Stratographic relations and source areas of ash-flow sheets of the Black Mountain and Stonewall...

JOURlqAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. B10, PAGES 8593-8602, SEPTEMBER 30, 1984

STRATIGRAPHIC RELATIONS AND SOURCE AREAS OF ASH-FLOW SHEETS OF THE BLACK MOUNTAIN AND STONEWALL MOUNTAIN VOLCANIC CENTERS, NEVADA

Donald C. Noble, 1 Thomas A. Vo•el, 2 Steven I. Weiss, 1 John W. Erwin, 1 Edwin H. McKee, and Leland W. Younker 4

Abstract. Recent work has resulted in major changes in the stratigraphic correlations and source-area assignments of ash-flow sheets of the late Miocene Black Mountain and Stonewall Moun-

tain volcanic centers, southern Nevada. Ash-flow tuffs exposed around Stonewall Mountain have different trace element patterns and thermoremanent magnetization directions than do ash-flow sheets of the Thirsty Canyon Tuff, erupted from the • Black Mountain center, with which they were previously correlated. Hafnium concentrations are particularly useful in distinguishing the mega- scopically and petrographically similar tuffs of the two centers. The ash-flow sheet overlying the Spearhead Member in the Stonewall Mountain area, herein named the Civet Cat Canyon Member, can be traced northward •ontinuously from outflow facies south of Stonewall Mountain to near-vent facies on the southern flank of Stonewall Moun-

tain. This,,in conjunction with distribution, thickness, and facies relations show that these units are the outflow sheets of th e Stonewall Mountain volcanic center; they are herein assigned to a new formation, the Stonewall Flat Tuff of late Miocene age. Tuffs previously mapped near Black Mountain as the Labyrinth Canyon Member (herein abandoned) are correlated on the basis of trace-element, paleomagnetic, petrographic, and radiometric age data with the type Spearhead Member at Goldfield. Tuffs previously assigned to the Spearhead Member in the BlaCk Mountain 9r•a are reassigned to the Pahute Mesa Member Inew) of the Thirsty Canyon Tuff. The Stonewall Mountain volcanic center is thus slightly younger than the Black Mountain center, and the period of time over which the Black Mountain center Was ac-

tive probably was appreciably shorter than was previously inferred. Recognition of the Stone- wall Flat Tuff as a separate stratigraphic and genetic entigy enables us to explain a number of geological inconsistencies, indluding the marked asymmetric distribution previously attributed to the Thirsty'Canyon Tuff. Moreover, the inferred original volume of the Thirsty Canyon Tuff is significantly reduced, increasing the importance of lava relative to tuff in the eruptive history of the Black Mountain center.

1Mackay School of Mines, University of Nevada - Reno.

2Department of Geology, Michigan State University.

3U.S. Geological Survey. 4Earth Science Department, Lawrence Livermore

National Laboratory.

Copyright 1984 by the American Geophysical Union.

Paper number 4B0153. 0148-022 7/84/004B-0153505.00

Introduction

The southern Nevada volcanic field, of middle to late Miocene age, is one of the more intensely studied and better understood in the World.

Work, largely by geologists of the U.S. Geologi- cal Survey, has lead to the identification of more than 20 major ash-flow sheets and at least 8 collapse calderas [e.g., Eckel, 1968; Ekren et al., 1971;.Byers et al., 1976]. This is an ideal area in which to study the evolution of high- level silicic magma system s.

We have focused on the Black Mountain volcanic

center for a detailed g•ological, geochemical and petrological •tudy. This center has four ash- flow sheets of varying composition that are closely associated with a number of units of lava that exhibit large variations in chemical composition. In addition, units of the Black Mountain center are exceptionally well exposed and Pre- served.

The purpose of this paper is to present evidence for new'stratigraphic correlations and source-area assignments for certain ash-flow units of the Black Mountain and Stonewall Moun-

tain volcanic centers (Figure 1). These new interpretations are based on field relations, trace element and paleomagnetic "fingerprinting," and radiometric age data. Some ash flows tha• were previously thought to have erupted from the Black Mountain center are now related to the

slightly younger?Stonewall Mountain center. As a result, the volumes of two of the members of

the Thirsty Canyon Tuff have beenereduced by approximately one-half, and the probable time span over which the Black Mountain volcanic center was

active has been appreciably shortened.

Black Mountain Volcanic Center

The Black Mountain volcanic center was mapped by R. L. Christiansen and D.C. Noble, largely during 1963 [Christiansen and Noble, 1968; Noble and Christiansen, !968]. Summaries of the geology and petrology of the center have been given by Noble et al. [1964], Christiansen and Noble [1965], Noble and Christiansen [1974], Noble and Parker [1974], Christiansen [1979], Vogel et al. [1982; 1983a, b], and Noble et al. [1983].' The center, although probably the smallest of the Southern Nevada volcanic field, has a remarkably complex history. At least four discrete ash-flow sheets of moderate size, each given member status within the Thirsty Canyon Tuff, were erupted, as were a large number of units of intermediate to silicic lava that interfinger with the ash-flow sheets in a complex fashion. Two episodes of caldera collapse can be directly demonstrated; two others are inferred.

