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I '1 
Geology Along the 

Northwest Border Zone 

of the Idaho Batholith, 

Northern Idaho 

By ANNA HIETANEN 

Stratigraphy, correlation, and 

some petrologic aspects of the 

metamorphic rocks northwest of 

the Idaho batholith 

U.S. GEOLOGICAL SURVEY BULLETIN 1608 
DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE 1984 

For sale by the 

Distribution Branch, Text Products Section 

U.S. Geological Survey 

604 South Pickett St. 

Alexandria, VA 22304 

Library of Congress Cataloging in Publication Data 

Hietanen, Anna Martta, 1909-

Geology along the northwest border zone of the Idaho 

batholith, northern Idaho. 

(U.S. Geological Survey Bulletin 1608) 

Bibliography: p. 16-17 

Supt. of Docs. no.: I 19.3:1608 

1. Rocks, metamorphic. 2. Batholiths-Idaho. I. Title. II. Series: 

United States. Geological Survey. Bulletin 1608. 

QE75.B9 no. 1608 557.3s 84-600041 

[QE475.A2] [552' .4] 
CONTENTS 

Abstract 1 

Introduction 1 

Earlier work 2 

Stratigraphy, lithology, and correlation 2 

Belt Supergroup 2 

Boehls Butte Formation 5 

Structures 12 

Metamorphism 13 

Age of plutonism and metamorphism 14 

References cited 16 

FIGURES 

1. Index map of northern Idaho 1 

2. Geologic map of the St. Joe-Clearwater region 6 

3. Geologic map of area along the east contact of the Boehls Butte Formation 

(Canyon fault) 10 

TABLES 

1. Generalized stratigraphic sequence of the Belt Supergroup northwest of the 

Idaho batholith 3 

2. Generalized section of the Boehls Butte Formation 5 

3. Composition of garnet and FeO/MgO ratios in biotite, chlorite, and cordierite 

in massive anorthosite and its foliated border zone 9 

4. Strontium-isotopic ratios in anorthosite 12 

Contents III 

GEOLOGY ALONG THE NORTHWEST BORDER 

ZONE OF THE IDAHO BATHOLITH, 

NORTHERN IDAHO 

By Anna Hietanen 

Abstract 

The metasedimentary rocks bordering the Idaho 

batholith on the northwest are correlative with the five 

lowest formations of the Belt Supergroup in the Coeur 

d'Alene area, to the north. These rocks are underlain by 

the Boehls Butte Formation, exposed in an uplifted block 

between the North Fork and Little North Fork of the 

Clearwater River. The metamorphic grade of the rocks of 

the Belt Supergroup in the study area ranges from 

greenschist facies near the St. joe River in the north to 

sillimanite grade of amphibolite facies near the Idaho 

batholith in the south. In the schist of the Boehls Butte 

Formation, all three aluminum silicates-kyanite, andalusite, and sillimanite-occur together with staurolite, cordierite, garnet, muscovite, and biotite. Lenses of twoplagioclase anorthosite occur in this schist. 

The study area is along the west margin of the Precambrian continent and within the area where two arcuate 

segments of Cordilleran trends meet. Owing to the complexity of this position and several episodes of deformation, the structures are intricate. In the southern part of 

the study area, fold axes and lineations parallel the northeastward trends that are prevalent in Permian to jurassic 

rocks to the south and southwest. In the northern part of 

the study area, northwestward trends are predominant, 

and northeast-trending fold axes and lineations occur locally. These northwestward trends are present as a second 

lineation in the southern part of the area. These structures 

could have been formed in response to the collision and 

accretion of Permian and early Mesozoic units to the continent. Locally, east-plunging fold axes and lineations 

occur in addition to either northwestward or northeastward trends. In the uplifted Boehls Butte block, this N. 

70'-90' W.-trending lineament is a distinct structural feature. In some places it is older, but in other places 

younger, than the northwest- and northeast-trending fold 

axes and lineations. Folds on north-south-trending axes 

occur near some faults. 

INTRODUCTION 

This report is based on petrologic and structural 

studies during 1950-64, 1966, and 1968 in western 

Clearwater County and in southern Shoshone County 

south of the St. 1 oe River, Idaho. Its purpose is to 

record the new data and reconsider the correlation, 

stratigraphy, and some petrologic aspects of the 

metamorphic rocks. Revision of correlation of the oldest metamorphic rocks is supported by fieldwork done 

in 1966 and 1982 and by new information on their constituent minerals. The study area encompasses about 

6,000 km2 between long 115°20' and 116°20' W., and lat 

46°15' and 47°15' N. (fig. 1). 

INDEX MAP OF IOAHO 

Area of figure 2 

0 100 200 KILOMETERS 

0 50 100 MILES 

Figure 1. Index map of northern Idaho. Area of figure 1 is 

shaded in small inset. Area of figure 2 is marked by 

diagonal lines. Study area described previously (Hietanen, 

1962, 1963a, b, c, 1967, 1968) is encircled with broken line. 

Outline of northern part of the Idaho batholith is shown 

by dotted line. 

Introduction 
The tectonic position of the study area, along the 

west margin of the Precambrian continent and within 

an area where two arcuate segments of late Paleozoic 

and early Mesozoic trends meet (Hietanen, 1981), has 

resulted in a complex structural pattern. These arcuate 

segments probably were controlled by the shape of the 

west margin of the North American Continent during 

the time of accretion of Permian and Triassic units. 

During Proterozoic time, a deep embayment that extended into northern Idaho and western Montana provided a basin into which the shelf sediments of the Belt 

Supergroup were deposited. Harrison (1972) bracketed 

the deposition from 1,450 to 850 m. y. ago. The Proterozoic folding, trending N. 70°-90° W., probably reflects the shape of the depositional basin and, later, the 

late Paleozoic and early Mesozoic structural trends 

developed parallel to the arcuate margin of the continent. Structures in the southern arc trend from westem Idaho through Oregon and the Klamath Mountains 

to the Sierra Nevada in California (Hietanen, 1981). 

The northern arc trends through Washington to British 

Columbia. Units of Paleozoic and Mesozoic rocks, 

many of them exotic in origin, collided during platetectonic movements against the arcuate continental 

margin. Parts of these Paleozoic and Mesozoic units 

were accreted to the continent in both the northern and 

southern arcs, but other parts probably were subducted with the oceanic plate that carried them, thus 

giving rise to the extensive Mesozoic plutonism along 

the continental margin. 

EARLIER WORK 

Brief reconnaissance work in Clearwater County 

was done by Anderson (1930), who distinguished two 

units of metasedimentary rocks: The gneissic rocks 

near the Idaho batholith and the Belt Supergroup north 

of lat 46°40'. Ross and Forrester (1947) followed this 

subdivision on their geologic map of the State of 

Idaho. In southern Shoshone County, Calkins and 

Jones (1911) mapped the five lowest formations of the 

Belt Supergroup: the Prichard, Burke, Revett, St. Regis, and Wallace Formations. Umpleby and Jones 

(1923) divided the Wallace Formation into three map 

units and combined the quartzites of the Revett and 

Burke Formations into a single unit that they showed 

to overlie the Prichard Formation. Wagner (1949) 

mapped and described the Burke, Revett, St. Regis, 

Wallace, and Striped Peak Formations on the south 

slope of the St. Joe Mountains north of the St. Joe 

River. Tullis (1944) suggested that the quartzite and 

schist in Latah County could be correlated with the 

lower formations of the Belt Supergroup. Johnson 

(1947) described the igneous metamorphism near 

Orofino. 

