Age, construction, and exhumation of the midcrust of the Jurassic Bonanza arc, Vancouver Island, Canada

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Citation for this paper:

Canil, D., Johnston, S.T., Larocque, J., Friedman, R., & Heaman, L.M. (2013). Age,

construction, and exhumation of the midcrust of the Jurassic Bonanza arc,

Vancouver Island, Canada. Lithosphere, 5(1), 82-91.

https://doi.org/10.1130/L225.1

UVicSPACE: Research & Learning Repository

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Faculty of Science

Faculty Publications

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Age, construction, and exhumation of the midcrust of the Jurassic Bonanza arc,

Vancouver Island, Canada

D. Canil, S.T. Johnston, J. Larocque, R. Friedman, and L.M. Heaman

2013

© 2012 Geological Society of America.

This article was originally published at:

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82 www.gsapubs.org | Volume 5 | Number 1 | LITHOSPHERE

INTRODUCTION

One of the proposed mechanisms for the growth of continental crust is by the accretion of island arcs (Hamilton, 1994). A conundrum arises in the origin of continental crust by this mechanism, however, in that mantle-derived magma fed to arcs is basaltic, yet accretion of island arc crust has somehow resulted in an andesitic continental crust. One explanation is that differentiation occurs within the arc, and there is delamination or loss of mafi c material through its base, where material is returned to the mantle (Kay and Mahlburg Kay, 1993). Dynami-cal constraints likely confi ne delamination to only a subset of arcs (Jull and Kelemen, 2001). A more recent idea is that crust can be relaminated, from below, due to subduction erosion or sedi-ment subduction (Hacker et al., 2011).

The mass balance of crustal growth in arcs in part involves knowing the thickness and bulk composition of all parts of the arc crust with its age and evolution. An important interface for within-arc differentiation is the middle crust, situated between the better-exposed upper parts of the arc crust and the lesser-preserved lower crust. In modern arcs, the middle crust is never exposed, but seismic studies defi ne it as having wave speeds of 6–6.5 km/s, with a thickness that varies greatly from ~4 km to 16 km, both

spatially and temporally (Calvert et al., 2008). A seismic distinction may be inaccurate, how-ever, because of poor correlation of seismic velocity and composition (Behn and Kelemen, 2006), and considerable heterogeneity in many exhumed midcrustal sections (Kawate and Arima, 1998; Rioux et al., 2010).

In terms of arc magma genesis, the middle crust may fi lter magmas rising to feed higher-level plutons, may be cannibalized by upward-rising magmas (Otamendi et al., 2009), or may even act as a “sponge” for water or other ele-ments in differentiating arc magmas by crystal-lization of amphibole or oxides (Davidson et al., 2007; Larocque and Canil, 2010). It is unclear the extent to which the addition of new material versus recycling of older material is involved in the formation of this intermediate layer in arcs. Magmatic “fl are-ups” occur in more long-lived arcs, which may relate to changing tectonic parameters (slab rollback) or to the growth of the arc (Ducea, 2001). Thus, age and volume relationships for the middle crust are neces-sary to better characterize the mass and tempo of magma production and differentiation within arcs (Ducea, 2001).

In this study, we characterize the thickness and age of a midcrustal section of heteroge-neous plutonic rocks of the Jurassic Bonanza arc exposed on Vancouver Island (Fig. 1). Age information for midcrustal rocks of the arc con-strain the consanguinity of volcanism and

plu-tonism of an arc, and the longevity of the arc and its magma fl ux rate. We examine whether any of these observations defi ne a polarity to the arc, or correlate with those of Jurassic arcs exposed farther north in Queen Charlotte Island and Alaska (Talkeetna).

GEOLOGY

Most of Vancouver Island is underlain by the Wrangellia terrane, which consists of a base-ment of Devonian Sicker Group volcanic and plutonic rocks, succeeded by Permian Buttle Lake carbonates, and Triassic Karmutsen Pla-teau basalt–Quatsino limestone, and clastic rocks (Muller, 1977). The Jurassic Bonanza arc intrudes or overlies these basement units of Wrangellia and is unconformably overlain by Late Cretaceous sedimentary rocks of the Nanaimo Group (Fig. 2). Wrangellia has an overall northwesterly structural grain. Major north-trending anticlinoria, including Cowichan and Buttle Lake, are attributed to Late Creta-ceous transpression overprinted by high-angle brittle deformation that resulted in homoclinal fault-bounded blocks (Nixon and Orr, 2007b; Yorath et al., 1999).

The Jurassic Bonanza arc forms a 15-km-thick crustal section and is divided from lower to higher structural level into the West Coast com-plex, the Island plutonic suite, and the Bonanza volcanics (Fig. 2). The West Coast complex has

Age, construction, and exhumation of the midcrust of

the Jurassic Bonanza arc, Vancouver Island, Canada

D. Canil1,_{*, S.T. Johnston}1_{, J. Larocque}1_{, R. Friedman}2_{, and L.M. Heaman}3

1_{SCHOOL OF EARTH AND OCEAN SCIENCES, UNIVERSITY OF VICTORIA, 3800 FINNERTY ROAD, VICTORIA, BRITISH COLUMBIA V8W 3P6, CANADA}

2_{PACIFIC CENTRE FOR ISOTOPIC AND GEOCHEMICAL RESEARCH, DEPARTMENT OF EARTH AND OCEAN SCIENCES, THE UNIVERSITY OF BRITISH COLUMBIA, 6339 STORES ROAD,}

VANCOUVER, BRITISH COLUMBIA V6T 1Z4, CANADA

3_{DEPARTMENT OF EARTH AND ATMOSPHERIC SCIENCES, UNIVERSITY OF ALBERTA, EDMONTON, ALBERTA T6G 2E3, CANADA}

ABSTRACT

We present new U-Pb zircon ages for heterogeneous mafi c-felsic rocks of the West Coast complex, a midcrustal plutonic component of exposed Jurassic arc-crustal section on Vancouver Island. We examine the timing of juvenile plutonic crust production, and consanguinity of volcanism and plutonism in this arc. The midcrustal plutons were emplaced between 193 Ma and 174 Ma, contemporaneous with the upper-crustal (<10 km depth) plutonic component of the arc (Island plutonic suite). A 12 km thickness of plutonic arc crust was built as a series of sheets at a rate of ~0.003 km3_yr–1_{. Deformation and emplacement were contemporaneous, and there are no correlations among age,} dif-ferentiation (peridotite to granite), or structural level of plutons in the arc. The age range and a weak eastward younging age polarity of the Jurassic arc section on Vancouver Island match that of the Talkeetna arc-crustal section in Alaska, suggesting that the two arcs are correlative and evolved by either forearc erosion above of an east-dipping slab, or slab rollback during west-dipping subduction.

