I 

I 

Plutonism and Orogeny in 

North-Central WashingtonTiming and Regional Context 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 989 
Plutonism and Orogeny in 

North-Central WashingtonTiming and Regional Context 

By KENNETH F. FOX, JR., C. DEAN RINEHART, and JOAN C. ENGELS 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 989 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1977 
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary 

GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

First printing 1977 

Second printing 1983 

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22.304 
CONTENTS 

Page 

1\bstract--------------------------------------------------- 1 

Introduction ---.- ______ ____ __ ______ _ ___ __________ _____ __ ____ 1 

Geologic provinces----------------------------------------- 2 

Geologic history ------------------------------------------- 3 

Permian and Triassic history--------------------------- 3 

Jurassic to Paleocene history --------------------------- 8 

Discordant ages ----------------------------------- 12 

1\ge of Omineca crystalline belt_ ____________________ 13 

Eocene to Miocene history ----------------------------- 15 

Page 

Orogenic implications of geologic history ------------------- 16 

Previous views on orogeny in the Cordillera ------------- 16 

Late Triassic orogeny and plutonism ------------------- 16 

1\ssociation of deformation of the Monashee Group and 

generation of gneiss domes with Cordilleran thrust 

faulting--------------------------------------------- 17 

Laramide orogeny, formation of gneiss domes, and thrust 

faulting _______________________________ ._ __ ____ ___ __ __ 18 

Discussion_________________________________________________ 20 

References cited ------------------------------------------- 24 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map showing orogenic provinces in Washington and southern British Columbia-------------------------------- 1 

2. Geologic map of Okanogan Highlands and vicinity showing radiometric ages ---------------------------------------- 4 

3. Geologic map of part of northeastern Washington and southern British Columbia------------------------------------ 6 

4. Map showing distribution of late Paleozoic and Mesozoic stratified rocks of the eugeosynclinal province---------------- 8 

5. Graph showing age determinations within the study area projected to an east-west transect -------------------------- 12 

6. Graph showing Mesozoic and Cenozoic age determinations within the region plotted with respect to the approximate 

boundaries of the Omineca crystalline belt and projected to the 49th parallel ------------------------------------ 13 

7. Map showing distribution of late Mesozoic and early Cenozoic thrust belts and areas of medium-to high-grade metamorphic rock, including gneiss domes in western United States and southwestern Canada ------------------------ 18 

8. Diagrams showing stages in the development of the Omineca crystalline belt---------------------------------------- 22 

TABLES 

Page 

TABLE 1. Sources of radiometric age data in figures 2, 3, 5, and 6 ________________ ---- ____ ---- ____ __ __ __ ____ __ __ ______ __ __ ____ 8 

2. Interpretation of radiometric age determinations in part of Okanogan Highlands ------------------------------------ 9 

III 

PLUTONISM AND OROGENY IN NORTH-CENTRAL WASHINGTONTIMING AND REGIONAL CONTEXT 

By KENNETH F. Fox, JR., C. DEAN RINEHART, and JoAN C. ENGELS 

ABSTRACT 

Bedrock in north-central Washington comprises (1) weakly to 

moderately metamorphosed eugeosynclinal rocks of Permian, Triassic, and possibly Jurassic ages, (2) high-grade polymetamorphic 

rocks-gneiss, schist, and amphibolite-that are, at least in part, 

more highly metamorphosed derivatives of the rocks of the previous 

category, (3) Mesozoic and Cenozoic plutonic rocks, and (4) Cenozoic 

lavas and continental sedimentary deposits. 

A review of the radiometric ages from north-central Washington 

suggests a complex history of plutonism and metamorpP.ism beginning in Late Triassic and extending to Eocene time. However, ages of 

coexisting minerals from single samples typically are moderately to 

highly discordant. The discordance reaches a maximum along a zone 

flanking the Okanogan gneiss dome on the west. This dome forms 

the southwestern extremity of the Omineca crystalline belt, a 

north-trending orogenic subprovince about 250 km (150 mi) long and 

55 km (35 mi) wide in British Columbia and Washington characterized by the presence of Shuswap (Monashee Group) terrane, 

gneiss domes, and allied metamorphic rocks. 

The Okanogan gneiss dome and the Shuswap are believed to be 

products of metamorphism and deformation, in part at least, ofLate 

Cretaceous age. The development of the gneiss domes indicates that 

metamorphism within the Omineca reached sufficient intensity at 

depth to cause incipient anatexis and mobilization of the infrastructure. The discordance west of the gneiss dome and the Omineca belt 

is attributed to weak thermal metamorphism that developed above 

and west of the zone of more intense metamorphism. 

Ages from within the Okanogan gneiss dome range from Late 

Cretaceous (lead-uranium, zircon) to Eocene (K-Ar, biotite and muscovite; fission track, apatite). Their discordance is attributed to slow 

cooling after the climax of metamorphism in the Late Cretaceous. 

Orogeny and plutonism in the north-central area of Washington 

began almost simultaneously during the Late Triassic with folding 

of the Permian and Triassic bedded rocks and their intrusion by the 

195-million-year-old Loomis pluton. Thermal events subsequent to 

Late Triassic cannot be linked with specific orogenic deformation, 

except for deformation associated with the hypothesized Late Cretaceous metamorphism and mobilization of the Shuswap and the 

gneiss domes within it. This event is temporally associated with 

westward-directed overthrusting along the Shuksan thrust to the 

west, attributed to mid-Cretaceous orogeny, and with eastwarddirected overthrusting to the east along the Cordilleran thrust belt, 

attributed to the Laramide orogeny, which ended in the Late Cretaceous or early Tertiary. The Shuswap terrane and associated 

gneiss domes appear to occupy the axial zone between convergent 

thrusts that show an aggregate crustal contraction of possibly 250 

km (150 mi). 

These relations suggest a genetic model as follows: The Permian 

volcanic and pyroclastic rocks of the region were probably deposited 

in island-arc and back-arc basins located east of an east-dipping 

subduction zone. We speculate that in Late Triassi<:, the continental 

plate overrode a rise system embedded in the oceanic plate, after 

which the eugeosynclinal prisms were invaded by calc-alkalic magmas derived through partial melting of upper mantle and lower crust 

within a zone of high heat flow above the overridden rise. The residue remaining in the zone of partial melting probably became 

progressively more dense as the hyperfusible part was removed and 

in Late Cretaceous catastrophically sank ·into the asthenosphere, 

forming a short-lived convection cell whose axis lay beneath the 

Okanogan region. The overlying crust was dragged toward this axis 

which caused thrust faulting to the east (Cordilleran thrusts) and 

west (Shuksan thrust) and thickening of the crust over the cell bystacking of thrust sheets and plastic flow. Concurrently, elements of 

the infrastructure were mobilized and penetrated higher levels in 

the crust, with their culminations forming the Okanogan gneiss 

dome and other gneiss domes within the Shuswap. After the demise 

of the convection cell in latest Cretaceous, the thickened crust isostatically rebounded. Upper crustal levels over the Omineca were 

rapidly eroded away, and elements of the Late Cretaceous infrastructure (the gneiss domes and the Shuswap) were exposed in the 

Eocene. 

INTRODUCTION 

Bedrock in north-central Washington (fig. 1) comprises (1) weakly to moderately metamorphosed 

eugeosynclinal rocks of Permian, Triassic, and possibly 

Jurassic age, (2) high-grade polymetamorphic rocksgneiss, schist, and amphibolite-that are, at least in 

part, more highly metamorphosed derivatives of the 

rocks of the previous category, (3) Mesozoic and 

Cenozoic plutonic rocks, and (4) Cenozoic lavas and 

continental sedimentary deposits that patchily overlie 

the older rocks. The lavas are products of regional 

episodes of volcanism, the older of which is believed to 

be Eocene and the younger late Miocene to early 

Pliocene. The age of plutonism and metamorphism in 

the Okanogan area is bracketed by the Permian to 

Triassic age of the eugeosynclinal host rocks and the 

early Cenozoic age of the overlying continental 

deposits. Fossils are so rare in the eugeosynclinal 

deposits, however, that the older boundary is quite imprecise. 

Previously reported radiometric ages of rocks from 

within the study area (fig. 2; table 1) range from Late 

Triassic to Eocene and taken collectively indicate that 

the history of magmatism and metamorphism has been 

1 
2 PLUTONISM AND OROGENY IN NORTH-CENTRAL WASHINGTON 

0 100 200 MILES 

0 100 200 KILOMETERS 

FIGURE 1.-Index map showing orogenic provinces in Washington 

and southern British Columbia. Modified and extended mainly 

from Wheeler (1970, p. 1). 

complex. However, the age of many of the plutons in 

the study area, including some that have been studied 

intensively, has not been unambiguously determined. 

These rocks typically show substantial discordance between age determinations on mineral pairs from individual samples and discordance from sample to sample 

within individual plutons. In general, we assume that 

this discordance reflects varied retention of fission 

tracks, argon, rubidium, strontium, lead, or uranium, 

either during protracted cooling after initial emplacement of the pluton or varied retention during and after 

later ther~al metamorphism. The temperature of a 

cooling mineral, at which the rate of loss of the 

radiogenic daughter element by diffusion becomes negligible compared to the rate of its accumulation, is 

defined as the critical blocking temperature (Macintyre and others, 1967, p. 822). This temperature may 

be a function of various factors, notably grain size, the 

pressure of argon within the host but external to the 

dated mineral, and the length of time during which 

that particular temperature is maintained. The blocking temperature for radiogenic argon is higher in 

hornblende than in coexisting biotite, and as a result, 

in thermally metamorphosed terranes the hornblende 

commonly yields an older age than the coexisting biotite (Hart, 1964; Hanson and Gast, 1967). The concept of 

a critical blocking temperature can also be extended to 

the rate of erasure of fission tracks relative to their rate 

of formation. Thus, the minimum age of crystallization 

of plutonic rocks yielding discordant K-Ar or fissiontrack ages is here assumed to be the older of a mineral 

pair. 

This paper refines the plutonic and metamorphic 

history of part of the Okanogan Highlands, mainly 

through review of the radiometric ages and consideration of the implications of their discordance, and 

relates this history to the depositional and orogenic 

history of the region. The eastern part of the region 

includes stratified rocks of Precambrian and early 

Paleozoic age that are not found within the study area. 

The distribution of these rocks, along with that of the 

more widespread late Paleozoic and Mesozoic stratified 

rocks, and their various dynamically metamorphosed 

derivatives provides the basis for dividing the region 

into depositional and orogenic provinces. We define 

these provinces and then discuss the Mesozoic and 

Cenozoic geologic history of the study area, where possible in the broader context of the geologic history of 

the region. 

We are grateful toR. G. Yates, who introduced us to 

the geologic problems of the region and who provided 

advice and encouragement throughout the duration of 

the study. We are also indebted toM. D. Crittenden for 

numerous stimulating discussions on tectonic problems of the region. 

GEOLOGIC PROVINCES 

The region encompassed by figure 3 comprises parts 

of two depositional provinces of differing ages. The 

pre-Tertiary stratified rocks of that part of 

northeastern Washington lying roughly east of long 

118° W. are chiefly Precambrian and lower Paleozoic 

miogeoclinal (Dietz and Holden, 1966) rocks, in contrast to those to the west, which are upper Paleozoic 

and Mesozoic eugeosynclinal rocks. 

In Washington, the contacts between the eugeosynclinal rocks and the miogeoclinal strata to the east are 

obscured by faulting and folding along much of their 

extent (Yates and others, 1966, p. 54). According to 

Campbell (1964), the eugeosynclinal deposits in the 

Hunters quadrangle and vicinity (south of Kettle 

·Falls, Wash.) have been thrust eastward over the 

miogeoclinal rocks. In British Columbia however, 

rocks of the eugeosyncline (Milford Group) concordantly 

but unconformably overlie rocks probably belonging to the miogeocline (Lardeau Group, Cairnes, 

1934, p. 36-38). 

In addition to the depositional provinces discussed 

above, the southern British Columbia and northcentral Washington region can also be assigned to 

three orogenic provinces, the Purcell foldbelt, the 

Omineca crystalline belt, and the Columbian intermontane belt (fig. 1). 
GEOLOGIC HISTORY 3 

The Omineca crystalline belt is distinguished chiefly 

by the presence of medium- to high-grade gneiss and 

schist of the Monashee Group of the Shuswap terrane 

of Jones (1959). Several gneiss domes have been identified within the rocks of the Monashee Group and its 

probable correlatives. The gneiss. domes include among 

others the Valhalla and Passmore domes in British 

Columbia (Reesor, 1965) and the Okanogan dome in 

Washington (Fox and Rinehart, 1971) (fig. 1). 

The eastern limit of the Monashee Group and 

Monashee-like rocks within the region serves as the 

boundary with the Purcell foldbelt, a~d the western 

limit serves as the boundary with the Columbian intermontane belt. The contact between the Purcell 

foldbelt and the Omineca crystalline belt (figs. 1, 3) 

nearly coincides with the contact between rocks. of the 

eugeosynclinal facies and the rocks of the miogeoclinal 

facies. 

