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RESEARCH ARTICLE

Petrological and geochemical constraints on the origin

of adakites in the Garibaldi Volcanic Complex, southwestern

British Columbia, Canada

Julie Fillmore &Ian M. Coulson

Received: 15 October 2010 /Accepted: 30 April 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract The Garibaldi Volcanic Complex (GVC) is located

in southwestern British Columbia and comprises two related

but distinct volcanic fields: the Garibaldi Lake and the Mount

Garibaldi volcanic fields. The rocks of the GVC range frombasalt to rhyolite, and analyses of samples from both fields

distinguish these as adakites. The GVC magmas have high

Sr/Y, Mg#, and Al2O3; low K2O/Na2O; and fractionated rare

earth element compositions. Models of adakite genesis fall

into two main groups: slab melting and non-slab melting.

Adakites generated by slab melting commonly occur from

young subducting crust (25 Ma) and are felsic partial melts

of the subducting slab that interact with the mantle wedge

during ascent. Non-slab melting models vary widely and

include basalt fractionation, assimilation, fractional crystalli-

zation processes and partial melting of mafic lower crust. Data

from the GVC are too limited to fully elucidate the mecha-

nisms of adakite genesis; however, the petrographical and

geochemical characteristics of the GVC rocks in this study

do not refute an origin by slab partial melts. Variations in trace

elements that reflect non-adakitic values (e.g., low La, low Cr)

are likely the result of magma mixing at shallow depths within

the magma reservoirs of each center, for which there is min-

eralogical and textural evidence. The adakite rocks of the

GVC share geochemical traits akin to both low-SiO2adakite

(LSA) and high-SiO2adakite (HSA) groups, though addition-

al data are needed to investigate whether LSA- or HSA-type

dominates within the GVC, and by extension, which should

be the preferred model of adakite genesis.

Keywords Adakite . Petrogenesis . Subduction-related

magmas . Garibaldi Volcanic Complex

Introduction

Adakites are a group of intermediate to felsic igneous rocks

formed in subduction zones involving relatively young, hotoceanic lithosphere (25 Ma). Since the introduction of the

term adakiteby Defant and Drummond (1990), the petro-

genesis of this suite of rocks has been the subject of contro-

versy. Adakites are named after magnesian andesites first

described by Kay (1978) from Adak Island in the Aleutians.

These rocks are believed to be the result of partial melting of

subducted ocean crust creating typically sodic slab melts.

Partial melting of basalt to generate adakite is supported by

experimental work (e.g., Rapp et al.1999) and evidenced in

natural rocks from subduction zones (e.g., Schiano et al.

1995). Adakites are characterized by specific geochemical

features (Sr/Y40 and La/Yb20), and as studies into mag-

ma genesis continued, an ever widening group of different

models was suggested. Consequently, a wide range of mag-

ma compositions from different tectonic environments have

been classified as adakites. Models that explain the forma-

tion of adakites fall into two main groups: genesis from slab

partial melts and magmas generated by various methods that

can reproduce the distinctive adakite chemistry (which are

not necessarily subduction related). Further ambiguity arises

when a suite of rocks are classified as adakites (which may

not follow the parameters outlined by Defant and

Drummond 1990) and are subsequently found not to be

related to slab melting. This leads to the conclusion that

adakites are not slab melts and has provided the means of

using adakite to characterize many different rock types

under the one name.

The Garibaldi Volcanic Complex (GVC) is comprised of

two fields, the Mount Garibaldi Volcanic Field (MGVF) in the

south and the Garibaldi Lake Volcanic Field (GLVF) in the

north. The GVC is Quaternary in age and is located in south-

ern British Columbia (Figs. 1and2). Geochemical attributes

of andesite and dacite rocks that comprise the GVC suggest

Editorial responsibility: M. A. Clynne

J. Fillmore (*) : I. M. Coulson

Solid Earth Studies Laboratory, Department of Geology,

University of Regina, Regina, SK S4S 0A2, Canada

e-mail: [email protected]

Bull Volcanol (2013) 75:730

DOI 10.1007/s00445-013-0730-5

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that they are the result of slab melting, under the definition put

forth by Defant and Drummond (1990). Interactions between

these melts and the overlying mantle wedge are also evident in

their major and trace element compositions, a refinement

made to the slab melt model by Martin et al. (2005). This

study aims to: (1) present the first, complete whole-rock

geochemistry for the MGVF as well as the first, complete rare

earth element (REE) geochemistry of the intermediate rocks in

the GLVF and (2) demonstrate that this chemistry identifies

the rocks of the GVC as adakites.

Regional geology

The Garibaldi Volcanic Belt (GVB) extends from the

CanadaUSA border northward into British Columbia for

approximately 140 km (Fig. 1; Sherrod and Smith 1990).

The GVC lies within the southern portion of the GVB

between the towns of Whistler and Squamish and comprises

two fields: the GLVF in the north and the MGVF in the

south (Figs. 1 and 2). The volcanic rocks from the GLVF

and the MGVF range in composition from basalt to rhyolite

and have been previously interpreted to be the result of

hydrous melting of the mantle wedge above the Juan de

Fuca Plate, which is subducting beneath the North American

Plate, and subsequent fractionation at various depths during

ascent (Green 1977, 1981, 1990; Green and Harry 1999;

Green and Sinha2005; Green et al. 1988). Basaltic volca-

nism in the GVC is thought to be related to the decreased

volatile content of the Juan de Fuca Plate and results in

lower degrees of melting under higher pressures and tem-

peratures (Green2006). The decreased slab flux is attributed

to both a northward decrease in plate movement and plate

age (Riddihough 1981, 1984; Green 1990; Wilson 2002).

The younger, more buoyant Explorer Plate separated from

the Juan de Fuca Plate at 4 Ma (Wilson 2002; Audet et al.

2008), and this deviation may relate to an increase in

Quaternary volcanism in the GVC.

Subduction of the Explorer Plate beneath North America

is slower than that of Juan de Fuca, and it has been sug-

gested that the Explorer Plate is undergoing capture by the

North American Plate (Audet et al.2008). The difference in

subduction rates has caused a region of extension and slab

thinning along the Nootka Fault zone (Fig. 1), a transform

fault that fractured in response to an interval of ridge prop-

agation and reorientation (Riddihough1984; Wilson1988;

Madsen et al. 2006) and created the Explorer and Juan de

Fuca plates. Independent movement of the Explorer Plate

northward relative to the northeasterly movement of the

Juan de Fuca Plate suggests that the subducted portions of

the plates have separated in addition to the oceanic portions

(Madsen et al. 2006). This segmentation coupled with the

relative subduction vectors of the plates has resulted in a

change in mantle flow. Recent studies (Madsen et al. 2006;

YukonTerritory

British Columbia

Garibaldi volcanic belt

Queen CharlotteTransform Fault

ExplorerPlate

Juan de

Fuca Plate

GVC (see Fig. 2)

Washington0 100

km

200

Squamish

Whistler

Pacific Plate

Alaska

Alberta

NWT

North American Plate

Nootka fault zone

Fig. 1 Location map of the

GVC in southwestern British

Columbia, with plate

boundaries highlighted and

relative plate motions of the

Juan de Fuca and Explorer

plates indicated. Map modified

after Hickson et al. (1999) and

Madsen et al. (2006)

730, Page 2 of 23 Bull Volcanol (2013) 75:730

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The MGVF and the GLVF sit unconformably on the

Coast Crystalline Complex, which is a series of meta-

morphosed quartz diorite and granodiorite plutons of

Cretaceous age. The MGVF is comprised of Mt.

