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Home , Anahim Volcanic Belt, Cheslatta Lake, Porphyritic, Queen Charlotte Basin, Stock (geology)

ISOTOPIC AND GEOCHEMICAL EVIDENCE FOR A RECENT TRANSITION IN MANTLE CHEMISTRY BENEATH THE WESTERN CANADIAN CORDILLERA

by

Christian D. Manthei

A Prepublication Manuscript Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements for the Degree of

MASTER OF SCIENCE

In the Graduate College THE UNIVERSITY OF ARIZONA 2009

1 Abstract

New petrologic, geochemical and isotopic data are reported from a suite of and flow samples collected from sites within the western Canadian Cordillera.

Samples range in age from Eocene to , and document a significant transition in mantle chemistry that occurred sometime after 10 Ma. Eocene to late emplaced as dikes within the contain abundant hornblende, are enriched in large ion lithophile elements (LILE; Ba, Rb, K), have negative high field strength element (HFSE; Nb, Ta) anomalies, and were likely derived from lithospheric

87 86 mantle ( Sr/ Sr = 0.70353 – 0.70486; εNd = +2.5 - +5.7). By contrast, Quaternary lava flows have lower LILE concentrations, positive Nb-Ta anomalies, and were likely

87 86 generated by upwelling asthenosphere ( Sr/ Sr = 0.70266 – 0.70386; εNd = +7.4 - +8.8).

A regional comparison of numerous mafic rocks from western Canada that are also

Eocene to Quaternary in age indicates that the transition in mantle chemistry after 10 Ma was pervasive and widespread, and was not limited to the present study area. This transition occurred c.a. 40 Ma after the cessation of Cordilleran arc magmatism in central

British Columbia, suggesting that large-scale transitions in mantle chemistry beneath magmatic arcs may occur on the order of tens of millions of years after the final subduction of oceanic lithosphere, in this case as a result of lithospheric thinning by continental extension.

Key Words: Coast Mountains, mantle chemistry, radiogenic isotopes, continental mafic magmatism, continental extension

2 1. INTRODUCTION

Continental basaltic magmatism provides a unique opportunity to study the geochemical nature and protracted evolution of the subcontinental mantle [e.g., Allègre et al., 1981; Hawkesworth et al., 1993; Carlson et al., 2005]. While the preserved volcanic cover and long-term intrusive construction of plutons and document the magmatic history of arcs, post-subduction mafic magmatism is one of the few tools available to geochemists to study the mantle during and after the cessation of arc- magmatism. Major, trace element and isotope compositions of mafic provide information on the affinity of melt sources (i.e. lithosphere vs. asthenosphere), metasomatic processes, and pathways of melt migration into the [e.g., Pearce and

Cann, 1973; Winchester and Floyd, 1977; Hawkesworth et al., 1990; Schmidt and Poli,

2005]. Augmentation of the ’s chemistry by crustal contamination during ascent and/or emplacement must be identified and ruled out, if the magmas are to be representative of a primary mantle source [Hawkesworth et al., 1990; McDonough and

Sun, 1995]. Identifying potential contamination may be difficult in some cases, because the composition of assimilated mafic lower crust [Rudnick and Fountain, 1995] may resemble the lithospheric mantle [Farmer, 2005].

Continental basalts are typically generated during crustal extension (e.g., Basin and Range Province, East Africa), hot spot magmatism (e.g., Snake River Plain), and wet-melting associated with a magmatic arc (e.g., Cascadia, Andes). Basaltic rocks from these regimes can be used to resolve the chemical nature of the subcontinental mantle within a time-integrated framework [Farmer et al., 2002; Cousens and Bevier, 1995].

Evaluating the geochemical nature of the subcontinental mantle is important for our

3 understanding of the evolution of magmatic arcs and orogenic belts [e.g., Kay et al.,

1994; Wernicke et al., 1996].

In this study, major, trace element and isotopic compositions of mafic dike and lava flows from western (52.3° – 55.3°N; Fig. 1), accompanied by newly acquired 40Ar/39Ar geochronology, provide a time-integrated geochemical history of the mantle since the demise of the Cordilleran arc (~48 Ma). Data from this study are compared to regional basalts from British Columbia and the Yukon Territory, also ranging in age from Eocene to Quaternary, to demonstrate that two geochemically distinct mantle-sources produced spatially widespread basaltic magmatism in western

Canada. A regional tectonic synthesis indicates that the onset of dextral transform motion along the Queen Charlotte system, and intermittent periods of highly oblique divergence and convergence between the North American and Pacific plates in the early

Eocene [Engebretson et al., 1985; Stock and Molnar, 1988], drove continental extension in western Canada. This period of extension has been documented by the opening of large, normal fault-bounded basins [Rohr and Dietrich, 1992; Rohr and Currie, 1997;

Dostal et al., 2001] and widespread mafic magmatism since the Eocene.

1.2 Cordilleran Tectonic Framework

Canada’s western Cordillera is composed of several allochthonous and discrete crustal fragments that were outboard of the paleo-Laurentian continent and accreted to the margin from to Eocene time [Coney et al., 1980; Monger et al.,

1982; Crawford et al., 1987, 1999; Colpron et al., 2007; Gehrels et al., 2009]. Figure 1 shows the location of the Stikine , Coast Mountains Batholith, the metamorphic

4 pendants and terranes of the Central Gneiss Complex, the Gravina belt and the amalgamated Alexander-Wrangellia terrane, respectively. Readers are referred to Coney et al., [1980], Monger et al., [1982], Colpron et al., [2007], and Gehrels et al. [2009] for comprehensive reviews of the Cordilleran terranes and their interactions through time.

1.3 Jurassic – Eocene Magmatic Arc

Cordilleran arc magmatism was active in western Canada from to early Tertiary time [Armstrong, 1988; van der Heyden, 1992], and occurred coevally with terrane accretion and greenschist to amphibolite facies [Monger et al.,

1982; Hollister, 1982; Gehrels and Saleeby, 1987; Crawford and Crawford, 1991;

Stowell and Crawford, 2000]. The plutons of the Coast Mountains Batholith (CMB) were assembled over this ~120 Ma period, and presently represent the largest exposed batholithic complex in [Barker and Arth, 1984]. The batholith, occasionally referred to as the Coast Plutonic Complex, can be traced from the northwestern United States, through British Columbia, the Yukon Territory and into southeastern Alaska, comprising a total distance of ~1,700 km [Roddick and Hutchinson,

1974; Gehrels et al., 2009].

The igneous rocks of the CMB range in composition from gabbroic through leucogranite, although the majority of plutons are or [Gehrels et al.,

2009]. They are dominantly mantle-derived [Samson et al., 1989; Cui and Russell, 1995], but may contain ~10 – 50% recycled crustal material [Samson et al., 1991]. Periods of high magmatic flux (35 – 50 km3/my per km) at 160-140 Ma, 120-78 Ma and 55-48 Ma

[Gehrels et al., 2009] correspond to periods of expedited batholith construction, and are

5 documented by numerous U-Pb crystallization ages of zircon from CMB plutons

[Crawford et al., 1999; Gehrels et al., 2009]. The lack of plutonic zircon ages that post- date the early – middle Eocene (~48 Ma), is consistent with field, stratigraphic and geochronological data that indicate Cordilleran arc magmatism along the Canadian margin ceased during this time [Armstrong, 1988; van der Heyden, 1992].

In southern British Columbia, the Garibaldi and Pemberton belts (Fig. 1) form a discrete chain of magmatic edifices that are the northernmost extension of the active

Cascade arc in the northwestern United States. The Garibaldi belt was active until very recently [c.a. 0.1 Ma; Armstrong, 1988; Coish et al., 1998], likely as a result of the ongoing subduction of the [Coish et al., 1998].

1.4 Eocene – Quaternary Mafic Magmatism

Numerous studies have focused on Eocene – Quaternary basalts (and similar mafic rocks) that are widely distributed throughout the western Canadian Cordillera.

