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 mafic dike
and lava flow samples collected from sites within the western Canadian Cordillera.
Samples range in age from Eocene to Quaternary, and document a significant transition
in mantle chemistry that occurred sometime after 10 Ma. Eocene to late Miocene basalts
emplaced as dikes within the Coast Mountains Batholith 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
mantle (87Sr/86Sr = 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
generated by upwelling asthenosphere (87Sr/86Sr = 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 batholiths 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 arcmagmatism. Major, trace element and isotope compositions of mafic magmas provide
information on the affinity of melt sources (i.e. lithosphere vs. asthenosphere),
metasomatic processes, and pathways of melt migration into the crust [e.g., Pearce and
Cann, 1973; Winchester and Floyd, 1977; Hawkesworth et al., 1990; Schmidt and Poli,
2005]. Augmentation of the magma’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 British Columbia (52.3° – 55.3°N; Fig. 1), accompanied by
newly acquired
40
Ar/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 fault 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 terranes and
discrete crustal fragments that were outboard of the paleo-Laurentian continent and
accreted to the margin from Jurassic 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 terrane, 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 early Jurassic to
early Tertiary time [Armstrong, 1988; van der Heyden, 1992], and occurred coevally with
terrane accretion and greenschist to amphibolite facies metamorphism [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 North America [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 tonalite or granodiorite [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 postdate 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 Juan de Fuca plate [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 plateau-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 North American plate [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 Volcanic Belt [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 andesites) are also shown in Figure 1. A goal of this
work will be to compare the geochemistry of these regional basalts to newly acquired
basalt 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
40
Ar/39Ar
geochronology, were performed on nineteen mafic dike and cinder cone 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
40
Ar/39Ar Geochronology
Determination of
40
Ar/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, GJP81 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
40
Ar/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
146
Nd/144Nd = 0.7219. New CHUR values from unequilibrated chondrites
(143Nd/144NdMEAN = 0.512630) provided by Bouvier et al. [2008], yielded εNd values that
deviated from values calculated using 143Nd/144NdCHUR = 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
40
Ar/39Ar Geochronology
Step-heating plots (see Appendix 1) of cumulative
39
Ar 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 phenocrysts 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-porphyritic 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, basanites, basalts, and trachybasalts 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 MT05110 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.
Miocene basalts define ranges of 87Sr/86Sr = 0.70353 – 0.70486,
143
Eocene to
Nd/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
Sr/86Sr (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.
40
Ar/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 postemplacement 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 felsic (77 wt.% SiO2)
composition. A subset of granodiorite, quartz diorite 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.
isotopically primitive (depleted in
These basalts have positive Nb-Ta anomalies, are
87
Sr, enriched in
143
Nd 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
87
Sr 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 volcanism 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 Chilcotin Group 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.
87
Sr/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 Anahim Volcanic Belt
(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
87
Sr/86Sr of 0.70317 and 0.70320, respectively. This range is not as depleted in
87
Sr 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 tectonics 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 Rift), 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 Garibaldi Volcanic Belt (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 Oligocene –
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]. Ridge 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 Neogene –
Quaternary. Bevier [1989] used Sr and Pb isotopes to conclude that the AVB is a hotspot track of primitive, formerly sub-oceanic mantle. Late Miocene samples from her
study are some of the most depleted with respect to
87
Sr 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
from lithospheric mantle (87Sr/86Sr = 0.70353 – 0.70486;
25
εNd = +2.5 - +5.7).
By
contrast, the Quaternary lava flows are anhydrous, have lower LILE
concentrations, positive Nb-Ta anomalies, and were likely generated by
upwelling asthenosphere (87Sr/86Sr = 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 NbTa, 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 %.
cumulative
39
Plateau ages were determined by looking for steps of
Ar released of overlapping age, and are reported, where available. Step-
integrated ages are reported in cases of non-ideal plateaus. 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.
40
References Cited
Abraham, A.C., D.M. Francis, and M. Polvé (2005), Origin of recent alkaline lavas by
lithospheric thinning beneath the Northern Canadian Cordillera, Can. J. Earth
Sci., 42, 1073-1095.
Allègre, C.J., B. Dupré, B. Lambret, and P. Richard (1981), The subcontinental versus
suboceanic debate: Lead-Neodymium-Strontium isotopes in primary alkali basalts
from a shield area, the Ahaggar Volcanic suite, Earth Planet. Sci. Lett., 52, 85-92.
Anderson, R.G., J. Resnick, J.K. Russell, G.J. Woodsworth, M.E. Villenueve, and N.C.
Grainer (2001), The Cheslatta Lake suite: Miocene mafic, alkaline magmatism in
central British Columbia, Can. J. Earth Sci., 38, 697-717.
Armstrong, R.L. (1988), Mesozoic and early Cenozoic magmatic evolution of the
Canadian Cordillera, Geol. Soc. Spec. Publ., 218, 55-91.
Barker, F., and J.G. Arth (1984), Preliminary results, Central Gneiss Complex of the
Coast Range batholith, southeastern Alaska: the roots of a high-K, calc-alkaline
arc?, Phys. Earth Plan. Int., 35, 191-198.
Beck, S.L., and G. Zandt (2002), The nature of orogenic crust in the central Andes, J.
Geophys. Res., 107, 2230.
Bevier, M.L. (1981), The Rainbow Ridge, British Columbia: a Miocene peralkaline
shield volcano, J. Volcan. Geol. Res., 11, 225-251.
