Low-Temperature 

Reaction Zones and 

Alpine Ultramafic Rocks 

of California, Oregon 

and Washington 

GEOLOGICAL SURVEY BULLETIN 1247 

Low -Temperature 

Reaction Zones and 

Alpine Ultramafic Rocks 

of California, Oregon 

and Washington 

By ROBERT G. COLEMAN 

GEOLOGICAL SURVEY BULLETIN 1247 

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

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

William T. Pecora, Director 

Library of Congress catalog-card! No. GS 67-282 

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

Washin~ton, D.C. 20402 - Price 25 cents (paper cover) 
CONTENTS 

Page 

Abstract__________________________________________________________ 1 

Introduction______________________________________________________ 1 

Alpine serpentinites of the Pacific coast, regional setting________________ 4 

Petrology of reaction zones_________________________________________ 8 

Rodingites____________________________________________________ 8 

Progressive metasomatism in reaction zones_______________________ 14 

Myrtle Creek, Oreg_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 14 

Baldy Mountain, Oreg_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 18 

lngalffi Peak, ~ash________________________________________ 22 

Twin Sisters, 23 

Cape San Martin, Calif____________________________________ 28 

Summary of Petrology_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 31 

Petrogenesis __________________________________________________ -· . _ _ 33 

Pressure and temperature conditions of metasomatism_________________ 42 

Tectonic setting of serpentinite emplacement and associated metasomatism___________________________________________________________ 44 

References cited___________________________________________________ 46 

FIGURE 1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

ILLUSTRATIONS 

Location map showing the distribution of ultramafic rocks 

and location of sample localities in California, Oregon, and 

Page 

~ashington__________________________________________ 6 

Field sketches of rodingites from Oregon, ~ ashington, and 

California, showing general shape and structure__________ 10 

Field sketch of locality at Myrtle Qreek, Oreg., showing 

sampling locations and relationships __________________ -_ 15 

Semilogarithmic plot of progressive chemical variation of 

samples from the Myrtle Creek area, Oregon____________ 17 

Field sketch of locality at Baldy Mountain, Oreg., showing 

locations of analyzed specimens and general relationships__ -19 

Semilogarithmic plot of the progressive chemical variation 

of samples from the Baldy Mountain area, Oregon______ 21 

Field sketch of locality at lngalffi Peak, ~ash., showing locations of analyzed specimens and general relationships____ 23 

Semilogarithmic plot of the progressive chemical variation of 

samples from the Ingalls Peak area, ~ ashington_ _ _ _ _ _ _ _ 25 

Field sketch of locality at Twin Sisters, ~ash., showing locations of analyzed specimens and general relationships ____ -- 26 

III 
IV CONTENTS 

Page 

FIGURE 10. Semilogarithmic plot of the progressive chemical variation 

of samples from the Twin Sisters area, Washington_ _ _ _ _ _ 2 7 

11. Field sketch of locality at Cape San Martin, Calif., showing 

locations of analyzed specimens and general relationships__ 28 

12. Cape San Martin reaction zone against sheared serpentinite__ 31 

13. Semilogarithmic plot of the progressive chemical variation of 

samples from Cape San Martin, Calif__________________ 32 

14. ACF diagrams showing the common minerals associated with 

rodingites_ _ _ __ _ _ __ __ _ _ _ __ ____ ___ __ ___ __ __ _ ____ _ _ ____ 36 

15. Progressive changes in composition within reaction zones 

plotted on ACF diagrams______________________________ 38 

16. Activity diagrams illustrating possible fields of stability for 

minerals present in reaction zones_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 39 

TABLES 

Page 

TABLE 1. Mineral assemblages and occurrences of rodingite_ _ _ _ _ _ _ _ _ _ 12 

2. Chemical and spectrographic analyses of rodingites and 

associated serpen tini tes _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 13 

3. Chemical and spectrographic analyses of samples from Myrtle 

Creek area, Oregon (loc. 9) _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 16 

4. Chemical and spectrographic analyses of samples from Baldy 

Mountain area, Oregon (loc. 10)________________________ 20 

5. Chemical and spectrographic analyses of samples from Ingalls 

Peak and Twin Sisters area, Washington (loc. 11, 12) _ _ _ _ 24 

6. Partial chemical and spectrographic analyses of metasomatized graywacke from Cape San Martin, Calif. (loc. 13) _ _ _ _ 29 
LOW-TEMPERATURE REACTION ZONES AND ALPINE 

ULTRAMAFIC ROCKS OF CALIFORNIA 

OREGON, AND WASHINGTON 

By ROBERT G. CoLEMAN 

ABSTRACT 

Metasomatism of va·ried rock type,s associated with alpine-type serpentinites is 

widespread in the Pacific coast area. This metasomatism is related to the process 

of serpentinization and the tectonic history of the ultramafic emplacement. Such 

diverse rock types as graywacke, gabbro, dacite, and basalt have been involved 

in the metasomatism. Originally, such metasomatized mafic rocks were named 

"rodingites," but a more general term such as "reaction zones" should be used, 

and such terms as "rodingite," "garnetized gabbro," "granat-vesuvian fels," 

"skarn," and "ophisperite" should be categorized as phases of a single process. 

The reaction zones are developed along the serpentinite-country rock contacts 

and in tectonic inclusions and dikes contained within serpentinite. Hydrogarnet 

is the predominant mineral, but idocrase, diopside, prehnite, xonotlite, chlorite, 

and nephrite also are characteristic minerals produced during the metasomatic 

alteration of igneous rocks. In altered sedimentary rocks and silicic igneous 

rocks, albite, potassium feldspar, and a fibrous actinolite are often associated 

with prehnite. Chemical analyses and mineralogical data of serpentinites and 

calcium metasomatites are presented for 42 specimens from ultramafic bodies in 

Oregon, California, and Washington. The metasomatic alteration is produced 

by a loss of silica to the serpentinite and a concomitant enrichment of calcium 

and magnesium within the reaction zone. Increase in density, addition of water, 

and frequent association with the blueschist facies metamorphism indicate 

that the alteration occurred under conditions of elevated pressure and high 

activity of water. The pressure-temperature conditions during metasomatism 

are estimated from the known experimental stability fields of those minerals 

formed during the alteration, the maximum temperature being approximately 

500°0 and the minimum about 250°0, with pressures probably in excess of 

4,000 bars. Such a pressure-temperature environment requires an unusual 

geologic environment as the combination of relatively low temperature and high 

pressure is not characteristic of igneous intrusions or load metamorphism. 

This report suggests that the reaction zones develop at the time when large 

portions of ultramafic mantle are tectonically emplaced into the tbase of the 

crust or are moved tectonically higher into the crust. Serpentinization of the 

ultramafic material is contemporaneous with alteration and tectonism. Thus, 

the alteration represents a tectonic contact metamorphism and is not related to 

igneous activity. 

INTRODUCTION 

Studies concerning the origin of alpine serpentinites. have led to the 

observation that metasomatic reaction zones are related to the emplacement and formation of these serpentinites. The purpose of this 

paper is to point out the widespread metasomatism in the alpine 

1 
2 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

serpentinites that form elongate belts along the Pacific coast of the 

United States and to explain its origin. The geologic setting and petrologic character of the metasomatism appear to be related to the tectonic 

history of these serpentinites, and a tentative theory on their formation will be advanced. The author first became interested in this problem while studying the jadeite deposits within the New Idria serpentinite mass in the central part of the Diablo Range of California 

(Coleman, 1961). Here the association of calcium-rich rocks with 

jadeite and albite presented an unusual mineral assemblage. Several 

field seasons were spent reconnoitering California, Oregon, and Washington serpentinites to ascertain the distribution and relationships of 

these peculiar reaction zones. In 1962, a year was spent in New Zealand 

studying similar reaction zones from the Great IDtramafic Belt (Coleman, 1966). Additional fieldwork on this same problem has been 

carried out on the alpine serpentinites of Australia and New Caledonia. The field evidence and petrologic information accumulated 

from these studies, together with a survey of the literature, clearly 

suggest that metasomatism is nearly ubiquitous as a contact reaction of 

ultramafic bodies with the country rock and provide important information relating the formation of serpentinites to the tectonic 

history. 

Rocks from these reaction zones have been given various names, 

depending upon their composition and textures. "RDdingite" was introduced to describe altered gabbros in the Dun Mountain serpentinites 

of New Zealand (Bell and others, 1911; Grange, 1927). The name is 

taken from the Roding River that drains much of the Dun Mountain 

ultramafic belt. Over the years the term "rodingite" has been employed 

to describe similar rocks from other serpentinites : Benson ( 1913, 

1915, 1917, 1918), Australia; Turner (1930), South Westland, New 

Zealand; Wells, Hotz, and Cater ( 1949), southwestern Oregon; Cater 

and Wells (1953), northern California; Leech (1953), southwest 

British Columbia; Suzuki ( 1953), Japan; Bloxam ( 1954) , Scotland; 

Baker ( 1959), Tasmania; Bilgrami and Howie ( 1960), Pakistan ; 

Majer ( 1960), northwest Syria; Schlocker ( 1960), central California; 

Chesterman ( 1960), northern California; Seki and l{uriyaga wa 

(1962), Japan. 

The rodingites described by these authors are made up predominantly of calcium-rich silicates such as hydrogarnet, idocrase, and diopside and are probably derived from altered gabbros or basalts associated 

with the ultramafics. Other authors have called these rocks garnetized 

gabbros (Miles, 1950), granat-vesuvian fels (Murigoci, 1900), skarns 

(Korzshinskii, 1958) , ophispherites ( J a:ffe, 1955), and kalksilikatfels 

(Muller, 1963). Related to the rodingites, and contained within or in 
INTRODUCTION 3 

contact with alpine ultramafics, are masses consisting predominantly 

of sodium-aluminum silicates and having only thin borders of calciumrich silicates. The jadeite deposits of Japan (I wao, 1953), Burma 

(Lacroix, 1930; Bleek, 1907), and California (Coleman, 1961) are 

albite-jadeite masses contained within serpentinites and showing 

calcium-rich borders. Reed (1959) has described metasomatized argillites from the Dun Mountain serpentinites, and further work in New 

Zealand has brought to light metasomatized graywackes and limestones (Coleman, 19·66). Chesterman (1960) has described calcium 

metasomatism of graywackes in contact with serpentinites. Various 

kinds of rocks showing metasomatism in contact with or contained 

within serpentinites are neither uncommon nor surprising. 

Metasomatism as used in this paper refers to a change in the bulk 

composition of the rocks in the reaction zone between serpentinite and 

country rock or associated mafic rocks. This bulk-composition change 

is reflected by the appearance of a different mineral assemblage 

(Thompson, 1959). 

All the metasomatic rocks associated with the serpentinites described 

in the literature referred to above and in this paper have similar characteristics, which may be summarized as follows: 

1. The metasomatic reaction zones are within or in contact with serpentinite and have not been observed i~ unserpentinized ultramafic rocks or in the high-temperature aureol~s of such rocks. The 

reaction zones are developed in diverse rock types in contact 

with the serpentinite. 

2. The reaction zones develop at the contact between tectonic inclusions, dikes and layers of mafic rocks, and surrounding country 

rock and the serpentinite. Most of these reaction zones have been 

involved in those tectonic movements that have acted on the 

serpentinite. Some of the original reaction zones and associated 

rock have been broken and transported tectonically during and 

after serpentinization. Synkinematic brecciation and mylonitization in the reaction zones indicates that metasomatism accompanies deformation. 

3. The chemical changes of the metasomatized rocks in the reaction 

zones trend toward a similar chemical composition. In the country rock, all the zones are undersaturated with respect to silica 

and show increases in calcium and, to a lesser extent, magnesium. In metasomatized gabbros and basalts where the chemical 

changes are well knmvn, the reaction zone contains Si02, Al203, 

CaO, and H 20 in approximately the molecular proportions 

3Ca0 · Al203 · 2Si02 · H 20. Those reaction zones derived from 

more silicic rocks, such as sandstone and granitic rocks, often 
4 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

contain a feldspar-rich zone or core and have a calcium-rich zone 

at the serpentinite contact. The serpentinite at the contact is also 

metasomatized and may consist of chlorite or nephritic actinolite. 

4. The reaction zones are localized and cannot be considered as related to any metasomatism on a. regional scale. There is no dir·ect 

relationship between the size of the ultramafic mass and the intensity or amount of metasomatic alteration. Local alteration 

zones with these particular calcium-rich mineral assemblages 

have not been reported from other geologic environments .. 

ALPINE SERPENTINITES OF THE PACIFIC COAST, 

REGIONAL SETTING 

The distribution of the ultramafic rocks in California, Oregon, and 

Washington is shown in figure 1. Certain faults and some of the 

ultramafics obviously are spatially related. Clark ( 1960) has shown 

that the locations of serpentinites are directly related to the fault 

systems of the foothills area of California and that the shape of the 

serpentinite masses are usually narrow and elongate parallel to the 

strike of the fault. Irwin ( 1964) postulated that certain California 

and Oregon ultramafics represent large overthrust sheets. Careful 

documentation of thrusting involving serpentinites has also been given 

by Brown (1964) for the northern Coast Ranges of California. Other 

areas of ultramafic rocks, notably in northern California and southwestern Oregon, that have irregular outlines and crosscutting relationships with the enclosing rocks 1nay have preserved intrusive 

contacts. Serpentinites associated with normal faults have not been recorded in the Pacific coast region. Several large diapiric serpentinite 

emplacements along the crests of anticlinal ridges in the Diablo Range 

of central California. are well documented. Although some serpentinites are associated with strike-slip faulting, serpentinit·e masses along 

the continental edge of the Pacific coast apparently are more commonly 

related to thrusting or diapiric piercements. 

The ultramafic masses in northern Washington, although less 

elongate than those in California, are mostly related to major faults. 

Thayer ( 1963) has stated that the eastern Oregon ultramafics are 

igneous intrusions but that postemplacement movement and serpentinization have affected their original position and shape. The degree 

of serpentinization of ultramafic rocks in the Western United States 

is related directly to the tectonic history of each individual mass. 

Certain ultamafic masses having a complex tectonic history, such 

as the New Idria, Calif., serpentinite, are completely serpentinized. 

Geologic evidence suggests that the New Idria mass has moved dia-
ALPINE SERPENTINITES OF THE PACIFIC COAST 5 

pirically upward, in successive tectonic steps, from middle Cretaceous 

to the present (Coleman, 19·61 ; Eckel and Myers, 1946). 

In contrast, certain ultra.mafics of the Klamath Mountains appear 

to have been emplaced as igneous intrusions during the Late Jurassic 

and show only partially serpentinized borders (Lipman, 1964). The 

Twin Sisters mass of intrusive "Tertiary?" (Ragan, 1963) dunite 

in northern Washington consists almost entirely of unaltered dunite 

with only a very thin skin of serpentinite (100 yd thick) developed 

at its contact with the country rock; this mass appears to have undergone diapiric movement. The primary igneous age 1 of the Pacific 

coast ultramafics therefore remains in doubt, as tectonism has allowed 

many of these intrusions to move after their original emplacement 

within the crust of the earth. 

