Petrogenesis of the 

Ultramafic Complex 

at the Blashke Islands, 

Southeastern Alaska 

By GLEN R. HIMMELBERG, ROBERT A. LONEY, 

and JOHN T. CRAIG 

U.S. GEOLOGICAL SURVEY BULLETIN 1662 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1986 

For sale by the 

Distribution Branch, Text Products Section 

U.S. Geologicai.Survey 

604 South :Pickett St. 

Alexandr.ia, VA 22304 

Library of Congress Catalog Card No. 85~600638 
CONTENTS 

Abstract 1 

Introduction 1 

Age and geologic setting 1 

Distribution and description of rock types 2 

Dunite 2 

Wehrlite 2 

Olivine clinopyroxenite 5 

Gabbro 5 

Mineral chemistry 5 

Olivine 6 

Pyroxene 6 

Hornblende 6 

Plagioclase and chromite 6 

Petrogenesis 9 

Reference cited 12 

FIGURES 

1. Map of southeastern Alaska showing relation of the Duke-Kluk~an ultramafic belt to the major 

tectonostratigraphic framework 3 

2. Geologic map of the peridotite-gabbro suite at the Blashke Islands showing locations of samples 4 

3. Ternary diagram showing compositions of pyroxenes in the peridotite-gabbro suite at the Blashke Islands 9 

4. Variation diagram of Si02 versus Al203 in clinopyroxene in the Blashke Islands and Duke Island 

intrusions 12 

5. Variation diagram of Al1

V versus Ti02 in clinopyroxene in the Blashke Islands and Duke Island intrusions 12 

6. Variation diagram of Ti versus (Ca+Na) in clinopyroxene in the Blashke Islands and Duke Island intrusions 12 

TABLES 

1. lUGS classification, cumulus minerals, and postcumulus minerals in analyzed samples from the peridotitegabbro suite at the Blashke Islands 7 

2. Chemical compositions and structural formulas of olivine in the peridotite-gabbro suite at the Blashke 

Islands 7 

3. Chemical compositions and structural formulas of clinopyroxene in the peridotite-gabbro suite at the 

Blashke Islands 8 

4. Chemical compositions and structural formulas of orthopyroxene in the peridotite-gabbro suite at the 

Blashke Islands 9 

5. Chemical compositions and structural formulas of hornblende in the peridotite-gabbro suite at the Blashke 

Islands 10 

6. Chemical compositions and structural formulas of plagioclase in the peridotite-gabbro suite at the Blashke 

Islands 11 

7. Chemical compositions and structural formulas of chromite in the peridotite-gabbro suite at the Blashke 

Islands 11 

III 
Petrogenesis of 

at the Blashke 

the Ultramafic C·omplex 

Islands, Southeastern Alaska 

By Glen R. Himmelberg\ Robert A. Loney, and john ·T. Craig1 

Abstract 

The peridotite-gabbro suite at the Blashke 

Islands is one of more than 30 Alaskan-type ultramafic 

complexes exposed along a linear trend in southeastern 

Alaska. The exposed peridotite-gabbro suite is 

concentrically zoned and about 3.5 km in diameter. 

The center of the intrusion is dunite, with wehrlite, 

olivine clinopyroxenite, and gabbro occurring progressively outward. The ultramafic rocks have textures 

that are characteristic of cumulates. A contact 

aureole approximately 100 m wide surrounds the 

intrusion; the temperature of recrystallization at the 

contact was about 800 °C. The peridotite-gabbro suite 

at the Blashke Islands differs from many other 

Alaskan-type complexes by having minor amounts of 

orthopyroxene and plagioclase in some ultramafic 

rocks and by not having magnetite + clinopyroxene 

cumulates. Field, petrographic, and mineral chemistry 

data indicate that the peridotite-gabbro suite at the 

Blashke Islands originated by crystallization differentiation of a subalkaline magma. Accumulation probably 

occurred in a subvolcanic magma chamber, and the 

concentric structure was probably caused by upward 

diapiric emplacement. 

INTRODUCTION 

The peridotite-gabbro suite exposed at the 

Blashke Islands, and herein informally called the 

Blashke Islands peridotite-gabbro suite, is one of more 

than 30 similar bodies exposed along a linear trend in 

southeastern Alaska· (fig. 1). These bodies generally 

have been recognized as a separate class of ultramafic 

instrusions and are referred to as Alaskan-type ultramafic complexes. Most of these complexes have the 

following characteristic features (Taylor, 1967; 

Jackson and Thayer, 1972): (1) intrusive contacts 

having country rocks marked by strong thermal 

aureoles, (2) rock types that are mostly dunite, 

wehrlite, olivine clinopyroxenite, magnetite-rich 

clinopyroxenite, and, in some bodies, gabbro; (3) crude 

concentric zoning in some of the larger bodies with 

dunite in the central-most parts and gabbro in the 

1Present address: Beparbnent of Geology, 

Uhiversity of Missouri, Cblumbia, MD 65211 

outer-most parts; and (4) principal mineral associations 

in the ultramafic rocks consisting of olivine, diopsidic 

pyroxene, magnetite, and hornblende; orthopyroxene 

and plagioclase are characteristically absent. The 

Blashke ·Islands peridotite-gabbro suite differs from 

the above general description in not hav1ng any 

magnetite-rich clinopyroxenite and by having trace 

amounts of orthopyroxene and plagioclase in some 

ultramafic rocks. 

Suites of ultramafic, gabbroic, and even dioritic 

to granodioritic rocks, similar to the Alaskan-type 

complexes, also have been described in the Klamath 

Mountains and western Sierra Nevada, California 

(James, 1971; Snoke and others, 1981, 1982) British 

Columbia (Findlay, 1969; Clark, 1980), th~ Urals 

(Noble and Taylor, 1960; Taylor and Noble, 1960; 

Taylor, 1967), and Venezuela (Murray, 1972). Although 

these bodies and the Alaskan-type ultramafic 

complexe~ ?i.ffer. in detail, they have many petrologic 

charactertsttlcs m common, and they are believed to 

have been formed in a rifted ensimatic arc petrologictectonic environment (Snoke and others, 1982). Thus 

studies of the petrogenesis of these bodies can be 

instrumental in reconstructing complex petrological 

processes operating at high crustal levels in arc environments. 

The Blashke Islands peridotite-gabbro suite was origi~ally mapped and described by Buddington and 

Chapm (1929). More complete descriptions were given 

by Kennedy and Walton (1946) and Walton (1951). The 

only Alaskan-type ultramafic bodies, except for the 

Blashke Islands peridotite-gabbro suite, that have been 

studied in any detail are those at Union Bay (Ruckmick 

and Noble, 1959) and Duke Island (Irvine, 197 4) in 

Alaska, and at Tulameen (not shown in fig. 1) (Findlay 

1969) in British Columbia. ' 

AGE AND GEOLOGIC SETTING 

Preliminary K-Ar dating by Lanphere and 

Eberlein (1966) suggests an age of 110 Ma for the 

Blashke Islands peridotite-gabbro suite. They also 

obtained similar ages (100-110 Ma) for the Duke Island 

Union Bay, and Klukwan ultramafic complexes of 

Taylor (1967) (fig. 1). These ages, which come from 

widely separated parts of the linear belt of Alaskantype ultramafic complexes called the Klukwan-Duke 

belt (Brew and Morrell, 1983), suggest that the 

complexes were intruded nearly simultaneously. 

