o ,_. :: •. ~. ~. t. . ;. t,r:.

GL04060 :- ( ' ,'~ ... , ', "" , :::" F : "

Nash et a1. " -: o~rl-nul/ ;-, t-: :cd C::<)Qcs ~ ':": .. !T ', ':.';"':; LAST t. ;' ',::: ,--_ _ :...:.....:c ......

Volcanism Of The Black Rock Desert and Twin Peaks Areas, West-Central Utah I _I ___ .... _____ _

.... _; i r W. P. Nash, H. R. Crecraft, and S. H. Evans, Jr. ,

r--L i i

~- .

-----_.,--------,---------, Department of Geology and' Geophysics

University of Utah, Salt Lake City, UT 84112

J. I IITRODUCT I ON

West Central Utah has been the site of episodic volcanism over the past 10 million years. The distribution, age and lithology of the various volcanic centers are shown in Figure 1. The region contains basalt and rhyolite lavas, the bimodal association characteristic of the Basin and Range province in the late Cenozoic period. The current model for this biomodal association is that in areas of high regional heat flow such as the Basin and Range, granitic melts are produced in the lower crust by the convective transfer of heat due to the injection of mantle derived basaltic magma.

The distribution of volcanic activity in space and time is illustrated in Figure 2. Quaternary volcanic centers extend from the Mineral Mountains in the south to Crater Springs in the North. The most recent activity has been in the Black Rock Desert, where all the volcanism has taken place in the last 1.5 million years.

BLACK ROCK DESERT Geologic Setting--The Black Rock Desert has

been the scene of periodic volcanism and abundant normal faulting over the past 1.5 million years. The petrography and age relations of basalts have been described by Condie and Barsky (1972) and Hoover (1974). Isherwood (1967) performed a regional gravity survey, and detailed gravity and magnetic surveys, have been completed by Serpa (1980). Spring and well waters in the region have been analyzed by Cleary (1978). Ages of the volcanic units are given in Table I, and whole rock chemical analyses are given in Table 2.

One of the striking features of the Black Rock Desert is abundant normal faulting. All of the major lavas including the young Ice Springs flows have been faulted. Evidence of faulting would be more extensive except that many scarps were erased by the action of Lake Bonneville . Faulting trends generally north~south, with southern faulting tending north-northeast and northern faulting ' tending north- northwest. The majority of the motion is downthrown to the west, and the volcanism lies along the eastern margin of a large graben which constitutes a topographic low containing Clear Lake. The western side of the graben terminates in the Cricket Mountains, 30 km to the west, and noma I fau It i ng dOl·mthrOlm to the east is evident in the Deseret volcanic

, field. The location of volcanic vent s in ,the

!

"

; ,

" i !' I.

"

8U.CK WOUHTAIHS

Fig. 1. Distribution and ages of volcanic rocks in west-central Utah. Tr = Tertiary rhyolite, Tb = Tertiary basalt, Qr ,. Quaterna~y

, rhyolite, Qb = Quaternary basalt. Ages are g1ven in parentheses in millions of years except where not ed otherwise. The location of Known Geothermal Resource Area (KGRA) administrative units are shown in stippled pattern .

L... _ .• ------ ---_._-- -- - '- - '--'--- - .- , - .- - - .-~- .-

I

~ .

LJ Na~1t e.t a 1.

' ...... :.' 1' " "-, ........ ~ ", ., d r·. ··l ·r .. .., ,...:"..-t r : .',"! , .. -,J '~Bla:cIiHOck~e~eri\a~been ' :c~~t;~l 'l ed 'by thi s----'

major structural discontinuity along the eastern : margin of the Black Rock Oesert graben. Hegional !

faulting is aligned along the edge of the graben which is well defined by gravity data (Serpa, 1980). 11ultiple vents in the Black Rock, Tabernacle, and Pavant fields are aligned along north-south directions; a line from Black Hock Volcano to White Mountain passes directly through a Holocene tufa field, intersecting the active

~ (j , 1 ;-'"1 :'- .; (: rf." : r. ; : ~ ~ ~r; : I ..... ~ r:;. : c::--':; I.AST ;o .... \ .' .. ~

hypersthen'e-' set -i nil fi iJ'e" gra fnecj 'a'nd- glass rfdl groundmass. There are occasional inclusions of large embayed feldspar often associated with