,

8593

8594 Noble et al.: Black and Stonewall Mountain Volcanic Centers

117000 ' ,

OLDFIELD

• ..- •o • .. o• CUPRITE ,' • '• • • DISTRICT' •A'.. O • Oe

. .. ........ % .• -.•o• .... • • ; • Stonewall B -• • • % t c = • • Mount Helen

• ', z Uoun•,n • ' •.,

' •_•' •PAHUTE

• ,• Ob;•d,an • P ME

% • B .... • :',.•• AHU•E SA

, • 7 "-.-•o •

•. [. H•p show•n• location o• •[ac• and S•onewa[[ Houn•a•n volcanic cen•e•s and lo-

calities •e•e•ed •o •n •ex•, •ea• so[•d •nd•ca•es •opo•aph•c •m o• •[ac• Houn•a•n caldera, •eav7 dashed and do•ed [•ne a•ound S•one- •a[[ Houn•a•n •nd•ca•es •n•e•ed approximate location of bu•ed •opo•aph•c •m of S•onewa[[ Houn•a•n caldera; so[•d [•ne sou•h o• S•onewa[[

•u•s o• •he C•ve• Ca• Canyon Hembe• (see •he [e•e• "A" •nd•ca•es •7pe [oca[•7 o• S•one- wail •[a• •u• and C•ve• Ca• CanTon Hembe•s; •nd•ca•es •Tpe [oca[•7 of •ahu•e Hesa Hembe• of

Thirsty Canyon Tuff

The evolution of the stratigraphic nomenclature of the Thirsty Canyon Tuff is shown in Table 1. Noble et al. [1964] named and described the Thirsty Canyon Tuff for exposures at its type area northeast of Black Mountain. They subdivid- ed the formation into five formal members --

Spearhead (bottom), Trail Ridge, Dry Lake, Gold Flat, and Labyrinth Canyon Members -- and an in- formal upper member (top). The Dry Lake Member, consisting of non-welded to partly welded ash- flow tuff containing very large blocks of pumice, was not mapped in its type area by Rogers et al. [1968]; its rocks were assigned by them to the lithologically similar Trail Ridge Member because a complete cooling break could not be shown to exist between it and the Trail Ridge. At present it is not clear whether or not the Dry Lake Member is a valid stratigraphic unit and, if it is, whether it predates or postdates the Trail Ridge Member. The demonstration of a complete cooling break between the upper and lower parts of the Spearhead Member later required recognition of a new unit, the Rocket Wash Member of the Thirsty Canyon Tuff, equivalent to the lower part of the Spearhead Member of Noble et al. [1964] [Noble et al., 1968] (Table 1).

Subdivision of the Thirsty Canyon Tuff was based on field relations in the vicinity of Black Mountain [e.g., Ekren et al., 1966; Christiansen and Noble, 1968; Noble and Christiansen, 1969;

Noble et al., 1968]. During mapping of the Northern Nellis Bombing and Gunnery Range in 1962 [Ekren et al., 1971], a sequence of two young, very well exposed, and undeformed ash-flow cooling units were recognized east of Stonewall Moun- tain (Figure 1). These tuffs could be traced continuously to exposures south of Stonewall Mountain•and were correlated on the basis of

lithology with tuffs exposed north of Stonewall Mountain. The lower of the two cooling units was graced continuously to the north to exposures in the vicinity of Goldfield that had been named the Spearhead R•yolite by Ransome [1909]. Subse- quently, the tuffs east of Stonewall Mountain were correlated with ash-flow sheets exposed in the vicinity of Black Mountain. • This correlation was based largely on the remarkable physical and petrographic similarities of the ash-flow units in the two areas, as it was not possible to trace continuously the units between the two areas. For these reasons, the Spearhead and Trail Ridge Members of the Thirsty Canyon Tuff, with sources in the Black Mountain area, were believed to ex- tend well•.•orth of Stonewall Mountain. This distribu•ion pattern is shown by Noble et al. [1964], Ekren et al. [1971], Cornwall [1972] and Albers and Stewart [1972]. The results of radiometric dating of tuffs from the Black Mountain and Goldfield areas [Kistler, 1968; Armstrong et al., 1972] lent support to this interpretation.

Nevertheless, there were several difficulties with the areal extent of the Thirsty Canyon Tuff as reported by Noble et al. [1964]. The distribution of the Spearhead and Trail Ridge Members was very asymmetric. In particular, the Spear- head extended for•'•ore than 100 km northwest of

the Black Mountain caldera, but only 35 km to the south, in spite.•f the fact that at the time of eruption of the Thirsty Canyon Tuff elevations decreased markedly to the south from the Black Mountain area (Figure 1). Moreover, ash-flow units deposited on the west flank of Mount Helen and hills to the north exhibit thickness and dis-

tribution patterns that are strongly suggestive of derivation from the west rather than from the

southeast, and 10+ m of air-fall tuff is present beneath the Spearhead east of Stonewall Mountain, whereas very little air-fall material is present in the vicinity of Black Mountain. In addition, there were problems with facies relations. For example, south of Stonewall Mountain tuffs previously mapped as Trail Ridge Member locally thicken to almost 300 m, contain blocks of lithic material more than a meter in diameter, and include an upper sequence of biotite-bearing ash flows lithologically dissimilar to any of the flows observed in the vicinity of Black Mountain.

Characterization, Discrimination, and Correlation of Ash-Flow Units

Trace Element Fingerprinting

In 1981 and 1982, as part of a systematic re- study of the chemistry of rocks of the Black Mountain volcanic center, we analyzed numerous specimens from outcrops mapped as the Spearhead and Trail Ridge Members. Chemical data were obtained largely by instrumental neutron'activation methods at the Lawrence Livermore National Labor-

atory and by induction-coupled plasma spectro-

Noble et al.: Black and Stonewall Mountain Volcanic Centers 8595

TABLE 1. Evolution of Stratigraphic Nomenclature of the Thirsty Canyon and Stonewall Flat Tuffs

Noble et al. [1964] Noble et al. [1968] Rogers et al. [1968]

