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Front cover. Balanced Rock in Chiricahua National Monument, composed of outflow facies Rhyolite Canyon Tuff.

Back cover. The feature known as The Fingers is located on the north side of Cave Creek and is composed of 

aphyric, high-silica rhyolite lava.
Geochemistry and Geochronology of Middle Tertiary 

Volcanic Rocks of the Central Chiricahua Mountains, 

Southeast Arizona

By Edward A. du Bray, Lawrence W. Snee, and John S. Pallister

Professional Paper 1684

U.S. Department of the Interior 

U.S. Geological Survey
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 

Version 1.0

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Suggested citation:

du Bray, E.A., Snee, L.W., and Pallister, J.S., 2004, Geochemistry and geochronology of middle Tertiary volcanic rocks

of the central Chiricahua Mountains, southeast Arizona: U.S. Geological Survey Professional Paper 1684, 57 p.

ISBN 0-607-95559-7
Ill

Contents

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

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

Acknowledgments........................................................................................................................................ 2

Sampling and Analytic Methods................................................................................................................ 2

Petrographic and Strati graphic Characteristics..................................................................................... 7

Pre-Turkey Creek Caldera Rocks...................................................................................................... 7

Rocks Associated with the Turkey Creek Caldera ....................................................................... 11

Post-Turkey Creek Caldera Rocks................................................................................................... 12

Geochemistry.............................................................................................................................................. 13

Classification...................................................................................................................................... 13

Within-UnitGeochemical Variation................................................................................................ 15

Pre-Turkey Creek Caldera Rocks........................................................................................... 18

Rocks Associated with the Turkey Creek Caldera .............................................................. 18

Post-Turkey Creek Caldera Rocks.......................................................................................... 19

Geochemistry- and Petrography-Based Stratigraphic Distinctions......................................... 19

Lavas ........................................................................................................................................ 19

Ash-Flow Tuffs and Other Pyroclastic Flow Deposits ........................................................ 22

Petrogenetic Implications................................................................................................................23

Petrogenetic Evolution of the Turkey Creek Caldera Magmatic System ................................. 27

Geochronology............................................................................................................................................31

Miscellaneous Units.......................................................................................................................... 31

Pre-Turkey Creek Caldera Rocks.................................................................................................... 40

Rocks Associated with the Turkey Creek Caldera ....................................................................... 50

Post-Turkey Creek Caldera Rocks................................................................................................... 51

Concluding Remarks.................................................................................................................................. 51

References Cited........................................................................................................................................ 54

Figures

1. Index map showing location of central Chiricahua Mountains, Ariz............................................3

2. Simplified correlation chart of map units and identification of studied volcanic rock units... 10

3. Total alkali-silica variation diagram showing compositions of volcanic rocks......................... 13

4. Abundance diagrams of selected major oxides and trace elements......................................... 14

5. Chondrite-normalized extended trace-element diagrams............................................................ 15

6. Diagrams showing Stratigraphic versus compositional variation among volcanic rocks....... 20

7. Trace-element-tectonic setting discrimination variation diagrams showing

average compositions......................................................................................................................... 24

8. Chondrite-normalized extended trace-element diagram showing average compositions..... 26

9. Ternary variation diagram showing average relative proportions of rubidium,

potassium, and strontium ................................................................................................................... 27

10. Chondrite-normalized rare earth element diagrams showing average compositions............ 28

11. Diagrams of 40Ar/39Ar age spectra..................................................................................................... 32
IV

Tables

1. Compositions of volcanic rocks of the central Chiricahua Mountains........................................... 5

2. Summary of 40Ar/39Ar age-spectrum results from the central Chiricahua Mountains................. 8

3. 40Ar/39Ar data for volcanic rocks of the central Chiricahua Mountains........................................ 42

4. Diagnostic age, petrographic, and geochemical features of middle Tertiary volcanic

rocks of the central Chiricahua Mountains....................................................................................... 52
Geochemistry and Geochronology of Middle Tertiary 

Volcanic Rocks of the Central Chiricahua Mountains, 

Southeast Arizona

By Edward A. du Bray, Lawrence W. Snee, and John S. Pallister

Abstract

Middle Tertiary volcanic rocks of the central Chiricahua 

Mountains in southeast Arizona are the westernmost constituents of the Eocene-Oligocene Boot Heel volcanic field of 

southwestern New Mexico and southeastern Arizona. About 

two dozen volumetrically and stratigraphically significant 

volcanic units are present in this area. These include largevolume, regionally distributed ash-flow tuffs and smaller 

volume, locally distributed lava flows. The most voluminous 

of these units is the Rhyolite Canyon Tuff, which erupted 26.9 

million years ago from the Turkey Creek caldera in the central 

Chiricahua Mountains. The Rhyolite Canyon Tuff consists 

of 500-1,000 cubic kilometers of rhyolite that was erupted 

from a normally zoned reservoir. The tuff represents sequential eruptions, which became systematically less geochemically evolved with time, from progressively deeper levels of 

the source reservoir. Like the Rhyolite Canyon Tuff, other 

ash-flow tuffs preserved in the central Chiricahua Mountains 

have equivalents in nearby, though isolated mountain ranges. 

However, correlation of these other tuffs, from range to range, 

has been hindered by stratigraphic discontinuity, structural 

complexity, and various lithologic similarities and ambiguities. 

New geochemical and geochronologic data presented here 

enable correlation of these units between their occurrences 

in the central Chiricahua Mountains and the remainder of the 

Boot Heel volcanic field.

Volcanic rocks in the central Chiricahua Mountains are 

composed dominantly of weakly peraluminous, high-silica 

rhyolite welded tuff and rhyolite lavas of the high-potassium 

and shoshonitic series. Trace-element, and to a lesser extent, 

major-oxide abundances are distinct for most of the units 

studied. Geochemical and geochronologic data depict a time 

and spatial transgression from subduction to within-plate 

and extensional tectonic settings. Compositions of the lavas 

tend to be relatively homogeneous within particular units. In 

contrast, compositions of the ash-flow tuffs, including the 

Rhyolite Canyon Tuff, vary significantly owing to eruption

from compositionally zoned reservoirs. Reservoir zonation is 

consistent with fractional crystallization of observed phenocryst phases and resulting residual liquid compositional evolution. Rhyolite lavas preserved in the moat of the Turkey Creek 

caldera depict compositional zonation that is the reverse of 

that expected of magma extraction from progressively deeper 

parts of a normally zoned reservoir. Presuming that the source 

reservoir was sequentially tapped from its top downward, 

development of reverse zonation in the rhyolite lava sequence 

may indicate that later erupted, more evolved magma contains 

systematically less wallrock contamination derived from the 

geochemically primitive margins of its incompletely mixed 

reservoir.

New 40Ar/39Ar geochronology data indicate that the 

principal middle Tertiary volcanic rocks in the central Chiricahua Mountains were erupted between about 34.2 and 26.2 

Ma, and that the 5.2 m.y. period between 33.3 and 28.1 Ma 

was amagmatic. The initial phase of eruptive activity in the 

central Chiricahua Mountains, between 34.2 and 33.3 Ma, 

was associated with a regional tectonic regime dominated by 

subduction along the west edge of North America. We infer 

that the magmatic hiatus, nearly simultaneous with a hiatus of 

similar duration in parts of the Boot Heel volcanic field east 

of the central Chiricahua Mountains, is related to a period of 

more rapid convergence and therefore shallower subduction 

that may have displaced subduction-related magmatic activity 

to a position east of the present-day Boot Heel volcanic field. 

The hiatus also coincides with a major plate tectonic 

reorganization along the west edge of North America that 

resulted in cessation of subduction and initiation of transform 

faulting along the San Andreas fault. The final period of magmatism in the central Chiricahua Mountains, between 28.1 and 

23.2 Ma, appears to be coincident with rapid westward retreat 

of the subducting slab hinge line and consequent redevelopment of an asthenospheric mantle wedge, probably associated 

with foundering of the Farallon plate beneath western North 

America. Shortly thereafter, magmatism ceased in the central 

Chiricahua Mountains as the position of extension-related 

magmatism rapidly shifted westward to the Great Basin.
Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

Introduction

This study is an outgrowth of investigations of the Turkey 

Creek caldera (du Bray and Pallister, 1991; Pallister and du 

Bray, 1997; du Bray and others, 1997), the principal volcanic 

edifice of the central Chiricahua Mountains east-southeast of 

Tucson, Ariz. (fig. 1). The Turkey Creek caldera is an Oligocene volcanic center that formed during eruption of the 26.9- 

Ma Rhyolite Canyon Tuff and partial evacuation of an underlying rhyolitic to dacitic magma chamber (Marjaniemi, 1969; 

du Bray and Pallister, 1991). Caldera evolution involved 

three distinct phases that concluded in a span of no more than 

200,000 years: (1) eruption of 500-1,000 km3 of Rhyolite 

Canyon Tuff and attendant caldera collapse, (2) resurgent 

intrusion of dacite porphyry and eruption of consanguineous 

dacite porphyry lava flows, (3) renewed eruption of high-silica 

rhyolite as lava flows. The majority of the rocks that form the 

topographic margin of the caldera are older, middle Tertiary 

volcanic rocks, and are principally rhyolite ash-flow tuffs. The 

Turkey Creek caldera is the westernmost and youngest source 

of regionally distributed ash-flow tuff sheets that are part of 

the Boot Heel volcanic field described by Mclntosh and Bryan 

(2000).

During geologic mapping of the Turkey Creek caldera 

and its eruptive products, du Bray and others (1997) defined 

numerous pre-caldera volcanic rock units and mapped their 

distributions. Earlier attempts to define stratigraphic relations 

in isolated parts of the central Chiricahua Mountains (Raydon, 

1952; Enlows, 1951, 1955; Fernandez and Enlows, 1966; 

Drewes, 1982; Bryan, 1988; Drewes and Brooks, 1988) had 

resulted in significant stratigraphic uncertainty and volcanologic ambiguity. Because none of the characteristics of these 

older units had been synthesized, we collected stratigraphic, 

petrographic, geochemical, and geochronological data for the 

older rocks; these data provide a framework for our own studies as well as provide data essential in correlating these rocks 

with their equivalents throughout southeastern Arizona and 

southwestern New Mexico.

In this report, we synthesize all available petrographic 

and stratigraphic data for volcanic rocks of the central Chiricahua Mountains. In addition, we present and discuss geochemical data for about two dozen volcano-stratigraphic rock units 

along with geochronologic data for 14 of these units. These 

units have significant implications for middle Tertiary volcanic stratigraphy in southeastern Arizona and adjacent southwestern New Mexico. Many of the units are ash-flow tuff. 

Because of their emplacement mode, the ash-flow tuffs are 

considerably more widely distributed than the lava flow units 

present in our study area. The distribution of the lava flow 

units is probably limited to the central Chiricahua Mountains; 

their utility in stratigraphic correlation is probably similarly 

restricted.

In order to construct a comprehensible framework for the 

volcanic rocks under study, we divided them into three groups. 

Because the Turkey Creek caldera dominates the geology of 

the area, we defined the first group as "rocks associated with

the Turkey Creek caldera." The two other logically identifiable groups are therefore referred to as "pre-Turkey Creek 

caldera rocks" and "post-Turkey Creek caldera rocks." In the 

discussions that follow, rocks are assigned to one of the three 

groups, as appropriate, and data and interpretations ordered 

accordingly.

In this report, we present a broad array of geologic data 

to refine knowledge of the central Chiricahua Mountains in 

particular and the Boot Heel volcanic field in general. First, 

we present basic stratigraphic setting and petrographic data 

acquired from the literature and from our own geologic 

investigations of the mountain range. Subsequently, in the 

geochemistry section, we present geochemical data, apply 

classification schemes, evaluate within-unit compositional 

variation, and establish diagnostic, between-unit compositional 

characteristics. The geochemistry section concludes with an 

analysis of the geochemical evolution of the Turkey Creek caldera and its magmatic components. Next, new geochronologic 

data are presented in order to refine complex stratigraphy-age 

relations among middle Tertiary volcanic rocks of the central 

Chiricahua Mountains. In the report's concluding section, 

we synthesize all the data in order to constrain the large-scale 

magmatic-tectonic environment in which the middle Tertiary 

volcanic rocks of the area were erupted, and evaluate how this 

regime evolved through the middle Tertiary time frame.

Acknowledgments

Our field work was facilitated by the cooperation and 

assistance of the Southwestern Research Station (SWRS) of 

the American Museum of Natural History, Chiricahua National 

Monument, and the University of Arizona. We especially 

thank Wade and Emily Sherbrooke, Pam Limberger, and 

Christina Schwartz of SWRS for assistance and for providing 

a stimulating research environment. Dick Armstrong, Carol 

Kruse, Chuck Milliken, David Moore, and Alan Whalon provided accommodations and assistance during our work in the 

national monument. We thank Joe Austin, Carol Hudson, 

Billie and Jean Riggs, Jim Riggs, and Robin Riggs for providing access to their land. We thank D.B. Yager for preparing 

geochronology mineral separates and for conducting most 

of the trace-element analyses. R. A. Yeoman ably conducted 

argon analyses. Reviews by WC. Shanks, C.A. Nutt, and 

D. A. John improved this study.

Sampling and Analytic Methods

Petrographic and stratigraphic relations presented in 

the next section of this report are expanded versions of map

Figure 1 (facing page). Location of central Chiricahua Mountains, Ariz. Letters show collection sites for samples whose ages 

were determined by the 40Ar/39Ar method (tables 2 and 3).
Methods Analytic and Sampling 

109°07'30" 15' 22'30" 109°30'

—————————————————I 32°07'30"

Contact

caldera Creek Turkey of margin Topographic 

caldera Creek Turkey of margin Structural 

Fault

EXPLANATION ' 

deposits surficial Quaternary 

ROCKS CALDERA CREEK POST-TURKEY 

rocks extrusive Tertiary 

WITH ASSOCIATED ROCKS OLIGOCENE 

CALDERA CREEK TURKEY 

deposits Moat 

extrusive and dikes, ring intrusion, Resurgent 

equivalents 

Tuff Canyon Rhyolite 

ROCKS CALDERA CREEK PRE-TURKEY 

rocks volcanic Tertiary 

rocks intrusive Tertiary 

rocks Paleozoic and Mesozoic 

rocks Proterozoic 

32°00'

37'30" 0 31

geologic of Limit 

\ mapping 

geologic of Limit 

mapping

of Limit 

geologic

mapping

geologic of Limit 

mapping

- 52'30" 

A 

B 

C 

D 

E 

F 

G 

H 

I

J 

K 

L 

M 

N 

O 

P 

Q 

R 

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P475

201769

201771

201765

201570

201996

201580

P3

P272C

201587

201538

202156

202154

202151

P652

P650A

DY91-36

DY92-54

202064

DY91-11

202057

DY91-77

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Arizona Mountains, Chiricahua Central Rocks, Volcanic Tertiary Middle 

central the of map geologic the on included descriptions unit 

contain and 1997) others, and Bray (du Mountains Chiricahua 

in included that than information petrographic detailed more 

selected of sections thin Standard descriptions. unit map the 

petrographic a using examined and prepared were samples 

were unit stratigraphic each of samples Multiple microscope. 

and synthesized were observations resulting the examined; 

accordingly. developed descriptions petrographic 

stratigraphic tuff ash-flow each of samples Numerous 

estab- to order in analyzed chemically and collected were unit 

essential is procedure This ranges. compositional their lish 

res- magma zoned from derived tuffs ash-flow of sampling in 

of range full the Establishing 1981). 1979; (Hildreth, ervoirs 

units tuff ash-flow the of each within zonation compositional 

employed be to is composition if required is area study the of 

tuff ash-flow Most correlation. stratigraphic of tool a as 

deter- specifically having our without collected were samples 

sampled the within position stratigraphic relative their mined 

reason- been have to sampling our consider we Although unit. 

define not do data our that possible is it comprehensive, ably 

geochemical oriented vertically of extent and nature full the 

unit's the because units studied the of some within variation 

the by represented be not may extent stratigraphic complete 

systematically to able were we contrast, In samples. collected 

Canyon Rhyolite facies outflow of sections complete sample 

sample resulting Their Canyon. Horseshoe of tuff and Tuff 

and position, stratigraphic relative to referenced well are suites 

within variation compositional defines fully chemistry their 

units. these 

collected samples to pertains presented geochemistry The 

for sites collection showing map A sites. 500 than more at 

Complete (1997). others and Bray du in appears samples these 

series a in contained are samples these for results analytical 

du and 1993) 1992b; (1992a; others and Bray du by reports of 

through- collected Samples 1995). (1994; Pallister and Bray 

all of coverage areal representative provide area study the out 

sample Each considered. are compositions whose units the 

xenolithic obvious all and outcrop the at coarsely crushed was 

strontium, rubidium, of Abundances removed. were clasts 

for determined were barium and niobium, zirconium, yttrium, 

fluorescence X-ray dispersive energy by samples these of all 

for determined were abundances Major-oxide spectrometry. 

abun- trace-element additional and samples, 230 than more 

by determined were samples these of 100 almost in dances 

of number The analysis. activation neutron instrumental 

were data geochemical of types various the which for samples 

1. table in unit, stratigraphic by listed, is obtained 

1989; (Boden, tuffs ash-flow of studies Compositional 

cognate of analysis on relied have 1987) Mahood, and Fridrich 

represent to considered are these because inclusions pumice 

tuffs Tertiary middle the of most Because magma. quenched 

and indurated are Mountains Chiricahua centra] the of 

the to flattened are pumices welded, densely to moderately 

matrix. tuff enclosing from inseparable are they that extent 

compositional this for fragments pumice collect to Therefore, 

(1965) Lipman possible. routinely not was study 

ash- Tertiary middle of compositions the that demonstrated 

cases some in are pumice) (including vitrophyre tuff flow 

includ- processes, postmagmatic during modified considerably 

blocks pumice devitrified Consequently, devitrification. ing 

of are area study our of tuffs the in contained those as such 

compositional The studies. compositional in utility uncertain 

that demonstrated (1995) Bray du by tuffs ash-flow of study 

reliable highly are samples whole-rock from obtained data 

correlation and characterization in utility considerable of and 

strati- tuff ash-flow high-silica large-volume, lithic-poor, of 

units. graphic 

we samples whole-rock of types the that aware are We 

that sorting physical to subject are analyzed and collected 

compositions. chemical affecting for potential known has 

potential of magnitude the determine not did we However, 

elutria- shard glass from resulting variation compositional 

emplacement ash-flow during winnowing or sorting and tion 

distributions. pumice and ash, phenocryst, affect may that 

here presented compositions bulk the although Consequently, 

of some values, magmatic reflect broadly to believed are 

from result also must variation compositional observed the 

compositional of coherence The dynamics. emplacement 

that suggests range mountain the of rocks volcanic for data 

reason- compositions magma represent here presented data 

chemical intra-ash-flow of origin the of Regardless well. ably 

tuff ash-flow individual for compositions distinctive variation, 

correla- regional for value great of be to proven have units 

variation, compositional sectoral of effects the Finally, tion. 

from samples between overlap compositional by indicated as 

the evaluate further To minor. be to seem area, the throughout 

volcanic of compositions variation, sectoral of effects potential 

col- samples from established completely more be could rocks 

mountain isolated other in distributions their throughout lected 

region. this of ranges 

analytical in determined were compositions chemical All 

Colo. Denver, in Survey Geological U.S. the of laboratories 

fluores- X-ray by determined were abundances Major-oxide 

I.E. (analysts, 1987) others, and (Taggart techniques cence 

Stew- K.C. and Robb, E.G. Siems, D.F. Bartel, AJ. Taggart, 

0.8, to adjusted were ratios FeO) as iron FeO:FeO*(total art). 

percent 100 to recalculated were abundances major-oxide and 

spectros- fluorescence X-ray Energy-dispersive free. volatile 

sources excitation radioisotope Am 241 and Cd 109 using copy, 

abundances determine to used was 1982), Bray, du and (Elsass 

du E.A. and Yager D.B. (analysts, Ba and Nb, Zr, Y, Sr, Rb, of 

discussed are data these of accuracy and precision the Bray); 

(1992). Quick and Yager and (1989) Sargent and Sawyer by 

Sc, Zn, U, Th, Ta, Sb, Hf, Cs, Cr, Ni, Co, of abundances The 

determined were Lu and Yb, Tm, Tb, Gd, Eu, Sm, Nd, Ce, La, 

and (Baedecker analysis activation neutron instrumental by 

R.B. and Budahn, J.R. Knight, R.J. (analysts, 1987) McKown, 

Vaughn).

and biotites, 8 sanidines, (21 separates mineral Thirty 

20 representing samples twenty-three from hornblende) 1 

analyzed were 2) table 1; (fig. region the of units stratigraphic 

splits Two technique. heating incremental Ar 39 Ar/ 40 the by 
Sampling and Analytic Methods 

Table 1. Compositions of volcanic rocks of the central Chiricahua Mountains.

[Major-oxide data in weight percent (normalized to 100%, anhydrous). Trace-element data in parts per million. Ferrous iron/total iron as FeO 

adjusted to 0.8. ND, not detected at the indicated abundance. Averages and standard deviations given for n analyses. NA, not analyzed]

Pre-Turkey Creek caldera rocks

n=

SiO2

A1A

Fe203

FeO

MgO

CaO

Na^O

K20

TiO2

P2 0,

MnO

n=

Rb

Sr

Y

Zr

Nb

Ba

n—

Co

Ni

Cr

Cs

Hf

Sb

Ta

Th

U

Zn

Sc

La

Ce

Nd

Sm

Eu

Gd

Tb

Tm

Yb

Lu

Ba/Nb

Ba/Ta

La/Nb

Tim

16

62.58±2.19

17.06±0.51

1.21±0.17

4.37±0.60

2.22±0.78

3.89±1.08

3.64±0.57

3.83±1.52

0.80±0.15

0.27±0.05

0.08±0.03

24

152±81

489±109

30±7

189±20

11±3

840±220

3

15.0±1.5

13.9±3.49

18.1M.9

7.69±4.18

5.31±0.35

3.39±5.29

0.96±0.17

14.5±4.1

3.58±0.84

59.1±4.4

10.2±1.6

36.9±2.4

74.3±4.1

33.3±4.1

6.38±.82

1.34±0.22

5.44±0.39

0.77±0.10

0.41±0.03

2.59±0.15

0.39±0.02

76.4

840

3.3

Thl

6

76.97±2.03

12.61±0.86

0.25±0.05

0.89±0.20

0.36HU3

0.93±0.85

1.32±0.43

6.48±1.41

0.15±0.02

ND(O.Ol)

0.03±0.02

15

314±102

177±147

40±14

107±8

17±2

517±290

0

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

30.6

NA

NA

Tjg

10

73.02±1.17

14.59±0.81

0.49±0.05

1.77±0.19

0.43±0.13

0.56±0.32

2.60±0.72

6.10±1.27

0.32±0.04

0.08±0.03

0.03±0.01

13

312±106

122±32

35±12

174±32

14±4

818±163

0

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

56.4

NA

NA

Tkr

0

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

3

419±25

26±6

43±1

144±9

27±5

54±11

0

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

2.00

NA

NA

Tc

5

77.28±0.66

12.20±0.37

0.20±0.05

0.72±0.17

0.12±0.07

0.27±0.19

2.54±1.51

6.46±2.45

0.17±0.04

ND(O.Ol)

0.04±0.03

11

459±147

30±17

51±11

181±24

32±3

35±15

3

0.166±0.07

4.00±6.93

0.567±0.98

55.7±78.5

7.60±0.59

1.54±2.35

2.70±0.21

29.6±1.3

7.31±0.55

60.4±7.9

2.78±0.57

45.5±8.8

99.9±23.1

38.8±12.9

8.44±3.11

0.393±0.24

8.82±-

1.20±0.32

0.874±-

5.35±0.07

0.777±0.02

1.09

12.9

1.43

Tfre

6

73.27±0.53

14.91±0.25

0.26±0.02

0.94±0.07

0.38±0.24

1.20i0.23

4.01±0.25

4.80±0.10

0.19±0.01

ND(O.Ol)

0.04±0.02

6

172±11

249±22

29±6

163±4

15±2

829±53

2

0.910±0.14

8.30±5.23

1.04±0.27

6.52±2.28

5.35±0.33

0.133±0.02

1.24±0.06

17.2±0.9

3.88±0.29

36.6±0.9

2.95±0.13

46.8±2.9

91.5±9.2

32.6±3.5

5.39±0.52

0.767±0.05

4.31±0.91

0.609±0.05

0.407±0.02

2.56±0.13

0.368±0.04

56.6

670

3.19

Thcl

9

75.94±1.30

12.49±0.69

0.28±0.06

1.02±0.23

0.15±0.03

0.21±0.13

2.14±0.98

7.50±1.36

0.25±0.09

O.OliO.02

0.02±0.03

23

486±118

40±18

51±10

247±88

32±7

197±162

1

0.211

ND(0.5)

1.65

14.9

7.44

2.74

2.56

29.0

8.12

87.5

2.41

40.2

82.0

28.8

5.65

0.393

5.31

0.801

0.748

4.79

0.719

6.13

77.0

1.25

Theu

12

69.75±1.94

15.64±0.90

0.47±0.12

1.87±0.16

0.42±0.13

0.91±0.69

3.34±0.79

6.85±1.25

0.58±0.10

0.12±0.03

0.05±0.01

25

251±73

143±82

55±14

640±182

23±5

1496±651

2

2.20±0.39

17.2±3.0

1.08±1.86

5.84±3.45

17.3±1.1

0.277±0.12

1.40±0.09

15.6±1.0

2.72±0.21

66.6±10.6

6.19±0.48

67.1±4.2

140±16

58.3±2.9

11.0±0.7

3.18±0.29

8.9 1±-

1.31±0.09

0.696±-

4.40±0.13

0.646±0.02

64.9

1066

2.91

from each of two of the sanidine separates (fig. 1; 201765 

and 201769) were analyzed in order to evaluate analytical 

reproducibility for samples analyzed during the several years 

that geochronologic investigations were conducted. Mineral 

separates were prepared, after crushing, grinding, and sieving, 

by magnetic separator, mica-table, and heavy liquid methods; 

grains ranged in size between 60 and 120 mesh (250-125 fim). 

Separates were handpicked to greater than 99 percent purity.

Individual samples were cleaned with reagent-grade acetone, 

alcohol, and deionized water in an ultrasonic bath, air-dried, 

wrapped in aluminum capsules and sealed in silica vials along 

with monitor minerals before irradiation. Samples were irradiated in two irradiation packages, one in 1995 and an earlier 

one in 1988, at two separate TRIGA research reactor facilities. 

