Bull Volcanol (2011) 73:479–496 DOI 10.1007/s00445-010-0418-z

RESEARCH ARTICLE

Volcanology and petrology of Mathews , northern , : 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 (e.g., Ash Mountain, South Tuya, Tuya ) as well as the data collected at together provide constraints on dominant NCVP rock type. Crystallization scenarios calculat- paleoclimate conditions during formation of the edifice. The ed with MELTS account for variations between whole rock and basaltic tuya was produced via glaciovolcanism in glass compositions via low pressure fractionation. The northern British Columbia, Canada, and is located within the presence of olivine microphenocrysts and the absence of Tuya (59.195°N/130.434°W), which is part of pyroxene phenocrysts constrain initial crystallization pressures the northern Cordilleran volcanic province (NCVP). The to less than 0.6 GPa. The eruption of Mathews Tuya occurred edifice comprises a variety of lithofacies, including columnar- between 0.718±0.054 Ma and 0.742±0.081 Ma based on jointed , , massive dikes, and volcaniclastic 40Ar/39Ar (weighted mean age of 0.730 Ma). rocks. Collectively these deposits record the transition from an The age determinations provide the first firm documentation explosive subaqueous to an effusive subaerial eruption for large (>700 m thick), pre-Fraser/Wisconsin in environment dominated by Pleistocene ice. As is typical for north-central British Columbia ~0.730 Ma, and correlate in , the volcaniclastic facies record multiple fragmentation age with glaciovolcanic deposits in Russia (e.g., Komatsu et processes including explosive, quench and mechanical al. Geomorph 88: 352-366, 2007) and with marine isotopic fragmentation. All samples from Mathews Tuya are olivine- evidence for large global ice volumes ~0.730 Ma. plagioclase porphyritic alkali olivine . They are minera- logically and geochemically similar to nearby glaciovolcanic Keywords Mathews . Tuya . Cordilleran . Glaciation . centers from the southeastern part of the Tuya volcanic field Pleistocene . NCVP. AOB

Editorial responsibility: M.A. Clynne Introduction 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. Volcanoes that erupt in glacial environments produce unique features that can be used to constrain paleo-environmental * B. R. Edwards ( ) conditions at the time of eruption. Such glaciovolcanic Department of Earth Sciences, Dickinson College, Carlisle, PA 17013, USA products have been documented worldwide, but are especially e-mail: [email protected] prevalent in (e.g., Kjartsansson 1943; van Bemmelen and Rutten 1955; Sigvaldason 1968;Jones1969; Tuffen et J. K. Russell al. 2002; Hoskuldsson et al. 2006; Schopka et al. 2006; Volcanology and Petrology Laboratory, Earth & Ocean Sciences, University of British Columbia, Jakobsson and Guðmundsson 2008; Skilling 2009), western Vancouver, BC, Canada North America (e.g., Mathews 1947;HoareandCoonrad 1978; Allen et al. 1982; Moore et al. 1995;Hickson2000; K. Simpson Lescinsky and Fink 2000; Dixon et al. 2002; Edwards et al. Geoscience BC, #440-890 West Pender Street, 2002, 2006, 2009;Kelmanetal.2002; Bacon and Lanphere Vancouver, BC V6C 1J9, Canada 2006), and (e.g., Skilling 1994; Smellie and 480 Bull Volcanol (2011) 73:479–496

Skilling 1994; Wilch and McIntosh 2007; Smellie et al. compositions of whole rock and glass samples. Most of the 2008; Smellie 2009). Tuyas, also referred to as ‘ field-based observations were made in July/Aug 1995, with a mountains’ (e.g. Mathews 1947; van Bemmelen and Rutten brief reconnaissance visit in 2009. Secondly, we present a 1955), represent one of two uniquely distinctive glaciovol- model for the physical and chemical evolution of the canic landforms. The term ‘tuya’ was coined 60 years ago by that integrates the observed stratigraphic relationships and W.H. Mathews, based on his landmark work describing the the petrological character of the volcanic rocks. Lastly, we stratigraphy and morphology of basaltic volcanoes in the use new 40Ar/39Ar geochronometry to constrain the regional Tuya-Teslin region of northwestern British Columbia paleoclimatic conditions in the area surrounding Mathews (Mathews 1947). There, he encountered numerous, small, Tuya and, in conjunction with previously published data, to apparently young, volcanic edifices hosting a variety of provide a preliminary view from glaciovolcanic constraints enigmatic features. Most of the volcanoes are basaltic in for the distribution of Pleistocene glaciers in the Canadian composition, are steep-sided and many have “flat” tops. Cordillera. Mathews recognized that these volcanic edifices shared common stratigraphic elements, including: pillow and , massive to bedded deposits of fragmented glassy Geological setting (‘’), and capping, massive basaltic lava. He proposed the term ‘tuya’ for these volcanoes and The Tuya volcanic field of the northern Cordilleran volcanic interpreted their morphology and attendant volcanic lithofa- province (NCVP) is an extensive area of Neogene to Recent cies as indicative of volcanic eruptions from beneath and alkaline volcanism formed from transtensional stresses con- within Pleistocene ice sheets. He also recognized that the centrated within the western Canadian Cordillera after transition from pillow lava and volcanic to subaerial subduction of the Farallon and Nazca plates (Fig. 1; Edwards lava flows was an intrinsically important marker horizon for and Russell 1999, 2000;Madsenetal.2006). Due to the establishing the minimum thickness of the confining ice, thus spatial and temporal coincidence of NCVP volcanism and providing critical information about paleo-environmental Pleistocene glaciations, including the Cordilleran conditions extant during the eruptions. Jones (1969)named (CIS), a diverse range of glaciovolcanic landforms and these transitions ‘passage zones’, which have recently been deposits have been documented within the NCVP (e.g., described in detail by Smellie (2006). Allen et al. 1982;Souther1992; Moore et al. 1995;Hickson Relatively few studies subsequent to Mathews (1947) 2000; Dixon et al. 2002; Edwards et al. 2002, 2006,2008). have examined the formation of basaltic tuyas in detail (e. More than 30 individual volcanic centers/deposits are g., Smellie and Skilling 1994; Moore et al. 1995; Werner mapped as part of the Tuya Formation in the and Schmincke 1999; Skilling 2009), and even fewer have 1:250,000 map sheet (Fig. 1b; Watson and Mathews 1944; attempted to integrate stratigraphic, petrological, and Gabrielse 1970); at least 20 of those deposits show evidence geochronologic data (Werner and Schmincke 1999). Strati- for glaciovolcanism. Mathews (1947) gave brief descriptions graphic studies of flat-topped volcanoes formed by glacio- of the petrology and volcanology of six tuyas, including volcanism derive mainly from work in Iceland (van what has been informally referred to as ‘Mathews Tuya’. Bemmelen and Rutten 1955; Sigvaldason 1968; Jones Several studies have subsequently been published on the 1969; Werner and Schmincke 1999; Tuffen et al. 2002; volcanology, petrology and geochemistry of the volcanic Skilling 2009), Antarctica (e.g., Smellie and Skilling 1994; centers that form part of the Tuya Formation (Allen et al. Skilling 1994; Smellie et al. 2008), and British Columbia, 1982; Moore et al. 1995; Simpson 1996; Dixon et al. 2002; Canada (e.g., Mathews 1947; Allen et al. 1982; Moore et al. Wetherell et al. 2006; Simpson et al. 2006), but none have 1995; Dixon et al. 2002). Tuyas likely formed in other done more than briefly mention ‘Mathews Tuya’. Work by glacially covered, volcanically active regions of Earth in the Allen et al. (1982) and Moore et al. (1995) gave brief field past (e.g. Russia; Komatsu et al. 2007), and are also and geochemical descriptions of Tuya Butte, Ash Mountain thought to be present on Mars (e.g., Allen 1979; Chapman and South Tuya (Fig. 1c; Table 1). Dixon et al. (2002) et al. 2000); their presence and stratigraphy can be used to presented data on volatile behavior and geochemistry of constrain planetary paleoclimates (e.g., Smellie 2009). Tanzilla Butte, a glaciovolcanic edifice immediately south- Here we present an integrated study of the formation of east of Tuya ; they concluded that ice was up 1 km thick Mathews Tuya, which is located in the Tuya volcanic field of at the onset of eruption. Wetherell et al. (2006)andSimpson northern British Columbia (59.195°N/130.434°W; Fig. 1; et al. (2006) documented subaerial components of the Tuya Mathews 1947; Allen et al. 1982; Moore et al. 1995). The Formation, which appear to be less common than glacio- purpose of this paper is threefold. First, we describe the volcanic deposits. and petrology of Mathews Tuya, including descrip- Gabrielse (1970) inferred that the area underlying the tions of the critical lithofacies and measurements of chemical Tuya Formation had been affected by regional ice sheets, Bull Volcanol (2011) 73:479–496 481

