AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY 

Instructions on ordering publications of the U.S. Geological Survey, along with prices of the last offerings, are given in the current-year issues of the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Survey publications released prior to the current year are listed in the most recent annual "Price and Availability List" Publications 

that are listed in various U.S. Geological Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and 

Availability List" are no longer available. 

Prices of reports released to the open files are given in the listing "U.S. Geological Survey Open-File Reports," updated monthly, which is for sale in microfiche from the U.S. Geological Survey, Books and Open-File Reports Section, Federal Center, Box 

25425, Denver, CO 80225. Reports released through the NTIS may be obtained by writing to the National Technical Information 

Service, U.S. Department of Commerce, Springfield, VA 22161; please include NTIS report number with inquiry. 

Order U.S. Geological Survey publications by mail or over the counter from the offices given below. 

BY MAIL 

Books 

Professional Papers, Bulletins, Water-Supply Papers, Techniques 

of Water-Resources Investigations, Circulars, publications of general interest (such as leaflets, pamphlets, booklets), single copies of Earthquakes 

& Volcanoes, Preliminary Determination of Epicenters, and some miscellaneous reports, including some of the foregoing series that have gone 

out of print at the Superintendent of Documents, are obtainable by mail 

from 

U.S. Geological Survey, Books and Open-File Reports 

Federal Center, Box 25425 

Denver, CO 80225 

Subscriptions to periodicals (Earthquakes & Volcanoes and 

Preliminary Determination of Epicenters) can be obtained ONLY from 

the 

Superintendent of Documents 

Government Printing omce 

Washington, D.C. 20402 

(Check or money order must be payable to Superintendent of Documents.) 

Maps 

For maps, address mail orders to 

U.S. Geological Survey, Map Distribution 

Federal Center, Box 25286 

Denver, CO 80225 

Residents of Alaska may order maps from 

Alaska Distribution Section, U.S. Geological Survey, 

New Federal Building - Box 12 

101 Twelfth Ave., Fairbanks, AK 99701 

OVER THE COUNTER 

Books 

Books of the U.S. Geological Survey are available over the 

counter at the following Geological Survey Public Inquiries Offices, all 

of which are authorized agents of the Superintendent of Documents: 

• WASHINGTON, D.C.--Main Interior Bldg., 2600 corridor, 

18th and C Sts., NW. 

• DENVER, Colorado--Federal Bldg., Rm. 169, 1961 Stout St. 

• LOS ANGELES, California--Federal Bldg., Rm. 7638, 300 N. 

Los Angeles St. 

• MENLO PARK, California--Bldg. 3 (Stop 533}, Rm. 3128, 

345 Middlefield Rd. 

• RESTON, Vlrginia--503 National Center, Rm. 1C402, 12201 

Sumise Valley Dr. 

• SALT LAKE CITY, Utah--Federal Bldg., Rm. 8105, 125 

South State St. 

• SAN FRANCISCO, California--Customhouse, Rm. 504, 555 

Battery St. 

• SPOKANE, Washington--U.S. Courthouse, Rm. 678, West 

920 Riverside Ave .. 

• ANCHORAGE, Alaska--Rm. 101, 4230 University Dr. 

• ANCHORAGE, Alaska--Federal Bldg, Rm. E-146, 701 C St. 

Maps 

Maps may be purchased over the counter at the U.S. Geological Survey offices where books are sold (all addresses in above list) and 

at the following Geological Survey offices: 

• ROLLA, M.lssouri--1400 Independence Rd. 

• DENVER, Colorado--Map Distribution, Bldg. 810, Federal 

Center 

• FAIRBANKS, Alaska--New Federal Bldg., 101 Twelfth Ave. 
K-Ar Ages of Jurassic to Tertiary 

Plutonic and Metamorphic Rocks, 

Northwestern Utah and Northeastern Nevada 

By DAVID M. MILLER, JOHN K. NAKATA, and 

LINDA L. GLICK 

Isotopic data are used to interpret emplacement ages for 

plutons and ages for metamorphic episodes within a region 

encompassing the northwest corner of Utah and adjacent 

eastern Nevada and southern Idaho 

U.S. GEOLOGICAL SURVEY BULLETIN 1906 
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication 

is for descriptive purposes only and does not imply 

endorsement by the U.S. Government 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1990 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Miller, David M. (David MacArthur), 1951-

K-Ar ages of Jurassic to Tertiary plutonic and metamorphic rocks, northwestern Utah and northeastern Nevada I by David M. Miller, John K. Nakata, 

and Linda L. Glick. 

p. cm.-(U.S. Geological Survey bulletin; 1906) 

"Isotopic data are used to interpret emplacement ages for plutons and 

ages for metamorphic episodes within a region encompassing the northwest 

corner of Utah and adjacent eastern Nevada and southern Idaho." 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:1906 

1. Intrusions (Geology) -Utah. 2. Intrusions (Geology) -Nevada. 3. Potassium-argon dating. 4. Rocks, Metamorphic-Utah. 5. Rocks, MetamorphicNevada. I. Nakata, John K. II. Glick, Linda L. Ill. Title. IV. Series. 

QE75.B9 no. 1906 557.3 s-dc20 89--600324 

[OE611.5.U6] [552'.3'09792] CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Acknowledgments 2 

Methods. 2 

Results 2 

Jurassic plutons 4 

Newfoundland Mountains 5 

Crater Island 7 

Cretaceous plutons 9 

Metamorphic rocks 11 

Toano Range 13 

Raft River Mountains 13 

Tertiary plutons 13 

Toano Range 14 

Bovine Mountains 14 

Summary and conclusions 14 

References cited 16 

FIGURES 

1. Index map showing location of study area and major Mesozoic and Cenozoic 

tectonic elements of the Great Basin 2 

2. Simplified geologic map showing distribution of pre-Cenozoic strata and of 

Mesozoic and Cenozoic plutons and metamorphic rocks within study area and 

K-Ar sample sites 3 

3. Map showing generalized geology of pre-Cenozoic rocks at Crater Island 

(northern Silver Island Mountains) and K-Ar sample sites 8 

4. Generalized geologic map of Miocene and older rocks of northern Toano 

Range, showing K-Ar sample sites 10 

5. Age spectra diagram for muscovite from the Toano Springs pluton 12 

6. Histogram of conventional K-Ar and comparable U-Pb, Rb-Sr, and 40Ar/39Ar 

ages for igneous rocks in five mountain ranges: Newfoundland, Silver Island, 

Pilot, Toano, and Grouse Creek 15 

TABLES 

1. K-Ar data for Jurassic igneous rocks 6 

2. K-Ar data for Cretaceous and Tertiary igneous and metamorphic rocks 11 

3. 40Ar/ 39Ar analytical data for muscovite sample B86TR-111 from the Toano 

Springs pluton l2 

Contents Ill 

K-Ar Ages of Jurassic to Tertiary 

Plutonic and Metamorphic Rocks, 

Northwestern Utah and Northeastern Nevada 

By David M. Miller, John K. Nakata, and Linda L. Glick 

Abstract 

Conventional K-Ar ages for biotite, muscovite, sericite, 

and hornblende, in conjunction with ages obtained using 

other dating methods, are used to infer ages for pluton 

emplacement and metamorphism in northwestern Utah and 

adjacent Nevada and Idaho. Eleven K-Ar ages from four 

Jurassic plutons and related dikes are used together with 

data from previous studies to develop a model explaining the 

chronology of pluton emplacement. Most K-Ar biotite cooling 

ages for the igneous rocks are 155 to 150 Ma, whereas U-Pb 

zircon ages are 165 to 160 Ma. According to our model, all 

Jurassic plutons were emplaced during a short time interval, 

about 165 to 160 Ma; compositionally related dikes probably 

were emplaced shortly after, but K-Ar data constrain them to 

the interval 165 to 152 Ma. Some dikes in the Newfoundland 

and Silver Island Mountains were sericitically altered about 

149 to 148 Ma. The approximately 10-m.y. difference between emplacement, given by U-Pb zircon ages, and the 

K-Ar biotite cooling ages we interpret as due to prolonged 

cooling in an upper crust widely heated by the plutonic rocks. 

A conventionally determined K-Ar age and one done by 

the 40Ar/ 39Ar method support previous assertions that a 

biotite-muscovite granite in the Toano Range, the Toano 

Springs pluton, was emplaced during the Late Cretaceous. 

We estimate that emplacement occurred about 85 to 80 Ma. 

Micas in surrounding greenschist-facies metasedimentary 

rocks probably grew during Jurassic metamorphism, but 

their isotopic ages widely record this Cretaceous plutonic 

event. Our two K-Ar cooling ages for metamorphic muscovite 

combined with two that were previously reported range from 

76 to 61 Ma. A metamorphic-biotite sample yielded a 94-Ma 

K-Ar cooling age, which is interpreted as representing incomplete resetting during the emplacement of the Toano Springs 

pluton. 

A Tertiary pluton in the northern Toano Range yielded a 

K-Ar biotite age of 37 Ma, which is consistent with ages from 

other, compositionally similar, Tertiary plutons in the area. 

Single K-Ar age determinations for mica schist in the western 

Raft River Mountains (30 Ma) and hornblende gabbro in the 

southern Grouse Creek Mountains (49 Ma) require corroborating data for their full interpretation. 

Manuscript approved for publication, July 28, 1989. 

The K-Ar ages reported here, combined with previously 

reported ages, date all known plutons in Box Elder County, 

Utah, and eastern Elko County, Nevada. Emplacement ages 

inferred from the combined isotopic data document the 

history of plutonism in this part of the northeastern Great 

Basin as (1) scattered but widespread plutonism during 

restricted intervals of the Late Jurassic and late Eocene, and 

(2) rare plutonism during the Late Cretaceous and middle 

Oligocene. 

INTRODUCTION 

The pioneering K-Ar studies of R.L. Armstrong 

(Armstrong and Hansen, 1966; Armstrong and Hills, 

1967; Armstrong, 1970, 1976; Armstrong and Suppe, 

1973) established Mesozoic and Cenozoic ages for both 

scattered plutons and regionally metamorphosed rocks 

throughout the eastern Great Basin. Several later studies 

in the northeastern Great Basin added to and refined the 

earlier data, documenting that many plutons were emplaced during restricted intervals of the Late Jurassic and 

late Eocene and that a few plutons are Late Cretaceous 

and middle Oligocene in age (Compton and others, 1977; 

Moore and McKee, 1983; Miller and others, 1987; Miller 

and others, 1988). These later studies and Snoke and 

Miller (1988) also confirmed that regional metamorphism in the northeastern Great Basin was late Mesozoic 

and middle Tertiary in age. 

In the Great Basin, sparse Mesozoic plutons intruded Proterozoic to Mesozoic strata far to the east of 

the Mesozoic Sierra Nevada batholith but west of the 

Cretaceous to early Tertiary frontal fold-and-thrust belt. 

The Mesozoic plutons probably were emplaced during 

regional contraction. In contrast, Tertiary plutons in this 

same region were emplaced during widespread crustal 

extension, most recently manifested in the characteristic 

structure of the Basin and Range province (fig. 1 ). These 

Mesozoic and Tertiary plutonic pulses mark magmatic 

events in widely differing tectonic and probably divergent 

thermal regimes, and these plutons provide data for 

inferring crustal and mantle conditions during the different tectonic events. 