Basalts are preserved as large -forming lava flows [e.g., 3,300 km3 Chilcotin

Group Basalts; Bevier, 1983], small-volume cinder-cone and fissure eruptions [e.g.,

Carignan et al., 1994; Francis and Ludden, 1990], and dikes that intrude the igneous rocks of the CMB and metamorphic rocks of the Cordilleran terranes [Irving et al., 1992;

Rohr and Currie, 1997; Crawford et al., 2005; Rusmore et al., 2005]. Increased mafic magmatism beginning at ~48 Ma corresponds with the initial stages of transform motion along the Queen Charlotte Fairweather fault system (Fig. 1), and intermittent periods of highly oblique divergence and convergence between the North American and Pacific plates [Engebretson et al., 1985; Stock and Molnar, 1988; Irving et al., 1992]. The

6 opening and subsidence of large extensional basins during the Miocene [Rohr and

Dietrich, 1992; Rohr and Currie, 1997] occurred concurrently with shear-distribution in the [Rohr and Currie, 1997], increased mantle heat-flow [Edwards and Russell, 2000] and the emplacement of mafic dike swarms that trend perpendicularly to the direction of extension [Irving et al., 1992].

The delineation of separate volcanic fields in the western Cordillera has been approximated in Figure 1, after the work of several authors. Major volcanic fields include the Northern Cordilleran Volcanic Province [Edwards and Russell, 1999, 2000], the Anahim [Bevier, 1979, 1989] and the back-arc basalts of the Chilcotin

Group [Bevier, 1983] and Challis-Kamloops Belt [e.g., Dostal et al., 2001]. Volcanic belts and formations that are smaller in volume, or are dominantly composed of more evolved compositions (i.e. basaltic ) are also shown in Figure 1. A goal of this work will be to compare the geochemistry of these regional basalts to newly acquired geochemical data from the Coast Mountains, in an attempt to characterize the regional mantle chemistry beneath western Canada since the Eocene.

2. METHODS AND TECHNIQUES

Major, trace and isotope geochemical analyses, along with 40Ar/39Ar geochronology, were performed on nineteen mafic dike and samples collected from sites within, and in the vicinity of, the Coast Mountains Batholith (52.3° –

55.3°N, Fig. 1) in an attempt to characterize the geochemical nature of the underlying mantle.

7 2.1 40Ar/39Ar Geochronology

Determination of 40Ar/39Ar ages was performed on whole rock samples, except for GJP-25, which was dated using hornblende separates. Field observations indicated that young basaltic lava flows are post-glacial in age, and are referred to as the

Quaternary basalts. Samples were irradiated at the USGS TRIGA Reactor in Denver,

Colorado along with GA1550 biotite standard (98.79 ± 0.54 Ma) [Renne et al., 1998], flux monitors to calculate J-factors, and K2SO4 and CaF2 salts to calculate correction factors for interfering neutron reactions. Ages of samples GJP-15, GJP-25, GJP-80, GJP-

81 and GJP-87 were determined at the New Mexico Geochronology Research Laboratory using Mass Spec software [Deino, 2001], and remaining ages were determined at the

University of Arizona. A detailed description of the 40Ar/39Ar procedure that was used can be found in Appendix 1.

2.2 Major and Trace Elements

Whole-rock samples were powdered in an Al2O3-lined container. Major element analysis by XRF (x-ray fluorescence) was performed at Activation Laboratories in

Ancaster, Ontario. The samples were fused in Pt crucibles following Norrish and Hutton

[1969] and analyzed on a Philips PW-2400 sequential XRF analyzer with Philips Super Q software. The G-16 standard was used to calibrate results, and the detection limit has been reported at ~0.01 wt. %.

Trace element analyses were performed on powdered whole-rock samples at the

University of Saskatchewan. Samples were digested in a HF/HNO3 solution for three

8 days and combined with a Na2O2 sinter. ICP-MS analytical procedures for trace elements followed Jenner et al. [1990] and Longerich et al. [1990].

2.3 Radiogenic Isotopes

Bulk rock powders were fully digested in HF/HNO3 (4:1) and combined with mixed Rb-Sr and Sm-Nd spike solutions. Standard cation exchange column techniques were used to separate Rb-Sr and the bulk of REE with AG50W cation resin, followed by second-stage column purification in HDEHP coated Teflon beads to separate Nd and Sm

[Patchett and Ruiz, 1987]. Sm-Nd and Rb-Sr isotope ratios were measured by isotope dilution thermal ionization mass spectrometry on VG 354 and Sector 54 mass spectrometers at the University of Arizona. Strontium and neodymium ratio measurements were calibrated using NBS-987 and La Jolla [Lugmair and Carlson, 1978] standards, and results can be found in Table 4. Analyses were normalized to 86Sr/88Sr =

0.1194 and 146Nd/144Nd = 0.7219. New CHUR values from unequilibrated chondrites

143 144 ( Nd/ NdMEAN = 0.512630) provided by Bouvier et al. [2008], yielded εNd values that

143 144 deviated from values calculated using Nd/ NdCHUR = 0.512638 by <0.2, and the latter value was used in all εNd calculations. Sample/blank ratios were >>1,000 for all samples.

3. RESULTS

3.1 40Ar/39Ar Geochronology

Step-heating plots (see Appendix 1) of cumulative 39Ar released from the Coast

Mountains basalts yielded plateau and step-integrated ages (Table 1, Appendix 1) that range from Eocene to late Miocene in age, and indicate that magmatism was sporadic but

9 essentially continuous during this time. Two magmatic gaps at 31.9 to 20 Ma and 10.3

Ma to recent Quaternary correspond to periods in which there are no mafic samples that were collected. The youngest Quaternary sample erupted c.a. 250 years B.P. The

Eocene – Miocene samples are dikes that intrude the plutonic and metamorphic rocks of the accreted terranes, and the Quaternary basalts are lava flows that were deposited in post-glacial valleys (Fig. 1). No correlation between sample location and age is observed: the Quaternary basalts were collected from sites that were in the vicinity of some of the older dikes at the south and north of the study area (Fig. 1).

3.2 Mineralogy/Petrography

Phenocryst mineralogy of the Eocene to Miocene (45 – 10 Ma) basalts is dominated by euhedral/subhedral hornblende and plagioclase, with minor amounts of orthopyroxene, clinopyroxene and biotite (Figs. 2A-D). Eocene dikes (samples GJP-25,

GJP-15; Figs. 2A-B) contain abundant hornblende that are either containing inclusions of subordinate minerals (Fig. 2A), or are elongated laths surrounded by interstitial plagioclase (Fig. 2B). Hornblende and biotite are less prevalent in the

Miocene basalts (e.g., samples GJP-53, GJP-80; Figs. 3C-D), but phenocrysts are still sometimes relatively large (~2 mm; Fig 2D). All Eocene to Miocene samples contain at least trace (≥%5) amounts of hornblende and/or biotite.

A marked distinction in mineralogy and crystal habit is noticed between the

Eocene to Miocene basalts and the Quaternary basalts (Figs. 2E-F). These youngest basalts consist mainly of seriate- euhedral plagioclase laths, and contain minor

(~5-10%) subhedral olivine, ortho- and clinopyroxene, and some samples contain minor

10 (≤2%) nepheline. Olivine phenocrysts are occasionally large in these samples (Fig. 2F).

No hydrous mineral phases are observed in the Quaternary basalts.

3.3 Major Elements

All Coast Mountains samples are characterized by their major element chemistry as tephrites, , basalts, and on a SiO2-alkalis diagram (Table 2, Fig.

3A; Le Bas et al., 1986). Two outlying samples, MT05-133 and GJP-30, plot within the foidite (Na2O + K2O = 9.8%) and basaltic trachyandesite (Na2O + K2O = 5.3%) classifications, respectively. These samples contain anomalous SiO2 (MT05-133 = 42%

SiO2; GJP-30 = 52% SiO2) when compared to the other basaltic samples in this data set

(45 - 50% SiO2). The majority of samples are alkaline, although some samples plot close to, and one plots below, the alkaline/tholeiitic dividing line on an AFM diagram (Fig. 3B)

[Irvine and Baragar, 1971]. Three samples have abnormally elevated LOI (loss on ignition) percentages (Table 2; 5.7 - 7.8%) compared to the other samples (0.76 – 3.6%).

All samples range in Mg# from 50 to 78, which is positively correlated with Ni and Cr concentration, but not with sample age. The Quaternary basalts are dominantly nepheline-normative (1-2% nepheline), but samples MT05-107, MT05-108 and MT05-

110 contain minor hypersthene.

