopic ratios for rock samples from the Long Valley Volcanic Field, Calif. [References: 1, R.W. Kistler and R.A. Bailey, (unpub. data, 1978); 2, Ormerod (1988); 3, Kelleher and Cameron (1990); 4, Chaudet (1986); 5, Christensen and DePaolo (1993); ten (1981); 8, Kistler (1993). Do., ditto]
POSTCALDERA SEQUENCE

Caldera suite

DP 73–1 (14) 1 Trachybasalt-----------------------------------W. moat flow---------------------------------------------- 0.10 50.92 0.70632 – – – – 

M 72–92 (15) 1 Hornblende-biotite trachydacite ---------Earthquake Fault flow --------------------------------- .15 68.71 .70629 – – – – DP73–140 (16) 1 Biotite rhyolite--------------------------------Mammoth Mountain ------------------------------------ .05 70.55 .70630 – – – – 

D85–59 2 Basalt--------------------------------------------S caldera moat -------------------------------------------- .20 51.22 .70599 – – – – 

L82–107 2 do--------------------------------------------Devils Postpile(?) ---------------------------------------- .60 51.83 .70603 .51252 – – LV–10 5 do--------------------------------------------S. caldera moat ------------------------------------------- .06 – – .70625 – – 19.2

TBQob–1 6 Older basalt------------------------------------Buttresses -------------------------------------------------- – – 46.45 .70646 .51242 18.9TBQob–2 6 do-------------------------------------------- do-------------------------------------------------------- – – 47.02 .70640 .51239 18.9

HLQpb 6 Basalt--------------------------------------------Horseshoe Lake------------------------------------------- – – 49.12 .70670 .51247 19.1

TFQpb 6 do--------------------------------------------Lake Mamie outlet (Twin Falls) --------------------- – – 49.05 .70667 .51248 19.2

PBQa–1 6 Trachyandesite--------------------------------Pumice Butte (northeast)------------------------------- – – 57.13 .70628 .51253 19.1

PBQa–2 6 do--------------------------------------------Pumice Butte (central)---------------------------------- – – 56.83 .70630 .51252 19.1

PBQa–3 6 do--------------------------------------------Pumice Butte (southwest) ----------------------------- – – 56.64 .70629 .51251 – – 

RMQa 6 do--------------------------------------------Mammoth Pass ------------------------------------------- – – 57.59 .70633 .51253 19.2

DPQpb 6 do--------------------------------------------Devils Postpile (Postpile flow)----------------------- – – 53.92 .70613 .51229 19.1

DPQaq 6 Dacite -------------------------------------------Devils Postpile (Rainbow Falls flow) -------------- – – 66.88 .70636 .51256 19.2SSQab 6 Basalt--------------------------------------------Soda Springs (Devils Postpile vent cone) --------- – – – – .70596 .51256 19.2

MMQpb 6 do--------------------------------------------SW. caldera moat ---------------------------------------- – – 50.85 .70629 .51255 19.2DCQgb–1 6 Trachyandesite--------------------------------Sawmill Road near Dry Creek------------------------ – – 55.14 .70623 .51253 19.1BSQpb 6 Basalt--------------------------------------------Big Springs ------------------------------------------------ – – 50.78 .70633 .51252 19.2

LMQa–1 6 do--------------------------------------------NW. caldera moat---------------------------------------- – – 51.71 .70620 .51243 19.2

LMQa–3 6 do-------------------------------------------- do-------------------------------------------------------- – – 51.14 .70620 .51253 19.2

HCQpb 6 do--------------------------------------------S. caldera moat (Hot Creek) -------------------------- – – 49.31 .70591 .51255 19.1

MLQab 6 do--------------------------------------------S. caldera moat (Mammoth Creek) ----------------- – – 51.48 .70620 .51250 19.2LVQpb 6 do--------------------------------------------N. caldera moat------------------------------------------- – – 50.83 .70634 .51247 19.2DCQab 6 do--------------------------------------------Deadman Creek------------------------------------------- – – 53.21 .70628 .51247 19.2

Corehole Inyo-4 (depth in feet)

73.8 6 Trachyandesite--------------------------------Group I------------------------------------------------------ 0.15 58.31 0.70638 0.51251 19.1

127.8 6 do-------------------------------------------- do-------------------------------------------------------- – – 57.17 .70631 .51252 19.1405.8 6 do--------------------------------------------Group II----------------------------------------------------- – – 55.99 .70626 .51248 19.1

503.4 6 Basalt--------------------------------------------Group IV --------------------------------------------------- – – 50.14 .70621 .51252 19.2

527.0 6 Trachyandesite--------------------------------Group III --------------------------------------------------- – – 54.21 .70620 .51252 19.1

540.4 6 do--------------------------------------------Group II----------------------------------------------------- – – 55.77 .70622 .51247 19.1

686.6 6 do-------------------------------------------- do-------------------------------------------------------- – – 55.01 .70617 .51253 19.1

883.3 6 Basalt--------------------------------------------Group III --------------------------------------------------- – – 53.05 .70618 .51251 – – 

2,123.5 6 do--------------------------------------------Group V ---------------------------------------------------- – – 50.50 .70596 .51259 19.1

2,130.4 6 do-------------------------------------------- do-------------------------------------------------------- – – 50.28 .70598 .51257 19.22,159.1 6 do-------------------------------------------- do-------------------------------------------------------- – – 50.81 .70597 .51259 19.1

2,209.0 6 do--------------------------------------------Group IV --------------------------------------------------- .415? 51.40 .70597 .51259 19.2

Age SiO2 87Sr/86Sr 143Nd/144Nd 206Pb/20

Sample Reference Rock type Locality (Ma) (wt pct) ratio ratio ratio 

Table 6—Continued
Pao–3 3 Rhyolite-----------------------------------------Paoha Island ----------------------------------------------- – – 71.70 .70633 .51265 – – 

SIERRAN GRANITOIDS RCKg 6 Granitoid ---------------------------------------Near Red Cones ------------------------------------------ – – 70.49 0.70779 0.51228 18.96 

DPKg 6 do--------------------------------------------W. of Devils Postpile ----------------------------------- – – 70.84 .70751 .51223 18.9

BSJg 6 do--------------------------------------------Big Springs ------------------------------------------------ – – 73.37 .70666 .51261 19.29 

Mzmv 6 Metavolcanic----------------------------------Ritter roof pendant (near Agnew Meadows) ----- – – 65.23 .72396 .51238 18.94 R–99 8 Evolution Basin alaskite ------------------------------------------------------------------------------------ 86 – – .71566 .512479 – – 

MG–1 8 Lamarck granodiorite---------------------------------------------------------------------------------------- 88 – – .70743 .512431 – – 

PAL–3 8 Inconsolable granodiorite----------------------------------------------------------------------------------- 107 – – .70666 .512368 – – 

PAL–61 8 Tinemaha Plateau--------------------------------------------------------------------------------------------- 109 – – .70818 .512215 – – 

FD–20 8 W. Sierra trondjhemite -------------------------------------------------------------------------------------- 112 – – .70529 .512571 – – 

JB–1 8 Bass Lake tonalite -------------------------------------------------------------------------------------------- 118 – – .70737 .512448 – – CL–1 8 Academy quarry----------------------------------------------------------------------------------------------- 125 – – .70405 .512884 – – FD–3 8 Paiute Monument --------------------------------------------------------------------------------------------- 130 – – .70926 .512163 – – 

PAL–8 8 Tinemaha Pluton ---------------------------------------------------------------------------------------------- 169 – – .71065 .512180 – – 

PAL–53x 8 do------------------------------------------------------------------------------------------------------------- 169 – – .70812 .512195 – – PAL–30 8 McMurray Meadows----------------------------------------------------------------------------------------- 171 – – .70773 .512299 – – 

PAL–72 8 do------------------------------------------------------------------------------------------------------------- 171 – – .71683 .512196 – – MT–1 8 Wheeler Crest granite---------------------------------------------------------------------------------------- 210 – – .70987 .512224 – – 

Age SiO2 87Sr/86Sr 143Nd/144Nd 206Pb/20

Sample Reference Rock type Locality (Ma) (wt pct) ratio ratio ratio 

Mono-Inyo suite D85–56 2 Younger basalt--------------------------------Red Cones-------------------------------------------------- <0.01 50.61 0.70516 0.51276 19.1

RCQyb–1 6 do-------------------------------------------- do-------------------------------------------------------- <0.01 49.98 .70521 .51277 19.2

RCQyb–2 6 do-------------------------------------------- do-------------------------------------------------------- <0.01 50.40 .70531 .51277 19.2

JLQa 6 Trachybasalt-----------------------------------June Lake--------------------------------------------------- – – 53.34 .70616 .51258 19.2

BPQyb 6 Younger basalt--------------------------------Black Point ------------------------------------------------ – – 50.55 .70538 .51267 19.2

BP83–1 3 do-------------------------------------------- do-------------------------------------------------------- .013 50.40 .70545 .51272 – – 

M14–1B 3 Andesite enclave -----------------------------Mono Craters---------------------------------------------- – – 57.90 .70526 .51278 – – 

Neg–RD2 3 Dacite -------------------------------------------Negit Island------------------------------------------------ – – 65.00 .70631 .51253 – – 

Table 6—Continued
a considerable body of evidence supports the associations of 

OIB-like basalts with an asthenospheric-mantle source and of 

enriched IAB-like basalts with a lithospheric-mantle source in 

the Western Great Basin.

Long Valley Basalt Sources

Many precaldera basalts in the Long Valley Volcanic 

Field satisfy the criteria for primitive lavas—that is, high 

MgO contents (8–10.5 weight percent), high MgO/FeO ratios 

(Mg#>60), and high compatible-element contents (>140 

ppm Ni, >200 ppm Cr) (Bacon and others, 1997). Precaldera 

basalts that meet these criteria include (1) all of the ultrapotassic Central Sierran basalts of Van Kooten (1980), (2) most 

East Sierran basalts (samples S1–S7, table 2), and (3) some 

of the more mafic Basin and Range basalts (samples B1, B2, 

B4, table 3). In contrast, only a few postcaldera basalts meet 

or approach these primitive criteria—for example, the Buttresses and Horseshoe Lake flows (samples C28, C27, table 

4), which closely resemble precaldera basalts, and, possibly, 

the Early Rhyolite basalt enclaves (unit Qer, figs. 4–6; sample 

C1, table 4), which contains 7.1 weight percent MgO, 60 ppm 

Ni, and 215 ppm Cr. The remaining postcaldera basalts are too 

deficient in MgO, Ni, and Cr (probably as a result of olivine 

fractionation at shallow crustal levels before eruption) to be 

considered primitive. The most mafic units, however, could 

have been parental to most other postcaldera mafic lavas; these 

few parental basalts include (1) the Early Rhyolite aphyric 

basalt enclaves (sample C1, table 4), (2) corehole Inyo-4 group 

V basalts (Vogel and others, 1994, tables 2, 3), (3) west-moat 

basalt (sample C3, table 4), (4) the south-moat Hot Creek 

basalt flow (sample HCQpb; Cousens, 1996, table 2), (5) the 

Devils Postpile Soda Spring flow (sample SSQab; Cousens, 

1996, table 3), (6) Black Point basalt (sample M2, table 5), 

and (7) Red Cones basalt (sample M1, table 5). As a group, 

they have higher Sr- and lower Nd-isotopic ratios than all 

other postcaldera basalts and plot closer to the isotopic range 

for OIB and MORB (fig. 20B).

Precaldera Basalts

Nearly all precaldera basalts, of both the Sierran and 

Basin and Range suites, have primitive-mantle-normalized 

trace-element signatures attributable to derivation from a 

lithospheric-mantle source (fig. 10). Van Kooten (1980, 1981) 

concluded that the ultrapotassic and potassic Central Sierran basaltic lavas were derived by 1.0- to 2.5-percent partial 

melting of alkali- and alkaline-earth-enriched, phlogopite- and 

garnet-bearing, pyroxene-rich lherzolite in the lithospheric 

mantle at 100- to 125-km depth. Van Kooten (1980) noted that 

at the few Central Sierran vents where stratigraphic relations 

are unambiguous, K2O content decreases upward in successive 

flows, suggesting an origin by increasing degrees of partial 

melting rather than fractional crystallization, which would 

have produced the opposite trend. Although source depletion 

might explain this decrease, it seems unlikely at such small 

degrees of partial melting. Van Kooten (1980) also noted the 

marked eastward decrease in K2O content within potassic 

Central Sierran lavas (fig. 9) and suggested, on the basis of 

the nonsystematic covariance of K2O with REE contents, that 

this regional change in K2O content was a source characteristic rather than a function of degree of partial melting. Thus, 

the local stratigraphic change in K2O content reflects inplace 

degree of partial melting, whereas the geographic variation 

in K2O content reflects a lateral change in composition of the 

lithospheric-mantle source. This conclusion is reinforced by 

Petrogenesis 53

1 0

1

10

100

1000

 OIB global average

 Central Sierran basalt (#12) 

Rb Ba Th Ta Nb K La Ce Nd S r P Zr Ti Tb Y Yb Lu

B EXPLANATION

Range of Basin and Range

OIB-like basalts

10

1

10

100

1000

IAB (Okmok)

Average WGB < 5 Ma

Rb Ba Th Ta Nb K La Ce Nd Sr P Zr Ti Tb Y Yb Lu

A EXPLANATION

R O C K / P R I M I T I V E - M A N T L E R AT I O

Compositional range of

East Sierran basalts

Figure 23. Mantle-normalized trace-element profiles of typical 

lithospheric- and asthenospheric-mantle-derived melts in the 

Basin and Range Province. A, Compositional range of East Sierran 

basalts in comparison with average Western Great Basin (WGB) 

lithospheric-mantle-derived basalt (Fitton and others, 1991) and 

primitive island-arc basalt (IAB) from Okmok, Aleutian Islands 

(sample ID16 of Nye and Reid, 1986). B, Central Sierran alkaliolivine basalt (sample 12 of Van Kooten, 1980) in comparison with 

OIB global average (Sun and McDonough, 1989) and compositional range of Basin and Range OIB-like basalts from Lunar 

Craters (Lum and others, 1989) and the Coso Volcanic Field (Novak 

and Bacon, 1986).
54 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

gradual eastward changes in the SiO2 content, Mg number, 

CaO/MgO and Ce/Yb ratios, and Y content of basaltic lavas 

from the Central Sierran through East Sierran to Basin and 

Range suites (fig. 9). These systematic lateral changes (particularly in Ce/Yb ratio, which is sensitive to garnet content 

in the source as well as to lithosphere thickness; Ellam, 1992) 

suggest a progressive change in source-rock composition in 

the lowermost lithospheric mantle—from garnet peridotite 

beneath the central Sierra Nevada to spinel peridotite beneath 

the Basin and Range—that, in turn, implies eastward thinning of the lithospheric mantle across the transition between 

the Sierra Nevada and Basin and Range Provinces, consistent 

with geophysical evidence for such eastward thinning across 

the East Sierran frontal fault zone (Carder, 1973; Mavko and 

Thompson, 1983; Wernicke and others, 1995; Fliedner and 

others, 1996).

Although the slightly more radiogenic Sr-isotopic composition of Central and East Sierran basalts relative to Basin and 

Range basalts (fig. 21A) could indicate greater contamination 

by crustal materials in passage through the thicker Sierran 

lithosphere, this effect is difficult to reconcile with the regional 

west-to-east decrease in K2O content (fig. 9). Central Sierran 

alkali-olivine basalt (sample 12 of Van Kooten, 1980), with 

its OIB-like signature and chemical and isotopic similarities 

to other OIB-like basalts of the Basin and Range Province 

(fig. 22A), supports derivation from an asthenospheric-mantle 

source, although its relatively high Ba, Th, and Sr contents and 

low Yb and Lu contents (fig. 11) could indicate cryptic crustal 

contamination (facilitated by its probable high temperature) 

during passage to the surface. The presence of OIB-like basalt, 

even in small amounts, indicates that at 3.8 Ma, asthenospheric-mantle source material existed above 80-km depth 

beneath the central Sierra Nevada and that magmas from both 

lithospheric- and asthenospheric-mantle sources were being 

tapped simultaneously and in close proximity.