STRATIGRAPHY, LITHOLOGY, AND 

CORRELATION 

Belt Supergroup 

The Belt Supergroup formations of Calkins and 

Jones (1911) and Umpleby and Jones (1923) in southem Shoshone County continue southward into Clearwater County, where high-grade metamorphism has 

obliterated the characteristic colors and relict 

sedimentary features-such as ripple marks, mud 

cracks, raindrops, crossbedding, and channels-that 

in the Coeur d'Alene area serve as distinguishing features for the formations. The change from low-grade 

muscovite-chlorite schist and quartzite with relict 

sedimentary structures into coarse-grained high-grade 

schist, quartzite, and gneiss takes place through progressive zones of regional metamorphism. The isograds 

indicating the metamorphic grade cut across the 

stratigraphic layers (Hietanen, 1967, 1968, 1969b). 

Each of the formations consists of interbedded schist 

and quartzite in varying proportions, and preserves its 

characteristic lithology and stratigraphy through the 

metamorphic zones. Because of the absence of relict 

sedimentary structures, the contacts between the formations of the metamorphosed Belt Supergroup as 

shown on my geologic maps (Hietanen, 1962, 1963a, b, 

c, 1968) may differ from the contacts used in subdivision of unmetamorphosed Belt rocks. Therefore, a 

short account of essential lithologic features in each 

formation of the metamorphosed Belt follows. The five 

oldest formations-the Prichard, Burke, Revett, St. 

Regis, and Wallace-were mapped in the eastern part 

of the area (Hietanen, 1968, pls. 1, 2). The Prichard 

Formation consists of a lower schist unit, a middle 

quartzite unit of one or two layers of light-gray foliated 

quartzite, and an upper schist unit (table 1). 

The schist of the Prichard Formation is a coarseto medium-grained muscovite-biotite-garnet schist, 

with fine- to medium-grained interlayers rich in quartz 

and plagioclase. Biotite and muscovite laminae separate individual thin ( 1-4 em thick) beds in the interlayers. Kyanite occurs in places in the northern part of 

the study area, and sillimanite in the southern part. 

The middle quartzite unit consists of two layers of 

foliated white to light-gray medium-grained muscovite 

quartzite with minor biotite and occasional garnetbearing beds, and an intervening layer of muscovitebiotite-garnet schist ranging from almost zero to 100m 

in thickness. 

Coarse-grained light-gray distinctly bedded and 

foliated quartzite that overlies the upper schist unit of 

the Prichard Formation was mapped as the Burke 

Formation. In the quartz-rich layers, individual beds 

of coarse-grained quartzite, 3 to 20 em thick, are sepa2 Geology Along the Northwest Border Zone of the Idaho Batholith, Northern Idaho 
Table 1. Generalized stratigraphic sequence of the Belt Supergroup northwest of the Idaho batholith 

llnit Estimated thickness (1n) Rock type 

Upper schist 200 

IIpper quartzite ?00 

In southern part of study area, garnet-mica schist 

containing aluminum silicates, with some thin 

interbeds of biotite-plagioclase gneiss; not 

exposed in northernmost part of study area. 

In western part of study area near the Elk River, 

abundant thin-berlrled biotite gneiss, biotite 

quartzite, and schist, interbedded with scapolitebearing diopside gneiss; in southern part of study 

area, mainly diopside-plagioclase gneiss containing some biotite and hornblende gneiss; not exposed in 

northernmost part of study area. 

~Ia 11 ace Formation 

800-2.,600 

Lower schist 

Lower quartzite 

St. Regis 

Formation 

20fl-300 

200-1,900 

100-300 

200-500 

In northern part of study area, fine-grained 

muscovite-biotite schist; grain size coarsens southward; garnet, staurolite, kyanite, and 

sillimanite appear, in that order, with increasing 

temperature and pressure. 

White granular quartzite, overlain in northern part 

of study area by thin-bedded gray biotite quartzite, 

biotite-granofels, and phlogopite-bearing carbonate 

yranofels gneiss (locally containing scapolite and 

actinolite), anrl in southern part of study area by 

interbedded diopside gneiss and biotite gneiss; the jio~side ~neiss, biotite granofels, and carbonate 

granofels contain scapolite. 

Garnet-mica schist with some layers of biotite 

gneiss or biotite quartzite. 

Ravalli 

Group 

Revett 

Formation 500-2,100 

Thick-bedded· coarse-grained pure quartzite; 

muscovite laminae separate individual beds. 

Burke 

Formation 

llpper schist 

200-1 ,300 

600-900 

Iipper ?00 rn is medium-grained biotite schist; lower 

part is micaceous quartzite, with some thick beds 

of pure quartzite. 

Garnet-mica schist, with layers of medium- to darkgray thin-bedded fine-grained biotite quartzite in 

upper part. 

Prichard 

Formation 

1,470-l,300 

White to light-gray beds of medium-grained granular 

to foliated muscovite-bearing quartzite with 

intervening layer of schist; uppermost part 

consists of thin-bedded biotite-rich quartzite. 

Middle quartzite 70-200 

Lower schist 800-1,200 

rated by thin micaceous layers. The upper part of this 

formation consists of garnet-mica schist with interbedded quartzite. 

The schist of the Burke Formation is overlain by 

thick-bedded white to very light gray, medium- to 

coarse-grained quartzite of the Revett Formation. 

Thick beds of pure quartzite are separated from one 

another by paper-thin micaceous laminae that in places 

contain some plagioclase (An10). The St. Regis FormaGarnet-mica schist containing some biotite quartzite; 

locally rich in kyanite. 

tion, stratigraphically above the Revett Formation, 

consists of medium- to coarse-grained garnet-mica 

schist with interbedded fine- to medium-grained 

micaceous quartzite. 

The Wallace Formation overlies the schist of the 

St. Regis Formation. The lowest part of the Wallace 

consists of medium-grained white granular quartzite 

with some thin light-gray quartzite layers. Thinbedded gray quartzite with interbedded thin micaceous 

Stratigraphy 1 Lithology 1 and Correlation 3 
layers is the major rock type above the white granular 

quartzite. Toward the top of the unit, scapolite and 

carbonate-bearing layers are common (Hietanen, 

1967). Prisms of actinolite, hornblende, and diopside 

crystallized in carbonate-bearing layers in the kyanite . 

zone and higher grade zones to the south. The schist 

unit that overlies the quartzite is mineralogically different throughout the study area. Another quartzitegneiss unit and schist unit overlie the lower schist unit 

in the southern part of the study area. 

The schist unit overlying the quartzite of the Wallace Formation in the northeastern part of the study 

area southeast of A very is a fine-grained dark 

muscovite-biotite schist in which metamorphism increases to the south through successive zones, as 

shown by the isograds in figure 2. Along the St. Joe 

River west of A very and to the south, the Ravalli 

Group (the Burke, Revett, and St. Regis Formations) 

is exposed under the lower quartzite unit of the Wallace Formation. The schist of the Prichard Formation, 

with its interbedded layers of quartzite, underlies the 

Ravalli Group (Hietanen, 1967). The continuity with 

correlative formations to the southeast is interrupted 

by the Roundtop pluton and by an uplifted block of 

pre-Beltian rocks, the Boehls Butte Formation (fig. 2). 

The lithology and thickness of the formations in the 

two parts of the study area are similar, but the degree 

of metamorphism is lower in the northern part. 

A series of faults belonging to the White Rock 

fault zone (fig. 2) separates high-grade rocks of the 

Prichard Formation from low-grade rocks of the Wallace Formation to the west in the northwestern part of 

the study area (fig. 2). The Wallace Formation there 

consists mainly of a gray thin-bedded fine-grained 

micaceous quartzite in which ripple marks, mud 

cracks, crossbedding, and channel features are preserved. Interbedded are layers rich in carbonate minerals and scapolite (Hietanen, 1967) and some units 

consisting of white to very light gray or beige granular 

quartzite and others rich in muscovite and biotite. A 

very fine grained dark-gray to black scapolite-bearing 

muscovite-biotite schist overlies the quartzite unit of 

the Wallace Formation conformably on the east. Near 

Huckleberry Mountain, this low-grade schist unit is 

separated from high-grade schist of the Prichard Formation by the northern part of the White Rock fault. 

