‘LA.UR -83-2229 DE93 015217 TITLE:GEOCHEMICAL .../67531/metadc695313/...‘LA.UR -83-2229 LA-UR--83-2229 DE93 015217 TITLE:GEOCHEMICAL SIMILARITIES BETWEEN VOLCANIC UN!TS AT YUCCA

‘LA.UR -83-2229

LA-UR--83-2229

DE93 015217

TITLE:GEOCHEMICAL SIMILARITIES BETWEEN VOLCANIC UN!TS AT YUCCAMOUNTAIN AND PAHUTE MESA: EVIDENCE FOR A COMMON MAGMATICORIGIN FOR VOLCANIC SEQUENCES THAT FLANK THE TIMBER MOUNTAINCALDERA

WTHOIW: R. G. Warren, ES..

SUBMITTEDTO: 2nd Containment Symposium2-4 August 1983Kirtland AFBAlbuquerque, NM

DISCLAIMER

This report WMS prcpnrcd us ●r uccount of work qmrwwrod by an ngency of drc Unital S{ma(krvcrhmen!. Ncilhor Ihc iJnllcd SInlU Govcrnmont nor wry ●gency thereof, nor wry of theircmploycm, mukos WY wnrrwrly, cxprem or implied, or nmumob nny Iegd Iinhillty or rOSlXVVMl-billiy for the mxxrncy, complctonec:, or um[ulncm of wry informslhn, ~pparmrm, produoI, orproccu dinclrwcd, or reprcaents thnt ho UM would rml Infringe privnmly ownod rishtm. Refer.encctracin mmy npocific~~rciml PWIUCL p~,~r -i=hy trndc n-lr~em~rklmmwfrncturcr, or olherwitc drva nul nwcamrily wn-tihrtc or imply itn cndorwnoni, rcamr-

n!cnduliwn, III I’mwring lIy i he 1 Jnikd SLulca (hwcrruwcm or wry qcncy lhcrcof, The wiowmwvd opinionn uf mulhm~ CP prcwxl herein rlu rrwl ncwwrily stulr or rclkl thooo of Ik

(Jnitcrl SIUICK I hwcrrrmcnl or mry u~cncy Ihcrcof.

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LOSAllaIIIITIOSl...l,...,.ew.~~~..~~~.~LosAlamos NationalLaboratory

wlull[fl

GEOCHEMICAL SIMILARITIES BETWEEN VOLCAdIC UNITS

AT YUCCA MOUNTAIN AND PAHUTE MESA:

EVIDENCE FOR A Cf)M?4014MAGMATICORIGINFOR VOLCANIC SEQUENCES

THAT FLANK THE TIMBER MOUNTAIN CALDERA

R. G. Warren

Los Alamos National LaboratoryLos Alamos, NM 87545

ABSTRACT

Chemical compositions have been determined for sanidlne, plagioclase,

hiotite, and hornblende phenocrysts by electron microprobe for a comprehensive

set of samples of Crater Flat Tuff and tuffs of Calico Hills. Most of these

samples were obtained from drill holes at Yucca Mountain. Samples of tuffs

and lavas of Area 20, obtained from locations at Pahute Mesa, have similarly

been subjected to microprobe analysis. Complete modal petrography has been

determined for all samples.

Biotite and hornblende in the samples from both Yucca Mountain and Pahute

Mesa have Fe-rich compositions that contrast strikingly with Fe-poor COMpOSi-

t~ons In the overlying paintbrush Tuff and the underlying Lithic Ridge Tuff at

Yucca Mountain. Each unit from Yucca Mountain has distinctive compositicms

for both sanfdine and plagioclase that very closely match compositions for a

corresponding unit identified within the lower, middle and upPer portions of

the Area 20 tuffs and lavas from Pahute Mesa. Each of these paired units

probably originated fran a conmron parental magma and was erupted contempor-

aneously or nearly so.

Each pair of units with matching phenocryst chemistries has a similar,

but not identical set of petrographic characteristics. The petrographic

differences, as well as small differences in phenocryst chemistry, result from

a zonal distribution of phenucrysts within the parent magma chamber and

eruption through earlier un!ts that differ markedly between Yucca Mountain and

Pahute Mesa.

INTRODUCTION

During the past two years, intensive research into the geochemistry of

the volcanic rocks of Pahute Mesa has been pursued at Los Alamos. The purpose

of thts research has been to establish the usefulness of m~jor, minor, ~nd

trace element geochemistry (fletermlned mostly by neutron actlvotlo~ andlysis,

or NAA) and mineral chemistry !deterinlnedby electron microprobe) in establish-

ing stratlgraphlc correlatioris among units In Pahute Mesa drill holes. This

geochefn!cal approach has been highly successful and allows the definition of

units based on chemistry and petrography rather than lithology; units so

defined are termed “Petrologic Units.” A single Petrologic Unit may Includ<

s?vcral llthologlc units, as seen in Table 1. For example, the Pyroxb,,~

bearing RhyolIte of the Scrugham Peak Quadrangle (Petrologic Unit symbol TRW)

1

Table 1.

Pahutemesaandvicinity

YuccaMountain&ndvicinity

Nomenclature and symbols for Petrologic Units discussed in this report. (Units withfn eact arealisted in stratlgraphic successfcm, youngest unit at the top. Complete stratfgraphic secuencenot given.)

Petrologic Lltholo91cUnitsymbol

TMRu

TMR1

TPcu

TRPP

TRA(u1’

TRA( U2

TRA(p)

TRA(m)

TRA(lr)

TRA( SW)

TCT

rTH 1

TH2

TCP

1

4 TCT

TLR

ITTA

llB

TTC

Unit-asymbol Subunf t

——Tmr quartz ltlte

Tmr rhyol 1 te

Tpc quartz latlte

‘r!li‘p(Tb”

Trau, Trab maflc-poor

Tral ,b Trab mafic-rich

Tral,c Trab

Trab

Trat

Tsw

Tp(Tbl

Tht, Thl

Tht, Thl

Tcp

Tcb

Tc t

rlr

1 ta

Ttb

Ttc

kafic-pmr

maffc-rich

Volcanlc VolcanicUnit Group

Rafnicr Hess P&ber

Tlva Caryon Member

Pyrnxene-bearing rhyoliteof Scrugham Peak Quadrangle

upper

upper

plagloclase-rich

middle

lithic-rich

Stockade Uash Tuff

Tram Member

Tlrber Mountain Tuff

Paintbrush luff

TuffsandlavasOfhrea 20

Crater Flat Tuff

tuffs and lavas of Calfco Hills

Prow Phgs Homhr

Bullfrog MemMr

I

Crater Flat Tuff

Tram Mamhr

Ltthlc Ridge Tuff

Unit A, USU-G1

Unit B, USH-G1

Unitt, USH-G1

~ PrcsQnt comon usag~ for NrS {olcanfc units.c Upper part of lowor Iavas (Rigurc 5 tn Pyurs et al., 1976a).

Lowor part of loww lavas (Figurt 5 In Byor8 at a}., 1976c).

Includes the Pyroxene-bearfng Lava of the Scrugham Peak Quadrangle (Lfthologfc

Unit symbol Trpp), bedded tuff that underlies the Trpp unit, and the local ash-

flow tuff of the ;~lntbrush Tuff (Tpl Lfthologfc Unft). All three of these

lfthologic unfts have fdentfcal mineral compositions and very sfmilar

(although not fdentfcal) phenocryst contents that contrast strikingly wfth

those of borderfng units. Symbols for Petrologic Units (Table 1) are utfllzed

extensively throughout thfs paper. These symbols are fully capitalized and

therefore distfnct from symbols for lfthologic units, which have only the

first letter capitalized (compare symbols In Table 1).