GEOLOGIC HISTORY 

PERMIAN AND TRIASSIC HISTORY 

The eugeosynclinal province within the area of figure 

3 is chiefly floored by sparsely fossiliferous rocks, probably oflate Paleozoic age. Within the study area (fig. 2), 

these rocks have been assigned to the Anarchist Group 

(Rinehart and Fox, 1972, p. 8-11), which is composed of 

variably metamorphosed complexly interfingering and 

intergrading deposits of argillite, siltstone, sharpstone 

conglomerate, graywacke, sandstone, and limestone. 

Deposits exclusively of volcanic origin-metamorphosed 

lava or pyroclastic material-are also present but are 

not abundant. However, clasts of volcanic rock are a 

minor but apparently widespread constituent of the 

coarser grained clastic rocks, such as the sharpstone 

conglomerate and graywacke, suggesting that material 

of volcanic origin may be a significant constituent of the 

finer grained clastic rocks as well. The Anarchist is believed to have an overall thickness in excess of 4,500 m 

(15,000 ft). Fossils are rare. Those found, which are 

probably restricted to a narrow stratigraphic interval 

in the upper middle of the group, have been dated as 

Permian, perhaps Late Permian (Rinehart and Fox, 

1972, p. 9-10; Waters and Krauskopf, 1941, p. 1364). 

The Anarchist is therefore, at least in part a temporal 

equivalent of the lithologically similar Cache Creek 

Group of south-central British Columbia (fig. 4), from 

which fossils of Mississippian(?), Pennsylvanian, and 

Permian ages have been reported (Cockfield, 1961, 

p. 9-11). 

Within the northern part of the study area (fig. 2), the 

Kobau Formation and Palmer Mountain Greenstone 

overlie the Anarchist Group disconformably or along a 

slight unconformity (Rinehart and Fox, 1972, p. 11-12, 

22). The Palmer Mountain Greenstone consists of 

metavolcanic rocks estimated to be locally at least 2,100 

m (7,000 ft) thick, consisting chiefly of greenstone and 

metadiabase. The formation probably originated as a pile 

of mafic extrusive rocks-a volcano-into which mafic 

magma was abundantly intruded. 

The Kobau Formation is composed of an unfossiliferous sequence-locally at least 3,600 m (12,000 ft) 

thick-of phyllite, greenstone, and massive metachert. 

The lower parts of the Koba u apparently interfinger 

with the upper part of the Palmer Mountain, leading to 

the hypothesis that the Koba u originated ((as a belt of 

marine deposits that formed satellitic to, and partly as a 

result of, the volcanic activity represented by the 

Palmer Mountain Greenstone" (Rinehart and Fox, 

1972, p. 22). The maximum possible age of the Kobau 

and Palmer Mountain, as established by the fossils in 

the subjacent Anarchist, is Permian, probably Late 

Permian. Okulitch (1973, p. 1514) rejected this conclusion and suggested instead that the Kobau Formation 

as mapped by us is in part older than the Anarchist 

because the Kobau contains rootless isoclinal folds that 

he believed were related to a deformational event that 

predates the Anarchist. ln his opinion (p. 1516), the 

Kobau is mid-Paleozoic. Our mapping (Rinehart and 

Fox, 1972) revealed no evidence supporting his speculations, and in our view the disconformable or unconformable contact ofKobau over Anarchist is well established. 

In the northwest corner of the region, near Kamloops 

(fig. 4), the Cache Creek Group is overlain along an 

erosional unconformity by the Nicola Group, a sequence 

of volcanic rocks composed mainly of greenstone and 

minor sedimentary rocks containing Triassic-locally 

Late Triassic-fossils (Cockfield, 1961, p. 8-15). The 

Nicola crops out over an extensive area within southcentral British Columbia. The stratigraphic positions of 

the Nicola over the Cache Cre·ek and Kobau over the 

Anarchist thus seem comparable and suggest that these 

two chiefly volcanic and volcaniclastic assemblagesNicola and Kobau-are correlative. However, Dawson 

(1879, p. 87B) considered the rocks now referred to the 

Kobau (along the route of his reconnaissance between 

the Similkameen and Okanogan Rivers and a few 

kilometers north of the international boundary) to be 

Cache Creek, although he recognized that the sequence 

contained members resembling the Nicola. In view of 

the relations noted above, the Kobau could be either the 

temporal equivalent of the Nicola-differing from the 

Nicola of the type area in that volcaniclastic rocks and 

bedded chert predominate over lava-or alternatively, 

older than the Nicola but not present or not recognized 

in the type areas of the Nicola and the Cache Creek. 

In the southern part of the study area, the Anarchist 

is unconformably overlain by the Cave Mountain Formation (Rinehart and Fox, 1976 ), a sequence of 
4 PLUTONISM AND OROGENY IN NORTH-CENTRAL WASHINGTON 

Base from U.S. Dept. Commerce, Sectional 

Aeronautical Chart, Kootenai, Edition of 

Nov., 1951, revised Dec. 1968 0 

0 

10 MILES 

10 KILOMETERS 

Geology compiled by K. F. Fox, Jr., 

and C. D. Rinehart, 1973 

FIGURE 2.-Geologic map of Okanogan Highlands and vicinity showing radiometric ages. 
GEOLOGIC HISTORY 5 

weakly to highly metamorphosed limestone, dolomite, I Columbia, originally described by Camsell (1910) that 

siltstone, slate, and capping basaltic volcanic rock. One also contains Late Triassic fossils. The nomenclature 

member contains fossils dated as Late Triassic (Misch, and lithology of these rocks have been reviewed by Rice 

1966, p. 118). These rocks are similar to a sequence of (1947, p. 12-14), who considers them to be part of a 

inter layered quartzite and limestone at Hedley, British dominantly sedimentary facies of the Nicola. Within 

EXPLANATION 

SURFICIAL DEPOSITS (Quaternary) - Alluvium and glacial drift 

COLUMBIA RIVER BASALT GROUP (upper Miocene) 

VOLCANIC ROCKS(Eocene) - Dacitic, andesitic, and minor trachytic lava flows 

and hypabyssal intrusive rocks 

EPICLASTIC AND VOLCANICLASTIC ROCKS (Eocene)- Sandstone, graywacke, 

tuff, conglomerate, and shale 

PARAGNEISS, ORTHOGNEISS, AND ASSOCIATED GRANITIC ROCKS OF THE 

OKANOGAN GNEISS DOME (age of metamorphism and mobilization probably 

late Mesozoic) - Includes similar rocks of the Monashee Group of the Shuswap 

terrane of Jones (1959) in British Columbia.A, paragneiss; B, orthogneiss and 

associated granitic rocks 

GRANITIC ROCKS (Triassic to Tertiary) -Includes granodiorite, quartz monzonite, 

quartz diorite, diorite, monzonite, and minor nepheline syenite, shonkinite, malignite, alkali pyroxenite, peridotite, hornblendite, and serpentinite. Interplutonic 

contacts shown where known. Kruger Alkalic Complex identified by "K" 

EUGEOSYNCLINAL DEPOSITS (Permian to Cretaceous) - Greenstone, greenschist, 

slate, phyllite, schist, metawacke, metasharpstone conglomerate, marble, and metachert 

GRANITOID GNEISS OF OKANOGAN RANGE (age unknown) - Trondhjemitic 

orthogneiss and paragneiss 

-----?--

Contact 

----············ Fault 

Queried where projected through unmapped areas Dotted where concealed 

KEY TO RADIOMETRIC AGES 120° 119° 

(Ages compiled from sources 12 1 9 8 1. Bostock (1940) listed in table 1) 

4if 2. Fox (1970) 

Locality L - 618 A7 3 . Fox (unpublished mapping) • K-Ar age 6 15 2 3 11 4. Fox and Rinehart (unpublished mapping) 

Biotite 179 ~ 5. Hawkins (1968) 

Muscovite [51.2) 6. Hibbard (1971) 

7. Huntting and others (1961) Hornblende (194) 8. Little (1957) 13 14 PI agioclase 51 .1 p 9. Little (1961) 

Fission track age 4 10. Menzer (1964, 1970) 

Apatite (53)a 

~ 11. Pearson (1967) 

Sphene (78) s 12. Rice (1947) 

13. Rinehart (unpublished mapping) Allanite (59)allan 14. Rinehart and Fox (1970) 

Zircon (90)z 15. Rinehart and Fox (1972) 

Epidote (63)epi 7 16. Roberts, R. J., and Hobbs, S. W. 

Rb-Sr age 

(unpublished mapping) 

~ I I 17. Staatz and others (1971) 48' 

Pb-a age {no} SOURCES OF AREAL 

GEOLOGIC DATA 

FIGURE 2.-Continued. 
6 PLUTONISM AND OROGENY IN NORTH-CENTRAL WASHINGTON 

Base from U.S. Dept. Commerce, Navigational 

Chart E 16 Edition 5, 1968 

0 15 30 MILES 

0 15 30 KILOMETERS 

Geology compiled by K. F. Fox, Jr., 

and C. D. Rinehart, 1973 

FIGURE 3.-Geologic map of part of northeastern Washington and southern British Columbia showing distribution of localities at which 

age determinations have been made. 
• 

II 

[ ................. ! 

i:::::::::::::::::::::::::::::::::. 

Radiometric age locality 

Compiled from sources listed in tables 

1 and 2 

121° 120° 51° 

7 

50° 

25 

49° 

12 

480 

GEOLOGIC HISTORY 

EXPLANATION 

SEDIMENTARY, VOLCANIC, AND LOW-GRADE METAMORPHIC ROCKS 

BASALT (upper Miocene and lower Pliocene) 

EPICLASTIC SEDIMENTARY ROCKS AND VOLCANIC ROCKS (Eocene)-

Basaltic, andesitic, dacitic, rhyolitic, and trachytic lava flows, hypabyssal intrusive rocks, pyroclastic rocks, sandstone, graywacke, shale, and conglomerate 

EUGEOSYNCLINAL DEPOSITS (Cambrian? to Cretaceous) - Sandstone, graywacke, argillite, conglomerate, lava flows, pyroclastic rock, greenstone, greenschist, 

slate, phyllite, schist, meta wacke, metasharpstone conglomerate, marble, and metachert 

MIOGEOCLINAL DEPOSITS (Precambrian to Mississippian?) -Limestone, dolomite, argillite, slate, phyllite, schist, marble, quartzite, siltite, and minor greenstone 

and conglomerate 

MEDIUM- AND HIGH-GRADE METAMORPHIC ROCKS 

PARAGNEISS, SCHIST, AND AMPHIBOLITE (Age not specified) 

MONASHEE.GROUP OF SHUSWAP TERRANE OF JONES (1959), AND 

SIMILAR ROCKS (age of metamorphism probably Mesozoic) - Paragneiss, orthogneiss, and associated granitic rocks, probably in part originating as diapiric intrusions. 

Includes rock within Thor-Odin, Pinnacles, Valhalla, Passmore, and Okanogan gneiss 

domes 

PLUTONIC ROCKS 

GRANITIC ROCKS (Triassic to Tertiary) - Includes granodiorite, quartz monzonite, 

syenite, monzonite, quartz diorite, diorite, granite, and minor nepheline syenite, 

shonkinite, malignite, alkali pyroxenite, gabbro, peridotite, dunite, hornblendite, 

and serpentinite 

-----?-- ----···-········ Boundary of orogenic province Contact Fault 

7 

Queried where projected through 

unmapped areas Dotted where concealea 

119° 118° 117° 

1. Becraft (1966) 20. Monger (1968) 

2. Bostock ( 1940) 21. Muessig (1967) 30 3. Bostock (1941 a) 22. Parker and Calkins (1964) 14 4. Bostock (1941b) 23. Pearson (1967) 

5. Campbell and Raup (1964) 24. Reesor (1965) 

6. Church (1971) 25. Rice (1947) 

7. Cockfield (1961) 26. Rinehart (unpublished 

e 8. Fox (1970) mapping) 

9. Fox and Rinehart 27. Rinehart and Fox (1970) 

(unpublished mapping) 28. Rinehart and Fox (1972) 15 10. Hawkins (1968) 29. Roberts, R. J. and 

16 11. Hibbard (1971) Hobbs, S. W., (unpub12. Huntting and others (1961) lished mapping) 

13. Hyndman (1968b) 30. Ross (1970) 

14. Jones (1959) 31. Savage (1967) 

15 . Little (1957) 32. Staatz (1964) 

31 16. Little (1960) 33. Staatz and others (1971) 

12 17. Little (1961) 34. Tabor and others (1968) 

18. Little (1962) 35. Yates (1964) 

19. Menzer (1964, 1970) 

SOURCES OF AREAL GEOLOGIC DATA 

FIGURE 3.-Continued. 
8 PLUTONISM AND OROGENY IN NORTH-CENTRAL WASHINGTON 

TABLE I.-Sources of radiometric age data compiled 

in figures 2, 3, 5, and 6 

F1gure References 

2,5 

3,6 

Cannon, 1966 

Engels, 1971 

Engels, Tabor, Miller, and Obradovich, 1976 

Fox, Rinehart, and Engels, 1975 

Fox, Rinehart, Engels, and Stern, 1976 

Hawkins, 1968 

Hibbard, 1970, 1971 

Mathews, 1964 

Menzer, 1970 

Naeser, Engels, and Dodge, 1970 

Rinehart and Fox, 1972 

Rinehart and Fox, 1976 

All above, plus the following: 

Baadsgaard, 1961 

Engels, 1975 

Hills and Baadsgaard, 1967 

Leech, Lowden, Stockwell, and Wanless, 1963 

Lowdon, 1960, 1961 

Lowdon, Stockwell, Tipper, and Wanless, 1963 

Nguyen, Sinclair, and Libby, 1968 

Preto, White, and Harakal, 1971 

Roddick and Farrar, 1971, 1972 

Roddick, Farrar, and Procyshyn, 1972 

Sinclair and White, 1968 

Tabor, Engels, and Staatz, 1968 

Wanless, Stevens, Lachance, and Rimsaite, 1965 

Wanless, Stevens, Lachance, and Edmonds, 1968 

White and others, 1967 

Yates and Engels, 1968 

the study area (fig. 2), both the Anarchist Group and 

Kobau Formation were strongly folded along northnorthwest axes and afterwards intruded and 

metamorphosed by the Loomis pluton, a quartz 

diorite-granodiorite batholith. The Loomis pluton is 

the oldest pluton so far recognized within the study area 

(fig. 2). K-Ar ages of 194 m.y. and 179 m.y. were measured on coexisting hornblende and biotite (table 2), respectively. The older of these is regarded as a 

minimum age-and probably close to the true age of 

the batholith, in view of the modest amount of the discordance. 