1 
The relation of the Klukwan-Duke belt of ultramafic intrusions to the regional geologic framework of 

southwestern Alaska is shown in figure 1 (modified 

from Berg and others (1978), Brew and Ford, (1984a), 

and· Brew and others, (1984). The main units from west 

to east are as follows: (1) the Chugach terrane, 

composed of mostly flysch and the rest melange 

consisting of Cretaceous metaflysch and mafic metavolcanic rocks; (2) Wrangellia terrane, composed of 

Permian and Triassic graywacke, limestone, and mafic 

metavolcanic rocks; (3) the Alexander belt, composed 

of coherent, barely metamorphosed Ordovician through 

Triassic graywacke turbidites, limestone, and volcanic 

rocks; (4) the Gravina belt, variably metamorphosed 

and deformed Upper Jurassic to middle Cretaceous 

flysch and intermediate to mafic volcanic rocks; (5) 

the Mainland belt composed of commonly 

metamorphosed and deformed graywacke, pelite, 

marble, and intermediate volcanic rocks of the Coast 

plutonic-metamorphic complex of Brew and Ford 

(1984a); the age is generally unknown, but fossiliferous 

Carboniferous, Permian, and Triassic rocks are present 

locally. 

As can be seen in figure 1, the Klukwan-Duke 

ultramafic belt parallels and overlaps the Gravina belt 

on both sides, and overlaps the Alexander belt on the 

west and the Mainland belt on the east. Most of the 

ultramafic complexes intrude in or near the Gravina 

belt. Important examples that intrude the Alexander 

belt are the Blashke Islands peridotite-gabbro suite, 

and to the southeast the Salt Chuck complex of Clark 

and Greenwood (1972) and the Duke Island complex of 

Irvine (1974). The Klukwan ultramafic complex 

intrudes the Mainland belt along trend to the northwest, beyond the end of the Gravina belt in 

southeastern Alaska. It is evident that- the present 

boundaries of the Gravina belt were not important 

controls on the movement of magma; instead, it is 

likely that the chief control was a slightly younger 

fracture system, probably a tensional one, that only 

generally paralleled the Gravina belt. 

The part' of the Alexander belt intruded by the 

Blashke Islands peridotite-gabbro suite consists of the 

Descon Formation, which ranges in age from Early 

Ordovician through Early Silurian (Eberlein and 

Churkin, 1970). Except where locally metamorphosed 

as in the contact zone of the Blashke Islands 

peridotite-gabbro suite, the formation is unmetamorphosed and consists mostly of interbedded graywacke, 

conglomerate, limestone, shale, and mafic and intermediate volcanic rocks. 

DISTRffiUTION AND DESCRIPTION OF ROCK TYPES 

The rock classification used in this report is that 

recommended by the lUGS Subcommission on the 

Systematics of Igneous Rocks (1973). Most of the 

rocks have cumulus textures, thus the textural terminology originally established by Wager and others 

(1960) and Jackson (1967), and more recently redefined 

by Irvine (1982) is used. 

The exposed peridotite-gabbro suite on the 

Blashke Islands is concentrically zoned and is about 3.5 

km in diameter (fig. 2). The center of the intrusion is 

dunite; wehrlite, olivine clinopyroxenite, and gabbro 

2 

occur progressively outward. Serpentinization of the 

olivine-rich rocks ranges from about 25 to 75 percent 

but original textures are still evident. Contacts 

between the rock types are generally sharp and almost 

vertical, although locally they are gradational. The 

contacts commonly show no evidence of one rock 

intruding another except for local olivine clinopyroxenite dikes in dunite and gabbro apophyses in 

olivine clinopyroxenite (Kennedy and Walton, 1946). 

Walton (1951) also reported dikes of clinopyroxenite 

with andradite garnet and dikes of hornblendite and 

anorthosite. 

The country rocks are domed and dip steeply 

away from the intrusion (Walton, 1951). Although the 

gabbro shows no chill zone against the country rocks, a 

contact aureole approximately 100 m wide surrounds 

the intrusion. Most of the rocks within the aureole are 

massive, fine-grained hornfels with a well-developed 

granoblastic polygonal texture. The maximum-phase 

metamorphic mineral association observed is plagioclase-hornblende-orthopyroxene-clinopyroxene-biotiteoxide; this association indicates recrystallization 

under conditions of the pyroxene-hornfels facies. 

Chemical compositions of metamorphic orthopyroxene 

and clinopyroxene in sample 27 projected onto the 

Lindsley (1983) 1-atm geothermometer yield temperatures of formation of 800 °C for clinopyroxene and 

950 °C for orthopyroxene. The clinopyroxene 

temperature is probably the better value since it is 

less sensitive to analytical error. 

Dunite 

Dunite, approximately 2 km in diameter, makes 

up the core of the peridotite-gabbro suite (fig.2) and is 

the most abundant rock type exposed. It forms 

massive, homogeneous exposures with no macroscopic 

internal structures. The dunite is an olivine-chromite 

cumulate. Clinopyroxene is a minor post-cumulus 

phase (less than 10 percent) within several meters of 

the contact with wehrlite, otherwise it is not present. 

The olivine commonly occurs as 2-3 mm anhedral 

grains with mutually interfering, gently curved grain 

boundary segments; euhedral and subhedral grains are 

also present but are less common. Some olivines show 

broad kink bands, but generally there is no evidence of 

strain. 

Chromite commonly makes up less than 2 

percent of the rock. It occurs as approximately 0.5 

m m euhedral grains intergranular to and enclosed in 

olivine. In some samples, sparse chromite grains in a 

wispy planar alignment drape over and continue 

through oivine grains-a textural relation that suggests 

adcumulus growth of olivine around cumulus chromite 

"layer". Similar wispy chromite layers are present in 

the Duke Island Alaskan-type ultramafic complex 

(Irvine, 1974). 