Postcaldera Basalts

Cousens (1996), in his study of Long Valley postcaldera 

mafic lavas, concluded that the more mafic (>7 weight percent 

MgO) parental basalts were derived from either a less enriched 

lithospheric-mantle source or (as he preferred) contributions 

from a mafic lower-crustal source, possibly gabbroic-dioritic 

rocks deep in the Sierra Nevada batholith. He further concluded that the less mafic basalts evolved predominantly by 

fractional crystallization and the trachyandesites by assimilation of Sierran granitoids – in agreement with earlier studies 

of the corehole Inyo-4 samples by Vogel and others (1994). 

Cousens added that although shifts toward lower Sr- and 

higher Nd-isotopic ratios in the younger postcaldera basalts 

suggest increasing involvement of an asthenospheric-mantle 

component (presumably OIB type), incompatible trace-element ratios and Pb-isotopic data do not support such a conclusion. The results of the present study generally agree, but 

possible alternative basalt sources are discussed below.

Buttresses and Horseshoe Lake Basalts

The postcaldera Buttresses and Horseshoe Lake flows 

(samples C27, C28, table 4) mineralogically and chemically 

resemble precaldera East Sierran basalts and have generally 

similar trace-element profiles (fig. 14). Cousens (1996, p. 

27686), on the basis of this similarity and probably influenced 

by designation of the Buttresses flows as “Older Basalt” (Bailey, 1989), considered them collectively to be the oldest basalts 

in the postcaldera sequence, possibly older than the Bishop 

Tuff. Geomorphic and stratigraphic relations, however, as noted 

above, indicate that the Buttresses flows may be as young 

as 130 ka and that the Horseshoe Lake flows were erupted 

between 100 and 25 ka and so are considerably younger than 

the oldest postcaldera basalts penetrated in corehole Inyo-4 and 

the basalt enclaves in the Early and Moat Rhyolites.

Though mineralogically and chemically similar to 

precaldera basalts, the Buttresses and Horseshoe Lake flows 

are distinctive in their relatively lower K and Rb contents and 

higher LREE and Sr contents (fig. 14). Particularly distinctive 

are their low mantle-normalized K/La ratios (<1), a feature 

also characteristic of most other Caldera basalts (figs. 14B, 

14C). Such low normalized K/La ratios also are characteristic of some other Pleistocene basalts in the Basin and Range 

Province, for example, the Big Pine basalts (Ormerod and 

others, 1991) and the Crater Flat, Nev., basalts (Bradshaw and 

Smith, 1994, fig. 2; Yogodzinski and others, 1996, fig. 4b), as 

well as some Miocene-Pliocene basalts of the Rio Grande Rift, 

Colo.-N.Mex. (group 3 lavas of Leat and others, 1988). Such 

lavas constitute a subtype of IAB-like Basin and Range basalt 

in which low normalized K/La ratios appear to be a source 

characteristic. Ormerod and others (1991), in their analysis of 

the source of the Big Pine lavas, suggested that such relative 

K (as well as Rb) depletion is due to breakdown of phlogopite 

into amphibole in the lithospheric mantle; they speculated that 

K depletion may be related to previous (Miocene–early Pliocene) potassic volcanism in the Sierra Nevada Province. For 

Long Valley Caldera basalts, such K depletion could be related 

to the generation of potassic basalts of the precaldera Central 

and East Sierran suites. Low K and Rb contents might also 

indicate tapping of less enriched or more refractory domains 

higher in the lithospheric mantle, consistent with Ormerod 

and others’ (1991) conclusion that the Big Pine basalts were 

derived from spinel lherzolite rather than garnet lherzolite.

Caldera Basalts

As noted above, most Caldera basalts have low mantlenormalized K/La ratios (<1) but also more flattened, less spiked 

normalized trace-element profiles (less enriched in LILEs 

and LREEs and less depleted in HFSEs) than the Buttresses 

and Horseshoe Lake basalts (figs. 14A–14C) and most precaldera basalts (fig. 10). Their profiles closely resemble those of 

mafic-lower-crustal global averages (figs. 24A, 24B; Taylor 

and McLennan, 1985; Rudnick and Fountain, 1995), suggesting that the Caldera basalts may have been derived by nearly 
complete melting of mafic lower crust. Glazner and others 

(1991) proposed such a mechanism for the chemical variation 

in the Amboy and Pisgah Craters basalts of the Mojave Desert, 

suggesting that they were produced by varying amounts of 

mixing of mafic lower-crustal to midcrustal partial melts with 

asthenospheric-mantle melt and that the youngest eruptive 

rocks represent “nearly pure mafic crustal melts.” Circumstantial evidence supporting this hypothesis was provided by 

Hanchar and others (1994) in their study of mafic-lowercrustal xenoliths from a mid-Tertiary dike swarm in the Paiute 

Mountains, southeastern California, which confirmed that the 

mafic lower crust beneath the Mojave Desert is, indeed, compositionally similar to the global average. Primitive Caldera 

basalts, with their relatively low Sr- and high Nd-isotopic ratios 

(

87Sr/86Sr=0.70521–0.70598, 143Nd/144Nd=0.51258–0.51277), if 

derived predominantly from mafic lower crust, would require a 

relatively young, unradiogenic mafic lower crust.

Cousens (1996) rejected the involvement of OIB-type 

(Geronimo or Cima type) asthenospheric-mantle melt as a 

mixing component in the formation of postcaldera Long Valley 

basalts, concluding that its incompatible-element content is too 

high and its Pb-isotopic ratio too low. Cousens also rejected 

MORB-type melt as a possible mixing component, on the basis 

of its low K/La ratio. However, the high K/La ratio of some 

Mono-Inyo basalts is due to low REE contents, not high K 

contents—a characteristic of MORB melts. Thus, MORB (both 

normal [N–MORB] and enriched [E–MORB]) melts as source 

components are further evaluated here. Although MORB lavas 

are not recognized in the Long Valley area, they do occur in 

adjacent regions: N–MORB lavas occur in the Cenozoic Coast 

Range of western California, where Johnson and O’Neil (1984) 

inferred that they welled up into the “slab gap” during formation of the San Andreas transform fault; E–MORB is closely 

approximated by the high-Al2O3 olivine tholeiite (HAOT) lavas 

of the Cascade Range of northeastern California and the northwestern Basin and Range Province of eastern Oregon (Bacon 

and others, 1997). N–MORB and E–MORB (HAOT), in addition to their low or depleted incompatible-element contents, 

have Pb contents about a tenth of those of OIB, and so Pb-isotopic mixtures would be dominated by the lithospheric-mantle 

or mafic-lower-crustal melt contribution. The calculated traceelement profile of a 50:50 mix of lithospheric-mantle-derived 

East Sierran basalt (represented by K-depleted sample S1, table 

2) and average N–MORB (Sun and McDonough, 1989) yields 

a normalized trace-element profile nearly identical to that of 

most primitive Caldera basalts (fig. 24C), as well as to that of 

the mafic-lower-crustal global average.

Mono-Inyo Basalts

Mono-Inyo basalts have primitive-mantle-normalized 

trace-element signatures closely resembling that of the maficlower-crustal global average (fig. 24B). They differ from Caldera basalts in their higher mantle-normalized K/La ratios (>1), 

but this feature is due to their relatively lower LREE contents 

rather than to higher K contents (fig 14D)—a relation accountable for by mixing of greater proportions of LREE-depleted 

MORB melt with lithospheric-mantle or mafic-lower-crustal 

melt. As noted above, the low Pb content of MORB would not 

significantly alter the Pb-isotopic ratio of the mixture. Moreover, consistent with their lower Sr- and higher Nd-isotopic 

ratios, Mono-Inyo basalts also have slightly less radiogenic 

207Pb/204Pb and 208Pb/204Pb ratios (figs. 22B, 22C), consistent 

with involvement of a larger N–MORB component than in 

Caldera basalts. The primitive-mantle-normalized trace-element profile of Mono-Inyo basalts (fig. 24C) can be approximated by a calculated 60:40 mix of average MORB (Sun and 

McDonough, 1989) and lithospheric-mantle-derived East Sierran basalt (represented by K-depleted sample S1, table 2).

Thus, mixing of MORB with locally derived lithosphericmantle melts can approximate the compositions of both 

Caldera and Mono-Inyo basalts. The tectonic relations for 

possible involvement of MORB in the origin of postcaldera 

Long Valley basalts are discussed below in the section entitled 

“Tectonomagmatic Model.”

Additional perspective on the sources of Long Valley basalts 

is provided by figures 25 (Ta/La versus La/Sm ratios) and 26 

(Th/Yb versus Ta/Yb ratios), where Long Valley basalts are compared with the mafic-lower-crustal global average and N–MORB, 

E–MORB, and OIB compositions and their local equivalents.

In figure 25A,

5

 OIB-type lavas from the Coso, Lunar 

Craters, Cima, and Amboy-Pisgah Volcanic Fields in the 

Basin and Range Province form a vertical array, with a wide 

range of Ta/La ratios in the middle of the diagram. Note that 

Central Sierran alkali-olivine basalt (sample 12) plots within 

this OIB array. Purported E–MORB-type (HAOT) lavas from 

the Cascade Range and eastern Oregon plot in a subparallel 

vertical array at lower La/Sm ratios and an equally wide range 

of Ta/La ratios between N–MORB and E–MORB global averages. Precaldera Long Valley basalts (shaded field, without data 

points) form a subhorizontal array in the lower middle of the 

diagram. Most Caldera basalts overlap this precaldera array at 

similar Ta/La ratios but at generally low La/Sm ratios. Relative 

to Caldera basalts, Mono-Inyo basalts have higher Ta/La and 

lower La/Sm ratios and are skewed toward the HAOT array and 

toward average N–MORB, E–MORB, OIB, and mafic-lowercrustal compositions, suggesting possible involvement of any 

one of these components. Mixing lines between a typical precaldera basalt (sample S1, table 2), inferred to be representative of 

lithospheric-mantle melt, and N–MORB, E–MORB, OIB, and 

mafic-lower-crustal global averages are plotted in figure 25B. 

Mono-Inyo June Lake (JL, sample M4, table 5) and Black Point 

(BP, sample M2, table 5) basalts lie along the lower-crustal 

mixing line (LC–S1) at ratios of 60:40 and 75:25, respectively, whereas Red Cones basalt (RC, sample M1, table 5) lies 

between the N–MORB and E–MORB mixing lines (N–S1 and 

E–S1, respectively), at a ratio of about 75:25. Although these 

plots should be viewed cautiously because of the scatter in Long 

Petrogenesis 55

5

Ta/La ratio, rather than the more commonly plotted Nb/La ratio, is used 

because for many of the basalts Nb data are unavailable. Furthermore, Ta 

content can be determined with good accuracy and precision.
56 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

Valley data points, the more mafic parental Caldera basalts and 

Mono-Inyo basalts fall within this array of mixing lines.

A plot of Th/Yb versus Ta/Yb ratios (fig. 26) is instructive in that it separates the effects of mantle-enrichment 

processes from those of crustal contamination and mixing 

(Pearce, 1982). Because mantle-enrichment events affect Th 

and Ta contents equally, the vector for the mantle array has a 

slope of 1; however, crustal-contamination and mixing events 

affect Th more profoundly than Ta contents, and so the vector 

for such processes is subvertical. Using Yb content as a normalizing factor largely eliminates the effects of partial melting 

and fractional crystallization. Thus, most OIB-like Western 

Great Basin basalts, including those of the Coso, Lunar Craters, Cima, and Amboy-Pisgah Volcanic Fields, lie within the 

mantle array near the OIB global average. Note, however, that 

OIB-like alkali-olivine basalt (sample 12) of the Long Valley 

Central Sierran suite plots close to, but above, the mantle array 

on a subvertical vector, suggesting contamination by crustal 

Figure 24. Primitive-mantle-normalized trace-element profiles of postcaldera basalts in comparison with representative global basalt 

types. A, Trace-element profiles of average global basalt types: ocean-island basalt (OIB), enriched midocean-ridge basalt (E–MORB), 

normal ocean-ridge basalt (N–MORB) (Sun and McDonough, 1989), mafic lower crust (LC) (Rudnick and Fountain, 1995), and average 

basalt derived from Western Great Basin (WGB) lithospheric mantle (Fitton and others, 1991). B, Trace-element profiles of primitive Caldera basalt (Unit Qer mafic enclaves) and Mono-Inyo basalt (Red Cones basalt) in comparison with mafic-lower-crustal global averages 

(1, Taylor and McLennon, 1985; 2, Rudnick and Fountain, 1995). C, Trace-element profile of primitive Mono-Inyo basalt (Red Cones basalt) 

in comparison with calculated 60/40 mix of N–MORB and lithospheric-mantle melt (represented by East Sierran basalt sample S1, table 2).

rocks, possibly Sierran granitoids. Other precaldera basalts 

cluster in a field well above the mantle array and in the general 

range for Big Pine and most other lithospheric-mantle-derived 

Basin and Range basalts. Caldera basalts form a vertical array 

between the precaldera basalt field and the mantle array, in 

the general vicinity of the mafic-lower-crustal global average 

(LC), with the most mafic parental basalts clustering close 

to the lower-crustal average at the edge of the mantle array. 

Mono-Inyo basalts of June Lake (JL) and Black Point (BP) 

fall within the Caldera array, but Red Cones basalt (RC) lies 

well to the left on an apparent subvertical vector, suggesting 

derivation from mantle magma intermediate in composition 

between N–MORB and E–MORB and possibly contaminated 

by or mixed with a lithospheric-mantle component, in general 

agreement with the mixing lines suggested in figure 25B.

Note where the Long Valley dacites plot in figure 26. 

Precaldera East Sierran dacites fall within the field of Sierra 

Nevada granitoids, whereas Basin and Range dacites plot 

within the field of precaldera basalts. Postcaldera dacites/

trachydacites also plot within the precaldera basalt field at 

somewhat lower Th/Yb and Ta/Yb ratios. These relations and 

supporting isotopic evidence suggest that the East Sierran 

dacites were derived by fusion of Sierran granitoids and Basin 

and Range dacites mainly by fractionation of associated mafic 

magmas. Anomalously, postcaldera dacites/trachydacites 

appear to be associated with precaldera basalts.

Evolution of the Precaldera Sequence

Central and East Sierran Suites

Precaldera Central Sierran, East Sierran, and Basin and 

Range basalts of the Long Valley Volcanic Field have chemical 10 0.1

1

10

100

1000

Rb Ba Th Ta Nb K La Ce Nd Sr P Zr Ti Tb Y Yb Lu

A

OIB

N-MORB

E-MORB

WGB Lithosphere

LC

10

0.1

1

10

100

1000

Average lower mafic crust (2)

Average lower mafic crust (1)

Red Cones basalt

Unit Qer mafic enclaves

Rb Ba Th Ta Nb K La Nd Ce Sr P Zr Ti Tb Y Yb Lu

B EXPLANATION

ROCK/PRIMITIVE-MANTLE RATIO ROCK/PRIMITIVE-MANTLE RATIO

ROCK/PRIMITIVE-MANTLE RATIO

0 10 0.1

1

10

100

1000

Red Cones basalt

East Sierran lithosphere

N-MORB

60/40 mix of N-MORB/lithosphere

Rb Ba Th Ta Nb K La Ce Nd Sr P Ti Zr Tm Y Yb Lu

C EXPLANATION
and isotopic signatures indicating that most of the basalts were 

derived mainly from lithospheric mantle, though probably 

from different depths or chemical domains within it. Contrasting chemical trends within these suites, however, suggest that 

the chemically more evolved trachyandesites and dacites of 

each suite were generated by substantially different magmatic 

processes.

