GEOLOGICAL SURVEY CIRCULAR 727 

Summary of Miscellaneous Potassium-Argon 

Age Measurements, U.S. Geological 

Survey,Menlo Park, California, 

for the Years 1 972-7 4 

Summary of Miscellaneous Potassium-Argon 

Age Measurements, U.S. Geological 

Survey,Menlo Park, California, 

for the Years 1 972-7 4 

Compiled and edited by A. l. Berry, G. B. Dalrymple, M.A. Lanphere, and J. C. 

Von Essen, in cooperation with H. C. Berg, R. H. Campbell, S. H. B. Clark, H. R. 

Cornwall, M.D. Crittenden, Jr., Bela Csejtey, Jr., G. D. Eberlein, K. F. Fo>·, Jr., 

W. E. Hall, B. C. Hearn, Jr., J. M. Hoare, D. M. Hopkins, E. M. MacKev~tt, Jr., 

R. May, T. P. Miller, J. C. Moore, C. D. Rinehart, and D. W. Scholl 

G E 0 l 0 G I C A l 5 U R V E Y C I R C U l A R 727 
United States Department of the Interior 

THOMAS S. KLEPPE, Secretary 

Geological Survey 

V. E. McKelvey, Director 

Library of Congress Cataloging in Publication Data 

Summary of miscellaneous potassium-argon age measurements, U.S. Geological Survey, 

Menlo Park, California, for the years 1972-74. 

(Geological Survey Circular 727) 

Bibliography: p. 12-13. 

Supt. of Docs. no.: I 19.16:727 

1. Potassium-argon dating. 2. Geology-The West. 3. Geology-Alaska. I. Berry, A. L. II. 

Series: United States. Geological Survey Circular 727. 

QE75.C5 no. 727[QE508] 557.3'08s[551.7'01] 75-619435 

·-'~ion to Branch of Distribution U.S. Geological Survey, 1200 South Eads Street, Arlingtor, VA 22202 
CONTENTS 

Page 

Abstract 1 

Introduction -------------------------------------------------------------------------- 1 

Alaska-------------------------------------------------------------------------------- 1 

Arizona ------------------------------------------------------------------------------ 9 

California ---------------------------------------------------------------------------- 9 

Idaho -------------------------------------------------------------------------------- 10 

lJtah --------------------------------------------------------------------------------- 10 

VVashington --------------------------------r----------------------------------------- 10 

Referencescited --------------------------------------------------------------------- 12 

Summary of ¥iscellaneous Potassium-Argon Age 

Measurements, U.S. Geological Survey, Menlo Park, California, 

for the Years 1972-7 4 

Compiled and edited by A. L. Berry, G. B. Dalrymple, M. A. Lanphere, and J. C. Von Essen, in cooperation with H. C. 

Berg, R. H. Campbell, S. H. B. Clark, H. R. Cornwall, M.D. Crittenden, Jr., Bela Csejtey, Jr., G. D. Eberlein, K. F. 

Fox, Jr., W. E. Hall, B. C. Hearn, Jr., J. M. Hoare, D. M. Hopkins, E. M. MacKevett, Jr., R. May, T. P. Miller, J. C. 

Moore, C. D. Rinehart, and D. W. Scholl 

ABSTRACT 

Potassium-argon age measurements are reported for 61 

mineral separates and rock samples from Alaska, Arizona, 

California, Idaho, Utah, and Washington. The age report for 

·each sample gives location, analytical data, and a brief geologic 

interpretation. 

INTRODUCTION 

This compilation contains the results of 

potassium-argon age measurements completed by 

the Geochronology (Menlo Park) Project for other 

Survey geologists during the years 1972-74. It is 

not a compilation of all age measurements made in 

the Menlo Park laboratory during that period, 

only those for which the scientific participation by 

Branch of Isotope personnel was insufficient to 

warrant a joint publication. The purposes of the 

compilation are to make these data available as 

soon as possible, to ensure uniform and complete 

publication of analytical results, and to provide an 

easy reference that can be used, in lieu of 

publication of complete analytical results, by the 

geologist when writing reports. Most of the age 

measurements made by the Branch of Isotope 

Geology are published in topical reports authored 

or co-authored by branch personnel; a bibliography of these publications is available on request 

from the Chief, Branch of Isotope Geology, U.S. 

Geological Survey, Denver Federal Center, Denver, Colo. 80225. 

Conventional argon measurements were made 

by isotope dilution mass spectrometry. Potassium 

was determined by flame photometry using 

lithium metaborate fusion. Detailed descriptions 

of the analytical methods are given by Dalrymple 

and Lanphere (1969) and by Ingamells (1970). The 

techniques for 40Ar/39Ar measurements are described by Dalrymple and Lanphere (1971). Constants used in the data reduction are A.E=0.585x 

10-10 yr-1, A,B=4.72x10-10 yr-1, 4°K/K total= 

1.19x 10-4mole/mole. 

Potassium measurements were made by L. B. 

Schlocker. Argon measurements and data reduction were made by A. H. Atkinson, A. L. B€rry, G. 

B. Dalrymple, S. J. Kover, M. A. Lanphere, B. M. 

Myers, and J. C. Von Essen. 

Users of this compilation are cautioned that the 

data are presented with only a brief description of 

the geologic setting and with little or no interpretation of the geologic significance or meaning of 

the calculated age. It is strongly advised that the 

geologist cited in the sample description be 

consulted before an age measurement is cited in 

another publication or otherwise used. Qu~stions 

concerning technical details of the age analyses 

should be directed to the Geochronology Project, 

Branch of Isotope Geology, U.S. Geologic:1 1 Survey, 345 Middlefield Road, Menlo Park, Calif. 

94025. ALASKA 

68 ABg 247 

(Biotite)299±9 m.y. 

Description and location: Central Metlakatla 

pluton. Foliated leucocratic quartz diorite. (Lat 

55°04.2' N., long 131 °33.8' W.; Ketchikan A-5 

quadrangle, Alaska, from outcrop just above 

high-tide line on the west shore of ':':'amgas 

Harbor, Annette Island.) 

Analytical data; K20=8.81, 8.83 percent, 40Arrad 

/gm=4.228x 10 -9moles,40Arrad140 Artotal =0.94. 
Geologist: H. C. Berg, U.S. Geological Survey, 

Menlo Park. Calif. 

Collected by: Berg. 

Geologic setting and comment: Metlakatla Peninsula, a southwestern projection of Annette 

Island, is underlain by a structurally complex 

assemblage of ultramafic and metamorphosed 

bedded and intrusive rocks. The premetamorphic age of the recrystallized bedded rocks is 

unknown. They are intruded by two foliated 

plutons, the South Metlakatla pluton and the 

Central Metlakatla pluton. Because the dated 

mineral was metamorphic biotite, the resultant 

age may be intermediate between the crystallization age of the pluton and one or more later 

thermal events or may even date the thermal 

events. The ultramafic body, which is in fault 

contact with the enclosing metamorphic units, is 

thought to correlate with upper Mesozoic zoned 

ultramafic bodies in nearby areas (Taylor and 

Noble, 1960; Lanphere and Eberlein, 1966). The 

assemblage is in fault contact with better dated 

rocks on mainland Annette Island that range in 

age from Silurian or older to Jurassic and 

Cretaceous. To this time, efforts to correlate the 

two have not been successful. 