This Paper

Thirsty Canyon Tuff

Upper member

Labyrinth Canyon Member

Gold Flat Member

Dry Lake Member

Trail Ridge Member

Spearhead Member

upper part

lower part

Thirsty Canyon Tuff

Upper member

Labyrinth Canyon Member

Gold Flat Member

Trail Ridge Member

Spearhead Member

Rocket Wash Member

Stonewall Mountain Volcanic Center

Stonewall Flat Tuff*

Civet Cat Canyon Member*

Spearhead Memberñ

Black Mountain Volcanic Center

Thirsty Canyon Tuff

Upper member

Gold Flat Member

Trail Ridge Member

Pahute Mesa Member*

Rocket Wash Member

*Indicates new stratigraphic name. tThe Spearhead Member is also Present in the Black Mountain area, where it was formerly map-

ped as the Labyrinth Canyon Member of the Thirsty Canyon Tuff (see text).

metry by Barringer Magenta, Ltda. [Vogel et al., plot there is some indication that both the tuffs 1984]. The specimens analyzed are largely indi- from Black Mountain and those from Stonewall vidual glassy •pumice blocks taken from a number Mountain form continuous trends. Hafnium is par- of outcrops, but include some glassy and devitri- ticularly useful in distinguishing rocks of the field whole-rock specimens. On simple inspection, two volcanic centers. Not only are Hf concentra- it was apparent that the resultant trace element data comprised two populations corresponding to tuffs from the general vicinities of Black Moun- tain and Stonewall Mountain• respectively. Tuffs from units in the vicinity of Black Mountain (that are herein retained in the Thirsty Canyon Tuff) have generally higher concentrations of such elements as Hf, Th, Rb, Be, and REE than do tuffs exposed in the vicinity of Stonewall Moun- tain and Goldfield (Figures 2-5). Tuffs previously mapped as the Labyrinth Canyon Member of the Thirsty•CanY0 n Tuff near Black Mountain have trace element contents statistically indistin- guishable from those of the Spearhead exposed in the vicinity of its type locality near Goldfield and in the general area of Stonewall Mountain.

The geochemical variations of Th, Hf, and Rb are especially diagnostic when the tuffs from the two volcanic centers are compared. As shown in Figure 2, there is no overlap of the tuffs from Stonewall Mountain (Spearhead and Civet Cat Canyon Members) and Black Mountain (Pahute Mesa

tions markedly different for the tuffs of the two centers, but they are also similar for the two ash-flow sheets of each center, irrespective of whether the tuffs are subalkaline or slightly peralkaline (see below). Moreover, Hf contents show little or no change with degree of differen- tiation within the individually compositionally variable units.

On plots of Th versus Rb (Figure 3), the tuffs from Black Mountain and Stonewall Mountain form

two distinct trends. Along with the differences in Hf content noted above, this would lend cre- dence to an interpretation in which the magmas of the two centers are genetically distinct•from each other, with the initial chemical differences

representing differences in source materials and (or) magma generation and evolution.

Paleomagnetic Measurements

Thermoremanant magnetization (TRM) directions for specimens from each of the units in question

and TrailRidge Members). Tuffs from each center were determined to corroborate the revised stra- have a limited range of Hf concentrations, with tigraphic interpretations. Data were obtained on those fr• TM the Stonewall center having the lower a computerized Schoenstadt fluxgate magnetometer values. •horium concentrations of rocks of both at the Mackay School of Mines by using standard centers phow a large range of values, with the progressive alternating-current demagnetization tuffs from the Stonewall center having lower techniques supplemented by thermal demagnetiza- values as well as a more limited range. On this tion procedures [Weiss, 1984].


6O 6o

5o

4o

lO

i,,,, /

-- v \

0 a Vl

W /

/ \

/o / '-'

0 ' ' 2'0 30 40 0 1'0 Hf(ppm)

Fig. 2. Contrast of ,Th and Hf abundances for tuffs of the Black Mount&in and Stonewall Moun- tain volcanic cente•rs. On this and subsequent diagrams open circles and squares represent rocks of the Spearhead and Civet Cat Canyon Members, respectively, of thee Stonewall Flat Tuff, and closed circles indicJ•e specimens from an exposure of the Spearhead Member within the Black Mountain caldera previously mapped as Labyrinth Canyon Member of the Thirsty Canyon Tuff. Open triangles and diamonds represent specimens of the Pahute Mesa and Trail Ridge Members, respectively, of the Thirsty Canyon Tuff. Inverted open triangles represent pumice blocks from exposures of a unit that overlies the Pahute Mesa Member

and is overi.ain by the Gold Flat Member south of Mount Helen that was mapped as Dry Lake Member of the Thirsty Cany, on Tuff by Noble et al. [1964] and as Trail Ridge Member by Rogers et al. [1968]. DAshed lines outline fields defined by specimens from the two centers.

5o

4o

A AA A A

O0 i i i i 1 O0 200 300 400

Rb(pprn)

Fig. 3. Contrast of Th and Rb abundances for tuffs of the Black Mountain and Stonewall Moun- tain volcanic centers.

500

Stratigraphic Changes in the Thirsty Canyon Tuff

The new information requires several changes in the stratigraphic nomenclature of the Thirsty Canyon Tuff. The ash-flow sheet previously mmpped as the Spearhead Member in the Black Moun- tain area by Christiansen and Noble [1968] and Noble and Christiansen [1968] and mapped as the upper part of the Spearhead Member by Ekren et al. [1966] is here renamed the Pahute Mesa Member for exposures on Pahute Mesa and retained in the Thirsty Canyon Tuff. A type locality is desig- nated at hill 5431 (latitude 37ø07'45"N, longitude 116ø32'35"W) directly south of Rocket Wash (Figure 1), where the unit overlies the Rocket

The results are summarized in Table 2. The unusual shallow south-directed reverse TRM re-

,

ported by Nd•le et al. [1968] for tuffs mmpped as Spearhead Member in the vicinity of Black Mountain was confirmed, and a very similar direction was found for the overlying Trail Ridge Member. The Spearhead Tuff exposed in the vicinity of Gold- field and Stonewall Mountain possesses a steep, north-directed normal TRMvector and the overlying ash-flow sheet (Civet Cat Canyon Member) a steeply inclined reverse TRM direction. Tuffs mapped as the Labyrinth Canyon Member possess the same TRM direction as t•e type spearhead.