The relations within the study area (fig. 2) indicate 

that deposition· of eugeosynclinal volcaniclastic sediments, carbonate sediments, and minor intercalated 

lavas was terminated during the Permian. After an 

interval of minor tilting, and possibly of minor erosion, 

voluminous lavas were erupted and deposited along 

with associated pyroclastic sediments. This phase of 

volcanism was terminated in Late Triassic by uplift and 

strong folding, followed by batholithic intrusion about 

195 m.y. ago. 

JURASSIC TO PALEOCENE HISTORY 

The folded Kobau Formation and Anarchist Group 

were beveled and buried beneath the Ellemeham For0 50 MILES 

I 

0 50 KILOMETERS 

FIGURE 4.-Late Paleozoic and Mesozoic stratified rocks of the 

eugeosynclinal province in northeastern Washington and southern British Columbia. Contrasting patterns used to aid in distinction of map units. Data modified slightly from same sources as 

figure 3. 

mation, a unit composed of a basal member of 

greenstone and tuffaceous metasiltstone, a middle 

member of monolithologic greenstone conglomerate, 

and an upper conglomeratic member composed of 

heterogeneous fragments of metamorphic rock. TheEllemeham Formation is locally at least 850 m (3,000 ft) 

thick (Rinehart and Fox, 1972, p. 22-25). The Ellemeham is younger than the subjacent Kobau, probably younger than the Late Triassic Loomis pluton, and 

is clearly older than 157 m.y., the age of biotite (sample 

L620R, table 2) in a hornfels formed at the contact of a 

crosscutting alkalic intrusive body; hence it appears to 

be of Late Triassic or Jurassic age. 

With the possible exception of the Ellemeham Formation, there are no known depositional units of Jurassic 

to Paleocene age within the study area. Rocks older 

than Ellemeham were eroded to levels deep enough to 

expose the plutonic rocks, then buried by the lavas, 

pyroclastic deposits, breccia, and conglomerate of the 

Ellemeham. 

Deposition was interrupted in the central and western parts of the eugeosynclinal province during the Late 

Triassic tectonism, but on the east flank of the province, 
GEOLOGIC HISTORY 9 

TABLE 2.-lnterpretation of radiometric age determinations in part of northern Okanogan Highlands 

Unit 

Swimptkin Creek pluton. 

Andesite of White- stone Mountain. 

Sample 

"0-419 

L-657 

Andesite-dacite plug L-590 

Do-------------- L-147 Dacite of Carter C-550 Mountain. Marron Formation (volcanics). 

Dacite of Twin Peaks. 

AK-112 

C-559 

Method Mineral 

Mineral of age determination 1 

age 

(m.y.) 

Biotite-------- K-Ar ------ 48.0±1.5 

Hornblende .... . do _________ 48.2=1.5 

Hornblende .... K-Ar _____ _ 49.1±1.8 

____ do ------------do______ 51.4±2.6 ____ do ------------do...... 52.1±2.3 ____ do ------------do______ 45.1±2.0 

____ do ------------do...... 49 

____ do ------------do______ 42.9±1.3 

Sanpoil Volcanics .... OBP-65-01 Plagioclase _________ do _____ _ 51.1 

Okanogan gneiss dome. 0-36A 

0-37A 

0.38A 

0-131 

0-176D 

0-176E 

0-296D 

0-425 

Coyote Creek pluton __ 0-424B 

Conconully pluton __ C-555 

T-156 

OK-1 

OK-4 

OK-1 

OK-4 

Trond}\iemitic gneiss T-153 ofTiffany Mountain. 

T-154 

T-155 

("Trond}\iemitic OK-3 gneiss"; Menzer, 

1970) OK-6 

OK-3 

OK-6 

Biotite-------- K-Ar 

Muscovite ____ K-Ar 

Allanite ______ F.T --------

Biotite ________ ------------ Allanite F.T 

Apatite F.T 

Hornblende .... K-Ar _____ _ 

Epidote ------ F.T --------

Sphene ________ F.T --------

Hornblende .... K-Ar ------

Zircon ________ 206Pbt•asu __ 207Pbf233U __ •ospbf232Th __ 

Biotite-------- K-Ar ------ Hornblende --------do _____ _ 

Biotite ------------do ..... . 

49.3±1.6 

49.8±1.6 

59±6 

51±5 

49.0±2.2 

53.8±1.6 

51.2=1.6 

50.8±1.6 

54.8±1.7 

59±2 

53±5 

58.1±1.7 

63±3 

66±7 

49.3±1.7 

87.3 

100.0 

94.0 

50.0±1.5 

53.9±1.6 

46.0±1.4 

Biotite ________ K-Ar _______ .49.1±1.5 

Biotite-------- K-Ar _____ _ Hornblende --------do _____ _ 

Biotite ------------ do _____ _ 

Hornblende --------do _____ _ 

Apatite ______ F.T -------- Sphene ------------ do _____ _ Zircon -------- Pb-a _______ _ 

Apatite ------ F.T --------

Apatite, ~ biotite. Whole rock, Rb-Sr muscovite, 

biotite. 

Biotite-------- K-Ar ------

Hornblende ________ do _____ _ 

Biotite ------------ do _____ _ Muscovite---------- do _____ _ Biotite ------------do _____ _ Muscovite---------- do _____ _ 

Apatite ______ F.T 

Zircon -------- Pb-a _______ _ Apatite ______ F.T -------- Zircon ______________ do _____ _ 

Whole rock, J biotite. Rb-Sr ____ do_______ isochron 

62.5±2.2 

72.8±4.6 

78.8±2.4 

81.2=2.4 

89±9 

84±8 

90±20 

94±12 

81.1±0.8 

88.5±2.7 

93.5±2.8 

91.7±2.8 

88.5±2.7 

108±3 

94.6±2.8 

76±8 

90±10 

92±9 

90±9 

104.2=0.5 

Inferred age Source of of event mineral age (m.y.J 

Fox, Rinehart, Engels, Intrusion, rapid cooling at and Stern, 1976. -48 do. 

Rinehart and Fox, 1972. Volcanism, -50. 

do. 

do. Rinehart and Fox, 1976. 

Mathews, 1964, p. 465. 

Engels, Tabor, Miller, and Obradovich, 1976. 

do 

Fox, Rinehart, Engels, and Stern, 1976. 

do. :l':laeser, Engels, and Dodge, 1970. 

do. Fox, Rinehart, Engels, and Stern, 1976. 

do. 

do. 

do. 

do. Naeser, Engels, and Dodge, 1970. 

do 

Fox, Rinehart, Engels, and Stern, 1976. Naeser, Engels, and Dodge, 1970 

do. Fox, Rinehart, Engels, and Stern, 1976. 

do. 

do. 

do. 

do. 

do. 

do. 

Fox, Rinehart, Engels, and Stern, 1976 

Rinehart and Fox, 1976. do. Engels, Tabor, Miller, and Obradovich, 1976 do. Menzer (1970). do. 

do. 

do. 

do. 

Engels, Tabor, Miller, and Obradovich, 1976. do. 

do. 

do. 

do. 

do. 

Menzer (1970>. do. 

do. 

do. 

do. 

Intrusion and metamor-. phism in Late Cretaceous; 

uplift and slow cooling to retentivity threshold of Ar in biotite at -50; unro- ofing of dome after 50. 

-49(?) 

Intrusion prior to 80, probably at about 90. Local thermal metamorphism at or subsequent to 63. 

Intrusion prior to intrusion of Old Baldy pluton (129 or before) 

Remarks 

Swimptkin Creek pluton cuts Okano- gan gneiss dome; contains xenoliths of linea ted gneiss of dome and of low- grade metamorphic rocks believed to have been part of roof of dome when cut by Swimptkin Creek. See New- comb (1937). Contains autoliths and fine-grained marginal phases. 

Volcanic rocks cut qr overlie associated sedimentary rocks that contain probably Eocene flora (Rinehart and Fox, 

1972, p. 61). Although volcanic rocks fringe gneiss dome, they do not di- rectly overlie it. Intercalated con- glomerate contains abundant detrital granitic rock, which northeast of dome resembles rocks of gneiss dome. 

Samples 0-131, 0-296 of alkalic border phase (monzonite and syenodiorite) at north edge of dome, interpreted as metasomatite formed during 

emplacement of dome. Sample 0-176D and 0-176E are of layered 

paragneiss, and remaining samples are of granodioritic gneiss. Volcanic rocks unconformably overlie country rocks dynamically metamorphosed 

during forceful intrusion of dome; 

therefore, volcanics are younger than age of emplacement of dome. The 206Pbt•asu age of87.3 m.y. is the most precise of the Pb-U ages. The cata- clastic textures of the rocks of the gneiss dome indicate that pervasive and penetrative deformation con- tinued well after most mineral con- stituents including zircon had crys- tallized. We conclude that deforma- tion and mobilization terminated in Late Cretaceous, after about 87 m.y. ago. Younger ages of other minerals reflect slow cooling. It is less likely, in our opinion, that discordance could be due wholly or in part to thermal metamorphism of gneiss dome at -50 m.y. 

Granodiorite porphyry of Coyote Creek pluton bounds Okanogan gneiss dome on southeast, not delineated separately in figure 2. Probably younger than gneiss dome, since only 

slight cataclasis apparent, but it is possible that granodiorite porphyry is marginal phase grading inward to gneissic quartz diorite of gneiss dome. 

Menzer (1970, p. 576) concludes that this event is 81 m.y. old, relying on the Rb-Sr isochron. However, the F.T. and Pb-a ages average 89, suggesting that the pluton is older than 81. The discordance between the hornblende and biotite K-Ar ages at C-555 and T-156, and between hornblende at C-555 and T-156, and between hornblende at C-555 and the mineral ages ofOK-1 and OK-4, probably re- flect metamorphism by a later ther- mal event. A steep east-west gradient 

t5~~C:~~o.Z1i,sa~b~[!~e~et~~~ and T-156 (fig. 2), suggesting meta- morphism by a nearby source to the east concealed in the subsurface. 

Trond}\iemitic gneiss is enclosed in, and · is thus older than Old Baldy pluton ("granodioritic gneiss," Menzer, 

1970, p. 576). Discordancy possibly due to cumulative metamorphism re- lated to intrusion of Conconully plu- ton about 90 m.y. ago and to Late Cretaceous and early Tertiary ther- mal event. Menzer ascribes its anomalously younger isotopic age, relative to Old Baldy pluton, as pos- sibly due to "penetrative rock defor- mation that pervades the unit" (p. 577>. ThuS'far, however, our observa- tions reveal no consistent differences between the two units in age of de- formation, its intensity, or its style. 
10 PLUTONISM AND OROGENY IN NORTH-CENTRAL WASHINGTON 

TABLE 2.--lnterpretation of radiometric age determinations in part of northern Okanogan Highlands-Continued 

Method Mineral Inferred age of event Unit Sample (m.y.) 

Old Baldy pluton ____ T-152 

("Granodioritic OK-5 gneiss" of Menzer, 

1970). 

Cathedral batholith __ H-1 

(2) 

(2) 

Anderson Creek pluton. 

Aeneas Creek pluton. 

L-451C 

AC39 

C-139 

Evans Lake pluton __ C-554 

of age age Mineral determination1 (m.y.) 

Biotite ________ K-Ar ------

Hornblende ________ do _____ _ Sphene ________ F.T _______ _ 

Whole rock, biotite. Rb-Sr isochron. 