Wehr lite 

Wehrlite occurs as a discontinuous unit (as much 

as 100 m wide) in contact with the western part of the 

dunite or as arc-shaped lenses within the olivine clinopyroxenite (fig. 2). All exposures are massive and 

homogeneous; contacts between wehrlite and either 

dunite or olivine clinopyroxenite are generally abrupt, 

although gradational contacts are present. 
PACIFIC OCEAN 

EXPLANATION 

hSSJ 

tt~{}jt~l 

ew 

r :. :. ·: =: ·:: = ~ ~ 

fZ):~j 

Chugach terrane 

Wrangellia terrane 

Alexander belt,dashed 

where overlaps 

Klukwan-Duke belt 

Gravina belt 

Mainland belt,dashed 

where overlaps Klukwan· 

Duke belt 

Boundary of Klukwan·Duke ultramafic. belt 

Boundary of tectonostratigraphic 

terranes and belts 

YUKON 

BRITISH 

0 50 KILOMETERS 

I I 

--

KETCHIKAN 

Figure 1. Southeastern Alaska, showing relation of the Duke-Klukwan ultramafic belt to the major 

tectonostratigraphic framework. Map compiled from Berg and others (1978), Brew and others (1984), and 

Brew and Ford (1984a). Dots indicate location of Alaskan-type ultramafic complexes. 

3 
4 

0 .5 

Kbd 

~SOd 

SOd 

"-Kbc 

1.0 KILOMETER 

EXPLANATION 

Peridotite· gabbro suite at Blashke Islands (Cretaceous) - Consists of: 

Dunite- Composed of olivine and less than 2 percent chromite. Contains as much as 5 

percent clinopyroxene near wehrlite or olivine clinopyroxenite contact 

Kbw Wehrlite- Composed of olivine clinopyroxene and chromite. ·Mineral ratios are extremely 

variable 

Kbc Olivine clinopyroxenite - Composed of clinopy_roxene and olivine. Locally may contain 

Kbg I 

SOd .• :::d 

28. 

hornblende 

Gabbro - Transitional from a clinopyroxene gabbro at the ultramafic contact to a 

hornblende gabbro at the country rock contact 

Descon Formation (Lower Silurian through Lower Ordovician)- Graywacke, 

conglomerate. limestone. shale. volcanic rocks. Contact- metamorphosed (mostly 

hornfels) part of unit SOd indicated by stipple 

Contact 

Sample locality - Numbers refer to tables 1 · 7 

Figure z. Geologic map of the peridotite-gabbro suite at the Blashke Islands showing location of 

analyzed samples (numbers refer to tables 1 through 7). Map modified from Walton (1951). 
The wehrlite is characterized by cumulus olivine 

and chromite (less than 1 percent) and 15-25 percent 

postcumulus clinopyroxene. The clinopyroxene 

commonly forms oikocrysts, as much as 1 em in size, 

that enclose approximately 1 mm, subhedral to 

rounded olivine grains. Olivine not poikilitically 

enclosed in clinopyroxene is 2-3 mm in size and. generally anhedral with gently curved grain boundary 

segments indicating adcumulus growth. Some olivine 

grains show broad kink bands. Chromite occurs as 

small (less than 0.5 mm) euhedral and subhedral grains 

dispersed throughout the rocks either as intergranular 

grains or included in olivine and clinopyroxene. Rare 

samples contain a trace of interstitial, postcumulus 

plagioclase (now rodingite). 

Olivine clinopyroxene 

Olivine clinopyroxenite forms a unit, as much as 

about 700 m wide, that encircles the dunite (fig. 2). 

The olivine clinopyroxenite is composed mostly of 

cumulus clinopyroxene and olivine. The olivine 

generally makes up about 10-15 percent of the rock 

although locally it may be as much as 50 percent. 

Other minerals include postcumulus hornblende, which 

may make up as much as 10 percent of the rock, but 

generally it is less than 1 percent, and traces of 

biotite, plagioclase, orthopyroxene. chromite, and 

magnetite are present in some samples. 

Clinopyroxene grain size is variable. Most of the 

grains are 5-7 m m in size although locally, with no 

regular spatial relation, a finer grained 

clinopyroxenite, with 2-3 mm size grains, occurs. 

Some individual gra_ins of the coarser clinopyroxenite 

are as large as 1.5 em. The clinopyroxene generally 

forms an anhedral granular texture with mutually 

interfering grain boundaries that are either gently 

curved, sutured, or coarsely irregular. Local areas 

within a sample have grain boundaries that are straight 

with 120° intersections. Simple twins and internal 

replacement of clinopyroxene by magmatic hornblende 

are common; exsolved orthopyroxene in clinopyroxene 

is not common. There is no evidence of strain. 

The olivine is generally of smaller grain size (1-2 

m m) than the pyroxene, but it usually occurs as groups 

of grains with curved grain boundary segments. Hornblende is mostly intergranular, but it also partly 

replaces clinopyroxene along margins and cleavage 

planes. 

Gabbro 

The gabbro unit is as much as 300 m wide and 

forms the outermost part of the peridotite-gabbro 

suite (fig. 2). Much of the unit is charaGterized by a 

cent·imeter-scale flow banding that is manifested by 

variable concentrations of plagioclase and mafic 

·minerals. Mafic 'dikes and xenoliths in the gabbro 

range in size from millimeters to tens of meters. 

Specific rock types of the gabbro unit range 

gradationally and irregularly, from olivine-bearing 

hornblende-pyroxene gabbro and gabbronorite near the 

contact with olivine clinopyroxenite to hornblende 

gabbro toward the outer contact with the country 

rock. In the hornblende-pyroxene gabbro and gabbronorite, the maximum clinopyroxene content is about 35 

percent, the maximum orthopyroxene content is about 

5 percent, hornblende content may be as much as 25 

percent-<>nly rarely is it less than 5 percent. Olivine 

commonly makes up less than 1 percent of these 

rocks. As the amount of hornblende increases the· 

amount of pyroxene decreases, and in the hornblende 

gabbros, clinopyroxene is absent or restricted to small 

relict cores in some hornblende grains. Other primary 

minerals include magnetite (as much as about 15 

percent), biotite, and interstitial quartz in some 

samples, and accessory sphene and apatite. 

The gabbros consist of 2:..3 mm grains that are 

subhedral to anhedral granular. The hornblende is 

generally brown; in some samples individual grains 

grade irregularly outward to green. In the hornblendepyroxene gabbro and gabbronorite, the hornblende is 

present as thick mantles around pyroxene, as internal 

replacements of pyroxene, and as discrete grains; in 

the hornblende gabbro, it is present as prismatic and 

subophitic grains, some of which have relict clinopyroxene granules. Orthopyroxene-magnetite 

symplectites, which are commonly associated with 

olivine, probably are a result of a late stage magmatic 

reaction: 

olivine +melt =orthopyroxene +.magnetite 

Plagioclase in the gabbro shows marked concentric 

zoning. Some hornblende gabbros have plagioclase 

with partly altered, embayed cores, and broad clear 

rims that probably reflect reaction of original plagioclase primocrysts with residual liquid. 

Secondary minerals include epidote, chlorite, 

actinolite, calcite, white mica, prehnite, and hydrogrossular. Small amounts of pyrrhotite and chalcopyrite are present in the olivine clinopyroxenite and 

gabbro. 

