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

Wrangellia is one of the most outboard of the major northern Cordilleran terranes. It occupies two separate regions: south-central Alaska and southwestern Yukon (northern Wrangellia) and Haida Gwaii and Vancouver Island (southern Wrangellia) (Figure 1 inset). It was originally defined as a coherent terrane based on characteristic thick piles of Triassic flood basalts, the Nikolai Greenstone of northern Wrangellia and Karmutsen Formation of southern Wrangellia, that overlie Paleozoic arc-related sequences [1, 2]. Paleozoic sequences of southern Wrangellia comprise the Upper Devonian to Lower Permian Sicker and Buttle Lake Groups [3–7]. Those in northern Wrangellia comprise the Carboniferous to Lower Permian Skolai Group, including the Station Creek and Hasen Creek formations [8–13].

Until recently, the records of Paleozoic arc activity in southern versus northern Wrangellia were considered to be divergent, with Sicker volcanism confined to the Late Devonian [7] and Station Creek volcanism poorly constrained as Carboniferous [11], with Pennsylvanian plutonism [12]. An Early Mississippian U-Pb age from the base of the Station Creek Formation [13] and an extensive database of Devonian to Permian U-Pb and microfossil ages from the Sicker Group [14] now support consideration of the Sicker and Station Creek as parts of a single, evolving arc system, such as proposed by Beranek et al. [15], mainly based on data from northern Wrangellia.

The tectonic settings of northern and southern Wrangellia are also distinct. Northern Wrangellia lies adjacent to the older Alexander terrane and has been linked to it by Late Devonian gabbro complexes [13] and Pennsylvanian plutonic suites that crosscut the terrane boundary [12, 15]. Southern Wrangellia is isolated from other terranes by faults, by the Late Jurassic Coast Plutonic Complex, and by seaways, which render its Paleozoic tectonic context highly enigmatic. Moreover, the Sicker Group of Vancouver Island has been modeled as a Late Devonian nascent arc succession that developed on oceanic crust [4–7]. The lowest exposed rock unit, the lower Duck Lake Formation, is a pile of N-MORB to E-MORB basalts [7]. As such, the Sicker arc has been modeled as developing in an intraoceanic setting as a possible extension of the Skolai arc, but with no direct connection to older terranes [15].

This study presents U-Pb ages and Hf isotope compositions of detrital zircons from upper Paleozoic strata of southern Vancouver Island. These data are then used to refine interpretations on the tectonic evolution of southern Wrangellia. First, the main grain populations will provide additional information on the nature and duration of Paleozoic arc-related igneous activity. Second, minor grain populations can be used to test for the possible presence of pre-Late Devonian basement to this part of Wrangellia. We analyzed nine samples from southern Vancouver Island, including five samples from the Fourth Lake Formation of the Buttle Lake Group and four samples from the Comox Formation at the base of the Upper Cretaceous Nanaimo Group where it directly overlies Paleozoic strata (Figure 1). Nanaimo Group samples were analyzed primarily to gather additional information about the Paleozoic history of Vancouver Island via primary and second-cycle zircons potentially preserved in these rocks.

The late Paleozoic arc complex at the base of northern Wrangellia has been linked to the adjacent Alexander terrane in terms of probable basement and tectonic coevolution [13, 15]. This section begins with a description of that older crustal fragment. The Alexander terrane extends over thousand kilometers from the St. Elias Mountains on the Yukon-Alaska border, through southeast Alaska and along the north and central coast of British Columbia (Figure 1 inset). It is a composite of two terranes with different pre-Permian histories, the main Craig subterrane and the much smaller Admiralty subterrane of Admiralty Island, southeast Alaska [15]. In northwestern Yukon/east-central Alaska, Craig terrane volcanic, carbonate, and siliciclastic strata are subdivided into the Donjek (Upper Cambrian-Middle Cambrian), Goatherd Mountain (Ordovician-Silurian), and Icefield (Upper Silurian-Triassic) assemblages [16–18]. Detrital zircons from the Donjek assemblage show peaks at ca. 477 Ma, reflecting local arc/back-arc volcanism, and ca. 565-760, 1000-1250, 1450, and 1650 Ma, from Baltican cratonal sources and Timanian volcanic arcs of the eastern Arctic region [17]. The lower Icefield assemblage is of Late Silurian to Middle Devonian age and resembles Old Red Sandstone successions of the Caledonides [18]. Consistent with this interpretation, main detrital zircon populations are 390-490 Ma, with Precambrian Hf model ages that reflect progressive Silurian-Devonian orogenesis [15, 18]. The Craig subterrane evolved into a passive margin environment from Late Devonian to Early Pennsylvanian time [15].

The oldest rocks in northern Wrangellia belong to the Steele Creek gabbro complex (Figure 2) along Duke River fault, which separates it from the Alexander terrane [13]. Southwest of the fault, the Mt. Constantine gabbro complex intrudes the Silurian-Devonian Bullion Creek limestone within the Alexander terrane [13] (Figure 2). Both complexes show N-MORB, nonarc geochemistry, and their U-Pb ages agree within error at ca. 363.5 Ma [13]. These data support the hypothesis that northern Wrangellia initiated as a Late Devonian arc that rifted away from the Alexander terrane due to slab rollback [13]. The Station Creek Formation records development of that arc-back-arc system, beginning in the Early Mississippian according to a ca. 353 Ma U-Pb age of a felsic tuff in the lower part of the formation [13] (Figure 2). The Station Creek Formation comprises a lower unit of mafic flows overlain by mafic to intermediate volcaniclastic strata [8] with arc type as well as BABB and N-MORB and E-MORB geochemical signatures [13, 19]. A suite of Late Pennsylvanian plutons intrudes both the Station Creek Formation and the Icefield Ranges assemblage of Alexander terrane, and one of them, the Barnard Glacier pluton, cuts the terrane boundary [12, 15]. The Barnard Glacier suite is interpreted as the product of melting due to slab breakoff after Alexander terrane lithosphere entered the subduction zone of the Skolai arc [15] The Station Creek Formation passes upwards into the Lower Permian sedimentary Hasen Creek Formation (Figure 2). Near the Duke River fault, the Station Creek Formation is absent, and the Hasen Creek nonconformably overlies the Late Devonian Steele Creek gabbro complex [13]. The Hasen Creek Formation represents postsubduction clastic sedimentation following the Alexander-Wrangellia collision.

