The Nature of 

Batholiths

By WARREN HAMILTON and W. BRADLEY MYERS

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 554-C

A survey of features of U. S. batholiths leads 

to the interpretation that these complexes are 

generally thin and that they crystallized 

beneath covers consisting largely of their own 

volcanic ejecta

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D-C. 20402 - Price 30 cents (paper cover)
CONTENTS

Abstract.._ _____---__--_-_-_-_---__________________

Introduction.______________________________________

Regional descriptions and interpretations ______________

Sierra Nevada batholith-________________________

Idaho batholith_______________________________

Boulder batholith.______________________________

Volcanic ash___________________________________

Tertiary plutons of Cascade Range _______________

Aleutian Islands._______________________________

Tertiary igenous rocks of Col orado________________

Tertiary igneous rocks of Basin and Range province. 

St. Francois Mountains batholith.________________

Page

Cl

2

2

2

5

6

9

9

11

11

12

12

Regional descriptions and interpretations Continued 

New England Appalachians.___________________

Erosion intervals_____________________________

Strontium isotopes.___________________________

Origin and emplacement of batholiths____-_-__--_--_

The environment of batholiths _________________

Origin of granitic magmas._________-_-___-__--

Emplacement of batholiths______________ ____

Batholiths and metamorphism________________

Batholiths and thrust faulting._________________

Batholiths and younger structure ___________________

Selected references.______________-_-___-------_---

Page

C13

15

16

17

18

18

21

23

24

25

26

ILLUSTRATIONS

FIGURE 1. Graph of velocities of compressional waves in 

a longitudinal section beneath the eastern 

Sierra Nevada______________________

2. Geologic and crustal section through the 

Sierra Nevada of California____________

3. Geologic map of part of the north end of the 

Boulder batholith, Montana ___________

4. Diagrammatic longitudinal geologic and 

crustal section through the Boulder batholith, Montana._______________________

Page

C4

10

Page 

FIGURE 5. Geologic and metamorphic map of New

England_________________.. C13

6. Sketch showing metamorphic zones and 

granitic rocks of a part of southeastern 

New Hampshire................______ 14

7. Sketch showing initial ratios of Sr87/Sr89 in 

Paleozoic and Mesozoic batholithic 

granitic rocks____________--_-------_- 16

8. Diagram showing pressure-temperature phase 

relationships in material of anhydrous 

basaltic composition._________________ 19

in

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

THE NATURE OF BATHOLITHS

By WARREN HAMILTON and W. BRADLEY MTERS

ABSTRACT

A survey of features of batholiths in the United States is 

interpreted to indicate that batholiths generally are thin, 

having spread out laterally at shallow depth, and that many 

of them reach the surface and crystallize beneath a cover of 

their own volcanic ejecta. It is inferred also that the magmas 

originate in the lower crust or upper mantle at depths greater 

than any ever exposed by erosion. Such conclusions agree 

with those reached by many geologists, but disagree with the 

concepts that batholiths are masses of great thicknesses, form 

beneath deep cover of metamorphic rocks, and crystallize from 

melts mobilized at the levels exposed in gneissic and migmatitic terranes.

Successively older Phanerozoic batholiths display in a broad 

way successively deeper sections into batholithic complexes. 

Mid-Tertiary batholiths are largely still capped by their volcanic crusts. The Late Cretaceous Boulder batholith of Montana is only a few kilometers thick; it spread across a floor 

of prebatholithic rocks, and preserves discontinuously its roof 

of almost exclusively volcanic rocks, which are contemporaneous and consanguineous with the plutonic ones. The Idaho 

batholith, largely of middle Cretaceous age, has a few small 

areas of possible volcanic roof rocks, although the correlative 

Sierra Nevada batholith has none remaining; but the more 

than a million cubic kilometers of volcanic ash in Cretaceous 

strata in the continental interior has no apparent source other 

than such batholiths. Seismic and gravity data indicate the 

Sierra Nevada batholith to be probably thin, and gravity data 

indicate the same for the Boulder batholith. The large Mesozoic batholiths were unroofed within a small fraction of a 

geologic period after their formation; since unroofing, they 

have been incised but not greatly eroded, and shallow depths 

are indicated. Late Cenozoic uplift correlates with Mesozoic 

batholithic rock type, apparently increasing with the proportion of radioactive components; the present crustal roots of 

the batholiths are responsible for the uplifts, are more mafic 

than the exposed rocks, and must somehow have formed because of the overlying batholiths. The uplift is now resulting 

in selective erosion of Mesozoic batholiths.

Batholiths are abundant in Precambrian and Paleozoic terranes, but probably none of these batholiths are as large as 

the great late Mesozoic batholiths of western North America. 

One possible interpretation of this contrast is that most preMesozoic batholiths have been selectively eroded away. 

Gneiss terranes such as the sillimanite "plateau" of New

Hampshire may form beneath batholiths, as plutons of magma 

rise bubblelike and displace heated wallrocks, which flow 

downward and beneath the plutons and become intensely 

metasomatized and injected. The metamorphic gradients 

flanking many gneiss terranes are far too steep to be explained in terms of geothermal heat conducted from the mantle, and the heat may have been introduced in magmas that 

largely rose through the crust and coalesced into surflcial 

batholiths.

The largest Phanerozoic batholiths are partly in eugeosynclinal terranes, and the tectogene hypothesis of melting in 

downbuckled geosynclines is based on this association. Many 

Phanerozoic batholiths, however, intrude miogeosynclinal and 

platform sedimentary rocks and Precambrian basement rocks, 

and even have formed in the oceanic environment of island 

arcs. The tectogene hypothesis cannot be applied to such 

noneugeosynclinal batholiths if batholiths have a common 

cause, it cannot be a tectogene. Strontium isotope data indicate 

that granitic magmas are melted from rocks poorer in rubidium than are exposed basement rocks and thus are derived 

from the lower crust or upper mantle or (hi eugeosynclines) 

from volcanic materials derived in turn from such sources.

Batholithic and silicic-volcanic magmas become in general 

more silicic and more potassic as the continental crust becomes thicker, so the lower crust may be increasingly involved 

in melting as its depth increases. The magmas produce batholiths capped by volcanic fields in some places but produce 

high-alumina volcanic fields alone in others, depending upon 

local factors. Laboratory high-pressure data require that 

high-alumina batholithic and volcanic magmas be equilibrated 

with crystals above the depth at which basaltic rock undergoes pressure-phase transformation to eclogite; this can be 

achieved by partial melting, differentiation, or assimilation. 

Zone melting whereby volatile components rising in response 

to pressure gradients within magmas lower the melting temperature of the roof while forcing crystallization low in the 

chamber can cause great assimilation; indeed, very little of 

the final high-level magma need represent material present in 

the initial melt.

Much deformation conventionally ascribed to either crustal 

compression or gravity sliding may be due to the shouldering 

aside of wallrocks by rising batholith magmas. Batholiths, 

once formed, resist fragmentation by younger structures and 

hence greatly influence the subsequent deformation of their 

regions.

Cl
C2 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

INTRODUCTION

Batholiths are composite masses of granitic rocks 

having areas ranging from tens of square miles to tens 

of thousands of square miles. Some batholiths that 

cut sharply across their wallrocks and that are surrounded by contact-metamorphic aureoles clearly 

formed from magmas intruded from greater depths. 

Other batholiths are largely concordant and lie within 

terranes of uniformly high-grade gneisses, and the 

origin of such batholiths and the source of heat for 

metamorphism of the associated gneisses are less 

obvious.

We attempt no broad review here of the descriptive 

features of batholiths; Buddington (1959) has done 

that ably, and the reader is referred to his work. We 

note features of batholiths and related rocks in the 

United States which appear to need explanation in 

any general theory of the origin and emplacement of 

batholiths. Much interpretation is incorporated with 

the individual descriptions in the first section of the 

paper. General synthesis and speculation follow in 

the second section. The rationale developed is that 

batholiths form from magmas generated in the upper 

mantle and lower crust, beneath any depths exposed 

by erosion; that pluton magmas rise in detached 

balloonlike forms through the crust, frequently reaching the earth's surface, and coalesce into shallow and 

fairly thin complexes; and that gneissic terranes form 

in the zones, beneath the final batholiths, through 

which the plutons rise, as wallrocks flow beneath the 

rising magmas and are heated and metasomatized by 

them. Elements of this synthesis have been suggested by other geologists, who are cited in the appropriate places. The third section of the paper discusses 

the influence of batholiths, once formed, upon the subsequent structural evolution of their regions.

Rock names applied on the basis of petrographic 

criteria to intermediate volcanic rocks generally connote compositions markedly more mafic and calcic 

than are connoted by names applied to plutonic rocks 

of identical chemical compositions. One is likely to 

think of andesite and diorite, dacite and quartz diorite, 

and rhyodacite and granodiorite as being of the same 

compositions, but this is not generally true. For 

example the postbatholithic andesites of Mount 

Rainier contain 60 to 64 percent SiO2 and 1.6 to 1.9 

percent K2O (Fiske and others 1963, table 2); plutonic rocks of the same composition would be classed 

petrographically as granodiorite or at least quartz 

diorite. Chemical analyses rather than petrographic 

names should be employed when comparisons are made 

between intermediate plutonic and volcanic rocks.

REGIONAL DESCRIPTIONS AND INTERPRETATIONS 

SIERRA NEVADA BATHOLITH

The Sierra Nevada batholith is known best across 

its central part (Bateman, 1965; Bateman and others, 

1963; Calkins, 1930; Ernst Cloos, 1932, 1935a, 1935b; 

Durrell, 1940; Hamilton, 1956; Krauskopf, 1953; 

Macdonald, 1941; Moore, 1963; Rinehart and Ross, 

1964; D. C. Ross, 1958; Sherlock and Hamilton, 1958; 

and others). Bateman, Clark, Huber, Moore, and 

Rinehart (1963, pi. 1) compiled a geologic map of the 

central Sierra. The batholith is 55 to 110 kilometers 

wide, has an exposed length of 650 km, and is a composite of plutons of Late Jurassic and Cretaceous ages 

(Kistler and others, 1965). Large plutons are elongate parallel to the northwesterly regional strike and 

small plutons lie between and within them. Younger 

plutons in many places cut sharply across structures 

of older plutons but in some places are nested concordantly inside them. Adjacent plutons can be of 

markedly different compositions within the spectrum 

quartz diorite-granodiorite-quartz monzonite-alaskite. 

The larger plutons and the bulk composition tend to 

become more silicic and richer in alkalis eastward; the 

dominant rock types are mafic calcic quartz diorite 

and granodiorite in the west and leucocratic granodiorite and quartz monzonite in the east, but exceptions are numerous on both sides.

Contacts between plutons and wallrocks and between adjacent plutons are typically so sharp that a 

hand specimen can be taken across them, although 

dike-injection zones make some contacts gradational 

at mapping scales and contact migmatites are extensive 

along some contacts with metamorphic rocks. Contacts may be quite irregular in detail but generally 

are broad curves at map scale. Thin discontinuous 

screens of metamorphic rocks locally separate plutons, 

and large pendants of metamorphic rocks separate 

groups of plutons.

The batholith is bounded by irregular belts of 

lightly to moderately metamorphosed Paleozoic and 

Mesozoic sedimentary and volcanic rocks. The proportion of mafic to silicic volcanic rocks is higher in 

the western border belt than in the eastern belt. The 

rocks of the western border belt in the central Sierra 

dip steeply toward the batholith and consist of long 

fault blocks of which those nearest the batholith contain the oldest rocks, although in each block the 

youngest rocks tend to be on the side toward the 

batholith; the eastern border belt of the central Sierra 

dips steeply in either direction but its rocks become 

in a gross way younger westward toward the batholith 

(Bateman and others, 1963). One pluton of the west
THE NATURE OF BATHOLITHS C3

part of the batholith was interpreted by Ernst Cloos 

(1932, 1935a) to have sent a flat tongue westward 

over its wallrocks. He (1935a, 1935b) emphasized 

that the flow-structure domes of some plutons demonstrated the batholith to have spread laterally, pushing 

its wallrocks aside, across the central Sierra.

Assimilation of mafic metamorphic rocks into mobile granitic magmas has been demonstrated in many 

places and probably has contributed much to the more 

mafic and calcic character of the western plutons, for 

mafic metavolcanic rocks are abundant in the western 

border belt, and widespread assimilation is shown 

along contacts. Static granitization has nowhere been 

found on more than a very small scale. Both regional 

and contact metamorphosed rocks show metamorphic 

grade and intensity decreasing systematically away 

from contacts: the primary source of heat for metamorphism to assemblages of higher temperature facies 

than greenschist was intruded granitic magma (Bateman and others, 1963; Durrell, 1940; Macdonald, 

1941).

The flow structures of most plutons show them to 

have risen as units past their granitic and metamorphic wallrocks. Injection complexes of gently 

dipping dikes at some contacts demonstrate vertical 

stretching of wallrocks (Bateman, 1965, p. 116; Moore, 

1963; Sherlock and Hamilton, 1958). Offset belts of 

metamorphic rocks indicate in some places shouldering aside by rising plutons (Bateman and others, 

1963; Moore, 1963; Kinehart and Koss, 1964) but elsewhere wallrock belts are truncated irregularly by plutons. The presence of contact breccias along some 

contacts indicates that stoping was operative during 

late stages of intrusion, but the general absence of 

xenoliths away from contacts seems to be evidence 

against the process as the dominant mode of emplacement. Much detailed mapping apparently demonstrates that the intrusion of the plutons was dominantly forcible (Bateman, 1965, p. 115-123; Bateman 

and others, 1963, p. 44).

Evidence for a very shallow depth of crystallization 

of Cathedral Peak Quartz Monzonite a large pluton 

of very coarse grained leucocratic rock was summarized by Evernden (1965). The relation between variations in potassium-argon ages and in elevation led 

him to conclude that the pluton was emplaced no 

deeper than 1 km, and possibly as shallow as 4 km.