Introduction 
As part of a regional tectonic study of northwestern 

Utah, northeastern Nevada, and southernmost Idaho, we 

have conducted a systematic K-Ar dating program for 

igneous and metamorphic rocks. In this paper we present 

data for several previously undated plutons and dike 

swarms, including a heretofore-unreported pluton in the 

Toano Range. We also compare previously reported 

geochronologic data with data reported here to interpret 

the intrusive ages for several other plutons. Our data 

further define the two main pulses of magmatism (Jurassic and Tertiary) in northwestern Utah; initially recognized by Armstrong and Suppe (1973), these magmatic 

episodes were later more narrowly dated, yielding intrusive ages of about 165 to 160 Ma and about 40 to 35 Ma 

(Moore and McKee, 1983; Miller and others, 1987). We 

also document a single pluton in the Toano Range that 

belongs to a group of less common Late Cretaceous 

plutons widely scattered to the south and west of our 

study area (see Miller and others, 1988). 

Acknowledgments 

Many individuals have contributed generously to 

our geochronologic studies, including Wendy Hillhouse 

and Victoria Todd, who contributed K-Ar dating; Larry 

Snee, who performed 40Ar/ 39Ar analysis; and P. Klock, 

Lauresno Espos, David Vivit, S. MacPherson, Marsha 

Dyslin, and Sarah Pribble, who provided potassium analyses. Rick Allmendinger and Terry Jordan continually 

Figure 1. Location of study area and major Mesozoic and 

Cenozoic tectonic elements of the Great Basin. Most Mesozoic plutons lie within the Mesozoic magmatic arc (stippled), 

but in parts of the Great Basin, there are scattered plutons far 

east of this belt. Approximate east- and west-side limits of 

major Cenozoic extension in Great Basin indicated by line 

with hachures; line with sawteeth (on upper plate) is east 

front of Mesozoic and early Cenozoic frontal thrust belt. 

stimulated our thinking on the geology of the region, and 

their geologic mapping and sample collection contributed directly to this study. Lydia K. Fox collaborated with 

studies at Crater Island and Sharon L. Gusa assisted with 

fieldwork there and in the Toano Range. Discussions 

with Elizabeth L. Miller, Phillip B. Gans, James E. 

Wright, and Arthur W. Snoke made us much more aware 

of geologic and geochronologic relations elsewhere in the 

Great Basin. 

METHODS 

Most of the K-Ar age determinations were made in 

the Stable Isotope Laboratories of the U.S. Geological 

Survey at Menlo Park, Calif., using the methods described by Dalrymple and Lanphere (1969). Argon was 

extracted on an ultra-high-vacuum system by fusion. 

Reactive gases were scrubbed by an artificial molecular 

sieve, Cu-CuO, and Ti metals. The spectrometry was 

performed on a Nier-type, 15-cm radius, 60°-sector spectrometer and a multichannel, 23-cm radius, 60° -sector 

mass spectrometer, both operated in static mode. Argon 

isotopic composition was determined by standard isotopic-dilution procedures. 

Potassium was analyzed by flame photometry at 

the U.S. Geological Survey analytical laboratory in Menlo Park using the procedure described in Cremer and 

others (1984). All analyses were duplicated to check 

precision. 

For samples that were analyzed once, their reported errors (as ± range) generally are ± 2.5 percent, which 

is a laboratory-determined conservative overall analytical 

precision, including errors resulting from sample preparation to spectrometry. A few older analyses were determined by different methods and have slightly higher 

errors (±3 percent). Some samples were analyzed twice. 

For these, we retained the conservative laboratory error, 

although several showed excellent replication and their 

precision is therefore underestimated. 

In cases where we cite previously reported K-Ar 

data, those that used constants other than the constants 

of Steiger and Jager's (1977), we have recalculated and 

annotated them as such. The table of Dalrymple's (1979) 

was used for recalculating the ages. 

RESULTS 

Ages reported here are primarily from samples 

collected from the Newfoundland Mountains westward 

to the Toano Range (fig. 2). Compton and others (1977) 

described two plutons in the northern part of this area, 

Caroon (1977) and Allmendinger and Jordan (1984) 

described the pluton in the Newfoundland Mountains, 

and Miller (1984) briefly described many others in the 

2 K-Ar Ages of Jurassic to Tertiary Plutonic and Metamorphic Rocks, Utah and Nevada 
:::a 

CD • c 

if 

w 

n>-n 

::J -· a.CC c (')"" 

co CD 

::JN 

o· 

N 

ocn 

c=r ~~r 12.."Q. 

C:=t> g[ 

(/J(Q 

O>CO 

::JO 

0.() 

3<0 co'< 

... o n>-

3~ oc: 

'""0. -g.'< 

-·0> o..., 

am o- "0 (/J;r 

. 0 

(/J::J 

~~-

ceo 

0Q. 

;r -· 

0~ :E :J. ::JC"' c: Q) ::::!". 

'""0 CD::J 

n>o 

"0-

"0"0 a.., 

x<p 

-· (') 3m 

f!::J 

me N 1' Q. )>0 

..,~ 

~DJ 

3ot 

12..~ ma. 

Y.!.o 

~3: CD 

~ 

~ ('';" 

0 

u; 

"' 0 

Goshute 

~~ 

Valley 

"'· q~ 

/////// 

'//////// 

V///,£4 ~'////// ///////////////// l. ,,,, ,,~///////// 

///////~H////////// ////////////////// 

- ////// ///// -Mou~! /// /// /////////////// 

// ~/ / L-/ //// ,•vr4·

1;.,/ ////////////////// 

() 

& 

~ 

//////~vS////////////////// 

//////////////////////// 

/////////////////////// 

////////////////////// 

~/////////////////// 

~//////////////// 

~ //////////// 

//////////// 

//////////// 

'/////////// 

//////////~ 

cz:;;. 

~ 

~ 

!!:J 

~ s:;--t 

0~ C:o 

Z)> 

~~ z 

/ ?' /// ~/ ///,? 

// ./"ELKO COUNTY~ 

"'0 

Ill 

en 

en 

~/////~~~//I /////~1'/// 

• 

:x> eno:g 

Ill ::::5 .... 3 (1) 0 

%Q ~· 

en~~ (1) !!l. 

";; )>--

.... Q 

I ~ P1f.:1~D 

~ t:-2 ~di b ~ (") 

~ g :r .... ~ ~ (1) .... 

Ill I "'C) "'ill ..., en O:r 

~·~ 

~ (1) 

"0 ~if .... (ti Ill 

' 3 (")0 

(1) .... 

::::5"0 o2: 

N (") EJ ~ 

c ~en:::l 

"@ 9J 

Q~~ ::::5 ao 0 Ill .... N"'I"' 0 '<: :r 

-Sa~ ~a. 

c... 

c 

~ 

~· 

"0 

c 

0 

iil 

0 

(1) 

(1) 

~ 

0 

c 

en 

"0 

c 

0 

iil 

_____ J 

~~ -

.....; 

:1 

Qj' 

-2 

"0 

c 

0 

iil 

"tl 

s: 

2 

~ 

a 

2 

~ 

~ 

l~ 

I~ 
Pilot Range and Toano Range. The geochronology of 

plutons in the central Silver Island Mountains was detailed by Moore and McKee (1983) and in the Pilot 

Range by Hoggatt and Miller (1981) and Miller and 

others (1987). Most of our work reported herein focuses 

on the Toano Range and the Silver Island and Newfoundland Mountains. All of these mountain ranges 

contain one or more Jurassic plutons, the Toano Range 

contains a Cretaceous pluton, and Tertiary plutons are 

exposed in the Toano and Pilot Ranges and the Grouse 

Creek Mountains (fig. 2). 

Metamorphic rocks are found as contact aureoles 

around the plutons and also in broader regions characterized by regional metamorphic assemblages and fabrics 

in the Silver Island Mountains and Pilot and Toano 

Ranges (Miller, 1984) and the Grouse Creek and Raft 

River Mountains (Compton and others, 1977). Supplementing similar work on Mesozoic metamorphism in the 

Pilot Range (Miller and others, 1987), we have studied 

the regionally metamorphosed rocks in the Toano 

Range. 

We interpret most K-Ar ages as cooling ages 

because the minerals cool slowly following thermal 

events and do not cease diffusion instantaneously. The 

K-Ar ages therefore measure a period of temperature 

decline following such events as (1) peak metamorphism, 

(2) intrusion, (3) thermal disturbance, or ( 4) a combination of these. Unlike K-Ar methods, U-Pb systematics for 

plutonic zircon yields ages closely approximating emplacement ages because zircon is highly refractory and 

daughter products are not gaseous. Where possible, we 

have compared our K-Ar data to examples for which 

U-Pb zircon data and K-Ar cooling ages are both available. 

In the absence of U-Pb zircon data, pluton-emplacement ages may be approximated from K-Ar ages on 

minerals with widely different blocking temperatures by 

calculating a cooling rate and then extrapolating this rate 

to emplacement temperatures. Unfortunately, blocking 

temperatures are not closely determined. Acknowledging 

this uncertainty, we use the following estimated blocking 

temperatures: hornblende, about 500±50 oc (Spear and 

Harrison, 1989); coarse muscovite, about 350±50 oc 

(Purdy and Jager, 1976; Harrison and others, 1989); 

biotite, about 325 oc (Harrison and others, 1989), and 

sericite, a slightly lower temperature than muscovite. 

Ages for hornblende, with its high blocking temperature, 

are necessary to constrain the cooling rate of a pluton. 

Rather than strictly applying the poorly constrained 

blocking temperatures when interpreting K-Ar data, we 

use the data as qualitative measures of decreasing temperature, from hornblende, to muscovite and sericite, to 

biotite. Cooling rates also may be approximated using 

heat-flow calculations that employ parameters such as 

original pluton temperature, size, and depth, and initial 

wallrock temperature. 

Calculating cooling rates has proved difficult in 

northern Utah. Miller and others (1987) demonstrated 

that in the Pilot Range excess argon was in most or all 

cases retained in hornblende. For this reason, we did not 

systematically study hornblende from granites. The sizes 

and shapes of the plutons are inadequately constrained 

by geologic and geophysical data. Although the maximum temperatures of the host rocks are known from 

conodont-alteration indexes, the times that the rocks 

attained those temperatures are not known. Because of 

these limitations, we were unable to establish precise 

cooling rates for the plutons. 

Jurassic Plutons 

The ages of Jurassic plutons in the Toano and Pilot 

Ranges are well-constrained by U-Pb zircon ages-162 

Ma for the Toano Range (J .E. Wright, oral commun., 

1986) and 165 to 155 Ma for the Pilot Range (Miller and 

others, 1987). Jurassic ages for diorite and granodiorite 

(K-Ar ages of 174 Ma on hornblende and 160 Ma on 

biotite) in the Silver Island Mountains just to the east 

were established by Moore and McKee (1983). 

In the Pilot Range, leucocratic muscovite-biotite 

granite occurs as networks of dikes within metamorphosed lower Paleozoic rocks. These dikes, in addition to 

common aplite and pegmatite dikes, and a broad metamorphic halo surrounding the area in which they lie 

(Miller and others, 1987) suggest that one or more large 

plutons lie below the area. Discordant U-Pb data were 

interpreted by Miller and others (1987) as indicating an 

emplacement age between 165 and 155 Ma. 