The oldest rocks on Vancouver Island record the early evolution of the southern Wrangellia arc. Included are Late Devonian–Carboniferous rocks of the Sicker Group and Mississippian–Permian strata of the Buttle Lake Group (Figures 1 and 3), which are exposed in the Buttle Lake and Cowichan uplifts of central and southern Vancouver Island (Figure 1) [4–6, 14, 20–22].

Primarily, volcanogenic strata of the Upper Devonian (to lowermost Mississippian?) Sicker Group are exposed in the Cowichan and Buttle Lake uplifts (Figure 1). Although stratigraphic nomenclature differs between the two uplifts, overall similarities in the sections support correlations; therefore, they are combined in a single column in Figure 2.

The oldest exposed unit, the Duck Lake Formation, is restricted mainly to the southern part of the Cowichan uplift [4, 5, 7] It is absent in the Buttle Lake uplift. It comprises aphyric to plagioclase-phyric pillowed and massive basalts with minor tuff and chert and local felsic bodies near its top. Basalts of the lower Duck Lake Formation are tholeiites of E-MORB affinities, whereas the upper part of the formation comprises high-potassium calc-alkaline basalt and basaltic andesite with dacite dikes and felsic tuffs [4–7]. It represents the inception of arc volcanism in southern Wrangellia. A dacite flow near the top of the formation in its type area has yielded an LA-ICPMS age of $366.6±0.7$ Ma [14].

The Nitinat Formation conformably overlies the Duck Lake Formation (Figure 2) [7]. It comprises augite-plagioclase phyric calc-alkaline basalt to basaltic andesite breccias and flows, volcanic sandstone and siltstone, and cherty tuffs [4–6]. The Price Formation in the Buttle Lake uplift, which comprises plagioclase-augite phyric andesite flows and volcaniclastic deposits [23], is considered correlative with the Nitinat Formation [7].

Uppermost Sicker Group units are the predominantly volcaniclastic McLaughlin Ridge and Myra formations, in the southern Cowichan and Buttle Lake uplifts, respectively (Figure 2) [7, 23]. Igneous compositions range from mafic to felsic, with intermediate compositions most common [4–7, 23]. Rhyolites are locally abundant, in some cases associated with volcanogenic massive sulphide deposits. Myra Formation rhyolites have been dated as $366±4$ Ma [24] and $361.5±2.5$ Ma (LA-ICPMS, Ruks [14]). McLaughlin Ridge rhyolites have yielded LA-ICPMS ages that span the Devonian-Mississippian boundary, between $363.0±6.7$ and $353.1±3.4$ Ma [14]. The coeval, cogenetic Saltspring Intrusive Suite yielded ages between $360.7±2.4$ and $355.0±1.5$ Ma [7, 14, 25, 26]. Granitoids of the Saltspring Intrusive Suite are transitional to calc-alkaline, primarily metaluminous, and yield trace element pattern characteristic with rocks generated within an oceanic arc. However, there are minor occurrences of peraluminous granitoids, which are more commonly associated with continental arcs [14]. McLaughlin Ridge and Myra formations represent a partially subaerial magmatic arc (Figure 2) [4–6].

The presence of volcanogenic massive sulphide occurrences associated with small rhyolitic centers is consistent with the extensional submarine arc to back-arc environments that host modern seabed massive sulphide deposits [27]. A unique occurrence of higher radiogenic lead within the lower accumulations of VMS deposits in the Buttle Lake uplift indicates a contribution of lead from a more evolved source [28–30].

Along with higher radiogenic lead levels in early VMS deposits and granitoids with peraluminous occurrences, a third piece of evidence from early Wrangellia indicative of an evolved component are $ƐNdt=360$ values of mafic to felsic rocks of the Sicker Group yielding values from +5.9 to +4.5 [14]. These values are less radiogenic than the depleted mantle reservoir at 360 Ma of +9.2 and are interpreted to be isotopically juvenile to intermediate. This led to the interpretation that sediment with an evolved isotopic composition enriched magmas during early phases of juvenile arc construction [4–6, 14, 31].

The Sicker Group is overlain by the mainly sedimentary lower Buttle Lake Group, including the Fourth Lake Formation in the southern Cowichan uplift and the Thelwood and Flower Ridge formations in the Buttle Lake uplift (Figures 1–3) [7, 23].

The Thelwood Formation includes fine-grained, thin-bedded siliceous and tuffaceous strata and penecontemporaneous mafic sills. It is overlain by amygdaloidal plagioclase-pyroxene phyric basalt lapilli tuff, breccia, and flows of the Flower Ridge Formation [23], which is correlated with basalts in the Fourth Lake Formation [6].

The Fourth Lake Formation, the target unit for this study (Figures 2 and 3), consists of a 100-200-meter-thick sequence of radiolarian ribbon chert that is overlain by cherty siltstone, thin siltstone-argillite beds, and thinly bedded fine- to medium-grained interlayered sandstone and mudstone (Figures 4(a), 4(b), and 4(e)–4(g)). The Fourth Lake Formation represents a marginal-basin assemblage that developed with coeval VMS-type deposits in the back-arc of southern Wrangellia [4–6, 14].

The Fourth Lake Formation has yielded Early to mid-Tournaisian and Middle to Late Pennsylvanian conodonts [7] and one collection of Early Permian radiolaria [14]. Previous detrital zircon U-Pb ages from the Fourth Lake Formation yield grains that range in age from 355 Ma to 294 Ma with dominant peak ages of 320, 312, and 304 Ma (Figures 1 and 5) [14]. A heterolithic lapilli tuff from the northern portion of the Cowichan uplift, within the Fourth Lake Formation, yields three distinct populations with age ranges of 339-337 Ma, 322-309 Ma, and ca. 295 Ma, the latter of which is interpreted to represent the depositional age of the tuff [14].