With increasing metamorphism to the south, all 

the rocks in all formations became coarser grained, 

and such minerals as actinolite, hornblende, and diopside crystallized in the carbonate-bearing quartzitic 

layers, whereas garnet, staurolite, and kyanite appeared in the aluminum-rich schist layers. First appearances of these index minerals are shown by isograds (fig. 2; Hietanen, 1967, pl. 1) that transect the 

south-southeast-trending structures which wrap 

around the northwest corner of the Idaho batholith. 

South of the East Fork of Potlatch Creek, the schist 

units are recrystallized to coarse-grained garnetsillimanite schist, with interbedded layers of laminated 

biotite-plagioclase schist and gneiss (Hietanen, 1963b). 

The quartzitic units contain layers rich in diopside and 

plagioclase, or in biotite and plagioclase. Recrystallized white granular quartzite can be recognized in 

several localities in the canyon of the North Fork of 

the Clearwater River, which cuts through folded and 

faulted sillimanite-grade rocks of the Wallace Formation for more than 50 km. Farther southeast, in the 

Headquarters quadrangle, sillimanite-garnet schist, 

quartzite, and gneiss are the major rock types in the 

Wallace Formation (Hietanen, 1962). 

Massive coarse-grained quartzite that I mapped as 

the Revett Formation is folded and faulted with the 

Wallace Formation east of the mouth of the Little 

North Fork of the Clearwater River, in the vicinity 

where the north-northwestward trends tum to the east, 

wrapping around the southwest corner of the uplifted 

Boehls Butte block (fig. 2). As mapped, this quartzite 

sequence probably includes beds that may belong to 

the Burke Formation, a formation which was not identified in the southern part of the area (Hietanen, 1963a, 

b, c; 1967, pl. 1). The schist above the Revett was 

mapped as the St. Regis Formation. 

The metasedimentary rocks included in the rocks 

of the Idaho batholith east of Pierce (Hietanen, 1963c) 

consist mainly of diopside and biotite gneiss, with interbedded garnet-mica schist that locally contains sillimanite. These rocks are similar to the Wallace Formation in the Headquarters quadrangle and near Elk 

River. They are underlain by rather thin units of schist 

and coarse-grained quartzite, mapped as the Revett 

Formation (Hietanen, 1963c, pl. 2). 

Anderson (1930) named the metamorphic rocks 

near Orofino (fig. 1) the "Orofino Series" and 

suggested that they may be the lowest part of the 

Prichard Formation not exposed elsewhere or, alternatively, that they may be much younger than the Belt 

Supergroup. 

Hietanen (1962, pl. 3, 4) mapped as the Revett 

Formation the coarse-grained quartzite exposed near 

Dent (northeast of Orofino, fig. 1), on Huckleberry 

Butte, and along Orofino Creek at Lime Mountain 

(east of Orofino). The thin-bedded biotite-hornblende 

and biotite-diopside gneiss with scapolite-bearing 

layers overlies the Revett Formation at Lime Mountain and near Dent. The stratigraphic position and 

lithology of this thin-bedded gneiss-quartzite sequence 

suggest that it is equivalent to the lower quartzite unit 

of the Wallace Formation. A similar thin-bedded 

schist-and-gneiss sequence is exposed 4 to 5 km east of 

Orofino; its possible equivalence to the lower 

4 Geology Along the Northwest Border Zone of the Idaho Batholith, Northern Idaho 
quartzite-gneiss unit of the Wallace Formation was 

suggested by Hietanen (1962, p. 46-48). This correlation was adopted on later maps (Hietanen, 1967, pl. 1) 

on which the metamorphic rocks near Orofino are 

shown as the Wallace Formation and the gneissic 

plutonic rocks as gabbro, quartz diorite, and tonalite. 

Metamorphosed peridotite and gabbro exposed in the 

canyon of the Clearwater River west of Orofino probably mark the west margin of the Proterozoic formations. Layers of metatuff, some containing euhedral 

plagioclase crystals, near Peck, 13 km east of Orofino, 

are similar to the Permian rocks to the south. 

Boehls Butte Formation 

The rocks in an uplifted block south of the Roundtop pluton (fig. 2) differ mineralogically and chemically 

from the Belt Supergroup and were named the Boehls 

Butte Formation (Hietanen, 1969b). The major rock 

types are coarse-grained aluminum silicate-rich schist, 

layered and massive anorthosite, and quartzite that 

locally contains calcium silicates (table 2). 

The Boehls Butte Formation is separated from the 

lower part of the Belt Supergroup by faults and igneous masses. The Orphan Point fault, the large northsouth-trending fault near the center of the study area 

(fig. 2), separates the Boehls Butte Formation from 

schist of the Prichard Formation on the west. A wide 

migmatized zone along the south border marks the 

North Fork fault (fig. 2) between schist of the Boehls 

Butte Formation and the Wallace Formation to the 

south. Roundtop pluton and associated gabbro bodies 

are along the north margin of the Boehls Butte Formation. 

On the east side, no definite contact was located 

during early geologic mapping of the Boehls Butte area 

(Hietanen, 1963a) because of an absence of exposures 

and the similarity of the schist and quartzite on either 

side of the faults. Additional fieldwork (fig. 3), however, shows that the Boehls Butte Formation is, indeed, bounded by a fault on the east. Southwest of 

Miller Peak (fig. 3), a long narrow body of pyroxene 

gabbro, and plagioclase porphyry north of the gabbro, 

occur along a fault that separates white coarse-grained 

muscovite-bearing quartzite and strongly deformed 

muscovite-biotite schist of the Boehls Butte Formation 

on the west from light -gray medium-grained thinbedded quartzite and interbedded garnet-mica schist of 

the Prichard Formation on the east. To the south, 

deeply weathered gneissic plagioclase-rich 

hornblende-biotite tonalite occurs along this fault 

zone, which farther south passes just west of Indian 

Dip, follows Delate Creek to the southeast, and 

crosses the Little North Fork of the Clearwater River 

at the mouth of Minnesaka Creek. This fault zone is 

Table 2. Generalized section of the Boehls Butte Formation 

Unit Rock type Thickness (m) 

Upper schist----- t~a rnet-b i ot i te-muscovite schist, in part 800 

laminated with thin biotite-plagioclasequartz layers; locally rich in kyanite. 

Ouartzite-------- Thin-bedded fine-grained biotite quartzite 300-ROO 

and coarse-grained white to very light 

gray foliated quartzite; interbedded 

locally are lime silicate rocks, 

phlogopite- or biotite-plagioclase 

rocks, and kyani te-andal us i te-si ll imanite 

schist containing plagioclase. 

Fe 1 dspathi zed Coarse-grained kyanite-andal usite- 0-200 

schist. sillimanite-garnet-mica schist containing 

plagioclase (An 25_45 ) and cordierite 

or staurol1te. 

Anorthosite------ Layered and fo 1 i a ted or massive bimoda 1 or 800-2,500 

trimodal plagioclase rock. In the layered 

rocks, white to light-gray layers consisting 

of large oval grains of andesine (An40-45) with inclusions and groundmass of bytown1te 

and anorthite ( An 80_95 ) are interbedded 

with thin layers nch in aluminum silicates, 

hiotite, and muscovite, or in hornblende. 

Massive parts are composed of coarse-grained 

bimodal andesine-bytownite rocks or trirnoda l 

labradorite rocks, with anorthite-andesine 

inclusions. Layers of bytownite-anorthite 

rocks containing garnet and quartz or 

containing hornblende occur along contacts. 