In the Toano Range (fig. 2), the Silver Zone Pass 

pluton (named by Pilger, 1972) intruded greenschistfacies Proterozoic and Cambrian strata at Silver Zone 

Pass. The pluton is composed of subequigranular, hornblende-biotite granodiorite (rock-classification scheme 

of Streckeisen, 1976) containing light-brown-weathering 

feldspar, light-gray quartz, and about 15 percent mafic 

minerals, which are evenly distributed through the rock. 

Biotite in places forms distinctive oikocrysts. Accessory 

minerals include epidote, calcite, chlorite, and opaque 

minerals, with lesser amounts of sphene, apatite, and 

zircon. Small mafic xenoliths occur throughout the pluton, and hornblende gabbro bodies (as much as 9m2 in 

surface exposure) of uncertain origin ocoor in a few 

places. 

Broad metamorphic aureoles and tight anticlines 

and foliated strata lie adjacent to the pluton, but the 

intrusive contact is generally sharp. These characteristics 

we interpret as representing a lower-epizonal intrusive 

setting. A region about 8 km by 17 km surrounding the 

exposed pluton has a strong geophysical expression that 

Grauch and others (1988) interpreted as probably related to the subsurface part of the pluton. These inferred 

4 K-Ar Ages of Jurassic to Tertiary Plutonic and Metamorphic Rocks, Utah and Nevada 
subsurface dimensions exceed surface exposures by 

about 2 km on the east and north and by at least 10 km 

on the southwest. A few northeast- to northwest-trending 

dikes consisting of fine-grained hornblende- and biotitebearing granite to granodiorite intruded the adjacent 

rocks. Aphanitic felsic dikes, mafic dikes, and quartz 

veins also cut the Silver Zone Pass pluton. Isotopic study 

of the Silver Zone Pass pluton shows that it lacks a 

significant crustal component (Farmer and DePaolo, 

1983), suggesting rapid ascent through continental crust. 

K-Ar dating of biotite sampled near the center of 

the pluton has yielded a range of ages (all recalculated 

using new constants): 152±8 Ma (Coats and others, 

1965), 133±3 Ma (Armstrong and Suppe, 1973), and 127 

Ma (McDowell, 1971). Of these K-Ar data, the two 

younger ages were obtained from impure samples. McDowell (1971) reported 24 percent chlorite and 10 

percent hornblende in the sample that yielded an age of 

127 Ma. Armstrong and Suppe (1973) reported somewhat-low ~0 for biotite (average 6.96 percent) in the 

sample that yielded an age of 133 Ma. Of the three K-Ar 

ages, we consider the 152-Ma age to most accurately 

represent the K-Ar cooling age for the pluton. U-Pb 

analysis of two size fractions of zircon yielded discordant 

results, but the smaller size was essentially concordant at 

162 Ma (J.E. Wright, oral commun., 1986), which we 

interpret as the emplacement age. The 10-m.y. difference 

between emplacement and K-Ar cooling ages we interpret as representing protracted cooling of a pluton emplaced into greenschist-facies rock. 

Newfoundland Mountains 

The Newfoundland stock of Caroon (1977) crops 

out in the northern Newfoundland Mountains (fig. 2). It 

is about 5.5 km in diameter, although a prominent 

positive aeromagnetic anomaly (Zietz and others, 1976) 

centered on it and extending eastward some 10 km may 

indicate a large subsurface extent for the stock and 

related intrusive bodies. The stock intruded an unmetamorphosed Cambrian and Ordovician miogeoclinal sequence consisting mainly of carbonate rocks, with 

subordinate shale and quartzite. Along the margin of the 

pluton, strata are highly metamorphosed in a zone about 

150 m wide (Caroon, 1977). The pluton is zoned from 

central equigranular and porphyritic monzogranite, to 

equigranular granodiorite, to peripheral porphyritic 

monzogranite (Caroon, 1977). All of the rocks generally 

contain subequal amounts of biotite and hornblende, 

totaling 5 to 13 percent, and minor sphene and magnetite. Several sets of dikes are spatially associated with the 

pluton. One set, composed of monzogranite porphyry 

that appears to be chemically related to the pluton 

(Caroon, 1977), cuts the pluton and emanates widely into 

country rocks. 

Two of the K-Ar analyses reported here were used 

by Caroon (1977) to establish a Jurassic age for pluton 

intrusion and three other ages that we report were used 

by Allmendinger and Jordan (1984) to establish a Jurassic or older age for thrust and normal faulting. Two 

samples of the peripheral porphyritic monzogranite were 

analyzed by W.C. Hillhouse and V.R. Todd: biotite from 

the first sample (NFL-1) yielded an age of 153.2±4.6 

Ma, and hornblende from the second (NFL-2) yielded an 

age of 147.7±4.4 Ma (table 1). The hornblende sample 

contains about 2 percent chlorite and biotite, perhaps 

causing the slightly (but not significantly) younger age 

than that given by biotite (Caroon, 1977). Alternatively, 

hornblende may have been too refractory to completely 

release argon during fusion. This peripheral porphyriticmonzogranite phase of the pluton grades into the other 

two main phases of the Newfoundland stock, and the ages 

therefore suggest a minimum crystallization age of about 

150 Ma for the pluton. 

Three samples (collected by R.W. Allmendinger 

and T.E. Jordan) from monzogranite porphyry dikes 

related to the pluton yielded ages roughly in accord with 

those for the pluton (table 1). Biotite from two dikes 

yielded ages of 150.8±4.5 Ma (sample 81B69, table 1) 

and 143.7±4.4 Ma (sample T82-47, table 1). Both dikes 

show chloritic alteration, and chlorite may have been 

present in the samples analyzed. A sericitized monzogranite porphyry dike (sample RA82-2, table 1) yielded 

a K-Ar age of 149.1±3.7 Ma on sericite. These mica ages 

were interpreted by Allmendinger and Jordan (1984) as 

minimum ages for emplacement of the dikes. They used 

these ages, in conjunction with_ field relations, to constrain the age of two thrust faults, several high- and 

low-angle normal faults, and folds spatially associated 

with the pluton as being Jurassic or older. 

The K-Ar cooling ages do not allow us to precisely 

establish the time of emplacement for the Newfoundland 

stock and related dikes, but it probably is at least as old 

as the oldest (153-Ma) cooling age. Caroon (1977) 

estimated depth of emplacement at 6 km based on 

stratigraphic thickness. The depth actually may have 

been less than this, because he did not attempt to adjust 

for a major Late Pennsylvanian unconformity in the 

Newfoundland Mountains (Allmendinger and Jordan, 

1984) or for pre-Late Jurassic structure and erosion. 

Metamorphic zones 150 m wide and a wider zone of 

ductile deformation around the pluton are consistent 

with a lower-epizonal emplacement. At 6-km depth, 

preemplacement temperatures may have been about 200 

°C, assuming a thermal gradient of about 30 °C/km. 

Rapid cooling of granite to biotite closure temperature 

would not be expected, since the pluton is at least 5.5 km 

in diameter. We estimate that the pluton probably required several million years to cool to biotite Ar-retention temperatures. A K-Ar age of 153 Ma for the 

Newfoundland stock is similar to the 152-Ma K-Ar 

Results 5 
en 

" )> 

... 

> co Table· 1. K-Ar data for Jurassic igneous rocks CD 

(I) 

0 [rad, radiogenic; *, analyses done by W.C. Hillhouse and V.R. Todd; ---, not applicable. Replicate-analysis results shown in parentheses. Decay constants -c.. 

c from Steiger and Jager (1977)] ... 

I» 

(I) 

(I) 

c; Sample Rock type Mineral Mean~O 40Arrad 40Arrad Replicate Age Sample site 

S" sampled dated (wt pet) (pet) (1 Q-10 mol/g) calculations (Ma) 

';;} (Ma) ~ Lat (N) Long (W) ;· 

-< 

-a NFL-1. ............. -41 o 14' -113°22' Granite Biotite* 8.55 95.39 19.7 --- 153 .2±4.6 c 

S' 

NFL-2 .............. -41 °14' -113°22' Hornblende* .66 73.55 ::s Granite 1.45 --- 147.7±4.4 c;· 

I» 81B69 ............... 41°10'43" 113°23'07" Granodiorite Biotite 7.05 87.03 15.96 150.7 150.8±4.5 ::s 

Q. 

s: dike (81.00) (15 .97) (150.8) 

! 

3 T82-47 ............. 41°07'53" 113°23'33" Granodiorite Biotite 7.79 85.77 16.54 141.8 143.7±4.5 0 

-a dike (86.39) (17 .00) (145 .6) :::T 

c;· 

::0 RA82-2 ............ 41°11'16" 113°22'15" Serici tized Sericite 10.42 92.20 23.31 --- 149.1±3.7 0 dike () 

~ 

.Y' 

c: M84CI-65 ......... 41°06'55" 113°45'15" Granodiorite Biotite 8.87 85.82 20.06 150.6 150.3±3.8 Dr 

:::T (34.74) (19.98) (150.0) I» 

::s 

Q. SI-1 .................. 41°02'17" 113°46'13" Quartz Biotite* 7.13 95.86 16.3 152.2±4.6 z --- CD monzonite < I» 

Q. 

I» M85CI-14 ......... 41°03'07" 113°44'13" Granite Biotite 7.74 88.04 17.96 --- 154.2±3. 8 

F84SI-58 .......... 41°01'13" 113°46'43" Granodiorite Biotite 8.45 75.65 19.70 155.0 152.2±3.8 

dike (84.08) (18.96) (149.4) 

M85CI-27 ......... 41°03'50" 113°47'15"' Granodiorite Biotite 7.22 89.64 16.78 --- 154.6±3.9 

dike 

M84CI-48 ......... 41°04'33" 113°44'48" Sericitized Sericite 5.29 92.70 11.77 --- 148.1±3.7 

dike 
biotite age (Coats and others, 1965) for the lithologically 

similar Silver Zone Pass pluton, although the latter 

pluton was emplaced within a wider metamorphic zone. 

The U-Pb zircon age of 162 Ma for the Silver Zone Pass 

pluton leads us to infer a 165- to 160-Ma time of 

emplacement for the Newfoundland stock. Because the 

dikes probably intruded metamorphic rocks still at high 

temperatures, their K-Ar cooling ages may not closely 

correspond to time of emplacement. In addition, the 

K-Ar results from dikes probably record effects of subsequent alteration. Sericitic alteration at or shortly before 

about 149 Ma apparently followed pluton emplacement 

by 10 to 15 m.y. 

Crater Island 

Dioritic to granitic plutons crop out in the central 

and northern Silver Island Mountains (fig. 2) and were 

first mapped and described by W.L. Anderson and F.E. 

Schaeffer (see Schaeffer, 1960). Although Schaeffer and 

Anderson tentatively assigned the plutons a Tertiary age, 

two of them in the central Silver Island Mountains and 

one in the northern Silver Island Mountains gave Jurassic 

K-Ar ages (Armstrong and Suppe, 1973; Moore and 

McKee, 1983). We did detailed field studies of the 

plutons at Crater Island in the northern Silver Island 

Mountains (fig. 3) and did additional analyses on selected 

samples from them. 