Overlaying the Fourth Lake Formation is the Mount Mark Formation, a massive limestone unit rich in marine fossils (Figures 2 and 3) [32]. The contact between the Mt. Mark and underlying strata is diachronous: it contains faunal assemblages of Middle Pennsylvanian and Early Permian age [7], overlapping those in the Fourth Lake Formation (Figures 2 and 3). Conformably overlying the Mount Mark Formation is a succession of clastic and volcaniclastic rocks of the St. Mary’s Lake Formation (Figures 2 and 3), interpreted to be Early Permian in age [6, 21].

Extensive U-Pb dating of igneous rocks at the northern end of the Cowichan uplift near Port Alberni and on northwestern Vancouver Island (Bedingfield uplift and Dragon property, Figure 1; Ruks [14]) has shown that mainly volcaniclastic strata assigned in previous mapping [4] to the Sicker Group are largely time-equivalent with the sedimentary Fourth Lake Formation in the classic South Cowichan and Buttle Lake Paleozoic sections (Figure 2). Here, we assign these younger strata to a newly recognized informal upper part of the Sicker Group. In all of these areas, the volcanic strata are directly overlain by limestones of the Mt. Mark Formation (Figure 2) [4, 14].

Near Port Alberni, in the northernmost Cowichan uplift, felsic volcanic and tuffaceous rocks yield detrital zircon LA-ICPMS ages from $351.3±2.1$ Ma to $334.9±2.2$ Ma, and rhyolite porphyry bodies yield LA-ICPMS ages from $352.8±3.1$ Ma to $335.0±2.9$ Ma, with a single result of $293.6±1.6$ Ma (Figure 1) [14]. Felsic volcanic rocks associated with VMS deposits in the Bedingfield uplift yield igneous zircon LA-ICPMS ages of $312.4±3.5$ Ma, $308.3±3.2$ Ma, and $305.6±4.1$ Ma (Figure 1) [14]. Similar VMS-associated felsic rocks in the Dragon property yield U-Pb zircon ages from 311 Ma to 300 Ma and are overlain by felsic tuffs with ages of ca. 293 Ma and 291 Ma (Figure 1) [14]. Nd isotope data for igneous rocks of this age yield values ranging from +7.0 to +4.3 at an age of ca. 300 Ma. A single sample with an age of ca. 317 Ma yields a $ƐNdt$ value of +6.1 [14]. These values are marginally more juvenile than rocks of the Sicker Group.

An apparent northward migration of arc magmatism is observed from the pattern of U-Pb ages and geographic positions of igneous rock units on southern Vancouver Island [14]. Igneous zircon U-Pb ages from rocks of the Sicker Group in the Cowichan uplift peak at an age of 358 Ma, whereas rocks of the Buttle Lake Group in the Bedingfield uplift and Dragon property peak at an age of 305 Ma (Figures 1 and 5). A considerable lack of ages ranging from 334 to 314 Ma between southern and northern uplifts on Vancouver Island has been interpreted as a “magmatic gap” in the arc system of southern Wrangellia [14].

Paleozoic rocks in northern and southern portions of Wrangellia are overlain by ~4-7 km of the Triassic Nikolai-Karmutsen flood basalts [1]. A younger, latest Triassic-Jurassic arc is represented by stratified volcanic-sedimentary rocks of the Bonanza Group and associated Island Intrusions in southern Wrangellia [21, 33–36]. Upper Cretaceous sediments of the Nanaimo Group accumulated on and adjacent to the older stratigraphic units of southern Wrangellia in a convergent margin basin west of the Coast Mountain Batholith [37–44].

Paleozoic rocks on Vancouver Island are overlain unconformably by ca. 3.5 km of ca. 230-225 Ma submarine flows and pillow basalts of the Karmutsen Formation (Figures 1 and 3) [2, 24, 45]. Gabbroic rocks related to Karmutsen basalts yield U-Pb zircon ages of ca. 228-226 Ma from outcrops on Saltspring Island [26].

The youngest widespread igneous assemblage on Vancouver Island is latest Triassic-Jurassic in age and consists of arc-type rocks of the Bonanza Group, Island Intrusions, and West Coast Crystalline Complex (WCC) (Figures 1 and 3) [33, 36]. The WCC contains gabbroic to dioritic plutonic rocks, migmatites, amphibolites, and metasedimentary rocks and preserves the deepest section of the Bonanza arc. Rocks of the WCC have yielded U-Pb zircon ages ranging from ca.190 to 177 Ma [33, 35], a Rb-Sr age of 151 Ma [33], and K-Ar ages of 172 and 163 Ma [33]. The midcrustal section is preserved in batholiths and felsic intrusions of the Island Intrusive Suite (Figure 1) [35, 36], which have yielded U-Pb zircon crystallization ages from ca.175 to 168 Ma, ⁴⁰Ar-³⁹Ar cooling ages of ca.176 and 166 Ma, and a K-Ar age range of 181-152 Ma [46, 47]. The Bonanza Group consists of ~2,500 meters of lava flows, pyroclastic flows, thin interbedded sedimentary units, and minor low-grade metamorphic rock that are interpreted to be the volcanic equivalents of the Island Intrusions (Figure 1) [34, 36, 48–50]. These rocks have yielded a U-Pb zircon age range from ca. 202 Ma to 165 Ma [35, 51].

Whole-rock geochemical data from volcanic rocks of the Bonanza Group indicate that melts were primarily mantle derived, but that moderately intermediate to juvenile geochemical signature record varying contributions from older rocks on Vancouver Island (e.g., Sicker Group, Buttle Lake Group, and Karmutsen flood basalts) [36].

The equivalent latest Triassic-Jurassic arc sequence in southern Alaska is referred to as the Talkeetna arc system [52].

Paleozoic through Jurassic rocks on Vancouver Island are overlain unconformably by ca. 5 km of Upper Cretaceous nonmarine to deep-marine basinal strata of the Nanaimo Group (Figures 1 and 3) [38–42, 44, 53]. The basal unit of the Nanaimo Group is the Comox Formation, which contains various macrofossil assemblages that indicate a Turonian to Coniacian depositional age [54–57]. A varying thickness of 0-350 m is reported for the Comox Formation, which consists primarily of poorly bedded pebble to boulder conglomerate. Clasts are angular to rounded, and compositions indicate derivation from older strata on Vancouver Island (Figures 4(c) and 4(d)). The Comox Formation is interpreted to consist of fluvial sediments trapped by localized topographic highs during deposition, as well as deltaic deposits containing large incised fluvial channels [43, 44, 53, 57].