LITHOSPHERE; v. 5; no. 1; p. 82–91 | Published online 26 October 2012 doi: 10.1130/L225.1

*E-mail: dcanil@uvic.ca.

Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/5/1/82/3723740/82.pdf by Univ Victoria McPherson Library Serials user

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been interpreted as a part of the middle crust (DeBari et al., 1999a; Larocque and Canil, 2008) and is a heterogeneous mixture of mostly diorite and quartz diorite, with lesser granodiorite cross-cut by intrusions of leucotonalite. In the diorite, recent studies have recognized decameter- to rarely kilometer-scale bodies of conformable but discontinuous hornblende gabbro, pyroxenite, olivine hornblendite, and peridotite with cumu-late textures (Figs. 3A and 3B) (Larocque, 2008; Larocque and Canil, 2007, 2010; Marshall et al., 2007). Intrusive relationships in the West Coast complex are heterogeneous on all scales (Fig. 3C), and levels of strain vary within meters in outcrop (Figs. 3D and 3E; Isachsen, 1987).

The Island plutonic suite occurs as a series of unfoliated quartz diorite to alkali feldspar granite plutons, with rare diorite and gabbro. The contact between rocks previously mapped as the Island plutonic suite and the West Coast complex is not defi ned, and to some degree the distinction between these two units is obscure as is discussed further herein in light of new age information in this study.

The Bonanza volcanics are pillowed and massive fl ows of aphanitic basalt, andesite, and dacite with pyroclastic deposits. Based on geological and geobarometric constraints, the structural section of the Bonanza arc con-sists of 2.5 km of Bonanza volcanics, 10 km of Island plutonic suite rocks, and a less con-strained thickness of 10–18 km for the West Coast complex (Canil et al., 2010). The entire section was thus likely 20–25 km thick prior to structural thinning to its current 15 km.

Published U-Pb ages for rocks of Bonanza arc vary between 202 and 168 Ma (Nixon and Orr, 2007a) with a weak eastward young-ing trend on Vancouver Island (DeBari et al., 1999a). There is no information on the age of mafi c and ultramafi c rocks in the West Coast complex.

SAMPLES

We selected seven samples of the West Coast complex from three regions (Fig. 1; Table 1) to investigate age relationships between samples

N

50 km

Figure 1. Map of Vancouver Island showing the three major units of the Jurassic Bonanza arc, and fault traces, from BC Map Place (Massey et al., 2005) and Canil et al. (2010), and sample locations and U-Pb ages (to nearest 1 Ma) of Bonanza arc plutons in this study as well as those from previ-ous work (Breitsprecher and Mortensen, 2004; DeBari et al., 1999b; Fecova et al., 2008). Place names: TO—Tofi no; PR—Port Renfrew; VIC—Victoria.

Figure 2. Stratigraphic section for Wrangellia on Vancouver Island, emphasizing plutonic units of the Bonanza arc (after Larocque and Canil, 2010).

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CANIL ET AL.

Figure 3. Outcrop photos of West Coast complex plutons: (A) Conformable layers of hornblende peridotite (Per) in diorite (Dior), Lens Creek, Port Renfrew. (B) Conformable layering (from left to right) of diorite (Dior), pegmatoid hornblende gabbro, and pyroxenite (Px), Grierson Mainline, Port Renfrew. (C) Heterogeneous intrusive relations amongst different diorite phases, Browns Mountain, Port Renfrew. Width of photo is 1 m. (D) Outcrop showing contact between massive and gneissose diorite, Mount Tolmie, Victoria. (E) Layering and heterogeneous strain in melano-and leucodiorite layers, with vertical lineations defi ned by hornblende crystals at center in melanodiorite layer, Grierson mainline, Port Renfrew. U-Pb age sample DC0507 was taken from this outcrop. (F) Decimeter-thick mylonite showing sheath folds defi ned by felsic layers (see arrow) within diorite gneiss, Mount Tolmie, Victoria.

TABLE 1. SAMPLE LOCATIONS AND DATA

n o i t p i r c s e D N M T U E M T U n o i t a c o L e l p m a S

DCMD04 Victoria 473955 5370850 Massive medium-grained quartz diorite

CS10-109 Catface 028895 5460930 Massive medium-grained quartz monzonite, in drill core DC10-02 Catface 028437 5460801 Massive medium-grained quartz monzonite

DC0637 Port Renfrew 385327 5391375 Massive medium-grained granodiorite

JL06-110A Port Renfrew 400129 5387005 Massive medium-grained melanodiorite, intruded by JL06-110B JL06-110B Port Renfrew 400129 5387005 Massive medium-grained leucotonalite dike

DC0507 Port Renfrew 392231 5386355 Foliated, sheared medium-grained leucotonalite, foliated

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with and without fabrics, and between the inter-preted upper and middle parts of the arc crust. Port Renfrew

Rocks of the West Coast complex in the Port Renfrew area were mapped at 1:20,000 scale (Larocque, 2008). Most of the region is underlain by massive quartz diorite, diorite, and hornblende gabbro that in places has layering or a deformation fabric, or is crosscut by leucoton-alite intrusives. Hornblende gabbro, pyroxenite, and peridotite occur both as conformable layers in the diorite (Figs. 3A and 3B) and as blocks, schlieren, or xenoliths (Fig. 3C; Larocque and Canil, 2007, 2010). Both massive and heavily strained diorite gneisses are recognized, con-taining mylonite with a foliation that parallels the general northwest strike and shows top-to-the-north shear sense indicators (sheath folds and lineations; Figs 3E and 3F).