After irradiation, the samples were progressively degassed 

in a double-vacuum resistance furnace in a series of 11 to 16
Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

Table 1. Compositions of volcanic rocks of the central Chiricahua Mountains.—Continued

M =

SiO2

A1A

FeO

MgO

CaO

Na^O

K2O

TiO2

PA

MnO

n=

Rb

Sr

Y

Zr

Nb

Ba

M

Co

Ni

Cr

Cs

Hf

Sb

Ta

Th

U

Zn

Sc

La

Ce

Nd

Sm

Eu

Gd

Tb

Tm

Yb

Lu

Ba/Nb

Ba/Ta

La/Nb

Pre-caldera 

rocks

Tjj

6

77.57*0.21

12.40*0.20

0.22*0.04

0.78*0.14

0.31*0.08

0.37*0.12

3.07*0.62

5.08*0.49

0.15*0.03

ND(O.Ol)

0.05*0.02

8

304*8

26±7

41*5

175*25

34*3

33*41

5

0.287*0.09

2.08*3.81

1.35'±0.80

8.43*1.68

7.16*0.26

0.530*0.46

2.88*0.13

34.0*1.7

6.69*0.83

56.2*15.6

2.31*0.12

39.2*6.7

74.2*7.9

26.3*4.2

5.54±1.13

0.156*0.02

5.18*0.78

0.83 ±0.08

0.809*0.06

5.27*0.37

0.770*0.04

0.98

11.5

1.16

Rocks associated with the Turkey Creek caldera

Trcb

6

77,08*0.44

11.98*0.50

0.35*0.03

1.26*0.11

0.02*0.04

0.07*0.06

2.25*0.40

6.84*0.50

0.10*0.00

ND(O.Ol)

0.05*0.01

18

553*55

24*4

78±11

283*14

59*3

43*14

0

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

0.72

NA

NA

Trcl

20

77.21*0.39

12.08*0.16

0.34*0.03

1.21*0.11

0.16*0.10

0.29*0.17

3.55±0.27

4.96±0.12

0.12*0.02

ND(O.Ol)

0.07*0.01

33

423*25

22±6

75*27

289±16

61*5

20*12

6

0.297*0.23

2.69*4.33

0.403*0.50

9.20*1.67

11.5*0.7

0.248*0.06

5.40*0.32

48.3*4.3

11.2*1.1

79.4*13.2

1.95*0.08

51.3*15.3

132*30

44.6*15.9

10.3*4.1

0.103*0.04

9.97*3.96

1.78*0.72

1.32*0.43

8.42*2.70

1.23*0.36

0.33

3.66

0.84

Trcm

17

77.50*0.26

12.10*0.12

0.31*0.03

1.10*0.10

0.09*0.09

0.23*0.07

3.47*0.20

5.02*0.33

0.12*0.02

ND(O.Ol)

0.06*0.02

47

394*24

22*8

74*18

278*10

57*4

24*18

13

0.426*0.32

2.50*2.71

0.693*0.78

6.38*0.57

11.0*0.7

0.225*0.07

4.88*0.48

48.1*2.9

9.29*1.41

63.5*13.8

2.09*0.34

63.5*16.6

146±20

58.9±18.6

13.3*4.4

0.146*0.05

13.4*4.3

2.17*0.72

1.53*0.39

9.53*2.34

1.33*0.30

0.42

4.95

1.11

Trcu

9

76.09*0.68

12.50*0.26

0.40*0.08

1.55*0.11

0.24*0.14

0.34±0.13

3.32*0.72

5.25*0.27

0.22*0.01

0.01*0.03

0.06*0.02

15

285*23

42*13

65*7

366*31

42*5

77*29

4

0.870*0.27

1.20*2.40

1.28*0.89

4.70*0.54

12.1*0.9

0.228*0.07

3.73*0.14

41.7*1.5

8.49*0.69

61.7*11.1

3.50±0.25

96.5*11.4

224*27

83.8*8.8

15.2*2.1

0.264*0.04

12.7*1.3

1.95*0.18

1.17*0.15

7.38*0.44

1.06*0.07

1.84

20.5

2.32

Trci

39

76.59*1.16

12.32*0.59

0.42*0.05

1.49*0.18

0.16*0.13

0.23*0.14

3.10*0.71

5.43*0.71

0.21*0.05

ND(O.Ol)

0.03*0.03

67

332*100

38*14

59*11

383*68

43*7

77*38

12

0.573*0.71

2.83*3.69

1.15*1.48

6.25*3.74

13.2*1.2

0.905*0.77

3.79*1.20

42.8*6.4

7.89*2.44

70.4*14.3

3.85±1.08

100*38

220*80

82.5*28.7

13.7*3.9

0.334*0.24

11.3*2.6

1.66*0.41

1.08*0.09

6.74*0.62

0.969*0.10

1.76

20.1

2.30

Trcf

7

76.65*1.36

11.99*0.67

0.41*0.06

1.48*0.23

0.11*0.08

0.06*0.04

2.12*1.33

6.95*1.61

0.21*0.04

ND(O.Ol)

0.03*0.03

15

397*122

34*7

64*8

395*59

42*4

94*88

3

0.248*0.03

ND(0.5)

0.693*0.60

7.52*1.43

11.1*0.5

1.06*0.27

3.70*0.54

40.2*2.8

8.20*0.30

61.9*11.3

2.80*0.03

87.8*7.4

180*18

67.3*3.7

13.2*0.5

0.212*0.03

10.4*0.3

1.63*0.06

1.08*0.07

6.95*0.67

0.987*0.09

2.27

25.5

2.11

Tdpl

24

65.42*1.39

15.54*0.42

0.97*0.08

3.49*0.28

1.38*0.40

2.84*0.74

3.77*0.49

5.26*1.07

0.92*0.09

0.34*0.03

0.09*0.02

60

208*66

227*54

48*8

490*89

26±3

794±117

10

8.84*1.35

22.3*3.6

16.5*6.2

11.7*22.0

11.6*1.7

0.244*0.14

1.92*0.25

22.5*3.01

4.59*0.94

77.9*7.3

9.50*0.60

80.1*11.1

175*27

72.3*10.7

12.0*1.8

2.00*0.22

10.1*1.4

1.42*0.15

0.740*0.08

4.59*0.47

0.669*0.08

31.1

412

3.14

individual, 20-minute-long temperature steps to a maximum 

temperature of 1,650°C. All analyses were done in the Argon 

Laboratory, U.S. Geological Survey, Denver, Colo. Decay 

constants are those of Steiger and Ja'ger (1977). The standard 

used in these age determinations was hornblende MMhb-1 

with a K-Ar age of 520.4 Ma (Samson and Alexander, 1987). 

Apparent ages were calculated using decay constants recommended by Steiger and Ja'ger (1977). The determination of

whether the individual apparent ages yielded a "plateau" was 

made using the critical value test of Dalrymple and Lanphere 

(1969) following the plateau definition of Fleck and others 

(1977). Plateaus that pertain to more than 50 percent of the 

gas produced during heating were achieved for all but seven 

of the mineral separates. Plateau dates were calculated using 

a weighted mean, where weighting is by the inverse of the 

analytical variance (Taylor, 1982).
Petrographic and Stratigraphic Characteristics

Table 1. Compositions of volcanic rocks of the central Chiricahua Mountains.—Continued

Rocks associated with the Turkey Creek caldera

n=

SiO2

A1A

FeA

FeO

MgO

CaO

Na20

K2 0

TiO2

PA

MnO

n~

Rb

Sr

Y

Zr

Nb

Ba

«=

Co

Ni

Cr

Cs

Hf

Sb

Ta

Th

U

Zti

Sc

La

Ce

Nd

Sm

Eu

Gd

Tb

Tm

Yb

Lu

Ba/Nb

Ba/Ta

La/Nb

Tdpi

16

65.88±2.73

15.29±0.57

0.97±0.20

3.51±0.74

1.40±0.48

2.59±0.88

3.80±0.32

5.20±0.56

0.94±0.21

0.33±0.09

0.09±0.03

25

204±62

206±48

47±8

474±81

24±4

763±96

9

9.35±2.83

13.2±9.3

9.46±3.63

5.02±2.34

11.8±0.6

0.268±0.02

2.10±0.18

25.7±5.7

4.53±0.74

77.1±7.9

9.45±1.84

83.8±10.6

178±14

72.3±7.4

12.U1.3

1.92±0.23

10.2±0.9

1.47±0.14

0.779±0.09

4.87±0.53

0.721±0.08

31.6

363

3.47

Ttp

2

64.00±5.00

15.99±0.72

1.02±0.37

3.68±1.34

1.66±0.72

2.46±2.16

4.43±0.07

5.38±0.91

0.99±0.30

0.32±0.18

0.08±0.05

4

206±29

152±117

63±4

839±120

34±4

266±182

0

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

7.74

NA

NA

Tmrb

6

73.73±0.62

14.01±0.41

0.39±0.01

1.41±0.03

0.46±0.08

1.46±0.30

3.41±0.47

4.76±0.77

0.28±0.02

0.07±0.04

0.03±0.02

17

265±57

162±66

39±6

175±25

15±3

667±109

6

2.31±0.24

8.23±3.00

2.26±0.83

4.91±2.87

5.68±0.39

0.098±0.02

1.45±0.13

27.8±2.5

3.72±0.48

39.6±6.5

3.79±0.29

52.2±5.8

114±15

46.3±7.4

8.44±1.33

1.01±0.08

7.50±1,17

1.09±0.16

0.606±0.08

3.69±0.50

0.524±0.07

45.4

460

3.55

Tmrl

12

76.55±0.47

12.8U0.26

0.27±0.02

0.98±0.07

0.17±0.02

0.55±0.24

3.84±0.52

4.59±0.88

0.18±0.01

ND(O.Ol)

0.06±0.01

64

351±70

34±14

59±14

220±39

40±8

81±18

7

0.395±0.05

5.18±2.51

0.398±0.44

22.2±19,4

8.00±0.41

0.222±0.07

3.65±0.18

38.9±2.9

8.95±1.29

69.9±24.2

2.53±0.22

70.5±9.3

158±18

61.8±5.5

11.4±0.9

0.487±0.03

10.7±0.7

1.73±0.13

1.18±0.04

7.46±0.22

1.07±0.05

2.01

22.1

1.76

Tmr2

9

77.32±0.55

12.62±0.33

0.22±0.01

0.78±0.04

0.08±0.06

0.19±0.17

3.65±0.48

4.93±0.79

0.15±0.03

0.01±0.03

0.06±0,02

27

413±55

18±7

48±12

93±7

53i7

14±9

7

0.370±0.42

11.2±22.0

0.587±0.65

11.1±0.7

8.62±1.21

0.242±0.06

4.81±0.80

51.4±12.8

11.2±2.5

57.8±11.8

2.29±0.43

36.5±7.3

92.1±24.9

24.2±7.6

5.02±1.88

0.189±0.05

4.9U2.82

0.880±0.33

0.853±0.33

6.6U1.37

0.996±0.22

0.27

2.96

0.68

Tmr3

3

77.45±0.09

12.31±0.16

0.24±0.01

0.86±0.05

O.lliO.Ol

0.15±0.07

3.57±0.03

5.12±0.09

0.15±0.01

ND(O.Ol)

0.03±0.01

6

391±14

20±4

38±12

196±6

51±2

12±5

2

0.223±0.01

2.40±0.00.

8.9U1.29

7.10±1.13

8.20±0.21

0.176±0.02

4.54±0.16

44.9±0.6

8.11±1.95

64.3±26.3

2.04±0.01

25.0±0.9

56.8±14.0

15.8±3.0

3.34±0.75

0.176±0.02

3.37±0.28

0.598±0.11

0.652±0.08

4.53±0.23

0.694±0.02

0.23

2.57

0.49

Post-caldera rocks

Ts

4

77.64±0.35

12.09±0.33

0.25±0.10

1.04±0.15

0.09±0.07

0.41±0.16

3.05±0.27

5.24±0.41

0.15±0.01

ND(O.Ol)

0.05±0.01

10

407±44

34±20

58±14

161±31

42±6

90±106

0

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

2.14

NA

NA

Trdp

4

76.79±0.40

12.73±0.20

0.22±0.07

0.95±0.08

ND(O.IO)

0.34±0.10

3.98±0.05

4.81±0.09

O.lliO.Ol

ND(O.Ol)

0.07±0.02

12

555±65

17±3

69±30

150±13

64±16

20±14

0

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

0.31

NA

NA

Petrographic and Stratigraphic 

Characteristics

In the descriptions that follow, a three- to four-letter map 

unit code is identified for each of the Stratigraphic units in 

order to aid their identification in the tables and figures. Map 

unit symbols designated by du Bray and others (1997) on the 

geologic map of the Turkey Creek caldera match those used 

here. The first letter in each of these map unit symbols, T,

denotes their Tertiary age. A simplified correlation of map 

units and a set of abbreviated rock unit identifiers are provided 

in order to define Stratigraphic relations among the various 

volcanic rock units that were studied (fig. 2).

Pre-Turkey Creek Caldera Rocks

Basement rocks on which middle Tertiary volcanic rocks 

were deposited include interlayered Mesozoic sedimentary
8 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

Table 2. Summary of 40Ar/39Ar age-spectrum results from the central Chiricahua Mountains.

[Preferred ages in bold; % data in Type of apparent age column indicates percentage of total gas included in plateau steps; --, apparent 

age, isochron. initial 40/36, not calculated]

Sample No.; 

unit symbol

Unit Mineral Apparent 

age (Ma)

Type of Isochron 

apparent age age

Initial 

40/36

Pre-Turkey Creek caldera rocks

DY91-77

202057 

Thl

DY91-11 

Tjg

Dacitic rocks of Halfmoon Valley, 

intrusion.

Rhyolite tuff of High 

Lonesome Canyon.

Lower member of the 

rhyolite of Joe Glenn

Biotite

Sanidine

Biotite

73.23+0.11

34.16±0.17

33.81+0.08

Excess argon 74.6+0.6 

Saddle-low

Plateau 

(76.5%)

Plateau - 

(91.1%)

276+19

-

--

Ranch.

P650A Pyrodastie rocks of Biotite 

Rucker Canyon, 

rhyolite lava.

202064 Pyroclastic rocks of Biotite 

Rucker Canyon.

33.32±0.07 Plateau 

(83.5%)

33.21+0.09

202064 Pyroclastic rocks of 

Rucker Canyon.

201570 Granodiorite of Mackey 

Canyon.

Sanidine 33.04+0.04

Hornblende 32.08+0.20

201570 Granodiorite of Mackey Biotite 

Canyon.

P272C Rhyolite lava of Cave

Tc Creek.

P475 Rhyolite of Erickson

Tfre Ridge.

P475 Rhyolite of Erickson Biotite 

Tfre Ridge.

P652 Jesse James Canyon

Tjj Tuff.

201771 Jesse James Canyon

Tjj Tuff.

30.62+0.15

Sanidine 28.10+0.12

Sanidine 27.89+0.09

28.24+0.08

Sanidine 27.52+0.06

Sanidine 27.59+0.06

202156 Lower member of the Biotite 27.62+0.10 

Thcl tuff ofHorseshoe 

Canyon.

202156 Lower member of the Sanidine 

Thcl tuff of Horseshoe 

Canyon.

202154 Latite of Darnell Peak Sanidine 27.58+0.08

Plateau 

(68.9%)

Weight 

average

Plateau 

(50.8%) 

excess Ar

Plateau 

(59.1%)

Plateau 

(89.3%)

Plateau 

(78.5%)

Plateau

(65.5%)

Plateau

(61.3%)

Plateau

(55.8%)

Plateau 

(88.8%)

No plateau

Plateau

(77.9%)

33.04±0.04 

31.42+0.30

310+7 

309+6

27.711-0.02 270+10

25.42+0.10 297+4

and volcanic rocks. These Mesozoic rocks are underlain by 

Paleozoic marine sedimentary rocks that were deposited on 

a basement composed of Proterozoic granitoid rocks. These 

Mesozoic, Paleozoic, and Proterozoic rocks are not discussed 

further in this report.

In many parts of the central Chiricahua Mountains, the 

Tertiary volcanic rocks described herein are directly underlain by intermediate-composition lava flows, flow breccias.

and near-source pyroclastic rocks (Tim) that probably were 

erupted from a field of coalescing composite volcanoes. These 

dacitic to andesitic rocks, which denote the onset of middle 

Tertiary volcanism in this area, are dark greenish gray to 

greenish black and maroon where oxidized and form massive, 

locally densely jointed and fractured outcrops. These rocks 

are aphyric to sparsely porphyritic; glassy flow margins are 

preserved in some places. Phenocrysts form trachytic or
Petrographic and Stratigraphic Characteristics 

Table 2. Summary of 40Ar/39Ar age-spectrum results from the central Chiricahua Mountains—Continued

Sample No.; 

unit symbol

Unit Mineral Apparent 

age (Ma)

Type of Isochron 

apparent age age

Initial 

40/36

Rocks associated with the Turkey Creek caldera

201769 

Trcl

201769 

Trcl

DY91-36 

Trcm

201765 

Trcu

201765 

Trcu

201587 

Tdpi

P3 

Tdpl

201996 

Tmrb

201996 

Tmrb

201538 

Tmrl

201580 

Tmrl

Lower member of the 

Rhyolite Canyon 

Tuff.

Lower member of the 

Rhyolite Canyon 

Tuff.

Middle member of the 

Rhyolite Canyon 

Tuff.

Upper member of the 

Rhyolite Canyon

Tuff.

Upper member of the 

Rhyolite Canyon 

Tuff.

Dacite porphyry, 

intrusion.

Dacite porphyry, lava 

flow.

Turkey Creek caldera 

moat lava, biotite 

rhyolite lava.

Turkey Creek caldera 

moat lava, biotite 

rhyolite lava.

Turkey Creek caldera 

moat lava, unit 1 

rhyolite lava.

Turkey Creek caldera 

moat lava, unit 1 

rhyolite lava.

Sanidine

Sanidine

Sanidine

Sanidine

Sanidine

Sanidine

Sanidine

Biotite

Sanidine

Sanidine

Sanidine

26.97+0.09

26.93+0.12

27.03+0.11

—

26.94+0.12

26.84±0.17

26.97+0.13

27.11+0.06

26.74±0.05

26.93+0.17

26.64+0.13

Plateau 27.09+0.04 

(60.8%)

Plateau 

(90.8%)

Plateau 27.14+0.03 

(75.0%)

No plateau 26.98+0.04

Plateau 

(91.3%)

Weight 26.90+0.04 

average

Minimum 27.44±0.15 

age step

Plateau 26.63+0.04 

(67.8%)

Plateau 

(65.6%)

Weight 26.89±0.05 

average

Plateau 

(94.3%)

309±2

—

297+2

309+2

—

296±2

303+2

366+15

-

298+1

"

Post-Turkey Creek caldera rocks

DY92-54

202151 

Trdp

P5

Rhyolite lava

Rhyolite lava of 

Dobson Peak.

Rhyolite of Packsaddle 

Mountain.

Sanidine

Sanidine

Sanidine

26.35±0.08

26.20± 0.07

23.23± 0.06

Plateau

(65.9%)

Plateau 

(62.8%)

Plateau 

(65,0 %)

-

-

-

intergranular intergrowths of plagioclase, pyroxene, hornblende, and biotite in a devitrified or aphanitic groundmass; 

accessory phases include Fe-Ti oxide minerals and apatite. 

The groundmass is variably altered to clay minerals, Fe-Ti 

oxide minerals, zeolites, and calcite.

Intermediate-composition volcanic rocks are only present 

at the base of the Tertiary section in the map area; rocks of this 

type are not interstratified with younger, voluminous

rhyolitic volcanic rocks. The onset of rhyolitic volcanism 

denotes the development of large, low-density magma reservoirs in the shallow crust. Once established, these reservoirs 

likely inhibited the buoyant ascent and eruption of additional 

intermediate-composition magma. Consequently, solidified 

masses representing unerupted parts of the rhyolitic reservoirs 

are probably underplated by a considerable volume of solidified, intermediate-composition magma petrologically similar
10 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

EXPLANATION

	POST-TURKEY CREEK CALDERA ROCKS 

Trdp Rhyolite lava of Dobson Peak 

Ts Pyroclastic deposits and lava flows of Swede Peak 

	ROCKS ASSOCIATED WITH THE

	TURKEY CREEK CALDERA

Tmr3 Unit 3 rhyolite lava of the Turkey Creek caldera

Tmr2 Unit 2 rhyolite lava of the Turkey Creek caldera

Tmr1 Unit 1 rhyolite lava of the Turkey Creek caldera

Tmrb Biotite rhyolite lava of the Turkey Creek caldera

Ttp Trachyte porphyry lava

Tdpi Dacite porphyry intrusion of the Turkey Creek caldera

Tdpl Dacite porphyry lava of the Turkey Creek caldera

Trcf Lava-flow-like phase of the Rhyolite Canyon Tuff

Trci Intracaldera facies Rhyolite Canyon Tuff

Trcu Upper member, outflow facies Rhyolite Canyon Tuff

Trcm Middle member, outflow facies Rhyolite Canyon Tuff

Trcl Lower member, outflow facies Rhyolite Canyon Tuff

Trcb Basal member, outflow facies Rhyolite Canyon Tuff

	PRE-TURKEY CREEK CALDERA ROCKS 

Tjj Jesse James Canyon Tuff

Thcu Upper member of the tuff of Horseshoe Canyon

Thcl Lower member of the tuff of Horseshoe Canyon

Tf re Rhyolite of Erickson Ridge

Trr Pyroclastic rocks of Rucker Canyon

Tc Rhyolite lava of Cave Creek

Tkr Rhyolite lava of Krentz Ranch

Tjg Lower member of the rhyolite of Joe Glenn Ranch

Thl Rhyolite tuff of High Lonesome Canyon

Tim Intermediate-composition lava flows

Ash-flow tuff

Mixed lava flow and pyroclastic flow deposit

Lava flow

Intrusive

Figure 2. Simplified correlation of map units and identification of studied volcanic rock units.

to the intermediate-composition volcanic rocks preserved at 

the base of the Tertiary section in the map area.

The oldest regionally extensive ash-flow tuff in the 

central Chiricahua Mountains was erupted from an unknown 

source; it is the pale-orange to yellowish-gray rhyolite tuff 

of High Lonesome Canyon (Thl). This ash-flow tuff, first 

described by Drewes and Brooks (1988), is crystal poor 

(contains about 5 percent crystals), pumice and lithic rich, and 

weakly to moderately welded. Phenocrysts include approximately equal amounts of partly resorbed and embayed quartz 

and subhedral to anhedral sanidine, lesser amounts of anhedral 

albite and Fe-Ti oxide minerals, and trace amounts of oxidized 

biotite, all in a devitrified ash- and shard-rich matrix. In exposures near the mouth of Rucker Canyon, the tuff consists of 

two cooling units separated by a 1-m-thick volcanic sandstone. 

Andesitic lithic fragments are common. In the study area, 

the tuff is restricted to a small area south of the Turkey Creek 

caldera.

The lower member of the rhyolite of Joe Glenn Ranch 

(Tjg) is grayish-orange-pink to pale-red-purple ash-flow tuff 

erupted from an unknown source. Only the lower member of 

the rhyolite of Joe Glenn Ranch, first described by Drewes 

and Brooks (1988), is present in the area studied by du Bray

and others (1997). The tuff, which overlies the rhyolite tuff of 

High Lonesome Canyon, is crystal rich (2CMK) percent), 

pumice and lithic poor, and weakly to moderately welded. 

Crystals are resorbed quartz, subhedral sanidine, subhedral albite, and anhedral oxidized biotite, and include trace 

amounts of anhedral Fe-Ti oxide minerals and zircon in a 

devitrified ash matrix. This unit may be correlative with 

Faraway Ranch Formation member 3 of Fernandez and 

Enlows (1966). In the central Chiricahua Mountains, the tuff 

is restricted to small areas north and south of the Turkey Creek 

caldera.

The rhyolite lava of Krentz Ranch (Tkr), defined by 

Drewes and Brooks (1988), is composed of light-gray to 

pale-orange-gray, nearly aphyric, massive to flow-laminated, 

high-silica rhyolite lava that contains interbedded, cogenetic 

pyroclastic flow deposits. These lava flows, vitrophyric in 

some places, probably represent a field of coalesced rhyolite 

domes. Sparse phenocrysts, which are in a massive, variably 

devitrified groundmass, include subhedral sanidine, resorbed 

quartz, and trace amounts, especially in vitrophyre, of zircon, 

Fe-Ti oxide minerals, and hornblende. The rhyolite lava of 

Krentz Ranch underlies an area of less than 10 km2 about 160 

km southeast of the center of the Turkey Creek caldera.
Petrographic and Stratigraphic Characteristics 11

The rhyolite lava of Cave Creek (Tec), defined by 

Raydon (1952), is light gray to pale orange gray and reddish 

orange, crystal-poor to aphyric, and massive to flowlaminated. Phenocrysts are mostly sanidine and quartz, but 

trace amounts of hornblende, biotite, magnetite, and titanite 

are preserved, especially in glassy rocks. The groundmass of 

the rhyolite is a devitrified intergrowth of quartz and feldspar 

that is spherulitic or granophyric in some places. The rhyolite 

of Cave Creek, which overlies andesite to dacite lava flows in 

the east part of the study area, forms numerous flow domes 

and contains interbedded, cogenetic pyroclastic flow deposits. 

These rocks probably overlie the small vents from which they 

were erupted. Bryan (1988) identified and mapped lower, 

middle, and upper members of the rhyolite in the Cave Creek 

watershed.

The rhyolite of Erickson Ridge (Tf re), as described by 

Pallister and others (1994), is light-gray (devitrified) to black 

(glassy) biotite rhyolite. It contains phenocrysts of oscillatoryzoned, subhedral oligoclase-albite (3-7 percent) and biotite 

(1-2 percent). Accessory to trace titanite forms euhedral 

phenocrysts. The rhyolite forms small lava domes and lobate 

flow-layered lava flows having black glassy carapace breccias 

and minor interbedded, cogenetic pyroclastic flow deposits. 

The rhyolite overlies andesite lava flows and forms extensive 

overlapping flow domes that probably overlie and conceal 

their small vents. The lava flows are probably correlative 

with Faraway Ranch Formation member 7 of Fernandez and 

Enlows (1966), whereas interbedded pyroclastic flow deposits 

are probably correlative at least in part with Faraway Ranch 

Formation member 6 of Fernandez and Enlows (1966). The 

rhyolitic lava flows of the Faraway Ranch Formation, which 

underlie an area of about 5 km2 about 5 km west of Sugarloaf 

Mountain and north of the Turkey Creek caldera, probably 

represent a field of coalesced rhyolite domes.

The tuff of Horseshoe Canyon (Bryan, 1988) crops out 

extensively east and southeast of the Turkey Creek caldera and 

consists of upper (Thcu) and lower (Thcl) members separated 

by a plagioclase-sanidine porphyry sill known as the latite 

of Darnell Peak (Bryan, 1988). This tuff, erupted from the 

Portal caldera (Bryan, 1988), is gray- to orange-weathering, 

densely welded, and strongly zoned. The lower member is 

zoned from a thin basal unit composed of high-silica rhyolite 

tuff with moderate crystal content (10-15 percent) to crystalrich (20-35 percent) trachyte tuff containing phenocrysts of 

sanidine, quartz, plagioclase, biotite, clinopyroxene, titanite, 

and Fe-Ti oxide minerals. The upper member has moderate 

crystal content (10-20 percent) and is composed of low-silica 

rhyolite tuff that contains phenocrysts of sericitized sanidine, 

quartz, and accessory or trace biotite. The tuff of Horseshoe 

Canyon is correlative with tuff of Price Canyon (Drewes and 

Brooks, 1988) and the Eagle Cliffs member of the rhyolite of 

Cave Creek (Raydon, 1952).

The Jesse James Canyon Tuff (Tjj), as described by Pallister and others (1994), erupted from an unknown source; 

it is light-gray or pinkish-gray, typically lithic poor, moderately crystal rich (approximately 10 percent), biotite-bearing

quartz-sanidine rhyolite ash-flow tuff. This unit is similar 

to middle and lower members of Rhyolite Canyon Tuff but 

is distinguished by trace amounts of biotite and titanite, an 

absence of clinopyroxene, a higher ratio of sanidine to quartz, 

less evolved chemistry, and Stratigraphic position. The Jesse 

James Canyon Tuff is correlative, in part, with the welded tuff 

of Rucker Canyon (Drewes and Brooks, 1988). In the study 

area, the tuff is restricted to small areas north and south of the 

Turkey Creek caldera.

Rocks Associated with the Turkey Creek 

Caldera

The Rhyolite Canyon Tuff, as redefined by Pallister and 

others (1994), consists of intracaldera and outflow facies. 

Drewes (1982) provided a synopsis for the correlation between 

the Rhyolite Canyon Formation, parts of which were redefined to the Rhyolite Canyon Tuff, and earlier nomenclature 

established for these rocks by Enlows (1955) and Fernandez 

and Enlows (1966). The tuff, whose eruption caused collapse 

of the Turkey Creek caldera, is the volumetric ally dominant 

eruptive product of the caldera (du Bray and Pallister, 1991). 