. ~artz. . The andesite unit is overlain directly by

basalts termed Beav~r Ridge I by Hoover (1974) who dated them at 0.9 m.y. These lavas are generally coarse grained and have a diabasic texture. The lavas occupy the central portion of

thennal springs at .. 1ts north and sout~ . ends •. , .. ___ _ Beaver Hidge, are cut by numerous normal faults, and have been modified by 'ancient Lake ' Bonneville. The distribution of these lavas is

"Petrolo9y~-Compositionally the basalts are tholeiitic in affinity, yet lack low calicum pyroxenes in the mode. All of the basalts are hypersthene nonnative. Most contain nonnative olivine with the exception of the Black Rock Volcano and Sunstone Knoll lavas which contain nonnative quartz. The single andesite is

I calc-alkalic, and the rhyolite of White Mountain has characteristics of highly evolved rhyolites, including high total alkalies and a relatively flat rare earth pattern, with a pronounced negative europium anomaly. . The oldest volcanic unit is the 1.5 million year old andesite exposed along the western side of Beaver Ridge. This unit is distinctively richer in silica than the other mafic lavas of

_:tJ)e_~J-,!c;~ .Rock Oese}"J ____ Tjle .Jav.as cont.~in . __ . _ _._~uhedral phenocrysts of plagio~lase. augite and , -. .....

AREA. b "-1

_ ._ill t-.. ..

;'.1 r --;; ~'-' --...,

probably significantly larger than now exposed, as lake depqsits veneer a large portion of the area. .

Beaver Ridge II lavas were erupted onto Beaver Ridge I . lavas 0.54 m.y. ago (Hoover, 1974). The lavas contain large plagioclase phenocrysts (1 cm) in a fine grained groundmass which contains interstitial quartz. · They resemble Black Rock Volcano lavas which were erupted approximately 5 km to the southeast 0.67 m.y. a~o (Hoover,1974). The Black Rock lavas are chemically and mineralogically identical to the lava at Sunstone Knoll, erupted 40 km to the northwest.

White Mountain (0.4 m.y.) is the youngest rhyolite yet dated in Utah. The dome was at one time under Lake Bonneville, and has been modified

~y_t.!:l.e .la.keL_Jre pumiceous, carapace has been

I ,..., ..

D I

I c

°1 'I

! • -.'1 ..... 1 ....

· 0 .... •..... n •••• I11 ....

' el--__ l = CI.',I' ... , ....

C M ••• ' •••• Mllil

CJ

,.," , ..... 2

---_ ... l1li11 .,,1 l1li ••• •

C,.l" ",'ft,. 0-

oL ______________________ ~ _____ ~CNi::~~~~~~w:":":.'~ .. ___________ <~.~ .. ~,.~.I ____________ _:~ .OUTH ..

• r

I !

:1

I i i I.

I

i,i

F' 2 Space-time Distribution of volcanic units projected to a n~~th-~outh line extending from the Thomas Range to the Black Mountains . i Open Hectangles are rhyol ite, .. ~~~d rectangles are bas_a_l_t_. ______________ ---'

----'---_. __ ._ .

I --_._--_. __ .-------'-------------

LJ -;"" ... ' : .... ('\:')'1' ;· ... r' ,,~ 'I. ~1 ~ ," r:: 1:-• '1 ,'-

• .' ' .• : !~ ,I":'~'~' ,NMh,et~l~L:":'!"' __ :. T--~~:""'~ ~ ..... :;:-- ;·~f·~: .:. ': '. ," ", ~-:',b~;~:1 r. "(1":',_ _ ___ . .'r..~~~!.~ ... ~!:_~~~~d. ~~~i:·_~._:.~~-':"':·~.~.:';-':~~ .:.'4.:.' ~_'.::.-'::':' -eroded'away';'"'iild'heach depOsi1icomposedof--"""--~ :Uonneville or at a time of low water level and cobbles of perlite are developed on the : :are accordingly less than 12,000 years old. southeastern edge of the dome. A fairly well __ :Holmes (1978) determined an age of 7,000 i 1,100' defined terrace has been developed on the dome at years by thermal luminescence dating. Initial the 4760 foot level and portions of the dome are eruption produced tuff cones, a larger one currently concealed by wind blown sands. The central to the lava field, and a smaller cone at volume of the dome is estimated to have been 0.1 the northern margin. The tuffs contain xenoliths km3• of lake Boneville mudstones and gravels as well