Seismic-refraction data indicate that the Mohorovicic discontinuity lies 40 or 45 km below sea level 

beneath the crestal region of the Sierra Nevada near 

the 39th parallel (Eaton, 1963) and about 50 km 

beneath the highest part of the crest farther south 

(L. C. Pakiser, written commun., 1964). Integration

of gravity data with this seismic information requires 

that the greater part of the crustal thickening represented by the deepening of the Mohorovicic discontinuity is in crustal rocks that are markedly denser 

than the granitic rocks of the exposed batholith. The 

gravity data published by Thompson and Talwani 

(1964), for example, when considered with the seismic 

model, suggest that the Sierra Nevada batholith near 

the 39th parallel is a tabular structure with a thickness of at most 8 km.1 The negative Bouguer gravity 

anomalies are not sufficiently large to permit both 

granitic rocks and the entire crust to be thick; as the 

crust is demonstrably thick, the granitic rocks cannot be.

From his preliminary interpretation of explosion 

seismic waves passing longitudinally through the 

eastern Sierra Nevada, Jerry P. Eaton (oral commun., 

1965) concluded that velocities increase downward in 

about the manner indicated schematically in figure 1. 

We are much indebted to Eaton for permission to 

incorporate this information. Rocks with a velocity 

appropriate for silicic granitic rocks such as those 

dominating the surface exposures apparently extend 

no deeper than about 10 km. We interpret the 

velocity increase near this depth to occur beneath the 

thin batholith, and suggest that the underlying highvelocity (6.4 km per sec) rocks are metasomatized 

schist and gneiss displaced beneath the plutons of the 

batholith as they rose toward the surface. The depthvelocity fields of common types of igneous rocks are 

shown in the figure for comparison.

Other seismologists have drawn different interpretations from seismic data. Mikumo (1965) suggested 

that low-velocity granitic rocks need extend little 

deeper than sea level, where they give way downward 

to denser (Fp 6.3 km per sec) rocks. He achieved 

close agreement between measured Bouguer gravity 

and gravity calculated from a model in which the 

entire crust beneath the Sierra Nevada, from the surface to a Mohorovicic discontinuity reaching 46 km, 

has a density of 2.80 gm per cm3 . (Surface rocks 

lighter than this are, however, exposed throughout 

much of the region.)

Press and Biehler (1964), on the other hand, inferred that there is a velocity inversion within the 

upper crust and that rocks with a velocity appropriate 

to granite extend to great depth. They studied arrival 

times of P waves from nuclear explosions in the 

western Pacific, and found that arrivals at Tinemaha 

and Reno (both short distances east of the Sierra

i Thompson and Talwani assumed that the Mohorovicic discontinuity liea no deeper than 32 km beneath the Sierra crest. This asaumption is contradicted by the seismic data; and even so, they could 

fit no more than about 12 km of granitic rocka to the gravity model.
C4 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

20

40

60

Batholith

Metasomatized schist 

and gneiss

Lower crust 

(gabbro and amphibolite)

__ MOHOROVICIC __ 

DISCONTINUITY

Mantle 

(oiivine-rich gabbro)

VELOCITY, IN KILOMETERS PER SECOND

FIGUBE 1. Velocities of compressional waves (heavy lines) in a longitudinal section beneath the eastern Sierra Navada, from 

an unpublished interpretation by Jerry P. Eaton. Depth-velocity fields of representative types of igneous rocks are generalized from Birch's (1960) data. An interpretation of the velocities is shown at the right.

Nevada mountain block, but near the axis of the 

gravity minimum associated with the Sierra and its 

batholith) were about 0.8 sec later than they would 

have been if the structure of the crust and upper 

mantle was the same throughout California as it is at 

Pasadena. If this delay is due (as they assumed) to 

low-velocity granitic rocks beneath the Sierra Nevada, 

then the batholith has a deep root, and a thickness of 

granite and granodiorite of approximately 37 km is 

indicated (Press and Biehler, 1964, p. 2987-2988).

The interpretation by Press and Biehler is not 

however in accord with the local seismic-refraction 

data (such as that illustrated by fig. 1), and the P-wave 

delays they found can be interpreted alternatively in 

terms of variations within the upper mantle. To 

eliminate the possibility that variations in thickness of 

the lower crust could account for the arrival-time 

delays, Press and Biehler (1964, equation 10) analyzed 

the data for fit to these relationships:

(Ag)=4:2Ah( P-Pf) = -

in which (Ag) is the difference in slab-equivalent 

Bouguer gravity, in milligals, between the reference

station (Pasadena) and the seismograph station; 

Ah is the difference, in kilometers, in depth of the 

Mohorovicic discontinuity; (p p') is the difference in 

density, in grams per cubic centimeter, above and 

below the discontinuity; (PU) is the wave delay, in 

seconds; and a and a' are the velocities of P waves 

above and below the discontinuity. Press and Biehler 

showed that, given reasonable velocities, the expression 

on the right yields values of (Ag) approximately 50 

percent too high. The middle expression, however, 

provides calculated values agreeing with the observed 

ones: the reasonable figures of Ah 22 km and (p p') = 

0.3 gm per cm3 , for example, yield (A#)=280 mgal, the 

same as that observed. The conflict can be resolved 

by the interpretation that the wave delay is due to 

lower velocities in the upper mantle beneath the Sierra 

Nevada than beneath coastal California, and that the 

batholith is thin, rather than thick as Press and 

Biehler assumed.

Heat-flow data also indicate the Sierra Nevada 

batholith to be thin. Thus, granodiorites west of the 

crest of the central Sierra produce by radioactive 

decay about 10 microcal per gram per year, yet heat
THE NATURE OF BATHOLITHS C5

flow in a deep core hole is only 1.3 microcal per sq cm 

per sec; if the entire flux came from granodiorite like 

that in the hole, the granodiorite could be only 15 km 

thick (Lachenbruch and others, 1966; see also Wollenberg and Smith, 1964). The more mafic granitic rocks 

of the western part of the batholith produce about 21/£> 

microcal per gram per year, and heat flow in a deep 

core hole there is only 1.3 microcal per sq cm per sec, 

equivalent to the heat production in 30 km of the 

local rock (Lachenbruch and others, 1966). As much 

of the heat must in fact come from greater depths, 

batholithic rocks like those near the surface must be 

much thinner than these limits. Five heat-flow determinations farther north in the Sierra batholith average only 0.9 microcal per sq cm per sec, half the value 

in the Great Basin to the east (Roy and Blackwell, 

1966), leading to the same conclusion that the batholith is thin.

We conclude that the crustal root beneath the high 

part of the Sierra Nevada is largely of rocks markedly 

more mafic and heavier than the exposed quartz monzonite and leucogranodiorite, and that the batholith 

is limited to the upper part of the crust.

Figure 2 shows a section through the central Sierra 

Nevada. The interpretation incorporates seismic and 

gravity data, and illustrates conclusions developed in 

this paper.

IDAHO BATHOLITH*

The Idaho batholith of central Idaho has an exposed 

length of 400 km and a width of 130 km and is surrounded by regionally metamorphosed rocks (Hamilton, 1963a, b; Larsen and Schmidt, 1958; Reid, 1959; 

Ross, 1963; Schmidt, 1964; and others). Massive 

granodiorite and quartz monzonite underlie two main 

regions, one in the southwest part of the granitic terrane and the other in the northeast; elsewhere there 

is much schist and gneiss interspersed with the granitic rocks of the batholith. Quartz diorite and 

trondhjemite are widespread in the gneissic western 

border zone of the batholith. The culminating intrusions of the Idaho batholith occurred about the middle 

of Cretaceous time according to lead-alpha determinations on zircons (Jaffe and others, 1959).

It is possible that the Idaho batholith formed without a roof, its plutons having reached the surface and 

erupted a volcanic capping, beneath which magma 

crystallized more slowly. The Casto Volcanics in the 

east-central part of the region of the batholith can be 

interpreted speculatively to be remnants of this 

volcanic cap. The Casto is undated, variably altered, and contact metamorphosed but generally nonschistose intermediate lavas and pyroclastics intruded 

by the Idaho batholith (Leonard, 1962; Ross, 1934).

*See note on page C30.

Great Valley Sierra Nevada batholith

Gneisses formed as wall rocks flowed f 

downward and beneath rising plutons

Owens Valley Basin and Range province

J\ ' Gneisses of upper crust 

^ mostly Precambrian

Gabbro and amphibolite of lower crust

Olivine-rich gabbro of upper mantle

Vp5s7.9 km per sec 

(divine eclogite lies deeper)

VERTICAL AND HORIZONTAL SCALE

10 20 30 40

_J________I________I________I

50 KILOMETERS 

_J

EXPLANATION

Upper Cretaceous and Cenozoic 

sedimentary rocks

Granitic rocks Metamorphic rocks

FIGURE 2. Geologic and crustal section through the Sierra Nevada of California, along the 37th parallel. Adapted from Hamilton and Pakiser (1965). Plutons of granitic magma, melted in upper mantle and lower crust, rose through crust and 

coalesced at surface to form Sierra Nevada batholith. In the Basin and Range Province, Paleozoic sedimentary rocks 

moved along bedding-plane thrust faults, then broke into normal-fault blocks.

232-376 67-
C6 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Deformation of the volcanics is only moderate. Typical dips are about 25°, and the rocks apparently lie 

unconformably upon highly deformed metasedimentary and metavolcanic rocks of which some at least 

are of late Precambrian age. Ross and Leonard both 

assumed the Casto to be part of the Paleozoic and 

Mesozoic eugeosynclinal suite, much older than the 

batholith, and perhaps of Permian age; if this assumption is correct, so is their conclusion that therje 

has been no major deformation or regional metamorphism since pre-Permian time. In the western border 

zone of the batholith, however, the fossiliferous Upper 

Triassic Martin Bridge Limestone was extremely deformed, highly metamorphosed, and intruded by granitic rocks which in turn were intruded by the main 

plutons of the Idaho batholith (Hamilton, 1963a, b), 

and the geometry of the deformation that accompanied the regional metamorphism indicates a genetic 

relation to the batholith. The absence of comparable 

deformation in the Casto Volcanics might indicate 

that they postdate the emplacement of the batholith 

and yet the volcanics are intruded by the batholith. A 

possible explanation is that the Casto Volcanics 

formed by extrusion of lava from the Idaho batholith 

itself; if so, at least part of the batholith formed with 

no roof other than a crust of its own ejecta.

Leonard (1963) and Leonard and Stern (1966) 

have, however, presented further evidence which they 

regarded as indicating that major deformation in central Idaho long predated the Idaho batholith. Highly 

deformed upper Precambrian metamorphic rocks were 

intruded there by syenite whose lead-uranium and 

lead-thorium calculated age is about 600 million years. 

(One potassium-argon hornblende age of only 93 m.y. 

apparently represents metamorphism by the Cretaceous batholith.) Leonard considered the syenite to 

postdate the major folding of the enclosing rocks, and 

our conjectures in the preceding paragraph are wrong 

if he is correct. The syenite, however, is grossly concordant to the structures of its Precambrian wallrocks, and has been variably crushed and recrystallized; so it is possible alternatively that the syenite 

was intruded into the old rocks before, rather than 

after, their metamorphism and deformation and thus 

that the syenite provides a maximum rather than 

minimum age for that event.

BOULDER BATHOLITH

The Late Cretaceous Boulder batholith of southwestern Montana is a composite mass 100 km long and 

50 km wide, and consists of plutons of granodiorite, 

quartz monzonite, and other granitic rocks (Becraft 

and others, 1963; Klepper, 1950; Klepper and others, 

1957; Knopf, 1963, 1964; Kuppel, 1963; Smedes,

1962). The batholith lies far to the east of the general 

belt of late Mesozoic metamorphism and batholith 

formation. The batholith appears to be a floored 

sheet, roofed only by its own volcanic ejecta.

The roof rocks of the batholith are exposed in many 

areas and consist of dacite, rhyodacite, and quartz 

latite. (Considerable "andesite" has been reported in 

the literature also, but published analyses of such rock 

are clearly of dacite and rhyodacite.) The granitic 

and volcanic rocks are of the same age within the 

Late Cretaceous insofar as the radiometric and isotopic age determinations of the granitic rocks can be 

compared with the paleontological dates of the volcanics. The volcanic rocks have a maximum thickness 

of 2 or 3 km and are mostly pyroclastic. They are 

broken only by warps and normal faults in most areas, 

and are but slightly altered except where converted to 

hornfels near the contact with the batholith.

Younger plutons within the batholith are in general 

more silicic and richer in alkalis than are older ones. 

A similar irregular compositional age progression 

within the volcanic roof rocks, silica and alkalis increasing upward, has been reported on the basis of 

field and petrographic studies, although it is not 

apparent in the few published chemical analyses of 

the rocks involved. Granitic and volcanic rocks 

broadly overlap in composition but the granitics are 

in bulk composition more silicic and more potassic 

than are the volcanics.

The present erosion surface everywhere is probably 

within 2 km above or below the position of the original roof of the batholith, for the subhorizontal contact 

with volcanic rocks is exposed in most regions about 

the batholith. Presumably the original relief of the 

roof was still less, as Cenozoic deformation has much 

affected western Montana, and the initial top of the 

batholith may have been almost horizontal. Eoofrock lavas lap across the edge of the batholith onto 

the older wallrocks. A few small masses of contactmetamorphosed sedimentary rocks, probably of prevolcanic Mesozoic units, are present locally in the 

roof complex and perhaps represent rafts of floor 

rocks. Lower Eocene quartz latite lies upon eroded 

granitic rocks (Smedes and Thomas, 1965), and shows 

that the batholith was exposed by erosion very soon 

after its formation and that magmatism continued 

into Tertiary time.