The outflow facies rocks have been divided into basal (Trcb), 

lower (Trcl), middle (Trcm), and upper (Trcu) members. The 

intracaldera facies (Trci) is dominated by a thick accumulation 

of homogeneous tuff but also includes a lava-flow-like phase 

(Trcf). All parts of the Rhyolite Canyon Tuff are petrographically similar, with the exceptions noted following. The unit 

is light-gray to reddish-brown, high-silica rhyolite ash-flow 

tuff that contains 7-35 percent phenocrysts; phenocrysts are 

almost entirely quartz and sanidine. Sanidine forms lathshaped crystals typically 1-4 mm long but locally as long as 1 

cm in the upper member of the Rhyolite Canyon Tuff. Quartz 

typically is rounded and embayed, and grains are 1-3 mm in 

diameter. The tuff also contains accessory Fe-Ti oxide minerals and trace augite, hornblende, zircon, apatite, and allanite. 

The lava-flow-like phase in the uppermost exposures of intracaldera tuff is distinguished by large (0.5-1 cm), mostly lath 

shaped crystals of perthitic sanidine, large subhedral or partly 

resorbed quartz phenocrysts, an apparent absence of eutaxitic 

structure, and a few lithic inclusions. The intracaldera facies 

is reddish brown, red, pink, orange, or gray and lithic-poor 

to lithic-rich (<5-20 percent). The Rhyolite Canyon Tuff is 

correlative with the tuff of Shake Gulch and the tuff of Bruno 

Peak (Drewes and Brooks, 1988).

Dacite porphyry was emplaced as a resurgent intrusion 

(Tdpi) in the center of the collapsed Turkey Creek caldera. 

Its extrusive equivalent, dacite porphyry lava flows (Tdpi), 

was erupted onto the floor of the evolving moat of the Turkey 

Creek caldera shortly after eruption of the Rhyolite Canyon 

Tuff. Dacite porphyry that forms lava flows is petrographically and compositionally similar to dacite porphyry of the 

resurgent intrusion, except that clinopyroxene is its predominant mafic mineral and its groundmass is much finer grained. 

Glassy margins, brecciated in places, are exposed locally
12 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

between flows at shallow stratigraphic levels. The dacite porphyry is gray to tan, massive, and highly jointed. Its groundmass grades from coarse cuneiform granophyre, most common 

at the lowest exposed levels of the resurgent intrusion, through 

medium- to fine-grained granophyre higher in the intrusion. 

The dacite porphyry contains megacrysts (5 mm to >3 cm 

across) of alkali feldspar and plagioclase, and small (typically 

1 cm across) hornfels inclusions. Alkali feldspar, commonly 

zoned, forms overgrowths on plagioclase, and is exsolved 

variably to microperthite; cores of some alkali feldspar phenocrysts are resorbed. Plagioclase phenocrysts (1-3 mm) are 

zoned from albite rims to andesine cores. The dacite porphyry 

also contains glomerocrysts of albite-andesine and phenocrysts 

or microphenocrysts of sanidine, quartz, biotite, hornblende, 

clinopyroxene, and Fe-Ti oxide minerals, and trace amounts of 

apatite, zircon, and titanite; phenocryst assemblages are highly 

variable. Phenocrysts of partly resorbed quartz are present 

locally, and groundmass quartz is abundant in granophyre.

Trachyte porphyry lava (Ttp) crops out locally at the base 

of the volcanic section preserved in the moat of the Turkey 

Creek caldera. The unit consists of red to reddish-brown or 

orange trachyte lava with intersertal to granophyric groundmass textures. The lava contains large phenocrysts (2 mm to 

>1 cm) of sanidine that have dusty reaction rims. Phenocrysts 

also include small crystals composed of Fe-Ti oxide minerals, oxyhornblende, clinopyroxene, and plagioclase. Strained 

quartz xenocrysts are present in some samples. The trachyte 

porphyry forms very restricted outcrops in the southeastern 

part of the Turkey Creek caldera.

A sequence of rhyolite lava flows and minor associated pyroclastic flow deposits, the Fife Canyon Volcanics of 

Latta (1983), are stratigraphically above dacite and trachyte 

porphyry lava flows in the moat of the Turkey Creek caldera. These rocks, with a total preserved volume of about 60 

km3 , probably were erupted from vents along the buried ring 

fracture collapse system of the Turkey Creek caldera. The 

presence of biotite in the basal rhyolite of this sequence is 

distinctive. The upper three rhyolite lava flows, units 1 through 

3, from oldest to youngest, are petrographically indistinguishable, nearly aphyric, high-silica rhyolite lava flows separated 

by thin pyroclastic flow deposits.

The biotite rhyolite (Tmrb) forms flows and domes 

on the north flank of the Turkey Creek caldera. The lava is 

gray to brownish- or yellowish-gray (devitrified) or black 

(glassy), moderately phenocryst rich (5-20 percent) rhyolite 

that contains plagioclase, sanidine, quartz, biotite, Fe-Ti oxide 

minerals, and trace amounts of zircon and monazite. Plagioclase also forms small (<1 mm), oscillatory-zoned (andesine 

cores) crystals; a xenocrystic origin is suggested by resorption, 

wormy glass inclusions, and its occurrence in small crystal 

clots, commonly with biotite. Perlite, locally preserved at the 

basal contact, is spherulitic in some outcrops. Flow interiors 

are devitrified and locally granophyric.

Unit 1 lava (Tmr1) consists of light-gray to reddish-gray 

or brown rock, most of which is flow layered and intricately 

flow folded—although some exposures are massive. The lava

is devitrified, except at its base, where perlitic glass locally 

contains spherulitic zones and geodes. It is typically aphyric 

or crystal poor (<5 percent) and contains sanidine, quartz, and 

Fe-Ti oxide minerals, along with trace amounts of plagioclase, 

hornblende, and clinopyroxene. Carapace breccia is exposed 

locally at margins of lava flows. Flow interiors are recrystallized to granophyre and contain vapor-phase quartz and 

feldspar in amygdules.

Unit 2 lava (Tmr2) is light-gray to reddish-gray, phenocryst-poor rhyolite lava that is flow layered and intricately 

flow folded, locally massive, and aphyric or sparsely (0-2 

percent) porphyritic. It contains phenocrysts (<1 mm) of 

sanidine, quartz, and Fe-Ti oxide minerals; accessory biotite 

and zircon are present in some samples. The lava is devitrified, except at its base, where black or green glassy breccia 

or flow-layered perlite is locally exposed; spherulitic and 

axiolitic (with respect to flow layers) devitrification and 

granophyric recrystallization are common.

Unit 3 lava (Tmr3) is light-gray to reddish-gray, typically 

aphyric, flow-layered and folded rhyolite. It contains trace 

amounts of sanidine phenocrysts and biotite microphenocrysts. 

Spherulitic and axiolitic (with respect to flow layers) devitrification and granophyric recrystallization are common features 

of this unit.

Post-Turkey Creek Caldera Rocks

Pyroclastic deposits and lava flows of Swede Peak (Ts), 

as defined by Drewes and Brooks (1988), consist of white to 

tan high-silica rhyolite. The rhyolite is commonly crystal rich 

(>20 percent); phenocrysts are sanidine, quartz, albite, and 

oxidized biotite and trace amounts of Fe-Ti oxide 

minerals, titanite, and zircon. Vitroclastic groundmass in 

unwelded pyroclastic deposits is composed of variably 

devitrified ash, glass shards, and crystal and lithic fragments. 

Abundances of pumice and glass shards are variable within 

and between individual pyroclastic flows. Abundance of lithic 

fragments also is widely variable. The great thickness of 

these areally restricted deposits near Swede Peak and decreasing thicknesses in surrounding areas suggest vent(s) near this 

feature southeast of the Turkey Creek caldera.

The rhyolite lava of Dobson Peak (Trdp) is light-gray to 

reddish-gray, flow-layered and intricately flow folded, highsilica rhyolite lava that is crystal poor (1-4 percent); 

phenocrysts are resorbed quartz, sanidine, and subordinate 

albite and trace amounts of Fe-Ti oxide minerals and biotite. 

Its groundmass commonly is devitrified and recrystallized 

to an aggregate of microscopic spherulites and granophyre. 

Thickness relations are similar to those of pyroclastic deposits 

and lava flows of Swede Peak, which, together with 

compositional and temporal similarities, imply cogenesis and 

eruption from associated conduits. The rhyolite underlies an 

area of about 5 km2 about 15 km southeast of the center of the 

Turkey Creek caldera.
Geochemistry 13

Geochemistry

Classification

The International Union of Geological Sciences (IUGS) 

classification of volcanic rocks (Le Bas and others, 1986) was 

applied to the compositions of volcanic rocks of the central 

Chiricahua Mountains (table 1); most of the units studied are 

rhyolite (fig. 3). Because the resurgent intrusion in the Turkey 

Creek caldera, its associated moat-filling lava flows, and the 

immediately overlying lava flows contain less than 20 percent 

normative quartz and greater than 9 percent Na,O+K^O and 

64—66 percent SiO2 , they are alkaline and can be classified 

as trachyte using the criteria of Le Bas and others (1986). 

However, in the case of the resurgent intrusion and its associated lava flows, we have chosen to apply the name dacite, the 

subalkaline variant of trachyte, because their compositions are 

transitional between alkaline and subalkaline compositions as 

defined by Irvine and Baragar (1971) and Le Bas and others 

(1986), and to emphasize their consanguinity with the other 

subalkaline products of the Turkey Creek caldera. That this 

igneous system is not intrinsically alkaline is another reason

that we have elected not to use the name trachyte; our choice 

of dacite is in accord with the absence of alkali-rich minerals, 

such as aegirine and alkali amphiboles, that are characteristic 

of alkaline series rocks. Because the trachyte porphyry lava 

flows (Ttp) that immediately overlie the dacite porphyry 

flows have compositions (fig. 3) that are alkaline, even by 

the definition of Irvine and Baragar (1971), we do apply the 

name trachyte to these rocks. However, all of the rhyolites are 

subalkaline, as defined by Irvine and Baragar (1971), and all 

but the upper member of the tuff of Horseshoe Canyon, the 

rhyolite of Erickson Ridge, the lower member of the rhyolite 

of Joe Glenn Ranch, and the biotite rhyolite lava of the Turkey 

Creek caldera moat sequence contain >76 percent SiO^, and 

are therefore high-silica rhyolite. Most volcanic rocks in the 

central Chiricahua Mountains are weakly peraluminous (alumina saturation indices range from 1.03 to 1.11); the exception 

is the dacite and trachyte porphyries, which are metaluminous 

(alumina saturation indices 0.91-0.92). The peraluminous 

tendencies may reflect minor, posteruptive alkali loss relative to alumina rather than primary magmatic characteristics. 

The lower member of the rhyolite of Joe Glenn Ranch and the 

rhyolite tuff of High Lonesome Canyon are distinctly more 

strongly peraluminous (alumina saturation indices 1.23 and 

1.16, respectively).

15

EXPLANATION

Pre-Turkey Creek caldera rocks 

Rocks associated with the Turkey Creek caldera 

Post-Turkey Creek caldera rocks

Figure 3. Average (table 1) total 

alkali-silica variation diagram showing 

compositions of volcanic rocks in the 

central Chiricahua Mountains, Ariz. 

International Union of Geological 

Sciences classification grid (Le Bas 

and others, 1986) is also shown.

35 40 45 50 55 60 65

SiO2 , IN WEIGHT PERCENT

70 75
14 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

As is typical for most calc-alkaline rocks, abundances of 

A12O3 , total iron, MgO, CaO, P2O5 , and TiO, decrease with 

increasing SiO, in volcanic rocks of these mountains, whereas 

abundances of Na^O, K2O, and MnO show no clear relationship with SiO2 (fig. 4). Variation patterns for Na^O, K0O, 

and MnO are poorly developed, largely because most of the 

volcanic rock units studied are composed of high-silica rhyolite; variation within this narrowly defined SiO2 compositional 

range is relatively limited. Exceptions to this generalization 

relate to the abundances of Na,O and K2O. Within the group

of high-silica rhyolites, those units with 76-77.6 percent SiO2 , 

Na,O abundances vary from about 1.3 to 4.0 weight percent, 

whereas K9O abundances vary from about 4.6 to 7.5 weight 

percent. A significant geochemical trait of volcanic rocks of 

the area is their elevated K,O abundances (fig. 4); all of the 

units are members of either the high-potassium calc-alkaline 

or the shoshonitic series of Ewart (1982). The K2O/Na2O 

ratio for all of the studied units is >1.0, which indicates that 

all of these units are potassic as defined by Le Bas and others 

(1986).

2 16

UJ

DC 15

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QJ 13

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2 12

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CJ

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*~ 5 0.4

UJ

1 0.2

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2 4

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DC

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o)|- 1.0

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to 1- 1.5

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QJ 1.0

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DC

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"1————I————T

A 

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Shoshonitic series

High-K (calc-alkaline) series

60 62 64 66 68 70 72 74 76 78 

Si02, IN WEIGHT PERCENT

EXPLANATION 

A Pre-Turkey Creek caldera rocks

• Rocks associated with the Turkey Creek caldera

• Post-Turkey Creek caldera rocks

80

60 62 64 66 68 70 72 74 76 78 

Si02, IN WEIGHT PERCENT

80

Figure 4. Abundances (table 1) of selected major oxides and trace elements of volcanic rocks in the central Chiricahua Mountains, Ariz.
Geochemistry 15

Within-Unit Geochemical Variation

In order to better define the volcanic units, we used 

several analytical methods to study the character and extent 

of within-unit compositional variation, as portrayed by all

available geochemical data (du Bray and others, 1992a; 1992b; 

1993; du Bray and Pallister, 1994; 1995). We first created a 

series of extended trace-element diagrams (Thompson and 

others, 1983) to evaluate within-unit compositional variation; 

for each volcanic unit a separate diagram (fig. 5) displays

1,000

100

10

0.1

1,000

100

10

0.1

RHYOLITE LAVA OF CAVE CREEK (Tc)

I I I I I

"I——I——I——I——T "1——T

LOWER MEMBER OF THE TUFF OF 

HORSESHOE CANYON (Thcl)

RHYOLITE OF ERICKSON RIDGE (Tfre)

UPPER MEMBER OF THE TUFF OF 

HORSESHOE CANYON (Thcu)

I__I J__L

Rb Ba Th K Nb Ta La Ce Sr Nd P Sm Zr Hf Ti Tb Y Tm Yb

1,000 -

100

10

0.1

JESSE JAMES CANYON TUFF (Tjj)

I I

Figure 5 (above and following pages). Chondritenormalized (modified from Thompson and others, 1983) 

extended trace-element diagrams showing compositions 

of volcanic rocks in the central Chiricahua Mountains, 

Ariz.; plots portray data for the indicated stratigraphic 

unit. Trace elements are arranged in order of increasing 

geochemical compatibility to the right. A, pre-Turkey Creek 

caldera rocks (this page).

Rb Ba Th K Nb Ta La Ce Sr Nd P Sm Zr Hf Ti Tb Y Tm Yb
16 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

1,000

100

10 -

0.1

UNIT 3 RHYOLITE LAVA OF THE 

TURKEY CREEK CALDERA (Tmr3)

UNIT 2 RHYOLITE LAVA OF THE 

TURKEY CREEK CALDERA (Tmr2)

I I I I I I I l I I

1,000

100

10

0.1

UNIT 1 RHYOLITE LAVA OF THE 

TURKEY CREEK CALDERA (TmrD

BIOTITE RHYOLITE LAVA OF THE 

TURKEY CREEK CALDERA (Tmrb)

l I I I I I I I I I

1,000

100

10

0.1

DACITE PORPHYRY INTRUSION OF THE 

TURKEY CREEK CALDERA (Tdpi)

J__L I I

DACITE PORPHYRY LAVA OF THE 

TURKEY CREEK CALDERA (Tdpi)

i i i i i i i i i i i i i i i i i i i i I I I I I I I I J——I——I——I——I——I——I——I——I——i 

Rb Ba Th K Nb Ta La Ce Sr Nd P Sm Zr Hf Ti Tb Y Tm Yb Rb Ba Th K Nb Ta La Ce Sr Nd P Sm Zr Hf Ti Tb Y Tm Yb

B
Geochemistry 17

1,000

100 -

LAVA-FLOW-LIKE PHASE OF THE 

RHYOLITE CANYON TUFF (Trcf)

\ I I T I

INTRACALDERA FACIES 

RHYOLITE CANYON TUFF (Trci)

< 

CO

1,000

100

10

< 

CO

0.1

1,000

100

10

UPPER MEMBER, OUTFLOW FACIES 

RHYOLITE CANYON TUFF (Trcu)

I I

LOWER MEMBER, OUTFLOW FACIES 

RHYOLITE CANYON TUFF (Trcl)

0.1 I I

MIDDLE MEMBER, OUTFLOW FACIES 

RHYOLITE CANYON TUFF (Trcm)

Rb Ba Th K Nb Ta La Ce Sr Nd P Sm Zr Hf Ti Tb Y Tm Yb

Figure 5—Continued (above and facing page). Chondritenormalized (modified from Thompson and others, 1983) 

extended trace-element diagrams showing compositions of 

volcanic rocks in the central Chiricahua Mountains, Ariz.; 

plots portray data for the indicated stratigraphic unit. B, 

rocks associated with the Turkey Creek caldera.

Rb Ba Th K Nb Ta La Ce Sr Nd P Sm Zr Hf Ti Tb Y Tm Yb
18 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

trace-element abundances for all samples analyzed by instrumental neutron activation analysis. Additional graphical 

portrayal of within-unit compositional variation, for a significantly greater number of samples (those for which trace-element abundances were determined by energy-dispersive X-ray 

fluorescence spectroscopy), appears in figure 6. Finally, the 

magnitude of calculated standard deviations relative to mean 

abundances (table 1) for each unit was considered. These 

data evaluations indicate that individual volcanic rock stratigraphic units of the central Chiricahua Mountains, especially 

units composed of lava flows, are relatively homogeneous. 

However, some of the ash-flow tuff units are characterized by 

considerable within-unit compositional variation. Moderate, 

systematic compositional variation characteristic of these units 

is consistent with eruption from normally zoned reservoirs 

(Hildreth, 1981). These observations suggest that neither flow 

sorting, elutriation, incorporation of exotic material, nor other 

factors significantly skewed the whole-rock composition of 

these samples from magmatic values.

Pre-Turkey Creek Caldera Rocks

Volcanic rocks erupted prior to Turkey Creek caldera 

formation display variable amounts of within-unit compositional inhomogeneity. The rhyolite tuff of High Lonesome 

Canyon has considerable variation especially with respect to 

abundances of SiO2 , K?O, Ba, Sr, and Rb; given the preferential partitioning of these components into feldspars (Hanson, 

1978), these variations probably indicate varying amounts 

of feldspar fractionation. Similarly, because of preferential 

partitioning of Sr and Ba into feldspar (Hanson, 1978), the 

considerable variation of Sr and Ba abundances in samples of 

the lower member of the rhyolite of Joe Glenn Ranch probably 

results from variable amounts of feldspar fractionation.

The rhyolite lavas of Cave Creek, Krentz Ranch, and 

Erickson Ridge, and the Jesse James Canyon Tuff display limited within-unit compositional variation. Of these, the rhyolite 

lava of Cave Creek displays the most significant variation; 

moderate variation in Na^O, K.,0, and Rb abundances may 

result from non-isochemical devitrification processes. Lipman 

(1965) showed that post-magmatic effects can significantly 

modify primary compositions of glassy volcanic rocks; alkali 

element abundances are most strongly affected. As glassy 

rocks interact with ground water, they become hydrated and 

alkali elements are susceptible to leaching. Light REE (rare 

earth element) abundances in samples of the Jesse James Canyon Tuff are moderately variable, although the tuff is otherwise quite homogeneous.

Samples collected without specific regard for vertical 

position in both members of the tuff of Horseshoe Canyon 

indicate considerable compositional inhomogeneity (table 1). 

Samples of the lower member of the tuff of Horseshoe Canyon 

contain variable abundances of SiO9 , Na2O, K2O, Rb, and Ba, 

which again may reflect varying amounts of alkali feldspar 

fractionation. In contrast, samples of the upper member of 

the tuff of Horseshoe Canyon contain variable abundances of

SiO,, CaO, Na2O, K,O, Rb, Sr, Zr, and Ba; these variations 

probably reflect control by plagioclase, biotite, and zircon, the 

principal residences of these components (Hanson, 1978). A 

reconnaissance evaluation of the nature and extent of vertical 

zonation within these two units was conducted by a systematic 

sampling of a continuous vertical section through these units 

along a ridge west of Dripping Spring, on the south side of 

Sulphur Draw in the Portal Peak 7.5' quadrangle. The thick 

section of the lower member of the tuff of Horseshoe Canyon 

along the ridge seems to be intact, complete, and representative of the unit, whereas the upper member of the tuff of 

Horseshoe Canyon section is relatively thin here and may be 

incomplete. Geochemical abundances in a suite of samples 

from the lower member of the tuff of Horseshoe Canyon 

vary up section as follows: SiO2 decreases from 75.3 to 74.5 

weight percent, Na?O increases from 1.25 to 2.98 weight 

percent, K9O decreases from 8.97 to 7.31 weight percent, Rb 

decreases from 571 to 362 ppm, Sr increases from 54 to 99 

ppm, Zr increases from 262 to 467 ppm, and Ba increases 

from 210 to 647 ppm. Geochemical abundances in a suite of 

samples from the upper member of the tuff of Horseshoe Canyon vary up section as follows: SiO., decreases from 73.4 to 

71.8 weight percent, Na?O increases from 3.79 to 3.92 weight 

percent, K2O decreases from 5.80 to 5.66 weight percent, Rb 

decreases from 270 to 142 ppm, Sr increases from 73 to 104 

ppm, Zr increases from 411 to 609 ppm, and Ba increases 

from 714 to 1,243 ppm. Geochemical variation depicted by 

these two suites of samples has the same polarity, is essentially 

overlapping and continuous, and is consistent with the tuff 

having originated as a series of eruptions from progressively 

deeper levels of a single, normally zoned reservoir.

Rocks Associated with the Turkey Creek Caldera

Volcanic units associated with the Turkey Creek caldera 

display limited inter- and intra-unit compositional inhomogeneity and zonation. Intracaldera facies Rhyolite Canyon 

Tuff is characterized by moderate compositional variation. 

Compositional variation within the intracaldera facies probably reflects physical incorporation of relatively large and 

variable amounts of exotic lithic fragments from the caldera's 

topographic wall during collapse. The lava-flow-like phase of 

intracaldera facies tuff is characterized by restricted compositional variation. In the lower and middle members of outflow 

facies Rhyolite Canyon Tuff, compositional variation, especially of REE abundances, is moderate, whereas that within 

the basal and upper members is relatively limited. Compositional variation among the four outflow facies members of 

the Rhyolite Canyon Tuff is continuous and systematic; as a 

group, the four members exhibit a trend of decreasing geochemical evolution with time. In particular, the upper member 

is characterized by lower abundances of SiO,,, Rb, Y, Nb, Ta, 

Th, and U, and higher abundances of total iron, MgO, CaO, 

TiO,, Sr, Zr, Ba, and Eu.

Compositional data for a large number of dacite porphyry 

samples collected from widely distributed sites indicate that
Geochemistry 19

this unit is relatively homogeneous. Trachyte porphyry lava 

flows in the moat of the Turkey Creek caldera generally display even less within-unit compositional variability, although 

SiO,, CaO, Sr, and Ba abundances are highly variable and may 

reflect varying degrees of feldspar fractionation.

Of the rhyolite lavas in the moat of the Turkey Creek caldera, those in unit 2 lava display the greatest amount of withinunit compositional variation; in particular, REE abundances 

vary considerably within this unit (table 1). Compositional 

variation among the caldera's four rhyolite lava units indicates 

that the upper three units are very distinct relative to the basal 

biotite-bearing rhyolite lava, are considerably more evolved, 

and depict a trend toward increasing evolution with time. In 

particular, the basal lava is a low-silica rhyolite, whereas the 

upper three are all high-silica rhyolites. In addition, the basal 

biotite rhyolite is characterized by higher abundances of total 

iron, MgO, CaO, TiO9 , Sr, and Ba, and lower abundances of 

Rb, Y, Zr, Nb, Th, and U than the three high-silica rhyolites. 

The compositions of the two youngest high-silica rhyolites, 

lavas of units 2 and 3, are indistinguishable. However, these 

two rhyolites are compositionally distinct relative to the underlying unit 1 lava. The most diagnostic distinction pertains to 

barium abundances. Lavas of units 2 and 3 contain 10-20 ppm 

Ba, whereas unit 1 lava contains 60-100 ppm Ba. Other distinctions include higher SiO, and lower MgO, CaO, and REE 

abundances in lavas of units 2 and 3 relative to unit 1 lava.

Post-Turkey Creek Caldera Rocks

Volcanic rocks erupted following Turkey Creek caldera 

formation display limited within-unit compositional inhomogeneity. The most pronounced compositional variation in 

samples of the pyroclastic deposits and lava flows of Swede 

Peak is that of yttrium, whose abundances range from 37 to 90 

ppm. Abundances of Ba and Sr are also somewhat variable in 

this unit, which suggests that these samples may have experienced varying amounts of feldspar fractionation. In samples 

of the rhyolite lava of Dobson Peak, Y, Nb, and Ba abundances 

vary considerably. Abundances of Y and Nb are probably controlled by accessory minerals, whereas Ba abundance variations suggest that magma represented by these samples may 

have experienced differing amounts of biotite and feldspar 

fractionation.

Geochemistry- and Petrography-Based 

Stratigraphic Distinctions

As demonstrated by Hildreth and Mahood (1985), various 

combinations or subsets of field, petrographic, paleomagnetic, 

or geochronologic data considered along with major-oxide 

and trace-element abundance data may facilitate Stratigraphic 

identification and correlation. Compositional ranges of major 

oxides and trace elements in samples of central Chiricahua 

Mountains volcanic rocks (fig. 6) were used to aid stratigraphic determinations when other data were either

insufficient or ambiguous. This approach is especially useful 

for instances in which sparse exposure or structural dismemberment resulted in limited Stratigraphic context. Because 

macroscopic features are sufficient to distinguish lava flows 

from tuffs, in most cases, compositional and petrographic 

comparisons presented herein are made separately for lava 

flows and tuffs; data for lava flows are compared only to data 

for other lava flow units, whereas data for tuffs were compared 

only to data for other tuff units.

