Bull Volcanol (2011) 73:479–496
DOI 10.1007/s00445-010-0418-z
RESEARCH ARTICLE
Volcanology and petrology of Mathews Tuya, northern
British Columbia, Canada: glaciovolcanic constraints
on interpretations of the 0.730 Ma Cordilleran paleoclimate
Benjamin R. Edwards & James K. Russell &
Kirstie Simpson
Received: 12 June 2009 / Accepted: 5 October 2010 / Published online: 11 November 2010
# Springer-Verlag 2010
Abstract Petrological, volcanological and geochronological
data collected at Mathews Tuya together provide constraints on
paleoclimate conditions during formation of the edifice. The
basaltic tuya was produced via Pleistocene glaciovolcanism in
northern British Columbia, Canada, and is located within the
Tuya volcanic field (59.195°N/130.434°W), which is part of
the northern Cordilleran volcanic province (NCVP). The
edifice comprises a variety of lithofacies, including columnarjointed lava, pillow lava, massive dikes, and volcaniclastic
rocks. Collectively these deposits record the transition from an
explosive subaqueous to an effusive subaerial eruption
environment dominated by Pleistocene ice. As is typical for
tuyas, the volcaniclastic facies record multiple fragmentation
processes including explosive, quench and mechanical
fragmentation. All samples from Mathews Tuya are olivineplagioclase porphyritic alkali olivine basalts. They are mineralogically and geochemically similar to nearby glaciovolcanic
centers from the southeastern part of the Tuya volcanic field
Editorial responsibility: M.A. Clynne
Electronic supplementary material The online version of this article
(doi:10.1007/s00445-010-0418-z) contains supplementary material,
which is available to authorized users.
B. R. Edwards (*)
Department of Earth Sciences, Dickinson College,
Carlisle, PA 17013, USA
e-mail: edwardsb@dickinson.edu
J. K. Russell
Volcanology and Petrology Laboratory, Earth & Ocean Sciences,
University of British Columbia,
Vancouver, BC, Canada
K. Simpson
Geoscience BC,
#440-890 West Pender Street,
Vancouver, BC V6C 1J9, Canada
(e.g., Ash Mountain, South Tuya, Tuya Butte) as well as the
dominant NCVP rock type. Crystallization scenarios calculated with MELTS account for variations between whole rock and
glass compositions via low pressure fractionation. The
presence of olivine microphenocrysts and the absence of
pyroxene phenocrysts constrain initial crystallization pressures
to less than 0.6 GPa. The eruption of Mathews Tuya occurred
between 0.718±0.054 Ma and 0.742±0.081 Ma based on
40
Ar/39Ar geochronology (weighted mean age of 0.730 Ma).
The age determinations provide the first firm documentation
for large (>700 m thick), pre-Fraser/Wisconsin glaciers in
north-central British Columbia ~0.730 Ma, and correlate in
age with glaciovolcanic deposits in Russia (e.g., Komatsu et
al. Geomorph 88: 352-366, 2007) and with marine isotopic
evidence for large global ice volumes ~0.730 Ma.
Keywords Mathews . Tuya . Cordilleran . Glaciation .
Pleistocene . NCVP . AOB
Introduction
Volcanoes that erupt in glacial environments produce unique
features that can be used to constrain paleo-environmental
conditions at the time of eruption. Such glaciovolcanic
products have been documented worldwide, but are especially
prevalent in Iceland (e.g., Kjartsansson 1943; van Bemmelen
and Rutten 1955; Sigvaldason 1968; Jones 1969; Tuffen et
al. 2002; Hoskuldsson et al. 2006; Schopka et al. 2006;
Jakobsson and Guðmundsson 2008; Skilling 2009), western
North America (e.g., Mathews 1947; Hoare and Coonrad
1978; Allen et al. 1982; Moore et al. 1995; Hickson 2000;
Lescinsky and Fink 2000; Dixon et al. 2002; Edwards et al.
2002, 2006, 2009; Kelman et al. 2002; Bacon and Lanphere
2006), and Antarctica (e.g., Skilling 1994; Smellie and
480
Skilling 1994; Wilch and McIntosh 2007; Smellie et al.
2008; Smellie 2009). Tuyas, also referred to as ‘table
mountains’ (e.g. Mathews 1947; van Bemmelen and Rutten
1955), represent one of two uniquely distinctive glaciovolcanic landforms. The term ‘tuya’ was coined 60 years ago by
W.H. Mathews, based on his landmark work describing the
stratigraphy and morphology of basaltic volcanoes in the
Tuya-Teslin region of northwestern British Columbia
(Mathews 1947). There, he encountered numerous, small,
apparently young, volcanic edifices hosting a variety of
enigmatic features. Most of the volcanoes are basaltic in
composition, are steep-sided and many have “flat” tops.
Mathews recognized that these volcanic edifices shared
common stratigraphic elements, including: pillow lavas and
breccias, massive to bedded deposits of fragmented glassy
basalt (‘hyaloclastite’), and capping, massive basaltic lava.
He proposed the term ‘tuya’ for these volcanoes and
interpreted their morphology and attendant volcanic lithofacies as indicative of volcanic eruptions from beneath and
within Pleistocene ice sheets. He also recognized that the
transition from pillow lava and volcanic breccia to subaerial
lava flows was an intrinsically important marker horizon for
establishing the minimum thickness of the confining ice, thus
providing critical information about paleo-environmental
conditions extant during the eruptions. Jones (1969) named
these transitions ‘passage zones’, which have recently been
described in detail by Smellie (2006).
Relatively few studies subsequent to Mathews (1947)
have examined the formation of basaltic tuyas in detail (e.
g., Smellie and Skilling 1994; Moore et al. 1995; Werner
and Schmincke 1999; Skilling 2009), and even fewer have
attempted to integrate stratigraphic, petrological, and
geochronologic data (Werner and Schmincke 1999). Stratigraphic studies of flat-topped volcanoes formed by glaciovolcanism derive mainly from work in Iceland (van
Bemmelen and Rutten 1955; Sigvaldason 1968; Jones
1969; Werner and Schmincke 1999; Tuffen et al. 2002;
Skilling 2009), Antarctica (e.g., Smellie and Skilling 1994;
Skilling 1994; Smellie et al. 2008), and British Columbia,
Canada (e.g., Mathews 1947; Allen et al. 1982; Moore et al.
1995; Dixon et al. 2002). Tuyas likely formed in other
glacially covered, volcanically active regions of Earth in the
past (e.g. Russia; Komatsu et al. 2007), and are also
thought to be present on Mars (e.g., Allen 1979; Chapman
et al. 2000); their presence and stratigraphy can be used to
constrain planetary paleoclimates (e.g., Smellie 2009).
Here we present an integrated study of the formation of
Mathews Tuya, which is located in the Tuya volcanic field of
northern British Columbia (59.195°N/130.434°W; Fig. 1;
Mathews 1947; Allen et al. 1982; Moore et al. 1995). The
purpose of this paper is threefold. First, we describe the
geology and petrology of Mathews Tuya, including descriptions of the critical lithofacies and measurements of chemical
Bull Volcanol (2011) 73:479–496
compositions of whole rock and glass samples. Most of the
field-based observations were made in July/Aug 1995, with a
brief reconnaissance visit in 2009. Secondly, we present a
model for the physical and chemical evolution of the volcano
that integrates the observed stratigraphic relationships and
the petrological character of the volcanic rocks. Lastly, we
use new 40Ar/39Ar geochronometry to constrain the regional
paleoclimatic conditions in the area surrounding Mathews
Tuya and, in conjunction with previously published data, to
provide a preliminary view from glaciovolcanic constraints
for the distribution of Pleistocene glaciers in the Canadian
Cordillera.