The volcanic rocks also form the wallrocks for most 

of the east margin of the batholith. The contact is 

steep and irregular. Deformation of the wallrock 

volcanics has been more severe than that of the roofrock ones, and the wallrock volcanics have gentle to 

moderate dips and are broken by many faults.
THE NATURE OF BATHOLITHS C7

The north contact of the batholith is semiconcordant 

to Paleozoic and prevolcanic Mesozoic strata which 

dip southward beneath the granitic mass (fig. 3; 

Knopf, 1963; Smedes, 1962). Dips in the wallrocks 

tend to steepen toward the contact with the batholith. 

The contact forms in plan three large cusps, concave 

toward the batholith; the western and central cusps 

appear in figure 3. The cusps are synclinal sags and 

are separated by sharp anticlines. The western cusp 

is 25 km across, and the batholith is in contact with 

middle and upper Paleozoic beds. The eastern two 

cusps are 20 and 10 km wide, and the granitic rocks 

lie against Cretaceous and Jurassic strata. Relatively 

dense (specific gravity about 2.8) granodiorite lies 

along the central and eastern cusps and forms an outcrop belt 0 to 4 km wide, south of which is less dense 

(about 2.71) quartz monzonite. A subhorizontal sheet 

of still lighter granophyre lies discordantly above the 

heavy granodiorite, in the same structural position 

occupied by nearby remnants of volcanic roof rocks 

(fig. 3). We interpret the general parallelism of the 

granodiorite-quartz monzonite contact to the margin 

of the batholith as suggesting that the granodiorite is 

part of a sheet which dips southward beneath the 

quartz monzonite.

The south contact of the batholith is against Precambrian crystalline rocks and Paleozoic and Mesozoic strata. Available data indicate the contact to 

be complex but are too meager to permit satisfactory 

generalization beyond the observation that long segments of the contact are semiconcordant to wallrock 

formations whose tops lie toward the north. The west 

contact of the batholith is hidden beneath Cenozoic 

deposits but probably is largely or entirely against 

Paleozoic and prevolcanic Mesozoic strata.

When it is viewed on a broad scale such as that of 

the geologic map of Montana (Ross and others, 1955), 

the Boulder batholith is seen to occupy a structural 

depression. The batholith is surrounded mostly by 

Mesozoic and Paleozoic strata, whereas Precambrian 

rocks are extensively exposed in other parts of the 

same tectonic province elsewhere in Montana. The 

batholith fills a basin.

The roof of the west-central part of the batholith is 

against an almost constant stratigraphic level in the 

overlying volcanic rocks through a broad area 

(Ruppel, 1963, p. 37). Contacts between variant granitic rocks in the batholith are partly sharp and 

partly gradational but are in general subparallel to 

the roof contact, and successively lower units tend to 

be succcessively coarser grained; fine-grained quartz 

monzonite several hundred feet thick typically lies 

between the volcanic roof rocks and the coarser quartz

monzonite of the interior of the batholith (Ruppel, 

1963, p. 32, 37).

Lawson (1914) long ago suggested that the Boulder 

batholith was a floored sheet, intruded between the 

Cretaceous volcanic rocks and the older rocks beneath. 

Barrell (1907, p. 166) made similar suggestions still 

earlier but thought them improbable. Ruppel (1963) 

suggested that the west-central part of the batholith 

was a floored sheet because the subhorizontal contacts 

between plutons are strong evidence for horizontal 

flow of the intrusive magma. The concordance of the 

northern contact to the inward-dipping right-side-up 

Mesozoic and Paleozoic section indicates that sector 

also to be floored. The near lack of prevolcanic rocks 

along the east margin of the batholith suggests that 

there, too, the granitic rocks lie wholly above the prevolcanic section. Prevolcanic rocks are virtually 

lacking in the roof.

The Boulder batholith is capped almost exclusively 

by its own volcanic ejecta and is better regarded as an 

extrusive complex, of which the volcanic rocks form 

the upper part and the granitic rocks the lower, than 

as an intrusive complex. The batholith magma 

flowed, in effect a gigantic mantled lava flow, across 

a broad basin whose subsidence may have been due to 

the withdrawal of magma from depth. Presumably a 

crosscutting batholith (in the customarily limited 

sense of the term) within the basin, or several stocks 

and small batholiths, served as magma conduits. The 

magma formed volcanic rocks where it erupted to the 

surface and granitic rocks where it crystallized beneath an insulating crust of its own ejecta. The capping crust was thickened by eruption over the top, by 

injection into it of dikes and sills (which are abundant 

in the volcanic roof), and by crystallization along the 

bottom of such rocks as granophyre and fine-grained 

quartz monzonite.

Granitic rocks intrude the volcanic rocks wherever 

the two are in contact, but this does not require the 

granitic rocks to be wholly younger than the volcanic 

ones: it indicates that granitic rocks formed where 

magma crystallized beneath insulating cover. The 

generally offset compositional ranges of volcanic and 

granitic rocks the more silicic half of the volcanic 

rocks having the same general composition as the less 

silicic half of the granitic rocks may indicate that 

the overlap in age consisted largely of the younger 

half of the volcanic series and the older half of the 

granitic series. If this is correct, then thick volcanic 

rocks formed across the entire basin before much 

magma spread laterally between volcanics and floor; 

however, the hidden deep part of the batholith may 

well consist of relatively mafic rocks of the same com-
o

oo

112°00'

5

£> i A s

EXPLANATION

Surf icial. deposits

Kq \/ *

. Granitic rocks

Rocks of the Boulder batholith and its satellites: Kgd, 

granodiorite; Kgp, granophyre; Kmn, monzonite; 

Kqm, quarts: monzonite; Kgq, granodiorite and 

quartz monzonite, undivided

Sedimentary rocks

pCu, pCm, and pCI: upper, middle, and lower formations, respectively, of the local succession

Contact

Anticline 

Showing trace of axial plane and plunge of axis

< <

? ! =

0

1 1

0

1

5 MILES 

i i i 1

5 KILOMETERS 

> i 1

Syncline 

Showing trace of axial plane and plunge of axis

30 70 

_J_ -cH

Strike and dip of beds Strike and dip of 

overturned beds

FIGURE 3. Geologic map of part of the north end of the Boulder batholith, Montana. Generalized from Knopf (1963) ; nomenclature of granitic rock types is

that of this paper rather than of Knopf.

O 

O
THE NATURE OF BATHOLITHS C9

position and age as the more mafic volcanics. A reasonable picture is that intrusion and extrusion largely 

overlapped in age and that the volcanic crust was 

thickened and broadened as granitic rocks crystallized beneath it. Repeated injections of magma beneath the volcanic cover are indicated by the many 

contacts between plutons. In the west-central part 

of the batholith, successive plutons flowed laterally.

Successive episodes of subsidence are suggested by 

the geometry along the north margin of the batholith. 

The granodiorite south of Helena (fig. 3), for example, may have crystallized after its magma filled the 

cuspate embayment in the wallrocks, and then both 

granodiorite and walls may have subsided more. The 

deepened cusp was then filled by the magma which 

formed the quartz monzonite to the south and above 

the granodiorite, if, as we infer, dips between the 

plutonic units decrease into the batholith. The aggregate thickness of the units is probably markedly less 

than might be inferred from extrapolation of dips of 

the north contact of the batholith.

A crust of volcanic rocks perhaps 2 km thick floated 

upon granitic magma over a region of about 7,000 

square kilometers. Barrell (1907, p. 166) implied this 

long ago. Whether or not all the roof floated at any 

one time is not yet certain. The warping and normal 

faulting of the volcanic pile may have occurred largely 

as a result of its vertical and horizontal jostling and 

stretching while afloat.

Many of the points discussed here and in subsequent 

sections are illustrated by the diagrammatic section 

through the Boulder batholith (fig. 4).

A gravity survey across the north end of the 

Boulder batholith shows no abrupt decrease in gravity 

corresponding to the margin of the granitic mass, 

which thus cannot there have great thickness (Eenick, 

1965). The data indicate that the surveyed part of 

the batholith is very thin at its north edge and 

thickens only gradually southward to a thickness of 

about 5 km, 15 km south of the north margin. 

(Renick's model shows the batholith thickening to 6 

or 7 km, but 5 km provides a better fit with the 

observed gravity.) An unpublished gravity survey of 

a larger area of the batholith and surrounding region 

is said to indicate the batholith to be thinner than 

15 km (Biehler, 1966).

The Boulder batholith lies in a miogeosyncline 

not in a eugeosyncline. Most Mesozoic batholiths of 

western North America occur at least partly within 

eugeosynclines, but such an environment is obviously 

not necessary for their formation; and an explanation 

for the origin of batholiths cannot properly apply to 

a eugeosynclinal setting alone. Such matters are

discussed in the subsequent section on "The environment of batholiths."

VOLCANIC ASH

Mesozoic strata of the western interior United 

States contain 1 or 2 million cubic kilometers of clay 

altered from volcanic ash blown in from sources 

nearer the Pacific Ocean. The varicolored siltstones 

and claystones of the Upper Triassic Chinle Formation of the southern part of the Colorado Plateaus 

consist largely of montmorillonitic clay derived from 

latitic or quartz latitic ash and of mixed-layer clay 

derived from rhyolitic ash, and relic shards and volcanic minerals are common (Schultz, 1963). Clay in 

the Middle and Upper Jurassic Carmel Formation 

and Upper Jurassic Morrison Formation of the Colorado Plateaus also is largely of volcanic-ash origin, 

and relic volcanic textures are widespread (Keller, 

1962; Schultz and Wright, 1963). Far more voluminous still are the volcanic-ash clays (montmorillonite 

and mixed-layer clay) of the marine Upper Cretaceous shales of the Colorado Plateaus and Great 

Plains (Schultz, 1965; Tourtelot and others, 1960; 

Leonard G. Schultz, oral commun., 1965). The volume 

of Upper Cretaceous strata of the western interior is 

approximately 4 million cu km (Gilluly, 1963); of 

this total, perhaps one-fourth or even one-half is 

altered volcanic ash, to judge by detailed studies of 

the Pierre Shale and reconnaissance studies of other 

formations (Schultz, oral commun., 1965).

Volcanoes were active throughout much of the Late 

Triassic and the Jurassic near the Pacific margin of 

North America, whereas known Cretaceous volcanoes 

were less widespread and were farther inland (Gilluly, 

1965). Granitic rocks of Triassic age are uncommon, 

those of Late Jurassic and Early Cretaceous age are 

widespread, and those of Late Cretaceous age are 

abundant and form large parts of the great Sierra 

Nevada and Idaho batholiths (Gilluly, 1963, 1965; 

but see Kistler and others, 1965). Unroofed batholiths may have provided much of the pyroclastic 

material preserved in the strata of the western interior, and the Late Cretaceous batholiths may have 

been the source of the greatest part of that material. 

The numerous middle Cretaceous bentonites of Wyoming came primarily from dacitic and quartz latitic 

sources farther west, presumably volcanoes above the 

Idaho batholith (Slaughter and Earley, 1965).

TEBTIAEY PLUTONS OF CASCADE RANGE

The Cascade Range of Washington is among the 

regions where middle or late Tertiary plutons are 

known to have broken through to the surface and produced volcanic piles. Fiske, Hopson, and Waters
Qh-* 

O

ssw

Gravity-thrust plate of roof 

volcanic rocks

Normal faults, formed by jostlingand stretching of floating roof, 

covered by younger volcanic 

rocks of roof

Caldera formed by collapse accompanying 

voluminous eruption of 

welded tuff

NNE,

Gravity-thrust plate of roof 

volcanic rocks, formed 

by sliding off inflated 

dome

Volcanic rocks lap across 

prebatholith rocks

Upper crust, composed mostly of 

silicic and intermediate gneisses

Floor rocks contorted by crowding e-> Thrust faults formed by push 

from spreading batholith

ranite remnants from magmas 

that rose through crust to form 

the batholith and its volcanic roof

Lower crust, composed mostly of 

amphibolitic and gabbroic gneisses

Low-velocity (Vp = 7.8 km per sec) upper mantle, 

composed of olivine, pyroxene, amphibole, plagioclase, and possibly spinel. Eclogite transformation 

occurs beneath the depths shown

O

o

10 5 0 

I I I I I 1 I I I I I

VERTICAL AND HORIZONTAL SCALE 

10

40 50 KILOMETERS

FIGXTEE 4. Diagrammatic longitudinal geologic and crustal section through the Boulder batholith, Montana. The batholith and its roof are of Late Cretaceous age, and the stratified rocks range in age from late Precambrian to early Late Cretaceous.
THE NATURE OF BATHOLITHS Cll

(1963, p. 40-63) showed that the 250-sq-km Tatoosh 

pluton of granodiorite and allied rocks in the Mount 

Eainier area produced explosive eruptions through its 

roof. Rapid loss of heat and volatiles resulted in 

distinctive crystallization features vertically alined 

vesicles, miarolitic vugs, explosively brecciated rocks, 

local granophyre, aphanite, and vitrophyre, and transitions between pluton and volcanic plugs in the 

upper few hundred feet of the pluton in vent areas. 

Eapid crystallization of the entire pluton, despite its 

granitic texture, is indicated by the absence of flow 

banding and lineation: "there was no prolonged interval during which the material moved as a viscous 

crystal mush" (Fiske and others, 1963, p. 46).

The much larger Snoqualmie Granodiorite batholith, farther north in the Washington Cascades, also 

deroofed itself explosively (E. E. Fuller, unpub. 1925 

thesis, as cited by Fiske and others, 1963, p. 59). 