The presence of detrital serpentine debris within Meszoic and. 

Tertiary sedimentary rocks of California attests to the long exposure 

of ultramafic rocks at the surface. Abundant ultramafic debris in the 

basal member of the Swauk Formation of Late Cretaceous and Paleocene age in the Mount Stuart area of Washington indicates a 

Paleocene or earlier age for some of the Washington ultramafics. In 

eastern Oregon, Thayer (1963) has given a Triassic age to the ultramafics of the Strawberry Mountains; but 20 miles to the west along 

the South Fork of the John Day River, highly sheared serpentinites 

appear to diapirically invade upper Miocene basalts of the Columbia 

River Group. The occurrences suggest that the Pacific coast ultramafic rocks are probably emplaced during the deformational events 

which initiated metamorphism and folding of their surrounding 

eugeosyclinal sediments. This concept is not new, and was advocated 

by Hess ( 1955) to explain the distribution of alpine ultramafics 

throughout the world. Ultramafics showing only minor serpentinization and crosscutting relationships, such as the central core of the 

Klamath Mountains, probably represent conditions of emplacement 

in the initial and early stages of tectonism. Those more highly serpentinized and elongated ultramafics associated with major faults 

have undergone more than one period of deformation. Each successive 

period of orogeny has further serpentinized these ultramafics, allowing further diapiric movement to take place as a result of expansion 

and decrease of density at several times during the process (Milovanovic and Karamata, 1960; Ducloz and V uagnat, 1962). 

A careful search for high-temperature aureoles (1100°-1130°C; 

see Jackson, 1961) produced by magmatic emplacement along many 

contacts of Pacific coa,st ultramafic masses was not successful. That. 

1 Primary igneous age as used here refers to the period of time when a. melt of ultramafic 

composition crystallized. 

269-731 0-67-2 
6 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

WASHINGTON 

-------

OREGON 

Tectonic map of the United States 

(U.S. Geol. Survey and Am. Assoc. 

Petroleum Geologists, 1962) 

' ' "' 

NEVADA 

' ' "' ' ' 

EXPLANATION 

Ultramafic rocks 

Including gabbro, serpentinite, dunite, and peridotite 

Major fault 

Dashed where approximately located; dotted 

where projected 

® 

Sample locality 

Localities listed in table on 

opposite page 

0 100 MILES 

FIGURE 1.-Distribution of ultramafic rocks and location of sample localities in 

California, Oregon, and Washington. Data from U.S. Geological Survey and 

American Association of Petroleum Geologists (1962). 
ALPINE SERPENTINITES OF THE PACIFIC COAST 7 

such contact effects do exist elsewhere has been suggested by MacKenzie ( 1960), Green (1964), and Challis ( 1965). Yet even these 

authors find that the observed contact metamorphism is not commensurate with the concept of a completely molten peridotite or 

dunite magma. For example, Lipman (1964, p. 199) states of the 

Klamath Mountains: "* * * the evidence accumulated indicates that 

the ultramafic pluton initially had the form of a gently dipping sheet, 

was intruded at temperatures of 500° to 700°C by flowage and recrystallization of an essentially crystalline mass concurrently with 

metamorphism of adjacent country rock to assemblages of the almandine amphibolite facies." Thayer and Brown (1964) described mafic 

a.nd ultramafic contact effects as follows: "The only contact-metamorphic effects were slight to moderate recrystallization, without 

development of foliation, a.long contacts of gabbro and diorite 

with country rock. Continued strong deformation after intrusion is 

shown by widespread brecciation and albitization of gabbro, quartz 

diorite, and Paleozoic rocks, and by diapiric movement of serpentine 

that engulfed blocks of albite granite." 

Contacts of the highly sheared Pacific coast serpentinites usually 

show no evidence of high-temperature thermal metamorphism; however, in some ·areas well-developed reaction zones are formed within 

the country rock. These zones resemble narrow contact aureoles observed around igneous intrusions except that the mineral assemblages 

are characteristic of low temperatures and metasomatism is extensive. 

This same metasomatic alteration also affects various tectonic inclusions engulfed within the serpentinite and mafic dikes and layers 

within the ultramafic masses. The low-temperature metasomatic zones 

in country rock are localized and are not proportional to the size of 

the ultramafic masses. The irregular distribution of the metasomatic 

zones of any one mass or from mass to mass precludes analogies to 

high-temperature contact metamorphism. The low-temperature 

metasomatic zones may mask earlier high-temperature contact effects. 

KEY TO SAMPLE LOCALITIES SHOWN IN FIGURE 1 

Map Sample Location Quadrangle Long w. LatN. 

No. 

------

1 7-RD-60-A Eureka mine road _________ Pearson Peak, Oreg _____________ 123°45' 41°20' 

2 18-RD-60 Chetco Pass road _________ Pearson Peak, Oreg ______________ 123°49' 42°171 

3 24-RD-60-C Illinios River bed _________ Pearson Peak, Oreg _____________ 123°46' 42°18' 

4 38-RD-60-5 Myers Creek ______________ Gold Beach, Oreg _______________ 1240Z5' 42°18' 

5 7-RD-61-A Salmon Creek _____________ Burro Mountain, Calif__ ________ 1?1°21' 35°48' 6 8-RD-61-1 Morro Creek ______________ Cayucos, CaliL ____________ ----- 120°46' 35°25' 7 51-RD-61-E Sultan Basin ______________ Index, Wash ____________________ 121°32' 48°00' R 52-RD-61-A Cypress Island ____________ Anacortes, Wash ________________ 122°40' 48°35' 

9 32-RD-61-1 Myrtle Creek _____________ Roseburg, Oreg _________________ 123°19' 43°02' 

10 41-RD-61-A Baldy Mountain __________ John Day, Oreg _________________ 118°47' 44°18' 11 48-RD-61 Ingalls Peak ______________ Mount Stuart, Wash ____________ 120°59' 47°29' 

12 53--RD-61-D Twin Sisters ______________ Hamilton, Wash ________________ 121 °55' 48°37' 

13 3--RD-61 Cape San Martin _________ Cape San Martin, Calif_ ________ 121°28' 35°53' 
8 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

Challis (1965) has described a "high" temperature aureole at Red 

Mountain in New Zealand, where a later metasomatism has been 

superposed. Such effects could be expected in the Pacific coast ultra.-

mafics, but they have not been discovered. The following sections 

describe the occurrence of selected calcium metasoma.tites and relate 

their petrologic nature to the tectonic period of serpentinization and 

emplacement of ultramafics. 

PETROLOGY OF REACTION ZONES 

Descriptions of calcium metasomatites from many locations (fig. 1) 

of ultramafic bodies unrelated to each other are given here to demonstrate a similarity in mineralogy, chemistry, and genesis. The discussion is divided into two sections; the first describes small individual 

rodingite masses, and the second describes the progressive nature of 

the reaction zones on various rock types. 

RODINGITES 

Included in this discussion are those dense white calc-silicate rocks 

that typically contain hydrogarnet, diopside, idocrase, and chlorite 

and that represent the metasomatism of ga})bros or basalts. These 

rocks invariably occur as isolated masses within sheared serpentinites 

and are identical with the rodingites originally described by Bell, 

Clarke, and Marshall (1911) from the Dun Mountain serpentinites 

of New Zealand. 

The diversity in shape and size of several typical rodingites is illustrated in figure 2. These masses were originally tabular bodies and 

may represent ·altered dikes of mafic rock cutting the ultramafics. 

Material from Cypress Island was originally a coa.rse-grained gabbro 

dike (fig. 2.A) that had been broken and distended within a. narrow 

serpentinized band of peridotite. In contrast, fine-grained Morro 

Creek rodingite, having an ellipitical and rounded shape, is included 

in extremely sheared se,rpentine (fig. 2F) and could be a fragment of 

a former mafic dike or, more likely, a tectonic inclusion of basalt. 

The outer surfaces of the rodingites often are slickensided, which indicates movement within the serpentinite, so all these rodingites have 

undergone some tectonic transport and some distension or fragmentation. A typical rodingite may only be a foot or more in width 

and rarely is more than 10--15 feet long. 

In serpentinites exhibiting alteration of mafic dikes to rodingites, 

a progressive change of shape of mafic dikes from tabular 

to isolated discoids accompanies the progressive alteration to rodingite 

and tectonic transport in the serpentinite. The outer portion of these 

rodingites usually consists of a nearly black rind of chlorite or, 
PETROLOGY OF REACTION ZONES 9 

in some places, nephrite (fig. 2B, 0, F), which contrasts with the 

nearly white calc-silicate core. These rinds are similar to the "blackwall" described by Chidester ( 1962, p. 65). The small Morro Creek 

rodingite is exceptional in that chlorite is present all the way into 

its center. The calc-silicates forming the central portion of rodingites 

are extremely fine-grained mixtures, and only rarely can modal data 

or textures be ascertained. Metasomatized tectonic inclusions of the 

more silicic igneous rocks or sediments are similar in their textures 

and structures to rodingites, but their central parts are usually rich 

in feldspar (Coleman, 1961). The more silicic inclusions are more 

commonly present along the serpentinite borders, whereas rodingites 

derived from preexisting mafic dikes or. layers are rather evenly distributed throughout the serpentinite masses. 

Microscopic examination of rodingites commonly reveals an igneous 

texture dharaoteristic of basalts or grubbros, and thus the original rock 

type can be 'ascertained; however, cataclastic deform31tion may completely obscure any evidence of the original textures. 

Mineralogical composition of rodingites is very simple and is 

dominated by hydrogarnet, diopside, idocrase, and prehnite (table 1). 

Where original textures are preserved, brown isotropic hydrogarnet, 

idocrase, or prehnite is pseudomorphous after feldspar. Ferromagnesian minerals alter to chlorite and tremolite or, in some rocks 

such as the Sultan Basin and Cypress Island rodingites, the pyroxenes 

are only partially altered to chlorite. Diopside seems to replace 

feldspars as well -as ·the ferromagnesians. The atypical more silicic 

Myers Creek material contains prehnite, pectolite, and barium-rich 

pota;ssium feldspar. These rocks ·.are so extremely fi11e grained ·and 

poorly crystallized that optical mineralogy is often not diagnostic. 

Identification of the mineral assemblages was determined by .X-ray 

diffraction of the whole rack and of density fractions separated by 

heavy liquids. 

The associated serpentinites exhibit no textural or mineralogical 

characteristics that distinguish them from other typical rocks within 

the surrounding ultramafic mass. Thin section and X-ray study of the 

associated serpentinites, however, does reveal the presence of diopside, 

andradite, and nephrite. These minerals are secondary and replace the 

host serpentine as veins or as porphyroblastic crystals. The chlorite rim 

surrounding the rodingites progressively replaces the serpentine and 

retains some of the original textures of the serpentine. Determination 

of the serpentine mineral species revealed no particular polymorph 

characteristic of the enclosing serpentinites (table 1). Hostetler, 

Coleman, Mumpton, and Evans (1966) have shown that clinochrysotile 

and lizardite are the common polymorphs of alpine-type serpentinites. 
10 REACTION ZONES AND ALPINE ULTRAMAFIC_ ROCKS 

1 FOOT 

L__j 

1 FOOT 

1 FOOT 

FIGURE 2.-Field sketches of rodingites from Oregon, Washington, and California, showing general shape and structure. 

A, Cypress Island (vertical section) (52-RD-61-A). 

B, Illinois River bed (horizontal section(2) 4-RD-60-C). 

C, Chetco Pass road (vertical section(1)8-RD-60). 
PETROLOGY OF REACTION ZONES 

L__j 

1 FOOT 

1 FOOT 

EXPLANATION 

Rodingite or 

Ca-metasomatite 

Chlorite or 

nephrite borders 

X 

Serpentinite 

Ultramafic rocks 

Approximate position of sample 

D, Eureka mine road (vertical section) (7-RD-6D-A). 

E, Sultan Basin (horizontal section) (51-RD-61-E). 

F, Morro Creek (horizontal section)(S-RD-61-1). 

See fig. 1 for location, table 1 for mineral assemblages, and 

table 2 for chemical and spectrographic analysis.) 

11 
12 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

TABLE 1.-Mineral assemblages and occurrences of rodingites 

Mineral 

assemblage Associated 

Sample Location (in order of Texture Occurrence serpentinites decreasing 

abundance) 

---

1 Eureka mine Hydrogarnet, Original vari- Dikelike ellipsoid Clinochrysotile, Road, chlorite, diop- olitir texture in sheared ser- lizardite, chroOreg. side, sphene. partially pre- pentinite. Width mite, magne- served. =2 ft, length =5 tite, garnet. ft. 

2 Chetco Pass Diopside, hydro- Cataclastic. Ellipsoidal inclu- C linochrysotile, Road, garnet, chlorite. sion in sheared orthochrysotile, Oreg. serpentinite. brucite, mag- Major diameter 3 netite. 

ft, minor diam- eter 2ft. 

3 Illinois River Hydrogarnet, Cataclastic. Tabular dikelike Clinochrysotile, bed, Oreg. idocrase, diop- mass within magnetite, or- side, chlorite. sheared serpenti- thochrysotile, nite. Width 2 ft, brucite, garnet, length 8ft. pyroxene. 4 Myers Creek, Prehnite, pecto- Cataclastic. Rounded tectonic Clinochrysotile, Oreg. lite, barium- inclusion in orthochrysotile, rich orthoclase. sheared serpenti- magnetite. nite. =5 ft in 

diameter. 

5 Salmon Hydrogarnet, Original ophitic Small rounded Antigoritelike Creek, chlorite, diop- basaltic tex- mass within serpentine, bruCalif. side, sphene. ture pre- sheared serpenti- cite, magnetite. 

served. nite. =1.5 ft in 

diameter. 

6 Morro Creek, Chlorite, preh- Cataclastic. Small rounded Clinochrysotile, Calif. nite, sphene. mass within lizardite, brusheared serpenti- cite, carbonate, 

nit e. = 1 ft in magnetite. diameter. 7 Sultan Basin, Hydrogarnet, Gabbroic. Flattened ellipsoid Clinochrysotile, Wash. chlorite, diop- within sheared amphibole, 

side, augite. serpentinite. magnetite. Width 2 ft, length 6ft. 

8 Cypress Albite, zoisite, Gabbroic. Dikelike tabular Lizardite, bruIsland, pyroxene, nat- mass in partially rite, olivine, 

Wash. rolite, tober- sheared massl.ve magnetite. 

morite, chlorite. serpentinite. 

Width =1ft, 

length =5 ft. 