Geological setting
The Tuya volcanic field of the northern Cordilleran volcanic
province (NCVP) is an extensive area of Neogene to Recent
alkaline volcanism formed from transtensional stresses concentrated within the western Canadian Cordillera after
subduction of the Farallon and Nazca plates (Fig. 1; Edwards
and Russell 1999, 2000; Madsen et al. 2006). Due to the
spatial and temporal coincidence of NCVP volcanism and
Pleistocene glaciations, including the Cordilleran Ice Sheet
(CIS), a diverse range of glaciovolcanic landforms and
deposits have been documented within the NCVP (e.g.,
Allen et al. 1982; Souther 1992; Moore et al. 1995; Hickson
2000; Dixon et al. 2002; Edwards et al. 2002, 2006, 2008).
More than 30 individual volcanic centers/deposits are
mapped as part of the Tuya Formation in the Jennings River
1:250,000 map sheet (Fig. 1b; Watson and Mathews 1944;
Gabrielse 1970); at least 20 of those deposits show evidence
for glaciovolcanism. Mathews (1947) gave brief descriptions
of the petrology and volcanology of six tuyas, including
what has been informally referred to as ‘Mathews Tuya’.
Several studies have subsequently been published on the
volcanology, petrology and geochemistry of the volcanic
centers that form part of the Tuya Formation (Allen et al.
1982; Moore et al. 1995; Simpson 1996; Dixon et al. 2002;
Wetherell et al. 2006; Simpson et al. 2006), but none have
done more than briefly mention ‘Mathews Tuya’. Work by
Allen et al. (1982) and Moore et al. (1995) gave brief field
and geochemical descriptions of Tuya Butte, Ash Mountain
and South Tuya (Fig. 1c; Table 1). Dixon et al. (2002)
presented data on volatile behavior and geochemistry of
Tanzilla Butte, a glaciovolcanic edifice immediately southeast of Tuya Lake; they concluded that ice was up 1 km thick
at the onset of eruption. Wetherell et al. (2006) and Simpson
et al. (2006) documented subaerial components of the Tuya
Formation, which appear to be less common than glaciovolcanic deposits.
Gabrielse (1970) inferred that the area underlying the
Tuya Formation had been affected by regional ice sheets,
Bull Volcanol (2011) 73:479–496
Fig. 1 Maps showing the location of Mathews Tuya. a Digital
elevation model (DEM) of the
Canadian Cordillera showing
the location of the Tuya volcanic
field in the northern Cordilleran
volcanic province (NCVP;
Edwards and Russell 2000). The
NCVP comprises predominantly
mafic, pre-Holocene volcanic
centers (red dots), with minor
Holocene activity (yellow dots)
and relatively few centers with
intermediate to felsic lavas
(green dots). Also shown are
other major Neogene to Recent
volcanic provinces (e.g.,
Wrangell, Anahim, Wells Grey,
Chilcotin and Cascade) situated
in the Canadian Cordillera. b
Distribution of the Quaternary
Tuya Formation (orange) in the
Jennings River 1:250,000 map
sheet (NTS 104O; Gabrielse
1970), including Mathews Tuya.
Abbreviations demarcate selected volcanic centers near Mathews Tuya, including Mount
Josephine (MJ), Tuya Butte
(TB), South Tuya (ST), Caribou
Tindar (CT), and Ash Mountain
(AM). c DEM of the region
surrounding Tuya Lake, showing the locations of volcanic
features mentioned in the text.
Stars and numbers indicate peak
elevations for centers discussed
in the text
481
a
Nome
20km Lake
0
Jennings
Lakes
b
30
Gabrielse
Cone
AM
ST
TB
West Tuya
Lava Field
30
c
Ash
Mountain
15
High Tuya
Lake
CT
Mathews
Tuya
MJ
Tuya
Lake
131 00
59 00 N
130 00 W
30
2100m
N
Caribou
Tindar
1850m
1650m
Tuya
Mount
Butte
Josephine
NW
Mathews
Tuya
1775m
South
Tuya
SE
1120m
0
and found evidence of tuyas to the south that have been
glaciated, but which appear to have formed on top of
glaciated bedrock, implying that the Tuya Formation could
span more than one glacial episode (Gabrielse 1998). As
glacial erratics, eskers, kame and kettle topography,
moraines and glacial lake deposits are widespread throughout the area, at least one ice sheet (i.e. the Last Glacial
Maximum CIS) probably covered all of the peaks in the
area at some point during the Pleistocene, followed by
potentially multiple episodes of alpine and valley glaciation
during and after retreat of the ice sheets (Gabrielse 1998).
Simpson (1996) presented the first detailed petrological
examination of Mathews Tuya and a preliminary analysis of
its stratigraphy and physical evolution. This work builds on
that study, reassesses its main conclusions, and aims to
place the detailed knowledge of the physical and chemical
5km
evolution of Mathews Tuya into a regional framework in
order to provide constraints for interpretation of paleoclimate conditions during formation of the tuya.
Physical and petrological characteristics of Mathews
Tuya
Mathews Tuya rises to an elevation of 1780 m above sea level,
with a relief of ~330 m, and has the characteristic flat top that
distinguishes tuyas from most other volcanic landforms
(Figs. 2 and 3). It is ~3 km long, elongated northeastsouthwest, and has a maximum exposed width of ~1.5 km.
The edifice is bounded on the south by a large (4.5 km
wide), northeast-trending U-shaped valley, which extends for
several kilometers northeast into the Cassiar Ranges and
482
Bull Volcanol (2011) 73:479–496
Table 1 Whole rock geochemical compositions (weight percent) and normative mineralogy for samples from Mathews Tuyaa
Sample
KAS2
KAS3A
KAS3B
KAS11
KAS12A
KAS12B
KAS14A
KAS14B
SiO2
TiO2
Al2O3
FeO(T)
MnO
MgO
CaO
Na2O
45.55
3.25
14.28
13.28
0.16
7.14
8.05
3.85
47.32
2.34
14.32
12.55
0.17
8.53
8.90
2.98
47.33
2.35
14.36
12.37
0.17
8.39
8.88
3.09
45.45
3.25
14.34
13.32
0.16
6.77
7.98
3.95
45.39
3.24
14.30
13.34
0.16
6.97
7.83
3.95
46.06
3.26
14.40
13.39
0.16
6.87
7.79
4.03
47.03
2.42
14.34
12.60
0.17
8.39
8.66
3.33
46.97
2.48
14.36
12.56
0.17
8.20
8.79
3.15
0.055
0.01
0.045
0.058
0.002
0.038
0.032
0.040
1.14
0.47
98.73
b.d.
1.18
0.47
98.59
b.d.
2.01
0.80
98.03
0.43
2.01
0.80
97.98
0.40
2.04
0.78
98.78
b.d.
1.29
0.52
98.75
b.d.
1.35
0.53
98.57
b.d.
0.010
0.003
6.74
23.24
22.33
1.07
15.46
24.37
4.44
1.12
6.97
23.06
21.83
1.67
15.79
23.74
4.46
1.12
11.88
17.06
15.47
8.86
15.74
21.01
6.18
1.88
11.88
17.12
15.36
8.83
15.20
21.63
6.15
1.88
12.06
18.24
15.18
8.59
15.27
21.46
6.19
1.85
7.62
21.68
20.37
3.52
15.82
23.95
4.60
1.24
7.98
21.62
21.06
2.73
15.74
23.52
4.71
1.26
K2O
1.81
P2O5
0.78
Total
98.14
LOI
0.61
Normative Mineralogyb
Or
10.70
Ab
18.25
An
16.34
Ne
7.76
Di
15.38
Ol
21.78
Il
6.16
Ap
1.84
1σ
a
analytical work was done at McGill University Geochemical Laboratories; b normative mineralogies are in weight percent and were calculated using an
Fe2+ /Fe3+ ratio constrained by fO2 =QFM
southwest into the Kawdy Plateau. The north side of the
edifice hosts a small cirque, and opens northward to a
smaller (1.5 km wide) U-shaped valley. The top of the
edifice is buttressed part way up on its south side by the
Parallel Creek batholith (Fig. 2b), which comprises biotite
granite and quartz monzonite (Fig. 2; Gabrielse 1970).