Previous samples analyzed by LA-ICPMS techniques from the Comox Formation (samples CO1 and CO2 of Matthews et al. [42]) from the east-central coast of Vancouver Island (Figure 1) yield peak ages of 154-153 Ma 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 of 167 Ma (Figure 6). A source from the central Coast Mountain Batholith is the preferred provenance interpretation by Matthews et al. [42] for zircon grains with ages from 150 to 80 Ma. This interpretation is consistent with previous studies indicating that Late Jurassic to mid-Cretaceous plutons of the Coast Mountain Batholith contributed significant sediment to the Nanaimo Group [39, 40, 58, 59].

Fieldwork focused primarily on collecting samples for geochronologic analysis. The sampling strategy was targeted at precise localities of the Fourth Lake Formation containing fine- to medium-grained sandstone fractions, guided by descriptions and coordinates of Massey [5, 6]. Recent forest clear cuts provided additional exposure of previously unknown outcrops of the Fourth Lake Formation (Figure 4(b)). Isolating millimeter to centimeter thick sandstone layers within the Fourth Lake Formation proved difficult considering the interbedded nature of these samples, which lead to the collection and processing of entire chunks of interbedded sandstone, mudstone, and/or chert (Figures 4(a) and 4(e)–4(g)). During collection of the Fourth Lake Formation, in the southern Cowichan uplift, field relations of the Comox Formation unconformably overlaying the Fourth Lake Formation and observations of clasts resembling the Fourth Lake Formation prompted sample collection (Figures 4(c) and 4(d)). These observations lead to the hypothesis that the Comox Formation was likely to yield detrital zircon grains that would provide insights into the Paleozoic and Early Mesozoic evolution of southern Wrangellia.

Zircon extraction was performed at the Arizona LaserChron Center (http://www.laserchron.org) using methods described by Gehrels et al. [60], Gehrels and Pecha, [61], and Pullen et al. [62]. Primary steps included crushing/pulverizing, usage of a Wilfley table, Frantz magnetic separator, and heavy liquids. Grains from each sample were poured in a 1-inch epoxy mount alongside fragments of U-Pb zircon standards (FC-1, SL2, and R33) and Hf zircon standards (Mud Tank, Temora-2, FC-1, 91500, Plesovice, R33, and SL2). Mounts were polished with fine sandpaper and finished with a 1 μm diamond polish. All sample mounts were imaged using cathodoluminesence (CL) and backscatter electron (BSE) methods. Samples were cleaned with a 2% HNO₃ and 1% HCL solution prior to isotopic analysis. CL and BSE images were utilized to select analytical points, avoiding complex internal structures and nonzircon grains.

U-Pb analyses were conducted by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) using a Photon Machines G2 excimer laser connected to an Element2 single-collector ICPMS. One spot was analyzed per grain using a laser beam diameter of 15 or 20 microns. The number of analyses per mount ranged from 109 to 380, depending on the abundance of zircon extracted from each sample. Results of U-Pb zircon analyses are reported in DR Table 3.

Lu-Hf analyses were conducted using a Photon Machines G2 excimer laser connected to an NU Plasma HR multicollector ICPMS, utilizing analytical methods described by Cecil et al. [63] and Gehrels and Pecha [61]. Hf analyses were conducted with a 40-micron spot centered over the preexisting pit evacuated by U-Pb analyses. Hf analyses were conducted on grains representative of each age group in each sample. The average uncertainty for analyses is 1.4 epsilon units (2σ). External precision, based on analysis of standards from the same mounts as unknowns, is 1.2 epsilon units (2σ). Analytical parameters are reported in DR Table 2. To assist in interpretations of Hf data, $ƐHft$ values (at the time of crystallization) within 5 units of depleted mantle (DM) are referred to as juvenile in composition, values between 5 and 12 units below DM are considered moderately juvenile (intermediate), and values >12 units below DM are considered evolved (following Bahlburg et al. [64]).

Sample 17AVI02, an interbedded chert and fine-grained sandstone (Figure 4(a)), yields 313 U-Pb ages with a single group ranging from 365 to 310 Ma and a peak age of 339 Ma (Figure 5). $ƐHft$ values from 36 analyses yield a dominantly juvenile signature, ranging from +17 to +5, with two analyses yielding more evolved values of -15 and -11 (Figure 6).

Sample 17AVI03 (see Figure 4 caption and Table 1 for sample details) yields 380 U-Pb ages with a dominant range of ages from 371 to 313 Ma and a peak age of 344 Ma (Figure 5). This sample also contains older grains displaying minor age peaks of 1202, 1015, 691, 668, 597, 572, 536, 442, and 402 Ma (Figure 5). 17AVI03 also yielded several single ages of 2679, 2133, 1665, and 1465 Ma (Figure 5). Grains representative of the 344 Ma peak age yield predominantly juvenile $ƐHft$ values, ranging from +16 to +5, whereas a single grain yields an evolved $ƐHft$ value of -17. $ƐHft$ values for eight zircon grains that fall within an age range of 714 to 442 Ma are intermediate to evolved, with values ranging from +2 to -19. Six zircon grains older than 1000 Ma yield intermediate to evolved $ƐHft$ values ranging from +6 to -7 (Figure 6).

Sample 17AVI04, an interbedded mudstone and sandstone cobble collected near sample 17AVI03, yields 313 U-Pb ages with a dominant range of 364-316 Ma and a peak age of 336 Ma (Figure 5). This sample yields four older grains with ages of 2802, 941, 492, and 374 Ma (Figure 5). Grains within the main age group yield juvenile to intermediate $ƐHft$ values of +15 to +6. Older grains, from oldest to youngest, yield $ƐHft$ values of -1, 0, +4, and +15 (Figure 6).

Sample 17AVI05 (see Figure 4 caption and Table 1 for sample details) yields 109 U-Pb ages with a dominant group of 359-311 Ma and a peak age of 331 Ma (Figure 5). Two grains older than 359 Ma yield ages of 2749 Ma and 1612 Ma (Figure 5). $ƐHft$ values for zircon grains from the main cluster yield primarily juvenile values ranging from +15 to +5. The two older grains yield $ƐHft$ values of +7 and +3, respectively (Figure 6).