We sampled: massive melanodiorite (JL06– 110A), and crosscutting leucotonalite intrusive dikes (JL06–110B) in one outcrop; foliated medium-grained leucodiorite layers (DC0507) adjacent to a mylonite zone showing a mineral lineation defi ned by hornblende crystals (Fig. 3E); and massive medium-grained hornblende-biotite granodiorite containing mafi c enclaves (DC06037).

Catface Peninsula

Rocks mapped as both the Island plutonic suite and the West Coast complex occur along the western and northern edge of Catface Pen-insula, north of Tofi no (Fig. 1; Isachsen, 1987). We sampled massive medium-grained horn-blende-biotite quartz monzonite along the west-ern fl ank of the peninsula in outcrop (DC1002) and in drill core from the Catface ore deposit (CS10–109) (Smith et al., 2012) (Fig. 1). Victoria

Both heterogeneous massive diorite and dio-rite gneiss with a general northwest-striking fab-ric in the Victoria region (Fig. 3D) are crosscut in several different areas by north-striking leucoton-alite dikes. We sampled massive medium-grained diorite near Mount Douglas (DCMD04) (Fig. 1). METHODS

U-Pb zircon dating was performed for six samples at the University of British Colum-bia using procedures described in Scoates and Friedman (2008) with a modifi cation after Mundil et al. (2004). Two of these samples underwent chemical abrasion pretreatment, and

four others underwent physical abrasion (listed in Table 2). U-Pb zircon dating for one other sample was done at the University of Alberta following procedures in Heaman et al. (2002). There are no studies of intercalibration between these two laboratories. All errors are quoted at

the 2σ or 95% level of confi dence (Table 2).

Iso-topic dates were calculated using the decay

con-stants 238_{U = 1.55125 × 10}–10_and235_{U = 9.8485 ×}

10–10_yr–1_{(Jaffey et al., 1971). The reported ages}

are based on weighted averages of 206_Pb/238_U

data for a number of fractions in each sample. RESULTS

Zircons were recovered from all samples as both fragments and euhedral prisms. The Th/U ratios of zircons vary between ~0.2 and 0.5, consistent with an igneous origin (Table 2). In general, the majority of zircon fractions are concordant or nearly concordant, with little if any inheritance, with the exception of sample DCMD04 from Mount Douglas, described next. There is no correlation of age with composition (i.e., mafi c vs. felsic) or fabric (foliated or mas-sive) for any of the samples in our data set. Sample DCMD04

Two multifragment colorless zircon frac-tions from sample DCMD-04 (massive quartz diorite) have intermediate U contents (310 and 269 ppm, respectively) and similar Th/U (0.45 and 0.42, respectively) (Table 2). Fraction 1 is

concordant and has a 206_Pb/238_{U date of 181.1}

± 0.4 Ma (Fig. 4A). Fraction 2 is discordant and likely refl ects the presence of an Archean inherited Pb signal in the cores of this zircon population. A reference line constructed to pass through these two analyses yields a lower inter-cept date of 181.3 ± 0.4 Ma, interpreted to be the emplacement time of this quartz diorite. Sample CS10-109

Five fractions from sample CS10–109 (mas-sive quartz monzonite) have intermediate U contents (172–448 ppm) but somewhat variable Th/U (0.29–0.41; Table 2). All fi ve fractions are concordant, with a cluster of four that have sig-nifi cant mutual overlap and one slightly younger grain. A weighted average of the four strongly

overlapping 206_Pb/238_{U dates gives an age of}

184.46 ± 0.41 Ma (mean square of weighted deviates [MSWD] = 0.28) (Fig. 4B).

Sample DC10-02

All fi ve fractions from this sample (massive quartz monzonite) have high U contents (721–

1512 ppm) but broadly similar Th/U (0.28–0.37; Table 2). Three concordant and overlapping

analyses (Fig. 4C) give a weighted 206_Pb/238_U

age of 186.20 ± 0.19 Μa (MSWD = 1.5).

Sample DC0637

Four fractions from this sample of mas-sive granodiorite have intermediate U contents (109–255 ppm) and Th/U of 0.38–0.97 (Table 2). All four fractions are concordant (Fig. 4D)

and give a weighted 206_Pb/238_{U age of 192.62 ±}

0.38 Ma (MSWD = 0.98). Sample JL06-110A

Four fractions from this sample all have low U (24–68 ppm), refl ecting the high color index of this melanodiorite (shown in Fig. 3E), and a narrow range of Th/U of 0.34–0.40 (Table 2). Four concordant analyses produce a weighted

206_Pb/238_{U age (Fig. 4E) of 174.75 ± 0.42 Ma}

(MSWD = 1.05). Sample JL06-110B

This sample is a leucrocratic dike crosscut-ting JL06–110A. Four zircon grains have variable

U contents of 117−1003 ppm and Th/U ratios of

0.21–0.39 (Table 2). A weighted mean 206_Pb/238_U

age of 174.72 ± 0.38 Μa (MSWD = 0.50) is based

on two concordant and overlapping analyses. The two other analyzed grains give discordant results

with 206_Pb/238_{U ages that are distinctly older for}

one and younger for the other. Sample DC0507

Four fractions from this leucocratic layer in deformed diorite gneiss (shown in Fig. 3E) have very high U contents (972–3266) and Th/U ratios of 0.42–0.50 (Table 2). All four analyzed fractions are concordant and form two groupings of overlapping pairs. Because these grains were air abraded and not chemi-cally abraded, and have a high U content, we interpret the older concordant and overlapping

analyses with a weighted mean 206_Pb/238_{U age}

of 193.90 ± 0.20 Ma (MSWD = 0.025) as the age if emplacement. The other two fractions are interpreted to have undergone minor Pb loss. DISCUSSION

Distinction between Upper and Middle Crust

With the exception of sample DCMD04, we observe little if any inheritance in U-Pb data for samples of the West Coast complex (Table 2;

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CANIL ET AL.