Pyroxene diorite in cupolas of the batholith grades 

upward through volcanic plugs into andesite flows and 

pyroclastics. Here also, resulting dehydration produced chill effects high in the batholith. Miocene 

plutons of quartz diorite and granodiorite crop out 

near five of the northern volcanoes of andesite and 

dacite in the Cascade Range, and their presence suggests that the volcanic magmas came from the plutons, 

which remained active at depth (Hopson and others, 

1966).

The Tertiary volcanic and intrusive rocks of the 

Cascades in part were erupted through pre-Cenozoic 

crystalline complexes but in part have probably 

formed where only oceanic crust existed in Mesozoic 

time. In northern Washington and in southern Oregon and northern California the Cascade igneous rocks 

cut and overlie the granitic and metamorphic rocks of 

the late Mesozoic orogens. These older rocks strike 

southeastward in the north and northeastward in the 

south so that northwestern Oregon and southwestern 

Washington are on the Pacific side of all exposed and 

projected pre-Cenozoic rocks (Carey, 1958, fig. 56; 

King, 1959, p. 161). We infer that there is no preCenozoic continental crust within this tectonic embayment. The central Cascades (including Mount 

Rainier) and the Coast Ranges of most of Oregon and 

Washington have apparently been built upon oceanic 

crust as a Cenozoic addition to North America.

ALEUTIAN ISLANDS

The island arc of the Aleutians consists of young 

volcanoes built upon platforms of middle Tertiary and 

older submarine lavas and pyroclastics. Andesite 

dominates both suites, but basalt and dacite are common. Many of the old rocks are variably spilitized,

but their prealteration compositions were identical to 

the young rocks. (The geology of the islands has been 

described by various authors, under the general title 

"Investigations of Alaskan Volcanoes," in the many 

chapters of U.S. Geological Survey Bulletin 1028.)

The older complexes are intruded by Tertiary granitic rocks on a number of islands. These are limited 

to stocks and smaller masses except on Unalaska 

Island, where three small batholiths occur (Drewes 

and others, 1961, p. 610-634 and table 1). About 70 

percent of the batholithic rocks consists of granodiorite. The remainder ranges from quartz gabbro to 

light-colored quartz monzonite. The granodiorite is 

chemically the same as andesites and dacites of both 

older and younger volcanic sequences and presumably 

is but an intrusive manifestation of the same igneous 

activity.

The Aleutian Islands extend across the North 

Pacific. No evidence requires that continental crust 

existed there before the onset of andesitic island-arc 

volcanism. The mechanism of generation of the 

magmas from an oceanic mantle is discussed in the 

second major section of this paper.

TERTIARY IGNEOUS ROCKS OF COLORADO

Numerous stocks of lower Tertiary granitic rocks 

dominated by quartz monzonite and granodiorite form 

a chain trending northeastward across western Colorado. The rocks intrude Precambrian plutonic rocks 

and the thin overlying Paleozoic and Mesozoic platform sedimentary rocks.

The great middle (and late?) Tertiary volcanic pile 

of the San Juan Mountains of southwestern Colorado 

is formed largely of rocks equivalent in composition 

to quartz diorite and granodiorite. Large calderas 

and other magmatic collapse structures occur throughout a region of at least 7,000 sq km (Luedke and Burbank, 1963; Steven and Ratte, 1963; Thomas A. 

Steven, oral commun., 1965); Tertiary batholiths 

must lie hidden beneath the volcanic cover of this 

area. No data are available to suggest whether the 

granitic rocks are mostly in a floored complex above 

the pre-Tertiary rocks.

Western Colorado was a stable platform region 

during late Precambrian(?), most of Paleozoic, and 

early Mesozoic times, but during Cretaceous and Cenozoic times it has been the site of moderate deformation. Western Colorado now has the highest regional 

elevation in the conterminous United States, and has 

a thick crust, high heat flow, and low-velocity upper 

mantle. The heat flow and mantle velocity distinguish 

the region from the still-stable platform east of the 

Rocky Mountains, and presumably the abnormal heat 

flow and mantle velocity, and the crustal thickening.
C12 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

are products of Cretaceous and younger changes and 

are related genetically to the deformation and igneous 

activity of western Colorado. A geosynclinal environment is not essential for the generation of granitic 

magmas: the controlling events occur in the lower 

crust or upper mantle, not in the upper crust.

TERTIARY IGNEOUS ROCKS OF BASIN AND RANGE 

PROVINCE

Silicic volcanic rocks (mostly welded tuffs) and 

intrusive porphyries and granitic stocks of similar 

compositions formed throughout broad parts of the 

Basin and Kange province of the Western States during middle Tertiary time. Presumably the intrusives 

fed the extrusives. Blank (1963) found a porphyry 

in southwestern Utah that broke through its roof 

after partial crystallization and produced quartz latite 

welded tuffs. Gilluly (1932, p. 69) found that in the 

Oquirrh Kange of central Utah, silicic stocks and 

porphyries cut the volcanic pile that probably was 

erupted from the same magma chambers, and similar 

relationships have been reported elsewhere; but in 

general, wallrocks of stocks consist of pre-Tertiary 

rocks.

The Tertiary silicic volcanic rocks of Nevada and 

western Utah have a total volume of approximately 

120,000 cu km (Mackin, 1960, p. 83). This is equivalent to about half the volume of the Sierra Nevada 

batholith if we are correct in assigning a thickness of 

approximately 8 km to the batholith. Most of this 

Great Basin material was melted beneath a miogeosynclinal terrane, and part of it was melted beneath 

an unstable platform environment.

ST. FRANCOIS MOUNTAINS BATHOLITH

The St. Francois Mountains of southeastern Missouri expose a Precambrian complex in which a roof 

of extrusive rhyolite and quartz latite was intruded 

by leucocratic granite, quartz monzonite, and granophyre (Bridge, 1930, p. 59-64; Dake, 1930, p. 26-44; 

Ha worth, 1895; Hayes, 1961; Kobertson, 1966; Snyder 

and Wagner, 1961). Granophyric and granitic rocks 

are about 1,300 million years old (Alien and others, 

1959; Tilton and others, 1962). According to Anderson (1962; oral commun., 1965), the roof rocks above 

the plutons of granophyre and granite are chiefly 

welded tuff and tuffaceous sedimentary rocks and are 

about 2 km thick. Despite their age and their intrusion by granite, the volcanic rocks have generally low 

dips, and are broken only by minor normal faults 

which developed contemporaneously with volcanism. 

The volcanic rocks have not been metamorphosed 

regionally, although some have been weakly silicified 

and albitized (propylitized?), and they contain wellpreserved primary volcanic fabrics such as collapsed 

pumice lapilli and shards. Granophyre tends to lie 

with gently dipping contacts between volcanic rocks 

above and coarse granitic rocks beneath, and therefore 

apparently represents a rapidly crystallized roof 

facies of the batholith. In bulk composition and compositional variation, granitic and granophyric rocks 

are similar to the dominant volcanic rocks. Rocks of 

all three types are mostly red, and are moderately 

alkalic and low in alumina. Minor sodic dacites 

present low in the volcanic succession, however, are 

less silicic than are any of the exposed silicic intrusive 

rocks (Anderson, 1962). Sheets and irregular intrusive masses of basalt and diabase cut both intrusive 

and extrusive silicic rocks.

We infer the volcanic rocks to be consanguineous 

with the intrusive granitic and granophyric rocks. 

The petrological similarities are great, and the volcanic rocks lack any evidence of regional deformation 

and metamorphism to suggest that they represent a 

wallrock terrane intruded by the batholith. If our 

inference is correct, then the batholith is roofed only 

by its own ejecta. The volcanic crust solidified from 

magma erupted from the molten interior of the complex. Granophyre crystallized first beneath the volcanic crust, and its formation thickened the insulating 

crust beneath which the granitic rocks then solidified. 

The cover above the coarse plutonic rocks when they 

crystallized was probably not thicker than 3 km. The 

earliest volcanic rocks included types less silicic than 

the magmas which crystallized at the exposed high 

levels of the intrusive complex.

The red color of most of the granite, granophyre, 

and rhyolite and their chemical character intermediate 

between normal calc-alkaline silicic rocks and the 

moderately alkalic silicic differentiates of lopoliths 

(Anderson, 1962; compare with Hamilton, 1960) 

might permit the inference that the St. Francois rocks 

are the silicic caprocks of a gabbroic lopolith, but 

gravity surveys indicate that the Precambrian complex in the St. Francois Mountains is batholithic 

rather than lopolithic. The Duluth and Mellen lopoliths of the Lake Superior region and the Wichita 

lopolith of Oklahoma (and the probable lopoliths 

forming a buried chain trending southwestward from 

Duluth through Minneapolis and Omaha to Abilene) 

are marked by great positive Bouguer gravity anomalies, with a relief of 100 mgals or more and steep 

gravity slopes. There is no such anomaly in the 

St. Francois region. (The lopolithic gravity anomalies are perhaps the most remarkable features shown 

on the United States gravity map by Woollard and 

Joesting, 1964.)
THE NATURE OF BATHOLITHS CIS

NEW ENGLAND APPALACHIANS

The Paleozoic erogenic terrane of the Appalachian 

system is best known in New England. Granitic rocks 

are widespread in this region but there are no great 

batholiths comparable to the late Mesozoic batholiths 

of western North America. Some reasons are suggested here for the contrasts between the Paleozoic 

and Mesozoic terranes of opposite sides of the 

continent.

Successive episodes of geosynclinal sedimentation 

and volcanism, deformation, metamorphism, and granitic intrusion have been superimposed complexly in 

New England, and many problems remain unsolved. 

Despite the overlapping of events, the major episodes 

of metamorphism and intrusion appear in a general 

way to be younger in the west than in the east and 

intermediate in the medial belt (fig. 5). Discussion

here is restricted to that medial belt, whose analysis 

appears particularly relevant to the topic of this 

report. We are particularly indebted to Wallace M. 

Cady and Robert H. Moench for discussions clarifying 

New England geology. Summary published reports 

include those of Billings (1956); Billings, Rodgers, 

and Thompson (1952); and Goldsmith (1964).

The medial belt of New England is formed largely 

of sedimentary rocks and subordinate volcanic rocks, 

of Silurian and Devonian ages, which were metamorphosed and widely intruded by granitic masses during 

the Devonian. In New Hampshire, the belt is a sillimanite-grade "plateau" of highly metamorphosed 

rocks, injected by granitic plutons which generally are 

concordant in both structure and mineralogy to the 

wallrocks, although structurally discordant granites 

also are present in the northeastern part of the belt.

EXPLANATION

Granitic rocks

Mostly Devonian in New Hampshire and central belt 

of Maine; mostly Devonian and Carboniferous in 

Rhode Island, eastern Massachusetts, and southeastern Maine

Eugeosynclinal rocks

Miogeosynclinal rocks Eugeosynclinal rocks

Includes gneiss domes of 

Devonian (?) Oliverian 

Plutonic Series in New 

Hampshire

Basement plutonic rocks

Sillimanite isograd

Ticks on side containing

sillimanite

Garnet isograd

Dots on side containing

garnet

100 MILES 

J

100 KILOMETERS

FIGURE 5. Geologic and metamorphic map of New England. Generalized from Goldsmith (1964).

232-376 67 3
C14 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

The high-grade belt is bounded on both sides by steep 

metamorphic gradients (figs. 5 and 6) which correspond approximately in some places, although not 

in others, to the contact zones with older (Cambrian 

and Ordovician) eugeosynclinal rocks. Outside the 

metamorphic gradients are less metamorphosed rocks 

whose pattern is much complicated by granitic intrusions of various ages and by superimposed metamorphisms which produced large areas of garnet-grade 

rocks but only small areas of sillimanite-grade rocks.

The sillimanite plateau of New Hampshire gives 

way northeastward along the strike in Maine to a 

terrane of low-grade metamorphosed Silurian and 

Devonian rocks. These are intruded by crosscutting 

masses of granitic rocks, mostly Devonian, and 

middle- and high-grade metamorphism is limited to 

narrow contact aureoles surrounding the intrusions. 

The regional biotite isograd, not shown in figure 5, 

crosses Maine northeast of the garnet isograd shown; 

most of the medial belt in Maine consists of chloritezone rocks.

Structures within the high-grade part of the ISTew 

England medial belt are extremely complex. Lithologic units swirl, intersect, branch, and pinch out in 

patterns of great fluidity, and dips typically are gentle 

to moderate (Billings, 1955; Goldsmith, 1963, 1964). 

Structures are refolded isoclinally and irregularly 

(Goldsmith, 1961). Metasomatic changes and granitic 

injections are widespread. The metamorphic gradients 

bounding the sillimanitic terrane are locally so steep 

that biotite and sillimanite isograds are only about 

1 km apart (fig. 6; Billings, 1955), although 8 km is a 

more common distance.

Petrologists in general agree that the biotite isograd 

represents a temperature of 200° to 300°C, whereas 

sillimanite forms near 600° to 700°C, or at about the 

temperature of granitic magma. Biotite and sillimanite isograds thus represent a temperature difference of about 400°C, and the temperature gradient in 

response to which they formed in New Hampshire was 

as steep as 300°C per km in the plane of the present 

ground surface. The common assumption that increasing metamorphism is due to increasing depth (for 

example, Turner, fig. 77, in Fyfe and others, 1958) is 

inadequate to explain such gradients (Hamilton, 

1963a, p. 90). No model based upon heat conducted

72° 71°

High-grade belt (sillimanite "plateau") 

of central New Hampshire

EWJTAMPSHIRE 

MASSACHUSETTS

5 MILES

J

5 'KILOMETERS

Granitic rocks, mostly Devonian

EXPLANATION

High grade Medium grade Low grade 

(sillimanite) (garnet, staurolite) (biotite, chlorite)

Metamorphic rocks, Ordovician to Devonian

FIGUKE 6. Metamorphic zones and granitic rocks of a part of southeastern New Hampshire. A steep metamorphic gradient, 

defined by the sillimanite, staurolite, garnet, and, in part, biotite isograds, separates high-grade and low-grade terranes. 