Reference: Berg 

68 ABg 727 

(Biotite)178±5 m.y. 

Description and location: South Metlakatla pluton. Foliated quartz diorite and diorite. (Lat 

55°01.3' N., long 131°34.1' W.; Ketchikan A-5 

quadrangle, Alaska, from outcrop just above 

high-tide line, south coast of Metlakatla Peninsula, Annette Island, about 1 mile west of mouth 

ofTamgas Harbor.) 

Analytical data: K20=8.83, 8.80 percent, 40Arrad 

/gm = 2.428 X 10-9 moles, 40Arradf40 Artotal 

=0.94. 

Geologist: H. C. Berg, U.S. Geological Survey, 

Menlo Park, Calif. 

Collected by: Berg. 

Geologic setting and comment and reference: See 

68 ABg 247. 

70 ACs 423 

(Hornblende) 33.9±2.0 m.y 

Description and location: Unnamed felsic to intermediate hypabyssal rocks. Dacite porphyry. 

(Lat 61°03.9' N., long 149°47.8' W.; Anchorage 

quadrangle, Alaska.) 

2 

Analytical data: K20 = 0.130, 0.133 percent, 

40Arrad/gm = 6.649 X 10-12 m'lles, 40Arradl 

40Artotal =0.17. 

Geologist: S. H. B. Clark, U.S. Geological Survey, 

Reston, Va. 

Collected by: Clark. 

Geologic setting and comment: Dike or sill adjacent 

to strongly deformed low-grade metamorphic 

rocks of the McHugh Complex of Late Jurassic 

or Cretaceous age. Other similar s:'llall intrusive 

bodies, including dike swarms, cut the Valdez (?) 

Group of Jurassic (?) and Cretc~eous (Maestrichtian) age in the western C:rugach Mountains. 

Reference: Clark ( 1972). 

72 ACy 127 

(Biotite) 64.8±2 m.y.; (Hornblende) 6~.7±2 m.y. 

Description and location: Unnamed pluton within 

the Talkeetna Mountains batholith. Coarsegrained biotite-hornblende tonalite. (Lat 

62°08.8' N., long 149°18.5' W.; Talkeetna Mts. 

quadrangle, Alaska.) 

Analytical data: biotite, K20 = 9.30 percent, 

40Arradlgm = 9.055 X 10-10 moles, 40Arradl 

40Artotal =0.87; hornblende, K20=0.781, 0.783 

percent, 40Arradl gm = 7.371 X 10-11 moles, 

40 Ar rad I 40 Artotal = 0. 7 8. 

Geologist: B. Csejtey, Jr., U.S. Geological Survey, 

Menlo Park, Calif. 

Collected by: Csejtey. 

Geologic setting and comment: The tonalite forms 

a separate pluton within the western part of the 

Talkeetna Mountains batholith. The new ages 

(see also 72 ACy 117) suggest that the bulk of the 

complex is made up of rocks of Late Cretaceous 

and early Tertiary age rather thar Jurassic age, 

as postulated by earlier workers. As most 

mineral despoits in south-centrcl Alaska are 

associated with Late Cretaceous and early 

Tertiary plutons (Reed and Lanphere, 1969), the 

Talkeetna Mountains batholith may be an area 

favorable for mineral exploration. 

References: Csejtey (197 4); Reed and Lanphere 

(1969). 

72 ACy 117 

(Biotite) 65.6±2 m.y.; (Hornblende) 6:1.0±2 m.y. 

Description and location: Pluton (eastern pluton) 

within Talkeetna Mountains batholith. 

Coarse-grained foliated biotite-hornblende 
quartz diorite. (Lat 62°09.0' N., long 149°13.5' 

W.; Talkeetna Mts., A-5 quadrangle, Alaska.) 

Analytical data: biotite, K20 =9.33 percent, 

40Arradlgm=9.207xl0-10 moles, 40Arradl 

40 Artotal =0. 79; hornblende, K20 = 1.043, 1.042 

percent, 40Arradlgm=9.869x 10-11 moles, 

40 Ar radf40 Artotal =0. 77. 

Geologist: B. Csejtey, Jr., U.S. Geological Survey, 

Menlo Park, Calif. 

Collected by: Csejtey. 

Geologic setting and comment: The quartz diorite 

constitutes a member in an intrusive series 

within the Talkeetna Mountains batholith. 

Reference: See 72 ACy 127. 

66 AE 91 

(Amphibole) 438± 13 m.y. 

Description and location: Unnamed. Heterogeneous assemblage of altered mafic volcanic rocks. 

(Lat 55°42.5' N., long 133°09.5' W.; Craig C-4 

quadrangle, Alaska.) 

Analytical data: K 2 0= 0.676, 0.672 percent, 1 

40 Ar radlgm=4.904xl0-10 moles, 4oArradl 

40 Ar total =0.51. 

Geologist: G. D. Eberlein, U.S. Geological Survey, 

Menlo Park. Calif. 

Collected by: G. D. Eberlein and M. Churkin, Jr. 

Geologic setting and comment: Age measurement 

was made on pyrogenic amphibole-bearing phase 

of a heterogeneous assemblage of epidotized 

mafic volcanic rocks (mainly breccia) believed to 

represent part of an early Paleozoic volcanic 

island arc terrane that furnished volcanic 

material to adjacent Early Silurian and possible 

Late Ordovician arc troughs. The age data (see 

also 65 AE 31) indicate that these rocks are 

coeval with part of the Descon Formation and 

are consistent with paleontologic evidence from 

limestone interbedded with the volcanic rocks 

that underlie Staney Cone (see also 65 AE 31 ). 

Reference: Churkin and Eberlein (1975). 

65 AE 31 

(Amphibole) 442±13 m.y. 

Description and location: Unnamed. Heterogeneous assemblage of altered mafic volcanic rocks. 

(Lat 55°37.2' N., long 133°00.2' W.; Craig C-4 

quadrangle, Alaska.) 

Analytical data: K20 = 0.596, 0.593 percent, 

40 Arrad /gm= 4.379X10 -10 moles, 40 Arrad/ 

40 Ar total =0.85. 

3 

Geologist: G. D. Eberlein, U.S. Geological Survey, 

Menlo Park, Calif. 

Collected by: G. D. Eberlein and M. Churkin, Jr. 

Geologic setting and comment and reference: See 

66 AE 91. 

71 AE 67 

(Amphibole) 281±9 m.y. 

Description and location: Unnamed. HE,terogeneous assemblage of altered mafic volcanic rocks. 

(Lat 55°32.0' N., long 132°51.5' W.; Craig C-3 

quadrangle, Alaska.) 

Analytical data: K 2 0 = 0.651, 0.655 percent, 

40 Ar rad /gm = 2.921 X 10 -10 moles, 40 Ar rad I 

40 Ar total= 0. 79. 

Geologist: G. D. Eberlein, U.S. Geological Survey, 

Menlo Park, Calif. 

