Prepared in cooperation with the National Park Service

Geochronology of Plutonic Rocks and their Tectonic 

Terranes in Glacier Bay National Park and Preserve, 

Southeast Alaska

U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1776-E
COVER

Mount Fairweather (15,300 ft or 4,663 m) in the far distance beyond the peaks and glaciers that surround (and conceal) 

John Hopkins Inlet. The photo was taken from the complex metamorphosed Paleozoic rocks and Cretaceous plutons 

east of Reid Inlet looking across the Tarr Inlet suture zone and over the dominant metamorphosed Mesozoic rocks of the 

Fairweather Range. Mount Fairweather is underlain by layered gabbroic rocks of Tertiary age.
Geochronology of Plutonic Rocks and their 

Tectonic Terranes in Glacier Bay National 

Park and Preserve, Southeast Alaska

By David A. Brew, Kathleen E. Tellier, Marvin A. Lanphere, Diane C. Nielsen, 

James G. Smith, and Ronald A. Sonnevil

Prepared in cooperation with the National Park Service

Professional Paper 1776–E

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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Suggested citation:

Brew, D.A., Tellier, K., Lanphere, M.A., Nielsen, D.C., Smith, J.G., and Sonnevil, R.A., 2014, Geochronology 

of plutonic rocks and their tectonic terranes in Glacier Bay National Park and Preserve, southeast Alaska 

in Dumoulin, J.A., and Galloway, J.P., eds., Studies by the U.S. Geological Survey in Alaska, 2008–2009: U.S. 

Geological Survey Professional Paper 1776–E, 18 p., http://dx.doi.org/10.3133/pp1776E.

ISSN 2330-7102 (online)
 iii

Contents

Abstract...........................................................................................................................................................1

Acknowledgments.........................................................................................................................................1

Introduction.....................................................................................................................................................1

Subregions and Tectonic Terranes......................................................................................................... 3

Nonplutonic Rocks of the Subregions........................................................................................................6

Plutonic Rocks................................................................................................................................................6

Composition and Geochronology of Rocks in the Plutonic Belts ..........................................................6

Descriptions of the Plutonic Rocks.............................................................................................................6

Rocks of Oligocene Age (33.9 to 23.0 Ma).........................................................................................6

Quartz Monzonite and Quartz Syenite of the Tkope Belt.......................................................6

Rocks of Eocene and Oligocene Age (55.8 to 23.0 Ma).................................................................10

Gabbros of the Crillon-La Perouse Mafic Belt ......................................................................10

Granodiorite, Quartz Diorite, and Granite of the Muir-Fairweather Felsic-Intermediate 

Belt..................................................................................................................................11

Rocks of Eocene Age (55.8 to 33.9 Ma) ...........................................................................................11

Granite and Granodiorite of the Sanak-Baranof Belt ..........................................................11

Rocks of Paleocene Age (65.5 to 55.8 Ma) .....................................................................................11

Tonalite Stock in Johns Hopkins Inlet ....................................................................................11

Rocks of Early Cretaceous Age (145.5 to 99.6 Ma) .......................................................................13

Granodiorite and Tonalite of the Muir-Chichagof Belt.........................................................13

Rocks of Late Jurassic Age (161.0 to 145.5 Ma) ............................................................................13

Leucotonalite in Johns Hopkins Inlet .....................................................................................13

Rocks of Jurassic Age (201.6 to 145.5 Ma) .....................................................................................14

Sheared Diorite and Gabbro of the Lituya Belt.....................................................................14

Discussion.....................................................................................................................................................14

Regional Magmatic Belts ..................................................................................................................14

Distribution and Origin of the Major Plutonic Belts in Glacier Bay National Park ...................15

Conclusions...................................................................................................................................................15

References Cited..........................................................................................................................................15

Figures

1. Map showing locations of tectonic terranes and major faults in and near Glacier Bay 

National Park and Preserve........................................................................................................2

2. Map showing locations of plutonic belts in Glacier Bay National Park and Preserve ..............7

3. Map showing individual plutonic bodies in Glacier Bay National Park and Preserve...............8

4. QAP diagrams showing modal compositions of Tertiary plutonic rocks in Glacier Bay 

National Park and Preserve, southeast Alaska.....................................................................12

5. QAP diagrams showing modal compositions of Cretaceous plutonic rocks in Glacier Bay 

National Park and Preserve, southeast Alaska.....................................................................13
iv 

Tables

1. Compositions and ages of rocks in the major plutonic belts in Glacier Bay National Park 

and Preserve .................................................................................................................................4

2. Potassium-argon ages of plutonic rocks from Glacier Bay National Park and Preserve..........9

3. Summary of major chemical compositional information for intrusive rock suites in Glacier 

Bay National Park and Preserve, southeast Alaska.............................................................10
Geochronology of Plutonic Belts and their Tectonic 

Terranes in Glacier Bay National Park and Preserve, 

Southeast Alaska

By David A. Brew, Kathleen E. Tellier, Marvin A. Lanphere, Diane C. Nielsen, James G. Smith, and Ronald A. 

Sonnevil

Abstract

We have identified six major belts and two nonbelt 

occurrences of plutonic rocks in Glacier Bay National Park 

and Preserve and characterized them on the basis of geologic 

mapping, igneous petrology, geochemistry, and isotopic 

dating. The six plutonic belts and two other occurrences are, 

from oldest to youngest: (1) Jurassic (201.6–145.5 Ma) diorite 

and gabbro of the Lituya belt; (2) Late Jurassic (161.0–145.5 

Ma) leucotonalite in Johns Hopkins Inlet; (3) Early Cretaceous 

(145.5–99.6 Ma) granodiorite and tonalite of the MuirChichagof belt; (4) Paleocene tonalite in Johns Hopkins Inlet 

(65.5–55.8 Ma); (5) Eocene granodiorite of the Sanak-Baranof 

belt; (6) Eocene and Oligocene (55.8–23.0 Ma) granodiorite, 

quartz diorite, and granite of the Muir-Fairweather felsicintermediate belt; (7) Eocene and Oligocene (55.8–23.0 Ma) 

layered gabbros of the Crillon-La Perouse mafic belt; and 

(8) Oligocene (33.9–23.0 Ma) quartz monzonite and quartz 

syenite of the Tkope belt. The rocks are further classified 

into 17 different combination age-compositional units; some 

younger belts are superimposed on older ones. Almost all 

these plutonic rocks are related to Cretaceous and Tertiary 

subduction events.

The six major plutonic belts intrude the three southeast 

Alaska geographic subregions in Glacier Bay National Park and 

Preserve, from west to east: (1) the Coastal Islands, (2) the Tarr 

Inlet Suture Zone (which contains the Border Ranges Fault Zone), 

and (3) the Central Alexander Archipelago. Each subregion 

includes rocks assigned to one or more tectonic terranes.

The various plutonic belts intrude different terranes in 

different subregions. In general, the Early Cretaceous plutons 

intrude rocks of the Alexander and Wrangellia terranes in the 

Central Alexander Archipelago subregion, and the Paleogene 

plutons intrude rocks of the Chugach, Alexander, and 

Wrangellia terranes in the Coastal Islands, Tarr Inlet Suture 

Zone, and Central Alexander Archipelago subregions.

Acknowledgments

B.E. John (University of Wyoming), H.C. Berg (USGS, 

retired), P.J. Haeussler (USGS), and L.J. Patrick Muffler 

(USGS, emeritus) contributed extremely constructive technical 

reviews. J.G. Weathers and S. Mayfield provided graphics 

support. J. Dumoulin was a welcome and effective editor. We 

are most indebted to our 1966 and 1975–1977 field-mapping 

colleagues, as well as to the field geologists who preceded and 

followed them in the study of the park and preserve.

Introduction

This report documents the ages and compositions of the 

plutonic rocks in Glacier Bay National Park and Preserve 

(fig. 1) and notes the subregions and tectonic terranes in 

which they occur. The same rocks and tectonic terranes are 

present in the adjacent areas, but Glacier Bay contains some 

of the best-studied examples. The detailed studies reported 

here were confined to all but the northwesternmost part of 

the park and preserve; additional information provided by G. 