3.4 Trace and Rare Earth Elements

Trace element concentrations (Table 3) are normalized to chondritic values of

McDonough and Sun [1995] and are shown in Figure 4. Differences in high field strength element (i.e., Nb, Ta) behavior are observed between the Eocene to Miocene

11 basalts and the Quaternary basalts. Eocene - Miocene basalts (Fig. 4A) have a negative

Nb-Ta anomaly, contrasting with slightly positive Nb-Ta anomalies observed in the

Quaternary basalts (Fig. 4B). An anomalous sample, GJP-30, is Miocene in age (10 Ma) and has a slightly positive Nb-Ta anomaly.

The Eocene – Miocene basalts have elevated concentrations of large ion lithophile elements (LILE; Table 3, Figure 4), which are concentrated in slab-derived material [e.g.,

Hawkesworth et al., 1993], and are mobilized during mantle melting. Some Eocene – late Miocene samples (e.g., MT05-133, MT04-143) are significant enriched in these elements (Fig. 4). A subset of Eocene – Quaternary samples (GJP-15, MT05-154,

MT05-143) are enriched in Ni (73 – 238 ppm), Cr (250 – 455 ppm) and have elevated

Mg-values (68-78).

Chondrite-normalized [McDonough and Sun, 1995] rare earth element (REE) patterns are shown in Figures 4C-D. REE patterns of basalts within each time period are broadly similar, and only minor variation is observed. (La/Yb)N of samples MT05-133 and MT05-143 is 51, where all other samples range from (La/Yb)N = 4 – 17. Sample

MT05-156 has a small, positive Eu anomaly (Eu/Eu* = 1.3).

3.5 Radiogenic Isotopes

Radiogenic isotope data are presented in Table 4 and Figure 5. Eocene to

Miocene basalts define ranges of 87Sr/86Sr = 0.70353 – 0.70486, 143Nd/144Nd = 0.512736

– 0.512922 and εNd = +2.5 - +5.7, that are distinctly different from the isotopic compositions of the Quaternary samples. These youngest basalts demonstrate lower

87 86 Sr/ Sr (0.70266 – 0.70386) and higher εNd (+7.4 - +8.8). Sample MT05-154 was

12 collected from a flow that had a few entrained crustal xenoliths, and has a higher 87Sr/86Sr ratio (0.70386) than other Quaternary samples (0.70266 – 70309) as a result. Although minor crustal interaction may have occurred, low SiO2 content (46%), elevated Mg# (72) and concentrations of Ni (170 ppm) and Cr (250 ppm) seem to preclude extensive fractionation and assimilation at crustal levels.

4. DISCUSSION

4.1 Transition in Mantle Chemistry

The mafic samples that were emplaced as dikes between 45-10 Ma in the Coast

Mountains Batholith are distinguished from the Quaternary lava flows on the basis of mineralogy, whole-rock geochemistry, and isotopic composition. Basalts from these two periods have similar major element compositions, consistent with the conclusion that only minor crustal assimilation may have taken place (MT05-154, MT05-156), but they have contrasting trace element and isotopic composition. 40Ar/39Ar geochronology has provided a time-integrated framework within which the evolution of the mantle beneath the Canadian Cordillera can be examined. Data from this study indicate two time periods for which distinctly different geochemical and isotopic signatures of the subcontinental mantle are observed.

4.1.1 Eocene – Miocene Basalts

The presence of amphibole and biotite in the Eocene – Miocene samples, as well as elevated concentrations of strontium, barium and LREE, indicate that they were likely generated in a hydrated lithospheric mantle that previously underwent metasomatism by

13 slab-derived fluids [e.g., Hawkesworth et al., 1990, 1993; Francis and Ludden, 1995].

Anomalously high LOI values (e.g., MT05-133 = 7.81%) are likely related to post- emplacement alteration, but Eocene – Miocene samples all demonstrate elevated LOI values compared to the Quaternary samples, which is consistent with presence of amphibole and biotite. Some Eocene - Miocene samples also appear to contain accumulated amphibole xenocrysts, and were possibly injected with subduction-related fluids prior to crystallization.

Eocene to Miocene basalts preserve a pronounced negative Nb-Ta anomaly, a fingerprint that is typical of arc rocks [e.g., Morris and Hart, 1983]. It is possible that samples GJP-25 and GJP-30 have slightly positive Nb-Ta anomalies due to the inclusion of residual amphibole which carried elevated concentrations of the high field strength elements [e.g., Coltorti et al., 2007]. Our trace element data require the presence of an

Nb and Ta-depleted (i.e. arc) mantle-source beneath western Canada until at least the production of the last Miocene basalt at 10 Ma.

In order to characterize the source of the Eocene – Miocene basalts as arc-related mantle, it is appropriate to compare them to the mafic plutonic end-members of the Coast

Mountains Batholith. Girardi et al. [2008] have presented new major, trace and isotopic data of several plutonic samples from the present study area, ranging in age from early

Jurassic to early Eocene, and from mafic (45 wt.% SiO2) to (77 wt.% SiO2) composition. A subset of granodiorite, and gabbroic plutons have εNd values ranging from +5.7 to +6.7, and represent the most primitive mantle-derived compositions from the Coast Mountains Batholith presented in that study. Other major, trace and isotope data collected from various CMB plutons [Cui and Russell, 1995] are

14 consistent with the conclusion that the Eocene – Miocene Coast Mountains basalts resemble the mafic end-members of the Coast Mountains Batholith, and that they were generated in a common arc-related mantle lithosphere.

4.1.2 Quaternary Basalts

During the past 10 Ma, the Cordilleran arc signature observed in the Eocene -

Miocene basalts was lost, and basalts that erupted during the Quaternary were generated in a different mantle source. These basalts have positive Nb-Ta anomalies, are isotopically primitive (depleted in 87Sr, enriched in 143Nd relative to chondrite), and are more primitive than the most mafic plutonic end-members from the study area [Girardi et al., 2008]. Minor contamination with crustal 87Sr may have occurred (e.g., MT05-154) but does not appear to be extensive [e.g., Russell and Hauskdóttir, 2000]. Samples with elevated Mg# and concentrations of Ni and Cr range in age from 45 Ma – 0 Ma, and show no correlation with age, and variations are likely related to the extent of fractionation that occurred prior to crystallization.

4.2 Regional Mafic Magmatism

A comparison between mafic samples presented in this study and regional volcanic suites from western British Columbia and the Yukon Territory indicates that the post 10-Ma transition in mantle chemistry is not unique to our study area. Regional volcanic fields are generally basaltic, but show large variation in compositional range due to bimodal dominantly located in northwestern British Columbia, the Yukon

Territory [Carignan et al., 1994; Cousens and Bevier, 1995; Edwards and Russell, 1999;

15 Dostal et al., 2001], and in the Garibaldi-Pemberton belt of southern British Columbia

[Green and Henderson, 1984; Coish et al., 1998].

Averaged trace element and REE patterns of 240 Tertiary mafic samples, plotted by region, are shown in Figure 6. Although specific ages of regional samples are sometimes not available, the authors who originally presented these data have determined the appropriate epochs that the samples belong to, usually by stratigraphic relationships.

A sub-set of Eocene to Pliocene samples that are included in Figure 6, however, have been dated (e.g., the basalts) [Bevier 1983].

Located to the north of our study area, in British Columbia, the Yukon and southeastern Alaska, numerous volcanic complexes ranging in age from 20 Ma – c.a. 200 yr B.P., have been collectively referred to as the Northern Cordillera Volcanic Province

(NCVP) by Edwards and Russell [1999, 2000] (Fig. 1), and have previously been referred to as the Stikine Volcanic Belt [e.g., Souther and Yorath, 1991]. A comparison of trace element abundances of major NCVP volcanic fields is shown in Figure 6. Mt.

Skukum and Bennett Lake are dominantly andesitic in composition, but they are interpreted as mantle-derived [Morris and Creaser, 2003]. Eocene – Miocene NCVP basalts demonstrate similar LILE enrichments and Nb-Ta depletions as the Eocene – late

Miocene basalts from the Coast Mountains (this study; Fig. 4), and post-10 Ma NCVP basalts have positive Nb-Ta anomalies and lower LILE concentrations.