Chemical and spectrographic analyses of eight rodingites are given 

in table 2. The field sketches in figure 2 show the approximate position 

of the samples taken for analysis and mineralogical study. Low silica 

content and high calcium content characterize rodingites; this composition places the rodingites in a rather unusual chemical group as they 

also contain an unusual amount of water. The Morro Creek specimen 

is chemically distinct because it consists predominantly of chlorite. The 

presence of chlorite in nos. 2, 5, and 6 (table 2) also accounts for the 

higher magnesium content. In general, the silica content of the rodingites approaches that of the serpentinite when metasomatism is at its 

maximum. The serpentinites associated with the rodingites commonly 

show increases in both calcium and aluminum, indicating that these 

elements move out into the serpentinite during metasomatism. The 

chlorite and nephrite rims represent the complete reaction of serpentinite in the zone of alteration. In some places, however, postmetasomatic 

tectonic movement of rodingites has put them in contact with serpen-
-'I 

~ ~ ,_. 

1 

r 

~ 

TABLE 2.-Chemical and spectrographic analyses of rodingites and associated serpentinites 

[Rapid rock analysis performed by methods similar to those described by Shapiro and Brannock (1956); analysts: PaulL. D. Elmore, S.D. Botts, Gillison Chloe. Quantitative 

spectrographic analysis by Robert Mays. Chlorine analysis by Joseph Budinsky and Irving May) 

1 2 3 4 5 6 7 8 

---------

7-RD- 7-RD- 18-RD- 18-RD- 24-RD- 24-RD- 38-RD- 38-RD- 7-RD- 7-RD- 8-RD- 8-RD- 51-RD- 51-RD- 52-RD- 52-RD60-A 60-B 60 60 60-C 60-D 60-5 60-A 61-1 61-3 61 61-3 61-B 61-B 61-A 61-B 

Roding- Serpen- Roding- Serpen- Roding- Serpen- Roding- Serpen- Roding- Serpen- Roding- Serpen- Roding- Serpen- Roding- Serpenite tinite ite tinite ite tinite ite tinite ite tinite ite tinite ite tinite ite tinite 

-

Chemical analyses (weight percent) 

Si02- ___ --- _______ ---- 40.2 40.5 47.3 39.0 43.2 40.6 55.6 41.4 30.8 41.1 34.6 40.2 39.8 42.9 44.0 34.4 

A.hOa_ ---------------- 10.5 1.6 3. 7 5.4 8.6 1.8 9.8 1.5 18.5 1.1 15.5 .89 20.8 7.5 27.5 .3(] Fe20at ________________ 12.2 6.8 6.4 5.5 11.8 6.6 2.0 6. 7 13.4 6. 5 8.6 7.4 3.9 11.2 .69 7.4 

MgO ----------------- 3.1 37.0 13.9 37.2 5. 7 36.6 . 78 36.8 14.4 38.1 22.7 37.3 5.8 23.5 1.0 41.1 

CaO ------------------ 32.7 1.1 26.8 .05 27.9 1.9 20.5 . 21 13.2 .17 5. 7 .23 25.7 7.3 13.8 • 3~ 

N a20 __ --------------- .07 .04 .07 .04 . 21 .03 6.2 .05 .05 .03 .12 .05 .14 .34 6.3 .os 

020-- ---------------- . 01 .01 . 01 . 01 .03 .02 2.6 .02 .01 . 01 . 01 . 01 .22 .03 .18 .04 

H20+ ---------------- . 76 12.1 1.5 12.3 1.5 12.0 2. 7 12.6 7.8 11.7 10.5 12.7 2.8 6.2 4.2 13.2 

H20----------------- .10 .88 .11 .87 .15 .83 .31 . 92 .35 . 79 . 93 .84 .22 .43 .43 1.4 

Ti02.---------------- . 56 . 03 . 72 . 03 . 63 . 03 .13 . 02 . 79 .03 . 58 . 02 .17 . 31 .03 . 0~ 

MnO __ --------------- .15 .12 .15 .11 .22 .09 .06 .08 1.2 .11 .23 .08 .14 .14 .02 .l:l 

------------------------------------------------ TotaL __________ 100.35 100.18 100.66 100.51 99.94 100.50 100.68 100.30 100.50 99.66 99.47 99.72 99.69 99.85 98.15 98.3Q 

------------------------------------------------ Specific Gravity ______ 3.59 2. 70 3.29 2.83 3.44 2. 75 2.81 2.87 3.07 2. 70 2.81 2. 76 3.25 2.98 2. 78 2. 6~ 

Spectrographic analyses, minor elements (parts per million) 

)_--- --------------- 19 90 33 80 30 90 6 100 40 70 34 75 17 90 <4 9(] 

N L ------------------- 25 2000 140 2400 26 2000 29 2400 36 2200 46 2000 80 1000 16 200(] 

r -------------------- 4 2000 200 2000 20 2200 32 4200 26 2800 20 2200 75 1800 20 320(] 

·-------------------- 40 9 50 10 50 9 <4 <4 75 10 44 8 14 20 <4 <4 

-------------------- 20 <10 90 <10 40 <10 80 <10 40 <10 50 <10 <10 <10 <10 <l<J 

-------------------- 12 <4 <4 <4 120 <4 100 20 26 <4 50 <4 120 45 420 4 

:t.--- --------------- <4 <4 <4 <4 <4 <4 1100 8 <4 <4 90 4 130 10 190 <4 

i~ ~ = = == = = == = === == = == = 

<20 50 <20 <20 <20 <20 30 <20 <20 150 280 70 70 40 <20 <2C 

40 150 30 30 20 80 70 330 40 170 130 560 90 90 830 1500 

z 

--

1 Total Fe given as Fe20a. 

1. Eureka mine Road, Oreg. 

2. Chetco Pass Road, Oreg. 

3. Illinois River bed, Oreg. 

4. Myers Creek, Oreg. 

5. Salmon Creek, Calif. 6. Morro Creek, Calif. 7. Sultan Basin, Wash. 

8. Cypress Island, Wash. 

'"d 

t;t 

t-3 

l;Jj 

0 

t'l 

0 

~ 

0 

~ 

pj 

t;t 

~ ~ 

0 

z 

IS 

~ Ul 

1--L 

~ 
14 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

tinite that shows no alteration. Rodingites have distinctly higher 

specific gravity than serpentinite or the protolith from which they are 

derived. A more complete discussion of the chemical and spectrographic analyses is given in the section on chemical petrology. 

PROGRESSIVE METASOMATISM :fN REACTION ZONES 

To demonstrate that the metasomatism in the reaction zones was 

progressive and affected rocks other than basalts and gabbros, a 

series of samples were taken from reaction zones adjacent to the 

serpentinite contact out to apparently unaltered rock. Most of these 

samples are inclusions or dikes within serpentinites, but one represents the contact of serpentinite against country rock (Cape San 

Martin). These samples are isolated examples and cannot be, as yet, 

related to a comprehensive theory regarding reaction zones and history of alpine serpentinites. Nonetheless, the reaction zones are widespread and should offer some clues to the serpentinite problem. 

MYRTLE CREEK, OREG. (loc. 9, fig. 1) 

A large tabular mass of dacite within sheared serpentinite is 

exposed in a roadcut along U.S. Highway 101, 1 mile north of Myrtle 

Creek on the south bank of the Umpqua River. The serpentinite is an 

elongate mass that strikes northeast and appears to be in fault contact to the east with the Dothan Formation of Jurassic age. Diller 

( 1898) considered the dacite to be intrusive into the Dothan but 

assigned no definite age to the dacite. A careful study of his map indicates that nearly all the dacite within the serpentinite of the Roseburg 

area can be interpreted as tectonic inclusions, whereas the other 

dacites are shown as intruding the Dothan Formation. 

The altered porphyritic dacite mass at Myrtle Creek is tabular and 

is approximately 116 feet thick. The fact that this body is truncated 

by the serpentinite at the top and is associated with numerous smaller 

inclusions of the same rock type completely surrounded by serpentinite indicates that the dacite is a large transported inclusion, even 

though it may have originally been a dike that intruded the ultramafic rock. 

The contact between the serpentinite and altered dacite is sharply 

delineated by a reaction zone of dense white rock which is about 3 

feet thick on both borders (fig. 3). The outer 6 inches of the altered 

zone is a highly altered serpentinite whose original texture is retained 

but that now consists predominantly of diopside and calcite and minor 

amounts of serpentine and chromite (table 3, Nos. 6, 7). This altered 

serpentinite is sheared against the unaltered serpentinite (table 3, 

No. 8), indicating some postmetasomatic tectonic movement. A white 
PETROLOGY OF REACTION ZONES 15 

Dacite 

50 FEET 

~l:GURE 3.-Field sketch (vertical section) of locality at Myrtle Creek, Oreg., 

showing sampling locations and relationships between dacite and serpentinite. 

Explanation is the same as shown in figure 2. Numbers are .those shown for 

samples in figure 4 and table 3. 

dense rock with only faint porphyritic texture makes up the conspicuous metasomatic zone in the dacite (table 3, Nos. 4, 5). A specimen from the hanging-wall side (No. 5) is a fine-grained cataclastic 

mixture of hydrogarnet, xonotlite, calcite, and prehnite that is cut by 

1 inch monomineralic veins of xonotlite, prehnite, and pectolite. 

Chemical and optical analyses of the pectolite were given earlier by 

Coleman (1961). 

On the footwall side, alteration is less intense, and the altered dacite, 

consisting of albite, prehnite, amphibole, and calcite, retains its original 

porphyritic texture. Late 0.5-inch-thick prehnite veins are present in 

this zone. The texture of the rock is definitely porphyritic away from 

both altered zones, and a series of samples (table 3, Nos. 1, 2, 3) were 

taken to determine the nature of the alteration. In texture, Nos. 1 and 

2 clearly represent partially altered dacite that consists of albite, chlorite, muscovite, quartz, hornblende, and calcite; in addition, No. 2 contains epidote and sphene. Original feldspar has altered to albite, muscovite, and calcite, and only minor amounts of igneous hornblende remain. The whole rock has undergone a low-grade metamorphism 

which may be part of the reaction zones seen at the borders or may be 

an earlier change. Intermediate to these slightly altered rocks of the 

dacite interior is sample 3, in table 3; this rock contains no quartz, and 

part of the groundmass feldspar is replaced and transected by small 

monomineralic veins of prehnite. The serial change within the inclu-
16 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

TABLE 3.-Chemical and spectrographic analyses of samples from Myrtle Creek 

area, Oregon (loc. 9) 

[0, tested for but not found; ______ , not tested for. Rapid rock analyses performed by methods similar to those described by Shapiro and Brannock (1956); analysts: Paul L. D. Elmore, S. D. Botts, Gillison 

Chloe, L. Artis, J. Glenn, D. Taylor, II. Smith. Quantitative spectrographic analysis by Robert Mays. 

Chlorine analysis by Joseph Budinsky and Irving May] 

Dacite Metasomatites Altered serpen- Serpentinites tinite 

1 2 3 4 5 6 7 8 

32-RD- 32-RD- 32-RD- 32-RD- 32-RD- 32-RD- 32-RD61-3 61-4 61-5 61-2 61-6 61-1 61-7 

Chemical analyses (weight percent) 

SiOt----------------- 62.5 67.2 56.1 53.7 39.4 41.0 41.3 39.7 

AbOa--- ------------- 17.6 16.9 14.9 14.6 12.1 2.3 7.8 1.9 

Fe20a------- --- _ ----- . 77 .80 .39 .28 .88 1.6 .80 FeO } I 7.4 _________________ 3.3 1.8 2.1 3.2 1.2 2.5 2.0 

MgO ___ -- ----------- 1. 6 .90 1.3 4.4 1.4 19.9 8.6 36.7 cao ______ ----------- 3. 7 3.2 12.7 14.1 38.4 20.9 31.5 .26 

Na20----- ---- _______ 4.5 4. 7 7.5 5.0 .11 0 0 .05 

K20----------------- 1.2 1. 4 .27 .07 0 0 0 .01 

H20+---- ____ ------- 2.2 1.3 2.3 2.1 3.3 4.6 3.2 12.5 

H20---------------- .21 .30 .24 .29 .45 .32 .28 1.1 Ti02-- ______________ . 51 .31 .32 .41 .25 .02 .16 .04 

P205----------------- .14 .09 .07 .14 .06 0 .02 MnO ---------- ________________ .16 .13 .17 .13 .18 .13 .15 .14 

C02----------------- 1.4 1.0 1. 5 1.5 2.2 6.6 4.2 ---------- --------------------- Total __________ 99.79 100.03 99.86 99.92 99.93 99.87 100.01 99.80 ------------------------ Specific gravity ______ 2. 72 2. 73 2. 77 2.87 3.01 3.00 3.06 2. 72 

Spectrographic analyses, minor elements (parts per million) 

co __________________ 7 <4 <4 8 <4 28 34 100 

Nf. _____ ------------- <4 <4 <4 35 <4 1,500 1,600 2,000 cr ___________________ 6 6 6 32 10 1,100 1,400 2,800 

80------------------- 4 <4 <4 10 <4 <4 4 11 zr ___________________ 140 140 90 110 90 <10 <10 <10 sr ___________________ 400 400 140 600 75 80 60 <4 Ba. _____________ --- _ 800 650 90 380 <4 10 8 <4 

B-------------------- <20 40 2,000 310 160 <20 210 180 CL __________________ 40 50 40 50 40 50 30 40 

1 Total iron as Fe20a. 

1. Slightly altered dacite 50 ft from contact; consists of albite, chlorite, muscovite, quartz, hornblende, 

and calcite. 

2. Slightly altered dacite 41 ft from contact; consists of albite, quartz, chlorite, muscovite, hornblende, 

epidote, sphene, and calcite. 

3. Metasomatically altered dacite 6 in. from contact; consists of albite, prehnite, chlorite, muscovite, epidote, sphene. 

4. Calcium metasomatite near contact; consists of albite, prehnite, amphibole, calcite. 

5. Calcium metasomatite from contact; consists of hydrogarnet, xonotlite, calcite, prehnite. 6. Altered serpentinite from contact; consists of diopside, serpentine, calcite, chromite. 

7. Altered serpentinite from contact; consists of diopside, serpentine, calcite. 

8. Serpentinite (unaltered) approximately 35 ft from altered dacite; consists of antigoritelike material, 

clinochrysotile, magnetite. 

sion demonstrates a progressive metasomatism. None of the described 

metasomatic minerals or textures of the dacite can be related to the 

Igneous processes. 

Chemical analyses of the alteration sequence, when plotted to show 

variation in the elements as related to the reaction zone, reveal systematic changes in bulk com position (table 3; figs. 3, 4). Samples 1 and 

2 are assumed to represent the original bulk composition of the dacite; 
1-

z 

LJ.J 

u 

a:: 

LJ.J 

10 

a. 1.0 

II 

(.!) 

w 

~ 

0.1 

a- __ 

\ 

\ 

PETROLOGY OF REACTION ZONES 

REACTION ZONE 

--.. --<>-------o.. SiO ............ ~ ........ 