In summJr•, the TRM data corroborate our con- clusions that the ash-flow tuffs exposed in the

2 • , ' ' 0.5 1 2 5 10

Sc (ppm)

vicinity of Stonewall Mountain are different from Fig. 4. Contrast of Yb and Sc abundances for those near Black Mountain. tuffs of the Spearhead and Pahute Mesa Members.


20 ,

2 i i i i 50 '1 O0 200 500

La(ppm)

Fig. 5. Contrast of Be and La abundances for tuffs of the Spearhead and Pahute Mesa Members.

Wash Member [Noble et al., 1968]. At this locality the Pahute Mesa Member is about 70 m thick and consists of three depositional units [Noble et al., 1968]. The lower unit consists of nonwelded to slightly welded high-silica rhyolite tuff containing a homogeneous population of pumice fragments as much as 10 cm in diameter whereas the upper two units consist of partly to densely welded tuff containing pumice fragments similar to those in the lower unit and blocks of

non-vesiculated to slightly vesiculated low-

silica rhyolite as much as 50 cm in diameter. Noble et al. [1968] give more detailed descrip- tions and locations for other representative sections in the area south of Black Mountain.

The term Spearhead Member is properly applied to the ash-flow sheet to which the type Spearhead [Ransome, 1909] belongs, which is herein reassigned to a new formation, the Stonewall Flat Tuff.

The name Labyrinth Canyon is here abandoned because tuffs mapped as Labyrinth Canyon Member of the Thirsty Canyon Tuff near Black Mountain are identified as being distal portions of the Spearhead Member.

Stonewall Flat Tuff

As discussed above, the distribution, facies relations, trace element abundances and paleomagnetic signatures of the two ash-flow sheets exposed in the vicinity of Stonewall Mountain and Goldfield show that these units do not belong to the same volcanic center as the Thirsty Canyon Tuff.

The name Stonewall Flat Tuff is thus here applied to tuffs of the Stonewall Mountain volcanic center (Figure 1) that previously have been mapped by Ekren et al. [1971] as the Spearhead and Trail Ridge Members of the Thirsty Canyon Tuff. The type locality is at latitude 37ø32'39" N, longitude 116ø56'29"W in Pack Rat Canyon near the southeastern margin of Stonewall Flat

TABLE 2. Summary of Paleomagnetic Data for Ash-Flow Units of the Stonewall Mountain and Black Mountain Volcanic Centers

Stratigraphic Unit Number of Mean TRM* • 95 Sites

D I

Stonewall Flat Tuff

Civet Cat Canyon Member 11 206.3 ø -49.0 ø 4.2 •

Spearhead Member 5 12.9 • +58.0 • 2.9 •

Spearhead Memberñ 2 357.5 ø +63.7 • 7.3 •

Thirsty Canyon Tuff

Trail Ridge Member 5 177.7 • -12.9 • 5.4 ø

Unnamed unitõ 2 172.8 • -12.5 • 5.8 •

Pahute Mesa Member 5 178.5 • -7.1 ø 7.0 •

*D is declination, measured clockwise from north; I is inclination, measured from the horizontal. Positive sign indicates projection on the lower hemisphere (normal polarity); negative sign indicates projection on upper hemisphere (reversed polarity).

ñTuffs within the Black Mountain caldera mapped as Labyrinth Canyon Member of the Thirsty Canyon Tuff by Noble and Christiansen [1968] and Christiansen and Noble [1968].

õGlassy ash-flow tuffs south of Mount Helen mapped as Dry Lake Member of the Thirsty Canyon Tuff by Noble et al. [1964] and Trail Ridge Member by Rogers et al. [1968]. These tuffs, which overlie tuffs of the Pahute Mesa Member and underlie the Gold Flat Member of the Thirsty Canyon Tuff, may belong either to the Pahute Mesa Member or to the Trail Ridge Member, or may constitute a separate unit of the Thirsty Canyon Tuff.


TABLE 3. New Potassium-Argon Age Determinations on Specimens of the Spearhead Member of the Stonewall Flat Tuff

Specimen K20, 4OAr*, 4OAr*, % Age, Ma Number - 11

wt. % kmo/kg x 10

1' 6.58 6.0148 45.5 6.3 + 0.2

2ñ 6.66 6.0176 40.7 6.3 _+ 0.2

Analyses made in laboratories of the U.S. Geological Survey, Menlo Park, California, under the supervision of E. H. McKee.

Constants: _•e = 0.581x 10 -10 yr -1 l B = 4 962x 10 -10 yr -1 40K/K(total) = 1.167 x 10 kmo/kmo.

*Sanidine phenocryst separate from glassy pumice fragments from ash- flow tuffs within the Black Mountain caldera (latitude 37ø17'42•VN, longitude 116ø40'24"W) previously mmpped as the Labyrinth Canyon Member (herein abandoned) of the Thirsty Canyon Tuff. Field Number LABCAN081.

ñSanidine phenocryst separate from partially welded, glassy ash-flow tuff from the lower part of the Spearhead Member south of Goldfield, Nevada (latitude 37•40'14"N, longitude 117•13'14"W). Field Number 83G-SPEAR.

(Figure 1), for which the formation takes its name. The Stonewall Flat Tuff is also well ex-

posed and readily accessible between the Cuprite district and Goldfield and at various localities

southwest of Stonewall Mountain (Figure 1). The Stonewall Flat Tuff consists of two mem-

bers: the Spearhead Member (reassigned from the Thirsty Canyon Tuff) and an overlying member, herein named the Civet Cat Canyon Member. Each member consists of an ash-flow sheet composed of a number of ash flows and includes the associated

underlying air-fall deposits. Both members are

phenocrysts, largely sodic sanidine with minor amounts of quartz and sodic plagioclase and traces of iron-rich clinopyroxene, fayalite, sodic amphibole, and Fe-Ti oxides. The tuffs are slightly peralkaline and very highly evolved, with Eu/Eu* of less than 0.01 and extremely low concentrations of Sr, Mg, Ba, Co, and Ni [Noble and Parker, 1974; Noble et al., 1979].

