U-Pb and Hf Analyses of Detrital Zircons from Paleozoic and Cretaceous Strata on Vancouver Island, British Columbia: Constraints on the Paleozoic ...
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GeoScienceWorld
Lithosphere
Volume 2021, Article ID 7866944, 20 pages
https://doi.org/10.2113/2021/7866944
Research Article
U-Pb and Hf Analyses of Detrital Zircons from Paleozoic and
Cretaceous Strata on Vancouver Island, British Columbia:
Constraints on the Paleozoic Tectonic Evolution of
Southern Wrangellia
Daniel Alberts ,1 George E. Gehrels ,1 and Joanne Nelson 2
1
Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA
2
British Columbia Geological Survey, Victoria, British Columbia V8W 9N3, Canada
Correspondence should be addressed to Daniel Alberts; dalbert720@yahoo.com
Received 23 February 2020; Accepted 25 January 2021; Published 26 February 2021
Academic Editor: Sarah Roeske
Copyright © 2021 Daniel Alberts et al. Exclusive Licensee GeoScienceWorld. Distributed under a Creative Commons Attribution
License (CC BY 4.0).
Wrangellia is a late Paleozoic arc terrane that occupies two distinct coastal regions of western Canada and Alaska. The Skolai arc of
northern Wrangellia in south-central Alaska and Yukon has been linked to the older, adjacent Alexander terrane by shared Late
Devonian rift-related gabbros and also by Late Pennsylvanian postcollisional plutons. Late Devonian to Early Permian Sicker arc
rocks of southern Wrangellia are exposed in uplifts on Vancouver Island, southwestern British Columbia, surrounded by
younger strata and lacking physical connections to other terranes. Utilizing the detrital zircon record of Paleozoic and
Cretaceous sedimentary rocks, we provide insight into the magmatic and depositional evolution of southern Wrangellia and its
relationships to both northern Wrangellia and the Alexander terrane. 1422 U-Pb LA-ICPMS analyses from the Fourth Lake
Formation (Mississippian–Permian) reveal syndepositional Carboniferous age peaks (344, 339, 336, 331, and 317 Ma), sourced
from the Sicker arc of southern Wrangellia. These populations overlap in part known ages of volcanism, but the Middle
Mississippian cumulative peak (337 Ma) documents a previously unrecognized magmatic episode. Paleozoic detrital zircons
exhibit intermediate to juvenile ƐHf ðtÞ values between +15 and +5, indicating that southern Wrangellia was not strictly built on
primitive oceanic crust, but instead on transitional crust with a small evolved component. The Fourth Lake samples yielded 49
grains (3.4% of the total grains analyzed) with ages between 2802 Ma and 442 Ma, and with corresponding ƐHf ðtÞ values
ranging from +13 to -20. In age—ƐHf ðtÞ space, these grains fall within the Alexander terrane array. They were probably derived
from sedimentary rocks in the basement of the Sicker arc. By analogy with northern Wrangellia, this basement incorporated
rifted fragments of the Alexander terrane margin as the combined Sicker-Skolai arc system advanced ocean-ward due to slab
rollback in Late Devonian to Early Mississippian time. Ultimately, data from detrital zircons preserved in the Fourth Lake
Formation provides significant information allowing for an updated tectonic model of Paleozoic Wrangellia.
1. Introduction Wrangellia and Karmutsen Formation of southern Wrangel-
lia, that overlie Paleozoic arc-related sequences [1, 2]. Paleo-
Wrangellia is one of the most outboard of the major northern zoic sequences of southern Wrangellia comprise the Upper
Cordilleran terranes. It occupies two separate regions: south- Devonian to Lower Permian Sicker and Buttle Lake Groups
central Alaska and southwestern Yukon (northern Wrangel- [3–7]. Those in northern Wrangellia comprise the Carbonif-
lia) and Haida Gwaii and Vancouver Island (southern erous to Lower Permian Skolai Group, including the Station
Wrangellia) (Figure 1 inset). It was originally defined as a Creek and Hasen Creek formations [8–13].
coherent terrane based on characteristic thick piles of Until recently, the records of Paleozoic arc activity in
Triassic flood basalts, the Nikolai Greenstone of northern southern versus northern Wrangellia were considered to be
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2 Lithosphere
Knight inlet
Bute inlet
129°W, 50°N
Coast Plutonic Complex
Tertiary Rocks
Nanaimo
Eocene Crescent terrane
Dragon Property =
Eocene Pacific Rim terrane Bedingfield Uplift =
Buttle Lake Uplift =
Cretaceous Nanaimo Group
Cowichan Uplift =
Wrangellia Terrane Comox Samples
Jurassic Island Intrusions/ This Study
Fourth Lake Samples
West Coast Crystalline Complex Saltspring
North Samples
Jurassic Bonanza Group Island
South Samples Ruks, 2015
Triassic Karmutsen Formation Detrital Samples
Samples CO1 and CO2 Victoria
Devonian Sicker/
Matthews et al., [2017] Thrust Fault 100 km
Carboniferous Buttle Lake Groups 125°W, 48°N
Figure 1: Generalized geologic map of Vancouver Island (modified from Massey et al. [82, 83]). Inset map indicates relevant terranes of the
northern Cordillera, location of study area, and general locations for three subdivisions of the Alexander terrane: SEM: Saint Elias Mountain
region [17, 18]; SE: Alaska [65, 66, 71]; and BIM: Banks Island assemblage [65]. Abbreviations for displayed tectonic components on inset
map: YA: Yakutat; CH: Chugach; CPC: Coast Plutonic Complex; PE: Peninsular; WR: Wrangellia; sWR: southern Wrangellia; nWR:
northern Wrangellia; AX: Alexander. Sample locations for this study are denoted with squares. Colored circles are used to represent samples
from Ruks [14]. Yellow hexagon represents sample location for samples from Matthews et al. [42]. Dashed ellipses mark the area of uplifts.
divergent, with Sicker volcanism confined to the Late Devo- to E-MORB basalts [7]. As such, the Sicker arc has been
nian [7] and Station Creek volcanism poorly constrained as modeled as developing in an intraoceanic setting as a possible
Carboniferous [11], with Pennsylvanian plutonism [12]. An extension of the Skolai arc, but with no direct connection to
Early Mississippian U-Pb age from the base of the Station older terranes [15].
Creek Formation [13] and an extensive database of Devonian This study presents U-Pb ages and Hf isotope composi-
to Permian U-Pb and microfossil ages from the Sicker Group tions of detrital zircons from upper Paleozoic strata of south-
[14] now support consideration of the Sicker and Station ern Vancouver Island. These data are then used to refine
Creek as parts of a single, evolving arc system, such as pro- interpretations on the tectonic evolution of southern Wran-
posed by Beranek et al. [15], mainly based on data from gellia. First, the main grain populations will provide addi-
northern Wrangellia. tional information on the nature and duration of Paleozoic
The tectonic settings of northern and southern Wrangel- arc-related igneous activity. Second, minor grain populations
lia are also distinct. Northern Wrangellia lies adjacent to the can be used to test for the possible presence of pre-Late Devo-
older Alexander terrane and has been linked to it by Late nian basement to this part of Wrangellia. We analyzed nine
Devonian gabbro complexes [13] and Pennsylvanian plu- samples from southern Vancouver Island, including five
tonic suites that crosscut the terrane boundary [12, 15]. samples from the Fourth Lake Formation of the Buttle Lake
Southern Wrangellia is isolated from other terranes by faults, Group and four samples from the Comox Formation at the
by the Late Jurassic Coast Plutonic Complex, and by seaways, base of the Upper Cretaceous Nanaimo Group where it
which render its Paleozoic tectonic context highly enigmatic. directly overlies Paleozoic strata (Figure 1). Nanaimo Group
Moreover, the Sicker Group of Vancouver Island has been samples were analyzed primarily to gather additional infor-
modeled as a Late Devonian nascent arc succession that mation about the Paleozoic history of Vancouver Island via
developed on oceanic crust [4–7]. The lowest exposed rock primary and second-cycle zircons potentially preserved in
unit, the lower Duck Lake Formation, is a pile of N-MORB these rocks.