Lower schist----- Andalusite-kyanite-sillimanite-biotite- 400-600 

muscovite schist, locally containing 

cordierite and staurolite. Contains 

plagioclase and small lenses of anorthosite. 

continuous with the Canyon fault, mapped earlier 

along this creek (Hietanen, 1968, pl. 1). The Canyon 

fault continues more than 30 km farther to the eastsoutheast (Childs, 1982) and crosses the North Fork of 

the Clearwater River at the mouth of Kelly Creek, 10 

km east of the map area (fig. 2). Near Canyon Ranger 

Station, this major fault separates the Prichard Formation on the north from the Wallace Formation on the 

south (fig. 2; Hietanen, 1968, pl. 1). To the northwest, . 

along Dog Creek, the Prichard Formation continues on 

the northeast side of the Canyon fault, but on the 

southwest side of the fault lies the Boehls Butte 

Formation-not the Wallace-separated from the Wallace Formation by the Beaver Creek pluton (fig. 2; 

Hietanen, 1968, pl. 1). Porphyritic dike rocks, 

pyroxene gabbro, and amphibolite are common along 

this fault. 

In the north, the Canyon fault is cut off by the 

Roundtop pluton but seems to continue on the pluton's 

northwest side as north- and northwest-trending faults 

that cut the Ravalli Group and Wallace Formation (fig. 

2; Hietanen, 1967, pl. 1). 

The schist of the Boehls Butte Formation is 

coarse-grained muscovite-biotite-garnet schist; near 

the anorthosite masses it contains plagioclase 

(Anz5-45), all three aluminum silicates (andalusite, 

kyanite, and sillimanite), and locally as much as 40 

percent cordierite (Hietanen, 1956, 1963a, 1969b). 

Staurolite occurs with the aluminum silicates in the 

northern part. The proportion of plagioclase and its 

anorthite content increase toward the anorthosite 

masses (Hietanen, 1969b, p. 378). The upper part of 

Stratigraphy, lithology, and Correlation 5 
Q, 

= 0 

~ 

~ 

rJ) = 

EXPLANATION 

Columbia River Basalt Group 

Dikes and siUs 

Range from gabbro to granite In composition 

Plutonic rocks related to Idaho batholith 

Range from quam diorite to granite In composition 

Garnet amphibolite 

Wallace Formation 

Thin-bedded calcareous quamlte and schist 

Ravalli Group 

St. Regis, Reuett, and Burke Formations, undivided 

r.=:-l 

t=:.J 

Prichard Formation 

Garnet-mica schist and noncalcareous quamlte. 

Shown by dots In the central part 

Boehls Butte Formation 

bbq, quamlte with layers rich In dlopslde and actinolite 

bba, anorthosltewlth two kinds qfplagloclase-bytownlte 

and andesine-and hornblende 

bbs, coarse garnet-mica schist with abundant aluminum 

silicates 

Contact 

Dashed where approximately located or Inferred 

Fault 

Dashed where approximately located or inferred 

Anticline 

* Syncline 

Showing plunge 

15 

__J_ 

Strike and dip of beds 

__2'5 

Strike and dip of beds and plunge of lineation 

~ 

Isograd 

~ 

Isobar 

Teeth on higher pressure side 

u 

0 

N 

0 

c::: 

g 

c::: 

Q. 

~ IS 

47°00' 

Figure 2. Geologic map of St. joe-Clearwater region, showing the uplifted Boehls Butte Formation and metamorphic 

zones. A, kyanite-andalusite zone; B, zone where kyanite, andalusite, and sillimanite occur together; C, sillimanitestaurolite zone; D, kyanite with staurolite or with sillimanite; E, kyanite-almandite zone. Modified from Hietanen 

(1969b). 

6 Geology Along the Northwest Border Zone of the Idaho Batholith, Northern Idaho 
50' 40' 

.St Joe River 0 5 10 15 20 KILOMETERS 

10 MILES 

.. 47°00' 

Figure 2. Continued. 

Stratigraphy, Lithology, and Correlation 7 
the schist in a syncline between two southern anorthosite masses is a distinctly laminated coarse-grained 

garnet-mica schist. Thin (1-3 mm thick) layers that 

consist of plagioclase (An25-as) and quartz are separated by biotite-muscovite laminae. Thick beds of this 

laminated schist are well exposed on the south slope of 

Smith Ridge. Abundant kyanite in elongate grains with 

irregular outlines and ranging from 0. 5 to 3 mm in 

length occurs in strongly foliated biotite-muscovite 

schist at the mouth of Minnesaka Creek (fig. 3). Muscovite schist on the west side of the Canyon fault south 

of the Roundtop pluton (figs. 2, 3) shows a strong postcrystallization deformation and rolling. Thin sections 

show an intensive wrinkling and strongly bent large 

muscovite flakes among strained quartz grains. 

The quartzite of the Boehls Butte Formation includes coarse-grained pearly-white to very light gray 

muscovite-bearing layers and fine-grained gray 

biotite-rich layers. Large clear grains of quartz are 

scattered in grayish-white muscovite-bearing quartzite 

west of Miller Peak and east of Monumental Buttes 

(fig. 3). Locally, calc-silicate-bearing quartzite and 

gneiss occur in the coarse-grained white layers; the 

largest occurrence is at Cedar Creek (fig. 2). 

Three large and several small lens-shaped masses 

of anorthosite are interlayered with the aluminumsilicate-rich schist (Hietanen, 1963a). Exposures along 

new logging roads show that the western part of the 

middle body is continuous with the southward extension of the northernmost mass (Nord, 1973). Most 

rocks of the middle and southern masses are distinctly 

layered and consist of white to light-gray layers, 5 to 50 

em thick, of andesine-bytownite rock and thin dark 

layers that are rich in biotite and muscovite or in 

hornblende (Hietanen, 1963a). In the light-colored 

layers, calcic plagioclase, commonly An85_95 (Nord, 

1973), forms a fine-grained groundmass between large 

oval andesine (An4o-45) grains and occurs as irregularly 

shaped and lamellalike inclusions in andesine. Both 

types of plagioclase occur also in the thin dark layers, 

which locally contain all three aluminum silicates, with 

muscovite and biotite and such rare assemblages as 

kyanite-hornblende and kyanite-anthophyllite-cordierite. 

Anorthosite in the northernmost large body is 

coarse grained and massive, and consists of andesine 

or labradorite with inclusions and interstitial grains of 

bytownite and (or) anorthite. Many of the inclusions 

have kyanite centers. Generally, fewer interstitial 

small grains of bytownite or anorthite occur between 

the large labradorite crystals than between the andesine grains. 

Nord (1973) made a detailed electron-microprobe 

study of the plagioclase in the two northern masses 

and found three distinct compositions in lath-shaped 

labradorite crystals: the host grains consist of An55-65, 

inclusions of An9o- 95 , and rims of A~o--45 around the 

inclusions. These crystals are trimodal and contain 

numerous rimmed inclusions of calcic plagioclase, 

several of which include kyanite or andalusite. In some 

localities, the lath-shaped and subhedral labradorite 

crystals are twinned according to the Carlsbad, albite, 

and complex twin laws, as is typical of plagioclase 

crystallized from a magma. It may be that an anatectic 

labradorite magma was formed locally by melting of 

the metasedimentary parent rocks and that calcium 

from andesine rims combined with aluminum and silicon to form anorthite shells around the remnants of 

Al-silicate. 

Electron-microprobe analyses of other constituent minerals in several specimens I collected 

(Hietanen, 1963a) from massive anorthosite and its 

foliated and layered border zone were kindly provided 

by Leonid Perchuk. Calculation of these (table 3) 

shows that the composition of garnet varies widely. In 

the massive anorthosite on Rocky Run Creek (loc. 