The Petrologic Unft has been particularly successful fn the recognition

and fdentfffcatfon of correlatable horfzons within bedded tufts and lavas

beneath Pahute Mesa. Such lithologfes generally show poor correlations

between drill holes (ff correlation is attempted at all), due to Intertonguing

of lavas wfth bedded tuffs and to misidentfffcatlons which often result due to

a lack of distinctive hand-sample characteristics among bedded tuffs and

lavas. However, early results indicate that Petrologic Units do not show

significant thickness variations where such lfthologfes fnterffnger. For

example, the TRA(u1) Petrologic Unit fn U19aS consfsts cf 229 m of lava and 21

m of bedded tuff but fn U19q, only 444 m distant, it cnnslsts of 34 m of lava

and 213 m of bedded tuff (Warren, in prep.). Although the thicknesses of the

Indlvfdual lithologfc components differ strfkfngly, the TRA(u1) unft fs 250 m

thfck fn U19aS and 247 m thick in U19q and the correlctfon of this Petrologic

Unft between these adjacent drill holes fs excellent. It fs lfkely that lavas

simply represent near-source eruption of magma, and tuffs tend to “ffll in”

the paleotopography for some dfstance away from thfs source.

The purpose of this report is to demonstrate that the Petrologic Unit

provides a powerful means to correlate volcanic units by relatlng units that

occur In widely separated areas cf the NTS. This correlation cauld not be

achieved using llthologic untts, due to strlkfng dlsslmflarltles between

ltthologles wlthln the volcanic sequences correlated between the two areas.

These volcanfc sequence$ are the tuffs and lavas of Area 20 of the Pahute Mesa

area and the tuffs and lavas of Calfco iIIlls and the Crater Flat Tuff of Yucca

Mountain and vtcfnfty (E!yers et al., 1976a; Carr et al., In press; see Figure

1 for locations). In order to establlsh the correlation of volcanic sequences

between these areas, ft Is necessary to suhdlvlde the tuffs and l~vas of Area

20 l~to members, and to demonstrate the petrograpnfc and chemical equivalence

3

—. ..._ .. .●

✎✎� � ✎ ✎✍�

i117W \“--”-+.%

I

m i“”” 7

\ \

/? W! Wa,,,,,,,?.,,.,, i;;1 ,,,”4,,,,,J,, 1‘~., ●,““’’”Mm- (-’’-’””‘1,, ,,,,?y4,:: “x /’

I ““w

...,,,,,,! .. ‘ ,,,,,,,,,,,,!,‘1‘-.,,,,..+ . +9. :.,.-,.’.., .,,.,,.. ‘.,.. ’., ,, ;,.,f,, ~.’mhtm;”.’,’“-, ,<“; ‘“%

,.’..,}.,,.“.,...,.

%S!!?!?b “’i!;’]W* ,,

/.,.-=”’i

-. -.. -/——. —-— -m -1

Fig. 1Location of sample sites . Outcrop Iocatlons are open symbo; s, drf11 holelocatlons are closed symbols. Volcanlc units of NTS are present throughoutregion except In stippled areas. Exact locations are given for samples southof Timber Mountain In Warren et al. (In prep.), and for samples of the TRA(u1)unit In Table 2. Locations for samples of other units north of TimberMountain arc Tpb: RW19f; TRA(u2) : Ue20f, RH19f; TRA(P): Ue19pp Ue19x;TRA(p)?: Ue20f; TRA(m): Ue19p, U19ab, Ue19fS; TRA(l r): RW19r, Ue19p, U19ab,Ue191; TRA(sw): RW19r, RU18a; TCT: RW18a. Mineral chemical data me avail-able for all samples frfnn each of these locatlons (USW-G2 and lJe25b-lh fromBroxton et al., 1982), and mudal petrographic data are available for alllocations (except USW-G2 and Ue25b-lh. )

4

—

of tuffs and lavas that comprise

section Is devoted to these topics.

.

a single Petrologic Unit. The following

DESCRIPTION OF PETROLOGIC UNITS OF THE TUFF5 AND LAVAS OF AREA 20

The tuffs and lavas of Area 20 (Byers et al., 1976a) are here divided

into four major Petrologic Units, tenned the upper, plagioclase--icll, middle,

and lower members (Table I). The upper member is further subdivided into an

upper, mafic-poor portion [symbol TRA(u1)] and a lower, mafic-rich portion

[symbol TRA(IJ2)I. The lower member includes highly lichic-rich tuff [symbol

TRA(lr)] av4 the Stockade Wash Tuff [TR4(sw)]. An additional Petrologic Unit

related to the tuffs and lavas of Area 20 iS present beneath the Stockade Wash

Tuff at Rattlesnake Ridge (see Fig. 1 for location), southeast of Pahute Mesa.

On the basis of a single sample, this unit (symbol TCT) is correlated with the

Tr~m Member of the Crater Flat Tuff (Tct).

Each Petrologic Uilit is defined on the basis of well-defirled range in

certain petrographic and mineral chemical parameters; the most useful and

reliable of these is the sanidine composition. Other silicic- to inte~ediate-

composition lavas also occur within the TRA sequence; preliminary study

Indicates that they are associated with the uppermost portions of the TRA!P)

and/or TRA(lr) units. These lavas include the olivine latite of Itawich Vdlley

(Sargent et al., 1966), and~sites of U19e and U19g, and a iava misidentified

as the pre-Pah lnva in U19aj. These lavas are not treated in this report.

6elow, sutnnary tables of petrographic and mfneral compositional data are

given for individual samples of the TRA(u1) unit to illustrate their ranges

within a Petro’iogic Unit (Tables 2-5), but only combined data for all samples

of other Petrologic Units are presented. Tables 6-11 include data for each

Petrologic Unit of the tuffs and lavas of Area 20, as well as its correlative

unit from Yucca Mountain. In each of these tables, data for Petrologic Units

appear in groups; the Petrologic Unit at the bottom of

Yucca Mountain and all those above it are correlat~ve

Mesa and vicinity. Petrologic Units increase in age

each table. For comparison, Table 11 also contains

younger and older than the tuffs and Iavas of Area 20.

each group is found at

units found at Pahute

towards the bottom of

data for units both

5

TRA(u1) Petrologic Unit

The upper, maflc-poor member of the tuffs and lavas of Area 20 consists

of ash-flow tuff, bedded tuff, and lava. The TRA(u1) unit is present in mosr

drill holes in Pahute Mesa (where penetrated), and attains a thickness

exceeding 1000 m in Ue20h. Petrographic results are given for twelve samples

of this unit in Table 2, including

samples of bedded tuff, and a sing’

results for a single sample of the

presented at the bottom of Table 2,

(in sequence) three samples of lava, eight

e sample of ash-flow tuff. For comparison,

tuff of Blacktop Buttes (Tpb unit) is also

This unit is positioned stratigraphically

within the Paintbrush Tuff by Byers et al. (1976a), but has petrographic and

chemical characteristics identical to those of the TRA(u1) unit, and

strikingly different from those of other Paintbrush Tuffs. Field work is in

proqess by Byers and Warren to investigate the possibility that the presently

accepted stratigraphlc position of the Tpb unit is incorrect.

All samples of the TRA(ul) unit show a consistently low phenocryst

content, ranging from 1.4 to 4.5%, and con~istently high proportion of quartz

phenocrysts. They also have a strikingly low content, of mafic m!nerals, iron-

titanium oxides, and accessory minerals. Bi6tite is the nnly mafic mineral

present irI most samples. Ilmenite, sphene, and perrierite (all abundant in

most units of the Paintbrush Tuff and many other ~olcanic units of the NTS)

were not found in any of these samples, but allanite is present in many

samples. Except for the presence of appreciable hornblende, pyroxene, and

apatite in sample RW19r-7, there are no significant differences among the

primary mineral contents of the samples of TRA(I!l) unit (Table 2).

Sanidine compositions, represented by their orthoclase (Or, KAlSi308) +

celsian (Cn, BaA12Si208) molecular percents (mol%) and BaO contents (Table 3)

are also very similar for all samples of the TRA(u1) unit. Compared to other

units of tho NTS, sanidine of the TRA(u1) unit is potassium-rich and barium-

poor. There fs a slight, but consistent, decrease in the Or+Cn content of

sanidine stratigraph!cally upward at each drill core location, from Or+Cn ■ 69

in the stratigraphically lowest sample at each location to a value as

potassium-poor as Or+Cn r 65 in Ue20e-1. The sanidine compositions near the

base of the TRA(u1) unit match those of the underlying TRA(u2) unit, and those

near the top match those at the base of the overlying Topopah Spring Member of

the Paintbrush Tuff (M~rren et al., in prep.; Caporuscio et al., 1982).