Biotite-------- K-Ar _____ _ 

____ do ------------ do _____ _ 

---- do ------------ do _____ _ 

90.1±2.7 

98.5±3.0 

78±8 

129.0±1.8 

97.7±2.9 

94.3±2.8 

98 

Biotite-------- K-Ar ______ 100.1±3.0 

Hornblende ________ do ______ 115.6±3.0 

Biotite ------------ do ______ 106.5±6.8 

Biotite-------- K-Ar ------ 92.7±6.6 

Biotite-------- K-Ar ______ 88.8±2.8 

Osoyoos pluton ______ Osoyoos #2 Biotite---~---- K-Ar ______ 49.0±1.5 

Blue Goat pluton ____ C-246D 

Quartz diorite gneiss OK-2 ·(Menzer, 1970). 

Toats Coulee L-591 pluton. 

Biotite-------- K-Ar ------ 99.0±3.0 Hornblende ________ do ______ 141.8±8.2 

Apatite ------ F.T -------- Sphene ------------ do _____ _ Zircon _ _ _ _ _ _ _ _ Pb-a _______ _ 

Whole rock Rb-Sr biotite. isochron. 

Biotite-------- K-Ar _____ _ 

Hornblende -------- do _____ _ 

56±6 

72±7 

170±20 

82:6±0.3 

151±5 

170±5 

Source of mineral age 

Engels, Tabor, Miller, and Obradovich, 1976. do. Menzer (1970). 

do. 

Engels, Tabor, Miller, and Obradovich, 1976. Hawkins, 1968. Misch (cited by Hawkins, 

1968). 

Intrusion at 129 or before. 

Intrusion at -95(?). 

Engels, Tabor, Miller, Intrusion at -115. and Obradovich, 1976. do. Hibbard, 1971 

Rinehart and Fox, 1976. Intrusion prior to 93. 

Rineha1't and Fox, 1976. Intrusion before 89. 

Remarks 

The Old Baldy pluton is cut by the Con- conully pluton. Discordancy due to cumulative metamorphism including that associated with intrusion of Con- conully pluton about 90 m.y. ago and that due to Late Cretaceous and early 

Tertiary regional thermal event. 

K-Ar ages of biotite provisionally ac- cepted, since no younger rocks known m vicmity, and sample locations appear to be west of area of influence of Late Cretaceous and early Tertiary 

regional thermal event. 

Intrudes Loomis pluton (Rinehart and Fox, 1972). No younger plutons known in vicinity; therefore, dis- cordancy between biotite and hornblende ages attributed to Late Cretaceous and early Tertiary re- gional thermal event. 

Well within area of influence of Late Cretaceous and early Tertiary re- gional thermal event; therefore, bio- tite age probably substantially less than actual age of intrusion ofpluton. 

Well within area of influenct of Late Cretaceous and early Tertiary re- gional thermal event; therefore, bio- tite age probably substantially less than actual age of intrusion of pluton. 

Fox, Rinehart, Engels, Intrusion before 50 m.y. ago. Well within area of influence of Late and Stern, 1976. Cretaceous and early Tertiary re- gional thermal event; therefore, biot- ite age probably substantially less than actual age of intrusion of pluton. 

Dynamic metamorphism along south border attributed to intrusion of Okanogan gneiss dome, therefore 

older than dome. 

Rinehart and Fox, 1976. do. 

Menzer, 1970. do. 

do. 

do. 

Rinehart and Fox, 1972. 

do. 

Intrusion at or before 142. 

Intrusion before 170. 

Intrusion between 170 and 195. 

Discordance attributed to metamorph- ism during Late Cretaceous and early Tertiary regional thermal event. 

Discordance attributed to cumulative effect of metamorphism during Late Cretaceous and early Tertiary re- gional event and metamorphism as- sociated with intrusion of Conconully 

pluton about 90 m.y. ago. Menzer im- plies that this rock is a marginal facies of a granodwritic gneiss unit (1970, p. 573); latter is Old Baldy pluton of this paper. 

Intrudes Loomis pluton (Rinehart and Fox, 1972). Discordance attributed to cumulative effect of metamorphism 

during Late Cretaceous and early 

Tertiary regional thermal event and metamorphism associated with in- trusion of Cathedral batholith about 

95 m.y. ago. 

if the age of the Slocan Group overlaps the Late Triassic 

and Early Jurassic as indicated by Little (1960, p. 56-

57), deposition there must have been roughly concurrent with deformation and plutonism in the central 

and western parts-possibly contemporaneous with deposition of the Ellemeham Formation in the study area. 

In Middle and probably Late Jurassic time, however, 

deposits were accumulating on both flanks of the province. Those on the east side were primarily lavas and 

related rocks (Rossland Group; Little, 1960, p. 62r-71; 

Frebold and Little, 1962, p. 3-9), whereas those on the 

west side (Ladner and Dewdney Creek Groups) were 

primarily marine volcaniclastic sediments derived from 

erosion of the Nicola terrane in the central part of the 

region (Coates, 1970, p. 150-151) and deposited in a 

subsiding fault-bounded trough (White, 1959, p. 77), the 

Methow graben. 

Sedimentation in the western part of the eugeosynclinal province within the region (fig. 3) continued 

through the Early Cretaceous marked by the appear-
GEOLOGIC HISTORY 11 

TABLE 2.--lnterpretation of radiometric age determinations in part of northern Okanogan Highlands-Continued 

Method Mineral of age age 

Unit Sample Mineral determination• (m.y.) Source of mineral age 

Inferred age of event 

(m.y.) Remarks 

Shankers Bend alkalic complex. 

I.r-620R Biotite-------- K-Ar ______ 157.4±4.7 Engels, Tabor, Miller, Intrusion before 157. Sample of hornfelsed rock of El- lemeham Formation at contact; 

Similkameen com- posite pluton. 

(Comprises Similka- meen batholith (Daly, 1906, 1912> and Kruger Al- kalic Complex 

(Daly, 1906, 1912; 

Campbell, 1939; 

Fox, Rinehart, and Engels, 1975\. 

l.r-277Y Biotite ________ K-Ar ______ 74.8±2.2 

(hornfelsed 

Koban at Hornblende -------- do .....• 166.7±5.0 contact) 

lr-277W Muscovite---------- do •....• 135.7±4.1 (pegmatitealaskite 

dike cuts pyroxenite 

phase) 

l.r-277Z Biotite------------- do ••.... 140.9±4.2 (pyroxenite 

____ do ------------ do...... 83.4±2.5 phase) 

l.r-589C 

(granodiorite 

interior Hornblende --------do .••... 155.5±4.7 phase) 

l.r-301 

(shonkin1te 

alkalic 

border 

phase) 

Biotite ....•. ____ .. do ....•• 69.9±2.1 

Hornblende ________ do ...... 170.9±5.1 

(hastingsite). 

l.r-618 Biotite ------------do ...... 3 70.9±2.1 (granodiorite, 

interior Hornblende -------- do ...•.• 3 177.2±5.3 phasel 

l.r-704 

(granodionte, 

interiOr 

phasel 

W-65-1 

(Kruger 

syenite; 

shonkinite 

from al- kalic bor- der phasel 

••.. do ------------do .....• 115.8±3.6 

Hornblendeaugite 

mixture. 

..•. do ..•.•. 152±9 

Loomis pluton .....• l.r-498A Biot1te ________ K-Ar ______ 179±5 

Hornblende --------do...... 194±6 

and Obradovich, 1976. 

Fox, Rinehart, and En- gels, 1975. 

do. 

do. 

do. 

do. 

do. 

do. 

Engels, 1971. 

do. 

do. 

do. 

Fox, Rinehart, and En- gels, 1975. do. 

Cannon, 1966. 

Rinehart and Fox, 1972 

do. 

Chopaka Intrusive CG10 Actinolitic K-Ar ______ 190.5±15.6 Hibbard, 1971. 

Complex of Hibbard (1971). 

1F.T., fission track. 2Sample number unknown. 

hornblende. 

acorrected for cross-contammation'ofbiot1te and hornblende in mineral separates (see Engels, l971l. 

Intrusion between 191 (iso- chron age of hornblendes; see Fox, Rinehart, and Engels, 1975) and 177; thermal metamorphism between 70 and 50. 

Intrusion about 195 m.y. 

To!:~~r;ar:i~eirewiilii~ri!i:e~ i~fi~~ enced by Late Cretaceous and early 

Tertiary regional thermal event. El- lemeham Formation probably Upper 

Triassic, Lower Jurassic, or Middle Jurassic. 

The batholith is zoned, grading outward from quartz monzonite and granodiorite to monzonite. The mon- zonite in turn grades outward to malignite and shonkinite of the bor- dering Kruger Alkalic Complex. This gradation suggests the batholith and alkalic complex are roughly coeval. 

The complex sharply crosscuts wali- rock of the Kobau Formation, of Permian or Triassic age. Discordance attributed to Late-Cretaceous or early Tertiary regional thermal metamorphism. 

Discordance attributed to cumulative effect of metamorphism during Late Cretaceous and early Tertiary re- gional thermal event, and to metamorphism associated with in- trusion of the Toats Coulee between 195 and 170 m.y. ago. and the meta- morphism associated with intrusion of Cathedral batholith about 95 m.y. ago. 

Metamorphism about 195 Probably the age of metamorphism as- m.y. ago. sociated with intrusion of the nearby Loomis pluton. 

ance of material eroded from sources west of the basin as 

well as from the east and by the appearance of granitic 

detritus (Coates, 1970, p. 151). The central part of the 

province, which includes the study area (fig. 2), was 

apparently positive and was the site of continued 

plutonism through the Jurassic and Early Cretaceous. 

By mid-Cretaceous most of the province had become 

positive, and the area of plutonism had spread eastward 

and westward through the entire extent of the province 

within the region (fig. 3). 

Within the study area (fig. 2), the Late Triassic 

Loomis pluton is cut by the Toats Coulee pluton on the 

west and the Anderson Creek pluton on the east. 

Coexisting hornblende and biotite from the Toats 

Coulee were dated at 170 m.y. and 151 m.y., respectively, and coexisting hornblende and biotite from the 

Anderson Creek were dated at 115.6 m.y. and 100.1 

m.y., respectively (sample L-451C, fig. 2 and table 2). 

The discordance between ages of coexisting minerals in 

other plutons increases dramatically to the north and 
12 PLUTONISM AND OROGENY IN NORTH-CENTRAL WASHINGTON 

east of the Loomis. The discordance between 

hornblende and biotite ages from some samples of the 

Similkameen batholith and the coeval Kruger Alkalic 

Complex is as much as 106 m.y., hornblende being invariably the older of the mineral pair. In some samples 

of the Similkameen and Kruger, hornblende shows 

maximum ages of 155 to 177 m.y. (table 2). 

In fact, all the other ·plutons within the study area 

(fig. 2) from which coexisting minerals have been dated 

show significant discordance, except for the Swimptkin 

Creek pluton of Eocene age, which yielded hornblende 

and biotite ages of 48.2 m.y. and 48.0 m.y., respectively. 

The data summarized in table 2 suggest that the 

plutonism in the study area, which had begun with 

intrusion of the Loomis pluton, continued sporadically 

through the Jurassic, Cretaceous, and early Tertiary. 

DISCORDANT AGES 

The discordancy between apparent ages of coexisting 

minerals reaches a maximum in a zone that borders the 

Okanogan gneiss dome on the west (fig.5). --

The Okanogan gneiss dome occupies 2,500 km2 (950 

mi2 ) within the study area (fig. 2). Rocks of the gneiss 

dome previously have been considered-at least in 

part-as part of the Colville batholith (Pardee, 1918; 

Waters and Krauskopf, 1941; Staatz, 1964). The gneiss 

dome consists chiefly of layered paragneiss, augen 

gneiss, and granodiorite, marked over most of the area 

by a penetrative cataclastic fabric whose chief elements 

are a very regular low-dipping foliation and a persistent 

west-northwest-trending lineation. Country rocks adjacent to the dome at the western, northwestern, and 

southwestern contacts have been crushed and broken, 

and rocks of the dome at and near its upper surface 

along the western contact are mylonitized. 

The mylonitization and gneissic fabric of the gneiss 

dome and the brecciation of the wallrock were attributed by Waters and Krauskopf (1941) to emplacement 

of the mass as a protoclastic batholith. However, Snook 

(1965, p. 775) concluded that these features resulted 

from development of a distributed flat thrust at depth 

within a regionally metamorphosed sedimentary or 

volcanic terrane that was later upfaulted against the 

lower grade metamorphic rock on the west. We have 

suggested a- third alternative-that the gneiss dome 

formed through metamorphism of volcanic and 

sedimentary strata at depth, a metamorphism that 

culminated in the mobilization and diapiric emplacement of the dome (Fox and Rinehart, 1971). 

Ages from within the gneiss dome are themselves 

discordant, with the 21 age determinations previously 

reported (Fox and others, 1976) ranging from 100 m.y. 

( 207Pb/235U, zircon) down to 46.0 m.y. (K-Ar, biotite). 

(/) 

a: 

ct 

w 

> 

"'" 0 

(/) 

z 

WEST 

0 

50 

0 100 . It 

:::i 

..J 

~ 

~ 

ui 

C) 

ct 150 

~~ I 

EAST 

c 

. 

. l . . .. . ~-I 

. I 

, l . 