The Pavant lava field can be divided into 3 volcanic units. The oldest unit, Pavant Lavas, consists of pahoehoe basalt flows erupted from several north-south trending vents. These lavas are overlain by thick flows of Pavant Ridge basalt. The internal structure of these basalts suggests that they were aa flows. The youngest volcanic unit 1s the large sideromelane and palagonite tuff cone, Pavant Butte. Pavant Butte was erupted through lake Bonneville, and is composed entirely of pyroclastic material, the subaerial portions of which have been converted to palagonite. Low lying outcrops of palagonite on the southeast margin of Pavant Butte are evidence of an earlier tuff cone which was destroyed either by Lake Bonneville or the

.construction of the present tuff cone. Because of uncertainties in age dating (Hoover, 1974), the volcanism of the Pavant field can only be constrained to the period of 130,000 to 30,000 years ago. The early Pavant Lavas consist of phenocrysts of olivine and plagioclase set in a quench textured groundmass of the same phases together with clinopyroxene, apatite and iron-titanium oxides. Plumose aggregates as well as swallowtailed coreless grains indicate rapid cooling of the groundmass characteristic of thin flows and subaqueous flows. Pavant Ridge lavas are coarser grained and diabasic in texture,

as Beaver Ridge basalts. Following construction ,of the tuff cones, lavas were erupted from the central region, which subsequently subsided to

'form a 20 m deep crater 0.4 km in diameter. lava was transported by an extensive network of lava

,tubes and numerous pit craters dot the surface of ,the field. The Tabernacle lavas are distinct in 'hand specimen, containing large phenocrysts of olivine and plagioclase. In addition, the groundmass contains clinopyroxene, iron-titanium oxides, apatite, sandine and glass.

The youngest volcanic episode in the Black Rock Desert makes up the Ice Springs volcanic

,field. Morphological aspects suggest that the .volcanism took place within'the last 1,000 years. ·A carbon 14 date (Valastro et al., 1974) yielded an age of 800 i 170 years b.p. The date was obtained on a root in a soil under the flow.

'However, it is not certain that the root was

_...re.sembling the_lavas of Beaver Ridge. .._ •• _ . The lavas of the Tabernacle field were

:charred, and the date may not represent the age 'of the flow; noneless the date is consistent with :the morphology. The lavas are perfectly fresh, :and vegetation is supported only where wind blown isands have been deposited in low lying areas. lava flows emanated from a set of nested cinder :and agglutinate cones in the southeastern portion 'of the field. Lavas and pyroclastic deposits ~contain abundant fragments of partially fused ;gneiss and lake sediments. The lavas contain :sparse phenocrysts of olivine and plagioclase set 'in a fine grained groundmass of olivine, 'clinopyroxene, orthopyroxene, plagioclase, :j~on-titanium oxides and apatite.

_e.r:up.t~d ~eJthet_after .. the demi se. oCL.ak~~~_

,

Table 1. Ages of Black Rock Desert Volcanic Rocks.

Volcanic Unit Average Age Reference

Ice Springs basalt

Tabernacle basalt

Pavant lava field

800t170 y.

7000=1100

30,OOO!l30,OOO y. 220,OOOt260,OOO y.

White Hountafn rhyolite 0.43:.07 m.y.

Beaver Ridge II bas! It

Black Rock Volcano

Beaver Ridge I Basalt

Beaver Ridge Andesite

0.40:.02 m.y.

0.54!.04 m.y.

0.67±.07 m.y.

0.92±.05 m.y.

1.43::.11 "'.Y. 1 .54.t.22 "'.y.

1. Vahstro, et al., 1974 2. Hoover, 1974 3. Condie and Barsky, 1972 4. lipman, et a1., 1978 S. University of Utah date 6. Holmes, 1978.