— (1983), Implications of chemical and isotopic composition for petrogenesis of
Chilcotin Group basalts, British Columbia, J. Petrol., 24, 207-226.
41
— (1989), A lead and strontium isotopic study of the Anahim volcanic belt, British
Columbia: Additional evidence for widespread suboceanic mantle beneath
western North America, Geol. Soc. Am. Bull., 101, 973-981.
Bevier, M.L., R.L. Armstrong, and J.G. Souther (1979), Miocene peralkaline volcanism
in west-central British Columbia - Its temporal and plate-tectonic setting,
Geology, 7, 389-392.
Bouvier, A., J.D. Vervoort, and P.J. Patchett (2008), The Lu-Hf and Sm-Nd isotopic
composition of CHUR: Constraints from unequilibrated chondrites and
implications for the bulk composition of terrestrial planets, Earth Planet. Sci.
Lett., 273, 48-57.
Breitsprecher, K., D.J. Thorkelson, W.G. Groome, and J. Dostal (2003), Geochemical
confirmation of the Kula-Farallon slab window beneath the Pacific Northwest in
Eocene time, Geology, 31, 351-354.
Carignan, J., J.N. Ludden, and D.M. Francis (1994), Isotopic characteristics of mantle
sources for Quaternary continental alkaline magmas in the northern Canadian
Cordillera, Earth Planet. Sci. Lett., 128, 271-286.
— (1996), On the recent enrichment of a subcontinental lithosphere: A detailed U-Pb
study of spinel lherzolite xenoliths, Yukon, Canada, Geochim. Cosomochim. Acta,
60, 4241-4252.
Carlson, R.W., D.G. Pearson, and D.E. James (2005), Physical, chemical and
chronological characteristics of continental mantle, Rev. of Geophys., 43, 1-24.
Coish, R.A., L.M. Kretschmar, and J.M. Journeay (1998), Geochemistry of the Miocene
Mount Noel Volcanic Complex, British Columbia and comparison with the
Columbia River basalt, J. Volcan. Geol. Res., 83, 269-285.
42
Colpron, M., J.L. Nelson, and D.C. Murphy (2007), Northern Cordilleran terranes and
their interactions through time, GSA Today, 17, 4-10.
Coltorti, M., C. Bonadiman, B. Faccini, M. Grégoire, S.Y. O'Reilly, and W. Powell
(2007), Amphiboles from suprasubduction and intraplate lithospheric mantle,
Lithos, 99, 68-84.
Coney, P.J., D. Jones, and J. Monger (1980), Cordilleran suspect terranes, Nature, 288,
329 - 333.
Cousens, B.L., and M.L. Bevier (1995), Discerning asthenospheric, lithospheric, and
crustal influences on the geochemistry of Quaternary basalts from Iskut-Unuk
rivers area, northwestern British Columbia, Can. J. Earth Sci., 32, 1451-1461.
Crawford, M.L., and W.A. Crawford (1991), Magma emplacement in a convergent
tectonic orogen, southern Revillagigedo Island, southeastern Alaska, Can. J.
Earth Sci., 28, 929-938.
Crawford, M.L., W. Crawford, and J. Lindline (2005), 105 Million years of igneous
activity, Wrangell, Alaska to Prince Rupert, British Columbia, Can. J. Earth Sci.,
42, 1097-1116.
Crawford, M.L., L.S. Hollister, and G.J. Woodsworth (1987), Crustal deformation and
regional metamorphism across a terrane boundary, Coast Plutonic Complex,
British Columbia, Tectonics, 6, 343-361.
Crawford, M.L., K.A. Klepis, G. Gehrels, and C. Isachsen (1999), Batholith emplacement
at mid-crustal levels and its exhumation within an obliquely convergent margin,
Tectonophysics, 312, 57-78.
43
Cui, Y., and J.K. Russell (1995), Nd-Sr-Pb isotopic studies of the southern Coast Plutonic
Complex, southwestern British Columbia, Geol. Soc. Am. Bull., 107, 127-138.
Deino, A.L. (2001), Users manual for Mass Spec v. 5.02: Berkeley Geochronology
Center Special Publication, 1a, 119
Dostal, J., K. Breitsprecher, B.N. Church, D.J. Thorkelson, and T.S. Hamilton (2003),
Eocene melting of the Precambrian lithospheric mantle: Analcime-bearing
volcanic rocks from the Challis-Kamloops Belt of South Central British
Columbia, J. Volcan. Geol. Res., 126, 303 - 326.
Dostal, J., B.N. Church, P.H. Reynolds, and L. Hopkins (2001), Eocene volcanism in the
Buck Creek Basin, Central British Columbia (Canada): Transition from arc to
extensional volcanism, J. Volcan. Geol. Res., 107, 149-170.
Dostal, J., D.A. Robichaud, B.N. Church, and P.H. Reynolds (1998), Eocene ChallisKamloops volcanism in Central British Columbia: an example from the Buck
Creek Basin, Can. J. Earth Sci., 42, 951-963.
Downey, M.E., R.L. Armstrong, and R.R. Parrish (1980), K-Ar, Rb-Sr and fission track
geochronometry of the Bock’s Brook Stock, Kluane Ranges, southwestern Yukon
Territory, Geol. Sur. Can. Spec. Publ., 80-1B, 189-193.
Ducea, M.N. (2002), Constraints on the bulk composition and root foundering rates of
continental arcs: A California perspective, J. Geophys. Res., 107, 2304-2316.