\ 

\ 

\ 

\ 

\ \ 

1 AND 2 

AVERAGE 

\ 

\ 

'o 

3 

\ 

\ 

\K2 0 

\ 

\ Na 2 0 

\ 

\ 

\ \ 

\ 

~ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

4 5 

SAMPLE 

\ 

\ II \; 

6 AND 7 

AVERAGE 

8 

FIGURE 4.-Semilogarithmic plot of progressive chemical variation of 

samples from the Myrtle Creek area, Oregon. Sample numbers are those 

given in table 3 and figure 3. The abscissa does not represent horizontal 

distance between samples. Points are placed equidistant to exhibit 

chemical variations. 

17 
18 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

samples 3-5, progressively metasomatized dacite; sample 8, the original bulk composition of the serpentinite; and samples 6 and 7, metasomatized serpentinite. Calcium enrichment of calcium in the altered 

dacite and serpentinite indicates that this element was a mobile component during alteration and was fixed in the reaction zone. The magnesium is erratic and may have migrated from the serpentinite toward 

the reaction zone in the dacite. The alkalis, as plotted, indicate considerable mobility during metasomatism; potassium is progressively 

depleted, and sodium appears to concentrate in the reaction zone away 

from the serpentinite contact. The altered serpentinite is enriched in 

aluminium and the altered dacite depleted. Desilication of the altered 

dacite is obvious; however, there is some silica enrichment in the altered 

serpentinite. The increase in specific gravity and water content of 

sample 5, compared with the original dacite of 1 and 2 signifies highwater pressures during metamorphism. 

BALDY MOUNTAIN, OREG. (loc. 10, :D.g. 1) 

The hornblende gabbro from Baldy Mountain ( SW%,NE% sec. 31, 

T. 14 S., R. 33 E., John Day, Oreg., 15-minute quadrangle) is part of 

the Canyon Mountain ultramafic complex in eastern Oregon (Thayer, 

1963). The hornblende gabbro dike intrudes and contains inclusions 

of partly serpentinized pyroxene-rich peridotite. The dike is metasomatized and shows a marked asymmetric alteration, more intense 

on the footwall (fig. 5). This same peridotite generally forms a somewhat tabular zone between much larger areas of olivine-rich peridotite and gabbro-norite which Thayer (1963) considers to be Triassic 

In age. 

The enclosing peridotite (table 4, No.5) is only partially serpentinized (10-15 percent by volume) and contains inclusions of plastically 

deformed mafic gabbro that is not related to the hornblende gabbro 

dike (table 4, No. 4) . The peridotite consists of approximately 25 

percent fresh and altered olivine and 75 percent pyroxene with the 

serpentinized areas generally confined to the olivine grains. Minor 

amounts of amphibole were detected by X-ray diffraction of the whole 

rock but were not seen in a thin section. 

The interior of the gabbro dike (table 4, No. 1 ; fig. 5, No. 1) consists of greenish -brown hornblende and calcic plagioclase ( An5o), 

having a hypidiomorphic texture, and a minor amount of pyroxene. 

Ma,gnetite is the dominant a.ccessory mineral. Incipient replacement of 

the plagioclase by prehnite is the only indication of alteration. A sample near the footwall contact (fig. 5. No. 2) is altered (table 4, No. 2) 

and consists of hydrogarnet, prehnite, diopside, and minor amounts of 

sphene, chlorite, and primary amphibole. The original texture is 

retained but plagioclase is completely replaced by hydrogarnet and 
PETROLOGY OF REACTION ZONES 

8 FEET 

FIGURE 5.-Field sketch (horizolllta:l section) of locality at Baldy Mountain, Oreg., showing locations of 

analyzed specimens and general relationships of 

specimens (table 4). Explanation is the same as 

shown in figure 2. 

I 

19 

prehnite; the amphibole is replaced by diopside. Numerous monomineraiic veins and patches of prehnite cut the gabbro. At the contact 

on the footwall side of the dike (fig. 5, No. 3), the hornblende gabbro 

ha.s been metasomatized to a dense white fine-grained rock (table 4, 

No.3) similar in appearance to the rodingites described earlier. Original igneous minerals of the hornblende ga.bbro'.have been completely 

replaced by hydrogarnet, diopside, prehnite, and chlorite, leaving only 

a faint tra.ce of the preexisting textures. This petrogTaphic evidence 

reveals a progressive metasomatism of the dike from the footwall side 

toward the center. This same alteration, but less intense, is developed on 

the hanging wall side of the dike. Replacement features and vein 

patterns indicate that the alteration followed the igneous emplacement, and there is no textural evidence relating 1netasomatism to the 

igneous event. From textural evidence, the source of the altering solutions appears to have been the peridotite, and the solutions probably 
20 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

TABLE 4.-Chemical and spectrographic analyses of samples from Baldy Mountain 

area, Oregon (loc. 10) 

[Rapid rock analysis performed by methods similar to those described by Shapiro and Brannock (1956); 

analysis: PaulL. D. Elmore, S D. Botts, Gillison Chloe. Quantitative spectrographic analysis by Robert 

Mays. Chlorine analysis by Joseph Budinsky and Irving May] 

Si02---------------

AJ203--------------

Fe20a 1---··-- _-- -- _--

MgO _ - _ ----------- Cao _______________ N a20 ______________ 

K20- - - ---- -- -- -- - - H20 + -------------

H20- ------------- Ti02--------------- MnO ___ - __ --------

Total ________ 

Specific Gravity _____ 

Altered gabbro 41-RD-61-C 

Metasomatites 

41-RD-61-B 141-RD-61-A 

2 4 

Chemical analyses (weight percent) 

47.8 39. 4 43.6 

14.5 15.4 10. 8 

11.8 12. 0 9. 7 

6. 3 8. 2 5. 6 

13. 0 18. 1 26. 5 

2. 4 . 30 . 12 . 16 . 03 . 01 

2. 1 4. 4 1.9 . 18 . 25 . 48 

1.5 1.7 1.2 . 20 . 20 . 16 

99. 94 99. 98 100. 07 

3. 04 3. 14 3. 34 

Peridotite 4- Altered mafic 

RD-61-E gabbro 41-

RD-61-D 

4 

-

48. 5 40.8 

2. 9 16. 3 

6. 8 7. 4 

21. 5 11.8 

15. 9 17.9 . 13 . 12 . 02 . 03 

3. 3 5. 3 . 36 . 32 . 14 . 24 . 13 . 15 

99. 68 100. 36 

3. 12 3. 05 

Spectrographic analyses, minor elements (parts per million) 

Co _______________ _ N i ________________ _ Cr ________________ _ Sc ________________ _ Zr ________________ _ Sr ________________ _ Ba _______________ _ B _________________ _ Cl ________________ _ 

1 Total Fe given as Fe20a. 

37 

70 

20 

60 

100 

240 

20 

180 

270 

44 

85 

16 

65 

120 

4 

7 

150 

110 

30 

70 

12 

31 

90 

<4 

<4 

220 

40 

36 

240 

1400 

55 

<10 

<4 <4 

180 

90 

40 

90 

16 

55 

<10 

30 <4 

160 

10 

1. Slightly altered gabbro from the central port10n of the dike; consists of hornblende, plagioclase, prehnite, magnetite. 2. Partially altered gabbro from the contact; consists of hydrogamet, prehnite, diopside, amphibole, sphene, chlorite, plus primary amphibole. 3. Complptely altered gabbro at the contact; consists of hydrogamet, diopside, prehnite, chlorite. 

4. Peridotite country rock enclosing gabbro; consists of clinopyroxene, olivine, orthopyroxene, serpentine. 

Sample taken about 3 ft from gabbro dike. 

5. Small inclusion of altered mafic gabbro in peridotite; consists of prehnite, chlorite, pyroxene, idocrase, 

h ydrogamet. Spatial relationships of the samples an~ shown in figure 5. 

were related to the incipient serpentinization. Increases in water 

content and density towa.rd the altered zone suggest a high activity of 

water combined with fajrly substantial total pressures. 

A plot of the serial chemical changes illustrates the probable movements of certain major components (fig. 6). The alkalis are noticeably 

22 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

depleted relative to the slightly altered hornblende gabbro (No. 1) in 

the most intensely altered zone, and they, show more than a tenfold 

difference between the altered contact and the center of the dike. The 

lack of newly formed alkali-bearing min~rals within the dike or 

peridotite leaves open the question of the direction of movement for 

the alkalis. Alumina is also depleted in the reaction zone (No.3) and 

may follow the alkalis, but the abundance of new calcium-aluminum 

silicate indicates that alumina is partly fixed in the metasomatic zone. 

The enriched calcium in the altered zone is interpreted as being derived 

from the serpentinized peridotite. Silica shows only moderate depletion during alteration and magnesium is erratic. 

INGALLS PEAK, WASH. (loc. 11) 

Smith (1904) described a large arcuate ultramafic mass surrounding Mount Stuart in the central part of the Cascade Mountains in 

Washington. Although the ultramafic intrusion is of uncertain age, it 

appears to invade upper Paleozoic metamorphic rocks, and a basal 

conglomerate consisting of ultramafic debris is present in the Swauk 

Formation of Late Cretaceous and Paleocene age resting unconformably on the ultramafics. Large areas of the ultramafic intrusion 

are partially serpentinized and contain porphyritic granodiorite dikes 

jn the vicinity of Ingalls Peak. Some of these dik&S, particularly in the 

highly serpentinized areas, have well-developed reaction zones at their 

contacts. 

One of these dikes (fig. 7) was sampled to determine the nature of 

the alteration. This particular dike is approximately 5 feet in width 

and nearly vertical, forming a prominent exposure along a ridge crest. 

The contact between the dike and serpentinite is fairly sh~rp but is 

sheared. The partially a.ltered granodiorite (table 5, No. 2) from ti1e 

center of the dike has retained its porphyritic igneous texture. The 

feldspar is albite, and in some areas it is partially replaced by hydrogarnet and analcite. Secondary zoisite forms irregular patches throughout the rock. The small amount of pale-green amphibole present may 

be a primary igneous ferromagnesian mineral. For comparison of bulk 

composition, an ana.lysis from Smith (1904) of an unaltered dike from 

this area is given (table 5, No. 1) . At the contact of the altered dike, 

an intense reaction zone approximately 6 inches wide has developed 

(table 5, No. 3). The intensely altered rock has retained none of its 

original igneous textures and consists of a new mineral assemblage : 

hydrogarnet, albite, and analcite and minor amounts of prehnite, 

diopside, and tremolite. 
PETROLOGY OF REACTION ZONES 

5 FEET 

FIGURE 7.-Field sketch (vertical section) of locality at Ingalls 

Peak, Wash., showing locations of analyzed specimens and 

general relationships of dike and serpentinite. Explanation is 

the same as shown in figure 2. Numbers are the same as those 

given in figure 8 and table 5. 

23 

A plot of selected elements from the bulk analyses of the Ingalls 

Peak material illustrates a similarity to the trends recorded for the 

same elements in the Myrtle Creek and Baldy Mountain, Oreg., reaction zones (fig. 8, table 5). Calcium enrichment in the reaction zone of 

the granodiorite has resulted in a mineral assemblage dominated by 

calc-silicates. The concentration of silica, as well as that of potassium, 

decreases toward the serpentinite. The concentration of sodium increases toward the center of the dike but becomes depleted toward the 

contact. The aluminum remains nearly constant from the center of the 

dike to the reaction zone, but magnesium shows a slight depletion in 

the reaction zone. Although the reaction zone does not have consistently 

higher water content than the granodiorite, its specific gravity is 

higher than that of the granodiorite or the serpentinite. 

TWIN SISTERS, WASH. (loc. 12, fig. 1) 

The Twin Sisters ultramafic mass has only a narrow rim of serpentinite surrounding its nearly unaltered dunitic core (Ragan, 1963). 

Nonetheless, along the southern boundary, where the Nooksack River 
24 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

TABLE 5.-Chemical and spectrographic analyses of samples from Ingalls Peak and 

Twin Sisters area, Washington (Zoe. 11, 12) 

[Rapid rock analysis performed by methods similar to those described by Shapiro and Brannock (1956); 

analysts: PaulL. D. Elmore, S.D. Botts, Gillison Chloe. Quantitative spectrographic analysis by Robert 

Mays. Chlorine analysis by Joseph Budinsky and Irving May] 

INGALLS PEAK TWIN SISTERS 

Altered Reaction Serpentin- Altered 

Granodio- granodio- zone 48- ite 48- gabbro 53-

rite rite 48- RD-61-B RD-61-C RD-61-D 

RD-61-A 

1 2 3 4 1 

Chemical analyses (weight percent) 

Si02-------------- 63.78 57.6 48.8 40.9 49.6 

AhOa. ----------- 16.39 18.0 17.8 2.8 15.9 

Fe,Oat. ---------- 4.19 4.8 3.2 7.4 9.6 

~;3_---~========== 3.27 3.3 2.0 37.8 6.3 

4.07 4.4 22.5 2.1 11.2 

Na20 _ ----------- 3.84 8.8 3.2 .08 4.3 K,o ______________ 2.03 .40 .04 .01 .31 

HsO+------------ 1.82 2.3 1.6 8.0 .27 HsO-____________ .22 .21 .25 .40 .15 

TiOs------------- .44 . 51 .35 .07 .64 

MnO. ----------- .05 .07 .05 .14 .16 

TotaL _____ 100.10 100.32 99.79 99.70 98.43 

Specific Gravity __ ------------ 2. 70 3.13 2.86 3.06 

Spectrographic analyses, minor elements (parts per million) 

Co _______________ ------------

NL _ ------------- ------------ Or __ ------------- ------------ Sc ________________ ------------

Zr ________________ ------------

Sr ... ~------------ ------------ Ba _______________ ------------ B ________________ ------------

CL.-------------- ------------

1 Total Fe given as Fe20a. 

12 

34 

34 

18 

110 

500 

2400 

30 

100 

8 

18 

12 

12 

70 

120 

65 

200 

50 

INGALLS PEAK 

85 

2000 

3800 

13 

<10 

<4 

<4 

130 

130 

26 

60 

140 

44 

50 

500 

1400 

140 

200 

Reaction 

zone 53-

RD-61-D 

2 

38.6 

14.3 

9.1 

5.4 

28.8 

.09 

.01 

2.8 

.12 

.60 

.13 

99.95 

3.42 

28 

50 

120 

48 

60 

65 

<4 

130 

70 

Serpentinite sa--

RD-61-F 

3 

38.6 

1.1 

9.6 

34.9 

3.6 

.04 

.02 

11.4 

1.0 

.05 

.16 

100.11 

2. 73 

110 

800 

1000 

17 

<10 4 

<4 

90 

460 

I. Unaltered porphyritic-granodiorite west of Mount Stuart area near Ingalls Peak, Smith (1904). Analyst: H. N. Stokes. Other elements given were P20s 0.11, SrO trace, BaO 0.08, LiO trace, S trace, Fe2031.12, 

FeO 2.76. 

2. Partially altered porphyritic-granodiorite from center of dike; consists of albite, amphibole, zoisite, hydro· 

garnet, analcite. 