Garibaldi and its subsidiary vents, Dalton Dome and

Atwell Peak, Opal Cone and the Ring Creek andesite

flow, and Columnar Peak and the andesite flows of Paul

Ridge. Recent activity in this field began at approximate-ly 700 ka with the eruption of hornblende andesite flows

atop pre-1,300 ka hornblende andesite and basaltic an-

desite. Increased volcanism occurred between 260 and

220 ka with the eruption of hornblendeorthopyroxene

andesite at Columnar Peak, followed by hornblende

orthopyroxene dacite flows and minor pyroclastic materi-

al from Mt. Garibaldi (Green et al. 1988). The composite

dacite cone of Dalton Dome formed later but before the

belt was overridden by glacial ice. Post-100 ka, dacitic

pyroclastic flows were erupted from Atwell Peak atop

the glacial ice as well as additional dacite flows from

Dalton Dome. These flows and the west flank of AtwellPeak collapsed following glacial retreat (Mathews 1952,

1958; Green 1990). The most recent volcanism in the

GVC was the eruption of the Ring Creek andesite flow

from Opal Cone between 10.7 and 9.3 ka (Brooks and

Friele 1992), which extends for some 17.5 km south

around Paul Ridge and then west toward Squamish

River (Fig. 2).

Quaternary volcanic centers in the GLVF include Black

Tusk, Cinder Cone, Clinker Peak, Mt. Price, and The

Table, as well as the Cheakamus Valley Basalts which

were erupted from an unknown centre. Timing and erup-

tive products from both fields have been summarized by

previous authors (Green 1977, 1981, 1990; Green et al.

1988) and are briefly outlined below. The oldest activity

in the GLVF was at Black Tusk and Mt. Price with

episodic volcanism beginning at 1,300 ka. The rocks of

Black Tusk are hornblende andesite and orthopyroxene

andesite flows. The oldest rocks of Mt. Price are a series

of hornblende andesite and andesite flows followed by the

formation of the hornblendebiotite andesite satellite cone

along Garibaldi Lake. Volcanism ended in the Mt. Price

area with the eruption of the Barrier and Culliton Creek

andesite flows from Clinker Peak at 100 ka. Activity at

Cinder Cone began post 100 ka with the formation of a

tuff ring and the eruption of the basaltic andesite of

Desolation Valley, followed by the Helm Creek basalt

flow. The Table formed at 100 ka, when hornblende

andesite magma erupted beneath the Cordilleran Ice

Sheet and melted its way upward to form a steep, flat

topped tuya (Mathews 1951; Green 1981). The olivine-

bearing Cheakamus Valley Basalts were erupted post-

100 ka, and eruptions continued episodically to approxi-

mately 34 ka (Green 1977; Green et al. 1988).

Petrography

MGVF

Ring Creek andesite

Four samples were collected from the Ring Creek andesite:

two taken proximal to Opal Cone (09JF007, 09JF008) andtwo taken approximately 2 km from the flow terminus

(10JF022, 10JF023; see Fig. 2). The mineralogy of the

proximal Ring Creek andesite differs from that of the distal

portion of the flow (first noted by Sivertz1976), and hence,

the petrography will be described separately.

1. Proximal Ring Creek andesite

The andesite is porphyritic; main phenocrysts are

plagioclase (15 %), followed by hornblende (10 %)

and augite (2 %). Quartz occurs in trace amounts

(0.1 %). Biotite (3 %) is present as rare large xenocrysts

(Fig.3a). Plagioclase occurs in two size populations; thelarger phenocrysts are approximately 2 mm in size and

the smaller less than 1 mm. The majority of the plagio-

clase crystals are subhedral, equant to tabular; more

rarely, these form glomeroporphyritic aggregates.

Several features are exhibited in plagioclase that in-

cludes sieve textures, resorption of grain margins and

in some of the larger crystals, seritization. An equal

proportion of plagioclase phenocrysts, however, are

inclusion-free and pristine. Hornblende phenocrysts

are second to plagioclase in abundance and range in size

from less than 0.5 up to 1 mm. The majority of the

crystals exhibit various disequilibrium textures includ-

ing fibrous cores of clinopyroxene and destabilization

rims of opaque oxides along the crystal margins

(Fig. 3b). Biotite occurs as subhedral xenocrysts up to

1 mm in size with rare crystals up to 3 mm. The edges of

biotite crystals are diffuse and poorly defined with rims

showing alteration to a mass of fine-grained opaque

minerals. The larger biotite xenocrysts are heavily

embayed and have sieve-textured cores. With few ex-

ceptions, these xenocrysts display extensive replace-

ment by opaque oxide phases; resorption of grain

boundaries is also common. Augite occurs as prismatic

to equant crystals up to 3.5 mm in size. The margins of

the phenocrysts are resorbed and contain abundant in-

clusions of apatite and oxides. The crystals are greenish

brown and not distinctly pleochroic. Only a few quartz

crystals have been identified in this part of the flow. The

phenocrysts are anhedral and less than 0.5 mm in size

and exhibit resorption along the grain margins. The

groundmass of the proximal Ring Creek andesite is

approximately equal parts crystallites and brown glass.

Plagioclase, augite, altered hornblende, and oxide


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a b

c d

e f

hg

i j

k l

Fig. 3 Thin section

photomicrographs of the

MGVF and GLVF rocks.a

Biotite xenocryst from the

proximal portion of the Ring

Creek andesite. Cross-polarized

light, FOV 5 mm.bHornblende

phenocrysts from the proximal

portion of the Ring Creek

andesite; note the dark, veryfine-grained reaction rims along

the crystal margins. Plane-

polarized light, FOV 3 mm.c

Mafic xenolith comprised of

plagioclase and orthopyroxene

in the distal portion of the Ring

Creek andesite. Cross-polarized

light, FOV 3 mm. d Weak flow

banding observed in the distal

portion of the Ring Creek

andesite. Note,lower left, the

quartz xenocryst with a reaction

rim of fine-grained augite.