Ducea, M.N., and J.B. Saleeby (1996), Buoyancy sources for a large, uprooted mountain
range, Sierra Nevada, California: Evidence from xenolith thermobarometry, J.
Geophys. Res., 101, 8229-8244.
44
— (1998), Crustal recycling beneath continental arcs: silica-rich glass inclusions in
ultramafic xenoliths from the Sierra Nevada, California, Earth Planet. Sci. Lett.,
156, 101-116.
Edwards, B.R., and J.K. Russell (1999), Northern Cordilleran Volcanic Province: A
northern Basin and Range?, Geology, 27, 243-246.
— (2000), Distribution, nature, and origin of Neogene-Quaternary magmatism in the
northern Cordilleran volcanic province, Canada, Geol. Soc. Am. Bull., 112, 12801295.
Eiché, G., D. Francis, and J.N. Ludden (1987), Primary alkaline magmas associated with
the Quaternary Alligator Lake Volcanic Complex, Yukon Territory, Canada,
Contrib. Mineral. Petrol., 95, 191-201.
Engebretson, D.C., A. Cox, and R.G. Gordon (1985), Relative motions between oceanic
and continental plates in the Pacific Basin, Geol. Soc. Spec. Publ., 206, 1-59.
Farmer, G.L. (2005), Continental basaltic rocks, in The Crust edited by R.L. Rudnick,
Vol. 3 Treatise on Geochemistry, edited by H.D. Holland and K.K. Turekian, pp.
85-121, Elsevier-Pergamon, Oxford.
Farmer, G.L., A.F. Glazner, and C.R. Manley (2002), Did lithospheric delamination
trigger late Cenozoic potassic volcanism in the southern Sierra Nevada,
California?, Geol. Soc. Am. Bull., 114, 754-768.
Francis, D. (1987), Mantle-melt interaction recorded in spinel lherzolite xenoliths from
Alligator Lake Volcanic Complex, Yukon, Canada, J. Petrol., 28, 569-597.
45
Francis, D.M., and J.N. Ludden (1990), The mantle source for olivine nephelinite,
basanite and alkaline olivine basalt at Fort Selkirk, Yukon, Canada, J. Petrol., 31,
371-400.
— (1995), The signature of amphibole in mafic alkaline lavas, and study in the northern
Canadian Cordillera J. Petrol., 36, 1171-1191.
Gehrels, G., M. Rusmore, G. Woodsworth, M. Crawford, C. Andronicos, L. Hollister,
P.J. Patchett, M. Ducea, R. Butler, K. Klepeis, C. Davidson, R. Freidman, J.
Haggart, B. Mahoney, W. Crawford, D. Pearson, and J. Girardi (2009), U-Pb
geochronology of the Coast Mountains Batholith in north-coastal British
Columbia: constraints on age and tectonic evolution, Geol. Soc. Am. Bull., in
press.
Gehrels, G.E., and J.B. Saleeby (1987), Geologic framework, tectonic evolution and
displacement history of the Alexander Terrane, Tectonics, 6, 151-173.
Girardi, J.D., P.J. Patchett, M.N. Ducea, G.E. Gehrels, C.D. Manthei, D.M. Pearson,
M.E. Rusmore, G.J. Woodsworth, J. Fan, R.W. Kerrich, J.T. Thole, and K.R.
Wirth (2008), Elemental and Isotopic Evidence for Positive and
Negative Feedback Mechanisms Governing Magmatic Flux in the Coast
Mountains Batholith, British Columbia, Eos Trans. AGU, 89(53), Abstract
V33A-2204
Green, N.L., and P. Henderson (1984), Rare earth element concentrations in Quaternary
volcanic rocks from Southwestern British Columbia, Can. J. Earth Sci., 21, 731736.
Hamilton, T.S., and J. Dostal (1993), Geology, geochemistry and petrogenesis of middle
Tertiary volcanic rocks of the Queen Charlotte Islands, British Columbia
(Canada), J. Volcan. Geol. Res., 59, 77-99.
46
— (2001), Melting of heterogeneous mantle in a slab window environment: examples
from the middle Tertiary Masset basalts, Queen Charlotte Islands, British
Columbia, Can. J. Earth Sci., 38, 825-838.
Harder, M., and J.K. Russell (2006), Thermal state of the upper mantle beneath the
Northern Cordilleran Volcanic Province (NCVP), British Columbia, Canada,
Lithos, 87, 1-22.
— (2007), Basanite glaciovolcanism at Llangorse mountain, northern British Columbia,
Canada, Bull. Volcan., 69, 329-340.
Hawkesworth, C., K. Gallagher, J.M. Hergt, and F. McDermott (1993), Mantle and slab
contributions in arc magmas, Ann. Rev. Earth Planet. Sci., 21, 175-204.
Hawkesworth, C., P.D. Kempton, N.W. Rogers, R. Ellam, and P.W. van Carlsteren
(1990), Continental mantle lithosphere, and shallow level enrichment processes in
the Earth's mantle, Earth Planet. Sci. Lett., 96, 256-268.
Hole, M.J., G. Rogers, A.D. Saunders, and M. Storey (1991), Relation between alkalic
volcanism and slab-window formation, Geology, 19, 657-660.
Hollister, L.S. (1982), Metamorphic evidence for rapid (2 mm/yr) uplift of a portion of
the Central Gneiss Complex, Coast Mountains, B.C., Can. Mineral., 20, 319-332.