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Lithosphere 3
2. Geological Background Near the Duke River fault, the Station Creek Formation is
absent, and the Hasen Creek nonconformably overlies the
2.1. Alexander Terrane. The late Paleozoic arc complex at the Late Devonian Steele Creek gabbro complex [13]. The Hasen
base of northern Wrangellia has been linked to the adjacent Creek Formation represents postsubduction clastic sedimen-
Alexander terrane in terms of probable basement and tec- tation following the Alexander-Wrangellia collision.
tonic coevolution [13, 15]. This section begins with a descrip-
tion of that older crustal fragment. The Alexander terrane 2.3. Paleozoic Stratigraphy and History of Southern
extends over thousand kilometers from the St. Elias Moun- Wrangellia. The oldest rocks on Vancouver Island record
tains on the Yukon-Alaska border, through southeast Alaska the early evolution of the southern Wrangellia arc. Included
and along the north and central coast of British Columbia are Late Devonian–Carboniferous rocks of the Sicker Group
(Figure 1 inset). It is a composite of two terranes with differ- and Mississippian–Permian strata of the Buttle Lake Group
ent pre-Permian histories, the main Craig subterrane and the (Figures 1 and 3), which are exposed in the Buttle Lake and
much smaller Admiralty subterrane of Admiralty Island, Cowichan uplifts of central and southern Vancouver Island
southeast Alaska [15]. In northwestern Yukon/east-central (Figure 1) [4–6, 14, 20–22].
Alaska, Craig terrane volcanic, carbonate, and siliciclastic
strata are subdivided into the Donjek (Upper Cambrian- 2.3.1. Sicker Group. Primarily, volcanogenic strata of the
Middle Cambrian), Goatherd Mountain (Ordovician-Silu- Upper Devonian (to lowermost Mississippian?) Sicker Group
rian), and Icefield (Upper Silurian-Triassic) assemblages are exposed in the Cowichan and Buttle Lake uplifts
[16–18]. Detrital zircons from the Donjek assemblage show (Figure 1). Although stratigraphic nomenclature differs
peaks at ca. 477 Ma, reflecting local arc/back-arc volcanism, between the two uplifts, overall similarities in the sections
and ca. 565-760, 1000-1250, 1450, and 1650 Ma, from Balti- support correlations; therefore, they are combined in a single
can cratonal sources and Timanian volcanic arcs of the east- column in Figure 2.
ern Arctic region [17]. The lower Icefield assemblage is of The oldest exposed unit, the Duck Lake Formation, is
Late Silurian to Middle Devonian age and resembles Old restricted mainly to the southern part of the Cowichan uplift
Red Sandstone successions of the Caledonides [18]. Consis- [4, 5, 7] It is absent in the Buttle Lake uplift. It comprises
tent with this interpretation, main detrital zircon populations aphyric to plagioclase-phyric pillowed and massive basalts
are 390-490 Ma, with Precambrian Hf model ages that reflect with minor tuff and chert and local felsic bodies near its
progressive Silurian-Devonian orogenesis [15, 18]. The Craig top. Basalts of the lower Duck Lake Formation are tholeiites
subterrane evolved into a passive margin environment from of E-MORB affinities, whereas the upper part of the
Late Devonian to Early Pennsylvanian time [15]. formation comprises high-potassium calc-alkaline basalt
and basaltic andesite with dacite dikes and felsic tuffs [4–7].
2.2. Paleozoic Development of Northern Wrangellia and Its It represents the inception of arc volcanism in southern
Interactions with Alexander Terrane. The oldest rocks in Wrangellia. A dacite flow near the top of the formation in
northern Wrangellia belong to the Steele Creek gabbro com- its type area has yielded an LA-ICPMS age of 366:6 ± 0:7
plex (Figure 2) along Duke River fault, which separates it Ma [14].
from the Alexander terrane [13]. Southwest of the fault, the The Nitinat Formation conformably overlies the Duck
Mt. Constantine gabbro complex intrudes the Silurian- Lake Formation (Figure 2) [7]. It comprises augite-
Devonian Bullion Creek limestone within the Alexander ter- plagioclase phyric calc-alkaline basalt to basaltic andesite
rane [13] (Figure 2). Both complexes show N-MORB, nonarc breccias and flows, volcanic sandstone and siltstone, and
geochemistry, and their U-Pb ages agree within error at ca. cherty tuffs [4–6]. The Price Formation in the Buttle Lake
363.5 Ma [13]. These data support the hypothesis that north- uplift, which comprises plagioclase-augite phyric andesite
ern Wrangellia initiated as a Late Devonian arc that rifted flows and volcaniclastic deposits [23], is considered correla-
away from the Alexander terrane due to slab rollback [13]. tive with the Nitinat Formation [7].
The Station Creek Formation records development of that Uppermost Sicker Group units are the predominantly
arc-back-arc system, beginning in the Early Mississippian volcaniclastic McLaughlin Ridge and Myra formations, in
according to a ca. 353 Ma U-Pb age of a felsic tuff in the lower the southern Cowichan and Buttle Lake uplifts, respectively
part of the formation [13] (Figure 2). The Station Creek For- (Figure 2) [7, 23]. Igneous compositions range from mafic
mation comprises a lower unit of mafic flows overlain by to felsic, with intermediate compositions most common [4–
mafic to intermediate volcaniclastic strata [8] with arc type 7, 23]. Rhyolites are locally abundant, in some cases
as well as BABB and N-MORB and E-MORB geochemical associated with volcanogenic massive sulphide deposits.
signatures [13, 19]. A suite of Late Pennsylvanian plutons Myra Formation rhyolites have been dated as 366 ± 4 Ma
intrudes both the Station Creek Formation and the Icefield [24] and 361:5 ± 2:5 Ma (LA-ICPMS, Ruks [14]). McLaugh-
Ranges assemblage of Alexander terrane, and one of them, lin Ridge rhyolites have yielded LA-ICPMS ages that span
the Barnard Glacier pluton, cuts the terrane boundary [12, the Devonian-Mississippian boundary, between 363:0 ± 6:7
15]. The Barnard Glacier suite is interpreted as the product and 353:1 ± 3:4 Ma [14]. The coeval, cogenetic Saltspring
of melting due to slab breakoff after Alexander terrane litho- Intrusive Suite yielded ages between 360:7 ± 2:4 and
sphere entered the subduction zone of the Skolai arc [15] The 355:0 ± 1:5 Ma [7, 14, 25, 26]. Granitoids of the Saltspring
Station Creek Formation passes upwards into the Lower Intrusive Suite are transitional to calc-alkaline, primarily
Permian sedimentary Hasen Creek Formation (Figure 2). metaluminous, and yield trace element pattern characteristic
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4 Lithosphere
250 252
Lopingian Cowichan south, Cowichan north (Alberni)
Northern Wrangellia Northern Alexander
(a) Buttle Lake (b) Vancouver Is. NW (c) (d)
260 259
Guadalupian
LEGEND
PERMIAN
270 St. Mary Lake
St. Mary Lake
273
Limestone
280
F
Cisuralian
F F Hasen U
F Mt. Mark Sedimentary
290 Mt. Mark
Creek U
U
U Donjek
suite
299 Volcaniclastic
300
PENNSLVANIAN
Upper U U
BUTTLE LAKE GROUP
SKOLAI GROUP
F
307 F U U U
F U Rhyolite, felsic tuff
310 Middle UpperBarnard Barnard
U Glacier Glacier
315 U
Flower Ridge Station suite suite
Volcanic rocks
320 Lower
Sicker (arc tholeiite,
323 Creek
calc-alkalic)
Icefield assemblage
Upper Group
330
331 (undivided) Lower Mafic volcanic rocks,
MISSISSIPPIAN
Fourth Lk., U gabbro (N-MORB,
Middle UU
U Station E-MORB, BABB)
340 Thelwood U
Creek
Felsic plutons
347 U
350
Lower U U Mt.