Cross-polarized light, FOV

5 mm. e, fGeneral textureobserved (cross-polarized light,

FOV 3 mm) and concentrically

zoned hornblende phenocrysts

(cross-polarized light, FOV

3 mm) present within the

Columnar Peak dacite. g

Plagioclase-rich nature of the

Paul Ridge andesite, with

rounded olivine (lower right).

Cross-polarized light, FOV

5 mm. h Olivine (second-order

blue) being replaced by fibrous

orthopyroxene (yellow) in Paul

Ridge andesite sample. Cross-

polarized light, FOV 3 mm.iStrongly altered biotite crystal

with a rim of oxide phases in

the Barrier andesite. Plane-

polarized light, FOV 3 mm.j

Glomeroporphyritic hornblende

in the Barrier andesite. Plane-

polarized light, FOV 3 mm.k

Strongly resorbed and

disaggregated orthopyroxene

phenocryst in the Black Tusk

andesite. Cross-polarized light,

FOV 3 mm.l Coarser-grained

crystal clot in the Black Tusk

andesite. Cross-polarized light,

FOV 3 mm

Bull Volcanol (2013) 75:730 Page 5 of 23, 730

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phases comprise the majority of the crystallites. Local

flow banding is evident around the larger phenocrysts.

2. Distal Ring Creek andesite

The distal Ring Creek andesite is less porphyritic

than the proximal portion and contains approximately

20 % total phenocrysts. The mineralogy of the distal

part of the flow differs from the proximal portion in thatthe only phenocryst phases present are plagioclase and

augite. Xenoliths of mafic-intermediate cumulate inclu-

sions that host orthopyroxene occur rarely. The inclu-

sions are heavily corroded and partially melted

(Fig.3c). Plagioclase, again occurring in two size pop-

ulations, is the most abundant phenocryst (15 %),

followed by augite (5 %). The larger plagioclase crystals

are up to 3.5 mm in size, the smaller approximately

1 mm; all crystals are equant to tabular and subhedral

and display complex zonation and various degrees of

resorption. Sieve-textured crystals are less common than

the plagioclase in the proximal portion of the flow butoccur mainly in the larger grains that also contain con-

centric trails of melt inclusions along their margins.

Augite phenocrysts are smaller than plagioclase, com-

monly less than 1 mm in size. The crystals are equant

and subhedral to euhedral; some grains display simple

twinning as well as glomeroporphyritic aggregates with

plagioclase. Rare, altered, orthopyroxene crystals (up to

2 mm in size) likely derived from the mafic-intermediate

cumulate xenoliths are present but have been almost

completely altered to chlorite and opaque oxides. Rare

quartz is also present occurring as anhedral crystals that

appear in disequilibrium with the surrounding melt, in

exhibiting reaction rims of fine-grained, radiating clusters

of augite (Fig. 3d). This is in contrast to the proximal

portion of the flow where the quartz appears to be prima-

ry. The groundmass is predominantly crystallites of pla-

gioclase with lesser amounts of augite and brown glass.

The groundmass displays local, weakly developed flow

foliation.

Columnar Peak

Four samples from the orthopyroxene-hornblende dacite of

Columnar Peak (09JF009, 09JF010, 10JF019, and 10JF020)

were examined as part of this study. This unit is described as a

series of flows by Green (1977). The dacite is porphyritic with

10 to 15 % phenocrysts (Fig. 3e). Plagioclase is the most

abundant phase (7 %), followed by hornblende (6 %) and

orthopyroxene (

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exhibits alteration to brown iddingsite along crystal edges

and fractures. Embayments are also common in the crystals.

A few quartz xenocrysts have been identified; the quartz is

anhedral, usually exhibiting weakly uneven extinction. The

quartz xenocrysts exhibit reaction rims and are surrounded

by small augite crystals radiating outward. The groundmass

is mainly brown glass with microlites of plagioclase and

opaques.

GLVF

Barrier andesite

Four samples (09JF004, 09JF005, 09JF006, 09JF012) were

collected from the Barrier andesite lava flow along the

northern shore of Garibaldi Lake. This flow is porphyritic,

with 10 to 15 % phenocrysts. Plagioclase is the most abun-

dant (10 %), followed by approximately equal amounts of

hornblende and biotite (23 % each). Quartz phenocrysts

occur rarely (

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Table1

Majorandminorelementcompositionofinvestigatedsamplesfromth

eGaribaldiVolcanicComplex

GaribaldiLakevolcanicfield

Mt.Garibaldivolcanicfield

BF

BF

BF

BF

BT

RC

RC

RC

RC

CP

CP

PR

PR

PR

Sample

09JF004

09JF00

5

09JF006

09JF012

10JF016

09JF007

09JF008

10JF022

10JF023

09JF009

09JF010

09JF011

10JF017

10JF018

SiO2

59.74

60.16

59.35

59.34

59.58

61.86

60.34

62.46

62.57

64.76

64.56

58.51

56.31

57.46

TiO2

0.61

0.65

0.67

0.64

0.63

0.57

0.57

0.58

0.5

5

0.39

0.41

0.65

0.96

0.78

Al2O3

18.52

18.72

18.80

18.41

18.37

17.71

17.97

17.18

17.52

17.11

17.19

16.90

18.51

18.61

Fe2O3(T)

5.56

5.40

5.74

5.64

5.87

4.82

4.34

4.78

4.6

6

3.63

3.88

5.93

8.06

6.90

MnO

0.09

0.09

0.10

0.10

0.10

0.09

0.08

0.09

0.0

8

0.08

0.09

0.10

0.14

0.12

MgO

2.92

2.76

2.88

3.05

3.05

2.37

2.24

2.52

2.4

0

1.81

1.95

4.96

3.42

3.61

CaO

6.22

6.13

6.36

6.27

6.09

5.78

5.28

5.51

5.6

5

4.39

4.52

6.62

6.39

6.70

Na2O

4.62

4.72

4.69

4.61

4.47

4.41

4.29

4.30

4.3

9

4.44

4.46

4.19

4.61

4.37

K2O

1.19

1.26

1.19

1.22

1.25

1.39

1.37

1.53

1.4

9

1.71

1.64

1.14

0.89

1.16

P2O5

0.25

0.28

0.28

0.26

0.24

0.26

0.24

0.25

0.2

4

0.16

0.16

0.21

0.33

0.28

LOI

0.37

0.20

4.69

0.46

0.10

0.78

3.15

0.63

0.6

3

1.48

0.77

1.02

0.21

0.05

Mg#

51.2

50.5

50.1

52.0

51.0

49.6

50.8

51.6

51.1

49.9

50.1

62.6

45.9

51.4

K2O/Na2O

0.26

0.27

0.25

0.26

0.28

0.32

0.32

0.36

0.3

4

0.39

0.37

0.27

0.19

0.27

K/Rb

678

648

619

686

788

628

653

660

668

674

706

713

881

733

SiO2/MgO

20.5

21.8

20.6

19.5

19.5

26.1

26.9

24.8

26.1

35.8

33.1

11.8

16.5

15.9

Total

100.20

100.46

100.26

100.08

99.85

100.13

99.97

99.93

100.27

100.07

99.73

100.34

99.91

100.15

TotalironreportedasFe2O3.