200L---------------------------------~ 

0 10 20 MILES 

I 

0 10 20 Kl LOME TEAS 

FIGURE 5.-Plot of age determinations (exclusive of uraniumthorium-lead ages of sample 0-176E, table 2) within study 

area (fig. 2) projected to east-west transect. Ages of coexisting 

minerals connected by vertical lines. Rb-Sr ages disregarded 

where less than fission-track ages of coexisting apatite, 

sphene, or zircon. Westward extremity of Okanogan gneiss 

dome at point C. 

The terminal stages of metamorphism and emplacement of the dome are believed to have been Late 

Cretaceous, after which the dome cooled slowly 

through the successive temperature thresholds (blocking temperatures) of the dated minerals, which included zircon, sphene, epidote, allanite, hornblende, 

apatite, muscovite, and biotite. The possibility that 

these minerals were variably degraded as a result of 

later mild reheating of the dome during a thermal 

event associated with regionwide Eocene volcanism 

can be evaluated through comparison of age data from 

elsewhere within the region. 

A compilation of age data shows that Tertiary ages 

are abundant within the three orogenic provinces represented within the region (figs. 1, 3)-from east to 

west, the Purcell foldbelt, the Omineca crystalline belt, 

and the Columbian intermontane belt. However, preTertiary ages, which are abundant within both the 

Purcell foldbelt and the Columbian intermontane belt, 

are strikingly rare in the Omineca crystalline belt. The 

contrast is apparent in figure 6, a plot of ages projected 

to an idealized east-west transect across the region 

shown in figure 3. 
GEOLOGIC HISTORY 13 

121o ~OMINECA CRYSTALLINE BELT~ 117o 

o~--------------------------~--------------------.-~~--------------------.o 

. . .. I VJ 

~ 50 

.I . - . ,;. ~· : . ! . . .. ._, ,, :.~ ·~· 

·' . ·= . . . . • I • I • • 

:. .. ~ ... . .... • • •.• I . . . I lf" 50 

w 

> 

IL. 

0 

VJ 

z 100 

0 

.J 

.J 

~ 

~ 160 

w 

(!) 

<( 

200 

. .. . . . . ·. ·. 

.. 

·r.· 

. : .. , J . . .· .. ": :~. . . 

.. 

. 

., .. 

.. .. 

. ' 

. . . . • . 

... . . . . 

. ... . .· 

.. 

. 

' : 100 

200 

0 15 30 MILES 

I I I I 

0 15 30 KILOMETERS 

FIGURE G.-Mesozoic and Cenozoic age determinations within region shown in figure 3 plotted with respect to the 

approximate boundaries of the Omineca crystalline belt and projected to the 49th parallel. 

Apparently the zone of discordant ages parallel to 

the west side of the Okanogan gneiss dome (fig. 5) is 

part of a regional pattern of discordance associated 

with the Omineca crystalline belt. 

AGE OF OMINECA CRYSTALLINE BELT 

The age of the Omineca crystalline belt refers to the 

ages of the lithologic elements by which it is definedthe Monashee Group of the Shuswap terrane of Jones 

(1959) (and correlative rocks) and the gneiss domes 

included within that terrane. Three ages must be considered: (1) the age of the parent material; (2) the age 

of metamorphism, deformation, and mobilization; and 

(3) the age of cooling through the blocking temperatures of the dated minerals. 

The Okanogan gneiss dome is one of a family of 

gneiss domes located within the Shuswap-or 

Shuswap-like-terrane of the Qmineca. These include, 

from north to south, Frenchman's Cap (Wheeler, 1965), 

Thor-Odin (Froese, 1970; Reesor and Moore, 1971), 

Pinnacles (Reesor and Froese, 1968), Valhalla (Reesor, 

1965), and Passmore (Reesor, 1965) gneiss domes (figs. 

1, 3). 

As redefined by Jones (1959), the Shuswap, in addition to the Monashee Group, also includes on the west 

flank two outliers of low-grade metamorphic rocks 

named the Mount Ida Group and the Chapperon 

Group. Their lower metamorphic grade and lack of 

evidence of profound tectonic involvement set them in 

marked contrast with rocks of typical Monashee terrane, and for that reason they are here considered part 

of the Columbian intermontane belt, rather than part 

of the Omineca. The Mount Ida Group is also lithologically dissimilar to the Monashee Group (Jones, 1959, 

p. 30). Thus, the Mount Ida and Chapperon Groups 

may not share a common age and origin with the 

Monashee Group. Hence, age constraints imposed on 

the Mount Ida and Chapperon Group do not necessarily apply to the Monashee. We refer to the K-Ar biotite 

ages of 135, 140, and 140 m.y. (samples GSC 61-1, 2, 3; 

Lowdon and others, 1963) from rocks of the Mount Ida 

Group and to the fact that the Mount Ida and Chapperon both are unconformably overlain by the Permian 

Cache Creek Group-the Mount Ida at the 

Glenemma(?) unconformity (Jones, 1959, p. 48) and 

the Chapperon at the Salmon River and Dome Rock 

unconformities (Jones, 1959, p. 28, 29)-thus placing 

firm younger age limits on both groups. Again, these 

restrictions do not place a younger limit on either the 

age of the parent material or the age of metamorphism 

and mobilization of the gneiss domes or the Monashee 

Group. 

The material composing the Frenchman's Cap dome 

lithologically resembles part of the late Precambrian, 

Cambrian, and post-Cambrian succession (Wheeler, 

1965, p. 9), as does that of the Thor-Odin (Froese, 1970, 

p. 173; Reesor and Moore, 1971, p. 113). Reesor (1970, 

p. 85) suggests that Pinnacles dome may be composed 

in part of rocks equivalent to the Milford Group (Mississippian to Triassic?), and Giovanella (1968) suggests 
14 PLUTONISM AND OROGENY IN NORTH-CENTRAL WASHINGTON 

that the Malton gneiss may include small bodies of late 

Precambrian Kaza rocks. Reesor (1970, p. 85) notes 

that changes in sedimentation from Windemere pelite, 

grit, and pebble conglomerate to Cambrian quartzite, 

pelite, and limestone can be tentatively identified by 

parallel changes in composition of rocks within the 

gneiss domes. Thus, the age of the material composing 

some of the domes appears to be late Precambrian to 

late Paleozoic. 

The age of mobilization and emplacement of the 

domes has not been narrowly bracketed. Rocks referred to the Shuswap Series east of the Frenchman's Cap 

dome in the !(Clachnacudainn Salient" are cut by a 

stock giving a K-Ar biotite age of 110 m.y. (Wheeler, 

1965, p. 15), and north-trending joints in the dome itself are occupied by lamprophyre dikes dated at 41 m.y. 

(Wheeler, 1965, p. 16). 

Unconformable contacts of the Cache Creek Group 

over the Monashee Group at Lavington and at B. X. 

Creek were reported by Jones (1959, p. 4 7-48), but the 

contacts have been reinterpreted as probable faults by 

Preto (1965). The Monashee Group, however, is patchily overlain by early Tertiary epiclastic deposits and 

volcanic rocks (Jones, 1959, p. 52). 

The Monashee Group has been contact metamorphosed by a 69-m.y.-old pluton in the Nakusp area 

(Hyndman, 1968b, p. 68--69). Hyndman (1968a) concluded that the age of deformation of the Monashee 

Group in the N akusp area was the same as that of the 

adjacent rocks of the Triassic Slocan Group because the 

orientation of the most prominent axis of folding of 

schistosity and cleavage is similar in both groups. 

Hyndman noted that the foliation in the Slocan predates contact metamorphism by Cretaceous plutons 

and therefore concluded that the age of this foliation, 

and hence the age of metamorphism and deformation 

of both the Slocan and Monashee Groups, is Jurassic. 

A contrary view-namely that the main deformation 

of the Monashee Group did not involve the Slocan 

Group-has been put forward by Ross (1970). He 

suggests that in the Kootenay arc three phases of 

deformation can be discerned, each producing morphologically distinct folds and associated foliation and 

lineation. The earliest folds are isoclinal and formed 

easterly verging allocthonous nappes cored with 

Shuswap gneiss. Since in his opinion these folds affect 

only pre-Slocan rocks, he believes they are of preTriassic age. They were refolded during a second phase 

of deformation, also of pre-Triassic age, and again during a third and final phase of deformation. The folds 

produced in the third phase of deformation are open 

structures that involve rocks in the Slocan Group, as 

well as older rocks. Since the deformed Slocan is intruded by the undeformed Nelson batholith, isotopically dated at about 164 m.y. (Nguyen and others, 

1968), this third phase of deformation is bracketed between Late Triassic and Early Jurassic. Ross attributes the·.refolded isoclinal folds of rocks of the Slocan 

Group, reported by Fyles (1967, p. 34-35) and 

Hyndman (1968a), to local deformation associated with 

emplacement of the Kuskanax and Nelson batholiths. 

The K-Ar ages of 17 biotite samples from the Valhalla gneiss dome range from 11 to 66 m.y. (Reesor, 

1965, p. 51), and K-Ar ·ages of both biotite and 

hornblende from the core zone of the Thor-Odin gneiss 

dome range from 60 to 70 m.y. (Reesor, 1970, p. 86). 

K-Ar ages of 15 samples of the Shuswap Series 

(Monashee Group) reported by the Geological Survey 

of Canada are as follows: 

Age Dated ~~:g~~ Reference in m.y. mineral 

62 Biotite GSC 60--1 Lowdon, 1961 

57 Do. 61-4 Lowdon, Stockwell, Tipper, 

and Wanless, 1963 

52 Do. 61-5 Do. 

71 Do. 61-6 Do. 

102 Do. 61-7 Do. 

81 Muscovite 61-8 Do. 

89 Biotite 62-36 Leech, Lowden, Stockwell, and 

Wanless, 1963 

64 Do. 62-35 Do. 

70 Do. 62-44 Do. 

65 Muscovite 62-45 Do. 

73 Do. 62-46 Do. 

76 Biotite 62-47 Do. 

81 Do. 62-48 Do. 

61 Hornblende 66-43 Wanless, Stevens, Lachance, 

and Edmonds, 1968 

79 Do. 66-44 Do. 

The apparent cooling age of the Monashee Gro't1p 

thus spans an interval from middle Cretaceous to 

Eocene, overlapping the cooling age of the gneiss 

domes. However, the average of the ten biotite ages 

listed above of about 72 m.y. significantly exceeds that 

of the six biotite ages from the Okanogan. gneiss dome 

(table 2), which range from 46 m.y. to 56.8 m.y. and 

average about 50 m.y. 

One striking feature of the Omineca crystalline belt 

is the contrast in metamorphic grade between the 

Monashee and Monashee-like rocks and the neighboring Permian and Triassic metavolcanic and 

metasedimentary rocks. Jones (1959, p. 130) notes that 

!(unmetamorphosed Cache Creek rocks in many places 

lie in direct contact with sillimanite gneisses of the 

Monashee Group***." The transition from fine-grained 

greenchist facies country rock to coarsely recrystallized amphibolite facies rocks of the Okanogan gneiss 

dome is also abrupt. 

Rocks typical of the Okanogan dome near this con-
GEOLOGIC HISTORY 15 

tact are mylonitized quartz-oligoclase-orthoclasebiotite gneiss or diopside-hornblende-plagioclasebiotite gneiss, or granodiorite. Alumino-silicate minerals are rare, but locally, toward the interior of the 

dome, the gneiss is composed of sillimanite, cordierite, 

orthoclase, biotite, garnet, quartz, and plagioclase and 

in places contains accessory muscovite. This assemblage is referable to the sillimanite-cordieritemuscovite-almandine subfacies of the amphibolite 

facies, Abukuma-type facies series (Winkler, 1967, 

p. 121), indicating metamorphism under moderate 

pressures, estimated by Winkler (1967, p. 187) as 3-5 

kilo bars. 

Country rocks at the west contact show little evidence of contact metamorphism, a point emphasized by 

Waters and Krauskopf (1941, p. 1376). However, there 

is a metamorphic halo from 100m (110 yd) to at least 2 

km (1.2 mi) wide bordering the dome on the north and 

northeast, within which garnet, biotite, and staurolite 

are present. Nevertheless it is clear from the contrast 

in metamorphic grade at the contact that the rocks of 

the dome could not have been metamorphically formed 

in situ. 

The rocks 10 km (6 mi) west of Curlew (fig. 3) are an 

exception to the above generalization that there is a 

sharp contrast in metamorphic grade at contacts of the 

Shuswap (Monashee Group) and its country rock. 

Parker and Calkins (1964, p. 5--24) described these 

rocks as the metamorphic rocks of Tenas Mary Creek 

and suggested their correlation with the Shuswap 

(Monashee Group). The Tenas Mary Creek rocks form a 

homoclinal sequence estimated by them to be about 

5,200 m (17 ,000 ft) thick that dips northward about 25°. 

The sequence shows a gradual stratigraphic upward decrease in degree of metamorphism from orthoclasequartz-oligoclase gneiss at the base, through 

sillimanite-, cordierite-, and muscovite-bearing schist 

and gneiss, to sericite-, chlorite-, and biotite-bearing 

phyllite at the top, where the sequence is conformably 

overlain by Permian to Triassic greenstone. 