Sample 17AVI08 was collected furthest to the north in the southern Cowichan uplift and consisted of interbedded chert and fine-grained sandstone with a distinctive black color (Figure 4(g)). This sample yields 307 U-Pb ages with a prominent group of 361-281 Ma and a peak age of 317 Ma. There are three single-grain ages of 1025, 273, and 270 Ma (Figure 5). Hf isotope results from 41 zircon crystals of 361-281 Ma yield juvenile to intermediate $ƐHft$ values of +17 to +2. The single older grain of 1025 Ma yields a $ƐHft$ value of +5 (Figure 6).

As shown in Figure 5, our five samples from the Fourth Lake Formation yield peak ages of 344, 339, 336, 331, and 317 Ma, that young northward, and produce a combined peak age of 337 Ma. The Hf isotopic compositions of these Paleozoic zircon grains record derivation from juvenile to intermediate signatures, with most $ƐHft$ values in the range of +17 to +9. Grains older than 371 Ma comprise 3.4% of the total grains analyzed. $ƐHft$ values of these older grains range from intermediate to highly evolved, with a range of +5 to -20 (Figure 6).

Sample 17AVI06 was collected near the unconformable contact between the Fourth Lake and Comox Formations and contains large pebble-sized clasts of the Fourth Lake Formation within a medium-grained sandy matrix (Figure 4(c)). Processing of entire conglomerate chunks yielded 118 detrital zircon U-Pb ages with prominent peak ages at 341 Ma and 202 Ma and subordinate peak ages of 196 and 159 Ma (Figure 7). Single-grain ages are 263, 223, 128, and 87 Ma. Grains within the main age groups yield juvenile $ƐHft$ values ranging from +15 to +6 (Figures 8 and 9). Three Hf analyses were conducted on grains within the subordinate younger age group and yield $ƐHft$ values ranging from +13 to +9. Two single grains with ages of 263 Ma and 87 Ma yield $ƐHft$ values of +10 and +13, respectively (Figure 9).

Sample 17AVI07A, exposed due to recent clear cuts in the southern Cowichan uplift, had relatively smaller pebble-sized clasts of the Fourth Lake Formation within a finer sandy matrix (Figure 4(d)). This sample yielded 314 U-Pb ages that belong to two different groups. The older group ranges from 217 to 184 Ma, with a peak age of 194 Ma, whereas the younger group yields an age range of 180-159 Ma, with a peak age of 166 Ma (Figure 7). Two single grains yielded ages of 363 Ma and 360 Ma. $ƐHft$ values from 33 zircon grains of 217-159 Ma range from +13 to +5, whereas the two older grains observed in this sample yield $ƐHft$ values of +13 and +10 (Figures 8 and 9).

Sample 17AVI07B, collected at the top of the outcrop where sample 17AVI07A was taken, yields 314 U-Pb ages with two equally prominent age peaks of 197 Ma and 167 Ma and a minor peak age of 83 Ma (Figure 7). A scattering of single grains yields older ages of 365, 356, 349, and 321 Ma (Figure 7). $39ƐHft$ values from sample 17AVI07B were generated, with ƐHf_(t) values for the two prominent age groups yielding $ƐHft$ values ranging from +14 to +7 (Figures 8 and 9). $ƐHft$ analyses of the younger ca. 83 Ma grains yield values of +7 (Figures 8 and 9). Older ages, encompassing grains from 365 to 321 Ma, yield $ƐHft$ values ranging from +13 to +9 (Figures 8 and 9).

Sample 17AVI09, a poorly sorted pebble to cobble conglomerate was sampled from a road cut near the city of Nanaimo (Figure 1 and Table 1), yields 312 U-Pb ages with dominant peak ages of 152 Ma and 87 Ma (Figure 7). A minor peak of 165 Ma is observed on the old side of the dominant 152 Ma age group (Figure 7). Older ages include a subordinate group of 360-340 Ma with a peak age of 356 Ma, several single-grain ages between 308 Ma and 09 Ma, and four single grains of 1712, 529, 396, and 381 Ma (Figure 7). Of the 142 grains analyzed for Hf isotopic data, ca. 152 Ma grains yield $ƐHft$ values ranging from +16 to +6 (with a single grain yielding an evolved value of -11), ca. 165 Ma grains yield $ƐHft$ values of +14 to +6 (with a single grain yielding a value of -6), and ca. 87 Ma grains yield $ƐHft$ values ranging from +13 to +8 (Figures 8 and 9). Zircons within the age range of 360-340 Ma yield $ƐHft$ values ranging from +11 to +5 (Figures 8 and 9), whereas 308-209 Ma grains have $ƐHft$ values ranging from +15 to +3 (Figures 8 and 9). Single grains with ages of 529 Ma and 396 Ma yield positive $ƐHft$ values of +1 and+9, respectively (Figures 8 and 9).

A single sample (07JBM06) of the Comox Formation outcropping on Saltspring Island (Figure 1) was collected by Brian Mahoney and is included in this study. This sample yields dominant age peaks of 367, 196, and 155 Ma (Figure 7). Six individual grain ages older than 1000 Ma are 2678, 1828, 1134, 1072, 1040, and 1037 Ma (Figure 7). Younger grains that are less than 110 Ma yield ages of 108 and 87, with two ages at 81 Ma (Figure 7). Hf isotope data are not available from this sample.

Collectively, Comox samples yield peak ages of 365, 195, 166, 153, and 91 Ma (Figure 7). Most $ƐHft$ values from these grains range from +15 to +5 (Figures 8 and 9).

As shown in Figure 5, most of the U-Pb ages of detrital zircons from the Fourth Lake Formation (five from this study and two from Ruks [14]) show considerable overlap with the igneous U-Pb ages from rocks of the Sicker Group and Buttle Lake Group. Additionally, detrital zircon peak ages and cumulative peak ages of igneous zircon ages are apparently young northward. Maximum peak ages in the range of ca. 360-340 Ma are associated with Sicker Group magmatism, predominately observed in the Cowichan uplift, whereas 320-300 Ma peak ages are sourced from Buttle Lake Group igneous rocks commonly found in the Bedingfield uplift and Dragon property to the north (Figure 1). This younging trend is most likely due to the progressive rifting of the arc shifting magmatic centers northward along with northerly propagation of depositional centers.