6

[021cfmlmzo u YcM; allVilues in p~

Mafic Prwlocry!ts FQ-TI011=S Accessory ■fnerils

-tot Hbld Cpx Opx 01 & Ar? Htim lm ~h 1 Per Zr Ot,

E*TS M POIIST cOlilSTUSJ or conporK nts, x

-M ■

&rlm FelstcIrm Polmts Ptbtno- Relttlwt t fclsici(Dzl cMCtU Voids Pmtce Lltbics trysts u I P

—— .— —— .———— —— ——— .— —

0

02

0

0

0

0

0

0

0

0

0

0

—— ——— —— —

o 10

000 23

00

02

05

07

00

00

00

08

0 27

0 92

6K@-1 Trm

1.k&-l-1715 Trw

-352S Tr~

Ut9r-7 Trab

M20g-5 Treb

ue19p-16a Trab

-1652 Treb

lJe191-ml Tr&

w-2474 Trab

ll19~16M Trab

-1 740-5d Trab

uE&-1-3155 Tr*

9M9f-L2 Tpb

:f6Mult.c2Pc#P~*lLs

o

c

c

o

0

c

c

c

04

4s

c

0

11

1

B

b

b

b

b

b

b

b

C1.*

61

S+,zc

c1 .D2C

c1c1c1ZcZc

61

Zc .61

Zc

61

3.4

22

4.0

17

12

5.2

11

2.5

2.3

2.6

z-i

2.0

8.1

C.o

0.0

0.0

1.4

3.9

11

1.1

0.4

14

13

6.4

1.2

3.0

1.9

1.5

4.5

18* M“ 48*

44* 3P 17*

46 42 12

0

00

0

0

0

0

0

0

0

0

0

0

9 14

6 16

026

33 3

0.6 0.3

23

25

03

05

00

25

0 0.2

97

M

100

41m

o

0

0

0 0.6 0n4 595

617 457

523 528

14S 435

724 595

153 559

123 535

640 475

626 515

153 310

234 w

534 - ~m

617 504

0

0

0

3300

.30 0

440 0

00

00

210 29

0

0

0

0

0

0

0

13

0

0

0

0

4.1

1.4

2.3

2.1

2.5

2.2

2.1

3.1

42* 42* 16*

63* 23” 14*

52” 31* 11”46* u’ 1P

51* 43” 6“

33* 479 ZtV

39* 33* 20*

39* 26* 35*

37

0

0

0

00

0

0

0

0

0

0

0

0

0

0

600 0

100 0

120 0

120 0

93 0

140 12b

32 0

lm o

440

280

82

170

77

350

62

110

0

0

0

0

0

0

0

0

0

0

0

0

00

0

0

0

7

12

2a

44

22

23

25

Mo2.6 28* 29” 43* 140 o

2@3 o2.1 35* ’28” 37* 410 71 0rNt

Explzmatim of S-IS:

L1*IosIcrniC: Trm = mmer tiollcs It-a of brea 20: Ireb ● ~ w asts-flow Mffs of AR4 20: Tob ■ luff ofCl=k~ Battes ““ -

. .

~le ~: c - &fll core; de = cuttlfbps -t am repmsent4tiwe of lftilog; e = outcrop lnewsdJ Ststecnordl MM, In inters: MOg-; 28519W. 1157=; 3M09-5 2ZM161M, 17601~; W19r-7 2WM?M. lWW.2E;Msf-lz -1-. 17%4X). wrs foi- core ssd cuttl~s ftiicata ~le &pth, tm f-t.

Table 3. Frequency distribution of Or+Cn and BaO contents for sanidlne phenocrysts Inlndlvldual samples of TRA(u1) and single sample of Tpb unit (RM19f-12). (Each valuerepresents the number of analyses that have orthoclase+celslan (Or+Cn) end-membercontents, and barium oxide wefght contents, wfth~n the interval Indicated. Symbol Sfor rock type defined In Table 2.)

Sample no.Rocktype

Rli20g-3

-5

RU19r-7

Ue19p-1600

-1652

Ue19x-2301

-2474

U19itb-1650-60

-1740-50

Ue20e-1-1715

-3155

-3525

lw19f-12

1

b

b

b

b

b

b

b

b

1

nwt

I

nut

Or+Cn, Ml% BaO, wt%72 70 68 66 64 6Z 60 58 0.0 0.3 0.6 0.9 1.2

2

4

s

,8

I I I I

1713

10 1

5 41

77

14 1

59

9

1 13

13 1

2 12

95

7231

2

1

1

3I I I

-T--

L-.

12

12

11

13

13

12

11

12

12

13

12

3

2

1

22

1

1

2

3

2

2

1

11

10 1

13 1I I I

Plagioclase compositions, represented by their anorthite (An, CaA12Si208)

molecular percents (mol%) (Table 4) also show a well-defined range for all

samples of the TRA(u1) unit. Compared to other units of the NTS, plagioclase

of the TRA(u1) Jnit is moderately calcium-rich (particularly for a mafic-poor

unit) and has a relatively narrow compositional range. The~e is a slight, but

consistent, d~,rease in che An content of plagioclase strati~raphically upward

at each location that parallels the change in sanidine compositions (in both

cases the chemical change can alternatively be regarded as dn increase in

sodium content stratigraphically upward). Similarly, plagioclase compositions

near the base of the TRA(u1) unit match those of the underlying TRA(u2) unit,

and those near the top match those within the rhyolitic portion of the

overlying Topopah Spring Tuff (Harren et al., in prep.; Broxton et al., 1982).

Biotite compositions, represented by their molecular Mg/(Mg+Fe) Conte,lts

(“Mg number, M Mg*), show a well-defined range for all samples of the TRA(u1)

unit (Table 5). Blotite compositions are very similar among all Petrologic

8

---

Table 4. Frequency dlstrfbutfon of AII contents for DldglocldSe ph’-ocrysts

Sample no.

and”single sample of Tpb unit (Rli19f-12). “(E;ch value” r.,resentsanorthite (An) end-member contents within the interval indicated.Table 2.)

Rocktype

RU20g-3

-5

RU19r-7

Ue19p-1600

-1652

Ue19x-230:

-2474

U19ab-1650-60

-1740-50

Uc20e-1-1715

-3155

-3525

RU19f-12

1

b

b

b

b

b

b

b

b

1

nut

1

nwt

in individual

—. ——

samples of TRA(uI)the number of anaiyses that have

Symbols for reck type defined in

An, Ml%8 iO 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 45 50 55 60

l--

1

I I I I

14

112

1 22

3 10

15

17

15

16

14

184

14

115I I

I I I I

71

91

71

7

4

221

331

8

621

402

33I 1

1

1

1

1

1 I I 1

1

1

I

1

Units of the tuffs and lavas of Area 20, and ?i~ striki~gly magnesium-poor

(Mg*@.4) compared to most NTS units.

Petrographic results for the TRA(u1) unit are summarized as median values

in Table 6. Mineral compositional data discussed above are combined for all

samples of the TRA(u1) unit in Table 7 for sanidine, in Table 8 ior plaglo-

clase, in Table 9 for biotite, and in Table 10 for hornblende. Dominant

values for the compositional parameters represented in Tables 7-10 are given

in Table 11; the dominant value is equivalent to the statistical mode.

TRA(u2) Petrologic Unit

The upper, mafic-rich member of the tuf;s and lavas of Area 20 consists

of ash-flow tuff, bedded tuff, and lava. The TRA(u2) unit is about 230 m

thick In iJe20f but is absent in Ue19p. Samples of the TRA(u2) unft contain

the same primary minerals as those of the TRA(u1) unit, except that the

9

Table 5.

Sample no.

——.