T

ABLE 2. U-Th-Pb ISOT

OPIC DA T A s e g a ci p ot o sI s oit ar e p ot o si ci n e g oi d a R sr et e m ar a p l a n oit i s o p m o C Sample † W eight § (mg) U # (ppm) Th** U Pb # (ppm) 206 Pb* †† (× 1 0 –1 3 mol) 206 Pb* †† (mol %) Pb* †† Pb c Pb c †† (pg) 206 Pb §§ 204 Pb 208 Pb ## 206 Pb 207 Pb ## 206 Pb % err*** 207 Pb ## 235 U % err*** 206 Pb ## 238 U % err*** Corr . coef. 207 Pb ††† 206 Pb ±* ** 207 Pb ††† 235 U ± *** 206 Pb ††† 238 U ± *** % discordant DCMD04 8. 4 – 0 2. 0 0 1. 1 8 1 5. 0 5. 0 8 1 7. 5 9. 2 7 1 0 6 4. 0 0 1. 0 9 4 8 2 0. 0 7 2. 0 6 4 9 1. 0 0 2. 0 3 5 9 4 0. 0 6 1 3 6 0. 4 1. 9 0 5 4. 0 0 1 3 4 0 0. 0 z 1 2. 3 2 0 2. 0 0 5. 2 8 1 2. 0 4. 6 8 1 9. 1 6. 6 3 2 0 0 8. 0 1 1. 0 1 7 8 2 0. 0 3 1. 0 5 1 0 2. 0 7 0. 0 1 9 0 5 0. 0 7 6 5 4 0. 6 1. 8 0 2 4. 0 9 6 2 5 0 0. 0 z 2 CS10-109 A 0.004 235 0.291 8.1 1.134 94.13% 5 5 .8 315 0 .093 0.05007 1 .82 0.2003 1.97 0.02901 0 .40 0.468 1 98.4 42.2 185.4 3 .3 184.35 0.73 7.1 B 0.005 172 0.338 5.9 1.041 94.84% 5 4 .7 359 0 .107 0.04928 1 .77 0 .1970 1 .94 0 .02899 0.53 0.437 1 61.3 41.4 182.6 3 .2 184.20 0.96 –14.2 C 0.005 448 0.408 14.2 2.714 97.70% 13 5.3 805 0.129 0.04953 1 .02 0 .1983 1 .17 0.02904 0 .45 0.519 173.0 23.7 183.7 2.0 184.55 0 .82 –6.7 D 0.004 321 0.304 1 0.6 1.364 9 5.83% 7 4.9 443 0 .096 0.04961 1 .43 0 .1989 1 .58 0.02907 0 .46 0 .452 176.9 3 3.4 1 84.2 2.7 1 84.72 0 .84 – 4.4 E 0.003 295 0.268 9.6 1.064 95.86% 7 3 .8 447 0 .086 0.05028 2.28 0.1998 2.49 0.02882 0.69 0.435 2 08.0 52.8 184.9 4.2 183.15 1.25 1 1 .9 DC10-102 A 0.002 1512 0.366 45.7 3.685 9 9.13% 3 3 2 .7 2135 0.1 17 0.04986 0 .47 0 .2010 0 .60 0 .02923 0.31 0.621 1 88.7 1 1 .0 186.0 1 .0 185.75 0.56 1.6 B 0.002 828 0.281 24.9 2.024 9 8.74% 2 2 2 .2 1466 0.089 0 .04984 0.76 0.2015 0.80 0.02932 0 .13 0.422 187.7 17.6 186.4 1.4 1 86.26 0 .24 0 .8 C 0.002 783 0.338 2 4.5 1 .921 98.05% 1 5 3 .1 951 0 .108 0.05018 0.68 0.2035 0 .76 0 .02942 0.18 0.547 2 03.5 15.9 188.1 1 .3 186.90 0.32 8.1 D 0.002 721 0.313 2 2.9 1 .762 97.32% 1 0 4 .0 691 0 .100 0.05007 1.01 0.2023 1 .09 0 .02931 0.19 0.540 1 98.0 2 3.4 1 87.1 1.9 1 86.25 0.34 5.9 E 0.002 798 0 .297 24.1 1.933 9 8.48% 1 9 2 .5 1213 0.096 0 .05025 0.58 0.2013 0.65 0.02905 0 .15 0.560 206.8 13.4 186.2 1 .1 184.61 0 .27 10.7 DC0637 A 0.002 150 0 .324 5.5 0 .381 94.30% 5 1 .9 323 0 .101 0.04886 5 .01 0.2049 5.34 0.03042 0 .52 0.648 1 41.2 1 17.7 1 89.3 9 .2 193.15 0.99 –36.8 B 2 0 .004 157 0 .41 1 5.2 0.792 9 7.94% 1 4 1 .4 895 0 .129 0.04924 1 .92 0.2053 2.09 0.03024 0.46 0.455 1 59.3 44.9 189.6 3.6 1 92.04 0 .88 – 20.6 C 2 0 .003 255 0.424 8.9 0 .966 96.24% 8 3.1 490 0.135 0 .04997 1.39 0.2091 1.51 0.03034 0.30 0.502 193.8 32.2 1 92.8 2 .7 192.69 0.56 0.6 D 2 0 .005 109 0.383 3.8 0 .692 96.27% 8 2.2 494 0.122 0 .05016 1.49 0.2097 1.64 0.03033 0.46 0.449 202.2 34.6 1 93.3 2 .9 192.60 0.88 4.7 JL06-1 10A B 0 .005 64 0.335 2.1 0.365 9 4.16% 5 1.8 3 17 0.104 0 .04823 3.22 0.1827 3.35 0.02747 0 .35 0.429 1 10.4 76.0 170.4 5 .3 174.72 0 .60 – 58.3 C 0 .006 73 0.396 2 .2 0.502 96.92% 9 1 .3 599 0.125 0 .04909 6.00 0.1868 6.38 0.02759 0.57 0.696 152.2 140.5 1 73.9 10.2 175.45 0 .98 –15.2 E 0 .013 237 0 .354 6.9 3.513 9 8.12% 1 5 5 .5 982 0.1 14 0.05006 3 .52 0.1889 3.81 0.02737 0 .81 0.450 197.6 81.8 175.7 6.2 1 74.08 1 .38 1 1.9 F 0 .005 68 0.339 2.1 0.387 96.71% 9 1 .1 562 0 .108 0.04958 3 .47 0 .1876 3 .71 0 .02745 0.49 0.539 1 75.2 80.9 174.6 5 .9 174.57 0.84 0.4 JL06-1 10B A 0 .01 1 353 0.360 9.8 4 .434 99.53% 6 2 1 .7 3942 0.1 1 4 0 .04949 0.25 0.1871 0.447 0.02742 0.34 0.829 171.3 5 .88 1 74.2 0 .7 174.37 0.58 –1.8 B 0 .014 1 1 7 0 .250 3.1 1 .816 99.45% 51 0.8 3354 0.080 0 .04986 0.28 0.1833 0.413 0 .02666 0 .26 0 .753 188.4 6 .43 170.9 0.6 169.62 0.44 10.0 C 0 .009 1003 0.393 2 8.1 1 0.302 9 9.58% 6 9 3.6 4379 0.125 0 .04942 0.28 0.1865 0 .439 0.02737 0.30 0.778 1 67.7 6 .52 1 73.7 0.7 1 74.09 0.51 –3.8 D 0 .010 352 0.214 9 .5 4.051 99.59% 6 8 1 .4 4508 0 .068 0.04980 0.31 0.1894 0 .579 0.02758 0.46 0.841 1 85.7 7.32 176.1 0 .9 175.37 0.80 5.6 DC0507 B 2 0 .006 1705 0.468 54.4 1 3.019 9 9.59% 7 3 4 .4 4470 0 .150 0.05017 0.49 0.21 12 0.56 0.03053 0 .16 0.555 2 02.7 1 1.4 194.6 1 .0 193.88 0.31 4.4 C 2 0 .007 972 0 .421 31.2 8.658 9 9.06% 3 1 6 .8 1967 0.134 0 .04998 0.18 0.2104 0.28 0.03054 0 .14 0 .834 193.8 4.2 193.9 0 .5 193.92 0.27 0.0 D 2 0 .006 1919 0 .497 61.2 14.627 99.75% 122 3.0 7 435 0 .158 0.04990 0 .12 0 .2097 0 .27 0.03048 0 .20 0.912 190.1 2.8 1 93.3 0.5 193.54 0 .39 –1.8 E 2 0 .003 3266 0.466 1 03.2 12.432 9 9.78% 1 34 2.3 8250 0.148 0.04989 0.14 0.2094 0 .30 0 .03044 0.23 0.899 1 89.9 3.2 1 93.0 0 .5 193.28 0.43 –1.8

