Northern Cordilleran Volcanic Province Research Articles

Pleistocene to Holocene volcanism in the Canadian Cordillera

The Canadian Cordillera hosts numerous Pleistocene and Holocene volcanoes and volcanic deposits, including a number of volcanoes that have erupted within the last several hundred years. The nature and composition of volcanic edifices and deposits are diverse and dictated by the complex configuration of tectonic plates along the western margin of British Columbia and the thermal structure of the underlying mantle. Our modern knowledge of these is built upon more than a century of field- and increasingly, laboratory-based studies. We recognize five distinct volcanic domains within the Cordillera that are distributed across British Columbia, the Yukon Territory, and easternmost Alaska. These include the Wrangell Volcanic Belt, the Northern Cordilleran Volcanic Province, the Anahim Volcanic Belt, the Wells Grey-Clearwater Volcanic Field, and the Garibaldi Volcanic Belt representing the northern extension of the Cascade Volcanic Arc. Volcanism in the Canadian Cordillera spans the full range of explosive to effusive behaviours, encompasses the suite of common volcanic chemical compositions (alkaline to calc-alkaline and nephelinite to peralkaline rhyolite), and is expressed by long-lived stratovolcanoes, shield volcanoes, and calderas, as well as shorter-lived tephra cones and associated lava flows. The range in tectonic settings (subduction to extension), eruption environments (subaerial–subaqueous–cryospheric), and topographic variability make volcanism within the Canadian Cordillera as diverse as anywhere on Earth, yet it is also the least studied. Here, we summarize the current state of knowledge concerning volcanism within the Canadian Cordillera and conclude with thoughts on research areas that merit further effort, namely glaciovolcanism and volcanic hazards.

Characterization of Hydrothermal Alteration in Palagonitized Deposits Using Short-Wave Infrared Spectroscopy and X-Ray Diffraction Methods

Abstract In this contribution, we evaluate the applicability of short-wave infrared spectroscopy to characterizing mineral assemblages in palagonitized glaciovolcanic edifices at Cracked Mountain, a basaltic volcano within the Garibaldi Volcanic Belt, and Kima Kho, a basaltic tuya within the Northern Cordilleran Volcanic Province. Second-order evaluation of the approach was established through comparison with data obtained by semi-empirical X-ray diffraction methods. Reflectance spectra show an increase in the depth of water and hydroxyl bands in samples with increasing amounts of hydrated minerals and decreasing amounts of amorphous materials (as determined through X-ray diffraction), indicating that the relative strengths of H2O- and OH-related absorption features may be used as a proxy for the degree of palagonite alteration (hydrated minerals crystallized from basaltic glass). In addition, the full width at half maximum of the OH- and H2O-related absorption bands decreased with the formation of zeolites, indicating that the full width at half maximum of OH- and H2O-related features may be used to estimate the degree of crystallinity across the progressive palagonitization process. Finally, short-wave infrared spectroscopy revealed a decrease in band depth of water-related absorption features with no change in the full width at half maximum along the devitrification or alteration process that converts analcime to chabazite to wairakite, indicating that spectroscopy may be used to identify the final dehydration and cementation stages of palagonitization. Results show that the short-wave infrared spectroscopy method is more robust in identifying poorly crystalline hydrated samples, while X-ray diffraction methods are better suited to understanding the crystalline components of palagonite. Short-wave infrared spectroscopy is a remote sensing technique that has proven to successfully characterize the state of H2O in hydrated clay-rich material and thus may serve as an invaluable tool in identifying stages of palagonitization not only on subglacial edifices on Earth but also on off-planet environs, including the Martian surface.