Collected by: G. D. Eberlein and M. Churkin, Jr. 

Geologic setting and comment: Mineral dated is 

from same unit aR 66 AE 91 and 65 AE 31. The 

calculated age is about 35 percent younger than 

the amphibole ages of these samples and may 

reflect thermal resetting during emplc~ement of 

the hornblende quartz diorite of Pin Peak, 

exposed about 0.8 km northeast (see 71 AE 68). 

Reference: none. 

71 AE 68 

(Amphibole) 112±3 m.y. 

Description and location: Quartz diorite of Pin 

Peak. Quartz diorite. (Lat 55°32.5' N., long 

132°50.7' W.; Craig C-3 quadrangle, Alaska.) 

Analytical data: K2 0 = 0.843, 0.841 percent, 

40 Ar rad /gm = 1.432 X 10 - 10 moles, 40 Arrad I 

40 Ar total =0.67. 

Geologist: G. D. Eberlein, U.S. Geological Survey, 

Menlo Park, Calif. 

Collected by: Eberlein. 

Geologic setting and comment: Dated naterial is 

from pegmatitic phase of the hornblende quartz 

diorite of Pin Peak intrusive into a h~terogeneous assemblage of andesitic to basaltic volcanic 

rocks, mainly breccia, the age of which ranges 

from late Middle Ordovician (Cara-:locian) to 

Upper Silurian (Ludlovian) on the basis of 

paleontologic evidence and other potassiumargon age measurements (see 65 AE 31 and 66 

AE 91 comments). This date confrms midCretaceous calcalkaline plutonism on central 

Prince of Wales Island with associated base-
metal mineralization and establishes a 

minimum age for the volcanic host rocks. 

Reference: none. 

69 ACk 1008 

(Biotite) 65.6±2 m.y. 

Descr.iption and location: Unnamed granitic intrusive rock. (Lat 60°29.5' N., long 158°4' W.; 

Taylor Mountain, B-6 quadrangle, Alaska, 

northeast sill of Shotgun Hills.) 

Analytical data: K2 0 = 8.65, 8.69 percent, 

40 Ar rad/ gm=8.556x10 -lO moles, 40Arrad/ 

40 Ar totfll = 0. 78. 

Geologist: J. M. Hoare, U.S. Geological Survey, 

Menlo Park, Calif. 

Collected by: J. M. Hoare and A. L. Clark. 

Geologic setting and comment: Intrudes the Kuskokwim Group (Early and Late Cretaceous). 

The biotite age is younger than other dates 

obtained on plutonic intrusive rocks in southwest Alaska. It is approximately the same as 

ages determined on hypabyssal and volcanic 

rocks in the Yukon-Koyukuk region. 

Reference: none. 

73 AHp 47 

(Basalt) 2.84±0.14 m.y. 

Description and location: Unnamed basalt. Tertiary and Quaternary lava flows of western 

Alaska. (Lat 65°25.3' N., long 166°32.6' W.; 

Nome quadrangle, Alaska, from an erosional 

escarpment at top of east wall of California 

River valley, 9 km above the river mouth.) 

Analytical data: K20=0.859, 0.865, 0.867, 0.875 

percent, 40 Ar rad /gm = 3.633 x 10 -12 moles, 

40 Ar rad I 40 Ar total =0.40. 

Geologist: D. M. Hopkins, U.S. Geological Survey, 

Menlo Park, Calif. 

Collected by: Hopkins. 

Geologic setting and comment: Basaltic lava flow 

resting on an erosion surface planed across 

limestone of early Paleozoic or Precambrian age 

and truncated on its seaward side by the 

shoreline ofthe York Terrace. The York Terrace 

at this point has an Anvilian (early Pleistocene) 

mollusk fauna. The terrace extends seaward to 

the wave-cut scarp of the Lost River Terrace of 

Sangamon age. The age of the lava flow provides 

a maximum limit on the age of the York Terrace 

and on the age of the Anvilian transgression. 

The age of the flow also indicates the time of a 

4 

major reorganization of the regioral drainage 

resulting from blocking by volcanic accumulations. 

References: Steidtman and Cathcart (1922); 

Sainsbury (1967); Hopkins, Rowland, Echols, 

and Valentine (1974). 

70 AMk 15D 

(Hornblende) 64.2±2 m.y.; (Biotite) 55.5±2 m.y. 

Description and location: Unnamed plutonic rock. 

A late phase of the Coast Rang~ batholith. 

Leucogranodiorite. (Lat 59°35.1' N., long 

136°01.8' W.; Skagway, C-3 quadrangle, 

Alaska.) 

Analytical data: hornblende, K20= 1.593, 1.595 

percent, 40 Ar rad lgm = 1.538x 10-10 moles, 

40 Ar .rad I 40 Ar total = 0. 70; biotite, K2 0=8. 70, 

8.73 percent, 40Arrad/gm=7.250x10-10 moles, 

40 Ar rad I 40 Ar total = 0.46. 

Geologist: E. M. MacKevett, Jr., U.S. Geological 

Survey, Menlo Park, Calif. 

Collected by: MacKevett. 

Geologic setting and comment: Probably youngest 

phase of Coast Range batholith in ro.arby area. 

A medium- to coarse-grained granitic rock that 

contains sodic plagioclase, quartz, microcline, 

minor amounts of hornblende and biotite, and 

rare minor accessories and alteration products. 

Forms a small stock with locally gradational, 

but generally obscure, contacts with older 

batholithic rocks. 

Reference: MacKevett, Robertson, and Winkler 

(1974). 

70 AMk 14A-2 

(Biotite) 111±3 m.y. 

Description and location: Unnarro,d pluton. 

Hornblende-biotite granodiorite. (Lat 59°16.6' 

N., long 136°26.3' W.; Skagway, B-4 quadrangle, Alaska.) 

Analytical data: K 2 0= 7.70, 7.73 percent, 

40 Ar d /gm = 1.306 x 10 -9 moles, 40 Ar rad I 40 ra 

Artotal =0.91. 

Geologist: E. M. MacKevett, Jr., U.S. Geological 

Survey, Menlo Park, Calif. 

Collected by: MacKevett. 

Geologic setting and comment: Intrudes 

metamorphosed Paleozoic rocks that are part of 

the Alexander terrane (Berg and others, 1972). 

A fine- to medium-grained foliated granitic rock 

that contains plagioclase (mainly oligoclase), 

quartz, K-feldspar, biotite, and minor 
hornblende and minor accessory minerals. 

Reference: MacKevett, Robertson, and Winkler 

(1974). 

70 AMk lOB 

(Hornblende) 119±4 m.y. 

Description and location: Unnamed pluton. Diorite or amphibolite. (Lat 59°21.2' N., long 

136°20. 7' W.; Skagway, B-4 quadrangle, 

Alaska.) 

Analytical data: K 20=0.417 percent, 4°Arrad I 

gm=7.587 X 10 -ll moles, 40 Ar rad I 40 Artotal = 

0.66. 

Geologist: E. M. MacKevett, Jr., U.S. Geological 

Survey, Menlo Park, Calif. 