Mg#=

molarMg/(Mg+Fe)100

,whereFe=Total

FeasFeO

BFBarrierflow(andesite),

BTBlackTusk(andesite),

RC

RingCreekflow(andesite),

CPColumnarPeak(dacite),

PRPau

lRidge(basalticandesite/andesite)


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Table2

Traceandrareeartheleme

ntcompositionofinvestigatedsamplesfrom

theGaribaldiVolcanicComplex

GaribaldiLakevolcanicfield

Mt.Garibaldivolcanicfield

BF

BF

BF

BF

BT

RC

RC

RC

RC

CP

CP

PR

PR

PR

Sample

09JF004

09JF005

09JF006

09JF012

10JF016

09JF007

09JF008

10JF022

10JF

023

09JF009

09JF010

09JF011

10JF017

10JF018

Sc

d/l

11

d/l

d/l

11

11

d/l

10

11

10

d/l

14

15

8

V

97

96

99

99

98

85

86

84

81

63

63

110

133

121

Cr

32

29

29

31

21

32

32

18

17

21

23

132

18

31

Ni

43

34

38

39

56

28

30

30

26

42

30

103

30

48

Cu

41

37

41

39

75

41

49

59

50

22

51

71

66

100

Zn

33

34

30

35

28

30

23

26

25

14

17

45

53

40

Ga

17.1

17.8

17.9

17.9

18.9

17.6

18.0

18.4

18.7

15.4

16.3

17.0

20.0

19.1

Rb

14.6

16.2

16.0

14.9

13.2

18.5

18.1

19.4

18.7

21.4

19.5

13.3

8.4

13.1

Sr

1,000.3

957.8

982.4

1,023.5

909.8

1,068.4

1,078.6

1,012.2

1,02

6.8

752.0

784.8

957.9

838.7

868.7

Y

11.9

13.1

12.5

11.4

12.0

11.9

10.9

13.2

12.6

9.5

10.1

11.8

17.1

14.0

Zr

90.2

101.8

101.9

92.5

91.5

117.9

115.7

119.7

111.7

91.5

100.5

104.2

91.4

101.1

Nb

5.3

7.3

7.4

5.6

3.3

4.4

4.7

4.5

4.2

4.3

4.2

4.0

3.8

3.7

Ba

441.1

477.0

450.6

441.5

436.3

478.0

469.5

561.9

549.4

580.7

558.8

390.7

467.1

457.9

Pb

2.2

2.8

3.3

4.4

6.5

4.6

3.0

7.3

8.2

d/l

4.7

3.5

6.6

7.3

Th

1.4

1.6

1.5

1.4

1.3

1.9

1.9

2.1

2.1

2.7

2.6

2.7

0.8

1.4

U

0.6

0.7

0.6

0.6

0.6

0.7

0.7

0.8

0.8

1.1

1.1

0.9

0.3

0.5

La

12.01

13.46

13.68

12.03

10.73

14.87

15.24

17.13

16.9

0

13.02

12.85

15.11

11.54

12.03

Ce

26.61

28.80

29.27

26.21

24.00

32.56

33.40

37.25

36.3

7

27.09

26.83

32.12

26.54

27.27

Pr

3.58

3.77

3.86

3.53

3.27

4.25

4.39

4.87

4.73

3.30

3.30

4.04

3.78

3.74

Nd

15.11

15.79

16.09

14.78

14.32

17.48

17.82

20.11

19.4

6

12.90

12.90

16.46

17.49

16.63

Sm

3.22

3.19

3.24

3.05

3.09

3.31

3.36

3.83

3.65

2.36

2.42

2.98

4.02

3.56

Eu

1.03

1.06

1.09

0.99

1.00

1.03

0.99

1.07

1.08

0.75

0.76

0.98

1.35

1.12

Gd

2.68

2.96

2.86

2.73

2.80

2.72

2.40

3.05

2.94

1.93

1.98

2.48

4.00

3.48

Tb

0.39

0.42

0.42

0.39

0.41

0.39

0.36

0.42

0.41

0.29

0.30

0.36

0.58

0.48

Dy

2.33

2.48

2.47

2.22

2.37

2.24

2.07

2.39

2.30

1.71

1.73

2.11

3.42

2.87

Ho

0.47

0.47

0.49

0.45

0.46

0.43

0.39

0.46

0.45

0.34

0.34

0.43

0.68

0.57

Er

1.25

1.32

1.33

1.27

1.33

1.20

1.13

1.27

1.27

0.94

0.98

1.18

1.98

1.58

Tm

0.19

0.19

0.19

0.17

0.19

0.17

0.16

0.19

0.18

0.14

0.14

0.17

0.28

0.23

Yb

1.26

1.29

1.31

1.19

1.25

1.19

1.11

1.28

1.19

1.01

1.01

1.17

1.85

1.53

Lu

0.19

0.20

0.20

0.18

0.19

0.18

0.17

0.19

0.18

0.15

0.16

0.18

0.28

0.22

Sr/Y

83.9

73.4

78.7

89.5

75.8

89.9

98.6

76.7

81.5

79.2

78.0

81.5

49.0

62.1

La/Yb

9.53

10.43

10.44

10.11

8.58

12.50

13.73

13.38

14.2

0

12.89

12.72

12.91

6.24

7.86

Elementalconcentrationsexpressed

inpartspermillion

d/lbelowdetectionlimit

,BFBarrierflow,

BTBlackTusk;RC

RingCreekflow

,CPColumnarPeak

,PRPaulRidge


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were prepared from fusion blanks. Oxide corrections on

the middle and heavy REE were made offline using

oxide production rates determined daily from single

REE standard solutions. Rock-sample detection limits

(based on three times the background standard deviation)

are 10 ppb for La through Pr and 5 ppb for Nd through

Lu. A set of three internal laboratory reference materials

are fused and run with each batch of samples to evaluatelong-term precision. Precision was additionally evaluated

through repeat measurements of samples, including re-

peat fusions and dilution; it is better than 3 % relative

standard deviation in all cases. Accuracy was evaluated

using a series of six standard reference materials (SRMs)

that span the sample concentration range, prepared using

the same procedure as the samples. Our determinations

agree with the accepted values for these SRMs with

discrepancies of less than 5 %.

