Differentiation of a Gabbro Sill 

in the Oregon Coast Range by
Differentiation of a Gabbro Sill 

in the Oregon Coast Range by 

Crystallization-Zone Settling

By NORMAN S. MacLEOD

GEOLOGICAL SURVEY P R O F E S S I O N A L P A P E R 1165

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1981
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Doyle G. Frederick, Acting Director

Library of Congress catalog-card No. 81-6000^9

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

Washington, D.C. 20402
CONTENTS

Page Page

Abstract ........................................................................................................ 1 Order of crystallization ........................................................................ 8

Introduction ................................................................................................ 1 Petrochemistry ........................................................................................ 8

Geologic setting ........................................................................................ 1 Composition of the chilled border ............................................ 8

General features ........................................................................................ 2 Composition of rocks from the interior of the sill .............. 9

Petrography ................................................................................................ 3 Compositional variations with height ............................................ 9

Chilled basalt .................................................................................... 3 Bulk composition of the sill ................................................................ 10

Gabbro .................................................................................................. 3 Residual liquid compositions .............................................................. 12

Ferrogranophyre ................................................................................ 3 Chemical variation ................................................................................ 14

Pegmatite ....................:....................................................................... 6 Process of differentiation .................................................................... 15

Granophyre ........................................................................................ 6 Summary .................................................................................................... 21

Mineralogical data .................................................................................... 6 References cited ...................................................................................... 22

Textural relations ...................................................................................... 7

ILLUSTRATIONS

	Page

FIGURE 1. Map showing distribution of Oligocene gabbro sills in Euchre Mountain and Valsetz quadrangles, Oregon .................. 2

2. Diagrammatic section showing distribution of rock types in gabbro sill ..................................................................................... 3

3-12. Diagrams showing:

3. Composition of pyroxene from Stott sill ................................................................................................................................................ 7

4. Vertical variation in modes, mineral compositions, major oxides, and oxide ratios in lower gabbro zone .................. 10

5. Amount of major oxides in rocks and bulk residual liquids at various stages in solidification of gabbro sill .......... 14

6. Silica variation for rocks from gabbro sill ............................................................................................................................................ 15

7. FMA variation ...........................................................................................................................................................................^ 15

8. Progressive solidification of gabbro sill ................................................................................................................................................ 17

9. Inferred crystallization and settling of mineral in lower zone of crystallization .................................................................. 18

10. Calculated settling rates of minerals ...................................................................................................................................................... 18

11. Proportion of liquid that added to rock duplicates composition of residual liquid ................................................................ 20

12. Relative distance that olivine could settle as a function of height .............................................................................................. 20

TABLES

Page

TABLE 1. Modal, mineralogical, and chemical data for rocks from the lower gabbro zone .......................................................................... 4

2. Chemical analyses and norms of rocks from the gabbro sill ................................................................................................................ 4

3. Chemical analyses of chilled gabbro .............................................................................................................................................................. 9

4. Average compositions of rocks .......................................................................................................................................................................... 11

5. Compositions of successive residual liquids ................................................................................................................................................ 12

6. Comparison of calculated residual liquids to compositions of mixtures of analyzed rocks and silicic liquids .................... 13

III

DIFFERENTIATION OF A GABBRO SILL

IN THE OREGON COAST RANGE 

BY CRYSTALLIZATION-ZONE SETTLING

BY NORMAN S. MACLEOD

ABSTRACT

At Stott Mountain in the Oregon Coast Range, an iron-rich 

granophyric gabbro sill, 90 to 229 m thick, is strongly 

differentiated but shows no rhythmic layering or crystal 

lamination. Gabbro in the lower half is progressively more mafic 

upward from the base to the center of the sill. Above the center the 

rocks are progressively more felsic, and ferrogranophyre and 

granophyre are abundant. At the top of the sill, gabbro like that 

near the base forms a thin zone below the upper chilled margin. 

The bulk composition of the sill and the composition of the chilled 

margins are virtually identical. Thus differentiation occurred in 

place without significant assimilation of country rock or influxes 

of magma of different composition.

Differentiation probably resulted from settling of crystals that 

were precipitating in two zones that moved inward from the 

bottom and top of the sill as it solidified. In the lower zone, crystals 

formed over a thermal gradient and settled according to their 

height of nucleation within the zone and their density and size. As 

the lower crystallization zone progressed continuously upward, 

differentiation became progressively more pronounced because 

the thermal gradient within the zone decreased, consequently 

increasing the thickness of the zone and the time available for 

settling. The minerals changed composition upward because an 

increasingly greater proportion of relatively silicic residual liquid 

was displaced upward out of the lower zone and thus was not 

available for reaction. In the upper zone crystals also settled, but 

they were apparently resorbed as they passed into the higher 

temperature liquid interior of the sill. The ferrogranophyre and 

granophyre in the upper part of the sill formed from late residual 

magma.

INTRODUCTION

A gabbro sill in the central part of the Oregon 

Coast Range contains rocks that range widely in 

composition; for example, their SiO2 content ranges 

from 51 to 76 percent, and Fe2O3+FeO from 2 to 18 

percent. The bulk composition of the sill is nearly the 

same as that of the chilled margins; thus 

differentiation must have occurred in place. The 

chemical, modal, and mineralogic composition of 

rocks collected at intervals in the lower half show 

that the sill is progressively more mafic upward from 

the base to the center. This part of the sill is massive 

with no crystal layering or lamination, and rock 

textures are not obviously cumulate. Nevertheless, 

the cryptic vertical change in composition appears to

be the result of crystal settling. In succeeding 

sections of this report the petrography, mineralogy, 

and textures of the rocks, the order of crystallization 

of the minerals, and the vertical modal, mineralogic, 

and chemical variations are described. In the last 

section they are shown to be consistent with differentiation by crystal settling within crystallization 

zones that moved progressively toward the sill's 

interior as it cooled and solidified.

This report is a product of geologic mapping and 

petrochemical studies of volcanic and intrusive rocks 

of the Oregon Coast Range carried out by P. D. 

Snavely, Jr., and the author. Snavely kindly made 

available several chemical analyses of the gabbro 

sill which he obtained prior to this study, as well as 

analyses of related sills farther south. I am indebted 

to him for providing the opportunity for this study 

and for his encouragement and advice.

GEOLOGIC SETTING

Granophyric gabbro sills underlie many of the 

higher mountains in a 5,000-km2 area of the Oregon 

Coast Range between lat 43°45' N. and 45° N. 

(Snavely and Wagner, 1961; Baldwin, 1964; Snavely 

and others, 1976). Three K-Ar ages on minerals from 

these sills range from 29.9 to 31.6 million years 

(Tatsumoto and Snavely, 1969; P. D. Snavely, Jr., 

unpub. data) and indicate an Oligocene age. The sills 

are typically 120 to 170 m thick; the thickest, at 

Marys Peak, near Corvallis, Oreg., is about 335 m 

(Roberts, 1953; Baldwin, 1955; Clark, 1969).

The granophyric gabbro sill near Stott Mountain, 

the subject of this report, is the northernmost of this 

group of sills (MacLeod, 1969). It was formerly more 

widespread, but now crops out only as erosional 

remnants on higher ridges and peaks in the Valsetz 

and Euchre Mountain 15-minute quadrangles (fig. 1). 

No feeder dikes were found near Stott Mountain, but 

two large dikes to the south (fig. 1) are similar in 

composition to the Stott sill and either may formerly 

have been connected to it.

The gabbro sill intrudes sandstone and siltstoneof
DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE

123°30'

figure 1 ] OREGON

44 45'

Figure 1. Distribution (cross-hatched areas) of Oligocene gabbro sills, dikes, and inclined sheets in Euchre Mountain (Snavely and 

others, 1976) and Valsetz (Baldwin, 1964; MacLeod, 1969) 15-minute quadrangles, Oregon. Dashed line outlines Stott Mountain 

sill; dikes and inclined sheets that may have fed Stott sill trend west-northwest and north-northeast from Diamond Peak. Solid 

circle shows location of samples described in table 1.

the middle Eocene Tyee Formation and siltstone 

with interbedded tuff, basaltic sandstone, and 

glauconitic sandstone of the Yamhill Formation of 

middle and early late Eocene age. These deep-water 

marine sedimentary rocks overlie basalt breccias, 

pillow flows, tuffs, and massive flows of the lower 

and middle Eocene Siletz River Volcanics, which are 

the oldest rocks exposed in the Coast Range (Snavely 

and others, 1968).

The Stott Mountain sill ranges in thickness from 

90 to about 220 m; several small sills, less than 1 m to 

about 45 m thick, occur in a few places above and 

below the main sill. Locally the sill becomes an 

inclined sheet with discordance to sedimentary 

bedding of 5° or more.

The sedimentary rocks are baked as far as 10 m 

from the sill. Hornfels with incipient andalusite and, 

less commonly, cordierite porphyroblasts have 

resulted from baking of siltstone in a few places; 

sandstone appears bleached, but only its matrix minerals are recrystallized. No evidence of assimilation 

of these sedimentary rocks was found within the sill.

GENERAL FEATURES

The chilled borders of the sill range from 0.5 m to 3 

m in thickness. The chilled basalt grades inward to 

fine- and medium-grained gabbro. The interior of the 

sill is divided into three intergradational zones (fig. 

2). The thick lower and thin upper gabbro zones are 

separated by a silicic zone composed of ferrogranophyre, granophyre, and pegmatite. The sill 

was systematically sampled from base to top along a 

small ravine on the north side of Stott Mountain (fig. 

1) where the sill is about 140 m thick. Here the lower 

gabbro zone is about 98 m thick, the silicic zone, 

about 35 m, and the upper gabbro zone is about 7 m 

thick.

Gabbro in the lower and upper zones is massive in 

appearance. Field and thin-section studies show that 

it is not rhythmically layered and that it lacks crystal 

lamination.

The silicic zone is heterogeneous. It is composed
PETROGRAPHY

Gabbro

Granophyre, 

pegmatite, 

ferrogranophyre 

silicic gabbro as 

gradational 

masses and 

tra regressive 

___bodies

Gabbro

} Baked sedimentary rocks 

> Chilled border 

Upper gabbro zone

Silicic zone

'50 METERS

L- 0 

Approximate scale

Chilled border

Baked sedimentary rocks

Figure 2. Distribution of rock types in gabbro sill.

principally of ferrogranophyre but also contains 

granophyre, pegmatite, and silicic gabbro. Transgressive bodies composed of ferrogranophyre, 

granophyre, and pegmatite occur in the upper part of 

the silicic zone, and some extend into the upper 

gabbro zone. Most of the transgressive bodies 

parallel the upper contact of the sill, extend laterally 

less than 50 m, and are 2-10 m thick. In detail they 

are very irregular and commonly bifurcate. Contacts 

of the bodies with the enclosing rock are sharp but 

show no evidence of chilling. The transgressive 

bodies are interpreted as resulting from injection of 

late residual magma produced by differentiation 

within the sill into its upper part. The limited 

horizontal extent of individual bodies suggests that 

the residual magma was derived nearby.

PETROGRAPHY

CHILLED BASALT

The chilled borders of the sill are composed of darkgray very fine grained basalt. The grain size 

gradually increases from less than 0.1 mm for larger 

crystals at the contact to about 1 mm at 3 m from the 

contact. The chilled basalt has an intersertal or 

intergranular and variolitic texture. Most is aphyric, 

but some contains rare plagioclase microphenocrysts less than 1 mm long. The chilled basalt is 

composed of fine radiating sheaves of plagioclase 

and minute grains of titaniferous ferroaugite, actinolite, biotite, opaque minerals, and clay minerals.