Adapted from Billings (1955).
THE NATURE OF BATHOLITHS C15

from deep levels in a static system can account for 

more than a small part of such metamorphism.

The apparent alternative is that the heat was introduced in magmas from hotter deeper levels. (Barrell, 

1921, reached this conclusion long ago.) This requirement negates the speculation that the gneisses and 

migmatites of the sillimanite belt represent spontaneous partial melting due to tectonic depression into 

levels heated to melting temperatures by heat conducted from the base of the crust. The granitic 

component added to the complex came from markedly 

greater depths than those exposed.

Exposed granitic masses are distributed irregularly 

through the gneiss belt. Had they alone carried the 

heat upward, the metamorphic effects should be related concentrically to them, but this is not the situation. The belt is generally bounded by a straight and 

steep metamorphic gradient, irrespective of the local 

distribution of exposed plutons within the belt. Some 

might interpret these relationships as indicating the 

presence of a continuous great batholith beneath the 

exposed gneisses, but so steep are the flanking metamorphic gradients that such a batholith would have 

to be very near the surface, and it is unreasonable to 

postulate that the top of the batholith could everywhere be at a uniform shallow depth without being 

exposed.

The known factors of structure and metamorphism 

agree with the hypothesis that the gneiss belt of the 

New England Appalachians formed beneath a batholith analogous to that of the Sierra Nevada. Molten 

plutons may have risen through the gneisses and 

coalesced above them in a thin, shallow batholith. 

Granitic melts are much lighter than metamorphic 

rocks even solid granitic rocks are lighter than most 

high-grade metamorphic rocks and must rise through 

them wherever the buoyancy of the plutons exceeds 

the strength of the metamorphic rocks. The extremely 

plastic and irregular deformation shown by the 

gneisses indicates that their structures did not form 

by simple response to systematic regional compression 

but rather that the rocks flowed about in complex 

patterns of rising, sinking, under-flowing, and overflowing. This in turn suggests that the dominant 

causes of flow were gravitational instability and differences in plasticity, as Kosenfeld (1960) concluded 

on the basis of detailed analysis of microscopic and 

macroscopic structures. The heating of the metamorphic rocks and their copious injection by granitic 

fluids caused them to flow readily outward, downward, 

and beneath the rising plutons. Much of the granitic 

material was enveloped and trapped in the gneisses, 

but most of it rose through to coalesce higher into

batholiths, which have since been eroded away. The 

steep metamorphic gradients bounding the gneissic 

terrane may mark the margins of the region through 

which the plutons rose.

If this rationale is correct, then the northeastern 

limit of the sillimanite plateau in the medial belt also 

represents the limit of initially nearly continuous granitic plutons in the overlying batholithic complex that 

has since been largely eroded away. Crosscutting 

granitic masses are more abundant near the alongstrike transition from high-grade to low-grade terranes than they are anywhere else in the medial belt 

(fig. 5); this is consistent with the suggestion that a 

thin batholith, initially continuous to the southwest 

but since largely eroded away, here gave way northeastward to a terrane of scattered smaller plutons. 

The lack of high-grade regional metamorphism in 

most of the medial belt in Maine is evidence that there 

is no batholith of regional extent beneath the belt 

there, and also suggests that there was never a batholith of regional extent above the levels now exposed by 

erosion.

Four heat-flow measurements in the Devonian granitic rocks indicated fluxes of only 1.2 to 1.7 microcalories per square centimeter per second (Birch and 

Roy, 1965); considered with the probably high radioactivity of these rocks, this low heat flow indicates 

the granites to be thin.

The northern Appalachian region was intruded during Mesozoic time by the stock and small batholiths 

of alkalic rocks of the White Mountains magma 

series. The relation between observed heat flow and 

measured radioactivity in these White Mountains 

rocks is such that rocks of high radioactivity, like 

those exposed at the surface, can extend only a few 

kilometers downward (Birch and Roy, 1965).

EROSION INTERVALS

Batholiths have crystallized and then been exposed 

by erosion within such short intervals of time in a 

number of places that it is difficult to visualize the 

granitic rocks as having formed at depths of many 

kilometers. Only a small fraction of a geologic 

period separates rocks deformed and intruded by 

batholiths from beds resting unconformably upon the 

same batholiths. The Baja California batholith, for 

example, intruded upper Lower Cretaceous strata and 

is overlain unconformably by middle Upper Cretaceous beds (Alien and others, 1960). Rocks as 

young as middle Upper Jurassic were metamorphosed 

and intruded by stocks and small batholiths in the 

Klamath Mountains of northwestern California, and 

the resulting crystalline complex is overlain unconformably by high Upper Jurassic beds (Irwin, 1960).
C16 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Zircon of middle and Late Cretaceous age, from 

batholithic rocks farther west, is abundant in sandstone throughout the Upper Cretaceous sequence of 

Montana and Wyoming (Houston and Murphy, 1962, 

1965), and thus shows regional exposure of the batholiths to erosion within a very short interval after 

crystallization.

STRONTIUM ISOTOPES

Of the stable isotopes of strontium, some Sr87 is 

formed radiogenically by the decay of Rb8T, whereas 

the remaining Sr87 and all Sr86 and Sr88 are nonradiogenic. The ratio of radiogenic to nonradiogenic 

strontium in a magma thus provides an index to the 

ratio of rubidium to strontium in the source rocks 

from which the magma was derived; and as these two 

elements behave quite differently chemically the rubidium being generally associated with potassium, the 

strontium with calcium valuable information can be

gained from this ratio. The strontium-isotope method 

of tracing geologic processes was developed by, among 

others, Hurley and his associates (Hurley and others, 

1962), and has been applied by them (Fairbairn and 

others, 1964a, b; Hurley and others, 1965) and by 

many more, including Hedge and Walthall (1963) 

and Zartman (1965). In abundance, Sr8T is comparable to Sr86, whereas Sr8T is far less abundant than 

Sr88 ; hence analyses are generally made for the ratio 

Sr87/Sr86.

The young basalts of Hawaii and Iceland have 

Sr8T/Sr86 ratios ranging only from about 0.7023 to 

0.7045 (fig. 7; Hedge and Walthall, 1963; Powell and 

others, 1965). The mantle sources from which their 

magmas were melted do not vary much in Sr/Bb 

ratios. Stony meteorites had an Sr8T/Sr86 ratio of 

about 0.698 at the time they formed (Hedge and Walthall, 1963; Pinson and others, 1965). No igneous 

rocks of any age yet analyzed have initial ratios that

0.73

0.72

o

£0.71

0.70

0.69

Change in ratio of granodiorite formed at 3 x 109 

years having Sr 87 /Sr86 = 

0.702, Rb = 0.01 percent, 

and Sr = 0.03 percent

Sierra Nevada and British Columbia 

granitic rocks

Most Precambrian rocks have 

present ratios between 0.71 and 

0.73. Rocks rich in K-feldspar 

or mica may have much higher 

ratios; 0.8 is common

^Recent continental basalts 

and most rhyolites

Young oceanic basalts 

(Hawaii and Iceland)

Achondrite

3 2 

TIME BEFORE PRESENT, IN 109 YEARS

FIGURE 7. Initial ratio of SrSVSr86 in Paleozoic and Mesozoic batholithic granitic rocks. Because Sr87 includes that 

strontium produced by radiogenic decay of Rb87, and Sr86 

and the remaining Sr87 are not radiogenic, the ratio Sr87/ 

Sr86 is a function of the Rb/Sr ratio in the rocks from 

which the magmas were melted. The initial Sr^/Sr86 

ratios of granitic rocks, like those of most continental 

basalts and rhyolites, are a little higher than those inferred 

for the mantle source regions of oceanic basalts, but much

lower than those of most Precambrian basement rocks. 

The granitic magmas might represent mantle melts which 

assimilated crustal rock, or melts of rock whose Sr/Rb 

ratio was intermediate between that of the mantle and 

that of Precambrian basement rocks. Data from Fairbairn, Hurley, and Pinson (1964a, b), Hedge and Walthall 

(1963), Hurley, Bateman, Fairbairn, and Pinson (1965), 

Pinson, Schretzler, Beiser, Fairbairn, and Hurley (1965), 

and Powell, Faure, and Hurley (1965).
THE NATURE OF BATHOLITHS C17

fall as much as 0.001 below the line between the ratios 

of meteorites and the ratios of the lower limit of 

young basalts: initial ratios in all igneous rocks fall 

along the line or are above it by less than 0.005, or at 

the most 0.008. The slope of this line indicates an 

Sr/Kb ratio in the source region of oceanic basalt of 

about 50:1, and this ratio is the virtual lower limit for 

the source region of all igneous rocks (Hedge and 

Walthall, 1963). This Sr/Kb ratio of 50:1 is similar 

to, or a little lower than, that of tholeiite. Tholeiite 

must form by the melting of a large proportion of its 

mantle source rocks, which thus cannot be ultramafic.

The Sr87/Sr86 ratios of Recent continental volcanic 

rocks overlap those of oceanic basalts and range 

upward only to about 0.71 (Hedge and Walthall, 

1963). The ratios in continental rhyolites and basalts 

overlap throughout their ranges: the source rocks 

from which the basalt and rhyolite magmas are 

derived have about the same Sr/Rb ratios, and there 

is little basis in these data for the common assumption 

that basalt magmas are melted from deeper and much 

more mafic rocks than are rhyolite magmas.

Precambrian basement rocks have in general markedly higher ratios of rubidium to strontium than have 

the mantle source rocks of oceanic basalts, and hence 

the Sr87/Sr86 ratio of average continental rock has 

increased more rapidly during geologic time than has 

the ratio in the mantle. (A line in fig. 7 illustrates 

the changing ratio in a typical Precambrian granodiorite whose Rb87 is decaying to Sr87 .) Even average 

Precambrian rocks markedly more mafic than those 

most likely to be partially melted to produce granitic 

magmas have present Sr87/Sr86 ratios of about 0.720 

(Faure and others, 1963). If silicic crustal rocks were 

melted directly to form any magmas, then the resulting igneous rocks should instead have initial Sr8T/Sr86 

ratios plotting high above the achondrite-oceanic 

basalt zone of figure 7.

Granitic rocks crystallized from magmas whose initial Sr87/Sr86 ratios were slightly higher than those 

of the mantle source regions of oceanic basalts but 

markedly lower than those of Precambrian plutonic 

rocks (fig. 7). The initial Sr8T/Sr86 ratio in the Paleozoic granitic rocks of Nova Scotia averaged 0.708 and 

only ranged from 0.705 to 0.711 (Fairbairn and others, 

1964a), and ratios in the upper Mesozoic granitic 

rocks of southeastern British Columbia and the Sierra 

Nevada batholith were mostly from 0.706 to 0.708 

(Fairbairn and others, 1964b; Hurley and others, 

1965). The average ratio in Precambrian rocks in 

Paleozoic and Mesozoic time would have been at least 

0.715, and many felsic Precambrian rocks would have 

had ratios of 0.8 to 1.0 and more.

Similar relationships hold for granitic rocks of all 

ages. Thus granitic rocks, 1,000 to 1,120 m.y. old, of 

the Llano Uplift of Texas had markedly lower 

Sr87/Sr86 ratios at the time of their crystallization 

than did their wallrocks (Zartman, 1965). Tertiary 

granitic rocks of Colorado and Washington had initial 

Sr87/Sr86 ratios of about 0.705, and some Precambrian 

granitic rocks, 2,400 to 2,800 m.y. old, of the Precambrian shield had ratios of only 0.701 to 0.703 

(Hedge and Walthall, 1963). Some small Paleozoic 

granites also had very low ratios, such as 0.703 

(Czamanske, 1965).

As the various authors cited have emphasized, these 

ratios of radiogenic to nonradiogenic strontium in 

granitic rocks preclude the possibility that granitic 

magmas are wholly melted from mantle rock like that 

which yields oceanic basalts, and also preclude the 

possibility that they are wholly melted from silicic 

crustal rocks like those exposed in Precambrian terranes. The source materials for granitic magmas are 

richer in rubidium (of which the radioactive isotope 

decays to Sr87 ) than are oceanic mantle rocks, but they 

are poorer in rubidium than are exposed crustal rocks. 

Obviously the magmas either are melted in an environment of intermediate composition, or else are hybrid 

products of melts combined from contrasting rocks.

The strontium-isotope data considered alone permit 

the speculations that granitic magmas are derived in 

the lower (intermediate) continental crust; that they 

represent mixed mantle and crustal materials; or that 

they are melted from eugeosynclinal volcanic rocks 

and volcanogenic sedimentary rocks. The eugeosynclinal-melting hypothesis is attractive for some granitic rocks but obviously is not applicable to the 

Boulder batholith (which formed in a miogeosyncline) 

or to the granitic rocks of southeastern California, 

Colorado, and elsewhere, which were intruded into 

Precambrian basement complexes.

The ratios between the various stable and radiogenic 

isotopes of lead and their parental uranium and 

thorium in granitic rocks provide another isotopic 

method for placing limits on the composition of the 

source rocks from which the magmas were melted. 

The lead relationships are being investigated by Bruce 

R. Doe (written commun., 1965), who finds that the 

lead isotopes generally support conclusions similar to 

those reached by the strontium-isotope researchers.

OBIGIN AND EMPLACEMENT OF BATHOLITHS

The examples cited in this paper are all from the 

United States. The local interpretations made could 

be supported further by other North American examples, and further yet by numerous examples from
CIS SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

other continents. The explanation of the examples 

is accordingly integrated here into a general theory of 

the characteristics of batholiths.