†Sample DCMD04 was done at the University of

Alberta employing the techniques of Heaman (2002), and all others were done at the

University of British Columbia, using techniques modifi

ed after those reported

in Scoates and Friedman (2008).

A, B, etc., are labels for fractions composed of single zircon grains or for two grain fraction

s, where listed; 1z, 2z are multigrain fractions. Grains from CS10-109 and DC10-102

were annealed and chemically abraded after Mattinson (2005) and Scoates and Friedman (2008). Zircons from other samples were ai

r abraded.

§Nominal fraction weights for chemically abraded zircons estimated from grain dimensions, adjusted for partial dissolution durin

g chemical abrasion. Others weighed on a microbalance.

#Nominal U and total Pb concentrations subject to uncertainty in photomicrographic estimation of weight and partial dissolution

during chemical abrasion.

**Model

Th/U ratio calculated from radiogenic

208 Pb/ 206 Pb ratio and 207 Pb/ 235 U age. ††Pb* and Pb c

represent radiogenic and common Pb, respectively; mol %

206

Pb* with respect to radiogenic, blank and initial common Pb.

§§Measured ratio corrected for spike and fractionation only

. Mass discrimination of 0.23%/amu based on analysis of NBS-982; all D

aly analyses.

##Corrected for fractionation, spike, and common Pb; up to 4.0 pg of common Pb was assumed to be procedural blank:

206 Pb/ 204 Pb = 18.50% ± 1.0%; 207 Pb/ 204 Pb = 15.50% ± 1.0%; 208 Pb/ 204 Pb = 38.40% ± 1.0% (all uncertainties 1 σ

). Excess over blank was assigned to initial common Pb with Stacey and Kramers (1975)

model Pb composition at the age of the rock.

***Errors are 2

σ

, propagated using the algorithms of Schmitz and Schoene (2007).

†††

Calculations are based on the decay constants of Jaf

fey et al. (1971). 206 Pb/ 238 U and 207 Pb/ 206

Pb ages were corrected for initial disequilibrium in

230

Th/

238

U using

Th/U [magma] = 3.