Results

Nine samples were analyzed from the MGVF: three samples

of the Paul Ridge andesite, two from the Columnar Peak

dacite, and four samples of the Ring Creek andesite (two

from the proximal part of the flow and two from the distal

portion). Another four samples from the Barrier andesite in

the GLVF and one sample from Black Tusk were also

analyzed (see Tables 1 and 2). The geochemistry of the

GLVF is well documented in several other studies (Green

1977,1981,1990; Green and Henderson1984), and select

data from these studies have been included for comparison

in this work. The investigated MGVF and GLVF rocks are

sub-alkaline in character ranging in composition from ba-

saltic andesite to dacite, with the bulk of samples falling in

the andesite field (Figs. 4 and 5). All the GVC samples

define similar trends as individual centers in several major

and trace element variation diagrams (Figs. 4 and 6). The

volcanic products from each centre are characterized by

decreasing Fe2O3, CaO, and V with increasing SiO2. MgO

is increasing in the Paul Ridge andesite with increasing

SiO2, as does TiO2in the Barrier andesite. All other centers

in the GVC have decreasing MgO and TiO2with increasing

SiO2. Al2O3 decreases only slightly as SiO2 increases with

alumina concentrations spanning a range of about 1.5 wt.%.

Ni, Cr, and Na2O do not vary significantly with increasing

SiO2, forming relatively flat trends. K2O and Rb contents

generally increase with SiO2 for all centers, as does Sr for

the Paul Ridge and Black Tusk andesites. Sr in the Barrier

andesite increases to approximately 60 wt.% SiO2and then

decrease sharply; a similar trend is seen for The Table

andesite at approximately 58 wt.% SiO2. Sr decreases with

increasing SiO2 for the Ring Creek andesite and the

Columnar Peak dacite. The Mg#

s of all the GVC rocks

exhibit a relatively narrow range (46 to 52), except for the

Paul Ridge andesite, where one sample has Mg# of 62.

Primitive mantle-normalized multi-element spider dia-

grams show that all centers have LILE enrichment and

NbTi-negative anomalies, typical of subduction-related

rocks (Fig.7a and b). The Paul Ridge andesite rocks exhibit

the strongest depletion of Th, Rb, U, and other incompatible

elements and the highest MREE-HREE abundances of allthe GVC rocks. One andesite from Paul Ridge (09JF011;

see Tables1and2) has higher MgO, TiO2, Ni, and Cr values

than the other samples. Chondrite-normalized REE spider

diagrams (Fig.8a and b) for the GVC volcanic rocks display

enrichment of LREE over HREE and lack any significant

Eu anomalies. The andesites of Black Tusk display the

lowest LREE/HREE fractionation of all the GVC volcanic

rocks with La/YbNaveraging 5.9. The Ring Creek andesite

samples exhibit the highest fractionation with La/YbNrang-

ing from 8.5 to 9.6. All of the GVC rocks have similar

MREE/HREE fractionation with ratios of 1.5 to 1.9.

On a Sr/Y versus Y diagram, all of the GVC rocks plotwithin the adakite field but fall outside of this field on the

La/Yb versus Yb diagram, having values typical of normal

arc-rocks (Fig. 6fg). The accepted minimum value for

La/Yb as an adakitic indicator ranges from as low as

8 (Drummond and Defant 1990) up to 20 and greater

(Castillo et al. 1999; Richards and Kerrich 2007). The

La/Yb values for the GVC have a range of6 (Paul Ridge

andesite) to 14.2 (Ring Creek andesite), plotting in the lower

end of the adakite field. When plotted against SiO2, both

Sr/Y and La/Yb appear to increase for the Paul Ridge

andesite rocks in contrast to the other GVC centers, for

which Sr/Y decreases with SiO2 but La/Yb increases (not

shown). Other adakitic indices (Sr, Na2O, Al2O3) do not

show definitive trends with fractionation indices (SiO2, Ni,

Cr), suggesting that the chemistry is not controlled exclu-

sively by fractionation processes (Chiaradia 2009). The

GVC rocks may also be divided into the low-SiO2 and

high-SiO2 adakite (LSA and HSA) groups on the basis of

geochemical characteristics outlined for adakitic rocks by

Martin et al. (2005). On a K/Rb versus SiO2/MgO diagram,

the Paul Ridge and Black Tusk andesite rocks exhibit high

K/Rb relative to SiO2/MgO and form a sub-vertical trend

(Fig. 9a) indicative of LSA; previously published data for

The Table andesite samples also follow this trend. The Ring

Creek andesite and the Columnar Peak dacite have lower

K/Rb values and plot sub-horizontally. The Barrier andesite

samples appear transitional, plotting at the intersection be-

tween the LSA and HSA trends. This may relate to the

slightly elevated Rb contents of Barrier andesite samples

and their SiO2values, which lie at the boundary between the

LSA and HSA groups. The LSA and HSA groupings are

still evident, though not as well defined in the Sr-

K/Rb-(SiO2/MgO)*100 ternary diagram (Fig. 9b). Here,


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only the Columnar Peak dacite is distinctly HSA and The

Table andesite rocks are clearly LSA.

Discussion

The rocks of the GVC exhibit geochemical characteristics

that favor their classification as adakites. Adakites are

characterized by 56 wt.% SiO2, 15 wt.% Al2O3, usually

3 wt.% MgO (rarely6 wt.%), 18 ppm Y, 1.9 ppm Yb

and Sr contents >400 ppm and were originally believed to

be associated with the subduction of young oceanic crust

(25 Ma; Defant and Drummond 1990). Since the in-

troduction of the term adakite, this definition has been

expanded to now include a wide range of compositions

and geological settings based on certain geochemical

Fig. 4 Harker diagrams illustrating variations in major element chemistry

with increasing SiO2 (oxide wt.%). Key: RCRing Creek, CPColumnar

Peak,PRPaul Ridge,BTBlack Tusk,BFBarrier flow, open squares, open

circles, and crossesrepresent previously published data for the Black Tusk,

Barrier flow, and The Table centers, respectively (data derive from Green

1977,1981; Green and Henderson1984). All data have been recalculated

to 100 % on an anhydrous basis


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indicators, specifically high Sr/Y (>40) and La/Yb

(>20) (e.g., Castillo et al. 1999; Xu et al. 2000). This

has led to some confusion in the literature and a gen-

eral misuse of the term adakite, such that any rock

type from any geological environment that has high

Sr/Y and/or high La/Yb has been called adakite or

adakite-like. To make valid use of adakite as a petro-

genetic indicator, a suite of geochemical and petrolog-

ical features as well as spatial associations is required.

Defant and Drummond (1990) first introduced the term

adakite and the criterion by which they are defined.

Hence, it is to this definition that the rocks of the

GVC have been evaluated.

Adakite Genesis

The majority of the models for adakite genesis fall into

two main categories: slab melting and non-slab melting.