A similar correlation is observed between the Eocene Challis-Kamloops belt,

Miocene Mt. Noel complex, and the Eocene – Miocene Coast Mountains basalts. Minor variation in REE patterns of regional samples (Fig. 6C-D) indicate broadly similar REE

16 compositions of source regions, and the lack of significant HREE/LREE fractionation precludes garnet as a residual phase.

Isotopic data that are available from Tertiary volcanic centers in western British

Columbia and the Yukon provide additional support for a transition in mantle chemistry towards primitive isotopic composition, occurring after 10 Ma. Figure 7 shows 87Sr/86Sr and 143Nd/144Nd compositions of regional basalts, and 87Sr/86Sr correlated to sample age, where ages are available. 87Sr/86Sr ranges from 0.70267 – 0.70657 and 143Nd/144Nd from

0.512288 – 0.513183. Quaternary basalts from this study are among the most primitive

(depleted in 87Sr, enriched in 143Nd) in the region. Challis-Kamloops samples [Dostal et al., 2001], and the outlying NCVP sample (Fig. 7A) were likely contaminated by lower crustal material.

Located at the southern terminus of our study area, the

(AVB) forms a 500-km-long linear chain of mafic edifices that is more or less perpendicular to the north-south striking accreted terranes of the western Cordillera

[Bevier, 1989]. Isotopic studies of the AVB and associated complexes have been presented by Bevier [1989], and a subset of these samples has been determined to be middle – late Miocene in age (Fig. 7). Two samples dated at 6.6 and 6.7 Ma, have initial

87Sr/86Sr of 0.70317 and 0.70320, respectively. This range is not as depleted in 87Sr as the majority of the Quaternary samples presented here (0.70266 – 0.70309), so this may suggest that the Quaternary Coast Mountains magmas were generated in a source that was still not in place by ~7 Ma, and that this transition was potentially quite recent.

17 4.3 Melting-Depth Constraints from Mantle Xenoliths

Numerous occurrences of mantle xenoliths exposed in the mafic rocks of western

Canada have been observed [Cousens and Bevier, 1995; Anderson et al., 2001; Harder and Russell, 2007], and described chemically [e.g., Francis, 1987; Carignan et al., 1996;

Shi et al., 1998; Russell and Hauskdóttir, 2000; Peslier et al., 2000, 2002; Harder and

Russell, 2006]. Xenoliths are dominantly spinel lherzolite in composition [Francis, 1987;

Carignan et al., 1996; Peslier et al., 2000], with subordinate occurrences of spinel harzburgite [Francis, 1987; Peslier et al., 2000; Harder and Russell, 2006], and rare dunite [Peslier et al., 2000].

Cordilleran xenoliths have been used by many workers in an attempt to resolve the composition [e.g., Francis, 1987; Peslier et al., 2000] and thermal structure [Shi et al., 1998] of the Cordilleran mantle lithosphere. Analytical and model results are generally consistent with one another, and indicate that Cordilleran xenoliths have originated from a variable depth range within the spinel stability field [20 – 80 km; Shi et al., 1998], and that some were generated in a region that was ~200°C hotter than the surrounding mantle [Shi et al., 1998]. This anomalously hot mantle likely extends from

400-500 km in depth, and appears to be composed of mobilized asthenospheric mantle

[Shi et al., 1998]. These results are consistent with those of Peslier et al. [2002], who determined that the Cordilleran lithosphere is composed of Cr-diopside bearing spinel lherzolite, which represents residues of 5-10% melting and experienced intermittent metasomatism by subduction related fluids.

The formation of residual harzburgite is consistent with the thermodynamic calculations of Francis [1987], and the occurrences of spinel harzburgites in regional

18 Cordilleran basalts [e.g., Francis, 1987; Carignan et al., 1996; Shi et al., 1998; Peslier et al., 2000, 2002; Harder and Russell, 2006]. Fractionation of LREE and HREE in the

Coast Mountains basalts (Fig. 5) is not indicative of generation in a garnet-bearing source, and the elevated LREE and incompatible element concentrations of the Eocene –

Miocene dikes likely reflect fluxes of slab-derived material [e.g., Hawkesworth et al.,

1990, 1993; Pearce and Peate, 1995]. Moreover, the thickness of the Cordilleran crust since the late Eocene has been estimated to be ~34 km [Crawford and Crawford, 2005;

Hollister and Andronicos, 2006], implying that the subcrustal mantle was within the spinel lherzolite stability field. The following discussion provides an overview of

Cordilleran since the Eocene (Fig. 8), and how these processes may have facilitated the thinning of the Cordilleran lithosphere, and the introduction of the anomalously hot mantle described by Shi et al. [1998].

4.4 Cordilleran Tectonics and the Transition in Mantle Chemistry

Widespread regional magmatism in western Canada has been studied extensively in an attempt to elucidate the tectonic mechanisms that contributed to ~40 Ma of nearly continuous mafic magmatism after the demise of the Cordilleran arc [Coish et al., 1998].

Continental basalts are typically generated in extensional environments (e.g., Basin and

Range Province, East African ), hot spots (e.g., Snake River Plain) or by wet melting associated with a magmatic arc (e.g., Andean, Cascades arcs), and basalts generated in each of these tectonic regimes have been identified in western Canada [e.g., Edwards and

Russell, 1999, 2000]. In the discussion that follows, regional volcanic fields are divided into the tectonic environments that they have been associated with in the literature.

19 Regardless of the tectonic regime responsible for their eruption, the basalts preserve a signature of the magmatic arc until ~10 Ma, at which point the Cordilleran mantle lithosphere was critically thinned, and the asthenospheric mantle was introduced.

4.4.1 Back-arc extension

At the time of the demise of the Kula-Farallon slab along the central to northern

Canadian margin (~48 Ma), subduction of the Juan de Fuca plate continued to the south, producing the magmas of the (3.8 – 0.1 Ma) and Pemberton

Volcanic Belt (29 – 6.8 Ma) through Miocene time [Coish et al., 1998]. Mafic eruptions associated with the Garibaldi and Pemberton belts include the Mount Noel Volcanic

Complex (MNVC) [Coish et al., 1998] and the volumetrically significant Chilcotin

Group basalts (3,300 km3) [Bevier, 1983]. Both groups of basalts have been attributed to an upwelling plume as a result of back-arc extension that was coeval with arc-magmatism

[Bevier, 1983; Coish et al., 1998]. The Garibaldi/Pemberton belt itself is not considered, because ongoing subduction of the Juan de Fuca plate produced andesitic magmas [e.g.,

Green and Henderson, 1984) until ~0.1 Ma [Coish et al., 1998]. The Challis-Kamloops volcanic belt is located immediately to the east of our study area in the Stikine Terrane, and also underwent extension, basin development (e.g., Buck Creek basin), and coeval mafic magmatism [Dostal et al., 2001].

Although these groups were generated in an extensional regime in the back-arc region, trace element patterns are distinguishable by age (Fig. 6). The MNCV basalts are

~19.6 Ma [Coish et al., 1998], and show significant LILE enrichment and Nb-Ta depletion. The Chilcotin basalts were intermittently erupted during late –

20 Quaternary time, with dominant activity occurring after 10 Ma [Anderson et al., 2001;

Coish et al., 1998]. Challis-Kamloops rocks are Eocene in age and display similar trace and REE patters to other samples that erupted during this period.

4.4.2 Tertiary slab-windows

Since the Eocene, the opening and subsequent migration of one or more

Cordilleran slab-windows has been important in the tectonic evolution of the western

Cordillera [Thorkelson and Taylor, 1989]. These widening “gaps” in subducting lithosphere allow underlying mantle material to buoyantly rise and melt by decompression [Hole et al., 1991], and can promote extension in the overlying crust

[Edwards and Russell, 1999, 2000]. subduction (Kula plate; Fig. 8A) may have also caused asthenospheric upwelling along the Canadian margin. Comprehensive reviews of slab-window evolution and associated magmatism in the western Cordillera have been reported by Thorkelson and Taylor [1989], Breitsprecher et al. [2003],

Madsen et al. [2006] and Edwards and Russell [1999, 2000], but will not be discussed at length here. Data from volcanic suites (Figs. 6 and 7) that have been attributed to the passage of a slab-window include, Challis-Kamloops [Hole et al., 1991; Breitsprecher et al., 2003; Madsen et al., 2006], and the forearc volcanics of the Miocene Masset formation [Madsen et al., 2006; Hamilton and Dostal, 2001]. These basalts have trace element and isotopic compositions consistent with generation in a LILE enriched, and

HFSE depleted mantle (Fig. 6).