3. Completely altered porphyritic-granodiorite from altered edge of dike; consists of hydrogarnet. albite. 

analcite. prehnite, diopside. tremolite. 

4. Serpentinite 172 ft from dike; consists of serpentine, brucite, olivine, orthopyroxene. 

TWIN SISTERS 

1. Partially altered gabbro from center of inclusion: consists of hornblende and blue-green actinolite, hydrogarnet. zoisite, albite, calcic plagioclase. 2. Completely altered gabbro from rim of inclusion; consists of hydrogamet, pyroxene, idocrase, prehnite. 

3. Serpentinite 5 ft from inclusion; consists of lizardite, clinochrysotile, brucite, pyroxene, magnetite. 

has exposed the enclosing serpentinite, numerous metasomatized tectonic inclusions are present within the serpentinite and as house-size 

boulders in the river where it cuts the ultramafic mass. Ragan (1963) 

refers to these inclusions as "exotic" blocks and suggests that they 

may be related basement rocks of pre-Carboniferous age. An altered 

mafic inclusion in place was selected for study; its diameter is approximately 8 feet (fig. 9). A sample from the center of the mass (table 5, 
1-

z 

UJ 

(.) 

a:: 

UJ 

10 

0.. 1.0 

1-

J: 

(!) 

w 

~ 

0.1 

PETROLOGY OF REACTION ZONES 

GRANODIORITE ALTERED REACTION SERPENTINITE 

GRANODIORITE ZONE 

o-. _____ '0 Si02 

----------o..__ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

~ 

\ K 2 0 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

~ 

3 

SAMPLE 

............. _--a 

\ 

\ 

\ 

\ Na 2 0 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

4 

FIGURE 8.-Semilogarithmic plot of the progressive 

chemical variation of samples from the Ingalls 

Peak area, Washington. Sample numbers are those 

given in table 5 and figure 7. The abscissa does 

not represent horizontal distance between samples. Points are placed equidistant to exhibit 

variations. 

25 
26 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

01 

Gabbro 

5 FEET 

FIGURE 9.-Field sketch (horizontal section) of locality at Twin 

Sisters, Wash., showing locations of analyzed specimens and 

general relationships between altered gabbro and serpentinite. 

Explanation is the same as shown in figure 2. Numbers are 

those used in table 5 and figure 10. 

No. 1) has a faintly recognizable ga;bbroic texture, but clear evidence 

of replacement is everywhere present in the ·mass. Original brown 

hornblende is retained, but its edges are being replaced by a feathery 

bluish-green actinolite. Original calcic plagioclase is. being replaced 

by albite, and the albite and relict calcic plagioclase areas, in turn, are 

invaded by hydrogarnet and zoisite. A peripheral white dense zone 

(table 5, no. 2) approximately 6-8 inches thick surrounds the mafic 

mass. In the reaction zone, nearly all the original igneous textures have 

been obscured by intense alteration. Hydrogar~et, diopside, idocrase, 

and prehnite are the predominant minerals. This serial alteration can 

be seen in the inclusion by the change in color from a dark gabbro in 

the center to the nearly white gabbro at the outer boundary. The serpentinite contact is sharp and consists of highly sheared material. The 

serpentinite consists of lizardite and clinochrysotile and minor amounts 

of brucite. Original relict primary pyroxene is still present, but the 

olivine is completely serpentinized. 

A semilogarithmic plot (fig. 10) shows that chemical changes in 

this sequence are nearly the same as those previously discussed. Smaller 

amounts of alkalis are in the outer zone than in the less altered core, 
PETROLOGY OF REACTION ZONES 

tz 

LJ.J 

u 

0:: 

w 

a.. 

100 

10 

t- 1.0 

I 

(.!) 

w 

~ 

0.1 

0....~-~~02 

~-o--------o 

2 

SAMPLE 

3 

FIGURE 10.-Semilogarithmic plot of the 

progressive chemical variation of sam-

.ples from the Twin Sisters area, Washington. Sample numbers are those given 

in table 5 and figure 9. The abscissa does 

not represent horizontal distance between samples; points are placed equidistant to exhibit chemical variations. 

27 
28 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

whereas in the central part, where some sodium enrichment may have 

taken place, albite is replacing the original calcic plagioclase. The reaction zone is strongly enriched in calcium; both magnesium and 

aluminum remain nearly the same, except for the abrupt change in 

the serpentinite. Silica shows a decrease of 11 weight percent from 

the center to the edge of altered gabbro. Again, the reaction zone contains more water than the parent gabbro and, yet, has a higher specific 

gravity than the rocks on either side. 

CAPE SAN MARTIN, CALIF. (loc. 13, :O.g, 1) 

A serpentinite body within a shear zone has reaetion zones with 

numerous rock types in contact with and as inclusions within it. The 

well-known semiprecious nephrite jade found along the beaches of 

this area has been formed along these same serpentinite contacts 

( Crippin, 1951). Where California Highway 1 cuts through a small 

ridge near the Cape San Martin beacon, a large mass of Franciscan 

graywacke is contained within the serpentinite. At the ·contact, a 2-footthick dense fine-grained reaction zone formed within the graywacke 

(fig. 11). This type of alteration is nearly identical with that described 

:Graywacke 

1 1 FOOT 1 

1 ~ 

40ft from 

contact 

FIGURE 11.-Field sketch (vertical section) of locality at Cape 

San Martin, Calif., showing locations of analyzed specimens 

and general relationships of the graywacke and serpentinite. 

Explanation is the same as shown in figure 2. Numbers are the 

same as those shown in figure 12 and table 6. 
PETROLOGY OF REACTION ZONES 29 

by Chesterman (1960) at Leech Lake Mountain, Calif. To determine 

the mineralogical and chemical changes related to this occurrence, a 

series of samples were taken from the contact 40 feet out into nearly 

unaltered graywacke (table 6, Nos.1-7). 

TABLE 6.-Partial chemical and spectrographic analyses of metasomatized graywacke 

from Cape San Martin, Calif. (loc. 13) 

[0, tested for but not found. Rapid rock analysis performed by methods similar to those described by Shapiro and Brannock (1956); analysts: Paul L. D. Elmore, S. D. Botts, Gillison Chloe, L. Artis, J. 

Glenn, D. Taylor; quantitative spectrographic analysis by Robert Mays) 

3-RJ-61-513-RD~1-413-RD~1-313-RD4_61-213-RJ-61-13-RJ-61-13-Rri-61- 1(3) 1(2) 1(1) 

2--------------

Oa. ----------- Oa_ ----------- ) ______________ 

o _____________ 

o_------------ o tO------------ ______________ 

O+ ------------ 0-------------

)2-------------

)s_- -----------

o_ ----------- 2--------------

Total. _____ 

Sp ecific gravity __ 

Co_--------------

NL __ ------------ Cr ___ ------------ Zr_. ______________ _ 

Sr __ -------------- Ba ____ -----------

Chemical analyses (weight percent) 

67.3 62.6 59.1 47.3 46.3 

13.9 15.2 14.1 16.8 14.8 

.89 .85 . 74 . 31 .25 

4.1 4. 5 4.6 3.9 4.5 

2.6 3.0 3.3 3.0 4. 7 

2.4 4.0 7.9 23.1 24.9 

4.1 4.6 4.3 .27 .05 

1.1 2.8 3.6 .61 .03 

2.1 1.3 1.2 2.9 3.1 

.15 .11 .16 .20 .22 . 72 . 70 . 76 . 75 .63 

.13 .10 .10 .11 .09 

.10 .08 .09 .09 .10 

.10 <.05 .05 .11 .28 

99.69 99.89 100.00 99.45 99.95 

2.66 2. 72 2. 78 2.94 3.02 

Spedrographic analyses, minor elements (parts per million) 

11 

40 

180 

130 

260 

220 

14 

30 

160 

120 

480 

1300 

12 

30 

210 

150 

700 

2300 

12 

30 

310 

100 

280 

300 

14 

20 

410 

90 

330o 

20 

46.7 

13.2 

.67 

4.3 

5.6 

25.3 

.09 

.00 

2.9 

.14 

.62 

.08 

.12 

.30 

100.02 

3.03 

11 

30 

190 

90 

180 

<20 

47.7 

10.8 

.58 

5.2 

11.6 

19.4 

.13 

.06 

3.2 

.18 

. 79 

.12 

.11 

<.05 

99.92 

3.03 

20 

210 

440 

10 

220 

20 

1. Slightly altered graywacke 40ft from contact. Veins of potassium feldspar only evidence of metasomatism. 

2. Partially altered graywacke""3 ft from contact. Amphibole, potassium feldspar, and chlorite metasomatic 

minerals. 

3. Metasomatized graywacke 1.5 ft from contact. Prehnite, amphibole, potassium feldspar, chlorite (relict 

quartz, feldspar, epidote). 

4. Metasomatized graywacke 1 ft from contact. Prehnite, diopside, calcite, potassium feldspar, chlorite. 

5. Metasomatized graywacke 6 in. from contact. Prehnite, diopside. 

6. Metasomatized graywacke 4 in. from contact. Prehnite, diopside, calcite, potassium feldspar. 

7. Metasomatized graywacke at contact. Amphibole, diopside, prehnite, pumpellyite. 

The nearly unaltered graywacke (table 6, No. 1) shows no evidence 

of widespread alteration and consists primarily of clastic grains of 

quartz, albite, and lithic fragments set in a pasty matrix consisting 

of chlorite and fine-grained detritus. Occasional detrital grains of 

epidote, apatite and zircon are present. Very small veins (less than 

one-fourth inch in diameter) consisting of quartz and potassium 

feldspar are the only indication of alteration; such potassium feldspar 

veins have not been found in graywac.kes unrelated to serpentinite 

contacts. The next sample, taken 3-5 feet :from the serpentinite contact 

(table 6, No. 2) is grayish white . and assumes a flinty appearance. 
30 ·REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

Even though the original clastic texture is clearly retained in this 

specimen, the thin section reveals finely fibrous actinolite, potassium 

feldspar, and chlorite replacing parts of the fine-grained groundmass 

and occurring as veins. Approximately 1.5 feet from the contact 

(table 6, No. 3) the original characteristics of the graywacke are 

gone, and at the contact the graywacke is a light-gray flinty rock. 

Staining of this rock for potassium feldspar by using the technique 

of Chayes (1952) revealed approximately 30 volume percent. 

Thin-section study shows a detrital structure, but much of the rock 

is replaced by prehnite, potassium feldspar, fibrous amphibole, and 

minor amounts of chlorite. At 1 foot from the contact (table 6, No. 4) 

the rock is dense and white and appears to be completely altered; 

it consists predominantly of prehnite and diopside with only minor 

amounts of potassium feldspar, chlorite, and carbonate (fig. 12). 

In thin section, the clastic texture is still faintly visible, but none 

of the original detrital grains remain. The final sample, taken from 

right at the contact out to about 6 inches from the contact was sawed 

into three equal parts (table 6, Nos. 5-7) to determine any small-scale 

contact variations. These three specimens consist predominantly of 

prehnite and varying amounts of diopside and actinolite; aragonite 

and calcite form irregular patches. Pumpellyite is restricted to certain 

small areas of this contact rock. Generally, the clastic textures are 

completely eradicated, but occasional patches retain the original texture. Irregular pods and lenses of a fibrous tremolite-actinolite occupy 

zones between the serpentine and calcium metasomatite; none of these 

were of gem quality, but their presence at this point clearly suggests 

that much of the nephrite in this area may have formed in a manner 

similar to that suggested earlier for certain New Zealand nephrites 

(Coleman, 1966). 

A plot of the chemical changes (fig. 13) shows the same trend as 

are shown in plots of the alteration of the mafic igneous rocks. The 

alkalis are definitely concentrated away from the reaction zone. Samples 2 and 3 may indicate the position of a potassium feldspar metasomatic front. The K20 and Na20 are undoubtedly derived from the 

breakdown of the preexisting detrital feldspars and mica, whereas 

the calcium must be from the ultramafic. The barium very clearly 

follows the potassium, and preliminary checks indicate that the 

barium is contained within the secondary potassium feldspar. Aluminum shows only minor variations in the altered graywacke and must 

not be mobile, whereas the silica gradient may indicate movement 

towards the ultramafic. The abundance of diopside in the contact 

rocks and increased magnesium in the reaction zone show that magnesium migrates from the serpentinite into the graywacke. Changes in 
PETROLOGY OF REACTION ZONES 31 

water content toward the contact are erratic, but specific gravity shows 

a remarkably consistent increase. 

SUMMARY OF PETROLOGY 

The foregoing discussion has detailed the metasomatism of rocks 

referred to as rodingites and of rocks whose original parentage 

ranges from graywacke to gabbro. By using chemical and textural 

evidence, the alteration and metasomatism of these rocks were shown 

to have occurred within or against serpentinites or partially serpentinized ultramafics. The alteration appears to have produced a volumefor-volume replacement, and no evidence was found to relate this 

FIGURE 12.-Cape San Martin reaction zone against sheared serpentinite. Hammer 

is resting on serpentinite with point toWtards thin nephrite zone. Light-colored 

and jointed rock to right of hammer is equivalent to Nos. 5-7 in figures 11 

and 13, and in table 6. Dense fine-grained rock on right margin of photograph 

is equivalent to No. 4. Photograph by Robert A. Loney. 
32 REACTION ZONES AND . ALPINE ULTRAMAFIC ROCKS 

fz 

w 

u 

c:: 

w 

10 

CL LO 

fI 

<.9 

w 

s: 

0.1 

ALTERED GRAYWACKE 

o-------o-------o-- Si02 

-----o-------o-------o 

..,../\\ 

f'/ 

I \. 

I \ I . I \ 

I ' 

I \ 

I '\ 

\ 

\ 3 4 5, 6, AND 7 AVERAGE 

SAMPLE SERPENTINITE 

FIGURE 13.-Semilogarithmic plot of the progressive chemical variation of samples from Cape San Mart:in, Oalif. Sample numbers are those given in table 6. 

The abscissa does not represent horizontal distance between samples; points 

are placed equidistant to exhibit chemical variations. 
PETROLOGY OF REACTION ZONES 33 

~Iteration to solutions resulting from igneous intrusion of the dike 

rocks or of the ultramafics. The fact that such metasomatism is always 

within or adjacent to serpentinized rocks strongly indicates that the 

alteration is a result of this proximity rather than being related to 

some cryptic or obscure igneous body or to the late stages of cooling 

in the ultramafic bodies themselves. 

The bulk chemistry, to be discussed further, reveals that this alteration process is metasomatic and is accomplished by solutions whose 

compositions are similar. Such solutions have repeatedly produced 

calcium metasomatism in the country rock of widely divergent ages 

and produced chlorite, amphibole, or diopside in the adjoining 

serpentinites. 

The mineral aHsemblages are nearly identical in rocks of similar bulk 

composition, an indication that such metasomatism must have occurred 

at analogous pressures and temperatures. The following section will 

discuss the mineral assemblages and chemical relationships. 