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0.02835 0.02845 0.02855 0.02865 0.02875 0.02885 0.191 0.193 0.195 0.197 0.199 0.201 0.203 207_Pb/235_U 206 Pb / 238 U 180.4 180.8 181.2 181.6 182.0 182.4 182.8 183.2 Intercepts at 181.26±0.36 [±0.45] & 2989±530 Ma MSWD = 0.000 Mt. Douglas DCMD04 0.0285 0.0287 0.0289 0.0291 0.0293 0.190 0.194 0.198 0.202 0.206 207_Pb/235_U 206 Pb / 238 U 182 183 184 185 186 Wtd. Mean 206_Pb/238_{U age} 184.46 ± 0.41 Ma; (2σ) MSWD = 0.28; n=4 Catface Peninsula CS10-109 0.0289 0.0291 0.0293 0.0295 0.198 0.200 0.202 0.204 0.206 207_Pb/235_U 206 Pb / 238 U 184 185 186 187 188 Wtd. Mean 206_Pb/238_{U age} 186.20 ± 0.18 Ma; (2σ) MSWD = 1.5; n=3 Catface Peninsula DC10-102 0.0300 0.0302 0.0304 0.0306 0.185 0.195 0.205 0.215 0.225 207_Pb/235_U 206 Pb / 238 U 191 192 193 194 Port Renfrew Glad - ML DC0637 Wtd. Mean 206_Pb/238_{U age} 192.62 ± 0.38 Ma; (2σ) MSWD = 0.98; n=4 0.0270 0.0272 0.0274 0.0276 0.0278 0.17 0.18 0.19 0.20 207_Pb/235_U 206 Pb / 238 U 172 173 174 175 176 177 Port Renfrew Lens Creek JL06-110A Wtd. Mean 206_Pb/238_{U age} 174.75 ± 0.42 Ma; (2σ) MSWD = 1.05; n=4 0.0262 0.0266 0.0270 0.0274 0.0278 0.180 0.182 0.184 0.186 0.188 0.190 0.192 207_Pb/235_U 206 Pb / 238 U 170 172 174 176 178 Port Renfrew Lens Creek JL06-110B Wtd. Mean 206_Pb/238_{U age} 174.72 ± 0.38 Ma; (2σ) MSWD = 0.50; n=2 0.0303 0.0304 0.0305 0.0306 0.208 0.209 0.210 0.211 0.212 0.213 207_Pb/235_U 206 Pb / 238 U 192.8 193.2 193.6 194.0 194.4 Port Renfrew Greirson DC0507 Wtd. Mean 206_Pb/238_{U age} 193.90 ± 0.20 Ma; (2σ) MSWD = 0.025; n=2

Figure 4. U-Pb concordia plots for samples

in this study. Ellipse errors are plotted at 2_σ.

Dashed lines parallel to the concordia curve

show 2σ limits from decay constant errors.

Shaded ellipses are data used for 206_Pb/238_U

age calculations, and unshaded ellipses indi-cate data not used; details in text. U-Pb ana-lytical data are listed in Table 2. MSWD—mean square of weighted deviates.

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CANIL ET AL.

Fig. 4), consistent with juvenile sources as

evi-denced by ε_Nd values of between 5 and 7 for this

unit (Andrew et al., 1991; Larocque, 2008). The U-Pb ages from this study, when combined with those from previous studies, show a range in age from ca. 193 Ma to 168 Ma for plutonic rocks in the Bonanza arc (Fig. 5). Ages older than 184 Ma are not recognized in the Island plu-tonic suite, and those younger than 174 Ma are not recognized in the West Coast complex, but a clear and defi nitive age distinction between these two units is not obvious.

The barely resolvable age distinction between Island plutonic suite and West Coast complex plutons is paralleled by a subtle dis-tinction in their bulk compositions. The major-element bulk composition of rocks mapped as the West Coast complex broadly overlaps the Island plutonic suite in three different regions of Vancouver Island (Fig. 6). The Island plu-tonic suite has no mafi c/ultramafi c rocks, and a notably greater abundance of felsic rocks, enriched in incompatible elements with positive Zr anomalies, whereas the West Coast complex and its mafi c/ultramafi c cumulates have positive Sr anomalies and trace-element patterns identi-cal to the Bonanza volcanic rocks (Fig. 7).

The only consistent fi eld distinctions are that the Island plutonic suite plutons never show a foliation and, unlike the West Coast complex, commonly exhibit intrusive contacts with the Triassic Karmutsen lavas. The Triassic supracrustal rocks are interpreted as the upper ~10 km of the crust through which Bonanza arc magmas transited to feed the volcanic arc. These fi eld observations can be used to quali-tatively infer that Island plutonic suite plutons occur stratigraphically higher than those of the West Coast complex (DeBari et al., 1999b), an inference corroborated by hornblende barom-etry, which shows crystallization pressures less than 300 MPa for Island plutonic suite plutons and greater than 300 MPa for the West Coast complex (Canil et al., 2010). A caveat in this interpretation, however, is that few samples of the West Coast complex have the mineral-ogical criteria amenable to quantitative Al-in-hornblende barometry (An < 35 plagioclase, K-feldspar–bearing; Anderson, 1996; Anderson and Smith, 1995). In intermediate to mafi c bulk compositions, the Ti and Al in amphibole are mostly temperature- and pressure-dependent, respectively (Ernst and Liu, 1998), and can be used as semiquantitative thermobarometers for the bulk compositions present in the West Coast complex. At a given Ti content, the Al in amphi-bole of West Coast complex plutons is distinctly higher than in Island plutonic suite plutons, suggestive of a higher pressure of crystalliza-tion than those of Island plutonic suite plutons

0 5 10 15 20 150 160 170 180 190 200 210 Sedimentary Basins Age (Ma) 0 2 4 6 8 10 12 Vancouver Island

West Coast Complex UPb Island Plutonic Suite UPb Island Plutonic Suite Ar

0 1 2 3 4 5 6 7 8 Alaska West Talkeetna East Talkeetna Alaska Penin. Chugach

Figure 5. Histograms of age compilations for (A) Jurassic arc intrusives on Vancouver Island, (B) Jurassic arc intrusives in southern Alaska, and (C) detrital zircons in post-Jurassic sedimen-tary units (Nanaimo Group and Longarm For-mation) outboard of, or onlapping, Wrangellia. Note the age similarities for intrusives of the West Coast complex and Chugach mountains, and Island plutonic suite and the Talkeetna Mountains–Alaska Peninsula, respectively. Also notable is the almost complete lack of detrital zircons within older ages for the West Coast complex (older than 180 Ma) in post-Jurassic sedimentary units. Data sources: this study; Breitsprecher and Mortensen (2004); DeBari et al. (1999a); Fecova et al. (2008); and Rioux et al. (2007, 2010). SiO2 wt% 0 2 4 6 8 10 12 14 40 45 50 55 60 65 70 75 80