One of the main differences between Paleozoic igneous versus Paleozoic detrital records of southern Wrangellia is the abundance of detrital zircon ages within the 334-314 Ma gap observed in igneous ages reported by Ruks [14]. Three samples contain dominant peak ages of 331 Ma (sample 17AVI05), 320 Ma (08TR017, from Ruks [14]), and 317 Ma (17AVI08) (Figure 5), and all of these samples were collected further north in the Cowichan uplift and near the city of Nanaimo (Figure 1). Considering the low-energy depositional environment of the Fourth Lake Formation and the presence of Sicker Group and Buttle Lake Group aged zircons, these ages must have been derived from igneous rocks of southern Wrangellia. This suggests that southern Wrangellia was highly active throughout the Carboniferous, contradicting the previously interpreted magmatic gap of 334–314 Ma. A proposed location for these igneous bodies would be somewhere between the Buttle Lake and Cowichan uplifts based on the perceived trajectory of arc migration to the north as proposed above (Figure 1). However, these rocks are most likely covered beneath the widespread Karmutsen basalts (Figure 1).

The Hf isotope data acquired in this study and the Nd isotope data from Ruks [14] can also be used to compare the detrital and igneous records. Converted $ƐNdt$ to $ƐHft$ values from Ruks [14] yield values for ca. 360 Ma samples ranging from +11 to +9, +11 for a single rock with an age of ca. 317 Ma, and +12 to +8.7 for rocks with ages of ca. 300 Ma (Figure 6). As shown on Figure 6, $ƐHft$ values for detrital zircons from the Fourth Lake Formation yield a similar age range of +17 to +10 (Figures 5 and 6). In addition, both igneous and detrital zircons are interpreted to show a decrease in the abundance of intermediate $ƐHft$ values from 325 Na to 290 Ma (Figure 6). The occurrence of similar U-Pb ages and Hf isotope values in both data sets supports the interpretation that strata of the Fourth Lake Formation were sourced from the Sicker Group and igneous rocks of the Buttle Lake Group (Figure 6).

The Fourth Lake Formation contains 49 grains (3.4% of the total grains analyzed) with ages between 2802 Ma and 442 Ma and with corresponding $ƐHft$ values ranging from +13 to -20 (Figure 6). These grains, plus the occurrence of some 360-300 Ma grains with more evolved $ƐHft$ signatures, suggest that sources for strata of the Fourth Lake Formation also included pre-mid-Paleozoic rocks or sediments with pre-mid-Paleozoic grains. The lack of evidence for such components of >380 Ma in samples from the Sicker Group and Buttle Lake Group [14] raises the possibility that some grains in the Fourth Lake Formation were sourced from rocks that are not currently part of southern Wrangellia.

One potential source area for these older grains is the Alexander terrane, given that previous workers have suggested that the Wrangellia and Alexander terranes were near, or even built partially on, each other during late Paleozoic time [12, 13, 15], and that some portions of the Alexander terrane contain older and more evolved crustal components [17, 18, 65]. Three distinct portions of the Alexander terrane have yielded U-Pb ages and Hf isotope data, including the Saint Elias Mountain region [17, 18], Southeast Alaska [63, 66], and the Banks Island assemblage [65] (Figure 1). Of these regions, connections with the Banks Island assemblage and the Saint Elias Mountain region are considered most likely given the geologic, geochronologic, isotopic, and paleomagnetic evidence that Vancouver Island was within proximity of northern Wrangellia, thusly within proximity of the Alexander terrane, prior to Early Cretaceous time [65–67]. Although Precambian–early Paleozoic grains within Wrangellia cannot be traced to one individual entity of the Alexander terrane due to the relatively low percentage of these grains in our samples, they do fit well within detrital zircon populations observed in the Alexander terrane.

The Comox Formation yields Paleozoic peak ages of 367, 356, and 341 Ma (Figure 7). With the inclusion of Comox samples from Matthews et al. [42], a cumulative Paleozoic peak age for the Comox Formation is 364 Ma with a range of ages encompassing rocks of the Sicker and Buttle Lake Groups (Figure 7). This suggests that zircons in these strata were derived directly from the Sicker and Buttle Lake Groups or recycled from the Fourth Lake Formation. The latter interpretation is preferred for detrital zircons analyzed in this study based on the occurrence of clasts derived from the Fourth Lake Formation observed in these samples (Figures 4(c) and 4(d)). Sample 07JBM06 yields the oldest Paleozoic peak age at 367 Ma, which is consistent with derivation from nearby rocks of the Sicker Group and the Saltspring Intrusive Suite, which is of similar age and representative of first cycle deposition of zircons into the Comox Formation (Figures 1 and 7). Conglomeratic samples of the Comox Formation located southeast of Vancouver Island in the San Juan Islands similarly yield several Sicker Group aged detrital zircons and a minor amount of 400-500 Ma ages [53]. As shown in Figure 8, Paleozoic zircons from the Comox Formation yield $ƐHft$ values of +15 to +5, which directly overlap the juvenile values of strata of the Fourth Lake Formation and igneous rocks of the Sicker arc.

The similarity of U-Pb ages, $ƐHft$ values, and previous detrital zircon interpretations of the basal Comox Formation in the San Juan Islands suggests that Paleozoic zircons were derived in large part from Paleozoic rocks of the Sicker arc and overlying Fourth Lake Formation.

The Comox Formation yields 12 single-grain ages ranging from 277 to 223 Ma (Figure 7) with corresponding $ƐHft$ values ranging from +12 to +3 (Figure 8). The source of these grains 277-223 Ma is uncertain given the lull in magmatism in southern Wrangellia during this time period. The only magmatism recorded in southern Wrangellia during this age range is the eruption of the Karmutsen flood basalts, which occurred from 232 to 225 Ma [2], but zircon is rare in basaltic rocks.

The abundance of 217-160 Ma detrital zircons (Figure 7) indicates that strata of the Comox Formation may have been shed from latest Triassic-Jurassic igneous rocks of the West Coast Crystalline Complex, Island Intrusive Suite, and/or Bonanza Group, which are widespread on Vancouver Island (Figure 1). These rocks yield U-Pb ages of 202-165 Ma [33, 35, 51] and juvenile to intermediate isotopic signatures [36], which are quite similar to Hf isotope values from the Comox Formation (Figures 8 and 9).