U F
Constantine Diorite, granodiorite
359 complex
SICKER GP.
360 McLaughlin Ridge, U U-Pb age
U Steele
U Myra U U U
U Creek
DEVONIAN
Famennian u. Duck Lk.; Nitinat, PriceU UU F Fossil
370 Upper Duck Lake, Nitinat complex age
372 Lower Duck Lake
F
(ML Dev) Bullion
Frasnian Creek
380 limestone
383
Figure 2: Comparative stratigraphic columns from (a, b) southern Wrangellia on Vancouver Island, (c) northern Wrangellia, and (d)
northern Alexander terrane. Vancouver Island data are from Juras [23], Massey [4–6], Yorath et al. [7], and Ruks [14]. Northern
Wrangellia and Alexander data are from Israel et al. [13] and Beranek et al. [15]. Timescale is from Cohen et al. [84].
with rocks generated within an oceanic arc. However, there are enriched magmas during early phases of juvenile arc
minor occurrences of peraluminous granitoids, which are construction [4–6, 14, 31].
more commonly associated with continental arcs [14].
McLaughlin Ridge and Myra formations represent a partially 2.3.2. Buttle Lake Group. The Sicker Group is overlain by the
subaerial magmatic arc (Figure 2) [4–6]. mainly sedimentary lower Buttle Lake Group, including the
The presence of volcanogenic massive sulphide occur- Fourth Lake Formation in the southern Cowichan uplift
rences associated with small rhyolitic centers is consistent with and the Thelwood and Flower Ridge formations in the Buttle
the extensional submarine arc to back-arc environments that Lake uplift (Figures 1–3) [7, 23].
host modern seabed massive sulphide deposits [27]. A unique The Thelwood Formation includes fine-grained, thin-
occurrence of higher radiogenic lead within the lower accu- bedded siliceous and tuffaceous strata and penecontempora-
mulations of VMS deposits in the Buttle Lake uplift indicates neous mafic sills. It is overlain by amygdaloidal plagioclase-
a contribution of lead from a more evolved source [28–30]. pyroxene phyric basalt lapilli tuff, breccia, and flows of the
Along with higher radiogenic lead levels in early VMS Flower Ridge Formation [23], which is correlated with
deposits and granitoids with peraluminous occurrences, a basalts in the Fourth Lake Formation [6].
third piece of evidence from early Wrangellia indicative of The Fourth Lake Formation, the target unit for this study
an evolved component are ƐNdðt=360Þ values of mafic to felsic (Figures 2 and 3), consists of a 100-200-meter-thick sequence
rocks of the Sicker Group yielding values from +5.9 to +4.5 of radiolarian ribbon chert that is overlain by cherty siltstone,
[14]. These values are less radiogenic than the depleted thin siltstone-argillite beds, and thinly bedded fine- to
mantle reservoir at 360 Ma of +9.2 and are interpreted to be medium-grained interlayered sandstone and mudstone
isotopically juvenile to intermediate. This led to the interpre- (Figures 4(a), 4(b), and 4(e)–4(g)). The Fourth Lake Forma-
tation that sediment with an evolved isotopic composition tion represents a marginal-basin assemblage that developed
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Lithosphere 5
66
Nanaimo Group
17AVI06
17AVI07A
17AVI07B
Cretaceous 17AVI09
145
Jurassic
Bonanza Volcanics/Island Intrusions
/West Coast Crystalline Complex
201
Quatsino and Parson Bay Fms
Triassic Karmutsen Flood Basalts
252
Permian
St. Mary’s Lake Fm
Mount Mark Fm
299
Pennsylvanian Buttle Lake
17AVI08 Group
323
17AVI05 Fourth Lake Fm
17AVI04
Mississippian 17AVI03
17AVI02
359
McLaughlin Ridge Fm
Sicker
Nitinat Fm Group
Duck Lake Fm
Devonian
419
Intermediate to
Sandstone
Felsic Volcanic rock
Limestone Mafic Volcanic rock
Interbedded fine-grained
Saltspring Intrusion
clastic rock
Chert Basalt
Figure 3: Generalized stratigraphic column and interpreted sample positions (based on maximum peak ages, illustrated for stratigraphic
context) for strata from Vancouver Island, B.C. (adapted from Massey and Friday, [20]). Stratigraphic nomenclature is from Massey [5].
Timescale is from Cohen et al. [84]. Stratigraphic column ages are displayed in millions of years.
with coeval VMS-type deposits in the back-arc of southern portion of the Cowichan uplift, within the Fourth Lake For-
Wrangellia [4–6, 14]. mation, yields three distinct populations with age ranges of
The Fourth Lake Formation has yielded Early to mid- 339-337 Ma, 322-309 Ma, and ca. 295 Ma, the latter of which
Tournaisian and Middle to Late Pennsylvanian conodonts is interpreted to represent the depositional age of the tuff [14].
[7] and one collection of Early Permian radiolaria [14]. Overlaying the Fourth Lake Formation is the Mount
Previous detrital zircon U-Pb ages from the Fourth Lake For- Mark Formation, a massive limestone unit rich in marine
mation yield grains that range in age from 355 Ma to 294 Ma fossils (Figures 2 and 3) [32]. The contact between the Mt.
with dominant peak ages of 320, 312, and 304 Ma (Figures 1 Mark and underlying strata is diachronous: it contains faunal
and 5) [14]. A heterolithic lapilli tuff from the northern assemblages of Middle Pennsylvanian and Early Permian age
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6 Lithosphere
(a)
(b)
(c) (d)
(e) (f) (g)
Figure 4: Select outcrop and hand sample photos from the (a, b, e–f) Fourth Lake Formation and the (c, d) Comox Formation. Further
sample descriptions are displayed in Table 1. (a) 17AVI02 hand sample displaying interbedding of chert and medium-grained sandstone.
(b) Outcrop of sample 17AVI05 consisting of chert and interbedded layers of silt to fine-grained sandstone. (c) Large pebble clast of
interbedded chert and sandstone, indicative of the Fourth Lake Formation, within the Comox Formation (sample 17AVI06). (d) Smaller
pebble clast of interbedded chert and sandstone, indicative of the Fourth Lake Formation, within the Comox Formation (sample
17AVI07A). (e) Outcrop photo from sample 17AVI03 displaying layering of chert and interbedded fine-grained sandstone. (f) Close-up
outcrop photo of sample 17AVI05, a layer of silt to fine sandstone at the centimeter scale. (g) Hand sample photo of sample 17AVI08,
centimeter scale beds of fine- to medium-grained sandstone interbedded with chert, with a distinctive black color.
[7], overlapping those in the Fourth Lake Formation 2.3.3. Mississippian-Early Permian Sicker Group Strata,
(Figures 2 and 3). Conformably overlying the Mount Mark Alberni (North Cowichan); Northwest Vancouver Island.