Collected by: MacKevett. 

Geologic setting and comment: Forms an intervening zone between Cretaceous granodiorite and 

metamorphosed Paleozoic rocks. Probably represents an early (border) phase of the Cretaceous 

pluton that has partly assimilated and invaded 

wallrock. A fine-grained foliated rock that 

consists mainly of plagioclase and hornblende, 

in addition to alteration products, chiefly epidote. 

Reference: MacKevett, Robertson, and Winkler 

(1974). 

70 AMk SA-2 

(Hornblende) 105±3 m.y.; (Biotite) 108±3 m.y. 

Description and location: Unnamed pluton. 

Hornblende-biotite quartz diorite. (Lat 59°16.7' 

N., long 136°15.6' W.; Skagway, B-4 quadrangle, Alaska.) 

Analytical data: hornblende, K20=0.857, 0.860 

percent, 40 Ar rad I gm = 1.374x1o-lO moles, 

40 Ar rad I 40 Ar total= 0. 72; biotite, K 2 0 = 8.27, 

8.33 percent, 40 Ar rad lgm=l.363 x 10 - 9 moles, 

40 Arrad I 40Artotal =0.85. 

Geologist: E. M. MacKevett, Jr., U.S. Geological 

Survey, Menlo Park, Calif. 

Collected by: MacKevett. 

Geologic setting and comment: Part of a large 

pluton that intrudes Paleozoic metamorphic 

rocks. A medium-grained rock that consists of 

plagioclase (zoned andesine-oligoclase), quartz, 

minor K-feldspar, fairly abundant hornblende 

and biotite, and lesser amounts of minor 

accessories and alteration products. 

Reference: MacKevett, Robertson, and Winkler 

(1974). 

5 

70 AWk 6A 

(Biotite) 29.3± 1 m.y. 

Description and location: Unnamec pluton. 

Biotite-hornblende granodiorite. (Lat 59°15.9' 

N., long 136°01.1' W.; Skagway, B-3 quadrangle, Alaska.) 

Analytical data: K 2 0 = 7.20, 7.25 percent, 

40 Ar rad I gm=3.147x10-10 moles, 40 Ar rad I 

40 Ar total =0.60. 

Geologist: E. M. MacKevett, Jr., U.S. Geological 

Survey, Menlo Park, Calif. 

Collected by: G. R. Winkler, U.S. Geological 

Survey, Menlo Park, Calif. 

Geologic setting and comment: Part of a pluton 

that intrudes metamorphosed Paleozoi~ rocks. A 

fine-grained granodiorite with local mafic clots; 

locally fractured and altered. Consists of 

plagioclase, quartz, K-feldspar, hornblende, and 

biotite. Less abundant minor access~1ries and 

alteration products, including prehnite. Isotopic 

dates on this and similar rocks (see als'l 70 AMk 

5D, 3A, 3C, 2A) document previously unrecognized Tertiary granitic plutons in the Skagway 

B-3 quadrangle. These plutons are probable 

counterparts of similar granitic masr~s in the 

northern part of Glacier Bay National Monument. 

Reference: MacKevett, Robertson, and Winkler 

(1974) 

70 AWk 5D 

(Biotite) 30.6± 1 m.y. 

Description and location: Unnamed plutC'll. Biotite 

granodiorite. (Lat 59°15. 7' N., long 136°00.0'W.; 

Skagway, B-3 quadrangle. Alaska.) 

Analytical data: K2 0 = 7.10, 7.16 percent, 

40Arrad /gm= 3.246x 10 -10 moles, 40 Ar rad I 

40Ar total =0.69. 

Geologist: E. M. MacKevett, Jr., U.S. Geological 

Survey, Menlo Park, Calif. 

Collected by: MacKevett. 

Geologic setting and comment: Part of a pluton 

that intrudes Paleozoic metamorphic: rocks. A 

fine-grained granodiorite that contains a few 

mafic clots. See 70 A Wk 6A. 

Reference: MacKevett, Robertson, and Winkler 

(1974). 

70 AMk 3C 

(Hornblende) 33.0± 1 m.y. 

Description and location: Unnamed pluton. 

Hornblende-biotite quartz diorite. (Lat 59°19.1' 
N., long 135°48.7' W.; Skagway B-3 quadrangle, 

Alaska.) 

Analytical data: K.20=0.480 percent, 40Arrad/gm 

=2.360x10 -11 moles, 40Arrad ; 40Artotal=0.35. 

Geologist. E. M. MacKevett, Jr., U.S. Geological 

Survey, Menlo Park, Calif. 

Collected by: MacKevett. 

Geologic setting and comment: Part of a pluton 

that intrudes Paleozoic metamorphic rocks. A 

weakly gneissic medium-grained quartz diorite. 

Contains abundant quartz and plagioclase, 

chiefly andesine, less abundant hornblende and 

biotite, and minor accessories and alteration 

products. See 70 AMk 6A. 

Reference: MacKevett, Robertson, and Winkler 

(1974). 

70 AMk 3A 

(Biotite) 22.7±1 m.y. 

Description and location: Unnamed pluton. 

Biotite-hornblende quartz diorite. (Lat 59°19.8' 

N., long 135°48. 7' W.; Skagway, B-3 quadrangle, Alaska.) 

Analytical data: K20=8.10, 8.68 percent, 40Arrad 

/gm= 2.928 X 10-10 moles, 40Arradf40Artotal 

=0.20. 

Geologist: E. M. MacKevett, Jr., U.S. Geological 

Survey, Menlo Park, Calif. 

Collected by: MacKevett. 

Geologic setting and comment: Part of a Tertiary 

pluton that intrudes Paleozoic metamorphic 

rocks. The sample is a deformed locally cataclastic medium-grained gneissic quartz diorite. As 

its site is near the Chilkat River-Lynn Canal 

fault, the isotopic age may be too low because of 

cataclasis and related effects of faulting. See 70 

AMk6A. 

Reference: MacKevett, Robertson, and Winkler 

(1974). 

70 AWk 2A 

(Hornblende) 111±3 m.y.; (Biotite) 110±3 m.y. 

Description and location: Unnamed pluton. 

Hornblende-biotite quartz diorite. (Lat 59°27.8' 

N., long 136°18.1' W.; Skagway, B-4 quadrangle, Alaska.) 

Analytical data: hornblende, K20 = 0.933, 0.935 

percent, 40Arradlgm = 1.574 x 10-10 moles, 

40Arrad140Artotal = 0.87; biotite, K20 = 8.87 

percent, 40Arrad/gm = 1.480 X l0-9 moles, 

40Arrad f40Artotal =0.93. 

Geologist: E. M. MacKevett, Jr., U.S. Geological 

Survey, Menlo Park, Calif. 

6 

Collected by: G. R. Winkler, U.S. Geological 

Survey, Menlo Park, Calif. 

Geologic setting and comment: The sample is from 

a large pluton that intrudes Paleozoic metamorphic rocks. It is medium grained and contains 

abundant plagioclase (andesine) and quartz, 

fairly abundant hornblende and biotite, and 

minor quantities of K-feldspar, ac~essory minerals, and alteration products. 