Lithofacies
Mathews Tuya is a basaltic volcano that comprises coherent
and fragmental lithofacies including: massive and jointed
lavas and dikes, pillow lava, and associated volcaniclastic
rocks (Figs. 2b, 3 and 4a–g). Outcrops at lower elevations
are sparse and are restricted to stream cuts on the north,
south and lower west faces, and sporadic outcrops
elsewhere (Fig. 2). Much of the low-lying landscape
surrounding the tuya is basaltic rubble; however, the
stratigraphy in these areas has likely been significantly
disrupted by permafrost, which is evident on the north and
northeast slopes. At higher elevations outcrop is more
abundant and forms prominent cliffs. Granitic cobbles and
boulders occur locally on most of the upper surfaces of the
edifice (Fig. 4h).
All of the lithofacies comprise black, variably vesicular (0–
30% vesicles), fine-grained, and olivine-phyric basaltic rocks
and clasts. Olivine microphenocrysts are up to 2 mm in
maximum dimension and comprise between 10 and 25% of
the rock; they are dominantly skeletal with some perfect
euhedral edges and locally contain melt inclusions (Figs. 5a,
c, g). Plagioclase laths are generally less than 1 mm long and
comprise 15 to 20% of the groundmass, which also contains
olivine, titanaugite and variable amounts of glass. Plagioclase laths are typically randomly oriented but locally form
trachytic textures (e.g., Figs. 5a and c). Oxides occur in all of
the samples, and are predominantly contained within olivine
microphenocrysts. Rare crustal xenoliths up to 1 cm are
present and locally show incipient melting.
Coherent
Flat-lying, columnar jointed basaltic lavas cap the edifice
(Figs. 2 and 4a). At least three distinct, stacked lava flows
are present and combine to form a total thickness that
ranges between ~20 and >80 m. The west side of the tuya is
~40 m higher in elevation than the eastern side and the
lavas are also thicker on the west side. Columnar jointing is
typically fine (<0.5 m), well developed, and locally highly
irregular in orientation (Fig. 4a). Columnar jointed lavas are
also found as isolated structures extending down the slopes
of the northwest, northeast and south central flanks, locally
Bull Volcanol (2011) 73:479–496
Fig. 2 Aerial view and
simplified geology of Mathews
Tuya. a Aerial photograph
showing relatively flat-topped,
high standing, oval shape of
Mathews Tuya (MT). Dashed
white line delineates the presumed original extent of MT
volcanic deposits. b Map
showing the general distribution
of coherent and volcaniclastic
lithofacies at Mathews Tuya.
Solid line denotes trace of
cross-section (A-A′; see Fig. 9b)
through the edifice. The
locations of samples described
in the text are denoted with
black stars and field sample
numbers
483
a
0
1.5km
N
b
1700
KAS12a,b
A
Undifferentiated
colluvium
KAS11
KAS9,10
KAS2
KAS7
KAS3a,b
KAS4 KAS5 KAS6
Undifferentiated
basaltic colluvium
Coherent Ol-phyric
basaltic facies
KAS8a,b
Volcaniclastic
facies
KAS14a,b
Interbedded coherent
and volcaniclastic facies
A’
Parallel Creek
Batholith granitoids
Oblique Creek Formation,
undifferentiated gneiss
and schist
1300
0
defined contact
approximate contact
1.5km
Contour interval = 20 m
with radial joint orientations perpendicular to inferred flow
directions (Fig. 4b, d). The eastern contact between
coherent basalt and granitic basement is distinct, sharp
and clearly visible on aerial photography, as a southwest
trending lineament.
Two 60 cm wide, south trending, steeply dipping,
massive dikes (200°/86°E, 180°/88°E) intrude coarse
volcanic breccia in the lower section of the north flank,
and locally show glassy margins (Fig. 4c). Dikes may also
be present in the upper wall of the north-facing cirque.
Exposures of pillow lava are most abundant on the
northwestern flank (Figs. 3a and 4d) where they commonly
occur with associated, interstitial volcanic breccia. Individ-
N
ual pillows range from 0.5 to ~1 m in diameter, up to 3 m in
length, and generally plunge moderately to steeply to the
northwest downslope. Pillows of similar size with no
interstitial volcaniclastic deposits are locally present lower
on the northwest flank.
The coherent lithofacies are interpreted to include
subaerial and ice-confined columnar jointed lavas, subaqueous pillow lava, and syn-eruption dikes. The variably
oriented and finely columnar-jointed lavas are consistent
with irregular cooling surfaces and rapid cooling. The radial
jointing displayed by some of the lava lobes is consistent
with lava tubes or emplacement into ice-confined tunnels.
In addition, the lavas do not extend great lateral distances
484
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a
b
c
d
Fig. 3 Morphology of Mathews Tuya. a View looking to the south at
the northern side of the tuya. Note the flat top on the northern side of
the edifice. b Traced line-diagram of a, with approximate boundaries
of geological units. See the legend in Fig. 2 for color code of specific
units for b and d. c Aerial view looking to the north showing the south
side of the edifice, with the Cassiar Mountains in the background. d
Traced line-diagram of c, with approximate boundaries of geological
units
from the main volcanic edifice, which further supports a
hypothesis of ice confinement. The flat-lying, capping
columnar-jointed lavas are interpreted to have erupted
subaerially.
The presence of pillow lava is indicative of subaqueous
deposition (e.g., Moore 1975; Walker 1992). The dipping
geometry of the pillow lava tubes and association with
pillow-fragment breccia is consistent with formation of a
lava-fed delta (e.g., Swanson 1970; Skilling 2002) that was
prograding to the northwest.
The massive basaltic dikes crosscut the stratigraphy in
the lower part of the edifice and their exposure at the
surface confirms that an undetermined amount of the tuya
has been eroded locally.
highly vesicular (scoriaceous) lava. The mineralogy of the
clasts is the same as that of the coherent facies, except that
the groundmass has more glass, and the vitric clasts are
variably palagonitized (e.g., Fig. 5c and f). The coarsegrained lithofacies is poorly-sorted, matrix-supported and
contains breccia-sized fragments from 2 to 120 cm in
maximum dimension (Figs. 4e and f). The scoriaceous
clasts are subrounded to angular and the non-vesicular/
poorly vesicular clasts are typically blocky, angular and
commonly have curviplanar clast margins. Rare granitic
xenoliths up to 5 cm in size occur within some clasts, and
microlites in some clasts terminate at clast margins. The
matrix consists of angular, less than 2 mm, variably
vesicular glassy fragments and olivine crystal fragments.
Well-preserved bubble wall shards are common (Fig. 5d).
The coarse-grained lithofacies is massive to very crudely
bedded and locally occurs as lenses within the fine-grained
lithofacies (Figs. 4f and g). The fine-grained lithofacies has
identical components to the coarse-grained lithofacies
except that fragments are sand-sized (<2 mm). This
lithofacies is also thickly laminated to thinly bedded
(Fig. 4g). The pillow-fragment breccia also has the same
components as the coarse-grained lithofacies with the
addition of pillow fragments and locally this unit can
contain angular granite clasts (Fig. 4d). This lithofacies is
Volcaniclastic
Volcaniclastic lithofacies, defined in the general sense of
Fisher (1961) to include all clastic materials derived from
dominantly volcanic sources without implications for
fragmentation mechanisms, are generally orange-brown in
colour, monomictic, and basaltic. The volcaniclastic lithofacies are subdivided into three distinct units: 1) coarsegrained; 2) fine-grained; and 3) pillow-fragment breccia.