Formation is a succession of clastic and volcaniclastic rocks Extensive U-Pb dating of igneous rocks at the northern end
of the St. Mary’s Lake Formation (Figures 2 and 3), inter- of the Cowichan uplift near Port Alberni and on northwestern
preted to be Early Permian in age [6, 21]. Vancouver Island (Bedingfield uplift and Dragon property,
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Lithosphere 7
Location on VI
317
17AVI08
(n = 307) 49.07, -124.38
(8xVE)
331
17AVI05
(n = 109) (8xVE) 48.87, -124.07
336
17AVI04
(n = 313) 48.90, -124.18
(8xVE)
339
17AVI02
(n = 313) 49.00, -124.42
344
17AVI03
(n = 380) 48.90, -124.19
(8xVE)
Fourth Lake Formation 337
(n = 1422) (8xVE)
312
08TR019 (Ruks, 2015)
n = 61 49.17, -124.09
320 08TR017 (Ruks, 2015)
304
353 n = 26 49.17, -124.10
305
Ruks, (2015) North
(N = 11, n = 180)
358
Ruks, (2015) South
(N = 32, n = 565)
358
Ruks, (2015)
309
(N = 44, n = 832)
250 300 350 400 800 1200 1600 2000 2400 2800
Detrital zircon age (Ma)
Figure 5: Normalized age distribution diagram for detrital zircons from the Fourth Lake Formation and igneous zircon U-Pb ages from Ruks
[14] separated into two curves based on geographic location as expressed in Figure 1. Two detrital samples (08TR017 and 08TR019) from
Ruks [14] are separated from igneous samples in order to compare with detrital samples in this study. Main peaks are noted for each
sample in millions of years. Number of samples within Ruks [14] distributions denoted with “N,” number of analysis in each curve
denoted with “n.” Proportions of >400 Ma ages have been vertically exaggerated by a factor of eight relative to
8 Lithosphere
2.4. Mesozoic Rocks. Paleozoic rocks in northern and southern Vancouver Island (Figures 4(c) and 4(d)). The Comox
portions of Wrangellia are overlain by ~4-7 km of the Triassic Formation is interpreted to consist of fluvial sediments
Nikolai-Karmutsen flood basalts [1]. A younger, latest trapped by localized topographic highs during deposition,
Triassic-Jurassic arc is represented by stratified volcanic- as well as deltaic deposits containing large incised fluvial
sedimentary rocks of the Bonanza Group and associated channels [43, 44, 53, 57].
Island Intrusions in southern Wrangellia [21, 33–36]. Upper Previous samples analyzed by LA-ICPMS techniques
Cretaceous sediments of the Nanaimo Group accumulated from the Comox Formation (samples CO1 and CO2 of
on and adjacent to the older stratigraphic units of southern Matthews et al. [42]) from the east-central coast of
Wrangellia in a convergent margin basin west of the Coast Vancouver Island (Figure 1) yield peak ages of 154-153 Ma
Mountain Batholith [37–44]. and 94-91 Ma (Figure 6). Sample CO2 also yields 50 grains
that range in age from 204 Ma to 163 Ma, with a peak age
2.4.1. Karmutsen Formation. Paleozoic rocks on Vancouver of 167 Ma (Figure 6). A source from the central Coast Moun-
Island are overlain unconformably by ca. 3.5 km of ca. 230- tain Batholith is the preferred provenance interpretation by
225 Ma submarine flows and pillow basalts of the Karmutsen Matthews et al. [42] for zircon grains with ages from 150 to
Formation (Figures 1 and 3) [2, 24, 45]. Gabbroic rocks 80 Ma. This interpretation is consistent with previous studies
related to Karmutsen basalts yield U-Pb zircon ages of ca. indicating that Late Jurassic to mid-Cretaceous plutons of the
228-226 Ma from outcrops on Saltspring Island [26]. Coast Mountain Batholith contributed significant sediment
to the Nanaimo Group [39, 40, 58, 59].
2.4.2. Bonanza Group. The youngest widespread igneous
assemblage on Vancouver Island is latest Triassic-Jurassic 3. Methods
in age and consists of arc-type rocks of the Bonanza Group,
Island Intrusions, and West Coast Crystalline Complex Fieldwork focused primarily on collecting samples for geochro-
(WCC) (Figures 1 and 3) [33, 36]. The WCC contains gab- nologic analysis. The sampling strategy was targeted at precise
broic to dioritic plutonic rocks, migmatites, amphibolites, localities of the Fourth Lake Formation containing fine- to
and metasedimentary rocks and preserves the deepest section medium-grained sandstone fractions, guided by descriptions
of the Bonanza arc. Rocks of the WCC have yielded U-Pb zir- and coordinates of Massey [5, 6]. Recent forest clear cuts pro-
con ages ranging from ca.190 to 177 Ma [33, 35], a Rb-Sr age vided additional exposure of previously unknown outcrops of
of 151 Ma [33], and K-Ar ages of 172 and 163 Ma [33]. The the Fourth Lake Formation (Figure 4(b)). Isolating millimeter
midcrustal section is preserved in batholiths and felsic intru- to centimeter thick sandstone layers within the Fourth Lake
sions of the Island Intrusive Suite (Figure 1) [35, 36], which Formation proved difficult considering the interbedded nature
have yielded U-Pb zircon crystallization ages from ca.175 to of these samples, which lead to the collection and processing of
168 Ma, 40Ar-39Ar cooling ages of ca.176 and 166 Ma, and a entire chunks of interbedded sandstone, mudstone, and/or
K-Ar age range of 181-152 Ma [46, 47]. The Bonanza Group chert (Figures 4(a) and 4(e)–4(g)). During collection of the
consists of ~2,500 meters of lava flows, pyroclastic flows, thin Fourth Lake Formation, in the southern Cowichan uplift, field
interbedded sedimentary units, and minor low-grade meta- relations of the Comox Formation unconformably overlaying
morphic rock that are interpreted to be the volcanic equiva- the Fourth Lake Formation and observations of clasts resem-
lents of the Island Intrusions (Figure 1) [34, 36, 48–50]. bling the Fourth Lake Formation prompted sample collection
These rocks have yielded a U-Pb zircon age range from ca. (Figures 4(c) and 4(d)). These observations lead to the hypoth-
202 Ma to 165 Ma [35, 51]. esis that the Comox Formation was likely to yield detrital zir-
Whole-rock geochemical data from volcanic rocks of the con grains that would provide insights into the Paleozoic and
Bonanza Group indicate that melts were primarily mantle Early Mesozoic evolution of southern Wrangellia.
derived, but that moderately intermediate to juvenile geo- Zircon extraction was performed at the Arizona Laser-
chemical signature record varying contributions from older Chron Center (http://www.laserchron.org) using methods
rocks on Vancouver Island (e.g., Sicker Group, Buttle Lake described by Gehrels et al. [60], Gehrels and Pecha, [61],
Group, and Karmutsen flood basalts) [36]. and Pullen et al. [62]. Primary steps included crushing/pul-
The equivalent latest Triassic-Jurassic arc sequence in verizing, usage of a Wilfley table, Frantz magnetic separator,
southern Alaska is referred to as the Talkeetna arc system [52]. and heavy liquids. Grains from each sample were poured in a
1-inch epoxy mount alongside fragments of U-Pb zircon
2.4.3. Nanaimo Group. Paleozoic through Jurassic rocks on standards (FC-1, SL2, and R33) and Hf zircon standards
Vancouver Island are overlain unconformably by ca. 5 km (Mud Tank, Temora-2, FC-1, 91500, Plesovice, R33, and
of Upper Cretaceous nonmarine to deep-marine basinal SL2). Mounts were polished with fine sandpaper and finished
strata of the Nanaimo Group (Figures 1 and 3) [38–42, 44, with a 1 μm diamond polish. All sample mounts were imaged
53]. The basal unit of the Nanaimo Group is the Comox For- using cathodoluminesence (CL) and backscatter electron
mation, which contains various macrofossil assemblages that (BSE) methods. Samples were cleaned with a 2% HNO3
indicate a Turonian to Coniacian depositional age [54–57]. A and 1% HCL solution prior to isotopic analysis. CL and
varying thickness of 0-350 m is reported for the Comox For- BSE images were utilized to select analytical points, avoiding
mation, which consists primarily of poorly bedded pebble to complex internal structures and nonzircon grains.