Magmatic origins and storage conditions for the historic eruption of Tseax Volcano, British Columbia, Canada

The Northern Cordilleran Volcanic Province (NCVP) is the most active volcanic belt in Canada, yet there are few detailed petrologic studies of the individual volcanoes. Tseax volcano is the southernmost volcanic centre of the NCVP, the second youngest (~1700 CE) eruption in Canada, and led to one of Canada's worst natural disasters killing up to ~2000 Nisga'a First Nation people. We present a conceptual model for the origins, evolution and pre-eruptive storage of the Tseax magma which erupted in ~0.5 km3 of volcanic material, mainly in the form of valley-filling lava flows. All Tseax products are alkali Fe- Ti- rich basanite-to-tephrite having trace element distributions similar to the other mafic NCVP lavas. Phenocrysts consist of plagioclase, olivine and titanomagnetite, often forming glomerocrysts suggesting co-crystallisation of these 3 mineral phases. Clinopyroxene is never observed as a phenocryst phase. The lavas have low Mg#’s implying they are fractionated relative to a mantle-derived ‘parent’ magma. Trace elements indicate the magma derives from melting of a fertile mantle source (i.e., (Nb/K)n ~ 1), most likely the upper asthenosphere. There is no evidence in the Tseax volcanic rocks for magma mixing or lithospheric contamination during ascent. The phenocryst assemblage suggests rapid ascent of a low viscosity magma to <5.5 km where titanomagnetite becomes the first phase on the liquidus (~ 1133 °C). There the magma stalled for a very short period of time under PT conditions where clinopyroxene did not reach saturation. Based on the size of the plagioclase phenocrysts, the magma stalled for less than a year and cooled down to 1094–1087 °C prior to eruption.

Pargasite-bearing vein in spinel lherzolite from the mantle lithosphere of the North America Cordillera

Basanite lavas near Craven Lake, British Columbia, host a spinel lherzolite xenolith containing cross-cutting veins with pargasitic amphibole (plus minor apatite). The occurrence of vein amphibole in spinel lherzolite is singular for the Canadian Cordillera. The vein crosscuts foliated peridotite and is itself cut by the basanite host. The amphibole is pargasite, which is the most common amphibole composition in mantle peridotite. Rare earth element concentrations in the pargasite are similar to those for mafic alkaline rocks across the northern Cordilleran volcanic province (light rare earth elements ∼50× chondrite and heavy rare earth elements ∼5× chondrite). Two-pyroxene geothermometry suggests that the vein and host peridotite were thermally equilibrated prior to sampling by the basanite magma. Calculated temperature conditions for the sample, assuming equilibration along a model steady-state geotherm, are between 990 and 1050 °C and correspond to a pressure of 0.15 GPa (∼52 ± 2 km depth). These conditions are consistent with the stability limits of mantle pargasite in the presence of a fluid having XH2O &lt; ∼0.1. The pargasite vein and associated apatite provide direct evidence for postaccretion fracture infiltration of CO2–F–H2O-bearing silicate fluids into the Cordilleran mantle lithosphere. Pargasite with low aH2O is in equilibrium with parts per million concentrations of H2O in mantle olivine, potentially lowering the mechanical strength of the lithospheric mantle underlying the Cordillera and making it more susceptible to processes such as lithospheric delamination. Remelting of Cordilleran mantle lithosphere containing amphibole veins may be involved in the formation of sporadic nephelinite found in the Canadian Cordillera.

Volcanology and petrology of Mathews Tuya, northern British Columbia, Canada: glaciovolcanic constraints on interpretations of the 0.730 Ma Cordilleran paleoclimate

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 columnar-jointed 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 olivine-plagioclase porphyritic alkali olivine basalts. They are mineralogically and geochemically similar to nearby glaciovolcanic centers from the southeastern part of the Tuya volcanic field (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 40Ar/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.