Civet Cat Canyon Member

The Civet Cat Canyon Member, here named for well exposed at the type locality. The Stonewall Civet Cat Canyon, is well exposed between Civet Flat Tuff is considered to be of late Miocene age Cat Canyon and Pack Rat Canyon (Figure 1), where on the basis of two new potassium-argon age de- it overlies the Spearhead Member and caps the low terminations of 6.3 + 0.2 Mm on phenocrystic sani- mesa at the southeastern end of Stonewall Flat. dine from tuffs of the Spearhead Member (Table 3), one from an exposure near Goldfield, and one from the type locality of the herein abandoned Labyrinth Canyon Member. These determinations agree well with the age of 6.3 Ma reported by Kistler [1968] (recalculated by using new constants) for the Labyrinth Canyon Member.

Spearhead Member

As defined by Ransome [1909], the type locality of the Spearhead Member is at Spearhead Point west of Goldfield. The member consists of flows

of rhyolitic ash-flow tuff. In most areas the unit is moderately to densely welded, but in the vicinity of Civet Cat Canyon (Figure 1) one or more flows of slightly welded to nonwelded tuff are present at the top of the unit. In addition, southwest of Stonewall Mountain the unit in many places is composed largely or entirely of nonwelded to slightly welded tuff. Where moderately to densely welded the unit typically is red, brownish red, brown, or gray. The member consists of rhyolitic tuff containing 5% to 8%

The type locality is in Pack Rat Canyon at latitude 37•32'39"N, longitude 116•56'28"W. At the type locality the member is approximately 25 m thick; directly south of Stonewall Mountain it reaches a maximum thickness of about 300 m. A

much greater thickness is probably present locally on Stonewall Mountain. Original areal extent, centered on Stonewall Mountain, is at least 2000 km'. Everywhere the member overlies the Spear- head Member and is the youngest rock unit present.

The Civet Cat Canyon Member consists of at least four distinct depositional units that cool- ed and crystallized together to form a single cooling unit. The tuffs contain from less than 2% to more than 25% phenocrysts, mainly sanidine- anorthoclase, with lesser amounts of biotite, clinopyroxene, Fe-Ti oxides, and olivine, the latter now pseudomorphed by aggregates of iron- rich (?) phyllosilicates. In many places the tuffs have undergone granophyric crystallization and at most localities granophyrically crystallized and devitrified tuffs are red to reddish

brown, reflecting the presence of fine-grained


hematite in the groundmass produced by oxida- tion attendant on the loss of hydrogen gas during cooling and crystallization.

Rocks of the Civet Cat Canyon Member range in composition from high-silica rhyolite to trachyte, but largely consist of rhyolite containing about 10% phenocrysts, which is less evolved chemically that that of the Spearhead Member. The local presence of groundmass acmite and (or) sodic amphibole indicates a slightly peralkaline chemistry.

Southwest of Stonewall Mountain the member

contains abundant lithic fragments that locally exceed a meter in diameter. A distinctive unit

of shard tuff containing virtually no lithic material and from less than one to several per- cent phenocrysts overlies the main body of the unit southwest of Stonewall Mountain. An upper sequence of flows, at present recognized only south of Stonewall Mountain, contains about 10% to 20% phenocrysts, consisting mainly of alkali feldspar with cores of anorthoclase and sodic plagioclase, clinopyroxene, Fe-Ti oxides, and abundant biotite. Welded pyroclastic rocks of trachytic composition are present on the south flank of Stonewall Mountain.

No radiometric ages are presently available for the Civet Cat Canyon Member.

Source of the Stonewall Flat Tuff

Several lines of evidence indicate that the

two ash-flow sheets of the Stonewall Flat Tuff

are outflow sheets of the Stonewall Flat volcanic center.

As described by Foley [1978], Stonewall Moun- tain is composed of a very thick sequence of welded tuffs that are commonly very lithic rich and locally have undergone marked flowage and (or) deformation. These tuffs are intruded by irregular bodies of intermediate to silicic composition and in many localities dip outward from the center of Stonewall Mountain [Foley, 1978; Weiss, 1984]. The general aspect is that of the resurgent dome of a collapse caldera. Although the Stonewall Mountain volcanic center was originally considered to be older than many or all of the centers of the southern Nevada volcanic field

[Ekren, 1968], tuffs and lavas from Stonewall Mountain have yielded K-Ar ages [Foley, 1978] slightly older than, but probably within the limits of analytical uncertainty of, the ages reported here for the Spearhead Member.

The distribution of the ash-flow sheets of the

Stonewall Flat Tuff is symmetrical with respect to Stonewall Mountain. Moreover, directly south of Stonewall Mountain, the Spearhead Member is cut by a scalloped, north-facing scarp (Figure 1) -- interpreted as the topographic rim of the caldera that formed on eruption of the Spearhead Member -- that in turn is overlain, with consid- erable compaction dip, by tuffs of the Civet Cat Canyon Member [Noble et al., 1983; Weiss, 1984]. North of this scarp the Civet Cat Canyon Member abruptly thickens to almost 300 m and can be traced continuosuly northward onto the southern flank of Stonewall Mountain [Noble et al., 1983], where it incorporates more mafic near-vent units within its upper part [Weiss, 1984]. Finally, the Civet Cat Canyon Member locally contains very coarse lithic fragments in exposures directly southwest of Stonewall Mountain.