Reference: MacKevett, Robertson, and Winkler 

(1974). 

71 AMk 26A 

(Hornblende) 142±4 m.y. 

Description and location: Unnamed pluton. Finegrained biotite hornblende granoiiorite. (Lat 

61 °44.4' N., long 143°49.6' W.; McCarthy, G-8 

quadrangle, Alaska.) 

Analytical data: K20=0.813 percent, 40Arrad/gm 

=1.766x10-10 moles, 40Arradi40Ar 1tai=0.76. 

Geologist: E. M. MacKevett, Jr., U.P. Geological 

Survey, Menlo Park, Calif. 

Collected by: MacKevett. 

Geologic setting and comment: Part of a pluton 

that cuts the Nikolai Greenstone (':':'riassic) and 

Skolai Group (Permian). 

Reference: none. 

71 AMk 19C 

(Hornblende) 145±4 m.y. 

Description and location: Unna:rr-'ed pluton. 

Hornblende granodiorite. (Lat 61°3 7.6' N., long 

143°39. 7' W.; McCarthy, C-8 ouadrangle, 

Alaska.) 

Analytical data: K20 = 0.680, O.f.S33 percent, 

40Arradlgm = 1.516 X 10-10 moles, 40Arradl 

40 Al\otal =0. 77. 

Geologist: E. M. MacKevett, Jr., U.f:. Geological 

Survey, Menlo Park, Calif. 

Collected by: MacKevett. 

Geologic setting and comment: Part of a pluton 

that cuts the Nikolai Greenstone (Triassic). A 

fine- to medium-grained granodiorite that contains abundant plagioclase (andesine), fairly 

abundant quartz, K-feldspar, and hornblende, 

and minor amounts of other minerals. 

Reference: none. 

70 AMk 61c 

(Hornblende) 3.8±0.8 m.y. 

Description and location: Unnamed Cenozoic felsic 

intrusive rock. Porphyrite hornblende-biotite 
dacite. (Lat 61 °37.6' N., long 143°39.3' W.; 

McCarthy, G-8 quadrangle, Alaska.) 

Analytical data: K20=0A68 percent, 40Arrad/gm 

=2.588x10-12 moles, 40Arradf40Artotal=0.05. 

Geologist: E. M. MacKevett, Jr., U.S. Geological 

Survey, Menlo Park, Calif. 

Collected by: MacKevett. 

Geologic setting and comment: A shallow pluton 

that may be related to the Wrangell Lava 

(Miocene to Holocene). Cuts the lower part of the 

Wrangell Lava and older rocks. Porphyritic, 

with plagioclase and mafic phenocrysts in a very 

fine grained felty groundmass. 

Reference: none. 

70 AMk 600-2 

(Hornblende) 142±4 m.y. 

Description and location: Unnamed pluton. 

Hornblende granodiorite. (Lat 61°33. 7' N., long 

143°39.6' W.; McCarthy, C-8 quadrangle, 

Alaska.) 

Analytical data: K20=0.800 percent, 40Arrad/gm 

=1.746x10-10 moles, 40Arrad140Artotal=0.77. 

Geologist: E. M. MacKevett, Jr., U.S. Geological 

Survey, Menlo Park, Calif. 

Collected by: MacKevett. 

Geologic setting and comment: A granitic pluton 

that cuts Triassic rocks, is cut by Cenozoic 

hypabyssal plutons, and is nonconformably 

overlain by Lower Cretaceous sedimentary 

rocks. 

Reference: none. 

70 AMk 60B 

(Hornblende) 134±7 m.y. 

Description and location: Unnamed pluton. 

Hornblende granodiorite. (Lat 61°32.6' N., long 

143°42.4' W.; McCarthy, C-8 quadrangle, 

Alaska.) 

Analytical data: K20 = 0. 705, 0. 704 percent, 

40Arrad/gm = 1.448 X 10-10 moles, 40Arradl 

40 Artotal =0.30. 

Geologist: E. M. MacKevett, Jr., U.S. Geological 

Survey, Menlo Park, Calif. 

Collected by: MacKevett. 

Geologic setting and comment: See 70 AMk 60D-2. 

70 AMa 226 

(Hornblende) 56.5±2 m.y.; (Biotite) 56.4±2 m.y. 

Description and location: Unnamed pluton. Quartz 

monzonite porphyry. (Lat 62°22.0' N., long 

7 

153°47.5' W.; McGrath, B-2 quadrangle, 

Alaska.) 

Analytical data: hornblende, K20 = 1.0~7, 1.057 

percent, 40Arrad/gm = 9.003 x 10-11 moles, 

40Arrad140Artotal =0.52; biotite, K20=7.44, 7.30 

percent, 40Arrad/gm = 6.232 X 10-10 moles, 

40Arradf40Artotal =0. 79. 

Geologist: R. May, U.S. Geological Survey, Menlo 

Park, Calif. 

Collected by: May. 

Geologic setting and comment: Mediun-grained 

quartz monzonite prophyry that contains abundant one-half-inch-long phenocrysts of orthoclase. This small pluton is exposed in the Sheep 

Creek drainage 11 miles (18 km) south of 

Farewell, Alaska, and intrudes limestone, 

shale, and phyllite of early Paleo-:>:oic age. 

Concordant ages on coexisting biotite and 

hornblende and similarities in both chemical 

and modal composition indicate that the pluton 

is genetically associated with the undifferentiated series of intrusive rocks of Late Cretaceous and early Tertiary age which l' ave been 

identified in this part of the Alaska Range by 

Reed and Lanphere (1973). 

Reference: Reed and Lanphere (1973). 

67 AMa 226 

(Hornblende) 57.6±2 m.y.; (Biotite) 56.6±~ m.y. 

Description and location: Unnamed plutcTt. Quartz 

monzonite porphyry. (Lat 62°22.0' N., long 

153°47.5' W.; McGrath, B-2 qu~drangle, 

Alaska.) 

Analytical data: hornblende, K20 = 1.0~(), 1.057 

percent, 40Arradlgm = 9.151 x 10-11 moles, 

40Arradf40Artotal=0.73; biotite, K20=7.23, 7.22 

percent, 40Arradl gm = 6.138 x 10-10 moles, 

40Arradl 40Artotal=0.86. 

Geologist: R. May, U.S. Geological Surw~y, Menlo 

Park, Calif. 

Collected by: May. 

Geologic setting and comment and reference: See 

70 AMa 226. 

70 AMm 158b 

(Hornblende) 92.8±2.6 m.y.; (Biotite) 88.3±1.5 m.y. 

Description and location: Darby pluton. Quartz 

monzonite. (Lat 64°45.2' N., long 162°252.2' W.; 

Solomon quadrangle, Seward Peninsula, 

Alaska.) 
Analytical data: hornblende, K20 = 0.893, 0.8961 long 162°48' W.; Sanak Island, False Pass 

percent, 40Arradlgm = 1.250 X 10-10 moles, quadrangle, Alaska.) 