Presuming that the processes of metamorphism and 

penetrative deformation were at least in part simultaneous with emplacement of the Okanogan gneiss 

dome, the contact relations with the Triassic Cave 

Mountain Formation and the Eocene sedimentary and 

volcanic rocks place older and younger limits on the 

age of these processes. The cataclastic texture prevalent throughout much of the dome suggests that these 

processes continued well after most mineral constituents, including zircon, had crystallized. Thus, the 

87 -m.y. (206Pb/238U) age of zircon from one sample (table 

2) is regarded as an older limit on the terminal age of 

penetrative deformation and mobilization of the gneiss 

(Fox and others, 1976). 

The distribution pattern of the Eocene volcanic 

rocks, which are widely scattered over the entire 

region shown in figure 3, does not coincide with that of 

the Omineca crystalline belt or its bordering zone of 

highly discordant ages. Therefore we see no correlation 

between the present outcrop areas of the Eocene rocks 

and the belts of discordant ages and conclude that the 

thermal event presumably associated with the formation of these volcanic rocks is not a factor, except 

perhaps locally, in the origin of the discordancies. 

Thus, the Shuswap-like rocks of the Omineca crystalline belt probably cooled through the successive blocking temperatures for the minerals dated in Late 

Cretaceous and early Tertiary as a result of uplift and 

progressive unroofing by erosion. 

The many gneiss domes and the Shuswap 

(Monashee) are products of an intense regional deformational and metamorphic event that was localized 

along the Omineca crystalline belt. It follows that the 

discordance in the ages of plutonic rocks bordering the 

Omineca is at least in part a manifestation of thermal 

degradation caused by reheating during this deformational and metamorphic event. Locally, however, the 

effect of contact metamorphism by younger plutonic 

rocks seems to be a factor in the origin of the discordance. For example, east of the Omineca crystalline belt 

in northern Washington and Idaho, the biotite and 

muscovite ages of pre-Tertiary plutonic rocks show 

increasing degradation towards areas of extensive 

Eocene plutonic rocks (Miller and Engels, 1975). 

The Omineca metamorphic event could have begun in 

British Columbia prior to 110 m.y. ago, if the K-Ar age 

determination of biotite in the stock cutting the Shuswap in the Clachnacudainn Salient reported by 

Wheeler (1965, p. 15) is accurate. In the Okanogan area, 

that event persisted into the Late Cretaceous, when 

both the mobilized elements of the metamorphosed 

rocks and also a thermal front of sufficient intensity to 

affect the K-Ar age of some minerals in the rocks of the 

terrane west of the belt reached high levels in the crust. 

The metamorphic event had been concluded and the 

high-grade metamorphic rocks had cooled through the 

blocking temperature of biotite by the end of the Cretaceous in British Columbia and by Eocene in 

Washington. 

EOCENE TO MIOCENE HISTORY 

The Tertiary volcanic rocks of the region (fig. 3) have 

been shown by Mathews and Rouse (1963), Rouse and 

Mathews (1961), Mathews (1964), and Hills and 

Baadsgaard (1967) to include two groups, one Eocene 

and the other late Miocene and early Pliocene. Most of 

the Eocene volcanic rocks within the region were 
16 PLUTONISM AND OROGENY IN NORTH-CENTRAL WASHINGTON 

formed during a thermal episode climaxing about 50 

m.y. ago. 

The Eocene episode began with roughly contemporaneous deposition in local basins of arkose, wacke, 

and conglomerate. At the base, these deposits consist of 

quartzo-feldspathic material eroded from nearby 

sources, but higher in the section these deposits contain 

pyroclastic material in increasing proportions. The 

sedimentary beds are typically overlain by pyroclastic 

rocks and lava flows and intruded by the hypabyssal 

intrusive equivalents of the volcanic rocks. The Eocene 

rocks were deposited on a profound angular unconformity beveled on older rocks. 

Eocene volcanic rocks in the southeastern and northeastern parts of the study area within Wasington include both the flows of quartz latite, rhyodacite, dacite, 

and interbedded tuff of the Sanpoil Volcanics and their 

rhyodacitic and quartz latitic hypabyssal intrusive 

equivalents, the Scatter Creek Formation (Muessig, 

1962; Staatz, 1964; Parker and Calkins, 1964; Muessig, 

1967). Coextensive rocks in British Columbia include 

andesite, trachyandesite, and sodic trachyte of the 

Marron Formation (Monger, 1968). Correlative volcanic rocks to the west include dacitic and andesitic 

flows and their hypabyssal intrusive equivalents, which 

are patchily distributed along the axis of the Okanogan 

Valley and also occupy a large tract farther west 

underlying the much dissected summit plateau of the 

Okanogan Range. 

A fossil flora in sedimentary deposits cut by intrusive 

equivalents of-the volcanic rocks northwest of Oroville 

is regarded as indicating a probable early Eocene age by 

J. A. Wolfe (in Rinehart and Fox, 1972, p. 61). The fossil 

flora of the O'Brien Creek Formation, a volcaniclastic 

unit that underlies the Sanpoil Volcanics near Republic 

(fig. 3), a few kilometers east of the study area, resembles 

the 'Eocene flora of Alaska and of the lower part of the 

Puget Group of Washington, whereas the fossil flora of 

the Klondike Mountain Formation, which overlies the 

Sanpoil, is Oligocene and Miocene(?) (R. W. Brown, in 

Muessig, -1967). 

K-Ar ages of volcanic rocks within the study area, 

previously reported by Rinehart and Fox (1972), 

Mathews (1964), and Engels, Tabor, Miller, and 

Obradovich (1976) range from 42.9 to 52.1 m.y. 

In the Republic area, Eocene deposition was accompanied by penecontemporaneous faulting and subsidence of a north-northeast-trending graben (Muessig, 

1967, p. 95-96). There and elsewhere in the region the 

Eocene rocks were folded and faulted, partially eroded, 

and later buried in places by basalt during the late 

Miocene. The late Miocene and early Pliocene rocks 

include the plateau basalts of the Columbia River 

Basalt Group, which overlaps the south edge of the 

region, and its erosional outliers, and other smaller 

patches of basalt distributed over the remainder of the 

region. 

OROGENIC IMPLICATIONS 

GEOLOGIC HISTORY 

PREVIOUS VIEWS ON OROGENY IN THE CORDILLERA 

Phanerozoic orogeny in the Cordillera was visualized 

by Gilluly (1965) as a virtually continuous, albeit locally episodic, process. King, while holding that in general orogeny !!was concentrated in a succession of 

episodes during each orogenic phase***" (1969b, p. 45), 

concluded that in the northern Cordillera (north of the 

49th parallel), Mesozoic time !!was one of nearly continuous orogeny from place to place in the eugeosynclinal area***" (1969b, p. 67-68). King concludes that 

climactic middle Mesozoic orogeny in the central Cordillera (for example, the Sierra Nevada) was in Late 

Jurassic but was either earlier or later in other parts of 

the eugeosynclinal belt. Also, it was !!progressively 

younger eastward across the fold belt, and in the 

miogeosynclinal area there is no clear separation between a middle Mesozoic orogeny and a terminal 

Mesozoic orogeny; most of the events were at intermediate times***" (1969b, p. 70). 

White (1959) attributes Mesozoic and early Cenozoic 

deformation in British Columbia to three orogenies: the 

Cassiar orogeny, which occurred between Permian and 

Late Triassic (p. 72); the Coast Range orogeny, whose 

effects first appear in the earliest Jurassic, culminate in 

the Early Cretaceous, and persist well into the Late 

Cretaceous (p. 78); and the Rocky Mountain orogeny, 

which :culminated in Paleocene (p. 98). White later 

modified these views, stating (White, 1966, p. 187) that 

!!much of Mesozoic was a time of what might be termed, 

!continuous-intermittent orogeny,' when orogenic 

events of one sort or another were happening in one part 

or another of the Western Cordillera throughout a span 

of perhaps 150 million years." He further states (p. 189) 

that uit is doubtful that Tertiary orogeny was separated 

from Mesozoic orogeny by any significant period of crustal stability *** ." Within the study area, the Cassiar 

orogeny may be responsible for the unconformable contacts ofKobau over Anarchist (Rinehart and Fox, 1972), 

but the degree of deformation is apparently slight, 

probably of epeirogenic rather than orogenic proportions. No pre-Late Triassic plutonic rocks have been 

discovered. 

LATE TRIASSIC OROGENY AND PLUTONISM 

The Upper Triassic Nicola Group is cut by the 

Guichon batholith (White, 1959, p. 87), now known to be 

about 200 m.y. old (White and others, 1967). Within the 

study area (fig. 2), strongly folded rocks of the Kobau 
OROGENIC IMPLICATIONS OF GEOLOGIC HISTORY 17 

Formation, a probable correlative of the Nicola Group, 

are similarly cut by the Loomis pluton, believed to be 

about 195 m.y. old (Rinehart and Fox, 1972; this report, 

table 2). Thus, orogeny (as defined in Gary and others, 

1972, p. 500) probably occurred simultaneously in 

southern British Columbia and northern Washington 

during Late Triassic. This event evidently has no 

chronological counterpart in the Sierra Nevada or in 

the Klamath Mountains of California. The earliest 

comparable Mesozoic event in the Sierra Nevada was in 

Early or Middle Triassic, and the next was in Early or 

Middle Jurassic (Evernden and Kistler, 1970). Mesozoic 

plutonism in the Klamath· Mountains began in early 

Middle Jurassic, where it ushered in the Nevadan 

orogeny (Lanphere and others, 1968, p. 1047, 1050). 

The ages of plutonic rocks in the Okanogan region 

suggest that the eugeosynclinal province underwent 

sporadic plutonism without a clearcut break between 

the Late Triassic and early Tertiary. However, the 

plutonism subsequent to Late Triassic and prior to 

emplacement of the Okanogan gneiss dome cannot be 

linked with orogenic deformation in the Okanogan 

region by means of the evidence at hand. 

ASSOCIATION OF DEFORMATION OF THE 

MONASHEE GROUP AND GENERATION OF 

GNEISS DOMES WITH CORDILLERAN THRUST FAULTING 

The most important thermal and deformationalhence, orogenic-event within the region was the 

metamorphism climaxed by mobilization of large segments of the lithosphere within and adjacent to the 

Omineca crystalline belt. Understanding of this event 

seems pivotal to an understanding of the geologic history of the region. 

The deformation and mobilization of the Shuswap 

(Monashee Group) and correlative rocks may be linked 

to thrusting along the Rocky Mountain thrust belt to 

the east and along the Shuksan thrust belt of the northern Cascade Range to the west (fig. 7). According to 

Bally, Gordy, and Stewart (1966, p. 36&-372), 

deformation proceeded from west to east, beginning 

with mobilization of gneiss domes and thrusting in the 

interior in Late Jurassic to Early Cretaceous and, except for later uplift, ending with the final stages of 

thrusting in the Rocky Mountains Foothills province in 

the Eocene. Price and Mountjoy (1970) postulated contemporaneous metamorphism and mobilization of the 

gneiss domes and thrusting in the Rocky Mountain 

thrust belt to the east. In their view, the origin of the 

Shuswap and the thrust belts can be explained by 

"progressive buoyant upwelling and lateral spreading 

of a hot mobile infrastructure beneath a relatively passive suprastructure*** and equivalent progressive 

northeasterly growth of the foreland thrust belt that 

forms the Rocky Mountains" (Price and Mountjoy, 1970, 

p. 7). 

The crystalline core of the northern Cascade Range is 

flanked to the west and east by major fault zones, the 

Shuksan thrust and associated faults on the west and 

the Ross Lake fault zone on the east (Misch, 1966). The 

Shuksan thrust is part of Misch's (p. 128) ~~midCretaceous Northwest Cascades System." The age of 

this westward thrusting is bracketed between uthat of 

the Nooksack Group (early Early Cretaceous) and that 

of the Chuckanut Group (late Late Cretaceous and 

Paleocene)" (McTaggart, 1970, p. 138, 144). 

The terrane between these two major faults consists 

mainly of the Cascade River Schist and the Skagit 

Gneiss, believed by Misch (1966, p. 103, 113) to :Ilave 

been metamorphosed before the Shuksan thrusting of 

the mid-Cretaceous orogeny in pre-Jurassic or probably 

pre-Mesozoic time. However, McTaggart (1970, p. 143) 

suggested that the Skagit migmatites and their correlative in Canada, the Custer Gneiss, might be Early to 

middle Cretaceous in age and closely related to the 

mid-Cretaceous orogeny. Mattinson (1972, p. 3778-

3779) obtained ages ranging from 90 to 60 m.y. on 

sphene and zircon from the Skagit Gneiss and correla-

. tive rocks by Pb-U and Ph-Ph methods, indicating that 

the Skagit metamorphism was probably middle to Late 

Cretaceous. In view of the relative ages of Skagit 

metamorphism and Shuksan thrusting, implied by 

Misch (1966), it is unlikely that the Shuksan thrusting 

is older than Late Cretaceous. 