One of the striking differences between the East Sierran 

and Basin and Range suites is the predominance of compositional gaps in the East Sierran suite, in comparison with the 

more nearly continuous secular trend in the Basin and Range 

suite (figs. 7, 8). Although compositional gaps in lava suites 

are commonly attributed to fractionation mechanisms in 

closed magmatic systems (Grove and Donnelly-Nolan, 1986; 

Bacon and Druit, 1988; Brophy and others, 1996), trends of 

trace-element and, particularly, REE versus SiO2 contents in 

East Sierran mafic lavas (figs. 8B, 8C) indicate that basalt 

fractionation did not lead to the generation of most associated 

trachyandesites. The nearly straight-line relation between the 

gap-separated East Sierran basalt, trachyandesite, and dacite 

clusters for most elements (figs. 7A, 8A–8C) is consistent with 

magma mixing of silicic and mafic end members. Magma 

mixing is further supported by (1) the abundance of mafic 

enclaves in East Sierran dacites, in comparison with their 

relative paucity in Basin and Range dacites; (2) the presence 

in Sierran trachyandesites of resorbed plagioclase and augitehaloed quartz xenocrysts, commonly of volcanic (magmatic) 

rather than Mesozoic plutonic origin; (3) resorption and 

reaction textures in olivine and augite phenocrysts; (4) steep 

fractionation trends in East Sierran basalts that do not project 

toward trachyandesitic compositions; (5) the crossover pattern 

on normalized trace-element and REE diagrams for East Sier0 2 4 6 8 10 12

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Central Sierran basalt

Caldera basalt

Mono-Inyo basalt 

Coso basalt (Pleistocene)

Cima basalt

N-MORB (N)

Amboy-Pisgah basalt

Mafic lower crust (LC)

Cascades HAOT

Ta / L a R AT I O

OIB

LC

N

A

BP

RC

1 2

EXPLANATION

JL

E

OIB

E-MORB (E)

0 2 4 6 8

0.00

0.02

0.04

0.06

0.08

S1+ N-MORB (N)

East Sierran basalt S-1 

Caldera basalts

Mono-Inyo basalts

S1+ E-MORB (E)

S1+ OIB

S1+ lower crust (LC) 

L a / S m R AT I O

N

OIB E

8 0

6 0 4 0

8 0

6 0

8 0

8 0

6 0

4 0

100

100

S 1

7 0

RC

B P

J L

B EXPLANTION

9 0

7 0 9 0

9 0

7 0

9 0

7 0

5 0

LC

Figure 25. Ta/La versus La/Sm ratios. A, Precaldera (gray area) and postcaldera basalts in 

relation to other Basin and Range basalts and 

Cascades high-Al2O3-olivine tholeiites (HAOT), 

as well as average ocean-island basalt (OIB), 

normal midocean-ridge basalt (N–MORB), 

enriched midocean-ridge basalt (E–MORB), 

and mafic lower crust (LC). B, Mixing lines 

between lithospheric-mantle melt (represented 

by East Sierran basalt, sample S1, table 2) and 

global averages for OIB, N–MORB, E–MORB, 

and mafic lower crust. (Note change in scale.) 

Note that June Lake (JL) and Black Point (BP) 

basalts lie on mafic-lower-crust mixing curve 

and that Red Cones (RC) basalt lies between 

N–MORB and E–MORB mixing curves, far to 

left of mafic-lower-crust mixing curve.

Petrogenesis 57
58 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

ran trachyandesites (fig. 12), which is permissive of mixing 

of extreme end members; (6) calculated basalt fractionation 

trends that do not yield compositions resembling associated 

trachyandesites; and (7) least-squares mixing calculations, 

using local basalt and dacite as end members, that yield 

compositions close to those of East Sierran trachyandesites. 

The observed range of trachyandesitic compositions (55–60 

weight percent SiO2) indicates basalt:dacite mixing ratios 

ranging from 38:62 to 67:33; this limited compositional range 

and, particularly, the relatively small gap between basalts and 

trachyandesites may be a function of the contrasting physical 

properties of the end members and the vagaries of the mixing 

process (Sparks and Marshall, 1986; Frost and Mahood, 1987). 

Some East Sierran trachyandesites, however, may have been 

generated from basalt by assimilation of Mesozoic granitoids, 

many of which are compositionally similar to precaldera East 

Sierran dacites. This mechanism is supported by (1) the close 

spatial and temporal association and superposed stratigraphic 

relations of some basalts and trachyandesites in the field; (2) 

the textural similarity of basalts and trachyandesites, both of 

which are finely porphyritic; and (3) basalt-trachyandesite 

sequences commonly showing continuous upsection changes 

in phenocryst compositions (Chaudet, 1986), in spite of the 

compositional gap between them. However, mixing calculations of basalt with local selected granitoid compositions 

produce less satisfactory solutions than mixing with coeval 

East Sierran dacitic compositions.

The higher K2O, Rb, Th, Ta, Zr, and REE contents and 

lower Na2O content of East Sierran dacites relative to Basin 

and Range dacites (fig. 8) supports the generation of East 

Sierran dacites by fusion of an incompatible-element-enriched 

crustal source rock. East Sierran dacites are too radiogenic in 

Sr (fig. 21) to have been generated by fractionation of East 

Sierran basalts; involvement of a more radiogenic component, such as midcrustal to upper-crustal Mesozoic granitoids 

(87Sr/86Sr=0.70666–0.70987), seems required. As plotted in 

figure 26 (Th/Yb versus Ta/Yb ratios), precaldera East Sierran dacites fall within the array of Sierra Nevada granitoids, 

suggesting fusion of such rocks as a possible origin; however, 

the marked depletion of P, Zr, Ti, Y, and HREEs in the dacites 

suggests probable late-stage fractionation of hornblende, along 

with apatite, zircon, and Fe-Ti oxides.

Figure 26. Th/Yb versus Ta/Yb ratios for precaldera and postcaldera lavas, showing relation to “mantle array” (Pearce, 1982) and other Basin and Range lavas and related rocks. Gray area, compositional 

range of lithospheric-mantle-derived precaldera basalts; pink area, range of upper-crustal Sierran 

granitoids. OIB-like Basin and Range basalts cluster within mantle array near OIB global average; 

OIB-like sample 12 plots above mantle array, probably owing to crustal contamination. Caldera basalts 

and Mono Inyo basalts (JL, BP) plot between lithospheric-mantle-derived basalts and mafic-lowercrust global average (LC), whereas Red Cones (RC) basalt plots to left toward MORB (E, N) compositions. Note that precaldera Basin and Range dacites and most postcaldera dacites plot within range 

of lithospheric-mantle-derived basalts, whereas precaldera East Sierran dacites plot within range of 

Sierran granitoids.

0.01 0.1 1 10

0.01

0.1

1

10

100

OIB global avarage 

Lunar Craters basalts

Caldera basalts

Mono-Inyo basalts

Coso basalts (Pleistocene)

Cima basalts

Amboy-Pisgah basalts

Caldera dacites

Basin and Range dacites

East Sierran dacites

Mafic lower crust (LC)

N-MORB (N), 

Central Sierran basalt (12)

Ta / Y b R AT I O 

T h / Y b R AT I O 

LC

OIB

Mantle array

E

12

N

RC BP

JL

Crustal contamination 

Within-plate enrichment

EXPLANATION

E-MORB (E)

Sierran granitoids
Basin and Range Suite

The more nearly continuous arcuate trend of Basin and 

Range lavas on SiO2-variation diagrams (figs. 7, 8A, 8B) suggests an evolution controlled initially by partial fusion of a 

heterogeneous upper-mantle source to produce both basalt and 

trachyandesite, followed by crystal fractionation to produce 

dacite. The increase in LREE contents with increasing SiO2

content from basalt to trachyandesite (fig. 8C) suggests generation by partial fusion of a mafic pyroxene- and/or hornblende-bearing source. The following decrease in both LREE 

and HREE contents from trachyandesite to dacite suggests 

fractionation of hornblende and plagioclase. The absence of a 

dacite Eu anomaly (fig. 12B) may result from compensating 

positive and negative Eu anomalies in hornblende and plagioclase, respectively (Zielinski, 1975; Zielinski and Lipman, 

1976). However, although fractionation of hornblende and plagioclase can account for most major-element trachyandesitedacite trends, it does not readily account for many traceelement trends. The overall decrease in mantle-normalized 

trace-element contents with increasing SiO2 content (fig. 12A) 

seems to require partial fusion of some mafic to intermediatecomposition source rock, probably accompanied or followed 

by fractionation of hornblende and plagioclase. Again, as plotted in figure 26, precaldera Basin and Range dacites fall within 

the field of precaldera basalts, consistent with an origin by 

fractionation of associated Basin and Range basalts or partial 

fusion of mafic lithospheric mantle.

Summary

In the Sierra Nevada Province, (1) Central and East Sierran basaltic lavas were derived by partial melting of lithospheric-mantle sources—ultrapotassic Central Sierran lavas 

by small degrees of partial melting of a phlogopite-bearing 

garnet peridotite near the base of a 100- to 125-km-thick upper 

mantle, as proposed by Van Kooten (1980), and East Sierran 

basalts by greater degrees of partial melting of garnet-poor 

peridotite at shallower mantle levels; (2) East Sierran dacites 

were derived by fusion of granitoid crustal rocks with relatively high incompatible-element contents, and subsequently 

modified by fractionation of hornblende and accessory minerals; and (3) most East Sierran trachyandesites were derived by 

mixing of coeval basaltic and dacitic end members, and some 

by assimilation of Mesozoic granitoids.

In the adjacent Basin and Range Province, chemical and 

mineralogic relations imply that (1) basaltic lavas were derived 

by partial melting of a thinned lithospheric mantle composed 

of spinel lherzolite and (2) trachyandesites and dacites evolved 

mainly by fractionation of hornblende and plagioclase.

The rare occurrence in the Central Sierran suite of a 

mafic nodule-bearing alkali-olivine basalt (sample 12, fig. 11) 

with OIB-like chemical and isotopic signatures indicates that 

asthenospheric-mantle magma was involved in the evolution 

of the Long Valley Volcanic Field at an early stage. Asthenospheric-mantle magma may have been present at depth in 

greater volume than is suggested by its eruptive volume, and 

may have provided the heat for melting of lithospheric-mantle 

and crustal sources to generate the precaldera basaltic through 

dacitic magmas. The slightly LILE enriched composition of 

this alkali-olivine basalt may be due to small degrees of partial 

melting or to contamination by upper-crustal rocks facilitated 

by its higher temperature relative to lithospheric mantlederived lavas. In general, however, most precaldera mafic lavas 

show little contamination by upper-crustal rocks, probably 

because during early stages of development of the volcanic 

field when extension was rapid, mafic magmas traveled rapidly to the surface and seldom stalled at crustal levels.

Evolution of the Postcaldera Sequence

Caldera and Mono-Inyo Suites

Basalt

The most mafic, parental basalts of the postcaldera 

sequence—including the mafic enclaves in units Qer and Qmr 

(fig. 2), a few early west-moat flows of the Caldera suite, and 

the Red Cones and Black Point units of the Mono-Inyo suite—

have modified IAB-like, mantle-normalized trace-element signatures, with significantly lower LILE contents, higher HFSE 

contents (fig. 14), and lower Sr- and higher Nd-isotopic ratios 

(fig. 20) relative to lithospheric-mantle-derived precaldera 

basalts (fig. 18). Their trace-element signatures (figs. 14, 24) 

closely resemble those of lower-mafic-crustal global averages 

(Taylor and McLennon, 1985; Rudnick and Fountain, 1995), 

suggesting that these parental basalts were generated by partial 

melting of a mafic-lower-crustal source, although such characteristics may also be explained by mixing of LREE-depleted, 

MORB-like (particularly N–MORB) mantle melts with 

lithospheric-mantle-derived melts. This alternative explanation 

is supported, particularly for the Mono-Inyo basalts, by their 

relatively low Sr- and high Nd-isotopic ratios (fig. 20B), lower 

incompatible-element and LREE contents (fig. 14D), and 

progressively decreasing LREE contents over time, suggesting 

progressively greater input of an N–MORB-like component.

Trachyandesite

The more evolved trachybasalts and trachyandesites of the 

Caldera and Mono-Inyo suites have undergone varying degrees 

of contamination and commingling with more silicic materials. Studies of the small-volume mafic enclaves in postcaldera 

Early and Moat Rhyolites and in Mono-Inyo rhyolites (Varga 

and others, 1990; Kelleher and Cameron, 1990) provide welldocumented evidence of mixing and commingling with their 

host rocks at shallow crustal levels. The more voluminous 

Caldera basalt and trachyandesite flows, particularly those 

in the west moat, have had a more complex history involving the successive incorporation of two different magmatic 

contaminants at midcrustal to upper-crustal levels, as well 

recorded in the chemical variations (fig. 27) of corehole Inyo-4 

Petrogenesis 59
60 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

basalt-trachyandesite groups I through V (Vogel and others, 

1994), wherein each successive group represents a separate 

magma batch that first was modified by assimilation of basement granodiorite (between-group variation) and subsequently 

evolved by crystal fractionation (within-group variation). 

However, although most within-group major-element variations 

can be explained by crystal fractionation, the marked increases 

in Ba and K2O contents (Vogel and others, 1994, fig. 13) cannot but instead require preferential ingestion of Ba and K2O 

from a Ba+K2O-rich source. Vogel and others (1994) convincingly demonstrated that the bulk assimilation of Ba-rich Moat 

Rhyolite cannot account for these increases; however, selective 

diffusion of Ba and K at basalt/rhyolite interfaces, a process 

demonstrated experimentally by Watson (1982) and cited for 

similar Ba and K2O enrichment in Coso basalts (Bacon and 

Metz, 1984), may be a likely enrichment mechanism. Moat 

Rhyolite sanidine phenocrysts (15 volume percent) contain 

12,000 to 15,000 ppm Ba (R.A. Bailey, unpub. data, 1978) and 

typically are strongly resorbed, probably as a result of heating 

during basalt influx into the postcaldera residual Long Valley 

silicic-magma chamber. In addition to the sensible heat of the 

injected basalt, some of the thermal energy for melting of these 

Moat Rhyolite sanidine phenocrysts and for Ba and K diffusion 

and assimilation may have been provided by crystallization 

and separation of olivine and Fe-Ti oxides, as suggested by 

strong Ni and Cr depletion in most such contaminated intracaldera basalts (fig. 16). These variations in Ba and K2O contents 

suggest a two-stage, dual-level process whereby (1) betweengroup major-element variations were generated by assimilation 

of Mesozoic granodiorite during upward passage through the 

midcrust (or, alternatively, by mixing with postcaldera dacitic 

magma; see next subsection), and (2) marked within-group Ba 

45 50 55 60 65 70 75 80

0

500

1000

1500

2000

2500

3000

Moat rhyolites

Early rhyolites

Inyo Craters rhyolites

Mono Craters rhyolites

Sierran granitoids

Caldera dacites

I

II

III

IV

Unit Qer-Qmr mafic enclaves V

Mono-Inyo mafic enclaves

Caldera basalt-andesite

Ba CONTENT, IN PARTS PER MILLION 

 K-feldspar 

V-IV

III

II

I

mixing

Qer

Qmr

Krv

TR wc

(10,000 ppm Ba) EXPLANATION

Corehole 

Inyo-4 groups

A

45 50 55 60 65 70 75 80

0

1

2

3

4

5

6

I

II

III

IV

V

Caldera dacites

Sierran granitoids 

K-feldspar

biotite

Krv

TR wc

B

IV, V 

III

II

I EXPLANATION

Corehole 

Inyo-4 

groups

SiO2 CONTENT, IN WEIGHT PERCENT

K2O CONTENT, IN WEIGHT PERCENT

(10 weight percent K2O)

(8 weight percent K2O)

Figure 27. SiO2-variation diagrams 

illustrating chemical evolution of Caldera 

basalts and trachyandesites (groups I–V; 

Vogel and others, 1994). A, Ba versus SiO2

contents, showing two-stage Ba enrichment: (1) moderate between-group Ba 

enrichment by granitoid assimilation or 

dacite mixing, and (2) marked within-group 

Ba enrichment by selective diffusion from 

Ba-rich sanidine in Moat Rhyolite magma 

at margin or roof of residual Long Valley 

silicic-magma chamber. Note that trends 

for all postcaldera rhyolites project along 

lines that suggest derivation from dacite. 

Krv, Round Valley Peak granodiorite; TRwc, 

Wheeler Crest granite, both Sierran plutons 

cut by Long Valley ring-fault zone. B, K2O 

versus SiO2 contents of groups I–V, showing similar but less marked within-group K 

enrichment, but between-group K enrichment consistent with dacite mixing.
and K2O enrichment was achieved by selective diffusion from 

sanidine-rich Moat Rhyolite magma at the base or margin of 

the residual Long Valley silicic-magma chamber.