PETROGENESIS 

As shown in the previous sections, mineralogical determinations 

and chemical analyses indicate analogous metasomatic alteration 

within both tectonic inclusions and country rock contacts associated 

with serpentinite. The highly sheared serpentinites of the Pacific coast 

often contain rodingites and metasomatized tectonic inclusions and 

contacts that could have formed only through a very complex history. 

The presence of cataclastic structures along many of the metasomatized 

serpentinite contacts indicates that tectonic movement has accompanied or followed the metasomatism. This tectonic movement is considered to be responsible, in part, for allowing the egress of fluids and 

for replenishing the supply of reactants. Thus the metasomatism is 

dependent on structure, fluids, and rock composition, a relationship 

considerably more complex than that at typical igneous contacts. It 

is not surprising, therefore, to find an erratic distribution of such 

metasomatism associated with serpentinization. The tectonic instability of serpentinite bodies further complicates the history, as postemplacement movements may displace serpentinites upwards in the 

earth's crust. Because this upward migration takes places under differing pressure and temperature conditions, the serpentinite may 

leave behind the original contacts, incorporate additional tectonic 

inclusions, and form additional new metasomatic contacts. Thus, 

metasomatic rocks formed as a result of several distinct tectonic periods 

may be closely associated within highly sheared serpentinite. 

The metasomatic alteration of basalts, gabbros, and sediments, 

though involving only small areas of rocks, seems to be a process 
34 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

characteristic of serpentinization. The most common alteration product, rodingite-found mostly as inclusions or dikes within serpentinites-has been described from numerous ultramafic areas in the world, 

and the chemistry and mineralogy of these rocks are now fairly well 

known. In contrast to the widespread occurrence of rodingites, metasomatized sediments associated with ultramafics have only rarely been 

reported, perhaps beeause they have not been looked for (Lodochnikov, 

1933; Chesterman, 1960; Coleman, 1961 ; and Reed, 1959). 

The chemical changes during this metasomatism have a similar 

trend for all rock types, although the initial rock eomposition exerts 

a minor influence on the end product. Rocks initially containing considerable amounts of silica and alkalis, such as the sediments or silicic 

igneous rocks, may, after metasomatism, contain enriched areas of 

albite or potassium feldspar away from the reaction zone, in the 

cores of inclusions or less altered areas of country rock. Metasomatism 

in the reaction zone of both igneous and sedimentary rocks is characterized by enrichment in calcium and, in some rocks, magnesium, and 

by apparent movement of the alkalis away from the contact. Furthermore, the original unaltered rock always loses silica. The important 

point is that desilication and calcium metasomatism are universal in 

these altered rocks. 

Intense metasomatism in rocks next to the serpentinites produces 

a zone of hydrated calcium-aluminum silicate that has various amounts 

of magnesium. In the serpentinite, this same metasomatism produces 

calcium-magnesium hydrates with various amounts of aluminum. _ 

The rodingites analyzed in this paper and the minerals commonly 

found are plotted on an ACF diagram (fig. 14A). For comparison, 

a collection of rodingites and associated minerals from other parts 

of the world is plotted on a similar diagram (fig. 14B) . These two 

diagrams closely overlap, and even though three-phase tielines are 

not drawn, most of the rocks can be enclosed within triangles by 

using diopside and chlorite as two phases and either zoisite, prehnite, 

grossular, or idocrase as the third phase, depending on the total 

calcium content. It must be emphasized that equilibrium is rarely 

attained in any of these rocks, and as metasomatism increases, their 

composition changes in the manner illustrated in figure 15. The mafic 

rocks describe fairly straight descent toward the calcium corner of 

the diagram (fig. 15A). The more siliceous rocks demonstrate a looping line of descent towards the calcium corner of the diagram (fig. 

15B). The lower loop of the curve reflects the formation of alkali 

:feldspars in the intermediate stages of alteration. Although local 
PETROGENESIS 35 

equilibrium may be obtained during metasomatism, as long as reaction 

takes place, the trend of alteration is toward more calcic phases. 

Some insight can be had regarding the process of metasomatism if 

we consider mainly the calcium-bearing silicates. 

Mineral stability diagrams can be constructed to relate these various 

phases, assuming, of course, that the mineral compositions and the 

other parameters remain constant. For this discussion, local equilibrium is assumed and pressure-temperatures are considered constant. It 

is also assumed that the original mineral affected by metasomatism is 

anorthite, that the activity of water is high and constant, and that the 

activity of aluminum and magnesium are constant. With these qualifications, it can be shown that a progression from anorthite to prehnite 

or zoisite to grossular (hydrogrossular) can take place with increasing 

activity of calcium (fig. 16A). At lower activities of silica, zoisite may 

appear instead of prehnite, but an increase in silica activity at certain 

fixed values of calcium activity could invert zoisite to prehnite. Such 

concentration variations may explain the presence or absence of zoisite 

and prehnite in the early stages of metasomatism, as these two minerals 

are not usually found in coexistence. If the same reasoning is used but 

silica activity is fixed and alumina is mobile, a mineral stability 

diagram of these same minerals demonstrates further relationships 

(fig. 16B). An increase of calcium activity apparently allows a change 

directly from prehnite or zoisite to hydrogrossular. If the alumina 

activity is lowered, prehnite could alter to xonotlite when the calcium 

activity increases. This reaction must reflect the progression at Myrtle 

Creek, Oreg., where prehnite has been replaced by hydrogarnet or 

xonotlite. 

The metasomatic alteration of the serpentinite in these 'reaction zones 

is not fully demonstrated in this paper, but the progressive alteration 

of serpentinite shows the same dependence on the activities of silica 

and calcium. Again, if we make the same assumptions-the activity 

of water is high and constant and the activity of magnesium is constant-the dependence of calcium and magnesium minerals on the 

relative activities of calcium and silica can be diagramed (fig. 160}. 

The formation of chlorite from serpentine depends on an increase of 

both silica and alumina, but this is not shown on the diagrams. If the 

silica and calcium activities are increased, serpentine and chlorite will 

react to form tremolite. Where the calcium activity is high relative to 

silica, diopside will form instead of tremolite; where silica activity 

exceeds that of calcium, talc forms from chlorite and serpentine. 

The mineral assemblages present within the various reaction zones 

can be related to changes in the concentrations of calcium, silica, and 
36 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

A 

A 

•Cypress Island, table I 

lng~~:~ak, •Sultan Basin, table I 

: •:~~ld~s~~~~·t~a:,~e 45 

•Illinois River, table I 

•Salmon Creek. table I 

• Morro Creek, table I 

•Chetco Pass. table 1 

CL-------------------------e---------~~----------~F 

Pee, Xon and. CC Di Tm 

A 

B 

C~Pe-c-.X~o-n_a_n_d_C_C ____________________ eD~i----------~T~m--------------~F 

FIGURE 14.-ACF diagrams showing the common minerals associated with rodingites. A, Rodingi,tes of Oregon, Washington, and 

California. B, Rodingites described in the literature. Zo, zoisite; 

Pre, prehnite; Gro, gros8ularite or hydrogarnet; Pee, pectolite; 

Xon, xonotlite; CC, calcite or aragonite; I do, idocrase; Di, diopside; 

Tm, tremolite-actinolite; Chi, chlorite. 
PETROGENESIS 37 

alumina as shown above. These chemical relationships are consistent 

with the observed mineral assemblages and reinforce the concept that 

the reaction zones result from metasomatic interchange of elements 

between serpentine and country rock. 

KEY TO MINERAL A:SSEIMBLAGES AND LOCATION'S IN FIGURE 14B 

1. Hydrogarnet-prehnite-zoisite rodingite; Girvan-Ballantrae complex, Ayrshire, Scotland (Bloxam, 1954). 2. Diopside (diallage)-hydrogarnet-vesuvianite rodingite; Eulaminna, Mount Margaret goldfield, Western Australia (Miles, 1950). 3. Pyroxene-hydrogarnet-vesuvianite rodingite; Eulaminna, Mount Margaret gold field, 

Western Australia (Miles, 1950). 4. Hydrogarnet-prehnite-xonotlite-diopside rodingite; Hindubagh, Pakistan (Bilgrami 

and Howie, 1960). 5. Hydrogarnet-prehnite-xonotlite-diopside rodingite; Hindubagh, Pakistan (Bilgrami and 

Howie, 1960). 6. Pectolite-hydrogarnet-thomsonite rodingite; Clear Creek, San Benito County, Calif. 

(Coleman, 1961). 7. Diopside (diallage)-hydrogarnet rodingite; Dun Mountain, New Zealand (Bell, Clarke, 

and Marshall, 1911). 8. Prehnite rodingite; Valley of the !Serpentine, Dun Mountain, New Zealand (Bell, Clarke, 

and Marshall, 1911). 9. Prehnite rodingite; Long Gully, Dun Mountain, New Zealand (Bell, Clarke, and 

Marshall, 1911). 

10. Diopside (diallage)-hydrogarnet rodingite; Champion Mine, Dun Mountain, New Zealand (Bell, Clarke, and Marshall, 1911). 11. Diopside (diallage)-hydrogarnet rodingite; Code's Point, Dun Mountain, New Zealand 

(Bell, Clarke, and Marshall, 1911). 

12. Dio·pside (diallage)-hydrogarnet rodingite; Lee Valley, Dun Mountain, New Zealand 

(Bell, Clarke, and Marshall, 1911). 

13. Di()pside (diallage)-hydrogarnet rooingite; Lee Valley, Dun Mountain, New Zealand 

(Bell, Clarke, and Marshall, 1911). 14. Diopside-hydrogarnet rodingite; Motueka River, Gordon Survey District, •New Zealand 

(Grange, 1927). 

15. Diopside-hydrogarnet rodingite; Motueka River, Gordon Survey District, New Zealand 

(Grange, 1927). 

16. Idocrase-hydrogarnet-prehnite-zoisite rooingite; Anderson's Creek, Beaconsfield, County Devon, Northern Tasmania (Baker, 1959). 

17. Hydrogarnet-diopside-idocrase rodingite; Ishikari Province, Hokkaido, Japan (Suzuki, 

1953). 

18. Hydrogarnet-diopside-idocrase rodingite; Ishikari Province, Hokkaido, Japan (Suzuki, 

1963). 

19. Prehnite-diopside-hydrogarnet rodingite; Cap Bassit, Northwest Syria (Majer, 1960). 

20. Brown hornblende-prehnite rodingite; Hindubagh, Pakistan (Bilgrami and Howie, 

1960). 

21. Zoisite, brown hornblende, chlorite, sphene, center of ophispherite; Col'des Gets (Chablais, Haute Savoie), France (Jaffe, 1955). 22. Clinozoisite, prehnite, albite, chlorite, clinopyroxene, hornblende, central part of altered 

gabbro; Hayachime ultramafic complex, Kitakami Mountains, northern Japan ( Onuki, 1963). 

23. Idocrase, chlorite, diopside, so-called vesuvianvels ; Piz Lunghin, Maloja, Switzerland 

(Muller, 1963). 

24. Diopside, grossular, rodingite; Piz Lunghin, Maloja, Switzerland (Muller, 1963). 

25. Rodingite composition calculated from analyses of diallage and hydrogrossular (Bell, Clarke, and Marshall, 1911, p. 32). 

26. Dense rooingite from near Trigonometrical Station, R. K. Lee Valley, Dun Mountain, 

New Zealand (Bell, Clarke, and Marshall, 1911, p. 33). 

27. Fine-grained rodingite, Code's Point, Dun Mountain Tramway, Dun Mountain, New 

Zealand (Bell, Clarke, and Marshall, 1911). 28. Fine-grained rooingite, Champion Level, Champion Mine, Dun Mountain, New Zealand 

(Bell, Clarke, and Marshall, 1911, p. 33). 

29. Fine-grained rodingite; near Dun Mountain, New Zealand (Bell, Clarke, and Marshall, 

1911, p. 33). 

30. Fine-grained rodingite, Long Gully-Lee Valley, Dun Mountain, New Zealand (Bell, Clarke, and Marshall, 1911, p. 33). 

31. Dense white grossularite-diop·side rock; half a mile south-southeast of cairn at 2602 ft. ; 1lh miles south of Mount Glennie, Motueka River Valley, Gordon Survey District, 

Red Hills, New Zealand (Grange, 1927, p. 165). 

32. Dense white grossularite-diopside rock ; near head of Motueka River, Gordon Survey District, Red Hills, New Zealand (Grange, 1927 p. 165). 

33. Gabbro wi-th prehnite-zoisite aggregate, Motueka Valley, 114 miles south-southeast of 

Mount Glennie, Gordon Survey District, Red Hills, New Zealand (Grange 1927, 

p. 165). 

34. Hydrogrossular; Champion Creek, near old copper mine, Waimea Survey District, Dun 

Mountain area, New Zealand (Hutton, 1943, p. 174). 

35. Hydrogrossular; Roding River, New Zealand (Bell, Clarke, and Marshall, 1911, p. 32). 

3•6. Hydro.gross_!l~ar; Roding Riv;er, New Zealand (Bell, Clarke, and Marshall. 1911, p. 32). 

37. Prehmte-zo1s1te aggregate m altered gabbro; southwest branch of Wairoa River. 

Gordon Survey District, Red Hills, New Zealand (Grange 1927, p. 165). · 
38 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

A 

Zo 

Pre 

Gro 

I do 0 

Table 4 

A 

c~-------------------------e----------~--------------~F Pee, Xon and CC Tm 

B 

Gro 

0 1do 

0~ 

Table 3, No 3 

Di 

A 

Table 3, No 2 

0 

Table 3, No 1 

0 

Table 6, No 1 

~~/x --~ <;,'<>(\ YX Table 6, No 2 

c~-

Table 6, No 3 

Chi 

c~--------------------------B-----------~--------------~F Pee, Xon and CC Di Tm 

FIGURE 15.--Progressive changes in composition within reaction 

zones plotted on ACF diagrams. A., Gabbroic rocks reacting to 

form rodingites. B, High siliC'a rocks (graywacke, San Martin 

and dacite, Myrtle Creek) reacting to form calcium-rich metasomatic borders. Zo, zoisite; Pre, prehnite; Gro, grossularite or 

hydrogarnet ; Pee, pectolite; Xon, xonotlite; CC, calcite or aragonite; Ida, idocrase; Di, diopside; Tm, tremolite-actinolite; Chi, 

chlorite. 
N 

+ro 

8 

tlO 

..Q 

PETROGENESIS 

Si02 , H 2 0 constant Al 20 3 , H2 0 constant 

Hydrogarnet 

(idocrase) 

N~ 

+ro 

8 

tlO 

..Q 

Anorthite 

log [AI203 ] log [Si02] 

A B 

MgO, H 20 constant 

Diopside 

Tremolite 

Serpentine 

Talc 

log [Si02] 

c 

FIGURE 16.-Activity diagrams illustrating possible fields of stability for 

minerals present in reaction zones. 