Fig. 1 Maps showing the loca- Nome Jennings tion of Mathews Tuya. a Digital a 0 20km Lake elevation model (DEM) of the b Canadian Cordillera showing 30 the location of the Tuya volcanic Gabrielse field in the northern Cordilleran Cone volcanic province (NCVP; Edwards and Russell 2000). The NCVP comprises predominantly AM CT High Tuya 15 mafic, pre- volcanic Lake ST centers (red dots), with minor Mathews TB Tuya Holocene activity (yellow dots) West Tuya MJ Lava Field and relatively few centers with Tuya intermediate to lavas Lake (green dots). Also shown are 59 00 N other major Neogene to Recent 30 131 00 30 130 00 W 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 c Ash 2100m sheet (NTS 104O; Gabrielse Mountain 1970), including Mathews Tuya. Abbreviations demarcate select- N ed volcanic centers near Math- Caribou ews Tuya, including Mount Tindar Josephine (MJ), Tuya Butte Mathews (TB), South Tuya (ST), Caribou 1850m NW Tindar (CT), and Ash Mountain Tuya (AM). c DEM of the region surrounding , show- 1775m ing the locations of volcanic 1650m South features mentioned in the text. Tuya Stars and numbers indicate peak Tuya elevations for centers discussed Mount Butte SE in the text Josephine

1120m 0 5km and found evidence of tuyas to the south that have been evolution of Mathews Tuya into a regional framework in glaciated, but which appear to have formed on top of order to provide constraints for interpretation of paleocli- glaciated bedrock, implying that the Tuya Formation could mate conditions during formation of the tuya. span more than one glacial episode (Gabrielse 1998). As glacial erratics, , and topography, and deposits are widespread through- Physical and petrological characteristics of Mathews out the area, at least one ice sheet (i.e. the Last Glacial Tuya Maximum CIS) probably covered all of the peaks in the area at some point during the Pleistocene, followed by Mathews Tuya rises to an elevation of 1780 m above sea level, potentially multiple episodes of alpine and glaciation with a relief of ~330 m, and has the characteristic flat top that during and after retreat of the ice sheets (Gabrielse 1998). distinguishes tuyas from most other volcanic landforms Simpson (1996) presented the first detailed petrological (Figs. 2 and 3). It is ~3 km long, elongated northeast- examination of Mathews Tuya and a preliminary analysis of southwest, and has a maximum exposed width of ~1.5 km. its stratigraphy and physical evolution. This work builds on The edifice is bounded on the south by a large (4.5 km that study, reassesses its main conclusions, and aims to wide), northeast-trending U-shaped valley, which extends for place the detailed knowledge of the physical and chemical 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 1σ

SiO2 45.55 47.32 47.33 45.45 45.39 46.06 47.03 46.97 0.055

TiO2 3.25 2.34 2.35 3.25 3.24 3.26 2.42 2.48 0.01

Al2O3 14.28 14.32 14.36 14.34 14.30 14.40 14.34 14.36 0.045 FeO(T) 13.28 12.55 12.37 13.32 13.34 13.39 12.60 12.56 0.058 MnO 0.16 0.17 0.17 0.16 0.16 0.16 0.17 0.17 0.002 MgO 7.14 8.53 8.39 6.77 6.97 6.87 8.39 8.20 0.038 CaO 8.05 8.90 8.88 7.98 7.83 7.79 8.66 8.79 0.032