21

4.4.3 Hot Spot Magmatism

The eruption of isotopically primitive magmas throughout the Anahim Volcanic

Belt during the latest Miocene [Bevier, 1989] provides additional evidence that there was pervasive upwelling mantle activity in western British Columbia from the –

Quaternary. Bevier [1989] used Sr and Pb isotopes to conclude that the AVB is a hot- spot track of primitive, formerly sub-oceanic mantle. Late Miocene samples from her study are some of the most depleted with respect to 87Sr observed in the western

Cordillera, but are still not as depleted as some of the Quaternary basalts presented here

(Fig. 7B). The AVB data suggest that the primitive source of the Quaternary Coast

Mountains basalts was not in place by ~7 Ma.

4.4.4 Delamination and Delamination Magmatism

The foundering of large blocks of cold mantle lithosphere or residual arc material has been shown to be a necessary process in the evolution of Cordilleran-style magmatic arcs [Kay and Kay, 1993; Kay et al., 1994; Ducea and Saleeby, 1996]. Bulk composition calculations [Ducea, 2002] indicate that the formation of a batholith with average tonalitic composition requires an ultramafic, eclogitic counterpart that is likely delaminated into the mantle, driving surface uplift and sub-crustal heat flow [Ducea and

Saleeby, 1996; Schott and Schmeling, 1998]. Recent work by Girardi et al. [2008] has used variations in (La/Yb)N of Coast Mountains plutonic rocks to document at least three periods of high-flux magmatism from 160-140 Ma, 120-80 Ma, and 60-50 Ma that may be linked to the rapid removal of garnet-rich arc residues beneath the Coast Mountains.

22 This process has been shown elsewhere to induce the eruption of mafic magmas as a result of decompression of the asthenospheric mantle [Kay et al., 1994].

Although strong evidence has been presented by Girardi et al. [2008], we are currently unable to suggest that the Eocene - Quaternary basalts were generated as a result of delamination. Models that predict rapid lithospheric response to delamination suggest root removal rates of 25-40 km3/km Myr [Ducea, 2002], which is consistent with field and geochemical studies of Cordilleran batholiths [i.e. Sierra Nevada; Ducea and

Saleeby, 1996]. The Coast Mountains basalts were periodically produced since Eocene time, implying that delamination occurred much more slowly than the rate mentioned above, or that it was achieved in a piecemeal fashion. The latter scenario may be valid, and “drip” models involving the removal of small blobs of eclogitic material have been successfully applied to the Andean arc [Beck and Zandt, 2002]. Small-scale delamination may have generated the Coast Mountains basalts, but any unique geochemical signature produced by this processes (i.e. asthenospheric upwelling) is likely masked by crustal extensional and extension-related magmatism that has been documented in the western

Cordillera since the Eocene.

4.4.5 Cordilleran Extension

Oblique subduction of the Kula/Farallon plate [Engebretson et al., 1985; Stock and Molnar, 1988; Crawford et al., 1999], along with thermal weakening of the crust

[Dostal et al., 2001], likely contributed to the collapse of the Cordilleran orogenic belt, and the development of numerous extensional grabens in central and southern British

Columbia [e.g., Dostal et al., 2001] during the Eocene, continuing through the Miocene.

23 Basin development occurred coevally with alkaline basaltic magmatism near the margin of the North American plate [Rohr and Currie, 1997; Dostal et al., 2001; Edwards and

Russell, 1999, 2000], and the emplacement of N-S trending mafic dike swarms [Irving et al., 1992].

Edwards and Russell [1999, 2000] argue that the volcanic fields of the NCVP are attributable to rifting along the continental margin of North America, and periods of net extension within the North American plate correlate with the most voluminous periods of magmatism in the NCVP. Trace element data of Neogene NCVP samples indicate that magmas were likely generated in an LILE enriched, and Nb-Ta depleted source (Fig.

6A), whereas post-10 Ma NCVP magmas resemble Coast Mountains Quaternary basalts, and likely tapped a common asthenospheric source.

4.5 Regional Tectonics and Mafic Magmatism

Eocene through Quaternary mafic volcanism in western Canada can be explained by upwelling material as a result of significant crustal extension, and hot spot activity that has periodically occurred since the cessation of arc magmatism (Fig. 8). Distributed shear along the Queen Charlotte fault system promoted extensional basin development and coeval mafic magmatism along the plate margin. The Pemberton and Garibaldi belts were likely active during times of highly oblique transform motion, or rejuvenated convergence, along the Queen Charlotte fault, driving back-arc extension. Slab window migration and hot-spot weakening of the crust also facilitated extension, and the emplacement of mafic magmas near the plate margin.

24 Regional basalts that are older than the late Miocene clearly record the presence of an enriched mantle source, most likely the mantle lithosphere associated with the

Cordilleran arc. In contrast, young regional basalts fingerprint a previously unseen, primitive source that resembles sub-oceanic mantle [Bevier, 1989]. The transition in mantle chemistry beneath the western Cordillera occurred ~40 Ma after the demise of the

Jurassic-Eocene Cordilleran arc, indicating that the geochemistry of continental basaltic magmas produced along paleo-subduction margins may fingerprint an arc-source, even if significant time has elapsed since the final subduction of the oceanic slab.

5. CONCLUSIONS

A suite of nineteen mafic dike and lava flow samples, referred to as the Coast

Mountains basalts, were collected from within, and in the vicinity of, the Coast

Mountains Batholith (52.3°-55.3°N), in an attempt to resolve the chemical nature of the underlying mantle. Major, trace element and isotopic data, combined with newly acquired 40Ar/39Ar of the mafic samples has permitted the following conclusions:

1. The basalts range in age from Eocene to the recent Quaternary, with dominant

mafic activity in the Coast Mountains occurring during two time periods: from

Eocene to late Miocene time (45 – 10 Ma), and recent Quaternary time.

2. The Eocene – Miocene basalts contain abundant hornblende, are enriched in large

ion lithophile elements, have negative Nb-Ta anomalies, and were likely derived

87 86 from lithospheric mantle ( Sr/ Sr = 0.70353 – 0.70486; εNd = +2.5 - +5.7). By

25 contrast, the Quaternary lava flows are anhydrous, have lower LILE

concentrations, positive Nb-Ta anomalies, and were likely generated by

87 86 upwelling asthenosphere ( Sr/ Sr = 0.70266 – 0.70386; εNd = +7.4 - +8.8).

3. This transition in mantle chemistry corresponded to the tapping of a previously

unseen, isotopically primitive asthenospheric mantle source, which has been

documented in several regional volcanic provinces that range in age from late

Miocene to recent time. Regional Eocene – Miocene basalts are depleted in Nb-

Ta, enriched in LILE, and resemble the Coast Mountains basalts (this study)

produced during this time. The presence of hot asthenospheric mantle beneath

western Canada is also consistent with studies of Cordilleran spinel-bearing

xenoliths.

4. The mafic rocks of the western Canadian Cordillera were likely generated during

times of oblique transform motion, and corresponding periods of highly oblique

convergence and divergence along the Queen Charlotte fault system, which

distributed shear along the plate margin and favored continental extension. Slab

window and hot spot migration have also facilitated mafic magmatism during the

Tertiary. Regional basalts indicate that post-subduction transitions in mantle

chemistry may occur on the order of tens of millions of years after the final

subduction of oceanic lithosphere.

26 APPENDIX 1: 40Ar/39Ar Procedure, Age Determination and Step-Heating Plots

Whole rock chips and separated hornblende (sample GJP-25) for 40Ar/39Ar analysis were irradiated at the USGS TRIGA Reactor, Denver, Colorado along with GA1550 biotite (98.79 ± 0.54 Ma) [Renne et al., 1998] flux monitors to calculate J-factors, and

K2SO4 and CaF2 salts to calculate correction factors for interfering neutron reactions.