Distribution of Thirsty Canyon and Stonewall Flat Tuffs

As discussed above, the Thirsty Canyon and Stonewall Flat Tuffs are distributed approximately symmetrically around Black Mountain and Stone- wall Mountain, respectively. However, with the exception of the Spearhead Member within and north of the Black Mountain caldera [Noble and Christiansen, 1968], the two formations are not in contact. This reflects in part the presence of topographic highs -- Mount Helen, Obsidian Butte, and hills and ridges in the vicinity of the Pahute triangulation station (Figure 1) -- that acted as barriers restricting movement of ash flows of the Thirsty Canyon Tuff to the north and northwest and of ash flows of the Stonewall

Flat Tuff to the south, southeast, and east.

Discussion

On the basis of geochemistry, field studies, paleomagnetic measurements, and K-Ar ages, we have revised our interpretations of the distribution, relative ages, and source areas for certain of the young ash-flow units of the southern Nevada volcanic field. These revisions modify our understanding of the volume of the eruptive products and the nature and timing of volcanic activity of the Black Mountain and Stonewall Mountain centers.

The estimated original volumes inferred for the two ash-flow sheets of the Thirsty Canyon Tuff, the Pahute Mesa, and Trail Ridge Members, must be reduced by about 50%. Detailed recalcu- lations have not been made, but the estimates of Christiansen [1979] for combined Spearhead plus Pahute Mesa and Trail Ridge plus Civet Cat Canyon Members lead to estimates of about 50 to 75 and

25 km 3 for the Pahute Mesa and Trail Ridge Mem- bers, respectively. These revised volumes are petrogenetically important because there appears to be a relation between the volume of ash-flow

eruptions and the amount of drawdown of mmgma within the chamber such that, in general, smaller ash-flow eruptions will include a greater propor- tion of the magma that is present in the chamber [Smith, 1979]. Furthermore, with the reduced volumes of the two members, the lavas, which are voluminous and exhibit a wider range in chemical composition than do the tuffs [cf. Noble and Christiansen, 1974; Vogel et al., 1983a], become relatively more important in understanding the chemical evolution of the Black Mountain center.

The length of time for the evolution of the Black Mountain center was originally inferred to be on the order of 1.5 Ma, on the basis of ages obtained on the Rocket Wash and Labyrinth Canyon Members by Kistler [1968]. With the reassignment of rocks previously mapped as Labyrinth Canyon Member to the Stonewall Flat Tuff, this time span is probably considerably less. One further important observation is that, on the basis of their nearly identical anommlous TRM directions, the Pahute Mesa and Trail Ridge Members were ap- parently erupted during a small portion of the same magnetic polarity transition [Weiss and Noble, 1983]. If so, this means that the two ash-flow sheets were erupted within a period of less than about 5,000 years [Fuller et al., 1979], and perhaps within a period of less than


100 years [Weiss and Noble, 1983]. This provides a severe time constraint on models for magma evolution needed to explain the chemical and miner- alogical variations observed within the two units [Vogel et al., 1983b].

Relating the Spearhead Member to the Stonewall Mountain volcanic center and recognition of the peralkaline character of the Civet Cat Canyon Member adds a fourth center that has produced highly evolved, slightly to strongly peralkaline silicic magmas during middle and late Miocene time in the southern Great Basin. As with the

other centers [e.g., Noble, 1968], the Stonewall Mountain center probably erupted both subalkaline and peralkaline magmas during its period of activity. Moreover, the new stratigraphic relations in conjunction with additional petrographic work have resulted in changes in the chemical classification of two of the ash-flow sheets of

the Thirsty Canyon Tuff. Previous classification of the Spearhead Member as in part slightly peralkaline (comenditic) was based on analyses of tuffs from the Goldfield area [Noble and Parker, 1974]. However, thin-section study has yet to reveal either sodic amphibole or sodic pyroxene in vapor-phase crystallized or devitrified tuffs of the Pahute Mesa Member, and the unit is now

provisionally interpreted as consisting entirely of subalkaline rhyolite. Convernsely, new petrographic work on the Trail Ridge Member has re- vealed the presence of both groundmass and vapor- phase sodic amphibole and sodic pyroxene in several specimens. As a result, we now recognize a progressive change from subalkaline rhyolite (Rocket Wash and Pahute Mesa Members) to progres- sively more peralkaline compositions (Trail Ridge and Gold Flat Members) in successive ash-flow sheets of the Thirsty Canyon Tuff. Complementary chemical changes appear to exist in the associated lavas of the center [Noble and Parker, 1974; Vogel et al., 1983a]. Similar patterns of chemical evolution are present at the Silent Canyon and Kane Springs Wash centers [Noble et al., 1968; Noble, 1968; D.C. Noble, unpublished data, 1964-1971; Novak, this issue]. Finally, with the removal of the Labyrinth Canyon Member from the

Thirs•g Canyon Tuff, the range of initial 87Sr/8 Sr ratios determined for rocks of the Black Mountain center [Noble and Christiansen, 1974] is reduced from about 0.004 to about 0.002.

Both north and south of Black Mountain the

Thirsty Canyon Tuff is overlain by thin flows of basalt. These basalts previously have been provisionally considered to be related to Black Mountain magmatism [e.g., Noble and Christiansen, 1974; Noble and Parker, 1974]. However, north of Black Mountain these basalts overlie tuffs of the

Spearhead Member of the Stonewall Flat Tuff [Noble and Christiansen, 1968]. Some, and perhaps all, of the basalts therefore probably significantly postdate activity at the Black Moun- tain center.