Plafker and D.A. Brew was used to delineate and define the 

plutons in that part. (Only one pluton occurs in the area of 

expansion that belongs to a belt not observed elsewhere in the 

park and preserve.) We note that Glacier Bay Preserve is a 

small area in the northwest. First, we describe the geographic 

subregions and tectonic terranes because they are the hosts 

for the plutonic rocks, followed by the plutonic belts, and, 

briefly, discuss the relations between the two types of features. 

The significant nonplutonic rocks in the park were briefly 

described by Brew (1997).

This report contains the first description of the age 

and compositional characteristics of the plutonic rocks in 

Glacier Bay National Park and Preserve, including the data on 

petrography, geochronology, and chemical compositions, as 

well as an assignment of the rocks to the various subregions 

and tectonic terranes. Earlier studies used preliminary versions 

of some of this information (MacKevett and others, 1971; 

Brew and others, 1978; Brew and Morrell, 1983; Brew, 

1994, 1997). This report is intended primarily to provide new 

compositional and age information to geologists concerned 

with tectonics and magmatism along the northeastern 

Pacific Rim, to informed laypersons visiting the park, and 

to the Glacier Bay National Park and Preserve scientific and 

interpretative staffs.
2 Geochronology of Plutonic Belts and their Tectonic Terranes in Glacier Bay National Park and Preserve

Figure 1. Map showing locations of tectonic terranes and major faults in and near Glacier Bay National Park and Preserve. Dark gray areas indicate plutons that are labeled individually in figure 3. Blue dotted lines show the locations of the Grand Pacific Glacier (north) and the Brady Glacier (south) systems. Thin black dot-dashed line denotes the boundary of Glacier Bay National Park and Preserve. Terranes are outlined with a red dashed line and labeled in red. Contacts are dashed where inferred (underwater). JHI, Johns Hopkins Inlet.

30 KILOMETERS 20 10 0

5 10 15 MILES

58°15’

58°30’

58°45’

137°30’ 137° 136°30’ 136° 135°30’ 135°

59°15’

59°00’

138°

ALASKA

Map Area

CANADA

CANADA

UNITED STATES

STATES

UNITED 

Mount Fairweather

Mount Root

MountCrillon

Mount LaPerouse

Astrolabe Peninsula

Fairweather Range

Takhinsha Range

Chilkat Range

Glacier Bay Lituya Bay

GLACIER BAY NATIONAL PARK BOUNDARY

GLACIER BAY

NATIONAL 

PRESERVE

C E N T R A L A L E X A N D E R A R C H I P E L A G O S U B R E G I O N

L Y N N C A N A L - C H AT H A M S T R A I TL Y N N C A N A L - C H AT H A M S T R A I T

C O A S T A L I S L A N D S S U B R E G I O N

FAIRWEATHER - QUEEN CHARLOTTE FAULT

F A U L T Z O N EF A U L T Z O N E

BORDER RANGES FAULT

CHUGACH TERRANE

ALEXANDER 

TERRANE

JHI

ALEXANDER TERRANE

ALEXANDER TERRANE

WRANGELLIA TERRANE/

ALEXANDER TERRANE

WRANGELLIA TERRANE ANDTARR INLET SUTURE ZONE

WRANGELLIA 

TERRANE

Terrane

Boundary

YAKUTAT  TERRANE

Fairweather Range
Subregions and Tectonic Terranes 3

Glacier Bay National Monument was established in 1925 

and elevated to Glacier Bay National Park and Preserve in 

1980 as part of the Alaska National Interest Lands Act. The 

studies described here began in 1966 in anticipation that the 

monument might later be designated in part as wilderness 

area; the studies in 1975 to 1977 were done as the monument 

was being further evaluated for wilderness status. All studies 

were in full cooperation with the National Park Service.

Specifically, the studies that are the basis for these 

descriptions were conducted primarily during the complete 

field seasons of 1966, 1975, 1976, and 1977 and secondarily 

during subsequent, topically oriented investigations. Samples 

and measurements were collected at several thousand localities 

in the park, using utility runabouts for shoreline traverses and 

contract helicopters for higher-elevation spot landings and 

traverses. Rocks, stream sediment, and mineralized material 

were analyzed for trace-, minor-, major-element contents, and 

(in some samples) isotopic constituents. Several short drillcore samples were collected for paleomagnetic studies, with 

indeterminate results (Grommé, 1998). The regional geologic 

and mineral-resource information acquired was used in an 

earlier evaluation of undiscovered mineral resources in the 

monument (MacKevett and others, 1971; Brew and others, 

1978, 1991). Marvin A. Lanphere of the U.S. Geological 

Survey (USGS) provided all of the previously unpublished 

K-Ar-isotopic ages.

Subregions and Tectonic Terranes

The nonplutonic rocks in Glacier Bay National Park and 

Preserve (fig. 1) have been classified into both formally named 

and unnamed formations and groups, as well as into tectonic 

terranes (Brew, 1997, 2001) and into geographic subregions, 

most of which extend the length of southeast Alaska and 

include one or more terranes. The formations and groups in the 

Coastal Islands subregion (CIS) west of the Fairweather-Queen 

Charlotte Fault Zone (F-QCFZ) are assigned to the Yakutat 

terrane, those in the same subregion between the F-QCFZ 

and the Tarr Inlet Suture Zone (TISZ) to the Chugach terrane, 

those in the TISZ mostly to the Wrangellia terrane, and those 

eastward from the TISZ to the Lynn Canal-Chatham Strait 

Fault Zone (LC-CSFZ) to the Alexander terrane as part of the 

Central Alexander Archipelago subregion (CAAS). Rocks of the 

Wrangellia terrane also occur on older rocks of the Alexander 

terrane in the northeastern part of the park (fig. 1). All of these 

terranes and subregions belong to the Insular Superterrane 

of Monger and others (1982) that adjoins the Intermontane 

Superterrane along the crustal-scale Insular-Intermontane Suture 

Zone (IISZ), which lies well east of the park against the base of 

the Coast Mountains (outside area of fig. 1). The above terrane 

assignments are those of Brew (2001).

Almost all the plate-tectonic movements that juxtaposed 

both the terranes and superterranes occurred in Mesozoic time, 

from 200 to 70 Ma. These movements predated some, but not 

all, of the magmatic/plutonic events that resulted in the granitic 

and gabbroic intrusions that are now exposed at the surface in 

Glacier Bay National Park and Preserve. Current tectonic and 

seismic activity in Glacier Bay National Park and Preserve 

has no associated magmatism, but consists of right-lateral 

movements in the F-QCFZ, northward-convergent movements in 

the offshore Transition Fault (Plafker, 1987), and northeastwardconvergent movements at depth (without surface expression) in 

an east-northeast-trending zone across the northern part of the 

park and preserve (Brew and others, 1995). The well-known 

rapid postglacial uplift of Glacier Bay (Barnes, 1984; Brew and 

others, 1995; Larsen and others, 2004, 2005; Motyka and others, 

2007; Elliott and others, 2010) may be partly related to these 

cryptic movements, as well as to isostatic rebound.

The sequence of assembly of the tectonic terranes into 

their present configuration is generally interpreted to have 

been as follows, from west to east:

1. The Yakutat terrane was emplaced by right-lateral 

strike-slip movement on a precursor of the F-QCFZ. 

This terrane must have been at its present position 

before late Tertiary time because its younger formations 

contain Tertiary mafic-plutonic detritus eroded from the 

Chugach terrane adjoining it immediately to the east. 

The northward movement occurred after mid-Cretaceous 

time, as indicated by the presence in older parts of the 

Yakutat terrane of detritus that is derived from coeval 

granitic bodies that occur east of the F- QCFZ and to the 

south in British Columbia (Plafker, 1987).

2. The Chugach terrane was accreted to the Alexander terrane 

by subduction the mid- Cretaceous time. The subduction zone is preserved in the TISZ, which contains both 

deformed and undeformed Early Cretaceous plutons. Brew 

and Morrell (1978) interpreted juxtaposition of these two 

terranes to have occurred in two stages, with the leading 

edge of the first advancing terrane to now be missing.

3. The rocks of the Wrangellia and Chugach terranes 

that occur in the TISZ as fault- bounded slivers were 

emplaced by strike-slip movements whose sense is 

uncertain. A Jurassic pluton in the TISZ could be related 

to a coeval body in the St. Elias Mountains to the north 

on the west side of the TISZ (Hudson, 1979), suggesting 

that the movement was right-lateral. The rocks in the 

TISZ were also deformed later by apparent right-lateral 

movements on the Border Ranges Fault, which forms 

the east boundary of the TISZ (Smart and others, 1996).