boulder conglomerate. Clasts are angular to rounded, and U-Pb analyses were conducted by laser ablation induc-
compositions indicate derivation from older strata on tively coupled plasma mass spectrometry (LA-ICPMS) using
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Lithosphere 9
20
Fourth Lake Formation
DM (n = 1422)
15
10
5
Epsilon Hf
337 Ma Crustal evolution
CHUR
0
–5
ion
–10 olut
a l ev
ust
Cr
–15
(x5 V
VE)
–20
275 300 325 350 375 400 800 1200 1600 2000 2400 2800
Detrital zircon age (Ma)
Ruks, (2015)
17AVI03
Nd converted
17AVI08 17AVI04
17AVI05 17AVI02
Figure 6: U-Pb and ƐHf ðtÞ values from the Fourth Lake Formation. Converted ƐNDðtÞ values from Ruks [14] are displayed as triangles.
Lower curve is a cumulative probability curve for the Fourth Lake Formation samples from this study. Ages > 400 Ma are vertically
exaggerated by a factor of five relative to 12 units below DM are -19. Six zircon grains older than 1000 Ma yield intermediate
considered evolved (following Bahlburg et al. [64]). to evolved ƐHf ðtÞ values ranging from +6 to -7 (Figure 6).
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10 Lithosphere
Table 1: Sample descriptions, peak ages, # of analyses done, and locations from the Fourth Lake Formation and Comox Formation on
southern Vancouver Island.
# of U-Pb # of Hf
Sample Unit Rock type Prominent U − Pb age peaks < 400 Ma Lat/long
analysis analysis
Fourth Lake Interbedded chert and fine- 49.006,
17AVI02 339 Ma 313 36
Formation grained sandstone -124.42
Fourth Lake Interbedded chert and fine- 48.906,
17AVI03 344 Ma 380 58
Formation grained sandstone -124.1937
Fourth Lake Cobbles with interbedded 48.908,
17AVI04 336 Ma 313 36
Formation mudstone and sandstone -124.1937
Fourth Lake Sandstone-siltstone argillite 48.873,
17AVI05 331 Ma 109 32
Formation interbedded with chert -124.072
Fourth Lake Interbedded chert and fine- to 49.0672,
17AVI08 317 Ma 307 41
Formation coarse-grained sandstone -124.3869
Comox 48.873,
17AVI06 Pebble conglomerate 201 Ma, 342 Ma 118 26
Formation -124.077
Comox 49.044,
17AVI07A Pebble to cobble conglomerate 166 Ma, 194 Ma 314 33
Formation -124.431
Comox 49.043,
17AVI07B Pebble to cobble conglomerate 167 Ma, 197 Ma 314 39
Formation -124.426
Comox 49.2067,
17AVI09 Pebble to cobble conglomerate 87 Ma, 152 Ma, 165 Ma, 356 Ma 312 142
Formation -124.035
Sample 17AVI04, an interbedded mudstone and sand- of +17 to +9. Grains older than 371 Ma comprise 3.4% of the
stone cobble collected near sample 17AVI03, yields 313 U- total grains analyzed. ƐHf ðtÞ values of these older grains
Pb ages with a dominant range of 364-316 Ma and a peak range from intermediate to highly evolved, with a range of
age of 336 Ma (Figure 5). This sample yields four older grains +5 to -20 (Figure 6).
with ages of 2802, 941, 492, and 374 Ma (Figure 5). Grains
within the main age group yield juvenile to intermediate Ɛ
4.2. Comox Formation. Sample 17AVI06 was collected near
Hf ðtÞ values of +15 to +6. Older grains, from oldest to youn- the unconformable contact between the Fourth Lake and
gest, yield ƐHf ðtÞ values of -1, 0, +4, and +15 (Figure 6). Comox Formations and contains large pebble-sized clasts of
Sample 17AVI05 (see Figure 4 caption and Table 1 for the Fourth Lake Formation within a medium-grained sandy
sample details) yields 109 U-Pb ages with a dominant group matrix (Figure 4(c)). Processing of entire conglomerate
of 359-311 Ma and a peak age of 331 Ma (Figure 5). Two chunks yielded 118 detrital zircon U-Pb ages with prominent
grains older than 359 Ma yield ages of 2749 Ma and peak ages at 341 Ma and 202 Ma and subordinate peak ages
1612 Ma (Figure 5). ƐHf ðtÞ values for zircon grains from of 196 and 159 Ma (Figure 7). Single-grain ages are 263, 223,
the main cluster yield primarily juvenile values ranging from 128, and 87 Ma. Grains within the main age groups yield juve-
+15 to +5. The two older grains yield ƐHf ðtÞ values of +7 and nile ƐHf ðtÞ values ranging from +15 to +6 (Figures 8 and 9).
+3, respectively (Figure 6). Three Hf analyses were conducted on grains within the subor-
Sample 17AVI08 was collected furthest to the north in dinate younger age group and yield ƐHf ðtÞ values ranging
the southern Cowichan uplift and consisted of interbedded from +13 to +9. Two single grains with ages of 263 Ma and
chert and fine-grained sandstone with a distinctive black 87 Ma yield ƐHf ðtÞ values of +10 and +13, respectively
color (Figure 4(g)). This sample yields 307 U-Pb ages with (Figure 9).
a prominent group of 361-281 Ma and a peak age of Sample 17AVI07A, exposed due to recent clear cuts in
317 Ma. There are three single-grain ages of 1025, 273, and the southern Cowichan uplift, had relatively smaller pebble-
270 Ma (Figure 5). Hf isotope results from 41 zircon crystals sized clasts of the Fourth Lake Formation within a finer
of 361-281 Ma yield juvenile to intermediate ƐHf ðtÞ values of sandy matrix (Figure 4(d)). This sample yielded 314 U-Pb
+17 to +2. The single older grain of 1025 Ma yields a ƐHf ðtÞ ages that belong to two different groups. The older group
value of +5 (Figure 6). ranges from 217 to 184 Ma, with a peak age of 194 Ma,
As shown in Figure 5, our five samples from the Fourth whereas the younger group yields an age range of 180-
Lake Formation yield peak ages of 344, 339, 336, 331, and 159 Ma, with a peak age of 166 Ma (Figure 7). Two single
317 Ma, that young northward, and produce a combined grains yielded ages of 363 Ma and 360 Ma. ƐHf ðtÞ values from
peak age of 337 Ma. The Hf isotopic compositions of these 33 zircon grains of 217-159 Ma range from +13 to +5,
Paleozoic zircon grains record derivation from juvenile to whereas the two older grains observed in this sample yield
intermediate signatures, with most ƐHf ðtÞ values in the range ƐHf ðtÞ values of +13 and +10 (Figures 8 and 9).