Na2O 3.85 2.98 3.09 3.95 3.95 4.03 3.33 3.15 0.040

K2O 1.81 1.14 1.18 2.01 2.01 2.04 1.29 1.35 0.010

P2O5 0.78 0.47 0.47 0.80 0.80 0.78 0.52 0.53 0.003 Total 98.14 98.73 98.59 98.03 97.98 98.78 98.75 98.57 LOI 0.61 b.d. b.d. 0.43 0.40 b.d. b.d. b.d. Normative Mineralogyb Or 10.70 6.74 6.97 11.88 11.88 12.06 7.62 7.98 Ab 18.25 23.24 23.06 17.06 17.12 18.24 21.68 21.62 An 16.34 22.33 21.83 15.47 15.36 15.18 20.37 21.06 Ne 7.76 1.07 1.67 8.86 8.83 8.59 3.52 2.73 Di 15.38 15.46 15.79 15.74 15.20 15.27 15.82 15.74 Ol 21.78 24.37 23.74 21.01 21.63 21.46 23.95 23.52 Il 6.16 4.44 4.46 6.18 6.15 6.19 4.60 4.71 Ap 1.84 1.12 1.12 1.88 1.88 1.85 1.24 1.26 a analytical work was done at McGill University Geochemical Laboratories; b normative mineralogies are in weight percent and were calculated using an 2+ 3+ Fe /Fe ratio constrained by fO2=QFM southwest into the Kawdy . The north side of the and clasts. Olivine microphenocrysts are up to 2 mm in edifice hosts a small , and opens northward to a maximum dimension and comprise between 10 and 25% of smaller (1.5 km wide) U-shaped valley. The top of the the rock; they are dominantly skeletal with some perfect edifice is buttressed part way up on its south side by the euhedral edges and locally contain melt inclusions (Figs. 5a, Parallel Creek batholith (Fig. 2b), which comprises biotite c, g). Plagioclase laths are generally less than 1 mm long and granite and quartz monzonite (Fig. 2;Gabrielse1970). comprise 15 to 20% of the groundmass, which also contains olivine, titanaugite and variable amounts of glass. Plagio- Lithofacies clase laths are typically randomly oriented but locally form trachytic textures (e.g., Figs. 5a and c). Oxides occur in all of Mathews Tuya is a basaltic volcano that comprises coherent the samples, and are predominantly contained within olivine and fragmental lithofacies including: massive and jointed microphenocrysts. Rare crustal xenoliths up to 1 cm are lavas and dikes, pillow lava, and associated volcaniclastic present and locally show incipient melting. rocks (Figs. 2b, 3 and 4a–g). Outcrops at lower elevations are sparse and are restricted to stream cuts on the north, Coherent south and lower west faces, and sporadic outcrops elsewhere (Fig. 2). Much of the low-lying landscape Flat-lying, columnar jointed basaltic lavas cap the edifice surrounding the tuya is basaltic rubble; however, the (Figs. 2 and 4a). At least three distinct, stacked lava flows stratigraphy in these areas has likely been significantly are present and combine to form a total thickness that disrupted by permafrost, which is evident on the north and ranges between ~20 and >80 m. The west side of the tuya is northeast slopes. At higher elevations outcrop is more ~40 m higher in elevation than the eastern side and the abundant and forms prominent cliffs. Granitic cobbles and lavas are also thicker on the west side. Columnar jointing is boulders occur locally on most of the upper surfaces of the typically fine (<0.5 m), well developed, and locally highly edifice (Fig. 4h). irregular in orientation (Fig. 4a). Columnar jointed lavas are All of the lithofacies comprise black, variably vesicular (0– also found as isolated structures extending down the slopes 30% vesicles), fine-grained, and olivine-phyric basaltic rocks of the northwest, northeast and south central flanks, locally Bull Volcanol (2011) 73:479–496 483

Fig. 2 Aerial view and simplified geology of Mathews a Tuya. a Aerial photograph showing relatively flat-topped, high standing, oval shape of Mathews Tuya (MT). Dashed white line delineates the pre- sumed 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

0 1.5km N

b 1700

KAS12a,b Undifferentiated colluvium A KAS11 KAS9,10 Undifferentiated KAS2 basaltic colluvium Coherent Ol-phyric KAS7 KAS8a,b KAS3a,b basaltic facies KAS4 KAS5 KAS6 KAS14a,b Volcaniclastic facies Interbedded coherent and volcaniclastic facies A’ Parallel Creek Batholith granitoids Oblique Creek Formation, undifferentiated gneiss and schist defined contact

1300 approximate contact 0 1.5km

Contour interval = 20 m N with radial joint orientations perpendicular to inferred flow ual pillows range from 0.5 to ~1 m in diameter, up to 3 m in directions (Fig. 4b, d). The eastern contact between length, and generally plunge moderately to steeply to the coherent basalt and granitic basement is distinct, sharp northwest downslope. Pillows of similar size with no and clearly visible on aerial photography, as a southwest interstitial volcaniclastic deposits are locally present lower trending lineament. on the northwest flank. Two 60 cm wide, south trending, steeply dipping, The coherent lithofacies are interpreted to include massive dikes (200°/86°E, 180°/88°E) intrude coarse subaerial and ice-confined columnar jointed lavas, sub- volcanic breccia in the lower section of the north flank, aqueous pillow lava, and syn-eruption dikes. The variably and locally show glassy margins (Fig. 4c). Dikes may also oriented and finely columnar-jointed lavas are consistent be present in the upper wall of the north-facing cirque. with irregular cooling surfaces and rapid cooling. The radial Exposures of pillow lava are most abundant on the jointing displayed by some of the lava lobes is consistent northwestern flank (Figs. 3a and 4d) where they commonly with lava tubes or emplacement into ice-confined tunnels. occur with associated, interstitial volcanic breccia. Individ- In addition, the lavas do not extend great lateral distances 484 Bull Volcanol (2011) 73:479–496

a b

c d

Fig. 3 Morphology of Mathews Tuya. a View looking to the south at units for b and d. c Aerial view looking to the north showing the south the northern side of the tuya. Note the flat top on the northern side of side of the edifice, with the in the background. d the edifice. b Traced line-diagram of a, with approximate boundaries Traced line-diagram of c, with approximate boundaries of geological of geological units. See the legend in Fig. 2 for color code of specific units from the main volcanic edifice, which further supports a highly vesicular (scoriaceous) lava. The mineralogy of the hypothesis of ice confinement. The flat-lying, capping clasts is the same as that of the coherent facies, except that columnar-jointed lavas are interpreted to have erupted the groundmass has more glass, and the vitric clasts are subaerially. variably palagonitized (e.g., Fig. 5c and f). The coarse- The presence of pillow lava is indicative of subaqueous grained lithofacies is poorly-sorted, matrix-supported and deposition (e.g., Moore 1975; Walker 1992). The dipping contains breccia-sized fragments from 2 to 120 cm in geometry of the pillow lava tubes and association with maximum dimension (Figs. 4e and f). The scoriaceous pillow-fragment breccia is consistent with formation of a clasts are subrounded to angular and the non-vesicular/ lava-fed delta (e.g., Swanson 1970; Skilling 2002) that was poorly vesicular clasts are typically blocky, angular and prograding to the northwest. commonly have curviplanar clast margins. Rare granitic The massive basaltic dikes crosscut the stratigraphy in xenoliths up to 5 cm in size occur within some clasts, and the lower part of the edifice and their exposure at the microlites in some clasts terminate at clast margins. The surface confirms that an undetermined amount of the tuya matrix consists of angular, less than 2 mm, variably has been eroded locally. vesicular glassy fragments and olivine crystal fragments. Well-preserved bubble wall shards are common (Fig. 5d). Volcaniclastic The coarse-grained lithofacies is massive to very crudely bedded and locally occurs as lenses within the fine-grained Volcaniclastic lithofacies, defined in the general sense of lithofacies (Figs. 4f and g). The fine-grained lithofacies has Fisher (1961) to include all clastic materials derived from identical components to the coarse-grained lithofacies dominantly volcanic sources without implications for except that fragments are sand-sized (<2 mm). This fragmentation mechanisms, are generally orange-brown in lithofacies is also thickly laminatedtothinlybedded colour, monomictic, and basaltic. The volcaniclastic lith- (Fig. 4g). The pillow-fragment breccia also has the same ofacies are subdivided into three distinct units: 1) coarse- components as the coarse-grained lithofacies with the grained; 2) fine-grained; and 3) pillow-fragment breccia. addition of pillow fragments and locally this unit can All three lithofacies comprise fragments of non-vesicular to contain angular granite clasts (Fig. 4d). This lithofacies is Bull Volcanol (2011) 73:479–496 485