In simple terms, the slab melting model reads that young,

hot subduction zones retain enough residual heat in the

slab to allow it to melt at shallower than typical depths

(Defant and Drummond 1990). Adakite magmas may also

form in response to flat subduction (e.g., Gutscher et al.

2000) or as a result of slab edge melting at slab windows

(Thorkelson and Breitsprecher 2005). Slab partial melts

are dacitic in composition with a restite rich in garnet

amphibole but plagioclase-poor. The melts react with the

Fig. 5 a, b Total alkali versus

silica diagram with IUGS rock

designations for the GVC

center rocks (after Le Bas and

Streckeisen1991).bRepresents

an enlargement of the area

where GVC samples fall.

Dashed line represents the

alkalinesubalkaline boundary

from Macdonald (1968).Symbolsas in Fig. 4


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overlying mantle wedge during ascent, modifying their

major and compatible element chemistries (e.g., increased

Mg#, CaO, Ni, Cr, lower SiO2) but keeping diagnostic

trace element concentrations and ratios intact (Sr, Y, REE;

Moyen2009), which remain recognizable as having a slab

melt origin. Other models for adakite genesis that are not

associated with slab melting include partial melting of

mafic lower continental crust (Huang and He 2010),

fractional crystallization of basaltic magma containing

garnet (Macpherson et al. 2006; Coldwell et al. 2011)

and high pressure assimilation, fractional crystallization

(AFC) processes of mantle-derived melts, and magma

mixing (Chiaradia et al. 2009). The slab melt model

has been refined by Martin et al. (2005). Using a

database of >340 adakite analyses (previously compiled

by Martin and Moyen 2003), Martin et al. (2005) pro-

posed that adakites should exhibit the following

characteristics: >3.5 wt.% Na2O, K2O/Na2O ratios of0.42,

an Mg# of approximately 50, FeO+MgO+MnO+TiO2

7 wt.%, and relatively high Ni (24 ppm) and Cr

Fig. 6 (ag) Harker diagrams illustrating variations in trace element

chemistry for the GVC centre rocks, with increasing SiO2. Plots of

adakitic indices for GVC rocks: fSr/Y versus Y and g La/Yb versus

Yb. Key symbology as in Fig. 4; adakite and normal arc-rock fields

derive from Castillo (2006) and Richards and Kerrich (2007)


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(36 ppm). As a result, Martin et al. (2005) introduced two

different groups of adakite, a high-SiO2adakite (HSA) and a

low SiO2(LSA) adakite. HSA contains >60 wt.% SiO2, lower

MgO (0.5 to 4 wt.%), CaO+Na2O < 11 wt. %, and Sr contents

< 1100 ppm. LSA contain 10 wt.% CaO+Na2O, and Sr in excess of 1,000 ppm. These

two groups are also differentiated by other trace elements and

REE (Martin et al.2005). LSA have higher LREE contents, a

Fig. 7 a Primitive mantle-

normalized spidergrams for

GVC samples analyzed in this

study, and b for previously

published data. Data in (b)

derive from Green and

Henderson (1984). Primitive

mantle normalizing values are

from Lyubetskaya and

Korenaga (2007).Symbolsas inFig.4


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more pronounced positive Sr anomaly and are generally

Rb-poor compared with HSA. The petrogenesis of LSA

and HSA are both related to slab melting, but the

magma sources are thought to differ. Martin et al.

(2005) suggested that HSA are slab melts that have

assimilated mantle wedge peridotite during ascent, prior

to eruption. LSA are not primary slab melts, but the

result of partial melting of mantle wedge peridotite that

Fig. 8 a Chondrite-normalized

REE spidergram for GVC

samples analyzed in this study,

and b for previously published

data. Data in (b) derive from

Green and Henderson (1984).

Chondrite normalizing values

are from Sun and McDonough

(1989). Symbolsas in Fig.4


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has been metasomatized by slab-derived melts. Adakite

rocks as defined by Defant and Drummond (1990) are

classified as HSA.

Adakite Geochemistry in the GVC

The rocks analyzed in this study and previously published

data conform to virtually all of the adakitic geochemical

traits put forward by Defant and Drummond (1990) and

Martin et al. (2005). However, there are some variations in

trace and major element contents which illustrate a more

typical arc-like magma composition for the GVC. Thesevariations are likely the result of mixing between pre-

existing, HSA magmas and intruding non-adakitic magmas

in the small, intermittent magma chambers beneath the

various centers. Despite this mixing, trace element ratios

and REE concentrations indicative of slab partial melts are

preserved in the investigated GVC rocks. While determined

Sr/Y ratios for the GVC cover a wide range (38109;

Fig. 6f, Table 2), the majority of values fall between 75

and 90. Some of the previously published data (Black Tusk

and the Barrier flow, in particular) straddle the boundary

between adakite and normal arc-rocks, which appears to be

related to elevated Y contents reported in these earlier pub-lished data. Moreover, all of the GVC samples have La/Yb

ratios that lie in the normal arc-rock field and not in the

adakite field (Fig.6g). This is a function of the low La in the

GVC versus the Yb contents, which are typical of adakites

(

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andesite (09JF011) is considerably higher in SiO2, Ni, Cr,

and MgO content and lower in Fe2O3 than the other two

andesite samples analyzed. Hence, it is likely that this more

mafic sample did not have the same mixing components, or

perhaps underwent a stronger mixing process in the volcano

than the other Paul Ridge andesite samples. Green (1977)

noted that some minor pyroclastic material was present at

Paul Ridge, but individual flows were not identified. It ispossible that the more mafic sample taken from the northern

part of Paul Ridge may represent a pyroclastic component

that has a stronger basaltic character and may be more

geochemically similar to the intruding basaltic magma com-

pared with the other andesite samples.

For the Ring Creek andesite, there is no evidence of

interaction with a basaltic intruding magma and mixing

relationships are less clear, suggesting that the mixing com-

ponents were of a similar composition. The mineralogy of

the Ring Creek andesite varies from an augitehornblende

biotite-bearing assemblage in the proximal portion of the

flow to augite only in the distal portion. The proximalandesite also appears to contain primary quartz, where

quartz crystals in the distal Ring Creek andesite show

disequilibrium with the surrounding melt and hence, is

difficult to explain through a simple mixing process.

However, Sivertz (1976) in mapping the Ring Creek an-

desite and Opal Cone also noted this difference in miner-

alogy, concluding that the hydrous mineral assemblage in

the proximal Ring Creek andesite was identical to the

mineralogy of Opal Cone itself. It is plausible, therefore,

that the proximal Ring Creek andesite entrained material

from Opal Cone during eruption and inherited the horn-

blendebiotitequartz mineralogy.