According to Misch (1966, p. 128) yielding on the 

Shuksan and associated thrusts is southwestward on 

the south, westward near the Border, and northwestward north of it. Movement on the Ross Lake fault zone 

was primarily strike slip, although reverse in part with 

eastward-directed over-thrusting on the subsidiary 

Jack Mountain fault of up to about 10 km (6 mi) 

displacement (Misch, 1966, p. 134); The northern extension of the Ross Lake fault zone, the Hozameen fault, 

seems to be very steep (McTaggart, 1970; Coates, 1970). 

Slivers of tectonically emplaced ultramafic rock are associated with both the Shuksan and Ross Lake fault 

systems, causing Misch (p. 133, 134) to postulate extension of these faults to the base of the crust. In Misch's 

view (p. 136), a 40-km-wide (25-mi-wide) wedge of the 

crystalline core of the Cascades has been compressively 

uplifted between the two faults. 

Maximum offset of the Shuksan and associated 

thrusts was estimated as about 50--65 km (30--40 mi) 

(Misch, 1966, p. 128), whereas the cumulative offset on 

the Rocky Mountain thrusts was variously estimated as 

about 190 km (120 mi) by Bally, Gordy, and Stewart 

I 

(1966, p. 359) and at least 200 km (125 mi) by Price and 

Mountjoy (1970, p. 16). The Shuswap complex and as-
18 

0 

.... ~ 

PLUTONISM AND OROGENY IN NORTH-CENTRAL WASHINGTON 

0 .(Rincon Mountains 

('V Olf area 

::Tucson~~ 

0 100 200 300 MILES 

0 100 200 300 KILOMETERS 

sociated gneiss domes thus appear to occupy the axial 

zone between convergent thrusts that show an apparent 

aggregate crustal contraction of possibly 250 km (150 

mi). 

The Shuswap (Monashee Group) is but one of the 

several areas of gneiss domes, or of medium- to highgrade metamorphic rock, some exhibiting Late 

Cretaceous to early Tertiary cooling ages, that flank 

the interior (western) side of the Cordilleran thrust 

belt in British Columbia, Washington, Idaho, Nevada, 

and Utah (fig. 7). These also include the Wolverine 

complex; Wildhorse Canyon Complex (Dover, 1969); 

the metamorphic complexes in Idaho, Utah and 

Nevada grouped by Armstrong and Hansen (1966) into 

three areas, namely Raft River, Ruby Range, and 

Snake Range; and the metamorphic complex in the 

Rincon Mountains area of Arizona (Waag, 1969). We 

speculate that some of these metamorphic terranes are 

either (1) uplifted elements of an abscherungzone 

(Armstrong and Hansen, 1966), (2) diapiric tongues of 

gneiss, possibly originating at an abscherungzone and 

intruding supracrustal strata of the overthrust plate, 

or (3) gneiss domes forming through mobilization of 

infrastructure farther to the west in the hinterland. 

The metamorphic terranes appear to be both temporally and geographically associated with the Late 

Cretaceous to early Tertiary thrust belts. 

These relations compel the conclusion that the formation of the Shuswap (Monashee Group) and the 

gneiss domes is the regional expression of the widespread deformational and metamorphic events that 

culminated in the formation to the west of the 

westward-directed Shuksan thrust, and to the east, as 

advocated by Bailey, Gordy, and Stewart (1966) and 

Price and Mountjoy (1970), of the belt of easterly directed Rocky Mountain thrusts. 

LARAMIDE OROGENY, FORMATION OF GNEISS DOMES, 

AND THRUST FAULTING 

In concept, the geographic locus of a deformational 

event could gradually shift through time so that the 

beginning and ending of the event would not be 

everywhere the same age. However, few Mesozoic and 

FIGURE ?.-Distribution of (1) late Mesozoic and early Cenozoic 

thrust belts and (2) areas of medium- to high-grade metamorphic 

rock, including gneiss domes, that show late Mesozoic to early 

Cenozoic cooling ages and (or) structural fabrics suggesting extensive mobilization. Belts of Jurassic and Cretaceous magmatic 

activity modified after Kistler, Evernden, and Shaw (1971, 

p. 858). Thrust faults after King (1969a). Section A-A' shown in 

figure 8. 
OROGENIC IMPLICATIONS OF GEOLOGIC HISTORY 19 

Cenozoic deformational events have left a structural 

signature by which they can be correlated from region 

to region. The Laramide orogeny may be an exception. 

Eardley (1951, p. 285) noted that Mesozoic movements 

in the Rocky Mountains systems "although intense in 

some places, were generally precursory to the climactic 

ones at the close of the Cretaceous and in the early 

Tertiary." He indicated that the Laramide orogeny was 

"the great compressional disturbance that occurred in 

very late Cretaceous, Paleocene, and Eocene." It follows that thrusting of that approximate age along the 

nearly continuous Rocky Mountain thrust belt in 

Canada, the Montana disturbed belt, Idaho-Wyoming 

thrust belt, and the Sevier belt in Nevada and Utah, is 

referable to the Laramide orogeny, regardless of minor 

differences in age of thrusting from region to region. 

This deduction contravenes Armstrong's (1968, 

p. 451) conclusion that thrusting in the Sevier belt 

predates and therefore is not part of the Laramide 

orogeny. Noting that thrusting in the Sevier belt ended 

in the Campanian Age of the ·Late Cretaceous, he 

introduced the term Sevier orogeny for that deformational event and relegated the Laramide orogeny to 

the succeeding episode of morphogenic uplift ranging 

in age from the Maestrichtian or Campanian Ages of 

Late Cretaceous through the middle Eocene. We agree 

that the term Sevier orogeny should be retained but 

suggest that it is a regional correlative of the Laramide 

orogeny as defined by Eardley (1951). 

The deformational events that resulted in formation 

of the gneiss domes, Shuswap terrane, and thrust belts 

therefore include tectogenesis attributed to midCretaceous orogeny (Misch, 1966) on the west and to 

Laramide orogeny on the east. 

Price and Mountjoy (1970, p. 23) suggested that the 

thrusting and folding in the Canadian Rocky Mountains spanned the time interval from Late Jurassic to 

Paleocene or Eocene. Their older limit is apparently 

based on the supposition that orogenic development in 

the interior of the Rocky Mountains and deposition of 

the sequence of clastic wedges to the northwest had 

begun by Late Jurassic and on the premise that formation of the clastic wedges implies beginning of thrusting as well as orogeny. In their view, a minimum age 

for thrusting and doming of the gneiss of 111 m.y. is 

established by the age of porphyroblastic biotite (sample GSC 66-4 7, Wanless and others, 1968) from metamorphic rocks believed to be part of the metamorphic 

halo that envelopes the diapiric Malton Gneiss (see fig. 

7), more than 150 km (95 mi) north of the area represented in figure 3. However, the following K-Ar ages 

have been reported from the gneiss itself: 

Mineral Age, 

in m.y. ~~;::~;~ 

Biotite 72 GSC 65-24 Wanless, Stevens, Lachance, 

and Edmonds, 1967 

Biotite 53 67-43 Wanless, Stevens, Lachance, 

and Delabio, 1970 

Biotite 59 67-43 Do. 

Hornblende 114 67-44 Do. 

Muscovite 60 70-16 Wanless, Stevens, Lachance, 

and Delabio, 1972 

Biotite 66 70-17 Do. 

Biotite 57 70-18 Do. 

The cooling age of mica in the Malton Gneiss is 

roughly Paleocene, virtually equivalent to that of the 

gneiss domes to the west. 

Armstrong and Oriel suggested that thrusting in the 

Idaho-Wyoming thrust belt spanned the interval from 

Late Jurassic to Eocene (1965, fig. 19, p. 1861). Evidence of thrusting prior to latest Cretaceous apparently consists of the presence of a latest Jurassic to 

earliest Cretaceous conglomerate 24 km (15 mi) east of 

the trace of the Paris thrust fault. Armstrong and Oriel 

reasoned that if the conglomerate is a synorogenic deposit derived from source areas to the west, as is likely, 

then movement on the Paris thrust is dated as latest 

Jurassic and earliest Cretaceous (1965, p. 1859). 

Although the time at which the Laramide orogeny 

ended is relatively easily established by the cooling 

ages of the crystalline rocks and the termination of 

overthrusting, the time at which this orogeny began is 

more difficult to establish and is partly a matter of 

definition. We' suppose that the Shuswap (Monashee 

Group) and correlative rocks include metamorphosed 

equivalents of Precambrian, Paleozoic, and even 

Mesozoic supracrustal rocks that were converted to 

infrastructure-that is, were deeply buried, heated, 

metamorphosed, migmatized, and ultimately formed 

rheomorphic bodies that later were forced upward into 

the suprastructure, much as visualized by Campbell 

(1973, p. 1614). This process may have begun in places 

in the region as early as the Late Triassic, which may 

mark a significant milestone in the orogenic cycle. 

However, the gneiss domes are set apart not by migmatization or metamorphic grade, but by internal 

structure and contact relations suggesting rheomorphic flow and diapiric intrusion. The time at which 

wholesale upward and lateral flow of parts of the 

infrastructure began therefore marks the beginning of 

this deformational event in the Okanogan region. The 

evidence within this region for the time at which this 

phase began is nebulous, principally because of difficulty in discriminating effects of earlier phases of 

metamorphism and deformation. 
20 PLUTONISM AND OROGENY IN NORTH-CENTRAL WASHINGTON 

The earlier phases of plutonism and metamorphism 

may have been a necessary precursor to the later invasion of supracrustal rocks by gneiss domes and concomitant overthrusting. However, the diapirism and 

thrusting seemingly mark a radical, even catastrophic, 

change in the tenor of compressional tectonics and 

thereby stand in contrast to the previous history. The 

rough simultaneity in the cooling age of the gneiss 

domes and the age of terminal thrusting indicates a 

similar equivalence in the age of their birth, assuming 

that the thrusts and gneiss domes are genetically related and that both originate at least in part in response to a particular episode of regional compression. 

If the gneiss domes and thrusts are indeed roughly 

coeval, the probable older limit on the age of the 

Shuksan thrust of Late Cretaceous suggests a comparable older limit on the age of the gneiss domes. 

DISCUSSION 

A stable-and perhaps at times extensionalmargin bordered the continental plate during building 

of the late Precambrian and early Paleozoic depositional wedge now exposed in the miogeoclinal province 

(Stewart, 1972; Gabrielse, 1972, p. 533). The tectonic 

stability that marked this period was succeeded in late 

Paleozoic or early Mesozoic time by the strongly convergent regime that prevailed throughout much of the 

Mesozoic and Cenozoic, while large areas of oceanic 

plate were subducted beneath the western side of the 

North American plate (Hamilton, 1969b). 

The late Paleozoic Cache Creek strata may represent 

sediments deposited upon the oceanic plate seaward of 

an eastwardly dipping subduction zone (Monger and 

others, 1972, fig. 6). However, the temporal counterpart of these strata in the study area, the Anarchist 

Group, contains limestone and clastic detritus more 

compatible with sedimentation in a back-arc basin 

close to a nearby landmass than with sedimentation 

far from land at the abyssal depths of the ocean floor. 

In any case, the predominantly volcanic strata of the 

overlying Middle to Late T:riassic formations and their 

probable correlative in the study area, the Kobau Formation, mark the abrupt onset of a period of (1) extrusion of voluminous lavas, (2) intrusion of batholiths 

and lesser plutons, (3) strong folding, (4) uplift and 

subaerial erosion, and deposition of detritus in successor basins, and (5) regional metamorphism. Most of 

this activity can be ascribed to processes occurring 

above an eastwardly dipping subduction zone or zones, 

located along the continental margin to the west 

(Monger and others, 1972, p. 592--593). 

Perhaps the most enigmatic of the problems discussed in the preceding pages are those of the origin of 

the gneiss domes and the thermal metamorphism of 

the Omineca crystalline belt and the reason for the 

apparent association of these features with the Cordilleran thrust faults. 

In the Canadian Cordillera, the spatial and temporal 

association of the gneiss domes and thrust faults with 

the Mesozoic plutonic belts and with the presumed 

convergent plate boundary suggest that the gneiss 

domes and thrust faults are in some way related to 

Mesozoic subduction to the west (Campbell, 1973, 

p. 1618). But it is puzzling that south of the Canadian 

Cordillera the belt of gneiss domes and the track of the 

eastern thrust belts diverge from parallelism with the 

continental boundary (as marked roughly by the present coastline, fig. 7) and swing far inland of the Jurassic and Cretaceous belts of plutonic activity delimited 

by Kistler, Evernden, and Shaw (1971, p. 858). 

Coney (1972, p. 620) stresses the probable tectonic 

impact of the worldwide change in spreading directions 

that occurred about 80 m.y. ago. He proposed (1972, 

p. 619-620) that the Late Cretaceous orogeny in the 

Sevier belt of Armstrong (1968) resulted from failure of 

the leading edge of the North American plate as it was 

being driven northwestwardly over a Benioff zone entrained in the oceanic plate below. Later, a£ter the 

change in spreading directions, the early Tertiary 

orogeny resulted from failure of the leading edge of the 

North American plate as it was driven westwardly to 

southwestwardly, also over a subduction zone or zones 

entrained in the subjacent oceanic plate. The latter is a 

vital stipulation because it frees the process from the 

possible requirement that subduction-related 

phenomena occur reasonably near the margin of the 

overriding plate. 