Fig. 4 Field photographs showing large-scale deposit a b 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 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 sitting on the flank of the edifice

g h

associated with intact, moderately dipping pillow lobes on rounding or sorting) was observed. The pillow-fragment the northwest flank of the edifice. breccia contains fragments derived from the disintegration Within all three lithofacies the non- to poorly vesicular, of pillow lobes. The lack of jigsaw-fit textures, mixing of blocky, angular clasts with curviplanar clast margins are variably vesicular clasts and the bedding within some units interpreted to have formed by autoclastic processes (quench supports a syn-eruptive origin involving non-explosive and fragmentation and autobrecciation; Figs. 5b, c, g, h). The explosive -water interaction. Thus the volcaniclastic bubble wall shards and scoriaceous clasts, which are deposits are interpreted to include fragments produced by dominant throughout most of the sampled sections, are autoclastic (hyaloclastite +/- autobreccia) and pyroclastic interpreted to have formed by explosive fragmentation processes, which is typical of many glaciovolcanic deposits (Fig. 5d and e). Attrition of larger clasts during transport (e.g. Werner and Schmincke 1999). and deposition also likely contributed to the generation of The angular granite clasts locally found within the clasts, although no evidence for significant transport (e.g. 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 microphe- nocrysts and plagioclase groundmass. b Fluidally-shaped, vesiculated, pyroclast of vitro- phyric olivine basalt in a matrix of finer vitroclastic material (pore space is filled by second- ary cement). c Blocky-shaped, non-vesicular pyroclast of tra- chytic, 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 ba- saltic 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 The angular clast shapes are most consistent with the Parallel Creek batholith. The origins of the granitic last hypothesis. clasts are enigmatic but could include: 1) dropstones melted out of surrounding ice during the course of the Geochronometry eruption; 2) syn-eruption mixing of pre-existing glacial sediment with volcaniclastic materials; or 3) blocks Two samples of coherent lithofacies were analyzed using ejected during the establishment of the volcanic conduit. 40Ar/39Ar geochronometry to constrain interpretations of Bull Volcanol (2011) 73:479–496 487 the time of eruption and to enable correlation with the two outermost steps on a plateau are not significantly global climate record. Previous workers have assumed that different from the weighted-mean plateau age (at 2 σ, six or much of the Tuya volcanic field formed during the Fraser/ more steps only); (5) the outermost two steps on either side Wisconsin glaciation (e.g., Mathews 1947; Moore et al. of a plateau must not have nonzero slopes with the same 1995; Dixon et al. 2002), although to our knowledge this sign (at 2 σ, nine or more steps only). All analytical data inference is not constrained by geochronometry. are included in Supplementary Tables 2 (KAS3a) and 3 (KAS14b). 40Ar/39Ar methodology Geochemistry Bulk separates from two samples (KAS3a and KAS14b) were washed in acetone, dried, wrapped in aluminum foil Analytical methods and stacked in an irradiation capsule with similar-aged samples and neutron flux monitors (Fish Canyon Major element concentrations for all samples of lava were sanidine, 28.02 Ma; Renne et al. 1998). The samples were analyzed by X-ray fluorescence at Geochemical Laborato- irradiated on July 4 through July 6, 2006 at the McMaster ries, McGill University, Quebec. Sample preparation Nuclear Reactor in Hamilton, Ontario, for 90 MWH, with a followed the methods described by Cui and Russell neutron flux of approximately 3×1016 neutrons/cm2. (1995). Major element compositions were used to calculate Analyses (n=57) of 19 neutron flux monitor positions normative mineralogy for whole-rock samples (Table 1). produced errors of <0.5% in the J value. Estimates of analytical uncertainty derive from replicate The samples were analyzed on August 2 and 15, 2006, at analysis of blind samples (Table 1). the Noble Gas Laboratory, Pacific Center for Isotopic and Compositions of volcanic glasses were analyzed using a Geochemical Research, University of British Columbia, Cameca SX-50 electron microprobe at the University of Vancouver, BC, Canada. Samples were step-heated at British Columbia (cf. Russell and Hauksdottir (2001) for incrementally higher powers in the defocused beam of a methods). Microprobe work was performed to determine

10 W CO2 laser (New Wave Research MIR10) until fused. the composition of volcanic glass within clastic samples The gas evolved from each step was analyzed by a VG5400 and the composition of a glassy rind from one of the two mass spectrometer equipped with an ion-counting electron dikes (see Table 2 for averages; a full set of analyses are multiplier. All measurements were corrected for total system available in electronic format from the authors). Replicate blank, mass spectrometer sensitivity, mass discrimination, analyses were used to estimate analytical uncertainty. radioactive decay during and subsequent to irradiation, as well as interfering Ar from atmospheric contamination and Major elements the irradiation of Ca, Cl and K (isotope production ratios: ðÞ40 = 39 ¼ : : 37 =39 ¼ Ar Ar K 0 0302 0 00006, Ar Ar Samples of coherent lithofacies from Mathews Tuya all plot  Ca 39 as basalt, basanite (greater than 10% normative ol), 1416:4 0:5, 36Ar= Ar ¼ 0:3952 0:0004, Ca=K ¼ Ca hawaiite and , above the alkaline-subalkaline 37 39 1:83 0:01 ArCa= ArK ). divide of Irvine and Baragar (1971; Fig. 7a). Whole-rock samples are nepheline normative (Table 1) and plot in two 40Ar/39Ar results distinct groups (Figs. 7a and b). The first group plots on the hawaiite-basanite boundary, and comprises samples from Results of the analyses give crystallization ages of 0.718± lower in the stratigraphic section. The second group 54 Ma (KAS3a; initial 40Ar=39Ar ¼ 296:5 8:7) and comprises olivine basalt samples from higher in the section 0.742±81 Ma (KAS14b; initial 40Ar=39Ar ¼ 302 34), that plot well into the basalt field, as well as a dike that for a weighted mean age of 0.730 Ma (Fig. 6). The plateau crosscuts the lower stratigraphy and may be a feeder dike and correlation ages were calculated using Isoplot ver.3.09 for units higher in the section. (Ludwig 2003). Errors are quoted at the 2 σ (95% Analyses of glass samples from volcaniclastic lithofacies confidence) level and are propagated from all sources (samples KAS 8a, 8b, 15a) and a dike margin also plot in except mass spectrometer sensitivity and age of the flux two clusters (Fig. 7a). Clastic facies from relatively low in monitor. The best statistically-justified plateau and plateau the section plot as hawaiites/basanites, while the glass rim age were picked based on the following criteria: (1) three or of a feeder dike plots in the alkali basalt field. Glasses from more contiguous steps comprising more than 50% of the both clusters have lower Mg # than accompanying whole 39Ar; (2) probability of fit of the weighted mean age greater rock analyses (Fig. 7b); however even though the whole- than 5% (3) slope of the error-weighted line through the rock Mg # differs by five units, the glasses from both plateau ages equals zero at 5% confidence; (4) ages of the clusters have overlapping Mg #. The two clusters also have 488 Bull Volcanol (2011) 73:479–496