40Arradf40Artotal=0.81; biotite, K20=7.74, 7.87 Analytical data:K20=8.28, 8.32 percent, 40Arrad/ 

percent, 40Arradlgm = 1.042 X 10-9 moles, gm = 7.460 X 10-10 moles, 40Arradf40Artotal 

40Arrad140 Artotal=0.93. =0.85. 

Geologist: T. P. Miller, U.S. Geological Survey, Geologist: J. C. Moore, Department of Earth 

Anchorage, Alaska. Sciences, University of California, Santa Cruz, 

Collected by: Miller. Calif. 

Geologic setting and comment: Pluton intrudes Collected by: Moore. 

high-grade metamorphic complex ofPrecambri- Geologic setting and comment: The fqnak pluton 

an age on the west and is in probable fault intrudes the Shumagin Formatior (Maestrichcontact with a Devonian carbonate complex on tian) and crosscuts the major phase of isoclinal 

the east. overturned folding. The pluton vras probably 

Reference: Miller, Grybeck, Elliott, and Hudson intruded at a shallow depth (few kilometres). 

(1972). The age provides an upper limit on the deforma71 AMm 415A tion and uplift of the Shumagin Formation, 

(Biotite) 94.0±3.0 m.y. which itself is probably less than 70 m.y. old. 

Description and location: Darby pluton. Coarsegrained porphyritic quartz monzonite. (Lat 

64°57.0' N., long 162°19.2' W.; Solomon quadrangle, Seward Peninsula, Alaska.) 

Analytical data: K20=8.96, 8.96 percent, 40Arrad 

/gm = 1.275 X 10-9 moles, 40Arradf40Artotal 

=0.78. 

Geologist: T. P. Miller, U.S. Geological Survey, 

Anchorage, Alaska. 

Collected by: Miller. 

Geologic setting and comment and reference: See 

70 AMm 158b. 

71 AGk 259 

(Biotite) 93.9±3.0 m.y. 

Description and location: Nepheline syenite dike. 

(Lat 64°43.5' N., long 162°53.1' W.; Solomon 

quadrangle, Seward Peninsula, Alaska.) 

Analytical data: K20=7.12, 7.12 percent, 40Arrad 

/gm = 1.013 X 10-9 moles, 40Arradf40Artotal 

=0.68. 

Geologist: T. P. Miller, U.S. Geological Survey, 

Anchorage, Alaska. 

Collected by: D. Grybeck, U.S. Geological Survey, 

College, Alaska. 

Geologic setting and comment: Nepheline syenite 

dike cutting granodiorite of the Kachauik 

pluton which intrudes metamorphic rocks of 

Precambrian age. 

Reference: Miller, Grybeck, Elliott, and Hudson 

(1972). 

S-70 

(Biotite) 59.9±1.8 m.y. 

Description and location: Sanak pluton. 

Granodiorite or quartz diorite. (Lat 54°29' N., 

8 

Reference: Moore (1973). 

54 Sn 197 

(Hornblende) 11.1±3 m.y.; (Biotite) 3.4±:0.2 m.y. 

Description and location: Shaler pluton. 

Granodiorite. (Lat 53°37.7' N., long 166°56' W.; 

Unalaska Island, Aleutian Islandf.) 

Analytical data: hornblende, K20 = 0.353, 0.355 

percent, 40Arradlgm = 5.821 x l0-12 moles, 

40Arradf40Artotal =0.05; biotite, K20=7.49, 7.59 

percent, 40Arrad/ gm = 3.754 X JQ-11 moles, 

40 Ar radl 40 Artotal =0.32. 

Geologist: D. W. Scholl, U.S. Geological Survey, 

Menlo Park, Calif. 

Collected by: G. L. Snyder, U.S. Geolo~ical Survey, 

Denver,~ Colo. 

Geologic setting and comment: The fl-taler pluton 

is an equigranular medium- to c<Jarse-grained 

granodioritic stock or small batholith exposed in 

the Shaler Mountains of southern Unalaska 

Island. It intrudes Tertiary sedilnentary and 

volcanic rocks of the Unalaska Formation, 

which, based on desmostylid teeth, includes 

units as young as early Miocene. Tl'~ biotite age 

is much too young and probably reflects a 

postintrusion heating event related to the 

formation of Makushin Volcano, a nearby (29 

km) late Cenozoic stratocone. The hornblende 

age is probably close to the crystalHzation age as 

similar batholiths and stocks elsewhere along 

the Aleutian ridge crystallized between about 

12 and 16 m.y. ago. 

References: Drewes, Fraser, Snyder, and Barnett 

(1961); Marlow, Scholl, Buffingtor, and Alpha 

(1973). 
ARIZONA 

12-100 

(Biotite) 17.1± 1 m.y. 

Description and location: Unnamed tuff in San 

Manuel Formation. Airfall tuff. (Lat 33°2.1' N., 

long 110°59.4' W.; Kearny quadrangle, Pinal 

County, Ariz.) 

Analytical data: K20=7.48, 7.55 percent, 40Arrad 

/gm = 1.902 X 10-10 moles, 40Arrad/40Artotal 

=0.30. 

Geologist: H. R. Cornwall, U.S. Geological Survey, 

Menlo Park, Calif. 

Collected by: M. H. Krieger, U.S. Geological 

Survey, Menlo Park, Calif. 

Geologic setting and comment: This tuff bed is near 

the top of a tuffaceous sandstone and conglomerate unit in the San Manuel Formation, which 

is composed of alluvial and playa deposits in 

intermontane basins. 

Reference: Cornwall and Krieger (1974). 

CALIFORNIA 

69 c 24 

(Plagioclase) 14.6:±:1 m.y. 

Description and location: Volcanic strata of Zuma 

Canyon. Porphyritic basalt. (Lat 34°00.0' N., 

long 118°48.3' W.; Point Dume quadrangle, Los 

Angeles County, Calif., from the southern tip of 

Point Dume.) 

Analytical data: K20=0.247 percent, 40Arradlgm 

=5.365x1Q-12 moles, 40Arradf40Artotal=0.16. 

Geologist: R. H. Campbell, U.S. Geological Survey, 

Menlo Park, Calif. 

Collected by: Campbell. 

Geologic setting and comment: Overlain by very 

thin bedded to flaggy dolomitic marl, chert, and 

shale of the Monterey Formation, which contains foraminifers assigned to the Luisian Stage 

(middle Miocene). Apparently associated with 

volcanic rocks in nearby structural blocks that 

contain interbedded mudstone locally containing foraminifers assigned to the Relizian and 

Luisian Stages (middle Miocene). 

Reference: none. 

Haiwee-1 

(Biotite) 86.7±2.6 m.y. 

Description and location: Biotite-hornblende 

quartz monzonite. (Lat36°9.8' N., long 117°55.2' 

W.; T. 20 S., R. 37 E., sec. 25, 1 mile east of 

Haiwee Reservoir at altitude 4,320 ft. From drill 

9 

core Haiwee--1, 735- to 750-ft interval. lnyo 

County, Calif. 

Analytical data: K20=8.76, 8.80 percent, 40Arrad 

/gm = 1.151 X 10-9 moles, 40Arrad/40Artotal 

=0.89. 

Geologist: W. E. Hall, U.S. Geological Survey, 

Menlo Park, Calif. 