Basalt-Dacite Mixing

Although Vogel and others (1994) suggested Round Valley 

Peak granodiorite as the assimilant for between-group basalttrachyandesite compositional variations, chemically similar 

coeval dacitic magma is an equally, if not more likely, assimilant, requiring less thermal input and falling compositionally 

closer to the projected between-group mixing line than local 

Round Valley Peak granodiorite or Wheeler Crest granite (figs. 

27A, 27B). Petrographic evidence from the solitary intracaldera 

trachyandesite flow that plots within the postcaldera trachyandesite-dacite compositional gap (fig. 7C) also supports such 

mixing of basaltic and dacitic magma. As previously noted, this 

flow (sample C12, table 4), a 60-weight-percent-SiO2 “olivine 

quartz latite” (Rinehart and Ross, 1964, p. 57–60), contains 

relict magnesian olivine and augite phenocrysts from a basaltic 

component and biotite, hornblende, plagioclase, and partially 

resorbed augite-haloed quartz phenocrysts from a dacitic component in a thoroughly homogenized matrix within which no 

intact relict mafic enclaves are preserved. Least-squares mixing 

calculations, utilizing local basalt and dacite end members, 

indicate a basalt:dacite mixing ratio of 43:57.

Dacite

For most major elements and Ba, Th, and LREEs, postcaldera dacites are compositionally similar to precaldera East Sierran dacites (fig. 13). However, the much lower Sr-isotopic ratios 

of postcaldera dacites (fig. 21B) relative to local batholithic rocks 

and precaldera dacites (fig. 21A) indicate that the postcaldera 

dacites were likely derived by fractionation of their associated 

postcaldera basalts, which have similar Sr-isotopic ratios.

As noted above, porphyritic dacites of the Caldera 

and Mono-Inyo suites are mineralogically and chemically 

extremely diverse and probably evolved, except for the Mammoth Mountain complex, as small, isolated, unrelated magma 

batches. Although the contents of many major elements and Sr 

(figs. 13A, 13B) monotonically change with SiO2 content, suggesting that as a group they evolved by crystal fractionation, 

the porphyritic dacites were not necessarily derived from a 

common source or evolved in a single chamber. Postcaldera 

dacites (figs. 15A–15C), in comparison with precaldera East 

Sierran dacites (fig. 12A), are less enriched in Sr and less 

depleted in Ta, Nb, and Ti. These trace-element profiles of 

postcaldera dacites closely mimic those of the associated postcaldera basalts, suggesting an origin, at least partly, by basalt 

fractionation, consistent with their similar isotopic compositions. The crystal-poor and somewhat less silicic Mono-Inyo 

dacites (fig. 15C) have higher Sr, P, and Ti contents, and 

appear to be slightly less evolved, than the porphyritic dacites. 

Though erupted later than the porphyritic dacites, Mono-Inyo 

dacites probably represent the deeper melts from which the 

porphyritic dacites were fractionated.

Although the postcaldera low-SiO2 rhyolites of both 

the Caldera and Mono-Inyo suites (fig. 15D) are chemically 

similar in most respects, the Mono-Inyo rhyolites are more 

depleted in REEs, particularly HREEs, and have slightly larger 

negative Eu, Ti, Ta, and Nb anomalies, suggesting greater 

fractionation of plagioclase and, possibly, a cryptic Ti-bearing 

phase. These rhyolites most likely evolved by fractionation 

of coeval postcaldera dacites, also the likely origin of most 

other postcaldera rhyolites of the Long Valley Volcanic Field. 

Ba-SiO2 trends of all postcaldera rhyolite groups, including 

the Early Rhyolite, Moat Rhyolite, and Mono-Inyo high-SiO2

rhyolites (fig. 27A), have negative slopes projecting along a 

fractionation line from K-feldspar and (or) biotite through the 

field of postcaldera dacites. This postcaldera rhyolite Ba-SiO2

trend and a similar Al2O3-SiO2 trend projecting from plagioclase through dacite (fig. 28) suggest that the Mono-Inyo rhyolites and most other postcaldera Long Valley rhyolites evolved 

from coeval dacite by fractionation of feldspars, biotite, and, 

possibly, hornblende.

Summary

Postcaldera parental basalts of both the Caldera and 

Mono-Inyo suites appear to have been derived by nearly complete remelting of mafic lower crust or by mixing of N–MORB 

and lithospheric-mantle melts. The more evolved trachybasalts 

and trachyandesites formed by a two-stage process involving (1) assimilation of Mesozoic granitoids or mixing with 

coeval dacitic magma at midcrustal levels, followed by (2) 

major-element fractionation and selective diffusion of Ba 

and K during contact with sanidine-rich Moat Rhyolite or 

Mono-Inyo Craters rhyolitic magma at shallow crustal levels. 

The postcaldera dacites probably were largely derived by 

fractionation of trachybasalt-trachyandesite because they have 

Sr-isotopic compositions too low to have been derived by 

fusion of Mesozoic granitoids. Low-SiO2 rhyolites associated 

with the postcaldera dacites, as well as most other intracaldera 

mid- to high-SiO2 rhyolites, also were likely derived by crystal 

fractionation of coeval dacitic magma.

Tectonomagmatic Model

Crough and Thompson (1977) speculated, on the basis of 

low Rayleigh-wave velocities beneath the crest of the Sierra 

Nevada, that Cenozoic thinning of the lithospheric mantle 

beneath the range is the cause of its uplift. Wernicke and others (1995) and Fliedner and others (1996) reported, on the 

basis of results of a multidisciplinary geophysical study of an 

east-west transect across the southern Sierra Nevada, that the 

crust beneath the southern Sierra Nevada is 34 to 42 km thick, 

only marginally thicker than beneath the western Basin and 

Range Province (30–34 km). They proposed that the Sierra 

Nevada gravity low, rather than being due to a thick low-density crustal root (Bateman and Eaton, 1967; Oliver, 1977), is 

Tectonomagmatic Model 61
62 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

the result of a hot, buoyant, asthenospheric-mantle upwelling 

displacing lithospheric mantle beneath the range crest. Ducea 

and Saleeby (1996, 1998) presented evidence, on the basis of 

mafic and ultramafic xenolith studies, that the asthenosphere 

lies close to the base of the crust immediately east of the Sierra 

Nevada crest and that the upper mantle and lower crust have 

been thinned, the upper mantle by convective erosion and 

(or) conversion to asthenosphere and the lower crust possibly 

by detachment of its eclogitic root. Thompson and Zoback 

(1979) and Zandt and others (1995) postulated a similar mantle 

upwelling at the east edge of the Basin and Range Province, beneath the Colorado Plateau Transition Zone. Thus, at 

the structurally and volcanically active western and eastern 

margins of the Basin and Range Province, proposed asthenospheric-mantle welts rise close to the base of the crust to depths 

shallower than beneath the central part of the province (fig. 29).

Although Ducea and Saleeby’s (1996) conclusions are 

based on data mainly from the central Sierra Nevada and central Owens Valley, their model has yet to be confirmed by geophysical studies in the vicinity of Long Valley. Considerable 

uncertainty remains concerning the depth to the Moho beneath 

the central and northern Sierra Nevada (Savage and others, 

1994). Clearly, a multidisciplinary trans-Sierran geophysical 

transect in the vicinity of Long Valley, similar to that across 

the southern Sierra Nevada (Wernicke and others, 1995), 

would be desirable—for example, across Pine Creek Pass in 

the Sierra Nevada to Westgard Pass in the White Mountains.

Geophysical uncertainties notwithstanding, petrologic, 

geochemical, and isotopic relations within the Long Valley 

Volcanic Field suggest a magmatic and structural model that 

is consistent with lithospheric-mantle thinning and formation 

of an asthenospheric-mantle welt beneath the volcanic field. 

These data, together with geochronologic and tectonic relations along the Owens Valley Rift and the East Sierran frontal 

fault zone, make an asthenospheric-mantle welt beneath 

the Pliocene Sierra Nevada crest an attractive model for the 

localization and evolution of volcanism at Long Valley and, 

possibly, at the Big Pine and Coso Volcanic Fields as well. At 

range-front offsets like those at the Long Valley, Big Pine, and 

Coso Volcanic Fields (fig. 1), where extensional deformation 

is probably intensified, localized domical upwellings rising 

from the asthenospheric mantle would provide the heat for 

localization of lithospheric-mantle melting, subcrustal magma 

ponding, crustal intrusion, and surface eruptions. Such a 

scenario also is consistent with the observed distinctive Long 

Valley precaldera and postcaldera magma compositions. A 

tectonomagmatic model for the evolution of the Long Valley 

Volcanic Field is outlined below and illustrated in figure 30.

Rapid extension along the Owens Valley Rift and the East 

Sierran frontal fault zone at about 4 Ma initiated local thinning 

of the lithospheric mantle along the west edge of the alreadythinned lithosphere of the Basin and Range Province, giving 

rise to a north- to northwest-trending asthenospheric-mantle 

welt at the lithosphere-asthenosphere interface (fig. 30A). 

4 5 5 0 5 5 6 0 6 5 7 0 7 5 8 0

10

12

14

16

18

20

22

24

Moat rhyolites

Early rhyolites

Bishop Tuff

Inyo Craters rhyolites

East Sierran basalt-andesite

East Sierran dacites

L

E

plagioclase

EXPLANATION

Al

2

O

3 CONTENT, IN WEIGHT PERCENT

SiO2 CONTENT, IN WEIGHT PERCENT

Figure 28. Al2O3 versus SiO2 contents for East Sierran lavas (gray areas) and postcaldera rhyolites, 

showing how postcaldera rhyolites (including the Bishop Tuff) project along a line suggesting derivation from dacite by plagioclase fractionation. E and L, earliest- and latest-erupted compositions in 

the zoned Bishop Tuff.
Decompression melting of this rising, possibly convecting 

asthenospheric-mantle welt above 80-km depth and conductive heating, intrusion, and melting of the adjacent lithospheric mantle generated both asthenospheric and lithospheric 

melts that fed Central and East Sierran, as well as Basin and 

Range, basalts, although asthenospheric-mantle melt only 

rarely reached the surface. Most Central and East Sierran 

basalts were derived by small to moderate degrees of partial 

melting of garnet lherzolite at 60- to 100-km depth, whereas 

to the east, Basin and Range basalts were derived by greater 

degrees of melting of spinel lherzolite above 60-km depth. At 

about 3 Ma, slowing of regional extension inhibited passage 

of basalt to the surface and caused ponding of large volumes 

of lithospheric-mantle-derived melt at the base of and, possibly, in the lower crust. In the east half of the volcanic field 

(Basin and Range Province), partial fusion of the lithospheric 

mantle and, possibly, mafic lower crust, followed by fractionation of the generated melts, produced the earliest precaldera 

dacitic lavas. In the west half of the volcanic field (Sierra 

Nevada Province), mafic magmas rose to higher levels, melting lower-crustal to midcrustal rocks, generating the more 

highly evolved East Sierran dacites. At about 2.5 Ma, cessation of regional extension terminated the rise of all mafic and 

intermediate-composition magmas to the surface, resulting in 

increased accumulation of basalt in the lower crust and generation of large volumes of entrapped dacitic magma. Initially 

formed in relatively small, isolated chambers, this dacitic 

magma gradually coalesced into a megachamber, from which 

the Glass Mountain high-SiO2 rhyolite evolved. The heterogeneity of early, highly evolved Glass Mountain rhyolites 

(Metz and Mahood, 1991) between 2.1 and 1.1 Ma suggests 

formation in discrete cupolas or chambers above this dacitic 

megachamber; only after 1 Ma did they coalesce and homogenize to form a single, continuous rhyolitic magma chamber, 

from which late Glass Mountain flows and the Bishop Tuff 

were erupted.

Shortly before the eruption of the Bishop Tuff and the 

formation of Long Valley Caldera (fig. 30B), renewed regional 

extension caused rapid injection of basalt into the base of the 

Long Valley (Bishop Tuff) magma chamber. This basalt is 

visualized as spreading laterally in multiple sheets across the 

floor of the chamber, in a manner similar to that described 

by Wiebe (1993a) for the floored Devonian plutons now 

exposed along the Maine coast—this “hotplate” effect possibly triggering eruption of the Bishop Tuff. After the eruption of the Bishop Tuff and the collapse of the caldera, basalt 

continued welling up into the base of the partially emptied 

magma chamber, forming chilled blobs, sills, and dikes in 

the partially molten residual rhyolite. Sufficiently hydrous to 

vesiculate, these mafic blobs and disrupted dike and sill fragments rose buoyantly to the chamber roof, aided by turbulent 

churning and thinning of the chamber caused by eruption of 

the Bishop Tuff; they were then transported to the surface as 

mafic enclaves in the Early and Moat Rhyolites. By this time, 

760–500 ka, the asthenospheric-mantle welt had risen into 

the uppermost levels of the lithospheric mantle and impinged 

against mafic lower crust (fig. 30B), and so postcaldera basalt 

compositions reflect nearly complete melting of mafic lower 

crust or, possibly, mixing of lithospheric and asthenospheric 

mantle (N–MORB) melts.

The shift in asthenospheric-mantle-melt composition 

from OIB type at the onset of precaldera volcanism (4 Ma) 

to N–MORB type at the onset of postcaldera volcanism (0.7 

Ma) may have resulted from convergence and gradual overriding of the subducted East Pacific Rise spreading center by 

the North American continental plate, wherein the precaldera 

OIB component was derived from the mantle wedge east of 

the subducting oceanic plate and the postcaldera N–MORB 

0 0

50 50

100

1,000

DISTANCE, IN KILOMETERS

900 800 700 600 500 400 300 200 100

100

150 150

200

DEPTH, IN KILOMETERS

200

Great

Valley Sierra Nevada

LONG VALLEY

VOLCANIC FIELD

Basin and Range Colorado Plateaus

Upper crust

Lower crust

Lithospheric 

mantle

Asthenospheric mantle

MOHO

Figure 29. West-to-east cross section from the southern Sierra Nevada (lat 36.5° N.) across the Basin and Range Province to the 

Colorado Plateaus Province, showing inferred crust-lithosphere-asthenosphere relations (after Wernicke and others, 1995; Zandt and 

others, 1995). Asthenospheric-mantle welts rising through lithospheric mantle to near lower-crustal interface are conspicuous features 

at both west and east edges of the Basin and Range Province.

Tectonomagmatic Model 63
64 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

80 km

70

60

50

40

30

20

10

0 SL

Glass Mountain Basin and Range

basalt

Basin and Range

dacite

Sierran dacite

Sierran basalt

LOWER CRUST

MOHO

LITHOSPHERIC

MANTLE

Spinel

peridotite

Spinel

peridotite

MOHO

Garnet 

peridotite

Granulite

Sierran dacite

generation

Basin and Range

dacite generation

Basalt underplating

Precaldera rhyolite 

accumulation

ASTHENOSPHERIC MANTLE

Zone of precaldera basalt generation

2 - 1 Ma

3 - 2 Ma

PRECALDERA

A

CO2

Glass Mountain

Long Valley Caldera

Mammoth Mountain Mafic-enclave-laden Early Rhyolite

Horseshoe

Lake

Buttresses

80 km

70

60

50

40

30

20

10

0 SL Zone of longperiod earthquakes

Plexus of 

post-1989 basalt

intrusions

Zone of postcaldera dacite

generation

(250 - 50 ka)

LOWER CRUST

LOWER CRUST

MOHO

MOHO

LITHOSPHERIC

MANTLE

ASTHENOSPHERIC MANTLE

Ultramafic restite

accumulation

Zone of postcaldera basalt generation

Dacite-dominant volume

Semimolten rhyolite

Residual Bishop Tuff

magma chamber

Post-1980 basalt

intrusions

Early postcaldera

basalt intrusions

(700 - 60 ka)

LITHOSPHERIC

MANTLE

POSTCALDERA

Crystallized magma

B

Figure 30. West-to-east cross sections of the Long Valley Volcanic Field, Calif., showing inferred subcrustal relations. A, 

During precaldera evolution (4–1 Ma). B, From 700 ka to present. Dashed lines defining regions of generation of various 

magmas do not define shapes of individual magma bodies, which are unknown and probably much more complex.
component was derived from the slab gap west of the trailing 

edge of the sinking Farallon oceanic plate. The estimated convergence rate of 4 to 5.5 cm/yr for the past 4.5 Ma (Atwater 

and Molnar, 1973) implies a relative eastward migration of the 

slab gap of 120 to 165 km, suggesting that it has moved from 

beneath the Great Valley to beneath the eastern Sierra Nevada 

in the past 3 m.y.

