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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U-Pb and Hf Analyses of Detrital Zircons from Paleozoic and Cretaceous Strata on Vancouver Island, British Columbia: Constraints on the Paleozoic ...
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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U-Pb and Hf Analyses of Detrital Zircons from Paleozoic and Cretaceous Strata on Vancouver Island, British Columbia: Constraints on the Paleozoic ...
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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U-Pb and Hf Analyses of Detrital Zircons from Paleozoic and Cretaceous Strata on Vancouver Island, British Columbia: Constraints on the Paleozoic ...
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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U-Pb and Hf Analyses of Detrital Zircons from Paleozoic and Cretaceous Strata on Vancouver Island, British Columbia: Constraints on the Paleozoic ...
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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U-Pb and Hf Analyses of Detrital Zircons from Paleozoic and Cretaceous Strata on Vancouver Island, British Columbia: Constraints on the Paleozoic ...
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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U-Pb and Hf Analyses of Detrital Zircons from Paleozoic and Cretaceous Strata on Vancouver Island, British Columbia: Constraints on the Paleozoic ...
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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U-Pb and Hf Analyses of Detrital Zircons from Paleozoic and Cretaceous Strata on Vancouver Island, British Columbia: Constraints on the Paleozoic ...
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
U-Pb and Hf Analyses of Detrital Zircons from Paleozoic and Cretaceous Strata on Vancouver Island, British Columbia: Constraints on the Paleozoic ...
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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U-Pb and Hf Analyses of Detrital Zircons from Paleozoic and Cretaceous Strata on Vancouver Island, British Columbia: Constraints on the Paleozoic ...
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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U-Pb and Hf Analyses of Detrital Zircons from Paleozoic and Cretaceous Strata on Vancouver Island, British Columbia: Constraints on the Paleozoic ...
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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Lithosphere                                                                                                                                                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 to
12                                                                                                                                                        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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Lithosphere                                                                                                                                             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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14                                                                                                                                              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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Lithosphere                                                                                                                                               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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16                                                                                                                                                                     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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