6

2 3

4 5

2

2

2

5 5

.. --~- --- -------------'

I I

I

: In a detailed chemical study of the lava !field Lynch (1980) documented significant inter­'and intra-eruption chemical variations. .Elemental trends include increases in titanium, liron, phosphorus, calcium, and strontium, and decreases in Silicon, potassium, rubidium, nickel, chromium, and zirconrum with decreasing age. Argon analyses establish the existence of excess radiogenic argon in the basaltic cinder and in lava flows which yield apparent "ages" of 16 and 4.3 million years, respectively. Lynch (1980) concluded that the chemical variability

; was a combination of (1) 6-8% fractionation of . plagioclase, minor olivine and magnetite at : shallow depth, (2) less than 1% assimilation of silicic crustal basement rocks and, (3) interaction of compositionally similar magma pul ses. ,

Geothermal Aspects--Geological criteria commonly employed to delineate areas with geothermal potential include 1) silicic , volcanism, 2) volcanism less than about 1 m.y. ln age, and 3) evidence of thermal waters leaking to the surface. The Black Rock Desert area fulfills all three criteria. The White Mountain rhyol ite, although inSignificant in volume, was erupted

I 400,000 years ago. Basaltic volcanism has _____ " _. ':,?~-=~r.~e~ :pi so~~c~.~ lJ __ ~~e!, !_h:,_~~:~_l_~,':>:,'-!. _____ _

Nash et al. ~'.,. r()py I I ~., \... ' .. I I

~~ •• _:~~:. ~j ~.~~ L.~.~~, r ~ I,:" "/E i '::'.'~'~.( ~·~2~~.LJ_~~~n:;.c..·i ___ j .r-'

Table 2.' Chemical Analyse~ of Black Rock Desert Lavas.

i

I

'l ., ,.1 "

j .~ I

I I

S102

n02

A1 203 Fe203

FeO

MnO

MgO

CaO

Na20

K20

P205

CO2 Sum

-O=F

Total

·20

66.52

1.06

12.27

1.67

5.72

0.16

0.75

3.72 .

3.21

2.63

0.28

1.28

99.27

Key to Samples:

24

4B.65

1.49

16.40

1.97

8.89

0.19

7.29

10.24

3.30

0.58

0.38

0.15

99.53

20 Beaver Ridge andesite 24 Beaver RiBge I basalt

25

50.88

1.73

16.06

4.41

7.53

0.19

4.58

9.03

3.38

1.00

0.38

0.27

.--

99.84

25 Black Rock Volcano .basalt 26 Sunstone Knoll basalt 3 Pavant basalt

26

50.99

1.76

'15.46

5.90

6.32

0.20

5.05

8.78

3.34

0.92

0.39

0.37

99.48

i· "-i extending to as recently as about 1,000 years ; ago. Finally, mixing models for waters from ! Meadow Hot Spri ng suggest a hot water temperature • of 190°C to 230°C mixed with a significant 'fraction (86 to 90%) of cold water (Cleary,

1978). These features, combined with evidence of ·.active tectonism, suggest that the region : warrants cont i nued exp 1 orat i on for concealed heat ~ sources.

TWIN PEAKS VOLCANIC COI~PLEX

. , 1 I

The Twin Peaks volcanic complex is composed of domes, flows and pyroclastic material of

_r.hyol.iticcomposition. basalt flows, and a local

L_.

3

50.67

1.50

17.01

1.05

7.90 .

0.16

7.02

9.97

3.03

1.05

0.35

0.05

6

48.97

1.51

16.80

1.52

8.04

0.17

7.88

10.61

2.85

0.71

0.38

0.14

15

49.04

1.55

16.68

1.57

9.15

0.19

7.46

9.30

3.13

0.89

0.41

0.27

99.76 99.58 99.64

6 15 12

Hf1-3

Pavant Ridge basalt Tabernacle basalt Ice Springs basalt White Mountain rhyolite

Analyst: W. P. Nash·

12

48.99

2.04

14.99

1.39

9.53

0.19

7.83

9.67

2.81

0.99

0.61

·0.37

99.41

WM-3

73.16

0.17

13.37

0.41

1.29

0.16

0.58

1.12

4.34

4.49

0.09

0.09

0.00

0.39

0.05

99.71

0.16

99.55

!'lacustri nesequence. An i ni ti a 1 study of the : area including a geologic map, was made by Haugh ·(1978) who, without benefit of K-Ar age dating, : unfortunately misinterpreted the sequence of : eruptive events. Volcanic activity in the region spanned a period from 2.7 m.y. to less than 1 million years ago. Ages of the various volcanic units are given in Table 3, and a generalized

: geologic map is given in Figure 3. Whole rock 'chemical analyses of the silicic units are given . along with modes in Table 4.