Hollister, L.S., and C. Andronicos (2006), Formation of new continental crust in Western
British Columbia during transpression and transtension, Earth Planet. Sci. Lett.,
249, 29-38.
Irvine, T.M., and W.R.A Baragar (1971), A guide to the chemical classification of the
common volcanic rocks, Can. J. Earth Sci., 8, 532-548
47
Irving, E., J. Souther, and J. Baker (1992), Tertiary extension and tilting in the Queen
Charlotte Islands, evidence from dyke swarms and their paleomagnetism, Can. J.
Earth Sci., 29, 1878-1898.
Jenner, G.A., H.P. Longerich, S.E. Jackson, and B.J. Fryer (1990), ICP-MS; a powerful
tool for high-precision trace-element analysis in earth sciences; evidence from
analysis of selected U.S.G.S. reference samples, Chem. Geol., 83, 133-148.
Kay, R.W., and S.M. Kay (1993), Delamination and delamination magmatism,
Tectonophysics, 219, 177-189.
Kay, S.M., B. Coria, and J. Viramonte (1994), Young mafic back-arc volcanic rocks as
indicators of continental lithospheric delamination beneath the Argentine Puna
plateau, central Andes, J. Geophys. Res., 99, 24,323-24,339.
Le Bas, M.J., R.W. Le Maitre, A. Streckeisen, and B. Zanettin (1986), A chemical
classification of volcanic rocks based on the total alkali-silica diagram, J. Petrol.,
27, 745-750.
Longerich, H.P., G.A. Jenner, B.J. Fryer, and S.E. Jackson (1990), Inductively coupled
plasma-mass spectrometric analysis of geological samples; a critical evaluation
based on case studies, Chem. Geol., 83, 105-118.
Lugmair, G.W., and R.W. Carlson (1978), The Sm-Nd history of KREEP: Proceedings of
the Lunar and Planetary Science Conference, 9, 689-704.
Madsen, J.K., D.J. Thorkelson, R.M. Freidman, and D.D. Marshall (2006), Cenozoic to
recent plate configurations in the Pacific Basin: Ridge subduction and slab
window magmatism in western North America, Geosphere, 2, 11-34.
48
McDonough, W.F., and S.-S Sun (1995), The composition of the Earth, Chem. Geol.,
120, 223-253.
Monger, J., R.A. Price, and D.J. and Tempelman-Kluit (1982), Tectonic accretion and
the origin of the two major metamorphic and plutonic welts in the Canadian
Cordillera, Geology, 10, 70 - 75.
Morris, G.A., and R.A. Creaser (2003), Crustal recycling during subduction at the Eocene
Cordilleran margin of North America: a petrogenic study from the southwestern
Yukon, Can. J. Earth Sci., 40, 1085-1821.
Morris, J.D., and S. R. Hart (1983), Isotopic and incompatible trace element constraints
on the genesis of island arc volcanics from Cold Bay and Amak Island, Aleutians,
and implications for mantle structure, Geochim. Cosomochim. Acta, 47, 20152030.
Norrish, K., and J.T. Hutton (1969), An accurate x-ray spectrographic method for the
analysis of a wide range of geological samples, Geochim. Cosomochim. Acta, 33,
431-453.
Patchett, P.J., and J. Ruiz (1987), Nd isotopic ages of crust formation and metamorphism
in the Precambrian of eastern and southern Mexico, Contrib. Mineral. Petrol., 96,
523-528.
Pearce, J.A., and J.R. Cann (1973), Tectonic setting of basic volcanic rocks determined
using trace element analyses, Earth Planet. Sci. Lett., 19, 290-300.
Pearce, J.A., and D.W. Peate (1995), Tectonic implications of the composition of arc
magmas, Ann. Rev. Earth Planet. Sci., 23, 251-285.
49
Peslier, A., L. Reisberg, J. Ludden, and D. Francis (2000), Re-Os constrains on
harzburgite and lherzolite formation in the lithospheric mantle: A study of
Northern Canadian Cordillera xenoliths, Geochim. Cosomochim. Acta, 64, 30613071.
Peslier, A.H., D. Francis, and J. Ludden (2002), The lithospheric mantle beneath
continental margins: melting and melt-rock reaction in Canadian Cordillera
xenoliths, J. Petrol., 43, 2013-2047.
Renne, P.R., C.C. Swisher, A.L. Deino, D.B. Karner, T. Owens, and D.J. DePaolo
(1998), Intercalibration of standards, absolute ages and uncertainties in
40Ar/39Ar dating, Chem. Geol., 145, 117-152.
Roddick, J.A., and W.W. Hutchinson (1974), Setting of the Coast Plutonic Complex,
British Columbia, Pacific Geol., 8, 91-108.
Rohr, K.M., and L. Currie (1997), Queen Charlotte basin and Coast Mountains: Paired
belts of subsidence and uplift cause by a low-angle normal fault, Geology, 25,
819-822.
Rohr, K.M., and J.R. Dietrich (1992), Strike-slip tectonics and development of the
Tertiary Queen Charlotte Basin, offshore western Canada: evidence from seismic
reflection data, Basin Research, 4, 1-20.
Rudnick, R.L., and D.M. Fountain (1995), Nature and composition of the continental
crust: A lower crustal perspective, Rev. Geophys., 33, 267-309.