Collected by: J. A. Sass, U.S. Geological Survey, 

Menlo Park, Calif. 

Geologic setting and comment: Intrude~:· Keeler 

Canyon Formation of Pennsylvanian and Permian age and is overlain by the Coso Formation 

of late Pliocene or early Pleistocene ag~. 

Reference: none. 

H72-52 

(Obsidian) 0.088±0.013 m.y. 

Description and location: Obsidian of Borax Lake. 

Nonporphyritic rhyolite. (Lat 38°58.1' N., long 

122°38. 7' W.; Clear Lake volcanic fiehl, Clear 

Lake Highlands quadrangle, Calif. From massive obsidian rib on north side of central deepest 

part of pumice quarry (about 12 ft above quarry 

floor), southeast of intersection of Ar:-:-owhead 

Road and Ciwa Street, Clear Lake Parl-, Calif.) 

Analytical data: K20=4.74, 4.70 percent, 40Arrad 

/gm = 0.618 X 10-12 moles, 40Arradf40Artotal 

=0.091. 

Geologist: B. C. Hearn, Jr., U.S. Geological 

Survey, Reston, Va. 

Collected by: Hearn. 

Geologic setting and comment: Overlies an olivine 

dacite flow; obsidian flow retains son1e of its 

pumiceous carapace and is overlain by soil rich 

in human-worked obsidian fragments; reported 

by Anderson (1936) to be one of the mo~t recent 

flows in the Clear Lake volcanic field. 

References: Brice (1953); Anderson (193f). 

H72-1 

(Obsidian) 0.466:±:0.015 m.y. 

Description and location: Rhyolite of Thurston 

Creek. Porphyritic rhyolite. (Lat 38°55.4' N., 

long 122°41.8' W.; Clear Lake volcanic field, 

Clear Lake Highlands quadrangle, Calif. From 

obsidian rib with vertical flow banding on east 

side of narrows of bottom of canyon north of 

Manning Flat.) 

Analytical data: K20=4.75, 4.73 percent,.40Arrad 

/gm = 3.268 X 10-12 moles, 40Arrad140Artotal 

=0.33. 
Geologist: B. C. Hearn, 

Survey, Reston, V a. 

Collected by: Hearn. 

Jr., U.S. Geological Geologic setting and comment: Intrudes Milligen 

Formation (Devonian) and overlain by Challis 

Volcanics (Eocene). 

Geologic setting and comment: Oldest exposed 

obsidian in central part of Clear Lake volcanic 

field; assumed to overlie pyroxene dacite ofBrice 

(1953); overlain by porphyritic rhyodacites of 

Mount Konocti type. 

References: Brice (1953); Anderson (1936). 

H72-73DI 

(Obsidian) 0.536±0.016 m.y. 

Description and location: Rhyolite of Camelback 

Ridge. Porphyritic rhyolitic obsidian. (Lat 

38°54.4' N., long 122°4 7.9' W.; Clear Lake 

volcanic field, Kelseyville quadrangle, Calif. 

From west side of Bottle Rock roadcut, 90ft N. 7° 

W. from large pine tree on east side of road; 

sample from central horizontally banded part of 

S-shaped fold of banding.) 

Analytical data: K20=4.67, 4.70 percent, 40Arrad 

/gm = 3.716 X 10-12 moles, 40Arradf40Artotal 

=0.41. 

Geologist: B. C. Hearn, Jr., U.S. Geological 

Survey, Reston, V a. 

Collected by: Hearn. 

Geologic setting and comment: Apparently overlain by lake beds and terrace gravels of southern 

provenance; flow subsequently upfaulted to 

block former drainage from the south or southwest; overlies or is faulted against serpentine on 

southwest side; contiguous rhyolitic obsidian to 

the east overlies older dacite and andesite of 

southern part of Clear Lake field and is overlain 

by basalt ('pyroxene dacite" of McNitt (1968)). 

Reference: McNitt (1968). 

IDAHO 

A-230 

(Biotite) 81. 7±2.5 m.y. 

Description and location: Idaho batholith. Biotite-hornblende quartz monzonite. (Lat 43°27.6' 

N., long 114°21.5' W.; NWIJ4SE!J4 sec. 31, T. 2 N., 

R. 18 E., from outcrop at McCoy mine, Bellevue 

quadrangle, Blaine County, Idaho.) 

Analytical data: K20=6.86, 6.95 percent, 40Arrad 

/gm = 8.530 X 10-10 moles, 40Arradf40Artotal 

=0.89. 

Geologist: W. E. Hall, U.S. Geological Survey, 

Menlo Park, Calif. 

Collected by: Hall. 

Reference: none. 

UTAH 

72MC-ll7e 

(Basaltic hornblende) 570±7 I'1'.y. 

Description and location: Browns Hole Formation 

(volcanic member). Alkali trachyte clast from 

volcanic conglomerate intercalated with basaltic flows. (Lat 41 °19.6' N., long 111 °44'W.; north 

slope Middle Fork Ogden River, 5 miles northeast of Huntsville, Utah.) 

Analytical data: 40Ar/39Ar = 18.78, 37Ar/39Ar= 

9.617 corrected for 37Ar decay;T%= 35.1 days, 

36Ar/39Ar = 0.008111, J =0.02045, 40Arrad/ 

40Artotal =0.91. 

Geologist: M. D. Crittenden, Jr., U.S. Geological 

Survey, Menlo Park, Calif. 

Collected by: Crittenden. 

Geologic setting and comment: Date;;- the youngest 

presumably Precambrian unit in the allochthonous Huntsville sequence of northeastern Utah and southeastern Idab) (Crittenden 

and others, 1971). That sequence. begins at the 

base with diamictite (825--925? m.y.) and extends without apparent break irto the Cambrian. Browns Hole Formation re2ts on Mutual 

Formation (mainly purple quartzite) and is 

overlain by Geertsen Canyon Quartzite (uppermost unit of Brigham Group). Locality shown on 

Browns Hole quadrangle (Crittenden, 1972). 

References: Crittenden, Schaeffer, Trimble, and 

Woodward (1971); Crittenden (1972); Crittenden and Wallace (1973). 

WASHINGTON 

T-152 

(Hornblende) 98.5±3.0 m.y.; (Biotite) ~0.1±2.7 m.y. 

Description and location: Old Faldy pluton. 

Coarse- to medium-grained porpl:tyritic locally 

gneissic granodiorite. (Lat 48°~ 6. 7' N., long 

119°56.8' W.) 

Analytical data: hornblende, K20 = 1.265, 1.254 

percent, 40Arrad/gm =1.882 X 10-10 moles, 

40Arradf40Artotal =0.83; biotite, K20=9.06, 9.06 

percent, 40Arradlgm = 1.236 x 10-9 moles, 

40Arradf40Artotal =0.94. 

Geologists: C. D. Rinehart and K. F. Fox, Jr., U.S. 

Geological Survey, Menlo Park, Calif. 

10 
Collected by: Rinehart. 

Geologic setting and comment: A member of a 

composite batholith underlying the Okanogan 

Range. Intrudes the gneissic trondhjemite of 

Tiffany Mountain on the west and is intruded by 

the granodioritic Conconully pluton on the east. 