All three lithofacies comprise fragments of non-vesicular to
Bull Volcanol (2011) 73:479–496
Fig. 4 Field photographs
showing large-scale deposit
geometry and lithofacies. a
Finely jointed, massive lava
flow exposed on the
north-central flank of Mathews
Tuya. b Highly jointed lava lobe
on the northwestern flank of
Mathews Tuya. c Basal
lithofacies comprising dikes and
coarse basaltic breccia. The dike
is ~1.6 m wide for scale. d
Outcroppings at western end of
the tuya contain crudely dipping
beds of coarse and fine-grained
lithofacies interbedded with
pillow lava, capped by multiple
subaerial lava flows. e
Close-up view of matrix-poor,
poorly-sorted coarse-grained
lithofacies from base of the
northern side of the edifice. f
Close-up view of talus boulder
comprising matrix-rich,
palagonitized, poorly sorted
fine-grained lithofacies from
near the top of the south side of
the edifice. Lens cap is 6 cm is
diameter. g Outcrop view of
crude bedding and sorting in the
fine-grained lithofacies near the
upper south side of the edifice.
Field notebook is 19 cm long.
h Granitic glacial erratic sitting
on the flank of the edifice
485
a
b
g
h
associated with intact, moderately dipping pillow lobes on
the northwest flank of the edifice.
Within all three lithofacies the non- to poorly vesicular,
blocky, angular clasts with curviplanar clast margins are
interpreted to have formed by autoclastic processes (quench
fragmentation and autobrecciation; Figs. 5b, c, g, h). The
bubble wall shards and scoriaceous clasts, which are
dominant throughout most of the sampled sections, are
interpreted to have formed by explosive fragmentation
(Fig. 5d and e). Attrition of larger clasts during transport
and deposition also likely contributed to the generation of
clasts, although no evidence for significant transport (e.g.
rounding or sorting) was observed. The pillow-fragment
breccia contains fragments derived from the disintegration
of pillow lobes. The lack of jigsaw-fit textures, mixing of
variably vesicular clasts and the bedding within some units
supports a syn-eruptive origin involving non-explosive and
explosive magma-water interaction. Thus the volcaniclastic
deposits are interpreted to include fragments produced by
autoclastic (hyaloclastite +/- autobreccia) and pyroclastic
processes, which is typical of many glaciovolcanic deposits
(e.g. Werner and Schmincke 1999).
The angular granite clasts locally found within the
pillow-fragment breccia occur on the northwestern flank
486
Bull Volcanol (2011) 73:479–496
Fig. 5 Photomicrographs of
lithofacies from Mathews Tuya,
all taken in plane-polarized
light. a Trachytic, vitrophyric
basalt with olivine microphenocrysts and plagioclase
groundmass. b Fluidally-shaped,
vesiculated, pyroclast of vitrophyric olivine basalt in a matrix
of finer vitroclastic material
(pore space is filled by secondary cement). c Blocky-shaped,
non-vesicular pyroclast of trachytic, vitrophyric olivine basalt
with incipient palagonitization
along the clast rim. d Ash
fraction of vesicular basaltic
pyroclastic material with
bubble-rich, shard-like
pyroclasts. e Moderately sorted,
clast-supported, highly porous
deposit of highly vesicular basaltic tephra. f Close up view of
pyroclast in deposits shown in e,
showing relatively thicker areas
of palagonitization on smaller
clasts. g Fluidually-shaped,
glass-jacketed phenocrysts of
olivine. h Fluidally-shaped
pyroclast with vesicles that
range from equant (bottom
center) to highly stretched
(upper edge of clast)
of the edifice and appear to be locally derived from the
Parallel Creek batholith. The origins of the granitic
clasts are enigmatic but could include: 1) dropstones
melted out of surrounding ice during the course of the
eruption; 2) syn-eruption mixing of pre-existing glacial
sediment with volcaniclastic materials; or 3) blocks
ejected during the establishment of the volcanic conduit.
The angular clast shapes are most consistent with the
last hypothesis.
Geochronometry
Two samples of coherent lithofacies were analyzed using
40
Ar/39Ar geochronometry to constrain interpretations of
Bull Volcanol (2011) 73:479–496
the time of eruption and to enable correlation with the
global climate record. Previous workers have assumed that
much of the Tuya volcanic field formed during the Fraser/
Wisconsin glaciation (e.g., Mathews 1947; Moore et al.
1995; Dixon et al. 2002), although to our knowledge this
inference is not constrained by geochronometry.
40
487
two outermost steps on a plateau are not significantly
different from the weighted-mean plateau age (at 2 σ, six or
more steps only); (5) the outermost two steps on either side
of a plateau must not have nonzero slopes with the same
sign (at 2 σ, nine or more steps only). All analytical data
are included in Supplementary Tables 2 (KAS3a) and 3
(KAS14b).
Ar/39Ar methodology
Geochemistry
Bulk separates from two samples (KAS3a and KAS14b)
were washed in acetone, dried, wrapped in aluminum foil
and stacked in an irradiation capsule with similar-aged
samples and neutron flux monitors (Fish Canyon Tuff
sanidine, 28.02 Ma; Renne et al. 1998). The samples were
irradiated on July 4 through July 6, 2006 at the McMaster
Nuclear Reactor in Hamilton, Ontario, for 90 MWH, with a
neutron flux of approximately 3 × 1016 neutrons/cm2.
Analyses (n=57) of 19 neutron flux monitor positions
produced errors of <0.5% in the J value.
The samples were analyzed on August 2 and 15, 2006, at
the Noble Gas Laboratory, Pacific Center for Isotopic and
Geochemical Research, University of British Columbia,
Vancouver, BC, Canada. Samples were step-heated at
incrementally higher powers in the defocused beam of a
10 W CO2 laser (New Wave Research MIR10) until fused.
The gas evolved from each step was analyzed by a VG5400
mass spectrometer equipped with an ion-counting electron
multiplier. All measurements were corrected for total system
blank, mass spectrometer sensitivity, mass discrimination,
radioactive decay during and subsequent to irradiation, as
well as interfering Ar from atmospheric contamination and
the irradiation of Ca, Cl and K (isotope production ratios:
ð 40 Ar = 39 Ar ÞK ¼ 0:0302 0:00006, 37 Ar=39 Ar
¼
Ca
39
36
1416:4 0:5,
Ar= Ar
¼ 0:3952 0:0004, Ca=K ¼
Ca
1:83 0:01 37 ArCa =39 ArK ).
40
Ar/39Ar results
Results of the analyses give crystallization ages of 0.718±
54 Ma (KAS3a; initial 40 Ar=39 Ar ¼ 296:5 8:7) and
0.742±81 Ma (KAS14b; initial 40 Ar=39 Ar ¼ 302 34),
for a weighted mean age of 0.730 Ma (Fig. 6). The plateau
and correlation ages were calculated using Isoplot ver.3.09
(Ludwig 2003). Errors are quoted at the 2 σ (95%
confidence) level and are propagated from all sources
except mass spectrometer sensitivity and age of the flux
monitor. The best statistically-justified plateau and plateau
age were picked based on the following criteria: (1) three or
more contiguous steps comprising more than 50% of the
39
Ar; (2) probability of fit of the weighted mean age greater
than 5% (3) slope of the error-weighted line through the
plateau ages equals zero at 5% confidence; (4) ages of the
Analytical methods
Major element concentrations for all samples of lava were
analyzed by X-ray fluorescence at Geochemical Laboratories, McGill University, Quebec. Sample preparation
followed the methods described by Cui and Russell
(1995). Major element compositions were used to calculate
normative mineralogy for whole-rock samples (Table 1).