Rusmore, M.E., G.J. Woodsworth, and G.E. Gehrels (2005), Two-stage exhumation of
midcrustal arc rocks, Coast Mountains, British Columbia, Tectonics, 24, 1-25.
50
Russell, J.K., and S. Hauskdóttir (2000), Estimates of crustal assimilation in Quaternary
lavas from the Northern Cordillera, British Columbia, Can. Mineral., 39, 275297.
Samson, S.D., W. McClelland, P.J. Patchett, G. Gehrels, and R.G. Anderson (1989),
Evidence from neodymium isotopes for mantle contributions to Phanerozoic
crustal gneiss in the Canadian Cordillera, Nature, 337, 705-709.
Samson, S.D., W. McClelland, P.J. Patchett, and G.E. Gehrels (1991), Nd and Sr isotopic
constraints on the petrogenesis of the west side of the northern Coast Mountains
batholith, Alaskan and Canadian Cordillera, Can. J. Earth Sci., 28, 939-946.
Schmidt, M.W., and S. Poli (2005), Generation of mobile components during subduction
of oceanic crust, In: The Crust (ed. R.L. Rudnick) Vol. 3 Treatise on
Geochemistry edited by H.D. Holland and K.K. Turekian, p. 567-591, ElsevierPergamon, Oxford Univ. Press
Schott, B., and H. Schmeling (1998), Delamination and detachment of a lithospheric root,
Tectonophysics, 296, 225-247.
Shi, L., D. Francis, J. Ludden, A. Frederiksen, and M. Bostock (1998), Xenolith evidence
for lithospheric melting above anomalously hot mantle under the northern
Canadian Cordillera, Contrib. Mineral. Petrol., 131, 39-53.
Souther, J.G., and C.J. Yorath (1991), Neogene assemblages, in Geology of the
Cordilleran orogen, Canada edited by H. Gabrielse, and C.J. Yorath, Geological
Survey of Canada, Geology of Canada, 4, pp. 373-401
Steiger, R.H., and E. Jäeger (1977), Subcommission on geochronology: Convention on
the use of decay constants in geo- and cosmo-chronology, Earth Planet. Sci. Lett.,
36, 359-363.
51
Stock, J., and P. Molnar (1988), Uncertainties and implications of the late Cretaceous and
Tertiary position of North America relative to the Farallon, Kula and pacific
plates, Tectonics, 7, 1339-1384.
Stowell, H., and H. Crawford (2000), Metamorphic history of the Coast Mountains
orogen, western British Columbia and southeastern Alaska, in Tectonics of the
Coast Mountains, southeastern Alaska and British Columbia, edited by H.H.
Stowell, and W.C. McClelland, Geol. Soc. Spec. Publ., 343, 257-284.
Thorkelson, D.J., and R.P. Taylor (1989), Cordilleran slab windows, Geology, 17, 833836.
van der Heyden, P (1992), A Middle Jurassic to early Tertiary Andean-Sierran arc model
for the Coast Belt of British Columbia, Tectonics, 11, 82-97.
Wernicke, B., R. Clayton, M. Ducea, C.H. Jones, S. Park, S. Ruppert, J. Saleeby, J.K.
Snow, L. Squires, M. Fliedner, M., G. Jiraceck, R. Keller, S. Klemperer, J.
Luetgert, P. Malin, K. Miller, W. Mooney, H. Oliver, and R. Phinney (1996),
Origin of high mountains in the continents; the southern Sierra Nevada, Science,
271, 190-193.
Winchester, J.A., and P.A. Floyd (1977), Geochemical discrimination of different
magma series and their differentiation products using immobile elements, Chem.
Geol., 20, 325-343.
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 MT05143, 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
Figure 5. A:
εNd vs. 87Sr/86Sr and B: εNd vs Age (Ma) of the Coast Mountains basalts.
The Quaternary basalts have higher
εNd and lower 87Sr/86Sr 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:
143
Nd/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:
87
Sr/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
NCVP
(extends northward)
Gravina belt
kin
MT05-154
MT05-156
Masset Fm.