Between 4 and 1 Ma, the site of mafic-magma generation 

beneath the caldera rose 30 km as a result of extensional thinning and advective melting from the base of the lithospheric 

mantle (60–80-km depth) to the base of lower crust (30-km 

depth), requiring an average ascent rate of about 1 cm/yr, 

though certainly fluctuating in response to changing rates of 

regional extension. Gravitational unloading following partial 

emptying of the Bishop Tuff magma chamber and consequent 

isostatic rise of the subcauldron asthenosphere into the lower 

crust may have facilitated the sudden change in chemical signature of the postcaldera basalts and accelerated the influx of 

basalt into the residual Bishop Tuff magma chamber. By about 

400 ka, ponded subcauldron basalt, migrating laterally beneath 

the “shadow zone” of the residual Bishop Tuff magma chamber, rose up in caldera ring fractures or along the incipient 

Mono-Inyo Craters fissures system and began to be erupted in 

the west moat. Fractionation and random mixing of basalt with 

coeval midcrustal dacitic magma (produced by basalt fractionation) and with rhyolitic magma at the margins of the Long 

Valley magma chamber generated the intracaldera trachyandesites, as well as the extracaldera flows at Mammoth Pass, 

Pumice Butte, and Devils Postpile. At about 250 ka, low-SiO2

rhyolite and dacite began to be erupted at Mammoth Mountain 

and in the west moat. At the west edge of this zone of Caldera 

suite volcanism, the Buttresses and Horseshoe Lake basalts 

were generated by melting of lithospheric mantle at the steep 

asthenosphere interface (fig. 30B; see Ducea and Saleeby, 

1996). At about 30 ka, basalt began migrating northward along 

the Mono-Inyo Craters fissure system and gave rise to the 

Mono-Inyo lavas, including the dacites and low-SiO2 rhyolites 

of Mono Lake, as well as the high-SiO2 rhyolites of Mono 

Craters. For the past 5,000 years, basaltic to dacitic eruptions 

have been confined to the north and south extremities of the 

Mono-Inyo Craters Volcanic Chain. This distribution suggests, 

in agreement with teleseismic evidence (Achauer and others, 

1986; Dawson and others, 1990), that in the central part of the 

volcanic chain, where voluminous high-SiO2 rhyolite domes 

and flows have been erupted, a rhyolitic-magma chamber sufficiently large to produce a “shadow zone” may have inhibited 

upward passage of basaltic magma except at the extreme ends 

of the fissure system—at Black Point in the north and at Red 

Cones in the south.

The common occurrence of mafic enclaves in postcaldera 

rhyolitic members of the Long Valley Volcanic Field suggests 

that many, if not all, eruptions were triggered by injection 

of basalt into the root of the magmatic system (see Sparks 

and others, 1977). Such repeated influx of basalt, as noted 

by Lachenbruch and others (1976), is required to sustain its 

silicic-magma chamber in a semimolten state capable of producing eruptions over the past 1 m.y. Whether these eruptions 

were derived from a continuously maintained molten-magma 

chamber or by repeated remelting of previously congealed 

“granitic” parts of the chamber (Mahood, 1990) would depend 

on the frequency of basalt reinjection and the rate of regional 

extension, neither of which are presently well constrained.

The form of these rejuvenating basalt injections as 

multiple sheets at the base of the silicic-magma chamber is 

inferred from apparent subcauldron analogs of the Long Valley 

magma chamber observed in the Paleozoic and Mesozoic granitic plutons at Cordillera del Paine, Chile (Michael, 1991), on 

Mount Desert Island and Isle de Haute, Maine (Wiebe, 1993a, 

b, 1994; Chapman and Rhodes, 1992), and in Onion Valley, 

eastern Sierra Nevada (Sisson and others, 1996), where numerous thin sill-like sheets of basalt, commonly with associated 

mafic cumulate layers (Sisson and others, 1996; Wiebe, 1997), 

invaded the lower levels of these floored (then semimolten) 

plutons (for the mechanics of such intrusions, see Snyder and 

Tait, 1995, 1998). A detail further supporting this analogy is 

the marked Ni and Cr depletion in Long Valley intracaldera 

basalts, suggesting subcauldron ponding and formation of 

mafic cumulates at depth, like those evident in these ancient 

plutonic complexes.

This model of a rising asthenospheric-mantle welt may 

also apply to the Coso and Big Pine Volcanic Fields. At Coso, 

the difference in chemical signatures between Pliocene and 

Pleistocene basalt lavas suggests a shift from lithosphericmantle (IAB like) to asthenospheric-mantle (OIB like) sources 

approximately contemporaneously with Long Valley. At the 

smaller, shorter-lived Pleistocene Big Pine Volcanic Field, 

however, only lavas with modified lithospheric-mantle signatures are present, suggesting that an asthenospheric-mantle 

cupola, if present, has not yet risen to lower-crustal levels.

Portrayal of the Long Valley magma chamber as an 

asymmetric, floored, semilaccolithic body (fig. 30; see Bailey, 

1984; Bailey and Hill, 1990) is partly based on the form of 

the above-mentioned floored plutonic analogs but is mainly 

inferred from the work of Lachenbruch and others (1976), 

whose heat-flow studies indicate that no thermal source 

persisted beneath the east half of the caldera after its collapse. 

They speculated that the east half of the magma chamber was 

nearly completely emptied during the eruption of the Bishop 

Tuff and that during its collapse the chamber roof came to 

rest on the floor. In accord with the fact that post-Bishop Tuff 

intracaldera eruptions have been limited to the west half of 

the caldera, they further inferred that the deep magmatic root 

existed and has persisted only in the west half. An oblate, 

floored configuration of the chamber also is not inconsistent 

with seismic and geophysical studies, including gravity (Carle, 

1988), seismic refraction (Hill, 1976, Luetgert and Mooney, 

1985), teleseismic P-wave tomography and S-wave attenuation 

(Steeples and Iyer, 1976; Achauer and others, 1986; Dawson and others, 1990; Ponko and Sanders, 1994; Steck and 

Prothero, 1994; Sanders and others, 1995), and deformation 

analysis (Langbein and others, 1995). Although estimates of 

the depth, volume, and liquid fraction of the residual magma 

Tectonomagmatic Model 65
66 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

chamber based on these studies vary considerably, the bestconstrained estimates indicate a chamber with a roof at 5- to 

7-km depth and a floor at 11- to 15-km depth, a volume of 

500 to 1,500 km3, and a liquid fraction ranging from 5 to 36 

volume percent. The model here proposed also is generally 

consistent with the teleseismic-P-wave-tomography models 

of Dawson and others (1990) and Steck and Prothero (1994), 

who defined a low-density zone in the upper part of the Long 

Valley magmatic system (at 5–30-km depth) and a deeper, 

higher-density root zone (at 50–60-km depth). They inferred 

this high-density root zone to represent “depleted” mantle, 

presumably generated by prolonged distillation of the lithospheric mantle and gradual accumulation of mafic restite 

components in the lower part of the system over its 4-m.y. 

lifespan. Alternatively, this zone could represent the detached, 

sinking eclogitic Sierra Nevadan root postulated by Ducea and 

Saleeby (1996, 1998).

Recent Reactivation of Basaltic 

Magmatism

The foregoing model provides some insight into the 

tectonic processes driving the current (post-1980) seismic 

unrest at Long Valley. Since 1980, shallow earthquake swarms 

have been concentrated in three general areas: (1) the south 

moat of the caldera, (2) the High Sierra granitic terrain south 

of the caldera, and (3) beneath Mammoth Mountain. At present, little evidence exists for unrest beneath the northern part 

of the Mono-Inyo Craters Volcanic Chain, although briefly in 

1990, faint swarms of long-period earthquakes were detected 

beneath Pumice Valley, east of the chain (A.M. Pitt, oral commun., 1990). In two of these seismically active areas (the south 

moat in 1980 and 1983 and Mammoth Mountain in 1989), 

magma of undetermined composition is inferred to have risen 

in dikes to within 2 to 3 km of the surface (Julian, 1983; Aki, 

1984; Sanders, 1984; Savage and Cockerham, 1984; Hill and 

others, 1990; Langbein and others, 1995). Given the current 

seismicity and the postcaldera eruptive record, both areas are 

potential sites for future eruptions.

Mammoth Mountain was built between 250 and 50 ka 

by extrusion of 30 or more dacite domes and flows but has 

been inactive since. This 50-k.y. quiescence after a 200-k.y. 

active period in which the average interval between eruptions 

was about 7 k.y. suggests that its source magma chamber has 

congealed and is unlikely to produce dacitic eruptions in the 

foreseeable future. However, the mountain has been the site of 

eruptions later than its dacite dome-building phase. At about 

700 B.P., phreatic eruptions broke out from six or more vents 

on the north face of the mountain (Rinehart and Huber, 1965; 

Sorey and others, 1998)—probably forerunners of the 600-

B.P. basalt-triggered phreatic and rhyolitic eruptions at Inyo 

Craters (Varga and others, 1990; Mastin, 1991). Although the 

most recent basaltic eruptions in the vicinity of Mammoth 

Mountain occurred about 5 ka at Red Cones, 3 km southwest 

of the mountain, active mantle-CO2-emitting soil and fumaroles along fissures and faults on its north and south flanks 

suggest that basaltic magma is again rising at depth.

Rhyolitic eruptions chemically related to the residual 

Long Valley magma chamber built the Mammoth Knolls rhyolite dome complex between 150 and 100 ka in the southwest 

moat, but the latest southwest-moat eruption was basaltic, 

a 64-ka flow extending 3 km westward from the Mammoth 

Lakes townsite to Casa Diablo. Thus, both in the vicinity of 

Mammoth Mountain and in the southwest moat, the most 

recent eruptions have been basaltic. Coupled with the recent 

emission of mantle-derived CO2 and He and the occurrence of 

long-period earthquakes at 10- to 25-km depth in the vicinity 

of Mammoth Mountain, basalt again appears to be invading the roots of the magmatic system, including beneath the 

floor of the Long Valley rhyolitic magma chamber, causing 

the current resurgent uplift. Although mantle-derived CO2

has not been detected in fumaroles or soils on or peripheral 

to the resurgent dome (Sorey and others, 1998), its absence 

may be due to entrapment of magmatic CO2 below or within 

the rhyolitic-magma chamber. In contrast, the absence of a 

partially molten residual magma chamber beneath Mammoth 

Mountain allows unimpeded flow of magma through a plexus 

of dikes and sills to depths as shallow as 10 km and the release 

of magmatic gases to a shallower vapor-gas reservoir, thence 

to near-surface faults and fissures, and finally to widespread 

dispersal in surface soils (Gerlach and others, 1998; Sorey and 

others, 1998).

Future Volcanism and Hazards

The volume of magma (presumably basaltic) modeled to 

account for post-1979 inflation of the resurgent dome (neglecting any contribution from thermal expansion of the subcauldron rocks and the hydrothermal system) is about 0.3 km3

(Langbein and others, 1995), nearly 3 times that of the most 

recent individual postcaldera rhyolitic or basaltic eruptions. 

Little is known about the volume of injected magma required 

to trigger an eruption, virtually nothing is known about the 

volume of resurgence that may have preceded earlier postcaldera eruptions at Long Valley, and little is known from similar 

intracaldera eruptions elsewhere in the world. Although an 

influx of 0.3 km3

 of magma may seem insufficient to trigger an eruption of any significant size, any eruption, however 

small, could be highly disruptive locally, particularly should 

it occur in winter, when melting of heavy snow cover could 

generate swift-moving debris flows and mudflows (lahars). 

Given the long postcaldera record of sporadic eruptions, all of 

small volume, an eruption approaching the magnitude of the 

Bishop Tuff (600 km3

) is extremely unlikely. However, if the 

ratio of eruptive volume to total magma-chamber volume for 

large rhyolitic eruptions is about 1:10, as estimated by Smith 

(1979), then the theoretical eruptive output of a Long Valley magma chamber with a residual volume of 30 to possibly 
100 km3

 (Dawson and others, 1990) could reach several cubic 

kilometers. Such an eruption, though only a fraction of that 

of the Bishop Tuff, could seriously impact the region, as well 

as much of the Western United States. Although the threshold 

volume of new magma required to trigger such an eruption is 

unknown, at the present estimated rate of basalt influx (<∼0.02 

km3/yr), the required volume probably would take many 

decades, possibly centuries, to accumulate. All of the above 

estimates are fraught with large uncertainties, however, and 

there is no substitute for diligent, persistent monitoring during 

episodes of seismic and magmatic unrest.

Viewed in terms of episodicity, a rhyolitic eruption from 

the Long Valley magma chamber in the near future seems 

unlikely. Because the interval between postcaldera rhyolitic 

eruptions derived from the Long Valley magma chamber 

in the past 700 k.y. is about 200 k.y. and the latest eruption 

(Mammoth Knolls) was 100 ka, the next eruption might not 

be expected for about another 100 k.y.—a statistically tenuous 

estimate. Nonetheless, rhyolitic eruptions along the MonoInyo Craters Volcanic Chain have occurred at 250- to 700-year 

intervals over the past 5 k.y., the latest at Paoha Island between 

about A.D. 1700 and 1850 (Stine, 1987). Thus, the probability 

of an eruption from the Mono-Inyo Craters Volcanic Chain 

seems much higher and could be considered either overdue or 

unlikely for another 500 years. The relatively passive Paoha 

Island eruptions, which included uplift of lake-bottom sedimentary deposits by a rhyolite cryptodome and the formation 

of several small dacite cinder cones and a lava flow, probably 

posed little danger to humans; however, the 650-B.P. pyroclastic eruptions at Mono Craters and Inyo Craters probably did 

pose a serious threat to the Native American population at that 

time, although no reliable records or oral traditions of their 

observation or impact are known. None of these recent eruptions seriously impacted the sites of the present local population centers at Lee Vining, June Lake, or Mammoth Lakes, 

although 2 to 8 cm of ash and pumice fell on the outskirts of 

the present townsite of Mammoth Lakes (Miller, 1985).

In view of the current seismic unrest, the probability of 

a basaltic eruption from Long Valley Caldera seems much 

higher than of an explosive rhyolitic one; however, the recurrence intervals of recent basaltic eruptions in the Long Valley 

area is poorly known. Several basaltic eruptions occurred 

around the base of Mammoth Mountain and in Devils Postpile 

National Monument about 100 ka, near June Lake about 30 ka, 

at Black Point about 13 ka, and at Red Cones about 5 ka. The 

current (post-1980) seismic unrest in the vicinity of Mammoth 

Mountain may portend a similar small basaltic eruption—possibly west of the mountain near Devils Postpile or southeast 

of it in the Horseshoe Lake CO2-tree-kill area, both sites of 

former vents for relatively recent basaltic eruptions. However, 

with resurgent-dome inflation proceeding at an average rate of 

2 cm/yr but intermittently at short-term rates of as much as 20 

cm/yr (D.P. Hill, oral commun., 1998), an intracaldera eruption, either basaltic or rhyolitic (possibly triggered by basalt 

influx), cannot be dismissed. Nevertheless, such seismic unrest 

and apparent precursory activity within large calderas does 

not invariably culminate in an eruption; the worldwide record 

at large calderas (Newhall and Dzurisin, 1988) indicates that 

in 60 episodes of seismic unrest recognized in historical time, 

only 10 have produced eruptions.

References Cited

Achauer, Ulrich, Greene, Lizbeth, Evans, J.R., and Iyer, 

H.M., 1986, Nature of the magma chamber underlying the 

Mono Craters area, eastern California, as determined from 

teleseismic travel time residuals: Journal of Geophysical 

Research, v. 91, no. B14, p. 13873–13891.