Estimates of analytical uncertainty derive from replicate
analysis of blind samples (Table 1).
Compositions of volcanic glasses were analyzed using a
Cameca SX-50 electron microprobe at the University of
British Columbia (cf. Russell and Hauksdottir (2001) for
methods). Microprobe work was performed to determine
the composition of volcanic glass within clastic samples
and the composition of a glassy rind from one of the two
dikes (see Table 2 for averages; a full set of analyses are
available in electronic format from the authors). Replicate
analyses were used to estimate analytical uncertainty.
Major elements
Samples of coherent lithofacies from Mathews Tuya all plot
as basalt, basanite (greater than 10% normative ol),
hawaiite and trachybasalt, above the alkaline-subalkaline
divide of Irvine and Baragar (1971; Fig. 7a). Whole-rock
samples are nepheline normative (Table 1) and plot in two
distinct groups (Figs. 7a and b). The first group plots on the
hawaiite-basanite boundary, and comprises samples from
lower in the stratigraphic section. The second group
comprises olivine basalt samples from higher in the section
that plot well into the basalt field, as well as a dike that
crosscuts the lower stratigraphy and may be a feeder dike
for units higher in the section.
Analyses of glass samples from volcaniclastic lithofacies
(samples KAS 8a, 8b, 15a) and a dike margin also plot in
two clusters (Fig. 7a). Clastic facies from relatively low in
the section plot as hawaiites/basanites, while the glass rim
of a feeder dike plots in the alkali basalt field. Glasses from
both clusters have lower Mg # than accompanying whole
rock analyses (Fig. 7b); however even though the wholerock Mg # differs by five units, the glasses from both
clusters have overlapping Mg #. The two clusters also have
488
a
b
1.6
0.0038
σ
0.0034
0.0030
40
0.8
0.0026
0.4
0.0022
KAS3a
KAS3a
0.0
0
20
40
60
80
0.0
100
0.2
0.0018
40
d
σ
1.2
0.004
40
Ar/ Ar
0.003
0.8
36
Age (Ma)
0.6
Ar/ Ar
Cumulative Ar Percent
c
0.4
39
39
0.002
0.4
KAS14b
0.0
Ar/ Ar
Age (Ma)
1.2
36
Fig. 6 40Ar/39Ar geochronometry for samples from Mathews
Tuya. Analytical data for
both samples is included in
Supplementary Tables 2 and 3. a
Plateau diagram for sample
KAS3a, a largely holocrystalline
basaltic dike (0.742±0.081 Ma).
Note that the first step (less than
~0.5% of cumulative 39Ar) is
not shown for clarity, and height
of individual boxes equals 2 σ
errors. b Inverse isochron plot
for KAS3a showing all heating
steps. Ellipses show 2 σ errors. c
Plateau diagram for sample
KAS14b, a largely holocrystalline subaerial lava flow (0.718±
0.054 Ma). Note that the first
step (less than ~0.5% of
cumulative 39Ar) is not shown
for clarity, and height of individual boxes equals 2 σ errors. d
Inverse isochron plot for
KAS14b showing all heating
steps. Ellipses show 2 σ errors
Bull Volcanol (2011) 73:479–496
0
20
40
60
80
39
Cumulative Ar Percent
distinctive Ca/Al ratios (Fig. 7b). Thus the glass and rock
analyses show the parallel relationships between units
lower and higher in the section. Previously published data
from nearby glaciovolcanic edifices (Ash Mountain, South
Tuya, Tuya Butte; Moore et al. 1995) show similar general
trends between whole-rock and glass analyses (Fig. 7a), and
in general parallel trends for the stratigraphically higher
group from Mathews Tuya (Fig. 7c). Although Moore et al.
(1995) postulated the existence of two distinct geochemical
groups at Ash Mountain, South Tuya, and Tuya Butte, the
amount of geochemical separation of the two groups at
those three volcanoes is minimal in comparison to the
compositional variation at Mathews tuya (Fig. 7b versus c).
Phase saturation diagrams
Phase saturation diagrams for each of three samples (KAS 2
[WR group 1], KAS 3a [WR group 2], KAS8a [glass]) were
calculated using MELTS (Ghiorso and Sack 1995; Asimow
and Ghiorso 1998) and are used to constrain interpretations
of the conditions attending crystallization of the microphenocryst assemblage (Fig. 8). The saturation calculations
show the relative stability of major phases for a fixed melt
composition. The saturation diagrams are not true phase
diagrams, as the calculations to do not take into account
changes in liquid compositions resulting from crystallization.
Although Opx is never seen as a phenocryst or groundmass
100
KAS14b
0.0
0.2
0.4
39
0.6
0.8
0.001
1.0
40
Ar/ Ar
phase in the samples, Ol is present as microphenocrysts and
Ol, Pl and Cpx are ubiquitous in the groundmass of all nonvitrophyric samples (Simpson 1996).
Calculations using whole-rock analyses as liquid compositions compare favorably to crystallization sequences
inferred from petrography (Ol > Ol+Pl > Ol+Pl+Cpx), with
Ol being the liquidus phase down to mid- to lower crustal
pressures (0.4 to 0.6 GPa). The observed compositional
differences between the two groups only slightly alter the
estimated phase stabilities as a function of pressure, with
the high SiO2 group losing Ol as the liquidus phase at
slightly lower pressures than the low SiO2 group. The
addition of one weight percent H2O enlarges the stability
field of Ol, making it the liquidus phase to even greater
depths (0.6 to 0.8 GPa) for both groups. Calculations
indicate that the minimum liquidus temperatures for the
magmas were greater than 1200°C.
Computations using the glass composition yield significantly different results. For low pressure, anhydrous
conditions plagioclase is predicted as the stable liquidus
phase, followed by Cpx at pressures greater than
~0.35 GPa. However, the addition of one weight percent
H2O destabilizes Pl, so that Ol followed closely by Cpx
become the predicted low pressure, liquidus phases. The
predicted liquidus temperatures as a function of pressure for
the glass sample are taken to indicate that the lava erupted
at a minimum temperature of ~1100°C.