Sti
Alexander Wrangellia
MT05-143
MT05-148
MT05-151
GJP-87 Buck Creek
GJP-88
Basin
GJP-81
GJP-80
Chilcotin,
MT05-133
ran
Queen
e
Central
Gneiss
Complex
Charlotte
Basin
r
e Te
Study Area
GJP-53
Cheslatta Lake
Challis-Kamloops
Groups
Queen-Charlotte Fault
Pacific
Ocean
GJP-41a
GJP-41b
GJP-25
MT05-107
MT05-108
GJP-15
MT05-110
GJP-30
AV
B
Mt. Noel
Pe
mb
N
0
FIGURE 1
Christian Manthei
Fig1_2_CMB-draft.pdf
150 km
ert
o
(Ca n/Ga
sca riba
de ldi
Van
s)
cou
ver
Isla
nd
Figure 2
Christian Manthei
thin_sections_fig2BW.pdf
Na O+K2O (wt. %)
2
16
14
A
Phonolite
Tephriphonolite
12
MT05-133
10
8
6
Trachyte
PhonoTephrite
Trachyandesite
Foidite
Basaltic
trachyTephrite
andesite
Basanite Trachybasalt
GJP-30
4
Picrobasalt
2
0
35
40
45
Basalt
Trachydacite
Andesite
Rhyolite
Dacite
Basaltic
andesite
50
55
60
65
70
75
FeO*
Tholeiitic
Calc-Alkaline
B
Alk
FIGURE 3
Christian Manthei
TAS-AFM_Fig3_NOSYMS.pdf
MgO
1000
GJP-15
GJP-25
GJP-30
GJP-53
GJP-80
GJP-81
GJP-87
GJP-88
MT05-133
MT05-143
MT05-148
MT05-151
Eocene - Miocene
Sample/Chondrite
100
10
C
A
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
Pr
Nd Sm Eu
Gd
Tb
Dy
Ho
Er
Tm Yb
Lu
Quaternary
100
10
D
B
1
Rb
K
Ba Th
U
Nb Ta La Ce Sr Nd Sm Zr
Trace Element
FIGURE 4
Christian Manthei
Trace_REE_mysamples_Fig4.pdf
Ti Gd Y
La
Ce
REE
GJP-41a
GJP-41b
MT05-107
MT05-108
MT05-110
MT05-154
MT05-156
9
8
7
εNd
MT05-154
Quaternary
6
5
4
Eocene - Miocene
3
A
2
0.7020
0.7030
0.7040
87Sr/86Sr
0.7050
9
8
7
εNd
6
5
4
3
2
B
0
10
20
30
Age (Ma)
FIGURE 5
Christian Manthei
Isotopes_Mysamples_Fig5_NOSYMS.pdf
40
50
1000
Eocne - Miocene dikes (This Study)
Mt. Skukum (NCVP)
Bennett Lake (NCVP)
“Neogene Dikes” (Crawford et al., 2005)
Mt. Noel
Challis-Kamloops
Masset Formation
Eocene - late Miocene
Sample/Chondrite
100
10
C
A
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
Post-10 Ma
100
Iskut River (NCVP)
Chilcotin Group Basalts
“Recent Volcanics” (Crawford et al., 2005)
10
1
Quaternary lavas (This Study)
Ft. Selkirk (NCVP)
Alligator Lake (NCVP)
Hirschfield - Llangorse (NCVP)
D
B
Rb
K
Ba
Th
U
Nb
Ta
La
Ce
Sr
Nd Sm
Trace Element
FIGURE 6
Christian Manthei
Regional_Trace_REE_Fig6.pdf
Zr
Ti
Gd
Y
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
REE
Dy
Ho
Er
Tm
Yb
Lu
0.5132
Coast Mountains basalts (this study)
Quaternary
144Nd/143Nd
0.5130
Northern Cordilleran Volcanic Province
Masset Formation
Eocene - late Miocene
Challis-Kamloops
0.5128
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)
87Sr/86Sr
0.7070
0.7050
0.7030
B
0.7010
0
10
20
30
Age (Ma)
FIGURE 7
Christian Manthei
Regional_Isotopes_Fig7.pdf
40
50
60
A)
48 Ma
SSW
NNE
Early Queen Charlotte Fault
CMB
X
Pacific plate
Stikine terrane
Wrangellia
---
Ku
la
Sub-oceanic (asthenospheric)
mantle
B)
25 Ma
---
---
--Cordilleran mantle lithosphere
-----
---
pla
---
---
mantle wedge
te
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
QCF
X
Queen Charlotte Basin
Qu
lca
Eocene - Miocene dikes
o
yv
r
na
r
ate
s
nic
CMB
Pacific plate Wrangellia
C. Manthei
tectonicmodel.pdf
Back-arc basins
Stikine terrane
asthenospheric mantle
FIGURE 8
t
ua
Q
y
ar
n
er
ic
af
m
ws
flo
Table 1.
40
Sample
Ar/39Ar Geochronology
Latitude
Longitude
MT05-143
54.201
-129.172
GJP-25
52.622
-127.055
GJP-15
52.351
-127.202
GJP-87
53.896
-128.711
GJP-53
53.050
-129.624
GJP-80
53.433
-128.511
GJP-81
53.495
-128.630
MT05-133
53.466
-128.563
GJP-88
53.893
-128.711
MT05-148
54.136
-128.269
GJP-30
52.273
-127.603
MT05-151
54.004
-128.385
See Appendix 1 for step-heating plots and detailed 40Ar/39Ar procedure
Rock type
Sample type
basanite
basalt
basalt
basalt
basalt
trachybasalt
trachybasalt
foidite
basanite
basalt
b. trachyand.
basanite
whole rock
hornblende
whole rock
whole rock
whole rock
whole rock
whole rock
whole rock
whole rock
whole rock
whole rock
whole rock
40
Ar/39Ar age (Ma)
45.1
43.9
37.6
31.9
20.0
19.7
19.7
17.2
13.9
13.3
10.8
10.3
Error (±1σ)
0.9
0.3
0.4
1.7
0.5
0.3
0.3
0.2
0.3
0.5
0.3
0.3
Table 2. Major Element Analyses
Sample
Latitude
Longitude
MT05-143
54.201
-129.172
GJP-25
52.622
-127.055
GJP-15
52.351
-127.202
GJP-87
53.896
-128.711
GJP-53
53.050
-129.624
GJP-80
53.433
-128.511
GJP-81
53.495
-128.630
MT05-133
53.466
-128.563
GJP-88
53.893
-128.711
MT05-148
54.136
-128.269
GJP-30
52.273
-127.603
MT05-151
54.004
-128.385
GJP-41a
52.504
-128.706
GJP-41b
52.504
-128.706
MT05-107
52.297
-128.437
MT05-108
52.296
-128.437
MT05-110
52.264
-128.374
MT05-154
55.269
-129.200
MT05-156
55.112
-129.123
All data is reported as wt %.