MINERAL CHEMISTRY 

Olivine, clinopyroxene, orthopyroxene, 

hornblende, plagioclase1 and chromite were analyzed 

with an ARL EMX-SM electron microprobe; natural 

and synthetic minerals and oxides of known composition were used as standards. Corrections were made 

using the procedures of Bence and Albee (1968) and the 

correction factors of Albee and Ray (1970). Structural 

formulas for all minerals were calculated by 

normalizing tf+ number ~f cations. For pyroxene and 

chromite, Fe and Fe were calculated by assuming 

a perfectly stoichiometric formula. For hornblende, 

the chemical and crystal-chemical limits outlined by 

Robinson and others (1982) are satisfied in all cases by 

normalizing to a total of 13 cations exclusive of Ca, 

Na, and K. For some hornblende analyses, a suitable 

formula was also obtained by assuming all iron as. FeO 

or by normalizing to a total of 15 cations exclusive of 

Na and K. Thus the hornblende formulas presented are 

1use of trade nmnes in this report is for 

descriptive purposes only and does not 

constitute endorsement by the U.S. Geological 

Survey. 

5 
averages of the two calculations that fall wit~¥~ the 

ch~'llical and crystal chemical limits. The Fe and 

Fe in hornblende were then estimated by assuming a 

total of 46 charges. 

The lUGS classification of the analyzed samples, 

along with a listing of the cumulus and postcumulus 

minerals, are given in table 1. Locations of the 

samples are shown in figure 2. 

Olivine 

Chemical compositions and structural formulas 

of olivine in the Blashke Islands peridotite-gabbro suite 

are given in table 2. The compositional range of 

olivine within a rock unit is very small and shows no 

relation to position in the unit. There is, however, a 

systematic decrease in Fo content (100 

Mg/(Mg+Fe+Mn)) through the rock series dunite (90.2 

to 8.7 .5), wehrlite (87 .6 to 87 .2), olivine clinopyroxenite 

(87 .6 to 80.4), and gabbro (76.6) that is consistent with 

fractional crystallization. The Fo values, as well as 

minor element contents, are similar to those reported 

for other Alaskan-type ultramafic complexes 

(Ruckmick and Noble, 1959; Findlay, 1969; Irvine, 

1974; Clark, 1980). 

Pyroxene 

Chemical compositions and structural formulas 

of clinopyroxene and orthopyroxene are given in tables 

3 and 4 and are plotted in the conventional pyroxene 

quadrilateral (fig. 3). Most of the clinopyroxenes are 

diopside; the Ca-values (100 Ca/(Ca+Mg+Fe)) of clinopyroxene in the ultramafic rocks are high and range 

only from 49.9 to 46.7. Clinopyroxene in gabbro 

generally has equally high Ca-values although one 

sample is 43.3. The clinopyroxenes show a very 

limited iron enrichment. The Mg-value (100 

Mg/(Mg+Fe+Mn)) of clinopyroxene in the ultramafic 

rocks ranges from 94.6 to 86.3; clinopyroxene in 

gabbro extends the range to 75.2. The A120 3 content 

of clinopyroxene ranges from 0. 70 to 2.87 weight 

percent, it is lowest in postcumulus clinopyroxene in 

dunite but otherwise there is no relation to rock type. 

The Ti02 and Na2o contents of clinopyroxene range 

from 0.03 to 0.50 weight percent and 0.02 to 0.37 

weight percent respectively; the lowest values are in 

dunite and the highest values in gabbro. 

In terms· of CA:Mg:Fe variation the clino-

. pyroxene in the Blashke Islands peridotite-gabbro suite 

is similar to clinopyroxene in the Alaska-type 

complexes at Union Bay and Duke Island,. Alaska 

(Ruckmick and Noble, 1959; Irvine, 1974) and 

Tulameen, British Columbia (Findlay, 1969). The high 

Ca-values and limited iron enrichment trend are 

different from the pyroxene trends in tholeiitic intrusions (Wager. and Brown, 1968; Atkins, 1969; 

Himmelberg and Ford, 1976) but similar to pyroxene 

trends in alkalic intrusions (Irvine, 1974). In terms of 

Al203 content, however, the clinopyroxene in the 

Blashke Islands intrusion differs appreciably from the 

clinopyroxene in other Alaskan-type ultramafic 

complexes. In the Union Bay, Tulameen, and Duke 

Island intrusions, AI2o3 values in some clinopyroxenes 

are as low as in the Blashke Islands clinopyroxene, but 

most values are higher, commonly between 3.0 and 4.5 

6 

weight percent, and some values are as much as 6 

percent. Considering all chemical variations, the 

clinopyroxene in the Blashke Islands intrusion more 

closely resembles those in the Emigrant Gap mafic 

complex (James, 1971) in California and in the 

composite calc-alkaline pluton at Yakobi Island, 

Alaska (Himmelberg and Loney, 1985). 

Orthopyroxene in three gabbro samples of the 

Blashke Islands peridotite gabbro suite have Mg-values 

that range from 73.0 to 58.3 and AI2o3 contents that 

range from 1.25 to 2.08 weight percent. The orientation of the tie-line between clinopyroxene and orthopyroxene in sample 28 indicates that these two 

pyroxenes did not obtain or retain equilibrium compositions. Trace amounts of orthopyroxene were 

observed in samples of wehrlite and olivine clinopyroxenite but none was analyzed. Orthopyroxene has not 

been reported in the ultramafic rocks of other 

Alaskan-type ultramafic complexes but it is present in 

the Emigrant Gap, California, ultramafic rocks, and 

these rocks have many similarities with the Alaskantype complexes (James, 1971). Orthopyroxene is 

present in the gabbroic rocks at Duke Island, Alaska, 

but these rocks are older than the ultramafic complex 

(Irvine, 1974). 

Hornblende 

Chemical compositions and structural formulas 

of magmatic hornblende in one sample of olivine 

clinopyroxenite and six samples of gabbro are given in 

table 5. The Mg-value of hornblende in the olivine 

clinopyroxenite is 85.7; the Mg-value of hornblende in 

gabbro ranges from 77.6 to 63.7. Except for sample 

28, the Mg-value of hornblende is highest in the hornblende-pyroxene gabbros and lowest in the hornblende 

gabbros. There are small variations in the number ~! 

Si c'31Jons, t'V«t number of A-site cations, and the Fe 

/(Fe + A1 ) ratio. In sample 19 where the 

hornblende is gradationally zoned from a brown core to 

a green rim, the later crystallizing green hornblende is 

silicon-enriched. A similar chemical zonation is 

present in hornblende in gab,Pro of the Yakobi Island 

calc-alkaline pluton (Himmelberg and Loney, 1985). 

Plagioclase and chromite 

Plagioclase compositions and structural formulas 

given in table 6 are average compositions of unaltered 

grain cores. The compositions range from about 

An8.z 2 to An53 9. In general, the more anorthitic 

plagioclase is present in gabbro near the contact with 

olivine clinopyroxenite and the albite component 

increases outward. The albite-enrichment of plagioclase, however, does not exactly parallel the ironenrichment of amphibole. Most plagioclase show 

marked oscillatory zoning and distinct Na-enriched 

rims, which in some samples are albite. 