The chilled basalt is extensively altered. Plagioclase is clouded with fine-grained albite, chlorite, 

and (or) clay minerals that surround a few remaining 

relict patches of primary sodic labradorite. Former 

glass(?) is replaced by clay minerals, actinolite, and 

biolite.

GABBRO

The mafic rocks that form the bulk of the the sill are 

not readily classified and are here called gabbro to 

avoid a cumbersome nomenclature. TheSiO2 content 

of 51 to 59 percent is greater than that of most 

gabbros, but the rocks have an unusually high iron 

content (13 to 18 percent as FeO) that results in a 

color index between 30 and 40, and the plagioclase is 

mostly labradorite. Similar rocks from other areas 

have been variously called ferrogranophyre, 

granophyre, granophyric ferrodiabase, quartz 

dolerite, or leucogabbro.

The gabbro is medium dark gray and weathers to 

light gray or reddish brown. Grain size is typically 

0.3-1 mm, and the largest crystals are as much as 3 

mm across. The average grain size increases only 

slightly from the base toward the sill center. The 

texture is granophyric and hypidiomorphic-granular 

and locally slightly subophitic. The gabbro in both 

the lower and upper zones is massive, and neither 

layering nor crystal lamination is present. The 

gabbro changes composition vertically, as discussed 

later, but this variation is not obvious in the field.

The gabbro is composed of plagioclase, ferroaugite, 

iron-rich olivine, apatite, and opaque minerals 

surrounded by quartz and alkali feldspar (both as 

intergrowths and isolated crystals) and patches of 

phyllosilicate minerals (tables 1 and 2). Iron-rich 

hornblende locally occurs as overgrowths on 

ferroaugite; it is also associated with iron-rich 

nontronitic clay minerals, biotite, and chlorite which 

together form the phyllosilicate patches.

Olivine occurs in equilibrium with quartz in the 

gabbro as well as in all other rocks from the sill 

because it has a very high iron content (fo 19_2). 

Olivine also occurs with quartz in late-stage rocks of 

the Skaergaard intrusion, Greenland (Wager and 

Brown, 1968) and quartz-Fe-olivine-Ca-Fe-clinopyroxene are stable coexisting phases at moderately 

low pressure (Smith, 1971).

FERROGRANOPHYRE

The ferrogranophyre is mottled medium gray or 

brownish gray and is lighter colored than gabbro,
DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE

TABLE 1.—Modal, mineralogical, and chemical

Height above base 

of sill (meters) ~~ *-V2

Plagioclase ————— 33.0

Alkali feldspar 2 — 1.7

Py e 9.6

Hornblende —————— .3 

Phyllosilicates —— 11.6

Olivine (percent fo) 13.

Pyroxene : zv ——— 52.

Plagioclase 

(percent an

Fe2°3 —————————— 2 ' 9

"

MODAL ANALYSES

9 13-1/2 24-1/2 29 35 42-1/2 49 62-1/2 70 81 91

1.7 1.1 2.1 3.0 1.5 1.5 1.3 2.2 1.6 1.9 4.7

1.6 2.0 1.5 1.4 1.7 1.1 .1 .4 .2 .6 .7 

10.9 8.9 8.7 8.3 8.5 10.8 11.8 12.2 10.8 11.5 11.4

MINERAL DATA

12. 17. 18. - 17. 18. 18. 18. 19. 19.

51. 52. 50. 51. 51. 52. 51. 51. 52. 52. 50.

CHEMICAL ANALYSES (RECALCULATED HjO-FREE TO 100 PERCENT)

TABLE 2.—Chemical analyses and

Lower Upper

gabbro gabbro

Column number ——— 1234

Rock number —— —— 61-66 60-56 66-124 66-125

Alkali feldspar — 1 1 2 2

Pyroxene ——————— 13 11 9 10 

Olivine ———————— 9897 

Opaque minerals — 3 3 2 5 

Hornblende —————— 1121 

Phyllosilicates — 12 10 11 12

Olivine (percent fo) - 

Ny ———— 1.718 1.722 - 1.723

Gabbro

567

67-216 67-218 67-219

MODAL ANALYSES

333

9 10 10 

6 8 12 

232 

1 2 1/2 

12 7 7

MINERAL DATA

16 15 10

Silicic zone

Ferrogranophyre Pegmatite Granophyre

8 9 10 11 12 13 14 15

66-145 60-62 67-215 66-144 66-136 61-65 66-142 66-135

4 3 10 2 7

6 10 8 5 6 2 

75252 

22215 1/2 

1-311/2 

5 12 12 2 6 2

9 - - 4

2v ———— 51. 50. - 50. 51. 52. 50. 54. 53. 51. 54. 52-56

s) ————— 53.

CHEMICAL ANALYSES (RECALCULATED WATER-FREE TO 100 PERCENT)

A12 O3 ——————————— 13.1 12.9 13.9

20 .7 2.9 2.6

K2 0 1.5 1.5 1.6

13.3 

3.0 

12.3 

1.7 

6.9 

2.7 

1.5 

2.1

14.9 13.3 13.3 13.6 13.0 13.4 13.4 14.3 

4.0 3.3 3.5 2.7 5.6 4.0 5.3 1.8 

9.3 11.1 10.2 9.8 7.6 7.7 5.8 5.4 

1.3 1.4 1.1 .93 .49 .68 .53 .49 

6.7 6.2 5.9 5.4 4.5 4.4 4.6 3.8 

3.1 2.7 2.8 3.0 3.1 3.0 3.4 5.4 

1.6 1.7 1.9 2.2 2.1 2.2 2.5 2.0 

1.6 1.7 1.4 1.3 1.5 1.0 .81 .84

13.9 

1.B 

1.9 

.27 

.54 

6.2 

2.4 

.37 

.04

13.0 

2.7 

.53 

.05 

.33 

3.5 

4.1 

.29 

.00

12.6 

.38 

1.2 

.11 

.76 

4.1 

4.3 

.31 

.02
PETROGRAPHY

data for rocks from the lower gabbro zone

MODAL ANALYSES

Height above base 

of sill (meters) ~ 4 ' 1/2

Rock number ————— M203

9 13-1/2 24-1/2

M204 M205 M206

29

M207

NORMS (CALCULATED WITH Fe.

ab ——————————————— 24.5

fs ——————————— 2.8

hy:

25.4 22.8 22.8 

18.4 19.4 21.4

.5 .6 .4 

2.8 3.1 2.0

17.6 17.4 20.2 

2.1 2.0 2.3

1.7 1.7 2.2

22.8 

19.5

.7 

3.2

18.7 

2.2

2.0

35

M20

2o3 » 10

22. 

19.

3.

19. 

2.

2.

42-1/2

8 M209

49 62-1/2 70 81 91

M210 M211 M212 M213 M214

PERCENT TOTAL Fe OXIDE)

0 21.2 

9 20.9

7 .7 

1 3.0

5 19.7 

3 2.3

0 2.1

22.8 22.8 23.7 21.2 22.0 

20.6 19.2 21.3 21.0 20.8

.8 1.0 .8 .7 .6 

3.6 4.5 3.7 3.2 3.0

19.2 19.0 21.2 21.9 20.0 

2.4 2.4 2.5 2.6 2.4

2.1 2.0 1.6 2.4 2.0

CHEMICAL ANALYSES (ORIGINAL) 3

Si02 ————————————— 56.1

MgO —————————————— 1.4 

CaO —————————————— 5.8 

Na20 ———————————— 2.8

MnO ————————————— .26

2 Individual quartz and alkali 

3Chemical analyses were made

55.7 55.5 54.3

2.5 1.4 2.2

1.4 1.6 1.6

2.9 2.6 2.6

1.3 1.1 .91 

1.3 1.2 1.1 

1.6 1.7 1.8 

.70 .71 .91 

.25 .27 .28

99.5 99.9 100.0

2.2

1.9

2.6

.92

1.8 

.83 

.29

99.8

tzand

3.

1.

2.

1.

1.

100.

feldspar

1 3.2

8 2.0

5 2.4

2 1.4

8 1.8 

81 .85 

28 .30

0 99.9

counted sepa

3.5 2.9 3.5 2.6 3.7

2.0 2.1 2.1 2.2 1.8

2.6 2.6 2.7 2.4 2.5

1.4 1.3 1.5 1.1 1.4

1.9 2.1 1.5 2.2 1.8 

.85 .83 .65 1.0 .83 

.29 .31 .19 .32 .29

99.8 99.8 99.2 99.8 99.9

rately.

described in U.S. Geological Survey Bulletin 1144A, supplemented by atomic absorption. Modal analys 

sample end an additional 2,000 points for opaque minerals and apatite.

based on 2/000 point

norms of rocks from the gabbro sill

Column number ———

Rock number ————

Lower Uppar 

gabbro gabbro

1 2 3

61-66 60-56 66-124

4 5

66-125 67-216

Gabbro

6 7

67-218 67-219

NORMS (CALCULATED WITH Fe 2 0j

8.9 8.9 9.5

19.2 17.8 21.5 

3.0 4.8 3.1 

0.5 0.6 0.5 

2.7 4.7 2.8 

4.2 2.4 3.5

2.7 2.4 2.2

2.4 2.0 1.6

8.9 9.5

19.7 22.0 

4.0 3.0 

0.7 0.5 

3.6 2.8 

3.5 2.8

2.2 1.9

1.8 1.4

10.1 11.2

19.2 18.1 

3.0 3.1 

0.5 0.4 

2.8 3.0 

3.0 2.3

2.1 2.0

1.6 1.3

Perr

8

66-145

ogranophyre

9

60-62

Silicic

10

67-215

zone

Pegmatite

11 12

66-144 66-136

Granophyre

13 14

61-65 66-142

15

66-135

= 10 PERCENT TOTAL Fe)

13.0

17.1 

3.0 

0.4 

2.9 

1.9

1.8

1.0

12.4

15.4 

1.3 

0.1 

1.4 

1.1

1.9

1.4

13.0

16.6 

1.3 

0.1 

1.3 

1.6

1.7 

1.9 

.8

14.8 11.8

13.9 8.9 

3.0 3.7 

0.3 0.4 

3.1 3.6 

1.1 .8

1.6 1.0 

1.5 1.6 

.6 .4

14.2 24.2

2.4 1.6 

.7 .1

.5 .5 

.7 .6 

.1

25.4 

34.7 

3.3 

.2

.2 

.3 

1.9 

.2 

.6 

.1

CHEMICAL ANALYSES (ORIGINAL) 1

«2°3 —— - ——— —— -

H2 0- ————— ——————

2

P2°5 —— —— —— —— - 

MnO ———————————

Total —————————

'chemical analyst

4.0 3.2 3.3 

13.8 12.8 11.5 

1.8 1.2 1.5 

6.4 6.8 6.5

1.4 1.5 1.5 

1.3 1.2 1.2 

1.7 1.0 2.0 

2.6 2.1 2.0 

1.0 .83 .65 

.34 .31 .22

99.6 99.4 100.0

•s were nade by s. Botts, H.

2.9 3.9

1.6 1.3 

6.6 6.5

1.4 1.5 

1.3 1.5 

2.0 1.6 

2.0 1.5 

.73 .59 

.21 .26

99.5 99.9

Smith, G. Chloe, L.

3.2 3.4

1.4 1.1 

6.0 5.7

1.6 1.8 

1.5 1.3 

1.3 1.1 

1.6 1.4

.28 .25 

99.8 99.6

Art is, J. Kelsey, J.