Holocene tephras in lake cores from northern British Columbia, Canada

Sediment cores recovered from alpine and subalpine lakes up to 250 km apart in northern British Columbia contain five previously unrecognized tephras. Two black phonolitic tephras, each 5–10 mm thick, occur within 2–4 cm of each other in basal sediments from seven lakes in the Finlay River – Dease Lake area. The upper and lower Finlay tephras are slightly older than 10 220 – 10 560 cal year B.P. and likely originate from two closely spaced eruptions of one or two large volcanoes in the northern Cordilleran volcanic province. The Finlay tephras occur at the transition between deglacial sediments and organic-rich postglacial mud in the lake cores and, therefore, closely delimit the termination of the Fraser Glaciation in northern British Columbia. Sediments in Bob Quinn Lake, which lies on the east edge of the northern Coast Mountains, contain two black tephras that differ in age and composition from the Finlay tephras. The lower Bob Quinn tephra is 3–4 mm thick, basaltic in composition, and is derived from an eruption in the Iskut River volcanic field about 9400 cal years ago. The upper Bob Quinn tephra is 12 mm thick, trachytic in composition, and probably 7000–8000 cal years old. A fifth tephra occurs as a cryptotephra near the top of two cores from the Finlay River area and is correlated to the east lobe of the White River tephra (ca. 1150 cal year B.P.). Although present throughout southern Yukon, the White River tephra has not previously been documented this far south in British Columbia. The tephras are valuable new isochrons for future paleoenvironmental studies in northern British Columbia.

Cordilleran Intermontane thermotectonic history and implications for neotectonic structure and petroleum systems, British Columbia, Canada

Heat flow increases northward along Intermontane Belt in the western Canadian Cordillera, as shown by geothermal differences between Bowser and Nechako sedimentary basins, where geothermal gradients and heat flows are ∼30 mK/m and ∼90 mW/m2 compared to ∼32 mK/m and 70 –80 mW/m2, respectively. Sparse temperature profile data from these two sedimenatary basins are consistent with an isostatic model of elevation and crustal parameters, which indicate that Bowser basin heat flow should be ∼20 mW/m2 greater than Nechako basin heat flow. Paleothermometric indicators record a significant northward increasing Eocene or older erosional denudation, up to ∼7 km. None of the heat generation, tectonic reorganization at the plate margin, or erosional denudation produce thermal effects of the type or magnitude that explain the north–south heat flow differences between Nechako and Bowser basins. The more southerly Nechako basin, where heat flow is lower, has lower mean elevation, is less deeply eroded, and lies opposite the active plate margin. In contrast, Bowser basin, where heat flow is higher, has higher mean elevation, is more deeply eroded, and sits opposite a transform margin that succeeded the active margin ∼40 Ma. Differences between Bowser and Nechako basins contrast with the tectonic history and erosion impacts on thermal state. Tectonic history and eroded sedimentary thickness suggest that Bowser basin lithosphere is cooling and contracting relative to Nechako basin lithosphere. This effect has reduced Bowser basin heat flow by ∼10–20 mW/m2 since ∼40 Ma. Neither can heat generation differences explain the northerly increasing Intermontane Belt heat flow. A lack of extensional structures in the Bowser basin precludes basin and range-like extension. Therefore, another, yet an unspecified mechanism perhaps associated with the Northern Cordilleran Volcanic Province, contributes additional heat. Bowser basin’s paleogeothermal gradients were higher, ∼36 mK/m, before the Eocene and this might affect petroleum and metallogenic systems.

Dating young basalt eruptions by (U-Th)/He on xenolithic zircons

Accurate ages for young (e.g., Pleistocene) volcanic eruptions are important for geomorphic, tectonic, climatic, and hazard studies. Existing techniques can be time-consuming and expensive when many ages are needed, and in the case of K/Ar and 40Ar/39Ar dating, extraneous Ar often can limit precision, especially for continental basalts erupted through old lithosphere. We present a new technique for dating young basaltic eruptions by (U-Th)/He dating of zircons (ZHe) from crustal xenoliths. Single-crystal ZHe dates generally have lower precision than typical 40Ar/39Ar dates, but can be determined relatively easily on multiple replicate grain aliquots. We dated zircons from xenoliths from four volcanic centers in western North America: Little Bear Mountain, British Columbia (157 ± 3.5 [2.2%] ka weighted 95% confidence interval [CI], mean square of weighted deviates [MSWD] = 1.7) and Prindle Volcano, Alaska (176 ± 16 [8.9%] ka, MSWD = 13), in the northern Cordilleran volcanic province, and Fish Springs (273 ± 23 [8.6%], MSWD = 43) and Oak Creek (179 ± 8.1 [4.5%] ka, MSWD = 12), in the Big Pine Volcanic Field, California. All ZHe ages are either equivalent to or younger than previously determined K/Ar or 40Ar/39Ar ages, indicating the possibility of inherited 40Ar in some of the previous measurements. Zircons from upper crustal xenoliths in the Oak Creek and Fish Springs vents show poorer reproducibility and multiple apparent age distribution peaks, consistent with either intracrystalline U-Th zonation or <99.99% He degassing (assuming ca. 100 Ma pre-entrainment ZHe ages) of some zircons during magmatic entrainment. Removal of clear outliers in the older age-distribution peaks of the upper crustal xenoliths, most of which have extremely high U compared to other zircons of the same xenolith, improve the reproducibilities of Fish Springs to 4.7% (95% CI, MSWD = 4.8) and Oak Creek to 3.4% (95% CI, MSWD = 6.2). Coupled thermal and He diffusion modeling using appropriate xenolith sizes and magma temperatures and assuming published diffusion kinetics for zircon indicate that incomplete He degassing would require entrainment times <1 h. However, the observation of extremely high U in most zircons with older ages raises the possibility that zircons with high radiation dosages may have more retentive He diffusion characteristics.