The revised distribution of ash flows from the

Black Mountain center modifies our understanding of the nature of pyroclastic eruption and em- placement for this system. In general, the ash flows of the Rocket Wash, Pahute Mesa, and Gold Flat Members were not particularly mobile. With certain exceptions, the pyroclastic flows of the Pahute Mesa and Rocket Wash Members did not trav-

el great distances, and the flows appear to have

been controlled appreciably by preexisting topographic features. The many flows that comprise the Gold Flat Member were strongly channeled to the north and south of Black Mountain by the low north and south parts of the topographic rim of the Black Mountain caldera [Noble et al., 1964; Christiansen and Noble, 1968; Noble and Chris- tiansen, 1968].

However, a number of features can be used to infer a more energetic eruptive style for the Trail Ridge Member. The Trail Ridge Member is the only member of the Thirsty Canyon Tuff that possesses a significant volume of plinian air fall at its base, and near the Black Mountain caldera the air fall contains large blocks of lithic material. The distribution and thickness

of the Trail Ridge Members appears to have been less influenced than the other members by preexisting topography. Moreover, finely layered welded tuff possessing bedding features charac- teristic of surge deposits mantles the wall of the Black Mountain caldera and welded tuffs of

possible co-ignimbrite ash-fall origin are locally present beyond the caldera margin [Noble, 1974; D.C. Noble, unpublished data, 1963, 1964, 1982, 1983]. Finally, field evidence for many thin depositional pulses is present, particularly north and northeast of Black Mountain.

In summary, the revised stratigraphic picture of the Black Mountain and Stonewall Mountain vol-

canic centers, on the basis of geochemical and paleomagnetic signatures and field relations, provides better geological constraints on the latest major pulses of silicic volcanic activity in the southern Great Basin. The new data for

magma volume, duration, and energetics of volcanic activity, when coupled with the geochemical characteristics of the tuffs and lavas [Vogel et al., 1983a, b, 1984] will provide a basis for understanding the magmatic evolution of these volcanic centers.

Acknowledgments. This study has drawn on the accomplishments of a number of individuals and in particular E. B. Ekren, who carried out, coordin- ated, and (or) inspired much of the geologic mapping of the Northern Nellis Bombing and Gunnery Range and the Pahute Mesa area. The field and geochemical aspects of this study were largely supported by the Containment and Basic Energy Sciences sections of the Lawrence Livermore

National Laboratory and to a lesser extent by research grants from the Geological Society of America and the National Science Foundation. We

particularly appreciate the enthusiastic support of Harley W. Dickensheets, Chief, Project Manage- ment Branch, 554RG/DOX, without whose personal commitment of time and effort field work on the

Northern Nellis Bombing and Gunnery Range would have been impossible. W. C. Carr and R. B. Scott made helpful comments on an early version of the manuscript.

References

Albers, J. P., and J. H. Stewart, Geology and mineral deposits of Esmeralda County, Nevada, Bull. Nev. Bur. Mines Geol., 78, 80 pp., 1982.

Armstrong, R. L., H. Dick, and C. J. Vitalñano, K-Ar dates and strontium isotope initial ratios of some Cenozoic volcanic rocks from west-


central Nevada, Isochron West, •, 23-28 1972. Byers, F. M., Jr., W. J. Carr, P. P. Orkild,

W. D. Qdinlivan, and K. A. Sargent, Volcanic suites and related cauldrons of Timber Mountain

- Oasis Valley caldera complex, southern Nevada, U.S. Geol. Surv. Prof. Pap., 919, 70 pp., 1976.

Christiansen, R. L., Cooling units and composite sheets in relation to caldera structure, in

Ash-Flow Tuffs, edited by C. E. Chapin and W. E. Elston, pp. 29-42, Spec. Pap. Geol. Soc. Am., 180, 211 pp., 1979.

Christiansen, R. L., and D.C. Noble, Black Moun- tain volcanism in southern Nevada (abstract),

Spec. Pap. Geol. Soc. Am., 82, 246, 1965. Christiansen, R. L., and D.C. Noble, Geologic

map of the Trail Ridge quadrangle, Nye County, Nevada, U.S. Geol. Surv. Geol. Quad. Map, GQ-774, 1968.

Cornwall, H. R., Geology and mineral deposits of southern Nye County, Nevada, Bull. Nev. Bur. Mines Geol., 77, 49 pp., 1972.

Eckel, E. B. (Ed.), Nevada Test Site, Mem. Geol. Soc. Am., 110, 290 pp., 1968.

Ekren, E. B., Geologic setting of Nevada Test Site and Nellis Air Force Range, in Nevada Test Site, edited by E. B. Eckel, p. 11-19, Mem. Geol. Soc. Am., 110, 290 pp., 1968.

Ekren, E. B., R. E. Anderson, P. P. Orkild, and E. N. Hinrichs, Geologic map of the Silent Butte quadrangle, Nye County, Nevada, U.S. Geol. Surv. Geol. Quad. Map, GQ-493, 1966.

Ekren, E. B., R. E. Anderson, C. L. Rogers, and D.C. Noble, Geology of Northern Nellis Air Force Base Bombing and Gunnery Range, Nevada, U.S. Geol. Surv. Prof. Pap., 651, 91 pp., 1971.

Foley, D., The geology of the Stonewall Mountain volcanic center, Nye County, Nevada, Ph.D. dissertation, 139 pp., Ohio State University, Columbus, 1978.

Fuller, M., I. Williams, and R. A. Hoffman, Paleomagnetic records of geomagmetic reversals and the morphology of transitional fields, Rev. Geoph¾s. Space Phys., 17, 179-203, 1979.

Kistler, R. W., Potassium-argon ages of volcanic rocks in Nye and Esmeralda Counties, Nevada, in Nevada Test Site, edited by E. B. Eckel, pp. 251-261, Mem. Geol. Soc. Am., 110, 290 pp., 1968.

Noble, D.C., Kane Springs Wash volcanic center, Lincoln County, Nevada, in Nevada Test Site, edited by E. B. Eckel, pp. 109-116, Mem. Geol. Soc. Am., 110, 290 pp., 1968.