Accessory chromite in the Blashke Island~ ultramafic rocks is characterized by high Fe + /( Cr+ 

Al+Fe 3-f). ratios (0.19-0.26) (table 7) which are typical 

of chromite in Alaskan-type ultramafic complexes 

(Irvine, 196 7; 19 7 4). The Cr /( Cr+ AI) is also relatively 

high (0.71-0.51) and generally decreases from the 

dunite core outward. 
Table 1. lUGS classification, cumulus minerals, and postcumulus minerals in analyzed samples 

from the peridotite-gabbro suite at the Blashke Island. 

[Sample locations shown on figure 2. Cumulus and postcumulus minerals listed in order of decreasing abundance] 

Sample No. 

37 

5 

6 

10 

12 

47 

44 

13 

48 

14 

lUGS classification 

Dunitc 

Dunitc 

Dunite 

lJunite 

Dunitc 

Dunite 

Wehrlitc 

Wehr lite 

Olivine clinopyroxenite 

Olivine clinopyroxenite 

Olivine clinopyroxenite 

Cumulus minerals 

olivine-chromite 

olivine-chromite 

olivine-chromite 

olivine-chromite 

olivinc-chromite 

olivine-chromite 

olivine-chromite 

olivine-chromite 

clinopyroxene-olivine-chromite 

clinopyroxene-olivine 

clinopyroxene-olivine 

49 Gabbronitc non-cumulate 

15 Hornblende-pyroxene gabbro non-cumulate 

29 Hornblende-pyroxene gabbro non-cumulate 

28 Olivine-bearing hornblende gabbronorite non-cumulate 

19 Hornblende gabbro non-cumulate 

17 Hornblende gabbro non-cumulate 

27 Two-pyroxene hornfels 

Postcumulus minerals 

clinopyroxene 

clinopyroxene 

clinopyroxene 

clinopyroxene-plagioclase 

clinopyroxene 

hornblende 

Table Z. Chemical compositions and structural formulas of olivine in the peridotite-gabbro suite at the 

Blashke Islands. 

[Total iron as FeO; - , not detected] 

Sample No. 

Si02 

FcO 

MnO 

MgO 

CaO 

NiO 

Sum 

37 

42.0 

9.51 

49.4 

.17 

41.1 " 

10.5 

.02 

48.1 

.26 

Dunite 

41.4 

10.5 

.02 

48.2 

.21 

10 43 

41.1 40.8 

10.6 10.4 

.OJ 

48.6 47.9 

.05 .18 

12 

41.1 

11.9 

.07 

47.2 

.08 

.1 .09 .09 .10 .07 .04 

101.18 100.07 100.42 100.48 99.35 100.39 

Wehrl ite 

44 

41.4 

11.9 

.07 

47.2 

.08 

47 

40.3 

12.4 

.03 

47.5 

.09 

.11 .04 

100.76 100.36 

Formula per 3 cations 

Mn 

Mg 

Ca 

Ni 

100 Ms 

(Mg+Fe+Mn) 

1.017 

.193 

1.784 

.004 

.002 

90.2 

1.011 

.216 

.001 

1.764 

.007 

.002 

89.1 

1.015 

.215 

.001 

1.762 

.006 

.002 

89.1 

1.006 1.010 

.217 .215 

.001 

1.773 1.768 

.001 .005 

.002 .001 

89.1 89.2 

1.014 

.246 

.001 

1.736 

.002 

.001 

87.5 

1.019 

.245 

.001 

1. 731 

.002 

.002 

87.6 

0.994 

.256 

.001 

1.746 

.002 

.001 

87.2 

Olivine 

clinopyroxenite 

Gabbro 

13 

41.4 

12.0 

.02 

47.4 

.07 

.04 

40 14 15 

40.5 38.7 

12.7 17.9 

.20 .28 

46.2 41.9 

.47 .37 

100.93 100.07 99.15 

39.0 

20.9 

.77 

39.8 

.1 7 

.09 

100.73 

1.016 

.246 

.001 

1. 734 

.002 

.001 

87.6 

1.007 

.264 

.004 

1. 712 

.013 

86.5 

0.994 1.003 

.385 .449 

.006 .017 

1.605 1.525 

.010 .005 

.002 

80.4 76.6 

7 
00 

Table 3. Chemical compositions and structural formulas of clinopyroxene in the peridotite-gabbro suite at the 

Blashke Islands. 

[Total iron as FeO; Fe 3+, Fe 2+, Fe 2o3 , and FeO calculated by assuming stoichiometry. -, not detected; n.d., not determined] 

Sample No. 

Si02 

Al 2o3 

Ti0 2 

cr 2o3 

FeO 

MnO 

MgO 

CaO 

Na 2o 

Sum 

FeO 

Fe 2o

3 

New Sum 

Si 

AliV 

AlVI 

Ti 

Cr 

Fe3+ 

2+ Fe 

Mn 

Mg 

Ca 

Na 

100 Mg/(Mg+Fe+Mn) 

Ca 

Mg 

Fe 

Dunite 

12 43 10 

55.2 54.8 53.4 

.70 .91 1.49 

.10 .06 .03 

.17 .09 .05 

1.72 2.43 2.64 

.05 .06 .05 

17.6 17.4 17.9 

25.7 24.8 24.8 

.02 .03 .06 - --- --- 101.26 100.58 100.42 

1.72 2.43 

2.93 - --- 101.26 100.58 100.71 

1.980 1.981 1.927 

.020 .019 .063 

.010 

.003 

. 005 

.020 

.002 

.003 

.052 .073 

.002 .002 

.941 .938 

.988 .961 

.001 .002 

94.6 92.6 

49.9 48.7 

47.5 47.6 

2.6 3.7 

.001 

.001 

.084 

.002 

.963 

.959 

.004 

92.2 

47.9 

48.1 

4.0 

Wehr lite 

47 

53.2 

2.87 

.25 

n.d. 

3. 72 

.09 

16.7 

23.4 

.22 

100.45 

3.00 

.81 

100.54 

1.928 

.072 

.051 

.007 

n.d • 

.022 

.091 

.003 

.902 

.909 

.015 

88.6 

47.2 

46.9 

5.9 

Olivine 

c linopyroxeni te 

48 13 14 

53.8 54.3 52.3 

1.91 1.61 2.87 

.17 .13 .26 

.32 .30 .23 

2.95 3.45 4.41 

.11 .09 .13 

17.5 17.4 16.0 

23.7 23.6 23.7 

.17 .• 17 .18 - --- --- 100.63 101.05 100.08 

2.02 3.00 

1.03 .48 

2.98 

1.60 

100.73 101.08 100.25 

Formula per 4 cations 

1.942 1.956 1.911 

.058 .044 .089 

.023 

.005 

. 009 

.018 

.061 

.003 

.942 

.917 

.012 

91.1 

47.1 

48.4 

4.6 

.024 

.004 

.009 

.013 

.035 

.007 

. 007 

.044 

.091 .091 

.003 .004 

.934 .872 

.911 .928 

.012 .013 

89.7 86.3 

46.7 48.0 

47.9 45.1 

5.3 7 .o 

Gabbro 

28 17 15 29 49 

53.2 53.8 52.2 53.0 52.1 

2.12 1.86 2.24 1.98 2.35 

.24 .24 .30 .25 .50 

n.d. .30 - n.d. n.d. 