Previous researchers have suggested that Late Jurassic and Cretaceous detrital zircons from the Comox Formation were shed from the Coast Mountain Batholith [39, 40, 42–44, 58, 59]. More specifically, Matthews et al. [42] suggested derivation from the central CMB because of consistencies in timing and volume of magmatic flux events in the central CMB (160-140 Ma, 120-78 Ma, and 55-48 Ma; Gehrels et al. [68]) correlating to peak ages. In contrast, Huang [43] has suggested that Nanaimo detrital zircons were shed from the southern CMB given that the observed ages are more similar.

A comparison of our age and Hf isotopic data with the information available from the southern and central CMB is shown in Figure 9. We conclude that the age distributions from strata of the Comox Formation are not an excellent fit with either portion of the CMB, especially the lower proportion of 130-100 Ma ages and higher proportion of ~170-160 Ma ages in Comox samples. $ƐHft$ values of Comox zircons are also significantly more juvenile than zircons from the central CMB (Figure 9). We accordingly agree with previous workers that the Late Jurassic and Cretaceous zircons in the Comox Formation were likely shed from the CMB, but conclude that the available data do not support firm connections with either the southern or northern portions of the batholith.

Previous workers have suggested that Wrangellia originated at the margin of the Paleo-Pacific and Paleo-Arctic realms, developing within close proximity to the Alexander terrane from Devonian through Permian time [13, 15–18, 69–77].

Previously reported connections between the Alexander and Wrangellia terranes [13–15, 74, 77, 78] combined with our new data lead to the following integrated tectonic model for Paleozoic development of northern and southern Wrangellia (Figures 10(a)–10(d)).

According to Colpron and Nelson [74] and Nelson et al. [77], the Alexander terrane was extruded westward out of the Paleo-Arctic into the NE Paleo-Pacific realm. An east-dipping subduction zone was established at this time outboard of western Laurentia, with the overriding plate bearing the Alexander terrane [13]. This subduction zone was proposed to extend into an intraoceanic setting beyond the terrane boundary of Alexander, where it gave rise to the Skolai arc of northern Wrangellia and Sicker arc of southern Wrangellia [13, 15].

In northern Wrangellia, initiation of back-arc rifting is inferred from the presence of coeval nonarc gabbros in Wrangellia (Steele Creek complex) and in the Craig subterrane of the Alexander terrane (Constantine complex and related gabbro dikes) yielding ages of ca. 363 Ma [13]. These gabbros suggest a close connection between the Alexander terrane and northern Wrangellia.

Although such gabbros are not recognized in southern Wrangellia’s Sicker arc, Late Devonian nonarc basalt in the lower Duck Lake Formation may similarly represent the onset of back-arc rifting during subduction-initiated arc development of characteristic IAT, L-IAT, and minor E-MORB-type rocks of the Sicker Group [15, 20–22]. The Sicker Group preserves the beginning of the southern Wrangellia arc, ca. 370-365 Ma, on top of oceanic to transitional crust bordering the Alexander terrane. Relatively intermediate Hf isotope values from Late Devonian to Early Mississippian detrital zircons from the Fourth Lake Formation suggest that evolved older crust must have been present in order to influence the chemistry of the early stages of the Sicker arc. Further geochemical indicators for an evolved crustal component in early arc construction include VMS deposits in the Myra Falls area yielding elevated levels of radiogenic Pb isotopes [29, 30], depletion of radiogenic Nd in felsic volcanic and intrusive rocks of the Cowichan uplift, and the occurrence of peraluminous granitoids within the Saltspring Intrusive Suite [14].

There are two scenarios in which intermediate early arc magmas were generated in southern Wrangellia. (1) Subduction-related magmatism was contaminated with sediment derived from the nearby Alexander terrane from the upper crust and was consumed during subduction-related arc magmatism. (2) In the scenario we prefer, a transitional portion of the Alexander terrane crust underlays the nascent arc of southern Wrangellia and was since buried due to extensive arc construction. This would provide the older, more evolved crust required to produce rocks with intermediate to evolved geochemical signatures observed in the earliest phase of the Sicker arc. The presence of Alexander-like older crust would also provide a source of pre-Devonian zircons deposited into the Fourth Lake Formation throughout the Carboniferous.

In southern Wrangellia, slab rollback rifted the preexisting Sicker arc and formed a back-arc spreading center in its place, focused in the Alberni area between the Buttle Lake and south Cowichan areas [14]. Within the new back-arc rift region, local bimodal magmatism accompanied the emplacement of VMS-type deposits [14]. The earliest deposits of the Buttle Lake Group, ribbon cherts associated with the Fourth Lake Formation, are deposited in the new back-arc basin and on portions of the rifted fragments of the Sicker arc [14]. A modern analog to Paleozoic southern Wrangellia’s rifted arc-type setting is the Taupo-Tonga-Kermadec arc system in the southwest Pacific Ocean [15, 79]. Maximum arc activity in southern Wrangellia occurred in the Middle to Late Mississippian, as shown by a cumulative detrital zircon age peak of 337 Ma. Igneous rocks with these ages have not been previously recognized in Wrangellia. On Vancouver Island, they are probably covered by extensive younger formations. The southern Wrangellia arc was highly active throughout the Carboniferous base on the abundance of detrital zircons within this age range. Corresponding Hf isotope data are highly juvenile compared to earlier Wrangellian sources. The shift to exceptionally juvenile Hf values in the Sicker arc may reflect the progressive rifting of the arc northward coupled with a process similarly invoked to explain juvenile εNd signatures in volcaniclastic rocks of the Klinkit Group (late Paleozoic) that were deposited on older pericratonic strata of the Yukon-Tanana composite terrane in northern British Columbia and Yukon [80]. They proposed that the voluminous asthenospheric melts that fed Klinkit volcanism were rapidly and repeatedly emplaced along coated conduits, insulated from sources of crustal contamination [80].