THE ENVIRONMENT OF BATHOLITHS

Most large Phanerozoic batholiths lie at least partly 

within eugeosynclines, and this has led to the popular 

tectogene hypothesis, whereby batholiths are visualized as the products of downbuckling and partial 

melting of the geosynclinal fillings. Batholiths also 

form, however, in such other environments as miogeosynclines (for example, the Boulder batholith), unstable platforms (as in the San Juan Mountains of 

Colorado), and oceanic island arcs (including the 

Aleutian Islands). The Idaho batholith intrudes both 

eugeosyncline and miogeosyncline. Boundaries between eugeosynclinal and miogeosynclinal suites shift 

back and forth through time, and in California the 

boundaries are crossed at high angles by the Sierra 

Nevada batholith (Gilluly, 1963, 1965).

Precambrian basement rocks also are intruded by 

batholiths, in geosynclinal as well as nongeosynclinal 

settings. In southern California in the San Gabriel 

and Orocopia Mountains (Crowell and Walker, 1962), 

in the West Kiverside Mountains (Warren Hamilton, 

unpub. data, 1966), and elsewhere, large plutons of 

upper Mesozoic granitic rocks intrude Precambrian 

basement plutonic complexes as well as Paleozoic and 

Mesozoic metasedimentary and metavolcanic rocks.

The common association of batholiths with eugeosynclines seems best interpreted to indicate that the 

batholiths and the geosynclinal volcanic rocks have 

related origins, rather than that the batholiths form 

because the geosynclines are present. Batholiths 

form wherever temperatures are high enough at depth 

to melt the needed magmas, and eugeosynclines are 

only one setting in which these conditions are met.

ORIGIN OF GRANITIC MAGMAS

The discussions in previous sections indicate to us 

that all or most granitic magmas are generated at 

depths below any ever exposed by erosion. Strontiumisotope data preclude the possibility that most granitic (and silicic-volcanic) magmas could form by 

melting of silicic crustal rocks alone. Sources permitted by the isotope data include volcanic rocks and 

volcanic sedimentary rocks in geosynclines; gabbroic 

or amphibolitic rocks of the lower continental crust; 

and mantle rocks, provided some continental rock 

relatively high in rubidium is assimilated by rising 

magmas. Geologic, petrologic, and physiochemical 

reasoning provides much further information on the 

origin of the magmas.

Some granitic masses are so closely associated with 

high-alumina volcanic rocks that their magmas must 

have come from the same source. The granodiorite 

batholiths of the Cascade and Aleutian provinces 

share the compositional patterns of the volcanic rocks 

they intrude and of the volcanic rocks which overlie 

them, and appear to be but different aspects of continuing magmatism from the mantle. Pregranitic 

deformation of the older volcanic rocks in these 

provinces was slight so that major orogeny cannot be 

postulated to have intervened.

Some granitic magmas may form by melting of eugeosynclinal materials, but many cannot possibly form 

in this way. In the Boulder batholith and the eastern 

part of the Idaho batholith, for example, no volcanic 

piles were available. In other batholiths (for example, in southern California) the magmas intruded 

subgeosynclinal basement rocks and hence came from 

still deeper sources. The rocks formed from such 

demonstrably noneugeosynclinal magmas cannot be 

distinguished from those (for example, in the Sierra 

Nevada) for which a eugeosynclinal origin might 

otherwise be postulated. The hypothesis that granitic 

magmas are generated by downbuckling and partial 

melting of eugeosynclinal materials is certainly inapplicable in many places and may be wholly invalid.

If there is a single magma-generating mechanism, 

it must thus be more general than that of geosynclinal 

downbuckling. The mechanism must produce granitic magmas beneath diverse geologic environments. 

Quartz diorite, granodiorite, and quartz monzonite 

are the dominant products required in Phanerozoic 

terranes, but their relative abundance differs widely 

from region to region. (In some Precambrian orogens, 

potassic granite is dominant or at least abundant, but 

it is lacking or is much subordinate to less potassic 

rocks in nearly all Phanerozoic suites.) Quartz diorite 

and trondhjemite are the dominant types in some 

regions, and gabbro and diorite are present in many 

assemblages and are abundant in some.

Compositions inferred for the lower crust and upper 

mantle must be incorporated into the explanation. 

Seismic, heat-flow, and petrologic data provide clues. 

In the uppermost mantle, seismic waves generally 

travel faster beneath stable regions of the continental 

crust than beneath orogenically and volcanically active 

regions (Herrin and Taggart, 1962; Nuttli, 1963; 

Pakiser, 1963). Heat flow is in general higher in 

active regions than in stable ones (Lee and Uyeda, 

1965). These relations preclude the possibility that 

peridotite forms the uppermost mantle in both active 

and stable regions: pressures in the uppermost mantle 

are too low to invert olivine (Wentorf, 1959), so that
THE NATURE OF BATHOLITHS C19

density-phase transformations cannot be called upon; 

and neither can the variable hydration of peridotite 

to serpentine be postulated, for hydration would be 

suppressed in the high-temperature regions, and hence 

would correlate with heat flow in the direction opposite to that required by the seismic data. Pressure 

and temperature at the base of the continental crust 

in stable regions, but probably not in active ones, are 

appropriate for the eclogite transformation of plagioclase and low-alumina pyroxene to the denser phases 

of garnet and aluminous alkalic pyroxene (Yoder and 

Tilley, 1962, fig. 43). Velocity data determined experimentally at high pressure (Birch, 1960) permit the 

suggestion that the Mohorovicic discontinuity at the 

base of the continental crust represents a compositional change, from basalt above to basalt plus dunite 

below, and that in the low-velocity mantle and lowermost crust of active regions the basaltic phase is in 

low-density plagioclase and pyroxene or amphibole. 

In the high-velocity mantle, and probably also in the 

deepest crust, of stable regions the basaltic phase is in 

high-density pyroxene and garnet, but in the lowvelocity mantle of technically active regions the transformation to those dense eclogitic minerals occurs well 

within the mantle (Pakiser, 1965). Another possibility is that high-velocity mantle contains more 

olivine than does low-velocity mantle. Such a contrast might develop as basalt is wrung out of the 

upper mantle, residual materials being progressively 

more peridotitic and dunitic.

Kimberlite and its dense inclusions provide direct 

evidence for the existence of mantle rocks of basaltplus-olivine composition. Kimberlite consists of large 

corroded crystals of high-pressure minerals f orsterite, 

jadeitic clinopyroxene and aluminous orthopyroxene, 

pyrope and almandine, phlogopite, diamond, magnesioilmenite in a matrix of serpentine, chlorite, olivine, calcite, and phlogopite. Most kimberlite contains inclusions of dense rocks composed of varying 

combinations and proportions of heavy minerals like 

those of the corroded crystals (Holmes, 1937; O'Hara 

and Mercy, 1963; Wagner, 1914; Williams, 1932). 

Kimberlite has a peculiar bimodal composition of 

ultramafic components on the one hand and alkalic 

and volatile components on the other, and is probably 

a mixture of mantle rock like the dense inclusions 

with mantle-derived water, carbon dioxide, and alkalies. The average composition of the dense inclusions, and of kimberlite itself minus the 15 or 20 

percent of apparently added volatile and alkalic components, is approximately equal to a mixture of equal 

parts of tholeiitic basalt (in the mineralogic form of

high-pressure pyroxenes and garnet) and magnesian 

olivine.

If these inferences are correct, then the upper 

mantle and lower crust differ primarily in olivine 

content. As olivine is more refractory than any 

other major mineral in the rocks, it would not be 

melted appreciably; therefore, only the melting of the 

basaltic phase need be considered here, and such discussion may be equally applicable to upper mantle 

and lower crust.

High-pressure laboratory data show that the dense 

alkali-bearing aluminous pyroxenes and the garnet of 

basalt-composition pyroxenite and eclogite melt together over a rather narrow temperature range under 

both anhydrous and hydrous conditions at pressures 

appropriate to the upper mantle or deep continental 

crust (fig. 8; Cohen and others, 1966; Yoder and 

Tilley, 1962, tables 43-47). Tholeiite (basalt low in 

alkalies, moderately low in alumina, and saturated in 

silica) and olivine-tholeiite magma could originate by 

the nearly complete fusion, within such a narrow 

temperature range, of the pyroxenitic or eclogitic 

fraction of mantle rock. Partial melting under such 

conditions to produce granitic magmas is not however 

possible. No substantial quantity of silicic magma can 

be generated by direct melting at depths greater than 

that of the gabbro-to-eclogite pressure transformation 

if the meltable part of the mantle is of basaltic 

composition.

At pressures corresponding to the uppermost few 

miles of the crust, pyroxene and plagioclase melt or 

crystallize together over a limited temperature range

600 800 1000 1200 1400 

TEMPERATURE, IN °C

1600 1800

FIGUEE 8. Pressure-temperature phase relationships in material of anhydrous basaltic composition. The data plotted 

are those of Cohen, Ito, and Kennedy (1966), with additions consistent with the data of Yoder and TiUey (1962). 

Olivine or orthopyroxene or both would be present in rocks 

of appropriate compositions the liquidus and solidus 

curves are in fact those of olivine tholeiite but are not 

illustrated here. Cpx, clinopyroxene; gar, garnet; plag, 

plagioclase.
C20 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

(Yoder and Tilley, 1962, figs. 6, 27-30). Crustal temperatures, however, preclude melting at such shallow 

depths.

The melting behavior of basaltic rock at pressures 

appropriate to the lower crust or uppermost mantle, 

but shallower than the gabbro-eclogite transition, is 

quite different. Plagioclase at these intermediate pressures is a markedly lower temperature mineral than 

is either pyroxene or amphibole (Cohen and others, 

1966; Yoder and Tilley, 1962, figs. 27-30). The temperature range of crystallization or melting is broad 

and increases with pressure. Progressive melting 

under these shallower-than-eclogite conditions will 

thus produce highly feldspathic magmas progressively 

richer in calcic plagioclase and in mafic minerals. 

Such melting could produce the high-alumina basalt, 

andesite, and dacite which typify island arcs and 

eugeosynclines (Hamilton, 1964).

This mechanism might provide an explanation for 

the melting of intermediate and silicic magmas from 

rock of basaltic composition and thus from the lower 

crust or mantle. Mantle rocks of the same composition that yield tholeiitic basalt magma by partial 

melting beneath the eclogite transformation could 

yield high-alumina basalt, andesite, and dacite magma 

by less complete melting above it. (High-alumina 

basalt differs from tholeiite primarily in being richer 

in the components of calcic plagioclase.) The intimate association of tholeiite and high-alumina rocks 

in some island arcs and also in some continental 

environments can be explained easily in this way. 

The contrast between midoceanic volcanism (tholeiite 

on the sea floor, tholeiite plus alkaline olivine basalt 

on islands) and island-arc volcanism (tholeiite plus 

high-alumina volcanic rocks) can then be explained 

in terms of variations in pressure-temperature relations in the mantle beneath the contrasted provinces, 

without any need for variations in mantle compositions. The curves of actual temperature and of the 

melting temperature of the basaltic component are 

probably close together and subparallel within a considerable thickness of the upper mantle (MacDonald, 

1964, fig. 15), so that such variations in conditions 

need not be great.

Differentiation by partial crystallization of a 

magma within the crust or upper mantle, but above 

the eclogite transformation boundary, would produce 

liquids of the same compositions as would partial 

melting there, as O'Hara (1965) emphasized. The 

composition of a magma provides clues as to the 

depth of its last equilibration with crystals, but not 

as to whether this equilibration was achieved by partial melting or by partial crystallization.

These mechanisms of deep derivation of magmas 

by varying degrees of melting satisfy some of the 

conditions but not all imposed by the data. How, 

for example, do voluminous granitic magmas form 

that are almost devoid of mafic components? Why 

are silicic magmas nearly lacking in the ocean basins 

except in those places as, island arcs, and Iceland 

where abnormally thick volcanic crust is present? 

Kegional relationships yield further data to be considered.

Available data suggest parallel regional variations 

in compositions of eugeosynclinal volcanic rocks and 

the granitic rocks which intrude them. Magmatism 

of the type that produced the volcanic piles could 

also have produced the batholiths. The eastern part 

of the Sierra Nevada batholith consists largely of 

light-colored quartz monzonite and granodiorite, and 

Mesozoic metavolcanic rocks are mostly dacite and 

quartz latite (Rinehart and Ross, 1964, p. 30-38; 

Bateman and others, 1963, p. 6). The western part 

of the batholith consists in general of more mafic and 

calcic rocks, and the Mesozoic volcanic rocks are correspondingly also more mafic and calcic on the average 

(Bateman and others, 1963, p. 6). Similarly, metavolcanic rocks along the west side of the Idaho batholith are basalt, andesite, and dacite, all low in potassium, and the granitic rocks intrusive into them are 

quartz diorite and trondhjemite (Hamilton, 1963a, 

table 5 and pis. 1, 2). Metavolcanic rocks are lacking 

in the eastern part of the batholith, which is of 

granodiorite and quartz monzonite.

The western part of the belt of late Mesozoic batholiths of western North America is dominated by 

quartz diorite, and the central and eastern parts are 

dominated by granodiorite and quartz monzonite 

(Moore, 1959, fig. 2). Silica and potassium are higher 

inland, whereas calcium, iron, and magnesium are 

higher oceanward. One possible explanation of this 

difference is that when the batholiths were forming 

the Mohorovicic discontinuity lay deeper beneath the 

inland region than beneath the coastal one and that 

the inland magmas were melted largely within the 

lower crust whereas the coastal ones came from the 

upper mantle. An inference proceeding from this 

postulate and from factors noted previously is that 

although the lower crust and the meltable part of 

the upper mantle are both grossly basaltic in composition, the lower crust is richer in potassium and is 

less mafic.