Dating young basalt eruptions by (U–Th)/He on xenolithic zircons

Accurate ages for young (e.g., Pleistocene) volcanic eruptions are important for geomorphic, tectonic, climatic, and hazard studies. Existing techniques can be time-consuming and expensive when many ages are needed, and in the case of K/Ar and 40Ar/39Ar dating, extraneous Ar often can limit precision, especially for continental basalts erupted through old lithosphere. We present a new technique for dating young basaltic eruptions by (U-Th)/He dating of zircons (ZHe) from crustal xenoliths. Single-crystal ZHe dates generally have lower precision than typical 40Ar/39Ar dates, but can be determined relatively easily on multiple replicate grain aliquots. We dated zircons from xenoliths from four volcanic centers in western North America: Little Bear Mountain, British Columbia (157 ± 3.5 [2.2%] ka weighted 95% confidence interval [CI], mean square of weighted deviates [MSWD] = 1.7) and Prindle Volcano, Alaska (176 ± 16 [8.9%] ka, MSWD = 13), in the northern Cordilleran volcanic province, and Fish Springs (273 ± 23 [8.6%], MSWD = 43) and Oak Creek (179 ± 8.1 [4.5%] ka, MSWD = 12), in the Big Pine Volcanic Field, California. All ZHe ages are either equivalent to or younger than previously determined K/Ar or 40Ar/39Ar ages, indicating the possibility of inherited 40Ar in some of the previous measurements. Zircons from upper crustal xenoliths in the Oak Creek and Fish Springs vents show poorer reproducibility and multiple apparent age distribution peaks, consistent with either intracrystalline U-Th zonation or <99.99% He degassing (assuming ca. 100 Ma pre-entrainment ZHe ages) of some zircons during magmatic entrainment. Removal of clear outliers in the older age-distribution peaks of the upper crustal xenoliths, most of which have extremely high U compared to other zircons of the same xenolith, improve the reproducibilities of Fish Springs to 4.7% (95% CI, MSWD = 4.8) and Oak Creek to 3.4% (95% CI, MSWD = 6.2). Coupled thermal and He diffusion modeling using appropriate xenolith sizes and magma temperatures and assuming published diffusion kinetics for zircon indicate that incomplete He degassing would require entrainment times <1 h. However, the observation of extremely high U in most zircons with older ages raises the possibility that zircons with high radiation dosages may have more retentive He diffusion characteristics.

Thermal state of the upper mantle beneath the Northern Cordilleran Volcanic Province (NCVP), British Columbia, Canada