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by guestLithosphere 11
94 Matthews et al., 2017
153 167 CO2 (n = 207)
91 Matthews et al., 2017
154 CO1 (n = 248)
367 (10x VE) 07JBM06
155 (n = 460)
196
(10x VE) 17AVI09
152
87 (n = 312)
165
356
167 197 17AVI07B
(n = 314)
166 17AVI07A
(n = 314)
194
341 17AVI06
202
(n = 118)
167 (10x VE) 17AVI Comox
195 (n = 1055)
86 152 341
166 (10x VE) Comox Formation
91 153 195 (n = 1970)
365
0 50 100 150 200 250 300 350 400 450 500 550 600 .9 1.2 1.5 1.8 2.1 2.4 2.7 3.0
Detrital zircon age (Ma)
Figure 7: Normalized age distribution diagram for detrital zircons from the Comox Formation compared to Comox Formation samples from
Matthews et al. [42] (CO1 and CO2). Main peaks are noted for each sample in millions of years. Proportions of >600 Ma grains have been
vertically exaggerated by a factor of ten relative to12 Lithosphere
20
DM
15
10
5
Epsilon Hf
CHUR
0
n
evolutio
Crustal
–5
–10
–15
–20
167
Comox Formation
(n = 1055)
195
86 152
341
337 Fourth Lake Formation
(n = 1422)
0 100 200 300 400 500 600
Detrital zircon age (Ma)
Fourth Lake Formation
Comox Formation
Figure 8: U-Pb and ƐHf ðtÞ values from the Fourth Lake Formation and the Comox Formation analyzed in this study. Lower curves are
cumulative normalized probability curves for each formation. Upper plot shows ƐHf ðtÞ values for all samples analyzed. Reference lines on
the Hf plot are as follows: DM—depleted mantle, calculated using 176 Hf /177 Hf 0 = 0:283225 and 176 Lu/177 Hf 0 = 0:038512 [85];
CHUR—chondritic uniform reservoir, calculated using 176 Hf /177 Hf = 0:282785 and 176 Lu/177 Hf = 0:0336 [87]. Black arrows show
interpreted crustal evolution trajectories assuming present-day 176 Lu/177 Hf = 0:0093 [64, 85, 88].
1134, 1072, 1040, and 1037 Ma (Figure 7). Younger grains tion (five from this study and two from Ruks [14]) show con-
that are less than 110 Ma yield ages of 108 and 87, with two siderable overlap with the igneous U-Pb ages from rocks of
ages at 81 Ma (Figure 7). Hf isotope data are not available the Sicker Group and Buttle Lake Group. Additionally, detri-
from this sample. tal zircon peak ages and cumulative peak ages of igneous zir-
Collectively, Comox samples yield peak ages of 365, 195, con ages are apparently young northward. Maximum peak
166, 153, and 91 Ma (Figure 7). Most ƐHf ðtÞ values from ages in the range of ca. 360-340 Ma are associated with Sicker
these grains range from +15 to +5 (Figures 8 and 9). Group magmatism, predominately observed in the Cowi-
chan uplift, whereas 320-300 Ma peak ages are sourced from
5. Provenance of the Fourth Lake Formation Buttle Lake Group igneous rocks commonly found in the
Bedingfield uplift and Dragon property to the north
5.1. Paleozoic Zircons. As shown in Figure 5, most of the (Figure 1). This younging trend is most likely due to the
U-Pb ages of detrital zircons from the Fourth Lake Forma- progressive rifting of the arc shifting magmatic centers
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by guestLithosphere 13
20
DM
15
10
5
n
evolutio
Crustal
Epsilon Hf
CHUR
0
–5 166
195
–10
91 153 365
Comox Formation
n = 1970
–15
Central Coast Mountain Batholith
n = 3015
Southern Coast Mountain Batholith
n = 2822
0 50 100 150 200 250 300 350 400
Age (Ma)
Central CMB Magmatic flux curve
17AVI09 Age-distribution
17AVI07B Magmatic flux curve
17AVI07A Age-distribution
17AVI06
Figure 9: U-Pb and ƐHf ðtÞ values from the Comox Formation and central Coast Mountain Batholith ƐHf ðtÞ values are from Cecil et al. [63].
Filled grey curve is the cumulative age distribution for the Comox Formation, which includes samples from Matthews et al. [42]. The pink
age-distribution curve for the Central Coast Mountains is from Gehrels et al. [68] and Cecil et al. [63]. Filled purple age-distribution curve
for the Southern Coast Mountains is from Cecil et al. [89]. Dashed magmatic flux curves for the Central Coast Mountains [68] and
Southern Coast Mountain Batholith [89] are represented with dashed lines. Reference lines on the Hf plot are as follows: DM—depleted
mantle, calculated using 176 Hf /177 Hf 0 = 0:283225 and 176 Lu/177 Hf 0 = 0:038512 [86]; CHUR—chondritic uniform reservoir, calculated
using 176 Hf /177 Hf = 0:282785 and 176 Lu/177 Hf = 0:0336 [87]. Black arrows show interpreted crustal evolution trajectories assuming
present-day 176 Lu/177 Hf = 0:0093 [64, 85, 88].
northward along with northerly propagation of depositional of Sicker Group and Buttle Lake Group aged zircons, these
centers. ages must have been derived from igneous rocks of southern
One of the main differences between Paleozoic igneous Wrangellia. This suggests that southern Wrangellia was highly
versus Paleozoic detrital records of southern Wrangellia is active throughout the Carboniferous, contradicting the previ-
the abundance of detrital zircon ages within the 334-314 Ma ously interpreted magmatic gap of 334–314 Ma. A proposed
gap observed in igneous ages reported by Ruks [14]. Three location for these igneous bodies would be somewhere
samples contain dominant peak ages of 331 Ma (sample between the Buttle Lake and Cowichan uplifts based on the
17AVI05), 320 Ma (08TR017, from Ruks [14]), and 317 Ma perceived trajectory of arc migration to the north as proposed
(17AVI08) (Figure 5), and all of these samples were collected above (Figure 1). However, these rocks are most likely covered
further north in the Cowichan uplift and near the city of beneath the widespread Karmutsen basalts (Figure 1).
Nanaimo (Figure 1). Considering the low-energy depositional The Hf isotope data acquired in this study and the Nd
environment of the Fourth Lake Formation and the presence isotope data from Ruks [14] can also be used to compare
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by guest14 Lithosphere
the detrital and igneous records. Converted ƐNdðtÞ to ƐHf ðtÞ in these strata were derived directly from the Sicker and But-
values from Ruks [14] yield values for ca. 360 Ma samples tle Lake Groups or recycled from the Fourth Lake Formation.
ranging from +11 to +9, +11 for a single rock with an age The latter interpretation is preferred for detrital zircons ana-
of ca. 317 Ma, and +12 to +8.7 for rocks with ages of ca. lyzed in this study based on the occurrence of clasts derived
300 Ma (Figure 6). As shown on Figure 6, ƐHf ðtÞ values for from the Fourth Lake Formation observed in these samples
detrital zircons from the Fourth Lake Formation yield a sim- (Figures 4(c) and 4(d)). Sample 07JBM06 yields the oldest
ilar age range of +17 to +10 (Figures 5 and 6). In addition, Paleozoic peak age at 367 Ma, which is consistent with deri-
both igneous and detrital zircons are interpreted to show a vation from nearby rocks of the Sicker Group and the Salt-
decrease in the abundance of intermediate ƐHf ðtÞ values spring Intrusive Suite, which is of similar age and
representative of first cycle deposition of zircons into the
from 325 Na to 290 Ma (Figure 6). The occurrence of similar
Comox Formation (Figures 1 and 7). Conglomeratic samples
U-Pb ages and Hf isotope values in both data sets supports
of the Comox Formation located southeast of Vancouver
the interpretation that strata of the Fourth Lake Formation
Island in the San Juan Islands similarly yield several Sicker
were sourced from the Sicker Group and igneous rocks of
Group aged detrital zircons and a minor amount of 400-
the Buttle Lake Group (Figure 6).