Lavas

The aphyric character of the high-silica rhyolite lava of 

Krentz Ranch distinguishes it from crystal-bearing lavas of the 

study area. Relative to most other aphyric high-silica rhyolite 

lavas in the area, the rhyolite lava of Krentz Ranch (abbreviated as KR in fig. 6) is compositionally indistinct, as indicated 

by limited available geochemical data. Low niobium abundances for the rhyolite lava of Krentz Ranch relative to those 

of aphyric rhyolite lavas exposed in the moat of the Turkey

Figure 6 (following pages). Stratigraphic versus compositional 

variation among volcanic rocks in the central Chiricahua Mountains, Ariz. Analytical uncertainty for oxide or element is shown 

by error bar in bottom right corner of each plot. Units arranged 

from youngest (top) to oldest (bottom) in each plot. Triangle, preTurkey Creek caldera rocks; dot, rocks associated with the Turkey 

Creek caldera; square, post-Turkey Creek caldera rocks. Unit 

designations are abbreviated here as follows: 

DP, rhyolite lava of Dobson Peak 

SW, pyroclastic deposits and lava flows of Swede Peak 

R3, unit 3 rhyolite lava of the Turkey Creek caldera 

R2, unit 2 rhyolite lava of the Turkey Creek caldera 

R1, unit 1 rhyolite lava of the Turkey Creek caldera 

RB, biotite rhyolite lava of the Turkey Creek caldera 

TP, trachyte porphyry lava

PI, dacite porphyry intrusion of the Turkey Creek caldera 

PL, dacite porphyry lava of the Turkey Creek caldera 

RF, lava-flow-like phase of the Rhyolite Canyon Tuff 

Rl, intracaldera facies Rhyolite Canyon Tuff 

RU, upper member, outflow facies Rhyolite Canyon Tuff 

RM, middle member, outflow facies Rhyolite Canyon Tuff 

RL, lower member, outflow facies Rhyolite Canyon Tuff 

RO, basal member, outflow facies Rhyolite Canyon Tuff 

J J, Jesse James Canyon Tuff 

HU, upper member of the tuff of Horseshoe Canyon 

HL, lower member of the tuff of Horseshoe Canyon 

RE, rhyolite of Erickson Ridge 

CC, rhyolite lava of Cave Creek 

KR, rhyolite lava of Krentz Ranch 

JG, lower member of the rhyolite of Joe Glenn Ranch 

HI, rhyolite tuff of High Lonesone Canyon
20 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

DP

SW

R3

R2

R1

RB

TP

PI

PL

RF

Rl

RU

RM

RL

RO

JJ

HU

HL

RE

CC

KR

JG

HI

DP

SW

R3

R2

R1

RB

TP

PI

PL

RF

Rl

RU

RM

RL

RO

JJ

HU

HL

RE

CC

KR

JG

HI

DP

SW

R3

R2

R1

RB

TP

PI

PL

RF

Rl

RU

RM

RL

RO

JJ

HU

HL

RE

CC

KR

JG

HI

DP

SW

R3

R2

R1

RB

TP

PI

PL

RF

Rl

RU

RM

RL

RO

JJ

HU

HL

RE

CC

KR

JG

HI

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1 66 70 74 78 

Si0 2, IN WEIGHT PERCENT

12 14 16 

AI 203, IN WEIGHT PERCENT

2 4 

FeO*, IN WEIGHT PERCENT

0.5 1.0 1.5 2.0 

MgO, IN WEIGHT PERCENT

_ ——— 1 1 1 1 

• • • —

• •• • • ••• €••• • -

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CaO, IN WEIGHT PERCENT

II I — • —

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Na2 0, IN WEIGHT PERCENT

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K20, IN WEIGHT PERCENT

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0.5 1.0 

Ti0 2, IN WEIGHT PERCENT

^ I

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AMHAA Am MA

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-Jd

A Al AHAAA

0.1 0.2 0.3 0.4 

P2 0 5, IN WEIGHT PERCENT

0.1 

MnO, IN WEIGHT PERCENT

200 400 600 

RUBIDIUM, IN PARTS PER MILLION

200 400 

STRONTIUM, IN PARTS PER MILLION

A A AAMAA MM

A «JM A MAA A A A A

30 60 90 

YTTRIUM, IN PARTS PER MILLION

300 600 900 20 40 60 80 

ZIRCONIUM, IN PARTS PER MILLION NIOBIUM, IN PARTS PER MILLION

1,000 2,000 

BARIUM, IN PARTS PER MILLION
Geochemistry 21

A A A

4 8 12 

COBALT, IN PARTS PER MILLION

10 20 10 20 30 

CHROMIUM, IN PARTS PER MILLION NICKEL, IN PARTS PER MILLION SCANDIUM, IN PARTS PER MILLION

A A A A

A AA

A

40 60 80 

ZINC, IN PARTS PER MILLION TANTALUM, IN PARTS PER MILLION

5 10 15 

URANIUM, IN PARTS PER MILLION

20 30 40 50 

THORIUM, IN PARTS PER MILLION

A A

5 10 15 

HAFNIUM, IN PARTS PER MILLION

10 20 

CESIUM, IN PARTS PER MILLION

40 60 80 100 120 5 10 15 20 

LANTHANUM, IN PARTS PER MILLION SAMARIUM, IN PARTS PER MILLION

A A

1 2 3 

EUROPIUM, IN PARTS PER MILLION

5 10 15 20 

GADOLINIUM, IN PARTS PER MILLION

5 10 

YTTERBIUM, IN PARTS PER MILLION

05 10 15 

LUTETIUM, IN PARTS PER MILLION
22 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

Creek caldera and the rhyolite lava of Dobson Peak (DP in fig. 

6) are the most diagnostic geochemical feature of this unit. In 

contrast, neither petrographic nor compositional data for the 

rhyolite lava of Krentz Ranch are distinguishable from those 

of the rhyolite lava of Cave Creek (CC in fig. 6). Given the 

stratigraphic settings of the rhyolite lava of Krentz Ranch and 

the rhyolite lava of Cave Creek, exposed beneath the tuff of 

Horseshoe Canyon, these two map units, which are spatially 

separated by about 15 km, may be parts of a single stratigraphic and magmatic unit. The rhyolite lava of Krentz Ranch 

may represent a distal part of the rhyolite dome field that also 

includes the rhyolite lava of Cave Creek.

By extension of the preceding observations, the aphyric 

character of the high-silica rhyolite lava of Cave Creek distinguishes it from porphyritic lavas of the study area; however, the petrographic characteristics and composition of this 

rhyolite are essentially indistinguishable from those of other 

aphyric, high-silica rhyolite lavas exposed in the area. The 

sole exception to this generalization may be the low niobium 

content of the rhyolite lava of Cave Creek relative to that of 

the other aphyric rhyolite lavas. As described in the preceding paragraph, this characteristic is common to the rhyolite 

lava of Cave Creek and the rhyolite lava of Krentz Ranch and 

hints at a possible common source. Lacking stratigraphic 

context, with the exception of diagnostic niobium abundances, 

distinguishing the rhyolite lava of Cave Creek from the other 

aphyric high-silica lavas is difficult.

Relative to the high-silica rhyolite lavas in the area, the 

composition and petrography of the rhyolite of Erickson Ridge 

(RE in fig. 6) are quite distinctive. The phenocryst content of 

the rhyolite of Erickson Ridge is distinctly greater than that of 

any other rhyolite exposed in the central Chiricahua Mountains, and the presence of 1-2 percent biotite is diagnostic. In 

contrast to most other rhyolite lavas in the area, the rhyolite 

of Erickson Ridge has low silica abundances that are similar 

to those of the biotite rhyolite of the Turkey Creek caldera. 

The rhyolite of Erickson Ridge is further distinguished by 

higher FeO*, CaO, Sr, Ba, and Eu abundances and lower Rb, 

Ta, Th, and U abundances. The rhyolite of Erickson Ridge is 

most easily distinguished from the biotite rhyolite lava of the 

Turkey Creek caldera (RB in fig. 6) by a Rb/Sr ratio less than 

1, higher Ba abundances, and lower Th abundances.

Dacite porphyry of the Turkey Creek caldera has petrographic and compositional features that distinguish it from all 

other volcanic rocks in the study area. Dacite that forms lava 

flows (PL in fig. 6) and the resurgent intrusion (PI in fig. 6) 

in the caldera is greenish gray to tan and contains distinctive 

alkali feldspar megacrysts. The most distinctive geochemical characteristics of the dacite porphyry include FeO*, MgO, 

CaO, TiO2 , P2O5 , Ba, Co, Cr, Ni, Sc, and Eu abundances that 

are elevated relative to abundances in the majority of the central Chiricahua Mountains volcanic rocks, which are considerably more evolved and contain significantly lower abundances 

of these components.

Trachyte porphyry (TP in fig. 6) forms an areally 

restricted set of outcrops characterized by distinctive sanidine

phenocrysts. The trachyte porphyry is distinguished from the 

dacite porphyry by its distinctive reddish-brown color and 

finer grained groundmass. The composition of the trachyte 

porphyry is similar to that of the dacite porphyry and so is 

similarly chemically distinct relative to all other volcanic rocks 

of the central Chiricahua Mountains. Elevated abundances of 

Na?O, Y, Zr, and Nb and lower abundances of Sr and Ba in the 

trachyte porphyry distinguish the trachyte porphyry from the 

dacite porphyry.

Of the four rhyolite lavas present in the moat of the Turkey Creek caldera, only the oldest of these, the biotite rhyolite 

lava (RB in fig. 6), is petrographically distinct. The biotite 

rhyolite contains phenocrysts of biotite, feldspar, and quartz 

that distinguish it from all other central Chiricahua Mountains 

rhyolite lavas, except the rhyolite of Erickson Ridge. The 

other three moat rhyolite lavas are virtually aphyric and macroscopically indistinguishable from one another and similarly 

cannot be distinguished petrographically from the aphyric 

rhyolite lavas of Krentz Ranch, Cave Creek, and Dobson Peak. 

The composition of the biotite rhyolite in the moat of the Turkey Creek caldera is distinguished from that of other rhyolites, 

except the rhyolite of Erickson Ridge, by lower SiO0 , Rb, 

Nb, Ta, U, Th, Hf, and heavy REE abundances and by higher 

FeO*, MgO, CaO, TiO9 , Sr, Ba, Co, Sc, and Eu abundances. 

The biotite rhyolite in the moat of the Turkey Creek caldera is 

distinguished from rhyolite of Erickson Ridge by a Rb/Sr ratio 

greater than 1, lower Ba abundances, and higher Th abundances. Compositions of the three aphyric moat rhyolites are 

almost indistinguishable from those of other aphyric rhyolites 

in the central Chiricahua Mountains, except that the moat lavas 

have higher Nb abundances than the rhyolite lavas of Cave 

Creek and Krentz Ranch, and lower Rb and Nb abundances 

than the rhyolite of Dobson Peak. Compositions of the youngest two rhyolite lavas (R2 and R3 in fig. 6) in the moat of the 

Turkey Creek caldera (lavas of units 2 and 3) are indistinguishable. They can only be differentiated in a stratigraphic context 

relative to the thin tuff that separates them. The composition 

of unit 1 lava (Rl in fig. 6) is distinct from that of units 2 and 

3 lavas; unit 1 lava is distinguished by lower SiO2 , Nb, Ta, 

and Th abundances and higher CaO, Ba, light REE, and Eu 

abundances.

The rhyolite of Dobson Peak (DP in fig. 6) is an aphyric 

rhyolite that is macroscopically indistinguishable from other 

aphyric rhyolites in the map area. Its aphyric character 

distinguishes the rhyolite lava of Dobson Peak from rhyolite 

of Erickson Ridge and biotite rhyolite lava in the moat of the 

Turkey Creek caldera. Elevated Rb and Nb abundances distinguish the rhyolite lava of Dobson Peak from all other rhyolites 

of the area.

Ash-Flow Tuffs and Other Pyroclastic 

Flow Deposits

The low phenocryst content and presence of trace 

amounts of oxidized biotite distinguish the rhyolite tuff 

of High Lonesome Canyon (HI in fig. 6), the oldest of the
Geochemistry 23

regionally exposed ash-flow tuffs, from most other tuffs of 

the central Chiricahua Mountains. In addition, the rhyolite 

tuff of High Lonesome Canyon is characterized by unusually 

low zirconium and Na0O abundances relative to other highsilica rhyolite ash-flow tuffs of the study area. Although the 

composition of the rhyolite tuff of High Lonesome Canyon is 

otherwise nondistinct, these features are probably sufficient to 

be diagnostic.

Several features of the lower member of the rhyolite of 

Joe Glenn Ranch (JG in fig. 6) help to distinguish this tuff 

from others exposed in the central Chiricahua Mountains. It 

is characterized by a relatively high crystal content (from 20 

to 40 percent) and distinctive, 1-3 mm wide, pseudohexagonal biotite crystals. The lower member of the rhyolite of Joe 

Glenn Ranch also is characterized by higher FeO*, TiO0 , and 

Ba abundances, and lower Nb abundances than other central 

Chiricahua Mountains tuffs.

The relatively crystal rich lower member of the tuff of 

Horseshoe Canyon (HL in fig. 6) contains diagnostic biotite 

phenocrysts in addition to quartz and sanidine. Geochemical characteristics diagnostic of the lower member of the 

tuff of Horseshoe Canyon include elevated K,O, Ba, and Eu 

abundances and low Zr, Nb, and Th abundances, relative to 

other central Chiricahua Mountains high-silica rhyolite tuffs. 

In contrast, the relatively crystal poor upper member of the 

tuff of Horseshoe Canyon (HU in fig. 6) also is characterized by biotite phenocrysts as well as by relatively elevated 

A12O3 , FeO*, TiO2 , P2O5 , Sr, Zr, Ba, Co, Ni, Cr, Sc, Hf, and 

Eu abundances, and low SiO,, Nb, Ta, U, and Th abundances; 

another characteristic of the upper member of the tuff of 

Horseshoe Canyon is its dramatic within-unit compositional 

variation. As such, the upper member of the tuff of Horseshoe 

Canyon is probably the most geochemically distinct ash-flow 

tuff exposed in the study area. Its most distinguishing features 

relative to the lower member of the tuff of Horseshoe Canyon 

are its higher A10O3 , FeO*, TiO2 , Ba, and Eu abundances, and 

lower SiO.,, U, and Th abundances.

The Jesse James Canyon Tuff (JJ in fig. 6) is macroscopically almost indistinguishable from the Rhyolite Canyon 

Tuff. Its only diagnostic petrographic characteristic is the 

presence of trace amounts of biotite. The composition of the 

Jesse James Canyon Tuff is distinguished from those of other 

ash-flow tuffs of the central Chiricahua Mountains, including the Rhyolite Canyon Tuff, by its slightly lower FeO*, Zr, 

Ta, Th, and Hf abundances, none of which distinguish it from 

the lower member of the tuff of Horseshoe Canyon. Slightly 

lower K,O, Rb, Ba, and Eu abundances of the Jesse James 

Canyon Tuff may distinguish it from the lower member of the 

tuff of Horseshoe Canyon.

The Rhyolite Canyon Tuff, the youngest regionally 

extensive ash-flow tuff in the central Chiricahua Mountains, 

is a high-silica rhyolite tuff that has been subdivided into six 

geologic map units. All these units contain diagnostic quartz 

and sanidine phenocrysts and, in contrast to most of the other 

ash-flow tuffs present in the study area, only rarely contain 

macroscopically identifiable mafic silicate minerals. The four

outflow facies members are macroscopically indistinguishable. The intracaldera facies member is distinguished from 

the outflow facies members by its reddish-brown color and 

considerably greater lithic fragment content. The lava-flowlike phase is similar in appearance to intracaldera facies tuff, 

but it includes unbroken 0.5-1 cm sanidine phenocrysts, lacks 

eutaxitic structure, and contains very few lithic fragments. 

The basal (RO in fig. 6), lower (RL in fig. 6), and middle 

(RM in fig. 6) members of the Rhyolite Canyon Tuff form a 

geochemically distinct subset of the Rhyolite Canyon Tuff, 

whereas the upper (RU in fig. 6), intracaldera (RI in fig. 6), 

and lava-flow-like phase (RF in fig. 6) form another. Elevated 

Zr and Nb abundances distinguish all parts of the Rhyolite 

Canyon Tuff from almost all other tuffs in the area. Zirconium 

abundances of 280^00 ppm in the Rhyolite Canyon Tuff are 

unusually elevated for subalkaline rhyolites and are distinctive relative to abundances of similar volcanic rocks exposed 

in the western United States. Although Zr abundances for the 

Rhyolite Canyon Tuff slightly overlap those of both members 

of the tuff of Horseshoe Canyon, Nb abundances of the former 

distinguish it from the latter two units. Similarly, elevated 

abundances of Ta, U, Th, Hf, and the heavy REE distinguish 

the Rhyolite Canyon Tuff from other study area tuffs. As a 

group, the basal, lower, and middle members of the Rhyolite 

Canyon Tuff are distinguished by higher SiO,,, Rb, Nb, and Ta 

abundances and lower TiO0 , Zr, Ba, and La abundances relative to the upper member, intracaldera facies, and lava-flowlike phase of the Rhyolite Canyon Tuff. The lower and middle 

members of the Rhyolite Canyon Tuff are essentially indistinguishable (a distinctive parting, and in some places a distinctive white ash deposit at the top of the lower member of the 

outflow facies tuff can be used to mark the boundary between 

these units), whereas the basal member of the Rhyolite Canyon Tuff is distinguished by higher K0O and Rb abundances. 

The upper member of the Rhyolite Canyon Tuff and the 

intracaldera facies are indistinguishable, but the lava-flow-like 

phase is distinguished by its higher K0O and Rb abundances. 

The pyroclastic deposits and lava flows of Swede Peak 

(SW in fig. 6) are present in a relatively limited area in the 

southeast part of the central Chiricahua Mountains. Distinctive macroscopic characteristics of this unit include the 

fact that thick sections are composed of numerous separate 

pyroclastic flow deposits that (1) are essentially unwelded,

(2) contain variable and frequently abundant lithic fragments,

(3) are crystal rich (including phenocrysts of quartz, sanidine, 

albite, and biotite), and (4) are interbedded with volcanic 

sandstone, as well as cogenetic high-silica rhyolite lava flows. 

Fortunately, these features are diagnostic; compositional characteristics of the pyroclastic deposits and lava flows of Swede 

Peak are not diagnostic.

Petrogenetic Implications

Pearce and others (1984) recognized that granitoid rocks 

generated in various tectonic settings have distinctive
24 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

geochemical signatures. Trace-element abundance variations 

in coeval volcanic and plutonic rocks generated in a given 

terrane should be similar. Consequently, compositions of 

volcanic rocks from the central Chiricahua Mountains can 

be compared to the trace-element-tectonic setting diagrams 

developed by Pearce and others (1984). Trace-element data 

for most of these rock units plot in the within-plate field, 

near its boundary with the volcanic arc field (fig. 7). These 

trace-element characteristics indicate that genesis of magmas 

represented by this region's volcanic rocks involved a significant crustal component and processes different than those that 

result in trace-element characteristics diagnostic of subduction-related processes. Compositions plotting in or near the 

volcanic arc field may indicate that the associated magmas 

had somewhat distinct sources and (or) evolved by different 

genetic processes.

Gill (1981) determined that Ba/Nb, Ba/Ta, and La/Nb 

ratios of modern arc rocks are >26, >450, and 2-7 respectively, whereas Pearce and others (1984) indicated that values 

of these ratios are generally «(much less than) 26, «450, 

and <3, respectively, for within-plate granites. For example, 

Ba/Nb and La/Nb ratios for subduction-related Tertiary volcanic rocks of the Bolivian Altiplano are about 30 and 3, respectively (du Bray and others, 1995), whereas Ba/Nb, Ba/Ta, and 

La/Nb ratios for Late Proterozoic within-plate granites of the

Arabian Shield are about 2, 5, and, 1 respectively (du Bray 

and others, 1988). Most, but not all, of central Chiricahua 

Mountains volcanic rock units (table 1) have values of Ba/Nb, 

Ba/Ta, and La/Nb that are <3, <50, and <2, respectively; these 

values are more like those of within-plate rocks than subduction-related arc rocks. However, the biotite rhyolite lava in 

the moat of the Turkey Creek caldera, dacite porphyry, upper 

member of the tuff of Horseshoe Canyon, rhyolite of Erickson 

Ridge, lower member of the rhyolite of Joe Glenn Ranch, and 

rhyolite tuff of High Lonesome Canyon consistently yield arclike values for these ratios.

Average chondrite-normalized extended trace-element 

patterns (fig. 8) for volcanic units of the study area are gently 

negatively sloping, and have superposed, well-developed negative Ba, Sr, P, and Ti anomalies; these patterns also include 

weakly developed, negative Nb-Ta anomalies. A striking 

feature of trace-element patterns for volcanic rocks of the area 

is their parallelism and relatively limited compositional range. 

The greatest amounts of compositional variation are among 

phosphorus, titanium, barium, and strontium, which probably 

reflect simple fractionation of varying amounts of apatite (P), 

iron-titanium oxide minerals (Ti), and plagioclase (Ba and 

Sr). The shape, slope, and abundance levels of chondrite-normalized extended trace-element patterns for volcanic rocks 

of the area are somewhat distinct from those characteristic of

EXPLANATION

Pre-Turkey Creek caldera rocks 

Rocks associated with the Turkey Creek caldera

0.1 -

0.01

1 10 

YTTERBIUM, IN PARTS PER MILLION

100

Figure 7 (alongside and facing page).

Trace-element-tectonic setting 

discrimination variation diagrams 

showing average compositions of volcanic rocks of the central Chiricahua 

Mountains, Ariz. Tectonic settingcomposition boundaries from Pearce 

and others (1984); compositions from 

table 1. A tantalum versus ytterbium. 

B, niobium versus yttrium.
Geochemistry 25

subduction-related arc volcanic rocks. In particular, for the 

rocks under study, the magnitudes of negative Ba, Sr, P, and Ti 

anomalies are greater, negative Nb-Ta anomalies are smaller, 

and, as indicated by heavy REE abundances that are greater 

than those of subduction-related arc rocks (fig. 8), the patterns 

are less steeply sloping. Negative Nb-Ta anomalies are considered (Wood and others, 1979; Gill, 1981; Pearce and others, 

1984) a hallmark of subduction-related, arc volcanic rocks. 

The weakly developed nature of Nb-Ta anomalies in the 

central Chiricahua Mountains rocks (fig. 8) further emphasizes 

the transition from subduction-related magmatism, characteristic of the oldest volcanic rocks in the area, to within-plate 

magmatism epitomized by younger volcanic rocks in this area. 

A measure of the magnitude of the negative Nb-Ta anomaly 

derives from the chondrite-normalized K/Nb ratio, (K/Nb)CN . 

The average value of (K/Nb)CN for 18 middle Tertiary, 

demonstrably subduction-related ash-flow tuffs of southeastern Nevada is 8.0±1.4 (du Bray, 1995), whereas values of this 

ratio are <4 for most of the volcanic rocks of the study area. 

Rocks with (K/Nb)CN values >4 are the biotite rhyolite lava in 

the moat of the Turkey Creek caldera (7.8), dacite porphyry 

(4.5), both members of the tuff of Horseshoe Canyon (7.1 and 

5.6), rhyolite of Erickson Ridge (7.9) and rhyolite lava of Cave 

Creek (4.9), the lower member of the rhyolite of Joe Glenn 

Ranch (10.1), and the rhyolite tuff of High Lonesome Canyon

(9.3); this ratio increases among progressively older rocks. All 

these compositional features suggest that volcanic rocks of the 

study area are principally of the within-plate type, but record 

the transition from an older subduction-related setting to a 

younger, within-plate setting. Furthermore, compositions of 

most volcanic rocks in the area are remarkably similar to the 

average obsidian composition for igneous systems situated in 

continental interior settings (Macdonald and others, 1992).

Zirconium abundances for most of the study area volcanic rocks are commensurate with the experimentally determined zirconium saturation threshold of 100-200 ppm for 

subalkaline compositions at typical magmatic temperatures 

(Watson and Harrison, 1983). Thus, magmas represented by 

these rocks did not equilibrate with peralkaline liquids, nor 

did they equilibrate at a temperature greater than about 860°C. 

Zirconium abundances in rocks derived from the Turkey Creek 

caldera and both members of the tuff of Horseshoe Canyon 

are variably elevated and imply final pre-emption equilibration at greater than about 860°C and (or) an association with 

magma having an alkaline affinity (Watson and Harrison, 

1983). Of these units with elevated zirconium abundances, 

rhyolite lava flows in the moat of the Turkey Creek caldera 

have the lowest zirconium abundances (175-220 ppm) and 

appear to have been least affected by the processes that caused 

pronounced zirconium enrichments in the other caldera-related

1,000

o 100

10

B

WITHIN PLATE

VOLCANIC ARC

OROGENIC

EXPLANATION

• Pre-Turkey Creek caldera rocks

• Rocks associated with the Turkey Creek caldera

• Post-Turkey Creek caldera rocks

_________________I_______________

10 100 

YTTRIUM, IN PARTS PER MILLION

1,000
26 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

1,000

cc 

Q

z 

o

I

o

EXPLANATION 

Pre-Turkey Creek caldera rocks 

Rocks associated with the Turkey Creek caldera

FigureS. Chondrite-normalized 

extended trace-element diagram 

showing average compositions of 

volcanic rocks in the central Chiricahua Mountains, Ariz. Trace elements 

are arranged in order of increasing 

geochemical compatibility to the right. 

Modified from Thompson and others 

(1983).

Rb Ba Th K Nb Ta La Ce Sr Nd Y Tm Yb

rocks. The remaining rock units that have unusually high 

zirconium abundances are also characterized by elevated Na^O 

and especially K?O abundances and were previously identified 

(fig. 4) as members of the shoshonite series defined by Ewart 

(1982). These elevated zirconium abundances may be related 

to elevated alkali abundances in magmas represented by the 

associated volcanic units, because elevated alkali abundances 

stabilize zircono-alkali silicate complexes (Watson and Harrison, 1983), which inhibit zircon nucleation, fractionation, and 

zirconium concentrations from reaching maximum thresholds 

in silicate liquids.

Feldspar-melt distribution coefficients for strontium, 

potassium, and rubidium (Hanson, 1978) are such that feldspar fractionation preferentially removes strontium and then 

potassium from the melt phase, relative to rubidium, causing 

residual liquids to become progressively enriched in rubidium 

relative to potassium and strontium. In volcanic rocks of the 

central Chiricahua Mountains, rubidium abundances increase 

systematically relative to those of strontium and potassium (fig. 9). Rubidium enrichment in most of these units 

is significantly better developed than in subduction-related 

volcanic arc rocks, including, for example, ash-flow tuffs of 

southeastern Nevada (du Bray, 1995), and is strikingly similar

to that characteristic of highly evolved, within-plate granites 

of the Arabian Shield (du Bray and others, 1988). Rb/Sr ratios 

range from a low of 0.69 in the rhyolite of Erickson Ridge to 

a highly evolved value of 32 in the rhyolite lava of Dobson 

Peak; the average Rb/Sr ratio for the 23 volcanic units is 11. 

These ratios contrast dramatically with those for subduction-related dacite to rhyolite ash-flow tuffs of southeastern 

Nevada, whose average Rb/Sr ratio is 0.68 (du Bray, 1995).

Average chondrite-normalized REE patterns for 

volcanic rock units of the study area (fig. 10A) mimic 

relations portrayed by chondrite-normalized extended traceelement patterns, in that the REE patterns are remarkably 

parallel and define a relatively narrow compositional range. 

The light REE parts of the patterns are moderately negatively 

sloping, whereas the heavy REE parts are essentially flat; the 

patterns include small to moderately well developed negative 

europium anomalies that probably indicate modest to considerable amounts of feldspar fractionation (Hanson, 1978). 

Chondrite-normalized La/Lu ratios range from 3.73 to 13.2 

and average 8.1 ±3.3; units with geochemical characteristics 

most like those of subduction-related arc volcanic rocks have 

the steepest patterns. Total REE contents range from 110 to 

444 (average 276±103) ppm.
Geochemistry 27

Petrogenetic Evolution of the Turkey Creek 

Caldera Magmatic System

Hildreth (1979, fig. 13) documented systematic compositional variation within the Quaternary Bishop Tuff (associated 

with the Long Valley caldera of east-central California), one 

of the first ash-flow tuffs for which systematic compositional 

variation was well characterized. In particular, cooler, earlier 

erupted parts of Bishop Tuff, extracted from the top of the 

magma reservoir, have lower light REE abundances and higher 

heavy REE abundances than hotter, later erupted material 

extracted from deeper within the reservoir. In addition, each 

sequentially erupted fraction of Bishop Tuff magma is characterized by a progressively smaller negative europium anomaly. 

These abundance variations for sequential eruptions from a 

zoned reservoir yield a clockwise rotation of chondrite normalized REE patterns and smaller negative europium anomalies. Hildreth (1981) also demonstrated that a broad suite of 

additional incompatible trace elements is more abundant in 

earlier erupted parts of the Bishop Tuff, whereas compatible 

trace-element abundances are greatest in its latest erupted 

parts. This same type of compositional variation is preserved 

among outflow facies members of the Rhyolite Canyon Tuff.

Sanidine, clinopyroxene, and zircon are the principal phenocrysts in the Rhyolite Canyon Tuff and thus are most likely 

to have controlled trace-element abundance variations in the 

evolving liquid represented by the tuff. Mineral-melt distribution coefficients (KD 's) for these minerals in rhyolitic melts are 

as follows (Hanson, 1978, 1980):

Sanidine: light REE KD 's (0.02-0.04) are greater than 

heavy REE KD 'S (0.006); except Eu (1-2). Values for KD 'S of 

Sr, Ba, and Rb are about 4, 6, and 0.7, respectively. Sanidine 

fractionation causes counterclockwise REE pattern rotation 

(light REE depletion), results in negative europium anomaly 

development, increases overall REE and Rb abundances, and 

decreases Sr and Ba abundances in the residual liquid.