The geologic framework of Crater Island is similar 

to that of the Newfoundland Mountains 25 km to the 

east. Cambrian to Permian miogeoclinal strata dip moderately westward and are cut by several classes of faults: 

(1) small-displacement low-angle faults (not shown on 

fig. 3), (2) east-striking high-angle dip-slip faults, and (3) 

north-northeast- to north-northwest-striking high-angle 

dip-slip faults. Several faults of the second and third 

classes are cut by the plutons and dikes; the faults have 

been described in detail by Miller (1990) and Miller and 

others (1990). These intrusive rocks share many modal, 

chemical, and textural characteristics and are considered 

to represent a magmatically related suite. Unifying features include overlapping modal and chemical compositions, broadly co linear variations of oxides with respect to 

Si02 , similar xenolith suites, ubiquitous dikes, and features such as broad metamorphic aureoles indicating a 

lower-epizonal level of emplacement. 

The igneous suite at Crater Island forms two 

composite plutonic centers, one at the north end of 

Crater Island and the other at Donner-Reed Pass (fig. 3); 

part of another small pluton of this suite forms a promontory 1.6 km northeast of Donner-Reed Pass. Several 

dike swarms and individual dikes related to the igneous 

suite are scattered throughout Crater Island. The dikes 

range widely in composition and texture but seem to 

mirror the compositional variations seen in the plutonic 

rocks. 

The plutonic center underlying about 15 km2 of the 

northern part of Crater Island was referred to as the 

"North stock" and the "Sheepwagon stock" by Schaeffer 

(1960). The bulk of the pluton consists of light-gray, 

porphyritic biotite granodiorite to monzogranite with a 

modal mafic-minerals content of 10 to 18 percent, but it 

is intruded by and grades over a short distance into 

porphyritic, fine-grained granodiorite to monzogranite 

(Miller and Glick, 1986). Sparse pegmatite and aplite 

dikes cut the pluton. The pluton carries common mafic 

inclusions. A metamorphic aureole about 1 km wide 

surrounds the southwestern part of the pluton. Within 

this contact aureole are Ordovician strata that were 

foliated and folded into a tight anticline by pluton emplacement (Allmendinger and others, 1984; Miller and 

others, 1990). A K-Ar cooling age of 150.3±3.8 Ma on 

biotite (sample M84CI-65, table 1) from the porphyritic 

granodiorite establishes the northern plutonic center as 

probably Jurassic in age. 

The composite pluton underlying much of southern Crater Island near Donner-Reed Pass (fig. 3) is 

composed of two phases-quartz monzonite and subordinate dark monzodiorite-of the Crater Island Quartz 

Monzonite. Anderson (see Schaeffer, 1960) named the 

pluton as two geographically separate stocks-"Crater 

Island stock" and "South stock" -but recognized that 

they represent a single igneous body. Zietz and others 

(1976) depicted a prominent aeromagnetic high centered 

just west of, and encompassing, outcrops of the Crater 

Island Quartz Monzonite. The pattern suggests that the 

magnetite-bearing pluton occupies at least 80 km 2 of the 

shallow subsurface, extending at least 10 km west of 

Donner-Reed Pass. 

The quartz monzonite phase of the Crater Island 

Quartz Monzonite is homogeneous and consists of biotite-hornblende quartz monzonite to monzogranite. Mafic minerals range from 12 to 24 modal percent. 

Metamorphosed Permian strata form an anticline parallel to the north margin of the pluton. Armstrong and 

Suppe (1973) dated a biotite and hornblende mixture, 

containing 10 percent chlorite, from the Crater Island 

Quartz Monzonite (their sample YAG-88), yielding an 

age of 143±3 Ma (recalculated using new decay constants). The mixture's average K2 0 concentration was 

2.75 percent, so it must have contained only a small 

proportion of biotite. Owin~ to the impurity of this 

sample from a region for which many coexisting mineral 

pairs yield discordant ages, the significance of this K-Ar 

age is highly uncertain. We consider that our K-Ar age of 

152.2±4.6 Ma on biotite (sample SI-1, table 1) from the 

Crater Island Quartz Monzonite better represents the 

cooling age of the pluton and thus suggests a Jurassic age 

of emplacement. 

The dark monzodiorite phase forms the northwestern and southeastern parts of the Crater Island pluton. 

This phase is biotite-augite monzodiorite to monzonite 

Results 7 
113°47'30" 113°45'00" EXPLANATION 

~ Granitic rocks (Jurassic) 

41° 

05' 

00" 

41° 

02' 

30" 

~ 

~ 

Donner-Reed Pass 

IPM ~ 2 KILOMETERS 

41° 

oo·~~-J-------L------~~~~~~~~----L-------------------~ 

ooCrater Island Quartz Monzonite 

p\::\{] Quartz monzonite phase 

!(:··.P,:':::.J 

v:~>M..:.xj 

L.Ps.<"J 

LJ:2j 

~ 

rg£·:A 

t:r:·:l 

Porphyritic monzogranite 

Porphyritic granodiorite 

Limestone and sandstone (Permian) 

Conglomerate, shale, and limestone 

(Pennsylvanian and Mississippian) 

Limestone and dolomite 

(Devonian and Silurian) 

Dolomite, quartzite,and 

limestone (Ordovician) 

Silty limestone and dolomite 

(Ordovician and Cambrian) 

Silty limestone (Cambrian) 

--+-+- Igneous dike 

Contact 

_t_ Fault-Ball and bar on downthrown 

side; dashed where approximate; 

dotted where concealed 

..........,. Dike-filled fault 

+ Anticline 

• M84CI-65 

[150.31 

K-Ar sample site- Shows 

sample number and age in Ma; 

biotite ages in brackets [ ], 

sericite ages in parentheses ( ) 

Figure 3. Generalized geology of pre-Cenozoic rocks at Crater Island (northern Silver Island Mountains) and K-Ar sample 

locations. Location of map area is shown in figure 2. Geology simplified from Glic~ and Miller (1986), Miller and Glick (1986), 

Miller (1990), and Miller and others (1990). 

8 K-Ar Ages of Jurassic to Tertiary Plutonic and Metamorphic Rocks, Utah and Nevada 
containing rare olivine. The rock in many places exhibits 

sharp contacts with the quartz monzonite phase, but in 

some places a compositionally transitional quartz monzodiorite intervenes. The transitional phase appears to 

grade into monzodiorite, but locally it and the monzodiorite form inclusions within the quartz monzonite. These 

relations indicate that the monzodiorite phase is older 

than the quartz monzonite phase, but gradational rock 

types and petrographic and petrologic affinities indicate a 

close relationship between the two, so the monzodiorite 

phase is inferred to be Jurassic in age. 

Porphyritic biotite monzogranite bearing about 10 

percent mafic minerals crops out over less than 1 km2 at 

the south end of a promontory 1.6 km east of the Crater 

Island Quartz Monzonite (fig. 3). Although no Paleozoic 

strata are exposed between this small body and the 

Crater Island pluton, the different rock types indicate 

that the bodies may represent different intrusions. Biotite 

from the porphyritic monzogranite (sample M85CI-14, 

table 1) yielded a K-Ar cooling age of 154.2±3.8 Ma, 

suggesting that the body is approximately coeval with 

other plutons at Crater Island. 

Other than the few aplite and pegmatite dikes that 

are closely associated with plutons, igneous dikes at 

Crater Island may be divided into three general groups: 

(1) common granodioritic dikes occurring as swarms 

cutting the Crater Island pluton and most sedimentary 

rocks, (2) mafic dikes occurring sparsely in sedimentary 

rocks, and (3) rare felsite dikes cutting sedimentary rocks 

and granodiorite dikes. These dikes mirror compositional types seen in the plutons, and our dating shows that 

they were emplaced at approximately the same time as 

the plutons. 

We have dated the granodioritic dikes from three 

locations. Near Donner-Reed Pass, a swarm of hornblende-biotite granodiorite dikes cutting the Crater Island pluton yielded a K-Ar cooling age of 152.2±3.8 Ma 

on biotite (sample F84SI-58, table 1), indicating that the 

dikes were emplaced shortly after the pluton and that the 

pluton and dikes shared a similar cooling history (compare biotite sample SI-1 (table 1) from the same pluton: 

152.2±4.6 Ma). Virtually identical granodioritic dikes 

cut sedimentary rocks north of the Crater Island Quartz 

Monzonite. Biotite from a fine-grained, hornblendebiotite granodiorite dike intruding a normal fault (sample M85CI-27, table 1) yielded a K-Ar cooling age of 

154.6±3.9 Ma, which is consistent with the interpretation 

that these dikes are coextensive with dikes in the Crater 

Island pluton. In a few locations, these dikes are highly 

sericitized. One such dike in central Crater Island yielded 

a K-Ar cooling age of 148.1±3.7 Ma on sericite (sample 

M84CI-48, table 1). Since the blocking temperature for 

sericite is higher than for biotite, these data indicate that 

the dike may have cooled less quickly than others we 

dated or, more likely, that it was altered after emplacement. 

As with the Newfoundland stock, we interpret the 

K-Ar ages for Crater Island plutonic rocks as cooling 

ages and therefore minimum ages for emplacement. By 

analogy with the Silver Zone Pass pluton, we infer that 

the K-Ar ages represent protracted cooling, suggesting 

crystallization perhaps 165 to 160 Ma. The narrow range 

of cooling ages for several plutons and dikes suggests that 

they were all emplaced during a short timespan and 

cooled synchronously. Ages for unaltered dikes should 

closely approximate their emplacement ages if the dikes 

intruded cool rock, for the dike minerals should quickly 

cool to Ar-retention temperatures. The 155-Ma age for a 

dike cutting sedimentary rocks probably most closely 

approximates the crystallization age. However, conodont-alteration indexes of 5 (Miller and others, 1990) for 

Ordovician strata several kilometers to the northeast and 

far from any mapped pluton indicate maximum temperatures of 300 to 400 oc (Epstein and others, 1977). Even 

if somewhat-lower temperatures obtained in Permian 

rocks near the dated dike at the time of intrusion, 

thermal decay to biotite Ar-retention temperatures may 

have been delayed. If cooling was prolonged by 2 to 3 

m.y., then the measured age of 154.6±3.9 Ma (sample 

M85CI-27, table 1) would imply that the dikes were 

emplaced within the interval161 to 153 Ma. At the other 

extreme, cooling of the dikes may have been controlled 

by the cooling of voluminous nearby plutons and, like the 

plutons, may have been as long as 10 m.y. Sericitic 

alteration of dikes about 148 Ma apparWJ.tly took place 

about 5 to 15 m.y. after their emplacement. 

Cretaceous Plutons 

Cretaceous plutons are absent in northwestern 

Utah (Moore and McKee, 1983; Miller and others, 1987) 

and sparse in northeastern Nevada. A single pluton of 

presumed Cretaceous age, the Toano Springs pluton 

(Pilger, 1972) is present in the Toano Range (fig. 4). This 

pluton is composed of homogeneous muscovite-phenocrystic biotite-muscovite monzogranite. On fresh exposures, large books of weakly aligned muscovite are 

prominent. Minor accessory minerals include garnet, 

apatite, sericite, and very minor opaques. Common garnet-muscovite pegmatite and aplite dikes and randomly 

oriented quartz veins cut the pluton. The common pegmatite and aplite dikes suggest a water-rich vapor phase, 

at least in the late stages of crystallization, in strong 

contrast with the Jurassic granitoids, which contain few 

aplite dikes and rare pegmatites. Aligned mica defines an 

indistinct foliation in the monzogranite and at one locality subparallels the southeast margin of the pluton. 

Surface exposures of the pluton indicate a minimum size 

of 7 by 3 km, but it could extend more widely in the 

subsurface; studies by Grauch and others (1988) identified no geophysical expression associated with the pluton. 

Results 9 
0 

<! 