Frequency distribution of molecu:ar Mg/(Mg+Fe) contents for biotite phenocrysts in individualsamples of TRA(u1) and single samDle of TDb unit (RW19f-12). (Each value represents the number ofanalyses that have molecul~r t4g/(Mg+Fe) end-member contents within the Interval indicated. Symbolsfor rock type defined In Table 2.)

Rocktype

Mg/(Mg+Fe).30 .32 .34 .36 .38 .40 .42 .44 .46 .48 .50 .52 .54 .56 .58 .60 .62

RU20g-3

-5

RU19r-7

Ue19p-1600

-1652

Ue19x-2301

-“474

U19ab-1650-60

-1740-50

Ue20e-1-1715

-3155

-3525

RW19f-12

1

b

b

b

b

b

b

b

h

1

nwt

1

nut

I I I I

2

1

11

4

2

2

1

1

1

11

1

1

4

i

4

4

3

4

2

5

1

,1

1

1

8

11.

I I I I

115

I I I I I I I 1 I 1 I

concentrations of these minerals are much higher (Table 6). The complete

I

1

I

absence of all other mafic minerals despite a high concentration of biotite

(up to 3.6% in one sample) is particularly distinctive.

Sanidine (Table 7) and plagioclase (Table f3) compositions of the TRA(u2)

unit are very similar to those at the base of the TRA(u1) unit, as previously

noted. Biotite of the TRA(u2) unit has a slightly, but significantly higher

Mg content than biotite of the TRA(u1) unit. Compared t’ other NTS units

(Table 11), sanidine is highly potassium-rich and moderately barium-rich, and

plagioclase is calcium-rich.

-m petrologic Unit

The plagioclase-rich member of the tuffs and lavas of Area 20 consists of

ash-flow tuff, bedded tuff, and lava. The stratigraphic positicn of the

TRA(p) unit is well established in Ue19p between the TRA(u1) and TRA(m) units,

10

5.2

3.0

2.52

0.7

3.0

3.7

9.0

2.0

c.?

z;

S.n

0.5

1.4

6.3

2.3

2.7

2.4

12

25

9.s

13

4.3

9.6

6.9

5.0

12

12

11

lts.jor ccqawnts. %

Uder Fel SIC Relative \ felsics Raflc phenocryscs (ppmv)Petrologic of ~Fnw-

untt Lmatfon -Ies Ltthlcs trysts G K P—. . .

T2A(lsll m 12

Tpe m i

Tti E m 5

m(p) m 3

TM(p)-l m 3

m(m) m 5

TIP m 22

d

4T’AA(’:! m 10

T-M(S9) m zTC6 m 35

TCT m 1

TC1 m 21

—44

35

52

35

32

16

1!

5

13

29

22

Is

m

33

ii-m22

29

14

20

25

67

44

47

50

36

39

33

—17

37

27

26

51

64

60

a

43

25

n

45

32

M

Eliot Hbld Cpx OpI 01 M Arf—— —.—. ——

155 0 0

410 71 0

770 0 0

6WO0

woo

MMOOO

61m 48M O

lm 170 0

25000

56000

la w 15

mmao

4um9 o

35M 0 0

000070C0

o 000

0000

0000

0000

0 000

Ooco

t 000

0060

0 0 9 0.5

0000

0000

0000

Fe-Tt oxides(pm]

H* 11=—.

130 02200

:25

1300 0

1lm

8200

1500 0

4a02

m

370 0

410 0

Hal

770 0

Im

o0

00

0c

o00

00

4

92

0

0

510

00

5

40

0

99

0

000

00

00

00

00

0

4

0

Sph All Per Ap 2r

T

o0

1

9

0.2

130

17

20

170

0e

2

16

26

77

52

4

7

4

33

53

49

lW

12

25

19

11

34

28

?5

~Petrolqlc of Or+cn. Ml& ho, Wti

Umi t Lmtf- ~les 7C 64 w 40 M 0 0.6 :.2 1.8 2.4

II 1 I II 1 w I I I 1- 1-1 II I .-r I I l-l

1 11

12

1

11

lli13

I

1

i T-T lT 1~1362742

n 1

67 11

61711621

3642964

20-..

51

269;

70 8 11

10 10 1

mid) Fn 12~

11?148205

1P m 1 ,3 3

m 10 ( 82s

22

2

99

2 11

1 1

1 535

13

259:

24

15

1TtA[mz)

Tn2

1

m(p)

m(p)?

mFH

3

3

3732

1032

3143i

1 ! 21 74

1355

3793:

m(m)

TcP

7

33il

13”;

111

s20

962< 71111

15 13 2

451299626 3 1

W(lr)

TM(9)

m

10

2

26

mnm

mm

1

2s ,111 m 157 43 aI

but its position with respect to the TRA(uZ) unit is uncertain. A 500-m-thick

unit in Ue20f has been identified as the TRA(p)? unit (see Tables 7-11); this

unit directly underlies the TRA(u2) unit. Samples of the TRA(p) unit

characteristically have consistently high plagioclase contents, and most

contain abundant hornblende in addition to biotite (see Table 6).

Sanidine (Table 7) and plagioclase (Table 8) compositions of the TRA(P)

unit are slgnfffcantly more sodium-rfch than those of the TRA(u2) unit, and

sanfdine fs considerably more potassium-rfch than sanidfne of the underlying

TRA(m) unft. However, a sample of the TRA(p) unft cannot be confidently

distinguished from one of the TRA(lr) unft on the basis of mineral chemistry

(compare unfts In Tables 7-10). Furthermore, some individual samples of the

TRA(lr) unit thdt have relatively hfgh plagfoclase contents and relatively low

contents of lfthic fragments are petrographically simflar to samples of the

TRA(p) unft, and so a distinction between thc~e units based on petrography is

also uncertain. The TRA(lr) and TRA(p) units can be confidently distinguished

only if th~’ strat~grdphic relatfon to the distinctive TRA(m) unit Is known.

TRA(m) Petrologic Unit

The middle member of the tuffs and lavas of Area 20 consists of ash-flow

tuff and bedded tuff. Lavas have not presently been recognized for the TRA(m)

unit, which consists mostly of very fine (a$h fall) tuff where characterized.

The unit fs 65 m thick in Ue19p. The TRA(m) unit fs distfnct due to a con-

sistently and distinctly low content of quartz and high content of sanfdine

(Table 6). In additfon, samples of the TRA(m) unft exhfbit extremely low

biotite contents, but contain hornblende contents that approximately equal

that of biotite.

Sanidlne (Table 7) and plagioclase (Table 8) compositions of the TRA(m)

unit are relatively sodium-rich. Sanldine compositions are particularly

rllstlnct from those of any other TRA unit (Table 7). In addition, one sample

of the TRA(m) unit contains an appreciable content of Fe-rich orthopyroxene

(Mg* w.3). Such orthopyroxcne is rarely found in NTS rocks and has been

presently ~dentified only In single samples of the TRA(P), Tpb, and TCP units.

TRA(lr) Petrologic Unit

The lithic”.richmember of the tuffs and lavas of Area 2(Iconsfsts of non-

to partially welded ash-flow tuff. The TRA(lr) unit fs widely distributed

13

T#bl@ 8. Froqumcy al ttribution of An contonts for pl~gloclasc phenocrysts In sawlesYucca tiufi~tln, and RittlesnJke Rtdge. (Each value represents themanber of●nd~r contents within the intww~l indtcated. Results c~ined for tllYucca Muntttn fr~ Uarren et *I., in prep.).

Ntierof An, Dolt

Unit 5e~les Lecttfon 10 20 30 40

of Petrologic Untt! of Pahute !lega.anti ses that have anortht te (An)

zscmp ●s of ●ach untt. Data for

so 60 70

I I 1 I

i 2 21 63

115

1 6 11 25

5 11

61577

3 13 1P

923144

91711

I I 1 1

\71851

33 1

!9311

381s52

713964

712

21 1

121

14231

1 I I I

211

322

112S3

12121

131

1 21

13242

—— —.I I I 1 I I I I

1

k

/211

1 I 1 I

1

1

1

1

1

12

TRA(U1)

Tpb

nil

12 m

1 m

8 YM

1

1

1

1

19

1

Ill

3213

112

11

11

1113

TRA(u2)

7n2

3 Pm

3 m

TAA(P)m(p)?