4.58 5.79 7.96 7.76 8.47 

.12 .43 .48 . 56 .41 

16.3 14.7 14.5 14.2 15.2 

23.4 23.4 22.4 22.3 21.2 

.16 .36 .35 .48 .37 - --- --- --- --- 100.12 100.88 100.43 100.53 100.60 

4.05 

.58 

5.79 5.96 6.77 

2.23 1.08 

6.40 

2.31 

100.17 100.88 100.66 100.62 100.84 

1.944 1.969 1.926 1.954 1.916 

.056 .031 .074 .046 .084 

.035 

.007 

.016 

.049 

.007 

. 009 

.124 .177 

;004 .013 

.888 .802 

.916 .918 

.011 .026 

86.0 80.8 

47.1 48.4 

45.7 42.3 

7.2 9.3 

.023 

.008 

.062 

.184 

.015 

.797 

.885 

.025 

75.3 

45.9 

41.3 

12.8 

.040 

.007 

n.d • 

.030 

.018 

.014 

n.d • 

.064 

.209 .197 

.017 .013 

.780 .833 

.881 .835 

.034 .026 

75.3 75.2 

46.4 43.3 

41.1 43;2 

12.6 13.5 

Hornfels 

27 

53.0 

.88 

.16 

n.d. 

9.97 

.35 

14.1 

21.0 

.24 

99.80 

9.51 

.50 

99.84 

1.982 

.018 

.021 

.004 

n.d • 

.014 

.298 

.011 

.786 

.841 

.025 

70.9 

43.4 

40.5 

16.1 
Table 4. Chemical compositions and structural 

formulas of orthopyroxene in the peridotite-gabbro 

suite at the Blashke Islands. 

[Total iron as FeD; Fe3•, Fe ~, Fe 2o3 , and FeD calculated by 

assuming stoichiometry] 

Sample No. 

Si02 

Al 2

03 

T102 

FeD 

NnO 

MgO 

CoO 

Na 2o 

Sum 

15 

54.1 

2.08 

• 21 

16.2 

.90 

25.9 

1.55 

.04 

1Cl0.98 

15.7 

.55 

Gabbro 

49 

54.1 

1.66 

.20 

16.6 

.77 

25.9 

1.34 

.04 

100.61 

16.0 

.67 

28 

52.9 

1. 25 

.13 

23.7 

.83 

20.6 

1.47 

.05 

100.93 

23.7 

Hornfels 

27 

52.2 

0.76 

0.15 

24.4 

0.76 

19.7 

l. 71 

0.07 

99.75 

24.0 

0.46 

New Sum _____ 10]_:~__!2.2.:_6...;.8 _____ 1...;.0...;..0...;.. 9...;.3 __ 9;...9...;.'..;;..81;.... 

Formula per 4 cations 

Si 

Al IV 

AI VI 

1'1 

Fe3+ 

Fe ~ 

Nn 

Mg 

Ca 

No 

1 .943 

.057 

.031 

.006 

.015 

.472 

.027 

1 .387 

.060 

.003 

100 Mg/(Mg+Fe+~n)73.0 

3.1 

71.7 

~5.1.' 

1 .953 

.047 

.024 

.005 

.16 

.485 

.022 

1.393 

.052 

.003 

72.7 

2.7 

71.6 

25.7 

1 • 971 

.029 

.026 

.004 

.738 

.026 

1 .144 

.Cl59 

.004 

60.0 

3.0 

58.9 

38.0 

---·--------------------

Co 

EXPLANATION 

• Ounito 

o Wohrlito 

+ Olivine clinopyroxenite 

t::. Gabbro 

• Contact hornfels 

Mg 50 

1. 977 

.023 

.011 

.004 

.013 

.760 

.024 

1.112 

.069 

.005 

58.3 

3.5 

56.9 

39.6 

Fe 

Figure 3. Compositions of pyroxene in the peridotitegabbro suite at the Blashke Islands. T~e lines connect 

clinopyroxene and orthopyroxene in the same sample. 

PHTROGBNFSIS 

Early investigators of Alaskan-type ultramafic 

complexes (Ruckmick and Noble, 1959; Taylor and 

Noble, 1960, 1969; Taylor, 1967) argued that the 

principal rock types in the complexes represent 

successive intrusions of a series of distinctly different 

ultramafic magmas. This interpretation is generally 

no longer accepted. The Alaskan-type ultramafic 

complexes are now recognized to be cumulates that 

originated by crystallization differentiation of basaltic 

magmas (Murray, 1972; Irvine, 1974). Questions 

remain, however, whether the parental basaltic 

magmas were alkaline (Irvine, 1974) or subalkaline 

with the resulting production of calc-alkaline liquids 

(Murray, 1972). 

In the case of the Blashke Islands peridotitegabbro suite, the data strongly suggest that the 

parental magma was subalkaline in composition. The 

crystallization order of minerals in the ultramafic 

rocks, as indicated by the succession of cumulus phases 

in the principal rock types, was olivine + minor 

chromite (dunite 8.nd wehrlite) followed by olivine + 

clinopyroxene ± minor chromite (olivine 

clinopyroxene). This sequence is the same as that of 

other Alaskan-type ultramafic complexes (Irvine, 

197 4), but it is not diagnostic of either alkaline or 

subalkaline magma fractionation trends, as it can 

occur in both. Notably absent in the Blashke Islands 

sequence of rocks but present ·in other Alaskan-type 

ultramafic complexes such as at Duke Island, Union 

Bay, Klukwan, Snettisham, and Percy Islands in Alaska 

and Tulameen in British Columbia are 

magnetite+clinopyroxene cumulates (magnetite-rich 

clinopyroxenites). Irvine (1974) argued that these 

cumulates could be a result of crystallization from a 

magma of low silica activity, and thus represent 

fractionation along an alkaline trend. Instead, in the 

Blashke Islands intrusion the crystallization of olivine 

clinopyroxenite is followed by crystallization of 

gabbro; much of the gabbro contains orthopyroxene 

and clinopyroxene and also commonly contains interstitial quartz, indicating fractionation along a 

subalkaline, silica-saturated trend. 