Noble, D.C., Road log a•d trip guide Black Moun- tain volcanic center, in Guidebook to the geology of four Tertiary volcanic centers in central Nevada, p. 21-26, Rep. Nev; Bur. Mines Geol., 19, 72 pp., 1974.

Noble, D.C., and R. L. Christiansen, Geologic map of the southwest quarter of the Black Moun- tain quadrangle, Nye County, Nevada, U.S. Geol. Surv. Misc. Geol. Inv. Map, 1-562, 1968.

Noble, D.C., and R. L. Christiansen, Black Moun- tain volcanic center, in Guidebook to the geology of four Tertiary volcanic centers in central Nevada, pp. 27-34, Rept. Nev. Bur. Mines Geol., 19, 72 pp., 1974.

Noble, D.C., and D. F. Parker, Peralkaline silicic volcanic rocks of the western United

States, Bull. Volc., 38, 803-827, 1974.

Noble, D.C., R. E. Anderson, E. B. Ekren, and J. T. O'Connor, Thirsty Canyon Tuff of Nye and Esmeralda Counties, Nevada, U.S. Geol. Surv. Prof. Pap., 475-D, D24-D27, 1964.

Noble, D.C., G. D. Bath, R. L. Christiansen, and P.O. Orkild, Zonal relations and paleo- magnetism of the Spearhead and Rocket Wash Members of the Thirsty Canyon Tuff, southern Nevada, U.S. Geol. Surv. Prof. Pap., 600-C, 1968.

Noble, D.C., K. A. Sargent, H. H. Mehnert, E. B. Ekren, and F. M. Byers, Jr., Silent Can- yon volcanic center, Nye County, Nevada, in Nevada Test Site, edited by E. B. Eckel, pp. 65-75, Mem. Geol. Soc. Am., 110, 290 pp., 1968.

Noble, D.C., W. Rigot, and H. R. Bowman, Geo- chemistry of rare-earth elements in silicic glasses from some highly differentiated ash- flow tuffs and lavas, in Ash-Flow Tuffs, edited by C. E. Chapin and W. E. Elston, pp. 77- 85, Spec. Pap. Geol. Soc. Am., 180, 211 pp., 1979.

Noble, D.C., T. A. Vogel, S. I. Weiss, and J. W. Erwin, Integrated field, trace-element, and paleomagnetic determination of the stratigraphic relations, distribution, and source areas of ash-flow sheets of the Black Mtn. and Stonewall

Mtn. volcanic centers, Nevada (abstract), Eos Trans. AGU, 64, 336, 1983.

Novak, S. W., Eruptive history of peralkaline and metaluminous rhyolites, and related trachyte lavas of the Kane Spring Wash volcanic center, Nevada, J. Geophys, Res., 89, this issue.

O'Connor, J. T., R. E. Anderson, and P. W. Lipman, Geologic map of the Thirsty Canyon Quadrangle, Nye County, Nevada, U.S. Geol. Surv. Geol. Quad. Map, GQ-524, 1966.

Ransome, F. L., The geology and ore deposits of Goldfield, Nevada, U.S. Geol. Surv. Prof. Pap., 66, 1909.

Rogers, C. L., E. B. Ekren, D.C. Noble, and J. E. Weir, Geologic map of the northern half of the Black Mountain quadrangle, Nye County, Nevada, U.S. Geol. Surv. Misc. Geol. Inv.

Map, 1-545, 1968. Smith, R. L., Ash-flow magmatism, in Ash-Flow

Tuffs, edited by C. E. Chapin and W. E. Elston, pp. 5-27, Spec. Pap. Geol. Soc. Am., 180, 211 pp., 1979.

Vogel, T. A., D.C. Noble, L. W. Younker, K. G. Knauss, and F. J. Ryersom, The Black Mountain volcanic center: Preliminary report on eruptive history and magmatic evolution (abstract), Geol. Soc. Am. Abstr. Programs, 14, 638, 1982.

Vogel, T. A., D.C. Noble, and L. W. Younker, Chemical evolution of a high-level magma system: The Black Mtn. volcanic center, southern Nevada (abstract)• Eos Trans. AGU, 64, 336, 1983.

Vogel, T. A., L. W. Younker, and D.C. Noble, Contrasting patterns of chemical evolution of two closely related ash-flow sheets (abstract), Eos Trans. AGU, 64, 884, 1983.

Vogel, T. A., D.C. Noble, and L. W. Younker, Chemical evolution of a high level magma system: The Black Mountain volcanic center,

southern Nevada, Univ. of Calif. Rep., Lawrence Livermore Natl. Lab., in press, 1984.

Weiss, S. I., Geologic and paleomagnetic studies of the Stonewall Mountain and Black Mountain


volcanic centers, southern Nevada, M.S. thesis, Univ. of Nevada - Reno, Reno, 1984.

Weiss, S. I., and D.C. Noble, Paleomagnetic constraints on the rate and duration of magmatic evolution of the•Black Mountain volcanic cen-

ter, southern Nevada (abstract), Eos Trans. AGU, 64, 884, 1983.

J. W. Erwin, D. C. Noble, and S. I. Weiss, Mackay School of Mines, University of Nevada- Reno, Reno, NV 89557.

E. H. McKee, U.S. Geological Survey, 345

Middlefield Road, Menlo Park, CA 94025. T. A. Vogel, Department of Geology, Michigan

State University, East Lansing, MI 48824. L. W. Younker, Earth Science Department,

Lawrence Livermore National Laboratory, Liver- more, CA 94550.

(Received August 15, 1983; revised January 3, 1984;

accepted January 19, 1984.)

Stratographic relations and source areas of ash-flow sheets of the Black Mountain and Stonewall...

Documents

Transcript of Stratographic relations and source areas of ash-flow sheets of the Black Mountain and Stonewall...