4. The Border Ranges Fault forms the boundary between 

deformed rocks of the TISZ and the generally less 

deformed rocks of the Alexander and Wrangellia terranes to the east. The Ordovician and Silurian rocks in 

the lower part of the Alexander terrane are regionally 

deformed, but the Devonian and younger rocks are 

interpreted to be part of a coherent, less deformed, but 

vertical-fault-disrupted, stratigraphic succession with 

unconformities.
4 Geochronology of Plutonic Belts and their Tectonic Terranes in Glacier Bay National Park and Preserve

Table 1. Compositions and ages of rocks in the major plutonic belts in Glacier Bay National Park and Preserve. 

[CIS, Coastal Islands subregion; TISZ, Tarr Inlet Suture Zone; CAAS, Central Alexander Archipelago subregion; B, biotite; H, hornblende; M, muscovite. Age 

ranges are from the Geological Society of America (2009)]

Major composition of rocks 

in belt

Map label

on figure 3

Other compositional

types Basis of age assignment

Body 

numbers

on figure 3

Subregion and 

terrane locations

ROCKS OF OLIGOCENE AGE (33.9 to 23.0 Ma)

Quartz monzonites and quartz syenites of the Tkope belt

Andesite stocks Tsan None Geologic inference 37, 59 CAAS, Alexander

Hornblende quartz monzonite Tgqm None Geologic inference 56 CAAS, Alexander

Porphyritic quartz syenite Tgqp Pyroxene-quartz syenite, alkali 

granite

Geologic inference and cuts 

the Tggd unit

44, 50 CAAS, Alexander

ROCKS OF EOCENE AND OLIGOCENE AGE (55.8 to 23.0 Ma)

Layered gabbros of the Crillon-La Perouse mafic belt

Pyroxene gabbro Tgbp Olivine gabbro, norite, gabbronorite, 

troctolite, anorthosite

40Ar/39Ar age: 28.0±8.0 Ma 

(Loney and Himmelberg, 

1983)

3, 11 CIS, Chugach

Magnetite-bearing

pyroxene gabbro

Tgbm Gabbronorite, norite, anorthosite, 

pyroxenite, dunite

14, 17, 18 CIS, Chugach

Hornblende and pyroxene 

gabbro

Tgbh None 6 CIS, Chugach

Pyroxene-hornblende gabbro Tgbg Diabase 35, 46, 57, 

61

CAAS, Alexander

ROCKS OF EOCENE AND OLIGOCENE AGE (55.8 to 23.0 Ma)—Continued

Granodiorite, quartz diorite, and granite of the Fairweather-Muir felsic-intermediate belt

Biotite granodiorite Tggd Hornblende-biotite granodiorite 44.4±1.3 Ma (B)

40.8±1.2 Ma (H)

40.2±1.2 Ma (B)

37.5±1.1 Ma (H)

37.3±1.1 Ma (B)

30.7±0.9 Ma (B)

2, 8, 10, 43, 

45, 52

CIS, Yakutat, 

Chugach, CAAS, 

Alexander

Garnet-biotite granodiorite Tggg None 12, 13, 15, 

19, 20, 21

CIS, Chugach

Biotite-hornblende quartz 

diorite

Hornblende-biotite granite

Tggg

Tggr

Biotite-hornblende diorite

Biotite-hornblende tonalite

Hornblende-biotite, granodiorite, 

tonalite

K-Ar age:.

1±0.8 Ma (B) 

(Table 2)

K-Ar ages:

34.3±1.0 Ma (B)

32.4±1.0 Ma (B)

27.6±0.8 Ma (B)

 (Table 2)

40Ar/39Ar age:

33.1±0.3 Ma (B)

(Smart and others, 1996)

4, 5, 7, 16

9, 22, 26, 

27, 30, 33, 

34, 42

CIS, Chugach

CIS, Chugach, 

CAAS, Alexander, 

TISZ, Wrangellia

ROCKS OF EOCENE AGE (55.8 to 33.9 Ma)

Granite and granodiorite of the Sanak-Baranof belt

Muscovite-biotite granite Tegr Muscovite-biotite granodiorite K-Ar age in mid-50s (G. 

Plafker, unpub. data)

None CIS
Subregions and Tectonic Terranes 5

Table 1. Compositions and ages of rocks in the major plutonic belts in Glacier Bay National Park and Preserve.—Continued

Major composition of rocks 

in belt

Map label

on figure 3

Other compositional

types Basis of age assignment

Body 

numbers

on figure 3

Subregion and 

terrane locations

ROCKS OF PALEOCENE AGE (65.5 to 55.8 Ma)

Tonalite stock at Johns Hopkins Inlet 

Hornblende-bearing biotite 

zone tonalite

Tgto Quartz diorite K-Ar age:

58.6±5.2 Ma (H)

(Decker and Plafker, 1982)

64 Tarr Inlet Suture

ROCKS OF EARLY CRETACEOUS AGE (145.5 to 99.6 Ma)

Granodiorite and tonalite of the Muir-Chichagof belt

Biotite-hornblende grandiorite Kggd Biotite-hornblende quartz, diorite, 

quartz mono-diorite, tonalite, granite, 

diorite

115±3.4 Ma (B)

111±3.0 Ma (B)

107±3.2 Ma (H)

(Table 2)

28, 36, 38, 

39, 40, 41, 

47, 48, 49, 

54, 58, 60, 

62

CAAS, Alexander

Biotite-hornblende zone 

tonalite

Kgto Biotite-hornblende grandiorite K-Ar ages:

 119±8 Ma (H)

116±3.5 Ma (H)

110±3.3 Ma (H)

 96.7±5.5 Ma (H)

(Table 2)

40Ar/39Ar ages:

123.3±0.4 Ma (H)

(Smart and others, 1996)

120.8±0.6 Ma (H)

(Roeske and n others, 1992)

3, 24, 25, 

29, 31,3 2, 

51, 5

Tarr Inset Suture 

Zone, CAAS, 

Alexander

ROCKS OF LATE JURASSIC AGE (161.0 to 145.5 Ma)

Leucotonalite in Johns Hopkins Inlet

Muscovite-bearing zone 

leucotonalite

Jgto Muscovite-bearing trondjhemite 40Ar/39Ar ages: Zone

162±0.4 Ma (M)

(Roeske and others, 1992)

152.7±0.4 Ma (M)

(Smart and others, 1996)

63 Tarr Inlet Suture

ROCKS OFJURASSIC OR LATE JURASSIC (?) AGE (201.6 to 145.5 Ma)

Sheared diorite and gabbro of the Lituya belt

Hornblende diorite Jgdi Hornblende quartz diorite, hornblende quartz monzo-diorite, hornblende tonalite hornblende gabbroGeologic inference; similarity to dated units to south 

near Sitka, Alaska

1 CIS, Yakutat
6 Geochronology of Plutonic Belts and their Tectonic Terranes in Glacier Bay National Park and Preserve

with the numbers used to identify them in the tables and text. 

Almost all of these outlines represent intrusive contacts; 

however, some of the westernmost bodies on the map (Nos. 

1 and 2, fig.3) and those in the TISZ (Nos. 30, 31, 63, 64, 

and part of No. 23) are bounded by faults that are too small 

to show on in figure 3. The country rocks include a variety 

of sedimentary, volcanic, and metamorphic rocks ranging in 

original age from Ordovician through Tertiary. Most of the 

extensive metamorphism is related to the Cretaceous intrusive 

rocks, and broad outcrop areas of hornfels are present, 

especially in the Alexander terrane (Seitz, 1959; Brew and 

others, 1992; Dusel-Bacon and others, 1996).

As noted above, the plutonic rocks are classified into 

17 different age- compositional units, using the geological 

time scale of Walker and Geissman (2009) and the modal 

classification scheme of Streckeisen (2002). As summarized in 

table 1, the rocks range in age from Jurassic through Oligocene 

and in composition from gabbro to granite and syenite. We 

discuss eight different age groups below, from youngest to 

oldest. The new age data presented in this report, together 

with the supporting data, are listed in table 2. The available 

major-chemical data are summarized in table 3, and modalcompositional data for some of the major plutonic suites are 

plotted in figure 4 and 5. With the exception of the Eocene and 

Oligocene gabbros of the Crillon-La Perouse belt, the younger 

rocks are generally more silicic than the older ones. 