Following a 2 to 3 week cooling period to allow for the decay of short-lived isotopes, samples were loaded into the arms of a glass storage tree above a double-vacuum, resistance-heated furnace and heated to 120° at the same time that the entire extraction line was baked for 48 hours at 220°C. Getters and furnace were independently degassed near the end of the bake-out. Samples were then dropped into the furnace and argon was extracted from each sample using a computer controlled step-heating routine. The temperature of the furnace is estimated to be accurate to ± 20°C. Each heating step had a duration of 12 minutes followed by a cool down to 500°C prior to advancing the gas into two successive gettering stages for argon purification. The argon was then admitted into a

VG 5400 mass spectrometer, where it was ionized and detected by a VG electron multiplier and digitized with a Keithley 617 Electrometer. Data collection and processing were accomplished using the computer program Mass Spec [Deino, 2001]. The decay constants used were those recommended by Steiger and Jäger [1977]. Baseline values were subtracted and the isotopic measurements then were regressed to time zero using standard linear regression techniques. Additional corrections and associated uncertainties were applied to account for blanks, machine discrimination, atmospheric contribution, and interfering isotopes produced in the reactor from Ca, K and Cl present in the samples.

Ages cited do not include uncertainties in J factors or decay constants, which generally

27 range from 0.5 to 0.8 %. Plateau ages were determined by looking for steps of cumulative 39Ar released of overlapping age, and are reported, where available. Step- integrated ages are reported in cases of non-ideal . Samples GJP-80 and GJP-81 are shown on the same step-heating plot, were collected in the vicinity of each other, and are interpreted as Miocene in age.

28

GJP-15

29

GJP-25 (Hornblende)

30

GJP-30

31

GJP-53

32

GJP-80 and GJP-81

33

GJP-87

34

GJP-88

35

MT05-133

36

MT05-143

37

MT05-148

38

MT05-151

39 Acknowledgements

This work was funded by NSF EAR-0309885 to Ducea, Gehrels, Patchett and G.

Zandt (Univ. of Arizona), and a ChevronTexaco research grant to Manthei. Early editions of this manuscript benefited from insightful comments and reviews from J.

Tepper (Univ. of Puget Sound) and A. Cartwright (Univ. of Arizona). This work is dedicated to a teacher and mentor of Manthei, J. Stewart Lowther (Univ. of Puget

Sound), who passed away in March 2008.

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52 Figure Captions

Figure 1. Locations of mafic dikes and lava flows collected for geochemical analyses.

Eocene to Miocene dikes locations are represented by the shaded polygons and

Quaternary lavas are the dark, filled polygons. Major regional volcanic provinces indicated by cross-hatched pattern are Eocene to Quaternary in age, and are after Coish et al. [1998], Edwards and Russell [2000], and Madsen et al. [2006]. AVB = Anahim

Volcanic Belt. Geologic map is modified after Gehrels et al. [2009].

Figure 2. A-D: Photomicrographs (4x; plane-polarized light) of representative Eocene to late Miocene basalts and E-F: Quaternary basalts. Figure 2F is in cross-polarized light

(XPL).

Figure 3. A: Total alkalis vs. silica diagram [Le Bas et al., 1986] of Coast Mountains basalts. B: Alkalis-Iron-Magnesium diagram [Irvine and Baragar, 1971].

Figure 4. Trace element abundances for (A) Eocene - Miocene basalts and (B)

Quaternary basalts. REE compositions of (C) Eocene – Miocene basalts; (D) Quaternary basalts. Negative Nb-Ta anomalies in the Eocene – Miocene basalts are contrasted with positive Na-Tb anomalies in the Quaternary basalts. Eocene – Miocene samples MT05-

143, MT05-133 and GJP-88 are slightly enriched in LREE, and depleted in HREE relative to the Quaternary basalts. Samples are normalized to chondritic values of

McDonough and Sun [1995].

53 87 86 Figure 5. A: εNd vs. Sr/ Sr and B: εNd vs Age (Ma) of the Coast Mountains basalts.

87 86 The Quaternary basalts have higher εNd and lower Sr/ Sr than the Eocene – Miocene basalts.

Figure 6. Trace element abundance patterns for (A) regional Eocene – late Miocene basalts and (B) regional post-10 Ma basalts. REE compositions of (C) regional Eocene –

Miocene basalts and (D) regional post-10 Ma basalts. All samples are normalized to chondritic values of McDonough and Sun [1995]. Negative Nb-Ta anomalies are observed in the Eocene to Miocene basalts (A), whereas the Quaternary basalts display positive Na-Tb anomalies (C). Dashed lines are inferred, where trace element data is unavailable. Chilcotin magmatism was active from ~31-1 Ma, but dominantly occurred post-10 Ma [Coish et al., 1998]. Plotted lines are averaged basalt compositions from the

GEOROC database: Northern Cordilleran Volcanic Province (NCVP): Mt. Skukum

(n=20) [Morris and Creaser, 2003], Bennett Lake (n=20) [Morris and Creaser, 2003], Ft.

Selkirk (n=24) [Francis and Ludden, 1990], Alligator Lake (n=19) [Eiché et al., 1987],

Hirschfield – Llangorse (n=15) [Francis and Ludden, 1995], and Iskut River area (n=18)

[Cousens and Bevier, 1995; Russell and Hauskdóttir, 2000]; Challis-Kamloops Volcanic

Belt: Buck Creek Basin (n=21) [Dostal et al., 1998; Dostal et al., 2001), Penticton Group

(n=15) [Dostal et al., 2003]; and Mt. Noel Volcanic Complex (n=39) [Coish et al., 1998];

Masset Formation (n=20) [Hamilton and Dostal, 2001]; Chilcotin Group: Cheslatta Lake

(n=20) [Anderson et al., 2001] and Chilcotin basalts (n=3) [Anderson et al., 2001];

Neogene Dikes and Recent Volcanics: west-central British Columbia (n=6) [Crawford et al., 2005].

54 Figure 7. A: 143Nd/144Nd vs. 87Sr/86Sr of Tertiary – Quaternary mafic samples collected from sites in the western Canadian Cordillera. Data compiled from the GEOROC database: Northern Cordilleran Volcanic Province [Francis and Ludden, 1990; Carignan et al., 1994; Cousens and Bevier, 1995; Abraham et al., 2005], Challis-Kamloops [Dostal et al., 2001, 2003], Masset Formation [Hamilton and Dostal, 1993, 2001]. B: 87Sr/86Sr vs. age of Tertiary mafic samples compiled from the Anahim Volcanic Belt [Bevier,

1981, 1989], Challis - Kamloops [Dostal et al., 2001, 2003] and Kluane Ranges

[southwestern Yukon; Downey et al., 1980].

Figure 8. The emplacement of asthenospheric mantle beneath the western Canadian

Cordillera was largely due to extension in the North American plate, initiated by oblique dextral motion along the Queen Charlotte Fault. Eocene – Miocene dike swarms were emplaced during early stages of extension (A) and continued as large, normal fault bounded basins (e.g. Queen Charlotte, Buck Creek) developed through Miocene time (B).

At the present day, the Coast Mountains are underlain by asthenospheric mantle (C), which has been sampled by the Quaternary lava flows presented here, and numerous mafic rocks from the western Cordillera. Cross-sections are simplified, and limited to the study area in Figure 1. CMB = Coast Mountains Batholith; QCF = Queen Charlotte

Fault.