2.6

.91 

5.3

2.1 

.79 

1.7 

1.3

.17 

99.9

Glenn, P. El

5.4

.47 

4.3

2.0 

1.4 

2.0 

1.4

.24 

99.4

more, and I

3.9

.66 

4.3

2.1 

1.2 

1.4 

1.0

.22

99.8

. Barlow

5.1 1.8

.52 .48 

4.5 3.7

2.4 2.0 

1.0 .59 

1.8 1.2 

.79 .82

.14 .04

100.0 99.9

using methods des«

13.8 13.3 

1.8 2.6 

1.9 .52 

.27 .05 

.54 .52

2.4 4.0 

.41 .81 

.85 1.6 

.37 .28

.06 .09

100.6 100.3

:ribed in U.S. Geologi

12.5 

.38 

1.2 

.11 

.75 

4.1 

4.2 

.16 

.69 

.31

.06 

99.8

cal

Survey Bulletin 1144A, supplemented by atomic absorption 

2C - 0.2. 

3C - 2.2.
DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE

owing largely to the greater abundance of interstitial 

quartz and alkali-feldspar. Grain size is variable 

even in individual samples, but larger crystals are 

typically 1-3 mm long. The rock is composed of the 

same minerals as the gabbro but in different 

proportions (table 2). Because of the greater 

abundance of late-formed granophyric intergrowths 

of quartz and alkali-feldspar, crystals of plagioclase, 

pyroxene, and olivine tend to be more euhedral than 

in gabbro.

Plagioclase is less abundant in the ferrogranophyre than in the gabbro. Crystal-core 

compositions are similar, but progressively zoned 

rims are larger in the ferrogranophyre and the bulk 

plagioclase is more sodic. Olivine and apatite are 

also less abundant than in gabbro, but the content of 

opaque minerals and hornblende is about the same. 

Variable amounts of phyllosilicates (iron-rich clay 

minerals, chlorite, and biotite) occur with quartzfeldspar intergrowth. Olivine is fayalitic, and the 

pyroxene is ferroaugite or ferrohedenbergite.

PEGMATITE

Small bodies of pegmatite, a coarse-grained (as 

large as 5 cm) variety of ferrogranophyre, occur 

within the silicic zone. They form masses that grade 

into ferrogranophyre and also form transgressive 

bodies. Some pegmatite contains as much as 17 

percent olivine, but most contains less than 7 

percent. Olivine and ferroaugite are very rich in iron 

(table 2). Plagioclase displays marked progressive 

zoning and ranges from sodic labradorite or 

andesine in crystal cores to calcic oligoclase on the 

rims.

GRANOPHYRE

Granophyre, which constitutes less than 5 percent 

of the sill, occurs as large masses that grade into 

ferrogranophyre, as transgressive bodies as much as 

10 m thick that cut ferrogranophyre, and as small 

veins. The granophyre is light gray or brownish 

gray, and the grain size is typically less than 0.1 mm. 

Texture is micrographic (granophyric) to aplitic. In 

addition to quartz and alkali-feldspar, the 

granophyre typically contains about 10 percent sodic 

plagioclase, 0.3 percent each of ferrohedenbergite 

and olivine or its alteration products, and less than 

0.5 percent opaque minerals. Most of the olivine is 

completely altered to clay minerals, but the high 

Fe/Mg ratios of these rocks suggest that it was 

fayalite.

MINERALOGICAL DATA

The approximate compositions of the pyroxenes 

were determined from optical properties using the

diagram of Carmichael (1960). Analyzed pyroxenes 

from other chemically similar Coast Range sills have 

optical properties and chemical compositions that fit 

this diagram better than other published diagrams 

relating composition and optical properties. The 

pyroxene ranges in composition from ferroaugite to 

ferrohedenbergite; no calcium-poor pyroxene was 

noted in any of the rocks. The pyroxene becomes 

progressively richer in iron (higher refractive index, 

Ny) in the series gabbro-ferrogranophyre-granophyre (tables 1 and 2).

The trend in pyroxene composition (fig. 3) parallels that of Ca-Fe pyroxenes from the Skaergaard 

intrusion (Brown and Vincent, 1963) but, at least on 

the basis of optical properties, is slightly more calcic. 

The trend is considerably more calcic than that of CaFe pyroxenes from the Red Hill Dolerite of Tasmania 

(McDougall, 1961). All these trends, however, are 

toward a final composition of about Ca40- 43Fe60_ 57 on 

the ferrosilite-wollastonite join, a composition 

similar to that of synthetic ferrohedenbergite in 

equilibrium with quartz and fayalite at pressures of 1 

and 2 kbar (Lindsley and Munoz, 1969).

Olivine compositions were determined on mineral 

separates using the X-ray diffraction method (d130) of 

Yoder and Sahama (1957). The olivine in all rocks is 

rich in iron, as would be expected considering that it 

occurs in equilibrium with quartz. In gabbro the 

olivine is fo 12_i 9 (ferrohortonolite), and in ferrogranophyre, pegmatite, and granophyre it is fo4. 9 

(fayalite). In some granophyre olivine may be richer 

in iron than fo4, but it is quantitatively minor, 

extensively altered, and consequently difficult to 

separate. Olivine is richer in iron than coexisting 

pyroxene in all rocks (fig. 3). Olivine is variably 

altered to iron-rich nontronitic clay minerals

Plagioclase compositions were determined by 

universal stage using the albite twin law curves of 

Uruno (1963). In two gabbro samples plagioclase 

crystal cores are an56; in all others they range only 

from an51 to an54. Cores constitute about 70 volume 

percent of individual crystals and are faintly 

oscillatory zoned; zonal differences are generally less 

than anj. The progressively zoned outer rims are 

sodic andesine to calcic oligoclase. Plagioclase cores 

are an45_ 51 in ferrogranophyre, an30_ 5i in pegmatite, 

and an2. 10 in granophyre; progressively zoned rims 

in ferrogranophyre and pegmatite reach oligoclase. 

The structural state of plagioclase is intermediate in 

all rocks. Some plagioclase is altered to clay minerals 

along crystal cracks, particularly in more silicic 

rocks. In most rocks, however, alteration is slight.

All euhedral quartz crystals have /3 quartz shape; 

no evidence of former tridymite was seen. The
TEXTURAL RELATIONS

Ferrohedenbergite

80 90 

Olivine (%

100

fa)

Figure 3. Inferred composition of pyroxene from Stott sill based on optical properties. Analyzed pyroxenes are labeled x; tie lines connect 

coexisting pyroxene and olivine pairs, wo, wollastonite; en, enstatite; fs, ferrosilite; fo, forsterite; fa, fayalite. Skaergaard (Sk) and 

Red Hill Dolerite (RH) pyroxene trends are shown by dashed lines.

composition of intergrown feldspar is difficult to 

determine optically because of its fine grain size.

TEXTURAL RELATIONS

The rocks contain subhedral to euhedral crystals of 

Fe-olivine, Ca-Fe pyroxene, plagioclase, opaque 

minerals, and apatite that are surrounded by 

intergrown quartz and alkali feldspar and patches of 

phyllosilicate minerals. The resulting granophyric 

and hypidiomorphic-granular texture changes from 

the most mafic gabbro to granophyre. In the former, 

the earlier minerals tend to be more commonly 

subhedral, and the amount of quartz-feldspar 

intergrowth is low (as little as 16 percent). In 

ferrogranophyre and granophyre, the earlier 

minerals tend to be more euhedral because the lateformed quartz-feldspar intergrowth is more 

abundant.

Cores of olivine, pyroxene, plagioclase, opaque 

minerals, and apatite crystals in gabbro of the lower 

zone are considered to be cumulus in origin. Rims on 

these minerals are interpreted as adcumulus 

growths, and the quartz-feldspar intergrowth, 

phyllosilicate minerals, and hornblende are 

intercumulus. Crystal settling is inferred to have 

occurred but produced neither crystal layering nor

lamination, for reasons discussed later.

Ferroaugite in gabbro is equant to irregular in 

shape, is in places molded in subophitic fashion on 

smaller plagioclase laths, and projects with euhedral 

form into quartz-feldspar intergrowth. In some 

ferrogranophyre and pegmatite, the ferroaugite or 

ferrohedenbergite occurs as sets of parallel elongate 

crystals that poikilitically enclose plagioclase along 

their ragged borders; single crystals reach lengths of 

as much as 5 cm. Individual sets interlock with sets 

with a different orientation, producing a randomly 

ordered mosaic. Olivine with the same habit also 

occurs in these same rocks. This texture may be the 

result of nucleation at points within sets of interconnected Fe-Si polymers in the magma.

Olivine crystals are subhedral or anhedral where 

they are in contact with plagioclase and subhedral to 

euhedral against pyroxene; some olivine occurs as 

small crystals enclosed in pyroxene. Although they 

clearly overlap in their period of crystallization, 

plagioclase occurs as small crystals within olivine 

whereas the reverse relation is rarely seen; where 

observed it may result from two-dimensional view in 

thin section. Olivine is euhedral where bordered by 

quartz-feldspar intergrowth and, because of its ironrich composition, has no reaction relation with
DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE

quartz.

The opaque minerals, about 90 percent of which are 

ilmenite and the remainder pyrrhotite, are typically 

much smaller (commonly 0.1-0.2 mm) than the other 

minerals except apatite. They occur enclosed in all 

other minerals except apatite and are particularly 

abundant within olivine and pyroxene crystals. The 

opaque minerals contain inclusions only of apatite. 

Most of the ilmenite appears homogeneous in 

polished section; only rarely are exsolution lamellae 

developed. Pyrrhotite occurs as small irregular 

masses within ilmenite and as small isolated grains.

Apatite occurs principally as 0.05 by 0.4 mm 

euhedra, but some very small, highly elongate 

crystals are also found within quartz-feldspar 

intergrowth. The larger apatite crystals occur within 

all other minerals and appear to be the first phase to 

begin crystallization; smaller crystals in intergrowth 

crystallized late.

Quartz-feldspar intergrowth and phyllosilicate 

patches fill in between crystals of plagioclase, 

pyroxene, olivine, opaque minerals, and apatite. The 

quartz-feldspar intergrowth consists of wavy 

parallel quartz rods with cuneiform cross sections 

that are enclosed in alkali-feldspar. Viewed parallel 

to quartz rods, some intergrowth appears myrmekitic; viewed perpendicular to the rods, it is micrographic. In gabbro the quartz rods are typically 

0.01-0.02 mm wide and 0.2-0.3 mm long; in 

ferrogranophyre and pegmatite they reach 0.2-0.3 by 

0.8-1 mm. Quartz and alkali-feldspar make up 

approximately equal proportions, but the intergrowth is so fine that it is difficult to measure their 

relative proportions.

The intergrowth is generally finer grained where 

bordered by plagioclase, pyroxene, and olivine than 

it is near centers of intergrowth. Near centers, quartz, 

and less commonly feldspar, occurs as subhedral to 

euhedral crystals in optical continuity with some of 

the surrounding intergrowth and is euhedral where it 

extends into phyllosilicate patches. Boundaries of 

intergrowth with plagioclase, pyroxene, and olivine 

are sharp, and no replacement of earlier minerals is 

indicated.