3 Pm

3 Pm

TR4(m)

KP5 M

16 w

22645130

141i61

3 35 127 79 16

2

~3322

153466

44 1

TRA(lr)

IRA(W)Tce

10 Pm

2 RR

29 m

21543

11

‘66646

L

2

10 14 14 13 13Lla31

1 1

6155

1:1

944i l!-111

1 1

1

531311

TCTTCT

1 RR

26 w! L 12532

L

Table 9. Frequency distribution of molecular 74g/(Mg+Fe) contents for blotfte phenocrystsIn samples o? Petrologic un~ts of Pahuta 14es#, Yucca Hountafn, and RattlesnakeRld9e. (Each value reDrasents the number of #nalYses that havcmolecultr14g/iM +FQ) end-met

?v contents wfthtn the fntervai fndfcated. Refults Cdfnd

for a 1 stmples of each unft. Data for YUCCR Wuntaln from blarren et al., Inprtpo).

Numberof

S4mp 1es.—

12

1

7

6

6

3

3

4

6

10

2

20

1

le

I ~ molacultr Mg/(Mg+Fe)

Location .30 .40 . 5U .60 .70Unit

TRA(u1)

Tpb

TH 1

TRA(u2)

TH2

TilA(p)

TRA[p)?

TRA(m)

Tcp

TRA(lr)

TRA( SW ]

TCB

TCT

TC T

I I 1 I I I I t3 2 13 24 6 10 2

I I I I

121

22

135

11

11

2

PM

PM

YM

PM

YM

PM

PM

PM

YM

PM

RR

YM

RR

YM

1

1

1

211

151

6 lJ 13 21

701?4

1 11 4799

232 3178

3 1171

?133 1

1 2 635

1 72712 4 4 1 3

14 5J

35”’I 3441

211

14 S022271341.-L

14

throughout Pahute Mesa. It Is >400 m thick In Ue19p and about 500 m thick in

Ue20f. Except ne~r the uppermost portion of the unit, samples of the TRA(lr)

unit consistently contain e~ceptionally high contents of lithic fragments

(Table 6) derived almost exclusively from the underlying peralkal ine units of

the Silent Canyon area. These lithic fragnents are unusually poorly sorted;

their sizes range down to microscopic, as ~llustrated In Figure 2, and up to

meters. Their presence provides a highly distinct petrographic characteristic

of the TRA(lr) unit. Samples of the TRA(lr) unit characteristically contain a

low to moderate content of felsic phenocrysts with plagioclase subordinate to

sanidine and quartz (Table 6). The biotite content is low, and hornblende is

absent. However, near both the top and bottom of the unit, samples have pheno-

cryst contents as high as 26%, plagioclase is dominant, and both biotite and

hornblende are abundant. “

Two very distinct compositions occur for sanldine In samples of the

TRA(lr) unit; one with a dominant Or+Cn value of 62 mol% and the other with an

Or+Cn value of 40 mol% (see Table 7). The former composition is similar to

danlnant compositions for other TRA units (see Tables 7,11), and represents

the true phenocryst chemistry. The latter composition matches those for

santdine In samples of the underlying peralkallne rocks and for sanidlne

phenocrysts within the lithic fragments of the TRA(lr) unit; sanldine grains

with th~se compositions are clearly xenocrysts. No other unit of the NTS is

known to contain sanidlne xenocrysts In such abundance. Plagioclase

compositions (Table 8) are not unusual compared to other NTS urlits (Table 11),

and both biotite (Table 9) and hornblende (Table 10) compositions are

magnesium-poor (Table 11), typical of the tuffc and rhyolltes of Area 20.

TRA(sw) Petrologic Unit

The TRA(SW) Petrologic llnlt is equivalent to the Stockade Hash Tuff(7) of

Byers et al. (1976d) and mly additionally include bedded tuff stratigraph-

Ically bounding the ash-flow coollng unit. TWC samples of the Stockade Wash

Tuff were examined; these were collected from locations (RW19r and RU18a, Fig.

1) that Byers et al. (1976a) consider probably not strati graphically

equivalent. The Strdtigraphic position of the TRA(sw) unit in the Quartet

Dome quadrangle (Sargent et al., 1966) has been confidently established at

lccation RW19r, where it directly underlies the TRA(lr) Petrologic Unit

(“conglomerate” of Sargent et al., 1966). The petrogr~phy and mineral

15

a.

b.

Fig. 2Photomlcrographs of sample Ue19p-2204, TRA(lr) unit. Note the abundantfragnents (mottled appearance), particularly those with very small}“ieldof view 1.9 x 2.8 m. (a) reflected Ilght, (b) transmitted l{ght.

16

llthlcsizes.

.

Table 10.

Unit

TRA(u1)

Tpb

TRA(p)

TRA(p)?

TRA(m)

TCP

TRA(lr)

TRA( SW)

TCB

TCT

within the IntervalYucca Pbuntain frm.,

Numberof

Samples Location

Frequency distribution of molecular Mg/(Mg+Fe) contents for Ilornblelide phenocrysts In

samples of Petrologic units of Pahute mesa, Yucca Mountiln, and Rattlesnake Ridge. (Eachvalue represents the nmber of analyses that have u?oleculer Mg/(Mg+Fe) ●nd-~er contents

Indfcated. Results canblned for tll smoles of each unit. Dat# forWarren et al., in prep.].

—r

molecular Mg/(Mg+Fe).30 .40 .50 .60 .70

I PM

1 PM

3 PM

2 PM

4 PM

2 YM

4 PM

2 RR

4 Ym

1 RR

chemistry of

(Hlnrichs et

Quartet Dome

these areas

TRA(p) unit.

r~

1

I I I I

1 2

34

2

11384

11

212

11

1

I 1 Ill I i

I I 1 I

11

2

355

10712

11

11

1422 1

351

75

I I I I1

1 I

I I I I

1

11

1

-J-LJ-l-

the sample at locatlon Rli18a from the Ammonia Tanks quadrangle

al,, 1967) Identically matches the sample of this unit from the

quadrangle. Nonetheless, the correlation of the unit between

Is considered uncertain due to a possible confusion wtth the

TCT Petrologic Unit (Rattlesnake Ridge Location)-—A single sample of bedded tuff from Rottleznake Ridge (locatlon In Figure

1) is tentatively correlated with the TCT unit of Yucca 140untaln. This thin

(18 m thick) bedded tuff Is bounded stratlgraphlcally below by the Grouse

Canyon Member of the Belted Range Tuff and abuve by the TRA(sw) unit. It iS

conceivable that the TRA(sw) unit at this locatlon correlates with the TRA(p)

rather than the TRA(lr) unit [see discussion of TRA(sw) unit]; In tt?ls case

the sample of bedded tuff might represent the base of the TRA(p) unit. Such

lb considered unllkcly because the petrography and mineral chemistry of this

sample of TCT unit differs distinctly from all known or tentatively assigned

samples of TRA(p) unit (see Tables 6-11).

.

17

Table 11. Dominant values for compositional parameters of phenocrysts for Petrologic Units ofPahute Mesa Yucca Mountain, and Rattlesnake Rid ●. (The dominant value isequlvalent to

!rthe statlst{cisl mode for the frequency dlstribut on of anal ses represented in Tables7-10. S~bols for all Petrologic Units defined in Table 1.

Maflc MineralsSanidine Plagioclase

MoltAn

Numberof

Samples

Molecular Hg/(14wF@)Ml%

OrtCnw tzBaO

PetrologicUnit

.Location Blot

——Hbld

0.00, 1.13b 19, 24b

1313

l~c6

13

202020

2524

1713

1211

15

;:

H

Ii

:!