West Coast Complex Bonanza Volcs. Island Plutonic Suite

MgO wt%

Figure 6. Comparison of major-element abun-dances in igneous rocks of the Bonanza arc. Ultramafi c rocks from the Port Renfrew area are omitted to eliminate bias. Data sources: Carson (1973); DeBari et al. (1999a); Larocque and Canil (2010); Massey (1992a, 1992b, 1992c). 0.1 1 10 100 1000 Ba Th K Nb Ta La Ce Sr Nd P Sm Zr Ti Y Yb sample/PUM WCC BON 0.1 1 10 100 1000 Ba Th K Nb Ta La Ce Sr Nd P Sm Zr Ti Y Yb sample/PUM WCC cumulates IPS

A

B

Figure 7. Trace-element patterns for igneous rocks of the Bonanza arc normalized to primitive upper mantle (PUM) of McDonough and Sun (1995). (A) Patterns with positive Zr anomalies and no Sr anomalies distinguish the Island plutonic suite (IPS) from the West Coast complex (WCC) and its mafi c/ultramafi c cumulates. (B) Nearly identical trace-element patterns (positive Sr anomalies) are observed for the Bonanza (BON) volcanics and the West Coast complex. Data sources: DeBari et al. (1999a) and Larocque and Canil (2010).

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(Fig. 8). Thus, one reason for unfoliated Island plutonic suite plutons is that temperatures suf-fi cient to enhance synintrusive deformation existed only within the warmer middle crust of the Bonanza arc, comprised of what is now the West Coast complex.

In the Victoria area, the West Coast com-plex was traditionally divided into separate units as the massive “Wark diorite” and foli-ated “Colquitz gneiss” (Muller, 1983). Both of these units are broadly diorite and can occur in the same outcrop. Rocks of the West Coast complex having a strong fabric (e.g., DC0507; Figs. 3B and 3F) are the same age as those that are massive with no foliation (DC0637). These observations require that strain during intrusion has been focused into other regions causing a gneissosity, and leaving some parts of the crust with no fabric. The strain partition-ing occurred on all scales durpartition-ing intrusion (Fig. 3B; see also Isachsen, 1987).

Leucotonalites crosscut less-evolved diorites that are concordant or comingled with ultra-mafi c rocks at Lens Creek (JL06110; Figs. 3A and 3B). The leucotonalites and diorites here have nearly identical U-Pb ages within the same outcrop (Fig. 4), thus providing a minimum age for the ultramafi c rocks of 174 Ma. As both Island plutonic suite and West Coast complex plutons overlap ages inferred for Bonanza arc volcanic rocks, it is evident that all parts of the crust were magmatically active during construc-tion of the Bonanza arc.

Magmatic Flux Rates

Jurassic plutons cover an area of ~11,000

km2_{on Vancouver Island (Fig. 1). Constraints}

from stratigraphy, metamorphic grade, and Al-in-hornblende barometry show that the West Coast complex and Island plutonic suite com-prise ~12 km in total of plutonic crust emplaced into supracrustal rocks of Wrangellia (Canil et al., 2010). If magmatism occurred over a

28 m.y. interval (Fig. 5), the 1.32 × 105_km3_of

arc plutonic magma was emplaced at a rate of

0.004 km3_yr–1_{, which is within uncertainty of}

other estimates for large batholiths of the Sierra

Nevada and elsewhere (0.001 km3_yr–1_{; Davis et}

al., 2011; Miller et al., 2011). This emplacement rate is consistent with incremental emplacement of arc crust by plutons and explains the lack of any obvious contacts between different units of the West Coast complex and Island plutonic suite in the fi eld (Glazner et al., 2004).

Arc Polarity and Relation to the Talkeetna Arc in Alaska

Figure 9A shows the U-Pb ages for West Coast complex and Island plutonic suite samples plotted versus distance eastward from the west-ernmost terrane-bounding faults in Wrangellia on Vancouver Island (the San Juan and West Coast fault). The latter reference point was used to account for sinistral movement along the San Juan fault, which offsets units of the West Coast complex in southern Vancouver Island (Fig. 1). Plutons younger than 170 Ma only occur in the east. As noted earlier by Debari et al. (1999), an age polarity is weak, but it matches that of the Talkeetna arc in south-central Alaska (Fig. 5). In Alaska, plutons in the Chugach mountains are coeval with the ages found in the West Coast complex plutons. Plutons of the Talkeetna moun-tains, north of the Chugach, are coeval with the Island plutonic suite intrusions. The eastward (landward)-younging age polarity that charac-terizes intrusions of the Bonanza arc is weak, but it matches that of the northward (landward)-younging trend exhibited by plutons of the Tal-keetna arc over a similar age range. Plutons in the Haida Gwaii (Queen Charlotte) Islands also match the ages of the Island plutonic suite, but there is insuffi cient sample coverage to recog-nize any pattern in polarity there (Fig. 9A).

There are some notable differences between the Talkeetna and Bonanza arcs. The Bonanza arc was built upon preexisting Devonian to Triassic supracrustal rocks, but no evidence of a pre-Jurassic basement is recognized in the Talkeetna arc (DeBari et al., 1999). The latter

also shows a shift in ε_Nd isotopes in intrusive

rocks to lower, more-evolved values to the north

(landward) (Rioux et al., 2010), whereas no such trend is observed for limited data of the Bonanza arc upper-crustal rocks (Fig. 9B). The

εNd values in plutons of the West Coast complex

show no extensive involvement of preexisting basement, whereas there is a trend of

decreas-ing ε_Nd westward for upper-crustal components

(Bonanza volcanics, Island plutonic suite) in the Bonanza arc, suggesting some assimilation or contamination in the upper crust to the west.

The similar age range and polarity between the Talkeetna and Bonanza arcs are easiest to explain if these two segments were part of a single, origi-nally continuous magmatic arc. The plate setting may have varied from a simple island arc in one portion (Talkeetna) trending along strike onto an arc built upon a preexisting Devonian–Triassic arc–oceanic plateau (Bonanza).