~ 

0 

,_ 

Pass 

EXPLANATION 

W Volcanic rocks (Tertiary) 

Granitic rocks 

[E2J 

~ 

~ 

~ 

Castle Park pluton (Tertiary) 

Toano Springs pluton (Cretaceous) 

Silver Zone Pass pluton (Jurassic) 

Unmetamorphosed carbonate rocks 

(Devonian to Cambrian) 

Metamorphosed limestone (Cambrian) 

Metamorphosed clastic rocks 

(Cambrian and Proterozoic) 

Contact 

..........._ Thrust fault-Teeth on hanging wall 

..J.L...J.L.. Low-angle normal fault- Double bars 

on hanging wall 

MBOTR-01 • 

(62.9) 

0 

High-angle normal fault- Bar and 

ball on downthrown side; dotted 

where concealed 

K-Ar sample site- Shows sample 

number and age in Ma; biotite ages 

in brackets [ ], sericite ages in 

parentheses ( ) 

2 3 4 KILOMETERS 

Figure 4. Generalized geology of Miocene and older rocks, northern Toano Range, and K-Ar sample sites. Location 

of map area shown in figure 2. Map after Glick (1987) and D.M. Miller and L.L. Glick (unpub. mapping, 1987). 

10 K-Ar Ages of Jurassic to Tertiary Plutonic and Metamorphic Rocks, Utah and Nevada 
Table 2. K-Ar data for Cretaceous and Tertiary igneous and metamorphic rocks 

[rad, radiogenic; *, analyses done by W.C. Hillhouse. Decay constants from Steiger and Jager (1977)] 

Sample Sample site 

Lat (N) Long (W) 

M80TR-01 ........ 41 °00'10" 114°18'11" 

B84TR-112 ...... 40°58'56" 114°18'05" 

B85TR-135 ...... 40°58'12" 114°18'18" 

B84TR-167 ...... 41°01'13" 114°16'39" 

M84RR-99 ........ 41 °54'00" 113°42'36" 

B85TR-131 ..... .41 °07'02" 114°13'47" 

81B1 ................ 41°27'08" 113°41'20" 

Rock type 

sampled 

Aplite 

Quartzite 

Schist 

Schist 

Schist 

Granite 

Gabbro 

The pluton intruded upper Proterozoic metamorphic 

rocks but shows little evidence for ductile deformation of 

wallrocks. We infer an epizonal setting for emplacement. 

Aphanitic dikes appear to emanate extensively 

from the Toano Springs pluton into the adjacent Paleozoic rocks. These felsic dikes are finer grained than the 

rocks of the pluton but characteristically contain muscovite. Similar aphanitic dikes cut the Silver Zone Pass 

pluton to the south. In both areas, these dikes may 

represent a late phase of the emplacement cycle that 

produced the Toano Springs pluton. 

Coiwentional K-Ar methods have yielded Cretaceous and Tertiary ages for the Toano Springs pluton. 

Lee and Marvin (1981) reported K-Ar ages (their sample 

451) of 71.7±2.0 Ma for primary muscovite and 

15.4±0.5 Ma for biotite with a K2 0 content of 6.10 

percent. (According to D.E. Lee (written commun., 

1985), Lee and others (1981) had reported an erroneous 

K-Ar biotite age of 128 Ma for this pluton.) Lee and 

others (1984) attributed the young biotite age to alteration to hydrobiotite and considered the muscovite age to 

approximate the time of emplacement. Lee and others 

(1986) supported the presumed Late Cretaceous age by 

comparing the Toano Springs body with better dated and 

chemically and isotopically similar granite bodies in 

east-central Nevada. We obtained a younger K-Ar muscovite age of 62.9±1.9 Ma (sample M80TR-01, table 2) 

from an aplite dike within the pluton. This muscovite age 

and that reported for the main body of the pluton by Lee 

and others (1984) indicate cooling during Cretaceous 

and early Tertiary (Paleocene) time. However, Miller 

(1984) inferred a Jurassic age for this pluton based on 

compositional similarity with the two-mica Miners Spring 

Granite in the Pilot Range. 

We have been unable to extract enough zircon 

from the pluton for U-Pb age determinations because the 

grains are both sparse and small (Lee and others, 1986), 

but 40Ar/ 39Ar analysis of muscovite sample B86TR-111 

Mineral 

dated 

Muscovite* 

Muscovite 

Muscovite 

Biotite 

Muscovite 

Biotite 

Hornblende* 

Mean~O 

(wt pet) 

10.76 

9.9 

9.95 

9.54 

10.44 

7.15 

1.24 

76.24 

93.88 

93.42 

86.73 

74.81 

80.50 

61.84 

40Arrad 

(1Q-10 moVg) 

9.93 

10.13 

8.95 

13.33 

4.54 

3.84 

.90 

Age 

(Ma) 

62.9±1.9 

69.7±1.7 

61.4±1.5 

94.5±2.4 

29.9±0.7 

36.9±0.9 

49.2±1.2 

(table 3) yielded a simple spectrum with a plateau age of 

77.1±0.3 Ma (fig. 5). Because the degassing spectrum 

shows no evidence of thermal disturbance, we consider 

this age to be a cooling age following crystallization. The 

probable deep (8-10 km) stratigraphic level of intrusion 

indicates a preintrusion temperature of wallrocks of at 

least 250 to 300 oc, suggesting a prolonged cooling of the 

pluton. We guess that the emplacement age is roughly 85 

to 80 Ma. The K-Ar muscovite age of 71.7 Ma (Lee and 

Marvin, 1981) is crudely compatible with our 40Ar/ 39Ar 

age, but our younger age (62.9 Ma) on muscovite from 

the aplite dike is not. The disparity in ages may have 

resulted from (1) analytical errors, (2) dike emplacement 

about 15 m.y. after pluton emplacement, or (3) posternplacement thermal disturbance. In view of current data, 

we favor the first alternative. Compositional similarity 

between pluton and dike rocks disfavors the second 

alternative, and the simple Ar-release spectrum for the 

pluton disfavors the third alternative. The new data 

indicate that the Toano Springs pluton is Late Cretaceous in age and that it is unrelated to Jurassic muscovite-bearing granite in the Pilot Range. 

Metamorphic Rocks 

Metamorphic rocks of Mesozoic age are widespread in the Toano and Pilot Ranges (Glick, 1987; 

Miller and others, 1987) and metamorphic rocks of 

Tertiary age and probable Mesozoic age are widespread 

in the Grouse Creek and Raft River Mountains (Compton and others, 1977; Snoke and Miller, 1988). In the 

Pilot Range, metamorphic rocks achieved peak temperatures at about 155 Ma and last cooled through K-Ar 

blocking temperatures for biotite and muscovite during 

Late Cretaceous time (Miller and others, 1987). 

We have studied metamorphic rocks in the Toano 

Range similar to those in the Pilot Range. Similar but 

Results 11 
lower grade metamorphic rocks occur in the central 

Silver Island Mountains, although they have not yet been 

studied. The local contact-metamorphic zones around 

Jurassic plutons at Crater Island and in the Newfoundland Mountains were caused by pluton emplacement and 

presumably are of the same age as the plutons. We 

obtained a single K-Ar age for schist in the Raft River 

Mountains as well. 

Table 3. 40Ar/ 39Ar analytical data for muscovite sample B86TR-111 from the Toano Springs pluton 

[Analyses done by L.W. Snee; machine sensitivity at analysis, 9.7x1o-13 molN. Sample weight, 30 mg. Decay 

constants from Steiger and Jager (1977). Age of monitor mineral (Minnesota hornblende), 520.4 Ma. Sample site: 

lat. 40° 59'05" N., long. 114° 18'17" W. Irradiation parameter, 1=0.007208±0.25 percent. at, atmospheric; Ca, 

calcium derived; Cl, chlorine derived; K, potassium derived; rad, radiogenic. Total-fusion age, 77 .4±0.3 Ma] 

Temperature 

(OC) 

«S 

~ 

wCJ 

<l: 

Iz 

w 

a: 

<l: 

0.. 

0.. 

<l: 

500 

600 

700 

750 

800 

850 

900 

950 

1000 

1050 

1100 

1150 

1200 

85 

80 

75 

r 

70 

0 

-

--

36Arat 

(volts) 

0.00177 

.00111 

.00412 

.00431 

.00802 

.02595 

.01227 

.01416 

.00920 

.00778 

.00265 

.00037 

.00031 

F=i 

~800 

I 

10 

37Area 

(volts) 

0.00135 

.00130 

.00104 

.00083 

.00079 

.00000 

.00092 

.00207 

.00095 

.00096 

.00145 

.00121 

.00113 

850 

I I 

20 30 

38Arcl 

(volts) 

0.00078 

.00003 

.00017 

.00005 

.00011 

.00024 

.00019 

.00021 

.00016 

.00008 

.00000 

.00000 

.00002 

900 

I 

40 

I 

50 

0.01997 

.04342 

.14175 

.18649 

.36211 

1.18863 

1.16762 

.98690 

.63485 

.69538 

.57287 

.01677 

.00592 

40Arrad 

(volts) 

0.12226 

.24376 

.83835 

1.13308 

2.24166 

7.32528 

7.05584 

5.96381 

3.84904 

4.24311 

3.46577 

.09403 

.03290 

Plateau age 77.1±0.3 Ma 

(67.4% of gas) 

950 1000 

I I I 

60 70 80 

39ARK RELEASED, CUMULATIVE PERCENT 

1050 

Age 

(Ma) 

77 .91±0.81 

71.57±0.30 

75.31±0.32 

77 .32±0.32 

78.75±0.33 

78.41±0.33 

76.91±0.32 

76.92±0.32 

77 .16±0.32 

77 .65±0.32 

77 .00±0.32 

71.47±1.00 

70.80±2.39 

1100 

I 

90 100 

Figure 5. Age spectra diagram for muscovite sample 886TR-111 from the Toano Springs pluton. Temperatures corresponding to major steps in Ar-release spectrum are shown in degrees Celsius. Vertical width of steps indicates analytical error for 

apparent-age data. 

12 K-Ar Ages of Jurassic to Tertiary Plutonic and Metamorphic Rocks, Utah and Nevada 
Toano Range 

Upper Proterozoic and Cambrian rocks in the 

lower plate of a detachment fault (fig. 4) underwent 

greenschist-facies metamorphism and local penetrative 

deformation. The upper Proterozoic rocks form a thick 

sequence of metamorphosed orthoquartzite, quartz 

wacke, feldspathic wacke, siltstone, and rare calc-silicate 

rock. Although all of these were similarly metamorphosed, the metasiltstone contains the best mineralogic 

indicators of metamorphic grade. Unlike the Proterozoic 

rocks, evidence of metamorphism is not apparent in all of 

the Cambrian rocks. The Cambrian graphitic phyllite, 

limestone, and siltstone may not have bulk compositions 

favorable for growth of facies-indicative minerals. 

Greenschist metamorphism of the upper Proterozoic and Cambrian rocks is defined by widespread muscovite, biotite, chlorite, and minor tremolite. 

Amphibolite-facies mineral assemblages are recognized 

at only one locality, 1.3 km northeast of the Toano 

Springs pluton. There, coexisting muscovite, biotite, staurolite, and garnet indicate almandine-amphibolite facies. 