Bull Volcanol (2011) 73:479–496
489
Table 2 Average rock and glass compositions with standard deviations and computed mineral saturation temperatures for anhydrous and hydrous
conditions
Sample
KAS3aa (n=10)
47.26
SiO2
TiO2
3.42
Al2O3
16.18
Cr2O3
0.02
FeO(T)
12.45
MnO
0.15
MgO
4.44
CaO
8.33
Na2O
4.56
K2O
2.25
P2O5
0.93
Totalf
100.00
Calculated Parameters
Tliq (dry)g
Phase
Tliq (wet)h
Phase
Fe2O3i
FeOi
1139
Pl (An64)
1113
Ol (Fo73)
2.297
10.382
1σ
KAS8ab (n=10)
0.18
0.06
0.15
0.01
0.12
0.03
0.07
0.07
0.12
0.05
0.07
48.77
3.08
14.58
0.04
12.85
0.19
4.86
9.48
3.91
1.56
0.68
100.00
1σ
0.13
0.05
0.18
0.02
0.19
0.03
0.07
0.08
0.05
0.02
0.05
1152
Pl (An65)
1118
Cpx (Di46)
2.326
10.756
KAS8bc (n=5)
48.01
3.21
15.72
0.01
12.32
0.15
4.47
8.62
4.57
2.08
0.84
100.00
1143
Pl (An64)
1111
Ol (Fo73)
2.290
10.258
1σ
0.36
0.18
0.15
0.01
0.19
0.04
0.21
0.41
0.24
0.19
0.10
KAS15d (n=10)
47.86
3.14
15.96
0.02
12.30
0.15
4.70
8.53
4.48
2.03
0.83
100.00
1145
Pl (An65)
1120
Ol (Fo74)
2.280
10.247
1σ
0.25
0.06
0.15
0.02
0.14
0.03
0.07
0.08
0.10
0.05
0.15
WRe (n=8)
47.12
2.87
14.56
nd
13.13
0.17
7.78
8.49
3.60
1.63
0.65
100.00
1σ
0.75
0.47
0.04
0.48
0.01
0.78
0.48
0.46
0.41
0.16
1224
Ol (Fo81)
1216
Ol (Fo81)
2.187
11.162
a
hyaloclastite; b dike selvage; c dike selvage; d hyaloclastite; e average of whole rock analyses from Table 1; f average analyses normalized to 100 weight
percent; g all calculations done at P=1 MPa using MELTS (Ghiorso and Sack 1995; Asimow and Ghiorso 1998); h ‘wet’ = 0.5 wt% H2O based on estimates
for Ash Mountain from Moore et al. (1995); i all calculations done at fO2=QFM
Discussion
Chemical evolution of Mathews Tuya
We infer, based on the compositional similarities with
alkaline basalts from throughout the northern Cordilleran
volcanic province (NCVP) and calculated liquidus temperatures, that the formation of Mathews Tuya was initiated by
partial melting in the asthenosphere (e.g., Edwards and
Russell 2000; Dixon et al. 2002). Edwards and Russell
(1999) suggested that the NCVP results from far-field plate
tectonic forces that, combined with access to nonsubduction-modified mantle via slab windows (Madsen et
al. 2006), produced a magma-charged lithosphere susceptible to periodic tapping caused by fluctuations in the local
stress field driven by ice loading and unloading (e.g.
Edwards et al. 2002). Other workers (e.g., Jull and
McKenzie 1996; MacIennan et al. 2002; Sigvaldason et
al. 1992) have suggested that isostatic changes caused by
formation/degradation of ice sheets in Iceland led to
increased eruption frequencies. While testing such a
hypothesis in the Tuya volcanic field is not possible with
present datasets, the dominance of glaciovolcanic products
in a volcanic field that also has subaerial volcanism (e.g.
Simpson et al. 2006; Wetherell et al. 2006) is consistent
with causal linkages between ice sheet dynamics and
volcanism. Work to test the hypothesis more rigorously by
determining eruption ages for 30 of the centers in the
volcanic field is presently in progress.
Phase equilibrium modeling via MELTS (Fig. 8) indicates that the onset of crystal growth in the parental melt
took place at less than 18 km depth based on the
observation of Ol as the dominant microphenocryst. The
lack of associated peridotite xenoliths, which are relatively
common in NCVP lavas (cf. Edwards and Russell 2000;
Harder and Russell 2005; Edwards et al. 2006), and the
relatively low Mg Numbers for lava samples are both
consistent with the parental magma having ponded temporarily within the crust and fractionating slightly prior to
eruption; this hypothesis is similar to that of Moore et al.
(1995). However, Moore et al. (1995) suggested that
chemical differences between what they termed ‘tholeiitic’
and ‘alkalic’ rock types at three volcanoes adjacent to
Mathews Tuya (Ash Mountain, Tuya Butte and South
Tuya; Fig. 1) resulted from eruption of magmas from two
different source regions during the course of a single
glaciovolcanic eruption, both of which had been stored at
shallow depths. In their model, initial eruption of overlying
‘tholeiitic’ magma allowed for vesiculation and eruption of
the ‘deeper’ alkalic magma. We disagree with their model
490
Bull Volcanol (2011) 73:479–496
from two perspectives. Firstly, although the samples from
Mathews Tuya also show two slightly different chemical
compositions (e.g. Fig. 7b), the mineralogy of the samples
is the same, and can be related to low pressure fractionation
accompanying changes in activity of H2O; the geochemical
variations seen at Mathews Tuya are far greater than those
reported by Moore et al. (1995). Secondly, we do not see
the predicted rapid change to dominantly magmatic
fragmentation that would result from sudden depressurization of a deeper magma body. The transition from quench
to magmatic fragmentation in volcaniclastic lithofacies does
not appear to be abrupt at Mathews Tuya.
a
2
Weight percent Na O + K O
7
2
6
5
4
3
2
44
46
48
50
52
Weight percent SiO2
54
Physical evolution of Mathews Tuya
b
1kb dry
0.70
MT WR
Molar Ca/Al
MT GL
0.60
0.50
1 kb 'wet'
0.40
30
35
40
45
50
Mg Number
55
60
c
Molar Ca/Al
0.70
AM WR
AM GL
ST WR
ST GL
1kb dry
0.60
0.50
TB WR
TB GL
1 kb 'wet'
0.40
30
40
50
Mg Number
60
70
Fig. 7 Geochemical characteristics for volcanic rocks and glasses
from Mathews Tuya and surrounding volcanic centers. a TAS diagram
(after LeMaitre 2002) showing published rock (WR) and glass (GL)
compositions from edifices within the Tuya volcanic field: Mathews
Tuya (MT, this study), Ash Mountain (AM, Allen et al. 1982; Moore
et al. 1995), South Tuya (ST, Allen et al. 1982; Moore et al. 1995),
and Tuya Butte (TB, Allen et al. 1982; Moore et al. 1995). Small gray
symbols show the range of compositions from samples throughout the
NCVP (cf. Edwards and Russell 2000). Alkaline-subalkaline division
(solid black line) from Irvine and Baragar (1971). b Compositions of
rocks and glasses from Mathews Tuya (MT) are plotted as ratios of
molar Ca/Al versus Mg# and compared against model liquid lines of
descent for low pressure crystallization of anhydrous and hydrous
melts (see Tables 1 and 2 for compositions; LLD calculations from
MELTS for b and c). c Compositions of rocks and glasses from Ash
Mountain (AM), South Tuya (ST), and Tuya Butte (TB) are plotted as
ratios of Ca/Al versus Mg# and compared against model liquid lines
of descent for low pressure crystallization of anhydrous and hydrous
melts
Mathews Tuya erupted along the crest of a ridge separating
a small U-shaped, glacier-filled valley to the north of the
tuya from a much larger U-shaped, ice-filled valley to the
south ~0.730 Ma (Fig. 9a). Thus, the volcano has an
asymmetric appearance, with the south wall of the valley
acting as a buttress along the south side of the edifice
(Fig. 9b). Initial venting of the tuya melted a cavity in the
overlying ice and produced an ice-confined, water-filled
chamber (englacial lake) beneath at least 500 m of ice (e.g.,
Tuffen 2007). Evidence for the production of a lava-fed
delta is consistent with eruption in an ice-dammed lake,
which is more likely to form and stabilize during an
eruption beneath relatively ‘thick’ ice (e.g. Smellie and
Skilling 1994). The eruption appears to have followed the
‘classic’ tuya development model that has been described in
detail by a number of previous workers (e.g. Jones 1969;
Skilling 1994; Werner and Schmincke 1999; Fig. 9c): initial
eruption of coherent pillow lava, followed by increasingly
more-explosive eruptions as the volcanic pile grew with
respect to the confining water, until the vent was sealed
from water and effusion of coherent lava into the iceconfined lake built a lava delta, separated from capping
subaerial lava flows by a passage zone (Mathews 1947;
Jones 1969; Smellie 2006).
The basal succession is overlain by coarse and fine clastic
lithofacies that contain dominantly vesicular clasts (Fig. 9c).