Correlation of the Bonanza and Talkeetna arcs has been suggested before, but tectonic interpretations of the polarity vary. One model

0 1 2 3 4 5 6 7 8 0 20 40 60 80 100 120 140 160 Epsilon Nd Distance eastward (km) Chugach Ta lkeetna QCI 160 170 180 190 200

West Coast Complex Island Plutonic Suite

Age (Ma)

A

B

4 6 8 10 12 14 0 0.5 1 1.5 2 Amphiboles

West Coast Complex Island Plutonic Suite

300 MPa, 650 °C Al2

O3

wt%

TiO2 wt%

Figure 8. Plot of the Al

2O3 and TiO2 (wt%) of amphiboles from West Coast complex and Island plutonic suite (data from Canil et al.,

2010). Note higher Al₂O₃ at a given TiO₂ in

amphibole in rocks from West Coast complex, consistent with a higher pressure of crystalliza-tion at a given temperature. Dashed line shows

the minimum Al₂O₃ in amphibole in a basaltic

composition for pressures and temperatures above 300 MPa and 650 °C, respectively (after Ernst and Liu, 1998).

Figure 9. Summary of (A) U-Pb ages and (B) ε_Nd

values for igneous rocks of the Bonanza arc plot-ted relative to the distance eastward from the westward terrane-bounding fault in Wrangellia. Also shown is the age range for arc plutons in Alaska and the Queen Charlotte Islands (QCI). Samples plotted in A and B are not identical or from the same studies. Data sources in A are as in Figure 5 as well as Anderson and Reichen-bach (1991); data sources in B are Andrew et al. (1991) and Larocque (2008).

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CANIL ET AL.

shows the age progression northward in the Tal-keetna as due to north-dipping subduction and removal of the forearc to a south-facing arc by tectonic erosion, at a rate observed in modern

subduction zones of ~3 mm yr–1_{, and consuming}

between 70–130 km of forearc over the course of the arc’s magmatic history, with a matching northward migration of arc magmatism (Clift et al., 2005). As in the case of Talkeetna arc, tec-tonic erosion of the forearc to west of an east-dipping, west-facing Bonanza arc could explain eastward migration of the arc plutons eastward from 193 to 165 Ma. An alternative interpreta-tion of the age polarity is that the Bonanza- Tal-keetna arc faced east to north (Reed et al., 1983), and that the decreasing ages to the east and north resulted from trench rollback and the related accretion of material to the upper plate at the trench (Fig. 10). Signifi cant post-Jurassic large-scale, margin-parallel translation of parts of Wrangellia and adjacent terranes, and removal of any forearc assemblage along strike make testing of these two disparate polarity models diffi cult. The stacking of the Bonanza crust along west-dipping, east-verging mylonite zones, however, is consistent with a model of accretionary growth and eastward arc migration within an eastward-facing arc (e.g., Johnston, 2008).

Exhumation of Middle Crust in the Bonanza Arc

In the Victoria and Port Renfrew areas, com-ponents of the West Coast complex form tabu-lar sheets bound by high-strain, brittle-ductile mylonite zones characterized by top-to-the-east kinematic indicators (Fig. 3F). The mylonites are overprinted by a later brittle deformation. A similar structural style is recognized to the north in the Nootka Sound region (Fecova et al., 2008), but there the mylonite zones are characterized by subhorizontal lineations and kinematic indicators indicative of strike-slip transport. These features suggest the plutons that make up the West Coast complex may have intruded one another as sheets, with massive textures, as has been proposed for the Island plutonic suite (Canil et al., 2010), but that the sheets have been subsequently stacked along high-strain discontinuities, induced at high temperatures in the middle crust. Postintrusive structural stacking explains the inconspicuous contacts between the different intrusive phases, and juxtaposition of middle- and upper-crustal components of the Bonanza arc.

There are no Late Jurassic sedimentary sequences deposited that show an immedi-ate record of denudation of the Bonanza arc. The oldest sandstones of the Late Cretaceous Nanaimo Group, exposed along eastern

Van-couver Island, and in correlative sandstones of the Longarm Formation, found east of and offshore of the Haida Gwaii (Queen Charlotte) Islands, largely originated from the Coast belt in the east, and the Wrangellia terrane, which at the time was an unknown distance to the west (Mus-tard, 1994). Detrital zircons in the Nanaimo and Longarm sedimentary packages have no popu-lations older than 180 Ma. The latter ages are distinct to the westernmost portions of West Coast complex in the Bonanza arc (Fig. 9A), suggesting that the deeper and older (older than 180 Ma) parts of the middle crust of the arc were not yet uplifted and dissected until post–Late Cretaceous time. Indeed, fi ssion-track and other cooling ages on Vancouver Island (England et al., 1997) suggest that the Bonanza arc was not fully uplifted and dissected until Eocene time (Fig. 5), likely commensurate with the forma-tion of the Cowichan fold-and-thrust belt, itself related to an outboard collision of the Crescent terrane (Johnston and Acton, 2003).

SUMMARY

Our results place some constraints on the nature and temporal evolution of the middle crust within an active arc. We show that there is only slight or subtle chemical, petrological, and geochronological distinction between the upper-crustal Island plutonic suite intrusions and the midcrustal West Coast complex in the

Juras-sic Bonanza arc. The middle crust is the feed-ing zone, active at the same time, for intrusions and extrusions that grow the upper crust, but by existing at higher temperatures, it is weaker and focuses more strain. Indeed, the distinguish-ing factor between the West Coast complex and Island Plutonic suite is strain: Middle-crustal rocks experienced signifi cant synmagmatic strain partitioning, whereas upper-crustal intru-sions largely lack evidence of synintrusive strain. Synintrusive fabric development in the middle crust may render the middle and lower crust prone to removal, and explain the preferential preservation of massive, unfoliated upper crust. ACKNOWLEDGMENTS

We sincerely thank G. Pearson for introducing us to the Port Renfrew area, and P. Hetherington for his support. D. Marshall kindly directed us to some unpublished ages. We appreciate reviews by M. Rioux and an anonymous reviewer. Research funding was provided by Emeralds Field Resources, Geoscience BC, and Natural Sciences and Engineering Research Council (NSERC) of Canada (to Canil).

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