K-Ar analysis of metamorphic biotite and muscovite from greenschist-facies rocks east of the main mass 

of Toano Springs pluton have yielded Cretaceous and 

Tertiary ages of 94.5±2.4 Ma (biotite sample B84TR167), 69.7± 1.7 Ma (muscovite sample B84TR-112), and 

61.4± 1.5 Ma (muscovite sample B85TR-135) (table 2). 

These ages bracket the results of Lee and others (1980), 

who reported K-Ar muscovite ages of 75.6±2.1 and 

72.5±2.0 (their samples 512 and 511) from Proterozoic 

metaquartzite near the Toano Springs pluton. 

Although metamorphic minerals from these rocks 

primarily yield Cretaceous apparent ages, field evidence 

suggests Jurassic metamorphism. Penetrative fabrics in 

the Cambrian rocks are truncated by the Late Jurassic 

Silver Zone Pass pluton (Glick, 1987) and its dikes, 

neither of which shows a penetrative fabric or any other 

sign of deformation. These relations indicate that metamorphic fabrics in the vicinity of this Late Jurassic pluton 

formed during, or more likely prior to, its emplacement. 

Since the metamorphic fabrics in wallrocks around 

the Toano Springs pluton appear to be continuous with 

those cut by the Silver Zone Pass pluton and therefore 

Jurassic in age, one or more processes could have 

produced the Cretaceous apparent ages: (1) a protracted 

period of cooling, (2) long-term burial at elevated temperature followed by rapid tectonic denudation, and (3) 

resetting caused by emplacement of the Toano Springs 

pluton. According to the first hypothesis, the ages represent cooling over a protracted interval subsequent to the 

Jurassic metamorphic event. If so, cooling took place 

over more than 90 m.y., during which time the rocks 

remained deep or temperature decline was otherwise 

retarded. However, studies pertaining to the Pilot Range 

and other parts of the northeastern Nevada and northwestern Utah region (Miller and others, 1987) indicate 

surprisingly consistent mica cooling ages of about 80 to 

65 Ma. This regionwide concurrence favors the hypothesis of tectonic denudation to rapidly curtail Ar diffusion 

in micas. However, for the Toano Range, two of our 

K-Ar ages yielded by metamorphic minerals and those 

reported by Lee and others (1980) also approximate 

cooling ages for the Toano Springs pluton. These relations indicate that local heating (by intrusion of the 

pluton) and subsequent cooling (perhaps during regional 

denudation) were most likely responsible for the Cretaceous ages. That the resetting by the pluton was only 

local is indicated by the biotite cooling age of 94.5±2.4 

Ma (sample B84TR-167, table 2). It may be that the 

pluton extends shallowly under metamorphic rocks to the 

south, causing widespread resetting of Jurassic micas, but 

has a steep north border along which the resetting was 

restricted to a much narrower zone. 

Raft River Mountains 

The Grouse Creek and Raft River Mountains were 

metamorphosed during the Tertiary (Compton and others, 1977). However, to the north, in the Albion Mountains, rocks that are strikingly similar to those in the 

Grouse Creek and Raft River Mountains show evidence 

of both Mesozoic and Cenozoic metamorphism (Armstrong, 1976). The continuity of these three mountain 

ranges (fig. 2) suggests that Mesozoic metamorphism in 

the Raft River and Grouse Creek Mountains may have 

been overprinted pervasively by Tertiary metamorphism 

(Miller and others, 1983; Snoke and Miller, 1988). 

The metavolcanic(?) muscovite schist member of 

the schist of the Upper Narrows (Compton, 1972) was 

sampled in the western Raft River Mountains (fig. 2) at 

a location about 12 km south of the Almo pluton of 

Armstrong and Hills' (1967). They dated this pluton at 

30 Ma by the Rb-Sr method. Our K-Ar age on muscovite 

from the schist member (sample M84RR-99, table 2) is 

29.9±0.7 Ma. The similarity of our result to the 30-Ma 

Rb-Sr age for the Almo pluton may reflect complete 

resetting caused by heat from the pluton. Alternatively, 

the similarity may be coincidental and our data may 

reflect cooling from any one of several possible metamorphic events. The age roughly fits biotite-age contours 

extrapolated southward from the Albion Mountains (see 

fig. 8 of Armstrong, 1976), reinforcing the assumption 

that there were coeval thermal events in the Albion and 

Raft River Mountains. 

Tertiary Plutons 

Late Eocene granitoids have been dated in several 

parts of northern Utah. A late Eocene granodiorite 

pluton in the southern Grouse Creek Mountains (fig. 2) 

Results 13 
yielded a Rb-Sr age of 38.2±2.0 Ma (Compton and 

others, 1977). A K-Ar age of 23.9 Ma-recalculated 

from Armstrong's (1970) data using new decay constants-for this pluton suggests either different ages of 

intrusion for different lobes or a complex cooling history 

(Compton and others, 1977). Granodiorite from the Pilot 

Range yielded a U-Pb zircon age of38.9±0.9 Ma (Miller 

and others, 1987). Similar plutons (granodiorite to monzogranite), small bodies, and dikes in the Pilot Range 

yielded 37- to 30-Ma K-Ar biotite ages, which Miller and 

others (1987) interpreted as cooling following emplacement at about 39 Ma. 

Toano Range 

The Castle Park pluton, first reported by Glick 

(1987), is located in the northeasternmost Toano Range 

(fig. 4). The pluton is composed of subequigranular 

monzogranite that contains about 10 percent of small 

(0.5-mm) and large (5-mm) books of rectangular and 

hexagonal biotite. The 0.8-km2 pluton is poorly exposed 

and intrudes Ordovician and Silurian rocks predominantly consisting of dolomite and quartzite. Spatially associated dikes considered to be genetically related to the 

pluton intrude Silurian and Devonian dolomite and cut 

north-striking faults (Glick, 1987). The pluton may be a 

small part of a much more extensive intrusive body or 

group of bodies in the subsurface, judging from geophysical data. Grauch and others (1988) depicted a magnetic 

anomaly that could be the expression of a pluton in the 

area where Glick (1987) identified the Castle Park 

pluton. Because no igneous rocks were identified on the 

geologic map used by Grauch and others, they only 

inferred the presence of a buried pluton. The anomaly 

identified by Grauch and others (1988) encloses about 25 

km2 centered on the outcrops Glick (1987) identified, 

and it extends from there about 10 km to the west and 

thereon in a south-southwest direction for several tens of 

kilometers. Even if the part of this anomaly that coincides 

with Goshute Valley (fig. 2) west of the Toano Range is 

caused by other rocks such as Tertiary basalt, the part of 

the anomaly coinciding with the northern Toano Range is 

much larger than the exposed pluton and suggests a large 

body in the subsurface. The pluton has a narrow metamorphic aureole, suggesting that it was emplaced at a 

much-shallower depth than the Mesozoic plutons farther 

south in the Toano Range. 

Biotite (table 2, B85TR-131) from this pluton 

yielded an apparent K-Ar age of36.9±0.9 Ma. This K-Ar 

age resembles those for the lithologically similar late 

Eocene plutonic rocks in the neighboring Pilot Range, so 

we infer an emplacement age for the Castle Park pluton 

similar to the 39-Ma U-Pb zircon age for the Bettridge 

Canyon Granodiorite in the Pilot Range. This inference 

yields a 1- to 3-m.y. interval for cooling the pluton to 

biotite blocking temperatures following emplacement, 

which is consistent with a shallower emplacement than is 

characteristic of the Jurassic plutons. 

Bovine Mountains 

Along the south flank of the Bovine Mountains 

(fig. 2), which lie south of the 38-Ma (Rb-Sr) Immigrant 

Pass intrusion of Compton and others' (1977), abundant 

hornblende gabbro boulders locally lie on a beach terrace 

of Lake Bonneville. The gabbro somewhat resembles 

local mafic phases of the Immigrant Pass intrusion, but 

definite outcrop was not found. Hornblende from this 

gabbro gave a K-Ar age of 49.2±1.2 Ma (sample 81B1, 

table 2). In view of the uncertainty that all of the 

Immigrant Pass pluton is 38 Ma (Compton and others, 

1977, p. 1248) and because of the uncertain relationship 

of the dated gabbro to that pluton, it is difficult to 

evaluate the age. The age may indicate an intrusion older 

than the Immigrant Pass, or may result from retention of 

excess Ar by hornblende such as was documented by 

Miller and others (1987) for a late Eocene pluton in the 

Pilot Range. 

SUMMARY AND CONCLUSIONS 

K-Ar biotite ages for Jurassic plutons and dikes in 

northwestern Utah and northeastern Nevada cluster at 

155 to 150 Ma (fig. 6), corroborating the peak of K-Ar 

ages at about 155 Ma reported by Armstrong and Suppe 

(see fig. 4 of Armstrong and Suppe, 1973) for a much 

wider area. We suggest that our K-Ar ages represent 

slow cooling following widespread emplacement of plutons about 165 to 160 Ma, based primarily upon the 

162-Ma U-Pb zircon emplacement age for one pluton, 

the Silver Zone Pass pluton (J.E. Wright, oral commun., 

1986). This restricted emplacement age is also strongly 

supported by the 165- to 155-Ma U-Pb zircon age of the 

Miners Spring Granite in the Pilot Range (Miller and 

others, 1987) and by similar U-Pb zircon ages of about 

160 Ma for three plutons in the southern Snake Range in 

east-central Nevada (Miller and others, 1988). Most 

Jurassic plutons we have studied were emplaced in 

lower-epizonal settings and are compositionally similar, 

suggesting that they all had similar cooling histories. 

K-Ar apparent ages younger than the 155- to 150-Ma 

group (fig. 6) include those reported for the Silver Zone 

Pass pluton (133 and 127 Ma) and the Crater Island 

Quartz Monzonite (143 Ma). These anomalously young 

ages probably result from impure samples, since other 

K-Ar ages on these same plutons are within the 155- to 

150-Ma peak. 

Dikes cutting the Jurassic plutons and wallrocks 

also yield 155- to 150-Ma K-Ar biotite ages, with the 

exception of an altered dike (sample T82-47, table 1) 

near the Newfoundland stock (144 Ma). The dikes in14 K-Ar Ages of Jurassic to Tertiary Plutonic and Metamorphic Rocks, Utah and Nevada 
truded rocks that at some point were fairly hot, as 

indicated by elevated conodont-alteration indexes (Harris and others, 1980) and their proximity to the previously 

emplaced plutons. If the rocks were hot at the time of 

dike emplacement, cooling to the Ar-retention temperature may have been prolonged compared to the rapid 

cooling of a narrow dike injected into cold rock. We 

cannot accurately date the time of emplacement of the 

dikes, but the similarity of both their compositions and 

K-Ar ages to those of the plutons suggest emplacement 

soon after the plutons solidified and almost certainly 

before 153 Ma. Sericite in some highly altered dikes 

yielded younger ages than biotite in unaltered dikes. 

Since sericite has a higher blocking temperature than 

biotite, alteration of these dikes apparently took place 

about 150 to 148 Ma, well after emplacement. 

If all Jurassic plutons in the Newfoundland and 

Silver Island Mountains and Toano Range were intruded 

within a short timespan, the cooling ages of plutonic and 

7 

6 

Cf) 5 

w 

dike rocks indicate that roughly 10 m.y. was required to 

cool plutons and nearby wallrocks to Ar-retention temperatures for biotite. No data indicate that the plutons 

were emplaced deeper than the 6- to 8-km depths 

indicated by stratigraphic thicknesses. We suggest that 

voluminous plutons heated a broad region of the shallow 

crust from about 165 and 160 Ma and that the thermal 

decay in this region was protracted because of the 

enormous volume of plutonic rocks. 