TMRu a

PMb

;:

PM

PMPMYM

PMYM

PmPM

PMYM

PhiRRYM

RRYM

Yn)?!YMYM

6

;

:

9

12

1;

37

33

2:

102

35

131

16826

60 0.62 0.64

TMR 1 upper1ower

O.OO0.00 0.60 0.72

0.650.70

TPCU upperlower

TRPP

4936

0.050.05

0.680.66

0.6549 0.75

TRA(u1)TbTh

0.120.090.10

0.37

0.350.38

0.460.39

TRA(U2)TH2

6971

0.500,71

0.440.43

6361

0.360.25

0.430.42

0.470.44

TRA(m)TCP

5353

0.050.14

0.370.42

0.37

62, 40b6261

0.13, O.oob0.290.56

0.370.400.40

0.430.440.44

TRA( 1 r)TRA( SW)TCB

TCTTCT

6967

0.270.55

0.420.42

TLRTTATTBTTc

0.67l!: 0.5566 0.9672 3.4

0.590.550.590.6?

~ !-ocatlons throughoutc Bimodal population

K-rfch anorthoclase.

NTS

Ca-rich plagfoclase (mantl@d by K-rich feldspar) also present.

18

other Petrologic Units.—Most other Petrologic Units discussed in this report (Table 1) consist of

well-described ash-flow coolfng units, mostly from Yucca Mountain (location

shown in Figure 1). Each such Petrologic Unit also includes strat+graphfcally

bounding bedded tuff that can be petrogruphically and chemically related to

the ash flow, but this added tuff is so volumetrically insignificant that the

distinction between Petrologic and Lith~logic Units is unimportant.

Descriptions of units younger than the TRA units are found in Byers et al.

(1976a), and descriptions for Petrologic Units of Yucca Mountain ar. ontained

in Warren et al. (in prep.).

COMPARISON OF TRA PETROLOGIC U!.ITSOF PAHUTE MESA WITH UNITS OF YUC,TAMOUNTAIN

The occurrence of substantial petrographic variations within single

‘Iithologic units of the NTS are well documented. Many NTS ash-flow cooling

units have mafic-r!ch caprocks (e.g., Lipman et al. 1966; Byers et al.,

1976a) . Substantial petrographic variations are now well documented for ash-

flow cooling units that do not h~ve mafic-rich caprocks; for example, the

upper porticn of the TCB ur,ltof USW-G1 is quartz-rich, but the lower pcrtion

IS quartz-poor (Carr et al., in press; Warren et al., in prep.). The TPCU

unit is a single cooling unit only 22 m thick in Ue19p, but the phenocryst

content differs by a factor of 3.5 between top afld bottom of the unit and

mafic mineral and trace mineral contents differ by an order of magnitude or

more (Warren, in prep.). It is clear that primary minerals (phenocrysts) are

not homogeneously distributed within most mag!nas. Furthermore, It might be

etl’~rted that the upper portion of a magma miqht concentrate volatiles.

Explosive release of these volatlles would result in eruption of the upper

portion of the magma as tuff, whereas the lower, more volatile-depleted

portion of the magma would tend to erupt more quietly, as a lava flow. Such

an eruption would result In both tuff and lava from the same magma.

Historical eruptions In such a manner are known (tiildreth and Drake, 1983).

Additionally, lary-vol~rne eruptions of tuff will occur ne~r-source as massive

and poorly scrted ash tlows, but os well sorted (bedded) tuffs farther from

the source (see, for example, Figure 8 in Sheridan, 1979). Overprinted on

these primary features

features of compaction

tilthough eruption of a

due to emplacement mechanics are the Tonal cooling

and crystallization described by Smith (1960). Thus ,

gfvcn magma might provide a time-stratigraphic marke”,

19

the recognition of this “pulse” might be very difficult due to verticai and

lateral variations both in the lithology and iil the primary mineral content in

the erupted matsrial.

Fortunately, the mineral chemistry of most eruptive sequences of the NTS

has bsen found to be unaffected by petrographic and lithologic variations.

For example, suost.antial petrographic variations occur both vertically and hori-

zontally wtthin the TCB unit (W~rren et al., +n prep.). Although samples from

this unit were obtained from numerous locations, separated by as much as 55

km, coinnosicions for sanidine are identical at all locations within the small

analytical uncert~lnty of about 1 mol% Or+Cn. Similar results have been

obtained for other minerals, particularly biotite and plagioclase. In

gener~al, each ucit II?S a unique set of mineral compositions that differs from

those of all other NTS units cnd serves as an invaluable aid in its recogni-

tim. The similarity in mineral chemistries between the TRA(p) and TRA(sw)

units noted in their descriptions is an unusual, and highly significant

exception. It should be noted, however, that not all units show as remarkable

a vertical and ?~teral consistency in mineral compositions as does the TCB

unit. There are sigllific~nt systematic vtiriat!ons in sanidine composition

within the Topopah Spring (Broxton et al., 1982) iind Rainier Mesa rhyolites

that could be employed to define stratigraphi; levels within these units.

Comparison of Mineral Chemistry

Although there are small (but significant) differences among individual

units, all TRA units have distl~ictively ilg-poor mafic minerals. In ccntrast,

mafic minerals in urltts of the overlying Paintbrush Tuff and all~ed lavas of

Pahute Mesa havq characteristically Mg-rich compositions (see Table 11). The

peralkallne rocks that underlie the TRA sequence are extremely biotite-poor

(most samples lack biotite) and Instead contain 01ivine and clinopyroxene

phcnocrysts that irlmany cases are nearly pure Fe end members.

At Yucca Mountain, mafi( miner?ls In the Crater Flat Tuff and overlying

tuffs and lavas of Calico Hills have Mg-poor compositions that are identical

to those of the TRA units of Pahute Mesa (see Tables 9-11). The overly+ng

Paintbrush Tuffs, except for the Topopah Spring rhyolite, have characteris-

tically Mg-rich maflc minerals identical to those in the Paintbrush Tuffs and

allied lavus of Pahute Mesa. The Crater Flat Tuff of Yucca Mountain, however,

overlies the Lithic Ridge Tuff And associated lavas rather than the

20

peralkaline rocks that underlie the TRA units of Pahute Mesa. The Lithic

Ridge and older units are generally plagioclase-rich units (Warren et al., in

prep. ! that bear Mg-rich mafic minerals (Table 11). It Is clear that the TRA

units of Pahute Mesa and the Crater Flat and Calico Hills tuffs of Yucca

Mountain have compositions for mafic minerals that are indistinguishable from

each other hut are highly different from those of bounding volcanic groups.

In contrast to mafic mineral compositions, sanidine and plagioclase

compositions dfffer distinctly among units, an~ can be compared unit-by--unit

thrcugbout the stratigraphic columns of Pahute Mesa and Yucca Mountafn (Figure

). Sanidine compositions (Table 7) a)~ Identical within the small analytical

uncertainties for each pair of units in the stratigraphic sequence at both

locfltions. All compositions of fndividu~l phenocrysts that differ substan-

tially from those of the daninant composition (Table 11) are due to xenocrysts

PAHUTE MESA

I- ///YUCCA MOUNTAIN ,/

M

/’Palntbruah ,/luffs

/TH 1 _ _ —.— — - -

TH2

u’K,

- — —.. _ _

TCP \\

<TCB \\’\

\~\

\\ -.=

TCT \\

\VM ---— — — _

TLR

PdntlwmhTuffab AHbd IAVn

TRA (uI)

TRA (u21”,----------

TRA (P)*

TRA (It)

[ TCT

TSCP

* R, IMIvomratlgr@k rmhm uncmmln

Fig. 3Correlation diagram between Petrologic Units of Yucca Mountain and PahuteMesa. (symbols for units defined in Table 1. TSCP ■ peralkaline rock of theSilent Canyon area. Greatest known or inferred thickness for each unitIllustrated. )

21

or to the occurrence of narrow sodium-rich rims (see Figure 4) produced from

late-stage deuteric alteration (Warren et al., in prep.). Matching units,

stratigraphically downward ,unit present at Fahute Mesa listed first), are

TRA(u1) and TH1; TRA(u2) and TH2; TRA(m) and TCP; TRA(lr) and TCB; and TCT and

TCT . A unit equivalent to the TRA(p) unit of Pahute Mesa is not known for

Yucca Mountain. Plagioclase compositions (Table 8) match in similar fashion.