1696), grossularite containing 7.7 percent almandite 

and 0.7 percent pyrope occurs with garnet that is a 

mixture of almandite, pyrope, grossularite, and spessartite (sample 1696, table 3). Pink grossularite in 

layered andesine-bytownite and anorthite rocks along 

the border zone 3 to 4 km east of Orphan Point contains 14 percent almandite and less than 1 percent each 

of pyrope and spessartite (sample 1352, table 3). The 

associated minerals are hornblende, diopside, and 

zoisite (see Hietanen, 1963a, f1

ig. 40). Garnet in micaceous layers that contain staurolite and aluminum silicates at this locality (see Hietanen, 1963a, figs. 39, 41) 

is almandite containing some pyrope and grossularite 

(sample 1353, table 3). Large garnets in border-zone 

rocks rich in biotite, chlorite, and kyanite near Monumental Buttes (sample 1319) are composed of about 60 

percent almandite, 28.5 percent pyrope, and 11.4 percent grossularite. Large garnets at Goat Mountain contain 10 percent spessartite (sample 971). 

The FeO/MgO ratios in biotite, chlorite, and cordierite vary considerably (table 3). In sample 1353, 

biotite and chlorite are relatively rich in iron, whereas 

in sample 971 both of these minerals and the associated 

garnets are rich in magnesium. Cordierite in the massive anorthosite is an iron-rich variety, whereas most 

cordierite in the border-zone rocks is rich in magnesium. 

Staurolite containing about 1 weight percent MgO 

occurs in the schist under the northernmost anorthosite mass and in its border zone. Garnet in this schist 

(spec. 1368) is almandite containing 9 percent pyrope, 

15 percent grossularite, and 1 percent spessartite. Associated biotite and chlorite are rich in iron, and their 

respective FeO/MgO ratios are 5.42 and 5.97. 

8 Geology Along the Northwest Border Zone of the Idaho Batholith, Northern Idaho 
Table 3. Composition of garnet and FeO/MgO ratios in biotite, chlorite, and cordierite in massive anorthosite and its 

foliated border zone 

[Calculated from electron-microprobe analyses by L. Perchuk] 

Rock type---- Massive anorthosite Layered border zone Border zone Plagioclase-biotite- rich in biotite of anorthosite and Al-silicates kyanite-garnet 

Location------ Rocky Run Creek 3 km east of Orphan Point Monumental Buttes Goat Mountain 

Sample--------

Mineral-------

Almandite-----

Pyrope--------

Grossul arite--

Spessart ite---

Garnet 

64.67-52.05 

17.73-20.90 

13.55-25.09 

4.05-1.96 

1696 

Grossularite 

7.70 

.fi6 

91.51 

.13 

Biotite----------------------------------------

Chlorite------ 1.58 

Cordierite---- 1.71 

1353 

Garnet 

79.70-62.90 

6.86-18.89 

10.91-25.03 

2.53-0.0 

Average 

1.93 

3.69, 2.24 

Layers and lenses of bytownite anorthosite, containing either garnet, biotite, and quartz or hornblende 

and biotite, are interbedded with the aluminumsilicate-rich garnet-mica schist beneath the large anorthosite masses and along their border zones. Some of 

these grade to two-plagioclase anorthosite by replacement of part of the bytownite by andesine. A 

medium-grained light-bluish-gray bytownite anorthosite with layers of foliated hornblende-bytownite 

gneiss is exposed along a tributary to the West Fork of 

Cedar Creek. The possibility of an igneous origin of 

the bytownite anorthosite was discussed earlier 

(Hietanen, 1963a, p. 69). 

The long thin bodies of garnet amphibolite and 

hornblende-plagioclase gneiss in the southern part of 

the northernmost large anorthosite mass could represent cogenetic gabbroic rocks. However, only one 

kind of plagioclase-labradorite-occurs with the 

hornblende, garnet, and quartz in the garnet amphibolite, which locally cuts the anorthosite and thus is 

younger. The relict sedimentary structures, such as 

paper-thin micaceous laminae and the large amount of 

aluminum silicates in the anorthosite, also speak 

against an igneous origin. _ 

Juras ( 197 4) chose shear differentiation and exsolution of igneous labradorite into anorthite and andesine 

to explain the unique textures of the two-plagioclase 

anorthosite. The possibility of exsolution and arguments against it were discussed earlier by Hietanen 

(1963a, p. 65). One of the main arguments against an 

1352 

Grossularite 

14.24 

.78 

84.11 

.87 

FeO/MgO ratios 

1319 

Garnet 

61.12-59.43 

?9 .OR-?7 .69 

10.07-12.69 

.o 

1.16 

.59 

971 

Garnet 

50.00 

24.13 

15.98 

9.89 

.84 

.54 

49.16 

25.96 

14.84 

10.04 

igneous origin and later exsolution is the large variation in andesine/calcic-plagioclase ratio, resulting in 

notable inhomogeneity of bulk composition in various 

parts of the masses. Moreover, if exsolution of labradorite to andesine and anorthite were possible under 

certain physicochemical conditions, then one would 

expect to find it in some other anorthosite occurrences 

and in some gabbroic and noritic rocks. Because this is 

not the case, we should view with scepticism any 

mode of origin that involves exsolution until such a 

possibility is proven in the laboratory or found 

elsewhere in nature. 

The distinct layering, the abundance of aluminum 

silicates, the presence of two or three types of plagioclase with unique textures, the occurrence of lenses 

of bytownite anorthosite containing quartz, garnet, 

and hornblende along the contacts of large masses, the 

varying composition of garnet, and the abundance of 

secondary andesine in the aluminum-silicate-rich 

garnet-mica schist of the contact aureoles are features 

different from those of any other known anorthosite. A 

metasedimentary origin with later addition of sodium 

to form andesine is therefore preferred for these 

layered anorthosite masses (Hietanen, 1963a, 1969b; 

Nord, 1973; G. L. Nord, Jr., written commun., 1976). 

The potassium-poor igneous rocks related to an early 

phase of the Idaho batholith could have been the 

source of the added sodium. The layered garnet- and 

hornblende-bearing bytownite anorthosite, which occurs along the border zones of the two northern 

Stratigraphy, Lithology, and Correlation 9 
0 

~ 0 ,..... 

~ '¢ 

I{) 

(/) 

a:: 

w 

1-

w 

~ 

0 

....J 

:;;;:: 

'<t 

(/) 

w 

....J 

~ 

N 

(") 

10 Geology Along the Northwest Border Zone of the Idaho Batholith, Northern Idaho 
~ 

a 

~-

Col 

"C 

:r 

~ 

!:': 

~ 0 

0 

~ Col 

&. 

(") 

~ 

~ 

a 

25' 

= 

.... 

.... 

11 

EXPLANATION 

~ 

~ 

Quartz monzonite 

~ 

~ 

Granite porphyry 

~ 

~ 

Pyroxene gabbro 

[2] 

Amphibolite and garnet amphibolite 

~pq 

Prichard Formation 

ps, coarse-grained muscovite-biotite schist with 

garnetiferous beds 

pq, thin-bedded light-gray muscovite-biotite quartzite 

with minor garnet 

bbq 

Boehls Butte Formation 

bbs, coarse-grained garnet-mica schist with kyanite, 

andalusite, and sillimanite near the anorthosite 

bbq, coarse-grained white to very light gray muscovite 

quartzite with minor biotite and fine-grained thinbedded gray biotite-muscovite quartzite 

bbl, lime-silicate rocks, consisting of dlopside, tremolite, anorthite, zoisite, and quartz 

bba, anorthosite; white to light-gray, layered to 

massive andesine-bytownite or anorthite rock with 

minor hornblende 

Contact 

Dashed where approximately located 

Fault 

Dashed where approximately located 

220 _._!0 

Inclined Vertical 

Strike and dip of beds and plunge of lineation 

40 

~ 

Strike and dip of foliation and plunge of lineation 

>- IX 

<C 

i= 

IX 

LL.I 

1-

(,/) 

u::l 

-o 01.1.1 Nu 

LL.IIJ,J 

~~ 

I-IX Ou 

IXO 

Q..lu 

0 

N 

0 

IX 

LL.I 

1-

0 

IX 

Q.. 