Clinopyroxene: heavy REE KD 'S (1-2) are slightly 

greater than light REE KD 's (0.5-1). Values for KD 'S of Sr, Ba, 

and Rb are less than 1. Clinopyroxene fractionation causes 

slight clockwise REE pattern rotation (heavy REE depletion), 

results in overall REE depletion, and increases Sr, Ba, and Rb 

abundances in the residual liquid.

Zircon: heavy REE KD 's (as high as 300) are significantly greater than light REE KD 's (about 1). Values for KD 's 

of Sr, Ba, and Rb are less than 1. Zircon fractionation causes 

dramatic clockwise REE pattern rotation (heavy REE

K/100

HIGHLY EVOLVED GRANITES 

(du Bray and others, 1988)

ASH-FLOW TUFFS OF THE SOUTHERN 

GREAT BASIN (du Bray, 1995)

EXPLANATION 

A Pre-Turkey Creek caldera rocks 

• Rocks associated with the Turkey Creek caldera 

Post-Turkey Creek caldera rocks

V V V V V V

Figures. Ternary variation diagram 

showing average relative proportions 

of rubidium, potassium, and strontium in volcanic rocks of the central 

Chiricahua Mountains, Ariz.

Rb Sr
28 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

1,000

EXPLANATION

Pre-Turkey Creek caldera rocks 

Rocks associated with the Turkey Creek caldera

Figure 10 (alongside and facing 

page). Chondrite-normalized (modified 

from Anders and Ebihara, 1982) rare 

earth element diagrams. A, average 

compositions of volcanic rocks in the 

central Chiricahua Mountains, Ariz. B, 

compositions of the lower (Trcl), middle 

(Trcm), and upper (Trcu) members of 

outflow facies Rhyolite Canyon Tuff. 

C, compositions of the biotite rhyolite 

(Tmrb)and units 1 (Tmr1), 2(Tmr2), 

and 3 (Tmr3) of rhyolite lava preserved 

in moat of Turkey Creek caldera.
Geochemistry 29

1,000

0.1

depletion), results in overall REE depletion, and increases Sr, 

Ba, and Rb abundances in the residual liquid.

REE abundance variations within outflow facies members 

of the Rhyolite Canyon Tuff (fig. 105; table 1) are similar to 

those of the Bishop Tuff as well as those of many other highsilica rhyolite systems. The upper member of outflow facies 

Rhyolite Canyon Tuff is the least geochemically evolved of 

the three members, and it therefore is considered to represent a 

composition similar to that from which the other two members 

could have evolved. REE patterns for sequentially erupted 

members of outflow facies Rhyolite Canyon Tuff rotate in a 

clockwise sense, have progressively smaller negative europium anomalies, and depict compositional evolution of earlier 

erupted members from later erupted members. Abundances of 

other trace elements also vary consistently relative to stratigraphic position within the Rhyolite Canyon Tuff. In particular, early erupted Rhyolite Canyon Tuff, represented by stratigraphically lower deposits, contain higher abundances of Rb, 

Y, Nb, Ta, Th, and U, and lower abundances of Sr, Zr, and Ba. 

Consequently, evolution of trace-element abundance variations 

within the reservoir represented by outflow facies Rhyolite 

Canyon Tuff is qualitatively consistent with fractionation of 

the phenocryst phases contained therein. Sanidine crystallization and fractionation from the least evolved, parental magma, 

represented by the upper member of the Rhyolite Canyon Tuff, 

caused the residual liquid to contain lower light REE

abundances relative to heavy REE abundances, lower Sr, Eu, 

and Ba abundances, and higher Rb abundances. Clinopyroxene crystallization and fractionation caused the residual liquid 

to contain slightly lower REE abundances overall. Zircon 

crystallization and fractionation caused the residual liquid to 

contain lower Zr abundances and partially counteracted heavy 

REE enrichment caused by sanidine fractionation. These 

mineral-melt distribution relations and crystal fractionation 

processes caused the residual liquid to differentiate to more 

evolved compositions and more evolved liquids accumulated 

at the top of the reservoir prior to eruption. Subsequent topdown eruptions from the normally zoned reservoir represented 

by the Turkey Creek caldera and its eruptive products depict 

compositional gradients that developed in the reservoir as a 

consequence of these crystal liquid equilibria.

The four rhyolite moat lava units (units 1,2, and 3 and 

the biotite rhyolite), considered together, are zoned from less 

evolved (biotite rhyolite) to more evolved (unit 3) compositions, which is the reverse of that which results from progressive, top-down magma extraction from a normally zoned 

reservoir. Each subsequently erupted unit is more evolved 

than the previously erupted unit. For example, the abundances 

of compatible elements, such as Ti, Ba, and Sr, are greater in 

the biotite rhyolite relative to those in the three successively 

erupted rhyolite lavas of units 1, 2, and 3, in which their abundances are progressively lower. In contrast, abundances of
30 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

incompatible elements, such as Rb, Th, and Nb, are lowest in 

the first erupted rhyolite lava and progressively higher in subsequently erupted moat rhyolite lavas (table 1). As described 

by Duffield and Ruiz (1992) for the Oligocene Taylor Creek 

Rhyolite of New Mexico, this type of zoning is a logical 

consequence of a reservoir's being progressively contaminated by incorporation of variable amounts of geochemically 

less evolved rock. In an incompletely mixed reservoir, with 

increasing distance downward, away from the upper, enclosing roof rock interface, the amount of incorporated wallrock, 

and therefore contamination, is predictably less. Progressive, 

top-down magma extraction from a reservoir so contaminated 

yields eruptions of progressively more evolved compositions. 

Reversed compositional variation preserved by the rhyolite 

lavas in the moat of the Turkey Creek caldera suggests that the 

upper parts of their source reservoir were progressively less 

evolved as a consequence of having incorporated relatively 

greater amounts of roof rock. To identify other geochemical 

processes by which the observed reversed geochemical zonation preserved in the four rhyolite moat lava units could have 

developed is difficult.

REE abundance variations (fig. 10C) between the biotite 

rhyolite and unit 1 lava imply that wallrock contaminant 

unmixing was not the sole control on the compositional evolution depicted by these rocks. The biotite rhyolite has slightly 

lower REE abundances, has a slightly less well developed 

negative Eu anomaly, and is characterized by a REE pattern slightly less steeply negatively sloped than that of unit 1 

lava. In order for unit 1 lava to have evolved from the biotite 

rhyolite, overall REE enrichment, which requires removal of 

material with REE abundances lower than those of the biotite 

rhyolite, must have occurred. However, the biotite rhyolite 

REE composition could not have evolved to that of unit 1 lava 

by unmixing of a lithologic contaminant, because all of the 

lithologies plausibly present within the intracaldera environment have overall REE abundances greater than those of the 

biotite rhyolite (fig 10A); their separation from the biotite 

rhyolite would have caused REE depletion in unit 1 lava 

relative to the biotite rhyolite. Fractionation of phenocrysts 

characteristic of the biotite rhyolite, sanidine and biotite, could 

have caused REE abundances to evolve to those of unit 1 lava. 

As described in the second paragraph of this section, REE 

mineral-melt distribution coefficients for sanidine are such that 

sanidine fractionation causes counterclockwise REE pattern 

rotation, results in negative europium anomaly development, 

and increases overall REE abundances in the residual liquid. 

Similarly, REE KD 'S for biotite (0.23-0.44) are uniformly low, 

so that biotite fractionation causes REE enrichment (Hanson, 

1980). Consequently, fractionation of sanidine and biotite 

phenocrysts from the biotite rhyolite could have caused the 

residual liquid, represented by unit 1 lava, to become REE 

enriched, to have a more well developed negative Eu anomaly, 

and to cause slight counterclockwise REE pattern rotation.

Compositional evolution from unit 1 lava to unit 2 lava 

and from unit 2 lava to unit 3 lava was quite different than the 

evolution from the biotite rhyolite to unit 1 lava. In particular,

REE abundances in units 2 and 3 lavas are successively lower, 

negative Eu anomalies are of constant magnitude, and chondrite-normalized REE patterns are slightly counterclockwise 

rotated relative to that for unit 1 lava. This type of REE depletion requires removal of material with REE abundances greater 

than that of unit 1 lava. Because unit 1 lava has REE abundances between 100 and 10 times chondrite values, phenocryst 

fractionation seems improbable because very few common 

minerals contain sufficiently elevated REE abundances. Alternatively, addition of material, possibly including xenocrysts, 

with REE abundances less than that of unit 1 lava may have 

caused the observed REE abundance variations. However, the 

aphyric character of these rocks indicates that crystals were 

not added. As described in the previous paragraph, all of the 

lithologies plausibly present within the intracaldera environment are inappropriate contaminant additions, because they 

have REE abundances equal to or greater than those of unit 1 

lava (fig. 10A); their addition to unit 1 lava would have caused 

REE enrichment in units 2 and 3 lavas relative to unit 1 lava. 

In contrast, removal of one of these lithologic contaminants 

could have caused the observed, systematic REE depletion.

Constraints on the rock unit(s) responsible for the 

hypothesized contamination of the moat rhyolite reservoir 

include the following:

1. The near contemporaneity of Turkey Creek caldera formation with eruption of the moat rhyelites,

2. The intracaldera-scale structural and lithologic configuration that prevailed immediately following Turkey Creek 

caldera formation, and

3. The compositions of rocks that may have been involved in 

the hypothesized contamination.

The fact that the Rhyolite Canyon Tuff and the moat 

rhyolite lavas are of essentially the same age (du Bray and 

Pallister, 1991) indicates that the rocks that contaminated the 

moat rhyolite reservoir must have been present in the intracaldera environment immediately following caldera formation. 

Structural relations immediately following caldera formation 

(du Bray and Pallister, 1991) indicate that major amounts of 

intracaldera Rhyolite Canyon Tuff and dacite porphyry dominated the shallow crust in the environs of the moat rhyolite 

reservoir. Other rocks that may have enclosed and hosted the 

caldera, and therefore may have been available as possible 

constituents of the shallow crustal roof for the moat rhyolite reservoir, include Tertiary volcanic rocks erupted before 

development of the Turkey Creek caldera. Volumetrically 

dominant among these are the intermediate composition lava 

flows (Tim) that are presumed to have been erupted from a set 

of coalescing stratovolcanoes and onto which younger, more 

evolved outflow ash-flow tuffs and lava were subsequently 

erupted.

The fact that negative Eu anomalies characteristic of 

rhyolite lavas of units 1, 2, and 3 are of constant magnitude 

indicates that the material being removed was characterized 

by, at most, a very small negative Eu anomaly; removal of 

material with a significant negative europium anomaly would 

have caused progressively less contaminated moat rhyolite to
Geochronology 31

have a progressively smaller negative anomaly. Of the rocks 

present in the central Chiricahua Mountains, only the upper 

member of the tuff of Horseshoe Canyon, the dacite porphyry 

of the Turkey Creek caldera, and the intermediate-composition lava flows at the base of the Tertiary volcanic section have 

small negative Eu anomalies (fig. 10). Geologic relations suggest that the upper member of the tuff of Horseshoe Canyon is 

not present in significant volumes, if at all, in the subsurface 

of the intracaldera environment; it is therefore an unlikely candidate for the unmixing lithologic contaminant. In contrast, 

geologic relations suggest that both the dacite porphyry and 

the intermediate lava flows were plausibly significant constituents of the intracaldera environment and therefore are likely 

candidates for the material that is inferred to have variably 

contaminated the rhyolite moat lava reservoir; top-down tapping of this variably but systematically contaminated reservoir 

could have produced the observed reversed compositional 

zonation.

Consideration of broader geochemical systematics is 

consistent with the dacite porphyry and the intermediate lava 

flows as plausible contaminant lithologies. Mixing considerations require that for moat lava compositional variation to 

be a consequence of variable contamination involving magma 

and an exotic constituent, the trace-element compositions of 

the contaminated products (the biotite rhyolite, unit 1 lava, 

and unit 2 lava) must be intermediate between those of the two 

parental components (uncontaminated rhyolite, represented 

by unit 3 lava and a combination of contaminants, represented 

by the dacite porphyry and the intermediate lava flows). An 

examination of compositional data for these six lithologies 

(table 1) indicates that, in general, the compositions of the 

biotite rhyolite and rhyolite lavas of units 1 and 2 are in fact 

intermediate between those of unit 3 lava and the inferred 

contaminants. However, these relations are not perfectly well 

developed. For example, zirconium abundances among these 

six units indicate that other processes, perhaps including minor 

fractionation, contributed to and modified compositional relations that seem largely to derive from variable magma contamination. In summary, compositional considerations indicate 

that the reversed compositional zonation depicted by the moat 

rhyolite lavas is consistent with the rhyolite reservoir's having 

being progressively roofward contaminated through entrainment and assimilation of dacite porphyry and intermediatecomposition lava flow inclusions. Additional consideration 

of REE data suggests that evolution of the most contaminated 

moat rhyolite (the biotite rhyolite) to unit 1 lava also involved 

selective phenocryst fractionation.

Geochronology

Most of the samples whose ages were determined 

are among those used in the volcanic rock stratigraphic/ 

composition investigation, a principal focus of this study. 

However, several samples (referred to as the miscellaneous

units) of other igneous rocks in the study area were collected 

and dated by the 40Ar/39Ar method in order to augment the 

existing time-stratigraphic framework for this region. These 

samples and the map units that they represent, from oldest 

to youngest, are identified; and new geochronologic data are 

presented. Subsequently, geochronologic data and interpretations are presented for the principal volcanic rocks, from oldest to youngest. ^Ar/^Ar results are presented as age spectra 

(fig. 11); ages are summarized in table 2; analytical data are in 

table 3.

Miscellaneous Units

Sample DY91-77 (fig. 1, locality V) represents a hypabyssal dacite intrusion, about 16 km south of the Turkey 

Creek caldera; the sample site is within the dacitic rocks of 

Halfmoon Valley mapped by Drewes and Brooks (1988). The 

age spectrum for biotite from this sample (fig. 11A) is complicated, does not include a plateau, and shows the contrasting effects of both excess argon and apparent argon loss in 

the low-temperature steps. The isochron age for 80 percent 

of released 39Ar, representing the least disturbed part of the 

age spectrum, is 74.6±0.6 Ma with (40Ar/36Ar)t = 276±19. 

Although disturbed, the age spectrum for this sample confirms 

the presence of Cretaceous-age igneous rocks in the central 

Chiricahua Mountains; igneous rocks of this age have not been 

previously identified in this area.

Samples P650A and 202064 (fig. 1, localities P and S) 

were analyzed in order to establish the age of tuff and lavas 

that constitute the pyroclastic rocks of Rucker Canyon (du 

Bray and others, 1997), which are correlative, in part, with the 

welded tuff of Rucker Canyon (Drewes and Brooks, 1988). 

Although volumetrically significant, the petrographic and 

compositional characteristics of these rocks have not been 

included in the present study, because these rocks, largely 

composed of many separate pyroclastic flows, are lithologically heterogeneous and contain large amounts of exotic lithic 

fragments that would render definition of their petrography 

and composition somewhat meaningless. The age spectrum 

for biotite from sample P650A (fig. IIB), a rhyolite lava 

interbedded in the pyroclastic flows, shows the effects of a 

minor amount of excess argon in the low-temperature steps 

but yields a plateau age of 33.32±0.07 Ma. The age spectra for 

biotite and sanidine from a sample (fig. 11C and D, respectively) of the pyroclastic flows (202064) also show the effects 

of excess argon in the low-temperature steps, but both yield 

relatively undisturbed results. The biotite gives a plateau age 

of 33.21±0.09 Ma. The sanidine gives a weighted mean age of 

33.04±0.04 Ma and an identical isochron age with (40Ar/36Ar)j 

= 310±7, reflecting the relatively minor amount of excess 

40Ar in this sample. The similarity of ages for the biotite 

separates from the pyroclastic flow and lava samples suggests 

that emplacement of pyroclastic flows and lava flows was 

approximately simultaneous. The best age estimate for these 

rocks, based on the three available analyses, is 33.27±0.08
80

75

70

65

60

55

50

DY91-77 Biotite 

Dacite intrusion

Isochron age = 74.6 ± 0.6 Ma 

(40Ar/36Ar). = 276±19

0 10 20 30 40 50 60 70 80 90 100 

PERCENT 39Arv RELEASED

40

35

30

25

20

202064 Biotite

Pyroclastic rocks of Rucker Canyon

. Plate

10 20 30 40 50 

PERCENT 39Ar R

CD 

<

40

35

30

25

20

P650A Biotite

Pyroclastic rocks of Rucker Canyon

Plateau age = 33.32 ± 0.07 Ma

B

10 20 30 40 50 60 70 80 90 100 

PERCENT 39ArK RELEASED

40

35

30

25

20

202064 Sanidine

Pyroclastic rocks of Rucker Canyon

F

Isochron age = 33.0

( 40Ar/36Ar): = 3

10 20 30 40 50 

PERCENT 39Aiv R
40

35

30

25

20

201570 Hornblende 

Granodiorite of Mackey Canyon

Isochron age = 31.42 ± 0.30 Ma 

(40Ar/36Ar). = 3og ± 6

Plateau age = 32.16 ± 0.20 Ma

0 10 20 30 40 50 60 70 80 90 100 

PERCENT 39An, RELEASED

40

35

T AG

co CD

ro

CJl

20

202154 Sanidine 

Latite of Darnell Peak

Plateau ag

0 10 20 30 40 50 

PERCENT 39An, R

40

35

30

25

20

201570 Biotite

Granodiorite of Mackey Canyon

Plateau age = 30.62 ±0.15 Ma

0 10 20 30 40 50 60 70 80 90 100 

PERCENT 39An, RELEASED

40

35

r>o in

r>o CD

DY92-54 Sanidine

Rhyolite lava

I

————— Platea

0 10 20 30 40 50 

PERCENT 39An, R

Figure 11 (above and following pages). 40Ar/39Ar age spectra for volcanic rocks of the central Chiricahua Mountains, Ariz. View letter

identifier in upper left of each view and also are the linkto discussion in text.
40

35 -

30 -

25 •

20

40

35

30

25

20

P5 Sanidine / 

Rhyolite of Packsaddle Mountain

1 —^

J + ——————— Plateau age -23.23 ±0.06 Ma ——————— »•

) 10 20 30 40 50 60 70 80 90 10 

PERCENT 39ArK RELEASED

202057 Sanidine J 

Rhyolite tuff of High Lonesome Canyon

-J ——— * ————— Plateau age - 34.16 ± 0.17 Ma —————————— »•

35

CO

^

APPARENT AGE

ro o

CJl 0

o zu c

fU

35

CD 

^

LLJ

CD

< 

£ 30

LLJ 

QC

<c

Q_ 

<

25 

on

1

-

DY91-11 Biotite 

Lower member of the rhyolite of Joe Glenn Ranc

-J

•< ———————————— Plateau age -33.8

10 20 30 40 50 

PERCENT 39ArK R

P272C Sanidine 

Rhyolite lava of Cave Creek

0 10 20 30 40 50 60 70 80 90 100 

PERCENT 39ArK RELEASED

0 10 20 30 40 50 

PERCENT 39ArK R
^ CD 

* z. 

» — CD 

T * 

c < 

* CD" 

?• § 

*. 03 

.= Z5 

> O. 

^S 03 

> s 

QJ O3 

9:

< oo' 

o_ o 

s 1 

1 1 

O Z5~ 

O rH7r CD 

oo x 

o • 

O

37;

o'

OJ 3- 

c

O3

^ 

O

c

i-H-

—zs'

y

>

' '_______ T~l ^~^J^

-O 33 

.& =T 

cn ~o 

OD CD 

> 

3D

m 

m 

GO

F

CD 

CD

o 

o

CD 

OO 

T3 

O

Z5

a.

c/) X T.^ 

CD =• ^ 

3 a j£ 

o§ •sjf 

CO

>

3D

m

GO

cn CD 

cn cn 

isj cn 

CD 

CO C_ 

2. 1 

Q- CD 

ABojouojijooag
40

35 -

30 -

25

20

202156 Biotite Q

Lower member tuff of Horseshoe Canyon

.

-

P

n

TML«

+ ——————————————— Plateau age - 27.62 ± 0.10 Ma —————————— »

i i i i i i i i i

tu

35

LU

CD

£ 30

LU

DC

<c

Q_

25

on

201769 Sanidine 

Lower member Rhyolite Canyon Tuff

.

-

1 -< ———— Plateau age 

i i i i i 

10 20 30 40 50 60

PERCENT 3!%v RELEASED

70 80 90 100 0 10 20 30 40 50 

PERCENT 39Ar, R

40

35

30

25

20

202156 Sanidine

Lower member tuff of Horseshoe Canyon

Isochron age = 25.42 ± 0.10 Ma 

(40Ar/36Ar)j = 297 ± 4

10 20 30 40 50 60

PERCENT 39An, RELEASED

70 80 90 100

40

35 -

30 -

25

20

201769 Sanidine 

Lower member Rhyolite Canyon Tuff

P ——————————— 

1 •* ———————————————— Plateau age - 

10 20 30 40 50 

PERCENT 39An, R
35

30

25

20

40

35

DY91-36 Sanidine

Middle member Rhyolite Canyon Tuff

Plateau age = 27.03 ± 0.11 Ma

201765 Sanidine

Upper member Rhyolite Canyon Tuff

Isochron age = 26.98 ± 0.04 Ma 

(40Ar/36Ar) = 309 ± 2

10 20 30 40 50 60 70 80 90 100 

PERCENT 39ArK RELEASED

V

10 20 30 40 50 60 70 80 90 100 

PERCENT 39ArK RELEASED

40

35

30

25

2°

40

35

30

25

201765 Sanidine

Upper member Rhyolite Canyon Tuff

Plateau age = 26.9

20

201587 Sanidine

Dacite porphyry intrusion — Turkey Creek caldera

Isochron ag

( 40Ar/3

0 10 20 30 40 50 

PERCENT 39An, R

0 10 20 30 40 50 

PERCENT 39ArK R

Figure 11—Continued. 40Ar/39Ar age spectra for volcanic rocks of the central Chiricahua Mountains, Ariz. View letters A through DD 

upper left of each view and also are the linkto discussion in text.
35

30

25

20

40

35

30

25

20

P3 Sanidine

Dacite porphyry lava in moat of Turkey Creek caldera

Minimum age step = 26.97 ± 0.13

201996 Biotite

Biotite rhyolite lava in moat of Turkey Creek caldera

Isochron age = 26.71 ± 

( 40Ar/36Ar): = 366±15

0 10 20 30 40 50 60 70 80 90 100 

PERCENT 39ArK RELEASED

35 -

30 -

25

20

40

35

30 -

25 -

0 10 20 30 40 50 60 70 80 90 100 

PERCENT 39Ar, RELEASED

20

201996 Sanidine 

Biotite rhyolite lava in moat of Turkey Creek calde

-

-

P— ,

<< ————————— Plateau age -26.74 

) 10 20 30 40 50 

PERCENT 39ArK R

201538 Sanidine 

Rhyolite lava of unit 1 in moat of Turkey Creek cald

-

Isochron age = 2

(40Ar/36Ar)

0 10 20 30 40 50 

PERCENT 39Ar, R
40

35

CD

<

30

25

20

201580 Sanidine

Rhyolite lava of unit 1 in moat of Turkey Creek caldera

Plateau age = 26.64 ±0.13 Ma

cc

40

35

CD

<

30

25

0 10 20 30 40 50 60 70 80 90 100 

PERCENT 39ArK RELEASED

20

202151 Sanidine

Rhyolite lava of Dobson Peak

Plateau age = 26.20 ± 

0 10 20 30 40 50 

PERCENT 39ArK R

Figure 11—Continued. 40Ar/39Arage spectra for volcanic rocks of the central Chiricahua Mountains, Ariz. View letters A through DD 

upper left of each view and also are the linkto discussion in text.
40 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

Ma, the average of the two biotite ages. In the study area, the 

pyroclastic rocks of Rucker Canyon represent the last manifestation of a short pulse of volcanic activity that began with 

eruption of the tuff of High Lonesome Canyon. This magmatic pulse ended at about 33.3 Ma and was followed by a 5.2 

m.y. eruptive hiatus, which ended with eruption of the rhyolite 

lava of Cave Creek.

Sample 201570 (fig. 1, locality E) is from the granodiorite of Mackey Canyon (Pallister and others, 1994), also 

known as the granodiorite of Thus stock (Drewes and others, 

1995). The granodiorite is medium to coarse grained and 

contains subequal amounts of biotite and hornblende, which 

together constitute about 10 percent of the rock. Previous 

K-Ar analyses yielded ages of 30.9±1.2 Ma (Marvin and 

others, 1978, no. 100) and 29.59±0.90 Ma (Shafiqullah and 

others, 1978, no. 20). The age spectrum for hornblende from 

sample 201570 (fig. 11£) shows evidence of excess argon in 

the low- and high-temperature steps but gives a plateau age 

of 32.16±0.20 Ma and an isochron age 31.42±0.30 Ma with 

(•WAr/36Ar)j = 309±6. Considering the effect of excess argon on 

the plateau age, we interpret the isochron age as more likely 

reflecting the crystallization age of the hornblende. The age 

spectrum for biotite from this sample (fig. 11F) is slightly 

disturbed but gives a plateau age of 30.62±0.15 Ma. Given the 

magnitude of uncertainties characteristic of K-Ar analyses and 

argon retention problems typical of biotite, we consider that 

the 40Ar/39Ar hornblende age for sample 201570 represents the 

best approximation of the age of this intrusion and the biotite 

age may reflect a brief period of cooling to the argon-in-biotite 

retention temperature. The presence of coarsely holocrystalline rocks, such as the granodiorite of Mackey Canyon, nearly 

adjacent to volcanic rocks as little as 5 m.y. younger implies 

rapid uplift, perhaps involving faulting, and unroofing of the 

terrane that includes the granodiorite prior to the onset of 

volcanism in this area at about 28 Ma.

Sample 202154 (fig. 1, locality M) represents the latite of 

Darnell Peak (Bryan, 1988), which forms a sill that intruded 

between the upper and lower members of the tuff of Horseshoe 

Canyon. The sill, which according to the nomenclature of Le 

Bas and others (1986) is composed of trachyte (11 percent 

Na,O+K,O at 68.6 percent SiOJ, is crystal rich and contains 

distinctive feldspar and biotite phenocrysts. The age spectrum 

(fig. 11G) for sanidine from this sample is simple, although 

slight effects of argon loss are evident in the low-temperature 

steps and excess argon in the two highest temperature steps. It 

gives a plateau age of 27.58±0.08 Ma. The age of the sill is 

indistinguishable from that of the lower member of the tuff of 

Horseshoe Canyon, which indicates that the sill was emplaced 

immediately after tuff emplacement. Bryan (1988) suggested 

that the considerable thickness of tuff of Horseshoe Canyon, 

southeast of the Turkey Creek caldera, forms an intracaldera 

accumulation within what he called the Portal caldera. In this 

context, the sill might represent resurgent magmatism within 

the Portal caldera and emplacement of less evolved magma 

up into the thick intracaldera tuff accumulation from lower 

within a zoned magma reservoir whose eruption created the

Portal caldera. The stratigraphic and compositional relations 

between the sill and the tuff indicate that the reservoir from 

which these products were erupted was strongly and discontinuously zoned, a feature that is known to have prevailed in the 

adjacent and 0.5 m.y.-younger Turkey Creek caldera as well.

Sample DY92-54 represents an isolated rhyolite lava that 

overlies outflow facies Rhyolite Canyon Tuff about 13 km 

southwest of the Turkey Creek caldera (fig. 1, locality R). The 

age spectrum for sanidine from this sample (fig. 1 IH) yields a 

plateau with an apparent age of 26.35±0.08 Ma. The age for 

this sample indicates that the underlying Rhyolite Canyon Tuff 

here is at least 26.35 Ma. The age and stratigraphic position 

of this rhyolite are similar to those of the rhyolite of Dobson 

Peak, exposed about 16 km to the east. The rhyolite represented by DY92-54 could be correlative with the rhyolite of 

Dobson Peak. If such a correlation were substantiated, then 

the lava dome field represented by rhyolite of Dobson Peak 

must have been considerably more extensive than its present 

erosional remnants.