Descriptions of the Plutonic Rocks

The descriptions given below summarize the main 

characteristics of the rocks in the various plutonic belts.

Rocks of Oligocene Age (33.9 to 23.0 Ma)

Quartz Monzonite and Quartz Syenite of the 

Tkope Belt

The Oligocene age assignment of the quartz monzonite 

and quartz syenite of the Tkope belt (figs. 2 and 3) is based 

on the lithologic similarity of these rocks to those dated at 

33–23 Ma to the north across the United States-Canada border 

(MacKevett and others, 1974; Dodds and Campbell, 1988). 

The hornblende-quartz monzonite (unit Tgqm, fig. 3) 

occurs in one small stock of faintly magmatically foliated, 

slightly porphyritic fine- to medium-grained hornblende-quartz 

monzonite that has associated gold and copper mineralization. 

Neither age nor chemical data are available for this 

assignment or for that of the following unit (tables 1, 3). 

A single stained-slab mode shows the sample to be quartz 

monzonite.

The porphyritic quartz syenite (unit Tgqp, figs. 3, 

4A) includes two slightly different groups of plutons: one 

Nonplutonic Rocks of the Subregions

Brew (1997) and D.A. Brew and G. Plafker (unpublished 

data, 2002) described the nonplutonic rocks of the various 

subregions and tectonic terranes in greater detail than is 

appropriate here. Briefly, in the Coastal Islands subregion, the 

Yakutat terrane consists mostly of Jurassic through Tertiary 

metasedimentary and metavolcanic rocks, and the Chugach 

terrane of Jurassic and Cretaceous metasedimentary rocks. The 

TISZ consists of Triassic metasedimentary and metavolcanic 

rocks, and the Alexander terrane in the Central Alexander 

Archipelago subregion of Ordovician through Triassic low- to 

high-grade metasedimentary and metavolcanic rocks. Fossils 

are generally rare except in parts of the Tertiary section in the 

Yakutat terrane and in Permian and some Devonian rocks of 

the Alexander terrane.

Plutonic Rocks

The granitic and gabbroic rocks of Glacier Bay National 

Park are classified into six plutonic belts, from west to east: the 

Sanak-Baranof, Lituya, Crillon-La Perouse, Muir- Fairweather, 

Muir-Chichagof, and Tkope belts; other areas of rocks are 

assigned to those belts, but are geographically distant from 

them (fig. 2), and two areas of nonbelt rocks (both in the Johns 

Hopkins Inlet area) are known. The three main plutonic belts, 

the Crillon-La Perouse, Muir-Fairweather, and Muir-Chichagof, 

trend generally north to south; all of these belts are interpreted 

to be related to Cretaceous and Tertiary subduction events. The 

plutonic rocks are classified into 17 different age-compositional 

units (fig. 3) ranging in age from Jurassic (201.6 Ma), through 

Cretaceous (ca.145.5–65.5 Ma), to Tertiary (65.5–23.0 Ma; 

Geological Society of America, 2009) and in composition from 

syenite to gabbro (table 1) on the basis of modal analyses of 

specimens, some of which are shown in figures 4 and 5.

In general, the Early Cretaceous plutons intrude rocks 

of the Alexander and Wrangellia terranes, and the Paleogene 

plutons intrude rocks of the Yakutat, Chugach, Alexander, 

and Wrangellia terranes. The plutonic age-compositional 

distribution patterns are approximately beltlike, and 

some belts of different age overlap. Also, in general, the 

compositions of Early Cretaceous plutons are dominantly 

foliated tonalite, quartz diorite, diorite, and granodiorite; 

those of Paleogene plutons vary, including gabbro, 

granodiorite, and minor granite; and those of Oligocene 

plutons are dominantly quartz syenite and quartz monzonite.

Composition and Geochronology of 

Rocks in the Plutonic Belts

The outlines of the intrusive bodies in Glacier Bay 

National Park and Preserve are shown on figure 3, together 
Composition and Geochronology of Rocks in the Plutonic Belts 7 30 KILOMETERS 20 10 0

5 10 15 MILES

58°15’

58°30’

59°00’

58°45’

59°15’

138° 137°30’ 137° 136°30’ 136° 135°30’ 135°

ALASKA

Map Area

LITUYA BELT

MUIR-FAIRWEATHER BELT

CRILLON-LA PEROUSE BELT

MUIR-CHICHAGOF BELT

MUIR-CHICHAGOF BELT

MUIR-CHICHAGOF BELT

MUIRFAIRWEATHER BELT

MUIRFAIRWEATHER 

BELT

MUIRCHICHAGOFBELT

MUIRCHICHAGOF

BELT

TKOPE BELT

T

KO

PE B

ELT

TKOPE BELT

SANAK-BARANOF BELT 

MUIR-FAIRWEATHER BELT

CANADA

UNITED STATES

Figure 2. Map showing locations of plutonic belts in Glacier Bay National Park and Preserve. The blue doted lines show the locations of the Grand Pacific (north) and the Brady Glaciers (south) systems. The thin black dot-dashed line denotes the boundary of Glacier Bay National Park and Preserve.
8 Geochronology of Plutonic Belts and their Tectonic Terranes in Glacier Bay National Park and Preserve

Figure 3. Map showing individual plutonic bodies in Glacier Bay National Park and Preserve. Black numbers correspond to individual plutons identified in the text; numbers in red show locations of the U.S. Geological Survey K-Ar ages listed in table 2. Labeled, but unnumbered, plutons in northwestern part of map area are those for which we lack detailed information. Blue dotted lines show the locations of the Grand Pacific (north) and the Brady Glaciers (south) systems. Thin black dot-dashed line is the boundary of Glacier Bay National Park and Preserve.

30 KILOMETERS 20 10 0

5 10 15 MILES

12

13

22

15 23

25

26

27

23 25

28

29

30

5

2

1

4

3

7

6

11

9

9

64 63 32

32

34 35

36

37

36

41

47

46

48

55

55

62

57

56

59

60

42

43

39

39

38

40

44

45

61

36

58

50

51

54 NUN

MTN

BODY

PORPOISE ISLAND

BODY

52

49

33

31

31 10

31 8

24

24

14

16

18

20

21

19

17

9

6

11

10

12

5

18

8

2

1615

4

21

13

17

14

7

19

21

13

17

14

7 3 20

1

19

9 6

11

10

12

5

18

8

2

1615

4

21

13

17

14

7

19

21

13

17

14

7 3 20

1

19

9 6

11

10

12

5

18

8

2

1615

4

21

13

17

14

7

19

21

13

17

14

7 3 20

1

19

58°15’

58°30’

59°00’

58°45’

137°30’ 137° 136°30’ 136° 135°30’ 135°

59°15’

138°

ALASKA

Map Area

CANADA

STATES

UNITED 

Mt. Fairweather

Mt. Root

Mt.Crillon

Mt. LaPerouse

Astrolabe Peninsula

Fairweather Range

Takhinsha Range

Chilkat Range

Glacier Bay Lituya Bay

L Y N N C A N A L - C H AT H A M S T R A I T

L Y N N C A N A L - C H AT H A M S T R A I T

FAIRWEATHER - QUEEN CHARLOTTE FAULT

F A U L T Z O N EF A U L T Z O N E

BORDER RANGES FAULT

Fairweather Range

CANADA

UNITED STATES

Tsan

Tsan

Tggd Tggr

Tsan

Tggd

Tggd

Tggd

Tggd

Tggd

Tggd

Tggd

Tggd

Tggd

Tggd

Tggd

Tggd

Tggr

Tggr

Tggr

Tggr

Tggr

Tggr

Tggr

Tggd

Tggg

Tggg

Tggg

Tggg

Tggg

Tgqd

Tgqd

Tggr

Tgbp

Tgbm

Tgbm

Tgbm

Tgqd

Tgbm

TgbhTgbg

Tgbg

Tgbg

Tgbg

Tgbg

Tegr

Tgto

Tegr

Kggd

Kggd

Kggd

Kggd

Kggd

Kggd

Kggd

Kggd

Kggd Kggd

Kggd

Kggd

Kggd

Kggd

Kggd

Kggd

Kggd Kggd

Kggd

Kggd

Kggd

Kgto

Kgto

Kgto Kgto

Kgto

Kgto

Kgto Kgto

Kgto

Kgto

Tgbp

Tgbp

Tgbh

Kgto

Kgto

Kgto

Kgto Kgto

Kgto

Tgqm Kgto

Kgto

Jgto

Jgto

Tgto

Jgdi

Jgdi

Tgqm

Tgqp

Tgqp

Tgqp
Composition and Geochronology of Rocks in the Plutonic Belts 9

Map 

no.