40 39 Fig. 6 Ar/ Ar geochronome- ab try for samples from Mathews 0.0038 Tuya. Analytical data for 1.6 σ both samples is included in 0.0034 Supplementary Tables 2 and 3. a Ar/ Ar 1.2 40 36 Plateau diagram for sample KAS3a, a largely holocrystalline 0.0030 basaltic dike (0.742±0.081 Ma). 0.8 Note that the first step (less than 0.0026 39 ~0.5% of cumulative Ar) is (Ma) Age not shown for clarity, and height 0.4 0.0022 of individual boxes equals 2 σ KAS3a KAS3a errors. b Inverse isochron plot 0.0 0.0018 for KAS3a showing all heating 0 20 40 60 80 100 0.0 0.2 0.4 0.6 39 40 steps. Ellipses show 2 σ errors. c 39 Cumulative Ar Percent Ar/ Ar Plateau diagram for sample c d KAS14b, a largely holocrystal- line subaerial lava flow (0.718± σ 0.004 0.054 Ma). Note that the first 1.2 step (less than ~0.5% of Ar/ Ar 640 36 cumulative 39Ar) is not shown 0.003 for clarity, and height of indi- 0.8 vidual boxes equals 2 σ errors. d

Inverse isochron plot for (Ma) Age KAS14b showing all heating 0.4 0.002 steps. Ellipses show 2 σ errors KAS14b KAS14b 0.0 0.001 0 20 40 60 80 100 0.0 0.2 0.4 0.6 0.8 1.0 39 39 40 Cumulative Ar Percent Ar/ Ar

distinctive Ca/Al ratios (Fig. 7b). Thus the glass and rock phase in the samples, Ol is present as microphenocrysts and analyses show the parallel relationships between units Ol, Pl and Cpx are ubiquitous in the groundmass of all non- lower and higher in the section. Previously published data vitrophyric samples (Simpson 1996). from nearby glaciovolcanic edifices (Ash Mountain, South Calculations using whole-rock analyses as liquid com- Tuya, Tuya Butte; Moore et al. 1995) show similar general positions compare favorably to crystallization sequences trends between whole-rock and glass analyses (Fig. 7a), and inferred from petrography (Ol > Ol+Pl > Ol+Pl+Cpx), with in general parallel trends for the stratigraphically higher Ol being the liquidus phase down to mid- to lower crustal group from Mathews Tuya (Fig. 7c). Although Moore et al. pressures (0.4 to 0.6 GPa). The observed compositional (1995) postulated the existence of two distinct geochemical differences between the two groups only slightly alter the groups at Ash Mountain, South Tuya, and Tuya Butte, the estimated phase stabilities as a function of pressure, with amount of geochemical separation of the two groups at the high SiO2 group losing Ol as the liquidus phase at those three volcanoes is minimal in comparison to the slightly lower pressures than the low SiO2 group. The compositional variation at Mathews tuya (Fig. 7b versus c). addition of one weight percent H2O enlarges the stability field of Ol, making it the liquidus phase to even greater Phase saturation diagrams depths (0.6 to 0.8 GPa) for both groups. Calculations indicate that the minimum liquidus temperatures for the Phase saturation diagrams for each of three samples (KAS 2 were greater than 1200°C. [WR group 1], KAS 3a [WR group 2], KAS8a [glass]) were Computations using the glass composition yield signif- calculated using MELTS (Ghiorso and Sack 1995;Asimow icantly different results. For low pressure, anhydrous and Ghiorso 1998) and are used to constrain interpretations conditions plagioclase is predicted as the stable liquidus of the conditions attending crystallization of the micro- phase, followed by Cpx at pressures greater than phenocryst assemblage (Fig. 8). The saturation calculations ~0.35 GPa. However, the addition of one weight percent show the relative stability of major phases for a fixed melt H2O destabilizes Pl, so that Ol followed closely by Cpx composition. The saturation diagrams are not true phase become the predicted low pressure, liquidus phases. The diagrams, as the calculations to do not take into account predicted liquidus temperatures as a function of pressure for changes in liquid compositions resulting from crystallization. the glass sample are taken to indicate that the lava erupted Although Opx is never seen as a phenocryst or groundmass 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) 1σ KAS8ab (n=10) 1σ KAS8bc (n=5) 1σ KAS15d (n=10) 1σ WRe (n=8) 1σ

SiO2 47.26 0.18 48.77 0.13 48.01 0.36 47.86 0.25 47.12 0.75

TiO2 3.42 0.06 3.08 0.05 3.21 0.18 3.14 0.06 2.87 0.47

Al2O3 16.18 0.15 14.58 0.18 15.72 0.15 15.96 0.15 14.56 0.04

Cr2O3 0.02 0.01 0.04 0.02 0.01 0.01 0.02 0.02 nd FeO(T) 12.45 0.12 12.85 0.19 12.32 0.19 12.30 0.14 13.13 0.48 MnO 0.15 0.03 0.19 0.03 0.15 0.04 0.15 0.03 0.17 0.01 MgO 4.44 0.07 4.86 0.07 4.47 0.21 4.70 0.07 7.78 0.78 CaO 8.33 0.07 9.48 0.08 8.62 0.41 8.53 0.08 8.49 0.48

Na2O 4.56 0.12 3.91 0.05 4.57 0.24 4.48 0.10 3.60 0.46

K2O 2.25 0.05 1.56 0.02 2.08 0.19 2.03 0.05 1.63 0.41

P2O5 0.93 0.07 0.68 0.05 0.84 0.10 0.83 0.15 0.65 0.16 Totalf 100.00 100.00 100.00 100.00 100.00 Calculated Parameters Tliq (dry)g 1139 1152 1143 1145 1224 Phase Pl (An64) Pl (An65) Pl (An64) Pl (An65) Ol (Fo81) Tliq (wet)h 1113 1118 1111 1120 1216 Phase Ol (Fo73) Cpx (Di46) Ol (Fo73) Ol (Fo74) Ol (Fo81) Fe2O3i 2.297 2.326 2.290 2.280 2.187 FeOi 10.382 10.756 10.258 10.247 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 g h percent; all calculations done at P=1 MPa using MELTS (Ghiorso and Sack 1995; Asimow and Ghiorso 1998); ‘wet’ = 0.5 wt% H2O based on estimates i for Ash Mountain from Moore et al. (1995); all calculations done at fO2=QFM