The phyllosilicate patches consist predominantly 

of clay minerals (probably iron-rich nontronite) with 

some chlorite(?) and iron-rich biotite. Ilmenite, 

pyrrhotite, and hornblende occur within some 

patches. The iron-rich clay minerals are not stable at 

magmatic temperatures (Ernst, 1966) so the patches 

were probably originally iron-rich glass that 

devitrified to clay minerals as the rocks cooled. This 

glass may have been immiscible with the residual 

liquid that formed the quartz-feldspar intergrowth,

but if so, later crystallization has obscured any 

textural indications of immiscibility. However, 

immiscibility has been demonstrated for liquids with 

bulk compositions similar to those of the Stott 

Mountain phyllosilicates plus quartz-feldspar intergrowth (Roedder, 1951; McBirney and Nakamura, 

1974; McBirney, 1975; and Irvine, 1975). Also, a3-mwide basalt dike south of Stott Mountain that is 

composition ally similar to the gabbro in the sill, and 

probably related to it, contains an iron-rich glass 

(chlorophaeite) and silicic glass in the residuum that 

do show textural relations of immiscibility similar to 

those reported by De (1974).

Iron-rich hornblende occurs principally as a 

marginal replacement product of pyroxene, probably 

as a result of reaction of pyroxenes with the residual 

fluids that ultimately formed the phyllosilicate 

patches.

ORDER OF CRYSTALLIZATION

The order of crystallization of the minerals in 

gabbro is inferred from textural relations. During 

much of the crystallization, plagioclase, olivine, 

ferroaugite, opaque minerals, and apatite formed 

together. However, of inclusion-bearing crystals, 

most larger plagioclase crystals contain only apatite 

and opaque minerals in their cores; olivine crystals 

contain small crystals of plagioclase, apatite, and 

opaque minerals; and pyroxene crystals contain 

small crystals of plagioclase, olivine, opaque 

minerals, and apatite. Opaque minerals contain 

apatite; crystals of apatite do not contain inclusions. 

Quartz-feldspar intergrowth and phyllosilicate 

patches fill volumes between the other minerals. 

These relations indicate that the order of first 

crystallization of the individual minerals in each 

rock was apatite, then opaque minerals, plagioclase, 

olivine, and pyroxene. Quartz-feldspar intergrowth 

and the phyllosilicate patches formed later.

Determining the order of first crystallization of 

apatite on the basis of texture may be misleading 

owing to its ubiquitous occurrence as euhedra in most 

rocks, including those from the Stott sill. However, 

apatite crystals that are only partially enclosed in 

other early-formed minerals commonly are larger 

where they protrude. Also, the high P2O5 content of 

the rocks (typically 0.7 percent) is consistent with 

apatite as an early liquidus phase.

PETROCHEMISTRY

•COMPOSITION OF THE CHILLED BORDER

Four analyses of chilled border rocks all have 

nearly the same chemical composition (table 3) 

except that differing amounts of alteration of
COMPOSITIONAL VARIATIONS WITH HEIGHT

TABLE 3.—Chemical analyses of chilled gabbro

[Chemical analyses were made by S. Botts, H. Smith, G. Chloe, L. Artis, 

J. Kelsey, J. Glenn, P. Elmore, and I. Barlow using methods described in 

U.S. Geological Survey Bulletin 1144A, supplemented by atomic absorption.]

Chemical analyses recalculated H2O-free to 100 percent

Column number——

Average of 

columns 1-4

Rock number———— S68 S84 M202 M220

Si02 ————————— 57.1 57.2 57.2 57.9 57.4

A1 2 03 ———————— 13.3 13.4 13.3 13.4 13.4

Fe2°3 ———————— 3>1 4 * 2 2 ' 1 2 ' 3 2 ' 9

Fe0 —————————— 12.3 10.7 13.4 12.4 12.2

MgO —————————— 1.5 1.2. 1.4 1.7 1.5

CaO —————————— 4.8 5.2 5.2 4.8 5.0

Na20 ————————— 4.9 3.2 2.9 4.2 3.8

K2 0 —————————— .41 1.9 2.1 .83 1.3

Ti02 ————————— 1.7 1.9 1.7 1.7 1.7

P2O5 ————————— .71 .72 .67 .70 .70

MnO —————————— .23 .31 .24 .28 .27

Chemical analyses (original)

Si02 ————————— 54.8 55.4 55.5 55.7

A1 2 03 ———————— 12.8 13.0 12.9 12.9

Fe2 03 ———————— 3.0 4.1 2.0 2.2

FeO —————————— 11.8 10.4 13.0 11.9

MgO —————————— 1.4 1.2 1.4 1.6

CaO —————————— 4.6 5.0 5.0 4.6

Na2 0 ————————— 4.7 3.1 2.8 4.0

K2 0 —————————— .39 1.8 2.0 .80

H2 0- ————————— .75 .38 .72 1.4

H2CH- ————————— 2.7 2.0 1.8 2.2

Ti02 ————————— 1.6 1.8 1.6 1.6

P2 O5 ————————— .68 .70 .65 .67

MnO —————————— .22 .30 .23 .27

C02 —————————— .05 .28 .05 .05

feldspar and former glass(?) produce variation in the 

CaO, Na2O, and K2O content. The analyzed chilled 

samples are from sites as much as 8 km apart, and 

one is from a 7-m-thick, very fine grained sill below 

the main sill. The magma was apparently uniform in 

composition over a large area. The chilled rocks are 

unusual in that they have a very high total iron 

oxide, TiO2, and P2O5 content considering their 

relatively silicic nature.

COMPOSITION OF ROCKS FROM THE INTERIOR OF 

THE SILL

The rocks in the sill show a large variation in 

chemical composition (tables 1 and 2). SiO2 ranges 

from 51.7 to 76.1 percent, and total iron oxide from 1.6 

to 18.4 percent. The combination of low MgO content 

(maximum 2.3 percent) and relatively high iron oxide 

content in all of the rocks produces an unusually high 

mafic index (100x(FeO+Fe2O3)/(FeO+Fe2O3+MgO)) 

that ranges from 88 to 98. Compared to the chilled 

margins, the least silicic rocks are enriched in total 

iron oxide, MgO, CaO, TiO2 and P2O5 and depleted in 

Na2O and K2O. In contrast, the granophyres are

enriched in alkalies and silica, depleted in other 

major oxides, and have a granitic composition.

The norms were determined on water-free analyses 

after recalculating the variable Fe2O3/(FeO+Fe2O3) 

to 1/10 which approximates that of the least oxidized 

rock. A low initial oxidation state is indicated by an 

absence of magnetite in these iron-rich rocks; the 

increase in the Fe2O3/FeO ratio resulted from 

deuteric alteration. All rocks in the sill are quartznormative. Modal quartz is greater than normative 

quartz (even when calculated as volume percent) 

because some of the normative hypersthene is 

represented in the rock by olivine and quartz. The 

normative pyroxenes plot in the iron-rich side of the 

pyroxene quadrilateral (fig. 3) in the "forbidden 

zone." Under moderately low pressure, liquids of this 

composition crystallize to Ca-Fe clinopyroxene, 

fayalitic olivine, and quartz; calcium-poor pyroxene 

is not stable (Smith, 1971).

COMPOSITIONAL VARIATIONS 

WITH HEIGHT

The modal, mineralogical, and chemical data 

indicate that the lower gabbro zone has a cryptic 

vertical variation in composition. For instance, the 

proportions of minerals in the zone vary systematically upward from the base to the center of the 

sill (fig. 4A-D). Olivine, opaque minerals, apatite, 

and plagioclase increase in abundance upward, 

quartz and alkali feldspar decrease, and ferroaugite, 

phyllosilicates, and hornblende are nearly constant 

(fig. 4A). Olivine, opaque minerals, apatite, and 

plagioclase are most abundant near the center of the 

sill.

Mineral compositions also change vertically (fig. 

4B). Ferroaugite shows a slight progressive increase 

in Mg/Fe ratio (shown by slight decrease in Ny) from 

the base to a maximum near the center even though 

the amount of ferroaugite does not change 

significantly. Olivine is also progressively richer in 

magnesium, ranging from fo 12_ 13 near the base to fo 19 

near the center. Plagioclase crystals are zoned, so it is 

difficult to determine their vertical variation in bulk 

composition,. However, crystal cores in the lowest two 

samples are an51 , whereas near the center of the sill 

most are an 54. This apparent upward increase in core 

anorthite content is paralleled by an upward 

increase in normative an/an+ab from 42-43 near the 

base to 46-50 near the sill center.

The chemical composition of the rocks also 

changes vertically (fig. 4C-D). From the base to the 

center of the sill, total iron oxide , MgO, CaO, TiO2, 

and P2O5 increase; A12O3 remains nearly constant;
10 DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE

Silicic zone Silicic zone

100-

10 20 30 10 20

PYROXENE OLIVINE

A

0 5 0 2 10 20 30 40 20 30 40

OPAQUES APATITE PHYLLO PLAGIOCLASE QUARTZ+ALKALI

,5t!TM!Str FELDSPAR

Center ., sill

'

0 10 201.7251.72092 90 88 

OLIVINE % FO PYROXENE MAFIC INDEX

M212 

M211

M210 

M209 

M208 

M207 

M206

M205 

M204 

M203

40 50

NORM

Figure 4. Vertical variation in lower gabbro zone. A, Modes. B, 

Mineral compositions and chemical data. C, Major oxides. 

D, Oxide ratios.

and SiO2, Na2O, and K2O decrease. The differentiation index (Q+or+ab), mafic index, and felsic 

index decrease systematically from the base to the 

center of the sill (fig. 4B-D). The differentiation index 

parallels the change in amounts of quartz and alkali 

feldspar, the mafic index in part reflects the change 

in Mg/Fe of ferroaugite and olivine, and the change 

in felsic index correlates with plagioclase composition and amount of alkali feldspar.

The modal, mineral, and chemical data indicate 

that composition changes systematically from the 

base to above the center of the sill and that the center 

is most mafic (femic). Most other differentiated 

dolerite or gabbro sills are most mafic at a much 

lower position, usually nearer the base than the 

center.

Most rocks in the silicic zone in the area where the

50 55 6012 14 12 14 16 1 

Si0 2 AI2 0

C

Na2 0+K 2 0

10&

80

60

40

20

Silicic zone

I I I I I I

0 1

TlO;

D

0.5 1 0.3 0.4 0.5 40 50 60 04 0.5 20 30 40

FeO Na 2 0+l<20 Q+or+ab an di+hy+mt+il 

FeO+Fe 2 03 Na 2 0+K 2 0+CaO an+aD 

(Felsic index)

M212 

M211

M210 

M209

M207 

M206

M205 

M204 

M203

rocks of table 1 and figure 4 were collected are 

weathered and only four were suitable for analysis 

(table 2, cols. 5, 6, 7, and 10); three are silicic gabbro 

and one is ferrogranophyre. The weathered rocks 

there and elsewhere are mostly ferrogranophyre. The 

remaining analyses of ferrogranophyre, pegmatite, 

and granophyre shown in table 2 are from the silicic 

zone at other locations at and near Stott Mountain. 

They show that rocks in the silicic zone contain more 

SiO2, and K2O and less total iron oxide, MgO, CaO, 

TiO2, and P2O5 than does the rest of the sill. Elements 

that are enriched in the silicic zone are those that are 

impoverished in the lower gabbro zone compared to 

the chilled margins.