Sample P5 (fig. 1, locality W) represents the rhyolite of 

Packsaddle Mountain (Drewes and Brooks, 1988), an isolated, 

small, garnet-bearing rhyolite plug about 16 km south of the 

Turkey Creek caldera. Drewes and Brooks (1988) presented 

a K-Ar age of 22.9±0.8 Ma (R.F. Marvin, H.H. Mehnert, and 

E.L. Brandt, written commun., 1985) on sanidine from this 

unit. The age spectrum for sanidine from this sample is simple 

(fig. 117) and gives a plateau age of 23.23±0.08 Ma, which 

confirms that this is one of the youngest Tertiary-age igneous 

rocks known in the central Chiricahua Mountains.

Pre-Turkey Creek Caldera Rocks

Sample 202057 (fig. 1, locality U) represents the rhyolite tuff of High Lonesome Canyon, a regionally distributed 

ash-flow tuff for which no geochronologic data have been 

previously reported. The age spectrum for sanidine from this 

sample is relatively simple (fig. 117), although some effects of 

argon loss are evident in the low-temperature steps; it gives a 

plateau age of 34.16±0.17 Ma, which we interpret to represent 

the eruption age of the tuff. The rhyolite tuff of High Lonesome Canyon is thus the oldest ash-flow tuff in the study area 

with a well-established age. Unfortunately, the age of the 

tuff does not match that of any other regionally distributed 

ash-flow tuff from the Boot Heel volcanic field. As such, the 

source, distribution, and stratigraphic correlation of this unit 

remain unknown.

Sample DY91-11 (fig. 1, locality T) represents the lower 

member of the rhyolite of Joe Glenn Ranch, another regionally 

distributed ash-flow tuff. A sanidine K-Ar age of 29.6±1.9 

Ma has been reported for this unit, but the collection site for 

the analyzed sample is uncertain (Marjaniemi, 1969; Drewes, 

1982). Drewes and Brooks (1988) presented a fission track 

(zircon) age of 30.4±3.0 Ma for this unit. The age spectrum 

for biotite from this sample is relatively simple (fig. \\K), 

although it shows minor disturbance in the low-temperature
Geochronology 41

steps; it gives a plateau age of 33.81±0.08 Ma. The new 

geochronologic data indicate that the lower member of the 

rhyolite of Joe Glenn Ranch was therefore erupted within 

about 350,000 years of the eruption of the rhyolite tuff of High 

Lonesome Canyon. Like the tuff of High Lonesome Canyon, 

the lower member of the rhyolite of Joe Glenn Ranch does not 

have an age correlative among units of the Boot Heel volcanic 

field. Consequently, the source, distribution, and stratigraphic 

correlation of this unit also remain unknown.

Sample P272C (fig. 1, locality I) represents the rhyolite 

lava of Cave Creek which forms an extensive flow dome field, 

possibly including the rhyolite lava of Krentz Ranch, in the 

eastern part of the central Chiricahua Mountains. Mclntosh 

and Bryan (2000) reported an 40Ar/39Ar age of 27.76±0.16 Ma 

for this unit. The age spectrum for sanidine from this sample 

is relatively simple (fig. 11L), although some effects of excess 

argon are evident in both the low- and high-temperature steps; 

it gives a plateau age of 28.10±0.12 Ma. This age indicates 

that following eruption of the pyroclastic rocks of Rucker Canyon at about 33.3 Ma, no volcanic rocks were erupted in the 

study area until the rhyolite lava of Cave Creek was erupted 

about 28.1 Ma. These three rock units therefore define a magmatic hiatus of about 5.2 m.y. in the area.

Sample P475 (fig. 1, locality A) represents the rhyolite of 

Erickson Ridge, which forms a set of coalesced rhyolite lava 

domes in the western part of the central Chiricahua Mountains; 

Drewes (1982) reported a biotite K-Ar age of 28.7±1.0 Ma for 

this rhyolite. Age spectra for coexisting sanidine and biotite 

are relatively simple, although the effects of excess argon and 

argon loss are evident, especially in the low-temperature steps, 

in spectra for both minerals. The sanidine (fig. 11M) gives a 

plateau age of 27.89±0.09 Ma, whereas the biotite (fig. UN) 

gives a discordant plateau age of 28.24±0.08 Ma. Harlan and 

others (1998) and Kellogg and others (1994) have suggested 

that age discordance between coexisting sanidine and biotite 

is common in volcanic rocks and that in some cases biotite 

yields 40Ar/3gAr ages older than those indicated by coexisting 

sanidine. We consider the sanidine age to be more representative and reliable, because sanidine is less susceptible to the 

diversity of geologic processes that can disturb argon systematics in biotite and the age spectrum of the biotite clearly 

is more affected by excess argon. Our preferred age for the 

rhyolite of Erickson Ridge is 27.89±0.09 Ma, which, given the 

analytical uncertainties, suggests that the rhyolite of Erickson 

Ridge is the same age as the rhyolite lava of Cave Creek. As 

such, the re-initiation of volcanism in the region shortly before 

28 Ma and following the 5.2 m.y. hiatus is dominated by the 

voluminous, effusive rhyolite lava dome field development 

preserved in the rhyolites of Erickson Ridge, Cave Creek, and 

Krentz Ranch.

Samples 201771 and P652 (fig. 1, localities C and O) 

represent the Jesse James Canyon Tuff. The two samples were 

collected and their ages determined to help verify the inferred 

correlation of these two isolated exposures of similar ash-flow 

tuff. Age spectra for sanidine from both of these samples are 

relatively simple, although that for sample P652 shows some

effects of excess argon in the low- and high-temperature steps. 

The spectrum for sample P652 (fig. 110) gives a plateau age 

of 27.52±0.06 Ma, whereas that for sample 201771 (fig. IIP) 

gives a plateau age of 27.59±0.06 Ma. An isochron analysis 

of 201771 yields an apparent age of 27.63±0.04 Ma, which is 

identical to the plateau age, with (40Ar/36Ar): = 270±10. That 

these two plateau ages are statistically indistinguishable substantiates the inferred correlation between these exposures of 

biotite-bearing high-silica rhyolite tuff exposed immediately 

beneath outflow facies Rhyolite Canyon Tuff in the northern 

and southern parts of the central Chiricahua Mountains. These 

ages suggest another correlation. Biotite-bearing high-silica 

rhyolite tuff exposed in the eastern and southeastern parts of 

the study area, and immediately beneath the Rhyolite Canyon 

Tuff, is the tuff of Horseshoe Canyon, which, as described 

following, has a preferred age of 27.62±0.10 Ma. All three 

of these ages are statistically indistinguishable. Therefore, 

because the Jesse James Canyon Tuff and the lower member 

of the tuff of Horseshoe Canyon occupy identical stratigraphic 

positions and are the same age, it seems probable that the Jesse 

James Canyon Tuff is the eruptive correlative of part, if not all, 

of the lower member of the tuff of Horseshoe Canyon. The 

near indistinguishability of geochemical data for these two 

units further substantiates the inference that these two units 

are, at least in part, volcanologic and therefore stratigraphic 

correlatives.

Sample 202156 (fig. 1, locality L) represents the lower 

member of the tuff of Horseshoe Canyon, a regionally distributed rhyolite ash-flow tuff. Bryan (1988) summarized the 

scant K-Ar age data available for this unit. He documented 

the attempt by Marjaniemi (1969) to determine the age of 

the tuff by K-Ar analysis of biotite, which resulted in an age 

of 26.4±0.7 Ma. Given that the tuff of Horseshoe Canyon is 

stratigraphically beneath the Rhyolite Canyon Tuff, and that 

the age of the latter (see next section) is now known to be 26.9 

Ma, we conclude that the earliest attempts to date the tuff of 

Horseshoe Canyon resulted in a slightly too young age. Using 

the 40Ar/39Ar method and having analyzed 17 samples of the 

tuff, Mclntosh and Bryan (2000) established its age as 27.6 

Ma. Age spectra for sanidine and coexisting biotite from 

sample 202156 give significantly different ages. The spectrum 

for the biotite separate is relatively simple (fig. IIQ), although 

it shows some evidence of excess argon and argon loss in the 

low-temperature steps; it gives a plateau age of 27.62±0.10 

Ma. The spectrum for the sanidine separate (fig. 1 IR) is 

significantly disturbed and is characterized by a progressive 

stepping-up pattern in the low-temperature steps, and then by 

a more gradual stepping-down pattern in the medium- to hightemperature steps. The sanidine analysis gives an isochron 

age of 25.42±0.10 Ma with (40Ar/36Ar)r = 297±4. Given the 

preceding comments concerning early K-Ar analyses of this 

unit, clearly this age is significantly too young and therefore 

meaningless. Consequently, our preferred age for the lower 

member of the tuff of Horseshoe Canyon is 27.62±0.10 Ma, 

which is in good agreement with the findings of Mclntosh and 

Bryan (2000). This age is also consistent with intrusion of the
42 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

Table 3. Ar/ Ardata for volcanic rocks of the central Chiricahua Mountains.

Temp

(

o p\ C)

4V 3V 40 39 2/ 39 37 Ar.,/ Ar., Ar/ /

\r2'31 %4°ArD

n

%39Ar 45/ Apparent age '

(Maat±1o)

Pre-Turkey Creek caldera rocks

Sample: DY 91-77, biotite from dacite intrusion, J = 0.007306, sample wt =

Latitude: 31 °38'04"N.

800

900

950

1000

1050

1100

1150

1200

1250

1300

1350

1400

1500

.33645

.56337

.60808

.46566

1.0832

1.6785

1.7835

1.8113

2.1122

2.3476

4.4704

4.1792

3.7589

.10407

.14027

.13195

.09300

.20101

.29570

.30919

.31623

.36772

.41295

.78842

.73630

.65902

62.4 mg

Longitude: 109°22'40"W.

3.233

4.016

4.609

5.007

5.389

5.676

5.768

5.728

5.744

5.685

5.670

5.676

5.704

30.5

49.0

53.4

62.6

74.5

83.1

87.4

88.5

89.2

86.3

85.6

89.0

90.6

2.3

3.1

2.9

2.0

4.4

6.5

6.8

6,.9

8.1

9.1

17.3

16.2

14.5

42.11 ±

52.17 ±

59.74 ±

64.82 ±

69.67 ±

73.31 ±

74.47 ±

73.96±

74.16 ±

73.41 ±

73.23 ±

73.30 ±

73.65 ±

.07

.33

.25

.45

.18

.29

.21

.11

.11

.11

.11

.11

.11

No plateau; total gas age = 71.46 ± 0.15 Ma

Isochron age (steps 7-13) = 74.6 ± 0.6 Ma; (""Ar^Ar): = 276 ± 19

Sample: 202057, sanidine from rhyolite tuff of High Lonesome Canyon, J = 0.007288, sample wt = 40.2 mg

Latitude: 31°43'07"N.

850

950

1050

1100

1150

1200

1250

1300

1350

1400

1450

1500

1600

.0712

.21400

.56327

.56931

.64957

.75008

1.0224

.89940

.86144

.90951

.82037

.99938

.70223

.0310

.08730

.22273

.22136

.25317

.28761

.38878

.34377

.32854

.34579

.31169

.37954

.26936

Longitude: 109°22/ 10"W.

2.29

2.451

2.529

2.572

2.566

2.608

2.630

2.616

2.622

2.630

2.632

2.633

2.607

36.2

85.3

74.7

95.4

96.2

97.7

98.3

97.8

98.4

98.6

98.1

98.2

94.8

.9

2.5

6.4

6.4

7.3

8.3

11.2

9.9

9.5

10.0

9.0

10.9

7.8

30. ±

31.94±

32.95 ±

33.50±

33.42±

33.97±

34.25 ±

34.07 ±

34.15 ±

34.25 ±

34.28 ±

34.29 ±

33.95 ±

2

.30

.33

.07

.10

.31

.17

.15

.16

.07

.16

.22

.09

Total gas age = 33.89 ± 0.19 Ma

Plateau age (steps 6-13) = 34.16 ± 0.17 Ma (for 76.5 percent of the gas produced during heating)

Latitude

800

900

1000

1050

1100

1150

1200

1250

1300

1350

1400

1450

1500

1550

1650

: 31°43'45"N.

.0228

.17104

.12715

.09694

.12553

.22775

.27804

.40447

.60641

.86287

1.3297

1.5075

2.1946

2.1471

1.7231

.0077

.06623

.05194

.03672

.04807

.08666

.10413

.15428

.23109

.32912

.50658

.57509

.83986

.82183

.65907

Longitude: 109°22'42"W.

2.96

2.582

2.448

2.640

2.611

2.628

2.670

<£*.\J^,^ 9 £99 __"*~~

2.624 ——

2.622 —

2.625

2.621

2.613

2.613

2.615

35.9

49.8

29.6

50.1

58.0

69.3

79.1

83.1

85.0

84.1

81.4

79.1

78.1

74.8

71.2

.2

1.5

1.1

.8

1.1

1.9

2.3

3.4

5.1

7.3

11.2

12.7

18.6

18.2

14.6

38. ±

33.4 ±

31.6 ±

34.1 ±

33.7 ±

34.0 ±

34.5 ±

33.9 ±

33.90 ±

33.87 ±

33.91 ±

33.86±

33.75 ±

33.75 ±

33.77 ±

5

.6

.7

.8

.5

.8

.3

.2

.05

.05

.10

.05

.06

.07

.06

Total gas age =33.81 ± 0.12 Ma

Plateau age (steps 8-15) = 33.81 ± 0.08 Ma (for 91.1 percent of the gas produced during heating)

Sample: P650A, biotite from lava in pyroclastic rocks of Rucker Canyon, J = 0.007273, sample wt =58.3 mg 

Latitude: 31°45'17"N. Longitude: 109°23 /18"W.

800 .0119 .0046 2.585 ----- 12.0 .1 34. ± 7 

900 .0606 .0220 2.749 —- 22.5 .5 36. ± 2 

1000 .0131 .0043 3.023 ----- 19.1 .1 39. ± 9 

1100 .25449 .09350 2.722 ----- 43.2 2.0 35.36 ± .10 

1200 .78074 .30083 2.595 ----- 79.7 6.4 33.73 ± .11 

1250 .91525 .35400 2.585 ----- 85.0 7.5 33.61 ± .12 

1300 1.4587 .56869 2.565 —— 83.2 12.0 33.35 ± .08 

1350 2.0019 .78026 2.566 —— 75.5 16.5 33.35 ± .05 

1400 2.5372 .99023 2.562 —— 70.3 20.9 33.31 ± .05 

1450 2.4490 .95833 2.555 —- 70.7 20.3 33.22 ± .06 

1600 1.6833 .65491 2.570 —- 73.6 13.8 33.41 ± .11 

Total gas age = 33.42 ± 0.10 Ma 

Plateau age (steps 7-11) = 33.32 ± 0.07 Ma (for 83.5 percent of the gas produced during heating)
Geochronology 43

Table 3. 40Ar/39Ar data for volcanic rocks of the central Chiricahua Mountains. ]/—Continued

Temp

/o pi ( C)

40 2/ 35 ArR V l\ 4VVn l\

39Ar/37Ar2-3/ %4°ArR % Ar Apparent age ' 

(Ma at±1o)

Pre-Turkey Creek caldera rocks (continued)

Sample:

Latitude

850

950

1050

1150

1200

1250

1300

1350

1400

1450

1500

1550

1650

202064, sanidine from pyroclastic rocks of Rucker Canyon, J = 0.00721 1 , sample wt = 58.3 mg

: 31°43'45"N.

.11280

.39018

.73384

1.6880

1.8456

1.7673

2.0964

2.5270

2.6241

2.8236

2.8690

2.2979

.32974

.08128

.15653

.28528

.66193

.72179

.69141

.82207

.99487

1.0284

1.1014

1.1192

.89018

.13024

Longitude: 1 09 ° 23 '01 "W.

1.388

2.493

2.572

2.550

2.557

2.556

2.550

2.540

2.552

2.564

2.564

2.581

2.532

60.8

76.0

88.7

97.9

98.3

98.3

98.2

98.3

98.6

99.0

98.7

98.7

86.0

.9

1.8

3.3

7.6

8.3

8.0

9.5

11.5

11.8

12.7

12.9

10.3

1.5

17.96 ±

32.14±

33.16 ±

32.87 ±

32.96 ±

32.95 ±

32.87 ±

32.74 ±

32.89 ±

33.04±

33.04±

33.27 ±

32.64 ±

.29

.14

.29

.09

.06

.07

.06

.05

.05

.05

.06

.05

.45

No plateau; total gas age = 32.81 ± 0.08 Ma

Weighted mean age (steps 3-12) = 33.04 ± 0.04 Ma; (^AsP'Ath = 310 ± 7

Sample: 202064, biotite from pyroclastic rocks of Rucker Canyon, J = 0.007291, sample wt = 36.1 mg

Latitude: 31°43'45"N.

800

900

1000

1100

1150

1200

1250

1300

1350

1400

1450

1550

.0119

.05414

.21606

.49025

.44536

.64025

.44545

.67533

.72277

.94128

1.1522

1.7176

.0044

.03385

.09988

.19608

.17204

.24651

.16993

.26433

.28346

.36886

.45211

.67533

Longitude: 1 09 "23 '01 "W.

2.729

1.599

2.163

2.500

2.589

2.597

2.621

2.555

2.550

2.552

2.549

2.543

13.2

13.5

20.7

48.5

61.2

67.4

70.4

66.9

64.8

72.7

79.1

84.5

.1

1.1

3.4

6.6

5.8

8.3

5.7

8.9

9.6

12.4

15.2

22.8

36. ±

20.91 ±

28.23 ±

32.59 ±

33.73 ±

33.84±

34.15 ±

33.29 ±

33.23±

33.26 ±

33.21 ±

33.15 ±

5

.15

.38

.05

.49

.12

.33

.07

.14

.07

.07

.07

Total gas age = 33.00 ± 0.14 Ma

Plateau age (steps 8-12) = 33.21 ± 0.09 Ma (for 68.90 percent of the gas produced during heating)

Sample: 201570, biotite from granodiorite of Mackey Canyon, J = 0.007202, sample wt = 44.7 mg

Latitude: 31°58'21"N.

500

700

750

800

850

900

950

1000

1050

1150

1300

.0428

3.8412

1.6872

1.0393

.99738

1.4464

3.5422

4.9721

1.0862

.0158

.0205

.0294

1.61536

.70572

.43799

.41005

.60469

1.49360

2.09179

.45726

.0064

.0058

Longitude: 109°14'04"W.

1.45

2.378

2.391

2.373

2.432

2.392

2.372

2.377

2.375

2.45

3.52

10.5

270

257

186

86

105

130

214

59

4.41

.68

2.8

64.4

86.3

84.8

82.3

88.0

90.9

92.4

90.1

20.1

12.7

.4

20.6

9.0

5.6

5.2

7.7

19.0

26.6

5.8

.1

.1

1.9 i 2

30.63 ± .10

30.80± .11

30.57 ± .25

31.33 ± .32

30.81 ± .18

30.55 ± .08

30.62 ± .08

30.60 ± .24

32. ±13

45. ±16

Total gas age = 30.64 ± 0.14 Ma

Plateau age (steps 6-9) = 30.62 ± 0.15Ma (for 59.1 percent of the gas produced during heating)

Sample: 201570, hornblende from granodiorite of Mackey Canyon, J = 0.007173, sample wt = 416.4 mg 

Latitude: 31°58'21"N. Longitude: 109°14'04"W.

500 .0737 .0097 7.6 .27 119 3 96. ± 6 

600 .13673 .04461 3.065 1.01 27.9 1.2 39.23 ± .74 

700 .20655 .07720 2.676 2.14 28.2 2.1 34.29 ± .79 

750 .19676 .08145 2.416 1.73 55.3 2.2 30.99 ± .85 

800 .23621 .09726 2.429 .97 60.0 2.7 31.16 ± .26 

850 .24370 .10093 2.415 .44 55.3 2.8 30.98 ± .58 

900 .44623 .17561 2.541 .13 60.1 4.8 32.59 ± .36 

950 2.0647 .81976 2.519 .09 61.0 22.6 32.30 ± .15 

1000 2.1878 .87670 2.496 .10 66.7 24.2 32.01 ± -09 

1050 1.6697 .66814 2.499 .09 64.7 18.4 32.05 ± .21 

1100 .74820 .29713 2.518 .08 70.1 8.2 32.29 ± .26 

1150 .93612 .36408 2.571 .07 57.9 10.0 32.97 ± .24 

1200 .04219 .01489 2.83 .08 7.4 .4 36. ± 4 

1350 .0145 .0010 14.0 .09 3.5 0.0 173. ±44 

Total gas age = 32.53 ± 0.24 Ma

Plateau age (steps 7-11) = 32.16 ± 0.20 Ma (for 78.2 percent of the gas produced during heating) 

Isochron age (steps 7-11) = 31.42 ± 0.30 Ma; (^Ai^At^ = 309 ± 6
44 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

Table 3. 40Ar/39Ar data for volcanic rocks of the central Chiricahua Mountains. 17—Continued

4fi ji qq ji 4(1 •sq j/ -)Q fj 21/ 40 39 45 

Temp Ar./' Ari/ Arp' Ar k- Ar' Ar % ArR % Ar Apparent age 

(°C)_______________________ ______________________________(Maat±1o)

Pre-Turkey Creek caldera rocks (continued)

Sample: P272C, sanidine from rhyolite lava of Cave Creek, J =

Latitude: 31 0 53'26"N.

500

600

700

800

850

900

950

1000

1050

1150

1250

.48519

.37072

.89466

1.4218

1.1244

1.1607

1.8113

2.3568

1.63749

.36169

.09375

.19673

.16771

.41087

.65488

.51623

.53686

.83389

1.07976

.74934

.16462

.04219

0.007213, sample wt = 64 mg

Longitude: 109°09'25"W.

2.466

2.211

2.177

2.171

2.178

2.162

2.172

2.183

2.185

2.197

2.22

26.6

28.0

31.0

32.4

32.8

32.8

29.8

32.5

29.8

16.9

18.7

16.6

53.4

91.2

93.5

93.6

94.2

94.9

95.4

94.1

74.5

38.7

3.7

3.1.

7.7

12.2

9.6

10.0

15.6

20.2

14.0

3.1

.8

31.81 ±

28.54 ±

28.11 ±

28.03 ±

28.12 ±

27.92 ±

28.05 ±

28.18 ±

28.21 ±

28.4 ±

29. ±

.20

.52

.25

.13

.15

.14

.11

.11

.10

.5

2

Total gas age = 28.26 ± 0.17 Ma

Plateau age (steps 3-9) = 28.10 ± 0.12 Ma (for 89.3 percent of the gas produced during heating)

Sample: P475, sanidine from rhyolite of Erickson Ridge, J = 0.007303, sample wt = 62.9 mg

Latmide

850

950

1050

1150

1200

1250

1300

1350

1400

1450

1500

1550

1600

1700

: 32°00'25"N.

.0411

.21870

.65125

1.6564

1.5143

1.4656

1.7516

1.3516

.92589

.54858

.55001

.32143

.0533

.0281

.0240

.09741

.30046

.77897

.71406

.68496

.81850

.63031

.43440

.25939

.25775

.14595

.0217

.0058

Longitude: 109°21'58"W.

1.713

2.245

2.168

2.126

2.121

2.140

2.140

2.144

2.131

2.115

2.134

2.202

2.46

4.86

33.4

62.3

79.9

96.1

96.5

97.1

97.4

97.1

94.3

88.3

83.7

75.8

47.5

30.1

.5

1.9

5.8

15.1

13.8

13.2

15.8

12.2

8.4

5.0

5.0

2.8

.4

.1

22.4 ±

29.34 ±

28.33 ±

27.80 ±.

27.72±

27.97 ±

27.98 ±

28.03 ±

27.86 ±

27.65 ±

27.90 ±

28.78 ±

32. ±

63. ±

1.0

.27

.22

.05

.05

.12

.05

.18

.09

.15

.24

.45

3

5

Total gas age = 27.99 ± 0.13 Ma

Plateau age (steps 4-9) = 27.89 ± 0.09 Ma (for 78.5 percent of the gas produced during heating)

Latitude: 32 ° 00 '25 "N.

800

900

950

1000

1050

1100

1150

1200

1250

1300

1350

1400

1450

1550

.02905

.13580

.14824

.20741

.32763

.39272

.35925

.22424

.67488

.93652

1.3938

1.8408

1.5656

1.7362

.03459

.07591

.06720

.08754

.14511

.17782

.16389

.10199

.30564

.42528

.63856

.84285

.72078

.80073

Longitude: 109°21'58"W.

.840

1.789

2.206

2.369

2.258

2.209

2.192

2.199

2.208

2.202 ——

2.183

2.184

2.172

2.168

11.7

20.6

50.9

71.8

77.9

80.1

81.3

77.6

83.9

83.6

83.9

84.0

84.6

86.8

.8

1.7

1.5

1.9

3.2

3.9

3.6

2.2

6.7

9.3

13.9

18.4

15.7

17.5

11.0 ±

23.2 ±

28.6 ±

30.7 ±

29.29 ±

28.65 ±

28.44 ±

28.53 ±

28.65 ±

28.57 ±

28.32 ±

28.34 ±

28.18±

28.13 ±

.6

.5

.7

.6

.06

.16

.33

.31

.09

.12

.09

.05

.07

.11

Total gas age = 28.20 ± 0.14 Ma

Plateau age (steps 11-14) = 28.24 ± 0.08 Ma (for 65.5 percent of the gas produced during heating)

Sample: P652, sanidine from Jesse James Canyon Tuff, J = 0.007264, sample wt = 69.2 mg

Latitude

850

950

1050

1150

1250

1300

1350

1400

1450

1500

1550

1650

: 31°45'05"N.

.0105

.07493

.28184

.61017

1.3877

1.2584

1.5180

1.5856

1.6488

1.7162

1.7090

.41918

.0037

.03117

.12708

.28434

.65676

.59257

.71705

.74795

.78151

.80214

.77221

.18834

Longitude: 109°23'51"W.

2.86

2.404

2.218

2.146

2.113

2.124

2.117

2.120

2.110

2.139

2 213 ——

2^226 ——

38.8

89.2

54.7

96.6

97.4

97.4

97.5

98.7

98.1

98.9

98.5

96.6

.1

.5

2.2

5.0

11.5

10.4

12.6

13.1

13.7

14.1

13.5

3.3

37. ±

31.2 ±

28.8 ±

27.90 ±

27.48 ±

27.62 ±

27.53 ±

27.57 ±

27.44 ±

27.82 ±

28.77 ±

28.93 ±

15

1.4

.4

.10

.04

.10

.04

.09

.04

.05

.04

.12

Total gas age = 27.85 ± 0.08 Ma

Plateau age (steps 5-9) = 27.52 ± 0.06Ma ('for 61.3 percent of the gas produced during heating)
Geochronology 45 

Table 3. 40Ar/39Ar data for volcanic rocks of the central Chiricahua Mountains. 1 —Continued

Temp

(

o p\ C)

V

39. 21 

K W 3V7Ar2"3/ %4°ArR %39Ar 45/ Apparent age '

(Ma at± 1o)

Pre-Turkey Creek caldera rocks (continued)

Sample:

Latitude

950

1050

1150

1200

1250

1300

1350

1400

1450

1500

1600

201771, sanidine from Jesse James Canyon Tuff, J = 0.007077, sample wt = 82

: 32°00'44"N.

.11532

.53649

1.1595

.87493

1.0288

.49043

1.8168

2.4049

2.0050

1.7793

1.7859

.0539

.23574

.53383

.40236

.47172

.22584

.83416

1.10278

.91537

.81286

.81488

Longitude: 109°21'43"W.