A similar postulate can be made for the melting of 

magmas during middle and late Cenozoic time 

throughout the Western United States. The ratio of 

potassium to sodium in high-alumina rocks of any
THE NATURE OF BATHOLITHS C21

given silica content tends to increase with altitude 

and hence with crustal thickness. This is demonstrated by the data plotted by Moore (1962, figs. 2, 3): 

the K/Na ratio tends to increase as Bouguer gravity 

becomes increasingly negative.2 (Bouguer gravity 

in general correlates broadly with regional surface 

altitude and with crustal thickness.) The bulk composition of the high-alumina volcanic suites also 

tends to become more silicic with increasing altitude. 

This can be seen by considering the many provincial 

plots of Moore's figure 1 as frequency-distribution 

diagrams. These relationships can be explained if 

the magmas are melted entirely in the mantle where 

the Mohorovicic discontinuity is relatively shallow 

and partially or entirely within the lower crust where 

it is deep. Gradations in magma types reflect gradations in crustal thickness and hence might be due 

either to melting together of both crustal and mantle 

materials in various proportions or to assimilation of 

deep-crust material in magmas rising from the mantle. 

Gilluly (1965, p. 28) made similar suggestions.

A mechanism which would permit generation of a 

magma in the mantle, followed by great assimilation 

of crustal materials in the rising magma, and which is 

capable of producing wholly leucocratic rocks, would 

appear to satisfy all the requirements set forth. Such 

a mechanism is available if we combine Dickson's 

concept of zone melting with that of partial melting 

within the upper mantle and lower crust.

Part of the rise of any magma must be accomplished by zone melting (Dickson, 1958), and this 

could result in profound modification of the original 

magma by contamination. The pressure gradient to 

which any magma of appreciable vertical extent is 

subjected must cause migration of the most volatile 

components toward the top of the chamber. This 

results in a lowering of the melting temperature at 

the top and a raising of the melting temperature at 

the bottom, so that roof rocks are melted and incorporated into the melt, whereas the basal magma is 

forced to crystallize. The rising magma becomes 

progressively enriched in the lowest melting components of both the initial magma and the rocks through 

which it passes. The energy needed to melt the roof 

comes from crystallization at the base. The energy 

loss as cold rocks are heated is partly compensated 

for by the lowering of temperatures of fusion as 

pressure decreases with rise of magma to higher

2 Moore plotted high-alumina and other provinces together. The relation bet-ween the K/Na ratio and gravity anomalies becomes more 

regular -when only high-alumina provinces are considered, and so provinces of tholeiite, olivine basalt, basalt and rhyolite, and highly alkaline rocks which all may originate beneath the eclogite boundary 

are logically omitted.

232-376 67 1

levels, and by the rise of magma through a graded 

or layered crust which becomes less refractory upward.

The final magma resulting from such a rise by 

zone melting could contain only a trifling quantity 

of material present in the initial melt: an energy 

envelope has risen through the crust, but only the 

most volatile of the initial components are present 

in the final magma. Magma can be mobilized beneath 

the crust, and yet the final high-level igneous rock 

can consist largely of components derived from the 

crust. The correlations between compositions of granitic rocks and the compositions of the columns through 

which they rose can thus be easily explained, as can 

the leucocratic character of many granitic rocks and 

the lead and strontium isotopic relationships in them.

The effects of differentiation, both by fractional 

crystallization and by upward migration of the less 

refractory components, must further complicate the 

evolution of the magmas. Zone melting of course 

represents a combination of assimilation and differentiation.

The mechanisms suggested here require that the 

crust be continually growing as material is added to 

it from the mantle. The general restriction of potassic batholiths to terraiies more than 1,000 m.y. old 

perhaps indicates that the mantle and crust became 

by that time so differentiated that potassium-rich 

magmas could no longer be generated in great volume 

from the mantle.

Silicic and high-alumina magmas form primarily 

in long belts, initially either oceanic or continental, 

whose tectonic mechanisms are not yet apparent; but 

the magmas can form anywhere in the high-heat-flow 

regions of the continents.

EMPLACEMENT OF BATHOLITHS

Batholith magmas are melted in the lower crust 

and upper mantle. The buoyant magma masses rise 

and probably become completely detached from their 

zones of melting. Overlying rocks are heated and 

displaced outward, then sink and flow beneath the 

rising plutons, becoming intensely metasomatized and 

injected by granitic material. Much additional granitic material is enveloped by the flowing gneisses, 

joins in their irregular motion, and crystallizes in 

concordant foliated sheets.

Such interpretations have been made by other geologists. Hans Cloos (1923) concluded that many batholiths are connected with deep sources by dikelike 

channels rather than by full-size chambers and that 

concordance and discordance are determined by the 

structure of the country rock. Chamberlin and Link 

(1927) suggested that batholiths are shaped like 

sheets, tongues, and mushrooms, which have narrow
C22 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

feeders and spread out high in the crust. Lane (1931, 

p. 823) wrote of batholiths as "intrusions on the 

surface" flat mushrooms, reaching the surface in 

some places and spreading out between basement and 

overlying sediments in other places, fed by relatively 

narrow conduits. Bott and Smithson (1966) concluded from gravity analyses that granitic plutons 

extend typically to depths of only about 10 km.

The plutons of magma rise until crystallization, 

forced by loss of heat and volatiles, prohibits further 

flow, or until they reach the surface. As the plutons 

stiffen, heating of the wallrocks decreases, metasomatism virtually stops, and thin aureoles of thermal 

metamorphism result. Wallrock screens are dragged 

upward and pushed outward by rising and expanding 

plutons, and roofs are raised. The deformation of 

the metamorphic rocks may be due largely to emplacement of the plutons.

The larger and more numerous the plutons, the 

higher they should rise. Many must reach the surface 

and produce voluminous volcanic eruptions. Any 

pluton that reaches the surface over a wide area is 

roofed only by its own volcanic ejecta, and granitic 

textures develop as the magma crystallizes beneath 

its insulating volcanic cover.

Large batholiths are formed by the coalescence of 

many plutons and may be less than 10 km thick and 

may be unroofed over broad areas. The plutons rise 

a minimum of perhaps 40 km before reaching their 

site of final crystallization, which is generally within 

a few kilometers of the surface. The loss of volatiles 

by eruption through roof rocks must stop the rise 

of many plutons, but it is difficult to visualize any 

mechanism which could prevent the broad surface 

breakthrough of many other large masses of light 

magma.

The exposed parts of the Boulder and St. Francois 

batholiths are covered almost entirely by consanguineous volcanic rocks, and such a relationship may be 

common. Volcanic crusts may float on broad plutons 

of batholithic magma.

The Boulder batholith probably is floored by a 

downbowed surface of prevolcanic rocks. This shape 

is indicated by the way the flanking rocks dip right 

side up beneath the batholith on the north and probably on the south; the shape is also indicated by the 

horizontal flow shown by internal structure in the 

west part of the batholith, although the west contact 

is hidden. The sagging presumably accompanied the 

withdrawal of magma at depth and its eruption at 

the surface. Near both east and west margins of the 

batholith, the volcanic roof rocks dip monoclinally 

outward, perhaps in response to lifting by the growing batholith. The final major eruptions of magma 

through the preigneous rocks apparently were primarily intrusive; the first eruptions were certainly volcanic. Extrusion through the volcanic crust and intrusion beneath it probably occurred simultaneously 

during much of the history of the complex: new 

injections of magma beneath the crust fed eruptions 

at the surface so that crust and batholith thickened 

simultaneously, although the volcanic rocks spread far 

beyond the batholith. The sharp contacts between 

some plutons in the batholith show sporadic intrusion 

and crystallization and it cannot yet be said how much 

of the batholith was molten at any one time. The 

size of the vents through the prevolcanic rocks, which 

fed the shallow batholith and its volcanic cover, is 

not known, but the great area of the complex suggests 

that they were large.

The central part of the Sierra Nevada batholith is 

flanked by steeply dipping metamorphic rocks whose 

tops face inward toward the batholith. If this structure is related to the batholith, rather than being an 

old structure fortuitously intruded by the granitic 

magmas, then conceivably it has an explanation similar to the structure flanking the Boulder batholith. 

The Sierran plutons may also have reached the surface 

and spread laterally as the wallrocks were pushed 

outward and downward. If this is so, the volcanic 

cover once above the plutons has been removed by 

erosion. The overlapping ages of granitic rocks and 

wallrocks complicate any interpretation, and some of 

the metavolcanic wallrocks may have formed as roof 

rocks of early plutons.

The largest Phanerozoic batholiths now exposed are 

of late Mesozoic age. This fact suggests either that 

this was a period unique in the earth's history or else 

that batholiths are features of the uppermost crust 

and are so thin that 200 million years' erosion commonly suffices to remove them. We prefer the latter 

alternative. Most gneiss terranes may have formed 

beneath batholiths since removed by erosion, and the 

steep metamorphic gradients bounding some gneiss 

terranes may mark the limits of the belts through 

which the plutons rose. There are, however, many 

large Paleozoic and Precambrian batholiths, so either 

such batholiths were less eroded than their largest 

correlatives, or else they extended to greater depths 

than we infer.

The rise of plutons can be likened to that of salt 

domes. Magma and salt both rise because they are 

lighter than the rocks above them, although the density differential is probably in general greater for 

magma and metamorphosed wallrocks than for salt 

and indurated sedimentary rocks. Isolated salt plugs
THE NATURE OF BATHOLITHS C23

form where the supply of salt is small, whereas salt 

megadikes form where the supply is great. The 

varied salt structures demonstrated by drilling in 

northern Germany and described by Trusheim (1960) 

are strikingly similar geometrically to various Late 

Cretaceous and early Tertiary igneous complexes in 

western Montana. An inference to be drawn from 

the salt-dome analogy is that plutons of magma do 

not mark the locations of hot spots in the mantle, 

but rather that magma is generated over broad regions 

at depth and coalesces into masses whose spacing is 

controlled by the supply of magma and whose position and shape are controlled by structural features 

of the crust. Batholiths form where the supply of 

magma is so great that the masses coalesce and rise 

toward the surface as large plugs and megadikes.

BATHOLITHS AND METAMOEPHISM

The deformation and heating shown by metamorphic rocks may be largely products rather than 

causes of magmatism. The steep metamorphic gradients of many terranes of regionally metamorphosed 

rocks cannot be quantitatively explained as due to 

conducted geothermal heat, but can be easily understood as produced by contact metamorphism on a 

regional scale. Gneisses and migmatites, ascribed by 

many writers to anatexis (partial melting in place 

due to ultrametamorphism), may instead be produced 

largely by metasomatism and injection by rising 

plutons. The highly plastic flow patterns of gneisses 

may form while wallrocks sink and close beneath 

rising plutons. The contortions in low-grade metamorphic rocks flanking high-level batholiths also may 

have been produced largely by rising plutons.

Similar suggestions have been made by many 

authors. Sederholm (1919, p. 250) wrote of "Kontaktmetamorphose regionaler." Barrell (1921, p. 255) 

concluded that "batholithic invasion is * * * one of 

two major factors in dynamometamorphism." Buddington and Chapin (1929, p. 50) used the phrase 

"contact metamorphism on a regional scale."

The metamorphic rocks of the east-central Sierra 

Nevada include mineral assemblages typical of the 

contact-metamorphic hornblende hornfels facies and 

the regional-metamorphic almandine amphibolite facies of Turner (in Fyfe and others, 1958). Bateman, 

Clark, Huber, Moore, and Einehart (1963, p. D 11) 

assumed that this combination indicates that the rocks 

formed at a depth whereat the conditions of contact 

metamorphism were giving way downward, because 

of increasing pressure, to conditions of regional metamorphism; and they accepted Turner's (fig. 107 m 

Fyfe and others, 1958) guess that this transition

might occur at a pressure corresponding to a depth 

of about 20 km and specified this as the depth of 

crystallization of the Sierran plutons. (Interpretations based on granitic rock compositions led Bateman 

and others to infer a water pressure of 5 kbars, equivalent to about the same depth.) The contrasting 

metamorphic mineral assemblages, however, occur in 

rocks of different compositions, and we regard this as 

evidence that the mineralogical contrasts reflect compositional differences rather than some critical pressure and that the distinction made between contact 

and regional metamorphism has no meaning here. 

The metamorphic grade and degree of recrystallization of the wallrocks decrease away from granitic 

contacts (Bateman and others, 1963, p. D 11), and 

the regional-type metamorphic rocks clearly owe the 

heat of their metamorphism to the nearby intrusive 

plutons. The pressure assigned the almandine amphibolite facies by Turner was based on the assumption that rocks of that facies were heated by conducted geothermal heat. If such an assumption is 

accepted, great depth must be postulated even if a 

steep thermal gradient is also assumed; but as this 

heat demonstrably came from intruded magmas, and 

not from deep burial, the assumption is not valid. 

Hamilton (1963a, p. 89-93) raised numerous objections to the depth-zone correlations and to the oversimplified applications of facies concepts that are still 

prevalent in the literature of metamorphic petrology. 

Not all medium- and high-grade metamorphism 

can be explained as regional effects of batholithic intrusion, however. The Blue Eidge province of the 

southern Appalachians, for example, comprises a metamorphic plateau of general staurolite- or kyanite-zone 

rocks with local highs of sillimanite-zone rocks (for 

example, Bryant, 1962, p. 20-23). Upper Precambrian rocks were metamorphosed progressively to 

these grades during early or middle Paleozoic time, 

when the older Precambrian basement rocks (which 

make up most of the Blue Eidge) were metamorphosed retrogressively to the same grades. The metamorphic slope bounding the plateau is rather gentle: 

the biotite and kyanite isograds are separated by an 

average distance of 20 km in the Great Smoky Mountains (Hadley and Goldsmith, 1963, pi. 3). Known 

Paleozoic igneous activity in the province is limited 

to small dikes and to large and small pegmatites in 

scattered areas. Inasmuch as effects of metasomatism 

are limited to these areas, passage of plutons upward 

through the entire exposed terrane cannot reasonably 

be postulated, nor is there evidence to suggest the 

presence of a hidden batholith beneath the entire 

province. Severe deformation accompanied the meta-
C24 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

morphism, and it is likely that the temperatures of 

deep burial were much increased by deformational 

heating.