 Body 

no.

Map 

unit Sample no. Latitude Longitude Mineral, %K2

O

Rock type

Average

%K2

O

40Arrad

(moles/gm)

40Arrad

40Artot

Age

(Ma)

1 36 Kggd 66ABd272A 58°56'54" 136°22'54" Hornblende, 0.623, 0.626

Sphene-biotite-hornblende

grandiorite

0.6245 1.035 x 10-10 0.72 112±3.4

2 36 Kggd 66ABd147 58°52'18" 136°03'48" Hornblende, 0.635, 0.636 

Sphene-biotite-hornblende

grandiorite

0.6355 5.752 x 10-10 ? 1 0.73?1 109±3.3

3 36 Kggd 75KB025A 58°59'15" 135°24'42" Hornblende, 0.654, 0.651

Sphene-biotite-hornblende

grandiorite

0.6525 1.041 x 10-10 0.35 107±3.2

4 40 Kggd 66ABd297B 59°11'15" 136°20'06" Biotite, 7.14, 7.24

Hornblende, 0.739, 0.741

Biotite-hornblende grandiorite

7.19

0.740

1.224 x 10-9

1.237 x10-10

0.71

0.23

115±3.4 

113±3.4

5 23 Kgto 66ABd632B 58°21'51" 136°22'45" Hornblende, 1.00, 1.01

Biotite-hornblende tonalite

1.005 1.649 x 10-10 0.83 110±3.3

6 25 Kgto 75CN121A 58°30'22" 136°39'39" Hornblende, 0.196, 0.198

Biotite-hornblende tonalite

0.197 1.545 x 10-11 0.39 53.7±1.6

7 32 Kgto 75BJ121A 58°58'03" 136°52'34" Hornblende, 0.786, 0.790 

Pyroxene-biotite-hornblende

quartz diorite

0.788 1.279 x 10-10 0.81 109±3.3

8 55 Kgto 66ABd359B 58°40'18" 135°54'00" Hornblende, 0.500, 0.505

Biotite-hornblende quartz

diorite

0.5025 8.703 x 10-11 0.79 116±3.5

9 13 Tggg 76DB147A 58°26'09" 136°56'57" Biotite, 8.24, 8.29

Muscovite, 8.33, 8.47

Garnet-muscovite-biotite

grandiorite

8.26

8.40

8.485 x 10-11

2.022 x 10-10

0.23

0.66

7.12±0.2 

16.6±0.5

10 15 Tggg 66AFd458 58°37'33" 136°53'27" Biotite, 7.63, 7.78

Muscovite, 9.96, 9.98

Muscovite-biotite granite

7.705

9.97

3.618 x 10-10

5.719 x 10-10

0.55

0.69

32.3±1.0

39.4±1.2

11 21 Tggg 76DB123A 58°20'15" 136°40'18" Biotite, 8.06, 7.97

Muscovite-garnet-biotite

tonalite

8.015 4.456 x 10-10 0.38 38.2±1.1

12 21 Tggg 76CN034A 58°17'56" 136°39'48" Biotite, 8.99, 9.12

Garnet-biotite grandiorite

9.055 4.007 x 1010 0.60 30.5±0.9

13 8 Tggd 66ABd699 58°50'10" 137°08'00" Biotite, 1 analysis

Hornblende, 0.615, 0.625

Magnetite-biotite-hornblende

monzonite

9.00

0.620

4.008 x 10-10

3.684 x 10-11

0.70

0.52

30.7±0.9 

40.8±1.2

14 10 Tggd 77BJ038A 59°02'25" 137°07'22" Biotite, 6.60, 6.63

Magnetite-biotite-hornblende

tonalite

5.97 3.865 x 10-10

2.649 x 10-11

0.43

0.34

44.4±1.3 

37.5±1.1

15 45 Tggd 75BJ105A 58°59'38" 135°55'37" Biotite, 6.86, 6.90

Chlorite-biotite-hornblende

grandiorite

6.615 3.592 x 10-10 0.61 37.3±1.1

16 45 Tggd 66ABd353 59°01'57" 135°57'21" Biotite 6.86, 6.90

Chlorite-biotite-hornblende

grandiorite

6.88 4.022 x 10-10 0.55 40.2±1.2

17 9 Tggr 66AFd404 58°52'21" 137°02'54" Biotite 8.52, 8.62

Hornblende-biotite granite

8.57 4.031 x 10-10 0.84 32.4±1.0

18 22 Tggr 66ABd564B 58°26'54" 136°31'21" Biotite 6.75, 6.77

Hornblende-biotite granite

6.76 3.369 x 10-10 0.80 34.3±1.0

19 33 Tggd 66AOv1442 59°01'52" 136°51'24" Biotite 7.28, 7.34

Hornblende-biotite granite

7.31 2.923 x 10-10 0.80 27.6±0.8

20 46 Tbgb 76DB330G 58°59'37" 136°24'34" Hornblende 0.172, 0.173

Hornblende gabbro

0.1725 9.449 x 10-12 0.33 37.6±1.1

21 7 Tggd 77BJ029A 58°53'27" 137°21'58" Biotite, 9.22, 9.23 Hornblende-biotite grandiorite9.225 3.768 x 10-10 0.60 28.1±0.8

1

The age of sample represented by map number 2 is considered to be correct, but the data given are incorrect; the original data have been lost, so the correction cannot be 

made. The constants used for all of these determinations have been retrospectively calculated by F.H. Wilson (U.S. Geological Survey) and found to be those of Steiger and 

Jäger (1977).

Table 2. Potassium-argon ages of plutonic rocks from Glacier Bay National Park and Preserve. See figure 3 for locations and table 1 for summary. 

[Ages calculated with constants in use at the time of analysis (1967-1978). All analyses by M.A. Lanphere, U.S. Geological Survey]
10 Geochronology of Plutonic Belts and their Tectonic Terranes in Glacier Bay National Park and Preserve

subunit that contains no pyroxene and has potassium feldspar 

phenocrysts that are perthitic, and another that contains 

pyroxene and plagioclase-feldspar phenocrysts that are 

extensively replaced and rimmed by potassium feldspar. 

Otherwise, the two subunits are similar. The pyroxene-free 

subunit consists of porphyritic, alkali feldspar-quartz syenite 

to quartz syenite, is greenish-gray to pink, and has a color 

index (mafic-mineral content) of 1 to 3. Two modes plot as 

alkali feldspar granite and as quartz syenite. One chemical 

analysis is available for this unit (table 3), but no isotopic-age 

data are available from within the Glacier Bay region. 

A third unit included here ofhornblende andesite (Tsan, 

fig. 3) that occurs as intrusive stocks and dikes consists of gray 

or greenish-gray porphyritic andesite.

Rocks of Eocene and Oligocene Age (55.8 to 23.0 

Ma)

Gabbros of the Crillon-La Perouse Mafic Belt

All of the gabbros of the Crillon-La Perouse mafic belt 

(figs. 2, 3) are conspicuously layered at a scale of one to tens 

of centimeters, and several gabbros include cumulate phases. 