~· 

,';t~;';~Ln i T ( O,';N 

•' 

··.; 

l',, 

\ 

\' 

. ' , 

·<ll 

. '" " 

~~ $I r.. ~~ 30 '" ,, ~ . ••. ;·)'/\', ,,.~ 4~· ', 

") \ / ·. ,J;" .. '' ~-"'::: :, 'L,,,,,, t/'',\ ', ' ,J:~'9' . ' , .. , ' ~ -,- ' ' ' 

,·--'":·-···-.·T, ·_I' ·.·f,l cf:' ~~-.· __ ;.J ' . ~ Dip ~()' I· 20' ' ' 

. "'1·\- i j 10,._'\0. ·',i 

~-: 0 '. N.-?<l ,I 

·~ '\ 

80 

.-nro.· --~ -.. . .·. i.l!O-' 30 -t !, \ I 

115°45' 

, .. ,

\:~'' 

~ ; /f~ ,.. 

~. '. '• ' .. ..,, '.,'_,. I ~~ ·:_ I'> :,) } -(j!JY .. # 0 l t• 'I i&.~'Y Q ·\ ', A, 

Getav\!ay : ~ ·~ .f~ 1 -tff"<J' ' I I • ' .,.... ' ~ •' r$7_ ~'l··~... . . ""·~---~~ ' ': ~ ,, Jf~~~~ •- --~-~, ~3'5/•of.)f' ) ~~... 40 " , . ., r~,. ' "'" " ' ; 0; .. ~~..z· '-"~ • ~-._,>I ~,:.:~ , 20 4 f~\i..i' '~t:~ .·. 14 ,.,,~\:~. 

:f'·-~~4 A?ulj~·~~-~ .....-;,.., 

' /~~:: 

SMTfH-: 

---. '.E.~cl'~~': f-. r-.20 30' 

E A .f( 

ips 

"' I 

\60 

46°55' 

~~.f.._ 

115°40' 

Figure 3. Fault contact (Canyon fault) between the Boehls Butte and Prichard Formations in the vicinity of Miller Peak and Indian Dip . 
masses and in many small bodies and thin layers in the 

underlying schist, could have formed from interbedded 

calcareous and aluminum-rich shale. Migration of 

aluminum from pelitic layers over a short distance to 

calcareous shale would produce calcic plagioclase, 

part of which was later replaced by andesine. Excess 

calcium would have entered the hornblende that is 

common in all layers. None of the ilmenitic iron ores 

that are commonly associated with igneous anorthosites occur in this area. Partial melting and mobilization could account for the massive appearance of parts 

of the northern masses. 

The strontium-isotopic ratios (table 4) in the massive anorthosite are similar to those in the layered 

anorthosite; both average 0.7046. In the bytowniterich border-zone rocks, however, this ratio is higher, 

averaging 0.7061 and supporting the contention of a 

mixed origin for these anorthosite masses. 

Table 4. Strontium-isotopic ratios in anorthosite 

[Analyst, Stanley A. Heathl 

Sample 87 sr;86sr Average 

Massive anorthosite 

Northern body-------- 1799 0.7049 } 1814 • 7039 

Middle body---------- 1419 .7039 

0.7046 

Layered anorthosite 

Southern body-------- 605 • 7040 

600 .7049 } 609 .7049 

.7046 

Border zone--------------- 1362 .7064 

11i47 • 7058 } .70fil 

STRUCTURES 

Multiple sets of structures are apparent in the 

study area; two or three sets of folds and lineations can 

be observed in many outcrops. Lineations are parallel 

to and interchangeable with the fold axes, and so the 

fold axes in a given outcrop are represented by lineations in nearby outcrops. Four major trends of fold 

axes and parallel lineations are common: westerly (N. 

70°- 90° W.), northwesterly, northeasterly, and northsouth. The age relations between these four major sets 

of folds are complex. In the central part of the study 

area and in the Boehls Butte block, large gentle folds 

with easterly plunges are modified by southeastplunging northwest-trending folds, which are therefore 

younger. In many outcrops in the northern part of the 

study area, overturned and recumbent folds of outcrop 

size on northwesterly axes are. refolded by westnorthwest- or northeast-trending folds, whereas in 

many other outcrops, axial planes of small overturned 

west-northwest- and northeast-trending folds are refolded by northwest-trending folds. I concluded earlier 

(Hietanen, 1961c, 1963b, 1968) that these three sets of 

folds were formed simultaneously over a long period. 

Folding on north-south axes may be related to faulting 

and is in places older than the northwest-trending 

folds, but some late folding and doming on north-south 

axes also occurs. 

One of the oldest (Proterozoic?) sets of fold axes 

trends N. 70°-90° W. and thus parallels the long axis of 

the basin into which the sediment of the Ravalli Group 

was deposited (Harrison, 1972; Armstrong, 1975). 

Armstrong's (1975) suggestion that the south limit of 

this depositional basin was a short distance to the 

south of the south border of Clearwater County agrees 

with the thinning of the Ravalli Group toward the 

south in the study area (Hietanen, 1963c, pl. 2). Late 

folds on N. 70°-90° W. axes also are common and may 

represent rejuvenation of the earlier structures. 

In the uplifted Boehls Butte block, the distribution 

of sheetlike masses of anorthosite (Hietanen, 1963a) 

shows a gentle folding on east-plunging axes. The two 

large northern masses are exposed in the nose of a 

double-crested anticline, and the southernmost mass 

that is exposed along the canyon of the North Fork of 

the Clearwater River has an anticlinal structure (fig. 2). 

The laminated biotite-muscovite-garnet schist on 

Smith Ridge-stratigraphically above the anorthosite 

and quartzite overlying it-forms a syncline between 

the two anticlines (fig. 2). In the central part of the 

Boehls Butte block, these structures have been modified by northwest folding, and in the northwestern and 

southern parts by gentle doming . 

The schist that is exposed to the west of the two 

large northern anorthosite masses underlies them. 

Reid and others (1973) reported a pre-Beltian 206 Ph/ 207 Pb age of 1,625 m.y. for the anorthosite at Goat 

Mountain and a somewhat older age of 1,665 m.y. for 

the underlying schist at O'Donnell Creek. Quartzite 

overlying the anorthosite to the east is folded on an 

east-plunging axis and locally has an earlier wrinkling 

parallel to south-plunging axes. The schist farther east, 

west of Indian Dip and near Getaway Point (fig. 3), and 

that east of Smith Ridge are folded on southeastplunging axes. On Smith Ridge, the laminated coarsegrained garnet-biotite-muscovite schist is folded and 

wrinkled on axes that plunge 5°-15° E., and these folds 

are overturned to the south. An older wrinkling on the 

north-south axis occurs locally. 

Juras (1974) made a detailed field study of folds in 

the anorthosite area and suggested nine episodes of 

folding, from oldest to youngest: (1) North-southtrending recumbent folds, (2) east-west to northwesttrending recumbent isoclinal folds, (3) northwesttrending tight flexural flow folds, (4) northeast12 Geology Along the Northwest Border Zone of the Idaho Batholith, Northern Idaho 
trending tight to recumbent subisoclinal folds, (5) 

north-south-trending tight to recumbent folds, (6) 

north-south-trending large closed to tight flexural 

folds, (7) northeast-trending gentle to closed parasitic 

folds, (8) west-northwest-trending large concentric 

flexural-slip folds, and (9) northwest-trending chevronlike folds. 

The schist of the Prichard Formation on the east 

side of the Canyon fault, north of Indian Dip, contains 

a set of early folds strongly overturned to the westwith amplitudes of about 1 m-on south-plunging axes, 

and a later set of larger folds on southeast-plunging 

axes (fig. 3). East of Indian Dip, the major fold axes 

plunge 20°-40° SE. 