Geologic History--Volcanism began in the ·,Twin Peaks area with the eruption of the Coyote Hills rhyolite 2.7 m.y. ago. This unit has an exposed volume of 1 km3 and occurs in the

'::" .. southwestern portion ~f Coyote Hills and in the

--------------------------~

I

I I I

I I

i I

I

I j

I

r I

LJ '"77tlr"'P'l rrr.' ,1 ..., ""' t;o;-;.;.C I • ~U. . Nash et a 1. I ".

:ocd-nu:n:')I!('.'d ~ilqes: ;· ... u i ~. ;i~ ~; LAST j\;M\~E

,

~-

I I J

o I M'L'S

GEOLOGY MAP OF TWIN PEAKS AREA, MILLARD COUNTY, UTAH

II--""T--'"-.--r-I •• o • KILO"ET,-...

Oal - AlluvIum

Ob - Black Rock B ••• ,t

Tnt - South TwIn Peak Rhyollt.

Trmd - MId Dome Rhyolite

Trnt - North TwIn Pe.k Rhyollt.

Tb - Basalt

TI - Lake lImeaton.

Trcm - Cudahy Mine UnIt

Trch - Coyote Hilla Rhyolite

Ob d1\ JZ~ 0: '1}267

~Qb

0.1

x CORE

Fig. 3. Geologic map of the Twin Peaks volcanic complex after Haugh (1978) with modifications by., Crecraft and Nash.

. Trcm~ . tj

Tb

'-- '.-

-.---------

.;

;i

, '

D . : ., .. [I F t. :: ::"

N·Hfi. et 1 " ' . 1 _ . .. " . ... . : . . . " ~; .•. • , r .. . - .. , •.•. : . .• , . •. [ ..J ' • 'j b ' . UTH,..~·C"LA-TrJ\.~,... . . • , __ , ___ :,_" ~_, c.,-·n_·_n : J 0:! : [, ~ Ij ,, [np_':_:: ___ --. [0(. ·num. (~ r80 L) ;.; r; esJ A Un . ., ~ . "'.' VI::'

lIbh J . k-AI" Agu If T.,h h.", Lt,,,

$nol. Unit A9f (a. ,.)

CC79-4 Ihtk Aoe" .... It 1.12rl).01

«17_' $.olin Tlift", re ... rh.ro1tl. 1.)5,0.08

«71-to "t. eo.. '1'1)'01", I.ShO. OI

«77-4 Morth l"tn , .... rhyolite 2.1"0.08

CC77.3Q ttorth T"tn ,,. .. r"')'OIU. I.O.O.M

W1,..eU T •• " 'uh Null 2.62.0. 35

W19-8l ,,,I,, ,uts wuh 2.II.O.S!

«77. It C~hy "t", UnU obs,jjl.1'I 2.41.0. 12

«17-1' Cllckhy ",,,t U"It obstclf.n 1.54t O.09

«77_' C41d.hy "'nt Untt obsldl." 2.n.0.l0

«19-. Co,ott Hflh rh.roltu Z.U!O.09

«19·2 Coyott HUh rhyolitf Z. f1tO.l0

«71·11 (o)"Oti Htlh rh101". %.14.0.10

for lna.yt'ul dettfh on .v' dAlt' IU h • .,. et al. (1980)

L:- ··_ .. · .. -.. . ,I

Ttblt 4. Chft'llul A~lyu, Ind tbdu of Reprtunuth,

lW;: Pukl Rh~l1t.. 20 ,

AGE

1"":"' 1 If"" oJ I ;. ~ ... c , I l:":IU

I -L

.J

st02 70.91 75.B3 73.B3 74.99 76.10

TtO,

Al,03

r·203 roO ""0 1190 COO

"',0 KZO P

20

5 H

200

HzO­r CO2 5 .... .

.{l·r Toul

0.47

Tl.47

1.17

1. 53

0.05

0.53

2.09

3. 50

5.19

0.12

0. 55

0.01

0.15

0. 44

100.19

0.06

l00. Tl

0.0

0 . 11

12.B6

0.21

0.66

0.05

0.07

0.14

3.93

4. 96

0.02

~.35

0.04

0.20

0.00

l!lO . O6

0.08

99 . 98

tr

0.30 0.26 0.01\

13.04 1'.97 1' . 30

0.77 1.06 0.81

1.19 0.24 0.40

0.02 0.00 0.01

0 . 34 0.20 0.16

1.30 1.20 0.89

3.70 2.95 3.20

4.72 4.93 5.18

0.09 0.05 0.01

0.49 0.35 0.11

0.04 0. 06 0.01

0.21 0.14 0.14

0.02 0.16 0.05

100.06 99.56 99.54

0.09 0.06 0.06

99.97 99.50 99.48

10.4 · 9.7 7.0

!