BULK COMPOSITION OF THE SILL

The bulk composition of a differentiated sill must 

approximate the original chilled margin composition 

if chemical variation resulted from closed-system 

differentiation of magma in place. As with many
BULK COMPOSITION OF THE SILL 

TABLE 4.—Average compositions of rocks

11

Column number ———

A1 2°3 

FeO + 0.9 Fe2O3 -

Na2o —— —— —— ——

P2°5

&*«,S^

Lower 

gabbro 

zone

54.7

•10 0

16.0

1 9 y 

6Q

2.7

1.4

1 Q

• OJ

.29

Upper 

Silicic gabbro Chilled

•) •) O

zone zone gabbros

CA C CC C CT A

D4.O 3O.3 D/.4

13.6 13.5 13.4 

9.6 14.9 14.8

0.67 1.5 1.5

3.9 6.0 5.0

3 "7 *5 Q "SB

2.5 1.6 1.3 

1 n 1 Q 1 i

• -J J . / U . / U

•I/ • /3 • ^ /

Lowest 

three 

gabbros

57.3 

13.4

14.2

1.5 

60

2 Q

1 . /

1.6

.71

.27

Bulk 

composition

13.4

14.4

1

C.

6

4

2 Q

1 . /

1.7

.70

*

Average of analyses in table 1 weighted according to interval thickness represented by

each analysis.

2Average of all analyzed rocks in zone.

Average of four chilled gabbro analyses listed in table 3.

4 Average of rocks 203-205, table 1.

Weighted average of three zones according to interval thickness represented by each 

zone.

gabbro and dolerite sills, however, the original 

composition of the chilled margins of this sill has 

been modified slightly by deuteric alteration and 

weathering. Alteration has caused variations in 

alkalies and CaO but little change in the other major 

oxides. Alteration effects are best seen by comparing 

analyses of the chilled margin rocks to those of littlealtered rocks from close to the chilled margins (table 

4, cols. 4 and 5).

The bulk composition of the sill can be calculated 

from compositions and thicknesses of the three zones 

that make up the sill. The composition of the lower 

gabbro zone, which forms 70 percent of the sill, can be 

determined accurately because rocks within the zone 

show a systematic vertical variation in composition 

defined by 12 analyzed rocks. The bulk composition 

of this zone (table 4, col. 1) was obtained by adding 

the compositions of the individual intervals 

weighted according to the interval thickness.

The composition of the silicic zone, which forms 

about 25 percent of the sill, is not so readily obtained 

because of its heterogeneity. The average composition of all analyzed rocks from the silicic zone is 

assumed to be approximately equal to the bulk 

composition of this zone. This average composition 

(table 4, col. 2) is similar to an analyzed 

ferrogranophyre (table 2, col. 10) that is typical of the 

most abundant rock in the silicic zone.

The bulk composition of the sill (table 4, col. 6) was 

determined by adding appropriate proportions of the 

average composition of each zone. For instance, 70 

percent of the average SiO2 of the lower zone plus 25 

percent of the average SiO2 of the silicic zone and 5 

percent of the SiO2 of the upper gabbro zone equals 

the bulk SiO2 composition of 57.3 percent SiO2.

The average chilled margin composition (table 4, 

col. 4) is virtually identical to the calculated bulk 

composition. This result indicates that the variations
12 DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE 

TABLE 5.—Compositions of successive residual liquids

[Residual liquid compositions were determined using bulk composition of sill (table 4, col. 6) equal to 

composition of sill at time of emplacement (0 percent solidified) and successively subtracting compositions 

or rocks starting at the base of the sill and progressing inward with each rock analysis weighted according 

to interval thickness it represents. For purposes of the calculation the average gabbro analysis from the 

upper gabbro zone was used for all rocks in that zone. The average composition of all rocks in the silicic 

zone is presumed to approximately equal the composition of the residual liquid when the silicic zone first 

began to form (75 percent solidified) and the average granophyre to approximately equal the liquid at 95-100 

percent solidified.]

Percent solidified- 0 20 25 30

Si02 ——————————— 57.3 57.4 57.5 57.7 5

A1 2°3 —————————— 13 * 4 13 * 4 13 ' 4 13 ' 4 1

FeO + Fe203 ————— 14.4 - 14.3 14.2 14.1 1

MgO ————————————— 1.6 1.6 1.6 1.6

CaO ————————————— 6.1 6.1 6.0 6.0

Na 2O ———————————— 3.0 3.0 3.0 3.0

K20 ————————————— 1.7 1.7 1.7 1.7

P205 ———————————— .70 .69 .67 .66

NORMS (Calculated with Fe 2(

or ———————————— 10 .1 10 .-| -i 0.1 10 .1 -i 

ab ———————————— 25.4 25.4 25.4 25.4 '<

wo ———————— 3.2 3.2 3.1 3.1

di: en ———————— .6 .6 .6 .6

hy: fs ———————— 17.4 !7. 2 17.3 17.2 1

il ———————————— 3.2 3.2 3.0 3.0

ap ———————————— -1.7 -1.6 -1.6 1.6

in composition of the sill resulted from differentiation of magma in place. Had there been multiple 

injection of magma (for which there is no evidence), 

then either the magmas were similar in composition 

or the amount of later magma of different 

composition was small. Furthermore, no significant 

amount of sedimentary rocks could have been 

assimilated after the chilled margins formed.

RESIDUAL LIQUID COMPOSITIONS

Residual liquid compositions were determined by 

subtracting the compositions of successive intervals 

of rock starting at the margins of the sill and 

progressing toward the interior. The bulk residual

35 43 50 59 67 75 95-100

>8.0 58.5 59.4 60.3 62.3 64.6 74.5

3.4 13.4 13.5 13.5 13.6 13.6 13.2

3.9 13.6 13.1 12.4 11.1 9.6 2.8 

1.5 1.4 1.3 1.2 .94 .67 .14

5.9 5.7 5.4 5.1 4.6 3.9 .54

3.1 3.1 3.2 3.3 3.4 3.7 4.6

1.7 1.8 1.9 2.0 2.2 2.5 3.6

1.6 1.5 1.5 1.4 1.2 1.0 .32

.65 .62 .58 .56 .45 .33 .02

X = 10 percent total iron)

3.1 13.7 14.5 15.5 17.9 19.7 30.8 

0.1 10.6 11.2 11.8 13.0 14.8 21.3

>C 9 9C 9 971 97 Q 9Q Q "^ 1 ^ ")Q Q

7.6 17.3 16.9 16.1 15.4 13.1 2.6

3

1 1 Q 9 C. 9 ^ 1 Q 17 -»

.5 .5 .4 .4 .3 .2

2

Q 97 9/1 99 1Q 17 —

3.2 3.0 2.8 2.6 2.1 1.5 .4

6.8 16.7 16.1 15.4 14.0 12.1 4.0

2

n on 1 Q 1 Q 1 a 1 A A

3 ft 1 Q OQ 97 9 ^ 1 Q ft

1.5 1.5 1.4 1.3 1.1 .8

liquids can be calculated only for the interval during 

which the lower and upper gabbro zones solidified 

because these show systematic changes in rock 

compositions whereas the order of formation of rocks 

within the silicic zone is poorly known. Compositions 

of the residual liquids are shown in table 5. Although 

the calculated residual liquid compositions define the 

gross changes in the bulk magma during 

solidification, they do not necessarily represent the 

liquid compositions near the sites of crystallization 

because of crystal growth and settling and because 

the liquid in the sill could not have been perfectly 

homogenized. Furthermore, the residual liquid 

compositions that were calculated are those of the
RESIDUAL LIQUID COMPOSITIONS 

TABLE 6.— Comparison of calculated residual liquids to compositions of mixtures of analyzed rocks and silicic liquids

13

Percent solidified ——

A1 2°3 ——- —— —— -——

FeO + Fe 2O3 ————————

MgO ————————————

Na2O ——————————————

P P2°5O — — __-._«-._....-._-.___

MnO ————————————

Percent solidified ——

CJ-in ________________

A12°3

MgO ———————————————

Na2O ——————————————

K r\ _

13 P2°5O _________________

MnO ————————————

13-1/2

0

205 

Liq. 2

57.3 

13.4 

14.4 

1.6 

6.1 

3.0 

1.7 

1.7 

.70 

.26

10

.062

205 

Calc. 3

57.3 

13.4 

14.1 

1.6 

6.4 

2.8 

1.8 

1.7 

.71 

.27

49

24-1/2

0

206 

Liq.

57.4 

13.4 

14.3 

1.6 

6.1 

3.0 

1.7 

1.7 

.69 

.25

19

.278

206 

Calc.

57.4 

13.7 

14.4 

1.4 

5.9 

2.9 

1.6 

1.6 

.80 

.26

0

207 

Liq.

57.5 

13.4 

14.3 

1.6 

6.1 

3.0 

1.7 

1.7 

.68 

.25

62-1/2

37-1/2

0.

210 

Liq.

693

210

Calc.

58.2 58.2 

13.4 13.5 

13.8 13.6 

1.5 1.5 

5.8 5.9 

3.1 3.1 

1.7 1.8 

1.6 1.6 

.64 .65 

.24 .25

1

211 

Liq.

59.1 

13.5 

13.3 

1.3 

5.5 

3.2 

1.9 

1.5 

.60 

.22

48

.018

211 

Calc.

59.1 

13.3 

13.1 

1.4 

5.7 

3.2 

1.9 

1.5 

.60 

.24

1

212

Liq.

59.7 

13.5 

13.5 

1.3 

5.3 

3.2 

1.9 

1.5 

.57 

.22

29

22

.287

207 

Calc.

57.5 

13.3 

14.2 

1.7 

6.2 

2.9 

1.7 

1.6 

.73 

.27

70

53

.264

212 

Calc.

59.7 

13.6 

13.6 

1.4 

5.4 

3.3 

1.8 

1.3 

.49 

.18

0

208 

Liq.

57.6 

13.4 

14.2 

1.6 

6.0 

3.0 

1.7 

1.6 

.67 

.25

2

213 

Liq.

61.1 

13.5 

13.5 

1.1 

4.9 

3.4 

2.1 

1.3 

.51 

.21

35

27

.371

208 

Calc.

57.6 

13.3 

14.3 

1.6 

6.0 

2.9 

1.8 

1.7 

.69 

.26

81

62

.600

213 

Calc.

61.1 

13.5 

13.5 

1.1 

4.8 

3.4 

2.1 

1.4 

.52 

.21

42-1/2

32-1/2

0.467

209 

Liq.

57.9 

13.4 

14.0 

1.6 

6.0 

3.0 

1.7 

1.6 

.66 

.25

91

70

209 

Calc.

57.9 

13.5 

14.0 

1.7 

6.0 

2.9 

1.8 

1.6 

.70 

.27

5.670

214 

Liq.

63.1 

13.6 

13.6 

.85 

4.4 

3.5 

2.3 

1.1 

.41 

.19

214

Calc.

63.1 

13.6 

13.6 

.85 

4.4 

3.5 

2.3 

1.1 

.41 

.19

Factor is the amount of liquid with composition of the silicic zone (SiO2=64.6) added to each rock to

match the SiO2 of the calculated mixture with the residual liquid composition.

o

Interpolated values from table 5.

SiO~ of residual liquid and calculated mixture are by definition the same, all other oxides of the 

mixture are calculated using the factor determined to balance the SiO contents.

bulk liquid plus all crystals suspended in it.

The residual liquid compositions can be closely 

duplicated by combining each analyzed rock and an 

appropriate proportion of liquid more silicic than the 

calculated residual liquid. In table 6, the residual 

liquid compositions are compared to calculated

mixtures of each rock and an appropriate amount of 

liquid whose composition is that of the average silicic 

zone rock (table 4, col. 2). The calculations were made 

so as to duplicate the SiO2 content of the residual 

liquid (as for example, for rock 211, SiO2 of rock = 53.5, 

SiO2 of residual liquid at the time of formation of rock
14 DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE

211 = 59.1, as extrapolated from table 5, SiO2 of silicic 

zone = 64.6, thus

— 1(53.5) —— T7— + x(64.6) i —— - = 

1+x

1 , , ni Q .1, and x = 1.018.