A, Ca (activity)-Al20 3 (activity) for hydrated calcium-aluminum silicates. 

B, Ca (activity)-Si02 (activity) for hydrated calcium-aluminum silicates. 

C, Ca (activity)-Si02 (activity) for calcium and magnesium silicates. 

All compositions considered fixed with pressure and temperature 

constant but not specified. 

39 

The metasomatized mafic igneous rocks (rodingites) have very 

consistent mineral assemblages as listed below: 

1. Hydrogarnet and chlorite. 

2. Hydrogarnet, prehnite, and chlorite. 

3. Hydrogarnet, diopside, and idocrase. 

4. Hydrogarnet, idocrase, and chlorite. 
40 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

Hydrogarnet is the dominant mineral in the rodingites. Assuming 

that hydrogarnet is related. to the breakdown of the original calcic 

plagioclase and does not result from the breakdown of ferromagnesians, 

which alter to chlorite and actinolite, a series of reactions can be 

written that may represent the chemical nature of the metasomatism. 

3(CaO·Al203·2Si02) +Ca+2+2H20 ~ 4Ca0·3Al203·6Si02·H20+2H+t 

anorthite zoisite 

4Ca0·3Al203·6Si02· H20 + 5Ca +2 + 13H 0~ 

zoisite 

3(CaO·Al20 3·1.5Si02·3H20) + 1.5Si02+ IOH+1 

hydrogarnet 

These reactions illustrate the nature of the metasomatism as being 

driven to completion by an increase in the activity of calcium and 

water. Other reactions can be written to include diopside, prehnite, 

and idocrase, but in general the same relations hold. 

In the more siliceous altered sediments and igneous rocks from 

reaction zones, five different mineral assemblages develop in the most 

intensely altered zones, but in general there is an overall resemblance 

to the rodingites. 

1. Albite, potassium feldspar, and tremolite-actinolite. 

2. Tremolite-actinolite. 

3. Diopside, tremolite-actinolite, prehnite, and chlorite. 

4. Tremolite-actinolite, hydrogarnet, and diopside. 

5. Hydrogarnet, prehnite, and xonotlite. 

Again, using the calcium silicates common in these rocks, a series 

of reactions can be written to illustrate the trend of the metasomatism. 

Chlorite and actinolite-tremolite are assumed to not have any effect 

on these hypothetical reactions. 

1.5(CaO·Al203·2Si02) +0.5Ca+++ 1.5H2o~ 

anorthite 

2CaO·Al20 3·3Si02.H20 + H+ + 0.5Al20a. 

prehnite 

2CaO·Al20a·3Si02.H20 +4Ca ++ + Al20 3 + 6H20~ 

prehnite 

2(3Ca0·Al:.03·1.5Si02·3H20) +8H+ 

hydrogarnet 

2CaO·Al20a·3Si02.H20+Ca+++3H2o~ 

prehnite 

3CaO·Al203·1.5Si02·3H20+4H+ ~ 

hydrogarnet 

3(CaO.Si02·H20)+Al'.JOa+2H+ 

xonotlite 

0.5(CaO.Si02·H20) +Al20a+ 14H20 

xonotlite 
PETROGENESIS 41 

The nature of these reactions indicates that in addition to caloium 

and water, aluminum is important in certain situations. Previous bulk 

analyses have shown that in certain places aluminum has been depleted 

in the reaction zones and, therefore, should be considered as a mobile 

element. All these hypothetical reactions are oversimplified because 

it is difficult to encompass all the phases related to a reaction zone 

undergoing progressive metasomatism. 

Calcium can be released from pyroxene during serpentinization. 

The average CaO content for 23 peridotites is 3.5 weight percent, and 

for 19 dunites, 0.75 weight percent (Poldervaart, 1955). The average 

CaO content determined from 26 analyzed serpentines (Faust and 

Fahey, 1962) is 0.08 weight percent. Even though these 26 samples 

are nearly monomineralic serpentinites, they are close enough to 

whole-rock serpentinites to illustrate the decrease of calcium. The 

serpentinites ~in table 4 are from reaction zones, and their high 

calcium content indicates metasomatism. The difference in calcium 

content between the dunite-peridotites and the serpentinites is considerable and shows that calcium is released during serpentinization. 

Much of the calcium may combine with C02 to form carbonate, but 

some could migrate to mafic dikes, tectonic inclusions, and contact 

rocks to raise the activity of calcium in the reaction zones. This 

concept of release of calcium during serpentinization has been expressed by Hess and Otalora (1964), who observed that hydrogarnet 

formed in veins within serpentine. Silica is needed to produce serpentine according to the reaction. 

3Mg2Si04 + 4H20 + Si02 ~ 2H4Mg3Si209 

olivine serpentine 

if it is assumed that brucite is not forming. The silica activity will 

usually decrease toward the serpentinite bodies, and movement of 

silicia would be in the same direction. In certain localities the silicia 

content of the metasomatic rocks is less than that of the enclosing 

ultramafics, but in all localities the parents of the metasomatites had 

iRitial silica contents greater than the enclosing ultramafics. 

Phillips and Hess ( 1936) described similar metasomatic alterations 

( 1936) of siliceous country rocks in contact with serpentinite in the 

Appalachian belt. They showed that the country rock was depleted 

in silica toward the serpentinite and that the serpentinite was depleted 

in magnesium toward the country rock. Monomineralic bands of 

biotite, talc, and tremolite form at these contacts. Read ( 1934) described small zoned ultramafic bodies within the metamorphic rocks 

of Unst, Shetland Islands. Here the migration of silica was toward 

the serpentinite, and magnesium migrated outward to the enclosing 
42 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

country rock. The ensuing sequence from the center of the ultramafic 

bodies outward is as follows: 

antigorite~talc~actinolite~chlorite~ biotite. 

Coleman (1961) described a distinct zoning for those tectonic inclusions containing jadeite and enclosed within the serpentinite mass 

of New Idria, Calif. Francis ( 1955, 1956) found zoned bodies at Glen 

Urquhart, Scotland, that have reacted with the serpentinites and are 

depleted in silica and enriched in magnesium and calcium. Comparison 

of the mineral zonation formed at these widely separa.ted occurrences 

and under different pressure-temperature conditions reveals strong 

similarities. Juxtaposition of serpentinite or ultramafic with other 

rocks of higher silica content in a pressure-temperature environment 

that would allow reactions to take place produces a distinctive suite 

of contact metasomatic minerals. These metasomatic minerals occur 

in distinct zones or produce monomineralic or bimineralic rocks. The 

suite of minerals formed depends on three factors: composition of 

rock in contact with the serpentinite; tectonic setting; and pressuretemperature conditions during reactions. Further work is needed before a classification can be established for reaction zones characterizing 

different environments, but the mineral assemblages probably could 

be used to establish the pressure-temperature conditions of formation. 

PRESSURE AND TEMPERATURE CONDITIONS OF 

METASOMATISM 

The metasomatic rocks are in direct contact with or completely 

surrounded by serpentinite, but direct contact of the metasomatites 

with unaltered dunite or peridotite was not observed by the author 

on the Pacific coast of the United States or in New Zealand, nor has 

it been reported in the literature. Thus, it is safe to predict that these 

metasomatic reactions must take place at temperatures below the 

breakdown of olivine and (or) pyroxene to serpentine. Bowen and 

Tuttle (1949) have said: "Serpentine cannot be present in any layer 

of the earth's crust whose temperature is normally above the decomposition temperature of serpentine under the pressure prevailing." 

The reaction 

serpentine~ forsterite +talc+ vapour 

fixes the serpentine breakdown at approximately 500°C at 1 kilobar; 

at higher total pressures, the breakdown temperature would increase 

only a few degrees. Additional work is needed to determine the stability of the various serpentine polymorphs, as Bowen and Tuttle were 

able to produce only one serpentine polymorph, chrysotile. The field 

relations of antigorite indicate that it may be more stable in a higher 
PRESSURE AND TEMPERATURE CONDITIONS OF METASOMATISM 43 

pressure-temperature field than chrysotile. Recently Hostetler and 

others (1966) showed that brucite is commonly present in olivine-rich 

ultramafics and pointed out that its presence indicates that serpentinization may have taken place at temperatures 100°C lower than 

the generally accepted 500°C. 

A nearly direct estimate of temperature can be obtained by consideration of the hydration state of the hydrogarnet in these metasomatic rocks. Yoder ( 1950) and Carlson ( 1956) showed that the 

silica and water content of hydrogarnet is related to its temperature of 

formation. The silica content of hydrogarnet increases with temperature up to 800°C, at which temperature grossularite is stable, whereas 

fully hydrated hydrogrossular is stable at 220°-226°C. These reactions 

are not pressure dependent; thus, if the hydration state of a hydrogarnet can be determined, its temperature of formation can be estimated. Hydrogarnets associated with jadeite and rodingite in the 

New Idria, Calif., serpentinite were found to indicate a possible 

temperature range from 240° to 360°C (Coleman, 1961). Later work 

by the author on hydrogarnets from New Zealand shows a temperature range of formation between 290° and 450°C. Caution is necessary 

in deducing temperature from relative hydration states as Zabinski 

( 1964) has shown that idocrase is often present as cryptic intergrowth 

within hydrogarnet. Further mineralogical work on the Pacific coast 

hydrogarnets is needed before temperature estimates can be established for these, but on the basis of the New Idria and New Zealand 

estimates, we can probably say that the Pacific coast hydrogarnets 

formed within the temperature range of 240° to 490°C. 

A third independent check on temperature of formation is afforded 

by the presence of xonotlite in the metasomatized rocks of the Pacific 

coast. The experimental work by Buckner, Roy, and Roy (1960) and 

by Flint, McMurdie, and Wells (1938) on the stability of xonotlite 

and wollastonite under hydrothermal conditions has shown that at 

400°C and atmospheric pressure wollastonite will form from xonotlite 

by dehydration, or from a gel of appropriate composition, and that 

at elevated pressures 30,000 psi wollastonite will form at 430°C. The 

presence of xonotlite suggests, thus, that in some area·s of metasomastism, the temperatures are probably within the same range indicated by the hydrogarnets, and both temperature estimates indicate 

that the metasomatism takes place in the temperature range where 

serpentine is stable. 

Estimates of pressure during metasomatism are difficult to make 

and are necessarily approximate. In California, however, there is 

evidence that much of the calcium metasomatism develops contemporaneously with the glaucophane schist facies metamorphism (Chesterman, 1960; Coleman, 1961). 
44 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

The presence of aragonite and jadeite in the New Idria reaction 

zones indicates that at temperature around 400°C the pressure would 

have to be about 9 kilobars. Of course, where these high-pressure 

minerals are not present, adequate estimates are difficult to make. 

Further temperature evidence can be obtained from the recent experin1ental work by Bell and England ( 1964) on the aragonite-calcite 

inversion. They have shown that aragonite is not quenchable at high 

temperatures, and if aragonite does form below 10 kilobars, the temperatures must be on the order of 200° to 300°C. The general increase 

of density of the rocks in the metasomatic reaction zones and the 

substantial water content indicate that the total pressure is high and 

that the water pressures are nearly equal to the total pressure. The 

pressure is estimated to be usually in excess of 4,000 bars. 

The tectonic evidence for incremental upward diapiric movement 

of ultramafics indicate an environment wherein metasomatic reactions 

took place at various depths, and the inferred pressure range is therefore only approximate. 

TECTONIC SETTING OF SERPENTINITE EMPLACEMENT 

AND ASSOCIATED METASOMATISM 

There is very little convincing evidence that ultramafics in the Pacific coast region were emplaced as high-temperature igneous intrusions 

in the earth's crust. In many localities, large areas of ultramafj.cs have 

no associated gabbros, and in these areas serpentinite is always in 

contact with the country rock rather than peridotite or dunite. 

The tectonic nature of the contacts and the. associated low-tempe.rature-high-pressure reaction zones are rather strong evidence suggesting that emplacement of some ultramafics coincides with major tectonic 

events. The close association of the Pacific coast serpentinite belts with 

thrust faulting and other evidences of intense tectonism demonstrate 

that the ultramafics were emplaced in the upper crust as solid intrusions rather than as magmas (Bailey and others, 1964). The mechanical instability of sheared serpentinites is so great that repeated tectonic 

events following the initial emplacement promote their upward migration within the earth's crust. This still does not preclude the possibility 

that the peridotites and dunites crystallized from a magma within the 

mantle before being transported upward. 

The concept of a solid intrusion of ultramafics is not new; Benson 

( 1926) postulated this mechanism for the New Zealand ultramafics. 

More recently, Hess ( 1955) summarized various problems concerning 

serpentinites and island arcs and demonstrated that the latter may 

represent an early development of an alpine-type mountain system. 

Furthermore., he showed that the distribution of alpine-type ultramaf-
TECTO~C SETT~G 45 

ics throughout the world follows an a.rcuate pattern and that, in the 

early stages, these rocks occupy continental margins. Thus, the favorable areas of ultramafic emplacement were along the continental margins, where the crust is thin and theM-discontinuity is only 10-20 kilometers below the ocean bottom. 

If parts of the original peridotite mantle are contained within the 

ultramafics, the igneous formation of the ultramafics could have been 

much earlier than the tectonic event that emplaced them into the earth's 

crust. Textures of the primary ultramafics are igneous but are often 

overprinted by a deformational fabric. The low incidence of hightemperature contact metamorphism associated with many ultramafics 

in the crust is compelling evidence against a magmatic emplacement in 

their present setting. The universal serpentinization-partial along 

borders of large ultramafic masses or complete in the thinner massescharacterizing the tectonic belts demonstrates the chemical instability 

of dunite-peridotites within the crust. Bowen and Tuttle (1949) have 

given experimental evidence that precludes an ultramafic melt having 

a water content high enough to allow autometamorphism of the crystallized inelt to serpentine. 

Assuming that the Pacific coast ultramafics may have formed in the 

mantle as peridotite and later been transported upward along a major 

circum-Pacific fault, such peridotites would first intrude geosynclinal 

sediments at considerable depths (10-20 km) in an environment of 

high pressure and relatively low temperature (approximately 300°-

4000C), depending on heat flow, composition of the sediments. and 

state of equilibrium between the sediments and peridotites. The invasion of water into the peridotite would cause serpentinization and 

resultant expansion. A local increase in pressure could be produced 

by expansion, assuming that the rocks had enough strength to sustain 

the local increase in pressure (Coleman and Lee, 1962). The alteration 

of peridotite to serpentinite was shown to be exothermic by Bennington (1956); thus, temperatures would rise in the area of serpentinization. The preserved contacts of serpentinite and metasomatized sedimentary rocks as described earlier show that metasomatism must take 

place after the major faulting of peridotite from the mantle up into 

the base of thick geosynclinal accumulations. Continued upward faulting would bring more water into contact with the ultramafic, and 

movement of the incompetent serpentinized parts would allow slabs 

of. country rock to become included within the serpentinite. During 

the dynamic emplacement, contact metasomatism would result because 

of the elevated temperatures ( 400°-500°C) and chemical activity of 

water, silica, and calcium at the contact. 
46 REACTION ZONES AND ALPINE ULTRAMAFIC ROCKS 

REFERENCES CITED 

BaHey, E. H., Irwin, W. P., and Jones, D. L., 1964, Franciscan and related rocks, 

and their significance in the geology of western California : California 

Div. Mines and Geology Bull. 183, 177 p. 