Mg# = ([MgO]/[MgO+FeO])*100
b. trachyand. = basaltic trachyandesite
Age (Ma)
Rock type
SiO2
TiO2
Al2O3
FeO(T)
MnO
MgO
CaO
Na2O
K2 O
P 2 O5
LOI
Total
Mg#
45.1 ± 0.9
43.9 ± 0.3
37.6 ±0.4
31.9 ± 1.7
20.0 ± 0.5
19.7 ± 0.3
19.7 ± 0.3
17.2 ± 0.2
13.9 ± 0.3
13.3 ± 0.5
10.8 ± 0.3
10.3 ± 0.3
0
0
0
0
0
0
0
basanite
basalt
basalt
basalt
basalt
trachybasalt
trachybasalt
foidite
basanite
basalt
b. trachyand.
basanites
trachybasalt
trachybasalt
trachybasalt
trachybasalt
trachybasalt
Basalt
trachybasalt
45.77
47.49
45.72
46.22
49.31
50.03
50.24
41.79
44.76
47.37
52.12
46.89
49.77
49.57
49.17
49.07
48.40
46.12
46.74
1.90
3.10
3.28
2.02
1.52
1.42
2.82
1.59
1.90
1.48
1.84
2.35
2.55
2.59
2.45
2.41
2.63
2.07
3.68
12.52
14.86
12.78
17.21
18.19
16.78
14.26
13.21
15.52
16.64
15.42
16.19
15.63
15.60
15.48
15.48
16.59
14.74
14.54
10.69
14.88
15.28
12.25
9.84
9.49
13.88
9.40
11.25
10.29
10.27
10.66
14.28
14.32
14.23
14.35
13.17
12.66
16.45
0.14
0.18
0.16
0.19
0.15
0.15
0.23
0.12
0.17
0.18
0.16
0.16
0.20
0.20
0.20
0.20
0.15
0.18
0.23
9.77
6.72
8.29
6.25
5.83
4.92
4.45
4.19
5.25
5.45
5.23
4.90
4.84
4.88
4.38
4.40
4.45
8.26
4.23
8.56
8.31
7.87
8.82
9.27
7.55
8.30
8.63
8.11
9.45
8.29
5.30
7.93
7.97
7.70
7.67
8.29
9.30
7.64
3.46
3.68
3.41
3.72
3.60
3.94
3.84
5.15
4.26
3.10
3.75
3.61
4.05
4.07
3.96
3.92
3.79
2.82
3.89
2.84
1.02
1.24
0.84
0.93
2.32
1.25
4.65
1.66
0.93
1.51
2.73
1.56
1.55
1.52
1.50
1.27
0.90
1.83
1.22
0.34
0.47
0.64
0.38
0.70
0.69
1.38
1.21
0.55
0.35
0.96
0.85
0.84
0.92
0.91
0.56
0.37
1.19
1.56
1.28
1.74
2.70
1.40
2.97
0.76
7.81
5.99
3.65
2.27
5.70
<0.01
<0.01
< 0.01
< 0.01
0.26
0.64
< 0.01
98.49
101.86
100.25
100.85
100.41
100.27
100.71
97.93
100.09
99.11
101.21
99.46
101.65
101.59
99.03
98.96
99.57
98.10
99.06
78
64
68
67
70
67
56
64
65
68
67
65
57
57
55
55
57
72
50
Table 3. Trace Element Analyses
MT05143
GJP25
GJP15
GJP87
GJP53
GJP80
GJP81
MT05133
GJP88
MT05148
GJP30
MT05151
GJP41a
GJP41b
MT05107
MT05108
MT05110
MT05154
MT05156
Element
Rb (ppm)
Ba
Th
U
Nb
Ta
Sr
Zr
Gd
Y
39
1885
12.22
3.51
29.5
1.34
1870
242
7.87
19
19
356
1.04
0.29
10.7
0.70
916
143
5.77
19
30
287
1.19
1.73
18.8
1.16
497
174
6.64
19
12
379
1.26
0.45
19.0
0.91
794
163
6.38
29
12
319
2.34
0.73
16.9
0.87
632
144
4.74
23
33
870
4.45
1.59
20.7
0.92
920
166
5.44
22
15
740
1.61
0.60
27.6
1.49
428
176
7.97
34
39
1563
4.21
1.56
30.4
1.33
2410
125
4.05
13
16
586
2.37
0.77
28.1
1.27
1254
164
6.93
24
10
464
1.68
0.68
15.0
0.67
662
156
6.39
29
51
305
4.65
1.81
36.3
2.26
559
218
7.14
34
101
786
2.20
1.61
21.8
1.18
708
228
7.67
31
31
351
4.31
1.25
60.6
3.59
558
245
9.25
36
30
353
4.15
1.06
60.1
3.54
519
242
9.22
35
24
360
4.07
1.40
58.4
3.34
451
243
9.45
35
30
359
4.03
1.44
53.8
3.30
453
247
9.38
35
20
312
2.88
0.51
42.1
2.53
529
198
6.88
26
12
318
1.48
0.53
22.0
1.33
760
136
5.33
21
23
872
2.50
0.92
39.6
2.21
470
195
9.81
34
La
Ce
Pr
Nd
Sm
Eu
Tb
Dy
Ho
Er
Tm
Yb
Lu
Ni
Cr