500 Ma ages [53]. As shown in Figure 8, Paleozoic zircons
from the Comox Formation yield ƐHf ðtÞ values of +15 to
5.2. Neoproterozoic to Paleozoic Zircons. The Fourth Lake
Formation contains 49 grains (3.4% of the total grains ana- +5, which directly overlap the juvenile values of strata of the
lyzed) with ages between 2802 Ma and 442 Ma and with cor- Fourth Lake Formation and igneous rocks of the Sicker arc.
responding ƐHf ðtÞ values ranging from +13 to -20 (Figure 6). The similarity of U-Pb ages, ƐHf ðtÞ values, and previous
These grains, plus the occurrence of some 360-300 Ma grains detrital zircon interpretations of the basal Comox Formation
with more evolved ƐHf ðtÞ signatures, suggest that sources for in the San Juan Islands suggests that Paleozoic zircons were
derived in large part from Paleozoic rocks of the Sicker arc
strata of the Fourth Lake Formation also included pre-mid- and overlying Fourth Lake Formation.
Paleozoic rocks or sediments with pre-mid-Paleozoic grains.
The lack of evidence for such components of >380 Ma in 6.2. Early to Late Triassic Zircons. The Comox Formation
samples from the Sicker Group and Buttle Lake Group [14] yields 12 single-grain ages ranging from 277 to 223 Ma
raises the possibility that some grains in the Fourth Lake For- (Figure 7) with corresponding ƐHf ðtÞ values ranging from
mation were sourced from rocks that are not currently part of +12 to +3 (Figure 8). The source of these grains 277-
southern Wrangellia. 223 Ma is uncertain given the lull in magmatism in southern
One potential source area for these older grains is the Wrangellia during this time period. The only magmatism
Alexander terrane, given that previous workers have sug- recorded in southern Wrangellia during this age range is
gested that the Wrangellia and Alexander terranes were near, the eruption of the Karmutsen flood basalts, which occurred
or even built partially on, each other during late Paleozoic from 232 to 225 Ma [2], but zircon is rare in basaltic rocks.
time [12, 13, 15], and that some portions of the Alexander
terrane contain older and more evolved crustal components 6.3. Late Triassic to Middle Jurassic Zircons. The abundance
[17, 18, 65]. Three distinct portions of the Alexander terrane of 217-160 Ma detrital zircons (Figure 7) indicates that strata
have yielded U-Pb ages and Hf isotope data, including the of the Comox Formation may have been shed from latest
Saint Elias Mountain region [17, 18], Southeast Alaska [63, Triassic-Jurassic igneous rocks of the West Coast Crystalline
66], and the Banks Island assemblage [65] (Figure 1). Of Complex, Island Intrusive Suite, and/or Bonanza Group,
these regions, connections with the Banks Island assemblage which are widespread on Vancouver Island (Figure 1). These
and the Saint Elias Mountain region are considered most rocks yield U-Pb ages of 202-165 Ma [33, 35, 51] and juvenile
likely given the geologic, geochronologic, isotopic, and paleo- to intermediate isotopic signatures [36], which are quite
magnetic evidence that Vancouver Island was within prox- similar to Hf isotope values from the Comox Formation
imity of northern Wrangellia, thusly within proximity of (Figures 8 and 9).
the Alexander terrane, prior to Early Cretaceous time [65–
67]. Although Precambian–early Paleozoic grains within 6.4. Late Jurassic to Cretaceous Zircons. Previous researchers
Wrangellia cannot be traced to one individual entity of the have suggested that Late Jurassic and Cretaceous detrital zir-
Alexander terrane due to the relatively low percentage of cons from the Comox Formation were shed from the Coast
these grains in our samples, they do fit well within detrital Mountain Batholith [39, 40, 42–44, 58, 59]. More specifically,
zircon populations observed in the Alexander terrane. Matthews et al. [42] suggested derivation from the central
CMB because of consistencies in timing and volume of mag-
6. Provenance of the Comox Formation matic flux events in the central CMB (160-140 Ma, 120-
78 Ma, and 55-48 Ma; Gehrels et al. [68]) correlating to peak
6.1. Paleozoic Zircons. The Comox Formation yields Paleo- ages. In contrast, Huang [43] has suggested that Nanaimo
zoic peak ages of 367, 356, and 341 Ma (Figure 7). With the detrital zircons were shed from the southern CMB given that
inclusion of Comox samples from Matthews et al. [42], a the observed ages are more similar.
cumulative Paleozoic peak age for the Comox Formation is A comparison of our age and Hf isotopic data with the
364 Ma with a range of ages encompassing rocks of the Sicker information available from the southern and central CMB
and Buttle Lake Groups (Figure 7). This suggests that zircons is shown in Figure 9. We conclude that the age distributions
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by guestLithosphere 15
from strata of the Comox Formation are not an excellent fit isotopes [29, 30], depletion of radiogenic Nd in felsic volcanic
with either portion of the CMB, especially the lower and intrusive rocks of the Cowichan uplift, and the occur-
proportion of 130-100 Ma ages and higher proportion of rence of peraluminous granitoids within the Saltspring Intru-
~170-160 Ma ages in Comox samples. ƐHf ðtÞ values of sive Suite [14].
Comox zircons are also significantly more juvenile than zir- There are two scenarios in which intermediate early arc
cons from the central CMB (Figure 9). We accordingly agree magmas were generated in southern Wrangellia. (1)
with previous workers that the Late Jurassic and Cretaceous Subduction-related magmatism was contaminated with sed-
zircons in the Comox Formation were likely shed from the iment derived from the nearby Alexander terrane from the
CMB, but conclude that the available data do not support upper crust and was consumed during subduction-related
firm connections with either the southern or northern por- arc magmatism. (2) In the scenario we prefer, a transitional
tions of the batholith. portion of the Alexander terrane crust underlays the nascent
arc of southern Wrangellia and was since buried due to
7. Discussion extensive arc construction. This would provide the older,
more evolved crust required to produce rocks with interme-
7.1. Tectonic Implications for Paleozoic Wrangellia. Previous diate to evolved geochemical signatures observed in the ear-
workers have suggested that Wrangellia originated at the liest phase of the Sicker arc. The presence of Alexander-like
margin of the Paleo-Pacific and Paleo-Arctic realms, devel- older crust would also provide a source of pre-Devonian zir-
oping within close proximity to the Alexander terrane from cons deposited into the Fourth Lake Formation throughout
Devonian through Permian time [13, 15–18, 69–77]. the Carboniferous.
Previously reported connections between the Alexander
and Wrangellia terranes [13–15, 74, 77, 78] combined with 7.1.2. Mississippian (354-323 Ma; Figure 10(b)). In southern
our new data lead to the following integrated tectonic model Wrangellia, slab rollback rifted the preexisting Sicker arc
for Paleozoic development of northern and southern Wran- and formed a back-arc spreading center in its place, focused
gellia (Figures 10(a)–10(d)). in the Alberni area between the Buttle Lake and south Cow-
ichan areas [14]. Within the new back-arc rift region, local
7.1.1. Late Devonian to Earliest Mississippian (370-355 Ma; bimodal magmatism accompanied the emplacement of
Figure 10(a)). According to Colpron and Nelson [74] and VMS-type deposits [14]. The earliest deposits of the Buttle
Nelson et al. [77], the Alexander terrane was extruded west- Lake Group, ribbon cherts associated with the Fourth Lake
ward out of the Paleo-Arctic into the NE Paleo-Pacific realm. Formation, are deposited in the new back-arc basin and on
An east-dipping subduction zone was established at this time portions of the rifted fragments of the Sicker arc [14]. A mod-
outboard of western Laurentia, with the overriding plate ern analog to Paleozoic southern Wrangellia’s rifted arc-type
bearing the Alexander terrane [13]. This subduction zone setting is the Taupo-Tonga-Kermadec arc system in the
was proposed to extend into an intraoceanic setting beyond southwest Pacific Ocean [15, 79]. Maximum arc activity in
the terrane boundary of Alexander, where it gave rise to the southern Wrangellia occurred in the Middle to Late Missis-
Skolai arc of northern Wrangellia and Sicker arc of southern sippian, as shown by a cumulative detrital zircon age peak
Wrangellia [13, 15]. of 337 Ma. Igneous rocks with these ages have not been pre-
In northern Wrangellia, initiation of back-arc rifting is viously recognized in Wrangellia. On Vancouver Island, they
inferred from the presence of coeval nonarc gabbros in are probably covered by extensive younger formations. The
Wrangellia (Steele Creek complex) and in the Craig subter- southern Wrangellia arc was highly active throughout the
rane of the Alexander terrane (Constantine complex and Carboniferous base on the abundance of detrital zircons
related gabbro dikes) yielding ages of ca. 363 Ma [13]. These within this age range. Corresponding Hf isotope data are
gabbros suggest a close connection between the Alexander highly juvenile compared to earlier Wrangellian sources.