Aki, Keiiti, 1984, Evidence for magma intrusion during the 

Mammoth Lakes earthquakes of May 1980 and implications 

of the absence of volcanic (harmonic) tremor: Journal of 

Geophysical Research, v. 89, no. B9, p. 7689–7696.

Asmerom, Yemane, Jacobsen, S.B., and Wernicke, B.P., 1994, 

Variations in magma source regions during large-scale 

continental extension, Death Valley region, Western United 

States: Earth and Planetary Science Letters, v. 125, no. 1–4, 

p. 235–254.

Atwater, Tanya, and Molnar, Peter, 1973, Relative motion of 

the Pacific and North American plates deduced from seafloor spreading in the Atlantic, Indian, and South Pacific 

oceans, in Proceedings of conference on tectonic problems 

of the San Andreas Fault system: Stanford University Publications in Geological Sciences, v. 13, p. 136–148.

Bacon, C.R., 1986, Magmatic inclusions in silicic and intermediate volcanic rocks: Journal of Geophysical Research, v. 

91, no. B6, p. 6091–6112.

Bacon, C.R., Bruggman, P.E., Christiansen, R.L., Clynne, 

M.A., Donnelly-Nolan, J.M., and Hildreth, Wes, 1997, 

Primitive magmas at five Cascade volcanic fields; melts 

from hot, heterogeneous sub-arc mantle: Canadian Mineralogist, v. 35, no. 2, p. 397–423.

Bacon, C.R., and Druit, T.H., 1988, Compositional evolution of the zoned calcalkaline magma chamber of Mount 

Mazama, Crater Lake, Oregon: Contributions to Mineralogy 

and Petrology, v. 98, no. 2, p. 224–256.

Bacon, C.R., and Metz, Jenny, 1984, Magmatic inclusions in 

rhyolites, contaminated basalts, and compositional zonation 

beneath the Coso Volcanic Field, California: Contributions 

to Mineralogy and Petrology, v. 85, no. 4, p. 346–365.

Baedecker, P.A., and McKown, D.M., 1987, Instrumental 

neutron activation analysis of geochemical samples, in

Baedecker, P.M., ed., Methods for geochemical analysis: 

U.S. Geological Survey Bulletin 1770, p. H1–H14.

References Cited 67
68 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

Bailey, R.A., 1982, Other potential eruption centers in California; Long Valley-Mono Lake, Coso, and Clear Lake 

volcanic fields, in Martin, R.C., and Davis, J.F., eds., Status 

of volcanic prediction and emergency response capabilities 

in volcanic hazard zones of California: California Division 

of Mines and Geology Special Publication 63, p. 17–28.

Bailey, R.A., 1983, Mammoth Lakes earthquakes and ground 

uplift; precursors to possible volcanic activity?: Earthquake 

Information Bulletin, v. 15, no. 3, p. 88–101.

Bailey, R.A., 1984, Chemical evolution and current state of 

the Long Valley magma chamber, in Hill, D.P., Bailey, R.A., 

and Ryall, A.S., eds., Proceedings of Workshop XIX; active 

tectonic and magmatic processes beneath Long Valley caldera, eastern California: U.S. Geological Survey Open-File 

Report 84–939, p. 24–40.

Bailey, R.A., 1989, Geologic map of Long Valley caldera, 

Mono-Inyo Craters volcanic chain, and vicinity, eastern 

California: U.S. Geological Survey Miscellaneous Investigations Series Map I–1933, 11 p., scale 1:62,500, 2 sheets.

Bailey, R.A., Dalrymple, G.B., and Lanphere, M.A., 1976, 

Volcanism, structure, and geochronology of Long Valley 

caldera, Mono County, California: Journal of Geophysical 

Research, v. 81, no. 5, p. 725–744.

Bailey, R.A., and Hill, D.P., 1990, Magmatic unrest at Long 

Valley caldera, California, 1980–1990: Geoscience Canada, 

v. 17, no. 3, p. 175–179.

Barker, P.F., 1995, Tectonic framework of the East Scotia Sea, 

in Taylor, Brian, ed., Backarc basins; tectonics and magmatism: New York, Plenum, p. 281–314.

Bateman, P.C., and Eaton, J.P., 1967, Sierra Nevada Batholith: 

Science, v. 158, no. 3807, p. 1407–1417.

Beanland, Sarah, and Clark, M.C., 1994, The Owens Valley 

fault zone, eastern California, and surface faulting associated with the 1872 earthquake: U.S. Geological Survey 

Professional Paper 1982, 29 p.

Benson, L.V., Burdett, J.V., Kashgarian, Michaele, Lund, S.P., 

Phillips, F.M., and Rye, R.O., 1996, Climatic and hydrologic oscillations in the Owens Lake basin and adjacent 

Sierra Nevada, California: Science, v. 274, no. 5288, p. 

746–751.

Benson, L.V., Currey, D.R., Dorn, R.I., Lajoie, K.R., Oviatt, 

S.W., Robinson, S.W., Smith, G.I., and Stine, S.W., 1990, 

Chronology of expansion and contraction of four Great 

Basin lake systems during the past 35,000 years: Paleogeography, Paleoclimatology, Paleoecology, v. 78, p. 241–286.

Bischoff, J.L., Fitzpatrick, J.A., and Rosenbauer, R.J., 1993a, 

The solubility and stabilization of ikaite (CaCO3·6H2O) from 

0° to 25°C; environmental and paleoclimatic implications for 

thinolite tufa: Journal of Geology, v. 101, no. 1, p. 21–33.

Bischoff, J.L., Menking, K.N., Fitts, J.P., and Fitzpatrick, 

J.A., 1997, Climatic oscillations 10,000–155,000 yr B.P. 

at Owens Lake, California, reflected in glacial rock flour 

abundance and lake salinity in core OL–92: Quaternary 

Research, v. 48, no. 3, p. 313–325.

Bischoff, J.L., Stine, Scott, Rosenbauer R.J., Fitzpatrick, J.A., 

and Stafford, T.W., Jr., 1993b, Ikaite precipitation by mixing 

of shoreline springs and lake water, Mono Lake, California, 

USA: Geochimica et Cosmochimica Acta, v. 57, no. 16, p. 

3855–3865.

Bradshaw, T.K., Hawkesworth, C.J., and Gallagher, Kerry, 

1993, Basaltic volcanism in the southern Basin and Range; 

no role for a mantle plume: Earth and Planetary Science 

Letters, v. 116, no. 1–4, p. 45–62.

Bradshaw, T.K., and Smith, E.I., 1994, Polygenetic Quaternary 

volcanism at Crater Flat, Nevada: Journal of Volcanology 

and Geothermal Research, v. 63, no. 3–4, p. 165–182.

Brophy, J.G., Dorais, M.J., Donnelly-Nolan, J.M., and Singer, 

B.S., 1996, Plagioclase zonation styles in hornblende gabbro inclusions from Little Glass Mountain, Medicine Lake 

Volcano, California; implications for fractionation mechanisms and the formation of composition gaps: Contributions 

to Mineralogy and Petrology, v. 126, no. 1–2, p. 121–136.

Bursik, M.I., and Gillespie, A.R., 1993, Late Pleistocene glaciation of Mono Basin, California: Quaternary Research, v. 

39, no. 1, p. 24–35.

Carle, S.F., 1988, Three dimensional gravity modeling of the 

geologic structure of Long Valley Caldera: Journal of Geophysical Research, v. 93, no. 11, p. 13237–13250.

Carder, D.S., 1973, Trans-California seismic profile, Death 

Valley to Monterey Bay: Seismological Society of America 

Bulletin, v. 63, no. 2, p. 571–586.

Chapman, Marshall, and Rhodes, J.M., 1992, Composite 

layering in the Isle au Haute igneous complex, Maine; 

evidence for periodic invasion of mafic magma into an 

evolving magma reservoir: Journal of Volcanology and 

Geothermal Research, v. 51, no. 1–2, p. 41–60.

Chaudet, R.E., 1986, The petrology and geochemistry of precaldera magmas, Long Valley Caldera, eastern California: 

Blacksburg, Virginia Polytechnic Institute, M.Sc. thesis, 34 

p.

Chouet, B.A., 1996, Long period volcano seismicity; its 

source and use in eruption forecasting: Nature, v. 380, no. 

6572, p. 309–316.

Christensen, J.N., and DePaolo, D.J., 1993, Time scales of 

large volume silicic magma systems; Sr isotopic systematics 

of phenocrysts and glass from the Bishop Tuff, Long Valley, 

California: Contributions to Mineralogy and Petrology, v. 

113, no. 1, p. 100–114.
Christensen, M.N., 1966, Late Cenozoic crustal movements 

in the Sierra Nevada of California: Geological Society of 

America Bulletin, v. 77, no. 2, p. 163–182.

Council, T.C., and Bennett, P.C., 1993, Geochemistry of ikaite 

formation at Mono Lake, California; implications for the 

origin of tufa mounds: Geology, v. 21, no. 11, p. 971–974.

Cousens, B.L., 1996, Magmatic evolution of Quaternary mafic 

magmas at Long Valley Caldera and the Devils Postpile, 

California; effects of crustal contamination on the lithospheric mantle-derived magmas: Journal of Geophysical 

Research, v. 101, no. B12, p. 27673–27689.

Crough, S.T., and Thompson, G.A., 1977, Upper mantle origin 

of Sierra Nevada uplift: Geology, v. 5, no. 7, p. 396–399.

Curry, R.R., 1971, Glacial and Pleistocene history of the 

Mammoth Lake Sierra, a geologic guidebook: Missoula, 

University of Montana, Department of Geology Geological 

Series Publication, v. 2, no. 11, 47 p.

Davies, G.R., Halliday, A.N., Mahood, G.A., and Hall, C.M., 

1994, Isotopic constraints on the production rates, crystallization histories and residence times of pre-caldera silicic 

magmas, Long Valley, California: Earth and Planetary Science Letters, v. 125, no. 1, p. 17–37.

Davies, G.R., and Halliday, A.N., 1998, Development of 

the Long Valley rhyolitic magma system; strontium and 

neodymium isotope evidence from glasses and individual 

phenocrysts: Geochimica et Cosmochimica Acta, v. 62, no. 

21–22, p. 3561–3574.

Dawson, P.B., Evans, J.R., and Iyer, H.M., 1990, Teleseismic 

tomography of the compressional wave velocity structure 

beneath the Long Valley region, California: Journal of Geophysical Research, v. 95, no. B7, p. 11021–11050.

Ducea, M.N., and Saleeby, J.B., 1996, Buoyancy sources for a 

large, unrooted mountain range, the Sierra Nevada, California; evidence from xenolith thermobarometry: Journal of 

Geophysical Research, v. 101, no. B4, p. 8229–8244.

Ducea, M.N., and Saleeby, J.B., 1998, The age and origin of a 

thick mafic-ultramafic keel from beneath the Sierra-Nevada 

Batholith: Contributions to Mineralogy and Petrology, v. 

133, no. 1–2, p. 169–185.

Eaton, G.P., 1982, The Basin and Range Province; origin and 

tectonic significance: Annual Review of Earth and Planetary 

Sciences, v. 10, p. 409–440.

Eichelberger, J.C., Vogel, T.A., Younker, L.W., Miller, C.D., 

Heiken, G.H., and Wohletz, K.H., 1988, Structure and stratigraphy beneath a young phreatic vent; South Inyo Crater, 

Long Valley Caldera, California: Journal of Geophysical 

Research, v. 93, no. B11, p. 13208–13220.

Ellam, R.M., 1992, Lithosphere thickness as a control on 

basalt geochemistry: Geology, v. 20, no. 2, p. 153–156.

Farmer, G.L., Glazner, A.F., Wilshire, H.G., Wooden, J.L., 

Pickthorn, W.J., and Katz, Marvin, 1995, Origin of late 

Cenozoic basalts at the Cima volcanic field, Mojave Desert, 

California: Journal of Geophysical Research, v. 100, no. B5, 

p. 8399–8415.

Farrar, C.D., Sorey, M.L., Evans, W.C., Howle, J.F., Kerr, 

B.D., Kennedy, B.M., King, C.Y., and Southon, J.R., 1995, 

Forest-killing diffuse CO2 emission at Mammoth Mountain, 

eastern California as a sign of magmatic unrest: Nature, v. 

376, no. 6542, p. 675–678.

Feuerbach, D.L., Smith, E.I., Walker, J.D., and Tangeman, 

J.A., 1993, The role of the mantle during crustal extension; 

constraints from geochemistry of volcanic rocks in the Lake 

Mead area, Nevada and Arizona: Geological Society of 

America Bulletin, v. 105, no. 12, p. 1561–1575.

Fitton, J.G., James, D.E., Kempton, P.D., Ormerod, D.D., and 

Leeman, W.P., 1988, The role of lithospheric mantle in the 

generation of late Cenozoic basic magmas in the Western 

United States, in Menzies, M.A., and Cox, K.G., eds., 

Oceanic and continental lithosphere; similarities and differences: Oxford, U.K., Clarendon Press, p. 331–349.

Fitton, J.G., James, D.E., and Leeman, W.P., 1991, Basaltic 

magmatism associated with Late Cenozoic extension in the 

Western United States; compositional variations in space 

and time: Journal of Geophysical Research, v. 96, no. B8, p. 

13693–13711.

Fliedner, M.M., Ruppert, S.D., Malin, P.E., Park, S.K., Jiracek, 

G.R., Phinney, R.A., Saleeby, J.B., Wernicke, B.P., Clayton, 

R.W., Keller, G.R., Miller, K.C., Jones, C.H., Luetgert, J.H., 

Mooney, W.D., Oliver, H.L., Klemperer, S.L., and Thompson, G.A., 1996, Three-dimensional crustal structure of the 

southern Sierra Nevada from seismic fan profiles and gravity modeling: Geology, v. 24, no. 4, p. 367–370.

Frost, T.P., and Mahood, G.A., 1987, Field, chemical, and 

physical constraints on mafic-felsic magma interaction 

in the Lamarck Granodiorite, Sierra Nevada, California: 

Geological Society of America Bulletin, v. 99, no. 2, p. 

272–291.

Gerlach, T.M., Doukas, M.P., McGee, K.A., and Kessler, Richard, 1998, Three-year decline of magmatic CO2 emissions 

from soils of a Mammoth Mountain tree kill; Horseshoe 

Lake, California, 1995–1997: Geophysical Research Letters, v. 25, no. 11, p. 1947–1950.

Gilbert, C.M., 1938, Welded tuff in eastern California; Geological Society of America Bulletin, v. 49, no. 12, pt. 1, p. 

1829–1862.

Gilbert, C.M., 1941, Late Tertiary geology southeast of Mono 

Lake, California: Geological Society of America Bulletin, v. 

52, no. 6, p. 781–815.

References Cited 69
70 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

Gilbert, C.M., Christensen, M.N., Al-Rawi, Yehya, and Lajoie, 

K.R., 1968, Structural and volcanic history of Mono Basin, 

California-Nevada, in Coats, R.R., ed., Studies in volcanology; a memoir in honor of Howel Williams: Geological 

Society of America Memoir 116, p. 275–329.

Glazner, A.F., and Farmer, G.L., 1992, Production of isotopic 

variability in continental basalts by cryptic crustal contamination: Science, v. 255, no. 5040, p. 72–74.

Glazner, A.F., Farmer, G.L., Hughes, W.T., Wooden, J.L., and 

Pickthorn, W.J., 1991, Contamination of basaltic magma by 

mafic crust at Amboy and Pisgah Craters, Mojave Desert, 

California: Journal of Geophysical Research, v. 96, no. B8, 

p. 13673–13691.

Grove, T.J., and Donnelly-Nolan, J.M., 1986, The evolution 

of young silicic lavas at Medicine Lake Volcano, California; implications for the origin of compositional gaps in 

calc-alkaline series lavas: Contributions to Mineralogy and 

Petrology, v. 92, no. 3, p. 281–302.

Halliday, A.N., 1990, Reply to comment of R.S.J. Sparks, H.E. 