Baker, George, 1959, Rod.ingite in nickeliferous serpentinite. near Beaconsfield, 

northern Tasmania: Geol. Soc. Australia Jour., v. 6, pt. 1, p. 21-35. 

Bell, J. M., Clarke, E. de C., and Marshall, P., 1911, The geology of the Dun 

Mountain subdivision, Nelson: New Zealand Geol. Survey Bull. 12, p. 

1-71. 

Bell, P.M., and England, J. L., 1964, High-pressure differential thermal analysis 

of a fast reaction with CaCOa: Carnegie lust. Washington Year Book 63, 

Ann. Rept. Geophys. Lab., p.176-179. 

Bennington, K. 0., 1956, Role of shearing stress and pressure in differentiation 

as illustrated by some mineral reactions in the system MgO-SiO:rH20: 

Jour. Geology, v. 64, no. 6, p. 558-577. 

Benson, W. N., 1913, 1915, 1917, 1918, The geology and petrology of the great 

serpentine belt 6f New South Wales: Linnean Soc. New South Wales, Proc. 

v. 38, p. 490-517, 569-596, 662-724; v. 40, p. 121-173, 540-624; v. 42, p. 223-

283,693-700; v. 43, p. 3.20--300,593-690. 

---1926, The tectonic conditions accompanying the intrusion of basic and 

ultrabasic igneous rocks: [U.S.] Natl. Acad. Sci. Mem., v. 19, 1st mem., 

90p. 

Bilgrami, S. A., and Howie, R. A., 1960, The mineralogy and petrology of a 

rodingite dike, Hindubagh, Pakiston: Am. Mineralogist, v. 45, nos. 7-8, p. 

791-801. 

Bleek, A. W. G., 1907, Die Jadeitlagerstiitten in Upper Burma: Zeitschr. f. prakt. 

Geologie, v. 15, p. 341-365. 

Bloxam, T. W., 1954, Rodingite from the Girvan-Ballantrae complex, Ayrshire: 

Mineralog. Mag., v. 30, no. 227, p. 5zs.--.528. 

Bowen, N. L., and Tuttle, 0. F., 1949, The system MgO-SiO:rH20 : Geol. Soc. 

America Bull., v. 60, no. 3, p. 439-460. 

Brown, R. D., Jr., 1964, Thrust-fault relations in the northern Coast Ranges, 

California, in Geological Survey research 1963: U.S. Geol. Survey Prof. 

Paper 475-D, p. D7-D13. 

Buckner, D. A., Roy, D. N., and Roy, Rustum, 1960, Studies in the system 

CaO-AhOa-Si02-H20 [pt. 2], the system CaSiOa-H20: Am. Jour. Sci., v. 258, 

no. 2, p. 132-147. 

Carlson, E. T., 1956, Hydroga,rnet formation in the system lime-alumina-silicawater: U.S. Natl. Bur. Standards Jour. Research, v. 56, p. 327-335. 

Oater, F. W., Jr., and ·wens, F. G., 1953, Geology and mineral resources of the 

Gasquet quadrangle, Oali.fornia-Oregon: U.S. Geol. Survey Bull. 99·5-C, 

p. 79-133. 

Cha1lis, G. A., 1965, The origin of New Zealand ultramafic intrusions: Jour. 

Petrology [London], v. 6, no. 2, p. 322-364. 

Chayes, Felix, 1952, Notes on 1the -staining of potash feldspar with sodium 

cobaltinitrite in thin section: Am. Mineralogist, v. 37, nos. 3-4, p. 337-340. 

Chesterman, 0. W., 1960, Intrusive ultrabasic rocks and their metamorphic 

relationships at Leech Lake Mountain, Mendocino County, California: 

Internat. Geol. Cong., 21st, Copenhagen 1960, Proc., pt. 13, p. 208--215. 

Chidester, A. H., 1962, Petrology and geochemistry of selected talc-bearing 

ultramafic rocks and adjacent country rocks in north-central Vermont: 

U.S. Geol. Survey Prof. Paper 345, 207p. 
REFERENCES CITED 47 

CJ.iark, L. D., 1960, Foothills fauJ.it system, western Sierra NeV'ada, California: 

Geol. Soc. America Bull., v. 71, no. 4, p. 483-496. 

Coleman, R. G., 1961, Jadeite deposits of the Clear Creek area, New Idria di·strict, 

San Benito County, California: Jour. Petrology [London], v. 2, no. 2, p. 

209-247. 

--- 1966, New Zealand serpentini,tes and associated metasomatic rocks: New 

Zealand Geol. Survey Bull., n.s. 76, 102 p. 

Coleman, R. G., and Lee, D. E., 1962, Metamorphic aragonite in the glaucophane 

schists of Cazadero, California: Am. Jour. Sci., v. 260, no. 8, p. 577-595. 

Crippin, R. A., Jr., 1951, Nephrite jade and associated rocks of the Cape San 

M·artin region, Monterey County, California: California Div. Mines Spec. 

Rept.lO-A, 14 p. 

Diller, J. S., 1898, Description of the Roseburg quadrangle· [Oregon] : U.S. Geol. 

Survey Geol. .Atlas, FoUo 49. 

Ducloz, Ch., and Vuagnat, M., 1962, A propos de l'age des serpentinites de Cuba: 

Archives Sci. [Geneva], v. 15, fasc. 2, p. 309-332. 

Eckel, E. B., and Myers, W. B., 1946, Quicksilver deposits of the New Idria 

district, San Beni:to and Fresno Counties, Oalifornia: California Jour. 

Mines and Geology, v. 42, no. 2, p. 81-124. 

F1aust, G. T., and Fahey, J. J., 1962, The serpentine-group minerals: U.S. Geol. 

Survey Prof. Paper 384-A, 92 p. 

Flint, E. P., McMurdie, H. F., and Wells, L. S., 1938, Formation of hydrated 

calcium silicates ·at elevated tempe~atures and pressures: U.S. Natl. Bur. 

Standards Jour. Research, v. 21, p. 617-638. 

Francis, G. H., 1955, Zoned hydrothe~mal bodies in the serpentinite mass of Glen 

Urquhart (Inverness-shire) : Geol. Mag. [Great Britain], v. 92, no. 6, 

p. 433-447. 

--- 1956, The serpentini:te ma·ss in Glen Urquhart, Inverness-S'hi·re, Scotland: 

Ain. Jour. Sci., v. 254, no. 4, p. 201-226. 

Grange, L. I., 1927, On the "Rodingi•te" of Nelson: New Zealand Inst. Trans., 

v.58,no.l-2,p.l60-166. 

Green, D. H., 1964, The metamorphic aureole of the peridotite at the Lizard, 

Cornwall: Jour. Geology, v. 72, no. 5, p. 543--563. 

Hess, H. H., 1955, Se~entines, orogeny, and epeirogeny, in Poldervaart, A., ed., 

Crust of the ea:rth-a symposium : Geol. Soc. America Spec. Paper 62, 

p. 391-407. 

Hess, H. H., and Otalora, Guillermo, 1964, Mineralogical chemical composition of 

the Mayagiiez serpentini•te cores, in A study of serpentinite: Natl. Acad. Sci.-

Natl. Research Council Pub. 1188, p. 152-168. 

Hostetler, P. B., Coleman, R. G., Mumpton, F. A., and Evans, B. W., 1966, 

Brucite in alpine serpentinites: Am. Mineralogist, v. 51, p. 75-98. 

Hutton, C.O. 1943, Hydrogrossular, a new mineral of the garnet-hydTogarnet 

series: Royal Soc. New Zealand Trans., v. 73, p. 174-180. 

Irwin, W. P., 1964, Late Mesozoic orogenies in the ultramafic belts of northwestern California and southwestern Oregon, in Geological Survey research 

1964: U.S. Gf"ol. Survey Prof. Paper 501-C, p. Cl-C9. 

Iwao, Shuichi, 1953, Albitite and associated jadeite rock from Kotaki district, 

Japan; a study in ceramic raw material: Japan. Geol. Survey Rept. 153, 25 p. 

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

zone of the Stillwater complex. Montana: U.S. Geol. Survey Prof. Paper 358, 

106 p. 
48 REACTION ZONES AND ALPINE UL'fRAMAFIC ROCKS 

Jaffe, F. C., 1955, Les ophiolites et les roches connexes de la region du col des 

Gets: Schweizer. Mineralog. petrog. Mitt., v. 35, pt. 1, p. 1-150 [including 

English and Italian summaries]. 

Korzhinskii, A. F., 19·58, Skarn formations from the ultrabasic rock massive 

of Ospinsk in the eastern Sayan Range: Vseoyuz. Mineral. Obshch. Zapiski, 

v. 87, p. 327-347. 

LaCroix, Alfred, 1930, La jadeite de Birmanie--les roches qu'elle constitute ou 

qui l'accompagnent, composition et origin : Soc. fran~aise mineralogie Bull., 

v. fi3, p. 216-254. 

Leech, G. B., 1953, Geology and mineral deposits of the Shulaps Range, southeastern British Columbia : British Columbia Dept. Mines Bull., no. 32, 54 p. 

Lipman, P. W., 1964, Structure and origin of an ultramafic pluton in the Klamath 

Mountains, California : Am. Jour. Sci., v. 262, p. 199--222. 

Lodochnikov, \V. N., 1933, Serpentines, serpentinites and the petrological problems connected with them: Problems Soviet Geol., v. 2, no. 5, p. 119-150. 

MacKenzie, D. B., 1960, High-temperature alpine-type peridotite from Venezuela: 

Geol. Soc. America Bull., v. 71, no. 3, p. 303-318. 

Majer, Vladimir, 1960, Rodingite ·of Cape Bassit, Northwest Syria: Neues Jahrb. 

Mineral. Monatsh., v. 48, p. 85-88. 

Miles, K. R., 1950, Garnetized gabbros from the Eulaminna district, Mt. Margaret 

goldfield: Western Australia Geol. Survey Bull. 103, no. 2, p. 108-130. 

Milovanovic, Branislav, and Karamata, Stevan, 1960, Uber den diapirismus 

serpentinischer Massen: Internat. Geol. Cong., 21st, Copenhagen 1960, Rept., 

pt. 18, p. 409--417. 

Muller, Von Peter, 1963, Kalksilikatfelse im serpentin des Piz Lunghin bei Maloja: 

Chemie der Erde, v. 22, p. 452--464. 

Murigoci, G., 1900, ranat- und Vesuvianfels aus dem Serpentin von Paringu: 

Bucarest Soc. Sci. Bull. 9, p. 568-612, 764-831. 

Onuki, Hitoshi, 1963, Petrology of the Hayachine ultramafic complex in the 

Kitakami mountainland, northern Japan: Tohoku Univ. Sci. Repts., 3d ser, 

v. 8, no. 2, p. 241-295. 

Phillips, A. H., and Hess, H. H., 1986, Metamorphic differentiation at contacts 

between serpentine and siliceous country 'rocks: Am. Mineralogist, v. 21, no. 3, 

p. 333--362. 

Poldervaart, A., 1955, Chemistry of the earth's crust, in Poldervaart, A., ed., 

Crust of the earth-a symposium: Geol. Soc. American Spec. Paper 62, 

p. 119--145. 

Ragan, D. M., 1963,. Emplacement of the Twin Sisters dunite, Washington: 

Am. Jour. Sci., v. 261, no.-6, p. 549--565. 

Read, H. H., 1934, On zoned associations of antigorite, talc, actinolite, chlorite, 

and biotite in Unst, Shetland Islands: Mineralog. Mag., v. 23, no. 145, 

p. 519--540. 

Reed, J. J., 1959, Soda-metasomatized argillites associated with the Nelson 

ultramafic belt: New Zealand Jour. Geology Geophysics, v. 2, no. 5, 

p. 905-915. 

Schlocker, Julius, 1960, Rodingite from Angel Island, San Francisco Bay, 

California in Geological Survey research 1960. U.S. Geol. Survey Prof. 

Paper400-B, p. B311-B312. 

Seki, Yotaro, and Kuriyagawa, Sakiko, 1962, Mafic and leucocratic rocks associated with serpentinite of Kanasaki, Kanto mountains, central Japan: 

Japanese Jour. Geology and Geography, v. 33, no. 1, p. 15--32. 
REFERENCES CITED 49 

Smith, G. 0., 1904, Description of the Mount Stuart quadrangle [Washington]: 

U.S. Geol. Survey Geol. Atlas, Folio 106, 10 p. 

Shapiro, L., and Brannock, W. W., 1956, Rapid analysis of silicate rocks: U.S. 

Geol. Survey Bull. 1036-C, 37 p. 

Suzuki, Jun, 1953, On the rodingitic rocks within the serpentinite masses of 

Hokkaido: Hokkaido Univ. Fac. Sci. Jour., v. 8, no. 4, p. 419-430. 

Thayer, T. P., 1963, The Oanyon Mountain Complex, Oregon, and the Alpine mafic 

magma stem in Geological Survey research 1963: U.S. Geol. Survey Prof. 

Paper 475-C, p. C82-085. 

Thayer, T. P., and Brown, C. E., 1964, Pre-Tertiary orogenic and plutonic intrusive activity in central and northeastern Oregon: Geol. Soc. America Bull., 

v.75,p.1255-1262. 

Thompson, J. B., Jr., 1959, Local equilibrium in metasomatic processes, in 

Abelson, P. H. ed., Researches in geochemistry: New York, John Wiley 

& Sons, p. 427--457. 

Turner, F. J., 1930, The metamorphic and ultrabasic rocks of the Lower Cascade 

Valley, South Westland: New Zealand Inst. Trans., v. 61, p. 170--201. 

U.S. Geological Survey and American Association of Petroleum Geologists, 

prepared by a committee, Cohee, G. V., chm., 1962, Tectonic map of the 

United States, exclusive of Alaska and Hawaii: scale 1 : 2,500,000. 

Wells, F. G., Hotz, P. E., and Oater, F. W., Jr., 1949, Preliminary description 

of the geology of the Kerby quadrangle, Oregon : Oregon Dept. Geology and 

Mineral Industries Bull. 40, 23 p. 

Yoder, H. S., 1950, Stability relations of grossularite: Jour. Geology, v. 58, no. 3, 

p. 221--253. 

Zabinski, W., 1964, On the occurrence of a dispersed vesuvianite phase in some 

so-called hydrogrossulars : l' Acad. Polonaise Sci. Bull., des Ser Sci. geol. 

geogr., v. 12, no. 3, p. 149-155. 

U. S. GOVERNMENT PRINTING OFFICE : 1967 0 - 269-731