Discussion with causal linkages between ice sheet dynamics and volcanism. Work to test the hypothesis more rigorously by Chemical evolution of Mathews Tuya determining eruption ages for 30 of the centers in the volcanic field is presently in progress. We infer, based on the compositional similarities with Phase equilibrium modeling via MELTS (Fig. 8) indi- alkaline basalts from throughout the northern Cordilleran cates that the onset of crystal growth in the parental melt volcanic province (NCVP) and calculated liquidus temper- took place at less than 18 km depth based on the atures, that the formation of Mathews Tuya was initiated by observation of Ol as the dominant microphenocryst. The partial melting in the asthenosphere (e.g., Edwards and lack of associated peridotite xenoliths, which are relatively Russell 2000; Dixon et al. 2002). Edwards and Russell common in NCVP lavas (cf. Edwards and Russell 2000; (1999) suggested that the NCVP results from far-field plate Harder and Russell 2005; Edwards et al. 2006), and the tectonic forces that, combined with access to non- relatively low Mg Numbers for lava samples are both subduction-modified mantle via slab windows (Madsen et consistent with the parental magma having ponded tempo- al. 2006), produced a magma-charged lithosphere suscep- rarily within the and fractionating slightly prior to tible to periodic tapping caused by fluctuations in the local eruption; this hypothesis is similar to that of Moore et al. stress field driven by ice loading and unloading (e.g. (1995). However, Moore et al. (1995) suggested that Edwards et al. 2002). Other workers (e.g., Jull and chemical differences between what they termed ‘tholeiitic’ McKenzie 1996; MacIennan et al. 2002; Sigvaldason et and ‘alkalic’ rock types at three volcanoes adjacent to al. 1992) have suggested that isostatic changes caused by Mathews Tuya (Ash Mountain, Tuya Butte and South formation/degradation of ice sheets in Iceland led to Tuya; Fig. 1) resulted from eruption of magmas from two increased eruption frequencies. While testing such a different source regions during the course of a single hypothesis in the Tuya volcanic field is not possible with glaciovolcanic eruption, both of which had been stored at present datasets, the dominance of glaciovolcanic products shallow depths. In their model, initial eruption of overlying in a volcanic field that also has subaerial volcanism (e.g. ‘tholeiitic’ magma allowed for vesiculation and eruption of Simpson et al. 2006; Wetherell et al. 2006) is consistent the ‘deeper’ alkalic magma. We disagree with their model 490 Bull Volcanol (2011) 73:479–496

a from two perspectives. Firstly, although the samples from 7 Mathews Tuya also show two slightly different chemical compositions (e.g. Fig. 7b), the mineralogy of the samples 6 is the same, and can be related to low pressure fractionation

22 accompanying changes in activity of H2O; the geochemical 5 variations seen at Mathews Tuya are far greater than those reported by Moore et al. (1995). Secondly, we do not see 4 the predicted rapid change to dominantly magmatic fragmentation that would result from sudden depressuriza- 3 tion of a deeper magma body. The transition from quench Weight percent Na O + K to magmatic fragmentation in volcaniclastic lithofacies does 2 not appear to be abrupt at Mathews Tuya. 44 46 48 50 52 54 Weight percent SiO2 Physical evolution of Mathews Tuya b 0.70 1kb dry MT WR Mathews Tuya erupted along the crest of a separating MT GL a small U-shaped, -filled valley to the north of the tuya from a much larger U-shaped, ice-filled valley to the 0.60 south ~0.730 Ma (Fig. 9a). Thus, the volcano has an asymmetric appearance, with the south wall of the valley 0.50 acting as a buttress along the south side of the edifice (Fig. 9b). Initial venting of the tuya melted a cavity in the Molar Ca/Al 1 kb 'wet' overlying ice and produced an ice-confined, water-filled 0.40 chamber (englacial lake) beneath at least 500 m of ice (e.g., 30 35 40 45 50 55 60 Tuffen 2007). Evidence for the production of a lava-fed Mg Number c delta is consistent with eruption in an ice-dammed lake, 1kb dry AM WR which is more likely to form and stabilize during an 0.70 AM GL eruption beneath relatively ‘thick’ ice (e.g. Smellie and ST WR ST GL Skilling 1994). The eruption appears to have followed the ‘ ’ 0.60 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 0.50 Molar Ca/Al TB WR eruption of coherent pillow lava, followed by increasingly 1 kb 'wet' TB GL more-explosive eruptions as the volcanic pile grew with respect to the confining water, until the vent was sealed 0.40 30 40 50 60 70 from water and effusion of coherent lava into the ice- Mg Number confined lake built a lava delta, separated from capping Fig. 7 Geochemical characteristics for volcanic rocks and glasses subaerial lava flows by a passage zone (Mathews 1947; from Mathews Tuya and surrounding volcanic centers. a TAS diagram Jones 1969; Smellie 2006). (after LeMaitre 2002) showing published rock (WR) and glass (GL) The basal succession is overlain by coarse and fine clastic compositions from edifices within the Tuya volcanic field: Mathews lithofacies that contain dominantly vesicular clasts (Fig. 9c). 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), Several hypotheses might explain the transition from quench and Tuya Butte (TB, Allen et al. 1982; Moore et al. 1995). Small gray to magmatic/phreatomagmatic fragmentation: (1) if the water symbols show the range of compositions from samples throughout the level in the subglacial cavity remained constant, the NCVP (cf. Edwards and Russell 2000). Alkaline-subalkaline division hydrostatic pressure overlying the active part of the edifice (solid black line) from Irvine and Baragar (1971). b Compositions of rocks and glasses from Mathews Tuya (MT) are plotted as ratios of would decrease as the edifice grew in height; (2) as the molar Ca/Al versus Mg# and compared against model liquid lines of eruption changed from initially effusion dominated to descent for low pressure crystallization of anhydrous and hydrous explosion dominated, the increased efficiency of heat transfer melts (see Tables 1 and 2 for compositions; LLD calculations from (e.g., Guðmundsson 2003) could cause an increase in 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 melting, leading to increasing cavity underpressures and ratios of Ca/Al versus Mg# and compared against model liquid lines higher explosivity (e.g., Tuffen 2007); (3) buildup of of descent for low pressure crystallization of anhydrous and hydrous meltwater in the sub-ice cavity could have led to catastrophic melts drainage of the cavity, causing a rapid pressure decrease. Bull Volcanol (2011) 73:479–496 491

Fig. 8 Liquidus and pseudo- liquidus pressure-temperature a 1400 1400 relationships calculated for three K A S3 a dr y OP X K A S3 a 1w t% H 2O

OP X T rock compositions from ) e C

1300 1300 m o

Mathews Tuya at anhydrous and (

O L p e

O L e

r CP X hydrous (1 weight percent H2O) CP X r u a t t conditions: a Sample KAS3a; b a 1200 1200 u r r e

Sample KAS2; c Sample PL e p

PL ( o KAS8a. The curves are m C e

1100 1100 ) consistent with a sequence of T crystallization of Ol > Cpx >> Pl at high pressures, and Ol > 1000 1000 Cpx ~ Pl at lower pressures. 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Orthopyroxene is predicted to be P r es s ur e ( GP a) P r es s ur e ( GP a ) stable at the base of the crust but would not crystallize at lower b 1400 1400 pressures because of its reaction KAS2 dry K A S2 1 wt % H 2O

relationship with olivine. The OPX T e C)

1300 OP X 1300 m main effect of water is to o p depress the saturation tempera- OL CPX O L e r

CP X a ture of plagioclase relative to all 1200 1200 t u r

other phases e

PL ( o C

1100 1100 ) Temperature ( PL

1000 1000 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Pressure (GPa) P r es s ur e ( GP a ) c 1400 1400 K A S8 a dr y K A S8 a 1w t% H 2O T ) e C

1300 1300 m

o CP X (

CP X p e e r r

u OP X a t t a 1200 1200 OP X u r PL r e e

p OL O L ( o m C e PL

1100 1100 ) T

1000 1000 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 P r es s ur e ( GP a) P r es s ur e ( GP a )