Data are presented for a suite of peridotitic mantle-derived xenoliths collected from basanite lavas within the Llangorse volcanic field, northwest British Columbia. The xenoliths comprise spinel lherzolite and subordinate spinel harzburgite. Two-pyroxene thermometry based on the Brey and Köhler (1990) [Brey, G.P., Köhler, T., 1990. Geothermobarometry in four-phase lherzolites II. New thermobarometers, and practical assessment of existing thermobarometers. Journal of Petrology, 31, 1353–1378.] calibration was applied to 44 xenolith samples. The resulting geothermometry define minimum (800–850 °C) and maximum (1050–1100 °C) temperatures of equilibration for the xenolith suite which are estimates of thermal conditions in the underlying lithospheric mantle. We take the minimum temperatures as indicative of the maximum MOHO temperature; the maximum xenolith temperatures provide a minimum temperature for the transition from lithospheric to asthenospheric mantle. The geothermometry data are combined with published heat flow data to produce a set of model geotherms for this portion of the northern Cordillera. The model geotherms constrain the thickness of the mantle lithosphere in the northern Canadian Cordillera to between 16 and 30 km, corresponding to depths to the lithosphere/asthenosphere boundary of 52–66 km. We show that this model is consistent with an underlying convecting asthenospheric mantle with an average temperature and viscosity of 1210–1250 °C and 10 19.4 Pa s, respectively. We conclude by exploring the implications this model has for the source regions of alkaline magmas erupted over this portion of the northern Cordilleran volcanic province.

ESTIMATES OF CRUSTAL ASSIMILATION IN QUATERNARY LAVAS FROM THE NORTHERN CORDILLERA, BRITISH COLUMBIA

The region between the Iskut and Unuk rivers and immediately south of Hoodoo Mountain in northwestern British Columbia is host to eight distinct occurrences of Quaternary volcanic rocks comprising alkali olivine basalt and minor hawaiite. The cen ters range in age from 70, 000 to ~150 years B.P. This volcanic field, called herein the Iskut volcanic field, lies along the southe rn boundary of the Northern Cordilleran volcanic province (NCVP) and includes sites identified as: Iskut River, Tom MacKay Creek, Snippaker Creek, Cone Glacier, Cinder Mountain, King Creek, Second Canyon and Lava Fork. The lavas are olivine- and plagioclase-phyric, contain rare corroded grains of augite, and commonly entrain crustal xenoliths and xenocrysts. Many crustal ly derived xenoliths are partially fused. Petrological modeling shows that the major-element compositional variations of lavas within the individual centers cannot be accounted for by simple sorting of the olivine and plagioclase phenocrysts or even cryp tic fractionation of higher-pressure pyroxene. Textural and mineralogical features combined with whole-rock chemical data suggest that assimilation of the underlying Cordilleran crust has affected the evolution of the relevant magmas. Mass-balance models ar e used to test the extent to which the within-center chemical variations are consistent with coupled crystallization and assimila tion. On the basis of our analysis of four centers, the average ratio of assimilation to crystallization for the Iskut volcanic field is 1:2, suggesting that assimilation has played a greater role in the origins of NCVP lavas than recognized previously.

Distribution, nature, and origin of Neogene-Quaternary magmatism in the northern Cordilleran volcanic province, Canada

The northern Cordilleran volcanic province encompasses a broad area of Neogene to Quaternary volcanism in northwestern British Columbia, the Yukon Territory, and adjacent eastern Alaska. Volcanic rocks of the northern Cordilleran volcanic province range in age from 20 Ma to ca. 200 yr B.P. and are dominantly alkali olivine basalt and hawaiite. A variety of more strongly alkaline rock types not commonly found in the North American Cordillera are locally abundant in the northern Cordilleran volcanic province. These include nephelinite, basanite, and peralkaline phonolite, trachyte, and comendite. The most MgO-rich nephelinites, basanites, and alkaline basalts from throughout the northern Cordilleran volcanic province show trace element abundances and isotopic compositions that are consistent with an asthenospheric source region similar to that for average oceanic island basalt and for post-5 Ma alkaline basalts from the Basin and Range. Our petrologic observations help constrain the origin of northern Cordilleran volcanic province magmatism as well as lithosphere changes between the four major basement terranes that underlie the province. Results from phase equilibria calculations and the spatial distributions of volcanic rock types and magmatic inclusions are more consistent with the existence of thicker lithosphere beneath Stikinia, which underlies the southern part of the northern Cordilleran volcanic province, than beneath the Cache Creek and Yukon-Tanana terranes, which underlie the northern part of the northern Cordilleran volcanic province. Our results support a model for initiation of northern Cordilleran volcanic province magmatism due to incipient rifting of the northern Cordillera, driven by changes in relative plate motion between the Pacific and North American plates ca. 15–10 Ma.