Two-pyroxene gabbros also occur at Duke Island, 

Alaska (fig. 1), but field evidence clearly indicates 

that they are older than the ultramafic complex 

(Irvine, 1974). At Blashke Islands, however·, there is no 

field evidence that indicates the outer zone of gabbro 

is of a different age than the ultramafic rocks. On the 

contrary (1) the spatial distribution of rock types in 

the gabbro zone-pyroxene-bearing gabbro near the 

contact with olivine clinopyroxenite, gradationally 

progressing to hornblende gabbro at the periphery; (2) 

the regular decrease in Mg/(Mg+Fe) ratio in minerals 

in the dunite, olivine clinopyroxenite, and gabbro; and 

(3) the occurrence of trace amounts of postcumulus 

orthopyroxene and plagioclase in some ultramafic 

rocks all indicate that the gabbro is genetically related 

to the ultramafic rocks by crystallization differentiation. The above evidence, as well as the 

clinopyroxene compositions, makes the Blashke Islands 

peridotite-gabbro suite more like the zoned mafic 

complex at Emigrant Gap, California, than it is like 

other Alaskan-type ultramafic complexes. However, 

the Emigrant Gap mafic complex is further differenti9 
10 

Table 5. Chemical compositions and structural formulas of hornblende in the 

peridotite-gabbro suite at the Blashke Islands. 

[Total iron as FeO; Fe 3+, Fe 2i, Fe 2o3 , and FeO determined by the procedures outlined by 

Robinson and others (1982). n.d., not determined. g, green rim, b, brown core] 

Olivine Gabbro 

--------~l2E~~~i~t~e---------------------------------------------------------

Sample No. 

Si02 

At 2o3 

Ti02 

FeO 

MnO 

MgO 

CaO 

Na 2o 

K20 

Sum 

14 

45.1 

12.2 

1.00 

8.46 

.1 0 

16.2 

12.5 

1. 58 

.50 

97.64 

49 

43.6 

11.7 

2.73 

11 .5 

.23 

14.2 

12.0 

2.35 

n.d. 

98.31 

15 

43.1 

13.2 

2.67 

11.6 

.30 

13.6 

11.9 

2.52 

.40 

99.29 

29 

44.1 

11.3 

2.19 

12.5 

.38 

13.3 

11.8 

2.29 

.55 

98.41 

19g 

50.8 

5.43 

.88 

13.5 

.52 

14.4 

11.8 

.99 

• 50 

98.82 

19b 

43.4 

11.8 

2.57 

13.6 

.42 

12.5 

11.2 

2.26 

.49 

98.24 

17 28 

45.3 43.1 

9. 46 11.6 

1.81 2.16 

14.4 15.6 

.40 .26 

12.7 12.0 

11.9 11.6 

1.44 2.43 

.72 n.d • 

98.13 98.7 5 

FeO 4.72 7.06 9.75 10.9 11.8 11.1 12.2 12.0 

Fe 2o3 4.14 4.93 2.03 1.73 1.90 2.81 2.50 4.02 

New S ll:.:.:m:__ _____ ~9-~8..:.. . ..:..04:....._ __ _..:..9..:..8..:... 8=-0=------=-9..:..9..:... 4_7;__ __ ~9.8. 5 4 _____ 9;:...9;;..;•:..;;0:.:2:...._ ___ 9;;..;8;;..;.:.:5:.:5 _____ 9;;..;8;;..;•;;..;4:.:3 __ __:_9 9.;....:..;. 1;;..;7_ 

Mg 

c 

2+ Fe 

Mn 

Ca 

Na 

B 

Na 

K 

A 

100 Ng 

(Mg+Fe 2++Mn) 

6.430 

1.570 

8.000 

.476 

.108 

.444 

.532 

3.440 

5.000 

.031 

.013 

1.909 

.047 

2.000 

.389 

.092 

.481 

85.7 

6.272 

1. 728 

8.000 

.253 

.296 

.533 

.849 

.028 

3.040 

4.999 

1.848 

.152 

2.000 

.502 

n.d. 

.502 

77.6 

Formula 

6.193 

·1.808 

8.000 

.429 

.289 

• 2.20 

1.146 

2.916 

5.000 

.029 

.036 

1 .835 

.100 

2.000 

.603 

.074 

.677 

70.7 

6.421 

1.579 

8.000 

.361 

. 240 

• 190 

1.322 

2.887 

5.000 

.010 

.047 

1 .842 

.1 01 

2.000 

.546 

.103 

.649 

67.7 

7.284 

.716 

8.000 

.202 

.095 

.205 

1. 416 

.003 

3.079 

5.000 

.060 

1.816 

• 124 

2.000 

.151 

.092 

.243 

67.6 

6.340 

1.660 

8.000 

.370 

.282 

.309 

1.318 

2. 721 

5.000 

.037 

.050 

1.753 

.160 

2.000 

.481 

.092 

.573 

65.9 

6.643 6.291 

1.357 1. 709 

8.000 8.000 

.274 .292 

.200 .239 

• 276 .442 

1.477 1.431 

2.773 2.618 

5.000 5.000 

.012 

.050 

.032 

.032 

1.869 1.818 

.069 .118 

2.000 2.000 

.341 • 571 

.135 

.476 .571 

64.3 63.7 
Table 6. Chemical compositions and structural 

formulas of plagioclase in the peridotite-gabbro suite 

at the Blashke Islands. 

ated to yield appreciable diorite, tonalite, and granodiorite (James, 1971). The suppression of crystallization 

of substantial amounts of orthopyroxene and 

plagioclase in the ultramafic rocks might be due to an 

elevated PH 0 in the magma (Murray, 1972; Allen and 

Boettcher, 1 ~7 8). 

[Total iron as Fe 2o3 . n.d., not determined] 

____ Gabbro ------------

Samplt> No. 49 15 17 28 29 19 

Several authors (Kushiro, 1960; LeBas, 1962; 

Nisbet and Pearce, 1977; Leterrier and others, 1982) 

have been successful in relating the chemical composition of igneous clinopyroxenes to the chemical nature 

of their host rocks. Clinopyroxenes that crystallized 

from alkaline magmas differ from those that 

crystallized from subalkaline magmas by generally 

having higher contents of calcium, titanium, and 

aluminum. The proportions of titanium and aluminum 

in clinopyroxene increase as the host magma changes 

from subalkaline through alkaline to peralkaline. The 

aluminum and titanium contents in clinopyroxene in 

the Blashke Islands intrusion are appreciably lower 

than in clinopyroxene in the Duke Island, Union Bay, 

and Tulameen ultramafic complexes. Plotting the 

compowons of clinopyroxene in Si02 versus Al20 3 and Al versus Tio2 diagrams (figs. 4 and 5) shows 

that all the Blashke Islands clinopyroxenes and most of 

the Duke Island clinopyroxenes fall within the field of 

subalkaline rocks as defined by LeBas (1962), although 

the Duke Island clinopyroxenes clearly trend toward 

more alkaline compositions. Clinopyroxene compositions plotted on the (Ca+Na) versus Ti discrimination 

diagram of Leterrier and others (1982) (fig. 6) fall in 

an area of low counting frequency overlap of the 

subalkaline and alkaline basalt fields and are thus 

equivocal. 