55 Coast Mountains Batholith

Gravina belt NCVP (extends northward)

Alexander - MT05-154 S t i k i n e T e r r a n e Masset Fm. Wrangellia MT05-156

Study Area

MT05-143 Central MT05-148 Gneiss MT05-151 Queen GJP-87 Complex Buck Creek Charlotte GJP-88 GJP-81 Basin Basin GJP-53 GJP-80 MT05-133 Chilcotin, Cheslatta Lake Challis-Kamloops Groups GJP-41a GJP-41b GJP-25 AVB Queen-Charlotte Fault MT05-107 Pacific MT05-108 MT05-110 GJP-15 Ocean GJP-30

Mt. Noel

Pemberton/Garibaldi (Cascades) N

Vancouver Island

0 150 km

FIGURE 1 Christian Manthei Fig1_2_CMB-draft.pdf Figure 2 Christian Manthei thin_sections_ g2BW.pdf

16 A 14

Tephri- 12 phonolite MT05-133

O (wt. %) 10

2 Phono- Tephrite Trachy- Trachydacite O+K 8 2 Foidite

a Basaltic trachy- N Tephrite andesite 6 Basanite Trachy- basalt GJP-30 4 Andesite Basalt Basaltic 2 Picro- andesite basalt

0 35 40 45 50 55 60 65 70 75

FeO*

Tholeiitic

Calc-Alkaline B

Alk MgO

FIGURE 3 Christian Manthei TAS-AFM_Fig3_NOSYMS.pdf 1000 GJP-15 Eocene - Miocene GJP-25 GJP-30 GJP-53 GJP-80 100 GJP-81 GJP-87 GJP-88 MT05-133 MT05-143 10 MT05-148 MT05-151

A C

1 Rb K Ba Th U Nb Ta La Ce Sr Nd Sm Zr Ti Gd Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu GJP-41a GJP-41b Sample/Chondrite Quaternary MT05-107 MT05-108 MT05-110 100 MT05-154 MT05-156

10

B D

1 Rb K Ba Th U Nb Ta La Ce Sr Nd Sm Zr Ti Gd Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Trace Element REE

FIGURE 4 Christian Manthei Trace_REE_mysamples_Fig4.pdf

9

8 MT05-154 Quaternary 7

6 εNd 5

4 Eocene - Miocene

3 A

2 0.7020 0.7030 0.7040 0.7050 87Sr/86Sr

9

8

7

6 εNd 5

4

3 B

2 0 10 20 30 40 50 Age (Ma)

FIGURE 5 Christian Manthei Isotopes_Mysamples_Fig5_NOSYMS.pdf 1000 Eocne - Miocene dikes (This Study) Eocene - late Miocene Mt. Skukum (NCVP) Bennett Lake (NCVP) “Neogene Dikes” (Crawford et al., 2005)

100 Mt. Noel Challis-Kamloops Masset Formation

10

A C

1 Rb K Ba Th U Nb Ta La Ce Sr Nd Sm Zr Ti Gd Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Quaternary lavas (This Study) Post-10 Ma Ft. Selkirk (NCVP) Alligator Lake (NCVP) Sample/Chondrite Hirschfield - Llangorse (NCVP) 100 Iskut River (NCVP) Chilcotin Group Basalts “Recent Volcanics” (Crawford et al., 2005)

10

B D 1 Rb K Ba Th U Nb Ta La Ce Sr Nd Sm Zr Ti Gd Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Trace Element REE

FIGURE 6 Christian Manthei Regional_Trace_REE_Fig6.pdf

0.5132 Coast Mountains basalts (this study) Quaternary Northern Cordilleran Volcanic Province 0.5130 Masset Formation Eocene - late Miocene Challis-Kamloops

Nd 0.5128

143 Nd/

144 0.5126

0.5124 A

0.5122 0.7010 0.7030 0.7050 0.7070 0.7090 87Sr/86Sr

Coast Mountains basalts (this study) 0.7090 Anaheim Volcanic Belt Challis-Kamloops Kluane Ranges (Southwestern Yukon) 0.7070

Sr

86

Sr/ 0.7050

87

0.7030 B

0.7010 0 10 20 30 40 50 60 Age (Ma)

FIGURE 7 Christian Manthei Regional_Isotopes_Fig7.pdf A) 48 Ma

SSW NNE

Early Queen Charlotte Fault

X CMB

Pacific plate Wrangellia Stikine terrane ------Cordilleran mantle lithosphere ------

Kula plate

mantle wedge Sub-oceanic (asthenospheric) mantle

B) 25 Ma SSW NNE

Basin development, magmatism Extension, N-S mafic diking QCF CMB X

Pacific plate Wrangellia Stikine terrane ------Cordilleran- - - mantle lithosphere- - -

upwelling asthenosphere

C) Post-10 Ma

Eocene - Miocene dikes QCF Quaternary mafic flows X Back-arc basins Quaternary volcanics CMB Queen Charlotte Basin

Pacific plate Wrangellia Stikine terrane

asthenospheric mantle

FIGURE 8 C. Manthei tectonicmodel.pdf 40 39 Table 1. Ar/ Ar Geochronology

Sample Latitude Longitude Rock type Sample type 40Ar/39Ar age (Ma) Error (±1σ) MT05-143 54.201 -129.172 basanite whole rock 45.1 0.9 GJP-25 52.622 -127.055 basalt hornblende 43.9 0.3 GJP-15 52.351 -127.202 basalt whole rock 37.6 0.4 GJP-87 53.896 -128.711 basalt whole rock 31.9 1.7 GJP-53 53.050 -129.624 basalt whole rock 20.0 0.5 GJP-80 53.433 -128.511 whole rock 19.7 0.3 GJP-81 53.495 -128.630 trachybasalt whole rock 19.7 0.3 MT05-133 53.466 -128.563 foidite whole rock 17.2 0.2 GJP-88 53.893 -128.711 basanite whole rock 13.9 0.3 MT05-148 54.136 -128.269 basalt whole rock 13.3 0.5 GJP-30 52.273 -127.603 b. trachyand. whole rock 10.8 0.3 MT05-151 54.004 -128.385 basanite whole rock 10.3 0.3 See Appendix 1 for step-heating plots and detailed 40Ar/39Ar procedure

Table 2. Major Element Analyses

Sample Latitude Longitude Age (Ma) Rock type SiO2 TiO2 Al2O3 FeO(T) MnO MgO CaO Na2O K2O P2O5 LOI Total Mg# MT05-143 54.201 -129.172 45.1 ± 0.9 basanite 45.77 1.90 12.52 10.69 0.14 9.77 8.56 3.46 2.84 1.22 1.56 98.49 78 GJP-25 52.622 -127.055 43.9 ± 0.3 basalt 47.49 3.10 14.86 14.88 0.18 6.72 8.31 3.68 1.02 0.34 1.28 101.86 64 GJP-15 52.351 -127.202 37.6 ±0.4 basalt 45.72 3.28 12.78 15.28 0.16 8.29 7.87 3.41 1.24 0.47 1.74 100.25 68 GJP-87 53.896 -128.711 31.9 ± 1.7 basalt 46.22 2.02 17.21 12.25 0.19 6.25 8.82 3.72 0.84 0.64 2.70 100.85 67 GJP-53 53.050 -129.624 20.0 ± 0.5 basalt 49.31 1.52 18.19 9.84 0.15 5.83 9.27 3.60 0.93 0.38 1.40 100.41 70 GJP-80 53.433 -128.511 19.7 ± 0.3 trachybasalt 50.03 1.42 16.78 9.49 0.15 4.92 7.55 3.94 2.32 0.70 2.97 100.27 67 GJP-81 53.495 -128.630 19.7 ± 0.3 trachybasalt 50.24 2.82 14.26 13.88 0.23 4.45 8.30 3.84 1.25 0.69 0.76 100.71 56 MT05-133 53.466 -128.563 17.2 ± 0.2 foidite 41.79 1.59 13.21 9.40 0.12 4.19 8.63 5.15 4.65 1.38 7.81 97.93 64 GJP-88 53.893 -128.711 13.9 ± 0.3 basanite 44.76 1.90 15.52 11.25 0.17 5.25 8.11 4.26 1.66 1.21 5.99 100.09 65 MT05-148 54.136 -128.269 13.3 ± 0.5 basalt 47.37 1.48 16.64 10.29 0.18 5.45 9.45 3.10 0.93 0.55 3.65 99.11 68 GJP-30 52.273 -127.603 10.8 ± 0.3 b. trachyand. 52.12 1.84 15.42 10.27 0.16 5.23 8.29 3.75 1.51 0.35 2.27 101.21 67 MT05-151 54.004 -128.385 10.3 ± 0.3 basanites 46.89 2.35 16.19 10.66 0.16 4.90 5.30 3.61 2.73 0.96 5.70 99.46 65 GJP-41a 52.504 -128.706 0 trachybasalt 49.77 2.55 15.63 14.28 0.20 4.84 7.93 4.05 1.56 0.85 <0.01 101.65 57 GJP-41b 52.504 -128.706 0 trachybasalt 49.57 2.59 15.60 14.32 0.20 4.88 7.97 4.07 1.55 0.84 <0.01 101.59 57 MT05-107 52.297 -128.437 0 trachybasalt 49.17 2.45 15.48 14.23 0.20 4.38 7.70 3.96 1.52 0.92 < 0.01 99.03 55 MT05-108 52.296 -128.437 0 trachybasalt 49.07 2.41 15.48 14.35 0.20 4.40 7.67 3.92 1.50 0.91 < 0.01 98.96 55 MT05-110 52.264 -128.374 0 trachybasalt 48.40 2.63 16.59 13.17 0.15 4.45 8.29 3.79 1.27 0.56 0.26 99.57 57 MT05-154 55.269 -129.200 0 Basalt 46.12 2.07 14.74 12.66 0.18 8.26 9.30 2.82 0.90 0.37 0.64 98.10 72 MT05-156 55.112 -129.123 0 trachybasalt 46.74 3.68 14.54 16.45 0.23 4.23 7.64 3.89 1.83 1.19 < 0.01 99.06 50 All data is reported as wt %. Mg# = ([MgO]/[MgO+FeO])*100 b. trachyand. = basaltic trachyandesite