Fig. 9 Schematic regional and a Mathews local sections for Mathews NW SE Tuya Tuya. a Schematic regional 1800 m 1800 m topographic profile NW-SE showing the position of 1600 m 1600 m Mathews Tuya with respect to a 1400 m 1400 m smaller U-shaped valley to the 1200 m 1200 m north and a much larger 0 km 1 km 2 km 3 km 4 km 5 km 6 km 7 km 8 km

U-shaped valley to the south b NW SE (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 492 Bull Volcanol (2011) 73:479–496

As the edifice grew above the level of the lake the eruption the edifice lava cap, and which still contains the remnants changed from subaqueous to subaerial. Lavas erupted subaer- of what appears to be a small rock glacier. The extent of ially flowed off the emergent portion of the volcano, entered erosion at the tuya is consistent with it having experienced the englacial lake and formed a pillow-lava delta comprising prolonged periods of glacial erosion since ~0.730 Ma. intact pillow lobes and pillow breccia (e.g., Skilling 2002). We can infer the presence and minimum thickness of ice Exposures of the pillow-lava delta are restricted to the west- extant at ~0.730 Ma surrounding Mathews Tuya from the northwest flank of the volcano so the true extent and height of the passage zone and the local bedrock topogra- geometry of this lava delta is not known. Continued phy (Fig. 9a). The passage zone occurs ~300 m above the magmatic fragmentation may also have been intermittent or inferred base of the edifice on the north side, and ~600 m synchronous with effusive eruptions during this time, but was above the valley floor on the south side. Smellie (2006) likely subordinate. Subaerially erupted lavas advanced over suggested that passage zones may demarcate the ice- the deltaic deposits, producing a relatively flat-lying passage transition in glaciers; thus the minimum ice thickness to the zone capped by subhorizontal, subaerial lava. north of the vent location was ~400 m, while the minimum On the northwestern flanks, a series of radially jointed lava ice thickness to the south was ~700 m. Given the geometry mounds outcrop midway down the slope. The origin of these of the deeper valley on the south side of the tuya, we would features and timing relative to the other eruptive deposits infer, based on a parabolic ice thickness model (Equation 5, remains uncertain. However, radial jointing, concave geom- p. 242 of Paterson 1994), that the eruption which formed etries, and apparent downslope dips are consistent with Mathews Tuya occurred within a regionally extensive formation as lava tubes within the delta or as lavas emplaced glacier or ice sheet. However, at present we cannot say into ice-bounded cavities along the volcano-ice contact (e.g., with certainly if the enclosing ice mass was a ~0.730 Ma Loughlin 2002). At present no pillow lava has been found manifestation of the Cordilleran Ice Sheet, which, during associated with these lavas, so they may have formed after the Last Glacial Maximum, is inferred to have been 2–3km the bounding englacial lake drained. in maximum thickness. We can only say with certainty that Based on edifice morphology, its eruption age of the region surrounding Mathews Tuya was partly ice ~0.730 Ma, and the presence of glacial erratics on the top covered ~0.730 Ma. and flanks of the edifice, it seems likely that glacial erosion has significantly modified the original form of Mathews Regional volcanological comparisons Tuya. In all of northern British Columbia, extensive cirque glaciation is evident on most north-facing slopes. Mathews As many as 20 volcanoes in the Tuya volcanic field may be Tuya also has a north-facing cirque that has almost bisected glaciovolcanic in origin; however, only three other edifices

Ash Mountain South Tuya Mathews Tuya Tuya Butte

2100 m Unconsolidated 2100 m 2100 m 2100 m volcaniclastic facies comprising 2000 m scoriaceous ash, 2000 m Consolidated 2000 m 2000 m lapilli and rare volcaniclastic 1900 m bombs (up to 2 m). 1900 m facies comprising 1900 m 1900 m Cross-cutting dikes scoriaceous ash, Subaerial basaltic are locally bulbous lapilli and pillow lava flows with 1800 m 1800 m 1800 m 1800 m and pillow-like. fragments. Local vertical to radial Subaerial basaltic lenses of pillow columnar jointing. lava flows with 1700 m 1700 m lava, dikes, and 1700 m 1700 m Pillow lava Consolidated vertical to radial layers of volcanic massive to bedded to likely subaerial eruption with minor columnar jointing. 1600 m 1600 m bombs. 1600 m volcaniclastic facies.1600 m interlayered Foreset bedded Transition zone from subaqueous volcaniclastic Local lenses of pillow lava, dikes, volcaniclastic facies Pillow lava 1500 m facies. Upsection1500 m 1500 m and lava tubes/sills. 1500 m containing broken and clastic facies is with minor intact isolated pillows ? more abundant interlayered and massive(?) lava. 1400 m 1400 m 1400 m Pillow lava 1400 m and pillows are volcaniclastic and massive Pillow lava more vesicular. facies. with minor ? lava(?) with 1300 m 1300 m 1300 m ? minor 1300 m interlayered interlayered volcaniclastic 1200 m 1200 m 1200 m volcaniclastic 1200 m ? facies locally facies, locally cross-cut by cross-cut by dikes. dikes.

Fig. 10 Comparative stratigraphic sections for glaciovolcanic edifices tite. General stratigraphic information for Ash Mountain, South Tuya in the Tuya volcanic field. Sections are drawn relative to their present and Tuya Butte was derived from Mathews (1947), Allen et al. (1982), day elevations. Shaded field denotes the inferred transition from and especially Moore et al. (1995) as well as reconnaissance by the subaqueous to subaerial facies as marked by the highest elevation of authors. Lithofacies colors same as in Fig. 2 pillowed lavas or deposits containing pillow fragments or hyaloclas- Bull Volcanol (2011) 73:479–496 493