(La/Yb)n
89.9
182.7
21.2
73.8
10.97
2.72
0.76
3.94
0.65
1.60
0.20
1.21
0.16
238
455
51
15.5
39.9
5.9
26.4
6.27
1.99
0.72
4.11
0.70
1.85
0.23
1.46
0.20
66
161
7
18.1
49.0
7.2
32.5
7.52
2.23
0.83
4.39
0.75
1.94
0.24
1.50
0.19
73
341
8
23.0
54.7
7.2
28.8
6.22
1.96
0.93
5.60
1.10
3.16
0.45
2.81
0.39
34
59
6
18.7
40.8
5.1
20.3
4.33
1.45
0.69
4.30
0.83
2.30
0.32
2.29
0.30
56
121
6
38.9
80.7
9.4
34.0
5.96
1.79
0.67
3.97
0.75
2.09
0.29
1.85
0.25
44
91
14
23.4
55.8
7.5
32.1
7.47
3.02
1.16
6.86
1.36
3.64
0.49
3.14
0.43
20
58
5
66.6
133.3
14.0
42.8
5.30
1.39
0.38
2.07
0.36
0.96
0.12
0.89
0.11
29
49
51
48.9
108.7
13.0
47.9
8.10
2.37
0.85
4.74
0.85
2.28
0.29
1.96
0.26
41
85
17
27.7
63.4
8.1
31.5
6.40
1.84
0.90
5.70
1.15
3.30
0.48
3.12
0.43
34
115
6
26.8
60.3
7.7
30.3
6.96
1.72
1.06
6.51
1.30
3.74
0.52
3.42
0.47
32
132
5
34.4
81.0
10.5
40.2
7.90
2.44
1.02
6.07
1.15
3.22
0.45
2.90
0.38
27
31
8
36.1
76.6
9.5
38.6
8.97
3.02
1.30
7.66
1.41
3.71
0.51
3.07
0.44
45
115
8
35.1
76.2
9.4
38.1
8.98
2.91
1.30
7.42
1.36
3.71
0.50
3.11
0.43
44
120
8
35.4
77.5
9.7
38.7
9.01
2.98
1.28
7.52
1.40
3.68
0.50
3.18
0.40
38
70
8
35.6
77.8
9.8
39.7
8.96
3.00
1.33
7.76
1.47
3.79
0.51
3.11
0.43
43
90
8
25.6
55.3
7.0
27.9
6.36
2.27
0.95
5.58
1.04
2.80
0.38
2.25
0.32
48
79
8
16.4
37.9
5.1
21.2
5.05
1.71
0.73
4.39
0.81
2.19
0.28
1.78
0.24
168
250
6
29.3
66.2
8.7
36.8
8.97
3.82
1.30
7.60
1.36
3.55
0.46
2.78
0.37
16
10
7
Table 4. Radiogenic Isotope Data
Sample
Age (Ma)
MT05-143
GJP-25
GJP-15
GJP-87
GJP-53
GJP-80
GJP-81
45.1 ± 0.9
43.9 ± 0.3
37.6 ±0.4
31.9 ± 1.7
20.0 ± 0.5
19.7 ± 0.3
19.7 ± 0.3
MT05-133
GJP-88
MT05-148
GJP-30
MT05-151
GJP-41a
GJP-41b
MT05-107
MT05-108
MT05-110
MT05-154
MT05-156
17.2 ± 0.2
13.9 ± 0.3
13.3 ± 0.5
10.8 ± 0.3
10.3 ± 0.3
recent
recent
recent
recent
recent
recent
recent
a
87
86
87
86
a
147
144
Sr/ Sr
2σ ±
0.0588
0.0781
0.1746
0.0435
0.0572
0.1039
0.1039
0.704864
0.704506
0.704391
0.704184
0.704014
0.704458
0.703525
16
14
13
15
13
15
10
0.0892
0.1445
0.1370
0.1089
0.1308
0.1085
0.1408
0.0475
0.0381
0.0429
0.2732
0.4460
0.1743
0.1693
0.1807
0.1973
0.1435
0.1155
0.1388
0.704577
0.704581
0.704404
0.704037
0.704069
0.702676
0.702687
0.702662
0.702702
0.702796
0.703861
0.703093
13
16
14
18
15
17
14
15
11
17
15
13
0.0736
0.1040
0.1241
0.1368
0.1191
0.1386
0.1390
0.1378
0.1387
0.1496
0.1435
0.1448
Rb/ Sr
Initial isotopic ratios, relative to Sr NBS-987 standard 0.710229 ± 11, and
Nd La Jolla standard 0.511869 ± 9.
b
εNd = [(143Nd/144Nd)sample/(143Nd/144Nd)CHUR – 1] x 104; present-day value of
(143Nd/144Nd)CHUR = 0.512638
Sm/
Nd
143
144
2σ ±
εNdb
0.512736
0.512848
0.512890
0.512855
0.512890
0.512779
0.512922
8
6
7
11
5
10
8
2.5
4.4
5.2
4.6
5.1
3.0
5.7
0.512749
0.512748
0.512808
0.512870
0.512886
0.513054
0.513067
0.513053
0.513072
0.513058
0.513015
0.513090
6
10
8
9
9
14
8
7
8
7
10
11
2.4
3.0
3.5
4.6
5.0
8.1
8.4
8.1
8.5
8.2
7.4
8.8
Nd/
a
Nd