Si02 48.1 47.1 48.7 50.2 49.4 

Al 2o3 33.6 33.5 32.4 32.6 31.9 

Fe 2o3 n.d. .45 .48 n.d. .37 

MgO n.~. .06 .06 n.~. .07 

CaO 17.2 17.4 16.0 15.1 15.3 

Na 2o 2.03 2.10 2.82 3.16 3.26 

K2 0 . 04 • 1 0 . 1 2 . 06 . 04 

Sum 100.97 100.71 100.58 101.12 100.34 

Mg 

Ca 

Na 

K 

An 

Ab 

Or 

For~ula per 20 cations 

8.736 8.575 8.846 9.045 8.970 

7.192 7.188 6.937 6.923 6.827 

n.d. .062 .066 n.d. .051 

n.d. .016 .016 n.d. .019 

3.347 3.394 3.114 2.915 2.977 

. 715 . 741 .993 1.104 1.148 

.009 .023 .028 .014 .009 

82.2 81.6 75.3 72.3 72.0 

17.6 17.8 24.0 27.4 27.8 

.2 .6 .7 .3 .2 

53.8 

29.2 

.16 

.04 

11 .6 

5.37 

.18 

100.35 

9.653 

6.175 

.022 

.011 

2.230 

1.868 

.041 

53.9 

45.1 

1.0 

Table 7. Chemical compositions and structural formulas of chromite in the peridotitegabbro suite at the Blashke Islands. 

[Total iron was determined, Fe 2o3 , FeO, Fe3+, and Fe£+ calculated by assuming stoichoimetry] 

Dunite Wehrlite Olivine 

---------------------------- _____ c,;_linopyroxenite 

Sample No. 

Cr 2o3 

Al 2o

3 

Ti02 

Fe 2o3 

FeO 

MnO 

MgO 

Sum 

Cr 

Al 

Mg 

Cr/ R3+ 

Al/ R3+ 

Fe3+; R)+ 

Mg/(Ng+Fe 2+) 

37 

45.8 43.6 

12.0 11.9 

.38 .44 

13.2 15.2 

17.7 18.7 

1. 26 1. 40 

9.93 ~ 

100.27 100.27 

1.190 

.465 

.009 

.327 

.487 

.035 

.487 

.600 

.235 

.165 

.500 

1.141 

.464 

.011 

.380 

.519 

.039 

.445 

.575 

.234 

.191 

.462 

6 

40.2 

11.2 

.57 

18.2 

20.8 

1.29 

7.71 

99.97 

1.070 

.444 

.014 

.460 

.587 

.037 

.387 

.542 

.225 

.233 

.397 

43 10 

38.9 31.6 

11.4 20.0 

.44 .60 

19.9 17.5 

21.2 20.6 

1.27 1.03 

7.72 9.38 

100.83 100.71 

12 

30.1 

18.6 

.96 

19.8 

21.8 

1.23 

8.38 

100.87 

Formula per 3 cations 

1.027 

.449 

.011 

.500 

.592 

.036 

.384 

.520 

.227 

• 2 53 

.393 

0. 795 

.750 

.014 

.420 

.548 

.028 

.445 

.405 

.382 

.214 

.448 

0.767 

.706 

.023 

.480. 

.587 

.034 

.403 

.393 

.361 

.246 

.407 

44 

28.8 

18.7 

.83 

20.5 

22.6 

1.12 

7.85 

100.40 

0.739 

.715 

.020 

.500 

.615 

.031 

.380 

.378 

.366 

.256 

.382 

13 

33.4 

16.4 

.58 

17.4 

24.5 

1.19 

5.76 

99.23 

0.886 

.649 

.015 

.440 

.688 

.034 

.288 

.449 

.329 

.223 

.295 

11 
N 

0 

CJ) 

1-

z 

w 

u 

a: 

UJ 

Cl.. 

1-

I 

~ 

UJ 

3 

54 

52 

50 

48 

• • • • 

•• • • 

••• 

Sub-alkaline 

46~--L---~--~--~--~--~--._~ 

0 2 4 6 8 

WEIGHT PERCENT AI20 3 

Figure 4. Si02 versus Al2o3 in clinopyroxene in the 

Blashke Islands (dots) and Duke Island (circles, Irvine, 

1974) intrusions. Compositional field boundaries 

(dashed lines from LeBas (1962). 

......... ....... Per-alkaline ........ ........ ........ ....... ......... 

12 ......... ........ ........ ....... ....... 

<{ ........ ......... 

...J 

..... ,; ~ 

......................... 

~ 

0: 

0 

u.. 8 

0 Normal alkaline 

0: 

w 

......... 

................. ........ a.. 

::: ........ 

........................... 

........ 

<{ ................ ........ ......... 00 0 

4 

........ • 

•• 

Sub-alkaline 

• 

0~----L-----L-----~----~----~--~ 

0 1 2 3 

WEIGHT PERCENT Ti 0 2 

Figure 5. AliV versus Ti02 in clinopyroxene in the 

Blashke Islands (dots) and Duke Island (circles, Irvine, 

1974) intrusions. Compositional field boundaries 

(dashed lines from LeBas (1962). 

12 

. 0.15 

0.10 

<{ 

.....J 

::J 

~ 

0: 

0 

u.. 

a: 

w 

a... 

1- 0.05 

Tho I e i it i c and 

calc-alkaline 

0.6 0.7 0.8 0.9 

(Ca +Na) PER FORMULA 

1.0 

Figure 6. Ti versus (Ca+Na) in clinopyroxene in the 

Blashke Islands (dots) and Duke Island (circles, Irvine, 

1974) intrusions. Counting frequency curves (contours) 

and compositional field boundary (straight line) are 

from Leterrier and others (1982) • 

Collectively the field, petrographic, and mineral 

chemistry data indicate that the parent magma of the 

Blashke Islands peridotite-gabbro suite was subalkaline 

and significantly dissimilar from the parent magma 

that crystallized the Duke Island, Union Bay, and 

Tulameen ultramafic complexes, and probably other 

Alaskan-type ultramafic complexes. The data also 

suggest, as proposed by Irvine (1974) for Alaskan-type 

complexes in general, that the Blashke Islands peridotite-gabbro suite accumulated as a stratiform body by 

crystallization differentiation in a subvolcanic magma 

chamber and achieved its concentric-zoned structure 

by upward diapiric emplacement. The occurrence of 

Alaskan-type complexes that crystallized from parent 

magmas of different compositions is compatible with 

the occurrence of volcanic rocks in southeastern 

Alaska of subalkaline and alkaline compositions (Brew 

and others, 1966; Brew, 1968; Berg and others, 1972; 

Irvine, 1973; Ford and Brew, 1977) . 

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13 
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14 

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