! i I

I i

_ ~.r.J::... ~ z::'Io •. L' ..cc~==--_____ -=--:-___ .., northern and southern portion of North Twin Peak. The unit contains plagioclase, small amounts of

.sanidine, ortho- and clinopyroxene, and Fe-Ti oxides. Chemically this unit is distinguished by

f>./art; Sandtnt ,1'9iocl.se 8tottte

1.0 tr 11 . B tr

5.2 11.5 7.1 I Tl.5 11.0 4.1

:Si02 contents ranging from 70.9 to 71.6%. : The next series of eruptions produced the ' relatively voluminous Cudahy Mine unit which ;consists of obsidian, felsite and :volcanoclastics. Its exposed volume is about 2 :km3, but a minimum 80 m thickness of a chemically ' related tuff . encountered in a drill hole 10 km ' south of Twin Peak suggests than an additional 8 km3 or more· may be buried. The eruption of this tuff resulted in local subsidence and the

'development of a tuffaceous lacustrlne sequence. .The lacustrine unit is approximately 200 m thick and overlies fluvial sediments. The lower portion of the lacustrine unit is 'rich in tuffaceous material derived from eruption of the Cudahy Mine Unit. As the eruption cycle waned,

;limestones became more dominant in the upper :portion of the sequence. During the late stages :of lacustrine ' sedimentation, basalts flowed into the lake, and subsequently the whole unit was

:capped by voluminous basalt flows which have been :dated at 2.5 m.y. .

Following this period of basaltic eruption, ' the North Twin Peak rhyolite dome was formed; it :has an exposed volume of 0.4 km3• Chemical and .mineralogical variety within the North Twin Peak rhyol itesuggests that thi s unit may consi st of two or more discrete flows. ' Si02 content ranges from 71.9 to 73.8%. Clinopyroxene is a dominant mafic mineral in the less silicic varieties, whereas biotite is predominant in the more silicic portions. Composite aggregates of plagioclase, pyroxene, and biotite, and graphic intergrowths of quartz and sanidine, which ,,:re believed to be xenolithic, indicate that

. throughout this period the magma remained well mi xed .

The Mid Dome magma comprises the small hill between North and South Twin Peaks; its exposed volume is 7xl0-4 km3• The unit is thought to be

_ ch,ron()10gic.allyintermediate between North and ;

0.0 tr '.0 1.1 0.4 i 0.9 0.0 0. 0 0.0 0.0 0.6 0.0 0.0 .0.0 0.0 0.7 tr 0.5 0. 2 0.6 I Cl t nopYl""Ount

Orthopyroxene O.'des Ground.l\ass 85.0

Key to INlyus: 15 CDyoto Hills

>99.5 68.4 66. 5 BO. B

.1 20 ~td DofIt

19 . Cudahy "tne: Unit 4 North r"fn Pelk

8 South Twt II Pu k

Anahsts : W. P. NlSh, H. Cr-ecrlft, R. F ~ Holmes

. 1

. !

.... .. - - -- '- - -.. -- - _.- - .- .. _. : . -: .. -- ._ - :---. - - ---: ... - - ... .• - - -I

L South' Tw'i n' Peaks" on the 'basi s of petrography and cherni stry.

South Twin Peak rhyolite was erupted through the lacustrine and basalt sequence, and has a volume of 0.3 km3• South Twin Peak appears to be the youngest rhyolite in the Twin Peaks field •. Quartz, sanidine, plagioclas~, 'and biotite are present. The phenocryst content is highly variable ranging from 3% to 20%. The silica content for the one sample analyzed is 76.2%.