Mixing can result from convective overturn initiated by

intrusion of hotter magma from depth, but there is no evi-

dence of mixing between compositionally distinct magmas

in the Ring Creek andesite. Magma chamber overturn be-

tween magmas of a similar composition can be caused if the

intruding magma has a high upward momentum (Turner and

Campbell1986), or was significantly hotter. The Ring Creek

andesite flow is unusually extensive for an intermediate to

felsic composition, approximately 17 to 18 km in length

(Sivertz 1976; Green 1977; Brooks and Friele 1992). It

represents predominantly effusive volcanism with no evi-

dence of any pyroclastic component associated with the

flow. Sivertz (1976) described lapilli and block fragments

of dacitic composition, but this material is only found within

Opal Cone. The large extent of the Ring Creek andesite flow

and the lack of any significant pyroclastic material might

suggest that intrusion of a large volume of relatively fast

moving magma (the proximal andesite) of similar composi-

tion into pre-existing, cooling magma (the distal andesite)

within the magma chamber created turbulence that facilitat-

ed entrainment of wall-rock material from Opal Cone as the

eruption proceeded. It is not known to what extent the

hornblendebiotitequartz mineralogy is present in the

Ring Creek andesite further south; access to the center

portion of the flow is very limited (Sivertz1976).

In the Black Tusk andesite, mixing relationships are not as

evident as for other centers within the GVC. The lava is

essentially aphyric, but the phenocrysts that are present are

strongly resorbed. This suggests that an intruding lava waslikely significantly hotter than the pre-existing magma, which

results in the near-total resorption of the phenocrysts and

obscures any other meltcrystal relationships. Additionally,

the intruding magma appears to be a very similar composition

to the resident magma; both are comprised of the same min-

eralogy. This may indicate that the volume of the intruding

magma was large, in order to superheat the system and

allow for the disaggregation of all the phenocrysts. Mixing

in the Barrier andesite was likely between that of a dacitic

pre-existing magma and an andesitic intruding magma. This

is evidenced by reaction rims of augite on quartz pheno-

crysts and strongly embayed biotite crystals. Hornblendeappears to be in equilibrium with the intruding andesitic

lava; there are few disequilibrium features in the pheno-

crysts except for rims of opaques along the grain margins,

and these may be the result of hornblende being removed

from its stability field during ascent. At Columnar Peak,

both the pre-existing and intruding magmas appear to be

compositionally alike. There are disequilibrium features in

the plagioclase and hornblende phenocrysts, but the intrud-

ing magma generally contains the same mineralogy, with

the exception of minor orthopyroxene (

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Interaction with mantle peridotite

The Ni, Cr, and Mg# values for the GVC adakite rocks are

generally higher than normal andesite and dacite (20 ppm,

2025 ppm, and 42, respectively) and, due to the lack of

evidence of a basaltic mixing component for the majority of

the GVC (with the exception of the Paul Ridge andesite),

these values likely reflect the interaction of HSA magmaswith mantle peridotite during ascent. Increasing the Mg#,

Ni, and Cr concentrations in slab melts by assimilation of

peridotite (as described for HSA by Martin et al.2005) is an

unlikely process for the GVC. Typical mantle peridotite

contains 3,200 ppm Cr, 2,300 ppm Ni, and 42 wt.%

MgO (Sigurdsson et al. 2000). To obtain the values ob-

served in the GVC adakites, assimilation of peridotite would

be strongly limited (1 % assimilation to obtain the Cr

values of GVC). Furthermore, peridotite begins to melt at

approximately 1,200 C (at lower pressures) and exceeds

that of the dacitic (900 C) slab melt, precluding any

significant partial melting of the peridotite. A more likelyprocess would be zone refining, whereby the ascending

HSA magma gains Ni, Cr, and MgO by diffusion. This

process enriches the adakite magma in mafic components

and lowers SiO2, but preserves the incompatible element

ratios of the slab-melt, as the diffusion rates of incompatible

elements (e.g., REE, Y) would be too slow to significantly

modify the ascending magma (Wilson 1989) and obscure

the slab-melt signature.

LSA versus HSA

Geochemical characteristics distinguish the Columnar Peak

dacite and the Ring Creek andesite rock as mainly HSA and

the Paul Ridge andesite and the Black Tusk andesite as

predominantly LSA. Previously published data from The

Table andesite are also considered and fall within the LSA

field. The Barrier andesite rocks appear transitional between

the two groups. LSA are best differentiated from HSA in

terms of K/Rb versus SiO2/MgO, and Sr-

K/Rb-(SiO2/MgO)*100 diagrams (see Fig. 9a and b). LSA

and HSA define an almost perpendicular relationship on the

K/Rb versus SiO2/MgO diagram, with LSA plotting at

higher values of K/Rb whereas K/Rb ratios for HSA remain

relatively uniform. On the Sr-K/Rb-(SiO2/MgO)*100 terna-

ry diagram, the relationship for the GVC rock is less clear;

HSA forms a group close to the (SiO2/MgO)*100 apex and

LSA plots towards the center of the plot. The Black Tusk

and Paul Ridge andesites fall clearly in the LSA field in

Fig. 9a, but both centers plot closer to the HSA field in

Fig. 9b. Other discrimination diagrams that compare LSA

and HSA are shown in Fig.10acand illustrate the lack of a

definite separation of LSA and HSA groupings in the GVC.

The dashed fields and insets representing LSA and HSA in

Figs. 9 and 10 are from the data presented in Martin et

al. (2005), which included analyses that had extreme

values (K/Rb up to 3,000, average Sr> 2,000 ppm, Nb

up to 20 ppm). For the range of values in the GVC

(K/Rb660880, Sr7501,300 ppm, Nb8 ppm),

LSA and HSA generally plot in the same space, creating

further confusion.

On a primitive mantle-normalized spider diagram (Fig. 7a

b), LSA differs from HSA by lower Rb and higher Nb values,

with a positive Sr anomaly and no Ti anomaly (Martin et al.

2005). The GVC rocks generally characterized as LSA (Paul

0

2

4

6

8

10

45 50 55 60 65 70 75 80

MgO

SiO2

0

5

10

15

20

25

45 55 65 75

Nb(ppm)

SiO2

0

500

1000

1500

2000

2500

3000

3500

0 5 10 15

Sr(ppm)

CaO + Na2O

MgO%

SiO %2

8

6

4

2

045 50 55 60 6 5 7 0 75

LSA

HSA

Nb ppm

20

15

10

5

045 50 55 60 65 7 0 75 SiO %2

LSA

HSA

r ppm3000

2500

2000

1500

1000

500

00 5 10 (CaO+Na O%)2

LSA

HSA

Fig. 10 aMgO versus SiO2,bNb versus SiO2, andc Sr versus CaO+

Na2O discriminant diagrams illustrating the variability of HSA and

LSA compositions in the GVC dataset. Insetplots in (ac) are modi-

fied from Martin et al. (2005).Symbolsas in Fig. 4


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Ridge and Black Tusk andesite rocks) have lower Rb, but also

lower Nb and similar Sr to HSA, as well as a negative Ti

anomaly. Similarly, the Ring Creek andesite and the

Columnar Peak dacite, which are predominantly HSA, have

higher Nb and Sr (on average), typically a characteristic of

LSA. The Barrier andesite has Rb values between the Paul

Ridge and Columnar Peak rocks, but also the highest Nb

contents of all GVC rocks (up to 8 ppm). The Table andesitesamples are the only rocks which exhibit all of the LSA

characteristics; the andesite rocks have the highest Nb and Sr

with no Ti anomaly (Fig. 7b). The Table andesite rocks also

consistently plot in the LSA fields in Figs. 9and10.