2.138

2.276

2.172

2.174

2.181

2.172

2.178

2.181

2.190

2.189

2.192

91.1

69.7

98.1

98.3

98.2

95.6

98.3

98.6

98.9

98.7

98.2

.8

3.7

8.3

6.3

7.4

3.5

13.0

17.2

14.3

12.7

12.7

.3mg

27.1 ± 1

28.82 ±

27.52 ±

27.55 ±

27.63 ±

27.51 ±

27.59 ±

27.63 ±

27.75 ±

27.73 ±

27.77 ±

.0

.13

.06

.08

.10

.04

.04

.04

.08

.05

.04

Total gas age = 27.69 ± 0.07 Ma

Plateau age (steps 3-8) = 27.59 ± 0.06 Ma (for 55.8 percent of the gas produced during heating)

Isochron age (steps 3-11) = 27.63 ± 0.04 Ma; (^Ar/^ArX = 270 ± 10

Sample: 202156, biotite from tuff of Horseshoe Canyon, J = 0.007167, sample wt = 36.1 mg

Latitude: 3l°4S'4l"N. Longitude: 109°

800 .0011 .0047 .24

900 .0959 .0523 1.833

1000 .2390 .1209 1.976

1050 .2098 .09215 2.276

1100 .39023 .17880 2.183

1150 .56686 .26338 2.152

1200 .87541 .40572 2.158

1250 .78254 .36346 2.153

1300 .76022 .35360 2.150

1350 .89572 .41781 2.144

1400 .63020 .29524 2.134

1450 .63368 .29453 2.151

1600 2.5219 1.1780 2.141

Total gas age = 27.53 ± 0.26 Ma

07'54"W.

1.3 .1 3. ±13

27.9 1.3 23.6 ± .6

39.3 3.0 25.4 ± .8

74.6 2.3 29.3 ± .7

81.5 4.4 28.08 ± .23

85.4 6.6 27.69 ± .13

88.0 10.1 27.76 ± .05

89.0 9.0 27.70 ± .08

86.3 8.8 27.66 ± .05

80.9 10.4 27.58 ± .22

70.8 7.3 27.46 ± .15

63.5 7.3 27.68 ± .04

69.7 29.3 27.55 ± .04

Plateau age (steps 6-13) = 27.62 ± O.lOMa (for 88.8 percent of the gas produced during heating)

Sample: 202156, sanidine from tuff of Horseshoe Canyon, J

Latitude: 31°48'41"N. Longitude: 109°

750 .00577 .00112 5.163

850 .47512 .29321 1.620

950 1.5388 .85001 1.810

1000 .58965 .30905 1.908

1050 .74928 .39174 1.913

1100 .66632 .34314 1.942

1150 .58334 .29686 1.965

1200 .56378 .28303 1.992

1250 .62872 .31318 2.008

1300 .69119 .34868 1.982

1350 .71097 .36393 1.954

1400 .64823 .33239 1.950

1450 .64756 .33920 1.909

1500 1.4407 .74702 1.929

1600 1.9596 1.0212 1.919

= 0.007283, sample wt = 64.1 mg 07'54"W.

8.1 0.0 67. ±13

44.1 4.7 21.16 ± .10

84.9 13.6 23.63 ± .07

74.2 5.0 24.90 ± .08

87.4 6.3 24.96 ± .07

86.6 5.5 25.33 ± .13

82.3 4.8 25.64 ± .14

75.1 4.5 25.98 ± .05

67.7 5.0 26.18 ± .24

62.2 5.6 25.86 ± .24

62.6 5.8 25.49 ± .15

63.5 5.3 25.44 ± .16

63.6 5.4 24.91 ± .21

64.3 12.0 25.16 ± .08

56.4 16.4 25.04 ± .05

No plateau; total gas age =24.91 ± 0.1 1 Ma 

IsocWn age (steps 4-15) = 25.42 ±0.10 Ma; ("Ar/^Arfc = 297 ± 4

Sample: 202154, sanidine from dacite sill of Darnell Peak, J

Latitude: 31°48/32/'N. Longitude: 109°

850 .02393 .04891 .489

900 .33132 .18543 1.787

950 .69998 .33580 2.085

1000 .88721 .41841 2.120

1050 .91390 .43340 2.109

1100 1.0905 .51235 2.128

1150 1.3174 .61978 2.126

1200 1.8225 .85116 2.141

1250 2.5723 1.2051 2.135

1300 1.6878 .79301 2.128

1350 1.4432 .68061 2.121

1400 1.1857 .55472 2.137

1450 1.0973 .50892 2.156

1500 1.8167 .85044 2.136

1550 .86657 .39935 2.170

1650 .09555 .04191 2.280

Total gas age = 27.34 ± 0.09 Ma

= 0.00722, sample wt = 1 12.5 mg 08'09//W.

8.2 .6 6.36 ± .09

59.2 2.2 23.12 ± .24

91.5 4.0 26.95 ± .11

78.4 5.0 27.41 ± .08

95.5 5.1 27.26 ± .08

97.3 6.1 27.51 ± .04

97.6 7.3 27.48 ± .04

98.5 10.1 27.68 ± .04

98.3 14.3 27.59 ± .10

97.6 9.4 27.51 ± .10

96.8 8.1 27.41 ± .04

96.7 6.6 27.63 ± .08

90.0 6.0 27.87 ± .22

69.2 10.1 27.61 ± .04

47.4 4.7 28.04 ± .11

21.3 .5 29.46 ± .29

Plateau age (steps 6-14) = 27.58 ± 0.08 Ma (for 77.9 percent of the gas produced during heating)
46 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

Table 3. 40Ar/39Ar data for volcanic rocks of the central Chiricahua Mountains. 17—Continued

4fl 71 "?q 71 40 V\ 71 Wi T7 2 3/ 40 39 . 4,5/ WArB^ Ar/' 4UArJ3 Ar// J9Arr Ar' ' % Ar %J3Ar Apparent age

•C) (Maat±1a)

Rocks associated with the Turkey Creek caldera

Sample: 201769, sanidine from Rhyolite Canyon Tuff-lower member, J = 0.007129, sample wt = 70.3 mg

Latitude: 32°00'46//N.

500

600

700

800

850

900

950

1000

1050

1150

1300

1375

.2131

.55956

1.9368

4.2472

3.4981

2.7618

2.6773

1.5311

1.9843

.93865

.16154

.0330

.1072

.25904

.91631

2.0124

1.6581

1.3112

1.2700

.7292

.93790

.43976

.07816

.0158

Longitude: 109°21'46"W.

1.99

2.160

2.114

2.110

2.110

2.106

2.108

2.100

2.116

2.134

2.067

2.09

50.6

81.5

101

100

100

103

100

89.9

94.7

89.4

39.4

6.37

18.0

58.1

94.5

97.8

98.0

97.6

88.9

92.6

95.8

91.0

60.2

20.5

1.1

2.7

9.4

20.7

17.0

13.5

13.0

7.5

9.6

4.5

.8

.2

25.4 ±

27.6 ±

26.98 ±

26.94 ±

26.93 ±

26.89 ±

26.91 ±

26.81 ±

27.01 ±

27.24 ±

26.4 ±

27. ±

.5

.3

.09

.09

.08

.09

.13

.26

.11

.15

1.1

7

Total gas age = 26.94 ± 0.13 Ma

Plateau age (steps 3-9) = 26.93 ± 0.12Ma (for 90.8 percent of the gas produced during heating)

Sample: 201769, sanidine from Rhyolite Canyon Tuff-lower member, J = 0.007172, sample wt = 55.1 mg

Latitude: 32°00'46"N.

850

950

1050

1100

1150

1200

1250

1300

1350

1400

1450

1500

1600

.00011

.11672

.58507

.76053

1.2153

1.2029

.71461

1.0934

1.0051

.78775

.72266

.51758

.38239

.00249

.06072

.27485

.35883

.57467

.57099

.34090

.52163

.47980

.37560

.34272

.24488

.17886

Longitude: 109°21'46"W.

0.04

1.922

2.129

2.119

2.115

2.107

2.096 ——

2.096

2.095

2.097

2.109

2.114

2.138

0.0

61.6

75.1

97.8

99.2

98.3

97.3

98.2

98.2

96.8

87.3

89.5

91.6

.1

1.4

6.4

8.3

13.3

13.2

7.9

12.1

11.1

8.7

7.9

5.7

4.1

0. ±16

24.7 ± 1

27.33 ±

27.22 ± .

27.16 ± ,

27.05 ± ,

26.92 ±

26.92 ±

26.90 ±

26.93 ±

27.08 ±

27.14 ± ,

27.4 ± ,

.2

.04

.12

.09

.04

.17

.09

.06

.10

.08

.12

.5

Total gas age = 27.02 ± 0.56 Ma

Plateau age (steps 6-11) = 26.97 ± 0.09 Ma (for 60.8 percent of the gas produced during heating)

Isochron age (steps 3-13) = 27.09 ± 0.04 Ma; (^Ai^Ai^ = 309 ± 2

Sample: DY91-36, sanidine from Rhyolite Canyon Tuff-middle member, J = 0.0071 15, sample wt = 64.7 mg

.1 39. ± 5 

1.2 26.8 ± 1.1 

4.6 27.24 ± .18 

11.8 27.21 ± .07 

12.3 27.08 ± .06 

15.2 27.04 ± .10 

13.4 27.07 ± .0,4 

13.0 26.86 ± .15 

12.4 27.05 ± .14 

8.7 27.07 ± .17 

5.4 27.22 ± .06 

2.0 27.7 ± .6 

Total gas age = 27.09 ± 0.12 Ma

Plateau age (steps 5-10) = 27.03 ± 0.1 1 Ma (for 75.0 percent of the gas produced during heating) 

Isochron age (steps 2-11) = 27.14 ± 0.03 Ma; (^Ar/^ArX = 297 ± 2

Sample: 201765, sanidine from Rhyolite Canyon Tuff-upper member, J = 0.0071 11, sample wt = 61.3 mg

Latitude 

850

950

1050

1150

1200

1250

1300

1350

1400

1450

1500

1600

: 31°44'24"N. 

.0069 .0023

.11256

.43294

1.0974

1.1427

1.4135

1.2435

1.1975

1.1498

.80364

.49925

.18656

.05347

.20245

.51373

.53753

.66592

.58527

.56797

.54152

.37822

.23369

.08566

Longitude: 109°24'06"W. 

3.07 ----- 11.8

2.105

2.139

2.136

2 126 — —

2^123

2.125

2.108

2.123

2.125

9 X> . -i-2f. 1 _J \J __-MMTM.

2.178

82.6

71.5

98.7

98.5

98.2

98.9

98.2

98.7

97.9

95.7

89.9

Latitude

500

600

700

800

850

900

950

1000

1050

1150

1250

: 32°00'47 //N.

.11023

.25977

1.1332

2.0009

2.0728

1.9046

2.4967

2.4659

3.6877

1.2413

.17004

.05888

.12202

.53671

.94622

.97892

.89961

1.1773

1.1679

1.7425

.58051

.08177

Longitude: 109°19'23"W.

1.87

2.129

2.111

2.115

2.117

2.117

2.121

2.111

2.116

2.138

2.08

31.9

44.2

54.0

54.5

53.5

53.1

53.7

56.0

57.2

53.7

32.8

6.5

36.0

82.9

92.6

94.5

95.3

95.4

95.6

96.6

91.2

57.5

.7

1.5

6.5

11.4

11.8

10.8

14.2

14.1

21.0

7.0

1.0

23.9 ±

27.1 ±

26.88 ±

26.93 ±

26.96 ±

26.96 ±

27.00 ±

26.88 ±

26.95 ±

27.22 ±

26.5 ±

1.2

.4

.19

.14

.13

.14

.08

.08

.07

.15

.8

Total gas age = 26.94 ± 0.13 Ma

Plateau age (steps 3-9) = 26.94 ± 0.12 Ma (for 89.8 percent of the gas produced during heating)
Geochronology 47

Table 3. 40Ar/39Ar data for volcanic rocks of the central Chiricahua Mountains. 17—Continued

Temp 

CO

r 2/ 4VH2/ 39Ar/37Ar2'3/ 

N UN

% 4V ,39Ar Apparent age 

(Maat± 10)

4,5/

Rocks associated with the Turkey Creek caldera (continued)

Sample: 201765, sanidine from Rhyolite Canyon Tuff-upper member, J = 0.007262, sample wt = 110.8 mg

Latitude: 32° 00 '47"N.

850

950

1050

1150

1200

1250

1300

1350

1400

1450

1500

1550

1650

.0304

.11556

.36519

.99459

1.0537

1.3150

1.5626

2.4327

2.3763

2.4623

1.7665

1.4424

.42926

.0155

.05246

.17234

.47862

.51024

.63665

.76140

1.1826

1.1468

1.1872

.84937

.69194

.20363

Longitude: 109°19/23"W.

1.96

2.203

2.119

2.078

2.065

2.065

2.052

2.057

2.072

2.074

2.080

2.085

2.108

10.7

38.1

62.5

90.7

94.1

95.0

95.8

97.5

98.4

98.4

97.7

96.8

86.9

.2

.7

2.2

6.1

6.5

8.1

9.7

15.0

14.5

15.0

10.8

8.8

2.6

26. ±

28.63 ±

27.55 ±

27.02 ±

26.85 ±

26.86 ±

26.69 ±

26.75 ±

26.94 ±

26.97 ±

27.04 ±

27.10 ±

27.41 ±

4

.92

.04

.18

.09

.10

.04

.04

.07

.06

.05

.04

.08

No plateau; total gas age = 26.94 ± 0.08 Ma

Isochron age (steps 3-13) = 26.98 ± 0.04 Ma; (wAi/36Ai\ = 309 ± 2

Latitude

500

600

700

800

850

900

950

1000

1050

1150

1250

: 31° 5531 "N.

.10369

.51280

.87410

1.38411

1.57630

1.39669

2.53404

3.07313

4.92151

1.93264

.36452

.04227

.22671

.40221

.64407

.73405

.65051

1.19844

1.44533

2.27911

.82957

.15550

Longitude: 109°16'22"W.

2.453

2.262

2.173

2.149

2.147

2.147

2.114

2.126

2.159

2.330

2.344

4.76

.96

4.90

11.3

14.6

18.9

21.7

19.4

23.1

17.8

7.66

5.9

40.7

76.9

85.4

81.9

79.1

72.9

74.9

78.3

65.4

43.0

.5

2.6

4.7

7.5

8.5

7.6

13.9

16.8

26.5

9.6

1.8

31.2 ±

28.8 ±

27.7 ±

27.41 ±

27.38 ±

27.38 ±

26.97 ±

27.12 ±

27.54 ±

29.69 ±

29.9 ±

.9

.3

.3

.13

.14

.13

.13

.08

.08

.12

.5

Total gas age = 27.66 ± 0.12 Ma; no plateau

Tmin = 26.97 ± 0.13 Ma (950°C); isochron age = 27.44 ±0.15 Ma; = 303 ± 2

Latitude: 31°52'25"N.

500

600

700

800

850

900

950

1000

1050

1150

1250

.57035

.79823

1.8771

3.1487

2.1960

1.33200

2.4557

4.0596

8.4536

.8959

.23026

.27009

.37890

.89706

1.49111

1.03651

.62910

1.17063

1.94263

4.01325

.42092

.10900

JL M. J J --~--- C3 -——- ——— ,—

Longitude: 109° 22'

2.112

2.107

2.092

2.112

2.119

2.117

2.098

2.090

2.106

2.129

2.112

20.2

14.9

14.7

22.4

27.4

26.0

20.3

18.0

19.8

8.03

5.83

04 "W.

25.1

60.0

85.0

92.6

92.0

91.4

87.6

85.8

88.3

78.7

63.8

2.2

3.1

7.3

12.1

8.4

5.1

9.5

15.7

32.5

3.4

.9

26.9 ±

26.86 ±

26.68 ±

26.92 ±

27.01 ±

26.99 ±

26.75 ±

26.64 ±

26.86 ±

27.13 ±

26.9 ±

—— (_,

.4

.15

.10

.09

.10

.11

.08

.07

.07

.21

.6

No plateau; total gas age = 26.84 ± 0.09 Ma; weighted mean age (all steps) = 26.84 ± 0.17 Ma 

Isochron age (all steps) = 26.90 ± 0.04 Ma; (40Ar/^sAr)r = 296 ± 2

Sample: 201996, sanidine from biotite rhyolite lava, 

Latitude: 31°56'19"N. Longitude:

850

950

1050

1100

1150

1200

1250

1300

1350

1400

1450

1500

1550

1650

.16089

.28121

.70405

.86402

1.0600

1.8335

2.5035

2.7035

2.37204

2.9481

3.0153

3.4135

1.3569

.29582

.08517

.13080

.33878

.42025

.5218

.8949

1.2220

1.3184

1.1574

1.4344

1.4667

1.6531

.65337

.13464

1.889

2.150

2.078

2.056

2.031

2.049

2.049

2.051

2.050

2.055

2.056

2.065

2.077

2.197

J = 0.007278, sample wt = 121.7 mg 109°19'15"W.

20.4

52.1

75.6

91.1

93.6

96.4

96.3

97.2

97.0

96.1

96.8

97.6

94.7

73.2

.7

1.1

3.0

3.7

4.6

7.8

10.7

11.5

10.1

12.5

12.8

14.5

5.7

1.2

24.63 ±

28.01 ±

27.08 ±

26.79±

26.47 ±

26.70 ±

26.70 ±

26.72 ±

26.71 ±

26.78 ±

26.79 ±

26.91 ±

27.06 ±

28.62 ±

.20

.41

.04

.17

.12

.04

.04

.04

.12

.04

.04

.04

.05

.27

Total gas age = 26.80 ± 0.06 Ma

Plateau age (steps 6-11) = 26.74 ± 0.05 Ma (for 65.6 percent of the gas produced during heating)
48 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

Table 3. 40Ar/39Ar data for volcanic rocks of the central Chiricahua Mountains. 17 — Continued

Temp 40Ar 2/ 392/ Ar W3/ %39Ar Apparentage

i.« , i (Maat±1o)

4,5/

Rocks associated with the Turkey Creek caldera (continued)

Sample: 201996, biotite from biotite rhyolite lava, J = 0.007148, sample wt = 74.4 mg

T rt-fj-fn/Jfn- "* "" 0 f ft f\ //-VT T • , i + n m~i t n. ti f ff-r-TT

800

900

950

1000

1050

1100

1150

1200

1250

1300

1350

1400

1450

1550

: 31°56'

.0684

.0999

.0328

.0546

.0946

.27051

.42437

.60224

.95712

1.5605

1.8422

2.9339

2.2666

1.5975

r !9"N.

.0265

.0340

.0167

.0207

.04424

.12193

.19741

.28319

.45167

.74214

.87078

1.3834

1.0724

.75188

Longitude: 109° 19

2.587

2.935

1.967

2.640

2.139

2.219

2.150

2.127

2.119

2.103

2.116 13.6

2.121 83.3

2.114 33.1

2.125

'15"W,

26.1

42.9

13.8

20.9

62.2

84.3

89.7

92.3

94.0

93.7

91.3

87.8

85.9

91.4

.4

.6

.3

.3

.7

2.0

3.3

4.7

7.5

12.3

14.5

23.0

17.8

12.5

33. ±2

37.4 ±1.0

25. ±3

33.7 ± .6

27. ±2

28.38 ± .10

27.51 ± .13

27.22 ± .09

27.12 ± .08

26.91 ± .06

27.08 ± .04

27.14 ± .04

27.05 ± .04

27.19 ± .10

Total gas age = 27.24 ± 0.10 Ma

Plateau age (steps 11-14) = 27.11 ± 0.06 Ma (for 67.8 percent of the gas produced during heating)

Isochron age = 26.71 ± 0.02 Ma; (40Ai/36Ai\ = 366 ±15

Sample: 201538, sanidine from rhyolite moat lava-unit 1, J = 0.007168, sample wt = 51.6 mg

Latitude;

500

600

700

800

850

900

950 

1000 

1050 

1150 

1300

31°52'24 //N.

.33103

.46422

1.2383

1.7972

1.8730

1.2565

2.0174

2.6956

2.4154

.73347

.1081

.15699

.22179

.58556

.85737

.88533

.60105

.97148

1.2888

1.1484

.34740

.0477

Longitude: 

2.109

2.093

2.115

2.096

2.116

2.090

2.077

2.091

2.103

2.111

2.266

109° 

7.95

13.5

19.1

23.3

23.0

23.7

21.6

23.7

23.3

23.6

20.9

17WW. ' 

9.6

40.6

80.1

90.2

90.8

90.8

92.1

95.0

94.0

62.6

25.0

2.2

3.1

8.2

12.1

12.4

8.5

13.7

18.1

16.1

4.9

.7

No plateau, total gas age = 26.94 ± 0.13 Ma; weighted mean age (steps 1-10) = 26.93 ± 0. 

Isochron age = 26.89 ± 0.05 Ma; (40Ar/36Ar)I = 298 ± 1

27.1 ± 

26.9 ± 

27.14 ± 

26.90 ± 

27.15 ± 

26.83 ± 

26.66 ± 

26.84 ± 

26.99 ± 

27.10 ± 

29. ±2 

.17 Ma

.6

.3

.14

.12

.16

.09

.10

.10

.09

.12

Sample:

Latitude

500

600

700

800

850

900

950

1000

1050

1150

1250

201580, sanidine from rhyolite moat lava-unit 1, J = 0.007143, sample wt = 57. : 31°55'20"N. * ' ' ------- ••—

.27438

.47682

1.2090

1.5634

1.8546

1.5144

1.5239

2.9275

2.0132

.63156

.0872

.11329

.23254

.58283

.74834

.88798

.73336

.73312

1.40960

.96080

.30099

.0432

3mg

Longitude: 109°17'39"W.

2.422

2.050

2.074

2.089

2.089

2.065

2.079

2.077

2.095

2.098

2.02

7.96

6.49

8.11

11.0

14.0

15.8

15.2

16.2

16.3

8.83

5.28

13.6

52.9

90.6

94.3

95.8

94.9

94.2

96.5

95.1

83.8

40.7

1.7

3.4

8.6

11.1

13.2

10.9

10.9

20.9

14.2

4.5

.6

30.94 ±

26.23 ±

26.53 ±

26.72 ±

26.71 ±

26.41 ±

26.59 ±

26.57 ±

26.80 ±

26.84 ±

26. ±

.47

.22

.08

.15

.09

.23

.12

.08

.09

.18

1

Total gas age = 26.69 ± 0.13 Ma

Plateau age (steps 3-10) = 26.64 ± 0.13 Ma (for 94.2 percent of the gas produced during heating)

Post-Turkey Creek caldera rocks

Sample: DY92-54, sanidine from rhyolite lava, J = 0.007237, sample wt = 74.5 mg Latitude- °'1 ° •"•"•""'" T •• • -__---.--.. —

850

950

1050

1150

1200

1250

1300

1350

1400

1450

1500

1550

1650

: 31°42'

.0134

.13173

.57428

1.0428

1.0567

1.1476

1.6901

1.5556

1.3902

1.2786

1.4064

1.4905

.60600

10"N.

.0056

.06109

.27719

.51297

.52834

.56920

.83390

.76632

.68581

.62862

.68959

.73136

.29152

Longitude: 109°29'50"W.

2.40

2.156

2.072

2.033

2.000

2.016

2.027

2.030

2.027

2.034

2.039

2.038

2.079

14.7

89.0

74.8

97.6

96.8

97.7

98.5

99.0

98.8

98.9

98.9

98.2

95.7

.1

.9

4.2

7.8

8.0

8.6

12.7

11.6

10.4

9.6

10.5

11.1

4.4

31. ±6

28. ±2

26.85± .18

26.35 ± .05

25.92 ± .07

26.13 ± .18

26.27 ± .09

26.31 ± .04

26.27 ± .10

26.36 ± .10

26.43 ± .06

26.41 ± .06

26.94 ± .06

Total gas age = 26.36 ± 0.10 Ma

Plateau age (steps 7-12) = 26.35 ± 0.08 Ma (for 65.9 percent of the gas produced during heating)
Geochronology 49 

Table 3. 4QAr/39Ar data for volcanic rocks of the central Chiricahua Mountains. 17—Continued___

Temp 40ArR 2/ 39Ar/ 4°ArR/39Ar/ 3W'3/ %4°ArR %39Ar Apparent age4'5/

/o n\ " K H K h

( 1)__________________________________________________________(Ma at± 1o)

Post-Turkey Creek caldera rocks (continued)

Sample: 202151, sanidine from rhyolite lava of Dobson Peak, J = 0.00702, sample wt = 66.7 mg 

Latitude: 31°46'36"N. Longitude: 109°13'51"W.

850 .0016 .0037 .42 —- 2.8 .1 5. ± 8 

950 .28556 .14540 1.964 ----- 82.7 2.2 24.70 ± .36 

1050 .87253 .42507 2.053 —— 77.4 6.4 25.81 ± .18 

1150 1.1464 .55507 2.065 —— 97.0 8.4 25.97 ± .06 

1200 1.1399 .54601 2.088 -—- 97.7 8.3 26.25 ± .11 

1250 1.2690 .61365 2.068 ----- 97.2 9.3 26.00 ± .10 

1300 1.4994 .71792 2.089 —— 98.3 10.9 26.26 ± .10 

1350 1.5883 .76260 2.083 —— 98.8 11.6 26.19 ± .04 

1400 1.6971 .81162 2.091 —— 99.1 12.3 26.29 ± .04 

1450 1.4332 .68728 2.085 —- 98.5 10.4 26.22 ± .04 

1500 1.1885 .56667 2.097 ----- 98.4 8.6 26.37 ± .06 

1600 1.5862 .75997 2.087 —- 95.0 11.5 26.24 ± .05 

Total gas age = 26.13 ± 0.10 Ma 

Plateau age (steps 5-10) = 26.20 ± 0.07 Ma (for 62.8 percent of the gas produced during heating)

Sample: P5, sanidine from rhyolite of Packsaddle Mountain, J = 0.006809, sample wt = 95 mg 

Latitude: 32°36'45"N. Longitude: 109°25'20"W.

850 .00712 .00282 2.524 5479.74 17.5 0.0 30.74± 19.84 

850 .0071 .0028 2.52 —— 17.5 0.0 31. ± 20 

950 .18435 .10729 1.718 —— 62.8 1.4 20.98 ± .22 

1050 .56527 .29758 1.900 —— 69.4 4.0 23.18± .07 

1150 1.1616 .61307 1.895 ----- 95.0 8.3 23.13 ± .09 

1200 .77133 .40730 1.894 ----- 95.1 5.5 23.11 ± .09 

1250 .91981 .48465 1.898 ----- 95.6 6.5 23.16± .11 

1300 .72999 .37911 1.926 ----- 97.2 5.1 23.50 ± .10 

1350 .80739 .42362 1.906 —- 97.2 5.7 23.26 ± .07 

1400 .87471 .45882 1.906 —— 97.3 6.2 23.27 ± .06 

1450 1.1131 .58198 1.913 —— 97.5 7.8 23.34 ± .10 

1500 1.7836 .94134 1.895 —— 96.2 12.7 23.13 ± .05 

1550 4.6120 2.42217 1.904 —- 96.9 32.6 23.24 ± .04 

1600 .48109 .25010 1.924 —— 91.1 3.4 23.47 ± .06 

1650 .12014 .05782 2.08 ----- 87.6 .8 25.3 ± .7 

Total gas age = 23.22 ±0.08 Ma 

Plateau age (steps 8-12) = 23.23 ± 0.06 Ma (for 65.0 percent of the gas produced during heating)

Mineral separates were prepared after crushing, grinding, and sieving by magnetic separator, mica-table, and heavy 

liquid methods; grains ranged in size between 60 and 120 mesh (250-125 um). Individual samples ranged in mass from 

36 to 416 mg. For irradiation an aluminum canister was loaded with six quartz vials each of which was loaded with 

samples and standards. Standards were placed between every two unknowns as well as at the top and bottom of each 

vial. Each standard was degassed to release argon in a single 20-minute long heating step at 1,250°C. Each sample was 

degassed stepwise in a series of 11 to 16 individual temperature steps for 20 minutes each. All analyses were done in the 

Argon Laboratory, U.S. Geological Survey, Denver, Colo. Decay constants are those of Steiger and Jager (1977). The 

standard for these experiments is hornblende MMhb-1 with percent K=l .555, 40ArR =1.624x10'9 mole/g, and K-Ar age = 

520.4 Ma (Samson and Alexander, 1987).

2/ Abundances of "Radiogenic 40Ar" and "K-derived 39Ar" are reported in volts measured on a Mass Analyser Products 

215 rare-gas mass spectrometer using the Faraday cup. Conversion to moles can be made using a sensitivity of 

9.736xlO"13 moles argon per volt of signal. Detection limit at the time of this experiment was 2x10"17 moles argon. 