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 emanating radially from the 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 character- ized 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 Fig. 11 Summary of geochronometric constraints on timing of North subaerial fire fountaining was the main late-stage eruptive American glaciation and NCVP glaciovolcanism. Data for approxi- mate relative global ice volumes are derived from 18O/16O for benthic style. Tuya Butte and Mathews Tuya are capped by subaerial foraminifera from Shackleton et al. (1990). Information on the North lavas, which is consistent with effusive eruptions being American ice record are derived from Bowen et al. (1986), and dominant in the final stages of eruptive activity as well. 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; Implications for pleistocene cordilleran paleoclimate Edziza), Edwards et al. (2002; Hoodoo), Edwards et al. (2006; Bell- Irving), and this study Glaciovolcanism is critically important for recording changes in Pleistocene paleoclimate conditions, and glacio- have been previously described in any detail (Fig. 1): Tuya volcanic products provide a means for further documenting Butte (Mathews 1947; Allen et al. 1982; Moore et al. the location and timing of continental ice, which can be 1995), South Tuya (Moore et al. 1995), and Ash Mountain compared with paleoclimate proxies from the marine (Mathews 1947; Allen et al. 1982; Moore et al. 1995). isotopic record and the continental glaciological record We use the published stratigraphic information on Tuya (Fig. 11). Detailed volcanological/geochronological studies Butte, Ash Mountain, South Tuya in conjunction with our in Antarctica (e.g. Smellie et al. 2008) show local increases stratigraphic description of Mathews Tuya and our own in maximum ice sheet thicknesses through time. In northern reconnaissance work at Ash Mountain and South Tuya to British Columbia, the eruption ages for a number of give a stratigraphic comparison of glaciovolcanic deposits glaciovolcanic deposits are now known and can be used within this region of the Canadian Cordillera (Fig. 10). All to further constrain interpretations of the Cordilleran four of the volcanoes are inferred to have 200–300 m thick paleoclimate, although distinguishing between local and successions of pillow lava at their bases (Mathews 1947; regional glacial events can be problematic (e.g., Edwards et Moore et al. 1995). At South Tuya and Ash Mountain, al. 2009). The glaciovolcanic origins of Mathews Tuya basal platforms of pillow lava are well exposed, while at confirm the existence of pre-Last Glacial Maximum (LGM) Tuya Butte and Mathews Tuya basal exposures are limited. glaciation at ~0.730 Ma, during a period of high ice volume Overlying the pillow lava facies are unconsolidated, as implied by the marine record. Geochronological evi- massive to bedded volcaniclastic facies. The volcaniclastic dence of tuya formation in the Tuva Republic, Russia, facies are monomictic (basaltic), containing scoriaceous appears to corroborate an expansion of northern hemisphere 494 Bull Volcanol (2011) 73:479–496 glaciers ~0.740 Ma (Yarmolyuk et al. 2001 as reported in Allen CC, Jercinovic MJ, Allen JSB (1982) Subglacial volcanism in – Komatsu et al. 2007). The location of the Tuya volcanic north-central British Columbia and Iceland. J Geol 90:699 715 Asimow PD, Ghiorso MS (1998) Algorithmic modifications extending field also corresponds with one of the two inferred areas of MELTS to calculate subsolidus phase relations. Am Mineral maximum thickness for the CIS during the LGM (Peltier 83:1127–1131 1994). However, few deposits remain in this area to provide Bacon CR, Lanphere MA (2006) Eruptive history and geochronology pre-LGM evidence for regionally extensive glaciation, of and the Crater Lake region, . Geol Soc Am Bull 118:1331–1359 other than glaciovolcanic deposits such as those exposed Bowen DQ, Richmond GM, Fullerton DS, Sibrava V, Fulton RJ, at Mathews Tuya. Although previous workers inferred that Velichko AA (1986) Correlation of quaternary glaciations in the Tuya area glaciovolcanism occurred during the LGM, our Northern hemisphere. Quat Sci Rev 5:509–510 work clearly documents the existence of earlier glaciovol- Chapman MG, Allen CC, Guðmundsson MT, Gulick VC, Jakobsson SP, Lucitta BK, Skilling IP, Waitt RB (2000) Volcanism and ice canic activity. Stratigraphic comparisons of glaciovolcanic interactions on Earth and Mars. In: Zimbelman JR, Gregg TKP deposits in the Tuya volcanic field combined with detailed (eds) Environmental effects on volcanic eruptions. Kluwer geochronologic studies currently in progress for other Academic/Plenum Publishers, New York, pp 39–74 locations within the Tuya volcanic field will soon allow Cui Y, Russell JK (1995) Magmatic origins of calc-alkaline intrusions from the Coast Plutonic Complex, southwestern British Colum- for much better correlations between North American bia. Can J Earth Sci 32:1643–1667 continental and marine climate records. Dixon JE, Filiberto JR, Moore JG, Hickson CJ (2002) Volatiles in basaltic glasses from a in northern British Columbia (Canada): implications for ice sheet thickness and mantle volatiles. In: Smellie JL, Chapman MG (eds) Volcano-Ice Conclusions interaction on Earth and Mars, Geological Society. Special Publications 202, London, pp 255–271 Mathews Tuya records continuous volcano-ice interactions Edwards BR, Russell JK (1999) Northern Cordilleran Volcanic – starting from initially sub-ice/subaqueous conditions to Province: a northern Basin and Range? Geol 27:243 246 Edwards BR, Russell JK (2000) The distribution, nature, and origin of subaerial conditions. Textural analysis of volcaniclastic Neogene-Quaternary magmatism in the Northern Cordilleran deposits show changes in proportions of clasts derived from Volcanic Province, northern Canadian Cordillera. Geol Soc Am quench versus explosive fragmentation processes and Bull 112:1280–1295 provides key information on the transitions between effusive Edwards BR, Russell JK, Anderson RG (2002) Subglacial, phonolitic volcanism at volcano, northwestern Canadian and explosive activity in glaciovolcanic eruptions. Although Cordillera. Bull Volcanol 64:254–272. doi:10.1007/s00445-002- geochemically similar to nearby volcanoes, Mathews Tuya 0202-9 shows the most extreme compositional diversity yet found in Edwards BR, Evenchick CA, McNicoll V, Nogier M, Wetherell K the Tuya volcanic field, which appears to be explained by low- (2006) Overview of the volcanology of the Bell-Irving volcanic district, northwestern Bowser Basin, British Columbia: new pressure crystallization. Its eruption age of ~0.730 Ma means examples of alpine glaciovolcanism from the northern Cordille- that it provides the first record of Cordilleran ice in northern ran volcanic province. Current Research, Geological Survey of British Columbia at that time and appears to correlate well Canada (GSC) Paper 2006-A3 with marine proxies indicative of relatively high ice volumes Edwards BR, Skilling IP, Cameron B, Lloyd A, Haynes C, Hungerford J (2009) Evolution of an englacial volcanic ridge: as well as ~0.740 Ma glaciovolcanic deposits in Russia. tindar, volcanic complex, NCVP, British Columbia, Canada. J Volcanol Geotherm Res 185. doi:10.1016/j.jvolgeores.2008.11.015 Acknowledgments We thank several people for general assistance Fisher RV (1961) Proposed classification of volcaniclastic sediments with fieldwork in the Tuya area, including Jim Reed, Chris Price, Blake and rocks. Geol Soc Am Bull 72:1409–1414 Parker and Tark Hamilton. Funding for initial fieldwork in 1995 was from Gabrielse H (1970) Geology of the Jennings River map-area, British LITHOPROBE. During the preparation of the manuscript BRE was Columbia (104-O): Geol Surv Canada Paper 68-55, 37 p supported in part by NSF-EAR 0439707, and a 2009 spot check was Gabrielse H (1998) Geology of the Cry Lake and Dease Lake Map funded in part by NSF-EAR 0910712; JKR was supported by the NSERC Areas, north-central British Columbia. Geol Surv Canada Bull Discovery Grants program. Tom Ullrich of the Pacific Center for Isotopic 504, 147 p 40 39 & Geochemistry Research at UBC made the Ar/ Ar determinations Ghiorso M, Sack RO (1995) Chemical mass transfer in magmatic and interpretations. Helpful comments by C. Bacon and M.T. processes. IV. A revised and internally consistent thermodynamic Guðmundsson improved the clarity and presentation of the manuscript; model for the interpolation and extrapolation of liquid-solid MTG particularly helped us clarify our thoughts on the implications for equilibria in magmatic systems at elevated temperatures and local versus regional glaciations. 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