The pyroxene gabbro (unit Tgbp, fig. 3) comprises two 

bodies in the FairweatherRange: one at Mount Fairweather

Unit name Map symbol(s) # of 

samples

Chemical

classification

SiO2

 range or 

value,

in percent

SiO2

 gap(s),

in percent

Intrusive rocks of Oligocene age

Rocks of the Tkope plutonic belt Tgqp 1 Alkalic, 70.1

peraluminous

NA NA

Intrusive rocks of Oligocene age

Rocks of the Fairweather-felsic-intermediate plutonic belt Tggd, Tggg, Tggr, Tgqd 13 Calc-alkalic,

subaluminous

57.5–75.3 60.2–65.2

Rocks of Eocene age

Rocks of the Crillon-La Perouse mafic plutonic belt Tgbp, Tgbg 30 Calc-alkalic,

subaluminous

38.8–51.2 None

Intrusive rocks of Paleocene age

Tonalite stock on south side of Johns Hopkins Inlet Tgto 0 No data No data No data

Intrusive rocks of Early Cretaceous age

Intrusive rocks of the Muir-Chichagof belt Kggd, Kgto 9 Calc-alkalic,

metaluminous

43.8–66.8 43.8–50.3

Intrusive rocks of Late Jurassic age

Leucotonalite stock Jgto 0 No data No data No data

Intrusive rocks of Early Jurassic age

Sheared intrusive rocks west of Fairweather Fault Jgdi 0 No data No data No data

(body 3, fig. 3) and another at Mounts La Perouse and

Crillon (body 11), the first of which was studied by Plafker 

and MacKevett (1970) and E. Redman (written commun., 

1985) and the second by Himmelberg and Loney (1981) 

and Loney and Himmelberg (1983). The most common 

rock types in these two bodies are olivine gabbro, norite, 

gabbronorite, troctolite, and anorthosite. Ultramafic rocks 

are rare except in the basal part of the La Perouse body, 

where peridotite, feldspathic peridotite, olivine gabbro, 

harzburgite, feldspathic harzburgite, gabbro, troctolite, and 

anorthosite occur as cumulates. Chemically, the rocks are 

subaluminous and calc-alkalic. Sulfides and ilmenite are 

locally concentrated at the contact of the La Perouse-Crillon 

body with hornfelsed Cretaceous graywacke country rock; 

one such concentration, the Brady Glacier nickel-copper 

deposit, is a world-class nickel resource (Brew and others, 

1978; Himmelberg and Loney, 1981). The age data (table 

1) are from the report by Loney and Himmelberg (1983), 

who evaluated all the earlier dating efforts, including those 

of Hudson and Plafker (1980); the chemical analyses (table 

3) are from the reports by Plafker and MacKevett (1970) 

and Himmelberg and Loney (1981). No detailed modal 

information is available.

Magnetite-bearing pyroxene gabbro (unit Tgbm, fig. 3) 

occurs in three bodies south of the La Perouse-Mt. Crillon 

body. The most detailed study was by Rossman (1963). 

The bodies consist of light-gray to very dark gray, fine- to 

Table 3. Summary of major chemical compositional information for intrusive rock suites in Glacier Bay National Park and Preserve, 

southeast Alaska. 

[Data is from Plafker and MacKevett (1970), Himmelberg and Loney, (1981), and D.A. Brew (unpub. data)] 
Descriptions of the Plutonic Rocks 11

coarse-grained, magnetite-bearing two-pyroxene gabbro 

interlayered with gabbronorite, norite, anorthosite, pyroxenite, 

and dunite. The body on the Astrolabe Peninsula (body 18, 

fig. 3) is well exposed and readily accessible; the others 

(bodies 14, 17) are small and poorly exposed. The chemical 

analyses (table 3) are from Rossman (1963). No detailed 

modal information in available.

The hornblende gabbro and pyroxene gabbro(unit 

Tgbh, fig. 3) consists of two poorly known bodies of mediumto very coarse-grained hornblende gabbro and pyroxene 

gabbro. Both bodies are in the Fairweather Range: one at high 

elevation at Mount Orville that is almost inaccessible (body 

6, fig. 3) and the other at low elevation but small and poorly 

exposed (body 17, fig 3). The rocks are brownish-gray to 

green and have a color index of 20 to 80.

The pyroxene-hornblende gabbro (unit Tgbg, fig. 3) 

comprises dikes and small stocks in the vicinity of Muir Inlet 

and in the Chilkat Range that consist of fine-grained pyroxenehornblende gabbro and diabase. The rocks are generally 

medium gray to dark brown and have a color index of 40 to 

90. Chemically, a sample (table 3) is similar to samples of the 

two-pyroxene gabbro (unit Tgbp). Although these rocks are 

distant from the main part of the Crillon-La Perouse mafic 

belt, they are similar in general composition and structure to 

the magnetite- bearing pyroxene gabbro (unit Tgbm, fig. 3) 

described above and probably do not represent a separate belt 

of gabbroic rocks.

The rocks in the Crillon-La Perouse mafic belt are reported 

to have an 40Ar/39Ar age of 28.0±8.0 Ma (tables 1, 2).

Granodiorite, Quartz Diorite, and Granite of the 

Muir-Fairweather Felsic-Intermediate Belt

The granodiorite, quartz diorite, and granite of the 

Muir-Fairweather felsic-intermediate belt comprises three 

subunits: (1) the biotite granodiorite (unit Tggd) consists of 

six bodies (table 1) spread across the map area, composed 

of nonfoliated light- to medium-gray, hornblende-biotite 

granodiorite, tonalite, and quartz monzodiorite with color a 

index of 6 to 24 (bodies 43, 45, 52); (2) slightly foliated very 

light red (pink) to gray biotite-hornblende granodiorite quartz 

diorite, tonalite, and monzodiorite form a second subunit 

(bodies 8, 10); and (3) locally foliated, medium- to coarsegrained, very light gray to light-gray hornblende-biotite 

granodiorite having a color index of 10 to 18 and containing 

inclusions of diorite, amphibolite, granodiorite, and tonalite 

(body 2). All of the foliations present are interpreted here to 

be subsolidus features. All modal analyses plot in or close to 

the granodiorite field (fig. 4B).

The garnet-biotite granodiorite (unit Tggg, fig. 3) 

consists of six plutons clustered in the southern part of the 

Fairweather Range (table 1, fig. 5A). The plutons intrude 

rocks of the Chugach terrane, and all locally have weak 

subsolidus foliation. The rocks are generally medium- to 

coarse-grained and contain biotite and garnet; muscovite and 

hornblende are locally present. Modal compositions of the 

various bodies plot in the granodiorite and quartz diorite fields 

(fig. 4C).

Three plutonic bodies in the northern part of the 

Fairweather Range (figs. 1, 3) and one plutonic body (No. 

16, fig. 3) in the southern part are assigned to the biotitehornblende-quartz diorite (unit Tgqd; table 1). Hornblende 

and biotite generally occur in approximately equal 

proportions, and the rocks have a color index averaging 30. 

Stained-slab-counted samples range from tonalite to diorite 

(fig. 4D).

The hornblende-biotite granite (unit Tggr, fig. 3) 

comprises six bodies (table 1, fig 6) in the Alexander terrane, 

one in the Chugach terrane just west of the Tarr Inlet Suture 

Zone (body 9, fig. 3), and one in that zone (body 30, fig. 3).

The rocks are medium- to coarse-grained grained, very 

light-gray to light-gray, hornblende-biotite granite and 

granodiorite, together with smaller amounts of tonalite and 

quartz monzonite (fig. 6). Color indices range from 1 to 20, 

and most of the bodies have weak subsolidus foliations, 

particularly near their margins. Stained-slab-counted samples 

range from granite to tonalite. All of these units have isotopic 

ages that range from 44.4±1.3 Ma to 27.6±08 Ma (K-Ar on 

biotite; tables 1, 2).

Rocks of Eocene Age (55.8 to 33.9 Ma)

Granite and Granodiorite of the Sanak-Baranof 

Belt

The Sanak-Baranof belt (figs. 2, 3) comprises a single 

large pluton (unit Tegr) of Eocene age that occurs west of the 

Fairweather Fault in the Yakutat terrane. According to the 

description by D.A. Brew and G. Plafker (unpublished data, 

2002), the pluton is composed of fine- to coarse-grained, 

equigranular to seriate, hypidiomorphic, massive to foliated, 

locally potassium-feldspar poikilitic muscovite-biotite 

granite and granodiorite, with common myrmekite. No 

modal or chemical data are available. The pluton occurs in 

country rocks of the Yakutat group. G. Plafker (unpublished 

data, 2002) established that K-Ar ages on this family of 

plutons farther north are closely concordant and range from 

46.8 to 51.1 Ma. This pluton is considered part of the SanakBaranof plutonic belt (Hudson, 1983), which has no other 

representatives in Glacier Bay National Park and Preserve.