All other oxides of the mixture were calculated using 

1 part of rock 211 and 1.018 parts of composition like 

that of the silicic zone rock). Similar results can be 

obtained using liquids more or less silicic than the 

average silicic zone rock. The similarity of the 

mixtures to the residual liquid compositions 

indicates that removal of relatively silicic liquid 

residues from crystal accumulations can produce the 

observed rock compositions.

CHEMICAL VARIATION

The composition of the analyzed rocks and calculated successive bulk residual liquids are plotted 

on a solidification diagram (Wager, 1960) in figure 5. 

It shows the composition of the rocks that formed at 

various stages during the solidification of the 

magma and the corresponding gross changes in the 

magma's bulk composition.

The successive residual liquids began to differ 

appreciably from the initial composition when the 

sill was about 20 percent solidified. With respect to 

the initial bulk composition, the liquid was enriched 

in SiO2, Na2O, and K2O and depleted in all other 

major oxides except A1 2O3, which remained nearly 

constant. The liquid progressively changed 

composition until the last liquid crystallized and, at 

this end stage, differed greatly in composition from 

the initial liquid. The liquid trends are smoother than 

the rock trends because the former were determined 

using successive rock analyses, which tend to 

smooth out the small scatter shown by the rock 

analyses. Much of the scatter in this and succeeding 

diagrams is within the range of analytical error for 

rapid-rock analyses. The CaO content of the rocks 

reached its maximum when the sill was 50 percent 

solidified, and FeO+Fe2O3, MgO, TiO2, andP2O5 were 

maximum when the sill was about 60 percent 

solidified. This stage in the solidification corresponds to the formation of the upper part of the 

lower gabbro zone and approximately to the 

crystallization of rock M213. The rock composition 

trends for all oxides cross the bulk composition lines 

at a time corresponding to 75 percent solidification. 

Thus rocks that formed at about this stage (for 

example, table 2, col. 6) may have compositions 

similar to the bulk composition and the chilled 

margins, although much different from the residual 

liquid from which they crystallized.

o 

ex.

O 

O

X

O

20 40 60 80 

SOLIDIFICATION, IN PERCENT

100

Figur§ 5. Solidification diagram showing amount of major oxides 

in rocks and bulk residual liquids at various stages in 

solidification of gabbro sill. Rock trends are shown by a 

solid circle and liquid trends by an open circle. Horizontal 

lines show bulk composition of sill; vertical line at 75 percent 

solidification indicates beginning of formation of silicic 

zone.
PROCESS OF DIFFERENTIATION 15

MgO

CaO 4

Na 2O

I I I

60 70 

SILICA CONTENT, IN PERCENT

Figure 6. Silica variation for rocks from gabbro sill. Bulk composition shown by x, residual liquid compositions are shown 

by solid lines, and composition of residual liquid at time of 

crystallization of gabbro M213 is shown by open circle.

The rocks show a concomitant increase in Na2O 

and K2O, relatively constant A12O3, and decrease in 

total iron, MgO, CaO, TiO2, and P2O5 with respect to 

increase in silica (fig. 6). The rock analyses plot along 

smooth curves except for those with high SiO2 

content, which show some scatter, especially with 

respect to Na2O and K2O (total alkalies, however, 

plot along a smooth curve). As the sill solidified, the 

newly formed rocks had a progressively lower silica 

content starting at 57 percent SiO2 (chilled margin 

and bulk composition) and decreasing to 52 percent

Figure 7. FMA (F=FeO+0.9Fe2O3+MnO, M=MgO, A=N2O+K2O) 

diagram of analyzed rocks and residual liquids of Stott sill. 

Open circle shows composition of residual liquid at time of 

formation of rock M213; solid line shows variation in 

composition of residual liquids; and dashed line Sk shows 

Skaergaard trend.

SiO2 (sample M213, table 1) when the sill was about 

60 percent solidified. After that, the rocks were 

successively enriched in SiO2 until formation of the 

granophyre with 76 percent SiO2.

The analyzed rocks all have high Fe/Mg ratios and 

plot along a nearly linear trend on an FMA diagram 

(fig. 7). Unlike many other differentiated dolerite or 

gabbro sills, the rocks from the Stott Mountain sill 

show no marked early change in the uniformly high 

Fe/Mg ratio. On the basis of 200 analyses of rocks 

from related sills in the Oregon Coast Range (P. D. 

Snavely, Jr., and N. S. MacLeod, unpub. data), 

however, this unusual F-A trend is characteristic, 

and none of the sills show an F-M trend. The bulk 

composition of the Stott sill, with its high Fe/Mg 

ratio, is much like that of late-stage rocks of many 

differentiated dolorite sills. Rock compositions 

changed initially to lower relative alkali content and 

slightly lower Fe/Mg ratios until about 40 percent 

residual liquid remained, at which time sample M213 

crystallized, and then showed a marked relative 

increase in alkalies. In contrast, the residual liquid 

trend is toward continuously higher alkali content.

PROCESS OF DIFFERENTIATION

Any explanation of the processes of differentiation 

of the gabbro sill must take into account the 

following:

(1) The sill is thin compared to other gabbro, 

diabase, or dolerite sills that show similar degrees of
16 DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE

differentiation.

(2) Agreement between the calculated bulk composition and the chilled margin composition shows that 

no large amount of material could have been 

assimilated by the magma within the sill and that 

there could not have been large additions of magma 

of different composition. The sedimentary rocks 

marginal to both the Stott Mountain sill and the 

related Marys Peak sill also have compositions 

(Snavely and others, 1969) grossly different from the 

sill rocks (that is, the sedimentary rocks have more 

MgO and A12O3). Also, no field evidence was found 

for assimilation of sedimentary country rock or 

multiple intrusion. Furthermore, lead-isotope ratios 

of gabbro, granophyric diorite, pegmatite, and 

granophyre from the Marys Peak sill are uniform 

and differ from isotope ratios of the sedimentary 

rocks intruded by the sill (Tatsumoto and Snavely, 

1969).

(3) Neither rhythmic layers nor crystal lamination 

were observed. The gabbro is uniformly homogeneous in gross aspect even though the lower half of 

the sill shows vertical chemical and mineralogic 

variations.

(4) Differentiation of the magma produced 

asymmetric trends in rock compositions, mineral 

proportions, and mineral compositions. Rocks show 

a progressive increase in MgO, total iron oxide, CaO, 

TiO2, and P2O5, and decrease in SiO2 and alkalies, 

from the base toward the center of the sill. 

Plagioclase, olivine, opaque minerals, and apatite 

increase in abundance, and pyroxene and olivine are 

progressively richer in magnesium over the same 

interval. The most mafic rock occurs somewhat 

above the center of the sill, and silicic rocks occur at a 

high level.

(5) The sill was intruded under a thick cover of 

sedimentary rocks in middle Oligocene time. Much of 

the cover has been eroded, but farther west, Eocene 

and lower Oligocene marine sandstone and siltstone 

have a thickness of about 1.5 km (Snavely and 

others, 1976), and the sill was intruded near the base 

of this sequence. Owing to the insulating cover, 

conductive heat loss should have been about the 

same from the base and top of the sill during 

solidification (Jaeger, 1957, p. 312), unless 

convection, crystal settling, or some other process of 

heat transport was involved. Convection and crystal 

settling both cause asymmetric heat loss resulting in 

final solidification in the upper part of a sill rather 

than at its center (Irvine, 1970).

(6) The chilled margins are aphyric, and the 

magma was therefore near or above liquidus 

temperature.

(7) The last minerals to crystallize in early-formed 

rocks have bulk compositions grossly similar to the 

composition of late-formed rocks.

(8) The parent magma had an unusual composition 

(that is, high SiO2, P2O5, and total iron, and low 

MgO).

A differentiation process that appears to account 

for the distribution of rock types, the mineralogic and 

chemical variations, and absence of rhythmic layers 

and crystal lamination combines the progressive 

inward crystallization proposed by Hess (1960) with 

crystal settling within zones of crystallization at the 

floor and roof of the liquid interior of the sill. 

Differentiation as a result of crystal settling within 

crystallization zones has been discussed by Jackson 

(1961), Wilshire (1967), and Irvine (1970). Richter and 

Moore (1966) report such a transient downwardmoving zone of crystallization beneath the 

developing crust of the Kilauea lava lake.

According to the suggested model, as the sill 

solidified, one crystallization zone moved continuously upward from the base and another downward 

from the top (fig. 8). Inferred relations in the lower 

zone of crystallization are shown in figure 9. The 

order of nucleation of the minerals shown by textural 

relations suggests that the minerals first nucleated 

at different heights within the zone. Thus, the top of 

the lower zone contained magma plus apatite 

crystallites, the first mineral phase to nucleate. 

Progressively lower parts contained (1) magma, 

apatite, plus opaque minerals; (2) magma, apatite, 

opaque minerals, plus plagioclase; (3) magma, 

apatite, opaque minerals, plagioclase, plus olivine; 

(4) magma, apatite, opaque minerals, plagioclase, 

olivine, plus ferroaugite; and (5) at the base of the 

zone, apatite, opaque minerals, plagioclase, olivine, 

ferroaugite, plus quartz-feldspar intergrowths. Even 

though the various minerals first nucleated at 

different heights in the zone of crystallization, in 

successively lower positions they continued to grow 

and react with residual liquid.

The early-formed minerals in the lower zone of 

crystallization were denser than the enclosing 

magma and thus would have settled. In order 

broadly to define relative settling velocities (fig. 10), 

the approximate viscosity and density of the residual 

bulk magmas were calculated from their chemical 

compositions using the methods of Shaw (1972) and 

Bottinga and Weill (1970), respectively. Crystals 

were assumed to have the same volumes as they do in 

the rocks but, for simplification, were assumed 

spherical; Jackson (1971) shows that elongate 

crystals will settle about 10 percent more slowly than 

spheres of the same volume. According to the model,
PROCESS OF DIFFERENTIATION 17

_t

/.: •>>>•

•r •

^

*<%

^'s$ 

•v.'-. ||

"if-f-/'^

«« 7r«n

.Z X -4>

: "f ifl»: :K|:.':

<-Si!-'.'-'.

: t X\;'N\-;\

— 1 '^'fy \^\\/\

Time of

initial 

intrusion

f.v: »; Baked-sedimentary 

rocks

Successively later times

iT^T^jrfrrV. Baked sedimentary

rocks 

Time of

final 

solidification

Figure 8. Progressive solidification of gabbro sill. Horizontal lines delimit upper and lower zones of crystallization; arrows

indicate direction of solidification.

the crystal size would increase downward in the 

lower crystallization zone; if the crystals grew 

continuously in size during settling, their initial 

velocity would be much lower and average settling 

velocity would be only one-third that of crystals with 

constant size, assuming constant viscosity. Thus, 

the calculated rates for various temperatures and 

liquid compositions (fig. WA, B) are somewhat 

greater than the actual rates would have been if 

crystals settled as individuals. Settling of crystal 

aggregates formed as a result of heterogeneous 

nucleation (Campbell, 1978), on the other hand, 

would be more rapid than settling of individuals.

Numerous laboratory experiments show that 

crystallization of basaltic magmas commonly occurs 

over a 100°-200°C temperature interval. No data are 

available for rocks with compositions similar to 

those in the sill, but a similar temperature interval 

appears reasonable; considering the unusual composition, the interval may even have been greater. The 

temperature interval over which the various 

minerals crystallized and settled would change as 

the lower crystallization zone migrated upward 

because the bulk residual magma became progressively more silicic in composition.