Several hypotheses might explain the transition from quench
to magmatic/phreatomagmatic fragmentation: (1) if the water
level in the subglacial cavity remained constant, the
hydrostatic pressure overlying the active part of the edifice
would decrease as the edifice grew in height; (2) as the
eruption changed from initially effusion dominated to
explosion dominated, the increased efficiency of heat transfer
(e.g., Guðmundsson 2003) could cause an increase in
melting, leading to increasing cavity underpressures and
higher explosivity (e.g., Tuffen 2007); (3) buildup of
meltwater in the sub-ice cavity could have led to catastrophic
drainage of the cavity, causing a rapid pressure decrease.
Bull Volcanol (2011) 73:479–496
a
1400
1400
Temperature (oC)
KAS3a dry
KAS3a 1wt% H2O
OPX
OPX
1300
1300
OL
OL
CPX
CPX
1200
1200
PL
PL
1100
1100
1000
1000
0.0
b
0.2
0.4
0.6
Pressure (GPa)
1.0 0.0
0.8
0.2
0.4
0.6
0.8
1.0
Pressure (GPa)
1400
1400
KAS2 1wt% H2O
KAS2 dry
OPX
1300
1300
OPX
OL
CPX
OL
CPX
1200
1200
PL
1100
1100
Temperature (oC)
Temperature (oC)
Temperature (oC)
Fig. 8 Liquidus and pseudoliquidus pressure-temperature
relationships calculated for three
rock compositions from
Mathews Tuya at anhydrous and
hydrous (1 weight percent H2O)
conditions: a Sample KAS3a; b
Sample KAS2; c Sample
KAS8a. The curves are
consistent with a sequence of
crystallization of Ol > Cpx >>
Pl at high pressures, and Ol >
Cpx ~ Pl at lower pressures.
Orthopyroxene is predicted to be
stable at the base of the crust but
would not crystallize at lower
pressures because of its reaction
relationship with olivine. The
main effect of water is to
depress the saturation temperature of plagioclase relative to all
other phases
491
PL
1000
1000
0.0
c
0.2
0.4
0.6
Pressure (GPa)
1.0 0.0
0.8
0.4
0.6
0.8
1.0
Pressure (GPa)
1400
1400
KAS8a 1wt% H2O
KAS8a dry
1300
1300
CPX
CPX
OPX
1200
PL
OL
OPX
1200
PL
1100
OL
1100
1000
1000
0.0
Fig. 9 Schematic regional and
local sections for Mathews
Tuya. a Schematic regional
topographic profile NW-SE
showing the position of
Mathews Tuya with respect to a
smaller U-shaped valley to the
north and a much larger
U-shaped valley to the south
(see Fig. 1). b Schematic
cross-section NW-SE (see
Fig. 2) illustrating the
relationship between volcanic
stratigraphy and underlying
basement rocks as well as
variations in volcanic lithofacies
Temperature (oC)
Temperature (oC)
0.2
0.2
a
0.4
0.6
Pressure (GPa)
1.0 0.0
0.8
0.2
0.4
0.6
0.8
1.0
Pressure (GPa)
Mathews
Tuya
NW
SE
1800 m
1800 m
1600 m
1600 m
1400 m
1400 m
1200 m
1200 m
0 km
b
1 km
NW
2 km
3 km
4 km
5 km
6 km
7 km
8 km
SE
492
Bull Volcanol (2011) 73:479–496
Transition zone from subaqueous
to likely subaerial eruption
As the edifice grew above the level of the lake the eruption
changed from subaqueous to subaerial. Lavas erupted subaerially flowed off the emergent portion of the volcano, entered
the englacial lake and formed a pillow-lava delta comprising
intact pillow lobes and pillow breccia (e.g., Skilling 2002).
Exposures of the pillow-lava delta are restricted to the westnorthwest flank of the volcano so the true extent and
geometry of this lava delta is not known. Continued
magmatic fragmentation may also have been intermittent or
synchronous with effusive eruptions during this time, but was
likely subordinate. Subaerially erupted lavas advanced over
the deltaic deposits, producing a relatively flat-lying passage
zone capped by subhorizontal, subaerial lava.
On the northwestern flanks, a series of radially jointed lava
mounds outcrop midway down the slope. The origin of these
features and timing relative to the other eruptive deposits
remains uncertain. However, radial jointing, concave geometries, and apparent downslope dips are consistent with
formation as lava tubes within the delta or as lavas emplaced
into ice-bounded cavities along the volcano-ice contact (e.g.,
Loughlin 2002). At present no pillow lava has been found
associated with these lavas, so they may have formed after
the bounding englacial lake drained.
Based on edifice morphology, its eruption age of
~0.730 Ma, and the presence of glacial erratics on the top
and flanks of the edifice, it seems likely that glacial erosion
has significantly modified the original form of Mathews
Tuya. In all of northern British Columbia, extensive cirque
glaciation is evident on most north-facing slopes. Mathews
Tuya also has a north-facing cirque that has almost bisected
Ash Mountain
2100 m
2000 m
1900 m
1800 m
1700 m
1300 m
1200 m
1200 m
1500 m
1400 m
Regional volcanological comparisons
As many as 20 volcanoes in the Tuya volcanic field may be
glaciovolcanic in origin; however, only three other edifices
South Tuya
Unconsolidated 2100 m
volcaniclastic
facies comprising
scoriaceous ash, 2000 m
lapilli and rare
bombs (up to 2 m).1900 m
Cross-cutting dikes
are locally bulbous
1800 m
and pillow-like.
1700 m
Pillow lava
with minor
1600 m
interlayered
volcaniclastic
facies. Upsection1500 m
clastic facies is
? more abundant
1400 m
and pillows are
more vesicular.
1300 m
1600 m
the edifice lava cap, and which still contains the remnants
of what appears to be a small rock glacier. The extent of
erosion at the tuya is consistent with it having experienced
prolonged periods of glacial erosion since ~0.730 Ma.
We can infer the presence and minimum thickness of ice
extant at ~0.730 Ma surrounding Mathews Tuya from the
height of the passage zone and the local bedrock topography (Fig. 9a). The passage zone occurs ~300 m above the
inferred base of the edifice on the north side, and ~600 m
above the valley floor on the south side. Smellie (2006)
suggested that passage zones may demarcate the ice-firn
transition in glaciers; thus the minimum ice thickness to the
north of the vent location was ~400 m, while the minimum
ice thickness to the south was ~700 m. Given the geometry
of the deeper valley on the south side of the tuya, we would
infer, based on a parabolic ice thickness model (Equation 5,
p. 242 of Paterson 1994), that the eruption which formed
Mathews Tuya occurred within a regionally extensive
glacier or ice sheet. However, at present we cannot say
with certainly if the enclosing ice mass was a ~0.730 Ma
manifestation of the Cordilleran Ice Sheet, which, during
the Last Glacial Maximum, is inferred to have been 2–3 km
in maximum thickness. We can only say with certainty that
the region surrounding Mathews Tuya was partly ice
covered ~0.730 Ma.
?
Mathews Tuya
Tuya Butte
2100 m
2100 m
Consolidated
volcaniclastic
facies comprising
scoriaceous ash,
lapilli and pillow
fragments. Local
lenses of pillow
lava, dikes, and
layers of volcanic
bombs.
2000 m
2000 m
1900 m
1900 m
Subaerial basaltic
lava flows with
1800 m
vertical to radial
columnar jointing.
1700 m
Pillow lava
with minor
interlayered
volcaniclastic
facies.
1500 m
Fig. 10 Comparative stratigraphic sections for glaciovolcanic edifices
in the Tuya volcanic field. Sections are drawn relative to their present
day elevations. Shaded field denotes the inferred transition from
subaqueous to subaerial facies as marked by the highest elevation of
pillowed lavas or deposits containing pillow fragments or hyaloclas-
1800 m
1700 m
1600 m
1400 m
1300 m
1200 m
Consolidated
massive to bedded
volcaniclastic facies. 1600 m
Local lenses of
pillow lava, dikes,
and lava tubes/sills. 1500 m
Pillow lava
and massive
lava(?) with
? minor
interlayered
volcaniclastic
facies, locally
cross-cut by
dikes.