Table 3. Trace Element Analyses

MT05- GJP- GJP- GJP- GJP- GJP- GJP- MT05- GJP- MT05- GJP- MT05- GJP- GJP- MT05- MT05- MT05- MT05- MT05- 143 25 15 87 53 80 81 133 88 148 30 151 41a 41b 107 108 110 154 156 Element Rb (ppm) 39 19 30 12 12 33 15 39 16 10 51 101 31 30 24 30 20 12 23 Ba 1885 356 287 379 319 870 740 1563 586 464 305 786 351 353 360 359 312 318 872 Th 12.22 1.04 1.19 1.26 2.34 4.45 1.61 4.21 2.37 1.68 4.65 2.20 4.31 4.15 4.07 4.03 2.88 1.48 2.50 U 3.51 0.29 1.73 0.45 0.73 1.59 0.60 1.56 0.77 0.68 1.81 1.61 1.25 1.06 1.40 1.44 0.51 0.53 0.92 Nb 29.5 10.7 18.8 19.0 16.9 20.7 27.6 30.4 28.1 15.0 36.3 21.8 60.6 60.1 58.4 53.8 42.1 22.0 39.6 Ta 1.34 0.70 1.16 0.91 0.87 0.92 1.49 1.33 1.27 0.67 2.26 1.18 3.59 3.54 3.34 3.30 2.53 1.33 2.21 Sr 1870 916 497 794 632 920 428 2410 1254 662 559 708 558 519 451 453 529 760 470 Zr 242 143 174 163 144 166 176 125 164 156 218 228 245 242 243 247 198 136 195 Gd 7.87 5.77 6.64 6.38 4.74 5.44 7.97 4.05 6.93 6.39 7.14 7.67 9.25 9.22 9.45 9.38 6.88 5.33 9.81 Y 19 19 19 29 23 22 34 13 24 29 34 31 36 35 35 35 26 21 34

La 89.9 15.5 18.1 23.0 18.7 38.9 23.4 66.6 48.9 27.7 26.8 34.4 36.1 35.1 35.4 35.6 25.6 16.4 29.3 Ce 182.7 39.9 49.0 54.7 40.8 80.7 55.8 133.3 108.7 63.4 60.3 81.0 76.6 76.2 77.5 77.8 55.3 37.9 66.2 Pr 21.2 5.9 7.2 7.2 5.1 9.4 7.5 14.0 13.0 8.1 7.7 10.5 9.5 9.4 9.7 9.8 7.0 5.1 8.7 Nd 73.8 26.4 32.5 28.8 20.3 34.0 32.1 42.8 47.9 31.5 30.3 40.2 38.6 38.1 38.7 39.7 27.9 21.2 36.8 Sm 10.97 6.27 7.52 6.22 4.33 5.96 7.47 5.30 8.10 6.40 6.96 7.90 8.97 8.98 9.01 8.96 6.36 5.05 8.97 Eu 2.72 1.99 2.23 1.96 1.45 1.79 3.02 1.39 2.37 1.84 1.72 2.44 3.02 2.91 2.98 3.00 2.27 1.71 3.82 Tb 0.76 0.72 0.83 0.93 0.69 0.67 1.16 0.38 0.85 0.90 1.06 1.02 1.30 1.30 1.28 1.33 0.95 0.73 1.30 Dy 3.94 4.11 4.39 5.60 4.30 3.97 6.86 2.07 4.74 5.70 6.51 6.07 7.66 7.42 7.52 7.76 5.58 4.39 7.60 Ho 0.65 0.70 0.75 1.10 0.83 0.75 1.36 0.36 0.85 1.15 1.30 1.15 1.41 1.36 1.40 1.47 1.04 0.81 1.36 Er 1.60 1.85 1.94 3.16 2.30 2.09 3.64 0.96 2.28 3.30 3.74 3.22 3.71 3.71 3.68 3.79 2.80 2.19 3.55 Tm 0.20 0.23 0.24 0.45 0.32 0.29 0.49 0.12 0.29 0.48 0.52 0.45 0.51 0.50 0.50 0.51 0.38 0.28 0.46 Yb 1.21 1.46 1.50 2.81 2.29 1.85 3.14 0.89 1.96 3.12 3.42 2.90 3.07 3.11 3.18 3.11 2.25 1.78 2.78 Lu 0.16 0.20 0.19 0.39 0.30 0.25 0.43 0.11 0.26 0.43 0.47 0.38 0.44 0.43 0.40 0.43 0.32 0.24 0.37 Ni 238 66 73 34 56 44 20 29 41 34 32 27 45 44 38 43 48 168 16 Cr 455 161 341 59 121 91 58 49 85 115 132 31 115 120 70 90 79 250 10 (La/Yb)n 51 7 8 6 6 14 5 51 17 6 5 8 8 8 8 8 8 6 7

Table 4. Radiogenic Isotope Data

87 86 87 86 a 147 144 143 144 a b Sample Age (Ma) Rb/ Sr Sr/ Sr 2σ ± Sm/ Nd Nd/ Nd 2σ ± εNd MT05-143 45.1 ± 0.9 0.0588 0.704864 16 0.0892 0.512736 8 2.5 GJP-25 43.9 ± 0.3 0.0781 0.704506 14 0.1445 0.512848 6 4.4 GJP-15 37.6 ±0.4 0.1746 0.704391 13 0.1370 0.512890 7 5.2 GJP-87 31.9 ± 1.7 0.0435 0.704184 15 0.1089 0.512855 11 4.6 GJP-53 20.0 ± 0.5 0.0572 0.704014 13 0.1308 0.512890 5 5.1 GJP-80 19.7 ± 0.3 0.1039 0.704458 15 0.1085 0.512779 10 3.0 GJP-81 19.7 ± 0.3 0.1039 0.703525 10 0.1408 0.512922 8 5.7 MT05-133 17.2 ± 0.2 0.0475 0.704577 13 0.0736 0.512749 6 2.4 GJP-88 13.9 ± 0.3 0.0381 0.704581 16 0.1040 0.512748 10 3.0 MT05-148 13.3 ± 0.5 0.0429 0.704404 14 0.1241 0.512808 8 3.5 GJP-30 10.8 ± 0.3 0.2732 0.704037 18 0.1368 0.512870 9 4.6 MT05-151 10.3 ± 0.3 0.4460 0.704069 15 0.1191 0.512886 9 5.0 GJP-41a recent 0.1743 0.702676 17 0.1386 0.513054 14 8.1 GJP-41b recent 0.1693 0.702687 14 0.1390 0.513067 8 8.4 MT05-107 recent 0.1807 0.702662 15 0.1378 0.513053 7 8.1 MT05-108 recent 0.1973 0.702702 11 0.1387 0.513072 8 8.5 MT05-110 recent 0.1435 0.702796 17 0.1496 0.513058 7 8.2 MT05-154 recent 0.1155 0.703861 15 0.1435 0.513015 10 7.4 MT05-156 recent 0.1388 0.703093 13 0.1448 0.513090 11 8.8 a Initial isotopic ratios, relative to Sr NBS-987 standard 0.710229 ± 11, and Nd La Jolla standard 0.511869 ± 9. b 143 144 143 144 4 εNd = [( Nd/ Nd)sample/( Nd/ Nd)CHUR – 1] x 10 ; present-day value of 143 144 ( Nd/ Nd)CHUR = 0.512638