Two sets of folds can be observed in many outcrops in the northeastern part of the study area, as 

described previously (Hietanen, 1961b, c, 1968). Large 

overturned and recumbent folds on southeast-plunging 

axes have axial-plane cleavages that are refolded on 

south-southwest- or northeast-plunging axes. In some 

places, mica flakes have recrystallized parallel to the 

axial planes of these later small folds, which have 

wave lengths of 20 to 30 em. On Goat Ridge and south 

of Black Mountain, large folds strongly overturned to 

the north have east-plunging axes, and their later small 

folds have northeast-plunging (on Goat Ridge) or 

south-plunging axes. Two sets of small folds whose 

axes plunge east-southeast and northwest have twisted 

the flanks of the large north-south-trending folds east 

of The Nub (Hietanen, 1968, fig. 13). North- or southplunging lineation and a few folds on axes parallel to it 

occur with two other sets of folds. Gentle open folds 

on north-plunging axes occur locally along the North 

Fork of the Clearwater River. Many large blocks bordered by faults seem to form structural units in which 

the trends and roles of major and secondary fold axes 

differ from those in neighboring blocks (Hietanen, 

1968, p. 26). The style of folding changes with the 

grade of metamorphism and with the type of material 

folded (Hietanen, 1961a, c). In places, round flexuralslip folds are common in quartzite layers interbedded 

with thick layers of schist that, instead of folding, 

show intense wrinkling (Hietanen, 1968, p. 25). 

In the western part of the study area, the axes of 

major folds parallel the northwesterly regional trends, 

and their plunges are either northwestward or southeastward, indicating later doming. The flanks of large 

folds are straight, and the hinges are tight; many 25°-

300 -slope hillsides represent dip slopes. Lineation 

parallels the axes of small earlier folds, many of which 

are overturned to the northwest and plunge southwest 

or northeast. In some places, three sets of folds can be 

observed: (1) Small overturned folds with wavelengths 

of 10 to 100 em on northeast axes that plunge 10° SW., 

(2) later large mappable folds on east-west axes, and 

(3)' still later large open folds on northwest axes that 

plunge 10° SE. 

In the southern part of the study area, two sets of 

folds commonly have northeast- and east-plunging 

axes. A strong northeasterly lineation also occurs in 

the igneous rocks near Orofino (Hietanen, 1962, pl. 4). 

The northeasterly lineation in these rocks parallels the 

Permian structural trends at the north end of a major 

Paleozoic and early Mesozoic structural zone that can 

be traced from California through Oregon to Idaho 

(Hietanen, 1981). 

Several active periods of plate convergence, each 

with associated deformation, are known in the northem as well as the southern arcs of the Cordillera during 

Paleozoic and Mesozoic time. It is probably all these 

tectonic events along the continental margin that make 

the structures exceptionally complex in this area, 

which lies within the convergence of the two arcs. 

METAMORPHISM 

The Belt Supergroup rocks to the north of the 

study area were metamorphosed to greenschist facies 

during Middle Proterozoic time, about 1,200 m. y. ago 

(Hobbs and others, 1965). Several episodes of recrystallization resulted from later periods of deformation 

and associated igneous activity. In the study area, the 

degree of metamorphism increases toward the Idaho 

batholith (Hietanen, 1961a,b, 1963b, 1968), and the 

K-Ar ages of biotite and muscovite (76 and 57 m.y., 

respectively) in the kyanite-sillimanite zone north of 

the batholith are within the age range of the intrusion. 

The lower units recrystallized at higher pressure and 

temperature, as shown by the displacement ofisograds 

by younger faults (fig. 2; Hietanen, 1967). A good 

example of this displacement is in the northwestern 

part of the study area, where fine-grained biotite-grade 

rocks of the Wallace Formation on the west side of the 

White Rock fault are juxtaposed against coarsegrained staurolite-kyanite schist of the Prichard Formation on the east (Hietanen, 1963b, 1967). This feature could have been accentuated by a gentle ·dip of 

isograd surfaces due to a possible extension of the 

batholith and its forerunners to the north, under the 

present exposed surface. 

In the northern part of the study area, two 

episodes of metamorphism-a synkinematic and a 

postkinematic--can be recognized in thin sections 

and, in places, mapped in the field (Hietanen, 1968). 

Higher temperatures during the second episode moved 

the isograds farther north. Pseudomorphs of kyanite 

after staurolite and zonal textures in garnets resulted 

from the increase in temperature during the second 

episode. Biotite, garnet, and some plagioclase are 

Metamorphism 13 
characteristic additional constituents in this kyanitestaurolite zone and in the higher grade zones to the · 

south. Muscovite is common in the sillimanite zone 

next to the Idaho batholith; this association indicates 

temperatures below 640°C. Occurrence of potassium 

feldspar with sillimanite in a few places indicates that 

only locally did temperatures exceed 640°C during recrystallization. In this highest grade zone, wollastonite 

crystallized with grossularite and diopside in some calcareous layers of the Wallace Formation near Dent 

(Hietanen, 1967, p. 24). 

The pre-Beltian Boehls Butte Formation contains 

andalusite in addition to kyanite and sillimanite. In 

many places andalusite crystallized from kyanite, but 

in other places all three aluminum silicates seem to 

occur together in equilibrium, indicating pressures and 

temperatures close to the triple point, here estimated 

to occur at a pressure of 0.5 GPa (5 kbar) and a temperature of 580°C (Hietanen, 1956, 1968, 1969a). A 

pressure of0.5 GPa would indicate that about 18 km of 

overlying rocks was removed by erosion after the recrystallization_of these pre-Beltian rocks. 

The absence of andalusite in the Belt Supergroup 

rocks in the study area indicates that the rocks of the 

kyanite-sillimanite zone recrystallized at pressures 

higher than the pressure at the triple point but lower 

. than that of the staurolite-kyanite-sillimanite association. The stability field of staurolite overlaps that of 

sillimanite near the triple point of kyanite, andalusite, 

and sillimanite; and the rare staurolite-kyanitesillimanite association was found near Orofino (Hietanen, 1959). A few small crystals of staurolite and bundles of sillimanite needles occur with kyanite and gedrite (aluminous anthophyllite containing 18 weight percent Al203) in rocks exposed 6 km east of Orofino. 

This mineral assemblage indicates temperatures at the 

boundary of the stability fields of kyanite and sillimanite and within the stability field of staurolite. The 

pressures during recrystallization were higher than the 

pressure at the triple point. Cordierite, which is stable 

at the pressure and temperature of the triple point in 

the Boehls Butte area, was not found near Orofino. 

Aluminous anthophyllite containing 12 weight percent 

Al203 occurs with kyanite and garnet in gneissic rocks 

on the Little North Fork of the Clearwater River 

(Hietanen, 1963a, p. 12). Seams of cordierite separate 

anthophyllite and kyanite on Monumental Buttes. 

Nord (1973) calculated the slopes of cordieriteproducing reactions there and estimated the temperature and pressure of crystallization at 580°C and 0.5 

GPa, close to the temperature and pressure of the triple point. Gamet in garnet-amphibole rocks is almandine containing 18 to 42 percent pyrope and 3 to 17 

percent grossularite (Hietanen, 1959, 1962, table 13; 

1963a, table 4). All these relations indicate maximum 

temperatures of about 650°C and pressures less than 

0.6 GPa in the highest grade zone ne!t !() th~b~!holith. 

The calcium mica margarite occurs in schist of the 

Wallace Formation at the south border of the Boehls 

Butte quadrangle. It forms white bundles with muscovite and probably replaces sillimanite, which occurs in 

similar bundles in nearby outcrops. Skeletal crystals of 

hematite and prisms of rutile replace biotite in the 

margarite-muscovite schist (Hietanen, 1963a, p. 24-

26). 

The west contact of the batholith dips about 45° E. 

At the time of its emplacement, therefore, the 

metamorphic rocks west of the border were structurally under the igneous rocks of the batholith, which in 

the study area consist of coarse-grained hornblendebiotite-quartz diorite (Hietanen, 1962, pl. 2). The Belt 

Supergroup rocks along this western contact zone 

show metasomatic changes due to the introduction of 

mainly water, iron, magnesium, calcium, and sodium 

and the removal of silicon. Tbis exchange of material 

has resulted in local crystallization of secondary 

hornblende, biotite, and plagioclase in quartzite and 

biotite schist (Hietanen, 1962). Addition of water into 

diopside-plagioclase quartzite and gneiss changed 

diopside to hornblende in many places in the Headquarters quadrangle. 