Corrections were made for volume, mass fractionation, trap current, radioactive decay of 37Ar and 39Ar, and interfering Ai 

isotopes. Mass discrimination in our mass spectrometer is routinely determined by measuring the 40Ar /36Ar ratio of 

atmospheric argon; our measured value is generally 298.9 the accepted value is 295.5. The calculated discrimination is 

applied to all samples and standards equally. Production ratios measured on pure K2SO4 and CaF2 salts irradiated with 

the samples were used to correct for irradiation-produced 40Ar (from K) and *Ar (from Ca). Corrections for Cl-derived 

3SAr were determined using the method of Roddick (1983). Production ratios determined for samples irradiated in the 

1995 TRIGA reactor experiment are: (40Ar/39Ar)K =7.92xlO-3,(3SAr/39Ar)K = 1.309x10-2,(37Ar/39Ar)K =l.8x10"4, 

C6Ai /"Ar^ = 2.68x10 , (39Ar /37Ar)Ca = 6.85x10'4, (38Ar ^Ai)c& = 4.4x10"5 . Production ratios were not directly 

determined for samples irradiated in 1988; ratios suggested bv Dalrymple and others (1981) were 

used. Analytical data for "Radiogenic 40Ar" and "K-derived 39Ar" are calculated to five places; "40ArE/39Arj," is 

calculated to three decimal places. "Radiogenic 40Ar", "K-derived 39Ar", and "40Ars/:'9ArK" are rounded to significant 

figures using associated analytical precisions. Apparent ages and associated errors were calculated from unrounded data 

and then rounded using associated errors. —, "Ar below detection; no ratio calculated. 

3/To calculate apparent K/Ca ratios, divide ^Ar/^Ar value by 2.

4/ Uncertainties in calculations for the apparent date of individual steps for a sample were calculated using the equations 

of Dalrymple and others (1981).

5/ The reproducibility of split gas fractions from selected monitors was used to calculate imprecision in J values This 

imprecision is generally 0.1%, 1 sigma. J values for each sample were interpolated from adjacent standards. The 

uncertainty in each apparent age includes the uncertainty in the J value.
50 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

tuff by the latite of Darnell Peak (Bryan, 1988), as described 

previously, for which our determined age is 26.58±0.08 Ma. 

The age of the upper member of the tuff of Horseshoe Canyon 

has not been determined, although its age is bracketed between 

the ages of the latite of Darnell Peak (27.6 Ma), which 

intrudes the tuff, and that of the overlying Rhyolite Canyon 

Tuff (26.9 Ma).

Rocks Associated with the Turkey Creek 

Caldera

Several previous attempts have been made to accurately 

establish the age of the Rhyolite Canyon Tuff. Drewes (1982) 

summarized earlier K-Ar age determinations reported by 

Marjaniemi (1969) and Marvin and others (1978) for parts of 

the Rhyolite Canyon Tuff. These ages range from about 24.7 

to 25.6 Ma, and are all young relative to new ^Ar/^Ar data 

presented herein. Mclntosh and Bryan (2000) reported three 

40Ar/39Ar ages for the Rhyolite Canyon Tuff and have established their preferred age as 26.8 Ma. We determined the ages 

of all three of the volumetrically significant members of outflow facies Rhyolite Canyon Tuff using the ^Ar/^Ar method.

Sample 201769 (fig. 1, locality B) represents the lower 

member of the Rhyolite Canyon Tuff. Two splits of this 

sanidine separate, as well as of the sanidine separate that 

represents the upper member (201765; fig. 1, locality D), 

were analyzed several years apart in order to evaluate the 

reproducibility of ages determined during the several years 

that geochronologic investigations were conducted. Both age 

spectra for sample 201769 are simple (fig. 115 and T) and give 

statistically indistinguishable plateau ages of 26.97±0.09 and 

26.93±0.12 Ma, respectively. An isochron age for one yields 

a statistically identical age of 27.09±0.04 Ma with a slightly 

elevated (40Ar/36Ar)j of 309±2. The average of the two sanidine ages, 26.95±0.07 Ma, is our preferred age for the lower 

member of the Rhyolite Canyon Tuff.

Sample DY91-36 (fig. 1, locality Q) represents the 

middle member of the Rhyolite Canyon Tuff. The age spectrum for sanidine from this sample is simple (fig. 11 U) and 

gives a plateau age of 27.03±0.11 Ma. Sample 201765 (fig. 

1, locality D) represents the upper member of the Rhyolite 

Canyon Tuff; as just indicated, two splits of this separate were 

analyzed. The age spectrum for one of these splits (fig. 11V) 

shows some evidence of excess argon, particularly in the lowtemperature steps, is broadly U-shaped, and does not include 

a plateau age; the isochron age for this split is 26.98±0.04 Ma 

with (40Ar/36Ar)I = 309±2, which reflects the presence of minor 

excess argon. The spectrum for the other split (fig. 11 W) is 

simple and gives a plateau age of 26.94±0.12 Ma. The ages 

for the two splits are statistically indistinguishable, and their 

average, 26.96±0.06 Ma, is our preferred age for the upper 

member of the Rhyolite Canyon Tuff. .Data for two splits 

each of samples 201765 and 201769 indicate that variation of 

analytical data acquired over several years has not adversely 

impacted analytical precision or accuracy. The five analyses

of sanidine from Rhyolite Canyon Tuff indicate that, within 

analytical uncertainty, each of the principal ash-flow tuffs that 

constitute this unit is the same age and that they must have 

been erupted in rapid succession. The absence of erosional 

breaks or other significant discontinuities between the members further indicates rapid eruption and emplacement of these 

pyroclastic flows.

Samples 201587 and P3 (fig. 1, localities J and H, 

respectively) represent dacite porphyry that was emplaced, 

after eruption of the Rhyolite Canyon Tuff and caldera collapse, as a resurgent intrusion in the core of the Turkey Creek 

caldera (201587) and as lava flows in the caldera's evolving 

moat (P3). The age spectrum for sanidine for sample 201587 

is slightly disturbed (fig. 1 IX) and does not yield a plateau. 

The weighted average age for this sample is 26.84±0.17 Ma 

and the isochron age is 26.90±0.04 Ma with (^Ar/^Ar^ = 

296±2. The age spectrum for sanidine from sample P3 is also 

disturbed (fig. 11F), shows some evidence of excess argon, 

and does not yield a plateau. The isochron age for this sample 

is 27.44±0.15 Ma with an elevated (^Ar/36^ = 303±2. 

The characteristic saddle-shaped spectrum results from the 

presence of excess argon. Others (Lanphere and Dalrymple, 

1976) have interpreted the age of the lowest age step defining the saddle to best represent a maximum age estimate. In 

this case, the lowest age step in the saddle is 26.97±0.13 Ma, 

which is indistinguishable from the age of sanidine from 

sample 201587. Given this assessment of the 40Ar/39Ar data 

for samples of the dacite porphyry, our best estimate of its 

age is 26.90±0.04 Ma, which is the isochron age for sample 

201587. These age data indicate that dacite porphyry intrusion and extrusion followed Rhyolite Canyon Tuff eruption 

and caldera collapse within less than 100,000 years, which is 

consistent with dacite porphyry lava flows being interbedded 

with lava-flow-like phase intracaldera Rhyolite Canyon Tuff 

in exposures north and west of John Long Canyon (fig. 1) (du 

Bray and others, 1997).

Sample 201996 (fig. 1, locality F) represents the biotite 

rhyolite lava, the first of the rhyolite lavas erupted into the 

moat of the Turkey Creek caldera; both biotite and sanidine 

separates were analyzed. The age spectrum for biotite from 

this sample is relatively simple (fig. HZ), although it shows 

some evidence of excess argon in the low-temperature steps. 

It gives a plateau age of 27.11±0.06 Ma and an isochron age 

of 26.71±0.02 Ma with a significantly enhanced (^Ar/^Ar^ of 

366±15. The plateau age is implausible given the well-defined 

26.9 Ma age for Rhyolite Canyon Tuff on which this rhyolite 

was deposited; the isochron age is more geologically reasonable. The age spectrum for sanidine from this sample is relatively simple (fig. 11AA), although it also shows some slight 

evidence of excess argon; it gives a plateau age of 26.74±0.05 

Ma. This age is consistent with the 26.9 Ma age for Rhyolite 

Canyon Tuff and 26.9 Ma dacite porphyry on which the rhyolite was deposited.

Samples 201538 and 201580 (fig. 1, localities K and 

G) represent unit 1 rhyolite lava (Tm r 1) in the moat of the 

Turkey Creek caldera; a sanidine separate from each of these
Concluding Remarks 51

samples was analyzed. The age spectrum for sanidine from 

sample 201538 is relatively simple (fig. 1155) but is disturbed 

and does not include a plateau; it gives a weighted-mean age 

of 26.93±0.17 Ma and an isochron age of 26.89±0.05 Ma with 

(^Ar/^Ar), of 298±1. This apparent age is slightly too old 

to be consistent with the 26.7 Ma age of the biotite rhyolite 

on which unit 1 rhyolite was deposited. The age spectrum 

for sanidine from sample 201580 is relatively simple (fig. 

11CC), although it shows some evidence of excess argon in 

the low-temperature steps; it gives a plateau age of 26.64±0.13 

Ma. This age seems slightly too young given that this sample 

is from the same unit as sample 201538 (26.9 Ma). Averaging the isochron age for sample 201538 and the plateau age 

for sample 201580 yields a preferred age of 26.77±0.12 Ma 

for unit 1 lava. Given the analytical uncertainties associated 

with the ages determined for the biotite rhyolite (201967) 

and the unit 1 lava (201538 and 201580), all of these ages are 

statistically indistinguishable. Perhaps the best reconciliation 

of the 40Ar/39Ar data for the rhyolite moat sequence would 

result in an age estimate of 26.7 Ma for the biotite rhyolite 

and unit 1 lava. Because units 2 and 3 lavas—the final rhyolite 

lavas erupted into the moat of the Turkey Creek caldera—are 

so crystal poor, no mineral separates could be prepared 

and their ages were not determined. Consequently, the full 

duration of volcanism associated with the caldera cannot be 

closely defined. However, the lack of significant erosional/ 

depositional break indicates that these moat lavas were erupted 

in a relatively short time interval, perhaps as little as 10,000 

years.

Post-Turkey Creek Caldera Rocks

Sample 202151 (fig. 1, locality N) represents the rhyolite 

lava of Dobson Peak, which is the youngest volumetrically 

significant volcanic unit in the central Chiricahua Mountains. 

The age spectrum for sanidine from this sample is relatively 

simple (fig. 1 IDD) and gives a plateau age of 26.20±0.07 Ma. 

Because this unit overlies unit 1 lava, the youngest rocks associated with Turkey Creek caldera volcanism must be at least 

this old. Consequently, the full eruptive cycle associated with 

the Turkey Creek caldera lasted no more than 700,000 years, 

from 26.9 to no less than 26.2 Ma, and all middle Tertiary volcanic activity in the central Chiricahua Mountains apparently 

ended about 26.2 Ma.

Concluding Remarks

Geochemical and petrographic data presented here show 

that volcanic units of the central Chiricahua Mountains are 

individually distinctive. These data can be used to corroborate 

stratigraphic identifications made with other data; they can 

also be used to identify units when either available data are 

insufficient or stratigraphic context is absent or ambiguous. 

Diagnostic age, petrographic, and geochemical features for

middle Tertiary volcanic rocks of the region are summarized 

in table 4. In many cases, macroscopically observable features 

of the area's volcanic rocks, including contained phenocrysts 

(mineral, size, and abundance), textures (degree of welding or 

flow features), and (or) lithic or pumice content (size, number, 

and composition), are sufficient to enable rapid, field-based 

stratigraphic identification. When these features are insufficient, other simple techniques, such as transmitted light 

microscopy, can in many cases be used to rapidly eliminate 

uncertainties. When more detailed data are required to determine or confirm stratigraphic identity, geochemical, geochronologic, or paleomagnetic data may be necessary. Of these, 

geochemical data, especially trace-element data obtained by 

energy-dispersive X-ray fluorescence analysis of rock powders, are probably the least expensive and most rapidly obtainable type of data for establishing stratigraphic identity. Traceelement data demonstrate that almost all the stratigraphic units 

of this area have characteristic geochemical features diagnostic 

of their stratigraphic identity. Combining geochemical data 

with macroscopically observable rock features results in 

virtually certain identification of the study area rocks, which 

enhances correlation of isolated or ambiguous occurrences of 

these rocks throughout the Boot Heel volcanic field.

The preponderant volume of volcanic rock preserved in 

the range is ash-flow tuff, although a significant volume of 

lava is present as well. The overwhelmingly dominant source 

of volcanic rocks preserved in the area is the Turkey Creek 

caldera. Between 500 and 1,000 km3 of Rhyolite Canyon 

Tuff is present, and approximately 60 km3 of caldera-related 

rhyolite lava is preserved in the caldera's moat. Middle Tertiary volcanic activity is dominated by eruption of subalkaline 

rhyolite, most of which is high silica and weakly peraluminous. One of the most diagnostic compositional features of 

these rocks is their elevated K2O abundances. All of the area's 

volcanic rocks are members of either the high-potassium calcalkaline or the shoshonitic series. Volcanic rock within-stratigraphic-unit compositions are relatively homogeneous. The 

principal exception to this is among the voluminous outflow 

facies ash-flow tuffs, in which there is considerable compositional inhomogeneity. Sections of the Rhyolite Canyon Tuff 

and the tuff of Horseshoe Canyon are relatively strongly zoned 

in ways that appear to reflect progressive, top-down tapping 

of normally zoned magma reservoirs. Most of the observed 

zonation is consistent with crystal-liquid fractionation of the 

phenocryst phases that are characteristic of each unit. Compositional variation within lavas is relatively minor.

Compositional variation among eruptive products of the 

Turkey Creek caldera reflects a contrast between magmatic 

processes in the reservoirs that produced the normally zoned 

Rhyolite Canyon Tuff and those that subsequently produced 

the reversely zoned moat rhyolite sequence. As described 

in previous sections, geochemical zonation preserved within 

the principal outflow facies members of the Rhyolite Canyon 

Tuff is consistent with systematic magma extraction from 

a normally zoned reservoir. Considering the geochemical 

characteristics of these rocks, the phenocrysts they contain,
52 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

Table 4. Diagnostic age, 

Mountains.

[ND, not determined]

petrographic, and geochemical features of middle Tertiary volcanic rocks of the central Chiricahua

Unit Unit 

symbol

Age, 

in Ma

Petrographic 

characteristics

Geochemical 

characteristics

Pre-Turkey Creek caldera rocks

Intermediate-compo- Tim ND 

sition lava flows.

Rhyolite tuff of High Thl 34.16±0.17 

Lonesome Canyon.

Lower member of the Tjg 33.81±0.08 

rhyolite of Joe Glenn 

Ranch.

Rhyolite lava of Krentz Tkr ND 

Ranch.

Rhyolite lava of Cave Tc 28.10±0.12 

Creek.

Rhyolite of Erickson Tfre 27.89±0.09 

Ridge.

Tuff of Horseshoe Canyon Thcl 27.62±0.10 

lower member.

Tuff of Horseshoe Canyon Thcu ND 

upper member.

Jesse James Canyon Tjj 27.56±0.04 

Tuff.

Dark greenish gray. Plagioclase, 

hornblende and pyroxene phenocrysts. Variable crystal content.

Quartz and sanidine phenocrysts; 

5 percent crystals. Pumice and 

lithic poor.

Biotite, feldspar, and quartz 

phenocrysts; 20-40 percent 

crystal.

Aphyric. Massive to flow banded 

Aphyric. Massive to flow banded

Biotite and feldspar phenocrysts; 

4-10 percent crystals.

Biotite, quartz, and sanidine 

phenocrysts; 20-35 percent 

crystals.

Biotite, quartz, and sanidine 

phenocrysts; 10-20 percent 

crystals.

Biotite, sanidine and quartz 

phenocrysts; 10 percent crystals.

Dacite with high FeO*, MgO, CaO, TiO2, 

P2O5, Sr, Ba, Eu, Co, Ni, Cr, and low 

Rb, Nb} and Ta.

Unusually low Zr and Na^O relative to 

other study area rhyolite tuffs.

Higher FeO*, TiO2, and Ba and lower Nb 

abundances than other study area rhyolite tuffs.

Low Nb abundances relative to other 

aphyric rhyolite lavas.

Low Nb abundances relative to other 

aphyric rhyolite lavas.

Low-silica rhyolite with high FeO*, CaO, 

Sr, Ba and Eu and low Rb, Ta, Th, and

U abundances.

High K,O, Ba, and Eu and low Zr, Nb, 

and Th abundances relative to other 

study area rhyolite tuffs.

High A1,O3, FeO*, TiO2, P2O5 ,Sr, Zr, Ba, 

Co, Ni, Cr, Sc, Hf, and Eu and low 

SiO2, Ta, U, and Th abundances relative to other study area rhyolite tuffs.

Low FeO*, Zr, Ta, Th, and Hf abundances 

relative to other rhyolite tuffs of the 

study area. Low abundances of K2O, 

Rb, Ba, and Eu distinguish it from Thcl.

Rocks associated with the Turkey Creek caldera

Rhyolite Canyon Tuff 

basal member.

Rhyolite Canyon Tuff 

lower member.

Rhyolite Canyon Tuff 

middle member.

Rhyolite Canyon Tuff 

upper member.

Rhyolite Canyon Tuff 

intracaldera facies.

Rhyolite Canyon Tuff 

lava-flow-like phase.

Dacite porphyry 

lava.

Trcb ND Sanidine and quartz phenocrysts;

7-35 percent crystals.

Trcl 26.95±0.07 Sanidine and quartz phenocrysts;

7-35 percent crystals.

Trcm 27.03±0.11 Sanidine and quartz phenocrysts;

7-35 percent crystals.

Trcu 26.96±0.06 Sanidine and quartz phenocrysts;

7-35 percent crystals.

Trci ND Sanidine and quartz phenocrysts;

7-35 percent crystals. Red-brown 

color. Abundant lithic fragments.

Trcf ND Sanidine and quartz phenocrysts;

7-35 percent crystals. Sanidine 

phenocrysts 0.5 to 1 cm long. 

Lacks pumice and lithic fragments.

Tdpl 26.97±0.13 Feldspar phenocrysts 5 mm to

3 cm long. Granophyric to 

glassy groundmass.

High Zr, Nb, Ta, U, Th, and Hf abundances 

relative to all study area tuffs. High SiO^ 

Rb, Nb, and Ta and low TiO2, Zr, Ba, and 

La abundances relative to Trcu, Trci, and 

Trcf. High K2O and Rb abundances 

relative to Trcl and Trcm.

High Zr, Nb, Ta, U, Th, and Hf abundances 

relative to all study area tuffs. High SiO2 , 

Rb, Nb, and Ta and low TiO2, Zr, Ba, and 

La abundances relative to Trcu, Trci, and 

Trcf.

High Zr, Nb, Ta, U, Th, and Hf abundances 

relative to all study area tuffs. High SiO2 , 

Rb, Nb, and Ta and low TiO2, Zr, Ba, and 

and La abundances relative to Trcu, Trci, 

and Trcf.

High Zr, Nb, Ta, U, Th, and Hf abundances 

relative to all study area tuffs.

High Zr, Nb, Ta, U, Th, and Hf abundances 

relative to all study area tuffs.

High Zr, Nb, Ta, U, Th, and Hf abundances 

relative to all study area tuffs. High K2O 

and Rb abundances relative to Trcu and 

Trci.

High FeO*, MgO, CaO, TiO2, P2O5 , Ba, Co, 

Cr, Ni, Sc, and Eu abundances.
Concluding Remarks 53

Table 4. Diagnostic age, petrographic, and geochemical features of middle Tertiary volcanic rocks of the central Chiricahua 

Mountains.—Continued

[ND, not determined]

Unit Unit 

symbol

Age, 

in Ma

Petrographic 

characteristics

Geochemical 

characteristics

Rocks associated with the Turkey Creek caldera (continued)

Dacite porphyry 

intrusion.

Trachyte porphyry 

lava.

Biotite rhyolite lava 

Unit 1 lava

Unit 2 lava, 

UnitS lava

Tdpi 26.90±0.04 Feldspar phenocrysts 5 mm to

3 cm long. Granophyric 

groundmass.

Ttp ND Sanidine crystals 2 mm to 1 cm

long.

Tmrb 26.74±0.05 Biotite phenocrysts; 5 to 20 percent crystals.

Tmrl 26.77=0.12 Aphyric. Flow banded

Tmr2 ND 

Tmr3 ND

Aphyric. Flow banded

Aphyric. Flow banded. 

Spherulitic.

High FeO*, MgO, CaO, TiO2 , P2O5 , Ba, Co,

Cr, Ni, Sc, and Eu abundances.

High NaA Y, Zr, and Nb and low Sr and 

Ba abundances relative to the dacite 

porphyry.

Low SiO2, Rb, Nb, Ta, U, Th, and Hf, 

and high FeO*, MgO, CaO, TiO2 , Sr, 

Ba, Co, Sc, and Eu abundances. Rb-Sr 

ratio >1 distinct from that of Tfre.

Higher Nb abundances than Tc and 

Tkr, lower Rb and Nb than Trdp. 

Lower SiO2 Nb, Ta, Th and higher 

CaO, Ba, and Eu abundances than 

Tmr2andTmr3.

Higher Nb abundances than Tc and 

Tkr, lower Rb and Nb than Trdp.

Higher Nb abundances than Tc and 

Tkr, lower Rb and Nb than Trdp.

Post-Turkey Creek caldera rocks

Pyroclastic deposits 

and lava flows of 

Swede Peak.

Rhyolite lava of 

Dobson Peak.

Ts

Trdp

ND Quartz, sanidine, albite, and biotite phenocrysts; >20 percent 

crystals. Abundant lithic 

fragments.

26.20±0.07 Nearly aphyric

No diagnostic geochemical features.

Higher Rb and Nb abundances than 

other study area rhyolite lavas.

pertinent mineral-melt distribution coefficients, and crystalliquid fractionation processes, the source reservoir likely had 

an initial composition similar to that of the geochemically 

least evolved (last erupted) part of the outflow sequence, 

and developed more evolved (first erupted) parts. Gradients 

within the reservoir apparently resulted from crystallization 

and fractionation of small amounts of clinopyroxene, sanidine, 

and zircon, the principal phenocrysts characteristic of the 

reservoir's least evolved part. In contrast, sequential evolution of the moat rhyolite lavas from initial, relatively primitive 

rhyolite to final, highly evolved rhyolite appears to have been 

principally controlled by variable contamination of a homogeneous reservoir by relatively primitive lithologic contaminants derived from the reservoir's roof and walls. In addition, 

evolution of the first erupted rhyolite (the biotite rhyolite) to 

the subsequent batch of rhyolite (unit 1 lava) requires fractionation of biotite and sanidine, the principal phenocrysts in the 

biotite rhyolite, to yield a composition like that of unit 1 lava. 

Subsequently erupted, more evolved, moat rhyolite lava (units 

2 and 3 lavas) represents a composition similar to that of unit 

1 lava that contains progressively less contamination derived

from intermediate-composition lava flows and dacite porphyry 

that enclosed the source reservoir.

New 40Ar/39Ar analyses provide accurate and precise 

absolute ages for many volcanic rock units whose ages were 

previously poorly known or unknown. These new ages are 

useful in stratigraphic correlation both within and beyond 

the central Chiricahua Mountains. For example, the new age 

data suggest that the Jesse James Canyon Tuff and the lower 

member of the tuff of Horseshoe Canyon are part of the same 

stratigraphic unit. Geochemical data are entirely consistent 

with this interpretation. The precision and small uncertainties 

associated with the new ages also help constrain the timing 

and nature of the volcanologic events that controlled middle 

Tertiary geologic evolution in this part of the Boot Heel 

volcanic field. Voluminous volcanic deposits, largely 

composed of regionally distributed ash-flow tuffs erupted from 

various caldera sources, were erupted between 34.2 and 26.2 

Ma. A distinctive eruptive hiatus between 33.3 and 28.1 Ma 

indicates that magmatic activity in this region was not 

continuous but was confined to at least two discrete pulses 

separated by an approximately 5.2 m.y. hiatus. Mclntosh
54 Middle Tertiary Volcanic Rocks, Central Chiricahua Mountains, Arizona

and others (1992) defined a hiatus of 3.2 m.y., between 28.9 

and 32.1 Ma, within volcanic rocks of the large MogollonDatil volcanic field, immediately north of the Boot Heel 

field. Mclntosh and Bryan (2000) defined a hiatus of 

5.1 m.y., between 27.6 and 32.7 Ma, for the Boot Heel 

volcanic field itself; they also indicated that the geographic 

evolution of the Boot Heel field was time transgressive, with 

magmatism sweeping from east to west across the field. 

Magmatism associated with the Turkey Creek caldera, the 

largest and best preserved volcanic system in the central 

Chiricahua Mountains, as well as the westernmost and 

youngest major component of the Boot Heel field, seems to 

have been confined to a narrow time window. The oldest 

rocks associated with this system, Rhyolite Canyon Tuff, 

were erupted 26.9 Ma, whereas the youngest, moat rhyolite 

lavas, have ages between 26.7 and 26.8 Ma. Consequently, 

all eruptions from the Turkey Creek caldera occurred in as 

little as 200,000 years. Volumetrically significant middle 

Tertiary volcanic activity seems to have ended about 26.2 

Ma with eruption of the rhyolite lava of Dobson Peak, the 

stratigraphically youngest volcanic unit preserved in the study 

area.

The age and composition of these volcanic rocks help 

to define the large-scale tectonic processes that were active 

along the western margin of North America and that controlled 

the geologic evolution of the Boot Heel volcanic field during 

middle Tertiary time. As suggested by du Bray and Pallister 

(1991) and substantiated herein, the geochemical characteristics of the area's volcanic rocks indicate genesis that changes 

from a subduction or arc-related tectonic regime to a withinplate, extensional regime. The transitional geochemistry of 

these rocks, as well as the periodicity and east-to-west sweep 

of magmatism within the Boot Heel volcanic field during its 

35 to 27 Ma history, may reflect a sequence of events that 

includes (1) subduction-related magmatism between 35 and 

33 Ma, (2) amagmatism related to rapid, low-angle subduction (Coney and Reynolds, 1977) between 33 and 28 Ma, (3) 

renewed magmatism and the beginning of extensional tectonics between about 28 and 27 Ma when subduction diminished 

or ceased and the downgoing Farallon plate began to founder 

and became more steeply inclined, and (4) magmatic re-initiation in response to restoration of an asthenospheric mantle 

wedge beneath this region (Coney and Reynolds, 1977; 

Armstrong and Ward, 1991). We interpret the approximately 5 

m.y. magmatic hiatus in the central Chiricahua Mountains and 

the Boot Heel volcanic field as resulting from a combination 

of shallow subduction and the regional tectonic reorganization 

that began along the west edge of the North American plate 

during this time (Atwater, 1970). The final, brief period of 

magmatism terminated as subduction (and therefore subduction-related magmatism) ended with the initiation of strike-slip 

faulting along western North America (Atwater, 1970). In 

the study area, magmatism ceased as the center of extension-related magmatism rapidly shifted westward to the Great 

Basin, where bimodal basalt-rhyolite magmatism became 

dominant.

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Manuscript approved for publication October 17, 2003 

Published in the Central Region, Denver, Colorado 

Graphics by authors and Springfield and Springfield 

Photocomposition by Gayle M. Dumonceaux 

Edited by L.M. Carter

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