Our K-Ar ages for the Toano Springs pluton 

strengthen its previous Late Cretaceous age assignment 

by Lee and Marvin (1981). A simple ~r/ 39Ar plateau 

age on muscovite from this pluton, giving a cooling age of 

about 77 Ma, indicates an emplacement age somewhat 

before 77 Ma. Although the depth of emplacement and 

size of the pluton and the temperature history of the 

wallrocks are all poorly constrained, we speculate that 

the pluton was emplaced in lower-greenschist facies 

rocks and therefore probably cooled slowly. Our best 

eJ 4 

<( K-Ar ages 

u.. 3 

0 2 

a: 

w 

co 

~ 

::::> 

;~ z 

0 

I (R] I ~ Other ages 

[A) 

20 40 60 80 100 

AGE, Ma 

EXPLANATION 

K-Ar ages 

Biotite (cooling) 

Biotite (reset) 

Sericite or muscovite (cooling) 

Sericite or muscovite (reset) 

Hornblende (cooling) 

Hornblende (reset) 

120 140 

Other ages 

U-Pbzircon 

Rb-Sr isochron 

40Ar/39Ar 

160 180 

Figure 6. Histogram of isotopic ages for intrusive igneous rocks in five mountain ranges: 

Newfoundland, Silver Island, Pilot, Toano, and Grouse Creek. Conventional K-Ar data are com~iled 

from this report and reports cited herein; other types of isotopic ages (U-Pb, Rb-Sr, and 40Ar/ 9Ar) 

are included for comparison. These latter ages generally more closely approximate the time of 

intrusion, whereas conventional K-Ar ages in this region generally record cooling or resetting by 

later thermal events. U-Pb zircon data for the Miners Spring Granite, interpreted by Miller and others 

(1987) to indicate an intrusion age between 165 and 155 Ma, are represented at 160 Ma. 

Summary and Conclusions 15 
estimate for the emplacement age of the Toano Springs 

pluton is about 85 to 80 Ma. 

Greenschist-facies metamorphic rocks surrounding the Toano Springs pluton were first metamorphosed 

during the Jurassic, as recorded by fabrics truncated by 

the 162-Ma Silver Zone Pass pluton. However, these 

metamorphic rocks typically yield Cretaceous to Paleocene K-Ar mica ages, most probably due to a combination of heating by the Toano Springs pluton and possible 

regional cooling, as proposed by Miller and others 

(1987). One K-Ar biotite cooling age of 94 Ma suggests 

that resetting of Jurassic K-Ar ages for metamorphic 

minerals was not complete in part of the northern Toano 

Range. Miller and Gans (1989) suggested that muscovite-bearing Late Cretaceous granitoids in east-central 

Nevada typically created wide metamorphic halos, regional in character, because the plutons and associated 

fluids efficiently transferred heat from the lower crust. 

Their model agrees with the evidence for widespread 

metamorphism around the Toano Springs pluton. Although Miller and Gans (1989) established that Late 

Cretaceous metamorphism was the major Mesozoic 

thermal event in east-central Nevada, geologic and geochronologic data presented by Miller and others (1987) 

for the Pilot Range indicate a more prominent Jurassic 

than Cretaceous metamorphism at lat. 41° N. The model 

for thermal transfer by wet granites (Miller and Gans, 

1989) can account for this apparent geographic distinction in age of major metamorphism in the eastern Great 

Basin. The most extensive and highest grade metamorphic rocks in the northeastern Great Basin are in the 

southern Pilot Range, where they are spatially associated 

with muscovite-bearing Jurassic granite and pegmatite 

(Miller and others, 1987). According to this model, 

muscovite-bearing granites-whatever their ageproduce regional-metamorphic zones; the preponderant 

age of metamorphism in any region of the eastern Great 

Basin is simply a function of the age of the wet granites. 

This model can be compared with other models for the 

origin of regional metamorphism by determining in detail 

the pressure-temperature-time histories for rocks in the 

Pilot and Toano Ranges. 

A small exposure of a large subsurface pluton in 

the northern Toano Range yielded a 37-Ma K-Ar biotite 

age, on which basis we combine it with other plutons in 

the area that indicate an approximately 39-Ma intrusive 

event (Miller and others, 1987). Plutons representing this 

intrusive event evidently were emplaced at shallow 

depths. They have narrow metamorphic auroeles, and 

their K-Ar ages are about 2 to 5 m.y. younger than U-Pb 

and Rb-Sr ages, indicating rapid cooling to Ar-retention 

temperatures for biotite. Hornblende from a gabbro 

possibly representing a phase of the 38-Ma Immigrant 

Pass intrusion in the Grouse Creek Mountains yielded a 

K-Ar age of 49 Ma, but this age may reflect excess Ar 

retention. 

Our data support the conclusion of Armstrong and 

Suppe's (1973) and Moore and McKee's (1983) that 

most of the scattered plutons in northwestern Utah and 

northeastern Nevada are Jurassic and Tertiary in age. As 

suggested by Miller and others (1987), the Jurassic 

intrusive event may have been brief but widespread in the 

area. Moreover, Jurassic plutons of a similarly restricted 

age range are numerous over a much larger area, including east-central Nevada (Miller and others, 1988), central 

Nevada (Roberts and others, 1971), and western Nevada 

(Dilles and Wright, 1988). 

The Cretaceous Toano Springs pluton in the 

Toano Range (fig. 4) apparently marks the northeastward extent of plutons of Late Cretaceous age in the 

Great Basin; the plutons are scattered to the southwest in 

east-central Nevada (Miller and others, 1988) and central Nevada (Roberts and others, 1971; Snoke and Miller, 

1988). An earliest Cretaceous K-Ar biotite age of 138.4 

Ma (revised with new constants) reported from a small 

pluton 50 km north of the Toano Range (Slack, 1974) 

also may represent Cretaceous magmatism, but its apparent age falls between more firmly dated plutonic 

events and needs to be verified. The 138.4-Ma age is 

similar to some analytically dubious ages for the Jurassic 

Silver Zone Pass pluton. 

All Tertiary plutons we have studied in northwestern Utah and many to the south and east in Utah (Moore 

and McKee, 1983) are latest Eocene in age: Most K-Ar 

biotite cooling ages range from 37 to 35 Ma, and one 

U-Pb zircon age is 39 Ma. However, somewhat-younger 

(Oligocene) plutons occur in a metamorphic core complex setting in the Albion (Armstrong and Hills, 1967) 

and Grouse Creek (Compton and others, 1977) Mountains. Volcanic rocks in this part of northwestern Utah 

yield 44- to 33-Ma K-Ar ages (Compton, 1983; Moore 

and McKee, 1983; Miller, 1984; Miller and Glick, 1986), 

probably corresponding to the latest Eocene plutonic 

event. These data add to those of Armstrong and Suppe's 

(1973, fig. 4) and of many other workers' (see Roberts 

and others, 1971; Compton and others, 1977; Miller and 

others, 1988) that suggest a widespread intrusive and 

extrusive event in late Eocene time. Dating these plutons 

by the U-Pb zircon method is required to more closely 

restrict their emplacement ages and to determine cooling 

rates and the effects of possible younger thermal events. 

REFERENCES CITED 

Allmendinger, R.W., and Jordan, T.E., 1984, Mesozoic structure of the Newfoundland Mountains, Utah: Horizontal 

shortening and subsequent extension in the hinterland of 

the Sevier belt: Geological Society of America Bulletin, v. 

95, p. 1280-1292. 

Allmendinger, R.W., Miller, D.M., and Jordan, T.E., 1984, 

Known and inferred Mesozoic deformation in the hinterland of the Sevier belt, northwest Utah, in Kerns, G.J., and 

16 K-Ar Ages of Jurassic to Tertiary Plutonic and Metamorphic Rocks, Utah and Nevada 
Kerns, R.L., Jr., eds., Geology of northwest Utah, southern 

Idaho and northeast Nevada: Utah Geological Association 

Publication 13, p. 21-34. 

Armstrong, R.L., 1970, Geochronology of Tertiary igneous 

rocks, eastern Basin and Range province, western Utah, 

eastern Nevada, and vicinity, U.S.A.: Geochimica et Cosmochimica Acta, v. 34, p. 203-232. 

---1976, The geochronometry of Idaho-Pt. 2, Metamorphic rocks: Isochron/Wes~ no. 15, p. 1-33. 

Armstrong, R.L., and Hansen, E.C., 1966, Cordilleran infrastructure in the eastern Great Basin: American Journal of 

Science, v. 264, p. 112-127. 

Armstrong, R.L., and Hills, F.A., 1967, Rb-Sr and K-Ar 

geochronologic studies of mantled gneiss domes, Albion 

Range, southern Idaho, U.S.A.: Earth and Planetary Science Letters, v. 3, p. 114-124. 

Armstrong, R.L., and Suppe, John, 1973, Potassium-argon 

geochronometry of Mesozoic igneous rocks in Nevada, 

Utah, and southern California: Geological Society of 

America Bulletin, v. 84, p. 1375-1392. 

Caroon, H.F., 1977, Petrochemistry and petrography of the 

Newfoundland stock, northwestern Utah: Stanford, Calif., 

Stanford University, Ph.D. dissertation, 241 p. 

Coats, R.R., Marvin, R.F., and Stern, T.W., 1965, Reconnaissance of mineral ages of plutons in Elko County, Nevada, 

and vicinity, in Ch. D of Geological Survey research 1965: 

U.S. Geological Survey Professional Paper 525, p. D11-

D15. 

Compton, R.R., 1972, Geologic map of the Yost quadrangle, 

Box Elder County Utah, and Cassia County, Idaho: U.S. 

Geological Survey Miscellaneous Geologic Investigations 

Map I-672, 7 p. of text and 2 map sheets, scale 1:31,680. 

---1983, Displaced Miocene rocks on the west flank of the 

Raft River-Grouse Creek core complex, Utah, in Miller, 

D.M., Todd, V.R., and Howard, K.A., eds., Tectonic and 

stratigraphic studies in the eastern Great Basin: Geological 

Society of America Memoir 157, p. 271-279. 

Compton, R.R., Todd, V.R., Zartman, R.E., and Naeser, C.W., 

1977, Oligocene and Miocene metamorphism, folding, and 

low-angle faulting in northwestern Utah: Geological Society of America Bulletin, v. 88, p. 1237-1250. 

Cremer, M.J., Klock, P.R., Neil, S.T., and Riviello, J.M., 1984, 

Chemical methods for analysis of rocks and minerals: U.S. 

Geological Survey Open-File Report 84-565, 149 p. 

Dalrymple, G.B., 1979, Critical tables for conversion of K-Ar 

ages from old to new constants: Geology, v. 7, p. 558-560. 

Dalrymple, G.B., and Lanphere, M.A., 1969, Potassium-argon 

dating-Principles, techniques, and applications to geochronology: San Francisco, Calif., Freeman, 258 p. 

Dilles, J.H., and Wright, J.E., 1988, The chronology of early 

Mesozoic arc magmatism in the Yeringtor1 district uf 

western Nevada and its regional implications: Geological 

Society of America Bulletin, v. 100, p. 644-652. 

Epstein, A.G., Epstein, J.B., and Harris, L.D., 1977, Conodont 

color alteration-An index to organic metamorphism: U.S. 