Although the TRA(lr) unit of Pahute Mesa contains sanidine whose

composition (dominant Or+Cn value = 62, Tables 7 and 11) matches the TCB unit

of Yucca Mountain, it also contains a population of sodium-rich sanidine

xenocrysts that do not occur in the TCB unit. Additlcnally, there are

significant differences in the barium contents of Sanidine for many paired

units, moS*. notably between the TRA{lr) and TCB units (see Table 11).

Discussion o’ these differences is deferred to a following section.

Comparison of Lithology and Petrography

The primary mineral contents of the TRA units of Pahute Mesa are quite

similar to their paired unit of Yucca Mountain (Table 6). At both locations,

Fig. 4Transmitted light photomicrograph (crossed nicols), of sodium-enriched rim ofsanidlne in sample CFLSM-5, TCB unit. Field of view 0.24 x 0.35 nun.

22

these units contain abundant quartz, and variable amounts of biotite, horn-

blende, and allanite. Pyroxene, sphene, and perrierite rarely occur in

samples of these units. These petrographic characteristics contrast

strikingly with those of stratigraphically bounding units of both areas (Byers

et al., 1976a; Warren et al., in prep.; Warren, in prep.). The match in

primary mineral contents is excellent for several of the paired units of

Figure 3, particularly for the TRA(u1) and TH1 units.

The most substantial Iithologic and petrographic differences between

paired units occur between the TRA(lr) and TCB units. Samples of these units

are grossly- dissimilar in hand sample; the TRA(lr) unit is exceptionally

lithic-rich and generally non-welded whereas the TCB unit is quite lithic-poor

and always occurs as a well-defined c~oling unit. Portions of the TCB cooling

unit may be densely welded or even vitrophyric. Differences in the primary

mineral contents of these units ar’” not as great, but nonetheless they are

appreciable. Most conspicuous is the much higher .,mdian biotite content of

the TCB unit (0.20%) compared to that of the TRA(lr) ~n’lt (0.06%). However,

highly phenocryst- and mafic-rich zones occur within the TRA(lr) but not

within the TCB unit. Thus, the average phenocryst and mafic contents of the

TRA(lr) unit are considerably higher than median values, and averages probably

match much better for the two units than the medians. The TRA(m) unit of

Pahcte Mesa also differs appreciably in lithology and primary mineral content

from the TCP unit of Yucca Mountain. The TRA(m) unit generally has a low tu

moderate content of phenocrysts, mostly saniriine (Table 6), and occurs as a

well-sorted (bedded) tuff where presently characterized. It is distinctly

quartz-poor. The TCP unit contains a considerable) higher phenocryst content,

and occurs as a well-defined ash-flow cooling u~lit. Both units, however,

contain a highly distinctive, magnesium-poor orttopyroxene

striking correlation of primary mineral contents.

DISCUSSION OF RESULTS: EVIDENCE FOR A COMMON MAGMATIC

that provides a

ORIGIN OF

VOLCANIC SEQUENCES THAT FLANK ‘HE TIMBER MOUNTAIN CALDERA

Each TRA unit of Pahute Mesa and correlative unit of Yucca Mountain

(Figure 3) contains Mg-puor mafic minerals, consisting primarily of biotite

and/or hornblende. The chemistry of these minerals differs distinctly from

those of bounding volcanic groups, and suggests

location were derived from closely similar magmas

that the sequences at each

or from the same magma. By

23

contrast, the chemistry of sanidine and plagioclase differs markedly between

successive Petrologic Units. These large chemical differences within each

stratigraphic succession are parallel at each location. It is considered

htghly unlikely that separate magmas could evolve in such a manner to produce

the observed correspondence in feldspar chemistry. The mineral chemical data

thus strongly indicates that each of the paired units of Yucca Mountain and

Pahute Mesa was derived from the same magma.

It is unlikely that Petrologic Units that include lavas have been

emplaced very far from their source; such units include the TRA(u1), TRA(u2),

and TRA(p) units of Pahute Mesa and the TH1 and TH2 units of Yucca Mountain.

Thick exposures of the TH ~nits occur north of Yucca Mountain and at Calico

Hills (Byers et al., 1976b). If these lavas ascended directly upward from a

magma chamber, a very large magma chamber must have been present during

eruption of the TH and TRA(u) units if indeed these units represent magma from

the same chamber. The distribution of TRA and TH lavas (shown in Figure 5)

may then represent the approximate limits of this magma chamber. The Timber

Mountain Caldera, a much younger feature, is loca~ed near its center. Lavas

intermediate in age between the tuffs and lavas of Area 20 and the Timber

Mountain Tuff have also been correlated iicross the Timber Mountain Caldera

(see Figure 19 in Byers et al. , 1976a). This suggests that the Timber

Mountain area has been the center of a sequence of chemically different magma

systems throughout an extensive period of time. It is probable that these

magma systems, which ~roduced a thick sequence of NTS volcanic rocks including

the tuffs and lavas of Calico Hills, tuffs and lavas of Area 20, Paintbrush

Tuff and Timber Mountain Tuff are related as a single, large evolving magma

system centered beneath Timber Mour~tain. A model that might describe such a

magma system is described by Hlldreth (1981). Each pair of unfts fcund on

opposite sides of Timber Mountain might have been erupted from a central

source, the present location of Timber Mountain, or from separate sources. If

the former is true, Pahute Mesa and Yucca Mountain are marginal to a major

structure (e.g., caldera) associated with eruption of these units. The latter

possibility, eruption from separate sources, IS consistent with presently

accepted views. In this case, the Crater Flat Tuffs and tuffs and lavas of

Area 20 were erupted from structures that flank Timber Mountain (Carr, 1982;

Orkild et al., 19LP). It is likely that the magma chamber was not fully

emptied during the tinal stages of erupt~on and the remaining llq[ld h~s

24

11

TI

-...*w

,,, *,

\

%,3PN [ .’

i ..- 1

i #-la;

L m 00W0ARY———— —0,,*I,,, :,

‘o,,,,,/10,,,

,,, ,. .,,,,,, :%,/,,,,,,,,1’

,1

‘. :-’ %,,rt...b1 ,.. .,,

“\... :.,, ! ,.

J,’ ,,

...! ,,!’ ,,,

i . .‘J:,.! .. vWa R81

:. ,,

i \\‘,,

,, lb., . ..,,, ,,”

,,,!,,,,,7:,... “’....,.,,,.,!,,,,,‘, .%‘!.,,,1

J,,

“\

r

1 “.,’

— 1’ ,, ...

Fig. 5Loc~tlon of lavas of the TRA(u) unit (cllagonal pattern north of TimberMountain) and of the TH unit (same pattern south of Timber Mountain). Non-welded tuff shows CI wider dlstrlbutlon for both units. The single magmachamber Inferred to be the source of both lavas Is centered beneath TimberMountafn; the d~strlbutlon of the lavas may approximate the location uf theouter portion of a magma chamber.

w

37%J

solidified to fornr a large pluton beneath Timber Mountain. If the size and

shape of the magma chamber did not change substantially through time, then the

distribution of lavas shown in Figure 5 may also approximate the subsurface

location of such a pluton, which presumably would be a granitlc body.

The nearly identical mineral chemistry for units erupted from opposite

sides of this hypothetical magma chamber indicates that chemical equilibrium

between phenocrysts and liquid is closely maintained throughout an

extraordinarily large volume of magma. This is true even where substantial

variations tif phenocryst concentrations occur within a unit. This indicates

that the magma has an inhomogeneous distribution of phenocrysts, a~d the

concentration obsei~ed withi~ a unit depends upon the precise position that

the sample occupied as a liquld

variations in phenocryst content

and water pressure variations.

chemistry, but the data at hand

in the magma chamber. Probable causes for

throughout a magma chamber are temperature

Such factors might also af’ect the mineral

indicate that for many units they do not.