BATHOLITHS AND THRUST FAULTING

Many thrust faults are temporally and spatially 

related to batholiths. The relationship might be 

ascribed to various factors, but one that must be 

considered in each such relationship is that the intrusion of a batholith is responsible for the thrusting. 

Keith (1923, p. 365-375), among others, has proposed 

such mechanisms, which are worthy of serious evaluation even though they are not currently popular.

The belt of late Mesozoic batholiths of the Western 

United States is flanked on both sides by broad terranes deformed by thrust faults during late Mesozoic 

and very early Cenozoic time. On the east is the 

Laramide thrust belt, whose east margin is coincident 

with that of geosynclinal strata and trends southsoutheastward through western Montana, southward 

across westernmost Wyoming, south-southwestward 

across western Utah and southern Nevada, and intersects the crystalline terrane in southeastern California. 

The belt widens again in southern Arizona and trends 

into Mexico. The distance of the east margin from 

the Idaho, Sierra Nevada, and related batholiths 

ranges from 50 km in southeastern California to as 

much as 500 km across Nevada and western Utah. 

(The maximum width of the belt may have been 

less than 300 km when it formed, however, for the 

wide sector may have undergone great extension during the formation of basin-and-range structures and 

of volcanic piles during Cenozoic time.) The geosynclinal assemblage is shingled by thrust faults eastward from the edge of the batholiths (although interpretation is clouded by the widespread late Paleozoic 

thrust faults present at least in central Nevada). 

Major thrust faults generally are nearly parallel to 

bedding of the rocks. Faults in the west and central 

parts of the belt tend to be subhorizontal or gently 

folded, and many carry younger strata over older 

strata, whereas major faults in the east generally dip 

westward and carry older rocks over younger. Thrusting probably preceded folding in most regions, and 

most of the folding is better regarded as an effect of 

the sliding that produced the thrusting than as a 

cause of thrusting. At least in western Wyoming and 

southeastern Idaho, thrusts in general become younger 

eastward within the belt (Kubey and Hubbert, 1959, 

p. 188). The thrust-belt rocks are not metamorphosed 

except close to the batholiths on the west. Individual 

faults have horizontal displacements reaching at least 

40 km, as proved by older-over-younger exposures in 

southeasternmost British Columbia, and the aggregate

displacement across the belt may reach several times 

this amount. Basement rocks do not appear in the 

overthrust sheets.

The lack of basement rocks in the overthrust sheets, 

despite great displacements, requires that the exposed 

faults are confined to the geosynclinal fill and do not 

reach the basement. It has been inferred from this 

relationship, in the Western States and elsewhere, 

that the strata slid off some highland arch but the 

thrust-faulted terrane extends westward to the batholithic belt, so such a hypothetical highland cannot 

be placed anywhere east of the batholiths. Possible 

explanations are that the blanket slid eastward off 

the batholiths themselves or that the rising and spreading batholiths shouldered the sedimentary rocks eastward. The lack of basement rocks in the thrust 

sheets would require that, in the latter explanation, 

the batholiths expanded eastward above the basement 

rocks.

The thrust belt is particularly irregular in the 

region of the Boulder batholith and neighboring small 

batholiths which lie within the belt in southwestern 

Montana. (See the geologic map of Montana: Eoss 

and others, 1955.) The east margin of the belt 

swings from a southwestward trend to a southeastward trend to define a curve rudely concentric to the 

Boulder batholith and 50 km or less from it. Trends 

of thrust faults within the belt are very irregular 

because early structures have been deformed by later 

ones which have a tendency toward concentricity 

about the batholith. Jumbled thrusts trend northward a similar distance west of the batholith. One 

possible explanation is that shouldering by the batholith produced much of the thrusting. As the batholith thickened, rocks of its inward-sloping floor 

would be forced plastically outward. Another alternative is that once formed, the stiff plate of the batholith greatly influenced the response of the sedimentary terrane to thrusting due to other causes. Both 

processes may have operated together.

We interpret published geologic maps of southwestern Montana as indicating that a relatively simple 

system of east-directed thrust faults had been formed, 

perhaps by the shouldering action of the Idaho batholith, before the eruption of the Boulder batholith 

and its neighbors at the end of Cretaceous time. The 

younger batholiths superimposed complex local patterns of outward thrusting upon this eastward-thrust 

terrane. We interpret the structure of the northern 

Flint Creek Kange (McGill, 1965), for instance, as 

recording the superposition of generations of folds 

and thrusts in this sequence: (1) Eastward thrusting 

from the direction of the Idaho batholith to the west.
THE NATURE OF BATHOLITHS C25

(2) westward thrusting from the Boulder batholith 

to the east, and (3) northward crowding by the small 

Philipsburg and Royal batholiths to the south (see 

Mutch and McGill, 1962).

Thrusting west of the batholiths is of comparable 

extent but different type. Great Late Cretaceous 

overthrusts carry variably metamorphosed eugeosynclinal materials, intruded by stocks and small batholiths during Cretaceous (?) and Jurassic, and possibly older, orogenies, westward over similar rocks and 

over little-metamorphosed Upper Jurassic and Cretaceous deposits. Such overthrusts occur throughout 

the Klamath Mountains of northwestern California 

and southwestern Oregon (Irwin, 1964) and in western Idaho (Hamilton, 1963b), and may be present in 

northwestern Washington (Hamilton, 1963b; Misch, 

1952, and Barksdale, 1960, make other interpretations). South of the Klamath Mountains, throughout 

the length of the Coast Ranges of California, an 

eastern facies of miogeosynclinal Cretaceous and 

Upper Jurassic strata has apparently been thrust 

westward over the eugeosynclinal Franciscan facies of 

rocks of the same age, the fault extending into the 

basement rocks beneath the eastern facies (Brown, 

1964; Irwin, 1964). Displacements on single faults 

may exceed 100 km in the Coast Ranges and in Washington. Sheets of serpentine and peridotite lie along 

many of the faults.

These western thrust faults broke the basement 

rocks and tapped sources of ultramafic material, and 

therefore are very different from the thrusts east of 

the batholiths. The age of major thrusting approximately coincides with the age of major plutonism, 

so the two processes may have been related genetically, 

although it is difficult to visualize a satisfactory 

mechanism linking them.

There was much thrusting of sedimentary rocks at 

least in Nevada during late Paleozoic and early Mesozoic intervals. No batholiths of corresponding age 

are known in the region (Gilluly, 1965, p. 23). Great 

thrust faults can form without assistance from batholiths.

BATHOLITHS AND YOUNGER STRUCTURE

Batholiths, once formed, influence subsequent deformation because of their mechanical strength. The 

largest unbroken mountain masses in the Western 

States the Sierra Nevada and the mountains of central Idaho are carved from those parts of the batholithic belt which contain the highest proportion of 

granitic rocks and the smallest proportion of metamorphic rocks. The fault zone bounding the Sierra 

Nevada block on the east cuts obliquely through the

batholithic belt, which curves northeastward from the 

northern Sierra and southeastward from the southern 

Sierra, but the fault zone approximates the eastern 

limit of almost continuous granitic rocks within the 

belt. The crystalline belt contains a higher proportion of metamorphic rocks along strike to both the 

northeast and southeast on the east side of the fault 

zone, where it is broken into numerous large and small 

basin-and-range blocks. Similarly Baja California 

and the Peninsular Ranges of southwestern California 

are composed of almost continuous batholithic rocks, 

which here form the west part of the belt; the east 

part, in southeastern California and western mainland Mexico, contains abundant metamorphic rocks, 

and is broken into many fault blocks. The large 

basin-and-range fault blocks striking northwestward 

into central Idaho abruptly lose structural relief and 

vanish as they intersect the Idaho batholith. Other 

fault blocks lie north, west, and east of the batholith, 

but no large ones break it (Hamilton, 1962). The 

Coast Range of British Columbia is another massive 

mountain block consisting largely of batholithic rocks. 

The history of batholithic mountains indicates that 

the presence of batholiths markedly influences uplift 

and erosion. Consider the Sierra Nevada batholith, 

which was sufficiently raised within Late Cretaceous 

time that its roof rocks (whether mostly volcanic, as 

we suggest, or metamorphic, as most geologists would 

infer) were eroded away before Eocene time. Mountains were still present in early Tertiary time, for 

middle Eocene strata in the foothills lap upon a 

bedrock surface whose local relief was at least 350 m 

(Alien, 1929, p. 382), and very coarse Eocene and 

Oligocene river boulder gravels are preserved in fossil 

valleys throughout the northern Sierra (for example, Lindgren, 1911, pis. 4, 21). The west base of the 

mountain block has remained near sea level throughout Tertiary time, and marine and nonmarine deposits of various ages lap onto it. The rest of the 

block has apparently undergone both subsidence and 

uplift within the Tertiary but not profound erosion. 

The widespread preservation of lower Tertiary deposits in the northern part of the Sierra shows that 

ridge surfaces there are not far from the early Tertiary surface, although there has been subsequent 

uplift and tilting. Lower Tertiary deposits are lacking in the central and southern Sierra, but presumably there also middle and late Cenozoic erosion has 

been limited largely to incision. During middle Tertiary time the Sierra Nevada was relatively low, and 

Miocene floras of the northern Sierra crest region 

indicate an altitude much lower than the present 

one (Axelrod, 1957). By the end of the Pliocene,
C26 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

local relief in the crest region reached at least 2,000 m 

(Dalrymple, 1964), so its altitude must have been as 

high or nearly as high as it is now. Much of the 

faulting that outlines the present Sierra block has 

occurred within late Pliocene and Pleistocene time 

(Dalrymple, 1964), and thus may have consisted primarily of the downdropping of blocks (such as Lake 

Tahoe and Owens Valley) on the east rather than of 

the upraising of the Sierra block itself.

The late Cenozoic uplift of the Sierra Nevada 

must be related to the deep downbulge in the Mohorovicic discontinuity beneath it. The range is high 

because the root is deep. Had this root formed at 

the same time as the batholith, the range would long 

ago have reached maximum altitude by rising isostatically; instead, therefore, much of the downbulge 

must be a late Cenozoic development. (Christensen, 

1966, reached the same conclusion.) Sierran silicic 

batholithic rocks have a high rate of production of 

radioactive heat, which increases markedly with potassium content (Wollenberg and Smith, 1964), and 

perhaps this property is responsible for the root 

growth and uplift. The highest part of the 650-kmlong Sierra Nevada is the 300-km segment, between 

Sonora Pass and Olancha, in which quartz monzonite and silicic granodiorite form most of the eastern part of the block. The lower crest north and 

south of this segment, and the middle-altitude region 

west of the high segment of the crest, consist largely 

of granodiorite, and the western foothills consist 

mostly of quartz diorite and metamorphic rocks. 

Eadioactive decay would retard cooling, particularly 

of the crestal silicic granodiorite and quartz monzonite; thus a partial barrier to conduction of heat 

from the mantle would be formed, and heating of 

the lower crust and upper mantle would result. 

Movement of crust or mantle materials, change in 

pressure-phase mineralogy or hydration state, or partial melting might have resulted from this deep heating and so enlarged the Sierra root.

Other late Mesozoic batholiths also tend to form 

mountain masses now standing higher than the surrounding nonbatholithic terranes, and the highest regions within the batholithic mountains tend to be 

those with the most silicic and potassic rocks. In the 

Coast Range of British Columbia, for example, the 

inland belt of quartz monzonite and granodiorite 

stands on the average about 1 km higher than the 

coastal belt of quartz diorite.

Regional or global events unrelated to the batholiths may of course have caused such observed relationships. It does appear, however, that batholiths 

are uplifted selectively two or more times, and if so

they must also be eroded selectively more than neighboring terranes. Batholiths perhaps cause their own 

destruction. The general scarcity of great preMesozoic batholiths is explicable in such terms. So 

are such along-strike transitions as that from the 

gneiss terrane of New Hampshire to the low-grade 

complex of Maine: the batholith that formed above 

the gneisses caused greater uplift and erosion there 

than in Maine, where the plutons were not numerous 

enough to have such an effect.

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C28 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

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THE NATURE OF BATHOLITHS C29

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C30 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

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NOTE

The discussion of the Idaho batholith in this report (p. C5-C6) was based in 

part upon the interpretation by Ross (1934) that the Casto Volcanics was 

intruded by the Cretaceous batholith. This interpretation was disproved during 

the 1966 field season, when all rocks assigned by Ross to the Casto west of the 

Middle Fork of the Salmon River were mapped by Frederick W. Cater, Warren 

Hamilton, Benjamin F. Leonard 3d, Raymond L. Parker, and Edwin V. Post. 

These rocks were found to be that part of the Challis Volcanics (of Eocene 

age as dated by Axelrodf) which has been altered or converted to hornfels 

by contact raetaraorphism by a Tertiary batholith of granite and quartz raonzonite. (Ross recognized that the young batholith was Tertiary, but he did 

not see its contacts with the volcanic rocks and did not recognize that fresh 

Challis grades into the altered rocks which he called Casto.) The "Casto 

Volcanics" apparently does not exist in the sense intended by Ross. This finding 

negates the argument in this paper that the Casto might be part of the extrusive 

cover beneath which the Idaho batholith crystallized but it is wholly in accord 

with the general concepts developed here, for the quartz latite welded tuffs 

of the Challis form the roof beneath which the Tertiary batholith crystallized. 

The tuffs presumably formed largely as the ejecta of that batholith.

t Axelrod, D. L., 1966, Potassium-argon ages of some western Tertiary floras : Am. Jour. 

Sci., v. 264, p. 497-506.

o