In contrast with the arc-rift to back-arc environment represented by Mississippian Sicker Group volcanic rocks, in northern Wrangellia, basalts and basaltic andesites of the Station Creek Formation were more likely the products of arc-axial magmatism. They range as old as ca. 352 Ma, as shown by a U-Pb age determination on a rhyolite near the exposed bottom of the section. At present, there is no other absolute age constraint on Station Creek volcanism. It may well have persisted through the Middle Mississippian interval documented in this study.

During this time interval, Sicker Group magmatic centers continue to migrate towards the northwest (modern coordinates) [14]. Deposition of low-energy marine sediments of the Fourth Lake and Thelwood Formation continued. Northward-younging detrital zircon peak ages in the Fourth Lake Formation also reflect the direction of magmatic and depositional migration.

Volcanism of the Skolai arc continued through this time interval, preserved in arc-type rocks of the upper Station Creek Formation [15]. During this time, the arc system in northern Wrangellia is thought to have undergone a subduction reversal, with evidence in changing chemistry from the lower to upper Station Creek igneous rocks [13, 15]. A reversal in subduction is also inferred in southern Wrangellia, marked by the migration of volcanism from the Alberni area in the Mississippian to northwestern Vancouver Island by ca. 312 Ma [14]. This change in subduction broke along the extinct Late Devonian spreading center and initiated the encroachment of the Alexander terrane towards northern Wrangellia [13, 15].

Northern Wrangellia and Alexander collided, as the Alexander block entered the Wrangellia subduction zone [15]. The collision led to the exhumation of basement gabbros and stitched the northern portion of Wrangellia and Alexander together, indicated by emplacement of the Barnard Glacier porphyry (ca. 307 Ma), Centennial Ridge pluton (304 Ma), and the Logan Glacier porphyry (307 Ma) within both terranes and across their boundary [15]. The Early Permian Hasen Creek Formation was deposited as a clastic wedge atop the now-defunct Station Creek arc; Hasen Creek conglomerates were also deposited over the exhumed gabbros [13].

Arc-related magmatism continued in southern Wrangellia, with a northwesterly migration of locally bimodal magmatism into the Bedingfield uplift and at the Dragon property (northwestern Vancouver Island; Figure 1), where 312-300 Ma volcanic rocks and associated VMS deposits are recognized [14]. The volcanic succession is overlain by ~290 Ma tuffs, the final volcanic deposits recorded in southern Wrangellia [14]. Deposition of the Fourth Lake and Thelwood formations continued in a back-arc setting. The lack of deformation and postcollisional plutons suggest that southern Wrangellia was largely unaffected by the collisional events of northern Wrangellia and the Alexander terrane.

After collision, the subduction zone shifted behind the Alexander terrane, as observed in emplacement of the Donjek Glacier Batholith (286-284 Ma), the Steele Glacier pluton (291 Ma), and other granitoids with ages from 290 to 280 Ma in the Alexander terrane [15]. In Early Permian time (~280 Ma), a fragment from the Alexander terrane, the Admiralty subterrane, clogged the subduction zone, which finally shuts off the arc magmatism within the Alexander terrane [15, 80]. Limestones of the Mt. Mark Formation on Vancouver Island record the cessation of arc-related volcanism of southern Wrangellia. Fossiliferous Lower Permian limestones of the Pybus Formation overlap the suture between the Craig and Admiralty subterranes in southeastern Alaska [81]. These coeval limestone formations in southern Wrangellia and Alexander terrane indicate that they shared a quiescent postcollisional tectonic regime and depositional environment.

This study presents the first robust detrital zircon analysis of Paleozoic strata from southern Wrangellia. Detrital zircons in sedimentary rocks of the Fourth Lake Formation primarily record penecontemporaneous magmatic activity of the Sicker arc. Cumulative detrital zircon U-Pb ages peak at 337 Ma. This broad Middle Mississippian peak lies between U-Pb igneous age clusters of 352-335 Ma and 312-300 Ma identified by Ruks [14]. The combined data represents a series of closely spaced magmatic episodes in the Sicker arc system between Late Devonian and Early Permian time. Each episode, illustrated in our updated tectonic model, reflects a significant change in the arc and back-arc configuration in southern Wrangellia, which in turn were caused by changes in subduction geometry and eventually subduction polarity. Progressively younger peak ages are observed from sample locations from south to north, suggesting volcanic centers in southern Wrangellia migrated northward during back-arc basin development.

Mississippian and younger detrital zircons yield predominately juvenile Hf isotopic signatures, whereas intermediate Hf signatures are observed in Late Devonian zircons derived from the nascent Sicker arc. We propose that Late Devonian zircons with intermediate $ƐHft$ values formed in Sicker arc-related magmas that assimilated minor amounts of crust from an Alexander terrane-like source. Incorporation of an older, more evolved component from the Alexander terrane is further supported by the presence of detrital zircon grains older than 380 Ma with highly variable Hf signatures. This suggests the presence of a Precambrian to lower Paleozoic crustal component, similar in profile to detrital zircon populations in the Alexander terrane. A key result of this study is that although southern Wrangellia is not connected to the Alexander terrane by common geological elements and stitching plutons as is northern Wrangellia [13, 15], the detrital zircon ages and Hf isotopic signatures presented here provide a strong case for early Alexander terrane linkages. Thus, all of Wrangellia and the Alexander terrane can be considered as parts of a coevolving tectonic system, beginning with the birth of Wrangellia marked by the inception of the Sicker and Skolai arcs. We recommend a study similar to this one in northern Wrangellia which is aimed at U-Pb age distributions and Hf isotopic data, which could be compared to results from southern Wrangellia to provide an integrated analysis of the combined Sicker-Skolai arc–back-arc system.

Data tables of data generated from this study are included in supplementary files. Ruks [14] data is also included.

The authors declare that they have no conflicts of interest.

Field work for this project was supported by NSF EAR 1347375. Laboratory analyses were supported by NSF EAR 1649254 to the Arizona LaserChron Center. Thanks are due to Graham Nixon for presubmission comments and edits. Lithosphere reviewers Grant Lowey and Sarah Roeske made many helpful comments that improved the manuscript. We thank Rebecca Alberts for assistance in the field and Mark Pecha, Nicky Giesler, Chelsi White, Kojo Plange, Clay Kelty, Gayland Simpson, Heather Alvarez, and Ken Kanipe for assistance with the laboratory analyses and logistics.