Huppert, and C.J.N. Wilson on “Evidence for long residence 

times of rhyolitic magma in the Long Valley magmatic 

system; the isotope record in precaldera lavas of Glass 

Mountain”: Earth and Planetary Science Letters, v. 99, no. 

4, p. 390–394.

Halliday, A.N., Mahood, G.A., Holden, Peter, Metz, J.M., 

Dempster, T.J., and Davidson, J.P., 1989, Evidence for long 

residence times of rhyolitic magmas in the Long Valley 

magmatic system; the isotopic record in precaldera lavas of 

Glass Mountain: Earth and Planetary Science Letters, v. 94, 

no. 3–4, p. 274–290.

Hanchar, J.M., Miller, C.F., Wooden, J.L., Bennett, V.C., 

and Staude, J.-M.G., 1994, Evidence from xenoliths for a 

dynamic lower crust, eastern Mojave Desert, California: 

Journal of Petrology, v. 35, no. 5, p. 1377–1415.

Heumann, Arnd, and Davies, G.R., 1997, Isotopic and chemical 

evolution of the post-caldera rhyolitic system at Long Valley, 

California, Journal of Petrology: v. 38, no. 12, p. 1661–1678.

Hildreth, Wes, 1979, The Bishop Tuff; evidence for the origin 

of compositional zoning in silicic magma chambers, in 

Chapin, C.E., and Elston, W.E., eds., Ash-flow tuffs: Geological Society of America Special Paper 180, p. 43–72.

Hill, D.P., 1976, Structure of Long Valley Caldera, California, 

from a seismic refraction experiment: Journal of Geophysical Research, v. 81, no. 5, p. 745–753.

Hill, D.P., 1996, Earthquakes and carbon dioxide beneath 

Mammoth Mountain, California: Seismological Research 

Letters, v. 67, no. 1, p. 8–15.

Hill, D.P., Bailey, R.A., and Ryall, A.S., 1985a, Active tectonic and magmatic processes beneath Long Valley caldera, 

eastern California; an overview: Journal of Geophysical 

Research, v. 90, no. B13, p. 11111–11120.

Hill, D.P., Ellsworth, W.L., Johnston, M.J.S., Langbein, J.O., 

Oppenheimer, D.H., Pitt, A.M., Reasenberg, P.A., Sorey, 

M.L., and McNutt, S.R., 1990, The 1989 earthquake swarm 

beneath Mammoth Mountain, California; an initial look at 

the 4 May through 30 September activity: Seismological 

Society of America Bulletin, v. 80, no. 2, p. 325–339.

Hill, D.P, Wallace, R.E., and Cockerham, R.S., 1985b, Review 

of evidence on the potential for major earthquakes and 

volcanism in the Long Valley-Mono Craters-White Mountains regions of eastern California: Earthquake Prediction 

Research, v. 3, no. 3–4, p. 571–594.

Hilton, D.R., 1996, The helium and carbon isotope systematics 

of a continental geothermal system; results of monitoring 

studies at Long Valley Caldera (California, U.S.A.): Chemical Geology, v. 127, no. 4, p. 269–295.

Huber, N.K., 1981, Amount and timing of late Cenozoic uplift 

and tilt of the Central Sierra Nevada, California—evidence 

from the Upper San Joaquin River Basin: U.S. Geological 

Survey Professional Paper 1197, 28 p.

Huber, N.K., and Rinehart, C.D., 1967, Cenozoic volcanic 

rocks of the Devils Postpile quadrangle, eastern Sierra 

Nevada, California: U.S. Geological Survey Professional 

Paper 554–D, p. D1–D19.

Johnson, C.M., and O’Neil, J.R., 1984, Triple junction magmatism; a geochemical study of Neogene volcanic rocks in 

western California: Earth and Planetary Science Letters, v. 

71, no. 2, p. 241–262.

Julian, B.R., 1983, Evidence for dyke intrusion earthquake 

mechanisms near Long Valley Caldera, California: Nature, 

v. 303, no. 5915, p. 323–325.

Kelleher, P.C., and Cameron, K.L., 1990, The geochemistry 

of the Mono Craters-Mono Lake volcanic complex, eastern 

California: Journal of Geophysical Research, v. 95, no. B11, 

p. 17643–17659.

Kempton, P.D., Fitton, J.G., Hawkesworth, C.J., and Ormerod, 

D.S., 1991, Isotopic and trace-element constraints on the 

composition and evolution of the lithosphere beneath 

the southwestern United States: Journal of Geophysical 

Research, v. 96, no. B8, p. 13713–13735.

Kistler, R.W., 1993, Mesozoic intrabatholithic faulting, Sierra 

Nevada, California, in Dunn, G.C., and McDougall, K.A., 

eds., Mesozoic paleogeography of the Western United 

States; II: Los Angeles, Society of Economic Paleontologists and Mineralogists, Pacific Section Field Trip Guidebook, v. 71, p. 247–261.

Kistler, R.W., and Peterman, Z.E., 1973, Variations in Sr, Rb, 

K, Na, and initial 87Sr/86Sr in Mesozoic granitic rocks and 
intruded wall rocks in central California: Geological Society 

of America Bulletin, v. 84, no. 11, 3489–3512.

Kistler, R.W., and Peterman, Z.E., 1978, Reconstruction of 

crustal blocks of California on the basis of initial strontium 

isotopic compositions of Mesozoic granitic rocks: U.S. 

Geological Survey Professional Paper 1071, 17 p.

Koeppen, R.P., 1983, Petrochemical evolution of Mammoth Mountain, Long Valley Caldera, Eastern California [abs.]: Eos (American Geophysical Union Transactions), v. 64, no. 45, p. 889.

Lachenbruch, A.H., Sass, J.H., Munroe, R.J., and Moses, T.H., 

Jr., 1976, Geothermal setting and simple heat conduction 

models for the Long Valley Caldera: Journal of Geophysical 

Research, v. 81, no. 5, p. 769–784.

Lajoie, K.R., 1968, Late Quaternary stratigraphy and geologic 

history of Mono Basin, eastern California: Berkeley, University of California, Ph.D. thesis, 271 p.

Langbein, J.O., Dzurisin, Daniel, Marshall, Grant, Stein, 

R.S., and Rundle, J.B., 1995, Shallow and peripheral volcanic sources of inflation revealed by modeling two-color 

geodimeter and leveling data from Long Valley Caldera, 

California, 1988–1992: Journal of Geophysical Research, v. 

100, no. B7, p. 12487–12495.

Langbein, J.O., Hill, D.P., Parker, T.N., and Wilkinson, S.K., 

1993, An episode of reinflation of the Long Valley Caldera; 

eastern California; 1889–1991: Journal of Geophysical 

Research, v. 98, no. B9, p. 15851–15870.

Lange, R.B., Carmichael, I.S.E., and Renne, P.R., 1993, Potassic volcanism near Mono Basin, California; evidence for 

high water and oxygen fugacities inherited from subduction: 

Geology, v. 21, no. 10, p. 949–952.

Larson, R.C., Menard, H.W., and Smith, S.M., 1968, Gulf of 

California; a result of ocean-floor spreading and transform 

faulting: Science, v. 161, no. 3843, p. 781–784.

Leat, P.T., Thompson, R.N., Morrison, M.A., Hendry, G.L., 

and Dickin, A.P., 1988, Compositionally-diverse Miocene–

Recent rift-related magmatism in NW Colorado; partial 

melting and mixing of mafic magmas from three different 

asthenospheric and lithospheric mantle sources, in Menzies, 

M.A., and Cox, K.G., eds., Oceanic and continental lithosphere; similarities and differences: Oxford, U.K., Clarendon Press, p. 351–377.

Le Bas, M.J., Le Maitre, R.W., Streckeisen, Albert, and Zanettin, B.A., 1986, Chemical classification of volcanic rocks 

based on the total alkali-silica diagram: Journal of Petrology, v. 27, no. 3, p. 745–750.

Leeman, W.P., 1982, Tectonic and magmatic significance of 

strontium isotopic variations in Cenozoic volcanic rocks 

from the western United States: Geological Society of 

America Bulletin, v. 93, no. 6, p. 487–503.

Leeman, W.P., and Harry, D.L., 1993, A binary source model 

for extension-related magmatism in the Great Basin, 

western North America: Science, v. 262, no. 5139, p. 

1550–1554.

Le Maitre, R.W., ed., 1989, A classification of igneous rocks 

and glossary of terms: Oxford, U.K., Blackwell Scientific, 

193 p.

Luetgert, J.H., and Mooney, W.D., 1985, Crustal refraction 

profile of the Long Valley Caldera, California, from the January 1983 Mammoth Lakes earthquake swarm: Seismological Society of America Bulletin, v. 75, no. 1, p. 211–221.

Lum, C.C.L., Leeman, W.P., Foland, K.A., Kargel, J.A., and 

Fitton, J.G., 1989, Isotopic variations in continental basaltic 

lavas as indicators of mantle heterogeneity; examples 

from the Western U.S. Cordillera: Journal of Geophysical 

Research, v. 94, no. B6, p. 7871–7884.

Mahood, G.A., 1990, Second reply to comment of R.S.J. 

Sparks, H.E. Huppert, and C.J.N. Wilson on “Evidence for 

long residence times of rhyolitic magma in the Long Valley 

magmatic system; the isotope record in the precaldera lavas 

of Glass Mountain”: Earth and Planetary Science Letters, v. 

99, no. 4, p. 395–399.

Mankinen, E.A., Grommé, C.S., Dalrymple, G.B., Lanphere, 

M.A., and Bailey, R.A., 1986, Paleomagnetism and K-Ar 

ages of volcanic rocks from Long Valley Caldera, California: Journal of Geophysical Research, v. 91, no. B1, p. 

633–652.

Mastin, L.G., 1991, The roles of magma and ground water in 

the phreatic eruptions of Inyo Craters, Long Valley Caldera, California: Bulletin of Volcanology, v. 53, no. 8, p. 

579–596.

Matthes, F.E., chairman, 1939, Report of Committee on Glaciers, April 1939: American Geophysical Union Transactions of 1939, pt. 4, p. 518–523.

Matthes, F.E., 1960, Reconnaissance of the geomorphology and glacial geology of the San Joaquin Basin, Sierra 

Nevada, California: U.S. Geological Survey Professional 

Paper 329, 62 p.

Mavko, B.B., and Thompson, G.A., 1983, Crust and upper 

mantle structure of the Northern and Central Sierra Nevada: 

Journal of Geophysical Research, v. 88, no. B7, p. 5874–

5892.

Menzies, M.A., 1989, Cratonic, circumcratonic and oceanic 

mantle domains beneath western United States: Journal of 

Geophysical Research, v. 94, no. B6, p. 7899–7915.

Menzies, M. A., Leeman, W.P., and Hawkesworth, C.P., 1983, 

Isotope geochemistry of Cenozoic volcanic rocks reveals 

mantle heterogeneity below western USA: Nature, v. 303, 

no. 5914, p. 205–209.

References Cited 71
72 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

Metz, J.M., and Bailey, R.A., 1993, Geologic map of Glass 

Mountain, Mono County, California, U.S. Geological Survey Miscellaneous Investigations Series Map I–1995, scale 

1:24,000.

Metz, J.M., and Mahood, G.A., 1985, Precursors to the Bishop Tuff 

eruption; Glass Mountain, Long Valley caldera, California: Journal of Geophysical Research, v. 90, no. B11, p. 11121–11126.

Metz, J.M., and Mahood, G.A., 1991, Development of the 

Long Valley, California, magma chamber recorded in precaldera rhyolite lavas of Glass Mountain: Contributions to 

Mineralogy and Petrology, v. 106, no. 3, p. 379–397.

Michael, P.J., 1991, Intrusion of basaltic magmas into a crystallizing granitic magma chamber; the Cordillera del Paine 

Pluton in southern Chile: Contributions to Mineralogy and 

Petrology, v. 108, no. 4, p. 396–418.

Miller, C.D., 1985, Holocene eruptions of the Inyo Volcanic 

Chain, California; implications for possible eruptions in 

Long Valley caldera: Geology, v. 13, no. 1, p. 14–17.

Miller, C.D., Mullineaux, D.R., Crandell, D.R., and Bailey,

R.A., 1982, Potential hazards from future volcanic eruptions 

in the Long Valley-Mono Lake area, east-central California 

and Nevada—a preliminary assessment: U.S. Geological 

Survey Circular 877, 10 p.

Moore, J.G., and Dodge, F.C.W., 1980, Late Cenozoic 

volcanic rocks of the southern Sierra Nevada, California; 

I, Geology and petrology: Geological Society of America 

Bulletin, v. 91, no. 9, p. 515–518.

Muffler, L.J.P., and Williams, D.L., 1976, Geothermal investigations of the U.S. Geological Survey in Long Valley, 

California, 1972–1973: Journal of Geophysical Research, v. 

81, no. 5, p. 721–724.

Mukasa, S.B., and Wilshire, H.G., 1997, Isotopic and traceelement compositions of upper mantle and lower crustal 

xenoliths, Cima volcanic field, California; implications for 

evolution of the subcontinental lithospheric mantle: Journal 

of Geophysical Research, v. 102, no. B9, p. 20133–20148.

Newhall, C.G., and Dzurisin, Daniel, 1988, Historical unrest at 

large calderas of the world: U.S. Geological Survey Professional Paper 1855, 2 v.

Nielsen, D., Mahood, G.A., Halliday, A.N., and Essene, E.J., 

1991, Petrogenesis of precaldera mafic and intermediate lavas, Long Valley, CA [abs.]: Geological Society of 

America Abstracts with Program, v. 23, no. 2, p. 83.

Novak, S.W., and Bacon, C.R., 1986, Pliocene volcanic rocks 

of the Coso Range, Inyo County, California: U.S. Geological Survey Professional Paper 1383, 44 p.

Nye, C.J., and Reid, M.R., 1986, Geochemistry and primary 

and least fractionated lavas from Okmok Volcano, central 

Aleutians; implications for arc magmagenesis: Journal of 

Geophysical Research, v. 91, no. B10, p. 10271–10287.

Oliver, H.W., 1977, Gravity and magnetic investigations of the 

Sierra Nevada batholith, California: Geological Society of 

America Bulletin, v. 88, no. 3, p. 445–461.

Ormerod, D.S., 1988, Late- to post-subduction magmatic 

transitions in the western Great Basin, U.S.A.: U.K., Open 

University, Ph.D. thesis, 331 p.

Ormerod, D.S., Hawkesworth, C.J., Rogers, N.W., Leeman, 

W.P., and Menzies, M.A., 1988, Tectonic and magmatic 

transitions in the western Great Basin, USA: Nature, v. 333, 

no. 6171, p. 349–353.

Ormerod, D.S., Rogers, N.W., and Hawkesworth, C.J., 1991, 

Melting of the lithospheric mantle; inversion modelling of 

alkali-olivine basalts from the Big Pine volcanic field, California: Contributions to Mineralogy and Petrology, v. 108, 

no. 3, p. 305–317.

Pakiser, L.C., 1970, Structure of Mono Basin, California: Journal of Geophysical Research, v. 75, no. 20, p. 4077–4080.

Peacock, M.A., 1931, Classification of igneous rock series: 

Journal of Geology, v. 39, no. 1, p. 54–67.

Pearce, J.A., 1982, Trace element characteristics of lavas from 

destructive plate margins, in Thorpe, R.S., ed., Andesites; 

orogenic andesites and related rocks: New York, John Wiley 

& Sons, p. 525–548.

Phillips, F.M., Zreda, M.G., Smith, S.S., Elmore, David, Kubik, 

P.W., and Sharma, Pankaj, 1990, Cosmogenic chlorine-36 

chronology for glacial deposits at Bloody Canyon, eastern 

Sierra Nevada: Science, v. 248, no. 4962, p. 1529–1532.

Phillips, F.M., Zreda, M.G., Benson, L.V., Plummer, M.A., 

Elmore, David, and Sharma, Pankaj, 1996, Chronology for 

fluctuations in Late Pleistocene Sierra Nevada glaciers and 

lakes: Science, v. 274, no. 5288, p. 749–751.

Pitt, A.M., and Hill, D.P., 1994, Long-period earthquakes in 

the Long Valley caldera region, eastern California: Geophysical Research Letters, v. 21, no. 16, p. 1679–1682.