terrane and northern Wrangellia. The shift to exceptionally juvenile Hf values in the Sicker
Although such gabbros are not recognized in southern arc may reflect the progressive rifting of the arc northward
Wrangellia’s Sicker arc, Late Devonian nonarc basalt in the coupled with a process similarly invoked to explain juvenile
lower Duck Lake Formation may similarly represent the εNd signatures in volcaniclastic rocks of the Klinkit Group
onset of back-arc rifting during subduction-initiated arc (late Paleozoic) that were deposited on older pericratonic
development of characteristic IAT, L-IAT, and minor E- strata of the Yukon-Tanana composite terrane in northern
MORB-type rocks of the Sicker Group [15, 20–22]. The British Columbia and Yukon [80]. They proposed that the
Sicker Group preserves the beginning of the southern Wran- voluminous asthenospheric melts that fed Klinkit volcanism
gellia arc, ca. 370-365 Ma, on top of oceanic to transitional were rapidly and repeatedly emplaced along coated conduits,
crust bordering the Alexander terrane. Relatively intermedi- insulated from sources of crustal contamination [80].
ate Hf isotope values from Late Devonian to Early Mississip- In contrast with the arc-rift to back-arc environment rep-
pian detrital zircons from the Fourth Lake Formation suggest resented by Mississippian Sicker Group volcanic rocks, in
that evolved older crust must have been present in order to northern Wrangellia, basalts and basaltic andesites of the Sta-
influence the chemistry of the early stages of the Sicker arc. tion Creek Formation were more likely the products of arc-
Further geochemical indicators for an evolved crustal com- axial magmatism. They range as old as ca. 352 Ma, as shown
ponent in early arc construction include VMS deposits in by a U-Pb age determination on a rhyolite near the exposed
the Myra Falls area yielding elevated levels of radiogenic Pb bottom of the section. At present, there is no other absolute
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by guest16 Lithosphere
Late Devonian - earliest Miss. Mississippian Late Miss. - Early Pennsylvanian Late Pennsylvanian - Early Permian
(370-355 Ma) N (354-323 Ma) (322-315 Ma) (314-280 Ma)
R
nW
R R BGP
nW nW
R CRP
nW LGP
SC
P
SG
sWR AX
MC AX BVMS DGB
R
AX sW R
AX sW
AVMS AXa
R
sW
R R R
sW sW sW
= Nascent nWR arc = Old spreading center = Fourth Lake and Thelwood Frm.
= Transient AX crust = Fourth Lake Formation
(a) (b) (c) (d)
Figure 10: Plate-tectonic reconstruction of Paleozoic Wrangellia during the (a) Late Devonian to earliest Mississippian (370-355 Ma), (b)
Mississippian (354-323 Ma), (c) Early Pennsylvanian (322-315 Ma), and (d) Late Pennsylvanian to Early Permian (314-280 Ma).
Abbreviations: AX: the Alexander terrane; AXa: Admiralty subterrane of Alexander terrane; nWR: northern Wrangellia; sWR: southern
Wrangellia; AVMS: Alberni VMS deposits; BVMS: Bedingfield uplift and Dragon property VMS deposits; BGP: Barnard Glacier porphyry;
CRP: Centennial Ridge pluton; LGP: Logan Glacier porphyry; DGB: Donjek Glacier Batholith; SGP: Steel Glacier pluton.
age constraint on Station Creek volcanism. It may well have within both terranes and across their boundary [15]. The
persisted through the Middle Mississippian interval docu- Early Permian Hasen Creek Formation was deposited as a
mented in this study. clastic wedge atop the now-defunct Station Creek arc; Hasen
Creek conglomerates were also deposited over the exhumed
7.1.3. Early Pennsylvanian (322-315 Ma; Figure 10(c)). Dur- gabbros [13].
ing this time interval, Sicker Group magmatic centers con- Arc-related magmatism continued in southern Wrangel-
tinue to migrate towards the northwest (modern lia, with a northwesterly migration of locally bimodal mag-
coordinates) [14]. Deposition of low-energy marine sedi- matism into the Bedingfield uplift and at the Dragon
ments of the Fourth Lake and Thelwood Formation contin- property (northwestern Vancouver Island; Figure 1), where
ued. Northward-younging detrital zircon peak ages in the 312-300 Ma volcanic rocks and associated VMS deposits are
Fourth Lake Formation also reflect the direction of magmatic recognized [14]. The volcanic succession is overlain by
and depositional migration. ~290 Ma tuffs, the final volcanic deposits recorded in south-
Volcanism of the Skolai arc continued through this time ern Wrangellia [14]. Deposition of the Fourth Lake and Thel-
interval, preserved in arc-type rocks of the upper Station wood formations continued in a back-arc setting. The lack of
Creek Formation [15]. During this time, the arc system in deformation and postcollisional plutons suggest that south-
northern Wrangellia is thought to have undergone a subduc- ern Wrangellia was largely unaffected by the collisional
tion reversal, with evidence in changing chemistry from the events of northern Wrangellia and the Alexander terrane.
lower to upper Station Creek igneous rocks [13, 15]. A rever- After collision, the subduction zone shifted behind the
sal in subduction is also inferred in southern Wrangellia, Alexander terrane, as observed in emplacement of the Don-
marked by the migration of volcanism from the Alberni area jek Glacier Batholith (286-284 Ma), the Steele Glacier pluton
in the Mississippian to northwestern Vancouver Island by ca. (291 Ma), and other granitoids with ages from 290 to 280 Ma
312 Ma [14]. This change in subduction broke along the in the Alexander terrane [15]. In Early Permian time
extinct Late Devonian spreading center and initiated the (~280 Ma), a fragment from the Alexander terrane, the
encroachment of the Alexander terrane towards northern Admiralty subterrane, clogged the subduction zone, which
Wrangellia [13, 15]. finally shuts off the arc magmatism within the Alexander ter-
rane [15, 80]. Limestones of the Mt. Mark Formation on
7.1.4. Late Pennsylvanian to Early Permian (314-280 Ma; Vancouver Island record the cessation of arc-related volca-
Figure 10(d)). Northern Wrangellia and Alexander collided, nism of southern Wrangellia. Fossiliferous Lower Permian
as the Alexander block entered the Wrangellia subduction limestones of the Pybus Formation overlap the suture
zone [15]. The collision led to the exhumation of basement between the Craig and Admiralty subterranes in southeast-
gabbros and stitched the northern portion of Wrangellia ern Alaska [81]. These coeval limestone formations in south-
and Alexander together, indicated by emplacement of the ern Wrangellia and Alexander terrane indicate that they
Barnard Glacier porphyry (ca. 307 Ma), Centennial Ridge shared a quiescent postcollisional tectonic regime and depo-
pluton (304 Ma), and the Logan Glacier porphyry (307 Ma) sitional environment.
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