Isotopic concentrations and effects of crustal interactions

Stern and Kilian (1996) noted that the effects of crustal

interactions were present in adakite rocks from the Austral

Volcanic Zone (AVZ), and these decreased southward in the

belt as the angle of subduction became more orthogonal.

This resulted in negligible interaction of the Cook Islandadakites with crustal material and hence, their Sr, Nd, Pb,

and O isotopic compositions more closely resemble MORB

(and by extension, slab partial melt) values. Crustal contam-

ination has been suggested to contribute to the chemistry of

the GVC lavas (Green1990; Green and Henderson1984),

specifically in the MGVF, based upon 87Rb/86Sr and87

Sr/86

Sr isotopic data. Green (1990) stated that the Mount

Garibaldi rocks contain a significant Rb-rich crustal compo-

nent based on higher 87Rb/86Sr isotopes than the GLVF and

that these values reflected AFC processes combined with

contamination from crustal xenoliths and mixing with

anatectic melts during ascent. While mixing of melts is

present in the MGVF, there is little evidence for the incor-

poration of crustal xenoliths in any rocks examined as part

of this study. Rare xenoliths are present in the Ring Creek

andesite but are mafic to intermediate in composition and

would not significantly modify Rb values. Lower Rb values

in the GLVF (418 ppm) was suggested by Green (1990) to

reflect a depletion of LILE in the source region and less

crustal interaction than the Mount Garibaldi rocks. For the

MGVF rocks in this study, the Rb concentrations are com-

parable to that determined by Green (1990) for the GLVF

(average 14.5 ppm). Similarly, the high 87Rb/86Sr values are

in samples that are rhyodacite to rhyolite in composition,

which is more felsic than the rocks examined in this study.

Green (1990) did not specify the exact sample locations, but

rhyodacite is present in Mount Garibaldi itself and may be

the location represented by the data presented by Green

(1990). Determination of isotopic data was not within the

scope of this work, and any conclusions on the effects of

crustal interaction in the GVC cannot be fully elucidated.

Similarly, while the above interpretations can account for

the geochemical variations seen in the GVC, it must be

noted that future, more detailed studies with a larger dataset

may argue against these hypotheses.

Origin of the GVC Adakites

The lack of a clear distinction between LSA and HSA

groups and limited data preclude a complete assessment of

adakite genesis within the GVC. However, data from theGVC can be compared with existing models for adakite

genesis and, as such, can provide some insight into possible

magmatic processes occurring in the development of the

GVC rocks. It is likely, based upon the data examined in

this study, that partial melting of the subducting Juan de

Fuca Plate played an important role in the generation of

GVC magmas. An adakite signature is ubiquitous across

GVC rocks (Sr>750 ppm, Yb

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adakitic chemistry (Sr/Y and La/Yb) can be obtained via

AFC and/or partial melting, these processes are likely not

the dominant ones occurring in the GVC. Average thickness

of continental crust in the GVC is approximately 35 km

(Perry et al. 2002); typically this is too thin for garnet to be

stable at the base of the crust and, therefore, a control on

HREE contents (e.g., Stern and Kilian 1996). However,

Garrido et al. (2006) have suggested that restitic garnetmay occur in the roots of island arcs at these depths in

response to dehydration melting of amphibole-bearing plu-

tonic rocks. Schiano et al. (2010) used trace element model-

ling on a database of 700 rocks from the Ecuadorian Andes

to illustrate that mixing was the major control on their

evolution. By plotting the ratios of compatible and incom-

patible elements, fractional crystallization, partial melting,

and mixing processes can be distinguished from each other

(Allgre and Minster1978; Schiano et al.2010). By plotting

an incompatible element versus the ratio of that incompati-

ble element and a compatible element (e.g., Rb versus

Rb/V), mixing and fractional crystallization will form acurved trend whereas partial melting forms a linear trend

(Fig. 11a). In the GVC, the Black Tusk and Paul Ridge

andesites show a clear mixing or fractional crystallization

process; the Ring Creek and Barrier andesites only show a

slight curved trend. The limited data from the Columnar

Peak dacite cannot show a curved versus linear correlation,

but in the GVC as a whole, illustrates a curved array. To

isolate mixing from fractional crystallization, a companion

plot is needed where the incompatible/compatible element

ratio is plotted against 1/compatible element (1/V versus

Rb/V, Fig.11b). On this companion diagram, mixing creates

the linear trend and partial melting and fractional crystalli-

zation plots as the curve; all the GVC lavas plot as lineartrends. Figure 11 also illustrates that the mixing trends

reflect magma mixing and not mixing of sources. Partial

melting of a heterogeneous source would significantly mod-

ify the incompatible/compatible element ratio, whereas ra-

tios of incompatible elements only would not be affected

(Langmuir et al. 1978; Schiano et al. 2010). This suggests

that the mixing relations observed in the GVC occurred in

the magma chamber (or chambers) beneath each center after

separation from their solid source and argues against signif-

icant fractional crystallization processes controlling the

chemistry of the GVC adakites.

Conclusions

Petrographic and geochemical examination of the interme-

diate rocks erupted at some of the centers that comprise the

Fig. 11 Incompatible/

compatible element ratio plots

for:aRb versus Rb/Vandb1/V

versus Rb/V distinguishing

mixing from both partial

melting and fractional

crystallization processes. Inset

schematics are modified after

Schiano et al. (2010). In plota,

a curved trend is generated by

either mixing or fractional

crystallization where a linear

trend is indicative of partial

melting. To isolate the mixing

relationship, a companion plot

must be used in tandem. The

linear trend in plotb illustrates

the dominance of mixing in the

GVC rocks and not fractional

crystallization (or partial

melting). Symbolsas in Fig. 4


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GVC illustrate that the andesites and dacites are adakitic in

character. Volcanic rocks from Paul Ridge, the Ring Creek

flow, and Columnar Peak in the MGVF were compared with

rocks from the Black Tusk and the Barrier flow in the GLVF

as well as previously published data. All the rocks from the

GVC exhibit Sr/Y>40, low Yb (

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