Crystals nucleating in the upper part of the 

crystallization zone would not settle until they 

reached a size large enough to overcome the yield 

strength of the magma. Upon reaching sufficient 

size, they would begin to settle, and the rate would 

increase as their size increased at progressively 

lower positions in the zone. At even lower positions, 

the rate would decrease because the remaining 

residual liquid at successively lower positions would 

have had a continuously higher viscosity (more 

silicic composition and lower temperature). To a 

certain extent, the downward decrease in settling 

rate caused by increasing viscosity may have been 

counteracted by an increasing tendency for crystals 

(because of higher crystal content) to settle as 

aggregates at a rate higher than they would as 

individuals.

The combined effect of changing temperature and 

liquid composition (arbitrarily related) on settling 

rate are shown diagrammatically in figure 10C. 

Without liquidus temperature determinations it is 

not possible to speculate on the thickness of the 

crystallization zone nor on the heights within the 

zone at which the various minerals would nucleate. 

Figure 10C does indicate, however, that the settling
18 DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE

EARLY

I 

Sill

LATER

.0

|a

u _

llilllll

llllllll

lllpllll

HI

III ^111

lit '2 111

llll^fll

•y&i?£y?&£ii

$$i$i$l&ir

';'>r'^;

j'i' o''i-i

'+~>~> ° r- "-'

^fesr^fe-

-ap

-op

-pl 

-ol 

-f

< —

top1

ap

op

———— > plolfHighest

nucleation of:

^

| Liquid

'::-::^;-':'-;:^'::-::-::-'^-:^:-

jjjiljiiiij:

R^^^%j A^

'i^vV^j-

^f O ^" r

^Sfe^

^J

O

D

^g

£

"•*•

Q) O

C CM

o o

N Si

lllization S•easing 

o jj

w . . jz

U

/

Chilled base —— > ^ 

- Sediment ——— > -

WvV,

'f^-

Si

S §

>

-E * %£ \

Figure 9. Inferred crystallization and settling of minerals in lower 

zone of crystallization at two different periods in solidification of gabbro sill, ap, apatite; op, opaque minerals; pl, 

plagioclase; ol, olivine; f, ferroaugite.

rates would have decreased by an order of magnitude 

from the upper to the lower part of the zone even if it 

represented a temperature interval of only 100°C. 

The settling rates would have been so low near the 

base even for crystal aggregates that the crystals 

would effectively become frozen in position at a 

height higher than the point at which an intact 

crystal framework (about 50 percent crystals) would 

itself inhibit further settling. Thus in the absence of a 

well-defined floor and because of the extremely low 

settling velocity in the lower parts of the zone, crystal 

lamination could not result from crystal settling, nor 

would the resulting textures be obviously cumulate. 

The calculated settling rates appear to be generally 

consistent with the model of crystallization zone 

settling, considering the apparent orders of 

nucleation of the minerals and the amount of upward 

increase they show in the rocks of the lower gabbro

1300 -

^ 1000

58

62

66

70

74

57/1300 

57/1200 

64/1100

T=1100°C

74/1000

.001 .01 .1 1 10 100 

SETTLING VELOCITY, IN METERS PER YEAR

1000

Figure 10. Calculated settling rates of apatite (ap), plagioclase (pl), 

opaque minerals (op), olivine (ol), and ferroaugite (f), as a 

function of temperature and silica content of residual liquid. 

A, Decrease in settling rate with decrease in temperature. 

Calculated viscosity ranges from 6.7xl02p at 1300°C to 

3.6xl04p at 1000°C and density increases from 2.56 to 2.60 

over same interval. B, Decrease in settling rate as a function 

of increasing SiO2 content of residual liquid with viscosity 

of least silicic liquid of 7.8xl03p increasing to 2.3xl06 for 

most silicic and density decreasing from 2.58 to 2.33 over 

same range. C, Marked decrease in settling rate that results 

from decreasing temperature and increasing silica content 

of liquid. Settling rates were calculated based on constant 

crystal size; actual settling rates would have shapes shown 

by dashed line if crystal growth were taken into account.

zone. Apatite, the first phase to nucleate, shows the 

greatest relative upward increase in amount in the 

lower gabbro zone. As the calculated settling velocity 

for apatite is very low (small crystal size), for it to 

have accumulated in large amount by settling 

requires that it settle from a very high position 

relative to the other minerals. Thus, for much of the 

zone, only apatite could have been present in the 

magma. Apatite crystals commonly do occur 

enclosed in olivine so they would have in part settled 

as aggregates at a higher rate, but much of the 

apatite must have settled as individual crystals 

because it increases in amount upward more than 

does- olivine. Early crystallization of apatite is 

consistent with the very high P2O5 content of the 

magma (bulk composition has 0.7 percent P2O5). 

Opaque minerals nucleated after apatite. Because
PROCESS OF DIFFERENTIATION 19

they have a much higher calculated settling rate, yet 

increase in abundance upward nearly as much as 

apatite, the height at which opaque minerals became 

large enough to settle must have been far below that 

of apatite.

Plagioclase nucleated after apatite and opaque 

minerals, thus at a lower position in the 

crystallization zone, but its calculated settling 

velocity is very low (low density contrast with the 

magma), consistent with the minor upward increase 

it shows. The calculated settling velocity for 

plagioclase is the least reliable, for its density 

contrast with the liquid is so low that slight 

differences between the actual and calculated 

magma density greatly affect settling rate. However, 

aggregate grains including plagioclase would settle 

at rates higher than those calculated.

Olivine nucleated next and increases upward more 

than plagioclase because it has a settling rate an 

order of magnitude greater. Ferroaugite, the last 

cumulus phase to nucleate, has the highest 

calculated settling velocity yet remains about 

constant in amount throughout the lower gabbro 

zone. Apparently at its low position of nucleation and 

growth in the zone, the viscosity of the remaining 

residual liquid was high, and ferroaugite settled 

much less than the other minerals. Even though 

ferroaugite does not significantly change in 

abundance upward, it nevertheless must have settled 

to some extent because the bulk residual magma 

composition was changing: the absence of 

ferroaugite settling would correspond to an upward 

decrease in ferroaugite. Compaction of the crystal 

mush in the lowest part of the crystallization zone 

may have been sufficient to increase the proportion 

of ferroaugite slightly compared to the amount 

produced by an increasingly more silicic magma 

with less potential ferroaugite.

Residual magma displaced upward by crystal 

settling within the lower zone would be depleted in 

components of the early-formed minerals and less 

dense than the magma overlying the zone of crystallization. The displaced residual magma may have 

diffused into the overlying magma in response to 

density, chemical, and thermal gradients. That part 

of the residual magma which remained trapped in 

the lower part of the zone ultimately formed 

adcumulus rims on crystals and the intercumulus 

quartz-feldspar intergrowth and phyllosilicate 

patches.

The phyllosilicate patches initially decrease in 

abundance upward and then increase toward the 

center of the sill to a value similar to that near the 

base. This increase may result from a possible

immiscible relation between the iron-rich liquid from 

which these patches were formed and the silicic 

residual melt that crystallized as intergrowth. 

Perhaps the residual liquids within the lower crystallization zone were initially displaced before 

immiscibility could occur, whereas during later 

stages the residual liquid unmixed and only the light 

silicic residual liquid was displaced upward.

On the basis of the calculated magma viscosity 

and the thickness of the sill, it seems likely that 

magma within the interior of the sill would convect 

(Bartlett, 1969, fig. 1). No evidence indicating 

convection was found, but the lower crystallization 

may have lain quiescent below the convecting 

interior, such as Jackson (1961, p. 97) postulated for 

the Stillwater Complex of Montana. Convection in 

the interior of the sill would have tended to 

homogenize displaced residual magma with magma 

in the interior.

When the lower zone of crystallization was near 

the base of the sill, crystal settling would have been 

minor because the zone was thin (steep thermal 

gradient) and its base (surface of complete solidification) moved rapidly upward. The early crystals in 

the zone would have reacted with entrapped residual 

interstitial liquid, and the bulk composition of the 

rock would be virtually unchanged from that of the 

original magma. As a sill cools, isotherms progress 

inward from the contacts approximately in 

proportion to the square root of time (Jaeger, 1957, p. 

79). Therefore, the distance between any two 

isotherms, such as bounded the lower zone of 

crystallization, would increase as they migrated 

upward (figs. 8 and 9). The distance from top to 

bottom of the zone would increase even though the 

bounding isotherms would change as the zone 

migrated upward as a result of changing residual 

magma composition. Consequently, because 

increasing time would be available as the 

crystallization zone moved upward, the efficacy of 

crystal settling would increase and progressively 

more residual liquid would be displaced.

Such a process is suggested by the data of table 6 in 

which residual liquid compositions are closely 

approximated by mixtures of analyzed rocks and 

relatively silicic liquids. The proportion of displaced 

silicic liquid (factor of table 6) increases with the 

degree of solidification and height (fig. 11). Although 

the compositions of the displaced residual liquids are 

not known and probably changed as the zone of 

crystallization progressed upward, reasonable 

compositions more silicic than the residual liquid at 

the time each rock formed can produce similar results 

with different amounts.
20 DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE

100

80

60

40

20

i i i i i —

^^« ^ ^

^

/

./

- i

'

- 1

~ij 

-i

^

r

»

i i i i i

80

60

i—

LU

0 

QC

LU 

Q.

40 (*

3LIDIFICATION,

CO

20

n

colUU

DC

LU

LU 

^

z 80

_T_i

CO

fe 60

LU

CO

CO 

•-1-1 >in > 40

o

CO

hi 20

LU

31

n

1 23456 

FACTOR

Figure 11. Proportion (factor) of liquid with composition of average 

silicic zone rock that added to rock duplicates composition of 

residual liquid (see table 6).

The relative distances that crystals of olivine could 

settle within the crystallization zone as a function of 

height above the base of the sill are shown in figure 

12. At progressively greater height, the zone of 

crystallization would be progressively thicker, and 

thus more time would be available for individual 

crystals to settle. Thus, the efficacy of settling 

increases as the zone moves upward and as its 

thickness increases. At the last stage of formation of 

the lower gabbro the effect of settling decreases as 

the increasing viscosity of the bulk residual magma 

in the interior begins to have a strong influence on 

settling rate. Although the curve shown is based on 

calculated distances of settling, no scale for distance 

is shown on figure 12 because no data are available 

on the probable temperature interval over which 

olivine would have been on the liquidus. For 

simplification in the calculation, the bounding 

isotherms were assumed to remain the same, 

whereas they actually would have decreased upward

RELATIVE DISTANCE 

SETTLED

Figure 12. Relative distance that olivine could settle as a function 

of height within sill. As crystallization zone moved upward 

it became progressively thicker allowing more time for 

settling. Late decrease in settling rate results from increasing viscosity of residual liquid.

as the bulk liquid became more silicic.

The loss of relatively silicic residual liquid 

(displaced upward) and the addition of settled 

crystals would cause the bulk composition (crystals 

plus remaining liquid) of the lower part of the 

crystallization zone to be more refractory. Therefore, 

the crystals would have a more refractory 

composition when crystallization was completed at 

successive intervals than if no settling had occurred. 