The cause of the change in spreading directions is 

unclear; in our opinion, it is as likely that the orogeny 

caused the change in spreading directions as vice 

versa. We do not dispute the geographic association of 

the tectonic and plutonic features of Laramide orogeny 

with the generally convergent plate boundary to the 

west, but this observation falls short ofbeing a solution 

to the gneiss dome and orogeny problem. 

Before a genetic mechanism can be suggested that 

could account for the association of a convergent plate 

boundary with the Laramide orogeny, the question 

must be answered as to whether or not the cumulative 

displacement on the Shuksan and Rocky Mountain 

thrust systems represents actual crustal shortening. 

Price and Mountjoy (1970, p. 18) hypothesized that 

the sole of the Rocky Mountain thrusts climbed the 

westward-dipping upper surface of the Precambrian 

crystalline basement in response to gravity loading in 

the vicinity of what is now the Omineca crystalline belt. 
DISCUSSION 21 

The gravity loading resulted from the upwelling of 

mobile material of the infrastructure, and in their view 

no crustal shortening is required. However, the Shuswap (Monashee Group), which probably represents both 

the upwelled material and its wrapping of supracrustal 

rocks and other supracrustal rocks metamorphosed to 

like degree in situ, spans only 55--105 km (35--.65 mi). 

Furthermore, the high-grade metamorphic rocks of possible diapiric origin are not continuous elements of the 

hinterland along their north-south trend west of the 

Rocky Mountain thrust zone. If crustal shortening were 

not a factor, the dilatational features in the hinterland 

that would be required to account for the cumulative 

offset on the thrusts-in places approaching perhaps 

250 km (150 mi)----do not appear to be present. Therefore, we agree with Bally, Gordy, and Stewart (1966,.p. 

360) that crustal shortening must be invoked to explain those offsets. 

The alternative-that the thrust faults represent 

gravitational gliding from hypothetical highlands 

located west of the thrust belts within the 

hinterland-has been advocated by Mudge (1970). The 

arguments against this hypothesis have been recently 

summarized by Price (1971), with special reference to 

the disturbed belt of northwestern Montana, and by 

Armstrong (1972), with special reference to the Sevier 

orogenic belt, and are, in our view, compelling. 

The required compressional mechanism apparently 

causes regional crustal contraction and concomitant 

mobilization and upward injection of infrastructure. 

Possibly the compressive stresses and high heat flow 

that caused the thrust faulting and ~iapirism of gneiss 

could result from opposing drag of the continental 

lithosphere by converging convection cells in the 

underlying asthenosphere. This hypothesis has been 

offered as an explanation of the bilateral symmetry 

exhibited by certain paired belts of post-Paleozoic 

thrust faults in part of the Cordillera by Burchfiel and 

Davis (1968). According to their interpretation, the 

magnitude of required crustal shortening in certain 

segments, such as that across the 40-km-wide ·(25-miwide) span between the east-dipping Shuksan thrust 

and the vertical or west-dipping Ross Lake fault zone 

(fig. 7), cannot be accommodated except by swallowing 

of crustal material at depth. They concluded that crustal shortening of large magnitude is required across the 

thrust complexes and suggest that convergence of subcrustal convection cells was the responsible orogenic 

mechanism. 

Although we believe that the Ross Lake fault zone is a 

relatively minor feature and that the contractional axis 

must lie well to the east, below the Omineca crystalline 

belt, the convection cell hypothesis does provide a believable thrusting mechanism. In this model, the basement rocks at either side of the bilateral thrust beltsShuksan and Cordilleran-are dragged toward and 

under the centrally located allochthon by a frictional 

couple with the converging convection cells in the mantle below. The crust would be expected to thicken along 

the axis and to either side of the cells through folding, 

stacking of thrust sheets, and plastic flow. This thickening is presumably counterbalanced by crustal shortening. The buoyancy of the crust would probably prevent its 

being swallowed by the mantle at the vortex of the cell. 

The model is unsatisfactory as it stands, however, 

because no triggering force, reason for localization, or 

physical basis for the convection is provided. These defects can perhaps be overcome by considering the possible ramifications of the ideas articulated by Vine 

(1966, p. 1411), Palmer (1968, p. 341), and Kistler, 

Evernden, and Shaw (1971, p. 864--866), whereby, in 

the Mesozoic, the westward-drifting North American 

plate overrode an ancestral East Pacific rise locked between underthrusting oceanic plates. 

Palmer suggested (1968, p. 344) that the Mesozoic 

plutonism and volcanism of the Cordillera was attributable to heat emanating from the underthrust rise 

and that the Cordilleran thrusts formed during uthe 

terminal throes of convection before motion from the 

overridden crust was stifled***." 

Kistler, Evernden, and Shaw (1971) suggested that the 

loci of magmatic activity in the western United States 

defined linear belts and that these belts were progressively displaced to the southeast from Jurassic to 

Cretaceous time (fig. 7). They hypothesized (1971, 

p. 864--866) that these magmatic belts originated as 

North America moved northwestward across a linear 

heat source analogous to an oceanic rise. But Rutland 

(1973, p. 829) rejected this hypothesis, noting that ''the 

similarity of timing and spatial relationships of 

plutonism in North America, South America, and Japan 

surely demands a common explanation***." He concluded that the plate relationships postulated by Kistler, Evernden, and Shaw (1971) could not have ocurred 

in all these areas. 

In our view, what all these areas have in common is 

an intermittently convergent continental plate--oceanic 

plate relation during the Mesozoic and Cenozoic. This 

relation could result in either subduction-zone related 

plutonism and volcanism of the Andean type (Hamilton, 1969a) or in plutonism and volcanism related to 

overriding of an oceanic rise. Both types would be related in space in that they occur inland from the convergent' plate boundary; they might also appear regionally 

to be related in time, as periods of strong convergence 

would be most likely to end in overriding of a rise system. 
22 PLUTONISM AND OROGENY IN NORTH-CENTRAL WASHINGTON 

The oceanic rise system has been considered by some 

(for example, Dietz and Holden, 1970, p. 4941) as a 

passive element in the plate tectonic model-simply a 

fracture along which magma upwells to fill the opening 

that would otherwise remain while the oceanic crust to 

either side is rafted or pulled away. For the rise system 

to exert the profound tectonic and magmatic influence 

here postulated, it must instead represent an active 

element-perhaps the crustal expression of a linear 

zone of high heat flow within the mantle that is not 

readily extinguished even though underthrust beneath 

a continental plate. 

We speculate that in the study area overriding of the 

rise system marked the beginning of a transition from 

arc-type eugeosynclinal deposition and volcanism to an 

orogenic phase, with the area of magmatism-formerly 

restricted to the area of subduction-related volcanism 

immediately inland from the convergent plate 

boundary-now diffused along and to either side of the 

axis of the former rise system (fig. 8). Magmas generated during this phase were presumably the products of 

differential melting of mantle material, modified by 

assimilation of material from the sialic crust of the 

overlying continental plate. 

Thus as the rise system is overriden production of 

basaltic magmas gives way to production of intermediate and felsic magmas; the latter are the products 

of varying degrees of fractional melting of mantle and 

assimilation of sial. Repeated extraction of this salic 

differentiate from the mantle and possibly the lower 

crust above the overridden rise system could leave an 

increasing accumulation of mafic residue. Possibly the 

build-up of this dense residue of infusibles could produce a mass that is gravitationally unstable relative to 

nearby areas within the mantle that lie outside of the 

zone of fractional melting. Ultimately, at some place 

along the axis of the overridden rise system, the weight 

of this more dense residue would overcome the plastic 

limit or strength of adjacent mantle, would sink, and 

the mantle to either side would converge catastrophically toward it. The overlying sial would be dragged 

toward the axis of the sinking mass but not swallowed 

because of its inherent buoyancy. Once started the cell 

could extend itself lengthwise along much of the overridden rise system, until it terminated against parts of 

the rise system so recently overridden that no significant instability existed. 

In summary, we speculate that the convergence of 

oceanic and continental plates at a subduction zone 

would ultimately result in the overriding of an oceanic 

rise system by the continental plate, continuing a cycle 

whose succeeding phases would be dominated in turn by 

(1) eugeosynclinal sedimentation, (2) orogenesis with 

plutonism, (3) extreme compression with failure by 

overthrusting, possibly attended by mobilization and 

diapiric intrusion of the infrastructure, forming gneiss 

domes, and (4) the relaxation of compression, with uplift 

through isostatic rebound. This is basically an orogenic 

magmatic cycle similar to that advocated by DeSitter 

(1964, p. 397) and Rutland (1973, p. 828). The reality of 

the concept of a tectonic cycle has been questioned by 

Coney (1970). We hypothesize that this cycle is not only 

rationalized by but is indeed a necessary consequence of 

the plate tectonic model. 

The Omineca crystalline belt may represent part of 

the relatively rigid center of the Cordilleran-Cascade 

allochthon, bridging the axis of sinking upper mantle. 

We speculate that the special features of the Omineca 

province-gneiss domes and the metamorphic rocks of 

the Monashee Group-originated through high heat 

flow and the forcing of hot plastic crustal material of the 

infrastructure upward along this axis during midCretaceous to Eocene compression. Presumably, the 

postulated contraction of the continental plate along 

FIGURE 8.-Stages in the development of the Omineca crystalline belt (sections drawn along line A-A' in fig. 7). A, In 

Permian and Early Triassic time, volcanic and pyroclastic deposits accumulated in island-arc and back-arc basin 

environments. To the west, an east-dipping subduction zone formed the boundary between oceanic and continental plates. A rise embedded within the oceanic plate was gradually nearing the continental plate. B, In Late 

Triassic and Jurassic time, the thickened eugeosynclinal prisms were invaded by calc-alkalic magmas derived 

through partial melting of the upper mantle and lower crust within a zone of high heat flow above the overridden 

rise. The residue (dotted area on figure) remaining in the zone of partial melting became progressively more dense 

as the hyperfusible part was removed. 

In Late Cretaceous time (C), after continued calc-alkalic plutonism through the Early Cretaceous, the dense 

residue occupying the source area of the magmas catastrophically sank into the asthenosphere, forming a shortlived convection that dragged overlying crust toward the axis of the cell. The crust thickened over the cell, 

through stacking of thrust sheets and plastic flow. Mobilized elements of the infrastructure penetrated higher 

levels in the crust, with their culminations forming gneiss domes. 

With the demise of the convection cell in latest Cretaceous, the thickened crust isostatically rebounded. Upper 

crustal levels over the Omineca were rapidly eroded away, exposing elements of the Late Cretaceous infrastructure (the gneiss domes and the Monashee Group) in the Eocene. 
DISCUSSION 

A. PERMIAN AND EARLY TRIASSIC VOLCANISM 

A 

EUGEOSYNCLINAL PROVINCE 

Island arc Back arc MIOGEOCLINAL PROVINCE 

-------------------- --=~~ASTHENOSPHERE =--------==- ------- - __31sotherms 

----------------- --- ------------------

------ --------------- ----- --- ----------

--- MESOSPHERE 

B. LATE TRIASSIC AND JURASSIC PLUTONISM AND VOLCANISM 

C. LATE CRETACEOUS THRUST FAULTING, METAMORPHISM, AND MOBILIZATION OF INFRASTRUCTURE 

A ~ NEW OCEANIC -1 r- CRUST 

ROSS LAKE 

FAULT ZONE OMINECA 

CRYSTALLINE 

f- BELT -1 

0 100 200 MILES 

0 100 200 KILOMETERS 

23 
/ 

24 PLUTONISM AND OROGENY IN NORTH-CENTRAL WASHINGTON 

the Cordillera must have coincided with a reciprocal 

addition of new oceanic crust elsewhere, either through 

accelerated spreading rates at existing oceanic rises or 

through the formation of new rises. 

The convulsive orogenic climax, represented in the 

study area by the emplacement of the Okanogan gneiss 

dome, was evidently followed in Late Cretaceous and 

early Tertiary by uplift and rapid erosion-and consequent cooling of the high-grade metamorphic rocks 

through the successive blocking temperatures of the 

dated minerals. This was followed in the Eocene by the 

postorogenic intrusion of hypabyssal plutons and the 

extrusion of related volcanic rocks. The Swimptkin 

Creek and Coyote Creek plutons are probably representatives of the postorogenic plutons. The Eocene deposits fringing the gneiss domes have been warped and 

locally faulted within the study area. To the east, they 

form a thick sequence of sedimentary deposits and interstratified volcanic rocks occupying the Republic graben. The north-northeast trend of the graben suggests 

that the former east-southeast to east-northeast direction of compression that had prevailed in the study area 

from the Late Triassic (Rinehart and Fox, 1972) became 

an axis of tension in the Eocene. 

The final magmatic event in the region, the extrusion 

ofnonorogenic late Miocene and early Pliocene plateau 

basalts, introduced a profound change in chemical composition of magma, from the more silicic and more 

highly differentiated Upper Triassic and post-Triassic 

igneous rocks. 

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