Ponko, S.C. and Sanders, C.O., 1994, Inversion of P and S

wave attenuation structure, Long Valley Caldera, California: Journal of Geophysical Research, v. 99, no. 2, p. 

2619–2635.

Raleigh, C.B., Sieh, Kerry, Sykes, L.R., and Anderson, D.L., 

1982, Forecasting southern California earthquakes: Science, 

v. 217, no. 4565, p. 1097–1104.

Reid, M.R., and Ramos, F.C., 1996, Chemical dynamics of 

enriched mantle in the Southwestern United States; thorium 

isotope evidence: Earth and Planetary Science Letters, v. 

138, no. 1–4, p. 67–81.
Rinehart, C.D., and Huber, N.K., 1965, The Inyo Crater Lakes—

a blast in the past: California Division of Mines and Geology 

Mineral Information Service, v. 18, no. 9, p. 169–172.

Rinehart, C.D., and Ross, D.R., 1964, Geology and mineral 

deposits of the Mount Morrison quadrangle, Sierra Nevada, 

California: U.S. Geological Survey Professional Paper 385, 

106 p.

Rudnick, R.L., and Fountain, D.M., 1995, Nature and composition of the continental crust; a lower crustal perspective: 

Reviews of Geophysics v. 33, no. 3, p. 267–309.

Russell, I.C., 1886–87, Quaternary history of Mono Valley, 

California: U.S. Geological Survey Annual Report 8, pt. 1, 

p. 261–394.

Ryall, A.S., Jr., and Ryall, Floriana, 1981, Spatial-temporal 

relations in seismicity preceding the May 1980, Mammoth 

Lakes, California, earthquakes: Seismological Society of 

America Bulletin, v. 71, no. 3, p. 741–760.

Ryall, A.S., Jr., and Ryall, Floriana, 1983, Spasmodic tremor 

and possible magma injection in Long Valley caldera, eastern California: Science, v. 219, no. 4591, p. 1432–1433.

Sampson, D.E., and Cameron, K.L., 1987, The geochemistry 

of the Inyo volcanic chain; multiple magma systems in the 

Long Valley region, eastern California: Journal of Geophysical Research, v. 92, no. B10, p. 10403–10421.

Sanders, C.O., 1984, Location and configuration of magma 

bodies beneath Long Valley, California, determined from 

anomalous earthquake signals: Journal of Geophysical 

Research, v. 89, no. B10, p. 8287–8302.

Sanders, C.O., Ponko, S.C., Nixon, L.D., and Schwartz, E.A., 

1995, Seismological evidence for magmatic and hydrothermal structure in Long Valley caldera from local earthquake 

attenuation and velocity tomography: Journal of Geophysical Research, v. 100, no. B5, p. 8311–8326.

Saunders, A.D., Norry, M.J., and Tarney, John, 1988, Origin 

of MORB and chemically-depleted mantle reservoirs; trace 

element constraints, in Menzies, M.A., and Cox, K.G., eds., 

Oceanic and continental lithosphere; similarities and differences: Oxford, U.K., Clarendon Press, p. 415–445.

Savage, J.C., and Clark, M.M., 1983, Magmatic resurgence in 

Long Valley caldera, California; possible cause of the 1980 

Mammoth Lakes earthquakes: Science, v. 217, no. 4559, p. 

531–533.

Savage, J.C., and Cockerham, R.S., 1984, Earthquake swarm 

in Long Valley, California, January 1983; evidence for dike 

intrusion: Journal of Geophysical Research, v. 89, no. B10, 

p. 8315–8324.

Savage, J.C., Lisowski, Michael, Svarc, J.L., and Gross, W.K., 

1995, Strain accumulation across the central Nevada seismic 

zone, 1973–1994: Journal of Geophysical Research, v. 100, 

no. B10, p. 20257–20269.

Savage, M.K., Li, Li, Eaton, J.P., Jones, C.H., and Brune, J.H., 

1994, Earthquake refraction profiles of the root of the Sierra 

Nevada: Tectonics, v. 13, no. 4, p. 803–817.

Sharp, R.P., and Birman, J.H., 1963, Additions to classical sequence 

of Pleistocene glaciations, Sierra Nevada, California: Geological 

Society of America Bulletin, v. 74, no. 8, p. 1079–1086.

Shearman, D.J., and Smith, A.J., 1985, Ikaite, the parent mineral of jarrowite-type pseudomorphs: Geologists’ Association of London Proceedings, v. 96, no. 4, p. 305–314.

Sherburne, R.W., ed., 1980, Mammoth Lakes, California, 

earthquakes of May 1980: California Division of Mines and 

Geology Special Report 150, 141 p.

Sheridan, M.F., 1965, The mineralogy and petrology of the 

Bishop Tuff (Pleistocene of California): Stanford, Calif., 

Stanford University, Ph.D. thesis, 193 p.

Sieh, Kerry, and Bursik, M.I., 1986, Most recent eruption of 

the Mono Craters, eastern Central California: Journal of 

Geophysical Research, v. 91, no. B12, p. 12539–12571.

Sisson, T.W., Grove, T.L., and Coleman, D.S., 1996, Hornblende gabbro sill complex at Onion Valley, California, and a 

mixing origin for the Sierra Nevada Batholith: Contributions 

to Mineralogy and Petrology, v. 126, no. 1–2, p. 81–108.

Smith, Genny, 1993, Mammoth Lakes Sierra; handbook for 

road and trailside: Mammoth Lakes, Calif., Genny Smith 

Books, 220 p.

Smith, G.I, Bischoff, J.L., and Bradbury, J.P., 1997, Synthesis 

of the paleoclimate record from Owens Lake core OL–92, 

in Smith, G.I., and Bischoff, J.L., eds., An 800,000-year 

paleoclimatic record from corehole OL–92, Owens Lake, 

southeast California: Geological Society of America Special 

Paper 317, p. 143–160.

Smith, R.L., 1979, Ash-flow magmatism, in Chapin, C.E., and 

Elston W.E., eds., Ash-flow tuffs: Geological Society of 

America Special Paper 180, p. 5–27.

Smith, R.L. and Bailey, R.A., 1968, Resurgent cauldrons, in

Coats, R.R., ed., Studies in volcanology; a memoir in honor 

of Howel Williams: Geological Society of America Memoir 

116, p. 613–662.

Snyder, Don, and Tait, Stephen, 1995, Replenishment of 

magma chambers; comparison of fluid-mechanic experiments with field relations: Contributions to Mineralogy and 

Petrology, v. 122, no. 3, p. 230–240.

Snyder, Don, and Tait, Stephen, 1998, The imprint of basalt 

on the geochemistry of silicic magmas: Earth and Planetary 

Science Letters, v. 160, no. 3–4, p. 433–445.

References Cited 73
74 Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California

Sorey, M.L., Evans, W.C., Kennedy, B.M., Farrar, C.D., Hainsworth, L.J., and Hausback, B.P., 1998, Carbon dioxide and 

helium emissions from a reservoir of magmatic gas beneath 

Mammoth Mountain, California: Journal of Geophysical 

Research, v. 103, no. B7, p. 15303–15323.

Sorey, M.L., Kennedy, B.M., Evans, W.C., Farrar, C.D., and 

Suemnicht, G.A., 1993, Helium isotope and gas discharge 

variations associated with crustal unrest in Long Valley 

caldera, California: Journal of Geophysical Research, v. 98, 

no. B9, p. 15871–15880.

Sparks, R.S.J., Huppert, H.E., and Wilson, C.J.N., 1990, Comment on “Evidence for long residence times of rhyolitic 

magma in the Long Valley magmatic system; the isotopic 

record in precaldera lavas of Glass Mountain” by A.N. Halliday, G.A. Mahood, P. Holden, J. M. Metz, T.J. Dempster, 

and J.P. Davidson: Earth and Planetary Science Letters, v. 

99, no. 4, p. 387–389.

Sparks, R.S.J., and Marshall, L.A., 1986, Thermal and 

mechanical constraints on mixing between mafic and silicic 

magmas: Journal of Volcanology and Geothermal Research, 

v. 29, no. 1–4, p. 99–124.

Sparks, R.S.J., Sigurdsson, Haraldur, and Wilson, Lionel, 

1977, Magma mixing; a mechanism for triggering acid 

explosive eruptions: Nature, v. 267, no. 5609, p. 315–318.

Steck, L.K., and Prothero, W.A., Jr., 1994, Crustal structure 

beneath Long Valley caldera from modeling of teleseismic 

P wave polarizations and Ps converted waves: Journal of 

Geophysical Research, v. 99, no. B4, p. 6881–6898.

Steeples, D.W., and Iyer, H.M., 1976, Low–velocity zone 

under Long Valley as determined from teleseismic events: 

Journal of Geophysical Research, v. 81, no. 5, p. 849–860.

Stine, Scott, 1987, Mono Lake; the past 4000 years: Berkeley, 

University of California, Ph.D. thesis, 615 p.

Suemnicht, G.A., and Varga, R.J., 1988, Basement structure 

and implications for hydrothermal circulation patterns in the 

western moat of Long Valley Caldera, California: Journal of 

Geophysical Research, v. 93, no. B11, p. 13191–13207.

Sun, S.S., and McDonough, W.F., 1989, Chemical and isotopic 

systematics of oceanic basalts; implications for mantle composition and processes, in Saunders, A.D., and Norry, M.J., 

eds., Magmatism and the ocean basins: Geological Society 

of London Special Publication 42, p. 313–345.

Taggart, J.E., Jr., Lindsay, J.R., Scott, B.A., Vivit, D.V., Bartel, 

A.J., and Stewart, K.C., 1987, Analysis of geologic materials by wavelength-dispersive X-ray fluorescence spectrometry: U.S. Geological Survey Bulletin 1770, p. E1–E19.

Taylor, S.R., and McLennan, S.M., 1985, The continental 

crust; its composition and evolution: Oxford, U.K., Blackwell Scientific, 312 p.

Thompson, G.A., and Zoback, M.L., 1979, Regional geophysics of the Colorado Plateau: Tectonophysics, v. 61, no. 1–3, 

p. 149–181.

Thompson, R.N., Morrison, M.A., Dicken, A.P., and Hendry, 

G.L., 1983, Continental flood basalts...arachnids rule OK?, 

in Hawkesworth, C.J., and Norry, M.J., eds., Continental 

basalts and mantle xenoliths: Nantwich, U.K., Shiva Publishing, p. 158–185.

Thompson, R.N., Morrison, M.A., Hendry, G.L., and Parry, 

S.J., 1984, An assessment of the relative roles of crust and 

mantle in magma genesis; an elemental approach: Royal 

Society of London Philosophical Transactions, ser. A, v. 

310, no. 1514, p. 549–590.

Turner, S.P., and Hawkesworth, C.J., 1995, The nature of the 

sub-continental mantle; constraints from the major-element 

composition of continental flood basalts: Chemical Geology, v. 120, no. 3–4, p. 295–314.

Van Kooten, G.K., 1980, Mineralogy, petrology, and geochemistry of an ultrapotassic basaltic suite, central Sierra Nevada, 

USA: Journal of Petrology, v. 21, no. 4, p. 651–684.

Van Kooten, G.K., 1981, Pb and Sr systematics of ultrapotassic and basaltic rocks from the central Sierra Nevada, 

California: Contributions to Mineralogy and Petrology, v. 

76, no. 4, p. 378–385.

Van Kooten, G.K., Leeman, W.P., and Menzies, M.A., 1985, 

Lithospheric peridotites and pyroxenites in alkaline basaltic rock 

from the Sierra Nevada, California, USA [abs.]: Eos (American 

Geophysical Union Transactions), v. 66, no. 18, p. 414.

Varga, R.J., Bailey, R.A., and Suemnicht, G.A., 1990, Evidence for 600 year-old basalt and magma mixing at Inyo 

Craters volcanic chain, Long Valley Caldera, California: 

Journal of Geophysical Research v. 95, no. B13, p. 21441–

21450.

Vogel, T.A., Woodburne, T.B., Eichelberger, J.C., and Layer, 

P.W., 1994, Chemical evolution and periodic eruption of 

mafic lava flows in the west moat of Long Valley caldera, 

California: Journal of Geophysical Research, v. 99, no. B10, 

p. 19829–19842.

Ward, P.L., 1991, On plate tectonics and the geological evolution of southwestern North America: Journal of Geophysical 

Research, v. 96, no. B7, p. 12479–12496.

Watson, E.B., 1982, Basalt contamination by continental crust; 

some experiments and models: Contributions to Mineralogy 

and Petrology, v. 80, no. 1, p. 73–87.

Welhan, J.A., Poreda, R.J., Rison, William, and Craig, Harmon, 1988, Helium isotopes in geothermal and volcanic 

gases of the western United States; 1. Regional variability 

and magmatic origin: Journal of Volcanology and Geothermal Research, v. 34, no. 3–4, p. 185–199.
Wenrich, K.J., Billingsley, G.H., and Blackjoy, B.A., 1995, 

Spatial migration and compositional changes of Miocene–Quaternary magmatism in the Western Grand Canyon: Journal of Geophysical Research, v. 100, no. B7, p. 

10417–10440.

Wernicke, B.P., Clayton, R.W., Ducea, M.N., Jones, C.H., 

Park, S.K., Ruppert, S.D., Saleeby, J.B., Snow, J.K., 

Squires, L.J., Fliedner, M.M., Jiracek, G.R., Keller, G.R., 

Klemperer, S.L., Leutgert, J.H., Malin, P.E., Miller, K.C., 

Mooney, W.D., Oliver, H.W., and Phinney, R.A., 1995, Origin of high mountains in the continents; the southern Sierra 

Nevada: Science, v. 271, no. 5246, p. 190–193.

Wiebe, R.A., 1993a, Basaltic injections into floored silicic 

magma chambers: Eos (American Geophysical Union 

Transaction), v. 74, no. 1, p. 1–3.

Wiebe, R.A., 1993b, The Pleasant Bay layered gabbro-diorite, 

coastal Maine; ponding and crystallization of basaltic injections into a silicic magma chamber: Journal of Petrology, v. 

34, no. 3, p. 461–489.

Wiebe, R.A., 1994, Silicic magma chambers as traps for basaltic magmas; Cadillac Mountain intrusive complex, Mount 

Desert Island, Maine: Journal of Geology, v. 102, no. 4, p. 

423–437.

Wiebe, R.A., 1997, Fe-rich tholeiitic liquids and their cumulate products in the Pleasant Bay layered intrusion, coastal 

Maine: Contributions to Mineralogy and Petrology, v. 129, 

no. 4, p. 255–267.

Wilson, C.J.N., and Hildreth, Wes, 1997, The Bishop Tuff; 

new insights from eruptive stratigraphy: Journal of Geology, 

v. 105, no. 4, p. 407–439.

Wilson, C.J.N., and Hildreth, Wes, 1998, Hybrid fall deposits 

in the Bishop Tuff, California; a novel pyroclastic depositional mechanism: Geology, v. 26, no. 1, p. 7–10.

Yogodzinski, G.M., Naumann, T.R., Smith E.I., Bradshaw, 

T.K., and Walker, J.D., 1996, Evolution of a mafic volcanic 

field in the central Great Basin, south central Nevada: 

Journal of Geophysical Research, v. 101, no. B8, p. 17425–

17445.

Zandt, George, Myers, S.C., and Wallace, T.C., 1995, Crust 

and mantle structure across the Basin and Range-Colorado 

Plateau boundary at 37°N latitude and implications for 

Cenozoic extensional mechanism: Journal of Geophysical 

Research, v. 100, no. B6, p. 10529–10548.

Zielinski, R.A., 1975, Trace element evaluation of a suite of 

rocks from Reunion Island, Indian Ocean: Geochimica et 

Cosmochimica Acta, v. 39, no. 5, p. 713–734.

Zielinski, R.A., and Lipman, P.W., 1976, Trace-element variations at Summer Coon volcano, San Juan Mountains, Colorado, and the origin of continental-interior andesite: Geological Society of America Bulletin, v. 87, no. 10, p. 1477–1485.

References Cited 75
G US S

Printed on recycled paper

Bailey—Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California—U.S. Geological Survey Professional Paper 1692