Structure--The structure of the Twin Peaks area has been dominated by east-west Basin and . Range extension. Faulted basalt flows in and around the .Twin Peaks area indicate that east-west extension has continued up until as recently as 1,000 years ago, ~onsistent with present-day seismicity. Deformation within the field is dominated by the development of a doubly plunging anticline extending from the east si~e of South Twin Peak southward for 12 kilometers. A maximum of 400 m of uplift along the anticiine ' has e~posed the lacustrine sequence in its eroded core. The timing of the formation of the . anticline is vncertain; it may be resurgent doming related to the emplacement of Twin Peaks ma~nas, or it may have occurred after the cessation of rhyolitic volcanism and be associated with the emplacement of basaltic magma; two vents are located along the axis of the anticline. These basa lts have not yet been dated, but they represent the youngest volcanism

__ i~the field and post-date the deformation . 'r • • - - ~ \. :. ~ : ~ :. ~: ~, ~ {" l j :-- .-~ i j ":.' ~~ : i ~ ~ i :l::

:_ --------------_.- ._--_ .. _-_._ . ..: '--- .-...:....------ ---------------'

i ,

\ .

~.U i · ~C;-.' .) Lt.S: r·;..: .. ··=: '. ' , I. '! - ,, ; ! :--r. - -· ' e~ ~ ~ ~ . : . ~ ~ ,e-.----.. - - . -- - - -- -.- . .. -- . . -- .-.. ---- . .. - . .. --~--

I I L ...

;( +"' .'1" ., .. , .. . .-,,1 n il {1 p- ' Na,sh .et al. , '. ': -: '. . , :: I~J. ~. , J , I .. 1 .1 . •• ~ .' • • ,. l . "'""'I ,_ •• " ' .. ....

. 1979). " This is a significant anomaly which may be related to the local uplift and younger ~afic

-' ... volcanism. _

_" ' Hhyol ite petrogenesis--Crustal thi nning --anc( concommittent injection of basalt into the lower crust caused partial ' melting followed by the diapiric rise of magma of the composition of the Coyote Hills rhyolite. Heat input into the ' -' - ,\ (: i; d r .. ..:; ;:. C::r.JT::R r: Q

ACKNmllEDGMENTS magma, probably by injection of basalt, was required to maintain the magma above its solidus for periods beyond 50,000 years the estimated time for crystallization of the Twin ' Peaks magma chamber (Carrier and Chapman, 1980). ~_! _ Differentiation over the 100,000-200,000 year __ .. __

This study was supported by U.S. Geological Survey grant 14-08-0001-G-343 and Department of Energy contract DE-AC07-80ID12079. R. F. Holmes assisted in the chemical analyses of the Twin Peaks lavas. period following emplacement of the Coyote Hills

magma resulted in a chemically zoned magma :i .. L; . ,.~. , __ __ . 1 chamber, the stagnant upper· portion of which was

erupted to form the Cudahy Ni ne unit. The chemical gradients were remarkably similiar to i those for the Bishop Tuff, California, magma l REFERENCES chamber and are due to liquid state J ' 1"--'· . differentiation (Smith, 1979; Hildreth, 1979; Carrie~, rio l., 1979, Gravity and heat flow Crecraft and Nash, 1980). Crystal settling was I studies at Twin Peaks: an area of late of only minor influence in the chemical evolution : : Tertiary silicic volcanism in Millard of the Cudahy mine unit. Highly evolved silicic ; ·1 County, Utah: M.S. Thesis, Univ. U~ah, 120 magmas _i n the roof zones of magma chambers are [ p. characterized by enrichment in Si, Na, Pb, Y. Sb. : Carrier, D. L., and Chapman, D. S., 1980, ·Gra-Cs, U, Th and heavy rare earth elements, and; vity and thermal models of the Twin Peaks depletion in Mg, Co, Fe, Sr, Ba and light rare magma system, west-central Utah: Geothermal earth elements. Details of the chemistry of the . . Resources Council .Trans., in press. Twin Peaks magmas are given by Crecraft et al. ' Chapman, D. S., Blackwell, O. D., Parry, W. T., (1980). Sill. W. R., Ward, S. H. and Whelan, J. A.