Such an upward increase in refractory components 

of minerals is shown by the upward increase in 

Mg/Fe in olivine and ferroaugite, the slight absolute 

increase in Ca/Na in plagioclase, and the increase in 

Mg/Fe and Ca/Na of rocks from the base toward the 

center of the sill. As the rocks became progressively 

more mafic, the bulk residual magma became more 

silicic.

If crystallization zone settling was the major
SUMMARY 21

process of differentiation of the gabbro sill, the 

residual magma, displaced by settling crystals, must 

have been mixed with overlying magma. The 

displaced residual liquid would have had a lower 

density (more silicic composition) than the overlying, 

probably convecting, magma and thus may have 

risen buoyantly and have been homogenized with it. 

If the displaced residual liquid was not mixed, but 

settling still occurred, a strong chemical gradient 

would have been locally established and chemically 

graded rhythmic layers probably would have been 

produced (Jackson, 1961, p. 94-98).

The upper zone of crystallization progressed 

continuously but slowly downward during solidification of the sill (fig. 8). In contrast to the lower zone, 

settling in this upper zone would result in removal of 

crystals from the zone. These crystals would sink 

toward the interior of the sill into magma hotter than 

in the upper zone of crystallization where they 

formed. Therefore, the settled crystals would be 

progressively resorbed during their downward 

transport if sufficient heat was available.

Heat released in the upper zone by crystallization 

would be removed from the interior of the sill by 

resorption. Thus crystallization, settling, and 

resorption probably would produce a net upward 

transport of heat, and as a result, high temperatures 

would be maintained near the top of the sill, causing 

it to solidify less rapidly from the top than from the 

base (Irvine, 1970). This process would largely 

account for the smaller thickness of the upper 

compared to the lower gabbro zone. The crystals that 

settled from the upper zone would, upon being 

resorbed, add refractory components to the interior 

magma. At the top of the upper zone of crystallization, the residual magma would have an increased 

viscosity (cooler and more silicic as a result of 

downward settling of early minerals) and settling 

would be impeded; thus this zone would move slowly 

downward.

If the crystals that settled downward from the 

upper crystallization zone had not been resorbed in 

the interior, but had settled through the sill into the 

lower zone of crystallization, a rhythmic layering or 

sharp increase in minerals with a high settling rate 

(for example, olivine) would be expected in the lower 

part of the sill. No such rhythmic layering or marked 

increase in any of the early minerals was observed. 

In addition, the first crystals to settle through the sill 

to the bottom would have had the more refractory 

composition, but no such compositions ,are found in 

the lowest part of the sill. Indeed, the most refractory 

mineral compositions occur near the center of the sill.

Because the upper zone progressed downward

much less rapidly than the lower zone moved 

upward, the displaced silicic residual magma 

accumulated high in the sill where it formed the rocks 

of silicic zone. The variations exhibited by rocks in 

the silicic zone most likely are the result of several 

processes including filter pressing (compaction of 

crystal mush), crystal settling, and possibly alkali 

diffusion. Crystal settling would have been much 

less efficient in producing compositional changes in 

the magma in this zone because viscosity was much 

greater (more silicic magma) than at an earlier stage 

when the lower gabbro zone was formed. Transgressive ferrogranophyre and granophyre bodies 

probably are the products of magma that was filter 

pressed from a somewhat lower part of the silicic 

zone (where the last part of the sill, other than the 

transgressive bodies, crystallized). The magma was 

injected into the upper part of the silicic zone and in 

places into the upper gabbro zone, perhaps along 

vertical and horizontal joints. The wide variation in 

Na2O/K2O in the granophyres, whose composition is 

otherwise similar, suggests that alkali diffusion may 

have occurred during the last stages of crystallization.

SUMMARY

Crystal settling in zones of crystallization that 

moved inward from the margins of the Stott sill as it 

solidified appears to account for the variation in 

mineralogy and petrochemistry in the sill. In the 

lower zone of crystallization, the amount of settled 

individual minerals was controlled by their height of 

nucleation and settling rate. Viscosity increased 

downward so that settling rates were very low near 

the base of the zone and no well-defined floor was 

present. Thus lamination or layering was not 

produced. The amount of settling increased as the 

zone moved upward because the zone became 

increasingly thicker and more time was available for 

settling. Liquid displaced upward by settled crystals 

was probably homogenized with magma in the 

interior of the sill. Crystals in the upper zone of 

crystallization settled downward into the interior of 

the sill where they were resorbed. As a result of the 

effects of heat of crystallization and resorption, high 

temperatures were maintained in the upper part of 

the sill, and the upper crystallization zone moved 

downward much more slowly than the lower 

crystallization zone moved upward. Thus the last 

part of the sill to solidify was in the upper part.

Differentiation by settling in zones of crystallization should be particularly pronounced where the 

constituent minerals crystallize over a wide temperature range. Such a wide temperature range may be
22 DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE

much more important for this type of differentiation 

than thickness of the sill and consequent greater 

time of solidification. Also important is the magma 

viscosity because of its effect on settling rate. Other 

sills much thicker than the Stott sill may have 

undergone a similar history but show much smaller 

degrees of fir action ation.

REFERENCES CITED

Baldwin, E. M., 1955, Geology of the Mary's Peak and Alsea

quadrangles, Oregon: U.S. Geological Survey Oil and Gas

Investigations Map OM-162, scale 1:62,500. 

___ 1964 Geology of the Dallas and Valsetz quadrangles,

Oregon: Oregon Department of Geology and Minerals Industries Bulletin 35 (revised), 56 p. 

Barlett, R. W., 1969, Magma convection, temperature distribution,

and differentiation: American Journal of Science, v. 267, p.

1067-1082. 

Bottinga, Y., and Weill, D. F., 1970, Densities of liquid silicate

systems calculated from partial molar volumes of oxide

components: American Journal of Science, v. 269, p. 169-182. 

Brown, G. M., and Vincent, E. A., 1963, Pyroxenes from the late

stages of fractionation of the Skaergaard intrusion, east

Greenland: Mineralogical Magazine, v. 32, p. 511-543. 

Campbell, I. H., 1978, Some problems with the cumulus theory:

Lithos, v. 11, p. 311-323. 

Carmichael, I. S. E., 1960, The pyroxenes and olivines from some

Tertiary acid glasses: Journal of Petrology, v. 1, p. 309-336. 

Clark, H. C., 1969, Remnant magnetization, cooling history, and

paleomagnetic record of the Marys Peak sill, Oregon:

Journal of Geophysical Research, v. 74, p. 3143-3160. 

De, Aniruddha, 1974, Silicate liquid immiscibility in the Deccan

traps and its petrogenetic significance: Geological Society

of America Bulletin, v. 85, p. 471-474. 

Ernst, W. G., 1966, Synthesis and stability relations of ferrotremolite: American Journal of Science, v. 264, p. 37-65. 

Hess, H. H., 1960, Stillwater Igneous Complex, Montana: Geological Society of America Memoir 80, 230 p. 

Irvine, T. N., 1970, Heat transfer during solidification of layered

intrusions—1, sheets and sills: Canadian Journal ofEarth

Sciences, v. 7, no. 4, p. 1031-1061. 

____ 1975, The silica immiscibility effect of magmas: Carnegie

Institute of Washington Year Book 1974-1975, p.484-492. 

Jackson, E. D., 1961, Primary textures and mineral associations in

theultramaficzoneof theStillwatercomplex, Montana: U.S.

Geological Survey Professional Paper 358, 106 p. 

____ 1971, The origin of ultramafic rocks by cumulus processes:

Mineralogical Magazine, v. 48, p. 138-174. 

Jaeger, J. C., 1957, The temperature in the neighborhood of a

cooling intrusive sheet: American Journal of Science, v. 255,

p. 306-318. 

Lindsley, D. H., and Munoz, J. L., 1969, Subsolidus relationships

along the join Hendenbergite-Ferrosilite: American Journal

of Science, v. 267A, p. 295-324. 

McBirney, A. R., 1975, Differentiation of the Skaergaard intrusion:

Nature, v. 253, p. 691-694. 

McBirney, A. R., and Nakamura, Yasuo, 1974, Immiscibility in

late-stage magmas of the Skaergaard intrusion: Carnegie 

Institute of Washington Year Book 1973-1974, p. 348-352.

McDougall, lan, 1961, Optical and chemical studies of pyroxenes 

in a differentiated Tasmanian dolerite: American Mineralogist, v. 46, p. 661-687.

MacLeod, N. S., 1969, Geology and igneous petrology of the 

Saddleback area, central Oregon Coast Range: Santa 

Barbara, University of California, Ph. D. thesis, 205 p.

Richter, D. H., and Moore, J. G., 1966, Petrology of Kilauea-Iki lava 

lake, Hawaii: U.S. Geological Survey Professional Paper 

537-B, 26 p.

Roberts, A. E., 1953, A petrographic study of the intrusive at 

Marys Peak, Benton County, Oregon: Northwest Science, v. 

27, no. 2, p. 43-60.

Roedder, Edwin, 1951, Low-temperature liquid immiscibility in the 

system K2O-FeO-Al2O3-SiO2 : American Mineralogist, v. 36, 

p. 282-286.

Shaw, H. R., 1972, Viscosities of magmatic silicate liquids—an 

empirical method of prediction: American Journal of 

Science, v. 272, p. 870-890.

Smith, Douglass, 1971, Reconnaissance experiments with compositions on the join Fs88En15-wollastonite in the pyroxene 

quadrilateral: Carnegie Institute of Washington Year Book, 

1970-1971, p. 125-129.

Snavely, P. D., Jr., and Wagner, H. C., 1961, Differentiated 

gabbroic sills and associated alkalic rocks in the central 

part of the Oregon Coast Range, Oregon, in Geological 

Survey Research 1961: U.S. Geological Survey Professional 

Paper 424-D, p. D156-D161.

Snavely, P. D., Jr., MacLeod, N." S., and Wagner, H. C., 1968, 

Tholeiitic and alkalic basalts of the Eocene Siletz River 

Volcanics, Oregon Coast Range: American Journal of 

Science, v. 266, p. 454-481.

Snavely, P. D., Jr., MacLeod, N. S., and Rau, W. W., 1969, Geology 

of the Newport area, Oregon: Ore Bin, v. 31, nos. 2 and 3, p. 

25-71.

Snavely, P. D., Jr., MacLeod, N. S., Wagner, H. C., and Rau, W. W., 

1976, Geologic map of the Cape Foulweather and Euchre 

Mountain quadrangles, Oregon: U.S. Geological Survey 

Miscellaneous Investigations Series Map 1-868, scale 

1:62,500..

Tatsumoto, M., and Snavely, P. D., Jr., 1969, Isotopic composition 

of lead in rocks of the Coast Range, Oregon and 

Washington: Journal of Geophysical Research, v. 74, 

p. 1087-1100.

Uruno, K., 1963, Optical study of the ordering degree of plagioclase: Tohoku University Science Reports, 3d series, v. 8, no. 

2, p. 171-220.

Wager, L. R., 1960, The major element variations of the layered 

series of the Skaergaard intrusion [Greenland] and a reestimation of the average composition of the hidden layered 

series and of the successive residual magmas: Journal of 

Petrology, v. 1, no. 3, p. 364-398.

Wager, L. R., and Brown, G. M., 1968, Layered igneous rocks: 

Edinburgh and London, Oliver and Boyd, 588 p.

Wilshire, H. G., 1967, The Prospect alkaline diabase-picrite 

intrusion, New South Wales, Australia: Journal of 

Petrology, v. 8, p. 97-163.

Yoder, H. S., Jr., and Sahama, T. G., 1957, OlivineX-ray determinative curve: American Mineralogist, v. 42, p. 475-491.