Rocks of Paleocene Age (65.5 to 55.8 Ma)

Tonalite Stock in Johns Hopkins Inlet

A small stock (body 64, fig. 3) of hornblende-bearing 

biotite tonalite (unit Tgto, fig. 3) occurs in the TISZ on the 
12 Geochronology of Plutonic Belts and their Tectonic Terranes in Glacier Bay National Park and Preserve

A P

Q

A

Q

Quartz-rich

granitoid

Granodiorite

Granite

Alkalifeldspar

granite

Quartz

alkali

feldspar

syenite

Quartz

syenite

Quartz

monzonite

Quartz

monzodiorite

Quartz

diorite

Diorite

Tonalite

Quartz-rich

granitoid

Granodiorite

Granite

Alkalifeldspar

granite

Quartz

alkali

feldspar

syenite

Quartz

syenite

Quartz

monzonite

Quartz

monzodiorite

Quartz

diorite

Diorite

Tonalite

P

C.

D.

A P

Q E.

Quartz-rich

granitoid

Granodiorite

Granite

Alkalifeldspar

granite

Quartz

alkali

feldspar

syenite

Quartz

syenite

Quartz

monzonite

Quartz

monzodiorite

Quartz

diorite

Diorite

Tonalite

A P

Q

Quartz-rich

granitoid

Granodiorite

Granite

Alkalifeldspar

granite

Quartz

alkali

feldspar

syenite

Quartz syenite

Quartz

monzonite

Quartz 

monzodiorite

Quartz

diorite

Diorite

Tonalite

Quartz-rich

granitoid

Granodiorite

Granite

Alkalifeldspar

granite

Quartz

alkali

feldspar

syenite

Quartz

syenite

Quartz

monzonite

Quartz

monzodiorite

Quartz

diorite

Diorite

Tonalite

A P

Q

A.

B.

Figure 4. QAP diagrams (after Streckeisen, 2002) showing modal 

compositions of Tertiary plutonic rocks in Glacier Bay National 

Park and Preserve, southeast Alaska. A, Units Tgqm and Tgqp

of the Tkope belt (4 modes); B, unit Tggd of the Fairweather-Muir 

belt (32 modes); C, unit Tggg of the Fairweather- Muir belt (21 

modes); D, unit Tgqd of the Fairweather-Muir belt (6 modes); E, 

unit Tggr of the Fairweather-Muir belt (15 modes).
Descriptions of the Plutonic Rocks 13

south side of Johns Hopkins Inlet. No modal or chemical 

analyses are available. A Paleocene K-Ar age of 58.6±5.2 Ma 

was reported for the unit (table 1; Decker and Plafker, 1982).

Rocks of Early Cretaceous Age (145.5 to 99.6 Ma)

Granodiorite and Tonalite of the Muir-Chichagof 

Belt

The Muir-Chichagof belt (figs. 2, 3; table 1) is one of 

several plutonic belts in southeast Alaska described by Brew 

and Morrell (1983); its rocks are the most extensive of any belt 

in the Glacier Bay region (fig. 2).

The biotite-hornblende granodiorite (unit Kggd) consists 

of 13 plutonic bodies whose postemplacement foliations 

consistently parallel those of the host rocks except for local 

divergences near contacts. Migmatites are locally present, 

and border zones consist of mafic rocks that are diked and 

brecciated by leucocratic granodiorite. The plutons are generally 

medium-grained, very light gray to gray; and have a color index 

of 4 to 30, mostly 15 and 25. Modal analyses are plotted in 

figure 5A. Isotopic ages of these rocks range from 115±3.4 Ma 

(K-Ar on biotite) to 107±3.2 Ma (K-Ar on hornblende; tables 1, 

2).

The biotite-hornblende tonalite (unit Kgto, fig. 3) 

includes two slightly different groups of plutons that have been 

combined because their differences appear to be insignificant. 

The unit was labeled "tonalite" in the field, on the basis of 

its color index and apparent quartz content; modal analyses, 

however, suggest that quartz diorite may actually be the most 

common rock type (fig. 5B).

One subgroup in the biotite-hornblende tonalite consists of 

four bodies in the Alexander terrane that range in composition 

from granite through diorite; they are medium- to coarsegrained, very light to medium gray, and have color indices 

from 6 to 40, but most commonly 20–30 (bodies 23, 24, 25, 29, 

fig. 3). Another subgroup consists of four bodies, also in the 

Alexander terrane (bodies 31, 32, 51, 55, fig. 3), in which the 

most common rock types are foliated white to dark-gray-green 

biotite-hornblende-quartz diorite and tonalite, but ranging in 

composition from granodiorite to diorite (fig. 5B.) and in texture 

from fine- to coarse-grained, averaging medium grained. Color 

indices range from 3 to 68 and average 25. 

Inclusions are common near body margins and 

consist of hornblende diorite and amphibolite that parallel 

postemplacement foliations. K-Ar and 40Ar/39Ar ages for this 

unit range from 123.3±0.4 Ma to 96.7±5.5 (tables 1, 2).

Rocks of Late Jurassic Age (161.0 to 145.5 Ma)

Leucotonalite in Johns Hopkins Inlet

A small stock (body 63, fig. 3) of muscovite-bearing 

leucotonalite (unit Jgto, fig. 3) occurs in the TISZ. No modal 

A P

Q

A

Q

Quartz-rich

granitoid

Granodiorite

Granite

Alkalifeldspar

granite

Quartz

alkali

feldspar

syenite

Quartz

syenite

Quartz

monzonite

Quartz

monzodiorite

Quartz

diorite

Diorite

Tonalite

Quartz-rich

granitoid

Granodiorite

Granite

Alkalifeldspar

granite

Quartz

alkali

feldspar

syenite

Quartz

syenite

Quartz

monzonite

Quartz

monzodiorite

Quartz

diorite

Diorite

Tonalite

P

A.

B.

Figure 5. QAP diagrams (after Streckeisen, 2002) showing modal 

compositions of Cretaceous plutonic rocks in Glacier Bay National 

Park and Preserve, southeast Alaska. A, Unit Kggd of the MuirChichagof belt (49 modes); B, unit Kgto of the Muir-Chichagof belt 

(6 modes).
14 Geochronology of Plutonic Belts and their Tectonic Terranes in Glacier Bay National Park and Preserve

or chemical analyses are available. Jurassic 40Ar/39Ar ages of 

162±0.4 Ma and 152.7±0.4 have been reported for this unit 

(table 1).

Rocks of Jurassic Age (201.6 to 145.5 Ma)

Sheared Diorite and Gabbro of the Lituya Belt

The sheared diorite and gabbro of the Lityua belt 

consists of one elongate northwest- trending body of 

hornblende diorite (unit Jgdi, fig. 3) north of Lituya Bay 

(body 1, fig. 3). Most of the east edge of the body is the 

Fairweather Fault. Younger biotite granodiorite (unit Tggd, 

fig. 3) intrudes part of this body. The pluton is foliated owing 

to hypersolvus deformation and is highly sheared and altered; 

the only available modal analysis indicates that the sample is 

quartz monzodiorite, but field and thin-section examinations 

indicate that most samples are composed of diorite, quartz 

diorite, or tonalite and have a color index from 15 to 70, 

mostly 20 to 30. Hornblende-augite gabbro or diorite is 

common as a border phase along the western margin. No 

isotopic age or chemical analyses are available for the body. 

A Jurassic age is inferred from the pluton's pervasive foliation 

and alteration which are like those in lithologically and 

structurally similar bodies near Sitka on Baranof Island to 

the south that are dated at about 190 Ma (S.M. Karl, written 

commun., 2005).

Discussion

The plutonic belts in Glacier Bay National Park and 

Preserve are parts of regional magmatic belts of varying extents 

in southeast Alaska that reflect different episodes of tectonic 

activity. Various names have been applied to these belts by 

different authors; the following paragraphs trace the evolution 

of these names for the benefit of the interested reader.

Regional Magmatic Belts

Stocks and small plutons of the Oligocene age quartz 

monzonite and quartz syenite of the Tkope belt of this report 

compose part of the Tkope-Portland Peninsula regional belt of 

Brew (1994). The origin of this magmatism is discussed below. 