The North Twin Peak rhyolite was apparently 1979, Regional heat flow and geochemical ' erupted after stagnation of the stratified roof . studies in southwest Utah: U. S. Geo. Surv •• zone collapsed, or from below this upper stagnant ' Contract 14-08-0001-G-341, Univ. Utah. 119 zone. large phenocryst sizes attest to a long : p. period of crystallization and yet the magma t Cleary, M. D., 1978, Description and interpreta-contained xenolithic material and so appears to tion of chemical geothermometry as applied have been well mixed. Major element trends from to Utah spring and well waters: M. · S • . North Twin Peak through the Mid Dome to South Thesis, Univ. Utah, 73 p. Twin Peak rhyolite indicate that crystal settling Condie, K. and Barsky, C., 1972, Origin of was the primary control on major chemical ! . Quaternary basalts from the Black Rock

i evolution in contrast to the earlier sequence in I Desert region, Utah: Geol. Soc. Amer. Bull. which liquid state differentiation in a . : v. 28, p. 333-352. non-convecting roof zone dominated. A I Crecraft. :H. R. and Nash, W. P., 1980, significantly shorter interval between eruptions ' liquid-state differentiation in silicic of North Twi n Peak and South Twi n Peak rhyo 1 i tes magmas: Geo I. Soc ~ Amer.Ann. Meet. Abs. suggests that the thermal input into the magma with Programs, in press. had ceased; this may also have been responsible Crecraft, H. R., Nash, W. P.i and Evans, S. for the collapse of the chemically and thermally : H.Jr.,1980, Petrology, geochronology and stratified roof zone prior to eruption of the " chemical .evolution of the Twin Peaks North Twin Peak rhyolite. rhyolite domes, Utah: Topical Report, Dept.

Geothermal Aspects--Conductive cooling . i Energy Contract .OE -AC07-80IOI2079, Univ. models for silicic magmas at Twin Peaks (Carrier Utah, 200 p. and Chapman, 1980) indicate that any magmatic . Evans. S. H., Jr., Crecraft, H. R •• and Nash, W. heat present 2.5 m.y. ago would have long been P., 1980, K/Ar ages of silicic volcanism in dissipated. On the other hand resurgent doming the Twin Pe~ks/Cove Creek Domes area, and the eruption of younger basalt indicated that southwestern Utah: Isochron/Jest, in press. there may have been renewed thermal inputs into Haugh, G. R., 1978, Late Cenozoic; cauldron the local area. related silicic volcanism in the Twin Peaks

Chemical analyses of well water·s from the -area, Millard County, Utah: Brigham Young northern portion of the area do not indicate the Vniv. Geol. Studies, v. 25, p. 67-82 . presence of thermal waters (Cleary, 1978) . Hildreth, E .• W. , 1979, The Bishop Tuff: Evidence Similarly, heat flow measurements from the for the origin of compositional zonation in northern and central regions yield an average silicic magma chambers: Geol. Soc. Amer . heat flow of 96mW/m2 (Carrier, 1979) which ·is Spec . Paper 180, p. 43- 75. ne~r the mean for eastern Basin and Range heat ' Holmes, R. F., 1978, Thermolumi nsecence dat ing of' flow of 90 ± 10 m'rl/m2 (Chapman et al . , 1979) . In ' Qua t ernary basalts : continental basalts from contrast, two dr ill holes at t he sout hern end of the eastern margin of the Basin and Range the plunging ant icline yielded heat flow values province, Utah and Arizona: M. S. Thesis of 451 ± 55 and 404 ± 46 mW/m2 (Cha pman et al .. . Bri gham Young Uniy •• 91 p •

. " .: :-,;'-L-: :.': ~ ',::.' -. : .: : ' ,", :?

7

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• N4Sh et al. . · . ·~ T - , . . :-:: r , • .' ~ ,; ' ·': "',·:; ·.'~'1d ~ '. : ) . . " -:: '

~Hociver, J. D.; -1974;-Periodic Quate·rnary - - - -.~~ ·volcanism in the Black Rock Desert, Utah: -'- ., Brigham Young Univ. Geol. Studies, v. 21, p. 2-:12.

Isherwood, W. F., 1967, Regional gravity surveys of parts of Millard, Juab and Sevier Counties, Utah: M. S. Thesis, Univ. Utah, 31 p.

Lynch, W. C., 1980, Chemical trends in the Ice Springs basalt, Black Rock Desert, Utah: M. S. Thesis, Univ. Utah, 86 p. .

Serpa, L. F., 1980, Detailed gravity and aeromag­netic surveys in the Black Rock Desert area, Utah: M. S. Thesis, Univ. Utah, 210 p.

Smith, R. L., 1979, Ash-flow magmatism: Geol. Soc. Amer. Spec. Paper 180, p. 5-27.

Valastro, S., Jr, Davis, E. M., and Varela, A.

-.

G., 1972, Texas at Austin Radiocarbon Oates IX: Radiocarbon, v. 14, p. 461-486.

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