Geological Survey Professional Paper 995, 27 p. 

Farmer, G.L., and DePaolo, D.J., 1983, Origin of Mesozoic and 

Tertiary granite in the Western United States and implications for pre-Mesozoic crustal structure-Pt. 1, Nd and 

Sr isotopic studies in the geocline of the northern Great 

Basin: Journal of Geophysical Research, v. 88, p. 3379-

3401. 

Glick, L.L., 1987, Structural geology of the northern Toano 

Range, Elko County, Nevada: San Jose, Calif., San Jose 

State University, M.S. thesis, 141 p. 

Glick, L.L., and Miller, D.M., 1986, Geologic map of the Lucin 

4 NW Quadrangle, Box Elder County, Utah: Utah Geological and Mineral Survey Map 93, 4 p. of text and 2 map 

sheets, scale 1:24,000. 

Grauch, V.J.S., Blakely, R.J., Blank, H.R., Oliver, H.W., Plouff, 

Donald, and Ponce, D.A., 1988, Geophysical delineation 

of granitic plutons in Nevada: U.S. Geological Survey 

Open-File Report 88-11, 7 p. of text and 2 map sheets, 

scale 1:1,000,000. 

Harris, A.G ., Wardlaw, B.R., Rust, C.C., and Merrill, G.K., 

1980, Maps for assessing thermal maturity (conodont 

color-alteration index maps) in Ordovician through Triassic rocks in Nevada and Utah and adjacent parts of Idaho 

and California: U.S. Geological Survey Miscellaneous Investigations Series Map I-1249, 2 sheets, scale 1:2,500,000. 

Harrison, T.M., Spear, F.S., and Heizler, M.T., 1989, Geochronologic studies in central New England-Pt. 2, PostAcadian hinged and differential uplift: Geology, v. 17, p. 

185-189. 

Hoggatt, W.C., and Miller, D.M., 1981, K-Ar ages of intrusive 

rocks of the .Pilot Range, Nevada and Utah: Isochron/ 

West, no. 30, p. 21-22. 

Lee, D.E., Friedman, Irving, and Gleason, J.D., 1984, Modification of oD values in eastern Nevada granitoid rocks 

spatially related to thrust faults: Contributions to Mineralogy and Petrology, v. 88, p. 288-298. 

Lee, D.E., Kistler, R.W., Friedman, Irving, and Van Loenen, 

R.E., 1981, Two-mica granites of northeastern Nevada: 

Journal of Geophysical Research, v. 86, p. 10607-10616. 

Lee, D.E., and Marvin, R.F., 1981, Markedly discordant K-Ar 

ages for coexisting biotite and muscovite from a two-mica 

granite in the Toano Range, Elko County, Nevada: Isochron/West, no. 32, p. 19. 

Lee, D.E., Marvin, R.F., and Mehnert, H.H., 1980, A radiometric-age study of Mesozoic-Cenozoic metamorphism in 

eastern White Pine County, Nevada, and nearby Utah: 

U.S. Geological Survey Professional Paper 1158, p. C1-

C28. 

Lee, D.E., Stacey, J.S., and Fischer, L.B., 1986, Muscovitephenocrystic two-mica granites of northeastern Nevada are 

Late Cretaceous in age, in Peterman, Z.E., and Schnabel, 

D.C., eds., Shorter contributions to isotope research: U.S. 

Geological Survey Bulletin 1622, p. 31-39. 

McDowell, F.W., 1971, K-Ar ages of igneous rocks from the 

Western United States: Isochron/West, no. 2, p. 1-16. 

Miller, D.M., 1984, Sedimentary and igneous rocks of the Pilot 

Range and vicinity, Utah and Nevada, in Kerns, G.J., and 

Kerns, R.L., Jr., eds., Geology of northwest Utah, southern 

Idaho and northeast Nevada: Utah Geological Association 

Publication 13, p. 45-63. 

--1990, Geologic map of the Lucin 4 SW Quadrangle, Box 

Elder County, Utah: Utah Geological and Mineral Survey 

Map 130, 13 p. of text and 2 map sheets, scale 1:24,000. 

Miller, D.M., Armstrong, R.L., Compton, R.R., and Todd, 

V.R., 1983, Geology of the Albion-Raft River-Grouse 

References Cited 17 
Creek Mountains area, northwestern Utah and southern 

Idaho: Utah Geological and Mineral Survey Special Studies 59, p. 1-62. 

Miller, D .M., and Glick, L.L., 1986, Geologic map of the 

Lemay Island Quadrangle, Box Elder County, Utah: Utah 

Geological and Mineral Survey Map 96, 9 p. of text and 2 

map sheets, scale 1:24,000. 

Miller, D.M., Hillhouse, W.C., Zartman, R.E., and Lanphere, 

M.A., 1987, Geochronology of intrusive and metamorphic 

rocks in the Pilot Range, Utah and Nevada, and comparison with regional patterns: Geological Society of America 

Bulletin, v. 99, p. 866-879. 

Miller, D.M., Jordan, T.E., and Allmendinger, R.W., 1990, 

Geologic map of the Crater Island Quadrangle, Box Elder 

County, Utah: Utah Geological and Mineral Survey Map 

125, 16 p. of text and 2 map sheets, scale 1:24,000. 

Miller, E.L., and Gans, P.B., 1989, Cretaceous crustal structure 

and metamorphism in the hinterland of the Sevier thrust 

belt, western U.S. Cordillera: Geology, v. 17, p. 59-62. 

Miller, E.L., Gans, P.B., Wright, J.E., and Sutter, J.F., 1988, 

Metamorphic history of the east-central Basin and Range 

province-Tectonic setting and relationship to magmatism, in Ernst, W.G., ed., Metamorphic and crustal evolution of the Western United States: Englewood Cliffs, N.J., 

Prentice-Hall, p. 650-682. 

Moore, W.J., and McKee, E.H., 1983, Phanerozoic magmatism 

and mineralization in the Tooele 1 o by 2° quadrangle, 

Utah, in Miller, D.M., Todd, V.R., and Howard, K.A., eds., 

Tectonic and stratigraphic studies in the eastern Great 

Basin: Geological Society of America Memoir 157, p. 

183-190. 

Pilger, R.H., Jr., 1972, Structural geology of part of the 

northern Toano Range, Elko County, Nevada: Lincoln, 

Nebr., University of Nebraska, M.S. thesis, 65 p. 

Purdy, J.W., and Jager, Emilie, 1976, K-Ar ages of rockforming minerals from the central Alps: Padua, Italy, 

University of Padua, Institute of Geology and Mineralogy, 

Memoir 30, 31 p. 

Roberts, R.J., Radtke, A.S., and Coats, R.R., 1971, Goldbearing deposits in north-central Nevada and southwestem Idaho, with a section on Periods of plutonism in 

north-central Nevada by M.L. Silberman and E.H. McKee: 

Economic Geology, v. 66, p. 14-33. 

Schaeffer, F.E., ed., 1960, Geology of the Silver Island Mountains, Box Elder and Tooele Counties, Utah, and Elko 

County, Nevada: Utah Geological Society, Guidebook to 

the Geology of Utah No. 15, 185 p. 

Slack, J.F., 1974, Jurassic suprastructure in the Delano Mountains, northeastern Elko County, Nevada: Geological Society of America Bulletin, v. 85, p. 269-272. 

Snoke, A.W., and Miller, D.M., 1988, Metamorphic and tectonic history of the northeastern Great Basin, in Ernst, 

W.G., ed., Metamorphic and tectonic evolution of the 

Western United States: Englewood Cliffs, N.J., PrenticeHall, p. 606-648. 

Spear, F.S., and Harrison, T.M., 1989, Geochronologic studies 

in central New England-Pt. 1, Evidence for pre-Acadian 

metamorphism in eastern Vermont: Geology, v. 17, p. 

181-184. 

Steiger, R.H., and Jager, Emilie, comps., 1977, Subcommission 

on geochronology-Convention on the use of decay constants in geo- and cosmochronology: Earth and Planetary 

Science Letters, v. 36, p. 359-362. 

Streckeisen, Albert, 1976, To each plutonic rock its proper 

name: Earth Science Reviews, no. 12, p. 1-33. 

Zietz, Isidore, Shuey, Ralph, and Kirby, J.R., Jr., 1976, Aeromagnetic map of Utah: U.S. Geological Survey Geophysical Investigations Map GP-907, scale 1:500,000. 

18 K-Ar Ages of Jurassic to Tertiary Plutonic and Metamorphic Rocks, Utah and Nevada 


SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS 

Periodicals 

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly). 

Technical Books and Reports 

Professional Papers are mainly comprehensive scientific reports of 

wide and lasting interest and importance to professional scientists and engineers. Included are reports on the results of resource studies and of 

topographic, hydrologic, and geologic investigations. They also include 

collections of related papers addressing different aspects of a single scientific topic. 

Bulletins contain significant data and interpretations that are of lasting scientific interest but are generally more limited in scope or 

geographic coverage than Professional Papers. They include the results 

of resource studies and of geologic and topographic investigations; as well 

as collections of short papers related to a specific topic. 

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrogeology, 

availability of water, quality of water, and use of water. 

Circulars present administrative information or important scientific 

information of wide popular interest in a format designed for distribution 

at no cost to the public. Information is usually of short-term interest. 

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS 

publications, and they are also available for public inspection at 

depositories indicated in USGS catalogs. 

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at 

depositories. They are a nonpermanent form of publication that may be 

cited in other publications as sources of information. 

Maps 

Geologic Quadrangle Maps are multicolor geologic maps on 

topographic bases in 7 1n.- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1 :62,500) showing bedrock, surficial, or engineering geology. Maps generally include brief texts; some maps include structure 

and columnar sections only. 

Geophysical Investigations Maps are on topographic or planimetric 

bases at various scales; they show results of surveys using geophysical 

techniques, such as gravity, magnetic, seismic, or radioactivity, which 

reflect subsurface structures that are of economic or geologic significance. 

Many maps include correlations with the geology. 

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 7 1/2-minute quadrangle photo geologic maps on planimetric bases 

which show geology as interpreted from aerial photographs. Series also 

includes maps of Mars and the Moon. 

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, stratigraphy, and structural relations in certain coal-resource areas. 

Oil and Gas Investigations Charts show stratigraphic information 

for certain oil and gas fields and other areas having petroleum potential. 

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on various subjects such as 

environmental studies or wilderness mineral investigations. 

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetri<-bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; principal scale 

is 1:24,000 and regional studies are at 1:250,000 scale or smaller. 

Catalogs 

Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available under 

the conditions indicated below from the U.S. Geological Survey, Books 

and Open-File Reports Section, Federal Center, Box 25425, Denver, 

CO 80225. (See latest Price and Availability List) 

"Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche. 

"Publications of the Geological Survey, 1962- 1970" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche. 

"Publications of the U.S. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche. 

Supplements for 1982, 1983, 1984, 1985,1986, and for subsequent 

years since the last permanent catalog may be purchased by mail and 

over the counter in paperback book form. 

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by mail 

and over the counter in paperback booklet form only. 

"Price and A vailabillty List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet form only. 

Selected copies of a monthly catalog" New Publications of the U.S. 

Geological Survey" available free of charge by mail or may be obtained 

over the counter in paperback booklet form only. Those wishing a free 

subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 

National Center, Reston, VA 22092. 

Note.--Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the 

prices charged may differ from the prices in catalogs, announcements, 

and publications.