Most of the bodies in that belt are in the Skagway quadrangle 

north of the Glacier Bay region (Wilson and others, 1994) and 

farther north in British Columbia (Dodds and Campbell, 1988).

The Eocene and Oligocene gabbros of the Crillon-La 

Perouse mafic belt of this report were included in the 

Fairweather-Baranof belt by Brew and Morrell (1983). This 

plutonic belt is confined to the Glacier Bay region except 

for its extension on Yakobi Island and vicinity to the south 

(Himmelberg and others, 1987).

The Eocene and Oligocene granodiorite, quartz diorite, 

and granite units of the Muir-Fairweather felsic-intermediate 

belt of this report correspond to part of the FairweatherBaranof belt of Brew and Morrell (1983) and Brew (1994) 

and to the Glacier Bay regional belt of Brew (1994). The 

felsic-intermediate belt has analogs to the north in the 

Yakutat and Skagway quadrangles (Wilson and others, 1994). 

Brew and Morrell (1983) considered the rocks of the felsicintermediate belt to be part of the Sanak-Baranof belt of 

Hudson (1983); however, newer (and better) age controls and 

other data suggest that the bodies in that belt may be slightly 

older and are all oceanward of those of the Muir-Fairweather 

felsic-intermediate belt (Bradley and others, 1993).

The late Early Cretaceous granodiorite and tonalite of the 

Muir-Chichagof belt of this report, which extends southward 

through southeast Alaska (Brew and Morrell, 1983) and into 

western British Columbia near Prince Rupert (Hutchison, 

1982; Woodsworth and others, 1989), is considered (Monger 

and others, 1982) to be the roots of a volcanic-plutonic 

arc associated with the juxtaposition of the Insular and 

Intermontane Superterranes. The volcanic part of the arc has 

been eroded from all the area underlain by intrusive rocks, 

but its distant products are preserved as part of the Gravina 

overlap-assemblage rocks to the east (Berg and others, 1972; 

Brew and Ford, 1998). Immediately south of Glacier Bay 

National Park and Preserve, on the northwestern part of 

Chichagof Island, a significant number of plutons of Jurassic 

age occur in the belt. These plutons cannot be distinguished 

from the bodies of mid-Cretaceous age on the basis of field 

criteria alone (Loney and others, 1975; Johnson and Karl, 

1985; Karl and others, 1988). Similarly, the Muir-Chichagof 

belt extends northward into British Columbia and Yukon 

Territory, where the compilation by Dodds and Campbell 

(1988) indicates that both Cretaceous and Jurassic plutonic 

bodies are present; there, also, the noncoeval bodies cannot 

be distinguished on the basis of field criteria (R.B. Campbell, 

oral comm., 1985). This belt includes some plutons east of 

Glacier Bay that were assigned to the Chilkat-Prince of Wales 

province by Sonnevil (1981). All of these late Early Cretaceous 

rocks were assigned to the Nutzotin-Chichagof belt by Hudson 

(1979, 1983). We have assigned or reassigned all of the plutons 

described in this report to the belts that are described here.

As part of how the plutonic belts in Glacier Bay National 

Park and Preserve fit into previous, but now-discarded, 

regional-belt nomenclature, we note that Hudson (1979, 

1983) and Brew and Morrell (1983) suggested that a distinct 

belt of Middle Jurassic plutons extended into the Glacier 

Bay region from the north and continued southward into the 

northeastern part of Chichagof Island. Hudson assigned the 

Jurassic rocks to his Tonsina-Chichagof belt and Brew and 

Morrell (1983) termed it the Muir-Chichagof Belt II. All of 

these plutons are now included in the late Early Cretaceous 

Muir-Chichagof belt, even though some may be Jurassic. No 

Jurassic plutons have been recognized in this belt south of 

Baranof Island (south of Glacier Bay) in southeast Alaska.
References Cited 15

two major fault zones and one crustal-scale tectonic boundary, 

present an intricate web of diverse geologic features. The 

plutonic belts profoundly influence our understanding the park 

and preserve. The oldest belt, consisting of Jurassic plutons, 

is poorly preserved and may have been generated during 

tectonic subduction events whose record has otherwise been 

lost. We (and other workers) interpret the extensive belt of 

northwest-trending Early Cretaceous plutons to have resulted 

from eastward and northeastward Late Jurassic and Cretaceous 

subduction of now-vanished parts of the Chugach terrane 

beneath the Alexander terrane long before the initiation of the 

major Fairweather-Queen Charlotte Fault in the western part 

of the park. The Tarr Inlet Suture Zone is a remnant of the 

original subduction zone that evolved into the Border Ranges 

Fault system. Only one pluton in Glacier Bay National Park and 

Preserve belongs to the regionally important Eocene SanakBaranof belt that represents ridge-subduction-related slabwindow magmatism along the northeastern Pacific Rim. The 

later Tertiary plutons of the Fairweather-Muir belt contrast with 

the earlier Tertiary ones because the later plutons are slightly 

more felsic and alkalic and because their distribution crosses 

the tectonic grain of the region. These younger plutons may be 

related to subduction of the Resurrection Plate, but alternatively 

they may be the deeper and northwesternmost expression of the 

Kuiu-Etolin volcanic-plutonic belt of central southeast Alaska 

(Brew and others, 1979).

Distribution and Origin of the Major Plutonic 

Belts in Glacier Bay National Park

The distribution of the plutonic belts in Glacier Bay 

National Park and Preserve is mapped in figure 2. The granitic 

rocks of the late Early Cretaceous Muir-Chichagof belt are 

dominant in the central and eastern part of the park and 

preserve, in the Alexander and Wrangellia terranes. These 

rocks are clearly related to the major accretionary events that 

joined the Chugach terrane to the Alexander terrane to form 

the Insular Superterrane, which was subsequently joined to the 

Intermontane Superterrane to the east.

Granitic rocks of the Eocene and Oligocene MuirFairweather felsic-intermediate belt are dominant to the west in 

the Chugach terrane in the Fairweather Range; they also occur 

as scattered outliers in the eastern part of the park and preserve, 

in the Alexander terrane. They distinctly postdate (44±1.3 Ma 

to 27.6±0.8 Ma) the Eocene plutons that make up the SanakBaranof belt of Hudson (1983), which are all dated to about 

50 Ma. The rocks are probably not the simple expression of 

the ridge-subduction-related slab-window magmatism that 

formed the Sanak-Baranof belt (Bradley and others, 1993; 

2003). The gap of about 20 m.y. between the ages of these 

two belts is difficult to explain, except to interpret the younger 

rocks as related either to a cryptic tectonic event or to a delayed 

expression of some earlier subduction event, which would 

likely be the later stages of the subduction of the recently 

recognized Resurrection tectonic plate in the northeastern 

Pacific (Haeussler and others, 2001, 2003; Madsen and others, 

2003). This subduction event may be the slightly later, obscure 

event that was vaguely hypothesized by Brew (1994). The 

presence of layered gabbroic rocks of the Crillon-La Perouse 

belt that slightly postdate the felsic rocks in essentially the 

same location suggests that both the felsic and mafic magmas 

were a late-generated product of the same event. The gabbroic 

and felsic rocks may even be a deeper and northwesternmost 

expression of the Kuiu-Etolin volcanic-plutonic belt of central 

southeast Alaska (Brew and others, 1979).

Rocks from almost all of the plutonic belts occur in 

or near the TISZ (fig. 1), as do the small and uncommon 

Paleocene and Late Jurassic bodies. As first recognized by 

Plafker and Campbell (1979), the TISZ contains the Border 

Ranges Fault. Almost all the intrusive rocks in the TISZ are 

strongly sheared, and some are altered much more than is 

common in other localities. We interpret the small bodies in 

the TISZ to be slices that have been transported there from 

unknown areas.

Conclusions

The Ordovician through Tertiary country rocks of Glacier 

Bay National Park and Preserve are laced with belts of plutonic 

rocks. The resulting complexities, together with the presence of 

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Menlo Park Publishing Service Center, California

Manuscript approved for publication March 20, 2013

Edited by George Havach and Claire M. Landowski

Design and layout by Cory Hurd
Brew and others—Geochronology of Plutonic Belts and their Tectonic Terranes in Glacier Bay National Park and Preserve—Professional Paper 1776-E

http://dx.doi.org/10.3133/pp1776E/

ISSN 2330-7102 (online)