Most metasomatic changes west of the batholith 

are postkinematic, and the secondary minerals commonly occur along structural surfaces that facilitated 

migration of the material introduced. Thus, postkinematic elevated temperatures resulted in an 

isochemical second episode of recrystallization in the 

outer part of the contact aureole of the batholith, and 

in crystallization of metasomatic minerals in the inner 

part of the aureole in places where new material was 

introduced from magmatic sources. It is not known 

whether the replacement of bytownite by andesine in 

the anorthosite in the Boehls Butte area took place 

during this or an earlier metasomatic phase. The 

source of the added sodium here could have been a 

northward extension of an early igneous activity below 

the present ground surface. The occurrence of many 

small igneous masses of tonalite, many of them gneissic, and plagioclase pegmatite bodies to the northwest 

of the batholith supports this suggestion. 

AGE OF PLUTONISM AND METAMORPHISM 

Igneous activity in the study area may have 

started during Proterozoic time in the form of small 

gabbroic sills and dikes. In Clearwater County and the 

southernmost part of Shoshone County, these sills and 

dikes have been recrystallized to amphibolite and garnet amphibolite. Most of these intrusions parallel the 

14 Geology Along the Northwest Border Zone of the Idaho Batholith, Northern Idaho 
bedding in the schist and quartzite, but some parallel 

faults. These sills and dikes are common in the Boehls 

Butte and Prichard Formations but occur only sparsely 

elsewhere. The major minerals in the amphibolite are 

hornblende, plagioclase, quartz, and, in most sills, 

garnet. Sphene and magnetite are common accessory 

minerals. The texture of the rock is granoblastic or 

lepidoblastic. The foliation parallels the foliation of the 

enclosing host rock, and the hornblende prisms parallel the local lineation. The dikelike bodies of amphibolite along fault zones rarely contain garnet and contain 

very little quartz and biotite. In contrast, the sill-like 

amphibolite bodies in the garnet-mica schist contain 

abundant quartz, garnet, and biotite; contacts with the 

host are gradational. In the anorthosite masses, amphibolite occurs as long layerlike bodies that grade to 

hornblende gneiss. 

The northwestern part of the Idaho batholith is 

exposed in the southeastern part of the study area 

(Hietanen, 1962, 1963c). Three large and several small 

satellitic bodies intrude the Belt Supergroup and the 

Boehls Butte Formation. Two distinct differentiation 

series are distinguished among the rocks of the 

batholith and its satellitic bodies: (1) An older quartz 

diorite-tonalite-plagioclase pegmatite series and (2) a 

younger quartz monzonite-granite-granite pegmatite 

series. The rocks of the quartz diorite series contain 

less than 15 percent normative potassium feldspar and 

are cut by rocks of the quartz monzonite series that 

contain as much as 26 percent normative potassium 

feldspar. For the northern part of the Idaho batholith 

(the Bitterroot lobe), Armstrong (1975) reported K-Ar 

ages ranging from 38 to 61 m.y. and lead-alpha ages 

ranging from 60 to 100 m.y. but probably not much 

older than 80 m.y. The K-Ar age of biotite in a sample 

of granite 1.6 km east of Bungalow was determined to 

be 43 m.y. (Hietanen, 1969a). The K-Ar age of biotite 

in quartz monzonite from the Roundtop pluton was 

given by Reid and Greenwood ( 1968) as 41 m. y. Structural relations show that the rocks of the quartz 

· diorite-tonalite suite are older than these (Hietanen, 

1963c). Armstrong and others (1977, fig. 2A) reported 

K-Ar ages of 71 m.y. for biotite, and 82 m.y. for 

hornblende, from a hornblende-biotite-quartz diorite 

near Orofmo. In the tonalite along the Clearwater 

River south of Greer (5.6 km north of Kamiah), the 

K-Ar age for biotite is 115 m.y., and for hornblende 

124 m.y (McDowell and Kulp, 1969; McDowell, 1971). 

It is not known whether this tonalite and associated 

quartz diorite near Greer (Hietanen, 1962, pl. 4) is a 

satellitic intrusion or part of the quartz dioritic border 

zone of the batholith that is exposed 10 km to the east 

(Hietanen, 1963c). Farther south, near McCall and 

south of Cascade, McDowell and Kulp (1969) reported 

biotite ages of 73 and 77 m. y. for the granodioritic and 

tonalitic western border zone of the batholith, respectively. 

A 206 Pbf! 07 Pb age on zircons from quartz diorite 

near Orofino was given as 283 m. y. by Reid and others 

( 1973). The difference between the biotite and zircon 

ages supports the concept that older sedimentary material was incorporated into the igneous rocks near 

Orofino, as suggested by Hietanen (1962). Reid and 

others' (1973) report of zircon ages of 1,473 m.y. for 

quartz diorite in the Headquarters quadrangle ('•type 

locality"; loc. 292 of Hietanen, 1962) suggests that 

older, zircon-bearing rocks (Belt Supergroup) were 

also incorporated into this quartz diorite. 

In the Belt Supergroup, the metamorphic grade 

increases over a distance of 40 km from the biotite 

zone near St. Joe River, through the garnet, staurolite, 

kyanite, and sillimanite-muscovite zones, to the 

sillimanite-potassium feldspar zone south of the North 

Fork of the Cle·arwater River near the batholith 

(Hietanen, 1967). The K-Ar age of biotite in a 

kyanite-garnet schist is 76 m.y., and of muscovite in 

the same rock 57 m.y. (Hietanen, 1969a). These dates 

are within the age range of the intrusive rocks of the 

Idaho batholith. K-Ar ages for biotites in the Boehls 

Butte Formation are within the same range: 61 m.y. for 

biotite in cordierite-kyanite-andalusite-sillimanite 

gneiss 1 km east of Boehls Butte (loc. 912) and 46 m.y. 

for biotite in kyanite-garnet-plagioclase gneiss next to 

the anorthosite at Goat Mountain (loc. 971) (Hietanen, 

1969a). This similarity indicates that the micaceous 

minerals recrystallized during the intrusion of rocks 

related to the Idaho batholith in this pre-Beltian basement. 

The approximate age of pyroxene gabbro in the 

study area can be deduced from the ages given above. 

The occurrence of many dikes and sills of pyroxene 

gabbro along fault zones indicates that these bodies are 

contemporaneous with, or postdate, the faulting. The 

White Rock fault displaces isograds and thus is 

younger than the recrystallization which occurred during the interval76-46 m.y. ago. All the faults are cut by 

the 41- to 43-m.y.-old plutons and thus predate these 

plutons. These relations delimit the time of faulting 

and of intrusion of the pyroxene gabbro to between 76 

and 41 m.y. ago. 

The possible continuation of the Idaho batholith 

and its forerunners farther northward below the presently exposed ground surface gives rise to a wide contact aureole in which the uplifted lower stratigraphic 

units show considerably higher metamorphic grade 

than the upper ones. Gentle dips of some isograd surfaces and local introduction of elements from magmatic sources support this contention. As a rule, the 

metasomatic transformations, such as the introduction 

of sodium to produce andesine from bytownite in the 

Age of Plutonism and Metamorphism 15 
anorthosite of the Boehls Butte area, took place during 

intrusion and at lower pressures than those that prevailed during the synkinematic regional metamorphism. Alteration of some kyanite to andalusite probably resulted from the release of pressure and may have 

occurred during the feldspathization of the schist. 

Kyanite alone occurs in the garnet-mica schist in the 

eastern part of the Boehls Butte block, where no 

sodium was introduced. 

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References Cited 17 

GPO 787-o42/120