Geochronology of Neogene alkaline volcanic rocks (Miles Canyon basalt), southern Yukon Territory, Canada: the relative effectiveness of laser 40Ar/39Ar and K-Ar geochronology

Miles Canyon basalt is an informal term used to describe numerous exposures of young alkaline olivine basalt flows in southern Yukon. The volcanic rocks are part of the Northern Cordilleran volcanic province. K-Ar and Ar-Ar whole-rock dates indicate that the Miles Canyon succession of flows at the Whitehorse Rapids are clearly Late Miocene in age (ca. 8.4 Ma). The largest exposure of the Miles Canyon basalt occurs at the Alligator Lake volcanic complex where two nearly concordant Pliocene Ar-Ar dates indicate eruption at ca. 3.2 Ma. K-Ar analyses from other sites yield dates of 2.4 and 7.1 Ma and indicate an episodic Neogene volcanic history for the region. There is no evidence of Quaternary or postglacial volcanism. The dates are older than assumed by previous workers, and in some cases the K-Ar dates are strongly discordant from Ar-Ar determinations. More accurate Ar-Ar determinations may result from the method's ability to select smaller amounts of better material for analysis. Excess 40Ar was not encountered. As a result, the accuracy of any single or several discordant K-Ar determinations for Neogene subaerial volcanic rocks, particularly low-K rocks such as basalts, should be questioned and resulting interpretations made with caution. Models accounting for the eruption of the Northern Cordilleran volcanic province lavas have typically relied upon extension along north-trending faults that were generated by stresses along the continental margin. However, we consider a slab window model which better accounts for the initiation and distribution of northern Cordilleran Neogene volcanic activity.

Northern Cordilleran volcanic province: A northern Basin and Range?

INTRODUCTIONThe western margin of North America hosts adiverse assemblage of large Cenozoic igneousprovinces (Fig. 1 inset). The temporal,spatial,andpetrological characteristics of these provinces areattributed to a variety of magmatic processes andtectonic environments, including lithosphericextension (e.g., Basin and Range), migration ofmantle plumes (e.g., Snake River Plain), and sub-duction (e.g., Cascade magmatic arc). However,Neogene alkaline volcanic rocks in the Cordilleraof northwestern Canada, between 55° and 64° N(Fig. 1), currently lack a clear linkage to tectonicprocesses. These volcanic centers constitute thenorthern Cordilleran volcanic province and repre-sent a significant gap in our understanding of theCenozoic tectonomagmatic activity in westernNorth America (Fig. 1 inset).Our objective is to demonstrate that an exten-sional tectonic model adequately explains thepetrologic, radiometric, and tectonic data forNeogene-Quaternary volcanic rocks in the north-ern Cordillera. Furthermore, the model we pro-pose is consistent with models for Neogene-Quaternary interactions between the Pacific andNorth American plates.NORTHERN CORDILLERAN VOLCANICPROVINCEThe northern Cordilleran volcanic provinceencompasses a broad area of Neogene to Qua-ternary volcanism approximately 1200 km longand 400 km wide in the northern part of west-ern Canada (Fig. 1). The majority of the vol-canoes within this region formed east of theDenali fault system and west of the Tintinatrench (Fig. 1). Volcanic expressions of magma-tism in the province range from large basalticplateaus and stratovolcanoes to individual cin-der cones, lava flows, and eroded vents.PetrologyNeogene to Quaternary volcanic rocks withinthe northern Cordilleran volcanic province arealkaline and chemically bimodal. At least 70% ofthe volcanic rocks comprise alkali olivine basaltand hawaiite (Fig. 2A). Basaltic lavas are gener-ally olivine and plagioclase phyric with plagio-clase, olivine, titanaugite, and magnetite in thegroundmass.The province also hosts a variety of other alka-line and peralkaline rock types that are uncom-mon in the North American Cordillera, such asnephelinite, basanite, phonolite, trachyte, and