There is good evidence (Warren, in preparation) that mixing of magmas occurs

at the margi~s of the magma system. This is not an equilibrium process and

non-equilibrium conditions are evident in units affected by magma mixing.

The crystallization sequerce is preserved, however, by variations In the

bariu:llcontents of sanidine. Sanidine has a marked tendency to concentrate

barium {Leeman and Phelps, 1981; Vanson, 1978), which readily substitutes for

potassium because these elements are very large ions of s’nilar size. As

crystallization proceeds, changing physiochemical conditions require the

equilibrium composition of sanidine to change. The primary change usually

requires progwssive replacement of orthoclase component (KAlSi308) by albite

component (N jC8) as temperature decreases. This Is rapidly accomplished

under magnati, dltlons simply by exchang~ of Na with K. Early crystallized

sanidine contains relatively high Da concentrations compared to sanidine

fornred later. However, equilibrium for 13a between early and late sanidines

requires an exchange of celslan component (BaA12SiZ08) with albite. This

equilibrium cannot be attained simply by exchange of Ba with Na, because it

also requires a coupled substitution of Al with Si. The latter elements are

present in a much more tightly bound (tetrahedral) structural site wlt;,ln

sanidine, and require considerably greater energies for exchange.

Consequently, although the Na and K exchange is rapid and complete up to the

time of eruption and reflects equilibrium, Da exchange Is limited or does not

26

occur. This process Is here termed “limited equilibrium.” Thus, Portions of

the magma that began to crystallize sanidine earliest will have the highest Ba

contents, and those that formed sani line latest will have the lowest. Thus ,

the differing Ba contents for sar,id!ne between paired Petrologic Units of

Yucca Mountain and Pahute Me$a simply relate to the crystallization history of

the magma, ind not to inherent canposltional differences between the units.

The distribution ana concentration of volatiles (primarily water) within

the megma chamber Is very important, because they certainly provide the

driving force of the eruption. Tile higher their concentration, the more

violent an eruption might be expected. The element cesl~m concentrates

strongly In the upper portion of the magma that produced the Bishop Tuff

(Hildreth, 1979); this element. shows an extremely strong association with

volatiles. Ceslum concentrations In the TRA(lr) un~t are extraordinarily

high, generally more than an order of magnitude higher than those of other NIS

units, Including the TCB unit (Warren, In prep.). The cesium contents suggest

that volatlles concentrated strol~gly in Lhc portion of the magma chamber that

erupted the TRA(lr) unit, but not In the portion of this same magma chamber

that erupted the TCB unit. These volatiles drove an unusually violent

eruption of the TRA(lr) unit that resulted In the tncorporatlon of remarkably

lhrge and poorly sorted lithlc frhgments within the TRA(lr) unit.

CONCLUSIC’NS AND FUTURE RESEARCH

A very strtklng geochemlcal similarity, ba-nd on chemistry of primary

mineral phases, has been demonstrated between volcanic groups that fl~nk the

Timber Mountain Caldcra. It Is concluded that paired units wlthln the two

locations were derived from the same magma. Llthologlc and petrographic

differences between units flanklng Timber Mountain that have simflar mineral

chcmlstrles are ~ttributable to an Inhomogeneous distribution of phenocrysts

and volatile: within the magma. The dlsti’ibutlon of paired units that contain

thick lavas Indicates that the magma body was very large and centered on

Timber Mountain. The similarities In maflc mineral compositions among an

entire group of units suggest that each successive unit represents the

evolution of a single, large magma system.

It Is the author’s bclluf th~t, thn hypothesized magma system centered at

Timber Mountain was also associated with the eruption of the oldest known NTS

volcanic rocks. The nllncral chcml~try Is presently being Invcstlgated for

such units of Pahute Mesa for comparison wlch well-characterized units of

Yucca Mountain (Warren et al., in prep.). The Petrologic Unit Introduced here

serves as a valuable correlative tool for beddsd tuffs, and is particularly

valuable for drill holes that do not penetrats the ash-flow cooling units used

as marker teds. Volcanic units In other areas of the NTS (e.g., Yucca Flat)

can probably be related to those of Pahute Mesa and Yucca Mountain by utll-

Izing the Petrologic Unit. Such use Is required to correlate older, poorly

exposed units effectively over a large region such as the NTS due to mark~d

llthologic differences that may occur among such units at different locations.

ACKNOWLEDGMENTS

This work has built on 20 years of geologic mapping and study, largely by

geologists of the U.S. Geological Survey but also geologists of several other

organizations. Many of their names are found among the references cited. The

correlations described herein could not have been accomplished without the

basic stratlgraphlc framework developed by previous NIS workers.

David E. Broxton, Frank M. 8yers, Jr., and B. W. Smith provided very

timely and thoughtful reviews. The advice and counsel of Frank M. Byers, Jr.,

has been very helpful. Roland C. Hagan and Lois F. Grltzo have greatly aided

in the microprobe analyses. The ability of Marcia A. Jones to produce complex

tables without error and with remarkable speet Is simply astounding.

Similarly, the ability of David A. Mann and Tlno Lucero to create wonderful

probe sections from r~ks that fall apart In one’s hands Is remarkable. Thfs

research has been vigorously supported by Thomas A. Weaver and Wayne A.

Morris, past and present group leaders of the ESS-2 (Geochemistry) group, and

by Jack H. House, program leader for containment. I am vary gratefbl to all

for their generous contribution.

REFERENCES

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Dyers, F. M,, Jr.. W. J. Carr, P. P. Orklldp W. D. Qulnllvan and K. A.Sar cnt,

!“Volcanlc Suiteh and Related Cauldrons of Timber Mountain - Oasis

Val ey Caldera Complex, Southern Nevada,” U.S. Geol. Surv. Prof. Paper 919,69 pp. (1976a).

—.

28

Byers, F. M., Jr., U. J. Carr, R. L. Christianson, P. W. Lipman, P. P. Orkildpand W. D. Quinlivan, “Geologic Map of the Timber Mountain Calder. ~,rea, NyeCounty, Nevada,” U.S. Geol. Surv., Denver, CO, Map 1-891, scale 1:48,000(19?6b).

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.—

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Lipman, P. U., R. L, Christi~nscn, and J. T. O’Connor, “A UompositlonallyZoned Asli-Flow Sheet in Louthorn Nevada,” U.S. GGO1. Surv, Prof. Paper5i?4-F,47 pp. (1966)..— ---

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Sar!cnt, K. A., S, J. Luft, A. 1).Glhbons,tho Quartet Demo Quadrangle, Nye County,C(),Mnp 60-496, SCOIC 1:?4,000 (1966).

~()

Test Sitp, Ncvad~~” in ~a u. Eckc]fun.Memoir 110, 77-06 (1Y68).---

aad Do Lt Hoover, “(icologic Map ofNcvadti,” U.S. Gco1. Surv., l)cnvcr,

Sheridan, H. F., “Emplacement of Pyroclastic Flows: A Review, ” in Ash FlowTuffs, C. E. Chapln and W. E. Elston, eds.,_~Geol. Sot. &ner., Inc.,

Geol, Sot. Amer. Special paperBoulder, CO), pp. 125-136 (1979).

Smith, R. L., “Zones and Zonal Variations In Welded Ash Flows,” IJ.S. Geol.Surv. Prof. Paper 354-F, p. 149-159 (1960).

Warren, R. G., “Use of Petrology and Geochemistry to Define Petrographic UnitsIn Drill Holes Ue19p, Ue19p-1, and Ue19ab, Southeastern Pahute Mesa, Nevadalest Site,” Los Alamos National Labor~tory report (In preparation).

Warren, R. G., F. M. Dyers, Jr., and F. A. Caporusclo, “Petrography andMlnerdl Chemistry of Units of the Topopah Spring, Calico Hills, and CraterFlat Tuffs, and Older Volcanlc Units, with Emphasis on Samples from DrillHole USW-G1, Yucca Mountain, Nevada Test Slto,” Los Alamos NationalLaboratory report (in preparation).

w