Probably the same as unnamed granodiorite 

gneiss (sample OK-5) referred to in Menzer 

(1970). See also T-153, T-154, T-155, T-156. 

Reference: Menzer (1970). 

T-153 

(Hornblende) 93.5±2.8 m.y.; (Biotite) 88.5±2.7 m.y. 

Description and location: Gneissic trondhjemite of 

Tiffany Mountain. Trondhjemite, mediumgrained weakly gneissic leucocratic rock (color 

index 6). (Lat 48°39.8' N., long 119°57.9' W.) 

Analytical data: hornblende, K20 = 0.580, 0.576 

percent, 40Arrad/gm = 8.196 X 10-11 moles, 

40Arradf40Artotal=0.51; biotite, K20=9.01, 9.00 

percent, 40Arrad/gm = 1.206 x 10-9 moles, 

40Arradf40Artotal =0.93. 

Geologist: C. D. Rinehart and K. F. Fox, Jr., U.S. 

Geological Survey, Menlo Park, Calif. 

Collected by: Rinehart. 

Geologic setting and comment: Probably the 

largest member of a composite batholith underlying the Okanogan Range; appears to be older 

than plutons adjacent on the east; relations to 

other members of the batholith not known as 

yet, for this unit may underlie as much as 1,300 

km2, mostly north, west, and south of the area 

studied. Probably the same as unnamed trondhjemitic gneiss (samples OK-3 and OK-6) 

referred to in Menzer (1970). See also T-152. 

T-154 

(Muscovite) 88.5±2.7 m.y.; (Biotite) 91.7±2.8 m.y. 

Description and location: Gneissic trondhjemite of 

Tiffany Mountain. Trondhjemite, mediumgrained weakly gneissic leucocratic rock (color 

index 6). (Lat 48°29.5' N., long 119°56.4' W.) 

Analytical data: muscovite, K20 = 10.26, 10.34 

percent, 40Arrad/gm = 1.379 x 10-9 moles, 

40Arradf40Artotal=0.77; biotite, K20=9.34, 9.30 

percent, 40Arrad/gm = 1.295 x 10-9 moles, 

40Arradf40Artotal =0.94. 

Geologists: C. D. Rinehart and K. F. Fox, Jr., U.S. 

Geological Survey, Menlo Park, Calif. 

Collected by: Rinehart 

Geologic setting and comment: See T-153. 

11 

T-155 

(Muscovite) 94.6±2.8 m.y.; (Biotite) 108±3 m.y. 

Description and location: Gneissic trondhjemite of 

Tiffany Mountain. Trondhjemite, mediumgrained virtually directionless leucoc¥atic rock 

(color index 5). (Lat 48°35.8' N., long 120°4.5' 

W.) 

Analytical data: muscovite, K20 = 10. 71, 10.78 

percent, 40 Arradlgm = 1.540 X 10-9 moles, 

40Arradf40Artotal=0.67; biotite, K20=9.37, 9.41 

percent, 40Arrad/gm = 1.543 X 10-9 moles, 

40Arradf40Artotal =0.93. 

Geologists: C. D. Rinehart and K. F. Fox, Jr., U.S. 

Geological Survey, Menlo Park, Calif. 

Collected by: Rinehart. 

Geologic setting and comment: See T-153. 

T-156 

(Hornblende) 81.2±2.4 m.y.; (Biotite) 78.8±~.4 m.y. 

Description and location: Conconully pluton. 

Medium-grained locally seriate p<Jrphyritic 

massive granodiorite. (Lat 48°35. 7' N., long 

119°52.1' W.) 

Analytical data: hornblende, K20=0.ffl1, 0.567 

percent, 40Ar rad I gm=6.914x10-ll moles, 

40Arrad; 40Artotal = 0.61; biotite, K~O = 9.16, 

9.16 percent, 40Arradlgm=l.089x10 - 9 moles, 

40 Ar rad/ 40 Ar total =0.90. 

Geologists: C. D. Rinehart and K. F. Fox, Jr., U.S. 

Geological Survey, Menlo Park, Calif. 

Collected by: Rinehart. 

Geologic setting and comment: Probably the 

youngest member in the area studied of a 

composite batholith underlying the Okanogan 

Range; intrudes metamorphic rocks on the east, 

a hybrid granitoid unit on the north, and the Old 

Baldy pluton on the west. Probably the same as 

unnamed granodiorite and quartz 1nonzonite 

(samples OK-1 and OK-4) referred to in Menzer 

(1970). See also T-152. 

C-559 

(Hornblende) 42.9± 1.3 m.y. 

Description and location: Dacite of Twin Peaks. 

Dark gray dense massive dacitic lava flow with 

fairly abundant hornblende crystals as long as 

5 mm set in an aphanitic matrix. (L2't 48°41.2' 

N., long 119°45.5' W.) 

Analytical data: K 2 0 = 0.556, 0.564 percent, 

40Arrad/gm=3.587x10-11 moles, 40Arrad/ 

40 Ar total =0.27. 
Geologists: C. D. Rinehart and K. F. Fox, Jr., U.S. 

Geological Survey, Menlo Park, Calif. 

Collected by: Rinehart. 

Geologic setting and comment: The dacite is near 

the base of a volcanic pile, probably a few 

kilometres thick, that unconformably overlies 

batholithic rocks. Except for glacial drift and 

alluvium, it is the youngest geologic unit in the 

area. 

References: Hibbard (1971); Rinehart and Fox 

(1976). 

H-1 

(Biotite) 97.7±2.9 m.y. 

Description and location: Cathedral pluton. 

Quartz monzonite, coarse-grained, massive, 

locally miarolitic, leucocratic. (Lat 48°52.3' N., 

long 120°01.0' W.) 

Analytical data: K 2 0 = 8.88, 8.87 percent, 

40Arrad /gm=l.316x10 -9 moles, 40Arrad I 

40 Ar total =0.90. 

Geologists: C. D. Rinehart and K. F. Fox, Jr., U.S. 

Geological Survey, Menlo Park, Calif. 

Collected by: Rinehart. 

Geologic setting and comment: One of the 

youngest, largest, and more homogeneous 

plutonic members of the composite batholith 

that underlies the Okanogan Range. It intrudes 

granitoid gneisses adjacent on both east and 

west. It occupies about 400 km2 (150 mi2) in the 

United States and an area perhaps half that size 

in Canada. About 115 km2 (44 mi2) of the 

eastern part of the pluton was mapped by 

Hibbard (1971), and a few tens of km2 of the 

western part was mapped by Hawkins (1968); 

this age is concordant with a potassium-argon 

age reported by Hawkins (1968, p. 1789). 

References: Hibbard (1971); Hawkins (1968). 

REFERENCES CITED 

Anderson, C. A., 1936, Volcanic history ofthe Clear Lake area, 

California: Geol. Soc. America Bull., v. 47, no. 5, p. 

629--664. 

Berg, H. C., 1972, Geologic map of Annette Island, Alaska: U.S. 

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12 

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