1400 m
1300 m
1200 m
Subaerial basaltic
lava flows with
vertical to radial
columnar jointing.
Foreset bedded
volcaniclastic facies
containing broken and
intact isolated pillows
and massive(?) lava.
Pillow lava
with minor
interlayered
volcaniclastic
? facies locally
cross-cut by
dikes.
tite. General stratigraphic information for Ash Mountain, South Tuya
and Tuya Butte was derived from Mathews (1947), Allen et al. (1982),
and especially Moore et al. (1995) as well as reconnaissance by the
authors. Lithofacies colors same as in Fig. 2
Bull Volcanol (2011) 73:479–496
Fig. 11 Summary of geochronometric constraints on timing of North
American glaciation and NCVP glaciovolcanism. Data for approximate relative global ice volumes are derived from 18O/16O for benthic
foraminifera from Shackleton et al. (1990). Information on the North
American ice record are derived from Bowen et al. (1986), and
includes data interpreted as indicating mountain glaciation (MTN),
Cordilleran ice sheet (CIS) and Laurentide ice sheet (LIS) deposits.
Geochronometry for NCVP volcanic centers is from Souther (1992;
Edziza), Edwards et al. (2002; Hoodoo), Edwards et al. (2006; BellIrving), and this study
have been previously described in any detail (Fig. 1): Tuya
Butte (Mathews 1947; Allen et al. 1982; Moore et al.
1995), South Tuya (Moore et al. 1995), and Ash Mountain
(Mathews 1947; Allen et al. 1982; Moore et al. 1995).
We use the published stratigraphic information on Tuya
Butte, Ash Mountain, South Tuya in conjunction with our
stratigraphic description of Mathews Tuya and our own
reconnaissance work at Ash Mountain and South Tuya to
give a stratigraphic comparison of glaciovolcanic deposits
within this region of the Canadian Cordillera (Fig. 10). All
four of the volcanoes are inferred to have 200–300 m thick
successions of pillow lava at their bases (Mathews 1947;
Moore et al. 1995). At South Tuya and Ash Mountain,
basal platforms of pillow lava are well exposed, while at
Tuya Butte and Mathews Tuya basal exposures are limited.
Overlying the pillow lava facies are unconsolidated,
massive to bedded volcaniclastic facies. The volcaniclastic
facies are monomictic (basaltic), containing scoriaceous
493
lapilli and ash +/- broken pillows and rare intact pillows. At
South Tuya and Ash Mountain some volcaniclastic facies
contain distinctive fluidally-shaped clasts or bombs (Moore
et al. 1995). Basaltic dikes cross-cut the stratigraphy at all
centers, and appear to uphold ridges emanating radially
from the summit of Ash Mountain. Mathews Tuya and
Tuya Butte have characteristic flat tops defined by capping
subaerial lava flows that overlie lava-fed deltas, while Ash
Mountain and South Tuya are more conical in shape with
the highest stratigraphic units comprising poorly characterized volcaniclastic deposits.
The varied morphology of the volcanoes within the area
suggests that they do not all share the same eruption
histories. Mathews Tuya and Tuya Butte have capping lava
flows; South Tuya and Ash Mountain do not. Neither
Mathews (1947) nor Moore et al. (1995) speculated as to
why capping flows were not erupted on South Tuya and
Ash Mountain, but at least three plausible explanations
exist: (1) the two eruptions may have ended before building
above the level of an englacial lake; (2) the bounding ice
might have been too thick to form an open cavity; or (3) the
capping lava flows may have been removed by erosion,
although this is considered least likely.
The absence of a capping basalt flow, and presence of an
upper conical edifice of scoriaceous lapilli and ash, including
fluidal bombs on South Tuya and Ash Mountain suggest
subaerial fire fountaining was the main late-stage eruptive
style. Tuya Butte and Mathews Tuya are capped by subaerial
lavas, which is consistent with effusive eruptions being
dominant in the final stages of eruptive activity as well.
Implications for pleistocene cordilleran paleoclimate
Glaciovolcanism is critically important for recording
changes in Pleistocene paleoclimate conditions, and glaciovolcanic products provide a means for further documenting
the location and timing of continental ice, which can be
compared with paleoclimate proxies from the marine
isotopic record and the continental glaciological record
(Fig. 11). Detailed volcanological/geochronological studies
in Antarctica (e.g. Smellie et al. 2008) show local increases
in maximum ice sheet thicknesses through time. In northern
British Columbia, the eruption ages for a number of
glaciovolcanic deposits are now known and can be used
to further constrain interpretations of the Cordilleran
paleoclimate, although distinguishing between local and
regional glacial events can be problematic (e.g., Edwards et
al. 2009). The glaciovolcanic origins of Mathews Tuya
confirm the existence of pre-Last Glacial Maximum (LGM)
glaciation at ~0.730 Ma, during a period of high ice volume
as implied by the marine record. Geochronological evidence of tuya formation in the Tuva Republic, Russia,
appears to corroborate an expansion of northern hemisphere
494
glaciers ~0.740 Ma (Yarmolyuk et al. 2001 as reported in
Komatsu et al. 2007). The location of the Tuya volcanic
field also corresponds with one of the two inferred areas of
maximum thickness for the CIS during the LGM (Peltier
1994). However, few deposits remain in this area to provide
pre-LGM evidence for regionally extensive glaciation,
other than glaciovolcanic deposits such as those exposed
at Mathews Tuya. Although previous workers inferred that
Tuya area glaciovolcanism occurred during the LGM, our
work clearly documents the existence of earlier glaciovolcanic activity. Stratigraphic comparisons of glaciovolcanic
deposits in the Tuya volcanic field combined with detailed
geochronologic studies currently in progress for other
locations within the Tuya volcanic field will soon allow
for much better correlations between North American
continental and marine climate records.
Conclusions
Mathews Tuya records continuous volcano-ice interactions
starting from initially sub-ice/subaqueous conditions to
subaerial conditions. Textural analysis of volcaniclastic
deposits show changes in proportions of clasts derived from
quench versus explosive fragmentation processes and
provides key information on the transitions between effusive
and explosive activity in glaciovolcanic eruptions. Although
geochemically similar to nearby volcanoes, Mathews Tuya
shows the most extreme compositional diversity yet found in
the Tuya volcanic field, which appears to be explained by lowpressure crystallization. Its eruption age of ~0.730 Ma means
that it provides the first record of Cordilleran ice in northern
British Columbia at that time and appears to correlate well
with marine proxies indicative of relatively high ice volumes
as well as ~0.740 Ma glaciovolcanic deposits in Russia.
Acknowledgments We thank several people for general assistance
with fieldwork in the Tuya area, including Jim Reed, Chris Price, Blake
Parker and Tark Hamilton. Funding for initial fieldwork in 1995 was from
LITHOPROBE. During the preparation of the manuscript BRE was
supported in part by NSF-EAR 0439707, and a 2009 spot check was
funded in part by NSF-EAR 0910712; JKR was supported by the NSERC
Discovery Grants program. Tom Ullrich of the Pacific Center for Isotopic
& Geochemistry Research at UBC made the 40Ar/39Ar determinations
and interpretations. Helpful comments by C. Bacon and M.T.
Guðmundsson improved the clarity and presentation of the manuscript;
MTG particularly helped us clarify our thoughts on the implications for
local versus regional glaciations. Editorial suggests by M. Clynne and J.
White helped to improve the clarity of the text and figures.
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